Synthesis and Structural Design of Graphene, Silicon and Silicon‐Based Materials Including Incorporation of Graphene as Anode to Improve Electrochemical Performance in Lithium‐Ion Batteries

Silicon emerges as a candidate for advancing lithium ion batteries with important roles in various applications ranging from portable electronics to electric vehicles. However, despite its theoretical capacities silicon faces challenges such as unstable cycling and limited rate performance. This thorough review examines developments in improving the electrochemical performance of silicon and graphene within the context of lithium ion batteries. The focus lies on strategies for designing and synthesizing composite materials that incorporate silicon particularly when combined with graphene. Structural aspects like particle size, morphology and porosity are carefully optimized to harness the potential of silicon based anodes and graphene. The review highlights the effects resulting from these tailored design approaches, including key factors such as capacity retention, cycling stability and rate capability of the resulting anode materials. By exploring these design paradigms this review offers a comprehensive perspective on the transformative capabilities of silicon, graphene and silicon/graphene composites. It does not highlights recent advancements but also outlines future directions for innovation and practical applications. This compilation of progress contributes to the understanding of how silicon based anodes, in lithium ion batteries have evolved from small‐scale implementations to catalyzing advancements in energy utilization.


Introduction
Silicon (Si) holds a prominent place in the periodic table as the second most abundant element on earth, making up ≈28% of the planet's crust.Its abundance and exceptional physical properties have rendered it indispensable in various fields, particularly in the realm of modern electrical engineering.While Si has undoubtedly played a transformative role in microelectronics and digital technology, the growing demand for energy storage solutions in the face of climate change and the transition to renewable energy sources has prompted a shift in focus.In recent years, researchers and engineers have been exploring alternative applications for Si beyond its traditional use in electronic devices.One such groundbreaking prospect lies in the realm of energy storage systems, particularly in Li-ion batteries (LIBs).
The development of LIBs in recent years has transformed energy storage technology, having an impact on many industries including portable electronic gadgets, electric vehicles, and large-scale storage systems.Because of their large capacity, lengthy lifespans, and environmental friendliness, LIBs have been widely adopted.However, there is an urgent need to create next-generation LIBs with even better energy and power densities due to the fast rising demands on the energy market.Due to their small theoretical capacity (372 mAh g −1 ) and relatively low energy density (150 Wh kg −1 ), commonly utilized anode materials like graphite fall short of fulfilling these requirements. [1,2][5][6] Li 4 Ti 5 O 12 stands out for its exceptional cycling stability and prolonged operational lifespan due to minimal volume fluctuations during lithium insertion and extraction processes.However, its relatively low electronic conductivity results in significant polarization losses at higher cycling rates, leading to suboptimal rate performance. [7]On the other hand, transition metal oxides, offer higher operating voltages and superior safety compared to graphite and lithium titanate anodes.Additionally, they exhibit diverse chemical valence states and morphological attributes. [8,9]Despite these advantages, the significant volume variations during cycling and resultant stability issues are notable drawbacks.In contrast, transition metal sulfides have garnered increasing attention for their high theoretical capacity, cost-effectiveness, and environmental friendliness. [10,11]hese sulfides typically feature a voltage platform surpassing 1.0 V compared to graphite anodes, mitigating the risk of lithium dendrite formation and enhancing safety.Moreover, they boast substantially higher theoretical specific capacity and electrical conductivity than Li 4 Ti 5 O 12 .However, the pronounced structural reorganization during delithiation induces significant volume changes in transition metal sulfides, leading to material pulverization and detachment from the current collector.Moreover, active material loss and electrolyte side reactions further compromise performance.Additionally, the inadequate ionic and electronic conductivity of transition metal sulfides, particularly in the discharged state (Li 2 S), hinders their high-power capabilities.[14][15] One compound that has shown great promise as a graphite substitute is silicon. [16]ue to the fact that one Si atom can form a connection with 3.75 lithium ions (Li 3 .75 Si), Si has a theoretical maximum capacity of 3589 mAh g −1 at room temperature, which is ten times more than graphite.This exceptional attraction of Si as an anode material stems from this property. [17][20] Significantly, Si is extensively available in nature as silicate minerals, rice husks, straw, bamboo stalks, and sugar cane.Utilizing Si resources for energy storage systems has significant benefits for developing next-generation LIB electrodes and promoting environmentally friendly sustainable growth.
Despite enormous potential of Si, a number of inborn problems prevent it from being used in commercial applications.The significant volume expansion (>300%) that Si experiences during lithiation, which causes Si powder to develop as a result of mechanical stress buildup, is a particularly noteworthy problem. [21]urthermore, volume growth leads to the separation of the conductive material on the active Si surface, which has a negative impact on the electrochemical performance. [22]A solid-electrolyte interphase (SEI) is created on the surface of the silicon electrode during the initial lithiation step as a result of electrolyte breakdown. [23]While allowing lithium-ion conduction, SEI limits electron flow, reducing electrolyte consumption and enhancing the cycle performance of the LIBs. [24]However, the volume expansion of Si contributes to the production of an unstable SEI that is prone to fracturing and re-forming on the Si surface.This unstable SEI causes the electrolyte to eventually run out since it consumes lithium ions continuously and has a lower initial Columbic efficiency (ICE). [25]Si is a semiconductor material with limited conductivity (10 −3 S.cm −1 at 25°C), faces conductivity challenges due to its stable crystal structure and occasional unstable configurations. [26]esearchers have explored various strategies to address these above-mentioned challenges.35] Graphene, a remarkable 2D material composed of a single layer of carbon atoms arranged in a hexagonal lattice structure, has garnered considerable attention in various scientific fields due to its extraordinary properties. [36]Graphene have been incorporated into anode materials to serve as an ideal conductive matrix for enhancing the performance of anode materials in LIBs.When combined with Si, Si/G composites emerge as promising solutions to overcome the limitations of Si as a standalone anode material.Integrating Si into graphene allows these composites to capitalize on the synergetic advantages offered by both materials.[39] This feature mitigates the detrimental effects of Si's volume changes, ultimately enhancing the cycling stability and rate capability of Si/G composites, making them highly desirable for high-performance LIBs. [40,41][44] Additionally, Si/graphite composites, where Si is combined with a graphite buffer layer, have demonstrated promising results in improving discharge capacity. [45]The endeavor to design Si/G composites with structural stability and efficient utilization of the graphene buffer layer has garnered significant attention and led to enhanced discharge capacity.These advancements in Si/G composite materials hold great potential for further enhancing the performance of next-generation LIBs. [46]any articles discussed the use of Si-based anodes, each focusing on different aspects of this promising technology.Some articles delve into the challenges and potential of Si anodes while others explore the combination of Si with carbon based materials like graphene, graphite or general carbon structures highlighting the benefits of these composites.Additionally certain reviews specifically focus on the methods used to fabricate anodes and provide a comprehensive overview of the various approaches found in literature.On the hand, there are alternative reviews that do not go into intricate details about fabrication methods but instead analyze the electrochemical performances in depth.49] This review aims to provide an overview of recent developments in Si-based anode materials for LIBs, covering Si architectural design, Si alloy structure management, SiO x and composite structures, surface engineering impacts, pre-lithiation, and binders effects on the electrochemical performance of Si anode materials.The review will also go through approaches for characterization of Si-based materials, structural control of Si-graphene composites, mechanisms for SEI production.This Reproduced with permission. [21]Copyright 1999-2023 John Wiley & Sons.
review attempts to shed light on the capacity retention, cycling stability, and rate capability of Si-based anode components by exploring these methods, offering helpful insights into the creation of high-performance Si-based anode materials for microbatteries and other applications.

Si Anode for LIBs
Among all the alloy anodes, Si is considered as one of the most promising materials that could enhance the capacity storage of the next-generation LIBs.The electrochemical reaction of lithium with silicon form four alloys of Li x Si y (Li 12 Si 7 , Li 7 Si 3 , Li 13 Si 4 , and Li 22 Si 5 ), theoretically resulting in different voltage-plateaus during the galvanostatic voltage curve. [50]These transformations occur only at high temperature, with notably the Li 22 Si 5 phase is particularly noteworthy for its potential capacity of 4200 mAh g −1 .At room temperature, the crystalline silicon transform into amorphous lithium-silicon alloy during the first lithiation step.Only a single long lower plateau appears in the voltage curve instead of multi-plateau. [51,52]The complete lithiation of silicon form a metastable crystalline Li 15 Si 4 phase at potential lower than 50 mV versus lithium. [53,54]and tend to transform back to the amorphous form during the charging process and is partially transformed back to the amorphous form during charging.The Li 15 Si 4 phase offer a theoretical capacity of 3589 mAh g −1 that is tenfold greater than that of conventional graphite.In addition, an ultimate volumetric capacity of 9786 mAh cm −3 higher than that of lithium metal itself. [55]Also, Si anodes exhibit a lithiation potential at 0.5 V vs. Li/Li + , which is very convenient for stable open-circuit-voltage cell that prevents undesirable Li plating and dendrite formation on anode (0.05 V vs. Li/Li + for conventional graphite anode). [55,56]Finally, Si is non-toxic, very abundant in earth and benefit from maturity of processes and methods that has been actively developed these last decades, as for the production of microelectronics and solar cells.
Until today, tremendous efforts are still needed for the industrialization of silicon-based anodes in LIBs.There are two main challenges associated with the use Si.The first one is related to the huge mechanical strain undergone by Si upon lithium uptake (>300%) (Figure 1). [21]This causes i) crushing and pulverization of Si anode and results in the loss of electric contact be-tween the current collector and Si anode.ii) the repetitive formation/degradation of stable solid electrolyte interphase (SEI) layer on Si upon cycling which than consume a significant amount of Li ions from the electrolyte (Figure 2). [23,24]iii) Delamination causes separation within the Si anode, leading to decreased stability and ion transport, ultimately compromising the performance of the lithium-ion battery. [57]The second challenge is the limited electronic conductivity of Si (10 −5 to 10 −3 S cm −1 ) and the ionic conductivity of Li + (10 −14 to 10 −13 cm 2 s −1 ) is another limiting factors that impact the electrochemical kinetics. [26,58]These abovementioned phenomenological effects are the main contribution of the low columbic efficiency and the fast capacity fading observed after only few cycles, thus limiting the Si based anodes battery rate performances.
Several methods have been developed to reduce capacitance fading during cycling and the irreversible ability of Si-based anode particles in the first cycle.Downsizing the Si to nanometer scale has been proved as an effective method for the purpose.Si can be used in different dimensionalities ranging from nanoparticles (zero-dimension), nanowires or nanotubes or micro-nanoporous structures (one-dimension) and nanofilms (two-dimension). [60]Each of these nanostructures show a critical dimension below which the anode can perform without cracks.[63][64][65][66][67][68][69] The nanostructures can thus clearly alleviate the strain in Si anodes but they can also enhance the fast charge transfer kinetics because the conduction paths for Li + and electrons is shorten.Finally, the high surface to volume ratio of nanostructures tends to increase the effective surface area at the interface between the Si anode and the electrolyte, thus reducing the internal resistance of the battery.
For instance, Si NPs can be produced by ball milling of Si powders has retained much attention as a cheap process in comparison to a high cost synthesis process such as by chemical vapor deposition (CVD) of silane SiH 4 precursor (2 euros kg −1 for ball milling in comparison to 100 euros kg −1 for CVD). [70,71]From industrial point of view, the use of nanoparticles still pose several challenges because of volatility, inhalation hazard and explosion risks.In attempt to still get benefits from nano-domains, M. Gauthier et al. [72] proposed to ball-mill micron powders to submicronic particles containing crystalline domain of ≈10 nm.By using submicron particles, the toxicity and the handling risks can be alleviated.The electrodes were made out of ball-milled silicon particles in a mixture of binders exhibit an irreversible capacitance of 1170 mAh g −1 after more than 600 cycles and with a columbic efficiency above 99% (Figure 3b).These high electrochemical performances were attributed to the presence of nanocrystalline domain inside the submicron particles and the large concentration of grain boundaries that fasten the Li + diffusion pathways in the Si anode (Figure 3a).Finally, the authors claim that milled Si particles offer higher packing density compared to Si NPs, which will be beneficial for achieving higher volumetric energy density.
Another strategy to circumvent the effect of volume change in Si nanoparticle is the use of Si-C composite material in the form of core-shell, hollow-shell and yolk-shell structures. [73]As an example of these composite anodes, A. Magasinski et al. [74] developed an elegant strategy to make porous Si-C material using  [59] Copyright 2020 Elsevier B.V. a granulation bottom-up assembly.Here, annealed carbon-black dendritic particles are coated by amorphous Si NPs (10 to 30 nm in diameter) using CVD of silane (SiH 4 ) at low pressure (Figure 4a).Followed by simultaneous granulation during carbon deposition step, which assembled initial nanostructures into rigid spheres with an average diameter of 26 μm, and containing open interconnected internal channels.Anodes made out of these porous Si-C composite anode exhibits exceptional electrochemical performances such as a specific capacity of 1950 mAh g −1 at C/20 (Figure 4b) (six times higher than that of conventional graphitic anodes) and a volumetric capacity of 1270 mAh cm −3 at C/20 (620 mAh cm −3 for graphitic anodes).
Si Nanowire (Si NWs) battery electrodes offer a solution to mitigate significant volume fluctuations and minimize capacitance decline.They have the capacity to absorb substantial strain without undergoing pulverization.Additionally, these electrodes establish effective electronic connectivity and conduction while enabling rapid lithium insertion due to their short lithium insertion distances, with theoretical capacity ≈3000 mAh g −1 . [75]Ge et al. [76] by direct etching of boron-doped silicon wafers they fabricated Si NWs which proved a very good long term electrochemical performance, it showed a capacity ≈2000 mAh g −1 over 250 cycles at 2 A g −1 due to the large pore size and high porosity for porous silicon.Therefore, this synthesis allowed reaching high capacity by applying high current rates.Liu et al. [77] they achieved Si NWs carbon textiles anodes by coating the carbon textiles with Si NWs that already dissolved in ethanol to be homogeneous suspension, it showed a high capacity ≈2950 and 1500 mAh g −1 over 200 cycles at 250 and 1.25 μA g −1 ,respectively (Figure 5).This structure provides several unique features.The 1D Si NWs decrease the exchange resistance for Li ions between Si NWs and electrolyte, due to many Si NWs attached to the C substrate it can obtain outstanding electronic conductivity.
2D-Si electrodes such as a single crystal Si monolayer named silicene is another exploratory path for anode electrodes, owing to its unique layered structure.The associated volume-change rate of 13% for single layer Si could alleviate the problems of cracks and repeated formation of SEI undergone on bulk silicon Cycling performance of milled millimetric Si and nanosized Si-based electrodes, showing discharge capacities and coulombic efficiencies.Reproduced with permission. [72]Copyright 2023, Royal Society of Chemistry.to the theoretical capacity of graphite.Reproduced with permission. [74]opyright 2010, Nature Materials.
electrodes.The high surface to volume ratio in 2D-Si is beneficial for enhancing the interfacial charge transfer reactions and promoting fast Li + diffusion that occur predominantly on the surface, unlike the bulk Si where Li + storage occurs mainly in the volume of Si.Yet, the benefits of having high specific surface area (SSA) in 2D-Si are at the expense of the fast oxidation of silicon, pushing forward scientists to develop new methods to passivate the extreme surface of these 2D-Si nanostructures. [78]inally, the allotropic structure of 2D-Si hybridized in sp 3 make the 2D-Si very difficult to exfoliate from bulk Si as for what it have been done for graphene material. [79]82][83][84][85] Silicon nanorods (Si NRs) have garnered considerable attention in the realm of anode materials for LIBs.Hieu et al. [86] focused on addressing the limitations of using silicon as an anode material for LIBs.They employed the metal-assisted chemical etching (MACE) method to create stress-dissipative Si NRs on a copper substrate, enhancing the anode's performance.The study revealed that the modified Si NRs exhibited superior cycling stability and improved electrochemical performance (2445 mAh g −1 after 25 cycles at 0.25 mA cm −2 ).This advancement was attributed to the optimal void structure of the silicon anode, enabling better stress absorption and providing free vacancies.Also, Zhou et al. [87] focused on the synthesis of mesoporous Si NRs using magnesiothermic reduction and their application as anode materials for LIBs.The researchers demonstrated the successful fabrication of the novel mesoporous Si NRs and characterized their structure and properties using various analytical techniques.The electrochemical testing of the mesoporous Si NRs as an anode material revealed promising results, showing su-perior electrochemical performance (1038 mAh g −1 after 170 at 0.2 A g −1 ) compared to porous silicon networks and bulk silicon.
Yu et al. [80] reported for the first time on the synthesis of 2D Si using DC arc discharge using bulk Si as the raw material (anode) and tungsten rod (cathode) under an atmosphere mixture of hydrogen and argon.They noticed that the use of hydrogen alone and its ionization to H + promote the evaporation of bulk Si into atomic state, which they combine isotropically to form Si NPs.The introduction of Ar + ions together with H + promote the growth of 2D Si nanosheets (Si NSs) and nanoribbons instead of nanoparticles (Figure 6a).In their collision with Si atoms, Ar+ ions tend to minimize the total system energy thus impeding the growth of Si in the <111> direction and resulting in the growth of plate-like 2D Si NSs.Furthermore, these produced 2D Si NSs were mixed with binding agents and their electrochemical performances were investigated for use as anode electrodes.The charge capacity shows a value of 441.7 mAh g −1 after 40 cycles at 100 mA g −1 with columbic efficiencies above 94%, indicating excellent reversible lithium storage capabilities (Figure 6b).Moreover, these 2D Si electrodes show good capacity retention and reversibility, demonstrating effective strain buffering capabilities during lithium intercalation.
Liu et al. [88] proposed a new scalable synthesis to produce fewlayer silicene using liquid oxidation followed by exfoliation of calcium disilicide (CaSi 2 ) in the presence of iodine (I 2 ) and acetonitrile (CH 3 CN).The iodine is used to oxidize the (Si 2n ) 2n− layers of the CaSi 2 into neutral Si 2n layer, and the CH 3 CN play the role of the solvent for iodine and the reaction by-product namely the CaI 2 (Figure 7a).When explored as anode electrode, these few-layer silicene material exhibits a high reversible discharge capacity of 721 mAh g −1 at 100 mA g −1 after 100 cycles (Figure 7b).Interestingly, the silicene nanosheets show excellent stability with capacity retention surprisingly increased after 1800 cycles at 1 A g −1 .This can be related to further exfoliation of fewlayer silicene into monolayer silicene during continuous lithium insertion/ desertion cycles.
Since the creation of the unsteady SEI remains a significant concern.Wu et al. [89] a new type of nanomaterial, double-walled Si-SiO x nanotubes (DWSi NTs), was introduced to address the issue of SEI formation in LIBs.These DWSi NTs were created using an innovative electrospun nanofiber templating method, resulting in continuous Si NTs coated with an outer layer of oxide (Figure 8a).The electrochemical performance of these DWSi NTs surpassed that of Si NTs lacking an oxide layer and Si NWs, exhibiting high specific charge capacities (2970 mAh g −1 at C/5, 1000 mAh g −1 at 12 C), remarkable cycle life (6000 cycles with 88% capacity retention at 12 C), and exceptional rate capability (up to 20 C).These enhanced electrochemical features were attributed to the distinct structure of the Si NTs.The rigid SiO x coating on the outer surface prevented Si expansion while facilitating the passage of Li + ions.Additionally, the high aspect ratio of the continuous NTs prevented the wetting of the electrolyte inside the NTs, ensuring a stable SEI formation.This was confirmed by SEM images and impedance measurements, highlighting the stability of the SEI even after 2000 cycles (Figure 8b,c).
Yao et al. [90] detailed the production of interconnected silicon hollow spheres (Si HSs) via the CVD technique, using hard SiO 2 spheres as templates.These Si HSs, featuring an inner radius of ≈175 nm and an outer radius of ≈200 nm, demonstrated Reproduced with permission of. [77]Copyright 2023, Springer Nature Limited.
exceptional electrochemical capabilities.The study observed an impressive initial reversible capacity of 2725 mAh g −1 at a rate of 0.1 C, along with a discharge capacity of 1420 mAh g −1 at 0.5 C over 700 cycles.Unfortunately, the entire CVD process, which involves the use of toxic SiH4 and SiO 2 spheres, is impractical for large-scale production.
The template method uses three steps to form 2D-Si.The first step consists in filling a sacrificial template made from specific materials such as Mg with SiO 2 .Next, a magnesiothermal reduction process is realized at temperature of 500-950 °C, much lower than the temperature used in conventional carbothermal reduction process requiring temperatures above 2000 °C.The magne-Figure 6. a) Formation mechanism schematic for Si NSs and silicon spherical NPs using the dc arc discharge method under hydrogen and hydrogen/argon atmospheres, respectively.b) Cycling performance of the anode at a steady current density of 100 milliamps per gram (mA g −1 ) within the voltage range of 0.01 to 1.2 V. Additionally, the reversible capacity of graphene under the same experimental conditions.Reproduced with permission. [80]Copyright 2023, Royal Society of Chemistry.
siothermal reduction step forms a composite made of silicon and magnesia (MgO).The last is finally etched away in hydrochloric acid (HCl) thus leaving a silicon replica with high SSA.The final morphology of porous 2D-Si (SSA and pore volumes) can be optimized by adjusting magnesiothermal reduction process parameters such as the temperature, ramp time, total time of chemical reaction, molar ratio of reactants.For better process control, a heat scavenger additive may be used together with Mg such NaCl.Indeed, the enthalpy of magnesiothermal reduction process is exothermic.The excess of heat generated render i) the control of temperature inaccurate, ii) the decrease of the yield by the formation of side-reaction material such as magnesium silicide (Mg 2 Si), iii) the increase of Si particles size through sintering. [91]ing et al. [92] reported on the fabrication of porous silicon membrane from rice husk (Figure 9a).To retrieve high quality silica material from the rice husk, silica powder obtained from the calcined rice husk using two protocols including HClleaching (h-silica) and water-rinsing (w-silica), obvious different morphologies for these two materials were observed.The authors showed that h-silica was composed of homogenous interconnected nanosized particles with irregular shapes and a diameter of 25 nm, whereas w-silica was composed of particles that were more than 10 times larger than those in h-silica with similar shapes and arrangements.The anode material can retain a considerably high reversible capacity of 1220.2 mAh g −1 at a specific discharge-charge current of 1000 mA g −1 after 100 cycles (Figure 9b,c).The magnesiothermic reduction process is carried out at a temperature of 750 °C for 2 h.Sodium alginate is used as a binder to improve the electrochemical performance of the Si NPs.
Zhang et al. [93] presented a novel and cost-effective method for converting high-purity silicon sawdust waste from the photovoltaic industry into silicon nano-plates (BM Si) suitable for use as anode material in LIBs.The ball milling process generates a thin SiO 2 layer on the BM Si, which helps buffer the volume changes during cycling.BM Si exhibits a high initial capacity of 2196 mAh g −1 and maintains a stable capacity of 1480 mAh g −1 after 100 cycles at 100 mA g −1 , outperforming commercial Si NPs.Additionally, BM Si shows high capacities at various current densities, recovering to 1488 mAh g −1 when the current density returns to 0.2 A g −1 .The approach offers a promising solution for sustainable solar energy development and LIBs advancement, Reproduced with permission. [88]Copyright 1999-2023, John Wiley & Sons.
providing economic and environmental benefits through effective Si waste recycling.
In Table 1, you can find a comparison of the different Si anodes mentioned.It provides information about fabrication method, particle size and their electrochemical performance.Each type of anode has its advantages, which have been explained before to understand why researchers have been working on them and publishing their findings.When we analyze the table, we can see that Si NWs synthesized using methods exhibit excellent electrochemical performance.They show consistent capacity throughout many cycles.On the hand, other materials like Si NPs or thin films show variations in performance depending on how they are fabricated.Notably, most other anode materials experience a loss of ≈50% of their capacity after being cycled for a long time.

Graphene as Anode for LIBs
Graphene has been clearly identified as one of the candidate to compete with the dominating graphitic material for manufacturing anode electrodes.Graphene is a 2D material that is composed of a single or few atomic layers of sp 2 bonded hexagonal carbon.[104] a remarkable surface area of 2630 m 2 g −1 and optical absorption of only 2.3%. [105,106]xceptional attributes of graphene, including its high electrical conductivity, large specific surface area, and remarkable mechanical strength, position it as a promising candidate for various Reproduced with permission. [89]Copyright 2023, Springer Nature Limited.The rice husk (waste) is used as the source of biosilica generated in rice by the unique biological process from silicic acid.3) This biosilica was transformed by a simple process into mesoporous silica, which is then 4) reduced by a magnesiothermic reaction into mesoporous Si and 5) used as a high-performance anode material for Li-ion batteries.b,c) Cycling and rate performance comparison for silicon anodes.Reproduced with permission. [92]Copyright 2023, Royal Society of Chemistry.
technological applications.In the context of LIBs, these properties become particularly relevant.For instance, the high electrical conductivity of graphene can be harnessed to improve the conductivity of electrodes within LIBs, facilitating faster charging and discharging processes. [107]Moreover, its large specific surface area enables better electrode-active material loading, thereby enhancing the battery's energy storage capacity. [108]urthermore, the exceptional mechanical strength of graphene can be utilized to reinforce the structure of LIBs, improving their durability and resistance to mechanical stress during operation and handling. [109]Additionally, the high thermal conductivity of graphene holds the potential to address heat dissipation challenges in LIBs, ensuring their stability and safety during usage. [110]Overall, the multifaceted properties of graphene offer an avenue for the development of high-performance LIBs with improved efficiency, longevity, and safety.
Notably, compared to traditional graphite, graphene exhibits a significantly higher theoretical capacity, the theoretical capacity of graphene is approximately twice that of graphite, with values ≈744 mAh g −1 for graphene and 372 mAh g −1 for graphite, offering the potential for greater energy storage capabilities in LIBs. [111]Additionally, its exceptional diffusion coefficient surpasses that of graphite, enabling faster and more efficient transport of lithium ions within the battery structure.When incorporated into anode materials, graphene serves as an outstanding conductive network, facilitating rapid electron transport throughout the electrode, thus enhancing the overall performance of the battery.Furthermore, unique structural properties of graphene allow the accommodation of volume expansion during lithiation, effectively mitigating the adverse effects of such volume changes and leading to improved cycling stability and rate capability of LIBs.Moreover, the diverse forms in which lithium can be in-corporated into graphene, including both sides and within the material's defects, offer opportunities for further exploration and optimization of LIB designs, emphasizing the versatility and potential of graphene in advancing the next generation of energy storage technologies. [112]articularly, Parvez et al. [113] introduced a rapid electrochemical process for mass-producing high-quality graphene sheets, enabling the creation of highly conductive graphene films on A4sized paper.These films were utilized in flexible supercapacitors with impressive capacitance and performance at different charge-discharge rates, highlighting the method's potential for large-scale high-quality graphene production.
Furthermore, self-heating can occur during short-circuiting or fast charging/discharging processes in LIBs, which may lead to cell rupture or raise serious safety concerns like fire and explosion.The pursuit of higher power density in practical LIB applications has made efficient heat removal an increasingly critical and challenging task.To address this, graphene, with its exceptionally high intrinsic thermal conductivity (e.g.≈5000 W m −1 K −1 for a single-layer graphene sheet at room temperature), has been incorporated into practical LIB packs to facilitate effective heat transfer.[116] This enhanced thermal conductivity efficiently restricts the temperature rise within the LIB pack, enhancing safety and performance.
Methods of producing high quality graphene have been extensively explored to meet the requirements of anode applications in LIBs.Various techniques have been developed for the large-scale synthesis of graphene, ensuring its availability for commercial applications.CVD is one of the prominent methods for producing high-quality graphene sheets.In this process, hydrocarbons are thermally decomposed on a metallic substrate, leading to the formation of graphene layers with controlled thickness and large surface area. [117]CVD allows scalable and continuous production, making it a suitable choice for industrial applications.Rana et al. successfully produced freestanding and flexible graphene films, ≈5-8 μm thick, through the CVD method for use as a battery anode.Notably, these graphene anodes, unlike typical ones, did not require current collectors or additives (Figure 10a).They demonstrated a specific capacity of 350 mAh g −1 , closely aligning with the theoretical value of graphite, and exhibited stable cycling performance for 50 cycles, even at a high C-rate (over 2 C) (Figure 10b).When applied in flexible LIBs under a bending radius of 10 mm, the anodes maintained stable initial capacity for over 40 cycles.This unique freestanding graphene, with a thickness of 5-8 μm, showcased excellent flexibility, attributed to its thinness and the absence of a need for a metal current collector, which, while also thin, cannot store lithium ions.
Several studies are currently examining the electrochemical capabilities of porous graphene anodes created using the templateassisted method.Zhu et al. [118] devised a sacrificial template technique, a cost-effective and scalable process, for synthesizing 3D porous graphene microspheres (3DGPM).The method involved mixing a suspension of polystyrene (PS) spheres with a solution of graphene oxide (GO), followed by spray-drying and calcination to produce the black 3DGPM.Additionally, they prepared reduced graphene oxide (rGO) through a similar process, omitting the addition of PS.The evaporation of gases during the decomposition of PS led to the development of partially open pores measuring 100-200 nm the graphene shells.This material exhibited excellent electrochemical performance, maintaining a capacity of 245.8 mAh g −1 at a discharge rate of 2 A g −1 even after 500 cycles.
In a separate study, Zhang et al. [119] devised a straightforward filtration method to produce a binder-free porous graphene film (PGF).Typically, porous graphene is combined with conductive additives and an insulating binder, and subsequently coated onto copper foil to create the anode electrode.This conventional approach often fails to establish a complete conductive network b) Cell discharge capacity plotted against the cycle number, illustrating data for two distinct current rates over a span of fifty cycles.Reproduced with permission. [117]Copyright 2023, Royal Society of Chemistry.
of graphene, leading to lower power and energy densities.To address this issue, the researchers proposed the use of a binder-free film of the active material.By increasing the amount of ferric nitrate, they observed a rise in pore size from 38 to 450 nm, accompanied by a decrease in the number of pores per unit area.This outcome was attributed to the larger size of iron oxide nanoparticles formed due to the decomposition of higher quantities of ferric nitrate.The higher pore density led to the formation of a more defective PGF.Notably, at a current density of 50 A g −1 , they achieved the highest initial capacity value of 1062 mAh g −1 , which decreased to 715 mAh g −1 after 50 cycles.
Wu et al. [120] utilized an elevated gas pressure technique to create perforations on graphene sheets by rapidly heating GO during the thermal reduction process.This cost-effective, rapid, and straightforward method involved heating graphite oxide powder in a furnace under an argon atmosphere to 1100 °C at varying rates.They manually applied an ultra-high heating rate of ≈18 000 °C min −1 to produce holey graphene (HGE).Because of functional group decomposition, CO 2 was generated between adjacent layers at a significantly faster rate than it could diffuse out, leading to the generation of very high internal pressure.This process induced the formation of pores in the graphene sheets and facilitated layer exfoliation.These occurrences influenced the SSA values, revealing that the SSA values increased with higher heating rates.Additionally, as the temperature ramp rate increased, both the size and number of holes on the HGE expanded.Ultimately, pore sizes ranging from 10 to 500 nm were observed on the HGE.Notably, samples prepared using higher heating rates demonstrated superior discharge capacity at all discharge rates, with HGE displaying the highest capacity.Even at a high discharge rate of 10 A g −1 , the HGE exhibited the highest capacity value of 141 mAh g −1 .
Liang et al. [121] developed 3D HGF using a cost-effective and scalable vacuum-induced drying (VID) method, after inducing pores on GO through a wet-chemical etching process involving treatment with H 2 O 2 .Upon reduction using sodium ascorbate, they obtained a holey graphene hydrogel (HGH).Notably, the VID strategy prevented volume shrinkage and cracking, resulting in the production of lightweight HGF with exceptional mechanical properties.Compared to freeze-drying and supercritical drying, the VID technique was deemed more suitable for large-scale graphene film production.The HGF anode, with a high mass loading of 4 mg cm −2 , demonstrated a remarkable areal capacity of 5 mAh cm −2 after 2000 cycles, with a Coulombic efficiency exceeding 99.9%.In contrast, the graphene electrode exhibited only an areal capacity of 0.29 mAh cm −2 after 2000 cycles.
Yong-Jian et al. [122] employed a method involving mechanical and ultrasonic dispersion, along with various dispersants such as poly (vinyl pyrrolidone), sodium lignin sulfonate, and sodium carboxymethylcellulose, to create a slurry for a graphene-based anode.This approach resulted in an initial discharge capacity of 1724 mAh g −1 and an efficiency of 74% at a constant current of 0.1 A g −1 .Moreover, the cell exhibited a capacity retention of 84% (1070.2mAh g −1 ) after 100 cycles with a current up to 0.2 A g −1 .
In 2008, The pioneering work of Yoo et al. [123] demonstrated the potential of graphene nanosheets (GNS) based materials in advancing lithium-ion storage technologies.They achieved this by controlling the layered structure of GNS through an exfoliation and reassembly process.The specific capacity of GNS was measured at 540 mAh g −1 .However, by incorporating carbon nanotubes (CNT) and C60 molecules into the GNS, the capacity was further improved to 730 and 784 mAh g −1 , respectively.This enhanced performance was attributed to the distinctive electronic structure of GNS compared to graphite and the provision of additional sites for Li + accommodation due to the expansion in the d-spacing of the graphene layers (Figure 11).
Furthermore, chemical rGO has gained popularity as an anode material due to its facile and cost-effective synthesis.The rGO material offers good electrical conductivity (12.02 S cm −1 -23.26 S cm −1 ) and surface area (527 m 2 g −1 ), although slightly lower than pristine graphene.This material can be readily prepared in bulk quantities, making it a promising candidate for practical anode applications in LIBs. [124,125]For example, Guex et al. [126] Synthesis graphene GO that have been chemically reduced to rGO using sodium borohydride (NaBH4).The electrical conductivity of the resulting rGO was systematically investigated with variations in both time and temperature (Figure 12a,b).The reduction process exhibited a substantial transformation during the initial 10 min, leading to an impressive five orders of magnitude enhancement in conductivity reaching 1500 S m −1 .Subsequently, the reduction rate witnessed a notable decline beyond the initial 10 min, culminating in a final rGO conductivity of 1500 S m −1 after a 24 h reaction period.This reduction Correlation between d-spacing and charge capacity in GNS families compared to graphite.Reproduced with permission. [123]Copyright 2023, American Chemical Society.
methodology showcased an unprecedented achievement in terms of conductivity values for the aqueous reduction of graphene oxide through the utilization of sodium borohydride as a mediator.They also presented a vacuum filtration method for producing rGO substrates with uniform thickness and high conductivity.The results pave the way for the use of rGO as a filler material in conductive polymer composites.Lian et al. [127] showed that chemically derived graphene has shown impressive reversible capacity, reaching up to 1264 mAh g −1 at low charge-discharge rates (e.g.50-100 mA g −1 ), which is approximately two times higher than conventional graphite anodes used in commercial LIBs.However, at high charge-discharge rates (500 mA g −1 or higher), the capacity experiences significant fluctuations.This behavior is closely related to surface side reactions, particularly the formation of SEI films.Additionally, oxygen release occurs during the delithiation process due to the decomposition of oxygen-containing functional groups, leading to partial oxidation of the electrolyte and inducing electrochemical instability in the electrode. [128]While thermal annealing can reduce the oxygen content in graphene films, the limited rate capability of chemically derived graphene remains a significant drawback.Efforts to address this limitation are crucial to further optimize the performance of graphene-based electrodes for high-rate applications in LIBs.
Among the various approaches to enhance the rate capabilities of graphene electrodes, the incorporation of nitrogen (N) and boron (B) into graphene structures has garnered significant attention.Wu et al. [129] successfully developed high-performance electrodes using chemically derived graphene doped with heteroatoms (N and B).The method starts with chemically derived graphene obtained from natural flake graphite powder through a chemical exfoliation process followed by thermal reduction.For N-doped graphene, the chemically derived graphene is heated at 600 degrees Celsius for 2 h in a mixture of ammonia (NH3) and argon (Ar).For B-doped graphene, the process involves heating the chemically derived graphene at 800 °C for 2 h in a mixture of boron trichloride (BCl3) and argon.At a low charge/discharge rate of 50 mA g −1 , the N-doped graphene exhibited a high capacity of 1043 mAh g −1 , and the B-doped graphene showed an even larger capacity of 1540 mAh g −1 (Figure 13a,b) these values greatly exceed those of the pristine graphene electrode (955 mAh g −1 ). [130]The doped graphene electrodes displayed exceptional performance, enabling rapid charge and discharge within seconds, while maintaining a significant capacity even at an ultrafast rate of 25 A g −1 (≈30 s to reach full charge).This remarkable performance is attributed to the disordered surface morphology, presence of heteroatomic defects, improved electrode/electrolyte wettability, increased intersheet distance, enhanced electrical conductivity, and thermal stability of the doped graphene.These factors facilitate rapid surface Li ion absorption, ultrafast Li + diffusion, and efficient electron transport, resulting in outstanding rate capability and long-term cyclability.
However, several challenges need to be addressed to fully harness its potential.One of the key challenges is the agglomeration of graphene sheets, which can lead to decreased specific surface area and prevent the efficient transport of lithium ions.Agglomeration can occur during the production process or during the charge-discharge process. [131,132]o overcome these challenges, researchers have turned to the concept of composites, combining graphene with other materials to create enhanced anode structures.Graphene composites offer a promising approach to tailor the properties of the anode material, reducing some of the limitations of pristine graphene.These composites can be engineered to maintain the high electrical conductivity and surface area of graphene while addressing other specific requirements, such as mechanical stability, enhanced lithium-ion diffusion, and improved structural integrity. [133]ne of the common approaches is to incorporate graphene into a hybrid structure with other nanomaterials, such as metal oxides, metal sulfides, or Si. [134]However, these materials often face challenges such as low electrical conductivity and poor capacity retention due to the pulverization process.Graphene, on the other hand, offers a solution by serving as a host for nanostructured electrode materials.It provides support for anchoring nanoparticles and acts as a highly conductive matrix, ensuring good contact between the electrode and the current collector.Additionally, graphene layers play a vital role in preventing volume expansion/contraction and the aggregation of nanoparticles during charge and discharge processes.By integrating inorganic nanostructures with graphene layers, the restacking of graphene sheets is reduced, preserving a high surface area. [130]137] For example, Reduced graphene oxide-wrapped Fe 3 O 4 composites have been prepared using a hydrothermal method followed by post-annealing (Figure 14a).The GNS/Fe 3 O 4 composite demonstrated significantly improved rate capability and stable cyclability compared to commercial Fe 3 O 4 and bare Fe 2 O 3 particles.Remarkably, even at a high current density of 1750 mA g −1 , the specific capacity of the rGO/Fe 3 O 4 composite remained at ≈520 mAh g −1 , representing ≈53% of the initial capacity (Figure 14b). [138]As a proof of concept, Cui et al. [139] adopted a two-step hydrothermal method to prepare Mn 3 O 4 NPs attached onto rGO sheets, forming Mn 3 O 4 /rGO composite (Figure 15a).This composite exhibited superior rate capability when tested as LIB anodes compared to bare Mn 3 O 4 NPs.The specific capacity of Mn 3 O 4 /rGO reached ≈390 mAh g −1 , even at a high current density of 1600 mA g −1 (Figure 15b), which surpasses the theoretical specific capacity of graphite.In contrast, the bare Mn 3 O 4 particles delivered a lower specific capacity of ≈100 mAh g −1 at a  (20, 40, 60, and 80 °C) after 3 water cleaning cycles and film formation.Square markers depict results using vacuum-assisted filtration, triangle markers show results for ambient evaporation over 24 h, and circles markers represent the conductance of GO/rGO as a freeze-dried powder formed into a film.b) The red line illustrates a rapid increase in conductivity within the first 16 min, while the blue line represents a slower increase after ≈20 min of reaction.Reproduced with permission. [126]Copyright 2023, Royal Society of Chemistry.
current density of 40 mA g −1 (Figure 15c).These results demonstrate the potential of these graphene-based composites for enhancing the performance of LIBs.
The concept of using rGO to encapsulate metal oxide composites has been extended to various combinations, such as rGO/Fe 2 O 3 , rGO/CuO, and rGO/CoO. [140]In this fabrication process, the surface of metal oxides is first modified with poly(allylamine hydrochloride) and then wrapped with GO.This approach has also been applied to organic molecular/rGO composites as anode materials.By grafting redox-active organic molecules onto rGO, researchers have successfully reduced the dissolution of these molecules into the electrolyte during charge and discharge processes, leading to significant improvements in their cycling stability.These findings highlight the versatility and potential of using rGO-based composites for enhancing the performance of various materials in lithium-ion batteries and beyond. [141]y combining graphene with Si, for instance, researchers can exploit the exceptional electrical conductivity and mechanical strength of graphene to counteract the volume expansion issues associated with Si during lithiation.This synergistic effect leads to improved cycling stability and enhanced performance of the anode in LIBs. [142]Furthermore, graphene-based composites can be tailored to achieve desired electrochemical properties, such as high capacity, fast charge-discharge rates, and long cycling life. [143]The compositional design and engineering of these composites allow for greater control over the interaction between graphene and the active material, optimizing the lithium-ion storage capacity and efficiency.
In Table 2, you can find a comparison of different graphene anodes.It covers information about how they are made and their electrochemical performance.Researchers have previously highlighted the benefits of each type of anode to explain why they put much effort into studying and publishing them.After analyzing the table, it becomes clear that doped graphene stands out for its electrochemical performance consistently maintaining its capacity even after multiple cycles.Copyright 2010, AmericanChemicalSociety.
Additionally, other anodes and fabrication methods show similar electrochemical performances.It is important to acknowledge that almost none of these methods are environmentally friendly but show good results in terms of cost effectiveness.

Si with Graphene
Following the pioneering success of utilizing graphenesupported SnO 2 NPs as anodes for LIBs. [144]Lee et al. [68] made a significant breakthrough in 2010 by introducing Si NPs/graphene (Si NPs/G) composites as LIB anodes.In their study, Si NPs with sizes below 30 nm were initially treated in air to form a silicon oxide layer before being combined with GO synthesized through graphite oxidation.Reduction of the mixture using 10% H2 in Ar at elevated temperatures resulted in the formation of Si/G nanocomposites.These newly developed Si/G nanocomposites exhibited an impressive initial capacity of ≈1950 mAh g −1 during the first discharge, with a remarkable capacity retention of ≈900 mAh g −1 after 120 cycles at a current rate of 1000 mA g −1 .Subsequently, Lu et al. [142] presented a novel approach, creating a free-standing Si/G composite film where GO was reduced using Hydrazine at mild elevated temperatures (80 °C), while the Si particles ranged in size from 3 to 80 nm.This composite demonstrated exceptional cycling  [139] Copyright 2023, American Chemical Society.performance, with a negligible loss ≈3% starting from the fifth cycle (≈820 mAh g −1 ) to the 100th cycle (≈800 mAh g −1 ).Xiang et al. [145] in 2011 prepared Si NPs/G and used for LIBs.They prepared graphene in two different methods, first one they used GO and the Si NPs/GO composite were then treated at 500 °C to reduce Si NPs/GO composites.In second method, they obtained graphene from thermal expansion of expandable graphite at 1050 °C.Normally reduced GO has poor cycling performance while the graphene sheets prepared by thermal reduction have much better cycling performance.Moreover, this is what the characterization showed.Then they prepared three different samples with different weight ratios between Si and GO SG1 (1:1), SG2 (1:2) and SG3 (1:3).The electrochemical characterization showed that SG1 Has the lowest capacity while SG2 and SG3 are approximately the same and close to the theoretical capacity of graphene (Figure 16).Additionally, Lu et al. [146] demonstrated the growth of Si NWs on graphene surfaces, utilizing Au NPs as catalysts.This approach yielded an initial capacity of ≈3500 mAh g −1 and a cycling performance exceeding 1500 mAh g −1 after 30 cycles at a current rate of 420 mA g −1 .Furthermore, Xin et al. [147] designed a 3D porous Si NPs/G nanocomposite structure using the Magnesiothermic Reduction method for synthesizing 3D Si (Figure 17a).This innovative architecture showcased a capacity retention of over 400 mAh g −1 at a high current rate of 5000 mA g −1 after 100 cycles (Figure 17b).Notably, the preparation of Si/G nanocomposites in these studies involved dispersion, mixing, and catalyst-assisted growth, with the electrochemical performance being highly reliant on the dispersion of nanostructured Si and substantial graphene loading (exceeding 30 wt%).
Later in 2012, Zhou et al. [148] succeeded in fabricating of Si NPs encapsulated in graphene, by using electrostatic self-assembly strategy.They let the negatively charged Si NPs adsorb polyelectrolyte poly(diallydimethylammonium chloride) (PDDA) which is positive charged, and assemble them with negatively charged GO followed by freeze drying, hydrofluoric acid (HF) treatment and thermal reduction to get Si NPs/G nanocomposite.Two layers of graphene sheets encapsulate Si NPs, creating a microsized composite with nanospaces between the Si NPs and the graphene sheet (Figure 18a).This led to renders an elastic buffer to accommodate the large volume change of Si NPs during lithiation and delithiation cycles.Si NPs/G nanocomposite gave 1205 mAh g −1 after 150 cycles which is high and more than 3.2 times higher than theoretical capacity of graphite (Figure 18b), even on different rates it showed very good capacity (Figure 18c).Another successful fabrication method done by Zhou et al. [149] Si NPs inserted into graphene by using freeze-drying and thermal reduction method.Freeze-drying to sublimate solvents under vacuum Reproduced with permission. [145]Copyright 2011, National University of Singapore. .Reproduced with permission. [147]opyright 2023, Royal Society of Chemistry.
to dry samples and maintain their microstructures and thermal reduction to reduce G to graphene.This method leads to nanospace between Si NPs and graphene sheets, which provides elastic buffer to accommodate the volume variation of Si NPs during lithiation and delithiation.It gave excellent cycling performance of the Si/thermally reduced graphene composites as cycling after 100 cycles the reversible capacity still as high as 1153 mAh g −1 .
Luo et al. [150] introduced a distinctive technique utilizing evaporation-induced capillary force to encapsulate Si NPs within heavily crumpled graphene sheets, forming a shell structure (Figure 19a).Despite a high weight ratio of graphene (40 wt%), this crumpled Si/G nanocomposite demonstrated an initial capacity of 1175 mAh g −1 and retained 86% of its capacity after 250 cycles (Figure 19b).The crumpled graphene architecture was deemed essential for accommodating the expansion/contraction of encapsulated Si, contributing to improved cycling stability.Subsequently, in 2013, Ji et al. [151] developed a graphene-encapsulated Si on Ultrathin-Graphite Foam (UGF) for LIB anodes.By modifying Si NPs with PDDA to wrap them in GO, Si NPs/GO were obtained and deposited onto UGF surfaces, followed by thermal annealing to yield Si NPs/G on UGF.This configuration displayed an initial capacity of ≈1000 mAh g −1 , retaining over 400 mAh g −1 after 100 cycles  [148] Copyright 1999-2023 John Wiley & Sons.
Figure 19.a) Schematic: aerosol-assisted capillary assembly of crumpled graphene-wrapped Si nanoparticles, where aerosol droplets containing Si and GO pass through a preheated furnace, with GO sheets wrapping Si particles upon evaporation, resulting in a crumpled graphene shell due to capillary stress.b) Electrochemical performance.Reproduced with permission of. [150]Copyright 2023, American Chemical Society.
at a current rate of 400 mA g −1 .Additionally, Ren et al. [152] succeeded in fabricating Si/G composite by using GNS which possess an open porous structure that impart flexibility to the material.This flexible porous system can be used as a confining structure, and they should produce pathways for transport of electrons and li ions, they used chemical vapor deposition process to deposit Si particles on the surface of GNSs.The full cell built successfully and prove a good performance on longterm since the capacity remain high 553 mAh g −1 even after 500 cycles.
Li and Zhi et al. [153] presented a self-supporting binder-free Sibased anode, encapsulating Si NWs with a dual adaptable apparel strategy.Overlapped graphene sheets were grown from Si NWs using CVD to form Si NWs/G nanocables, which were then dispersed in a graphene oxide aqueous solution and vacuum-filtered (Figure 20a).The resulting structure, Si-NWs/G/rGO, demonstrated outstanding high-rate cycling performance, retaining over 95% capacity for 50 cycles at a current rate of 840 mA g −1 and over 90% capacity for 100 cycles at a current rate of 2100 mA g −1 (Figure 20b,c).) Capacity and coulombic efficiency of Si NW/G/rGO electrode cycled at 210 mA g −1 for the first cycle and 2.1 A g −1 for 100 cycles.Reproduced with permission. [153]Copyright 2023, Springer Nature Limited.
Another noteworthy technique for fabricating Si/G nanocomposites involves pyrolysis, initiated by freeze-drying Si/GO suspensions.This process is followed by GO reduction at elevated temperatures to achieve sandwiched Si/G structures.An optimized loading ratio of Si NPs and graphene at 1:2 yielded promising cycling performance, maintaining a capacity of ≈600 mAh g −1 (96% initial capacity retention) after 200 cycles. [154,155]In 2014, Hu et al. [156] fabricated Si/G nanocomposite by using discharge-plasma assisted milling (P-milling).They confirmed that graphene nanosheets play a dual role: minimizing impedance via the stable SEI film and enhancing surface charge transfer through improved electrode conductivity thanks to the graphene nanosheet matrix.The P-milling Si/G nanocomposite anode had a very good capacity after 200 cycles where the discharge capacity loss was only 0.21% per cycle, capacity retention of 58.8% and coulombic efficiency above 99%.The reversible capacity was 1195 mAh g −1 at 0.2 mA cm −1 .Li et al. [157] introduced method to encapsulate Si microparticles (Si MPs) using cages of multilayered graphene.Si MPs should be broken into small Si NPs, which leads to losing in electrical contact and form additional SEI.They introduce the growth of a graphene cage to encapsulate the material for more stability.The cage must be highly conformal to let Si acquire the remarkable properties of graphene, so they developed synthesis approach using a dual-purpose Ni template, which provide void space and let serves catalyst for graphene growth (Figure 21a).After encapsulating Si MPs with Ni, the carburization process took place and then the Ni etched using FeCl 3 .The Raman spectroscopy reveals the highly graphitic nature (Figure 21b).The electrochemical analysis demonstrated exceptional and enduring capacity, maintaining a consistent 1400 mAh g −1 even after 300 cycles (Figure 21c).
Moreover, Chen et al. [158] demonstrated that by combining the advantages offered by Si/G nanocomposite and utilization of novel binder the electrochemical performance will improve.They prepared Si/G nanocomposites using high-energy ball milling followed by thermal treatment, and for the first time Xantham gum was employed as an advanced binder.Experiments proved that this method improved the electrochemical performance since after 50 cycles the capacity was 484 mAh g −1 which is much better than that of pure Si.Another successful fabrication of microsized silicon/carbon/graphene spherical composite, in 2017 by Pan et al. [159] They used the spray drying process to wrap the Si NPs by GO NSs, Graphene was reduced during the calcination process, and it was introduced along with carbon to create an active matrix capable of accommodating volume changes (Figure 22).Si NPs/rGO and carbon as anode material for LIBs shows great cycling performance, especially for long term cycling.After 50 cycles the reversible capacity kept stable 1599 mAh g −1 at 100 mAh g −1 .
In 2019, Mingru et al. [160] fabricated Si/G composite also by using spray-drying technology and subsequent thermal treatment to form the anode material.By increasing the content of graphene, Si NPs are uniformly enwrapped by graphene, while increasing of graphene will result in aggregate phenomenon, so it is clear that content of graphene will affect the morphology of the obtained Si/G material.This research indicated that graphene Figure 22.Schematic: synthesis of Si/C/rGO.Reproduced with permission. [159] Copyright 2023, IOP Publishing.could enhance capacity.However, reducing the graphene content increased capacity but decreased cyclic performance, emphasizing the need to find a balance between these factors.This Si/G composite shows good electrochemical performance with high specific capacity in the first cycle and good rate performance, even after 50 cycles the specific capacity still stable ≈600 mAh g −1 .
In addition, Zhang et al. [161] in 2019 succeeded in fabricating Si NPs and graphene composite films as an anode material for Li-ion batteries using CVD method.They achieved contact between Si NPs/G and the current collector copper foil without a binder which improved the energy density of LIBs (Figure 23a).They used multi-walled carbon nanotubes (MWCNTs) as a dispersing agent to improve the quality of the composite graphene film.The Raman spectra of Si NPs/G with varying concentrations of MWCNTs suggested that the composite film was supported (Figure 23b), yet excess MWCNTs increased the specific surface area, leading to a higher consumption of Li+ and the formation of the SEI film at the anode.This had a negative impact on the energy density of the LIB.Cyclic voltammetry diagrams indicated a recurring spike near 0 V in each cycle, likely due to lithium-ion intercalation into the silicon nanoparticles and graphene composite film electrodes.Despite this, the composite anode failed after just 6 cycles (Figure 23c).
Extensive research endeavors have been devoted to refining the synthesis techniques for Si/G anodes, often involving higher ratios of graphene content (>20 wt%). [155,162]Ye et al. [163] conducted a comprehensive investigation into the implications of increased graphene weight ratios within Si/G nanocomposites, studying ratios spanning 3:1, 2:1, 1:1, to 1:2.Impressively, the Si/G ratio of 3:1 exhibited a remarkable equilibrium between initial capacity and cycling performance.Likewise, Chabot et al. [155] meticulously examined the loading ratio between Si NPs and graphene, exploring ratios including 1:3, 1:2, 1:1, and 1:0.5.Interestingly, the Si/G composition with a ratio of 1:0.5 revealed a peak initial capacity of ≈2600 mAh g −1 , while the 1:1 ratio demonstrated optimal capacity retention of ≈1000 mAh g −1 after 100 cycles.The electrode loading density serves as a crucial parameter, exerting a significant influence on the overall energy density of the entire cell. [164]For traditional full cells, maintaining a minimal loading density of 2.5-3.5 mAh cm −2 (equivalent to 1 mg cm −2 of Si anode) is vital.However, applications requiring high energy density batteries necessitate an ideal loading density ranging from 7 to 10 mg cm −2 . [165]Maintaining a lower loading density of the anode can inadvertently escalate the overall cell weight, resulting in a decrease in the overall energy density, which is not conducive to the development of electric vehicles. [166]ollowing, In the year 2020 Kim et al. [167] presented a method for developing a standalone and binder free material for high energy density LIBs.They created a structure called rGO/Si/GA by incorporating Si/G microspheres into a porous graphene aerogel (Figure 24a).This composite material can be used directly as an LIB anode without the need for inactive materials like binders, conductors or metal substrates.By doing it offers advantages such as flexibility, lightweight properties, simplification of electrode manufacturing processes and cost reduction.The research demonstrated electrochemical performance and remarkable cyclic stability when compared to other anodes.Specifically, it showed a discharge capacity retention of 94% after 3 cycles and 75% after 50 cycles at a density of 0.1 C (Figure 24b).Furthermore, Zhang et al. [168] utilized cost effective ball milling to synthesize Si/G composites in an accessible manner.They addressed challenges like dispersion and weak connections commonly associated with hybrid materials by introducing amino and carboxyl groups that formed covalent linkages The structural alterations in the rGO/Si/GA composite anode was examined before and after lithiation/delithiation processes.b) The cyclic performance of the composite anodes was evaluated at a rate of 0.2 C over 50 cycles.Reproduced with permission. [167]Copyright 2024 Elsevier B.V.
between Si NPs and graphene.The resulting composite exhibited a structure where Si NPs were uniformly attached to the surface or embedded within the inter layers of graphene.When employed as electrodes in LIBs this combination demonstrates the ability to maintain a capacity of 1516.23 mAh g −1 after 100 cycles at a rate of 100 mA g −1 .
In an approach Imae et al. [169] utilized an environmentally friendly method that involved using tetraethyl orthosilicate and natural graphite as initial materials to create a composite of silicon and rGO.This composite exhibited charge discharge capacities and cycle characteristics compared to batteries with graphene or Si-based anodes.It is worth noting that this method does have one drawback that may limit its application in the industry, which is its relatively high cost.Moreover, Malik et al. [170] discussed a wet jet milling technique for producing a hybrid material consisting of Si and graphene.The use of wet jet milling enables largescale production of this material making it suitable for industrial purposes.This hybrid material is intended to be used as an anode in LIBs.When incorporated into lithium ion cells the composite exhibits exceptional storage capacity for lithium ions extended cycling stability and high rate capability (1763 mAh g −1 after 450 cycles with a columbic efficiency of 99.85% at a current density of 358 mA g −1 ).The physical and chemical properties of the ma-terial provide additional evidence for the connection between its electrochemical capabilities and how its structure changes over time.Furthermore, the outstanding performance of this material along with the ability to produce it on a large scale represents a major step forward, in achieving high capacity and energy dense LIBs.
In 2023, Lu et al. [171] made progress in the field of Si and graphene research.They achieved this by developing a structure consisting of a core/double layer carbon coating on silicon particles.Additionally, they established a conductive graphene network.The process involved ball milling followed by the introduction of GO and sucrose solutions.Hydrothermal treatment and carbonization techniques were applied to improve the dispersion of ZIF 8Si, GO and sucrose.The results were impressive demonstrating electrochemical performance, with high lithium storage capacity, cycling stability and rate capability.Notably the material they developed showed a retention rate of 89.5% over 240 cycles at a current density of 0.2 A g −1 .Also, Zhang et al. [172] discussed a study around the development of a Si/carbon/reduced-graphene composite featuring a honeycomb structure, designed to enhance the performance of LIBs.Constructed using Si NPs, acetylene black (ACET), and rGO (Figure 25a).The composite proves to be a formidable solution Reproduced with permission. [172]Copyright 2024 Elsevier B.V. for addressing the volumetric expansion of silicon during the charging and discharging processes.This unique structural design contributes to the notable improvement in both cycle and rate performance of the battery.The composite's efficacy is highlighted by its ICE, surpassing 85%, and its ability to maintain a commendable reversible capacity of 1004 mAh g −1 after 270 cycles at a current density of 1 A g −1 .Impressively, the cycling stability endures even under higher current densities of 2 and 3 A g −1 (Figure 25b,c), demonstrating the composite's resilience.Furthermore, in rate cycle tests, the composite exhibits a remarkable reversible capacity of up to 450 mAh g −1 at 5 A g −1 .
It is important to note that most of the methods mentioned usually come with costs and inherent risks of potential damage to equipment and samples.The use of graphene precursors can also pose health hazards highlighting the urgent need to shift toward nontoxic graphene sources.This highlights the challenge in the field, where cost implications and safety considerations are important concerns across various methodologies.Moreover, producing Si/G composite materials often involves fabrication processes.Ensuring a controlled and uniform distribution of graphene, within the silicon matrix can be difficult and optimizing these processes for large-scale production adds another layer of complexity further contributing to the multifaceted nature of challenges faced in advancing these materials (Table 3).
Upon exploring approaches to develop Si/G composite materials, we discover a promising avenue to enhance the performance of LIBs.Individually Si and Graphene electrodes exhibit electrochemical behaviors, which are influenced by the methods used in their fabrication.The intricate aspects of performance are closely tied to the mechanical structure and physical formation of these materials.Notably Si NWs demonstrate stability and capacity throughout cycling due to their strong structural integrity.Additionally doped graphene plays a role in improving capacity and stability by activating additional lithium ion sites and reducing energy losses caused by battery polarization effects.Optimizing the amount of Si in electrodes has proven effective in increasing overall battery capacity.The inclusion of graphene does not contributes to stabilizing Si performance but also results in battery materials that exhibit stable high rate performance and high ICE.By examining the evolution of Si/G LIBs over time, recent studies have achieved the capacity over multiple cycles.These advancements can be attributed to innovations such, as encapsulation of Si particles, which effectively accommodate volume expansion as well as cost effective fabrication methods that demonstrate exceptional performance.
In situ characterization techniques provide real time insights into mechanical and chemical changes during electrochemical processing which surpasses ex situ methods.However, it is important to mention that using in situ techniques can be more expensive and carries a risk of equipment and sample damage.Some graphene precursors like methane and toluene are harmful.Therefore, we should encourage the use of toxic graphene sources.The quality of graphene material greatly depends on the precursor used thus, we need to develop nontoxic low cost options that maintain high quality standards.Optimizing the contact between Si particles and graphene materials is crucial for enhancing the capacity of the Si/G electrode.Stronger physical contact between them improves performance over repeated cycles.The controlled design of spaces within the electrode helps minimize material loss and accommodates stress caused by changes in Si volume.
Proper management of the size of pores helps prevent the buildup of SEI within the pores when cycling, which allows for smooth movement of ions.Although increasing the weight ratio of Si/G can boost the capacity of a Si/G electrode, it is crucial to optimize this ratio based on various graphene materials and structural designs in order to achieve maximum and stable performance for practical usage.

Conclusion
Exceptional properties of graphene, including high electrical and thermal conductivity, mechanical strength, and large surface area, make it an attractive candidate for enhancing various components of LIBs.As an anode material, graphene serves as an excellent conductive matrix, mitigating volume expansion effects, improving cycling stability, and enabling high capacity.Its compatibility with cathodes and current collectors further augments LIB performance.Moreover, graphene's thermal conductivity contributes to safer and more stable LIB operation.
Si, with its impressive gravimetric and volumetric capacities, represents a promising avenue for next-generation LIBs.However, challenges stemming from its large mechanical strain during lithiation, pulverization, and formation of SEI layers have motivated diverse strategies.Nanostructured Si, through nanoparticles, nanowires, or 2D structures like silicene, offers solutions by accommodating volume changes and enhancing electrochemical kinetics.
Graphene and Si have emerged as critical materials in the pursuit of high-performance LIBs.The synergetic effect resulting from the integration of these materials has been found to significantly enhance the overall performance of the composite compared to standalone graphene or Si.The combination of outstanding properties of graphene, such as high electrical and thermal conductivity, mechanical strength, and extensive surface area, with the impressive gravimetric and volumetric capacities of Si, has led to substantial improvements in various aspects of LIBs.
One of the key benefits of this composite material is the effective accommodation of stress and volume changes, which effectively mitigates the detrimental effects of volume expansion, alleviates mechanical strain, and ensures enhanced cycling stability.Additionally, the presence of graphene as a conductive matrix facilitates the efficient and rapid transportation of electrons, leading to improved electrochemical kinetics and higher specific capacities.The incorporation of graphene also aids in the removal of agglomeration, ensuring a more uniform distribution of active material and enhancing the overall electrochemical performance.
Moreover, the integration of Si and graphene has been instrumental in enabling LIBs to function optimally at high current rates, thereby significantly improving their power capabilities.The conductive nature of the graphene matrix, coupled with the high capacity of silicon, allows for efficient electron transport and enables the battery to deliver high power outputs without compromising cycling stability or overall performance.
It is evident that the collaboration between graphene and Si in LIBs has the potential to revolutionize the landscape of energy storage technology.By addressing challenges related to stress accommodation, conducting matrix requirements, and agglomeration removal, this composite material paves the way for the development of safer, more efficient, and longer-lasting energy storage solutions.With further research and development, the continuous advancement of Si/G hybrid materials is poised to drive significant progress in the field of energy storage, fostering the development of a sustainable and efficient energy ecosystem for diverse applications, including electric vehicles, portable electronics, and large-scale energy solutions.

Figure 3 .
Figure 3. a) Schematic illustration of the nanocrystalline structure in milled Si powder, highlighting grain boundaries for enhanced Li diffusion.b)Cycling performance of milled millimetric Si and nanosized Si-based electrodes, showing discharge capacities and coulombic efficiencies.Reproduced with permission.[72]Copyright 2023, Royal Society of Chemistry.

Figure 4 .
Figure 4. a) Schematic illustration of hierarchical bottom-up assembly for Si-C nanocomposite granule formation: Annealed carbon-black dendritic particles undergo coating with Si nanoparticles.Subsequently, they assemble into rigid spheres with interconnected internal channels during carbon deposition.b) Reversible Li deintercalation capacity and coulombic efficiency of the C-Si granule electrode versus the cycle number, comparedto the theoretical capacity of graphite.Reproduced with permission.[74]Copyright 2010, Nature Materials.

Figure 5 .
Figure5.The voltage profiles of as-prepared Si NWs-carbon textiles electrodes are examined between 0.01-3.0V at a rate of 0.2 C and 1 C. Reproduced with permission of.[77]Copyright 2023, Springer Nature Limited.

Figure 8 .
Figure 8. a) Illustration of the method used to create DWSi NTs.b) SEM picture of DWSi NTs following 2000 cycles, revealing the NTs covered with a consistent, thin SEI layer.c) Impedance measurements for DWSi NTs after varying cycle counts, indicating minimal expansion of SEI during cycling.Reproduced with permission.[89]Copyright 2023, Springer Nature Limited.

Figure 9 .
Figure 9. a) 1,2)The rice husk (waste) is used as the source of biosilica generated in rice by the unique biological process from silicic acid.3) This biosilica was transformed by a simple process into mesoporous silica, which is then 4) reduced by a magnesiothermic reaction into mesoporous Si and 5) used as a high-performance anode material for Li-ion batteries.b,c) Cycling and rate performance comparison for silicon anodes.Reproduced with permission.[92]Copyright 2023, Royal Society of Chemistry.

Figure 10 .
Figure 10.a) Graphene films produced without any additives or the need for current collectors, designed for flexible lithium-ion battery (LIB) anodes.b)Cell discharge capacity plotted against the cycle number, illustrating data for two distinct current rates over a span of fifty cycles.Reproduced with permission.[117]Copyright 2023, Royal Society of Chemistry.

Figure 11 .
Figure11.Correlation between d-spacing and charge capacity in GNS families compared to graphite.Reproduced with permission.[123]Copyright 2023, American Chemical Society.

Figure 12 .
Figure 12. a) Conductance values at 25°C for samples prepared at varying temperatures(20, 40, 60, and 80 °C) after 3 water cleaning cycles and film formation.Square markers depict results using vacuum-assisted filtration, triangle markers show results for ambient evaporation over 24 h, and circles markers represent the conductance of GO/rGO as a freeze-dried powder formed into a film.b) The red line illustrates a rapid increase in conductivity within the first 16 min, while the blue line represents a slower increase after ≈20 min of reaction.Reproduced with permission.[126]Copyright 2023, Royal Society of Chemistry.

Figure 13 .
Figure 13.a) Cycling performance and coulombic efficiency of the N-doped graphene electrode.b) Cycling performance and coulombic efficiency of the B-doped graphene electrode.Reproduced with permission.[129]Copyright 2023, American Chemical Society.

Figure 17 .
Figure 17.a) 3D porous Si NPs/G nanocomposite for high performance Li-ion battery anodes.b) Li + extraction capacity vs. cycle number for Si/G nanocomposite at fixed Li + insertion rate (100 mA g −1 ) and varied Li + extraction rates (5 A g −1 and 10 A g −1 ).Reproduced with permission.[147]Copyright 2023, Royal Society of Chemistry.

Figure 18 .
Figure 18.a) Fabrication process of the Si NPs/G nanocomposite.b) Cycling performance comparison: Si NPs/G nanocomposite vs. pure Si NPs c) Rate capability of the Si NPs/G nanocomposite.Reproduced with permission.[148]Copyright 1999-2023 John Wiley & Sons.

Figure 20 .
Figure 20.a) Schematic of the fabrication and adapting of Si NWs/G/rGO.The fabrication process mainly includes chemical vapor deposition and vacuum filtration of an aqueous Si NW/G-GO dispersion followed by thermal reduction.b) Capacity retention comparison for various electrodes cycled initially at 210 mA g −1 and later at 840 mA g −1c) Capacity and coulombic efficiency of Si NW/G/rGO electrode cycled at 210 mA g −1 for the first cycle and 2.1 A g −1 for 100 cycles.Reproduced with permission.[153]Copyright 2023, American Chemical Society.

Figure 24 .
Figure24.a) The structural alterations in the rGO/Si/GA composite anode was examined before and after lithiation/delithiation processes.b) The cyclic performance of the composite anodes was evaluated at a rate of 0.2 C over 50 cycles.Reproduced with permission.[167]Copyright 2024 Elsevier B.V.
Saadaoui is a Research associate professor at the Center of Microelectronics in Provence of the Ecole des Mines de Saint Etienne (France).He received his Ph.D. in 2006 at the university of Paul Sabatier (Toulouse, France) in the field of microelectornics.His Ph.D. work done at the LAAS-CNRS consist in the development of RF devices technology on ultra-thin silicon membrane.Then, he joined Lina Sarro group in TU-Delft (The Netherlands) to work on the development of through silicon vias technology for 3D integration.His research is focused on the development of nanomaterials and inks, selective annealing technologies such as photonic sintering for printed electronics on flexible substrate.Thierry Djenizian is the head of the Flexible Electronics Department at the Ecole Nationale Supérieure des Mines de Saint-Etienne.In 2002, he received his Ph.D. degree in Materials Chemistry from the Swiss Federal Institute of Technology in Lausanne and the Friedrich Alexander University of Erlangen-Nuremberg.His research activities are mainly dedicated to the development of flexible and stretchable micropower sources for wearable technologies.He is one Conference Chair of Porous Semiconductors Science and Technology international conferences.

Table 2 .
Comparison of electrochemical performance of graphene and graphene-based materials as anode electrodes for LIBs.

Table 3 .
An overview of investigations concerning Si/G nanocomposite anodes.