Prelithiation Enhances Cycling Life of Lithium‐Ion Batteries: A Mini Review

During the last decade, the rapid development of lithium‐ion battery (LIB) energy storage systems has provided significant support for the efficient operation of renewable energy stations. In the coming years, the service life demand of energy storage systems will be further increased to 30 years from the current 20 years on the basis of the equivalent service life of renewable energy stations. However, the life of the present LIB is far from meeting such high demand. Therefore, research on the next‐generation LIB with ultra‐long service life is imminent. Prelithiation technology has been widely studied as an important means to compensate for the initial coulombic efficiency loss and improve the service life of LIBs. This review systematically summarized the different prelithiation methods from anode and cathode electrodes. Moreover, the large‐scale industrialization challenge and the possibility of the existing prelithiation technology are analyzed, based on three key parameters: industry compatibility, prelithiation efficiency, and energy density. Finally, the future trends of improvement in LIB performance by other overlithiated cathode materials are presented, which gives a reference for subsequent research.


Introduction
Global energy storage technology, especially the lithium-ion battery (LIB) energy storage system, has been rapidly developed in recent years.LIB energy storage has obvious economic advantages compared to other energy storage technology, and there is huge potential for technological improvements in the future.Therefore, China, America and the European Union all have taken it as a key emerging industry.By the end of 2020, the cumulative installed capacity of the global LIB energy storage system was approximately 13.1 GW, which accounts for 90% of the total installed capacity of new energy storage technology (excluding pumped storage), exhibiting a five-year compound growth rate of 107%.To cope with 1500 to 1800 GW new energy access by 2030, China needs to employ 150 GW new energy storage system to achieve power grid balance and efficient use of clean energy.At that time, large-scale energy storage technology will become the leading force for flexible regulation and auxiliary support of the new power system.It requires the energy storage power station not only to have the application functions, such as active grid support, large-scale peak shaving, frequency modulation, and voltage regulation, but also to service the same life as the renewable energy station, to achieve the best economic benefits.Based on the above requirements, LIB, as the core of the energy storage power station is also required to have a long life, high safety, highenergy efficiency, and low cost.
Currently, LIBs for energy storage commonly use graphite as the anode electrode and liquid organic electrolyte as the electrolyte.For the cathode materials, layered lithium transition metal oxides (LiNi x Co y Mn 1-x-y O 2 , hereafter referred to as NCM) have been mainly used by LG, Samsung (Korea), and Panasonic (Japan), which shows high-energy density.Therefore, it would reduce the coverage area and the installation cost of the energy storage power station under the same cell capacity.However, the challenges are transition metal dissolution and NCM particles cracking during the cycling, which leads to rapid performance degradation and severe safety concerns, and cannot meet the long service life demand of the energy storage power station for 20-30 years. [1]In China, lithium iron phosphate (LFP) is chosen as the primary cathode material by Contemporary Amperex Technology Co., BYD Company Ltd., Gotion High-Tech, and other companies.Different from the layered structure of NCM, the LFP crystal structure is an ordered olivine type.Moreover, the P-O strong covalent bond of PO 3À 4 polyanion supports the LFP crystal structure well, [2,3] which significantly reduces the impact of frequent lithiation and de-lithiation processes on LFP and greatly improves the cycle life of the cell.[6] Results show that the main factor for the capacity/voltage fade of the LFP battery is the active lithium consumption caused by anode side reactions, resulting in the continuous thickening of solid electrolyte interface (SEI) and growing of swelling force.Therefore, increasing the total amount of active lithium by prelithiation can not only help improve the battery's energy density but also significantly prolong the cell's service life, meeting the long-term service life requirements of new energy storage stations.
With the rapid development and application of LIBs in the field of electric vehicles and electrochemical energy storage, conventional chemical systems are unable to meet the increasing energy density and cycle life of LIBs.Prelithiation technology is used as an efficient solution to the above needs, [7][8][9] and shows a largely increased published paper number and citation in recent 5 years (Figure 1).Based on three key parameters: large-scale manufacturability, prelithiation efficiency, and the influence of energy density, this review systematically summarizes the industrialization possibility of the present prelithiation technology, [10,11] including anode chemical prelithiation, anode electrochemical prelithiation, anode physical contact prelithiation, and cathode additive prelithiation, to provide a reference for the practical application of follow-up research.

Anode Chemical Prelithiation
Chemical prelithiation can be divided into mechanical alloying method and solution prelithiation method.The former mainly realizes prelithiation at the active material level by ball milling or stirring reaction between molten lithium metal and active material under inert gas protection.The latter makes use of the potential difference between the prelithiation solution and the active material, which can spontaneously realize the prelithiation at both the active material level and electrode level.
As early as 2006, Sun et al. [12] combined hard carbon with Li 2.6 Co 0.4 N by high-energy ball milling in an inert atmosphere to synthesize pre-lithiated hard carbon composites (hard carbon/Li 2.6 Co 0.4 N).In the subsequent half-cell test, the coulombic efficiency was increased from 66.3% to 100% for the first time.Zhao et al. [13] synthesized a series of prelithiation materials (Li 22 Z 5 -Li 2 O) and their corresponding oxides of group IV elements (Z = Si, Ge, Sn, etc.) through the mechanical metallurgy method (Figure 2).The prelithiation compensates for the active lithium consumption in the SEI film-forming process, making the capacity of as-synthesized material close to the theoretical capacity.In addition to the benefits from the wide band gap between Li and Ge, Li x Ge shows high stability in the air environment, maintaining ~75% reversible capacity even after being exposed in air for 6 hours.Compared with the pure Li x Z, the O atom in Li 2 O passivation layer can strengthen the stability of Li atom in Li x Z, slow down the side reaction rate of Li x Z and further improve the environmental stability of Li x Z-Li 2 O. [14] Therefore, when ZO 2 is pre-lithiated by the mechanical metallurgy method, the dense Li 2 O passivation layer generated at high temperature is evenly distributed around the Li x Z nano domain, ensuring a reversible capacity retention of ~85% after exposure to air for 6 h.Despite metal lithium, LiH can also be used as a precursor for prelithiation by dehydrogenation-driven solid-state reaction. [15]When LiH was used for prelithiation of SiO, it played as the source for lithium silicate, resulting in a boost of initial coulombic efficiency to 90.5%.To further improve the long-term cycle life of LiH prelithiated SiO anode, Jeong et al. [16] proposed a double-buffer-phase embedded Si/TiSi 2 / Li 2 SiO 3 nanocomposite material prepared by phase-selective reaction of SiO with TiH 2 .In terms of the synthetic process, Yom et al. [17] studied the effect of heating rate on metallurgically pre-lithiated SiO active materials.XRD and DTA results showed that the reaction products generated at high temperature and high heating rate were similar to the SEI components formed in the constant current formation stage.The formation of Li 2 SiO 3 phase is only related to the synthesis temperature (≥350°C) and is independent of the heating rate, while the formation Xiaomei Liu is currently pursuing her doctoral degree at the Institute of Nuclear and New Energy Technology, Tsinghua University, under the supervision of Prof. Xiangming He.She received bachelor's and master's degree from the School of materials, Xi'an University of Technology.Her research focuses on the design and application of Li-ion power battery and energy storage battery. of Li 4 SiO 4 phase is related to both temperature (≥450 °C) and heating rate (≥14°C min −1 ).Li et al. [18] synthesized a series of thermodynamically stable Li x Si compounds (x = 4.4, 3.75, 2.33) by high-energy ball milling.The cycle stability and capacity of as-synthesized material are significantly improved after prelithiation.In addition, surface nitrogen doping also helps to improve the cycle stability of Li x Si compounds, and the capacity fading is reduced from 1.06% to 0.15% per cycle.However, due to the formation of Li x N y Si z phase without electrochemical activity, the initial capacity is reduced from 2610 mAh g −1 to 713 mAh g −1 .
Unlike the mechanical metallurgy method, which could only realize prelithiation at the material level, the solution method can also achieve prelithiation at the electrode level, [19] and the industrialization difficulty is a bit lower than the mechanical alloying method.In the 1990s, Takei et al. [20] reported the prelithiation of carbon black anode materials by lithium naphthalene compound.However, due to the difference in solvent systems, the passivation effect of SEI film formed by prelithiation is worse than that of electrochemical formed.Similarly, Scott et al. [21] used n-butyllithium to pre-lithiate carbon black and SEI could form on the surface of carbon black, reducing the active lithium loss in the formation process.However, it cannot improve the amount of intercalated lithium due to its high reduction potential (~1 V vs. Li/Li + ).Shen et al. [22,23] used lithium naphthalene (Naph-Li)/ethylene glycol dimethyl ether (DME) as the prelithiation solution to partially prelithiate sulfur polyacrylonitrile (S-PAN), Si and hard carbon, respectively.The energy density of Li 2 S-PAN/Li x Si full battery assembled by this method is as high as 710 Wh kg −1 , and the capacity retention is ~90.6% after 250 cycles at 100 mA g −1 current density.Unlike carbon black, Si, hard carbon, and other negative electrode materials, lithium titanate (LTO) has a lithium intercalation potential of ~1.5 V, which is much higher than that of lithium naphthalene/n-butyl lithium solution.Therefore, high stoichiometric prelithiation can be realized by adjusting the solvent and prelithiation time. [24]Li et al. [25] combined mechanical lithography with Naph-Li chemical prelithiation solution to enhance the structural stability and compensate for the active Li loss of Sn foil electrode in full cells.As expected, the undulating LiSn/Sn electrode displayed excellent cycle stability at 1 mA cm −2 for over 1000 h with a low overpotential below 15 mV.However, most commercial LIBs use graphite as the anode, of which lithium intercalated potential is around 0.1 V vs. Li/Li + .Hence, the methods above are difficult to be directly used in the present manufacturing process.To solve this problem, on the basis of lithium naphthalene system, Jang et al. [26] further introduced a branched chain to adjust the π bond lowest unoccupied molecular orbital (LUMO) energy of naphthalene, biphenyl, and other aromatics, and reduced the reduction potential of lithium/aromatics solution.The results showed that 2-methylbiphenyl (2-BP) can reduce the reduction potential to 0.13 V vs. Li/Li + , and the coulombic efficiency of SiO x anode can be increased to ~118% only by prelithiation for 30 min.Alternatively, Choi et al. [27] revealed that weakly solvated solutions could enhance the Li + -anion interactions and suppress the formation of free solvated ions during Li + desolvation, thereby mitigating solvated ions intercalation into graphite and enabling stable prelithiation.As a result, graphite-SiOx/NCM 622 combined full cell exhibits an energy density of 506 Wh Kg −1 , which is almost 98.6% of the ideal value.Although the use of organolithium compounds for chemical prelithiation has attracted increasing attention, the deep mechanism of organolithium involvement in chemical prelithiation has not been thoroughly explored.Yue et al. [28] demonstrated that lithium radical anions formed from the reaction of metal lithium and aromatic compounds are the active species for prelithiation process (Figure 3), while lithium aromatic compounds do not function in this process by monitoring the electrical conductivity change of lithiation solution during the lithiation duration.Furthermore, compared with dissociated species, associates show higher prelithiation activity but less uniform prelithiation.

Anode Electrochemical Prelithiation
Electrochemical prelithiation in form is similar to the structure of the half-cell that commonly used in the laboratory (Figure 4).With the active material to be treated as the cathode and lithium metal as the anode, the precise prelithiation can be achieved at the electrode level by controlling the cut-off voltage or direct external short circuit.After prelithiation, the electrode is disassembled and then reassembled with the conventional cathode to form a full battery.Kim et al. [29] adjusted the prelithiation degree of C-SiO x negative electrode by simply controlling the external short-circuit resistance and time.After a short circuit for  [13] Copyright 2017, Elsevier B. V. Half cell tests show that the SEI film-forming peak at 1.3 V disappears in first cycle, and the coulombic efficiency is increased from 40% to 97%.After further assembly with Li(NiCoMn) 1/ 3 O 2 into a full cell, the initial coulombic efficiency can reach 82%. [30]Hong et al. [31] prelithiated the silicon/carbon negative electrode by electrochemical method.The energy density of the full cell assembled with sulfur/mesoporous carbon cathode is as high as 590 Wh kg −1 .When the amount of the active Li increased from 20% to 50%, the capacity of the battery still maintained 650 mAh g −1 after 100 cycles.Despite compensating for the irreversible capacity loss, some interesting applications of electrochemical prelithiation have been reported recently.Wang et al. [32] reported direct regeneration of spent LFP by prelithiated graphite.As the main failure mechanism arises from the loss of Li + from LFP and the appearance of FePO 4 in the LFP cathode, the loss of Li + in the spent LFP cathode could be compensated when assembled with prelithiated graphite, which largely simplified the recycling process of spent LFP.Fu et al. [33] found that prelithiation can improve the sodium storage properties of commercial bulk Sb 2 S 3 (CSS).After prelithiation (LSS), the ratedetermining step of sodiation/desodiation kinetics changed from diffusion-controlled to capacitive process controlled.Thus, the sodiation/ desodiation kinetics of LSS is promising, which exhibits improved initial coulombic efficiency, stable cycling performance, and high rate capability.The electrochemical prelithiation can introduce an accurate amount of active Li into the anode and maintain the initial SEI components similar to that generated from the electrochemical cycling by electrolyte optimization.However, there are also some problems, such as complex electrode de-assembling and re-assembling operation, and moreover, the prelithiated anodes are moisture sensitive, largely liquid electrolyte consumption, and so on.Therefore, this method is more suitable for lab studies, and is challenging to be employed in large-scale manufacturing.

Anode Physical Contact Prelithiation
The form of physical contact prelithiation is similar to the electrochemical prelithiation, and the main difference between the two methods is whether an electron path is formed through the open circuit.Contact prelithiation mainly uses the potential difference between lithium metal and active material to realize the spontaneous lithium intercalation in the form of an internal short circuit.It is also the main prelithiation method that has been industrialized at present.Sun et al. [34] pressed the hard carbon electrode and thin metal lithium foil together under an inert atmosphere to prepare the prelithiated hard carbon composite electrode.After assembling with LiCoO 2 cathode, the full cell initial coulombic efficiency increased from 52% to 86%.Yao et al. [35] pre-lithiated graphene oxide/Si composite negative Figure 3. Illustration of chemical prelithiation mechanism.Copyright 2016, American Chemical Society.
Energy Environ.Mater.2023, 6, e12501 electrode with direct physical contact.The initial coulombic efficiency can be improved from 77.7% to 97.1% after only 5 min of contact.Berhaut et al. [36] investigated the real-time contact prelithiation process on Si/FeSi/graphite composite anode.Over lithiation, Li migrates entirely toward the silicon phase where it acts as a reservoir of Li due to its higher delithiation potential as compared to graphite.By controlling the thickness of Li-foil, approximately 50% of prelithiation degree was achieved, resulting in more than 1760 cycles at 100% depth-ofdischarge with <11% capacity loss at C/2.However, due to the porous structure and large surface roughness of the electrode, there is a problem of uneven current density distribution in the process of lithium intercalation in the external short-circuit method.Xu et al. [37,38] made the lithium metal undergo Newtonian-like fluid viscous creep by rolling, [39] so as to increase the contact area between the lithium metal and the electrode and improve the uniformity of prelithiation.Meng et al. [40] controlled the prelithiation rate by adding a buffer layer between the lithium metal and the electrode (Figure 5), resulting in a uniform prelithiation of SiO x .The modified buffer layer is synthesized by coating polybutyraldehyde (PVB) on the surface of carbon nanotube film.The impedance of the buffer layer can be adjusted by controlling the coating times, so as to control the lithium supplement rate.After assembling NCM622 into a full cell, the initial coulombic efficiency is ~87% at 0.1C, and the specific capacity increases from 134 mAh g −1 to 173 mAh g −1 with a capacity retention of ~77% after 200 cycles.A recently published work revealed that the electron channel played a key role in improving the Li utilization of contact prelithiation. [41]In view of this, using vacuum thermal evaporation enabled an adequate electron channel in the contact interface, which ensured a mitigated local current density and reaction kinetics, resulting in increased Li utilization from 65% to 91.0%.Moreover, they also find out that the Li utilization can be significantly affected by the electrolyte solvent. [42]When compared with ethylene carbonate (EC), dimethyl carbonate (DMC) induced a low-organic-content raw electrolyte interphase (REI) on Li source and anode, which behaves as a mixed ion/electron conductor.Therefore, electron channels can be maintained even when the Li source was completely wrapped by the DMC-derived REI, resulting in an outstanding Li source utilization of 92.8%.
Using lithium powder instead of lithium metal can also realize uniform prelithiation. [43,44]However, both lithium metal and lithium powder have high reaction activity and atmosphere sensitivity, which can easily cause safety risks.Referring to the actual manufacture process, in addition to shortening the time from prelithiation to assembly process, how to passivate the lithium metal is also of great significance.Cao et al. [45] passivated the lithium metal with polymethylmethacrylate (PMMA).After adding the liquid electrolyte, the PMMA was dissolved, and then the lithium metal was spontaneously intercalated into the anode.Under 30% relative humidity, the lithium metal without PMMA protection blackened within 2 min due to side reactions, while the lithium metal protected by PMMA remained stable for more than 2 h.Stabilized Li metal powder (SLMP) can also achieve the above functions.48] Pu et al. [49] prepared nano (<500 nm) SLMP by low-temperature ball milling under the protection of the ionic liquid (tetrabutyl phosphine bis(trifluoromethanesulfonyl) imide salt), which usually behaves a high melting point.As-synthesized SLMP showed good prelithiation efficiency in Si, SiO, and SnO 2 negative electrodes.Huang et al. [44] reported prelithiation of SiO anodes by using a mixture of SLMP, styrene butadiene rubber (SBR), and toluene as a relatively stable prelithiation reagent.Among them, SLMP acted as the main pre-lithiation reagent, while SBR could enhance the dispersion uniformity of SLMP in toluene.By simply controlling the volume of the mixture solution, ICE varying from 66% to 120% was achieved.Wang et al. [50] used silane coupling agent 3-(methacryloyloxy) propyltrimethoxysilane (MPS) to improve the stability of lithium metal.The Li-O-Si bond strengthened the protection of the MPS layer to Li metal.After exposing to air for 2 h, the material was re-assembled and showed no deterioration of the electrochemical performance.

Cathode Additives Prelithiation
Although the anode prelithiation process based on lithium metal has made great progress, the safety issue of lithium metal cannot be ignored.As an alkali metal with low potential and high reactivity, lithium reacts violently with water, which is easy to cause compatibility problems between lithium-ion battery and environment, solvent, and binder, thus greatly increasing the manufacturing cost.In contrast, lithium compensation at the cathode side is much simpler.The active Li can be introduced by only adding a small amount of prelithiation additives in the slurry process.Based on the prelithiation mechanism, the cathode prelithiation can be roughly divided into two types: direct overlithiation of cathode and high lithium content additives.The former uses the multivalent and multi-hole characteristics of transition metals to store non-stoichiometric Li + in the unoccupied lattice and compensate for the SEI consumption in the formation stage.As for the latter method, additives with high-donable lithium ions are desirable.The additive generally needs to meet some conditions, including low decomposition voltage (not exceeding the upper operating voltage), no secondary lithium intercalation in the discharge process, and no side reaction with electrolyte, active materials, and auxiliary materials.
As shown in Table 1, the overlithiated cathode materials of LIBs are commonly related to the manganate and phosphate series.[53] Aravindan et al. [54] used the oxygen octahedral site of LiMn 2 O 4 for prelithiation (~2.8 V vs. Li/Li + ), to improve the initial coulombic efficiency and cycle stability of LMO full  [40] Copyright 2019, American Chemical Society.
Energy Environ.Mater.2023, 6, e12501 cell assembled with α-Fe 2 O 3 .Gabrielli et al. [55] doped partially overlithiated Li 1+x Ni 0.5 Mn 1.5 O 4 into LiNi 0.5 Mn 1.5 O 4 to compensate the high active Li consumption of Si negative electrode in full cells.When the lithium-rich phase in Li 1+x Ni 0.5 Mn 1.5 O 4 is consumed (<3.0 V vs. Li/Li + ), Li 1+x Ni 0.5 Mn 1.5 O 4 simultaneously transformed into spinel phase LiNi 0.5 Mn 1.5 O 4 with stable electrochemical activity, resulting in an improvement of initial coulombic efficiency without deteriorating the energy density of the cell.[58] However, synthesizing materials with high stoichiometric lithium will inevitably produce a second crystal phase, which commonly shows no reactive activity.Betz et al. [59] recently reported the synthesis of overlithiated Li 1+x Ni 0.5 Mn 1.5 O 4 by lithium metal and 1-pentanol.By using this method, commercial LiNi 0.5 Mn 1.5 O 4 can be overlithiated with controllable lithium content through a one-step reaction, and this material can be stably stored at room temperature for at least 24 h.Song et al. [60] used Li 5 V 2 (PO 4 ) 3 as the cathode to solve the low initial coulombic efficiency of hard carbon.During the first charging process, overlithiated Li 5 V 2 (PO 4 ) 3 transformed into Li 3 V 2 (PO 4 ) 3 with a conventional stoichiometric ratio.The initial coulombic efficiency of the pouch cell assembled by this method increased to 96.7% with an energy density of 320 Wh kg −1 and a ~80% capacity retention even at −40 °C.In addition, ~100 mAh g −1 active lithium can also be supplemented by using the reduction electric pair of V 3+ / 2+ at ~1.7 V (vs.Li/Li + ) in the hydroxyphosphorus phase LiVPO 4 F, and then the stable cycle can be realized in the range of 3-4.5 V by using the V 3+ / 4+ redox electric pair. [61]he overlithiated cathode materials can compensate for the SEI consumption of the full cell, the widely used cathode materials of commercial LIBs are mainly NCM and LFP, especially for the ultra long-life energy storage batteries, which mainly are based on the LFP route.
Unfortunately, the above overlithiated cathode materials have no experience in large-scale manufacture and are difficult to be applied in the short term.If it is combined with LFP, there are some problems, such as low lithium supplement and low-energy density.On the contrary, though the lithium-rich additives cannot be directly used as the cathode material, it can release a much higher specific lithium-ion capacity by weight and volume than the existing cathode materials.When mixing with the existing mature system, it can significantly improve the energy density of the cell.Li x Y (Y = O, N, S) materials are considered to be the most commercial potential lithium-rich additives, and the lithium supplement capacity is generally >1000 mAh g −1 .Park et al. [68] found that the reversible capacity of Li 3 N is closely related to the particle size, and shows no electrochemical activity before ball milling.After ball milling, it exhibits a high reversible capacity of ~1400 mAh g −1 and a low decomposition potential of ~0.9 V vs. Li/Li + .When the LiCoO 2 mixed with 2% Li 3 N and assembled with SiOx/C@Si anode into a full cell, the reversible capacity increased by ~11% at the discharge rate of 0.2C.However, due to the poor electronic conductivity of Li 3 N, the discharge polarization increases, and the reversible capacity decreases rapidly with the increase of current density.When Li 3 N is coated on the surface of the LiCoO 2 electrode, its low conductivity issue can be alleviated, resulting in a slight reduction in the capacity retention even at 10C.In addition to low conductivity, Li 3 N also meets other problems, such as being sensitive to water and incompatible with polar solvents (NMP).Sun et al. [74] improved the stability of Li 3 N by forming Li 2 O and Li 2 CO 3 passivation layers on its surface, so that it can be compatible with low polarization solvents such as tetrahydrofuran (THF).Liu et al. [75] used the high dehydrogenation energy of N, Ndimethylformamide (DMF) instead of NMP as a solvent to stabilize Li 3 N slurry.Compared with Li 3 N, Li 2 O and Li 2 O 2 exhibit no compatibility issue with the existing system. [69,71]However, their high decomposition voltage (>4.7 V vs. Li/Li + ) usually face electrolyte decomposition risks.In addition, the O 2 produced by decomposition is easily dissolved in the electrolyte to form superoxide ions, affecting the cycling performance of the cell.Sun et al. [65] found that forming nanocomposites with transition metals (M/Li 2 O, M = Fe, Co, Ni, Mn, etc.) can reduce the decomposition voltage of Li 2 O to <4.0 V, while maintaining a high lithium supplement capacity (Figure 5).When LFP was mixed with 4.8% Co/Li 2 O, the reversible capacity increased by ~11% and showed better cycle stability.In addition to the abovementioned materials, Li 6 CoO 4 , [63,64,76] Li x C y O z , [77] Li 5 FeO 4 , [70,78,79] Li-Al alloy, [72] and other materials also show high lithium donable capacity.

Conclusion and Perspective
With the deepening of research and the improvement of manufacturing levels in recent years, the design of commercial LIB has become more and more mature.In this context, it is difficult to achieve a significant leap in battery performance only by adjusting the electrode materials and electrolytes.The emergence of prelithiation technology provides an effective mean to break the current cycle life and energy density bottleneck of LIBs.The implementation feasibility of each prelithiation strategy is summarized based on four dimensions, including atmosphere compatibility, manufacturing ability, prelithiation controllability, and energy density (Figure 6).
The ball milling or high-temperature melting involved in the mechanical alloying method is a high-energy process.Although the whole process is protected by inert gas, it is difficult to ensure the safety of large-scale manufacturing due to the strong reactivity of lithium metal and high requirements for equipments.Prelithiation by solution method can realize the lithium supplement at the electrode level, and the semi-quantitative control of the lithium content can be realized based on the prelithiation time.However, the existing prelithiation solution has high reduction potential, which can only form SEI on the surface of graphite, resulting in a relatively low prelithiation efficiency.Meanwhile, high toxic solvents, such as ether, biphenyl, and naphthalene are necessary, usually suffering safety risks.Generally speaking, the mechanical alloy method does not have the potential for industrialization, and the relevant research has decreased year by year.As for the solution method, the key to industrialization lies in whether the solvent system with lower reduction potential (<0.1 V vs. Li/Li + ) can be developed in the future, and then cooperate with the humidity control of the production line to realize the prelithiation of graphite anode.
Besides the above core shortage, another key parameter, adhesion force, which is usually dismissed in lab study cannot be ignored when facing large-scale manufacturing.Due to the high dosage of binder used in the mixing process (~5-10 wt% in the lab vs. 2-3 wt% in practice), the anode adhesion force is significantly higher than the commercial one.Therefore, brittleness deterioration of the electrode after solution prelithiation is not obvious, especially in a coin type or pouch type cell.However, when used in prismatic cells, the length of the electrode is extensively extended, and a lot of driving roller needs to be used to control the winding precision before the anode, cathode, and separator are assembled into a jelly roll.Thus, deterioration of the brittleness should be avoided to inhibit demolding of the active material from the current collector.
For the electrochemical prelithiation, the lithium supplementation could be precisely controlled by adjusting the cut-off voltage and charge current.Simultaneously, through the optimization of electrolyte additives, it can realize the regulation of SEI components to ensure that it is similar to that formed during the cycling process in the real battery.However, it should be mentioned that the cost of the electrolyte accounts for approximately 15%-20% of the total cost of LIBs.Such a large electrolyte requirement is usually associated with high cost, which is unacceptable in commercial applications.At the same time, the reassembly process of the prelithiated electrode is complex and the environmental requirements are high.Therefore, this prelithiation method is mainly suitable for laboratory-level study and not large-scale industrialization.
As one of the main prelithiation strategy that has been industrialized at present, physical contact prelithiation has the advantages of relatively low difficulty in large-scale manufacture and SEI components similar to that formed by the electrochemical method.In practical production, metal lithium can be directly rolled onto the surface of the electrode by controlling the humidity of the production line (<2 ppm).Such low humidity usually means extremely high production cost, which can only be diluted by scale manufacture.Besides, the prelithiation consistency is also of great significance, especially when the designed amount of prelithiated Li exceeds the SEI consumption at a certain N/P ratio.If the local thickness of rolled Li foil is higher than the designed value, the full cell will encounter a high risk of lithium plating after charging to 100% SOC.In addition, despite Li utilization already illustrated by Zhang's group, [41] another key problem is the considerable heat generated during the contact prelithiation process, which is commonly dismissed in lab studies.If there is no efficient and reliable cooling equipment, the prelithiation rate is relatively low.In summary, though contact prelithiation has been already industrialized by many top companies, many problems still need to be handled, such as passivation of lithium metal without affecting the lithium efficiency, prelithiation consistency improvement, Li utilization optimization, and so on (Figure 7).
Overlithiated cathode materials can supplement active lithium without sacrificing the energy density and rate performance of the cell.However, considering the safety, cost, and service life, the existing energy storage batteries, especially ultra long-life energy storage batteries, are mainly based on the LFP cathode route.It means that the manganese and lithium vanadium phosphate-based materials are challenging to be large-scale used in the short term.If the lithium ferromanganese phosphate (LMFP) can solve the problems of low conductivity and poor high-temperature stability, and then be introduced into mass production as the next generation of LFP products, overlithiated LMFP is possible to realize industrial application combined with the overlithiated strategy of manganese materials.Relatively speaking, lithium-rich additives are the most valuable technology to be explored at this stage.Only a small amount of additives can realize a supplement of 5-15% active lithium.For such materials, the application bottleneck is that the electrochemical inactive phase generated after prelithiation will deteriorate the energy density and the rate performance of the cell, thus affecting the power performance of the electric vehicle.In addition, the decomposition of lithium-rich additives is often accompanied by the generation of a large number of gases (O 2 , N 2 , CO 2 , etc.), which worsens the cell interface, and even causes breaking of the electrolyte bridge and induces lithium plating on the electrodes.Therefore, the follow-up research is significant if it can provide effective improvement measures, including carbon coating, low decomposition voltage, low residual lithium compounds, and gas radical capture agents.
Besides the problems mentioned above, the safety of LIBs with prelithiated electrode is also a noteworthy issue.According to our Reprinted with permission from Sun et al. [65] Copyright 2016, Nature Publishing Group.experience, we have not encountered safety issues in the prismatic cell after contact prelithiation yet.Prelithiated cells behave no obviously different from traditional LIBs in national standardized tests, such as GB/T 36276, UN 38.3, and UL 1642.The failure level of prelithiated LFP cells after overcharge, overdischarge, vibration, shock, drop, temperature cycling, and short tests are similar to unprelithiated ones.As for solution method and overlithiated additives, we suppose that the safety issue will not remarkably differ from contact prelithiated ones.However, the deepening study of the failure mechanism of prelithiated cells in the electrode level is still urgent, which is helpful for understanding whether there is an essential difference between prelithiated cells and unprelithiated ones.

Figure 2 .
Figure 2. a) The calculated binding energy of Li with Si, Ge, and Sn.b) Schematic diagram of group IV element prelithiation material synthesized by mechanical metallurgy.Reprinted with permission from Zhao et al.[13]Copyright 2017, Elsevier B. V.

Figure 1 .
Figure 1.Annual distribution of prelithiation-related papers published since 2016.Adapted from web of science, dated 20th December 2021.

Figure 5 .
Figure 5. a) Schematic diagram of contact prelithiation and b) buffer layer controlled contact prelithiation.Reprinted with permission from Meng et al.[40]Copyright 2019, American Chemical Society.

Figure 6 .
Figure 6.Lithium supplement capacity of different M/Li 2 O nanocomposites.Reprinted with permission from Sun et al.[65]Copyright 2016, Nature Publishing Group.

Figure 7 .
Figure 7.Comparison of each prelithiation strategy by radar map.

His research focuses on the design and application of advanced Li-ion power battery and energy storage battery.
Energy Environ.Mater.2023, 6, e12501