Design of high‐performance and sustainable Co‐free Ni‐rich cathodes for next‐generation lithium‐ion batteries

Great attention has been given to high‐performance and inexpensive lithium‐ion batteries (LIBs) in response to the ever‐increasing demand for the explosive growth of electric vehicles (EVs). High‐performance and low‐cost Co‐free Ni‐rich layered cathodes are considered one of the most favorable candidates for next‐generation LIBs because the current supply chain of EVs relies heavily on scarce and expensive Co. Herein, we review the recent research progress on Co‐free Ni‐rich layered cathodes, emphasizing on analyzing the necessity of replacing Co and the popular improvment methods. The current advancements in the design strategies of Co‐free Ni‐rich layered cathodes are summarized in detail. Despite considerable improvements achieved so far, the main technical challenges contributing to the deterioration of Co‐free Ni‐rich cathodes such as detrimental phase transitions, crack formation, and severe interfacial side reactions, are difficult to resolve by a single technique. The cooperation of multiple modification strategies is expected to accelerate the industrialization of Co‐free Ni‐rich layered cathodes, and the corresponding synergistic mechanisms urgently need to be studied. More effects will be aroused to explore high‐performance Co‐free Ni‐rich layered cathodes to promote the sustainable development of LIBs.


INTRODUCTION
2][3][4] Significantly, the global trend in the LIB market has shifted from portable electronics to the motor industry, mainly owing to transport electrification over the last decade.][7] With a forecast that at least 20% of road vehicles will be powered by electricity by 2030, EVs represent a rapidly growing market. 8Accordingly, the demand for highenergy density and low-cost LIBs is dramatically rising due to the fast expansion of electro-mobility worldwide.
Since Sony was the first to commercialize LiCoO 2 (∼140 mAh g −1 ) in 1991, Co has been applied in LIBs for a long time.0][11][12][13] However, Co is a rare metal accounting for just 0.0025% of the Earth's crust.Its price is easily affected by unexpected global events.The Democratic Republic of Congo is reported to be the source of about two-thirds of the world's Co, and Co mining there often involves the ethical issue of child labor. 4,14,15The scarcity of sulfuric acid needed to extract Co can also drive up the cost of Co.Moreover, Co plays a significant role in the production of catalysts, super alloys, magnets, pigments, and semiconductors. 7,16Therefore, the worldwide dependence on Co could intensify competition for raw materials and even lead to geopolitical conflicts, stimulating in-depth research into energy storage technologies without Co. 4,14,15ommercial Co-free LiFePO 4 possesses the merits of excellent thermal stability, superior long-term cycling capability and low price.However, the significant defects of LiFePO 4 are poor conductivity at low temperatures and a low voltage platform of 3.3 V, limiting the specific energy density. 14,15,17Although sulfur is a cheap cathode with a theoretical capacity of up to 1675 mAh g −1 , 15,18 inferior coulombic efficiency, significant volume changes and serious self-discharge rate impede the commercial application of lithium-sulfur batteries. 5The performance of LiNi 0.3 Mn 0.3 Co 0.3 O 2 (NMC333) is similar to that of LiCoO 2 , providing the benefits of high capacity and long cycle life.Nevertheless, the high Co content in NMC333 causes the issue of considerable expenses. 4Consequently, reducing Co content in NMC has been incremental, such as the NMC333 → NMC811 progression. 19,20NMC811 was developed to cut Co consumption and offer higher capacity. 14,15][26] EVs will be powered primarily by LIBs.Consequently, the demand for power LIBs will be 10-20 times higher than it is now by 2030 (Figure 1A).The cathodes usually make up about half of the current cost of LIBs.8][29] Due to the strong reliance on Co in the EV supply chain, EVs are prohibitively expensive at the production level for most consumers.Therefore, the high cost of battery packs has long been a barrier to the rapid development of the EV market. 4The current price of EVs has exceeded that of fuel vehicles, negatively impacting the sustainable development of LIBs. 4,15Co-free Ni-rich layered cathodes with higher specific energy are three times cheaper than commercial LiCoO 2 . 4,151][32] It is worth noting that Ni-rich layered cathodes can provide higher reversible capacity. 4,33Eliminating Co and increasing Ni content is a feasible approach to raise the energy density and reduce the price of LIBs. 4 Herein, we highlight the adverse roles of Co in Ni-rich layered cathodes and then summarize the recent challenges and advanced design strategies of Cofree Ni-rich layered cathodes.Based on the great potential of both increasing capacity and lowering price, we anticipate that the commercialization of Co-free Ni-rich layered cathodes will be achieved early and contribute to the sustainable development of LIBs systems to a new height.

Conventional viewpoint on the role of Co in layered cathodes
5][36] Ni is naturally unstable in the oxide's transition-metal (TM) layer due to its relatively strong magnetic moment.The triangular arrangement of three Ni 3+ cations (Figure 2) can result in "magnetic frustration" because there are always two opposing magnetic moments.Co 3+ is nonmagnetic.Its addition to the TM layer relieves magnetic frustration and creates a stable cathode.7][38][39] However, low-spin Co 3+ F I G U R E 1 (A) Growth of global electric vehicles (EVs) and batteries by 2030.Reproduced with permission. 29Copyright 2021, RSC.(B) Price trends of battery materials from 2010 to 2021.Reproduced with permission. 28Copyright 2022, Wiley-VCH GmbH.
F I G U R E 2 Magnetic frustration relieved situations when Li + or Co 3+ replaces Ni 3+ .Reproduced with permission. 35Copyright 2020, AAAS.
with unpaired electron spin in the d shell is favored for 90 • super-exchange. 34,40Therefore, Co can efficiently reduce the Li/Ni exchange 35,41,42 by both alleviating the magnetic frustration and diminishing the super-exchange intercalations between the TMs and antisite Ni 2+ in Li layers.Introducing more Co into the Ni-rich layered oxides can result in lower degree of Li/Ni disorder.However, the negative impacts of Co on Ni-rich cathodes must be highlighted, especially when designing high-energy cathodes.Compared with NMC622 and NM64, NC64 exhibits an almost perfect multilayer structure with negligible Li/Ni disturbance due to the high content of Co (Figure 3A-F).However, transmission electron microscopy (TEM) morphological investigations indicate that intraparticle fractures could be found along the c-axis of the NC64 particle (Figure 3G). 37Meanwhile, an irreversible lattice oxygen redox is overly activated by Co, leading to the release of oxygen and irreversible structural alterations.Inside the NC64 single particle, a significant number of nanovoids revealed the release of oxygen (Figure 3H). 374][45][46] NC82 has better cycling stability than NC64, further confirming the negative impacts of Co.The in situ X-ray diffraction (XRD) (Figure 3I,J) suggests that the lattice expansion/contraction in NC82 is significantly attenuated because of the pillar function of the Li/Ni disorder, illustrating the benefit of a certain amount (3%-7%) 37 of Li/Ni mixing on structural stability.

Adverse effects of Co on Ni-rich cathodes
Furthermore, the effect of Co on oxygen redox and oxygen release promotes the trend of the Ni-rich layered structure transforming to spinel and rock-salt phases. 47Unlike oxygen next to Ni (Ni 2+ , Ni 3+ , and Ni 4+ ), oxygen close to Co 3+ is more stable.The oxidation of Co 3+ to Co 4+ in the deep delithiated state leads to the release of molecular oxygen.Ni 2+ / 3+ , in contrast to Co 3+ , can be easily oxidized to the +4 state without producing oxygen molecules. 2herefore, the formation of oxygen vacancies can be accelerated by Co, which releases oxygen and heat. 41The Ni-rich cathodes can undergo a sequence of phase transitions: the original layered structure (H1), monoclinic (M), spinel (H2), and rock-salt (H3) phases. 48,49The intensity of the H2-H3 phase transition would increase during the cycling of the Co-containing cathode when the Ni content is higher than 60% (Figure 4A).The extent of microcracking increases rapidly because the abrupt shift in lattice volume induced by the H2-H3 phase transition leads to an increase in anisotropic strain. 48,50,51The electrolyte enters the interior of the Ni-rich cathode particles through channels formed by the microcracks to attack the electrode materials further.Due to the rich Ni content, the amount of active Ni 4+ also increases.Through a side reaction with the electrolyte, the unstable Ni 4+ is quickly reduced to Ni 2+ , releasing oxygen to create an impurity phase of rocksalt.This phase reduces the electrochemical performances of the Ni-rich cathodes owing to a rise in charge transfer impedance.Compared to the Co-free cathode (Figure 4B), Co-rich cathode (Figure 4C) shows larger lattice parameter changes when charging and discharging between 2.8 and 4.5 V, confirming that the addition of Co to Ni-rich layered structure can cause a more serious H2-H3 phase transition when operated under the same conditions. 41,52igure 4D provides the differential scanning calorimetric profiles of LiNi 0.94 Co 0.06 O 2 (NC), LiNi 0.90 Mn 0.05 Co 0.05 O 2 (NMC), and LiNi 0.93 Al 0.05 Ti 0.01 Mg 0.01 O 2 (NATM) cathode materials at the same state of charge.The Co-free NATM displays significantly improved thermal stability compared to the Co-containing cathodes. 53Consequently, Co is one of the critical factors leading to the thermal runaway of Ni-rich cathodes.
The structural evolution of the Co-rich and Co-free cathodes is illustrated in Figure 4E.Co suppresses Li/Ni mixing but induces significant changes in lattice parameters and intergranular cracking.Furthermore, Co continuously activates the unstable O redox, leading to oxygen release and an irreversible phase transition.The capacity degradation mechanism of Co-rich cathodes is mainly determined by these two factors.In contrast, structural stability is improved and oxygen release is inhibited by reducing Co with Mn substitution.As the pillar function of Li/Ni disorder, intergranular microcracks and irreversible phase transitions are absent from Co-free cathodes. 37In summary, Co in Ni-rich cathodes is responsible for structural degradation, deterioration of cycling performance, and thermal runaway. 41,50Expensive Co is also hindering the production of commercial batteries and becoming less attractive. 41Coupled with escalating mining royalties and political turmoil in Africa, researchers are working on reducing or even eliminating the use of Co in the Ni-rich layered cathodes.Co-free Ni-rich layered cathodes have attracted increasing attention. 54,55

CHALLENGES OF Co-FREE Ni-RICH LAYERED CATHODES
Co-free Ni-rich layered cathodes based on layered LNO have two competitive advantages: lower costs and increased energy density.Layered LNO (α-NaFeO 2 -type) with the R-3m space group has been prepared since the 1950s. 56Theoretically, the energy density of LNO is 275 mAh g −1 . 57Because LNO inherently mixes its cations, the formation of an LNO layered structure involves complex control of the synthesis process, such as an appropriate annealing atmosphere and temperature, 4 and it is nearly impossible to create LNO-based materials with theoretical stoichiometric ratios. 580][61] Poor capacity retention of LNO is believed to be primarily related to the excessive H2-H3 structural phase transition. 62,63LNO undergoes a severe H2-H3 phase transition when the cutoff voltage is above 4.3 V (Figure 5A-C). 62Differing at 4.1 and 4.2 V, considerable mechanical damage is observed from the scanning transmission electron microscopy (STEM) image of cycled LNO at 4.3 V (Figure 5D-F).Crack extension and mechanical failure are now increasingly recognized as the primary causes of high-voltage degradation of Ni-rich materials. 48s mentioned in Section 2, Co can accelerate the H2-H3 phase transition of the Ni-rich layered structure.Therefore, eliminating the content of Co in Ni-rich cathodes can greatly improve structural stability.5][66][67][68][69] To mitigate these problems, doping with Mn, Al, Mg, Ti, Fe, etc., elements, surface coating, electrolyte optimization,

Elemental doping
Elemental doping is an efficient technique for improving layered Ni-rich oxides, enabling excellent stability and long-term cyclability. 70Recent papers suggest that Co is less satisfactory for enhancing the electrochemical performances of Ni-rich layered cathodes than other dopants, including Mn, Al, Mg, Ti, Fe, etc. 66,71

Mn doping
Thermal runaway will happen when oxygen from the cathodes reacts with the flammable organic electrolyte. 54,72specially for Ni-rich cathode materials, Mn doping can greatly enhance cycling performance and thermal stability.LiNi 1-x Mn x O 2 (0.1 ≤ x ≤ 0.5) has been produced since 1998 and has shown more excellent stability than LNO. 73A comparative rate capability and sustainability analysis is performed between 2.7 and 4.3 V (Figure 6A,B).Increasing the content of Mn reduces discharge capacity while improving cycling stability.LiNi 0.9 Mn 0.1 O 2 displays an incredibly high capacity of 149 mAh g −1 at 10.0 C. 74 Several years later, LiNi 0.9 Mn 0.1 O 2 , LiNi 0.9 Co 0.1 O 2 , and LiNi 0.9 Co 0.05 Mn 0.05 O 2 were synthesized by Aishova et al., 75 and their physicochemical properties were compared.Although LiNi 0.9 Mn 0.1 O 2 exhibits 3.35% Li/Ni mixing, it has slightly higher cation disorder than LiNi 0.9 Mn 0.05 Co 0.05 O 2 (1.77% Li/Ni mixing) and LiNi 0.9 Co 0.1 O 2 (0.67% Li/Ni mixing).Surprisingly, LiNi 0.9 Mn 0.1 O 2 displays an initial discharge capacity of 236 mAh g −1 and an 88% capacity retention rate after 100 cycles at 0.5 C between 2. when LiNi 0.9 Mn 0.1 O 2 with higher Mn content is charged to 4.4 V. Furthermore, the fracture strength of LiNi 0.9 Mn 0.1 O 2 secondary particles is 175 MPa (Figure 6D).These results demonstrate that raising the Mn content in the Nirich cathode could improve the mechanical strength of the secondary particles.Because the better mechanical strength of LiNi 0.9 Mn 0.1 O 2 prevents the formation and extension of microcracks, the charge transfer resistance (R ct ) of LiNi 0.9 Mn 0.1 O 2 is only 16 Ω after 100 cycles (Figure 6E).Electrically unreactive Mn 4+ can reduce Jahn-Teller distortions. 74From a thermodynamic perspective, Mn 4+ firms the crystal structure by strengthening the bond between Mn and O. 32,76 Mn doping also stabilizes the surface of the cathode particles and alleviates the formation of interfacial impedance between electrolyte and cathode, thus improving the cycle performance. 76Accordingly, Mn, a low-cost and abundant element, can effectively inhibit thermal runaway and improve the safety of Co-free Ni-rich materials in terms of stabilized bulk and surface.Moreover, Mn is popularly employed as an efficient element to cooperate with other elements to further enhance the electrochemical performance of Co-free Ni-rich cathodes.

Al-Mn doping
Recently, dual or triple element doping has been expected to work synergistically to actualize amazing effects superior to those of single elements.8][79] Moreover, Mn will lead to more Li/Ni mixing, while Al can reduce it. 80Mn-Al doping enhances long-cycle capability and thermal stability.
Ni-based layered oxides could mix up to 30%-35% Mn without restructuring into a spinel phase. 81,82However, Al is typically restricted to 6% to prevent the formation of secondary phases. 25,79LiNi 1-x-y Mn x Al y O 2 (NMA), a brand-new substance for Co-free Ni-rich cathodes, has been proposed to replace LiNi 1-x-y Co x Mn y O 2 (NCM) and LiNi 1-x-y Co x Al y O 2 (NCA).The electrochemical results indicate that LiNi 0.883 Mn 0.056 Al 0.061 O 2 displays a capacity of 216 mAh g −1 and capacity retention of 82% after 1000 deep cycles at a voltage range of 2.5-4.2V in a pouch full cell. 19Remarkably, the fast discharge capability of NMA is quite close to that of NCA and NMC, and the rate performance of NMA is better than that of LiNi 0.9 Mn 0.1 O 2 . 75Further investigations 83 reveal that Mn-Al co-doped cathodes exhibit admirable structural reversibility during the H2-H3 phase transition and undergo mild surface reactions, 83 which confirms that the Mn-Al combination favors the cycling performance of Co-free Ni-rich layered oxides.

Mg-Mn doping
Mu et al. 84 discovered that Mg occupied the Li site while Mn was located in the Ni site by Rietveld refinement of neutron diffraction (Figure 7A,B).Mg's occupation of the Li site mitigates the Li/Ni mixing.Moreover, Mg 2+ is an effective pillar ion to relieve the internal stresses in the lattice during anisotropic lattice contraction at high voltages, thus enhancing the structural reversibility of the Ni-rich cathodes during phase transitions. 85,86The Mg-Mn-doped LNO cathode achieves a high capacity of 216 mAh g −1 and exhibits a superior capacity retention rate of 80% after 350 cycles. 84Furthermore, the Ni concentration was analyzed by X-ray fluorescence microscopy on the Li anode.In 100-200 electrochemical cycles, the concentration of Ni deposition is relatively high.It is worth mentioning that the Ni concentration of Mg-Mn-doped LNO after 100 cycles is lower than that of LNO after 50 cycles, indicating that Mg-Mn doping can inhibit Ni dissolution. 84

Fe-Al doping
Fe is an abundant and inexpensive element, and the ionic radius of Fe 3+ is comparable to that of Ni 3+ and Al 3+ (0.55 Å for Fe 3+ , 0.54 Å for Al 3+ , 0.56 Å for Ni 3+ ). 16Fe-Al co-doping significantly enhances structural stability and reduces Li/Ni mixing, contributing to improved electrochemical performance. 16a Co-free pouch battery with a high-performance NFA cathode and a graphite anode was also successfully fabricated.After 200 rounds of charging and discharging, the assembled pouch cell exhibits reasonable capacity retention of 72% at 1/3 C between 3.0 and 4.4 V (Figure 7C), similar to the electrochemical behavior of typical NCM and NCA-type cathodes with Co.

Ti-Mg doping
Ti diffusion in LNO has slow kinetics during hightemperature calcination.1][92] In the Ti-Mg-doped LNO, Mg 2+ is uniformly distributed, and the enrichment of Ti 4+ is on the top of the particles at about 3 nm (Figure 7D-F).The distribution of surface-enriched Ti can stabilize the oxygen on the surface and inhibit the formation of Li 2 CO 3 and NiO-type rock-salt phase, minimizing the negative interactions between the active substance and the electrolyte. 93he Ni concentration of the mated Li anode collected by the Ti-Mg-doped LNO cells after 50 cycles is only about 1/10 of that of the LNO (Figure 7G,H), indicating a marked restraining effect on TM dissolution.The Ti-Mg-doped LNO cathode exhibits greater rate capability than the LNO.After 300 electrochemical cycles at 1.0 C, this cathode maintains 85% of its initial capacity (Figure 7I).Additionally, Ti-Mg co-doped LNO was also synthesized by Wang et al. 55 Benefiting from the robust binding between oxygen and the doped Ti, the surface of doped LNO particles is stabilized by the segregation of Ti on its surface, thus preventing surface-initiated fissures from extending to the core of the particles.As the doped Ti and Mg are also distributed inside the particles, the cracks emerging from the interior of particles are reduced.

Al-Ti-Mg doping
Cui et al. 53 systematically investigated Co-free and Mn-free NATM cathode materials with a Co-containing cathode NC and a Mn-bearing cathode NMC.Powder XRD was applied to analyze the structure of the lithiated cathodes.The as-prepared samples all exhibit a well-defined rhombohedral α-NaFeO 2 crystal structure with the R-3m space group (Figure 8A).Benefiting from the absence of Mn 4+ and the incorporation of Al 3+ , the NATM exhibits no evidence of considerable Li/Ni mixing.Using a high-angle annular dark-field STEM, the surface lattice reconstruction of the NATM, NMC, and NC cathodes after 800 cycles was scanned (Figure 8B).The formed layers are approximately 5-8 nm and 10-15 nm on the NMC and NC particles.However, the thickness of the reconstructed layer on NATM is just 2-3 nm, suggesting that the appropriate proportional mix of Al, Ti, and Mg will substantially contribute to the surface stability of LNO-based cathodes.The H2-H3 structural phase transition of NATM is alleviated without compromising the rapid discharge performance (Figure 8C,D).The long-cycle capability of batteries with NATM cathodes paired with graphite anodes in pouch battery packs was evaluated.The NATM cathode still maintains 82% capacity retention after 800 deep cycles, which far exceeds the NMC (60%) and NC (52%) cathodes (Figure 8E).The NATM material is more resistant to electrolyte attacks than the Co-containing cathode (Figure 8F), which is probably attributed to the surface of enriched Ti and the impact of Al on surface stability.Moreover, the Mg 2+  and NMC secondary particles are entirely broken into tiny pieces after 800 cycles (Figure 8G), while the better particle integrity of NATM can reduce side reactions between the cathode and electrolyte.Accordingly, reasonable multiple doping can play the role of each element and show a better doping effect. 53

Surface coating
Surface coating is a straightforward method to improve the safety and cycling stability of the cathode.The coating layer acts as a barrier and shield between the electrolyte and the active component, greatly reducing the acidic corrosion from the electrolyte and essentially impeding TM from dissolving from the cathode during the cycle. 94he surfaces of commercial cathodes NMC and NCA are usually coated with Al 2 O 3 , TiO 2 , SiO 2 , AlPO 4 , ZrO 2 , etc. [95][96][97][98][99][100][101][102] Nevertheless, the capacity of the cathode is limited by typical coating materials such as Al 2 O 3 , SiO 2 , etc., because of the low Li + and electron conductivity. 103
of a high-capacity Ni-rich core with the excellent structural and thermal stability of a low-Ni-content shell. 104The core portion of the Ni-rich content experiences abrupt lattice contraction during the emergence of the H3 phase, while the relatively delayed H3 phase transition in the shell layer of the low Ni content prevents drastic changes in cell size, thereby inhibiting the extension of fissures in the particle core to the surface and reducing side reactions between the active components and the electrolyte.Preliminary studies on cathode materials with LNO as the core and Mg, Al, and Mn added coating shells were conducted.Using a Co-free precursor engaged in a 750 C reaction with LiOH, Liu et al. 103 synthesized core-shell cathode with 1 µm Ni 0.8 Mn 0.2 (OH) 2 as the shell layer and 16 µm Ni(OH) 2 as the core (Figure 9A).The relatively low Ni content and extended cycle life of the shell protects the high specific capacity Ni-rich core from direct contact with the electrolyte, enabling the core-shell construction to exhibit better cycle life than the Ni-rich core alone. 103The specific capacity of the cathodes lithiated at 750 C and 800 C is slightly higher than that of NCM811, and the 750 C lithiated core-shell material demonstrates a similar cycling performance to that of NCM811.However, the 800 C lithiated core-shell cathode shows an even worse cycling performance. 103Subsequently, they systematically studied the relationships between synthesis temperature, coating composition, and the impact of coating layer thickness on mechanical performance, reversible capacity, and capacity retention rate of the core-shell material.They found that selecting the heating temperature of the core-shell cathode was critical.Excessively high temperatures will lead to core-shell interdiffusion, and insufficient temperature can trigger more Li/Ni mixing. 105The best material they synthesized delivered a capacity of ∼200 mAh g −1 at 0.2 C and a capacity retention rate of 93% after 100 cycles. 105n a recent study, Park et al. 106 proposed a nanostructured concentration gradient cathode with a Ni-rich core and a 2 µm thick Mn-rich shell (Figure 9B), named Ns-NM90.Figure 9C,D shows that the secondary particles of the conventional LiNi 0.9 Mn 0.1 O 2 (NM90) cathode consist of relatively large, almost equiaxed, randomly oriented primary particles.In contrast, the secondary particles of the Ns-NM90 are composed of closely spaced rod-shaped primary particles arranged in a radial direction to form a spoke-like structure.The unique morphology of Ns-NM90 is attributed to the distribution of secondary particles' Ni-Mn composition, creating a driving force elongating the primary particles due to the gradual change in TM concentration.The advanced structure of the Ns-NM90 enhances the mechanical strength and reduces the internal strains caused by heterogeneous charge states.The initial discharge capacity of Ns-NM90 is approximately 233 mAh g −1 .
Because the restriction of the microcracks inside the secondary particles can effectively avoid electrolyte erosion, Ns-NM90 retains 79.5% of its initial capacity after 2000 cycles under 3.0 C charge and 1.0 C discharge conditions.Moreover, the synergistic modification contributes to the excellent thermal stability of the Ns-NM90 cathode.The as-prepared Co-free layered cathode is both fast-charging and durable and is more economically viable than commercial Co-containing cathodes, making it suitable for use in EVs to replace the current NMC, NCA, etc. 106

Surface coating combined with doping
The conductive coating can improve the conductivity of the cathodes, enhance the diffusion of Li + on the material surface and reduce the cathode/electrolyte interface resistance. 107,108Recently, polypyrene (PPy) coating layer featuring attractive flexibility and conductivity was employed on LiNi 0.95 Mn 0.05 O 2 surface to promote the electrochemical performance. 109It is worth noting that the reversible capacity and capacity retention of the PPy-coated LiNi 0.95 Mn 0.05 O 2 have been greatly improved, especially at high rates.Bare and PPy-coated LiNi 0.95 Mn 0.05 O 2 after 100 cycles between 2.7 and 4.5 V were investigated by scanning electron microscopy (SEM) and TEM.Obvious mechanical cracking can be detected on the surface of uncoated LiNi 0.95 Mn 0.05 O 2 (Figure 9E).Conversely, PPy-coated sample still displays integrated secondary particles with an undamaged spherical morphology (Figure 9F).The rough surface of the uncoated cathode (Figure 9G) is induced by serious electrolyte corrosion.Whereas, the PPy-coated LiNi 0.95 Mn 0.05 O 2 maintains a smooth surface with well-defined boundaries (Figure 9H).These results demonstrate that coating a flexible and conductive polymer (Figure 9I) on Co-free Ni-rich cathodes is a practical strategy to achieve admired structural stability and excellent rate performance.1][112][113][114] Yoon et al. 115 conducted an experiment in which Zr 4+ above their solubility limit was injected during the formation of Ni(OH) 2 precursors.In the lithiation state, Zr 4+ is added to the LNO particles.Excessive Zr ions diffuse to the particle surface and interact with LiOH to spontaneously create a stable self-passivated Li 2 ZrO 3 layer of 5-10 nm (Figure 9J).This thin Li 2 ZrO 3 cladding serves as a stable and impedance-reducing electrolyte interface layer, allowing the coated Zr-doped LNO to achieve improved thermal stability and capacity retention.Moreover, Zr doping stabilizes the structure in the deeply charged state and postpones the harmful phase transition.The smooth transition instead of the abrupt contraction in the c-axis enables the particles to better absorb the internal strain caused by the anisotropic lattice contraction.When cycled between 2.7 and 4.3 V, the initial discharge capacity of the Li 2 ZrO 3 -coated Zr 4+ -doped LNO is 233 mAh g −1 , which is significantly higher than that of the currently reported NMC and NCA cathodes.The capacity retention of the Li 2 ZrO 3 -coated Zr 4+ -doped LNO after 100 cycles at 0.5 C reaches 86%, better than the cycling performance of the LNO cathode (74%) under the same conditions (Figure 9K).

Electrolyte modification
Due to the high content of Ni in Co-free Ni-rich layered cathodes, the increase in highly oxidative Ni 4+ potentially aggravates the electrolyte decomposition, resulting in battery failure and complicated surface chemistry reactions of cathode electrolyte interphase (CEI). 116,117Furthermore, corrosive by-products such as hydrofluoric acid (HF) of conventional Li hexafluorophosphate (LiPF 6 )/carbonate electrolytes can damage the surface of Ni-rich cathodes and cause additional loss of TM and oxygen.These aging processes are accelerated when the operating voltage or temperature is raised. 118,119Since the degradation occurs mainly on the surface of the cathode, the breakdown rate depends primarily on the CEI. 51A thick CEI caused by severe surface degradation during cycling can lead to an increase in interfacial impedance. 120At the same time, a thin and electrolyte corrosion-resistant CEI will facilitate the kinetics of Li + transport and slow the aging rate at elevated operating voltages or temperatures to achieve long cycling life of the Co-free Ni-rich layered cathodes. 121,122Particularly, it is more practical to form a thin and electrolyte corrosion-resistant CEI via electrolyte modification.Deng et al. 123 created a unique CEI with fluoride (F) and boron (B) enrichment on LNO (Figure 10A) by adding small amounts of lithium difluoro(oxalate)borate (LiD-FOB) to the fluorinated electrolyte and the modified cathode maintained a high capacity retention of over 80% at a high cutoff voltage of 4.4 V after 400 deep cycles (Figure 10B).The formation of F-and B-rich interfacial phases on the surface of the LNO cathode (Figure 10C-E) is critical to the integrity of the LNO during lithiation and delithiation.The modified CEI contains less C-O and C═O, suggesting that the electrolyte decomposes less and that the DFOB-ligand promotes more lithium oxalate and ROCO 2 Li in the CEI.Therefore, the formed CEI is robust and thin, capable of reducing the dissolution and irreversible conversion of Ni into the undesired NiO rocksalt phase by shielding the surface from interactions with by-products of electrolyte oxidation at high voltages.In addition, by inhibiting Li-solvent co-intercalation, F-and B-rich solid electrolyte interphase (SEI) achieves steady Li + intercalation and deintercalation in the layered structure of graphite.This modification strategy considerably improves the long-cycle performance of the LNO without the expense of its energy density.
Based on a readily available and inexpensive LiPF 6 , partly saturated electrolyte (LSE) with restricted solubility in carbonate solvents was proposed by Manthiram and coworkers. 124After 600 cycles at a high cutoff voltage of 4.4 V, the LSE-modified cathode exhibits 81% of its initial capacity, the best cycle performance ever for an LNO cathode.According to experimental findings, the formed CEI also has less organic and more inorganic components.Further characterization proved that the CEI contained LiF, Li x PF y O z , and LiNi x F y O z .The in situ formation of CEI with high LiF content is believed to be an ideal shield to protect the LNO surface from further reactions with the electrolyte without reducing the ion transport kinetics.Additionally, the degraded LiPF 6 species, regarded as Li x PF y O z , has been shown to effectively depress the dissolution of TM and enhance the stability of electrodes.These components safeguard LNO against attack from HF and other free radical groups in the electrolyte, thereby reducing oxygen loss and surface reconstruction into the disordered phases.Moreover, LSE favors establishing a robust and inorganic-rich SEI on the Li metal anodes, resulting in better cycling stability of batteries using LSE.
Zhang et al. 125 designed an electrolyte of LiFSI, dimethyl carbonate, ethylene carbonate (EC), and 1,1,2,2tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether (TTE) at a molar ratio of 1:2:0.2:3,labeled as EC-LHCE.LiNi 0.96 Mg 0.02 Ti 0.02 O 2 (NMT) cathode and graphite anode paired with EC-LHCE as the electrolyte display 97.2% capacity retention after 200 cycles at a high charging voltage of 4.4 V. Uniform and thin electrolyte interfaces are formed on both the surface of the NMT cathode and the graphite anode circulating in the EC-LHCE.Deeper decomposition of LiFSI salt and more TTE are enabled by EC-LHCE, resulting in a protective SEI with fluorinated specials and ion-conducting Li salts on the graphite anode surface.The stable CEI formed in EC-LHCE also contains more Li salts (such as LiF, LiNO x , and Li x SO y ), thus contributing to more efficient Li + diffusion and effectively inhibiting cathode corrosion and TM dissolution.Consequently, there is hardly any phase change in the surface region of the NMT cathode after cycling, and the cycled surface structure is comparable to that of the original NMT, which reveals that EC-LHCE can significantly suppress the degradation of the NMT cathode.
Owing to the catalytic effect of the highly oxidized TM ions, particularly Ni 4+ , 126,127 the EC molecules tend to dehydrogenate on the surface of cathode particles.EC molecules also tend to react with the oxygen released from the Ni-rich cathode at high temperatures, resulting in thermal runaway. 83,128Pan et al. 129 prepared EC-free electrolyte with high ionic conductivity, low battery interface impedance, and excellent Li passivation ability.The EC-free electrolyte is based on 1.5 M LiPF 6 -ethyl methyl carbonate (EMC) with the addition of 20 wt% fluoroethylene carbonate (FEC) and 1 wt% lithium 4,5-dicyano-2-(trifluoromethyl)imidazole (LiTDI).FEC usually undergoes rapid degradation during storage.LiTDI acts as a moisture scavenger additive to mitigate the hydrolysis of LiPF 6 and improves the storage life of FEC-containing electrolytes. 130The EC-free electrolyte can prevent side reactions on the Ni-rich cathode due to the absence of the highly oxidizable EC, leading to a reduction in the thickness of the CEI.The electrolyte containing the FEC additive effectively passivates the graphite anode, and fewer Ni 2+ are solubilized into the electrolyte and deposited on the graphite anode.Adding LiTDI to an ECfree electrolyte containing FEC allows for the formation of a thicker but excellent Li + -conductive SEI.In addition, doping with Mn and Al decreases the catalytic oxidation of Ni 4+ , hindering electrolyte decomposition and structural reconstruction.Therefore, LiNi 0.9 Mn 0.05 Al 0.05 O 2 exhibits discharge capacity of 158 mAh g −1 after 500 cycles at 1.0 C with 80% capacity retention in the optimized electrolyte. 129hen, Kim et al. 33 prepared a Co-free Ni-rich LIB with superior cycle performance using a polyimide/polyvinylpyrrolidone (PP) surface-coated NMT combined with advanced localized high-concentration electrolyte (LHCE) modification.The LHCE consists of an electrolyte of LiFSI, 1,2-dimethoxyethane, TTE, and FEC in a molar ratio of 1:1.1:3:0.2,aiming to enhance the performance of graphite||NMT battery.The doping of Mg and Ti reduces cation mixing and impedes the release of singlet oxygen from NMT at high voltages, thereby reducing electrolyte decomposition.The PP coating largely alleviates the contamination of the cathode surface induced by residual Li compounds such as LiOH and Li 2 CO 3 and effectively inhibits side reactions between the cathode particle surface and carbonate-based electrolyte (Figure 10F).The electrolyte-modified SEI contains Li 3 N, Li x S y , Li dithionate, sulfates, and amines, many of which are highly ionically conductive. 131Mg-Ti doping and PP coating make the cathode surface more stable and reduce the side reactions between the cathode and electrolyte, resulting in a thin CEI.Consequently, SEI with high ionic conductivity and thin CEI depresses impedance.Benefiting from the PP coating, Mg-Ti doping, and the combination of advanced electrolyte modification, the graphite||NMT battery presents an 86.7% capacity retention rate after 500 cycles (Figure 10G).
In summary, differing from many improving methods that make synthesis more complex, electrolyte modification can boost electrochemical performances without sacrificing the energy density of the cathode and significantly raising the cost.It is facile to combine with other modification methods.

Single-crystal technology
Single-crystal materials can form more uniform particles and have a less fragile structure than polycrystalline materials, allowing more excellent compression resistance during cycling. 132,133Due to anisotropic shrinkage and expansion during the electrochemical cycle, intergranular cracks are easily formed in the polycrystalline secondary particles.Furthermore, the graded structure of secondary particles with grain boundaries will produce more severe microcracks and speed up the side reactions at the electrode-electrolyte interface. 134,135Single-crystal Co-free Ni-rich cathode has not been extensively researched.
Recently, Dai et al. 136 prepared both the single-crystal and polycrystalline LiNi 0.9 Mn 0.1 O 2 (noted as SC-NM91 and PC-NM91, respectively).Similar to the previously reported NM91, both cathodes show an initial coulombic efficiency of around 85% and an initial discharge capacity of nearly 200 mAh g −1 at 0.1 C. Nevertheless, SC-NM91 achieves a capacity retention rate of 85.3% when cycled at 1.0 C for 300 cycles, while under the same conditions, only 65.8% of the capacity is retained by PC-NM91.SC-NM91 maintains 95.8% of its initial capacity after 65 cycles, while the PC-NM91 displays a faster capacity decay with 83.69% capacity retention.Cross-sectional SEM images exhibit cracks and severe surface pulverization of PC-NM91 particles cycled under different conditions after 200 cycles (Figure 11A).Under the same circumstances, SC-NM91 particles maintain high mechanical integrity and therefore have excellent voltage tolerance and thermal stability.The improved performance of SC-NM91 can be attributed to the less dissolution of TM and a more robust structure resisting cracks and surface pulverization.Moreover, on the surface of SC-NM91 particles, fewer electrolyte breakdown products provide a uniform and thin CEI, enhancing the cathode/electrolyte interface's durability and reducing electrolyte degradation caused by the cathode (Figure 11B).However, the synthesis of single-crystal materials generally requires higher temperatures to drive the precursors so that primary particles with different orientations can fuse and eventually grow into single-crystal particles.Crystallization is usually slow and residual grain boundaries may be present, leading to degradation of electrochemical properties. 24,137,138Recent studies have revealed that the molten salt synthesis method promotes the rapid formation of layered structures from precursors at lower calcination temperatures.The surface of the precursor becomes sub-stable due to the strong polarizing forces provided by the molten salt, facilitating the insertion of Li + into the layers of the precursor to form layered oxides and reducing the formation energy of the stabile layered structure.The experiments by Liu et al. 24 provide an in-depth study of material synthesis from the perspective of controlling the kinetic path of the reaction, providing a new idea for accelerating the development of high-performance Co-free Ni-rich single-crystal layered oxides.
Although single-crystal materials may enhance cycling stability, their capacity and reversibility are limited by the poor diffusion kinetics of Li + .The narrow interlayer spacing in the single-crystal material 30,139,140 can also result in structural flaws such as particle rupture, particle plane slide, and spatial stress inhomogeneity during the electrochemical response.Fortunately, this structural damage can be reduced by doping and surface coating the particles with heterogeneous materials. 141It is essential for industry and academia to promote the production of single-crystal particles more practically and affordably.

CONCLUSION AND OUTLOOK
Co-free Ni-rich layered cathodes deliver the advantages of high specific capacity, superior coulombic efficiency, and attractive price.However, they suffer from TM dissolution, structural degradation, microcrack formation, and surface-side reactions.We review the recent design strategies of Co-free Ni-rich layered cathodes, including elemental doping, surface coating, electrolyte optimization, on the electrochemical and safety performances of the battery.The high reactivity of the Ni-rich cathode and electrolyte can greatly damage the structure and interface of the cathode.Therefore, suitable electrolyte modification is required to achieve better performance.(4) Due to their stable structure and robust mechanical strength, single-crystal materials exhibit outstanding thermal stability and cycling performance.However, the cost and complexity of preparation of single-crystal materials are higher than those of polycrystalline materials, and they suffer from the problem of low Li + diffusion rate.Singlecrystal layered Co-free Ni-rich materials are still at an early stage of development and need further optimization and research.
Accordingly, no single modification strategy can solve all of the problems of Co-free Ni-rich cathodes.The favorite application of Co-free Ni-rich cathodes improved by a single method may be only in the energy storage system due to their higher capacity and lower price but unsatisfactory stability.The co-modification of multiple strategies can suppress the phase transition of the structure as well as reduce unnecessary side reactions on the electrode surface, showing much more potential than using a single strategy.Table 1 summarizes the electrochemical performances of Co-free Ni-rich cathodes after modification.As shown in Table 1, Co-free Ni-rich layered cathodes modified by a combination of classical methods are fast-charging and durable, making them suitable for use in both the energy storage systems and EVs.Notably, co-modification with element doping and an optimized electrolyte is a successful strategy.Nevertheless, the synergistic mechanisms of multiple modification strategies urgently need to be deeply studied in the future.
Co-free Li-rich layered cathodes also feature higher energy density and lower cost.Recent studies on Co-free Li-rich layered cathodes show promising progress.Xu et al. 142 found a pre-activation approach for chemically modified Li 1.2 Ni 0.2 Mn 0.6 O 2 with CaCl 2 ⋅6H 2 O, which altered the state of the lattice oxygen and contributed to excellent chemical reversibility.Yao et al. 143 developed a Li-rich layered cathode with low-strain and no Co.The cathode material was created through surface integration, bulk phase doping, and concentration gradient modification.The volumetric strain of the as-prepared cathode is only 1.05% between 2.0 and 4.8 V, close to the critical value of zero strain.The low strain makes the cathode more resistant to cracking and can effectively inhibit TM migration, interfacial reactions, and structural degradation.Moreover, oxygen defects have been linked to structural degradation and reduced cycling performance in Co-free Ni-rich layered cathodes.However, the impact of oxygen defects on Li-rich layered oxides remains uncertain.According to the results of Zhang et al., 144 the presence of nanoscale oxygen defects increases the surface area exposed to the electrolyte, promoting the electrochemical activation process and enhancing the capacity.Although they exhibit unsatisfactory rate performance due to the poor electrical conductivity of the Li 2 MnO 3 component, as well as inferior oxygen redox and surface kinetics, Co-free Li-rich layered cathodes are attractive candidates for grid-level energy storage systems due to their high capacity and competitive cost. 145,146Both Ni-and Li-rich layered cathodes are moving toward being Co-free for the sustainable development of LIBs.

A C K N O W L E D G M E N T S
This work was financially supported by the National Natural Science Foundation of China (51108455 and 52106264), the Civil Aviation Safety Capacity Building Fund (ADSA2022026), the LiaoNing Revitalization Talents Program (XLYC2008013), and the Liaoning Province Applied Foundation Research Program Project (2023JH2/101300215).

C O N F L I C T O F I N T E R E S T S TAT E M E N T
The authors declare no conflict of interest.Dr. Gang Wu is an Associate Editor of SusMat and a coauthor of this article.To minimize bias, he was excluded from all editorial decision making related to the acceptance of this article for publication.

First,
Co exacerbates the changes in lattice characteristics and speeds up the formation of intralattice microcracks due to the suppression of Li/Ni disorder.To explore the interactions between Co content and structural characteristics in Ni-rich cathodes, Liu et al. 37 synthesized LiNi 0.6 Co 0.4 O 2 (NC64), LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622), LiNi 0.6 Mn 0.4 O 2 (NM64), and LiNi 0.8 Co 0.2 O 2 (NC82).

F I G U R E 3
High-energy X-ray diffraction (HEXRD) and Rietveld refinement of LiNi 0.6 Co 0.4 O 2 (NC64) (A), LiNi 0.6 Mn 0.2 Co 0.2 O 2 (NMC622) (B), and LiNi 0.6 Mn 0.4 O 2 (NM64) (C).Transmission electron microscopy (TEM) images observed at atomic level with 1 nm scale bars (the bright columns with the uniform arrangement should be arranged as transition metals [TMs], and the dark columns between the columns of the TM should be arranged as Li layers) of NC64 (D), NMC622 (E), and NM64 (F).TEM images of NC64 with 100 nm (G) and 20 nm (H) scale bars after 100 cycles.The 2D contour plots of in situ X-ray diffraction (XRD) during the structural evolution of NC64 (I) and LiNi 0.8 Co 0.2 O 2 (NC82) (J).Reproduced with permission. 37Copyright 2021, Springer Nature Limited.
7 and 4.4 V, showing superior reversible capacity and cycling performance compared to the two Co-containing cathodes.At 60 C, LiNi 0.9 Mn 0.1 O 2 presents a higher initial discharge capacity than at 30 C and a capacity retention rate of 93% after 100 cycles.The crosssections of the three cathodes in the first charged state are exhibited in Figure 6C.No obvious microcrack appears F I G U R E 6 (A) Cycling stability and capacity retention of LiNi 1-x Mn x O 2 (0.1 ≤ x ≤ 0.5).1-20 cycled at 0.2 C and 21-120 cycled at 0.5 C. (B) The rate capability of LiNi 1-x Mn x O 2 (0.1 ≤ x ≤ 0.5).Reproduced with permission. 74Copyright 2013, ACS Appl.Mater.Interfaces.(C) Cross-sectional scanning electron microscopy (SEM) images of the first charged LiNi 0.9 Co 0.1 O 2 (NC90), LiNi 0.9 Co 0.05 Mn 0.05 O 2 (NCM90), and LiNi 0.9 Mn 0.1 O 2 (NM90).Average microcompression test results (D) and R ct (E) of the NC90, NCM90, and NM90.Reproduced with permission. 75Copyright 2019, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

F I G U R E 7
(A) Neutron diffraction (ND) pattern with Rietveld refinement of the Mg-Mn-doped LiNiO 2 (LNO).(B) Atomic structure of the Mg-Mn-doped LNO.Reproduced with permission.84Copyright 2020, ACS Appl.Mater.Interfaces.(C) Charge/discharge cycling performance evaluation of the LiNi 0.85 Fe 0.052 Al 0.091 O 2 (NFA).Reproduced with permission.89Copyright 2020, Elsevier B.V. Scanning transmission electron microscopy (STEM)-energy dispersive spectrometer (EDS) plots of composition scale (D) and concentration scale (E and F) of selected particles.X-ray fluorescence microscopy (XFM) on the Li countered with Mg-Ti-doped LNO (G) and LNO (H) cathodes operated at 0.3 C after 50 cycles between 2.5 and 4.4 V (the colors represent the Ni concentration).(I) Cycling performances of the Mg-Ti-doped LNO between 2.5 and 4.4 V. Reproduced with permission.90Copyright 2019, ACS Chem.Mater.
in the Li layers postpones the migration of Ni from the TM sites to the Li sites and the corresponding layered-to-spinel phase transition.The strong bond energy of the Al-O, Ti-O, and Mg-O bonds hinders oxygen release, resulting in better structural stability of the NATM than the Co-containing cathodes.Due to anisotropic lattice distortion, most NC F I G U R E 8 (A) Powder X-ray diffraction (XRD) patterns of pristine LiNi 0.94 Co 0.06 O 2 (NC), LiNi 0.90 Mn 0.05 Co 0.05 O 2 (NMC), and LiNi 0.93 Al 0.05 Ti 0.01 Mg 0.01 O 2 (NATM).(B) High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of the primary particles of NC, NMC, and NATM (scale bars: 5 nm).(C) dQ/dV curves of the third cycle.(D) Rate performance test results.(E) Performance test results of pouched full cells during 800 cycles.(F) Overlap maps of selected secondary ion fragments (scale bars: 5 µm).(G) Scanning electron microscopy (SEM) images of cathodes (scale bars: 20 µm).Reproduced with permission. 53Copyright 2021, Wiley-VCH GmbH.

F I G U R E 1 1
(A) Scanning electron microscopy (SEM) images of polycrystalline LiNi 0.9 Mn 0.1 O 2 (PC-NM91) (I and II) and single-crystal LiNi 0.9 Mn 0.1 O 2 (SC-NM91) (III and IV) after 200 cycles between 3.0 and 4.5 V at 1.0 C. SEM images of PC-NM91 (V and VI) and SC-NM91 (VII and VIII) after 200 cycles between 3.0 and 4.3 V at 2.0 C. (B) Schematic illustration of the mechanism by which the single-crystal structure mitigates the structural degradation and performance deterioration of LiNi 0.9 Mn 0.1 O 2 .Reproduced with permission.136Copyright 2022, ACS Sustainable Chem.Eng. and single-crystal technology.Different improving methods have their merits and shortcomings: (1) element doping improves structural stability.Large quantities of inexpensive elements are verified to enhance cycling performance and thermal stability.However, the proportion of active components in the cathode material decreases when inactive ingredients are added, which is not conducive to realizing high specific capacity.In addition, elemental doping has no apparent effect on inhibiting electrolyte corrosion and decomposition or reducing the interface resistance of the batteries.(2) The surface coating, similar to elemental doping, raises the inactive weight in the cathode and consequently decreases the specific capacity.The surface coating strategy can effectively protect the surface of the cathode, reduce moisture and electrolyte attack, and inhibit surface phase change and oxygen loss.However, the surface coating cannot fundamentally inhibit structural transformation in the bulk of Co-free Ni-rich layered materials.(3) The electrolyte is the diffusion medium for Li + , displaying a significant effect TA B L E 1 Electrochemical performances of Co-free Ni-rich cathodes after modification.
Among them, the LiNi 0.8 Fe 0.05 Al 0.15 O 2 obtained by the sol-gel method demonstrates acceptable rate capability and maintains approximately 80% of the initial capacity after 100 cycles.Overcoming the challenge of simultaneous co-precipitation of Al, Ni, and Fe ions, 16,88Muralidharan et al.16designed LiNi 0.8 Fe y Al z O 2 (NFA) to improve structural stability and deliver long cycle life. Despie apparent differences in rate and cycling performances, all the NFA samples exhibit a discharge capacity of about 200 mAh g −1 at 0.05 C.