Analysis and Recent Progress of Interface in Layered Ni-Rich Cathode Materi-als:A Review
In recent years,due to the urgency and necessity of achieving"carbon peaking and carbon neutrality"goals,lithium-ion batteries had received great attentions as green and clean energy storage devices.Because of the advantages of high energy density,low cost,long cycle life and no memory effect,they were widely used as energy storage devices in various large equipment,especially hy-brid vehicles and electric vehicles(HEVs and EVs).However,the mileage limitation of EV had become the biggest obstacle to the popularity of EV industry.The energy density of EV battery packs and batteries needed to reach 235 Wh·kg-1 and 350 Wh·kg-1,re-spectively,and the cathode material needed to provide an energy density of 800 Wh·kg-1 or more to achieve a similar mileage range compared to conventional vehicles.Even the commercial Li-ion cathode materials such as LiFePO4,LiMn2O4 and LiNi1/3Co1/3Mn1/3O2(NCM111)could not meet this demand.Layered high nickel cathode materials(Ni-rich materials,Ni≥60％)had attracted much atten-tion due to their high theoretical specific energy,high operating voltage,good cyclic performance and rate capability.However,the material suffered from severe interfacial instability problem,leading to poor cyclic performance and safety issues,restricting the wide application of the material.This research reviewed recent studies and summarized the factors leading to the interfacial instability of Ni-rich cathode materials were as the follows:gas releasing(O2,CO and CO2),Li/Ni disordering,irreversible phase transition on materi-al surfaces,dissolution of transition metals(Mn,Co,Ni),formation of secondary particle microcracks,lithium residue and active material consumption due to electrolyte decomposition.To solve these problems,modification approaches included:surface coating,concentration gradient design,metal elemental doping,single crystallization and electrolyte modification.(1)Surface coating strategy could effectively reduce the erosion of the electrolyte on the cathode material,enhancing the stability of the interface and extending the cycle life of the battery.Common coating materials included:inert compounds,lithium-ion conductors,high molecular polymers,car-bon materials,composite materials,etc.Various coating cladding had its own advantages and disadvantages.(2)Concentration gradi-ent design represented for Ni concentration gradient design mostly,including surface doping and core-shell structure.Due to the insta-bility of Ni3+,Ni4+on the surface of Ni rich material particles,it was necessary to dope Ni ions from the core to the surface with a de-creased concentration to obtain high capacity and high interfacial stability of Ni rich materials.However,this strategy was facing the problems of complex processes,high costs and poor consistency.(3)Metal element doping mainly included Li-site doping and Ni-site doping.The former one aimed to support or expand the lithium-ion transport channels,alleviating the formation of microcracks caused by the drastic changing in lattice parameters during cycling and reducing interfacial side reactions;the latter one aimed to stabilize the laminar structure,improving M-O(M=transition metal)bonding energy and reducing the formation of oxygen vacancies.Both could improve the structural stability and enhance the electrochemical properties of Ni rich materials.However,both ways had the potential to cause a reduction in the initial capacity of batteries,so the elemental doping amount was usually small,thus improvement in electro-chemical properties was limited.(4)Single crystallization:Compared to polycrystalline secondary particles,single crystalline parti-cles had better mechanical properties and there were no grain boundaries between particles.Furthermore,due to its high orientation,the problem of cracking could be inhibited during cycling and alleviating side reactions with the electrolyte,thus improving the interfa-cial stability and electrochemical properties of Ni-rich materials.The current difficulty in single crystallization of Ni-rich materials was crystal size control.(5)Electrolyte modification:the purpose of electrolyte modification was to reduce interfacial side reactions and to regulate the formation and composition of chemical-electrochemical interface(CEI)films.Compared to surface coating strategy,which created artificial CEI layers on the surface of secondary particles,electrolyte modification was more convenient and efficient,less cost-ly,avoiding rapid degradation of highly unstable interfaces in air.A common means of doing this was to add multifunctional additives,facilitating the formation of a homogeneous and dense CEI film on the surface of the material,such as vinylene carbonate(VC),prop-1-ene-1,3-sultone(PES)and succinic anhydride(SA).In general,Ni-rich materials had the following problems:(1)the complexity and variability of interfacial problems,making it difficult to fully understand the connections between various problems;(2)a single conventional modification method might not be suitable for Ni-rich materials,which required minor modification or combination of dif-ferent modification methods;(3)the electrolyte as an integral part of the interface had been neglected.Therefore,future research should focus on the following aspects:(1)enhancing theoretical studies:gained an in-depth understanding of the connections between various interface problems by using advanced characterization equipment and combining with theoretical calculations;(2)combing multiple modification strategies to co-build stable and robust interfacial layers;(3)enhancing theoretical studies of electrolytes and optimized electrolyte formulation,etc.
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