Ni and Co Segregations on Selective Surface Facets and Rational Design of Layered Lithium Transition‐Metal Oxide Cathodes

The chemical processes occurring on the surface of cathode materials during battery cycling play a crucial role in determining battery's performance. However, the understanding of such surface chemistry is far from clear due to the complexity of redox chemistry during battery charge/discharge. Through intensive aberration corrected STEM investigation on ten layered oxide cathode materials, two important findings on the pristine oxides are reported. First, Ni and Co show strong plane selectivity when building up their respective surface segregation layers (SSLs). Specifically, Ni‐SSL is exclusively developed on (200)_m facet in Li–Mn‐rich oxides (monoclinic C2/m symmetry) and on (012)_h facet in Mn–Ni equally rich oxides (hexagonal R‐3m symmetry), while Co‐SSL has a strong preference to (20?2)_m plane with minimal Co‐SSL also developed on some other planes in Li–Mn‐rich cathodes. Structurally, Ni‐SSLs tend to form spinel‐like lattice while Co‐SSLs are in a rock‐salt‐like structure. Second, by increasing Ni concentration in these layered oxides, Ni and Co SSLs can be suppressed and even eliminated. The findings indicate that Ni and Co SSLs are tunable through controlling particle morphology and oxide composition, which opens up a new way for future rational design and synthesis of cathode materials.     

High‐Voltage‐Driven Surface Structuring and Electrochemical Stabilization of Ni‐Rich Layered Cathode Materials for Li Rechargeable Batteries

Layered lithium–nickel–cobalt–manganese oxide (NCM) materials have emerged as promising alternative cathode materials owing to their high energy density and electrochemical stability. Although high reversible capacity has been achieved for Ni‐rich NCM materials when charged beyond 4.2 V versus Li⁺/Li, full lithium utilization is hindered by the pronounced structural degradation and electrolyte decomposition. Herein, the unexpected realization of sustained working voltage as well as improved electrochemical performance upon electrochemical cycling at a high operating voltage of 4.9 V in the Ni‐rich NCM LiNi_0.895Co_0.085Mn_0.02O₂ is presented. The improved electrochemical performance at a high working voltage at 4.9 V is attributed to the removal of the resistive Ni²⁺O rock‐salt surface layer, which stabilizes the voltage profile and improves retention of the energy density during electrochemical cycling. The manifestation of the layered Ni²⁺O rock‐salt phase along with the structural evolution related to the metal dissolution are probed using in situ X‐ray diffraction, neutron diffraction, transmission electron microscopy, and X‐ray absorption spectroscopy. The findings help unravel the structural complexities associated with high working voltages and offer insight for the design of advanced battery materials, enabling the realization of fully reversible lithium extraction in Ni‐rich NCM materials.     

Understanding the Rate Capability of High‐Energy‐Density Li‐Rich Layered Li1.2Ni0.15Co0.1Mn0.55O2 Cathode Materials

The high‐energy‐density, Li‐rich layered materials, i.e., xLiMO₂(1‐x)Li₂MnO₃, are promising candidate cathode materials for electric energy storage in plug‐in hybrid electric vehicles (PHEVs) and electric vehicles (EVs). The relatively low rate capability is one of the major problems that need to be resolved for these materials. To gain insight into the key factors that limit the rate capability, in situ X‐ray absorption spectroscopy (XAS) and X‐ray diffraction (XRD) studies of the cathode material, Li_1.2Ni_0.15Co_0.1Mn_0.55O₂ [0.5Li(Ni_0.375Co_0.25 Mn_0.375)O₂·0.5Li₂MnO₃], are carried out. The partial capacity contributed by different structural components and transition metal elements is elucidated and correlated with local structure changes. The characteristic reaction kinetics for each element are identified using a novel time‐resolved XAS technique. Direct experimental evidence is obtained showing that Mn sites have much poorer reaction kinetics both before and after the initial activation of Li₂MnO₃, compared to Ni and Co. These results indicate that Li₂MnO₃ may be the key component that limits the rate capability of Li‐rich layered materials and provide guidance for designing Li‐rich layered materials with the desired balance of energy density and rate capability for different applications.     

Lithium‐ and Manganese‐Rich Oxide Cathode Materials for High‐Energy Lithium Ion Batteries

Layered lithium‐ and manganese‐rich oxides (LMROs), described as xLi₂MnO₃·(1–x)LiMO₂ or Li_1+yM_1–yO₂ (M = Mn, Ni, Co, etc., 0 < x <1, 0 < y ≤ 0.33), have attracted much attention as cathode materials for lithium ion batteries in recent years. They exhibit very promising capacities, up to above 300 mA h g^?1, due to transition metal redox reactions and unconventional oxygen anion redox reaction. However, they suffer from structural degradation and severe voltage fade (i.e., decreasing energy storage) upon cycling, which are plaguing their practical application. Thus, this review will aim to describe the pristine structure, high‐capacity mechanisms and structure evolutions of LMROs. Also, recent progress associated with understanding and mitigating the voltage decay of LMROs will be discussed. Several approaches to solve this problem, such as adjusting cycling voltage window and chemical composition, optimizing synthesis strategy, controlling morphology, doping, surface modification, constructing core‐shell and layered‐spinel hetero structures, are described in detail.     

Stabilized Co‐Free Li‐Rich Oxide Cathode Particles with An Artificial Surface Prereconstruction

Li‐rich metal oxide (LXMO) cathodes have attracted intense interest for rechargeable batteries because of their high capacity above 250 mAh g^?1. However, the side effects of hybrid anion and cation redox (HACR) reactions, such as oxygen release and phase collapse that result from global oxygen migration (GOM), have prohibited the commercialization of LXMO. GOM not only destabilizes the oxygen sublattice in cycling, aggravating the well‐known voltage fading, but also intensifies electrolyte decomposition and Mn dissolution, causing severe full‐cell performance degradation. Herein, an artificial surface prereconstruction (ASR) for Li_1.2Mn_0.6Ni_0.2O₂ particles with a molten‐molybdate leaching is conducted, which creates a crystal‐dense anion‐redox‐free LiMn_1.5Ni_0.5O₄ shell that completely encloses the LXMO lattice (ASR‐LXMO). Differential electrochemical mass spectroscopy and soft X‐ray absorption spectroscopy analyses demonstrate that GOM is shut down in cycling, which not only stabilizes HACR in ASR‐LXMO, but also mitigates the electrolyte decomposition and Mn dissolution. ASR‐LXMO displays greatly stabilized cycling performance as it retains 237.4 mAh g^?1 with an average discharge voltage of 3.30 V after 200 cycles. More crucially, while the pristine LXMO cycling cannot survive 90 cycles in a pouch full‐cell matched with a commercial graphite anode and lean (2 g A^?1 h^?1) electrolyte, ASR‐LXMO shows high capacity retention of 76% after 125 cycles in full‐cell cycling.     

Spinel‐Layered Core‐Shell Cathode Materials for Li‐Ion Batteries

In an attempt to overcome the problems associated with LiNiO₂, the solid solution series of lithium nickel‐metal oxides, Li[Ni_1–xM_x]O₂ (with M = Co, Mn, Al, Ti, Mg, etc.), have been investigated as favorable cathode materials for high‐energy and high‐power lithium‐ion batteries. However, along with the improvement in the electrochemical properties in Ni‐based cathode materials, the thermal stability has been a great concern, and thus violent reaction of the cathode with the electrolyte needs to be avoided. Here, we report a heterostructured Li[Ni_0.54Co_0.12Mn_0.34]O₂ cathode material which possesses both high energy and safety. The core of the particle is Li[Ni_0.54Co_0.12Mn_0.34]O₂ with a layered phase (R3‐m) and the shell, with a thickness of < 0.5 μm, is a highly stable Li_1+x[CoNi_xMn_2–x]₂O₄ spinel phase (Fd‐3m). The material demonstrates reversible capacity of 200 mAhg‐1 and retains 95% capacity retention under the most severe test condition of 60 °C. In addition, the amount of oxygen evolution from the lattice in the cathode with two heterostructures is reduced by 70%, compared to the reference sample. All these results suggest that the bulk Li[Ni_0.54Co_0.12Mn_0.34]O₂ consisting of two heterostructures satisfy the requirements for hybrid electric vehicles, power tools, and mobile electronics.     

Site‐Selective In Situ Electrochemical Doping for Mn‐Rich Layered Oxide Cathode Materials in Lithium‐Ion Batteries

Various doped materials have been investigated to improve the structural stability of layered transition metal oxides for lithium‐ion batteries. Most doped materials are obtained through solid state methods, in which the doping of cations is not strictly site selective. This paper demonstrates, for the first time, an in situ electrochemical site‐selective doping process that selectively substitutes Li⁺ at Li sites in Mn‐rich layered oxides with Mg²⁺. Mg²⁺ cations are electrochemically intercalated into Li sites in delithiated Mn‐rich layered oxides, resulting in the formation of [Li_1?xMg_y][Mn_1?zM_z]O₂ (M = Co and Ni). This Mg²⁺ intercalation is irreversible, leading to the favorable doping of Mg²⁺ at the Li sites. More interestingly, the amount of intercalated Mg²⁺ dopants increases with the increasing amount of Mn in Li_1?x[Mn_1?zM_z]O₂, which is attributed to the fact that the Mn‐to‐O electron transfer enhances the attractive interaction between Mg²⁺ dopants and electronegative O^δ? atoms. Moreover, Mg²⁺ at the Li sites in layered oxides suppresses cation mixing during cycling, resulting in markedly improved capacity retention over 200 cycles. The first‐principle calculations further clarify the role of Mg²⁺ in reduced cation mixing during cycling. The new concept of in situ electrochemical doping provides a new avenue for the development of various selectively doped materials.     

In Situ Probing and Synthetic Control of Cationic Ordering in Ni‐Rich Layered Oxide Cathodes

Ni‐rich layered oxides (LiNi_1–x M_x O₂; M = Co, Mn, …) are appealing alternatives to conventional LiCoO₂ as cathodes in Li‐ion batteries for automobile and other large‐scale applications due to their high theoretical capacity and low cost. However, preparing stoichiometric LiNi_1–x M_x O₂ with ordered layer structure and high reversible capacity, has proven difficult due to cation mixing in octahedral sites. Herein, in situ studies of synthesis reactions and the associated structural ordering in preparing LiNiO₂ and the Co‐substituted variant, LiNi_0.8Co_0.2O₂, are made, to gain insights into synthetic control of the structure and electrochemical properties of Ni‐rich layered oxides. Results from this study indicate a direct transformation of the intermediate from the rock salt structure into hexagonal phase, and during the process, Co substitution facilities the nucleation of a Co‐rich layered phase at low temperatures and subsequent growth and stabilization of solid solution Li(Ni, Co)O₂ upon further heat treatment. Optimal conditions are identified from the in situ studies and utilized to obtain stoichiometric LiNi_0.8Co_0.2O₂ that exhibits high capacity (up to 200 mA h g^?1 ) with excellent retention. The findings shed light on designing high performance Ni‐rich layered oxide cathodes through synthetic control of the structural ordering in the materials.     

Improved Cycling Stability of Li[Ni0.90Co0.05Mn0.05]O2 Through Microstructure Modification by Boron Doping for Li‐Ion Batteries

Boron‐doped Li[Ni_0.90Co_0.05Mn_0.05]O₂ cathodes are synthesized by adding B₂O₃ during the lithiation of the hydroxide precursor. Density functional theory confirms that boron doping at a level as low as 1 mol% alters the surface energies to produce a highly textured microstructure that can partially relieve the intrinsic internal strain generated during the deep charging of Li[Ni_0.90Co_0.05Mn_0.05]O₂. The 1 mol% B‐Li[Ni_0.90Co_0.05Mn_0.05]O₂ cathode thus delivers a discharge capacity of 237 mAh g^?1 at 4.3 V, with an outstanding capacity retention of 91% after 100 cycles at 55 °C, which is 15% higher than that of the undoped Li[Ni_0.90Co_0.05Mn_0.05]O₂ cathode. This proposed synthesis strategy demonstrates that an optimal microstructure exists for extending the cycle life of Ni‐rich Li[Ni_1‐x‐yCo_xMn_y]O₂ cathodes that have an inadequate cycling stability in electric vehicle applications and indicates that an optimal microstructure can be achieved through surface energy modification.     

Insight of a Phase Compatible Surface Coating for Long‐Durable Li‐Rich Layered Oxide Cathode

Li‐rich layered oxides (LLOs) can deliver almost double the capacity of conventional electrode materials such as LiCoO₂ and LiMn₂O₄; however, voltage fade and capacity degradation are major obstacles to the practical implementation of LLOs in high‐energy lithium‐ion batteries. Herein, hexagonal La_0.8Sr_0.2MnO_3?y (LSM) is used as a protective and phase‐compatible surface layer to stabilize the Li‐rich layered Li_1.2Ni_0.13Co_0.13Mn_0.54O₂ (LM) cathode material. The LSM is Mn? O? M bonded at the LSM/LM interface and functions by preventing the migration of metal ions in the LM associated with capacity degradation as well as enhancing the electrical transfer and ionic conductivity at the interface. The LSM‐coated LM delivers an enhanced reversible capacity of 202 mAh g^?1 at 1 C (260 mA g^?1) with excellent cycling stability and rate capability (94% capacity retention after 200 cycles and 144 mAh g^?1 at 5 C). This work demonstrates that interfacial bonding between coating and bulk material is a successful strategy for the modification of LLO electrodes for the next‐generation of high‐energy Li‐ion batteries.     

Ni‐Rich/Co‐Poor Layered Cathode for Automotive Li‐Ion Batteries: Promises and Challenges

To pursue a higher energy density (>300 Wh kg^?1 at the cell level) and a lower cost (<$125 kWh^?1 expected at 2022) of Li‐ion batteries for making electric vehicles (EVs) long range and cost‐competitive with internal combustion engine vehicles, developing Ni‐rich/Co‐poor layered cathode (LiNi_1?x?yCo_xMn_yO₂, x+y ≤ 0.2) is currently one of the most promising strategies because high Ni content is beneficial to high capacity (>200 mAh g^?1) while low Co content is favorable to minimize battery cost. Unfortunately, Ni‐rich cathodes suffer from limited structure stability and electrode/electrolyte interface stability in the charged state, leading to electrode degradation and poor cycling performance. To address these problems, various strategies have been employed such as doping, structural optimization design (e.g., core–shell structure, concentration‐gradient structure, etc.), and surface coating. In this review, five key aspects of Ni‐rich/Co‐poor layered cathode materials are explored: energy density, fast charge capability, service life including cycling life and calendar life, cost and element resources, and safety. This enables a comprehensive analysis of current research advances and challenges from the perspective of both academy and industry to help facilitate practical applications for EVs in the future.