Lithium ion batteries (LIBs) have received worldwide attention as power sources for electric vehicles (EVs) and portable energy storage. [1] However, research to address the limited cycle life, rate capability, energy density, safety concern and cost issue of commercialized LiCoO 2 (LCO), LiNi x Mn y Co z O 2 (NMC), LiNi 0.80 Co 0.15 Al 0.05 O 2 (NCA) and LiFePO 4 (LFP) is still ongoing for their large-scale application in EVs and the electricity grid. [1b] The lithium-and manganese-rich (LMR) layered structure cathode materials xLi 2 MnO 3 ·(1−x)LiMO 2 (M = Ni, Co, Mn or combinations) have entered the spotlight becauseThe lithium-and manganese-rich (LMR) layered structure cathodes exhibit one of the highest specific energies (≈900 W h kg −1 ) among all the cathode materials. However, the practical applications of LMR cathodes are still hindered by several significant challenges, including voltage fade, large initial capacity loss, poor rate capability and limited cycle life. Herein, we review the recent progress and in depth understandings on the application of LMR cathode materials from a practical point of view. Several key parameters of LMR cathodes that affect the LMR/graphite full-cell operation are systematically analyzed. These factors include the first-cycle capacity loss, voltage fade, powder tap density, and electrode density. New approaches to minimize the detrimental effects of these factors are highlighted in this work. We also provide perspectives for the future research on LMR cathode materials, focusing on addressing the fundamental problems of LMR cathodes while keeping practical considerations in mind. Center. Cho' current research is focused on high-energydensity cathode and anode materials and their direct implantation in ful-cell systems, as well as metal-air batteries and redox flow batteries for energy storage.Ji-Guang (Jason) Zhang is a Laboratory Fellow of the Pacific Northwest National Laboratory. He is the group leader for PNNL's efforts in energy storage for transportation applications and has 25-year experience in the development of energy storage devices, including Li-ion batteries, Li-air batteries, Li-metal batteries, Li-S batteries, and thin-film solid-state batteries.

Battery industries and research groups are further investigating LiCoO2 to unravel the capacity at high‐voltages (>4.3 vs Li). The research trends are towards the surface modification of the LiCoO2 and stabilize it structurally and chemically. In this report, the recent progress in the surface‐coating materials i.e., single‐element, binary, and ternary hybrid‐materials etc. and their coating methods are illustrated. Further, the importance of evaluating the surface‐coated LiCoO2 in the Li‐ion full‐cell is highlighted with our recent results. Mg,P‐coated LiCoO2 full‐cells exhibit excellent thermal stability, high‐temperature cycle and room‐temperature rate capabilities with high energy‐density of ≈1.4 W h cc−1 at 10 C and 4.35 V. Besides, pouch‐type full‐cells with high‐loading (18 mg cm−2) electrodes of layered‐Li(Ni,Mn)O2 ‐coated LiCoO2 not only deliver prolonged cycle‐life at room and elevated‐temperatures but also high energy‐density of ≈2 W h cc−1 after 100 cycles at 25 °C and 4.47 V (vs natural graphite). The post‐mortem analyses and experimental results suggest enhanced electrochemical performances are attributed to the mechanistic behaviour of hybrid surface‐coating layers that can mitigate undesirable side reactions and micro‐crack formations on the surface of LiCoO2 at the adverse conditions. Hence, the surface‐engineering of electrode materials could be a viable path to achieve the high‐energy Li‐ion cells for future applications.

The surface modifi cation of micrometersized active materials has been considered to be one of the most effective methods to enhance the electrochemical performance of Li-rich cathode materials because it can effectively improve active material's electrochemical performance without decreasing the energy density of the active material. The sol-gel method (classifi ed as a bottomup method) has been commonly used to provide various surface treatments for the Li-rich cathode materials. This includes the formation of the surface coating layers (metal oxides, metal phosphates and metal fl uorides) on the cathode [ 6 ] and synthesis of heterostructured cathode materials with a core-shell structure and concentration-gradient surface. [ 7 ] However, these approaches have not been a comprehensive solution because the Li 2 MnO 3 phase in the Li-rich cathode material, which was reported to be a major cause for the instability of the surface structure and electrochemical performance, [ 1 ] has not been successfully stabilized. For this reason, these previous studies showed only limited improvements that still have high initial irreversible capacity and working voltage decay problems. [ 6,7 ] Furthermore, they have the experimental diffi culty of optimizing the surface layer with a uniform thickness, which yielded localized coating products on the host cathode ( Figure 1 a). [ 8 ] The localized coating material of the cathode surface acted as an insulator layer, resulting in the poor electrochemical performance. It has been challenging to optimize the coating thickness on the nanoscale using the sol-gel method. Therefore, the search for new surface treatment methods has been continued to fi nd an effective strategy for the Li-rich cathode material.Here, we propose a new surface modifi cation method using chemical activation and new insight of voltage decline for improvement of Li-rich cathode material's intrinsic problems. The new surface modifi cation consists of 1) construction of continuous electron pathways on the bare particle's surface by wrapping its surface with a few layers of rGO to enhance its surface electronic conductivity [ 9 ] and suppress metal dissolution during cycles (Supporting Information Figure S2a) and 2) formation of the chemically activated layer via the chemical treatment to stabilize the Li 2 MnO 3 phase on the surface (Figure 1 b). Remarkably, this surface treatment method using hydrazine is a very promising apprach in that it stabilizes the surface Li 2 MnO 3 phase and produces an extremely high initial CoulombicThe Li-rich cathode materials have been considered as one of the most promising cathodes for high energy Li-ion batteries. However, realization of these materials for use in Li-ion batteries is currently limited by their intrinsic problems. To overcome this barrier, a new surface treatment concept is proposed in which a hybrid surface layer composed of a reduced graphene oxide (rGO) coating and a chemically activated layer is created. A few layers of GO are fi rst coated on the surface...

Li-rich layered metal oxides have attracted much attention for their high energy density but still endure severe capacity fading and voltage decay during cycling, especially at elevated temperature. Here, facile surface treatment of Li 1.17 Ni 0.17 Co 0.17 Mn 0.5 O 2 (0.4Li 2 MnO 3 ·0.6LiNi 1/3 Co 1/3 Mn 1/3 O 2 ) spherical cathode material is designed to address these drawbacks by hybrid surface protection layers composed of Mg 2+ pillar and Li-Mg-PO 4 layer. As a result, the surface coated Li-rich cathode material exhibits much enhanced cycling stability at 60 °C, maintaining 72.6% capacity retention (180 mAh g −1 ) between 3.0 and 4.7 V after 250 cycles. More importantly, 88.7% average discharge voltage retention can be obtained after the rigorous cycle test. The strategy developed here with novel hydrid surface protection effect can provide a vital approach to inhibit the undesired side reactions and structural deterioration of Li-rich cathode materials and may also be useful for other layered oxides to increase their cycling stability at elevated temperature.

Li-rich materials are considered the most promising for Li-ion battery cathodes, as high energy densities can be achieved. However, because an activation method is lacking for large particles, small particles must be used with large surface areas, a critical drawback that leads to poor long-term energy retention and low volumetric energy densities. Here we propose a new material engineering concept to overcome these difficulties. Our material is designed with 10 μm-sized secondary particles composed of submicron scaled flake-shaped primary particles that decrease the surface area without sacrificing rate capability. A novel activation method then overcomes the previous limits of Li-rich materials with large particles. As a result, we attained high average voltage and capacity retention in turn yielding excellent energy retention of 93% during 600 cycles. This novel and unique approach may furthermore open the door to new material engineering methods for high-performance cathode materials.

Lithium-excess 3d-transition-metal layered oxides (Li1+xNiyCozMn1−x−y−zO2, >250 mAh g−1) suffer from severe voltage decay upon cycling, which decreases energy density and hinders further research and development. Nevertheless, the lack of understanding on chemical and structural uniqueness of the material prevents the interpretation of internal degradation chemistry. Here, we discover a fundamental reason of the voltage decay phenomenon by comparing ordered and cation-disordered materials with a combination of X-ray absorption spectroscopy and transmission electron microscopy studies. The cation arrangement determines the transition metal-oxygen covalency and structural reversibility related to voltage decay. The identification of structural arrangement with de-lithiated oxygen-centred octahedron and interactions between octahedrons affecting the oxygen stability and transition metal mobility of layered oxide provides the insight into the degradation chemistry of cathode materials and a way to develop high-energy density electrodes.

A practical solution is presented to increase the stability of 4.45 V LiCoO 2 via high-temperature Ni doping, without adding any extra synthesis step or cost. How a putative uniform bulk doping with highly soluble elements can profoundly modify the surface chemistry and structural stability is identified from systematic chemical and microstructural analyses. This modification has an electronic origin, where surface-oxygen-loss induced Co reduction that favors the tetrahedral site and causes damaging spinel phase formation is replaced by Ni reduction that favors octahedral site and creates a better cation-mixed structure. The findings of this study point to previously unspecified surface effects on the electrochemical performance of battery electrode materials hidden behind an extensively practiced bulk doping strategy. The new understanding of complex surface chemistry is expected to help develop higherenergy-density cathode materials for rechargeable batteries.

Only a very limited amount of the high theoretical energy density of LiCoO as a cathode material has been realized, due to its irreversible deterioration when more than 0.6 mol of lithium ions are extracted. In this study, new insights into the origin of such low electrochemical reversibility, namely the structural collapse caused by electrostatic repulsion between oxygen ions during the charge process are suggested. By incorporating the partial cation migration of LiNiO , which produces a screen effect of cations in the 3b-Li site, the phase distortion of LiCoO is successfully delayed which in turn expands its electrochemical reversibility. This study elucidates the relationship between the structural reversibility and electrochemical behavior of layered cathode materials and enables new design of Co-rich layered materials for cathodes with high energy density.