Facile Solid-State Synthesis to In Situ Generate a Composite Coating Layer Composed of Spinel-Structural Compounds and Li3PO4 for Stable Cycling of LiCoO2 at 4.6 V.

Due to its high energy density, high-voltage LiCoO2 is the preferred cathode material for consumer electronic products. However, its commercial viability is hindered by rapid capacity decay resulting from structural degradation and surface passivation during cycling at 4.6 V. The key to achieving stable cycling of LiCoO2 at high voltages lies in constructing a highly stable interface to mitigate surface side reactions. In this study, we present a facile in situ coating strategy that is amenable to mass production through a simple wet-mixing process, followed by high-temperature calcination. By capitalizing on the facile dispersion characteristics of nano-TiO2 in ethanol and the ethanol dissolubility of LiPO2F2, we construct a uniform precoating layer on LiCoO2 with nano-TiO2 and LiPO2F2. The subsequent thermal treatment triggers an in situ reaction between the coating reagents and LiCoO2, yielding a uniform composite coating layer. This composite layer comprises spinel-structured compounds (e.g., LiCoTiO4) and Li3PO4, which exhibit excellent chemical and structural stability under high-voltage conditions. The uniform and stable coating layer effectively prevents direct contact between LiCoO2 and the electrolyte, thereby reducing side reactions and suppressing the surface passivation of LiCoO2 particles. As a result, coated LiCoO2 maintains favorable electronic and ionic conductivity even after prolonged cycling. The synergistic effects of spinel-structured compounds and Li3PO4 contribute to the superior performance of LiCoO2, demonstrating a high capacity of 202.1 mA h g-1 (3.0-4.6 V, 0.5 C, 1 C = 274 mA g-1), with a capacity retention rate of 96.7% after 100 cycles.

Conformal Coating of a High-Voltage Spinel to Stabilize LiCoO2 at 4.6 V.

The ever-growing demand for portable electronic devices has put forward higher requirements on the energy density of layered LiCoO2 (LCO). The unstable surface structure and side reactions with electrolytes at high voltages (>4.5 V) however hinder its practical applications. Here, considering the high-voltage stability and three-dimensional lithium-ion transport channel of the high-voltage Li-containing spinel (M = Ni and Co) LiMxMn2-xO4, we design a conformal and integral LiNixCoyMn2-x-yO4 spinel coating on the surface of LCO via a sol-gel method. The accurate structure of the coating layer is identified to be a spinel solid solution with gradient element distribution, which compactly covers the LCO particle. The coated LCO exhibits significantly improved cycle performance (86% capacity remained after 100 cycles at 0.5C in 3-4.6 V) and rate performance (150 mAh/g at a high rate of 5C). The characterizations of the electrodes from the bulk to surface suggest that the conformal spinel coating acts as a physical barrier to inhibit the side reactions and stabilize the cathode-electrolyte interface (CEI). In addition, the artificially designed spinel coating layer is well preserved on the surface of LCO after prolonged cycling, preventing the formation of an electrochemically inert Co3O4 phase and ensuring fast lithium transport kinetics. This work provides a facile and effective method for solving the surface problems of LCO operated at high voltages.

Raising the Intrinsic Safety of Layered Oxide Cathodes by Surface Re-Lithiation with LLZTO Garnet-Type Solid Electrolytes.

Battery safety concerns are becoming more and more prominent with the increasing demands of lithium-ion batteries (LIBs) with higher energy density. The greatest threat to battery safety derives from the easy release of oxygen from the high-capacity layered oxide cathodes at highly delithiated states and subsequent exothermic reactions with reductive agents in batteries. Herein, it is demonstrated that solid electrolyte Li6.5 La3 Zr1.5 Ta0.5 O12 (LLZTO) can supply lithium ions to re-lithiate the charged LiCoO2 at elevated temperatures. Such a re-lithiation process can lower the state-of-charge of LiCoO2 , and thus, inherently postpones its structural decomposition and the associated release of oxygen during the heating process. The LiCoO2 /graphite full cell with 1 wt% addition of LLZTO demonstrates remarkably enhanced safety performances. This work proposes a strategy that through the adoption of solid electrolytes to solve safety issues raised from both flammable liquid electrolytes and high capacity cathodes, to achieve intrinsically safe LIBs or solid-state batteries.