Surface And Interfacial Properties Of High Voltage Layered Oxide Cathode Materials For Lithium Batteries

Along with the rapid development of electric vehicles,intelligent electronic devices and other fields,the demand for high energy density lithium-ion batteries with long cycle life and high safe is increasing.Employing positive electrodes with high voltage and high specific capacity is effective to improve the energy density of batteries.Layered cathode materials have attracted widespread attention due to their high theoretical specific capacity.However,before the large scale application at high voltage,several problems and challenges need to be solved,especially the phase transition at the interface with electrolyte,transition metal dissolution,oxygen precipitation,electrolyte oxidation and decomposition,which seriously restrict the development of high energy density lithium batteries.In order to improve the electrochemical and safety performance of layered cathode at high voltage,this paper starts with the cathode electrolyte interface?CEI?,revealing the influence of crystal structure and element composition of cathode materials on CEI under high voltage.Then indicate which structure and composition of the cathode surface is more conducive to the formation of a stable CEI.Based on these results,surface coating was conducted to modify the cathode/electrolyte interface and improve its stability at high voltage.In addition,considering that solid-state metal lithium batteries can further improve energy density and safety performance,which is the development direction of lithium-ion batteries,the interface between layered cathode and solid-state electrolyte has also been preliminarily studied in this paper.Firstly,the effect of crystal structure and element composition on CEI were studied by X-ray photoelectron spectroscopy.The results show that the composition and thickness of CEI on layered and spinel cathode materials are similar when charging to4.8 V.while the thickness of CEI changes inversely after discharging to 3 V.The increase of CO₃ content in layered cathode CEI results in the thickening of CEI,which is related to the active oxygen group(O₂^n-)produced by charge transfer of charged lattice oxygen.The dissolution of C and O containing species in spinel CEI leads to the thinning of CEI.In addition,the effect of 4d transition metal Ru on CEI is mainly reflected in the low voltage region and immersion stage.Upon any cathode,the dynamic evolution of CEI at higher voltage is more significant.The instability of CEI at high voltage leads to the continuous decomposition of electrolyte and the continuous growth of CEI.However,the CEI of spinel structure containing 3d transition metal is relatively more stable at a high voltage.Therefore,the spinelization of surface structure for high voltage cathode will contribute to the stability of the interface.Secondly,the surface of LiCoO₂ and LiNi_0.6Co_0.2Mn_0.2O₂?NCM622?cathode was coated with solid electrolyte Li_1.4Al_0.4Ti_1.6?PO₄?₃?LATP?to improve its electrochemical and safety performance at high voltage.The coating conditions were also explored and optimized.For LATP coated LiCoO₂ and NCM622 materials,the optimum sintering temperatures are 700?and 600?,respectively.At this time,the in-situ reaction of the coating generates lithium ion conductor Li₃PO₄ and spinel structure Co₃O₄?or rock salt structure NiO?,which can provide double protection for the interface.Compared with the original unmodified material,the LATP modification layer can significantly improve the cyclic stability,rate performance and safety performance of the cathode material at high voltage.Further experiment shows that a more stable and uniform CEI layer at the LiCoO₂ interface can form with the LATP modified layer.The modified CEI inhibits the electrolyte decomposition,Co dissolution and surface structure rearrangement side reactions,thus ensures the stability of the high voltage LiCoO₂ interface.Finally,the interfacial stability of high energy density cathode with organic polymer and inorganic oxide solid electrolyte was studied.For LiCoO₂ and polyethylene oxide?PEO?solid electrolyte,the performance of LiCoO₂/PEO/Li battery cycled at 4.2 V rapidly decays and the interface charge transfer impedance significantly increases due to the spontaneous oxidation and decomposition of PEO above 4 V.The LATP coated LiCoO₂ can reduce the oxidation decomposition of PEO,stabilize the interface impedance of LiCoO₂,and improve the cycle stability significantly.For the chemical stability under high temperature between different structure oxide cathodes and solid electrolytes.When heated together with LATP,Li⁺can be easily took off from layered cathode,especially the lithium-rich cathode,which leads to the formation of spinel or rock-salt structure.While LATP gains Li⁺to form more stable Li₃PO₄.Eventually LATP and layered structure completely disappear at 650?.At the same time,the high nickel content and the lithium-rich structure increase the instability of the layered cathode material at high temperatures.When sintering together Li_6.75La₃Zr_1.75Ta_0.25O₁₂?LLZO?,despite some interface by-products generated,well layer structure can still maintain even at 800?.But Li⁺in LLZO tends to take off at high temperatures,which leads to a drop in conductivity.When heated with LLZO,spinel cathode gets the Li⁺from LLZO to from lithium-rich layered structure Li₂MnO₃begins at 500?promoting the sustained delithiation of LLZO and leading to La₂Zr₂O₇phase formation.While sintering with LATP,spinel cathode is relatively stable,but it also converts to LiMnPO₄ above 650?.