Research On Synthesis And Optimization Of Electrochemical Performance For High-voltage Lithium Nickel Manganate Oxide Cathode Material

The development and use of high-voltage cathode materials is of great significance for improving the energy density and power performance of lithium-ion batteries.Lithium nickel manganate oxide,a kind of typical high-voltage cathode materials,has attracted significant attention in recent years.However,the commercial application of lithium nickel manganate oxide is still rarely reported due to the rapid decay of capacity caused by its interface instability and structural degradation.Therefore,it is crucial to have a deeper understanding of the intrinsic relationship between the structure and electrochemical performance of lithium nickel manganate oxide,and to develop high-performance interface modification methods and efficient means for suppressing side reactions.This paper focus on three perspectives of bulk phase structure regulation,particle surface modification,and electrode performance optimization,revealing the phase evolution process during the synthesis of lithium nickel manganate oxide and the key steps affecting the structure of lithium nickel manganate oxide.Further,optimization strategies through room-temperature surface modification and PVDF binder modification were taken to improve the electrochemical performance of lithium nickel manganate oxide.From the prospect of the regulation of bulk phase structure of lithium nickel manganate oxide and in view of the long-standing problem of disordered and ordered regulation of lithium nickel manganate oxide,we studied the phase formation and evolution mechanism during the sintering process of lithium nickel manganate oxide and further determined the key steps controlling the formation of lithium nickel manganate oxide and affecting transition metal cation ordering.Besides,the optimized strategies of synthesis are proposed,which can significantly improve the ordering degree and electrochemical performance of the active material.The phase composition of lithium nickel manganate oxide synthesized by conventional solid-state method at different temperatures was investigated.It was found that the difference in phase formation temperature of LiMn₂O₄ and LiNi O₂ resulted in a slow formation of the key intermediate Li_xNi_yMn_zO,thereby affecting the crystallization and transition cation ordering of the product.Vein with strong exothermic peaks at 300-400℃was introduced as thermal regulators to promote the progress of key reaction steps,resulting in the synchronous formation of LiMn₂O₄ and LiNi O₂ at 300℃and further complete transformation into disordered LiNi_0.5Mn_1.5O_4-δat 400℃.The content of ordered phases in O-LNMO was also significantly increased.O-LNMO exhibits excellent rate performance and cyclic stability.The discharge capacity of O-LNMO is 96.9m Ah/g at 100 C.After 1000 cycles at 1 C under room temperature,the reserved capacity of O-LNMO is 89.60%.From the prospect of surface structure regulation of lithium nickel manganate oxide,lithium nickel manganate oxide coated with LiF（RT-LiF@LNMO）was prepared by extraction method at room temperature and the cycling stability and rate performance of lithium nickel manganate lithium were improved.The effect of different growth conditions on the structure of the coating layer,LiF-lithium nickel manganate oxide interface structure,lithium nickel manganese oxide bulk phase structure,and electrochemical performance were investigated.On the basis of ion bonding with the transition metal on the surface of lithium nickel manganate oxide,LiF continuously crystallizes and forms a uniform coating layer in RT-LiF@LNMO.After high-temperature treatment on the basis of room temperature coating（HT-LiF@LNMO）,LiF coating layer no longer forms Ni/Mn-F bonds with lithium nickel manganate oxide,and a small amount of Ni-F bonds are formed by the F element entering oxygen vacancies when Li_yNi_1-yO dissolves into the disordered lithium nickel manganese oxide lattice.Meanwhile,the content of ordered phase also increased in HT-LiF@LNMO.The continuous growth of crystalline LiF does not hinder the lithium ion transport at the interface,while providing a stable and high-speed interface transport channel;Meanwhile,LiF can inhibit the interfacial side reaction between lithium nickel manganate oxide and electrolyte.Therefore,RT-LiF@LNMO has optimal cycling stability and rate performance.After 250 cycles at 1 C at elevated temperature,the reserved capacity of RT-LiF@LNMO is 85.00%.The discharge capacity of RT-LiF@LNMO is 89.1 m Ah/g at 60 C.From the prospect of the optimization of electrode structure and performance,solid electrolyte Li_6.4La₃Zr_1.4Ta_0.6O₁₂（LLZTO）with inherent alkalinity and the ability to induce the partial dehydrofluorination of PVDF was added to the electrode slurry to modify PVDF and improve the lithium ion conductivity of the binder.The experimental results suggest that the process of partial dehydrofluorination of PVDF by LLZTO does not affect the physical and chemical properties of the active substance,and the mixture of LLZTO and PVDF can remain stable in the electrolyte.Modified PVDF can react with acid,and the first-principles calculations revealed that the reaction between HF and LLZTO had higher chemical reactivity than that between HF and LiNi_0.5Mn_1.5O₄,which can effectively inhibit the corrosion of HF in lithium nickel manganate oxide.The results of cyclic voltammetry and electrochemical impedance spectroscopy showed that the ionic conductivity of the modified PVDF and LLZTO was twice higher than that of PVDF.Therefore,the modified cathode LNMO-LLZTO has a discharge capacity of 102.62 m Ah/g with a capacity retention of 82.42%after1000 cycles at room temperature,and a discharge capacity of 100.87 m Ah/g at 40C.