Research On Interface Modification Of Ternary Cathode Materials For Lithium Ion Batteries

Lithium-ion batteries are increasingly used in large-scale energy storage,aerospace and other fields.Its performance in terms of energy density,cycle life,safety,and production cost need to be further optimized.However,the interface reaction between the materials and the electrolyte is violent during the cycling process,resulting in irreversible phase transition,dissolution of transition metal ions and increase of charge transfer impedance.Here,we propose two methods of electrolyte additive modification and positive electrode material surface composite modification to enhance the interface stability of the ternary positive electrode materials and improve the electrochemical performance of the battery.LiPO₂F₂ is used as an electrolyte additive to improve electrochemical performance by inhibiting interfacial reactions.The modification effects of LiNi_0.8Co_0.1Mn_0.1O₂（NCM811）、LiNi_0.5Mn_1.5O₄（spinel）and Li_1.14Ni_0.133Co_0.133Mn_0.544O₂（LRNCM）cathode materials were respectively explored.The modification effect of LiPO₂F₂ on NCM811 is limited,which can only improve the long cycle stability of the battery;LiPO₂F₂ can improve the specific discharge capacity of spinel materials by about 15 mAh·g^-1.After 280 cycles at 3 C,the capacity retention rate increased from 80.1%to 89.3%;The electrochemical performance improvement effect of LRNCM material is the best.After 500 cycles at 3 C,the charge transfer resistance of the battery is reduced by 80%.the capacity retention rate is increased from 53.6%to 85.0%,and the discharge specific capacity is increased by about 30 mAh·g^-1 at the rate of 0.1 C-10 C.The modification mechanism of LiPO₂F₂ was investigated based on the modification results of LRNCM material.LiPO₂F₂ participates in the formation of a dense and stable electrochemical interfacial film,restraining the side reaction and maintaining the stability of the electrode material structure.After 500 cycles at 3 C,the rupture of the materials is reduced,the transformation from the layer phase to spinel the interphase to the spinel is inhibited,the dissolution of transition metal ions is reduced,and the decomposition products of electrolyte on the surface of the material are reduced.These are the reasons for the improvement of electrochemical performance.The Li_1.2Ni_0.13Co_0.13Mn_0.54O₂（LRNCM-2）materials were modified by induced phase transformation and in-situ composite coating to form a layer@spinel@MoO3|PPy composite structure.The spinel phase transition layer of 4-8 nm was induced by phosphomolybdate hydrate.Meanwhile,pyrrole monomer was polymerized in situ on the surface of materials to form composite coating layer with its decomposition products.The spinel phase can participate in the electrochemical reaction in the cycling process and provide reversible capacity and the composite coating can enhance the interface stability of the material structure.The discharge capacity of the material annealed at 640℃increases about 20 mAh·g^-1 at 1 C,and the capacity retention rate increases from 76.7%to 93.0%after 200 cycles at 3 C.