Interface Modification And Performance Optimization Of High Voltage Lithium Cobalt Oxide And Ternary Cathode Materials

Since commercialization of LIBs,layered LiCoO₂ has being dominating the cathode market due to its easy production and good cycling stability.In recent years,the focus of research in LiCoO₂ has shifted to the application at high voltages,which can increase the degree of Li⁺utilization in the lattice structure and thus push up the limit of its capacity and energy density.Unfortunately,pure LiCoO₂ is not stable and its rate capability also becomes very poor under such high-voltage conditions.Therefor the present performances of lithium ion batteries still need to be improved.On the other hand,layered NCA has developed rapidly in the field of power batteries for its high capacity and excellent cycling stability.However since its inherent defects,this cathode materials are not yet widely used.Surface coating has long been an important strategy to improve the electrochemical performances of electrode materials for Li-ion batteries.In this work,surface coating is used to improve the electrochemical perfo rmances of LiCoO₂ and NCA(LiN i_0.8Co_0.15Al_0.05O₂)cathode materials.?1?In this work,commercial LiCoO₂ is modified with a glassy B₂O₃ by solution mixing with H₃ BO₃ followed by post-calcination in order to enhance its high-voltage electrochemical performance.The glassy B₂O₃ coating/additive is believed to serve as an effective physicochemical buffer and protection between LiCoO₂ and liquid electrolyte,which can suppress the high-voltage induced electrolyte decomposition and active material dissolution.During the early cycling and due to the electrochemical force,the as-coated B₂O₃ glasses which have 3D open frameworks tend to accommodate some mobile Li⁺and form a more chemically-resistant and ion-conductive lithium boron oxide?LBO?interphase as a major component of the solid electrolyte interphase?SEI?,which consequently enables much easier Li⁺diffusion/transfer at the solid-liquid interfaces upon further cycling.Due to the synergetic effects of B₂O₃ coating/modification,the high-voltage capacity and energy density of the B₂O₃-modified LiCoO₂ cathode are promisingly improved by 35%and 30%after 100 cycles at 1C within 3.0-4.5 V vs.Li/Li⁺.Meanwhile,the high-rate performance of the B₂O₃-modified electrode is even more greatly improved,showing a capacity of105 mAh g^-1 at 10 C while the bare electrode has dropped to no more than 30 mAh g^-1under this rate condition.?2?In this work,an amorphous Li₃PO₄?LPO?layer,which is a poor electronic conductor but good ionic conductor,is coated directly on NCA(LiN i_0.8Co_0.15Al_0.05O₂)composite electrodes by magnetron sputtering.The battery performances of the electrodes are studied at both 2.8-4.3 V vs.Li/Li⁺and 2.8-4.5 V vs.Li/Li⁺.The LPO sputter-coating allows significant improvement of the electrode's cycling stability at both voltages.With an optimum coating thickness of⁴0 nm,the electrode's capacity after 100 cycles at 1 C can reach 182.4 mAh g^-1?90.16%retention?and 202.8 mAh g^-1?86.52%retention?at 4.3 V and 4.5 V,which are improved by 15%and 22%,respectively,compared to those of the bare NCA electrode.More impressively,the rate capability and temperature capability are also greatly enhanced by LPO-coating.The remarkable improvement of the LPO-coated electrodes is mainly attributed to the high chemical stability and temperature-enhanced electrochemical activity of the LPO coating layer,which synergistically serves as a physiochemical protection layer and an efficient pathway for Li⁺transport.