Preparation And Electrochemical Performance Of Li₅FeO₄ Coated LiCoO₂

Lithium cobaltite（LiCoO₂）still plays an important role in the cathode materials market of lithium ion batteries for consumer electronic products due to its high theoretical specific capacity(1363 m Ah cm^-3).However,due to the attack of trace HF in the electrolyte and the instability of its structure,the capacity of LiCoO₂ decreases in cycling.In addition,high capacity anode materials such as silicon and hard carbon have a serious drawback that their first charge-discharge coulomb efficiency is very low.This drawback reduces the capacity of the whole battery.These two issues greatly limit the comprehensive performance of the whole battery.In order to solve these issues,this paper proposes an approach of making Li₅FeO₄ coating on the surface of LiCoO₂ particles.A process with two-steps（precipitate and calcinate）was adopted to prepare Li₅FeO₄ coated LiCoO₂.The results show that when the Li₅FeO₄ coated LiCoO₂ prepared at a calcinating temperature of 750 ℃ and through a precipitant of Na OH solution,the cycle performance of prepared Li₅FeO₄ coated LiCoO₂ is obviously better than the original LiCoO₂.The capacity of lithium release from Li₅FeO₄ is as high as 710 m Ah g^-1.We fixed the content of Li₅FeO₄ coating to be 6 wt.% and 9 wt.% by changing the proportion of raw materials.The Li₅FeO₄ coating improves the cycle performance and rate performance of LiCoO₂ in the voltage range of 3-4.3 V（vs.Li/Li⁺）.But the cycle performance becomes worse when the charging cut-off potential is set at 4.4 V and 4.5 V.The mechanism for improved cycle performance and rate performance by Li₅FeO₄ coating was studied by electrochemical impedance spectroscopy.The results show that the Li₅FeO₄ coating can inhibite the side reaction at the electrode/electrolyte interface,suppress the growth of SEI film and slow down the increase of electrode impedance with cycling.Accordingly,Li₅FeO₄ coating promotes the cycle performance.Moreover,impedance spectra in the low frequency region show that the lithium-ion diffusion coefficient of Li₅FeO₄ coated LiCoO₂ is higher than that of the original LiCoO₂.As a result,the rate performance is improved by the Li₅FeO₄ coating.Overall,the Li₅FeO₄ coating on LiCoO₂ particles achieves the dual function of improving cycle performance and compensating lithium in one cell.This strategy is also be applicable to other cathode materials.