Synthesis And Surface Modification Of Cathode Materials For Lithium Ion Battery

LiCoO2is the earliest commercialized and the most widely used cathode material for lithium ion battery due to its fairly good electrochemical performance, simple method of synthesis and excellent stability of manufacturing. However, the relative scarcity of Co metal leads to the expensive cost of LiCoO2. Its cost is several to more than ten times than other materials such as LiMn2O4、LiFePO4and LiNi1/3Co1/3Mn1/3O2. Besides, as for commercial LiCoO2powders, there are still some shortcomings such as the low practical capacity, poor anti-overcharge and safety performance, low charge-discharge rate capability. The main reason is that the crystal structure of LiCoO2materials assumes irreversible phase transformation as the lithium ions deintercalate in the charging process. In order to overcome the problems mentioned above, this paper mainly involves three aspects:(1) The precursor, Ni-Co hydroxy-carbonate, was synthesized by colloid mill method with Na2CO3as the precipitator. LiNi0.8Co0.202powders, which have a good electrochemical performance and sphere-like microscopy, were prepared by calcining the ball-milling mixture of the precursor and LiOH under appropriate conditions. The average particle size of so-obtained LiNi0.8Co0.2O2materials is about5μm. Under2.75～4.4V (vs. Li+/Li) charge-discharge voltage range and0.2mA/cm2current density, the LiNi0.8Co0.2O2sample prepared at optimized conditions has the highest discharge capacity of173mAh/g and a capacity retention ratio of95.7%after30cycles. LiNi0.8Co0.2O2is one of the emulous materials to substitute LiCoO2.(2) LiCoO2was surface modified with low-cost, environmentally friendly and nano-sized FePO4coating material by a simple coprecipitate method in the water system. The FePO4coating improves the anti-overcharge cycling performance, stabilizes the crystal structure and enhances the safety property. When the discharge cutoff voltage is2.75V, the charge cutoff voltage is4.3V and4.4V, and the discharge rate is1C, the initial discharge capacity of3wt.%FePO4-coated LiCoO2is146and155mAh/g, and the400th capacity retention is88.7%and82.5%, respectively.(3) LiMn2O4material was mechanically doped with3wt.%FePO4-coated LiCoO2. Using the mixed material (weight ratio, coated LiCoO2:LiMn2O4=15:85, signed as CoMn1585) as the cathode material and MCMB as the anode material, the initial discharge capacity is103.7mAh/g at2.75-4.3V charge-discharge voltage range and1C rate. When the discharge rate is increased to3C and5C, the discharge capacity retention is98.6%and95.7%, respectively. Through3C-10V overcharge and short-circuit experiments, it is found that the safety performance of MCMB/CoMn1585is the best.