Research On The LiNi_0.5Co_0.2Mn_0.3O₂ Cathode Material In Lithium-ion Batteries

LiNi_1/3Co_1/3Mn_1/3O₂ and its ramifications with α-NaFeO₂ layered-structure have been considered to be promising cathode materials for rechargeable lithium batteries due to its intrinsic characteristics, such as high capacity, stable cyclability, relatively low cost and better safety performance than currently used LiCoO₂ material. Among them, LiNi_0.5Co_0.2Mn_0.3O₂（referred as NCM523） has attracted much attention because of the improvement of specific capacity and the reduction of cost contributed by its high Ni content. The presented paper studied the influences of Li/TM（TM=Ni+Co+Mn） stoichiometric ratio and sintering technology on the physical, chemical and electrochemical performances of NCM523 material. After that, the effects of precursor particle size were studied on the properties of NCM523 material. And XRD Rietveld refinement, scanning electron microscopy（SEM）, BET specific surface area（SSA） analysis, particle distribution analysis, complexometry, atomic absorptionflame emission spectrometer and other electrochemical tests were performed to determine the physical/ chemical and electrochemical properties of those LiNi_0.5Co_0.2Mn_0.3O₂ materials.Li2CO₃ and Ni_0.5Co_0.2Mn_0.3（OH）₂ hydroxides（industrial grade, provided by the Xi’an Huijie Industrial CO., LTD.） with different particle sizes（3 μm, 6 μm and 9 μm, precursors denoted as PRE-D3, PRE-D6 and PRE-D9） were used as the raw materials to synthesis NCM523 materials by the solid-state reaction process. The sintering condition was set as 800 ℃12 h for the study of impact of Li/TM stoichiometric ratio on the physical and chemical properties of NCM523 materials with different precursor particle size（referred as NCM523-D3, D6 and D9）. The elemental analysis showed that the Li/TM ratio of different NCM523-D3 samples is close to that of their own raw material mixtures. XRD results stated that no impurity phase in those materials could be found, however the pH test of NCM523 solution indicated the formation of residual alkali on the NCM523 surface. The electrochemical test showed that the sample with 10% Li excess owns the best capacity, cycling and rate performance compared with other samples, and its initial charge/discharge capacity reach 200.0/172.6 mAh g-1, with an average discharge capacity of 155.7, 145.1, 128.2 and 97.5 mAh g-1 for 1 C, 2 C, 5 C and 10 C, respectively.The optimal synthesis temperature for sample NCM523-D6 and-D9 were reconfirmed to be 800℃, with the Li excess of 10%. Their average capacity for the first 10 cycles reaches 151.68 mAh g-1 and 146.05 mAh g-1, respectively, and a capacity retention of 104.9% and 102.4% was attached on those samples after 100 cycles. The excess of Li element was fixed at 10% and the sintering condition was set as 800 ℃12 h to study the influences of precursor particle on the NCM523 material. Physical and electrochemical characterizations demonstrated that sample NCM523-D3 has the best layered structure compared with others, and with the decrease in precursor particle size, the cation mixing degree shows up a decrease trend while the specific surface area increases from 0.236 m2 g-1 to 0.937 m2 g-1. Sample NCM523-D3 presents a better rate capability than NCM523-D6 and-D9, whose average capacity for 1 C, 2 C, 5 C and 10 C reaches 155.7, 145.1, 128.2 and 97.5 mAh g-1, respectively. Although the CV and EIS tests show that NCM523-D3 has the maximal impedance among all samples, the bigger crystallite dimension and larger SSA of NCM523-D3 are the key factors affecting the outstanding capacity and rate performance of the material.