Preparation And Properties Of Li[Ni,Co,Mn]O₂ Cathode Material For Lithium Ion Battery

Nowdays, LiCoO₂ is the most widely used cathode material in commercial lithium-ion batteries. Its high toxicity, poor safety and high cost are still issues of great concern and suitable alternatives have always been pursued. Layered Li[Ni,Co,Mn]O₂, has been extensively studied as a promising cathode material for lithium ion battery, because of its higher reversible capacity, milder thermal stability, lower cost than commercially used LiCoO₂. In this dissertation, solid state method and co-precipitation method were adopted to synthesize Li[Ni,Co,Mn]O₂ materials. The preparation processes, electrochemical performance, the electrode/electrolyte interphase layer (SEI layer) and the surface modification of Li[Ni,Co,Mn]O₂ were further studied.LiNi_1/3Co_1/3Mn_1/3O₂ was synthesized by solid state method. The effects of calcination method, temperature and time and Li/M ratios on the performance of LiNi_1/3Co_1/3Mn_1/3O₂ were studied by X-ray diffraction (XRD), scanning electron microscope (SEM) and charge-discharge tests. Results demonstrated that the materials synthesized by multi-step calcination showed better electrochemical performance. Calcination temperature, Li/M ratios and calcination time showed synergistic effect on the performance of materials. The optimal condition to synthesize LiNi_1/3Co_1/3Mn_1/3O₂ was Li/M=1.09, calcination at 900℃for 12 h after precalcination at 500℃for 5 h in air. The initial discharge capacity of the as-prepared LiNi_1/3Co_1/3Mn_1/3O₂ was 177 mAh·g-1 at a current density of 20 mA·g-1 over the voltage range of 2.8⁴.4 V and the capacity retention after 40 cycles was 94.9%.LiNi_1/3Co_1/3Mn_1/3O₂ was also synthesized by co-precipitation method. The effects of precursor [Ni_1/3Co_1/3Mn_1/3(OH)2] synthesis process and calcination conditions on the morphology and electrochemical performance of LiNi_1/3Co_1/3Mn_1/3O₂ were studies by SEM and charge-discharge tests. The optimal precursor synthesis process was pH=12.0, the amount of ammonia _0.5mol·L-1, and the rate of dripping 4mL·min-1. The optimal calcination method was Li/M=1.05, calcination at 900℃for 12 h after precalcination at 500℃for 5 h in air. The initial discharge capacity of the as-prepared LiNi_1/3Co_1/3Mn_1/3O₂ was 183 mAh·g-1 at a current density of 20 mA·g-1 over the voltage range of 2.8⁴.4 V and the capacity retention after 40 cycles was 97.3%. The properties of the material obtained by co-precipitation method were better than those obtained by solid state method. Surfactant, immersing in nitrate solution, F- associated chelating agent and calcination after tabletting were attempted to improve the morphology and tap density of the final products. Results showed that surfactant and F- associated chelating agent could only improve the morphology of the precursor. Immersing in solution could improve the tap density but deteriorate the electrochemical performance of the material. Tabletting before high-temperature calcination could significantly increase the tap density of the material without deteriorating the electrochemical performance. The content of Co in the material was optimized. In the LiCO₂xNi_0.5-xMn_0.5-xO₂ series material, if the Co content was too high or too low, the performance of the material would get worse. The optimal Co content was 2x=0.2⁰.33.The effects of the electrolyte and the charge-discharge regime on the morphology and the components of SEI layer formed on the surface of LiNi_1/3Co_1/3Mn_1/3O₂ material were studied by SEM, Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS). The electrolyte decomposed seriously and the amount of SEI layer was larger when LiNi_1/3Co_1/3Mn_1/3O₂ was cycled in the electrolyte with pure DMC (dimethyl carbonate) or DEC (diethyl carbonate) solvent. The decomposition of DMC and DEC could be inhibited and the amount of SEI layer was little after EC (ethylene carbonate) or PC (propylene carbonate) was added to the electrolyte. The SEI layer formed in pure DMC electrolyte was thicker and more compact, and the cycle performance of LiNi_1/3Co_1/3Mn_1/3O₂ was better than that in other electrolyte. The SEI layer formed in the electrolyte with pure DEC, EC/DEC, PC/DEC, EC/DMC and PC/DMC solvents was not compact, and the cycle performance of material got worse with increasing in the amount of SEI layer. The cycle performance of LiNi_1/3Co_1/3Mn_1/3O₂ was better in the electrolyte containing DMC than that in the electrolyte containing DEC. The effects of charge-discharge regimes on the SEI layer on the cathode materials were also significant. Increasing the end-of-charge voltage, the charge/discharge current, the temperature and the cycle number could make the decomposition of electrolyte more seriously and promote the formation of SEI layer. Surface modifications of Li[Ni,Co,Mn]O₂ material by carbon coating, Ag additive and metal oxide coating were respectively adopted to improve the electrochemical perforance of the material. In this dissertation, LiNi_1/3Co_1/3Mn_1/3O₂ was carbon-coated at 400℃in air using polyvinyl alcohol (PVA) as carbon source. XRD patterns indicated that the crystal structure of LiNi_1/3Co_1/3Mn_1/3O₂ was not destroyed during the carbon coating process. Transmission electron microscope (TEM) images confirmed that carbon existed on the surface of LiNi_1/3Co_1/3Mn_1/3O₂ particles. Charge-discharge tests showed that the electrochemical performance of LiNi_1/3Co_1/3Mn_1/3O₂ was improved by carbon coating. The LiNi_1/3Co_1/3Mn_1/3O₂/C composite with the optimal carbon content of 1.0 mass% showed the best cycle performance (the capacity retention after 40 cycles was 96.3%) and the best rate capability. The electrochemical impedance spectrum (EIS) results showed that the improved electrochemical performance caused by carbon coating was mainly attributed to the decrease of the charge transfer resistance. Ag was added to LiNi_1/3Co_1/3Mn_1/3O₂ material by the thermal decomposition of AgNO3, which could also improve the electrochemical performance of LiNi_1/3Mn_1/3Co_1/3O₂. XRD results indicated that Ag existed as elementary substance. The results of cyclic voltammetry and XPS indicated that Ag was unstable in the commercial electrolyte at high potential. XRD, SEM and XPS were used to study the electrodes after charge/discharge cycles, and a novel viewpoint was proposed: Ag additive not only enhanced the electrical conductivity of the material and lower the polarization of the cell, but also properly increased the layer spacing of LiNi_1/3Mn_1/3Co_1/3O₂ after repeated charge/discharge cycles and promoted more compact and protective SEI layer formed on the surface of LiNi_1/3Mn_1/3Co_1/3O₂, which were beneficial to the electrochemical performance of LiNi_1/3Mn_1/3Co_1/3O₂. ZnO was coated on the surface of LiNi_0.5Co_0.25Mn_0.25O₂ powders by a sol-gel method. XRD results indicated that the lattice structure of LiNi_0.5Co_0.25Mn_0.25O₂ was not changed distinctly after surface coating and the expansion of lattice parameters after coating was due to the fact that part of Zn2+ might dope into the lattice of the material during the heat treatment process. SEM, energy dispersive spectroscopy (EDS) and XPS proved that ZnO dispersed on the surface of LiNi_0.5Co_0.25Mn_0.25O₂ particles. Charge and discharge tests showed that the cycle performance and rate capability were improved by ZnO coating, however, the initial capacity decreased dramatically with increasing the amount of ZnO. The XPS spectra of the electrodes after charge/discharge cycles showed that ZnO coated on LiNi_0.5Co_0.25Mn_0.25O₂ promoted the decomposition of the electrolyte at the early stage of charge-discharge cycle to form more stable SEI layer, and it also can scavenge the free acidic HF species from the electrolyte. The electrochemical impedance spectroscopy (EIS) results showed ZnO coating could suppress the augment of charge transfer resistance upon cycling, which guarantees a better electrochemical performance.