A Study On Synthesis And Coating Modification Of LiNi_1/3Co_1/3Mn_1/3O₂

In recent years, LiNi1/3Co1/3Mn1/3O2(abb. NCM) was regarded as the possiblesubstitution for LiCoO2, because of its low cost, high specific capacity, betterthermodynamic stability and lower toxicity. Layed NCM was prepared via hydroxideco-precipitation, and the optimal condition of synthesis process was also explored.Surface-coating through carrier transfer method was adopted to improve elechemicalperformance. The structure, morphology and electrochemical performances ofsurface-coating materials were characterized via XRD, SEM, charge-dischargecycling, cyclic voltammetry (CV) and electrochemical impedance spectroscopy(EIS).NCM was prepared via the hydroxide co-precipitation using ammonia ascomplex agent, and the optimal conditions were that the molar rotio of NH3andM(M=Ni+Co+Mn) was0.1, pH value was11.5, the reaction time was12h andcalcined at900℃.XRD showed that the phase of NCM belonged to hexagonalα-NaFeO2layered structure, and the morphology of surface was smooth and compactfrom SEM. The initial discharge special capacity of NCM was156.3mAh·g-1in2.8-4.3V, while the discharge special capacity of the50thcycle was144.5mAh·g-1, andthe capacity retention was92.5%, which indicated that the cathode material had bettercycle performance. But the high rate discharge performance was not satisfied at5Crate with93.6mAh·g-1, which was only60%of that at0.2C rate.The surface of NCM was modified with SnO2or TiO2by carrier transfer method.The result indicated that the surface coating did not effect its α-NaFeO2layeredstructure. SEM showed that SnO2or TiO2had coated the surface of cathode materialuniformly, which reduced direct contact with electrolyze and supressed the sideeffects.2%SnO2or TiO2-coated NCM exhibited the best electrochemicalperformance. The initial discharge capacity of S2-NCM and T2-NCM were169.4mAh·g-1and156.9mAh·g-1at0.2C rate, and the capacity retention were95.2%and94%after50cycles in2.8-4.3V, while at1C and5C rates, the discharge capacitywere139.4mAh·g-1,116mAh·g-1and134.4mAh·g-1,111.8mA·hg-1, respectively. Theresults showed that the cycle performance and rate performance were improved via surface modification.TiO2coated-NCM synthesized via carrier transfer method exhibited betterperformance. The initial discharge capacity of T-NCM (directly mixing) and T2-NCM(coated via carrier transfer method) were153.3and156.9mAh·g-1at0.2C rate in roomtemperature, and the capacity retention were82.6%and94%after50cycles in2.8-4.3V, while at1C and5C rates, the discharge capacity were112.1mAh·g-1,124.7mAh·g-1and96.5mAh·g-1,111.8mAh·g-1, respectively. The result indicated thatTiO2coated-NCM synthesized via carrier transfer method showed the best cycleperformance and rate performance. EIS result showed that impedance of TiO2coated-NCM synthesized via carrier transfer method suppressed the increasing ofelectrochemical impedance during cycling.