Electrochemical performance of Li(NMC)O2 cathode materials for Li-ion batteries

Abstract

Li-ion batteries are widely used in portable electronic industry, and have been considered for large scale applications in electric or hybrid electric vehicles. The rapid development in electronics industry calls for better batteries of higher energy storage, lower capacity decay, and lighter weight. A better understanding on materials’ electrochemistry would help us optimize the battery’s performance. This project is therefore focused on the electrochemical performance of a LiNiyMnzCo1-y-zO2 type cathode material. LiNiyMnzCo1-y-zO2 powder was successfully synthesized through co-precipitated hydroxide route, with a resultant composition of LiNi0.36Mn0.29Co0.35O2 determined from EDX. The particle morphology was examined by SEM. It was found that the particles possessed a polyhedral geometry with an average size of 1 μm. XRD analysis on lattice parameters, I003/I104 parameter and R-factor showed a well layered structure in this material with little cation mixing between Li+ and Ni2+. Both particle morphology and well indexed diffraction pattern pointed to the formation of highly crystallized and pure compound. This material’s electrochemical properties were characterized with various electrochemical measurements. Cyclic voltammetry was performed at different scan rates. The oxidation and reduction peaks were found to be at ~3.8 V and ~3.7 V, respectively, which corresponds to the redox reaction of Ni2+/Ni4+. No Mn3+ was found in this compound due to a lack of redox peak at 3.0 V; and this confirms that the major oxidation state of Mn ion is 4+. Cycling charge-discharge performance was carried at two rates: 0.5 C (80 mA/g) and 2 C (320 mA/g). The corresponding discharge capacity was 154 mAh/g and 143 mAh/g at 0.5 C and 2 C rate, respectively. By the end of the 30th cycle, the discharge capacity decays to 137 mAh/g and 122 mAh/g, respectively. The Coulombic efficiency after the first cycle was over 100%. A linear capacity fading model was adopted for this material, and the capacity decay rate was 0.52 % per cycle. Li-ion chemical diffusion coefficient was determined as a function of cell voltage using both GITT and PITT techniques, yielding DLi of ~10-10 cm2/s from GITT and ~10-11 cm2/s from PITT. This range is consistent with the DLi obtained using CV curves at different scan rates, which yielded a value of 2.4×10-11 cm2/s. The equilibrium voltage vs. amount of lithium extraction showed a change of slope at 30% lithium de-intercalation. Finally, a brief investigation on crystal structural change after 30 cycles at 0.5 C and 2 C rates was carried out using XRD, which revealed severe cation mixing and low hexagonal ordering upon cycling by analyzing the I003/I104 parameter, R-factor and the c/a ratio. The increase of (110) over (108) peaks intensity might infer an irreversible Li loss which accounts for the deterioration of the layered structure.