Structural, thermal and electrochemical properties of chromium-substituted LiNi0.8Co0.2O2

LiCr_yNi_0.8−yCo_0.2O₂ compositions, where y=0.000, 0.010, 0.025, 0.040, 0.050, 0.075 and 0.100, were synthesized via a conventional ceramic route. X-ray diffraction studies indicated cation mixing for the compositions with y ≥ 0.05. Cyclic voltammetric studies revealed that the systems were reversible only when y was lower than 0.05. High levels of substitutions with Cr resulted in highly irreversible systems, either due to cation mixing or the displacement of the substituent ions to the lithium inter-slab regions, or both. The charge-discharge characteristics of LiCr_yNi_0.8−yCo_0.2O₂ were similar to those of the unsubstituted material over ten cycles. All the other substituted compositions showed much lower capacities and reduced cyclability. LiCr_0.025Ni_0.775Co_0.2O₂ gave a first-cycle capacity of 169 mAh/g in the 3.0 to 4.4 V window at a 0.1 C rate, fading to 156 mAh/g in the tenth cycle. Differential scanning calorimetric studies revealed that substituting with chromium produced no benefit to thermal stability. The structural, thermal and electrochemical properties of the pristine and Cr-substituted LiNi_0.8Co_0.2O₂ compositions are discussed.