Effects of cation mixing on lithium intercalation into and deintercalation from $Li_{1-5}Al_yNi_{1-y}O_2$ (0≤y≤0.25) electrodes

Abstract

Electrochemical lithium intercalation into and deintercalation from $Li_{1-δ}Al_y Ni_{1-y}O_2$ (0≤y≤0.25) electrodes were investigated in 1 M $LiClO_4$-propylene carbonate (PC) solution by using galvanostatic intermittent titration technique (GITT), X-ray diffractometry (XRD), and potentiostatic current transient technique. The open circuit potential transient of $Li_{1-δ}Al_y Ni_{1-y}O_2$ (y = 0) electrode determined from GITT showed three potential plateaux at 3.65, 4.00, and $4.20 V_{Li/Li+}$ due to the coexistence of two phases: hexagonal phase H1 and monoclinic phase M, phase M and hexagonal phase H2, and phase H2 and hexagonal phase H3, respectively, as confirmed in the analysis of the XRD patterns. The interlayer spacings of H3 were determined to be smaller than other phases, H1 and H2, by $3×10^{-2} nm$. This shrinkage, caused by the irreversible migration of nickel ion into lithium vacant sites, led a noticeable capacity loss in the charge-discharge curve. The potential plateau at $4.20 V_{Li/Li+}$ became narrower with aluminum content, y, and finally it disappeared at y = 0.25. This indicated that aluminum ion substituted for nickel ion stabilizes the layered structure and hence improves the reversibility of the lithium intercalation and deintercalation. Potentiostatic current transients were obtained from the $Li_{1-δ}Al_yNi_{1-y}O_2$ electrodes by application of large potential step encountering the potential plateau at $4.20 V_{Li/Li+}$. Both cathodic and anodic current transients displayed three-stage behaviours, which are responsible for a phase transformation of H3 to H2 phase and vice versa, respectively. Moreover, the total charges transferred during the lithium intercalation and deintercalation corresponding to the potential steps were decreased with cycling. From these experimental results, it is suggested that lithium transport through the $Li_{1-δ}Al_yNi_{1-y}O_2$ (y = 0; 0.05; 0.15) electrodes was strongly affected by the cation mixing.