Quantitative Analysis for Evaluating the Exothermic Reaction of LiNiO2-Derivatives with Nonaqueous Solvent

Abstract

LiNiO2-derivatives exhibit reversible lithium extraction / insertion reaction without the destruction of their core structure, namely topotactic reaction, and have been considered as a positive electrode for lithium-ion batteries with respect to high specific capacity and low risk for natural resources in comparison to LiCoO2. Since electrochemically-oxidized forms of Li1-xMO2 (M: Ni, Co, etc.) are not stable phases with respect to chemical thermodynamics, those may convert to stable phases [1-3]. Such stable phases localized in the material grains are one of the main reasons for deterioration of lithium-ion batteries after long-term cycling especially at 60–80ºC [1,2]. Analogous to such materials degradation, exothermic reaction of Li1-xMO2 with nonaqueous solvent occurs at elevated temperature, leading to safety issues for lithium-ion batteries. In this paper, exothermic reactions of Li1-xNi0.8Co0.15Al0.05O2 (NCA) with nonaqueous solvent were examined by differential scanning calorimetry (DSC), and were described numerically in terms of onset temperature and exothermic heat of reactions. Mg-substitution in NCA is also discussed therefrom, i.e., LiNi0.75Co0.15Al0.05Mg0.05O2(NCA-Mg).The lithium cells of pelletized NCA or NCA-Mg were fabricated and charged to specific capacities at 20°C. The pellet was taken out of the cell in an Ar-filled glove box, and a piece of crushed pellet with nonaqueous solvent was sealed in a DSC pan. The exothermic signals, a heat flow in W g-1, were measured at a heating rate of 5ºC min-1.Figure 1(a) displays the DSC profiles for NCA as a function of x in Li1-xMO2. NCA has two-types of exothermic signals in the temperature ranges of 150–250°C and 300–400°C. Pristine x = 0 sample starts to react with nonaqueous solvent at around 300°C. On oxidation, the exothermic signal at around 300°C decreases whereas the new signal appears at around 200°C. Highly-oxidized sample exhibits increased exothermic signals with lowered onset-temperature from 200 to 150°C. NCA-Mg shown in Fig. 1(b) also exhibits a similar trend to NCA in terms of onset temperature and exothermic heat of reactions except the remarkable change in the shape of exothermic signals for x = 0.78 in NCA-Mg, which is derived from structural change upon oxidation [4].In order to compare exothermic reactions of NCA-Mg with those of NCA quantitatively, exothermic heats of reactions were estimated by integrating the exothermic signals with time and are shown in Fig. 2. In a former half of oxidation, 0 < x < 0.5 in Li1-xMO2, exothermic signals in 150–250°C increase whereas those in 300–400°C decrease. Consequently, exothermic heats of reactions become constant at about 1000 J g-1. Then, in a latter half, 0.5 < x < 0.95, exothermic signals in 150–250°C continuously increase, therefore exothermic heat of reactions increases linearly. As shown in Fig. 2, exothermic reaction changes at x = 0.5 in Li1-xMO2, and exothermic heats of reactions of NCA-Mg just follow those of NCA in this temperature range.From these results together with TG, XRD, XAFS, and electrochemical data, similarities and differences in the exothermic reaction between NCA and NCA-Mg are described, and the effect of Mg substitution is discussed. References : [1] Sasaki et al., J. Electrochem. Soc., 156, A289 (2009).[2] Muto et al., J. Electrochem. Soc., 156, A371 (2009).[3] Makimura et al., J. Electrochem. Soc., 159, A1070 (2012).[4] Sasaki et al., J. Electrochem. Soc., 158, A1214 (2011).