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

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 Li_1-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 Li_1-xMO₂ with nonaqueous solvent occurs at elevated temperature, leading to safety issues for lithium-ion batteries. In this paper, exothermic reactions of Li_1-xNi_0.8Co_0.15Al_0.05O₂ (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., LiNi_0.75Co_0.15Al_0.05Mg_0.05O₂(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 Li_1-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 Li_1-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 Li_1-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).