Li Pre-Doping of SiO Electrodes Using Li-Naphthalenide Solution for Next-Generation Batteries

Abstract

SiO is regarded to be one of the promising anode materials to replace graphite in lithium-ion batteries (LIB) because of its higher specific capacity than graphite and better cycle stability than Si [1]. However, the large irreversible capacity is a serious problem which consumes charged capacity in vain during initial cycles. Therefore, Li pre-dope to dose Li to SiO beforehand has been proposed. In this study, our previous achievement on Li pre-doping to Si electrodes was much helpful. The highlights of the achievement are as follows [2, 3]. Li-naphthalenide (Li-NTL) was selected as a Li pre-doping agent.Li pre-doped capacity was much dependent on the solvent of Li-NTL solution.2-methyltetrahydrofuran (MeTHF) delivered the largest pre-doped capacity. Hence, we conducted this study on Li pre-doping to SiO electrodes by referring to the achievement on Si electrodes.SiO electrodes coated on Cu foil consisted of SiO, Ketjen Black, and polyimide binder in the ratio of 80 : 5 : 15 (m/m). Two types of SiO powder with an average particle size of 5 μmφwere used: carbon-coated SiO denoted as C-SiO and bare SiO which were purchased from OSAKA Titanium technologies Co., Ltd. We adopted 0.5 M NTL/MeTHF as a standard pre-doping solution based on our result of Li pre-doping to Si. SiO electrodes were confined in an air-tight cells with the pre-doping solution for 24 h to be pre-doped. The electrodes were taken out of the cells, rinsed in DMC, and used to assemble coin cells with Li counter electrodes, separators and electrolyte solution of 1.0 M LiPF6 / EC + DMC (1:1, v/v) + 10 wt% FEC. All of these processes were conducted in an Ar-filled glove box. Discharge/charge cycle test mode was constant-curent at 0.1 mA cm−2 between 0.02 and 1.5 V at 30 °C. Figure 1 shows charge/discharge performance of C-SiO electrodes with and without Li pre-doping. In Figure 1a, the OCV of the Li pre-doped electrode was as low as 0.2 V, which was much lower than that of the electrode without pre-doping, exceeding 2.0 V, and the charge capacity was nearly missing. However, the discharge capacity was not less than that of the electrode without pre-doping. This result reveals that SiO can be pre-doped effectively by Li-NTL/MeTHF, which is the same as Si. Figure 1b shows dQ/dV analysis results, in which the anodic curves for the with and without pre-doping are nearly identical. However, almost no cathodic curve is observed for the with pre-doping as shown in Figure 1a, while a sharp peak at 0.46 V assigned to formation of L x SiO y is observed for the without pre-doping [4]. A dashed curve inserted in the Figure 1b is the cathodic curve for the second cycle of the with pre-doping to complement the missing first cycle one. This curve shows no peak at 0.46 V and suggests formation of L x SiO y during pre-doping. We further analyzed the pre-doping reaction to elucidate the mechanism based on the result shown above. The detail of the results will be shown and discussed in our presentation.[1] M. N. Obrovac, V. L. Chevrier, Chem. Rev., 114, 11444 (2014).[2] M. Saito et al., s of PRiME 2020, A02-0410 (2020).[3] M. Saito et al., J. Electrochem. Soc., 166, A5174 (2019).[4] C. P. Grey et al., J. Am. Chem. Soc., 141, 7014 (2019). Figure 1