Electronic Conductivity Measurements for Li x Ni0.45Mn1.5Cr0.05O4 (0 < x < 2) Thin Films

Abstract

Spinel-structured Li x Ni0.45Mn1.5Cr0.05O4 (LNM) is known as a 5-V-class cathode material [1]. The LNM shows the redox reactions of Ni2+/3+/4+ in the 5 V region with x from 1 to 0. In addition, the redox reactions of Mn3+/4+ take place in the 3 V region with x from 1 to 2. As has been reported by West et al., stable charge-discharge reactions in the 3-5 V region occurs in case of thin-film all-solid-state battery (SSB) using LNM film electrodes, which utilize the capacity of LNM up to 250 mAh g-1 [2]. During these charge-discharge reactions, several phase transition reactions occur and 3 V region shows hysteresis at room temperature. Measuring the electronic conductivity (σ) of LNM is one of the effective approaches to understand this hysteresis mechanism around 3 V region because the σ of LNM will depend on the Li composition and crystal structure. Although electronic conductivity variations of LNM (0<x<1) has been already reported [1], wide compositional range (0<x<2) of the LNM has not been investigated at all. This study investigates the variation of σ with x in LNM films from 0 to 2 as SSB. We used interdigitated array (IDA) electrodes with patterned Pt comb electrodes on quartz substrates for the σ measurements [3]. Thin films of LNM with the thickness of 40 nm were deposited onto the IDA electrodes by pulsed laser deposition (PLD). In case of the conductivity measurements using SSB, both an amorphous Li-P-O (LPO) film and a lithium phosphorus oxynitride glass (LiPON) film were sequentially deposited on the LNM film by PLD and radio frequency (RF) magnetron sputtering, respectively. After that, thin film of Li was deposited on the LiPON film by vacuum evaporation on the LiPON film. Lithium insertion/extraction reactions of LNM films were carried out by the CV at 0.5 mV s-1, and the voltages were maintained at desired points for the σ measurement until the current density decreased below to 10 nA cm-2. After that, the σ in LNM film was measured in-plane direction by electrochemical impedance spectroscopy (EIS). All these experiments were conducted at room temperature (25°C) in an Ar-filled grove box. Figure 1 shows a CV of SSB and σ measured after the voltage of the SSB is maintained at numbered potentials during the CV. The CV started from 3.7 V (No. 1) and was swept to 2.0 V-5.0 V-3.7 V. At initial voltage of 3.7 V, the σ was 1.5 × 10-4 S cm-1. The σ then slightly decreased after the lithium insertion peak A (No. 3) and drastically decreased at 2.0 V after the peak B (No. 4). The decreased σ partially recovered at anodic scan after the peak A’ (No. 6) and the σ gradually increased until the peak B’ (No. 7&8). At higher voltage region (No. 9-14), the σ showed reversible change against Li+ insertion/extraction reactions and finally returned to initial value at 3.7 V (No. 15). Wide-range variations of the σ of LNM film during the charge-discharge reactions were successfully measured as SSB by using IDA electrodes. Same measurements were carried out in liquid electrolyte, but the hysteresis of σ appeared different manner. Based on these data, lithium insertion/extraction reaction at 3-5 V region of LNM will be discussed in the poster session. Reference 1. Z. M. Rosenberg, A. Huq, J. B. Goodenough, and A. Manthiram, Chem. Mater., 27, 6934 (2015). 2. W. C. West, Y. Ishii, M. Kaneko, M. Motoyama, and Y. Iriyama, ECS Electrochem. Lett., 3, A99 (2014). 3. M. Nishizawa, T. Ise, H. Koshika, T. Itoh, and I. Uchida, Chem. Mater., 12, 1367 (2000). Figure 1