Intrinsic Electrochemical Characteristics of LiNi0.5Mn1.5O4 Spinel Synthesized By Flux Method
Authors: Nishikawa Kei , Zettsu Nobuyuki , Teshima Katsuya , Kanamura Kiyoshi 
Published: February 2020

Lithium-ion batteries (LIBs) with high gravimetric and volumetric energy density are very important key devices for a power supply of plug-in hybrid electric vehicles (PHEVs) and electric vehicles (EVs). Much higher energy density is extensively demanded to increase the mileage per charge. Current LIBs utilize the composite electrodes which include active materials, binders, conductive agents, and current collector. The electrochemical characteristics of the composite electrode must be affected by the “composite” state (kinds of materials, mixing ratio, thickness, and tap density). It is difficult to distinguish the electrochemical characteristics of only active materials from that of the composite electrode. However, the intrinsic properties of active materials are very important to evaluate the electrode performance and understand the mechanism of the charging and discharging reaction. In this study, the single particle measurement technique was applied to study the intrinsic electrochemical characteristics of LiNi0.5Mn1.5O4 spinel (LNMO) synthesized by the flux method. This LNMO particles are aggregates of primary particles with from sub-micro to several micro-meter size, as shown in Fig.1. Fig.1 shows that the primary particles of LNMO are crystalline particles. This research will discuss the relationship between the electrochemical characteristics and the crystallinity, e.g. shape, orientation, and surface crystal plane. The details of the single particle measurement equipment were described elsewhere [1]. The two-electrode electrochemical cell was placed on the stage of an optical microscope. Glass coating Au wire (f =10 mm) was used as the micro-probe for the measurement. The electrolyte was mainly 1M LiPF6-EC:PC=1:1. In some measurement, FEC and other solvent were added to study the side reaction. The charging and discharging tests were conducted in the range from 3.4 V to 4.9 V vs. Li metal counter electrode by using a SP-150 (Biologic) with low current probe. After the electrochemical measurements, the measured particle was picked up by micro-tweezers and transferred to micro-Raman, SEM, FIB-SEM and TEM. This transfer procedure was done by using a transfer vessel in order to prevent the atmosphere exposure.

Fig.2 shows the discharge rate characteristics of one LNMO particle. The discharge currents were changed from 0.2 nA to 2 nA while the charging current was fixed to be 0.2 nA. If the discharge capacity at 0.2 nA is assumed to be full capacity of the particle, 0.2 nA is corresponding to about 11 C-rate. Therefore, 2.0 nA is about 110 C-rate. The discharge capacity at 110 C-rate was kept over 80% of the capacity at 0.2 nA. The LNMO particles have excellent discharge rate characteristics although the initial irreversible capacity was quite high. In this presentation, the detailed results about not only electrochemical characteristic but also Raman, SEM and TEM analysis will be delivered.

References

[1] K. Dokko, N. Nakata, K. Kanamura, J. Power Sources
189, 783 (2009)

Figure 1