Electrochemical Measurements for Rechargeable Batteries Using MCoO2 (M=Li, Na) Single Positive Electrode Particle
Authors: Saito Takahiro , Nakamura Tatsuya , Kobayashi Takeshi , Seki Shiro 
Published: February 2020

Background

　Rechargeable batteries including lithium-ion battery have been used as many power sources. Electrode active materials react as intercalate/de-intercalate component of metal cation, which are governing battery properties. Moreover, electrode fabrication process, porosity and thickness influence for battery performances, such as rate capabilities. So far, many studies were carried out on composite electrode containing electric conductive additives and binder and thin-film electrodes. Therefore, it is desirable to analyze for single electrode active material particle (species, quantities). Single particle electrochemical measurement is important to understand essential battery materials. Therefore, we developed a microelectrode manipulation. By applying this technique, we can obtain essential responses independently from the reaction at the active material and electrolyte interface.

　Among them, in addition to LiCoO2 which is a general positive electrode active materials for rare metal of Li batteries, active material for Na battery which is abundant resource on earth and used as alternative material of NaCoO2 were subjected to electrochemical measurements. By obtaining the intrinsic resistance and capacity of a single particle, we will accomplish for design of unified guidelines for battery materials being developed from various perspectives, and being expected to propose the optimal electrode / electrolyte material.

In this study, electrochemical properties of active materials for lithium (LiCoO2) and sodium (NaCoO2) batteries were investigated by single particle electrochemical measurements.

Experiments

　The electrochemical measurements were carried out in the argon-filled glovebox (-70℃ dew point). In the electrochemical measurement, glass capillary coated Pt wire (φ 20 μm) was used for microelectrodes. A Pt microelectrode (20 μm diam.) was directly attached to a LiCoO2 particle (10-20 μm diam. Fig.1) into electrolyte using a micromanipulator under optical microscope observation, then the electrochemical measurements were carried out. 1M-LiPF6/propylene carbonate solution was used as electrolyte. Li foil was used as negative (counter) electrode. In sodium battery systems, 1M-NaTFSA/propylene carbonate solution was used as electrolyte. Na foil was used as negative (counter) electrode. Also, the pasted cell using mixture of LiCoO2-acetyleneblack-PVdF (weight ratio 85:6:9) as a positive electrode was fabricated to compare battery properties under the same voltage condition.

　All electrochemical measurements were carried out at room temperature. We evaluated cyclic voltammetry (CV), constant current charge and discharge (CDC), and electrochemical impedance spectroscopy at several potentials (3.8-4.4 V).

Results & Discussion

　Fig.2 shows nyquist plots for LiCoO2 (22.4 μm diam.) and NaCoO2 (23.0 μm diam.) particles. We detected up to MΩ order resistance in a frequency range from 20 kHz to 10mHz. Also, comparing the response frequencies corresponding to the time constants, since the respective values are substantially equal, it can be presumed that the same resistance component was detected.

　Comparing another Nyquist plots for LiCoO2, it is shown that the response frequencies of the first semicircular arc of the single particle and the second semicircular arc of the pasted cell are almost same ca. 5Hz. Also we assumed that the first semicircular arc of the pasted cell is the resistance component derived from the nagative electrode. The resistance component responding the same frequency is considered to be derived from the interface at the positive electrode active material / enectrolyte solution.

　In addition, as a result of the CDC, the observed capacity of single particle was about 120 mAhg-1, and pasted battery was 160 mAhg-1. When we hope to determing the mass of a single particle, we should know particle size and density. Although the particle size affects the error of the capacity, we could calculate the error range of ±20％, but the unit of capacity was not changed with mAhg-1.

Summary

We constructed a system for measuring electrochemical characteristics of positive electrode active material single particles. By measuring the electrical response, it was possible to measure with accuracy similar to a conventional pasted batteries. In addition, we compared AC impedance measurement results. It was possible to measure the resistance component of single particle with high accuracy and showed the possibility to clarify the reaction process at positive electrode interfaces.

Figure 1