Electrochemical Performance of LiNi0.5Mn0.5O2 Electrode in All-Solid-State Battery
Authors: Okochi Rin , Arachi Yoshinori 
Published: December 2024

Introduction

Oxide-based all-solid-state batteries (Ox-ASSBs) have been attracting attention as next-generation secondary batteries because of their high safety and high energy density. Recently, Ox-ASSBs have been reported by use of LISICON-type Li3.5Ge0.5V0.5O4(LGVO) as a solid electrolyte and layered NaCl-type LiCoO2 and LiNi1/3Mn1/3Co1/3O2 as positive electrodes. We have focused on Li2MnO3-based positive electrodes without Co, as shown in Li4/3-2/3x
Mn2/3-1/3x
NixO2 (0<x<1/2). LiNi0.5Mn0.5O2(x=1/2, LNMO) combines high capacity and thermal stability. It is well known that disordering of Li and Ni in the layered NaCl-type structure occurs during sintering and that oxide ions involve in charge compensation at higher voltages. Accordingly, the electrochemical properties vary depending on the amount of Ni substitution and excess Li relative to the transition metal in conventional Li-ion battery. Therefore, we have investigated an Ox-ASSB battery using LNMOs as a positive electrode, specifically focusing on effects of both Li, Ni-disordering and their sintering temperatures on the performance while controlling chemical compositions precisely.

Experimental

LNMO were prepared by co-precipitation method. The precursor Ni and Mn hydroxides ware prepared from Ni(NO3)2･6H2O and Mn(NO3)2･6H2O. The precursor and LiOH･H2O were mixed to a specified chemical composition ratio, formed under pressure, and calcined at 1073, 1173, 1273 K to obtain LNMO. The solid electrolyte LGVO was synthesized by sintering a mixture of Li4GeO4 and Li3VO4 at 973 K. The LNMO composite cathode layer on LGVO was prepared by applying a composite LNMO cathode slurry, drying, and sintering at 1023K for 6 hours The solid electrolyte LGVO and cathode material LNMO contain 5 % Li3BO3 as a sintering aid. The crystalline phase was identified by powder X-ray diffraction measurement by CuKα radiation. The chemical bonding state was analyzed by X-ray photoelectron spectroscopy Electrochemical testing was performed by LNMO/ LGVO / sulfide solid electrolyte Li10GeP2S12 / In-Li cell (Φ10mm) at 323 K. The rate of constant current was 1/100 C in the 2.0-(3.6~4.0) V.

Results and discussion

The samples sintered at 1073, 1173, and 1273 K showed a single phase of LiNi0.5Mn0.5O2. The I
(003)/I
(104) intensity ratio of Bragg peak, which is an index of the degree of disordering of Li and Ni, increased with increasing sintering temperature. The disordering of Li and Ni in the layered NaCl-type structure occurred with increasing sintering temperature. Fig. 1 shows the charge-discharge measurement results of Ox-ASSB using LNMO (x=1/2) sintered at 1173 K. Followed by a single phasic reaction, a plateau is observed around 3.9 V, the discharge capacities are 175 mAhg-1 at the end-of-charge voltage of 3.6 V and more than 250 mAhg-1 at 4.0 V. Compared to conventional LiB using a liquid electrolyte, this LNMO shows a higher specific capacity, suggesting a possibility to have stable charge-discharge behaviors with LGVO solid electrolyte. These results will be discussed based on the analysis of the cell resistance and the chemical bonding states by XPS measurements before and after charging and discharging. We will present results of other Li-rich positive electrodes in Ox-ASSBs while focusing on a role of oxide ions during and charging-discharging.

Figure 1