All-Solid-State Lithium Secondary Batteries Using NiPS3 Electrode and Li2S-P2S5 Solid Electrolyte
Authors: Fujii Yuta , Sutoh Yusaku , Miura Akira , Higuchi Mikio , Tadanaga Kiyoharu 
Published: February 2020

All-solid-state lithium secondary batteries using non-flammable solid electrolytes are expected to have high safety, durability, and reliability, and the increased number of studies have been undertaken to investigate their applicability as next-generation power sources. Notably, all-solid-state batteries using sulfide-based solid electrolytes with high Li-ion conductivity have attracted much attention. Glasses or glass-ceramics of Li2S-P2S5 system have been actively investigated as the solid electrolyte.  In order to improve the performance of these Li2S-P2S5containing cells, the development of high-capacity electrodes with favorable electrode/electrolyte interfaces is mandatory. 

On the other hand, a layered material NiPS3 can be reversibly intercalated with lithium, and its potential as an electrode active material has been studied.  However, the potential use of NiPS3as an electrode active material within the framework of all-solid-state batteries using sulfide-based solid electrolyte has not been sufficiently investigated so far.

In this study, we focused on the ability of NiPS3 as an electrode active material in all-solid-state batteries using sulfide-based electrolytes. An all-solid-state Li-In/80Li2S∙20P2S5/ NiPS3 cell was assembled, and the charge-discharge performances of this cell, using NiPS3-80Li2S∙20P2S5composite electrode, were investigated [1].

The XRD pattern proved that the single phase of NiPS3 was obtained by heating a mixture of nickel powder, red phosphorus, and sulfur in an evacuated quartz tube.  The sizes of the NiPS3 particles, measured from scanning electron microscopy images, ranged from a few to 100 micrometers. Since NiPS3 particles of several hundred micrometers were included in the ground sample, a 100 μm sieve was used, and the NiPS3 particles with sizes smaller than 100 μm were used as the active material for fabricating all-solid-state batteries with a sulfide-based solid electrolyte.

In the all-solid-state Li-In/80Li2S∙20P2S5/ NiPS3 cell, the open circuit voltage of the cell was 1.8 V.  The cell was first discharged to a capacity of 216 mAh g-1, which corresponds to the insertion of 1.5 mole Li+ per Ni, under a current density of 64 µA cm-2 at room temperature. Then, the cell was charged to 2.5 V (vs. Li-In). The cutoff voltage was 0.8-2.5 V (vs. Li-In) from the second cycle onwards. After the first cycle, the charge capacity was 102 mAh g-1, roughly corresponding to the insertion of 0.7 mole Li+ per Ni. The all-solid-state cell exhibited reversible behavior after the first cycle, and a stable reversible capacity for 30 cycles. During the first cycle, the cell showed a discharge plateau at about 1.2 V (vs. Li-In). This plateau, which increases for subsequent cycles, would be identical to that observed for Li/NiPS3 cells using conventional liquid electrolytes. As it has been previously reported that the discharge potential in this kind of systems is influenced by the crystalline state of NiPS3, the above mentioned change in the discharge plateau can be due to the change in the electrode crystallinity with cycling.

In the cycle performance study of the all-solid-state cell, the all-solid-state cell exhibited a capacity of about 80 mAh g-1 for 30 cycles; this capacity accommodates the insertion of about 0.5 molar Li+ per Ni. The all-solid-state cell showed this stable reversible capacity for 30 cycles. Consequently, our results demonstrate that NiPS3 can be efficiently utilized as an electrode active material in all-solid-state batteries with sulfide-based solid electrolytes.

[1] Y. Fujii, A. Miura, M. Higuchi, and K. Tadanaga, submitted.