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.