Synthesis of submicron-sized NiPS₃ particles and electrochemical properties as active materials in all-solid-state lithium batteries

Abstract

Submicron-sized NiPS₃ particles were synthesized by heating fine Ni powder (<100 nm), red phosphorus, and sulfur at 673 K for 48 h, and their electrochemical properties as cathodes in sulfide-based all-solid-state lithium batteries were investigated using discharge/charge measurements, X-ray diffraction (XRD) measurements, electrochemical impedance spectroscopy, and X-ray absorption near-edge structure spectroscopy (XANES). Batteries using submicron-sized NiPS₃ as a cathode active material exhibited a 10th discharge capacity of 147 mAh g⁻¹, which was larger than that obtained with 10–100 micron-sized NiPS₃ (∼90 mAh g⁻¹). The XRD patterns of the composite cathode before and after discharge/charge suggested that Li₄P₂S₆ was formed irreversibly (2NiPS₃ + 4Li⁺ + 4e⁻ → Li₄P₂S₆ + 2Ni), and this irreversible reaction would reduce the capacity of the cathode. The XANES spectra suggested the oxidation/reduction of Ni during discharge–charge cycles, but the change in the spectra during the cycles was considerably small if one assumed the oxidation/reduction of Ni²⁺/Ni⁰.