What happens structurally and chemically during sodium uptake and release by Ni2P2S6: a combined X-ray diffraction, X-ray absorption, pair distribution function and MAS NMR analysis

Abstract

The layered compound Ni₂P₂S₆ was electrochemically characterized for application as an anode material in sodium-ion batteries (SIBs). A high reversible capacity of 621 mA h g⁻¹ at 1 A g⁻¹ was achieved after 190 cycles. The investigation of the complex reaction mechanism of the conversion reaction was performed applying complementary techniques including X-ray powder diffraction, pair distribution function analysis, X-ray absorption spectroscopy, ¹⁹F/²³Na/³¹P MAS NMR, TEM and nano-EDX. The results highlight that Na uptake for up to 5 Na per formula unit (f.u.) led to reduction of Ni²⁺ to metallic Ni nanoparticles and concomitant formation of an intermediate compound Na₄P₂S₆. Increasing the Na content to 12 Na per f.u. generates nanocrystalline Na₂S, which is accompanied by the loss of the long-range order of the pristine sample. In the completely discharged state elemental Ni and Na₂S are present, but in contrast to literature reports, no evidence for the formation of Na_xP phases was found. During the charge process, Ni₃S₂ is formed upon the release of ∼11.7 Na per f.u.