Towards excellent electrical conductivity and high-rate capability: A degenerate superlattice Ni3(S)1.1(S2)0.9 micropyramids electrode

Abstract Degenerate semiconductor is very highly desired in energy conversion and storage technologies due to its metal-like conduction behaviors. This is the first time the doping S2 in Ni3S2 lattice into chemically homogeneous Ni3(S)1.1(S2)0.9 superlattice structure is proposed to induce a degenerate characteristic towards excellent electrical conductivity and high-rate capability. In this study, a series of the chemically homogeneous S2-doped Ni3(S)1.8(S2)0.2, Ni3(S)1.6(S2)0.4, Ni3(S)1.3(S2)0.7, and Ni3(S)1.1(S2)0.9 micropyramid arrays on Ni foam were synthesized by reacting the Ni foam and alkaline sulfur aqueous solution in different S22- concentrations. The perfect Ni3(S)1.1(S2)0.9 superlattice structure corresponds to the periodic S–Ni–S2 atom arrangements in whole crystal lattice, which endows a degenerate characteristic of metal-like electrical conductivity to significantly improve the electrochemical performance. The bulk series resistance (Rs) value is only 0.62 Ω, while the charge-transfer resistance (Rct) is nearly 0 Ω in the superlattice Ni3(S)1.1(S2)0.9 electrode. As a cathode material for application in lithium ion batteries (LIBs), a very high specific capacity of 874 mAh g−1 is achieved at current density of 200 mA g−1. Remarkably, it still holds a high capacity of 565 mAh g−1 at current density of 500 mA g−1, indicating its superior high-rate capability. This study reveals that the periodic S–Ni–S2 atom arrangements in crystal lattice is a key factor in determining the superlattice structure, high specific capacity, and the dynamic behaviors of electron/ion transport.