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Octahedral Ni3+ Substitution-Induced D-Band Center Modulation Enables Fast and Fully Reversible Conversion Kinetics
Ni Fang1,2, Huihui Yuan1,2, Xiaorong Dong1,2
1The State Key Lab of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai, P.R. China.
Abstract:
Conversion-type sulfides offer high capacities for sodium storage but are limited by sluggish redox kinetics, incomplete reconversion, and large volume changes. Here we show that these limitations can be addressed through octahedral-site Ni3+ substitution in spinel nickel-cobalt sulfides. By selectively enriching Ni3+ in the octahedral sites of NiCo2S4 to form Ni(Co1- xNix)2S4 (NNCS) hollow nanospheres, the d-band center is downshifted from -1.49 to -1.80 eV and the metal-sulfur d-p hybridization is strengthened, with the d-p energy separation narrowing from 0.289 to 0.103 eV. These electronic-structure changes lower antibonding-state occupancy and facilitate electron transfer during bond breaking and reformation, thereby enabling fast and reversible conversion. Ex situ HRTEM, SAED, XPS, and XRD directly show that NNCS undergoes nearly complete reconversion upon charge, in contrast to the residual metallic species observed in the control sample. The electrode accordingly delivers an initial Coulombic efficiency (ICE) of 91%, 632 mAh g-1 after 800 cycles at 5 A g-1, and 500 mAh g-1 after 900 cycles at 10 A g-1. When assembled into a full cell with Na3V2(PO4)3, the capacity retention after 56 cycles reaches 91.3%.
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