Understanding electrochemical structural degradation of NiCo2S4 nanoparticles in aqueous alkaline electrolyte

Relationship between long-cycle life and crystal structural stability of electrode materials in aqueous electrolyte is of vital importance to accurately understand the electrochemical energy storage mechanism. As a classical non-oxide, spinel NiCo2S4 nanoparticles@carbon nanotube (CNT) sponge, easily synthesized by one-pot hydrothermal method, are used to clarify the relationship between the structural degradation and electrochemical performance in 1 M KOH electrolyte against long-cycle charge/discharge process. Herein, NiCo2S4 nanoparticles@CNT sponge as the electrode is constructed into the asymmetric pseudocapacitor. The performance in asymmetric pseudocapacitor, such as higher redox reaction kinetics, retention capacitance and longer cycling stability, is superior to that of symmetric device. Intriguingly, the changes in the specific capacities, capacitance contribution ratios and capacitance retentions, clearly demonstrate that NiCo2S4 nanoparticles have suffered from the crystal structural degradation during charge/discharge procedures. NiCo2S4 particles are dissociated from CNT and gradually reconstructed to the nanosheets deposited on the carbon nanotube wall, which is completely different from pristine morphology of NiCo2S4 nanoparticles@CNT sponge. Further, spinel phase of NiCo2S4 particles have been partially degraded and cracked into the new and stable derivatives of hexagonal NiOOH and cubic Co3S4. This degradation is supposed that the bonds of Ni-S on or near surface of spinel NiCo2S4 nanoparticle are preferentially broken down by OH− ions driven by charging/discharging potential, and a certain amount of nickel hydroxide ions gather to form the hexagonal NiOOH. Meanwhile, Co-S ion pairs partially evolve into the tetrahedral configurations, which are reconstructed into cubic Co3S4 with the adjacent Co-S octahedral coordination. The mixed multivalence states of Ni2+/3+ and Co2+/3+ are assigned to the origin of redox reaction kinetics and the capacitance contribution against long-cycle operation process. This work provides a method for evaluating the relationship between electrochemical properties and crystal degradation of non-oxide electrode materials.