Voltage-induced phase transformation: key to enhanced cycling stability in NiS pseudo-supercapacitors

Abstract

Pseudocapacitive materials have been intensively investigated for their high specific capacitance, yet most suffer from inferior rate capability and cycling stability in energy storage. The key to addressing these drawbacks is to enhance the electronic/ionic conductivity and structural robustness of such materials. Herein, a 3D hierarchical porous carbon (HPC) skeleton with interconnected macropores was adopted as the growth substrate, and high-capacitance NiS was in situ grown within its interconnected pores to fabricate a high-performance pseudocapacitive composite. Results show that the HPC not only facilitates rapid ion transport via its porous architecture and constructs a 3D conductive network for boosted electrical conductivity, but also exerts a nanoconfinement effect to effectively suppress NiS agglomeration. Further mechanistic studies reveal that rapid cycling decay mainly stems from over-discharge induced phase transition of NiS to β-Ni(OH)₂, and a potential-controlled phase transition strategy is validated to markedly improve cycling performance. The asymmetric supercapacitor assembled with the optimized composite and activated carbon delivers a maximum energy density of 51.1 Wh kg⁻¹ and a power density of 21 566.6 W kg⁻¹, with its capacitance retention after 10 000 cycles increased from 7.9% to 62%. This work provides a feasible strategy and mechanistic insight for improving the cycle life of pseudocapacitive devices by suppressing electrode side reactions, as well as a method to determine their optimal operating voltage window.