Robust carbon nanotube-interwoven KFeSO4F microspheres as reliable potassium cathodes

Abstract

Orthorhombic iron-based fluorosulfate KFeSO₄F represents one of the most promising cathode materials due to its high theoretical capacity, high voltage plateau, unique three-dimensional conduction pathway for potassium ions, and low cost. Yet, the poor thermostability and intrinsic low electronic conductivity of KFeSO₄F challenge its synthesis and electrochemical performance in potassium-ion batteries (PIBs). Herein, we report, for the first time, judicious crafting of carbon nanotubes (CNTs)-interwoven KFeSO₄F microspheres in diethylene glycol (DEG) (denoted KFSF@CNTs/DEG) as the cathode to render high-performance PIBs, manifesting an outstanding reversible capacity of 110.9 mAh g^-1 at 0.2 C, a high working voltage of 3.73 V, and a long-term capacity retention of 93.9% after 2000 cycles at 3 C. Specifically, KFSF@CNTs/DEG microspheres are created via introducing CNTs into the precursors DEG solution at relatively low temperature. Notably, the strong binding of the ether groups in DEG retards the nucleation and growth of KFSF, leading to in situ formation of microspheres with CNTs interwoven within KFSF crystals, thereby greatly enhancing electronic conductivity of KFSF. Intriguingly, the remarkable electrochemical performance of KFSF@CNTs/DEG cathode is found to stem from the massively exposed (100) plane and uniform interpenetration of CNTs inside KFSF microsphere. More importantly, in situ X-ray diffraction and electrochemical kinetics study unveil outstanding structural stability and high K⁺ diffusion rate of KFSF@CNTs/DEG. Finally, the KFSF@CNTs/DEG//graphite full cell displays a large energy density of ∼243 Wh kg^-1. Such simple route to KFSF@CNTs/DEG highlights the robustness of creating inexpensive CNTs-interwoven polyanionic cathodes for high-performance PIBs.

Keywords: Carbon nanotube-interwoven; Cathodes; Crystal plane; Iron fluorosulfate; Potassium-ion batteries.