An Ultra-stable Sodium Dual-ion Battery Based on S/Se Co-doped Covalent Organic Framework Anode with 12,000 Cycles Under Lean Electrolyte

Abstract

Sodium dual-ion batteries (SDIBs) employing covalent organic frameworks (COFs) as anode exhibit enormous application prospect and attract widespread attention for large-scale energy storage and conversion owing to the advantages of environment-friendly, cost-effective, and high-safety. However, achieving long-term cyclic stability and capacity retention at high current densities remains a huge challenge due to inherent low conductivity and kinetic limitations as well as severe solubility in electrolytes. Rational design of COFs with high conductivity, rapid redox kinetics, and structural stability is the key to improving their performance in SDIBs. Herein, a kind of S/Se co-doped COF is successfully designed and synthesized in which the co-doping effect significantly enhances stability and conductivity, improves the compatibility with electrolyte, facilitates the fast ion and electron transfer, and strengthens the adsorption capability for Na⁺. As anticipated, the constructed SDIBs not only deliver a high discharge capacity of up to 167.2 mAh g⁻¹ and can cycle stably for 700 cycles at 2 C without degradation, but also achieve an ultra-long cycle life of 15,000 cycles at 20 C, with a capacity decay rate as low as 0.00045% per cycle. Moreover, a capacity retention of 89.2% corresponding to the normal electrolyte dosage and 12,000 ultra-stable cycles are realized even under lean electrolyte and demonstrate excellent fast-charging performance with extremely low self-discharge rate. The structural evolution of electrodes and the energy storage mechanism of DIBs are further revealed by in-situ characterization and theoretical calculations. This work expands the versatility of designing COFs with high redox activity, emphasizes the importance of heterogeneous element doping, and sheds novel insights on the construction of COFs-based materials for efficient Na⁺ storage.