From Na2FePO4F/CNT to NaKFePO4F/CNT as advanced cathode material for K-ion batteries

The Na2FePO4F/CNT composite investigated in this work was prepared by an optimized and easily scalable spray-drying process, resulting in an excellent initial discharge specific capacity of 123 mAh/g at C/15 when cycled against sodium. This NFPF/CNT composite was able to intercalate potassium ions reversibly with a promising specific capacity of 80 mAh/g after optimization of the electrolyte composition. Electrochemical desodiation was the key factor to reach outstanding performance as a cathode material in K-ion batteries, with the NaKFePO4F/CNT composite leading to a stable capacity of 114 mAh/g at C/15 rate (99% of the theoretical capacity), one of the highest capacity values reported for K-ion technology.