Auto-generated iron chalcogenide microcapsules ensure high-rate and high-capacity sodium-ion storage

Sodium-ion batteries (SIBs) are regarded as promising alternative energy-storage devices to lithium-ion batteries (LIBs). However, the trade-off of between energy density and power density under high mass-loading conditions restricts the application of SIBs. Herein, we synthesized an FeSe@FeS material via a facile solid-state reaction. A microcapsule architecture was spontaneously achieved in this process, which facilitated electron transport and provided stable diffusion paths for Na ions. The FeSe@FeS material exhibits a high capacity retention (485 mA h g^-1 at 3 A g^-1 after 1400 cycles) and superior rate capability (230 mA h g^-1 at 10 A g^-1 after 1600 cycles) in the half-cell test. Furthermore, superior cycling stability is achieved in the full-cell test. The high mass-loaded FeSe@FeS electrodes (8 mg cm^-2) realize a high areal capacity retention of 2.8 mA h cm^-2 and high thermal stability.