Stepwise induced passivation film formation on the surface of spherical hard carbons enabling ultra-stable potassium ions storage

Potassium-ion storage devices are seen as a form of prospective substitute for large-scale electrochemical energy storage because of the low cost and the fast charge transport in electrolytes. However, the rosy scenario is compromised by developing ultra-stable electrode materials to ensure long-term cycling performance. Herein, with sucrose as the carbon source, we construct two carbon materials, namely hard carbon spherules (HCS) and active hard carbons (AHCS), as anode and cathode materials, respectively, to fabricate high-performance potassium-ion hybrid capacitors (PIHC). Benefiting from the advantageous ionic conductivity and the comparatively stable passivation film, HCS bonding with sodium alginate delivers superior rate performance and high pseudocapacitive proportion. In situ electrochemical impedance spectroscopy and distribution of relaxation time analysis combined with ex situ scanning electron microscope images reveal that the formation of solid electrolyte interface on the surface of the HCS during the initial discharge can be explained as a gradually forming process. As expected, the HCS//AHCS PIHC device delivers a high energy density of 116 Wh kg−1 at a power density of 218 W kg−1 and exhibits an ultra-stable lifespan over 20000 cycles. This study provides a worthy reference for the value-added utilization of biomass in innovative potassium-based energy storage technology.