Surface oxo-functionalized hard carbon spheres enabled superior high-rate capability and long-cycle stability for Li-ion storage

Hard carbon is emerging as a highly promising material for power-demanded energy storage devices. Recently, introducing heteroatom such as oxygen is confirmed to be available to improve the capacity. However, it remains a significant challenge to achieve simultaneously superior high-rate capability and long-term cycling stability. Here we demonstrate that the surface oxo-funcitionalized hard carbon spheres (o-HCS, 4.2 m2 g−1) derived from mild oxidative approach enhance pseudocapacitance lithium-ion storage with improved Li-ion diffusivity and thus exhibit high-rate capacity and long cycle life in both Li-ion batteries and Li-ion capacitors. The o-HCS electrode delivers specific capacity of around 275 mAh g−1 at 744 mA g−1 (2C) and capacity retention of above 92.0% and about 86.5% after 1100 and 1700 cycles, respectively. Impressively, it delivers above 110 mAh g−1 at extreme high current density of 14.88 A g−1 (40C). As well, o-HCS electrode in Li-ion capacitor shows a capacitance of 34.8 F g−1 (corresponding to 48.5 Wh kg−1 and 3.6 kW kg−1) at the current density of 3720 mA g−1, and after 7000 cycles the capacity retention is 96% (∼0.057% decay per cycle).