Revealing molecular-level surface redox sites of controllably oxidized black phosphorus nanosheets

Bulk and two-dimensional black phosphorus are considered to be promising battery materials due to their high theoretical capacities of 2,600？mAh g？1. However, their rate and cycling capabilities are limited by the intrinsic (de-)alloying mechanism. Here, we demonstrate a unique surface redox molecular-level mechanism of P sites on oxidized black phosphorus nanosheets that are strongly coupled with graphene via strong interlayer bonding. These redox-active sites of the oxidized black phosphorus are confined at the amorphorized heterointerface, revealing truly reversible pseudocapacitance (99% of total stored charge at 2,000？mV？s？1). Moreover, oxidized black-phosphorus-based electrodes exhibit a capacitance of 478？F？g–1 (four times greater than black phosphorus) with a rate capability of ~72% (compared to 21.2% for black phosphorus) and retention of ~91% over 50,000 cycles. In situ spectroelectrochemical and theoretical analyses reveal a reversible change in the surface electronic structure and chemical environment of the surface-exposed P redox sites