Highly Flexible Hydrogen Boride Monolayers as Potassium-Ion Battery Anodes for Wearable Electronics.

The rapid development of wearable electronics has revealed an urgent need for low-cost, highly flexible, and high-capacity power sources. In this sense, emerging rechargeable potassium-ion batteries (KIBs) are promising candidates owing to their abundant resources, low cost, and lower redox potential in nonaqueous electrolytes compared to lithium-ion batteries. However, the fabrication of flexible KIBs remains highly challenging because of the lack of high-performance flexible electrode materials. In this work, we investigated the mechanical properties and electrochemical performance of a recently developed hydrogen boride (BH) monolayer as a high-performance anode material on the basis of density functional theory formalism. We demonstrated that (i) BH presents ultralow out-of-plane bending stiffness, rivaling that of graphene, which endows it with better flexibility to accommodate the repeated bending, rolling, and folding on wearable device operation; (ii) high in-plane stiffness (157 N/m along armchair and 109 N/m along zigzag) of BH makes the electrode stable against pulverization upon external and internal strains. More importantly, a BH electrode delivers a low voltage of ∼0.24 V in addition to desired K-ion affinity and hopping resistance, which remains very stable with the bending curvature. Emerged H vacancies in electrodes were found to improve both the K-ion intercalation and K-ion hopping, yielding a high theoretical capacity (1138 mAh/g), which was among the highest reported values in the literature for K-ion anode materials. All of the presented results suggested that a BH electrode could be used as a brand-new flexible and lightweight KIB anode with high capacity, low voltage, and desired rate performance.