Highly Flexible Hydrogen Boride Monolayer as Potassium-ion Battery Anodes for Wearable Electronics.

The rapid development of wearable electronics has revealed an urgent need of low-cost, high-flexible, and high-capacity power sources. In this sense, emerging rechargeable potassium-ion batteries (KIBs) are promising candidate owing to their abundant resources, low-cost and lower redox potential in non-aqueous 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 recently developed hydrogen boride (BH) monolayer as a high-performance anode material based on density functional theory (DFT) formalism. We demonstrated that (i) BH presents ultra-low out-of-plane bending stiffness, rivaling that of graphene, which endows it with better flexibility to accommodate the repeat bending, rolling and folding on wearable device operation; (ii) high in-plane stiffness (157 N/m along armchair and 109 N/m along zigzag) in BH makes the electrode stable against pulverization upon external and internal strains. More importantly, BH electrode delivers low voltage of ~0.24 eV in addition to desired K-ion affinity and hopping resistance which remains very stable with the bending curvature. Emerged H vacancies in electrode 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 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.