Revealing the key factors affecting the anode performance of metal-ion batteries: a case study of boron carbide monolayers

Abstract

The continuous performance improvement of anode materials is of paramount importance for the development of metal-ion batteries. Discovering the factors restricting the anode materials' performance is a key prerequisite for designing applicable materials. Herein we propose that the BC₅ hexagonal ring and the shortest B–B distance in graphene-like BC_x (x = 6, 8, 9, and 11) monolayers are responsible for their migration path/barrier and theoretical capacity of metal-ion batteries including lithium, sodium, and potassium. The center of the BC₅ ring is the best optimal adsorption site because of the redistribution of charge caused by the B atoms. The interconnected BC₅ rings become the best optimal path for metal ion migration. Interestingly, their migration energy barrier is proportional to the adsorption energy difference between different BC₅ rings. In this context, the adsorption energy difference becomes an effective descriptor for identifying the optimal migration path. Additionally, the greater the nearest-neighbor B–B distance, the higher the theoretical capacity. These monolayers show the best performance for K-ion batteries, with migration barriers ranging from 0.04 to 0.08 eV and theoretical capacities from 1294 to 1755 mA h g⁻¹. The high capacity arises from the surplus electrons situated between the K atom layers, which act as anionic electrons, promoting multilayer adsorption. Our study makes a significant step toward the design of high-performance anode materials with higher target directionality and lower computational costs.