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

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 BCx (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. This study has employed boron carbide monolayers to reveal the key factors affecting the anode performance of metal-ion batteries.