First-principles design of a borocarbonitride-based anode for superior performance in sodium-ion batteries and capacitors

Abstract

Three fundamental challenges for the development of technologically relevant sodium-ion batteries (SIB) and sodium-ion capacitors (SIC) are the lower cell voltage, decreased ionic-diffusivity and larger volume of sodium-ions relative to their lithium-ion analogues. Using first-principles computation, we show that two-dimensional B_xC_yN_z with nitrogen-excess trigonal B_xN_z-domain (T_N) meets the requirements of a superior anode for SIB. Variation in the shape of the B_xN_z-domain and B–N charge-imbalance in B_xC_yN_z results in tunable anodic properties. Monolayer T_N-sheet can store Na(Li) up to Na_2.2C₆(Li_1.8T₆) composition, which corresponds to a specific capacity as high as 810(668) mA h g⁻¹ for SIB(LIB). The average open circuit voltage is ∼1.25 V vs. Na/Na⁺ for a wide range of chemical stoichiometries of Na_xT_N, which is also beneficial to the overall cell-voltage. The enhanced electronic transport and fast diffusion kinetics of the Na-ions is particular for the T_N-anode, which can result in high power efficiency in SIB, even better than that of graphite electrode in conventional LIB. Charge-storage upon layer-wise accumulation of Na-ions on the T_N surface is also appealing for application to sodium-ion capacitors, as an alternative to lithium-ion capacitors. These features are in contrast to conventional layered materials, where the voltage drops quickly as Na-ions are removed from the matrix. Hence, this article may serve as a guide for designing borocarbonitride electrodes for SIB(SIC) with controlled experimental behaviour.