2D Electrides as Promising Anode Materials for Na-Ion Batteries from First-Principles Study

Searching for suitable anodes with good performance is a key challenge for rechargeable Na-ion batteries (NIBs). Using the first-principles method, we predict that 2D nitrogen electride materials can be served as anode materials for NIBs. Particularly, we show that Ca₂N meets almost all the requirements of a good NIB anode. Each formula unit of a monolayer Ca₂N sheet can absorb up to four Na atoms, corresponding to a theoretical specific capacity of 1138 mAh·g^-1. The metallic character for both pristine Ca₂N and its Na intercalated state Na_xCa₂N ensures good electronic conduction. Na diffusion along the 2D monolayer plane can be very fast even at room temperature, with a Na migration energy barrier as small as 0.084 eV. These properties are key to the excellent rate performance of an anode material. The average open-circuit voltage is calculated to be 0.18 V vs Na/Na⁺ for the chemical stoichiometry of Na₂Ca₂N and 0.09 V for Na₄Ca₂N. The relatively low average open-circuit voltage is beneficial to the overall voltage of the cell. In addition, the 2D monolayers have very small lattice change upon Na intercalation, which ensures a good cycling stability. All these results demonstrate that the Ca₂N monolayer could be an excellent anode material for NIBs.