First-principles Studies Of Sodiated Black Phosphorus Anode Materials For Na-ion Batteries

Sodium-ion batteries?NIBs?as an alternative to lithium-ion batteries?LIBs?have recently received great attentions because of the relatively high abundance of sodium.Searching for suitable anode materials has always been a hot topic in the field of NIB study.Recent reports show that phosphorus-based materials are potential as the anode materials for NIBs.Using first-principles calculations,herein,we study the atomic and electronic structures,diffusion dynamic,and intrinsic elastic properties of various Na-P(NaP₅,Na₃P₁₁,NaP,Na₃P)alloy compounds as the intermediate phases during Na extraction/insertion in phosphorus-based anode materials.It is found that all the crystalline phases of Na-P alloy phases considered in our study are semiconductors with band gaps larger than that of black phosphorus?BP?,indicating that the sodiation lowers the electric conductivity of the Na-P alloys.The calculations of Na diffusion dynamics indicate a relatively fast Na diffusion in these materials,which is important for good rate performance.In addition,the diffusion channels of sodium ions are one-dimensional in NaP₅ phase and three-dimensional in other three phases(Na₃P₁₁,NaP,Na₃P).Elastic constants are calculated by the energy-strain approach from firstprinciple calculations.The calculated elastic constants of NaP₅,Na₃P₁₁,NaP,Na₃P satisfy the Born's criteria,indicating that all four phases are mechanically stable.With the elastic constants,Young's modulus E,shear modulus G,bulk modulus B and Poisson's ratio ? of various Na-P alloy phases could be derived by using Voigt-Reuss-Hill averaging method.Based on the Pugh's ratio?B/G?,the results show that NaP₅,Na₃P₁₁,and NaP alloy phases are ductile,while brittle for the fully sodiated phase Na₃P.