Surface structure and their environment-dependent stability of NaMn2O4

Abstract

As cathode materials for alkali-ion batteries, sodium manganese oxides have been receiving considerable and continuous attention in recent decades. In this work, the structure and environment-dependent stability of NaMn2O4 surface were studied based on the first principles calculations. The surface stability diagram of NaMn2O4 involving various different terminations of (100), (110) and (111) surfaces was constructed, and the stability of these different terminations could be compared as a function of chemical environment. It is found that the (100)-MnO and (111)-ONa terminations are two more stable terminations under the investigated chemical conditions. And the surface energies of (110) surfaces are negative under the investigated chemical potential, hence, (110) surfaces are unstable. The surface energy of NaMn2O4 as a function of O chemical potential is also investigated under constant Na chemical potential. The structure relaxation indicates that the surface rumpling and surface reconstruction can affect the electronic structure of the surface, thereby reducing surface energy and stabilizing the surface. Furthermore, the Wulff shape of NaMn2O4 was also constructed based on Gibbs-Wulff theorem.

1. Introduction

In recent years, sodium manganese oxides has received considerable attention due to their fast ionic diffusivity, excellent electronic conductivity, easy synthesis, and reasonable production cost, which have been applied as cathode materials for alkali-ion batteries [1 −8]. Unlike the cubic spinel structure of LiMn2O4, the reported NaMn2O4 has an orthorhombic structure (space group Pnam) [9, 10], which is composed of eight-fold coordinated Na and distorted MnO6 octahedra. NaMn2O4 is a Mn3+/Mn4+ mixed-valence compound, in which Mn3+/Mn4+ is randomly located at octahedral sites [9]. And single crystal NaMn2O4 can be prepared under high-temperature and high-pressure conditions [9, 10]. In addition to the above orthorhombic structure, the cubic spinel structure of NaMn2O4 also exhibits good stability [11]. The structure of cubic spinel NaMn2O4 is identical to that of LiMn2O4, therefore, cubic NaMn2O4 also has great potential to become a good battery cathode material.

The structure of AMn2O4 (A=Li, Na) is not stable enough, which leads to surface reconstruction and unreasonable reactions with battery electrolytes [12 −15]. These LiMn2O4 surfaces have attracted widespread attention from experimental and theoretical points of view. It is reported that LiMn2O4 has a strong sensitivity to local surface structure in maintaining capacity. The LiMn2O4 particle dominated by (111) facet shows good resistance to Mn3+ dismutation, while (110) and (100) are relatively soluble. And the equilibrium LiMn2O4 often adopts an octahedral shape, indicating that (111) facet has good stability [14, 15]. Unfortunately, there is still limited research on NaMn2O4, and even research on the relationships between its capacity retention and surface structure has not been reported. Hence, it is significant to investigate the surface structure of NaMn2O4 for understanding the structural stability and battery performance.

First-principles calculations [16 −20] can overcome the difficulty of experimental preparation and surface index characterization, thereby providing a good comprehension for surface properties. In the present work, we performed the first principles calculations to study the surface stability and Wulff construction of NaMn2O4, and the Wulff shape was constructed according to the Gibbs-Wulff theorem.