Ab Initio Study of Sodium Insertion in the λ-Mn₂O₄ and Dis/Ordered λ-Mn_1.5Ni_0.5O₄ Spinels

Abstract

The main challenge of sodium-ion batteries is cycling stability, which is usually compromised due to strain induced by sodium insertion. Reliable high-voltage cathode materials are needed to compensate the generally lower operating voltages of Na-ion batteries compared to Li-ion ones. Herein, density functional theory (DFT) computations were used to evaluate the thermodynamic, structural, and kinetic properties of the high voltage λ-Mn2O4 and λ-Mn1.5Ni0.5O4 spinel structures as cathode materials for sodium-ion batteries. Determination of the enthalpies of formation reveal the reaction mechanisms (phase separation vs solid solution) during sodiation, while structural analysis underlines the importance of minimizing strain to retain the metastable sodiated phases. For the λ-Mn1.5Ni0.5O4 spinel, a thorough examination of the Mn/Ni cation distribution (dis/ordered variants) was performed. The exact sodiation mechanism was found to be dependent on the transition metal ordering in a similar fashion to the insertion behavior observed in the Li-ion system. The preferred reaction mechanism for the perfectly ordered spinel is phase separation throughout the sodiation range, while in the disordered spinel, the phase separation terminates in the 0.625 < x < 0.875 concentration range and is followed by a solid solution insertion reaction. Na-ion diffusion in the spinel lattice was studied using DFT as well. Energy barriers of 0.3–0.4 eV were predicted for the pure spinel, comparing extremely well with the ones for the Li-ion and being significantly better than the barriers reported for multivalent ions. Additionally, Na-ion macroscopic diffusion through the 8a-16c-8a 3D network was demonstrated via molecular dynamics (MD) simulations. For the λ-Mn1.5Ni0.5O4, MD simulations at 600 K bring forward a normal to inverse spinel half-transformation, common for spinels at high temperatures, showing the contrast in Na-ion diffusion between the normal and inverse lattice. The observed Ni migration to the tetrahedral sites at room temperature MD simulations explains the kinetic limitations experienced experimentally. Therefore, this work provides a detailed understanding of the (de)sodiation mechanisms of high voltage λ-Mn2O4 and λ-Mn1.5Ni0.5O4 spinel structures, which are of potential interest as cathode materials for sodium-ion batteries.

Double the Capacity of Manganese Spinel for Lithium‐Ion Storage by Suppression of Cooperative Jahn–Teller Distortion

“…All configurations have reached a convergence threshold of 0.01 eV Å −1 in force. [ 19 ] It should be noticed that the experimental voltage can be realized only by PBE without U . The usual U value 3.8 or 4.2 eV used in LiMn 2 O 4 calculations can't repeat the experimental average voltage 2.8 V and overestimates the average voltage to 3.5 eV.…”

“…The usual U value 3.8 or 4.2 eV used in LiMn 2 O 4 calculations can't repeat the experimental average voltage 2.8 V and overestimates the average voltage to 3.5 eV. [ 19,20 ]…”

Faradaic Electrodes Open a New Era for Capacitive Deionization

“…Reproduced with permission. [ 155 ] Copyright 2018, American Chemical Society. d) Cyclic voltammogram (CV) curves of λ ‐MnO 2 electrode at various scan rates in 1 m Na 2 SO 4 solution.…”

“…Some earlier work showed that the electrochemical sodiation of the λ ‐MnO 2 could generate topological deformations, resulting in a local phase transition from the spinel to O′3 layered structure. On the other hand, more recent studies [ 155–157 ] reported that reversible Na + insertion/extraction could be achieved by initially filling the interstitial 8a tetrahedral sites and 16c octahedral sites of λ ‐MnO 2 as displayed in Figure 7c. The CV curves in 1 m Na 2 SO 4 solution (Figure 7d) show that there are two pairs of oxidation/reduction peaks emerged, which could be ascribed to the insertion/extraction of Na + in the 8a and 16c sites of λ ‐MnO 2 , respectively.…”

An Ab Initio Study of Li/Ni-doped NaxMeO2 Cathode Material for Na-Ion Batteries

“…The charge/discharge dynamics, the specific role of each constituent element in the material, the electrical conduction properties, and the surface chemistry of these electrodes present several fundamental questions, despite numerous studies carried out so far. Density functional theory (DFT) calculations have proven to be a useful technique for evaluating the thermodynamic, structural, and kinetic properties of manganese oxide (MnO 2 ) with monoclinic [27] and spinel structure [28] for their use as cathode materials for NIBs. Therefore, we applied an advanced quantum mechanics (QM) method based on DFT [29,30] and combined with a statistical approach to evaluate the Na 0.85 Li 0.17 Ni 0.21 Mn 0.64 O 2 (NLNMO) material as P2-type layered oxide with the promising application as a cathode electrode in NIBs [31].…”