Controlling the Lithium Intercalation Voltage in the Li(Mn1-xNix)2O4 Spinel via Tuning of the Ni Concentration: a Density Functional Theory Study

Kemeridge T. Malatji,Nora H. de Leeuw,David Santos-Carballal,Umberto Terranova,Phuti E. Ngoepe

doi:10.17159/0379-4350/2021/v74a2

Abstract

ABSTRACT LiMn2O4 spinel is a promising cathode material for secondary lithium-ion batteries. Despite showing a high average voltage of lithium intercalation, the material is structurally unstable, undergoing lowering of the crystal symmetry due to Jahn-Teller distortion of the six-fold Mn3+ cations. Although Ni has been proposed as a suitable substitutional dopant to improve the structural stability of LiMn2O4 and enhance the average lithium intercalation voltage, the thermodynamics of the Ni incorporation and its effect on the electrochemical properties of this spinel material are not yet known. In this work, we have employed density functional theory calculations with a Hubbard Hamiltonian (DFT+u) to investigate the thermodynamics of cation mixing in the Li(Mn1_xNix)2O4 solid solution. Our results suggest LiMn1.5Ni0.5O4 is the most stable composition from room temperature up to at least 1000 K, in agreement with experiments. We also found that the configurational entropy is much lower than the maximum entropy at 1000 K, indicating that higher temperatures are required to reach a fully disordered solid solution. A maximum average lithium intercalation voltage of 4.8 eV was calculated for the LiMn1.5Ni0.5O4 composition, which is very close to the experimental value. The temperature was found to have a negligible effect on the Li intercalation voltage of the most stable composition. The findings reported here support the application of LiMn1.5Ni0.5O4 as a suitable cathode material for lithium-ion batteries, with a highly stable voltage of intercalation under a wide range of temperatures. Keywords: Spinel, equilibrium concentration, mixing thermodynamics, solid-state chemistry and lithium voltage of intercalation.

Highlights

The spinel-structured lithium manganese oxide LiMn2O4 (LMO), which can be and reversibly de-lithiated, is an environmentally acceptable compound with a low fabrication cost.[1]
We have employed density functional theory (DFT) calculations to investigate the effect of substituting nickel for manganese in the LiMn2O4 cathode material
Our results indicate that any small change in the amount of Ni will be reflected in the stability of the Li(Mn1–x Nix)2O4 spinel

Summary

Introduction

The spinel-structured lithium manganese oxide LiMn2O4 (LMO), which can be and reversibly de-lithiated, is an environmentally acceptable compound with a low fabrication cost.[1]. The crystal structure of LMO is severely degraded after a few operational cycles of lithiation and de-lithiation due to the strong Jahn-Teller (JT) distortions of the octahedrally coordinated high-spin Mn3+ cations, especially below the Verwey-like temperature (TV) of 283.5 K.4. The uneven occupation of the Mn3+ eg 2 state and the interaction with the oxygen p orbitals in LMO causes a tetragonal elongation of this cation in the direction of the dz[2] orbital which lowers its energy. The JT effect vanishes in the de-lithiated MnO2, as all the Mn cations become oxidized to the highly stable 4+ state with a half-filled electronic t2g level

Methods

Results

Conclusion