Unusual Mn coordination and redox chemistry in the high capacity borate cathode Li7Mn(BO3)3.

Julian Roos,M. Saiful Islam,Stephen M. Wood,Alexander Whiteside,Christopher Eames

doi:10.1039/c5cp02711j

Abstract

The recently discovered lithium-rich cathode material Li7Mn(BO3)3 has a high theoretical capacity and an unusual tetrahedral Mn(2+) coordination. Atomistic simulation and density functional theory (DFT) techniques are employed to provide insights into the defect and redox chemistry, the structural changes upon lithium extraction and the mechanisms of lithium ion diffusion. The most favourable intrinsic defects are Li/Mn anti-site pairs, where Li and Mn ions occupy interchanged positions, and Li Frenkel defects. DFT calculations reproduce the experimental cell voltage and confirm the presence of the unusually high Mn(V) redox state, which corresponds to a theoretical capacity of nearly 288 mA h g(-1). The ability to reach the high manganese oxidation state is related to both the initial tetrahedral coordination of Mn and the observed distortion/tilting of the BO3 units to accommodate the contraction of the Mn-O bonds upon oxidation. Molecular dynamics (MD) simulations indicate fast three-dimensional lithium diffusion in line with the good rate performance observed.

Highlights

The implementation of layered LiCoO2 as the cathode in rechargeable lithium-ion batteries heralded the revolution in portable electronics
Yamada et al.[17] demonstrated capacities of about 190 mA h gÀ1 for the monoborate system LiFeBO3, which is considerable compared to LiCoO2 and LiFePO4 with theoretical capacities of 272 and 170 mA h gÀ1 respectively.[7,26]
Analysis of the electronic structure reveals that on lithium extraction the spin on each Mn atom decreases from 5 to 2, indicating a d5–d2 transition corresponding to oxidation from MnII to MnV. These results suggest two important features: (i) the as-synthesised bulk structure remains in place during the first cycle. (ii) on the first cycle the +5 oxidation state of Mn is obtained

Summary

Introduction

The implementation of layered LiCoO2 as the cathode in rechargeable lithium-ion batteries heralded the revolution in portable electronics. The search for alternative cathode materials has generated considerable research activity,[1,2,3,4,5,6] for large-scale applications such as electric vehicles or grid storage. Significant interest has focused on materials such as olivinestructured LiFePO4,1–5,7 where the strong binding in the polyanion unit leads to a stable framework with long cycle life and high safety. Yamada et al.[17] demonstrated capacities of about 190 mA h gÀ1 for the monoborate system LiFeBO3 (theoretical capacity 220 mA h gÀ1), which is considerable compared to LiCoO2 and LiFePO4 with theoretical capacities of 272 and 170 mA h gÀ1 respectively.[7,26]

Methods

Results

Conclusion