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

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.

Anionic Redox in Rechargeable Lithium Batteries

“…The ultimate redox process was determined as a combination of Mn 3+ /Mn 4+ , O 2− /O − , and Mn 4+ /Mn 5+ , which revises the rule on the redox of manganese oxides. The same high‐valence Mn ions were also reported in Li 7 Mn(BO 3 ) 3 and Li 3 MnO 4 . However, the presence of Mn 5+ in Li 2 MnO 3 is only possible for 5‐coordinated Mn at the surface, as shown by Chen and Islam using magnetic‐moment calculations …”

Direct Visualization of the Reversible O2−/O− Redox Process in Li‐Rich Cathode Materials

“…The processes summarizing the aforementioned discussion are as follows: At first, Li + migrates out from Li layers with the charge compensation by Ni 2+/4+ redox couple, corresponding to ≈130 mA h g −1 . From then on, oxidation of oxygen starts to emerge for charging compensation till the end of the charging because Ni 2+ is totally oxidized to Ni 4+ and Mn 4+ cannot be oxidized to a higher valence state in octahedral site . Afterward, the oxidation of oxygen is the main responsible for charge compensation at the plateau of ≈4.5 V. Since about 1.1 Li + can be extracted from the structure, Li + continues to be removed out from Li layers and TM layers accompanied with appearance of peroxo OO bond.…”

Reversible Decomposition of Secondary Phases in BaO Infiltrated LSM Electrodes—Polarization Effects

“…Unfortunately, there are to the best of the author's knowledge no reports in literature on the electrical properties of Ba 3 Mn 2 O 8 . However, work described by Roos et al may provide insight into the electrical properties of Ba 3 Mn 2 O 8 since Roos et al explain manganese ions in tetrahedral coordination as in Ba 3 Mn 2 O 8 can easily change from Mn 5+ to Mn 4+ and from Mn 4+ to Mn 3+ , which may contribute to the observed enhancement of the electrode's electrocatalytic properties. The suggestion that Ba 3 Mn 2 O 8 causes the reduced polarization resistance at OCV is made with the reservation that for each reporting in the literature, both the amount of Ba infiltrate and the LSM stoichiometry play a role in secondary phase formation and the possible formation of BaCO 3 , which may also influence the electrocatalytic properties .…”