LiMn₂O₄ (LMO) can deliver a reasonable discharge capacity of ~130 mAh g^-1, where the three-dimensional lithium ions diffusion channel in the spinel structure allows a rapid de-/intercalation reaction. There are several drawbacks of the spinel LMO including a severe capacity fade during cycling due to a Jahn-Teller (JT) distortion and Mn²⁺ dissolution. This Mn dissolution presumably starts at the particle surface; and therefore, it is essential to completely understand of the surface structure and its stability. We have recently constructed the thermodynamic stability diagram of LMO using density functional calculations (DFT) to investigate how stability of bulk LMO and LMO surfaces correlate as a function of oxygen and lithium chemical potentials.¹ We have also suggested that Li-excess environments and too high temperatures should be avoided upon synthesis to yield particles with less (001) LMO facet, i.e., the surface that is more prone to Mn dissolution.

   Furthermore, we have recently provided a novel strategy to suppress Mn dissolution and the JT distortion by investigating the spinel LMO (001) surface with a single layer of graphene using DFT.² Our theoretical calculation in Ref. 2 suggests that the interaction between the graphene sheet and the LMO (001) surface suppresses the Mn³⁺ disproportionation reaction into Mn²⁺ and Mn⁴⁺. While all surface Mn atoms at the LMO (001) have an oxidation state of +3, we find that the (001) Mn atom that chemically bonds with graphene adopts an electron configuration that has a clear +4 character with an empty e_g band. By analyzing the (001) LMO surface Mn-O bond distance, we observe that the JT distortion is reduced as the oxidation state of surface Mn shifts to Mn⁴⁺ in the presence of the graphene sheet. A chemical bonding between graphene and LMO on the (001) surfaces provides an underlying mechanism that electronically modifies the LMO (001) surfaces, and subsequently stabilizes them against Mn³⁺ disproportionation reaction and the JT distortion by converting (001) surface Mn³⁺ to Mn⁴⁺. Our advanced first-principles computations shed lights on the development of alternative hybrid Mn-cathode architecture approaches that can overcome the key shortcomings in these cathode materials.

References

[1] S. Kim, M. Aykol, and C. Wolverton. Phys. Rev. B, 92, 115411.

[2] L. Jaber-Ansari, K. P. Puntambekar, S. Kim, M. Aykol, L. Luo, J. Wu, B. D. Myers, H. Iddir, J. T. Russell, S. J. Saldaña, R. Kumar, M. M. Thackeray, L. A. Curtiss, V. P. Dravid, C. Wolverton, and M. C. Hersam. Adv. Energy. Mater. 5 (2015) 1500646

Acknowledgement

This computational research work was supported by Northwestern-Argonne Institute of Science and Engineering (NAISE) and the Dow Chemical Company.

Figure Caption: Hybrid Cathode Architectures: Graphene-stabilized LiMn₂O₄ (001) Li-terminated Surface.