Ab-Initio Energetics of Electrochemical Ion Insertion into Manganese Oxides

The ion insertion redox chemistry of manganese oxides has diverse applications in energy storage, catalysis, and chemical separations. Unique properties derive from the assembly of Mn-O octahedra into polymorphic structures that can host protons and non-protonic cations in interstitial sites. Despite many experimental reports targeting specific applications, a comprehensive understanding of ion insertion in Mn oxides remains elusive. In this work, we use density functional theory to study the electrochemistry of AxMnO2 (where A = H+, Li+, Na+, K+, Mg2+, Ca2+, Zn2+ & Al3+) in aqueous and non-aqueous electrolytes. We develop an efficient computational scheme demonstrating that Hubbard-U correction has a greater impact on calculating accurate redox energetics than choice of exchange-correlation functional. Using PBE+U, we find that non-protonic cation insertion into MnO2 depends on the oxygen coordination environments inside a polymorph but that when H+ is present, the driving force to form hydroxyl bonds is generally stronger. Only three ion-polymorph pairs are thermodynamically stable within water’s voltage stability window (Na+ and K+ in 𝛼-MnO2, and Li+ in λ-MnO2), with all other aqueous ion insertion relying on metastability. Al3+ insertion into the 𝛿, R, and λ polymorphs may enable the full 2-electron redox of MnO2 at high voltage, but electrolytes must be designed to impede formation of insoluble precipitates and facilitate ion desolvation. We also show that water co-insertion stabilizes small ions in 𝛼-MnO2, while solvation energies and kinetic effects dictate water insertion in 𝛿-MnO2. Taken together, these findings rationalize experimental reports of mixed ion insertion mechanisms in aqueous batteries and highlight promising design strategies for safe, high energy density electrochemical energy storage.