Transition Metal Substitution of Hollandite α-MnO₂: Enhanced Potential and Structural Stability on Lithiation from First-Principles Calculation

Abstract

Hollandite α-MnO2, consisting of manganese–oxygen octahedra, has recently attracted attention due to its high theoretical capacity, yet it suffers capacity degradation during repeated (de)­lithiation. Here we use a new conceptual approach to substitute one of the Mn in the tunnel wall via the form of Mn0.875M0.125O2 (M = Ti, V, Cr, Nb, Ru), aiming to increase the lithiation potential and attain the theoretical capacity via the enhanced structural stability, with the ultimate goal of improved capacity retention upon repeated (de)­lithiation. A bottom-up screening using density functional theory (DFT) was performed to identify the effect of the transition metal substitution on lithiation of α-MnO2. The calculations reveal that substitution with electron-accepting Cr ions results in a more significant increase in the lithiation potential of MnO2 than the other substituents. In terms of structural stability, both Cr and Nb are capable of effectively stabilizing the tunnel structure of α-MnO2 under increased levels of lithiation, thus providing the opportunity for significant increases in the cyclability and delivered capacity. Our study not only discovers the new lithiation pathway and intermediates at the atomic level but also develops the key concepts to optimize the lithiation potential and structural durability for future α-MnO2-based materials. This approach opens a new avenue for materials design of 1D tunnel structured materials for use as stable host frameworks for electrochemical ion (de)­insertion.

Unraveling the Dissolution‐Mediated Reaction Mechanism of α‐MnO2 Cathodes for Aqueous Zn‐Ion Batteries

“…To gain some understanding on the possible origin of surface dissolution of MnO 2 during the discharge, we purposely selected the nanorods that had undergone relatively low level dissolution even at the depth of discharge of ≈40% (i.e., three electron equivalents). [21] Mn 3+ ion is the strong Jahn-Teller active species that induces its structural distortion. [22] In our system, as we expect either Zn 2+ or H + ion insertion on the surface, the guest ion may more likely be H + ions than Zn 2+ ions as no clear Zn signal was detected from the surface.…”

“…This change in interplanar spacing may be attributed to distortion of a MnO 2 unit cell due to Mn valence reduction from guest ion insertion as in the case of lithiation of α‐MnO 2 . [ 21 ] Mn 3+ ion is the strong Jahn–Teller active species that induces its structural distortion. [ 22 ] In our system, as we expect either Zn 2+ or H + ion insertion on the surface, the guest ion may more likely be H + ions than Zn 2+ ions as no clear Zn signal was detected from the surface.…”

“…Notice, the average Mn valence of both the distorted and collapsed surfaces is 3.1 and 2.5, respectively. The values are much less than Li insertion into α‐MnO 2 , [ 21 ] implying that the structure can potentially undergo much larger Mn valence reduction before dissolution likely due to much smaller size of H + ions than Li + ions.…”

Local and Bulk Probe of Vanadium-Substituted α-Manganese Oxide (α-KxVyMn8–yO16) Lithium Electrochemistry

“…20−26 Density functional theory (DFT) has been used to explore transition-metal substitution of Mn in the tunnel wall for use as cathode materials in Li-ion batteries. 27 In each case, it was assumed that the substituted transition metal was oxidation state 4+ and indicated some promise for increased stability on lithiation. 27 Incorporation of V into the α-MnO 2 structure has been of particular interest due to its similar atomic radius (V 5+ , 0.53 Å) to Mn 4+ (0.54 Å), potential for electrochemical activity, and the lack of a Jahn−Teller distortion upon reduction of the vanadium ion.…”

The Effects of Vanadium Substitution on One-dimensional Tunnel Structures of Cryptomelane: Combined TEM and DFT Study

“…Consistent with the experimental observations, DFT simulations also reveal the substituted V ions to be in a slightly oxidized state and that upon lithiation V is reduced only slightly as compared to Mn. In a former theoretical paper, [5] it is predicted that there might be two possible structural evolution mechanism upon lithiation. In our ex-situ study on lithiated samples, we observed the evidence for both evolution mechanisms.…”