Asynchronous Crystal Cell Expansion during Lithiation of K⁺-Stabilized α-MnO₂

Abstract

α-MnO2 is a promising material for Li-ion batteries and has unique tunneled structure that facilitates the diffusion of Li(+). The overall electrochemical performance of α-MnO2 is determined by the tunneled structure stability during its interaction with Li(+), the mechanism of which is, however, poorly understood. In this paper, a novel tetragonal-orthorhombic-tetragonal symmetric transition during lithiation of K(+)-stabilized α-MnO2 is observed using in situ transmission electron microscopy. Atomic resolution imaging indicated that 1 × 1 and 2 × 2 tunnels exist along c ([001]) direction of the nanowire. The morphology of a partially lithiated nanowire observed in the ⟨100⟩ projection is largely dependent on crystallographic orientation ([100] or [010]), indicating the existence of asynchronous expansion of α-MnO2's tetragonal unit cell along a and b lattice directions, which results in a tetragonal-orthorhombic-tetragonal (TOT) symmetric transition upon lithiation. Such a TOT transition is confirmed by diffraction analysis and Mn valence quantification. Density functional theory (DFT) confirms that Wyckoff 8h sites inside 2 × 2 tunnels are the preferred sites for Li(+) occupancy. The sequential Li(+) filling at 8h sites leads to asynchronous expansion and symmetry degradation of the host lattice as well as tunnel instability upon lithiation. These findings provide fundamental understanding for appearance of stepwise potential variation during the discharge of Li/α-MnO2 batteries as well as the origin for low practical capacity and fast capacity fading of α-MnO2 as an intercalated electrode.

Manganese Dioxide As Rechargeable Magnesium Battery Cathode

“…Figure 2D showed that the redox potential for Li-intercalation is higher than Li-conversion, resulting in thermodynamically stable intercalation. The insertion of Li into α-MnO2 was directly observed in a recent report (Yuan et al, 2015). The redox potential of Mg intercalation is nearly the same as Li-intercalation.…”

“…However, it was revealed that the cycling of α-MnO2 in LIB is not stable due to asymmetric distortion of lattice, resulting in poor capacity retention (Thackeray, 1997;Ling and Mizuno, 2012;Yuan et al, 2015).…”

“…Several apparent characteristics were immediately noticed. First, no clear plateau was observed in the voltage profile and the voltage slope was increased from that of Li + -intercalation (Thackeray, 1997;Ling and Mizuno, 2012;Yuan et al, 2015). The average discharge voltage was typically around 1.5-1.6 V, far below that for Li + -intercalation even if the difference of anionic potential between Li/Li + and Mg/Mg 2+ (0.7 V in aqueous solution and may around 1 V in non-aqueous solution) was taken into account.…”

Nanofabrication strategies for advanced electrode materials

“…MnO 2 material has attracted significant interests in electrochemical energy storage devices, such as Li-ion battery, Zn-Mn battery, supercapacitor, owing to its low cost, environmental compatibility and abundant polymorphism [48][49][50]. MnO 2 material possesses multiple tunnel and layered crystal structures, i.e., α, β, γ, δ-phases, etc.…”

Green synthesis of nanosized manganese dioxide as positive electrode for lithium-ion batteries using lemon juice and citrus peel

“…Raman spectra were collected on a LabRam Evolution HR (HORIBA) spectrometer equipped with a Nd:YAG laser (523 nm, 1 mW). The final spectra consist of two independent acquisitions with a total acquisition time of 720 s. It is well known that large K + cations are inserted into the tunnel cavity (2e Wyckoff site) during synthesis of the -MnO 2 hollandite structure [41,42]. The presence of K + is the key parameter, which governs the properties of these materials.…”