Developing advanced cathode materials for sodium ion batteries (NaIBs) with stable host structure and reliable capacity performance during cycling is crucial for the commercialization of NaIBs.^[1] Nanostructured MnO₂ is one competitive cathode candidate due to its low cost, environmental friendliness, and various open-framed polymorphs to facilitate cation insertion and extraction.^[2] α-MnO₂, which features 2×2 tunnels with 4.6×4.6 Ų cavity, has recently become one research focus as such a tunneled structure in nanoscale was reported to be suitable for Na⁺ transport.^[3] However, α-MnO₂ suffers from structure instability, low coulombic efficiency and quick capacity decay during cycling, which prevent its further application to a large extent. The causes of these problems should be directly related to the repetitive tunnel-Na⁺interactions during (de)sodiation, which is currently poorly understood.

In this work, the (de)sodiation of α-MnO₂ nanowires is dynamically studied and well understood using in situ TEM with an open cell design. The morphology, phase and structure of the electrode are characterized and recorded in real time. It is found that the sodiation starts with Na⁺ intercalation into the 2×2 tunnels without tunnel degradation up to Na/Mn equaling 0.5 (Mn^3.5+). Then α-MnO₂ phase gradually evolves into an intermediate Na_0.5MnO₂ phase. Upon deep sodiation to Na/Mn equaling 1 (Mn³⁺), the structure totally collapses and degrades to polycrystalline Mn₂O₃ and Na₂O. The following (de)sodiation cycles confirm that the tunneled structure, once destroyed, can not be recovered, and the following cycles are dominated by the conversion reaction between Na_0.5MnO₂ and Mn₂O₃, whose reversibility is poor. 

The galvanostatic voltage profile at bulk level coin cells shows that the intercalation reaction featuring a slope profile happens between 4-1.4 V, while the conversion reaction featuring a plateau region is below 1.4 V. Based on this, the cycling voltage windows of the Na/MnO₂ coin cells are set to above 1.5 V to maintain the original tunneled structures of α-MnO₂ for reversible Na⁺(de)intercalation. This voltage control results in the prominent capacity retention of the batteries compared to those cycled below 1.5 V, whose capacity decays very fast. The coulombic efficiency is also obviously improved when the cells are cycled above 1.5 V.

References:

[1] Chem. Rev., 2014, 114, 11636–11682

[2] Nano Lett., 2015, 15, 2998–3007

[3] Nano Energy, 2016, 19, 382-390.