Abstract
Manganese dioxide (MnO 2 ) with different crystal structures has been widely investigated as the cathode material for Zn-ion batteries, among which spinel λ -MnO 2 is yet rarely reported because Zn-ion intercalation in spinel lattice is speculated to be limited by the narrow three-dimensional tunnels. In this work, we demonstrate that Zn-ion insertion in spinel lattice can be enhanced by reducing particle size and elucidate an intriguing electrochemical reaction mechanism dependent on particle size. Specifically, λ -MnO 2 nanoparticles (NPs, ~80 nm) deliver a high capacity of 250 mAh/g at 20 mA/g due to large surface area and solid-solution type phase transition pathway. Meanwhile, severe water-induced Mn dissolution leads to the poor cycling stability of NPs. In contrast, micron-sized λ -MnO 2 particles (MPs, ~0.9 μ m) unexpectedly undergo an activation process with the capacity continuously increasing over the first 50 cycles, which can be attributed to the formation of amorphous MnO x nanosheets in the open interstitial space of the MP electrode. By adding MnSO 4 to the electrolyte, Mn dissolution can be suppressed, leading to significant improvement in the cycling performance of NPs, with a capacity of 115 mAh/g retained at 1 A/g for over 500 cycles. This work pinpoints the distinctive impacts of the particle size on the reaction mechanism and cathode performance in aqueous Zn-ion batteries.
Highlights
Li-ion batteries (LIBs) dominate the commercial market in recent years, limited supplies of Li, high cost, potential safety issues, and environmental concerns may inhibit their future development [1,2,3]
Through X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray spectroscopy (EDX), we show that reducing particle size enhances Zn-ion insertion in spinel lattice, but at the same time, severe water-induced Mn dissolution leads to quick capacity degrading
We reveal the effects of particle size on the performance and reaction mechanisms of λ-MnO2 in aqueous Zn-ion batteries
Summary
Li-ion batteries (LIBs) dominate the commercial market in recent years, limited supplies of Li, high cost, potential safety issues, and environmental concerns may inhibit their future development [1,2,3]. The effect of particle size on Mg-ion intercalation in λ-MnO2 has been thoroughly studied, where we show that reducing particle size can enhance Mg-ion intercalation [16]. This effect can be attributed to the solid-solution type phase transition pathway of λ-MnO2 NPs from cubic to tetragonal phase during Mgion insertion, which minimizes lattice mismatch and energy penalty for phase transformation. We further illustrate the relations among the particle size, Zn-ion intercalation, side reactions, and the resulting electrochemical performance in ZIBs, by investigating λ-MnO2 NPs (~80 nm) and micron-sized particles (MPs) (~900 nm) as a model system. This paper highlights the complicated and nonmonotonic size effects of cathode materials on the reaction mechanism and electrochemical property of ZIBs, which offers guidelines for integrating nanoengineering and high-performance cathode materials design for aqueous ZIBs
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