Three-dimensional conductive interface and tip structure of MnO₂ electrode facilitate superior zinc ion batteries

Abstract: Manganese dioxide has been significantly utilized in zinc ion batteries (ZIBs). However, in the rechargeable battery system, the manganese dioxide cathode suffers from poor conductivity, volume expansion, and substance dissolution, resulting in low capacity and poor stability. Herein, a 3D frame structure MnO₂@CNTs cathode is proposed. In this system, the electrodeposited spherical MnO₂ was anchored and interlinked via the in-situ growth carbon nanotubes (CNTs) onto the carbon cloth. Benefiting the unique 3D frame structure, the MnO₂ structure crush problem and the pathway of the electronic and ions have been dramatically improved. The optimized MnO₂@CNTs cathode demonstrate a high capacity of 256.35 mAh g^-1 at 0.1 A g^-1 and exceptional cycling stability. Furthermore, in-situ Raman spectroscopy elucidate the energy storage mechanism of aqueous ZIBs (AZIBs). Moreover, COMSOL finite elements analysis demonstrates that the petal edge-riched nanostructures of MnO₂@CNTs generate a localized high electric field under constant current, accelerating ion/electron transfer. Our work explains the rationale for CNTs to improve the properties of MnO₂ cathodes, providing a new perspective for the design of high-performance batteries.

Schematic 1. The synthesis of MnO₂@CNTs.

Figure 1. SEM images of a) CNTs@CC. b) CNTs. c) MnO₂@CNTs after 5 min electrodeposition. d,e) MnO₂@CNTs after 40 min electrodeposition. f) MnO₂. g) TEM image. h) HRTEM image. i) Element mapping images of MnO₂@CNTs.

Figure 2 a) XRD patterns of MnO₂@CNTs and CC. b) XPS spectra of MnO₂@ CNTs. The fine XPS spectra for c) C 1s. d) O 1s. and e) Mn 2p.

Figure 3. Electrochemical performances of MnO₂@CNTs and MnO₂ electrodes: a) CV curves at 0.1 mV s⁻¹. b) GCD curves at 0.1 A g⁻¹. c) GCD curves at different current densities. d) rate performance. e, g) long cycle performance at 0.1 A g^-1 and 1 A g^-1. f) Ragone plot.

Figure 4. MnO₂@CNTs cathode a) CV curves of at different scan rates. b) log (i) versus log (v) plots at different peaks. c) Capacitive contributions at various scan rates. d) GITT curves. e) The ion diffusion coefficient of MnO₂@CNTs and MnO₂. f) Arrhenius plots for MnO₂@CNTs and MnO₂.

Figure 5. a-b) Free electron density distribution at the tip of MnO₂@CNTs and MnO₂. c-d) Computed electric field near the tip of MnO₂@CNTs and MnO₂. e-f) Zn²⁺ concentration at the tip of MnO₂@CNTs and MnO₂.

Figure 6. a-b) In-situ Raman spectral evolution of the MnO₂@CNT during the discharging and charging processes. c) SEM of MnO₂ after 1000th cycles; (d-e) SEM of MnO₂@CNTs after 1000th cycles.