Ultra-low diffusion barrier tetramethyl ammonium cation-intercalated layered MnO2 for high-performance aqueous zinc-ion batteries

Abstract

The distinctive layered configuration of δ-MnO₂ renders it an exceptionally promising candidate as a cathode material for aqueous zinc-ion batteries (ZIBs). However, its practical utilization is constrained by the sluggish diffusion kinetics of Zn²⁺ and the capacity degradation resulting from lattice distortions occurring during charge and discharge cycles. To address these challenges, we have developed TMA–MnO₂ by pre-intercalation of both tetramethylammonium ions (TMA⁺) and H₂O molecules. The incorporation of TMA⁺ within the interlayer of δ-MnO₂ results in the expansion of the interlayer spacing to 0.96 nm, thereby establishing a rapid diffusion pathway for Zn²⁺. At a rate of 0.2 A g⁻¹, the ZIB employing TMA–MnO₂ exhibits a specific capacity of 310.3 mA h g⁻¹. Furthermore, after 1000 cycles at 2 A g⁻¹, it maintains 91% of its initial capacity. Ex situ characterization techniques provide evidence of the co-intercalation mechanism of Zn²⁺/H⁺, concurrently indicating the stable presence of TMA⁺ as a structural support between layers during charge–discharge cycling. Density functional theory calculations provide evidence of the transformative impact of TMA⁺ introduction on the electron cloud density distribution within MnO₂. This alteration leads to an enhancement in material conductivity and a substantial reduction in the diffusion barrier for Zn²⁺, consequently facilitating rapid diffusion kinetics.