Interlayer modulation in layered δ-MnO2 for high-performance Ca-ion storage in aqueous batteries

Abstract

Aqueous Ca-ion batteries (ACIBs) are seen as a member of the emerging energy storage technologies owing to their cost-effectiveness and eco-friendliness. Nevertheless, the application of traditional cathode materials for ACIBs is limited by inadequate specific capacity and low cycle durability. Among multiple cathodes, manganese oxides with a high capacity in theory are proper to be excellent prospects for ACIBs. In this work, the interlayer spacing in layered δ-MnO₂ is effectively modulated via the pre-intercalation of metal ions (Zn²⁺ or Ca²⁺) into the layered lattice. Interestingly, the pre-insertion of Zn²⁺ into the layered δ-MnO₂ induces an unusual contraction of interlayer spacing, while the pre-insertion of Ca²⁺ shows an expansion of interlayer spacing. The different variation of layered δ-MnO₂ structure via various metal ions pre-insertion is considered to be a combined result of ionic size and electrostatic interactions. Electrochemical tests exhibit that the Zn–MnO₂ with the contracted interlayer spacing offers an increased reversible capacity of 138.5 mAh g⁻¹ at 0.5 A g⁻¹, which should be credited to the decreased Ca adsorption energy (E_ads) in Zn–MnO₂ according to the Density Functional Theory (DFT) calculations. In addition, Zn–MnO₂ cathode demonstrates a prolonged cyclic life of 1000 cycles at 2.0 A g⁻¹ with 85.4 % capacity retention, due to the relatively small volume variation during Ca²⁺ insertion/extraction in the electrochemical reactions. The stable Ca-ion full battery is constructed using Zn–MnO₂ cathode and PPTCDI (poly 3,4,9,10-perylentetracarboxylic diimide) anode, demonstrating a high capacity of 93.2 mAh g⁻¹ at 2.0 A g⁻¹ and outstanding cycling stability. This work introduces the effective interlayer modulation of the layered δ-MnO₂ via foreign metallic ion pre-intercalation to fulfill enhanced Ca²⁺ ion storage.