Bulk-to-surface co-modification of layered hydrated vanadate cathode for aqueous zinc ion batteries

Abstract

The major challenges of vanadium-based layered materials are their dissolution tendency and the instability of their bulk-phase structure, resulting in unsatisfactory cyclability, particularly at lower current densities. Herein, we propose a co-modification strategy of dual-ion doping and forming an in situ cathode-electrolyte interphase (CEI). The dual ions, consisting of an alkali-metal ion (Na⁺ or K⁺) and an alkaline-earth-metal ion (Ca²⁺ or Ba²⁺), stabilize the bulk phase. The latter forms a precipitate with SO₄²⁻ in the electrolyte as an in situ CEI with a balance of stability and pH adaptiveness. Based on the stabilized cathode from bulk phase to surface, Ca_0.56Na_1.19V₆O₁₆·4.09H₂O exhibits excellent cyclability, especially at lower current densities. The full cell retains 99.4% of capacity after 120 cycles at 0.2 A g⁻¹ and 25 °C while using Zn(OTF)₂ electrolyte. Moreover, it exhibits 84.5% capacity retention at 0.1 A g⁻¹ and −30 °C after 1000 cycles. The Zn²⁺/H⁺ intercalation mechanism was investigated by analytical characterizations and density functional theory (DFT) calculations, which implied that proton (de)intercalation is restrained at −30 °C, leading to the median discharge voltage increasing from 0.705 to 0.795 V. The co-modified cathode exhibits a significant performance in Zn(ClO₄)₂ electrolyte at 0.1 A g⁻¹ and −30 °C (90.8% capacity retention after 2000 cycles). The co-modification strategy provides a viable option for cathode design.