The role of aqueous electrolytes in the electrochemical performance of calcium vanadate cathode for Zn-ion storage

Abstract

In light of the growing needs for high-performing, safe, low-cost and eco-friendly energy storage systems, herein, the CaV<inf>2</inf>O<inf>6</inf>–based composite (CaVO400/C) is studied as a potential cathode host for the Zn²⁺ion storage in aqueous rechargeable zinc-ion batteries (AZIBs). The CaVO400/C composite, synthesized via malonic acid-assisted solution combustion method (including calcination at 400 °C), is subjected to thorough electrochemical and structural analyses. Its electrochemical performance in a half-cell configuration is evaluated in three different 3 M zinc aqueous electrolytes: Zn(NO<inf>3</inf>)<inf>2</inf>, ZnSO<inf>4</inf>and ZnCl<inf>2</inf>. Over a large number of CV cycles, the electrochemical behaviour of the composite is markedly different depending on the electrolyte (anion type and pH value). The most favourable electrochemical behaviour is observed in Zn(NO<inf>3</inf>)<inf>2</inf>, where high and quite stable redox activity is retained over 400 cycles. In contrast, the performance in ZnSO<inf>4</inf>is diminished by dissolution of active vanadium species during initial cycling (pH = 4) or prolonged cycling (pH = 2), while after cycling, the formation of Zn<inf>4</inf>(SO<inf>4</inf>)(OH)<inf>6</inf>·nH<inf>2</inf>O phase is detected on the electrode. The ZnCl<inf>2</inf>electrolyte leads to a complete loss of activity due to vanadium consumption: for pH = 5 - caused by irreversible transformation of the electrochemically active phase into the inactive one, Zn<inf>3</inf>(OH)<inf>2</inf>V<inf>2</inf>O<inf>7</inf>·2H<inf>2</inf>O, and for pH = 2 – caused by consecutive formation and dissolution of Zn<inf>3</inf>(OH)<inf>2</inf>V<inf>2</inf>O<inf>7</inf>·2H<inf>2</inf>O. It is suggested that formation of the proton-doped vanadium oxide species as the active phase, during cycling in Zn(NO<inf>3</inf>)<inf>2</inf>electrolyte, facilitates the efficient Zn²⁺storage in the CaVO400/C electrode, enabling insertion/deinsertion specific capacity of ∼167/145 and ∼119/115 mAh g⁻¹at 500 and 2000 mA g⁻¹, respectively, in a half-cell configuration.