Reversible Zn and Mn deposition in NiFeMn-LDH cathodes for aqueous Zn-Mn batteries

Introducing NiFeMn-Layered Double Hydroxide (LDH) as an innovative cathode material for Zn-Mn batteries, this study focuses on bolstering the electrochemical efficiency and stability of the system. We explored the effect of varying Zn/Mn molar ratio in the electrolyte on the battery's electrochemical performance and investigated the underlying reaction mechanism. Our results show that an electrolyte Zn/Mn molar ratio of 4 : 1 achieves a balance between capacity and stability, with an areal capacity of 0.20 mA h cm^-2 at a current of 0.2 mA and a capacity retention rate of 53.35% after 50 cycles. The mechanism study reveals that during the initial charge-discharge cycle, NiFeMn-CO₃ LDH transforms into NiFeMn-SO₄ LDH, which then absorbs Zn²⁺, Mn²⁺, and SO₄ ^2- ions to form a stable composite substrate. This substrate enables the reversible deposition-dissolution of Mn ions, while Zn ions participate in the reaction continuously, with most Mn- and Zn-containing compounds depositing in an amorphous phase. Although further optimization is needed, our findings provide valuable insights for developing Zn-Mn aqueous batteries, highlighting the potential of LDHs as cathode substrates and the pivotal role of amorphous compounds in the reversible deposition-dissolution process.