Impact of Na2MoO4 nanolayers autogenously formed on tunnel-type Na0.44MnO2

Abstract

We propose the coating of tunnel-type Na_0.44MnO₂ cathode materials with multi-functional Na₂MoO₄ nanolayers for use in rechargeable sodium batteries. Electro-conducting Na₂MoO₄ nanolayers (electrical conductivity of ∼10³ S cm⁻¹) are autogenously formed on the surface of Na_0.44MnO₂ particles through the reaction of (NH₄)₂MoO₄ with surface sodium residues via melt impregnation at 350 °C. The Na₂MoO₄-modified Na_0.44MnO₂ electrode delivers discharge capacities of ∼120.4 mA h (g-oxide)⁻¹ at 0.1C (12 mA g⁻¹) and 79.7 mA h g⁻¹ at 50C (6 A g⁻¹). Moreover, with continuous cycling at a rate of 60C (7.2 A g⁻¹), the Na₂MoO₄-coated Na_0.44MnO₂ electrode is able to retain a capacity of approximately 56 mA h g⁻¹ without notable capacity fading for 1000 cycles. This achievement is attributed to the presence of Na₂MoO₄ on the active materials, which facilitates electron transfer during electrochemical reaction in Na cells. More interestingly, Na₂MnO₄ undergoes two-step HF scavenging to finally form MoO_3−xF_2x layers via an intermediate of H₂MoO₄ (MoO₃·H₂O) layers. The surface layers protect the active materials from HF attack in the electrolyte. These multi-functional effects of the Na₂MoO₄ and MoO_3−xF_2x surface layers are responsible for the long-term cycle stability of the cathode material for ultra-high-rate sodium storage applications.