Aqueous Dual-Electrolyte Full-Cell System for Improving Energy Density of Sodium-Ion Batteries

Sodium-ion batteries (SIBs) are regarded as one of the most promising candidates for next-generation energy storage devices and have been gradually grasping market share for their low cost and similar reaction mechanism and production process as compared to lithium-ion batteries. However, the low energy density of SIBs restricts their practical applications. For example, regular full cells of a Prussian blue cathode and NASICON anode have only a low discharge capacity (about 77 mA h/g at 1 C). Taking into account the compatibility of the electrolyte and electrode materials, a novel strategy for a viable aqueous dual-electrolyte sodium-ion battery (ADESIB) has been proposed using Na₂SO₄ solution as the anolyte and redox-active sodium hexacyanoferrate Na₄Fe(CN)₆ solution as the catholyte to accommodate a NASICON NaTi₂(PO₄)₃ anode and Prussian blue Na₂NiFe(CN)₆ cathode. The capacity of Na⁺ ion deinsertion/insertion electrodes combined with the redox chemistry of the Na₄Fe(CN)₆ catholyte thus enhances overall charge storage and energy density. The ADESIB delivers a capacity of about 113 mA h/g at 1 C, showing a 43% improvement over batteries with a regular single Na₂SO₄ electrolyte. Additionally, the dual-electrolyte full-cell system is proved to reach a 84.7% capacity retention after 1000 cycles, mainly due to the synergy of the electrolytes in both sides. This pioneering research proposes an aqueous dual-electrolyte sodium-ion full cell, showing potential applications in a new sodium-ion full battery system.