Abstract:

Aqueous sodium-ion batteries show promising application in fields of large-scale storage of intermittent renewable energies owing to the earth-abundant sodium resources and incombustible aqueous electrolytes. Primary factors determining whether they can be commercially utilized are low cost and long lifetime. Among current electrode materials, NASICON-type NaTi₂(PO₄)₃ arouses wide interests as an anode material for aqueous sodium-ion batteries as it offers a high specific capacity, fast Na-transport ability and reasonable working potential, however, suffering from insufficient cycling performance caused by severe dissolution of active materials in traditional aqueous electrolytes. In this work, a functional sulfate electrolyte (2 mol·L^-1 Na₂SO₄ + 0.3 mol·L^-1 MgSO₄) was designed by coupling concentrated Na₂SO₄ salt and functional MgSO₄ additive to enhance the cycling stability of NaTi₂(PO₄)₃/C material. Experimental results from cyclic voltammetry and galvanostatic measurements suggest that the electrolyte can improve electrochemical reversibility and cycling performance of NaTi₂(PO₄)₃/C material relative to traditional electrolyte (1 mol·L^-1 Na₂SO₄). In specific, the material harvested a reversible capacity of 93.4 mAh·g^-1 and impressive capacity retention of 96.5% at the specific current of 100 mA·g^-1 in the functional sulfate electrolyte, but exhibited a reversible capacity of 88.6 mAh·g^-1 and much lower capacity retention of 72.1% in the traditional electrolyte. In order to explore intrinsic causes of the performance improvement, structural properties of the material before and after cycling were comparatively investigated by using X-ray diffraction and X-ray photon spectroscopy. It is found that the material showed excellent structural stability and formation of protective Mg-containing interfacial layer during cycling in the functional sulfate electrolyte. Both the raised electrolyte-salt concentration and functional MgSO₄ additive should be responsible for the enhanced structural stability. The high electrolyte-salt concentration could decrease electrochemical activity and widen electrochemical stability window of electrolyte solvents, while the MgSO₄ additive could timely capture the hydroxyl group resulting from water-solvent decomposition to prevent the alkalization of aqueous electrolytes and spontaneously form protective Mg(OH)₂ interfaces. As a result, the electrolyte could suppress the dissolution of active NaTi₂(PO₄)₃, thus, resulting in the enhanced structural stability and cycling performance. With an aim to further exhibit the feasibility for practical application, full aqueous sodium-ion batteries were assembled by coupling Na₂Ni[Fe(CN)₆] cathode, functional sulfate electrolyte and NaTi₂(PO₄)₃/C anode. Charge/discharge tests show that the battery could deliver a working voltage of 1.3 V and a reversible capacity of 84.2 mAh·g^-1 (calculated as the mass of active anode material) at the current of 100 mA·g^-1, achieving a specific energy of about 110 Wh·kg^-1. After being continuously charging and discharging for 500 cycles at the current of 500 mA·g^-1, it achieved high capacity retention of 80%. The results in this work suggest that designing functional additive-containing sulfate electrolytes is an effective strategy to fabricate low-cost, long-lifetime aqueous sodium-ion batteries.

Key words: aqueous sodium-ion batteries, anode material, sulfate electrolyte, structural stability, cycling performance