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Journal of Electrochemistry ›› 2021, Vol. 27 ›› Issue (6): 605-613.  doi: 10.13208/j.electrochem.210125

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Functional Sulfate Electrolytes Enable the Enhanced Cycling Stability of NaTi2(PO4)3/C Anode Material for Aqueous Sodium-Ion Batteries

Shu-Jin Li, Zhi-Kang Cao, Wen-Kai Wang, Xiao-Han Zhang, Xing-De Xiang*()   

  1. College of Chemistry & Chemical Engineering and Resource Utilization, Northeast Forestry University, Harbin 150040, Heilongjiang, China
  • Received:2021-01-25 Revised:2021-03-01 Online:2021-12-28 Published:2021-03-20
  • Contact: Xing-De Xiang E-mail:xiangxingde@nefu.edu.cn

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