水系钠离子电池具有钠资源丰富、成本低廉、安全可靠、维护简单等特点,在可再生能源规模储存领域具有重要应用前景。NASICON型NaTi₂(PO₄)₃具有可逆容量高、工作电位低、离子传输快等优点,是目前最受关注的水系钠离子电池负极材料。但是,该材料在传统的水系电解液中结构不稳定,循环性能不足。本论文通过调控Na₂SO₄浓度和引入MgSO₄添加剂,构建了一种新型硫酸盐功能电解液(2 mol·L^-1 Na₂SO₄ + 0.3 mol·L^-1 MgSO₄)。该电解液能够显著增强NaTi₂(PO₄)₃/C材料在充放电循环过程中的结构稳定性,从而提高其电化学可逆性和稳定性。电化学测试表明,NaTi₂(PO₄)₃/C基于该电解液在100 mA·g^-1条件下的可逆容量为93.4 mAh·g^-1,循环100次后容量保持率高达96.5%;基于该电解液构建的Na₂Ni[Fe(CN)₆]|NaTi₂(PO₄)₃/C电池可以稳定循环500次以上。本论文结合XRD、XPS等技术讨论分析了该电解液的功能作用机制,其研究结果为设计低成本高性能水系钠离子电池提供了新思路和实验基础。

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.