Assembly And Electrochemical Properties Of NaTi₂?PO₄?₃/C?LiMn₂O₄ Batteries Based Li⁺/Na⁺-Mixed Aqueous Electrolyte

Aquerous rechargeable batteries?ARBs?based on two types of shuttle ions not only enrich the battery family but also make it more available for obtaining ARBs with output potential higher than 1.2 V.In this thesis,NaTi₂?PO₄?₃/C?NTP/C?serving as an anode material for ARBs was prepared by a co-precipitation in combination with annealing method.The samples were characterized with X-ray diffraction,Raman,X-ray photoelectron spectroscopy,scanning electron microscopy,transmission electron microscopy,and thermogravimetric technique.Further,cycling voltammograms,galvanostatic charge and discharge,and electrochemical impedance spectroscopy techniques were employed to evaluate the electrochemical properties of the samples.Through a three-electrode system,NTP/C and LiMn₂O₄?LMO?were tested in the role of electrode materials for ARBs,using 1 M Li₂SO₄,1 M Na₂SO₄,and 0.5 M Li₂SO₄/0.5 M Na₂SO₄ as the electrolytes,respectively.Further,NTP/C?LMO full batteris were assembled and their electrochemical performance was examined in a two-electrode system.The results show:?1?The as-obtained NTP/C samples are featured with hierarchical structure,high specific area,and abundant mesopores.NTP/C annealed at 750°C?NTP/C-750?for 4h in nitrogen atmosphere exhibits superior electrochemical performance to those annealed at 700 and 780°C.?2?NTP/C-750 operating in 1 M Na₂SO₄ exhibits superior electrochemical performance to that operating in 0.5 M Li₂SO₄/0.5 M Na₂SO₄ and to that in 1 M Li₂SO₄.At current densities of 0.1,0.2,0.5,1,2 and 5 A g^-1,the gravimetric discharge capacity(C_dis)values of NTP-750 are 119.6,99.6,88.5,91.5,75.7 and 71.7 mA h g^–1,respectively.Compared with the C_dis at 0.1 A g^-1,the capacity retention of NTP/C-750at 5 A g^-1 is 59.9%.After 1000 cycles at 1 A g^-1,the C_dis retains 36.1 mAh g^–1,presenting capacity retention of 40.7%.?3?LMO operating in 1 M Li₂SO₄ exhibits superior electrochemical performance to that operating in 0.5 M Li₂SO₄/0.5 M Na₂SO₄ and to that in 1 M Na₂SO₄.At current densities of 0.1,0.2,0.5,1,2 and 5 A g^-1,the C_dis values of LMO are 104.7,102.5,98.6,93.6,85.7 and 70.1 mA h g^–1,respectively.Compared with the C_dis at 0.1 A g^-1,the capacity retention of LMO at 5 A g^-1 is 70.0%.?4?NTP/C?LMO operating in 0.5 M Li₂SO₄/0.5 M Na₂SO₄ exhibits superior electrochemical performance to that operating in 1 M Li₂SO₄.This result indicates the electrochemical performance of NTP/C?LMO is anode-dependent.?5?The C_dis values of NTP/C-750?LMO are 106.0,104.6,102.5,101.4,99.7,61.9 and 48.4 mA h g^–1 after 10 cycles at 0.2,0.5,1,2,5,10,and 20 C(1 C=148 mA h g^-1),respectively.The C_dis at 20 C is 45.7%relative to that at 0.1 C.After 220 cycles at 1 C,the capacity retention is 72.3%and the Coulombic efficiency is 97%.The average output potential of NTP/C-750?LMO is 1.31 V.Considering that the mass of active materials accounts for ca.60 wt%of the full batteries in practice,an energy density of ca.36 W h kg^-1 can be deliverd by NTP/C-750?LMO in practical application.In view of the inferior cycling stability of ARBs due to the dissolution of electrode,reactions between electrode and H₂O/O₂,and co-insertion of proton and metal ions into the electrode,it is highly desirous to raising the cycling stability of ARBs through a simple method.In the following section,we comparatively investigated the electrochemical performance of LMO operating in 1 M Li₂SO₄without and with carbon dots?CDs?,which carry rich oxygen-containing functional groups and serve as an additive in electrolyte.Preliminary results show that:?1?The presence of CDs in 1 M Li₂SO₄ can suppress Li⁺extraction from LMO and assist Li⁺insertion into LMO.As a result,the rate capability and cycling stability of LMO can be greatly improved.?2?The concentration of CDs in electrolyte was optimized and LMO exhibits superior electrochemical performance in 1 M Li₂SO₄ with CD concentration of 0.25mg mL^-1.At 0.1,0.2,0.5,1,2,5 and 10 A g^-1,the C_dis values of LMO operating in 1M Li₂SO₄ with 0.25 mg mL^-1 CDs are 116.3,115.9,114.2,110.0,105.2,80.6 and47.8 mA h g^–1.After 1000 cycles at 1 A g^-1,the C_dis of LMO operating with CDs retains 66.7 mA h g^–1,present capacity retention of 60.6%.The retained C_dis and the capacity retention are much higher than that of 8.9 mA h g^–1 and that of 7.6%for LMO operating without CDs.?3?The dramatic increase in cycling performance of LMO operating with CDs is attributed to the enhanced structural stability through inhibition of LMO dissolution during charge/discharge cycles.