| With the growing demand for portable electronic devices and electric vehicles,as well as dwindling renewable energy sources,high-efficiency energy storage devices have received a lot of attention.Lithium-ion batteries are widely used in daily life due to their high operating voltage,long cycle life and high energy density.However,traditional lithium-ion batteries generally use organic electrolyte,which has the disadvantages of being flammable,explosive and volatile,leading to safety hazards in the use of lithium-ion batteries.The use of solid electrolyte instead of electrolyte can fundamentally solve the safety problem.Polyethylene oxide（PEO）-based polymer solid-state electrolytes have high dielectric constants and lithium salt solubility,and are inexpensive and easy to process.However,the disadvantages of PEO,such as high crystallinity and lack of chain segment motility,lead to its low ionic conductivity at room temperature(only 10-6～10-8 S cm-1 at room temperature).The use of modification means can improve the room temperature ionic conductivity,but at the same time will lead to the deterioration of its mechanical properties.A block copolymer is a special polymer prepared by joining together two or more polymer chain segments with different properties.Block copolymer-based electrolytes decouple the mechanically supported phase from the ion-conducting phase,improving ionic conductivity while maintaining dimensional stability.In this paper,three block copolymer electrolytes were prepared and their physical and chemical properties,electrochemical performance and full-cell performance were investigated in detail as follows:（1）A triblock copolymer electrolyte PCL-PEO-PCL（TCBC）was synthesized from PEO andε-CL by ring-opening polymerization.With the introduction of PCL,the intensity of PEO characteristic peak decreased gradually and the crystallinity decreased significantly.The introduction of PCL improved the thermal stability of the electrolyte and the TGA showed that the thermal decomposition temperature was 276℃.TCBC20 has good electrochemical properties,with an ionic conductivity of 8.2×10-5 S cm-1 at 30°C and 4.1×10-4 S cm-1 at 70°C.The electrochemical window of TCBC20 is 3.8 V and the ion mobility number is 0.28.The initial discharge specific capacity of the Li Fe PO4/Li cell was 152.5 m Ah g-1 at 70°C and 0.1C multiplier,and the capacity retention rate was 91%after 200 cycles.After the gradient of the multiplication rate from 0.1 C(152.9 m Ah g-1)to 2 C(94.8 m Ah g-1)and then to 0.1 C(150.4 m Ah g-1),the capacity decreases only 1.6%,indicating that the battery has a good cycling performance as well as multiplication rate performance.（2）The pentablock copolymer electrolyte PS-PCL-PEO-PCL-PS（PCBC）was prepared by atom transfer radical polymerization.The introduction of PS not only improved the mechanical properties of the electrolyte as a rigid support phase,but also reduced the crystallinity of the copolymer electrolyte,which further improved the thermal stability of the copolymer electrolyte,and the thermal decomposition temperature of PCBC20＿30 was 360℃.The pale yellow PCBC-SPE film is a self-supporting electrolyte film that can regain its flat shape after folding and curling,and has excellent mechanical flexibility.PCBC20＿30 has good electrochemical properties,with ionic conductivity of 9.6×10-5 S cm-1 at 30℃and6.4×10-4 S cm-1 at 70℃.The electrochemical stability window of PCBC20＿30 is 4.0 V and the ion mobility number is 0.34.The initial discharge specific capacity of the Li Fe PO4/Li cell was 155.1 m Ah g-1 at 70°C and 0.1 C multiplier,and the capacity retention rate was 93.1%after 160 cycles.After a gradient of the multiplicity from 0.1 C(156.1 m Ah g-1)to 2 C(104.7m Ah g-1)and then to 0.1 C(153.4 m Ah g-1),the capacity decreased by only 1.7%.This indicates that the battery has excellent cycling performance as well as multiplicative performance.In a further study,the composite electrolyte PCBC@LLZTO was prepared by adding LLZTO to PCBC.At 30°C,the ionic conductivity of PCBC@LLZTO＿20 was 3.1×10-4 S cm-1.The initial discharge specific capacity of the Li Fe PO4/Li cell was 156.3 m Ah g-1at 30°C and 0.1 C multiplier,and the capacity retention rate was 91.6%after 100 cycles.（3）The pentablock copolymer electrolyte PS-PLA-PEO-PLA-PS（PDBC）was prepared by atom transfer radical polymerization.The peak melting points of PDBC20＿20,PDBC20＿30 and PDBC20＿40 were 59.84℃,60.67℃and 60.95℃,respectively,by DSC test.The TGA test results showed that PDBC20＿20 and PDBC20＿40 started to decompose at258°C,while PDBC20＿30 did not start to experience weight loss until 322°C.The ionic conductivity of PDBC20＿30 was 1.3×10-4 S cm-1 at 30°C and 2.1×10-3 S cm-1 at 70°C.The electrochemical stability window of PDBC20＿30 is 4.2 V and the ion mobility number is 0.36.The initial discharge specific capacity of the Li Fe PO4/Li cell was 152 m Ah g-1 at 70°C and0.1 C multiplier,and the capacity retention rate was 94.6%after 200 cycles.PDBC20＿30 has good interfacial stability to lithium metal electrodes,and the Li/PDBC20＿30/Li symmetric cell is stable at a current density of 0.1 m A cm-2 for more than 600 h at 70°C,and the overpotential remains stable below 0.1 V,and no lithium dendrite is generated.The initial discharge specific capacity of Li Ni0.6Co0.2Mn0.2O2/PDBC20＿30/Li（NCM622/PDBC20＿30/Li）battery at 70℃and 0.1 C was 168.8 m Ah g-1 with a coulombic efficiency of 88.5%;the capacity was maintained at 123.5 m Ah g-1 after 20 cycles with a capacity retention rate of73.2%.In a further study,the tri-salt system polymer electrolyte（TSBC）was prepared by adding Li BOB（5 wt%）,Li NO3（5 wt%）,and LITFSI（5 wt%）to PDBC20＿30.The initial discharge specific capacity of Li Fe PO4/TSBC/Li was 157.7 m Ah g-1 at 70°C and 0.1 C multiplier,and the discharge specific capacity remained 156.8 m Ah g-1 after 100 cycles,with a capacity retention rate of 99.4%,and the average coulomb efficiency were above 99.5%,and the polarization between charging and discharging platforms was extremely low（ΔV≈0.04 V）.The first discharge specific capacity of the NCM622/TSBC/Li battery at 70°C and0.1 C was 167.4 m Ah g-1 with a coulombic efficiency of 86.9%;the capacity was maintained at 144.1 m Ah g-1 after 20 cycles,with a capacity retention rate close to 86.1%. |
PDF Full Text Request