Design And Fabrication Of High Safety And Flexibie Polymer Electrolyte And Its Application In Lithium Batteries

Lithium-ion batteries are widely used in people’s daily life because of their high energy density,high working voltage,no memory effect and long operating life.However,the liquid electrolyte used in lithium-ion batteries have great potential safety hazards due to its leakage,combustion,and even explosion.Solid polymer electrolyte is composed of polymer and lithium salt,which can avoid the safety problems caused by the liquid electrolyte.However,the ionic conductivity of the solid polymer electrolyte is so low that is difficult for practical application,thereby the plasticizer is needed.Molecular plastic crystal material succinonitrile(SN),as a new-type plasticizer,has a plastic crystalline phase between solid phase and liquid phase.The plastic crystalline phase is solid but has a high ionic conductivity.However,SN is paste and has poor mechanical property.Therefore,high strength polymers are needed to improve its mechanical properties.Polyurethane acrylate(PUA),as a kind of prepolymer with hard and soft segments,has a higher tensile strength after curing,and the soft segments containing ether groups are also beneficial to the transportation of lithium ions.Therefore,PUA is chosen as the polymer matrix of the electrolyte in this paper.By compounding with plasticizer,a safe polymer electrolyte with high mechanical strength and high ionic conductivity was prepared.1.The polymer system showed incompatible "island-sea" structure and could not form film after curing when PUA,SN,TPO and lithium salt LiTFSI were blended.A certain amount of THF solvents were added into the system,then the THF was removed to form a polymer electrolyte at high temperature.With the addition of THF,the electrolyte system tends to be compatible gradually.The electrolyte was completely compatible by SEM observation when the weight-based concentration ratio of precursor mixture was THF/(SN+LiTFSI)=4wt./5wt.With the increasing of the compatibility,the electrolytes were from failure to film formation to the films with a tensile strength of 0.22 MPa,thereby the mechanical properties of the electrolytes have been improved significantly,and they can also show high flexibility and elasticity.At the same time,the ionic conductivity of the electrolyte with a better compatibility is 8.2 × 10-4 S/cm(20℃),the electrochemical stability window is 4.8V,and the ion mobility is 0.66.The electrolyte has good cycling and rate performance in LiFePO4/Li batteries at room temperature.After 50 cycles,the capacity retention rate is 99.2%,and discharge capacity is 97.7 mAh/g at 2C.2.PUA was solidified into high strength film and placed LiTFSI/SN solvent at the elevated temperature.Polymer is swellable and insoluble in liquid SN,which can adsorb a large amount of SN.When it returned to room temperature,SN was transformed from liquid phase to plastic crystalline phase,so as to improve the mechanical properties of the electrolytes.Compared with the electrolyte prepared by in-situ polymerization in the previous chapter,the electrolyte prepared by swelling method had a higher crosslinking density of 2.77 × 10-4 mol/cm3,a tensile strength of 6.5 MPa,an ionic conductivity of 1.63 × 10-3 S/cm3 an electrochemical stability window of 5.3V and a lithium ion transference number of 0.59.In LiCoO2/Li batteries,the discharge capacity is 119.8 mAh/g after 100 cycles,the cycle retention rate is 87.6%,the discharge capacity is 104.7 mAh/g at 2C rate and 25℃;the discharge capacity is 128.3 mAh/g after 100 cycles,the cycle retention rate is 89.4%,and the discharge capacity is 112.6 mAh/g at 2C rate and 55℃.After cycling at the elevated temperature,the battery was dismantled and the surface of the cathode was observed by SEM,and there were less impurities on the interface.LiCoO2/Li4Ti5O12 flexible battery with GPE-SN-IM electrolyte can withstand large bending folding and the electrical properties were not affected.3.As a reactive flame retardant,DOPO has a good flame retardant effect.DOPO was grafted onto the main chain of PUA to prepare flame retardant prepolymers.When the grafting rate of DOPO on PUA reached 39%,the electrolyte(FR-PE-4)was formed by combining DOPO-PUA with SN,Al2O3 and lithium salt LiTFSI.The overall flame retardant effect of the electrolyte could reach the V-0 level of UL-94 flame retardant standard,and the safety of the electrolyte was greatly improved.At the same time,the electrical properties of the electrolyte were not affected by the addition of flame retardant.The ionic conductivity of FR-PE-4 at room temperature is 2.66 × 10-4 S/cm,the electrochemical stability window is 5.07 V,the lithium ion transference number is 0.55.The discharge capacity of LiFePO4 FR-PE-4/Li4Ti5O12 battery is 107.4 mAh/g at 5C rate.At the same time,the discharge capacity of the battery has no reduction after a larger rate cycle.The capacity retention rate is 93.3%after 100 cycles at 0.2C ratio.In graphite/FR-PE-4/Li batteries,the capacity retention rate is 96.9%after 100 cycles at 0.5C ratio.LiFePO4/FR-PE-4/Li4Ti5O12 soft-packed batteries have a good high temperature performance,which can still work normally with no gas expansion at 150℃.4.On the basis of safety,the energy density of lithium-ion batteries is further improved.Lithium metal is used as the negative electrode material in lithium-ion batteries.However,dendrites will be formed on the surface of lithium metal during charging and discharging,which will pierce the electrolyte and cause short circuit of the batteries.Moreover,lithium metal is quite active,and SN will fail under high voltage or a long time environment.Based on this problem,the polymer electrolyte with high mechanical strength and safety can be prepared by electro spinning thermoplastic polyurethane(TPU)on glass fiber and then solidifying ionic liquid in the inner hole of the spinning.The electrospinning TPU on glass fibers can cover the large pore size of the fibers,thus forming layers of micro-pore,and the micro-pore size is less than 5 um.The electrolyte has high flexibility and 122.1 MPa tensile strength.The ionic conductivity is 3.5×10-3 S/cm(25℃),the electrochemical stability window is 5.57V,and the lithium ion transference number is 0.61.Moreover,the electrolyte is compatible with lithium metal and has no obvious calendar aging within 60 days.The electrolyte remained stable within 600 hours at a current density of 0.5mA/cm2 for lithium battery.It can also operate stably at the current densities of 0.5 mA/cm2,1 mA/cm2,2 mA/cm2 and 4 mA/cm2,respectively.In LiFePO4/Li battery,the discharge capacity can reach 103.1 mAh/g at 5C,and the capacity retention rate is 96.7%after 200 cycles.