Study On Modification Of PEO-based Solid Polymer Electrolyte By Phosphorus Flame Retardan

As one of the most important energy storage technologies,lithium-ion batteries account for a considerable share in the global electrochemical energy storage market.With the increasing demand of consumers for electrochemical energy storage,many problems have emerged in traditional liquid Li-ion batteries,such as poor safety and low energy density.In order to improve the safety performance of the batteries,using solid electrolyte instead of traditional organic liquid electrolyte is the most commonly used solution.Polymer solid electrolyte（PEO）has been widely studied because of its advantages such as good flexibility,small interface impedance and low price.Moreover,the electrochemical performance of PEO-based solid polymer electrolyte（PEO-SPE）modified by adding inorganic filler and lithium salt has also been greatly improved.However,as an organic material,PEO has inherent poor thermal stability.Although its safety performance has been greatly improved compared with liquid electrolytes,its flammability at high temperatures still limits the practical application range of PEO-based solid polymer electrolytes.In order to improve the safety performance of the polymer electrolyte,adding flame retardants is one of the most commonly used method.Ammonium polyphosphate（APP）is an inorganic flame retardant with high content of P and N.However,its poor compatibility in polymer materials limits its application in polymers,so it is necessary to try different ways to modify it and increase its compatibility in polymer materials.In this work,ion-exchange method and microencapsulation method were used to further modify it,and piperazine-modified ammonium polyphosphate（PA-APP）and titanium-coated piperazine-modified ammonium polyphosphate（Ti@PA-APP）were prepared and added to PEO-SPE as flame retardants,respectively.Flame retardant PEO-based solid polymer electrolytes（2 wt%PA-APP/PEO-SPE,2 wt%Ti@PA-APP/PEO-SPE）were prepared,and the effects of flame retardants on the electrochemical performance,thermal stability and mechanical strength of the electrolytes were investigated.It was found that the addition of 2 wt%PA-APP and Ti@PA-APP to the PEO-based solid polymer electrolyte could not only improve the thermal stability of the PEO-based solid polymer electrolyte（The heat release rate decreased by 74%and 71%,and the residual carbon content increased from 11.8%to 18.8%and 17.9%,respectively）,but also improved the electrochemical performance of the PEO-based solid polymer electrolyte.Compared with PEO-SPE,the ionic conductivity of 2 wt%PA-APP/PEO-SPE and 2 wt%Ti@PA-APP/PEO-SPE increased from 8.7×10^-5 S cm^-1 to 1.4×10^-4 S cm^-1 and 1.5×10^-4 S cm^-1 at 60 ^oC,and the electrochemical stability window was increased from 5.4 V to 5.6 V and 6 V,respectively.In addition,Li Fe PO₄//2 wt%PA-APP/PEO-SPE//Li,Li Fe PO₄//2 wt%Ti@PA-APP/PEO-SPE//Li after 100 cycles its capacity retention rate compared to Li Fe PO₄//PEO-SPE//Li from 87%increased to 96%and 98%,respectively.This indicates that the modified APP as a flame retardant added to PEO-SPE can play an effective lifting role in all aspects.2D black phosphorene nanosheets（BP）were prepared via an electrochemical cathodic expansion exfoliation method and added into the PEO based solid polymer electrolyte to obtain a flame-retardant PEO based solid polymer electrolyte（BP/PEO-SPE）.The results showed that the thermal stability of PEO-SPE could be significantly improved by the addition of appropriate amount of BP.the micro-scale combustion calorimetry test results showed that the peak value of the heat release rate（p HRR）of the membrane is reduced from 919.4 W g^-1 to 344.5 W g^-1 after addition of 0.7 wt%BP nanosheets.In addition,the electrochemical performance of 0.7 wt%BP/PEO-SPE is also significantly improved.Compared with PEO-SPE,the ionic conductivity was improved from 8.7×10^-5 S cm^-1 to 2.74×10^-4 S cm^-1 after addition of 0.7 wt%BP nanosheets at60 ^oC.The PEO-based solid polymer electrolyte membrane with addition of 0.7 wt%BP nanosheets exhibits a higher electrochemical stability window and enhanced interfacial compatibility with Li anode compared to the membrane without addition of BP nanosheets.The charge and discharge results showed that the discharge specific capacity of the Li Fe PO₄//0.7 wt%BP/PEO-SPE//Li increased by around 10 m Ah g^-1 at every corresponding rate compared with Li Fe PO₄//PEO-SPE//Li.This shows that the addition of a small amount of BP flame retardant will not only improve the electrochemical performance of the PEO-based solid polymer electrolyte,enhance its thermal stability and mechanical strength,but also improve the charge and discharge performance of the all-solid lithium-ion battery.