Preparation And Characterization Of PEO Based Li_1.5Al_0.5Ge_1.5?PO₄?₃ Solid Composite Electrolyte For All Soild State LiFePO₄ Lithium-ion Battery

All-solid-state lithium ion battery is a new type of high safety lithium secondary battery which attempted to avoid potential safety problems by replacing flammable liquid electrolytes with solid electrolytes.Solid electrolyte with good chemical stability and high ion conductivity are crucial for the commercialization of all-solid-state lithium ion battery.Solid composite electrolyte based on LAGP-PEO-Li X has a great prospect in application due to the combination of the fast ion conductor Li1+xAlx Ge2-x(PO4)3 with high ionic conductivity and the polymer electrolyte with good processability.The LAGP-PEO-LiX solid composite electrolyte was prepared using fast ion conductor Li1.5Al0.5Ge1.5(PO4)3(LAGP)and PEO-Li X by solution casting method.In this work,we have investigated the effect of the main factors,such as the content of LAGP,EO/Li mole radio and lithium categories,on electrochemical performance of solid composite electrolyte.The preparation technology of solid composite electrolyte was optimized during the preparation process.We also discussed the transport mechanism of Li-ion in solid composite electrolyte.Finally,we applied the solid composite electrolyte to LFP all-solid-state batteries.Firstly,LAGP-PEO-LiTFSI solid composite electrolytes were prepared with LAGP and Li TFSI as conductive components and PEO as the binder using solution casting method.The ratio of EO/Li with fixed to 13 when the ratio of PEO to LAGP was varied.The results showed that LAGP partially interacted to PEO-LiTFSI and uniformly distributed in the electrolyte.With increasing LAGP content,the amorphous regions of PEO rises up to a maximum value due to the coordination between the LAGP and PEO-LiTFSI.We found that three phases are generally present,namely a pure crystalline LAGP phase,all amorphous complexion PEO-Li TFSI phase and a transition phase consisting of lithium salt particles and amorphous PEO-Li TFSI.Electrochemical impedance analysis show that,the ionic conductivity of LAGP-PEO-Li TFSI solid composite electrolyte was dependent on temperature by Arrhenius relations and varies with the content of LAGP.The highest ionic conductivity and the processability of composite electrolyte were obtained when the optimized content of LAGP was fixed to6:4 in weight ratio.The influence of the molar ratio of EO/Li on the ionic conductivity of LAGP-PEO-Li TFSI solid composite electrolyte were investigated and the molar ratio was optimized.The weight ratio of LAGP:PEO was fixed to 6:4 and LiTFSI salt was adopted in this work.The highest ionic conductivity was obtained when EO/Li=13.The results suggested that there is an optimal value for the coordination number of LiTFSIand PEO.When EO/Li=13,the coordination number reached its maximum and the highest carrier concentration of Li+ ions is obtained.Meanwhile,low activation energy of Li+migration reached its minimum.Whereas the content of LiTFSI exceeds the limit of coordination with PEO,more contact ion pairs and rich salt crystals are generated,which hinder the transfer of ions.At the same time,it can also reduce the concentration of Li+ ions,which can decrease the ion conductivity.LAGP-PEO-Li X solid composite electrolyte with different lithium salts were prepared while EO/Li=8 and the weight ratio of LAGP:PEO is fixed to 6:4.The results show that the ability to inhibit the crystallization of the PEO and the concentration of mobile carrier Li+ source were improved by the lithium salt with bigger volume of anion..The size of anion volume can be sorted as follow: TFSI-1> BOB-1> Cl O4-1,therefore,the solid composite electrolyte containing LiTFSI has the highest ionic conductivity and the lowest activation energy of Li+ migration.The role of LAGP and the transport mechanism of Li-ion in solid composite electrolyte were analyzed using electrochemical impedance spectroscopy and morphology techniques.The results showed that Li+ions can go through the interface between ceramic particles and polymer.Compared with other ceramic fillers(SiO2,Al2O3),the addition of Li+ conducting LAGP improves ionic conductivity and electrochemical stability of PEO-based solid composite electrolyte.LAGP glass-ceramic improves solid composite electrolyte conductivity not only by enhancing the amorphous PEO phase,but also via its intrinsic conductivity.The influence of the conductivity of the composite cathode and the ionic conductivity of the solid composite electrolyte on the electrochemical properties of LFP all solid state batteries were studied.The results showed that when the composite cathode containing the same electrolyte as the solid composite electrolyte,it is helpful to obtain higher discharge capacity at 25 ?.After adding an appropriate amount of PVDF binder in composite cathode,the discharge capacity is slightly reduced,but it has the best cycle stability and rate performance.The cell with containing LAGP-PEO-Li TFSI-PVDF composite cathode had a capacity retentive ratio was 96%after 50 cycles when cycled at 0.05 C,and the stable capacity of 105mAh/g at 1C rate.In addition,active substance content in the composite cathode can reach 77%.The influence of the solid composite electrolyte on the electrochemical properties of Li/SCE/LFP cell is positively related to the ionic conductivity of itself.The higher the ionic conductivity,the better the cycle performance and the rate performance.At 60?,due to the ion conductivity of solid composite electrolyte and the electrochemical conductivity of composite cathode is greatly enhanced;the battery performance dependson the relative velocity and the stability of interface.