Research On Modification Of Polyethylene Oxide(PEO)-Based Polymer Electrolytes In Solid-state Lithium Metal Batteries

Safe and high energy density lithium-ion batteries(LIBs)have been widely used in portable electronic device,electric vehicles(EVs)and grid energy storage.However,the inherent limitations of LIBs limit the further improvement of energy density.Lithium metal(Li)has been regarded as one of the best candidate material for anode in terms of the highest theoretical capacity(3860 m Ah g^-1)and the lowest electrochemical potential(-3.04 V vs.standard hydrogen electrode).However,with the increase of energy density of lithium-based batteries,safety risks become more serious,such as electrolyte leakage,fire and explosion originating from combustible organic solvents(carbonates and ethers)in liquid electrolytes.The development of high performance electrolyte materials is of great significance to improve the safety and electrochemical performance of lithium metal batteries(LMBs).Solid-state polymer electrolyte have many advantages,such as low flammability,flexibility,easy processability,tolerance to shock and mechanical deformation compared with liquid electrolytes.Moreover,polymer electrolytes exhibit better interface contact and compatibility with Li compared with inorganic electrolyte.Therefore,the replacing conventional liquid electrolytes with solid-state polymer electrolytes has been considered as an effective method to solve these safety issues.Among various polymer matrix,polyethylene oxide(PEO)have attracted great interest for its good chemical compatibility with lithium metal,good solubility of lithium salt,easy processability and low cost.In 2011,French company Bollorélaunched electric vehicle(Bluecar)based on Li/PEO/Li Fe PO₄ battery.However,there are problems such as high battery operating temperature requirement(70?80?)and low energy density.In view of these existing problems of low ionic conductivity at room temperature,narrow electrochemical window and poor cycling performance of PEO-based polymer electrolytes,several feasible solutions are proposed,as follows:(1)Composite polymer electrolyte(CSPE-Ce-MOF)with excellent performance was prepared by solution casting method by combining Ce-MOF nanoparticles with rich catalytic active sites or defects with polyethylene oxide(PEO).Ce-MOF filler distributed in the PEO matrix is regared as the crosslinking center,which can prevent the recombination crystallization of PEO and reduces the crystallization ratio.A large number of open metal sites in Ce-MOF show abundant catalytic active sites or defects,making ether oxygen groups(EO)and lithium anions(TFSI^-)more likely to interact with Lewis acid sites on the inner and outer surface of Ce-MOF,releasing more free Li⁺.In addition,TFSI^-can be restricted in Ce-MOF nanopores to further promote the transport of Li⁺in electrolyte and improve the ionic conductivity and l Li⁺transference number.With the addition of 10 wt%Ce-MOF,the ionic conductivity of the CSPE at room temperature is 3.0×10^-5 S cm^-1,which is an order of magnitude higher than that before modification(PEO-Li TFSI,6.7×10^-6S cm^-1),the Li-ion transference number is0.75,and the electrochemical window reaches 4.5 V.The excellent flexibility and thermal stability of the CSPE effectively inhibit the growth of lithium dendrites,and can cycle at current density of 0.4 m A cm^-2 at 60?without short circuit.Li/Li Fe PO₄and Li/Fe F₃ all solid-state lithium metal batteries based on CSPE-Ce-MOF show excellent cycle stability and rate performance.The addition of Ce-MOF in polymer electrolytes is expected to stabilize the cathode side and alleviate the change of its volume and the dissolution of active materials,especially in the conversion reaction.(2)Composite polymer electrolyte(CSPE-EACe₂)with excellent performance was obtained by combining amorphous metal-polyphenol coordination polymers(AICPs,EACe₂)with defect enrichment and high surface energy with PEO.On the one hand,amorphous EACe₂ particles with high surface energy tend to adhere and self-assemble into a network structure with better penetration in PEO,helping to improve the overall mechanical strength of the electrolyte.On the other hand,EACe₂ with rich catalytic activity or defects has strong Lewis acid-base interaction with EO in PEO and anions in lithium salt,increasing the concentration of free Li⁺in electrolyte and enhancing the ionic conductivity of electrolyte.The ionic conductivity of the modified electrolyte at room temperature is 2.03×10^-5 S cm^-1 and Li⁺transference number is 0.47.In addition,the electrochemical window is raised to 4.8 V,and a stable protective layer rich in Li F,Li₂O and Li₂S is formed between the electrolyte and Li,which can effectively inhibit the growth of lithium dendrites and achieve stable deposition stripping for 8800 hours at 0.1 m A cm^-2 at 60?.Low electrolyte/electrode interface impedance(CSPE-EACe₂/Li,16?cm²;CSPE-EACe₂/Li Fe PO₄,20.5?cm²)can realize highly reversible deposition of Li/Li Fe PO₄ all solid-state lithium metal batteries.Even in Li/NCM811 battery system also shows good electrochemical performance.(3)Composite polymer electrolyte(CSPE-Ce F₃)was prepared by combining two-dimensional(2D)sheet Ce F₃ with PEO.Compared with fillers with other structures(0D and 1 D),2 D Ce F₃ fillers have a larger specific surface area,which can form ion transport network in polymer electrolyte,improving the ionic conductivity of the electrolyte.The modification mechanism of Ce F₃ filler for polymer electrolyte is shown in the following aspects.On the one hand,the larger interaction interface between 2D Ce F₃ filler and PEO can promote the conformational freedom of PEO chains,reduce the crystallinity of PEO,and improve the transport of Li⁺in electrolyte.On the other hand,the lithiophilic Ce F₃ nanosheets can realize the rapid adsorption and desorption of Li⁺,promote the dissociation of lithium salts,improve the free Li⁺concentration in the electrolyte,and realize the rapid transport of Li⁺in the electrolyte.The electrochemical properties of the modified polymer electrolyte are greatly improved:the ionic conductivity is increased to 3.08×10^-5 S cm^-1 at room temperature and the electrochemical window reaches 4.8V.Li/CSPE-Ce F₃/Li symmetric battery can stable cycle for more than 8000 hours at 0.1 m A cm^-2 at 60?,effectively inhibiting the growth of lithium dendrite.The batteries based on CSPE-Ce F₃ and different cathode materials:Li/CSPE-Ce F₃/Li Fe PO₄,Li/CSPE-Ce F₃/Fe F₃ and Li/CSPE-Ce F₃/NCM811all solid-state lithium metal batteries show good cycling and rate performance.(4)Green composite polymer electrolyte(CSPE-CD-MOF)with excellent electrochemical performance was obtained by combining"green"CD-MOF nanoparticles with a large number of hydroxyl groups with PEO.On the one hand,with the help of the ordered pores of"green"CD-MOF and a large number of hydroxyl groups on the surface,hydrogen bonds can be formed with PEO chains,which changes the arrangement of polymers and reduces the crystallinity of PEO.Meanwhile,the hydroxyl group of CD-MOF forms hydrogen bond with EO of PEO,which reduces the interaction strength between EO and Li⁺and promote Li⁺transport along the PEO chain.On the other hand,hydrogen bonds can also be formed between lithium anions(TFSI^-)and CD-MOF,which can bind TFSI^-to promote the disintegration of lithium salts and increase the proportion of free Li⁺in the electrolyte,thus improving the ionic conductivity and lithium ion transference number of the electrolyte.Ion conductivity of polymer electrolyte modified by CD-MOF is 2.75×10^-5 S cm^-1,which is an order of magnitude higher than that before modification and electrochemical window is raised to 5 V.The addition of CD-MOF effectively improved the inhibition of polymer electrolyte on lithium dendrite,and achieved stable Li⁺plating and stripping for 2000hours at 0.1 m A cm^-2 at 60?.The addition of CD-MOF filler into polymer electrolyte greatly improves the electrochemical performance of the batteries,especially for the conversion reaction battery system.