Study On Metal-Organic Frameworks/PEO Composite Solid Polymer Electrolyte And Its Solid-State Battery

With the continuous development of society,problems such as shortage of non-renewable energy and environmental pollution are becoming increasingly serious.It will be the inevitable trend of future development to develop green and sustainable lithium ion batteries with the booming of portable electronic equipment and electric vehicles.However,the limited energy density and hidden safety hazards of lithium ion batteries using liquid electrolyte in the commercial market limit their further development.To solve the energy density and safety problems,the development of all-solid lithium-metal batteries using solid electrolyte is of vital important.Solid polymer electrolyte is one of the most mature solid electrolytes,which has the advantages of easy manufacturing and good compatibility with electrode.However,it has not been largely commercialized due to the challenges of insufficient ionic conductivity at room temperature,low migration number of lithium ions and limited inhibition of lithium dendrite.In view of the shortage of polymer-based solid electrolyte,in this paper,two kinds of metal-organic framework materials were prepared and added as fillers with lithium salts into poly(ethylene oxide)-based electrolyte to prepare composite solid polymer electrolyte(CPE).Scanning electron microscope,energy dispersive X-ray spectroscopy,differential scanning calorimetry,Zeta potential,mechanical stretching,thermogravimetric analysis and Fourier transform infrared spectrometer were used to characterize the dispersibility of the filler and the structure,mechanical and thermal properties of the CPE.The electrochemical properties of the CPE were studied by electrochemical impedance spectroscopy and linear sweep voltammetry.Finally,the composite solid polymer electrolyte was applied to Li Fe PO₄and Li Ni_0.8Co_0.1Mn_0.1O₂all-solid lithium metal batteries.In this paper,a kind of metal-organic framework nanoparticle 2-methyl imidazole zinc salt(ZIF-8)with high specific surface area was firstly prepared,and a composite solid polymer electrolyte ZIF-8-CPE was then prepared by solution casting method with lithium bis(trifluoromethanesulphonyl)imide(Li TFSI)and PEO.The addition of ZIF-8 nano-fillers reduces the crystallinity of the electrolyte and improves its mechanical strength.The Lewis acid site on the surface of ZIF-8 can adsorb TFSI^-anion,which is beneficial to the dissolution of lithium salts.When adding ZIF-8 nanofillers at mass 15%relative to PEO,the ZIF-8-CPE exhibits the highest ionic conductivity,up to 4.1×10^-4S/cm at 60?,higher electrochemical stability window(4.9 V)and higher lithium ion transference number(0.36).Besides,the ZIF-8 can adsorb residual solvent molecules in electrolyte,which gave ZIF-8-CPE good stability against lithium,so it can be cycled for more than 300 h at a current density of 0.1m A/cm²in Li symmetric cells.The Li Fe PO₄all-solid-state lithium metal battery shows good rate capability and cycling performance.In detail,the cells delivers specific discharge capacities of 142.1,137.8,132.9,126.5 and 109.8 m Ah/g at 0.05,0.1,0.2,0.5 and 1.0 C ratio,respectively,which is measured at 60?.Also,When cycled at 0.5 C ratio for over 350circles,the highest specific discharge capacity is 131.2 m Ah/g and capacity remained is as high as 84.8%.On this basis,a metal-organic framework ZIF-90-g-IL grafted with imidazolium quaternary ammonium salt ionic liquid was further prepared,and employed as filler to prepare PEO-based polymer electrolyte.By grafting ionic liquid,the excellent properties of the nano-fillers were inherited,and the negative effects of adding liquid ionic liquid were avoided.The composite solid polymer electrolyte Z9IL-CPE has a high ionic conductivity,which can reach1.17×10^-4S/cm at 30?.The ZIF-90-g-IL nano-fillers in the electrolyte can chelate with the PEO chain segment,which improves the electrochemical stability window to more than 4.8 V,and enhances the lithium ion transference number to 0.44.In addition,the chelation of the nanofillers with PEO chain segments makes more uniform dispersion of the nanofillers,so the Z9IL-CPE can inhibit the growth of lithium dendrites well,and it can be cycled for more than700 h at 60? and 0.1m A/cm²current density in Li symmetric cells.Besides,the Li Fe PO₄all-solid-state lithium battery was assembled and obtained good rate performance and cycling performance,the battery can cycle stability for over 500 circles at 60? and 2.0 C ratio,with the highest specific capacity of 141.6 m Ah/g and a capacity retention of 71.3%.The excellent performance of Z9IL-CPE enabled it to cycle stability for 50 circles at 30? and 0.1 C ratio,with the specific capacity of 145.4 m Ah/g and a capacity retention of 96.8%,and it can adapt to Li Ni_0.8Co_0.1Mn_0.1O₂cathode materials.The application of Z9IL-CPE in pouch cell shows considerable cycling performance and excellent safety.The method of using metal organic framework as the filler to modify PEO-base solid polymer electrolyte in this article can improve the ionic conductivity,electrochemical stability window,lithium ion transference number and chemiccal stability to lithium metal,and the cell with modified PEO-base solid electrolyte can achieve high capacity,room temperature circulation,4.2 V high voltage circulation and practical application in pouch cell.This article provides a reference to the research of composite solid polymer electrolyte.