| With the increasing demand for high energy and power density energy storage device in modern society,traditional lithium ion batteries with carbon-based materials as anode materials gradually fails to meet the energy density of the practical application requirements.Lithium metal possesses a high theoretical specific capacity(3860 mAh g-1)and the lowest electrochemical potential?-3.04 V vs.SHE?,and therefore is one of the most promising anode materials for next-generation high energy density batteries.Recently,researchers have conducted in-depth research on lithium metal secondary batteries?LMSBs?using lithium metal as anode.However,the uneven deposition of lithium ions caused by the concentration polarization during charge/discharge process of the LMSBs based on non-aqueous electrolytes may result in lithium dendrites on the surface of the lithium anode,which leads to attenuation of performance and safety issues.In addition,the intrinsic instability,flammability and leakage of liquid electrolyte can also cause a series of safety issues of LMSBs.At present,the methods for inhibiting the formation and growth of lithium dendrites in LMSBs mainly include reducing the deposition current density,improving the uniformity and compactness of SEI,inhibiting lithium dendritic growth and prolonging the formation of the"ion depletion layer".Among them,the single ion conducting polymer electrolytes?SIPEs?with high ionic conductivity can alleviate the issue of concentration polarization,inhibit the formation of lithium dendrites and prevent the leakage of organic solvents,thus enhancing the cycling performance and security of LMSBs.So it is considered as one of the main ways to solve the issues existent in LMSBs and improve energy density.Therefore,the preparation of SIPEs and its application in LMSBs have attracted extensive attention from some researchers.Although the addition of organic solvents as plasticizers in SIPEs can improve the ionic conductivity of electrolytes at room temperature and enahnce the cycling stability of LMSBs,there is still a gap to meet the requirement of practical applications,and safety issues caused by organic solvents still exist.The research on solid SIPEs only stays on the discussion of basic electrochemical properties,and the ionic conductivity at room temperature is relatively low,which is not enough to support the operation of LMSBs at room temperature.In addition,the working temperature of the assembled LMSBs is too high,and the in-depth study of the practical application of solid SIPEs in LMSBs is still lacking,which is far from meeting the requirement of practical applications.In addition,it still lacks certain theoretical support of the mechanism of SIPEs inhibiting the formation and growth of lithium dendrites and improving the interfacial stability of batteries,which needs to be further explored.Therefore,further study of the structure-function relationship affecting the ionic conductivity of SIPEs and the mechanism of inhibiting the formation and growth of lithium dendrites for the preparation of polymer electrolytes with high ionic conductivity,high lithium ion transference number and high security performance and long cycle life may have theoretical guiding significance and value of practical application.In this paper,the molecular structure design and the membrane structure construction are used to improve the ionic conductivity of SIPEs and the interface stability,so as to obtain a novel SIPE membrane that can improve the stability and safety of LMSBs.The research content is as follows:In order to further improve the ionic conductivity of the SIPEs,a poly?arylene ether?-based single ion polymer conductor?LiPHFE?was designed and prepared,and the organic solvent?EC/DMC?was used as a plasticizer to enhance the dissociation and migration ability of lithium ion.Therefore the ionic conductivity was increased to 4.1×10-44 S cm-1 at room temperature.The effects of polymer chain structures?polyarylene ether and polyamide?on the properties of mechanical,electrochemical and battery performance were investigated.The thermal stability,mechanical properties and micromorphology of the electrolyte membranes were also discussed.Electrochemical properties and battery performance were tested and analyzed,respectively.The results indicate that the poly?arylene ether?blend film is superior to the polyamide.The assembled LiFePO4/Li and Li4Ti5O12/Li with LiPHFE film as an electrolyte and a separator were charged and released 800 times at 1 C with almost no capacity attenuation,showing good cycling stability.In view of the low ionic conductivity of LiPHFE membrane and the low discharge capacity of the assembled battery,a three-dimensional porous network structure was constructed in the preparation process of poly?arylene ether?SIPE membrane via microphase separation,so as to form a continuous ion transport channel to improve the migration rate of lithium ions.The structure-function relationship between the formation of pores and anionic polymer properties was also discussed.The results show that it is the polarity of polymer rather than carrier concentration that dominates the pore-forming in microphase separation process of polymer electrolyte membrane.When certain organic solvents?EC/DMC?were absorbed,the ionic conductivity of LiPBIE gel electrolyte reached 5.2×10-44 S cm-1 at 25 oC,and the results of lithium stripping/deposition stability test showed that the prepared polymer electrolyte was able to inhibit lithium dendrites.A battery of LiFePO4/Li incorporating the electrolyte film exhibits excellent rate performance and electrochemical stability.Considering that the existence of porous structure may reduce the mechanical properties of SIPEs membrane and limits the further improvement of ionic conductivity,nano-particles of LAGP,an inorganic electrolyte with high ionic conductivity and stability,are filled into the SIPEs to construct and organic-inorganic composite SIPE membrane.The introduction of LAGP nanoparticles not only enables the phase separation space between polymer chain and inorganic nanoparticles to form ion transport channels after the absorption of organic solvent?EC/DMC?,but also improves the carrier concentration of the system,thus further improving the migration rate of lithium ions.When the content of LAGP nanoparticles is 30%,the maximum ionic conductivity of the composite electrolyte film reaches 8.3×10-44 S cm-1?30 oC?.The results of lithium stripping/deposition stability test of lithium symmetric battery show that the presence of LAGP nanoparticles can improve the ability of the gel SIPE membrane to inhibit lithium dendrites growth,thus improving the interface stability.Therefore,the assembled LiFePO4/Li battery displays a discharge capacity of 141 mAh g-1at 1 C and a capacity retention rate of 93%after 900 cycles,which is much better than the commercial LiPF6 liquid electrolyte.In view of the flammability of organic solvents in gel SIPEs,polyethylene glycol?PEG?chain was introduced into SIPEs to replace the solvation and dissociation of lithium ions by organic solvents,so as to realize all-solid state SIPEs.The lithium bis?benzene sulfonyl?imide super-delocalization property serves as the carrier,while polyethylene glycol with different chain lengths?EO,Mw=200,400,600,800 or 1000?serves as the lithium ion conduction region.The alternating block copolymerization structure inhibites the agglomeration of ionic group and the ethoxylated chain,leading to the reduction of the crystallinity of the solid SIPE.The relaxation time of the polymer segment is shortened,thus enhancing th migration ability of lithium ion.When the molar ratio of[EO]/[Li+]is 23.7:1,the polymer electrolyte owes the lowest glass transition temperature and the highest ionic conductivity,and the ionic conductivity is6.61×10-6 S cm-1 at 30 oC and 2.24×10-6 S cm-1 at 100 oC,respectively.Then the composite single ion electrolyte membrane with good mechanical properties was prepared by filling the polymer electrolyte with glass fiber membrane.The stability test of lithium stripping/deposition indicated that the prepared composite single ion electrolyte membrane had a good ability to inhibit lithium dendrites.The assembled all-solid battery of LFP/Li with composite single-ion electrolyte membrane as electrolyte and membrane can exhibit good performance at operating temperatures as low as 40oC.The battery delivers 102 mA h g-1 at 0.1 C and is stabilized at 94 mA h g-1 after 200cycles with the capacity retention rate around 92.2%. |
PDF Full Text Request