| Over the past 30 years, solid polymer electrolytes（SPEs） have been intensively studied as electrolytes for application in solid-staterechargeable lithium metal batteries（LMBs）. This is mainly motivated by their superior advantages over conventional liquid electrolytes, such as good flame-resistance, excellent flexibility and low cost in design, functioning as a separator, as well asmitigating the lithium dendriticgrowth.So far, the conventional lithium bis（trifluoromethanesulfonyl）imide（Li[N（SO2CF3）2], Li TFSI） / poly（ethylene oxide）（PEO）electrolyte has been most extensively studied. This is mainly attributed to several intrinsic features of TFSI-, including（1） the high flexibility of –SO2–N–SO2– of TFSI-, being favorable for reducing the crystallinity of PEO matrix（plasticizing effect）;（2） the highly delocalized charge distribution of TFSI-, being pivotal for effectively reducing the interactions between Li+ and TFSI-, thus increasing the dissociation and solubility of Li TFSI in PEO matrix;and（3） excellent thermal, chemical and electrochemical stability, being required for stable electrolytes.Whilst, it has been reported that the membrane of the SPEs using Li TFSI is very sticky and difficult to process; and the ionic conductivity at ambient temperature（20–60 oC） is low（ca. 10-7–10-4 S cm-1） due to the semi-crystalline nature of PEO; andthe compatability of Li TFSI/PEO electrolyte with Li metal electrodeis relatively poor.In recent years, lithium bis（fluorosulfonyl）imide（Li[N（SO2F）2], Li FSI）,an analogue of Li TFSI, has been intensively studied as electrolyte salt for lithiumandlithium-ion batteries, because of its excellent compatibilitywith various electrodes（e.g., Li metal and graphite）in conventional liquid carbonates,room temperature ionic liquids, and ambient temperature molten salt electrolyte systems. It has been demonstrated that FSO2– group of bis（fluorosulfonyl）imide（[N（SO2F）2]–, FSI–） anion has a significant impact on the improvement of electrode/electrolyte interface, though the mechanism behind this improvement still remain to be understood well. Thus far, little has been done on FSI-based SPEs.With above known knowledge in minds, we wish to report the fundamental physicochemical and electrochemical properties of FSI-based SPEs. The main results of this work could be summarized as follow:1. To understand the effect of Li FSI on fundamental physicochemical and electrochemical properties of SPEs, the conventional SPEs system, lithium salt/PEO electrolyte, has been investigated.New SPEs comprising of Li FSI andhigh molecular weight PEO（number-averagemolecularweight（Mn） = 5 × 106 g mol-1） have been prepared and characterized, and are comparatively studied with the representative SPEs, Li TFSI/PEO, at a molar ratio of [EO]/[Li+] = 20. Their physicochemical properties have been investigated in terms of phase transition behavior, ionic conductivity, lithium-ion transference number（t Li+）, electrochemical stability, and with particular attention tothe interfacial behavior between Limetal electrode and SPEs. It has been demonstrated that（1） Li FSI/PEO electrolyte shows low glass transtion temperature and crystallinity;（2） the ionic conductivities of Li FSI/PEO electrolyte are higher than those of the Li TFSI/PEO electrolyte above 60 oC, and exceed 10–3 S cm–1 at 80 oC;（3） compared with the Li symmetric cell（Li |SPEs|Li）using Li TFSI/PEO electrolyte, the Li symmetric cellusing Li FSI/PEO electrolyte shows lower interface resistances（Ri） in rest condition（Ri = 79 Ω cm2（Li FSI/PEO） vs. Ri = 352 Ω cm2（Li TFSI/PEO） after storage at 80 oC for 30 days）;（4）the Li symmetric cellusing Li FSI/PEO electrolyte shows good electrochemical stability with Li metal electrode（stable cyclingof 253 hfor Li FSI/PEO vs. 55 hfor Li TFSI/PEOat 80 oC under a current density of 0.1 m A cm–2）;（5） the Li/Li Fe PO4 cell using Li FSI/PEO electrolyte exhibits good cycling performance and high coulombic efficiency at 80 oC.2. To overcome the low ionic conductivity of the semi-crystalline PEO-based SPEs in room temperature region of 20 to 60 oC, a new type of amorphous polymer, polymeric ionic liquid（PIL）, has been studied as polymer matrix for SPEs. Poly（1-vinyl-3-methylimidazolium） bis（fluorosulfonyl）imide（PVy MIM-FSI） has been prepared by polymerization of the ionic liquid monomer（ILM）, 1-vinyl-3-methylimidazolium bis（fluorosulfonyl）imide（Vy MIM-FSI）. Its structure and composition has been characterized by 1H and 19 F NMR, and viscosity-average molecular weight（Mv）. For comparison, the corresponding PIL, poly（1-vinyl-3-methylimidazolium） bis（trifluoromethanesulfonyl）imide（PVy MIM-TFSI）, is also prepared and characterized. The physicochemical properties of the binary composite electrolytes of Li FSI/PVy MIM-FSI with various concentrations of Li FSI have been comparatively studied with those of the corresponding Li TFSI/PVy MIM-TFSI system, in terms of DSC, TGA, ionic conductivity, and lithium-ion transference number（t Li+）. It has been shown that（1） while both types of the prepared PILs show significant decrease in glass transition temperature（Tg） upon addition of lithium salt, the values of Tgfor the composite electrolytes of Li FSI/PVy MIM-FSI are all significantly lower by 20-60 oC in magnitude than those of the corresponding Li TFSI/PVy MIM-TFSI composite ones, indicating much better plasticizing effect for FSIvs. TFSI-;（2）the ionic conductivities of Li FSI/PVy MIM-FSI are all higher by about 2 orders in magnitude than those of the corresponding Li TFSI/PVy MIM-TFSI with the same concentration of lithium ions;（3） the ionic conductivity of Li FSI/PVy MIM-FSI at 30 oC is higher than that of Li FSI/PEO（[EO]/[Li+] = 20）, suggesting thatamorphous nature of PILs is favorable for improving the ionic conductivity of SPEs.3. To obtain the PILs with desired physicochemical properties（e.g., good mechanical strength and high ionic conductivity）,novelpolymeric ionic liquids（PILs） based on alkyl（–CH2–） and alkyl ether（–O–） substituted ammoniums andperfluorinated sulfonimides（i.e., FSI-and TFSI-） have been synthesized and characterized. The PILs were synthesized by polymerization of the correspondingionic liquid monomers（ILMs）, andtheir structuresand compositions have been characterized by 1H and 19 F NMR, FTIR and viscosity-average molecular weight（Mv）. The physicochemical properties of both the ILMs and the PILs have been studied in terms of thermal stability, phase transition, and ionic conductivity. It has been shown that（1） all obtained ILMs and PILs reveal excellent thermal stabilities to greater than 250 oC;（2） the PILs containing alkyl ether side unit show significant decrease in glass transition temperature（Tg）, the Tgvalues of the alkyl ether based-PILs are all significantly lower by 10-77 oC in magnitude than those of the corresponding alkyl based ones;（3） alkyl ether based-PILs show ionic conductivity of4.0 × 10–6 S cm–1at 30 oC and 7.6 × 10–5 S cm–1at 60 oC, and outperform their ammoniumcounterparts with alkyl side chain that were synthesized as references.4. To investigate the physicochemical and electrochemical properties of SPEs using the above ammonium-based PILs as polymer matrix, novelsolid polymer electrolytes（SPEs） comprised of Li FSI and poly[N,N-dimethyl-N-[2-（methacryloyloxy）ethyl]-N-[2-（2-methoxyethoxy）ethyl]ammonium] bis（fluorosulfonyl）imide（P[C5O2NMA.11]FSI） have been prepared and characterized. For comparison, the corresponding SPEs, Li TFSI/poly[N,N-dimethyl-N-[2-（methacryloyloxy）ethyl]-N-[2-（2-methoxyethoxy）ethyl]am monium]bis（trifluoromethylsulfonyl）imide（P[C5O2NMA.11]TFSI）,havealso been prepared and characterized. Their physicochemical properties have been investigated in terms of phase transition behavior, ionic conductivity, lithium-ion transference number（t Li+）, electrochemical stability, and the interfacial behavior between lithium metal electrode and SPEs. It has been demonstrated that（1） the glass transition temperaturesof both Li FSI/P[C5O2NMA.11]FSI and Li TFSI/P[C5O2NMA.11]TFSI electrolytes gradually increase with increasing the concentration of lithium salt;（2） the ionic conductivities of Li FSI/P[C5O2NMA.11]FSI electrolyte are higher than those of the corresponding TFSI-based one;（3） the anodic stability of Li FSI/P[C5O2NMA.11]FSI electrolyte is 4.5 V（vs. Li/Li+）, which is well acceptable for 4 V class rechargeable LMBs;（4） the interfacialresistances of Li symmetric cell using Li FSI/P[C5O2NMA.11]FSI electrolyte are much lower than those using Li TFSI/P[C5O2NMA.11]TFSI electrolyte. |
