| The inherent low ionic conductivities of Zinc(Zn)-ion conducting solid polymer electrolytes(SPEs)limit their application into practical Zn-ion battery to some degrees.Therefore,it is necessary to design new Zn-ion conducting SPEs,including Zn salt and polymer matrix,with improved properties.Compared with traditional salts such as Zn Cl2,Zn SO4,Zn(Cl O4)2 and so on,Zn salts with fluorinated sulfonimide anions are highly desired and it is expected to get some excellent electrochemical performances to serve them as conducting salts of SPEs for Zn battery.Because these fluorinated anions have flexible molecular structure,highly delocalized negative charge and weakly coordinating ability,which are beneficial to improve the dissociation of Zn salts.Of all the polymer matrix reported,polyethylene oxide(PEO)is the most widely studied one for various SPEs over the past40 years.Because it has a low glass transition temperature(-64 oC)and highly flexible molecular chain.However,crystalline region existing in PEO hinders the migration of cation,thus causing low conductivity in ambient temperature region.On the other hand,poly(ethylene carbonate)(PEC)is amorphous and has strong ability to dissolve salts because of the high polarity of carbonate group(–OCO2–).And the dissociation of salt in PEC is therefore anticipated to be high.So PEC is highly expected to be utilized as new matrix to enhance the conductivity of SPEs.On the base of above consideration,we have prepared two kinds of Zn salts with fluorinated anions,including zinc trifluoroacetate(Zn(CF3CO2)2)and zinc bis(trifluoromethylsulfonyl)imide(Zn[N(CF3SO2)2]2,Zn(TFSI)2).The chemical structures and compositions of the as-prepared salts have been characterized by NMR and FT-IR.Then new Zn-ion conducting SPEs,Zn X2/PEO(X=CF3CO2,CF3SO3,TFSI),with various mole ratios of[Zn2+]:[O]have been prepared by solvent casting method.The dependence of ionic conductivities of Zn(TFSI)2)/PEO electrolytes with various molar ratios of[Zn2+]:[O]have been investigated and optimized.Three kinds of Zn salts,Zn X2(X=CF3CO2,CF3SO3,TFSI),are employed as conducting salts.And the fundamental physical and electrochemical properties of the corresponding PEO-based SPEs,including phase transition behavior,conductivity,Zn-ion transference number,anodic stability and Zn deposition behavior,have been comparatively studied.The new PEC-based Zn-ion conducting SPEs,Zn(TFSI)2)/PEC electrolytes,have also been prepared and the phase transition behaviors and conductivities of Zn(TFSI)2/PEC and Zn(TFSI)2/PEO have been comparatively studied for illustrating the effect of polymer matrix on the fundamental properties of SPEs.The main results of this work could be summarized as below:(1)All the electrolytes of Zn(TFSI)2/PEO with various molar ratios of[Zn2+]:[O]([Zn2+]:[O]=1:50?1:8,by molar ratio,the same meaning as below)have low glass transition temperatures(Tg=-20?-11 oC),which displays the highly chain segmental motion ability.And the electrolyte of Zn(TFSI)2/PEO at[Zn2+]:[O]=1:20 reaches the maximum conductivity(e.g.,?=1.20×10-3 S cm-1,at 100 oC).(2)Zn(TFSI)2,as Zn-ion conducting salt,displays much improved ion-conducting properties over the conventional ones including Zn(CF3CO2)2 and Zn(CF3SO3)2.Among the three PEO-based electrolytes,the Zn(TFSI)2/PEO electrolyte exhibits the highest conductivity in each measured temperature(25?100 oC),which is at least one order of magnitude higher than the other two ones(e.g.,at 100 oC,Zn(TFSI)2/PEO(1.20×10–3 S cm–1)vs.Zn(CF3CO2)2/PEO(1.67×10–5 S cm–1)and Zn(CF3SO3)2/PEO(1.12×10–4 S cm–1)).For the electrolyte of Zn(TFSI)2/PEO at[Zn2+]:[O]=1:20,it displays a value of 0.32 for Zn-ion transference number(t+)and a strong anodic stability up to 3.01 V vs.Zn/Zn2+.And the deposition/dissolution of Zn metal can be observed on the stainless steel electrode in the electrolyte.(3)Much better conductivities have been obtained for the electrolytes of Zn(TFSI)2/PEC vs.Zn(TFSI)2/PEO.In each measured temperature(25?100 oC),the highest conductivity of Zn(TFSI)2/PEC electrolyte is at least one to three orders of magnitude higher than that of the Zn(TFSI)2/PEO one.At 30 oC,the ionic conductivity of Zn(TFSI)2/PEC with[Zn2+]:[O]=1:4 is 1.55×10-4 S cm-1,which is two orders of magnitude higher than that of 1.70×10–6 S cm–1for Zn(TFSI)2/PEO([Zn2+]:[O]=1:20). |
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