Investigation On Novel Sulfonimide Lithium Salts:Synthesis,Characterization,and Application In Rechargeable Lithium Metal Batteries

In recent years,with the rapid development of electric vehicles and large-scale energy storage power grids,the lithium-ion batteries(LIBs),comprising nonaqueous electrolyte,intercalation cathode and graphitized carbon anode,are hard to meet the application requirements of the next-generation electric cars and smart power grids,due to its maximum energy density limited to about 350 Wh kg-1.Lithium(Li)metal has been studied as an anode electrode for rechargeable Li metal batteries(LMBs)worldwide,by virtue of its extremely high theoretical specific capacity(3860 mA h g-1),low molecular weight(6.94 g mol-1),and the lowest negative electrochemical potential(-3.040 V vs.the standard hydrogen electrode).Especially for advanced rechargeable LMBs,Li-sulfur(Li-S)and Li-air,have captured an unprecedented attention of academic research.Among various cathodes,the sulfur is a very promising one for the secondary battery for acquiring high energy density,due to its significant advantages of abundant resources,low cost,environmental benignity and high theoretical specific capacity(1675 mA h g-1).Theoretically,the energy densities of Li-S batteries can attain as high as 2600 Wh kg-1 or 2800 Wh L-1,based on weight or volume,respectively,which are 3-5 times higher than those of state-of-the-art LIBs,and could satisfy the application requirements of the next-generation electric cars and smart power grids.All of the above results suggest that the Li-S batteries would be a promising "green batteries" with excellent social benefits for application in practical large-scale scenarios.Electrolyte is a key component of Li-S batteries,and plays an important role in dictating the electrochemical performances of Li-S batteries,through functionalizing as Li-ion conductors between the two electrodes and as tailors of electrode/electrolyte.To date,earlier work on the liquid and solid polymer electrolytes(SPEs)for Li-S batteries has almost concentrated on lithium bis(trifluoromethanesulfonyl)imide(Li[(CF3SO2)2N],LiTFSI)as a conducting salt,owing to the unique feature of TFSI-anion,that is,a high degree of negative charge distribution,excellent flexibility of the structure,and good thermal,chemical and electrochemical stability.However,it is revealed that LiTFSI has certain drawbacks as a conducting Li salt for rechargeable LMBs,particularly its poor interfacial stability towards Li metal electrode.This has significantly reduced the actual cycling life of Li-S batteries.Therefore,it is extremely urgent to seek for suitable conducting Li salts for improving the stability of electrode/electrolyte interphases and performance of rechargeable LMBs.This has become the current hotspot in the research of the development of Li-S batteries.Additionally,with the safe accidents being usually reported in conventional LIBs,the safe concerns on the more reactive Li metal based,high-energy-density LMBs have attracted more attention.SPEs are becoming the research focus of electrolyte in rechargeable LMBs.This is mainly motivated by their superior advantages over conventional liquid electrolytes,including the enhanced safety,non-flammability,non-leakage,and excellent design flexibility,as well as mitigating Li dendritic growth.However,conventional SPEs formed by dissolving Li salt with a small anion in a polymer host,are dual-ion conductors,in which both cations and anions are mobile similarly to liquid electrolytes.In fact,rocking-chair-type Li batteries only involve the electrode reactions of intercalation/deintercalation and plating/stripping with Li cations,and the mobility of the anions can cause the concentration gradients.At this case,practical rechargeable LMBs using these conventional binary salt electrolytes will eventually result in premature failure of battery,due to the concentration gradients of the salt and cell polarization.Therefore,it is necessary to design and prepare the single Li-ion conducting solid polymer electrolytes(SLIC-SPEs),in which the anions are immobilized,and Li-ion transference number(tLi+)is increased to unit.Although the advantages of SLiC-SPEs in tLi+have been well understood,the low Li-ion conductivities and poor interfacial stabilities with Li metal electrode for most of the reported SLIC-SPEs are still a ceiling limiting the practical application of SLIC-SPEs in rechargeable LMBs.Therefore,it is extremely urgent to design new anion with potential ability for effectively overcoming the above two drawbacks.On the basis of the above research background and understanding,we wish to explore the systhesis and characterization of new sulfonimide Li salts,as well as their applications in rechargeable LMBs.This would be expected from two considerations of the conducting Li salts and SPEs to improve the present interfacial and safe problems of rechargeable LMBs.In order to understand the impact of anionic structure of Li salt on the stability of electrode/electrolyte interphases and ionic conductivity of electrolytes,new sulfonimide Li salt-based liquid and solid electrolytes for application in rechargeable LMBs(especially in Li-S batteries),have been investigated.The main results of this work could be summarized as below:(1)In order to overcome the problem of poor interfacial stability between the traditional conducting Li salt(LiTFSI)and Li metal.Herein,we try to explore a new perfluorinated sulfonimide salt,namely lithium(trifluoromethanesulfonyl)(n-nonafluorob-utanesulfonyl)imide(Li[(CF3SO2)(n-C4F9SO2)N],LiTNFSI),for application in liquid Li-S batteries.The differences for the interfacial stability towards Li metal electrode and performance of Li-S batteries in LiX-1,3-dioxolane(DOL)/dimethoxymethane(DME)(X=TNFSI,TFSI)electrolytes have been comparatively analyzed.It has been demonstrated that(a)Li-S batteries:the Li-S batteries with the LiTNFSI-based electrolyte can deliver good cycling and C-rate performances,which can maintain a capacity retention rate of 62%over 400 cycles.However,the Li-S batteries with the LiTFSI-based electrolyte display inferior performances(e.g.,a relatively low capacity retention rate of 54%over 400 cycles).Meanwhile,the soft-package Li-S batteries with the LiTNFSI-based electrolyte can exhibit a relatively high initial discharge capacity of 1102.3 mA h(note:design capacity is 1200 mA h)and maintain a relatively good cycling performance almost without capacity fading after 20 cycles at 0.1 C.Moreover,the solid-electrolyte-interphase(SEI)films formed on the Li metal anode in the LiTNFSI-based electrolyte are more stable than those formed in the LiTFSI-based one,which can be verified by the electrochemical impedance spectra(EIS),scanning electron microscopy(SEM)and X-ray photoelectron spectroscopy(XPS).This would be mainly attributed to the participation of the-C4F9 group(in LiTNFSI)in forming stable electrode/electrolyte interphases;(b)Li|Li cells:the Li|Li cells with the LiTNFSI-based electrolyte can maintain a superior cycling stability over 1200 hours at a current density of 0.5 mA cnm-2.In contrast,a short circuit in the Li|Li cells with the LiTFSI-based electrolyte is observed after 570 hours;(c)Li|Cu cells:the Li|Cu cells with the LiTNFSI-based electrolyte can afford an average coulombic efficiency(CE)of 97.9%(150 cycles)at a current density of 0.5 mA cm-2,which is comparable to that in the Li|Cu cells with the LiTFSI-based one(i.e.,CE?98.3%,150 cycles).(2)Liquid rechargeable LMBs have a potential safety hazard.Herein,we utilize SPEs to solve the problem.Novel perfluorinated sulfonimide salt-based SPEs,composed of the LiTNFSI and poly(ethylene oxide)(PEO)with the different molar ratios of EO/Li+ = 8,16,20,and 30,were prepared by a facile solution-casting technique.The physicochemical and electrochemical properties of the LiTNFSI/PEO blended polymer electrolytes were intensively investigated,in terms of their phase transition behavior,XRD characterization,thermal stability,ionic conductivity,Li-ion transference number,and anodic electrochemical stability.More importantly,the cycling stability of metallic Li electrode and cycling performances of Li|LiFePO4 and Li-S cells with the LirTNFSI/PEO(EO/Li+ =20)SPE,have also been comparatively investigated with the LiTFSI/PEO(EO/Li+ = 20)SPE.It has been demonstrated that(a)fundamental properties:the LiTNFSI-based SPE exhibits a relatively efficient ionic conductivity(3.69 × 10-4 S cm-1 at 90 ?)and enough thermal stability(Td = 374 ?);(b)Li|Li cells(60 0C):the Li|Li cells with the LiTNFSI-based SPE can maintain a superior cycling stability over 400 hours and show the negligible variations in total impedance during cycling at a current density of 0.2 mA cm-2.In contrast,a short circuit in the Li|Li cells with the LiTFSI-based SPE is observed after 200 hours,and the random fluctuations of diameter of the semicircle from the EIS spectra can also be observed;(c)Li|LiFeP04 cells(60 ?):the Li|LiFeP04 cells with the LiTNFSI-based SPE can display a relatively good cycling performance(i.e.,slow capacity decay)with capacity retention rate of 72%after the following 300 cycles at 1 C.However,the obviously inferior cycling performances(only cycling for 35 cycles)and the random fluctuations of capacity retention are observed in the Li|LiFePO4 cells with the LiTFSI-based SPE after 35 cycles.Moreover,the excellent C-rate capability of the Li|LiFePO4 cells with the LiTNFSI-based SPE can also be obtained,(d)Li-S cells(60 ?):the Li-S cells with the LiTNFSI-based SPE can deliver an excellent cycling performance almost without capacity fading even after 500 cycles at 0.5 C and effectively suppress the so-called shuttle effect.In contrast,the so-called shuttle effect cannot be effectively suppressed in the Li-S cells with the LiTFSI-based SPE.The electrode/electrolyte interphases formed in the LiTNFSI-based SPE are more stable than those formed in the LiTFSI-based one,which can be verified by the EIS,SEM and energy-dispersive X-ray spectroscopy(EDS).One of the best solutions to the concentration polarization caused by the dual-ion conducting SPEs is to prepare SLIC-SPEs.Herein,we design and prepare two kinds of SLIC-SPEs(e.g.,lithium poly[(4-styrenesulfonyl)(fluorosulfonyl)imide](LiPSFSI)/PEO and lithium poly[(4-styrenesulfonyl)(trifluoromethyl(S-trifluoromethylsulfonylimino)sulf-onyl)imide](LiPSsTFSI)/PEO).The physicochemical and electrochemical properties of the LiPSFSI/PEO and LiPSsTFSI/PEO SLIC-SPEs were investigated,in comparison with those of two similar SLIC-SPEs(e.g.,lithium poly(4-styrenesulfonate)(LiPSS)/PEO and lithium poly[(4-styrenesulfonyl)(trifluoromethanesulfonyl)imide](LiPSTFSI)/PEO).Furthermore,the impact of anionic structure of polymer Li salt on the stability of electrode/electrolyte interphases and ionic conductivity of SLIC-SPEs has been intensively investigated.It has been demonstrated that(a)the ionic conductivities of LiX-based SLIC-SPEs(X= PSFSI and PSTFSI,EO/Li+ = 20)are higher than those of the LiPSS-based SLIC-SPE(EO/Li+ = 20)over the whole temperature range,which would be attributed to more flexible and delocalized nature of the-SO2-N(-)-SO2-structure in PSFSI-and PSTFSI-anions,compared with that of the-SO3-structure in PSS-anion.More importantly,the LiPSFSI-based SLIC-SPE exhibits an excellent interfacial compatibility with Li metal electrode,compared with both those of LiX-based SLIC-SPEs(X= PSS and PSTFSI).This would be essentially attributed to the-SO2F group in the polyanion of LiPSFSI;(b)the neat LiPSsTFSI ionomer displays a low glass transition temperature(Tg = 44.3 ?).This would be attributable to highly flexibility and super-delocalized negative charge distribution of the-SO2-N(-)-SO(=NSO2CF3)-CF3 structure in PSsTFSI-anion,which impedes the motion of both Li+ cations and PSsTFSI-anions to rearrange orderly in space.Meanwhile,the ionic conductivities of the LiPSsTFSI/PEO(EO/Li+ = 20)SLIC-SPE are higher by about 1 order in magnitude than those of the LiPSTFSI/PEO(EO/Li+ = 20)one,and by 2-3 orders in magnitude than those of the LiPSS/PEO(EO/Li+ = 20)one over the whole temperature range.More importantly,the ionic conductivities of individual Li+ cations for the LiPSsTFSI-based SLIC-SPE are comparable to those for the classic ambipolar LiTFSI-based one above the melting point of PEO(? 70 ?).These outstanding properties of the LiPSsTFSI-based SLIC-SPE would make it promising as SPEs for rechargeable LMBs.