Electrochemical energy storage is one of the main societal challenges to humankind in this century. The performances of classical Li-ion batteries (LIBs) with non-aqueous liquid electrolytes have made great advances in the past two decades, but the intrinsic instability of liquid electrolytes results in safety issues, and the energy density of the state-of-the-art LIBs cannot satisfy the practical requirement. Therefore, rechargeable lithium metal batteries (LMBs) have been intensively investigated considering the high theoretical capacity of lithium metal and its low negative potential. However, the progress in the field of non-aqueous liquid electrolytes for LMBs has been sluggish, with several seemingly insurmountable barriers, including dendritic Li growth and rapid capacity fading. Solid polymer electrolytes (SPEs) offer a perfect solution to these safety concerns and to the enhancement of energy density. Traditional SPEs are dual-ion conductors, in which both cations and anions are mobile and will cause a concentration polarization thus leading to poor performances of both LIBs and LMBs. Single lithium-ion (Li-ion) conducting solid polymer electrolytes (SLIC-SPEs), which have anions covalently bonded to the polymer, inorganic backbone, or immobilized by anion acceptors, are generally accepted to have advantages over conventional dual-ion conducting SPEs for application in LMBs. A high Li-ion transference number (LTN), the absence of the detrimental effect of anion polarization, and the low rate of Li dendrite growth are examples of benefits of SLIC-SPEs. To date, many types of SLIC-SPEs have been reported, including those based on organic polymers, organic–inorganic hybrid polymers and anion acceptors. In this review, a brief overview of synthetic strategies on how to realize SLIC-SPEs is given. The fundamental physical and electrochemical properties of SLIC-SPEs prepared by different methods are discussed in detail. In particular, special attention is paid to the SLIC-SPEs with high ionic conductivity and high LTN. Finally, perspectives on the main challenges and focus on the future research are also presented.

To improve the safety of sodium (Na) batteries, we first report a new solid polymer electrolyte (SPE), composed of sodium (fluorosulfonyl)(n-nonafluorobutanesulfonyl)imide (Na[(FSO2)(n-C4F9SO2)N], NaFNFSI) and poly(ethylene oxide) (PEO), which is prepared by a facile solution-casting method. The NaFNFSI/PEO (EO/Na+ = 15) blended polymer electrolyte exhibits a relatively high ionic conductivity of 3.36 × 10−4 S cm−1 at 80 °C, sufficient thermal stability (>300 °C) and anodic electrochemical stability (≈4.87 V vs. Na+/Na) for application in solid-state Na batteries. Most importantly, the NaFNFSI-based SPE can not only deliver excellent chemical and electrochemical stability with Na metal, but can also display good cycling and current-rate performances for the Na|SPE|NaCu1/9Ni2/9Fe1/3Mn1/3O2 cell. All of these outstanding properties would make the NaFNFSI-based SPE promising as a candidate for application in solid-state Na batteries.

Solid polymer electrolytes (SPEs) would be promising candidates for application in high-energy rechargeable lithium (Li) batteries to replace the conventional organic liquid electrolytes, in terms of the enhanced safety and excellent design flexibility. Herein, we first report novel perfluorinated sulfonimide salt-based SPEs, composed of lithium (trifluoromethanesulfonyl)(n-nonafluorobutanesulfonyl)imide (Li[(CF3SO2)(n-C4F9SO2)N], LiTNFSI) and poly(ethylene oxide) (PEO), which exhibit relatively efficient ionic conductivity (e.g., 1.04 × 10–4 S cm–1 at 60 °C and 3.69 × 10–4 S cm–1 at 90 °C) and enough thermal stability (>350 °C), for rechargeable Li batteries. More importantly, the LiTNFSI-based SPEs could not only deliver the excellent interfacial compatibility with electrodes (e.g., Li-metal anode, LiFePO4 and sulfur composite cathodes), but also afford good cycling performances for the Li|LiFePO4 (>300 cycles at 1C) and Li–S cells (>500 cycles at 0.5C), in comparison with the conventional LiTFSI (Li[(CF3SO2)2N])-based SPEs. The interfacial impedance and morphology of the cycled Li-metal electrodes are also comparatively analyzed by electrochemical impedance spectra and scanning electron microscopy, respectively. These indicate that the LiTNFSI-based SPEs would be potential alternatives for application in high-energy solid-state Li batteries.Keywords: Li batteries; LiTNFSI; poly(ethylene oxide); polymer electrolytes; solid-state batteries

•LiFNFSI-based electrolyte does not decompose after storage at 85 °C for two weeks.•Decompositions of LiPF6-carbonate electrolyte would be initialized by the presence of trace amounts of HF and protic impurities.•SEI films formed on graphite anode in the LiFNFSI-based electrolyte are majorly dominated by the reductive products of FNFSI− anions.•Graphite/LiCoO2 cells with LiFNFSI show better higher-temperature storage, current-rate and cycling performances than those with LiPF6.Lithium (fluorosulfonyl)(n-nonafluorobutanesulfonyl)imide (LiFNFSI) is investigated as conducting salt to replace conventional used LiPF6 for lithium-ion batteries. The stabilities of electrolytes of 1.0 M LiFNFSI- and LiPF6-ethylene carbonate (EC)/ethyl methyl carbonate (EMC) are comparatively studied at both room (25 °C) and elevated temperatures (60 and/or 85 °C) by using NMR. It is found that the electrolyte of LiFNFSI does not decompose after storage at 85 °C for 14 days; however, both LiPF6 and carbonate solvents occur degradations even after storage at 25 °C, and degrade prominently with increasing the temperature. A new mechanism for continuous decompositions of both LiPF6 and carbonate solvents, being initialized by trace amounts of HF and protic impurities, has been suggested. The electrochemical performances of LiFNFSI for graphite/LiCoO2 Li-ion cells have been comparatively investigated with those of LiPF6 at both 25 and 60 °C, with particular attention to characterizing the electrode/electrolyte interphases formed on both electrodes by using electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS). It is demonstrated that LiFNFSI is advantageous over LiPF6 in the high-temperature storage, current-rate and cycling tests, particularly at elevated temperature; however, the impedances are all larger for the LiFNFSI-based cells than for the LiPF6-based ones. Analyses of XPS reveal that the chemical compositions of electrode/electrolyte interphases formed on both electrodes are highly dependent on the types of lithium salts. Particularly, on graphite anode, the solid-electrolyte-interphase (SEI) films formed in the LiFNFSI-based electrolyte are majorly dominated by the reductive products of FNFSI− anions, and are relatively stable at elevated temperature, while those formed in the LiPF6-based electrolyte are largely governed by the reductive products of carbonate solvents, and occur significant dissolutions and regrowth at elevated temperature. All above results suggest that the improved capacity retention for the cells with LiFNFSI is mainly attributable to the robust nature of the SEI films formed on graphite anode, and the superior stability and absence of HF contamination for the LiFNFSI-based electrolyte, while the rapid capacity fading of the cells with LiPF6 is essentially due to the decompositions and regrowth of SEI films on graphite anode, and the detrimental impact of HF and protic residues in the electrolyte of LiPF6.

A novel single lithium-ion (Li-ion) conducting polymer electrolyte composed of lithium poly[(4-styrenesulfonyl)(fluorosulfonyl)imide] (LiPSFSI) and poly(ethylene oxide) (PEO) exhibits a high Li-ion transference number (tLi+ = 0.90) and sufficient electrochemical stability for use in Li batteries. The ionic conductivities of the LiPSFSI/PEO blended polymer electrolytes are higher than those of the lithium poly(4-styrenesulfonate) (LiPSS)/PEO electrolyte and are comparable to those of the lithium poly[(4-styrenesulfonyl)(trifluoromethanesulfonyl)imide] (LiPSTFSI)/PEO electrolyte in the temperature range of 25–90 °C. More importantly, the complex of LiPSFSI/PEO exhibits excellent interfacial compatibility with the Li metal electrode compared to both those of the LiPSS/PEO and LiPSTFSI/PEO electrolytes.

•Lithium salt with a super-delocalized imide anion (Li[CF3SO(=NSO2CF3)2]) is prepared.•Li[CF3SO(=NSO2CF3)2] has better dissociation ability than Li[(CF3SO2)2N] in carbonate solvents.•Li[CF3SO(=NSO2CF3)2] can passivate Al in 3–5 V versus Li/Li+.•Graphite/LiCoO2 cells with Li[CF3SO(=NSO2CF3)2] show good cycling performance.Lithium salt with a super-delocalized imide anion, namely (trifluoromethane(S-trifluoromethanesulfonylimino)sulfonyl) (trifluoromethanesulfonyl)imide ([CF3SO(=NSO2CF3)2]−), [sTFSI]−), has been prepared and studied as conducting salt for Li-ion cells. The fundamental physicochemical and electrochemical properties of neat Li[sTFSI] and its carbonate-based liquid electrolyte have been characterized with various chemical and electrochemical tools. Li[sTFSI] shows a low melting point at 118 °C, and is thermally stable up to 300 °C without decomposition on the spectra of differential scanning calorimetry-thermogravimetry-mass spectrometry (DSC-TG-MS). The electrolyte of 1.0 M (mol dm−3) Li[sTFSI] in ethylene carbonate (EC)/ethyl-methyl-carbonate (EMC) (3:7, v/v) containing 0.3% water does not show any hydrolytic decomposition on the spectra of 1H and 19F NMR, after storage at 85 °C for 10 days. The conductivities of 1.0 M Li[sTFSI]-EC/EMC (3:7, v/v) are slightly lower than those of Li[(CF3SO2)2N] (LiTFSI), but higher than those of Li[(C2F5SO2)2N] (LiBETI). The electrochemical behavior of Al foil in the Li[sTFSI]-based electrolyte has been investigated by using cyclic voltammetry and chronoamperometry, and scanning electron microscope (SEM). It is illustrated that Al metal does not corrode in the high potential region (3–5 V vs. Li/Li+) in the Li[sTFSI]-based electrolyte. On Pt electrode, the Li[sTFSI]-based electrolyte is highly resistant to oxidation (ca. 5 V vs. Li/Li+), and is also resistant to reduction to allow Li deposition and stripping. The applicability of Li[sTFSI] as conducting salt for Li-ion cells has been tested using graphite/LiCoO2 cells. It shows that the cell with Li[sTFSI] displays better cycling performance than that with LiPF6.

•Synthesis of fluorosulfonimides are updated.•Applications of fluorosulfonimide anions in electrolyte materials are reviewed.•Electrolyte of fluorosulfonimide improves the performance of Li (Li ion) battery.Fluorosulfonimide anions, including bis(fluorosulfonyl)imide ([(FSO2)2N]−, FSI−) and (fluorosulfonyl)(perfluoroalkanesulfonyl)imide ([(FSO2)(n−CmF2m+1SO2)N]−, m = 1, 2, 4, 6, 8, etc.) anions, play an increasingly important role in electrolyte materials for rechargeable Li and Li-ion battery. Recent advances in the synthesis of fluorosulfonimides and their alkali metal salts are summarized. The main advances on the electrolytes of these anions for improving the electrochemical performances of rechargeable Li and Li-ion battery are reviewed, in terms of the types of electrolytes, including conventional liquid carbonate electrolytes, ionic liquid electrolytes, molten salt electrolytes, and solid polymer electrolytes.Recent progresses on the preparation of fluorosulfonimides, and their corresponding alkali metal salts and ionic liquids are summarized. Main advances on the electrolytes of these anions for improving the electrochemical performances of rechargeable Li and Li-ion battery are reviewed.

•Ionic liquid electrolytes containing various concentrations of lithium bis(fluorosulfonyl) imide (LiFSI) are characterized.•Their electrochemical properties on electrodes highly depend on the concentration of LiFSI.•High Li-ion reversibility in graphite electrodes are obtained in the LiFSI concentrated electrolytes.•SEI films on graphite electrodes mainly comprise reductive products of bis(fluorosulfonyl) imide anions.Binary electrolytes, comprising of lithium bis(fluorosulfonyl) imide (LiFSI) and ionic liquids (ILs) of N-methyl-N-propylpiperidinium bis(fluorosulfonyl) imide (PI13FSI) with various concentrations of LiFSI (i.e., LiFSI/PI13FSI in 0.05:1, 0.1:1, 0.2:1, 0.5:1, 0.8:1 and 1:1, by mole) have been investigated as electrolyte for Li-ion cells, in terms of phase behavior, thermal stability, density, viscosity, ionic conductivity, lithium-ion transference number, and electrochemical behaviors on Al, Pt, Ni, and composite natural graphite electrodes, with particular attention to the effect of concentration of LiFSI in PI13FSI on these properties. The stability of Al in the high potential region (3.0–5.0 V vs. Li/Li+) has been confirmed in these electrolytes using cyclic voltammetry, chronoamperometry and SEM morphology. The anodic stability of these electrolytes on Pt electrode has been little affected by addition of LiFSI. Li deposition/stripping on Ni electrode shows low columbic efficiencies (< 45%) in these electrolytes, due to the continuous reduction of FSI− anions and PI13+ cations. Reduction of FSI− anions for forming solid electrolyte interphase (SEI) films on the graphite is observed at ca. 2.0 V (vs. Li/Li+), followed by intercalation of Li+ ions and IL cations into graphite in these electrolytes at the first cathodic scan in CV measurements. The performances of SEI films formed on the graphite highly depend on the concentration of LiFSI, and a stable Li-ion conducting SEI film can only be formed in the electrolyte containing a high concentration of LiFSI. Li/natural graphite cell using LiFSI-PI13FSI (1:1, by mole) as electrolyte displays high specific capacities (> 360 mAh g−1) and columbic efficiencies (> 99%) after conditioning, except for a large irreversible capacity (139 mAh g−1) observed at the first cycle. Analyses of XPS and electrochemical impedance spectra reveal that a stable Li-ion conducting SEI film, mainly comprising reduction products of FSI− anions (e.g., LiF, LiOH, Li2SO3, and species containing NSO2-, FSO2-, and N−), has been formed on the graphite.

New solid polymer electrolytes (SPEs) comprising of lithium bis(fluorosulfonyl)imide (LiFSI) and high molecular weight poly(ethylene oxide) (PEO, Mv = 5 × 106 g mol−1) have been prepared and characterized, and are comparatively studied with the representative SPEs, Li[N(SO2CF3)2] (LiTFSI)/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 (tLi+), electrochemical stability, and with particular attention to the interfacial behavior between lithium electrode and SPEs. It has been demonstrated that the ionic conductivities of LiFSI/PEO electrolyte are higher than those of LiTFSI/PEO one above 60 °C, and exceeds 10−3 S cm−1 at 80 °C. The interface resistances of Li symmetric cell (Li metal | polymer electrolytes | Li metal) using LiFSI/PEO electrolyte are much lower than those using LiTFSI/PEO. The Li/LiFePO4 cell using LiFSI/PEO electrolyte exhibits good cycling performance at 80 °C. These outstanding properties of the LiFSI/PEO electrolyte make it attractive as SPEs for Li battery.

The binary eutectic mixture of lithium bis(fluorosulfonyl)imide (LiFSI) and potassium bis(fluorosulfonyl)imide (KFSI) with a molar ratio of xLiFSI: xKFSI = 0.41: 0.59 has been investigated as molten salt electrolyte for natural graphite/LiFePO4 lithium-ion cells at 80 °C. The electrochemical performances of the Li/natural graphite, Li/LiFePO4, and natural graphite/LiFePO4 cells using this molten salt electrolyte have been evaluated, in terms of cycling performances and electrochemical impedance spectra (EIS) at 80 °C. Both the Li/natural graphite and Li/LiFePO4 cells show good cycling performances. The initial specific capacities after conditioning for the natural graphite and LiFePO4 electrodes are 353 and 150 mAh g−1, respectively, and the corresponding capacity retention rates are 98% and 97%, respectively, after 100 cycles at 80 °C, except for a large irreversible capacity (162 mAh g−1) observed for the natural graphite at the first cycle for forming solid electrolyte interface (SEI) film. The natural graphite/LiFePO4 lithium-ion cell using this molten salt electrolyte shows a low capacity in the range of 71–86 mAh g−1, due to the large irreversible capacity (71 mAh g−1) at the first cycle, but it shows a good cycleability after the first cycle, and provides a specific capacity of 71 mAh g−1 after 100 cycles at 80 °C. The variations of cycling performances of these cells can be well correlated with their impedance evolution with cycling.

•Polymeric ionic liquids based on bis(fluorosulfonyl)imide are prepared.•Fundamental physical and ionic transport properties are measured.•Their composite electrolytes with lithium salt show low glass transitions.A new polymeric ionic liquid (PIL), poly(1-vinyl-3-methylimidazolium) bis(fluorosulfonyl)imide (PVyMIM-FSI), has been prepared by polymerization of the monomer of ionic liquids, 1-vinyl-3-methylimidazolium bis(fluorosulfonyl)imide (VyMIM-FSI). Its structure and composition have been characterized by 1H and 19F NMR, and viscosity-average molecular weight (Mv). For comparison, the corresponding PIL, poly(1-vinyl-3-methylimidazolium) bis(trifluoromethanesulfonyl)imide (PVyMIM-TFSI), is also prepared and characterized. The physicochemical properties of the binary composite electrolytes of lithium bis(fluorosulfonyl)imide (LiFSI)/PVyMIM-FSI with various concentrations of LiFSI have been comparatively studied with those of the corresponding lithium bis(trifluoromethylsulfonyl)imide (LiTFSI)/PVyMIM-TFSI system, in terms of DSC, TGA, ionic conductivity, and lithium ion transference number (tLi+). Both the FSI−- and TFSI−-based PILs are thermally stable up to 250 °C. While both types of the prepared PILs show a significant decrease in glass transition temperature (Tg) upon addition of lithium salt, the values of Tg for the composite electrolytes of LiFSI/PVyMIM-FSI are all significantly lower by 20–60 °C in magnitude than those of the corresponding LiTFSI/PVyMIM-TFSI composite ones, indicating much better plasticizing effect for FSI− vs. TFSI−. The ionic conductivities of LiFSI/PVyMIM-FSI are all higher by about 2 orders in magnitude than those of the corresponding LiTFSI/PVyMIM-TFSI with the same concentration of lithium ions.

A new family of polymeric ionic liquids (PILs) based on alkyl and alkyl ether substituted ammoniums and perfluorinated sulfonimides (i.e., bis(fluorosulfonyl)imide (FSI−), and bis(trifluoromethanesulfonyl)imide (TFSI−)) have been synthesized by polymerization of the corresponding ionic liquid monomers (ILMs). Their structures and compositions have been characterized by 1H and 19F 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. All the prepared ILMs and PILs reveal excellent thermal stabilities to greater than 250 °C. The PILs containing alkyl ether side unit show significant decrease in glass transition temperature (Tg), the values of Tg of the alkyl ether based-PILs are all significantly lower by 10–77 °C in magnitude than those of the corresponding alkyl based ones. The ionic conductivity of alkyl ether based-PILs in the best case increases up to 4.0 × 10−6 S cm−1 at 30 °C, and reaches 7.6 × 10−5 S cm−1 at 60 °C, and outperform their ammonium counterparts with alkyl side chain that were synthesized as references.

New class of hydrophobic ionic liquids based on bis(2,2,2-trifluoroethoxysulfonyl)imide ([(CF3CH2OSO2)2N]−, TFESI−) anion with various oniums, including N,N-dialkylimidazolium, tetraalkyl (ether-containing alkyl) ammonium, N-alkyl-N-methyl-pyrrolidinium, and -piperidinium, are prepared and characterized. Their physicochemical and electrochemical properties, including phase transitions, thermal stability, viscosity, density, ionic conductivity and electrochemical windows, have been determined. The influences of the structural variations in both the cation and anion on the above properties are extensively discussed. The key features of these new TFESI−-based ionic liquids are their low melting points (ranging from 9 to 44 °C) and/or low glass transition (between −74 and −61 °C), relatively low viscosities (44–426 cP at 25 °C), good thermal stability (Td > 270 °C), and wide electrochemical windows (ranging from 5.7 to 6.4 V).Highlights► Ionic liquids based on bis(2,2,2-trifluoroethoxysulfonyl)imide anion are prepared. ► Fundamental physicochemical and electrochemical properties are measured. ► They show low melting points or low glass transitions without melting. ► They show wide electrochemical windows.

Sixteen ionic liquids (ILs) based on two new asymmetric imide anions, (fluorosulfonyl)(polyfluorooxaalkanesulfonyl)imide {[(FSO2)(XCF2CF2OCF2CF2SO2)N]−, [5X3O-FOFSI]−, X = H and I}, are synthesized and characterized by in combination with various oniums, including 1,3-dialkylimidazolium, quaternary alkyl (or ether-functionalized alkyl)ammonium, N-alkyl-N-methyl-pyrrolidinium and -piperidinium. Their physicochemical properties are investigated, in terms of phase transitions, thermal stability, density, viscosity, ionic conductivity, electrochemical stability, and the capability as electrolytes to allow Li deposition/stripping. The influences of the structural variations in both the cations and anions, particularly introduction of an ether group into the anion, on the above properties are systematically studied. All the ILs are liquids at room temperature and show low glass transition temperatures, suggesting the high flexibility of the ether-functionalized imide anion. It has been demonstrated that only having an ether group into the polyfluoroalkyl chain of the imide anion affect neither the cathodic nor anodic stability of the anion and their ILs. The ILs based on the [5H3O-FOFSI]− anion have an enough electrochemical stability to allow Li deposition/striping, while those based on the [5I3O-FOFSI]− anion show relatively low cathodic stability due to the presence of a CI bond in the anion.Highlights► Ionic liquids based on (fluorosulfonyl)(polyfluorooxaalkanesulfonyl)imides are prepared. ► Fundamental physical and electrochemical properties are measured. ► They show low melting points or low glass transitions without melting. ► They show wide electrochemical windows.

New single lithium-ion conducting polymer electrolytes are prepared by a copolymerization of the two monomers, lithium (4-styrenesulfonyl)(trifluoromethanesulfonyl)imide (LiSTFSI) and methoxy-polyethylene glycol acrylate (MPEGA, CH2CHCO2(CH2CH2O)nCH3, n = 8) in various monomer ratios. The structures and compositions of the prepared lithium poly[(4-styrenesulfonyl)(trifluoromethanesulfonyl)imide-co-methoxy-polyethylene glycol acrylate] (Li[PSTFSI-co-MPEGA]) copolymers are characterized by 1H and 19F NMR, and gel permeation chromatography (GPC). For comparison, the corresponding blended polymer electrolytes comprising lithium poly[(4-styrenesulfonyl) (trifluoromethanesulfonyl)imide] (LiPSTFSI) and poly(ethylene oxide) (PEO) are also prepared and characterized. The fundamental properties of these two types of lithium-ion conducting polymer electrolytes are comparatively studied, in terms of phase transitions, thermal stability, XRD, ionic conductivities, lithium-ion transference numbers (tLi+), and electrochemical stabilities. Both types of the polymer electrolytes are thermally stable up to 300 °C. While both types of polymer electrolytes exhibit single lithium-ion conducting behavior with tLi+ > 0.9, the solid-state ionic conductivities of the Li[PSTFSI-co-MPEGA] copolymer electrolytes are all higher by 1–3 orders in magnitude than those of the blended ones, irrespective of the concentration of lithium ions. The highest ionic conductivities for the copolymer electrolytes are 7.6 × 10−6 S cm−1 at 25 °C and reach 10−4 S cm−1 at 60 °C, which are obtained at the ethylene oxide (EO) unit/Li+ ratio of 20.5.Highlights► Single lithium-ion conducting polymer electrolytes based on highly delocalized polyanions are prepared. ► Phase behavior and transport properties are measured. ► They show high lithium ion transference number approaching unity. ► They show high ionic conductivity at room temperature.

Twenty derivatives of N,N-dialkyl perfluoroalkanesulfonamides, n-CmF2m + 1SO2NRR′ (m = 1, 2, 4, 6, 8; R, R′ = CH3, CH2CH3 or CH2CH2OCH3), have been prepared and characterized. Their fundamental physicochemical properties, including melting point, density, viscosity and dielectric constant, are determined. The influences of structural variations in both the perfluoroalkyl and alkyl chains on the above properties are extensively studied. Among these N,N-dialkyl perfluoroalkanesulfonamides, 14 compounds are liquid at room temperature, and some of them exhibit low melting points, even lower than −40 °C. The empirical solvent polarity scales for these liquid perfluoroalkanesulfonamides, such as ‘energy of transition’ ET(30) values and Kamlet–Taft solvent parameters (π*, α, β), are also determined by solvatochromic measurements, and show good linear temperature dependence. The ET(30) values are in the range of 45.36–55.19 kcal mol−1 at 25 °C, comparable to those of short chain alcohols. The π* parameters for the compounds containing alkyl chain are in the range of 0.550–0.651 at 25 °C, being a little higher than those of short chain alcohols. All these prepared compounds show strong hydrogen-bond donating ability (α values ranging from 0.865 to 1.204 at 25 °C), comparable to that of short chain alcohols. Meanwhile, those containing alkoxyalkyl chain display strong hydrogen-bond accepting ability (β values ranging from 0.71 to 0.82 at 25 °C), comparable to that of short chain alcohols.Graphical abstractA series of 20 N,N-dialkyl perfluoroalkanesulfonamides, n-CmF2m + 1SO2NRR′ (m = 1, 2, 4, 6, 8; R, R′ = CH3, CH2CH3 or CH2CH2OCH3), are prepared and characterized. The influences of structure variation in both the perfluoroalkyl and alkyl chains on their fundamental physicochemical properties are extensively studied. Some of the liquid perfluoroalkanesulfonamides exhibit low melting points, even being lower than −40 °C. Both the dielectric constants and the empirical solvent polarity scales, such as ET(30) values and Kamlet–Taft solvent parameters (π*, α, β), indicate that these liquid perfluoroalkanesulfonamides show relatively high polarity comparable to that of short chain alcohols.Highlights► A series of N,N-dialkyl perfluoroalkanesulfonamides were synthesized in high quality and good yields. ► The influences of structure variation on their fundamental physicochemical properties were extensively studied. ► These compounds showed relatively low melting points even lower than −40 °C. ► These compounds showed high polarity comparable to that of short chain alcohols.

Lithium bis(fluorosulfonyl)imide (LiFSI) has been studied as conducting salt for lithium-ion batteries, in terms of the physicochemical and electrochemical properties of the neat LiFSI salt and its nonaqueous liquid electrolytes. Our pure LiFSI salt shows a melting point at 145 °C, and is thermally stable up to 200 °C. It exhibits far superior stability towards hydrolysis than LiPF6. Among the various lithium salts studied at the concentration of 1.0 M (= mol dm−3) in a mixture of ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3:7, v/v), LiFSI shows the highest conductivity in the order of LiFSI > LiPF6 > Li[N(SO2CF3)2] (LiTFSI) > LiClO4 > LiBF4. The stability of Al in the high potential region (3.0–5.0 V vs. Li+/Li) has been confirmed for high purity LiFSI-based electrolytes using cyclic voltammetry, SEM morphology, and chronoamperometry, whereas Al corrosion indeed occurs in the LiFSI-based electrolytes tainted with trace amounts of LiCl (50 ppm). With high purity, LiFSI outperforms LiPF6 in both Li/LiCoO2 and graphite/LiCoO2 cells.Research highlights▶ Lithium bis(fluorosulfonyl)imide (LiFSI) has been studied as conducting salt for nonaqueous liquid electrolytes for lithium-ion batteries. ▶ Lithium bis(fluorosulfonyl)imide (LiFSI) exhibits far superior stability towards hydrolysis than lithium hexafluorophosphate (LiPF6) and does not release hydrogen fluoride (HF). ▶ Pure lithium bis(fluorosulfonyl)imide (LiFSI) does not corrode Al, and Al corrosion is induced by trace amounts of chloride (Cl−) impurities present in it. ▶ Lithium bis(fluorosulfonyl)imide (LiFSI) outperforms lithium hexafluorophosphate (LiPF6) for lithium-ion batteries.

A novel lithium salt, lithium (fluorosulfonyl)(nonafluorobutanesulfonyl)imide (LiFNFSI), is investigated as a conducting salt to improve the high-temperature resilience of lithium-ion cells. It shows better thermal stability than LiPF6. The electrolyte having 1.0 M LiFNFSI in a mixture of ethylene carbonate (EC)/ethyl methyl carbonate (EMC) (3:7, v/v) shows high conductivity comparable to LiClO4, good electrochemical stability, and does not corrode aluminum. At both room temperature (25 °C) and elevated temperature (60 °C), the graphite/LiCoO2 cells with LiFNFSI exhibit better cycling performances than those with LiPF6. Particularly, at 60 °C, the capacity fading rate of the LiFNFSI-based cell without any additive is 37% after 100 cycles, while the cell with LiPF6 fails rapidly. These outstanding properties of LiFNFSI make it an attractive candidate to overcome the rapid capacity fading of lithium-ion batteries at elevated temperatures.

New functionalized ionic liquids (ILs), comprised of multi-methoxyethyl substituted quaternary ammonium cations (i.e. [N(CH2CH2OCH3)4−n(R)n]+; n = 1, R = CH3OCH2CH2; n = 1, R = CH3, CH2CH3; n = 2, R = CH3CH2), and two representative perfluorinated sulfonimide anions (i.e. bis(fluorosulfonyl)imide (FSI−) and bis(trifluoromethanesulfonyl)imide (TFSI−)), were prepared. Their fundamental properties, including phase transition, thermal stability, viscosity, density, specific conductivity and electrochemical window, were extensively characterized. These multi-ether functionalized ionic liquids exhibit good capability of dissolving lithium salts. Their binary electrolytes containing high concentration of the corresponding lithium salt ([Li+] >1.6 mol kg−1) show Li+ ion transference number (tLi+tLi+) as high as 0.6–0.7. Their electrochemical stability allows Li deposition/stripping realized at room temperature. The desired properties of these multi-ether functionalized ionic liquids make them potential electrolytes for Li (or Li-ion) batteries.New functionalized ionic liquids based on multi-methoxyethyl substituted quaternary ammonium cations and perfluorinated sulfonimide anions are introduced.

New hydrophobic ionic liquids based on (fluorosulfonyl)(pentafluoroethanesulfonyl)imide ([(FSO2)(C2F5SO2)N]−, FPFSI−) anion with various oniums, including imidazolium, tetraalkyl ammonium, pyrrolidinium, and piperidinium, were prepared and characterized. Their physicochemical and electrochemical properties, including phase transitions, thermal stability, viscosity, density, specific conductivity and electrochemical windows, were extensively characterized, and were comparatively studied with the corresponding ionic liquids containing the isomeric but symmetric TFSI− ([(CF3SO2)2N]−) anion. These new FPFSI−-based ionic liquids display low melting points, low viscosities, good thermal stability, and wide electrochemical windows allowing Li deposition/dissolution. All these desired properties suggest they are potential electrolyte materials for Li (or Li-ion) batteries.New ionic liquids based on (fluorosulfonyl)(pentafluoroethanesulfonyl)imide with various oniums are introduced.

A new series of low-melting, low-viscosity, hydrophobic ionic liquids based on relatively small tertiary sulfonium cations ([R1R2R3S]+, wherein R1, R2, R3 = CH3 or C2H5, R3 = CH2CH2OCH3, CH2CH2COOCH3, CH2CH2CN) and bis(fluorosulfonyl)imide (FSI−) anion have been prepared and characterized. The important physicochemical and electrochemical properties of these salts, such as melting point, glass transition, viscosity, density, ionic conductivity, thermal and electrochemical stability, have been determined. The influence of structure variation in the tertiary sulfonium cations on the above physicochemical properties is discussed. Among these new salts, some of them show the desirable properties including low-melting points, low viscosities, and high conductivities, to be selected as potential candidates as electrolytes in energy devices, and two salts are ionic plastic crystals.Ionic liquids and plastic crystals based on tertiary sulfonium and bis(fluorosulfonyl)imide were synthesized and characterized. Their properties including phase transitions, thermal stability, viscosity, conductivity, and electrochemical windows are significantly affected by the nature of the cation.

To improve the safety of sodium (Na) batteries, we first report a new solid polymer electrolyte (SPE), composed of sodium (fluorosulfonyl)(n-nonafluorobutanesulfonyl)imide (Na[(FSO2)(n-C4F9SO2)N], NaFNFSI) and poly(ethylene oxide) (PEO), which is prepared by a facile solution-casting method. The NaFNFSI/PEO (EO/Na+ = 15) blended polymer electrolyte exhibits a relatively high ionic conductivity of 3.36 × 10−4 S cm−1 at 80 °C, sufficient thermal stability (>300 °C) and anodic electrochemical stability (≈4.87 V vs. Na+/Na) for application in solid-state Na batteries. Most importantly, the NaFNFSI-based SPE can not only deliver excellent chemical and electrochemical stability with Na metal, but can also display good cycling and current-rate performances for the Na|SPE|NaCu1/9Ni2/9Fe1/3Mn1/3O2 cell. All of these outstanding properties would make the NaFNFSI-based SPE promising as a candidate for application in solid-state Na batteries.

Electrochemical energy storage is one of the main societal challenges to humankind in this century. The performances of classical Li-ion batteries (LIBs) with non-aqueous liquid electrolytes have made great advances in the past two decades, but the intrinsic instability of liquid electrolytes results in safety issues, and the energy density of the state-of-the-art LIBs cannot satisfy the practical requirement. Therefore, rechargeable lithium metal batteries (LMBs) have been intensively investigated considering the high theoretical capacity of lithium metal and its low negative potential. However, the progress in the field of non-aqueous liquid electrolytes for LMBs has been sluggish, with several seemingly insurmountable barriers, including dendritic Li growth and rapid capacity fading. Solid polymer electrolytes (SPEs) offer a perfect solution to these safety concerns and to the enhancement of energy density. Traditional SPEs are dual-ion conductors, in which both cations and anions are mobile and will cause a concentration polarization thus leading to poor performances of both LIBs and LMBs. Single lithium-ion (Li-ion) conducting solid polymer electrolytes (SLIC-SPEs), which have anions covalently bonded to the polymer, inorganic backbone, or immobilized by anion acceptors, are generally accepted to have advantages over conventional dual-ion conducting SPEs for application in LMBs. A high Li-ion transference number (LTN), the absence of the detrimental effect of anion polarization, and the low rate of Li dendrite growth are examples of benefits of SLIC-SPEs. To date, many types of SLIC-SPEs have been reported, including those based on organic polymers, organic–inorganic hybrid polymers and anion acceptors. In this review, a brief overview of synthetic strategies on how to realize SLIC-SPEs is given. The fundamental physical and electrochemical properties of SLIC-SPEs prepared by different methods are discussed in detail. In particular, special attention is paid to the SLIC-SPEs with high ionic conductivity and high LTN. Finally, perspectives on the main challenges and focus on the future research are also presented.