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.

Energy storage and conversion remain signifi cantly challenging to the research community. Among the candidates, lithium-ion batteries show great attraction and have been used in a wide range of applications, from small electronic devices, such as mobile phones and notebook computers, to increasing numbers of electric vehicles and large-scale energy storage equipments. [1][2][3][4][5][6] However, the relatively high cost of lithium resources shows the potential problems in terms of the long-term and large-scale applications of lithium-ion batteries. Lithium resources are limited; lithium makes up about 0.0065% of the earth ′ s crust and is unevenly distributed in South America. Thus, development of alternative storage devices is not only desirable but also necessary. Given this background, intense interest in the use of sodium-ion batteries particularly for largescale energy storage has recently been rekindled. Sodium, an element of electrochemical equivalence and proper potential, could be used as a substitute for lithium to meet the demands of rechargeable batteries. Furthermore, the sodium resources are considered to be unlimited and sodium salts widely exist in the sea. Therefore, sodium-ion batteries demonstrate the potential to substitute for lithium-ion batteries in the particular application in large-scale energy storage for renewable solar and wind power as well as smart grid. [ 7 , 8 ] Tremendous attention has been paid to sodium-ion batteries in recent years. Many electrode materials, such as Na x CoO 2 , [ 9 ] NaCrO 2 , [ 10 ] Na 1.0 Li 0.2 Ni 0.25 Mn 0.75 O δ , [ 11 , [ 17 ] hard carbon [ 13 , 18 , 19 ] and TiO 2 [ 20 ] have been investigated for application in sodium-ion batteries. Very recently, we reinvestigated the sodium ion insertion/extraction into/from Na 3 V 2 (PO 4 ) 3 with a NASICON structure. [ 21 ] The NASICON structure features a highly covalent three-dimensional framework that generates large interstitial spaces through which sodium ions may diffuse. [22][23][24] Our previous study was the fi rst to demonstrate that carbon coating can signifi cantly improve its sodium storage performance. [ 21 ] Carbon-coated Na 3 V 2 (PO 4 ) 3 electrodes show two fl at plateaus at 3.4 V and 1.6 V vs. Na + / Na, respectively. The voltage plateau located at 3.4 V is relatively higher than that of other cathode materials for sodium-ion batteries in recent reports. [9][10][11][12][13][14][15] However, the coulombic efficiency of the Na 3 V 2 (PO 4 ) 3 electrode in a half-cell is not as high as 99.5%, and does not even increase after the fi rst cycle, [ 21 ] likely because of the NaClO 4 /PC electrolyte used. Moreover, the storage capacity could also be enhanced by decreasing the carbon content of the composite and using optimized electrolyte system. In this contribution, Na 3 V 2 (PO 4 ) 3 /C nanocomposites with different carbon contents were prepared by a one-step solid state reaction and evaluated in different electrolyte systems. It was found that the sodium storage performance in terms of capacity...

In the fi eld of energy storage, lithium (Li)-ion batteries dominate the portable consumer electronic market because of their high energy density. Recently, however, the sodium (Na)-ion battery has again aroused interest for large-scale energy storage due to Na abundance. [1][2][3][4][5][6] Conventionally, the chemistry behind Li-ion and Na-ion batteries involves transition metal elements, [ 4 , 6 ] thus giving rise to problems of cost and environmental concern. Therefore, intensive efforts have been aimed at the development of new Li storage materials shifting from inorganic to organic compounds. [ 7 ] Numerous advantages exist in using organic materials as electrodes for energy storage, such as their tremendous chemical compounds, the tuning of the redox potential in a wide range, possible multi-electron reactions, the abundant resources from biomass, and ease in recycling. [7][8][9][10][11][12] A number of Li-containing organic compounds have been demonstrated to have high lithium storage capacity, good cycleability, and moderate rate capability, making them promising applications in Li-ion batteries. [ 7 , 10-12 ] However, there is no report on the use of such organic compounds in Na-ion batteries.In this contribution, a carboxylate-based organic material, disodium terephthalate (Na 2 C 8 H 4 O 4 ), is introduced as a novel anode material for low-cost room-temperature Na-ion batteries. To the best of our knowledge, this is the fi rst time that an organic compound is reported for the use as an anode material for Na-ion batteries. This material exhibits a low Na insertion voltage at 0.29 V vs. Na + /Na and a high reversible capacity of 250 mAh/g with excellent cycleability. It is found that Na storage performance can be further improved by a thin layer of Al 2 O 3 coating on the Na 2 C 8 H 4 O 4 electrode surface.Scheme 1 shows the molecular structure of Na 2 C 8 H 4 O 4 . The existence of two carbonyl groups allows for the insertion and deinsertion of two Na ions, corresponding to a theoretical capacity of 255 mAh/g. In order to confi rm the purity of the as-received Na 2 C 8 H 4 O 4 , nuclear magnetic resonance (NMR), Fourier transform infrared spectroscopy (FTIR), and X-ray diffraction (XRD) measurements were performed. Both NMR and FTIR results as shown in Figures S1 and S2 reveal the high purity of the sample. The Na 2 C 8 H 4 O 4 has an orthorhombic structure and can be indexed in space group Pbc21, according to JCPDS card No. 00-052-2146 ( Figure S3). [ 13 ] The lattice constants of Na 2 C 8 H 4 O 4 are a = 3.5480 Å, b = 10.8160 Å, and c = 18.9943 Å, and its lattice volume is V = 728.92 Å 3 . The XRD pattern of as-received Na 2 C 8 H 4 O 4 showed preferentially oriented (006) and (008) planes. Thus, refi ning the structure is diffi cult. This can be evidenced by the scanning electron microscopy (SEM) image in Figure 1 b, where the as-received sample is shown to have a fl ake-like structure with very large size near 100 μ m. To reduce the particle size and increase electronic conductivity, the sample wa...

A novel class of low-melting, hydrophobic ionic liquids based on relatively small aliphatic quaternary ammonium cations ([R(1)R(2)R(3)NR](+), wherein R(1), R(2), R(3) = CH(3) or C(2)H(5), R = n-C(3)H(7), n-C(4)H(9), CH(2)CH(2)OCH(3)) and perfluoroalkyltrifluoroborate anions ([R(F)BF(3)](-), R(F) = CF(3), C(2)F(5), n-C(3)F(7), n-C(4)F(9)) have been prepared and characterized. The important physicochemical and electrochemical properties of these salts, including melting point, glass transition, viscosity, density, ionic conductivity, thermal and electrochemical stability, have been determined and comparatively studied with those based on the corresponding [BF(4)](-) and [(CF(3)SO(2))(2)N](-) salts. The influence of the structure variation in the quaternary ammonium cation and perfluoroalkyltrifluoroborate ([R(F)BF(3)](-)) anion on the above physicochemical properties is discussed. Most of these salts are liquids at 25 degrees C and exhibit low viscosities (58-210 cP at 25 degrees C) and moderate conductivities (1.1-3.8 mS cm(-1)). The electrochemical windows of these salts are much larger than those of the corresponding 1,3-dialkyimidazolium salts. Additionally, a number of [R(F)BF(3)](-) salts exhibit plastic crystal behavior.

New cyclic quaternary ammonium salts, composed of N-alkyl(alkyl ether)-N-methylpyrrolidinium, -oxazolidinium, -piperidinium, or -morpholinium cations (alkyl = nC4H9, alkyl ether = CH3OCH2, CH3OCH2CH2) and a perfluoroalkyltrifluoroborate anion ([R(F)BF3]-, R(F) = CF3, C2F5, nC3F7, nC4F9), were synthesized and characterized. Most of these salts are liquids at room temperature. The key properties of these salts--phase transitions, thermal stability, density, viscosity, conductivity, and electrochemical windows--were measured and compared to those of their corresponding [BF4]- and [(CF3SO2)2N]- salts. The structural effect on all the above properties was intensively studied in terms of the identity of the cation and anion, variation of the side chain in the cation (i.e., alkyl versus alkyl ether), and change in the length of the perfluoroalkyl group (R(F)) in the [R(F)BF3]- ion. The reduction of Li+ ions and reoxidation of Li metal took place in pure N-butyl-N-methylpyrrolidinium pentafluoroethyltrifluoroborate as the supporting electrolyte. Such comprehensive studies enhance the knowledge necessary to design and optimize ionic liquids for many applications, including electrolytes. Some of these new salts show desirable properties, including low melting points, high thermal stabilities, low viscosities, high conductivities, and wide electrochemical windows, and may thus be potential candidates for use as electrolytes in high-energy storage devices. In addition, many salts are ionic plastic crystals.

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A novel single lithium-ion (Li-ion) conducting polymer electrolyte is presented that is composed of the lithium salt of a polyanion, poly[(4-styrenesulfonyl)(trifluoromethyl(S-trifluoromethylsulfonylimino)sulfonyl)imide] (PSsTFSI(-)), and high-molecular-weight poly(ethylene oxide) (PEO). The neat LiPSsTFSI ionomer displays a low glass-transition temperature (44.3 °C; that is, strongly plasticizing effect). The complex of LiPSsTFSI/PEO exhibits a high Li-ion transference number (tLi (+) =0.91) and is thermally stable up to 300 °C. Meanwhile, it exhibits a Li-ion conductivity as high as 1.35×10(-4)  S cm(-1) at 90 °C, which is comparable to that for the classic ambipolar LiTFSI/PEO SPEs at the same temperature. These outstanding properties of the LiPSsTFSI/PEO blended polymer electrolyte would make it promising as solid polymer electrolytes for Li batteries.