Water can be an attractive solvent for Li-ion battery electrolytes owing to numerous advantages such as high polarity, nonflammability, environmental benignity, and abundance, provided that its narrow electrochemical potential window can be enhanced to a similar level to that of typical nonaqueous electrolytes. In recent years, significant improvements in the electrochemical stability of aqueous electrolytes have been achieved with molten salt hydrate electrolytes containing extremely high concentrations of Li salt. In this study, we investigated the effect of divalent salt additives (magnesium and calcium bis(trifluoromethanesulfonyl)amides) in a molten salt hydrate electrolyte (21 mol kg lithium bis(trifluoromethanesulfonyl)amide) on the electrochemical stability and aqueous lithium secondary battery performance. We found that the electrochemical stability was further enhanced by the addition of the divalent salt. In particular, the reductive stability was increased by more than 1 V on the Al electrode in the presence of either of the divalent cations. Surface characterization with X-ray photoelectron spectroscopy suggests that a passivation layer formed on the Al electrode consists of inorganic salts (most notably fluorides) of the divalent cations and the less-soluble solid electrolyte interphase mitigated the reductive decomposition of water effectively. The enhanced electrochemical stability in the presence of the divalent salts resulted in a more-stable charge-discharge cycling of LiCoO and LiTiO electrodes.

To investigate physicochemical relationships between ionic radii, valence number and cationic metal species in electrolyte solutions, propylene carbonate with Li[N(SO 2 CF 3 ) 2 ], Na[N(SO 2 CF 3 ) 2 ], Mg[N(SO 2 CF 3 ) 2 ] 2 and Ca[N(SO 2 CF 3 ) 2 ] 2 were prepared. The temperature dependence of density, viscosity, ionic conductivity (AC impedance method) and self-diffusion coefficient (pulsed-gradient spin-echo nuclear magnetic resonance) was measured. The effects of cationic radii and cation valence number on the fluidity and transport properites (conductivity and self-diffusion coefficient) were analyzed. Research and development of lithium-ion batteries (LIBs) have focused on the efficient use of energy. Application fields of LIBs are spreading from portable commercial use (mobile phone and laptop PC) to large-scale energy systems (electric vehicle and accumulator for household use).1 Furthermore, the usages (utilities, needs, requirements and demands) of industrial-scaled electricity storage systems using LIBs are increasing for applications alongside renewable energy systems (photovoltaics and/or wind energy) and frequency regulation demands. However, resources and stock amounts of Li are limited. Another reactive cationic species, such as high Clarke number Na + (2.63, 6th) and divalent cations Mg 2+ (1.93, 8th) and Ca 2+ (3.39, 5th) have been reported as new cationic species for next-generation batteries (Li + : 0.0006, 27th). 2 The number of reports for battery operations using a Na (sodium) system is increasing, and they have focused on the research and development of electrode and electrolyte materials for, mainly, positive electrodes.3-5 Because of the differences of ionic radii between Li + (60 pm) and Na + (95 pm), understanding the effects of ionic radii on electrolyte properties is important. 6,7 In addition, comparison between monovalent cations and divalent cations (Mg 2+ : 65 pm, 8 Ca 2+ : 99 pm 9 ) is also important to understand the effects of valence number of cation on the electrolyte properties. In this study, an electrolyte solutions of propylene carbonate (PC) and N(SO 2 CF 3 ) 2 − ([TFSA] − ) anion-based metal (Li, Na, Mg and Ca) salts were prepared and evaluated by measuring their physicochemical properties. We investigate the dependence of static (density) and dynamic (macroscopic fluidity: viscosity, ionic mobility: ionic conductivity and microscopic ionic diffusivity: self-diffusion coefficient) properties of electrolytes on the cationic metal species of salts (ionic radii and valence number) and salt concentrations. We also analyze the intermolecular interactions of the cations with PC and [TFSA] − by ab initio molecular orbital calculations. Using the mearurements of static and dynamic properties and analysis of interactions, the expectations of innovative next-generation battery systems are discussed. ExperimentalMaterials.-PC (Kishida Chemical, battery grade) and metal cation salts (Table I) were used as the solvent and dissolved salts for electrolyte solutions, respectively...

Lithium−glyme solvated ionic liquids (Li−G SILs) and superconcentrated electrolyte solutions (SCESs) are expected to be promising electrolytes for next-generation lithium secondary batteries. The former consists of only the oligoether glyme solvated lithium ion and its counteranion, and the latter contains no full solvated Li + ion by the solvents due to the extremely high Li salt concentration. Although both of them are similar to each other, it is still unclear that both should be room-temperature ionic liquids. To distinctly define them, speciation analyses were performed with the Li−G SIL and the aqueous SCES to evaluate the free solvent concentration in these solutions with a new Raman/infrared spectral analysis technique called complementary least-squares analysis. Furthermore, from a thermodynamic point of view, we investigated the solvent activity and activity coefficient in the gas phase equilibrated with sample solutions and found they can be good criteria for SILs.

It has been reported that aqueous lithium ion batteries (ALIBs) can operate beyond the electrochemical window of water by using a superconcentrated electrolyte aqueous solution. The liquid structure, particularly the local structure of the Li + , which is rather different from conventional dilute solution, plays a crucial role in realizing the ALIB. To reveal the local structure around Li + , the superconcentrated LiTFSA (TFSA: bis(trifluoromethylsulfonil)amide) aqueous solutions were investigated by means of Raman spectroscopic experiments, high-energy X-ray total scattering measurements, and the neutron diffraction technique with different isotopic composition ratios of 6 Li/ 7 Li and H/D. The Li + local structure changes with the increase of the LiTFSA concentration; the oligomer ([Li p (TFSA) q ] (p-q)+ (q > 2) forms at the molar fraction of LiTFSA (x LiTFSA ) > 0.25. The average structure can be determined in which two water molecules and two oxygen atoms of TFSA anion(s) coordinate to the Li + in the superconcentrated LiTFSA aqueous solution (LiTFSA) 0.25 (H 2 O) 0.75 . In addition, the intermolecular interaction between the neighboring water molecules was not found, and the hydrogen-bonded interaction in the solution should be significantly weak. According to the coordination number of the oxygen atom (TFSA or H 2 O), a variety of TFSA − and H 2 O coordination manners would exist in this solution; in particular, the oligomer is formed in which the monodentate TFSA cross-links Li + .