Advanced High-Voltage Electrolyte Design Using Poly(ethylene Oxide) and High-Concentration Ionic Liquids for All-Solid-State Lithium-Metal Batteries.

Poly(ethylene oxide) (PEO)-based solid polymer electrolytes (SPEs) are among the most promising materials for solid-state lithium metal batteries (LMBs) due to their inherent safety advantages; however, they suffer from insufficient room-temperature ionic conductivity (up to 10-6 S cm-1) and limited oxidation stability (<4 V). In this study, a novel "polymer-in-high-concentrated ionic liquid (IL)" (PiHCIL) electrolyte composed of PEO, N-propyl-N-methylpyrrolidinium bis(fluorosulfonyl) imide (C3mpyrFSI) IL, and LiFSI is designed. The EO/[Li/IL] ratio has been widely varied, and physical and electrochemical properties have been explored. The Li-coordination and solvation structure has been explored through Fourier-transform infrared spectroscopy and solid-state magic-angle spinning nuclear magnetic resonance. The newly designed electrolyte provides a promisingly high oxidative stability of 5.1 V and offers high ambient temperature ionic conductivity of 5.6 × 10-4 S cm-1 at 30 °C. Li|Li symmetric cell cycling shows very stable and reversible cycling of Li metal over 100 cycles and a smooth dendrite-free deposition morphology. All-solid-state cells using a composite lithium iron phosphate cathode exhibit promising cycling with 99.2% capacity retention at a C/5 rate over 100 cycles. Therefore, the novel approach of PiHCIL enables a new pathway to design high-performing SPEs for high-energy-density all-solid-state LMBs.

Concentrated Nonaqueous Polyelectrolyte Solutions: High Na-Ion Transference Number and Surface-Tethered Polyanion Layer for Sodium-Metal Batteries.

Na metal is a promising anode material for the preparation of next-generation high-energy-density sodium-ion batteries; however, the high reactivity of Na metal severely limits the choice of electrolyte. In addition, rapid charge-discharge battery systems require electrolytes with high Na-ion transport properties. Herein, we demonstrate a stable and high-rate sodium-metal battery enabled by a nonaqueous polyelectrolyte solution composed of a weakly coordinating polyanion-type Na salt, poly[(4-styrenesulfonyl)-(trifluoromethanesulfonyl)imide] (poly(NaSTFSI)) copolymerized with butyl acrylate, in a propylene carbonate solution. It was found that this concentrated polyelectrolyte solution exhibited a remarkably high Na-ion transference number (tNaPP = 0.9) and a high ionic conductivity (σ = 1.1 mS cm-1) at 60 °C. Furthermore, the surface of the Na electrode was modified with polyanion chains anchored via the partial decomposition of the electrolyte. The surface-tethered polyanion layer effectively suppressed the subsequent decomposition of the electrolyte, thereby enabling stable Na deposition/dissolution cycling. Finally, an assembled sodium-metal battery with a Na0.44MnO2 cathode demonstrated an outstanding charge/discharge reversibility (Coulombic efficiency >99.8%) over 200 cycles while also exhibiting a high discharge rate (i.e., 45% capacity retention at 10 mA cm-2).

Li-Ion Transport and Solvation of a Li Salt of Weakly Coordinating Polyanions in Ethylene Carbonate/Dimethyl Carbonate Mixtures.

Electrolytes with a high Li-ion transference number (tLi) have attracted significant attention for the improvement of the rapid charge-discharge performance of Li-ion batteries (LIBs). Nonaqueous polyelectrolyte solutions exhibit high tLi upon immobilization of the anion on a polymer backbone. However, the transport properties and Li-ion solvation in these media are not fully understood. Here, we investigated the Li salt of a weakly coordinating polyanion, poly[(4-styrenesulfonyl)(trifluoromethanesulfonyl)amide] (poly(LiSTFSA)), in various ethylene carbonate and dimethyl carbonate mixtures. The highest ionic conductivity was unexpectedly observed for the lowest polar mixture at the highest salt concentration despite the low dissociation degree of poly(LiSTFSA). This was attributed to a unique conduction phenomenon resulting from the faster diffusion of transiently solvated Li ions along the interconnected aggregates of polyanion chains. A Li/LiFePO4 cell using such an electrolyte demonstrated improved rate capability. These results provide insights into a design strategy of nonaqueous liquid electrolytes for LIBs.

Ionic transport in highly concentrated lithium bis(fluorosulfonyl)amide electrolytes with keto ester solvents: structural implications for ion hopping conduction in liquid electrolytes.

Recent studies have suggested that a Li ion hopping or ligand- or anion-exchange mechanism is largely involved in Li ion conduction of highly concentrated liquid electrolytes. To understand the determining factors for the Li ion hopping/exchange dominant conduction in such liquid systems, ionic diffusion behavior and Li ion coordination structures of concentrated liquid electrolytes composed of lithium bis(fluorosulfonyl)amide (Li[FSA]) and keto ester solvents with two carbonyl coordinating sites of increasing intramolecular distance (methyl pyruvate (MP), methyl acetoacetate (MA), and methyl levulinate (ML)) were studied. Diffusivity measurements of MP- and MA-based concentrated electrolytes showed faster Li ion diffusion than the solvent and FSA anion, demonstrating that Li ion diffusion was dominated by the Li ion hopping/exchange mechanism. A solvent-bridged, chain-like Li ion coordination structure and highly aggregated ion pairs (AGGs) or ionic clusters e.g. Lix[FSA]y(y-x)- forming in the electrolytes were shown to contribute to Li ion hopping conduction. By contrast, ML, with greater intramolecular distance between the carbonyl moieties, is more prone to form a bidentate complex with a Li cation, which increased the contribution of the vehicle mechanism to Li ion diffusion even though similar AGGs and ionic clusters were also observed. The clear correlation between the unusual Li ion diffusion and the solvent-bridged, chain-like structure provides an important insight into the design principles for fast Li ion conducting liquid electrolytes that would enable Li ion transport decoupled from viscosity-controlled mass transfer processes.

Enhanced Electrochemical Stability of Molten Li Salt Hydrate Electrolytes by the Addition of Divalent Cations.

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-1 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 LiCoO2 and Li4Ti5O12 electrodes.