A review on liquid electrolyte design for LIBs operating under low-temperature (<0 °C) conditions. Covers various processes that determine performance below 0 °C and recent literature on electrolyte-based strategies to improve said performance.

In lithium-sulfur batteries, the growth of insulating discharge product Li2S film affects the cathode microstructure and the related electron as well as lithium ion transport properties. In this study, chemical reactions of insoluble lithium polysulfides Li2Sx (x = 1, 2) on crystal Li2S substrate are investigated by a first-principles approach. First-principles atomistic thermodynamics predicts that the stoichiometric (111) and (110) surfaces are stable around the operating cell voltage. Li2Sx adsorption is an exothermic reaction with the (110) surface being more active to react with the polysulfides than the stoichiometric (111) surface. There is no obvious charge transfer between the adsorbed molecule and the crystal Li2S substrate. Analysis of the charge density difference suggests that the adsorbate interacts with the substrate via a strong covalent bond. The growth mechanism of thermodynamically stable surfaces is investigated in the present study. It is found that direct Li2S deposition is energetically favored over Li2S2 deposition and reduction process.

The Li-S battery is a promising next-generation technology due to its high theoretical energy density (2600 Wh kg −1) and low active material cost. However, poor cycling stability and coulombic efficiency caused by polysulfide dissolution have proven to be major obstacles for a practical Li-S battery implementation. In this work, we develop a novel strategy to suppress polysulfide dissolution using hydrofluoroethers (HFEs) with bi-functional, amphiphlic surfactant-like design: a polar lithiophilic "head" attached to a fluorinated lithiophobic "tail." A unique solvation mechanism is proposed for these solvents whereby dissociated lithium ions are readily coordinated with lithiophilic "head" to induce self-assembly into micelle-like complex structures. Complex formation is verified experimentally by changing the additive structure and concentration using small angle X-ray scattering (SAXS). These HFE-based electrolytes are found to prevent polysulfide dissolution and to have excellent chemical compatibility with lithium metal: Li||Cu stripping/plating tests reveal high coulombic efficiency (>99.5%), modest polarization, and smooth surface morphology of the uniformly deposited lithium. Li-S cells are demonstrated with 1395 mAh g −1 initial capacity and 71.9% retention over 100 cycles at >99.5% efficiency-evidence that the micelle structure of the amphiphilic additives in HFEs can prohibit polysulfide dissolution while enabling facile Li + transport and anode passivation.

Stiff polymers with tunable self-healing and mechanical properties show promising potential in electrochemical devices.