Synergy of Weakly Solvated Electrolyte and LiF-Reinforced Interphase Enables Long-Term Operation of Li-Metal Batteries at Low Temperatures.

Hindered by the high diffusion energy barrier of Li+ in graphite anode layers, the low-temperature application of traditional Li-ion batteries is limited. Lithium metal without intercalation and with excellent specific capacity is expected to support battery operation at low temperatures. However, due to the low conductivity, high freezing point, and strong solvation energy of traditional carbonate electrolytes, the application of lithium-metal batteries at low temperatures remains challenged. In this paper, an all-ester-based ternary solvent electrolyte based on fluorinated carbonate and methyl acetate is developed to improve the cyclic efficiency of the Li-metal anode at subzero temperatures. Methyl acetate, with low viscosity and low freezing point, endows Li+ with efficient transfer in the bulk phase at low temperatures. Fluorinated cosolvent regulates the solvation structure, thereby facilitating Li+ desolvation while forming a LiF-rich solid electrolyte interphase. The electrolyte exhibits good compatibility with the Li-metal anode, as confirmed by the significantly reduced kinetic barrier of Li+ diffusion at the interface. The theoretical calculations suggest that anions occupy the dominant positions within the inner solvation sheath. The in situ/ex situ characterizations provide straightforward evidence of a dendrite-free Li-metal electrode during cycling. As a result, the symmetric Li||Li cell is able to cycle stably for thousands of hours at current densities of 0.5 mA cm-2 and 1 mAh cm-2. When paired with a LiFePO4 cathode, the battery at 0.2 C (1 C = 170 mA g-1) has a capacity retention of 95.4% after 200 cycles at -15 °C and 92.6% after 100 cycles at -20 °C, respectively.

Boosting Cathode Activity and Anode Stability of Lithium-Sulfur Batteries with Vigorous Iodic Species Triggered by Nitrate.

Lithium-sulfur (Li-S) battery with a sulfurized polyacrylonitrile cathode is a promising alternative to Li-ion systems. However, the sluggish charge transfer of cathode and accumulation of inactive Li on anode remain persistent challenges. An advanced electrolyte additive with function towards both cathode and anode holds great promise to address these issues. Herein, we present a new strategy to boost sulfur activity and rejuvenate dead Li simultaneously. In the polar electrolyte containing I2-LiNO3 additives, I3 -/IO3 - are triggered significantly by the reaction between NO3 - and I- ions. The I3 -/IO3 - are reactive to insulated Li2S product of cathode and inactive Li on anode, thus accelerating the conversion reaction of sulfur and recovering Li sources back to battery cycling. The in situ/ex situ spectroscopic and morphologic monitoring reveal the crucial role of iodine in promoting Li2S dissociation and inhibiting dendritic Li growth. With the modified electrolyte, the symmetric Li||Li cells deliver a lifespan of 4000 h with an overpotential less than 12 mV at 0.5 mA cm-2. For Li-S cells, 100 % capacity retention up to thousands of cycles and enhanced rate capability are available. This work demonstrates a feasible strategy on electrolyte engineering for practical applications of Li-S batteries.