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Lithium is the most attractive anode material for high-energy density rechargeable batteries, but its cycling is plagued by morphological irreversibility and dendrite growth that arise in part from its heterogeneous “native” solid electrolyte interphase (SEI). Enriching the SEI with lithium fluoride (LiF) has recently gained popularity to improve Li cyclability. However, the intrinsic function of LiF—whether chemical, mechanical, or kinetic in nature—remains unknown. Herein, we investigated the stability of LiF in model LiF-enriched SEIs that are either artificially preformed or derived from fluorinated electrolytes, and thus, the effect of the LiF source on Li electrode behavior. We discovered that the mechanical integrity of LiF is easily compromised during plating, making it intrinsically unable to protect Li. The ensuing in situ repair of the interface by electrolyte, either regenerating LiF or forming an extra elastomeric “outer layer,” is identified as the more critical determinant of Li electrode performance. Our findings present an updated and dynamic picture of the LiF-enriched SEI and demonstrate the need to carefully consider the combined role of ionic and electrolyte-derived layers in future design strategies.

Formation of stable solid electrolyte interphases (SEI) that protect Li against continuous electrolyte reduction is one of the remaining challenges to enable safe, secondary high-energy Li batteries with minimal capacity loss. However, SEI formation pathways remain difficult to experimentally pinpoint, even with the most well-known carbonate electrolytes and graphite anodes, and especially on Li. Using a custom electrochemical cell coupled to a gas chromatograph (GC), dynamic gas-phase signatures of interphase reactions during a first Li plating step in EC/DMC were monitored as a function of cell chemistry and operational parameters. The operando nature of these experiments allows distinction to be drawn between gases formed chemically by the reaction of metallic Li and electrolyte, vs those evolved electrochemically, i.e., through electron-transfer and reaction with Li + . Quantification of gas evolution molar ratios during cycling enables determination of specific interphase reactions and their branching ratios dominating active SEI formation. We find that SEIrepair mechanisms are sensitive to the choice of the electrolyte salt (LiPF 6 /LiClO 4 /LiTFSI), solvent fluorination, and current density. In particular, SEIs resulting from solvent decarbonylation and/or decarboxylationleading to enhanced CO and/or CO 2 evolutionare the most stable, providing a simple and descriptive gas-phase signature indicative of high Coulombic efficiencies of Li plating/stripping.

Discovery of new electrochemical redox motifs is essential to expand the design landscape for energy-dense batteries. We report a family of fluorinated reactants based on pentafluorosulfanyl arenes ( R-Ph-SF 5 ) that allow for high electron-transfer numbers (up to 8-e − /reactant) by exploiting multiple coupled redox processes, including extensive S–F bond breaking, yielding capacities of 861 mAh·g reactant −1 and voltages up to ∼2.9 V when used as catholytes in primary Li cells. At a cell level, gravimetric energies of 1,085 Wh·kg −1 are attained at 5 W·kg −1 and moderate temperatures of 50 °C, with 853 Wh·kg −1 delivered at >100 W·kg −1 , exceeding all leading primary batteries based on electrode + electrolyte (substack) mass. Voltage compatibility of R-Ph-SF 5 reactants and carbon monofluoride (CF x ) conversion cathodes further enabled investigation of a hybrid battery containing both fluorinated catholyte and cathode. The hybrid cells reach extraordinarily high cell active mass loading (∼80%) and energy (1,195 Wh·kg −1 ), allowing for significant boosting of substack gravimetric energy of Li−CF x cells by at least 20% while exhibiting good shelf life and safety characteristics.

Réseau sur le stockage Electrochimique de l'Energie (RS2E), CNRS FR3459, 33 rue Saint Leu,

Capacity losses in Li anodes derive from the formation of electronically isolated Li0 and parasitic reactions that form the solid electrolyte interphase (SEI). Loss quantification has focused on Li0, but SEI losses increasingly dominate at high Coulombic efficiency (CE), becoming the loss mode to minimize. Here, we apply quantitative titration to track an array of key SEI phases: ROCO2Li, Li2C2, RLi, LiF, P-containing phases, and total Li loss, which teach new insights beyond inactive Li0. In 1 M LiPF6 EC/DEC, we demonstrate chemical resolution up to 71% of Li loss and 33% of SEI loss inventory. Expanding to additional carbonate electrolytes, ROCO2Li was consistently the major quantifiable SEI phase, but proportions were invariant with CE. Instead, Li2C2, a minor phase, exhibited clear inverse correlation with CE. These results demonstrate that, while minor phases often receive less focus and are harder to characterize, they can play governing roles in SEI function.

Despite being a leading candidate to meet stringent energy targets of Li-ion batteries, the lithium (Li) metal anode has yet to achieve Coulombic efficiency (CE) requirements for long cycle life...

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