Interfacial polymerization mechanisms assisted flame retardancy process of low-flammable electrolytes on lithium anode.

Thermal runaway is a hazardous risk, occurring more readily in high-energy-density lithium-ion batteries (LIBs), which leads to a rapid temperature rise and even combustion or explosion when using flammable electrolyte systems. Flame retardants (FRs), such as trimethyl phosphate (TMPa) and triethyl phosphate (TEP), are commonly utilized due to their effective flame suppression, low toxicity, and excellent thermal stability. However, the lack of in-depth understanding of the flame retardancy mechanism and solid electrolyte interphase (SEI) formation process has made the development of functional electrolytes difficult at present. In this study, we clarified the flame retardancy and interfacial reaction mechanisms of low-flammable TMPa localized high-concentration electrolytes (LHCE) using hybrid ab initio and reactive force field (HAIR) schemes. Long-term HAIR simulation reveals that phosphorous radicals produced by the decomposition of TMPa capture carbon radicals, encouraging their polymerization into low-flammable oligomers, while fluorine-containing solvents in the electrolyte capture hydrogen radicals and produce nonflammable hydrofluoric acid (HF). This synergistic flame retardancy mechanism provides essential atomic-level insights for the rational design of high-safety electrolytes in the future.

Temperature-dependent interphase formation and Li+ transport in lithium metal batteries.

High-performance Li-ion/metal batteries working at a low temperature (i.e., <-20 °C) are desired but hindered by the sluggish kinetics associated with Li+ transport and charge transfer. Herein, the temperature-dependent Li+ behavior during Li plating is profiled by various characterization techniques, suggesting that Li+ diffusion through the solid electrolyte interface (SEI) layer is the key rate-determining step. Lowering the temperature not only slows down Li+ transport, but also alters the thermodynamic reaction of electrolyte decomposition, resulting in different reaction pathways and forming an SEI layer consisting of intermediate products rich in organic species. Such an SEI layer is metastable and unsuitable for efficient Li+ transport. By tuning the solvation structure of the electrolyte with a lower lowest unoccupied molecular orbital (LUMO) energy level and polar groups, such as fluorinated electrolytes like 1 mol L-1 lithium bis(fluorosulfonyl)imide (LiFSI) in methyl trifluoroacetate (MTFA): fluoroethylene carbonate (FEC) (8:2, weight ratio), an inorganic-rich SEI layer more readily forms, which exhibits enhanced tolerance to a change of working temperature (thermodynamics) and improved Li+ transport (kinetics). Our findings uncover the kinetic bottleneck for Li+ transport at low temperature and provide directions to enhance the reaction kinetics/thermodynamics and low-temperature performance by constructing inorganic-rich interphases.

Preferential decomposition of the major anion in a dual-salt electrolyte facilitates the formation of organic-inorganic composite solid electrolyte interphase.

The performance of a lithium metal battery (LMB) with liquid electrolytes depends on the realization of a stable solid electrolyte interphase (SEI) on the Li anode surface. According to a recent experiment, a high-concentrated (HC) dual-salt electrolyte is effective in modulating the SEI formation and improving the battery performance. However, the underlying reaction mechanism between this HC dual-salt electrolyte and the lithium metal anode surface remains unknown. To understand the SEI formation mechanism, we first performed 95 ps ab initio Molecular Dynamics (AIMD) simulation and then extend this AIMD simulation to another 1 ns by using Hybrid ab Initio and Reactive Molecular Dynamics (HAIR) to investigate the deep reactions of such dual-salt electrolytes consists of lithium difluorophosphate and lithium bis(trifluoromethanesulfonyl)imide in dimethoxyethane (DME) solvent at lithium metal anode surface. We observed the detailed reductive decomposition processes of DFP- and TFSI-, which include the formation pathway of CF3 fragments, LiF, and LixPOFy, the three main SEI components observed experimentally. Furthermore, after extending the simulation to 1.1 ns via the HAIR scheme, the decomposition reactions of DME solvent molecules were also observed, producing LiOCH3, C2H4, and precursors of organic oligomers. These microscopic insights provide important guidance in designing the advanced dual-salt electrolytes for developing high-performance LMB.

Influence of Ag Metal Dispersion on the Catalyzed Reduction of CO2 into Chemical Fuels over Ag-ZrO2 Catalysts.

Metal/metal oxide catalysts reveal unique CO2 adsorption and hydrogenation properties in CO2 electroreduction for the synthesis of chemical fuels. The dispersion of active components on the surface of metal oxide has unique quantum effects, significantly affecting the catalytic activity and selectivity. Catalyst models with 25, 50, and 75% Ag covering on ZrO2, denoted as Ag4/(ZrO2)9, Ag8/(ZrO2)9, and Ag12/(ZrO2)9, respectively, were developed and coupled with a detailed investigation of the electronic properties and electroreduction processes from CO2 into different chemical fuels using density functional theory calculations. The dispersion of Ag can obviously tune the hybridization between the active site of the catalyst and the O atom of the intermediate species CH3O* derived from the reduction of CO2, which can be expected as the key intermediate to lead the reduction path to differentiation of generation of CH4 and CH3OH. The weak hybridization between CH3O* and Ag4/(ZrO2)9 and Ag12/(ZrO2)9 favors the further reduction of CH3O* into CH3OH. In stark contrast, the strong hybridization between CH3O* and Ag8/(ZrO2)9 promotes the dissociation of the C-O bond of CH3O*, thus leading to the generation of CH4. Results provide a fundamental understanding of the CO2 reduction mechanism on the metal/metal oxide surface, favoring novel catalyst rational design and chemical fuel production.

Multiscale Simulation of Solid Electrolyte Interface Formation in Fluorinated Diluted Electrolytes with Lithium Anodes.

Lithium metal batteries (LMBs) hold great promise in facilitating high-energy batteries due to their merits such as high specific capacity, low reduction potential, and so forth. However, the realizations of practical LMBs are hindered by severe problems such as undesirable dendrite growth, poor Coulombic efficiency, and so forth. A recently proposed fluorinated electrolyte based on 1 M lithium bis(fluorosulfonyl)imide (LiFSI) dissolved in designed fluorinated 1,4-dimethoxybutane (FDMB) solvent has attracted significant attention because of its excellent electrochemical performance that origins from its superior physical and chemical properties, especially its unique ability in forming a robust, stable solid electrolyte interphase (SEI). However, the detailed structure and reaction mechanism of the SEI formation in such a novel electrolyte remains unclear. In this work, we carry out a hybrid ab initio and reactive molecular dynamics (HAIR) simulation to investigate the elementary reactions that regulate the formation of the primitive SEI, paying special attention to the process that involves FDMB, the fluorinated solvent. HAIR simulation reveals that both FSI- anion and FDMB provide F that is adequate to form a uniformed LiF layer that resembles the inorganic inner layer (IIL) of the SEI. N and S radicals from the FSI- anion, which do not deposit on the electrode interface to form lithium-containing inorganic substances, promote the polymerization reaction of unsaturated carbon chains produced by FDMB defluorination, forming the organic outer layer (OOL) of the SEI. The combination of the LiF-rich IIL and polymer-rich organic OOL explains the superior performance of the FDMB-based electrolyte in the device. The detailed reaction mechanism and SEI observed in this work provide insights into the atomic scale for the rational design of F-rich electrolytes in the near future.

Au-activated N motifs in non-coherent cupric porphyrin metal organic frameworks for promoting and stabilizing ethylene production.

Direct implementation of metal-organic frameworks as the catalyst for CO2 electroreduction has been challenging due to issues such as poor conductivity, stability, and limited > 2e- products. In this study, Au nanoneedles are impregnated into a cupric porphyrin-based metal-organic framework by exploiting ligand carboxylates as the Au3+ -reducing agent, simultaneously cleaving the ligand-node linkage. Surprisingly, despite the lack of a coherent structure, the Au-inserted framework affords a superb ethylene selectivity up to 52.5% in Faradaic efficiency, ranking among the best for metal-organic frameworks reported in the literature. Through operando X-ray, infrared spectroscopies and density functional theory calculations, the enhanced ethylene selectivity is attributed to Au-activated nitrogen motifs in coordination with the Cu centers for C-C coupling at the metalloporphyrin sites. Furthermore, the Au-inserted catalyst demonstrates both improved structural and catalytic stability, ascribed to the altered charge conduction path that bypasses the incoherent framework. This study underlines the modulation of reticular metalloporphyrin structure by metal impregnation for steering the CO2 reduction reaction pathway.