Electrolytes and interphases

Electrolyte design on one page: the superconcentrated LiFSA/DMC network, a localized high-concentration electrolyte with an inert diluent, desolvation at graphite, the two-layer SEI on silicon, a CEI on NCM811, and manganese and nickel crossing from the cathode to the graphite. Every caption and number is quoted from the cited papers.

1. Superconcentrated: In superconcentrated 1 : 1.1 LiFSA/DMC, simulations show every DMC and every FSA⁻ coordinated to Li⁺: each FSA⁻ binds 2–3 Li⁺ and each Li⁺ 2–3 FSA⁻, a reinforced three-dimensional network. An FSA⁻ on one Li⁺ is a contact ion pair, on two or more an aggregate, and at 1 : 1.1 aggregates dominate. The electrolyte held back aluminium and transition-metal dissolution at around 5 V and cycled graphite without EC (Wang 2016). 2. Localized high-concentration: A localized high-concentration electrolyte, 1.2 m LiFSI in DMC/BTFE (1 : 2 by mol), cycled lithium metal without dendrites at 99.5% Coulombic efficiency and kept > 80% capacity after 700 cycles in Li||NMC cells, with low concentration, low viscosity and good wettability (Chen 2018). An inert solvent dilutes the high-concentration electrolyte; in simulations of LiFSI in DME/TFEO, FSI⁻ and TFEO release fluorine that builds a LiF-rich inner SEI layer (Lu 2022). 3. Desolvation: The ion-transfer barrier at graphite has two parts: desolvation of Li⁺ before it enters the graphene interlayer, and migration of the bare Li⁺ through the interphase. Desolvation is the major energy-consuming step: stripping the solvation sheath is rate-determining when a graphite anode charges (Xu 2010). 4. SEI on silicon: X-ray reflectivity on a Si wafer resolved two inorganic SEI layers: a bottom layer from lithiation of the native oxide and a top layer mainly of LiF, which grows during lithiation and contracts during delithiation; crystalline Si lithiates layer by layer, with a well-defined boundary between LiₓSi and c-Si (Cao 2019). With LiPF₆ in EC the SEI on Si is first mostly LEDC and LiF; with FEC it is LiF and poly(FEC), which changes little on cycling (Young 2015). 5. CEI on NCM811: With 1,3-propanesultone as an additive, the cathode–electrolyte interphase on NCM811 had an inorganic inner and an organic outer layer, mechanically stable and flexible, which relieved stress and kept the cathode intact (Li 2025). Much less is understood about interphases on positive electrodes than on negative ones; Li-rich layered oxides can make highly reactive species through oxygen anion redox (Gauthier 2015). 6. Metal crossover: LiPF₆ hydrolyses to HF, which forms an insoluble AlF₃ film on the aluminium current collector but speeds transition-metal dissolution from the cathode. Dissolved Mn²⁺ and Ni²⁺ (< 1% of the total) deposit on graphite, thicken its SEI by catalysing electrolyte reduction and use up lithium: > 50% capacity loss in 100 cycles. In LiNi₀.₅Mn₁.₅O₄|graphite cells at 40 °C the commercial electrolyte left 18% of the capacity after 100 cycles; superconcentrated LiFSA/DMC kept over 90% (Wang 2016).

Wang et al., “Superconcentrated electrolytes for a high-voltage lithium-ion battery”, Nat. Commun. 2016 (doi:10.1038/ncomms12032); Chen et al., “High-voltage lithium-metal batteries enabled by localized high-concentration electrolytes”, Adv. Mater. 2018 (doi:10.1002/adma.201706102); Lu et al., “DFT-ReaxFF hybrid molecular dynamics investigation of the decomposition effects of localized high-concentration electrolyte in lithium metal batteries: LiFSI/DME/TFEO”, Phys. Chem. Chem. Phys. 2022 (doi:10.1039/d2cp02130g); Xu et al., “Differentiating contributions to "ion transfer" barrier from interphasial resistance and Li+ desolvation at electrolyte/graphite interface”, Langmuir 2010 (doi:10.1021/la1009994); Cao et al., “Shedding X-ray light on the interfacial electrochemistry of silicon anodes for Li-ion batteries”, Acc. Chem. Res. 2019 (doi:10.1021/acs.accounts.9b00233); Young et al., “Hard X-ray photoelectron spectroscopy (HAXPES) investigation of the silicon solid electrolyte interphase (SEI) in lithium-ion batteries”, ACS Appl. Mater. Interfaces 2015 (doi:10.1021/acsami.5b04845); Li et al., “Electrolyte engineering to construct robust interphase with high ionic conductivity for wide temperature range lithium metal batteries”, Angew. Chem. Int. Ed. 2025 (doi:10.1002/anie.202414636); Gauthier et al., “Electrode–electrolyte interface in Li-ion batteries: current understanding and new insights”, J. Phys. Chem. Lett. 2015 (doi:10.1021/acs.jpclett.5b01727).

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