Solid-Electrolyte Interphase Control

• Solid-electrolyte interphase control is the targeted engineering of a nanometric, passivating layer at the electrode-electrolyte interface that regulates charge transfer and inhibits degradation.
• It employs advanced methods such as atomistic modeling, high-throughput computational screening, and in situ characterization to map reaction kinetics and optimize film structure.
• Chemical, physical, and electrochemical strategies are integrated to tailor SEI composition and morphology, improving ionic conductivity, cycle life, and safety in diverse battery systems.

Solid-electrolyte interphase (SEI) control concerns the directed formation, composition, structure, and evolution of the nanometric passivating layer that spontaneously arises at the interface of electrode and electrolyte in batteries. The SEI mediates charge transfer, suppresses undesired electrolyte decomposition, controls access of metal ions, determines impedance, and impacts cycle stability, rate performance, and failure modes (such as dendrite formation or graphite exfoliation) in diverse electrochemical systems. Advanced strategies for SEI control exploit atomistic insights, high-throughput computational screening, cryogenic and in situ structural characterization, multi-scale modeling, and targeted chemical engineering of interphases. The field integrates thermodynamics, surface chemistry, defect physics, and reaction–transport kinetics to rationally engineer the SEI as an active, multi-functional interface.

1. Atomistic Mechanisms of SEI Nucleation and Composition

SEI nucleation is initiated when the electrode Fermi level ($E_F$) surpasses the electrolyte stability window, driving electron transfer into electrolyte molecule LUMOs. The fundamental thermodynamic criterion is $E_F \leq E_{\text{LUMO}}(\text{solvent or anion})$, establishing the reduction of electrolyte constituents at the interface (Hasan et al., 28 Nov 2025). The redox event produces a nanometric film—electronically insulating, ionically conductive—whose precise molecular architecture depends on both the electrode material and electrolyte composition.

In Li-ion systems, classical reduction of ethylene carbonate (EC) yields a bilayer structure: an inorganic Li$_2$CO$_3$-rich inner region and an outer layer comprising polymeric (polycarbonate, PEO-like) and oligomeric species. Additives such as fluoroethylene carbonate (FEC) alter this trajectory by preferential defluorination, forming LiF and acetal/branched-polymer crosslinks, thus densifying and stabilizing the interphase network (Jin et al., 2018). In solid-state sulfide electrolytes, direct reduction can initially generate overlithiated, amorphous phases (e.g., $a$-Li$_{14}$PS$_5$Cl) which subsequently crystallize to a $\mathrm{Li}_2$S$\cdot$Li$_3$P$\cdot$LiCl solid solution (Chaney et al., 2024). These outcomes are mediated by the relative kinetics of electron transfer, diffusion, and defect formation.

High-throughput computational screening, exemplified by the “Redox Fingerprint Analysis” (RFPA) workflow (Husch et al., 2015), systematically generates and evaluates all possible low-electron-transfer SEI-forming reactions for each electrolyte candidate, ranking products by thermodynamic $\Delta G$, kinetic proxies (HOMO energies, Tanimoto coefficients), and solubility. This enables prediction of the most likely SEI constituents for arbitrary electrolyte libraries.

2. Multi-Scale Structure, Dynamics, and Transport in the SEI

SEIs are now understood as hierarchically structured, multi-component entities, with composition and transport properties modulated by morphological order and interface chemistry. Cryogenic electron microscopy has revealed, for example, that the Li|LiPON SEI adopts a three-layer mosaic configuration: successive sublayers of Li$_2$O, Li$_3$N, and Li$_3$PO$_4$ nanocrystallites embedded in a dense amorphous matrix, with graded N and P concentration profiles (Cheng et al., 2020). This structure is electronically insulating but enables facile Li$^+$ conduction through continuous ionic channels.

Scanning electron nanobeam diffraction (SEND) distinguishes ordered amorphous and mixed LiF nanocrystalline SEI types at Li-metal anodes (Koh et al., 6 May 2025). Short-range order in the SEI is directly correlated with high coulombic efficiency and dendrite suppression, whereas the presence of crystalline LiF crystallites elevates interfacial resistance.

Molecular dynamics, ab-initio, and machine-learned potentials uncover glassy trapping and hopping regimes for Li$^+$ migration, with marked differences in activation barriers, mean-squared displacements, and vibrational frequency signatures between SEI materials (e.g., Li$_2$EDC vs. EC) (Muralidharan et al., 2018). The strongest Li$^+$ trapping arises from rigid carbonate cages and high interfacial energy, limiting fast-ion transport and ultrafast charging capability.

Defect engineering at SEI heterophase interfaces (e.g., LiF/Li$_2$CO$_3$) can markedly enhance ionic mobility: lattice registry and strain lower migration barriers from 0.30 eV (bulk) to 0.10–0.22 eV interfacially, with Car–Parrinello MD revealing diffusion coefficients up to $D \approx 8 \times 10^{-11}$ cm$^2$/s at room temperature (Ahmad et al., 2020).

3. Control Strategies: Chemical, Physical, and Electrochemical Levers

Directed SEI formation leverages compositional tuning, processing parameters, and external fields.

• Electrolyte Chemistry: High-salt/concentrated electrolytes foster anion-derived, inorganic-rich SEIs with favorable short-range order (Koh et al., 6 May 2025). Additives like FEC or VC (5–15 vol%) promote sacrificial, early-stage reduction, rapid cross-linking, and LiF generation, enhancing mechanical and electronic passivation (Jin et al., 2018). Partial chalcogen substitution in sulfide SSEs (e.g., O for S in Li$_6$PS$_5$Cl) integrates in-situ formation of Li$_3$PO$_4$ AEI/CEI layers, benefitting both Li-metal compatibility and oxidation-resistance (Xu et al., 2020).
• Interface Engineering: Pre-coating metallic electrodes with ultrathin Li$_2$O, LiF, or phosphate films by atomic-layer deposition or CVD blocks direct organic contact, yielding robust, self-limiting SEIs (Leung et al., 2016). Buffer interlayers (e.g., LiNbO$_3$, LiPON) template amorphization and offer controlled chemical potential drops, stalling excessive reaction flux (Chaney et al., 2024, Ncube et al., 15 Nov 2025).
• Voltage and Current Protocols: Voltage-stepped or pulsed plating and slow, staged formation at moderate overpotentials generate preferred SEI layering and amorphization, facilitating thin, uniform, non-porous films with low defect density (Koh et al., 6 May 2025, Chaney et al., 2024). Real-time electronic control via the Quantum Continuum Approximation (QCA) enables tuning of the electrostatic barrier for Li$^+$ migration, minimizing activation energies for ultrafast charging (Campbell, 2024).
• Thermal and Pressure Processing: Post-deposition annealing at moderate temperatures (150–250 °C) and controlled stack pressure (0.1–1 MPa) adjust crystallite morphology, heal lattice strains, and preserve amorphous, highly conductive channels (Cheng et al., 2020, Ding et al., 12 Jun 2025).
• Transient Interphase Manipulation: Intentional exploitation of the Transient SEI (T-SEI)—precipitated salt films under fast metal dissolution—can “polish” metallic electrodes in situ, resulting in epitaxial, flat redeposited grains with reduced roughness and dendrite risk (Fuller et al., 2024). Key, however, is the rapid relaxation and dissolution of T-SEI, dictated by the interplay of current density, electrolyte concentration, and surface kinetics.

4. Modeling, Descriptor-Based Prediction, and Automated Screening

Machine-learned molecular dynamics and density functional approaches now form the backbone of predictive SEI control.

• Reaction-Diffusion Kinetics: Growth laws for SEI thickness, $L(t)$, follow $\sqrt{t}$ or logarithmic scaling, depending on whether transport is dominated by Li$^0$ interstitial diffusion, electron conduction, or tunneling (Horstmann et al., 2018, Single et al., 2018). The critical rate-limiting step for continued SEI growth and lifetime capacity fade in Li-ion cells is diffusion of neutral Li interstitials, not solvent transport or electron tunneling, as empirically validated by long-term open-circuit tests (Single et al., 2018).
• Unsupervised Phase Discovery: Clustering in atomic environment descriptor space (ACE+DPA) enables in situ identification of new crystalline disordered phases (e.g., Li$_2$S$_{0.72}$P$_{0.14}$Cl$_{0.14}$) that arise from the interplay of reaction kinetics and ionic interdiffusion but evade pure thermodynamic analyses (Ding et al., 12 Jun 2025).
• Automated Screening Workflows: The RFPA pipeline computes redox fingerprints for arbitrary electrolyte molecules, evaluates SEI-forming pathways by thermodynamic and kinetic proxies, and flags candidates with high risk of graphite exfoliation or unfavorable film properties (Husch et al., 2015).

5. Advanced Characterization, Interface Resilience, and Challenges

Next-generation SEI control requires integration of high-resolution and operando structural probes, as well as microstructural engineering.

• Cryogenic and Low-Dose Electron Microscopy: Avoiding ambient or beam-induced artifacts is critical for resolving nanoscale ordering and mosaic sublayer architecture (e.g., grain-boundary-free, fast-ion channels vs. defective LiF nanocrystallites) (Koh et al., 6 May 2025, Cheng et al., 2020).
• Spectroscopic Fingerprints and Quantitative Metrics: X-ray absorption spectroscopy (XAS) provides atomic-scale insights into SEI degradation (e.g., PS$_4$ distortion, S–S bond nucleation, PS$_3$ motif emergence), with well-defined energy shifts and spectral markers quantitatively correlating with impedance growth (Cao et al., 2023).
• Interphase Engineering at the Cathode: Blocked ionic interdiffusion via redox-inert, mechanically compliant buffer layers, such as LNTO, inhibits deleterious intermixing at high-voltage cathode|SSE interfaces but must balance adhesion and elasticity to prevent delamination (Ncube et al., 15 Nov 2025).
• Transferable Design Rules: Maximizing the inorganic fraction, achieving percolating low-barrier heterointerfaces, and employing additives or coatings for targeted electron-insulation and Li$^+$-conductivity are universally relevant strategies (Hasan et al., 28 Nov 2025). Achieving robust, dynamic, and self-healing SEIs under fast cycling remains an open challenge, particularly in hybrid battery|supercapacitor platforms.

6. Outlook and Unified Principles

SEI formation, structure, and evolution are governed by universal electrochemical principles: the crossing of stability windows, kinetic competition between reaction and diffusion, and the emergence of morphology from local chemistry and physical constraints. Advanced predictive frameworks now enable rational engineering of passivation layers tailored for specific device classes—batteries, supercapacitors, solid-state cells—by direct manipulation of composition, process, and operating regimes. Forthcoming integration of multiscale simulation, data-driven descriptor mapping, and closed-loop operando diagnostics heralds a new era of active SEI management for high-performance, long-lived electrochemical energy storage (Hasan et al., 28 Nov 2025).