Cathode Electrolyte Interphase Formation

• CEI is a complex interphase formed by electrolyte decomposition at the cathode, involving rapid solvent adsorption and low-energy ring-opening reactions.
• The formation process follows multi-step oxidation pathways, with reaction kinetics and composition sensitive to surface terminations, voltage, and electrolyte additives.
• Advanced operando characterization and computational modeling guide strategies—like tailored coatings and additive use—to enhance CEI stability and prolong battery cycle life.

The cathode electrolyte interphase (CEI) refers to the layer of inorganic and organic species formed at the interface between the cathode and the electrolyte in batteries, typically as a consequence of electrolyte decomposition induced by chemical or electrochemical processes. The CEI is a critical factor governing cathode stability, cycling performance, interfacial impedance, and overall degradation in high-energy systems—most notably in Li-ion, Na-ion, and Li-metal batteries. Its formation mechanism involves nuanced surface chemistry, interfacial charge transfer, multistep decomposition pathways, and is sensitive to local crystallography, surface terminations, electrolyte composition, temperature, and applied voltage.

1. Fundamental CEI Formation Mechanisms

The nucleation of CEI initiates primarily through chemical (sometimes non-electrochemical) reactions between electrolyte molecules and the cathode surface. In canonical Li(Nix,Mny,Co1–x–y)O2 (NMC) systems, ab-initio DFT studies using the CI-NEB method establish that ethylene carbonate (EC), a prevalent electrolyte solvent, undergoes a fast concerted ring-opening reaction on the bare cathode surface, with the rate independent of cell voltage (Xu et al., 2017). Upon initial contact, EC adsorbs via its carbonyl oxygen to exposed surface transition metal atoms (e.g., Ni, Mn, Co), facilitating subsequent tetrahedral intermediate formation. The critical Cc–OE bond (ethylene oxygen) rupture proceeds at a low energy barrier (e.g., 17 meV in the presence of coordinated Li⁺), occurring on a millisecond timescale.

For spinel oxides, multi-step decomposition is operational. EC is partially oxidized on the (001) surface of LiₓMn₂O₄ or LiₓNi₀.₅Mn₁.₅O₄, forming adsorbed organic fragments that act as initial passivators. Higher operational voltages (lower surface lithium) drive further oxidation of CEI species, culminating in CO₂ evolution and exposure of pristine oxide for continued degradation (Leung et al., 2020). The overall mechanism may be generalized as a sequence: solvent adsorption, partial oxidation, intermediate passivation, further oxidation, and at elevated potential, bond cleavage and gas release.

2. Reaction Pathways, Surface Chemistry, and Kinetic Barriers

The precise atomic configuration and terminations of the cathode modulate the decomposition kinetics and CEI composition. DFT-derived energy barriers for the EC ring-opening process are highly sensitive to –OH and –F surface terminations (Xu et al., 2017):

Termination	Site	Energy Barrier (meV)	Impact
None (bare)	-	17–290	Fast CEI
–OH (type I)	O-surface	150	Modest passivation
–F	metal	490	Medium passivation
–OH (type II)	transition M	860	Strong passivation

The presence of Li⁺, acting as a Lewis acid, can substantially reduce the activation barrier by stabilizing intermediates. The Arrhenius equation for reaction rate estimation is:

$$R = k_0\,\exp\left(\frac{-E_{\rm barrier}}{k_B T}\right) \times C_R \times (1-C_p)$$

where even minor variations in $E_{\rm barrier}$ (e.g., due to surface functionalization) yield orders of magnitude changes in CEI nucleation rates.

In spinel systems, application of hybrid PBE0 DFT calculations is critical for quantitatively accurate determination of reaction energetics and activation barriers; DFT+U systematically underestimates these parameters (Leung et al., 2020). The vibrational free energy contributions are estimated via:

$$\Delta \Delta A = \sum_i \left[ \hbar \omega_i + k_B T \ln \left( 1 - \exp\left(-\frac{\hbar \omega_i}{k_B T}\right) \right) \right]$$

3. CEI Composition, Voltage Dependence, and Multistep Oxidation

The final CEI structure and properties are contingent on the specific electrolyte, local voltage, cathode chemistry, and cycling protocol.

• In high-voltage spinel and layered oxides, CEI evolution involves a multi-step oxidation pathway:
  1. EC adsorption and partial oxidation, yielding organic surface fragment passivation.
  2. At increased potential (lower $x$), these fragments are further oxidized, leading to CO₂ gas release—characterized by higher activation barriers ($\Delta E^* \sim 1.05$ eV on Li₀.₆Mn₂O₄, $1.22$ eV on LNMO at moderate $x$), with rapid decomposition only above certain cutoff voltages (Leung et al., 2020).
  3. Native surface carbonate (Li₂CO₃) phase often acts as a precursor, itself requiring dissolution or oxidation before direct electrolyte-cathode contact and subsequent CEI formation.

• In NMC811, the proximity of cathode LUMO and electrolyte HOMO at high state-of-charge results in facilitation of electrolyte oxidation, leading primarily to LiF or LiF/LiOH-rich CEI layers at cutoff voltages of 4.5 V and 4.3 V, respectively (Gallegos-Moncayo et al., 2023). However, mechanical instability and non-uniform formation of inorganic-rich CEI compromise protective efficacy, resulting in capacity fade and ongoing surface degradation.

• Sodium-ion NCAM cathodes exhibit formation of insulating CEI composed principally of NaF, Na₂O, and NaCO₃ as a result of electrolyte oxidation and reaction with residual surface species (e.g., NaOH, CO₂), particularly at elevated potentials (Lecce et al., 2021). These compounds precipitate at the interface and accentuate polarization, crack formation, and loss of electrochemical activity, with the underlying crystal lattice remaining structurally intact.

4. Role of Electrolyte Chemistry, Additives, and Surface Modifications

Control over CEI formation is achievable via strategic modifications to the electrolyte or cathode surface:

• In solid-state systems based on oxygen-doped argyrodite (Li₆.₂₅PS₄O₁.₂₅Cl₀.₇₅), in situ CEI formation leads to a thin film composed primarily of Li₃PO₄, which serves as an electronically insulating yet ionically conductive buffer, reducing further parasitic reactions and stabilizing the cathode interface at high voltages (Xu et al., 2020). The oxidation potential is upshifted by ~1 V, as extracted from theoretical simulations.
• In ether-based high-voltage Li-metal batteries, the use of perfluorobutane sulfonate (LiPFBS) salt enables the predilection for PFBS⁻ anion oxidation at the cathode, forming an inorganic-dominant CEI (LiF, sulfur-oxides) and passivating the electrode before DME oxidation can commence (He et al., 26 Sep 2024). This approach results in improved interphase stability, lower impedance, and extended cycle life, contrasting with TFSI-based electrolytes where an organic-rich—hence less effective—CEI arises.
• Thermal instability and radical-driven degradation are addressed via additives such as dimethoxydimethylsilane (DODSi), which act as H⁺ and F* radical scavengers in LHCEs at elevated temperature (80 ℃) (Meng et al., 11 Jan 2024). This functionalization inhibits HF chain reactions, preserves CEI integrity, and enables superior cycling performance.
• Alumina (Al₂O₃) coatings significantly suppress electrolyte decomposition and mechanical breakdown at Co-rich cathode surfaces (Peiris et al., 10 Jun 2025). AIMD studies indicate reduction in EC dehydrogenation, formation of interfacial Al–O–C bonds, and enhancement of cleavage energy $W_{\rm sep}$, which quantifies mechanical resilience:

$W_{\rm sep} = \frac{E_A + E_B - E_{\rm total}}{S}$

Optimal performance is linked to tailored coating thickness (5–10 Å) and the crystallographic facet (e.g., high benefit on open (012) planes).

5. Experimental Insights: Advanced Characterization and Operando Dynamics

Recent breakthroughs utilize in situ and ex situ analytical methods to scrutinize CEI formation and dissolution:

• Real-time liquid ec-TEM imaging and GC/MS provide direct visualization of CEI nucleation, growth, and dynamic behavior under cycling (2.5–6 V vs. Li) (Gallegos-Moncayo et al., 14 Oct 2025). CEI deposit morphology varies with voltage window; high-voltage cycling yields micron-scale LiF/amorphous particles, while a moderate window generates thin (36 nm) amorphous films.
• Chemical pathways mapped via GC/MS confirm that EC/DMC oxidation drives HF and LiF formation, e.g.,

$$\text{LiPF}_6 + \text{H}_2\text{O} \rightarrow \text{LiF} + \text{POF}_3 + 2\,\text{HF}$$

and

$$\text{HF} + \text{LiOH} \rightarrow \text{LiF} + \text{H}_2\text{O}$$

.

• The dissolution of LiF is proposed to occur through a two-step process: generation of soluble intermediates during anodic oxidation, followed by reductive dissolution at lower potential. This identifies the applied potential window as a key determinative factor in CEI thickness, homogeneity, and persistence.

6. Rate-Limiting Steps, Cycle Life, and Design Implications

Despite the prompt initiation of solvent breakdown and CEI buildup, rate-limiting processes are more often linked to subsequent chemical or electrochemical processes occurring on/within the decomposed CEI layer. Surface passivation by strong terminations (e.g., –OH bonded to transition metals) can slow initial CEI formation to minutes per event. However, long-term evolution is dominated by further reactions involving EC decomposition products, irreversible gas evolution, and side reactions with additional electrolyte by-products (e.g., PF₅ in LiPF₆ systems, polysulfides, or oxidative attack in sodium cells) (Xu et al., 2017, Lecce et al., 2021).

The optimization window involves:

• Tuning cathode, electrolyte, and additive composition to favor stable, ionically conductive, and electronically insulating CEIs.
• Implementing coatings and artificial interphases (Al₂O₃, Li₃PO₄, LiF-rich layers).
• Dynamic control of cycling protocols, voltage windows, and temperature profiles to suppress undesirable multistep CEI evolution.

A plausible implication is that, for practical cycle lifetimes, management of CEI formation and suppression of ongoing interfacial and interphase reactions are critical, and are strongly dependent on atomic-resolution structural and chemical design.

7. Outlook and Open Research Directions

The field continues to advance refined mechanistic models integrating multiscale computational and experimental insights, accounting for surface orientation, defect chemistry, solvent/salt identity, and dynamic cycling effects. Open directions include:

• Extending operando analytical techniques to diverse chemistries and non-carbonate electrolytes to generalize CEI growth and stability principles.
• Exploring radical and acid scavenging additive efficacy under extreme conditions (temperature, potential window).
• Quantitative mapping of mechanical and chemical resilience imparted by artificial coatings and interphases.
• Elucidating dissolution–regrowth kinetics, especially for insulating species (LiF, NaF), to improve interphase regenerative capacity and prevent local passivation.

These advances are essential to achieve longer-lived, higher-energy batteries by controlling CEI formation, structure, and evolution. Careful calibration of interface chemistry, electrolyte design, and cathode engineering remains central in further suppressing degradation due to non-ideal CEI formation and dynamics.