Coulometric titration time analysis (CTTA) has recently emerged as a powerful technique for quantifying the rate of solid electrolyte interphase (SEI) growth and elucidating the underlying mechanisms governing degradation in solid-state batteries with argyrodite solid electrolytes. However, subsequent studies employing metallic interlayers and variable stack pressures have revealed that CTTA results are highly sensitive to the effective contact area between the solid electrolyte and the current collector. In this work, we further explore CTTA as a SEI characterisation technique to gain more information on its strengths and weaknesses. The findings highlight key challenges when comparing results across samples and propose strategies to mitigate these effects. Moreover, this study contextualises CTTA within the broader suite of SEI characterisation techniques, offering guidance on its optimal implementation and interpretation.

Hydroborate solid electrolytes are attracting increasing attention as alternatives to argyrodite electrolytes for all-solid-state lithium and sodium batteries. In this work, we first summarize recent progress in mixed-anion closo-hydroborate and closo-hydrocarborate electrolytes and derive criteria to select anion compositions that balance ionic conductivity, electrochemical stability, and interface compatibility. Building on a diffusion-limited interphase growth model, we quantitatively compare the interface resistance growth rate of Li3(CB11H12)2(CB9H10) and Li6PS5Cl in contact with lithium metal, lithiated silicon, and delithiated NMC811 by monitoring impedance as a function of time using electrochemical impedance spectroscopy. Despite its lower reductive stability, the hydroborate exhibits substantially slower interface resistance buildup in contact with lithium metal than the argyrodite, emphasizing that the transport properties of the solid electrolyte interphase, rather than the bulk electrolyte stability alone, govern long-term interfacial degradation. In contact with lithiated silicon, both electrolytes show markedly slower resistance growth, with the hydroborate showing higher stability than the argyrodite. In contact with delithiated NMC811, neither electrolyte shows any resistance growth at an open-circuit potential of 3.8 V vs. Li/Li[+]. At 4.1 V vs. Li/Li[+], the hydroborate electrolyte shows resistance growth due to electrolyte oxidation, despite its higher oxidative stability compared to the argyrodite, which shows no resistance growth at this potential. The cell resistance is dominated by the interface resistance at the cathode, highlighting the importance of protective coatings to prevent electrolyte oxidation. Our findings highlight closo-hydroborates and closo-hydrocarborates as promising electrolytes for high-energy solid-state lithium and sodium batteries and underline the critical importance of tailoring interphase composition and transport to unlock their full potential.

Quasi-solid-state lithium-sulfur batteries offer a promising route to combine the high energy density of lithium-sulfur chemistry with improved interfacial stability. However, their performance is limited by the formation of resistive solid-liquid electrolyte interphases at the interface between the solid electrolyte separator and the liquid catholyte. In this work, we investigate solid-liquid electrolyte interphase formation at the interface between an argyrodite-type sulfide solid electrolyte Li5.5PS4.5Cl1.5 and two representative liquid electrolytes: a conventional ether-based electrolyte (LiTFSI in DOL:DME) and an ionic liquid (LiTFSI in EMIMTFSI). Using a combination of time-resolved electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, and focussed ion beam scanning electron microscopy, we reveal substantial differences in interphase chemistry, morphology, and transport properties. The ether-based electrolyte undergoes continuous chemical reaction with the sulfide solid electrolyte, forming a thick, inhomogeneous, and highly resistive interphase (∼2000 Ω cm[2] after 250 h), driven in part by dissolution of polysulfide species. In contrast, the ionic liquid electrolyte forms a significantly thinner, layered interphase with a comparatively low area-specific resistance (∼150-220 Ω cm[2]), which remains constant over extended time and during electrochemical cycling. These findings demonstrate that choice of catholyte plays an important role in governing solid-liquid electrolyte interphase formation, and highlights ionic liquids as viable catholytes for stable, low-resistance interfaces in quasi-solid-state lithium-sulfur batteries with argyrodite-type solid electrolyte separators.

The stability of the solid electrolyte interphase (SEI) in sodium-ion batteries remains a critical challenge for achieving long cycle lifetimes. In hard-carbon anodes, the interplay between microstructure, solvation chemistry, and electrolyte decomposition dictates SEI evolution, yet the mechanistic link between electrolyte decomposition, gas evolution, and interphase formation remains poorly resolved. Here, we apply chip-based electrochemistry-mass spectrometry (EC-MS) with picomole-per-second sensitivity to monitor real-time gas evolution during SEI formation on hard carbon in different solvent systems, representing one of the first applications of EC-MS to sodium-ion cells. Carbonate-based (EC:DMC) electrolytes exhibit pronounced gas evolution, dominated by C2H4 during the first discharge, consistent with reductive decomposition of EC and the formation of sodium ethylene dicarbonate and Na2CO3 rich interphase species. In contrast, ether-based electrolytes (diglyme) show strongly suppressed gas evolution, with only trace C2H4 detected, alongside improved rate capability and enhanced cycling stability over 100 cycles at 0.5 C. These differences are attributed to solvent-dependent solvation and interfacial kinetics, arising from the lower effective desolvation barrier and higher reductive stability of diglyme relative to carbonate solvents, leading to the formation of a thinner and more ion-permeable SEI. Collectively, these findings establish a direct connection between solvent chemistry, gas evolution, and SEI formation in sodium-ion systems and demonstrate the utility of operando chip-based EC-MS for resolving electrolyte degradation pathways.

Fluorine additives in lithium metal batteries are often correlated with improved performance, yet lead to challenges associated with cost, health, and safety. In this work, we formulate a series of electrolytes that allow us to interrogate the physical and electrochemical properties of non-fluorinated and fluorinated anions (nitrate, bis(oxalato)borate, and hexafluorophosphate) on the reversible electrodeposition of Li metal via traditional surface characterization methods, spectroscopy, and electroanalytical techniques. When the salts are used in the moderate concentration regime (0.5 M Li), the solid electrolyte interphases (SEIs) generated during cycling are primarily solvent-derived and are statistically similar in terms of ionic transport properties. Nuclear magnetic resonance (NMR) in conjunction with transient voltammetry measurements indicate that Li metal CE is correlated with charge transfer, which is strongly influenced by Li[+] solvation structure. Specifically, we find that the nitrate anion displaces solvent molecules in the Li[+] solvation sphere, leading to tunable solvation/desolvation dynamics. We demonstrate that these metrics can be utilized to construct fluorine-free electrolytes containing lithium nitrate and lithium bis(oxalato)borate for high performance lithium metal anodes.

This study demonstrates the utility of the electrochemical-surface force balance (e-SFB) for observing solid electrolyte interphase (SEI) formation directly at an electrode surface. Through proof-of-concept measurements of lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) based electrolytes, absolute thickness and mechanical properties of SEIs are measured in situ before and after electrochemical cycling. The SEIs that formed in these systems were found to have thickness on the order of 100s of nm and elastic moduli on the order of 10s of MPa depending on cycling conditions and the nature of the solvent.

The growing demand for sustainable energy has intensified efforts to develop safer, high-performance batteries. Lithium metal offers exceptional energy density but its use is limited by safety concerns and short cycle life. Electrolyte additives such as LiNO3 are known to enhance battery performance, yet their specific mechanism remains unclear. Nuclear magnetic resonance (NMR) spectroscopy provides a powerful and non-destructive means to probe Li metal batteries, offering a unique insight into the Li species and interfacial processes. Among the different NMR methods, operando[7]Li NMR measurements of the Li-S battery enable time-resolved and quantitative monitoring of electrochemical Li metal deposition, thereby linking electrochemical performance to changes in the metallic Li environment. Complementarily, ex situ dynamic nuclear polarization (DNP) NMR experiments on Li metal microstructures provide detailed structural information about the interface between the metal and the solid-electrolyte interface (SEI). Together, these approaches provide a comprehensive picture of both the dynamic and structural aspects governing Li metal anode behavior. In this work, we systematically investigate the influence of LiNO3 concentration in the ubiquitous Li-S electrolyte, 1 M LiTFSI DOL : DME, on controlling anode performance and interfacial processes using a combination of operando[7]Li NMR and ex situ DNP NMR spectroscopy. DNP NMR spectroscopy reveals that LiNO3 distinctly modifies the inner SEI, correlating with improved cell performance. In contrast, operando[7]Li NMR shows that increasing LiNO3 concentrations only marginally affect Li deposition. Together, these results demonstrate that while LiNO3 enhances Li metal anode behavior in Li-S batteries, higher additive levels do not yield additional benefits. This combined NMR approach provides new insight into interfacial processes and supports rational electrolyte design for high-performance Li-S batteries.

The solid electrolyte interphase (SEI) is widely recognized as essential for enabling lithium metal electrochemistry, yet it is still commonly described using qualitative terms that obscure the underlying transport and mechanical parameters governing function. A recent conceptual shift treats the SEI as a functional solid-state electrolyte (SSE), enabling direct quantification of SEI ionic conductivity, electronic conductivity, and Li[+] transference number in separator-free architectures. Here, we extend this perspective from quantification to engineering by identifying three independent and experimentally addressable axes that control whether an SEI can operate as an SSE: (i) transport chemistry, tuned through electrolyte salt concentration to shift SEI formation toward anion-derived, inorganic-rich products; (ii) interfacial nucleation thermodynamics, tuned through current collector surface modification to regulate where lithium nucleates relative to conductive SEI residues; and (iii) mechanical robustness, tuned through the formation of ceramic nanofiller-reinforced composite SEI layers inspired by structural composite materials. Using separator-free Cu|SEI|Li cells, we show that increasing LiFSI concentration in DME from 4 M to 6 M can form an SEI that functions as an SSE with increased first-cycle coulombic efficiency (CE) from 32% to 85% even under an 8× higher current density, indicating that ion transport can be substantially improved by electrolyte-driven changes in SEI chemistry. Using the accumulated thick residual SEI (rSEI) harvested after cycling, we demonstrate that rSEI itself can sustain separator-free plating/stripping without short circuiting, and that replacing Cu with Au-coated substrates increases average CE from ∼15.8% to ∼70%, highlighting that nucleation energetics represent an independent design lever beyond bulk transport. Finally, incorporating ceramic nanoparticles (SiO2, TiO2, Al2O3) into chemically formed SEI yields nanofiller-reinforced SEI that enables separator-free Li/SEI/Cu cycling at practical current densities with a first-cycle CE of 82.7%, representing a substantial improvement over earlier SEI-as-SSE systems and supporting current densities ∼50× higher than baseline SEI-SSE operation. Together, these results define a multidimensional design framework in which the SEI emerges as an engineerable, self-assembled composite SSE whose performance is governed by coupled transport, nucleation, and mechanical constraints. This framework motivates a broader discussion: whether interphase engineering can blur the classical boundary between liquid-electrolyte batteries and solid-state batteries by enabling in situ-formed composite electrolytes that approach SSE-like functionality.

The fields of surface science and heterogeneous catalysis are often inherently scientific investigations of the same fundamental phenomena from different (but potentially complementary) perspectives. It is little wonder therefore that this juncture has for many years been a point of note for scientists in either area. Although their long-standing interaction has often helped to refine and shape thinking, it has also fostered a degree of mutual cynicism, encapsulated in the oft-heard choruses that "catalysis is too empirical to offer fundamental insight" and that "surface science is too idealized to mean anything in the real world." These concluding remarks attempt summarize aspects of the Faraday Discussion meeting entitled "Bridging the gap from surface science to heterogeneous catalysis" held at Burlington House, London, UK in April 2026. In particular, drawing on discussions with other attendees, I have tried to summarise some of the key themes: both progress overcoming gaps at the interface of these two fields and perhaps new gaps that emerge as a result of improved insight as the fields advance their own understanding and meet new challenges.

The electric double layer (EDL) determines the species present at the electrode-electrolyte interface (EEI) and the distribution of reactants that will form the solid electrolyte interphase (SEI). Knowledge of the species in the EDL at the onset of formation and the change in the EDL composition with time provides valuable insight into SEI formation mechanisms and improves control over the SEI composition and morphology. Despite this significance, the EDL remains challenging to study with length scales on the order of 10s of nanometers and a complex dependence on electrolyte formulation and electrode potentials. In this work, we use operando attenuated-total-reflectance (ATR) surface-enhanced infrared absorption spectroscopy (SEIRAS) to study the EDL in non-aqueous lithium electrolytes. We report operando ATR-SEIRAS on 1 M lithium hexafluorophosphate (LiPF6) in ethylene carbonate (EC) : ethyl methyl carbonate (EMC) 3 : 7 wt/wt and 1 M lithium bis(fluorosulfonyl)imide (LiFSI) in 1,2-dimethoxyethane (DME) in Li‖Cu half cells to observe the evolution of the species in the EDL as a function of cell voltage. We use these data to advance detailed models of the EDL evolution between 0 V and 3 V vs. Li/Li[+]. We then use this model to control the SEI composition for the ether-based electrolyte via a constant potential protocol. This work serves as an important step towards understanding the evolution of the EDL structure and how it determines the composition of the SEI in lithium-metal batteries during cycling.

Electrochemical impedance spectroscopy (EIS) is widely used to probe the solid electrolyte interphase (SEI) in alkali-metal batteries, yet the interpretation of impedance responses remains challenging due to the simultaneous influence of interfacial transport properties and evolving electrode morphology. Here, we systematically evaluate the capability of EIS to characterise SEI behaviour in symmetric lithium- and sodium-metal cells using carbonate and ether electrolytes. Time-resolved impedance measurements were combined with distribution-of-relaxation-times analysis, equivalent-circuit modelling, temperature-dependent EIS, and in situ NMR spectroscopy to track interfacial evolution during formation, cycling, and rest. EIS captures key interfacial changes, including resistance growth during open-circuit conditions and capacitance increases associated with expansion of the electrochemically active surface area during cycling. However, comparison with in situ NMR reveals that continuous microstructure formation and SEI accumulation can occur without being directly reflected in impedance-derived SEI parameters. Temperature-dependent analysis further shows that activation energies derived from resistance may include geometric contributions associated with surface-area changes, whereas analysis of the interfacial time constant provides a more surface-area-independent metric of SEI transport. These results demonstrate that while EIS is a powerful tool for monitoring interfacial evolution, impedance responses in metal-electrode systems must be interpreted carefully and supported by complementary techniques to distinguish intrinsic SEI properties from morphology-driven effects.