Engineering the solid electrolyte interphase as a solid-state electrolyte

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.