Measuring the buried interphase between solid electrolytes and lithium metal using neutrons

Interfaces are the key to next generation high energy batteries including solid state Li metal batteries. In solid state batteries, the buried nature of solid solid electrolyte electrode interfaces makes studying them difficult. Neutrons have significant potential to non destructively probe these buried solid solid interfaces. This work presents a comparative study using both neutron depth profiling (NDP) and neutron reflectometry (NR) to study a model lithium metal-lithium phosphorus oxynitride (LiPON) solid electrolyte system. In the NDP data, no distinct interphase is observed at the interface. NR shows a difference between electrodeposited, and vapor deposited LiPON -Li interfaces but finds both are gradient interphases that are less than 30 nm thick. Additional simulations of the LiPON-Li2O-Li system demonstrate that NDP has an excellent resolution in the 50 nm-1 mm regime while NR has an ideal resolution from 0.1 - 200 nm with different sample requirements. Together NDP and NR can provide a complementary understanding of interfaces between Li metal and solid electrolytes across relevant length scales.

💡 Research Summary

The paper addresses a critical challenge in solid‑state lithium‑metal batteries: the buried solid‑solid electrolyte/electrode interface that is difficult to probe with conventional techniques. The authors evaluate two neutron‑based, non‑destructive methods—Neutron Depth Profiling (NDP) and Neutron Reflectometry (NR)—using a model system consisting of lithium metal in contact with lithium phosphorus oxynitride (LiPON), a well‑studied thin‑film solid electrolyte.

NDP works by irradiating the sample with thermal neutrons (≈0.025 eV). The neutrons undergo the ^6Li(n,α)T reaction, producing α particles and tritons whose energy loss depends on the depth at which the reaction occurs. By measuring the energy distribution of the emitted particles, a depth‑resolved Li‑6 concentration profile can be reconstructed. The authors prepared six NDP samples: thin (≈100 nm) and thick (≈500 nm) LiPON layers deposited on Li metal, with and without an additional 50 nm Ni interlayer. The α‑spectra clearly show a high‑intensity Li metal peak and a lower‑intensity LiPON shoulder. Fitting the spectra yields LiPON thicknesses of ~77 nm (thin) and ~294 nm (thick), consistent with deposition parameters. However, no distinct interphase between LiPON and Li metal is observable; the data are equally well‑fit by models with or without a sub‑10 nm interlayer. When a Ni layer is inserted, the Ni‑rich region produces a noticeable dip between the Li and LiPON peaks, demonstrating that NDP can detect interlayers with a higher atomic number or sufficient thickness (≈50 nm). Simulations extending the method to artificial interphases of Li₂O, AuLi alloy, and Ni show that NDP can resolve interlayers only when they are on the order of 100 nm or more, corresponding to a Li content of ~10¹⁸ atoms cm⁻². Thus, NDP excels in profiling Li concentration over depth ranges from ~50 nm up to 1 mm, but its resolution is insufficient for naturally formed, nanometer‑scale solid‑electrolyte interphases.

NR, by contrast, measures the reflectivity of a cold neutron beam (5 × 10⁻⁵ eV–0.025 eV) at small incident angles. The reflected intensity as a function of momentum transfer yields Kiessig fringes that encode layer thickness, roughness, density, and composition via the scattering length density (SLD) profile. The authors performed NR on analogous Li/LiPON stacks prepared by two deposition routes: electrodeposited Li on LiPON (electrochemical) and vapor‑deposited LiPON on Li (physical). The reflectivity curves reveal two distinct interphases: a gradient interphase <10 nm thick for the electrodeposited case and a slightly thicker (~30 nm) gradient interphase for the vapor‑deposited case. Modeling demonstrates that NR is highly sensitive to interlayers in the 0.1–200 nm range, providing precise thickness and compositional gradients that NDP cannot resolve.

By juxtaposing experimental data and systematic simulations, the authors map the complementary resolution windows of the two techniques. NDP offers excellent depth resolution for relatively thick samples (50 nm–1 mm) and can average over large areas (tens of mm²), but it cannot distinguish sub‑10 nm interphases unless the interlayer contains a high‑Z element or is sufficiently thick. NR, on the other hand, delivers sub‑nanometer to hundreds‑nanometer resolution, but requires smooth, well‑defined interfaces and is limited to thinner films due to neutron penetration depth and reflectivity constraints.

The combined approach thus provides a multi‑scale toolbox for solid‑state battery research: NDP characterizes the overall Li distribution and detects relatively thick interlayers (e.g., intentional buffer layers), while NR resolves the fine structural and compositional details of the buried solid‑electrolyte/Li metal interphase. This synergy enables researchers to study realistic battery interfaces over macroscopic areas and relevant length scales, informing the design of stable, low‑impedance interphases essential for high‑performance solid‑state lithium‑metal batteries.