Probing lithium mobility at a solid electrolyte surface

Abstract

Solid-state electrolytes overcome many challenges of present-day lithium ion batteries, such as safety hazards and dendrite formation^1,2. However, detailed understanding of the involved lithium dynamics is missing due to a lack of in operando measurements with chemical and interfacial specificity. Here we investigate a prototypical solid-state electrolyte using linear and nonlinear extreme-ultraviolet spectroscopies. Leveraging the surface sensitivity of extreme-ultraviolet-second-harmonic-generation spectroscopy, we obtained a direct spectral signature of surface lithium ions, showing a distinct blueshift relative to bulk absorption spectra. First-principles simulations attributed the shift to transitions from the lithium 1 s state to hybridized Li-s/Ti-d orbitals at the surface. Our calculations further suggest a reduction in lithium interfacial mobility due to suppressed low-frequency rattling modes, which is the fundamental origin of the large interfacial resistance in this material. Our findings pave the way for new optimization strategies to develop these electrochemical devices via interfacial engineering of lithium ions.

Fig. 1. LLTO structure and experimental geometry.

a, Basic crystal structure of LLTO, consisting of alternating Li-rich and -poor layers and Ti and O octahedra. b, Calculated partial density of states (DOS) for LLTO and indicated transitions for the XUV-SHG probe. c, Overview of the experimental setup used for measurement of XUV-SHG data in reflection geometry. The inset shows the layered sample structure with repeating layers of LLTO and LCO. d, Schematic representation of an LCO–LLTO stack forming a prototypical battery, with the XUV-SHG process indicated on the top surface; note that LLTO is polycrystalline in the film measured despite the schematic representation. XFEL, X-ray free-electron laser. MCP, microchannel plate.

Fig. 2. Measured and numerically simulated linear and nonlinear response of LLTO at the lithium K-edge.

a, The measured imaginary part of the refractive index of LLTO (brown shaded area, left axis) agrees well with the numerically retrieved linear response (dashed brown line, right axis) around the Li K-edge that appears around 61 eV in LLTO. The calculated curve is an equal-weight linear superposition of response from both LLTO and LCO. See Supplementary Fig. 13 for the effect of including the LCO contribution. The slight disagreement of the linear response at higher energies could have stemmed from broadening due to unrealistic sample structures in the simulation, where an ideal crystal geometry is assumed. b, Experimentally derived second-order nonlinear susceptibility χ⁽²⁾(2ω) response across the Li K-edge (blue open squares, blue solid line for visual clarity). The computationally simulated second-order nonlinear susceptibility χ⁽²⁾(2ω) for a LLTO at the surface is denoted by solid triangles, and is in good agreement with the measurements. Vertical error bars correspond to errors in the quadratic fit of the second-order response; horizontal error bars are a result of energy jitter of the FEL for which 72,000 shots were collected at each photon energy. a,b, Double-sided arrow highlights the difference in peak positions between linear absorption (a) and second-harmonic response (b). c,d, Representative wave function of the resulting lithium atom core-excited states in the XUV-SHG spectrum at ~61 eV (c) and ~64 eV (d). We adopt the convention that the positive and negative phases of wave function are coloured beige and teal, respectively.

Fig. 3. Restricted lithium dynamics at LLTO surfaces.

a, Plot of the barrier for Li migration along the low-energy pathway (inset), resolved for Li atoms at the surface (blue curve) and in the bulk (brown curve). Lower barriers are calculated for bulk diffusion. b, Lithium vDoS energy distribution at the surface and in bulk LLTO. We found a reduction in the population of low-frequency rattling modes (50–100 cm^–1) for the surface lithium ion, which resulted in 40% reduction in entropy. The region in the dashed rectangle is enlarged in the bottom panel of g. c–f, Visualization of bulk LLTO vibrational modes at 62 cm^–1 (c), 70 cm^–1 (d), 89 cm^–1 (e) and 132 cm^–1 (f). Arrows indicate the general direction of atom displacement at a particular frequency. Li vibrational dynamics are coupled with cage breathing modes (c,f), as well as the optical longitudinal (d) and transverse (e) modes. g, vDoS of the LLTO cage (top) and Li (bottom) vibrations in the low-frequency range. c–f, The various vibrational modes are indicated. Surface cages show suppressed or blueshifted vibrational modes, which compromises Li interfacial dynamics.