All-solid-state lithium-ion batteries (ASSLIBs) replace the liquid electrolyte with a solid electrolyte (SE). Both the electrodes and the electrolyte are solids, so the interfaces are buried. Side reactions, poor contact, and lithium dendrites are hard to see directly. Synchrotron X-rays can do diffraction, absorption spectroscopy, and imaging in the same facility, which is why they are used to watch bulk ion transport and how a solid–solid interface changes during cycling.

The cases below are grouped as electrolyte, cathode interface, anode interface, and failure morphology. Each paper is recorded once. Two source articles used the same experiments and the same DOIs; they are combined here rather than written as two independent measurements. Chemical formulae are typeset as formulae. Sample abbreviations that the papers or the software left short are kept as written.

What the synchrotron is being asked to see

Compared with a laboratory X-ray source, synchrotron light is brighter, tunable, and easier to use in situ. The methods that recur in this set are:

Method What it mainly reads
Synchrotron X-ray diffraction (SXRD) phase, lattice, and weak crystalline signal inside an amorphous electrolyte
X-ray absorption fine structure (XAFS), including the near-edge structure (XANES) and the extended fine structure (EXAFS) valence, coordination, and bond length of a chosen element
Synchrotron microscopy and computed tomography (SX-CT) three-dimensional pores, cracks, contact, and dendrites
Hard X-ray photoelectron spectroscopy (HE-XPS) interfacial chemistry deeper than conventional XPS

Soft X-ray absorption sits closer to the surface. Hard X-ray absorption gives the average chemical state of that element in the sample. Small-angle X-ray scattering (SAXS) is used for nanopores, and pair-distribution functions for short-range order. The division of labour is the same across the source articles. They do not contradict one another.

What synchrotron XRD and XAS are drawn as reading

How an absorption spectrum is split from the near edge into the extended region

On an all-solid-state interface, the source articles collect the problems into three kinds: the electrolyte decomposes at high potential and blocks ions; solids make poor contact and leave micropores and cracks; stress and phase change during charge and discharge keep rebuilding the interface. A conventional source often lacks both penetration and resolution at once. A buried interface leans on high-penetration SXRD and X-ray absorption spectroscopy (XAS).

Sketch of a sulfide solid-electrolyte interface

Solid electrolytes: crystallisation, local disorder, and moisture

A solid electrolyte has to carry ions and block electrons. Laboratory XRD on a poorly crystalline fluoride often only says “amorphous”. It does not show whether that material forms a solid solution with a crystalline phase.

Annealing and crystallisation of LAGP

Safanama and co-workers followed the phase change of the lithium aluminium germanium phosphate glass-ceramic $\mathrm{Li_{1+x}Al_xGe_{2-x}(PO_4)_3}$ (LAGP) with in-situ SXRD. Crystalline peaks appear when the sample is heated to $578\ ^\circ\mathrm{C}$. The intensity of the split (214) and (300) peaks is used to argue that the anneal has to be raised to $750\ ^\circ\mathrm{C}$ and held longer before aluminium is incorporated effectively. Early crystallisation is mainly aluminium-poor $\mathrm{LiGe_2(PO_4)_3}$ (LGP). As the temperature rises, LGP decreases and a more uniform LAGP is left. 10.1039/c6ta00402d

In-situ SXRD during LAGP annealing

Local coordination in amorphous LZCFO

Shen and co-workers used XAFS on amorphous $\mathrm{Li_{2.5}ZrCl_5F_{0.5}O_{0.5}}$ (LZCFO). XANES shows that co-doping $\mathrm{F^-}$ and $\mathrm{O^{2-}}$ does not change the $\mathrm{Zr^{4+}}$ oxidation state, but makes the surroundings of Zr more disordered. The Fourier transform and wavelet transform of the EXAFS give $\mathrm{Zr\text{-}Cl}$ at about $2\ \mathrm{\AA}$ and $\mathrm{Zr\text{-}F/O}$ at about $1.5\ \mathrm{\AA}$, written as the coordination polyhedron $[\mathrm{ZrCl}_a\mathrm{F}_x\mathrm{O}_x]^{(a+x+2x-4)-}$. The source presents that local disorder, and the amorphous fraction, as the reason $\mathrm{Li^+}$ diffusion is faster. 10.1002/adfm.202408571

XANES and EXAFS of LZCFO

Vacancy-rich LNC and humid air

Li and co-workers used in-situ SXRD and XAFS on vacancy-rich $\mathrm{Li_9N_2Cl_3}$ (LNC). At about 20% relative humidity, SXRD shows peaks of $\mathrm{Li_4Cl(HO)_3}$ and $\mathrm{NH_4Cl}$. In a humid atmosphere, the Cl K-edge XANES grows an extra product peak near 2827.7 eV. In dry air, the edge position and shape stay put. The same DOI is broken across a line in one place and written as 10.1126/sciadv.adh4626 in another. The second form is the one used here.

SXRD and the Cl K-edge of LNC in humid air

Morphology of a compliant electrolyte and a sulfide electrolyte

Soft X-ray microscopy and SX-CT can look at pores and contact without taking the cell apart. After cycling, the interface of an $\mathrm{Li\text{-}In/CSF/Li\text{-}In}$ symmetric cell stays flat, with almost no pores or cracks. The source attributes that to a compliant CSF that can deform and dissipate stress. An LPSC-based symmetric cell, by contrast, develops volume change, pores, and dendrite-like structures because $\mathrm{Li^+}$ plating and stripping are uneven, and the local current and the stress at a crack tip rise with them. The same work also uses SX-CT to compare pores inside compacted electrolyte pellets and discusses microcracks in LPSC together with dendrite growth. The source does not expand the formulae of CSF or LPSC. 10.1002/adma.202401909

SX-CT of an Li–In symmetric cell

Synchrotron images of electrolyte and interface contact

Cathode interface: NMC811 and LGPS

Oxide electrolytes have high ionic conductivity and a wide electrochemical window, but they are hard and brittle and often need a high-temperature sinter to densify. Sintering causes cation interdiffusion. Sulfide electrolytes paired with oxide cathodes are a different combination.

Li and co-workers followed the interface of a composite electrode of $\mathrm{LiNi_{0.8}Mn_{0.1}Co_{0.1}O_2}$ (NMC811) and $\mathrm{Li_{10}GeP_2S_{12}}$ (LGPS) with in-situ XANES. The Ni K-edge changes reversibly, corresponding to $\mathrm{Ni^{2+}/Ni^{3+}}$ and $\mathrm{Ni^{3+}/Ni^{4+}}$. The S K-edge moves back and forth during charge and discharge. On discharge a new signal appears at 2472.5 eV, which the source assigns to $\mathrm{Li_2S}$ and takes as evidence of an interfacial side reaction. Shifts of the red and blue regions on the first-derivative plot are used to argue that the interface is incompatible: at high voltage the sulfide electrolyte and the oxide cathode undergo a parasitic reaction, the cathode surface reconstructs, and performance drops. 10.1021/acsenergylett.9b01676

Ni and S near-edge spectra at the NMC811/LGPS interface

Anode interface: LATP reduced by lithium

Liu and co-workers used HE-XPS on the interface between $\mathrm{Li_{1.3}Al_{0.3}Ti_{1.7}(PO_4)_3}$ (LATP) and lithium metal. Raising the photon energy reaches beyond 10 nm. Fresh LATP is mostly $\mathrm{Ti^{4+}}$, with a little $\mathrm{Ti^{3+}}$ attributed to slight reduction during preparation and storage. After cycling, more $\mathrm{Ti^{3+}}$ is still seen even at an excitation energy of 6 keV, which the source reads as lithium reducing titanium at the interface. After a $\mathrm{Li_3PO_4}$ coating, $\mathrm{Ti^{3+}}$ is detected only at 3 keV. The source takes that as the coating preserving $\mathrm{Ti^{4+}}$ deeper in, so that $\mathrm{Li^+}$ transport at the interface can continue. 10.1021/acsami.8b06366

Ti chemical state at the LATP/Li interface at different excitation energies

What each case is answering

System Method Main observation in the source DOI
LAGP glass-ceramic in-situ SXRD crystallisation starts at $578\ ^\circ\mathrm{C}$; aluminium is incorporated more evenly after $750\ ^\circ\mathrm{C}$ 10.1039/c6ta00402d
Amorphous LZCFO XANES, EXAFS $\mathrm{Zr^{4+}}$ valence is unchanged; $\mathrm{Zr\text{-}Cl}$ and $\mathrm{Zr\text{-}F/O}$ coordination appear 10.1002/adfm.202408571
$\mathrm{Li_9N_2Cl_3}$ in-situ SXRD, Cl K-edge humid air produces side products; the edge is stable in dry air 10.1126/sciadv.adh4626
CSF and LPSC symmetric cells microscopy, SX-CT the CSF interface keeps contact; LPSC develops pores, cracks, and dendrite-like structures 10.1002/adma.202401909
NMC811/LGPS in-situ XANES Ni redox is reversible; the S edge shows a $\mathrm{Li_2S}$ signal 10.1021/acsenergylett.9b01676
LATP/Li HE-XPS $\mathrm{Ti^{4+}}$ is reduced after cycling; the deeper signal weakens after a $\mathrm{Li_3PO_4}$ coating 10.1021/acsami.8b06366

A classification of ionising-radiation techniques is given in 10.14062/j.issn.0454-5648.20240837. That paper is a taxonomy of methods, not one of the battery measurements in the table.

Source statements about the light source, kept as written

One source describes the High Energy Photon Source (HEPS) as reaching an electron energy of 6 GeV, quotes a brilliance of about $10^{22}$, and gives a hard X-ray ceiling of 300 keV, a spatial resolution of 10 nm, and an energy resolution of 1 meV. The brilliance unit is broken up in the original wording, so it is not completed here and is marked unchecked. The same text describes a fourth-generation source as something that would raise experimental capability after it is built, and mentions femtosecond-to-picosecond pulses for time-resolved work. Those are source specifications, not a measurement on a particular solid electrolyte.

The European Synchrotron Radiation Facility (ESRF) appears in that text only as existing beamline experience. It does not add a new all-solid-state battery data set. A HiP-CT flowchart in the same material is not tied to a specific cell, so it is left out.