The search for safer, more powerful batteries hinges on seeing clearly what others cannot.
Imagine a powerful camera that transforms the subject it's photographing. Just as you're about to capture the perfect shot, the subject changes appearance under the bright studio lights. This is precisely the challenge facing scientists developing next-generation solid-state batteries using one of their most powerful diagnostic tools.
For years, researchers have relied on X-ray photoelectron spectroscopy (XPS) to examine the critical interfaces within batteries, believing they were seeing pristine images of chemical compositions. Recent revelations, however, have exposed a troubling reality: the tool itself may be creating the very features scientists thought they were discovering, potentially sending battery development down unproductive paths. This discovery forces us to reconsider what we thought we knew about the hidden world inside solid-state batteries.
The global push toward solid-state batteries represents a fundamental shift in energy storage technology. Unlike conventional lithium-ion batteries that contain flammable liquid electrolytes, solid-state batteries utilize solid electrolytes, eliminating the fire risk that has plagued everything from smartphones to electric vehicles 3 . This safety advantage comes with significant performance benefits, including higher energy density and the potential to use lithium metal anodes—a combination that could dramatically extend the driving range of electric vehicles and the battery life of electronic devices 1 .
Solid-state batteries can store more energy in the same volume compared to traditional lithium-ion batteries.
Elimination of flammable liquid electrolytes reduces fire risk in electronic devices and electric vehicles.
Central to many solid-state battery designs are solid polymer electrolytes (SPEs), particularly those based on poly(ethylene oxide) (PEO) and lithium salts like LiTFSI (lithium bis(trifluoromethanesulfonyl)imide) 1 5 . These materials offer flexibility, ease of processing, and the ability to suppress dangerous lithium dendrite growth 1 .
The performance of these electrolytes hinges on the complex solid electrolyte interphase (SEI) that forms where the electrolyte meets the lithium electrode. This nanoscale layer, though incredibly thin, can make or break a battery's performance, influencing everything from cycle life to charging speed .
For decades, scientists have turned to XPS as their primary tool for analyzing this critical interface. The technique allows them to identify chemical species on surfaces with high precision—or so they believed.
X-ray photoelectron spectroscopy (XPS) has long been the gold standard for surface analysis in battery research. The technique works by irradiating a sample with X-rays and measuring the energy of electrons that are ejected, providing a detailed chemical fingerprint of the surface composition. When properly functioning, it can identify different compounds, oxidation states, and chemical environments at the surface being analyzed.
XPS provides accurate, non-destructive analysis of surface chemistry in battery materials.
XPS radiation can degrade samples, creating artifacts that are misinterpreted as genuine interface chemistry.
The reliability of this technique has recently been called into question. A groundbreaking 2024 study published in the Journal of The Electrochemical Society revealed a critical flaw: the X-ray beam itself can degrade the samples researchers are trying to measure, particularly fluorinated compounds commonly used in lithium salts 2 . This photodecomposition phenomenon means that what scientists observe may be artifacts created during analysis rather than the actual chemical composition of the original surface.
The most widely used conductive salt in solid polymer electrolytes is lithium bis(trifluoromethanesulfonyl)imide (LiTFSI). This salt has become popular due to its excellent ionic conductivity and electrochemical stability 1 5 . Its molecular structure contains multiple carbon-fluorine bonds, which are particularly susceptible to breaking under X-ray irradiation. When these bonds break during XPS analysis, they can form lithium fluoride (LiF), a compound that has frequently been reported as a key component of the solid electrolyte interphase 2 .
| Material | Chemical Formula/Symbol | Primary Function | Note |
|---|---|---|---|
| Poly(ethylene oxide) | PEO | Polymer matrix | Provides medium for ion transport2 5 |
| Lithium bis(trifluoromethanesulfonyl)imide | LiTFSI | Conductive lithium salt | Source of lithium ions; prone to X-ray degradation2 |
| Lithium metal | Li | Anode material | High-energy-density electrode4 |
| Lithium fluoride | LiF | Decomposition product | Previously thought critical to SEI; may be analysis artifact2 |
Contains vulnerable C-F bonds that break under X-ray exposure
Lithium bis(trifluoromethanesulfonyl)imide
X-rays break C-F bonds, releasing fluorine that forms LiF
Artifact formation during analysis
The 2024 study conducted by Breuer and colleagues provided compelling evidence of XPS-induced artifacts in solid polymer electrolyte analysis 2 . Their systematic investigation followed a meticulous process to isolate and identify the root cause of the mysterious LiF detections that had been frequently reported in literature.
The research team prepared a popular solid polymer electrolyte composition consisting of LiTFSI salt embedded in a PEO matrix 2 . This combination represents one of the most widely studied systems in solid-state battery research. The experimental approach was carefully designed to track changes occurring specifically during XPS analysis:
The researchers prepared SPE films with controlled composition and ensured consistent surface quality for reliable measurements.
The fresh, unexposed surface of the polymer electrolyte was immediately analyzed upon being placed in the XPS instrument.
The same surface area was subjected to continuous X-ray radiation for varying durations while repeated measurements were taken.
The spectral data collected at different time points were systematically compared to track the emergence of new chemical species.
This time-resolved approach was crucial for distinguishing between compounds that were genuinely present on the original surface and those that formed as a result of X-ray exposure.
The findings were striking. Initial measurements of the fresh polymer electrolyte surface showed little to no detectable LiF 2 . However, after just a few minutes of X-ray exposure, significant amounts of LiF began to appear in the spectra. The LiF signals grew stronger with prolonged exposure, clearly demonstrating that the compound was forming during analysis rather than being present initially.
| Exposure Time | LiF Detection Level | LiTFSI Integrity | Interpretation |
|---|---|---|---|
| Initial (0-2 minutes) | Low or non-detectable | High | Represents actual surface composition |
| Short (2-10 minutes) | Emerging | Beginning to degrade | Early stage of photodecomposition |
| Extended (10+ minutes) | Strongly evident | Severely degraded | Advanced artifact formation |
The research identified that the carbon-fluorine bonds in the TFSI anion were particularly vulnerable to breaking under X-ray radiation 2 . Once freed, the fluorine atoms readily combined with lithium ions to form LiF, which was then detected by the same X-ray beam that created it.
The implications of this discovery extend far beyond a single laboratory finding. For years, the battery research community has operated under the assumption that LiF is a crucial component of a stable SEI layer 2 . Numerous studies have aimed to intentionally engineer interfaces rich in LiF, based largely on XPS evidence. The 2024 findings compel us to reconsider these interpretations and raise troubling questions about how many previous observations were influenced by measurement artifacts rather than representing genuine electrochemical processes.
The revelation demands a critical re-evaluation of previously published work on solid electrolyte interphases, particularly those relying exclusively on XPS data 2 .
Conclusions about the composition and structure of SEI layers in solid polymer electrolyte systems may need to be revisited, especially those emphasizing LiF as a primary component.
This discovery also highlights the broader challenge of analyzing dynamic interfaces in battery systems. The solid electrolyte interphase is not a static layer but a complex, evolving structure that changes during battery operation. When analytical tools themselves alter the sample, distinguishing genuine interface chemistry from measurement artifacts becomes exceptionally difficult.
Potentially hundreds of papers may need reinterpretation
Research based on XPS findings may have been misdirected
Battery optimization strategies may need revision
Years of research progress may need reassessment
In response to these challenges, researchers are now developing more robust analytical approaches that minimize or account for potential artifacts. The future of battery interface characterization lies in multimodal techniques and careful experimental design.
To overcome the limitations of XPS, scientists are increasingly turning to complementary analytical methods that are less susceptible to radiation damage:
Provides information about molecular vibrations and functional groups without X-ray damage 4 .
Minimal Sample DamageOffers complementary molecular information with different selection rules, allowing cross-verification of results 4 .
Non-destructiveProvides extremely surface-sensitive elemental and molecular information through ion sputtering rather than X-ray exposure 4 .
Minimal RadiationMeasures electrical properties of interfaces without causing significant degradation 4 .
Non-invasiveNo single technique provides a complete picture, but together they can build a more reliable understanding of interface composition and structure.
While XPS remains a valuable tool when used properly, researchers must now implement specific strategies to minimize misinterpretation:
| Research Reagent/Material | Primary Function | Analytical Consideration |
|---|---|---|
| LiTFSI salt | Provides lithium ions for conduction | Highly susceptible to X-ray degradation; requires minimal exposure2 |
| PEO polymer matrix | Forms electrolyte scaffold | Generally stable under analysis; may host contaminants5 |
| Lithium metal electrode | High-capacity anode | Reactive surface; requires careful handling4 |
| LAGP ceramic electrolyte | Ionic conductor in composites | Interface stability crucial for performance4 |
| LLZTO ceramic filler | Enhances ion conductivity | Can improve stability of polymer composites4 |
The discovery of XPS-induced artifacts in solid polymer electrolyte analysis represents both a challenge and an opportunity for the battery research community.
While it forces us to reconsider certain assumptions about solid electrolyte interphases, it also opens the door to more rigorous and reliable characterization methods.
As research continues, this more critical approach will ultimately accelerate the development of safer, more powerful solid-state batteries.
The path to revolutionary energy storage solutions requires not just advanced materials and clever engineering, but also analytical techniques that reveal rather than transform the truth about battery interfaces. By acknowledging and accounting for the limitations of our tools, we can build a more accurate understanding of the complex nanoscale world that determines battery performance—bringing us closer to the energy storage breakthroughs that will power our future.
The message for both scientists and the public awaiting better battery technology is clear: what we see depends critically on how we look, and sometimes the most important discovery is recognizing the limits of our own vision.
For those interested in exploring this topic further, the seminal research discussed here was published in the Journal of The Electrochemical Society in 2024, while additional context on solid polymer electrolytes can be found in Nanoscale (2025) and the Journal of Materials Chemistry A (2014).