Solid-State NMR Finds Its Place in Energy Storage Research – Understanding Paramagnetic Materials

Improving electrochemical energy storage is one of the major challenges the scientific community faces today. The search for new battery materials and technologies, however, together with the drive to improve performance and lower the cost, is not straightforward. Success is based on a comprehensive understanding of the underlying chemistries of the materials and the relationships between the components involved.

Paramagnetic materials and metals – characterized by the presence of unpaired or conduction electrons – exhibit unique electrochemical properties that make them ideal for use in energy storage and battery applications, and there are several analytical technologies that can be used to understand these materials.

A consensus has emerged that solid-state nuclear magnetic resonance (NMR) spectroscopy, in conjunction with electron paramagnetic resonance (EPR) spectroscopy, provides a specific picture that can help researchers and manufacturers improve battery lifetime and performance, ultimately leading to more advanced, sustainable, and cost-effective batteries.

In this article, we hear from three leading academic researchers about how their work using NMR spectroscopy at a reduced magnetic field is improving paramagnetic and metallic materials analysis and providing new insights into this fast-moving sector.    

The analytical landscape in battery research

EPR can be used to detect unpaired electrons in paramagnetic materials in the electrodes. By observing the free radical chemistry in the electrolyte causing electrode material degradation during charge and discharge cycles, EPR provides insights into material behavior under electrochemical conditions.

In contrast, NMR is primarily used to investigate the structure and dynamics of battery materials at atomic level (using for example ^6,7Li, ²³Na, ¹⁹F, ¹H), allowing researchers to extract information related to their properties and performance – identifying chemical changes that may occur during charge and discharge cycles, for example. NMR can detect decomposition products, track changes in electrode and electrolyte materials, and correlate chemical changes to battery capacity fade over time.

Whilst NMR spectroscopy has a central role in materials analysis, traditional high-field NMR spectroscopy (800 MHz and upwards) is considered a challenge for paramagnetic and metallic materials due to short relaxation times and signal broadening, as well as large chemical shifts that can push signals outside the range of high-field instruments.

Recently, reduced-field NMR instruments (200 MHz) have been employed as an alternative to high-field systems. Applying a lower magnetic field helps not only resolve some of the specific challenges encountered with paramagnetic and metallic materials when using high-field solid-state NMR, but also adds its own specific benefits.

Dr. Juan Miguel López del Amo, head of Solid-State NMR Laboratory at CIC energiGUNE, explained how working at lower fields provides benefits when analyzing paramagnetic materials: “It might seem counter-intuitive to use a low-field magnet because, generally, the higher the field, the better the outcome. That is not the case for paramagnetic materials. There are advantages to applying relatively low fields, including reduced broadening to better resolve spectra and the application of stronger pulses to observe signals. This allows us to see nuclei interactions clearly.”

Reduced-field solid-state NMR instrumentation can offer specific advantages when analyzing paramagnetic materials: 

• Reduced spectral distortion makes it possible to acquire sharper and more defined peaks
• Differentiating T₁ (longitudinal) relaxation times of different materials becomes more straightforward
• Detection of T₂ (transverse) relaxation signals that are otherwise lost at high fields can be achieved.

Professor Kent Griffith, assistant professor at the University of California, San Diego, described recent trends in NMR field strengths: “While there's been a move towards higher magnetic fields for better resolution in most NMR applications, low-field magnets such as a 200 MHz instrument are better suited to paramagnetic NMR. Low-field NMR, especially when combined with rapid spinning, enables experiments that are otherwise impossible on standard high-field instruments.”

Further advancements in low-field NMR spectroscopy are supporting its application in paramagnetic materials. Using low-field NMR has brought new benefits to Prof. Griffith’s work. “A challenge with low-field instruments for conventional diamagnetic materials is the lower sensitivity, since signal increases exponentially with field. However, paramagnetic materials relax rapidly so rapid pulsing and ‘summing up’ many spectra can address this issue,” he said.

Dr. López del Amo added: “NMR enables the use of paramagnetism as a source of structural insight rather than a limitation. Paramagnetic interactions are used to gain local structural information, electronic information, in the context of the elemental surroundings. This is a key for looking at materials such as lithium and sodium in situ – within a complete battery.” 

Driving advances in battery research

Improving electrode performance is important for battery development. Many common electrode materials in lithium-ion batteries rely on transition metals, e.g., manganese, nickel, cobalt and iron. These transition metal ions often possess unpaired electrons, making them inherently paramagnetic.

The paramagnetism of key battery components and the paramagnetic species formed during battery operation are valuable in understanding fundamental electrochemistry, diagnosing degradation mechanisms, and developing new and improved battery materials.

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New lithium-rich materials are emerging as a potential alternative to current cathode chemistries. Dr. Griffith and his team are currently using NMR to investigate. “We are working to understand the different local environments that exist in these new materials, where the excess lithium ions are initially in the structure,” he explained. “Understanding where excess lithium ions sit, and which ones are able to be removed upon charge cycles, are critical factors in developing high-energy and stable battery performance.”

In addition, unwanted side reactions, a mechanism of degradation, can lead to battery capacity fade and safety issues such as gas build-up. Solid-state NMR can monitor such chemical changes in batteries over extended periods, mapping the aging process during charge and discharge cycles under actual operating conditions.

Understanding dynamics in equilibrium and ion diffusion within battery structures is also supported by solid-state NMR. Dr. López del Amo provided an example. “In our work, NMR allows us to see how elements diffuse through the solid electrolyte interphase (SEI) to reach the electrodes or between the different composite materials. This is very important information for the battery community as it directly links to the performance, safety and lifespan of a battery.” 

The benefits of lower fields

Characterizing paramagnetic materials in batteries can be complicated because their composition, electron count, oxidation states, magnetic properties, electronic properties and ionic properties can change dramatically during charge and discharge cycles.

Importantly, as functional batteries contain large amounts of metals, including casing and current collectors, Dr. Elodie Salager, researcher at the Centre National de la Recherche Scientifique (CNRS) believes using reduced-field solid-state NMR is advantageous. “If you want to look at whole batteries that contain metallic parts, the problems of shielding and distortions are reduced when working at lower fields,” she noted.

Recent work with NMR in Dr. Salager’s lab has focused on operando studies in conditions close to industry protocols and especially rapid charging, a critical development area in the electric vehicle (EV) industry. Fast charging involves the transfer of high electrical current into the battery in a very short time, and the chemistry inside the battery cells during this process can encounter kinetic limitations that lead to “degradation reactions” and the creation of metallic lithium dendrites that compromise battery performance.

Whilst there are many parameters at play, Dr. Salager reiterates that NMR is highly effective in detecting these chemical changes early, with the potential to provide information to improve charging protocols and help prevent dendrites forming during fast charging or low temperature operation.

“To better understand the limitations in terms of charging rates and capacity, analytical techniques must be pushed to their limits to characterize, in situ and in a non-invasive and non-destructive way, the internal parts of batteries in operating conditions,” she said. “In our work we have found that combining in situ NMR spectroscopy and imaging enables visualization of the displacement of lithium fronts inside paramagnetic electrodes during battery operation, something that is critical to identifying limiting parameters in high-capacity and fast-cycling batteries.”

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The non-invasive NMR technique developed by Dr. Salager and her team also offers opportunities to study other devices that contain paramagnetic materials while they are operating.

Complementary nature of EPR and solid-state NMR

The combination of solid-state NMR and EPR provides researchers with a more comprehensive picture of a battery system under investigation. EPR focuses on the paramagnetic centers themselves to directly detect electronic spins and probe the vicinity of electrons, but it is “blind” to closed-shell materials like electrolytes.

NMR, on the other hand, examines the influence of the electrons on surrounding nuclei, making the two techniques highly complementary for understanding battery materials.

This combined approach helps in understanding local changes during charging and discharging, ion diffusion within and between structures and detecting low-concentration or disordered materials.

“Using NMR in addition to EPR is useful in battery analysis to understand, for example, how transition-metal ions in the cathode structure, such as iron and manganese, influence the electrochemically active lithium and sodium ions in the cathode,” Dr. López del Amo explained. 

Reduced-field NMR for tomorrow’s battery materials

Diversifying battery chemistries will be essential in the battery industry of the future to pave the way to application-specific performance characteristics, such as high-power density, extended cycling stability, or faster charging.

Insights derived from low-field NMR will contribute directly to strategies for enhancing recyclability and the circular economy of batteries. In parallel, software developments are reshaping data acquisition and interpretation. The integration of machine learning for automated and more accurate signal interpretation enhances experimental throughput and reliability of complex systems.

These future-looking innovations position reduced-field solid-state NMR as an essential technique for shaping new research into sustainable, high-performance and recyclable energy storage technologies.