Our scientific activity is directed  to batteries and supercapacitors and to thermal energy storage applications. Since most of the components of these systems are solids, the relevance of the application in the solid-state NMR is evident.

Given the variety of the investigated materials, the solid-state NMR platform at CIC energiGUNE has been very efficiently adapted to the investigation of ceramic materials, paramagnetic inorganic solids, metals and alloys, polymers, graphitic systems and composites formed by different materials.

NMR is a non-destructive quantitative technique that allows the structural determination of solid materials independently of their crystalline state. Thus, NMR is essential for the characterization of disordered solid materials such as polymers, semi-solid and amorphous inorganic systems.

The characterization of this type of components is crucial to understand surface degradation processes, interfaces and the structure of those materials. Solid-state NMR is also a local characterization technique, since it provides information on the atomic structure around the specific atoms. Consequently, this technique clearly reveals the presence of local structural defects that are not easily identifiable by diffraction methods.

Solid-state Nuclear Magnetic Resonance is also enormously versatile and accurate in the identification and quantification of dynamic processes. This is another feature widely exploited in our laboratory since kinetic processes are crucial in the electrochemical and molecular processes of energy storage materials.

Solid-state NMR as a key in the characterization of solid-state batteries.

The kinetics of lithium and sodium ions both within solid electrolytes and across the interfaces is one of the most important and least understood parameter in lithium and sodium batteries. In particular, the replacement of flammable liquid electrolytes with safer and more reliable solid electrolytes requires the development of high lithium conductivity materials (usually polymers, ceramics or composites of both) and optimal interaction with anodes and cathodes.

Solid-state NMR represents a unique tool whichallowsthe characterization of polymers such as the determination of dynamic lithium processes and their relationship with the kinetics of the anion and the polymer matrix in a quantitative way.

Our equipment also allows characterizations at variable temperatures, so that kinetic investigations allow us to obtain precise activation energy values. These activation energies obtained are related to the local mobilities of the atoms or ions investigated and are very important in understanding the experimental values of ionic diffusion obtained by other techniques.

Within the solid electrolyte research, composites generated by mixing polymers and lithium-conducting ceramics are attracting interest in the scientific community, with the idea of combining the processability of polymeric materials with the high ionic conductivity of ceramics.

In this field, the CIC energiGUNE NMR platform is carrying out intensive research and has been able to demonstrate its applicability to determine lithium exchange processes at the interface of polymers and ceramics. This research has recently gone a step further by proposing diffusion mechanisms at the interface.

Other dynamic processes of great importance in batteries are the kinetic processes of ion diffusion inside cathodes and anodes. These processes are also investigated in our NMR laboratory and the results correlate with the performances of the developed materials.

NMR of paramagnetic solids

One of the most relevant particularities of our NMR facilities is the presence of equipment optimized for the characterization of paramagnetic solid materials. The application of NMR for this type of materials has traditionally been unfeasible due to the strong interactions between these materials and the high magnetic fields used in NMR. These interactions usually result in low resolution signals from which it is not possible to extract useful information.

Our equipment includes low-field NMR equipment (200 MHz) and very high speed probes (1.3 mm) that allow Magic Angle Spinning (MAS) experiments up to 67 kHz. This combination allows us to detect, with adequate resolution, strongly paramagnetic materials including even large concentrations of iron. This equipment has been contributory in a large number of publications in the investigation of paramagnetic electrodes of both lithium and sodium. This work includes ex situ NMR characterization of the complex electrochemical processes that occur during battery cycling.

In situ characterization of batteries and supercapacitors

Our equipment dedicated to the research of electrochemical energy storage by NMR has taken an important step forward in recent years with the incorporation of a probe for the in situ characterization of batteries and supercapacitors. This equipment allows simultaneous measurement of the NMR signals of the different components of the electrochemical cells directly during their cycling inside the magnet. This step undoubtedly represents an advantage in understanding chemical and physicochemical processes that occur directly during electrochemical cycling and that are not easily isolated ex situ.

The investigation of solid-state NMR supercapacitors at CIC energiGUNE is also a central topic of our scientific interest. In this field of research, the main source of information is based on the interaction of the aromatic rings existing in the porous carbons of the electrodes used and the nuclei detected by NMR. The interaction of these aromatic components and the strong magnetic fields used in NMR generate local magnetic field perturbations that generate signal shifts depending on the arrangement of the atoms with respect to the pores of these carbons. Thus, the signals observed by NMR represent a clear measure of the accessibility of pores to different electrolytes used in our supercapacitors. This information is very useful for the development of these materials, especially if they are followed by the in situ characterizations available  at our facilities.

NMR for thermal energy storage processes

Also very relevant is the work that our platform is carrying out for thermal energy storage processes. In these systems, NMR measurements are used over a wide range of temperatures to understand the kinetic processes involved in the entropy and enthalpy changes that occur during certain phase changes. In particular, our research is focused on organic materials and the characterization of the molecular processes involved in the undercooling mechanisms observed in these materials.

Services to companies and collaborations with research centers

It is also important to highlight the collaborative vocation of our platform, which is open both to services to companies and to collaborations with other research centers. Moreover, in order to promote the access of public and private entities to our facilities, the NMR platform is part of STORIES, a European program for access to high added value facilities.

Finally, it is noteworthy the existing collaboration between our platform and the company Bruker Spain that has led us to be recognized by it in 2018 as a reference center for solid-state NMR at state level.