Li-Ion Batteries (LIBs) are widely used in electronic devices, electric vehicles, power grid storage, and other fields due to their small size, lightweight, high battery capacity, long cycle life, and high safety.
Electron paramagnetic resonance (EPR or ESR) technology can non-invasively probe the inside of the battery and monitor the evolution of electronic properties during the charging and discharging of electrode materials in real-time, thus studying the electrode reaction process close to the real state. It's gradually playing an irreplaceable role in the study of the battery reaction mechanism.
Composition and Working Principle of Lithium-ion Battery
A lithium-ion battery consists of four main components: the positive electrode, the negative electrode, the electrolyte, and the diaphragm. It mainly relies on the movement of lithium ions between the positive and negative electrodes (embedding and de-embedding) to work.
Fig. 1 Lithium-ion Battery Working Principle
In the process of battery charging and discharging, the changes of charging and discharging curves on the positive and negative materials are generally accompanied by various microstructural changes, and the decay or even failure of performance after a long time cycle is often closely related to the microstructural changes. Therefore, the study of the constitutive (structure-performance) relationship and electrochemical reaction mechanism is the key to improving the performance of lithium-ion batteries and is also the core of electrochemical research.
EPR (ESR) Technology in Lithium-ion Batteries
There are various characterization methods to study the relationship between structure and performance, among which, the electron spin resonance (ESR) technique has received more and more attention in recent years because of its high sensitivity, non-destructive, and in situ monitorability. In lithium-ion batteries, using the ESR technique, transition metals such as Co, Ni, Mn, Fe, and V in electrode materials can be studied, and it can also be applied to study the electrons in the off-domain state.
The evolution of electronic properties (e.g., change of metal valence) during the charging and discharging of electrode materials will cause changes in EPR (ESR) signals. The study of electrochemically induced redox mechanisms can be achieved by real-time monitoring of electrode materials, which can contribute to the improvement of battery performance.
EPR (ESR) Technology in Inorganic Electrode Materials
In lithium-ion batteries, the most commonly used cathode materials are usually some electrodeless electrode materials, including LiCoO2, Li2MnO3, etc. The improvement of cathode material performance is the key to improving the overall battery performance.
In Li-rich cathodes, reversible O redox can generate additional capacity and thus increase the specific energy of oxide cathode materials. Hence, the study of O redox has received much attention in the field of Li-ion batteries. There are still relatively few techniques to study the characterization of lattice oxygen redox reactions. For cathode materials, the stability of the cathode/electrolyte interface is closely related to the oxide species generated during the charging process, so it is necessary to study the chemical state of the oxidized O species. The EPR technique can detect the oxygen or peroxide species during the reaction, which provides technical support to study oxygen redox in Li-ion batteries.
Fig. 2 Chemical state of oxide O interpreted by EPR. (a, b) X-band EPR spectra of Na0.66[Li0.22Mn0.78]O2 at 50 K, in different charge and discharge states. Fig. a: the generation of (O2)n- (n=1, 2,3); Fig. b: the generation of captured molecular O2. Fig. c,d: Variable temperature EPR spectra under 4.5 V charging. It can be seen that (O2)n- is detected in the temperature range of 2-60 K, while molecular O2 can only be detected at the characteristic temperature of 50 K; Fig. e: Fine sweep EPR spectrum in the range of 5000-10000 G magnetic field; Fig. f: X-Band EPR spectrum of Na0.66[Li0.22Mn0.78]O2 at 50 K, 4.5 V charging state. (J. Am. Chem. Soc. 2021, 143, 18652−18664)
EPR (ESR) Technology in Organic Electrode Materials
In addition to inorganic materials, some organic small molecules or covalent organic framework materials (COFs) are also widely used in ion battery research. EPR spectroscopy can study the working principle of organic electrodes by non-destructive in situ, and monitor their redox reactions in real-time. As shown in Fig. 3, the formation and reduction of radicals during charge and discharge can be monitored using EPR technology. The regulation of the activity and stability of radical intermediates can be achieved by adjusting the thickness of two-dimensional COFs, thus providing a new breakthrough point for the design of new high-performance organic electrode materials for energy storage and conversion.
For conventional cigarettes, the presence of carbon-centered free radicals makes them detectable by EPR techniques. For modern e-cigarettes, the EPR technique allows the determination of free radicals generated during the inhalation of e-cigarettes and the quantification of the generation of EPFRs and the production of ROS in TPM, respectively.
Fig. 3 (a) Redox mechanism of free radical intermediates. (b) EPR spectra of COFs of different thicknesses before and after 30 cycles after discharge to 0.30 V. (c) EPR spectra of TSAQ samples before and after 30 cycles after discharging to 0.30 V. (d) EPR spectra of 4-12 nm thickness samples after immersion in electrolyte for different times. (e) NMR spectrum of 23Na after discharging the electrode to 0.05 V. (K. Am. Chem. Soc. 2019, 141, 9623−9628)
CIQTEK Electron Paramagnetic Resonance (EPR) Spectroscopy
The CIQTEK EPR (ESR) spectroscopy provides a non-destructive analytical method for the direct detection of paramagnetic materials. It can study the composition, structure, and dynamics of magnetic molecules, transition metal ions, rare earth ions, ion clusters, doped materials, defective materials, free radicals, metalloproteins, and other substances containing unpaired electrons, and can provide in situ and non-destructive information on the microscopic scale of electron spins, orbitals, and nuclei. It has a wide range of applications in the fields of physics, chemistry, biology, materials, industry, etc.