New research on energy storage at Diamond

Image credit: UL Research institute

In recent years, there has been a surge of interest in lithium-rich cathodes with a cation disordered rock salt (DRS) structure. DRS cathodes are relatively low cost, and their high first charge capacities offer tantalising promise for high-energy-density Li-ion batteries with capacities beyond 300 mAh g-1. However, DRS cathodes suffer from a large first-cycle irreversible capacity loss. Researchers used Diamond’s I15-1 and I20-scanning beamlines to understand the mechanisms of the large first charge capacity and the origin of the capacity loss. 

As lithium resources are scarce and lithium extraction can have potential environmental impacts, researchers are investigating alternative materials to be used for energy storage. Sodium ion batteries are an example of rechargeable batteiriesbatteries where sodium ions are used as charge carriers, instead of the standard lithium ions. 

Various materials can be used as electrodes in sodium-ion batteries (SIBs), and a comprehensive understanding of their charge storage mechanisms is essential for SIB development. Researchers from the Christian-Albrecht University of Kiel, in Germany, used X-ray Absorption Spectroscopy (XAS) on Diamond's B18 beamline as part of a rigorous study of the sodium storage properties of ultra-small Fe3S4 nanoparticles. This material exhibits excellent electrochemical performance as an anode material for SIBs. 

A team of researchers performed investigations at Diamond to understand the structural evolution of nanostructured DRS cathodes operando, they acquired PDF data at Diamond's 15-1 beamline using DRIX electrochemical cells.  

Researchers therefore investigated operando the evolution of the average structure, short-range ordering and charge during the electrochemical cycling of a DRS, using both spectroscopic and structural probes. 

Performing High Energy Resolution Fluorescence Detected (HERFD-)XANES operando at the I20-scanning beamline provided a better understanding of the charge compensation mechanism. 

The combined use of advanced PDF refinement methods and BVS mismatch mapping revealed the local cation ordering of Li-ions with battery cycling can perturb the percolating Li-diffusion network in DRS. Total scattering and XE show that the cation and lithium vacancies in the layered domain that form during cycling become less accessible in subsequent charge cycles.  

The trapping of Li-ions in short-range-ordered domains could be associated with the capacity fade of DRS and could be a significant source of capacity fade alongside contributions from oxygen redox irreversibility. 

Figure: Local structure and Li diffusion of LMTO DRS from PDF refinements with electrostatic potential constraints. Left: Magnified view of the short r-range of the PDF refinements. Right: refined average structural models (top) and supercells of atomistic disorder (bottom). Green, purple, blue and red spheres denote Li, Mn, Ti and O, respectively. The yellow isosurfaces represent the regions with an energy threshold suitable for Li-diffusion. This figure is taken from our published work (https://doi.org/10.1039/d2ta04262b)

Future research should investigate the formation of short-range ordering in DRS cathodes, whether it forms during ball milling to prepare the composite or the evolution of these domains with electrochemical cycling.  

The key to reducing capacity losses of DRS could be preventing the formation of layered domains during cycling or controlling their size. Potentially, this could be achieved via nanostructuration or by introducing electrochemically inactive dopants. 

Although the growth of layered domains can act as a trap for lithium, layered cathode materials have an inherently high capacity and rate capability. Further investigation into the interplay between the layered and DRS sublattices is needed, and the impact of short-range ordering should be optimised.  

This work provides insight into the design of better DRS cathodes and highlights the importance of local structures in the cyclability of battery materials. Furthermore, successful control of the coexistence of layered and DRS sublattices offers a novel route to electrode design, opening a new path to developing high-performance cathode materials. 

This research is an example of how multimodal, operando experiments across complementary techniques including HERFD-XANES, XES (Kβ main line, V2C), and X-ray Total Scattering (Bragg, XPDF), can aid the complete understanding of complex electrochemical processes in new battery materials.  

Previous research using X-ray diffraction and total scattering (pair distribution function analysis) had shown different structural phase transformations during the discharge and charge of the anode material. For a more detailed understanding, the researchers needed to analyse the local structure around the elements Fe and S and their oxidation states, in the pristine nanocrystalline material and during charging and discharging. Such element-specific information, combined with other techniques (to determine the crystallographic structure), can yield important insights into the reaction mechanism of a battery material during operation and allow optimisation of battery cells, by tailoring materials properties or by adjusting cut-off potentials. 

B18 provides high-quality XAS data, which allow precise and reliable determination of changes at the K-edges and yield fantastic k space spectra with high spectral resolution. During their experiments, the team gathered X-ray absorption near edge structure (XANES) spectra at the Fe and S K-edges and extended X-ray absorption fine structure (EXAFS) spectra at the Fe K-edge. Their results showed that the Na storage mechanism of this anode material can be attributed to cationic redox chemistry involving Fe. The experiments revealed the oxidation states of both elements at specific discharge/charge voltages. Using this information in combination with findings from other techniques (multi-method analysis), the researchers were able to explain the long-term cycle stabilities of Na/Fe3S4 cells during cycling to different lower cut-off potentials.  

XANES and EXAFS experiments yield insights into sodium storage mechanisms that are important to understanding and developing other electrode (anode and cathode) materials ex situ and for testing battery cells in operando during galvanostatic or potentiostatic measurements. The investigation of fundamental redox reactions in battery chemistry is a highly relevant topic to understand degradation reactions. Such studies can be used to find root causes for cell failure, precisely adjust battery cell limits and find optimal cycling conditions to improve the electrochemical performances and battery lifetime. 

Figure: Simplified reaction mechanism of the anode material nano-Fe3S4 during discharge and charge vs. Na+/Na in sodium-ion battery cells: X-ray absorption spectroscopy experiments at Diamond Light Source (beamline B18) revealed the cationic redox chemistry during Na storage, which involves Fe3+, Fe2+, and Fe0 products.

To find out more about the B18 beamline or discuss potential applications, please contact Principal Beamline Scientist Diego Gianolio: [email protected]  

To find out more about the I20 beamline or discuss potential applications, please contact Principal Beamline Scientist Shu Hayama:  [email protected] 

To find out more about the I15-1 beamline or discuss potential applications, please contact Principal Beamline Scientist Philip Chater: [email protected]