SAXS for Renewable Energy and Storage
Improve energy conversion, storage capacity, power delivery over longer lifetimes with higher safety profiles from lighter materials and a sustainable supply chain at a cheaper cost. There are high demands on scientists in this sector and the gains are marginal unless breakthroughs can be realised, such as using sodium ion and solid state cells.
Challenges
Ion transport is a trade-off, not a fix.
Faster ion transport requires more porosity or thinner materials, which trades away energy density and structural stability.
Degradation is varied, subtle, and easy to miss.
Mechanisms like cracking, SEI growth, dendrite formation and phase transformation progress at different rates. Catching them early requires sensitive techniques collecting frequent, statistically relevant data.
The most informative techniques rarely scale.
Minimal accessibility, laborious sample preparation and slow analysis times renders some of the most informative techniques impracticable to use on a regular basis.
Most testing destroys the sample.
Disassembling a cell exposes its materials to new conditions — which can change or mask the very property being measured.
What works at the bench doesn't always work at scale.
Manufacturing shifts properties from batch to batch, so validating consistency demands testing that’s fast, reproducible and sensitive.
Solution
With a dynamic range spanning five orders of magnitude, SAXS is a non-destructive technique that is sensitive to the performance properties of battery materials in operating environments at the micron to atomic scales.
Information obtained
Particle size and integrity
The particle size controls reaction kinetics through exposure of the surface area in and around the particle, under the high demands of battery performance these materials can crack and fail.
Porosity
This controls the surface area within a particle and the diffusion rates of electrolyte in and out thereby controlling reaction rates and the power density of electrochemical cells.
Solid electrolyte interphase growth
An important, protective layer that forms from initial decomposition of electrolyte and permits ion transport but prevents fluid transport. In operando SAXS monitors the growth of this layer during charge-discharge cycling to control capacity fade and observe any cracks that will accelerate this process.
Electrode volume expansion
In operando SAXS tracks the nanoscale structures for fractures that lead to capacity decay as the battery cells expand and contract from charge-discharge cycles to give insight into long-term durability.
Film thickness and integrity
Film thickness influences ion transport, electrical conductivity, light absorption, and catalytic activity, directly affecting the efficiency of batteries, fuel cells, electrolyzers, solar cells, and thin-film coatings. These properties may be controlled by measuring the thickness using SAXS.
Void formation in solid-state electrolytes
Voids form and grow due to imperfections in the contact between the electrolyte and electrode leading to increased resistance in the cell. Monitoring the voids with SAXS helps to develop robustness and longevity in these cells.
Dendrite formation
These structures can penetrate the separator in liquid state batteries causing short circuits and thermal runaway. SAXS identifies the formation and monitors growth of dendrites improving the safety of the cell.
Amorphous to crystalline phase transition
Amorphous phases are generally preferred as their disordered structure can host more ions, allows for free movement of ions and swells and contracts isotropically during cycling thereby reducing the propensity to crack the cell. SAXS gives early identification of this phase transition to support development and quality control.
Data example
In this example, operando SAXS/WAXS measurements of an Li-S cell show phase changes between different crystalline states and an amorphous state through electrochemical cycling. The q2 region in the SAXS data shows a subtle and reversible change in intensity attributed to Li₂S₂ formation. The heat maps show conversions between crystalline sulphur and LiS and amorphous sulphur.
Figure 1. Operando SAXS/WAXS measurements of a Li-S cell with solvating electrolyte. (a) SAXS intensity response during galvanostatic discharge at a C/10 rate. (b) Time-resolved contour map showing the relative SAXS intensity as a function of scattering vector q and cycling time, together with the corresponding electrochemical profile. The WAXS intensity evolution is shown on the right. Red lines mark the end of each discharge cycle.
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