Shining a light on batteries
Advances in microscopy accelerating battery research
We write a lot about the advances in battery materials and supply chains. However, without crucial characterisation tools R&D would be completely lost. We caught up with Cambridge spin out illumion to learn about charge photometry and why they think being able to ‘see’ into a battery this way is a game changer.
Microscopy is an essential component in the arsenal of the battery material researcher looking to understand the behaviour of next generation battery materials. It is used to study the morphology of battery materials, from assessing the form of active particles, to probing structural changes at the electrode and cell level.
Changes to particle and electrode morphology during battery use and over its lifetime can provide deeper understanding of performance and behaviour on the mechanistic level, facilitating optimisation of formulations and designs to produce improved battery performance.
Table 1 provides a top-level summary of two types of microscopy which can be applied in battery research, optical and electron:
However, like most techniques, electron microscopy and traditional optical microscopy have their limitations – particularly when it comes to addressing two of the central challenges facing those studying battery materials:
Non-equilibrium operating conditions of batteries
If you are interested in studying the mechanisms impacting performance or leading to degradation, operando techniques are required to capture the dynamic processes at play.
Not only do in-situ measurements require specialist and expensive sample holders for electron microscopes, but the technique is also restricted to slow charge/discharging of cells, with response times slower than optical microscopy. This is especially limiting for those involved in the development of next generation materials designed to be charged at faster rates.
Even traditional optical microscopes are not really set up for truly dynamic image capture and analysis of rapid processes, and the sample holders typically used are tricky to assemble and not realistic representations of cell operation.
Heterogeneity found within the electrodes
Assessing the degree of variation in electrochemical activity under different operating conditions requires a technique with single-particle resolution across an electrode.
Whilst it is possible to resolve detail on individual active particles using traditional microscopy methods, building up a statistical picture across many such particles, under different operating conditions, is much more difficult.
Introducing charge photometry
Charge photometry is a novel optical microscopy-based technique that addresses these two challenges. Recently developed by researchers at the University of Cambridge and now being delivered to the wider battery research community by illumion, charge photometry captures both the state-of-charge and morphology of individual active particles within an electrode on the millisecond scale, enabling rapid dynamic processes to be visualised in real time.
So, how does the technology work? It is based on the principle that the intensity of the scattered light is proportional to the sample polarizability. This in turn is determined by the sample’s electronic properties – which varies with the changing state-of-charge.
So, as the battery is charged and discharged, the intensity of the scattered light from the active particles in the electrode changes, reflecting each one’s individual state-of-charge. Indeed, it is even possible to observe variation in lithiation state within the individual particles – enabling visualisation of ion-gradients and ion-transport mechanisms.
The measurement itself is straightforward: the electrode stack is assembled into an optically accessible coin cell, which is then mounted into the bench-top instrument. The desired electrochemical testing protocol is set up in the software, and then applied to the cell during the measurement by the integrated cycler.
The varying intensity of the scattered light from the active particles in the coin cell is then detected during cycling, delivering operando insights into the changes in particle state-of-charge and morphology, alongside the synchronised electrochemistry. The general applicability of the technique means it is agnostic to the underlying battery chemistry, and that it can be applied to a wide range of next generation battery materials. This unlocks a variety of applications, for example:
Studying the mechanisms limiting charging performance
Probing the material degradation pathways leading to accelerated capacity loss
Determining how much material is active across the electrode and where performance is lost.
This provision of an operando, single-particle resolution technique enables the aforementioned challenges facing battery researchers to be tackled:
Studying the impact of fast charging conditions on degradation pathways
Cycling batteries at a faster rate can lead to accelerated degradation, so with the increasing demand for more rapidly charging batteries there is a pressing need to understand the causes of degradation under such conditions. Analysis of these dynamic, non-equilibrium processes is challenging using existing methods, but rapid image acquisition, allied with the use of realistic coin cell geometries, enables charge photometry to capture such processes in real-time.
Here, charge photometry was applied to niobium tungsten oxide (Nb14W3O44), a next-generation, high-rate anode material1. It was possible to observe not only the real-time formation of cracks across the active particles at higher charging rates up to 30C, but also the development of Li-ion gradients within the particles, that ultimately led to the degradation observed.
It was even possible to identify the subsequent formation of electrically disconnected fragments (c) – recognisable as more intense regions within the active particle, indicating the presence of trapped lithium when the battery was in a delithiated state – the presence of which can lead to increased capacity loss.
Determining the proportion of inactive particles within an electrode
Increased variation in electrochemical activity across an electrode typically results in diminished battery performance and cycling stability. Monitoring such heterogeneity across the electrode is challenging because it requires single particle resolution over a population of particles. However, this variation can be characterised using charge photometry, because the (de)lithiation rate of each individual active particle is captured from the observed changes to active particle intensity during cycling.
Above shows the intensity profiles for two Ni-rich NMC active particles, A and B, under two different cycling rates.2 At the slower C/3 rate, the particles exhibit synchronised delithiation rates. However, when increased to 2C, Particle B displays a lag in delithiation at the beginning of charge – in other words, the faster charging rate has induced an increase in electrochemical activity variation.
This approach can be extended to consider all active particles within the field of view, with the (de)lithiation rate data captured used to classify a population of active particles based on their relative performance. On the simplest level this can quantify the proportion of inactive particles within an electrode and provide a metric for characterising the performance of an electrode material under defined cycling conditions. Taken further, it can be used to drill down into the different (de)lithiation rate behaviours exhibited and, combined with the spatial knowledge of the active particles concerned, used to probe electrode-level electrochemical activity mechanisms.
Charge photometry: a valuable new addition to the analytical toolkit
In providing orthogonal insights into particle state-of-charge, morphology and electrochemistry in an operando, single-particle resolution measurement, charge photometry represents the next frontier in material characterisation.
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Merryweather et al., Nature Materials (2022)
Xu et al. Joule (2022)