Looking inside battery cells

It is common knowledge that batteries degrade with usage. While this is certainly due to a complex combination of effects, how these effects interact and how they could be mitigated is far from being well understood – and is cutting-edge research. An important step forward has just been published in the journal Energy and Environmental Science. Working with a commercial-grade battery, a team of researchers bringing industry and academia together used neutron and X-ray imaging to identify a concrete effect that degrades performance. Surprisingly, this particular effect does not come with age: it is there since the first time the battery is put to work.

Lithium-Ion batteries presently are the ubiquitous source of electrical energy in mobile devices, and the key technology for e-mobility and energy storage. Massive interdisciplinary research efforts are underway both to develop practical alternatives that are more sustainable and environmentally friendly, and to develop batteries that are safer, more performing, and longer-lasting – particularly for applications demanding high capacity and very dense energy storage. Understanding degradations and failure mechanisms in detail opens opportunities to better predict and mitigate them.

In the study, a team of researchers led by the CEA, the ILL and the ESRF in collaboration examined Li-ion batteries during their lifetime using state-of-the-art, non-intrusive imaging techniques available at neutron and X-ray sources, respectively the Institut Laue Langevin (ILL), the world’s most powerful neutron source, and the European Synchrotron (ESRF), the brightest synchrotron. Neutrons and photons are largely complementary. Neutrons are particularly good at seeing lithium and other light elements, while X-rays are sensitive to heavy elements, such as nickel and copper. Their sophisticated combination allowed to gain multidimensional information on the components and elements inside working battery cells.

The team identified macroscopic deformations in the wound structure of the copper current collector. The deformed areas already existed in fresh battery cells that only went through the initial activation cycle (the first charging-discharging cycle). Further investigations revealed that these defects are due to local accumulations of silicon occurring during electrode manufacturing. Upon activation, the largest agglomerates expand heavily, which leads to deformations in the current collector, wasting capacity before the cell ever went into use.

It was possible to determine how large these accumulations have to be to become a problem: cell structure and functioning is compromised for silicon agglomerations with a size above 50 microns. This is crucial information for both quality control and future developments. Erik Lübke, PhD student hosted at ILL and the main author of the study, summarises: “In fact, resources are wasted when this happens, and we have quantified the effects and understood their causes."

Full-field, high-resolution 3D transmission tomography enabled the inspection of the entire volume of the battery cell, revealing the presence of a number of defect features. These were more closely investigated at selected cross-sectional 2D slices. The neutron tomography scans (with simultaneous low intensity X-ray computed tomography scans) were carried out at the NeXT instrument of the ILL. Synchrotron X-ray tomography scans of the very same cells were then measured at the ESRF using two beamlines, BM05 and the high-energy ID31 beamline for phase-contrast and scattering tomography respectively.

At NeXT, 3D high resolution neutron tomography is coupled with X-ray tomography to image the entire cell. Erik Lübke explains that “X-rays give the basic structure, making it possible to know exactly where we are when we use neutrons to examine the spatial distribution of lithium in detail” benefiting from “the best neutron resolution you can get anywhere in the world”.

Selected parts of the cell were then examined in further detail using several different X-ray tomography techniques at the ESRF high-energy beamlines. Acquiring data during the battery charging process (a so-called operando experiment) made it possible to gather more information about the reaction dynamics in the defective regions: lithium diffusion is partly blocked there, and even when most of the cell is fully charged these areas remain without lithium in their centre.

To ensure the industrial relevance of the results, the team tested cylindrical silicon-based Li-Ion battery cells manufactured according to industry standards. Cells of this format are in commercial use in small electronic devices such as medical sensors, headphones, and smart devices. The size was however reduced for a better compatibility with the experimental requirements. Both fresh cells and aged ones (cycled over 700 times with roughly 50% remaining capacity) were imaged, in charged and discharged states. The different techniques were applied to the very same cells.

For this study, operating within the InnovaXN and Battery Hub frameworks in Grenoble was a clear advantage. The presence of the industrial partners VARTA and MCL made it possible to work with products and issues that are very close to the market, while having access to all the required know-how, expertise and experimental facilities, including the world leading power and state-of-the-art techniques of the largely complementary ILL and ESRF sources. While experiments were performed at the ILL and the ESRF, the Institute of Interdisciplinary Research of the CEA had a fundamental role in providing the bridges between know-how, techniques, methods for data acquisition and analysis.

This kind of approach is at the heart of the Battery Hub created in Grenoble by the CEA, the ESRF and the ILL as a European platform for battery diagnosis and investigations using standardised, integrated and multi-technique workflows. This work is also part of the PhD thesis that Erik Lübke is currently finishing in the framework of the InnovaXN project, an EU-funded doctoral training programme in which 40 PhD students tackled a variety of subjects driven by industrial challenges and exploiting the advanced characterisation techniques of the large-scale European facilities in Grenoble.

Partners: Institut Laue-Langevin, European Synchrotron Radiation Facility, CEA-IRIG, Grenoble (France), VARTA Innovation GmbH, Materials Center Leoben Forschung GmbH (Austria).

Institut Laue-Langevin (ILL) is the world-leading facility for neutron science and technology. Its high-flux reactor delivers the world’s most intense neutron beams to a state-of-the-art suite of more than 40 public instruments. Neutrons are a unique and powerful probe of materials and processes. ILL’s instruments, complemented by a comprehensive suite of labs and scientific support services, enable cutting-edge research across a wide range of scientific domains, including physics, chemistry, biology, and materials science. The ILL works closely with industry and develops an impactful program addressing societal challenges namely in health, energy, environment, and quantum technologies. Each year, the ILL hosts about 2500 researcher visits from 40 countries. The ILL is a Landmark Facility on the European Strategy Forum on Research Infrastructures (ESFRI) Roadmap, recognising its central role in the ecosystem of European research facilities. More: https://www.ill.eu

Interdisciplinary Research Institute of Grenoble (CEA-IRIG) is an institute of the Fundamental Research Division that employs 1100 people. Based in Grenoble, France, the institute covers a wide range of theoretical and experimental research in the fields of physics, chemistry, biology and instrumentation. The teams associated with other French research institutes are involved in 4 major thematic pillars: Biology-Health, Physics-Digital, Energy-Environment and Cryotechnologies. All of its R&D is supported by the exceptional research platforms with instruments at the cutting edge. Some of these are National Research Infrastructures serving the French and European communities, with the French light and neutron lines at ESRF and ILL (F-CRG: French Collaborative Research Group). The CEA-IRIG is a "knowledge factory" and a place of innovation to support societal transitions (digital, health and energy). For more information: https://irig.cea.fr/drf/irig/english

ESRF - The European Synchrotron Radiation Facility is the most intense source of synchrotron-generated light, producing X-rays 100 billion times brighter than the X-rays used in hospitals. These X-rays, endowed with exceptional properties, are produced at the ESRF by the high energy electrons that race around the storage ring, a circular tunnel measuring 844 metres in circumference. Each year, more than 9000 scientists from around the world come to Grenoble, to “beamlines”, each equipped with state-of-the-art instrumentation, operating 24 hours a day, seven days a week. Thanks to the brilliance and quality of its X-rays, the ESRF functions like a "super-microscope" which "films" the position and motion of atoms in condensed and living matter, and reveals the structure of matter in all its beauty and complexity. It provides unrivalled opportunities for scientists in the exploration of materials and living matter in many fields: chemistry, material physics, archaeology and cultural heritage, structural biology and medical applications, environmental sciences, information science and nanotechnologies. More: https://www.esrf.fr