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A Look into the Atomic World of Batteries

Internationaliseringsstipendium | 02/05/2016

Lithium-ion-crystals | Foto: The Science Photo Libary Limited, Scanpix

Rechargeable batteries are already an essential part of our everyday life as they power our smart-phones, tablets and laptops. However, there is still a huge demand for improving the technology. A research team at University of Southern Denmark lead by Assistant Professor Dorthe B. Ravnsbæk uses X-ray radiation to map out the structural changes on the atomic scale in the battery electrodes. Hereby, we can obtain unprecedented fundamental scientific knowledge, which can pave the way towards battery materials for e.g. storage of sustainable energy. 

The aim of this project has been to gain increased insights into the interplay between chemical composition, structural changes on the atomic level and battery performance of novel electrode materials for Li-ion batteries. This has been achieved by the development of a new tool for studying the material structure while the battery is in operation. The project has revealed that small changes to the chemical composition can alter the charge and discharge mechanism of the battery and improve the power performance significantly. This may form the basis for future material design criteria. Furthermore, the new tool for studying batteries under operation will enable us to deepen our fundamental understanding of a wide range of electrode materials.

The Technology Behind the Li-ion battery

In 1991, the first hand-held Sony video camera reached the marked, and thereby the first commercial available lithium-ion (Li-ion) battery. This type of rechargeable battery offered a much higher energy density than the nickel-cadmium batteries typical for that for time and enabled the development of hand-held electronics. Hence, Sony started what can be considered a small technological revolution. It is hard to imagine how the development we have experienced the past decades within smart-phones, tablets and laptops would have been possible without the invention of the Li-ion battery.

Now the technology of the Li-ion battery faces the next big challenge with the increasing demand for cheap, safe and efficient solutions for energy storage in hybrid- and electric-vehicles, stabilisation of the electrical grid and storage of sustainable energy. The markets of electric vehicles are increasing steadily, and in several places around the world large battery-parks for storage of wind- and solar-energy are being tested. Thus, these years a development towards larger and larger batteries is taking place – from watt scale, like in electronics, towards the megawatt scale in facilities for sustainable energy. With this, the use of Li-ion batteries will shift from electronics to becoming a fundamental part of the global energy society. Achieving this is a vital step in solving one of the 21st century’s grand challenges – sustainable and reliable sources and availability of energy. 

However, the future success of the Li-ion battery relies on a further improvement of the technology in terms of lifetime, safety and price. Especially the requirement for longer lifetime is essential for new larger scale applications. A battery for a typical smart-phone or laptop is not expected to have a lifetime of more than five years, while that in an electric vehicle or for use on the electrical grid has to be able to maintain the capacity even after 10 years of constant usage. Hence, there is still a huge need for fundamental scientific focus on the development of new electrode materials based on cheap elements and with a long lifetime. Furthermore, there is increasing focus on developing materials that allow fast charge and discharge without reducing the energy capacity of the battery, since flexibility in the power (watt) the battery delivers is of vital importance for future applications.

The essential components in a Li-ion battery are the electrolyte and the two electrodes: the cathode and the anode. When a battery is charged, the Li-ions are stored in the anode, which is typically made of graphite, where the Li-ions are intercalated between the carbon-layers, which constitutes graphite. When the battery is used, the Li-ions move from the anode into the cathode, which is typically made from an oxide or a phosphate. The electrolyte transports the Li-ions the direct way through the battery. In contrast, the electrolyte is not capable of transporting the electrons, which therefore force through the external circuit. Thereby a current is generated. The opposite process occurs during charging of the battery.

”X-Ray Movie” of a Running Battery

In order to develop improved electrode materials for future generations of Li-ion batteries, it is of vital importance to understand the fundamental processes that occur on the atomic level during charge and discharge. The reversibility and stability of these structural changes have a significant influence on the capacity, lifetime and safety of the battery.

Researchers are utilising a method called “X-ray diffraction” to study these changes. Here the electrode material is irradiated by a focused X-ray beam, which is reflected (diffracted) in specific angles by the electrode. The reflection angles depend on the distances between the atoms in the material, while the intensity of the reflected beam depends on which type of atoms the material is made from. Hence, each material has a unique ”X-ray fingerprint”. Even very small changes in the atomic structure of the material are observed as changes in the reflected X-ray beam.

Awards:

Dorthe Bomholdt Ravnsbæk has already received several awards for her research i.e. ”Danish Academy of Science – PhD award” 2012, ”Strategic Research Award” 2014 and ”Women in Science” 2015.

Traditionally, X-ray diffraction studies of electrode materials have been conducted by charging or discharging batteries to certain stages, disassembling the batteries, taking out the electrode and collecting the X-ray diffraction data from this. This is a time consuming process and a lot of electrode material is wasted. Furthermore, this approach has a tendency of providing a false interpretation of the charge and discharge mechanisms, as the conditions in the battery during charge and discharge typical are very far from the equilibrium condition that sets in when the process is stopped. One of the main objectives of this fundamental science project, carried out at MIT in Boston, was to develop methodologies and expertise to collect X-ray diffraction data from a battery while it is in operation. For these experiments Assistant Professor Dorthe B. Ravnsbæk utilised specially designed battery test cells together with a team of researchers from Argonne National Laboratory in the USA. The test cells are designed to allow the X-ray beam to penetrate through the entire battery. To achieve this, it is necessary to utilise X-rays with both a very high intensity and energy. The experiments are therefore carried out at international synchrotron facilities, which can deliver the required radiation. With this equipment, the researcher can take a high-resolution “X-ray picture” of the battery in 10-30 seconds. If such pictures are recorded continuously while the battery is running an “X-ray movie” is obtained, which can be transformed into a movie on how the atomic structure of the electrodes change during battery charge and discharge.

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X-ray diffraction measurements of batteries during operation. High intensity X-rays from a synchrotron penetrates the specially designed test-battery. The X-rays are reflected by the materials in the battery and recorded by an X-ray detector. Simultaneously, the battery is charged and/or discharged and the electrochemical signal is recorded. By combining the signals from the two data types, information, which is not achievable by other methods, is obtained. 

New Material Reveals Exciting Mechanism

One of the new electrode materials the researchers have been focusing on in this project is a phosphate based on iron and manganese (Mn0.4Fe0.6PO4). The structure of the material resembles that of iron phosphate (FePO4). Nanoparticles of iron phosphate have been utilised in commercial batteries for more than 10 years. Surprisingly, it turned out that besides increasing the potential (volt) of the battery, the incorporation of manganese also made the material capable of delivering a much higher capacity during very fast discharge.

By utilising X-ray diffraction, the researchers found that this occurs because the presence of manganese significantly alters the mechanism for the changes in the atomic structure that occurs during charge and discharge. Without manganese, the nanoparticles in the material are typically either Li-free or Li-filled, that is they consist of FePO4 or LiFePO4, while particles containing both phases are rare. With manganese in the material, the researcher found evidence of presence of particles containing both Li-rich and Li-poor phases, as well as formation of new phases stable over a wide range of Li-concentrations.

Through detailed analysis, it is now clear that the introduction of manganese means that the structural changes, which the electrode has to pass through during battery charge and discharge, can occur in several smaller steps. This effectively means that the changes can occur faster without formation of damaging defects in the material. The results further indicate that it is possible to prepare well-functioning electrodes composed of larger particles than traditionally utilised. This could have a huge impact on both the production costs and the effective energy density of future batteries. 

Support

The Carlsberg Foundation has supported the project ”Novel Nanomaterial for Improved Lithium Batteries”, which Dorthe B. Ravnsbæk has carried out as postdoc in collaboration with researchers from Massachusetts Institute of Technology (MIT) in USA from 2012 to 2014.

The fundamental knowledge achieved through this project has a high potential for leading to design of new nano-materials for improved batteries. This is due to the fact that it has already expanded the detailed understanding of how changes in material characteristics affect the charge and discharge mechanism and thereby the entire efficiency of the battery. Furthermore, the methodology for recording “X-ray movie” is a very powerful tool expected to provide researchers with an increased understanding of how batteries function on the atomic scale. Such knowledge will help pave the way for rechargeable batteries that will meet future demands for large-scale storage of sustainable energy.  

Professor Dorthe Ravnsbæk says:

“The grant from the Carlsberg Foundation allowed me to spend two years at the top-ranked Massachusetts Institute of Technology being a part of a world leading team within battery research. This has truly allowed me to take my research career to the next level.”

"Scientific Social Responsibility has been a natural part of the project as the increasing energy demand makes efficient cheap and safe energy storage one of the most important challenges for our society.”

Peer-reviewed Papers

Extended Solid Solutions and Coherent Transformations in Nanoscale Olivine Cathodes
D. B. Ravnsbæk, K. Xiang, W. Xing, O. J. Borkiewicz, K. M. Wiaderek, P. Gionet, K. W. Chapman, P. J. Chupas, and Y.-M. Chiang
Nano Lett., 2014, 14, 1484–1491
http://pubs.acs.org/doi/abs/10.1021/nl404679t

Na3Ti2(PO4)3 as a sodium-bearing anode for rechargeable aqueous sodium-ion batteries
Zheng Li, Dorthe B. Ravnsbæk, Kai Xiang, Yet-Ming Chiang
Electrochemistry Communications 2014, 44, 12–15
http://www.sciencedirect.com/science/article/pii/S138824811400109X

Characterization of Electronic and Ionic Transport in Li1-xNi0.8Co0.15Al0.05O2 (NCA)

Ruhul Amin, Dorthe Bomholdt Ravnsbæk, Yet-Ming Chiang
Journal of Electrochemical Society 2015, 162, A1163-A1169
http://findresearcher.sdu.dk/portal/files/111554387/2015_Amin_NCA_JECS.pdf