Energy is something that we all need and most of us depend upon electricity. We also need to store this electrical energy so that we can use it when and where we need it. We have become accustomed to mobile devices containing rechargeable batteries, we expect them to be rapidly re-charged and to have a lifetime as long as the device that they power.
Batteries for electric vehicles (EVs) also need to be robust and lightweight while having a large capacity, which is also a requirement for the off-grid storage of electricity produced from intermittent renewable sources such as solar or wind energy. All these things together mean that research into materials for batteries and energy storage is a hot topic. In this active research area, X-ray diffraction (XRD) offers a rare insight into the material changes at atomic scale, and at Malvern Panalytical we offer innovative solutions to these new challenges.
Batteries typically exploit chemical reactions involving charge transfer by ions. Ions in one electrode of the battery transfer to another electrode of the battery during use, with the release of electrons that flow around an external circuit. They can be recharged by connecting them to a source of electrons that reverses the chemical reaction. Globally many research centres are investigating the possibilities for different types of battery, using a variety of materials with different chemistries to support both the efficient and reversible ion transfer and to provide structural stability for the battery.
Ion exchange in a lithium-ion battery
The cathode accepts electrons from the external circuit and ions from the electrolyte and is ‘reduced’. Cathodes are often transition metallic oxides or phosphates. During discharge Li+ ions are ‘intercalated’ (incorporated) into the cathode material. The anode provides electrons to the external circuit and is ‘oxidized’ as a result. Anodes can be made from materials containing carbon/graphite or metal alloys. During discharge intercalated Li+ ions are released into the electrolyte and the electrons flow in the external circuit. The electrolyte is an ionic conductor but an electric insulator. It connects the two electrodes and provides the medium for charge transfer inside the cell. The electrolyte is often a non-conducting inorganic solvent containing a dissolved lithium salt, e.g. LiPF6 in propylene carbonate. The distance between the anode and cathode is made as small as possible.
When a battery has discharged an excess of lithium ions is held in the cathode. Upon charging the lithium ions transfer back into the anode. The schematic drawing in Fig.1 illustrates the principles of charge storage in a lithium battery, a typical example of an ‘electrochemical cell’.
A number of research themes related to materials research for battery technologies make use of X-ray methods:
Exploration of desirable structural parameters for the electrode materials
Cathode materials are usually synthesized using co-precipitation and solid-state fusion. Crystalline quality plays an important role in the overall performance of battery in terms of discharge capacity and lifetime. For example, good crystallinity tends to favour better cyclic stability, however a higher discharge capacity is observed in less crystalline or even amorphous materials. X-ray powder diffraction, especially in combination with pair distribution function (PDF) studies, is a useful technique for exploring crystallinity, lattice disruption and grain size.
Exploration of battery material combinations
A variety of cathode/ion/anode/electrolyte materials are investigated in order to optimize the energy costs and gains for new battery designs. The suitability of new materials as electrodes is dependent upon their crystal structures and how well ion species can be taken up and released by the electrode material. These crystal structures and the extent to which intercalation causes phase changes or strain in the host lattice with charge discharge cycle are often studied using in operando X-ray diffraction.
Learn more on how to monitor the quality of of your electrode materials with in-operando XRD from the recorded webinar in our knowledge center.
Exploration of the micro-structure of the battery components
Large improvements in charging and discharging speed and efficiency can be made by increasing the surface areas of the anodes and cathodes. Complex interpenetrating phases, and nanostructured surfaces are investigated, such as films of nanowires, nanoparticle arrays or porous materials. At the macroscopic level the electrodes may be sheets that are multi-layered and coiled around each other as is the case for AA or pencil batteries. X-Ray Diffraction (XRD), in-situ diffraction, Small Angle X-ray Scattering (SAXS), Pair Distribution Function (PDF) and computed Tomography (CT) can explore these micro- and macrostructures in great details.
Monitoring the crystalline quality of electrodes during battery use
The repeated transfer of charge carrying ions in and out of the electrode structures puts strains on the unit cells and hence the fabric of the electrodes or can even induce phase changes in the crystal structures. Deterioration of performance is most commonly due to the structural defects that can build up in the electrodes hindering the free movement of ions. Abrupt lattice parameter changes can sometimes also cause particle cracking irreversibly degrading discharge capacity. The non-destructive XRD methods can be used in combination with dedicated in situ stages to directly observe charge and discharge reactions in battery coin, pouch or the electrochemical cells.
Malvern Panalytical now provides a number of new products and enhancements to deal with the challenges of battery research. Hard (Mo and Ag) radiation is necessary for some of the in situ studies and here the new GaliPIX3D detector can facilitate in-operando diffraction, PDF and CT measurements all on one instrument. New in situ stage options e.g. for whole button cells and pouch cells enable measurements both in reflection and transmission. Together with enhancements to the Highscore Plus software suite that performs Rietveld and PDF analyses, they illustrate Malvern Panalytical’s commitment to this important research area.
Learn more about this topic from our application note ‘High-quality in operando X-ray diffraction analysis of pouch bag lithium-ion batteries’ in the knowledge center on our website: www.malvernpanalytical.com.