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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 2 - ARTIST (Advancing Rechargeable-Batteries Through In Situ Techniques)

Teaser

Summary

Energy storage in rechargeable batteries is a key technology in reducing our reliance on fossil fuels, with the aim of minimising global warming and its potentially disastrous effects. The search for new battery materials together with the drive to improve performance and lower the cost of rechargeable batteries presents significant challenges. Many of the most important physical processes that occur in rechargeable batteries occur at the interface between the solid electrodes and a liquid electrolyte. However, directly probing the reactions occurring at these interfaces is challenging due the bulk of material either side of these interfaces. This project aims to address this problem by developing new techniques based on using atomically thin graphene membranes to probe the solid-liquid interface with established X-ray spectrocopy and scanning probe microscopy techniques. The understanding of electrode-electrolyte interfaces in rechargeable batteries is critical to improving their performance, and thus this project is of broad significance to society, given the widespread use of rechargeable batteries in portable electronic devices such as laptops and mobile phones, as well as their increasing prevalence as power sources for zero-emission vehicles.

The overall objectives of this project are to:
- Develop in situ techniques to probe solid-liquid interfaces.
- Use these techniques to reveal the structural and chemical evolution of solid-liquid interfaces.
- Obtain a detailed understanding of the evolution of electrode materials in rechargeable batteries and how these materials can be improved.

Conclusions of the action:
- Developed a new approach to perform X-ray photoelectron spectroscopy of gases and liquids at atmospheric pressures using graphene membranes.
- Revealed the structure of the solid-electrolyte interphase formed on high-capacity silicon anodes in lithium ion batteries, and how electrolyte additives can reduce capacity fade over many charge/discharge cycles.
- Observed the evolution of silicon-based anode materials during lithium insertion, showing that using silicon oxide avoided the formation of undesirable phases.

Work performed

The work performed for this project was carried out at the Lawrence Berkeley National Laboratory in California (US) and at the Department of Chemistry at the University of Cambridge (UK). The focus has been on investigating how interfaces in electrochemistry and heterogeneous catalysis behave under reaction conditions. X-ray spectroscopy and scanning probe microscopy techniques that have been specially adapted to operate at ambient pressures (tens of mbar) have been applied to study the behaviour of solid-gas interfaces. This has included fundamental studies of the pressure-dependent adsorption of carbon dioxide and methanol on low-index faces of Cu single-crystals which serve as models for the complex facetted nanoparticles typically used in carbon dioxide reduction and methanol oxidation reactions. X-ray photoelectron spectroscopy has been used to obtain a precise account of the chemical species adsorbed on the Cu surface, the pressures at which they dissociate, and the corresponding oxidation state of the Cu catalyst. Scanning tunnelling microscopy has been used to observe the structures adopted by the adsorbed species on the catalyst surface, and how the catalyst surface restructures at pressures of up to 20 mbar.

A new technique has been developed that allows photoelectron spectroscopy to be performed at atmospheric pressures in liquid or gas environments (see Figure). This is based on using graphene membranes to separate the gas/liquid from the vacuum conditions needed for electron detection, whilst the graphene is thin enough to allow the transmission of photoelectrons from the high-pressure side. Methods have been developed for producing stable graphene membranes that allow these measurements to be performed, and protocols for depositing electrode and catalyst materials onto these membranes have been established. Special liquid and gas reaction cells have been designed to accommodate these graphene membranes and successfully tested at various synchrotron facilities. Work has also been undertaken on applying X-ray absorption spectroscopy (XAS) to study solid-liquid interfaces. A three-electrode liquid flow cell that incorporates a metal-coated silicon nitride membrane (100nm thick) as both X-ray transparent window and working electrode, has been designed that allows measurement under electrochemical control. These operando X-ray spectroscopy techniques have been used to study electrolyte decomposition and resulting formation of a solid-electrolyte interphase (SEI) on high capacity silicon anodes for lithium-ion batteries. The effect of common additives such as fluoroethylene carbonate on the formation and structure of the SEI have been revealed, improving our understanding of how SEI structure can be altered to minimise capacity fade during repeated cycling. To complement the powerful chemical information these X-ray spectroscopy techniques can provide, scanning probe microscopy has been applied to study the structure of solid-liquid interfaces. Operando NMR has also been used to investigate the Lithiation mechanisms of the Si and SiO as high-capacity anode materials for Li-ion batteries.

Final results

The studies of solid-gas interfaces provide new understanding of how initially flat Cu surfaces can drastically restructure in carbon dioxide environments of ~20 mbar due to Cu nanocluster formation. They thus provide new insights at the molecular level into the â€œself- poisoningâ€ and the role of step and kink sites in important industrial reactions catalyzed by Cu-based materials, such as the reverse water gas shift reaction and methanol synthesis. Furthermore they highlight the importance of in situ characterisation under realistic reaction conditions, as the catalyst structure and chemistry can be significantly changed simply by changing the pressure.

The first X-ray Photoelectron Spectroscopy measurements of nanoparticles in an atmospheric pressure reaction environment have been performed by using graphene membranes. It has been demonstrated that this technique can equally be used to detect ions in aqueous solutions and to observe the electrodeposition of cobalt onto the graphene membrane from a cobalt(II) sulfate solution. This has been developed in collaboration with synchrotron research facilities (e.g. ALS, Alba) opening up new instrumentation opportunities to the international scientific community. The studies of how the SEI forms and evolves on silicon anodes will inform the rational engineering of the SEI structure to achieve improved battery cycle life. Complementary NMR studies have revealed how Lithium is inserted into Si and SiO anodes, showing that in the later case a high concentration of Li can be inserted without causing the undesirable formation of Li15Si4 formation which is otherwise found to be detrimental to maintaining battery cycle life. The enhanced understanding of solid-liquid and solid-gas interfaces obtained using this technique will benefit industries important to developing a sustainable society such as electrochemical energy storage and catalysis.