Enhanced Imaging of Lithium Ion Battery Electrode Materials

This was Paper 963 presented at the Chicago, Illinois, Meeting of the IMLB, June 19–24, 2016. This paper is part of the Focus Issue of Selected Papers from IMLB 2016 with Invited Papers Celebrating 25 Years of Lithium Ion Batteries.In this study we present a novel method of lithium ion battery electrode sample preparation with a new type of epoxy impregnation, brominated (Br) epoxy, which is introduced here for the first time for this purpose and found suitable for focused ion beam scanning electron microscope (FIB-SEM) tomography. The Br epoxy improves image contrast, which enables higher FIB-SEM resolution (3D imaging), which is amongst the highest ever reported for composite LFP cathodes using FIB-SEM. In turn it means that the particles are well defined and the size distribution of each phase can be analyzed accurately from the complex 3D electrode microstructure using advanced quantification algorithms. The authors present for the first time a new methodology of contrast enhancement for 3D imaging, including novel advanced quantification, on a commercial Lithium Iron Phosphate (LFP) LiFePO4 cathode. The aim of this work is to improve the quality of the 3D imaging of challenging battery materials by developing methods to increase contrast between otherwise previously poorly differentiated phases. This is necessary to enable capture of the real geometry of electrode microstructures, which allows measurement of a wide range of microstructural properties such as pore/particle size distributions, surface area, tortuosity and porosity. These properties play vital roles in determining the performance of battery electrodes

Advanced 3D imaging and quantification of battery materials

Rising global demand for energy supply, storage and portability in a sustainable manner needs significant improvements to be made in the next generation of batteries, as rechargeable batteries for electric vehicles and energy storage applications are considered to be an effective solution for lower carbon electric mobility and balancing intermittent renewables in low carbon energy systems In particular because to their potential to store relatively high amounts of energy per unit volume (energy density).The need to develop better (i.e. lower cost and longer lifetime) batteries for these emerging applications drives the need to better understand battery performance, and this in turn is linked to a better understanding of the role of battery electrode microstructure. This will be improved through the ability to directly image changes as they occur within the battery which can be linked to battery degradation. Although the irreversible microstructural transformations in battery electrode structures is one of the key processes associated with battery degradation and failure, their mechanisms are poorly investigated, and existing data is not sufficient to draw clear guidelines for better electrode design. The performance of the battery is dependent on nano/micro-structure while during processing or operation microstructural evolution may degrade electrochemical performance. Degradation and failure mechanisms in electrochemical systems lead to poor cycle life, in particular dendritic growth and volume expansion of anode materials. These are important failure mechanisms in various battery systems. Understanding their 3D microstructure is essential for developing high performance batteries for electric vehicles and energy storage applications. Tomographic techniques allow for the direct 3D imaging and characterisation of complex microstructures from millimetres down towards nanometres and in real time during operation. The performance of the battery is dependent on the nano and micro-structure achieved during manufacture. Furthermore microstructural evolution during operation may degrade electrochemical performance. Here in this thesis, results from in-situ, in-operando 3D x-ray and ex-situ FIB-SEM tomography, enabling analysis of complex micro-structures during battery operation, are presented. 3D imaging of Zn dendrites formation, down to resolutions of tens of nanometres facilitating analysis at different scales especially of nano structures can provide exciting opportunities to study dendritic growth. This approach was found to be effective in understanding how dendrites growth at high resolutions and consequently that tomography coupled with modelling/experiments can provide new insights into degradation mechanisms. The growth of dendrites represents a limiting failure mechanism in some battery systems; in particular this can be a challenge in zinc-air batteries. Furthermore volume expansion during lithiation is another major failure mechanism. Tomographic techniques allow the direct 3D imaging and characterisation of complex microstructures, including the observation and quantification of dendrite growth and volume expansion. Moreover, in this thesis, a new methodology of contrast enhancement for multi modal 3D imaging, including novel advanced quantification, on a commercial Lithium Iron Phosphate (LFP) LiFePO4 cathode is presented. This enables higher focused ion beam – scanning electron microscope (FIB-SEM) resolution (3D imaging), which is amongst the highest ever reported for carbon containing electrode materials (e.g. composite LFP cathodes) using FIB-SEM. In turn it means that the particles are well defined and the size distribution of each phase can be analysed accurately from the complex 3D electrode microstructure using advanced quantification algorithms.Open Acces

Operando Visualization and Multi-scale Tomography Studies of Dendrite Formation and Dissolution in Zinc Batteries

Alternative battery technologies are required to meet growing energy demands and address the limitations of present technologies. As such, it is necessary to look beyond lithium-ion batteries. Zinc batteries enable high power density while being sourced from ubiquitous and cost-effective materials. This paper presents, for the first time known to the authors, multi-length scale tomography studies of failure mechanisms in zinc batteries with and without commercial microporous separators. In both cases, dendrites were grown, dissolved, and regrown, critically resulting in different morphology of dendritic layer formed on both the electrode and the separator. The growth of dendrites and their volume-specific areas were quantified using tomography and radiography data in unprecedented resolution. High-resolution ex situ analysis was employed to characterize single dendrites and dendritic deposits inside the separator. The findings provide unique insights into mechanisms of metal-battery failure effected by growing dendrites. Rechargeable metal batteries form the next generation of energy storage devices aiming to replace lithium-ion technology. Nonetheless, these batteries suffer from dendrites formation during repeated battery charging. Understanding how dendrites form is a key to building safer batteries. In the current work, we report multi-scale tomography studies of formation and dissolution of the dendrites in rechargeable zinc batteries. Using operando radiography we found that the dendrites form on surface inhomogeneities, with larger dendrites formed at higher current. The dendrite dissolution and regrowth results in formation of a porous metal network sparsely attached to the anode. The presence of separator does not prevent dendritic growth. Dendrites start growing inside, filling the submicron pores as dense deposits and penetrate the separator. They continue growing on top of the separator, forming a compact entangled network that cannot be re-dissolved back, causing permanent battery failure. This work emphasizes the importance of understanding the phenomena of dendrite formation occurring in rechargeable metal batteries. These batteries can reach quite a high specific energy but have inherently short life due to dendritic growth. The current work implements different tomographic methods to visualize dendritic growth in real time and to quantify various dendrite characteristics at submicron and nanoscale levels. The methodology presented here can be also extended to study the growth of other metal dendrites in aqueous and non-aqueous batteries