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From R&D to Recycling: Solving Challenges with Materials Characterisation

Our customers and applications teams are supporting the success of next generation batteries using different analytical techniques. Here we share selected case studies that cover raw materials checking, end product quality control, accelerating R&D, root cause failure analysis and improving safety.

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Dr Christian Lang, Managing Director of Oxford Instruments NanoAnalysis
Dr Christian Lang, Managing Director of Oxford Instruments NanoAnalysis

Developing New Polymer Electrodes for Solid-state Lithium ion Batteries 

Lithium-ion batteries with solid electrolytes are currently the focus of electrochemical research. Compared to liquid electrolytes, solid-state electrolytes achieve higher energy densities and increased safety. Polymer-based electrolytes are particularly promising, as they are not only flexible but also inexpensive to manufacture. In addition, they enable efficient wetting of the electrodes, so that volume differences between the electrodes can be compensated for when charging and discharging the cell.

However, the currently available polymer systems generally have insufficient ionic conductivity at room temperature, which is essential for a powerful battery. Composite materials made from a polymer matrix and particles with good ionic conductivity could remedy this deficit and combine the best properties of both materials.

It is assumed that the interface between the particles and the matrix plays a decisive role in the conduction mechanism of the lithium ions through the composite. A better understanding of the underlying processes can pave the way for the development of new, improved composite materials.

Kerstin Neuhaus and collaborators at the Helmholtz Institute Münster, working also with Forschungszentrum Jülich, have characterised the electrochemical properties of the interfaces between different polymer electrolytes containing nanoscale ceramic lithium containing particles (see figure 1) and two different mixed ionic-electronic conductive ceramic particles. They used data from Kelvin Probe Microscopy to characterise the Volta potential differences between the particles and the polymer and used impedance spectroscopy to characterise local conductivity variations and lithium transference numbers. This new characterisation approach can open the door to the optimisation of polymer-based lithium batteries.

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Figure 1: The distribution of the Volta potential of an ionically conductive LLZ particle in a MEEP polymer matrix shows a reduced potential at the interface and an increased potential within the particle. This indicates different lithium ion activities and different concentrations of the anions. Read their publication: "Buchheit et al., J. Electrochem. Soc. 168, 010531 (2021)”

Optimising Electrolyte R&D and Manufacture with Benchtop NMR Spectroscopy

The current electrolyte solutions in next generation batteries impact their energy/power density, lifespan, safety and cost. The individual components of electrolyte solutions are selected and optimised to achieve the desired final battery cell properties. In lithium batteries, precise analysis of electrolytes components is therefore required. Typically these include an organic solvent mixture, an inorganic lithium-containing salt and other additives, some of which are present at less than 2% concentration.

Benchtop Nuclear Magnetic Resonance (NMR) spectroscopy is ideal for identifying the structure and concentrations of unknown electrolyte materials as well as determining the relationships between their individual components and a reference sample. Furthermore, important physical parameters including individual diffusion coefficients and ionic conductivity can also be quantified using NMR. The technique is becoming increasingly valuable throughout the product lifecycle - from accelerating new formulation design through to quality control in manufacturing and root cause identification of both electrolyte and final cell failure.

Our X-Pulse broadband benchtop spectrometer uniquely enables the investigation and optimisation of all types of electrolytes. It is the only commercial benchtop spectrometer that can analyse all key chemical nuclei (H, C, F, B, P, Li, Na, Si) important for comprehensive battery electrolyte characterisation. Every analysis can be carried out directly and quickly in multiple locations on site or in a laboratory without the need for highly trained personnel or special solvents.

Figure 2: ¹⁹F 1D NMR spectrum of a failed lithium battery electrolyte chemically identifying breakdown products

Figure 2, shows how NMR identifies the causes of breakdown that led to decreased charge/discharge performance in a Lithium hexafluorophosphate, Li [PF6], based electrolyte containing a solvent mixture of ethyl methyl carbonate and ethylene carbonate. Firstly by comparing with a reference sample, it was determined that the solvent was not responsible for failure. The NMR spectrum in Figure 2 identifies that a hydrolysis reaction has taken place converting some of the original [PF6] to difluorophosphoric acid, OPF(OH)2 which had a molar ratio of 0.6 relative to the remaining (pure) lithium electrolyte species. There is additionally unwanted formation of Lithium fluoride (LiF), a known biproduct of this hydrolysis reaction.

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Determining Powder Particle Composition in Li-ion Battery Cathodes

Battery innovations for electric vehicles are a central element in the much anticipated mobility turnaround towards an emission-free future. Batteries must be long-lasting, safe, quickly rechargeable and, of course, have a high charging capacity. In addition to our mobility, batteries are also the key to energy storage from renewable energy sources (sun, wind, water, biomass, tides, and geothermal energy).

In order to improve performance, new materials with increased complexity are required, keeping the consistently high demands on safety and recyclability. Used batteries can be given a second life in large energy storage applications before they are ultimately recycled.

Effective material characterisation, from the extraction of raw materials to the end product, is of the utmost importance. Energy dispersive spectroscopy (EDS) plays a key role here, as it is a fast and non-destructive technique to identify elemental distribution with high sample throughput. Cathodes are usually made from a combination of nickel, cobalt, and manganese (NCM) materials, but aluminium is often also present. Due to fluctuations in costs and controversial mining practices, there is a desire to replace cobalt. New compositions are constantly being tested to find the optimal recipe. Quality assurance and control of elemental distribution in these powders is critical to ensuring material performance and lifespan. For example, determining the ratio of transition metals provides a powerful monitoring tool for controlling the composition of ternary cathode materials during production.

Figure 3: Automated particle analysis of lithium nickel cobalt manganate (NCM) with AZtecBattery. The ratio of transition metals in each ternary cathode particle is automatically calculated.

Our Ultim Extreme EDS detector was developed for operation at low acceleration voltages (e.g., 1-3 kV) and thus for recording high-resolution element distribution images. The use of very low acceleration voltages also enables highly surface-sensitive measurements without any surface damage. Due to the windowless design, it is very sensitive in the light element area and is currently the only EDS detector that can detect lithium. AZtecBattery software automatically performs the relevant quantitative analysis needed for battery materials.

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Identifying Causes of Failure with Raman Microscopy

Understanding structure-composition-property-performance relationships is fundamental in developing higher performance, longer-life, affordable Li-ion batteries. Correlative Raman Imaging and Scanning Electron Microscopy (RISE) is extremely powerful to visualise structural and chemical information, including: molecular composition, grain fractures, solid electrolyte interphase (SEI) formation and electrode degradation. High-resolution scanning electron microscopy (SEM) characterises electrode ultrastructure and energy-dispersive X-ray spectroscopy (EDS) detects incorporated elements. Li-containing molecules are identified by Raman spectra, which reveals localisation, concentration and polymorph changes.

Fig. 1a - 1c: Raman microscopy and SEM-EDS mapping investigation of 18650 cell LMO batteries

Using a WITec alpha300 confocal Raman microscope integrated with SEM, we examined cross sections of 18650 Li-ion battery cells before and after cycling. SEM-EDS of the new battery reveals the cathode consists of Co/Ni (pink) and Mn-rich parts (cyan) (Fig. 1a). Raman imaging additionally identifies the graphite (cyan) and amorphous carbon (blue) in the anode, and amorphous carbon and lithium with manganese oxides (MO, red) in the cathode (Fig. 1b). The separator layer of polyethylene (PE, green) between two layers of polypropylene (PP, yellow). During cycling (Fig. 1c), the uniaxial polymer chains deteriorate, appearing as bi-axial PP in the used battery and are likely to significantly reduce battery performance.

Fig. 2a - 2d: RISE microscopy analysis of the cathode of a fast-cycled NMC battery

We then analysed Lithium-Nickel-Manganese-Cobalt-Oxide (NMC) batteries that underwent fast recharging with 40% loss of capacity. Reduced performance often occurs from inhomogeneous electrode microstructure degradation. In the RISE image, the new charged cathode particles consist of uniform MO (Fig. 2a, b). Rapid cycling induced significant changes in particle lithiation with the Raman spectral peaks broadening and shifting (Fig. 2c). The RISE image of one cycled MO particle reveals a high level of inhomogeneity and degradation in the form of cracks (Fig. 2d).

This case study demonstrates the power of RISE microscopy for pinpointing causes of degradation occurring during cycling at both cathode and separator that reduce both battery lifespan and charge/discharge performance.

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