The widespread use of lithium-ion batteries (LIBs) in a multitude of industrial and private applications, like consumer electronics or automotive applications, amplifies the need of recycling and reutilization of their constituent components. This necessity is mainly economically driven by the prices for the applied metals, like cobalt and nickel, but in more recent times also for lithium. Furthermore, the dependency on unsustainable mining practices without closed loop recycling is another driver. In addition, the recycling of LIB components has been encouraged by government regulation and legislation because of possible risks to human health or the environment deriving from hazardous battery constituents in the environment, such as transition metals or electrolyte components.
Current LIB recycling procedures start with deactivation and shredding of the old battery modules. After discharge and dismantling of the electronic components like wiring, modules are shredded under inert atmosphere conditions to avoid thermal runaways. At this stage, the more volatile electrolyte residues are also removed. Afterward, hydrometallurgical procedures use pH-dependent precipitation of salts to recover valuable active and inactive materials like lithium, cobalt, or nickel (salts). If pyrometallurgical recovery is preferred, discharge and deactivation are not needed during the recycling process, but can be performed to further enhance the safety. The characterization of the LIB materials is mandatory to ensure the recycling process is effective. Evaluation, adjustment, and running of recycling procedures requires reliable and comprehensive information on the various recycled materials. To calculate recycling rates, elemental analysis and cell chemistry are needed. In addition, with LIB lifetimes of up to 20 years, the material flow of spent LIB cells will not only represent state of the art materials, but also various cell chemistries with different transition metal oxide compositions from previous development phases.
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Application of analytical methods for the characterization of end-of-life batteries
A state-of-the-art LIB normally consists of a graphitic anode and a cathode with layered lithium transition metal oxides (nickel, cobalt, manganese). Furthermore, a separator, soaked with the electrolyte, which consists of lithium hexafluorophosphate (LiPF6) dissolved in organic carbonate solvents is placed between the electrodes. For elemental analysis, inductively coupled plasma (ICP)-based methods provide the best parameters for the determination of all of the elements of interest. However, other methods like atomic absorption (AA) spectroscopy, x-ray-based methods like total reflection x-ray fluorescence (TXRF) or energy dispersive x-ray (EDX) spectroscopy can also provide important information about the composition of the recycled scrap materials.1
While the transition metal oxides and lithium are of primary interest in the recycled materials elements like phosphorus, sulfur and fluorine are also important because they provide information about possible electrolyte components and possible challenges during the recycling process. Fluoride is of special focus due its potential to hamper the downstream recycling processes like hydrometallurgy. Ion chromatography (IC) is applied to characterize these important anionic components of the scrap material. The addition of a combustion unit to the IC can simplify the sample preparation and allows a direct analysis of the recycling scrap. This can provide significant advantages since the recycling samples are quite inhomogeneous and this must be addressed during sample preparation and analytical measurements.
In addition to the elemental analysis, the analysis of organic compounds by gas chromatography (GC) and IC-based methods provide further insight, especially with regard to fluorinated compounds deriving from electrolyte decomposition. Analyses of electrolyte constituents and binder, and their degradation species enable conclusions regarding cell aging conditions and material aging history. Moreover, species that can interfere with the recycling process, like binder polymer residues, or create potential dangers, like hazardous fluorinated species, are identified. This allows effective countermeasures to be established during the recycling process. In addition to the identification of the originally used electrolyte components (solvent and conducting salt), marker molecules can provide information about additives even when these wholly reacted during the electrochemical operation of the battery, and are no longer present in their applied form. These investigations are carried out using both GC and liquid chromatography (LC) methods, often in combination with high resolution mass spectrometry (HR-MS). Pyrolysis GC/MS can identify polymer binders and electrolytes from fingerprint analysis from established databases from the pristine materials. In addition, the enrichment of samples via solid phase microextraction (SPME) prior to GC/MS analysis can further provide insights about compounds that are only present in very low concentrations.
Analytical methods during the recycling process
Fluorinated decomposition compounds, and in particular HF, are a special focus for pilot processes and plants, since their chemically aggressive nature and potential toxicity can seriously hamper or damage the industrial recycling approach. These compounds need to be removed from the recycling scrap during the recycling process. This can be done via thermal treatment or solvent extraction. However, the conducting salt and parts of the organic carbonate solvents are lost during these processes and lower the overall recycling efficiency. To help resolve these issues, sub- and supercritical extraction with carbon dioxide is providing some interesting benefits.2 The application of these extraction techniques with additional solvents demonstrated that a quantitative amount of the electrolyte, including the conducting salt and aging products, could be recovered from commercial test cells.
The mixture of anode and cathode materials, the so-called black mass, needs to be investigated intensively before further separation and further purification of the respective electrode materials. This is often done using thermal analysis to determine concentrations of components and the temperature at which organic residues are released from the black mass. Thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) are applied to determine these parameters. GC-based methods can be carried out afterwards to provide more information about the nature of the released compounds. In addition, elemental analysis is done to determine residual fluorine or sulfur concentrations and to provide feedback to see if additional thermal or extraction cleaning steps are needed.
Analysis of recycled materials and batteries
During the re-synthesis of the anode and cathode material, the chemical structure and purity of the materials needs to be verified before building new cells with the recycled material. The crystallinity of the graphite is of crucial importance when it is used as an anode material because it determines the electrochemical performance. The crystallinity can be influenced by high pressure carbon dioxide, which is used as both an extracting agent and in the exfoliation of layered graphite. RAMAN experiments provide the best data about the influence of CO2 on the obtained graphitic anode material. Furthermore, scanning electron microscopy (SEM) in combination with energy dispersive x-ray analysis (EDX) give the first indications about possible elemental impurities. ICP-based methods are then applied to determine the exact concentrations of any significant impurities, for example transition metal oxides from the cathode material. If these impurities are above a certain threshold, additional purification steps are needed before further processing. For the cathode material, both SEM-EDX and ICP-based methods are applied to investigate the purity of the material and determine if the needed stoichiometry is present. If required, the addition of lithium and transition metal salts can correct the stoichiometry. Finally, SEM and x-ray diffraction (XRD) verify the structural integrity and detect any possible changes of the re-built electrodes.
Once investigations with electrochemical tests on the performance of new cells built from recycled material, additional analytical tests investigate the electrolytes and any decomposition products. Similar to the investigation of end-of-life LIBs, mainly GC-based and IC-based methods are used. HR-MS and fragmentation approaches are needed for the identification of any unknown decomposition products. Standard detectors are used to quantify the amounts of the components after electrochemical cycling. Furthermore, coupling of chromatographic and ICP-based methods can obtain concentrations of otherwise difficult to analyze decomposition products, even when standards are not available. For the electrodes, surface sensitive methods like laser ablation, glow discharge, XPS, and others can provide insight about possible transition metal oxide loss from the cathode and migration to the anode due to structural changes or particle cracking. In addition, structural changes are determined by XRD, while general information about the bulk composition are generally determined by ICP-OES, ICP-MS, TXRF, or AA.
In general, while the recycling of LIBs is an engineered-based collection of different physical processes, analytical methods provide valuable and important information starting with the initial assessment of the recycling scrap and finally the characterization of the re-synthesized and new, recycled, LIBs.
References:
- Nowak S, Winter M. 2017. ‘Elemental Analysis of Lithium Ion Batteries.’ Journal of Analytical Atomic Spectrometry 32, Nr. 10: 1824. doi: 10.1039/C7JA00073A.
- Nowak S, Winter M. 2017. ‘The Role of Sub- and Supercritical CO2 as “Processing Solvent” for the Recycling and Sample Preparation of Lithium Ion Battery Electrolytes.’ Molecules 22 (3), Special Issue Green Chemistry: 403. doi: 10.3390/molecules22030403.