A state-of-the-art lithium-ion battery (LIB) consists of a negative electrode, commonly graphite-based, and a positive electrode, typically a lithium transition metal oxide such as LiCoO2 (lithium cobalt oxide, LCO) and LiMnO2 (lithium manganese oxide, LMO) coated on metal foil (the current collector, copper, or aluminium). Typically, mixtures of these materials are applied commercially with varying contents, (e.g., LiNi1/3Co1/3Mn1/3O2 (lithium nickel manganese cobalt oxides, NMC)). In between, a separator provides electronic insulation, which is wetted with liquid organic electrolyte. The electrolyte consists of a conducting salt. The most common electrolyte is lithium hexafluorophosphate (LiPF6) which is dissolved in a mixture of linear (e.g. dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC)) and cyclic organic carbonates (e.g. ethylene carbonate (EC)). Additives such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC) are commonly added to the electrolyte to improve the overall performance of LIBs.
Analysis of raw materials and components
It is mandatory to analyze and control the raw materials before these components can be assembled as a working battery. Depending on the component, a variety of analytical methods are employed to obtain the needed information about the purity of the material. Despite this importance, the lack of commercially available standards or reference materials is critical. Currently, only self-made standards are available. As a result, the analysis of LIBs demands a careful analytical approach.1 For the electrodes, the most important issues are elemental impurities, lithium stoichiometry, and chemical structure. Therefore, techniques like inductively coupled plasma (ICP) mass spectrometry or optical emission spectroscopy (ICP-MS, ICP-OES) are applied to determine the remaining impurities from the specific synthesis routes of the materials. For ICP analysis, the typical sample preparation methods involve acid digestion carried out with the help of analytical microwave devices. Since the incorporation of silicon into the anode material, direct methods without the need to liquefy the electrodes prior to analysis have gained more interest. Some direct methods that are of interest include electro thermal vaporization (ETV) and surface methods, which have the ability to sputter through the electrode material and perform a bulk analysis, like laser ablation (LA) and glow discharge (GD) techniques. The structural integrity of the electrodes is normally investigated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Changes in morphology and the distribution of the active material particles and their size are investigated by SEM. Structural changes, like phase identification, crystal structure, and microstructure in the electrodes are analyzed by XRD and compared to standard materials.
The electrolyte and its components also need to be carefully analyzed for both elemental and compound impurities from the synthesis of the conducting salt. Anions like chloride, fluoride, or bromide are common impurities in the electrodes. Ion chromatography (IC) is frequently applied to characterize both the halides and bigger anions like hexafluorophosphate or other applied salts. IC can also be used to analyze some cations of interest like lithium or sodium. While ICP-MS and high resolution ICP-OES instruments with the ability to detect wavelengths below 170nm are complementary, IC is normally the state of the art for these analyses.
For the solvents involved in the electrolyte, gas chromatography (GC) methods are the standard choice. GC followed by mass spectrometry (GC/MS) is performed to identify possible volatile and semivolatile chemical compound impurities. With high-resolution mass spectrometry and softer ionization approaches like chemical ionization, the exact mass and structure of the impurities are elucidated. For the quantitative analysis of the solvent and additive constituents, GC with a flame ionization detector (FID) is the technique of choice, since all of the chemical compounds of interest are accessible.
Analytical methods during formation
Once assembly is complete, the cells are subjected to a formation process that forms the protective layers or interphases between the electrodes and the electrolyte during the first charge and discharge cycle(s) of a battery cell. This occurs when the cell is charged and discharged for the first time. The conditions of the first charge are important for the initial interphase formation on the electrodes and better electrolyte distribution between the electrodes. The initial charge is performed at low currents followed by charging at slowly increasing currents according to the specific characteristics of the cell for one or two more cycles after formation. The investigation of the formation process is of enormous interest in research and has great impact on the application. Appropriate formation of these interphases—of the so-called solid electrolyte interphase (SEI) on the negative electrode and the cathode electrolyte interphase (CEI) on the positive electrode—is essential for long-term battery performance and safety of the cell. To accelerate the process, higher currents are applied, which result in a decrease in overall formation time, but also in increased outgassing due to additional electrolyte decomposition. Decomposition products from formation can include H2, CO, CO2, CH4, C2H4 and C2H6. Depending on the form of the cell, the amount of decomposition can be determined with the help of the Archimedes principle and investigation of the evolved gases can be done with GC-based methods. However, to safely and correctly quantitate some gases, like hydrogen, standards are prepared using gas-mixing devices and multi-gas standards. A thermal conductivity detector (GC-TCD) or systems with a barrier discharge ionization detector (BID) can perform the quantitative determination of the gas components. Depending on the setup and gases of interest, a combination of these detectors may be needed.
Characterization of complete cells
End of line or post-mortem analysis of cells is a known way to obtain information about the performance and possible failures during production and operation. Basically, the same methods can be applied as before and during production; however, two important things need to be considered. First, the cell needs to be opened to get the samples and the collection of the different components, especially the electrolyte. This is not an easy task. In addition, before opening any cell, it is important to know about the internal design, to avoid short circuits and hazards during cell opening. Therefore, X-ray photographs of the cell provide very useful information. Regarding the electrolyte, there are seldom liquid residues left. Most of the electrolyte sticks to the cell components surfaces or is incorporated into the deeper layers of the electrodes. Therefore, solvent extraction or centrifugation of the separator are steps required to remove the remaining electrolyte for analysis. A more recent approach for the extraction of the electrolyte from cells is the use of sub- supercritical CO2.2 This is comparable to the decaffeination of coffee, whereas the electrolyte is removed from the cell without damaging it. Afterwards, the electrolyte can be analyzed by GC- and IC-based methods. With regard to the electrode, a rinsing step prior to analysis is needed when the sample will be handled in the laboratory atmosphere to protect the surface from decomposition reactions between the electrolyte and atmospheric moisture. Similar to the electrolyte, the electrodes are then investigated by the same methods discussed above. In addition, surface sensitive methods like x-ray photoelectron spectroscopy (XPS) are applied. XPS can provide analytical data about the elemental composition and chemical bonding of the top several nanometers of the surface. XPS can also provide depth profiling to see how those components change through the surface layer.
In general, many different analytical methods can be applied to better understand the raw materials and components of a LIB. The crucial point for these analyses is always the sampling and sample preparation, especially with regard to quantitative investigations.
- 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.