A Guide to Sample Prep for Battery Analysis

How to Guide 1 A Guide to Sample Prep for Battery Analysis Emmanuel Batsa Tetteh, PhD Batteries currently power almost all electronic devices and power tools. They are a key enabling technology for electric vehicles and are increasingly considered to be the technology of choice for grid storage.1 A battery’s performance demands the highest-performing materials in the anode, cathode, electrolyte and separator. Battery material analysis and characterization is essential for ensuring optimal performance of all battery components during the stages of material selection, development and manufacturing; it is also essential to provide key parameters associated with battery manufacture, operation, health, degradation and failure. Battery analysis encompasses various methodologies. Thermal analysis, for instance, offers insights into material thermal stability and structural changes across temperature ranges. Rheology is employed to optimize battery slurry storage, mixing, coating and drying, to achieve uniform and defect-free electrode manufacturing. Mechanical analysis explores material structure–property relationships. Additionally, electrochemical analysis provides insights into the electrochemical performance of battery components within a functional battery cell. Advanced analytical methods including magnetic resonance, neutron and X-ray spectroscopies, focused ion beam (FIB), scanning electron microscopy (SEM) and transmission electron microscopy are employed to probe electrode microstructures. But for such analysis to afford useful results, it is important that proper care is taken during sample preparation. Understand analysis goals and design experiments accordingly Understanding the goal of the analysis is crucial for proper sample preparation. Batteries are composed of several components which together affect the battery’s performance, so it is important to define what will be analyzed. Even when only considering electrodes (cathodes or anodes), their formulation will drastically affect performance or degradation test results. Usually, it is necessary to re-test many possible combinations by evaluating all possible factor combinations across their entire range or changing one factor at a time while keeping the others constant.2 While the former would provide a full understanding of each factor’s impact, it requires a very high number of experiments. The latter strategy would only require a limited number of experiments; however, it would also limit understanding by preventing the observation of combined interactions. A good compromise between these two strategies is a Design of Experiments (DoE) protocol.2,3,4 This methodology provides a statistical understanding of the observed process through the creation of an empirical model, which is obtained by running an optimized set of experiments to identify factor impacts and combined interactions. A GUIDE TO SAMPLE PREP FOR BATTERY ANALYSIS 2 How to Guide Safety precautions and avoiding contamination Batteries contain hazardous materials and can be a risk for electric shock due to high voltages. Wearing appropriate personal protective equipment (PPE), including safety goggles, lab coats, gloves and, if necessary, respiratory protection is advised. Insulating gloves must be worn when assembling and disassembling high voltage batteries. Avoid short-circuits by properly insulating and isolating electrodes. Select PPE that is resistant to the chemicals or materials you’ll be working with. This is easier if you familiarize yourself with the relevant material safety data sheets (MSDS). Some materials can react with or degrade in the presence of battery electrolytes. During sample preparation, consistency is key because slight differences in the experimental setup or protocol could induce significant difference in results on similar cells.5 This difference could stem from ambient temperature and humidity or a change in circumstances, such as moving a sample or loosening a connection during the testing. The dryness of equipment, tools and components used is also critical to avoiding contamination. Uncontrolled moisture content in batteries can result in unstable active material structure, gas evolution and other safety issues.6,7 Periodically checking the moisture content of the glovebox where most of the sample prep and storage is done, as well as keeping organic solvents dry, is necessary. For example, some ether-based solvents are highly hygroscopic due to hydrogen-bond formation with water, and this moisture content can be increased with frequent use, even when stored within an inert gas glovebox.8 For prepared electrolytes and commercial electrolytes, checking the purity and moisture content periodically by Karl Fischer titration and NMR is highly recommended.9,10 Labeling and documentation Properly labeled samples and comprehensive documentation ensures traceability, transparency and the ability to reproduce results. Every label and documentation item must be clear and legible. You should consider assigning unique identifiers or codes to each sample for easy tracking. Labels should include essential information, such as the sample name, date, time, preparation steps and any treatments applied, in a standardized labeling format to ensure consistency across all samples. In the absence of a laboratory information management system (LIMS) or database, a simple spreadsheet can be used to organize sample data. Taking photographs of samples before and after preparation to visually document their appearance can also be important. You should ensure that any documentation remains accurate, unaltered and tamper-proof. For example, labels made with a permanent marker on vials are easily erased in the presence of ethanol and should be avoided. If working in a team, ensure that all team members understand your labels to some extent and have access to the documentation to promote collaboration and knowledge sharing. A GUIDE TO SAMPLE PREP FOR BATTERY ANALYSIS 3 How to Guide Electrode preparation for microscopy Microscopy techniques are useful for identifying cracks caused by the degradation characteristics of battery materials during the charge–discharge process or for analyzing structural changes due to the volume expansion of electrodes. Scanning electron microscope (SEM) is the most common microscopy technique used to observe failed sections of electrodes. However, it is difficult to analyze and quantify the size of particles or pores if the observed section is not even. Fracturing the sample using a knife is very simple and easy for sample preparation, but it can damage the interior of the specimen and so should be avoided. Furthermore, due to the rough surface and micro-cracking caused by cutting the sections in this way, pore size can be distorted and accurate analysis of the microstructure of the electrode material will be inhibited. Where electrode materials are too big to be analyzed and must be cut, extra processes are needed to smooth out the uneven section before structural or chemical composition analysis.11 Ion milling using a cross-section polisher (CP) and FIB can be used for this purpose. FIB, although complex and expensive, is useful for observing specific areas by finely controlling the gallium ion source. On the contrary, the large beam size of CP, which uses argon gas as its ion source, allows for large-area milling. Mechanical polishing should not be used as a large-area cross-section sampling preparation method because most anode and cathode materials are susceptible to moisture and have a porous structure. Electrolyte sampling The accurate sampling of electrolytes is essential for obtaining meaningful insights into battery behavior. Use clean, disposable syringes, pipettes or sampling tools to prevent contamination. The sampling equipment should be chemically compatible with the electrolyte to avoid unintended reactions. Additionally, ensure that the sampling method captures a representative portion of the electrolyte – the electrolyte can be agitated before sampling to ensure uniform composition. Appropriate points in the battery’s charging or discharging cycle must be selected for sampling to capture different states of the electrolyte. Determine the required sample quantity based on the analysis techniques you plan to use in order to avoid excessive sampling that might affect the battery’s performance. Analyze the electrolyte samples promptly if the analysis requires fresh samples. If not immediately analyzed, store samples according to relevant guidelines to prevent degradation. Gas sampling Use gas-tight containers, sampling bags or chambers specifically designed for collecting emitted gases. As with electrolyte sampling, select appropriate points in the battery’s charging or discharging cycle for gas sampling to capture relevant gas emissions. Using separate gas sampling equipment for each battery or cell can prevent cross-contamination between different samples. Determine the duration of gas sampling based on expected emission patterns and the sensitivity of your analysis methods, be it gas chromatography, mass spectrometry or gas sensors. A GUIDE TO SAMPLE PREP FOR BATTERY ANALYSIS 4 How to Guide Sample storage and transport Samples might be stored for several reasons, including for future analysis or to conduct long-term performance and degradation monitoring. When storing samples, avoid excessively hot and humid conditions, especially when batteries are fully charged. Using sealed containers or desiccants can help to maintain humidity levels. To decrease the risk of unintended chemical reactions and contamination, different battery materials should be stored separately in containers and packing materials that are chemically compatible with each sample. Containers should also be labeled with the date of storage and any relevant safety precautions. If samples are to be transferred between different individuals or locations, handlers should provide information about any potential hazards and safe handling procedures, while also documenting the chain of custody to maintain accountability. It is critical to be aware of and comply with all relevant transportation regulations and guidelines, especially if you are shipping across national borders, as battery material transport regulations may differ internationally. The accuracy and reliability of battery analyses hinge on sample preparation, method repeatability and the quality of analysis. Proper sample preparation is of utmost importance when working with battery materials to ensure that findings can be consistently reproduced. References 1. Barai A, Uddin K, Dubarry M, et al. A comparison of methodologies for the non-invasive characterisation of commercial Li-ion cells. Prog Energy Combust Sci. 2019;72:1-31. doi: 10.1016/j.pecs.2019.01.001 2. Rynne O, Dubarry M, Molson C, et al. Designs of experiments for beginners—a quick start guide for application to electrode formulation. Batteries. 2019;5(4):72. doi: 10.3390/batteries5040072 3. Mathieu R, Baghdadi I, Briat O, Gyan P, Vinassa J-M. D-optimal design of experiments applied to lithium battery for ageing model calibration. Energy. 2017;141:2108-2119. doi: 10.1016/j.energy.2017.11.130 4. Lee D-C, Lee K-J, Kim C-W. Optimization of a lithium-ion battery for maximization of energy density with design of experiments and micro-genetic algorithm. Int J of Precis Eng and Manuf-Green Tech. 2020;7(4):829-836. doi: 10.1007/s40684-019- 00106-4 5. Taylor J, Barai A, Ashwin TR, Guo Y, Amor-Segan M, Marco J. An insight into the errors and uncertainty of the lithium-ion battery characterisation experiments. J Energy Storage. 2019;24:100761. doi: 10.1016/j.est.2019.100761 6. Xu K. Nonaqueous liquid electrolytes for lithium-based rechargeable batteries. Chem Rev. 2004;104(10):4303-4417. doi: 10.1021/cr030203g 7. Yang D, Li X, Wu N, Tian W. Effect of moisture content on the electrochemical performance of LiNi1/3Co1/3Mn1/3O2 /graphite battery. Electrochimica Acta. 2016;188:611-618. doi: 10.1016/j.electacta.2015.12.063 8. Dai F, Cai M. Best practices in lithium battery cell preparation and evaluation. Commun Mater. 2022;3(1). doi: 10.1038/ s43246-022-00286-8 9. Schweiger H-G, Multerer M, Wietelmann U, Panitz J-C, Burgemeister T, Gores HJ. NMR determination of trace water in lithium salts for battery electrolytes. J Electrochem Soc. 2005;152(3):A622. doi: 10.1149/1.1859693 10. Meyer AS, Boyd CM. Determination of water by titration with coulometrically generated Karl Fischer reagent. Anal Chem. 1959;31(2):215-219. doi: 10.1021/ac60146a018 11. Kim J-Y, Jeong YW, Cho HY, Chang HJ. Alternative sample preparation method for large-area cross-section view observation of lithium ion battery. Appl Microsc. 2017;47(2):77-83. doi: 10.9729/AM.2017.47.2.77