This method can help identifying thermal decomposition mechanism and thermal properties of battery electron materials. This enables a further understanding of a thermal runaway event in a single cell. From this protocol, thermal properties of battery materials were derived more accurately by ensuring innate conditions from sample preparation to sample loading and by selecting fit-for-purpose parameters.
This technique extends towards the development of an improved thermal model to simulate thermal runaway in a single cell. This allows then a better assessment of battery safety performance to support, for example, the formulation of standards and regulations. This method provides a helpful insight to the thermal stability of materials.
This can be applied to study other energetic materials such as explosives, propellants, pyrotechnics, or novel materials. As the material is being heated over time, multiple spectra are being collected. Therefore, it is important to associate any phase transition with the correct GC-MS and FTIR spectra.
To begin, take a polymer separator disc of a diameter of 22 millimeters and a thickness of 25 micrometers and place it on top of the bottom part of the polypropylene insulation sleeve. Carefully press down the upper part of the insulation sleeve to assemble it and ensure that the separator is flat. Gather the necessary tools and materials for electrochemical cell assembly and insert them inside the glove box.
Weigh the electrode discs on a 4-digit analytical balance and record the values to determine the active material loading. Take 150 microliters of the electrolyte with the micropipette and put a drop on the separator facing the bottom part of the insulation sleeve. Insert the graphite anode with the help of the vacuum pick-up tweezer, followed by the lower plunger.
After turning the insulation sleeve, dispense the remaining electrolyte on the separator. Using a vacuum pick-up tweezer, insert the NMC cathode disc and place the upper plunger. Mount the assembly inside the cell core part.
Place the O-ring before fastening everything together with the bolt clamp. Remove the electrochemical cell from the glove box and place it inside the temperature chamber, then plug in the appropriate cables to connect the cell to the cycler. Run the electrochemical cycling process by selecting the file name of the protocol entering the corresponding current for C/20 C-rate and select the chamber number.
Afterwards, click on the Start button. After the cycling step, bring the electrochemical cell inside the glove box. Disassemble the cell and take out one electrode, then reassemble the cell to protect the remaining electrode from drying out.
Weigh the electrode using the precision balance and place it on fresh aluminum foil and fold the foil. To dry the electrode, place it in the transfer glove box antechamber under vacuum for two hours. When the weight has stabilized at x milligram plus minus 0.01 milligram, note the weight of the dried electrode.
Using tweezers and a spatula, scratch the disc to harvest the coated material for further characterization. For STA preparation, create a new method by opening the STA software and clicking on File, and then on New. Select the parameters under the Setup tab of the Measurement Definition window.
Go to the Header tab and select Correction to execute a correction run with an empty crucible for baseline correction. Write the name of the sample and select the file for the temperature and sensitivity calibration to be used for the run. Go to MFC gases and select Helium as a purge gas and protective gas.
Create the temperature program under the Temperature Program tab to define the heating and cooling process. Set the flow rate of helium to 100 milliliters per minute and 20 milliliters per minute for purge and protective gas respectively. Click on GN2 as cooling medium and STC for sample temperature control for all the segments of the temperature program, starting from the isothermal step at 5 degrees Celsius to the end of the heating segment.
Go to the Last Item tab and give a file name to this run. Use the precision balance and measure the weight of the empty crucible. Enter the crucible mass next to the name of the sample.
Open the silver furnace and place the crucible on the DSC/TG sample holder of the STA. Evacuate the furnace slowly to remove argon and refill it with helium at maximum flow rate. Repeat the evacuation refill at least two times to get rid of the argon coming from the glove box atmosphere when opening the furnace to place the crucibles.
After the evacuation and refilling, wait for 15 minutes to stabilize the weight. Using the temperature program, execute the correction run by pressing Measure. When the run is finished, take out the empty crucible.
Put 6 to 8 milligrams of the scratched material in the crucible. After weighing the sample in the crucible and recording the mass, seal the pan and lid using a sealing press. Open the correction run file by going to File and Open.
Under the Fast Definition tab, select Correction Sample as measurement type. Write the name and the weight of the sample and choose a file name. Go to the Temperature Program tab and activate the FT option for the isothermal step of 5 degrees Celsius and the heating segment to 590 degrees Celsius in order to launch FTIR gas monitoring for these two segments.
Click the GC box for the heating segment to trigger GC-MS analysis. Take a funnel, insert it into the Dewar of the mercury cadmium telluride detector port and carefully fill it with liquid nitrogen. Open the FTIR software.
On the Basic Parameter tab, load the TG-FTIR method called TGA.XPM. Check the interferogram by clicking on the Check Signal tab, then wait until the interferogram has stabilized before starting the thermal analysis. Turn on the vacuum pump line to draw evolved gaseous species from STA to FTIR and GC-MS.
Adjust the pumping rate to a stable flow, which is approximately 60 milliliters per minute. After loading the method in the GC-MS software, click on Run Method and fill in the sample name and data file name, then click OK, and Run Method. In the STA software, verify the temperature program, the gas flow, and make sure the GC-MS and FTIR options are enabled.
Go to the Last Items tab and give a file name to the sample for the STA and FTIR data. Press Measure and click on Start FTIR Connection to establish the connection between STA software and FTIR software. Once the connection is established, click on Tare to put the balance at zero and check the gas flow by selecting Set Initial Gases, then press the Start button to launch the run.
The discharge curve of the NMC 111 graphite electrochemical cell shows an anode potential of 50 millivolts, which confirms the absence of lithium plating. The thermal decomposition profile of anode material revealed a sharp endothermic peak in region 1 with no mass loss or gas generation. Region 2 shows a broad DSC heat decomposition in addition to minimal gas evolution and mass loss.
Carbon dioxide emission is seen around 100 degrees Celsius, but drops before 150 degrees Celsius, while ethylene carbonate starts to evaporate near 150 degrees Celsius. Region 3 displayed significant mass loss, gas evolution, and heat generation, shown by a sharp exothermic peak. Carbon dioxide, ethylene carbonate, phosphorus trifluoride, and ethylene were detected.
Region 4 shows a decreased amount of heat release with small, partly overlapping peaks, minor mass loss with gas traces of ethylene, and ethane, methane and propylene was observed. Increased heating rates resulted in higher peak temperature except for peak 1, where the maximum peak temperature shifts to lower values. Kissinger plots of peak 2 and peak 3 were used to calculate the kinetic parameters.
Reproducibility is paramount when assembling the electrochemical setup and when opening the cell for thermal analysis. Therefore, multiple repetitions by the same operator and following identical steps are needed. Other analytical techniques such as SEM-EDX or XRD can provide a deeper insight on the chemical composition of battery materials, and moreover, it can show its changes upon exposure to various environmental or electrochemical conditions.
This technique can help researchers to undertake the assessment of thermal properties of battery materials on a very systematic manner, while ensuring proper sample preparation.
