Today I met a customer inquiry: lithium battery graphite anode, I want to use TEM to see the thickness of SEI after the electrode piece cycle, how to prepare a sample? This article introduces in detail the whole process of TEM sample preparation of the SEI film thickness of lithium battery anode after cycling.
On the preparation of lithium battery graphite anode TEM samples. Observing the SEI film of the post-cycle pole piece is a very classic but meticulous task.
The general principles are:The original morphology and chemical state of the SEI membrane are maintained to the greatest extent to avoid artificially introduced damage or artifacts.
Below I will break down the steps and precautions for sample preparation for you in detail.
Analysis of core difficulties
SEI membranes are very fragile and have the following characteristics:
Sensitive to air and moisture: Components in SEI membranes (e.g., LiF, Li₂CO₃, ROLi, etc.) react rapidly with H₂O and O₂, leading to structural disruption and compositional changes.
Sensitive to electron beams: Under the high-energy electron beam irradiation of TEM, organic components are easily decomposed.
Low mechanical strength: Mechanical forces such as scraping and ultrasonic can easily shatter or peel off the graphite surface.
Therefore, the entire preparation process must revolve around solving these three major difficulties.
Step-by-step preparation process (recommended method)
Step 0: Preparation (Crucial!) ）
Tools: Glove box (H₂O and O₂ content less than 0.1 ppm), scraper, weighing flask, vial, pipette.
Solvent selection: This is the core question you ask. Choose high-purity, anhydrous, high-boiling alkane solvents, such as:
N-Hexane (n-Hexane)
Cyclohexane
Methylene chloride under argon protection (DCM, need to be used in glove boxes, volatilizes quickly)
Why Choose These Solvents?
Chemically inert: They do not react with the lithium salt components in the SEI membrane.
Insoluble SEI: The organic and inorganic components in SEI have very low solubility in these non-polar solvents.
Low surface tension: Easy to dry on TEM copper mesh, reducing SEI film wrinkling or rupture caused by capillary force during drying.
Avoid: Polar solvents such as NMP, water, ethanol, acetone, DMF, etc. They can dissolve or severely disrupt the SEI membrane structure.
Step 1: Remove the electrode pieces from the cell and wash them (in the glove box!) ）
Disassemble the battery: In the glove box, carefully disassemble the cycle of coin or pouch batteries.
Take out the electrode piece: Use tweezers to take out the negative electrode piece.
Cleaning Residual Electrolyte: This is a Crucial Step! The purpose is to wash away the remaining LiPF₆-based electrolyte on the surface of the electrode (which reacts with moisture to form HF and corrode SEI).
Place the pole piece in a small vial.
Add a sufficient amount of high-purity anhydrous DMC (dimethyl carbonate) or other carbonate solvent (e.g., EMC). DMC is an electrolyte solvent that effectively dissolves residual electrolyte, but is relatively gentle on the formed SEI film.
Gently shake or steep for tens of seconds to a minute. Do not ultrasound!
Take out the pole piece with tweezers, and you can quickly rinse it with clean DMC.
Place the cleaned electrode piece on the filter paper in the glove box to dry naturally. Ensure that the DMC is fully volatilized.
Step 2: Scrape the powder sample (in the glove box!) ）
Take a clean weighing flask or vial.
Gently scrape the surface of the negative electrode piece with a surgical blade. The action must be gentle, the purpose is to scrape off the graphite particles attached to the SEI film, rather than scraping off the current collector aluminum foil.
Collect the scraped small amount of powder into a vial. The amount of powder does not need to be large, just a little can be seen with the naked eye.
Step 3: Disperse the sample (in a glove box!) ）
Add 1-2 mL of your pre-prepared dispersion solvent (e.g., n-hexane) to the vial containing the powder.
About sonication: This is a step that requires extreme caution.
Principle: If you can, don't use it, use it for a short time and low power.
Risk: The strong ultrasonic cavitation effect can produce a huge local impact force that is enough to "shake" the SEI film off the graphite surface or cause it to shatter. This way you may see free SEI fragments under TEM rather than the topography that completely covers the graphite surface.
Recommended actions:
First, gently shake or shake the vial for a few minutes by simply wristing, relying on the shear force of the liquid to initially disperse. Many times this is enough.
If dispersion is not ideal, consider placing the vial in an ultrasonic cleaner in the glove box (if available) and ultrasonicating at the lowest power for 5-10 seconds. Take it out immediately to observe the dispersion. Never do ultrasound for a long time.
Step 4: Drop and transfer (in the glove box!) ）
Take a TEM copper mesh (ultra-thin carbon support film or microgrid is recommended for high-resolution imaging).
Pipette the upper dispersion (avoid aspirating large pellets at the bottom) and drop 1-2 onto the copper screen.
Wait for the solvent to volatilize completely inside the glove box.
Place the dried copper mesh in a dedicated TEM sample holder and seal it tightly, ensuring that there is as little air exposure as possible during the transfer from the glove box to the electron microscope.
Step 5: TEM observation
Quickly transfer the sealed sample to the TEM device.
When observing on the machine, try to use the Low Dose Mode to find the target area, and then perform high-resolution imaging to reduce the damage of the electron beam to the SEI film.
Look for those graphite sheets with clear edges, thin, and flat layers, and observe the thickness and structure of the SEI film at its edges.
Summary and review of key points
Inert environment: From battery removal to sample dropping, all operations must be completed in a glove box filled with high-purity argon gas. This is the first condition for success.
Correct solvent selection: DMC for cleaning, n-hexane/cyclohexane for dispersion. Avoid using any polar solvents.
Minimal mechanical force: Scraping should be light, ultrasound is strongly discouraged or extremely cautious. Prefer shaking dispersion.
Electron beam damage awareness: use low-dose techniques when observing.
Alternatives (if conditions are limited)
If you don't have a glove box, you can try using the "solid-state transfer method", but the success rate and protection against SEI will decrease:
Quickly disassemble the battery in an air environment and immediately scrape a small (millimeter-sized) fragment (including graphite, SEI, and partial current collector) directly from the pole with a blade.
Quickly clip this piece between the TEM copper mesh or gently clamp the edge of the fragment with the copper mesh, then quickly place the TEM sample rod and vacuum.
This method SEI inevitably reacts with air, but sometimes some basic topographic information can still be observed, although the thickness and true structure are no longer reliable.
