How to Measure Particle Size Distribution of Ternary Cathode Materials

Particle‑size distribution (PSD) is a critical physical index for ternary cathode materials (NCM/NCA). Accurate PSD measurement provides D10, D50, D90, D97, Dmax and span data to guide precursor synthesis, calcination, jet‑milling classification and finished‑product quality control. Laser diffraction is the mainstream industrial testing method for ternary powder. Improper sample preparation, dispersion parameters and instrument settings will bring large test deviation and mislead production adjustment. This article introduces commonly used testing methods, sample‑preparation workflows, key setting points, error sources and result validation for ternary cathode PSD measurement.

1. Main Testing Methods for Ternary Cathode PSD

1.1 Laser diffraction particle‑size analyzer (industrial mainstream)

Laser diffraction works based on light‑scattering principle. When laser passes dispersed powder particles, particles scatter light at different angles; instrument calculates particle‑size distribution according to scattering‑light pattern.

• Advantages: fast measurement, wide testing range, covers sub‑micron to dozens‑of‑micron range; can obtain full‑spectrum PSD including D10, D50, D97 and Dmax; suitable for batch incoming inspection and finished‑product offline testing.
• Two dispersion modes: wet dispersion and dry dispersion. For ternary cathode powder, dry‑dispersion method is widely adopted in lithium‑battery material factories.

Note: High‑nickel ternary materials are sensitive to moisture. Wet testing with water‑based solvent easily causes secondary agglomeration and distorts test results.

1.2 Scanning Electron Microscope (SEM)

SEM obtains particle morphology and size by image shooting.

• Advantages: Intuitive observation of secondary‑particle integrity, hard agglomerates and killer particles.
• Limitations: Only counts limited particles, poor statistical representativeness; cannot replace laser diffraction for routine batch‑to‑batch PSD reporting. Usually used for auxiliary verification when laser‑test results are abnormal.

1.3 Online inline particle‑size monitor

Installed on jet‑milling production line for continuous real‑time sampling. It feeds back D50, D97 and Dmax, supports early‑warning for PSD drift during production. Used for process trend monitoring rather than final laboratory‑grade inspection.

2. Standard Testing Procedure for Dry‑Laser Diffraction of Ternary Cathode

2.1 Sample management

• Sample collection: Collect representative samples from precursor, calcined clinker or finished NCM powder. Avoid taking only surface powder from the container.
• Moisture protection: High‑nickel NCM absorbs moisture quickly. Minimize air‑exposure time before testing; keep sample cup sealed. Moisture‑induced soft agglomeration will make D97 and Dmax falsely high.
• Sample quantity: Follow instrument requirement, ensure sufficient powder enters testing chamber, avoid insufficient sampling causing statistical deviation.

2.2 Dispersion parameter setting (key for accurate results)

Dry dispersion relies on compressed‑air impact to separate soft agglomerates, without breaking intact secondary particles.

1. Dispersion air pressure: The most critical parameter.
   • Too low pressure: Soft agglomerates are not dispersed; measured D97 and Dmax are higher than true value.
   • Too high pressure: Impact force breaks complete NCM secondary particles; test shows artificially finer PSD, higher fine‑particle proportion.
   • For most NCM grades, perform pressure‑gradient test to select suitable dispersion pressure. Stop increasing pressure when D50 no longer decreases obviously.
2. Feeding rate / sample injection speed: Adjust to maintain stable powder concentration within instrument obscuration‑rate window. Too high or too low obscuration brings test error.

2.3 Optical parameter setting

• Select proper optical model (Mie theory). Input real‑refractive‑index and absorption‑index parameters for ternary oxide material. Wrong refractive‑index setting will produce systematic deviation of PSD result.
• Clean instrument sample chamber before batch testing; avoid cross‑contamination from residual powder of previous sample.

2.4 Data output

Output complete indicators: D10, D50, D90, D97, Dmax, and calculate span:

Span = (D90‑D10) / D50

Span reflects width of particle‑size distribution. Pay special attention to Dmax / D99 index, which can reflect sporadic oversized killer particles that D97 cannot fully expose.

3. Common Sources of Measurement Error

1. Inappropriate dispersion air pressure: Under‑dispersion retains soft agglomerates; over‑dispersion shatters real secondary particles. This is the top cause of inconsistent test data between laboratories.
2. Sample moisture absorption: High‑nickel NCM absorbs water in ambient air, forms temporary agglomerates, test result shows falsely coarse PSD.
3. Cross‑contamination: Residual powder inside instrument from last test pollutes new sample.
4. Unrepresentative sampling: Taking partial segregated powder leads to result cannot reflect real batch status.
5. Improper Mie optical parameters: Wrong refractive index setting brings systematic offset for D10 and fine‑particle fraction.

4. Result Judgment and Auxiliary Verification

• When large deviation occurs between test result and production expectation: Do not directly adjust jet‑mill parameters. First check sampling method, dispersion pressure and instrument status. Repeat test 2‑3 times to confirm repeatability.
• If laser test shows normal D97 but Dmax occasionally spikes: Use SEM to check whether real giant agglomerates exist, instead of regarding it as instrument noise.
• Keep test records, including dispersion pressure, obscuration rate and instrument model, for traceability when comparing data across different labs.

5. Key Points for Special Samples

1. Precursor (nickel‑cobalt‑manganese hydroxide): Precursor is easy to agglomerate; optimize dry‑dispersion pressure, avoid over‑crushing real secondary particles.
2. Calcined sintered clinker: Contains hard sintered lumps. Dispersion air pressure cannot break hard sintered agglomerates; measured Dmax truly reflects hard‑lump status of clinker.
3. High‑nickel NCM811 / NCA: Strictly control sample exposure time in atmospheric environment to prevent moisture‑absorption agglomeration.

Laser diffraction dry‑dispersion method is the standard technique for measuring particle‑size distribution of ternary cathode materials. Test accuracy depends heavily on representative sampling, reasonable dispersion pressure, correct optical parameters and anti‑moisture sample handling. Operators should focus not only on D50, but also track D10, D97, Dmax and span value. SEM can serve as auxiliary morphological verification. Eliminating measurement error ensures PSD test data truly reflects powder status and provides reliable guidance for synthesis, calcination and jet‑milling classification processes.