In previous discussions, we explored the relationship between cell capacity, C-rate, and electrode density. A key parameter influencing electrode density—and thus overall cell performance—is the particle size distribution (PSD) of the active material.
Why PSD Matters
The PSD of active materials directly impacts their tap density and compactibility—two critical factors that determine how densely an electrode can be packed during calendaring. A higher tap density allows for denser coatings, which is essential when aiming for higher energy density in cells.
What Affects Tap Density?
Several parameters influence tap density:
- Shape of secondary particles: Smooth, spherical particles tend to pack more efficiently, resulting in higher tap density.
- PSD uniformity: A narrower PSD (smaller D90/D10) improves packing behaviour, again leading to a higher tap density.
- Particle size: Larger particles are generally more spherical, indirectly contributing to a higher tap density.
Capacity and PSD: Observations from NMC 18650 Cells
The graph below plots D10, D50, and D90 values against the capacities of various 18650 NMC cells.
The trend is clear: As capacity increases, the average particle size (D50) also increases.
High-capacity cells are typically designed for energy-oriented applications where high C-rates are less critical. The goal is to fit as much active material as possible while ensuring long cycle life. This requires:
- Denser electrodes
- Lower particle surface area (to reduce side reactions)
Lower-capacity cells are often optimized for power applications. Here, maximizing surface area is essential to support high C-rates. This leads to:
- Less dense electrodes
- Higher surface area per unit mass
Uniformity Index (UI) and Its Role
The Uniformity Index (UI), often calculated as D90/D10, is another useful metric. It reflects how narrow or wide a PSD curve is:
- Lower UI = narrower PSD → higher tap density → denser electrode → higher energy density
- Higher UI = broader PSD → lower tap density → less dense electrode → higher power density
The relationship between UI and capacity aligns with the observations above:
Higher-capacity cells tend to have lower UI values, while power-oriented cells show broader PSDs (higher UI).
“The path to energy begins at the particle level.”
Stay tuned for more information and analysis on cell design!
Kerem Buğra Batuş, Cell Design & Process Development
At AION Engineering, we accelerate lithium-ion cell development using AI-driven design and advanced technologies, delivering innovative, sustainable energy solutions for a better future
broader PSD → lower tap density → less dense electrode → higher power density
is this true?
A broader PSD generally indicates a relatively higher surface area. A higher surface area, in turn, allows for more lithium pathways, which typically translates to higher C-rate capability. This is, of course, a simplified approach, since materials with the same PSD can still exhibit different surface area values due to factors such as particle surface roughness. Nevertheless, the general reasoning remains valid.