1.2. Morphology & Composition (FE-SEM & EDS)
Morphology (FE-SEM)
Technique
Field-Emission Scanning Electron Microscopy (FE-SEM) is used to visualise the surface of battery materials at the nanoscale. By bombarding a sample with a focused electron beam it produces ultra-high-resolution images that reveal fine surface details.
Observation:
Particle size and distribution - are the particles uniform or varied in size?
Cracks, agglomeration, and coating quality - are particles clumping together, or are coatings evenly applied?
Surface texture - is the surface smooth or rough?
“The particle size, shape, and how tightly they pack together decides ion facilitation through the material and how mechanically stable it is during battery cycling (charge/discharge). ”
NCM v LFP
NCM cathodes are made up of secondary particles roughly 5-15 µm wide, each built from much smaller primary particles (100-200 nm). Their nearly spherical form allows them to pack tightly together, giving the material excellent energy density and mechanical strength. Think of it like stacking smooth marbles, they fit neatly, making the most of the available space.
LFP has a completely different architecture. Its olivine crystal structure comes with two key drawbacks: one-dimensional Li⁺ diffusion pathways and an electronically insulating framework. These characteristics limit both ionic and electronic transport within the material, making pristine LFP inherently less conductive than layered oxides like NCM. Its electronic conductivity is extremely low (around 10-9 S/cm, at room temperature), which significantly restricts charge transport across the electrode. To overcome this limitation, LFP must be manufactured with much smaller, nanosized particles to shorten Li⁺ diffusion distances and provide more conductive contact points between grains. The primary particles, typically rod- or platelet-shaped (0.1-1 µm), facilitate faster lithium insertion and extraction, enabling higher charge-discharge rates and reducing mechanical stress during cycling. Additionally, conductive carbon coatings or dopants are applied to enhance electron mobility and create efficient percolation networks throughout the electrode, collectively improving the overall electrochemical performance.
Elemental Composition (SEM-EDS)
Technique
Energy-Dispersive X-ray Spectroscopy (EDS), typically coupled with SEM, detects the unique X-ray emissions produced when a material is bombarded with electrons. This allows us to identify and quantify the elemental makeup of the sample, ensuring stoichiometric accuracy and uniform distribution.
Observation
Elemental uniformity and stoichiometry - Are all key elements evenly distributed?
Elemental ratio accuracy - Does the atomic ratio match theoretical values?
Contaminants or dopants - Are unwanted impurities present?
“Accurate structural composition ensures predictable electrochemical behaviour. A slight deviation in elemental ratio can alter conductivity, stability, or safety.”
NCM v LFP
NCM: The Ni: Co: Mn ratio controls electrochemical balance (read more in detail here).
Higher Ni content → increases capacity but reduces structural stability
Higher Co content → enhances conductivity but adds cost
Higher Mn content → improves thermal stability but lowers conductivity
A Fe:P atomic ratio close to 1 is consistent with the expected LFP composition, although EDS alone cannot confirm lithium content, phase purity or complete stoichiometry. Carbon is often detected as a coating residue that enhances electrical contact.
Impact on Cell KPIs
The cathode composition directly influences energy density, lifespan, safety, and cost.
Worked example: Interpreting LFP Morphology and Elemental Mapping
Figure 1. FE-SEM images of three LiFePO₄/C samples showing differences in particle morphology, surface texture, carbon distribution and agglomeration, together with corresponding EDS elemental maps for Fe, C, O and P. Panel (b) shows the most regular secondary-particle morphology and uniform distribution, while panel (a) contains visible amorphous-carbon-rich regions and panel (c) shows substantial agglomeration.
Note: Conventional SEM–EDS does not reliably detect lithium and cannot independently confirm LFP phase purity, elemental stoichiometry or carbon-coating thickness. XRD, XPS and TEM provide complementary information.
Image Source: Peng et al. 2023
Key observations
- Panel (a) shows irregular LFP particles approximately 0.8–1.2 µm in size, mixed with uneven carbon-rich regions.
- Panel (b) shows regular secondary particles with a rough but uniform surface.
- These secondary particles are built from capsule-like primary particles approximately 100 nm wide and 200 nm long.
- Panel (c) shows large agglomerates and a less controlled particle structure.
- The EDS maps show Fe, P, O and C distributed across the analysed particle regions.
What to look for
- Regular, well-defined secondary particles with limited large agglomerates.
- Small, evenly distributed primary particles appropriate for the intended material grade.
- A consistent particle-size distribution, reported using D10, D50 and D90 and checked across batches.
- As a broad commercial reference, published LFP grades span approximately 1–13 µm D50, depending on their intended application.
- An Fe:P atomic ratio close to 1:1, with uniform Fe, P and O distribution across several mapped regions.
- Uniform carbon distribution, with total carbon checked separately by TGA or combustion analysis. Public commercial examples commonly report approximately 0.9–2.1 wt% carbon.
Red flags
- Broad particle-size distribution or large agglomerates → poor electrode uniformity.
- Irregular secondary particles → inconsistent packing and slurry processing.
- Carbon-rich or carbon-poor regions → uneven coating or excess carbon.
- Large variation in the Fe:P ratio → possible compositional inconsistency.
- Unexpected elemental inclusions → possible contamination or incomplete synthesis.
- Using EDS alone to claim trace-impurity control → more sensitive bulk techniques are required.
Why it matters
- A controlled particle-size distribution supports uniform coating and efficient powder packing, improving electrode density and volumetric energy density.
- Well-controlled primary particles and carbon distribution support lithium-ion and electron transport, reducing polarisation and improving rate capability and power performance.
- Large agglomerates can create uneven porosity, electrolyte wetting and current distribution, increasing cell resistance and heat generation.
- Uneven composition or carbon coverage can reduce active-material utilisation, lowering usable capacity and energy.
- Local differences in transport and current density can accelerate resistance growth, capacity fade and cycle-life degradation.
- Consistent morphology and elemental distribution support reliable cell matching, production yield and batch-to-batch quality.
Edited by Muthu Krishna
