1.5. Nanostructure Imaging (TEM & HRTEM)
Nanostructure Imaging (TEM)
Technique
Transmission electron microscopy (TEM) passes electrons through an ultrathin sample to reveal particle morphology, internal structure and interfaces. High-resolution TEM (HRTEM) provides greater detail, allowing lattice fringes, crystal planes, defects and nanoscale coatings to be examined.
Observation
Particle and internal structure: Are primary particles, pores or internal boundaries visible?
Lattice fringes: Are they clear, ordered and consistent with the expected crystal planes?
Coating thickness and coverage: Is the surface layer thin, continuous and reasonably uniform?
Interfaces and defects: Are grain boundaries, dislocations, voids or amorphous regions visible?
“Nanostructure influences Li-ion transport and structural strength. Uniform coatings and well-ordered crystal lattices can reduce resistance and slow degradation.”
NCM v LFP
NCM: 2–10 nm coatings (LiAlO₂, Al₂O₃, etc.) can reduce direct contact with the electrolyte, suppress parasitic surface reactions and improve structural stability. The appropriate material and thickness depend on the NCM grade and coating process.
LFP: Uniform 2-5 nm carbon layers improve electronic conductivity and can reduce direct contact with the electrolyte, particle growth, surface oxidation and iron dissolution. A coating that is too thick or uneven may obstruct lithium-ion transport and add inactive mass.
Impact on Cell KPIs
Particle size, lattice order, interfaces and coating quality influence lithium-ion and electron transport. Well-controlled nanostructure can support lower resistance, better rate capability and longer cycle life, while cracks, defects or uneven coatings can increase polarisation, heat generation and capacity fade.
Worked example: Interpreting LFP Nanostructure, Lattice Fringes and Carbon Coating
Figure 1. TEM (g) and HRTEM (h) images of the carbon-coated LFP/C-60 sample from Peng et al. 2023.
Panel (g) shows connected, short rod-shaped LFP primary particles at the nanoscale, and the 100 nm scale bar provides a reference for their dimensions. Panel (h) provides a higher-magnification view of the selected region. The regular dark and light lines are lattice fringes, with a measured spacing of approximately 0.396 nm, consistent with the (210) lattice planes of orthorhombic LiFePO₄. A thin amorphous carbon layer approximately 2 nm thick is visible at the particle surface.
Note: TEM examines a very small amount of material, so coating thickness and structural quality should be checked across several particles and locations. The electron beam and sample-preparation process can also affect sensitive materials.
Key observations
- TEM shows connected, short rod-shaped LFP primary particles at the nanoscale.
- HRTEM shows clear, regularly spaced lattice fringes, indicating an ordered crystalline region.
- The measured lattice spacing is approximately 0.396 nm, consistent with the (210) lattice planes of olivine LFP.
- A thin amorphous carbon layer approximately 2 nm thick is visible around the particle surface.
What to look for
- Clear lattice fringes with spacings consistent with recognised LFP crystal planes.
- A thin and continuous carbon layer around the particle surface.
- Similar coating thickness and lattice structure across several particles and analysed areas.
- Limited cracks, voids, damaged regions or uncontrolled amorphous material.
- TEM results supported by XRD, Raman spectroscopy and bulk carbon analysis.
Red flags
- Blurred, broken or strongly distorted lattice fringes → possible disorder, strain, damage or an unsuitable imaging area.
- Thick or uneven carbon layers → possible obstruction of lithium-ion transport and excess inactive mass.
- Large uncoated regions → inconsistent electronic conductivity and surface protection.
- Cracks, voids or poorly connected particles → possible transport limitations and mechanical weakness.
- Drawing conclusions from a single particle → the image may not represent the bulk powder.
Why it matters
- A thin, continuous carbon coating improves electronic transport, supporting lower resistance and better rate capability and power.
- Nanoscale primary particles shorten lithium-ion transport paths, supporting capacity utilisation and high-rate performance.
- Thick or uneven carbon can obstruct ion transfer, add inactive mass and reduce energy density.
- Structural defects, cracks and poor interfaces can increase polarisation, resistance and heat generation.
- Uneven coatings or local defects can accelerate capacity fade and resistance growth.
- Consistent nanostructure supports reliable cycle life, cell matching and batch-to-batch quality.
Edited by Muthu Krishna
