1.3. Structural & Surface Chemistry (XRD & XPS)
Crystal Structure & Phase Purity (XRD)
Technique
X-ray Diffraction (XRD) determines how atoms are arranged in a material. By analysing Bragg reflections, we can identify crystalline phases, impurity peaks, and lattice distortions.
Observation:
Phase purity - are secondary or impurity phases present?
Peak sharpness and splitting - do they confirm structural order?
Lattice spacing - is it consistent with theoretical symmetry?
“The crystal lattice influences lithium-ion transport and long-term phase stability. Unintended structural disorder or secondary phases can reduce electrochemical reversibility”
NCM v LFP
NCM: Displays a layered α-NaFeO₂ (R3̄m) structure. The distinct splitting of the (006)/(102) and (108)/(110) reflections confirms excellent layered ordering, while the high I₍₀₀₃₎/I₍₁₀₄₎ intensity ratio indicates minimal Li⁺/Ni²⁺ cation mixing and stable lithium transport pathways.
LFP: Exhibits an orthorhombic olivine (Pnma) structure. The absence of characteristic Fe₂P or Li₃PO₄ impurity peaks confirms high crystalline phase purity relative to the reference standard.
Impact on KPIs
Crystal structure governs cycle life, rate performance, and safety. A stable lattice = durable, high-performing electrode.
Worked Example: Interpreting LFP Crystal Structure and Phase Purity
Figure 1. XRD patterns of three carbon-coated LFP samples prepared using different mixtures of phytic acid and phosphoric acid as the phosphorus source. LFP/C-0, LFP/C-60 and LFP/C-100 used 0%, 60% and 100% phytic acid, respectively; the remainder was phosphoric acid. These names describe the synthesis route and do not indicate carbon content or material grade.
Image Source: Peng et al. 2023
The x-axis, 2θ (degrees), is the diffraction angle, while the y-axis, intensity (a.u.), shows the relative strength of the diffracted X-ray signal. The measured patterns are shifted vertically so they can be compared clearly. The green pattern is the reference for orthorhombic olivine LiFePO₄. Labels such as (200), (101) and (111) are Miller indices, identifying the crystal planes responsible for each peak. The three measured patterns closely match the LFP reference.
Note: Matching peak positions and the absence of obvious extra peaks support good crystalline phase purity, but they cannot rule out small amounts of amorphous, poorly crystalline or low-concentration impurities. Peak intensity may also be influenced by crystal orientation, sample preparation and instrument settings.
Key observations
- The main peaks match the reference pattern for orthorhombic olivine LiFePO₄.
- The peaks are sharp and well defined, showing that the samples are crystalline.
- No obvious additional crystalline impurity peaks are visible.
- The three samples have similar patterns, showing that each synthesis route produced the same main LFP phase.
- Small differences in peak width or relative intensity may reflect changes in crystallite size, orientation or particle growth.
What to look for
- Peak positions that match a recognised LFP reference pattern, such as JCPDS/ICDD card No. 81-1173.
- No unexplained additional peaks from crystalline secondary phases.
- Sharp, repeatable peaks without unusual shoulders or asymmetry.
- Similar peak positions, widths and lattice parameters across production batches.
- Rietveld refinement where numerical phase content or lattice parameters are required.
Red flags
- Unexpected peaks → possible secondary phases, contamination or incomplete reaction.
- Large peak shifts → possible compositional change, lattice strain or calibration error.
- Broad or asymmetric peaks → possible structural disorder, strain, very small crystallites or overlapping phases.
- Large differences between batches → inconsistent raw materials, synthesis or heat treatment.
- Claiming exact phase purity from the absence of extra peaks → low-level or amorphous impurities may remain undetected.
Why it matters
- The correct LFP phase provides the intended electrochemically active material and supports usable capacity and energy.
- Inactive or unwanted phases reduce the active-material fraction and can lower cell capacity and energy density.
- Structural disorder or blocked lithium pathways can increase polarisation and reduce rate capability and power.
- Unstable or inconsistent structure can contribute to faster capacity fade and resistance growth.
- Consistent XRD results support predictable cycle life, cell matching and batch-to-batch quality.
Surface Chemistry (XPS)
Technique
X-ray Photoelectron Spectroscopy (XPS) probes the outer few nanometres of the material’s surface by measuring the binding energy of emitted electrons. This reveals oxidation states and surface composition — critical for understanding degradation.
Observation
Surface oxidation states - are Ni, Fe, or Co in correct valence states?
Surface contamination - Are unwanted surface species such as Li₂CO₃, LiF, metal fluorides or electrolyte-decomposition products present?
Coating integrity - Are the expected coating elements and bonding environments present, and are the results consistent across multiple analysed regions?
“Surface chemistry controls how the cathode interacts with the electrolyte, affecting side reactions and long-term stability.”
NCM v LFP
NCM: Shows Ni²⁺/Ni³⁺/Ni⁴⁺ ratios. An increase in Ni²⁺ or NiO-like layers indicates surface reconstruction and capacity fading.
LFP: Fresh, fully lithiated LFP should contain mainly Fe²⁺. Fe³⁺ may indicate surface oxidation or partial lithium loss. Comparing charged and discharged samples shows how the Fe²⁺/Fe³⁺ redox reaction changes during cycling. C 1s peaks indicate carbon-containing species on the surface, consistent with a carbon coating. However, TEM, Raman spectroscopy and bulk carbon analysis are needed to assess the coating structure, coverage and total carbon content.
Impact on Cell KPIs
Stable surface chemistry supports better capacity retention, lower resistance, and longer cycle life.
Worked Example: Interpreting LFP Surface Chemistry and Oxidation States
Figure 2. XPS survey and high-resolution spectra of the carbon-coated LFP/C-60 sample. Panel (a) is a broad survey scan showing the elements detected at the particle surface. Panels (b)–(e) examine the Fe 2p, P 2p, C 1s and O 1s regions in more detail. The The x-axis, binding energy (eV), indicates how strongly an electron is bound to an atom, and helps identify the element and its chemical state, while the y-axis, intensity (a.u.), shows the relative strength of the detected signal.
Image Source: Peng et al. 2023
The spectra show the expected Li, Fe, P, O and C surface signals. The Fe 2p region is consistent mainly with Fe²⁺, as expected for fully lithiated LFP. The P 2p and O 1s regions are consistent with phosphate, while the C 1s spectrum shows carbon-containing surface species associated with the LFP/C composite.
Note: XPS mainly examines the outer few nanometres of the particles, so it describes the surface chemistry rather than the full bulk composition.
Key observations
- The survey scan shows the expected Li, Fe, P, O and C surface signals.
- The Fe 2p spectrum is consistent mainly with Fe²⁺, as expected for fresh, fully lithiated LFP.
- The P 2p and O 1s signals are consistent with phosphate groups.
- The C 1s spectrum shows carbon-containing species at the particle surface, consistent with the LFP/C composite.
- No obvious unexpected elements are visible in the survey spectrum.
- Overall, the surface chemical states are consistent with LFP, although XRD is needed to confirm the bulk crystal structure.
What to look for
- Mainly Fe²⁺ in fresh, fully lithiated LFP powder.
- P and O signals consistent with the phosphate framework.
- No strong unexpected elemental or contamination signals.
- Similar peak positions and surface composition across several measured areas and production batches.
- A carbon signal consistent with the intended coating.
- TEM, Raman spectroscopy and bulk carbon analysis used alongside XPS to assess coating coverage, structure and total carbon content.
Red flags
- A substantial Fe³⁺ contribution in fresh LFP → possible surface oxidation or partial lithium loss.
- Unexpected elemental peaks → possible contamination, foreign material or synthesis residues.
- Strong carbonate or oxygenated-carbon signals → possible air exposure, contamination or surface oxidation.
- Large binding-energy shifts between samples → possible chemical change, charging effects or poor calibration.
- Large differences between measured areas or batches → non-uniform or unstable surface chemistry.
- Using one spectrum to claim a uniform carbon coating → measurements across several areas and complementary techniques are required.
Why it matters
- Battery reactions occur at the particle–electrolyte interface, so surface chemistry can strongly affect cell behaviour.
- Surface oxidation and contamination can increase charge-transfer resistance and cell DC resistance, reducing power and rate capability.
- Parasitic surface reactions can consume lithium and electrolyte, reducing first-cycle efficiency and usable capacity.
- An unstable cathode–electrolyte interphase can accelerate resistance growth and capacity fade.
- Poor or inconsistent carbon-surface chemistry can reduce electronic contact and active-material utilisation.
- Consistent surface chemistry supports predictable capacity, power, calendar life, cycle life and batch quality.
Edited by Muthu Krishna
