Part 3: Structural & Surface Chemistry - XRD & XPS
Crystal Structure & Phase Purity
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 atomic lattice dictates lithium-ion transport and long-term phase stability. Any distortion or impurity phase reduces electrochemical reversibility.”
NCM v LFP
NCM: Displays a layered α-NaFeO₂ (R-3m) structure. Clear (003)/ (104) splitting indicates minimal Li/Ni antisite mixing and stable Li+ pathways.
LFP: Exhibits an orthorhombic olivine (Pnma) structure. The absence of Fe2P or Li3PO4 impurity peaks confirm >95% phase purity.
Impact on KPIs
Crystal structure governs cycle life, rate performance, and safety. A stable lattice = durable, high-performing electrode.
Surface Chemistry
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 species (Li₂CO₃, HF salts) detected?
Coating integrity - are carbon or oxide layers uniform?
“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: Stable Fe²⁺/Fe³⁺ ratios ensure reversible redox reactions. Carbon peaks (C1s) confirm the presence of conductive surface coatings.
Impact on Cell KPIs
Stable surface chemistry ensures better capacity retention, lower resistance, and longer cycle life.
