Part 1: Materials Characterization 101 for Li-ion Cell Cathodes
Introduction
The success of Li-ion batteries arises from high energy density, power density, and long cycle life, achieved through precisely engineered electrochemical materials. A typical LiB cell comprises four essential components: anode, cathode, electrolyte, and separator. Among these, the cathode largely dictates the cell’s energy density, operating voltage, and thermal stability. In the current EV battery landscape, nickel-cobalt-manganese (NCM) and lithium iron phosphate (LFP) have emerged as the two most commercially dominant cathode chemistries. Though both belong to the lithium-ion family, they differ profoundly in their structure, electrochemical behaviour, and industrial application.
From a scientific perspective:
NCM adopts a layered α-NaFeO₂-type (R-3m) structure, capable of a theoretical capacity of ~275 mAh/g and practical capacities near 200 mAh/g. This translates to gravimetric energy densities of ~500-600 Wh/kg (material level) and cell-level values around 220-280 Wh/kg, making it the chemistry of choice for long-range EVs. However, it exhibits moderate thermal stability and cycle life typically below 2000 cycles, depending on nickel content and surface protection.
LFP, on the other hand, crystallizes in an olivine (Pnma) structure stabilized by strong Fe-O and P-O bonds. While its energy density is moderate (~150-180 Wh/kg at cell level), it offers exceptional thermal stability (stable up to ~400 °C), intrinsic safety, and an impressive lifespan of 3000-10,000 cycles. Its robust one-dimensional Li⁺ diffusion pathways support specific power levels up to 2000-5000 W/kg, making it ideal for high-rate and long-life applications such as buses, energy storage, and short-range EVs.
From an industrial standpoint, it’s often seen as a simple trade-off, NCM delivers more energy, while LFP provides greater thermal stability and longevity. But when we look deeper, beyond the surface of what industry people broadly perceive, the true explanation comes from fundamental science. So, what causes such contrasting characteristics between NCM and LFP? The answer lies in their crystal chemistry, surface structure, and electronic properties, which govern how lithium ions and electrons move, react, and age over time. The real key to next-generation battery innovation lies in fine-tuning their microstructure from atomic-scale ordering to surface coatings to optimize ion transport, electronic conductivity, and degradation resistance. In essence, understanding these structural nuances is what allows scientists and engineers to design the batteries that will power our future safely, efficiently, and sustainably.
Why Characterisation Matters
Electrochemical behaviour originates from atomic and microscale order. Parameters such as crystallinity, phase purity, particle morphology, defects, and surface chemistry dictate Li-ion mobility and structural resilience. Small changes like cation mixing, oxygen deficiency, or coating uniformity can dramatically alter cycle life or rate performance. To decode these relationships, researchers use a hierarchy of characterization tools, each probing different length scales:
X-ray diffraction (XRD): crystal structure and phase purity
Electron microscopy (SEM/TEM): particle morphology and coatings
X-ray photoelectron spectroscopy (XPS): surface chemistry and oxidation states
Raman/FTIR spectroscopy: lattice vibrations and bonding
BET surface area analysis: porosity and electrolyte access
Thermal analysis (DSC/TGA): safety and stability
Through these techniques, we can correlate structure-property-performance, guiding cathode design for optimal KPIs.
Battery Cell Key Performance Indicators (KPIs)
A cell’s performance ultimately comes down to five closely connected factors which are all dependent on the structure of its cathode. The KPIs are also affected by other factors including electrolyte composition, electrode design, cell design, and manufacturing quality, but in this article we shall focus on the cathode material itself.
The five essential KPI to decide the overall cell performance are,
Energy density tells us how much power a cell can pack into a given weight or space. It decides how far an electric vehicle can go or how long a device can run on one charge. This depends mainly on the cathode’s voltage and how many lithium ions it can hold and release during cycling.
Rate capability is about how fast the cell can charge or deliver power. It depends on how easily lithium ions and electrons move through the cathode and the electrolyte. Particle size, structure, and conductivity all matter here—materials with faster ion flow can charge quicker and deliver higher bursts of power.
Cycle life reflects how long a cell can keep performing before its capacity fades. Repeated charging and discharging cause structural stress and unwanted reactions that slowly wear materials down. A strong, stable cathode structure means the cell lasts longer.
Safety – While all chemistries store significant energy and can become hazardous under extreme conditions, nickel-rich materials like NCM are more prone to oxygen release and rapid heat generation. LFP, by contrast, possesses a more stable phosphate framework that resists oxygen release, offering superior thermal stability and a generally greater range of tolerance.
Cost ultimately determines how widely a cell can be adopted. Expensive metals like nickel and cobalt make NCM cells costly, while LFP relies on cheap, abundant elements such as iron and phosphorus. Although it stores less energy, its lower cost and longer life make it more economical over time.
The distinct characteristics of NCM and LFP cathodes can be compared across these five key performance indicators, highlighting how their chemistry and structure translate into real-world cell behaviour.
“Comprehending the relationship between cathode microstructure and each KPI enables a rational optimization of energy density, durability, and safety, guiding the development of next-generation lithium-ion batteries.”
