Battery Chemistry

The battery pack is the most expensive and heaviest component in an EV — typically 30-40% of vehicle cost and 25-30% of curb weight. Understanding battery chemistry is crucial for making informed decisions about EV design, performance, and safety.

All modern EVs use lithium-ion (Li-ion) batteries, but "lithium-ion" is an umbrella term covering many different chemistries. The choice of cathode material dramatically affects energy density, power capability, cycle life, safety, and cost.

Lithium-Ion Cell Structure

Every Li-ion cell has four main components:

1. Cathode (Positive Electrode)

• The chemistry differentiator — defines cell characteristics
• Made of lithium metal oxides (LFP, NMC, NCA, etc.)
• Lithium ions are stored here when the cell is charged
• Coated on aluminum foil current collector

2. Anode (Negative Electrode)

• Usually graphite (layered carbon structure)
• Silicon-doped graphite emerging for higher energy density
• Lithium ions intercalate between graphite layers during charging
• Coated on copper foil current collector

3. Separator

• Thin porous polymer membrane (PE or PP)
• Prevents electrical contact between electrodes
• Allows lithium ion transport through electrolyte
• Shutdown feature: Melts at ~130°C to stop ion flow

4. Electrolyte

• Lithium salt (LiPF₆) dissolved in organic solvent
• Conducts lithium ions between electrodes
• Must be stable across voltage range (0-4.2V)
• Flammable — major safety consideration

How a Li-ion Cell Works

$$\text{LiMO}_2 \rightarrow \text{Li}_{1-x}\text{MO}_2 + x\text{Li}^+ + xe^-$$

Lithium ions leave the cathode, travel through electrolyte, and insert into graphite anode. Electrons flow through external circuit (charger).

$$\text{Li}_x\text{C}_6 \rightarrow \text{C}_6 + x\text{Li}^+ + xe^-$$

Reverse process — lithium ions return to cathode, electrons power the motor.

The cell voltage depends on the electrochemical potential difference between cathode and anode materials. This is why different chemistries have different voltage ranges.

Cathode Chemistries Compared

The cathode material is what differentiates Li-ion variants. Each has tradeoffs:

LFP (Lithium Iron Phosphate)

• Excellent thermal stability and safety
• Long cycle life (ideal for commercial vehicles)
• No cobalt (ethical and cost benefits)
• Flat discharge curve (easy SOC estimation)

• Lower energy density (larger/heavier pack)
• Poor low-temperature performance
• Lower voltage (more cells needed for same pack voltage)

NMC (Nickel Manganese Cobalt)

Common variants: NMC111, NMC532, NMC622, NMC811 (numbers = ratio)

Higher nickel = higher energy density but lower stability. The industry is moving toward NMC811 for EVs to maximize range.

NCA (Nickel Cobalt Aluminum)

• Highest energy density among commercial cathodes
• Excellent power capability

• Lower cycle life than LFP/NMC
• Thermal stability concerns (requires sophisticated BMS)
• Contains cobalt

LTO (Lithium Titanate)

• Extremely fast charging (minutes, not hours)
• Excellent cycle life
• Operates well at -30°C to +55°C
• Very safe (no lithium plating risk)

• Very low energy density
• Higher cost
• Low voltage

OCV-SOC Relationship

The Open Circuit Voltage (OCV) of a cell varies with State of Charge (SOC). This relationship is fundamental for:

• SOC estimation in BMS
• Understanding cell behavior
• Designing charging algorithms

Mathematical Model

The OCV-SOC relationship can be modeled with various functions. A common empirical model:

$$OCV(SOC) = a_0 + a_1 \cdot SOC + a_2 \cdot SOC^2 + a_3 \cdot SOC^3 + \frac{a_4}{SOC} + a_5 \ln(SOC) + a_6 \ln(1-SOC)$$

The logarithmic terms capture the steep voltage changes at extreme SOC values (near 0% and 100%).

Key Observations

Cell Formats

Cells come in three main form factors:

Cylindrical (18650, 21700, 4680)

• 18650: 18mm diameter, 65mm length (laptop cells)
• 21700: 21mm × 70mm (Tesla Model 3, Lucid)
• 4680: 46mm × 80mm (Tesla's new tabless design)

Prismatic

• Rectangular metal case
• Capacity: 20-300 Ah per cell
• Used by: BYD, CATL, Samsung SDI

Pouch

• Soft aluminum laminate packaging
• Capacity: 10-100 Ah
• Used by: LG, SK Innovation

Chemistry Selection for Indian Market

Indian EV manufacturers are increasingly choosing LFP for these reasons:

• High ambient temperatures: LFP's thermal stability is advantageous
• Cost sensitivity: No cobalt reduces cell cost by 30-40%
• Cycle life: Important for commercial vehicles (autos, buses)
• Supply chain: Less dependence on cobalt supply chain

Tata Motors transitioned the Nexon EV to LFP (from NMC) in 2023, citing better thermal performance in Indian conditions.

Safety: Thermal Runaway

• Trigger: Internal short, overcharge, external heat
• Self-heating: Exothermic reactions at >80°C
• Separator failure: ~130°C separator melts, short circuit
• Cathode decomposition: Releases oxygen
• Electrolyte ignition: Organic solvents catch fire

LFP > LTO > NMC111 > NMC622 > NMC811 > NCA

Modern EVs use multiple safety layers:

• Cell-level: CID (current interrupt device), PTC
• Module-level: Thermal barriers, fuses
• Pack-level: BMS monitoring, cooling, firewall
• Vehicle-level: Crash protection, emergency disconnect

Key Takeaways

• LFP is safest and cheapest, but lowest energy density — ideal for India
• NMC811 offers highest energy density but requires careful thermal management
• NCA is used by Tesla for maximum range, but with sophisticated BMS
• LTO enables ultra-fast charging but sacrifices energy density
• OCV varies with SOC — this relationship is key for BMS algorithms
• Chemistry choice involves tradeoffs between energy, power, life, safety, and cost

What's Next

In the next lesson, we'll learn how individual cells are combined into battery packs — understanding series/parallel configurations, pack voltage classes (400V vs 800V), and how to calculate pack specifications from cell data.