Lithium-Ion Cell Chemistry and Pack Architecture
Electric vehicle battery packs are composed of thousands of individual lithium-ion electrochemical cells. Each cell generates electricity through lithium-ion migration between a cathode (positive electrode) and an anode (negative electrode) through a lithium-salt electrolyte. Cell chemistry choices define the pack's performance profile. Lithium Nickel Manganese Cobalt Oxide (NMC) chemistry provides a balanced combination of high energy density (~200 Wh/kg), good thermal stability, and reasonable cycle life (1,000–2,000 cycles to 80% capacity) — used in BMW i-series, Nissan LEAF (newer), and most passenger EVs requiring range. Lithium Iron Phosphate (LFP) chemistry sacrifices some energy density (~160 Wh/kg) for dramatically superior thermal stability (the cathode is chemically stable at high temperatures, reducing thermal runaway risk significantly), exceptional cycle life (3,000–6,000 cycles), and lower cost — used by BYD, Tesla Standard Range vehicles, and commercial fleet applications where longevity outweighs maximum range. Lithium Nickel Cobalt Aluminum Oxide (NCA), used by early Tesla vehicles, maximizes energy density (~250 Wh/kg) but requires more sophisticated thermal management. Cell form factors: cylindrical cells (18650 at 18mm × 65mm, 2170 at 21mm × 70mm used by Tesla) are manufactured efficiently and have built-in pressure relief vents; prismatic cells (rectangular, used by BMW) offer high packing density; pouch cells (flat flexible pouches used by many manufacturers) minimize packaging mass but require structural containment. Pack architecture: cells are connected in series to increase voltage and in parallel to increase capacity. A Tesla Model 3 Long Range uses a nominal 350V pack — 96 groups of cells in series × 3.6V per cell nominal. Higher voltage reduces current for the same power (P = V×I), reducing resistive losses in conductors and enabling thinner, lighter cables.