Inside a battery

Anode, cathode, electrolyte, separator and casing; different chemistries for different jobs.

Open the case

A battery does not create energy: it lends it

A rechargeable battery does not generate electricity out of nothing: it stores chemical energy in its materials and releases it, in a controlled and reversible way, through oxidation-reduction reactions between two electrodes separated by an electrolyte. When charging, an external electric current forces those reactions in reverse, returning ions to their original position and "recharging" the cell's chemical potential. That charge-discharge cycle can repeat hundreds or thousands of times, but each repetition slightly wears down the materials — the exact subject developed in this section's degradation leaf.

The five families of parts inside any cell

Inside a rechargeable cell, whether button-sized or the size of an electric-vehicle pack, there are always five types of components performing different functions. The cathode is the positive electrode during discharge, and its chemical composition defines much of the whole cell's behavior: it can contain lithium combined with iron and phosphate (the chemistry known as LFP), or lithium combined with nickel, manganese and cobalt (NMC), among other combinations. The anode, the negative electrode, is usually graphite; some more advanced cells incorporate silicon to increase capacity, and other completely different technologies use titanates or other materials. The electrolyte is the medium that lets ions move between the two electrodes; in most commercial lithium-ion cells it is a lithium salt dissolved in organic solvents, which are flammable if the cell is damaged. The separator is a thin porous polymer film that lets ions through but prevents direct electrical contact between the two electrodes — a catastrophic internal short circuit if that film fails. Finally, the current collectors and casing — copper, aluminum, steel, polymers — give mechanical structure, collect the generated current, and protect the assembly from the outside.

A complete electric-vehicle battery is not a single large cell: it is an assembly of hundreds or thousands of individual cells grouped into modules, with additional cooling systems to prevent overheating, structural protection against impacts, high-voltage wiring, and battery-management software that constantly monitors voltage, temperature and state of charge of each cell. Simply saying "it's a lithium battery" hides the vast majority of that real object.

Chemistry is not a race with a single winner

Each chemical battery family represents a different trade-off between energy density, cost, safety, longevity and material availability — there is no objectively "best" chemistry for every use. LFP (lithium iron phosphate) completely avoids nickel and cobalt, usually offers good durability and thermal stability — it is more fire-resistant than other chemistries — but has lower energy density, meaning it needs more mass and volume to store the same energy. NMC (nickel-manganese-cobalt) allows storing more energy per unit of mass, an important advantage when weight and range are critical — as in a long-range electric vehicle — in exchange for depending on nickel and cobalt, minerals with their own more complex extraction and oversight chains. Sodium-ion batteries, still emerging at commercial scale, reduce dependence on lithium by using a much more abundant element, and can fit well in stationary storage or in vehicles where energy density is less critical, though they do not automatically replace all current lithium-ion applications. Lead-acid batteries, the oldest rechargeable technology still in mass use, are heavy and contain a toxic metal, but have the most established and efficient recycling chain of any battery that exists today.

Choosing the right chemistry starts with the job, not the fashion

The relevant question is not which chemistry is the most advanced or the most fashionable, but what specific job the battery has to do. For a mobile phone, mass and volume are the absolute priorities. For an electric-vehicle pack, thermal safety, cycle life and total cost may matter more. For large-scale seasonal energy storage — storing summer solar electricity to use in winter —, a battery may simply be too expensive, however good it is for a few hours of range, and other storage technologies — developed in the rest of this section — may fit far better.

Start with the job, not the trendy chemistry

For a phone, mass and volume matter. For a home, safety, cycle count and total cost may matter more. For seasonal energy storage, a battery can be too expensive even if it is excellent for a few hours of range.

Questions before choosing or evaluating a battery:

  • What is the real usable capacity, not just the nameplate figure?
  • What is the peak power and safe operating temperature?
  • What is the expected calendar and cycle life, under real use conditions?
  • Is it repairable, or does a partial fault require replacing the whole pack?
  • Is there traceability of its materials' supply chain?
  • What thermal risk does that specific chemistry carry, and what protection does it include?
  • Is there an accessible collection and recycling channel when it reaches end of life?

A bigger battery is not always better: carrying capacity every day that is almost never used adds more materials, more weight and more energy cost from the very first kilometer or hour of use. The most prudent choice is usually the right cell, of just enough size — no more, no less — used for a long time, with guaranteed collection at end of life.

The numbers, with their source

Density versus longevity: the trade-off measured in cycles

LFP and NMC chemistries illustrate this leaf's central trade-off very concretely. LFP cells typically deliver 3,000 or more full cycles of useful life, with an annual degradation rate on the order of 1-1.5%, and can be charged to 100% daily without significant penalty — in exchange for lower energy density. NMC/NCA cells typically deliver between 1,000 and 1,500 full cycles, with somewhat higher annual degradation, on the order of 2-2.5%, but store more energy per unit of mass, which is why they still dominate electric vehicles where range and weight are critical.

Sources: chemistry-specific degradation comparisons gathered in MOTORWATT — EV Battery Degradation and Bonnen Batteries — EV Lithium Battery Lifespan.

What the typical factory warranty guarantees (and does not)

Most electric-vehicle manufacturers guarantee the battery will retain at least 70% of original capacity at 8 years or 160,000 km, whichever comes first. That figure is a minimum contractual threshold, not a description of when the battery "dies": under moderate conditions, NMC/NCA chemistries usually retain between 82% and 88% of their capacity at that point, and LFP between 88% and 92% — both above the guaranteed minimum, confirming that the warranty is a safety floor, not the central expectation.

What this table is missing

None of these cycle or degradation figures are universal: they depend on the specific thermal design of each pack, the battery-management software, the climate where the vehicle or equipment operates, and the real usage pattern — always charging to 100% and discharging to the minimum accelerates degradation in almost any chemistry, while more moderate use slows it down. This section's degradation leaf develops these factors in detail.

Sources