Electrochemistry Open LessonsSustainable Electrochemical Science & Technology, BRIN

Chapters / Chapter 6, Batteries / Lesson 6.1

6.1 How batteries work

How a rechargeable cell works, voltage, capacity and energy, the common lithium-ion chemistries, the SEI, and what comes beyond lithium-ion, with a theoretical capacity calculator.

About 25 minutesOpen to everyoneQuiz at the end

The parts of a cell

A rechargeable battery cell has two electrodes, a negative one and a positive one, separated by an electrolyte that conducts ions but not electrons, and a porous separator that keeps the electrodes apart. During discharge, the negative electrode is oxidised and gives electrons to the outer circuit, while ions move through the electrolyte to balance the charge. Charging drives both processes backwards.

A lithium-ion cell during discharge: lithium ions move from the graphite anode to the oxide cathode through the electrolyte while electrons flow through the outer circuitCuAlGraphite anodeSeparatorOxide cathode, e.g. LiCoO₂Li⁺ through the electrolyteloade⁻ during discharge
A lithium-ion cell during discharge. Lithium ions leave the graphite and enter the layered oxide, while electrons flow through the load. Both electrodes are hosts that take lithium in and out, which is why this is called a rocking-chair cell.

In battery language the negative electrode is often called the anode and the positive one the cathode, using their roles during discharge, even though their roles swap on charge.

Voltage, capacity and energy

  • Cell voltage is the difference between the potentials of the two electrodes, for example about 3.7 V for graphite against a layered oxide. Water decomposes above about 1.23 V, which is why lithium-ion cells use organic electrolytes.
  • Capacity is the charge the electrodes can store, in mAh or mAh/g. The theoretical capacity of a material follows from Faraday's law, Q = nF/(3.6M).
  • Energy is capacity times the average voltage, in Wh or Wh/kg. Energy per mass of the whole cell is much lower than per mass of the active material, because current collectors, electrolyte, separator and casing add weight.

Common lithium-ion chemistries

Positive electrodeStructureStrengthsTrade-offs
LiCoO2 (LCO)LayeredHigh voltage, denseCobalt cost, only about half the lithium usable
LiNixMnyCozO2 (NMC)LayeredHigh energy, less cobaltHigher Ni lowers stability
LiFePO4 (LFP)OlivineSafe, long life, no cobalt or nickelLower voltage, about 3.4 V
LiMn2O4 (LMO)SpinelCheap, fastMn dissolution at higher temperature

The negative electrode is usually graphite, often with some silicon added for more capacity. Silicon stores about ten times more lithium per gram than graphite but expands by about 300%, which cracks particles and the SEI.

The SEI and the first cycle

The electrolyte is not stable at the low potential of a lithiated graphite electrode. In the first cycles it is reduced on the surface and forms a thin solid electrolyte interphase (SEI). A good SEI lets lithium ions through but blocks electrons, so it stops further electrolyte reduction. Forming it consumes lithium, which is why the first-cycle Coulombic efficiency is below 100%. The quality of the SEI largely decides how long a cell lasts.

Beyond lithium-ion

Sodium-ion cells use cheaper, more abundant elements and are being commercialised. Lithium–sulfur, lithium-metal and solid-state cells promise more energy but face problems with dendrites, shuttle effects or interfaces. Aqueous zinc batteries are safe and cheap for stationary storage. The testing principles in Lesson 6.2 apply to all of them.

Check your understanding

Quiz for lesson 6.1

Pick one answer per question. You see the explanation straight away.

References and further reading

  1. J. B. Goodenough, K.-S. Park, The Li-ion rechargeable battery, a perspective, J. Am. Chem. Soc. 135 (2013) 1167–1176.
  2. G. Harper et al., Recycling lithium-ion batteries from electric vehicles, Nature 575 (2019) 75–86. doi:10.1038/s41586-019-1682-5
  3. J. M. Tarascon, M. Armand, Issues and challenges facing rechargeable lithium batteries, Nature 414 (2001) 359–367.