Grade by Grade
F. Electrochemistry · Electrochemistry

Batteries: Primary and Secondary Cells

F3.2 F3.3 F1.2 F2.6About 70 minutes
Learning goals
  • I can explain the difference between primary (single-use) and secondary (rechargeable) cells.
  • I can identify the anode, cathode and electrolyte, and write the half-reactions, for common batteries.
  • I can explain recharging as electrolysis.
  • I can analyse safety, health and environmental issues of batteries, including safe disposal in Ontario.

Words to know

battery
Strictly, two or more galvanic cells connected together; in everyday speech, also a single cell.
primary cell
A cell that cannot be practically recharged, because its reaction is not easily reversed (e.g. alkaline cells).
secondary cell
A rechargeable cell: an outside power supply can drive its reaction backwards (e.g. lead-acid, NiMH, lithium-ion).
dry cell
A cell with a moist paste electrolyte instead of a free liquid, such as the zinc-carbon and alkaline cells.
intercalation
The reversible insertion of ions (such as Li⁺) between the layers of a solid host (such as graphite).
energy density
The energy a battery stores per unit mass (W·h/kg) or volume.
thermal runaway
A dangerous chain reaction in which a damaged or overheated cell gets hotter and hotter, which can lead to fire.

The galvanic cell in your pocket

The zinc–copper cell with its beakers and salt bridge is great for learning, but you can't put it in a remote control. Real batteries pack the same parts (anode, cathode, electrolyte and separator) into a small, sealed, spill-proof can. In this lesson you will look inside the most common batteries, find the redox reactions, and see why some can be recharged and others cannot.

Primary cells (single use)

Zinc-carbon dry cell (the original "flashlight battery", now less common):

  • Anode: the zinc can. Zn → Zn²⁺ + 2e⁻
  • Cathode: manganese(IV) oxide packed around a carbon (graphite) rod, which collects electrons. 2MnO₂ + 2NH₄⁺ + 2e⁻ → Mn₂O₃ + 2NH₃ + H₂O
  • Electrolyte: a moist paste of ammonium chloride and zinc chloride. About 1.5 V.

Alkaline cell (most AA and AAA cells):

  • Same zinc and manganese(IV) oxide, but the electrolyte is potassium hydroxide, a base.
  • Anode: Zn + 2OH⁻ → ZnO + H₂O + 2e⁻
  • Cathode: 2MnO₂ + H₂O + 2e⁻ → Mn₂O₃ + 2OH⁻
  • Overall: Zn + 2MnO₂ → ZnO + Mn₂O₃. About 1.5 V. Powdered zinc gives a huge surface area, so alkaline cells deliver more current and last longer than zinc-carbon cells.

Button and coin cells: silver oxide cells (Zn + Ag₂O → ZnO + 2Ag, about 1.55 V) in watches; zinc-air cells (about 1.4 V) in hearing aids, which use oxygen from the air as the oxidizing agent; and lithium coin cells such as the CR2032 (about 3 V) in car key fobs and computer motherboards.

− terminal+ terminalsteel can: contact for the cathode (+)MnO₂ + graphite: CATHODEseparator soaked in KOH(aq)powdered zinc gel: ANODEbrass collector to the − endAnode:Zn + 2OH⁻ →ZnO + H₂O + 2e⁻Cathode:2MnO₂ + H₂O + 2e⁻→ Mn₂O₃ + 2OH⁻
Inside an alkaline AA cell. Powdered zinc in the centre is the anode; a manganese(IV) oxide mixture next to the steel can is the cathode; a separator soaked in potassium hydroxide lets OH⁻ ions move between them. The steel can connects to the + end.

Secondary (rechargeable) cells

A cell can be recharged only if its products stay on the electrodes in a form that can be turned back into reactants. During charging, an outside power supply pushes electrons the opposite way. The battery becomes an electrolytic cell and the non-spontaneous reverse reaction rebuilds the reactants.

Lead-acid (cars, backup power):

  • Anode (discharge): Pb + SO₄²⁻ → PbSO₄ + 2e⁻ (E° as a reduction: −0.36 V)
  • Cathode (discharge): PbO₂ + SO₄²⁻ + 4H⁺ + 2e⁻ → PbSO₄ + 2H₂O (+1.69 V)
  • Overall: Pb + PbO₂ + 2H₂SO₄ → 2PbSO₄ + 2H₂O. E°cell = 1.69 − (−0.36) = 2.05 V. Six cells in series make a 12 V battery.
  • Both electrodes become lead(II) sulfate, and sulfuric acid is used up, so the acid's density falls as the battery discharges. Charging reverses everything. Lead-acid batteries are heavy, but cheap and reliable, and in Canada most are collected and recycled.

Nickel-metal hydride (NiMH) (rechargeable AA cells, many hybrid cars): about 1.2 V. Anode: a metal alloy holding hydrogen (MH + OH⁻ → M + H₂O + e⁻). Cathode: NiOOH + H₂O + e⁻ → Ni(OH)₂ + OH⁻. NiMH replaced nickel-cadmium cells, because cadmium is toxic.

Lithium-ion (phones, laptops, power tools, electric vehicles): about 3.6 to 3.7 V per cell. Lithium has the most negative E° in the table (−3.04 V) and is the lightest metal, so lithium cells store a lot of energy for their mass.

  • Lithium ions are stored between the layers of graphite at the anode (intercalation, LiC₆) and move through an organic electrolyte to spaces in a metal-oxide cathode (for example LiCoO₂, or LiFePO₄ in lithium iron phosphate (LFP) cells).
  • Discharge, simplified: LiC₆ → C₆ + Li⁺ + e⁻ (anode) and CoO₂ + Li⁺ + e⁻ → LiCoO₂ (cathode).
  • Lithium ions shuttle back and forth between the electrodes, which is why this design is nicknamed the rocking-chair battery.
ANODE (−): graphiteCATHODE (+): metal oxideseparator+ electrolyteLi⁺dischargeLi⁺chargeLi⁺ stored betweengraphite layers (LiC₆)Li⁺ fills spacesin the oxide (LiCoO₂)
A lithium-ion cell. During discharge, Li⁺ ions leave the graphite anode and move through the electrolyte into the metal-oxide cathode while electrons travel through the device. Charging pushes them back into the graphite.
BatteryTypeAnode / cathodeElectrolyteCell voltageTypical use
Zinc-carbonprimaryZn / MnO₂NH₄Cl, ZnCl₂ paste1.5 Vlow-drain devices
AlkalineprimaryZn / MnO₂KOH1.5 Vremotes, toys, flashlights
Lithium coinprimaryLi / MnO₂organic3 Vkey fobs, watches
Lead-acidsecondaryPb / PbO₂H₂SO₄(aq)2.0 Vcar starter batteries
NiMHsecondarymetal hydride / NiOOHKOH1.2 Vrechargeable AAs, hybrids
Lithium-ionsecondaryLiC₆ / metal oxidelithium salt in organic solvent3.2–3.7 Vphones, laptops, EVs
Common batteries compared. LFP lithium-ion cells are at the low end of the lithium-ion voltage range (about 3.2 V).
Worked example

Example 1: Analyse a lead-acid battery

During discharge, which electrode is the anode, which is the cathode, and what happens to the sulfuric acid? What happens during charging?

  1. Discharge: lead (Pb) is oxidized to PbSO₄, so the lead plate is the anode (−). PbO₂ is reduced to PbSO₄, so the lead(IV) oxide plate is the cathode (+).
  2. Both half-reactions use sulfate ions and the cathode also uses H⁺, so H₂SO₄ is consumed and the electrolyte gets less dense.
  3. Charging: a power supply drives the reverse reaction, 2PbSO₄ + 2H₂O → Pb + PbO₂ + 2H₂SO₄. Now the plate where PbSO₄ is oxidized to PbO₂ is the anode, and it is positive (an electrolytic cell).

Anode: Pb. Cathode: PbO₂. Sulfuric acid is used up during discharge and regenerated during charging.

Worked example

Example 2: Energy stored

A phone's lithium-ion battery is rated 3.85 V and 4500 mA·h. How much energy does it store, in joules and in watt-hours?

  1. Charge: 4500 mA·h × 3.6 C/(mA·h) = 16 200 C.
  2. Energy = voltage × charge = 3.85 V × 16 200 C = 62 370 J (1 V × 1 C = 1 J).
  3. In watt-hours: 3.85 V × 4.500 A·h = 17.3 W·h. (Check: 62 370 J ÷ 3600 = 17.3 W·h.)

About 62 kJ, or 17.3 W·h. An electric car battery stores thousands of times more (often 50 000 to 100 000 W·h).

Safety, health and the environment (F1.2)

  • Coin cells and children: a swallowed lithium coin cell can cause serious internal burns within hours. Health Canada warns parents to keep them out of reach and to seek emergency care immediately if one is swallowed.
  • Lithium-ion fires: a damaged, crushed, overheated or poorly made cell can go into thermal runaway, because the organic electrolyte is flammable. Use the right charger, don't charge damaged or swollen batteries, and never put lithium batteries in the garbage or the blue box: they cause fires in garbage trucks and recycling plants.
  • Toxic metals: lead, cadmium, nickel and cobalt can harm people and ecosystems if batteries end up in landfills.
  • Disposal in Ontario: under Ontario's battery regulation (in force since 2020), battery producers must collect and recycle the batteries they sell. Many hardware stores, electronics stores and municipal depots have free drop-off bins. Tape the terminals of lithium and 9 V batteries before dropping them off, to prevent short circuits.
  • Recycling recovers valuable metals such as lead, nickel, cobalt and lithium, reducing the need for new mining.
Just for fun

Why are lithium-ion batteries such good dancers? They've been practising the same move for years: Li⁺ goes to the left, Li⁺ goes to the right, a thousand charge cycles, and they still call it the rocking chair.

Hands-on activity

Home battery audit and safe-disposal plan

You need: batteries already in your home, a digital multimeter if you have one, masking or electrical tape, and paper. Never open, crush or puncture a battery.

  1. Collect all loose batteries (not ones inside devices). Sort them: alkaline, rechargeable NiMH, lithium coin, lithium-ion packs, other.
  2. If you have a multimeter, measure a new alkaline AA and a used one. A fresh alkaline cell usually reads a little over 1.5 V; a cell reading well below about 1.2 V is close to dead for most devices. Explain the drop using what you learned in Calculating Cell Potentials and Predicting Spontaneity (reactants used up, approaching equilibrium).
  3. Find any swollen, leaking or damaged batteries. Put them in a separate container, away from anything flammable, for drop-off.
  4. Tape the terminals of lithium coin cells, lithium-ion packs and 9 V batteries.
  5. Look up the nearest battery drop-off location (your municipality's website or a store that sells batteries).
  6. Make a one-page "battery rules" sheet for your household: where batteries are stored (coin cells out of reach of small children), how to charge safely, and where used batteries go.

What to look for: students usually find more lithium batteries than they expected (old phones, vapes, toys, key fobs). Every one of them should go to a collection bin, not the garbage.

Your turn

1

What is the difference between a primary cell and a secondary cell?

2

In an alkaline cell, what is the anode and what is the oxidizing agent?

3

Write the overall discharge reaction for a lead-acid cell and calculate its E°cell.

4

When a rechargeable battery is being charged, is it a galvanic or an electrolytic cell?

5

Why does lithium make a high-voltage, lightweight battery?

6

Why should lithium-ion batteries never go into household garbage or the blue box?

7

A 12 V car battery contains how many lead-acid cells?

More practice: warm-up

1

What is the electrolyte in an alkaline cell?

2

What is the voltage of a NiMH cell?

3

What is "intercalation"?

4

Why were NiCd batteries largely replaced by NiMH?

5

What is the danger of a swallowed coin cell?

More practice: on your own

1

Explain why a zinc-air hearing-aid battery has a sticker over its holes that you remove just before use.

2

A rechargeable AA NiMH cell is rated 1.2 V and 2000 mA·h; an alkaline AA is 1.5 V and about 2500 mA·h. Compare the energy stored. Why might someone still choose NiMH?

3

Lead-acid batteries can release hydrogen gas when overcharged. Explain using electrolysis, and state the safety rule that follows.

4

Explain why an alkaline cell's voltage falls gradually as it is used.

5

LFP (lithium iron phosphate) cells have a lower voltage than cobalt-based lithium-ion cells. Give two reasons many electric vehicles and home storage systems use them anyway.

Challenge

1

A car's 12 V lead-acid battery delivers 60.0 A·h. What mass of lead is oxidized at the anodes during a complete discharge? (Pb 207.2 g/mol; remember the battery has 6 cells in series and the same charge passes through each.)

2

Write the charging half-reactions for a lead-acid cell, labelling which is oxidation and which is reduction, and state the sign of each electrode while charging.

3

A laptop battery pack has 3 lithium-ion cells in series (3.6 V each) and 2 such strings in parallel, each cell rated 3000 mA·h. Find the pack voltage, capacity and energy.

Check your understanding

Quiz

1.

Which battery is a primary cell?

2.

In an alkaline cell, which substance is oxidized?

3.

During charging, a rechargeable battery acts as:

4.

What is the standard potential of one lead-acid cell?

5.

In a lithium-ion battery during discharge, lithium ions move:

6.

What is the safest way to get rid of a used lithium-ion phone battery in Ontario?

For parents and teachers

Big idea (F3.2, F3.3, F1.2): every battery is one or more galvanic cells: an anode that is oxidized, a cathode that is reduced and an electrolyte between them. Rechargeable batteries can have their reaction reversed by electrolysis. Batteries must be stored, charged and disposed of safely.

How to help: Do the battery audit together and set up a household drop-off container. Ask your teen to explain what is being oxidized in an AA battery and why a phone battery can be recharged but an alkaline cannot (easily).

Common difficulties:

  • Thinking batteries "store electricity". They store chemical energy and convert it.
  • Mixing up anode and cathode during charging (the signs stay with the terminals; the roles swap).
  • Confusing capacity (A·h, a charge) with energy (W·h).

Safety: never open or puncture batteries; keep coin cells away from young children; take lithium batteries to collection points; charge with the correct charger, never leave swollen batteries in use.