Secondary Cells (DP IB Chemistry): Revision Note

Secondary cells

Secondary cells

• Secondary / rechargeable cells employ chemical reactions which can be reversed by applying a voltage greater than the cell voltage, causing electrons to push in the opposite direction

• There are many types of rechargeable cells, but common ones include:

  • Lead-acid batteries,

  • Nickel-cadmium / NiCad cells

  • Lithium cells

Lead-acid batteries

• A lead-acid battery is made up of six cells connected in series

• Each cell uses lead as the negative electrode and lead(IV) oxide as the positive electrode

• The electrolyte is sulfuric acid

• The half-cell reactions are

Pb (s) +  SO₄^2- (aq)  →   PbSO₄ (s)  +  2e^-                                                 E^θ = -0.36 V 

PbO₂ (s) +  4H⁺ (aq) +  SO₄^2- (aq) +  2e^- →  PbSO₄ (s)  + 2H₂O (l)         E^θ = +1.70 V

• The cell generates an EMF of about 2 V and the overall reaction is

PbO₂ (s) +  4H⁺ (aq) +  2SO₄^2- (aq) +  Pb (s) →  2PbSO₄ (s)  + 2H₂O (l)       E^θ_cell = +2.06 V

• In cars, six cells provide about 12 V

• While driving, the generator reverses the discharge reaction, regenerating the electrodes

• These batteries are designed to deliver a high current for a short period, ideal for starting engines

Disadvantages of lead-acid batteries

• Very heavy and bulky

• Contain toxic materials: lead and lead dioxide

• Sulfuric acid is highly corrosive

• Disposal and recycling pose significant environmental challenges

NiCad cells

• Nickel-cadmium cells are available in many standard sizes and voltages so they can replace almost any application of traditional zinc-carbon cells

• Although they are more expensive cells, the fact they can be recharged hundreds of times means they are commercially viable

• The negative electrode consists of cadmium and the positive electrode is made of a nickel(II) hydroxide-oxide system

• The half-cell reactions are

Cd (s) +  2OH^-  (aq) → Cd(OH)₂ (s)  +  2e^-                            E^θ = -0.82 V 

NiO(OH) (s) + H₂O (l) + e^- → Ni(OH)₂ (s) +   OH^-  (aq)         E^θ =  +0.38 V

• The overall reaction in the cell is

2NiO(OH) (s) + 2H₂O (l) + Cd (s) → 2Ni(OH)₂ (s) + Cd(OH)₂ (s)         E^θ = +1.2 V

Disadvantages of NiCad batteries

• Cadmium is toxic, making disposal hazardous

• Cells must be properly recycled to avoid environmental contamination

• NiCad batteries can suffer from the memory effect

  • If the cell is recharged repeatedly without being fully discharged, it may gradually lose capacity

  • This reduces performance and shortens their usable lifespan unless carefully managed

Lithium-ion cell

• Lithium-ion cells are common in phones, laptops, and other portable devices

• Lithium is chosen for its low density and high electrode potential

• The Nobel Prize in Chemistry (2019) was awarded to Goodenough, Whittingham, and Yoshino for developing this technology

Structure:

• The cell consists of:

  • Positive electrode: lithium cobalt oxide

  • Negative electrode: carbon

  • Electrolyte: solid polymer membrane (non-leaking and safer than liquid types)

    • The polymer electrolyte cannot leak since it is not a liquid or paste, which presents advantages over other types of cells

Lithium-ion cell

• The cell consists of a sandwich of different layers of lithium cobalt oxide and carbon

Lithium-ion discharge mechanism

• Lithium ions move between electrodes through the polymer electrolyte during charge/discharge

• The half-cell reactions on discharge are:

Li (s) →   Li⁺ (s)  +  e^–                                                                E^θ = -3 V 

Li⁺ (s)  + CoO₂ (s)  +  e^– →   Li ⁺ (CoO₂) ^– (s)                E^θ = +1 V

• The cell generates an EMF of between 3.5 V and 4.0 V and the overall reaction is

         Li (s)  + CoO₂ (s)  →   Li ⁺ (CoO₂) ^– (s)                         E^θ_cell ~ +3.5

Advantages of lithium-ion cells

• Lightweight and high voltage output

• No toxic heavy metals like lead or cadmium

• No memory effect (unlike NiCad cells), so topping up charge doesn't reduce capacity

Disadvantages of lithium-ion cells

• Degrade over time — cells lose capacity with repeated cycles

• Lithium supply is limited, raising concerns about long-term sustainability

• If not recycled, lithium becomes a lost resource

• Fire risk — lithium is reactive, and damaged cells can overheat or combust

Summary of primary and secondary cells

• This section compares key features of different cell types, with a focus on performance, environmental impact, and safety:

Primary cells

• Cheap to produce, lightweight, and have a long shelf life

• Cannot be recharged — disposal after one use contributes to landfill waste

• Only suitable for low-power devices due to limited current output

Fuel cells

• Produce lower emissions if hydrogen is used as the fuel

• More energy-efficient than combustion engines

• Hydrogen is flammable and must be stored under high pressure in heavy tanks

• Expensive to produce and can suffer from catalyst wear, leakage, or corrosion

• Generally provide a steady but low electrical current

Secondary (rechargeable) cells - general features

• Can be recharged multiple times, regenerating their chemical reactants

• Provide enough current for demanding applications like power tools and vehicles

Lead-acid batteries

• Can deliver large amounts of energy quickly — ideal for starter motors

• Heavy and bulky

• Contain toxic lead and corrosive sulfuric acid — disposal is environmentally challenging

Nickel-cadmium (NiCad) cells

• Longer life than lead-acid batteries

• Cadmium is highly toxic

• Produce relatively low voltage

• Expensive compared to alternatives

Lithium-ion cells

• Lightweight due to lithium’s low density

• Deliver a high voltage output (typically 3.5–4.0 V)

• Free from toxic metals like lead or cadmium

• Gradually lose capacity with repeated use

• Expensive and dependent on finite lithium resources

• Risk of overheating or fire, especially if damaged or improperly handled