Secondary Cells (SL IB Chemistry)

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

• Lead-acid batteries consist of six cells joined together in series
• The cells use lead metal as the negative electrode and lead(IV) oxide as the positive electrode
• The electrolyte is sulfuric acid

A lead-acid battery

A lead acid battery is made by placing negative lead and positive lead dioxide electrodes into the sulfuric acid electrolyte 

• 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 a commercial car battery, the six cells in series give a combined voltage of about 12 V
  • When the car is in motion, the generator provides a push of electrons that reverses the reaction and regenerates lead and lead(IV) oxide
• Lead-acid batteries are designed to produce a high current for a short period of time, hence their use in powering a starter motor in car engines
• The disadvantage of lead-acid batteries is that:
  • They are very heavy
  • They contain toxic materials: lead and lead(IV) oxide
  • The sulfuric acid electrolyte is very corrosive

• This presents challenges of disposal when lead-acid batteries come to the end of their useful life

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

• Cadmium is a toxic metal so the disposal of old NiCad cells is also an environmental issue

Lithium-ion cell

• Lithium ion cells power the laptop or mobile device you are probably reading this on
• The Noble Prize for Chemistry in 2019 was awarded to John B. Goodenough, M. Stanley Whittingham and Akira Yoshino for their work on lithium ion cells that have revolutionised portable electronics
• Lithium is used because it has a very low density and relatively high electrode potential
• The cell consists of:
  • A positive lithium cobalt oxide electrode
  • A negative carbon electrode
  • A porous polymer membrane electrolyte
• 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 lithium-ion cell consists of a positive lithium cobalt oxide electrode and a negative carbon electrode

• The cell consists of a sandwich of different layers of lithium cobalt oxide and carbon
• When the cell is charged and discharged the lithium ions flow between the negative and the positive through the solid electrolyte
• 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

• NiCad cells have a problem called the memory effect in which they gradually begin to lose their charge after repeated charge cycles when the cell is not fully discharged. The cells appear to 'remember' their lower state of charge
• Lithium-ion cells do not have this problem so can be topped up without any loss of charge
• Some of the problems with lithium-ion cells:
  • A global shortage of lithium is likely to make lithium-ion cells unsustainable as the current demand for lithium exceeds the supply
  • If cells are not recycled but thrown away in landfills, then a huge amount of lithium becomes lost to future generations
  • Reports of lithium-ion cell fires have raised concern about the safety of these batteries in electronic devices; it is a reminder to us that lithium is a very reactive element in Group 1 of the periodic table, which is why it has a high electrode potential

Summary of primary and secondary cells

Type of cell		Advantages	Disadvantages
Primary	General	Inexpensive Lightweight Long shelf life	Single-use which increases landfill  and causes environmental impacts Only delivers small currents
	Fuel cell	Reduced pollution if hydrogen used as fuel Hydrogen has a low density More efficient than combustion as more chemical energy converted to electrical energy	Safety issues with hydrogen gas Hydrogen must be transported using heavy containers Expensive Only delivers small currents Technical issues due to catalytic failures, leaks and corrosion
Rechargeable / secondary	General	Materials can be regenerated Can deliver high current
	Lead-acid	Can deliver large amounts of energy over short periods	Heavy mass Lead and sulfuric acid could cause pollution
	Cadmium-nickel	Longer life than lead-acid batteries	Cadmium is very toxic Produces a low voltage Expensive
	Lithium-ion	Low density of lithium No toxic heavy metals High voltage	Limited life span Expensive