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  • Batteries are devices which consist of one or more electrochemical cells
    • These produce electrical energy from chemical energy
  • To prevent waste, we use rechargeable batteries that can be reversibly oxidised and reduced
  • Based on the basic principles of the Galvanic Cell
    • Since the cell only has a limited potential, we can chain multiple cells together to create a higher voltage battery
    • \(E^\circ\) is the maximum potential output, reality is always lower
      • Based at 1M STP, so as concentrations drop, so too will the output voltage


  • Consist of three distinct regions:
    1. The anode (positive) terminal - oxidation occurs
    2. The cathode (negative) terminal - reduction occurs
    3. Electrolyte - a weak barrier that allows for electrons to be transferred from the anode to the cathode
      • Often just cardboard, soaked in an ion or ionic liquid
      • Can be a conductive polymer (Li ion)


  1. Cell voltage - the combination of the two half reactions \(E\circ_{cell}=E\circ_{reduction}−E\circ_{oxidation}\)
  2. Battery capacity - the amount of energy the battery can release at the specified voltage for a period of time (Ah - amp hours)
  3. Energy density - energy per unit mass (Ah/kg)
  4. Cut-off voltage - the lowest voltage that the battery can safely be discharged to (permanent damage may result is exceeded)
  5. Depth of discharge - the amount of energy that can be taken from the battery without resulting in loss of efficiency over time

Charging and Discharging

  • Changes in battery voltage results as it discharges. Can be used as a metric of battery state
  • The reactions are temperature dependent, so changes in temperature will effect the cell
  • Capacity tends to decrease as the charging rate increases
    • The chemistry is quite slow, so it can take time to take place
    • Forcing more electricity in will make it heat up, reducing the reaction effectiveness
  • Different battery chemistries require different charging profiles - different \(E^\circ\)

Battery Types

Lead Acid

  • Very heavy, low density, but effective and cheap. Useful where weight is not the issue
  • Anode \(\hskip{1cm}E^\circ=1.685\:V\)
\[ \ce{PbSO4_{(S)} + 5H2O_{(l)} <=> PbO2_{(s)} + 3H3O_{(aq)}+ + HSO4_{(aq)}− + 2e−} \]
  • Cathode \(\hskip{1cm}E^\circ=−0.356\:V\)
\[ \ce{PbSO4_{(S)} + 3H3O_{(aq)}+ + 2e− <=> Pb_{(s)} + HSO4_{(aq)}− + H2O_{(l)}} \]
\[ 1.685−(−0.356)=2.04\:V∗6 \text{cells} \]

Dry-Cell Batteries

  • Manganese is a cheap and relatively non toxic material
  • Anode \(\hskip{1cm}E^\circ=1.225\:V\)
\[ \ce{MnO2_{(s)} + 2e− + 4H+ <=> Mn_{(aq)}^{2} + 2H2O} \]
  • Cathode \(\hskip{1cm}E^\circ=−0.763\:V\)
\[ \ce{Zn_{(aq)}^{2+} + 2e− <=> Zn_{(s)}} \]
\[ 1.225−(−0.763)=1.988\:V \]

Li ion

  • Lithium is really useful because it has the highest oxidation potential
  • Anode
\[ \ce{xLiC6+ <=> xLi+ + xe− + xC6} \]
  • Cathode
\[ \ce{Li_{1−x} CoO2 + xLi+ + xe− <=> LiCoO2} \]
  • Overcharging - up to 5.3V leads to a secondary reaction happening synthesising Co(IV) oxide
    • Cannot easily be reversed
\[ \ce{LiCoO2 <=> Li+ + CoO2 + e−} \]
  • Over-discharging - saturates the lithium cobalt oxide
\[ \ce{Li + e− + LiCoO2 <=> Li2O + CoO} \]


  • Were a very popular rechargeable battery in the 80s/90s
    • Less popular now due to less toxic alternatives, particularly, NiMH and Li ion
    • Cd is very toxic to humans
  • Anode
\[ \ce{Cd + 2OH− <=> Cd(OH)2 + 2e−} \]
  • Cathode
\[ \ce{2NiO(OH) + 2H2O + 2e− <=> 2Ni(OH)2 + 2OH−} \]


  • Similar to NiCd but uses a metal alloy (less toxic) instead of Cd
    • 3x the energy of an equivalent size \(\ce{NiCd}\)
  • Anode
\[ \ce{Ni(OH)2 + OH− <=> NiO(OH) + H2O + e−} \]
  • Cathode
\[ \ce{H2O + M + e− <=> OH− + MH} \]


  • Exclude oxygen from the reaction, preventing build up of stable metal oxides
  • Allow for better mixing of the components of the cell
  • Non conducting solids become conducting when molten

Ionic Liquids

  • A salt in it’s liquid molten state

Polymer Electrolytes

  • Most polymers are used as insulators, however modern developments have resulted in conductive ones
  • New polymers also allow for the movement of ions through the matrix
  • Commonly used in Li ion batteries since they have good stability