# Batteries¶

• 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

## Region¶

• 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)

## Comparison¶

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¶

• 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
$\ce{Li + e− + LiCoO2 <=> Li2O + CoO}$

#### NiCd¶

• 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
$\ce{2NiO(OH) + 2H2O + 2e− <=> 2Ni(OH)2 + 2OH−}$

#### NiMH¶

• 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
$\ce{H2O + M + e− <=> OH− + MH}$

## Electrolytes¶

• 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