Batteries Chemistry Tutorial

Key Concepts

Battery: galvanic (voltaic) cells in a series

Types of cells used to produce batteries:

• Primary cells: cannot be recharged (can only be discharged)

  Examples: Leclanché, Alkaline cell, Lithium battery, Button cell

• Secondary cells: can be recharged (can be discharged and recharged)

  Examples: Lead-acid battery, Nickel-cadmium cell, Vanadium redox battery

During discharge of any cell:

• The cell is a galvanic cell (spontaneous redox reaction occurs).

  Chemical energy is converted to electrical energy.

• Oxidation occurs at the anode.

  Anode is negative (−).

• Reduction occurs at the cathode.

  Cathode is positive (+).

• Electrons flow from anode (-) to cathode (+).
• Maximum voltage that can be delivered by the cell during discharge is equal to the electrode potential for the spontaneous redox reaction.

  Under standard conditions, maximum voltage during discharge = E^o_(redox)

During recharge of a secondary cell:

• The cell is an electrolytic cell (non-spontaneous redox reaction occurs).

  Electrical energy is converted to chemical energy.

• Oxidation occurs at the anode.

  Anode is positive (+).

• Reduction occurs at the cathode.

  Cathode is negative (−).

• Electrons flow from anode (+) to cathode (-).
• The voltage that needs to be applied during recharge must be greater than the electrode potential for the spontaneous redox reaction.

  Under standard conditions, applied voltage > E^o_(redox)

Please do not block ads on this website.
No ads = no money for us = no free stuff for you!

Examples of Primary Cells

Leclanché or dry cell :

• cell diagram: Zn|(ZnCl₂),NH₄Cl|MnO₂,C
• Anode: zinc case

  Reaction at the anode:

  Zn(s) → Zn²⁺(aq) + 2e^-

• Cathode: carbon rod in contact with carbon and MnO₂

  Reaction at the cathode:

  NH₄⁺(aq) + MnO₂(s) + e^- → NH₃(aq) + MnO(OH)(s)

• Electrolyte: Paste of NH₄Cl, ZnCl₂ (acid electrolyte)
• Voltage: 1.5 volts
• Uses: low drain appliances such as radios, torches, liquid crystal calculators, toys

Alkaline cell :

• cell diagram: Zn,ZnO|KOH(paste)|MnO₂,C
• Anode: brass rod in contact with powdered zinc

  Reaction at the anode:

  Zn(s) + 2OH^-(aq) → ZnO(s) + H₂O(l) + 2e^-

• Cathode: steel case in contact with carbon and MnO₂

  Reaction at the cathode:

  MnO₂(s) + H₂O(l) + e^- → MnO(OH)(s) + OH^-(aq)

• Electrolyte: paste of 7 mol L^-1 KOH (alkaline electrolyte)
• Voltage: 1 volt
• Uses: High current or high total capacity appliances such as electronic photographic flash units, tape recorders, radios, shavers
  Can deliver higher current than Leclanché cell without severe voltage drop.

Mercury button cell

• Anode: steel cap in contact with powdered zinc

  Reaction at the anode:

  Zn(s) + 2OH^-(aq) → ZnO(s) + H₂O(l) + 2e^-

• Cathode: steel case in contact with powdered HgO

  Reaction at the cathode:

  HgO(s) + H₂O(l) + 2e^- → Hg(l) + 2OH^-(aq)

• Electrolyte: paste of KOH (alkaline electrolyte)
• Voltage: 1.3 volts
• Uses: watches, pacemakers, hearing aids, microphones

Silver button cell:

• cell diagram: Zn,ZnO|KOH (paste)|Ag₂O,Ag
• Anode: steel cap in contact with powdered zinc

  Reaction at the anode:

  Zn(s) + 2OH^-(aq) → ZnO(s) + H₂O(l) + 2e^-

• Cathode: steel case in contact with powdered Ag₂O

  Reaction at the cathode:

  Ag₂O(s) + H₂O(l) + 2e^- → 2Ag(s) + 2OH^-(aq)

• Electrolyte: paste of KOH (alkaline electrolyte)
• Voltage: 1.3 volts
• Uses: watches, pacemakers, hearing aids

  As cell discharges, concentrations do NOT change as reactants and products are solids.
  [OH^-] in electrolyte does NOT change.
  So, the voltage of this cell remains constant throughout its lifetime.

Lithium cell :

• cell diagram: Li|Li⁺(nonaqueous)|KOH(paste)|MnO₂,Mn(OH)₃,C
• Anode: lithium

  Reaction at the anode:

  Li → Li⁺ + e^-

• Cathode: carbon in contact with manganese(III),manganese(IV) electrode

  Reaction at the cathode:

  MnO₂ + 2H₂O + e^- → Mn(OH)₃ + OH^-

• Electrolyte: paste of KOH (alkaline electrolyte)
• Voltage:  
• Uses: auto-wind, auto-focus cameras
  Can deliver high current.
  Have long shelf-life.

Lithium iodide solid state cell :

• cell diagram: Li|LiI(s)|I₂(in polymer)
• Anode: lithium

  Reaction at the anode:

  Li(s) → Li⁺ + e^-

• Cathode: iodine

  Reaction at the cathode:

  ½I₂(s) + e^- → I^-

• Electrolyte: lithium iodide
• Voltage: 2 volts
• Uses: cardiac pacemaker, watches, calculators
  Lasts about 10 years

Examples of Secondary Cells

Lead-acid battery (or accumulator) :

• Cell diagram: Pb,PbSO₄|H₂SO₄(aq)|PbO₂,Pb
• Anode: lead plates

  Reaction at the anode:

  Pb(s) + HSO₄^-(aq) → PbSO₄(s) + H⁺(aq) + 2e^-

• Cathode: lead plates covered with PbO₂

  Reaction at the cathode:

  PbO₂(s) + 3H⁺(aq) + HSO₄^-(aq) + 2e^- → PbSO₄(s) + 2H₂O(l)

• Electrolyte: 4 mol L^-1 H₂SO₄
• Voltage: 2.1 volts per cell
• Uses: motor vehicle batteries

  As cell delivers current, lead sulfate deposits on electrodes and [H₂SO₄] falls.
  Six 2V cells connected in series make up a 12V battery.
  To obtain high current output from the battery, the surface area of the electrodes in contact with the electrolyte must be large and the plates close together.

Nickel-cadmium cell (NiCad) :

• Cell diagram: Cd,Cd(OH)₂|KOH|NiO₂,Ni(OH)₂,Ni
• Anode: grid covered with cadmium

  Reaction at the anode:

  Cd(s) + 2OH^-(aq) → Cd(OH)₂(s) + 2e^-

• Cathode: grid covered with nickel(III)hydroxide

  Reaction at the cathode:

  NiO(OH)(s) + H₂O(l) + e^- → Ni(OH)₂(s) + OH^-(aq)

• Electrolyte: KOH
• Voltage: 1.25 volts
• Uses: video cameras, phones, cordless drills, laptop computers