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Cell Chemistries

How Cells Work


Galvanic Action

In simple terms, batteries can be considered as electron pumps. The internal chemical reaction within the battery between the electrolyte and the negative metal electrode produces a build up of free electrons, each with a negative charge, at the battery's negative (-) terminal - the anode. The chemical reaction between the electrolyte and the positive (+) electrode inside the battery produces an excess of positive (+) ions (atoms that are missing electrons, thus with a net positive charge) at the positive (+) terminal - the cathode of the battery. The electrical (pump) pressure or potential difference between the + and - terminals is called voltage or electromotive force (EMF).


Different metals have different affinities for electrons. When two dissimilar metals (or metal compounds) are put in contact or connected through a conducting medium there is a tendency for electrons to pass from the metal with the smaller affinity for electrons, which becomes positively charged, to the metal with the greater affinity which becomes negatively charged. A potential difference between the metals will therefore build up until it just balances the tendency of the electron transfer between the metals. At this point the "equilibrium potential" is that which balances the difference between the propensity of the two metals to gain or lose electrons.

A battery or galvanic cell stores energy in chemical form in its active materials and can this convert this to electrical energy on demand, typically by means of an electrochemical oxidation-reduction (redox) reaction. (Note the generic name "redox" seems to have been appropriated by a recent flow battery design employing two vanadium redox couples).

Each galvanic or energy cell consists of at least three and sometimes four components

  1. The anode or negative electrode (the reducing or fuel electrode) which gives up electrons to the external circuit and is oxidised during the elecrochemical (discharge) reaction. It is generally a metal or an alloy but hydrogen is also used. The anodic process is the oxidation of the metal to form metal ions.

    ( LEO Lose Electrons - Oxidation)

  2. The cathode or positive electrode (the oxidising electrode) which accepts electrons from the external circuit and is reduced during the electrochemical (discharge) reaction. It is usually an metallic oxide or a sulfide but oxygen is also used. The cathodic process is the reduction of the oxide to leave the metal.
    (GER Gain Electrons - Reduction). Remember the mnemonic of the lion growling.
  3. The electrolyte (the ionic conductor) which provides the medium for transfer of charge as ions inside the cell between the anode and cathode. The electrolyte is typically a solvent containing dissolved chemicals providing ionic conductivity. It should be a non-conductor of electrons to avoid self discharge of the cell.
  4. The separator which electrically isolates the positive and negative electrodes.

The Discharge Process


When the battery is fully charged there is a surplus of electrons on the anode giving it a negative charge and a deficit on the cathode giving it a positive charge resulting in a potential difference across the cell.

When the circuit is completed the surplus electrons flow in the external circuit from the negatively charged anode which loses all its charge to the positively charged cathode which accepts it, neutralising its positive charge. This action reduces the potential difference across the cell to zero. The circuit is completed or balanced by the flow of positive ions in the electrolyte from the anode to the cathode.

Since the electrons are negatively charged the electrical current they represent flows in the opposite direction, from the cathode (positive terminal) to the anode (negative terminal).


Two Electrolyte Systems


The principles of the Galvanic cell can be demonstrated by the workings of the Daniell cell, a two electrolyte system.





The positive pole of the battery

Daniell Cell

The negative pole of the battery


Zinc loses electrons more readily than copper



Accepts electrons from the external circuit

Supplies electrons to the external circuit


Copper metal deposits on the cathode

Zinc goes into aqueous solution


The site of Reduction

The site of Oxidation



The half-cell with the highest electrode potential

The half-cell with the lowest electrode potential



Two electrolyte primary cell systems have been around since 1836 when the Daniell cell was invented to overcome the problems of polarisation. This arrangement illustrates that there are effectively two half cells at which the chemical actions take place. Each electrode is immersed in a different electrolyte with which it reacts. The electrode potential, either positive or negative, is the voltage developed by the single electrode. The electrolytes are separated from each other by a salt bridge or porous membrane which is neutral and takes no part in the reaction. By the process of osmosis, it allows the sulphate ions to pass but blocks the metalic ions.

This two electrolyte scheme allows more degrees of freedom or control over the chemical process.

Although more complex these cells enabled longer life cells to be constructed by optimising the electrolyte/electrode combination separately at each electrode.

More recently they have been employed as the basis for Flow Batteries in which the electrolytes are pumped through the battery, providing almost unlimited capacity.


Zinc is a very popular anode material and the chemical action above causes it to dissolve in the electrolyte.

The Daniell cell shown can be said to "burn zinc and deposit copper"

Note- The simple, single electrolyte cell can also be represented by two half cells. It can be considered a special case of a Daniell cell with the two electrolytes being the same.

The model of the cell as two half cells is used by electrochemists and cell designers to calculate electrode potentials and and characterise the chemical reactions within the cell. The cell voltage or electromotive force (EMF) for the external current derived from a cell is the difference in the standard electrode potentials of the two half cell reactions under standard conditions. But real voltaic cells will typically differ from the standard conditions. The Nernst equation relates the actual voltage of a chemical cell to the standard electrode potentials taking into account the temperature and the concentrations of the reactants and products. The EMF of the cell will decrease as the concentration of the active chemicals diminishes as they are used up until one of the chemicals is completely exhausted.

The theoretical energy available from the cell can be calculated using Gibbs free energy equation for the initial and final equilibrium states.

Fortunately such intimate knowledge of cell chemistry and thermodynamics is not usually required by the battery applications engineer.

Primary cells

In primary cells this electrochemical reaction is not reversible. During discharging the chemical compounds are permanently changed and electrical energy is released until the original compounds are completely exhausted. Thus the cells can be used only once.

Secondary cells

In secondary cells this elecrochemical reaction is reversible and the original chemical compounds can be reconstituted by the application of an electrical potential between the electrodes injecting energy into the cell. Such cells can be discharged and recharged many times.


Rechargeable Battery Action


The Charging Process

The charger strips electrons from the cathode leaving it with a net positive charge and forces them onto the anode giving it a negative charge. The energy pumped into the cell transforms the active chemicals back to their original state.

Choice of Active Chemicals

The voltage and current generated by a galvanic cell is directly related to the types of materials used in the electrodes and electrolyte.

The propensity of an individual metal or metal compound to gain or lose electrons in relation to another material is known as its electrode potential. Thus the strengths of oxidizing and reducing agents are indicated by their standard electrode potentials. Compounds with a positive electrode potential are used for anodes and those with a negative electrode potential for cathodes. The larger the difference between the electrode potentials of the anode and cathode, the greater the EMF of the cell and the greater the amount of energy that can be produced by the cell.

Electrochemical Series is a list or table of metallic elements or ions arranged according to their electrode potentials. The order shows the tendency of one metal to reduce the ions of any other metal below it in the series.

A sample from the table of standard potentials shows the extremes of the table.


Strengths of Oxidizing and Reducing Agents


Cathode (Reduction)

Standard Potential
E ° (volts)

Li + (aq) + e - -> Li(s)


K + (aq) + e - -> K(s)


Ca 2+ (aq) + 2e - -> Ca(s)


Na + (aq) + e - -> Na(s)


Zn 2+ (aq) + 2e - -> Zn(s)


Cu 2+ (aq) + 2e - -> Cu(s)


O 3 (g) + 2H + (aq) + 2e - -> O 2 (g) + H 2 O(l)


F 2 (g) + 2e - -> 2F - (aq)


The values for the table entries are reduction potentials, so lithium at the top of the list has the most negative number, indicating that it is the strongest reducing agent. The strongest oxidizing agent is fluorine with the largest positive number for standard electrode potential.

The table below shows some common chemicals used for battery electrodes arranged in order of their relative electrode potentials.

Anode Materials
Corner   Corner
Cathode Materials

(Negative Terminals)

(Positive Terminals)

BEST - Most Negative

BEST Most Positive

Lithium Ferrate
Magnesium Iron Oxide
Aluminum Cuprous Oxide
Zinc Iodate
Chromium Cupric Oxide
Iron Mercuric Oxide
Nickel Cobaltic Oxide
Tin Manganese Dioxide
Lead Lead Dioxide
Hydrogen Silver Oxide
Copper Oxygen
Silver Nickel Oxyhydroxide
Palladium Nickel Dioxide
Mercury Silver Peroxide
Platinum Permanganate
Gold Bromate

WORST Least Negative

WORST Least Positive

Cells using aqueous (containing water) electrolytes are limited in voltgage to less than 2 Volts because the oxygen and hydrogen in water dissociate in the presence of voltages above this voltage. Lithium batteries (see below) which use non-aqueous electrolytes do not have this problem and are available in voltages between 2.7 and 3.7 Volts. However the use of non-aqueous electrolytes results in those cells having a relatively high internal impedance.


Alternative chemical reactions

More recently new cell chemistries have been developed using alternative chemical reactions to the traditional redox scheme.

Metal Hydride Cells

Metal hydride cell chemistry depends on the ability of some metals to absorb large quantities of hydrogen. These metallic alloys, termed hydrides, can provide a storage sink of hydrogen that can reversibly react in battery cell chemistry. Such metals or alloys are used for the negative electrodes.The positive electrode is Nickel hydroxide as in NiCad batteries. The electrolyte, which is also a hydrogen absorbent aqueous solution such as potassium hydroxide, takes no part in the reaction but serves to transport the hydrogen between the electrodes.

Lithium Ion Cells

Rather than the traditional redox galvanic action, Lithium ion secondary cell chemistry depends on an "intercalation" mechanism . This involves the insertion of lithium ions into the crystalline lattice of the host electrode without changing its crystal structure. These electrodes have two key properties

  1. Open crystal structures which allow the the insertion or extraction of lithium ions
  2. The ability to accept compensating electrons at the same time

Such electrodes are called intercalation hosts.

In a typical Lithium cell, the anode or negative electrode is based on Carbon and the cathode or positive electrode is made from Lithium Cobalt Dioxide or Lthium Manganese Dioxide. (Other chemistries are also possible)

Since Lithium reacts violently with water, the electrolyte is composed of non aqueous organic Lithium salts and acts purely as a conducting medium and does not take part in the chemical action, and since no water is involved in the chemical action, the evolution of hydrogen and oxygen gases, as in many other batteries, is also eliminated.

Swing Cell


During discharge lithium ions are dissociated from the anode and migrate across the electrolyte and are inserted into the crystal structure of the host compound. At the same time the compensating electrons travel in the external circuit and are accepted by the host to balance the reaction.

The process is completely reversible. Thus the lithium ions pass back and forth between the electrodes during charging and discharging. This has given rise to the names "Rocking chair", "Swing" or "Shuttlecock" cells for the lithium ion batteries.


Variations on the Lithium technology are also used in primary cells which were originally developed for space and military applications. These include Lithium-thionyl chloride and Lithium-sulphur dioxide chemistries which use reactive electrolytes and liquid cathodes to obtain higher energy and power densities.

Alternative chemistries - Special flavours

Designing a better battery is not simply a matter of choosing a pair of elements with a larger difference in electrode potentials, there are many other factors which come into play. These may be: availability and cost of the raw materials, stability or safety of the chemical mix, manufacturability of the components, reversibility of the electrochemical reaction, conductivity of the components, operating temperature range and quite possibly the desire to circumvent some other manufacturer's patent. All of these considerations lead to the use a limited range of basic chemistries but with a wider variety of proprietary material formulations.


Over the years a wide range of cell chemistries and additives has been developed to optimise cell performance for different applications.

Alternative active compounds may be substituted to increase energy densities (See below), increase the current capacity, reduce internal impedance, reduce the self discharge, increase the terminal voltage, improve the coulombic efficiency or reduce costs.

Additional compounds may be incorporated to modify the behaviour of the active compounds to increase cycle life, to prevent corrosion or leakage, to control polarisation or to increase safety. These could include catalysts which may be used to promote or accelerate desired chemical actions such as recombination of the active chemicals in sealed cells. They could also include inhibitors which may be added to slow down or prevent unwanted physical or chemical actions such as the formation of dendrites.


Added to the range of available cell chemistries are the different cell capacities and physical constructions of the cells, the battery applications engineer thus has a wide variety of options from which to choose.


Energy Density

The energy density is a measure of the amount of energy per unit weight or per unit volume which can be stored in a battery. Thus for a given weight or volume a higher energy density cell chemistry will store more energy or alternatively for a given storage capacity a higher energy density cell will be smaller and lighter. The chart below shows some typical examples.

Relative Energy Density of Some Common Secondary Cell Chemistries

Energy Density

In general higher energy densities are obtained by using more reactive chemicals. The downside is that more reactive chemicals tend to be unstable and may require special safety precautions. The energy density is also dependent on the quality of the active materials used in cell construction with impurities limiting the cell capacities which can be achieved. This is why cells from different manufacturers with similar cell chemistries and similar construction may have a different energy content and discharge performance.

Note that there is often a difference between cylindrical and prismatic cells. This is because the quoted energy density does not usually refer to the chemicals alone but to the whole cell, taking into account the cell casing materials and the connections. Energy density is thus influenced or limited by the practicalities of cell construcion.

Supply of the Basic Chemical Elements

Worried about the availability of exotic chemicals and the effect future demand may have on prices?

The chart below shows the relative abundance of chemical elements in the earth's crust.

Abundance of elements

Source - U.S. Geological Survey Fact Sheet 087-02

Note - From the chart above Lithium is between 20 and 100 times more abundant than Lead and Nickel. The reason it is less common is that Lithium, being much more reactive than either metal, is not usually found in its free state, but is combined with other elements. By contrast Lead being less reactive is more often found in its free state and is easier to extract and purify. The heavy metals Cadmium and Mercury whose use is now deprecated because of their toxicity are 1000 times less common than Lithium.

Toxicity of Lithium

In case you wondered whether there were any toxic effects associated with Lithium, it is claimed that Lithium on the contrary has theraputic benefits. The soft drink "7Up" started life in 1929, two months before the Wall Street Crash, with the catchy name "Bib Label Lithiated Lemon-Lime Soda". "7Up" contained Lithium Citrate until 1950 when it was reformulated, some say because of Lithium's association with mental illness. Since the 1940s, Lithium in the form of Lithium Carbonate has been used successfully in the treatment of mental disorder particularly manic depression. As with most chemicals however, small doses may be safe or theraputic, but too much can be fatal.

Make your own battery at home or at school

See Homebrew Batteries for instructions on how to make a battery using simple materials available at home.

Practical Cell Chemistries

Some of the most common cell chemistries are described and the applications for which they are suitable if you follow the links below:-

Primary Cells

Secondary Cells


Unusual Batteries


Cell chemistry Comparison Chart

Alternative Energy Generation and Storage Methods



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