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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 (see below).

(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 is the reducing or fuel electrode. It 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 reducing agent to form metal ions.

    ( LEO Lose Electrons - Oxidation)

    Alternatively

    (OIL - Oxidation is Loss)

  2. The cathode or positive electrode is the oxidising electrode. It 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 oxidising agent (oxide) to leave the metal.
    (GER Gain Electrons - Reduction). Remember the mnemonic of the lion growling.
  3. Alternatively

    (RIG - Reduction is Gain) Alternative mnemonic - OIL RIG

  4. 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.
  5. Metal ions are metal atoms missing electrons and are thus positively charged. Particles missing electrons are called cations and during discharge they move through the electrolyte towards the cathode.

    Anions are atoms or particles with excess electrons and thus negatively charged. During discharge they are attracted towards the anode.

  6. 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).

 

The anode is the electrode through which electrons flow out of a polarised electrical device (or the electrode through which current flows in)

Mnemonic ACID = Anode Current Into Device (During discharge).

 

Two Electrolyte Systems

 

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

 

THE CATHODE

DANIELL CELL

THE ANODE

The positive pole of the battery

Diagram of 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

GER

LEO



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 metallic 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"


Redox Reactions and Half Cells

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 electro-chemists and cell designers to calculate electrode potentials and and characterise the chemical reactions within the cell. Reduction occurs at one half cell and oxidation takes place at ther other half cell. In a battery, both reactions take place simultaneously and the combined reaction is called a Redox reaction (Reduction and Oxidation)

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.

 

Corner
Rechargeable Battery Action (Much Simplified)
Corner

 

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.

 

The Periodic Table

Anode and cathode materials are chosen for their suitability as oxidizing or reducing agents. The relative reducing and oxidizing capabilities of the elements are indicated by the coloured arrow on the Periodic Table below. The strong reducing elements are grouped to the left, while the strong oxidizing elements are grouped to the right.

 

Periodic Table of the Elements

  • Groups
  • Elements within each individual group have the same number of "valence" electrons in their outer valence shell. Because the number of valence electrons determines how the atom reacts chemically with other atoms, elements within a particular group tend to have similar chemical properties.

    The outer electron shell can have up to eight electrons but with a full complement of eight electrons, as in the noble gases (group 18), there are no "free" electrons available to take part in chemical reactions hence the noble gases are chemically non-reactive or inert. Thus in the outer, valence shell of atoms there are effectively only seven possible valence electrons and each element has a unique characteristic number of electrons which determine its properties. The ways in which atoms react with other atoms, in other words their possible chemical reactions, are determined by the number of electrons in their valence shells.

    The most reactive elements are found at the left and right extremes of the table. They are the alkaline metals (group1) whose atoms have only one electron in their valence shells and the halogens (group 17) with seven valence electrons, just missing one electron from having a complete shell.

     

  • Periods
  • All the elements in any one period have the same number of electron shells or orbits which corresponds to the number of possible energy levels of the electrons in the atom. The period number corresponds to the number of electron shells.

    The number contained in each box in the table is the atomic number of the element which is the number of protons in the nucleus of each atom. Moving from left to right across the table from group 1 to group 18 within each period, the number of protons per atom increases by one from each element to the adjacent element.

 

Reducing agents (elements) have surplus electrons in their outer, valence shell which they donate in a redox reaction and hence become oxidised. The oxidising agents (elements) have a deficit of electrons in their valence shell which accepts electrons in the redox reaction and becomes reduced.

 

Electrochemical Series

The electrochemical series below 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.


Corner

Strengths of Oxidizing and Reducing Agents

Corner

Cathode (Reduction)
Half-Reaction

Standard Potential
E ° (volts)

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

-3.04

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

-2.92

Ca2+ (aq) + 2e- --> Ca(s)

-2.76

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

-2.71

Zn2+ (aq) + 2e- --> Zn(s)

-0.76

2H+ + 2e- --> H2

0

Cu2+ (aq) + 2e- --> Cu(s)

0.34

O3+ (g) + 2H+ (aq) + 2e- --> O2 (g) + H2O(l)

2.07

F2 (g) + 2e- --> 2F- (aq)

2.87


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 strength of the reduction or oxidising capacity of compounds is also indicated by their characteristic electrode potentials.


Available Energy

Chemical elements contain intrinsic electrochemical energy potential associated with the energy of the electrons in the outermost electron shell or valence band in the atom and whether in its current state it has a potential surplus or deficit of electrons. These outermost electrons, called the valence electrons, determine how the atom reacts chemically with other atoms. Atoms in which the valence shell is full tend to be chemically inert. Atoms with one or two valence electrons more than a closed shell are highly reactive because the extra electrons are easily removed to form positive ions (oxidation). Atoms with one or two valence electrons less than a closed shell are also highly reactive because of a tendency either to gain the missing electrons and form negative ions (reduction), or to share electrons and form covalent bonds. The lowest energy for a species is when its outer shell is fully occupied by electrons. The gaining or losing of electrons changes the energy level of the atom and it is this energy which is released as electrical energy during the discharge of a primary or secondary battery, or is absorbed when charging a secondary battery.

The energy available in an atom to do external work is called the Gibbs Free Energy and an indication of the magnitude of this potential energy realease is given by the electrode potential of the element. For a balanced reaction, this is expressed in the following equation:

 

ΔG = U0 n. F

Where

ΔG is The change in Gibbs Free Energy in Joules

U0 is the standard electrode potential in Volts (See table above)

n is the number of moles of electrons transferred in the cell reaction per mole of product

F is the Faraday constant in Coulombs per mole (the magnitude of electric charge per mole of electrons)

 

This equation is used to calculate the energy available from the redox reactions possible with various combinations of active chemicals.

The voltage or potential difference between an oxidation and reduction reactions arises from the different electrochemical potentials of the reduction and oxidation reactions in the battery. The electrochemical potential is a measure of the difference between the average energy of the outer most electrons of the molecule or element in its two valence states.

 

History

 

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

(Negative Terminals)

(Positive Terminals)

BEST - Most Negative

BEST Most Positive

Lithium Ferrate
Magnesium Iron Oxide
Aluminium 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 voltage 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.

 

See more about the choice of electrode materials in the page about New Battery Designs and Chemistries.

 

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 Lithium 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.


Lithium Ion Swing Cell Charge Discharge

 

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.

 

  • Solid Electrolyte Interface/Interphase (SEI)
    The SEI layer is essential for the stability of Lithium secondary cells using carbon anodes.
  •  

    The electrolyte reacts vigorously with the carbon anode during the initial formation charge and a thin passivating SEI layer builds up moderating the charge rate and  restricting current

    Lithium Ion Electrode SEI Layer

     

    The deposition of the SEI layer is an essential part of the formation process when the cells take their first charge.

    BUT the SEI layer increases the cell internal impedance and reduces the possible charge rates as well as the high and low temperature performance.

    Excessive heat can cause the protective SEI barrier layer to beak down allowing the anode reaction to restart releasing more heat leading to thermal runaway.

    The thickness of the SEI layer is not homogeneous and increases with age, increasing the cell internal impedance, reducing its capacity and hence its cycle life.

     

    Lithium Titanate Oxide (LTO) anodes do not react adversely with the commonly used electrolytes in Lithium Ion cells hence no SEI layer is formed nor is it needed in LTO cells. This allows new degrees of freedom in mofifying cell performance. See Lithium Cell Variants

     

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 for Typical Energy Cells

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 construction.


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 Chemical Elements

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


Note - From the chart above Lithium is between 20 and 100 times more abundant in terms of the number of atoms 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.

 

Lithium Consumption in EV and HEV Batteries

The Lithium content in a high capacity Lithium battery is actually quite small.

Taking a Lithium Cobalt cell as an example, the Lithium content in the LiCoO2 cathode material is only 7% by weight. The cathode material itself makes up between 25% and 33% of the battery weight so that the Lithium content of the electrode in the cell amounts to about 2% of the weight cell. In addition the electrolyte which accounts for about 10% of the battery weight also contains smaller amounts of dissolved Lithium so that the total Lithium content in a high energy battery is typically less than 3% by weight.

 

Lithium batteries used in EVs and HEVs weigh about 7 Kg per KWh and so that the Lithium content will be about 0.2 Kg per kWh. A typical EV passenger vehicle may use batteries with capacities between 30KWh and 50KWh so that the Lithium content will be about 6 Kg to 10 Kg per EV battery.

The capacity of HEV batteries is typically less than 10% of the capacity of an EV battery and the weight of Lithium used is correspondingly 10% less.

Thus 1 million EVs would consume less than 10,000 tons of Lithium (without recycling) and 1 million HEVs would consume no more than 1,000 tons

Considering the availabile supply of Lithium (see next section) there is more than enough Lithium available to satisfy the world demand for high energy automotive batteries.

 

Lithium Supplies

Lithium is the 31st most abundant element in the Earth’s crust with an abundance of 20 ppm. This compares with Lead (14 ppm), Tin (2.3 ppm), Cobalt (25 ppm) and Nickel (84 ppm). It is found in small amounts in nearly all igneous rocks and mineral springs with particularly large deposits in China, North America, Brazil, Chile, Argentina, Russia, Spain, and parts of Africa.

 

The current estimate of exploitable reserves (apart from recovery from seawater) is estimated as 28.4 million tons. In addition the earth’s 1.4 × 1021 kilograms of seawater contain a relatively high 0.17 ppm of Lithium which means that there are over 200 billion tons of Lithium in the world’s oceans.

The US Geological Survey reported the world production of Lithium in 2006 was 333,000 metric tons, slightly down on the previous year. China is expected to bring on production of 45,000 tons of Lithium per year from brine based facilities in 2010

 

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 therapeutic 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 therapeutic, but too much can be fatal.

See more about toxicity on the New Battery Designs and Chemistries page.


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.

 

New Battery Chemistries

See more about introducing new battery technology on the following page New Battery Designs and Chemistries

 

Battery Constuction

Information about the mechanical design of batteries can be found on the following pages:

 

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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