Share |

How cells work

Lithium batteries are an example of an Electrolytic Cell. An electrolytic cell is an electrochemical cell in which a non-spontaneous reaction is driven by an external source of current.

By comparison a Galvanic or Voltaic cell is an electrochemical cell that stores electricity as a result of spontaneous chemical reactions occurring inside and consists of two half cells joined by a salt bridge.

A lithium ion cell consists of two dissimilar electrodes, separated from each other by an electrolyte. An electrolyte is an ionic conductor and electronic insulator.  On discharge electrolytic cells convert chemical energy to electrical energy.  Battery packs can contain several, series or parallel strings of many individual cells.

The lithium battery is known as a rocking chair or swing battery due to charge carriers shuttling back and forth between two intercalating electrodes during the charge and discharge processes.

Cell Potential

In simple terms, batteries can be considered as electron pumps. The electrical (pump) pressure or potential difference between the positive and negative terminals is called voltage or electromotive force (EMF).  The free energy associated with the transfer of electrons around an external circuit and lithium ions between two intercalation electrodes is related to the difference in the chemical potential of lithium in the two electrodes, the higher the electrodes potential, the harder it is to remove a lithium from a site within host matrix.  On discharging a cell Li is transferred from a high energy state in the anode to a low energy configuration in the cathode.

Battery cells store energy chemically in its electro-active electrode materials.  This chemical energy can be converted into electrical energy on demand, typically by means of an electrochemical oxidation-reduction (redox) reaction.

Cell components

Lithium Battery cells consist of three main components :

The anode : on discharge gives up electrons to the external circuit and is oxidised during the electrochemical reaction. Most commercial cells currently employ a carbon/graphite based electrode; however metal or an alloy can also be used.
(Oxidation is loss of electrons - Oil)

The cathode : on discharge accepts electrons from the external circuit and is reduced during the electrochemical reaction. It is usually a transition metallic oxide or phosphate.
(Reduction is gain of electrons - Rig)

Redox = Oil-Rig.

The electrolyte (an ionic conductor but electronic insulator) separates the two electrodes and provides the medium for charge transfer inside the cell between the anode and cathode. The electrolyte is typically a non-aqueous inorganic solvent containing a dissolved Lithium salt, e.g. LiPF6 in propylene carbonate.

The Charge Process

Cells are generally constructed in the discharged state.  On charge the positive electrode, cathode, material is oxidized, Li+ ions are de-intercalated from the layered lithium intercalation host, e.g LiCoO2, pass across the electrolyte and are intercalated between the graphite layers in graphite by an electrochemical reduction reaction proceeding at the negative electrode.

The Discharge Process

When the cell is discharged, an oxidation reaction occurs at the negative electrode, Li+ ions are de-intercalated from the anode and migrate across the electrolyte to be re-intercalated into the cathode material, due to charge balance the equivalent number of electrons travel through the external circuit.  A simultaneous electrochemical reduction reaction proceeds at the positive electrode and accepts electrons from the external circuit, Li+ ions from the electrolyte, to reform the starting material. A change from electronic current to ionic current occurs at the electrode/electrolyte interface.

Operates by the principle of intercalation - the reversible insertion of a guest atom into a solid host structure without inducing a major disruption of the host material.

On Discharge :

Positive electrode (cathode) reduction reaction  : Li1-xCoO2 + xLi+ + xe- → LiCoO2

Negative Electrode (anode) oxidation reaction  : LiC6 → xLi+ + xC6 + e-

Overall reversible, Redox, cell reaction    : LiC6 + CoO2  C6 + LiCoO2