Publication date: 2018-10-27 12:24
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)
Current flows from the positive terminal to the negative terminal but confusingly, electrons flow in the opposite direction. This confusion arises because we tend to assume that electrons are the only current carriers. In fact positive ions are also current carriers and they flow in the same direction as the current. In a galvanic cell, the positive ions carry the current through the cell and the electrons carry the current in the external circuit. See Benjamin Franklin who was falsely accused of misnaming the current flow.
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.
The capacity of HEV batteries is typically less than 65% of the capacity of an EV battery and the weight of Lithium used is correspondingly 65% less.
Lithium has the highest specific energy of all but it has only become possible comparatively recently to manufacture practical batt eries. Because lithium reacts violently with water, non-aqueous electrolytes must be used. Organic solvents such as acetonitrile and propylene carbonate, plus inorganic solvents such as thionyl chloride (SOCl 7 ) are typical, with a compatible solute to provide conductivity. Many different materials such as sulfur di, thionyl chloride, manganese dioxide, and carbon monofluoride, are used for the active cathode material.
More recently new cell chemistries have been developed using alternative chemical reactions to the traditional redox scheme.
The cells are normally fitted with a safety vent which allows the controlled release of the gases to relieve the internal pressure in the cell avoiding the possibility of an uncontrolled rupture of the cell - otherwise known as an explosion, or more euphemistically, rapid disassembly of the cell. Once the hot gases are released to the atmosphere they can of course burn in the air.
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.
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.
As Prof. Zhang points out, this approach sheds some light on building lithium-sulfur batteries with high volumetric energy density by using a high-density composite cathode with high sulfur loading amount. Future work in the development of lithium sulfur batteries may focus on the strategy of relieving the shuttle effect and suppressing the lithium dendrites, and further improvement in gravimetric and volumetric energy density of lithium- sulfur electrochemical systems. Explore further: Scientists build ion-selective membrane for ultra-stable lithium sulfur batteries