Publication date: 2018-09-18 19:27
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. The potentials are chosen with reference to Hydrogen whose potential was arbitrarily defined as zero which results in positive and negative values of electrode potential. In reality they follow a progressive series covering a range of about 6 volts.
Or perhaps you caught the NOVA episode, which offered a slightly different take on America's "search for the super battery" that will power our smartphones, our laptops, and even our cars in the future?
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 graph above shows that Lithium Iron Phosphate cathodes do not break down with the release of oxygen until much higher temperatures and when they do, much less energy is released. The reason is that the Oxygen molecules in the Phosphate material have a much stronger valence bond to the Phosphorus and this is more difficult to break. The other cathode chemistries are based on Lithium metal oxides which have much weaker valence bonds binding the Oxygen to the metal and these are more easily broken to release the Oxygen.
Lithium Cobalt Oxide was the first material used for the cathodes in Lithium secondary cells but safety concerns were raised for two reasons. The onset of chemical breakdown is at a relatively low temperature and when the cathode breaks down, prodigious amounts of energy are released. For that reason alternative cathode materials have been developed. The diagram below shows the breakdown characteristics of several alternative cathode materials.
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
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.
More recently new cell chemistries have been developed using alternative chemical reactions to the traditional redox scheme.