The basis for a battery operation is the exchange of electrons between two chemical reactions, an oxidation reaction and a reduction reaction. The key aspect of a battery which differentiates it from other oxidation/reduction reactions (such as rusting processes, etc) is that the oxidation and reduction reaction are physically separated.
The reaction below is a reduction reaction in which a copper ion with a valance state of 2+ is reduced to copper metal, with a valence state of zero. Since a reduction reaction requires electrons, it will always have electrons as one of the reactants. The reduction reaction occurs at the cathode.
Even though many types of batteries exist with different combinations of materials, all of them use the same principle of the oxidation-reduction reaction. In an electrochemical cell, spontaneous redox reactions take place in two electrodes separated by an electrolyte, which is an ionic conductive and electrically insulated substance.
The electrons transferred in a redox reaction arise from the change of the valence state of materials in the redox reaction. If a material gives up or loses an electron, then its valance state becomes more positive (since an electron has a negative charge) and the reaction is called an oxidation reaction.
However, battery materials, especially with high capacity undergo side reactions and changes that result in capacity decay and safety issues. A deep understanding of the reactions that cause changes in the battery's internal components and the mechanisms of those reactions is needed to build safer and better batteries.
However, the sluggish sulfur reduction reaction (SRR) kinetics results in poor sulfur utilization, which seriously hampers the electrochemical performance of Li–S batteries. It is critical to reveal the underlying reaction mechanisms and accelerate the SRR kinetics. Herein, the critical issues of SRR in Li–S batteries are reviewed.