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Lithium - Secondary Batteries Electrochemical power sources, particularly batteries, have driven extensive research in the past few decades in an attempt to improvise systems of high energy density and long shelf life. Such concerns along with the advances in electrochemistry and materials science in the last thirty years contributed to the development of advanced lithium secondary batteries that are highly demanded in the electronic industry due to the need of miniaturized electronic appliances. Lithium secondary batteries consist of a substance capable of cyclic transfer of lithium ions between two electrodes. Lithium is used because it is the lightest metal; it provides the highest voltage and has the greatest energy density. In most cases, the battery consists of a lithium salt in a polymer host matrix. The negative electrode (anode) is a lithiated carbon and the positive electrode (cathode) is a partially lithiated compound. The cathode always consists of transition metal atoms like Co and Mn in a mixed valence state. Such an electrode combination is highly favorable as it allows rapid lithium movement accounting for the fact that the potential difference between both electrodes is close to the redox potential of lithium. As shown in the figure below, lithium ions migrate from the anode to the cathode during discharge and from the cathode to the anode while charging. The discharging transfer is very critical; importantly, the charge should be carried by the lithium ions of the salt and not the anions. If anions mobilize in the polymer matrix and carry charge, then they pair with the migrating lithium ions. This coupling causes salt to accumulate forming concentration gradients across the electrodes. Such an occurrence reduces the electrolyte's conductivity due to polarization losses leading to a low voltage upon discharge and a premature battery failure. Therefore, it is necessary to develop new materials that can overcome such a limitation; single ion conductors are the best choice.
Here at Clemson, novel perfluorinated alkyl lithium salts have been developed by Dr. DesMarteau's group. These salts are based on low molecular structure PEGs having attached sulfonic or sulfonyl imide pendants. These single ion conductors are good candidates for rechargeable lithium batteries since they contain the sulfonic/sulfonimide functionality that drives their acidity from the delocalization of charge over the O-S/O-S-N skeleton. This property is highly favorable for a single ion conductor because it allows minimal ion pairing. The idea of developing single ion conductors is to covalently bind the anionic moiety of the salt to a polyethylene glycol (PEG) backbone through a trifluorovinylether coupling.
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