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The Dendrite formation in Lithium batteries

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Theo Mok

on 12 August 2015

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Transcript of The Dendrite formation in Lithium batteries

At the lithium electrode, oxidation occurs, with the effect of turning to solid lithium from the anode into lithium ions, which then go into solution.
When the battery is recharged, the electron flow is reversed, and electrons can flow back into the lithium electrode, these electrons attract the positively charged lithium ions. once the lithium ions touch the lithium electrode, they relieve electrons can can turn back into lithium solid, which gathers on the outside of the lithium electrode.
The growth of the dendrites
The growth of the dendrites
At the Lithium Electrode
Lowering Battery life and number of recharge cycles
As described before, when a dendrite falls off the electrode, it can no longer take part in oxidation or reduction as it cannot gain or lose electrons to the anode or cathode. This makes the battery life of the battery lower as it cannot turn these solid dendrite pieces back into ions when recharging and thus there will be less reactants available to produce a voltage, and the battery will be used faster
In a lithium cell, there may be a charged semi-permeable membrane, which serves the same function as a salt bridge in a classical electrochemical cell, as in it helps balance out the charges cased by the loss or gain in electrons. It does this by only allowing the opposite charge through to the other half cell, for example if a half cell becomes negatively charged, the semi-permeable membrane will also be negatively charged, in order to attract positively charged ions through the membrane to balance out the charges, while repelling negative charge from inside the half cell.
Causing a short circuit
Eventually, a dendrite may get so long that it can pierce the membrane between the half cells and touch the electrode on the other side. This occurs if the dendrite growth goes on uncontrolled and does not fall off int he process. If it does manage to touch the other electrode, this completes the circuit and electrons can flow relatively unrestricted.

This can cause a large amount of heat being generated and eventually a short circuit, as the current holding wire may become so hot that it melts away. This can be very dangerous for the battery and surroundings, and may cause the casing of the battery to break or for it to explode violently
Using different types of electrolytes to hinder growth
In a study performed majorly by scientists from Stanford university in California, it was found that using electrolytes containing various poly-sulfide and lithium nitrate could help reduce dendrite growth by helping in the formation of a stable SEI layer, this was shown to improve the life cycle of the battery.

There are other ways that dendrite formation such as coating the electrode in other conductive materials, or by adding additives to the electrolyte, all of these things working together will help to make a safer, more eficient and longer lasting battery.
By maintaining a uniform layer of solid between the electrode and the electrolyte
By maintaining a uniform layer of solid lithium, ie. not having lithium dendrites but rather a flat surface, the negative effects of lithium dendrite grown can be reduced, or at least delayed for as long as possible. This layer is known as the solid-electrolyte interphase or SEI as it is formed between the solid and the electrolyte
The formation and growth of the dendrites
The dangers of dendrite growth
How dendrite formation can be hindered
The Dendrite formation in Lithium batteries
The problem with the lithium formation at the lithium electrode is that it does not solidify uniformly across the electrode. Instead, it can form long, tendril like lengths of lithium metal called dendrites. As these dendrites have a large surface area, new lithium ions turning into solid from solution may receive electrons from them, attaching themselves to the dendrite as they turn solid, lengthening the dendrite with each successive battery cycle.
When the lithium electrode is undergoing oxidation, it may not be the tip of the dendrite that turns into ionic form, the base of the dendrite may turn into ionic form, leaving the rest of the dendrite to break off and fall into the solution. Here it is unable to turn back into ionic form as it is no longer connected to he electrode and cannot lose electrons. This leaves leftover lithium stuck in solid form in the electrolyte, shortening the life of the battery
The electrochemical cell
The electrochemical cell is composed of two half cells, one containing the anode and the other the cathode. it cell produces an potential difference through the gain and loss of electrons that occurs when substances undergo oxidation and reduction,
In the lithium sulfur battery the lithium electrode undergoes oxidation an the electrons flow to the cathode. Also there is no salt bridge, but rather a semipermeable membrane, and oppositely charged ions can diffuse through it to balance out the charges. this is NOT shown in the diagram below

Li(s) --> Li2+(aq) +2e-
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