The basic principles of a lithium-ion battery are as follows –
When the battery is fully charged the negative electrode is full of lithium ions.
As soon as it is connected to an appliance these lithium ions begin to move through the electrolyte to the positive electrode and this chemical reaction creates the electrical charge at the terminals.
When there are no more lithium ions left on the negative plate the battery is flat and recharging is needed to reverse the process.
How a lithium-ion battery works
but in reality the process is not quite perfect. From the very first discharge and recharge cycle the capacity of the battery degrades.
There are four issues at play:
Solid Electrolyte Interface build up on the negative electrode
Electrolyte oxidation on the positive electrode
The Solid Electrolyte Interface and electrolyte oxidation
While a lithium-ion battery is being charged lithium oxide and lithium carbonate atoms form a film on the negative electrode known as a Solid Electrolyte Interface (SEI).
A similar film known as electrolyte oxidation occurs on the positive electrode if the battery is operated at a full or near full state of charge in temperatures of 104°F (40°C ) or above. Electrolyte oxidation is nothing new, the process is actively used to treat metals so that they do not rust. This is very popular in the yachting industry, but not welcome in a battery.
This heat may sound rare, but operating temperatures inside devices such as laptops can frequently go far above this limit. Its also common in some parts of the world as Nissan found out when their electric car owners living in hot areas such as Arizona sued because of poor battery capacity after relatively few discharge and recharge cycles.
Electrolyte oxidation also increases self-discharge rates which can explain why some batteries seem to need regular charging even when not in use.
These films are initially so thin that most lithium ions can pass through, but after progressive charging and discharging cycles, they gradually thicken. Each time the film thickens it reduces the number of lithium ions that can move between the plates, affecting the ability of the battery to fully charge as well as increasing the time it takes to recharge.
Various manufacturers have been able to reduce the build up of one or either films with the addition of certain chemicals to the electrolyte, but no one has managed to stop the process altogether.
When lithium-ion batteries are charged rapidly the rate at which ions are leaving the positive electrode and heading towards the negative electrode exceeds the rate at which the negative electrode can absorb the ions. In such cases the ions then become a metallic deposit on the electrode.
The positive electrode is especially poor at absorbing ions at low temperatures, so mix rapid charging and a cold environment and you get a recipe for rapid lithium plating.
Less ions around means less battery capacity. In more extreme circumstances the build up can cause the battery to short out and fail completely.
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