Lithium ion Batteries
Basic concept
Economies of scale means that the cost of Lithium ion batteries is falling quickly. Round-trip efficiency is initially 90%. The cost per kWh was cited as below $100 for first time in 2020. Average $137 per kWh.
Standard Lithium ion batteries contain solid electrode materials in contact with liquid electrolytes and separated by a membrane that allows the Li+ ions to pass through it, but which keeps the anode and cathode separate.
In Tesla car batteries these sandwiches of electrodes and separator membranes are all very thin layers, rolled up like a swiss-roll, to give huge anode and cathode areas in a small volume. The large area allows for faster charge and discharge and therefore higher power. However, because of this they may need active cooling to stop them from overheating.
There are several different types of lithium ion batteries, using different materials for the cathode – usually lithium and a transition-metal oxide, but all having basically the same way of working. Most of the designs use cobalt in their cathode material, which is a fairly scarce global resource.
One of the big problems with the current range of lithium ion batteries is that the electrolyte used is volatile and combustible, so that if the battery gets too hot, or any instability in the metal oxide leads to the loss of oxygen, the battery can spontaneously catch fire or even explode. The electrolyte also stops being very conductive when it drops below 0oC, so the batteries need active monitoring and temperature control to keep them working safely.
More technical details about how current Lithium ion batteries work and the different types.
Other designs of Lithium ion battery
Two possible alternatives to the standard lithium ion battery have been designed. Both claim higher energy density, faster charging and to have removed the problem of thermal runaway.
A sulphur-based lithium ion battery with an inorganic lithium ion electrolyte has been produced by a company called Innolith
This appears to be another version of a lithium ion battery, using the same materials for anode and cathode, but Innolith claims that because it uses a non-flammable inorganic electrolyte it does not suffer from the thermal run-away problems which plague conventional lithium ion battery designs.
Because they use the same anode and cathode materials as the more conventional lithium batteries, they will still be dependent on the global supply of cobalt for making the cathode.
They claim a higher energy density and faster charging ability compared with standard lithium ion batteries, but there is nothing quantitative on the company’s website.
All Solid State Batteries
Research has been ongoing into solid-state lithium ion batteries. In a solid state lithium ion battery there would be a “solid electrolyte” – some form of material which would allow lithium ions to pass through it between the electrodes, replacing the electrolyte, membrane combination in conventional Li+ ion batteries. In this sort of set-up instead of having to have the lithium diluted by graphite at the anode, you can use solid lithium metal, so the energy density of the battery can be far higher. With a conventional organic liquid electrolyte, you cannot use solid lithium as the cathode because tiny thread-like lithium “dendrytes” can form on the surface of the lithium which can pierce the membrane in the cell and lead to shorting and thermal runaway.
Various chemicals have been found that would work as a solid electrolyte, but there is then a problem with the conductivity at the interfaces between the anode and electrolyte, and the between the electrolyte and the cathode. During charge and discharge, the anode and cathode will change shape very slightly at the atomic level, as lithium ions move in and out of the anode and cathode structures, so even if there is a complete solid to solid contact when manufactured, the solid interfaces can become less conductive over time. The probable solution will be to put some form of conducting gel – something conductive but flexible/expandable, on the solid interfaces.
Toyota claim to have solved these technical issues, and are releasing an Electric Vehicle with an “All Solid State Battery” some time in 2021.
Recycling of Lithium ion batteries
Lithium ion batteries, used in electric cars, can be recycled for use in storage of energy for domestic PV installations.
Inside a lithium ion battery, each charge and discharge cycle slightly degrades the ion storage, so lithium batteries deteriorate over time. There is beginning to be a market in re-using “spent” lithium car batteries, where the energy density is really important, for domestic storage of PV systems where the energy density is far less important. If your PV storage battery, sitting in your garage weighs 100kg rather than 50kg you simply don’t care.
The charge rate for an worn-out lithium battery from a car is about right to store the energy produced from a domestic PV system.
Given the potential over-heating/thermal runaway problems with lithium ion batteries, and particularly with old lithium ion batteries, it is not sensible to use old electric vehicle lithium ion batteries in remote hot countries. However, for storage of relatively small amounts of spare PV in a cooler environment it might be a plausible solution.
References / external websites:
Innolith – sulphur based Lithium ion batteries
https://innolith.com/technology/
News article about Toyota’s Solid state battery