The quantity of lithium ion batteries has been predicted to grow considerably in the future, and therefore, guaranteeing their sustainable production is particularly important. The environmentally friendliness of the lithium ion batteries has been tried to enhance with several different methods. The substitution of harmful solvents used in the electrolytes has been tried, and if it hasn’t worked, the amount of solvents has been reduced as much as possible. Replacing the environmentally harmful chemicals in the electrode materials with more sustainable ones, such as LiFePO4, is also one approach. In addition, the recycling of the battery metals has been investigated, for example in the CloseLoop project.
The environmentally friendliness of lithium ion batteries can be enhanced also by lengthening their lifetime. The longer the battery works, the less often it needs to be replaced, and the less natural resources are consumed. At the moment, a typical mobile phone battery has a lifetime of 500-1000 cycles. In other words, the battery can be discharged and charged fully that many times. After this, the battery capacity has dropped to under 80 % of the initial capacity. The battery can be used also after the capacity drops below 80%, but it has to be charged more and more frequently after this.
One of the most common causes for lithium ion battery aging is the reaction of the electrolyte in the electrodes. This is an especially big problem in the carbonaceous electrodes, in which the electrolyte reacts forming a layer on top of the electrode. This layer is called SEI-layer (solid electrolyte interface) and it consists mostly of lithium based compounds. After a few first charge-discharge cycles, the growth of the layer is stabilized but it never stops. As the cycling continues for a long time, the SEI-layer both increases the cell impedance and consumes lithium. Very similar, although usually thinner, layers are formed also on the other electrode materials. Usually these are modified with different additives.
Many other factors affect the aging of lithium ion batteries as well. The storage and usage temperatures affect it clearly. For example, even the temperature slightly above 30 °C accelerates the aging compared to the room temperature. The charge and discharge currents determined by the manufacturer also affect the lifetime. One of the best methods to slow down the aging is to lower the charge and to raise the discharge cut-off voltages. In this manner, the battery is never charged full or discharged empty, and therefore the capacity of the battery decreases but the lifetime increases.
The above mentioned methods are clearly related to the use of the battery. The long-lasting materials are also investigated in the materials research. Typically, investigated topics include the aging mechanisms, and how to slow down or prevent the mechanisms from occurring. Typical means include for example electrolyte additives, doping of electrode materials and controlling electrode morphology. Although many solutions are found, the same ones rarely function for several different materials. Therefore, new options are being searched continuously.
Figure 1. Coated electrodes for material testing.