Electrolytes are materials through which charged particles can move. Seawater is an example of an electrolyte, as it contains dissolved salts, such as sodium chloride. Sodium chloride is made up of positively and negatively charged particles. As the salt is dissolved in water, the charged particles can move and so seawater will conduct electricity.
Figure 2: Simplified diagram showing how seasalt dissolving in water forms an electrolyte, seawater.
An exciting area of research is investigating polymers as electrolytes as these are generally less flammable and more stable than the liquid electrolytes used in lithium-ion batteries.
Polymers are long chains formed by joining together smaller units called monomers – like a paper-clip chain, daisy chain or beaded chain, as below.
Figure 3: A beaded necklace provides a good analogy of a polymer. Each bead is a monomer, or unit, and the joined up beads represent a polymer, or many connected units.
These chains are tiny: much, much smaller than what we can see! Polymers are used to make a wide variety of materials that you will be familiar with: such as food packaging, glasses lenses, and clothing. As some polymers are even able allow Li+ to flow through them, they can act as electrolytes.
Researchers have found that:
- Li+ can attach to oxygen atoms in some polymer chains.
- When the polymer chain moves, the Li+ can hop between different oxygen atoms.
- This hopping motion means that Li+ can move through polymers: the polymer is an electrolyte (Figure 4).
Figure 4: Diagram of a polymer electrolyte, showing how Li+ hops between oxygen atoms to move through the polymer. Red circles represent oxygen atoms; blue circles represent Li+.
Scientists are working to make new polymers which can conduct Li+, so that better electrolytes can be developed for batteries.4
Polymers can also be used in the electrodes. The electrodes consist of three main parts (Figure 5):
- Active particles, the material that the lithium is stored in.
- Conductive additives, particles which transport the electrons to the external circuit.
- Polymer binder, which acts as a glue to hold the particles together and ensures good contact of the electrode with the electrolyte.
Figure 5: A close-up diagram of an electrode, showing the three key components: the active particles store lithium, the conductive additive transports electrons to the external circuit, and the polymer binder acts as a glue to hold the components together.
For batteries which store lots of energy, new polymer binders need to be developed. When batteries charge and discharge, the lithium moves in and out of the electrodes: in high-energy batteries, this causes the electrode to swell and contract. With current binder technology, cracks form in-between the particles and the battery degrades.
Figure 6: A close-up of an electrode surface showing pitting and cracking.
Scientists are investigating new polymer binders which are elastic and sticky: this should allow close contact between the electrode parts which will improve battery lifetime.5
Researchers also want to understand how the behaviour of an electrolyte is altered when it is concentrated in these cracks (which are as small as the width of a human hair!) and so are investigating what happens when electrolytes are concentrated and 'squashed'.
Figure 7: A simplified diagram of a Surface Force Balance, showing how the lenses act to force the electrolyte particles closer together.