An introduction to Evonik products for batteries

Blog Archive | 8 minutes  | Author: Erin White , BSc.

Rechargeable batteries are something most of us couldn’t live without these days. The ability to store energy has allowed technological breakthroughs that would have otherwise been impossible and as we look to science and technology to solve one of our species most pressing crises, batteries could again form part of the answer.

The global climate crisis has demonstrated the need for clean, renewable energy but many of our current solutions rely on the weather which, as the British public knows all too well, is highly inconsistent. An energy solution that can only provide energy when it's sunny or windy will not be able to replace coal, so storing energy is important to provide consistent energy come rain or shine.

Another tool in the climate saving belt is electric vehicles and hybrid electric vehicles as they provide a lower climate impact than petrol and diesel-burning vehicles. However, again, to be able to compete with their less sustainable counterparts, batteries that can be quickly charged and last for a decent number of miles are required.

 

In this technical article:

 

What are the different types of batteries?

The emphasis here is on rechargeable batteries. For electric vehicles, these are very important, but there are many different types of batteries that can be categorised based on how the energy is stored. Batteries can be either physical or chemical and within the chemical battery category, there are different types depending on the chemical reaction utilised. Evonik offers several additives for batteries but to understand these, we need to understand the different components of a battery.

 

A flow chart showing the different types of batteries

Figure 1: There are many different types of batteries that can be categorised by the method used to store and transform the energy for use.

 

Chemical Batteries

The different types above all work in different ways, but many of the chemical rechargeable batteries have similar elements.

The energy is stored as chemical energy and converted to electrical energy for use by the device and is all controlled by the flowing of charged particles, which is how an electric current is defined. Charged particles can be either positively charged, or negatively charged, like electrons. Chemistry is often described as the study of electrons because understanding electrons goes a long way to understanding the way elements and compounds interact. In batteries, electrons as negatively charged particles, are also key.

To get charged particles to flow, they must be attracted from their source to a different destination - this is the job of the electrodes. Figure 2 shows a schematic of a battery. A positively charged electrode, a cathode, will attract electrons while the negatively charged electrode, the anode, is the source of the electrons. This occurs because the anode has an excess of electrons and undergoes oxidation to lose these, while the cathode has a deficiency of electrons and undergoes reduction to gain them. This branch of chemistry is therefore known as redox chemistry.

To harness the flow of electrons and prevent the battery from short-circuiting, the electrodes must be separated. However, as most will remember from school science, a circuit must be closed for a current to flow. These may appear to contradict but combining specific materials to create a system that exhibits strong ionic conductivity, but no electric conductivity can achieve both. In modern batteries, this is often done with a polyethene (PE) and silica separator and an electrolyte.

The electrolyte provides ionic conductivity and can help the movement of the positive ions.

 

Rechargeable batteries

Once all the negative ions have moved from the anode to the cathode the battery is considered empty or ‘dead’. However, this is not ideal for many modern applications so for rechargeable batteries the chemical reaction needs to be able to be reversed.

Whether the reaction can be reversed depends on what the battery is made from, but where it is possible, the charger will reverse the flow of negative ions to restore the excess at the anode. This can also be seen in Figure 2. This reaction however cannot be reversed an unlimited amount of times and so the battery will eventually need replacing which is known as the lifetime of the battery.

A schemtatic of a battery during discharge and charge.

Figure 2: The movement of the charged particles can be seen in this schematic of a battery where the flow of charged particles, ie the current, can be seen in the wires above the electrolyte.

 

Lead-acid batteries

Lead-acid batteries are widely used around the world, particularly as the battery in non-electric cars. For these, both electrodes are based on Lead (Pb) and the electrolyte is sulfuric acid, hence the name. During discharge, the lead anode loses electrons and undergoes oxidation via the following reaction.

Pb → Pb2+ + 2e-

While the lead dioxide cathode undergoes the following reduction reaction while it gains electrons.

PbO2 + 4H+ + 2e- → Pb2+ + 2H2O

Sulfate ions from the electrolyte get involved to produce lead sulfate as a precipitate by the following reaction on both electrodes.

Pb2+ + SO42- → PbSO4

This gives the following overall redox reaction for the cell during discharge. 

Pb + PbO2 + 4H+ + 2SO42- → 2PbSO4 + 2H2O

This is a reversible reaction as during recharge, the opposite reaction occurs.

 

Lithium-ion batteries

Lithium-ion batteries are another crucial battery for modern life. Commonly the battery found in portable devices such as modern smartphones, these batteries must be powerful and lightweight. The anode consists of an intercalated Lithium Graphite compound while the cathode consists of a lithium cobalt substrate and are connected by a Lithium electrolyte. The 

During discharge, the anode undergoes oxidation by the following half-reaction.

LiC6 → C6 + Li+ + e-

Simultaneously, the cathode undergoes reduction by the following half-reaction.

CoCO2 + Li+ + e- → LiCoO2

This gives an overall reaction that sees the lithium migrating from the anode to the cathode along with the electrons. 

LiC6 + CoO2 → C6 + LiCoO2

This again is reversible as the opposite occurs to recharge the cell. 

Lithium-ion polymer batteries

While similar to Lithium-ion, lithium-ion polymer batteries use a polymer electrolyte instead of the liquid electrolyte. The solid polymer electrolyte (SPE) can be either dry, gelled or porous with porous being the most modern form. This uses the same reactions as above in the lithium-ion cell. One big advantage to this technology is that they can easily be produced in any desired shape and so as devices get thinner and lighter. they are also becoming the chosen option for electric vehicles.

 

Evonik products for battery technology

Evonik offers several products for use in batteries to improve function and performance which are employed in the electrodes, separator and electrolyte.

Electrodes

For electrodes, Evonik offers carbon blacks such as PRINTEX® and HIBLACK® which improve conductivity and can limit crystal growth. They also provide colour which can be useful for differentiation between the two electrodes.

 

Separator and electrolyte

Traditionally this was liquid sulfuric acid, but with the requirements of modern batteries, spillage and leakage of sulfuric acid from the battery when not maintained upright meant this was no longer suitable. Fumed silica, like AEROSIL® fumed silica, can be added to create a sulfuric acid gel which gives it numerous preferable properties, but most notably for consumers it allows the battery to be tilted and used in other positions than upright.

The separator then is non-electrically conducting and gives a physical barrier between the electrodes. This physical barrier, however, needs to allow the ions through and so for lead-acid batteries, silica is added to the separator to give high porosity that enables the ions to flow freely and for Li-ion batteries, microporous polymer membranes among other polymer and composite solutions are used. The silica in lead-acid batteries needs to be tailor-made for which Evonik provide a few solutions, SIPERNAT 325 C, SIPERNAT 325 AP and SIPERNAT BG-2, that fulfil the needs. While for Li-ion batteries inorganic filer solutions such as AEROSIL and ceramic layers with AEROXIDE can be used for improved porosity and ion conductivity.

 

A grey electric car (a tesla) charging

Figure 3: Charging ports for electric cars are becoming more and more common. Greater access to charging ports is another improvement needed for wide spread use of electric vehciles.

 

Evonik additives can help the battery's performance in converting the energy from chemical to electrical energy by improving ionic conductivity of the electrodes and reducing the electrical resistance of the separator.; They can improve charging times by aiding the porosity of the separator; they can improve consumer usability by creating a gel electrolyte for on-the-go use. As new and exciting science is discovered and utilised, batteries will continue to improve in performance, charging times and lifetime making them more and more competitive against the less sustainable options.

If you wish to discuss anything raised in this article in further detail or your specific requirements, please get in touch with us and one of the technical team will be happy to help you.

Author: Erin White , BSc.

Erin studied at the University of York where she earned a BSc in Chemistry and has just completed a MSc in Atmospheric Chemistry. She has recently joined us and will be covering areas across all markets.