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The Chemistry of Metal-Ion Batteries Liam Critchley

(Source: AS - stock.adobe.com)

Many different types of batteries are in use today, and one which is the most notable is the lithium-ion (Li-ion) battery. The Li-ion battery is ubiquitous across technology, and its discovery has been central to the realization of many other technologies such as handheld devices, laptops, etc. Li-ion batteries’ significance in the technology world was further recognized when the scientists who developed the Li-ion battery won the 2019 Nobel Prize for Chemistry. While the importance of Li-ion batteries in modern-day technologies can’t be understated, they are, in fact, just one of the many batteries that fall within the metal-ion battery class, but their use has been more widespread because they are more efficient and safer. Behind all these battery innovations is quite a bit of chemistry―materials chemistry and electrochemistry―that enables these batteries to function.

The Different Basic Cell Chemistries

All metal-ion batteries are rechargeable batteries. When talking about the chemistry of metal-ion batteries, there is a lot of material chemistry involved in creating the batteries. Each metal-ion battery consists of two electrodes (anode and cathode), an electrolyte, a separator membrane, and an external electronic circuit. When discussing metal-ion batteries, four primary examples come to mind. The most obvious example is the Li-ion battery, but others include the aluminum-ion (Al-ion), sodium-ion (Na-ion), and the lithium-ion polymer battery (LiPo). The main areas where these four batteries differentiate are in the composition of the electrodes, electrolyte, and the ions that are the active charge carriers within the battery.

Li-ion batteries often have a lithium-based cathode, which can be made of a lithium oxide, lithium-layered oxide, or a polyanion material. The anode is carbon-based, but the materials here can vary quite a bit. The longstanding option is graphite, but recent developments have since seen the use of graphene and graphene-graphite hybrid electrodes, among various other carbon composites, as anode materials. The electrolyte is a liquid and a lithium salt that is effective at transporting lithium ions. The electrolyte is a complex mixture of various organic and non-aqueous constituents, such as ethylene (or diethylene) carbonate with LiPF6, LiAsF6, LiClO4, LiBF4, or LiCF3SO3 anion salts. Lithium-ion batteries are the most effective because the lithium ions can enter the electrodes, undergo electrochemical reactions effectively, and exit the electrode easily. This makes the charge and discharge cycles of the battery more efficient.

The composition of Na-ion batteries is not far removed from Li-ion batteries as both active ions have a single positive charge. In Na-ion batteries, sodium ions are the charge carriers, the cathodes are composite materials made of sodium transition metal oxides, and the anodes are typically made of amorphous carbon. The electrolytes can either be aqueous or non-aqueous in nature, but the sodium equivalents of the non-aqueous anion salt electrolytes found in Li-ion batteries are the most widely used, with NaPF6 being the common choice.

Al-ion batteries are not as well-established but show a great deal of promise. The main reason they are undergoing a significant amount of research is that aluminum carries three charges to lithium’s one, so this has the potential for much larger energies to be stored. But the large effective charge of the aluminum ions makes it harder for them to exit the electrodes once they have undergone electrochemical reactions and why we haven’t seen them used commercially yet. Aluminum-based materials and graphite tend to make up the anode and cathode respectively, with the electrolyte being a liquid made up of aluminum chloride species, but all these basic areas are currently under development and could change as new research yields more effective materials/chemical species.

The battery that is different from the others is the LiPo battery, which is a variation of the conventional Li-ion battery. Both electrodes are made of the same materials as Li-ion batteries, but the electrolyte is the main difference. Rather than being made of a non-aqueous liquid, the electrolyte is made from a polymer, so it has a gel-like (semi-solid) consistency but is still fluidic enough to transport lithium ions. Most of the electrolytes used in LiPo batteries are composed of solid polymers, such as poly(ethylene oxide) (PEO), poly(acrylonitrile) (PAN), poly(methyl methacrylate) (PMMA), or poly(vinylidene fluoride) (PVdF), which have been dispersed in an organic solvent to give it the required gel-like consistency..

How Metal-ion Batteries Work

Above we discussed the materials used to construct the electrodes and the materials used as the electrolyte. The basic working mechanism is the same for most metal-ion batteries, so it's easier to use one example rather than discussing the working principles of all the batteries. Li-ion is the best example; however, for the Al-ion and Na-ion batteries, the principles are the same if you substitute the Li ions for Al and Na ions respectively. It should be noted that the specific electrochemical reactions that occur at the anode and cathode are different for all the batteries. Hence there are far too many electrochemical reactions to mention the different reactions in detail.

There are two main mechanisms of a rechargeable battery: charge and discharge. The charge mechanism is the process by which the battery stores energy, and the discharge is when it releases it —e.g., when a device is on. When the battery is being charged, electrons from the power source charging the battery combine with the lithium ions in the cathode. This causes the ions to move through the electrolyte and the separator to the anode, where they enter via the molecular holes in the anode material—a process known as intercalation. Energy is then stored in the form of bound electrons in the lithium ions within the anode. When the device holding the battery is switched on, the anode undergoes an oxidation reaction that causes the lithium ions to exit the anode and move to (and intercalate in) the cathode. The stored electrons are then released to generate an electric current that powers the device. If a battery is not being used or charged, then the lithium ions desorb into the electrolyte medium between the electrodes.

Conclusion

Most metal-ion batteries work in a similar way, with the main differentiator being the chemistry that makes up the electrodes and the electrolyte, which in turn, affects the electrochemical reactions that occur within the battery. These electrochemical reactions can also differ for the same batteries using different electrolytes. Out of all the different metal-ion batteries, Li-ion batteries are the most effective because they can easily move between the electrodes and perform the necessary electrochemical reactions without getting stuck in the molecular holes of the electrodes. Aluminum batteries show a great deal of promise, but issues with the charge-heavy ions leaving the electrodes will have to be addressed first before they can be used commercially.



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Liam Critchley is a writer, journalist and communicator who specializes in chemistry and nanotechnology and how fundamental principles at the molecular level can be applied to many different application areas. Liam is perhaps best known for his informative approach and explaining complex scientific topics to both scientists and non-scientists. Liam has over 350 articles published across various scientific areas and industries that crossover with both chemistry and nanotechnology.

Liam is Senior Science Communications Officer at the Nanotechnology Industries Association (NIA) in Europe and has spent the past few years writing for companies, associations and media websites around the globe. Before becoming a writer, Liam completed master’s degrees in chemistry with nanotechnology and chemical engineering.

Aside from writing, Liam is also an advisory board member for the National Graphene Association (NGA) in the U.S., the global organization Nanotechnology World Network (NWN), and a Board of Trustees member for GlamSci–A UK-based science Charity. Liam is also a member of the British Society for Nanomedicine (BSNM) and the International Association of Advanced Materials (IAAM), as well as a peer-reviewer for multiple academic journals.


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