Powerful and Lightweight: Materials for Batteries

What are composite materials? How can the properties of fabric or metal be significantly improved? How are new materials created? Most modern gadgets rely on lithium-ion batteries. The materials used in these batteries determine how lightweight, efficient, durable, and reliable they will be.

A lithium-ion battery typically consists of a cathode made from an oxide or salt (like phosphate) containing lithium ions, an electrolyte (a solution containing soluble lithium salts), and a negative electrode (often graphite). The choice of electrode materials impacts the battery’s capacity and other characteristics. Thanks to advancements in materials science, batteries are becoming more energy-dense, reliable, and affordable.

New Cathodes

A notable example from the history of lithium-ion battery development is LiFePO4 or lithium iron phosphate. This material was first proposed in 1997 by John Goodenough as a cathode for lithium-ion batteries. This suggestion was initially met with scepticism, if not humour, because LiFePO4 is a dielectric with a conductivity of just 10^-9 Siemens per centimetre and a very low lithium diffusion coefficient—both weak points for a cathode material. All batteries then operated on oxide cathodes, which had much higher conductivity than LiFePO4. In the 1990s, battery manufacturers rejected lithium iron phosphate, which hadn’t been considered a cathode material for several years.

Eight years after LiFePO4 was first invented, another group of scientists came up with a solution to increase its electrical conductivity: creating a composite by reducing particle size and evenly coating them with a carbon layer just a few nanometers thick—LiFePO4/C. This was a groundbreaking approach: carbon, of course, is not a lithium-ion conductor, and the fact that a thin layer of amorphous carbon would not impede lithium intercalation was far from obvious. At that time, there were concerns about creating nanoparticles, as oxides like LiCoO2 react violently with nanoparticle-forming electrolytes, potentially leading to undesirable consequences. However, it turned out that LiFePO4 was utterly stable in the electrolyte. No matter how small the particles got, they remained stable with no side reactions. They did not release gases during the first battery charge, a common issue with oxide materials.

Reducing particle size made the low lithium diffusion coefficient less of a concern. Creating a composite by coating with carbon significantly improved the material’s conductive properties, with the conductivity of such a composite reaching around 10^-1 Siemens per centimetre.

This method has since been used to create various materials for lithium-ion, sodium-ion, and potassium-ion batteries and other power sources. It’s a convenient process: there’s no need to create a composite and coat it with something separately—just add inexpensive organic compounds during synthesis and calcine in an inert atmosphere. The carbon formed during this process prevents particle growth, encapsulating them. The result is a core of active material within a carbon matrix.

Today, LiFePO4-based batteries are high-power, safe devices with excellent cycling performance, as they are much more stable than oxide-based batteries. These batteries are widely used in electric vehicles—such as scooters, boats, cars, warehouse equipment, and autonomous power supply systems. 

The Ideal Battery Material

A good battery material should have a low molar mass. There is a relationship between the number of moles of a substance and the amount of charge it can store, and according to Faraday’s law, the more moles of a substance, the more electrons it can store. Therefore, the lower the molar mass, the better. In this regard, choosing between lithium fluoride and bromide would be much more advantageous because it has a much lower molar mass. The molar mass of LiFePO4 is moderate, with a theoretical capacity of 170 milliampere-hours per gram. This is a reasonable figure—not too high, not too low.

A good cathode material should also have high lithium-ion conductivity, diffusion coefficient, and electrical conductivity. Even if the material has poor conductivity, it can be addressed by reducing particle size and creating a carbon composite.

It is also crucial to consider how the material will behave in relation to the electrolyte: it must be a very poor catalyst for electrolyte decomposition. If the electrolyte decomposes on the surface of the material’s particles, this will increase the system’s resistance, and the electrolyte will eventually be depleted. For example, LiCoPO4 is relatively stable and has properties similar to LiFePO4, but the cobalt in this phosphate is active in oxidizing the electrolyte. While iron is a very poor oxidation catalyst, cobalt is too good a catalyst, making LiFePO4 much easier to work with. In contrast, a lithium-ion cell with LiCoPO4 degrades after a few cycles.

The potential of the material determines the battery’s energy density. It is useless if the cathode material has a potential of less than 3 V. If the potential is above 4.5 V, it is approached cautiously because high potential alone can initiate electrolyte decomposition. This issue can be addressed through targeted morphology design. For example, lithium-nickel-manganese spinel LiNi0.5Mn1.5O4 has a working potential of about 5 V—this high-voltage material is not yet widely used but holds great promise. It has been found that depending on the shape of the particles—whether cube, octahedron, or some more complex form—different degradation parameters emerge, meaning different levels of interaction with the electrolyte on various crystallographic faces of the material. This combination of crystallochemical and synthetic approaches can enhance the material’s properties and its stability concerning the electrolyte.

How New Materials Are Developed

Developers typically think by analogy. For instance, the material that served as the prototype for LiFePO4 is the mineral olivine, a silicate, not a phosphate: MgMnSiO4.

At first glance, what is the connection between LiFePO4 and MgMnSiO4? With extensive experience in crystal chemistry, we understand that magnesium and lithium cations are very similar—one is divalent, and the other is monovalent—so we can replace the magnesium cation with a lithium cation. The SiO4 and PO4 groups have tetrahedral shapes and can be easily substituted for each other as they have similar properties. Manganese and iron are similar and can be combined in any proportion. If we replace magnesium with lithium and substitute SiO4 with PO4 to compensate for the charge, we get LiFePO4.

Creators of new materials learn from nature. There are natural minerals and known phases, and we understand what needs to be done to turn a phase into an electrode material while using the same structure. For example, we know in which coordination a d-cation should be to work effectively: if it is in an octahedron, that is good; if it is in a tetrahedron, that is less optimal; and if it is in a pyramid, it is usually awful. In the previously mentioned example of LiCoBO3, the cobalt cation is in a pyramid, which led to failure.

Using knowledge of crystallochemical properties and electrochemical potentials, we can predict the potential of a metal in an oxide, phosphate, sulfate, or silicate. So far, computational methods do not allow us to determine how to create a material that does not exist in nature or to suggest a composition that has yet to be observed in nature. But if we propose a specific material with a particular composition to theoreticians who conduct these calculations, they can estimate its potential and stability during deintercalation. We can create something new by combining various methods and fields of science.

Directions for Developing New Materials

Currently, the most exciting direction is replacing lithium in batteries with sodium or potassium, leading to the development of sodium-ion and potassium-ion batteries. Within lithium-ion technology, there are attempts to develop materials with either higher potential or greater capacity. For oxides, this includes Li-rich NMCs, which feature an oxygen redox transition. Unlike traditional cathodes, these materials involve the oxidation and reduction of nickel, cobalt cations, and oxygen anions. There are also plans to explore redox transitions with oxygen and other anions, such as sulfur.

For polyanion materials, optimizing them themselves (for example, replacing LiFePO4 with LiMnPO4 to increase the voltage from 3.4 to 4 V) and finding an electrolyte suitable for even higher-voltage applications. This would allow the use of, for example, nickel compounds—phosphates, fluorophosphate, and others—with potentials exceeding.