Sodium Batteries – solutions for the Post-Lithium Era
Sodium is the sixth most abundant element in the earth's crust and 1000 times more abundant than lithium. Since it is an element of the first main group just like lithium, it has very similar properties. Therefore, scientists at Fraunhofer IKTS and Friedrich Schiller University Jena are designing electrochemical energy storage systems based on sodium compounds and elemental sodium.
The expansion of renewable energies is increasing the need for powerful and safe energy storage systems for stationary applications. Batteries used must be cycle-stable and safe over a service life of 15 years. Sodium-nickel-chloride batteries meet these requirement, are ecologically sustainable and are based on readily available raw materials such as common salt, aluminum oxide and nickel. The metallic raw materials can be reused through simple recycling. Sodium nickel chloride cells are commercially tubular manufactured and operated at hot temperatures around 300 °C.
However, high-temperature systems cannot serve the market of mobile utility and end-use devices. Sodium-ion batteries can enter this market to some extent, but they impose different conditions on electrode and electrolyte materials than lithium-ion batteries (LIB). For example, graphite, which is well established for LIB, is an unsuitable choice for the negative electrode and hard carbons are used as an alternative.
Contact: Michael Stelter
Anode materials
a) Sodium metal
Considering only the aspects of energy density and specific storage capacity, sodium metal (q = 1165 mAh/g) would unrivaled be the best choice for the negative electrode in sodium batteries. However, this electrode is far from easy to control in the solid state and poses a safety risk in combination with conventional liquid electrolytes. For example, dendrites could grow, short-circuiting both electrodes and inducing "thermal runaway." The solvability of this problem is controversial and the electrode is discussed for low-temperature sodium-sulfur and sodium-air batteries. For sodium-ion battery (SIB) research purposes, however, it serves as a viable counter and reference electrode in specially secured laboratory cells. In the molten state, the excellent properties of the light metal are used in stationary sodium-nickel chloride batteries and sodium-sulfur batteries. No elemental alkali metal will be used in the SIB, as is the case with the LIB. The development of Na+ storage materials suitable for this purpose is therefore a core requirement of SIB.
Product sheet: Sodium as an alternative to lithium in electrochemical energy storage systems (DE/EN)
b) Hard carbon
The use of graphite on the anode side of the lithium-ion battery (LIB) has significantly contributed to the success of the LIB. Not only does it have almost perfect electrochemical properties for high-energy lithium storage, but it is also an abundantly available and hence cheap material.
Contrary to the case of lithium, no substantial amounts of sodium can be accommodated in graphite, since these two elements do not form thermodynamically favorable compounds with each other. Hence, other carbon modifications have to be developed for the use in sodium-ion batteries (SIBs).
Non-graphitizing, often also called hard carbon due to its mechanical properties, is a disordered carbon allotrope, which does not adapt a graphitic structure, even at temperatures as high as 3000 °C. It is obtained from the thermal treatment of carbon-rich materials, oftentimes plant residues or other agricultural waste products. Therefore, it is basically unlimitedly available and can be considered a sustainable resource.
Due to the unique structural features of hard carbon, it is capable of storing large amounts of sodium at low voltages, enabling high cell potentials and energy densities in SIBs.
Current fundamental research is focused on sharpening the understanding of the impact of hard carbon structure on its sodium storage characteristics, in order to increase the sodium storage capacity, while minimizing capacity losses during cycling, particularly in the first sodiation process. This can be achieved by tuning the heat treatment procedure and incorporating additional synthetic steps that aim at optimizing the interaction between hard carbon, electrolyte, and sodium ions.
c) Coated electrode
For the application of the hard carbon in battery electrodes, it needs to be coated onto meta foils as a thin layer. Due to their less negative potential, SIB anode foils can be produced from aluminum rather than from copper, which in practice offers advantages in terms of cost and weight. The micrometer-sized hard carbon particles are firstly mixed with a conductive additive, usually finely ground and highly electrically conductive carbon blacks and a binder, often carboxymethyl cellulose or styrene-butadiene rubber to form a slurry. On the laboratory scale, a doctor blade is used to coat the metal foils batchwise, while slot-die coating is more commonly applied in a roll-to-roll process in larger scale manufacturing.
Institute for Technical Chemistry and Environmental Chemistry at FSU Jena
Fraunhofer Institute for Ceramic Technologies and Systems IKTS
