Fluorine-Free Salts for Sodium Battery Applications

Nowadays, technology of the renewable batteries is based on Li-ion technology. Due to development of such technologies, large growth of the marked demands on the batteries of this type is expected. It is due to the intensive impact put on technologies of the electric cars and large-scale batteries dedicated to storage of energy from renewable sources.

Unfortunately, global lithium resources, currently estimated at 29 million tonnes, are not sufficient to fulfill these demands. Therefore, studies on batteries which are alternative to lithium-based energy sources are conducted intensively. A natural candidate to replace the lithium is sodium. Standard potential of the Na/Na⁺ (‑2.71 V) is similar to the potential of Li/Li⁺ (-3.05 V). Sodium is the most abundant alkali metal on Earth’s crust and therefore the above-mentioned problems of the use of the lithium do not occur in case of sodium or Na-ion batteries. Moreover, sodium materials are cheaper than lithium.

The first successful attempt to use sodium in batteries took place in 1967, after the discovery of the ionic conductivity of the β-alumina (Na₂O·11Al₂O₃). Sodium cations were responsible for the ionic transport in this system. The ionic conductivity of this material was approximately 10 S/cm at 300°C. As β-alumina does not exhibit electronic conductivity, it was judged as a promising, ceramic electrolyte. On this basis, high temperature, sodium-sulfur battery (with sodium and sulfur as electrode materials) was constructed. The range of the operating temperatures for this battery was 300-400°C, OCV was between 2.1 V and 1.8 V (at the final stage of discharge). Though the good reversibility, the battery did not achieve the commercial success due to high operating temperature, problems related to the corrosion of the electrodes, low safety of use. Also the aspect of the overcharge control remained unsolved, which was of high importance because of very high internal resistance of the fully charged cell.

Simultaneously, studies on the Na/NiCl₂ battery were conducted. The design of this system was similar to the Na / S cell. The liquid sulfur cathode was replaced by solid nickel chloride. Additionally, an electrolyte intermediate (NaAlCl₄) was used in order to enhance contact between the solid cathode and solid electrolyte. This battery was characterized with better safety of use (mainly due to the fact that there was no metal sodium in the battery when it was installed- the anode material was produced at the anode during the first charging) and operated at lower temperatures (250-350°C). Also high OCV (about 2.5 V) was advantage of the proposed battery.

Na-NaSICON-S battery, designed in 2000, became real success. The system exhibited very high energy density. On the other hand, the problem of the high temperature of work at which the batteries are working (270-350°C) remained unsolved.

High-temperature sodium cells, despite the many benefits are not competitive in many applications. The construction of the power source for the portable device needs to be simple and safe. When application of the battery in the uninterruptible power source (UPS) is taken into consideration, the battery should work no later than 1 s after switching on. These expectations cannot be fulfilled by the high-temperature battery.

The idea of the low-temperature sodium battery may be realized by preparing the system which is analog of the lithium anode or Li-ion battery. The main issue deciding on the successful application of the sodium battery is stability of the interface between sodium or Na-ion anode and the electrolyte. This limits possible range of the solvents to the polyethers, as organic carbonates as well as their mixtures are unstable versus metallic sodium and Na-ion anode.

In the presentation, we present conductivity and CV results of the Na-based electrolytes. The compositions of the electrolytes were optimized in order to obtain system of highest conductivity. The obtained data were confronted with spectroscopic studies of the system in order to discuss the influence of the salt concentration and temperature on the formation of the various ionic agglomerates.