The environment and energy are one of the main areas of concern of the 21st century. Climate change associated with increasing CO2 in the atmosphere and the depletion of fossil resources, as well as reliance on politically unstable oil producing countries, requires a shift to an energy model based on renewable sources.
New Storage Batteries
Despite the increase of facilities powered with renewable energy sources, mainly wind and photovoltaic, the intermittency is a disadvantage compared to other sources and makes it necessary to develop systems of storage for renewable energy.
Electrochemical storage systems, such as batteries, are presented as the best solution. However, there are some technical difficulties in terms of efficiency, shelf life and cost of current storage technologies that explain why energy storage has not been widely applied.
Currently lithium-ion (Li-ion) batteries which have conquered the market for portable electronic devices, have become the leading candidate as an energy source in the next generation of electric and hybrid vehicles. These provide the highest energy density of all current rechargeable battery technologies; However, its high cost, low Li and technical limitations (extremely sensitive to high temperatures, overload, accumulated internal pressure, and total discharge intolerance) are the main drawbacks for its use in energy storage.
In this context, sodium-ion batteries (Na-ion), despite having a lower energy density, have the advantages to lead the next generation of batteries for stationary applications within Smart Grids, in which the limitations Volumetric values are lower and the cost becomes the critical parameter. Sodium is abundant and is next to lithium in the periodic table, so we can expect similarities in terms of technology and performance.
Na-ion batteries are particularly attractive because the same industrial infrastructures (same production technology) can be used as for Li-ion batteries and thus minimize the cost of technology transfer. Another immediate cost reduction is the possibility of using an aluminum foil as a current collector at both the anode and the cathode instead of copper (aluminum corrosion in Li-ion).
There are still many challenges to be resolved, especially in terms of shelf life and safety at the pre-industrial prototype level. Current results show that we are facing a competitive technology to any of the existing ones in terms of performance and cost.
Overview of Sodium-ion Batteries
During the 1980s, the development of Na-ion batteries was parallel to Li-ion batteries, but due to the better density / volume ratio, the latter has dominated the market for portable electronic devices since 1990. However, for stationary applications where energy density is not the most important parameter, Na-ion technology seems to be a very excellent future option, with an estimated cost of <0.1 € / Wh in 2020 (50% lower To Li-ion) and with the possibility of storing several MWh in reduced volumes (3 times lower than Pb-acid technology).
Positive electrodes for sodium-ion batteries
In recent years, many materials have been proposed as possible cathodes for Na-ion batteries, although the most promising ones in which more efforts are being focused are lamellar oxides, polyanionic compounds and Prussian blue and their analogs.
Among these materials are lamellar oxides and polyanionic compounds, based on phosphates, pyrophosphates, fluorophosphates, sulfates, etc .; Which offer great versatility for the development of new cathodic materials for Na-ion batteries. But let us see what the Prussian blue consists of.
One of the materials that has been attracting attention lately is known as Prussian Blue, AM [M'(CN) 6] (A = alkali metal, M = M ‘= generally Fe). Among its advantages, it is possible to emphasize its ease of synthesis, its friendly nature with the environment and its modulable 3D structure that facilitates the intercalation of ions. Among its disadvantages is a low coulombic efficiency due to the collapse that usually occurs in its structure along the cycle.
Negative electrodes for sodium-ion batteries
The identification of an anode with adequate voltage, high reversible capacity and high stability is necessary for the development of Na-ion batteries. Despite a wide variety of studies, from an industrial point of view, only the disordered coal has been considered as a promising anodic material for Na-ion batteries.
Disordered coals are the most studied anodic materials and provide a reversible capacity of ~ 300 mAh g-1 to ~ 0.1 V. Their excellent performance is compounded by the fact that they are low cost materials (<1 € / kg) converts them to one of the most attractive materials for anodes in Na-ion batteries.
On the other hand, it is also being investigated in Carbodiimides and in sodium-ion batteries in aqueous medium. One of the most promising approaches to cost reduction in Na-ion batteries is the use of aqueous electrolytes which also have higher ion mobility and greater safety than conventional organic electrolytes.
Due to the variety of advantages it has in terms of cost and availability, together with promising improvements in performance, durability and safety, it makes Na-ion technology a clear future option for energy storage in Smart Grids. In this aspect we are working, and therefore, the promising results obtained so far, make us think that this is possible.