Storage – New hybrid electrode

The steady increase in energy consumption, mainly based on fossil fuels, is causing an alarming increase in CO2 emissions, causing a major impact on climate change. The alternative could be the use of renewable energy sources, but their intermittency makes their use difficult.

Electrochemical storage systems in general, and batteries in particular, are presented as the best solution to this drawback. Researchers are working on the development of new materials for use in batteries, and in this way provide the market with promising technology as an alternative to lithium ion batteries for storage in low-cost Smart Grids with a better life cycle.

New Storage Batteries

The increasing deployment of renewable energy sources such as solar and wind energy requires a proportional increase in the energy storage capacity to integrate them into the electricity grid. The combination of these sources with the energy network is especially difficult due to the great and rapid variability in its production. Intermittent peaks or power drops should be smoothed for durations as short as a few seconds, while load balancing is necessary to counteract daytime fluctuations.

Therefore, economic energy storage with a rapid response, long service life, high power and high energy efficiency is required, which can be distributed through the network to allow a wide penetration of solar energy, wind energy and other variable energy sources.

Conventional energy storage technologies struggle to meet the needs of the network. Virtually all the energy storage capacity currently in the network is provided by pumped hydroelectric power, which requires an immense capital investment, depends on the location and suffers a low energy efficiency. The energy storage of compressed air also depends on the site and must be supported by a fossil fuel combustion plant. Mechanical flywheels offer high power and efficiency, but are too expensive.

On the other hand, several battery technologies have seen limited deployment on the network. Lead acid cells are the least costly, but have a limited depth of discharge, cycle length and efficiency. Sodium-sulfur, sodium-metal halide and redox flow batteries operate only at low rates and have low energy efficiency. Also, lithium and nickel / metal hydride batteries used in electric vehicles are currently too expensive for use on larger scales.

In recent years, many materials have been proposed as possible cathodes for Na-ion batteries, although the most promising ones and in which more efforts are being concentrated in the CICe are lamellar oxides, polyanionic compounds and “Prussian Blue” and its analogues.

We have explored a variety of new aqueous alkali-ion battery chemistries. These are potentially advantageous because of the safety, high ionic conductivity and low cost of aqueous electrolytes. Aqueous lithium-ion batteries using cathodic materials adopted from commercially available organic electrolyte cells have been explored, but in general have shown a limited shelf life. Aqueous sodium cells using a Na x MnO 2 cathode and a carbon capacitive anode have been shown to provide a long cycle duration, but have a limited speed capability. These aqueous technologies have been mainly limited by the development of anode materials having the correct potential and which are chemically stable to the desired electrolyte.

Recently, we have developed a family of open-structure nanoparticle materials with the Prussian Blue crystal structure. These materials have an open structure crystal structure containing large interstitial sites that allow the rapid insertion and extraction of Na + and / or K + with very little crystallographic deformation. For example, copper hexacyanoferrate (CuHCF) reacts with K + through a single-phase insertion reaction.

CuHCF electrodes are promising for network-scale energy storage applications due to their ultra-long life cycle (83% capacity retention after 40,000 cycles), high power (67% capacity at 80C), high efficiency energy and potentially a very low cost.

An anode to operate on the same electrolyte as the CuHCF cathode must be chemically stable in acidic solutions and preferably have a potential close to -0.1 V against the standard hydrogen electrode (SHE). In addition, a useful anode must have a very long life cycle and a high speed capability to adapt to the remarkable properties of the CuHCF cathode. In addition to K +, CuHCF can also react with alkali ions, such as Li + and Na +, so that an anode capable of reacting with any of these ions could be used. An intuitive option would be another Prussian Blue analogue with a reaction potential near SHE. However, the reduction of Prussian Blue to Everitt salt has too high potential (0.45 V vs. SHE) and other Prussian Blue analogs containing electrochemically active hexacanomanganate and hexacycchromate are chemically unstable.

The new class of anodes that are compatible with our CuHCF materials has an open structure in aqueous electrolytes. These anodes are based on a hybrid electrode that works by means of a fundamental new concept; That is, by combining an electrode material (polypyrrole, PPy), which is capable of a faradaic reaction to a fixed potential with a capacitive electrode (activated carbon, AC), the potential of the entire electrode can be controlled.

Fundamentally different from traditional battery and capacitive electrodes, our new hybrid electrode has the high-speed capability of an ultra-capacitor, but with the well-defined potential of a battery electrode. This hybrid electrode has attractive open circuit potential (OCP), tunable at -0.2 V versus SHE, a shallow loading / unloading profile and low self-discharge.

In addition, we demonstrate that a complete cell with this hybrid anode and a CuHCF cathode provides promising performance for large-scale stationary storage applications such as high power and energy efficiency, and a lifespan of thousands of cycles.

Prof. Mauro Pasta