Energy storage devices are key components for a successful and sustainable energy system. Some of the best materials and types right now are Lithium-ion/lithium-sulfur/lithium air cells, supercapacitors, and beyond. Research in this area has greatly improved electrode materials, enhanced electrolytes, and conceived clever designs for cell assemblies with the goal of increasing specific energy (Wh/kg) and pushing the power envelope (W/kg). But moving forward, new developments must lower cost, shrink size, and increase reliability and life, while also improving upon safety under temperature, vibration, overloading, and other harsh conditions. A major breakthrough is needed in higher capacity electrode materials and new cell technologies in order to keep pace with the needs of hand-held devices, electric vehicles (EV), alternative energy, energy harvesting and more.Batteries/cellsLithium-ionTechniques have been developed and are being refined for stretchable Lithium-ion batteries, replacing the graphite negative electrode with a silicon one in a Lithium-ion cell, new Lithium-Sulfur batteries with sulfur-carbon nanotubes as the cathode for fast charge-discharge, any many more efforts. The fact is these are all relatively small steps of progress in the right direction, but we still need that earth-shaking discovery to leap to the next storage platform. When that next technology will appear is unclear, but 2014 will certainly make good progress toward it.SupercapacitorsLow-cost battery solutions tend to have high ESR and subsequently large amounts of stored energy can’t be used when supplying pulse currents. Supercapacitors, in parallel with batteries, reduce the overall ESR and can increase battery life by as much as 300%. That’s really good, but still not good enough.New materials in supercapacitor electrodes
GrapheneEVs are perfect examples of a system in need of new energy storage technology. One promising new technology is graphene supercapacitors that can enhance the efficiency of regenerative braking in an EV. These devices charge quicker than Lithium-ion cells and discharge the stored energy fast, as well. The traditional supercapacitor charges and discharges too quickly and, hence, do not store much energy. The graphene supercapacitor, however, can store as much energy as a Lithium-ion battery. The graphene version has a specific capacitance of more than 150 Farads per gram and can store energy at a density of more than 64 watt-hours per kg at a current density of 100 to 200 watt-hours per kg. They can also be fully charged in 16 seconds, with repeated charging as many as 10,000 times without much reduction in capacitance.Manganese dioxideAn even more astounding new electrode material in supercapacitors is being developed by researchers at the Leibnitz Institute for Solid State and Materials Research in Dresden, Germany. Manganese dioxide is not really good in conducting electrical current, but when it is vaporized and reformed into thin, flexible strips, and thin layers of gold is connected to those strips, the supercapacitor stored more energy and provided more power per unit volume than the standard supercapacitor. More work will be done to reduce cost, as gold is expensive, but we can expect to see more ingenuity out of this effort in 2014. Figure 1. Manganese dioxide electrode in a supercapacitor (Image courtesy of Reference 1)Sometimes an added or improved component in the overall system design for storage will further improve the process. One example is ALD’s Supercapacitor Auto Balancing (SAB) MOSFETs that provide automatic active leakage current regulation and self-balancing of supercapacitors connected in a series stack. The addition of a simple, but uniquely designed, MOSFET can eliminate both overcharging and excessive power dissipation to preserve lifespan of the supercapacitor and battery components. Figure 2. SAB MOSFETs have unique electrical characteristics for active continuous leakage current regulation and self-balancing of stacked series-connected supercapacitors. They are designed to dissipate near zero leakage currents, eliminating extra power dissipation.Hybrid energy storageScientists are also exploring hybrid technologies that can bridge the gap between traditional batteries and the supercapacitor. This would create a device that could simultaneously store and deliver energy efficiently — a perfect EV power source. UCLA scientists are looking at niobium-oxide based material for this effort.What’s next?More clever designs in the overall energy storage circuit will certainly continue, but in order to have a major improvement in energy storage, we must have a major breakthrough in the materials used to fabricate the storage device itself beyond present Lithium-ion and standard supercapacitor materials and structures. Progress is expected toward this goal in 2014 and maybe even some new ideas about a different storage mechanism altogether. It will be an interesting progression toward that ideal energy storage device for sure.