There are still many challenges to overcome, to create a reliable electrochemical storage device with high energy and high power densities. Both battery and supercapacitor technologies contain gaps in their abilities and neither of them possess both a high energy and power density within a single device. A group of researchers from Sandia National Laboratories, New Mexico have adopted a different strategy by using a tethered array of manganese oxide nanoparticles bound to a gold current collector, to produce an energy storage materials that addresses many of the current issues surrounding energy storage devices.
Electrochemical energy storage devices are the main proprietor of storing charge when size and weight are critical factors. Both batteries and supercapacitors have made significant advancements in recent years, but are still lacking in fundamental areas such as energy density, power density and reliability. The main limitation in these electrochemical systems is the use of inactive material in the electrodes, where current technology only achieves an active mass of 70-80%. Poor life cycle and charge transfer, choice of electrolyte and degradation of the electrodes are just some of the other challenges that the current technologies face.
The researchers have created an alternate approach by using tethered manganese oxide nanoparticles, and although it doesn’t address every problem facing electrochemical energy storage today, it does provide a system that has a high reliability, energy density, and power density in a single device.
The researchers synthetically produced the manganese oxide (MnOx) nanoparticles, which are tethered to a gold current collector by conductive organic linkers that are less than 2 nm in length.
The researchers developed the scalable method using wet chemical techniques and a self-assembly approach. The researchers used MnO nanoparticles with a 4-aminothiophenol organic linkers, although varying nanoparticle compositions are possible based on the oxidation state of the manganese. The researchers used Fourier transform infrared spectroscopy (FTIR) and transmission electron microscopy (TEM) to characterise the material.
Unlike other systems, 99.9% of the material is active, excluding the current collector. By using organic linkers, the researchers produced a material which did not require any ‘inactive’ filler material and has produced a material that is virtually all active.
The tethered structure goes beyond normal conductivity mechanisms by limiting the distance of the charge that travels through the oxide material, by using the organic tethered links. In other materials, this is usually facilitated by carbon additives i.e. the filler molecules.
The material also addressed another common problem- poor charge and discharge cycles. By using freestanding nanoparticles, the active material can accommodate a large amount of strain and size changes. When cations are intercalated/deintercalated, the flexible structure of the material can allow for such changes, resulting in an improved charge/discharge mechanism compared to other electrochemical storage devices.
The array of manganese oxide nanoparticles is also highly tuneable and versatile. By changing the composition, and oxidation state, the array can be used for a range of electrochemical storage electrodes. MnO can be used as an anode for Li-ion batteries. MnO2 can be use as cathode for Li-ion and Li metal batteries, an electrode for an oxide supercapacitor, or as a catalyst for oxygen reduction in Li-air batteries. Manganese itself also possess two important factors- it is in abundance and is of low cost. Both factors make these materials an ideal candidate for commercial and large scale energy storage applications.
In short, the tethered nanoparticle array is modifiable and addresses many of the current limitations in electrochemical systems today. By producing a material that improves on current charge and discharge times, power densities, larger voltage windows and charge transfer times, the researchers have created a material that could potentially become a main player in the world of electrochemical energy storage.
Stevens T. E., Pearce C. J., Whitten C. N., Grant R. P., Monson T. C., Self-Assembled Array of Tethered Manganese Oxide Nanoparticles for the Next Generation of Energy Storage, Scientific Reports, 2017, 7, 44191