Higher-performance lithium-ion batteries, thanks to solid electrolytes

For the most common uses for the quality of a battery it is simply gudicata on the basis of its autonomy: the longer is able to ensure the operation of a device, more will be considered valid. But there are also other types of applications for which the autonomy is not the only important parameter: stable operation over a wide temperature swing, compactness and flexibility, rapid charge / discharge cycles. And a desired characteristic, definitely transverse to all types of application, is the safety of the device. Many energy storage technologies are no more than a compromise in favor of this or that characteristic, in relation to the type of application they are intended.

A collaboration between academic researchers and technicians Toyota, however, seems to promise the "holy grail" of all batteries: a more compact device of a lithium-ion battery, with a better energy density, speed charging of a supercapacitor and greater safety . All this would be possible by eliminating the liquid electrolyte present in most of the lithium-ion architectures currently used.

Generally speaking, the basic structure of the batteries is quite simple and involves two electrodes where the ions exchange electrons, separated by an electrolyte which allows the ions to move between the electrodes. These electrolytes have always been liquid since it can dissolve more easily the ions and enable them to move freely between the electrodes. Unfortunately often the cause of problems, even severe, is due to the losses of the liquid electrolyte. Untie this knot is complicated, moreover it appears rather laborious to dissolve the ions in a solid.

Not impossible, but until now it was possible to do it in a manner not so efficient. Over the years they have been developed few electrolytes in solid state, which, however, have not been able to express the same performance of currently existing technologies on the market, due to the difficulty that the ions are covering the solid electrolyte. A document, part of a project run by Toyota researchers and published in 2011, shows, however, that some particular solid electrolytes are able to conduct well enough lithium ions. It is solid with a crystalline structure capable of ordering the lithium ions in a row, giving rise to a movement almost as if it were inside a channel.

The problem is that all solid identified with this precise structure were found to be chemically unstable or involve the use of rather expensive raw materials. In recent years, however, the paper's authors have performed a series of experiments on some other materials that have shown very interesting properties. Especially one of them, marked by the complex formula Li9.54Si1.74P1.44S11.7Cl0.3, appeared to be a promising candidate for applications that require high current while another Li9.6P3S12 material, showed the best characteristics for the high-voltage cells. Both are chemically stable and do not require expensive raw materials for their realization. Their structure is similar to a three-dimensional grid and allows ions to move freely inside in all directions. These new materials are able to show a double conductivity than the solid electrolytes studied previously.

Researchers have constructed a series of batteries using these electrolytes, observing how they were able to operate in a temperature range from -30 ° C to 100 ° C, whereas the commercially available lithium-ion batteries are not able to operate at the two extremes of this interval. The batteries thus constructed have also shown a high-speed charge / discharge with a full cycle in less than seven minutes. At high temperatures, the download rate is confrotabile with that of supercapacitors. Finally, the energy density (weak point of supercapacitors) was comparable to that of lithium-ion batteries.

The chemical reactions that take place between the electrodes and this material when operating the first charge / discharge cycle immediately causes a deterioration of the capacity in the order of 10%. After 500 cycles, however, the batteries have been shown to be able to maintain 75% of original capacity, indicating that the loss after the first cycle remains relatively low.

The high performance solid electrolyte are due to its structure, which offers a sort of "preferential path" that the lithium ions can travel, and its three-dimensional network that allows ions to circumvent any defects. Inside the solid there are also many more of lithium ions per unit volume than is possible to obtain in a solution, about 20 times as much. The solid, also can not freeze and will not degrade chemically at high temperatures, for this offers a wide range of operation. And, finally, it is not subject to losses.

At this point it is necessary to match the electrolyte materials suitable for the realization of the electrodes, since it must be able to withstand great variations in the quantity of lithium contained in them without thinning or deteriorate in such a way as to cause structural failure. The researchers began testing different materials and hope to build even more efficient batteries with the correct combination of materials. In any case the use of a solid electrolyte can eliminate those common weak points at the base of the current batteries, as well as simplifying the structure and allow a greater energy density in the complex.