New battery concept unites advantages of battery and capacitor - oxygen vacancies in molybdenum trioxide prolong life
Pseudocapacitative material for fast current storage: orthorhombic structure of molybdenum trioxide.
Los Angeles (USA) - Even at high-speed charging stations, drivers of electric cars have to wait a good half hour before the batteries are sufficiently charged again. In the future, electricity storages consisting of so-called pseudo-capacitative materials could be filled much more quickly. With molybdenum trioxide, scientists have now optimized such a material in order to increase its storage capacity and significantly extend its service life. They present their basic work for a fast and efficient electricity storage in the trade journal "Nature Materials".
"We wanted to combine the large power density of batteries with the short charging times of capacitors in a power storage," says Bruce Dunn of the University of California in Los Angeles. Together with his colleagues, Dunn made use of the material molybdenum trioxide, which as an electrode material enables an energy density comparable to that of conventional lithium ion batteries. In addition, electricity from this material could be charged within minutes. The reason for this fast charging lies in the so-called pseudocapacity, in which ions as charge carriers attach to the electrodes via weak van der Waals bonds.
However, when the pseudocapacity memory was being charged, the structure of the molybdenum trioxide crystals changed so far, which allowed only a few charging cycles. This is precisely the problem solved by the researchers with an optimization of the material. In a microwave heated to 180 degrees Celsius, molybdenum oxide crystals were formed which lacked an oxygen atom in the crystal structure. This vacancy increased the electrical conductivity and provided extra space for lithium ions, which were exposed to weak bonds. Initial experiments showed that the undesirable phase change of the crystal structure could be avoided. Even after fifty charging cycles, the prototype lost only five percent of its storage capacity. Up to now, such current storage devices have lost half of their storage capacity after only a few charging cycles.
With this result, Dunn and colleagues were able to show that the effect of pseudo-capacitance is in principle suitable for the construction of powerful and extremely fast chargeable current storage devices. In further experiments, in addition to molybdenum trioxide, other metal oxide crystals with missing oxygen atoms could also be investigated for their current-storing properties. Thus the pseudo-capacity offers a tempting approach for novel quick-loading caches. Before series production, however, the number of charging cycles would have to be increased to 1000 at least without a large loss of storage capacity.