Tiny Capacitors Amp Up Energy Storage

We have come to a point where it seems like any new technology promising to be smaller and more portable should have an asterisk next to its claims. This is due to the fact that the tremendous advancements that have been seen in computing and sensing equipment have not been met with equivalent steps forward in the area of power storage and delivery. Sure, that new wearable health monitor may be the size of a dime, but if the battery is three times larger and needs to be recharged every few hours, it is still inconvenient to use.

Until battery technology can meet the challenges of powering today’s miniature devices, developers will continue experimenting with alternatives. Capacitors, in particular, have been utilized in an effort to fill this gap in capabilities. But while capacitors can supply energy at a much faster rate than a battery and are capable of being recharged an unlimited number of times without degradation, they store less energy per unit of weight.

A brighter future for our smallest of gadgets might be on the horizon, thanks to the work of a team of researchers at the Lawrence Berkeley National Laboratory. They have developed a new type of capacitor that is very small, yet retains the desirable properties of traditional capacitor technologies. These tiny capacitors are not lacking in energy density, however — they exhibit 9-times higher energy density and 170-times higher power density than even the best electrostatic capacitors. Using these miniaturized capacitors, energy storage and delivery could be built directly into individual microchips, greatly enhancing efficiency.

The team’s breakthrough was achieved by layering ultrathin sheets of hafnium oxide and zirconium oxide on top of one another. Crystalline films of these materials are grown by a process known as atomic layer deposition, which is widely used in manufacturing microchips today, so existing processes could be adapted to produce these capacitors without too much difficulty.

The key to the success of this approach involves creating a negative capacitance effect in the materials, which counterintuitively allows the capacitors to store more energy. By fine-tuning the composition of the films, the electric field balances the capacitor at the point between ferroelectric and antiferroelectric states, resulting in a negative capacitance effect. This state allows the material to become easily polarized with a small electric field, significantly increasing the charge storage capacity of the microcapacitor.

In order to scale up the energy storage capacity of the device, the film needed to be made thicker, but without upsetting the delicate antiferroelectric-ferroelectric balance that had been achieved. Ultimately, it was found that by inserting layers of aluminum oxide at regular intervals between the hafnium oxide and zirconium oxide layers, the microcapacitors could grow to 100 nanometers in thickness without losing their effectiveness.

As a final step, these capacitors were built into trenches cut into silicon, which would allow them to be integrated with existing electronic devices, where they could serve as a power delivery system. Looking ahead, the team plans to build these microcapacitors directly into the microchips that they are intended to power. With some additional work, this strategy could prove to be very important in edge computing and Internet of Things applications, in particular.