International Collaboration Provides New Insight on the Material Functions of Solid-State Devices

Consumer electronics may have come a long way in recent years, but one thing has remained constant: the need to charge a device’s battery.

Now, a collaborative team of researchers based in Porto, Portugal and Texas, United States hope to make this user-unfriendly requirement a thing of the past with the development of a new type of battery that can recharge itself without losing energy. 

The team’s research, which was published by AIP Publishing in Applied Physics Reviews,  combines negative capacitance and negative resistance inside the same cell to allow it to recharge without losing power. It has significant implications for increased power and long-term storage for batteries.

 

Combining Negative Capacitance and Negative Resistance

Negative capacitance occurs when a decrease in voltage across a material causes an increase in its electrical charge. In contrast, negative resistance is the opposite—this occurs when an increase in voltage across a material results in a decrease in the electrical charge running through it. 

 

New Insight on SSD Materials 

By enabling the two to co-exist in one battery is very significant because it unifies the theory behind solid-state devices such as batteries but also transistors, capacitors, and photovoltaics. The theory demonstrates that different materials in electrical contact can exhibit the properties of the combined materials rather than the individual ones.

In a battery, the open circuit potential difference between electrodes is due to an electrical need to align the Fermi levels. These are a measure of the energy of the least tightly held electrons within a solid, something that is also responsible for the polarity of the electrodes, said Helena Braga, an associate professor at the University of Porto and lead researcher working on the project.

 

A diagram of bistable energy landscape for a lithium-glass ferroelectric-electrolyte in contact with an aluminum-negative electrode. Image credited to University of Porto

 

A Very Simple Battery

"When one of the materials is an insulator or dielectric, such as an electrolyte, it will locally change its composition to form capacitors that can store energy and align the Fermi levels within the device," she added. 

A lithium-rich glass electrolyte was developed for the battery, which Braga described as “very simple”, because it uses only two different metals as electrodes and a lithium or sodium glass electrolyte between them. The glass electrolyte is capable of feeding both electrons with lithium ions, meaning that there is no need to use lithium metal in its design. By omitting lithium metal, they were able to achieve this simplified battery design. 

"The glass electrolyte we developed was lithium-rich, and so I thought that we could make a battery in which the electrolyte would feed both electrodes with lithium ions, on charge and discharge with no need for lithium metal," said Braga.

 

Self-Charging Capability

To make a more sustainable energy source, the device’s self-cycling can either be mitigated or stopped completely by configuring a negative resistance to happen. This is achieved by having the negative electrode of the same material as the positive ions of the electrolyte. This, Braga said, is what enables the device to self-charge without self-cycling; it increases the energy stored in it as opposed to losing it through heat dissipation. 

 

Suitable for Consumer Electronics

It is thought that this method could be employed in consumer devices and other energy-storage devices, such as capacitors, to help improve autonomy. 

Although the team’s technology cannot yet be applied to consumer devices—currently, it is suited to extremely low-frequency communications such as inverters or digital converters—it could be scaled for use in larger devices in the future.