New 'light-charging' battery charges all day — on sunlight or indoor lighting

A new battery that charges continuously — on sunlight during the day and indoor lighting at night — has been developed by researchers in South Korea and the United Kingdom.

A team led by Kwon Tae-hyuk, a professor in the Department of Chemistry at Ulsan National Institute of Science and Technology (UNIST), announced Wednesday that it had developed a modular light-charging battery structure capable of switching between daytime and nighttime charging configurations, in collaboration with a team led by Professor Michael De Volder at the University of Cambridge.

A light-charging battery combines power generation and power storage in a single unit, with electricity produced by a solar cell flowing directly into a battery. Because it charges using ambient light without an external power line, the technology has drawn attention as a self-contained power source for Internet of Things devices and wireless sensors — applications where running cables is difficult or frequent battery replacement is impractical.

In the newly developed modular design, the battery module remains fixed while only the solar cell module is swapped out to match the available light intensity. A sunlight module is connected during the day, while a module designed to produce high voltage even under weak light is used at night with indoor lighting. The battery itself uses a high-capacity lithium iron phosphate (LFP) cathode and a lithium metal anode.

In testing, the lithium metal battery fitted with the sunlight module charged to 70 percent in 10 minutes under standard solar conditions, achieving a discharge energy density of 327.8 mWh/g. The indoor lighting module also successfully charged and discharged using only light at 1,000 lux — typical office brightness. When light was applied while the battery was in use, the available capacity increased compared with operating in darkness.

The performance stems from a structural design tuned not only to the battery's storage capacity but also to the voltage required for charging.

The modular light-charging battery boosts solar cell voltage to the level needed for charging simply by changing connection points. One battery compartment containing an LFP cathode is connected in series with four solar cell compartments that add voltage, allowing the user to select from configurations of one, three, or five compartments. When charging the higher-voltage lithium metal battery, all five compartments are connected.

"This research establishes a design framework for lithium metal secondary batteries and light-charging secondary batteries that can charge across a wide range of illumination — from intense sunlight to weak indoor lighting," Kwon said. "Converting and storing the light from artificial lighting, which is currently mostly wasted, could be used to develop wireless, self-contained power sources that operate without external power lines or frequent battery replacements."

The findings were published in the August issue of Energy Storage Materials, an international journal in the energy field.