ULSAN — A modular battery technology that switches between sunlight and indoor lighting to charge around the clock has been developed. The advance is expected to accelerate commercialization of standalone power sources for Internet of Things (IoT) sensors that require neither external power lines nor periodic battery replacement.
The Ulsan National Institute of Science and Technology (UNIST) said on the 2nd that a research team led by Professor Kwon Tae-hyuk of the Department of Chemistry, working with Professor Michael De Volder's team at the University of Cambridge, has developed a modular photo-rechargeable battery architecture whose configuration can be changed according to light intensity.
Photo-rechargeable batteries combine the power-generating function of solar cells and the energy-storage function of batteries into a single device. Existing photo-charging systems had a limitation: when light intensity changed, the voltage produced by the solar cells no longer matched the battery's charging requirement, halting the charge or sharply cutting efficiency.
The team devised an approach that keeps the battery body fixed while swapping only the solar cell modules to suit light intensity. Solar cells that add voltage are wired in series next to a battery cell containing a lithium iron phosphate (LFP) cathode, allowing the voltage of one, three or five segments to be selected depending on light conditions. When charging a lithium metal battery that requires higher voltage, all five segments are connected to secure sufficient driving force for the charge.
The battery materials were also overhauled to raise energy density. In the conventional approach using an iodine-based liquid cathode electrolyte, the separator needed to prevent side reactions increased internal resistance and slowed charging and discharging. The team applied a solid LFP cathode requiring no separator, together with a lithium metal anode, sharply raising volumetric energy density. It also adopted a solid polymer (PEDOT) as the charge transport material under sunlight and a copper complex electrolyte under weak indoor lighting, maximizing generation efficiency.
In performance testing, the lithium metal battery fitted with the sunlight module charged rapidly to 70% in 10 minutes under standard solar conditions and recorded a discharge energy density of 327.8 mWh/g. Complete charging and discharging was also achieved using light alone at 1,000 lux, the illumination level of a typical office. The researchers also observed that available battery capacity increased when light was shone on the device while power was being drawn.
The technology is expected to find broad use in indoor energy harvesting, recovering lighting energy otherwise wasted inside buildings to power wireless sensors and IoT devices indefinitely. On a European basis in 2016, annual energy consumption by buildings accounted for roughly 40% of total energy consumption, and about 20% of the energy consumed in buildings went to lighting.
"By making the number of solar cell segments and the materials variable, we secured optimal driving force matched to the charging voltage of each battery material," said Kim Byung-man, a UNIST researcher and the paper's first author. "This will serve as a design strategy applicable to the various battery materials now in commercial use."
"By establishing a structure capable of stable charging under both indoor and outdoor light conditions, we have cleared a major obstacle to commercializing photo-rechargeable secondary batteries," said Professor Kwon of UNIST. "This can fundamentally resolve the maintenance cost problem of IoT devices in which battery replacement is difficult."
The findings were published in the August issue of Energy Storage Materials, an international journal in the field of energy storage. The research was supported by KEPCO and UK Research and Innovation (UKRI).
