Energy Conversion and Storage in a Li Photorechargeable Battery
Authors: Haro Marta , Ciria Ramos Isabel , Juarez-Perez Emilio J. 
Published: February 2024

The development of energy systems in which solar radiation acts as an energy input is the most attractive alternative for meeting global demand in a sustainable manner in the near future. Since photovoltaic technologies are limited by the daily and seasonal intermittencies inherent in solar radiation, they often work in tandem with batteries, which can store electrical energy as electrochemical energy to be supplied on demand. However, a monolithic device that integrates electrical energy production and storage by sharing a common electrode is not yet available, a challenge since 1976 when this concept was first reported1. But, it has been in the last decade when the interest in this type of systems has increased due to advances in portable electronics and the potential in applications such as microbots or for the IoT (internet of things). In these applications, the device shape and weight factor, portability and decentralization of production and energy storage are more important properties than the overall efficiency of the process.
The studies of photorechargeable batteries in the decade of 2010 were mainly based on dye-sensitized solar cells2, and extended to Li-ion batteries3. More recently, photorechargeable lithium batteries have been reported by using a semiconductor electrode with the dual functionality of light harvesting and electrochemical energy storage4, 5. However, a single material for both capacitive and light harvesting functions affects the system operation and hinders the interpretation of the physical processes because of the changes in the electrical properties of the semiconductor induced by the intercalation of ions.
In a previous study, a photocapacitive system was developed based on BiVO4 light harvester and PbOx nanoparticles performing the capacitive platform by redox pesudocapacitance6. Based on this idea, a new photoelectrode based on Cu2O-TiO2 heterostructured film is studied as photoelectrode in an adapted Li coin cell (inset figure 1)7. The optical and electrochemical characterization of the electrodeposited Cu2O and nanoparticulated TiO2 films were analyzed separately by UV-vis and UPS spectroscopy and cyclic voltammetry (CV). The analysis of these results depicts an energetic scheme of the heterostructure system that explains the cyclic voltammetry registered at different light powers (figure 1). Then, a Cu2O-TiO2/Li photobattery is fabricated and its performance studied. The fabricated photorechargeable lithium battery can accumulate 88% of the theoretical energy density of the TiO2 electrode (~150 mAh g-1 at 0.1 C discharge rate) using only white LED illumination. The overall efficiency of the output electric energy respect to the light energy emitted by the LED is 0.29%. Beyond the photorechargeable lithium battery, this study analyzes the mechanism of operation in order to provide enlightening design rules to guide the development of a monolithic photorechargeable battery technology.

Figure 1. Cyclic Voltammetry plots of the photorechargeable battery illuminated at different light powers (0, 50 and 100 mW/cm2) with a white LED. Inset: the adapted coin cell to allow light illumination of the photoelectrode.

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Acknowledgments

M.H. acknowledges the funding support from MCIN/ AEI/10.13039/501100011033 for the Ramón y Cajal fellowship (RYC2018-025222-I) and the project PID2019-108247RA-I00. E.J.J-P acknowledges the funding support from MCIN/AEI/ 10.13039/501100011033 and European Union NextGenerationEU/ PRTR (project grants PID2019-107893RB-I00 and EIN2020-112315, respectively).

Figure 1