Advancing Li–CO2 battery performance with photo-energized MoS2/CNT cathodes

The development of high-performance Li–CO₂ batteries is crucial for efficient energy storage solutions, especially in extreme environments. A team of researchers from the Nanjing University of Aeronautics and Astronautics, led by Prof. Laifa Shen, has demonstrated a significant breakthrough by integrating a MoS₂/carbon nanotube (CNT) photocathode, leveraging both photoelectric and photothermal effects to enhance electrochemical performance across a wide temperature range.

One of the primary challenges in Li–CO₂ batteries is the sluggish decomposition kinetics of Li₂CO₃ during charging, which leads to high overpotential and capacity loss. Shen has confirmed that photo-generated carriers effectively accelerate the electrochemical kinetics, thereby mitigating overpotential and improving battery efficiency. Under illumination, the charge overpotential at 0.02 mA cm^-2 is reduced by 0.62 V compared to the non-illuminated condition, highlighting the substantial influence of photovoltage compensation. As current density increases, the light-mitigated overpotential remains significantly lower, demonstrating the efficient charge transport and reaction facilitation enabled by the MoS₂/CNT photocathode.

The MoS₂/CNT-based Li–CO₂ battery exhibits remarkable improvements in energy efficiency and cycling stability. With full charge/discharge cut-off voltages of 2 V and 4 V, the illuminated battery achieves high area capacities of 4.88 and 4.21 mAh cm^-2, respectively, far surpassing the 0.40 and 0.10 mAh cm^-2 capacities of the non-illuminated counterpart. The morphology of discharge products further explains this enhancement—film-like Li₂CO₃ under illumination is more easily decomposed compared to stacked bulk Li₂CO₃ in the dark. Furthermore, a photo-responsive voltage shift from 2.68 to 2.90 V during discharge and a reduction from 3.78 to 3.54 V during charge highlight the efficient generation and transport of photo-excited carriers.

Beyond room-temperature applications, the photothermal effect of MoS₂/CNT significantly enhances battery performance at ultra-low temperatures. Under −30°C conditions, the infrared thermal imaging results reveal that the MoS₂/CNT cathode can elevate its temperature from −30°C to −12°C within 50 minutes under illumination. This localized heating reduces electrolyte viscosity and accelerates reaction kinetics, enabling stable battery operation in sub-zero environments. The Li–CO₂ battery with illumination at −30°C exhibits a higher discharge voltage (2.78 V) and lower charge voltage (3.60 V) compared to non-illuminated conditions at −12°C, proving the synergistic impact of photoelectric and photothermal effects.

Electrochemical impedance spectroscopy (EIS) and linear sweep voltammetry (LSV) further validate the efficiency of MoS₂/CNT photocathodes. At −30°C under illumination, the battery demonstrates significantly lower impedance and higher current density, confirming the acceleration of ion diffusion and reaction kinetics. The Tafel slope analysis reveals that the illuminated cathode achieves a remarkably low value of 16.42 mV dec^-1, compared to 61.96 mV dec^-1 at −12°C and 181.76 mV dec^-1 at −30°C without illumination. These results underline the crucial role of photo-energy in improving charge transfer dynamics and overall battery efficiency.

This work establishes MoS₂/CNT photocathodes as a game-changer for next-generation Li–CO₂ batteries. At room temperature, the photo-energized battery achieves an outstanding energy efficiency of 90.2%, significantly surpassing the 74.9% efficiency of the non-illuminated version. Under extreme cold conditions, the photoelectric and photothermal synergy enables a median charge voltage of just 3.4 V at −30°C, with an impressive round-trip efficiency of 86.6%. These advancements offer valuable insights into the development of high-performance photo-assisted energy storage systems capable of operating in diverse environmental conditions.