Design Of High-Efficiency Carbon Nitride Based Photocathode And Their Photo-Assisted Lithium-Sulfur Batteries

Lithium-sulfur（Li-S）batteries have been regarded as one of the most promising candidates for next-generation energy storage device because of its higher theoretical specific capacity(1675 mAh g^-1),energy density(2600 W h kg^-1)and natural abundance of low-cost sulfur.However,the shuttle effect caused by the dissolution and diffusion of soluble polysulfides seriously hinders the practical application of Li-S batteries.Fortunately,electrochemical catalysis with typical heterogeneous catalytic characteristics can effectively accelerate the conversion of polysulfides intermediates.Moreover,the introduction of photocatalysis through sunlight can further improve catalytic performance and more efficiently restrain the migration and shuttling of polysulfides.The photogenerated electrons and holes with strong redox ability are generated when the photocatalyst is illuminated.And during the discharge process,photogenerated electrons with strong redox ability can accelerated the conversion of sulfur to polysulfides and further to lithium sulfide.The strongly oxidizing photogenerated holes can promote the sulfur evolving reaction during the reverse process of charging.In other words,the introduction of photocatalysis can reduce the reaction energy barrier and reach a bidirectional catalytic process for the oxidation and reduction of sulfur,significantly shortening the existence time of polysulfides and inhibiting the shuttle effect.Hence,Solar energy is introduced into Li-S batteries to further improve the reaction kinetics.Based on this,the concrete research of this paper is as follows:1.Design and construction of bifunctional carbon nitride based photocathode for photo-assisted Li-S batteries:A photo-assisted Li-S battery is constructed by employing g-C₃N₄ nanosheets（CNNs）as a novel bifunctional catalyst to improve the shuttle effect.The CNNs can effectively capture sunlight,shorten the migration distance,and boost the separation efficiency of electrons and holes.Under light illumination,photogenerated electrons can enhance the reaction from S₈ to Li₂S.Especially,the electrons in CB are more likely to inject into the reaction from polysulfides to Li₂S because of the energy structure,in which demonstrating a differential catalysis capacity that has a more significant acceleration effect on the conversion of soluble polysulfides.In the opposite discharge process under illumination,Li₂S is oxidized gradually to S₈ driven the holes in the VB.In addition to the photocatalysis effect,the photoconductivity effect caused by the increase of carrier concentration due to the excitation of photogenerated carriers can further reduce the resistance,improve the migration rate of Li⁺,and boost the electrochemical reaction kinetics.As a result,compared to the dark system batteries,the battery with the CNNs photocathode exhibits lower polarization,higher deposition at 2.13 V,and enhanced specific capacity(up to 1500 mAh g^-1 at 0.2 C).Besides,the photo-assisted Li-S battery can deliver a high specific discharge capacity of 500 mAh g^-1 under illumination at 0.2 C without external voltage,demonstrating that the integration of light energy conversion and storage in photo-assisted Li-S battery.Combined E_acalculated by EIS fitting and CV curve at different light intensity to determine promotion mechanism of solar light.2.Design and construction of plasma mediated multifunctional photocathode for photo-assisted Li-S batteries:In order to further enhance the utilization of light and fully utilize the function of light energy,while improving the performance of Li-S batteries.Based on the previous work,the photocathode material was additionally optimized by loading plasma Au onto the surface of Nv-C₃N₄.On the one hand,the presence of nitrogen vacancies provides vivid adsorption and reaction sites for polysulfides.On the other hand,the Schottky barrier formed at the metal semiconductor interface effectively restrains the recombination of carries and improves the separation efficiency of carries,which further optimizes photocatalytic and photoconductive capabilities.Therefore,the battery has an ultra-low polarization voltage of less than 0.1 V at 0.2 C and a high deposition potential of 2.14 V.Moreover,the cell exhibits enhanced specific capacity(up to 1300 mAh g^-1 at 0.5 C).Simultaneously,the hot carries generated by plasma resonance release heat during the decay process,thereby achieving photothermal conversion,and raising the temperature of Li-S batteries to achieve a temperature rise of 20 ℃ under a solar intensity,which is conductive to reducing electrochemical impedance and promoting the migration rate of ions,while improving the battery’s capacity to cope with extreme environment such as low temperatures.Combined with the calculation of E_a and the measurement of CV curves under different light intensities at specific optical wavelengths,which explored the gain effects of photocatalysis and photothermal conversion respectively.