Design Of Carbon-Based/Metal Compound Composite Functional Materials And Their Application In Lithium-Selenium Batteries

Lithium-selenium（Li-Se）batteries have attracted great interest from researchers due to their high theoretical energy density.However,there are many challenges in the practical application of Li-Se batteries,leading to their commercialization being hindered.At present,the problems faced by the positive side of Li-Se batteries mainly include:low utilization of bulk Se;volume expansion and contraction problems between Se and discharge product lithium selenide due to density difference;dissolution diffusion of intermediate product lithium polyselenides in the electrolyte,etc.These problems lead to the collapse of electrode structure,poor cycling stability and other manifestations.Based on the above problems,this paper explores the promotion of the morphological structure and composition through the design of the Se cathode with a composite material of porous carbon and metal compounds as the host of Se,aiming to improve the electrochemical performance of Li-Se batteries,and the main research is as follows:（1）In order to alleviate the volume expansion of Se,we prepared ultrathin Mo Se₂nanosheet composites grown on hollow carbon spheres as the cathode host material for Li-Se batteries by sol-gel method and high-temperature pyrolysis process to obtain hollow porous carbon（PHCS）,and then an ultrathin layer of Mo Se₂ nanosheets（PHCS@Mo Se₂）was grown on the surface.They be used as a host material for Se.Their internal cavity structure of PHCS is large enough to relieve the volume expansion of Se,while Mo Se₂acts as an external defense line that can hinder the diffusion of polyselenides by physical and chemical effects,and the thin layer structure of Mo Se₂ nanosheets can enable the aggregation and full infiltration of electrolyte.（2）We report a three-dimensional hierarchical structure composed of porous carbon layers derived from the metal-organic framework with Ni（OH）₂ nanosheet shell as Se host.The 3D hierarchical structure looks like slices of toast stacked on top of each other and decorated with Ni（OH）₂ nanosheets.The toast improves the electronic conductivity of Se and serves as a shelter to provide sufficient free space for volume change.The Ni（OH）₂nanosheet shell act as a functional protective barrier to restrain the shuttle effects and achieve a shorter ion transport path.Consequently,the Li-Se batteries deliver remarkable cyclic performance over 2500 cycles at 3 C with 0.018%capacity decay due to the unique structure.In addition,the electrode also performs well in Na-Se batteries.（3）A strategy for the constructing of the cathode-electrolyte interface（CEI）film assisted by Al F₃ as a cathode host for Li-Se batteries is proposed and realized.A simple one-step method for the conversion of porous carbon containing Al particles to layered Al F₃（PC/Al F₃）was employed.The stable PC substrate with multidirectional channels alleviates the volume expansion and improves the electron conductivity during cycling.At the same time,the uniform dispersion of Al F₃ in the interlayer and intracavity of PC shows strong chemical adsorption toward polyselenides.Moreover,Al F₃ boosts the construction of the CEI film,which further serves as a blocked layer grown on the cathode interface to prevent subsequent side reactions.Hence,the PC/Al F₃-Se cathode presents a satisfying discharge capacity of 580 m Ah g^-1 at 5 C.This strategy provides an insight into facilitating the formation of CEI film toward highly stable Li-Se batteries.In summary,we have synthesized three different composites with the strategy of designing functionalized host from composites of carbon base and metal compounds and immersed Se into the prepared materials by melt-diffusion method.These composites significantly enhance the utilization of the active material,which not only can effectively alleviate the volume expansion due to density difference during charging and discharging,but also significantly inhibit the shuttling of polyselenides,thus significantly improving the performance of Li-Se batteries,and the proposed structures and strategies provide important insights for the wide application of Li-Se batteries.