A First-principles Study Of 2D Monolayer ?-GeSe As An Anode Material In Na/K Ion Batteries

In recent years,due to the rapid development of science and technology,the demand for energy is increasing.The continuing consumption of fossil energy and the environmental problems caused by it make people more and more eager for new energy.With the vigorous study of solar energy,tidal energy,hydrogen energy and nuclear energy,more and more clean energy has been put into formal use.Consequently,the problem of how to store energy arises at the historic moment.Lithium ion batteries?LIBs?have become an ideal energy storage device because of their high energy density,long cycle life and no pollution.With the wide use of lithium-ion batteries,the limited lithium resources in the crust hinder the further development of lithium-ion batteries.Because Na and K are in the same main group and have similar chemical properties with Li,sodium ion batteries?NIBs?and potassium ion batteries?KIBs?have attracted more and more attention.Graphite,a widely used anode material in LIBs,can not store Na and K with larger atomic radius because of its limited interlayer spacing,therefore,new NIBs and KIBs anode materials need to be studied.Due to the unique electronic structure of graphene,a variety of two-dimensional materials have been widely studied.Because of its high surface area and good ionic diffusivity,the two-dimensional material is expected to be an advantageous anode material in batteries.It is well known that the group IV and group V elements could react with Na to form binary compounds.A large number of germanium-based chalcogenides,such as two-dimensional?-GeSe,have been studied for their high stability,isotropy and environmental protection.In this paper,the theoretical capacity,open-circuit voltage and rate performance of two-dimensional?-GeSe as NIBs/KIBs anode material are calculated by first principles calculation method.However,the pristine two-dimensional?-GeSe is semiconductor,which seriously hinders the transmission of electrons on anode materials.Therefore,it was further compounded with boron-doped graphene BC₅ to form GeSe/BC5 heterostructure to improve the conductivity of the anode material and further improve the battery performance.The research is mainly divided into two parts:Firstly,through density functional theory calculation,it is found that two-dimensional?-GeSe is a promising anode material with theoretical specific capacity of353.65 mAh/g for NIBs and KIBs.Two-dimensional?-GeSe has low open-circuit voltages of 0.219 V?vs Na/Na+?and 0.030 V?vs K/K+?,which means that there will be a large working voltage between the cathode and anode electrodes,so NIBs and KIBs have high energy density.Moreover,the volume expansion rates of two-dimensional?-GeSe after inserting Na/K ions are 3.34%/17.26%respectively,which is relatively small compared with other two-dimensional materials.In addition,the diffusion barriers of Na and K on two-dimensional?-GeSe are 0.080/0.125 eV on Ge/Se side and 0.052/0.047 eV on Ge/Se side,respectively.However,the conductivity of two-dimensional?-GeSe is not good,which hinders the transmission of electrons and limits its rate performance.Secondly,two-dimensional?-GeSe is compounded with BC₅ with excellent conductivity and small mismatch to form a stable GeSe/BC₅ heterostructure.The theoretical specific capacities of using GeSe/BC₅ as the anode material for NIBs/KIBs both are 400.66 mAh/g,which is improved compared with two-dimensional?-GeSe.OCV is 0.574 V and 0.667 V respectively,which can inhibit dendrite growth and ensure the safety of the battery.By calculating the density of states of GeSe/BC₅,it is found that the conductivity of two-dimensional?-GeSe is significantly improved when combined with two-dimensional BC₅.The diffusion barriers of Na/K ions between GeSe/BC₅ layers are 0.302/0.098 eV,respectively.The diffusion barriers of Na/K ions on the outside surface of BC₅ are 0.219/0.106 eV,respectively.GeSe/BC₅ maintains a good charge-discharge rate with a notable increase in conductivity and a slight decrease in ionic diffusivity.The volumetric expansion rates of GeSe/BC₅ heterostructures inserted into Na/K ions were calculated to be 1.09%and 1.82%,respectively.Compared with two-dimensional?-GeSe as the anode material for NIBs/KIBs,the cyclic stability of GeSe/BC₅ heterostructures is better.The results indicate that GeSe/BC₅heterostructures,whether isolated two-dimensional?-GeSe or compounded with two-dimensional BC₅,can be used as anode materials for NIBs/KIBs with excellent performance.