First-principles Study On Design And Energy Storage Mechanism Of Phosphorus Anode For Lithium-ion Batteries

Phosphorus possesses the high theoretical capacity,relatively low and safe lithiation potential and good Li-ion conductivity,which is considered to be one of high-capacity and fast-charging anode materials with great research potential and application value for lithium-ion batteries.However,the huge volumetric expansion and dissolution of lithium polyphosphides during the charge and discharge processes reduce the cycling stability and high-rate performance,which limit its application.The researches on the material design and reaction mechanism of phosphorus anodes are crucial to improve the performance of batteries.Based on the first-principles calculations,the electrochemical performance and reaction mechanism of black phosphorus nanotubes（BPNTs）and green phosphorus（GP）were firstly investigated to provide research ideas and theoretical support for the design and optimization of phosphorus anodes.Then,the functional additives about their inhibitory effect on the dissolution of lithium polyphosphides and the promoting effect on the reaction kinetics were studied,to provide theoretical basis for the selection of functional additives and investigation of reaction mechanisms.The main research contents and conclusions are as briefly summarized in the following part.（1）Due to the anisotropy of black phosphorus,two kinds of BPNTs were taken into account in the present work by rolling up black phosphorene nanosheet along the armchair（called a-BPNTs（n₁,0））and zigzag（called z-BPNTs（0,n₂））directions,respectively.The effects of chirality,curvature,Li-storage concentrations and strain stress on the Li-storage performance of BPNTs were investigated.The results show that the（0,n₂）BPNTs have the best Li-storage capacity（649.0 m Ah/g）,compared with a-BPNTs（n₁,0）with similar diameter（432.7 m Ah/g）and two-dimensional black phosphorus（164.4 m Ah/g）.By analyzing the structural changes of BPNTs with different Li-intercalation concentrations or under different strain stress,it is found that z-BPNTs（0,n₂）exhibit good flexibility.In addition,the a-BPNTs（n₁,0）have excellent Li-ion conductivity,and the Li migration energy barrier along the c-axis direction is only 0.06 e V.Moreover,the structural stabilities and Li-ion conductivity of BPNTs are all related to curvature.When the curvature of BPNTs is less than 0.1,the structural stabilities of lithiated BPNTs are strong.However,the small curvature is not conducive to the Li-ion conductivity.（2）The Li-storage mechanism and properties of GP with different layers were investigated.It is found that single-layer（SL）GP is helpful to improve the theoretical specific capacity.The maximum theoretical specific capacity of SL GP is 432.7 m Ah/g,and the maximum theoretical specific capacities of double-layer（DL）and bulk GP are all 288.4 m Ah/g.The maximum specific capacity of bulk GP is higher than that of bulk black phosphorus（164.4 m Ah/g）,indicating that GP can be considered as an excellent anode material for Li-ion batteries with high specific capacity.In addition,GP also exhibits good Li conductivity as the Li-migration energy barrier in bulk GP along the zigzag direction is only 0.14 e V,which is smaller than 0.28 e V on graphene and0.23 e V on silicene.Moreover,studies find that the average lithiation voltage of GP decreases with the decreasing number of GP layers,indicating that the lithiation voltage can be reduced by reducing the thickness of GP layers.（3）The Li-containing compounds of Li F,Li₂O and Li₂S were selected as functional additives of phosphorus anode due to that they can keep stable during the charge and discharge processes.The effects of functional additives on the reaction mechanism for inhibiting the dissolution of lithium polyphosphides and on the reaction kinetics were mainly investigated.It is found that the adsorption of lithium polyphosphides on the surface of functional additives mainly depends on the strong chemical adsorption between Li in lithium polyphosphides and anions in functional additives.The strong chemical adsorption of lithium polyphosphides on the surface of functional additives can effectively inhibit the dissolution and shuttling of lithium polyphosphides.In addition,the functional additives can catalyze the delithiation process of high Li-intercalation concentration Li₃P and low Li-intercalation concentration Li P₇ and Li P₅,which is conducive to accelerate the reaction kinetics of delithiation and reduce the reaction overpotential.Among these three functional additives,Li₂O has the strongest adsorption of lithium polyphosphides and the lowest delithiation energy barrier of Li₃P,which make it the best functional additive for phosphorus anode.Based on first-principles calculations,the crystal structures,theoretical specific capacities,lithium intercalation voltages,Li-ion conductivity,as well as the action mechanism for functional additives of phosphorus anode have been studied in this paper.The relationship between crystal microstructures and intrinsic natures and macrostructures of electrode materials has been discussed,which provided research ideas and reliable theoretical basis for exploring and designing the high-performance phosphorus anodes.