Electrode Material Design And Energy Storage Mechanism For Lithium/Potassium Secondary Battery: A First-principles Study

Most of the world's energy consumption comes from fossil fuels,which are not renewable and are unevenly distributed across the planet.With the consumption of chemical fuels,the improvement of people's environmental awareness and the increase of energy demand,renewable energy sources such as solar,wind and tidal power are gradually developed and utilized.Because of the intermittent nature of these sources of energy,secondary battery?rechargeable battery?energy storage has become an integral part of making more efficient use of these renewable resources.However,with the device updating,the traditional lithium ion battery materials are more and more difficult to meet the increasing energy storage requirements in terms of energy density and cycle stability,so the research on the secondary battery system is very urgent.The traditional research is mainly based on experiments.However,with the continuous improvement of people's requirements on the performance of materials in all aspects and the deeper understanding of the reaction mechanism,the spatial scale of material research is also reduced,and the research on secondary batteries based on experiment alone cannot meet the needs of modern development.The first-principles calculation based on the density functional theory can simulate the materials at the atomic scale and predict the microstructure and properties of the materials.In this paper,the first-principles calculated was used to investigate the materials of lithium-and potassium-ion batteries.The basic scientific problems of secondary battery materials,such as crystal structure,phase transition,elastic properties,electrical conductivity,ion storage and migration mechanism,were explored from the atomic scale.While deeply understanding the operation mechanism of electrode materials,we also focus on screening and even designing potential secondary battery materials,providing practical theoretical guidance for the development of new materials,striving to improve the energy density and cycle life of the existing secondary battery system,and promoting the development of secondary battery field.Rechargeable lithium-ion batteries?LIBs?now dominate the portable energy storage market,appearing in many devices,including laptops,mobile phones,cameras and,most recently,electric cars.These batteries are well suited for these applications due to their reliable cycling characteristics and high energy density,which can be attributed to the high load ratio of lithium.Due to the non-uniform deposition of lithium metal cathode in the cycling process,lithium dendrites will grow,resulting in the decrease of coulomb efficiency and even safety problems.Researches on lithium ion battery mainly focus on the anode material.Although the traditional graphite material has excellent cycle life,the limitation of capacity cannot meet the demand of large-scale energy storage system gradually.A high capacity anode material requires a sufficient number of active sites to bind to lithium ions,or a specific lithium mechanism such as alloying or conversion reactions.The high degree transition metal sulfides,such as VS₄and TiS₄,belong to typical anode materials with high capacity.Because the conversion reaction of them is very complex,the experiment is difficult to directly observe their structures during lithiation.In order to solve these problems,we systematically predicted the intermediate phase structure of lithium VS₄ at the atomic scale for the first time,and further revealed the lithium mechanism during the conversion reaction.In addition,we were also committed to the development and design of new carbon anode materials to improve the capacity of battery while ensuring the cycling performance.The content of lithium in the earth's crust is only 20ppm.It has long been an important direction of secondary battery development to find an alternative to lithium ion batteries.Hence potassium-ion batteries have attracted wide attention.The main problem in potassium-ion batteries is the large size of potassium,which make it difficult to store or transport in electrode materials.In order to be comparable to lithium-ion batteries in ion storage and transport,the electrode material for potassium-ion batteries also requires a special structure or reaction type.For example,two-dimensional layered electrode materials have open space and rich chemosorption active sites.The organic-based electrode material is composed of organic molecules,which can spontaneously form suitable structures during the potassiation.A part of polyanion electrode materials in the potassium-ion battery can form a completely different structure from its lithium counterpart.These unique structures are very suitable for the storage and diffusion of large potassium ions.In view of these unique types,we selected representative materials and studied their electrochemical properties in potassium ion batteries through first-principles calculation,providing profound theoretical guidance for the reaction mechanism of negative electrode materials in potassium-ion batteries.The research contents of this paper are as follows:?1?The structural transformation of Li_xVS₄ electrode material during charging and discharging is illustrated by first principle calculation.The material underwent a two-phase transition from VS₄ to Li₃VS₄ in the initial stage of lithium,during which the anion redox reaction of S was accompanied.In the subsequent lithium process?x>3?,the material underwent a series of amorphous transformation.This is because the long-range migration of decomposition products in the decomposition process of material destroyed the long-range ordering of the material,during which V acted as the redox center.Li⁺mainly migrates between adjacent S atoms and has very low activation energy.These properties indicate that VS₄ is expected to be a potential cathode material with high capacity and high cycling performance in lithium-ion battery.?2?Based on the first-principles calculation,we observed the formation of a new structure during the?dis?charge T-carbon.This new structure belongs to the reported bct-C₁₆.Through molecular dynamics simulation,we predicted that T-carbon could be converted to bct-C₁₆ at 400 K.Bct-C₁₆ is a high temperature resistant material,which can maintain the topological stability of the structure at 2600 K.In the structure,the hybridization degree of s-p orbital of each carbon atom is lower than that of ordinary carbon materials.When used as the negative electrode of LIBs,bct-C₁₆ showed a two-phase transition C₃₂ to Li₈C₃₂ during lithiation,with a stable voltage platform of about0.40 V and the capacity of 558 mA?h?g^-1.Compared with the voltage of Li⁺deposition?0 V?,the voltage of 0.40 V is enough to prevent the growth of lithium dendrites.The interface of C₃₂ and Li₈C₃₂ is stable during the reaction process.Bct-C₁₆ can retain metallic properties during the lithium process.At different Li⁺concentrations,the Li⁺migration energy barrier in the material is about 0.5 eV,which allows Li⁺to achieve diffusion in bct-C₁₆.?3?The electrochemical properties of?-graphdiyne as anode material for potassium ion battery were studied by first principle calculation.The initial potassiation mainly depends on the ionic interaction between potassium and carbon,and then gradually transforms to the metallic interaction between different potassium.For?-graphdiyne monolayer,potassium ions can achieve two layers of chemisorbation on both sides of the material,corresponding to the theoretical capacity of 2870 mA?h?g^-1,and the potassium ion migration energy barrier is less than 0.25 eV.The trilayer?-graphdiyne has a theoretical capacity of 1700 mA?h?g^-1.The out-of-plane K⁺diffusion barrier is only 0.353 eV,while the in-plane migration between the two carbon layers has a large energy barrier of 0.666 eV,which leads to the unique"drop"migration characteristics of K⁺trilayer?-graphdiyne.The results show that?-graphdiyne is a potential anode material for potassium-ion batteries,with great capacity and good cycling stability.?4?Using the structure prediction software USPEX,we revealed a series of ground state structures of K_2+xC₆O₆ during potassiation.It is found that the insertion of K⁺can be divided into two phases,including two-phase transition from K₂C₆O₆ to K₃C₆O₆ and homogeneous reaction from K₃C₆O₆ to K₄C₆O₆.During the phase transition,the space groups of all stable structures are different,and organic molecules can spontaneously recombine with the change of K⁺concentration,providing a free volume change.A stable voltage platform first appeared in the material at 2.76 V,then gradually reduced to 1.06 and 0.74 V.The material shows a capacity of 217.8 mA?h?g^-1 at K₄C₆O₆.Bader charge analysis shows that C in the material acts as the redox center of the electrode reaction and O as the charge transfer medium between C and K.The flexible structural framework provides a feasible K⁺diffusion channel,so that the K⁺migration process has a low barrier.?5?The electrochemical properties of K_1-xVOPO₄ in potassium-ion battery were studied by first-principles calculation.The material can be divided into several processes during the extraction of K⁺.The first two-phase transition took place in the process of 0?x?0.5,and then transformed to the solid solution reaction at the stage of 0.5<x?0.625.The second two-phase transition occured at the stage of 0.625<x?0.75,and finally transformed to the solid solution reaction again at the stage of 0.75<x?1.The complete removal of K⁺from the material resulted in a volume change of only 6.6%,which contributed to the cycling stability of potassium-ion batteries.The density of states and Bader charge analysis revealed that both V and O were involved in the charge transfer process,in which V,as the redox center of KVOPO₄,played a major role in K⁺storage,and O became the charge transfer medium between V and K.In addition,at different K⁺concentrations,K⁺can be migrated in the one-dimensional channel of the material,and the energy barrier is only between 0.214 eV and 0.491 eV,which ensures good cyclic stability of the material.These works include the transition metal sulfides material with conversion reaction,the carbon material with two phase reaction,the two-dimensional carbon material,the polyanion material with a variety of reaction process,and the organic-based material with the alloy-like reaction.They cover almost all reaction types and material types for electrodes,which is of great significance to the understanding and development of secondary battery electrode materials.