Study On Transition Metal Sulfides/nitrides And Their Derived Heterostructures In Electrochemical Energy Storage

Recently,lithium-ion batteries（LIBs）have been applied to large-scale energy storage power systems due to the high energy density and high operating voltage,etc.Nevertheless,the low power density of LIBs has severely limited their application in high-power output devices.Supercapacitors（SCs）have successfully overcome this problem and have been applied in power systems and portable electronic devices.However,they are limited by low energy density compared to LIBs.Therefore,the co-development of LIBs and SCs can make them favorable to play their individual advantages to use new energy efficiently.Among many factors that affect the electrochemical performance of these two electrochemical energy storage（EES）devices,the design and construction of electrode materials are crucial.Recently,transition metal sulfides（TMSs）are used as electrode materials for LIBs and SCs due to their high specific capacity and rich morphology.However,the poor conductivity results in the inferior rate performance and cycling stability of TMSs;transition metal nitrides（TMNs）are also reported in LIBs and SCs because of their excellent conductivity and good thermal stability.Nevertheless,the specific capacity of TMNs is low due to severe agglomeration.Therefore,researchers usually modify TMSs or TMNs to enhance electrochemical performance.Based on this,these two materials are used in this thesis as electrode materials.Firstly,the construction of heterostructures has been used to enhance electrochemical performance;then,the origin of the differences in EES of electrode materials with various crystal structures was investigated.The main research contents are as follows:1)The electrochemical performance was optimized by introducing metallic Mo N into MoS₂ to construct the MoS₂/Mo N heterostructure.The experimental results show that the MoS₂/Mo N heterostructure features a specific discharge capacity of up to819.90 m Ah/g at 0.1 A/g,and the rate performance and cycling stability are much better than MoS₂.Density functional theory（DFT）results further demonstrate that the MoS₂/Mo N heterostructure shows a theoretical specific capacity of 836.74 m Ah/g at the maximum Li⁺adsorption concentration,which is mainly owing to the synergistic effect between MoS₂ and Mo N.In addition,the Mo N shows excellent conductivity and rate performance owing to its metallicity.Meaniwhile,MoS₂/Mo N heterostructure shows much better rate performance than MoS₂ due to the presence of built-in electric field and metallic Mo N as well as the small Li⁺migration barrier（0.17 e V）.Therefore,introducing metallic Mo N can improve electrochemical performance of MoS₂efficiently.2)According to the first work,we concluded that the hexagonal Mo N possesses excellent rate performance due to its metallicity.However,it is found that except for hexagonal Mo N,cubic Mo₂N also features metallicity.Therefore,taking cubic Mo₂N and hexagonal Mo N as research targets,we have systematically investigated the origin of the differences in rate performance and specific capacity of these two molybdenum nitride electrode materials with different crystal structures.The experimental results show that the hexagonal Mo N features better rate performance than the cubic Mo₂N,which is attributed to its higher conductivity（1.03×10⁵ S/m）and up to 93.79%capacity retention at 10 A/g.Meanwhile,the cubic Mo₂N possesses an electrochemical specific capacity of 201.40 F/g at 0.5 A/g due to the larger specific surface area and more surface-active sites,which is much higher than that of the hexagonal Mo N.DFT calculations further indicate that the hexagonal Mo N shows better rate performance than the cubic Mo₂N,which is attributed to the smaller electron effective mass（0.306m_e）and K⁺migration energy barrier（0.03 e V）.More importantly,the rigid band approximation（RBA）method proves that the difference in work function change（ΔWF）is the root cause of the difference in pseudocapacitance(C_redox)between the cubic Mo₂N and the hexagonal Mo N.It is due to the fact that the cubic Mo₂N shows a higher theoretical total capacity(C_total=219.39 F/g)because of its smallerΔWF.Besides,the RBA method calculations reveal for the first time that the proportions of C_redox are 86.45%and 84.57%for Mo₂N and Mo N,respectively,indicating that these two materials are typical pseudocapacitive electrode materials,and the pseudocapacitive behaviour of the cubic Mo₂N is more obvious.Hence,the above findings will provide a new research idea for screening high-performance pseudocapacitive electrode materials with different crystal structures.3)Based on the second work,we used another molybdenum nitride（Mo₃N₂）with more active-sites as research target,and systematically investigated its electrochemical performance.The experimental results show that the Mo₃N₂ electrode exhibits an electrochemical specific capacity of 188.10 F/g at 0.5 A/g,and conductivity of up to4.55×10⁴ S/m reveals its good rate performance.DFT calculations further suggest that the Mo₃N₂ electrode possesses a theoretical maximum specific capacity of 219.39 F/g under full K⁺coverage.In addition,the Mo₃N₂ electrode shows excellent rate performance owing to its metallicity and smaller K⁺migration energy barrier（0.26 e V）.Therefore,DFT calculations provide the strong theoretical support for the experimental results and make the results more reliable.4)Based on the modification of the first work,we constructed the Mn S/Ni Co₂S₄full-sulphide heterostructure by introducing metallic bimetallic sulfide（Ni Co₂S₄）into monometalic sulfide（γ-Mn S）to further improve the electrochemical performance.The experimental results indicate that the Mn S/Ni Co₂S₄ heterostructure shows a high electrochemical specific capacity of 2291 F/g at 1 A/g,which is much higher than that of Mn S（1878 F/g）and Ni Co₂S₄（1255 F/g）.Meanwhile,the Mn S/Ni Co₂S₄heterostructure shows outstanding rate performance owing to the small charge transfer resistance,and its capacity retention still remains at 67.69%after 3000 cycles.In addition,the Mn S/Ni Co₂S₄ heterostructure widens the potential window from 0.5 V to0.55 V in 1 M KOH electrolyte.DFT calculations further demonstrate that under the synergistic effect between Mn S and Ni Co₂S₄,the Mn S/Ni Co₂S₄ heterostructure exhibits much better electrochemical performance than Mn S and Ni Co₂S₄.Meanwhile,the Mn S/Ni Co₂S₄ heterostructure exhibits superior rate performance,which can be attributed to the presence of built-in electric field and metallic Ni Co₂S₄,as well as the low migration energy barrier（0.55 e V）of OH^-.Hence,the above results once again demonstrate that introducing metallic materials into transition metal sulfides to construct heterostructure can enhance electrochemical performance of EES devices effectively.