Study On Molybdenum-Based Electrode Materials For Li/Na-Ion Batteries

With the rapid development of society in recent years,the demand of energy storage systems with high energy density,large capacity and high power battery is becoming more and more urgent.The development of electrode materials with excellent performance is the key to manufacture high-performance rechargeable batteries,which also poses new challenges for researchers all around the world.Mo-based electrodes including:MoS2?MoSe2?MoO2?MoO3?LiMoO2?Fe2(MoO4)3 and so on,possess multiple valence states and exhibit rich chemistry,which are ideal candidates for electrochemical energy storage systems.However,when applied in practical battery systems,these materials usually suffer from low conductivity,large volume change during cycling,low capacity and bad rate capability.In this dissertation,we have studied to improve electrochemical performance of Mo-based materials including MoO2?MoSe2?MoXW1-xSe2 with the use of strategy including:carbon material compounding.oxygen defect introduction,morphology tunning and W-doped molybdenum-based materials.Besides.Li+/Na+ storage mechanisms have been dicussed based on XRD,EXAFS and Raman techonology.The details of this dissertation are summarized briefly as follows:1.We have synthesized amorphous carbon coated MoO2 uniformlly dispersed in three dimensional graphene aerogel by using solvo-themal method.The characterization results show that the size of the MoO2 range from 20 to 50 nm,and they were double-coated by thin amorphous carbon with thickness of 1-2 nm and graphene sheets.The hybrid electrode exibit a supior cycle performance and rate capability when appied in anode material for LIBs.When tested at current density of 80 mA g-1,the electrode delivered a discharge capacity of 1113 mAh g-1 after 60 cycles.Even at a high current density of 1.2 A g-1,a discharge capacity of 440 mAh g-1 can still be achieved.Thus,this work not only improves the electrochemical performance of MoO2 material as an anode material for LIBs,but also provides a new reference for the synthesis of carbon coated transition metal oxides.2.We have developed hydrothermal methods to synthesis flower-like MoO2 growing on graphene sheets,XANES,EXAFS and ESR data suggest there are rich oxygen vacancies in as prepared MoO2.When tested as anode materials,as prepared electrode material shows better electrochemical properties than the material without introduction of oxygen vacancies.Our ex-situ XRD and ex-situ XAFS illustrate that Li+ insertion result in the transition between MoO2 and Li2MoO4,and MoO2 fully converted to Li2MoO4 at the first 29th cycle,thus cause the increasing of the capacity.The change between MoO2 and Li2MoO4 was contributed to the introduction of oxygen vacancies.The proposed mechanism provides a new thinking for understanding the intercalation/deintercalation behavior of Li+ in oxygen vacancies-containing oxides.3.We have prepared semi-open MoSe2@ N-doped carbon hollow structure,through a complexation reaction between MoO42-and dopamine.The introduction of nitrogen-doped carbon improves the overall electrical conductivity of the material.Then,the semi-open hollow sphere increased the contact of the electrode material with the electrolyte.Moreover,the hollow structure can buffer the volume expansion during the insertion and extraction of sodium ions.Thus,the as-prepared electrode materials possess a discharge capacity of 378 mAh g-1 after 120 cycles at a current density of 200 mAh g-1.The surprising part is even tested at a current density of 20 A g-1,the electrode have delivered a discharge capacity of 199 mAh g-1.Besides.We implied in-situ Raman and ex-situ XRD technology to understanding the insertion and extraction process of Na+.The results suggested that during the discharge process,MoSe2 changed to Mo and Na2Se below 0.5 V.However,reversible reaction was not happened between MoSe2 and Mo and Na2Se.The in-situ Raman clearly shown Raman peaks of Se appeared during the chage process.In the subsequent cycles,the insertion and extraction process of Na+ turned into alloy reaction between Se and Na.4.We have designed hollow structure composite of W doped MoSe2 embedded on nitrogen dopped carbon sheetswith the use of a selenization process.By tuning the ratio of Mo and W,Mo1-XWXSe2 with different atomic ratios has been synthesized.When tested as anode material of LIBs,the Mo0.75W0.25Se2 delivered the best cycle and rate capability compared to pure MoSe2@C nanospheres,Mo1-XWXSe2 with other atomatic ratios.Even tested for 260 cycles at a current density of 0.5 A g-1,the Mo0.75W0.25Se2 electrode can still obtained a discharge capacity of 640 mAh g-1.The excellent electrochemical performance of W doped MoSe2 can attribute to:First,Mo1-XWXSe2 possessed larger space spacing in contrast to MoSe2,which is benefit for the transfer of electrons and ions.Then,voids in the material made as-prepared electrode contact better with electrolyte.What is more,the hollow structure alleviated the volume change of the electrode material effectively during the insertion and extraction of Li+.