Syntheses And Electrochemical Performances Of Novel Vanadium- And Germanium-based Anode Materials

The growing demand for renewable energy to replace traditional fossil fuels and associated large-scale energy storage systems drives the development of today’s battery technologies.As the trend-setter of rechargeable batteries,lithium-ion batteries（LIBs）dominate the market of portable electronic products and electric vehicles.However,there is incresing concern about the cost and resource availability of lithium.Sodium-ion batteries（SIBs）have similar electrochemical properties to LIBs,as well as the wide availability and accessibility of sodium.It is recognized as the most promising alternative energy for commercial LIBs at present.Nowadays,in order to meet the growing demand of energy storage market,higher requirements are put forward for the performance of energy storage equipment.The key to improve its performance lies in electrode materials.Therefore,the design and research of appropriate electrode materials plays a crucial role in realizing high-performance batteries.In this dissertation,novel anode materials for LIBs and SIBs are explored and studied.The concerning material systems include binary metal vanadates,organic-vanadium hybrids,and layered binary metal germanates.The main research contents and results are as follows:（1）Irregular shaped Mn V₂O₄ blocks are synthesized by an efficient sol-gel combined with calcination process for the first time,and then their electrochemical performance is investigated as an anode material for LIBs.GITT,CV and SEM are executed to ascertain the electrochemical kinetics characteristics and morphologies of Mn V₂O₄ electrodes with different cycles in greater depth.It shows that the increasing specific capacity of Mn V₂O₄electrode during the charging and discharging process is due to the pulverization of Mn V₂O₄blocks（activation process）.In this process,both surface capacitive-controlled and diffusion-controlled processes contribute to the improvement of the specific capacity,but surface capacitive-controlled plays a leading role in the early stage,and diffusion-controlled process in the later stage.Based on the CV curves and in situ XRD technique,the lithium storage mechanism of Mn V₂O₄ is recognized as the coexistence of conversion reaction and solid solution behavior.（2）The MnV₂O₆ nanoflakes are prepared by one-step hydrothermal method.When tested as an anode for LIBs,it delivers high reversible specific capacity,favorable rate capability and long-term cycling stability.The CV curves at different sweep speeds,morphology variations during cycling and Nyquist plots after different cycles are analyzed in detail.The results make known that the pulverization of nanoflakes in the cycling process will shorten the diffusion distance of lithium ions and make it easier to transport.In this process,as the number of cycles increase,the proportion of diffusion-controlled process contributing to the capacity is expanding.The lithium storage mechanism of Mn V₂O₆ involving conversion reaction and solid solution behavior has been affirmed by in situ XRD measurements.Additionally,the practical application of Mn V₂O₆ nanoflakes as anode material for LIBs is discussed.The NCM811//Mn V₂O₆ full cell can light a LED and exhibit excellent lithium storage performance.（3）Fe₂V₄O₁₃ with a horseshoe-like chain structure is fabricated by hydrothermal method combined with calcination process,and it is used as anode material of LIBs for the first time.Fe₂V₄O₁₃ electrode exhibits a high initial reversible specific capacity of 1213 m Ah g^-1 at 100m A g^-1 with a Coulombic efficiency of 77.0%,which should be derived from the distinctive structure and multivalent metal elements of Fe₂V₄O₁₃.In addition,the electrode also has excellent rate capability and cycle performance.It is worth noting that the Fe₂V₄O₁₃ electrode always manifests ever-increasing specific capacity after a period of attenuation regardless of whether it is cycled under low or high current density,which benefits from the continuous activation during cycling.In this process,the proportion of pseudocapacitance contributing to the capacity is increasing.（4）Vanadyl acetate（VO（CH₃COO）₂,VA）nanobelts,an organic-vanadium hybrid material,are synthesized by solvothermal method and used as anode material for LIBs firstly.Without extra conductive additives,the VA nanobelts electrode displays outstanding electrochemical performance after a period of activation.Based on the analysis of the electrochemical kinetic process and the morphology evolutions of the VA electrode during the cycles,it can be concluded that the improved electrochemical performance of VA electrode after discharge/charge cycles is predominantly ascribed to the pulverization of VA nanobelts.In this process,the increased capacity primarily derives from capacitive contribution.According to the in situ XRD test of the first two cycles and CV curves of similar shape,the lithium storage mechanism of VA is confirmed to be intercalation reaction.At the same time,we also calculate the lattice parameters in the process of charge and discharge.The results show that the volume expansion ratio of VA electrode is low during discharge/charge process,that is,the insertion/extraction of Li⁺ions have little effect on the VA structure and stress generation during electrochemical process,which is exceedingly vital for harvesting excellent lithium storage performance.（5）Layered Ni₃Ge₂O₅（OH）₄nanosheets is prepared by hydrothermal method and used as anode material of SIBs for the first time.The sizes and layers of Ni₃Ge₂O₅（OH）₄ nanosheets are regulated by controlling the hydrothermal reaction temperature,and the optimal synthesis temperature is confirmed according to HRTEM,BET and electrochemical performance characterization.Subsequently,Ni₃Ge₂O₅（OH）₄/KB nanocomposite is synthesized by introducing carbon source（Ketjen black,KB）in situ at this temperature.Compared with the unmodified Ni₃Ge₂O₅（OH）₄nanosheets,the Ni₃Ge₂O₅（OH）₄/KB nanocomposite demonstrates higher reversible specific capacity,better cycling stability,and rate capability.The excellent sodium storage performance can be attributed to the synergistic effects of bare Ni₃Ge₂O₅（OH）₄nanosheets and Ketjen black.Ketjen black,as a conductive matrix,is uniformly attached to the Ni₃Ge₂O₅（OH）₄ nanosheets,further increasing the specific surface area,improving electronic conductivity,and accommodating volume change during cycling.The sodium storage mechanism of Ni₃Ge₂O₅（OH）₄ is confirmed to be the coexistence of conversion reaction and alloying/de-alloying reaction by CV and ex situ XRD test.（6）CGH,CGH@C,CGH/rGO,and CGH@C/rGO are selectively prepared by hydrothermal method using graphene oxide（GO）and L-ascorbic acid as carbon sources,and they are used as anode materials for SIBs firstly.Compared with CGH,CGH@C and CGH/rGO,the stability of CGH@C/rGO composites has been greatly improved,which is attributed to the strong interaction between CGH,carbon,and graphene.Specifically,the CGH nanosheets are encapsulated in a carbon matrix derived from the decomposition of L-ascorbic acid to form irregular aggregates,and rGO acts as a conductive bridge for these aggregates to connect them.The existence of double carbon enhances the electronic conductivity,relieves the volume expansion aroused by sodiation/desodiation,shortens the pathway of electron/ion transportation,thereby further improving the reaction kinetics and endowing the material with remarkable cycling capability.In addition,the sodium storage mechanism of Co Ge O₂（OH）₂ as anode for SIBs is identified to be the coexistence of conversion reaction and alloying/de-alloying reaction by CV curves and in situ XRD.