Study On Electrochemical Energy Storage Of Molybdenum-Based Nanomaterials For Lithium-Ion Capacitors

Lithium-ion capacitor（LIC）,as a new device with dual characteristics of lithium-ion battery and supercapacitor,shows great potential in energy and power density.Therefore,it is of great significance to study complementary hybrid devices in the field of energy storage.Although the current research on LIC has achieved fruitful results,the slow Faraday reaction in the battery negative electrode and the fast non-Faraday adsorption and desorption process in the capacitive positive electrode cause a serious dynamic imbalance between the positive and negative electrodes of the device,which has caused many restrictions on the practical application of hybrid devices in the field of“green and sustainable”energy storage.Thus,if you want to build a high-performance lithium ion hybrid capacitor,you must find positive and negative electrode materials with better performance.In LICs,the negative electrode material as the core electrode should have the following characteristics:（1）low lithium storage potential and high reversible specific capacity;（2）high first-time Coulomb efficiency and obvious capacitive effect;（3）the negative electrode material can efficiently match the positive electrode in terms of rate and cycle performance.In view of the above,the research work of this paper will devote to the design and development of molybdenum-based compound negative electrode materials that can significantly improve the overall performance of lithium-ion capacitors.At the same time,the differences in the electrochemical performance of the single-phase four kinds of molybdenum-based compounds（Mo₂N,MoO₂,MoS₂ and MoSe₂）were investigated in terms of their electronegativity,conductivity and micro-morphology.Then,the molybdenum-based compounds（MoO₂ and MoS₂）with poor electrochemical properties were introduced into carbon materials to construct a multi-dimensional hetero-hybrid three-dimensional composite electrode structure to improve their electrochemical performance such as cycle and rate.Subsequently,the lithium storage mechanism,electrode dynamics analysis,and capacitance contribution of the four molybdenum-based anode materials were systematically studied.Finally,four kinds of molybdenum-based compounds with excellent performance were used as negative electrodes,and commercial activated carbon（AC）with high specific surface area,reasonable pore size distribution and high specific capacitance was used as positive electrodes to sequentially assemble and construct high-performance lithium-ion capacitor devices.The main research contents are as follows:（1）Mo₂N nanobelts rich in mesoporous structure and interlaced and interconnected were prepared by solvothermal method and nitridation method,such disordered and intersecting mesoporous structure is very favorable for the transport and diffusion of Li⁺in the electrolyte.As a result,the electrochemical performance and kinetic analysis of the assembled Mo₂N half-cell reveal that the Mo₂N electrode exhibits a dominant pseudo-capacitive energy storage characteristic and exhibits a discharge specific capacity of 299.3 mAh g^-1 after 400 cycles at0.1 A g^-1.Even with a current density of 5.0 A g^-1,there is still a reversible capacity of 119.0mAh g^-1.At the same time,it has a high reversible capacity of 115.3 mAh g^-1 after 1200 cycles at 1.0 A g^-1.Moreover,the Mo₂N//AC device constructed with a Mo₂N negative electrode and a hierarchical porous structure AC positive electrode with a large specific surface area and typical double-layer capacitor energy storage behavior has a working range of 4.5 V,an energy density of up to 139.75 Wh kg^-1,a power density of 11.25 kW kg^-1 and an excellent specific capacity of 70.14%after 2000 cycles is maintained.（2）A new strategy of synthesizing MoO₂@CNT@C nanoclusters with disordered structure was proposed by a two-step method of hydrothermal combined sintering.Its detailed micro-morphology is composed of clusters formed by a large number of loose nanoparticles and some nano-spheres with smooth surfaces and chaotic cross-linked coexisting structure of nanotube structure,which is more conducive to the formation of pores,and thus the ions in the electrolyte can be quickly transported at the material interface and the lithium ions can be quickly intercalated/de-intercalated.Glucose forms a carbon protective layer after carbonization,which can suppress the volume expansion of MoO₂ during the cycle,and the addition of CNT improves the conductivity of the overall electrode.The reversible capacity of the obtained material after charging and discharging for 300 cycles at a current density of 0.1 A g^-1 was 381.0mAh g^-1.MoO₂@CNT@C//AC LIC assembled with commercial activated carbon later has a maximum energy density of 48.29 Wh kg^-1,a maximum power density of 3600 W kg^-1,and an outstanding capacity retention rate of 82.26%after 6000 cycles.This research work not only improves the electrochemical performance of molybdenum dioxide negative materials for lithium ion capacitors,but also provides a new reference for the preparation of carbon-coated metal oxide composite electrodes.（3）MoS₂@C composite electrode material with a typical two-dimensional layered structure was prepared by solvothermal method and heat treatment process.The combination of this three-dimensional hollow structure and the strong Mo-O-C bond can well alleviate the strain caused by the volume change during the charge and discharge process,and the ultra-thin MoS₂ nanosheets and continuous carbon matrix ensure that the composite electrode has high electrochemical reactivity,fast charge transport,and significant contribution of pseudo-capacitance.The prepared MoS₂@C hybrid hollow ball electrode can maintain a high reversible capacity of 341.7 mAh g^-11 even at a high current density of 5.0 A g^-1,and shows good rate performance.When cycling at a current density of 0.5 A g^-1,the MoS₂@C hybrid hollow sphere composite electrode still has a reversible capacity of 433.6 mAh g^-11 after 800 cycles,showing excellent cycling performance of the composite electrode.In order to further explore the application potential of the MoS₂@C material,we assembled and tested the MoS₂@C negative electrode and AC positive electrode in a full cell and tested its electrochemical performance（MoS₂@C//AC LIC）.The constructed LIC is based on the excellent performance of the positive and negative electrodes and has a large potential window of 4.5 V.It exhibits a high energy density of 189.68 Wh kg^-1,an excellent power density of 11.25 kW kg^-1,and a good capacity retention rate of 72.12%after 3000 cycles.（4）Using MoSe₂ as the research object,a three-dimensional self-assembled hetero-structured MoSe₂ nanoflower was prepared using a simple,efficient,and mild hydrothermal system,and was first applied to LIC negative materials.Then,the mechanism of lithium storage reaction was studied by ex-situ XRD and XPS,and it was concluded that the reason for the increase in capacity in the later cycle was due to the precipitation of Se.Next,the kinetic analysis and electrochemical results indicate that the lithium storage mechanism of the MoSe₂nanoelectrode is the dominant pseudocapacitive controlled conversion behavior,and it can exhibit discharge specific capacity 641.4 mAh g^-11 after cycling 200 times at a current density of0.1 A g^-1.Moreover,the MoSe₂//AC hybrid device constructed by this material and a porous structure AC exhibits a large working potential window of 3.0 V,and brings high energy density and large power characteristics,that is,the highest energy density that can be achieved at a power density of 150 W kg^-1 is 78.75 Wh kg^-1,while at a high power density of 3600 W kg^-1,it can still maintain a high energy density of 39.10 Wh kg^-1,and excellent cyclic stability up to70.28%after 5000 cycles.