Study On Design Synthesis,Modification And Electrochemical Performances Of Germanate Anode Materials

At present,with the continuous improvement of human living standards and the continuous exhaust of existing traditional energy storage device,the need to develop a low pollution,high efficiency economical new energy is very urgent.Lithium-ion batteries are getting more and more attentions as a kind of energy storage device that meets the requirements in the scientific community.However,the popular anode material of commercial lithiumion batteries is mainly graphite anode,its theoretical specific capacity is only 372 m Ah g^-1,which is difficult to meet the needs of people's daily life.Therefore,it is very important to develop a new type of anode material with high specific capacity,high energy density and long cycle life to replace graphite anode for the development and application of lithiumion batteries.Existing studies have shown that germaniumbased materials used as anode materials have obvious advantages.To be compared with other highcapacity anode materials(such as Ti,Sn,etc.),the germanium has a higher theoretical specific capacity(1623 m Ah g^-1),and germaniumbased materials have a high lithium-ion diffusion coefficient and excellent conductivity.The ion diffusion speed determines the rate of the alloying reaction between lithium and germanium,and the conductivity determines the electron transfer rate.These properties lead to germanium-based material to have high power density,high current charge and discharge capability.However,germanium also has its shortcomings.For example,its high price makes it difficult to control the cost as an anode material for lithiumion batteries.Therefore,our aim is to synthesize some germanate materials as anode to reduce the cost of lithium-ion battery.It can also ensure the excellent electrochemical performances of anode material.In addition,some special purpose batteries(such as artificial pacemaker and batteries in the aerospace field etc.)can take on higher cost.If this aim can be achieved,it will be of great significance to develop new kind of anode materials.In this paper,a series of germanate anode materials(Zn₂GeO₄,Cd₂Ge₂O₆,Mn₂GeO₄,PbGeO₃)with different morphologies(such as nanowires,microrods and so on)were synthesized by a variety of methods.During the experiments,XRD,SEM,XPS,HRTEM and constant current charge and discharge methods were used to characterize the structure,morphology and electrochemical properties of the materials.The relationship between electrochemical performance and morphology,and the electrochemical reaction mechanism are explored,also.The above materials were matched with commcial LiFePO₄ to make up full battery(button),and the performances of full battery(button)were tested also to evaluate synthesized anode materials.The important conclusions have been arrived.The main details are as follows:(1)The Zn₂GeO₄ with different morphologies(such as nanowires,micro clusters and micro particles)were successfully synthesized by a mixed solvothermal method,and the influence of the calcination temperature and raw materials on the morphology were explored.The relation between electrochemical performance and morphology of the material was analyzed by XRD data fitting calculation,constant current charge and discharge,and electrochemical impedance spectrum(EIS)test.It was found that the Zn₂GeO₄ with morphology of nanowires had better electrochemical performance than those of other Zn₂GeO₄ with morphology of micro clusters or micro particles.The results show that ZGO-650 sample behaved best electrochemical properties among all nanowire samples.At a current density of 100 mA g^-1,the specific capacity can still maintain at 1246 m Ah g^-1 after 150 cycles.It behaved good rate performance also.The full battery behaved good cycle and rate performances too,so ZGO-650 can be used as promising anode material in lithium-ion battery system.(2)The Cd₂Ge₂O₆ with morphology of miro spindle was successfully prepared by hydrothermal method combined with calcination.It was found that p H value of reaction system had influence on morphology and electrochemical properties of the Cd₂Ge₂O₆.The glucose was used as carbon sources to modify Cd₂Ge₂O₆to obtain Cd₂Ge₂O₆/C composite.The XRD,SEM,HRTEM,TG and electrochemical techniques were used to characterize prepared composit.The results showed that the impedance of Cd₂Ge₂O₆/C was smaller than that of pure Cd₂Ge₂O_6,which can improve electrochemical properties of Cd₂Ge₂O₆ greatly.When the current density was 100 m A g^-1,the initial specific capacity arrived at 2121.9 m Ah g^-1,it can maintain at 727 m Ah g^-1 after 150 cycles and the rate performance of Cd₂Ge₂O₆ was also improved greatly.The Cyclic Voltammetry(CV)technique was used to discuss electrochemical behaviors and mechanisms.The full battery behaved good cycle and rate performancestoo.Therefore,Cd₂Ge₂O₆/C composite is a potential lithi?m ion anode material for future application.(3)The Mn₂GeO₄ samples with different morphologies(such as one-dimensional nanowires,two-dimensional nanoplatches and irregular nanoparticles)were successfully prepared by hydrothermal method combined with high temperature annealing.The XRD and SEM techniques were used to detect structure and morphology of Mn₂GeO₄ samples.The electrochemical performances of Mn₂GeO₄ samples with different morphologies were tested by constant current charge/discharge system.The experimental results showed that Mn₂GeO₄ sample with morphology of two-dimensional nanoplatches behaved best electrochemical properties among three Mn₂GeO₄ samples.Therefore,carbon nanotubes(CNTs)were used to composite with two-dimensional nanoplatches(Mn₂GeO₄)to improve its electrochemical properties.It was found that initial discharge specific capacity of Mn₂GeO₄/C composite arrived at 1847.9 mAh g^-1 and it could maintain at about 1001 mAh g^-1 after 100 cycles.The charge platform is about 1.2 V.The Mn₂GeO₄/C composite owned three-dimensional net structure,which can alleviate stress resulted from volume expansion of the electrode during discharge/charge process and keep the electrodes from collapsing.The Cyclic Voltammetry(CV)technique was used to discuss electrochemical reaction mechanisms.The full battery behaved good cycle and rate performancesalso.Therefore,Mn₂GeO₄ is a potential lithiumion anode material for future application.(4)The PbGeO₃ material were successfully prepared by a simple one-step hydrothermal method.The PbGeO₃ with morphology of nanorods was regulated and controlled by adjusting the concentration of ethylenediamine(EDA).The polypyrrole(PPy)was used as carbon sources to modify PbGeO₃ nanorods.The XRD,SEM and TEM techniques were used to characterize prepared composit.The content of carbon in the composite was tested by TG.The electrochemical performance was tested by battery comprehensive testing system.It showed that the electrochemical performance of the sample modified by carbon coating was obviously improved.At a current density of 100 mA g^-1,the initial specific capacity was 1715.7 mAh g^-1 and it can maintain at 887 mAh g^-1 after 150 cycles,and the rate performance was improved greatly also.And the electrochemical modification mechanism was studied cyclic voltammetry(CV)technique also.But the cycle and rate performances of full battery need to be improved further.Therefore,PbGeO₃/C is a promising lithi?m ion anode material,which is worthy to be developed and applied in lithium-ion battery system.(5)Cobalt germanate(Co₂GeO₄)was successfully synthesized by hydrothermal method combined with heat treatment.Subsequently,the morphology of the product was regulated and controlled by adding ethylenediamine tetraacetic acid(EDTA).The structure and morphology of the samples were characterized by XRD and SEM techniques.The constant current charge/discharge test results showed that the electrochemical performances of Co₂GeO₄ nano particles were significantly improved to be compared with micro Co₂GeO₄lumpy material.When the current density is 100 mA g^-1,the innitial specific discharge capacity was 1811.1 mAh g^-1.After 150 cycles,the specific discharge capacity still remained at 1198 mAh g^-1,and the rate performance was also significantly improved.The related electrochemical mechanisms were disscussed by cyclic voltammetry(CV)method.However,the full battery test results showed that the cycle and rate performances of full battery need to be improved further.