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Electrochemical Characterization Of Three Dimensional Nanofibers Electrode Fabricated By Electrospinning

Posted on:2009-01-07Degree:DoctorType:Dissertation
Country:ChinaCandidate:H W LuFull Text:PDF
GTID:1101360272958905Subject:Materials Physics and Chemistry
Abstract/Summary:PDF Full Text Request
The oil crisis forced people to find new alternative energy sources since 1960s. Lithium-ion battery with the significant advantages of high voltage,large capacity, high energy density and smooth discharge current have received widespread attentions and have been quickly commercialized.In the last decade,the continuing growth of integrated circuit(IC) and micro-electromechanical systems(MEMS) industries had an enormous impact on a higher demand of new embedded batteries. Lithium ion batteries use insertion processes for both the positive and negative electrodes,leading to the term 'rocking chair' battery.The resulting transport of Li ions between the electrodes,usually arranged in a parallel-plate configuration,is 1D in nature.To minimize power losses resulting from slow transport of ions,the thickness of the insertion electrodes,as well as the separation distance between them, is kept as small as possible.This approach may appear counterintuitive in the effort to produce a useful battery,because reducing the thickness of the electrode results in lower energy capacity and shorter operating time.Thus,battery design always trades of between available energy and the ability to release this energy without internal power losses.In recent years there has been the realization that improved battery performance can be achieved by reconfiguring the electrode materials currently employed in 2D batteries into 3D architectures.The general strategy of this approach is to design cell structures that maximize power and energy density yet maintain short ion transport distances.A lot of research works on the three-dimensional battery electrode materials and their performance tests.The realization of 3D design is still long way to go, mainly because many are unknown about the materials' physical and chemical properties in the 3D architecture.These length scales and geometries will determine the performance characteristics of 3D batteries based on these architectures.At present,only the component arrays of the periodic interdigitated electrode have been fabricated using lithographic or template methods,with aspect ratio about 10 to 100,which was not sufficient to present the advantages of 3D electrode.Another method known as electrospinning was chosen in this thesis to construct 3D electrode with longer and thinner 1D nanofiber.Electrospinning technology is a soft chemical method,which can fabricate large aspect ratio(up to 100000) organic or inorganic nanofibers materials.Comparing with other nanoscale material fabrication technology such as templates method and lithography method,the electrospinning technology is low cost equipped,easy to operate with high deposition rate and has good control of composition.In this paper,a homemade electrostatic spinning combined with sol-gel method was used to fabricate a number of metal oxide nanofibers.The nanofibers were then constructed into three-dimensional(3D) network structure and assembled into a lithium-ion battery as electrode,and their electrochemical performance was tested.Several 3D nanofibers electrode modification method were also investigated including doping,solid electrolyte coating and carbon nanotubes enhancement,some were found effective in improving the electrochemical properties for the 3D battery materials.In order to use the electrospinning method to fabricate large aspect ratio 3D electrode materials,Ti(OiPr)4-PVP system was first investigated for its better preparation properties.The Ti-PVP nanofibers were obtained by properly adjusted the concentration of the precursor and its electrospinning parameters.Three-dimensional network structure composed of the nanofibers was successfully built by a set of paralleled collectors.After annealing at 500℃,PVP was removed and anatase 3D-TiO2 nanofibers electrode was left with its morphology perfectly reserved,which showed discharge capability under large current density.The capacity was 153mAh/g in the first cycle,but it soon decreased in the following cycles.SEM illustrated that the 3D anatase TiO2 structure collapsed after lithium-ion intercalation and deintercalation.The collapsing is mainly because of the volume expansion induced stress and the poor uniformity of Li+ intercalation and deintercalation on the nanofibers surface during charge and discharge.A 3D battery design parameter U (U=(r2/L2)(μ/σ)(1/C)) was used to estimate the uniformity of Li+ insertion,the smaller of U the better of uniformity.According to this,a doping method was used to increase the conductance(σ) of 3D-TiO2.Sn(OiPr)4 was added into the precursor by 5%.The electronic conductivity of the SnO2 doped 3D-TiO2 can be improved with small polarization show in the charge and discharge curves.But its cycle performance is still poor,indicated that the structure stability was not improved by doping method.The 'zero strain' spinel Li4Ti5O12 was investigated considering the physical structure and interface stability of it during charge and discharge process.The lithium salt influenced the properties of precursor;a series of change had to be made by add acetic acid to enhance the complexity and strict control of aging time of the precursor. Based on these changes in precursor,three-dimensional structured nanofibers of Li-Ti-PVP were successfully fabricated for the first time.The nanofibers kept unchanged in length,and reduced to 100nm in diameter after 750℃annealing.The 3D Li4Ti5O12 electrode showed a good charge and discharge performance under large current density,the first discharge capacity under 4.5C was 167mAh/g,cyclic performance is about 27%better than the thin film Li4Ti5O12 anode.XRD and SEM images show that the structure of 3D spinel Li4Ti5O12 network keep constant during charge and discharge processes,which revealed the zero strain characteristics of Li4Ti5O12 nanofibers.Preliminary results showed that the Li4Ti5O12 is one of promise candidate anode materials for three-dimensional lithium-ion battery.The electrospinning of nanofibers method was used to investigate cathode materials.LiCoO2 was the first choice for its high discharge voltage,high conductance and good cycle ability.The precursor of LiCoO2 nanofibers was composed of Li(CH3COCHCOCH3) and Co(CH3COO)2·4H2O,which was very sensitive to electrospinning parameter and environment conditions.For this reason,former reported electrospun of LiCoO2 nanofibers had a large diameter in micrometer or sub micrometer scale.To get a better aspect ratio(lower the r2/12),the setting of the electrospinning equipments was adjusted to raise the temperature and controlled moisture when electrospinning,and proper annealing process was also applied.Thus, for the first time,3D architectures of layered LiCoO2 nanofibers with the sizes of 60nm to 80nm in diameter was prepared by an electrospinning method for 3D rechargeable lithium ion batteries.But electrochemical measurement showed poor performance compared with thin film LiCoO2.SEM observation also found structure collapsed after charge and discharge process.In order to resolve the structure instability induced by lithium intercalation and deintercalation,and to protect the electrode interface from the liquid electrolyte,a lithium phosphorous oxynitride(LiPON) layer was coated onto 3D structure.3D electrode of electrospun LiCoO2 nanofibers with a fully coating LiPON layer exhibited the discharge capacity of 120.4mAh/g with the loss 0.11%per cycle during the 100th cycle at the discharge rate of 0.05mA/cm2,and had a better rate capability and higher reversibility as compared with electrospun LiCoO2 nanofibers without LiPON layer.The electrochemical test under 0.10mA/cm2 and 0.15mA/cm2 also showed good rate capability.These results indicated that the effectiveness of a coating LiPON layer for application of LiCoO2 nanofibers in 3D rechargeable lithium-ion batteries.Further investigation of 3D cathode was focused on the low toxic and low cost manganese oxide materials.Proper electrospinning parameter like feeding rate and pH value adjust was found for metal acetic acid salt in the precursor,and shrink of nanofibers in diameter was successfully obtained.Three-dimensional architecture manganese oxide nanofibers with 50-70nm in diameter was constructed for the first time.The discharge potential of 3D manganese oxide nanofibers was above 2.5V. Collapsing of nanofibers did not happen during Li+ ion intercalation and deintercalation,showed good structure stability of the 3D electrode.The discharge capacity of the 3D manganese nanofibers could reach 160mAh/g.The reversible rate of capacity of 3D nanofibers architecture is larger than 99%during 50 cycles under different discharge rates.Carbon nanotubes(CNTs) have an excellent mechanical strength and electric conductance.Composites of CNTs and other thin film material showed relatively good mechanical and electrical properties.Former research illustrated that composites nanofibers of CNTs could be obtained by electrospinning with CNTs added in the precursor.However,it is difficult to get the CNTs paralleled placed in the nanofibers, unless the diameter of the nanofibers is in a very small scale.To further improve the structure stability and conductance of the 3D electrode materials,CNTs doping was used as a modification method of 3D nanofibers electrode.CNTs enhanced 3D nanofibers electrode was investigated for the first time in this thesis.Nickel oxide(NiO) nanofibers was chosen as the matrix of the nanofibers,because their diameter could be controlled under 40-50 nm with proper electrospinning procedures.Single-walled carbon nanotubes(SWNTs) enhanced NiO nanofibers were successful prepared by electrospinning with CNTs paralleled place inside or outside the nanofibers.Charge and discharge curves showed that undoped 3D-NiO has a large capacity at first discharge,and then it decreased.The capacity loss was more obvious under large discharge rates.Structure instability was clearly observed by SEM under 1C.Carbon nanotubes enhanced NiO-CNTs nanofibers can effectively improve the property of the three-dimensional architecture electrode in battery process by emerge from structural instability problems.At different energy densities such as 0.1 C,0.5C,1C and 2C,the reversible capacity of 3D NiO-CNTs were 1.6%,2.8%,20.6%and 43.3%large than the 3D NiO.The results show that carbon nanotubes doping is an effective way to enhance the three-dimensional electrode NiO.The above results may have some reference value for the exploration of physical and chemical properties of the cathode and anode materials in three-dimensional lithium-ion battery.
Keywords/Search Tags:Electrospinning, Nanofibers, Transitional Metal Oxide, Three-dimensional Lithium Ion Batteries
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