【作者】 陆海纬；

【导师】 李越生；

【作者基本信息】 复旦大学 ， 材料物理与化学， 2008， 博士

【摘要】 上世纪六、七十年代的石油危机迫使人们去寻找新的替代能源。锂离子电池、燃料电池和贮氢电池作为先进的致密能源正在逐步取代传统的化学电池。其中,锂离子电池因其具有高电压,高容量,大功率,放电平稳等优点受到广泛关注。最近十年,随着集成电路（IC）和微机电系统（MEMS）产业的不断发展,对与之匹配的新型嵌入式电池提出了更高的要求。然而,在传统的块体材料电极如LiCoO₂中,锂离子的脱嵌类似于一维（1D）运动,为了减小锂离子在块体材料中扩散速度过慢引起的能量损失,电池的电极厚度被设计得尽量小。但是这样的设计降低了电池的容量,缩短了电池工作时间,与工业的需求相违背。所以二维电池结构,总是需要在总容量大小与完全释放能量而无内部损失之间做出取舍。最近几年,人们发现可以通过将二维电池结构重新构置成三维电池的途径,可大幅提高电池的性能,三维（3D）电池的设计概念被提出并逐步受到关注。基本方法是通过设计电极的结构,缩短的离子扩散距离,进而提高电池的能量密度和充放电容量。单位面积容量较大和高速充放电能力较强是这种三维电池的最主要优点。目前,大量的研究工作集中在三维电池电极材料的制备方法和材料物理化学性能测试上。然而,3D概念的实现还存在许多困难,主要是由于对具有三维结构电池中材料的物理化学性能缺乏深入的认识。在三维电池结构中,电极材料的充放电容量、高速充放电能力和循环性能等电池特性由电极的几何形状和长宽尺寸决定,而电池尺寸和形状在很大程度上取决于它们各自的制备方法。目前的报道的模版法和光刻方法能够达到的电极材料长径比在几倍到数十倍之间,与薄膜电池接近不足以凸现电极材料的性能。本论文选择的静电纺丝技术（Electrospin）是一种软化学方法,利用静电纺丝制备可以制备大长径比的有机或无机纳米纤维材料。与模版法、光刻法等技术相比,静电纺丝技术可以制备直径小于100纳米,长度达到数毫米的一维纳米丝材料,同时具有成本低廉,操作简便,沉积速率高,组分结构容易控制等优点。本论文通过自建的静电纺丝装置,结合溶胶凝-胶法制备了一系列金属氧化物纳米丝,包括TiO₂,Li₄Ti₅O₁₂,NiO等负极材料和LiCoO₂,MnO₂等正极材料,并将纳米丝构建的三维网状结构电极装配入锂离子电池,进行了电化学性能测试。本论文还研究了三种纳米丝三维电极的改性手段,包括掺杂、固体电解质包覆和碳纳米管增强,并取得理想的效果。对于静电纺丝方法制备三维负极材料的研究,我们首先系统地研究了制备性能较好的异丙醇钛（Ti（OiPr）₄）-聚乙烯吡咯烷酮（PVP）体系。通过调整前驱体溶胶浓度和静电纺丝电压等参数成功地制备了含钛的PVP纳米纤维丝,并改进平行收集电极成功构建了纳米丝三维（3D）网状结构。500℃退火后得到的锐钛矿TiO₂纳米纤维丝三维网状电极被完好的保留下来,其中纳米丝长度达到数毫米,直径在200纳米以下。对该三维电极的性能测试显示出大电流密度下的放电行为,首次充放电容量达到153mAh/g,但3D-TiO₂的循环性能较差。SEM观察发现锂离子嵌入三维纳米丝电极后,锐钛矿TiO₂结构发生塌陷是造成容量不可逆的主要因素。结构塌陷的原因与锐钛矿TiO₂在嵌锂时的体积膨胀有关,也与纳米丝表面嵌锂不均匀有关。根据三维电极充放电表面电流一致性参数U（U=（r²/L²）（μ/σ）（1/C））的关系式,通过增大电极电导率σ的方法对三维电极进行改性。在前驱体中掺入Sn（OiPr）₄异丙醇锡,退火得到SnO₂掺杂的TiO₂纳米丝,由于纺丝条件的限制,Sn的掺入量被限制在5%以下。改性后的三维电极,放电平台由1.2V提高到1.7V,充电平台由2.2V下降到1.9V,电极充放电过电压现象明显减小,但是其循环性能未得到明显改善,主要原因是脱嵌锂时体积膨胀导致的坍塌未能得到改善。考虑到三维电极在锂离子脱嵌时的物理结构和界面稳定的重要性,本论文研究了“零应变”尖晶石Li₄Ti₅O₁₂纳米丝构建的三维电极。本论文中含锂氧化物的静电纺丝研究为目前文献中首次报道。由于锂盐改变了前驱体溶胶的性质,制备Li₄Ti₅O₁₂纳米丝需要加入乙酸等添加剂帮助络合,并延长溶胶的陈化时间。我们利用静电纺丝方法首次成功地制备了含Li盐和Ti盐的PVP纳米丝,750℃退火后得到尖晶石结构Li₄Ti₅O₁₂纳米丝三维电极,纳米丝直径减小至100纳米,长度保持在毫米级。由于尖晶石Li₄Ti₅O₁₂在脱嵌锂时体积变化在千分之一左右结构非常稳定,3D-Li₄Ti₅O₁₂电极在大电流密度下显示出良好的三维电极特性,4.5C下首次放电容量达到167mAh/g,每次循环容量损失小于1%,循环性能优于报道的薄膜Li₄Ti₅O₁₂负极材料约27%。XRD和SEM分析显示了3D-Li₄Ti₅O₁₂纳米丝的结构稳定性。综合对结构和性能的研究结果,表明了Li₄Ti₅O₁₂材料构建三维锂离子电池负极材料的可行性。静电纺丝制备含锂金属氧化物纳米丝的方法,被进一步应用于研究三维正极材料。LiCoO₂由于其高放电电位,高电导率和良好的循环性能,被作为首选的三维正极材料。静电纺丝对成丝参数极其敏感,由于制备LiCoO₂的前驱体溶胶含有乙酰丙酮锂（Li（CH₃COCHCOCH₃））和醋酸钴（Co（CH₃COO）₂·4H₂O）等物质,当制得的纳米级纤维丝直径小于500nm时,自溶现象十分明显,所以过去静电纺丝制备LiCoO₂纤维的直径都在1微米左右。为了得到长径比更大的LiCoO₂纳米丝,我们更加严格的控制环境温度、湿度和退火条件,对自建的静电纺丝实验装置进行改装,首次成功制备了直径在60-80纳米的层状结构LiCoO₂纳米纤维丝,并构建了三维LiCoO₂纳米丝网状电极。但是其放电容量与循环性能均远远低于薄膜材料。通过SEM观察放电后的纳米丝形貌,发现了LiCoO₂纳米丝三维电极的结构不稳定问题,其原因比较复杂,包括脱嵌锂时的应力导致结构塌陷和较大的电极与电解液表面使得Co元素在电解液中的溶解变得明显。为了避免这些问题,首次使用锂磷氮氧（LiPON）固体电解质包覆LiCoO₂对三维结构进行增强。LiPON包覆之后,层状LiCoO₂三维电极的放电容量达到120.4mAh/g,在0.05mA/cm²的放电电流下充放电100次,平均每次循环的不可逆容量为0.11%,与未包覆的纳米丝三维电极相比具有更高的放电容量和更好的循环性能。在更大的能量密度下,其循环性能同样良好。研究表明LiPON层包覆对于提高三维LiCoO₂纳米丝电极的电化学性能有很好的效果。为了进一步研究其他结构稳定、低毒、低成本的三维正极材料的电化学性能,本文对氧化锰纳米丝展开研究。首次使用静电纺丝制备了Mn（CH₃COO）₂·4H₂O-PVP为前驱体的含Mn-PVP纳米丝,调节酸碱度后纳米丝直径被控制在50-80纳米之间。利用此纳米丝构建三维网状结构电极,应用于锂离子二次电池正极。三维氧化锰纳米丝网状电极放电电位在2.5V以上,在嵌锂和脱锂的过程中表现出良好的结构稳定性。由于其长径比更大,放电容量可以达到160mAh/g,在不同大小的能量密度下,纳米丝三维电极的循环性能均十分优秀,平均每次循环的容量衰减在1%以下,可作为优良的3V左右的三维电池正极材料。碳纳米管具有优秀的机械强度和导电能力,采用碳纳米管掺杂的薄膜复合材料,显示出更好的力学和电学性能。过去的研究证明,在静电纺丝前驱体中加入碳纳米管可以得到碳纳米管掺杂的聚合物纳米丝。但是,只有当纳米丝的直径非常小时,才可以实现碳纳米管沿纳米丝轴向排列,从而将碳纳米管的优秀性能传递给纳米丝。为了进一步满足三维电极对材料导电率和结构稳定性的要求,本论文将碳纳米管掺杂作为一种新的改性手段应用到三维电极材料上,首次研究了静电纺丝制备碳纳米管（CNTs）增强的纳米丝三维电极。首先通过对静电纺丝电压、喷速等参数的进一步研究,首次成功制备直径在40nm左右的氧化镍（NiO）纳米纤维丝,以此为基础首次电纺了含轴向平行排列的单壁碳纳米管（SWNTs）的NiO纳米丝。研究发现未掺杂的NiO纳米丝首次放电容量较大,其后出现了容量衰减,特别是当放电速率较大时,衰减现象明显。1C下放电后的结构不稳定在SEM下能被清晰的观察到。碳纳米管增强的NiO-CNTs纳米丝能够有效改善三维电极在锂离子嵌入和脱出的过程中产生的结构不稳定问题,提高了高能量密度下的循环性能;在0.1C、0.5C、1C和2C的放电速率下,可逆容量比NiO分别提高1.6%、2.8%、20.6%和43.3%。研究结果表明碳纳米管掺杂是三维NiO纳米丝电极改性的有效途径。以上研究对探索三维锂离子电池的制备工艺、正极负极材料物理化学性能及其改性方法都有一定的参考价值和指导意义。更多还原

【Abstract】 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 100^th 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.更多还原