喷墨打印锂离子薄膜电极和复合超电容材料的制备及电化学性能研究

【作者】 赵尧敏；

【导师】 江志裕；

【作者基本信息】 复旦大学 ， 物理化学， 2006， 博士

【摘要】 随着微电子机械系统（MEMS）和超大规模集成电路技术（VLSI）的发展，对能源的微型化、集成化提出了越来越高的要求。民用电子器件如传感器、智能卡、便携式电子设备等众多领域的迅猛发展，也对化学电源的小型化、微型化和薄膜化提出了更高的要求。全固态薄膜锂离子电池因其良好的集成兼容性和电化学性能成为MEMS、VLSI、智能卡等能源微型化、集成化的最佳选择。近年来，人们一直在寻求锂离子薄膜微电池的最佳制备技术。超级电容器是介于传统电容器和电池之间的新型储能元件，它具有很高的功率密度、超长循环寿命以及能在低压下操作等特点。因此在大功率脉冲电源、电动车驱动电源等领域有广泛用途。另一方面，超电容虽然有非常优越的功率密度，但是最大的问题就是能量密度比较低。为了提高超电容的能量密度，负载型复合超电容材料也成为一大研究热点。本论文分为两大部分。第一部分，利用新颖的喷墨打印方法制备锂离子薄膜电极并对其电化学性能进行详细的表征和深入的研究。第二部分，将纳米水合钌氧化物粒子负载在介孔碳上制备出复合材料，并详细考察其作为超电容材料的电化学性能。本论文的主要研究结果如下：1．利用喷墨打印技术制备薄膜电极需要使用纳米尺寸的电极材料。本文利用P123嵌段共聚物表面活性剂作为结构导向剂，通过溶胶凝胶方法合成得到了用作锂离子电池正极材料的纳米单晶LiCoO₂和LiMn₂O₄、用作锂离子电池负极的SnO₂和Li₄Ti₅O₁₂纳米多晶粉体以及可用于锂二次电池正极的单晶V₂O₅。其中纳米单晶LiMn₂O₄和单晶V₂O₅是首次采用该方法合成。通过对材料的结构分析以及电化学性能测试，得出如下结论：用P123作为模板剂通过溶胶凝胶方法在750℃氧气氛下热处理20h合成了具有理想层状岩盐结构的纳米单晶LiCoO₂，首次放电比容量可达149mAh／g，50次循环后容量保持率约为80％：用P123作为模板剂通过溶胶凝胶方法在750℃热处理6h得到直径在50nm以下具有金红石四方相结构的纳米多晶SnO₂；用P123作模板剂450℃下热处理4h得到纳米TiO₂，然后用所制备的纳米TiO₂和LiOH·H₂O作为反应物、丙酮作为球磨介质湿法球磨辅助进行固相反应合成，得到了具有尖晶石结构的纳米Li₄Ti₅O₁₂粒子（氧气氛下900℃煅烧8 h），所制备的纳米Li₄Ti₅O₁₂具有非常优异的循环性能和较高的放电比容量，首次放电比容量可达155 mAh／g，首次库仑效率为98.3％。在后续的循环过程中充放电效率接近100％；首次用P123作为模板剂通过溶胶凝胶方法在750℃氧气氛下煅烧15 h，得到晶粒生长完美、粒径在100～300 nm之间的纳米单晶LiMn₂O₄，在充放电倍率分别为0.2C、1C和2C时首次放电比容量分别约为110mAh／g、103mAh／g和90mAh／g，说明所制备的纳米单晶LiMn₂O₄具有较为优良的倍率特性；首次用P123作为模板剂通过溶胶凝胶方法600℃热处理2h得到具有四方棱锥体形貌的单晶V₂O₅，在电流密度为20 mA／g、电位区间为3.5～1.5V循环时首次放电比容量约为390mAh／g。经过35次循环后放电比容量为186mAh／g。容量损失主要在前三个循环，主要是由于在深度嵌锂（放电）过程中发生了不可逆相转变。2．采用喷墨打印方法的关键是纳米粒子在分散体系中的稳定性。怎样使具有电化学活性的锂离子电池材料在水体系中十分均匀地稳定分散，是工艺成功的关键步骤。本文首次通过联合采用空间位阻型聚合物分散剂和湿法球磨工艺成功地解决了喷墨打印技术中的墨水制备问题，成功地制备出了可用于锂离子电池的薄膜LiCoO₂正极、薄膜SnO₂负极、薄膜Li₄Ti₅O₁₂负极，建立了方便快速的喷墨打印制备薄膜电极的方法，并对薄膜电极的形貌和电化学性能进行了深入的研究。3．用喷墨打印方法在商用铝箔上制备了厚度仅为1.27μm的薄膜LiCoO₂正极，在电位区间为3.0～4.2V、电流密度约为40μA／cm²时的可逆放电容量约为15μAh／cm².μm，但是这种直接喷墨打印出的LiCoO₂薄膜正极的充放电稳定性较差。主要原因是由于在墨水制备的过程中球磨工艺对其晶体结构造成了一定程度的破坏，另外Co在墨水体系中的溶出以及高分子分散剂对纳米粒子的包覆也对薄膜电极的电化学性能有一定的影响。试验表明球磨过程使得原来具有理想的层状岩盐结构的LiCoO₂材料中阳离子无序度明显增加，不可逆相变加剧，而且小粒径的LiCoO₂材料受球磨影响更为严重。对喷墨打印在蒸金铝箔上的LiCoO₂薄膜正极进行了轻微的后续热处理（450℃，30min）可使不可逆相变减弱，明显提高材料的电化学稳定性。在电流密度为20μA／cm²时薄膜LiCoO₂电极的首次放电容量为20.31μAh／cm^.μm(81mAh.g^-1)。充放电过程中库仑效率先是逐渐上升，10次后其充放电效率可接近100％，第50次循环时的放电比容量为18μAh／cm².μm(71 mAh.g^-1)，保持初次容量的87％。4．首次采用新颖的喷墨打印方法在商用铜箔上成功制备出了可用于锂离子电池负极的SnO₂薄膜电极，薄膜电极的厚度可以通过改变打印层数来进行调节。其中，单层打印电极的厚度约为770～780nm，10层打印电极在8 Mpa压延后的平均厚度约为2.3μm。打印薄膜SnO₂电极仍然具有四方相的金红石晶体结构。循环伏安曲线表明，峰电流和扫描速度呈现较好的线性关系，体现了薄层电极的重要特征。在电流密度为33μA/cm²时，薄膜电极的首次放电比容量高达812.7mAh／g，循环性能也得到了一定程度的改善，这主要是由于薄膜化可以在一定程度上缓解由于体积膨胀收缩而造成的活性物质的失活以及高度分散的导电剂乙炔黑在薄膜中起到的“缓冲基质”的作用。薄膜电极容量衰减的主要原因是纳米SnO₂粒子的团聚以及在循环过程的体积膨胀和收缩而造成的粉化现象。5．首次采用新颖的喷墨打印方法成功制备出了可用于锂离子电池负极的Li₄Ti₅O₁₂薄膜电极。在商用铜箔基底上采用喷墨打印方法制备了薄膜厚度非常均匀一致的Li₄Ti₅O₁₂薄膜电极。在电流密度为20.8μA／cm²时首次比容量可达172mAh／g，接近理论容量。50次循环后比容量约为107.8mAh／g，容量保持率为62.7％。在电流密度高达208μA／cm²时的放电比容量仍可达173mAh／g，体现了薄膜电极优越的倍率特性。存在问题是薄膜电极的充放电稳定性较差。对打印在金片基底上的Li₄Ti₅O₁₂薄膜在550℃下热处理90 min得到热处理薄膜Li₄Ti₅O₁₂电极，试验结果表明热处理薄膜电极的充放电稳定性显著提高。循环伏安测试表明峰电流和扫描速度呈现非常好的线性关系，体现了薄层电极的特征。经过后续热处理的薄膜Li₄Ti₅O₁₂电极和热处理LiCoO₂薄膜电极一样，存在一个“活化”阶段。经过300次循环后，薄膜电极的容量保持率约为峰值比容量（172 mAh／g）的88％，显示了优越的循环稳定性。薄膜电极的高比容量可以归因于以下两个方面：纳米尺寸的Li₄Ti₅O₁₂有利于活性物质的充分利用；高比表面Li₄Ti₅O₁₂的双电层电容贡献。6．用介孔氧化硅分子筛（SBA-15）作为硬模板，用蔗糖作为碳源通过两遍浸渍制备了有序介孔碳（MC）。采用液相吸附的方法将低量的RuO₂.xH₂O纳米粒子以溶胶的形式首次负载在介孔碳上得到RuO₂.xH₂O／MC复合材料（载Ru量为0.9～5.4wt％）并对其电容器性能进行了详细的研究。分析表明RuO₂.xH₂O／MC复合材料不仅比电容明显增加，而且还保持了介孔碳优越的倍率特性。当扫描速度为25 mVs^-1、电位区间-0.2～0.8V（vs.Ag／AgCl）、在0.1M H₂SO₄水溶液电解质体系中，载钌量为3.6％的复合材料比电容从115增加到181 F／g，增加幅度约为57％。通过扣除介孔碳在复合材料中的电容贡献可估算出水合钌氧化物的比电容高达1527 F／g，表明复合材料中水合钌氧化物具有很好的利用率。负载的RuO₂.xH₂O纳米粒子在介孔碳载体上呈现的高度分散的状态以及具有无定形水合物结构是其同时表现出高比电容和高利用率的主要原因。载钉量为3.6％的复合材料经过1000次循环后其比电容保持率仍为97.2％，表明复合材料具有优异的循环稳定性。更多还原

【Abstract】 With the development of microelectromechanical systems （MEMS） and very large-scale integration （VLSI）, there is an increasing requirement in the miniaturization and integration of power sources. The reduction in size and power requirement of electronic devices is the major driving force behind the development of all-solid-state thin-film batteries. Applications focus on the improvement of existing consumer and medical products, such as smart cards, sensors, portable electronic devices, as well as on the integration with electronic chips and microelectromechanical systems. With better integration compatibility and electrochemical performance, thin-film lithium ion battery becomes the optimal choice for miniaturization and integration of MEMS and VLSI power.Electrochemical （EC） capacitors, also called supercapacitors, are a kind of new-style energy-storing sources between conventional capacitors and batteries. They can provide high power densities and unusual cycleability and therefore are urgently needed for a number of technologically important systems. These systems include acceleration power for electric vehicles, electrical regenerative braking storage for electric drive systems, power assist to hybrid vehicles, starting power for fuel cells, pulse power for mobile telecommunication and other electronic devices which require high power to operate. In addition, when EC capacitors are coupled with batteries, they can reduce the peak power requirement, prolong the lifetime and reduce the energy requirement （or the size） of the battery. On the other hand, the big problem for the current supercapacitors such as carbon materials is their low energy density. Therefore, many researchers focus on the composite supercapacitor materials such as carbon-supported RuO₂ in order to increase the energy densities.This thesis includes two major parts. Firstly, thin-film electrodes including LiCoO₂ cathode, SnO₂ anode and Li₄Ti₅O₁₂ anode used for lithium ion batteries were successfully fabricated by a very novel and facile route of ink-jet printing technique. In addition, their structure, morphology and electrochemical performance were investigated in great detail. Secondly, RuO₂.xH₂O/MC composite materials obtained by loading small amount of amorphous hydrous ruthenium oxide nanoparticles on mesoporous carbon （MC） were fabricated and used for supercapacitors for the first time. Their electrochemical behaviors were also investigated.The main results are as follows.1. Nano-sized or single-crystalline materials including LiCoO₂, LiMn₂O₄, V₂O₅, SnO₂ and Li₄Ti₅O₁₂ were synthesized by using tri-block copolymer amphiphilic surfactant (EO₂₀PO₇₀EO₂₀, always abbreviated as P123) as a structure-directing agent through sol-gel process because nano-sized materials with good electrochemical activity are demanded during the subsequent ink-jet printing process. Among them, the nano-sized single-crystalline LiMn₂O₄ and single-crystalline V₂O₅ were synthesized for the first time by using this method. At the same time, the electrochemical performances of these materials were investigated.2. The key procedure for the ink-jet printing process is to obtain the stability of nano-sized materials in the dispersion system. The stable LiCoO₂, SnO₂ and Li₄Ti₅O₁₂"inks" containing conductive agent and binder were successfully prepared by employing both wet ball-milling technology and steric polymeric dispersant.3. Thin film LiCoO₂ electrode with the uniform thichness of 1.27μm was successfully prepared by using the novel facile and low-cost ink-jet printing technique onto the commercial Al substrate. The initial discharge capacity was 15μAh/cm².μm at charge current of 40 μA/cm² in the potential range of 3.0—4.2 V （vs. Li⁺/Li） . CV measurements showed the obvious phase transition and obvious capacity loss was also observed with respect to the as-printed thin film LiCoO₂ electrode without any post-annealing process. The reason for the capacity loss was attributed to both the crystalline structure change during the wet ball-milling process and the polymeric dispersant coating on the surface of the nano particles. Secondly, thin film LiCoO₂ was also ink-jet printed on the gold-coated Al foil and was then followed by a slight annealing process at 450°C for 30 min in order to improve the electrochemical performance. The electrochemical performance was obviously improved after this slight heatment. The initial discharge capacity of thin film LiCoO₂ electrode at a charge current of 20μA/cm² in the potential range of 3.0～ 4.2 V （vs. Li⁺/Li） was 20.31μAh/cm².μm (81 mAh.g^-1). The charge-discharge efficiency approached almost 100 % after 10 cycles. The discharge capacity was 18μAh /cm².μzm (71 mAh.g^-1), which is 87% of the initial capacity, after 50 charge-discharge cycles.Besides, the effects of three kinds of ball-milling processes on the crystalline structure and electrochemical performances of nano-sized LiCoO₂ were investigated in great detail. The ideal rock-salt structure of original nano-sized LiCoO₂ was obviously influenced by all these ball-milling processes employed in this paper. Thestructure of LiCoO₂ with smaller particle size was influenced more seriously.4. SnO₂ thin film electrodes on commercial Cu foil substrate as an anode for rechargeable lithium ion batteries were also successfully fabricatd by using ink-jet printing method for the first time. The distribution of as-printed thin film SnO₂ electrodes is smooth and uniform. The thickness can be adjusted by printing different layers. The thickness of monolayer is ca. 770-780 nm and the average thickness of the 10-layer film after compression is about 2.3 μm which was used for electrochemical measurements. The linear relationship between anodic peak current and the scan rate obtained by CV technique shows the characteristics of thin-film electrodes. High initial discharge capacity about 812.7 mAh/g was obtained at a constant discharge current density of 33 μA/cm² over a potential range of 0.05-1.2 V vs. Li⁺/Li and the cycle performance is improved because the conducting agent AB can also perform as a better matrix for nano-structured thin films. Aggregation and pulverization due to the large volume expansion/contraction during the alloy/dealloying process gives rise to capacity decay which can be concluded by comparison of SEM and TEM pictures of the thin films before and after the charge-discharge process.5. Thin-film Li₄Ti₅O₁₂ electrode, which can be used as an anode in lithium ion batteries, was successfully fabricated also employing the ink-jet printing technique for the first time.Firstly, the thin-film Li₄Ti₅O₁₂ electrode was ink-jet printed on commercial Cu foil without any post-annealling heat-treatment. The cross-sectional SEM image showed that the uniform thickness of monolayer ink-jet printing was about 700-800 nm. The sharp and symmetric reversible redox couples located at about 1.55V in CV curves corresponds to the spinel structure of Li₄Ti₅O₁₂. High initial discharge capacity about 172 mAh/g is obtained at a constant discharge current density of 20.8 μA/cm² over a potential range of 1.0-2.0 V vs. Li⁺/Li, which almost reaches the theoreticl capacity 175 mAh/g. Even at a very high current density of about 208 μA/cm², the initial discharge capacity was not decreased compared to that at the low current densities which showed the excellent rate capabilities of the thin-film electrodes. The capacity retention was about 62.7% after 50 cycles and the cycle stability was not so good.Secondly, thin-film Li₄Ti₅O₁₂ electrode was also ink-jet printed on Au plate and then followed by a post-annealing process at 550°C for 90min in order to improve theelectrochemical performance of thin film Li₄Ti₅O₁₂ electrodes. The cross-sectional SEM image showed that the uniform thickness of 10-layer ink-jet printing process was about 1.7～1.8μm. The linear relationship between peak current and the scan rate obtained by CV technique shows the characteristics of thin-film electrodes. Excellent cycle performance was observed at a constant discharge current density of 10.4 μA/cm² over a potential range of 1.0-2.0 V vs. Li⁺/Li. The capacity retention after 300 cycles was about 88% of the peak discharge capacity （172 mAh/g）. At the same time, high discharge capacity was obtained. The high discharge capacity can be attributed to both the thin-film characteristics and the double-layer capacity due to the highly dispersed nano Li₄Ti₅O₁₂.6. Amorphous hydrous ruthenium oxide/mesoporous carbon powders （RuO₂.xH₂O/MC） were prepared by liquid adsorption method. The mesoporous characteristics of mesoporous carbon and the high specific capacitance and highly electrochemical reversibility of RuO₂·xH₂O play a dominant role in the electrochemical properties of amorphous hydrous ruthenium oxide/mesoporous carbon （RUO2.XH2O/MC） composites.Electrochemical measurements showed that the RUO2.XH2O/MC composites prepared by loading small amount of RuO₂.xH₂O nanoparticles （ranged from 0.9 to 5.4 wt % Ru） on MC not only have an enhanced specific capacitance but also retain the ideal capacitive performance such as highly reversibility, excellent rate capability, and good stability of MC. The RuO₂.xH₂O/MC composite （3.6 wt.% Ru） exhibited an increase of the specific capacitance of approximately 57% （from 115 to 181 F/g） at the scan rate of 25mVs^-1in 0.1 M H₂SO₄ aqueous electrolyte within the potential range from -0.2 to 0.8V vs. Ag/AgCl. The specific capacitance of RuO₂.xH₂O was estimated to be 1527 F/g by subtracting the contribution from MC in the composite in case of 3.6 wt.% Ru loaded electrode at the scan rate of 25 mVs^-1, which indicates the high utility of the active material RuO₂.xH₂O. Cycle performance tests derived by CV measurements showed that 97.2% of capacitance retention for the RuO₂.xH₂O/MC composite （3.6 wt.% Ru） was observed after 1000 cycles.更多还原