锂离子电池纳米材料及其薄膜电极的制备与性能研究

【作者】 黄俊杰；

【导师】 江志裕；

【作者基本信息】 复旦大学 ， 物理化学， 2008， 博士

【摘要】 微电子器件以及超大规模集成电路的迅猛发展对微型和薄膜电源提出了更高的要求,全固态薄膜锂离子电池由于其高的比容量、优越的循环性能,被认为是最可能满足需求的微型电源之一。但是,目前文献上用的磁控溅射或激光溅射等制作技术具有设备复杂、成本高、制作慢、不易大规模生产等缺点。全固态锂离子薄膜电池的大规模制备给研究人员带来了极大的挑战。喷墨打印技术具有经济、高效、快速制膜等特点,对于薄膜电极和锂离子薄膜电池制备具有很大的吸引力。尖晶石Li₄Ti₅O₁₂放电时电位平稳、不生成SEI膜、“零”体积变化以及高的安全性,被认为是最有希望替代石墨碳成为大倍率锂离子电池的负极材料。但是,其实际应用受制于低的电子电导率。因此,如何提高Li₄Ti₅O₁₂的电子电导率实现其在大倍率锂离子电池中应用是一个重要的课题。本论文分为两部分,第一部分,制备锂离子电池正负极纳米材料,通过喷墨打印的方法制成薄膜电极,并对纳米材料及超薄电极的电化学性能进行了的表征和深入的研究。第二部分,通过对Li₄Ti₅O₁₂材料的结构调整,以及与碳纳米管复合两种方法,提高它的大电流充放电性能。本论文的主要研究结果如下:1.在本组前期研究的基础上,对喷墨打印工艺进行了新的重要改进。其一是改进了电极的打印“墨水”制备工艺,找到了一种新的高效分散剂Lomar D,使得仅通过超声分散就能得到稳定分散的打印“墨水”,省去了以前湿法球磨辅助分散的步骤。其二是省去了薄膜电极后续的热处理步骤,仅通过简单的热滚压过程就能获得结构稳定,性能良好的薄膜电极。2.通过溶胶凝胶法制备了LiCoO₂前驱体。750℃下合成的LiCoO₂（平均粒径为93 nm）适合作为制备薄膜电极的活性材料。用新的Lomar D分散剂制备了打印“墨水”,喷墨打印制得了厚度约为1.1μm的LiCoO₂超薄电极,LiCoO₂的担载量为0.30 mg/cm²。XRD和Raman光谱证明,经过超声分散和打印过程,薄膜电极中LiCoO₂的晶体结构保持完好,省去了后续热处理步骤。采用热滚压工艺克服了充放电循环过程中的薄膜开裂。超薄LiCoO₂电极在384μA/cm²（约为10.8 C）下充放电,放电容量为105 mAh/g。在电流密度为192μA/cm²（约5.4 C）下经过循环充放100次后,电池的放电容量仍保持在118 mAh/g。与首圈相比,容量仅减少5%。这种高倍率充放电稳定性可归因于电极极薄的厚度、低的内阻、纳米尺度的LiCoO₂以及稳定的薄膜电极结构。3.通过溶胶凝胶法制备了LiMn₂O₄前驱体,分析了处理温度对材料形貌结构及电化学性能的影响。结合喷墨打印的特点,选择750℃下合成的LiMn₂O₄（粒径为100 nm）作为制备超薄电极的原材料。喷墨打印方法制得的LiMn₂O₄薄膜电极表面致密,厚度在1.8μm左右,LiMn₂O₄的担载量为0.48 mg/cm²。CV、EIS以及恒电流充放实验表明Li⁺在薄膜电极中传输是一种半无限扩散行为,扩散系数为1.15×10^-11cm²/s。薄膜电极在100μA/cm²（2C）的电流密度下经过54周充放电后,容量仍可保持在104 mAh/g,单圈容量衰减仅为0.1%。这种优异的倍率和循环稳定性可归因于LiMn₂O₄晶体结构的完整、纳米粒径以及超薄的电极厚度。4.通过表面活性剂辅助溶胶凝胶法制备了Li₄Ti₅O₁₂的前驱体,研究了在O₂气氛中不同温度下热处理对Li₄Ti₅O₁₂形貌及粒子大小的影响。基于XRD、SEM以及充放电测试结果的分析,选择650℃处理得到的Li₄Ti₅O₁₂作为薄膜电极制备的材料,其平均粒径约为120 nm。通过喷墨打印法制得了Li₄Ti₅O₁₂薄膜电极。SEM显示薄膜电极表面平整致密,厚度约为1μm。用涂敷法获得的PEO（LiN（CF₃SO₃）₂）固体电解质膜,其厚度约为20-40μm。研究了Li/PEO（LiN（CF₃SO₃）₂）/Li₄Ti₅O₁₂（1μm）全固态电池的电化学性能,电池的平台电位在1.5 V左右,在20μA/cm² 35℃下放电容量可达到22μAh/cm²。5.首次制备了全固态聚合物电解质超薄锂离子电池。正负电极均为喷墨打印制得,电池为(Li₄Ti₅O₁₂（1μm）/PEO（LiN（CF₃SO₃）₂）/LiMn₂O₄（1.8μm）)。电池的工作电压为2.5 V,整个电池的厚度不超过30μm,在电流密度为20μA/cm²时,35℃下的放电容量为18μAh/cm²,并且具有良好的循环性能。6.采用以P123为高分子乳化剂,制备了正庚烷/乙醇的两相微乳体系。调整原材料Ti（OC₄H₉）₄和LiAC的量,制得了由纳米Li₄Ti₅O₁₂组成的不同形貌的材料。包括不同壁厚的Li₄Ti₅O₁₂空心球以及多孔Li₄Ti₅O₁₂。纳米颗粒组成的单层Li₄Ti₅O₁₂空心球（壁厚约100 nm）具有优异的电化学性能。在0.5 C下充放电,其放电容量为162 mAh/g,此材料甚至可用20 C进行充放电,容量仍可达95mAh/g。2 C下500圈充放电后,其放电容量仍有142 mAh/g,单圈容量损失仅为0.01%。这种高倍率充放电稳定性可归因于:纳米粒子有助于缩短充放电过程中锂离子的扩散距离;空心结构有益于增加电极材料和电解液接触的有效反应面积。7.在Li₄Ti₅O₁₂的制备过程中加入纳米碳管,N₂气氛下热处理获得了CNT-Li₄Ti₅O₁₂复合材料。XRD数据表明纳米碳管的存在并没有影响尖晶石Li₄Ti₅O₁₂的形成。TEM图表明CNTs在Li₄Ti₅O₁₂中分布很均匀,Li₄Ti₅O₁₂纳米粒子几乎都粘附在纳米碳管的周围。热重实验测得纳米碳管在复合材料中的重量百分比为8.2%。与相似过程制得的Li₄Ti₅O₁₂相比,CNT-Li₄Ti₅O₁₂复合材料中的Li₄Ti₅O₁₂具有更小的纳米粒径。复合材料在0.5 C下的放电容量为150 mAh/g。如果考虑到其中Li₄Ti₅O₁₂的实际含量（91.8%）,那么Li₄Ti₅O₁₂的放电容量可以高达163mAh/g,接近Li₄Ti₅O₁₂的理论放电容量（175 mAh/g）。复合材料在20 C下的放电容量仍可达106 mAh/g,相当于0.5 C时的70.1%。复合材料还具有很好的循环稳定性,5 C下充放电500圈其放电容量几乎不变。CNT-Li₄Ti₅O₁₂复合材料的这种优异的电化学性能可归因于Li₄Ti₅O₁₂小的纳米粒径,纳米碳管的复合极大地改进了材料的电子导电性;以及纳米碳管带来的复合材料电极的多孔性。更多还原

【Abstract】 With the fast development of microelectronic system and very large-scale integrations,there is an increasing requirement of micro driving power with excellent electrochemical performance.In the fields of micro power source, all-solid-state thin film lithium ion batteries has been viewed as one of the most promising micro powers due to its high specific capacity and excellent cycling performance.But the large scale fabrication of thin film electrode brings a big challenge in its application.In the thin film preparation field,ink-jet printing technique has its special advantages of economy,fast and high efficiency,which will be a promising way to large scale fabrication of thin film electrode in all solid state lithium ion batteries.Recently,the spinel Li₄Ti₅O₁₂ has been viewed as a promising alternative of graphite to be an anode material in lithium ion batteries.But,its’ application in high rate lithium ion batteries was hindered by its low electronic conductivity.Therefore, how to increase the electronic conductivity of Li₄Ti₅O₁₂ is a key solution for its high-rate application.This paper consists of two parts.In the first part,the sol-gel method was used to synthesize nano cathode/anode materials for lithium ion batteries.With these materials the thin film electrodes were prepared by ink-jet printing technique.Also, their electrochemical properties were investigated in detail.In the second part,the rate performance of Li₄Ti₅O₁₂ was improved by its structure modification or composing with carbon nano fibers.The main results were listed as following:1.With the aim to improve the quality of thin film electrode and to simply the processes,a new high efficient dispersant（Lomar D） was used to suspense the particles for preparing electrode printing ’ink’.Compare with our previous method it can save the wet ball milling process,and the consequent heat treatment process.2.Through a sol-gel process and the calcination at 750℃in O₂ atmaspher, nano-LiCoO₂ particles with the diameter of 93nm could be synthesized.Using ink-jet printing technique LiCoO₂ thin film with the thickness of 1.16μm was prepared after 30 iterative printings and hot rolling process.The loading amount of LiCoO₂ on the substrate of aluminum sheet was 0.30mg/cm².The influence of thermal rolling process on the electrochemical performance of thin film electrode was investigated.The thin film without thermal rolling has a rough surface and loose structure,and bad cycle capability.XRD and Raman spectra showed that the LiCoO₂ crystal structure maintained very well in thin film even after the ultrasonic and printing processes.After 100 cycles at 5 C it still kept stable.This thin film electrode presented a good rate capability and capacity retention.At current of 64μA/cm² （about 1.8 C）,the discharge capacity was 132mAh/g.It even could be charged-discharge at 384μA/cm²（about 10C） with a stable discharge capacity of 105 mAh/g. At the current rate of 192μA/cm²（5.4 C）,after 100 cycles the discharge capacity kept at 118 mAh/g,which was only 5%loss compare with the value in initial cycle. The good electrochemical performance of thin film LiCoO₂ electrode could be attributed to its extremely thin thickness,low inner resistance by carbon doping, nano particle size of LiCoO₂ and the stable film structure.3.Spinel LiMn₂O₄ was synthesized by a sol-gel method,the influence of calcination temperature on LiMn₂O₄ properties was investigated.The LiMn₂O₄ treated at 750℃with a particle size of about 100 nm was chosen for preparing printing ’ink’.The jet printed thin film electrode with thickness of about 1.8μm has a smooth surface.The loading amount of LiMn₂O₄ was 0.48mg/cm².Its electrochemical performance was investigated by CV,EIS and chronoampermetry methods.The Li⁺ diffusion behavior was performed as a semi-infinite diffusion process with the diffusion efficiency of 1.15×10^-11cm²/s.LiMn₂O₄ thin film electrode showed a good rate capability.At discharge current density of 100μA/cm²（2C）,the electrode presented a discharge capacity of 104mAh/g.After 54 cycles,the capacity loss was only 0.1%for per cycle.This good electrochemical performance could be attributed to its extremely thin thickness,good crystal structure,and the nano particle size of LiMn₂O₄.4.Spinel Li₄Ti₅O₁₂ was synthesized through a sol-gel method.Under O₂ atmosphere the influence of calcination temperature on the morphologies,size and the electrochemical properties of Li₄Ti₅O₁₂ was investigated.According to the results of XRD,SEM and charge/discharge tests,Li₄Ti₅O₁₂ prepared at 650℃with the diameter of about 120nm was selected as the raw material for preparing printing ink. The jet printed Li₄Ti₅O₁₂ thin film electrode with the thickness of about 1μm showed a smooth surface and a porous structure.The performance of all solid battery of Li/PEO（LiN（CF₃SO₃）₂）/Li₄Ti₅O₁₂（about 1μm ） was tested.The charge/discharge plateau was about 1.5 V.At the current density of 20μA/cm²,its discharge capacity was 22μAh/cm² at 35℃.5.An all solid thin film polymer electrolyte battery was prepared in first time. The battery consisted of jet printed thin film electrodes and a PEO(LiN（CF₃SO₃）₂ separator:Li4_Ti₅O₁₂（about 1μm ） / PEO(LiN（CF₃SO₃）₂（27μm）/LiMn₂O₄（1.8μm）. This thin film battery with the thickness of less than 30μm presented a work voltage about 2.5 V.At 35℃,it can be charge-discharged at current density of 20μA/cm² with a capacity of 18μAh/cm².6.Hollow spherical Li₄Ti₅O₁₂ was prepared by a macro emulsion method using P123 as emulsifier.Its frameworks built from many nano Li₄Ti₅O₁₂ particles with the size of 100nm.Different morphologies from hollow sphere to marco porous structure of material could be synthesized by adjusted the amount of Ti（OC₄H₉）₄ and LiAC in the two phases n-heptane/ethanol solvent system.Among them the mono layer hollow spherical material showed the best electrochemical performance.It can be charge-discharged at 20 C（3.4A/g） with the specific capacity of 95mAh/g. Besides its excellent rate capability,this material also presented good capacity retention:over 500 cycles at the charge and discharge rate of 2 C the specific capacity kept very stable as 140mAh/g,and only loss 0.01%for per cycle.The excellent electrochemical performance of hollow spherical Li₄Ti₅O₁₂ is mainly due to its stable hollow structure and the nano scale particles.7.CNTs（carbon nano tubes）/Li₄Ti₅O₁₂ composite was prepared by sol-gel method using Ti（OC₄H₉）₄,LiCH₃CO0·2H₂O and the n-heptane containing CNTs. The CNTs amount in the composite was about 8.2%.The characters of CNTs/Li₄Ti₅O₁₂ composite were determined by XRD,SEM,and TG methods.Its electrochemical properties were measured by charge-discharge cycling and impedance tests.TEM image shows that CNTs dispersed homogenously in CNTs/Li₄Ti₅O₁₂ samples and the nano Li₄Ti₅O₁₂ particles stickled on the surface of CNTs.Compared with spinel Li₄Ti₅O₁₂ prepared by similar way,the size of Li₄Ti₅O₁₂ particles in CNT-Li₄Ti₅O₁₂ composite was smaller.Experimental results showed that the CNTs/Li₄Ti₅O₁₂ composite presented an excellent rate capability and capacity retention.At 0.5 C it demonstrated a discharge capacity of 150mAh/g,if considered the weight content of Li₄Ti₅O₁₂ in composite,the nominal capacity of Li₄Ti₅O₁₂ should be 163mAh/g,which is close to the theoretical capacity of 175 mAh/g.At the charge-discharge rate of 5 C and 10 C,the discharge capacities of Li₄Ti₅O₁₂/CNTs were 145 and 135 mAh/g respectively.After 500 cycles at 5 C,the discharge capacity retained as 142mAh/g.Even it can be discharged at 20 C,the discharge capacity was 106mAh/g,which was equal to 70.1%of that at 0.5 C.The excellent electrochemical performance of CNTs/Li₄Ti₅O₁₂ electrode could be attributed to the improvement of electronic conductivity and the porous electrode structure caused by adding of CNT fibers and the nano size of Li₄Ti₅O₁₂ particles in the CNTs/Li₄Ti₅O₁₂ composite.更多还原