全固态薄膜锂离子电池新型电极材料的研究

【作者】 崔艳华；

【导师】 汪小琳；

【作者基本信息】 中国工程物理研究院 ， 核燃料循环与材料， 2010， 博士

【摘要】 随着微机电系统(MEMS)的发展,对微能源的需求日益迫切。全固态薄膜锂离子电池是迄今为止最适合用于MEMS器件上的一种可集成的微能源之一。薄膜电极材料对于电池的性能有着至关重要的影响,常用的嵌锂复合物如LiCoO2、石墨等电极材料虽然性能稳定,但电池比容量却很难提高,因此,发展新型大容量的薄膜电极是未来薄膜电池的一个重要的发展方向。本文采用脉冲激光沉积(PLD)方法获得了CuF2、MnF2、CoP3和InP四种薄膜锂电池电极材料,并采用X射线衍射(XRD)、扫描电子显微镜(SEM)与能量色散X射线分析(EDX)、透射电子显微镜(TEM)与选区电子衍射(SAED)、X射线光电子能谱(XPS)和恒电流充放电法、循环伏安法(CV)对其物理化学结构和电化学性能进行了表征,结果表明：(1)CuF2薄膜在2.8和2.0 V处的两个可逆的放电平台分别对应于部分Li进入CuF2晶格和更多的Li置换金属Cu,而得到LiF和纳米级的金属Cu,其后的可逆反应是纳米级Cu驱动下的LiF与CuF2之间的可逆转化。首次放电后较大的容量损失可能是由于相变和部分Cu不能再回到晶格中重新形成CuF2导致的,但是在前45次循环[1.0,4.0 V]的电化学窗口,CoF2薄膜仍然保持了544 mAh/g左右的可逆容量,每周容量衰减约为0.6%,放电平台稳定在1.8-2.0V之间,显示了较大的容量优势和较好的电化学稳定性。有意义的是,我们首次在其CV曲线上观察到了2.8 V以上的清晰的可逆氧化还原峰,并发现CuF2薄膜在前20次的[2.8,4.2V]之间的电化学反应中,放电平台保持在3.1 V左右,平均可逆容量为125mAh/g。这表明经PLD纳米化后的CuF2表现出2.8 V以上的电化学可逆性,并具有可利用的容量。(2)制备出良好的循环保持力、较大容量和极化小的MBF2薄膜电极。充放电特性显示纳米化的氟化锰负极薄膜的可逆容量达到530 mAh/g,相当于1mol MnF2可与1.84 mol Li发生反应。其循环伏安曲线中1.0 V和0.5 V处的一对可逆性较好的氧化还原峰,分别归属于LiF在过渡金属Mn的驱动下的可逆的分解和形成,并为SAED结果所证实。其反应机理是由首次放电生成的纳米金属Mn驱动LiF的可逆分解和形成,再一次证实了氟化物以金属为反应中心的机理特征。充放电实验表明这种负极薄膜的充放电平台差值比其他氟化物都小,表明其很适合用做负极材料。同时室温沉积条件便于和IC工艺兼容,有利于全固态薄膜锂离子电池与微芯片的集成。(3)CoP3薄膜中的金属在首次还原后,不再参与氧化还原反应,该机理体现出典型的以阴离子为中心的反应机理,这与前人关于CoP3粉末的机理研究一致。PLD方法制备的纳米薄膜放电后得到的纳米级金属颗粒高度分散,对于Li3P和LiP的电化学可逆反应起到了促进作用,因此CoP3薄膜表现出比粉体材料更好的电性能,在前25次循环中,薄膜可逆容量800mAh/g,且每次循环容量衰减率仅为0.05%。(4)首次将PLD制备的InP薄膜作为锂离子电池的负极材料,且表现出良好的电化学可逆性。InP在O到2V的反应过程如下：放电过程,首先是Li进入InP晶格,置换出In并生成Li3P；在0.3 V以下,In继续和Li发生合金化反应,生成InLi合金；充电过程中,InLi合金发生分解,在合金和去合金化过程中,是以In作为活性反应中心,之后在纳米金属In的驱动下,Li3P和InP发生可逆转化,这一步是以阴离子P3-为中心的反应。(5)作为对实验研究的补充,采用第一原理的理论计算方法对Li/CuF2体系结构与性能进行了研究,包括Li嵌入CuF2晶体前后的电子结构、嵌入途径和电化学性能参数。计算结果与已有的实验数据相当,即随着嵌入的Li原子增加而体系的导电性增强和计算所得的嵌入平均电势能与观测到实验值相近。本论文的研究表明PLD方法是获得新型性能优异锂电池薄膜电极材料的重要途径,新型氟化物和磷化物纳米薄膜由于其独特的反应机理和纳米效应而具有远大于传统材料的超大容量,显示出良好的发展前景。更多还原

【Abstract】 Micro-power sources are increasingly required with the development of microelectromechanical systems (MEMS).The advantages of all-solid-state thin film lithium ion battery (TFLB),high power densities, low self-discharge rate and long cycle life and easy to be fabricated in specific shape and size, make them be one of the most promising micro-power sources.Obviously, the battery performance is mostly influenced by its electrode properties,such as capacity, potential, etc. Because it is very difficult to enlarge the capacities of current lithium-inserting electrode of TFLB,for example, LiCoO2 and graphite, great efforts should be made to develop new thin film electrodes with higher capacities to satisfy the increasing requirements.Four kinds of thin film electrodes available for lithium batteries, CuF2, MnF2, CoP3 and InP, were successfully obtained by the pulsed laser deposition method (PLD).Their composition and structure were characterized by XRD, SEM, EDX, TEM, SAED and XPS,and their electrochemical porformance were also investigated by constant current charge/discharge method and cyclic voltammograms (CV), respectively. The results are listed as following:(1)There are two reversible discharge plateaus located on 2.8 and 2.0 V for CuF2 film, which respectively correspond to two processes:the inset of Li ions into CuF2 lattice and the appearance of nano-particles of Cu after their displacement by Li, and a reversible transformation between LiF and CuF2 activated by the nano-particles of Cu. Much loss of the capacity after the first discharge may be resulted from the phase change and partial Cu atoms not back to the lattice to reform CuF2. However, the reversible capacity of 544 mAh/g of CuF2 film is obtained during the first 45 cycles between the scope of [1.0,4.0V],in which the capacity decreases 0.6% per cycle with a stable discharge plateau of 1.8-2.0V. It suggests the CuF2 thin film fabricated by PLD could be used as the cathode film of the TFLB because of its high capacity and little fading. It is noticeable that a pair of reversible redox peaks above 2.8 V appears on its CV curve, and a discharge plateau located on 3.1V and an average capacity 125mAh/g in first 20 cycles during [2.8,4.2V].It indicates that the nano film of CuF2 fabricated by PLD shows good electrochemical reversibility when voltage is above 2.8V.(2) The MnF2 film fabricated by PLD shows good capacity retentivity, high reversible capacity, and lower polarization. The polycrystalline film of MnF2 has a reversible capacity of 530 mAh/g, implying one mol MnF2 reacts with 1.84 mol Li. There is a pair of redox peaks sited on 1.0 and 0.5 V on its CV curve, respectively corresponding to the reversible decomposition and formation of LiF activated by nano particles of metal Mn, which is confirmed by SAED results.Nano metal Mn appears after the first discharge and drives the reactions of LiF subsequently, which proves again the metal reaction center mechanism of fluoride.It shows there is the least difference in charge/discharge plateaus among all fluorides in experiments, suggesting good properties available for anode film. At the same time, it is an attractive way to deposit MnF2 at room temperature, which just satisfies the temperature required in IC technology.(3)A Li electrochemical reaction in CoP3 film takes place after the first metal reduction, which does not take part in the subsequent reactions. This result agrees with the previous research on powdery CoP3.Differently, the highly diffused nano metal in the film obtained by PLD can activate the reversible reaction between Li3P and LiP, in which there is an anion center mechanism.(4) InP film deposited by PLD was firstly applied as a electrode material for lithium battery with good electrochemical reversibility. The reaction of InP under the condition of 0 to 2 V includes:in discharge, Li is firstly inserted into LiP lattice and then In is displaced with the product of L13P, and successively, In reacts with Li to form InLi alloy below 0.3 V; in charge, InLi alloy decomposes.It is In which is the reaction center in the decomposition and the formation of the alloy. Afterwards, there are transformations between Li3P and InP, where P3" is the reaction center.(5)To complement the experiments, the structures and properties of Li/CuF2 system was theoretically investigated using the first principle method, including the electronic structure changes after Li lithiation, lithiation pathway and electrochemical parameters. The calculated results are comparable to the experimental data. That is, the conductivity of the system enhances with the increasing of Li atoms, and the calculated average potentials are close to the experimental.Summarily, this work shows that PLD is an important approach to excellent film electrode materials in lithium battery. There is an attractive perspective for novel nano film of fluoride and phosphide due to their large capacities resulted from their unique reaction mechanism and nano effect.更多还原