【作者】 胡仁宗；

【导师】 朱敏；

【作者基本信息】 华南理工大学 ， 材料加工工程， 2011， 博士

【摘要】 开发新型的高容量、高安全性、长寿命和价格低廉的负极材料是发展大型化和微型化锂离子电池的重点之一。具有高容量和适中嵌锂电位的Sn基合金被广泛研究作为锂离子电池的负极材料。但是,电极的容量和循环稳定性的矛盾还需要进一步解决。我们认为,多相多尺度复合结构的Sn基合金薄膜负极可以兼顾容量和循环两方面的性能。本文以发展薄膜锂离子电池负极为目的,主要制备出多种具有多相多尺度复合结构的Sn基合金薄膜负极,重点研究电极结构对循环性能改善的作用及机理。首先,以电子束蒸发沉积的Sn-Cu合金薄膜为研究对象,探讨多相多尺度结构对电极容量和循环性能的影响。未退火Sn/Cu薄膜负极虽具有较高的首次放电容量和库仑效率,但由于界面结合不好,循环稳定性差。200℃真空退火后,Sn/Cu薄膜形成Cu₃Sn/Cu₆Sn₅两相复合结构,循环稳定性明显改善。然而,退火处理导致薄膜的表面粗糙度增大,使得首次不可逆容量损失较大,Cu₃Sn/Cu₆Sn₅复合薄膜负极的可逆容量较小。在Cu集流体上直接沉积的Sn/Cu₆Sn₅薄膜负极具有多尺度多相复合结构,单晶Sn微颗粒分散在纳米Cu₆Sn₅相基体中。Sn/Cu₆Sn₅复合薄膜具有较高的Li⁺扩散系数和放电容量,以及优良的循环稳定性。这主要得益于两方面的作用:其一,纳米Cu₆Sn₅相基体及Sn微粒形成的复合结构有效地减缓电极的体积变化和应力释放;其二,在特定电位下,Sn和Cu₆Sn₅相不同时反应,因此两相起到相互保护和抑制体积过度膨胀的作用。其次,为了克服Sn基金属间化合物负极材料的不足,我们率先开展了互不溶Al-Sn合金的负极性能研究。电子束蒸发沉积的Al-Sn合金薄膜仍保持互不溶组织特性,并具有纳米Sn颗粒分散于Al基体中的两相复合结构。研究发现,在Al基体中均匀分散有适量的Sn相可以有效改善Li⁺在其中的扩散动力学。因此,Al-Sn薄膜具有优于纯Al和纯Sn薄膜负极的循环稳定性。不同成分的Al-Sn薄膜有不同的电化学性能,中Sn成分的Al-x wt%Sn（40≤x≤60）合金负极能同时兼顾容量、结构稳定性和嵌锂/脱锂反应动力学等方面的性能,因此它们具有良好的负极性能。Al-40%Sn薄膜负极中的Sn相有两种不同的形貌特征,形成特殊的多尺度纳米相复合结构,它的稳定可逆放电容量为600mAh/g。再次,我们研究了另一种互不溶体Sn-C复合薄膜负极的微观结构与电化学性能。以TiNi合金作为石墨的熔融媒介,可较容易实现非晶碳在薄膜中的电子束蒸发沉积,获得Sn-C-Ni复合薄膜负极。Sn-C-Ni复合薄膜中的活性物质颗粒具有多尺度与核/壳结构,其内核为单晶Sn微粒,外壳为纳米Sn和Ni粒子分散于sp²非晶碳中形成的复合物。它的首次不可逆容量较低,1/10C和12C的放电容量分别为1872和472 mAh/g;在1C倍率下40次循环后的放电容量仍超过600 mAh/g。多尺度核/壳结构有效提高了复合薄膜负极的结构稳定性和Li⁺扩散动力学;非晶碳基体及其中的Ni纳米粒子有效抑制了纳米Sn相的团聚,从而提高复合薄膜负极的循环稳定性。最后,我们首次将TiNi形状记忆合金与Sn基电极材料相结合,设计出多种结构的Sn-TiNi复合薄膜,尝试利用NiTi合金相的应力诱发相变与超弹性来实现Sn负极循环性能的改善。通过一步共溅射法,可制备出非晶TiNi与Sn相复合的薄膜,其微观结构为多尺度Sn颗粒均匀分散于非晶TiNi合金基体中。该结构薄膜负极具有优良的循环稳定性和突出的高倍率放电性能。1C倍率的稳定容量为520mAh/g,经过40次循环后15C的可逆容量仍高达372mAh/g。这是三方面综合作用的结果:其一,非晶TiNi相作为活性物质Sn及Li_xSn相的良好连接导体,且有效阻止纳米Sn颗粒的团聚。其二,复合薄膜中Sn相的纳米尺度有效缩短了Li⁺在其中嵌入和脱出的扩散路径。其三,Sn相的多孔结构有利于电解液在电极中的快速浸润和提高Li⁺的扩散动力学。采用分步溅射的方法,我们成功制备出三明治结构的B2-NiTi/Sn/a-TiNi（简称B2/Sn/a）薄膜负极,并用它验证了B2-NiTi相的超弹性对Sn电极循环性能的改善作用。电极放电时B2-NiTi层能发生应力诱发马氏体相变而产生超弹性。得益于B2-NiTi层的超弹性对Sn层体积膨胀效应的限制和容让作用,B2/Sn/a薄膜负极具有优良的循环稳定性和高倍率性能。0.7C和2.7C倍率充放电时,100次循环后电极的可逆容量分别为630和500mAh/g。B2-NiTi合金相改善Sn负极循环性能的作用机理可述如下:当电极放电时,Sn→Li_xSn转变产生的体积膨胀应力诱发NiTi发生马氏体相变和超弹性应变。因此,Sn和Li_xSn的应变能部分转化为NiTi合金的相变能和应变能,同时,容让Sn的部分体积膨胀,电极膜层的破坏程度小。当电极充电时,Li_xSn→Sn转变导致Li_xSn和NiTi相中的应力释放,马氏体相逆转变为母相,NiTi相的弹性应变回复对Sn相产生压缩的作用,使裂开的Sn闭合,抑制粉化。上述Sn和NiTi相变的协同作用克服了Sn负极的体积膨胀效应。基于相同的作用机理,NiTi形状记忆合金的应力诱发马氏体相变和超弹性也将可以用于改善Al、Si、Sb等金属基负极材料的循环性能。更多还原

【Abstract】 Developing new advanced anode materials with higher energy density, long life and improved safety is of great importance for both the large-scale and miniaturized lithium ion batteries （LIB）. Sn-based alloys have been widely studied as alternative anode materials for LIB due to their high theoretical capacity and moderate operation potential. However, their capacity and cycle performance should be further improved to meet the requirements for practical applications. Multiphase and multiscale structures have been demonstrated to be benefit to both of the capacity and cycleability of Sn-based anodes. This dissertation addresses the preparation and characterizations of various Sn-based thin films with multiscaled multiphase structures, emphasizing the influences of microstructure on the cycle performance and their mechanisms. The focus on film electrode is also aiming at development of all-solid-state thin film battery.Firstly, Sn-Cu thin films prepared by electron beam deposition （EBD） are discussed. The as-prepared Sn/Cu thin film anode has high initial discharge capacity and coulombic efficiency but poor cycleability. The Cu₃Sn/Cu₆Sn₅ composite structure, formed in the Sn/Cu thin film after annealing at 200℃in vacuum, exhibits obvious enhancement on the cycle performance. However, due to the large irreversible capacity loss associated with increase of surface roughness of annealed electrode, the Cu₃Sn/Cu₆Sn₅ thin film anode delivers small reversible capacity. Therefore, a Sn/Cu₆Sn₅ composite thin film has been directly prepared on the Cu foil by EBD, which has a structure of polyhedral micro-sized Sn grains uniformly dispersed in the Cu₆Sn₅ matrix. The Sn/Cu₆Sn₅ composite thin film anode has higher Li⁺ diffusion rate and discharge capacity, and better cycleability than those of the Cu₃Sn/Cu₆Sn₅ anode, which benefits from the nanostructure of Cu₆Sn₅ matrix and the different lithiation potentials of Sn and Cu₆Sn₅ phases. This demonstrates that the multiphase composite structure can improve electrochemical performance of the Sn-Cu alloy anodes.Secondly, in order to overcome the shortages of the Sn-based intermetallic anode materials, the immiscible Al-Sn alloys have been explored as lithium ion anode materials for the first time. Al-Sn thin films prepared by EBD have complex structures of Sn phases homogenously dispersed in the Al matrix, in which the Sn phases act as diffusion channels to enhance the Li⁺ diffusion kinetics. Thus, the cycle performance of Al-Sn thin film anodes is much better than those of the pure Sn and pure Al thin film anodes. It has been found that the composition of Al-Sn anodes has obvious influence on their cycle performance. The Al-x wt% Sn （40≤x≤60） thin film electrodes show a good balance among cycling ability, fast Li⁺ diffusion and acceptable capacity. In particular, the Al-40wt%Sn thin film anode has a unique multi-scale composite structure with faceted big Sn particles and Sn nanocrystallites, and its stable reversible capacity is about 600mAh/g.Furthermore, another immiscible system, Sn-C-Ni composite thin film anode, has been prepared by EBD using TiNi alloy as a reaction medium. The thin film has a multi-scale structure composed of lots of micro-sized core/shell particles, in which the cores are Sn single crystals and the shells are amorphous carbon with nano-size Sn and Ni particles dispersion inside. Both of the Sn and the sp2 amorphous carbonaceous shells react with lithium and give substantial contributions to its total high initial capacities of 1872mAh/g at 1/10C, 472mAh/g at 12C. The stable discharge capacity at 1C was more than 600 mAh/g after 40 cycles. These good performances are attributed to the enhanced Li⁺ diffusion kinetics and stability of structure of active materials, resulted from the multi-scale structure of Sn phases and the well coating of nanocomposite carbonaceous shells on the Sn cores as well as the dispersion of nano-size Sn and Ni particles in the amorphous carbon matrix.Finally, for the first time, the TiNi shape memory alloy has been combined with Sn to form different kinds of composite negative electrodes for lithium ion battery. The capacity decay of Sn-based anodes can be to overcome by utilizing the superelasticity of NiTi shape memory alloy. The Sn-TiNi composite thin film, which has unique microstructure of multi-scale Sn nanoparticles uniformly dispersed in amorphous TiNi matrix, has been prepared by one-step co-sputtering. It delivers a stable capacity of 520 mAh/g at 1C and 372mAh/g at high rate of 15C after 40 cycles, indicating good cycle performance and high-rate capability of the Sn-TiNi thin film anode, which is attributed to the following three reasons:ⅰ) the amorphous TiNi matrix acts as good conductors for the active Sn and Li_xSn phases, also effectively prevents the aggregation of Sn nanoparticles;ⅱ) the nano-size Sn phases decrease the path length for Li⁺ transport;ⅲ) the porous structure of thin film facilitates the electrolyte transportation and Li⁺ diffusion. A sandwich structured B2-NiTi/Sn/a-TiNi （named as B2/Sn/a） thin film has been prepared on stainless steel substrate by stepwise sputtering. The capacity decay of Sn anode is overcome by utilizing the superelasticity of B2-NiTi shape memory alloy to accommodate the volume expansion and constrain the pulverization due to Li-Sn alloying. Thus, the B2/Sn/a thin film anode has good cycleability and high-rate capability. The reversible capacities after 100 cycles were 630 and 500mAh/g at current rate of 0.7C and 2.7C, respectively. According to the results of electrochemical and microstructure characterization, we emphasize the mechanism, which the cycle performance of Sn electrode enhances by the superelasticity of B2-NiTi layer, as following. During discharge process, the volume of Sn phase expands due to Sn→Li_xSn and generates large stress, and this spontaneously induces martensitic transformation and superelasticity in the B2-NiTi layer. Thus, the stress in Sn phase can be well accommodated while its volume expansion can be constrained. At the subsequent charge process, the stress in B19′-NiTi and Li-Sn alloys releases due to Li_xSn→Sn. And consequently, the B19′phase transforms back to the B2 phase accompanying with closing of crack and contract of the volume of Sn phases by superelastic recovery of the NiTi matrix. The above interaction between Sn and NiTi shape memory alloy prevents cracking and pulverizing of Sn, and overcomes ultimately the capacity decay of Sn anode in lithium ion battery. We believe that shape memory alloys can also combine with other high capacity anodes, such as Si, Sb, Al and etc, and improve their cycle performance.更多还原