Multiscaled-multiphase Structure And Cycle Performance Of Sn-based Thin-film Anodes For Lithium-ion Batteries

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.