Cycle-stable Si-based Composite Anodes For Lithium-ion Battery

Developing a new generation of lithium ion batteries (LIB) with substantially enhanced energy density and safety is an urgent demand for a number of new energy technology applications ranging from portable electronics to electrical vehicles and to renewable energy storages. A critical issue for this technology development is to find safer and higher capacity anodes to substitute the graphite anodes used in current LIB technology, which have only a theoretical capacity of372mAh g-1. In this technological pursuit, Silicon has received great attention as a new anode material, because of its extremely high Li-storage capacity, which is ten times higher than the theoretical capacity of graphite anode. However, the commercial development and application of Si anodes is still seriously hindered by their poor cyclability resulted from the huge volumetric changes (>300%) during lithium insertion/extraction cycles. With an eye to developing cycle-stable and high capacity Si-based anodes, this PhD work takes three typical Si-based anodes, Si alloy, Si oxides and nano Si powder as the study objects and investigates the effects of nanosizing, alloying, inactive buffering and surface coating on their electrochemical performance, so as to find out an effective means for improving the cycling performance of these Si-based anodes. As a result, several kinds of highly cycleable Si-based composite anodes were developed successfully, including FeSi2/Si@C, SiC/SiO-Si/C and nano-Si/CP (conductive polymer). The main contents and results are summarized as follows:1. Preparation of Si-based alloy composite anode with long-term cycling stability. Considering the high hardness of Si alloy and the easy exfoliation of graphite under external force, A core-shelled FeSi2/Si@C nanocomposite was prepared by first high-energy milling FeSi2/Si alloy to obtain a nanosized alloy powder and then carbon-coating the surface of the FeSi2/Si alloy by further planetary milling the alloy powder with graphite to produce the core-shell nanoparticles. The electrochemical performance of the FeSi2/Si@C composite anode was optimized by adjusting the content of active Si in the inner core, the mass ratio of alloy to graphite, as well as the binder for electrode preparation. The experimental results from the gavanostatic charge-discharge measurements showed that the thus prepared FeSi2/Si@C composite anode using an optimized PAA binder exhibits a high Li-storage capacity of～1010mAh g-1and an excellent cyclability with94%capacity retention after200cycles. Even at a very high current of1000mA g-1, the composite anode can still deliver a reversible capacity of>700mAh g-1. Such an excellent cyclability obviously arises from the highly effective biphasic buffer in the FeSi2/Si@C structure. On the one hand, the highly dispersed FeSi2in the inner alloy core acts as an inert inactive phase to buffer the volume expansion of the Si lattices. On the other hand, The graphite-coated outer shell acts as both a conductive network and an additional confining buffer layer, providing enhanced electron conduction and a strong restriction for the volume change of the inner active Si cores during Li alloying and de-alloying cycling.2、Preparation of Si-based oxide composite anodes with stable cyclability. During the first lithiation process of Si oxide anodes, nano-sized amorphous silicon clusters is formed and surrounded by in situ generated products of lithium oxide and lithium silicate, which in turn, can act as buffer matrixes to alleviate the volume expansion of the active silicon clusters in the following Li alloying and de-alloying cycles. For this reason, Si oxide anodes usually exhibit improved Li alloying and de-alloying behaviors in some extent. To further stabilize the cycling performance of the Si oxides, we detailedly investigated the effects of nano-sizing, inactive buffering and surface carbon-coating on the electrochemical performance of SiO and SiO2anodes. The experimental results demonstrated that nanosizing of Si-based oxides can effectively improve the utilization of the active component and thereby achieve a higher Li-storage capacity. While inert buffering matrix and surface carbon-coating can greatly enhance the cycling stability of the Si-based oxide anodes. As an example, a SiC/SiO@C composite prepared by nano SiC-assistant ball milling the mixture of SiO and graphite exhibits high reversible capacity of1008mAh g-1, superior cycle stability with85%capacity retention after200cycles. 3. To investigate the influence of the surface carbon-coating on the cycling performance of the Si-based anodes, several types of SiO@C composites were prepared using different carbon source as surface coating layer, including sugar pyrolytic carbon, polyacrylonitrile (PAN) pyrolytic carbon and chemical vapor deposition carbon. Our experimental results demonstrated that a complete, uniform and dense carbon coating layer can effectively protect the active Si core from contact with electrolyte, thus considerably avoiding the destruction and reconstruction of the SEI film on the Si surface during cycling. As a result, a vapor-decomposition carbon-coated SiO-Co/C composite shows a considerably high capacity of1046mAh g-1and an excellent cycle performance with99%capacity retention after100cycles.4. To increase the initial coulombic efficiency of the Si-based oxide anodes, a number of Si-SiOx hybrid anodes were prepared by direct ball-milling Si with SiO2or SiO, so as to increase Si/O atom ratios in the composite anodes, thus achieving a high Li-storage capacity and enhanced coulombic efficiency. The experimental results showed that the Si-SiO2/C hybrid anode with a moderate Si/O atom ratio of1:0.4exhibits a high Li-storage capacity of1636mA h g-1and a good cyclability with75%capacity retention after200cycles at a high charge/discharge rate of1000mA g"1. Encouraged by the high performance of the Si-SiO2/C hybrid anode, we prepared a SiC/SiO-Si/C hybrid anode by SiC-assistant ball milling SiO-Si(x) mixture with graphite and tested its electrochemical performance. Our experimental results showed that SiC/SiO-Si/C hybrid anode can exhibit high Li-storage capacity and excellent cyclability with87%capacity retention after200cycles at a charge/discharge rate of500mA g-1. As the mass content of Si in the composite increases from8%to16%and32%, the initial coulombic efficiency increased from64%to65%and73%, respectively.5. A new strategy is proposed to enhance the long-term cyclability of Si anode by embeding nano-Si particles in a Li+-conductive polymer to form a Si/polymer composite with core-shell structure, in which nano-Si cores act as active Li-storage phase and the polymeric matrix serves not only as a strong buffer to accommodate the volume change, but also as a protection barrier to prevent the direct contact of Si surface with electrolyte, so as to maintain the mechanical integrity of Si anode and suppress the repeated destruction and construction of solid electrolyte interphase (SEI) on the Si surface. To realize this strategy, we chose two kinds of n-type PPP (polyparaphenylene) and PBT as polymer matrix to synthesize nano-Si/CP (conductive polymer) composites simply by ball-milling the Si nanoparticles with conductive polymer, respectively. The experimental results demonstrated that the nano-Si/PPP composite and nano-Si/PBT composite both deliver high capacity of>1000mAh g-1after1000cycles at a high current of3000mA g-1and an excellent rate capability with a considerably high capacity even at a very high rate of16A g-1, showing a great prospect for battery application.