Stabilization Of Silicon And Germanium Based High Capacity Anodes For Lithium Ion Batteries

Lithium-ion batteries (LIBs) have been the driver of the widespread application of portable electronics. As the electronic devices have been more powerful and versatile, the pressing demand for higher energy density batteries have led the extensive explorations of novel materials for high capacity electrodes. Despite several breakthroughs in literature, the current state of art LIBs still uses the battery chemistry developed in early 1990s.The keys challenges for switching to other high capacity electrodes have been high cost, non-linear scalability and poorer electrochemical performance. This dissertation focusses on development of high capacity electrodes in order to increase the energy density of the current state of art lithium ion batteries by using scalable synthesis techniques while investigating the structure-property-performance relations for different materials. Initial work focused on the examination of the role of morphology on the electrochemical performance of germanium anode for lithium ion batteries. This was followed by the study of mass loading on the electrochemical performance of high capacity germanium(Ge) anodes (Chapter 2). More than 100 cycles with high areal capacities (\u3e3 mAh/cm2) are observed with Ge for the first time. Analysis of electrode phase and morphology changes before and after cycling revealed the ability of Ge grains to maintain contact with each other, a key requirement for stable electrochemical performance. To exploit the intrinsic properties of Ge electrode as well as to reduce the amount of expensive Ge in the electrode, solid solutions of the Si and Ge with various compositions were synthesized by solid state synthesis and their performances were examined for LIBs (Chapter 3). Lattice strain was calculated to be highest for Si0.5Ge0.5 suggesting largest resistance to plastic deformation. As expected, Si0.5Ge0.5 offered highest specific capacity (~1560 mAh/g) and highest the capacity retention (87.5%) for 80 cycles. The promising results achieved in SixGe1-x electrode still required substantial amount (~72 wt%) of Ge. Therefore, with the objective of further reducing the fraction of Ge in the electrodes, thin layers of C and Ge were coated on Si to stabilize the electrochemical performance (Chapter 4). The silicon-germanium-carbon (Si@Ge@C) core shell electrodes synthesized at low temperature (580°C) offered superior electrochemical performance (~80% capacity retention after 200 cycles) owing to improved electron conduction and mechanical stability. Finally, a low temperature aluminothermic reduction technique for the synthesis of porous Si was developed and the effect of carbon coatings on the cyclic stability was investigated (Chapter 5)