Surface-modified silicon alloy anode materials with high capacity and good cycling stability for lithium-ion cells

Abstract

Rechargeable lithium-ion batteries have received a lot of attention in various applications such as electric vehicles and large-scale energy storage systems. To meet the recent demand for the above-mentioned applications, their cycling performance should be further improved in terms of energy density, cycle life and safety. As an anode material, silicon is a promising candidate to replace graphite due to its high capacity, low reduction potential and environmental friendliness. However, the implementation of Si-based anodes has been challenging due to the detrimental electrode pulverization resulting from large volume changes and the instability of the solid electrolyte interphase (SEI) layer formed on the electrode surface during cycling, which results in rapid capacity fading upon cycling. Various approaches have been suggested to address these problems, including synthesis of nano-sized Si materials with various morphologies, Si-based active/inactive composites, porous Si-based materials, Si/SiOx nanocomposites, embedding silicon in conductive carbon, utilizing highly adhesive and self-healing polymer binder materials. In this dissertation, I invest several ways to improve the discharge capacity and cycling stability of anode materials for lithium-ion batteries. In chapter 2, silicon alloys composed of silicon nanoparticles embedded in inert Cu-Al-Fe matrix phases were synthesized and encapsulated with reduced graphene oxide (rGO) nanosheets. Successful synthesis of the silicon alloys and their encapsulation with rGO were confirmed by X-ray diffraction, X-ray photoelectron spectroscopy and transmission electron microscopic analyses. The silicon alloy encapsulated with an optimal amount of rGO delivered an initial discharge capacity of 1140.7 mAh g-1 with good capacity retention (90.4% retention after 100 cycles) and exhibited excellent rate capability. We attributed this superior performance to encapsulation of the silicon alloy by rGO, which effectively accommodated large volume changes during cycling and provided continuous electronic pathways in the electrode. In chapter 3, we synthesize silicon nanoparticles embedded in the inactive Al4Cu9, AlFe and TiFeSi2 matrix phases, as an anode material. The silicon alloy material exhibits good high rate performance and delivers a high initial discharge capacity of 1459.3 mAh g-1 with capacity retention of 85.7% after 200 cycles at a current density of 300 mA g-1. The superior cycling performance of the silicon alloy compared to that of micro-sized pure silicon can be attributed to the unique structure of the alloy material. Here, the nano-sized silicon particles reduce the ionic diffusion path length and minimize volume expansion during lithiation, while the inactive matrix phases accommodate volume changes during repeated cycling and provide a continuous electronic conduction pathway to the silicon nanoparticles. In chapter 4, we demonstrate the effectiveness of double-layer coating of anode active materials for improving the cycling performance and the suppressing volume expansion of lithium-ion cells. A new silicon alloy material composed of silicon nanoparticles embedded in Al–Fe–Ti matrix phases was synthesized and characterized. Using this anode active material, double-layer coated with porous rGO and poly(3,4-ethylenedioxythiophene)-co-poly(ethylene glycol). The surface modified silicon alloy material exhibits good high rate performance and delivers a high initial discharge capacity of 1145.8 mAh g-1 with capacity retention of 73.1 % after 300 cycles at a current density of 615 mA g-1. The protective double layer formed on the surface of Si alloy materials effectively accommodate volume changes during repeated cycling and provide a continuous electronic conduction pathway to the Si alloy materials. This effective design strategy can be adopted to improve the cycling performance and suppress volume change of other anode materials used in lithium-ion batteries.