Engineering Si anodes for solving the interfacial issues in lithium ion batteries

Abstract

The demand for high-power and high-energy-density lithium ion batteries (LIBs) has been increasing to meet the ever-growing requirement for use in portable electronics and electric vehicles. Silicon (Si), associated with its natural abundance, low discharge voltage vs. Li/Li+, and ultra-high theoretical capacity (4200 mAh/g), has attracted considerable interests as an alternative to the graphite anode materials used most in conventional LIBs. However, Li alloying with Si is accompanied by a large volume change which induces cracking and rapid pulverization of Si-based anodes. Significant improvements in the anode's lifetime as well as charge-discharge rates have been obtained over the past few years by employing Si nanostructures. Up to date, the issues occurred at the interfaces of LIBs, named as interfacial issues, are still challenging the practical implementation of Si anodes. In this dissertation, we demonstrate two kinds of Si based nanostructures for solving the (i) anode and electrolyte and (ii) anode and current collector interfacial issue, respectively. To solve the anode and electrolyte interfacial issue, we demonstrate a facile method for synthesizing a novel Si membrane structure with good mechanical strength and three-dimensional (3D) configuration that is capable of accommodating the large volume changes associated with lithiation in LIB applications. Furthermore, the existence of the outer SiOx content shows rigid mechanical property and is treated as clamping layer to restrict the major volume expansion with inward direction rather than outward direction, hence to stabilize formation of solid electrolyte interface (SEI) layer. The membrane electrodes demonstrated a reversible charge capacity as high as 2414 mAh/g after 100 cycles at current density of 0.1 C, maintaining 82.3% of the initial charge capacity. The high performance of the Si membrane anode is assigned to their characteristic 3D features, which is further supported by mechanical simulation that revealed the evolution of strain distribution in the membrane during lithiation reaction. This study could provide a model system for rational and precise design of the structure and dimensions of Si membrane structures for use in high-performance LIBs. To solve the anode and current collector interfacial issue, we introduce graphene as an interfacial layer between the current collector and the anode composed of Si nanowires (SiNWs) to improve the cycling capability of LIBs. The atomically thin graphene lessened the stress accumulated by volumetric mismatch and inhibited interfacial reactions that would accelerate the fatigue of Si anodes. By simply incorporating graphene at the interface, we demonstrated significantly enhanced cycling stability for SiNW-based LIB anodes, with retentions of more than 2400 mAh/g specific charge capacity over 200 cycles, 2.7 times that of SiNWs on a bare current collector.