(Invited) Study of SEI on High Performance 3D SiNWs and Sinps Based Anode for Lithium Ion Batteries

In order to increase the energy density of the lithium battery, better anodes and cathodes are still required. Silicon has attracted much attention because its theoretical capacity is 4200mAhg⁻¹, an order of magnitude greater than that of graphite. Nevertheless, the main disadvantage of high-capacity anode materials is their very large volume expansion and contraction (~320% ) during Li insertion/de-insertion, followed by cracking and pulverization of the anode material. Si nanostructures have the advantage of a shorter diffusion distance for lithium species, which can improve the power performance of the battery. Studying of large surface capacity anodes (high Si loading) is almost nonexistent. Several degradation mechanisms are involved in the charge-discharge process of SiNWs-based anodes, including: 1) SEI thickness and resistance increase, 2) solvent and salt reduction which leads to drying out of the electrolyte and to precipitation of a "secondary" porous SEI, 3) large increase in battery impedance and reduced power, 4) breaking or disintegration of the SiNWs, 5) loss of SiNW contact to the current collector and 6) cracks in the electrode due to large volume expansion of the SiNWs. In this work, in addition to the synthesis and characterization of novel three-dimensional high-capacity SiNWs and SiNPs-based anodes, we focused on studying their degradation mechanisms.  We have been able to produce remarkably high loadings of 3-15 mAh/cm², very low irreversible capacity (of the order of only 10% for the 3-4 mAh/cm² samples), current efficiency greater than 99.5% and a fast charge–discharge rate (up to 2.7C (20mA/cm2) which is not common for silicon anodes). These properties meet the requirements of lithium batteries for portable and electric-vehicle applications. These SiNWs-based binder free anodes and the SiNPs anodes have been cycled for over 200 cycles, exhibiting a stable cycle life. Loss of capacity, at 0.1 mA/cm², is about 20% only indicating that at least 80% of the SiNWs and the SiNPs are still connected to the substrate. It was found that both the SEI and the diffusion impedance grow with cycling. The thickness of a freshly formed SEI on lithium or on other substrates such as nickel, copper, stainless steel and carbon is a few nanometers only (the tunneling range of electrons (Peled 1979). During open circuit voltage conditions, and further exacerbated on cycling, the thickness and the resistance of the SEI grow with time. It was found that all measured parameters for the 3D SiNWs anode: R_sei, ρ_sei and SEI thickness grow with cycle number. For example: the SEI thickness after one and eleven cycles is 22 and 72 nm respectively and SEI resistance, of lithiated SiNWs anodes, grows from 17.8 (first cycle) to over 109 ohms.cm² (after 71 cycles). The structure and composition changes of the SiNWs, the SiNPs and of the SEI will be reported.