Electrochemical Studies of Nanoncrystalline Mg[sub 2]Si Thin Film Electrodes Prepared by Pulsed Laser Deposition

Abstract

Electrochemically active thin films of Mg 2 Si in various film thicknesses of 30-380 nm have been prepared with the pulsed laser deposition technique. The thinnest film of 30 nm showed a highly stable cycling behavior at 0.1-1.0 V vs. Li, delivering capacity greater than 2000 mAh/g for more than 100 cycles. Though the film morphology became remarkably rougher with cycling, the films showed good stability. However, the first cycle irreversible capacity loss increased with film thickness. Therefore, lithium adsorption/desorption reaction forming Li-Si alloy at the Si-rich film surface is suggested as one of the sources of the large capacity of the 30 nm film. The superior capacity retention, when compared to porous electrodes of this alloy, may be attributed to a limited structural volume change in the two-dimensional film, shorter lithium diffusion path and enhanced conductivity from stainless steel substrate. The goals of this study are to promote the emerging need of thin film anodes for all solid-state microbatteries and clarify the capacity failure of powder intermetallic anodes for rechargeable lithium batteries.The search for anode materials to replace graphite in rechargeable lithium batteries has intensified over recent years due to concerns about safety during overcharge in the presence of organic electrolyte and a highly oxidizing cathode material such as LiCoO 2 . 1 Considerable effort has been devoted to searching for suitable alternative anodes among lithium binary alloys 2-4 and intermetallics 5-10 which operate a few hundred millivolts above metallic lithium. Magnesium silicide alloys have been studied for this application because Si has a good affinity for lithium. 11-16 Consensus regarding the electrochemical reaction between Li and Mg 2 Si has not been reached. Part of the reason may be due to the fact that Mg 2 Si is a semiconductor whose performance will be dependent on particle morphology and current density, and porous electrodes from alloyed powders show rapid capacity fade over the first ten cycles. It has been generally recognized that lithiation of the alloying metal elements takes place by significant volume change causing electrochemical-mechanical disintegration of particles during cycling. 5-15 Many attempts have been made to improve the electrochemical performance of the alloy anode materials by means of controlling the crystal structure to achieve a small lattice volume change and a particle morphology subject to a minimum mechanical stress, based on the study of capacity failure mechanisms of those alloy anodes. 17-20 Much can be learned regarding the performance of low conductivity materials when they are in thin film form. 16,[21][22][23]

Amorphous and nanocrystalline Mg2Si thin-film electrodes

“…Self-discharge reaction was certainly observed in the films, reflecting side reaction with the electrolyte forming a SEI layer. 5 The SEI layer usually forms during the initial cycle. Much of the 30 nm thick amorphous film may be utilized for the SEI layer preventing further growth of surface layer by the electrode-electrolyte reaction during cycling, resulting in stable cyclability.…”

“…5 The film thickness, determined using scanning electron microscopy, was 30 to 380 nm depending on deposition time. The 30 nm thick film was amorphous to X-ray diffraction, whereas the 380 nm thick film was poorly crystalline.…”

“…4 We have prepared Mg 2 Si thin film electrodes on a nanometer scale by pulsed laser deposition (PLD) and some films showed excellent cyclability to 200 cycles. 5 Our previous studies of the films have shown a strong dependence of the electrochemical properties on the film crystallinity and thickness, providing insight into the capacity failure mechanism of powder electrodes. Examination of the film micro-structure using transmission electron microscopy should be more helpful than X-ray diffraction.…”

Nanostructured magnesium silicide Mg2Si and its electrochemical performance as an anode of a lithium ion battery

“…The observed absence of certain cathodic peaks corresponding to the formation of Li-Si (0.25 and 0.09 V) and Li 2 MgSi (0.2V) phases when compared with the differential capacity plot is probably caused by kinetical hindering of the specific reactions [34] when the experimental conditions of the CV measurements are applied.…”

“…For the delithiation process, there are four anodic peaks related to the removal of lithium which are located at 0.19, 0.28, 0.46 and 0.6 V. The peaks at 0.19 and 0.28 V are related to the dealloying of Li-Mg [33] and Li-Si [34] alloys, respectively. At the same time, the deintercalation of lithium from Li 2 MgSi is associated with the peak at 0.6 V [14].…”