Effect of Surface Termination on Electrochemical Performance of Silicon Thin Films

Abstract

The surface termination layer of silicon-based lithium-ion anodes is a complex mixture of silicon oxides, hydroxides, and hydrides. The species present reflect the history of the electrode and combine to make each silicon electrode different in its electrochemical performance and reactivity with the electrolyte. This variability creates challenges for silicon-based anodes as it affects SEI formation and stability, columbic efficiency, and irreversible capacity. To elucidate some of the parameters that control surface functionality we used non-aqueous electrodeposition to produce silicon thin films on copper foams. These porous electrodes were studied using 29 Si MAS NMR to correlate the synthetic process and surface functionality with electrochemical performance. We found that the supporting electrolyte salt used has a significant effect on the electrodeposited silicon thin films electrochemical properties. Films deposited using tetrabutylammonium chloride (TBACl) were found to be more crystalline with much higher surface concentration of hydrides, hydroxides and oxides, have lower capacity, and much higher irreversible capacity than films deposited using tetraethylammonium chloride (TEACl). The role of supporting electrolyte salt is hypothesized to be related to the amount of free solvent in the reaction mixture as the TBACl salt has been reported to tie up significantly more solvent due its larger size. For many new LIB applications, the space available for the energy storage component in the end device is at a premium. This need is driving the search for new couples that can be designed, modified, or optimized to meet these volumetric goals. Among the materials under study, silicon is a promising alternative anode material to replace graphitic carbon in Li-ion batteries. Its attributes, including high specific capacity, both gravimetric (>3500 mAh/g) and volumetric (>2194 mAh/cm 3 ), and a low charge/discharge insertion/de-insertion voltage at room temperature make it desirable for many applications, however its large volume expansion (>300%) on cycling, poor capacity retention, and Coulombic inefficiencies have limited its use to date. 1,2 While many possible solutions have been proposed, including electrode surface modification, silicon dilution via creation of a composite anode, e.g. (Si-C), identifying new binders that control the expansion, or new electrode designs that contain the expansion, none alone have met the challenge. 3,4 While several studies have focused on the properties of silicon, its morphology, or its fabrication into an electrode, aspects of its electrochemical cycling, such as Coulombic inefficiencies, are related in part to the system level problem of how the silicon interface interacts with the electrolyte. Edström et al., utilizing Soft X-Ray Photoelectron Spectroscopy, was able to not only track SEI and surface changes within an electrochemical cell but was able to differentiate interfacial reaction products forming on the surface of underlying electrode materials.5 Philippe et al.,...

A Green and Low-Cost Approach to Synthesize Bamboo Charcoal@Si@C Composites as Anode for High-Performance Li-Ion Batteries

“…This indicates the existence of meso-and micropores. 21,49 While, for samples of Si and BC@Si@C, both present the typical type Ⅱ, with a sharp capillary condensation step in the high-pressure range, which indicated meso-and macropores character. 26,50 According to the test reported, the specific surface area of BC is as high as 330 m 2 g −1 , while the specific surface area of Si and BC@Si@C is much smaller, which are 41 m 2 g −1 and 53 m 2 g −1 , respectively.…”

Utilization of 29Si MAS-NMR to Understand Solid State Diffusion in Energy Storage Materials

“…In these examples, the spinel cation ordering was identified by diffraction methods, in-situ spectroscopic techniques, thermal properties, and electrochemical properties to correlate the structural differences to the electrochemical properties. In this report, the use of local-probe 29 Si Solid State Magic Angle Spinning (MAS) NMR spectroscopy is reviewed for its uses as a tool to better understand the role of annealing temperature, precursor, and sample history on the properties of energy storage materials (Blanc et al, 2013;Delpuech et al, 2016;Dogan and Vaughey, 2016;Michan et al, 2016). Specific examples from the literature in the fields of solid-state electrolytes, electrode formation, and silicon-based anode materials will be used to create a better understanding of the synthetic processes involved.…”

“…The locations of the 29 Si NMR peak shift values, reported relative to a tetramethylsilane (TMS) standard, are directly related to the shielding of the 29 Si nucleus by the electronic structure in its immediate environment. Therefore, 29 Si MAS NMR spectroscopy provides direct information about the structure of silicon compounds via measurements of the isotropic chemical shifts (Blanc et al, 2013;Delpuech et al, 2016;Dogan and Vaughey, 2016;Michan et al, 2016). The observed silicon chemical shift is influenced by: 1) the coordination number of the Si, 2) the nature of the nuclei in the first coordination sphere, and 3) the local symmetry of the silicon's environment.…”

In Situ Measurement of the Deformation and Elastic Modulus Evolution in Si Composite Electrodes during Electrochemical Lithiation and Delithiation

“…3579 mAh/g) and low lithiation potential, Si is considered to be one of the most promising materials to replace graphite as anode electrodes in the next-generation lithium-ion batteries. 1,2 However, the large volumetric changes of Si (∼270% when fully lithiated) during lithiation and delithiation are known to cause electrode damage and consequent performance degradation, 3,4 which is a bottleneck constraint for its future application. Therefore, experimentally systematic investigations of the Si electrode deformation during Li + insertion/extraction and internal mechanism analyses of its performance evolution have important scientific significance for the development of new lithium battery electrode materials.…”