Electrochemical Behaviors of Silicon Electrode in Lithium Salt Solution Containing Alkoxy Silane Additives

Abstract

The electrochemical behaviors of a silicon thin-film electrode in organic lithium salt solution were explored with focus on irreversible reactions of the first lithium charge and discharge cycling by using electrochemical quartz crystal microbalance ͑EQCM͒ combined with various electrochemical techniques. A considerable increase in mass of the silicon electrode was observed during lithium charging even before lithium absorption into the electrode, which is ascribed to the buildup of electrolyte reduction products on the silicon surface. Galvanostatic charge-discharge experiments combined with ac impedance spectroscopy demonstrate a significant overpotential growth and an aggravating capacity for the lithium charge and discharge cycling, and suggest they are due to the sedimentation of electrolyte reduction product. Additives containing alkoxy silane functional groups were evaluated as a passivation agent for lithium rechargeable batteries utilizing a silicon anode. The presence of additives in electrolyte suppressed the mass accumulation to the silicon electrode caused by irreversible electrolyte reductions and improved the electrode for cycle life. Electrochemical analyses associated with EQCM as a function of the number of alkoxy functional groups of the additives illustrate that the silicon electrode is passivated by chemical reaction of the alkoxy silane functional group of the additives with hydroxyl groups at the electrode/electrolyte interface, and this passivation improves the cycle life.Silicon as a negative electrode for lithium-ion batteries has been attracting significant interest because of its high specific capacity. 1-6 Silicon reacts with lithium to form Li 4.4 Si alloy, showing theoretical capacity of 4200 mAh/g. Capacity retention is the most important issue for utilizing the silicon-based negative electrode. 1,2,4 It has been generally accepted that the limited cycle life of the silicon electrode is ascribed to severe volume changes of the electrode. 5,6 Many researchers have focused on the structural modification of the silicon electrode to prolong the cycle life of the electrode. 3,4,6 In crystalline silicon, intermetallic phases are formed during lithium charging, leading to inhomogeneous volume expansions which can cause cracking and pulverization of the silicon-lithium alloy. 3,5 Reversible lithium cycling with little capacity loss is possible for amorphous silicon thin film in which the homogeneous volume expansion occurs. 5 To suppress the volume expansion during lithium charging, the silicon particle of the fine grain size uniformly distributed within a less active matrix, like graphite, has been proposed. 7 Organic lithium salt solution reduces electrochemically on the silicon electrode to form a solid electrolyte interphase layer on the electrode because the lithiation potential of the silicon electrode is far lower than 1.0 V Li/Li + . However, few studies on the interface between silicon electrode and organic electrolyte have been conducted. Unlike graphite electrode, it is reasonab...

Interfacial structural stabilization on amorphous silicon anode for improved cycling performance in lithium-ion batteries

“…In order to prevent the direct reaction between Si and electrolyte, some surface protection layer is necessary. We have considered silane molecules as an effective agent for protecting the surface of Si, since they can form siloxane with Si O Si molecular linkages by self-assembly condensation between their alkoxy groups and silanol groups of Si surface [11][12][13]. The presence of trace water in the electrolyte also assists the hydrolysis of silanes and polycondensation on neighboring silane molecules forming siloxane.…”

Stable silicon-ionic liquid interface for next-generation lithium-ion batteries

“…The onset of these rapid mass changes are visible in the voltage trace of each system through the 'potential overshoot' phenomenon, a known artefact of electrolyte decomposition in EQCM experiments (further explained in the Supplementary Information) 48 . The onset of this rapid mass change occurs earlier (20.5% lithiation) and at a significantly higher rate (reaching 73 g mol À 1 of e À at 57.8% lithiation) in a conventional organic electrolyte.…”

Additives for Functional Electrolytes of Li-Ion Batteries

“…If FEC indeed protects the Si anode through the VC mechanism, then the LiF generated through the first elimination step may have some positive effect on the Si anode as well. Other additives that work on the Si anode include LiDFOB [35], succinic anhydride [80], alkoxysilanes [108,118] and tris(pentafluorophenyl) borane [47].…”