On the Lithiation Mechanism of Amorphous Silicon Electrodes in Li-Ion Batteries

Abstract

Amorphous silicon is a high-capacity negative electrode material for use in advanced lithium-ion batteries. We investigated the mechanism of Li incorporation into and removal from this material during electrochemical lithiation and delithiation using a combination of in operando neutron reflectometry and ex situ secondary ion mass spectrometry. The results indicate that a heterogeneous lithiation mechanism is present for the first cycle and also for subsequent cycles during lithiation and delithiation, where a highly lithiated phase penetrates the silicon electrode. During the first lithiation half-cycle, a two-step process takes place, which is not present for delithiation and higher cycles. In the first step, a Li-poor phase penetrates the silicon electrode leading to about 10% of maximum capacity. Afterward, during the second step, a Li-rich phase moves into the electrode leading to complete lithiation in a slower process. The different phases are separated by a relatively sharp interface of only several nanometers. The Li-poor phase extended over the entire electrode is still present after delithiation in the form of irreversibly trapped Li.

Li Diffusion in Thin-Film LiySi Electrodes: Galvanostatic Intermittent Titration Technique and Tracer Diffusion Experiments

“…The observed results can be explained with the assumption that the small amount of Li introduced into the sample during a GITT pulse (δ x ) does not correspond to simple diffusion into Li 0.02 Si (solid solution). As shown in a recent publication of our group and others, , the electrochemical lithiation of amorphous silicon during the first cycle can be described by a two-phase mechanism with a sharp phase boundary between the Si electrode and a certain Li x Si phase (see Figure for illustration). According to our findings, , first, a Li-poor Li x Si phase penetrates the silicon electrode until the complete silicon is transformed, leading to about 10% of full capacity (with about x = 0.3).…”

“…Presently, Li-ion batteries (LIBs) are one of the systems of choice for advanced rechargeable battery technology due to their low weight and high energy density. − However, for optimization improvements in cycle life, safety, costs, and especially in specific electrode capacity and charging/discharging rate are necessary. In this context, silicon is a promising high-capacity negative electrode material due to its theoretical specific capacity of about 4 Ah/g. − During electrochemical lithiation, Li and Si form an alloy according to Si + y Li + + y e – ↔ Li y Si, a process that is correlated with an enormous volume expansion of up to 400% − and stress formation in the GPa range, both of which are counterproductive to a stable electrochemical performance. A further serious problem is initial capacity losses due to irreversible Li trapping .…”

Electrochemical Lithium Alloying Behavior of Guest-Free Type II Silicon Clathrates

“…As shown in Figure , this alloying reaction occurs at a higher voltage for the Na 1 Si 136 clathrate than for α-Si. It is also a higher voltage than that seen in Na-filled clathrates Na 24 Si 136 (type II) and Na 8 Si 46 (type I), which also alloy with Li to form amorphous phases beginning at around 0.10–0.15 V versus Li/Li + . , However, the alloying potential of Na 1 Si 136 is similar to the voltage seen in the lithiation of amorphous Si (0.25–0.30 V), − suggesting that these materials may be undergoing similar phase transformations (this will be discussed in further detail later). After the plateau at 0.24 V, the profile of Na 1 Si 136 becomes sloped until reaching the voltage cutoff at 10 mV.…”

“…Based on these results, we liken the alloying behavior in the type II Si clathrate to be similar to that in amorphous Si. Numerous mechanisms have been proposed for the first cycle reaction between Li and amorphous Si, with certain analyses showing two-phase reactions between pristine amorphous Si and a distinct amorphous lithium silicide phase, ,, while others show single-phase behavior, ,, depending on the sample conditions and electrode parameters. Despite these opposing descriptions, it is generally agreed that lithium insertion into amorphous Si beyond the first cycle follows solid-solution behavior.…”

Simulation of Spatial Strain Inhomogeneities in Lithium-Ion-Cells Due to Electrode Dilation Dependent on Internal and External Cell Structures

“…Especially with added silicon, the dilation of the anode during operation is increased significantly. Where graphite shows a dilation between 4.3 % and 13.2 % (dependent on the measurement method) [2][3][4][5][6][7], silicon shows a dilation of around 300 % during lithiation [8,9]. Also, cathode materials show a dilation of -0.8 % to 0.9 % (NMC111) [2,10,11] or -2 % to -1.9 % (LCO) during lithiation [3,12].…”