Field-Limited Migration of Li-Ions in Li-Ion Battery

Abstract

The migration of Li-ions in lithium-ion battery cannot be simply described by Fick's second law; the interactions among ionic migration, field, and stress need to be taken into account when analyzing the migration of Li-ions. Using the theory of thermal activation process, the flux for ionic migration under concurrent action of electric field and mechanical stress is found to be a nonlinear function of the gradient of electric potential and the gradient of stress. Electric field can either accelerate or retard the growth of the lithiation layer, depending on polarity of the field. Si-based materials have attracted great interest for possible use as anode materials due to high specific capacity of Si (3580 mAh/g). However, the specific volume change of Si during electrochemical cycling can reach to ∼400%, 2 which can lead to catastrophic failure of lithium-ion battery (LIB). Low-dimensional Si-structures have been studied in rechargeable LIBs 3-7 to improve the cyclic performance and structural durability of Si-based anodes. The results from the studies 3-7 have demonstrated that low-dimensional Si-structures have the potential to offer high reversible capacity and long service life.Ariel et al. 8 observed that the ability to store and deliver charge for electrochemical cells consisting of LiCoO 2 (cathode)/SiO 2 /Si(anode) was inversely proportional to current density. Zhou et al. 9 suggested that the driving force for Li-migration was the gradient of electric potential and the concentration gradient of Li ions. Recently, Liu et al. 10 noted that electric field accelerated the migration of the reaction front during the lithiation of Si nanowires. They pointed out that the quantitative information about the effects of electrical field on both Li diffusion and interface reaction was not available at the moment of their study. 10The theory of thermal activation process 11 has been applied to study diffusion in solids. 12 The purpose of this work is to analyze electromechanical effect on the lithium migration, using the theory of thermal activation process. We consider the lithium migration under concurrent action of electric field and mechanical stress as a thermal activation process and derive an expression for the flux of ions. Physical ModelConsider a system, which consists of an electric anode, an alloy layer, and electrolyte, as shown in Fig. 1. The alloy layer is bounded by an anode-alloy interface and an alloy-electrolyte interface, as demonstrated by Liu et al. 10 for the lithiation of Si nanowires. Due to the large diffusivity of Li-ions, it is believed that the migration of Li-ions through the alloy layer and the reaction of Li-ions with the atoms of active materials to form Li-compound at the anode-alloy interface determine the growth of the alloy layer.According to the theory of thermal activation process, 11 the probability of an ion moving from a stable state to another stable state is determined by the vibration frequency and the energetic fraction of ions which depends on the energy barrier, Q, betw...

Li-Ions Transport Promoting and Highly Stable Solid–Electrolyte Interface on Si in Multilayer Si/C through Thickness Control

“…So the Si–C interfaces effectively promote the diffusion kinetics of Li-ions along the D ⊥ in Si/C multilayer electrodes. It should be a result of the local polarization of lithiated Si (Li x Si) located at the Si–C interface under an applied electric field. − Li x Si alloy preferentially nucleates and grows at the Si–C interfaces, and its conductivity is much higher than that of Si. As schematically illustrated in Figure , introducing a conducting sphere into a static electric field causes the charges inside the sphere to quickly redistribute near the surface, so that the electric field inside the sphere becomes zero due to the polarization of the sphere, but the local external field becomes stronger.…”

“…The interfacial diffusion pathway of Li-ions differs from the surface-coating directed Li-ions transport and the Se nanowires confined in carbon shows a significantly improved rate capability and cyclic performance. Theoretical calculations combined with experimental research have shown that an unbalanced charge distribution induced a built-in electric field within the CuGeO 3 /graphene, SnS/SnO 2 , and MnO 4 /Bi 2 O 2 heterointerfaces that can boost Li-ions diffusion kinetics and promote rate capability. − The built-in electric field within heterointerfaces can either accelerate or retard the Li-ions diffusion, as reported by Yang …”

“…41−43 The builtin electric field within heterointerfaces can either accelerate or retard the Li-ions diffusion, as reported by Yang. 44 In this work, we use a ∼400 nm Si/C multilayer thin-film electrode prepared by a physical vapor deposition (PVD) method to investigate the effect of Si−C interfaces on Li-ions diffusion kinetics based on uncracked and cracked Si/C multilayer structure. The Si−C interfaces appear to effectively enhance the Li-ions diffusion.…”

Interface Engineering of Silicon/Carbon Thin-Film Anodes for High-Rate Lithium-Ion Batteries

“…To in-depth understand the advantage of multilayered Si/C anodes for high-rate LIB applications, three possible mechanisms are proposed as follows. (1) The enhanced local electric fields by Li x Si alloys formed at Si–C interfaces play a starring role in boosting the performance. It is reported that the Li x Si alloys prefer to nucleate and grow at Si–C interfaces under the applied electric field, resulting in the local polarization of Li x Si alloys at the interfaces. , As illustrated in Figure a, the conductive Li x Si alloy spheres in a static electric field can make the charge redistribution and local polarization near the sphere surfaces, leading to a zero electric field inside the spheres and a stronger local external field. As a result, the locally coupled built-in electric field promotes the Li + ion diffusion in the multilayered Si/C film anodes .…”

“…(1) The enhanced local electric fields by Li x Si alloys formed at Si−C interfaces play a starring role in boosting the performance. It is reported that the Li x Si alloys prefer to nucleate and grow at Si−C interfaces under the applied electric field, 58 resulting in the local polarization of Li x Si alloys at the interfaces. 59,60 As illustrated in Figure 6a, the conductive Li x Si alloy spheres in a static electric field can make the charge redistribution and local polarization near the sphere surfaces, leading to a zero electric field inside the spheres and a stronger local external field.…”