Understanding the Electrolyte Chemistry Induced Enhanced Stability of Si Anodes in Li-Ion Batteries based on Physico-Chemical Changes, Impedance, and Stress Evolution during SEI Formation

Abstract

Volume expansion/contraction of Si-based anodes during electrochemical lithiation/delithiation cycles causes loss in mechanical integrity and accrued instability of the solid electrolyte interphase (SEI) layer, culminating into capacity fade. Electrolyte additives like fluoroethylene carbonate (FEC) improve SEI stability, but the associated causes remain under debate. This work reveals some of the roles of FEC via post-mortem observations/analyses, operando stress measurements, and a comprehensive study of the impedance associated with the formation/evolution of SEI during lithiation/delithiation. Usage of 10 vol.% FEC as electrolyte additive leads to significant improvements in cyclic stability and Coulombic efficiency and facilitates smoother/compact/crack-free surface/SEI, in contrast to the cracked/pitted/uneven surface upon non-usage of FEC. Operando stress measurements during SEI formation reveal compressive stress development, followed by loss in mechanical integrity, upon non-usage of electrolyte additive, in contrast to insignificant stress development associated with SEI formation upon usage of FEC. The proposed electrochemical impedance spectroscopy model facilitates good fit with the impedance data at all states-of-charge, with the SEI resistance and capacitance exhibiting expected variations with cycling and the SEI resistance progressively decreasing with cycle number in the presence of FEC. By contrast, in the absence of FEC, severe fluctuations observed with the SEI resistance and capacitance indicate instability.

Modeling Rate Dependent Volume Change in Porous Electrodes in Lithium-Ion Batteries

“…6,7 As battery pack designs for electric vehicles increase in energy to meet range requirements and mitigate range anxiety 8 for consumers, higher energy density materials 9 such as silicon [10][11][12][13][14][15][16][17][18] containing materials have come into focus. Silicon and other high-capacity anode materials undergo significant volume change [19][20][21][22][23] during lithiation, [24][25][26] which can ultimately impact the performance of the battery pack and electric vehicle due to the engineering changes that must be made to account for the silicon volume change. Additionally, a desire exists to increase the speed 27 at which battery cell designs move from concept to production reality.…”

Ion Depletion Microenvironments Mapped at Active Electrochemical Interfaces with Operando Freezing Cryo-Electron Microscopy

“…Cryogenic electron microscopy (cryo-EM) has emerged as a solution to the latter problem, enabling multimodal studies of beam-sensitive electrode or electrolyte materials and their interfaces; − however, traditional cryo-EM sample preparations for battery materials are ex situ and time-consuming. Batteries must be disassembled under an inert atmosphere after electrochemical cycling, a process that takes on the order of 10 3 s and often damages or destroys solid–liquid interfaces by allowing the electrolyte to dry before freezing. ,, Even when care is taken to preserve and vitrify a layer of electrolyte for cryo-EM, , ion diffusion coefficients are in the range of 10 –6 cm 2 s –1 in typical battery liquid electrolytes and diffusion layers are expected to be 10 –6 –10 –5 m thick; , thus, operando ion concentration profiles should relax well before ex situ freezing occurs. This makes it impossible to directly visualize the local microenvironments and resulting kinetic limitations that underpin safety and stability in practical batteries by using existing characterization techniques.…”