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Title:Mechanics of deformation and fracture of silicon electrodes in high-capacity lithium-ion batteries
Author(s):Wang, Haoran
Director of Research:Chew, Huck Beng
Doctoral Committee Chair(s):Chew, Huck Beng
Doctoral Committee Member(s):Lambros, John; Geubelle, Philippe H.; Ertekin, Elif
Department / Program:Aerospace Engineering
Discipline:Aerospace Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Lithium ion batteries
Silicon electrode
Density functional theory
Molecular Dynamics
Multiphysics modeling
Finite element method
Abstract:Lithium ion batteries, a high energy density system, store energy by insertion of Li ions into solid electrodes. Silicon is one of the most promising electrode materials for high performance lithium ion batteries, since it has a specific capacity of ~10 times higher than conventional graphite electrodes. During lithiation, the Si electrodes form LixSi compounds, and undergo a huge volume expansion of ~300%. The inhomogeneous deformation during charge-discharge cycles will subject the Si electrodes to large stresses, massive cracking and subsequent loss of capacity. The objective of my Ph.D. dissertation is to elucidate the mechanisms of deformation and failure of the Si electrodes during the electrochemical cycling process, using atomistic simulations, comprising of density functional theory (DFT) calculations and molecular dynamics (MD) simulations. The specific objectives are to: (i) quantify the deformation behavior of the Si electrodes as a function of Li concentration, (ii) understand the mechanisms underlying the eventual delamination of the Si electrode from the current collector after a number of charge-discharge cycles, and (iii) elucidate the properties of the solid electrolyte interphase (SEI) formed between Si electrodes and the electrolyte, and understand how this interphase layer influences the lithiation-deformation behavior of Si electrodes. The stress-strain response of lithiated Si electrodes, LixSi, was calculated with DFT, and the underlying mechanisms explaining the brittle-to-ductile transition of LixSi with increasing x was uncovered. Results show that plasticity initiates at x < 0.5 with the formation of a craze-like network of nanopores separated by Si-Si bonds, while subsequent failure is still brittle-like with the breaking of Si-Si bonds. Transition to ductile behavior occurs at higher Li concentration of x ≥ 1 due to the increased density of highly stretchable Li-Li bonds, which delays nanopore formation and stabilizes nanopore growth. At a higher length scale, these changes in the bonding properties ultimately translate into significantly higher flaw tolerance of LixSi alloys, as revealed by large-scale MD simulations. Other studies have successfully demonstrated the mitigation of Si electrode cracking through the use of small-sized electrodes. However, the delamination of crack-free electrodes from the current collector was still reported after a number of charge cycles, resulting in the loss of electrical contact and subsequent capacity fade. To gain insights into this delamination process, ab initio calculations were used to reconstruct the interdiffused Li-Si-Cu interphase structure between the lithiated Si electrode and a Cu current collector. Under shear deformation induced by Si expansion, well-delineated and weakly bonded Si-Cu and Li-Cu crystalline atomic layers form within this interphase structure and then sliding can happen between the electrode and the current collector. This interfacial sliding will help release stresses introduced by the lithiation process. However, it can be terminated by the formation of LiSi3 compounds across the Si-Cu and Li-Cu atomic layers, causing the build-up of interfacial stresses and eventual delamination of the Si electrode from current collector. One other consequence of the huge volume expansion of LixSi electrodes during charge cycling is the cracking of the ~100 nm thick SEI layer. To understand how strain is transferred from LixSi to the SEI and induce cracking, DFT calculations were performed to quantify the stress-strain response of two major inorganic SEI components-LiF and Li2O-bonded to LixSi. Results show that LiF, effectively bonded on LixSi at x > 1, enables the entire LiF-LixSi interface structure to deform plastically by forming delocalized stable voids and thus can better accommodate the volume changes of the Si electrodes. In contrast, Li2O tightly bonded to LixSi is stiffer, and deforms rigidly across all x. These results explain the significantly improved ductility of SEI with higher LiF versus Li2O content per experimental observation.
Issue Date:2018-01-04
Rights Information:Copyright 2018 Haoran Wang
Date Available in IDEALS:2018-09-04
Date Deposited:2018-05

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