Kejie Zhao's blog

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Symposium on Li-ion batteries at the SES 50th annual meeting

Dear Colleagues,

Please consider to attend the symposium entitled  "Lithium ion batteries - when chemistry meets mechanics"  at the occasion of the Joint Society of Engineering Science (SES) 50th Annual Technical Meeting and ASME-AMD Annual Summer Meeting, July 28-31, 2013, at Brown University.

The description of the symposium is attached below.


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Fracture and debonding in lithium-ion batteries with electrodes of hollow core-shell nanostructures

In a novel design of lithium-ion batteries, hollow electrode particles coated with stiff shells are used to mitigate mechanical and chemical degradation.  In particular, silicon anodes of such core-shell nanostructures have been cycled thousands of times with little capacity fading.  To reduce weight and to facilitate lithium diffusion, the shell should be thin.  However, to avert fracture and debonding from the core, the shell must be sufficiently thick.


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Lithium-assisted plastic deformation of silicon electrodes in lithium-ion batteries: a first-principles theoretical study

Silicon can host a large amount of lithium, making it a promising electrode for high-capacity lithium-ion batteries.  Recent experiments indicate that silicon experiences large plastic deformation upon Li absorption, which can significantly decrease the stresses induced by lithiation and thus mitigate fracture failure of electrodes. These issues become especially relevant in nanostructured electrodes with confined geometries. Based on first-principles calculations, we present a study of the microscopic deformation mechanism of lithiated silicon at relatively low Li concentration, which captures the onset of plasticity induced by lithiation.


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Large Plastic Deformation in High-Capacity Lithium-ion Batteries

Evidence has accumulated recently that a high-capacity electrode of a lithium-ion battery may not recover its initial shape after a cycle of charge and discharge.  Such a plastic behavior is studied here by formulating a theory that couples large amounts of lithiation and deformation.  The homogeneous lithiation and deformation in a small element of an electrode under stresses is analyzed within nonequilibrium thermodynamics, permitting a discussion of equilibrium with respect to some processes, but not others.  The element is assumed to undergo plastic deformation when the stresses reach a yield condition.  The theory is combined with a diffusion equation to analyze a spherical particle of an electrode being charged and discharged at a constant rate.  When the charging rate is low, the distribution of lithium in the particle is nearly homogeneous, the stress in the particle is low, and no plastic deformation occurs.  When the charging rate is high, the distribution of lithium in the particle is inhomogeneous, and the stress in the particle is high, possibly leading to fracture and cavitation. 


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Inelastic hosts as electrodes for high-capacity lithium-ion batteries

Silicon can host a large amount of lithium, making it a promising electrode for high-capacity lithium-ion batteries.  Upon absorbing lithium, silicon swells several times its volume; the deformation often induces large stresses and pulverizes silicon.


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Fracture of electrodes in lithium-ion batteries caused by fast charging

During charging or discharging of a lithium-ion battery, lithium is extracted from one electrode and inserted into the other.  This extraction-insertion reaction causes the electrodes to deform.  An electrode is often composed of small active particles in a matrix.  If the battery is charged at a rate faster than lithium can homogenize in an active particle by diffusion, the inhomogeneous distribution of lithium results in stresses that may cause the particle to fracture.  The distributions of lithium and stress in a LiCoO2 particle are calculated.  The energy release rates are then calculated for the particle containing


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ES247: Fracture Mechanics, Fundamentals and Applications. 3rd Edition

The book I recommend for reading is Fracture mechanics: fundamentals and applications, by T.L.Anderson, 3rd edition, 2005. I first saw this book on the top list of reading materials of Brown U. When I have it I found so pleasent to read through it. Here is the short-list of its content

Chapter 1: Introduction: History and overview

Chapter 2:Fundamental concepts: linear elastic fracture mechanics

Chapter 3: Elastic-plastic mechanics

Chapter 4: Dynamic and time-dependent fracture

Chapter 5: Material behavior: Fracture mechanics in metals

Chapter 6: Fracture mechanics in nonmetals (engineering plastics, polymers, fiber-reinforced plastics, ceramics)

Chapter 7: Fracture toughness testing of metals


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ES241Presentation: General theory of finite deformation

Please see attachment for ES241 final presentation--general theory of finite deformation


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Kejie Zhao

Hi everyone, very glad to see you here. My name is Kejie Zhao, a first year phd student working in Prof.Zhigang Suo's group (www.seas.harvard.edu/suo). My concentration is solid mechanics with the same name of this course, it also signifies its importance to my future research. I graduate from Xi'an Jiaotong University in China before coming to Harvard. There I obtained my bachelor and master degree in Engineering Mechanics and Solid Mechanics respectively. I have taken several courses related to ES240 at undergraduate level, i.e., elasticity mechanics, plasticity, non-linear continuum mechanics etc.


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Multiaxial behavior of nanoporous single crystal copper: a molecular dynamics study

The stress-strain behavior and incipient yield surface of nanoporous single crystal copper are studied by the molecular dynamics (MD) method. The problem is modeled by a periodic unit cell subject to multi-axial loading. The loading induced defect evolution is explored. The incipient yield surfaces are found to be tension-compression asymmetric. For given void volume fraction, apparent size effects in the yield surface are predicted: the smaller behaves stronger. The evolution pattern of defects (i.e., dislocation and stacking faults) is insensitive to the model size and void volume fraction. However, it is loading path dependent.


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A movie of stress distribution on crack-tip based on MD method


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Question: Is the local energy dictating dislocation emission constant for single crystal?

Hi everyone, in my size dependence study, I find the local energy dictating dislocation emission is almost constant for varied sized samples, in given directions of single crystal. I don't know this is an interesing finding, or just a common sense. Will you give me some suggestion, Thank you!

Kejie


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A MD simulation of nano-void growth in Copper under uniaxial tension

Recentely we did a MD simulation work on the nano-void growth in Copper, welcome to my blog for any discussion..


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