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Review Article

Electrochemomechanics of Electrodes in Li-Ion Batteries: A Review

[+] Author and Article Information
Rong Xu

School of Mechanical Engineering,
Purdue University,
West Lafayette, IN 47906

Kejie Zhao

School of Mechanical Engineering,
Purdue University,
West Lafayette, IN 47906
e-mail: kjzhao@purdue.edu

1Corresponding author.

Manuscript received June 11, 2016; final manuscript received November 16, 2016; published online December 12, 2016. Assoc. Editor: George Nelson.

J. Electrochem. En. Conv. Stor. 13(3), 030803 (Dec 12, 2016) (9 pages) Paper No: JEECS-16-1079; doi: 10.1115/1.4035310 History: Received June 11, 2016; Revised November 16, 2016

A Li-ion battery is a system that dynamically couples electrochemistry and mechanics. The electrochemical processes of Li insertion and extraction in the electrodes lead to a wealth of phenomena of mechanics, such as large deformation, plasticity, cavitation, fracture, and fatigue. Likewise, mechanics influences the thermodynamics and kinetics of interfacial reactions, ionic transport, and phase transformation of the electrodes. The emergence of high-capacity batteries particularly enriches the field of electrochemomechanics. This paper reviews recent observations on the intimate coupling between stresses and electrochemical processes, including diffusion-induced stresses, stress-regulated surface charge transfer, interfacial reactions, inhomogeneous growth of lithiated phases, instability of solid-state reaction front (SSRF), as well as lithiation-modulated plasticity and fracture in the electrodes. Most of the coupling effects are at the early stage of study and are to be better understood. We focus on the elaboration of these phenomena using schematic illustration. A deep understanding of the interactions between mechanics and electrochemistry and bridging these interdisciplinary fields can be truly rewarding in the development of resilient high-capacity batteries.

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Figures

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Fig. 1

(a) Schematic of diffusion-induced stress in a free-standing spherical particle. Li insertion causes deformation of the host material, and the inhomogeneous distribution of Li induces a field of stress within the particle. The shell region (Li rich) is under compression and the core region (Li poor) is under tension. (b) Sketch of the stress effect on radial distribution of Li in the spherical particle. The stress gradient tends to homogenize the Li distribution. The red line represents the Li profile calculated by considering both effects of stress and Li concentration gradient, while the blue line represents the case that Li diffusion is driven only by the concentration gradient.

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Fig. 2

Free-energy diagram for the surface charge transfer process altered by the applied electrical potential and mechanical stresses. The activation energies of the redox reactions at equilibrium states are identical, ΔG0O = ΔG0R (black-solid lines). The chemical equilibrium will be broken by the electrical overpotential as well as mechanical stress. The electrical overpotential E − E0 promotes electron transfer and decreases the free energy of the oxidized state by F(E − E0) (red-dashed line). The tensile stress in the surface layer causes a change of elastic energy ΔW = σmΩ (blue-dashed line) and promotes the formation of neutral Li atoms. The change in the total free energy of the reduced state relative to the oxidized state is F(E − E0) − σmΩ.

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Fig. 3

(a) Design of mechanical energy harvester based on the stress-driven Li diffusion. Two identical partially lithiated Si films act as electrodes, separated by electrolyte-soaked polymer membranes (the size of the electrodes and electrolyte is not in scale). Bending-induced asymmetric stresses generate chemical potential difference, driving migration of Li ions from the compressed electrode (red) to the tensed side (blue). (b) Schematic of short circuit current density during the release of bending. The peak current is determined by the Butler–Volmer surface charge transfer process, and the subsequent state is controlled by Li diffusion in the electrodes. (Figure 3(a) is reproduced with permission from Kim et al. [87]. Copyright 2015 by the Nature Publishing Group.)

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Fig. 4

(a) Stress-regulated interfacial reaction during lithiation of a crystalline Si spherical particle. Stress modifies the free energy associated with the reaction that converts one Li and 1/x Si atoms into lithiated Si at the reaction front. The figure plots the evolution of the free energy contributed by mechanical stresses as a function of the position of the reaction front. The inset shows the schematic of the stress field within the particle (Reproduced with permission from Zhao et al. [29]. Copyright 2012 by the Electrochemical Society). (b) Experimental evidence of the stagnation of lithiation reaction due to the stress effect. Lithiation in the particles of different sizes significantly slows down at similar a/b ratios suggesting that stress gradually builds up as the a/b ratio decreases and retards the progression of the reaction front (Reproduced with permission from McDowell et al. [92]. Copyright 2012 by John Wiley and Sons).

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Fig. 5

(a) Schematic of inhomogeneous growth of lithiated phase due to the stress-regulated lithiation reaction. The lithiated phase of convex curvature develops a field of tensile stresses, facilitating Li transport through the lithiated material and promoting the interfacial reaction at the phase boundary. On the contrary, the lithiated phase of concave curvature is under a field of compressive stresses, retarding the electrochemical growth of the layer. (b) In situ TEM observation of the inhomogeneous growth of the SiO2 layer during the cycle of lithiation and delithiation. (Reproduced with permission from Zhang et al. [93]. Copyright 2014 by the American Chemical Society).

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Fig. 6

(a) Stress-induced instability of solid-state reaction front (SSRF) during lithiation of the ZnO nanowires. Li ions diffuse quickly on the nanowire surface, forming a thin wetting layer with typical thickness of a few nanometers. The primary SSRF propagates along the longitudinal direction of the nanowire away from the electrolyte. Lithiation induces a field of tensile stress in the lithiated shell, and compressive stress in the unlithiated core. The stress field regulates the lithiation reaction and breaks the planar SSRF into a curved interface. (b)–(e) In situ TEM observation on the evolution of the curved reaction front. The radius of curvature gradually decreases, and the core is eventually lithiated as the reaction front propagates (Reproduced with permission from Zhang et al. [96]. Copyright 2015 by Elsevier).

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Fig. 7

(a) Schematic illustration of concurrent Li insertion and plastic deformation. The host atom bonds are broken by the insertion of Li, and the valence state of matter is under dynamic change. (b) The stress needed to maintain a given shear displacement with ongoing chemical reaction is lower than that needed for pure mechanical load. (c) A yield function Q(s1,s2,s3,ζ) is sketched in the space of (sk,ζ); the condition Q = 0 defines the yield. (δeik, ΩδC) represents the increment of inelastic deformation driven by both the mechanical load sk and the chemomechanical load ζ. Under pure shear loading, plastic flow occurs when the shear stress sk reaches the yield strength τY. Under the condition of insertion without stress, inelastic volume change occurs when the chemomechanical load ζ reaches τY/√q.

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Fig. 8

(a)–(c) Schematic of the wait-and-go fracture behavior induced by the concurrent Li diffusion, Li weakening at the crack tip, and crack propagation. (d) Stress–strain curves of the SWCNTs containing a holelike defect. (e) Three stages of SWCNTs showing the corrosive fracture behavior of SWCNTs studied by molecular dynamics simulations (Reproduced with permission from Huang et al. [35]. Copyright 2013 by AIP Publishing LLC).

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