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Research Papers

Coupled Mechanical and Electrochemical Analyses of Three-Dimensional Reconstructed LiFePO4 by Focused Ion Beam/Scanning Electron Microscopy in Lithium-Ion Batteries

[+] Author and Article Information
Sangwook Kim

Mechanical and Aerospace
Engineering Department,
North Carolina State University,
R3311 Engineering Building 3,
Campus Box 7910 911,
911 Oval Drive,
Raleigh, NC 27695
e-mail: skim49@ncsu.edu

Hongjiang Chen

Mechanical and Aerospace
Engineering Department,
North Carolina State University,
R3311 Engineering Building 3,
Campus Box 7910 911,
911 Oval Drive,
Raleigh, NC 27695
e-mail: hchen30@ncsu.edu

Hsiao-Ying Shadow Huang

Mechanical and Aerospace
Engineering Department,
North Carolina State University,
R3158 Engineering Building 3,
Campus Box 7910 911,
911 Oval Drive,
Raleigh, NC 27695
e-mail: hshuang@ncsu.edu

1Corresponding author.

Manuscript received April 9, 2018; final manuscript received June 26, 2018; published online August 6, 2018. Assoc. Editor: Partha P. Mukherjee.

J. Electrochem. En. Conv. Stor. 16(1), 011010 (Aug 06, 2018) (7 pages) Paper No: JEECS-18-1032; doi: 10.1115/1.4040760 History: Received April 09, 2018; Revised June 26, 2018

Limited lifetime and performance degradation in lithium ion batteries in electrical vehicles and power tools is still a challenging obstacle which results from various interrelated processes, especially under specific conditions such as higher discharging rates (C-rates) and longer cycles. To elucidate these problems, it is very important to analyze electrochemical degradation from a mechanical stress point of view. Specifically, the goal of this study is to investigate diffusion-induced stresses and electrochemical degradation in three-dimensional (3D) reconstructed LiFePO4. We generate a reconstructed microstructure by using a stack of focused ion beam-scanning electron microscopy (FIB/SEM) images combined with an electrolyte domain. Our previous two-dimensional (2D) finite element model is further improved to a 3D multiphysics one, which incorporates both electrochemical and mechanical analyses. From our electrochemistry model, we observe 95.6% and 88.3% capacity fade at 1.2 C and 2 C, respectively. To investigate this electrochemical degradation, we present concentration distributions and von Mises stress distributions across the cathode with respect to the depth of discharge (DoD). Moreover, electrochemical degradation factors such as total polarization and over-potential are also investigated under different C-rates. Further, higher total polarization is observed at the end of discharging, as well as at the early stage of discharging. It is also confirmed that lithium intercalation at the electrode-electrolyte interface causes higher over-potential at specific DoDs. At the region near the separator, a higher concentration gradient and over-potential are observed. We note that higher over-potential occurs on the surface of electrode, and the resulting concentration gradient and mechanical stresses are observed in the same regions. Furthermore, mechanical stress variations under different C-rates are quantified during the discharging process. With these coupled mechanical and electrochemical analyses, the results of this study may be helpful for detecting particle crack initiation.

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References

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Figures

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Fig. 1

(a) Experimental setup for FIB-SEM, (b) a stack of 2D images, (c) 3D reconstructed microstructure of LiFePO4, and (d) final microstructure and mesh in comsolmultiphysics

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Fig. 2

Potential change during the discharging process under different C-rates

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Fig. 3

Concentration distribution at 0.6 C at (a) 1200 s, (b) 2400 s, (c) 3600 s, (d) 4800 s, (e) 5400 s (color legend), and (f) cross-sectional contour plot at 5400 s in cyclic legend

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Fig. 4

Concentration variation at three different points, P1, P2, and P3 (marked in Fig. 3) under various C-rates (i.e., 0.6 C, 1.2 C, and 2.0 C)

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Fig. 5

(a) Total polarization and (b) overpotential at the interface between the electrode and the electrolyte at different points under 0.6 C, 1.2 C, and 2.0 C

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Fig. 6

((a)–(c)) Contour plots of overpotential at the interface at different DoDs. ((d)–(f)) Contour plots of normalized concentration gradient at different DoDs. ((g)–(i)) Contour plots of von Mises stress distribution at different DoDs. C-rate = 0.6 C.

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Fig. 7

Maximum normalized von Mises stress variation during discharging at 0.2 C, 0.6 C, 1.2 C, and 2.0 C

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