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CURATOR

PhD student, Purdue University

PINBOARD SUMMARY

Operando nanoindentation to probe the concurrent mechanics and electrochemistry

Li-ion batteries play a central role in portable electronics and electric vehicles. Technological improvements in rechargeable batteries are being driven by the ever-increasing demand on materials of long lifespan, high energy density, fast charging and power output, low cost, and safe operation. In such systems mechanics and electrochemistry are intimately coupled. The electrochemical reactions between the host material and guest species induce deformation, stress, fracture, and fatigue. Likewise, mechanical stresses regulate the electrochemical potential and capacity of the batteries. In order to develop solutions to overcome the setbacks of mechanical and electrochemical degradation, it is crucial to understand how these two phenomena are coupled.

Few studies have been able to reliably determine and quantify change in mechanical behavior as a function of lithium content and electrochemical cycles. This scarcity is mostly due to the challenges associated with the complex microstructure of most electrode materials (composites, nano-to-micrometer scale) and the high sensitivity to moisture and oxygen.

In this work, we design an experimental platform (nanoindenter, home-developed fluid cell, electrochemical station, argon-filled glovebox) that allows for continuous measurement of mechanical properties (Young's modulus, hardness and creep exponent) of virtually any electrode material during controlled electrochemical operation. The system also allows for visual inspection of the electrode surface, tracking crack formation and growth. The ultimate goal of this research is to combine experimental observations with continuum theories to unravel the fundamental coupling between mechanics (stress, plasticity, and fracture) and electrochemistry (ion diffusion, charge transfer, and non-equilibrium chemical reactions).

10 ITEMS PINNED

Measurements of the fracture energy of lithiated silicon electrodes of Li-ion batteries.

Abstract: We have measured the fracture energy of lithiated silicon thin-film electrodes as a function of lithium concentration. To this end, we have constructed an electrochemical cell capable of testing multiple thin-film electrodes in parallel. The stress in the electrodes is measured during electrochemical cycling by the substrate curvature technique. The electrodes are disconnected one by one after delithiating to various states of charge, that is, to various concentrations of lithium. The electrodes are then examined by optical microscopy to determine when cracks first form. All of the observed cracks appear brittle in nature. By determining the condition for crack initiation, the fracture energy is calculated using an analysis from fracture mechanics. In the same set of experiments, the fracture energy at a second state of charge (at small concentrations of lithium) is measured by determining the maximum value of the stress during delithiation. The fracture energy was determined to be Γ = 8.5 ± 4.3 J/m(2) at small concentrations of lithium (~Li0.7Si) and have bounds of Γ = 5.4 ± 2.2 J/m(2) to Γ = 6.9 ± 1.9 J/m(2) at larger concentrations of lithium (~Li2.8Si). These values indicate that the fracture energy of lithiated silicon is similar to that of pure silicon and is essentially independent of the concentration of lithium. Thus, lithiated silicon demonstrates a unique ability to flow plastically and fracture in a brittle manner.

Pub.: 09 Oct '13, Pinned: 03 Jul '17