Journal Club for November 2017: In-situ Mechanics Experiments on Battery Materials

Kejie Zhao, School of Mechanical Engineering, Purdue University

A few past journal clubs [1-3] and recent review articles [4-7] highlight the rich coupling between mechanics and electrochemistry. The electrochemical reactions between the host material and guest species induce deformation, stress, fracture, and fatigue which cause ohmic and thermal resistance increase, and performance degradation. Meanwhile, mechanical stresses regulate mass transport, charge transfer, interfacial reactions, and consequently the potential and capacity of electrochemical systems. In this journal club, I would like to summarize a few experimental techniques to characterize the mechanics of electrodes in Li-ion batteries. One challenge is that the operation of batteries is very sensitive to the work environment – a trace of oxygen and moisture can cause numerous side reactions. In contrast, most mechanical test equipment is open system with limited capability of environment control.  As such, mechanics and electrochemistry are often characterized separately. A focus of discussion here will be on in-situ/operando mechanics experiments which can probe the chemomechanical behaviors of electrodes during the electrochemical reactions. Other in-situ techniques (time-resolved X-ray, in-situ NMR, in-situ Raman spectroscopy, synchrotron X-ray, etc) can be found in recent review articles.

In-situ wafer curvature measurements

The multi-beam stress sensor (MOSS) is a convenient and reliable tool to measure the stress evolution in thin-film electrodes during electrochemical cycles. This technique has been used to monitor the stress development in Ge, Si, metal oxides, and composite thin films [8-12]. The average stress in the film electrode can be deduced from the curvature of the substrate using Stoney's equation

where σ is the average stress in the film, hs and hf are the thicknesses of the substrate and film respectively, κ is the change in curvature of the substrate as a result of the stress, and Ms is the biaxial elastic modulus of the substrate. Note that knowledge of film properties other than film thickness is not required to calculate stresses.  The MOSS sensor employs an array of parallel laser beams to measure the curvature of a substrate. The array of laser beams allows simultaneous multi-point illumination and detection, which in turn greatly reduces measurement noise caused by fluid motion in the electrochemical cell or by ambient vibrations. The curvature of a substrate is calculated from a geometric relation

where d is the distance between two adjacent laser spots on the CCD camera, d0 is the initial distance between the laser spots, and L is the distance between the beaker cell and the CCD camera. Since the laser passes through the electrolyte and an optical window, refraction of the laser beams needs be taken into account. In the above equation, ne represents the refraction index of the electrolyte, and na represents the refraction index of air.

In-situ tension/bending in TEM

Since the first paper published in 2010, in-situ TEM diagnosis has been a powerful tool to quantify the real-time electrochemical process, the defect growth, and the chemomechanical instability of nanostructured electrodes. The experiments require the integration of a sample holder inside the TEM chamber. In a typical setup, a few nanowires are attached to a Au rod with conductive silver epoxy as the working electrode. Li metal is scratched by an electrochemically shaped W tip inside a glove box. The Au and W tips are mounted on the station of the holder. By manipulating the pizeo-driven stage with nanometer precision on the TEM-holder, the Li2O covered Li metal comes into contact with the single nanowire. Once a reliable contact was made, a bias voltage is applied to drive the lithiation reaction of the nanowire. By accurately controlling the W tip toward the lithiated nanowire, a junction was formed using in-situ electron beam induced deposition between the lithiated nanowire and the W tip. Uniaxial tension can be applied to the lithiated nanowire by a controlled displacement to pull the W tip away from the nanowire. In-situ bending experiments of individual nanowires before and after lithiation can also be performed by moving the W tip. The phase information can be obtained by the EDX and EELS spectra. In an previous example, Kushima et al. performed delicate tensile experiments of fully lithiated Si nanowires inside a transmission electron microscope [13], where an atomic force microscopy cantilever connected to a Li rod was used to conduct lithiation and subsequently apply tension to the nanowire. The in-situ TEM experiments can provide valuable information such as fracture strength and Young’s modulus of nanowires. But one drawback is that the electrochemical condition is difficult to control [14].

In-situ AFM experiments

AFM experiments have been employed in the characterization of battery materials,owing to the simplicity of the test [15-18], the resolution being suitable to the size of the electrode constituents, and the ability to access a range of material behaviors. In-situ AFM is an effective technique to examine the solid electrolyte interface (SEI) layer and the morphological evolution of electrodes during electrochemical cycles. In one example study, Yoon et al. tracked the volumetric expansion and the thickness of SEI of Si thin-film electrodes and explained the capacity hysteresis due to the coupling between the electrochemical potential and mechanical stresses [19]. In the setup, an atomic force microscope is integrated in an argon-filled glove box. The sample is fastened to the center of the “beaker” cell made of Teflon. An Ni wire is attached to the current collector, which serves as an electrical lead. The cell consisted of an annular strip of Li metal that acts as the counter and reference electrode. The cell is filled with electrolyte such that the sample and the Li metal are fully submerged in it. In the measurement of the SEI thickness, the cell is held at a given potential until the current became negligibly small, where the samples are considered to have reached a state of near equilibrium and the SEI would not grow any further. AFM scans across the Si strip are made after that equilibrium is reached at each potential. In order to minimize AFM tip damage to the sample surface and the SEI, scanning is carried out in the tapping mode. One another noteworthy study [20] using in-situ AFM investigated the distribution of elastic modulus of the SEI across the sample surface; close inspection of the load-displacements curves indicated that the SEI structure is highly heterogeneous, composed of a combination of multiple layers, hard particles, and bubbles.Application of in-situ AFM to study the mechanics of electrode materials, nevertheless, is relatively scarce since the electrode materials are in general of high mechanical strength.

Operando nanoindentation

Nanoindentation is a well-established technique to measure a variety of mechanical properties of materials at local positions. The experimental setup requires careful control of the stability of the surrounding environment, sample size and properties, surface condition, and tip size and geometry. Additional challenges are associated with the measurements of materials submerged in a fluid cell environment. When it comes to operando indentation in the course of electrochemical reactions, specific challenges, such as the volumetric change of electrodes during indentation, the substrate effect, structural degradation of the electrodes, and the interference of SEI need be addressed.

We recently develop a platform of operando indentation [21] by integrating a nanoindenter, a home-developed fluid cell, and an electrochemical station into an argon-filled glovebox. Nanoindentation tests are performed on the electrode submerged in the electrolyte solution as the cell is (dis)charged in an open configuration. We evaluate the influence of the custom work environment and confirm that the effects of the dielectric constant of Ar on the capacitance gauge of nanoindentation, the non-standard sample holder, and the buoyance and surface tension of the liquid electrolyte are negligible. The structural degradation can be mitigated by designing the size of the thin film and preserving the electrochemical window in a relatively narrow range. The volumetric change of electrodes during Li reactions causes electrochemical drift and can be treated by the conventional thermal drift correction procedure. The interference of SEI can be avoided by the selection of the electrolyte.

The above figure shows the exemplary results on the continuous evolution of the elastic modulus, hardness, and creep stress exponent of lithiated Si as a continuous function of Li concentration under open circuit as well as various charging rates. The elastic modulus and hardness of lithiated Si steadily decreases as Li reaction proceeds. Power-law relationship between the strain rate and hardness is obtained, with stress exponents of 50 for pristine Si, 22 for lithiated Si of a wide range of Li compositions, and 8 for pure Li. The creep behavior of Si changes dramatically during the first 5% of lithiation and remains mostly invariant in the subsequent lithiation process. Several observations are not understood. For instance, the creep of lithiated Si behaves like a step function – the stress exponent dramatically decreases upon the start of lithiation and drops to 22 for Li0.5Si. Afterwards, the stress exponent remains nearly constant for the Li composition over Li0.5Si. We also observe that the elastic modulus and hardness of lithiated Si for a given composition measured at different charging rates and under open circuit condition are nearly identical. This observation is contradictory to the prior theory on reactive flow which postulates that under the non-equilibrium chemical state, the chemical driving force for reactions in a solid perturbs the valence states of the reactants and enables a material to flow under a lower level of stress than that at the chemical equilibrium state [22,23]. The coupling of chemical reactions and plasticity has also been studied by recent theories of anisotropic compositional expansion and glassy relaxation [24,25]. Tihs coupling is not apparent in the operando indentation tests. The assessment, nevertheless, is not conclusive at this point.  Further investigation considering a wider span of charging rates and different materials is necessary to make more conclusive understanding of reactive flow.

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