Rapid, Non-Invasive Method for Quantifying Particle Orientation Distributions in Graphite Anodes

Abstract

For materials with 1D or 2D lithiation pathways and non-spherical particle shapes, knowledge of the crystallographic grain orientation in the particle and the active particle orientation in the porous electrode is important for quantifying battery performance. Here we study graphite anodes and show how X-ray diffraction based texture measurements can be used to quantify both the particle orientation, which develops during the coating and calendaring process, and the grain orientation within a specific type of graphite. This lab-based, non-invasive approach to study electrode structure and active particles can assist in engineering improved lithium ion batteries. In order to optimize a lithium ion battery, it is important to understand the lithiation pathways within the active material 1-5 as well as the lithium transport through the pore space of the electrode, which is governed by the shape and orientation distribution of particles in the electrode. 6,7 Here, we present X-ray diffraction (XRD)-based texture measurements as a quantitative, non-invasive method to characterize the crystallographic grain orientation of particles and the orientation of these particles within lithium ion battery porous electrodes. The importance of this structural information is highlighted in the case of graphite, which is the most widely used active material for negative lithium ion battery electrodes. The two-dimensional structure of graphite is responsible for the anisotropy of its electronic, ionic, and thermal conductivity as well as its mechanical properties. 8 (De)lithiation occurs along the (002) planes, causing directional vol-umetric expansion in the [001] direction, which can also translate to directional changes in electrode porosity as the graphite particles swell into the pore space during lithiation. 9 The two-dimensional nature of graphite is also reflected in the shape of graphite particles: they are naturally platelet-shaped. 10 Earlier work has shown that non-spherical particles align during porous electrode fabrication, leading to large tor-tuosity of the pore-phase, reducing effective transport of lithium in the electrolyte. 6 Tortuosity can be somewhat reduced by using spherical graphites, which often exhibit complex grain distributions depending on the manufacturing process. 10 For graphite anodes, the electronic and ionic conductivity of the graphite particles does not limit cell performance, 11,12 so most efforts have focused on quantifying and reducing tortuosity, which limits the fast charging of high energy dense cells. 13,14 To determine tor-tuosity of an electrode, numerical diffusion simulations can be run on a three-dimensional representation of the electrode, 15 or, because porous electrodes obey the Bruggeman relationship, 16 an estimation of tortuosity can be obtained from particle shape and orientation distributions within the electrode. 6,17 Current techniques used to quantify orientation distributions of particles in porous electrodes have drawbacks. Scanning electron mi-croscopy (SEM) cross-sectional images are destructive to the sample under investigation and can present ambiguities due to their 2D nature. 14 Focused-ion-beam (FIB)-SEM tomography provides quantitative 3D information but it can require elaborate sample preparation and is a time consuming measurement that can access only a small electrode volume. 14,18 X-ray tomography is data intensive and the instruments and computational resources may not be readily available to a large number of users. 19 In contrast, our XRD-based texture measurement approach presented here is performed with a laboratory XRD system on centimeter-* Electrochemical Society Member. z E-mail: vwood@ethz.ch size electrodes with no special sample preparation required. Furthermore , the experiments can be fast (<30 min) and the data generated is small (<200 kB), enabling rapid assessment of a large number of samples. While texture measurements have been used to quantify orientation of grains in metal sheets such as cold-rolled copper 20 or thin films, 21,22 here we show that this technique can be extended to porous electrodes. For active materials such as graphite that exhibit a preferred orientation at the single particle level, this technique can be used to extract the particle orientation distribution, the grain orientation distribution within the particles, and information about particle deformation and fracture during manufacturing. Experimental Measurement setup.-The concept of XRD texture measurements is to obtain the orientation distribution of crystal planes in a sample. To perform a texture measurement, the intensity of X-rays diffracted from a sample at a fixed scattering angle 2θ is recorded for all possible orientations. The 2θ angle is chosen to satisfy the Bragg condition of a crystal direction of interest. Here we measure the orientation of (002) planes in a graphite electrode, so 2θ = 26.53 • is chosen. Orientation is described by the elevation angle α and an azimuth angle β of the scattering vector k, which is normal to the crystal planes that contribute to the recorded intensity. Texture measurements can be obtained by either (1) keeping the source and detector fixed to probe the desired 2θ angle and tilting the sample using an Eulerian cradle, or (2) keeping the sample fixed and moving both the source and detector along coupled trajectories. We use the latter approach, which is shown in Figure 1a. 22 The electrode is positioned with the current collector parallel to the xy-plane. This requires an XRD system with an in-plane axis, which allows the detector to rotate out of the xz-plane. Discrete steps for α and β are chosen. To vary α, the diffraction plane sweeps from being orthogonal to the xy-plane at α = 0 • to parallel to the xy-plane at α= 90 •. Figure 1 shows the in-plane measurement position at α= 90 • , where the diffraction plane is parallel to the sample plane. At each α step, the sample stage is rotated to move the electrode around its surface normal (which is collinear with the z-axis of the goniometer) to allow scanning of β from 0 • to 360 •. A pole figure is produced by attributing the recorded X-ray intensity to points on a hemisphere defined by a unit vector with α and β and projecting the result stereographically onto the plane (Figure 1b). As described in the SI (part 2), the pole figure is corrected for defocusing, absorption, and background effects. The result is normalized and the intensity is represented in multiples of random distribution (m.r.d.). 23 An m.r.d. value of 1 corresponds to a random orientation while values higher than 1 indicate a preferred orientation.) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. 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