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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 43623−43630
Effect of Defects on Diffusion Behaviors of Lithium-Ion Battery Electrodes: In Situ Optical Observation and Simulation Le Yang,† Hao-Sen Chen,*,‡,§ Wei-Li Song,‡,§ and Daining Fang*,†,‡ †
China State Key Laboratory for Turbulence and Complex Systems, College of Engineering, Peking University, Beijing 100871, China ‡ Beijing Key Laboratory of Lightweight Multi-Functional Composite Materials and Structures and §State Key Laboratory of Explosion Science and Technology, Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing 100081, China
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S Supporting Information *
ABSTRACT: Lithium-ion batteries (LIBs) with high energy efficiency are urgently needed in various fields. For the LIBs electrodes, defects would be generated during the manufacturing processes and mechanical degradation and significantly impact the stability and performance of the LIBs. However, the effects of electrode defects on the electrochemical processes are still not clear. Herein, an in situ optical observation system is developed for monitoring the Li diffusion around the preintroduced defects in the commercial graphite electrodes. The experiments show that the gas-filled defects vertical to the direction of the Li diffusion would obviously decelerate the Li diffusion, whereas the electrolytefilled defects parallel to the direction of the Li diffusion would accelerate the Li diffusion. In addition, finite element analysis (FEA) suggests, consistent with the experiments, a nonuniform distribution of local Li concentration around the defect. The equivalent diffusivity obtained by the FEA is also dependent on the configuration of the defects. The diffusivities of the electrolyte-filled parallel defect and gas-filled vertical defect are 12.6 and 11.0%, respectively. For the gas-filled defects, the sizeeffect calculation manifests that the equivalent diffusivity would decrease with the enlarged defect size, and the shape of the defects would substantially impact the decrease rate. The results directly reveal the mechanisms of the defect-induced diffusion behavior change in the electrodes by the new equivalent two-dimensional experiments, and the equivalent diffusivity would be useful for optimizing the electrode designs in LIBs. KEYWORDS: lithium-ion battery, defect, diffusion, graphite anode, colorimetric method
1. INTRODUCTION
properties are directly related to the crap rate and battery cost.7,8 The mechanisms of the defect-induced performance degradation in LIBs are diverse and primarily linked to various electrochemical process and defect evolution. As the size scale of those manufactured defects is much larger than that of the voids in the porous electrodes, the interface defects would cause the contact loss between the current collectors and the electrodes, resulting in the increase in the interface resistance. Besides, the microstructure change that results from the inner defects evolution also affects the tortuosity of the electrode and hinders the Li diffusion. In general, all of these phenomena are related to the defect-induced path of the ion diffusion and electron conduction and will cause a nonuniform distribution of the electric field and Li concentration field, which would lead to Li plating or inactivation area in the electrodes.14,15
Rechargeable lithium-ion batteries (LIBs) are widely used in plenty of applications, including electric vehicles, consumer electronics, and grid-scale storage.1,2 The high energy efficiency of LIBs also contributes to the use of renewable energy sources such as solar, wind, and tidal.3,4 However, the critical parameters in current LIBs, including energy density, rate capability, and cycle life, still do not meet the practical requirements. One of the critical questions is the effect of electrode defects on the electrochemical processes. Defects, such as pinholes, cracks, and metal particle contaminations, would be inevitably introduced into the electrodes during the manufacturing processes and could be clearly detected by the nondestructive infrared thermography technique.5−8 Defects would also result from the mechanical failure of the composite electrodes (e.g., Si/C anode, as the next-generation electrode with high capacity and huge volume deformation).9−13 Those defects would severely impact the stability and performance, especially the rate capability and lifespan, of LIBs, and those © 2018 American Chemical Society
Received: September 3, 2018 Accepted: November 28, 2018 Published: November 28, 2018 43623
DOI: 10.1021/acsami.8b15260 ACS Appl. Mater. Interfaces 2018, 10, 43623−43630
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a, b) Schematic illustration of the simplification in the experiments. (c) Schematic illustration of the in situ battery. (d) The color change in graphite electrodes during lithiation observed by optical microscopy. The scale bar is 2 mm.
in the liquid phase. This transport behavior could be simplified as an equivalent diffusion process that is a synergistic result of the Li mole fraction in the solid phase (graphite) of the electrode and the Li concentration distribution in the liquid phase of the electrode, and it is consistent with the experimental phenomenon in the previous work.23,25 For the defects in the x−y section, the Li-diffusion flux in the zdirection was asymmetric, and the three-dimensional problems could be reduced to two-dimensional ones as shown in Figure 1b. Thus, the geometry of the x−y cross section could be magnified with the principle of equilateral proportion. On the basis of the above assumption, in situ experiments were developed for various defects with different filing media and geometrical morphologies. Then, the proper finite element analysis (FEA) was applied to quantitatively explain the experimental results, and the size effect of the defects was also calculated to reveal the mechanisms of the defect-induced diffusivity decrease.
Owing to the significant influence of the defect on the battery degradation, plenty of experiments and simulation studies were carried out to figure out the defect evolution inside the electrodes.16−19 However, the main goal of this work is to reveal the effect of the electrochemical process on the evolution of defects. The relation between the defects and the change in Li-diffusion behavior is still unclear. The difficulty is a lack of an efficient platform for the observation and characterization of the lithiation/delithiation processes by in situ experiments, especially during the electrochemical processes in the batteries. One of the possible methods to in situ observe the Li diffusion in the electrodes is the colorimetric method, which is widely applied to graphite electrodes. The Li intercalation into the crystal lattice of the graphite electrodes would change the visible absorption spectra of the LiCx compounds.20−25 The color of the areas around the random cracks is obviously different from that of other areas, which directly proves the influence of defects on the Lidiffusion process.24 However, this phenomenon was just incidental in the previous work, and the effects on the Lidiffusion behavior of the defects still need to be studied systematically. In this paper, different types of filling media in the defects were considered, including the primary electrolyte in the batteries and the gas that might be generated by the side reactions. As shown in Figure 1a, the electrodes in the traditional batteries cannot be directly observed by the optical equipment; therefore, an in situ optical battery was developed to realize the observation of the Li diffusion. However, the defect distribution in the electrodes are also difficult to detect on the electrode surfaces. Considering the size of the manufacturing defects is much bigger than the size of the graphite particle and the microvoid,5−8 the electrodes could be homogenized for mathematical simplicity, and the Li transport contains both the phase-separating diffusion in the solid phase and migration
2. EXPERIMENTAL SECTION 2.1. Materials and Electrochemical Conditions. Commercial graphite electrodes (Lishen) were used after drying in the vacuum oven at 80 °C for 24 h, and the lithium foil was used as a counter electrode. The size of the graphite electrodes was 4 mm long, 3 mm wide, and 104 μm thick, and elliptical defects with 500 μm long axis and 125 μm short axis were generated by micromachining to remove the electrodes materials from the current collector. The electrolyte consisted of 1 M LiPF6 in 1:1 dimethyl carbonate/ethylene carbonate. The in situ cell was assembled in a high-purity argon-filled glovebox (Mbraun Inc.) with H2O and O2 contents
