Article Cite This: J. Phys. Chem. C 2019, 123, 10237−10245 pubs.acs.org/JPCC Lithium Diffusion Mechanism through Solid−Electrolyte Interphase in Rechargeable Lithium Batteries Ajaykrishna Ramasubramanian, Vitaliy Yurkiv, Tara Foroozan, Marco Ragone, Reza Shahbazian-Yassar, and Farzad Mashayek* Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, Chicago, Illinois 60607, United States *S Supporting Information ABSTRACT: The composition, structure, and the formation mechanism of the solid−electrolyte interphase (SEI) in lithium-based (e.g., Li-ion and Li metal) batteries have been widely explored in the literature. However, very little is known about the ion transport through the SEI. Understanding the underlying ion diffusion processes across the SEI could lead to a significant progress, enabling the performance increase and improving safety aspects of batteries. Herein, we report the results of first-principles density functional theory calculations on the dominant diffusion pathways, energetics, and the corresponding diffusion coefficients associated with Li diffusion through the polycrystalline SEI. This paper is particularly concerned with the Li diffusion through the grain boundary (GB) formed between the three major inorganic components of the SEI, such as Li2O, LiF, and Li2CO3. It is found that Li diffusion occurs through the numerous open channels formed by the GB. The energetics and potential barriers vary significantly depending upon the structure of these channels, with the general trend being that Li diffusion in the GB is generally faster than in the neighboring crystalline regions within the grain interiors. In addition, the elastic properties of the GB are calculated allowing for more profound understanding of the SEI stability and formation. 1. INTRODUCTION component, multilayer structure”18 and the second model, 19 “ Lithium-based batteries, such as Li-ion batteries (LIBs) and Li developed by Shi et al., is called two-layer-two-mechanism diffusion”. Both models describe the SEI as a two-layer film metal batteries (LMBs), are widely used as power sources for a variety of applications starting from small portable (e.g., consisting of organic (outer layer close to the electrolyte) and inorganic (inner layerclose to the electrode). The main IURPKWWSVSXEVDFVRUJGRLDFVMSFFE phones, laptops, etc.) up to automotive, unmanned aerial ff vehicles and grids. However, still further improvements are di erence between these two models lies in the structure of the 1−3 inner layer. In particular, the first model describes it as a grain- expected for their widespread usage in high-power devices. ff During battery operation, a thin (nanoscale) solid−electrolyte structured layer, where ions can di use through the grains (i.e., the individual components of the SEI) and the GB formed 'RZQORDGHGE\81,92),//,12,6&+,&$*2DWRQ-XO\ interphase (SEI) film grows on the surface of an electrode as a result of the decomposition of an electrolyte. It is generally between these individual components. The second model 4−6 fi considers the inner layer as one structured sublayer, where ions believed that the SEI provides many bene cial functions to ff ff 19 the battery operation, one of which is by controlling Li uniform di use through it via the knock-o mechanism. Because the delivery to the electrode surface, where the charge-transfer inner (inorganic) layer is located directly at the surface of the negative electrode, the ion diffusion through this last layer reaction occurs. However, as the SEI further grows and ff expands, numerous defects and grain boundaries (GBs) are directly a ects the morphology of Li electrodeposition and fi ff governs the cycling performance of the battery. formed, leading to a signi cant anisotropic di usivity of Li 14,20,21 toward the electrode surface. This results in an uneven Li Although many studies have been performed to delivery to the electrode/SEI interface, leading to undesired understand parameters such as composition, morphology, phenomena, which cause performance decrease and possibly and thickness of the SEI and their growth mechanisms, very ff failure of a battery. These phenomena include random Li little e ort was made to understand the SEI as an ion transport ff electrodeposition and dendrite formation in LMBs and medium and the role of GB di usion. This is due, in part, to fi anisotropic diffusion/reaction and exfoliation in the case of the fact that the experimental veri cation of such theories is LIBs. very challenging. Moreover, most of the SEI components are The concept of the SEI was first introduced by Peled7 in highly reactive when exposed to contaminants, air, or − 1979 and further improved in their later works.8 10 With further contributions to the field of the SEI understand- Received: January 15, 2019 − ing,11 17 two major models have emerged explaining the SEI Revised: March 27, 2019 structure and composition. The first model is called “multi- Published: March 29, 2019 © 2019 American Chemical Society 10237 DOI: 10.1021/acs.jpcc.9b00436 J. Phys. Chem. C 2019, 123, 10237−10245 The Journal of Physical Chemistry C Article humidity.14 For these reasons, the ex situ characterization of components show a preference for the vacancy diffusion and the SEI becomes very difficult, whereas in situ experiments knock-off mechanisms. require specially designed tools and precise characterization Liu et al.25 have performed DFT calculations to evaluate the 22 ffi fl tools. Given the di culties of experimental techniques, in uence of two major SEI components (LiF and Li2CO3)on computational simulations,23 especially using ab initio density the Li surface energy. Their study shows that the LiF/Li − functional theory (DFT),24 27 have become a valuable tool to interface has higher electron tunneling energy barrier from the ff study the properties and di usion characteristics of the SEI. Li metal to SEI than the Li2CO3/Li interface. In the other Thus, in this study, we focus on the GB of the SEI employing study, Chen et al.31 calculated the Li migration energy barriers DFT in an effort to understand and evaluate the diffusion along major diffusion pathways and energy barriers of the three pathways and energetics of Li transport. Figure 1 illustrates the main components (Li2CO3,Li2O, and LiF) of the inner SEI layer using DFT. Yildrim et al.24 have applied first-principles calculation to study defect thermodynamics, the dominant diffusion carriers, and the diffusion pathways in crystalline LiF and NaF components of the SEI. They reveal that for both compounds, vacancy defects are energetically more favorable; therefore, they form more readily than interstitials because of the close-packed nature of the crystal structures. However, the vacancy concentrations are very small for the diffusion processes facilitated by defects. The works of Peled et al.,10 Christensen and Newman,.32 and recently Leung and Jungjohann.33 are the only studies providing information on details of diffusion through GB in the SEI. Peled et al.10 provided evidence for Li transport via the SEI GBs through impedance analysis and calculation of the Figure 1. Schematic illustration of the SEI envisioned in the present apparent SEI ionic resistance. Christensen and Newman,32 on work. Upper left picture depicts a LIM showing the SEI on the Li− the other hand, developed a mathematical model to simulate metal surface; upper right picture depicts a graphite-based LIB with the growth of the SEI and transport of lithium and electrons the SEI; lower picture shows the atomic structure of a representative through the film via vacancies and interstitials and also GB, which is a focus of this paper. Note: all drawings shown in this ff picture are not to scale merely for illustrative purpose. concluded that GBs and di usion through them might be an important factor. Leung and Jungjohann.33 have used the DFT ffi calculation to reveal that Li2O GBs of the SEI with su ciently large pores can accommodate Li0 atoms. Also, they showed − emphasis of this work showing a Li−metal cell highlighting the that Li metal nanostructures as thin as 12 Å are exaggerated SEI structure (red bubble-like structures in the thermodynamically stable inside Li2O cracks. Subsequently, fi upper left picture), a graphite-based anode Li-ion cell these Li nanostructures become Li laments. highlighting the exaggerated SEI structure (upper right To date, there is no work dedicated to the detailed ff picture), and the atomic structure of the GB formed between investigation of GB properties in the SEI and the Li di usion fi two representative components of the inner layer of the SEI through it. Thus, the present paper aims to ful ll this gap by (lower picture). Using the DFT method, this paper providing a computational framework and the data necessary ff concentrates specifically on the Li diffusion through the GB to understand Li di usion in the GBs of the SEI. of the inner layer and the stability of different GBs. The Methodology section provides the details of the Before developing the detailed concept of the Li diffusion computational methods that are utilized to capture the Li model through the SEI, it is helpful to review the aspects of the transport through the SEI. In Results and Discussion section, ff Li diffusion through the SEI individual components. The major the results from our DFT calculations for di erent GBs are components of the inner inorganic layer of the SEI in both the presented and are compared against the theoretical and ff Li metal and graphite negative electrodes are lithium fluoride experimental values that are calculated for the di usion (LiF), lithium oxide (Li O), and lithium carbonate (Li CO ) through the bulk crystal structures. In the Results section, 2 2 3 fi ff at temperatures in the range of 250−400 K. Benitez et al.28 our ndings on sti ness properties and stabilities of these fi ff have used the classical molecular dynamics (MD) simulations structures are rst examined; the results from di erent GB to study the Li-ion diffusivity in three main inorganic combinations in the structures of SEI are then discussed; and fi ff ffi components of the SEI.