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Synergetic Effects of Inorganic Components in Solid Electrolyte Interphase on High Cycle Efficiency of Lithium Ion Batteries Qinglin Zhang, Jie Pan, Peng Lu, Zhongyi Liu, Mark W. Verbrugge, Brian W. Sheldon, Yang-Tse Cheng, Yue Qi, and Xingcheng Xiao Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b05283 • Publication Date (Web): 18 Feb 2016 Downloaded from http://pubs.acs.org on February 23, 2016
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Nano Letters
Synergetic Effects of Inorganic Components in Solid Electrolyte Interphase on High Cycle Efficiency of Lithium Ion Batteries Qinglin Zhang, 1,2 ‡ Jie Pan, 2‡ Peng Lu, 1 Zhongyi Liu1, Mark W. Verbrugge, 1 Brian W. Sheldon, 3
Yang-Tse Cheng, 2* Yue Qi, 4* Xingcheng Xiao1 *
1
Chemical and Materials Systems Laboratory, General Motors Research and Development
Center Warren, MI 48090, USA 2
Department of Chemical & Materials Engineering, University of Kentucky, Lexington, KY
40506, USA 3
School of Engineering, Brown University, Providence, RI 02019, USA
4
Department of Chemical engineering & Material Science, Michigan State University, East
Lansing, MI 48824, USA
ABSTRACT
The solid electrolyte interphase (SEI), a passivation layer formed on electrodes, is critical to battery performance and durability. The inorganic components in SEI, including lithium carbonate (Li2CO3) and lithium fluoride (LiF), provides both mechanical and chemical
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protection, meanwhile controls lithium ion transport. Although both Li2CO3 and LiF have relatively low ionic conductivity, we surprisingly found that the contact between Li2CO3 and LiF can promote space charge accumulation along their interfaces, which generates a higher ionic carrier concentration and significantly improves lithium ion transport and reduces electron leakage. The synergetic effect of the two inorganic components leads to high current efficiency and long cycle stability.
One of the most significant challenges for current and future lithium ion batteries is to control the charge transfer at the electrode/electrolyte interface.1, 2 This issue is further complicated by the existence of an ultrathin interphase layer covering the electrode.1 This interphase layer is typically formed by electrolyte decomposition and called as the solid electrolyte interphase (SEI). However, the name of SEI does not fully capture its multi-functional nature. Ideally it should be: (1) electronic insulating (suppress the electrolyte reduction)3; (2) ionic conducting (enable the fast Li ion transport)1,
4, 5
; (3) mechanically “tough” (sustain the large strains in
electrodes during cycling)4, 6; and (4) chemically stable (long shelf- and cycle-life)4. Intensive research efforts have been devoted to designing an artificial SEI layer in order to control the electron and ion transport at the electrode/SEI/electrolyte interface. For example, inorganic coatings, e.g., oxides4, carbonates7, and fluorides8, and electrolyte additives, e.g., Fluoroethylene Carbonate (FEC)9-12, have been developed to modify the properties of SEIs to stabilize the electrode. These efforts motivated extensive studies on correlating the structure and property of each SEI component to the performance of the battery.1, 10, 11, 13 It has been believed that the shown that there are three major inorganic components in naturally formed SEI from
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liquid electrolyte decomposition is heterogeneous and the main inorganic components include: lithium carbonate (Li2CO3), lithium alkylcarbonate, lithium oxide (Li2O), and lithium fluoride (LiF).1,
3, 10, 13, 14
Their mechanical and transport properties have been investigated by
simulations15, 16 and their contributions in naturally formed SEI to the cycling performance have been revealed by experiments as well10, 11, 17. However, the complex mosaic and heterogeneous structural nature of naturally formed SEI has not been fully understood. For example, the ionic conduction through the multi-component SEI depends on various factors, such as interfacial defect chemistry, topological distribution of phases.18, 19 In a broader view of solid ionic conductors, it has been demonstrated that the bulk transport properties can be either increased or decreased dramatically, sometimes anomalously, by introducing heterogeneous structures and interfaces. For example, addition of small insulating particles into ionic conductors may, under certain conditions, enhance the ionic conductivity due to a space-charge layer effect.20 This was firstly observed when non-Li-conducting Al2O3 was mixed with LiI, resulting in a heterogeneous structure, where Li+ was adsorbed into the Al2O3 interface, which increases the Li vacancy (the diffusive carrier) concentration in LiI by orders of magnitude.20 Utilizing this effect and reducing the distance between the space-charge layers, multi-layer fast ionic conductors were achieved.21 On the other hand, the ionic conductivity in structures with heterogeneous interfaces can decrease due to the depletion of ionic carriers near the interface, such as Li+ conduction near the LiF/Al2O3 interface22, 23. As a result, whether SEI components have synergetic or antagonistic effects remains an open question. We believe that addressing this question can lead to a new avenue of designing artificial SEI design and precise control of electron and ion transport at the electrode/electrolyte interface.
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Two interesting SEI components, Li2CO3 and LiF serve as a good example to explore this question. In this study, we chose Li2CO3 and LiF as two representative components to show the effect heterogeneity in the SEI to the electrical conduction. Both components form crystalline phases on the electrode and they are stable in the SEI comparing with other phases, such as lithium alkylcarbonate.14 In addition, we believe that Li2CO3 is a close approximation to other carbonates, such as alkylcarbonate, to calculate the SEI properties. Theoretically, it has been shown that the dominant defects in a pure LiF coating on negative electrodes are Schottky pairs (cation and anion vacancy pair).15 Due to the low defect concentration and high transport barriers, LiF coating on negative electrodes was believed to be a good electronic insulator with poor ionic conductivity (~10-31 S/cm).15 On the other hand, the main defect in Li2CO3 in contact with negative electrodes is Li ion interstitials with its charge balanced by free electrons.16, 24 As a result, Li2CO3 can provide relatively high ionic conductivity (~10-8 S/cm) but containing a considerable electron concentration on negative electrodes.16,
24
Considering these drawbacks,
neither one of them alone has the desired properties as an ideal SEI. However, they may benefit from each other as they co-exist in SEI to provide improved ionic conduction and electronic insulation on negative electrodes. We designed an engineered SEI with specified amounts of Li2CO3 and LiF co-existing on silicon thin film electrodes, to identify the role of combining effect of various components in the SEI which can effectively passivate Li-ion battery electrodes and facilitate Li ion transport. Silicon thin film electrodes, below a critical thickness, provide a good platform to investigate the functions of artificial SEIs. They are not subject to larger lateral strains, and can thus avoid the severe mechanical degradation that occurs with silicon particles, such that we don’t need to consider the impact of cracking and additional SEI formation.4, 24 The relatively smooth surface
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of Si thin films enables us to carry out reliable characterization using many surface sensitive techniques, e.g., X-ray Photoelectron Spectroscopy (XPS). The composition of the engineered SEIs can be controlled by the deposition rates of Li2CO3 and LiF through changing the power of each Radio-Frequency of sputtering sources. These films have been characterized by XPS. Figure S1 in the Supporting Information shows the depth profiles of several engineered SEI films before and after cycling. Obviously, SEI1 has the highest concentration of F. The atomic percentage of F is around 15%. If counting the films as Li2CO3/Li2O and LiF, half of the inorganic components are LiF (by molar). SEI2 and SEI3 contain 15% and
