Journal of The Electrochemical Society, 161 (6) A1001-A1006 (2014) A1001 0013-4651/2014/161(6)/A1001/6/$31.00 © The Electrochemical Society Role of Lithium Salt on Solid Electrolyte Interface (SEI) Formation and Structure in Lithium Ion Batteries Mengyun Nie * and Brett L. Lucht **, z Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 02882, USA A comparative investigation of the different lithium salts on formation of the solid electrolyte interface (SEI) on binder free graphite anodes for lithium ion batteries has been conducted. The electrolytes investigated include 1 M LiPF 6 , LiBF 4 , LiTFSI, LiFSI, LiDFOB or LiBOB dissolved in ethylene carbonate (EC). The SEI has been investigated via a combination of spectroscopic and microscopic techniques. Transmission electron microscopy (TEM) allows direct observation of the SEI formed from the different electrolytes. Nuclear magnetic resonance (NMR) spectroscopy of D 2 O extracts are utilized to characterize the soluble species of SEI. XPS and FTIR provide additional elemental and functional group information for the SEI components. The SEI for all electrolytes contains lithium ethylene dicarbonate (LEDC), the primary reduction product of EC. In addition, the SEI for all electrolytes contain LiF except for the SEI generated from the LiBOB electrolyte. The SEI generated in the presence of LiBOB or LiDFOB electrolytes contain multiple oxalate containing species, including lithium oxalate (Li 2 C 2 O 4 ), and borates. © 2014 The Electrochemical Society. [DOI: 10.1149/2.054406jes] All rights reserved. Manuscript submitted February 24, 2014; revised manuscript received March 19, 2014. Published May 1, 2014. Lithium ion batteries dominate the consumer electronics market and are rapidly being introduced into the expanding electric vehi- cle (EV) market. The EV market requires batteries with high en- ergy density and excellent long term performance. The electrolytes, both lithium salts and solvents have a very important role in ca- pacity, efficiency, and calendar life of lithium ion batteries. Ini- tial investigations of electrolyte for lithium ion batteries included LiAsF 6 , LiClO 4, LiBF 4 and LiPF 6 , but due to the high solubil- ity in carbonates-based solvents, high ionic conductivity and rela- tively low price, 1,2 LPF 6 is the only commercially utilized lithium salt in lithium ion batteries. More recently, there has been inter- est in the development of novel lithium salts with superior prop- erties to LiPF 6 , including lithium bis-oxalato borate (LiBOB), 3 lithium difluorooxalato borate (LiDFOB), 4 lithium tetrafluoroox- alato phosphate (LiTFOP), 5,6 lithium bis(fluorosulfonyl) imide (Li[(FSO 2 ) 2 N], LiFSI), 7–9 and lithium bis(trifluoromethylsulfonyl) imide (Li[(CF 3 SO 2 ) 2 N], LiTFSI). 10,11 Investigations of these novel salts suggest that the salt has a role in the structure of the SEI formed on graphite. 12–18 We have previously reported the use of binder free graphite elec- trodes to develop a better understanding of the SEI and the role of the electrolyte in SEI structure and function. 19,20 Our investigation has now been expanded to include electrolytes containing LiPF 6 , LiBF 4 , LiTFSI, LiFSI, LiDFOB or LiBOB in EC. The absence of binder and presence of a single solvent electrolyte simplifies the analysis of the SEI components. After cycling, ex-situ analysis of the electrodes has been conducted via a combination of TEM, solution NMR spec- troscopy of electrode extracts, FT-IR, and XPS. The specially designed TEM experiment allows direct imaging of the graphite particles after cycling, while analysis of the D 2 O extracts provides information on the molecular structure of the SEI components. The results provide insight into changes in SEI composition resulting from the structure of the anions and confirm the role of both salt and solvent in SEI formation and structure. Experimental Preparation of binder-free graphite electrode and coin cells fabrication.— Binder free graphite (BF-G) electrodes were prepared by Electrophoretic Deposition (EPD) method in the same manner to our previously reports. 12,14 The EPD bath was prepared with SFG- 6 graphite particles (∼5 um, Timex) suspended in acetonitrile with 0.1% v/v trimethylamine (anhydrous, Fisher, Co). Utilizing this method, allows the preparation of electrodes without polymer binders ∗ Electrochemical Society Student Member. ∗∗ Electrochemical Society Active Member. z E-mail: blucht@chm.uri.edu (PVDF) or conductive carbon. The electrode is composed exclusively SFG-6 graphite particles with a theoretical capacity ∼372 mAh/g. The BF-graphite electrodes were vacuum dried for 24 h at 120 ◦ C. Coin cells (CR2032) were fabricated with BF-graphite electrodes, polypropylene separator (Celgard 3501), and lithium foil in high pu- rity Ar-filled glove box. Six different lithium salts have been investi- gated which fall into three categories: 1) LiPF 6 and LiBF 4 2) LiBOB and LiDFOB, 3) LiTFSI and LiFSI. The electrolytes were prepared (1 mol/L) with a single solvent, ethylene carbonate (EC, BASF) and the different lithium salts. The structures of the salts are summarized in Figure 1. Each coin cell contains 30 uL of electrolyte. Special in-house TEM coin cells were assembled containing binder-free graphite electrodes with copper TEM grids as previously reported. 19 Graphite particles were removed from the center of the BF graphite electrode to allow placement of the copper TEM grid. Dur- ing cell construction some of the particles shift from the BF graphite electrode and adhere to the copper TEM grid. Cell assembly was conducted in an Ar-atmosphere glove box (<1 ppm H 2 O). Electrochemical cycling.— Coin cells undergo a constant-current charge and discharge between 2.0 to 0.05 V on a ARBIN BT 2000 cycler with a current density of ∼50 uA/cm 2 which is approximately a C/20 rate at 25 ◦ C. Cycling was discontinued after the first lithiation and delithation. TEM imaging.— Cycled cells were disassembled in an Ar- atmosphere glove box (<1 ppm H 2 O). TEM grids were extracted from cycled coin cells and rinsed with anhydrous dimethyl carbonate (DMC, Acros) to remove residual electrolyte and dried overnight in a vacuum. The TEM grids were quickly transferred into the TEM chamber. Imaging was conducted using a JEOL JEM-2100F TEM LiTFSI O S O NLi F 3 C 2 O S O NLi F 2 LiFSI O BF 2 O O O LiDFOB O B O O O O O O O LiBOB Li Li Figure 1. Structures of salts investigated. ) unless CC License in place (see abstract). ecsdl.org/site/terms_use address. Redistribution subject to ECS terms of use (see 131.128.30.23 Downloaded on 2015-04-23 to IP