A2100 Journal of The Electrochemical Society, 159 (12) A2100-A2108 (2012) 0013-4651/2012/159(12)/A2100/9/$28.00 © The Electrochemical Society Electrochemical Analysis of Li-Ion Cells Containing Triphenyl Phosphate Ronald P. Dunn, a, ∗ Janak Kafle, a, ∗ Frederick C. Krause, b Constanza Hwang, b Bugga V. Ratnakumar, b, ∗∗ Marshall C. Smart, b, ∗∗ and Brett L. Lucht a, ∗∗, z a Department of Chemistry, University of Rhode Island, Kingston, Rhode Island 02881, USA b Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, USA The development and subsequent incorporation of flame retardant additives (FRAs) has become a priority for Li-Ion battery research and development. Triphenyl phosphate (TPP) was studied to ascertain the safety benefits and electrochemical performance when incorporated into a LiPF 6 /ethylene carbonate (EC)/ethyl methyl carbonate (EMC) electrolyte system. The flammability of electrolytes containing TPP was investigated via self-extinguishing time and flash point analysis. The electrochemical stability was studied by cyclic voltammetry (CV), battery cycling in graphite/LiNi 0.8 Co 0.2 O 2 cells, electrochemical impedance spectroscopy (EIS) and Tafel polarization. In order to better understand the role of TPP, ex-situ surface analysis of the cycled electrodes was conducted with X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Incorporation of TPP results in a moderate decrease in the flammability of the electrolyte with relatively minor detrimental effects on the performance of the cells and thus is a promising additive for lithium ion batteries. © 2012 The Electrochemical Society. [DOI: 10.1149/2.081212jes] All rights reserved. Manuscript submitted August 17, 2012; revised manuscript received September 21, 2012. Published October 20, 2012. Lithium-ion battery technology in recent years has proven itself as a dependable energy storage medium for commercial consumer electronics. Li-ion batteries offer higher operating cell voltage, higher energy density, longer cycle life and lower self-discharge. These ad- vantages make Li-ion cells superior to other rechargeable systems such as Ni-MH and Ni-Cd. Safety issues however remain a concern with today’s Li-ion batteries since the electrolyte is typically a blend of ethylene carbonate (EC) with ethyl methyl carbonate (EMC) with a lithium salt, such as lithium hexafluorophosphate (LiPF 6 ). These electrolyte solutions are flammable and a risk of thermal runaway is a concern. The main causes of Li-ion cell thermal runaway are attributed to both internal short via metallic dendrite accumulation and/or cell overcharge leading to destabilizing over-deliathiation of the cathode. 1–5 The potential for thermal runaway has led to efforts to reduce the fire risk and the propagation within Li-ion cells. Many of these efforts focus on the development, and subsequent incorporation, of flame retardant additives (FRA) into the electrolyte solution. A number of organophosphorus compounds have been investigated for lithium ion batteries. For example, various research groups have reported the use of trimethyl phosphate (TMP), 6 triphenyl phosphate (TPP), 3,7–11 tris(2,2,2-trifluoroethyl) phosphate, 12–14 and dimethyl methylphos- phonate (DMMP) 1,4,15 in lithium ion battery electrolytes. These ad- ditives are believed to result in lower electrolyte flammability due to the formation of a layer of char which protects the uncombusted condensed phase and/or the decomposition products serving as radical scavengers in the gas phase inhibiting combustion chain reactions. 16,17 Of the phosphate-based FRAs, triphenyl phosphate (TPP) is especially attractive since it has been reported to improve the safety of Li-ion cells under abuse conditions by lowering the flammability of elec- trolytes when incorporated in sufficient proportion, 18 and it has been observed to provide good life characteristics. 7,9 Some FRAs have been observed to disrupt the formation and sta- bility of the anode solid electrolyte interphase (SEI) layer, and are thus detrimental to the cycling performance of the cells. FRAs have also been investigated in combination with SEI forming additives such as lithium bis(oxalato)borate (LiBOB) or vinylene carbonate (VC) which generate a more stable anode SEI and limit the detrimental effects of the FRA. 7,9,19 A stable SEI layer is critical to the proper functioning of Li-Ion cells, allowing the intercalation and de-intercalation of Li + at the graphite anode and preventing further reduction of the electrolyte. 2 The formation mechanisms of the SEI and the role of the constituent ∗ Electrochemical Society Student Member. ∗∗ Electrochemical Society Active Member. z E-mail: blucht@chm.uri.edu solvents and salts in the solid electrolyte interphase (SEI) are currently under investigation. 20 This current research is focused upon the effort to inhibit flamma- bility of electrolytes via incorporation of FR additives while mitigat- ing their negative attributes and maximizing electrochemical perfor- mance. The drawbacks of FRA incorporation into lithium-ion batteries include increased discharge capacity fade and poor cycling perfor- mance at low temperatures. Loss of electrochemical performance in the presence of FR additives is commonly attributed to inadequate for- mation of a stable SEI on the surface of the anode, and in some cases undesirable properties of the cathode electrolyte interphase (CEI). 21,22 This investigation focuses on the effects of the incorporation of TPP on the flammability of the electrolyte, the conductivity of the electrolyte, the cycling performance of graphite/LiNi 0.8 Co 0.2 O 2 cells, electrode transfer kinetics, and electrode interphase structure in lithium ion batteries. Experimental Battery-grade carbonate solvents ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC), as well as lithium hexafluorophosphate (LiPF 6 ) salt, were obtained from No- volyte Technologies, Inc. Two different electrolytes 1.2 M LiPF 6 in EC/EMC (3:7 vol.%, BL1) and 1.0 M LiPF 6 in EC/EMC (2:8 vol.%, BL2) were obtained from Novolyte Technologies and utilized without further purification (water content was less than 50 ppm in all cases). Triphenyl phosphate (TPP) was obtained from Thermo Fisher Scien- tific at 99% purity and used as received. TPP containing electrolytes were prepared with a constant concentration of LiPF 6 and EC while EMC was replaced with TPP. Self-extinguishing time, or SET, of the electrolyte combinations was measured via a modified version of the procedure detailed by Xu and coworkers using commercial cotton swabs as the test wick. 1,12 The commercial cotton swab wicks were manufactured to a uniform diameter of 1 cm and were injected with 100 μL of electrolyte. The wick was placed in a fume hood with an air flow velocity of 90 ft/s and suspended at uniform height above a watch glass. Burning time was recorded with the use of a digital stopwatch. This procedure was performed on ten samples of each electrolyte and an average SET was calculated for each. The flash points of solvent blends incorporating TPP were mea- sured using a Pensky-Martens Closed Cup Flash Tester from Koehler Instrument Company. Solvent blends were mixed excluding Li salt to a total mass of 70 g and placed in a closed test cup. A motorized stirrer was used to enhance solvent evaporation within the closed cup and a propane supplied flame was dipped into the sample cup every 1 ◦ C to test for vapor ignition signaling the flash point of the sample.