The capacity fade mechanisms of LiCoO 2 /Si-alloy:graphite pouch cells filled with a 1M LiPF 6 EC:EMC:FEC (27:63:10) electrolyte were studied using galvanostatic cycling, electrochemical impedance spectroscopy on symmetric cells, gas-chromatography and differential voltage analysis. Analysis of the gas generated during the first cycle indicated that FEC reacts at the negative electrode following a 1-electron reduction pathway and other pathways that do not lead to the formation of gaseous products. An analysis of the electrolyte showed that FEC is continuously consumed during the first 80% of the first charge (formation cycle). Typical cells charged and discharged at 40 • C showed a gradual capacity loss for the first 250 cycles followed by a sudden capacity drop associated with a large polarization growth. Analysis of the electrolyte showed that this sudden failure is associated with the depletion of FEC. The capacity loss as well as the consumption of FEC prior to the sudden failure was fitted using a model that includes a time dependence and a cycle number dependence. The time dependence was associated with the thickening solid electrolyte interface at the surface of the negative electrode particles (Si-alloy and graphite) and the cycle number dependence was associated with the solid electrolyte interface repair at the surface of Si-alloy particles during repeated expansion and contraction. Cost reduction and an increase in the volumetric energy density of Li-ion cells can possibly be attained using silicon-based negative electrodes.1-3 While these benefits have been known for many years, commercial implementation remains limited due to silicon's inherent challenges.Si-based electrodes can suffer from particle pulverization upon cycling (e.g. micron sized silicon), 1,4-7 can have loss of electronic contact between particles after repeated expansion and contraction during cycling, 2,8-10 can have high irreversible first cycle coulombic efficiency 11,[12][13][14][15] and can have low coulombic efficiency (compared to graphite electrodes) during cycling. 1,16,17 The root cause of these challenges is the large volume expansion 1,18 of the material during lithiation and subsequent contraction during delithiation. Several issues have been solved through the use of either nanosized Si or nanostructured Si. For instance the use of nanosized Si particles solves some of the pulverization problems encountered by micron sized Si particles. 1,4,11,[19][20][21] However, nanosized Si still suffers from inter-particle contact loss during cycling as well as low coulombic efficiency due to large specific surface area. 1,16In a recent publication, Chevrier et al. 16 presented a rational way to design commercially relevant Si-based negative electrodes. They showed that confining nano-domains of silicon in an alloy matrix suppresses particle pulverization and greatly reduces the surface area of Si exposed to the electrolyte. This approach yields materials with lower first cycle irreversible capacity (IRC) loss, better coulombic e...

In an effort to better the understanding of the high voltage degradation of electrolytes in lithium ion cells, this work presents isothermal microcalorimetry results on LiNi 0.42 Mn 0.42 Co 0.16 O 2 (NMC442)/graphite pouch cells up to 4.7 V. The voltage and time dependent parasitic heat flow was determined for cells containing several electrolyte compositions based on carbonate solvents with several additive combinations, as well as a fluorinated carbonate solvent system. We have demonstrated that cells containing fluorinated carbonate-based electrolyte (1M LiPF 6 in 3:7 fluoroethylene carbonate: di-2,2,2-trifluoroethyl carbonate) showed a significantly decreased parasitic heat flow at voltages >4.4 V compared to ethylene carbonate-based cells, but limited advantage <4.4 V. However all cells, regardless of electrolyte composition, exhibited very large parasitic heat flows, and therefore parasitic reaction rates, at high voltages (>4. Existing and emerging applications using lithium ion batteries are demanding increased energy densities, decreased cell bulging due to gas production, fast charging capabilities, and longer lifetimes, etc, all while reducing cost. The calendar and cycle lifetimes of lithium ion cells are well known to be primarily affected by parasitic reactions between the electrodes and electrolyte.1,2 To extend these lifetimes, the rate and extent of parasitic reactions must be reduced. Therefore it is of utmost importance to be able to measure and quantify the impact of changes in cell components on these reactions in a rapid and precise manner that correlates to long term performance.One common and effective way of minimizing parasitic reactions and extending cell lifetimes is the use electrolyte of additives. Extensive studies have been carried out on such additives, and many comprehensive reviews can be found on the topic. 1,3,4 However, the role of electrolyte additives is extremely complex, is relatively poorly understood, and is the subject of much debate in the literature. Furthermore, in an effort to increase energy density, there has been a push to move to higher voltage systems. However, the typical carbonate solvents are oxidized at such high voltages, where the decomposition has been reported to begin as low at 4.0 V 5,6 and up to 4.5 V vs Li/Li + . 3,7-9 One class of solvents used to extend the stability window are fluorinated carbonates, the most common of which is fluoroethylene carbonate (FEC).3,10-15 Several other fluorinated cyclic and linear carbonates have been investigated as both full solvent systems and cosolvents.1,14-17 One such fluorinated linear carbonate investigated in this work is di-2,2,2-trifluoroethyl carbonate (TFEC) which has been shown to improve the protective passivation film that forms on the negative electrode. [17][18][19] It is clear that the voltage dependent impact of electrolyte additives and unconventional solvents on parasitic reactions is of particular interest, especially at high voltages.Recently, Downie et al. demonstrated that isothermal microc...

Isothermal microcalorimetry has been previously used to determine the voltage dependent parasitic heat flow for individual lithium ion cells. While a decrease in parasitic heat flow over time was observed, no attempts were made to quantify the time dependence. Here, by varying the current over narrow voltage ranges, the relative contributions of each of the components of the total heat flow were isolated as functions of time and state of charge. By fitting the measured total heat flow to a simple empirical model, the polarization and entropic heat flows were determined as a function of state of charge, while the heat flow resulting from parasitic chemical reactions was determined as functions of both state of charge and time. The time and state of charge dependent parasitic heat flow was determined for high voltage LiCoO 2 /graphite and Li[Ni 0.4 Mn 0.4 Co 0.2 ]O 2 (NMC442)/graphite pouch cells, with particular emphasis on high voltage operation. The effects of electrolyte additive blends containing combinations of vinylene carbonate, prop-1-ene sultone, tris(trimethylsilyl)phosphite, and methylene methanedisulfonate are also shown. ■ INTRODUCTIONApplications such as electric vehicles and grid-scale energy storage are putting an increased demand on lithium ion batteries to have higher energy densities and dramatically longer lifetimes than cells typically used in portable electronics. The calendar and cycle lifetimes are well known to be largely affected by parasitic reactions such as electrolyte oxidation occurring in the cell. To extend the lifetimes, these parasitic reactions must be reduced, and therefore, it is of utmost importance to be able to measure and quantify these reactions in a precise manner.One common technique for improving cell lifetimes by increasing coulombic efficiencies, increasing capacity retention, and reducing parasitic reactions is with the addition of electrolyte additives. 1−3 Extensive studies have been performed investigating and cataloging the benefits of electrolyte additives, including studies by Wang et al. 4,5 of 55 and 110 additive combinations. However, the mechanisms of how additives provide such benefits are not well understood and are frequently debated in the literature. Furthermore, transitioning to increasingly higher upper cutoff voltages has proved difficult as many additives and solvents are unstable at such high potentials, and the parasitic degradation of these components results in severely decreased lifetimes. It is therefore important to examine the voltage dependent impact of electrolyte additives on parasitic reactions.One technique well suited for the quantification of parasitic reactions is isothermal microcalorimetry. Many reported isothermal calorimetry studies on various lithium ion chemistries 6−15 interpret the measured heat flow based on the Newman energy balance, 16,17 a model which assumes that the heat flow due to side, or parasitic, reactions is negligible. However, with the introduction of microcalorimeters with nanowatt-scale sensitivities, it ha...

The use of electrolyte additives to form a passive solid-electrolyte interphase (SEI) at one or both electrodes is a common method for improving lithium-ion cell lifetime and performance. This work follows the chemical and electrochemical processes involved in SEI formation on graphite electrodes for two Lewis acid-base adducts, pyridine boron trifluoride (PBF) and pyridine phosphorus pentafluoride (PPF). The combination of experimental methods (electrochemistry, in situ volumetric measurements, gas chromatography, isothermal microcalorimetry, and X-ray photoelectron spectroscopy) with quantum chemistry models (density functional theory) provides new insight into the interfacial chemistry. PBF and PPF are reduced at ∼1.3 V vs. Li/Li + and ∼1.4 V, respectively. This is followed by radical coupling to form 4,4 -bipyridine adducts, hydrogen transfer to ethylene carbonate solvent molecules, and reduction of the solvent to produce lithium ethyl carbonate. The reduced bipyridine adducts, Li 2 (PBF) 2 Over the past decade, the predominant electrolyte solution used in lithium-ion cells has remained lithium hexafluorophosphate (LiPF 6 ) salt dissolved in some blend of organic carbonate solvents.1 This is not, however, an indication that advances in cell solution chemistry have stagnated, but it is rather a result of a major shift to focus on electrolyte additives. By adding just a few weight percent of the right compounds to the electrolyte solution before cells are filled, one can significantly improve charge-discharge cycling performance, extend calendar lifetime, decrease detrimental gas formation, and improve lithium-ion cell safety. [1][2][3][4][5] This move to electrolyte additives has the rather practical aspect that the battery industry can tweak performance with minimal changes to their existing supply chains for LiPF 6 and solvents.1 This article will focus on Li(Ni a Mn b Co 1−a−b )O 2 (NMC)/graphite cells, for which vinylene carbonate (VC), [6][7][8][9][10] prop-1-ene-1,3-sultone (PES), [8][9][10][11][12][13][14][15] methylene methane disulfonate (MMDS), tris(trimethylsilyl) phosphite (TTSPi), 16,17 and triallyl phosphate (TAP) 18,19 are among the best reported additives. Recently, Nie et al. introduced a series of Lewis acid-base adducts that may be used individually or as part of an additive blend. [20][21][22] In particular, pyridine boron trifluoride (PBF) and pyridine phosphorus pentafluoride (PPF) offer improved charge capacity retention after cycling at high temperature and high cell voltage. 20 It is these latter two additives that will be closely examined in this article, chosen because they are still quite new and, so far, relatively unstudied.If one is to optimize the solution chemistry in lithium-ion cells for various applications (high temperature, high power, etc.), it is clearly desirable to have thoroughly characterized the chemical and electrochemical reactions in a cell, including all those of the various electrolyte additives. Unfortunately, such a detailed understanding of the many proces...

Physical properties of LiPF 6 in ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC) electrolytes were studied by conductivity measurements, Fourier transform infrared spectroscopy (FT-IR) and differential thermal analysis (DTA). Conductivity measurements show that the addition of additive levels of FEC to EMC electrolyte can dramatically increase the conductivity of EC-free EMC electrolytes at low salt concentrations below 0.4 M. FT-IR results show that the added FEC hinders ion pair formation by competing with EMC to dissociate LiPF 6 resulting in increased conductivity in EMC electrolytes. Conductivity measurements show that the conductivity of DMC electrolytes decreases significantly below 0 • C due to the high melting point of DMC. Differential thermal analysis was used to determine the LiPF 6 -DMC phase diagram which then can be used to explain the conductivity results. The results presented here identify avenues by which EC-free electrolytes can be improved for use in practical Li-ion cells. Li-ion battery packs for electrified vehicles and grid energy storage require high energy density and long lifetime cells.1-3 One approach to increase the energy density of Li-ion cells is to adopt high potential positive electrodes. Layered Li[Ni x Mn y Co 1-x-y ]O 2 materials have been extensively investigated since they are cheaper alternatives to LCO and since they can function at high potentials. 4-9However, it is a great challenge to cycle high voltage NMC cells (above 4.3V) well. The use of new solvent blends and the introduction of electrolyte additives are two common ways to improve the lifetime of high voltage NMC Li-ion cells. 24-32 Therefore, it is important to study the physical properties of EC-free electrolytes to optimize the performance of cells where EC-free electrolytes are used.Differential scanning calorimetry (DSC) 29,32,33 and differential thermal analysis (DTA) 34 can be used to examine the phase diagrams of solvent blends and of electrolytes. Ding et al. showed that supercooling and superheating during DSC measurements for phase diagram determination could be dramatically reduced if carbon black and/or electrode powders were present in the samples during measurement. 33Recently Day et al. demonstrated that the DTA technique can be applied to an entire Li-ion cell and is a non-invasive in-situ method to probe the state of the liquid electrolyte within intact Li-ion cells. 34The DTA measurements provide information about the amount of liquid electrolyte remaining in the cell and about the electrolyte composition. The interpretation of the DTA results requires information about the phase diagram of the electrolyte of interest. However, the phase diagrams of LiPF 6 :ethyl methyl carbonate and LiPF 6 :dimethyl carbonate electrolytes have not yet been reported.In this report, the conductivity of LiPF 6 :EMC electrolytes with various salt concentrations were measured over a wide range of temperatures. Fourier transform infrared spectroscopy (FT-IR) was used to study the interactions between t...