Article pubs.acs.org/JPCC
Determination of the Time Dependent Parasitic Heat Flow in Lithium Ion Cells Using Isothermal Microcalorimetry L. E. Downie, S. R. Hyatt, A. T. B. Wright, and J. R. Dahn* Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia B3H 4R2, Canada ABSTRACT: 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 LiCoO2/graphite and Li[Ni0.4Mn0.4Co0.2]O2 (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.
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flow due to parasitic reactions can be easily measured.18−22 Recent work done by Downie et al.22 demonstrated that the parasitic heat flow as a function of voltage could be isolated for an individual cell. This was done by varying the current over narrow voltage ranges and fitting the resulting measured heat flow using a model where each of the three contributions of heat flow (polarization, changes in entropy, and parasitic heat) was a simple polynomial function of the state of charge. The fitting constants then gave the relative contributions of each term, with particular importance paid to the parameters associated with the parasitic heat flow. It was noted by Downie et al.22 that the extracted parasitic heat flow decreased with time. This reduction in parasitic heat over time is consistent with results from Smith et al.23,24 which showed that the charge consumption by the solid electrolyte interphase (SEI) layers on the negative and positive electrodes was dependent primarily on time and not cycle count, as well as the observed decay in open circuit heat flow over time by Downie et al.20 In this work, the time and state of charge dependencies of the parasitic heat flow are determined using a method similar to that of Downie et al.,22 as described above, where a time dependent model is proposed. In this model, over narrow voltage ranges the parasitic heat flow is assumed to comprise a polynomial function in relative state of charge multiplied by a linear time dependent term. This analysis is particularly useful for examining the behaviors of the parasitic heat flow as a function of voltage for a variety of electrolyte additives across many different cell chemistries, as well as determining how the parasitic heat flow evolves over time for various voltage ranges. As demonstrative examples, the parasitic heat flow as functions
INTRODUCTION Applications 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 chemistries6−15 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 has been shown that the heat © 2014 American Chemical Society
Received: September 3, 2014 Revised: November 10, 2014 Published: November 23, 2014 29533
dx.doi.org/10.1021/jp508912z | J. Phys. Chem. C 2014, 118, 29533−29541
The Journal of Physical Chemistry C
Article
noise level of about 10 nW, and a baseline drift from 0.00 μW of 4.4 V) an increased amount of parasitic heat flow was observed, but that parasitic heat flow decreased rapidly with time. Figure 9 shows the voltage and corresponding measured total heat flow as a function of time for the three identical NMC442/ graphite pouch cells containing 2% PES + 1% TTSPi + 1% MMDS discussed in Figure 8. Only the results from 4.1−4.3 V cycles at 1 mA (cycles 2, 4, 5, and 6 as shown in Figure 7) are shown. The left panels show the voltage and associated heat flow for all three cells during cycle 2, again showing that all
Figure 7. Panel a shows the voltage over the time frame of the variable rate experiments for a NMC442/graphite pouch cell containing control electrolyte (black, bottom axes) and 2% PES + 1% TTSPi + 1% MMDS (green, top axes). Panel b shows the corresponding measured total heat flow. Panel c shows the extracted time dependent parasitic heat showing the progression as a function of time. Panel d shows the extracted scale factor, (1 − LΔt), which multiplies the time independent parasitic heat.
rate of change of the parasitic heat flow is nearly identical for both cells for all voltage ranges, and for both cells, a slight increase for the highest voltage range, 4.4−4.5 V, is observed. This corresponds to an increased overall parasitic heat flow, so while there is an initial increase in heat flow, it is more rapidly reduced with time. Figure 8 shows the measured total heat flow as a function of voltage of three identical NMC442/graphite pouch cells
Figure 8. Measured heat flow for NMC442/graphite pouch cells containing 2% PES + 1% TTSPi + 1% MMDS during 1 mA charge (solid lines) and discharge (dashed lines) segments. Cycles 2, 4, 5, and 6 were charged and discharged between 4.1−4.3 V, while during cycle 3 each cell was charged to a different upper cutoff voltage: 4.3 V (black), 4.5 V (red), and 4.6 V (blue).
containing 2% PES + 1% TTSPi + 1% MMDS. The three cells went through initial cycling between 2.8−4.3 V at 10 mA, then began narrow range cycling at 1 mA between 4.1−4.3 V for 2 cycles. On the third 1 mA cycle, each cell went to a different upper cutoff voltage: 4.3 V (black), 4.5 V (red), and 4.6 V (blue). The remaining three cycles (cycles 4−6) were all done between 4.1−4.3 V at 1 mA to see the effect of one high voltage segment on the parasitic heat flow at lower voltages. For
Figure 9. Measured heat flow as a function of time (bottom panels) for a NMC442/graphite pouch cell containing 2% PES + 1% TTSPi + 1% MMDS for cycles 2, 4, 5, and 6 for cells that went through one cycle (cycle 3) to an upper cutoff voltage of 4.3 V (black), 4.5 V (red), and 4.6 V (blue). Corresponding voltages as a function of time are shown in the top panels. 29539
dx.doi.org/10.1021/jp508912z | J. Phys. Chem. C 2014, 118, 29533−29541
The Journal of Physical Chemistry C
Article
for Advanced Batteries. L.E.D. acknowledges financial support from the NSERC CREATE DREAMS (Dalhousie Research in Energy Advanced Materials and Sustainability) program at Dalhousie University. S.R.H. acknowledges financial support from NSERC in the form of a Warr Award. We thank Dr. Jing Li of BASF for providing the electrolyte solvents and salts used in this work. We also acknowledge Dr. Vincent Chevrier of 3M Co. for providing the software used to aid in data processing. Dr. Jens Paulsen and Dr. Xin Xia of Umicore (Korea) are thanked for providing pouch cells free of charge.
three cells were identical prior to the high voltage cycle. The right panels show the measured heat flow for the three 1 mA cycles after the cells were cycled to an upper cutoff voltage of 4.3 V (black), 4.5 V (red), and 4.6 V (blue). The t = 0 point is taken as the time at the start of cycle 4 for comparative purposes. As shown in Figure 8, the heat flows for the cells that went to higher voltages are initially significantly higher than the 4.3 V cell but decrease fairly quickly to nearly return to the level of the low voltage cell. At the start of cycle 4, the heat flow of the 4.6 V cell was 100 μW above that of the 4.3 V, but at the end of cycle 6, that difference was below 40 μW. From the results presented above, it is clear that the behavior of electrolyte additives is very different at high voltages and for different cell chemistries. While an additive blend such as 2% VC + 1% TTSPi + 1% MMDS was very beneficial at lower voltages (
