Safety Testing with Dreamweaver Nonwoven Nanofiber Separators: Comparing Shutdown Separators to Thermally Stable Separators

Transcription

1 Voltage (V) Safety Testing with Dreamweaver Nonwoven Nanofiber Separators: Comparing Shutdown Separators to Thermally Stable Separators Executive Summary 2.7 Ah Pouch cells of LiFePo4 and graphite were made with four different separators: micron polypropylene dry process (PP) micron ceramic coated polyethylene wet process 2.5 (PE) 2 3. Dreamweaver Silver 25 micron Dreamweaver Gold 25 micron. 1 Starting electrical performance was nearly identical between the :: cells before testing: :28:48 :57:36 1:26:24 1:55:12 PP Ceramic Voltage Hard Short: The cells were shorted between their tabs. The cells all performed the same, with the anode tab (4 mm tab) getting Silver Voltage Gold Voltage red hot, and the cells slowly heating and internally gassing until the seal broke. Overcharge: The cells were charged at 2C from 3% SOC. All uncoated cells failed at 5.55 V by outgassing, while the ceramic coated PE failed at 5.9 V. There were no flames, and the temperature rise was uniform between cells. Hot Box: Cells made with PP and PE/ceramic separators both had fast temperature rise and they also both experienced a catastrophic short after around 2 minutes. DWI Silver and Gold cells maintained operational voltage for 1 hour and survived without shorting. The short for the PE/ceramic cell occurred with onset at around 14 C, and for the PP cell at around 145 C. Both of the DWI cells rose to 17C, the oven temperature, without experiencing a short. Nail Penetration: Cells made with both PP and PE separators dropped immediately to a persistent V along with faster and higher temperature rise (up to 115 C) than the cells made with Gold (4 C) and Silver (6 C). The Dreamweaver cells, on the contrary, experienced only a small voltage drop of approximately 1 mv, with a slow rise thereafter as the cell discharged. The DWI cells continued to function throughout the test and retained 2/3 of their charge and capacity after completion of the test. Shutdown Function: In no test was a shutdown function observed to improve the performance in any of the cells. Figure 1 (Left) DWI Silver separator after test, with no change in shape or color and clean puncture. (Right) PP separator after test, with splitting and shrinkage near penetration and significant heat shrinkage at end of electrode. DWI C Hot Box Voltage vs Time Nail Penetration Voltage vs Time :: :7:12 :14:24 :21:36 PP Voltage DWI Silver Voltage PP Ceramic PE Voltage DWI Gold Voltage Shrinkage

2 Introduction As lithium ion batteries proliferate into larger and more diverse applications, safety becomes one of the key concerns with battery design. In particular, the ability to survive abuse testing is increasingly difficult with higher energy density electrodes, and with cells that have higher energy per cell. Many approaches are being taken, some of which will take years, or decades, to mature until they can be widely disseminated. Among these are nonflammable ionic liquid electrolytes, which remove the most flammable component of a cell, but are prohibitively expensive when used in the high concentrations needed to be non-flammable. Others include solid state batteries, which are commercial today with a requirement for an 8 C operating temperature, but which are not yet available for lower temperature usage on a commercial scale. A key component for safety design is the battery separator, which separates the electrodes but holds electrolyte while allowing ions to flow freely between electrodes through the electrolyte. However, cell failure under abuse testing today is often due to failure of the separators. Current technology separators are made from olefin polymers with low melting points between 135 C for PE and 165 C for PP. In attempts to mitigate the thermal failures that are experienced when cells approach these temperatures, manufacturers have coated the separators with a variety of materials, including primarily ceramic particles held together with polymer binders. Much has been made about the shutdown function in polyolefin lithium ion separators, which is said to shut off the ionic conductivity of the separator as the cell temperature is raised, eliminating the source of heat and shutting down the function of the cell prior to full thermal runaway. In the experiments presented here, lithium iron phosphate pouch cells have been prepared that are otherwise identical except for the substitution of four different separators: a polypropylene separator, a ceramic coated polyethylene separator, and Dreamweaver s nanofiber based Silver and Gold separators. The first two separators are shutdown separators, while the latter two are not, but have higher melting and thermal degradation points. The cells were tested for electrical performance and very little difference was seen between the cells. They were also tested for safety using hard short, overcharge, hot box and nail penetration tests. As will be discussed in detail later, there was no evidence of a shutdown function in the polyolefin separators, but significant evidence of higher thermal stability in the separators made from more thermally stable materials. Cell Preparation Pouch cells were made of dimensions 55 mm x 115 mm and nominal capacity 3 Ah. The electrodes were lithium iron phosphate (LFP) cathodes and graphite anodes, commercially produced by a Chinese battery producer. The cells were filled, sealed formed and tested according to the procedure in Table 1. Formation was done on a Maccor battery tester. All of the separators tested were 25 microns thick. The separators tested were: 25 micron polypropylene, dry processed 25 micron ceramic coated polyethylene, wet processed 25 micron Dreamweaver Silver nanofiber nonwoven 25 micron Dreamweaver Gold nanofiber nonwoven reinforced with Twaron For all of the tests except nail penetration, the cells were clamped between two pieces of cardboard to limit expansion due to outgassing prior to the pouch venting. Cell Cycle Dry separator 3 hrs under vacuum at 12 C (DWI) or 1 C (PP and PE/ceramic) Assemble cells Dry cells overnight at 1 C Fill Cells with electrolyte, wait >2 hrs Tap charge to 2. V 2. V 24 hrs Sit 24 hours (rest) Formation charge at C/6 rate to 3.6 V then CV to C/6 Discharge at C/6 to 2.5 V Charge at C/6 rate to 3.6 V then CV to C/6 Discharge at C/6 to 2.5 V Charge at C/6 rate to 3.6 V then CV to C/6 Discharge at C/6 to 2.5 V Charge cells at C/6 to 3% SoC Record voltage (24 hour rest period) Table 1 Cell build and formation procedure for cells included in this study.

3 Temperature (C) Discharge Capacity (mah) Electrical Tests Cells were characterized for rate capability by charging at C/6, and then performing three consecutive discharges at each of C/12, C/6, C/4, C/2, C, 2C and 3C. The average at each rate is shown for each separator type in Figure 1, below. For discharge times longer than C (1 hour), the cells performed similarly. At discharge times from 2C and 3C, the highest discharge capacity was measured for Silver, then PE/Ceramic, then Gold and the PP separator which performed nearly the same at 3C, with Gold outperforming at 2C. Safety Testing Hard Short Hard short tests were conducted by connecting the positive and negative electrodes with 14 gauge wire through a switch rated for 3 A. The cells were wired to record voltage across the electrodes and temperature on the surface of the cell for the duration of the test. During the test, the most immediate effect after the switch was switched was that the negative electrode tab heated and began to glow. These tabs, at 4 mm, were not sufficient to carry the load of the current. All of the cells performed similarly, rising within 3 minutes to above 1 C, reaching a peak temperature at 5 minutes, with the electrolyte outgassing and venting after about 15 minutes. There was a small difference in the rate of rise of the cells during the first three minutes as shown in Figure 3, which could be evidence of internal shorting around the negative electrode tab, as the rate of rise for the different cells is faster for those made with less thermally stable materials (PE/ceramic, PP) and slower for those made with more thermally stable materials (DWI Silver, Gold). However, all of the cells failed within the same time period and reached approximately the same temperatures, so no strong conclusions can be reached. Discharge Capacity vs. C-Rate Figure 2 Discharge capacity for cells tested at different rates from C/12 to 3C. 2 :: ::43 :1:26 :2:1 :2:53 :3:36 Overcharge The overcharge test was performed by starting with cells at 3% state of charge (SOC) and then charging at 2C until failure. Each cell was again wired to measure temperature and voltage over time. The voltage at the onset of failure was measured by observing the voltage rise and determining the first dip in voltage. The uncoated separators all failed at the same voltage: 5.55 V, while the ceramic coated PE separator failed at 5.9 V, significantly higher than the other three. Hot Box Hot box tests were performed in an oven at 17 C, with the cells similarly wired to measure voltage and temperature over the course of the test. All cells were tested for 1 hour or until failure. The PP and PE/ceramic cells both vented when the thermocouple read 125 C, and shorted when the thermocouple read 145 C. As with the hard short tests, the initial temperature rise was faster with the less thermally stable materials than with the more thermally stable materials, as is shown in the left side of Figure 4. Notice there is some slight perturbation C-rate PP PE/Ceramic Silver Gold Time (min) Celgard Ceramic Silver Gold Figure 3 Temperature rise during the first three minutes of the hard short test, showing slightly higher rates for less thermally stable separators.

4 Temperature (C) Voltage (V) in the temperature rise for the PP and PE/ceramic separator cells occurring around a cell temperature of C. It is possible that this could be attributed to the shutdown function temporarily slowing the internal heating of the cells. However, the cells quickly defeat this mechanism and resume their internal heating. The voltage over the course of the test is shown on the right side of Figure 4, where the shorts exhibited by the PP and PE/ceramic cells can easily be seen. It is interesting that the short for PP appears to be clean and complete with a relatively broad onset, while the short with PE/ceramic is initially sharper, but then oscillates significantly, as if the ceramic coating is playing a role, but is ineffective in blocking the short. Neither of the nanofiber based separators exhibited any venting or shorting, and the voltage remained stable over the course of the test within +/- 7 mv :: :7:12 :14:24 :21:36 :28:48 Time (min) PP PE/ceramic Silver Gold :: :14:24 :28:48 :43:12 :57:36 Time (min) PP PE/ceramic Silver Gold Figure 4 (Left) Initial temperature rise during the 17 C hot box test, showing some internal heating from polyolefin separators (PP and PE/ceramic) compared to nanofiber based separators (Silver, Gold). (Right) Voltage drop during 17 C hot box test, showing stable voltage for the duration of the test for nanofiber based separators, and hard shorts occurring for polyolefin separators around 3 minutes into the test. Nail Penetration Nail penetration tests were performed by placing a polished and cleaned iron nail into a holder over the cell, and then driving the nail through the cell with a hammer. The holder ensured a straight and clean penetration, which was in general achieved with a single stroke of the hammer. The cells were wired for temperature measurement above and below the cell near the area of nail penetration, and also for recording voltage. The results of the test are striking. First, the voltage of both of the polyolefin separators dropped immediately to zero and persisted there for the duration of the test. This is shown below in Figure 5. The voltage for the thermally stable separators dropped approximately 1 mv, and then underwent a slow rise in voltage over the next 2 minutes. The thermal results also reflect the difference in the character of the shorts generated by the nail penetration, and this is also shown in Figure 5, using the scale on the right. The PE/ceramic cell underwent the fastest temperature rise, rising within ninety seconds to a peak of 115 C, and then slowly dropping. This cell smoked from the penetration hole but did not catch fire. The PP cell had the next highest temperature rise, taking approximately 9 minutes to reach a peak of 74 C. The Silver cell fared only a little better, reaching a peak of 63 C in about the same time period.

5 Voltage (V) Temperature (C) The Gold cell showed the slowest and least temperature rise, peaking at 41 C after 12 minutes and then slowly dropping Nail Penetration Voltage Nail Penetration Temperature :: :7:12 :14: :: :7:12 :14:24 PP Voltage Ceramic PE Voltage PP Temp Ceramic PE Temp DWI Silver Voltage DWI Gold Voltage DWI Silver Temp DWI Gold Temp Figure 5 Voltage and temperature for cells undergoing a nail penetration test. The polyolefin separators voltage dropped immediately to zero and had the fastest temperature rise to the highest temperatures, compared to the thermally stable nanofiber based cells, which showed a voltage drop of only 1 mv and a slow temperature rise to much lower temperatures. One remarkable feature of these cells is the voltage behavior, in which the PP and PE/ceramic cells voltage immediately dropped to zero, indicating a hard short between the current collectors from the anode and the cathode, while the voltage for the Silver and Gold separators each dropped by approximately 1 mv, indicating that the short was likely between electrodes and did not short the current collectors themselves. In this case, as the portion of the cell around the nail began to be depleted of energy, the voltage at the tabs would rise slowly, which is what was observed, with a total voltage rise of ~5 mv over the course of the test, as measured from the lowest voltage after the nail penetrated until the final voltage. The cells that underwent nail penetration were then taken to a dry box and disassembled with photographs taken of every layer. It is worth examining these on a separator-by-separator basis. Dry Process PP Separator Autopsy Figure 6 shows pictures of the disassembled cells using PP separator, and also close up images of the holes created by the nail penetration. Two features are immediately evident. The first is the shrinkage indicated by both the exposed electrodes in the top two pictures and also the ripples in the separator. This exposed electrode and rippling was present in nearly every layer of the disassembled cell. It is surprising to see such prevalent shrinkage in a cell that only exhibited a maximum surface temperature of 73 C, and indicates that the temperature inside the cell likely reached much higher temperatures. The second feature is the eye-shaped holes created by the nail, as shown most clearly in the bottom two pictures. In both of these pictures, the separator shows splitting in addition to a round penetration. This can be understood when a piece dry processed PP separator is handled it is splitty, or extremely easy to split and tear in the machine direction. Whether this occurred when the nail penetrated, or after when the material heated and began to shrink is unknown. However it is certain to have caused additional shorting around the hole, allowing electrodes to come into contact.

6 A third feature is present in the upper right picture, in which it can be seen that the separator near the right edge of the electrode is clearer than a few centimeters into the cell. This is likely the shutdown feature that is often discussed in these cells. In this feature, as they shrink, the pores close up and disallow ions from transferring from one electrode to another in the region where the separator has shut down. About halfway between the edge of the electrode and the nail penetration hole is a white line, where this separator, still damp with electrolyte, likely still has full porosity. However, it is clear there is a morphological change in the separator at this point in the cell, delimiting two regions with likely different electrical properties. In any case, any shut down that occurred did not prevent internal heating and shorting, which is evident both near the nail hole and at the right edge of the cell. Figure 6 Photos taken during the autopsy of the cells made with dry processed PP separator that had undergone the nail penetration test. The top two photos show electrode exposed when the separator shrank. The bottom two show a splitting of the separator around the hole where the nail penetrated. Wet Process Ceramic Coated PE Separator Autopsy The disassembled cells using the wet process ceramic coated PE separator after having undergone the nail penetration test are shown in Figure 7. In contrast to the PP separator, it appears there is no macroscopic shrinkage of the film. There is also little or no evidence of any shutdown feature, with the film retaining it s milky white appearance that would be normal as it is still saturated with electrolyte. Indeed, it is difficult to imagine that a film that has been stretched in two dimensions would somehow collapse in the other without shrinking in the first two, and there is no evidence of this occurring, despite the external temperature of the cell reaching ~115 C, near the melting point of PE at 135 C.

7 However, the size and shape of the holes has changed dramatically. Near where the nail penetrated, there is significant melting and shrinkage. The splitting appears to be random in orientation compared to the orientation seen in the dry processed PP separator in Figure 6. There is significant cracking and charring of the separator around the holes, and significant exposed electrode and also exposed metal evident in both of the lower photos. These features, as well as the size of the hole in comparison to the PP separator, correlate well with the very fast thermal rise to a very high temperature compared to the cells made with other separators. Figure 7 Photos taken during the autopsy of the cells made with wet processed ceramic coated PE separator that had undergone the nail penetration test. The top two photos show little macroscopic shrinkage in the separator. The bottom two show significant shrinkage, splitting and melting of the separator around the hole where the nail penetrated. Dreamweaver Nanofiber Silver Separator Autopsy The autopsy photos from the cell made with Dreamweaver Silver nanofiber separator are shown in Figure 8. Very different from the other two separators, the top two pictures show that the separator has maintained its shape and morphology throughout the test, even continuing to show excellent wettability with electrolyte. There is in the pictures, very little evidence that a thermal event of any kind has occurred except just near where the nail penetrated. The nail penetration holes, shown in the bottom two pictures, both show some evidence of tearing of the separator as the nail drove through the layers. It appears that the separator did not pull back, though, as there is little evidence in either picture of exposed electrode or metal current collector. In the bottom right photo, the separator has charred to some extent, turning a dark brown in the area just on top of the nail penetration hole, evidencing significant heating of the nail as the cell was discharged. In other tests, charring of this separator occurs over 25 C,

8 and occurs first in the polyacrylonitrile nanofibers. In fact, other than the slight browning of the fibers around the nail penetration hole, the only evidence of damage to the separator is physical tearing where the nail penetrated, as compared to the significant melting, charring, shrinking and splitting present in the PP separator and the PE/ceramic separators. The photos showing no exposed metal and no exposed electrodes correlates well with the slow rise in temperature and also the ability of the cell to maintain its voltage (measured at the tabs) even after the nail had penetrated the cell. Figure 8 Photos taken during the autopsy of the cells made with Dreamweaver Silver separator that had undergone the nail penetration test. The top two photos show no macroscopic shrinkage in the separator. The bottom two show some tearing and a little charring of the separator around the hole where the nail penetrated. However, there was little evidence in any of the photos of exposed electrode, and no exposed metal. Dreamweaver Nanofiber Gold Separator Autopsy The photos from the autopsy of the cells made with Dreamweaver Gold para-aramid reinforced separator are shown in Figure 9. These photos show the least damage due to the test, and indeed exhibit no evidence of a thermal event, although some small level of charring has been observed around the nail penetration hole in a few photos when the cell was first heated to 5 C prior to performing the test. The stark truth is that the cells made with the Gold separator exhibited only physical damage on autopsy, with no evidence of a thermal event of any kind. This is completely different from the behavior of the cells made with polyolefin separators, which showed thermal damage in many different facets, including melting, shrinkage, charring and splitting. Again, for the Gold separator holes, there was no evidence in any of the pictures of exposed metal or

9 Capacity (mah) electrode, which again correlates well with the slow rise in temperature and also the ability of the cell to maintain its voltage (measured at the tabs) even after the nail had penetrated the cell. Figure 9 Photos taken during the autopsy of the cells made with Dreamweaver Gold separator that had undergone the nail penetration test. The top two photos show no macroscopic shrinkage in the separator. The bottom two show some tearing of the separator around the hole where the nail penetrated. However, there was no evidence in any of the photos of exposed electrode, and no exposed metal. Residual Charge and Post-test Capacity Prior to disassembly but after the removal of the nails, the cells were CC discharged at C/6 to 2.5 V, then CCCV charged at C/6 to 3.6 V, and then CC discharged at C/6 to 2.5 V. The results are shown in Figure 1. The cells made with the PE/Ceramic separator continued to be shorted, and showed no residual charge and no ability to hold a charge. This correlates well with the large holes present in the autopsy, and the significant exposed surface area of both electrode and metallic current collectors. The PP cell, on the other hand, showed a residual charge of 27 mah, indicating a 9% discharge during the test. When charged, the cell was able to charge to 137 mah, or about 5% total Nail Penetration Residual Charge & Capacity PP PE/ceramic Silver Gold Residual Charge End Capacity Figure 1 The residual charge and after-test capacity of the cells after the nail penetration test. The Silver and Gold cells retained about 7% of the charge after the test, and were able to give a discharge of 9% and 8% of their initial capacity.

10 capacity. The Silver and Gold cells, however, held 1913 and 243 mah respectively, with about 3% discharge during the test. They were able to charge up to 245 and 218 mah, about 9% and 8% of their initial capacity, respectively. This is further evidence of the cells undergoing a much lighter thermal event during the test, and exhibiting much less damage due to the test than the cells made with PP separator and PE/ceramic separators. Evidence of Shutdown Function There has been much industry discussion of the shutdown function of polyolefin cells, and how it improves the safety of lithium ion cells. In these tests, there were only two indications of a potential shutdown, and the effects were limited. The two indications were: Hot Box: a slight deflection of the thermal trajectory in the hot box test, as shown in Figure 4, at around C. This deflection did not endure and both cells failed in the test, while those cells made with the thermally stable separators (Silver and Gold) did not undergo a test even after one hour at 17 C. Nail Penetration Autopsy for PP: In the top right photo in Figure 6 there appears to be a change from opaque to transparent about halfway between the edge of the cell and the nail penetration site. This would correspond to a shutdown occurring in the region of high shrinkage, and porosity maintained in the region where the shrinkage is less evident. Note that the separator has shrunk along the edge of the shutdown area and is allowing direct electrode to electrode shorting to occur. This effect limits the usefulness of shutdown in PP separators. These two indications of a shutdown feature did not affect the outcome of the tests, and in fact the cells made with polyolefin separators outperformed the cells made with thermally stable separators in only one test, that of the overcharge test, and only for the ceramic coated separator. Likely the ceramic coating helped to prevent the formation of, or limit the size of dendrites formed in the final stages of overcharging. In all of the other tests, the cells made with polyolefin separators performed worse than those made with the thermally stable separators. Summary and Conclusions Abuse tests have been performed on 2.7 Ah lithium iron phosphate pouch cells made with four different separators: a standard polypropylene separator, a ceramic coated polyethylene separator, Dreamweaver s nanofiber based Silver separator, and Dreamweaver s nanofiber based Gold separator that is reinforced with para-aramid. The cells were all tested for hard short, overcharge, 17 C hot box and nail penetration. A summary of the results is as follows: Hard Short: The cells all performed the same, with the anode tab (4 mm tab) getting red hot, and the cells slowly heating and the electrolyte expanding until the seal broke. Overcharge: All cells failed at 5.5 V by outgassing. There were no flames, and the temperature rise was uniform between cells made with different separators. Hot Box: Cells made with PP and PE/ceramic separators both had faster temperature rise than either the DWI Silver or Gold cells. They also both experienced a catastrophic short after around 2 minutes, compared to the DWI Silver and Gold cells, which maintained voltage for 1 hour and never experienced a short. The short for the PE/ceramic cell occurred with onset at around 14 C, and for the PP cell at around 145 C. Both of the DWI cells rose to nearly the oven temperature without experiencing a short, and did not undergo thermal runaway. Nail Penetration: The nail penetration cells experienced likewise a significantly improved behavior in the Dreamweaver cells. Cells made with both olefin separators dropped immediately to zero voltage, which persisted throughout the test. The temperature rise was also faster and to higher temperatures (up to 115 C) than the cells made with Gold (4 C) and Silver (6 C). The Dreamweaver cells, on the contrary, experienced only a small voltage drop of approximately 1 mv, with a slow rise thereafter as the cell discharged. The DWI cells continued to function and retained 2/3 of their charge and capacity after completion of the test.

11 These results show a dramatic improvement in the safety performance of these cells when using separators that are more thermally stable, as compared with separators that have a shutdown function. Follow on experiments will need to be performed on cells with a larger capacity as well as cells with different cathode materials, such as NMC and NCA which generally have poorer safety than LFP.