Influences of Instant Voltage Drop Occurring during Insulation Test on

Feb 28, 2018 - Department of Materials Engineering, Tatung University, No. ... Center of Green Energy, Ming Chi University of Technology, 84 Gungjuan ...
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Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Influences of Instant Voltage Drop Occurring during Insulation Test on Performance of Lithium Ion Batteries Po-Han Lee,† Shun-Shiang Hsu,† Po-Han Huang,† Han-Jen Tseng,‡ Shuo-Chieh Chang,‡ and She-huang Wu*,†,§ †

Department of Materials Engineering, Tatung University, No. 40, Sec. 3, Zhongshan N. Rd., Taipei City 104, Taiwan Product Marketing Department, Test & Measurement BU, Chroma ATE, Inc., 66 Huaya First Road, Guishan, Taoyuan 333, Taiwan § Battery Research Center of Green Energy, Ming Chi University of Technology, 84 Gungjuan Rd., Taishan District, New Taipei City 24301, Taiwan ‡

ABSTRACT: Effects of partial discharge during an insulation test were investigated with jelly rolls of a single electrode pair and 500 mAh stackingtype cells by inspecting the separator and studying the capacity fade of the cells prepared with the tested 500 mAh jelly rolls. For comparison, 500 mAh cells with jelly rolls without insulation test or with separator having laser-engraved holes are also prepared. The cells prepared with jelly rolls with insulation test show higher capacity fading rate than cells comprised with jelly rolls without insulation test and lower fading rate than those assembled with jelly rolls with laser-engraved separator. Dark spots were observed on the negative electrodes at the corresponding position facing the defect spots on the separator caused by partial discharge or the laserengraved holes in a glovebox and became white after being moved into air accompanied with a temperature rise. Li2O was also detected with XPS in addition to Li2CO3, and higher Li content was found by LIBS at the spots. These results recommend that there is residual Li at the spots when cells were discharged due to local overlithiation. The accumulated residual Li at the spots may cause imbalanced lithium inventory and enhance capacity fade of the cells. KEYWORDS: Partial discharge, Insulation test, Separator defects, Li deposition, Internal short circuit

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INTRODUCTION Internal short circuit (ISC) can not only affect coulombic efficiency and the life of lithium ion batteries, but can be also a key factor of field failures and safety hazards. It can occur between negative and positive electrodes, Al current collector and negative electrode, Cu current collector and positive electrode, or Al and Cu current collectors.1,2 ISCs can be developed from various sources, such as manufacturing defects, dissolution/ deposition of positive electrode materials and current collectors, lithium plating, and dendrite formation. Though the probability of these failure incidents is estimated to be very low (1 in 5−10 million), the consequences of a failure caused by ISC in a Li-ion battery system can be catastrophic because they are difficult to detect and predict.1 Most of the reports are focused on the electrochemical-thermal models of the forced ISCs created by mechanical deformation of the cell, such as small nail penetration,3−5 indentation,4,6 surface indentation, or pinch test.4,7 It had been suggested that electrodeposited metallic lithium is a major cause of failure in lithium batteries,8,9 which may be caused by overcharging or charging cells at high rates especially at low temperature since the working potential of graphite is very close to that of metallic Li deposition.10,11 In order to screen out the potential ISC risks due to manufacturing defects, such as impurities on electrodes or separator, pin holes, © XXXX American Chemical Society

and thickness nonuniformity of separator, an insulation test is commonly carried out for the prepared dry jelly rolls by applying a high voltage for several hundred milliseconds. Jelly rolls can retain insulation after the test and are acceptable for follow-up processes, while those cannot retain insulation are considered as unqualified and have to be reassembled. In our preliminary experiments, instant voltage drop or so-called partial discharge (PD) might be detected from the V-t curves collected with a digital oscilloscope for jelly rolls that were charged to a high voltage for 200 ms and retained insulation after the test. According to the current screening criteria, these jelly rolls will be considered as passing the insulation test and can be moved to subsequent processes. In this study, the effects of the partial discharge on the separators and the cycling performance of the cells were investigated with single electrode-pair cells and 500 mAh NMC//MCMB pouch-type cells, respectively.

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EXPERIMENTAL SECTION

LiNi0.5Mn0.3Co0.2O2 (NMC532, Umicore Korea Ltd., Korea) and mesocarbon microbeads (MCMB, China Steel Chemical Co., Taiwan) were selected as cathode and anode materials in this study by mixing Received: November 20, 2017 Revised: January 26, 2018 Published: February 28, 2018 A

DOI: 10.1021/acssuschemeng.7b04351 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering with Super P (Timcal Co. Ltd., Switzerland), vapor grown carbon fiber (VGCF, Yonyu Co. Ltd., Taiwan), and Kynar PVDF (Arkema Inc., USA) in adequate amounts of NMP (Mitsubishi Chemical Co., Japan) at weight ratios of 92:3:1:4 for cathode and 92:2:0:6 for anode, respectively. The slurry of NMC was coated on aluminum foil, whereas the slurry of MCMB was coated on copper foil with a continuous coater, followed by drying under 120 °C for 24 h. According to the specific capacities of MCMB and NMC determined with coin-type cells and the negative/positive (N/P) loading ratio of 1.1:1 in capacity, the loadings of MCMB and NMC per cm2 were calculated and used for preparing negative and positive electrode tapes. Single-sided coated tapes were used for preparing single electrode-pair cells, while double-side coating was applied for electrodes of 500 mAh cells. After compression with high pressure for densification, the coated tapes were punched into sizes of 3.2 cm × 5.7 cm and 3.1 cm × 5.6 cm with terminal film for negative and positive electrodes, respectively. Single electrode-pair cells were assembled with a single-sided coated negative electrode, a single-sided coated positive electrode, and a piece of separator (microporous polyethylene separators, Asahi Kasei, Japan). After being sealed in Allaminated pouches, these cells were tested with a insulation tester (Model 11210-K, Chroma ATE Inc., Taiwan) by charging them with a testing voltage increasing from 200 V with an increment of 50 V until partial discharge is detected with a digital oscilloscope (TDS-2022B or TDS1002B, Tektronix Inc., USA) within testing duration of 200 ms. Then, the separators obtained from the tested cells were observed with an optical microscope (OM, Nikon OPTIPHOT-100, Japan) and a scanning electron microscopy (SEM, SU8020, Hitachi Ltd., Japan). For 500 mAh cells, 13 pieces of double-sided coated negative electrodes and 12 pieces of double-sided coated positive electrodes were sandwiched together with a folded separator on a homemade stacking machine, followed by welding the positive and negative terminals separately with nickel tabs and sealing with a preformed housing of Allaminated film to become a dry cell. Five of the prepared 500 mAh dry cells were used for insulation test by charging them up to a voltage higher than 500 V for 200 ms and increasing by 50 V each time until partial discharge was detected with a digital oscilloscope, but they all retained insulation after the test. After the insulation test, two of them were disassembled for separator examination with an optical microscopy, while another three dry cells were opened for filling with an electrolyte of 1 M LiPF6 in EC/DEC = 1:1 (vol.) and sealed with hot pressing after a 24 h rest in an Ar-filled glovebox to ensure complete wetting of the cells. The prepared pouch-type cells were connected with a battery tester (Model 17011, Chroma ATE Inc., Taiwan) for formation with 50 mA (C/10 rate) between 3.0 and 4.2 V at room temperature for 2 cycles, and then, the cells were cycled at a 0.5 C rate by charging with constant current (CC) mode followed by constant (CV) mode with a current limit of C/25 and discharging at CC mode between 3.0 and 4.2 V at 10 °C for 200 cycles. Three reference cells containing a 500 mAh jelly roll without being insulation tested and six cells consisting of a 500 mAh jelly roll prepared with a piece of defective separator with three laser-engraved 30 or 50 μm holes at the apex of a triangle, as shown in Figure 1, located at the 13th anode/cathode pair were also prepared for comparison. After cycling, the aged cells were disassembled at their discharged state in a glovebox. The collected negative electrodes were inspected carefully before and after removal from glovebox, and then, they were studied with a scanning electron microscopy, an X-ray photoelectron spectrometer (VG Scientific ESCALAB 250), and a laserinduced breakdown spectroscopy (J200 LIBS system, Applied Spectra, USA).

Figure 1. (a) Typical photo of a piece of separator with three laserengraved holes with diameters of 50 μm taken with back lighting. (b, c) OM photos of the upper and lower left holes, respectively. (d) SEM photo with inset of OM photo of lower right hole.

are shown in Figure 2(c) and (d), respectively. Carbonization seems to occur at the black spots caused by partial discharge. Black spots can be found with OM on the separators of the cells displayed partial discharge. However, no damage can be observed on the separators of the dry cells did not show any partial discharge after being charged to 800 V. These results may suggest that a partial discharge may induce at least one black spot on separator. Figure 3(a) and (b) show two partial discharges and their enlargement recorded with a digital oscilloscope during an insulation test on a 500 mAh cell at 550 V. Figure 3(c) illustrates the distribution of the black spots on the separator between various pairs of negative/positive electrodes, shown with different symbols, observed with an optical microscope. The diameters of the black spots range between 7 and 18 μm. It is also found that most of the black spots are located near the edge of the electrode probably due to the potentially lower resistance for partial discharge at the bare cutting surface of the Al current collector than any other part because the positive electrodes are slightly smaller in size than the negative electrodes. Figure 3(d) and (e) are typical photos of OM of the two black spots on the separator between the first negative/positive electrode pair. Figure 4 exhibits the average of the capacity retention studies at 0.5 C rate within cutoff voltages of 3.0 and 4.2 V at 10 °C after formation for three 500 mAh NMC//MCMB cells with partial discharge being detected during the insulation test. For comparison, the average of cycling studies of the three reference cells without an insulation test and those of six cells prepared with a jelly roll containing a piece of separator with three laserengraved holes of diameters of 30 or 50 μm are also shown in the figure. It can be found that these cells show good reproducibility up to 200 cycles except the cells prepared with jelly roll assembled with a piece of separator with three laser-engraved 50 μm holes that manifested significant discrepancy from each other after being cycled more than 150 cycles. The results recommend that cells with larger holes show a higher capacity fading rate and worse reproducibility. The insulation-tested cells manifest a capacity fading rate higher than the reference cells and lower than those cells with a piece of separator with three laser-engraved 30 μm holes. It can be attributed to the defect holes, caused by

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RESULTS AND DISCUSSION From the results of the insulation test for the dry single electrodepair cells, five cells displayed two to eight partial discharges and retained insulation after being charged to 700 V and kept at the voltage for 200 ms, but four cells did not show any partial discharge after being charged to 800 V for 200 ms. Figure 2 shows a typical example of a cell with two partial discharges detected. Three black spots were observed on its separator as shown in Figure 2(b); photos of spot No. 2 observed with OM and SEM B

DOI: 10.1021/acssuschemeng.7b04351 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 2. (a) Typical partial discharges recorded with a digital oscilloscope during insulation test of a single electrode-pair cell at 700 V. (b) Illustration of the black spots on the separator. (c) OM and (d) SEM photos of black spot No. 2.

Figure 3. (a) Screenshot of the partial discharge pattern with (b) the enlarged image recorded with a digital oscilloscope during the insulation test on a 500 mAh cell at 550 V. (c) Illustration of the black spots distribution on separators. (d, e) OM photos of the spots on separator between the first negative/positive electrode pair.

partial discharge showing diameters ranging between 7 and 18 μm, that are smaller than 30 μm.

After 200 cycles, the cells that have been insulation tested were disassembled at their discharged state in an Ar-filled glovebox for C

DOI: 10.1021/acssuschemeng.7b04351 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Dark spots on the negative electrode at the corresponding position facing the laser-engraved holes of the defective separator can be observed. In addition to the color changes of the dark spots into white, exothermic temperature rises were also detected when the negative electrodes were removed from the glovebox. A typical case of a quick temperature rise from a temperature near room temperature (23.6 °C) to 27.5 °C had been recorded by a thermal couple fixed on the back of the electrode near the spot when an anode was removed from an argon-filled glovebox. Figure 6(a) shows a typical photo of the negative electrode next

Figure 4. Average results of the capacity retention studies at 10 °C at a 0.5 C rate after formation of the 500 mAh pouch-type cells composed of jelly rolls with or without an insulation test and jelly rolls prepared with a piece of separator having three laser-engraved 30 or 50 μm holes.

inspection carefully. Dark spots were observed on the negative electrodes at the corresponding positions facing the black spots on separators caused by partial discharge. Figure 5(a) shows a

Figure 6. (a) Typical photo of the negative electrode next to the defective separator collected from a cell prepared with a piece of separator with three laser-engraved 50 μm holes after being cycled 200 cycles. (b−d) SEM photos of white spots.

to the separator with the laser-engraved holes (Figure 2(a)). The color change and heat evolution can be attributed to the reaction between residual lithium on the anode and oxygen. It suggests that lithium plating might occur on the anode next to the defects of the separator after charging/discharging cycling because of the relative higher local charging current densities at these sites than other sites next to the nondefective separator and accumulation of residual dead lithium. Cells prepared with a jelly roll containing a piece of separator with three laser-engraved 50 μm holes show higher capacity deviation after 150 cycles than those prepared with jelly rolls assembled with a piece of separator with three laser-engraved 30 μm holes and those have been insulation tested. It can be due to the cells with 50 μm laserengraved holes have larger defect area on the separator, higher deviation of defect areas since the laser-engraved holes are not perfect circles, and potentially higher variation in morphology of the accumulated dead lithium next to the holes than the other two kinds of cells. While the cells prepared with insulation-tested jelly rolls manifest higher capacity deviation than cells prepared with jelly rolls containing a piece of separator with three laserengraved 30 μm holes and can be attributed to the high variations in number, size, and degree of defect spots caused by partial discharge. However, the defect area of the separator caused by partial discharge is much lower than those of the three laserengraved 30 or 50 μm holes, such that cells prepared with jelly rolls with an insulation test show lower capacity fading rates than those prepared with jelly rolls containing a piece of separator with three laser-engraved holes. From the XPS study for the site near a white spot and other parts far from it for the negative electrode next to the separator

Figure 5. Typical photos of a negative electrode with (a) dark spots taken in an argon-filled glovebox and (b) after it was removed into atmosphere. (c, d) SEM photos of a white spot under different magnifications.

typical example of the dark spots on a piece of negative electrode at the corresponding positions facing the black spots on the neighboring separator caused by partial discharge during an insulation test. When the negative electrodes were taken from the glovebox to atmosphere, the black spots turn white gradually, as shown in Figure 5(b). One of the spots was observed with SEM with different magnifications as shown in Figure 5(c) and (d). Bursts can be found at the white spot and can be differentiated from other parts of the negative electrode easily. It may be attributed to the oxidation of the residual lithium at the dark spot due to lithium plating or overlithiation caused by the higher local current density during cycling at the carbonized spots than other parts of the separator. For comparison, the cells prepared with a piece of separator with three laser-engraved 50 μm holes were also disassembled in an Ar-filled glovebox at their discharged state after 200 cycles. D

DOI: 10.1021/acssuschemeng.7b04351 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Figure 7. (a) Illustration of the sites for XPS study. (b) XPS spectra of Li 1s at the site near the white spot and other parts of the negative electrode next to the separator with three laser-engraved 50 μm holes from the cell cycled 200 cycles.

Figure 8. Li LIBS mapping of a cycled anode next to the defective separator: (a) sites far from white spot and (b) site of white spot. (c, d) Surface layer of the depth profile of 3D mapping of (a) and (b), respectively.

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with three laser-engraved 50 μm holes, as shown in Figure 7, it can be found that Li2O is also detected in addition to Li2CO3 at the sites around the white spot, such as points 3, 4, and 5 in Figure 7(a), while only Li2CO3 which exists in the SEI layer can be found at the sites far from the white spots. The results of LIBS, shown in Figure 8, also show a higher Li concentration at the site of the white spot than sites far from it. These results show that there is residual lithium, that will become Li2O after moving out from the Ar-filled glovebox, on the negative electrode at the corresponding position of the laser-engraved holes of the defective separator after cycling that be attributed to the accumulation of dead lithium due to higher local charging/ discharging current densities at the holes than other sites. Residual lithium on the anode may induce imbalance of the lithium inventory upon cycling and enhance capacity fade.

CONCLUSION Partial discharge occurring during an insulation test will cause black spots or local carbonization on a separator with a diameter range between 7 and 18 μm near the edge of the cathode. The cells prepared with dry jelly rolls showing partial discharge during the insulation test exhibit a higher capacity fading rate than those without being insulation tested, but lower than those assembled with jelly rolls containing a piece of separator with three laserengraved 30 or 50 μm holes. From the observation of the color change of the dark spots on the anodes next to the defective separator collected from the cells prepared with jelly rolls with partial discharge during the insulation test and with a piece of laser-engraved separator accompanied with the results of SEM, XPS, and LIBS studies, it can be suggested that the defects caused by partial discharge during the insulation test can induce E

DOI: 10.1021/acssuschemeng.7b04351 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering accumulation of dead lithium at the corresponding sites of the anodes next to the damaged separator due to relative higher current density at these sites. The accumulation of dead lithium will lead to imbalance of the lithium inventory and increase capacity fading rate. Furthermore, the accumulated lithium may result in internal shortage of the cell after long-term cycling.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 886-919045888 (S.H. Wu). ORCID

Po-Han Lee: 0000-0002-5882-1376 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful for financial support from Chroma ATE Inc., instrumental support from the Instrumentation Center, National Taiwan University for ESCA analyses, and the LIBS study from Applied Spectra, USA and Rightek, Taiwan.

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REFERENCES

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DOI: 10.1021/acssuschemeng.7b04351 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX