Internal Shorts Background

Transcription

1 Internal Shorts Background Judy Jeevarajan, Ph.D. Underwriters Laboratories Inc. USA Battery Safety Council Forum 3 January 2017 and the logo are trademarks of LLC 2015

2 Charge / Discharge Characteristics of Li-ion Discharge Protocol: Varies from Constant current or power to pulse Discharge Charge Charge Protocol: Constant Current/Constant Voltage Typical Voltage Range 2.7 V to 4.2 V 1

3 Lithium-ion Batteries: Hazards Overcharge Extreme Thermal Environments High/Low temperatures/ Repeated Overdischarge/ Overdischarge followed by charge Internal Shorts External Shorts High and Low Impedance Manufacturing Defects Misuse in the Field 2

4 Challenges Faced at all Stages of Manufacturing and Usage with Li-ion Chemistry Material Powders Recycling Aging Cell Manufacturing: slurry prep, coating, calendaring, slitting, cell assembly, electrolyte filling, cell sealing, formation, final inspection System: Load and Charge (Maintain usage within manufacturer s spec for current, voltage and thermal environment Battery Manufacturing: Cell screening and matching, BMS design and test; lowest thermal gradient; usage within manufacturers spec for current, voltage and thermal environment Transportation 3

5 Nature of Defects Observed in Li-ion Cells Pristine electrode Salt Deposits? Tear Zero porosity Delamination Separator Al particles Iron particle Iron Anode Current collector 4

6 Overcharge/Overvoltage hazard Overcharge / OverVoltage Electrolyte Decomposition (formation of gases such as H 2, CO and CO 2 Lithium metal dendrites (can pierce the separator causing short circuit) Cathode Destabilization (release of Oxygen) High Temperatures Venting Fire Thermal Runaway 5

7 Challenges with Cell Safety Features Overcharged cell (fresh cell) with delayed CID activation Anode showing charring Burned material Cell with normal CID activation Cell header (underside) showing charring Anode of Fresh cell Anode surface of fresh (not cycled) overcharged and externally shorted cell 6

8 Overdischarge Hazard Discharging a cell below the manufacturer specified voltage is called overdischarge. Dissolution of copper current collector occurs during overdischarge Copper deposits on the cathode, anode and current collector forming short circuits; delamination of anode is significant due to loss of copper Cell is not usable any more, but it can give rise to other hazards depending on battery design (especially if cell level voltage monitoring and control are not provided) It can lead to lithium dendrite formation during subsequent charge. Decomposition of electrolyte producing gases occurs during overdischarge conditions. Undervoltage / High Rate Discharge Dissolution of Copper Current Collector Or Benign Internal Shorts 7

9 External Short Circuit Hazard Electrical shock to the cell or battery from external sources. Short circuits are of high and low impedance; common one is low impedance; high impedance shorts can cause significant hazards too. Very high temperatures, venting and fire are observed if not protected against this hazard cause Hard Carbon Cell with PTC (Single Cell Test) Anode of Fresh cell Anode surface of fresh (not cycled) externally shorted cell PTC Activation - occurs reliably Inadvertent activation of PTC can change internal resistance of cell Cell exhibited PTC activation 8

10 High and Low Temperature Hazards High Temperatures: Electrolyte decomposition and gas production Cathode and anode destabilization Can lead to venting and fire. Low Temperatures: Electrolyte viscosity increases Increases resistance for the flow of ions Can result in lithium metal dendrite formation 9

11 Major Challenge with Large Battery Designs Example: Thermal Gradient Deviations: Voltage, capacity, internal resistance/impedance Eventually safety is? BMS 1 BMS 2 BMS 3 Module 1 Module 6 Module 2 Module 5 Module 3 Module 4 Forced Air Cooling Inlets at one end of Battery 10

12 Observations from B787 Data from NTSB final report (NTSB/AIR-14/01; PB ) in the public docket ( 01.pdf) or a public conference presentation. 11

13 Cell DPA after APU Start Tests Melted separator at edge of center winding next to collector finger Intact separator material 12

14 Cell Dimensions and Strain on Rivets under Load and at Different temperatures Photo from NTSB report on Boeing 787 investigation 13

15 Rivet tests Cell can Areas where current collector from electrodes is attached 14

16 Battery Manufacturing Process Challenges Cell Screening and Cell Matching Stringent pass/fail for cell and battery tests Cell-level monitoring and control (voltage, temperature; battery level current) Battery Testing before leaving manufacturing facility Testing in relevant battery configuration Testing Under Relevant environmental conditions 15

17 Questions Is diligence used to look into manufacturing defects as well as to follow stringent design and test requirements? Is one factor being blown out of proportion compared to the other (defects versus design induced internal shorts)? Are batteries being designed to prevent formation of shorts in the field? Do we understand what type of defects can become a catastrophic hazard? Do we need to worry about all types of defects? Do we need to pay attention to cell and battery screening? Data exists in the literature that shows that defects and low quality cells and batteries show rapid performance degradation; but do these eventually lead to catastrophic failures? Forum will go through cell screening for internal shorts before assembly into a battery, screening for the formation of shorts in the field and latent field failures and methods to simulate internal shorts (provide a cell design s tolerance to an internal short) 16

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19 Internal Short Circuit Hazard Short circuit that occurs inside a cell is called internal short circuit Usually occurs due to defects inside the cell causing breakage of separator and consequently short circuit Or it can be caused if the cell is used outside the manufacturer s specification, wherein short circuits are created internal to the cell. High temperatures, venting and fire are observed. Internal Short Manufacturing Defect Field Failures Manufacturing Defect Manufacturing facility stringent screening; Subtle defects identified and screened out with stringent cell and battery tests. Field Failures Avoided by use within manufacturer s specification (I, V, T); stringent cell and battery selection and screening criteria; stringent monitoring and control (I, V, T); cell balancing, health checks (with issue- recognizable tests); good thermal design 18

20 Nature of Defects Commonly Observed with Li-ion Cells Salt Deposits? Tear Delamination Separator Pristine electrode Iron particle Zero porosity 19

21 Effect of Defects on Performance D. Mohanty et al Study (J Power Sources 312 (2016) 70-79) Agglomerates, pinholes, metal particle contaminants ( 2 mm); nonuniform coating (6mm; 2mm wide) Methods used: Electrochemical testing, IR thermography, microstructural characterization 20

22 Effect of Defects on Localized Plating Cannarella and Arnold (J Electrochem. Soc., 162 (7) A1365-A1373 (2015) Separator intentionally crushed to achieve pore closure. Conclusion: Local defects can create hot spots of local high electrochemical activity that can lead to lithium plating. Changes such as delamination of negative electrode expected to exhibit more pronounced localized phenomena. Effect of Defect on Electrochemical Characterization NASA Flight Program Test: AC Impedance on as received cells: 38.7 mohms compared to a range of 26 to 27 mohms for all other cells in the same batch. Cycling the cell, dropped the value within the range of other cells. 21

23 Self Discharge Study Test Results Example 1 Jeevarajan, et al., Battery Safety 2013 Found on every + electrode in every cell Al 6 mv Cells charged to 3.15 V before storage period Si, Al, Fe, Ca, Mg, O Cells charged to 3.15 V before storage period Al,Fe Found on some electrodes Cell with declining voltage underwent 75 cycles no drastic capacity change or catastrophic event 22

24 Self Discharge Study Test Results Example 2 Jeevarajan, et al., Battery Safety 2013 Clean Electrodes and Separator Cells charged to 3.15 V before storage period Cell with declining voltage underwent 75 cycles no drastic capacity change or catastrophic event 23

25 Recommendations Good understanding of cell and battery manufacturing process and quality control Stringent characterization and study with destructive analysis of samples from every lot Cell and battery screening with stringent pass/fail criteria 24

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27 Aerospace Corporation TOR (SMC-S-017) 1.4 Cell Manufacture and Test Cell manufacturing and testing shall be as defined below and data made available for review by the Procurement Authority upon request All flight cells shall be inspected at the following points with Procurement Authority participation unless otherwise agreed to by Contract: a) Electrode fabrication (coating, pressing, slitting) b) Stacking or winding process c) Tab to terminal weld d) Case Insertion e) Final cell Additional inspection points can be established as appropriate All cells shall be inspected at 1X to 10X magnification and meet the criteria below: a) The cell assembly (electrodes, separator, insulator and tabs) contains uniform tension, and compression to maintain the cell dimensions within drawing tolerance and to prevent interference that can induce a burr or damage to cell core or stack. b) Electrode stack contains no wrinkling, tearing and/or deformation. c) The positive electrode is covered by the active material of the negative electrode and is within acceptable tolerance and design criteria. d) The separator extends beyond the positive and negative electrode with margin and within design criteria. e) Tabs or connections within the electrical path are not loose or broken (including partially broken). f) Tab positioning are within acceptable limits to prevent electrode damage or short circuits. g) Tab lengths and strain relief are within design criteria. h) Corrosion is not present on cell materials. i) Foreign object debris (FOD) is not present inside cell. j) Native object debris (NOD) is not present inside cell. k) Weld/crimp alignment are within design criteria. l) Cell case cleanliness, scratches and dents are within design criteria. m) Cell safety devices, such as current interrupt, temperature control devices and/or vents, are correctly installed and meets design criteria. 26

28 Aerospace Corporation TOR (SMC-S-017) Inspection and in-process controls of anode/cathode manufacturing shall be implemented to verify that coating composition, thickness, weights and dimensions meet manufacturing specification requirements Records of process measurements and variability shall be maintained throughout the lot build and be made available to the Procurement Authority upon request Process controls shall be implemented to maintain lot-to-lot variation and rejections rates for critical process (such as coating, die-cut, slitting, pressing, electrode winding/assembly, electrolyte, electrolyte fill, post-fill voltage stability, each weld/crimp step, leak test, and formation) within family and acceptable manufacturing limits for the design Cell pressure testing shall be conducted on each flight lot per SMC-S-00016/TR-RS [Ref.1] and the following to demonstrate margin in the cell design so that structural failure does not occur before the design burst pressure is reached, and/or excessive deformation does not occur at the maximum expected operating pressure The cell case of each cell (100% of cases) in the battery flight unit if either a sealed container (<100 psi MEOP) or a pressurized vessel (>100 psi MEOP), shall go through one proof pressure test at 1.25 MEOP minimum for 5 minutes in accordance with AIAA S (metallic vessels) [Ref. 4], or in AIAA S [Ref. 5] for composite overwrapped vessels Cell dimension, case material and welds shall be inspected pre and post-test to verify no structural failure, leakage or excessive deformation does not occur All flight cells shall be electrically tested for cell capacity, energy, voltage profile, charge retention, and impedance at the conditions defined in paragraph , or and and meet specified requirements An AC impedance test at 1000 hertz shall be performed on all cells Test voltage limits, temperatures, test durations, state-of-charge and compressive force on cells shall match conditions for subsequent battery level tests as defined in paragraph All cell level acceptance test data shall be recorded in a time stamped format and provided to the Procurement Authority The flight lot of cells shall be screened to remove ±3-sigma outliers for the following minimum performance parameters: charge retention (open circuit charge decay), mass, impedance, and discharge capacity After removing outlier cells, the resultant ±3 sigma range for the following parameters as a percentage of the mean (±3-sigma range/mean) shall not exceed the following: a) Charge retention OCV (at end of test) (<1%) b) Mass (<2%) c) Capacity (<5%) d) Impedance (<15%) If the total number of cells that fail the acceptance criteria is greater than 15 percent of the lot, then the lot shall be rejected. (>3% for NASA standards for lot rejection) All cells shall be subjected to x-ray inspection prior to cell selection with established pass/fail criteria that include the following: 27

29 Aerospace Corporation TOR (SMC-S-017) Battery Capacity (or Energy) The battery capacity or energy shall be measured at nominal charge conditions to the maximum charge voltage, with cells in a balanced state, followed by a C/2 discharge rate to the minimum voltage level, for each of the following three temperature conditions. a) Maximum acceptance temperature b) Nominal acceptance temperature c) Minimum acceptance temperature The battery shall achieve capacity/energy stability within 1% for two consecutive cycles, upto 3 cycles maximum, for each condition in If the battery capacity test conditions are different than the cell level, an additional capacity cycle shall be performed using the cell level conditions for comparison Battery Charge Retention (Designed with active cell-level charge balance circuit that is electrically removed during battery ground storage) Battery charge retention shall be measured at 20 C in an open-circuit state starting at 100% state-of charge for at least 7 days to screen out potential internal shorts while electrode stack is under highest mechanical force and at a region with highest voltage sensitivity Any charge control electronics or voltage monitoring devices shall be fully disconnected during the open-circuit period to minimize stray currents Open circuit stand shall commence within 10 minutes following completion of charge termination Cell or bank voltages shall be monitored periodically over the test period, and the remaining battery capacity or energy measured using a C/2hr discharge rate to the minimum allowed voltage level at the end of the test period Pre- and post-battery unit level environmental test charge retention shall meet the design requirements the results compared to verify that charge retention does not degrade from previous cell module or battery level test data Time stamped data shall be provided for entire charge retention test starting at beginning of charge thru end-of-discharge. 28

30 Aerospace Corporation TOR (SMC-S-017) Battery Charge Retention (Designed with active cell-level charge balance circuit that is electrically connected during battery ground storage) Battery charge retention shall be measured at 20 C in an open-circuit state starting at 100% state-of charge for at least 7 days to screen out potential internal shorts while electrode stack is under highest mechanical force and at a region with highest voltage sensitivity Any charge control electronics or voltage monitoring devices mounted on the battery shall be configured similar to ground storage plans during the open-circuit period Open circuit stand shall commence within 10 minutes following completion of charge termination Cell or bank voltages shall be monitored periodically over the test period, and the remaining battery capacity or energy measured using a C/2hr discharge rate to the minimum allowed voltage level at the end of the test period Pre- and post-battery unit level environmental test charge retention shall meet the design requirements and the results compared to verify that charge retention does not degrade from previous cell, module or battery level test data Time stamped data shall be provided for entire charge retention test starting at beginning of charge thru end-of-discharge. 29

31 Aerospace Corporation TOR (SMC-S-017) Battery Charge Retention (Designed with active battery-level charge control/without cell balancing) Battery charge retention shall be measured at 20 C in an open-circuit state starting at 100% state-of charge for at least 30 days to screen out potential internal shorts while electrode stack is under highest mechanical force and at a region with highest voltage sensitivity Any battery charge control electronics or voltage monitoring devices shall be fully disconnected during the open circuit test period to minimize stray currents Open circuit stand shall commence within 10 minutes following completion of charge termination Battery, string voltages shall be monitored periodically over the test period, and the remaining energy measured using a C/2hr discharge rate to the minimum allowed voltage level at the end of the test period Pre- and post-battery unit level environmental test charge retention shall meet the design requirements and the results compared to verify that charge retention does not degrade from previous cell, module or battery level test data Time stamped data shall be provided for entire charge retention test starting at beginning of charge thru endof-discharge Battery Impedance The battery impedance shall be measured at the battery terminals at the nominal on-orbit cycling temperatures at 95%, 50%, and 20% states-of-charge via a step current change during discharge for 30 seconds or longer and shown to meet the design requirements. 30

32 Thank you 31 31