STRUCTURE, PROPERTIES, AND PERFORMANCE OF INORGANIC-FILLED SEPARATORS R. Waterhouse, J. Emanuel, J. Frenzel, D. Lee, S. Peddini, Y. Patil, G. Fraser-Bell, and R.W. Pekala The 29 th International Battery Seminar & Exhibit, March 12 15, 2012
COMPETING SEPARATOR TECHNOLOGIES Separator Design Company Limitations / Challenges Biaxially oriented PE separators High melting point polymers PP/PE/PP PVDF Coextrusion PI, PEEK crosslinked systems Tonen SK Asahi Celgard Ube Tonen Porous Power 135 C flow, 180 max meltdown Residual stress in melt Polymer oxidation 155-165 C flow Residual stress in melt Polymer oxidation Processing difficulties with high temp or crosslinkable polymers Heat resistant layers Inorganic coatings Electrodes Separator Ceramic separators PET inner layer Al2O3/SiO2 coating Matsushita LG Evonik High coating precision required Controlled particle size distribution PE layer shrinkage Poor mechanical properties (i.e., brittle) Dusting Expensive Nanofiber-based separators Electrospinning Polyimide Dupont Throughput Expensive Highly Filled separators ENTEK Asahi Sufficiently high loading levels Residual stress in polymer matrix 2
ENTEK APPROACH Overcome high temperature thermal shrinkage and mechanical integrity challenges in large format Li-ion batteries via (1) sufficiently high, inorganic filler loading levels (2) polymer crosslinking, and/or (3) heat treatment above polymer melting point Investigate highly filled systems using UHMWPE as polymer matrix Challenges can thickness range be achieved? what fillers? what loading level? does annealing / heat treatment work? 3
INORGANIC-FILLED SEPARATOR SCHEMATIC Pores Polymer fibrils Inorganic aggregates 4
INORGANIC / CERAMIC FILLERS Filler Type / Grade Surface Treatment availability Commercial Concerns Technical Concerns Alumina fumed high cost Al2O3 reduction at anode calcined activated Silica fumed precipitated gas generation from reaction of SiO2 with HF SiO2 reduction at anode Titania fumed lithium intercalation pigment Calcium Carbonate ground precipitated gas generation from reaction of CaCO3 with HF 5
SILICA VS ALUMINA Skeletal density SiO 2 2.15 g/cc Al 2 O 3 3.96 g/cc Fumed structures generally have lower fractal dimension than precipitated structures Loading level to achieve 3-dimensional inorganic network depends upon both the filler type and its dimensionality filler / polymer > 1 69.1 wt % silica 80.5 wt % alumina 6
7 PRODUCTION PROCESS SCHEMATIC
SURFACE SEM 67 wt % Fumed Al 2 O 3 8
SURFACE SEM 67 wt % Fumed Al 2 O 3 9
SEM --- MD FRACTURE 67 wt % Fumed Al 2 O 3 10
SEM --- MD FRACTURE 67 wt % Fumed Al 2 O 3 11
SEM --- MD FRACTURE Activated Alumina particles Lower cost, activated alumina particles do not breakdown and disperse uniformly during the extrusion process. 12
SURFACE SEM 69 wt % precipitated silica 13
SEM --- MD FRACTURE 69 wt % precipitated silica 14
PATHWAYS TO OPTIMAL FILLER DISPERSION Filler selection High surface area Low fractal dimension (wispy) weak inter-aggregate bonds surface chemistry Screw configuration Aggressive Pre-dispersion Wet milling 15
EXCELLENT DIMENSIONAL STABILITY Silica filled separator Roll ID Base roll Filler:PE Thickness (avg) 200 C Shrinkage % Gurley Puncture µm MD XMD sec/10ml gf /25 u DY110217.002 59 2.1 18.7 5.87 4 6.8 280 DY110221.001 263 2.3 25.3 4.67 4 7.8 230 DY110301.003 260 2.3 23.8 4.2 3.5 7.2 224 DY110302.002 260 2.3 23.3 4 3.5 7.4 244 DY110303.002 260 2.3 25.1 4.5 3.7 7.6 214 DY110218.002 64 2.6 21.3 6.5 2.67 5.3 192 Alumina filled separator Roll ID Base roll Filler:PE Thickness (avg) 200 C Shrinkage % Gurley Puncture µm MD XMD sec/10ml gf /25 u DY110131.025 202 2.7 25.4 5 1 12.7 391 DY110214.003 202 2.7 20.8 6.33 2.5 12 386 USABC Goal : < 5% shrinkage at 200 C Achieved with more than one formulations and different inorganic fillers 16
BENCHMARKING AGAINST OTHER INORGANIC SEPARATORS Alumina or silica coated onto PET carrier Ceramic coating on polyolefin separator 17
SEPARION BEFORE & AFTER SHRINKAGE AT 200 C < 5% MD & TD shrinkage Before 200 C Shrink Test 18 After 200 C Shrink Test
BENCHMARKING FILLED SEPARATOR VS. COATED SEPARATOR 12 micron PE separator dip coated with an Al 2 O 3 /PVA solution. Varying amount of alumina coating applied 19
SURFACE SEM Alumina coated separator 20
COATED SEPARATOR SHRINKAGE Loading A Loading B < 5% MD & TD shrinkage only achieved when coat wt. exceeded 33% Loading C Loading A Loading B Loading C 21
ALUMINA-COATED SEPARATOR AFTER 200 C OVEN TEST Filler penetration inside the separator could be responsible for improved thermal stability. Filler rich surface can interact with electrodes. PE + Al 2 O 3 PE only PE + Al 2 O 3 22
EXCEPTIONALLY LOW IMPEDANCE Rapid wetting with electrolyte Enhanced power capability and low temperature performance Roll ID Base roll Filler Filler:PE Thickness Areal Resistance Resistivity MacMullin Number Microns Ω-cm² Ω-cm Inorganic Filled Separators DY110217.002 59 Silica 2.1 19 0.59 308 2.6 DY110224.002 263 Silica 2.3 23.8 0.63 266 2.2 DY110224.004 261 Silica 2.3 22.8 0.85 373 3.1 DY110303.001 260 Silica 2.3 24.1 0.59 247 2.1 DY110218.002 64 Silica 2.6 20.5 0.48 232 1.9 DY110131.025 202 Alumina 2.7 25.9 1.06 410 3.4 DY110214.003 202 Alumina 2.7 22.3 0.88 396 3.3 Unfilled Separators Teklon HPIP Unfilled 25 2.15 869 7.2 Teklon Gold LP Unfilled 12 1.85 1460 12.2 Coated Unfilled Separator Coated Teklon Alumina 14 2.4 1668 13.9 USABC Goal : MacMullin # < 11 Achieved with all formulations 23
INORGANIC FILLER IMPROVES SEPARATOR WETTING Separator suspended in electrolyte in a graduated cylinder Wicking height at 30 minutes Wicking Rate: PR57, 2.1:1, silica-filled Separator Wicking Test 16 16 14 14 Wicking Distance (mm) 12 10 8 6 4 1 2 3 4 Wicking Height (mm) 12 10 8 6 4 2 0 0 10 20 30 40 50 60 70 Wicking Time (min.) 2 0 Microporous PE PR61 silica-filled 0 20 40 60 80 100 120 Wicking Time (min.) Wicking rate measurements are repeatable. Silica-filled separator rises at over twice the rate of porous PE. 24
CELL TESTING Build 18650 Cells American Lithium Energy Corp. 10 cells for each separator formulation 7-day OCV screening test Initial Performance Characterization 8 cells 1C discharge, room temp 1C discharge, -30 C HPPC, room temp. 2 cells reserved for future tests. Calendar Life Test 60 C, 100% SOC 4 cells each formulation Repeat RPT every 4 wks Cycle Life Test Room Temp., 1C with 2C pulse 4 cells each formulation 25
INORGANIC FILLER IMPROVES CELL CYCLE LIFE 18650 cells: NMC/graphite Room temperature, 100% DOD, 1C rate. Control cells: 80% of initial capacity at 1000-1100 cycles. Cells with silica-filled separator: 80% of initial capacity at 1600-2000 cycles. 18650 Cycle Capacity: Microporous PE Controls 18650 Cycle Capacity: Silica-filled Separators 1.6 1.6 1.4 1.4 1.2 1.2 Discharge Capacity (AH) 1 0.8 0.6 0.4 Average Fade = -24.6% 009 010 012 013 80% Cap. Discharge Capacity (AH) 1 0.8 0.6 0.4 2000 cycles Average Fade (3 cells) = -20.2% silica 19 silica 21 silica 23 silica 24 80% Series2 0.2 Microporous PE 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Cycle Number 0.2 Silica-filled separator 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Cycle Number Silica-filled separator increases cell cycle life compared to control (MP). More uniform electrolyte distribution more uniform electrode utilization. 26
CYCLE LIFE OF CELLS WITH DIFFERENT INORGANIC-FILLED SEPARATORS 1.6 Separator filler: fumed silica 1.6 Separator filler: fumed silica + alumina Discharge Capacity (AH) 1.4 1.2 1.0 0.8 0.6 0.4 1600 cycles Average Fade = -22.5% Discharge Capacity (AH) 1.4 1.2 1.0 0.8 0.6 0.4 2000 cycles Average Fade = -21.3% 0.2 0.0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Cycle Number 0.2 0.0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Cycle Number 1.6 Separator filler: alumina + titania 1.6 Separator filler: alumina Discharge Capacity (AH) 1.4 1.2 1.0 0.8 0.6 0.4 Cycle 1700 Average Fade = -23.7% Discharge Capacity (AH) 1.4 1.2 1.0 0.8 0.6 0.4 2200 cycles Average Fade = -20.9% 0.2 0.2 27 0.0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Cycle Number 0.0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 Cycle Number All four groups have better cycle life than the control. No filler combination appears better than precipitated silica.
INORGANIC FILLER REDUCES SELF-DISCHARGE 60 C Storage Test: Fully charged (4.2V), OCV checked daily, test every 4 weeks. 60 C Storage Test - Teklon Control: Cell OCV 60 C Storage Test - Silica-filled: Cell OCV 4.2 4.2 4.15 4.15 4.1 4.1 4.05 4.05 Open Circuit Voltage 4 3.95 3.9 3.85 US0005 US0006 US0007 US0008 Open Circuit Voltage 4 3.95 3.9 3.85 US0015 US0016 US0017 US0018 3.8 3.8 3.75 3.75 3.7 0 20 40 60 80 100 120 140 160 180 Days on Test 3.7 0 20 40 60 80 100 120 140 160 180 Days on Test Microporous PE Silica-filled separator Silica-filled separator reduces self discharge and capacity loss. 28
SUMMARY Free-standing, dimensionally stable, inorganic-filled separators were produced from precipitated silica and fumed alumina using UHMWPE as a binder These inorganic-filled separators exhibited < 5% shrinkage in both MD and TD after 1 hour at 200 C. Inorganic-filled separators have excellent wettability and ultralow impedance (MacMullin number < 3) that allows for high power capability and low temperature performance 18650 cells with inorganic-filled separators show good performance compared to control cells with a microporous polyethylene separator. Improved cycle life Lower self discharge Higher rate capability Preliminary cost models suggest that silica-filled separators can approach target price; however, cell drying step is likely required to gain performance benefits 29
ACKNOWLEDGMENT This material is based upon work supported by the Department of Energy, National Energy Technology Laboratory under Award Number DE-FC26-05NT42403 with the United States Advanced Battery Consortium (USABC). Disclaimer: This report was prepared as an account of work sponsored by an agency of the United States Government and USABC. Neither the USABC, the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof, or those of USABC. 30