New Separator Approaches for Lead Acid Batteries

Transcription

1 New Separator Approaches for Lead Acid Batteries R. Waterhouse, C. La, M. Warren, J. Kim, W. Self, D. Wandera, J. Frenzel, J. Norris, D. Lee, C. Rogers, E. Hostetler, and R.W. Pekala ENTEK International LLC ENTEK International LTD June 15, 2017

2 BATTERY SEPARATORS: PAST TO PRESENT A Brief History 1890 Wood separators introduced and remain in use until the 1950 s (Port Orford cedar, Douglas fir, redwood, etc.) Perforated, slit and punched rubber sheets Porous ebonite and microporous rubber 1950 s Resin-bonded cellulosic and synthetic fiber separators 1960 s Phenol-resorcinol-formaldehyde separators and sintered PVC 1970 s AGM introduced in Gates sealed lead acid cells s PE-silica separators and envelopes are developed and become the standard for SLI batteries. Perhaps no component of the lead-acid battery has experienced more innovation and change than the battery separator Chemically-treated wooden separators Gould rib design for wooden separators 2

3 DESIRED CHARACTERISTICS OF LEAD ACID BATTERY SEPARATORS Chemical stability: resists oxidation and attack by sulfuric acid Readily wettable by sulfuric acid High porosity (low acid displacement) Small pore size to prevent migration of lead particles and formation of dendrites Low resistance to ionic current flow in the electrolyte High mechanical strength to withstand vibration and penetration by grid wires Optimal stiffness: varies from very stiff to very flexible depending on the type of battery Capable of forming envelopes or sleeves around electrodes Capable of various physical geometries (ribs, corrugations, etc.) to define the spacing between electrodes, influence the distribution of the mechanical load on the electrode surface, or directing the movement of acid and gas bubbles during charging. Influence over the hydrogen evolution reaction through antimony control, amd/or extending the overpotential for hydrogen evolution on lead and carbon surfaces in the negative electrode. High purity (not introducing unwanted contaminants) Low impact on the recycling process Low cost No separator material can satisfy all of these requirements. But we still try. 3

4 WHAT S NEXT IN LEAD ACID BATTERY SEPARATORS/SEPARATION? Two Candidates Cast Films: Non-solvent Induced Phase Separation (NIPS) Polysulfone Polyether sulfone Cross-linked Polymer Gels: Polymer-immobilized Acid Polyacrylamide and variations 4

5 NONSOLVENT INDUCED PHASE SEPARATION (NIPS) The phase inversion induced by immersion precipitation consists of the following steps: 1. Polymer dissolution in single or mixed solvent 2. Casting: the polymer solution is deposited as a thin film on a suitable substrate or carrier film 3. Immersion: the cast film is immersed in a nonsolvent coagulation bath (e.g. water). The solvent diffuses into the bath while the nonsolvent diffuses into the polymer solution, causing the polymer to precipitate as a porous solid. 4. Post-treatments such as rinsing, annealing and drying

6 MATERIALS: POLYMERS, ADDITIVES, SOLVENTS Sulfone polymers are amorphous thermoplastics comprised of aromatic units separated with sulfone, isopropylidene or ether groups. PES, with the highest concentration of sulfone groups in the polymer repeating unit, has the highest water absorption ability hence the most hydrophilic. Readily formed into membranes with highly controllable pore size distribution, very high mechanical strength, stable at ph from 2-13, low levels of extractable materials. Additives Hydrophilic polymers Pore forming agents, improved wetting Polyacrylic acid (PAA) Structure enhancing agent Fumed Silica Filler, wetting agent Solvents NMP, DMF, DMAc, DMSO 6

7 NIPS: PROCESS VARIABLES Characteristics of the casting solution. Most important is the selection of a suitable solvent for the polymer. Polymer concentration determines the membrane porosity. Increasing polymer concentration in the casting solution leads to higher fraction of polymer and decreases average membrane porosity and pore size. Solvent / nonsolvent system. The solvent must be miscible with the nonsolvent (usually water). An aprotic polar solvent like N-methyl pyrrolidone (NMP), dimethyl formamide (DMF), dimethyl acetamide (DMAc) or dimethylsulfoxide (DMSO) is preferable for rapid precipitation. Additives. Examples of additives or modifiers added to the casting solution include pore forming agents, cosolvents with relatively high solubility parameter, the nonsolvent or a cross-linking agent. Hydrophilic additives can enhance not only membrane pore size but also membrane hydrophilicity. Casting surface. The material properties of the surface on which the dope solution is cast can influence the formation of a skin layer and the release of the membrane. Coagulation (nonsolvent) bath. This includes temperature of the bath and presence of quantities of the solvent. Exposure time of cast film before precipitation. This step has significant effects on the characteristics of the skin layer and the pore morphology of the resulting membrane. 7

8 CAST FILMS WITH RIBS ON BOTH SIDES A special casting process allows ribs to be formed on both sides of the separator. Diagonal positive side ribs Straight negative side ribs 8

9 MEMBRANE STRUCTURE: PES + PET CARRIER Top Surface Top Surface X-Section 9 Open pore structure from the surface throughout the X-section of the cast film Surface pores showing absence of surface-skin

10 SURFACE STRUCTURE: SOLVENT COMPARISON Solvent: NMP Solvent: DMF Due to the faster rate of solvent exchange between water and DMF (compared to water and NMP) during the phase inversion/polymer precipitation step, smaller pores are formed when DMF is used as a solvent compared to when NMP is used. 10

11 CAST FILM COMPARED TO PHENOLIC, PE AND AGM SEPARATORS Cast film AGM 11 Phenolic separator PE-silica separator

12 HG POROSIMETRY: CAST FILM COMPARED TO PE-SILICA Porosity Cast Film: PES/silica/PET 68% Entek PE-silica Standard 58% Narrow pore size distribution Pores from silica agglomerates Cast film separators, with silica additive, have a higher porosity than standard PE separators. The pore size distribution is bimodal, with pores defined by polymer precipitation and the silica agglomerates. 12

13 POROSITY AND RESISTANCE OF CAST FILM SEPARATORS Water Porosity Palico Resistance Cast films of polyethersulfone, polysulfone, and PVDF. with or without silica additive, demonstrate high porosity and low resistance. Comparison separators Resistance (mω-cm²/0.1mm BW) Porosity (%) ENTEK Standard (0.25) 37 58% ENTEK LR (0.25) 23 60% Phenolic (0.45) 25 70% 13

14 MECHANICAL PROPERTIES Puncture Strength (normalized to 0.1mm thickness) Tensile Strength (normalized to 0.1mm thickness) Cast films of polysulfone and polyethersulfone, with or without silica additive, demonstrate high mechanical strength. 14

15 CAST FILM SUMMARY Cast Film technology offers High strength with high porosity, inherent wettability, low resistance and submicron pore size. High purity (low trace metals, no residual oil) Design flexibility (can form many different rib profiles) What improvements are needed? Scale-up to a continuous process Next steps Demonstrate performance in a multi-cell lead acid battery. 15

16 CROSSLINKED POLYMER GEL FORMATION IN SULFURIC ACID O O acrylamide N H + NH 2 N H O (NH 4 ) 2 S 2 O 8 H 2 N H 2 N C O HN HN O C O H 2 N H 2 N O C O O HN HN C O O Polyacrylamide gels are widely used for macromolecule (proteins and nucleic acids) separations by electrophoresis. N,N'-methylene-bis-acrylamide Polymerization of monomer acrylamide and cross-linker bis-acrylamide can be initiated by ammonium persulfate to form a covalently crosslinked polymer gel in sulfuric acid. Lead 10% PA gel in H 2 SO 4 16

17 MECHANICAL STRENGTH: EFFECT OF POLYMER % Compression of gels made in 1.31sg H 2 SO 4 Modulus calculated from compression measurements 7.5wt% 15wt% 10wt% MMMMMMMMMMMMMM = IIIIIIIIIIII ttttttttttttttttt SSSSSSSSSS Slope is obtained by linear regression In all cases, r-squared is about 0.99 Compressive strength (modulus) increases as a function of polymer content in the gel. 17

18 ELECTRICAL RESISTIVITY: EFFECT OF POLYMER % AND CHEMISTRY Different gel chemistries were prepared in 1.28 s.g. H 2 SO 4. XPG Chem.1 : acrylamide/bisacrylamide XPG Chem. 2: bisacrylamide XPG Chem. 3: AMPS/bisacrylamide Resistivity measured by AC impedance technique at room temperature. Resistance increases with gel polymer concentration. XPG at 10% (w/w) polymer concentration and less exhibit lower resistivity than saturated AGM in 1.28 s.g. H 2 SO 4 AGM 1.28 s.g. H 2 SO 4 18

19 XPG CELL CYCLING 3 2,5 2 Capacity (Ah) 1,5 1 0,5 (Discharge/PbH005) AGM control (Discharge/PbH014) AGM control (Discharge/PAM051) 10%(w/w) XPG Chemistry 1 (Discharge/PAM070) 10%(w/w) XPG Chemistry 1 (Discharge/PAM071) 10%(w/w) XPG Chemistry 1 (Discharge/PAM072) 10% (w/w) XPG Chemistry 2 (Discharge/PAM073) 10% (w/w) XPG Chemistry 3 19 Polymerization of the preheated gel solution is completed within minutes of pour into the cell Cycle XPG cells exhibit lower initial capacity (Ah) compared to AGM control cells XPG cells capacity gradually increases in the first 20 cycles to reach plateau capacity

20 POLARIZATION DATA: WARM AND COLD 2,5 30 C Polarization Scans 2,5-18 C Polarization Scans Voltage at the End of 30s Pulse 2 1,5 1 0,5 AGM Average (4) XPG Average (4) XPG + PE Average (2) Voltage at the End of 30s Pulse 2 1,5 1 0,5 AGM Average (4) XPG Average (4) XPG + PE Average (2) Current Density (Amps/cm 2 ) Current Density (Amps/cm 2 ) Cells were built with 10% XPG, 10% XPG+PE separator, and AGM (flooded) Temperature seems to have a much smaller impact on the XPG cells than the AGM cells AGM cells had much lower voltage at the end of the pulse than the cells with XPG at -18 C 20

21 CELL RESISTANCE VS. TEMPERATURE (HIOKI TESTER) Resistance (mω) AGM 8% XPG 10% XPG 15% XPG Normalized Resistance AGM 8% XPG 10% XPG 15% XPG 39.10wt% H2SO wt% H2SO Acid resistivity from Bode, Lead Acid Batteries Temperature Temperature Resistance is measured with a Hioki BT3562 Battery HiTester After each temperature change the cells are allowed to acclimate for 1 hour before testing XPG cells start with slightly higher resistance at 30 C, but as temperature decreases the XPG has lower resistance than AGM The cell resistance measured agrees with the polarization test results 21

22 XPG PROCESS IN MULTI-ELECTRODE BATTERY CELLS Spacing electrodes A book of electrodes in original battery case Right after adding XPG solution XPG formation 22

23 XPG SUMMARY XPG technology offers Non-spillable electrolyte system Decreased resistance at low temperatures and improved CCA performance Prevention of acid stratification What improvements are need? Develop better methods for electrode spacing and gel filling Next steps Demonstrate recombination in a sealed cell. Demonstrate performance in a multi-cell XPG battery. 23

24 IN CONCLUSION History 1890 Wood separators introduced and remain in use until the 1950 s (Port Orford cedar, Douglas fir, redwood, etc.) Perforated, slit and punched rubber sheets Porous ebonite and microporous rubber 1950 s Resin-bonded cellulosic and synthetic fiber separators 1960 s Phenol-resorcinol-formaldehyde separators and sintered PVC 1970 s AGM introduced in Gates sealed lead acid cells s PE-silica separators and envelopes are developed and become the standard for SLI batteries. And Now, the Future? 2010 s Cross-linked polymer gels 2020 s Polyethersulfone cast films Thank you! 24