Challenges for VRLA Separator Technology

1. Challenges for VRLA Separator Technology A Review of Recent ALABC Research M.J.Weighall MJW Associates

2. VRLA Separator Properties • Gas transfer properties • High acid absorbency • High chemical purity • Oxidation stability • Acceptable manufacturing cost • Compressible

3. Premature Capacity Loss

4. Unified Mechanism to explain PCL-1 and PCL-2 CSIRO Energy Technology

5. ALABC Research • Importance of high plate group pressure • eliminate premature capacity loss (PCL) • Maintain +ve plate integrity • oppose expansion of +ve active material • Must set plate group pressure early in life and maintain throughout life.

6. Plate Group Pressure • Influenced by many factors: • Control of plate and separator thickness • Assembly process • Container design and materials • Separator properties • All glass • “Hybrid” - glass + other material • Alternative material • Compression devices • Internal compression plate • external clamping

7. The CSIRO Piston Cell

8. Separator Behaviour under PressureThe “ideal” AGM separator • Compressive Properties should be reproducible when dry, and the separator should recover to its original thickness when the compressive force is released. • Separator should not contract when wetted with acid and should show the same compressive properties as the dry separator. • The slope of the compression curve should accommodate small changes in plate thickness without an appreciable change in the compressive force

9. Typical Compression Curve for AGM Separator Acknowledgement: Hollingsworth & Vose Company

10. AGM Compression Curves • Separator thickness reduces with increasing applied force • up to and possibly beyond 50 kPa • Separator recovery is incomplete when applied force is released • does not return to original thickness • Wet separator tends to contract. • Lower thickness than dry separator for given applied force • Applied force in excess of 50 kPa may be needed to achieve a compression of 30%

11. Influence of AGM Separator Composition (1) • A separator with the same grammage but a different surface area will display different compression behaviour. • The separator mat with the higher surface area requires a higher applied pressure to achieve a given % compression • The separator with the high surface area may be better able to resist “crush” under high compression.

12. Influence of AGM Separator Composition (2) • The pore structure (coarse/ fine fibres) influences wicking speed, wicking height, stratification • Coarse fibres wick faster but to lower final height; greater risk of stratification. • Fine fibres wick slower but to greater final height; less risk of stratification • Fine fibres: lower hysteresis and better spring-back properties

13. EALABC Project BE7297 Task 6

14. Other Glass Separator Options? • Owens Corning have developed a bi-component glass fibre, trade marked “Miraflex™” • two different formulations of glass fibre have been engineered into a single filament • the resultant fibre is extremely compactable and able to recover its original form after being compressed. • Could the coarse and fine glass fibres used in the AGM separator similarly be engineered into a single filament? • Other innovative fibre manufacturing/ processing techniques?

15. Alternative Separators • Less compressible separators may maintain higher plate group pressure • Less tolerant of plate thickness variations • May need to be used in conjunction with compression plate. • May require other battery design or assembly changes. • Volume porosity may be lower than AGM

16. Ceramic Separator • Preliminary study carried out in current EALABC programme • This study showed that a completely rigid separator is unsuitable for the VRLA battery. • The separator needs to mould to the contours of the plate surfaces • The ceramic sheet is too brittle • The pore size is too large to inhibit short circuits

17. Daramic AJS • Tested in EALABC project BE97-4085 Task 1(b) • Almost incompressible. • Thickness remains almost constant up to 130 kPa • Lower porosity than AGM (82%) • Excellent cycle life under high applied pressure (80 kPa)

18. Daramic AJS Cycle Life

19. H & V SLGM • Studied by CSIRO in ALABC Project B-001.2 • Glass mat loaded with silica - this makes the glass mat more rigid and changes the compression/ recovery characteristics • Demonstrated the least compaction upon wetting with acid & smallest loss of thickness at 80 kPa.

20. Amer-Sil Composite Separator • EALABC Project BE97-4085 Task 1(a) • High porosity microporous separator interposed between 2 layers of AGM HIGHER BET SURFACE AREA AGM MATERIALAGAINST BOTH ELECTRODES MICROPOROUS POLYMER MEMBRANE(>80% VOLUME POROSITY, 3-5  PORE SIZE)ENCAPSULATED BETWEEN TWO LAYERS

21. Amer-Sil Proposal A Acknowledgement: BE97-4085 Task 1(a)

22. Cycle Life of Amersil Composite Separator (Proposal A)

23. Disadvantages of Alternative Separators • Inferior wicking properties • May wick to lower ultimate height • Prone to stratification • Problems more acute when separator is partly saturated • Lower porosity, therefore lower acid absorption • Lower oxygen diffusion/ recombination efficiency

24. The Oxygen Cycle • 100% recombination efficiency may not be desirable • Negative plate may lose capacity if the rate of local action in the negative electrode exceeds the rate of grid corrosion in the positive electrode • Modified separator may influence oxygen diffusion through the separator and recombination efficiency

25. Fast Charging • The ALABC Research Programme has shown that 80% of the charge of the VRLA battery can be restored in 15 minutes or less • The maximum rate of the internal oxygen cycle is likely to be lower in alternative separator materials than in AGM separators • This may restrict the maximum charging current for fast charge regimes

26. Keeping up the Pressure • Improved container designs and materials • Pre-compress and/ or pre-wet the separator • Advanced assembly techniques • Purpose designed compression plates

27. Conclusions (1) • PCL can be eliminated by maintaining a high plate group pressure (> 40 kPa) • The compression characteristics of microglass separators need to be improved e.g. by modifying the fibres and fibre blend • High % of fine fibres (high surface area) • Inclusion of silica (H & V SLGM) • Less compressible separators may extend battery life by maintaining a high plate group pressure • e.g. Daramic AJS

28. Conclusions (2) • Totally rigid (ceramic) separator is unsuitable • Control of oxygen transfer and recombination efficiency are needed • modify separator properties • interpose microporous sheet • Additional measures may be required to maintain plate group pressure

29. Future Research • Changes and improvements to the properties of the raw fibres • e.g. dual filament or multi-filament fibres • Further development of less compressible separator materials • Improved compression characteristics of the microglass separator • Improved understanding of the oxygen cycle and the mechanisms of oxygen transport through the separator

30. Acknowledgements • This paper is based mainly on the results of research sponsored by the Advanced Lead Acid Battery Consortium. (ALABC). The support of the ALABC in allowing this paper to be published is gratefully acknowledged.