Battery-grade PAA performance characteristics and application research practice

I. Characteristics and Advantages of Polyacrylate (PAA) Binders

1. Minimal Swelling in Electrolyte Solvents: Exhibits low swelling, maintaining structural integrity of electrode sheets during charge/discharge cycles.
2. High Proportion of Carboxyl Groups: The high density of polar carboxyl groups forms strong hydrogen bonds with hydroxyl-containing active materials, enhancing dispersion stability.
3. Continuous Film Formation: Creates a uniform film on material surfaces, improving contact between active materials and current collectors.
4. Excellent Mechanical Stability: Facilitates ease of processing during electrode manufacturing.
5. Enhanced SEI Formation and Cycling Performance: The high concentration of polar functional groups promotes hydrogen bonding with silicon material surfaces and aids in forming a stable Solid Electrolyte Interphase (SEI) layer, resulting in superior cycle life.

II. Development Challenges

Conventional PAA (Polyacrylic Acid) binder systems for electrodes typically utilize cross-linked PAA polymers as the anode binder. As a high-molecular-weight polymer, PAA offers excellent adhesion, dispersion stability, and corrosion inhibition. It stabilizes the network structure within the anode slurry, ensures uniform dispersion of active materials, and extends electrode sheet lifespan.

• However, the polar functional groups facilitate hydrogen bonding within the long molecular chains of PAA. This restricts free rotation of the chains, increasing their rigidity. Consequently, PAA-based electrode sheets generally exhibit poor toughness. This compromises their ability to withstand stresses induced by the volume expansion of active materials during cycling, hinders cell winding processes, and ultimately limits improvements in battery electrochemical performance.

III. Research Practices in Practical Applications of Battery-Grade PAA

1. Sodium-Ion Battery Hard Carbon Anodes

Manufacturers of hard carbon anodes for Sodium-Ion Batteries (SIBs) impose stringent requirements on PAA binders. A high-quality, highly flexible PAA binder is crucial for protecting the structural integrity of hard carbon anodes.

• In the current SIB hard carbon anode market, using substandard PAA binders significantly increases the risk of elevated internal resistance, negatively impacting battery efficiency and reliability. Conversely, a premium, highly flexible PAA binder effectively mitigates these issues.
• The electrochemical performance, conductivity, environmental adaptability, and corrosion resistance of the flexible PAA binder are also critical factors, directly influencing the quality of the final hard carbon anode product.
• Beyond inherent characteristics, practical application focuses heavily on performance parameters such as binder characteristics, solid content, adhesion strength, and pH level. These parameters directly correlate with the operational efficiency of the hard carbon anode.

2. Silicon-Based Anodes

Silicon-based lithium-ion battery anodes offer a specific capacity an order of magnitude higher than conventional graphite. However, forming stable silicon anodes is challenging due to significant volume changes during the electrochemical alloying/dealloying of silicon with lithium. Binder selection and optimization are vital for improving silicon anode stability. Most research utilizes Carboxymethyl Cellulose (CMC) and Polyvinylidene Fluoride (PVDF) binders.

• A significant body of experimental research indicates that pure PAA possesses mechanical properties comparable to CMC but contains a higher concentration of carboxyl functional groups. This enables PAA to act as a binder for Si anodes, delivering superior performance.
• Research further demonstrates the positive impact of carbon coating on anode stability. Carbon-coated Si nanopowder anodes (tested between 0.01 and 1 V vs. Li/Li+), incorporating PAA at levels as low as 15 wt%, exhibit exceptional stability over the first 100 cycles. These findings open new avenues for exploring novel binders like the Polyvinyl Alcohol (PVA) series.
• Crosslinking PAA with other materials represents a new development direction, including AA-CMC cross-linked binders, PAA-PVA cross-linked binders, PAA-PANI (Polyaniline) cross-linked binders, and EDTA-PAA binders.

3. PVA-g-PAA (PVA-grafted-PAA)

A novel water-soluble binder, PVA-g-PAA, is synthesized by grafting PAA onto the side chains of highly flexible PVA (Polyvinyl Alcohol). This functional group modification enhances the flexibility of the PAA binder system while leveraging PVA's excellent adhesion properties.

• This free-radical grafting polymerization introduces elasticity, compensating for the structural limitations of pure PAA binders.
• During electrode sheet fabrication, rolling compaction is performed continuously using varying roller pressures across defined length segments of the sheet. This process enhances sheet toughness, minimizing deformation, increasing electrode specific capacity, improving rate capability, and extending battery cycle life.

4. PAA Prelithiation (LiPAA)

The application of silicon-carbon (Si-C) materials imposes higher demands on anode binder and conductive agent systems. Traditional rigid PVDF binders are unsuitable for Si anodes. Acrylic PAA binders contain numerous carboxyl groups capable of forming hydrogen bonds with functional groups on Si surfaces, promoting SEI formation and significantly improving the cycle life of Si anodes. Thus, PAA binders are highly effective for Si anodes.

• Studies indicate that Lithium Polyacrylate (LiPAA) outperforms PAA itself, although the underlying reasons were unclear. Extensive research has been conducted to elucidate the mechanism behind LiPAA's superior performance.
• Electrodes composed of 15% nano-Si, 73% artificial graphite, 2% carbon black, and 10% binder (either PAA or LiPAA) were studied. After initial drying, a secondary drying step at 100-200°C was performed to remove residual moisture completely. Coin cell testing revealed capacities of ~790 mAh/g for LiPAA-based anodes versus ~610 mAh/g for PAA-based anodes.

Cycle performance curves of full cells using NMC532 cathodes

• Figure A: Cells with LiPAA binder show no significant correlation between cycle performance and secondary drying temperature. The NMC532 cathode delivered an initial capacity of 127 mAh/g at C/3, declining to ~91 mAh/g after 90 cycles.
• Figure B: Cells with PAA binder exhibit a clear dependence on secondary drying temperature (120°C red, 140°C gold, 160°C green, 180°C blue). While the 160°C dried PAA cell showed the highest initial capacity and the 120°C dried cell the lowest, the 160°C dried cell degraded fastest, reaching ~62 mAh/g after 90 cycles. The 140°C dried cell degraded slower, maintaining ~71 mAh/g.
• First-cycle Coulombic Efficiency (CE): LiPAA cells achieved ~84% (only the 200°C LiPAA cell was slightly lower at ~82%). Their Coulombic efficiency rapidly increased to ~99.6% within the first 5 cycles. PAA cells achieved ~80% first-cycle CE (only the 180°C PAA cell was significantly lower at ~75%), requiring ~40 cycles to reach 99.6% CE – markedly slower than LiPAA cells.
• Pulse discharge tests at 50% Depth of Discharge (DOD) revealed significantly lower internal resistance in LiPAA cells compared to PAA cells [Referenced Figure Below], with no apparent link to secondary drying temperature for LiPAA. In contrast, PAA cell resistance increased noticeably with higher secondary drying temperatures.

• Thermogravimetric Analysis (TGA) by Kevin A. Hays [Referenced Figure Below] on LiPAA and PAA anodes identified two main dehydration steps: 1) Free water removal (~40°C), 2) Adsorbed water removal (LiPAA ~75°C, PAA ~125°C). Additional weight loss peaks occurred for PAA between 140-208°C and LiPAA between 85-190°C, attributed to polymerization of some carboxyl groups releasing water [Referenced Reaction Below]. This reaction is less pronounced in LiPAA, where Li replaces H in ~80% of carboxyl groups.

• High-temperature polymerization of PAA carboxyl groups may weaken the interaction between PAA and Si, potentially explaining the poor cycle performance of high-temperature dried PAA anodes. However, peel strength tests showed that while PAA adhesion decreased with higher drying temperatures, it remained higher than LiPAA overall, suggesting other factors contribute to LiPAA's superior cycling.

Ⅳ. Conclusion

This study identifies poor electrochemical stability as a key factor limiting PAA's cycle performance. At low potentials, PAA undergoes partial conversion to LiPAA, generating hydrogen gas:

PAA + ... -> LiPAA + H₂

This reaction explains the lower first-cycle CE of PAA cells (~80%) compared to LiPAA cells (~84%), and the significantly longer time (~40 cycles vs. <5 cycles) required for PAA cells to achieve high Coulombic efficiency (99.6%).

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