● Abstract

1. Summarized the mechanism of the binder's role in the volume expansion of silicon anodes.

2. Proposed a solution using high-flexibility PAA binder to improve the performance of silicon anodes.

 

Current Status and Challenges of Silicon Anodes

With the increasing demand for range in new energy vehicles, lithium battery anode materials are also developing towards high specific capacity. Currently, graphite is the main material for lithium battery anodes, but the specific capacity of commercial graphite has approached its theoretical limit (372mAh/g) [1]. Silicon anode materials have a theoretical specific capacity of up to 4200mAh/g [2], which is more than 10 times that of graphite-based anode materials. The application of silicon materials can significantly enhance the energy density of lithium batteries, making it the most promising next-generation anode material.

However, the intrinsic defects of silicon anodes limit their commercial application, and the main challenges they face are:

1. Significant volume expansion during lithium intercalation and deintercalation (volume change > 300%);

2. Low initial charge-discharge efficiency;

3. Poor electronic conductivity;

4. Unstable SEI film.

The solutions to the above problems can be roughly divided into three directions:

1. Modifying silicon materials, such as size control, silicon-carbon composites, and pre-magnesium and pre-lithiation to obtain silicon anode materials with smaller expansion, better conductivity, and higher initial efficiency;

2. Selecting suitable binders to increase the adhesion between active materials and between active materials and current collectors, suppressing expansion and improving conductivity;

3. Developing compatible electrolyte formulations to form a stable SEI film.

As one of the essential materials for manufacturing silicon-based anodes, the content of binders in the electrode is very low (1.5%~3%), and their cost accounts for about 1%~3% of the total battery cost, but they play an irreplaceable role. They can shorten the Li+ transport pathway, directly participate in and affect key indicators such as the porosity, wettability, electronic/ionic conductivity, and electrochemical/mechanical stability of the electrode, providing a buffer for the volume expansion of silicon materials and stabilizing the structure of the electrode material. Additionally, due to the severe volume effect, silicon-based anode materials have higher requirements for the adhesion and flexibility of the binder.

Mechanism of Binder's Role in Silicon Anodes

The binder in the electrode mainly binds the active material particles and conductive agents together and adheres them to the current collector, maintaining the integrity of the electrode by ensuring sufficient contact between the active materials and the current collector during repeated charge and discharge cycles. As a type of polymer, the adhesion strength of the binder is strongly related to its chemical properties. Based on reversibility and adhesion strength, the interactions between polymer molecules can be divided into the following three categories:

1. Weak interactions (such as van der Waals forces), which have weak resistance to mechanical stress separation, high reversibility, and weak recovery driving force.

2. Strong interactions (such as hydrogen bonds or ion-dipole interactions), which have 'self-healing' properties after expansion, with moderate reversibility.

3. Covalent cross-linking, which results in plastic fracture after expansion and is irreversible.

These three types of interactions respond differently to external mechanical stress, as shown in Figure 1.

 

Figure 1. Schematic diagram of the three types of interactions between polymer molecules.

(Source: Zhihu @ Detection Center Huang Gong)

 

Based on the mechanism of the binder's role, in order to address the volume expansion issue of silicon anodes through the binder, the core of developing silicon anode binders lies in enhancing their strong interactions with silicon anodes, as well as improving the binder's own flexibility, self-healing ability, and adhesion.

 

Application of PAA Binders in Silicon Anodes

Polyacrylic acid (PAA) is one of the commonly used binders for silicon anodes in lithium-ion batteries. Compared to the traditional CMC+SBR scheme, the numerous polar groups in the PAA molecular chain form hydrogen bonds with the hydroxyl groups on the surface of silicon nanoparticles, creating a strong interaction that effectively overcomes the volume expansion of silicon nanoparticles. There are many reports in the literature regarding the application of PAA in silicon anodes, such as Komaba et al. [3] comparing four types of binders (CMC, PVDF, PVA, and PAA), finding that the electrochemical performance of the PAA electrode was significantly improved due to the stronger chemical or physical bonding between amorphous SiO and amorphous PAA.

However, the molecular structure of PAA contains a large number of carboxyl and hydroxyl groups, and the excessive hydrogen bonding between strong polar functional groups can hinder the free rotation of the molecular chain, resulting in poor flexibility of the polymer.This is unfavorable for withstanding the stress generated by the volume expansion of the active material, affecting the improvement of battery performance. Additionally, the linear long-chain structure of PAA makes it easy for the molecular chains to slide, leading to permanent deformation under stress and causing agglomeration of the active material, which affects the capacity performance of the active material. To address these issues, designing a new type of modified PAA polymer binder scheme is essential for improving lithium battery performance.

Performance of High-Flexibility PAA in Silicon Anodes

High Rui Power has been deeply engaged in the field of PAA aqueous binders for many years. In response to the current issues and challenges faced by silicon anode PAA binders, they have developed PAA products with high flexibility.

Its molecular structure is shown in Figure 2, containing both rigid and flexible segments, exhibiting a combination of rigidity and flexibility! Figure 3 shows a comparison of the flexibility of the adhesive film; our PAA adhesive film has good flexibility and can be folded 180° without breaking (film thickness ~1mm), while similar foreign competitors' products exhibit brittle fracture after being folded 180°.

 

Figure 2. 3D network molecular structure of rigid-flexible binder.

 

Figure 3. Comparison of adhesive film flexibility.

 

In addition to its good flexibility, it also has the following highlights in material design:

1. Rich carboxyl/cyanide and other active functional groups (functional groups such as cyanide, carboxyl, carbon-oxygen bonds form chemical bonds with silicon anodes, and the interaction of hydrogen bonds enhances adhesion, suppresses silicon anode rebound, and ensures the stability of silicon anodes during the cycling process);

2. Combination of hard and soft monomers (reasonable selection of hard/soft monomers balances key properties such as the flexibility of the gel film, mechanical strength, ionic conductivity, and the ability to compact the electrode);

3. Amphiphilic properties (by the action of amphiphilic surfactant functional groups, the surface tension of the slurry is regulated to solve the cracking problem of thick coatings. At the same time, it ensures that the electrode has good electrolyte wettability).

Key performance demonstration

1. Electrode rebound

 

Figure 4. Electrode rebound

 

The silicon anode electrodes prepared with our PAA binder showed a 0.5% reduction in rebound compared to competitors after 48 hours of rolling, and a 2.5% improvement after full charge compared to competitors.

2. Full charge interface

 

Figure 5. Full charge interface

 

Our PAA electrodes showed no delamination, lithium plating, purple spots, or wrinkling at the full charge interface, and the overall interface was quite good.

3. Cycle & storage life

 

Figure 6. 25°C cycle and 45°C storage life

 

The cells made with our PAA showed a 30% increase in lifespan at 80% SOH during 25°C cycling; at 45°C/100% SOC high-temperature storage, the capacity retention rate improved by 4% after 180 days.

References:

[1] XIAO Z, WANG C, SONG L, et al. Research progress of nano-silicon-based materials and silicon-carbon composite anode materials for lithium-ion batteries[J]. J Solid State Electrochem, 2022, 26(5): 1125-1136.

[2] YAN Z, JIANG J, ZHANG Y, et al. Scalable and low-cost synthesis of porous silicon nanoparticles as high-performance lithium-ion battery anode[J]. MT Nano, 2022, 18: 100-175.

[3] KOMABA S, SHIMOMURA K, YABUUCHI N, et al. Study on polymer binders for high-capacity SiO negative electrode of Li-ion batteries[J]. J Phys Chem C, 2011, 115(27): 13487 - 13495.

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