Because the thick coating is applied “thicker”, lithium ions need a longer transmission path, and the internal pores are complex and tortuous, resulting in poor high-rate cycling and rate performance of the battery. Therefore, pore formers are introduced to optimize the pores of thick electrodes.
The application of pore formers in thick electrodes of lithium batteries is a key technology to increase porosity and optimize the lithium ion transmission path. Especially in the design of thick electrodes, the reasonable distribution of porosity has an important influence on the rate performance and cycle life of the battery.
1.Common types of pore formers
- Soluble salt-pore formers: LiFSi, LiPO2F2, ammonium carbonate, etc.
- Solvent pore formers: n-methylpyrrolidone (NMP), water, ethanol, etc.
2.Usage
- Directly mix into the slurry: Such as soluble salt pore formers: LiFSi, LiPO2F2, ammonium carbonate, etc.
- Secondary spraying after coating: Such as solvent pore formers: n-methylpyrrolidone (NMP), water, ethanol, etc.
3.Working Principle
① Principle of pore-retaining position: During the preparation of the electrode slurry, the pore-forming agent is evenly dispersed in the slurry in the form of solid particles, occupying a certain spatial position. In the subsequent drying, heat treatment or solvent elution process, the pore-forming agent is removed, and the space originally occupied by it will be left to form pores.
For example, soluble salt pore-forming agents: LiFSi, LiPO2F2, ammonium carbonate, etc., are directly mixed into the electrode slurry, dried and dissolved in the electrolyte, and micropores are formed in situ.
② Qualitative adsorption filling and pore expansion principle: During the secondary spraying, the solvent-based pore former solution is atomized into tiny droplets and evenly sprayed on the electrode that has been coated and dried. The pore former solution will be preferentially adsorbed into the original pores of the electrode. In the subsequent rolling process, the surface pores of the electrode are filled with pore formers, which reduces the porosity of the surface layer, increases the hardness, and is difficult to be compressed. The electrode after rolling is then subjected to high temperature treatment, at which time the pore former will decompose.
The gas or volatile substances generated by the decomposition of the pore former escape from the electrode, leaving pores at the original position of the pore former, thereby achieving the purpose of pore formation.
Such as solvent-based pore formers such as n-methylpyrrolidone (NMP), water, ethanol, etc. After coating, spray the solvent for the second time to form a pore gradient through volatilization. For example, spray NMP on the positive electrode and water on the negative electrode to optimize the pore distribution
4.Advantages and disadvantages of pore-forming agents
Advantages:
- Reduce polarization: High porosity shortens the diffusion path of lithium ions, reduces polarization during charging and discharging, and improves rate performance.
- Extend cycle life: The pore structure alleviates electrode expansion to a certain extent, and the side is conducive to battery cell circulation.
Disadvantages:
- Pore-forming agent residue affects battery performance: Residues may form a passivation layer on the electrode surface, hindering lithium ion transmission. For example, if LiFSi and LiPO2F2 are not completely volatilized after dissolution, they may participate in the formation of SEI film and increase interfacial impedance. Some pore-forming agents (such as ammonium carbonate) may release gas when decomposed under high temperature or high pressure, causing battery expansion and even safety problems.
- Reduction in the proportion of active substances: Traditional pore-forming agents need to be directly mixed into the electrode slurry. The introduction of pore-forming agents may reduce the proportion of active substances and sacrifice the active substance loading of the electrode.
- Decreased mechanical strength and structural stability: If the pore-forming agent causes the pore unevenness to increase, it may cause cracks or fractures when the electrode is rolled. Pore-forming agent compatibility restrictions: Different electrode systems have higher requirements for the chemical stability of pore-forming agents, and customized design is required.
- Increased consistency unevenness: In the actual production process, the distribution uniformity of the pore-forming agent may be difficult to accurately control, which will lead to uneven pore structure inside the battery. The uneven pore structure will cause differences in the ion transfer rate and utilization rate of active materials inside the battery, thus affecting the consistency of the battery.
