● Highlights of this article

This article mainly discusses the life (cycle/storage) decay mechanism of the LFP-Gr system, and the method of improving its life performance through the design of PAA anode binder.

● Background of this article

In general, energy storage batteries have higher requirements for service life. The life of new energy vehicles is generally 5-8 years, while the life of energy storage projects is generally expected to be greater than 10 years. The number of cycles of the power battery is 1000-2000, and the number of energy storage batteries is generally required to be greater than 5000 times. Improving the life of lithium iron phosphate energy storage battery is of great significance to its application and promotion in the energy storage market.

attenuation mechanism of LFP-Gr system

Analysis of cycle life decay mechanism based on lithium iron phosphate battery. In the early stage of the cycle, the structure of graphite negative electrode and LFP positive electrode is relatively stable, and the main attenuation comes from the consumption of active lithium in the negative electrode. In addition, during the cycle, iron dissolution will occur in LFP positive electrode (iron deposit catalyzes the decomposition of electrolyte to thicken SEI film on the one hand, and on the other hand hinders the deintercalation of lithium ions, thus increasing the charge migration resistance and eventually leading to the attenuation of battery capacity), due to the deterioration of graphite expansion and the increase of expansion force, the porosity of the positive and negative pole pieces decreases rapidly, the electrolyte cannot infiltrate the inside of the pole piece, and the pole piece will gradually lose its activity, resulting in the rapid decay of cycle performance.

From the analysis of the attenuation mechanism, the main methods to improve the life of lithium iron phosphate batteries are:Reduce the loss of active lithium; inhibit the dissolution and deposition of Fe; inhibit the expansion of negative graphite.

Performance advantage of PAA in LFP system

PAA has been widely studied as a binder for lithium batteries. From the investigation of distribution morphology, the traditional negative electrode binder CMC SBR,PVDF system, the binder is distributed on the surface of the electrode material in the form of small particles. The new negative electrode binder PAA is an amorphous polymer (dispersion distribution), which can better cover the active material and form a buffer layer similar to SEI film. This buffer layer can not only inhibit the irreversible side reaction of electrolyte on the surface of graphite, moreover, the erosion of the graphite gap by the electrolyte component PC can be inhibited.

Therefore, PAA can better inhibit the loss of active lithium and improve the expansion of graphite. In addition, studies have shown that the buffer layer can effectively reduce the contact between iron ions in the electrolyte and the graphite electrode, thereby reducing the deposition of metal iron and improving the cycle performance of the battery. PAA has a large number of carboxyl groups and high cross-linking state, which has better cohesion and tensile strength, so it can better inhibit the expansion of graphite and maintain the integrity of the electrode structure.

From the above point of view,PAA has high application value in improving the life of energy storage batteries.

PAA binder problem point

The molecular structure of PAA has a large number of carboxyl and hydroxyl functional groups, which makes PAA very easy to absorb water. At the same time, the strong polar functional groups such as carboxyl group and hydroxyl group generate too much hydrogen bond and interaction, which will hinder the free rotation of the molecular chain, so that the flexibility of the adhesive is poor. The main problems in the preparation of lithium batteries are:The pole piece is brittle, the coating is easy to crack, and the pole piece is easy to have film release and powder drop in the production process. The above problems need to be optimized from the PAA product itself or the negative electrode formula.

negative electrode formula design

1. Use with traditional SBR: PAA has the problem of greater brittleness and can be considered to be used in conjunction with other binders with better toughness. SBR is currently a mature commercial binder product with better toughness and adhesion. The combination of PAA SBR can give full play to the advantages of PAA while improving the problem of greater brittleness.

2. Cancel or reduce the amount of CMC: PAA has a large number of carboxyl groups, and the molecular structure is linear long chain, which has good dispersion of graphite active materials. After the introduction of PAA into the negative electrode formula, the amount of CMC can be appropriately reduced, or CMC can be completely canceled.

3. PAA dosage design: After the amount of PAA increases, the brittleness of the pole piece increases, and the amount of PAA needs to be properly controlled. If the amount of PAA is designed to be high, it is necessary to choose PAA products with better flexibility after modification to reduce the risk of large brittleness of pole pieces. PAA modification, generally by introducing a suitable elastic chain structure or by means of substitution, free radical polymerization and other methods to adjust the distribution of carboxyl groups, in order to reduce the number of PAA inter-chain hydrogen bonds. At the same time, PAA itself can be moderately cross-linked with other polymers to enhance the capacity of the binder to withstand the volume change of the active material during charge and discharge.

It is reported that the flexibility of the modified PAA product can be significantly improved, it can completely replace SBR, the amount of PAA added can be> 2.5%, and the slurry and pole pieces can meet the processing performance requirements of lithium batteries.

References:

[1] Liu Menru, Ye Chengxi, Peng Libo, et al. Research progress on electrochemical properties of PAA binders in lithium batteries [J]. Materials Engineering, 2021, 049(002):21-31.DOI:10.11868/j.issn.1001-4381.2020.000532.