Inventory of solvents that wanted to replace NMP as cathode preparation material for lithium batteri

--NMP has been planned to replace, but never succeeded!

ZESHENG produces and supplies N-methyl-2-pyrrolidone (NMP) solvent, which is an organic solvent commonly used in the preparation of cathode materials for power batteries (lithium batteries) in the new energy industry. Because NMP (N-methyl-2-pyrrolidone) solvent is considered harmful to human body by EU, thus the use of it is restricted in some countries.

For this reason, the ZESHENG team has developed and produced a matching NMP recycling equipment (tailored to the needs of the battery factory). It can not only prevent NMP (N-methyl-2-pyrrolidone) waste liquid from flowing into the environment (environmental protection), but also prevent staff from accidentally coming into contact with NMP waste liquid (to protect staff health), and save unnecessary expenses for the battery factory (waste liquid can be recycled and purified for reuse).Kill three birds with one stone.

But even so, there are still many laboratories working to find organic solvents that can replace NMP (N-methyl-2-pyrrolidone). The editor today will take stock with you of the solvents that have wanted to replace NMP over the years. According to the editor's understanding, only water-based batteries are currently using water as the solvent; and the negative electrode of the battery using deionized water as the solvent. So what other solvents are trying to replace NMP's position in the jungle?

• Dimethylformamide (DMF) [content from the network, the specific source can not be verified]

The lower boiling point and higher ignition point of DMF can significantly reduce the energy consumption required for electrode drying and improve the safety of the production process.

In addition, the lower surface tension and viscosity of DMF can improve the wettability of the collector fluid and increase the concentration of solid materials in the cathode slurry.

To verify the suitability of DMF as an alternative to NMP, Rafal Sliz et al. from the University of Oulu, Finland, investigated DMF solvents using screen printing technique with NMC523 and NMC88 (NMC: LiNixMnyCozO2) as active substances.

The results show that DMF can effectively replace the NMP solvent commonly used in the NMC cathode fabrication process without affecting the battery performance.

The capacity retention of NMC523 and NMC88 cells using NMP squeegee coating and DMF screen printing was 87% and 90% after 1000 cycles.

In addition, using a low-boiling DMF solvent instead of NMP (N-methyl-2-pyrrolidone) can reduce the drying energy consumption of the cells by four times, resulting in a more environmentally friendly cell production process.

But in fact, is there a battery plant using DMF solvent instead of NMP solvent now? Not yet known, welcome comments from those in the know.

• Green solvent dihydroleuco-glucoside (Cyrene) (latest results from Nobel laureate Whittingham)

Dihydroleuco-glucoside (Cyrene) is a new bio-based solvent, derived from cellulose, first reported in 2014. It does not contain any nitrogen and sulfur heteroatoms in its molecule and is much more environmentally friendly than common polar, nonprotonic solvents.

Comparative studies have shown that the solvent properties of Cyrene are very similar to those of NMP, making it a promising solvent to replace NMP.

• Dimethyl Sulfoxide (DMSO)

Dimethyl sulfoxide (DMSO) was reported in 2020 as an alternative to NMP that could enable the preparation of cathodes by dissolving PVDF binders under low toxic conditions. However, the presence of sulfur in the DMSO structure makes it impossible to avoid potential contamination.

• Replacement of NMP (N-methyl-2-pyrrolidone) solvent with water

The replacement of NMP solvents with water not only solves the toxicity problem but also reduces the cost of cathode fabrication.

However, the aqueous pulping process also presents many challenges, which require water-soluble binders that can provide the same properties and stability as PVDF, such as high chemical and electrochemical stability, strong cohesion and adhesion, and flexibility of the coated electrodes.

Despite some reported good performance of aqueous prepared cathodes, they usually have ultra-low capacity; and once the mass loading is increased, the electrodes will crack, limiting the quest for high energy density of the batteries.

The second major problem facing aqueous slurry conditioning is that the cathode material will react with water.

Almost all current cathode materials can react with water, which is more severe for the currently popular nickel-containing transition metal oxide cathodes, and the reaction becomes more severe as the nickel content increases.

The reaction with water can lead to leaching of lithium from the active particle surface, resulting in loss of recyclable lithium, reduction of surface nickel, and even formation of rock salt layers on the surface. In addition, the reaction of NCM with water can lead to an increase in the pH of the slurry, which can corrode the aluminum collector, leading to many problems such as hydrogen release, loss of adhesion to the collector due to the formation of Al(OH)3, etc.

It has also been found that immersion of NMC in water may weaken the integrity of secondary particles, leading to inter-particle break-up into smaller agglomerates and even individual primary particles during mechanical mixing. Therefore, the aqueous sizing process is widely used for graphite anodes, but has not been adopted for cathodes.

The editor checked the relevant experiments, in fact, all of these solvents failed to completely replace NMP (N-methyl-2-pyrrolidone) in the jungle of battery cathode materials. In the next article, let's look at the experimental analysis together.