Mitigating the Impact of Thermal Binder Removal for Direct Li-Ion Battery Recycling

[NNadir]

The paper I'll discuss in this post is this one: Mitigating the Impact of Thermal Binder Removal for Direct Li-Ion Battery Recycling. (Bradley J. Ross, Michael LeResche, Donghao Liu, Jessica L. Durham, Erik U. Dahl, and Albert L. Lipson,* ACS Sustainable Chem. Eng. 2020, 8, 33, 12511–12515).

There is a widespread belief that energy storage is "green" because the stuff whose massive environmental and technical flaws it's supposed to mitigate, so called "renewable energy," is "green."

Unshakable and persistent faith in these beliefs is destroying the planetary atmosphere, since no amount of energy storage infrastructure built, nor the massive amounts of money and matter thrown at so called "renewable energy" has had any result on the acceleration of climate change or, for that matter, the acceleration of the use of dangerous fossil fuels that mostly drive climate change, or the accumulation of the dangerous fossil fuel waste, carbon dioxide, in that atmosphere which provides most of the mechanism for climate change.

That's a fact. Facts matter.

Whatever.

Happily the paper under discussion reports something that rather pleases me - since I often rail against the criminal social practices (human slavery) that drives the cobalt mining industry - that the amount of cobalt in lithium batteries is being reduced, not eliminated, but at least reduced. That's some good news. The bad news is that it makes the lithium battery industry even less economically viable than it already is - in fact it's not economically viable - meaning that the fate of lithium batteries is going to involve even larger landfills than those with which our party hardy generation has left behind, in total unbridled contempt for all future generations.

There is also widespread belief that hand waving assertions about "recycling" are something other than expressions of this same contempt: In my personal experience, the type most prone to this hand waving are generally bourgeois assholes who want to brag about their stupid solar cells on the roofs of their McMansions powering their Tesla electric car, something they do whenever they are confronted by the reality that in 25 years, all their solar cells and all their batteries are going to be one of the most intractable forms of toxic waste there is, electronic waste, this on a scale of hundreds of millions of tons.

Don't be recklessly brave and head over to the E&E forum to discuss this fact. Facts are not all that impressive there, at least among some of the clientele.

Nevertheless, facts matter. Facts matter.

Another one of the more intractable waste problems the world faces, among many, is the accumulation in the environment of compounds containing the (very strong) carbon fluorine bond. I've written a number of posts in this space about aspects of these compounds, chiefly in connection with perfluoroalkyl sulfonates, carboxylic acids and related compounds. These bonds are also of course, found in Teflon, in a material most of us have utilized in various ways - I have - and they are important components of certain classes of fuel cells, hydrogen fuel cells, where the fluoropolymer Nalfion plays an important role.

It turns out - and I didn't actually know this until yesterday - that fluoropolymers are components of lithium batteries.

That's what this paper is about: Fluoropolymers in lithium batteries.

The introductory cartoon:



From the introduction:

With the introduction of mass-produced electric vehicles, there is a growing need to find appropriate recycling methods before their batteries reach the end of life. Currently, battery recycling is not profitable and is solely reliant on the recovery of the various metals. Cobalt is the most expensive of these metals; therefore, it is the most profitable to recycle. However, cobalt is gradually being eliminated from Li-ion battery cathodes as a way to increase the nickel content, which increases the capacity of the cathode, lowers costs, and reduces reliance on sources of cobalt that utilize poor labor practices.(1?3) The reduction in cobalt content makes processes that only recover the metals less attractive. However, there is still substantial value in the cathode material if it can be recovered in an intact condition.(4) This type of recycling is termed direct recycling, but so far, there have not been any commercially successful processes for direct recycling.

Current battery recycling technology relies on methods that are similar to mining techniques. One such method is the use of pyrometallurgical processing, where the battery materials undergo carbothermal reduction after shredding. One of the byproducts of the this process is a slag, which is often discarded or used as aggregate though Li can be extracted from it.(5) The product of interest is a metal alloy that is refined using a variety of processes, such as hydrometallurgy, which yields high-purity individual metals or metal salts.(6) Another option is to utilize hydrometallurgy directly, which employs acids to dissolve the metals from the battery. These metals are then separated by various processes, and the metals or metal salts can be sold.(7?11)

A direct recycling process for Li-ion batteries requires different unit operations throughout the process to preserve the cathode materials while separating the cathode from other materials. Once the cathode is separated and removed from the aluminum foil, the conductive carbon black and adhesive binders must be removed. Typically, the binder used in lithium-ion batteries to adhere the particles to each other and to the aluminum foil is poly(vinylidene difluoride) (PVDF). Removal of PVDF is required, because relithiation and rejuvenation processes will likely require temperatures that decompose the polymeric binder, and removal allows for simple usage of recycled cathode in current manufacturing processes. Binders can be removed with the use of solvents, mechanical methods,(12) or thermal methods. There have been a number of reports utilizing thermal decomposition to remove binder from various cathode materials. These studies demonstrate the effectiveness of this method to remove PVDF and carbon black, but few report electrochemical performance data.(13?21) Lee et al. have reported the successful removal of PVDF binder from pristine LiNi0.8Mn0.1Co0.1O2 (NMC 811) with good electrochemical performance through the use of annealing at 780 °C without any additions.(14) In order to prevent deleterious effects, the removal of the PVDF and carbon black must be performed in a controlled manner that is optimized for that material. After binder removal, end of life Li-ion battery cathode materials will be partially delithiated.(22) There are a number of methods to relithiate cathode materials including solid state,(23) hydrothermal,(24) and electrochemical processes...(25)

The authors describe the goal (and presumably outcome) of their work:

...To address these challenges, we developed a thermal process that can mitigate the deleterious effects of binder removal through the addition of LiOH·H2O. A thermal process eliminates the need for a large amount of organic solvent that would be needed to adequately remove the binder. Thermogravimetric analysis with mass spectroscopy (TGA-MS) helped with elucidating when the materials are removed and the types of gases evolved during the process. These results were utilized to optimize the processing conditions and demonstrate effective binder removal. Furthermore, this process was extended to both remove binder and relithiate the cathode material in a single step. To understand why this process is effective, X-ray diffraction (XRD) and scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) were used for analysis.

Some experimental procedures used in the process:

LiNi0.333Mn0.333Co0.333O2 (NMC 111) powder was procured from Toda America Inc. In order to create a consistent delithiated cathode material for testing, chemical delithiation was utilized instead of electrochemical methods. The chemically delithiated NMC 111 was prepared by mixing the cathode in a solution of K2S2O8 at 50 °C for 15 h. The material was then washed with water and filtered followed by acetonitrile washing before drying. Inductively coupled plasma mass spectrometry analysis of the chemically delithiated material indicated that approximately 10% of the lithium was removed.

The electrode materials for binder removal were prepared using methods similar to cathode coating...

...Binder removal experiments were done using a muffle furnace (Nabertherm) with a 200 L/h flow of dry air. The 500 °C processing used a 0.5 °C/min ramp rate and immediate cooling at 2 °C/min. The 925 °C processing used a 0.5 °C/min ramp rate to 500 °C and then 2 °C/min to 925 °C for 8 h before cooling at 2 °C/min. LiOH·H2O (FMC) was ground and sieved using a 45 ?m sieve before being acoustically mixed (Resodyn LabRAM II) with the electrode material.

The heat here, of course, requires a lot of energy to recycle materials in this "energy storage" material.

A figure from this text:



The caption:

Figure 1. Thermogravimetric analysis with simultaneous mass spectroscopy of a sample consisting of 92 wt % NMC 111, 3 wt % PVDF, and 5 wt % carbon black.

Do not be deceived by the picoampere scale on the right. In a GC/MS coupled TGA (thermogravimetric analyzer) a tiny sample is thermally decomposed and then a tiny sample of the tiny sample of decomposition gases is injected into the gas chromatograph and detected by the mass spectrometer, the latter being an extremely sensitive device.

One of the decomposition products of the PVDF is unsurprisingly hydrogen fluoride gas.

In my career, when I was still in the lab, I got to work with some pretty hazardous chemicals, chromium VI, lead, mercury, phosgene, a war gas that was responsible for thousands upon thousands of death in the First World War, sodium cyanide, dimethyl sulfate, sodium metal, potassium metal, radioactive iodine, radioactive cobalt (57, not 60) and of course HF, hydrogen fluoride.

In all those years there was one, and only one, occasion where I was truly afraid for my life. Some guys who worked for me came to my office to tell me that a piece of apparatus on which I had worked had leaked pure liquid hydrogen fluoride all over the hood. When I went to look, it was a large amount, and fuming prodigiously. Since I had obviously failed to properly train and properly supervise them, given the outcome, and thus the route responsibility lay with me, I asked them to leave the lab, suited up and cleaned up the mess myself with copious amounts of calcium carbonate. I was literally shaking when I was done.

It was maybe, ten to fifty grams. Some oil refineries use this stuff on a ton scale, as a cracking catalyst. Just saying...

I'm sure that Nalfion manufacturing plants do as well...

And I guess, so do factories, making PVDF for "green" lithium batteries.

Nasty stuff, hydrogen fluoride, scary stuff, in my opinion, far worse than phosgene. (To be perfectly clear, I would recommend its use in nuclear fuel reprocessing, but that's another issue entirely.)

The authors in this paper claim that the hydrogen fluoride gas, however, reacts with the residual metals in the electrode undergoing recycling.

The decomposition products of the PVDF include CO2, H2O, and HF. The signal during the decomposition of PVDF at mass 19, indicating F, is likely HF that splits during the mass spectrometry ionization process. The carbon black then decomposed with the peak of decomposition around 500 °C as indicated by the strong release of CO2. Interestingly, at temperatures exceeding 900 °C, additional fluorine is released, which is accompanied by a mass loss. This is a strong indication that HF reacts with the cathode material. This fluorine release is likely accompanied by lithium loss and phase decomposition at high temperatures that would be expected to release anions from the structure. This is consistent with the weight loss seen at these high temperatures. With this information, we propose the following likely reactions:

So be it. Who am I to say no?

The goal is to directly recycle electrodes, and it is noted that reaction with HF can pull lithium out of the electrode, thus degrading its performance. Therefore the authors add quantities of lithium hydroxide to the reaction to prevent this from happening.

Hence the following figure:



The caption:

Figure 2. Electrochemical cycling rate performance between 2.7 and 4.3 V of NMC 111 cathode that was mixed with 3 wt % PVDF and 5 wt % carbon black, and then had these materials removed at 500 °C with or without added LiOH·H2O.

The performance of electrodes under various conditions:



The caption:

Figure 3. Electrochemical cycling rate performance of chemically delithiated NMC 111 cathodes as compared to the pristine material. The material without binder was mixed with the LiOH·H2O and annealed at 500 °C. The 500 and 925 °C were mixed with 3 wt % PVDF and 5 wt % carbon black before being mixed with LiOH·H2O and heat treated.

The caption:

Figure 4. SEM secondary electron (SE) and EDS elemental imaging of NMC 111 cathode after binder removal with 4 wt % LiOH·H2O.

A heating process to decompose the carbon black and PVDF binder can be an effective methodology for recycling Li-ion battery materials. This process benefits from the addition of a Li source to counteract the tendency of fluorine to pull out Li from the structure. The additional Li can react with the F and Mn from the NMC to form another phase. Additionally, a one-step process can be used to remove binder and relithiate the material; however, this requires higher temperatures than that in the simple binder removal. Although this process does somewhat degrade performance, it should be noted that these studies were utilizing 3 wt % PVDF, and commercial manufacturers typically use substantially less. A reduction in binder amount will lessen the detrimental impacts of binder removal. Overall, this process is a straightforward methodology to remove the PVDF binder and relithiate the cathode material and help enable direct recycling. An efficient direct recycling process has the potential to enable profitable recycling of Li-ion batteries thereby mitigating the effects of their disposal while reducing the costs of Li-ion batteries.

It's not clear that this process will make lithium battery recycling economically viable.

For all my whining about lithium batteries here and in other posts in this space, it should be said that I own a few computers with lithium batteries, so a claim to innocence would not stand scrutiny.

History will not forgive us, nor should it, and to be perfectly clear, I am part of "us."

I trust you're having a pleasant afternoon.