(100d) Facile Remanufacturing and Re-Lithiation for Spent Lithium-Ion Battery Electrodes

As the demand for Li-Ion batteries has increased, so have concerns about supply and environmental impact. From 2019 to 2020, global lithium demand increased by 18% to 56,000 tons. At that rate, a shortfall of 50 million tons would occur by 2050 based on proven global lithium reserves [1]. Additionally, the primary disposal method of spent batteries are landfills which could contaminate the environment with toxic heavy metals and organic electrolytes. To that end, we present a facile approach to remanufacturing applying Direct Recovery principles. Our approach maintains the underlying structure of old NMC-622 cathodes and combines it with fresh cathode slurry to fix the damage. A plasma treatment and washing process is used to remove degraded material before a fresh coat of slurry is applied. This allows us to restore performance to heavily degraded cells. To this end, we investigated different plasma treatments, treatment step order, slurry types and pressing to optimize our process. We determined that normal NMC-622 slurry with plasma treatment and pressing produced the best results in terms of Cell Viability (>80%) and Capacity Performance restoration on highly degraded cells. Furthermore, we also applied this approach to treating anode material where similar performance restoration of about 50% of pristine cells was all achieved.

The direct recovery remanufacturing approach has evolved and undergone optimization for NMC-622 cathodes and MCMB anodes, but basics were maintained. The general process is initial cell characterization where a baseline capacity is determined; disassembly where the constituent components (casing, electrodes, separator, spacer and spring) are separated; plasma treatment and washing where degraded material and electrolyte is removed; slurry treatment where fresh slurry fills in cracks and holes; a pressing process to make sure the new and old material has a strong connection before reassembly into Li half cells; Re-lithiation which reinserts Li ions into the treated crystalline structure of NMC622; and finally for cycle testing to determine performance.

The characterization process is critical because the old cells used for this process have been lying around the lab for indeterminate amounts of time. Determining the capacity performance of these cells allows for the establishment of two benchmarks, one based on the previous performance measured capacity, and one based on the practical capacity performance. The practical capacity for the NMC cells is 160 mAh/g and the initial mass of the NMC cathodes is assumed to be 10 mg (active material 8.55 mg). The previous performance benchmark is set by cycling the cells between their voltage limits of 2.8 V to 4.2 V for NMC for 5 charge and discharge cycles at a given C-rate, typically 0.1C. This characterization allows us to categorize cells broadly as Good (above 70% practical capacity), Poor (anything below 70% but above 0), and Zero (cells are flatlined or trapped because they never reach 4.2V).

The separation process is a delicate situation where the cell is opened using a crimping device. This allows for access to the electrode where the difficult process of removing the electrode occurs which can be difficult due to age and degradation. For the LMO half cells, the separator is stuck to the LMO and must be carefully cut and peeled away from each other without damaging the cathode. This process creates another level of classification based on how much damage the cathode underwent during the separation process. Currently referred to as Minimal, Moderate and Significant though this relies on the eye test instead of strict classification.

Plasma treatment should allow for the filling in of cracks to the electrode surface with fresh material. Initially work was attempted with PlasmaTech Plasma Wand where treatment occurred in the air. However, the power and plasma application could not be effectively controlled.

Therefore, a switch occurred to fully self-contained PlasmaTech PE200 Plasma Processing Machine at 100 W for 1 min in an Oxygen environment. First, the switch occurred because the plasma wand must be raster, so the plasma application is highly variable. Second, while the wand has a maximum output of 30 W that power could not be guaranteed over the course of the entire process or from cathode to cathode as each cell must be treated individually. Third, unwanted side reactions could occur due to air exposure instead of pure Oxygen. This plasma treatment was shown to have no impact on pristine cathodes under Oxygen or Argon environments. Thus, it can be assumed that Oxygen catalyzes reactions that remove only degraded materials from the surface.

After plasma treatment, the electrodes are washed with DMC before the drying process occurs at 80oC in a Vacuum Oven, overnight. This process should remove the LiPF6 electrolyte, Li salt and side reaction material from the surface creating a clean surface to apply fresh slurry. DMC washing of pristine cathodes showed no loss of mass after even five minutes of washing. Drying for this was done on a hot plate to speed up the process.

Surface repair is the application of slurry to fill in any cracks or holes on the surface of the cathode after plasma treatment. The slurry types considered are normal slurry (NMC622, Carbon Black, PVDF, NMP) and salted slurry (NMC622, Carbon Black, PVDF, NMP and LiTFSi) where the salt amount is 10% wt of the NMC6222. The Anode slurry consists of MCMB, Carbon Black, PVDF and NMP. The slurry is carefully applied with a thin tipped paint brush. Cathodes are placed into the vacuum oven and heated at 120oC for 16 hours.

The pressing stage applies 1 MPa of pressure to the electrodes for 30 mins at room temperature. This pressure can be maintained for at least one hour without damaging the surface of the cathodes for pristine cells. However, raising the pressure to 2 MPa results in damage to pristine cells. Additionally, diminishing returns mean that 30 mins is optimal for compressing the fresh slurry material onto the old material. Finally, cycle testing can occur for these cells.