Introducing a Bio-Degradable Binder for Aqueous Production of NMC111 Cathodes

Abstract

The LiNixMn1-x-yCoyO2 (NMC) is a widely used cathode material in lithium-ion batteries (LIBs) due to its high capacity. By enabling water-based cathode processing, the cost and environmental impact of LIBs will be reduced substantially. However, the water compatibility of Ni-containing materials has been problematic due to lithium (Li)-leaching, corrosion of the aluminium (Al) current collector, and lack of aqueous dissoluble binders. For the first time, we demonstrated that NMC111 cathodes with comparable specific capacities to the standard polyvinylidene fluoride/N-methyl-2-pyrrolidone (PVDF/NMP)-processed cathodes can be formulated in water using lignin as binder material. Rheology measurements revealed that less solvent is needed to obtain the same slurry viscosity when replacing the NMP solvent with water. Cycling voltammetry and differential scanning calorimetry revealed that lignin is electrochemically inactive between 2.5-4.5 V and thermally stable up to 152 oC, respectively. Drying the cathode coatings at 50 oC allowed for a controlled evaporation rate as surface cracks and binder migration detected using scanning electron microscopy diminished. The lignin binder provided strong cohesion forces to the carbon black (CB) and the NMC111 particles, and the use of carbon(C)-coated Al-foil (C-Al) further increased the coating's mechanical strength. Scratch tests revealed that calendaring magnified the nature of the initial mechanical strength, intensifying a poor adhesion of the coating to the Al-foil and strong cohesion between the particles. While calendering and pore removal improved the rate performance for PVDF-cathodes (85:10:5 wt % NMC:CB:binder), the rate capability of lignin-cathodes (80:11:9 wt %) decreased with lower porosity (from 53 to 0 %) and higher mass loading (9.7 mg/cm2). Particle deformation and extensive pore-blocking at high carbon contents created a Li+-transfer barrier across the cathode/electrolyte surface, decreasing the rate performance. Cathodes using a CMC/lignin-binder mix (2:7 wt% ratio) and C-Al foil (75 % capacity retention) electrochemically outperformed those with the commercial CMC/SBR mix and plain Al-foil (35 % capacity retention) at a high C-rate (5 C). Of all the aqueous produced cathodes, the uncalendered with pure lignin-binder, C-Al foil, dried at 50 oC, and a mass loading of 7.4 mg/cm2 showed the highest capacity retention at 5 C (60 %). Additionally, the lignin-cathodes have poor electrolyte wetting abilities and need longer exposure time before cycling and additional formation cycles compared to the PVDF-cathodes. Lignin-cathodes with the smallest lignin/CB matrix (90:5:5 wt% NMC:CB:binder) wetted for 35 days with 5 formation cycles at C/10 showed similar initial discharge capacity and capacity retention (154 mAh/g and 89 %) as 85:10:5 PVDF-based cathodes (153 mAh/g and 93 %) after 100 cycles at C/2.