High Areal Capacity Ni-Rich NMC Water-Based Electrodes with Excellent Long-Term Cycling Stability

Abstract

Research efforts in the last three decades have resulted in a steady increase in the energy density of Li-ion batteries (LIBs), leading to their integration into electric vehicles (EVs). Therefore, car manufacturers have increased the number of EVs in their fleets and successfully introduced them to the mass market, with 200M of EVs expected to be sold by 2025 [1] . In such a fast-growing market, the cost and the environmental impact of LIB production play major roles. A crucial step in battery manufacturing is the processing of anode and cathode active materials to produce electrode coatings. While commercial anode electrodes (i.e. graphite, silicon-graphite) are already water-based, cathode electrodes (i.e. LiNixMnxCoxO2 (NMC), LiNi0.5Mn1.5O4 (LNMO) etc.) are still largely processed using organic solvents, specifically N-methyl-2-pyrrolidone (NMP), with poly(vinylidene difluoride) (PVDF) as a binder. However, the NMP-PVDF combination has many disadvantages [2]. Firstly, the binder is considerably more expensive than water-soluble binders and not easy to dispose of at the end of the battery life. Secondly, NMP is also costly and toxic, and being a volatile organic compound (VOC), needs solvent recovery at commercial scale. On the other hand, water-soluble binders such as Na-carboxymethylcellulose (Na-CMC) and styrene butadiene rubber (SBR) are not only cheaper but also easier-to-recycle [3]. Additionally, the use of water as a solvent would remove the need for solvent recovery. It is then clear that switching to water-based processing for cathode materials would decrease cost and improve the environmental sustainability of LIB production. Nevertheless, aqueous cathode processing has many challenges [2], the major one being the instability of the active material in water. The leaching of Li+ ions gives rise to very high pH (~ 12) values which, in turn, leads to the corrosion of the current collector (aluminium) and gas evolution during coating. Consequently, electrodes usually show cracks and pinhole defects and exhibit poor flexibility. These issues are even more enhanced for thick coatings, which are needed to achieve high energy density cells. Therefore, the electrochemical performance of water-based cathode electrodes is generally poorer than that for NMP-based electrodes.In this work, the effect of additions of phosphoric acid (H3PO4) on slurry rheology, adhesion strength, electrode morphology and the electrochemical performance of Ni-rich NMC cathodes prepared via water-based processing was investigated. To effectively bridge the gap between lab-scale investigations and industrially applicable procedures, pH control of the slurries and roll-to-roll electrode coating were performed at the pilot scale, and only electrodes exhibiting commercially relevant areal capacities (~ 4 mAh cm-2), high active material contents (> 90%) and low binder amounts (3 wt.%) were investigated. Phosphoric acid was added in amounts between 1.5 and 3 wt.%, and a fast-mixing procedure was developed to minimize the contact time between the NMC particles and water. The pH evolution over time showed that at least 2 wt.% of H3PO4 is required to keep the pH below the corrosion threshold of aluminium (pH < 9) for 6-8 hours, which is a usual time scale for industrial electrode processing. All slurries showed shear thinning behaviour with the viscosity of the water-based slurries being slightly lower than that for NMP-based slurries at the shear rate used for coating. Optical micrographs of coated electrodes showed no evidence of defects or agglomerates and good homogeneity. The adhesion to the current collector decreased with increasing amount of acid, e.g., electrodes prepared using 3 wt.% H3PO4 showed cracks at the edges of the coating and delamination.The electrochemical performance of the electrodes was studied in full-cell configuration vs graphite. NMC cathodes prepared with 1.5 wt.% acid showed a specific capacity of around 180 mAh g-1 at C/10 and 165 mAh g-1 at C/3. The specific capacity of the cells decreased with increasing the acid amount. However, electrodes prepared with 1.5 wt.% and 2.25 wt.% acid showed excellent cycling stability, achieving a capacity retention above 90% after 500 cycles, which is comparable to NMP-based electrodes.