Enhanced Electrochemical Performance of NCM811 Cathodes with Functionalized PVDF Graft Copolymer Binders

Abstract

Nickel-rich Li(Ni x Co y Mn1–x–y O2) (x ≥ 0.6) (NCM) cathodes are regarded as the predominant cathode materials for next-generation Li-ion batteries due to their high specific energy density. However, undesired structural disruption and thermal instability were observed with the increase of Ni content e.g. Ni0.8Co0.1Mn0.1(NCM811), which are often attributed to the transition metal (TM) ion dissolution, phase change, oxygen release and microcracks formed on the secondary particles during cycling. Therefore, the rapid capacity loss and poor capacity retention have hindered the successful practical applications for NCM811 cathodes. To solve these abovementioned problems, many strategies have been proposed. One of most effective way is to design and explore functional and novel polymer binder materials. Although poly(vinylidene fluoride) (PVDF), as the benchmark binder in battery community, has been widely used for decades, lacking of functional groups and poor chemical interactions with the active particles makes it no longer the optimal choice for more challenging cathodes.Herein, to explore the possibilities and broaden the range of functional polymers for boosting the performance of NCM811 cathodes, we propose grafting from poly(vinylidene fluoride-co-chlorotrifluoroethylene) (PVDF-CTFE) to obtain PVDF-based polymers with a comb-like architecture, where the PVDF-CTFE was chosen as a substrate backbone to provide the initiating sites for further modification. Atom transfer radical polymerization (ATRP), the controlled radical polymerization technique was used to obtain well-controlled grafted chains with low dispersity and desired length and architecture. A random copolymer of poly(ethylene glycol) methyl ether acrylate and poly(tert-butyl acrylate) (PEGMEA-co-PtBA) was first grafted from the PVDF-CTFE backbone, then after hydrolysis of PtBA to poly(acrylic acid) (PAA), the PVDF-CTFE-g-PEGMEA-co-PAA graft copolymers were obtained. Benefiting from the ethylene oxide conductive segments in PEGMEA, the Li ion diffusion and transport was significantly improved. The chelation of TM ions to -COOH groups from PAA would effectively mitigate the TM dissolution and prevent phase transition of NCM811. To test the electrochemical performances of NCM811 cathodes prepared with the designed polymer binder, PVDF and PVDF-CTFE were compared as the control groups in this study. The rate performance of the Li|NCM811 cells showed that PVDF-CTFE-g-PEGMEA-co-PAA binder gave comparable discharge capacities to PVDF and PVDF-CTFE at 0.1C-2C (Figure 1a), however, under 4C with faster charging/discharging, the discharge capacities of PVDF-CTFE-g-PEGMEA-co-PAA were much higher than the two control groups. When increasing the NCM811 weight ratio from 85 wt% to 93 wt%, where the contribution from the conductive carbon is less, the differences were even more significant (Figure 1b). PVDF-CTFE-g-PEGMEA-co-PAA delivered higher capacity at all rates, especially at 2C and 4C, suggesting faster Li ion transport and enhanced electronic properties in the cathodes. Figure 1. Rate performance of Li|NCM811 cells with binder PVDF (grey), PVDF-CTFE (orange) and PVDF-CTFE-g-PEGMEA-co-PAA (green) in the voltage range of 2.9–4.3 V using NCM811/carbon black/binder weight ratio of a) 85/10/5 and b) 93/4/3. Figure 1