Structure–property relationship in phenothiazine-based hypercrosslinked organic electrode materials through porosity adjustment

Abstract

A series of phenothiazine-based hypercrosslinked p-type porous polymers were synthesized via a knitting polymerization method, incorporating increasing amounts of benzene as a co-monomer for a systematic observation of the porosity–electrochemical performance relationship. The resulting materials, denoted as IEP-29, IEP-29-b/4, IEP-29-b/2, and IEP-29-b, correspond to PTz/benzene ratios of 1/0, 1/0.25, 1/0.5, and 1/1, respectively. The inclusion of benzene, acting as a structure directing co-monomer, significantly increased the crosslinking density and accessible surface areas, which varied from 29 m² g⁻¹ (IEP-29, no benzene) to 586 m² g⁻¹ (IEP-29-b, 1/1 ratio). However, this also resulted in reduced theoretical capacity, which decreased from 112 mA h g⁻¹ (IEP-29) to 70 mA h g⁻¹ (IEP-29-b), due to the incorporation of non-electrochemically active benzene units. Electrochemical testing in Li-cells revealed that increased crosslinking improved capacity utilization and high-rate capability, despite a moderate decline in gravimetric capacity. This study further explored the effect of increasing electrode mass loading (up to 50 mg cm⁻²) on electrochemical performance. Remarkably, IEP-29-b, the most crosslinked analogue, exhibited near-linear areal capacity scaling with minimal loss in gravimetric capacity as mass loading increased. At 50 mg cm⁻², it achieved 3.5 mA h cm⁻² along with excellent rate performance and cycling stability, retaining 71% of its capacity after 500 cycles at 2C. Importantly, the moderately crosslinked analogue (IEP-29-b/4) offered an optimal balance between specific gravimetric and areal capacities, delivering record-high values of 72.9 mA h g⁻¹ (based on the total mass of the electrode) and 3.85 mA h cm⁻², respectively – among the highest reported for p-type polymer cathodes in lithium cells. This work presents a cost-effective, scalable route to develop crosslinked, porous p-type hyperbranched polymers as high-performance cathodes with enhanced electrochemical properties at both the material and electrode levels. Therefore, this strategy paves the way for more commercially viable, high-capacity energy storage solutions.