Cross-Linking Organic Cathodes Enhances Stability at the Expense of Ionic Accessibility

Abstract

We have investigated the fundamental impacts of network cross-linking density on organic cathode performance. Current battery technologies rely on transition metal oxide cathodes that suffer from significant availability, cost, environmental, and humanitarian drawbacks. This reality has inspired the exploration of organic cathode materials that host high theoretical capacities, are environmentally friendly, and are tunable by molecular design. These desirable features are typically accompanied by undesirable instability during battery cycling due to solubility in electrolyte, leading to diminished capacities. Cross-linking polymer electrodes presents one strategy to address dissolution challenges. Here, we synthesized variably cross-linked naphthalene diimide (NDI)-based networks to systematically study the effect of cross-linking density on organic electrode battery performance. NDI-networks were cast as composite electrodes, manufactured into coin cells with lithium metal anodes, and evaluated by galvanostatic cycling. We observed that increased cross-linking facilitated reversible redox-access to NDI units in the network, which correlated to increased stability and capacities. Cathodes with optimized cross-linking host an initial capacity of 106 mAh g-1 and >75 % capacity retention after 100 cycles with a C/10 rate. This contrasts with uncross-linked materials, which rapidly diminished in performance due to dissolution in the electrolyte, and more densely cross-linked materials, which suffered from limited ionic accessibility. These findings demonstrate that (1) cross-linking can improve organic electrode performance and (2) there are tradeoffs between cycling stability, capacity, and overall energy storage performance with cross-linking density. Future investigations will explore how network design and processing conditions can be leveraged to co-optimize organic cathode performance across these important metrics.