Cross-linking Effects in Organic Electrodes

Ani N. Davis¹, Kausturi Parui², Megan M. Butala², Austin M. Evans^1,2

1George and Josephine Butler Polymer Laboratory, Department of Chemistry, University of Florida, Gainesville, FL 32611, USA
2Department of Materials Science and Engineering, University of Florida, Gainesville, FL 32611, USA

Here, we study the fundamental impacts of network cross-linking density on organic cathode performance. Current battery technologies rely on transition metal oxide cathodes. These materials suffer from significant availability, cost, environmental, and humanitarian drawbacks. This reality has inspired interest in alternative cathode materials. Redox-active organic materials are attractive as cathodes because they host high theoretical capacities (>250 mAh g^-1), are environmentally friendly, and are tunable by molecular design. Unfortunately, these desirable features are typically accompanied by instability during battery cycling because organic electrodes dissolve in their electrolyte, which leads 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. Under optimal cross-linking conditions, an initial capacity of 106 mAh g^-1 and 78% capacity retention after 70 cycles with a C/20 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 power density with cross-linking density. Future investigations will explore how network design can be leveraged to co-optimize organic cathode performance across these important metrics.