In-situ Electropolymerized Metalporphyrin Cathode Materials for High Performance Organic Electrodes

The interest in organic electrode materials (OEM) for rechargeable batteries has increased tremendously over the last decade.¹ This development is caused by the inherent advantages which OEMs exhibit over commercially leading inorganic materials. Specifically, the accessibility of a broad range of redox potentials due to the molecular engineering of redox-active functionalities allows boosting the storage capacity of organics. Moreover, most materials can be easily synthesized from readily available precursors. This structural diversity leads to the application of organics in different battery cell chemistries including Li-, as well as post-lithium systems like Na-, K-, and Mg- batteries. Challenging aspects of OEM materials are high dissolution in aprotic electrolytes and therefore interlinked rapid decay in capacity.² Also low conductivity of OEMs remains an unresolved issue.¹<br/>One of the most promising representatives for organic electrode materials are π-conjugated heteroatom macrocycles like porphyrins. Using bipolar-type (<i>b</i>-type) porphyrins was considered as a conventional strategy to improve the capacity and discharge voltage of OEMs when compared to <i>p</i>-and <i>n</i>-type materials, respectively.³ Reported porphyrin materials display good electrochemical performances combined with fast charging kinetics due to a small HOMO-LUMO gap.⁴ However, like other organic electrodes, porphyrins often suffer from low conductivity and, consequently, require a significant amount of electrochemically inactive conductive carbon that occupies volume and mass without storing energy. We investigated [5,10,15,25 tetrakis(4-aminophenyl)-porphyrin] (TAPP) and its metal complexes as redox-active cathode materials to address the aforementioned issues. Interestingly, the lithium-ion cells prepared with a high content of CuTAPP active material (70 wt%) demonstrate a stable discharge capacity of ~120 mAh/g over 2000 cycles when cycling with a constant current density of 1000 mA/g. The material also showed a superior rate capability of ~ 60 mAh/g at 8 A/g. The results of DFT calculations and experimental evaluations indicate that the degree of planarity of the metalloporphyrins directly correlates to their cycling stability. On the other hand, the contribution from the central metal redox during the cycling is found to be the reason for the significantly higher performance of the Cu-complex. The findings show a general approach for facing common conductivity challenges of organic electrodes and open up a pathway for practical application in energy storage.<br/><br/>1. P. Poizot, J. Gaubicher, S. Renault, L. Dubois, Y. Liang and Y. Yao, <i>Chemical Reviews</i>, 2020, <b>120</b>, 6490-6557.<br/>2. H. Nishide, <i>Green Chemistry</i>, 2022, <b>24</b>, 4650-4679.<br/>3. J. Y. Shin, T. Yamada, H. Yoshikawa, K. Awaga and H. Shinokubo, <i>Angewandte Chemie</i>, 2014, <b>126</b>, 3160-3165.<br/>4. P. Gao, Z. Chen, Z. Zhao Karger, J. E. Mueller, C. Jung, S. Klyatskaya, T. Diemant, O. Fuhr, T. Jacob and R. J. Behm, <i>Angewandte Chemie</i>, 2017, <b>129</b>, 10477-10482.