Annotation

The project aims at solving the problem of unsatisfactory performance of the existing organic cathode materials for electrochemical power sources. The main task of the project is development of novel organic cathode materials for batteries and hybrid supercapacitors, possessing high electron conductivity and redox-capacitance, that should afford high specific capacitance and power, as well as fundamental studies on the charge transport in these materials.

The design concept of such materials is based on the previously obtained results of the synthesis of two materials of different nature, based on common structural motifs: polymeric nickel complex with salen-type ligand as a conductive backbone and groups containing free nitroxyl radicals for an additional redox capacity. Both types of active centers may function in the prototypes of electrochemical accumulators at high discharge rates [Macromolecular Chemistry and Physics 2017, 218, (24), 10.1002/macp.201700361; Electrochimica Acta 2019, 295, 1075-1084, 10.1016/j.electacta.2018.11.149]. Our novel material demonstrates the best performance among the redox-conductive polymers based on the nitroxyl radicals, able of functioning without the use of conductive additives [Electrochimica Acta 2019, 295, 1075-1084, 10.1016/j.electacta.2018.11.149; Batteries&Supercaps 2020, accepted, 10.1002/batt.202000220; Patent application RU2018143206], demonstrating the best performance in the class of redox-conducting polymers based on nitroxyl radicals.

The authors of the project are the only scientific group that has mastered the synthesis of TEMPO-salen polymers. However, the specific capacity of the materials obtained during the previous studies has not yet reached the theoretical limit characteristic of TEMPO-containing polymers (ca. 110 mAh/g). In addition, the fundamental aspects of the charge collection and transport in such structures are still undiscovered, which complicates the rational design of high-performance materials based thereof. Therefore, three new tasks were formulated, the solution of which will significantly improve the characteristics of batteries with organic cathodes.

The first task is devoted to the fundamental aspects of the action of the proposed materials as cathode materials. This task includes the following subtasks:

1. Examination of the intraunit charge transfer between the Ni-salen fragment and the attached nitroxyl fragments
2. Examination the role of the spin interaction between the nitroxyl fragments and the possibility for the interchain electron transfer involving this fragments
3. Determination of the charge transport parameters along the main chain of Ni-salen polymers

The second task of the project is focused mainly on the optimization of the molecular structure of proposed materials to achieve its highest energy density. One of the possibilities to increase further the specific capacity of such electrode materials is the reduction of the molecular weight of the linker. Another way to increase the capacity of the polymer is to increase the number of TEMPO groups per polymer unit.

The third task of the project is to increase the practical load of the proposed polymeric materials maintaining high conductivity and power density. For this purpose, composites of the novel redox-conductive carbon materials such as carbon nanotubes and graphene will be used.

Expected results

The project will provide the deep insight into the fundamentals of the mechanism of action, as well as practical evaluation of the materials based on the conductive polymers with appended nitroxyl fragments.

The synthetic part of the project will result in synthesis of a series of new polymer materials consisting of polymerized Ni-salen complexes bearing at least two redox-active TEMPO groups per one monomer unit. These new electrode materials will combine high electron conductivity (provided by the polymer Ni-salen complexes) with high redox capacitance (provided by the TEMPO groups). The anticipated results are expected to be at the cutting edge of worldwide science as can be evidenced by the high attention shown to this topic in high-impact journals (e.g. Chem. Mater. 2018, 30, 5169−5174, 10.1021/acs.chemmater.8b0177; Angew. Chem. Int. Ed. 2017, 56, 9856 –9859, 10.1002/anie.201705204, ACS Macro Lett. 2016, 5, 59−64, 10.1021/acsmacrolett.5b00811 etc.). In this context, it should be mentioned that the combination of Ni-salen conducting matrix with the redox-active TEMPO pendants was proposed for the first time in the pioneering works of the applicants of this project and, as it was shown during the preceding RSCF project (16-13-00038), it afforded materials surpassing the analogues by the exhibited charge/discharge rate and practical capacitance.

Taking into account the results of the project 16-13-00038, we suppose that the energy storage performance of the synthesized new materials will be sufficiently competitive to justify their practical applications in different fields of science and engineering. In particular, the remarkable feature of these materials should be the high range of operational temperature (including temperatures of -40°C and below). The development of the synthetic procedures proposed at the previous stage of the project should increase the specific capacitance of the materials to the practically acceptable value of 100 mAh/g and afford the scalability of the preparative techniques.

Investigation of electrochemical and spectroelectrochemical properties of the obtained materials will provide the way to rational design of redox-conductive polymers. The expected results of this part of the project will include the choice of the optimal linker configuration for efficient participation of both the pendant groups and conductive backbone in charge-discharge processes; estimation of maximal TEMPO-group load per repeat unit of the conducting polymer and influence of the structural factors on the charge transfer rate and redox capacity.

Collaboration with German partners will make possible in-deep understanding the charge transfer mechanism in the redox-conducting polymers due to the unique in situ EPR facilities, available at Free University of Berlin. This fundamental research, in combination with outstanding practical benefits of new material (flexibility, low weight, ease of treatment and low price of the polymer materials), ensures the high level of the expected results of the project.

The project at RSCF website Prof. Dr. Jan Behrends group

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