Polyphenylamine-Based Cathode Material for Ultrafast Charging Lithium, Sodium and Potassium Batteries
Authors: Obrezkov Filipp A. , Shestakov Alexander F. , Traven Valerii F. , Stevenson Keith J. , Troshin Pavel A. 
Published: February 2020

Organic redox-active compounds are considered as one of the most promising alternatives to conventional inorganic electrode materials. Organic cathodes, comprising of abundant elements (C, H, N, O), can be obtained from renewable resources and also have a number of other advantages. [1] In particular, batteries based on certain groups of organic materials have higher rate capabilities due to their fast oxidation-reduction kinetics and soft amorphous structure which facilitates ion diffusion. The diversity of organic molecules allows for tailoring their structure and properties to improve specific capacity, rate capability and operation stability of batteries.
Inorganic intercalation-type compounds are usually constrained by their crystal lattice to a certain ion (e.g. Li+) with a specific ionic radius. On the contrary, the vast majority of known organic cathode materials are, in principle, electroactive with respect to a variety of metal ions since they are soft solids and have simple redox chemistry. Therefore, there is also a considerable potential in the development of potassium, sodium, magnesium, zinc and aluminium ion batteries if appropriate organic electrode materials and electrolytes are provided. This feature opens up wide opportunities for switching from scarce Li+ to some more abundant metal ions such as Na+ or K+.
The recent flow of publications highlights four promising families of organic cathode materials represented by conjugated carbonyl compounds, free radical polymers, organosulfides, and conducting polymers. [2] The nitroxyl-based radical ion polymers and other redox-active conjugated polymers demonstrated impressive performances in terms of rate capabilities due to their high porosity, insolubility in conventional electrolytes and high operation voltages, while their capacity in batteries is typically limited by ~100 mA h g-1.
Polytriphenylamine [3] and its analogues represent emerging group of redox-active polymers, which are expected to deliver superior gravimetric and volumetric capacities as well as high rate capability and good operational stability. [4]

Herein, we report the synthesis and investigation of a novel redox-active poly(N,N’-diphenyl-p-phenylenediamine) (PDPPD) polymer obtained via Buchwald-Hartwig C-N cross-coupling reaction. PDPPD has a high density of redox-active amine groups enabling the theoretical specific capacity of 209 mA h g-1, which is nearly twice higher compared to all other materials of this family reported so far. The obtained polymer was evaluated as a cathode material for dual-ion batteries and demonstrated promising operation voltages of 3.5-3.7 V and decent practical gravimetric capacities of 97, 94 and 63 mA h g-1 in lithium, sodium and potassium half-cells, respectively, while being tested at the moderate current density of 1C. A specific capacity of 84 mA h g-1 was obtained for the ultrafast lithium batteries operating at 100C (full charge and discharge takes 36 seconds only), which is, to the best of our knowledge, the highest battery capacity reported so far for such high current densities. The PDPPD//Li batteries also showed promising stability reflected in 67% capacity retention after 5000 cycles.
The obtained results prove that the designed PDPPD polymer indeed represents a highly promising organic cathode material for metal-ion batteries. Further rational design and exploration of this family of compounds might result in the development of a new generation of organic redox-active materials for advanced energy storage devices. In particular, the demonstrated in this work ultrafast batteries are of special interest in the view of multiple applications in the “electric push” devices emerging at the interface between the metal-ion batteries and supercapacitors.

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