Conjugated Organophosphorus Materials As Electrodes for Organic Batteries

Abstract

The demand for environmentally benign, organic π-conjugated materials is constantly growing since they combine low cost and solution processability with light weight and mechanical flexibility. These features open up the opportunity to access new domains in contemporary organic electronics such as rechargeable organic batteries. The currently best established batteries are based on Li-ion technology and typically suffer from various disadvantages, for example (i) the use of expensive and toxic metal oxides (e.g. LiCoO2), (ii) slow kinetics of electrode reactions leading to extended charging times, and (iii) the danger of self-ignition resulting from heat generation during the Li-intercalation processes at the electrodes. To overcome these deficiencies, research is focused on the development of electrochemically active compounds for use in ‘plastic’ batteries. State-of-the-art organic electrodes feature well-known redox-active structural motifs that exhibit stable reduced or oxidized states, such as nitroxides,[1] quinones,[2] or viologens[3] (e.g., N,N’-dimethylviologen MV2+ ; cf. Figure). Recent research in the area of π-conjugated materials has shown that the incorporation of inorganic main group elements – in particular B, Si, and P – is an efficient strategy to obtain materials with intriguing properties for a host of most different practical applications.[4] Our group has of late designed new π-conjugated, ring-fused organophosphorus viologens such as the N,N’-dimethyl-2,7-diazadibenzophosphole oxide (phospha-MV2+ , cf. Figure) that show enhanced electronic characteristics for potential battery applications as compared to parent viologen MV2+ .[5] The materials’ properties of these compounds have been refined by further modifications of phospa-MV2+ in order to improve the applicability in cathodes and/or anodes. Half-cell cycling studies of fabricated electrodes against lithium metal shed light on the performance of the electrode materials with respect to their capacities under different C-rates for charging/discharging currents, cycle lives, and other performance criteria.