Thianthrene-Functionalized Polymers As High-Voltage Materials for Organic Cathode-Based Dual-Ion Batteries
Authors: Kolek Martin , Speer Martin E. , Jassoy Jean Jacques , Heine Jennifer , Winter Martin , Bieker Peter , Esser Birgit 
Published: February 2020

Organic materials are well known as electrode-active materials in rechargeable batteries.[
1
] Since the early 80s, conductive polymers have been utilized as electrodes in energy storage devices while undergoing doping/undoping processes.[
2
,
3
] Additionally, the substitution of polymers with functional groups or specialized molecules (localized redox centers), so called redox polymers, has been investigated.[
4
] One of the most studied functionalized polymers is poly (2,2,6,6-tetramethylpiperidinyloxy-4-yl methacrylate) (PTMA).[
5
] This TEMPO-radical-based poly­methacrylate network grants high rate capabilities, while providing a stable capacity of around 110 mAh g-1 with an oxidation potential of 3.6 V vs. Li/Li+.[
6
,
7
]

Nevertheless, such redox polymer-based systems still suffer from low specific capacities (~ 120 mAh g-1) compared to commercially available cathode battery materials (e.g. LiCoO2: 140 mAh g-1).[
1
,
8
] In order to optimize the electrochemical performance of redox polymers in terms of power and energy density, we have been investigating substituents for polymeric backbones with high working potentials, focussing on thianthrene as redox-active functional group. The interest on thianthrene is related to the high redox potential range of 4.01 – 4.10 V vs. Li/Li+ and the ability to form stabilized radical cations during oxidation.[
9
] Free thianthrene molecules have already been investigated as an overcharge protection additive in Lithium-ion batteries, but not as redox-active cathode materials for dual-ion batteries.[
10
,
11
] The general working principle of a thianthrene-based dual-ion battery is shown Figure 1. During charging (oxidation of thianthrene), anions are inserted into the polymer structure and the cations are intercalated into graphite, a so called dual-ion insertion process.[
11
]

In this work, the addition of thianthrene to a polymeric backbone and the study of the applicability as organic cathode material are presented. The resulting thianthrene-functionalized polymers exhibit oxidation potentials of above 4.10 V vs. Li/Li+ (Figure 2). This oxidation potential is, to the best of our knowledge, one of the highest published values for organic-based electrode materials.[
1
,
3
]

The synthesis of three different thianthrene-functionalized polymers with a polynorbornene backbone via ring-opening metathesis polymerization (ROMP) technique as well as their characterization using NMR spectroscopy will be shown. In addition, electrochemical investigations, as cyclic voltammetry (CV) and constant current cycling (CCC), are performed. CV results exhibit stable oxidation and reduction potentials of the polymers in solution and also when used as composite electrode. We could achieve capacities (related to anion insertion) up to 66 mAh g-1 depending on the electrolyte formulation. Combined with the high redox potential of the synthesized materials, a promising new class of functionalized polymers for application as organic cathode materials in rechargeable batteries is proposed.

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Figure 1