Mechanism of Charge/Discharge of Poly(vinylphenothiazine)-Based Li–Organic Batteries

Abstract

Organic electrode materials are among the promising next generation compounds for battery energy storage as a greener and cheaper alternative to transition-metal-based electrodes. A prominent class among them are redox polymers, which can reversibly store energy and can be capable of fast redox processes. Nevertheless, drawbacks are their often low specific energy and lifetime. A main challenge is their solubility in battery electrolytes, which is detrimental to cell performance. Herein, we discuss the solubility properties of a polyvinyl-based redox polymer with a methylphenothiazine side group (PVMPT) in organic-solvent-based battery electrolytes and generate new insights into the mechanism of the redeposition process of dissolved active material. We addressed the mechanistic studies of this “polymer–electrolyte cross-talk” with microscopic and spectroscopic methods. These findings are important for the molecular design of new organic electrode materials, since the redeposited polymer showed improved cycling performance and outstanding cycling stabilities. We herein aim to draw a bigger picture of the solubility of redox polymers and its consequences and motivate the scientific community to reconsider the common conception of the deteriorating nature of the solubility of organic battery electrode materials.

Unlocking Full Discharge Capacities of Poly(vinylphenothiazine) as Battery Cathode Material by Decreasing Polymer Mobility Through Cross‐Linking

“…This stands in contrast to the PVMPT-based cells previously investigated [29] and confirms the different mechanism at work here. While PVMPT dissolved in the electrolyte in its charged state and was redeposited onto the electrode upon discharging, allowing for significant rearrangement processes of the redox-active side groups and formation of π-π interactions, as shown in Figure 1, [29] cross-linked X-PVMPT remained embedded in the composite electrode during charge and discharge.…”

“…[24,29] This had been due to the formation of π-π interactions between redox-active phenothiazine groups in PVMPT, which were present to a lesser extent in X-PVMPT. Composite electrodes based on cross-linked X-PVMPT showed an enhanced specific capacity of up to 112 mAh g −1 , high cycling stability, and high rate capability.…”

“…[25][26][27][28] These interactions, however, also led to a reduction of the accessible specific capacity to half of the theoretical value, namely 56 mAh g −1 . [29] Upon reduction to oxidation state B, (re)deposition on the surface and inside the composite electrode occurred. In a previous study, we showed that this rearrangement was facilitated by the dissolution of PVMPT in oxidation state C in the battery electrolyte.…”

Natural Compounds Gallic Acid Derivatives for Long‐Life Li/Na Organic Batteries

“…As so far, various organic materials such as free radical‐, quinine carbonyl‐ (C=O), organosulfur‐, imine‐ (C=N) and azo‐ (N=N) compounds have been applied in LIBs and SIBs . The double bonds are reduced by Li and Na in wide potential window, enabling them working as the anode and cathode materials in LIBs and SIBs.…”

“…[16] As so far, various organic materials such as free radical-, quinine carbonyl-(C=O), organosulfur-, imine-(C=N) and azo-(N=N) compounds have been applied in LIBs and SIBs. [17][18][19][20][21][22][23][24][25][26][27][28][29] The double bonds are reduced by Li and Na in wide potential window, enabling them working as the anode and cathode materials in LIBs and SIBs. The carboxylic lithium and sodium salt groups are normally introduced to organic compounds to overcome their dissolutions in the electrolyte and promote the high-rate capability and cycle life.…”