Xylene‐Bridged Phosphaviologen Oligomers and Polymers as High‐Performance Electrode‐Modifiers for Li‐Ion Batteries

Abstract

of p-type or electron-donating materials, which have been well researched and include, but are not limited to, polyacetylenes, organosulfur compounds, paraquinones, tetracyanoquinodimethane, and polypyrrole composites. [ 6 ] While anode (n-type or electron-accepting) materials are far more rare in the fi eld of organic batteries, one particular class of compounds have appeared as potential candidates to develop fully organic and fl exible, lightweight batteries: compounds based on quaternized pyridine moieties, such as viologens (quaternized 4,4′-bipyridiniums). [ 7 ] Viologens are known to exhibit rapid and reversible electron transfers giving them great potential for use in a variety of applications such as, redox mediators, electrochromic devices, and now as electrode materials for organic batteries. Palmore and co-workers have recently reported a polymer anode material with pendant viologens, utilizing the fi rst reduction state of the viologen moieties for charge storage. [ 8 ] In 2011, our research group introduced a new class of viologen, the phosphaviologen, which is a phosphoryl-bridged 4,4′-bipyridine derivative with exceptionally strong electronaccepting properties. [ 9 ] Since the initial synthesis of the phosphaviologen scaffold, we have explored a variety of functionalizations at the nitrogen centers, including both benzyl [ 10 ] and aryl [ 11 ] substituents, to tune both the electronics and chromics of this species. With their enhanced electron-accepting properties (reduction thresholds 500 mV less than methylviologen for both reduction steps), we have chosen to investigate their properties as battery electrode materials.In this paper, we now report our initial contribution to the fi eld by applying our recently developed phosphaviologens P-MV and P-BnV ( Figure 1 ) as electrode materials in a hybrid organic/Li-ion battery setting. These initial studies focus on using the previously synthesized materials to test their viability as electrode materials in proof-of-concept devices via half-cell measurements versus Li-metal. While we do not provide large leaps in terms of overall capacity, our phosphaviologens (PV) utilize both redox steps in the charging/discharging cycles, allowing us to double the electron capacity per molecule compared to that of conventional viologens used as electrode materials. In an attempt to improve performance and stability, we have developed new synthetic methods to access phosphaviologen dimers, along with polymeric species that can be solution-processed as electrode materials in proof-of-concept Lithium-ion batteries are one of the most common forms of energy storage devices used in society today. Due to the inherent limitations of conventional Li-ion batteries, organic materials have surfaced as potentially suitable electrode alternatives with improved performance and sustainability. Viologens and phosphaviologens in particular, are strong electron-accepting materials with excellent kinetic properties, making them suitable candidates for battery applications. In this p...

Viologens and Their Application as Functional Materials

“…Like the parent viologen, these phosphole‐bridged viologens were predicted to be a promising anodic material due to their excellent electron‐accepting ability, kinetics and sustainability. In 2016, o ‐ and p ‐xylene‐based phosphaviologens (XMV) were investigated in a hybrid organic/Li‐ion half‐cell battery . Importantly however, and in contrast to previous viologen‐based batteries, we utilized both redox steps during charging and discharging, decreasing the mass per charge.…”

Phosphaviologen‐Based Pyrene‐Carbon Nanotube Composites for Stable Battery Electrodes

“…The electroactive phosphaviologen core was synthesized using our previously reported procedures . 1‐(Bromomethyl)pyrene was used as a quaternizing agent to synthesize pyrene‐functionalized phosphaviologen ( PY 2 PV ), and the bromide counterion was exchanged for the more electrochemically stable triflate anion by treating the product with an excess of methyl triflate (Figure ).…”

“…Like other related phosphaviologens, [33][34][35] PY 2 PV exhibits the highly reversible reductions and low-lying LUMO desired for materials in organic battery electrodes. Cyclic voltammetry on PY 2 PV shows two reversible reductions occurring at À 0.1 V and À 0.5 V vs. Ag/Ag + with the position and shape of these peaks showing little dependency on scan rate, behavior typical of fast and reversible redox kinetics.…”

“…The electroactive phosphaviologen core was synthesized using our previously reported procedures. [33][34][35] 1-(Bromomethyl)pyrene was used as a quaternizing agent to synthesize pyrenefunctionalized phosphaviologen (PY 2 PV), and the bromide counterion was exchanged for the more electrochemically stable triflate anion by treating the product with an excess of methyl triflate ( Figure 1). PY 2 PV was purified by placing the solid under vacuum remove methyl bromide and excess methyl triflate, then filtering and rinsing with acetone to yield a red solid.…”

“…Viologens and viologen derivatives have been used as electron acceptors in organic electrode materials thanks to their highly reversible reductions. [33,[36][37][38][39][40][41][42][43] Often only the first reduction is used, however, viologens can act as two-electron acceptors when both reductions are reversible. [36,38,39,42,43] Phosphaviologens exhibit the stable and reversible redox behavior of viologens, while the electron-withdrawing phosphoryl unit lowers the LUMO allowing them to operate at higher voltages vs. Li/Li + .…”