Polymer wiring of insulating electrode materials: An approach to improve energy density of lithium-ion batteries

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Abstract

The poor electronic conductivity of LiFePO4 has been one of the major issues impeding it from achieving high power and energy density lithium-ion batteries. In this communication, a novel polymer-wiring concept was proposed to improve the conduction of the insulating electrode material. By using a polymer with tethered “swing” redox active molecules (S) attached on a polymer chain, as the standard redox potential of S matches closely the Fermi level of LiFePO4, electronic communication between the redox molecule and LiFePO4 is established. Upon charging, S is oxidized at the current collector to S+, which then delivers the charge (holes) to the LiFePO4 particles by intermolecular hopping assisted by a “swing” – type motion of the shuttle molecule. And Li+ is extracted. Upon discharging, the above process is just reversed. Preliminary studies with redox polymer consisting of poly (4-vinylpyridine) and phenoxazine moiety tethered with a C12 alkyl chain have shown promising result with carbon-free LiFePO4, where effective electron exchange between the shuttle molecule and LiFePO4 has been observed. In addition, as the redox polymer itself could act as binder, we anticipate that the polymer-wiring concept would provide a viable approach to conducting-additive and binder free electrode for high energy density batteries.

Introduction

As one of the state-of-the-art power sources, lithium-ion batteries have been widely investigated in recent years, due to the increasing demands in portable electronic market and the emerging application in electric vehicles. For pursuing high energy and high power densities, numerous efforts have been taken to increase the capacity and rate capability by using advanced device configuration or electrode materials [1]. For instance, the amounts of binder and conducting additives have been rigorously optimized in the electrode sheet to maximize the fraction of active material. However, for olivine-type materials LiMPO4 (Mdouble bondFe, Mn, etc.), which are electronically insulating wide bandgap semiconductors [2], a large amount of conducting additives has to be added to secure acceptable electrochemical performance greatly reducing the energy density of batteries. The situation is even worse if nanostructured materials are used, since more conducting additives are needed to form a continuous conducting pathway of electrons. Meanwhile, more binder has also to be added due to the higher surface area of the material itself and any additives, further reducing the energy density of batteries.

There have been many approaches employed to address the above challenges [3], [4], [5], [6]. Recently, we reported two new strategies of molecular wiring [7], [8], [9] and redox targeting [10] to improve the electrochemical performance of carbon-free LiFePO4, whose electronic conductivity is ∼10−9 S/cm3. Molecular wiring of battery materials is based on our recent discovery of cross-surface charge transfer in self-assembled molecular charge transport layers on mesoscopic oxide films [11], [12], [13]. Such molecular charge transport layers can be employed to electrochemically address insulating battery materials. While intriguing, molecular wiring is constrained by the risk of desorption of the surface adsorbed redox mediator, and the low current output limited by the rates of cross-surface charge percolation and interfacial charge transfer. Redox targeting with free relay molecules dissolved in the electrolyte was then introduced to address these issues. The advantage of using a freely diffusing redox shuttle over molecular wiring is that it allows charge transport to proceed at a much faster rate, thus enhancing greatly the power output of the battery. However, the challenge in this case is that a special separator has to be used to prevent the redox shuttle molecules from reaching the counter electrode, which leads to self-discharge.

Here we introduce the concept of polymer wiring using tethered “swing” redox active molecules (S) attached on a polymer chain to electrochemically address the insulating LiFePO4. The principle is illustrated in Fig. 1. During charging S is oxidized at the current collector to S+, which then delivers the charge to the LiFePO4 particles by intermolecular hopping assisted by a “swing” – type motion of the shuttle. If the standard redox potential of S+ matches closely the Fermi level of LiFePO4, S+ will be reduced back to S by hole injection in the LiFePO4 particles, thus resulting in the oxidation of FeII to FeIII and the release of lithium ions, closely resembling molecular wiring and redox targeting. The advantage of resorting to the flexible long alkyl chain is that the redox species are attached via the polymer to the electrode material while at the same time being able to undergo a swing type translational motion assisting the charge exchange between neighboring molecules. The charge transport through mobile “swing” redox molecules combines therefore electron hopping and physical displacement of the redox shuttle. Applying a bounded diffusion model [14], the apparent diffusion coefficient becomesDapp=kexcr6(δ2+3λ2)where kex is the bimolecular rate constant for electron self-exchange, cr is the total concentration of redox species, δ is the characteristic charge hopping distance and λ is the distance across which the tethered redox center can move. Apparently Dapp is scalable to the free molecule diffusion rate, if cr and length of the tethers (or λ) become large enough. In addition to the redox center forming a continuous pathway for charge conduction, the polymer chain itself also provides a network binding the electrode material particles within the film.

Section snippets

Synthesis of the redox polymer

A. Synthesis of 10-(12-bromododecyl) phenoxazine: Sodium hydride (55% dispersion in mineral oil; 119 mg, 4.97 mmol) was stirred in dry THF under argon atmosphere. Phenoxazine (500 mg, 2.73 mmol) was added to a stirred suspension of the sodium hydride in THF. The mixture was stirred to form phenoxazine N-sodium salt for 2 h at 50 °C. 1,12-dibromododecane (8962 mg, 27.3 mmol) was added to the solution and stirred vigorously for 24 h at room temperature. The mixture was filtered and evaporated under

Results and discussion

The redox polymer used in this study consisted of poly (4-vinylpyridine) and phenoxazine moiety tethered with a C12 alkyl chain (PVP-DD-PXZ). Upon mixing with N-methyl pyrrolidone, it forms a homogeneous transparent film on the conducting substrate. From the cyclic voltammograms (CV) shown in Fig. 2a, the PXZ moiety presents the same oxidation potential as its free molecular counterpart, being ∼3.48 V (vs. Li+/Li) [9] which matches closely the potential of LiFePO4. Meanwhile, the colorless

Conclusion

In summary, a novel polymer-wiring concept is introduced to electrochemically address insulating battery materials. It provides an all-in-one solution to the problems encountered by molecular wiring and redox targeting. From the very preliminary study, the polymer-wiring approach has shown promising result. It is expected that, with engineered polymer backbone and swing redox centers, this strategy will pave the way to conducting-additive and binder free electrode for high energy density

Acknowledgement

We acknowledge financial support of this work by a CTI project (contract No. 7136.3 EPRP-IW). QW thanks Faculty of Engineering, National University of Singapore for a start-up Grant (No. R-284-000-064-133) support.

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