Short communicationHigh energy lithium batteries by molecular wiring and targeting approaches
Introduction
Lithium-ion batteries are the state-of-the-art power sources for portable electronic devices [1]. Lithium insertion materials are preferred over Li-metal, at the expense of energy storage capacity, for reasons of better cycle ability and safety. A notorious problem arises from the poor electronic conductivity of the currently used lithium insertion materials. Thus, the layer-structured LiM1O2 (M1 = Co, Ni, Mn, etc.), spinel-type Li2Mn2O4, Li4Ti5O12, and the olivine-type LiM2PO4 (M2 = Fe, Mn, etc.) which are typically poor conductors or large band gap semiconductors [2], alone cannot sustain the flow of electric current during the charging and discharging of the battery. Therefore, large quantities of conductive additives have to be used in practical electrode formulations to improve their conductivity, greatly reducing the energy density of the batteries.
We have recently discovered that self-assembled monolayers (SAMs) of redox active molecules on mesoscopic substrates exhibit two-dimensional conductivity if their surface coverage exceeds the percolation threshold [3], [4], [5]. Such molecular charge transport layers can be employed to electrochemically address insulating battery materials as shown in Fig. 1(a). The concept of molecular wiring of insulating battery materials has been successfully demonstrated, where the widely used olivine LiFePO4 was derivatized with a monolayer of 4-[bis(4-methoxyphenyl)amino]benzylphosphonic acid (3) [6]. At equilibrium, the electrochemical potential of the molecular hole transporter is equal to that of the olivine. During charging of the battery a positive polarization is applied to the electrode resulting in the oxidation of the molecule. This moves its redox potential above the Fermi level of the solid, thereby providing the driving force for hole injection from the oxidized molecule into the valence band of olivine. At the same time, Li+ is ejected. Conversely, during discharging the electron transfer from the current collector into the molecular charge transport film reduces its potential below the Fermi level of the olivine. As a consequence, electrons are injected from an occupied orbital of the molecule into the conduction band of the n-type FePO4, and Li+ is inserted concomitantly.
While intriguing, molecular wiring face stability risks due to possibility of desorption of the redox mediator from the insertion material during the charge and discharge cycle. In addition, the current output is limited by the rate of cross-surface charge percolation and interfacial charge transfer. In order to solve such problems, a new concept of redox targeting by freely diffusing relay molecules to address insulating or poorly conducting lithium insertion materials was proposed [7]. As indicated in Fig. 1(b), the molecular redox shuttle (S) is dissolved in the electrolyte of the cathodic compartment of the cell. During charging S is oxidized at the current collector to S+, which delivers the charge to local particles via bulk diffusion. Because the standard redox potential of S+ matches closely the Fermi level of olivine, S+ will be reduced back to S by hole injection in the LiFePO4 resulting in the oxidation of Fe(II) to Fe(III) and the release of lithium ions. By contrast, during the discharging process, S+ is reduced at current collector to S, which in turn delivers electron to FePO4. 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, enhancing greatly the power output of the battery, even though a special membrane has to be used to block the molecular shuttles from penetrating to the counter electrode compartment.
In this paper, we will present a serial of p-type redox molecules suitable for molecular wiring and redox targeting of insulating cathodic battery materials. By using different substitutes, the potentials can be fine-tuned and these molecules can be used for various cathodic materials for high energy lithium ion batteries.
Section snippets
Materials
The molecules studied in this paper have been listed in Table 1. The synthesis and characterization of molecules 1–3, 5, 6, 9, 10 will be published elsewhere, and those of molecules 4, 11–15 have been reported in literatures [3], [8], [9]. Molecules 7 and 8 were purchased from Acros and used as received.
LiFePO4 powder was synthesized by a solid state reaction and ball-milled to reduce the particle size [10]. The carbon-free electrode was prepared by mixing the LiFePO4 powder with 8 wt.% PVDF and
Electrochemical measurements
Voltammetric measurements employed a PC-controlled AutoLab® PSTA30 electrochemical workstation (Eco Chimie) with counter and reference electrodes of lithium foil. The electrolyte was 1 M LiPF6/EC + DMC (1:1, w/w).
Results and discussion
Despite of their difference in charge transport, where molecular wiring employs cross-surface charge percolation and redox targeting resorts to free diffusion of redox molecules, these two concepts are basically following the same principle of interfacial charge transfer between the redox molecule and the substrate material. In order to achieve fast charge transfer rate and consequently larger current output, these redox molecules should possess strong electronic coupling with the electrode
Conclusion
The structures, oxidation potentials and other electrochemical properties of triarylamine derivatives, phenothiazine/phenoxazine derivatives and transition metal complexes are presented in this study. All molecules show stable and reversible redox behavior upon polarization. By using proper substituents, these molecules can be used as promising mediators for redox targeting of insulating battery materials. Alternatively, these molecules can be assembled on the surface of battery materials in
Acknowledgements
We acknowledge financial support of this work by CTI project (contract No. 7136.3 EPRP-IW). We thank Mr. P. Comte for providing the mesoscopic TiO2 films.
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