Date Published: December 19, 2017
Publisher: John Wiley and Sons Inc.
Author(s): Zhaohui Wang, Ruijun Pan, Changqing Ruan, Kristina Edström, Maria Strømme, Leif Nyholm.
A bilayered cellulose‐based separator design is presented that can enhance the electrochemical performance of lithium‐ion batteries (LIBs) via the inclusion of a porous redox‐active layer. The proposed flexible redox‐active separator consists of a mesoporous, insulating nanocellulose fiber layer that provides the necessary insulation between the electrodes and a porous, conductive, and redox‐active polypyrrole‐nanocellulose layer. The latter layer provides mechanical support to the nanocellulose layer and adds extra capacity to the LIBs. The redox‐active separator is mechanically flexible, and no internal short circuits are observed during the operation of the LIBs, even when the redox‐active layer is in direct contact with both electrodes in a symmetric lithium–lithium cell. By replacing a conventional polyethylene separator with a redox‐active separator, the capacity of the proof‐of‐concept LIB battery containing a LiFePO4 cathode and a Li metal anode can be increased from 0.16 to 0.276 mA h due to the capacity contribution from the redox‐active separator. As the presented redox‐active separator concept can be used to increase the capacities of electrochemical energy storage systems, this approach may pave the way for new types of functional separators.
Lithium‐ion batteries (LIBs) play a major role in powering portable electronic devices and have also made their way into the electric vehicle (EV) market. However, to meet this market’s particular requirements for energy storage, the energy density, and power density of LIBs need to be further improved. This need has resulted in a search for new anode and cathode materials.1, 2, 3 The development of high capacity cathodes has received particular attention, since the capacities of the LIBs generally are limited by the capacities of the cathodes. Contemporary cathode materials (e.g., LiFePO4, LiCoO2, Li2MnO4) used in commercial LIBs thus have relatively low theoretical capacities ranging from 140 to 170 mA h g−1. While various new cathode materials and electrode‐engineering technologies have been developed to enhance the performance of the batteries,4, 5, 6, 7, 8 little effort has been made to improve the battery capacities by modifying other parts of the cells other than the electrodes.
The results have demonstrated that the present proof‐of‐concept redox‐active separator, which contains a mesoporous insulating NCF layer and a redox‐active PPy‐containing support layer, can be used to enhance the capacity of LIBs. The flexible redox‐active separator can readily be manufactured employing a straightforward paper‐making process, and the porous structure of the redox‐active separators yielded an ionic conductivity of ≈0.8 mS cm−1. The results indicate that ≈3 µm thin insulating NCF layer was sufficient to prevent short‐circuits in the LIBs and PPy was reduced into an electronically insulating form if the PPy layer encountered a lithium electrode. This finding suggests that the NCF layer can be made as thin as ≈3 µm without jeopardizing cell safety. By replacing a conventional separator with the redox‐active separator, the capacity of a proof‐of‐concept battery comprising a LiFePO4 cathode and a Li metal anode could be increased from 0.16 to 0.276 mA h. Based on the total volume (or weight) of the separator and cathode, the capacity could be increased from 18 µAh cm−3 (or 26 mA h g−1) to 67 µAh cm−3 (or 81 mA h g−1). Although the actual capacity contribution due to the redox‐active separator depends on the PPy mass loading in the redox‐active separator and that of the cathode, the results obtained with the proof‐of‐concept cells clearly indicate that the redox‐active separator concept holds considerable promise for use in high energy density thin‐film LIBs, and possibly also other electric energy storage devices.
Materials: The nanocellulose fibers were obtained from FMC Biopolymers (USA) while iron(III) chloride hexahydrate, sodium chloride, hydrochloric acid, pyrrole, and Tween‐80 were purchased from Sigma‐Aldrich and used without any further purification. The Solupor separator, LP40 (i.e., 1.0 m lithium hexafluorophosphate (LiPF6) in ethylene carbonate/diethyl carbonate (1/1, v/v), BASF), the nylon filter membrane (ø90 mm, 0.45 µm; Magna), and the Li foil (125 µm; Cyprus Foote Minerals) were purchased from the indicated suppliers and used as received. The other materials (e.g. LFP, deionized water, ethanol, Al foil, and Cu foil) were also bought from commercial manufacturers and used as received.
The authors declare no conflict of interest.