High Areal and Volumetric Capacity Li-Ion Battery Electrodes Prepared Via Particle/Polymer Electrospinning

Despite widespread commercial success, Li-ion battery technologies require significant improvements to keep pace with increasing energy demands. The scientific community has dedicated considerable time and resources to develop superior Li-ion battery materials. Nevertheless, today’s Li-ion battery performance is limited in terms of: (i) lithium capacity (ii) rate capabilities and (iii) cycle life.

Particle/polymer electrospinning is a robust platform which is utilized here to prepare high performance nanofiber electrodes for Li-ion batteries. The approach taken in the present study builds upon the recent work of Pintauro and co-workers who used particle/polymer electrospinning to fabricate hydrogen/air fuel cell electrodes [1, 2]. Potential advantages of electrospun anodes over conventional slurry cast electrodes for Li-ion batteries include: (i) a large electrode/electrolyte interfacial area for enhanced electrochemical oxidation/reduction kinetics, (ii) a controllable interfiber void volume to ensure good electrolyte infiltration into the electrode, and (iii) micron/sub-micron fibers with high nanoparticle content and short Li⁺ transport pathways in the radial fiber direction.

A primary focus of this study was to create anodes with high areal and volumetric capacities. These metrics are often under-emphasized in the battery literature, but they are of critical importance for practical battery applications. An electrode may perform well with a low areal capacity but become less useful at higher loadings due to large internal stresses and/or transport resistances (i.e., low Li⁺ and/or electronic conductivity). Furthermore, a low areal capacity increases the amount of inactive components (i.e., current collectors, separators, cell housing, etc.) required for full cell construction. New electrode architectures should be prepared with areal capacities comparable to that of commercial graphite anodes to assess their practicality for consumer devices [3]. Likewise, volumetric capacity is an important parameter when battery space is limited, such as electric vehicle applications.

Herein, we present two different anodes containing either titania or carbon as the active material with either poly(acrylic acid) or poly(vinylidene fluoride) as the binder (henceforth referred to as TiO₂/C/PAA and C/PVDF, respectively). SEM images of these fiber anodes are shown in Figures 1a and b. A typical fiber anode composition was: 40/25/35 wt% TiO₂/C/PAA or 65/35 wt% C/PVDF. The fiber diameter in the anode mats was in the range of 900 – 1,400 nm. For comparison, a C/PVDF slurry cast electrode is shown in Figure 1c.

To achieve high areal capacities, particle/polymer electrospinning was used to prepare thick TiO₂/C/PAA anodes, where multiple fiber mats were stacked to create an electrode of the desired areal loading. The areal capacity dependence on C-rate for electrospun titania-based anodes of three thicknesses (600µm – 1.4 mm) was evaluated. For the 1.4 mm anode, a remarkably high capacity of 2.5 mAh cm^-2 was achieved at 1C (vs. 3.9 mAh cm^-2 at 0.1C). These areal capacities are comparable to those of commercial graphite anodes (2.5-3.5 mAh cm^-2) [3].

As a second example of an electrospun nanofiber Li-ion battery anode, C/PVDF nanofiber electrodes were prepared with a high carbon content (65 wt%) and a high fiber volume fraction  (0.89). The 0.1C volumetric capacity of the nanofiber electrode was similar to that of a slurry cast anode of the same composition (98 vs. 110 mAh cm^-3, respectively). At a 1C charge/discharge rate, however, the volumetric capacity of the nanofiber anode exceeded that of the slurry cast anode (70 vs. 26 mAh cm^-3, respectively) due to improved Li⁺ transport rates. The excellent performance of the electrospun anodes is attributed to the interfiber voids which provide electrolyte intrusion throughout the electrode, a large electrode/electrolyte interface, and short Li⁺ transport pathways between the electrolyte and active material nanoparticles.

In this presentation, additional results will be presented on the fabrication and use of titania, carbon, and other electrospun Li-ion battery anode materials.

Acknowledgements

This work was funded in part by the National Science Foundation (NSF EPS-1004083) through the TN-SCORE program under Thrust 2.

References

1.         W. Zhang and P. N. Pintauro, ChemSusChem, 4, 1753 (2011).

2.         M. Brodt, R. Wycisk and P. N. Pintauro, Journal of The Electrochemical Society, 160, F744 (2013).

3.         E. C. Self, R. Wycisk and P. N. Pintauro, Journal of Power Sources, 282, 187 (2015).