Latex Binder for Five-Volt Spinel in Li-Ion Battery

Introduction

Electrode active materials and electrolyte solutions are of essential importance to demonstrate high capacity, high voltage, and high current performances with long cycle life for high energy lithium-ion batteries.  Binder is an inactive component in composite electrode.  However, we have shed light upon a binder functionality to maximize the performances of active materials [1,2].  Fluorine-containing polymers, such as poly(vinylidene difluoride) (PVDF), have been widely used as binder since practical lithium-ion batteries with PVDF binder were commercialized in 1991.  For electrode fabrication, PVDF has to be dissolved in toxic and volatile solvents such as N-methylpyrrolidone (NMP) to prepare a slurry in which all of powder electrode components are dispersed.  From the viewpoint of environmental compatibility, water soluble/dispersible binders have attracted interest in recent years [3].  Among them, latex-based binders, such as styrene-butadiene rubber (SBR) aqueous dispersion, are practically used for graphite negative electrodes.  Recently we reported the positive effect of SBR binder for LiCoO₂ composite electrode [1].  As the oxidation resistance is required for the positive electrode binders, the oxidizable SBR-latex binder is supposed to be difficult especially for higher-voltage electrode materials.  In this study, electrode performance of high-voltage spinel, LiNi_0.5Mn_1.5O₄operating above 4.5 V, is investigated with different binders to optimize its potential performance.

Experimental

Spinel-type LiNi_0.5Mn_1.5O₄ was synthesized by a conventional solid-state reaction.  Composite positive electrodes were prepared with different three binders, PVDF, SBR-latex/CMC, and acrylic rubber (AR)-latex/CMC.  Binder solution or dispersion was prepared with NMP solvent or water for PVDF or the latex binders, respectively.  The resultant slurry with different binders was cast onto Al foil and dried at 80 degree under vacuum for 24 h, followed by roll press.  Coin-type cells with lithium metal as a counter electrode and the positive electrode were assembled in an argon-filled glove box.  A electrolyte solution of 1.0 mol dm^-3 LiPF₆dissolved in EC : DMC (1 : 1 by volume) was utilized.  Electrochemical test of Li cells was carried out at room temperature.

Results and discussion

We should pay attention to Li⁺/H⁺ion exchange of positive electrode materials in the aqueous slurry because lithium is always contained in positive electrode materials, not in negative ones.  The ion exchange will result in not only deterioration of active materials but also corrosion of Al current collector because of LiOH contamination in the slurry.  It is confirmed that such deterioration and corrosion are not evidenced apparently in our observation.

Figure 1 shows the dependency of electrode performance on three different binders.  All of the electrodes exhibit high electrochemical activity above 4.5 V which is attributed to reversible lithium extraction/insertion for LiNi_0.5Mn_1.5O₄.  The discharge capacity is increased by the AR/CMC and SBR/CMC binders compared to that with PVDF.  This is due to better dispersive condition of electrode components in slurry and electrode.  Because of electrochemical oxidation of SBR, larger irreversible capacity is definitely observed for the SBR.  As shown also in Fig. 1, the required amount of binder can be reduced for the latex binders due to better binding ability which is confirmed by preliminary test of electrode preparation.

Figure 2 compares the variation in capacity retention and charge/discharge efficiency (capacity ratio) during twenty cycles.  Gradual capacity decay of ca. 15% is observed for the PVDF electrode.  Among them, the AR/CMC electrode delivers high discharge capacities with the highest coulombic efficiencies of 98.5% at 20th cycle.  Synergy effect of oxidation-resistant AR latex and surface modification with CMC thickener is believed to be responsible for the improvement.  We will further discuss details of the impact of latex-based binders to exploit five-volt spinel electrode performance from electrochemical, microscopic, and surficial analyses.

References

[1] N. Yabuuchi, S. Komaba et al., J. Electrochem. Soc., 162, A538 (2015).

[2] Z.-J. Han, S. Komaba et al., Phys. Chem. Chem. Phys., 17, 3683 (2015).

[3] S. F. Lux et al., J. Electrochem. Soc., 157, A320 (2010). 

Fig. 1.   First charge/discharge curves of LiNi_0.5Mn_1.5O₄ electrodes with different binders.

Fig. 2.   Discharge capacity (open symbols) and coulombic efficiency (filled symbols) of LiNi_0.5Mn_1.5O₄ electrodes with AR-latex/CMC (triangle), SBR-latex/CMC (squares), and PVDF (circles) binders.