Effect of the Interface Between Current Collector and LiNi0.5Mn0.3Co0.2O2 Composite Cathodes on the Electrode Performance

Introduction

Recently, there has been growing interest in switching fabrication of composite electrodes for lithium-ion batteries (LIBs) to aqueous systems due to advantages in low cost and environmental and safety concerns associated with N-methyl-2-pyrrolidone (NMP) handling ^1,2. Aqueous processing has been applied in manufacturing of graphite anodes, but remains challenging for the diverse array of LIB cathodes. Part of the challenges is the poor wetting of cathode slurry on current collector (Al foil), which is due to the the high surface tension of water (72.8 mN/m at 25^oC) compared to that of NMP (41.0 mN/m at 25^oC).³ In this work, current collectors with three different surface properties were applied for LiNi_0.5Mn_0,3Co_0.2O₂ (NMC532) coating. The effect of surface properties of current collectors on electrode performance was characterized by contact angle of NMC532 slurry on the current collectors, adhesion between the NMC532 coating and current collectors, and electrode performance.

Experimental

NMC532 aqueous suspension was mixed with a planetary mixer and coated by a slot-die coater. The cathode consists of 90wt% NMC532, 5 wt% Denka carbon black, 4 wt% water soluble latex binder, and 1 wt% carboxymethyl cellulose (CMC) as dispersant/binder. Al foils and carbon-coated Al foils were used as the current collector. One Al foil was treated by corona plasma for higher surface energy. The contact angles of NMC532 slurry on current collectors were measured using a goniometer. The adhesion between NMC532 cathode and current collectors was characterized by 180^o-peel test.  Electrode performance was evaluated in half coin cells. Half cells were assembled with NMC532 and Li metal as the cathodes and anodes, respectively. A Celgard 2325 separator was placed between the cathode and anode. The electrolyte was 1.2 M LiPF₆ in ethylene carbonate: diethyl carbonate (3/7 wt ratio). The cells were cycled between 2.5 and 4.2 V (VSP, BioLogic) for rate performance and cyclic performance.

Results

The contact angle of NMC532 slurry on the treated Al foil was dramatically decreased from 83.0^o to 28.5^o when the Al foil was treated by corona treatment, which was due to the increased surface energy on the treated Al foil. The adhesion between the NMC532 cathode and Al foils was also increased from 5.7 to 48.6 N/m after corona treatment on the Al foil.

Acknowledgment

This research at Oak Ridge National Laboratory (ORNL), managed by UT Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725, was sponsored by the Office of Energy Efficiency and Renewable Energy Vehicle Technologies Office (VTO) Applied Battery Research (ABR) subprogram (Program Managers: Peter Faguy and David Howell).

References 

(1) Li, J.; Daniel, C.; Wood, D. Journal of Power Sources 2011, 196, 2452.

(2) Wood Iii, D. L.; Li, J.; Daniel, C. Journal of Power Sources 2015, 275, 234.

(3) Li, J.; Rulison, C.; Kiggans, J.; Daniel, C.; Wood III, D. L. J. Electrochem. Soc. 2012, 159, A1152.