Electrochemical capacitors can provide both high energy storage capacity and fast charge-discharge speed, and are thus seen as a promising technology to fill in the gaps in energy needs not covered by devices such as batteries. Among the most widely investigated materials, transition metal oxides have been shown to exhibit high specific capacitance because of Faradaic redox activity, and long cyclic life due to their robust crystal structure. Cationic substitutions in ternary oxides have shown dramatic improvements over binary analogues, but the peak stoichiometry is often arrived at by heuristic chance. To fully optimize mixed-metal oxides there is an unmet need to understand how cationic substitution affects supercapacitor performance. Transition metal oxides are also hampered by low electronic conductivity, requiring the use of conducting additives such as carbon black. Organic-solution phase colloidal nanoparticle synthesis is seen as the prized method to integrate nano-engineered materials into industry because of the ability of its ability to finely control size dispersion and to easily produce a large amount of nanocrystals through industrially-friendly, scalable solution processing. However, conventional methods for incorporating colloidal nanoparticles into electrochemical storage electrodes involves additives such as polymeric binders to create strong attachments between the particles and current collector, and conductive carbon to increase electronic conductivity. The addition of binders and conductive carbon increases the weight by ~10%‒40% and reduces the porosity of the electrode, which diminishes the ability of the electrolytes to homogeneously percolate through the network.

In this talk I will report on the effect of cationic substitutions in Co_3-xMn_xO₄nanoparticles on electrochemical Li-ion energy storage, in electrodes assembled without polymeric or conducting additives.  We employ colloidal synthesis to produce Co-Mn oxide nanoparticles with various ratios of Co/Mn. The nanoparticles are assembled on copper substrates as supercapacitor electrodes using Electrophoretic Deposition. Using this technique, we successfully fabricate electrochemical capacitors with additive-free and binder-free transition metal oxides electrodes.

 We find that the sample with a Co/Mn ratio of unity (i.e., Co_1.5Mn_1.5O₄) has an anomalous increase in performance, outperforming all other stoichiometries by up to 20× for specific capacitance. The Co_1.5Mn_1.5O₄ eletrode shows an energy density of 26.6 Wh/Kg with a specific capacitance of 173.6 F/g. This nanoparticle electrode delivers the highest power density of 3.8 kW/Kg at a 5 A/g constant current discharge. The energy density and power density delivered by the optimal mixture is ~6× and ~3× higher, respectively, than that of pure Co₃O₄ electrodes.  The specific capacitance for the Co-Mn mixture is also ~4× better than the pure Co₃O₄ supercapacitor.  The Co_1.5Mn_1.5O₄composition also exceeds the Mn-rich composition performance in energy density (~16×), power density (~12×), and specific capacitance (~20×). This composition shows excellent stability with greater than 80% capacitance retention over 300 cycles.

To understand this anomalous increase in performance we analyze the mechanisms behind the charge storage. The cyclic voltamogram and charge-discharge analyses indicate that the capacitive charge storage is the significant contribution for total charge storage for the Co_1.5Mn_1.5O₄ electrode in all sweep rates. This is in contrast to Co₃O₄ electrodes which show that a majority of charge storage is diffusion-controlled. By characterizing the impedance behavior of the samples we find that the Co_1.5Mn_1.5O₄ exhibits higher charge transfer kinetics and better capacitor performance than both the Co₃O₄ and the Mn-rich end composition. Based on these impedance results the Co_1.5Mn_1.5O₄ electrode delivers its stored energy almost two times faster than Co₃O₄, and operates in a much more capacitive mode than does the Co₃O₄sample and the Mn-rich x=2.54 sample. Finally, we measure the oxidation states through XPS for the Mn and Co 2+ and 3+ ions. We find that the top-performing sample (Co_1.5Mn_1.5O₄) has both a high Co3+/Co2+ ratio and a high Mn3+/Mn2+ ratio, compared to other samples. We posit that this composition could be displaying peak performance because it maximizes the 3+ to 2+ redox activity of the cations, and also the ratio is not too high as to overwhelm electronic conduction that relies on equal 3+ and 2+ ions on equivalent sites to mediate polaron hopping.

Our work reveals a method to test cationic substitution in transition-metal oxides for nanoparticle supercapacitors, and shows how to assemble high-performing electrochemical electrodes without the use of additives, providing a new pathway for materials research towards supercapacitors

S.D. Perera, X. Ding, A. Bhargava, R. Hovden, A. Nelson, L.F. Kourkoutis, R.D. Robinson, “Enhanced supercapacitor performance for equal Co-Mn stoichiometry in colloidal Co_3-xMn_xO₄ nanoparticles, in additive-free electrodes,” Chem. Mater. 27, 7861 (2015) 10.1021/acs.chemmater.5b02106