Enhanced Supercapacitor Performance for Equal Co鈥揗n Stoichiometry in Colloidal Co3-xMnxO4 Nanoparticles, in Additive-Free Electrodes

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• 作者：Sanjaya D. Perera ; Xiaoyue Ding ; Anuj Bhargava ; Robert Hovden ; Andrew Nelson ; Lena F. Kourkoutis ; Richard D. Robinson
• 刊名：Chemistry of Materials
• 出版年：2015
• 出版时间：December 8, 2015
• 年：2015
• 卷：27
• 期：23
• 页码：7861-7873
• 全文大小：801K
• ISSN：1520-5002

文摘

Driven by the search for faster and more efficient energy storage systems, research efforts toward electrochemical capacitors, also known as supercapacitors, have dramatically increased in recent years. Transition metal oxides have been shown to exhibit both 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 trial and error. Transition metal oxides are also hampered by low electronic conductivity, requiring the use of conducting additives such as carbon black. In this work, we study the effect of cationic substitutions in Co<sub>3-xsub>Mn<sub>xsub>O<sub>4sub> nanoparticles on electrochemical Li-ion energy storage in electrodes assembled without polymeric or conducting additives. We use a hot injection synthesis to produce colloidal Co鈥揗n nanoparticles with various ratios of Co/Mn. Through electrophoretic deposition (EPD), we assemble the metal nanoparticles onto current collectors and then oxidize them, forming electrochemical capacitors without carbon additives or polymeric binders (鈥渁dditive-free鈥?nanoparticle electrodes). We find that the highest-performing Co鈥揗n mixture has a 1:1 ratio of Co to Mn and shows an energy density of 26.6 Wh/kg with a specific capacitance of 173.6 F/g. This nanoparticle electrode composition delivers a high 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 鈭?脳 and 鈭?脳 higher, respectively, than that of pure Co<sub>3sub>O<sub>4sub> electrodes. The specific capacitance for the Co鈥揗n mixture is also 鈭?脳 better than the pure Co<sub>3sub>O<sub>4sub> nanoparticle supercapacitor. This 1:1 composition exceeds the performance of the Mn-rich composition in energy density (鈭?6脳), power density (鈭?2脳), and specific capacitance (鈭?0脳). The optimum composition shows excellent stability with greater than 80% capacitance retention over 300 cycles. We attribute this peak-performance in the 1:1 equal mixture Co/Mn sample to an increased electronic conductivity for this stoichiometry and also find that this stoichiometry has both a high Co<sup>3+sup>/Co<sup>2+sup> ratio and a high Mn<sup>3+sup>/Mn<sup>2+sup> ratio, compared to other samples. This work could lead to advanced tailoring of electrochemical storage based on design principles of the interrelationship between oxidation state, stoichiometry, and redox charge storage.