Controllable Synthesis of CuO Anodes Via a Green Method for Lithium-Ion Batteries

Transition metal oxides (M_xO_y, M = Fe, Co, Ni, Mn, Cu, etc.), are promising anode materials for lithium-ion batteries (LIBs). These materials can react with lithium based on the reduction/oxidation reaction mechanism of M_xO_y + 2y Li = y Li₂O + x M, which exhibits large rechargeable capacities.^1-3 Among these transition metal oxides, copper oxide (CuO) is of particular interest for its inexpensiveness, good safety, large theoretical capacity (670 mAh g^-1.), and low toxicity.^4,5 However, CuO is a semiconductor with low electrical conductivity, which is unfavorable for charge transfer. In addition, CuO based electrodes suffer from significant volume expansion and dispersion of Cu particles in the Li₂O matrix during discharge-charge processes, which leads to severe mechanical strain and rapid capacity decay.

Improved electrochemical performance of CuO based anodes is required to meet the needs of next-generation LIBs. Tremendous efforts have been devoted to improving the performance of CuO based electrodes through nanostructured design, morphology control techniques, and hybridization of composite materials, resulting in materials with encouraging electrochemical properties and unique structure. For instance, CuO electrodes containing urchin-like particles were reported to deliver an electrochemical capacity above 560 mAh g^-1 after 50 cycles at a 150 mAg^-1 rate. 3D dendrite-shaped CuO hollow micro/nanostructures show enhanced performance. A type of hierarchical micro/nanostructured electrode was developed by fabricating Cu(OH)₂ using a facile NH₃ corrosion approach, yielding a structure with excellent cycling stability of 651.6 mAh g^-1 after 100 cycles at a 0.5 C rate, and a high-rate capability of 561.6 mAh g^-1at a 10 C rate.

All these techniques have been demonstrated as promising ways to fabricate CuO anode materials. However, the requirements of toxic raw materials, the expense of removing the template, and contamination by byproducts have limited large-scale applications. Thus, new synthetic strategies for fabricating CuO, especially viasimple and environment-friendly processes without surfactants or templates, are highly desired. The use of non-toxic chemical agents as templates, chelating agents, or reactants represents a possible pathway to greener and lower-cost synthesis of CuO anodes. Examples of such chemicals include malic acid, sodium citrate, and urea. Malic acid is a green organic material that is often used as an additive agent in foods and pharmaceuticals. Malic acid is also used as a chelating agent in electroplating and electroless plating due to its high ability for complexation of copper ions in solution. Sodium citrate is another example of a low-toxicity material that is commonly used as a chelator as well as a food additive. As a normal waste product of human metabolic processes with low toxicity, urea is one of the simplest organic compounds, which is employed as a ligand and ammonia source during the hydrothermal synthesis process of metal oxides.

In this work, leaf-like CuO, oatmeal-like CuO, and hollow-spherical CuO nanostructures with controllable morphologies were developed via a simple, green, and cost-effective method suitable for large-scale synthesis. Malic acid, sodium citrate, and urea were employed as ligands to control the synthesis of CuO nanostructures. In electrochemical measurements, the leaf-like CuO and oatmeal-like CuO nanostructured electrodes exhibit high reversible capacities while hollow CuO spheres possess enhanced reversible capacity after 55 cycles.

Acknowledgment

Financial support for this work has been provided by Los Alamos National Laboratory through Laboratory Early Career LDRD program.

Reference

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