Achieving high Li storage properties in copper oxide based hybrid anodes
Date of Issue2013
School of Materials Science and Engineering
With an ever increasing demand for efficient energy storage devices and while many micrometer sized bulk materials have failed to meet the high demands, fabrication of various complex nanostructures with controlled size, shape and composition have attracted increasing attention. However, due to the limiting intrinsic properties, not a single material could be developed which could meet all the necessary requirements, e.g., high energy density, good cycling stability; especially at high cycling rates and good rate capability. At this point, application of hybrid electrodes was proposed. In the present work, a rational design and development of nanostructured copper oxide based hybrid anodes in two categories of conversion and alloying-conversion hybrids is presented. The focus is to develop facile, green, template-free fabrication of free-standing hybrid anodes. Hybrids with explicit electrochemical properties are developed by using copper hydroxide as scaffold for growth. The aim of hybrid development is to benefit from multi-functionality or expose novel enhanced electrochemical properties and lead to hybrid anodes with high energy density, good cycling stability and rate capability with practical applications. Copper oxide is an important transition metal oxide with high theoretical capacity, high safety, inexpensive and environmentally abundant nature. Well aligned arrays of CuO nanoneedles is fabricated by anodization. CuO nanoneedle arrays demonstrated excellent performance in high current densities. At 1C rate the electrode delivered an excellent discharge capacity of 559 mAh/g, with 99.2% Coulombic efficiency after 200 cycles, while demonstrating perfect rate capabilities when tested to up to 50 C. Based on the above observation, copper oxide was selected as scaffold for fabrication of coaxial hybrids. A facile, two-step electrochemical method was used to fabricate free-standing Sb/Cu2Sb/Cu alloying hybrid electrodes grown directly on copper substrate. The hybrid is fabricated by electrodeposition of Sb on Cu(OH)2 nanoneedle arrays and is composed of numerous Sb nanoplate surrounding Cu2Sb/Cu core-shell nanoneedle arrays. The initial purpose was to fabricate Sb-CuO core-shell nanowire hybrids. However, electrodeposition of Sb on Cu(OH)2 scaffold resulted in reduction of the copper hydroxide and formation of Cu2Sb/Cu core-shell. Compared to pure Sb nanoplates, the Sb/Cu2Sb/Cu hybrids demonstrated lower initial capacity, while demonstrating improved cycling stability and enhanced rate capability to up to 20 C. In this hybrid, the free-standing Cu core provides 1D electronic pathway and structural spacer. While, the mesoporous Cu2Sb nanocrystals with direct contact to the copper core and strong structural relationship with the lithiated phase; Li2CuSb, provides a more stable electrochemical performance and enhanced rate capability. Next, electrochemical anodization and CVD growth was combined to develop a safe fabrication route for growth of free-standing Ge-Cu2O alloying-conversion hybrid with high theoretical capacity. To demonstrate the effectiveness of hybrid application Ge-Cu2O core-shell nanowire arrays were compared with Ge nanoparticles; grown directly on copper substrate. Ge-Cu2O hybrid demonstrated superior electrochemical properties at 0.5C and 1C rate and perfect rate capability when tested to up to 15C, while Ge nanoparticles demonstrated fast fading electrochemical properties. In-situ generation of GeO2 was observed during the cycling process of Ge-Cu2O, which can be explained based on the different electroactive response of hybrid’s building block towards Li+; with Cu2O undergoing conversion reaction and Ge undergoing alloying reaction, with the capability to react electrochemically with Li2O. It is suggested that the excellent electrochemical performance of Ge-Cu2O hybrid comes from their smart selection and unique hierarchical architecture. The Ge shell with high theoretical capacity, large surface area and short diffusion distance for Li+ lithiation\delithiation lead to delivery of large specific capacities. While, the one-dimensional Cu2O nanoneedle arrays with direct contact to the current collector serve both as structural spacer to buffer the strain during lithiation/delithiation and act as a good electron pathway to improve electronic conductivity. Finally, two-step electrochemical fabrication was applied to fabricate coaxial Fe3O4-CuO conversion hybrids grown directly on copper substrate. The electrodes are made of electrochemically coated mesoporous Fe3O4 nanocrystals, grown on free-standing CuO nanoneedle arrays. Two types of core-shell and hierarchical 3D network Fe3O4-CuO hybrids were fabricated by variation of electrodeposition conditions. Both Fe3O4-CuO hybrids were electrochemically tested at 1C rate and compared with pure Fe3O4 nanoflakes. It was observed that both Fe3O4-CuO hybrids have superior electrochemical performance at 1C rate compared to Fe3O4 nanoflakes. The hybrids were also tested for their rate capability; exposing excellent properties, while being tested to up to 15 C rates. It’s believed that the high capacity, perfect stability and ultrafast charging/discharging capability of the coaxial Fe3O4-CuO hybrids comes from the intelligent integration of two compatible components to free-standing mesoporous architectures. In these hybrids free-standing ultra-fast CuO nanoneedle arrays core provide 1D electronic pathway, structural spacer and superior rate capabilities, while the mesoporous Fe3O4 nanocrystals with high specific capacity provide active sites for lithium interaction and lead to high specific capacities at high cycling rates while maintaining excellent stability. The discussed findings highlights the many possibilities and advantages of template-free deposition routes and demonstrates the large opportunities of hybrid anodes to develop the next generation LIBs with superior electrochemical properties.