Structure and composition design of ternary metal oxide anodes for rechargeable lithium ion batteries
Date of Issue2016-03-21
School of Materials Science and Engineering
Rechargeable lithium ion battery (LIB) is one of the most promising energy storage systems with wide range of applications. With the development of portable electronics, hybrid electric vehicles and electric vehicles, the demand for LIBs with high specific gravimetric and volumetric energy densities, good safety and low cost is increasing. Transition metal oxide (TMO) is one of the most studied alternatives for replacing the commercial anode material (graphite) as the low theoretical capacity (372 mA h g-1) of graphite can barely meet the ever-increasing demand. By reacting with lithium through intercalation, conversion and alloy reactions, TMOs can offer high theoretical capacities. Moreover, the nature abundance and low in price make TMOs more appearing. However, their low electric conductivities and volume changes during reactions with lithium impeded TMOs from commercialization. Thus, scientific research and breakthroughs are required to make the commercialization of TMO anodes possible. Nanoengineering provides an answer to the technological bottleneck of the electrode materials. By fabricating nanoscale electrodes, the charge carrier transfer kinetics can be largely improved regarding the diffusion distance, contact area and strain relaxation. In this case, the rational design the nanostructures are the key factors. Moreover, to realize the full potential of TMOs, further tailor of properties through ‘heterogeneous nanostructures’ is another way to further enhance the electrochemical performance of the TMOs due to the combination of advantages from both constituents. Firstly, spinel type AFe2O4 hollow nanostructures (uniform NiFe2O4, ZnFe2O4 and CoFe2O4 hollow microcubes) are successfully prepared. The well-defined hollow structured AFe2O4 cubes are hierarchically assembled by numerous nanoparticles on a dimension of 15-30 nm. The nanoparticles at outer shell offer large electrode/electrolyte contact area and create a shorter diffusion distance for the charge carriers, ultimate utilizing the materials at inner shell. This is crucial for the good rate capability. And the hollow interior space and the voids between nanoparticles at outer shell provide the cushion to volume changes during the charge/discharge process, which is of paramount importance for the cycling stability. The impact of different metal ions at A site (AFe2O4) on electrochemical performance, such as the cycling stability, rate capability, working voltage and side effect during discharging, is investigated and discussed. Though stable cycling performance are achieved in all three materials at a high current densities, the electrochemical performance under high current densities still needs to be further improved, especially for CoFe2O4. Therefore, to achieve stable cycling performance and high capacity at high current density for CoFe2O4, the effect of heterostructures is investigated on top of the nanoengineering. A facile and effective MOF template method was used to prepare Co3O4/CoFe2O4 hollow cubes with uniform dimension and complex interior architectures. The molar ratio of Co3O4 and CoFe2O4 particles are 0.4: 0.6. The nanodimensional homogeneous distribution of both phases is achieved. The Co3O4/CoFe2O4 hollow cubes show great promise in lithium storage properties with enhanced stability and rate capabilities under high current densities, which are supported by the smaller shifts in voltage plateaus at different current densities and reduced charge carrier resistance. Based on above results, the ‘heterogeneous nanostructure’ concept is extended to conversion/alloy TMOs. As indicated in the first project that when an alloy reaction based metal ions is used at A site of AB2O4, capacity contribution from the side effect is less. Moreover an extra alloy reaction would inherent the material more specific capacity. And at the same time, in order to solve the problems raised by nanostructures, such as higher surface energy and increased grain boundaries, electrodes of Zn0.95Co0.05O/ZnCo2O4 hybrid 3D architectures are prepared. The Zn0.95Co0.05O/ZnCo2O4 heterostructure are porous flower-like nanostructures directly grown onto the macroporous nickel foam, which acts as an effective 3D conductive network of the current collector. The nickel foam is flexible and eliminates the usage of organic binding agent. The configuration of direct contact of the active material with the current collector is free of side-effects arising from the increased grain boundaries and compromised electronic conductivity by downsizing. The different degrees of porosity favor the fast kinetics of charge transfer. The electrodes of Zn0.95Co0.05O/ZnCo2O4 heterostructures show good cycling performance and rate capabilities. The capacity enhancement from the Zn alloy step is also discussed.