|M.Sc Student||Schwieger Jonathan|
|Subject||Study and Development of Copper Sulfate Based Cathode|
Materials for Rechargeable Lithium Batteries
|Department||Department of Materials Science and Engineering||Supervisor||Professor Ein-Eli Yair|
|Full Thesis text|
As lithium battery technology sets out to bridge the gap between portable electronics and the electrical automotive industry, cathode materials still stand as the bottleneck regarding performances. In the realm of highly attractive polyanion-type structures as high-voltage cathode materials, the sulfate group (SO4)2- possesses an acknowledged superiority over other contenders in terms of open circuit voltage arising from the inductive effect of strong covalent S-O bonds. In parallel, novel lithium insertion mechanisms are providing alternatives to traditional intercalation, enabling reversible multi-electron processes securing high capacities. Combining both of these advantageous features, the successful electrochemical reactivity of copper sulfate pentahydrate (CuSO4?5H2O) with respect to lithium insertion is reported here.
The demonstrated two-electron displacement reaction entails the extrusion of metallic copper and is shown to respond to optimization techniques, namely reductions in particle size and electrode thickness. By galvanostatic cycling and cyclic voltammetry, lithium insertion is shown to progressively evolve from a single-step process along both discharge and charge, to a full two-step process, owing to the faster kinetics at the nanometer scale. In the fully optimized scenario, this process occurs at the dual voltage of 3.2 V and 2.7 V followed by the reversible extraction of Li at 3.5 V and 3.8 V. Structural considerations reveal the crucial nature of the hydration water in CuSO4?5H2O in relation to its electrochemical reactivity by improving ionic diffusion thanks to a larger unit cell volume. When dehydrated to both a monohydrate and anhydrous state, the performances of the electrodes are considerably decreased.
Through post-mortem investigation of both the cathode and Li anode by ex-situ XRD and SEM, a reaction mechanism is established indicating the irreversible degradation of the cathode material to a monohydrate structure due to constitutional water loss during cycling. Associated with copper poisoning at the anode from dissolution at the cathode as well as poor electronic conductivity of the active material, the limited cyclability of the system observed at this stage is explained.
A comparison with other hydrated transition metal sulfates is made as well, illustrating the enhanced behavior of CuSO4?5H2O.