Synthesis and characterisation of new generation of cathode materials for lithium-ion and sodium-ion batteries

Abstract

Li-ion batteries are dominating the field of energy storage, in particular for use in portable devices such as mobile phones, cameras and more recently for electric vehicles. Low mass materials offering high specific capacity and also high power are thus required.

Improvement of the performance at high rates, by improvement of the kinetics, was performed on solid solutions of the type Li_2+2xFe_1-xSiO₄. They contain interstitial lithium ions whose presence was characterised by various diffraction techniques (powder X-ray and neutron diffraction) and electrochemistry. Rate capabilities were successfully improved, especially for the compounds containing a higher fraction of lithium ions in the structure: 70% theoretical capacity was reached at 500 mA g⁻¹ for the x=0.4 compound compared with only 45% for the x=0 composition.

In parallel, studies on alternative energy storage systems are being explored, such as sodium-based devices. Layered sodium manganese oxides with various Na stoichiometries have been studied. Selected characterisation techniques have been employed in order to study the behaviour of such materials upon cycling: powder X-ray, neutron and synchrotron diffraction as well as electrochemical techniques.

β-NaMnO₂ exhibits a complex structure and undergoes many structural transitions upon cycling albeit being highly reversible upon cycling. 190 mA h g⁻¹ were obtained at 10 mA g⁻¹ and 100 mAh g⁻¹ were obtained at 1000 mA g⁻¹. In addition, this material exhibits exceptional capacity retention at high rates (100% at 1000 mA g⁻¹).

Other layered compounds of the type Na_2/3Mn_1-xM_xO₂ (with M=Li,Al,Mg and 0≤x≤0.2) have been studied. Despite addition of electrochemically inactive ions in the structure lowering theoretical capacities, Li as a dopant was found to be electrochemically active. Although only 65% theoretical capacity was reached, capacity retention was ca. 100% after 20 cycles in the best case. Mg-doped compounds were found to sustain high current densities (ca. 150 mAh g⁻¹ obtained at 1000 mA g⁻¹). Cyclability was also enhanced on extended cycling: 100% capacity retention at 2000 mA g⁻¹ was observed.

Date of Award	24 Jun 2015
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Peter Bruce, FRS (Supervisor)