Tsikourkitoudi, Vasiliki P. (2016) Development of advanced nanomaterials for lithium-ion batteries. (PhD thesis), Kingston University, .

Abstract

The scope of the present study was to demonstrate the capability of Flame Spray Pyrolysis (FSP) process as a unique facility for the one-step synthesis of lithium titanate (Li[sub]4Ti[sub]5O[sub]12, LTO) nanoparticles with tailored properties. FSP offers a versatile technology to produce a wide range of high-purity oxide nanoparticles with desired properties. The ability of FSP to manipulate nanoparticles' properties was demonstrated by controlling operating conditions and selecting appropriate precursors. More precisely, the effect of FSP processing conditions on LTO properties were thoroughly investigated both experimentally in a pilot-scale reactor (production rates up to 1 kg h[sup]-1) and theoretically by the development of models describing particle dynamics in the spray flame. The main aim was to obtain LTO nanoparticles of different particle sizes. The produced nanoparticles were used as active materials for the fabrication of lithium-ion battery anodes and electrochemical characterisation was performed in order to examine the influence of the particles' physical properties on the battery performance. The control of the flame synthesis parameters is crucial, since the properties of the final product depend on the nanoparticles' size distribution, morphology, extent of agglomeration, as well as phase compostition. Initially, the influence of liquid feed properties (precursor concentration and solvent) on LTO physical properties was established. LTO particle size increazsed when the precursor concentration was increased due to particle concentration increase in the flame followed by the enhancement of particle collisions and hence particle growth. Moreover, high precursor concentration caused a variation of physical properties of the precursor mixture, affecting the atomisation process, and subsequently led to the formation of larger droplets. Larger droplets generated larger particle. Additionally, the choice of solvent for the dissolution of metal precursors was proven to be an important issue for LTO synthesis by FSP. The physical properties of the solvents in relation to metal precursor properties affected the formation of LTO nanoparticles. Inhomogenous particle size distribution was observed for LTO synthesised by a precursor mixture containing isopropanoil, due to its low boiling point inducing particle formation via droplet-to-particle mechanism, whereas pure 2-ethylhexanoic acid was used, LTO particles were formed by gas-to-particle route and had homogenous size distribution. The droplets generated during atomisation by the precursor solution of pure 2- ethylhexanoic acid had the largest diameter due to the high viscosity and density of the mixture. Despite this, the obtained nanoparticles were the smallest in comparison to those obtained from other precursor solutions. In such cases, the boiling point and specific combustion enthalpy of the solvents should be taken into consideration. Apart from the liquid feed properties, the effect of FSP operating conditions (O[sub]2 dispersion gas and precursor flow rate) were also investigated in the present study. By increasing the O[sub]2 dispersion gas rate, LTO nanoparticles' diameter was decreased due to a decrease of the droplet diameter. Particle sintering was also prevented due to the faster transport of primary particles through a shorter flame. An increase of the precursor flow rate at first increased and then decreased the LTO nanoparticles' size. The initial increase of particle size occurred due to a flame temperature increase. At higher precursor flow rates, the droplets disintegrated and generated many smaller fragmented droplets due to higher temperature, which subsequently formed smaller particles. Moreover, particle growth in the spray flames was studied theoretically, and numerical models were developed. The monodisperse model developed assumed that all primary particles had the same size. However, it overestimated the primary particle diameter values. Polydispersity was taken into consideration in the development of an additional model which was solved by the quadrature method of moments. The results obtained from the polydisperse model were closer to the experimental values, both for low and high production rates. Finally, the synthesised LTO nanoparticles were used as active materials in lithium-ion battery half cells and their electrochemical behaviour was elucidated, demonstrating the effect of the particles' physical properties on their electrochemical performance. LTO of particle size 18 and 21 nm showed the best electrochemical performance with capacity retention of almost 100% after 500 cycles, whereas the smallest particle deteriorated the electrochemical performance with a capacity loss of more than 60%.

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