Abstract
Lithium-ion (Li-ion) battery has been widely used in different applications. To further improve the energy and power density of Li-ion battery, approaches on developing new materials and by cell engineering are now being investigated by scientists. Conventional Li-ion battery electrodes are composite consist of active material, conductive carbon and polymer binder. Recently, another form of electrodes has been developed and reported by different groups. These electrodes are fabricated by thermal treatment of active material pellets and are called “sintered electrode”. Compared to composite electrodes, sintered electrodes have greater thickness, no carbon and polymer additives and thus can improve the energy density at the cell level. However, a thicker electrode also indicates that the ion transport in liquid phase becomes the limiting factor for high-rate charge/discharge. Therefore, it is necessary to understand the ion transport and further improve the cell performance.
In this thesis, we first focused on studying the ion transport of cells with sintered Li4Ti5O12 (LTO) anode and sintered LiCoO2 (LCO) cathode. The cells had different electrode thickness and were discharged at different rate/current density. For such thick electrodes, the energy density advantage was further established. The limitation to deliver capacity at high rate was clearly evident. However, the detailed evidences that gave rise to this resistance were challenging to assign. Thus, to study the Li+ distribution and track the movement of Li+ during charge/discharge process, operando neutron imaging was used and the results were further studied with numerical tools. These results confirmed our hypothesis that the ion transport in electrolyte phase was the limiting factor of the reduced capacity at high charge/discharge rate.
After understanding the transport limitation, we further move towards on mitigating the resistance. Different approaches have been investigated. The first approach was to focus on electrode engineering by controlling the microstructure using ice-templating. With that technique, electrodes with aligned pores were fabricated, and we further confirmed that the ion transport in electrolyte phase was facilitated. The second approach was to use high concentration/conductivity electrolyte to mitigate Li+ depletion during fast charge/discharge. Then the two methods were combined to further boost the cell performance. As a result, the cell with high concentration/conductivity electrolyte and ice templated electrodes showed 69 % improvement in discharge capacity retention compared to the cell with commercial electrolyte and non-templated electrodes.
At last, in addition to cell engineering, TiNb2O7 (TNO) anode material was synthesized and evaluated in sintered electrode system. TNO has a higher gravimetric and volumetric energy density than LTO used in previous experiments and can be an option to increase the energy density of the cell. Based upon the report, direct substitution of TNO for LTO can improve the anode capacity by 32 %. This results in either improved cell capacity or reduced electrode thickness which also mitigates the ionic transport resistance.
