Electrode Material Solutions for Large-Scale Lithium-Ion Batteries

Lithium-ion (Li-ion) batteries have become the power source of large-scale applications, such as electric or hybrid-electric vehicles and energy storage systems, in addition to their conventional use in small consumer electronics. However, the traditional electrode materials do not meet the requirements of safety and lifetime, which are emphasized in the large battery packs. Furthermore, for transportation purposes, a high energy density is needed. The aim of this thesis was to study different electrode materials from these perspectives. The lifetime of commercial Li-ion cells was studied under different cycling temperatures. Especially after prolonged cycling at elevated temperature, the graphite negative electrode was found to be not only the main source of aging but also a safety risk as dendritic Li depositions, a potential source of short-circuits, were observed. Possible reasons promoting Li plating could be the extensive passivation layer growth on graphite and the consequential cell drying and formation of gaseous components. A new alternative to the graphite negative electrode is Li4Ti5O12 which however decreases the cell voltage thus sacrificing the energy density. This sets a demand for novel, safe, high-voltage positive electrodes of which mixed Li(Fe1-yMy)PO4 (M = Co, Ni) materials were investigated in this work. A beneficial, mutual influence of the Fe and M occupying the same lattice site was observed as e.g. shift of redox potentials and changes in the delithiation/ lithiation reaction mechanisms. Especially the local environment of Fe3+ was affected by the substitution. An optimal composition for the Co substitution is around Li(Fe0.5Co0.5)PO4. In the case of the Ni substitution, on the other hand, the Ni2+/Ni3+ redox couple could not be reversibly activated, presumably due to lack of electrical conductivity or possible structural changes. Knowledge of the heat generation in a Li-ion cell is needed for accurate design of cooling systems and thus avoiding the undesirable cell temperature increase. The reversible part of heat generation depends on the entropy change in the electrode materials during the cell reaction, affected mainly by the arrangement of Li in the electrode lattices. In this work, the entropy change behavior was studied for LiFePO4, Li(Fe0.33Mn0.67)PO4, graphite, and Li4Ti5O12 electrode materials and their combinations. Specific materials and states of charge were determined to be unfavorable due to extra heat generation. Furthermore, the effect of Mn substituent was observed in the entropy change behavior of Li(Fe0.33Mn0.67)PO4. The impact of reversible heat generation was demonstrated in practice as a cooling effect during the discharge of a commercial Li-ion cell.