Abstract
Fundamental studies for a calcium ion battery and improvement of electrolytes for conventional lithium ion batteries by controlling the solvation structure of lithium ions were studied. In chapter 1, electrochemical behavior of graphite electrodes in organic solvents containing calcium salts was studied. The study showed that co-intercalation of calcium ions and solvent molecules took place in electrolytes used in the study, and showed the importance of de-solvation of calcium ion to realize a calcium ion battery. Furthermore, it was suggested that surface film control is critical for intercalation of calcium ions into graphite electrodes. Chapters 2-7 aim to improve electrolyte for conventional lithium ion battery in terms of low-temperature performance and non-flammability. Ethylene carbonate (EC) is used as a main organic solvent for lithium ion batteries, but has problems on ionic conductivity at low temperature and flammability. Propylene carbonate (PC) has advantage in low-temperature ionic conductivity, and trimethyl phosphate (TMP) has advantage in non-flammability, but those solvents are not compatible with graphite electrode because of exfoliation and degradation of graphite, respectively. In this thesis, it was made clear that intercalation of lithium ions can take place by controlling solvation structure of lithium ions by adding stronger Lewis acid, such as calcium ions, into PC-based electrolyte. Successful electrochemical intercalation of lithium ions in PC-based electrolytes is attributed to effect of calcium ions on the solvation structure of lithium ions. The similar effect of calcium ion addition on lithium ion intercalation was also observed in electrolytes containing TMP. The work also shows that co-solvents, such as dimethyl carbonate (DMC), can also affect intercalation reaction of lithium ions. Hence, lithium ion intercalation can take place by controlling both amount of calcium ion and co-solvent. The above approach for lithium ion intercalation enables to use organic solvents that were considered not to be used for lithium ion batteries, and opened for more options for solvents to improve performance of the batteries.
