On the Surface Chemistry of Cathode Materials in Li-Ion Batteries

The main cathode materials for Li batteries include the following systems: transition metal oxides and sulfides (MO x , MS x ), lithiated transition metal oxides and sulfides (Li x MO y , Li x MS y ), and LiMPO4 olivine compounds. There are also oxygen- and sulfur-based cathodes whose main solid components are carbonaceous materials. Most of these cathodes develop very rich surface chemistry that affects very strongly their electrochemical performance. The main reactions are acid–base reactions (with acidic solution species, HF, PF5, PF3O, etc.); nucleophilic reactions between the basic compounds and the electrophilic alkyl carbonate solvents; polymerization; possible oxidation of solution species; and dissolution of transition metal ions. The behavior of many cathodes in Li-ion batteries is controlled by surface-film formation, passivation phenomena, and Li-ion migration through solid electrolyte interphases formed on the active mass by spontaneous reactions. We describe herein major surface processes, techniques that can address and analyze them, as well as means to improve the performance of cathodes in Li-ion batteries by controlling their surface phenomena.