The rates of the two processes, as well as access of Li + ions in the electrolyte to electrode surface, dominate the maximum discharge current. As for discharge process, Li + ions are reinserted into the cathode materials and electrons from anode reduce transition metal ions in the cathode to a lower valence. Thus, the structural stability of cathode materials is a key during Li + extraction/insertion processes. In this regard, cathode materials that are stable enough over a wide composition range must be employed. While oxidation of the transition metals can retain charge neutrality in the compound, Li + removal/insertion usually causes phase changes and structural strain. Typically, transition metals of such cathode materials undergo an oxidation process to higher oxidation state when Li + ions are removed. Up to now, various cathode materials have been proposed and developed such as layered structure LiCoO 2, LiMnO 2, LiNiO 2, ternary LiNi 1−x−yMn xCo yO 2, spinel LiMn 2O 4 (LMO), spinel LiNi 0.5Mn 1.5O 4 (LNMO), and polyanion-based cathode including LiFePO 4 and Li 3V 2(PO 4) 3, as shown in Figure 1. Especially for cathode materials, they dominate the whole cell performance compared to graphite anode. The main reason is attributed to the significant improvement of battery materials and battery technology. The energy density of LIBs in 1991 is only 80 Wh kg −1 at a cell level and has been currently increased to 200–250 Wh kg −1 (at a cell level). Since lithium-ion batteries (LIBs) gained the commercial success in 1991, they have received considerable attention in various areas including consumer electronics, power tools, electric vehicles, and grid energy storage owing to their high energy density, low cost, long cycle life, and environmentally benignity. Finally, the thin-film Li-ion battery applications of these cathode materials are summed up toward next-generation flexible and high-energy devices. Furthermore, some modification strategies for these cathode materials have also been discussed for improving electrochemical performance. In the meanwhile, the existing drawbacks and limitations of various battery chemistries are also analyzed. This chapter firstly gives an overview of cathode materials including lithium-containing cathode (e.g., LiCoO2, LiMn2O4, LiFePO4, LiNi1−x−yMnxCoyO2, LiNi0.5Mn1.5O4) and lithium-free cathode (e.g., vanadium oxides) for LIBs in terms of specific capacity, energy density, working voltage, cycling life, and safety. However, the significant breakthroughs of electrochemical performance for electrode materials, electrolyte, and electrode/electrolyte interface are still highly desirable. Thin-film lithium-ion batteries (LIBs) have attracted considerable attention for energy storage device application owing to their high specific energy compared to conventional LIBs.
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