Research On Thin-film Lithium(-ion) Batteries

During recent years, with the rapid development of smart card, especially the appearance of super smart card with LCD keyboard and CPU, the needs of more and complex functions, the implanted power with excellent performance is urgently requested. Thin film lithium-ion batteries (TFLB) may be the only choice due to their ultra-thin thickness, flexible and high energy density. TFLB for smart card should be in keeping with the international standard of ISO7816 for smart card. Particularly, the thickness and flexibility are most important. Now TFLB can be successfully fabricated by various film-forming techniques (for example, polymer technical, physical deposited).A kind of TFLB has been successfully fabricated by adjusting some battery parameters, such as the ratio of anode vs cathode's capacity, thickness of cathode, the amount of electrolyte etc. It shows a good cycleability, for example, it retains 93% and 87% of the initial capacity after 300 cycles at 0.5C rate and at 1C rate, respectively, and passes the safety test. This TFLB can meet the requests of smart cards commonly. However the capacity declines rapidly at high rate and TFLB is difficult to be activated. It is found that adding more electrolytes, extending aging time and improving the formation conditions can accelerate activation distinctly. For the safety, the formation at low current and voltage is suggested.The thin-film lithium batteries have been also fabricated. It was found that their cycleability can be improved by using appropriate electrolyte or adding more electrolytes. However, there is still a problem that such batteries fail suddenly after several cycles. Based on dissecting batteries and analyzing using EIS, SEM, XRD, IR thermal imaging, it was found that part of lithium deposits irreversibly on the surface of lithium anode during cycle. The surfaces of new deposited lithium will react with electrolyte and form SEI layer, exhaust large amount of electrolytes, lead to an increase of impedances finally. On the other hand, the forming of dead lithium will increase the chance of micro-scale short circuit. Increased impedances and micro-scale short circuit induce large quantity of Joule heating to release when batteries cycled. As a result, the separator would shut down and the impedance of batteries rapidly raise and the batteries can't charge and discharge normally again.