Abstract:

The cathode film for lithium-ion batteries is fabricated using lithium iron phosphate powder on a production line. During the calendering process, the roll temperature is set to 25 ℃, 60 ℃, 80 ℃, 100 ℃, and 120 ℃, respectively, and the cathode film is rolled to a uniform thickness under consistent roll pressure. Subsequently, the cathode film is fashioned intoCR2032 button batteries for evaluation. Parameters such as thickness uniformity, film resistance, peeling force, thickness rebound, elongation, and scanning electron microscopy (SEM) analyses are assessed. Additionally, electrochemical impedance spectroscopy (EIS), 0.1—2C rate performance, and short-term 1C cycle performance are characterized to ascertain the impact of heated calendering on cathode film and cell performance, and to identify the optimal calendering temperature. Findings indicate that heated calendering does not affect the elongation of the cathode film; however, it enhances the uniformity of film thickness. The process also diminishes the resistance of the cathode film, with the optimal reduction observed at 100 ℃, approximately 2.1%. Moreover, heated calendering reduces thickness rebound, with the lowest rates at 60 ℃ and 100 ℃, approximately half of that at 25 ℃. The peeling force initially increases and then decreases with roll temperature, peaking near 60 ℃. SEM images reveal tighter binding of active materials postheated calendering compared to normal temperature processes, without grain breakage. Heated calendering effectively lowers both ohmic and charge-transfer impedance, with optimal results at around 100 ℃, correlating with film resistance observations. Enhanced rate performance is also noted in cathode films subjected to heated calendering, particularly at 100 ℃. However, no significant differences are observed in the cycle performance data over 100 cycles between cathode films produced via heated and normal calendering. Overall, heated calendering positively influences both film and cell performance, with the most beneficial effects at approximately 100 ℃.

Key words: lithium ion batteries, heated calendering, lithium iron phosphate, process window