In recent years, much effort has been made to identify new materials suitable for use as a positive electrode in rechargeable lithium batteries. LiFePO4 represents an excellent candidate: it is inexpensive, non-toxic, and environmentally benign. It also exhibits a theoretical specific capacity of 170 Ah kg?1. LiFePO4 is a natural product, known by the name of Triphylite. In 1997 Padhi et al. (J. Electrochem. Soc. 144, 1188–1194 ,1997) showed that lithium can be electrochemically extracted from LiFePO4 and inserted into FePO4 along a flat potential at 3.5 V versus Li. Two years later Ravet et al. (Improved iron based cathode material. in Proceeding of 196th ECS Meeting, Hawaii, 17–22 Oct 1999) reported a new synthetic route for LiFePO4 which improves the practical capacity and charge/discharge rate. Less than 1% in weight of an electronically conductive substance was added during the synthesis which led to the formation of conductive LiFePO4 particles with outstanding electrochemical properties. Electrochemical tests were made at 80°C using a polymer electrolyte. They claimed that the active material was able to deliver almost the full theoretical capacity when discharged at a current density as high as 170 A kg?1. Carbon was already used to improve the electrochemical performance of cathode materials for lithium-ion batteries. Bruce et al. prepared lithium manganese oxide by a low-temperature solution route which included the addition of a small amount of carbon to the solution. As a result, significant enhanced capacity retention was obtained on cycling. They supposed that the finely divided carbon between the particles ensured a more flexible composite cathode, better able to accommodate the volume changes on cycling (Bruce et al., J. Power Sour. 68, 19–23, 1997). In this chapter, we present the same beneficial influence on the electrochemical performance of a cathode prepared with a LiFePO4 synthesized in presence of high-surface area carbon-black (Prosini et al., Electrochim. Acta. 46, 3517–3523, 2001). The carbon was added to the LiFePO4 precursors before the formation of the crystalline phase. SEM micrographs confirmed that the addition of the fine carbon powder reduces the LiFePO4 grain size. Furthermore, carbon is uniformly dispersed between the LiFePO4 grains, ensuring a good electronic contact. Electrochemical tests showed that the presence of carbon enhanced the electrochemical performance, in terms of improved practical capacity and cyclability. The cathode was tested at various temperatures. The specific capacity was seen to increase on rising the cell temperature. The full capacity (170 Ah kg?1) was obtained when discharging the cell at 80°C and 17 A kg?1 (C/10) discharge rate. The cyclability of the material at room temperature was tested at 85 A kg?1 (C/3) discharge rate. The cell was cycled for over 230 cycles with an average specific capacity of about 95 Ah kg?1.
KeywordsSpecific Capacity Electrochemical Performance Composite Cathode Voltage Profile Lithium Manganese Oxide
- 2.N. Ravet, J.B. Goodenough, S. Besner, et al., Improved iron based cathode material. In Proceeding of 196th ECS Meeting, Hawaii, 17–22 Oct 1999Google Scholar