Biogenous Iron Oxide(BIOX)-As a High-Capacity Anode Material for Lithium Ion Batteries

Abstract

1.Introduction Biogenous iron oxide(BIOX) is produced by Leptothrix ochracea, aquatic bacteria living in the world. BIOX is amorphous Fe3+-based oxide containing Si and P, and takes tube-formed structure. In recent years, lithium-ion batteries have been widely used and intensively studied to meet an increasing variety of application in the future. Especially exploitation of high-capacity lithium-ion batteries is very important. The capacity of BIOX is approximately three times larger than that of graphite, so that BIOX is expected as a new high-capacity anode material for lithium ion batteries. BIOX belongs to the category of conversion anode materials and shows superior cycle and rate performance beyond normal conversion anode; however, the large first irreversible capacity remains in disadvantage for the practical use. In this study, we applied lithium pre-doping to BIOX electrode to eliminate the first irreversible capacity, and succeeded to provide the lithium ion battery with high capacity and high cycle performance.< 2.Experimental The BIOX cultured in Okayama university was firstly mixed with CNF (Carbon nanofiber) as a conductive agent, SBR (styrene-butadiene rubber) / CMC (carboxymethyl acid) as a binder (80:10:10 wt%) and water as a viscosity adjustor in an agate mortar. The slurry was coated on a copper foil, dried in air at 80ºC and vacuum-dried at 120ºC for 3h. The punched out electrode was pressed under the pressure of 2t. The first irreversible capacity was eliminated by contacting a lithium metal sheet to the electrode in an electrolyte (LiPF6in EC / DEC (1:1)). The charge-discharge behavior was measured in a coin type cell (2025) with Li metal as a counter electrode. The current rate was 0.1 C (the theoretical capacity of BIOX: 666mAh / g) and the voltage range was set between 50 and 3000mV 3.Results Figure 1 shows the voltage profiles of the first cycle for the as-prepared and Li- contacted electrodes. The former showed the first discharge capacity of 984mAh/g, and the following charged one of 712mAh/g. The first irreversible capacity is considerably large; the coulomb efficiency is only 72.0%. After the second cycle, almost 100% efficiency was obtained. On the other hand, the Li pre-doping by the contact of Li metal brought the high coulomb efficiency. The electrode of 30 s contacting of Li metal showed discharged capacity of 972mAh/g and charged one of 818mAh/g(coulomb efficiency:84.1% ). The first irreversible capacity due to the SEI formation on the nano-particles in BIOX may be compensable with the reaction of Li metal in advance. Figure 1