Secondary batteries with high energy density, high voltage, high charge capacity, high discharge capacity, and excellent cycling performance have recently been required for use in products with high power consumption, such as cellular phones, notebook computers, and digital cameras. Although nickel-hydrogen batteries, lead storage batteries, nickel-cadmium batteries, and the like have been used as secondary batteries, lithium ion secondary batteries are now attracting attention for use as secondary batteries with excellent charge capacity and discharge capacity.
A lithium (Li) ion battery is a secondary battery that uses Li-metal oxide (e.g., LiCoO2) as a cathode (positive electrode) and a carbon material (e.g., graphite) as an anode (negative electrode) to extract power through lithium ion transfer between the cathode and the anode via an electrolyte solution. FIG. 1 shows a schematic view. To improve the properties (i.e., to attain higher energy density), improvements in the main components, i.e., the cathode, the anode, and the electrolyte solution, are important. In particular, obtaining a cathode active material with higher voltage, higher capacity, and higher output, as well as obtaining an anode active material with higher capacity and higher output, are extremely important.
Graphite exhibits a low potential and can occlude Li ions between the layers (LiC6<=>6C+Li++e−), and is thus most widely used in commercially available products. However, graphite has a theoretical capacity of 372 mAh/g. Its low capacity and unsatisfactory output characteristics are considered disadvantages for applications such as for a power source for electric vehicles, which are expected to be widely used in the future. Recent reports reveal that transition metal oxides show high reversible capacity at a low potential, and various transition metal oxides have been studied. This is called a conversion reaction. The insertion of Li reduces the valence of the metal ion in metal oxide, and the oxide is eventually reduced to metal. In this manner, a large number of electrons can be used, achieving higher capacity (e.g., Co3O4+8Li<=>3Co+4Li2O) (Non-patent Documents 1 to 3).
Many reports regarding, in particular, iron oxide-based materials (e.g., Fe2O3) have been published since 2005 (Non-patent Documents 4 to 7). To evaluate the properties of an electrode as an anode, the reversible capacity at a low potential of 3.0 to 0 V is evaluated. A known iron oxide-based powder material shows a reversible capacity of about 500 to 800 mAh/g at a low current density of about 20 to 100 mA/g (Non-patent Documents 4 to 7).
However, all of these anode materials disclosed in the non-patent documents show unsatisfactory cycling performance, and thus have insufficient properties as anode materials. None of the non-patent documents suggest an anode material comprising an amorphous iron oxide, ferrihydrite, or lepidocrocite.
Patent Documents 1 and 2 disclose using iron oxide in the air electrode (cathode) of a lithium air battery. However, these documents nowhere disclose using iron oxide as the anode material of a lithium ion battery.
It has been reported that a lithium ion secondary battery with excellent discharge capacity and charge capacity can be provided by using, as an electrode, an iron oxide produced by an iron-oxidizing bacterium (hereinafter sometimes referred to as “biogenous iron oxide”) (Patent Document 3). However, in Patent Document 3, the biogenous iron oxide is used only as a cathode active material or as a cathode for a lithium ion secondary battery.