1. Field of the Invention
The present invention relates to ferrous phosphate (II) powders, lithium iron phosphate powders prepared therefrom, and methods for manufacturing the same. More specifically, the present invention relates to ferrous phosphate powders for preparing Li-ion batteries with reduced cost, lithium iron phosphate powders prepared therefrom, and methods for manufacturing the same.
2. Description of Related Art
As the development of various portable electronic devices continues, more and more attention focuses on the techniques of energy storage, and batteries are the main power supplies for these portable electronic devices. Among commercial batteries, small-sized secondary batteries are especially the major power supplies for portable electronic devices such as cell phones and notebooks. In addition, secondary batteries are applied to not only portable electronic devices, but also electric vehicles.
Among the developed secondary batteries, the lithium secondary batteries (also named as the Li-ion batteries) developed in 1990 are the most popular batteries used nowadays. The cathode material of the initial lithium secondary batteries is LiCoO2. LiCoO2 has the properties of high working voltage and stable charging and discharging voltage, so the secondary batteries which use LiCoO2 as a cathode material are widely applied to portable electronic devices. Then, LiFePO4 with an olivine structure and LiMn2O4 with a spinal structure were also developed as a cathode material for lithium secondary batteries. Compared to LiCoO2, the safety of the batteries can be improved, the charge/discharge cycles can be increased, and the cost can be further reduced when LiFePO4 or LiMn2O4 is used as cathode material of secondary batteries.
Although the batteries which use LiMn2O4 as cathode materials have low cost and improved safety, the spinal structure of LiMn2O4 may collapse during the deep discharge process, due to Jahn-Teller effect. In this case, the cycle performance of the batteries may further be decreased. When LiFePO4 is used as cathode material of batteries, the batteries also have the properties of low cost and improved safety. In addition, the capacity of LiFePO4 is higher than that of LiMn2O4, so the batteries made from LiFePO4 can further be applied to devices which need large current and high power. Furthermore, LiFePO4 is a non-toxic and environmentally friendly material, and also has great high temperature characteristics. Hence, LiFePO4 is considered as an excellent cathode material for lithium batteries. Currently, the average discharge voltage of the lithium batteries using LiFePO4 as a cathode material is 3.4˜3.7 V vs. Li+/Li.
A conventional structure of the Li-ion batteries comprises: a cathode, an anode, a separator, and a Li-containing electrolyte. The batteries perform the charge/discharge cycles by the lithium insertion and extraction mechanism, which is represented by the following equations (I) and (II).Charge: LiFePO4−xLi+−xe−→xFePO4+(1−x)LiFePO4  (I)Discharge: FePO4+xLi++xe−→xLiFePO4+(1−x)FePO4  (II)
When a charge process of the batteries is performed, Li ions extract from the structure of LiFePO4; and the Li ions insert into the structure of FePO4 when a discharge process is performed. Hence, the charge/discharge process of the Li-ion batteries is a two-phase process of LiFePO4/FePO4.
Currently, the LiFePO4 powders are usually prepared by a solid-state process. However, the property of the product is highly related to the sintering temperature of the solid-state process. When the sintering temperature is below 700° C., all the raw materials have to be mixed well. If the raw materials are not mixed well, Fe3+ impurity phase will be present in the LiFePO4 powders. When sintering temperature is below 600° C., the average grain size of the LiFePO4 powders will be smaller than 30 μm. However, if the sintering temperature is increased, the average grain size of the LiFePO4 powders will be larger than 30 μm. When the average grain size of the LiFePO4 powders is larger than 30 μm, a grinding process and a sieving process have to be performed to obtain powders with specific grain size between 1 μm to 10 μm, in order to be used for preparing Li-ion batteries. Hence, in the case that the LiFePO4 powders are prepared through a solid-state process, the grinding process and the sieving process have to be performed, which may increase the cost of the Li-ion batteries. In addition, the problem of large and non-uniform grain size of the LiFePO4 powders may also occur.
Therefore, it is desirable to provide a method for manufacturing micro-sized, submicro-sized, even nano-sized cathode materials of Li-ion batteries in a simple way, in order to increase the charge/discharge efficiency of the batteries and reduce the cost thereof.