Two Japanese institutes succeeded in synthesizing olivine structure (a kind of crystal structure) LiFePO4 (lithium ion phosphate) coated with a carbon film, which has great potential as a positive electrode material, into superfine particles of nanometer-order.
As LiFePO4 is inexpensive, it is attracting attention as a positive electrode material for large Li-ion batteries used in electric cars. However, a sudden drop of capacity after high-rate charging/discharging (flowing large current in a short period) is pointed out as one of the shortcomings of the material.
The National Institute of Advanced Industrial Science and Technology (AIST) and the Japan Society for the Promotion of Science (JSPS) improved this shortcoming by coating the olivine structure LiFePO4 superfine particles, the diameters of which were controlled to 20-40nm, with a 1-2nm carbon layer (semi-graphite film) similar to graphite.
The superfine particles retained a high capacity of 112mAh/g and 90mAh/g under high-rate charging/discharging of 30C and 60C, respectively. Furthermore, it retained the initial capacity of 165mAh/g after 1,100 charging/discharging cycles under a 100% charging degree.
It is difficult to use LiCoO2, which is widely used as the positive electrode material for Li-ion batteries, in electric cars. This is because of the raw material price and limited resources.
It will be a big step toward the realization of batteries for electric and hybrid cars, which are required to be inexpensive, offer high output and be highly stabile, if the olivine structure LiFePO4 composed of cheap iron and phosphate can be used as a positive electrode material for Li-ion batteries.
Nanostructure electrode materials reportedly improve the output of Li-ion batteries. The reasons include (1) the diffusion distance of Li-ions in active material is reduced, (2) the current density per unit area is reduced due to the increase in specific surface area, and (3) the cycle characteristics are improved through the relaxation of volume expansion during charging and discharging due to the nanopores. Reasons (1) and (2), in particular, are major factors in realizing improved output.
On the other hand, it was reported that the area of contact between electrolyte and the positive electrode is extremely large, resulting in the risks of firing in thermal releasing and deterioration of cycle characteristics.
The energy research division of AIST has been working on the research and development of nanostructure positive electrode materials, aiming to improve the output of next-generation Li-ion batteries intended for automobiles.
AIST has already succeeded in synthesizing nonporous materials and nanowires of titanium oxide and spinel-structure lithium manganate, which are attracting attention as materials for the negative electrode and the positive electrode, respectively. It has been indicated that outputs of Li-ion batteries are expected to improve by nanostructuring these materials.
AIST assumes that the causes of capacity deterioration in olivine structure LiFePO4, which is a promising positive electrode material, lie in slow diffusion of Li-ion in active materials and low electron conductivity. AIST has been carrying out research and development to rectify the problems, focusing on the refinement of the particles to a nanometer order and the carbon-coating of the refined particles.
It received a Scientific Research Subsidy from JSPS for part of the research.
To coat the particles with carbon, the organic compound needs to be carbonized. If a high-temperature treatment is employed for this purpose, however, the particles will grow unnecessarily, spoiling the efforts of refining the particles to a nanometer order. Furthermore, part of the surface can be left uncoated, if nanometer-order particles are aggregated.
Refinement to a nanometer order and complete carbon-coating of the surface were realized at the same time through this research.
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