A Japanese research group clarified the crystal structure of lithium iron silicate (Li2FeSiO4: lithium, iron, silicon and oxygen), a promising material for positive electrodes of Li-ion secondary batteries.
The structure was revealed by the group led by Atsuo Yamada, an associate professor at Interdisciplinary Graduate School of Science and Engineering of Tokyo Institute of Technology. It used techniques such as high resolution powder X-ray diffractometry (HR-XRD) based on high-intensity synchrotron radiation.
"Contrary to our expectations, we found that it has a one-dimensional, chain-like coupled structure composed of FeO4 and SiO4 tetrahedrons that rotate gradually and regularly in a long cycle," Yamada said.
The basic crystal structure of Li2FeSiO4 provides an important clue for designing positive electrodes with Li2FeSiO4, which consists only of abundantly available "ubiquitous elements." In other word, the research group paved the way for more affordable Li-ion secondary batteries.
The Li-ion secondary batteries currently mounted in mobile phones and notebook PCs mainly use lithium cobalt oxide (LiCoO2), which contains rare metal cobalt, for the positive electrode. In fear of short supply of raw cobalt, an alternative materials have been sought for some time. In addition, it has been pointed out that LiCoO2 generates oxygen gas (O2) in high temperature environments because of its low thermal stability.
Under these circumstances, it has been required to develop a positive electrode made of inexpensive ubiquitous elements for use in large-capacity batteries such as in electric vehicles, power storage systems, etc, for which cost is more important than energy capacity per volume.
Recently, A123 Systems, a Massachusetts-based venture firm, developed and commercialized a Li-ion rechargeable battery that uses lithium iron phosphate (LiFePO4; P = phosphate) for the positive electrode. The battery has already been utilized in electric power tools, hobby products, etc and is being imported into Japan for use in radio-controlled model planes and other products.
Compared with LiFePO4, which has an olivine-type crystal structure, Li2FeSiO4 is expected to realize a higher energy capacity of Li-ion secondary battery because it contains two lithiums. But the analyses of its crystal structure have not shown much progress so far.
The research group prepared a sample of Li2FeSiO4 by sufficiently mixing the powders of starting materials such as LiCO3, FeC2O4·2H2O and SiO2 and baking the mixture in an argon atmosphere for six hours at 800°C
"We made efforts to avoid a side reaction and increased the grain diameter to several hundred nanometers," Yamada said.
The group analyzed the sample powder by powder X-ray diffractometry, TEM-based electron diffractometry and other methods to clarify its complicated crystal structure. The structure thus revealed agreed well with the results of an electron density analysis, according to the group.
As a compound, Li2FeSiO4 is more stable than the pervasive LiCoO2 because the bonding force between Co and O is stronger in Li2FeSiO4.
"Li2FeSiO4 is superior to LiCoO2 not only because the combustion-supporting property, which allows the dissociation of oxygen atoms into oxygen gas at high temperature, is lower but also because the stability as a battery material is higher," Yamada said.
The latest achievement was part of the "R&D of Storage Systems for Smooth Connection to Supply System" conducted under the program called "Strategic R&D of Next Generation Storage Systems for Practical Use," which is promoted by Japan's New Energy and Industrial Technology Development Organization (NEDO).
The structure was revealed by the group led by Atsuo Yamada, an associate professor at Interdisciplinary Graduate School of Science and Engineering of Tokyo Institute of Technology. It used techniques such as high resolution powder X-ray diffractometry (HR-XRD) based on high-intensity synchrotron radiation.
"Contrary to our expectations, we found that it has a one-dimensional, chain-like coupled structure composed of FeO4 and SiO4 tetrahedrons that rotate gradually and regularly in a long cycle," Yamada said.
The basic crystal structure of Li2FeSiO4 provides an important clue for designing positive electrodes with Li2FeSiO4, which consists only of abundantly available "ubiquitous elements." In other word, the research group paved the way for more affordable Li-ion secondary batteries.
The Li-ion secondary batteries currently mounted in mobile phones and notebook PCs mainly use lithium cobalt oxide (LiCoO2), which contains rare metal cobalt, for the positive electrode. In fear of short supply of raw cobalt, an alternative materials have been sought for some time. In addition, it has been pointed out that LiCoO2 generates oxygen gas (O2) in high temperature environments because of its low thermal stability.
Under these circumstances, it has been required to develop a positive electrode made of inexpensive ubiquitous elements for use in large-capacity batteries such as in electric vehicles, power storage systems, etc, for which cost is more important than energy capacity per volume.
Recently, A123 Systems, a Massachusetts-based venture firm, developed and commercialized a Li-ion rechargeable battery that uses lithium iron phosphate (LiFePO4; P = phosphate) for the positive electrode. The battery has already been utilized in electric power tools, hobby products, etc and is being imported into Japan for use in radio-controlled model planes and other products.
Compared with LiFePO4, which has an olivine-type crystal structure, Li2FeSiO4 is expected to realize a higher energy capacity of Li-ion secondary battery because it contains two lithiums. But the analyses of its crystal structure have not shown much progress so far.
The research group prepared a sample of Li2FeSiO4 by sufficiently mixing the powders of starting materials such as LiCO3, FeC2O4·2H2O and SiO2 and baking the mixture in an argon atmosphere for six hours at 800°C
"We made efforts to avoid a side reaction and increased the grain diameter to several hundred nanometers," Yamada said.
The group analyzed the sample powder by powder X-ray diffractometry, TEM-based electron diffractometry and other methods to clarify its complicated crystal structure. The structure thus revealed agreed well with the results of an electron density analysis, according to the group.
As a compound, Li2FeSiO4 is more stable than the pervasive LiCoO2 because the bonding force between Co and O is stronger in Li2FeSiO4.
"Li2FeSiO4 is superior to LiCoO2 not only because the combustion-supporting property, which allows the dissociation of oxygen atoms into oxygen gas at high temperature, is lower but also because the stability as a battery material is higher," Yamada said.
The latest achievement was part of the "R&D of Storage Systems for Smooth Connection to Supply System" conducted under the program called "Strategic R&D of Next Generation Storage Systems for Practical Use," which is promoted by Japan's New Energy and Industrial Technology Development Organization (NEDO).
The crystal structure of lithium iron silicate (Li2FeSiO4) analyzed by the research group. The diagram shows a superlattice structure on which the position and the electron density of each constituent element are indicated. (Diagram courtesy of Atsuo Yamada)
The one-dimensional chain in the crystal structure, which is composed of FeO4 and SiO4tetrahedrons: (a) expected model, (b) one-dimensional chain analyzed by the group; FeO4 and SiO4tetrahedrons are located regularly in a rotating manner, (c) Structure diagram illustrating the basic lattice structure and the superlattice structure indicative of the crystal characteristics. (Diagram courtesy of Atsuo Yamada)
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