Patent Application
20120214067
1. Field of the Invention
The present invention relates to an inorganic material used as a negative electrode active material for a lithium ion secondary battery, and a method for producing the same.
2. Description of the Related Art
In recent years, different types of lithium ion secondary batteries have been developed. Carbon materials have been conventionally used as a negative electrode active material for a lithium ion secondary battery. However, lithium titanium complex oxide materials have recently been developed and attracting public attention. For example, a lithium ion secondary battery using LiCoO2 as the positive electrode active material and Li4Ti5O12 as the negative electrode active material has already been put to practical use.
Li4Ti5O12 is a material having a spinel crystalline structure and is capable of repeatedly absorb and release Li, and therefore Li4Ti5O12 can be used as an active material for a lithium ion secondary battery. Li4Ti5O12 absorbs and releases Li at a potential of about 1.5 V with respect to the standard oxidation-reduction potential of lithium (Li/Li+). Therefore, it is believed that where Li4Ti5O12 is used as a negative electrode active material in a lithium ion secondary battery, lithium metal is unlikely to deposit on the negative electrode even if there occurs a reaction overvoltage resulting from rapid charging, or the like, thus realizing a lithium ion secondary battery with a high degree of safety. It also offers desirable cycle characteristics because there is little lattice dilation from charging/discharging.
Materials have been developed, various properties of which are improved by adding an additive element to Li4Ti5O12. For example, Japanese Laid-Open Patent Publication No. 2000-277116 discloses a material in which a portion of Ti element of Li4Ti5O12 is substituted with a different element selected from V element, Nb element, Mo element and P element for the purpose of improving the output characteristics by increasing the electronic conductivity. However, the discharge capacity at high loads is 83% or less that at low loads, and it cannot be said that the discharge capacity is sufficient.
Japanese Laid-Open Patent Publication No. 2000-156229 discloses a material in which a portion of Ti element of Li4Ti5O12 is substituted with a transition metal element other than Ti element. Japanese Laid-Open Patent Publication No. 2000-156229 reports that the storage stability improves by substituting a portion of Ti element with various transition metal elements. However, the actual syntheses, the resulting production of intended lithium titanium complex oxides, and the property of the produced materials are only reported for cases in which the substituting element is B element, Co element or Zn element. Also, it provides no specific reference to the output characteristics or the electrode capacity density.
Japanese Laid-Open Patent Publication No. 2001-185141 discloses that Li4/3Ti5/3-xFexO4 (0≦x≦0.2) obtained by substituting a portion of Ti element of Li4Ti5O12 with Fe has an improved electronic conductivity as compared with Li4Ti5O12. With Li4Ti5O12 and Li4/3Ti5/3-xFexO4, however, the particle size of primary particle is small, and therefore the packing density in the electrode is low, resulting in a problem that the energy density as an electrode is small.
Embodiments of the present invention aim at solving at least one of the aforementioned problems in the art, and providing a negative electrode active material for a lithium ion secondary battery with a high packing density in the electrode, and a method for producing the same.
The negative electrode active materials for a lithium ion secondary battery according to the embodiments of the present invention contain a lithium titanium complex oxide having a composition expressed as Li4Ti5-x-yFexVyO12 (where 0<x≦0.3, 0<y≦0.05) or Li4Ti5-x-zFexBzO12 (where 0<x≦0.3, 0<z≦0.3). By substituting a portion of Ti element of Li4Ti5-xFexO12 (where 0<x≦0.3) with V element or B element, the primary particle size of the lithium titanium complex oxide increases, thereby increasing the packing density in the electrode and improving the capacity density as an electrode.
According to the embodiments of the present invention, the primary particle size of a lithium titanium complex oxide can be increased by substituting a portion of Ti element of Li4Ti5-xFexO12 (where 0<x≦0.3) with V element or B element. Therefore, it is possible to realize a lithium ion secondary battery with a large capacity density.
Negative electrode active materials for lithium ion secondary batteries according to embodiments of the present invention will now be described with reference to the drawings.
A negative electrode active material for a lithium ion secondary battery according to a first embodiment of the present invention will be described. The negative electrode active material for a lithium ion secondary battery of the present embodiment contains a lithium titanium complex oxide having a composition expressed as Li4Ti5-x-yFexVyO12 (where 0<x≦0.3, 0<y≦0.05). The lithium titanium complex oxide of the present embodiment is a compound in which a portion of Ti element of Li4Ti5-xFexO12 (where 0<x≦0.3) is further substituted with V element. Herein, x and y represent the amounts of substitution of Fe element and V element, respectively.
V element provides an effect of increasing the particle size of the lithium titanium complex oxide. Conventional lithium titanium complex oxide materials typically have primary particle sizes of 1 μm or less. Therefore, it was not possible to realize a large packing density when the electrode is formed by using such a lithium titanium complex oxide material.
In contrast, the present inventors have found that substituting a portion of Ti element with V element increases the primary particle size, thereby improving the packing property as an electrode of a lithium ion secondary battery. Thus, by further substituting Ti element with V element in a lithium titanium complex oxide having a composition expressed as Li4Ti5-xFexO12 (where 0<x≦0.3), it is possible to realize a lithium titanium complex oxide with a large primary particle size. Therefore, when the lithium titanium complex oxide of the present embodiment is used as a negative electrode active material for a lithium ion secondary battery, it is possible to realize a lithium ion secondary battery with a high capacity.
The lithium titanium complex oxide contained in the negative electrode active material for a lithium ion secondary battery of the present embodiment has a spinel crystalline structure. The crystalline structure can be confirmed by X-ray diffraction (XRD).
The amount x of Fe element added preferably satisfies 0<x≦0.3 in the lithium titanium complex oxide of the present embodiment. It has been shown by Japanese Laid-Open Patent Publication No. 2001-185141 that a lithium titanium complex oxide exhibits a desirable electronic conductivity only if a very small portion of Ti element is substituted with Fe. Therefore, x only needs to be greater than 0. An in-depth study by the present inventors has revealed that when x increases, the available discharge capacity of a lithium ion secondary battery using a negative electrode active material of the present embodiment tends to decrease, and it has been shown that the available discharge capacity decreases significantly when x exceeds 0.3. As will be discussed in the Examples section below, it is more preferred that the amount x of Fe element added satisfies 0<x 0.1 in view of the electrode capacity density.
The amount of V element added is preferably 0<y≦0.05. With a portion of Ti substituted with V element, it is possible to obtain the effect of increasing the primary particle size. The reason why the primary particle size increases is not clear. It is nevertheless believed that since the melting point of V2O5, which is the V source, is relatively low at 690° C., and V2O5 is therefore in a molten state during calcining, the diffusion of the V source is very fast, thereby facilitating the growth of particles of the lithium titanium complex oxide. On the other hand, if the amount of V element added exceeds 0.05, there occurs an oxide phase containing no Ti element, and it is then difficult to obtain a single phase of a lithium titanium complex oxide of the spinel crystalline structure. This can be confirmed by X-ray diffraction (XRD) measurement.
A lithium ion secondary battery using a negative electrode active material of the present embodiment with large amounts x and y of substitution of Fe element and V element shows a decrease in the available discharge capacity. That is, the available discharge capacity decreases in accordance with the amounts of substitution of Fe element and V element. However, when a lithium titanium complex oxide of the present embodiment is used as a negative electrode active material for a lithium ion secondary battery, the packing density in the electrode is increased, thereby improving the capacity density as an electrode, due to the effect of increasing the primary particle size, within the aforementioned range for the amounts x and y of substitution.
The lithium titanium complex oxide of the present embodiment may be in the form of primary particles or secondary particles, each being an aggregation of primary particles. In either case, it is preferred that the average particle size d (μm) of the primary particles is 1≦d≦5.
The average particle size of primary particles tends to increase as the amount of substitution of V element increases. Depending on the application, the average particle size d of the lithium titanium complex oxide may be 5 μm or more.
The lithium titanium complex oxide contained in the negative electrode active material for a lithium ion secondary battery of the present embodiment can be synthesized by mixing and calcining a compound containing the constituent elements. Specifically, for example, the production can be done through a step of weighing the Li source, the titanium oxide, the Fe source and the V source to such a proportion that Li, Ti, Fe and V will be in a ratio indicated by the composition formula and uniformly mixing the weighed materials together, and a step of calcining the mixture. As used herein, “uniformly” means that there is no significant unevenness in the distribution on the level of particles of the material.
The Li source may be LiOH or a hydrate thereof, Li2CO3, Li2SO4, LiF, Li2O, or the like. While the LiOH hydrate is typically a monohydrate (LiOH.H2O), LiOH hydrates of other levels of water content may by used. In view of the reaction temperature and the possibility of impurity residue, it is preferred to use LiOH or a hydrate thereof or Li2CO3. The titanium oxide may be one that has the rutile or anatase crystalline structure. In view of the reactivity, it is preferred to use one that has the anatase crystalline structure. The Fe source may be FeO, Fe2O3, Fe3O4, FeO2, α-FeOOH, Fe(OH)3, FeSO4, Fe2(SO4)3, or the like. In view of the reaction temperature, it is preferred to use Fe2O3 or α-FeOOH. It is preferred to use V2O5 as the V source.
The calcining may be done in an air atmosphere, an oxygen atmosphere, or an inert gas atmosphere such as nitrogen or argon. The calcining temperature depends on the Li source, the titanium oxide, the Fe source and the V source used. Where the respective preferred materials described above are used for the Li source, the titanium oxide, the Fe source and the V source, it is possible to obtain a lithium titanium complex oxide having a composition expressed as Li4Ti5-x-yFexVyO12 (where 0<x≦0.3, 0<y≦0.05) by calcining the mixture at a temperature of about 700° C. or more and about 1000° C. or less.
With the negative electrode active material for a lithium ion secondary battery of the present embodiment, since the lithium titanium complex oxide has a larger primary particle size than Li4Ti5-xFexO12 (where 0<x≦0.3) and since the packing density of the negative electrode active material at the negative electrode can be increased, it is possible to realize a lithium ion secondary battery with a high capacity.
A negative electrode active material for a lithium ion secondary battery according to a second embodiment of the present invention will be described. The negative electrode active material for a lithium ion secondary battery of the present embodiment contains a lithium titanium complex oxide having a composition expressed as Li4Ti5-x-zFexBzO12 (where 0<x≦0.3, 0<z≦0.3). The lithium titanium complex oxide of the present embodiment is a compound in which a portion of Ti element of Li4Ti5-xFexO12 (where 0<x≦0.3) is further substituted with B element. Herein, x and z represent the amounts of substitution of Fe element and B element, respectively.
B element, like V element, has the effect of increasing the particle size of the lithium titanium complex oxide. The present inventors have found that substituting a portion of Ti element with B element, as with V element, increases the primary particle size of the lithium titanium complex oxide, thereby improving the packing property as an electrode of a lithium ion secondary battery. Thus, by further substituting Ti element with B element in a lithium titanium complex oxide having a composition expressed as Li4Ti5-xFexO12 (where 0<x≦0.3), it is possible to realize a lithium titanium complex oxide with a large primary particle size. Therefore, when the lithium titanium complex oxide of the present embodiment is used as a negative electrode active material for a lithium ion secondary battery, it is possible to realize a lithium ion secondary battery with a high capacity.
As in the first embodiment, the lithium titanium complex oxide contained in the negative electrode active material for a lithium ion secondary battery of the present embodiment has a spinel crystalline structure. The crystalline structure can be confirmed by X-ray diffraction (XRD).
The amount x of Fe element added preferably satisfies 0<x≦0.3 in the lithium titanium complex oxide of the present embodiment. This is based on a reason similar to that of the first embodiment.
The amount z of B element added is preferably 0<z≦0.3. With a portion of Ti substituted with B element, it is possible to obtain the effect of increasing the primary particle size. The reason why the primary particle size increases is not clear. It is nevertheless believed that since the melting point of B2O3, which is the B source, is relatively low at 480° C., and B2O3 is therefore in a molten state during calcining, the diffusion of the B source is very fast, thereby facilitating the growth of particles of the lithium titanium complex oxide. It is believed that also when HBO3 is used as the B source, the growth of particles of the lithium titanium complex oxide is facilitated similarly because HBO3 decomposes into B2O3 at around 169° C.
On the other hand, if the amount of B element added exceeds 0.3, there occurs an oxide phase containing no Ti element, and it is then difficult to obtain a single phase of a lithium titanium complex oxide of the spinel crystalline structure. This can be confirmed by X-ray diffraction (XRD) measurement.
A lithium ion secondary battery using a negative electrode active material of the present embodiment with large amounts x and z of substitution of Fe element and B element shows a decrease in the available discharge capacity. That is, the available discharge capacity decreases in accordance with the amounts of substitution of Fe element and B element. However, when a lithium titanium complex oxide of the present embodiment is used as a negative electrode active material for a lithium ion secondary battery, the packing density in the electrode is increased, thereby improving the capacity density as an electrode, due to the effect of increasing the primary particle size, within the aforementioned range for the amounts x and z of substitution.
The lithium titanium complex oxide of the present embodiment may be in the form of primary particles or secondary particles, each being an aggregation of primary particles. In either case, it is preferred that the average particle size d (μm) of the primary particles is 1≦d≦11. The average particle size of primary particles tends to increase as the amount of substitution of V element increases. Depending on the application, the average particle size d of the lithium titanium complex oxide may be 11 μm or more.
As in the first embodiment, the lithium titanium complex oxide contained in the negative electrode active material for a lithium ion secondary battery of the present embodiment can also be synthesized by mixing and calcining a compound containing the constituent elements. Specifically, for example, the production can be done through a step of weighing the Li source, the titanium oxide, the Fe source and the B source to such a proportion that Li, Ti, Fe and B will be in a ratio indicated by the composition formula and uniformly mixing the weighed materials together, and a step of calcining the mixture.
The Li source may be LiOH or a hydrate thereof, Li2CO3, Li2SO4, LiF, Li2O, or the like. While the LiOH hydrate is typically a monohydrate (LiOH.H2O), LiOH hydrates of other levels of water content may by used. In view of the reaction temperature and the possibility of impurity residue, it is preferred to use LiOH or a hydrate thereof or Li2CO3. The titanium oxide may be one that has the rutile or anatase crystalline structure. In view of the reactivity, it is preferred to use one that has the anatase crystalline structure. The Fe source may be FeO, Fe2O3, Fe3O4, FeO2, α-FeOOH, Fe(OH)3, FeSO4, Fe2(SO4)3, or the like. In view of the reaction temperature, it is preferred to use Fe2O3 or α-FeOOH. It is preferred to use H3BO3 or B2O3 the B source.
The calcining may be done in an air atmosphere, an oxygen atmosphere, or an inert gas atmosphere such as nitrogen or argon. The calcining temperature depends on the Li source, the titanium oxide, the Fe source and the B source used. Where the respective preferred materials described above are used for the Li source, the titanium oxide, the Fe source and the B source, it is possible to obtain a lithium titanium complex oxide having a composition expressed as Li4Ti5-x-zFexBzO12 (where 0<x≦0.3, 0<z≦0.3) by calcining the mixture at a temperature of about 700° C. or more and about 1000° C. or less.
With the negative electrode active material for a lithium ion secondary battery of the present embodiment, since the lithium titanium complex oxide has a larger primary particle size than Li4Ti5-xFexO12 (where 0<x≦0.3) and since the packing density of the negative electrode active material at the negative electrode can be increased, it is possible to realize a lithium ion secondary battery with a high capacity.
Hereinafter, results of synthesizing negative electrode active materials for lithium ion secondary batteries, which are examples of the present invention, and examining various properties thereof, will be described.
The material powders of LiOH.H2O, TiO2, Fe2O3 and V2O5 were weighed so that the molar ratio Li/Ti/Fe/V is 4/(5−x−y)/x/y (x=0.01, y=0.01), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2, Fe2O3 and V2O5 were weighed so that the molar ratio Li/Ti/Fe/V is 4/(5−x−y)/x/y (x=0.01, y=0.05), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2, Fe2O3 and V2O5 were weighed so that the molar ratio Li/Ti/Fe/V is 4/(5−x−y)/x/y (x=0.05, y=0.05), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2, Fe2O3 and V2O5 were weighed so that the molar ratio Li/Ti/Fe/V is 4/(5−x−y)/x/y (x=0.3, y=0.01), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2, Fe2O3 and V2O5 were weighed so that the molar ratio Li/Ti/Fe/V is 4/(5−x−y)/x/y (x=0.3, y=0.05), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2, Fe2O3 and HBO3 were weighed so that the molar ratio Li/Ti/Fe/B is 4/(5−x−z)/x/z (x=0.01, z=0.01), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2, Fe2O3 and HBO3 were weighed so that the molar ratio Li/Ti/Fe/B is 4/(5−x−z)/x/z (x=0.01, z=0.3), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2, Fe2O3 and HBO3 were weighed so that the molar ratio Li/Ti/Fe/B is 4/(5−x−z)/x/z (x=0.05, z=0.05), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2, Fe2O3 and HBO3 were weighed so that the molar ratio Li/Ti/Fe/B is 4/(5−x−z)/x/z (x=0.3, z=0.01), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2, Fe2O3 and HBO3 were weighed so that the molar ratio Li/Ti/Fe/B is 4/(5−x−z)/x/z (x=0.3, z=0.3), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2, Fe2O3 and HBO3 were weighed so that the molar ratio Li/Ti/Fe/B is 4/(5−x−z)/x/z (x=0.1, z=0.01), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2, Fe2O3 and HBO3 were weighed so that the molar ratio Li/Ti/Fe/B is 4/(5−x−z)/x/z (x=0.1, z=0.3), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2, Fe2O3 and V2O5 were weighed so that the molar ratio Li/Ti/Fe/V is 4/(5−x−y)/x/y (x=0.1, y=0.01), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2, Fe2O3 and V2O5 were weighed so that the molar ratio Li/Ti/Fe/V is 4/(5−x−y)/x/y (x=0.1, y=0.05), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2, Fe2O3 and V2O5 were weighed so that the molar ratio Li/Ti/Fe/V is 4/(5−x−y)/x/y (x=0.05, y=0.02), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O and TiO2 were weighed so that the molar ratio Li/Ti is 4/5, and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2 and Fe2O3 were weighed so that the molar ratio Li/Ti/Fe is 4/(5−x)/x (x=0.01), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2 and Fe2O3 were weighed so that the molar ratio Li/Ti/Fe is 4/(5−x)/x (x=0.05), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2 and Fe2O3 were weighed so that the molar ratio Li/Ti/Fe is 4/(5−x)/x (x=0.1), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2 and Fe2O3 were weighed so that the molar ratio Li/Ti/Fe is 4/(5−x)/x (x=0.3), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2 and VO5 were weighed so that the molar ratio Li/Ti/V is 4/(5−y)/y (y=0.01), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2 and VO5 were weighed so that the molar ratio Li/Ti/V is 4/(5−y)/y (y=0.05), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2 and VO5 were weighed so that the molar ratio Li/Ti/V is 4/(5−y)/y (Y=0.1), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2 and HBO3 were weighed so that the molar ratio Li/Ti/B is 4/(5−z)/z (z=0.01), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2 and HBO3 were weighed so that the molar ratio Li/Ti/B is 4/(5−z)/z (z=0.05), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2 and HBO3 were weighed so that the molar ratio Li/Ti/B is 4/(5−z)/z (z=0.1), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2 and HBO3 were weighed so that the molar ratio Li/Ti/B is 4/(5−z)/z (z=0.3), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2 and HBO3 were weighed so that the molar ratio Li/Ti/B is 4/(5−z)/z (z=0.75), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
The material powders of LiOH.H2O, TiO2 and α-FeOOH were weighed so that the molar ratio Li/Ti/Fe is 4/(5−x)/x (x=0.3), and mixed together in a mortar. The material TiO2 used was one having an anatase crystalline structure and an average particle size of about 0.3 μm.
The mixture of the material powders was put in an Al2O3 crucible and calcined in an electric furnace in an air atmosphere. The calcining temperature was 850° C., and the calcining temperature holding time was 12 hours.
The calcined material was taken out of the crucible and crushed in a mortar to obtain an intended lithium titanium complex oxide.
Powder X-ray diffraction (XRD) measurements were done in order to confirm the crystalline structure of the lithium titanium complex oxides of Examples 1-15 and Reference Examples 1-14. An XRD measurement apparatus from Rigaku Corporation was used for the measurements.
These results confirmed that the lithium titanium complex oxides of Examples 1-15 and Reference Examples 1-7, 9-12 and 14 had a single spinel phase. The lithium titanium complex oxide of Reference Example 8 contains a small amount of an Li3VO4 phase, in addition to the spinel phase. The lithium titanium complex oxide of Reference Example 13 contains a small amount of an Li2B4O7 phase, in addition to the spinel phase.
These results showed that it is possible to obtain a lithium titanium complex oxide including a single spinel phase if the composition of the lithium titanium complex oxide is in the ranges of Examples 1-15 and Reference Examples 1-7, 9-12 and 14, i.e., 0<x≦0.3, 0<y≦0.05, 0<z≦0.3. It was also shown that it is not possible to obtain a single spinel phase when the amount y of V added is excessive as in Reference Example 8 or when the amount z of B added is excessive as in Reference Example 13.
With the measurement results of Reference Example 5 and Reference Example 14 being substantially equal to each other, it was shown that the same lithium titanium complex oxide is obtained whether Fe2O3 or α-FeOOH is used as the Fe source.
In the examples of the present invention, the average particle size of the primary particles was evaluated in terms of the “average particle size d” as defined below. We arbitrarily selected 30 particles from a SEM image to measure the primary particle sizes, and calculated the average thereof, thus estimating the “average particle size d”.
Typically, the “cumulative average particle size d50” in particle size distribution measurement is often used as the average particle size. However, the particle size distribution measurement is a measurement of the size of aggregated particles (secondary particles), as opposed to the size of primary particles. There is no correlation between the size of primary particles and the size of secondary particles. Therefore, the “average particle size d”, which is a unit representing the size of primary particles, is suitable for demonstrating the effects from the examples of the present invention.
A scanning electron microscope (SEM) was used to examine the “average particle size d” for the lithium titanium complex oxides of Examples 1-15 and Reference Examples 1-7 and 9-12, which had a single spinel phase. An apparatus from Hitachi High-Technologies Corporation was used.
Table 2 shows the average particle size d for the lithium titanium complex oxides of Examples 1-15 and Reference Examples 1-7 and 9-12, calculated from SEM images.
The average particle size d of the lithium titanium complex oxides of Reference Examples 1-5 is about 0.8 μm to about 0.9 μm, whereas the average particle size d of the lithium titanium complex oxides of Examples 1-15 and Reference Examples 6-7 and 9-12 is larger and about 1 μm to about 11 μm. These results showed that substituting Ti element only with Fe element provides no significant change to the primary particle size, whereas substituting Ti element with B element or V element provides the effect of increasing the primary particle size.
For the lithium titanium complex oxides of Examples 1-15 and Reference Examples 1-7 and 9-12, the pressed density was measured as a measure of the packing property when made into an electrode. A powder resistance measurement system from Mitsubishi Chemical Analytech Co., Ltd. was used for the measurement. The density under an applied pressure of 64 MPa was determined as the pressed density.
Table 2 shows the measurement results.
As compared with the pressed densities of Reference Examples 1-5 from about 2.1 g/cm3 to about 2.2 g/cm3, those of Examples 1-15 and Reference Examples 6-7 and 9-12 were larger and from about 2.3 g/cm3 to about 2.9 g/cm3.
The results of the average particle size and the pressed density showed the tendency that larger average particle sizes lead to larger pressed densities.
Electrodes were produced using the lithium titanium complex oxides of Examples 1-15 and Reference Examples 1-7 and 9-12 as the active material. The active material, a conductive material and a binder were weighed to a weight ratio of 85/10/5, and mixed together in a mortar. Acetylene black and PTFE were used as the conductive material and the binder, respectively. After mixing, the mixture was rolled out with a roller and punched into pellet-shaped electrodes.
Batteries were produced using these electrodes in order to examine properties as a negative electrode active material for a lithium ion secondary battery. Typically, in a lithium ion secondary battery, a lithium transition metal complex oxide (typically containing a transition metal such as Co, Mn or Ni) such as LiCoO2 is used as the positive electrode active material. In the examples of the present invention, however, a metal Li was used, instead of a common positive electrode active material, in the counter electrode, in order to examine the properties of the negative electrode active material per se, independent of the positive electrode active material. Methods like this are common in evaluating active materials.
Coin batteries were produced. Each of the electrodes produced in accordance with the examples and the reference examples was stacked with a separator impregnated with electrolyte and a metal Li plate in this order, and sealed in a coin-shaped case, obtaining a battery. The separator includes a PE microporous membrane from Asahi Kasei E-materials Corporation and a PP non-woven fabric from Tapyrus Co., Ltd., layered together in the order PP/PE/PP. The electrolyte included 1 mol/L of LiPF6 dissolved in a solvent mixed so that EC/EMC=⅓.
Batteries produced using the lithium titanium complex oxides of Examples 1-15 and Reference Examples 1-7 and 9-12 as active materials will be referred to as batteries containing the active materials of Examples 1-15 and Reference Examples 1-7 and 9-12, respectively.
Each produced battery was charged and then discharged so as to examine the available discharge capacity thereof. A charge-discharge system from Nagano Co., Ltd. was used for the charge-discharge test. The charge-discharge test was performed so that the voltage range was from 1 V to 3 V and the current rate was 0.02 C rate. Herein, 1 C rate is defined as a current value representing the discharge rate over 1 hour, and 0.02 C rate is the current value that is 0.02 time 1 C rate, i.e., a current value representing the discharge rate over 50 hours.
Table 2 shows the available discharge capacity measurement results obtained as described above for the batteries containing the active materials of Examples 1-15 and Reference Examples 1-7 and 9-12.
As can be seen from
The capacity density per volume of an electrode directly contributes to the energy density of the battery, and an improvement thereof has been sought for. The electrode capacity density is expressed as the product between the density of the active material in the electrode, the weight capacity density of the active material, and the discharge average voltage. The pressed density of the active material can be used as one measure of the density of the active material in the electrode, i.e., the packing property. The weight capacity density of the active material is the available discharge capacity measured as described above. Thus, in the examples of the present invention, as an evaluation measure of the electrode capacity density, the “electrode capacity density ρ” is defined and calculated as the product between the pressed density and the available discharge capacity.
Table 2 shows the results of the “electrode capacity density ρ” for batteries containing the active materials of Examples 1-15 and Reference Examples 1-7 and 9-12 obtained as described above.
It can be seen from
It was also shown that the “electrode capacity densities ρ” of batteries containing the active materials of Examples 1-15 with B element or V element added thereto, in addition to Fe element, were generally equal to or greater than those of Reference Examples 2-5 with no B element or V element added thereto. It is believed that this is because although the available discharge capacity decreases in accordance with the amounts of B element and V element added, the effect of increasing the pressed density has a significant contribution. Therefore, the “electrode capacity densities ρ” of the batteries containing the active materials of Examples 4, 5 and 9, in which B element or V element is further contained, are greater than that of Reference Example 1, even though the amount x of Fe element added is 0.3. The results of Reference Examples 6-7 and 9-12, with respect to Reference Example 1, also have a similar tendency.
Negative electrode active materials for lithium ion secondary batteries according to the embodiments of the present invention, when used as an electrode, give a high capacity density, and are useful as negative electrode active materials for lithium ion secondary batteries for mobile applications. They can also be used for applications such as large batteries, electric vehicles, etc.
A person of ordinary skill in the art will understand that various changes and/or modifications can be made to the prevent invention as illustrated by specific embodiments set forth above, without departing from the sprit and scope of the invention broadly defined herein. Thus, the embodiments are in any respect illustrative of the present invention and not limiting thereof. Other features and aspects of the present invention will be appreciated by a person of ordinary skill in the art upon reading and understanding the disclosure herein. It is clear that expected changes and modifications to such features, aspects, and reported results and examples are included within the scope of the present invention, which is only defined by the appended claims.
This application claims priority to U.S. Provisional Patent Application No. 61/442,958 filed Feb. 15, 2011, the disclosure of which is incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61442958 | Feb 2011 | US |
