WO2006057146A2

WO2006057146A2 - Method of producing electrode active material

Info

Publication number: WO2006057146A2
Application number: PCT/JP2005/020324
Authority: WO
Inventors: Shigeto Okada; Junichi Yamaki; Yasunori Okazaki; Motoshi Isono; Syoji Yokoishi
Original assignee: Toyota Jidosha Kabushiki Kaisha; National University Corporation Kyushu University
Priority date: 2004-11-25
Filing date: 2005-10-31
Publication date: 2006-06-01
Also published as: EP1812341A2; CA2583391A1; WO2006057146A3; KR20070086541A; CN101065322A; JP2006155941A

Abstract

An object of the present invention is to provide a method of more efficiently producing an electrode active material whose main component is a transition metal phosphate compound. According to the present invention, a method of producing an electrode active material is provided in which the main component thereof is a phosphate compound represented by the general formula: AxM(PO4) y (here, 0 ≤ x ≤ 2, 0 < y ≤ 2, A is an alkali metal, and M is a transition metal). This method comprises preparing a composition in the melted state that contains a source of M and a source of phosphorus (and also a source of A when 0 < x). This method is suitable as a method of producing an electrode active material whose main component is, for example, an olivine-type lithium iron phosphate compound.

Description

DESCRIPTION

METHOD OF PRODUCING ELECTRODE ACTIVE MATERIAL

Technical Field The present invention relates to a method of producing a phosphate compound represented by a general formula A_xM(Pθ₄)_y (wherein 0 < x < 2, 0 < y < 2, A is one element or two or more elements selected from alkali metals, and M is one element or two or more elements selected from transition metals) that can be used as an electrode active material of a secondary battery. In addition, the present invention relates to a non-aqueous electrolyte secondary battery that employs the electrode active material.

Background Art

Secondary batteries are known that are charged and discharged by means of cations such as lithium ions and the like traveling between a pair of electrodes. A typical example of this type of the secondary battery is a lithium secondary battery (typically a lithium ion battery). A material that can store/release cations can be employed as the electrode active material of the secondary battery. The term of the electrode active material in this specification and claims means a material that is used for forming an electrode of the secondary battery. The electrode may include conductive material(s) and binder(s) in addition to the electrode active material. In usual, the electrode active material is used with a metal piece connected to a terminal of the secondary battery. In general, the electrode active material is in the form of powders, and a paste including the electrode active powders is coated on the metal piece. The paste may include conductive powders and binder(s) in addition to the electrode active powders. The electrode active powders may be compressed to form the electrode. Various materials are being studied as anode active materials and cathode active materials of this type of the secondary battery. For example, Japanese Laid Open Patent Application Publication No. H9- 134724 discloses a non-aqueous electrolyte secondary battery containing an olivine-type iron phosphate compound represented by the formula LiFePO₄ as the anode active material. In addition, Japanese Laid Open Patent Application Publication No. 2000-509193 discloses an electrode active material composed of a Nasicon-type iron phosphate compound represented by the formula Li₃Fe₂(PO₄)₃. Other prior art references relating to electrode active materials include Japanese Laid Open Patent Application Publication No. H9- 134725, No. 2001-250555, No. 2002-15735, and No. H8-83606.

According to intensive study, it was proved that the phosphate compound (compound of transition metal and phosphate or compound of alkali metal, transition metal and phosphate) represented by the general formula A_xM(P0₄)_y (wherein 0 < x < 2, 0 < y < 2, A is one element or two or more elements selected from alkali metals, and M is one element or two or more elements selected from transition metals) could be used as the electrode active material of the secondary battery. In the following, the definition of x, y, A and M may be omitted when the above definition is applied.

Disclosure of Invention

In the prior art, the phosphate compound suitable for the electrode active material is produced by means of a solid state reaction (baking). A comparatively long period of time is generally needed in this type of solid state reaction. It would be useful if a more efficient method of producing the phosphate compound could be provided.

Accordingly, an object of the present invention is to provide a more efficient method of producing the phosphate compound represented by the general formula A_xM(Pθ₄)_y that can be adopted as the electrode active material.

Another object of the present invention is to provide a secondary battery comprising this type of the electrode active material.

The present inventors discovered that the phosphate compound represented by the general formula A_xM(PO₄)_y can be produced with better efficiency by slow cooling a melted composition of raw materials from a melted state (liquid state) to a solid state.

One invention disclosed herein relates to a method of producing a phosphate compound (compound of transition metal and phosphate or compound of alkali metal, transition metal and phosphate) represented by the general formula A_xM(PO₄)_J, (here, 0 < x < 2, 0 < y < 2, A is one element or two or more elements selected from alkali metals, and M is one element or two or more elements selected from transition metals). The production method comprises a step of preparing a composition of raw materials in the melted state. The raw materials contain a source of the A in A_xM(P0₄)_y, a source of the M in A_xM(P0₄)_y, and a source of phosphorus (P), when x in A_xM(PO₄)_y is not 00 (x ≠ 0), i.e., when x is greater than 00 (0 < x). When x in A_xM(PO₄)_y is 00 (x = 0), the raw materials contain the source of the M in A_xM(PO₄)_y and the source of phosphorus (P). This type of melted composition can be prepared by, for example, mixing a solid raw material containing a source of the M and a solid raw material containing a source of phosphorus, as well as a solid raw material containing a source of the A when x is greater than 00, and then heating the raw materials into the melted state. The production method further comprises a step of slow cooling the melted composition slowly. Here, "slow cooling" is a concept that is opposite to quenching, and means reducing the temperature comparatively slowly.

There may not be clear boundary between slow cooling and quenching. It is conformed that non¬ crystalline phosphate is produced when the melted compound is cooled rapidly, on the other hand, crystalline phosphate is produced when the melted compound is cooled slowly. That is, quick cooling (quench) whose cooling rate is faster than a predetermined speed grows non¬ crystalline phosphate and slow cooling whose cooling rate is slower than the predetermined speed grows crystalline phosphate.

The non-crystalline phosphate and crystalline phosphate can be distinguished by the X- ray diffraction pattern. The non-crystalline phosphate dose not show peaks in the X-ray diffraction pattern, on the other hand, crystalline phosphate shows peaks corresponding to crystal structure of the crystalline phosphate in the X-ray diffraction pattern.

According to this production method, a compound that exhibits useful characteristics as an electrode active material can be produced with better efficiency (e.g., in a shorter period of time). In one preferred aspect of the method, the melted composition and/or the raw materials for forming the melted composition contain(s) the A, the M, and phosphorus in an atomic ratio that is substantially 1 :1:1. The melted composition having this atomic ratio is suitable to produce a phosphate compound in which x and y in the formula A_xM(Pθ₄)_y are both substantially 1. In other words, the melted composition having this atomic ratio is suitable to produce the phosphate compound represented by the formula AMPO₄ that corresponds to the olivine type. AMPO₄ of the olivine type has superior characteristics for the electrode active material.

In another preferred aspect, the melted composition and/or the raw materials for forming the melted composition do(es) not contain the A. The melted composition and/or the raw materials contain(s) the M and phosphorus in an atomic ratio of substantially 1:1. The melted composition having this atomic ratio is suitable to produce a phosphate compound in which x in the formula A_xM(P0₄)_y is 0 and y in the formula A_xM(P0₄)_y is substantially 1. In other words, the melted composition having this atomic ratio is suitable to produce the phosphate compound represented by the formula MPO₄ that corresponds to the olivine type. MPO₄ of the olivine type also has superior characteristics for the electrode active material. Sources of the M may be compounds having M as a constituent element (hereinafter also referred to as " M compounds" ). M compounds may be selected from compounds in which the valence number of the M is higher than the valence number of the M in A_xM(Pθ₄)_y. In addition, M compounds may be selected from compounds in which the valence number of the M is equal to the valence number of the M in A_xM(P0₄)_y. Alternatively, M compound in which the valence number of the M is higher than the valence number of the M in A_xM(Pθ₄)_y may be used together with M compound in which the valence number of the M is equal to or less than the valence number of the M in A_xM(Pθ4)_y. According to the production method disclosed herein, the options for the sources of the M can be broadened to low reactivity oxide compounds that could not be used in the conventional solid state baking method. In particular, the merits of oxide raw materials are that they are generally less expensive than more reactive materials such as ammonium salts, acetates, oxalates, and the like. Further, oxide raw materials generate little malodorous or toxic reactive by-product gases. Thus, the production method disclosed herein is extremely efficient for shortening production time, reducing process costs, reducing raw material costs, and the like.

When M compound in which the valence number of the M is higher than the valence number of the M in A_xM(Pθ₄)_y is to be used as part or all of the source of the M, it is preferable that the composition in the melted state (melted composition) contain a reducing agent. In this way, the desired phosphate compound A_xM(Pθ₄)_y can be produced with good efficiency. Carbon powder can, for example, be used as the reducing agent, hi addition, instead of using the reducing agent, or in addition to the reducing agent, it is also useful to increase maximum temperature of the melted composition (hereinafter the maximum temperature means the highest temperature of the melted composition during the production process).

The invention disclosed herein can be applied for producing an electrode active material whose main component is a phosphate compound in which the M is primarily iron (Fe). For example, the invention is suitable for producing an iron phosphate compound A_xFe" (PC«₄)_y in which both x and y are 1 (typically, an olivine-type iron phosphate compound represented by AFe^π(Pθ₄)). The invention is also suitable for producing an iron phosphate compound A_xFe¹¹¹ (PO₄)_y in which x is 0 and y is 1 (typically, an olivine-type iron phosphate compound represented by Fe^m(PO₄)). A particularly preferred application is the iron phosphate compound in which both x and y in A_xM(Pθ₄)_y are substantially 1. According to the method disclosed herein, not only can a compound containing bivalent iron as a constituent element (e.g., FeO) be selected as a raw material (a source of iron) for producing the olivine-type phosphate compound having bivalent iron described above, but a compound containing trivalent iron as a constituent element (e.g., Fe₂O₃) can also be selected as a raw material. When trivalent iron oxide (Fe₂O₃ and the like) is to be used as part of or the entire source of iron, it is preferable to add the reducing agent (carbon powder) to the melted composition, and/or comparatively increase the maximum temperature of the melted composition. In this way, an electrode active material whose main component is the desired iron phosphate compound can be produced with good efficiency. The electrode active material obtained by any of the methods described above can be suitably employed as a constituent material of a secondary battery (typically a lithium ion secondary battery). The secondary battery described above comprises, for example, a first electrode (an anode or a cathode) having any of the electrode active materials described above, a second electrode (an electrode that is opposite to the first electrode, e.g., a cathode or an anode) having a material that will store/discharge cations, and a non-aqueous-type electrolyte or solid electrolyte.

Yet another invention disclosed herein relates to a secondary battery. This secondary battery comprises an anode having an electrode active material obtained by any of the methods described above. In addition, the secondary battery comprises a cathode having a material that stores/discharges alkali metal ions. For example, if the battery of the present invention is a lithium secondary battery, it will comprise a cathode having a material that stores/discharges lithium ions. Furthermore, the secondary battery comprises a non-aqueous-type electrolyte or a solid electrolyte. According to the present invention, a secondary battery having the aforementioned construction can be produced with good efficiency.

Another aspect of the invention disclosed herein is an anode active material for a secondary battery that is manufactured by any of the methods described above. A typical example of the active material of the present invention is an anode active material for a secondary battery whose main component is a substantially crystalline phosphate compound represented by A_xM(P0₄)_y.

Brief Description of Drawings

Figure 1 shows an X-ray profile of Sample 1.

Figure 2 shows an X-ray profile of Sample 2. Figure 3 shows an X-ray profile of Sample 3.

Figure 4 shows an X-ray profile of Sample 4.

Figure 5 shows an X-ray profile of Sample 5.

Figure 6 shows an X-ray profile of Sample 6.

Figure 7 shows an X-ray profile of Sample 7. Figure 8 shows a charge/discharge profile of Sample 1.

Figure 9 shows a graph showing a cycle characteristics of Sample 1.

Figure 10 shows a graph showing a rate characteristics of Sample 1.

Figure 11 shows a charge/discharge profile of Sample 2.

Figure 12 shows a graph showing a cycle characteristics of Sample 2. Figure 13 shows a graph showing a rate characteristics of Sample 2. Figure 14 shows a charge/discharge profile of Sample 3. Figure 15 shows a graph showing a cycle characteristics of Sample 3. Figure 16 shows a graph showing a rate characteristics of Sample 3. Figure 17 shows a charge/discharge profile of Sample 5.

Figure 18 shows a graph showing a cycle characteristics of Sample 5.

Figure 19 shows a cross-section schematically showing a structure of a measurement cell.

Best Mode for Carring Out the Invention Preferred embodiments of the present invention will be described below. Note that matters, other than those specifically referred to in the present specification, that are essential to practice the present invention will be understood as design matters to one of ordinary skill in the art based upon the prior art in this field. The present invention can be performed based upon the details disclosed in the present specification and the common technical knowledge in this field. The production method of the present invention can be applied to the production of an electrode active material whose main component is a transition metal phosphate compound represented by the general formula A_xM(PO₄)_J,. The M in A_xM(P0₄)_y is one element or two or more elements selected from transition metals. Specific examples of the M include iron (Fe), vanadium (V), titanium (Ti), and the like. The A in A_xM(P0₄)_y is one element or two or more elements (typically, one element) selected from alkali metals such as lithium (Li), sodium (Na), potassium (K), and the like, "x" is a number that satisfies 0 < x < 2 (typically, 0 < x < 2, however x may be 0). In addition, "y" is a number that satisfies 0 < y < 2 (y can not be 0). Because the electrochemical equivalent of the compound represented by A_xM(PO₄)_y is relatively small, a larger theoretical capacity can be achieved. According to the production method of the present invention, this type of useful electrode active material can be produced with good efficiency.

Electrode active materials that can be produced by means of the method disclosed in the present specification include an electrode active material whose main component is a compound in which the M of A_xM(PO₄)_y is primarily Fe. Preferably, about 75% or more of the M in A_xM(Pθ₄)_y is Fe. More preferably, about 90% or more of the M in A_xM(PO₄)_y is Fe. Even more preferably, substantially all of the M in A_xM(PO₄)_y is Fe. An electrode active material which is to be used in a secondary battery that stores/discharges electricity by means of lithium ions traveling between a pair of electrodes is preferably one whose main component is a compound in which x in A_xM(PO₄)_y is greater than 0, and the A in the formula is primarily lithium (Li). In addition, an electrode active material which is to be used in a secondary battery that stores/discharges electricity by means of sodium ions traveling between a pair of electrodes is preferably one whose main component is a compound in which x in A_xM(PO₄)_y is greater than 0, and the A in the formula is primarily sodium (Na).

The method disclosed herein can be applied for producing an electrode active material whose main component is a compound having the olivine type, in which x and y in A_xM(PO₄)_y are substantially x = y = 1 (a compound represented by AM^π(PO₄), e.g., LiFe^π(PO₄)). Alternatively, the method disclosed herein can be applied for producing an electrode active material whose main component is a compound having the olivine type, in which x and y in A_xM(PO₄)_y are substantially x = 0, y = 1 (a compound represented by M^IH(PO₄), e.g., Fe¹¹¹CPO₄)). In the production of the aforementioned olivine-type materials (in particular, olivine-type materials in which the M is primarily bivalent), particularly good effects will be exhibited by adopting the method of the present invention.

In addition, the method disclosed herein can also be applied for producing an electrode active material whose main component is a compound having the Nasicon type, in which x and y in A_xM(P0₄)_y are substantially x = y = 1.5 (A₃M^iπ ₂(PO₄)₃). Compounds represented by A₃M^iπ ₂(PO₄)₃ include, for example, Li₃Fe^πi ₂(PO₄)₃. The effects due to the adoption of the method of the present invention will be suitably exhibited even in the production of the Nasicon type material described above.

When the method disclosed herein is to be applied for producing an electrode active material whose main component is a compound in which x in A_xM(P0₄)_y is greater than 0 (i.e., 0 < x), a composition in the melted state (melt) will be prepared that contains a source of the M, a source of P (phosphorus), and a source of the A. Typically, the atomic ratio (molar ratio) of the M, P and A in the melt is the atomic ratio that corresponds to the electrode active material (A_xM(P0₄)_y) to be produced (target compound). For example, a solid composition of raw materials may be prepared that contains the M, P and A in an atomic ratio corresponding to the target compound, and the composition of raw materials may be melted. Normally, it is appropriate for the atomic ratio of the M, P and A to substantially match between the melt and/or the composition of raw materials and the target compound.

Compounds having any one of the M, P and A as a constituent element can be used respectively as a source of the M, a source of P, and a source of the A. In other words, a compound having the M as a constituent element (M compound) can be used as a source of the M. An oxide of the M or a compound that produces an oxide of the M by means of heating (a carbonate, hydrogen carbonate, acetate, oxalate, halogenate, hydroxide, or the like of the M) can, for example, be used as the M compound. A phosphorus compound having P as a constituent element can be used as the source of P. For example, an oxide of phosphorus or a compound that produces an oxide of phosphorus by means of heating (an oxide such as P₂O₅ and the like, and ammonium salts such as NH₄H₂PO₄, (NH₄)₂HPO₄, and the like) can be used. A compound having the A as a constituent element (A compound) can be used as a source of the A. For example, a salt of the A (a carbonate, hydrogen carbonate, acetate, oxalate, halogenate, hydroxide, and the like) can be used. Each source is composed of one compound or two or more compounds. When the A is primarily lithium, the source of lithium can be one compound or two or more compounds selected from lithium salts such as lithium carbonate (Li₂CO₃), lithium hydroxide (LiOH), and the like. Note that by selecting the compound that functions not only as a source of the A but also as a flux (e.g., Li₂CO₃), the melting point of the melt can be reduced.

Alternatively, a compound having any two or more elements amongst the M, P and A as constituent elements may be used as a source of the M, a source of P, and a source of the A. For example, a compound having P and the A as constituent elements can be used as a source of P and a source of the A, and a compound having the M as a constituent element can be used as a source of the M. Furthermore, a compound having the M, P and the A as constituent elements may be used as a common source of the M, P and the A. For example, one type or two or more types of a phosphorus compound represented by A_xM(P0₄)_y can also be employed as a source of the M, P and the A. Note that A_xM(Pθ4)_y used as a source may be crystalline or non-crystalline, or a mixture of crystalline and non-crystalline. Because the atomic ratio of each element (M, P and A) is easily adjusted, it is normally preferable to use at least three kinds of compounds, each compound having one element amongst M, P and A respectively, to prepare a composition of raw materials and/or a melt (composition of raw materials in a melted state).

A compound can be selected as the source of the M in which the valence number of the M in the compound is equal to the valence number of the M in the desired A_xM(PO₄^. For example, in order to produce an olivine-type electrode active material represented by AM^π(P0₄), a compound can be selected in which M is bivalent. In order to produce a Nasicon-type electrode active material represented by A₃M^IH ₂(PO₄)₃, a compound can be selected in which M is trivalent. When the M is an element having tendency that it is more stable in the trivalent state than in the bivalent state (e.g., iron), and the target material is an electrode active material in which the valence number of the M in A_xM(PO₄)_y is primarily 2, the production steps of the electrode active material can be carried out under reducing conditions in order to prevent the bivalent M source (e.g., FeO) from being oxidized and becoming trivalent.

In addition, a compound may be selected as the source of the M in which the valence number of the M in the source is higher than the valence number of the M in the target material. According to the method disclosed herein, even when the M has tendency that it is more stable in the trivalent state than in the bivalent state, and the M in the target material is primarily bivalent, still the selection of a trivalent M source is possible. For example, when the target material is an olivine-type material represented by LiFe^π(PO₄) containing bivalent Fe, Fe₂O₃ containing trivalent Fe can be used as the source of Fe. In other words, according to the method of the present invention, a trivalent M source can be used to produce a material having bivalent M as a constituent element (e.g., an olivine-type electrode active material represented by AM^U(PO₄)). This is important in the production of an electrode active material that is substantially composed from an olivine-type material in which, for example, the M in A_xM(P0₄)_y is primarily bivalent iron (Fe). This is because Fe₂O₃ is clearly less expensive to use as an iron source than FeO. The effect due to the adoption of the present invention (the effect of reducing raw material costs) will be suitably exhibited even when a portion of FeO is replaced with Fe₂O₃ as an iron source.

On the other hand, when an electrode active material is to be produced whose main component is a compound (M(P0₄)_y) in which the x in A_xM(P0₄)_y is substantially O, a composition in the melted state (a melt) that contains the aforementioned source of the M and source of phosphorus (P) should be prepared. A compound that is the same as the compound illustrated above when 0 < x can be appropriately selected and used as the M source and P source. For example, when the production of an olivine-type material represented by M^iπ(P0₄) is intended (i.e., y = 1), a compound can be selected in which the M is trivalent.

The aforementioned melt can typically be prepared by mixing together a raw material having the source of the M and a raw material having the source of P, as well as a raw material having the source of the A when x is greater than 0 and heating the composition of the mixed raw materials. For example, a powdered M source, a powdered P source, and a powdered A source, can be mixed together to prepare the composition of raw materials. The average particle diameter and particle diameter distribution of each source are not particularly limited. This is because the composition of raw materials is melted, therefore, the impact that the quality of the composition of the raw materials will have on the final product will be limited. It is preferable that the mixed state of each of these sources (typically powdered) is comparatively uniform. It is more preferable that the mixed state be substantially uniform. However, because the composition of the raw materials is melted, an electrode active material having sufficient uniformity for a practical use can be produced even if the uniformity of the raw materials is not that high. Thus, the management of the production parameters of the present method will be easier than in, for example, the conventional solid state reaction method. For example, it will be easy to manage the quality of the raw materials to be used (the sources of each element and the like), and the uniformity of the composition of raw materials (the mixed state of each source). This is useful in increasing production efficiency.

The method of melting (heating) the composition of raw materials is not particularly limited. Known heating means such as induction heating, heating by means of microwaves, and the like can be appropriately adopted.

The speed of the heating (the rate of increase in temperature) for melting the raw material composition is not particularly limited. A suitable heating rate can be adopted in accordance with the abilities and the like of the heating means to be used. However, when the heating rate is too slow, the production efficiency may be reduced. From this perspective, it is normally preferable for the heating rate to be about 60°C/h or higher, and more preferable for it to be about 150°C/h or higher.

The maximum temperature of the composition in the melted state (the melt) is not particularly limited, so long as the melted state of the composition will be achieved. For example, this is about 800 to 2000°C (preferably about 850 to 1800°C, more preferably about 900 to 1600⁰C), and is a temperature that can achieve the melted state. The lowest temperature that can achieve the melted state will differ according to the composition of raw materials (e.g., the types of the A and the M, the values of x and y, etc.). For example, when the A is primarily lithium, and the M is primarily Fe, the maximum temperature is preferably about 850 to 1800⁰C, and more preferably about 900 to 1600⁰C.

When the M is an element having tendency that it is more stable in the trivalent state than in the bivalent state, and the production of an olivine-type material in which the M is primarily bivalent (typically, an electrode active material whose main component is a compound in which both the x and y of A_xM(PO₄)_y are substantially 1) is intended, it is preferable that the aforementioned maximum temperature be comparatively increased. For example, it is preferable that the production parameters be set so that the maximum temperature is at least approximately 400⁰C (typically, about 400 to 800⁰C) higher than the minimum temperature that can achieve the melted state, and more preferably a temperature that is at least approximately 600⁰C (typically, about 600 to 800⁰C) higher. The amount of time while the composition of the raw materials is maintained in the melted state (the melt time) is not particularly limited. From the perspective of production efficiency, energy costs, and the like, it is normally suitable for the melt time to be about 24 hours or less (typically, about 5 minutes to 24 hours), and preferably about 6 hours or less (typically about 5 minutes to 6 hours). In addition, the amount of time while the melt is held at the aforementioned maximum temperature (the hold time)is also not particularly limited. From the perspective of increasing the uniformity of the target material and the like, it is normally suitable for the hold time to be about 30 seconds or greater (e.g., about 30 seconds to 2 hours), and preferably 1 minute or greater (e.g., about one minute to one hour). Alternatively, the temperature reduction (cooling or slow cooling) may be initiated immediately after heating up the composition to the maximum temperature.

Note that the method of preparing the composition in the melted state is not limited to the aforementioned method in which a pre-mixed (prepared) composition of solid raw materials is melted. For example, each solid source may be separately prepared, then may be separately melted and separately melted raw materials are mixed together to form the melted composition of the raw materials. More specifically, each source may be separately melted, and the sources in the melted state may be mixed together. Alternatively, a solid M source may be melted, then a solid A source may be added to the melted M source, and then a solid P source may be added thereto. A reducing agent may be included in the aforementioned melted composition. In particular, when M compound (e.g., M compound in which M is trivalent) in which the valence number of M is higher than the valence number of M in A_xM(P0₄)_y (e.g., bivalent) is used as a portion of or the entire M source, the reducing agent is preferably added to the melted composition. In addition, when M compound (e.g., an M compound in which M is bivalent) in which the valence number of M is the same as the valence number of M in A_xM(P0₄)_y (e.g., bivalent) is used as a portion of or the entire M source, the reducing agent is preferably added to the melted composition. In particular, when M is an element in which the trivalent state exhibits more stable than the bivalent state (e.g., iron), the reducing agent is preferably added to the melted composition. The reducing agent can better prevent a phenomenon in which the bivalent M in the M compound is oxidized to the trivalent M.

A carbonaceous material (e.g., a carbon powder such as acetylene black, ketjen black, graphite precursor, and the like) can be preferably used. Examples of other reducing agents that can be used in the method disclosed herein include saccharides, polypropylene, and the like.

The means of adding this type of reducing agent to the melted composition is not particularly limited. For example, each powdered source may be mixed together with a powdered reducing agent (carbon powder or the like) to prepare the raw material composition, and the mixture of raw materials may then be melted. Alternatively, the reducing agent may be added after each source is melted. The quantity of reducing agent that is added to the melted composition is not particularly limited, but when too little is added, the effect of using the reducing agent may not be sufficiently exhibited. On the other hand, if too much reducing agent is used, there may be unintended effects on the properties of the target material and battery performance may be deteriorated. The quantity of reducing agent can be, for example, about 2 g per 100 g of melted composition. The target material will be obtained by slow cooling and hardening the aforementioned melted composition. Slow cooling may be performed while managing the slow cooling so as to achieve a predetermined temperature profile, or the melted composition may be allowed to cool naturally. The aforementioned predetermined temperature profile may be one in which the temperature is gradually reduced at a fixed rate, one in which the temperature is reduced in steps, or one in which these are combined. Normally, reducing the temperature at a fixed rate (temperature reduction rate) is easy and is thus preferred. The temperature reduction rate in this situation can be, for example, about 600°C/h or less, preferably about 450°C/h or less, and more preferably about 300°C/h or less. There is a significant tendency for the target material to become highly crystalline when the temperature reduction rate is slowed. On the other hand, excessively slowing the temperature reduction rate may give rise to a reduction in production efficiency. From this perspective, it is normally suitable for the temperature reduction rate to be about 6°C/h or higher, preferably about 30°C/h or higher, and more preferably about 60°C/h or higher. Even when the temperature is reduced in steps, the average temperature reduction rate from the initiation of slow cooling to the completion of slow cooling is preferably in the aforementioned range.

In one aspect of the method disclosed herein, the melted composition is slow-cooled from the temperature at which the aforementioned composition is in the melted state (typically, from the maximum temperature) until the temperature at which the composition is hardened. Typically, slow cooling will be performed until the temperature of the composition is lowered to at least about 300°C or less (preferably about 100°C or less). It is preferable that slow cooling be performed until the temperature of the composition is almost at room temperature (e.g., about 60°C or less, preferably about 40°C or less).

When the M in A_xM(P0₄)_y has a character that it is more stable when a valence number is higher than the valence number of the M in A_xM(Pθ₄)_y (e.g., when M in A_xM(PO₄)_y is primarily bivalent and M is iron), at least a portion of the aforementioned production process can be performed in a non-oxidative atmosphere (typically, an inert gas atmosphere such as argon or an atmosphere that contains a reduction gas such as hydrogen (H₂) or the like). Normally, it is preferred that a process from the initiation of the slow cooling (temperature reduction) of the melted composition to the completion of the slow cooling thereof be carried out in a non- oxidative atmosphere. It is more preferable that a process from the melting of the solid raw materials to the completion of the slow cooling be carried out in a non-oxidative atmosphere, and even more preferable that a process from the preparation of the raw materials to the completion of the slow cooling be carried out in a non-oxidative atmosphere. The electrode active material obtained by means of any of the aforementioned production methods is typically substantially crystalline. For example, the electrode active material thus produced is a crystalline transition metal phosphate compound as noted above. In a preferred aspect, the electrode active material may be one whose main component is a crystalline transition metal phosphate compound having an olivine-type structure, or a crystalline transition metal phosphate compound having a Nasicon-type structure. Alternatively, the electrode active material may also be one that is primarily crystalline, but contains a non-crystalline portion. The fact that an electrode active material is crystalline (at least primarily crystalline), and the fact that the electrode active material has a defined crystalline structure (olivine type, Nasicon type, etc), can be confirmed by, for example, X-ray diffraction patterns. The X-ray diffraction pattern can generally be created by means of an X-ray diffraction device.

According to the method disclosed herein, a material can be produced that is composed of a substantially olivine single phase transition metal phosphate compound (or an electrode active material substantially formed from this material). Here, "olivine single phase" means that when, for example, the production of an olivine-type material represented by AM¹^PO₄) is contemplated, the material will not substantially contain trivalent M (e.g., trivalent Fe). When the presence of trivalent Fe in the X-ray diffraction profile of the material cannot be confirmed, "olivine single phase" is obtained. According to a preferred aspect, the aforementioned olivine single phase alkali metal, transition metal and phosphate compound (typically, a lithium iron phosphate compound and the like) can be produced with good efficiency. Lithium iron phosphate compound and the like can be produced not only from a bivalent iron source (e.g., FeO), but also from a trivalent iron source (e.g., Fe₂O₃).

An electrode active material, which was manufactured by the method disclosed herein, can function as an electrode active material of a secondary battery that generates voltage by means of the storing/releasing of various types of cations. The cations that can be stored/released by this type of active material include alkali metal ions such as lithium ions, sodium ions, potassium ions, cesium ions, and the like; alkaline earth metal ions such as calcium ions, barium ions, and the like; magnesium ions; aluminum ions; silver ions; zinc ions; ammonium ions such as tetrabutylammonium ions, tetraethylammonium ions, tetramethylammoniurn ions, triethylmethylammonium ions, triethylammonium ions, and the like; imidazolium ions such as imidazolium ions, ethylmethlimidazolium ions, and the like; pyridinium ions; hydrogen ions; tetraenium ions; tetxamethylphosphonium ions; tetraphenylphosphonium ions; triphenylsulphonium ions; triethylsulphonium ions; and the like. Preferred from amongst these are alkali metal ions, and lithium ions are particularly preferred. When the electrode active material produced by the methods disclosed herein are employed in the anode of a secondary battery, metals such as lithium (Li), sodium (Na), magnesium (Mg), aluminum (Al), and the like or alloys of the same, or carbonaceous materials and the like that can store/discharge cations can be employed as the active material of the cathode. An electrode having the aforementioned electrode active material can be suitably employed as the electrode of a secondary battery having various shapes, such as a coin type, cylinder type, square type, and the like. For example, the electrode active material can be compression-molded to form an electrode in the shape of a plate and the like. In addition, by adhering the aforementioned electrode active material to a collector composed of a conductive material such as metal sheet or the like, a plate- or sheet-shaped electrode can be formed. This type of electrode can, in addition to the electrode active material according to the present invention, also contain the same one or two or more types of materials in an electrode having a standard electrode active material, in accordance with need. Representative examples of this type of material include a conductive material and a binding agent. Carbonaceous materials such as acetylene black (AB) and the like can be employed as a conductive material. Organic polymers such as polyfluorovinylidene (PVDF), polytetrafluoroethylene (PTFE), polyfluorovinylidene- hexafmoropropylene copolymer (PVDF-HFP), and the like can be employed as a binding agent.

As the non-aqueous electrolyte, an electrolyte containing a non-aqueous solvent, and a compound having cations that can be stored/released by the electrode active material (supporting electrolyte) can be used.

An aprotonic solvent having carbonate, ester, ether, nitryl, sulfone, lactone, and the like can be employed as the non-aqueous solvent that forms the non-aqueous electrolyte, but is not limited thereto. For example, propylene carbonate, ethylene carbonate, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, 1, 2-dimethoxyethane, 1, 2-diethoxyethane, acetonitrile, propionitrile, tetrahydrofuran, 2-methyltetrahydrofuran, dioxane, 1, 3-dioxoran, nitromethane, N, N-dimethylformamide, dimethylsulfoxide, sulfolane, γ-butyrolactone, and the like. Only one type may be selected from these non-aqueous solvents, or a mixture of two or more types may be employed.

In addition, as the supporting electrolyte that forms the non-aqueous electrolyte, one type or two or more types can be employed that are selected from compounds containing cations that can be stored/released by the electrode active material, e.g., lithium compounds (lithium salts) such as LiPF₆, LiBF₄, LiN (CF₃SO₂)₂, LiCF₃SO₃, LiC₄F₉SO₃, LiC(CF₃SO₂)₃, LiClO₄ and the like when a lithium ion secondary battery is used. Several examples relating to the present invention will be described below, however the present invention is in no way limited to these examples.

(Experimental Example 1)

The present experimental example is one in which a Fe source having bivalent Fe (FeO) was employed to produce an electrode active material sample whose main component is a phosphate compound having bivalent iron.

FeO, P₂O₅, and LiOH were mixed together at a molar ratio of 1:0.5:1. This mixture (raw material composition) was heated to a maximum temperature of 1100⁰C at a rate of increase of 200°C/h in an Ar atmosphere to melt the same, and this temperature was maintained for 15 minutes. Next, this melted composition (melt) was slow-cooled from the HOO⁰C melted state down to room temperature at a rate of reduction of 200°C/h. The resulting product was milled by a standard method to obtain a sample (the resulting product and the milled product are hereinafter referred to as "Sample 1"), and was subjected to powder X-ray diffraction (XRD) measurements. An X-ray diffraction device (model number "Rigaku RJNT 2100HLR/PC") which can be obtained from Rigaku Corporation was employed for the measurements. The results are shown in Figure 1. As shown in Figure 1, only an X-ray diffraction profile having olivine-type characteristics was observed. From this, it was confirmed that Sample 1 obtained by means of the present experimental example is substantially crystalline, and is olivine (LiFePO₄) single phase.

(Experimental Example 2)

The present experimental example is another example in which a Fe source having bivalent Fe (FeO) was employed to produce an electrode active material sample whose main component is a phosphate compound having bivalent iron. FeO, P₂O₅, and Li₂CO₃ were mixed together at a molar ratio of 1 :0.5:0.5. Other than the fact that this mixture was employed as a raw material composition, and the fact that the time during which the HOO⁰C maximum temperature was maintained was 30 minutes, Sample 2 was obtained in the same manner as in Experimental Example 1. The results of the XRD measurements performed on Sample 2 in the same way as in Experimental Example 1 are shown in Figure 2. As shown in Figure 2, only an X-ray diffraction profile having olivine-type characteristics was observed, and it was confirmed that Sample 2 is substantially crystalline and olivine (LiFePO₄) single phase. From this, it was confirmed that Sample 2 is substantially crystalline, and is olivine (LiFePO₄) single phase.

(Experimental Example 3)

The present experimental example is one in which a Fe source having trivalent Fe (Fe₂O₃) was employed to produce an electrode active material sample whose main component is a phosphate compound having bivalent iron. Fe₂O₃, P₂O₅, and Li₂CO₃ were mixed together at a molar ratio of 1 :1 :1, and carbon powder (acetylene black, hereinafter also referred to as "AB") was also mixed therein as a reducing agent. The quantity of AB mixed therein was two parts by mass to a total of 100 parts by mass Of Fe₂O₃, P₂O₅, and Li₂CO₃. This was heated to a maximum temperature of HOO⁰C and melted as in Experimental Example 1, and the maximum temperature was maintained for 30 minutes. Next, this melt (containing AB as a reducing agent) was slow-cooled from the 1100⁰C melted state down almost to room temperature at a rate of reduction of 200°C/h. The resulting product was milled by a standard method to obtain Sample 3, and was subjected to XRD measurements in the same way as in Experimental Example 1. The results are shown in Figure 3. As shown in Figure 3, only an X-ray diffraction profile having olivine-type characteristics was observed, and it was confirmed that Sample 3 is substantially crystalline and olivine (LiFePO₄) single phase. (Experimental Example 4)

The present experimental example is another one in which a Fe source having trivalent Fe (Fe₂O₃) was employed to produce an electrode active material sample whose main component is a phosphate compound having bivalent iron.

Other than the fact that AB was not used as a reducing agent, a raw material composition was prepared in a manner identical to that in Experimental Example 3. This raw material composition was employed, and was heated (a rate of increase of 200°C/h, maximum temperature of 1000⁰C, holding time of 30 minutes), cooled (a rate of reduction of 200°C/h), and milled in the same way as in Experimental Example 3 in order to obtain Sample 4. XRD measurements were performed on Sample 4 in the same way as in Experimental Example 1. The results are shown in Figure 4. As shown in Figure 4, Sample 4 obtained by means of the production parameters of the present experimental example contained a trivalent Fe compound, and was not olivine (LiFePO₄) single phase. (Experimental Example 5)

The present experimental example is another example in which a Fe source having bivalent Fe (FeO) was employed to produce an electrode active material sample whose main component is a phosphate compound having bivalent iron.

FeO, P₂O₅, and LiOH were mixed together at a molar ratio of 1 :0.5:1. This mixture (raw material composition) was heated to 1500°C (maximum temperature) at a rate of 200°C/h in an Ar atmosphere to melt same, and this temperature was maintained for 5 minutes. This melt was cooled (slow-cooled) from the 1500°C melted state down to almost room temperature at a rate of reduction of 200°C/h. The resulting product was milled by a standard method to obtain Sample 5, and was subjected to powder X-ray diffraction (XRD) measurements. The results are shown in Figure 5. As shown in Figure 5, only an X-ray diffraction profile having olivine-type characteristics was observed, and it was confirmed that Sample 5 is substantially crystalline and olivine (LiFePO₄) single phase. Thus, even when the maximum temperature was raised from HOO⁰C to 1500°C, a sample composed of olivine single phase as in Experimental Example 1 was obtained.

(Experimental Example 6)

The present experimental example is another one in which a Fe source having trivalent Fe₂O₃ was employed to produce an electrode active material sample whose main component is a phosphate compound having bivalent iron.

Other than the fact that AB was not used as a reducing agent, a raw material composition was prepared in a manner identical to that in Experimental Example 3. This mixture (raw material composition) was employed, and was heated (a rate of increase of 200°C/h, maximum temperature of 1500°C, holding time of 5 minutes), cooled (a rate of reduction of 200°C/h), and milled in the same way as in Experimental Example 5 in order to obtain Sample 6. XRD measurements were performed on Sample 6 in the same way as in Experimental Example 1. The results are shown in Figure 6. As shown in Figure 6, only an X-ray profile having olivine-type characteristics was observed, and it was confirmed that Sample 6 is substantially crystalline and olivine (LiFePO₄) single phase. Thus, even when Fe₂O₃ was used as the Fe source, a sample composed of olivine single phase was obtained, without employing a reducing agent, by raising the melt temperature (maximum temperature) from HOO⁰C to 1500⁰C.

(Experimental Example 7) The present experimental example is another one in which a Fe source having trivalent Fe (Fe₂O₃) was employed to produce an electrode active material sample whose main component is a phosphate compound having trivalent iron.

Fe₂O₃ and P₂O₅ were mixed together at a molar ratio of 1 : 1. This mixture (raw material composition) was employed, and was heated (a rate of increase of 200°C/h, maximum temperature of 1500°C, holding time of 5 minutes), cooled (a rate of reduction of 200°C/h), and milled in the same way as in Experimental Example 6 in order to obtain Sample 7. XRD measurements were performed on Sample 7 in the same way as in Experimental Example 1. The results are shown in Figure 7. As shown in Figure 7, only an X-ray profile having olivine-type characteristics was observed, and it was confirmed that Sample 7 is substantially crystalline and olivine (LiPO₄) single phase.

(Characteristics test for Sample 1)

Sample 1 obtained by means of Experimental Example 1 was used to create a measurement cell.

In other words, Sample 1 was prepared as an electrode active material by milling it until it could not be felt on the fingertips (for convenience, the milled product is hereinafter referred to as "Sample 1 "). About 0.25g of the electrode active material was mixed together with about 0.089g of acetylene black (AB) as a conductive material (mass ratio: about 70:25). A planetary ball mill was used to mix the electrode active material and the AB. Into this mixture (the mixture of the electrode active material and AB) was added and mixed about 0.018g of polytetrafluoroethylene (PTFE) as a binding agent (mass ratio of electrode active material :AB: PTFE: about 70:25:5). This mixture was compression molded into a plate shape having a diameter of about 1.0 cm and a thickness of about 0.5 mm, and a test electrode was prepared.

Lithium foil having a diameter of 1.5 mm and a thickness of 0.15 mm was employed as the opposite electrode. A porous polyethylene sheet having a diameter of 22 mm and a thickness of 0.02 mm was employed as a separator. In addition, a non-aqueous electrolyte was used in which LiPF₆ was dissolved at a concentration of about 1 mole/liter in a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) having a volume ratio of 1 :1. These elements were combined in a stainless steel container, and as shown in Figure 19, a coin-type battery cell having a thickness of 2 mm and a diameter of 32 mm (2032 type) was constructed. In Figure 19, reference number 1 indicates the positive electrode (test electrode), reference number 2 indicates the negative electrode (opposite electrode), reference number 3 indicates the separator and electrolyte solution (non-aqueous electrolyte), reference number 4 indicates a gasket, reference number 5 indicates a positive electrode container, and reference number 6 indicates a negative electrode cover. The aforementioned cell, in which Sample 1 was employed as the electrode active material, and a planetary ball mill was employed to mix said electrode active material with the AB, is hereinafter referred to as "cell 1-B".

In addition, a test electrode was produced and a cell was constructed that are identical to the aforementioned test electrode and cell, except that a stone mill was employed instead of a planetary ball mill to mix the electrode active material, the AB, and the PTFE together. The aforementioned cell, in which Sample 1 was employed as the electrode active material, and a stone mill was employed to mix said electrode active material with the AB, is hereinafter referred to as "cell 1 -S".

Both the aforementioned cell 1-B and cell 1-S that were produced (i.e., the cell constructions) were then left as is for about 12 hours, after which a constant current charge/discharge test was performed on these cells. The test parameters were voltage control of 4.5 to 2.5 V (cell voltage), and current density of 0.2 mA/cm². The charge/discharge profile obtained by the first charge/discharge cycle is shown in the upper portion of Figure 8. The results show that cell 1-B for which a planetary ball mill was employed for mixing (the thick line in the figure) exhibited performance (the lithium ion use ratio) that is superior to that of cell 1 -S for which a stone mill was employed for mixing (the thin line in the figure). Accordingly, only the results for cell 1-B in the second cycle are shown in the lower portion of Figure 8.

Note that the reason the initial charging capacity is slightly lower than the discharge capacity is because Li is lost due to volatility in the melting process. However, it is not that the site is lost, but rather that the charge capacity of the missing Li is recovered in the second cycle and thereafter. A cycle test of cell 1-B was performed. In other words, a charge/discharge cycle was repeated with cell 1-B at the aforementioned test parameters (voltage control of 4.5 to 2.5 V, current density of 0.2 mA/cm²), and the specific capacity of each cycle was measured. The results obtained (the cycle characteristics) are shown in Figure 9.

Furthermore, the discharge capacity of cell 1-B in the first cycle was measured at a discharge rate of 0.1 mA/cm², 0.2 mA/cm², 0.5 mA/cm² and 1.0 mA/cm². The results obtained (the rate characteristics) are shown in Figure 10.

(Characteristics test for Sample 2)

A measurement cell was produced in the same way as cell 1-B (i.e., a planetary ball mill was used in the mixing of the electrode active material and the AB), except that Sample 2 was employed as the electrode active material instead of Sample 1. This measurement cell will be hereinafter referred to as "cell 2-B".

A constant current charge/discharge test and a cycle test were performed on cell 2-B with the same test parameters as the aforementioned test 1. The results obtained are respectively shown in Figure 11 and Figure 12. Furthermore, the discharge capacity of cell 2-B in the first cycle was measured at a discharge rate of 0.1 mA/cm², 0.2 mA/cm², 0.5 mA/cm² and 1.0 mA/cm². The results are shown in Figure 13.

(Characteristics test for Sample 3)

A measurement cell was produced in the same way as cell 1-B (i.e., a planetary ball mill was used in the mixing of the electrode active material and the AB), except that Sample 3 was employed as the electrode active material instead of Sample 1. This measurement cell will be hereinafter referred to as "cell 3 -B". A constant current charge/discharge test and a cycle test were performed on cell 3 -B with the same test parameters as the aforementioned test 1. The results obtained are respectively shown in Figure 14 and Figure 15. Furthermore, the discharge capacity of cell 3-B in the first cycle was measured at a discharge rate of 0.1 mA/cm², 0.2 mA/cm², and 0.5 mA/cm². The results are shown in Figure 16.

(Characteristics test for Sample 5)

A measurement cell was produced in the same way as cell 1-S (i.e., a stone mill was used in the mixing of the electrode active material and the AB), except that Sample 5 was employed as the electrode active material instead of Sample 1. This measurement cell will be hereinafter referred to as "cell 5-S".

A constant current charge/discharge test and a cycle test were performed on cell 5-S with the same test parameters as the aforementioned test 1. The results obtained are respectively shown in Figure 17 and Figure 18.

By means of these tests, it was confirmed that all of the Samples 1, 2, 3 and 5 obtained by means of the aforementioned experimental examples exhibited various useful characteristics as electrode active materials for a lithium secondary battery (typically, a lithium ion secondary battery).

Preferred embodiments of the present invention were described in detail above.

However, these are simply examples, and do not limit the scope of the patent claims. The technology disclosed in the scope of the patent claims includes various modifications and changes of the aspects illustrated above. In addition, the technological components described in the present specification or figures exhibit technological utility either independently or in various combinations, and are not limited by the combinations disclosed in the claims at the time of application. Furthermore, the technology illustrated in the present specification or the figures simultaneously achieves a plurality of objects, and has technological utility by achieving one object from amongst these.

Claims

1. A method of producing a phosphate compound represented by a general formula A_xM(P0₄)_y, wherein 0 < x ≤ 2, 0 < y < 2, A is one element or two or more elements selected from alkali metals, and M is one element or two or more elements selected from transition metals, comprising: a step of preparing a composition in a melted state, the melted composition containing a source of the A in the general formula when x in the general formula is greater than 0, a source of the M in the general formula and a source of phosphorus; and a step of slow cooling the composition from the melted state into a solid state.

2. The method of claim 1, wherein the step of preparing the melted composition comprises: a step of preparing the composition in the melted state, the melted composition containing the source of the M in the general formula and the source of phosphorus when x in the general formula is equal to 0.

3. The method of claim 1, wherein the step of preparing the melted composition comprises: a step of mixing a solid raw material containing the source of the A in the general formula when x in the general formula is greater than 0, a solid raw material containing the source of the M in the general formula and a solid raw material containing the source of phosphorus; and a step of heating the mixture of the solid raw materials into the melted state.

4. The method of claim 3, wherein the step of preparing the melted composition comprises: a step of mixing the solid raw material containing the source of the M in the general formula and the solid raw material containing the source of phosphorus when x in the general formula is equal to 0.

5. The method of claim 1 or 3, wherein the melted composition contains the A, the M, and phosphorus in a substantially 1:1 :1 atomic ratio.

6. The method of claim 2 or 4, wherein the melted composition contains the M and phosphorus in a substantially 1 :1 atomic ratio.

7. The method of any one of claims 1 to 6, wherein the source of the M contains a compound having the M as a constituent element and in which the valence number of M in the compound is higher than the valence number of the M in the general formula; and the melted composition contains a reducing agent.

8. The method of any one of claims 1 to 7, wherein the phosphate compound represented by the general formula is an olivine-type iron phosphate compound in which the M in the general formula is primarily bivalent iron;

9. The method of claim 8, wherein the melted composition contains Fe₂O₃ as the source of the M and a reducing agent.

10. The method of claim 7 or 9, wherein carbon powder is used as the reducing agent.

11. A secondary battery, comprising: an anode having the phosphate compound produced by the method defined in any one of claims 1 to 10; a cathode having a material that stores/discharges alkali metal ions; and a non-aqueous electrolyte or a solid electrolyte.

12. The secondary battery of claim 11 , wherein the alkali metal ions are lithium ions.