LITHIUM-CONTAINING TRANSITION METAL OXIDE AND LITHIUM ION SECONDARY CELL USING SAME

Abstract

The present invention provides, as a lithium-containing transition metal oxide, a substance which is given by the chemical compositional formula Li.sub.4M.sub.5O.sub.12 (M=Cr, Co, or Zr) and has a spinel-type crystal structure. Provided is a lithium ion secondary cell having a positive electrode configured from a lithium-containing transition metal oxide which has a spinel-type crystal structure and has the chemical compositional formula Li.sub.4M.sub.5O.sub.12 (M=Cr or Co). The present invention further provides a lithium ion secondary cell having a negative electrode configured from a lithium-containing transition metal oxide which has a spinel-type crystal structure and has the chemical compositional formula Li.sub.4M.sub.5O.sub.12 (M=Zr).

Claims

1. A negative electrode material for lithium ion secondary battery comprising: a lithium-containing transition metal oxide having a spinel-type crystal structure and having the chemical compositional formula Li.sub.4M.sub.5O.sub.12 (M=Zr).

2. A lithium ion secondary battery having: a positive electrode comprising a lithium-containing transition metal oxide having a spinel-type crystal structure and having the chemical compositional formula Li.sub.4M.sub.5O.sub.12 (M=Cr or Co); a negative electrode comprising the negative electrode material according to claim 1; and a liquid or solid electrolyte having lithium ionic conductivity.

3. A lithium ion secondary battery having: a positive electrode; a negative electrode comprising the negative electrode material according to claim 1; and a liquid or solid electrolyte having lithium ionic conductivity.

4. The lithium ion secondary battery according to claim 3, wherein the positive electrode comprises a lithium-containing transition metal oxide having a spinel-type crystal structure and having the chemical compositional formula Li.sub.4Cr.sub.5O.sub.12.

5. The lithium ion secondary battery according to claim 3, wherein the positive electrode comprises a lithium-containing transition metal oxide having a spinel-type crystal structure and having the chemical compositional formula Li.sub.4Co.sub.5O.sub.12.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0013] FIG. 1 is a view illustrating a crystal structure of Li.sub.7M.sub.5O.sub.12.

[0014] FIG. 2 is a view illustrating a crystal structure of Li.sub.4M.sub.5O.sub.12.

[0015] FIG. 3 is a view illustrating a volume invariable mechanism.

[0016] FIG. 4 is a schematic view of a bulk-type all solid cell.

DESCRIPTION OF EMBODIMENTS

[0017] The present inventors took notice on Li.sub.4Ti.sub.5O.sub.12 having a spinel-type crystal structure and found a principle of suppressing volumic change accompanied by charge/discharge by theoretically investigating the charge/discharge mechanism thereof. Based on the finding, the present inventors reached an idea of increasing the energy density while suppressing the volumic change accompanied by charge/discharge in Li.sub.4M.sub.5O.sub.12 having a spinel-type crystal structure. Investigations reaching the present finding are to be described below.

[0018] FIG. 1 illustrates a generally known crystal structure of a spinel-type Li.sub.4Ti.sub.5O.sub.12 by X-ray diffraction experiment. FIG. 1 illustrates more generally as an Li.sub.4M.sub.5O.sub.12 crystal structure. Oxygen 103 is coordinated by the number of six to titanium to form an octahedral TiO.sub.6 structure 101. The octahedral structures 101 are present by the number of 16 in a unit cell, and ⅙ of which is occupied by lithium to form LiO.sub.6. Oxygen 103 is coordinated by the number of four at the periphery of lithium 102 to form an LiO.sub.4 tetrahedral structure.

[0019] According to Non-patent Literature 1, Li.sub.4Ti.sub.5O.sub.12 occludes lithium to form Li.sub.7Ti.sub.5O.sub.12. The crystal structure of Li.sub.7Ti.sub.5O.sub.12 has also been known generally, which is illustrated in FIG. 2. FIG. 2 illustrates the structure as a more general crystal structure of Li.sub.4M.sub.5O.sub.12. It can be seen that the arrangement of the octahedral TiO.sub.6 structure 201 is identical with that of FIG. 1. On the contrary, lithium 202 has oxygen coordinated by the number of six at the periphery thereof, to form an octahedral structure. The coordinate structure is different from the lithium coordinate structure in Li.sub.4Ti.sub.5O.sub.12. Accordingly, it can be seen that the coordinate structure of lithium changes by intercalation of lithium in Li.sub.4Ti.sub.5O.sub.12 to change from the tetrahedral structure to the octahedral structure. Change of the coordinate structure is caused by the change of the position of lithium. The change of the position is transition from an 8a site to a 16c site in crystallographic expression (Non-patent Literature 1).

[0020] Change of valence number of titanium due to lithium intercalation is also known. Valence changes from tetra valance to tri valence partially in titanium. This is because lithium tends to become a monovalent ion and an electron emitted from lithium reduces titanium. Along with reduction of titanium, ionic radius of titanium increases from 0.61 Å to 0.67 Å. Since the ionic radius increases, the bonding distance between titanium and oxygen increases and, further, the volume of the TiO.sub.6 octahedron increases. When the present inventors examined the crystal structure, it has been found that while the volume of the octahedral structure 101 is 10.6 Å.sup.3, the volume of the octahedral structure 201 is partially 12.1 Å.sup.3 and it has been found that the volume increases by as much as 14%. Thus, it has been confirmed that titanium is reduced.

[0021] The present inventors considered that since the volume of the TiO.sub.2 octahedron increases along with lithium intercalation, the volume of the unit cell should also be increased. However, it is known that the volumic change of this material along with lithium intercalation is as small as 0.2%. Then, the present inventors examined the change of the crystal structure along with lithium intercalation as illustrated in FIG. 3. In the drawing, “A” shows Li.sub.4Ti.sub.5O.sub.12. In this case, lithium 301 at the tetrahedral position is present and the tetrahedral height is 2.75 Å. Further, titanium 303 has a tetra valence and a small radius. If titanium becomes trivalent along with lithium intercalation, the titanium radius increases as “B” in the drawing and the volume of TiO.sub.8 octahedron increases. Then, the LiO.sub.4 tetrahedron shrinks by being pushed from the surrounding to undergo large strain. Then, the present inventors considered that lithium 305 transmits to the octahedral position as “C” in the drawing to avoid increase in the strain. As a result of measuring the height of the LiO.sub.6 octahedron in the crystal structure for confirming the concept, the height was 2.63 Å. Since the height is lower by about 0.12 Å than the height of the tetrahedron, it undergoes no large strain due to expansion of TiO.sub.8 octahedron as “B”. In view of the consideration described above, the present inventors considered that since the lithium transferred to the octahedral position, volumic expansion of the unit cell due to the volumic expansion of the TiO.sub.6 octahedron was suppressed.

[0022] The present inventors further developed the concept and considered that electrode active materials of Li.sub.4M.sub.5O.sub.12 having such mechanism in which M was other than Ti or Mn could be formed. That is, the present inventors considered that there should be a material having a positional change of lithium along with intercalation and deintercalation of lithium and giving no significant effect of the volumic change of M0.sub.6 octahedron on the volumic of the unit cell.

[0023] However, increase or decrease of the ion radius due to oxidation/reduction of a transition metal M is strictly non-isotropic generally and volumic change of the unit cell cannot always be suppressed irrespective of the type of M. Accordingly, new active material Li.sub.4M.sub.5O.sub.12 cannot be deduced easily based on the active materials known so far.

[0024] Further, the potential of the active material containing a transition metal M is generally different depending on the crystal structure. Accordingly, the potential of a new active material Li.sub.4M.sub.5O.sub.12 not yet synthesized so far cannot be estimated easily.

[0025] In view of the above, the present inventors simulated active materials of a compositional formula Li.sub.4M.sub.5O.sub.12 having a spinel structure by using various M based on a first principle calculation capable of theoretically calculating the structure and the potential of a material at high accuracy, and conducted the first principle calculation for the volumic change and the potential thereof. Specifically, calculation was performed by using Cr, Co, Zr which are apart of the transition metals. The results are shown in Table 1.

TABLE-US-00001 TABLE 1 Relation between transition metal and volumic change and potential of Li.sub.4M.sub.5O.sub.12 Transition metal Cr Co Zr Volume upon charge (Å.sup.3) 432.4 418.5 561.3 Volume upon discharge (Å.sup.3) 433.6 422.7 558.5 Difference (%) 0.3 1.0 0.5 Potential 3.4 3.7 0.2

[0026] In a case where M=Cr, the volume in a state of Li.sub.4Cr.sub.5O.sub.12 was 432.4 Å.sup.3, whereas the volume in a state of Li.sub.7Cr.sub.5O.sub.12 was 433.6 Å.sup.3. The volumic change corresponds to 0.3% and volumic change hardly occurs in the same manner as Li.sub.4Ti.sub.5O.sub.12. Accordingly, Li.sub.4Cr.sub.5O.sub.12 has a property suitable as an aimed active material in view of the volumic change. Further, the potential of Li.sub.4Cr.sub.5O.sub.12 was calculated as 3.4 V. Since the value is higher than 3.0 V, this can be an active material capable of attaining an energy-density higher than that of Li.sub.4Mn.sub.5O.sub.12.

[0027] In a case where M=Co, the volume in a state of Li.sub.4Co.sub.5O.sub.12 was 418.5 Å.sup.3, whereas the volume in a state of Li.sub.7Co.sub.5O.sub.12 was 422.7 Å.sup.3. The volumic change corresponds to 1.0%, which is a sufficiently small volumic change. Accordingly, Li.sub.4Co.sub.5O.sub.12 has a property sufficient as the aimed active material in view of the volumic change. Further, the potential was calculated as 3.7 V for Li.sub.4Co.sub.5O.sub.12. Since the value is higher than the potential 3.0 V of Li.sub.4Mn.sub.5O.sub.1, this can be an active material for attaining a high energy density.

[0028] In a case where M=Zr, the volume in a state of Li.sub.4Zr.sub.5O.sub.12 was 561.3 Å.sup.3, whereas the volume for Li.sub.7Zr.sub.5O.sub.12 was 558.5 Å.sup.3. The volumic change corresponds to 0.5% and since this is a sufficiently small amount of change, the material is suitable as an aimed active material. Further, since the potential of Li.sub.4Zr.sub.5O.sub.12 is 0.2 V, when the material is used as a negative electrode, an energy density higher than that of Li.sub.4Ti.sub.5O.sub.12 can be attained.

[0029] In view of the above, M=Cr, Co, or Zr was found as electrode active material of extremely less volumic change accompanied by charge/discharge and capable of attaining a high energy density.

[0030] An actual method of manufacturing the active materials is to be described. The manufacturing method of Li.sub.4M.sub.5O.sub.12 (M=one of Cr, Co and Zr) is performed by usual firing of a powdery body. For example, the substance can be manufactured by a step of mixing, while pulverizing, a starting material containing a lithium source and a transition metal source mechanochemically and a step of firing the pulverized mixture.

[0031] As the lithium source, compounds forming lithium oxide by heating can be used and include, specifically, lithium hydroxide, lithium oxide, lithium acetate, basic lithium carbonate, lithium nitrate, lithium oxalate, lithium sulfate, lithium carbonate, and lithium chloride. Among them, lithium hydroxide and lithium carbonate are used particularly preferably.

[0032] As the cobalt source, compounds forming cobalt oxide by heating can be used. Specifically, they include cobalt oxide, cobalt hydroxide, cobalt sulfate, etc. Among them, cobalt oxide is particularly preferred. The chromium source can include chromium chloride, chromium oxide, chromium hydroxide, chromium sulfide, and chromium sulfate. The zirconium source includes, for example, zirconium oxide, zirconium chloride, and zirconium hydroxide.

[0033] Mechanochemical pulverization includes a wet mill method and a dry mill method that conduct pulverization while giving physical impact. Specifically, a ball mill, a vibration mill, attrition mill, a planetary mill, etc. can be used and use of the ball mill is preferred with a viewpoint of installation cost. By the pulverization treatment using the ball mill, particles of the lithium source and the transition metal source are pulverized and mixed, and the spinel-type solid electrolyte proposed by the present inventors can be manufactured. The time for the mixing treatment by mechanochemical pulverization is not particularly restricted and it is generally preferred within a range of 1 hour to 6 hours. In this embodiment, while description is to be made for a solid electrolyte, a liquid electrolyte may also be used.

[0034] Firing time is not particularly restricted, but it is performed preferably within a range of one hour to 48 hours. The temperature for firing the mixed material is, for example, in a temperature region of 900 to 1500° C. and preferably in a temperature range of 1000 to 1300° C. If the firing temperature is lower than 800° C., the firing time becomes longer. Further, if the firing temperature exceeds 1500° C., it is not preferred because the solid electrolyte is sintered strongly.

[0035] The firing atmosphere mainly comprises oxygen and may further contain one or more of materials selected from air, argon, nitrogen or the like. From an economical point of view, the firing atmosphere is the most preferably air. Further, the starting mixture may be fired in a state where an atmospheric gas is filled in a firing furnace, or may be fired in a gas stream of an atmospheric gas. The starting mixture is fired in a gas stream of an atmospheric gas preferably at a flow rate of 10 ml/min or more. With the steps described above, the positive electrode active material of this embodiment is obtained.

[0036] The process for producing an all solid lithium ion cell is performed preferably in accordance with a known method. The all solid lithium ion cell has, for example, as shown in FIG. 4, a positive electrode 401, a negative electrode 403, and an inorganic lithium solid electrolyte 402 present between the positive electrode 401 and the negative electrode 403 for conducting lithium ions, in which electrons are conducted by way of a collector plate 404 and a conductor wire 405 connected therewith, and they are connected to an external circuit (resistor) 406.

[0037] The positive electrode active material of a lithium ion cell in embodiments is preferably formed by mixing a positive electrode active material, a conduction aid, and a binder by using a solvent into a paste-like state, which is then coated and dried on the surface of a collector. The conduction aid may be any material so long as the material has electron conductivity and is less reactive with the positive electrode active material. Specifically, acetylene black, carbon, graphite, natural graphite, artificial graphite, needle coke, carbon nanotube, carbon nanohorn, graphene nanosheet, etc. can be used. As the binder, a polymeric material capable of binding the positive electrode active material and a conduction aid may be used preferably. Specifically, polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl chloride, ethylene-propylene diene polymer, styrene butadiene rubber, acrylonitrile butadiene rubber, fluoro rubber, polyvinyl acetate, polymethyl methacrylate, polyethylene, nitrocellulose, etc. can be used. Any solvent can be used so long as the solvent can mix the active material and the binder in a highly dispersed state. An electrode proposed by the present inventors can also be manufactured by a method, for example, of coating a slurry obtained by mixing the mixed powder described above with an organic solvent (for example, N-methyl methylpyrrolidone, toluene, cyclohexane, dimethyl formamide, dimethyl acetamide, methyl ethyl ketone, methyl acetate, methyl acrylate, diethyl triamine, N—N-dimethyl aminopropylamine, ethylene oxide, tetrahydrofuran, etc.) on a metal substrate made of aluminum, nickel, stainless steel, copper and the like, or on a conductive polymer.

[0038] The negative electrode of the lithium ion cell proposed by the present inventors may be formed in the same manner as for the positive electrode active material by mixing the negative electrode active material, the conduction aid, and the binder using a solvent, coating and drying the material formed into a pasty state on the surface of the collector. The negative electrode active material contains, for example, lithium.

[0039] The shape of the secondary cell is not restricted to a coin type, and may also be configured as a cylindrical shape formed by winding of the electrode, for example, configured by a 18650 type. Further, it may be configured by stacking the electrode into a square shape.

EMBODIMENTS

[0040] Typical embodiments are to be described. It is apparent that the invention proposed in the present specification is no way restricted by them. In the embodiments to be described later, the cell was manufactured and measured in a dry box under an argon atmosphere. For the cell, the first cycle was started from discharge and then charge/discharge were conducted.

First Embodiment

[0041] A chromium oxide powder (CrO.sub.2) and lithium hydroxide (LiOH) are filled at a ratio of Li:Cr=4:5 in a wet pulverizer, purified water is added such that the slurry solid concentration is charged by 30% by weight, treated for one hour by using beads of 1.00 mmφ under the condition at a number of rotation of 2500 rpm. Purified water is added to them to prepare a liquid dispersion mixture for spray drying at a solid concentration of 20% by weight. The dispersion is heat treated for 4 hours in a firing atmosphere of 800° C. by using an atmospheric furnace (argon gas stream). As a result, lithium-chromium composite oxide particles having a spinel structure in a composition of: Li:Cr=4.77:5.13 are obtained.

[0042] Then, they are charged in a mixer, pulverized for 30 seconds and then classified for one minute by using an electromagnetic sieve shaker under the condition of mesh opening of 45 μm and an amplitude of 3.0 mm, fractions passing through the sieve are recovered to prepare lithium-manganese-calcium composite oxide particles for positive electrode.

[0043] When the X-ray diffraction pattern is examined for fired particles, their space group belongs to Fd-3m(No. 227). It can be recognized therefrom that Li.sub.4Cr.sub.5O.sub.12 having a spinel structure is formed.

[0044] A coin-type lithium ion secondary battery as illustrated in FIG. 4 is assembled by using the present positive electrode active material. When a charge/discharge test is performed at a cut-off potential of 3.5 V or 2.0 V, a discharge capacity of 10 mAh/g at a discharge potential of 3.3 V can be confirmed. Since the discharge potential is higher than 3.0 V, it is confirmed that a necessary potential can be obtained.

Second Embodiment

[0045] A liquid containing 200 g of lithium carbonate suspended in 600 mL of purified water is prepared, to which gaseous carbon dioxide is blown to adjust pH to 7.5. On the other hand, 110 g of cobalt chloride is dissolved in 300 mL of purified water to prepare a solution. The solution was dropped to prepare a carbonate containing lithium and cobalt (mixture of lithium carbonate and cobalt chloride). The carbonate is washed and filtered by using a saturated solution of lithium carbonate so that lithium carbonate is not eluted, and then dried in a hot blow at 100° C.

[0046] The carbonates was used as the starting material and heated from a room temperature to 490° C. in an anhydrous air for 2 hours, kept at 490° C. for 3 hours, and heated to 800° C. over 4 hours. After keeping at that temperature for 24 hours, they are cooled spontaneously to obtain a lithium manganese composite oxide represented by the general formula Li.sub.4−xCo.sub.5O.sub.12 (where 0<x<0.2).

Third Embodiment

[0047] A carbonate precursor is prepared by a reactive crystallization method using a zirconium sulfate as a starting material. Each of the elements is uniformly dispersed in a particle of the carbonate precursor. Lithium carbonate is mixed to the carbonate precursor and when the mixture is subsequently fired at 900° C. for 4 hours, a positive electrode active material represented by Li.sub.4Zr.sub.5O.sub.12 is obtained. When a discharge test is performed on the present active material, a discharge plateau appears at 0.2 V.

LIST OF REFERENCE SIGNS

[0048] 101: Octahedral structure having a transition metal or lithium at the center [0049] 102: Lithium [0050] 103: Oxygen [0051] 104: Unit cell boundary [0052] 201: Octahedral structure having a transition metal or lithium at the center [0053] 202: Lithium [0054] 203: Oxygen [0055] 204: Unit cell boundary [0056] 301: Lithium at tetrahedral position [0057] 302: Lithium vacant hole at octahedral position [0058] 303: Tetravalent titanium [0059] 304: Trivalent titanium [0060] 305: Lithium at octahedral position [0061] 401: Positive electrode [0062] 402: Solid electrolyte [0063] 403: Negative electrode [0064] 404: Collector plate [0065] 405: Conductor wire [0066] 406: External resistor (resistor)