Skeleton-forming agent and negative electrode using same

Abstract

To provide a negative electrode of a lithium ion battery excellent in cycle life characteristics. The negative electrode for a lithium ion battery includes an Si-based material as an active material, wherein a skeleton-forming agent including a silicate having a siloxane bond or a phosphate having an aluminophosphate bond as an ingredient is present on the surface and inside of an active material layer, and the skeleton of the active material is formed with the skeleton-forming agent.

Claims

1. An electrode for a lithium ion battery, comprising a skeleton-forming agent for skeleton formation of an active material layer of the electrode, wherein the active material layer includes an active material capable of forming an alloy with lithium or an active material capable of occluding lithium ions and a binder, wherein the skeleton-forming agent is permeated into the active material layer such that the skeleton-forming agent is on a surface of the active material layer and in the active material layer, thereby covering the active material and the binder inside the active material layer, and a space is produced between active material particles in the active material layer, wherein the skeleton-forming agent includes a silicate having a siloxane bond as an ingredient, wherein the silicate of the skeleton-forming agent has a crystalline or amorphous structure represented by general formula A.sub.2O.nSiO.sub.2, and where A is at least one kind selected from Li, Na, K, a triethanol ammonium group, a tetramethanol ammonium group, a tetraethanol ammonium group, and a guanidine group, and n is greater than or equal to 1.6 and less than or equal to 3.9.

2. The electrode according to claim 1, having a layer including alkali-resistant inorganic particles on the active material layer.

3. The electrode according to claim 1, wherein the active material includes a Si-based material.

4. The electrode according to claim 3, wherein the Si-based material has a median diameter (D.sub.50) greater than or equal to 0.1 μm and less than or equal to 10 μm, and an oxygen content included in the Si-based material is 0.5 to 30 mass %.

5. The electrode according to claim 3, wherein the Si-based material exhibits a shape selected from the group consisting of a facet shape, a belt shape, a fiber shape, a needle shape, a flake shape, and a donut shape, and further includes a work-affected layer.

6. A lithium ion battery comprising the electrode according to claim 1.

7. An electric apparatus comprising the battery according to claim 6.

8. The electrode according to claim 1, wherein the electrode is a negative electrode.

9. The electrode according to claim 8, wherein the active material includes a Si-based material, the Si-based material has a median diameter (D.sub.50) greater than or equal to 0.1 μm and less than or equal to 10 μm, and an oxygen content included in the Si-based material is 0.5 to 30 mass %.

10. The electrode according to claim 9, wherein the Si-based material exhibits a shape selected from the group consisting of a facet shape, a belt shape, a fiber shape, a needle shape, a flake shape, and a donut shape, and further includes a work-affected layer.

11. A lithium ion battery comprising the electrode according to claim 8.

12. An electric apparatus comprising the battery according to claim 11.

13. The electrode according to claim 1, wherein the electrode is a negative electrode for a lithium ion battery including Si-based material in the active material.

14. The electrode according to claim 1, wherein A of the general formula is Li or Na, and n is greater than or equal to 2.0 and less than or equal to 3.5.

15. The electrode according to claim 1, wherein the active material layer is formed on a surface of a collector.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a schematic view showing a cross section of an electrode coated with a skeleton-forming agent according to an embodiment of the present invention.

(2) FIG. 2 is a view showing an example of coating with a skeleton-forming agent according to an embodiment of the present invention.

(3) FIG. 3 is a view showing an example of manufacturing processes for an electrode according to an embodiment of the present invention.

(4) FIG. 4 includes views showing the cycle life characteristics of electrodes according to an embodiment of the present invention.

(5) FIG. 5 includes views showing the relationships between the skeleton density and the discharge capacity of electrodes according to an embodiment of the present invention.

(6) FIG. 6 includes views showing the cycle life characteristics of electrodes according to an embodiment of the present invention.

(7) FIG. 7 includes views showing the relationships between the skeleton density and the discharge capacity of electrodes according to an embodiment of the present invention.

(8) FIG. 8 is a view showing the cycle life characteristics of electrodes according to an embodiment of the present invention.

(9) FIG. 9 is a view showing the cycle life characteristics of an electrode using a skeleton-forming agent including a surfactant according to an embodiment of the present invention.

(10) FIG. 10 includes views comparing electrodes coated with a skeleton-forming agent to uncoated electrodes according to an embodiment of the present invention.

(11) FIG. 11 includes views comparing electrodes coated with a skeleton-forming agent to uncoated electrodes according to an embodiment of the present invention.

(12) FIG. 12 is a view showing a cross section of an electrode coated with a skeleton-forming agent containing inorganic particles according to an embodiment of the present invention.

(13) FIG. 13 is a view showing initial charge/discharge curves of batteries using no separator according to an embodiment of the present invention.

(14) FIG. 14 shows SEM images of Si granulated bodies adopting a skeleton-forming agent according to an embodiment of the present invention.

(15) FIG. 15 is a view showing charge/discharge curves at different rates according to an embodiment of the present invention.

(16) FIG. 16 includes views showing the results of a peel test comparing an electrode coated with a skeleton-forming agent to an uncoated electrode according to an embodiment of the present invention.

(17) FIG. 17 is a view showing X-ray diffraction (XRD) patterns of silicate (Na.sub.2O.nSiO.sub.2) powder

(18) FIG. 18 is a view showing the cycle life characteristics of an electrode of a reference example including alkali metal silicate (Li.sub.0.05Na.sub.1.95O.2.8SiO.sub.2) as a binder, together with the electrode after 200 cycles.

(19) FIG. 19 shows copies of photographs showing collectors of conventional Si negative electrodes.

(20) FIG. 20 includes views showing the results of glow discharge emission spectroscopic analysis comparing au electrode coated/impregnated with a skeleton-forming agent to an uncoated/unimpregnated electrode.

(21) FIG. 21 shows molecular formulas of a silicate and a phosphate.

(22) FIG. 22 shows a SEM image of Si particles of a negative electrode active material according to an embodiment of the present invention.

MODE FOR CARRYING OUT THE INVENTION

(23) Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note however that the present invention is not limited to this embodiment. In particular, while an electrode of a lithium secondary battery (lithium ion battery) will be described as an example in this embodiment, the present invention is not limited to this electrode. Also, while an alkali metal silicate aqueous solution including Na.sub.2O.3SiO.sub.2 will be mainly described as an example of a skeleton-forming agent, the present invention is not limited to this.

First Embodiment

1. MANUFACTURING METHOD FOR ELECTRODE

(24) First, as an example of this embodiment, a manufacturing method for an electrode for a lithium ion battery will be described. Note that, as the electrode, there are a negative electrode and a positive electrode, which are mainly different from each other in their collectors and active materials, but the manufacturing methods for these electrodes are alike. Therefore, the manufacturing method for the negative electrode will be described hereinafter omitting appropriately the manufacturing method for the positive electrode.

(25) The negative electrode is manufactured by applying an electrode material to copper foil. First, 10 μm-thick rolled copper foil, for example, is produced to prepare copper foil wound into a roll shape. Also, as the electrode material of the negative electrode, artificial graphite obtained by burning a carbon derivative is mixed with a binder, a conductive auxiliary, etc. to form a paste. In this embodiment, as an example, PVdF is used as the binder, and acetylene black (AB) is used as the conductive auxiliary. The copper foil is coated with the electrode material, and, after drying, the pressure is regulated, to complete the negative electrode body.

(26) The positive electrode is manufactured by applying an electrode material to aluminum foil. As the electrode material of the positive electrode, a lithium-transition metal oxide is mixed with a binder, a conductive auxiliary, etc. to form a paste. In this embodiment, as an example, PVdF is used as the binder, and AB is used as the conductive auxiliary. Hereinafter, the positive electrode body and the negative electrode body may be collectively referred to as the electrode body, the copper foil and the aluminum foil may be collectively referred to as the collector, and the electrode material applied to the collector may be referred to as the active material layer in some cases.

(27) In this embodiment, the skeleton-forming agent used for the electrode body is prepared in advance. The skeleton-forming agent is produced by dry- or wet-purifying Na.sub.2O.3SiO.sub.2, an alkali metal silicate having a siloxane bond and adjusting the purified one by adding water. For example, the dry purification follows formula 4 below, and the wet purification follows formula 5 below. A surfactant is mixed in at this time. As an example of the skeleton-forming agent of this embodiment, the solid content concentration of Na.sub.2O.3SiO.sub.2 in the skeleton-forming agent is 5 mass %, and that of the surfactant is 0.04 mass %.
Na.sub.2CO.sub.3+3SiO.sub.2.fwdarw.Na.sub.2O.3SiO.sub.2+CO.sub.2↑  (4)
2NaOH+3SiO.sub.2.fwdarw.Na.sub.2O.3SiO.sub.2+H.sub.2O  (5)

(28) The skeleton-forming agent is then applied to the surface of each electrode body, to coat the active material layer. The application of the skeleton-forming agent is performed by a method of immersing the electrode body in a bath retaining the skeleton-forming agent, a method of dropping or applying the skeleton-forming agent to the surface of the electrode body, spray coating, screen printing, a curtain method, spin coating, gravure coating, and dye coating. The skeleton-forming agent applied to the surface of the electrode body permeates into the inside of the active material layer, entering gaps of the active material and the conductive auxiliary. The electrode body is then dried by hot air of 110° C. to 160° C., heating, etc. to harden the skeleton-forming agent. In this way, the skeleton-forming agent forms the skeleton of the active material layer.

(29) Finally, both electrode bodies, i.e., the negative electrode body and the positive electrode body, are cut into desired sizes, to complete the skeleton-formed electrodes.

(30) The electrode manufacturing method described above can be implemented by a manufacturing apparatus. The collector wound in a roll form is fed out and coated with the electrode material in an active material layer coating device. The electrode material is then dried by hot air in a first dryer. The resultant electrode body is coated with the skeleton-forming agent in a skeleton-forming agent coating device. The skeleton-forming agent is then dried by hot air in a second dryer and hardened. The resultant electrode is wound up into a roll form. Finally, the wound electrode is cut into a desired size.

(31) According to the electrode manufacturing method described above, electrodes having high strength, excellent heat resistance, and improved cycle life characteristics can be manufactured continuously. Also, using a silicate having a siloxane bond as the skeleton-forming agent, not as a binder, no creases or cracks occur on the copper foil, and no cracking, warping, or expansion due to generated gas occur in the active material layer, permitting use as the electrode.

(32) The thus-obtained positive electrode and negative electrode are joined together via a separator and sealed in the state of being immersed in an electrolytic solution, to obtain a lithium ion battery. The lithium ion battery having this structure can function as a lithium ion battery having good safety. The structure of the lithium ion battery is not specifically limited, but the present invention is applicable to the existing battery forms and structures such as laminated batteries and wound batteries.

2. CONFIGURATION OF ELECTRODE

(33) The negative electrode for a lithium ion battery manufactured by the manufacturing method described above includes the copper foil collector and the active material layer including the active material, the conductive auxiliary, and the binder, formed on the surface of the collector. The surface of the active material layer is coated with the hardened skeleton-forming agent, and the hardened skeleton-forming agent is also present inside the active material layer. The skeleton-forming agent inside the active material layer is present in gaps of the active material, the conductive auxiliary, and the binder so as to cover them. The density of the skeleton-forming agent in the active material layer is 0.7 mg/cm.sup.2 as an example, and preferably in the range of 0.1 to 3 mg/cm.sup.2.

(34) The negative electrode of tins embodiment includes Si particles in the active material. The Si particles may have a shape such as a facet shape, a belt shape, a fiber shape, a needle shape, and a flake shape, and the median diameter (D.sub.50) is greater than or equal to 0.1 μm and less than or equal to 10 μm. With the presence of the active material particles having these shapes, the active material particles are entangled with one another, achieving surface contact, and also an anchoring effect occurs between the active material particles, whereby the resultant electrode is resistant to collapse of the conductive network. Also, since the Si particles having the above shapes are bulky compared to particles having a spherical shape and an oval shape, space is produced between the active material particles. With the presence of such space in the active material layer, permeation of the skeleton-forming agent is prompted by the capillary phenomenon, permitting formation of the skeleton-forming body excellent in uniformity in the negative electrode.

(35) For a similar reason, permeation of the electrolytic solution is also promoted, improving the ion conductivity of the negative electrode material. Also, since the space serves as room for lessening the volume change occurring along with occlusion/release of Li, excellent cycle life characteristics are obtained.

(36) The Si particles contain oxygen in a proportion of 0.5 to 10 mass %. With this, the irreversible capacity is controlled, the energy density of the battery improves, and the stability of the active material increases.

(37) Further, the Si particles have a work-affected layer. The work-affected layer, which has generated residual distortion, causes residual stress, storing energy therein in the form of potential energy. For this reason, the work-affected layer portion is high in energy level compared to the crystalline Si, and has been activated. The Si particles having such a work-affected layer are therefore small in stress exerted at initial charge/discharge compared to the crystalline Si-based material, whereby pulverization of the active material can be prevented or reduced, and thus a longer life of the battery can be expected.

(38) The Si particles described above can be obtained by the following method, for example. Specifically, they can be manufactured by a process such as cutting, shaving, polishing, grinding, rubbing, and abrading of a silicon ingot as the active material. In this embodiment, as an example, a silicon ingot or massive silicon is cut with a diamond powder-supported wire saw having a wire diameter of 0.1 to 0.2 mm, to obtain cut powder produced at this cutting.

(39) As another method, Si particles can be obtained by reducing a silicon tetrachloride liquid with zinc gas at a high temperature. In this case, Si particles conforming to the Si according to the present invention are obtained although the number of spherical particles is somewhat large. Note that the actually produced Si particles had an average particle diameter of 1.3 μm as measured by a laser method, and fundamental particles had diameters of roughly 1 μm or less by SEM observation (see FIG. 22).

(40) On the other hand, the positive electrode includes the aluminum foil collector and the active material layer including the active material, the conductive auxiliary, and the binder, formed on the surface of the collector. The surface of the active material layer is coated with the hardened skeleton-forming agent, and the hardened skeleton-forming agent is also present inside the active material layer. The skeleton-forming agent inside the active material layer is present in gaps in the active material, the conductive auxiliary, and the binder so as to cover them. The density of the skeleton-forming agent in the active material layer is 0.5 mg/cm.sup.2 as an example, and preferably in the range of 0.1 to 3 mg/cm.sup.2.

(41) The electrode of this embodiment has high strength, excellent heat resistance, and improved cycle life characteristics. Also, as shown in the results of the nail penetration test described later, the nail penetration safety improves. Since the skeleton-forming agent is applied to the surface of the electrode by coating and immersion, the skeleton-forming agent is present in gaps so as to cover the active material, the conductive auxiliary, and the binder inside the active material layer. That is, in the case of using the skeleton-forming agent by kneading the agent in the binder, the skeleton-forming agent will be present in the active material layer with roughly no space inside. In the electrode of this embodiment, however, a certain number of gaps are present inside the active material layer. This allows expansion/contraction of the electrode, and can prevent or reduce occurrence of creases and cracks on the collector.

3. CONFIGURATION OF SKELETON-FORMING AGENT

(42) As described above, the skeleton-forming agent of this embodiment includes Na.sub.2O.3SiO.sub.2, an alkali metal silicate having a siloxane bond, in a solid content concentration of 7.5 mass % and a surfactant in 0.09 mass %, and the remainder is water. The surfactant is not essential, but, with this, the lyophilicity to the active material layer improves and the skeleton-forming agent permeates into the active material layer uniformly.

(43) While the skeleton-forming agent of this embodiment uses the alkali metal sodium silicate, Na.sub.2O.3SiO.sub.2, as the silicate, the silicate is not limited to this, but Na may be replaced with Li, K, a triethanol ammonium group, a tetramethanol ammonium group, a tetraethanol ammonium group, and guanidine group.

(44) Na is used because Na has high strength and excellent cycle life characteristics. Use of both Na and Li is possible, where the ion conductivity will improve by use of Li. In this case, Li<Na is preferable. With this, the resultant skeleton-forming agent will retain given strength and exhibit good ion conductivity. More specifically, Na is preferably contained in the range of 51% to 99% and Li in the range of 1% to 49%, more preferably Na in the range of 70% to 98% and Li in the range of 2% to 30%, with respect to the total of Na and Li being 100 mol %.

(45) In the skeleton-forming agent of this embodiment, although the coefficient of SiO.sub.2 is 3, but it is not limited to this, but may be greater than or equal to 0.5 and less than and equal to 5.0, preferably greater than or equal to 2.0 and less than and equal to 4.5, further preferably greater than or equal to 2.2 and less than and equal to 3.8.

(46) The skeleton-forming agent of this embodiment can be used for coating of an existing electrode. By this coating, an electrode having high strength, excellent heat resistance, and improved cycle life characteristics is obtained.

(47) The skeleton-forming agent of this embodiment can be used for coating of the surface of the separator. By this coating, a separator having high strength, excellent beat resistance, and improved cycle life characteristics is obtained.

4. EXAMPLES

(48) Hereinafter, examples in this embodiment in which the solid content concentration of the skeleton-forming agent and the skeleton density are changed variously will be described together with their tests, results, and effects.

(49) <Examination of n Number and Skeleton Density in Na.sub.2O.nSiO.sub.2>

Examples 1 to 18 and Comparative Example 1

(50) Tests on the cycle life characteristics observed when a Si—C granulated body was used as the electrode active material, and the solid content concentration of Na.sub.2O.nSiO.sub.2 as the skeleton-forming agent was changed in the range of 0 mass % to 20 mass % and the value of n was changed to 2.0, 2.5, and 3.0 under the conditions of 0.2 C-rate, a cutoff potential of 0.01 V to 1.2 V (vs. Li.sup.+/Li), and 30° C. The Si—C granulated body (D.sub.50=9.8 μm) was produced by spray-drying a suspension made of Si (D.sub.50=1.1 μm), artificial graphite (D.sub.50=1 μm) (Si:artificial graphite=29:71 mass %), and a granulating auxiliary under the conditions of a feeding rate of 6 g/min, a spray pressure of 0.1 MPa, and a drying temperature of 80° C. to 180° C.

(51) As the granulating auxiliary, polyvinyl alcohol (PVA, POVAL 1400) was used. PVA is contained by 1 mass % with respect to the solid content constituted by Si, the artificial graphite, and PVA being 100 mass %. The value affix denotes the median diameter by laser diffraction/dispersion particle size distribution measurement.

(52) Test electrodes (1.4 mAh/cm.sup.2) were manufactured by: coating copper foil with slurry made of the Si—C granulated body, AB, vapor-grown carbon fiber (VGCF), copper flakes, and PVdF (solid content ratio: 85:3:1:1:10 mass %) and regulating the pressure; immersing the resultant coated foil in an aqueous solution with a given skeleton-forming agent dissolved therein; and performing heat treatment at 160° C. As the counter electrode, metal lithium foil was used. As the separator, a glass nonwoven fabric (GA-100 manufactured by Toyo Roshi Kaisha, Ltd.) and a polyethylene (PE) microporous membrane (20 μm) were used. As the electrolytic solution, 1.0 M LiPF.sub.6/(EC:DEC=50:50 vol %, +1 mass % of viuylene carbonate) was used.

(53) Table 1 shows the solid content concentration of the skeleton-forming agent and the value of n of Na.sub.2O.nSiO.sub.2 for each of Examples 1 to 18 and Comparative Example 1. Each inorganic skeleton-forming agent was produced by preparing a mixture of Na.sub.2CO.sub.3 and SiO.sub.2 to have a composition shown in Table 1, melting the preparation by healing to 1000° C. or more; and, after cooling, dissolving the preparation in water.

(54) TABLE-US-00001 TABLE 1 Na.sub.2O•nSiO.sub.2 Solid content Skeleton concentration density Na.sub.2O•nSiO.sub.2 (mass %) (mg/cm.sup.2) Test example n = 2.0 0.5 0.04 Example 1 1 0.10 Example 2 2 0.14 Example 3 5 0.58 Example 4 10 0.73 Example 5 20 2.06 Example 6 n = 2.5 0.5 0.06 Example 7 1 0.10 Example 8 2 0.35 Example 9 5 0.58 Example 10 10 1.61 Example 11 20 5.27 Example 12 n = 3.0 0.5 0.22 Example 13 1 0.24 Example 14 2 0.38 Example 15 5 0.99 Example 16 10 1.29 Example 17 20 3.15 Example 18 — 0 0 Comparative Example 1

(55) FIG. 4 shows graphs of the cycle life characteristics of Examples 1 to 18 and Comparative Example 1. As is evident from FIG. 4, the life characteristics of the negative electrodes coated with the skeleton-forming agent (Examples 1 to 18) were greatly improved compared to those of the uncoated negative electrode (Comparative Example 1). Among others, especially excellent cycle life characteristics were exhibited when the solid content concentration of Na.sub.2O.nSiO.sub.2 was in the range of 2 to 10 mass %. Also, especially excellent cycle life characteristics were exhibited when the value of n in Na.sub.2O.nSiO.sub.2 was 3.0.

(56) FIG. 5 shows graphs of the relationships between the density of the skeleton-forming body (skeleton density) and the discharge capacity at each cycle in Examples 1 to 18 and Comparative Example 1. As is evident from FIG. 5, for the electrode using the Si—C granulated body (Si:artificial graphite=29:71 mass %) as the active material, a sufficient discharge capacity is obtained when the skeleton density is 0.04 to 3.15 mg/cm.sup.2. More preferably, the skeleton density is 0.1 to 2.5 mg/cm.sup.2.

Examples 19 to 36 and Comparative Example 2

(57) Examples 19 to 36 and Comparative Example 2 are similar to the previous tests (Examples 1 to 18 and Comparative Example 1) except that the composition of the Si—C granulated body was changed from Si:artificial graphite=29:71 mass % to 46:54 mass %, and the capacity density of test electrodes was set to 1.2 mAh/cm.sup.2. Table 2 shows the solid content concentration of the skeleton-forming agent and the value of n of Na.sub.2O.nSiO.sub.2 for each of Examples 19 to 36 and Comparative Example 2.

(58) TABLE-US-00002 TABLE 2 Na.sub.2O•nSiO.sub.2 Solid content Skeleton concentration density Na.sub.2O•nSiO.sub.2 (mass %) (mg/cm.sup.2) Test example n = 2.0 0.5 0.05 Example 19 1.0 0.08 Example 20 2.0 0.19 Example 21 5.0 0.55 Example 22 10 1.16 Example 23 20 3.45 Example 24 n = 2.5 0.5 0.07 Example 25 1.0 0.19 Example 26 2.0 0.29 Example 27 5.0 0.52 Example 28 10 1.24 Example 29 20 1.38 Example 30 n = 3.0 0.5 0.03 Example 31 1.0 0.16 Example 32 2.0 0.31 Example 33 5.0 0.93 Example 34 10 1.27 Example 35 20 2.84 Example 36 — 0 0 Comparative Example 2

(59) FIG. 6 shows graphs of the cycle life characteristics of Examples 19 to 36 and Comparative Example 2. As is evident from FIG. 6, the life characteristics of the negative electrodes coated with the skeleton-forming agent (Examples 19 to 36) were greatly improved compared to those of the uncoated negative electrode (Comparative Example 2). Among others, especially excellent cycle life characteristics were exhibited when the solid content concentration of Na.sub.2O.nSiO.sub.2 was in the range of 2 to 10 mass %. Also, especially excellent cycle life characteristics were exhibited when tire value of n in Na.sub.2O.nSiO.sub.2 was 3.0.

(60) FIG. 7 shows graphs of the relationships between the density of the skeleton-forming body (skeleton density) and the discharge capacity at each cycle in Examples 19 to 36 and Comparative Example 2. As is evident from FIG. 7, for the electrode using the Si—C granulated body (Si:artificial graphite=46:54 mass %) as the active material, a sufficient discharge capacity is obtained when the skeleton density is 0.03 to 3.45 mg/cm.sup.2. More preferably, the skeleton density is 0.1 to 2.5 mg/cm.sup.2.

Examples 3 to 40 and Comparative Example 3

(61) Examples 37 to 40 and Comparative Example 3 are similar to the previous tests (Examples 1 to 18 and Comparative Example 1) except that Si (D.sub.50=1.1 μm) was used as the active material, Li.sub.0.05Na.sub.1.95O.3SiO.sub.2 was used as the skeleton-forming agent, the capacity density of test electrodes was set to 3.0 mAh/cm.sup.2, and the charge/discharge test was made under the conditions of 0.1 C-rate and a cutoff potential of 0.01 V to 1.4 V (vs. Li.sup.+/Li). Table 3 shows the solid content concentration of the skeleton-forming agent and the skeleton density of the electrode for each of Examples 37 to 40 and Comparative Example 3. The inorganic skeleton-forming agent was produced by preparing a mixture of Li.sub.2CO.sub.3, Na.sub.2CO.sub.3 and SiO.sub.2 to have a composition shown in Table 3, melting the preparation by heating to 1000° C. or more; and, after cooling, dissolving the preparation in water.

(62) TABLE-US-00003 TABLE 3 Li.sub.0.05Na.sub.1.95O•nSiO.sub.2 Solid content Skeleton concentration density Li.sub.0.05Na.sub.1.95O•nSiO.sub.2 (mass %) (mg/cm.sup.2) Test example n = 3.0 0.5 0.12 Example 37 1.0 0.48 Example 38 2.5 0.90 Example 39 5.0 1.77 Example 40 — 0 0 Comparative Example 3

(63) FIG. 8 is a graph showing the cycle life characteristics of Examples 37 to 40 and Comparative Example 3. As is evident from FIG. 8, while the initial discharge capacity of the negative electrode coated with the skeleton-forming agent (Examples 37 to 40) exceeded 1800 mAh/g, that of the uncoated negative electrode (Comparative Example 3) was 56.7 mAh/g. As is found from comparing the negative electrodes coated with the skeleton-forming agent (Examples 37 to 40) to the uncoated electrode (Comparative Example 3), significant improvement was exhibited in the cycle life characteristics. Among others, especially excellent cycle life characteristics were exhibited when the solid content concentration of Li.sub.0.05Na.sub.1.95O.3SiO.sub.2 was in the range of 0.5 to 2.5 mass %. It was found that, for the electrode using Si as the active material, a discharge capacity exceeding 2000 mAh/g was obtained when the skeleton density was 0.12 to 0.90 mg/cm.sup.2.

(64) The batteries after charging/discharging (20 cycles) in Examples 37 to 40 and Comparative Example 3 were disassembled, and the test electrodes were observed. As a result, while the active material layer peeled off from the collector in Comparative Example 3, the active material layers did not peel off from the collectors and no creases or cracks were observed in the collectors in Examples 37 to 40. From these results, it became evident that the electrode coated and filled with the skeleton-forming agent was enhanced in the adhesiveness between the active material layer and the collector and unproved in electrode performance.

(65) <Examination of Surfactant>

Example 41 and Example 42

(66) Example 41 is similar to Example 10 except that an Si—C granulated body (Si:artificial graphite=29:71 mass %) was used as the active material, and, as the skeleton-forming agent, the solid content concentration of Na.sub.2O.nSiO.sub.2 was adjusted to 6 mass %, the value of n to 2.5, and the skeleton density to 0.84 mg/cm.sup.2.

(67) Example 42 is similar to Example 41 except that, as the skeleton-forming agent, a nonionic surfactant (registered trademark: Triton X-100) was added by 0.05 mass %.

(68) FIG. 9 is a graph showing the cycle life characteristics of the electrode having no surfactant added to the skeleton-forming agent (Example 41) and the electrode having a surfactant added to the skeleton-forming agent (Example 42) in comparison. As is evident from FIG. 9, the cycle life characteristics improve by adding a surfactant. This is because the lyophilicity of the skeleton-forming agent to the active material layer improves with the surfactant, succeeding in formation of a uniform skeleton in the active material layer. While the nonionic surfactant is used as the surfactant in this embodiment, the present invention is not limited to this, but an anionic surfactant, a cationic surfactant, an ampholytic surfactant, or a nonionic surfactant may be used.

(69) <Nail Penetration Safety>

Example 43 and Comparative Example 4

(70) The safety of the battery adopting the negative electrode using the skeleton-forming agent was tested. As the test method, the nail penetration test was performed in which a nail was allowed to penetrate a battery model, to examine the state of smoking and firing of the battery model. For the test, used was a 1.1 Ah battery model in winch the negative electrode, the separator, and the positive electrode were stacked forming a plurality of layers in an aluminum laminate casing and an electrolytic solution was sealed therein. The positive electrode (4.2 mAh/cm.sup.2) was manufactured by coating aluminum foil (20 μm) with slurry made of LiNi.sub.0.33Co.sub.0.33Mu.sub.0.33O.sub.2, AB, and PVdF and regulating the pressure, and then performing heal treatment at 160° C. The negative electrode (4.6 mAh/cm.sup.2) was manufactured by coating copper foil (10 μm) with slurry made of artificial graphite (D.sub.50=20 μm). AB, and an acrylic binder and regulating the pressure, and then performing heat treatment at 160° C.

(71) In Example 43, as shown in FIG. 2, the negative electrode was immersed in an aqueous solution with the skeleton-forming agent dissolved therein and then heat-treated at 150° C. As the skeleton-forming agent, used was an aqueous solution containing Na.sub.2O.3SiO.sub.2 and a nonionic surfactant (registered trademark: Triton X-100) in which the solid content concentration of Na.sub.2O.3SiO.sub.2 was 6 mass % and the solid content concentration of the surfactant was 0.05 mass %. The skeleton density of the electrode is 0.9 mg/cm.sup.2 for a single side. In this test, since the positive electrode and the negative electrode are subjected to both-sided coating, the skeleton density for both sides is 1.8 mg/cm.sup.2. A battery using a negative electrode uncoated with the skeleton-forming agent was used as Comparative Example 4.

(72) As the electrolytic solution, used was 1 M LiPF.sub.6/ethylene carbonate (EC):diethyl carbonate (DEC)=50:50 vol %+1 mass % of vinylene carbonate. As the separator, a polypropylene (PP) microporous membrane (23 μm) was used. In the nail penetration test, this battery model was charged at 0.1 C-rate up to 4.2 V, then an iron nail (ϕ3 mm, round) was allowed to penetrate through the battery at the center thereof at a speed of 1 mm/sec under the environment of 25° C., and the battery voltage, the nail temperature, and the casing temperature were measured.

(73) In the conventional battery model using the electrode having no skeleton-forming agent (Comparative Example 4), when the nail penetration was performed, the battery voltage reduced down to 0 V and a large amount of smoke was generated. This is because the separator melted down due to heat generation accompanying occurrence of short-circuiting inside the battery model.

(74) By contrast, in the battery model using the graphite negative electrode having the skeleton-forming agent (Example 43), when the nail penetration was performed, the voltage of 3 V or more was maintained, no smoking was generated, and the temperatures of the casing and the nail were 50° C. or less, hardly causing heat generation accompanying short-circuiting. This is considered because the skeleton present inside the active material layer of the negative electrode, coating the active material, etc., blocks migration of elections and thus prevents or reduces short-circuiting.

(75) <Electrode Peel Test>

Example 43 and Comparative Example 4

(76) A peel test was performed for the negative electrodes used for the batteries in Example 43 and Comparative Example 4. In the peel test, conforming to JIS K685, an adhesive tape (Scotch, No. 845, Book Tape) was pressure-bonded to the active material layer of the negative electrode with a 1 kg roller, and peeling was evaluated under the conditions of a tension rate of 300 mm/min and an angle of 180° in the environment of 25° C. FIG. 16 shows photographs of the negative electrodes and the adhesive tapes in Example 43 and Comparative Example 4 after the peel test. As is evident from FIG. 16, while the active material layer having peeled off adheres to the adhesive tape for the case of the negative electrode using no skeleton-forming agent in Comparative Example 4, the active material layer little adheres to the adhesive (ape for the negative electrode using the skeleton-forming agent in Example 43, exhibiting improvement in peel strength.

(77) <Examination of Negative Electrode Active Material>

Examples 44 to 59 and Comparative Examples 5 to 20

(78) Using various negative electrode active materials shown in Table 4, the effects with the presence/absence of the skeleton-forming agent were compared. Examples 44 to 59 were manufactured by: coating copper foil (10 μm) with slurry made of any of the negative electrode active materials shown in Table 4, AB, and PVdF and regulating the pressure; coating and impregnating the active material layer with the skeleton-forming agent with a spray gun; and performing heat treatment at 160° C. The solid content ratio of the electrode slurry was 88 mass % of the electrode active material, 4 mass % of AB, and 8 mass % of PVdF.

(79) As the skeleton-forming agent, used was an aqueous solution containing Li.sub.0.05Na.sub.1.95O.3SiO.sub.2 and a nonionic surfactant (registered trademark: Triton X-100) in which the solid content concentration of Li.sub.0.05Na.sub.1.95O.3SiO.sub.2 was 7.5 mass % and the solid content concentration of the surfactant was 0.03 mass %. In this test, the test electrode was subjected to single-sided coating.

(80) As the test battery, used was 1 M LiPF.sub.6/ethylene carbonate (EC):diethyl carbonate (DEC)=50:50 vol %+1 mass % of vinylene carbonate. As the separator, a polypropylene (PP/PE/PP) three-layer microporous membrane (25 μm) was used. A charge/discharge test was performed at a current density of 0.1 mA/cm.sup.2 and a test environment temperature of 30° C. The cutoff potential, the active material density of the electrode (single side), the electrode thickness, and the skeleton density (single side) are as shown in Table 4.

(81) Table 5 is a table showing the results of a cycle life characteristics test for the electrodes having skeleton formation (Examples 44 to 59). Table 6 is a table showing the results of the cycle life characteristics test for electrodes having no skeleton formation for comparison (Comparative Examples 5 to 20). As is evident from comparison between Table 5 and Table 6, the electrodes using Si, SiO, Ge, In, and Fe.sub.2O.sub.3 as the active material exhibited excellent cycle life characteristics by using the skeleton-forming agent. Among others, the electrodes using Si as the active material (Example 56 and Comparative Example 17) exhibited an especially significant difference. This is considered because, with establishment of the robust skeleton-forming agent in the active material layer, destruction of the electron conductive network occurring along with a volume change was prevented or reduced.

(82) By contrast, when using Ag, Ag.sub.2O, Sb, Sb.sub.2S.sub.3, SnO.sub.2, CuO, NiO, artificial graphite, and hard carbon, no large difference was observed in cycle life characteristics with the presence/absence of the skeleton-forming agent. It is considered that, since these active materials are small in volume change compared to Si, SiO, Ge, In, and Fe.sub.2O.sub.3, no definite difference was exhibited under the conditions of tins test.

(83) In Sn, however, the cycle life characteristics were worsened using the skeleton-forming agent. This is considered because Sn was high in dissolution rate compared to the skeleton-forming agent and unable to maintain the shape of the active material, whereby the capacity was reduced.

(84) TABLE-US-00004 TABLE 4 Active material Cutoff Electrode Skeleton density potential thickness density Active material (mg/cm.sup.2) (vs. Li.sup.+/Li) (μm) (mg/cm.sup.2) Sn 2.6 0.01~1.80 27 0.52 SnO 2.3 0.01~1.80 46 0.28 SnO.sub.2 2.4 0.01~1.80 27 0.47 Ge 2.2 0.01~1.40 37 0.59 Ag 2.2 0.01~1.00 37 0.62 Ag.sub.2O 2.1 0.01~1.00 38 0.69 Sb 1.9 2.00~0.50 41 0.98 Sb.sub.2S.sub.3 2.4 2.00~0.50 44 0.49 In 2.1 0.01~1.50 33 0.74 CuO 3.2 0.01~3.00 34 0.30 Fe.sub.2O.sub.3 2.7 0.01~3.00 34 1.06 NiO 4.2 0.01~3.00 39 0.73 Si 1.1 0.01~1.40 25 0.47 SiO 18 0.01~1.40 31 0.48 Artificial graphite 4.5 0.01~0.80 64 0.70 Hard carbon 5.3 0.00~1.00 76 0.40

(85) TABLE-US-00005 TABLE 5 Discharge capacity of active material (mAh/g) Experimental example Active material 1 cycle 2 cycle 10 cycle Example 44 Sn 378.9 44.9 8.0 Example 45 SnO 661.2 489.8 178.9 Example 46 SnO.sub.2 893.5 850.9 708.9 Example 47 Ge 460.7 556.42 558.7 Example 48 Ag 140.8 139.6 34.8 Example 49 Ag.sub.2O 253.3 263.0 233.9 Example 50 Sb 68.6 39.3 39.5 Example 51 Sb.sub.2S.sub.3 384.6 348.1 275.0 Example 52 In 325.4 146.4 52.6 Example 53 CuO 328.7 254.0 206.6 Example 54 Fe.sub.2O.sub.3 989.8 948.6 846.2 Example 55 NiO 630.7 563.6 471.1 Example 56 Si 2718.5 2670.1 2573.5 Example 57 SiO 809.9 607.0 226.8 Example 58 Artificial graphite 331.2 334.5 336.4 Example 59 Hard carbon 220.9 212.5 212.5

(86) TABLE-US-00006 TABLE 6 Discharge capacity of active material (mAh/g) Experimental example Active material 1 cycle 10 cycle 10 cycle Comparative Example 5 Sn 446.7 247.9 6.9 Comparative Example 6 SnO 765.0 654.6 250.8 Comparative Example 7 SnO.sub.2 732.0 620.3 430.5 Comparative Example 8 Ge 527.6 284.8 80.0 Comparative Example 9 Ag 125.2 124.2 37.6 Comparative Example 10 Ag.sub.2O 234.0 257.6 251.0 Comparative Example 11 Sb 178.2 29.4 25.3 Comparative Example 12 Sb.sub.2S.sub.3 410.7 378.9 293.9 Comparative Example 13 In 48.2 36.2 25.3 Comparative Example 14 CuO 266.9 214.8 124.7 Comparative Example 15 Fe.sub.2O.sub.3 805.8 330.3 123.1 Comparative Example 16 NiO 608.0 557.3 473.3 Comparative Example 17 Si 56 4.9 2.5 Comparative Example 18 SiO 836.1 519.0 14.1 Comparative Example 19 Artificial graphite 322.5 324.7 326.1 Comparative Example 20 Hard carbon 228.1 219.1 210.7
<Examination of Si and Graphite Blend Ratio>

Examples 60 to 63 and Comparative Examples 21 to 24

(87) The presence absence of the effects of the skeleton formation were compared among electrodes using a mixture of Si (1 μm) and artificial graphite (19 μm) as the negative electrode active material. Examples 60 to 63 were manufactured by: coating copper foil (10 μm) with slurry made of the negative electrode active materials shown in Table 7, AB, and PVdF and regulating the pressure; coating and impregnating the active material layer with the skeleton-forming agent with a spray gun; and performing heat treatment at 160° C.

(88) For comparison, electrodes that were not coated or impregnated with the skeleton-forming agent were manufactured (Comparative Examples 21 to 24). The solid content ratio of the electrode slurry was 88 mass % of the electrode active material, 4 mass % of AB, and 8 mass % of PVdF. As the skeleton-forming agent used was an aqueous solution containing Li.sub.0.05Na.sub.1.95O.3SiO.sub.2 and a nonionic surfactant (registered trademark: Triton X-100) in winch the solid content concentration of Li.sub.0.05Na.sub.1.95O.3SiO.sub.2 was 7.5 mass % and the solid content concentration of the surfactant was 0.03 mass %. In this test, the test electrode was subjected to single-sided coating.

(89) As the test battery, used was 1 M LiPF.sub.6/ethylene carbonate (EC):diethyl carbonate (DEC)=50:50 vol %+1 mass % of vinylene carbonate. As the separator, a polypropylene (PP/PE/PP) three-layer microporous membrane (25 μm) was used. A charge/discharge test was performed at a current density of 0.1 C-rate, a test environment temperature of 30° C., and a cutoff potential of 0.0 to 1.4 V (vs. Li.sup.+/Li). The skeleton density (single side) is as shown in Table 8. The capacity density of the test electrodes was set to 3.0 mAh/cm.sup.2.

(90) FIG. 10 includes graphs showing, in comparison, the cycle life characteristics of Examples 60 to 63 coated with the skeleton-forming agent and Comparative Examples 21 to 24 uncoated with the skeleton-forming agent. As is evident from FIG. 10, so large change was not observed for the electrodes having an Si amount less than or equal to 10 mass % with respect to the total amount of the active material included in the negative electrode being 100 mass % (Example 60, Example 61, Comparative Example 21, and Comparative Example 22). However, the life characteristics were evidently improved for the electrodes having an Si amount greater than or equal to 20 mass % (Example 62, Example 63, Comparative Example 23, and Comparative Example 24). In particular, for the electrodes having an Si amount of 50 mass % (Example 63 and Comparative Example 24), an overwhelming effect of improving the life characteristics was exhibited by skeleton formation.

(91) TABLE-US-00007 TABLE 7 Blend ratio of active material (mass %) Experimental example Artificial graphite Si Example 60 95 5 Example 61 10 10 Example 62 80 20 Example 63 50 50 Comparative Example 21 95 5 Comparative Example 22 90 10 Comparative Example 23 80 20 Comparative Example 24 50 50

(92) TABLE-US-00008 TABLE 8 Experimental example Skeleton density (mg/cm.sup.2) Example 60 0.32 Example 61 0.43 Example 62 0.53 Example 63 0.74 Comparative Example 21 — Comparative Example 22 — Comparative Example 23 — Comparative Example 24 —

5. ADVANTAGEOUS EFFECT OF THIS EMBODIMENT

(93) According to this embodiment described above, the following advantageous effects are obtained.

(94) By using the skeleton-forming agent including A.sub.2O.3SiO.sub.2 (A=Na, Li) for the electrode, it is possible to obtain an electrode having excellent heat resistance, high strength, and improved cycle life characteristics. Without the necessity of a tough collector, occurrence of creases and cracks on the collector can be prevented or reduced. It is unnecessary to use a binder large in irreversible capacity. Even if the election conductivity of the active material layer is high, heat generation due to internal short-circuiting can be reduced. Since having heat resistance against a temperature exceeding 1000° C., the skeleton-forming agent is not carbonated. Even an electrode including an alloy-based material having sharp volume change and a binder low in bonding strength can obtain good life characteristics. Expansion of the active material layer is small even coming into contact with a high-temperature electrolytic solution.

Second Embodiment

1. CONFIGURATION OF SKELETON-FORMING AGENT AND ELECTRODE

(95) Next, the second embodiment of the present invention will be described. The second embodiment is mainly different from the first embodiment in that ceramic is included in the skeleton-forming agent. FIG. 12 shows a cross-sectional image of the electrode of the second embodiment, in winch the skeleton-forming agent of the second embodiment includes ceramic powder or powder of a solid electrolyte excellent in alkali resistance.

(96) FIG. 3 shows an example of the manufacturing process, in which the surface of the electrode body is coated with the skeleton-forming agent of the second embodiment, whereby alumina in the skeleton-forming agent is accumulated on the surface of the active material layer, forming an electrically insulated ceramic layer or solid electrolyte layer, and also the skeleton-forming agent permeates into the active material layer.

(97) By the above process, a robust skeleton can be formed, and occurrence of peeling and cracking during drying can be prevented or reduced. Also, holes are formed from gaps between inorganic particles, imparting good lyophilicity with the electrolytic solution. Moreover, the ceramic layer serves as the separator, making it possible to constitute a battery without use of a separator separately.

2. EXAMPLES

(98) Examples in the second embodiment in which the composition of the skeleton-forming agent was changed variously will be described.

(99) [Examination of the Ratio of Na.sub.2O.3SiO.sub.2 to α-Al.sub.2O.sub.3]

Examples 85 to 87 and Comparative Example 23

(100) Table 9 shows the solid content compositions of the skeleton-forming agent used in Example 85 to 87. Also, an electrode using polyimide (PI) as the skeleton-forming agent was manufactured (Comparative Example 23). As α-Al.sub.2O.sub.3, powder having a median diameter (D.sub.50) of 0.95 μm as measured by the laser diffraction/scattering particle diameter distribution measurement was used. The solid content concentration of the skeleton-forming agent was set to 10 mass % when the solid content of Na.sub.2O.3SiO.sub.2 and α-Al.sub.2O.sub.3 was 100 mass %.

(101) TABLE-US-00009 TABLE 9 Solid content composition of skeleton-forming agent (mass %) Experimental example Na.sub.2O•3SiO.sub.2 α-Al.sub.2O.sub.3 PI Example 85 90 10 — Example 86 80 20 — Example 87 50 50 — Comparative Example 23 — — 100

(102) The negative electrode (4.0 mAh/cm.sup.2) was manufactured by: coating copper foil (10 μm) with slurry made of SiO, carbon black (CB), and an acrylic binder and regulating the pressure; coating and impregnating the active material layer with the skeleton-forming agent shown in Table 9 with a spray gun; and performing heat treatment at 160° C. The skeleton density of the electrode is 3.0 mg/cm.sup.2 for a single side. The solid content ratio of the negative electrode slurry was 90 mass % of SiO, 5 mass % of CB, and 5 mass % of Ihe acrylic binder. Note that the negative electrode had undergone electrochemical compensation of Li equivalent to the irreversible capacity.

(103) The positive electrode (2.0 mAh/cm.sup.2) was manufactured by coating aluminum foil (20 μm) with slurry made of LiFePO.sub.4, CB, and an acrylic binder and regulating the pressure, and then performing heat treatment at 160° C. The solid content ratio of the positive electrode slurry was 91 mass % of LiFePO.sub.4, 5 mass % of CB, and 4 mass % of the acrylic binder. For the test battery, as the electrolytic solution, used was 1 M LiPF.sub.6/ethylene carbonate (EC):diethyl carbonate (DEC)=50:50 vol %+1 mass % of viuylene carbonate.

(104) Note that no separator was used in this test. A charge/discharge test was performed at a test environment temperature of 60° C. and a current density of 0.1 C-rate. The cutoff potential was 4.0 V to 2.5 V.

(105) FIG. 13 shows initial charge discharge curves of test batteries of Examples 85 to 87. It is found that the batteries function without a separator. No battery test was performed for Comparative Example 23. The reason is that, since the active material layer was coated with PI, the electrode resistance increased, failing to electrochemically dope the active material layer with Li ions.

(106) <Examination of Particle Diameter of Alkali-Resistant Ceramic Particles>

(107) The surface of the electrode body was coated with the skeleton-forming agent including α-Al.sub.2O.sub.3 of which the particle diameter was changed as shown in Table 10 and dried, and the coating properties, lyophilicity binding properties, foamed state, aggregated stale, and precipitated state of the resultant electrodes were observed. As the skeleton-forming agent, an aqueous solution of Li.sub.0.05Na.sub.1.95O.3SiO.sub.2 and α-Al.sub.2O.sub.3 was used. The solid content concentration of the skeleton-forming agent was 25 mass % when the solid content of Li.sub.0.05Na.sub.1.95O.3SiO.sub.2 and α-Al.sub.2O.sub.3 was 100 mass %. Note that the particle diameter of inorganic particles shown in Table 10 is the median diameter (D.sub.50) by the laser diffraction/scattering particle diameter distribution measurement. As shown in Table 10, it is found that the particle diameter of ceramic particles is preferably in the range of 0.2 to 20 μm in the second embodiment.

(108) TABLE-US-00010 TABLE 10 Particle diameter of inorganic Coating Binding Foamed Aggregated Precipitated particles properties Lyophilicity properties state state state less than 0.2 μm ⊙ X X X ◯ ⊙ 0.2 μm or greater ⊙ ◯ ◯ ◯ ◯ ⊙ less than 1 μm 1 μm or greater ⊙ ⊙ ◯ ⊙ ⊙ ⊙ less than 10 μm 10 μm or greater ⊙ ⊙ ⊙ ⊙ ⊙ ◯ less than 50 μm 50 μm or greater ◯ ⊙ ⊙ ⊙ ⊙ ◯ 100 μm or less 100 μm or greater ◯ ⊙ ⊙ ⊙ ⊙ X 200 μm or less over 200 μm X ⊙ ⊙ ⊙ ⊙ X Coating properties: ⊙very good ◯good Xuneven Lyophilicity: ⊙very good ◯good Xhardly lyophilic Binding properties: ⊙very good ◯good Xeasily peel off from collector Foamed state: ⊙None ◯small foam observed Xlarge foam observed Aggregated state: ⊙hardly aggregate ◯normal Xeasily aggregate Precipitated state: ⊙slow ◯normal Xfast

3. ADVANTAGEOUS EFFECTS OF THIS EMBODIMENT

(109) According to this embodiment, the following advantageous effects are obtained.

(110) By coating the electrode surface with the skeleton-forming agent used in the first embodiment and powder (D.sub.50=0.2 to 20 μm) excellent in alkali resistance, a robust skeleton can be formed in the active material layer, and also occurrence of peeling and cracking during drying can be prevented or reduced. Holes are formed from gaps between inorganic particles, imparting good lyophilicity with the electrolytic solution. The ceramic layer serves as the separator, making it possible to constitute a battery without use of a separator separately.

Other Embodiments

(111) <Skeleton Formation of Active Material Granulated Body>

Examples 88 to 93

(112) A test on the production method for a Si granulated body having the skeleton-forming agent was performed.

(113) A Si granulated body was produced by spray-drying a suspension containing Si (D.sub.50=1 μm), AB, VGCF, and PI under the conditions of a liquid feeding rate of 5 g/min, a spray pressure of 0.1 MPa, and a drying temperature of 80 to 180° C., to obtain D.sub.50=5 to 8 μm. The solid content ratio of the suspension is as shown in Table 11.

(114) Thereafter, the Si granulated body having the skeleton-forming agent was produced by transferring the granulated body obtained by spray drying to a fluid bed, to coat the granulated body with particles using the skeleton-forming agent (Li.sub.0.2Na.sub.1.8O.nSiO.sub.2) adjusted to have a solid content concentration of 0.5 mass %. The skeleton-forming agent was adjusted to be 1 mass % with respect to the total of the granulated body obtained by spray drying and the skeleton-forming agent being 100 mass %.

(115) FIG. 14 shows SEM images of Si granulated bodies having the resultant skeleton-forming agent (Examples 88 to 93). It has been found that, in Example 88, since the amount of PI contained in the suspension is small, it is difficult to obtain a spherically granulated body, but, as the PI amount increases, a spherically granulated body becomes easily obtainable. It has also been found that, by adding fiber-shaped particles like VGCF, an echinus-shaped granulated body tends to be formed.

(116) TABLE-US-00011 TABLE 11 Solid content composition of suspension (mass %) Experimental example Si AB VGCF PI Example 88 97 1 1 1 Example 89 96 1 1 2 Example 90 93 1 1 5 Example 91 88 1 1 10 Example 92 78 1 1 20 Example 93 68 1 1 30

Aluminum Phosphate-Based Skeleton-Forming Agent (Example 94)

(117) The test electrode (4.0 mAh/cm.sup.2) was manufactured by: coating copper foil (40 μm) with slurry made of Si (1 μm), CB, and a PVdF binder and regulating the pressure: coating and impregnating the active material layer with the skeleton-forming agent with a spray gun; and performing heat treatment at 300° C. As the skeleton-forming agent, aluminum phosphate (Al.sub.2O.sub.3.3P.sub.2O.sub.5) was dissolved in water and adjusted to have a solid content of 5 mass %. The skeleton density of the electrode is 2.0 mg/cm.sup.2 for a single side. The solid content ratio of die negative electrode slimy was 90 mass % of Si, 4 mass % of CB, and 8 mass % of the PVdF binder.

(118) For the test battery, metal Li was used as the counter electrode. As the electrolytic solution, used was 1 M LiPF.sub.6/ethylene carbonate (EC):diethyl carbonate (DEC)=50:50 vol %+1 mass % of vinylene carbonate. As the separator, a glass nonwoven fabric (GA-100 manufactured by Toyo Rosin Kaisha, Ltd.) was used. In the charge/discharge test, charge/discharge was performed at a test environment temperature of 30° C. and at current densities of 0.1 C-rate, 0.5 C-rate, and 1.0 C-rate. The cuttoff potential was 1.0 V to 0.01 V. FIG. 15 shows charge/discharge curves at these rates in Example 94. It has been found that using aluminum phosphate (Al.sub.2O.sub.3.3P.sub.2O.sub.5) as the skeleton-forming agent, also, a stable irreversible capacity is obtained.

(119) <Evaluation as Binder>

Comparative Example 24

(120) A test using the skeleton-forming agent as a binder was performed.

(121) As the binder, an alkali metal silicate (Li.sub.0.05Na.sub.1.95O.2.8SiO.sub.2) was adjusted so that the solid content concentration be 40 mass % by adding water. The test negative electrode (2 mAh/cm.sup.2) was manufactured by coating copper foil (10 μm) with slurry made of Si (D.sub.50=1 μm), carbon black, and an inorganic binder and performing heat treatment at 150° C. The solid content ratio of the electrode slurry was 19 mass % of Si, 4 mass % of CB, and 76 mass % of the binder.

Comparative Example 25

(122) Comparative Example 25 was manufactured in a similar manner to Comparative Example 24 except that the binder was changed from the alkali metal silicate to an aluminum primary phosphate (Al.sub.2O.sub.3.3P.sub.2O.sub.5). Note that a commercially available reagent (produced by Aesar) was used as the aluminum phosphate.

Reference Example 1

(123) As the binder, an alkali metal silicate (Li.sub.0.05Na.sub.1.95O.2.8SiO.sub.2) and α-Al.sub.2O.sub.3 (D.sub.50=3 μm) were mixed at a ratio of 50:50 mass %, and water was added to the mixture so that the solid content concentration of the mixture be 40 mass %. The other conditions are similar to those in Comparative Example 24.

Reference Example 2

(124) As the binder, an aluminum phosphate (Al.sub.2O.sub.3.3P.sub.2O.sub.5) and α-Al.sub.2O.sub.3 (D.sub.50=3 μm) were mixed at a ratio of 50:50 mass %, and water was added to the mixture so that the solid content concentration of the mixture be 40 mass %. The other conditions are similar to those in Comparative Example 25.

(125) In Comparative Example 24, Si and the alkali metal silicate reacted with each other during mixing into slurry, causing generation of hydrogen gas and foaming of the slurry. Also, in Comparative Example 24 and Comparative Example 25, at temporary drying at 80° C., the active material layer expanded, failing to obtain a uniform electrode. Furthermore, in the heat treatment at 150° C. of the electrodes of Comparative Example 24 and Comparative Example 25, the volume of the active material layer greatly contracted, causing cracks in the active material layer and drop-off from the collector.

(126) By contrast, in Reference Example 1, although Si and the alkali metal silicate reacted with each other during mixing into slurry, causing generation of hydrogen gas and foaming of the slurry, the active material layer did not expand at temporary drying at 80° C. Also, in the heat treatment at 150° C. of the electrode, the volume contraction was prevented or reduced, and the phenomena such as cracks in the active material layer and drop-off from the collector did not occur, compared to Comparative Example 24.

(127) In Reference Example 2, no foaming of the shiny occurred because hydrogen gas was not generated during mixing into shiny, and the active material layer did not expand at temporary drying at 80° C. Also, in the heat treatment at 150° C. of the electrode, the phenomena such as cracks in the active material layer and drop-off from the collector did not occur, compared to Comparative Example 24.

(128) The reason why the active material layer expanded at 80° C. in Comparative Example 24 and Comparative Example 25 is considered that vaporized gas (water vapor) was confined in the active material layer at the time of drying of the slimy, causing expansion of the active material layer. It is nuttier considered that, at 150° C., due to great volume contraction originating from the alkali metal silicate and the aluminum phosphate, cracks occurred in the active material layer and the active material layer dropped off from the collector.

(129) By contrast, in Reference Example 1 and Reference Example 2, in which Al.sub.2O.sub.3 having no binding properties is included in the binder, it is considered that gas was emitted from between Al.sub.2O.sub.3 particles during electrode drying, preventing the active material layer from expanding. Further, it is considered that, at 150° C., Al.sub.2O.sub.3 prevented or reduced the volume contraction of the alkali metal silicate and the aluminum phosphate, preventing cracks of the active material layer and drop-off from the collector.

(130) The above phenomena are the same as the following one, for example: when mochi (glutinous rice cake) is heated, it expands since water vapor inside is confined in the mochi, but cookies made of wheat flour as a main ingredient are resistant to such expansion as is seen in mochi since water vapor can escape from gaps of the flour.

(131) The charge/discharge test was performed using the electrodes of Reference Example 1 and Reference Example 2. As the battery, manufactured was a half cell using 1 M LiPF.sub.6/EC:DEC=50:50 vol %+VC (1 mass %) as the electrolytic solution, a layered structure of a polyolelin microporous membrane (20 μm) and a glass nonwoven fabric (GA-100) as the separator, and metal Li as the counter electrode. The charge/discharge test was performed at an environment temperature of 30° C., a current density of 0.25 C-rate, and a cutoff potential of 1.5 V to 0.01 V.

(132) In comparison of the initial charge/discharge efficiency, the alkali metal silicate (Reference Example 1) was 71% and the aluminum phosphate was 67%, indicating that the alkali metal silicate exhibited better initial charge/discharge efficiency. This means that the alkali metal silicate is smaller in irreversible capacity than the aluminum phosphate.

(133) FIG. 18 shows the cycle life characteristics in Reference Example 1, together with an electrode photograph (on the side of the collector) after 200 cycles. The capacity maintenance rate after 100 cycles to the initial discharge capacity of 2609 mAh/g was 78%. It is considered that, by the formation of the skeleton of the robust inorganic binder (alkali metal silicate) in the active material layer, the conductive network became resistant to destruction due to expansion/contraction of Si, thereby exhibiting excellent capacity maintenance rate. The reason why even thin copper foil could be used without occurrence of distortion is considered that the strength of the inorganic binder (alkali metal silicate) is higher than copper.

(134) <Confirming Analysis of Skeleton-Forming Agent>

Reference Example 3

(135) A test of confirming that the skeleton-forming agent had permeated into die electrode was performed. A simulated electrode was manufactured by: coating copper foil (10 μm) with slurry made of Al.sub.2O.sub.3 (D.sub.50=9 μm), AB, and a PVdF binder and regulating the pressure: coating and impregnating the active material layer with the skeleton-forming agent with a spray gun; and performing heat treatment at 150° C. As the skeleton-forming agent, an alkali metal silicate (Na.sub.2O.3SiO.sub.2) was dissolved in water and adjusted to have a solid content of 8 mass %. Note that no surfactant was added. The solid content ratio of the simulated electrode shiny was 85 mass % of α-Al.sub.2O.sub.3, 5 mass % of CB, and 10 mass % of the PVdF binder.

Reference Example 4

(136) Reference Example 4 was the same as Reference Example 3 except that the electrode was not coated or impregnated with the skeleton-forming agent as comparison. FIG. 20 shows the results of glow discharge emission spectroscopic analysis (GDS) in Reference Example 3 and Reference Example 4. The GDS measurement conditions were a measured diameter of 4 mmϕ and an Ne gas pressure of 2000 Pa. The measured wavelength was 121 nm for H, 685 nm for F, 130 nm for O, 156 mu for C, 396 nm for Al, and 251 nm for Si. The time of the x-axis represents the sputtering time, which is an index corresponding to the depth of the active material layer (the direction from the electrode surface to tire collector). The strength of the y-axis represents the emission intensity, which is an index corresponding to the content of each element. As is evident from FIG. 20, while presence of Si was not confirmed in Reference Example 4, presence of Si in a deep layer was confirmed, although Si is not present uniformly from the surface to the inside, in Reference Example 3. From these results, it has been proved that, by applying the skeleton-forming agent to the electrode surface, the skeleton-forming agent can permeate into the inside of the active material layer.

(137) While the preferred embodiments of the present invention have been described with reference to the accompanying drawings, various additions, changes, or deletions can be made without departing from the spirit and scope of the present invention. For example, the concentrations and ratios such as the solid content concentration of the alkali metal silicate of the skeleton-forming agent are not limited to the values described in the embodiments. Also, in the above embodiments, although A.sub.2O.3SiO.sub.2 (A=Li, Na) was described as the alkali metal silicate, A is not limited to Li and Na, and the coefficient of SiO.sub.2 is not limited to 2 to 3. Likewise, the phosphate of the skeleton-forming agent is not limited to the aluminum primary phosphate (Al.sub.2O.sub.3.3P.sub.2O.sub.5), and the coefficient of P.sub.2O.sub.5 is not limited to 3. It is therefore to be understood that these modifications are also included within the scope of the present invention.