Secondary battery Number:7,150,941 from the United States Patent and Trademark Office (PTO) owispatent Provided is a secondary battery in which high energy density can be obtained and charging/discharging cycle characteristic can be improved. A positive electrode (13) and a negative electrode (15) are stacked with a separator (16) interposed therebetween, and are enclosed inside an exterior can (11) to which an electrolyte is injected. The negative electrode (15) contains a negative electrode material capable of occluding/releasing lithium in an ionic state. Thereby, lithium metal precipitates in the negative electrode (15) in a state where the open circuit voltage is lower than the overcharge voltage. In other words, lithium is occluded in an ionic state in a negative electrode material capable of occluding/releasing lithium in the beginning of charging, and then lithium metal precipitates on the surface of the negative electrode material thereafter during charging. The amount of precipitation of lithium metal is preferable to be from 0.05 to 3.0 times, both inclusive, the ability of charging capacity of the negative electrode material capable of occluding/releasing lithium. Thereby, a high energy density and an excellent cycle characteristic can be obtained.<H Secondary, battery, Secondary, batte, 7150941.html, Patent, pto, 7150941

Title: Secondary battery

Abstract: Provided is a secondary battery in which high energy density can be obtained and charging/discharging cycle characteristic can be improved. A positive electrode (13) and a negative electrode (15) are stacked with a separator (16) interposed therebetween, and are enclosed inside an exterior can (11) to which an electrolyte is injected. The negative electrode (15) contains a negative electrode material capable of occluding/releasing lithium in an ionic state. Thereby, lithium metal precipitates in the negative electrode (15) in a state where the open circuit voltage is lower than the overcharge voltage. In other words, lithium is occluded in an ionic state in a negative electrode material capable of occluding/releasing lithium in the beginning of charging, and then lithium metal precipitates on the surface of the negative electrode material thereafter during charging. The amount of precipitation of lithium metal is preferable to be from 0.05 to 3.0 times, both inclusive, the ability of charging capacity of the negative electrode material capable of occluding/releasing lithium. Thereby, a high energy density and an excellent cycle characteristic can be obtained.

Patent Number: 7,150,941 Issued on 12/19/2006 to Fujita, et al.

Inventors: Fujita; Shigeru (Kanagawa, JP), Akashi; Hiroyuki (Kanagawa, JP), Adachi; Momoe (Tokyo, JP)
Assignee: Sony Corporation (Tokyo, JP)

Appl. No.: 11/084,552

Filed: March 18, 2005

Related U.S. Patent Documents


Application Number	Filing Date	Patent Number	Issue Date
09856431		6884546
PCT/JP00/06181	Sep., 2000

Foreign Application Priority Data


Sep 20, 1999 [JP]			P11-266017

Current U.S. Class: 429/231.9 ; 429/231.1; 429/231.8; 429/231.95; 429/331; 429/332; 429/338

Current International Class: H01M 4/02 (20060101); H01M 10/40 (20060101); H01M 4/58 (20060101)

References Cited [Referenced By]

U.S. Patent Documents


5626981	May 1997	Simon et al.
5709968	January 1998	Shimizu
6025094	February 2000	Visco et al.
6884546	April 2005	Fujita et al.
2002/0076605	June 2002	Akashi et al.
2003/0008212	January 2003	Akashi et al.

Foreign Patent Documents


03137010	Jun., 1991	JP
03252053	Nov., 1991	JP
04162370	Jun., 1992	JP
04328278	Nov., 1992	JP
07326342	Dec., 1995	JP
08298117	Nov., 1996	JP
10003948	Jan., 1998	JP
11-31534	Feb., 1999	JP
11238515	Aug., 1999	JP

Primary Examiner: Crepeau; Jonathan
Attorney, Agent or Firm: Sonnenschein Nath & Rosenthal LLP

Parent Case Text

RELATED APPLICATION DATA

This application is a continuation of prior application Ser. No. 09/856,431 filed Jul. 12, 2001, now U.S. Pat. No. 6,884,546 incorporated herein by reference to the extent permitted by law, which is the U.S. National Stage Application under 35 U.S.C. 371 of International Application No. PCT/JP00/06181, filed Sep. 11, 2000, and claims foreign priority under 35 U.S.C. 119(a) and 365(b) to Japan Patent Application JP 11/266017 filed Sep. 20, 1999.

Claims

The invention claimed is:

1. A secondary battery comprising: a negative electrode; a positive electrode; an electrolyte, and a separator, wherein, the negative electrode includes a negative electrode material capable of occluding and releasing light metal in an ionic state while the light metal precipitates on the negative electrode in a state where an open circuit voltage is lower than an overcharge voltage, when the open circuit voltage of the battery is below overcharging voltages, the ratio (moles of light metal precipitated on the negative electrode)/(moles of light metal reversibly occluded in the material of the negative electrode) is at least 0.05/1 and at most 3/1.

2. A secondary battery as claimed in claim 1, wherein the light metal comprises lithium.

3. A secondary battery as claimed in claim 2, wherein lithium precipitates on the negative electrode when the open circuit voltage of the battery is at least 0 V and at most 4.2 V.

4. A secondary battery as claimed in claim 2, wherein a peak attributed to lithium ion and a peak attributed to lithium metal are obtained when measuring the negative electrode material in a full-charged state by a .sup.7Li polynuclear species nuclear magnetic resonance spectroscopy.

5. A secondary battery as claimed in claim 4, wherein the peak attributed to lithium ion measured in the full-charged state disappears when measuring the negative electrode material in a complete-discharged state by the .sup.7Li polynuclear species nuclear magnetic resonance spectroscopy.

6. A secondary battery as claimed in claim 1 wherein the light metal precipitates on the negative electrode material.

7. A secondary battery as claimed in claim 1, wherein the ability of charging capacity of the negative electrode material is 150 mAh/g and more.

8. A secondary battery as claimed in claim 2, wherein the negative electrode has a negative electrode mixture layer containing the negative electrode material and the thickness of the negative electrode mixture layer is from at least 10 .mu.m to at most 300 .mu.m.

9. A secondary battery as claimed in claim 1, wherein the negative electrode material contains 50 percent by weight and more of a negative electrode active material.

10. A secondary battery as claimed in claim 1, wherein the negative electrode contains a carbonaceous material as the negative electrode material.

11. A secondary battery as claimed in claim 1, wherein the positive electrode contains an oxide containing the light metal.

12. A secondary battery as claimed in claim 1, wherein the positive electrode contains metallic carbonate.

13. A secondary battery as claimed in claim 1, wherein the metallic carbonate is lithium carbonate.

14. A secondary battery as claimed in claim 1, wherein the electrolyte contains at least one of the group consisting of ethylene carbonate and propylene carbonate.

15. A secondary battery as claimed in claim 14, wherein the electrolyte contains a non-aqueous solvent which contains propylene carbonate with a concentration of less than 30 percent by weight.

16. A secondary battery as claimed in claim 14, wherein the electrolyte contains ethylene carbonate and propylene carbonate and a mass fraction of mixing ethylene carbonate to propylene carbonate (ethylene carbonate/propylene carbonate) is 0.5 and more.

17. A secondary battery as claimed in claim 1, wherein the electrolyte contains at least one of the group consisting of chain ester carbonate, 2,4-difluoroanisole, and vinylene carbonate.

18. A secondary battery as claimed in claim 17, wherein the electrolyte contains a non-aqueous solvent which contains 2,4-difluoroanisole at a concentration of 15 percent by weight and below.

19. A secondary battery as claimed in claim 17, wherein the electrolyte contains a non-aqueous solvent which contains vinylene carbonate with a concentration of 15 percent by weight and below.

20. A secondary battery as claimed in claim 1, wherein the electrolyte contains ethylene carbonate, propylene carbonate, dimethyl carbonate, and ethyl-methyl carbonate.

21. A secondary battery as claimed in claim 1, wherein the electrolyte contains LiPF.sub.6.

22. A secondary battery as claimed in claim 1, wherein the separator comprises one or more materials selected from polyolefins, polytetrafluoroethylene and ceramic porous films.

23. The secondary battery of claim 22, wherein the polyolefin is selected from polyethylene and polypropylene or a mixture thereof.

24. The secondary battery of claim 22, wherein the polyolefin is polyethylene.

25. A secondary battery comprising: a negative electrode; a positive electrode; and an electrolyte wherein, the negative electrode includes a negative electrode material capable of occluding and releasing light metal in an ionic state while the light metal precipitates on the negative electrode in a state where an open circuit voltage is lower than an overcharge voltage, when the open circuit voltage of the battery is below overcharging voltages, the ratio (moles of light metal precipitated on the negative electrode)/(moles of light metal reversibly occluded in the material of the negative electrode) is at least 0.05/1 and at most 3/1, and the electrolyte comprises one or more lithium salts and a non-aqueous solvent.

26. A secondary battery as claimed in claim 25, wherein the light metal comprises lithium.

27. A secondary battery as claimed in claim 25, wherein lithium precipitates on the negative electrode when the open circuit voltage of the battery is at least 0 V and at most 4.2 V.

28. A secondary battery as claimed in claim 25, wherein a peak attributed to lithium ion and a peak attributed to lithium metal are obtained when measuring the negative electrode material in a full-charged state by a .sup.7Li polynuclear species nuclear magnetic resonance spectroscopy.

29. A secondary battery as claimed in claim 25, wherein the peak attributed to lithium ion measured in the full-charged state disappears when measuring the negative electrode material in a complete-discharged state by the .sup.7Li polynuclear species nuclear magnetic resonance spectroscopy.

30. A secondary battery as claimed in claim 25, wherein the light metal precipitates on the negative electrode material.

31. A secondary battery as claimed in claim 25, wherein the ability of charging capacity of the negative electrode material is 150 mAh/g and more.

32. A secondary battery as claimed in claim 26, wherein the negative electrode has a negative electrode mixture layer containing the negative electrode material and the thickness of the negative electrode mixture layer is from at least 10 .mu.m to at most 300 .mu.m.

33. A secondary battery as claimed in claim 25, wherein the negative electrode material contains 50 percent by weight and more of a negative electrode active material.

34. A secondary battery as claimed in claim 25, wherein the negative electrode contains a carbonaceous material as the negative electrode material.

35. A secondary battery as claimed in claim 25, wherein the positive electrode contains an oxide containing the light metal.

36. A secondary battery as claimed in claim 25, wherein the positive electrode contains metallic carbonate.

37. A secondary battery as claimed in claim 25, wherein the metallic carbonate is lithium carbonate.

38. A secondary battery as claimed in claim 25, wherein the non-aqueous solvent is selected from ethylene carbonate and propylene carbonate or a mixture thereof.

39. A secondary battery as claimed in claim 25, wherein the non-aqueous solvent contains propylene carbonate at a concentration of less than 30 percent by weight.

40. A secondary battery as claimed in claim 25, wherein the non-aqueous solvent is a mixture of ethylene carbonate and propylene carbonate and a mass fraction of mixing ethylene carbonate to propylene carbonate (ethylene carbonate/propylene carbonate) is 0.5 and more.

41. A secondary battery as claimed in claim 25, wherein the non-aqueous solvent contains at least one or more chain ester carbonates.

42. The secondary battery of claim 41, wherein the chain ester carbonates are selected from diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, methyl propyl carbonate and mixtures thereof.

43. A secondary battery as claimed in claim 25, wherein the non-aqueous solvent contains 2,4-difluoroanisole at a concentration of at most 15 percent by weight.

44. A secondary battery as claimed in claim 25, wherein the non-aqueous solvent contains vinylene carbonate at a concentration of at most 15 percent by weight.

45. A secondary battery as claimed in claim 25, wherein the non-aqueous solvent contains one or more solvents selected from butylene carbonate, .gamma.-butyrolactone, .gamma.-valerolactone, butylene carbonate wherein one or more hydrogen atoms are replaced with fluorine atoms, .gamma.-butyrolactone wherein one or more hydrogen atoms are replaced with fluorine atoms, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyl tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, methylacetate, methylpropionate, acetonitrile, glutaronitrile, adiponitrile, methoxyacetonitrile, 3-methoxypropylenenitrile, N,N-dimethylformamide, N-methylpyrrolidinone, N-methyloxyazolidinone, N,N-dimethylimidazolidinone, nitromethane, nitroethane, sulfolane, dimethylsulfoxide, trimethyiphosphate or mixtures thereof.

46. A secondary battery as claimed in claim 25, wherein the lithium salts are selected from LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6, LiClO.sub.4, LiB(C.sub.6H.sub.5).sub.4, LiCH.sub.3SO.sub.3, LiCF.sub.3SO.sub.3, LiN(SO.sub.2CF.sub.3).sub.2, LiC(SO.sub.2CF.sub.3).sub.3, LiAlCl.sub.4, LiSiF.sub.6, LiCl, LiBr or mixtures thereof.

47. A secondary battery as claimed in claim 25, wherein the lithium salt is LiPF.sub.6.

48. A secondary battery as claimed in claim 25, wherein the concentration of the lithium salts in the non-aqueous solvent is at least 0.1 mol/dm.sup.3 and at most to 5.0 mol/dm.sup.3.

49. A secondary battery as claimed in claim 25, wherein the concentration of the lithium salts in the non-aqueous solvent is at least 0.5 mol/dm.sup.3 and at most 3.0 mol/dm.sup.3.

50. A secondary battery as claimed in claim 25, wherein the electrolyte is in a solid state.

51. An electric or electronic device comprising the secondary battery of claim 1.

52. An electric or electronic device comprising the secondary battery of claim 25.

Description

TECHNICAL FIELD

The invention relates to a secondary battery comprising a negative electrode, a positive electrode and an electrolyte, specifically, to a secondary battery in which light metal is used for an electrode reaction.

BACKGROUND ART

Recently, in accordance with development in electronic technology, a number of portable electron devices such as VTRs (video tape recorder) with a built-in camera, cellular phones and laptop computers have come into wide use, and miniaturization and weight-reduction of the devices have become the subject. Research and developing aimed at improving energy density of batteries used for portable power sources for the devices, specially secondary batteries, have been actively conducted.

Widely known secondary batteries of the related art are a lead battery, a Ni(nickel)-Cd(cadmium) battery, a lithium ion secondary battery in which a material such as a carbonaceous material capable of occluding/releasing lithium (Li) is used for a negative electrode, and a lithium secondary battery in which lithium metal is used for a negative electrode. A large expectation has been put on a secondary battery using a non-aqueous electrolyte, specifically on a lithium ion secondary battery, since the battery can obtain a higher energy density than the lead battery and the nickel-cadmium battery of the related art using an aqueous electrolyte, and its market has largely grown. Also, theoretical electrochemical equivalent of lithium metal in the lithium secondary battery is as large as 2054 mAh/dm.sup.3, which is equivalent to 2.5 times the graphite material used in a lithium ion secondary battery. Thereby, an excellent energy density higher than that of the lithium ion secondary battery can be expected and the lithium secondary battery has been actively studied.

However, while being capable of obtaining a large capacity, a lithium secondary battery has low charging/discharging efficiency. Also, it has a problem such as deterioration in the charging/discharging capacity when repeating charging and discharging, thereby having an insufficient charging/discharging cycle characteristic. Especially, the problem is noticeble when performing boosting charge by a large current for a short time. Therefore, it is difficult to perform boosting charge on the lithium secondary battery. Also, in the lithium secondary battery, lithium is consumed by repeating charging and discharging. Therefore, it is necessary for the lithium secondary battery to contain an excessive amount of lithium in advance. As a result, there is another problem such that the actual charging/discharging capacity cannot be made much larger.

The problems are directly due to the fact that lithium metal forming a negative electrode is pulverized during a dissolving/re-crystallizing process of lithium metal at the time of charging/discharging. As techniques for suppressing the pulverization, a variety of improvement methods are proposed as noted in "Lithium Batteries" (Edited by JEAN-PAUL GABANO, ACADEMIC PRESS, 1983, London, New York). For example, a lithium alloy such as a lithium-aluminum alloy is used as a negative electrode material, a variety of addictives are added to an electrolyte, or the surface of the lithium metal is slightly coated by a carbonaceous material. However, none of these methods are sufficient and it has been still difficult to put a lithium secondary battery in a practical use.

One of the reasons for having difficulties suppressing pulverization of lithium metal is that the volume of the negative electrode made of lithium metal largely changes at the time of charging/discharging. For example, as may be evident in the characteristic of manganese-lithium secondary batteries on the market, the amount of displacement is small in the distance between the positive electrode and negative electrode when charging/discharging is shallow, and the electrode reaction on the surface of the negative electrode tends to proceed homogeneously. On the contrary, when charging/discharging is deep, the amount of displacement is large in the distance between the positive electrode and negative electrode and the displacement phenomenon tends to proceed heterogeneously. Therefore, the distance between the positive electrode and negative electrode tends to be heterogeneous. These can be considered as the reasons for promoting pulverization of lithium metal at the time of charging/discharging.

According to the hypothesis, it is assumed that pulverization of the lithium metal can be suppressed by minimizing the displacement in the distance between the positive electrode and negative electrode as small as possible so that the charging/discharging cycle characteristic can be improved.

For example, one of the methods for minimizing the amount of displacement in the distance between the positive electrode and negative electrode is minimizing the amount of reaction of the negative electrode made of lithium metal. However, if this method is applied to a lithium secondary battery of the related art in which lithium metal is provided on a collector layer, the energy density of the battery is largely deteriorated. As a result, the significance of using lithium metal for a negative electrode, which initially has high electrochemical equivalent, becomes insignificant.

Another method for minimizing the amount of displacement in the distance between the positive electrode and negative electrode is to maintain the distance between the positive electrode and negative electrode constant by applying a pressure through providing a spring or the like. However, if a spring or the like with no electrode activity is provided inside the battery, the fraction of the volume of the electrode active material inside the battery becomes relatively low for the volume of the spring or the like provided. Thereby, the discharging density and the energy density of the battery deteriorates.

Therefore, development of a secondary battery in which the amount of displacement in the distance between the positive electrode and negative electrode can be minimized without deteriorating the characteristic capable of obtaining a high energy density has been demanded. In developing such a secondary battery, it is necessary to study the composition of an electrolyte in order to sufficiently utilize the capacity of the electrode in addition to studying the electrode material.

The invention has been designed to overcome the foregoing problems. The object of the invention is to provide a secondary battery in which a high energy density can be obtained and the charging/discharging cycle characteristic can be improved.

DISCLOSURE OF THE INVENTION

A secondary battery of the invention comprises a negative electrode, a positive electrode and an electrolyte wherein: the negative electrode contains a negative electrode material capable of occluding and releasing light metal in an ionic state while the light metal precipitates in the negative electrode in a state where the open circuit voltage is lower than the overcharge voltage.

A secondary battery of the invention comprises a negative electrode, a positive electrode and an electrolyte, wherein: the capacity of the negative electrode is expressed by the sum of the capacity which is obtained when occluding/releasing light metal in an ionic state and the capacity which is obtained when precipitating/dissolving light metal.

In a secondary battery of the invention, charging is performed by the shift of light metal ions from the positive electrode to the negative electrode via an electrolyte, and discharging is performed by the shift of light metal ions from the negative electrode to the positive electrode via an electrolyte. In the process of charging at this time, the light metal precipitates in the negative electrode when the open circuit voltage is lower than the overcharge voltage.

In another secondary battery of the invention, charging is performed by the shift of light metal ions from the positive electrode to the negative electrode via an electrolyte, and discharging is performed by the shift of light metal ions from the negative electrode to the positive electrode via an electrolyte. At this time, the capacity of the negative electrode is expressed by the sum of the capacity which is obtained when occluding/releasing light metal in an ionic state and the capacity which is obtained when precipitating/dissolving light metal.

Other and further objects, features and advantages of the invention will appear more fully from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section showing the configuration of a secondary battery according to a first embodiment of the invention.

FIG. 2 is a cross section showing the configuration of a secondary battery according to a first modification of the invention.

FIG. 3 shows the solid .sup.7Li polynuclear species nuclear magnetic resonance spectrum (solid .sup.7Li-NMR spectrum) of a negative electrode according to Example 2-1 of the invention.

FIG. 4 shows the solid .sup.7Li-NMR spectrum of a negative electrode according to Example 2-2 of the invention.

FIG. 5 shows the solid .sup.7Li-NMR spectrum of a negative electrode according to Example 2-3 of the invention.

FIG. 6 shows the solid .sup.7Li-NMR spectrum of a negative electrode according to Example 2-4 of the invention.

FIG. 7 shows the solid .sup.7Li-NMR spectrum of lithium chloride which is an outer standard substance.

FIG. 8 shows the solid .sup.7Li-NMR spectrum of lithium metal.

FIG. 9 shows the solid .sup.7Li-NMR spectrum of a negative electrode according to Comparative Example 2-2 of the invention.

FIG. 10 shows the solid .sup.7Li-NMR spectrum of a negative electrode according to Comparative Example 2-3 of the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In the following, embodiments of the invention will be described in detail by referring to the drawings.

[First Embodiment]

FIG. 1 shows the cross sectional configuration of a secondary battery according to a first embodiment of the invention. The secondary battery shown in FIG. 1 is what we call a coin-type. The secondary battery is formed by stacking a circular-plate positive electrode 13 enclosed in an exterior can 11 via a spacer 12 and a circular-plate negative electrode 15 enclosed in an exterior cup 14 with a separator 16 interposed therebetween. Inside the exterior can 11 and the exterior cup 14 are filled with an electrolyte which is an aqueous electrolyte, and peripheral edges of the exterior can 11 and the exterior cup 14 are sealed by caulking with an insulating gasket 18 in between.

The exterior can 11 and the exterior cup 14 are formed of, for example, iron (Fe) whose surface is plated with nickel (Ni), respectively. The spacer 12 is provided for adjusting the thickness of the battery and is made of, for example, copper (Cu).

The positive electrode 13 comprises, for example, a positive electrode mixture layer 13a, and a positive electrode collector layer 13b provided on the exterior cup 14 side of the positive electrode mixture layer 13a. The positive 13b is formed of a metallic foil such as aluminum (Al) foil. The positive electrode mixture layer 13a is formed containing, for example, a positive electrode active material, a conductive agent such as graphite, and a binder such as polyvinylidene fluoride.

As the positive electrode active material, a compound containing lithium as the light metal, such as lithium oxide or lithium sulfide, and an intercalation compound containing lithium are appropriate. Two and more kinds of these may be mixed to be used. Especially, it is preferable for the positive electrode active material to contain a lithium composite oxide which mainly includes Li.sub.xMO.sub.2 in order to obtain a higher energy density. M is preferable to be one and more kind of transition metals. Specifically, it is preferable to be at least one kind selected from the group consisting of cobalt (Co), nickel (Ni), manganese (Mn), iron (Fe), aluminum (Al), vanadium (V), and titanium (Ti). x varies depending on the charging/discharging state of the battery and is normally in the range of 0.05.ltoreq.x.ltoreq.1.10. Specific examples of such lithium composite oxides are Li.sub.xCoO.sub.2, Li.sub.xNiO.sub.2, Li.sub.xNi.sub.yCo.sub.1-yO.sub.2 and Li.sub.xM

n.sub.2O.sub.4 (where, x.apprxeq.1, 0<y<1).

The lithium composite oxide is prepared in the following manner. For example, carbonate, nitrate, oxide, or hydroxide of lithium and carbonate, nitrate, oxide, or hydroxide of transition metal are mixed in a desired composition and pulverized, and then calcined in an oxidizing atmosphere at a temperature within the range of 600 to 1000.degree. C., both inclusive.

It is preferable that the positive electrode mixture layer 13a contains, in regard to improving the charging/discharging capacity, the amount of lithium corresponding to 280 mAh and more of charging/discharging capacity per 1 g of the negative electrode active material in a steady state (e.g., after repeating charging/discharging about 5 times). It is more preferable to contain the amount of lithium corresponding to 350 mAh and more of charging/discharging capacity. However, all of the lithium does not have to be supplied from the positive electrode mixture layer 13a, that is, the positive electrode 13, but may be present throughout the battery. For example, lithium inside the battery can be supplemented by laminating lithium metal or the like on the negative electrode 15. The amount of lithium inside the battery can be quantitated by measuring the discharging capacity of the battery.

The positive electrode mixture layer 13a may further contain, for example, metallic carbonate such as lithium carbonate (Li.sub.2O.sub.3). It is preferable to contain metallic carbonate as described since it can further improve the cycle characteristic. This is considered to be due to the fact that part of metallic carbonate in the positive electrode 13 is dissolved and forms a stable coating on the negative electrode 15.

The negative electrode 15 comprises, for example, a negative electrode mixture layer 15a and a negative electrode collector layer 15b provided on the exterior can 11 side of the positive electrode mixture layer 15a. The negative electrode mixture layer 15a is formed containing a negative electrode material capable of occluding/releasing lithium, comprising light metal, in an ionic state as a negative electrode active material. The Occlusion of the light metal in an ionic state denotes a state where light metal exists taking the form of ion and a typical example thereof is the electrochemical intercalation of light metal in an ionic state to graphite. It is a concept different from precipitation of light metal in a metallic state. In the following, there may be a case where it is simply referred to as occlusion of the light metal in order to simplify the description. Examples of the negative electrode materials are a carbonaceous material, a metallic compound, silicon, a silicon compound, and a polymer material. One of these, or two and more kinds may be mixed to be used.

Examples of the carbonaceous materials are non-graphitizing carbon, graphitizing carbon, graphite, cracked carbon, cokes, glassy carbon, polymer organic compound calcined materials, carbon fiber and activated carbon. The cokes include pitch coke, needle coke and petroleum coke. The polymer compound calcined material is a material obtained by calcining a polymer material such as phenolic resin or furan resin to be carbonated at an appropriate temperature and some of which are classified into a non-graphitizing carbon or a graphitizing carbon. Examples of the metallic compounds are oxides such as SnSiO.sub.3 and SnO.sub.2 and examples of polymer materials are polyacetylene and polypyrrole.

As the negative electrode material, the ones having a charging/discharging potential relatively close to that of lithium metal is preferable. The reason is that the lower the charging/discharging potential of the negative electrode 15 is, the easier the high energy density of the battery can be obtained. Especially, the carbonaceous material is preferable since changes in the crystal structure which occur at the time of charging/discharging are very small so that a high energy density and an excellent cycle characteristic can be obtained. Specifically, graphite is preferable since the electrochemical is high so that a high energy density can be obtained. Also, non-graphitizing carbon is preferable so that an excellent cycle characteristic can be obtained.

Graphite with a true density of, for example, 2.10/cm.sup.3 and more is preferable and the one with 2.18/cm.sup.3 is more preferable. In order to obtain such a true density, it is necessary that the thickness of C-axis crystal of 002 plane is 14.0 nm and more. It is preferable that the interlayer spacing distance (002) plane is less than 0.340 nm and is more preferable to be within the range of 0.335 nm to 0.337 nm, both inclusive.

The graphite may be natural graphite or artificial graphite. Artificial graphite can be obtained by carbonating an organic substance, applying a heat treatment at a high temperature, and then pulverizing/classifying it. The heat treatment at a high temperature is performed by, for example, carbonating the material at 300.degree. C. to 700.degree. C. in an inert gas flow such as nitrogen (N.sub.2) if necessary, increasing the temperature to 900.degree. C. to 1500.degree. C. at a rate of 1.degree. C. to 100.degree. C. per minute and then calcining it by keeping the temperature for about 0 to 30 hours, and heating it to 2000.degree. C. and more, more preferably to 2500.degree. C. and more, and keeping the temperature for an appropriate length of time.

Coal or pitch can be used as an organic substance as the starting material. Examples of the pitches are coal tar, tar obtained by cracking ethylene bottom oil or crude oil at a high temperature, and tar obtained from asphalt or the like by performing distillation (vacuum distillation, atmospheric distillation, steam distillation), thermal polycondensation, extraction, chemical polycondensation and the like, pitch generated at the time of dry distillation of wood, polyvinyl chloride resin, polyvinyl acetate, polyvinyl butylate and 3,5-dimethyl-phenol resin. During carbonization, coal and pitch exist in a liquid state at a temperature of about 400.degree. C. at the maximum and the aromatic rings become a state of laminated orientation by being condensed and polycycled through maintaining the temperature. At about 500.degree. C. and more, semicokes, which is a precursor of solid carbon can be formed (a liquid phase carbonization process).

Examples of the organic substances are condensed polycyclic hydrocarbon compounds such as naphthalene, phenanthrene, anthracene, triphenylen, pyrene, perylene, pentaphen and pentacen, the derivatives thereof (e.g., their carboxylic acid, carboxylic anhydride, carboxylic imide), and the mixture thereof. Other examples are condensed heterocycle compounds such as acenaphtylene, indole, isoindole, quinoline, isoquinoline, quinoxaline, phthalazine, carbazole, acridine, phenazine and phenantolidine, and the derivatives and mixtures thereof.

Pulverization may be performed at any time during the process of increasing the temperature before/after carbonization and calcinations or before graphitization. In this case, lastly a heat treatment for graphitization is performed in a state of powder. However, in order to obtain graphite powder with high bulk density and fracture strength, it is preferable to apply a heat treatment after molding a starting material and pulverizing/classifying the obtained graphitized molded body.

For example, when forming the graphitized molded body, after cokes to be a filler and a binder pitch to be a molding agent or a sintering agent are mixed and molded, a calcining step of applying a heat treatment on the molded body at a low temperature of 1000.degree. C. and below and a pitch-impregnating step of impregnating a binder pitch which is dissolved in the calcined body are repeated a number of times. Then, a heat treatment at a high temperature is applied. The impregnated binder pitch is carbonated and graphitized in the above-mentioned process of heat treatment. In this case, the filler (cokes) and the binder pith are used as the starting material thereby being graphitized as a polycrystal body. Also, sulfur and nitrogen contained in the starting material generates gas at the time of the heat treatment, thereby forming micropores on the way through. Because of the pores, occluding/releasing of lithium becomes easier and processing efficiency becomes higher industrially. As the starting material for the molded body, a filler having a moldability and sintering characteristic by itself may be used. In this case, it is unnecessary to use a binder pitch.

The non-graphitizing carbon is preferable to have an interlayer spacing distance (002) plane of 0.32 nm and more and a true density of less than 1.70 g/cm.sup.3, and not to exhibit an exothermic peak at 700.degree. C. and more in the differential thermal analysis (DTA) in the air.

The non-graphitizing carbon can be obtained by performing a heat treatment on an organic substance at about 1200.degree. C. and then pulverizing/classifying it. The heat treatment is performed by, for example, carbonating the material at 300.degree. C. to 700.degree. C. (solid phase carbonating process) if necessary, increasing the temperature to a temperature from 900.degree. C. to 1300.degree. C. at a rate of 1 to 100.degree. C. per minute and then calcining it by keeping the temperature for about 0 to 30 hours. Pulverization may be performed before/after carbonization, or during the process of increasing a temperature.

Examples of the organic substances as the starting material are polymer and copolymer of furfuryl alcohol and furfural, or a furan resin which is a copolymer of the polymer and other resin. Other Examples are a phenol resin, an acryl resin, a vinyl halide resin, a polyimide resin, a polyamide-imide resin, a polyamide resin, a conjugate resin such as polyacetylene or polyparaphenylene, cellulose and cellulosic, coffee beans, bamboos, crustacea including chitosan, and bio-cellulose using bacteria. Still other examples are a compound obtained by bonding a functional group including oxygen (O) (so-called a oxygen cross-linking) to a petroleum pitch in which the fraction of the number of atoms of hydrogen (H) and carbon (C), H/C, is, for example, 0.6 to 0.8.

Preferably, the compound contains 3% and more oxygen, more preferably 5% and more (see Japanese Patent Application laid-open Hei 3-252053). This is due to the fact that the oxygen content influences the crystal structure of the carbonaceous material, and the property of the non-graphitizing carbon and the capacity of the negative electrode can be improved by containing the above-mentioned content and more of oxygen. The petroleum pitch can be obtained, for example, by performing distillation (vacuum distillation, constant pressure distillation, steam distillation), thermal polycondensation, extraction and chemical polycondensation on tar, asphalt and the like, the tar being obtained by cracking coal tar, ethylene bottom oil or crude oil at a high temperature. Examples of methods for forming oxidation cross-linking are a wet method in which aqueous solutions of nitric acid, sulfuric acid, hypochlorous acid or a mixed acid of these, and the petroleum pitch are activated, a dry method in which an oxidizing gas such as air or oxygen and the petroleum pitch are activated, and a method in which a solid reagent such as sulfur, ammonium nitrate, ammonium persulfate, or ferric chloride, and the petroleum pitch are activated.

The organic substances as the starting material are not limited to these but any other organic substances, which can be non-graphitizing carbonaceous material through a solid phase carbonization process by oxidation cross-linking process, may be applied.

In addition to the non-graphitizing carbonaceous material prepared using the above-mentioned organic substance as the starting material, it is also preferable to use a compound, disclosed in Japanese Patent Application laid-open Hei 3-137010, which mainly contains phosphorus (P), oxygen and carbon since it exhibits the property parameter mentioned above.

Incidentally, in the embodiment, the negative electrode material capable of occluding/releasing lithium does not include lithium metal and lithium alloy such as lithium-aluminum alloy, which function as a negative electrode active material when lithium is precipitated/dissolved. However, the secondary battery may contain lithium metal or lithium alloy in the negative electrode mixture layer 15a as the negative electrode active material. Also, however not shown in the figure, a metallic layer made of lithium metal or lithium alloy may be included in the negative electrode 15 separately from the negative electrode mixture layer 15a.

The negative electrode mixture layer 15a may be formed containing a binder such as polyvinylidene fluoride (PVDF), for example. The negative electrode collector layer 15b is formed of a metallic foil such as a copper (Cu) foil.

In the secondary battery, in the process of charging, lithium metal starts to precipitate in the negative electrode 15 when the open circuit voltage (that is, the battery voltage) is lower than the overcharge voltage. In other words, in the secondary battery, lithium metal precipitates in the negative electrode 15 in a state where the open circuit voltage is lower than the overcharge voltage, and the capacity of the negative electrode 15 is expressed by the sum of the capacity which is obtained when occluding/releasing lithium in an ionic state and the capacity which is obtained when precipitating/dissolving lithium metal.

The overcharge voltage herein means an open circuit voltage when the battery is overcharged, and indicates the voltage higher than the open circuit voltage of the "full-charged" battery, which is defined in page 6 of "GUIDE LINE FOR SAFETY EVALUATION ON SECONDARY LITHIUM CELLS" (SBA G1101) which is one of the guide lines appointed by JAPAN STORAGE BATTERY ASSOCIATION (BATTERY ASSOCIATION OF JAPAN), for example,. In other words, it means that the voltage higher than the open circuit voltage charged by a charging method used when obtaining the nominal capacity of each battery, a standard charging method, or a recommended charging method. Specifically, the secondary battery is full-charged when, for example, the open circuit voltage is 4.2 V and lithium metal precipitates on the surface of the negative electrode material capable of occluding/releasing lithium in one part within the range of the open circuit voltage from 0 V to 4.2 V, both inclusive.

Therefore, a peak attributed to lithium ion and a peak attributed to lithium metal can be obtained when measuring the negative electrode 15 (specifically, a negative electrode material capable of occluding/releasing lithium) in a full-charged state by a .sup.7Li polynuclear species nuclear magnetic resonance spectroscopy. On the other hand, the peak attributed to lithium ion disappears when measuring the negative electrode material in a complete-discharged state by the .sup.7Li polynuclear species nuclear magnetic resonance spectroscopy. The complete-discharged state corresponds to the case where there is no supply of the electrode reactant (lithium in the embodiment) from the negative electrode 15 to the positive electrode 13. For example, the secondary battery and the lithium-ion secondary battery according to the embodiment can be considered as being "completely discharged" when the open circuit voltage reaches 2.75 V.

Thereby, in the secondary battery, a high energy density can be obtained while the cycle characteristic and the boosting charging characteristic can be improved. With respect to precipitation of lithium metal in the negative electrode 15, it is the same as the lithium secondary battery of the related art using lithium metal or lithium alloy for the negative electrode. It may be due to the fact the following advantages obtained by precipitation of lithium metal in the negative electrode capable of occluding/releasing lithium.

First, with the lithium secondary battery of the related art, it is difficult to heterogeneously precipitate lithium metal, resulting in deterioration in the cycle characteristic. In the secondary battery of the invention, however, lithium metal can heterogeneously precipitate since the negative electrode capable of occluding/releasing lithium generally has a large surface area. Secondly, with the secondary battery of the related art, changes in volume by precipitating/dissolving of lithium metal are large, resulting in deterioration in the cycle characteristic. In the secondary battery of the invention, however, lithium metal precipitates in spaces between particles of the negative electrode material capable of occluding/releasing lithium so that changes in volume are small. Thirdly, with the secondary battery of the related art, the above-mentioned problems become larger, as the amount of precipitating/dissolving lithium metal becomes larger. With the secondary battery of the invention, however, the amount of precipitating/dissolving lithium metal is small for a battery with such a large capacity since occlusion/release of lithium by the negative electrode capable of occluding/releasing lithium contributes to the charging/discharging capacity. Fourthly, with the secondary battery of the related art lithium metal precipitates more heterogeneously when performing boosting charging, resulting in further deterioration in cycle characteristic. With the secondary battery of the invention, however, boosting charging can be performed since lithium is occluded in the negative electrode material capable of occluding/releasing lithium in the beginning of charging.

In order to more effectively obtain the advantages, it is preferable that the maximum amount of lithium precipitating in the negative electrode at the maximum voltage of the open circuit voltage before reaching overcharge voltage is from 0.05 to 3.0 times, both inclusive, the charging capacity of the negative electrode material capable of occluding/releasing lithium. The reason is that the same problems as those of the related art occur if the amount of precipitation of lithium is too large and a sufficient charging/discharging capacity cannot be obtained if the amount is too small. Also, for example, the discharging capacity of the negative electrode material capable of occluding/releasing lithium is preferable to be 150 mAh/g and more. The reason is that the larger the ability of occluding/releasing lithium is, the smaller the amount of precipitation of lithium becomes. The charging capacity of the negative electrode material can be obtained from the amount of electricity when charging the negative electrode, with the negative electrode material as the negative electrode active material, to 0 V by a constant current/constant voltage method with, for example, lithium metal being the opposite electrode. The discharging capacity of the negative electrode material can be obtained from the amount of the electricity when keeping discharging it to 2.5 V by a constant current method for 10 hours and more.

Furthermore, for example, the thickness of the negative electrode mixture layer 15a containing the negative electrode material capable of occluding/releasing lithium in the opposite direction to the positive electrode 13 is preferable to be from 10 .mu.m to 300 .mu.m, both inclusive. The reason is that the amount of lithium precipitating in the negative electrode material becomes heterogeneous in the thickness direction if the negative electrode mixture layer 15a is too thick, resulting in deterioration in cycle characteristic. The amount of precipitating lithium becomes relatively too large if the thickness is too thin so that the same problems as those of the secondary battery of the related art occur. In addition, for example, in the case where the negative electrode 15 contains a material as the negative electrode active material other than the negative electrode material capable of occluding/releasing lithium such as lithium metal or lithium alloy, it is preferable to contain 50 percent by volume and more of the negative electrode material capable of occluding/releasing lithium. The reason is that the problems of the lithium secondary battery of the related art cannot be sufficiently solved if the proportion of the negative electrode material capable of occluding/releasing lithium contained is small.

The separator 16 let lithium ions pass through while separating the positive electrode 13 and the negative electrode 15 and suppressing short circuit of the current, which occurs due to the contact of both electrodes. The separator 16 is formed of a porous film made of synthetic resin such as polytetorafluoroethylene, polypropylrne, or polyethylene, or a ceramic porous film. Also, it may have a configuration in which two and more kinds of porous films are laminated. Especially, a porous film made of polyolefin is preferable since it is excellent in suppressing short circuit and can improve safety of the battery by the shutdown effect. Specifically, polyethylene is preferable as the material for forming the separator 16 since it can obtain the shutdown effect at a temperature within the range of 100.degree. C. to 160.degree. C., both inclusive, and is also excellent in electrochemical stability. Polypropylene is also preferable. Other resins having chemical stability can be used by being co-polymerized or blended with polyethylene or polypropylene.

The porous film made of polyolefin can be obtained in the following manner. For example, a fused low-volatile solvent in a state of liquid is mixed with a fused polyolefin composite, thereby obtaining a solvent with high concentration of homogeneous polyolefin composite. The solvent is molded with a die and cooled down to obtain a gel sheet, and then stretched.

Examples of the low-volatile solvent are low-volatile aliphatic or cyclic hydrocarbon such as nonane, decane, decaline, p-xylene, undecane, and liquid paraffin. The proportion of mixing the polyolefin composite and the low-volatile solvent is preferably containing from 10 percent by volume to 80 percent by volume, both inclusive, of the polyolefin composite, more preferably containing from 15 percent by volume to 70 percent by volume, both inclusive, of the polyolefin composite provided the sum of both is 100 percent by volume. The reason is that if the volume of polyolefin composite is too small, imbibition occurs in the exit of the die or the neck-in becomes large at the time of molding so that molding of a sheet becomes difficult. On the other hand, if the volume of polyolefin is too large, preparation of the homogeneous solvent is difficult.

When molding the solvent with a high concentration of polyolefin composite by a sheet dice, it is preferable that a gap is, for example, from 0.1 mm to 5 mm, both inclusive. Also, it is preferable that the pressing temperature is from 140.degree. C. to 250.degree. C., both inclusive, and the pressing rate is from 2 cm/minute to 30 cm/minute, both inclusive.

Cooling is performed at least until the temperature at which the solvent galates and below. Examples of cooling methods are a method of making a direct contact with a cold wind, a coolant, or other cooling medium, and a method of making a contact with a roll cooled by a coolant. The solvent with a high concentration of polyolefin composite which is pushed out from the die may be taken back before or during the cooling process by a fraction of from 1 to 10, both inclusive, preferably 1 to 5, both inclusive. The reason is that if the fraction is too large, the neck-in becomes too large and breakings are easily generated when stretching the sheet, which is not preferable.

Stretching of the gel sheet is preferable to be performed by biaxial stretching with a tentering method, a roll method, a pressing method, or a combination of these methods after heating the gel sheet. At this time, simultaneous stretching in both longitudinal and lateral direction or sequential stretching maybe performed. However, the simultaneous two-dimensional stretching is specifically preferable. The stretching temperature is preferable to be the temperature, which is obtained by adding 10.degree. C. to the melting point of polyolefin composite, and more, and is more preferable to be within the range of the dispersing temperature of crystal, inclusive, to the melting point, exclusive. If the stretching temperature is too high, the effective molecular chain orientation by stretching cannot be obtained due to dissolution of the resin, which is not preferable. If the stretching temperature is too low, the degree of softening the resin is insufficient so that the sheet is easily broken when being stretched. Thereby, stretching with high magnification cannot be performed.

After stretching the gel sheet, it is preferable to wash the stretched film with a volatile solvent and then remove the low-volatile solvent remaining. After washing, the stretched film is dried by applying a heat or sending an air and the washing solvent is volatilized. Examples of the washing solvents are: hydrocarbon such as pentane, hexane, and hebutane; chlorine hydrocarbon such as methylene chloride and carbon tetrachloride; carbon fluoride such as ethane trifluoride; and ether such as diethyl ether and dioxane, which have a volatilizing characteristic. The washing solvents are selected according to the low-volatile solvent used, and are used alone or by being mixed. Washing is performed by a method of soaking the stretched film in a volatile solvent, a method of sprinkling the volatile solvent over the film, or a combined method of these. Washing is performed until less than 1 percent by volume of the low-volatile solvent remains in the stretched film to 100 percent by volume of polyolefin composite.

The electrolyte 17 is obtained by dissolving lithium salt as an electrolyte salt in a non-aqueous solvent. A non-aqueous solvent is a non-aqueous compound with, for example, 10.0 mPa.cndot.s intrinsic viscosity and below at 25.degree. C. It is preferable that the non-aqueous solvent contains at least either ethylene carbonate (EC) or propylene carbonate (PC) so that the cycle characteristic can be improved. It is more preferable to mix ethylene carbonate and propylene carbonate for use so that the cycle characteristic can be further improved.

However, when graphite is used for the negative electrode 15, the concentration of propylene carbonate in the non-aqueous solvent is preferable to be less than 30 percent by volume. Propylene carbonate exhibits relatively high reaction to graphite so that the characteristic is deteriorated if the concentration of propylene carbonate is too high. When the non-aqueous solvent contains ethylene carbonate and propylene carbonate, preferably the mass fraction (ethylene carbonate/propylene carbonate) of mixing ethylene carbonate with propylene carbonate in the non-aqueous solvent, which is the value obtained by dividing the content of ethylene carbonate by the content of propylene carbonate, is 0.5 and more.

It is also preferable that the non-aqueous solvent contains at least one kind of chain ester carbonates such as diethyl carbonate, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and methyl propyl carbonate. Thereby, the cycle characteristic can be improved.

It is preferable that the non-aqueous solvent further contains at least either 2,4-difluoroanisole (DFA) or vinylene carbonate (VC). The reason is that 2,4-difluoroanisole can improve the discharging capacity and vinylene carbonate can further improve the cycle characteristic. It is especially preferable to use the combination of both so that the discharging capacity and the cycle characteristic can be improved at the same time.

It is preferable that the concentration of 2,4-difluoroanisole in the non-aqueous solvent is, for example, 15 percent by volume and below. The reason is that the discharging capacity cannot be improved if the concentration is too high. It is preferable that the concentration of vinylene carbonate in the non-aqueous solvent is, for example, 15 percent by volume and below. The reason is that the cycle characteristic cannot be improved if the concentration is too high.

Furthermore, the non-aqueous solvent may include one kind, or two kinds and more selected from the group consisting of butylene carbonate, .gamma.-butylolactone, .gamma.-valerolactone, these compounds in which a part or all of the hydrogen base is replaced with fluorine base, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyl tetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, methylacetate, methylpropionate, acetonitrile, glutaronitrile, adiponitrile, methoxyacetonitrile, 3-methoxypropylenitrile, N,N-dimethylformamid, N-methylpyrrolidinone, N-methyloxzolidinone, N,N-dimethylimidazolidinone, nitromethane, nitroethane, sulfolane, dimethylsulfoxyde, and trimethylphosphate.

Examples of appropriate lithium salt are LiPF.sub.6, LiBF.sub.4, LiAsF.sub.6, LiClO.sub.4, LiB(C.sub.6H.sub.5).sub.4, LiCH.sub.3SO.sub.3, LiCF.sub.3SO.sub.3, LiN(SO.sub.2CF.sub.3).sub.2, LiC(SO.sub.2CF.sub.3).sub.3, LiAlCl.sub.4, LiSiF.sub.6, LiCL, and LiBr. One of these alone, or two and more kinds of these are mixed for use. Especially, LiPF.sub.6 is preferable so that a high ion conductivity can be obtained while further improving the cycle characteristic. The concentration of lithium salt to the non-aqueous salt is not specifically limited. However, it is preferable to lie within the range of 0.1 mol/dm.sup.3 to 5.0 mol/dm.sup.3, both inclusive, and is more preferable to lie within the range of 0.5 mol/dm.sup.3 to 3.0 mol/dm.sup.3, both inclusive. The reason is that the ion conductivity of the electrolyte 17 can be improved with the concentration within the above-mentioned range.

The secondary battery having such a configuration acts as follows:

When the secondary battery is charged, lithium ions are released from the positive electrode active material contained in the positive electrode mixture layer 13a, passed through the separator 16 via the electrolyte 17 and occluded first in the negative electrode material capable of occluding/releasing lithium contained in the negative electrode mixture layer 15a. When charging is continued, in the state where the open circuit voltage is lower than the overcharge voltage, the charging capacity goes beyond the charging capacity of the negative electrode material capable of occluding/releasing lithium so that lithium metal starts to precipitate on the surface of the negative electrode material capable of occluding/releasing lithium. Specifically, although it depends on the electrode material, lithium metal starts to precipitate on the surface of the negative electrode material capable of occluding/releasing lithium at a point where the open circuit voltage is within the range of 0 V and 4.2 V, both inclusive. Then, lithium metal continues to precipitate in the negative electrode 15 until the open circuit voltage reaches, for example, 4.2 V, that is when charging is completed. Thereby, the exteriority of the negative electrode mixture layer 15a changes from black to gold, and then to silver when using, for example, a carbonaceous material as the negative electrode material capable of occluding/releasing lithium.

Then, when the battery is discharged, first, lithium metal having precipitated in the negative electrode 15 is dissolved as ions, passes through the separator 16 via the electrolyte 17, and then is occluded in the positive electrode active material contained in the positive electrode mixture layer 13a. When discharging is further continued, ionic lithium occluded in the negative electrode material capable of occluding/releasing lithium in the negative electrode mixture layer 15a is released and occluded in the positive electrode active material.

In the secondary battery, lithium is occluded in the negative electrode material capable of occluding/releasing lithium in the beginning of charging, and lithium metal precipitates on the surface of the negative electrode material capable of occluding/releasing lithium at some midpoint of charging where the open circuit voltage is lower than the overcharge voltage. Therefore, both of so-called characteristics of a lithium secondary battery and a lithium-ion secondary battery of the related art can be obtained. In other words, a high energy density can be obtained while improving the cycle characteristic and boosting charging characteristic.

As described, the secondary battery according to the embodiment is formed to contain the negative electrode material capable of occluding/releasing lithium in the negative electrode 15 and precipitate lithium in the negative electrode 15 in the state where the open circuit voltage is lower than the overcharge voltage. Also, the capacity of the negative electrode 15 is expressed by the sum of the capacity which is obtained when occluding/releasing lithium in an ionic state and the capacity which is obtained when precipitating/dissolving lithium. Therefore, a high energy density can be obtained while improving the cycle characteristic and boost