Patent Description:
In recent years, battery-incorporating smart cards have been put into practical use. Examples of smart cards that incorporate primary batteries include credit cards provided with a one-time password display function. Examples of smart cards that incorporate secondary batteries include cards provided with fingerprint authentication and wireless communication functions, the cards having ICs for wireless communication, ASICs for fingerprint analysis, and fingerprint sensors. Batteries for smart cards generally have several requirements, such as a thickness of less than <NUM>, high battery capacity, low electrical resistance, high bending resistance, and high resistance to process temperature of batteries.

For such applications, thin lithium batteries with liquid electrolyte have been proposed. For example, PTL <NUM> (<CIT>) and PTL <NUM> (<CIT>) disclose film-packed batteries including an electrode stack that is contained and sealed in laminated-film containers, where the electrode stack includes collectors for positive electrodes, positive electrodes, separators, negative electrodes, and collectors for negative electrodes. The film-packed batteries disclosed in PTLs <NUM> and <NUM> are lithium primary batteries.

Powder-dispersed positive electrodes are widely known as layers of positive electrode active material for lithium secondary batteries (also referred to as lithium ion secondary batteries), and are usually produced by kneading and molding particles of lithium complex oxide (typically, lithium-transition metal oxide) and additives, such as binders or conductive agents. Such powder-dispersed positive electrodes contain a relatively large amount (e.g., about <NUM>% by weight) of binder that does not contribute to the capacity of battery, resulting in a low packing density of the positive electrode active material, i.e., lithium complex oxide. Accordingly, the powder-dispersed positive electrode should be further improved from the viewpoint of the capacity and charge/discharge efficiency. Some attempts have been made to improve the capacity and charge/discharge efficiency with positive electrodes or layers of positive electrode active material composed of sintered plate of lithium complex oxide. In this case, the positive electrode or the layer of positive electrode active material contains no binder; hence, high capacity and satisfactory charge/discharge efficiency can be expected due to a high packing density of lithium complex oxide. For example, PTL <NUM> (<CIT>) discloses a positive electrode for secondary lithium battery including a collector of the positive electrode and a positive electrode active material layer connected to the collector of the positive electrode with a conductive bonding layer therebetween. The layer of positive electrode active material is composed of a sintered plate of lithium complex oxide, and the sintered plate has a thickness of <NUM> or more, a porosity of <NUM> to <NUM>%, and an open pore rate of <NUM>% or more.

PTL <NUM> (<CIT>) discloses a secondary lithium battery including a mixed solvent consisting of <NUM> to <NUM>% by volume of ethylene carbonate and <NUM> to <NUM>% by volume of γ-butyrolactone as a non-aqueous solvent for an electrolytic solution in order to improve low-temperature discharge property.

<CIT> discloses a battery comprising a lithium cobaltate positive electrode plate; a negative electrode containing a lithium alloy; and an electrolytic solution containing LiBF<NUM> in a non-aqueous solvent composed of γ-butyrolactone.

A hot lamination process may be applied to production of cards. The production of cards by a hot lamination process is performed, for example, through pressing of a card substrate and a resin film at a temperature of <NUM> or higher (e.g., <NUM> to <NUM>) and bonding to each other. Accordingly, the use of hot lamination process is advantageous in a method of incorporating a thin lithium battery in a low-profile device, such as a smart card. In this case, it is conceivable that a thin lithium battery and a protective film are sequentially stacked on a card substrate and pressed at a high temperature of <NUM> or more. However, a conventional thin lithium battery with liquid electrolyte has insufficient heat resistance, resulting in swelling, breaking and increase in electrical resistance of the battery when the battery is heated to <NUM> or more. In a procedure of mounting a thin lithium battery on a printed wiring board, a reflow soldering process may be used. This process also involves heating to a high temperature, thereby a similar problem as described above may occur.

The present inventors have now found that by selective combination of a positive electrode plate which is a lithium complex oxide sintered plate, a negative electrode containing carbon and styrene butadiene rubber (SBR), and an electrolytic solution containing lithium borofluoride (LiBF<NUM>) in a non-aqueous solvent composed of γ-butyrolactone (GBL) and optional ethylene carbonate (EC), it is possible to provide a secondary lithium battery having superior heat resistance.

Accordingly, an object of the present invention is to provide a secondary lithium battery having superior heat resistance.

According to an aspect of the present invention, there is provided a secondary lithium battery, comprising:.

According to another aspect of the present invention, there is provided a method for manufacturing a battery-incorporating device, comprising the steps of:.

<FIG> schematically illustrates an exemplary secondary lithium battery in the present invention. The secondary lithium battery <NUM> shown in <FIG> includes a positive electrode plate <NUM>, a negative electrode <NUM>, and an electrolytic solution <NUM>. The positive electrode plate <NUM> is a lithium complex oxide sintered plate. The negative electrode <NUM> includes carbon and styrene butadiene rubber (SBR). The electrolytic solution <NUM> contains lithium borofluoride (LiBF<NUM>) in a non-aqueous solvent composed of γ-butyrolactone (GBL) and optional ethylene carbonate (EC). As described above, the positive electrode plate <NUM>, which is a lithium complex oxide sintered plate, the negative electrode <NUM> containing carbon and styrene butadiene rubber (SBR), and the electrolytic solution <NUM> containing lithium borofluoride (LiBF<NUM>) in the non-aqueous solvent composed of γ-butyrolactone (GBL) and optional ethylene carbonate (EC) are selectively combined, thereby a secondary lithium battery having superior heat resistance can be provided. Although multiple pieces of positive electrode plates <NUM> are shown in <FIG>, one piece of positive electrode plate <NUM> that is not divided into multiple pieces may be used in the present invention.

As described above, it is conceivable that hot lamination may be applied to incorporate a thin lithium battery into a low-profile device, such as a smart card, and reflow soldering may be used to mount a thin lithium battery on a printed wiring board. All of these procedures involve heating to a high temperature of <NUM> or higher, and a conventional thin lithium battery with liquid electrolyte has insufficient heat resistance, resulting in swelling, breaking and increase in electrical resistance of the battery when the battery is heated to <NUM> or higher. In contrast, the secondary lithium battery <NUM> in the present invention has superior heat resistance: Even when heated to <NUM> or higher, the battery does not swell or break and the electrical resistance of the battery does not increase. Such superior heat resistance is provided by selective combination of the specified positive electrode plate <NUM>, negative electrode <NUM>, and electrolytic solution <NUM> described above.

Accordingly, the secondary lithium battery <NUM> may be mounted on a substrate through, preferably a process involving heating to <NUM> or higher, more preferably a process involving heating by hot lamination or reflow soldering. In other words, according to another preferred embodiment of the present invention, a method of manufacturing a battery-incorporating device is provided comprising a step of preparing a secondary lithium battery and a step of mounting the secondary lithium battery on a substrate through, preferably a process involving heating to <NUM> or higher, more preferably a process involving heating by hot lamination or reflow soldering. In this embodiment, it is particularly preferred that the process involving heating is hot lamination and the battery-incorporating device is a battery-incorporating smart card. In any embodiment, a heating temperature is preferably <NUM> to less than <NUM>, more preferably <NUM> to less than <NUM>, further more preferably <NUM> to less than <NUM>, particularly more preferably <NUM> to less than <NUM>, most preferably <NUM> to less than <NUM>.

The positive electrode plate <NUM> is a lithium complex oxide sintered plate. The phrase "the positive electrode plate <NUM> is a sintered plate" indicates that the positive electrode plate <NUM> contains no binder because the binder disappears or burns off during firing even if the green sheet of the positive electrode contains the binder. Containing no binder in the positive electrode plate <NUM> has an advantage in that deterioration of the positive electrode due to the electrolytic solution can be avoided. For example, as disclosed in PTLs <NUM>, <NUM>, and <NUM>, a binder called polyvinylidene fluoride (PVDF) is widely used for a positive electrode in conventional lithium batteries, and this PVDF is highly soluble in γ-butyrolactone (GBL) used as the electrolytic solution in the present invention, resulting in a loss in function of the binder. In this regard, since the positive electrode plate <NUM> used in the present invention is a sintered plate containing no binder, the problem as described above does not occur. Particularly preferred lithium complex oxide constituting the sintered plate is lithium cobaltate (typically, LiCoO<NUM> (hereinafter, it may be abbreviated as LCO)). Various lithium complex oxide sintered plates or LCO sintered plates are known. For example, the sintered plate disclosed in PTL <NUM> (<CIT>) can be used.

According to a preferred embodiment of the present invention, the positive electrode plate <NUM>, i. e, the lithium complex oxide sintered plate, includes a plurality of primary grains composed of lithium complex oxide, and is also an oriented positive electrode plate in which the plurality of primary grains are oriented at a mean orientation angle of more than <NUM>° to <NUM>° to a plate face of the positive electrode plate. <FIG> illustrates an example of a cross-sectional SEM image perpendicular to the plate face of the oriented positive electrode plate <NUM>, and <FIG> illustrates an electron backscatter diffraction (EBSD) image on a cross-section perpendicular to the plate face of the oriented positive electrode plate <NUM>. <FIG> is an area-based histogram illustrating the distribution of orientation angles of the primary grains <NUM> in the EBSD image of <FIG>. In the EBSD image shown in <FIG>, discontinuity of the crystal orientation can be observed. In <FIG>, the orientation angle of each primary grain <NUM> is determined by the shade of color, and a darker shade indicates a smaller orientation angle. The orientation angle is a tilt angle between the (<NUM>) plane of each primary grain <NUM> and a plate face. In <FIG> and <FIG>, black portions inside the oriented positive electrode plate <NUM> indicate pores.

The oriented positive electrode plate <NUM> is an oriented sintered plate composed of multiple primary grains <NUM> bonded to each other. Each primary grain <NUM> is mainly in a platy shape, and may be formed in, for example, a cuboid shape, a cubic shape, and a spherical shape. The cross-sectional shape of each primary grain <NUM> may be a rectangle, a polygon other than a rectangle, a circle, an ellipse, or any other complicated shape.

Each primary grain <NUM> is composed of lithium complex oxide. The lithium complex oxide is an oxide represented by LixMO<NUM> (<NUM><x<<NUM>, M is at least one transition metal, and M typically contains at least one of Co, Ni and Mn). The lithium complex oxide has a layered rock salt structure. The layered rock salt structure is a crystal structure in which a lithium layer and a transition metal layer other than lithium are alternately stacked with an oxygen layer therebetween, i.e., a crystal structure in which a transition metal ion layer and a lithium single layer are alternately stacked with oxide ions therebetween (typically an α-NaFeO<NUM> structure, i.e., a structure in which transition metals and lithium metals are regularly disposed along the [<NUM>] axis of a cubic rock salt structure). Examples of lithium complex oxides include LixCoO<NUM> (lithium cobaltate), LixNiO<NUM> (lithium nickelate), LixMnO<NUM> (lithium manganate), LixNiMnO<NUM> (lithium nickel manganate), LixNiCoO<NUM> (lithium nickel cobaltate), LixCoNiMnO<NUM> (lithium cobalt nickel manganate), LixCoMnO<NUM> (lithium cobalt manganate), and particularly preferably, LixCoO<NUM> (lithium cobaltate, typically LiCoO<NUM>). The lithium complex oxide may contain at least one element selected from Mg, Al, Si, Ca, Ti, V, Cr, Fe, Cu, Zn, Ga, Ge, Sr, Y, Zr, Nb, Mo, Ag, Sn, Sb, Te, Ba. , Bi, and W.

As shown in <FIG> and <FIG>, a mean value of the orientation angles of primary grains <NUM>, i.e., a mean orientation angle, is more than <NUM>° to <NUM>°. This angle range provides the following various advantages. Since each primary grain <NUM> greatly tilts from the thickness direction of the positive electrode plate, the adhesiveness between the individual primary grains can be improved. Due to this configuration, the lithium ion conductivity between one primary grain <NUM> and another primary grain <NUM> adjacent on two ends of the one primary grain <NUM> in the longitudinal direction can be improved, resulting in an improvement in high rate property. In addition, the high rate property can be further improved. This further improvement is caused by smooth intercalation/deintercalation of lithium ions in the oriented positive electrode <NUM>, because expansion and contraction of the oriented positive electrode plate <NUM> proceeds more dominantly in the thickness direction than in the plate face direction and thereby lithium ions can intercalate and deintercalate smoothly.

The mean orientation angle of the primary grains <NUM> is determined by the following procedure. As shown in <FIG>, three horizontal lines which divide the oriented positive electrode plate <NUM> into four equal intervals in the plate thickness direction, and three vertical lines that divide it into four equal intervals in the plate face direction are drawn in an EBSD image that illustrates a <NUM> by <NUM> rectangular area observed at <NUM>-fold magnification. The mean orientation angle of the primary grains <NUM> is determined by arithmetically averaging the orientation angles of all the primary grains <NUM> that intersect at least one of the three horizontal lines and the three vertical lines. The mean orientation angle of the primary grains <NUM> is preferably <NUM>° or less, and more preferably <NUM>° or less from the viewpoint of a further improvement in high rate property. The mean orientation angle of the primary grains <NUM> is preferably <NUM>° or more, and more preferably <NUM>° or more from the viewpoint of a further improvement in high rate property.

As shown in <FIG>, the orientation angle of each primary grain <NUM> may be widely distributed from <NUM>° to <NUM>°, and most of the orientation angles are preferably distributed in a region of more than <NUM>° to <NUM>°. In other words, when the cross-section of the oriented sintered plate constituting the oriented positive electrode plate <NUM> is analyzed in the EBSD image, the total area of primary grains <NUM> that have the orientation angle of more than <NUM>° to <NUM>° to the plate face of the oriented positive electrode plate <NUM> (hereinafter, referred to as low-angle primary grains) is preferably <NUM>% or more, and more preferably <NUM>% or more of the total area of the primary grains <NUM> (specifically, <NUM> primary grains <NUM> used in calculation of the mean orientation angle) included in the cross-section. This configuration can increase the proportion of the primary grains <NUM> having higher mutual adhesiveness, thereby high rate property can be further improved. In addition, the total area of the low-angle primary grains having an orientation angle of <NUM>° or less is more preferably <NUM>% or more of the total area of the <NUM> primary grains <NUM> used in the calculation of the mean orientation angle. Furthermore, the total area of the low-angle primary grains having an orientation angle of <NUM>° or less is more preferably <NUM>% or more of the total area of the <NUM> primary grains <NUM> used in the calculation of the mean orientation angle.

Since each primary grain <NUM> is mainly platy, the cross-section of each primary grain <NUM> respectively extends in a predetermined direction as shown in <FIG> and <FIG>, and typically has a substantially rectangular shape. In other words, when the cross-section of the oriented sintered plate is analyzed in the EBSD image, the total area of primary grains <NUM> that have an aspect ratio of <NUM> or more among the primary grains <NUM> included in the analyzed cross-section is preferably <NUM>% or more, and more preferably <NUM>% or more of the total area of the primary grains <NUM> (specifically, <NUM> primary grains <NUM> used in calculation of the mean orientation angle) included in the cross-section. In detail, the configuration shown in the EBSD image of <FIG> can increase the mutual adhesiveness between the primary grains <NUM>, resulting in a further improvement of high rate property. The aspect ratio of the primary grains <NUM> is a value determined by dividing the maximum Feret diameter by the minimum Feret diameter of the primary grains <NUM>. The maximum Feret diameter is the maximum distance between two parallel straight lines by which the primary grains <NUM> are sandwiched in the EBSD image from cross-sectional observation. The minimum Feret diameter is the minimum distance between two parallel straight lines by which the primary grains <NUM> are sandwiched.

A plurality of primary grains constituting the oriented sintered plate preferably has a mean primary grain diameter of <NUM> or more. Specifically, the <NUM> primary grains <NUM> used in the calculation of the mean orientation angle has a mean grain diameter of preferably <NUM> or more, more preferably <NUM> or more, and further more preferably <NUM> or more. The number of grain boundaries between the primary grains <NUM> thereby decreases in the direction where lithium ions are conducted, resulting in an increase in overall lithium ion conductivity and thus a further improvement in high rate property. The mean grain diameter of the primary grains <NUM> is a value determined by arithmetically averaging equivalent circle diameters of respective primary grains <NUM>. The equivalent circle diameter indicates the diameter of a circle having the same area as each primary grain <NUM> in the EBSD image.

The compactness of the oriented sintered plate constituting the oriented positive electrode plate <NUM> is preferably <NUM>% or more, more preferably <NUM>% or more, further more preferably <NUM>% or more. The mutual adhesiveness thereby increases between the primary grains <NUM>, resulting in a further improvement in high rate property. The compactness of the oriented sintered plate can be calculated by binarizing the SEM image resultant from the observation with a SEM at <NUM>-fold magnification after polishing of the cross-section of the positive electrode plate with a cross-section polisher (CP). The mean equivalent circle diameter of pores formed inside the oriented sintered plate may be preferably <NUM> or less. Smaller mean equivalent circle diameter of pores can further increase the mutual adhesiveness between the primary grains <NUM>, resulting in a further improvement in high rate property. The mean equivalent circle diameter of pores is a value determined by arithmetically averaging equivalent circle diameters of <NUM> pores in the EBSD image. The equivalent circle diameter indicates the diameter of a circle having an area that is the same as that of each pore in the EBSD image. Each pore formed inside the oriented sintered plate may be an open pore connected to the outside of the oriented positive electrode plate <NUM>, although each pore does not preferably penetrate the oriented positive electrode plate <NUM> and thereby may be a closed pore.

The oriented positive electrode plate <NUM> has a thickness of preferably <NUM> or more, more preferably <NUM> or more, particularly more preferably <NUM> or more, most preferably <NUM> or more from the viewpoint of an increase in the active material capacity per unit area and an improvement in energy density of the secondary lithium battery <NUM>. Although the upper limit of the thickness is not particularly limited, the oriented positive electrode plate <NUM> has a thickness of preferably less than <NUM>, more preferably <NUM> or less, further more preferably <NUM> or less, particularly more preferably <NUM> or less, most preferably <NUM> or less, <NUM> or less, or <NUM> or less from the viewpoint of restraint in deterioration of battery properties due to repeated charge/discharge cycles (particularly, due to an increase in electrical resistance). In addition, the oriented positive electrode plate has a dimension of preferably <NUM> by <NUM> square or more, more preferably <NUM> by <NUM> to <NUM> by <NUM> square, further more preferably <NUM> by <NUM> to <NUM> by <NUM> square. In another expression, the oriented positive electrode plate has a dimension of preferably at least <NUM><NUM>, more preferably <NUM> to <NUM>,<NUM><NUM>, further more preferably <NUM> to <NUM>,<NUM><NUM>.

The negative electrode <NUM> includes carbon and styrene butadiene rubber (SBR). In detail, the carbon is a negative electrode active material, and the SBR is a binder. Examples of carbon include graphite, pyrolytic carbon, coke, fired resin, small mesophase spheres, and mesophase pitch, and preferred carbon is graphite. The graphite may be any of natural graphite and artificial graphite. Containing styrene butadiene rubber (SBR), which is a binder, in the negative electrode <NUM> is advantageous in avoiding deterioration of the negative electrode due to the electrolytic solution. For example, as disclosed in PTL <NUM>, a binder called polyvinylidene fluoride (PVDF) is widely used in a negative electrode of a conventional lithium battery, and this PVDF is greatly soluble in γ-butyrolactone (GBL) used in the present invention and loses its function as a binder. In this regard, since the styrene butadiene rubber (SBR) barely soluble in GBL is selectively applied in the negative electrode <NUM> used in the present invention, the problem as described above does not occur. Accordingly, the negative electrode <NUM> preferably contains no binder other than SBR (e.g., PVDF).

The electrolytic solution <NUM> is a non-aqueous solvent that contains lithium borofluoride (LiBF<NUM>). The non-aqueous solvent may be a single solvent composed of γ-butyrolactone (GBL) or a mixed solvent composed of γ-butyrolactone (GBL) and ethylene carbonate (EC). Since the non-aqueous solvent contains γ-butyrolactone (GBL), the boiling point is increased, and the heat resistance is greatly improved. From this viewpoint, the ratio by volume of EC:GBL in the non-aqueous solvent is preferably <NUM>:<NUM> to <NUM>:<NUM> (GBL ratio of <NUM> to <NUM>% by volume), more preferably <NUM>:<NUM> to <NUM>:<NUM> (GBL ratio of <NUM> to <NUM>% by volume), more preferably <NUM>:<NUM> to <NUM>:<NUM> (GBL ratio of <NUM> to <NUM>% by volume), particularly more preferably <NUM>:<NUM> to <NUM>:<NUM> (GBL ratio of <NUM> to <NUM>% by volume). Lithium borofluoride (LiBF<NUM>) dissolved in a non-aqueous solvent is an electrolyte having high decomposition temperature, and also results in a significant improvement in heat resistance. The electrolytic solution <NUM> has a LiBF<NUM> concentration of preferably <NUM> to <NUM> mol/L, more preferably <NUM> to <NUM> mol/L, further more preferably <NUM> to <NUM> mol/L, particularly more preferably <NUM> to <NUM> mol/L.

The electrolytic solution <NUM> further preferably contains vinylene carbonate (VC), and/or fluoroethylene carbonate (FEC) and/or vinyl ethylene carbonate (VEC) as an additive. VC and FEC each have superior heat resistance. Further addition of such an additive into the electrolytic solution <NUM> can form an SEI film having superior heat resistance on the surface of the negative electrode <NUM>, resulting in a further improvement in heat resistance of the secondary lithium battery <NUM>.

The secondary lithium battery <NUM> is further preferably provided with a separator <NUM>. The separator <NUM> is composed of preferably polyimide, polyester (e.g., polyethylene terephthalate (PET)) or cellulose, and more preferably polyimide. A separator composed of polyimide, polyester (e.g., polyethylene terephthalate (PET)) or cellulose has not only superior heat resistance as its own property, but high wettability to γ-butyrolactone (GBL) different from a separator composed of polyolefin, such as polypropylene (PP) and polyethylene (PE), that is widely used and has inferior heat resistance. The electrolytic solution <NUM> containing GBL can thus sufficiently penetrate (without being repelled) into the separator <NUM>. As a result, the heat resistance of the secondary lithium battery <NUM> can be further improved. A particularly preferred separator is composed of polyimide. Polyimide separators are commercially available and have a greatly complicated microstructure, and thereby have an advantage in more effective prevention or delaying of the growth of dendritic lithium deposited during overcharge and thus short circuiting. In contrast, cellulose separators are advantageous in less expensiveness than the polyimide separators.

The secondary lithium battery <NUM> has a thickness of preferably <NUM> or less, more preferably <NUM> to <NUM>, further more preferably <NUM> to <NUM>, particularly more preferably <NUM> to <NUM>. Such thickness ranges can cause a thin lithium battery suitable for being incorporated into a low-profile device, such as a smart card.

As shown in <FIG>, cell unit <NUM> and the electrolytic solution <NUM>, which are components of the secondary lithium battery <NUM>, are preferably wrapped and sealed with packaging films <NUM>. In other words, the secondary lithium battery <NUM> is preferably in the form of a so-called film-packed battery. In this battery, the cell unit <NUM> each includes a positive electrode plate <NUM>, a separator <NUM>, and a negative electrode <NUM>, and typically further includes a positive electrode collector <NUM> and a negative electrode collector <NUM>. The positive electrode collector <NUM> and the negative electrode collector <NUM> may be composed of any material, preferably copper foil. The positive electrode collector <NUM> is preferably provided with a positive electrode terminal <NUM> extending from the positive electrode collector <NUM>, and the negative electrode collector <NUM> is preferably provided with a negative electrode terminal <NUM> extending from the negative electrode collector <NUM>. In <FIG>, the secondary lithium battery <NUM> is illustrated as being a layered structure and a sealed structure having a certain amount of space to clearly show the existence of the electrolytic solution <NUM>, however such a space is desired to be minimized in an actual battery. The outer edges of the secondary lithium battery <NUM> are sealed by thermal bonding of the packaging films <NUM> to each other. Sealing by thermal bonding is preferably performed with a heat bar (also referred to as a heating bar) generally used in heat sealing.

The packaging film <NUM> may be a commercially available packaging film. The packaging film <NUM> has a thickness of preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, further more preferably <NUM> to <NUM>. The packaging film <NUM> is preferably a laminated film including a resin film and a metal foil, more preferably an aluminum-laminated film including a resin film and an aluminum foil. The laminated film is preferably provided with resin films on two faces of the metal foil, such as the aluminum foil. In this case, it is preferred that a resin film on one face of the metal foil (hereinafter, referred to as a surface protective film) be composed of a material having high reinforcing properties, such as nylon, polyamide, poly(ethylene terephthalate), polyimide, polytetrafluoroethylene, and polychlorotrifluoroethylene, and a resin film on the other face of the metal foil (hereinafter, referred to as a sealing resin film) be composed of a heat sealing material, such as polypropylene. Aluminum-laminated films having such a layer structure composed of a surface protective film/an aluminum foil/a sealing resin film are commercially available for lithium batteries, and most of the sealing resin films in the commercially available aluminum-laminated film have a two-layer structure of polypropylene resin. In general, this two-layer structure is composed of a main layer having a softening point of <NUM> to <NUM> and an adhesive layer having a softening point of <NUM> to <NUM> disposed on the outside of the main layer. However, since the adhesive layer having a softening point of <NUM> to <NUM> has a lower softening point than the main layer, the adhesive layer can be readily softened or fluidized by heating, resulting in poor heat resistance. Accordingly, from the viewpoint of an increase in heat resistance of the secondary lithium battery <NUM>, the following improvement a) or b) preferably should be achieved on the sealing resin film having the two-layer structure of polypropylene resin.

In order to prevent the packaging film <NUM> from being broken in a hot-pressing process, such as hot lamination, at least one end of the positive electrode collector <NUM>, the positive electrode terminal <NUM>, the negative electrode collector <NUM>, and the negative electrode terminal <NUM> may be covered with a protective tape. The covering can effectively prevent the breakage of the packaging film <NUM> due to burrs that may be formed at the edge of the cell unit. Preferred examples of the protective tape include a polyimide tape because of its superior heat resistance.

An oriented positive electrode plate or an oriented sintered plate preferably used in the secondary lithium battery of the present invention may be produced by any process, although preferably produced through the following steps: (<NUM>) preparation of LiCoO<NUM> template grains, (<NUM>) preparation of matrix grains, (<NUM>) preparation of green sheet, and (<NUM>) production of oriented sintered plate.

Co<NUM>O<NUM> raw material powder and Li<NUM>CO<NUM> raw material powder are mixed. The mixed powder is fired at <NUM> to <NUM> for <NUM> to <NUM> hours to synthesize LiCoO<NUM> powder. The resultant LiCoO<NUM> powder is pulverized in a pot mill into a volume-based D50 grain diameter of <NUM> to <NUM> to give platy LiCoO<NUM> grains capable of conducting lithium ions parallel to the plate face. The resultant platy LiCoO<NUM> grains are in a state of being readily cleaved along cleavage planes. The LiCoO<NUM> grains are cleaved by disintegration to produce LiCoO<NUM> template grains. Such LiCoO<NUM> grains may also be produced through several processes, such as a disintegration process after the grain growth in a green sheet prepared from LiCoO<NUM> powder slurry, a flux process, a hydrothermal synthesis process, a single-crystal growth process using a melt, and a sol-gel process.

In this step, the profile of primary grains <NUM> constituting the oriented positive electrode plate <NUM> can be controlled as follows.

Co<NUM>O<NUM> raw material powder is used as matrix grains. The Co<NUM>O<NUM> raw material powder may have any volume-based D50 grain diameter, for example, <NUM> to <NUM>, and is preferably smaller than the volume-based D50 grain diameter of the LiCoO<NUM> template grains. The matrix grains can also be prepared through heat treatment of Co(OH)<NUM> raw material at <NUM> to <NUM> for <NUM> to <NUM> hours. Co(OH)<NUM> grains or LiCoO<NUM> grains other than Co<NUM>O<NUM>. are used as the matrix grains.

LiCoO<NUM> template grains and matrix grains are mixed in a ratio of <NUM>:<NUM> to <NUM>:<NUM> to give a mixed powder. The mixed powder, a dispersion medium, a binder, a plasticizer, and a dispersant are stirred and defoamed under reduced pressure while mixing to prepare a slurry having a desired viscosity. The resultant slurry is then formed into a shaped material using a molding procedure capable of applying a shear force to the LiCoO<NUM> template grains. Through these steps, each primary grain <NUM> can be aligned to have a mean orientation angle of more than <NUM>° to <NUM>°. The molding procedure capable of applying a shear force to the LiCoO<NUM> template grains is preferably a doctor blade process. In the doctor blade process, the resultant slurry is formed on a PET film to prepare the shaped material, i.e., a green sheet.

The shaped material prepared from the slurry is placed on a zirconia setter and heated at <NUM> to <NUM> for <NUM> to <NUM> hours (primary firing) to give a sintered plate as an intermediate. This sintered plate is sandwiched between lithium-containing sheets (e.g., Li<NUM>CO<NUM>-containing sheets), placed on a zirconia setter, and heated (secondary firing) to prepare a LiCoO<NUM> sintered plate. In detail, a setter on which a sintered plate sandwiched between lithium-containing sheets is placed is disposed in an alumina sheath and fired at <NUM> to <NUM> for <NUM> to <NUM> hours in the air, and then the resultant sintered plate is further sandwiched between lithium-containing sheets and fired at <NUM> to <NUM> for <NUM> to <NUM> hours to produce a LiCoO<NUM> sintered plate. This firing step may be performed in two separate stages or in one stage. In a two-stage firing process, the firing temperature in the first stage is preferably lower than that in the second stage. The total amount of the lithium-containing sheet used in the secondary firing may be selected such that Li/Co ratio is <NUM>, the Li/Co ratio being the molar ratio of Li content in the green sheet and the lithium-containing sheet to Co content in the green sheet.

The present invention will be described in more detail by the following examples.

A secondary lithium battery <NUM> in the form of a film-packed battery schematically shown in <FIG> was produced by the procedures as shown in <FIG> and <FIG>. The details are as follows:.

A <NUM>-thick LiCoO<NUM> sintered plate (hereinafter referred to as an LCO sintered plate) was provided. The LCO sintered plate was manufactured in accordance with the method described above for manufacturing a lithium complex oxide sintered plate, and satisfied several preferred conditions for the lithium complex oxide sintered plate as described above. The sintered plate was cut with a laser processing tool into multiple positive electrode plates <NUM> each being a square of <NUM> by <NUM>.

Two aluminum-laminated films (manufactured by Showa Denko Packaging Co. , a thickness of <NUM>, three-layer structure composed of a polypropylene film/an aluminum foil/a nylon film) were provided as packaging films <NUM>. As shown in <FIG>, multiple positive electrode plates <NUM> were laterally disposed on one packaging film <NUM> with a positive electrode collector <NUM> (a copper foil having a thickness of <NUM>) therebetween to give a positive electrode assembly <NUM>. In this assembly, the positive electrode collector <NUM> was fixed to one packaging film <NUM> with an adhesive. The positive electrode collector <NUM> was provided with a positive electrode terminal <NUM> welded to and extending from the positive electrode collector <NUM>. In contrast, a negative electrode <NUM> (a carbon layer having a thickness of <NUM>) was disposed on the other packaging film <NUM> with a negative electrode collector <NUM> (a copper foil having a thickness of <NUM>) therebetween to give a negative electrode assembly <NUM>. In this assembly, the negative electrode collector <NUM> was fixed to the packaging film <NUM> with an adhesive. The negative electrode collector <NUM> was provided with a negative electrode terminal <NUM> welded to and extending from the negative electrode collector <NUM>. In addition, as shown in Table <NUM>, the carbon layer, which is the negative electrode <NUM>, was in the form of a coated layer containing a mixture of graphite as an active material and styrene butadiene rubber (SBR) as a binder.

A porous polyimide film (manufactured by Tokyo Ohka Kogyo Co. , a thickness of <NUM>, a porosity of <NUM>%) was provided as a separator <NUM>. As shown in <FIG>, the positive electrode assembly <NUM>, the separator <NUM>, and the negative electrode assembly <NUM> are stacked in sequence such that the positive electrode plate <NUM> and the negative electrode <NUM> face the separator <NUM> to produce a laminate <NUM> in which two outer surfaces were covered with packaging films <NUM> and the outer periphery of the packaging films <NUM> protruded from the outer edge of the cell unit <NUM>. The cell unit <NUM> (the positive electrode collector <NUM>, the positive electrode plate <NUM>, the separator <NUM>, the negative electrode <NUM>, and the negative electrode collector <NUM>) constructed in the laminate <NUM> had a thickness of <NUM>, and a rectangular shape with dimensions of <NUM> by <NUM>.

As shown in <FIG>, three edges A of the resultant laminate <NUM> were sealed. This sealing was performed by hot pressing of the outer periphery of the laminate <NUM> at <NUM> and <NUM> MPa for <NUM> seconds and thermal fusion bonding of the packaging films <NUM> (aluminum-laminated films) to each other at the outer periphery. After sealing of the three edges A, the laminate <NUM> was placed in a vacuum dryer <NUM> to remove moisture and dry the adhesive.

As shown in <FIG>, the laminate was transferred into a glove box <NUM>, and a gap was formed between a pair of packaging films <NUM> at one unsealed edge B remained in the laminate <NUM> in which the three edges A were sealed. An electrolytic solution <NUM> was injected from an injector <NUM> into the gap, and the edge B was then temporarily sealed with a simple sealer under a reduced pressure of <NUM> kPa. The electrolytic solution was prepared by dissolution of LiBF<NUM> in a concentration of <NUM> mol/L into a mixed solvent containing ethylene carbonate (EC) and γ-butyrolactone (GBL) in a ratio of <NUM>:<NUM> (by volume). The laminate having the edge B temporarily sealed as described above was subjected to initial charging and aging for seven days. After these procedures, an outer periphery (an end not including the cell unit) of the edge B sealed was cut off, and internal gas was discharged.

As shown in <FIG>, a fresh edge B' formed by cutting off of the temporary sealed portion was sealed under a reduced pressure of <NUM> kPa in the glove box <NUM>. This sealing was also performed by hot pressing of the outer periphery of the laminate <NUM> at <NUM> and <NUM> MPa for <NUM> seconds and thermal fusion bonding of the packaging films <NUM> (aluminum-laminated films) to each other at the outer periphery. As described above, the edge B' was sealed between a pair of packaging films <NUM> to produce a secondary lithium battery <NUM> in the form of a film-packed battery. The secondary lithium battery <NUM> was retrieved from the glove box <NUM>, and unnecessary portions on the outer periphery of the packaging film <NUM> were trimmed away to fix the shape as the secondary lithium battery <NUM>. The secondary lithium battery <NUM> was completed in which the four outer edges of the cell unit <NUM> were sealed between a pair of packaging films <NUM> and the electrolytic solution <NUM> was injected. The resultant secondary lithium battery <NUM> was in a rectangular shape having dimensions of <NUM> by <NUM>, and had a thickness of <NUM> or less and a capacity of <NUM> mAh.

The lithium secondary batteries prepared were heated at various temperatures (<NUM>, <NUM>, <NUM>, <NUM>, or <NUM>) shown in <FIG> for <NUM> minutes and pressed at a pressure of <NUM> MPa in a hot press system, and the following properties were then evaluated.

The heated lithium secondary batteries were visually observed to determine whether any change occurred in appearance of the battery. As shown in Table 1A, no change in appearance of the battery is observed at all the heating temperatures.

The Electrical resistance of each heated secondary lithium battery was measured by an AC impedance method using an electrochemical measurement system SP-<NUM> manufactured by BioLogic Science Instruments. The electrical resistance measured was calculated as a relative value where the resistance of the battery heated at <NUM> was <NUM>. The results are shown in Table 1A. No change in electrical resistance is observed in the batteries heated at all the temperatures compared with the battery heated at <NUM>.

Batteries were prepared and evaluated as in Example A1 except that a) a coated layer of a mixture of LiCoO<NUM> powder and polyvinylidene fluoride (PVDF) (hereinafter referred to as an LCO coated electrode) was used as the positive electrode instead of the LCO sintered plate, b) a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a ratio of <NUM>:<NUM> (volume ratio) containing <NUM> mol/L LiPF<NUM> was used as the electrolytic solution, and c) polyvinylidene fluoride (PVDF) was used as the negative electrode binder instead of SBR. The results are shown in Table 1A.

Batteries were prepared and evaluated as in Example A2 except that a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a ratio of <NUM>:<NUM> (volume ratio) containing <NUM> mol/L LiPF<NUM> was used as the electrolytic solution. The results are shown in Table 1A.

Batteries were prepared and evaluated as in Example A1 except that a) a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a ratio of <NUM>:<NUM> (volume ratio) containing <NUM> mol/L LiPF<NUM> was used as the electrolytic solution, and b) polyvinylidene fluoride (PVDF) was used as the negative electrode binder instead of SBR. The results are shown in Table 1A.

Batteries were prepared and evaluated as in Example A4 except that a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a ratio of <NUM>:<NUM> (volume ratio) containing <NUM> mol/L LiPF<NUM> was used as the electrolytic solution. The results are shown in Table 1A.

Batteries were prepared and evaluated as in Example A1 except that a mixed solvent of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a ratio of <NUM>:<NUM> (volume ratio) containing <NUM> mol/L LiPF<NUM> was used as the electrolytic solution. The results are shown in Table 1A.

Batteries were prepared and evaluated as in Example A1 except that a mixed solvent of ethylene carbonate (EC) and diethyl carbonate (DEC) in a ratio of <NUM>:<NUM> (volume ratio) containing <NUM> mol/L LiPF<NUM> was used as the electrolytic solution. The results are shown in Table 1A.

Batteries were prepared and evaluated as in Example A1 except that a) a coated layer of a mixture of LiCoO<NUM> powder and polyvinylidene fluoride (PVDF) (i.e., an LCO coated electrode) was used as the positive electrode instead of the LCO sintered plate, and b) polyvinylidene fluoride (PVDF) was used as the negative electrode binder instead of SBR. The results are shown in Table 1B.

Batteries were prepared and evaluated as in Example A1 except that a coated layer of a mixture of LiCoO<NUM> powder and polyvinylidene fluoride (PVDF) (i.e., an LCO coated electrode) was used as the positive electrode instead of the LCO sintered plate. The results are shown in Table 1B.

Batteries were prepared and evaluated as in Example A1 except that polyvinylidene fluoride (PVDF) was used as the negative electrode binder instead of SBR. The results are shown in Table 1B.

Batteries were prepared and evaluated as in Example A10 except that a mixed solvent of propylene carbonate (PC) and γ-butyrolactone (GBL) in a ratio of <NUM>:<NUM> (volume ratio) containing <NUM> mol/L LiBF<NUM> was used as the electrolytic solution. The results are shown in Table 1B.

Batteries were prepared and evaluated as in Example A1 except that a mixed solvent of propylene carbonate (PC) and γ-butyrolactone (GBL) in a ratio of <NUM>:<NUM> (volume ratio) containing <NUM> mol/L LiBF<NUM> was used as the electrolytic solution. The results are shown in Table 1B.

Claim 1:
A secondary lithium battery, comprising:
a positive electrode plate that is a lithium complex oxide sintered plate;
a negative electrode containing carbon and styrene butadiene rubber (SBR); and
an electrolytic solution containing lithium borofluoride (LiBF<NUM>) in a non-aqueous solvent composed of γ-butyrolactone (GBL), or composed of γ-butyrolactone (GBL) and ethylene carbonate (EC).