Patent Application
20130209870
The present invention relates to a non-aqueous electrolyte battery including a non-aqueous electrolyte and a flame retardant added to the non-aqueous electrolyte.
Non-aqueous electrolyte batteries that use a non-aqueous electrolyte such as lithium-ion secondary batteries provide a high energy density at a high voltage and have a reduced size and a reduced weight, and thus are widely used primarily as power sources for information terminals such as personal computers and cellular phones. Examples of the non-aqueous electrolyte used in the non-aqueous electrolyte batteries include a solution obtained by dissolving a supporting electrolyte such as LiPF6 in an aprotic organic solvent such as an ester compound and an ether compound. However, since the aprotic organic solvent, is flammable, the battery may be disadvantageously ignited or expanded when the battery generates an abnormal amount of heat. Therefore, in the field of the non-aqueous electrolyte batteries, it is requested to manufacture safe non-aqueous electrolyte batteries that are less likely to be ignited or ruptured.
The possibility of expanded use of the non-aqueous electrolyte batteries as power sources for large devices such as power sources for power storage and power sources for electric vehicles has recently been studied. Therefore, also for such application to power sources for large devices, it is necessary to provide safe non-aqueous electrolyte batteries that are less likely to be ignited or ruptured.
Patent Documents 1 to 5 disclose technologies for suppressing ignition or rupture of the non-aqueous electrolyte batteries by adding a flame retardant material to a non-aqueous electrolyte as technologies for enhancing the safety of the batteries. In all of the technologies, a phosphazene compound is used as the flame retardant.
The phosphazene compound used in the non-aqueous electrolyte batteries according to the related art has a chemical structure with a large amount of a halogen element (in particular, fluorine) to impart high flame retardance to the non-aqueous electrolyte. Therefore, the phosphazene compound has a low boiling point and is liquid at normal temperature because of its chemical structure.
If a phosphazene compound that, is liquid at normal temperature is added as a flame retardant to a non-aqueous electrolyte as in the technologies disclosed in Patent Documents 1 to 4, the flame retardant may be dissolved or dispersed in the non-aqueous electrolyte at normal temperature to increase the viscosity of the non-aqueous electrolyte and reduce the ion conductivity of the non-aqueous electrolyte. As a result, the battery characteristics (such as high voltage performance, a high discharge capacity, and large current discharge performance) may be disadvantageously reduced when the temperature inside the battery is not increased. In addition, when the temperature inside the battery is increased, the liquid flame retardant may be volatilized, from the non-aqueous electrolyte to decrease the amount of the flame retardant existing in the non-aqueous electrolyte. This may disadvantageously reduce the effect of rendering the non-aqueous electrolyte flame-retardant. Further, if the surface of a negative electrode is covered with a flame retardant composed of a phosphazene monomer as in the technology disclosed in Patent Document 5, the liquid phosphazene compound may be disadvantageously vaporized when the temperature is increased. Besides, the flame retardant coating formed on the negative electrode surface may degrade the ion permeability to increase the internal resistance of the battery. This may disadvantageously reduce the battery characteristics.
An object of the present invention is to provide a non-aqueous electrolyte battery capable of preventing ignition or rupture of the battery without, reducing the battery-characteristics.
Another object of the present invention is to provide a non-aqueous electrolyte battery capable of reliably imparting flame retardance to a non-aqueous electrolyte when the battery generates an abnormal amount of heat.
A still another object of the present invention is to provide a non-aqueous electrolyte battery including a non-aqueous electrolyte and a flame retardant in an amount enough to render the non-aqueous electrolyte flame-retardant.
The present invention improves a non-aqueous electrolyte battery including a non-aqueous electrolyte and a flame retardant added to the non-aqueous electrolyte to suppress ignition (combustion) of the non-aqueous electrolyte due to increased temperature inside the battery. In the non-aqueous electrolyte battery according to the present invention, a large number of flame retardant particles are added as the flame retardant to the non-aqueous electrolyte. The frame retardant particles are particles of a material that exists as a solid and does not perform a function of suppressing combustion when the temperature of the non-aqueous electrolyte is equal to or less than a reference temperature at which the non-aqueous electrolyte is likely to ignite and that is at least partially liquefied and performs a function of suppressing combustion when the temperature of the non-aqueous electrolyte is more than the reference temperature. That is, the flame retardant particles used in the present invention exists as a solid in the non-aqueous electrolyte when the battery is normal (when it is not necessary to perform a function of suppressing ignition of the non-aqueous electrolyte), and at least partially exists as a liquid in the non-aqueous electrolyte when the battery generates an abnormal amount of heat (when it is necessary to perform a function of suppressing ignition of the non-aqueous electrolyte).
According to the present invention, during normal times when the temperature inside the battery is not significantly increased, the solid flame retardant particles are not dissolved (or dispersed) in the non-aqueous electrolyte. Therefore, the viscosity of the non-aqueous electrolyte is not increased to reduce the battery characteristics when the battery is normal or in a use environment. When the temperature inside the battery is increased enough to ignite the non-aqueous electrolyte, all or some: of the flame retardant particles are liquefied, to be dissolved (or dispersed) in the non-aqueous electrolyte. Therefore, the flame retardant particles perform a function of suppressing ignition of the non-aqueous electrolyte when the battery generates an abnormal amount of heat. According to the present invention, only some of the flame retardant particles are liquefied, because of increased temperature inside the battery (and the flame retardant particles which have not been liquefied remain in the non-aqueous electrolyte as a solid), the whole flame retardant does not volatize (or vaporize) from the non-aqueous electrolyte immediately after being liquefied. This allows the flame retardant to exist in the non-aqueous electrolyte in an amount necessary to suppress ignition of the non-aqueous electrolyte when the battery generates an abnormal amount of heat.
The flame retardant particles used in the present invention preferably exist as a solid in the non-aqueous electrolyte when an internal temperature of the non-aqueous electrolyte battery is equal to or less than 90° C. This prevents use of a non-aqueous electrolyte that ignites at equal to or less than 90° C. The melting point of the flame retardant particles is preferably in the range of 90 to 120° C. Commonly used non-aqueous electrolytes have a pyrolysis temperature of about 150° C. which is higher than 120° C. Thus, if the flame retardant particles have a melting point of 90 to 120° C., most of the flame retardant particles are liquefied to perform a function of suppressing ignition before the temperature of the non-aqueous electrolyte reaches the pyrolysis temperature.
The flame retardant particles are preferably particles of a phosphazene compound. The phosphazene compound, because of its structure, has a tendency to capture (trap) oxygen in the non-aqueous electrolyte (for example, oxygen radicals released, from the positive electrode when the battery generates an abnormal amount of heat). Utilizing such a tendency, a thermal runaway reaction of the battery can be suppressed by adding particles of the phosphazene compound to the non-aqueous electrolyte.
Examples of the phosphazene compound suitable for use in the present invention include a cyclic phosphazene compound of formula (I):
(NPR2)n (I)
In the cyclic phosphazene compound of formula (I), preferably, n is an integer of 3 or 4, and R's are independently a halogen, an alkoxy group, an aryloxy group, or an amino group. In an example of the cyclic phosphazene compound of formula (I), the n may be an integer of 3, and four of the R's may be chloro groups and the remaining two R's may be aminomethyl groups. In another example of the cyclic phosphazene compound of formula (I), the n may be an integer of 3, and all the R's may be phenoxy groups. If such a phosphazene compound, is used, the particles of the phosphazene compound are not easily dissolved (or dispersed) in the non-aqueous electrolyte when the internal temperature of the non-aqueous electrolyte battery is equal to or less than 90° C., and at least some of the particles of the phosphazene compound are liquefied to be dissolved (or dispersed) in the non-aqueous electrolyte when the internal temperature of the non-aqueous electrolyte battery is more than 90° C.
When the phosphazene compound described above is used, the amount of the particles of the phosphazene compound added is preferably 3.5 wt % or more per 100 wt % of the non-aqueous electrolyte. If the amount of the phosphazene compound added is less than 3.5 wt % per 100 wt % of the non-aqueous electrolyte, combustion of the non-aqueous electrolyte may not be sufficiently suppressed. The upper limit of the amount of the phosphazene compound added is determined according to the properties and the price of the battery needed. In consideration of the improvement of the flame retardant performance of the non-aqueous electrolyte due to the addition of the phosphazene compound (flame retardant) and the manufacturing cost of the battery, the amount, of the phosphazene compound added is preferably less than 14.0 wt % per 100 wt % of the non-aqueous electrolyte.
Further, the average particle size of the particles of the phosphazene compound is preferably 20 μm or less. The phosphazene compound having an average particle size of 20 μm or less is changed from, a solid into a liquid at a high rate (liquefaction rate) when the internal temperature of the non-aqueous electrolyte battery is increased. The high liquefaction rate of the phosphazene compound (flame retardant) also increases the rate at which, the liquefied phosphazene compound (flame retardant) is further dissolved or dispersed into the non-aqueous electrolyte. As a result, the function of suppressing ignition of the non-aqueous electrolyte can be performed as the internal temperature of the non-aqueous electrolyte battery is increased. A phosphazene compound having an average particle size of more than 20 μm is changed from a solid into a liquid at a low rate (liquefaction rate) when the internal temperature of the non-aqueous electrolyte battery is increased. The lower limit of the average particle size of the particles of the phosphazene compound is not specifically limited. In the current state of the art, however, it is practically difficult to manufacture particles of a phosphazene compound having an average particle size of less than 5 μm. Therefore, the lower limit of the average particle size of the particles of the phosphazene compound may be determined, as 5 μm.
An embodiment of the present invention will be described in detail below.
First, a lithium-cobalt complex oxide (LiCoO2) was prepared as a positive active material of the positive electrode. The lithium-cobalt complex oxide, acetylene black, serving as a conducting agent, and polyvinylidene fluoride serving as a binding agent were mixed at a mass ratio of 90:5:5, and the mixture was dispersed in a solvent of N-methylpyrrolidone to prepare slurry. The slurry was applied to an aluminum foil serving as a positive current collecting member, and dried. After that, the aluminum foil was subjected to pressing to fabricate a positive electrode sheet. The positive electrode sheet was cut to a size of 10 cm×20 cm, and a current collecting tab formed from an aluminum, foil, was welded to the positive electrode sheet to fabricate the positive electrode 3.
First, artificial graphite was prepared as a negative active material. The artificial graphite and polyvinylidene fluoride serving as a binding agent were mixed at a mass ratio of 90:10, ana the mixture was dispersed in a solvent of N-methylpyrrolidone to prepare slurry. The slurry was applied to a copper foil serving as a negative current collecting member, and dried. After that, the copper foil was subjected to pressing to fabricate a negative electrode sheet. The negative electrode sheet was cut to a size of 10 cm×20 cm, and a current collecting tab formed from a nickel foil was welded to the cut sheet to fabricate the negative electrode 5.
A separator sheet, made of polyethylene was interposed, between the positive electrode and the negative electrode fabricated as described above. The positive electrode, the negative electrode, and the separator sheet were laminated to fabricate the laminated member 11 having a battery capacity of 8 Ah.
First, a mixed, solvent was prepared from 50 vol % of ethylene carbonate and 50 vol % of dimethyl carbonate. LiPF6 was dissolved in the mixed solvent to achieve a concentration of 1 mol/L to prepare an electrolyte solution. A cyclic phosphazene compound given below was appropriately added as a flame retardant to the prepared electrolyte solution to prepare the non-aqueous electrolyte 9.
In the examples, the following cyclic phosphazene compounds (phosphazene compounds A to G) were used. A phosphazene compound A is a cyclic phosphazene compound (having a melting point of 99° C.) of formula (I), in which, n is 3, four of all the R's are chloro groups, and the remaining two R's are aminomethyl groups. A phosphazene compound B is a cyclic phosphazene compound (having a melting point of 110 to 111° C.) of formula (I), in which n is 3 and all the six R's are phenoxy groups. A phosphazene compound C is a cyclic phosphazene compound (having a melting point of 20° C.) of formula (I), in which n is 3, five of all the R's are chloro groups, and the remaining one R is a phenoxy group. A phosphazene compound D is a cyclic phosphazene compound (having a melting point, of 90° C.) of formula (I), in which n is 3 and all the six R's are aminopropyl groups. A phosphazene compound E is a cyclic phosphazene compound (having a melting point of 120° C.) of formula (I), in which n is 3 and all the six R's are aminoethyl groups. A phosphazene compound F is a cyclic phosphazene compound (having a melting point of 132° C.) of formula (I), in which n is 3, two of all the R's are chloro groups, two R's are phenyl groups, and the remaining two R's are aminomethyl groups. A phosphazene compound G is a cyclic phosphazene compound (having a melting point of 145° C.) of formula (I), in which n is 3 and all the six R's are aminoethyl groups.
The fabricated laminated member 11 was inserted into an exterior member (which would later serve as the case 13) made of a heat seal film (aluminum lamination film) and having one open end, and the prepared non-aqueous electrolyte 9 was further injected into the exterior member. After that, the exterior member was evacuated, and the opening of the exterior member was quickly heat sealed to fabricate a non-aqueous electrolyte battery (lithium-ion secondary battery 1) having the structure of a flat laminated battery.
The flame retardance (battery safety) was evaluated for the non-aqueous electrolyte battery (laminated battery) fabricated as described, above. The flame retardance is evaluated by a nail penetration test. In the nail penetration test, first, a charge—discharge cycle was repeated twice at a current density of 0.1 mA/cm2 in a voltage range of 4.2 to 3.0 V in an environment at 25° C., and further the battery was charged to 4.2 V. After that, a nail made of stainless steel and having a shaft with a diameter of 3 mm was stuck in the center of a side surface of the battery at a speed, of 0.5 cra/s at the same temperature of 25° C. to examine whether or not the battery ignited (smoked) and whether or not the battery was ruptured or expanded.
The battery characteristics were evaluated for the fabricated non-aqueous electrolyte battery (laminated battery). The battery characteristics were evaluated by a high-rate discharge test. In the high-rate discharge test, first, a charge—discharge cycle was repeated under the same conditions as in the nail protrusion test described above, and the battery was charged to 4.2 V. After the battery was charged, a constant, current discharge was performed at a current, of 24 A to a final voltage of 3.0 V. The thus obtained discharge capacity was defined as the high-rate discharge capacity.
Particles of the cyclic phosphazene compound described above were added as a flame retardant (flame retardant particles) to the non-aqueous electrolyte, and the relationship between the amount of the cyclic phosphazene compound added and the flame retardance of the battery was examined. In the examples, first, the phosphazene compound A was used as the cyclic phosphazene compound, and the flame retardant retardance was evaluated for Experimental Examples 1 to 8 in which the amount of the phosphazene compound A added was varied. The amount of the phosphazene compound A added was represented in the unit of wt % of the phosphazene compound A per 100 wt % of the non-aqueous electrolyte. The results of the evaluation of the flame retardance are shown in Table 1 and
As shown Table 1 and
The phosphazene compound B was used as the cyclic phosphazene compound, and the flame retardance was evaluated for Experimental Examples 9 to 16 in which the amount of the phosphazene compound B added was varied. The amount of the phosphazene compound B added was represented in the unit of wt % of the phosphazene compound B per 100 wt % of the non-aqueous electrolyte. The results of the evaluation of the flame retardance are shown in Table 2 and
As shown in Table 2 and
Next, the relationship between the amount of the cyclic phosphazene compound, added and the battery characteristics was examined when particles of the cyclic phosphazene compound, were added as flame retardant particles to the non-aqueous electrolyte. In the examples, the phosphazene compound A was used as the cyclic phosphazene compound, and the battery characteristics were evaluated (high-rate discharge test) for Experimental Examples 17 to 24 in which the amount of the phosphazene compound A added was varied. Also in this case, the amount of the phosphazene compound A added was represented in the unit of wt % of the phosphazene compound A per 100 wt % of the non-aqueous electrolyte. In addition, the battery characteristics were represented as the high-rate discharge capacity (%) for examples in which the phosphazene compound A was added in an amount of 1.0 to 20.0 wt % (Experimental Examples 18 to 24) compared to the high-rate discharge capacity for an example in which no phosphazene compound A was added (Experimental Example 17), being defined as 100%. The results of the evaluation of the flame retardance are shown in Table 3 and
As shown in Table 3 and
Further, particles of the cyclic phosphazene compound were added as flame retardant particles to the non-aqueous electrolyte, and the relationship between the average particle size of the particles of the cyclic phosphazene compound and the flame retardance of the battery was examined. In the examples, the phosphazene compound. A was used as the cyclic phosphazene compound, and the flame retardant retardance was evaluated (nail protrusion test) for Experimental Examples 25 to 29 in which the average particle size of the particles of the phosphazene compound A added was varied. The amount of the phosphazene compound A added was determined as 3.5 wt % (the minimum amount to allow the battery to demonstrate the flame retardant) per 100 wt % of the non-aqueous electrolyte. The results of the evaluation of the flame retardant retardance are shown in Table 4 and
As shown in Table 4 and
In addition, particles of the cyclic phosphazene compound were added, as flame retardant particles to the non-aqueous electrolyte, and the relationship between the melting point of the particles of the cyclic phosphazene compound and the flame retardance of the battery was examined. In the examples, phosphazene compounds A to G (Experimental Examples 30 to 36) having different melting points were used as the cyclic phosphazene compound, and subjected to evaluation of the flame retardance (nail protrusion test) and evaluation of the battery characteristics (high-rate discharge test). The amount of any of the phosphazene compounds A to G added was determined as 3.5 wt % (the minimum amount to allow the battery to demonstrate the flame retardance) per 100 wt % of the non-aqueous electrolyte. In addition, the battery characteristics were represented as the high-rate discharge capacity (%) for each of the phosphazene compounds compared to the high-rate discharge capacity for the phosphazene compound A (Experimental Example 32) with good battery characteristics discussed above and shown in Table 3 and
As shown in Table 5 and
An embodiment and examples of the present, invention have been specifically described above. However, the present invention is not limited to the embodiment and the examples, and may be changed based on the technical concept of the present invention as a matter of course.
The present, invention employs a number of flame retardant particles made of a material that exists as a solid and does not perform a function of suppressing combustion when the temperature of a non-aqueous electrolyte is equal to or less than a reference temperature at which the non-aqueous electrolyte is likely to start combustion and that is at least partially liquefied and performs a function of suppressing combustion when the temperature of the non-aqueous electrolyte is more than the reference temperature. A large number of such flame retardant particles are added to the non-aqueous electrolyte. Therefore, it is possible to provide a non-aqueous electrolyte battery whose battery characteristics are not significantly reduced and which performs a function of suppressing ignition (rupture) of the non-aqueous electrolyte only when the temperature inside the battery is increased.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010--199036 | Sep 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/070255 | 9/6/2011 | WO | 00 | 4/30/2013 |
