CN1408127A - Composite carbon electrode for rechargeable lithium-based batteries - Google Patents

Abstract

The invention discloses a composition suitable for coating collectors in lithium ion batteries, including carbon fibers and electrically conductive particles coating the carbon fibers. The electrically conductive particles have an average particle size less than about 40 nm. Preferably, the average particles size is about 30 nm and has a specific surface area greater than about 1200 square meters per gram. Electrodes manufactured using a composition of the invention exhibit significantly improved electronic conductivity, in-cell capacity and cycling properties, generally thought to be due to enhanced binding properties that give an overall improved electronic conductivity.

Description

Composite carbon electrode for rechargeable lithium-based batteries

Background of invention

As the lithium-base battery that rechargable lithium ion cell is used, generally include the electrode of the part that can form the composite material that contains carbon fiber.General these carbon fibers are formed by a kind of mesophase pitch and comprise the graphitized carbon crystallite.This composite material also comprises the conductive coating additive that is used to improve conductivity usually, as carbon black, thereby has improved the performance of electrode.

The electrode composite material carbon black quantity that is used for lithium ion battery generally is about 5% of composite material weight.The average grain granularity of the carbon black that adopts is usually between 50nm-100nm.The specific area of particle generally is about 80 square metres of every grams or lower in the carbon black.For the electrolyte that suppresses to take place decomposes, generally to limit the specific area of carbon black in battery.

Use the low relatively carbon black of surface area that several shortcomings are arranged.One, in order to significantly improve the conductivity of carbon fiber-containing electrode, the necessary use amount of carbon black is generally many relatively.Thereby, the capacity of battery in the carbon black of high-load can reduce relatively, because the amount of corresponding active material, promptly the amount of celion is low.In addition, utilize the low relatively carbon black of surface area to suppress the electrolyte decomposition, however electrolyte still can decompose.Improve the rated capacity that interior battery capacity can reduce electrode by the amount that reduces carbon black.In addition, it is generally acknowledged the electrode that the carbon black that uses high surface has generation the first low cycle efficieny.

Therefore, need be applicable to the battery electrode composite material that overcomes or reduce the problems referred to above.And then need be applicable to the lithium ion cell positive composite material that overcomes or reduce the problems referred to above especially.

Summary of the invention

The present invention relates to a kind of composite material, this composite material comprises the conductive particle of carbon fiber and this carbon fiber of coating.The average grain granularity of conductive particle is less than 4 about 0nm.Preferred embodiment, the average grain granularity of conductive particle is about 30nm.In particularly preferred embodiments, the surface area of conductive particle is greater than about 1200 meters squared per gram (m ²/ g) and the average grain granularity be 30nm or littler.More preferably the surface area of conductive particle is at least 1500 meters squared per gram (m ²/ g).For example, conductive particle can be the combination of carbon black, metallic particles, metal oxide particle or any of these particle.

The present invention has several advantages.For example, when using the high relatively conductive particle of surface area, think the electrical conductance path that has produced quite high quantity, thereby cause comprehensive raising of electrode conductance rate.In addition, use the high relatively conductive particle of surface area to make the surface area of the graphitized carbon fibre that is contacted bigger relatively, make lithium ion in insertion and disengaging graphitized carbon fibre process, more reflecting point can be arranged thus.And the possibility that is touched by a zone in the graphitized carbon fibre of the synthetic defect sturcture isolation that produces is bigger, thereby makes this zone of the easier insertion of lithium ion.In addition, found that composite layer can better adhere to current collection basic unit.The result is that the interface display of coating and current collection layer goes out lower resistance and improved the mechanical stability of electrode.For example, the lithium ion battery that comprises composite material of the present invention is owing to higher insertion ability demonstrates higher specific capacity, owing to bigger material loading demonstrates the higher interior energy content of battery, demonstrate better rated capacity owing to electronic conductivity contact with big surface, think owing to mechanical stability with make the Undec high coverage of graphitized carbon fibre particle surface demonstrate better circulation ability.In addition, the advantage owing to using the high relatively conductive particle of specific area to bring can adopt these conductive particle percent by weight of obvious minimizing and corresponding increase amount of carbon fiber to design anode.

Moreover, to compare with the prescription that does not use carbon black, the used granule carbon black of the present invention can cause the carbon fiber in the lithium ion battery to have higher specific capacity usually.The raising of preferred specific capacity is consistent with high rated capacity.The capacity of interior battery is based on the total capacity that goes out of prescription total weight, and it depends on the relative weight of carbon fiber, polymer and electrode coated carbon black.Interior battery capacity is defined as follows:

ICC＝(SC)(CF)

Wherein, battery capacity (mAh/g) in the ICC=, SC=specific capacity (mAh/g), the relative populations (carbon fiber weight is divided by the weight sum of carbon fiber, polymer, carbon black) of carbon fiber in the CF=prescription.

Yet the carbon black that contains in the electrode coating is many more, and cycle efficieny will be low more for the first time.Because cycle efficieny influences the total capacity of lithium-based electrochemical battery for the first time, it is useful having the higher cycle efficieny first time.Cycle efficieny is low more for the first time, and the amount that adds the negative material in the battery must be many more, because negative pole provides initial lithium.If for the first time cycle efficieny is 100%, anodal capacity can promptly needn't be added excessive negative material by the accurate weighting capacity institute balance of negative pole.If cycle efficieny is less than 100% for the first time, must add excessive negative material to the anodal limit of battery.In ensuing circulation, efficient is usually near 100%.Cycle efficieny is low to be owing to having formed side reaction, such as formed the interface that contains lithium at electrode surface.In the practice, cycle efficieny should be higher than 85% for the first time.Usually, with comparing of the carbon black of low specific surface, the cycle efficieny first time of the every weight basis of high-ratio surface carbon black is low.Use carbon black still less to solve this shortcoming in the present invention.Because can make the amount of carbon black few, the cycle efficieny first time of optimization formula will be 85% or higher.(accompanying embodiment shows can obtain these efficient).

Brief Description Of Drawings

Fig. 1 is for applying the scanning electron micrograph of the graphitized carbon fibre of relative high-specific surface area carbon black in the composite material of the present invention.

Fig. 2 is for being coated with the scanning electron micrograph of the graphitized carbon fibre of relative low specific surface area carbon black in the comparative composite.

Detailed Description Of The Invention

Above-mentioned feature of the present invention and other details more illustrate with reference to accompanying drawing and will point out in the claims. It is indicative being interpreted as the specific embodiment of the invention, is not to be limitation of the present invention. Principal character of the present invention can be applicable to not break away from the numerous embodiments of the scope of the invention.

Composite of the present invention comprises carbon fiber and applies the conductive particle of this carbon fiber.

The applicable composite of carbon fiber is used to apply the anodal colelctor electrode of lithium ion battery. In one embodiment, carbon fiber is graphitized carbon fibre. In a preferred embodiment, fiber is the graphitized carbon fibre that is formed by mesophase pitch. Being more preferably carbon fiber is the graphitized carbon fibre that is formed by the mesophase pitch solvate. The example of preferred carbon fiber is Petoca M5161^Carbon fiber. The example of other the suitable carbon fiber U.S.5766 that Conoco Inc. published on June 16th, 1998 is described in 523, and its content is incorporated herein by reference. According to the specification of manufacturer, Petoca M5161^Have following feature:

Surface area---1.3m²/g

Typical average fibre diameter----10 μ m

Typical average fiber length----30 μ m and, the average-size of the Petoca M5161 fiber by light scattering method measuring distributes as follows:

Size accounts for 10% less than the fiber of 8.4 μ m

Size accounts for 50% less than the fiber of 18 μ m

Size accounts for 90% less than the fiber of 57 μ m.

The average length of suitable carbon fiber about between the 20-70 μ m and average diameter between about 5-20 μ m.In the composite material that comprises carbon fiber, conductive particle and polymeric binder, the percentage by weight of carbon fiber is about between the 85%-98%.The amount of the preferred carbon fiber that exists accounts between the 90%-97% of composite material weight.

Suitable conductive particle comprises, for example carbon black, metallic particles and conductive metal oxide particle.In one embodiment, conductive particle is the carbon black of average grain granularity less than about 40nm.One preferred embodiment in, to be the average grain granularity be about 10-20nm and the specific area carbon black greater than about 1500 meters squared per gram to conductive particle.Preferred especially conductive particulate materials is a high surface area material, such as Cabot black B lack Pearls ^2000.Cabot black B lackPearls ^2000 average grain granularity is 12nm, and its surface area is 1500m2/g.The percentage by weight of the conductive particle that exists in the composite material is generally between about 0.3%-8%.

The third composition comprises polymeric binder, and it is used for carbon fiber and conductive particle are bonded together, so that electrode has mechanical stability.The suitable polymer blend composition of composite material comprises Kynoar (PVDF), polyimides (PI) and Kynoar-hexafluoro cyclopropane (PVDF-HFP).Preferred polymers is for example Kureha Chemical Industry CompanyLtd., the PVDF that Solvay, ElfAtochem sell.The amount of polymer accounts for the 1%-10% of composite material weight usually in the composite material.

The method of synthetic composite material is included in the container carbon fiber component, conductive particle and polymer is merged and mix.In mixed process, can utilize suitable solvent medium to come dissolve polymer.In one embodiment, solvent is 1-Methyl-2-Pyrrolidone (NMP).Utilize suitable mechanical assistance means, for example iron ball promotes to mix.Blending ingredients is until forming uniform composite material.

The homogeneous mixture that obtains or slurries are coated on the suitable current-collecting member such as Copper Foil.Collector electrode is generally the Copper Foil of thickness between about 10 μ m-25 μ m.In a preferred implementation, the thickness of Copper Foil is about 20 μ m.For example, utilize doctor blade slurries can be applied into the coating of thickness range between about 152 μ m-508 μ m.Preferred thickness is about 254 μ m.Then coated collector electrode is heated to suitable temperature, between 110 ℃-175 ℃, keeps time enough according to appointment to evaporate all solvents.In one embodiment, the electrode that applies being heated to 150 ℃ approximately kept about 20 minutes.After the heat treatment, the gross thickness of electrode is between about 50 μ m-150 μ m usually.In a preferred implementation, anode has gross thickness and the current collection groundwork thickness of about 100 μ m.

Then electrode cutting is become the bar of suitable size.Then under room temperature and suitable pressure, suppress these with roll press.For example, Shi Yi pressure is at about 250kg/cm ²To 1500kg/cm ²Between, the diameter of roller is 5.08cm (2 inches).In a preferred implementation, roll press is in room temperature and be about 1000kg/cm ²Apparent pressure down compacting this.

After the compacting, the common thickness of electrode is between 30-100 μ m.In a preferred implementation, the thickness of the collector electrode after the compacting is about 80 μ m.

In a preferred embodiment, then that downtrodden electrode is dry under suitable condition, to remove residual solvent and moisture.In a preferred implementation, with electrode about 16 hours of 80 ℃ of following vacuumizes.

Electrode can be as negative pole in lithium-ion electric.

To the present invention be described in further detail by embodiment now.Unless other regulation, all marks and percentage are all by weight.

Embodiment

Embodiment 1

With 47 gram Petoca M5161 ^Carbon fiber (active material), 0.5 gram Cabot black B lack Pearls ^The PVDF/NMP solution of 2000 (conductive additives), 31.25 grams, 15% percetage by weight and 40 gram NMP add in 250 milliliters the jar that 50 iron balls (φ=1/4 inch) are housed.Shake sieve (paint shaking) to mixture mixing 40 minutes by paint vehicle.

The doctor blade that utilization has the thick wet coating of 250 μ m is coated on slurries on the Copper Foil (thickness～20 μ m).Coated electrode was heated 20 minutes down at 150 ℃.The thickness of electrode that has the copper collector electrode is generally 100 μ m.

Electrode is cut into 3.5 * 4cm ²Bar.Then at room temperature with 1000kg/cm ²Apparent pressure press these with roll press.After the compacting, the thickness of electrode that collector electrode is housed is generally 80 μ m.Repressed electrode is cut into the disk of diameter for about 0.75cm.The weight of active material is 15mg in the electrode wafer.With electrode about 16 hours of 80 ℃ of following vacuumizes to form dry electrode wafer.

With lithium paper tinsel (Aldrich) do negative pole, dry electrode wafer do anodal, the glass fibre disk makes isolator and EC/DNC (1: 1)-LiPF6 1M (EM Industries) has made chargeable coin cell (the CR2520 type of standard can obtain from Hohsen Corporationof Japan) as electrolyte.In being lower than the argon filling glove box of about 1ppm order of magnitude, water and oxygen level all operate.

Utilize C/5 and the C2 electric current coin cell that in the 0.000-2.000V voltage range, circulates.

Embodiment 2

Except conductive additive is that surface area is 254m ²/ g and grain graininess are the Cabot carbon black Vulcan XC 72R of 30nm ^Outward, all steps are similar to the aforementioned embodiment.The comparative example A

Except conductive additive is relative low surface area (surface area 80m ²The acetylene black C-100 of Chevron 100% compacting/g, grain graininess 42nm) ^Outside the material, all steps are similar to the aforementioned embodiment.

Table 1 has been listed the capacity of lithium ion battery difference in cyclic process of using above-mentioned carbonaceous additive.As can be seen, utilize Black Pearls ^2000 carbon blacks are that the lithium ion battery of conductive additive material has kept good specific capacity through repeatedly circulating.In addition, also can find out, use Black Pearls from table 1 ^The execution mode of 2000 carbon blacks has significantly improved specific capacity and rated capacity.

Table 1

Cycle-index		The 1st time	The 10th time	The 30th time	Capacity under (C2)/(C5) * 100
						Specific capacity (mAh/g)	Embodiment 1 Black Pearls 2000 	323	?320	?316	?100
Embodiment 2 Vulcan XC 72R 	309	?298	?264	?98
					Comparative example A C-100 		300	?278	?236	?89

Fig. 1 is the scanning electron micrograph that utilizes carbon fiber in the composite material of the present invention of the high relatively carbon black materials of specific area.Fig. 2 is the scanning electron micrograph that utilizes carbon fiber in the Comparative Examples composite material of the low relatively carbon black of specific area.To finding that than Fig. 1 and Fig. 2 compare with carbon fiber shown in Figure 2, the carbon black distribution layer of carbon fiber is more even among Fig. 1.Although do not wish to rely on the support of any particular theory, can believe that the improvement that the carbon black of Fig. 1 representative distributes has improved the conductance and the performance thereof of the lithium ion battery that utilizes composite material of the present invention.We think the distribution that improves and cause the improvement of performance to be to use surface area to be at least 250m ²/ g and grain graininess are no more than the result of 40nm carbon black materials.

Embodiment 3

To derive from 48.45 gram graphitized carbon fibres (active material), the 0.3 gram Cabot black B lack Pearls of mesophase pitch (Conoco Inc.) solvate ^The PVDF/NMP solution of 2000 (conductive additives), 8.33 gram 15wt% and 50 gram NMP add in 250 milliliters the jar that 50 iron balls (φ=0.64cm (1/4 inch)) are housed.Utilize paint vehicle to shake sieve with mixture mixing 40 minutes.

The doctor blade that utilization has the thick wet coating of 250 μ m is coated on slurries on the Copper Foil (thickness～20 μ m), and coated electrode was heated 30 minutes down at 130 ℃.The thickness of electrode that has the copper collector electrode is generally 100 μ m.

Electrode is cut into 3.5 * 4cm ²Bar.Then at room temperature with 1000kg/cm ²Apparent pressure suppress these with roll press.The thickness of electrode that collector electrode is housed after the compacting is generally 80 μ m.Repressed electrode is cut into the disk of diameter for～0.75cm.The weight of active material is 15mg in the electrode wafer.With electrode about 16 hours of 80 ℃ of following vacuumizes.

With lithium paper tinsel (Aldrich) do negative pole, dry electrode wafer do anodal, the glass fibre disk makes isolator and EC/DNC (1: 1)-LiPF6 1M (EM Industries) has made coin cell (CR2520 type) as electrolyte, in water and oxygen level are lower than the argon filling glove box of about 1ppm order of magnitude, all operate.

Utilize C/5 and the C2 electric current coin cell that in the voltage range of 0.000-2.000V, circulates.Comparative Examples B

To derive from 45.75 gram graphitized carbon fibres (active material), the 2.23 gram Chevron C-100 of mesophase pitch (Conoco Inc.) solvate ^The PVDF/NMP solution of acetylene black (conductive additive), 13.33 gram 15wt% and 75 gram NMP add in 250 milliliters the jar that 50 iron balls (φ=0.64cm (1/4 inch)) are housed.Shake sieve with mixture mixing 40 minutes by paint vehicle.All other steps are consistent with the foregoing description.

Table 2 has been summed up material and the performance characteristic of embodiment 3 and Comparative Examples B.

Table 2

		Embodiment 3	Comparative Examples B
				Prescription	Active material	Conoco carbon fiber 990585	Conoco carbon fiber 990585
Conductive additive	Black?Pearls 2000carbon?black	?C-100 Acetylene black
			Active material: conductive additive: the weight ratio of PVDF binding agent		96.9∶0.6∶2.5	?91.5∶4.5∶4
Battery capacity (mAh/g) in the 1st circulation		303					?280
			The 1st circulation coulombic efficiency (%)		90	?90
Battery capacity (mAh/g) in the 18th circulation		294					?284
			Battery capacity (mAh/g) in the 36th circulation		292	?283

With reference to table 2, each sample has all adopted optimum formula.That is to say that people are desirably between coulombic efficiency and capacity compromise usually.But table 2 shows that the battery of the present invention's preparation can have coulombic efficiency same as the prior art in the battery capacity in obtaining height.As shown in table 2, the battery of manufacturing of the present invention will have higher total capacity.Embodiment 3: carry out safety research by quickening calorimetry (ARC)

The initial temperature of exothermic reaction is the measurement standard of its fail safe in the lithium-base battery system.When exothermic reaction begins, the heat that reaction produces will elevate the temperature and increase with the speed that is defined as " self-heating speed ".Self-heating speed is high more, and the temperature increase is fast more.Sometimes will be called " thermal runaway " or " self-catalysis " with the exothermic reaction that uncontrollable mode is carried out.For battery manufacturers, thermal runaway is (especially when the low temperature) that will pay close attention to, because battery may explode in battery charging equipment user's hand fully.Therefore, for security reasons, has high as far as possible initial temperature and alap self-heating speed is useful.

For studying these phenomenons, with 0.5 gram carbon fiber powder (mesocarbon fiber solvate), 0.5 gram Black Pearls ^2000 powder and 1.0 gram PC-LiPF ₆The 1M electrolyte is packed in the titanium shuttle.Loading operation carries out in water and oxygen level are lower than the argon filling glove box of about 1ppm.Under argon atmospher, the titanium vessels that fills up is contained in ARC then ^On the 2000 accelerated heat analyzers (Arthur D.Little).Write down the heat release situation in the experimentation of 50 ℃ of beginnings, 350 ℃ of end.The firing rate that adopts is that 5 ℃/minute and per time of staying in step are 17 minutes.Following table 3 has been listed the initial temperature and the maximum self-heating speed of the exothermic reaction that obtains from experiment.

With 0.5 gram C-100 ^Powder replaces Black Pearls ^2000 powder have carried out another experiment with experiment as a comparison.All other experiment conditions are the same.As shown in the table, adopt BlackPearls ^The experiment of 2000 powder has higher initial temperature, can think in the lithium-ion battery systems safe material more thus.

Table 3

	Initial temperature	Maximum self-heating speed (℃/min)
			Black?Pearls 2000 powder	170	?54
C-100 Powder	135	?10

Equivalent

The people who knows this area can be appreciated that or can determine by normal experiment many equivalents of the specific embodiment of the invention that this paper describes in detail.The scope of following claim comprises these equivalents.

Claims (23)

1. electrode composite material comprises:

A) carbon fiber; With

B) the conductive particle coating of the described carbon fiber of coating, the average grain granularity of described conductive particle is less than about 40nm.

2. composite material as claimed in claim 1, wherein carbon fiber is a graphite fibre.

3. composite material as claimed in claim 1, the average grain granularity of wherein said conductive particle is about 30nm.

4. composite material as claimed in claim 1, the specific area of wherein said conductive particle be greater than about 1,500m ²/ g.

5. composite material as claimed in claim 1, the specific area of wherein said conductive particle are about 1,200m ²/ g-is about 1,500m ²/ g.

6. composite material as claimed in claim 1, the specific area of wherein said conductive particle is about 800m ²/ g-is about 1,500m ²/ g.

7. composite material as claimed in claim 1, the specific area of wherein said conductive particle is about 250m ²/ g-is about 1,500m ²/ g.

8. composite material as claimed in claim 1, wherein conductive particle comprises carbon black.

9. composite material as claimed in claim 1, wherein conductive particle comprises metallic particles.

10. composite material as claimed in claim 1, wherein conductive particle comprises metal oxide particle.

11. composite material as claimed in claim 1, wherein the amount of conductive particle accounts for about 0.5%-about 10% of described composite material weight.

12. as the composite material of claim 11, wherein the amount of carbon fiber accounts for about 85%-about 96% of described composite material weight.

13. as the composite material of claim 12, it also comprises a kind of polymers compositions.

14. as the composite material of claim 13, wherein the amount of polymers compositions accounts for about 1%-about 10% of described composite material weight.

15. as the composite material of claim 13, wherein said polymers compositions comprises Kynoar.

16. as the composite material of claim 13, wherein said polymers compositions comprises polyimides.

17. as the composite material of claim 13, wherein said polymers compositions comprises Kynoar-hexafluoro cyclopropane.

18. a positive pole that comprises carbon fiber and conductive particle composite material is arranged in the lithium-base battery:

Its improvement comprises that the average grain granularity of conductive particle is less than about 40nm.

19. as the composite material of claim 18, the specific area of wherein said conductive particle is greater than about 1,500m ²/ g.

20. as the composite material of claim 18, the specific area of wherein said conductive particle is about 250m ²/ g-is about 1,500m ²/ g.

21. comprise the positive pole of carbon fiber and conductive particle composite material:

Its improvement comprises that the average grain granularity of conductive particle is less than about 40nm.

22. as the composite material of claim 21, the specific area of wherein said conductive particle is greater than about 1,500m ²/ g.

23. as the composite material of claim 21, the specific area of wherein said conductive particle is about 250m ²/ g-is about 1,500m ²/ g.

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