EP2678891A1

EP2678891A1 - Electrode material having high capacitance

Info

Publication number: EP2678891A1
Application number: EP12702788.6A
Authority: EP
Inventors: Chuanqi TAO; Martin Miessen; Peter Pilgram; Stefano Passerini; Martin Winter
Original assignee: Evonik Litarion GmbH
Current assignee: Litarion GmbH
Priority date: 2011-02-23
Filing date: 2012-01-27
Publication date: 2014-01-01
Also published as: US20130337334A1; CN103518275A; WO2012113606A1; JP2014511546A; KR20140014142A; DE102011004564A1

Abstract

The invention relates to a silicon-carbon composite comprising at least one portion of hard carbon and one portion of silicon powder, said composite being obtained by virtue of the fact that under a noble gas atmosphere a) the hard carbon portion is treated at high energy at least once in a mechanofusion mixer, and b) afterwards the portion of silicon powder is added thereto and the portions are mixed together, or during step a) the portion of silicon powder is added thereto and the mechanofusion treatment is continued, and said composite being characterized in that the composite has an average particle size of less than or equal to 12 μm, a portion of hard carbon of 5 to 50% by weight and a portion of silicon powder of 5 to 50% by weight.

Description

Electrode material with high capacity

The invention relates to a microstructured silicon-carbon composite, and its use as an anode material for lithium-ion batteries.

Mobile and portable applications place ever greater demands on storage media for electrical energy. Although double-layer capacitors have high charge / discharge cycle numbers, the energy density is significantly lower than in rechargeable battery systems, especially lithium-ion batteries. Here, the theoretical capacity of the main graphite used is 372 mAh / g for a LiC ₆ composition. Even higher gravimetric energy densities can be achieved by substituting silicon for graphite; here the capacity for an alloy with the composition Li ₂₂ Si ₅ is 4200 mAh / g. However, an aforementioned complete lithiation of the silicon leads to a volume expansion of over 300%. The occurring mechanical stress pulverizes the silicon particles, whereby the electrical contact between silicon and the current-conducting

Components of a cell within a few charging / discharging cycles decreases sharply and so the silicon-based anode quickly loses its capacity.

A very comprehensive overview of causes and various approaches to overcoming the difficulties in using silicon as an active component in lithium ion batteries has been given by Kasavajjula, Wang and Appleby (Journal of Power Sources 163 (2007) 1003-1039).

The following approaches to overcome the problems resulting from volume expansion are named: - Pure Si micro- and nano-scale powder anodes

Si dispersed in an inactive matrix

Si dispersed in an active matrix

Si anodes with different binders

Si thin films

Neither the use of the smallest crystalline or amorphous Si particles, conventionally applied as a slurry or by Polyvinyhdenfluorid on the current collector, nor the use in intercalatable or non-intercalatable matrices that were chemically, physically or mechanically produced and processed with a variety of binder systems have to silicon-based electrodes of sufficient cycle resistance.

Common to all these teachings is the disadvantage of high irreversible capacitance and, for practical applications, low cycle numbers of charge and discharge operations.

The essays of CS. Wang, Wu, Thang, Qi, Li, Electrochem. Soc 145 (1998) 2751, and G.X. Wang, Yao, Liu, Electrochem. Solid State Lett. 7 (2004) A250, discuss that mixing and milling finely divided silicon powder with carbon in the ball mill by destroying graphite and MCMB structures results in an electrode material exhibiting very high irreversible capacities.

It was therefore an object of the present invention to provide a process for producing silicon-carbon composites and the silicon-carbon composite itself, which is suitable for use as an anode material having sufficient capacity and cycle stability in a lithium-ion battery.

Surprisingly, it has been found that when using hard carbon as the carbon source for a silicon-carbon composite, a pretreatment of the hard carbon fraction in the composite under argon in a high-energy mixer improves the subsequent composite formation with silicon powder. If, after this separate pretreatment of the hard carbon, the proportion of silicon powder in the mixer is added, the resulting mixture is again subjected to high energy treatment and further processed under protective gas, the result is a silicon-hard carbon composite with significantly improved

Cycle behavior.

High-energy mixing, also called mechanofusion in the context of the present invention, is a method of applying powder or powder mixtures with high mechanical energy. This is described, for example, in the article "Mechanofusion for High Performance Particles", in Process Engineering, cfi / Ber. DKG 79 (2002), No. 4. Mechanofusion causes an intensive decomposition of the particles of the powder or powder mixture by beating, bouncing , Shearing and breaking of the particles.The particles thus obtained are intensively mixed in the mechanofusion.As a result of mechanofusion, sub-micron particles are found under the particles, which are known to the expert as "guest particles" and a coating on the remaining, larger particles, also called "host particles" form, such an effect is already in 1989 in a work of Yokoyama in "Mechanofusion" of the

Micromeritics Laboratory, Hosokawa Micron Corp. Osaka, Japan, 1989. In the

The guest particles can coat each other and the surface of the host particles by coating Adhesion forces be bound. Depending on the extent of the energy input by means of mechanofiision and the type and size of the host or guest particles, the bonds between the guest or guest and host particles can also have arisen by chemical nature or by sintering. The apparatus for the mechanofiision is shown schematically in FIG. The "roundabout" is set in rotation at rotational speeds of 200 to 3100 rpm, so that the particles are pressed by centrifugal forces against the inner wall of the roundabout and pushed through the gap between "scraper blade" or "inner compression piece" and the inner wall Not shown in the figure are water cooling and protective gas connection.

The introduction of mechanical energy by scraper blades and inner compression pieces exerts a striking, bouncing, shearing, and refractive effect on the particles. The totality of these effects is so intense that heat energy also arises. Depending on the extent of the energy input and the type and size of the host or guest particles, the bonds between the guest or the guest and host particles can also have arisen from a chemical nature or by sintering.

In the context of the present invention, a complex combination of macroscopic and microscopic processes occurs, the effect of which strongly depends on the energy input. In mechanofiision, particles can be mixed, shaped in their morphology or even connected to each other or

be merged.

The invention thus relates to a silicon-carbon composite, which has at least one portion of hard carbon and a proportion of silicon powder, which is obtained by the fact that under noble gas

the atmosphere

a) the hard carbon content is treated at least once in a mechanofiision mixer high energy, and

b) then adding the portion of silicon powder thereto and mixing the portions, or during step a) adding the proportion of silicon powder thereto and continuing the mechanofiision treatment,

and characterized in that the composite has an average particle size of less than or equal to 12 μm, a proportion of hard carbon of from 5 to 50% by weight and a proportion of silicon powder of from 5 to 50% by weight.

By "mixing" is meant any process known to those skilled in the art, in which powder or powder mixtures are mixed.

The two-stage processing has the advantage that the silicon particles are distributed very evenly on the hard carbon particles and adhere better to them than according to prior art approaches. There is a very good electrical contacting of the silicon particles with the irregularly arranged stacks of graphene which are typical for hard carbon and are referred to in the context of the invention as "carbon matrix." Without being bound by theory, the inventors are of the opinion in that, due to the stable electrical contact with the carbon matrix, the high

Cycle resistance is achieved when used as the anode material. This ensures a high reversible capacity, and the battery made of this electrode material has a better cycle behavior.

The carbon matrix of the composite according to the invention has a pore structure optimally adapted to the morphology of the silicon particles. The adapted pore structure makes it possible ideally to compensate for the volume changes occurring in the charge / discharge process, which is equivalent to the lithiation / delithiation, without mechanical stress on the carbon matrix. The resulting from the volume expansion or contraction in the lithiation or delithiation

Mechanical stresses are dissipated in the pores of the carbon matrix. Although in the cyclization or in the charge / discharge partially strong volume changes in the crystalline and / or amorphous phases of the ion-intercalating particles occur, the binding of these particles with each other is obtained in the composite according to the invention. Thus, the electronic contact is preserved in the composite according to the invention. Another advantage of the silicon-carbon composite according to the invention is that it can be used both with and without further treatment steps for the preparation of anode slabs. Such anode slats may be solvent or water based and prepared by methods known to those skilled in the art. Therefore, the present invention also provides the use of the microstructured silicon-carbon composite according to the invention as the anode material in energy storage systems, as well as the use of the silicon-carbon composite obtained in the claimed method in energy storage systems. Furthermore, the subject matter is a lithium-ion cell which has the silicon-carbon composite produced by the process according to the invention.

Furthermore, the subject matter of the present invention is a lithium-ion battery which has the silicon-carbon composite produced by the process according to the invention.

The invention will be explained in more detail below.

The erfmdungsgemäße silicon-carbon composite is obtained under inert gas atmosphere. The continuous exclusion of oxygen in all steps is required because it avoids unwanted oxidation. However, molecular nitrogen as an inert gas is unsuitable because it reacts with silicon in the energy introduced by the mechanofusion process of hard carbon

unwanted silicon nitride. Preferred noble gases, under which the inventive silicon-carbon composite is obtained, are helium, neon, argon, krypton, xenon, of which argon is particularly preferred.

Preferably, silicon powder, available from Evonik Degussa GmbH, is used, which is produced by the thermolysis of silane and agglomerates having sizes less than or equal to 1 μιη, which are composed of round primary particles with diameters of 20 to 60 nm.

The mechanofusion in step a) preferably takes place from 2500 to 3100 rpm, more preferably at 3100 rpm, and for a time of 60 to 360 minutes, particularly preferably for 60 minutes. The proportion of silicon powder of the silicon-carbon composite according to the invention may preferably be added subsequently to step a), and the proportions mixed by continuing the mechanofusion treatment. This has the advantage of dramatically increasing the cycle life of the resulting composite in a lithium-ion battery. The silicon particles adhere better to the hard carbon particles than mixing the fractions by means of processes other than mechanofusion. The mechanofusion treatment in step b) is preferably carried out from 2500 to 3100 rpm, more preferably at 3100 rpm, and for a time of 60 to 360 minutes, particularly preferably for 60 minutes. The further advantage of this procedure is that the Si agglomerates are partially broken and the individual Si particles are incorporated into the surfaces of the hard carbon particles. Hard Carbon is a very hard but brittle substance that can be made from quite different organic precursor materials such as petroleum pitch, sugar, cellulose, phenolic resins and some more by pyrolysis in the absence of oxygen. Hard Carbon is characterized by good electrical conductivity, but also by high micro- and mesoporosity, as well as low

Fernordnungsgrad the graphene layers.

The irregularly arranged graphene layers can also be achieved by using very high

Do not arrange temperatures and therefore not graphite hard carbon. The closed microporosity, which is not directly connected to the outer surface of hard carbon, causes one opposite

Graphite with its density of 2.26 g / cm ³ significantly lower density of about 1.5 g / cm ³ , but also a very high irreversible capacity of the composite according to the invention in the first cycles, and a lower specific capacity compared to graphite. Hard carbon usually has water in the pores, which must be expelled before use by drying in vacuo at elevated temperature, preferably at 150 ° C and reduced to 1 mbar ambient pressure.

Particularly preferred is a ratio of the proportions by weight of hard carbon and silicon powder of 8: 2.

Possible aftertreatment steps include the further stabilization of the composite according to the invention by application of carbon and / or binder layers, which may also be of a metallic nature, in the manner known to the person skilled in the art. The silicon-carbon composite according to the invention is particularly suitable for use in rechargeable lithium-ion batteries, one of whose electrodes consists of a lithium-ion battery.

Transition metal mixed oxide of the general formula LiM0 ₂ , where M = at least one metal from the group Co, Ni, Mn, V and / or a lithium transition metal phosphate of the general formula LiMPC, with M = at least one metal from the group Fe, Mn, Co This electrode represents the positive electrode.

Example:

Hard carbon and silicon powder were in a weight ratio 8: 2 under argon in a planetary ball mill (Fritsch Pulverisette 6) with sialon grinding balls in a sialon grinding cup with Gassing lid intensively mixed (16 hours, 500 rpm). The material obtained was processed into anode material as described below and characterized electrochemically.

To 77% by weight of the resulting material was added 15% by weight of "Super P Conductivity Enhancer" available from Timcal, Switzerland, and 8% by weight sodium carboxymethylcellulose, available from

Chemical trade, for example, at Fluka, Buchs, Switzerland, given as an aqueous, deaerated with nitrogen solution, homogenized, degassed and applied with a squeegee gap of 100 μιη on a 14 μιη thick copper foil. The drying was carried out under nitrogen for 30 min at 80 ° C and 120 min at 110 ° C. The punched out 12 mm thick electrodes were post-dried overnight before the assembly of half-cells at 130 ° C in a vacuum and in the glove box with a contact pressure of 6.6 bar to half cells (so-called T-cells, Swagelok, USA) with lithium installed as counter and reference electrode. The separator used was Celgard 2320 (Celgard, USA) and the electrolyte used was a mixture of EC: EMC: DEC: VC = 29: 49: 20: 2 (Ferro Corp., USA). The electrochemical characterization was carried out with a Maccor S4000 Battery Tester (Maccor, USA). The cells were cycled in CCCV mode from 0.02 to 1.5 V, the turn-off points respectively. In the first charging step (lithiation) was charged with 0.1 C and discharged at 0.3 C (delithiation). The further loading and unloading steps were carried out with 0.5 C. Example 1

It was hard carbon used, which was made of petroleum pitch, obtained from Kureha Corp., Japan under the name Carbotron P. It had a mean particle diameter of 10 μιη on.

8 parts of hard carbon were separately treated with high energy prior to the addition of 2 parts of silicon powder at 3100 rpm for 60 minutes under argon in the AMS-Lab mechanical fusion mixer. After this first treatment stage, the addition of the silicon powder was carried out under argon. Hard carbon and

Silicon powder was high energy treated for a further 60 min at 3100 rpm in the Mechanofusion mixer, then discharged under argon. A scanning electron micrograph of silicon particles on a hard carbon particle of the composite according to the invention is shown in FIG. 2.

Subsequently, the composite according to the invention was processed to anode material as in the comparative example and characterized electrochemically. Example 2:

8 parts hard carbon and 2 parts silicon powder were used as in Example 1, in

Difference, however, mixed from the beginning together at 3100 rpm for 60 min under argon atmosphere.

The silicon-carbon composite according to the invention was then as in the

Comparative example processed to anode material and characterized electrochemically. Fig. 3 shows the specific capacity of the half-cell as a function of the number of cycles, each with the electrode material prepared from Example 1, 2 and the comparative example. Filled (black) symbols mark the on loading, empty symbols for the specific capacitance measured when the half cell is discharged.

Claims

claims:

1. silicon-carbon composite,

at least comprising a proportion of hard carbon and a proportion of silicon powder,

obtained by

that under noble gas atmosphere

a) the hard carbon content is treated at least once in a mechanofusion mixer high energy, and

b) then adding the portion of silicon powder thereto and mixing the portions, or during step a) adding the proportion of silicon powder thereto and continuing the mechanofusion treatment,

characterized,

that the composite has an average particle size of less than or equal to 12 μm,

a proportion Hard Carbon of 5 to 50 wt .-% and

has a proportion of silicon powder of 5 to 50 wt .-%.

2. silicon-carbon composite according to claim 1, wherein

the noble gas atmosphere is argon.

3. silicon-carbon composite according to one of the preceding claims, wherein

the proportion of silicon powder is then added to step a), and

the mechanofusion treatment is continued.

4. Use of the silicon-carbon composite according to one of claims 1 - 3

as an active component in a lithium-ion battery,

characterized,

that the composite is processed together with a conductivity enhancer and a binder into a paste which is applied to a current collector.

5. Lithium-ion battery, comprising the silicon-carbon composite according to one of

Claims 1 - 3