WO2013060422A1

WO2013060422A1 - Electrode material for lithium ion batteries and method for the production thereof

Info

Publication number: WO2013060422A1
Application number: PCT/EP2012/004256
Authority: WO
Inventors: Wilhelm Pfleging; Robert Kohler; Johannes PRÖLL
Original assignee: Karlsruher Institut für Technologie
Priority date: 2011-10-29
Filing date: 2012-10-11
Publication date: 2013-05-02
Also published as: DE102011120893B3; EP2771930A1

Abstract

The invention relates to an electrode material for lithium ion batteries in the form of a compressed layer (1) that has a layer thickness of between 10 μm and 1mm, said electrode material comprising at least 80% of an active material (3) that is made of a lithium ion conductor and is present as a particulate material, and a maximum of 20% of binding agent (5) and conductivity additives (4). At least one region that adjoins a surface of the layer (1) has a structure made of conical elements (8, 8', 8"...) that are located on a base made of the particulate material and are formed from locally removed material of the layer (1) via material transfer, the binding agent (5) and the conductivity additives (4) being retained along with the active material (3). The invention further relates to a method for producing the electrode material by irradiating such a compressed layer (1) with 100 to 10000 laser pulses (6) that each have an energy density of between 1.5 and 3 J/cm², whereby in at least one region that adjoins the surface of the layer (1) a structure is formed consisting of conical elements (8, 8', 8"...) that form on a base made of a particulate material to a height of between 20 μιτι and 150 μm, however not higher than the layer thickness, the binding agent (5) and the conductivity additives (4) being retained along with the active material (3) in the structure formed via material rearrangement. The electrode material has improved electrochemical properties and allows for high charging and discharging cycle stability.

Description

Electrode material for lithium-ion batteries and

Process for its preparation

The present invention relates to an electrode material for Lithi ^¬ to-ion batteries, and a method for its preparation.

New applications in the industry, especially for electric and / or hybrid vehicles, require powerful energy storage. These include in particular lithium-ion batteries.

For conventional lithium-ion batteries, electrodes of par ^¬ tikulären (granular) are produced materials. In A. Jossen and. eydanz, modern accumulators use properly, Inge Reichardt Ver ^¬ lag, Untermeitingen, ISBN 3-939359-11-4, the structure of such electrodes is described, wherein the particles typically have particle sizes in the range of 0.5 μπι to 10 μιη. The particles are mixed with a binder to form a viscous mass and applied to a preferably metallic substrate via a film casting process. The layers thus produced are then pressed in a rolling calender, the porosity of the electrode material, ie the space which is later occupied by the electrolyte, being set at 30% to 50%. The electrode material is subsequently connected to the counter electrode, a separator, the electrolyte and the cell casing to an electrochemical cell where it is provided with egg ^¬ nem electrolyte acts as a medium for the transfer of ions between the two electrodes.

There are many efforts to improve the electrochemical properties of the electrode materials for lithium-ion batteries. Thin-film electrodes are used as model systems for this purpose. These electrode layers are applied to a carrier substrate and are in the form of a compact crystalline layer without the ^¬ to administration of binder, conductive carbon black or graphite before. The production of such thin-film electrodes preferably takes place by means of high-frequency magnetron sputtering. The layer thicknesses of the thin film used Layer electrodes are in the range of 100 nm to a maximum of 5 μκι, typically from 500 nm to 3 μπ \. This is necessary, in particular to avoid critical layer stresses and too long coating time.

It is known that by reducing the diffusion paths of the Li ions, the electrochemical properties of the thin-film electrodes can be improved. A known solution to this is the magnification ^¬ tion of the surface of thin-film electrodes. In JW Long, B. Dunn, DR Rolison, et.al. Three Dimensional Battery Architectures, Chem. Rev. 104 (2004), pp 4463 et seq., A structuring of thin film electrodes to improve the electrochemical own sheep ^¬ th of the thin-film electrodes are disclosed. Here, the Trägersub ^¬ strat of the thin film is patterned, and the thin-film electrode formed thereon receives the predetermined structure.

Kohler, P. Smyrek, S. Ulrich et. al. Describe in Patterning and Annealing of nanocrystalline LiCo ₂ thin films, J. Optoelectronics Adv. Mat. 12, 2010, p 547-552, the laser-based production of rod or conical structures on thin-film electrodes with layer thicknesses of at most 3.5 μτ, which served as model systems. For these structured thin film electrodes had been an improvement ^¬ tion of the electrochemical properties was demonstrated.

US 2009/0202903 A1 discloses the production of surface structures in powdery materials with layer thicknesses of at least 200 μm by means of an ablative laser structuring method. A laser beam with a Gaussian intensity profile carries off the material during a melting and / or evaporation process in the region of the laser beam focus (laser ablation), while in untreated areas not irradiated by the laser, the material is retained. By suitable Laserscannstrategien can be produced in this way structural geometries with dimensions of 50-500 μιτι. Alternatively, other methods, such. As the micro mills, used, which are based on the same principle of operation, ie Material is removed, the desired structures are formed by the remaining, not eroded material.

US 2010/0035152 Al discloses an electrode material for lithium-ion batteries in the form of a layer having periodic structures, which were introduced by means of an Nd: YAG laser in such a way that material was thereby removed from the layer.

DE 699 01 178 T2 describes an electrode carrier which is produced from a metal sheet which has slight surface irregularities, which are produced in such a way by a mechanical method, in particular by means of a roller, that elevations and depressions are formed therein.

US 2009 / 0280407A1 discloses an electrode with a flat substrate, on which first protrusions are applied, on the flat surface of which there are respectively second protrusions. Both the first projections and the second projections were produced by electrolysis.

US 2011/0027650 A1 describes an anode in which columns of an electrode material for lithium-ion batteries, which are each provided with a layer of a polymer, are applied to a substrate. The columns were produced by means of an electron beam evaporator; the application of the polymer layer by immersion in a corresponding solution.

Based on this, the object of the invention is to propose an electrode material for lithium-ion batteries, which overcomes the disadvantages and limitations of the prior art. This material should have improved electrochemical properties compared to the listed electrode materials and in particular allow improved charging and Entladezyklenstabilität. Another object of the invention is to provide a method for producing this electrode material.

This object is achieved by an electrode material having the features of claim 1 and the method of claim 9. The respective subclaims indicate advantageous embodiments of the invention.

To achieve the object is struck with respect to the electrode material before ^¬ introduce a conical surface structure in an area adjacent to a surface of a layer of the electrode material region of locally removable material layer by the material assignment. The formation of conical structures in an electrode material comprising at least 80% of an active material of a lithium ion conductor, which is in the form of a particulate material, and at most 20% of binders and of conductivity additives, which are obtained in addition to the active material in the conical structures Although there is no longer any requirement for a further reduction in the mean diffusion path length of the lithium ions, since there already exists a large-scale grain structure comprising fine particles and small grains of material which may also be present as agglomerates. However, the surface structure according to the invention forms a hierarchical structure, ie a conical structure superimposed on the grain structure, which on the one hand better compensates for the mechanical degradation of the battery due to lithium incorporation and removal and on the other hand increases the active surface in the near-surface region. This in turn causes an increased lithium exchange with the electrolyte locally, ie in the near-surface electrode layer region. Furthermore, all chemical reactions take place in the electrode material according to the invention at the interface active material / electrolyte, which is why here an enlarged surface plays a central role. The surface is enlarged by the surface structure in the border area. For this purpose, in particular layer thicknesses are in the range of 10 ^μι η to 1 mm, preferably in the range of 20 to 150 μπι. The height of the conical elements has a value of 20 μιη to 50 μιη, but not more than the layer thickness. The half-width of the conical elements moves in the range of 1 μπι to 20 μιη, while the distance between two peaks of the conical elements has a value of 1 μιη to 50 μιη.

When applying a current to a lithium-ion cell electrochemical reactions at the interface of electrode material / electrolyte take place, and lithium ions are transported out of the electrode materials ^¬ rial into the electrolyte. This happens until a certain cell voltage is reached. When the lithium ions re-deposit into the host structure of the electrode material, the cell voltage decreases with the absorption of electrons. As a result more mali ^¬ ger charging and discharging the cell, ie repeated entry and exit of lithium ions in the host lattice of the electrode, volume expansion of the electrode particles are induced. This leads to damage (mechanical degradation) of the electrode material, which is reflected in the decrease in the capacity of the cell. The finer the coordination between particle size of the electrode material, adjusted porosity and wetting of the particles with electrolyte, the better the cycle stability of a particular electrode material. The mechanical degradation as well as the sub-optimal surface size and structure of the electrode material in the near-surface

Layer region is counteracted by a targeted structuring of porous electrode material. Suitable for this purpose are cone-shaped structures which on the one hand are mechanically stable and on the other represent an increase in surface area, which in turn locally effects an improved lithium exchange with the electrolyte. In this way, the Zyklierverhalten, so the behavior of the capacity as a function of the number of charging and discharging operations, stabilized in operation especially at short-term high charging and discharging. In a preferred embodiment, the electrode material according to the invention comprises active material, the Li (Ni _x Mn _y Co _z ) 0 ₂ compound with 0 <x <1, O ^ y ^ l, 0 <z <1 and x + y + z = 1 and which is provided in a particular embodiment with dopants (additions) of Al, Mg, Fe, i, Zn, Ga, Nb, F, B, Cr, and / or Y. In particular, the active material has a particulate shape with an average particle diameter of 0.5 μπι to 10 m, which is not thicker than the layer of electrode material.

In a particular embodiment, the layer of the electrode material on a substrate (carrier) made of a metal, in particular aluminum or copper, or applied from an alloy having a thickness of 5 μπι to 50 μιη, but at most the thickness of the electrode ^¬ material, and which preferably serves as Abieiter in an elec ^¬ trochemischen cell.

With regard to the method is proposed to solve the problem to produce the surface structure by laser irradiation of the electrode material. In order to produce the advantageous conical structures, suitable laser parameters must be set, since these determine the cumulative irradiation dose E, which is determined according to FIG

E = N * ε as a function of the emitted number of laser pulses N (number of pulses) calculated from the energy density ε of the laser beam per laser pulse and therefore can be specified independently of the laser device used.

The value of the cumulative exposure dose E is crucial to induce egg ^¬ NEN local removal of material as well as a material rearrangement of the electrode material, wherein partial re-particles are deposited from the material vapor on the material surface without entering a structural damage to the material that is detrimental to the functionality as a cathode. The re-deposits on the material surface are made such that the advantageous conical structures arise. According to the invention, the production of the electrode material is carried out by applying one in the form of a pressed layer with a

Layer thickness of 10 μιη to 1 mm provided electrode material comprising at least 80% of an active material of a lithium-ion conductor, which is present as particulate material, and at most 20% of the binder and conductive additives, with 100 to 10,000 La ^¬ serpulsen that each have an energy density of 1.5 J / cm ² to 3 J / cm ² . As a result of this treatment, in at least one region adjoining the surface of the layer, a structure is generated by material rearrangement which consists of conical (conical) elements which project at a height of from 20 μm to 50 μm, but at most the layer thickness, on a substrate form the particulate material and remain in addition to the active material, the binder and the conductivity additives. The cumulative irradiation dose E is thus in a range of 0.4 to 25 kJ / cm ² , preferably 1 to 10 kJ / cm ² .

Preferred pulse numbers for carrying out the process are in the range of 500 to 5,000 and the preferred power density is in the range from Be ^¬ 0.8 J / cm ² to 5 J / cm ^2. In particular, with energy densities in the range of 1.5 J / cm ² to 3.0 J / cm ² can produce conical microstructures.

Suitable laser devices for carrying out the procedural ^¬ Rens invention are lasers which have a homogeneous beam profile, in particular excimer lasers, solid-state, fiber and diode laser with homogenized beam profile and wavelength ranging from 150 nm to

400 nm, but especially such laser, with a Wavelength of 193 nm, 248 nm, 266 nm, 308 nm and 355 nm. In a preferred embodiment, laser pulses with a pulse length in the range of 500 ps to 500 ns are used.

An essential advantage of the invention is the applicability of the laser structuring on conventionally cast, consisting of powdered electrode material electrodes, which are currently used in lithium-ion batteries. The method can be scaled up to large FLAE ^¬ Chen and therefore also for industrial applicatio ^¬ gene, such as for lithium-ion batteries stability for the electric motors can be used advantageously.

The invention will be explained in more detail with reference to embodiments and the figures. Here are shown in detail:

Schematic representation of the invention

Manufacturing process by structuring a layer of unstructured composite electrode material by irradiation with laser pulses (under different laser parameters);

Scanning electron micrographs of conical surface structures in lithium cobalt oxide (LiCoC> ₂ );

Scanning electron micrograph (SEM image) of the electrode material according to the invention with conical surface structures (cross-sectional view);

Schematic representation of the structure a) of a thin-film electrode, b) of a laser-structured thin-film electrode and c) of an unstructured composite electrode (prior art) /

SEM image of the cross section of a conical element made of the electrode material according to the invention;

Specific discharge capacity of electrodes: unstructured (prior art) or structured according to the invention;

SEM images of a) unstructured electrode material (prior art) after 1000 electrochemical Cycles and b) according to the invention structured electrode material after 2000 electrochemical cycles.

In Fig. 1, the manufacturing method according to the invention is shown schematically. Referring to FIG. La) of the electrode material is for this purpose first a ^¬ μπι be in the form of a pressed thick film 1 having a thickness of 50 provided that at least 80% of an active material 3 made of a lithium-ion conductor, which as a particulate (granular) material is present, and at most 20% of conductivity additives 4, in particular Leitruß and graphite, as well as to binder 5 contains.

The active material 3 used here is powdered lithium cobalt oxide

(LiCo0 ₂ ), ie x = y = 0 and z = 1, or LiNii _{/ 3} Mni / 3Coi / 302 (NMC), ie x = y = z = 1/3. The active material 3 is mixed with a binder 5, preferably PVDF, and a solvent, preferably N-methyl-2-pyrrolidone (NMP), into a pourable viscous paste and preferably on a metallic substrate 2 serving as a current collector, preferably of aluminum or copper applied via a film casting process. The porosity of the electrode material is adjusted by means of a rolling calender to 30% to 50%, wherein the particle size of the electrode material in the range of 500 nm to 10 μπι moves.

According to Fig. Lb), the thick film 1 is removed from the electrode material 1 by irradiation with a homogeneous flat-top laser beam profile 6 in part. A large part of the removed active material deposits again in a process 7 on the surface which forms a base of the particulate material 1, thereby forming characteristic, conical (conical) structures 8 ', 8 ".

To carry out the process according to the invention, an excimer laser was preferably used. First, the described electrodes ^¬ material in the imaging plane of the laser optics was positioned, what JE but not necessary in every case. The invention Bear ^¬ processing of the material can be connected to ambient air and with process gases, preferably helium or argon, take place. . The excimer laser produces laser light with a wavelength of 248 nm for the production of the desired surface structures, the energy density was ^¬ represents, with an energy density of 1.5 to 3 J / cm ² has been found to be particularly suitable. Subsequently At ^¬ game set a pulse number of 1000 described here.

Then, the electrode material was be ^¬ aufschlagt with the laser radiation, either in continuous (Scan mode) or step-wise ^¬ processing (step-and-repeat mode). While the sample is moved continuously under the laser beam in the scan mode, an incremental offset takes place in the step-and-repeat mode after delivery of all laser pulses in order to process the sample over a large area. In the scanning process, therefore, the speed must be adjusted so that the required number of laser pulses, which is necessary for the structure production, reaches the irradiated area.

By suitable choice of the laser parameters can be size, lateral structure size and density (number of structures / cm ² ) of the conical structures 8, 8 ', 8''set .... This effect is shown schematically in FIG. 1c), which differs from FIG. 1b) in that fewer, but wider and higher conical structures 8 are produced here.

The influence of different laser parameters can be seen, for example, from FIG. 2, which shows scanning electron micrographs (SEM images) of surface structures according to the invention in the electrode material LiCoO ₂ , which are used in lithium-ion batteries. With the method according to the invention conical (conical) structures could be produced, the tip of which has a diameter of 0.5-3 μπι. The height of the structures produced here was in the range of about 5-100 pm. Structures with heights of 20-50 μm prove to be particularly advantageous. The physical effect of the process 7 of rearrangement and redeposition of ablated material can be seen in FIG. 3, which shows an SEM (cross-sectional view) of conical surface structures in individual NMC agglomerates. The conical structures 8, 8 ', 8 "were produced with homogeneous UV laser radiation from an excimer laser in the electrode material 1. In this case, it can be seen that as a result of the laser irradiation 6 in a region adjacent to a surface of the electrode material 1, a structure 8, 8 ', 8 "... of conical elements is impressed on the existing granular structure of active material 3 in the electrode material 1 which significantly increases the active area in the near-surface layer area. It can be seen that this conical element structure 8, 8 ', 8 "... is capable of processing high currents or stress peaks in lithium-ion cells.

Of great importance for fast charging or discharging of a lithium-ion cell is the fast lithium diffusion. Since this can be done very quickly in the electrolyte due to its high conductivity, the type of lithium diffusion in the solid state is crucial. For a high efficiency of the cell short diffusion paths are determining.

In Fig. 4, the prior art is shown schematically. Fig. 4a) shows the structure of a thin-film electrode, Fig. 4b) shows the structure of a laser-structured thin-film electrode and Fig. 4c) shows the structure of a composite electrode. Shown are in each case the substrate 2, the active material 3, the electrolyte 9, conductivity additives 4 and binder 5. The white arrows indicate typical diffusion paths of the lithium ions.

From a comparison between Fig. 4a) and Fig. 4b), it can be seen that the lengths of the diffusion paths of the lithium ions in a thin film can be significantly reduced by removing a portion of the active material at the surface of the thin film. On the other hand, in a composite electrode material 1, due to the porous structure filled with electrolyte, there are already inherently very short diffusion paths of the lithium ions. It is therefore not apparent from this finding from the known prior art how structuring of the electrode material 1 can take place and whether and, if so, what advantages such a method entails.

5 shows a SEM image at the cross section (transverse section)

a laser-structured electrode. Recognizable are the bright grains of active material 3. The darker regions are Conductive ^¬ keitsadditive 4, 5 and a binder encapsulant 10 is formed. As can be seen from FIG. 3, the conical structures 8 are formed from compact material (composite), which means that the additives 4, 5 present in addition to the active material 3 are retained in the conical structures 8. In this respect, therefore, surprisingly, no selective material removal takes place, as would be expected with composite materials.

The height of the structures produced can correspond to the layer thickness of the starting layer, and the rearrangement process even makes it possible to produce higher structures. However, structures with heights of 20-50 μτα are particularly advantageous.

The distances between the structures are in the range of

1-50 μιχι, preferably from 3 to 30 μπι. As the half width, i. the diameter of the structures at half structural height (FWHM) were values between 1 and 20 μm, preferably between 2 and 15 μm

reached .

FIG. 6 shows the specific discharge capacity of two LiCoC 2 -composite electrodes, of which one electrode (unfilled squares) consists of unstructured composite material, while the other electrode (filled squares) is an electrode material structured ^{according to the} invention. having . The first 10 cycles / g performed with egg ^¬ nem charge / discharge current of 28 mA, followed by 100 cycles of 140 mA / g. Finally, 10 cycles of 28 mA / g were examined. It can be clearly seen that the cycle stability is significantly improved by the present invention.

Fig. 7a) shows SEM images of unstructured electrode material (prior art) after 1000 electrochemical cycles, while shown in Fig. 7b) SEM images of the present invention structured electrode material after 2000 ^¬ electrochemical cycles. From this it can be seen that cracks form in the unstructured grain agglomerates after electrochemical cyclization, while no cracks were found in the layers structured according to the invention. Obviously, the present invention avoids mechanical degradation processes.

Claims

claims

1. electrode material for lithium-ion batteries in the form of a compressed layer (1) having a layer thickness of 10 μιτι to

1 mm, comprising at least 80% of an active material (3) made of a lithium-ion conductor, as a particulate

Material is present, and at most 20% of binder (5) and Leit ^¬ ability additives (4), characterized in that at least one of a surface of the layer (1) adjacent area a structure of conical elements (8, 8 ', 8''...), which are located on a substrate of the particulate material and from locally abraded material of the layer (1) are formed by Mate ralumlegung, wherein in addition to the active material (3) and the binder (5) and the conductivity additives (4) are preserved.

2. electrode material according to claim 1, characterized in that the pressed layer (1) has a layer thickness of 20 μπι to 150 μπι.

3. The electrode material according to claim 1 or 2, characterized in that the active material (3) a Li (Ni _x Mn _y Co _z ) 0 ₂ - compound, wherein 0 <x <1, 0 <y <1, 0 <z < 1 and

x + y + z = 1, contains.

4. electrode material according to one of claims 1 to 3, characterized ge ^¬ indicates that the particulate material has an average particle or Agglomeratdurchmesser of 0.5 μπι to 10 μιη, but at most of the layer thickness.

5. electrode material according to one of claims 1 to 4, characterized in that the height of the conical elements (8, 8 ', 8''...) has a value of 10 μιη to 50 μπι, but at most of the layer thickness, assumes.

6. electrode material according to one of claims 1 to 5, characterized in that the half-width of the conical elements (8, 8 ', 8' '...) has a value of 1 μπι to 20 μπι.

7. electrode material according to one of claims 1 to 6, characterized ge ^¬ indicates that the distance between two tips of the conical ^¬ shaped elements (8, 8 ', 8''...) assumes a value of 1 μπι to 50 μιη.

8. electrode material according to one of claims 1 to 7, characterized in that the layer (1) of the electrode material on a substrate (2) made of a metal or an alloy is applied, which has a thickness of 5 μπι to 50 μπι, but at most the thickness of the electrode material (1) possesses.

9. A method for producing an electrode material for lithium-ion batteries according to one of claims 1 to 8 by irradiating an in the form of a pressed layer (1) with a layer thickness of 10 μτα to 1 mm provided electrode material, the min ^¬ least 80% of a Active material (3) of a lithium ion conductor, which is present as a particulate material, and at most 20

% of binder (5) and of conductivity additives (4), with 100 to 10000 laser pulses (6), each having an energy density of 1.5 J / cm ² to 3 J / cm ² , whereby at least one of the surface The layer (1) adjacent region, a structure is generated, the conical elements (8, 8 ', 8''...), with a height of 20 μπι to 150 μπι, but at most the layer thickness, on a substrate form from the particulate material, wherein in the structure formed by material rearrangement in addition to the active material (3) and the binder (5) and the conductivity additives (4) are retained.

The method of claim 9, wherein for the laser pulses (6) a pulse length in the range of 500 ps to 500 ns is set.