DE102016225312A1 - Anode active material with glassy carbon coating - Google Patents

Abstract

The invention relates to a negative electrode active material of a lithium ion cell comprising an anode active material coated with a layer of glassy carbon (GC). Due to the GC coating, the active material forms a very stable SEI with high protection against decomposition of the electrolyte and degradation of the active material, which at the same time enables a good kinetics with regard to the transfer of lithium ions between the active material and the electrolyte. As a result, the life of the cell can be improved, and electrolyte solvents can be used, which would lead to rapid degradation in conventional anode active materials. Furthermore, the invention also relates to a negative electrode for a lithium-ion cell, and a lithium-ion cell comprising the negative electrode.

Description

Technical area

The invention relates to an anode active material having a glassy carbon coating, a negative electrode for a lithium-ion cell comprising the coated active material, and a lithium-ion cell comprising the negative electrode.

Technical background

At present, lithium and lithium ion batteries with liquid electrolyte, which essentially comprise a negative electrode (anode), a positive electrode (cathode) and an intermediate separator impregnated with a nonaqueous liquid electrolyte, are used in many mobile and stationary energy storage systems.

The anode or cathode each comprise at least one anode or cathode active material, which is optionally applied to a current collector using at least one binder and / or at least one additive for improving the electrical conductivity. The electrode structure of such a cell is generally porous, so that the contact area between the liquid electrolyte and the active material particles is as large as possible and an exchange of lithium ions is possible.

The anode active material generally has a very low electrochemical potential of less than 1 V over Li / Li +. Under these conditions, most of the eligible electrolytes are decomposed by reduction. After the first charge cycle, therefore, decomposition products of the electrolyte form one
SEI (solid electrolyte interphase) designated passivation layer on the anode, which inhibits further reactions between the electrolyte and the electrode.

Before the actual commissioning of the cell, the SEI is formed specifically in the so-called formation. The aim is to form an SEI that protects electrodes and electrolyte from further degradation and that changes as little as possible in the subsequent charge / discharge cycles. That the SEI should act as a passivation and prevent further electrolyte from being decomposed during formation of the cell after formation. On the other hand, the SEI must also have the highest possible permeability for lithium ions in order not to hinder the exchange of lithium ions between the active material and the electrolyte solution.

The liquid electrolyte used is a polar aprotic solvent in which a conducting salt such as LiPF _{6 is} dissolved. Carbonates such as ethylene carbonate (EC), propylene carbonate (PC), dimethlycarbonate (DMC), diethyl carbonate (DEC) or ethylmethyl carbonate (EMC) are usually used as solvents.

EC is preferred in view of the formation of the SEI and the high dielectric constant, which allows a good dissolving power for the conductive salt, but the melting point of EC is above room temperature. PC also has a high dielectric constant and a much lower melting point, and therefore can provide high dissolving power with low viscosity and good low temperature properties. However, PC can co-intercalate into the layered structure of the graphite active material, resulting in degradation of the electrode. Open-chain carbonates such as DMC, DEC, or EMC have low melting points and low viscosity, which allows for high ion mobility, but due to their lower dielectric constant, the solvency of the conductive salt is reduced.

In order to achieve acceptable properties both in terms of viscosity and ion mobility, especially at low temperature, as well as in terms of SEI formation and solvency for the electrolyte salt, mixtures of EC with PC and / or open-chain carbonates such as DMC, DEC are generally used or EMC used, optionally in the presence of other additives. On the one hand, however, since a high EC content increases the viscosity and on the other hand a high proportion of open-chain carbonates can degrade the solvency and SEI formation, such blends are typically compromises attempting to meet all the requirements in the stress field described above to become.

The resulting SEI usually has by far not 100% passivation, and it comes during the life of the cell to still progressive decomposition of the electrolyte. This is in practice a major cause of aging of lithium-ion cells. In addition, the production of multicomponent solvents is expensive, and optimization, for example, in terms of low-temperature behavior and ion mobility requires compromises with regard to the other requirements.

Another difficulty is the frequent kinetic inhibition of intercalation / de-escalation due to the SEI in the graphite active materials used. This can lead to high overvoltages, resulting in a poorer efficiency result and the risk of deposition of metallic lithium on the anode brings with it. Metallic lithium can cause internal short circuits and therefore represents a high security risk.

Disclosure of the invention

task

In view of the stress field described above, there is a need for an anode active material which allows SEI with good passivation without undue compromises in the formulation of the electrolyte solvent and which allows a good exchange rate for lithium ions between electrolyte and electrode.

According to the invention, this object is achieved by coating the active material with glassy carbon (Glassy carbon, GC). For example, GC can be made by pyrolysis of polymers, and has a glassy, amorphous structure with high hardness and rigidity compared to graphite. The ability to intercalate lithium is lower than that of graphite. Pure GC as the active material therefore has a lower energy density and a lower negative potential compared to graphite, but also a higher reaction kinetics and a lower sensitivity to degradation by side reactions such as co-intercalation of PC.

The invention is based on the idea of applying a GC layer to an anode active material, such as graphite, which acts as a protective layer and inhibits undesired side reactions, but at the same time has good exchange rates for lithium with high reaction kinetics. This allows the formation of a very stable SEI with high passivation and low overvoltage. As a result, the life of the cell and the stability against degradation can be increased, and fewer compromises are required in terms of the composition of the electrolyte solvent. For example, it is possible to use a solvent with a high PC content in view of a good solubility for the conductive salt and low viscosity, since the GC layer significantly reduces the risk of degradation due to co-intercalation of PC.

Summary of the invention

The present invention thus provides an active material for the negative electrode of a lithium ion cell comprising an anode active material coated with a layer of glassy carbon (GC).

As the anode active material, carbon materials other than GC, alloy formers and oxide conversion materials may be considered. Preferably, the anode active material is graphite.

Further aspects of the invention relate to a negative electrode for a lithium-ion cell, in which the coated active material, if appropriate in the presence of at least one binder and / or at least one additive for increasing the electrical conductivity, is applied to a current collector.

Another aspect of the invention relates to a lithium ion cell comprising the negative electrode.

Due to the GC coating, the active material according to the invention forms a very stable SEI with high protection against decomposition of the electrolyte and degradation of the active material, which at the same time enables a good kinetics with regard to the transfer of lithium ions between active material and electrolyte. As a result, the life of the cell can be improved, and it can also be used electrolyte solvents such as PC, which would lead to rapid degradation in conventional anode active materials.

list of figures

• 1 shows the potential profile during the galvanostatic cyclization of GC at 0.1 C according to Reference Example 1.
• 2 shows the corresponding potential profile for graphite.
• 3 shows the specific capacity of GC in the cyclization test according to Reference Example 2.

Detailed description

In the following, the individual features of the present invention will be described in more detail.

The terms "anode" and "negative electrode" are used synonymously in the present specification, as are the terms "cathode" and "positive electrode".

The volume-average particle diameters described below relate in each case to the primary particle diameters. They can be determined, for example, by laser scattering, in particular according to ISO 13320.

active material

Anode active materials usable in the invention are not particularly limited, and may be made of carbon materials other than GC, Alloy formers and oxidic conversion materials are selected.

For example, graphite such as natural graphite or synthetic graphite, hard carbon or soft carbon can be used as the carbon material. Preferably, graphite is used.

Due to the ordered layer structure, graphite has a high intercalation capacity and a low potential compared to other carbon materials. On the one hand, these properties cause a high energy density, but on the other hand, they can also lead to a susceptibility to degradation (for example by co-intercalation of solvent constituents such as PC) and to a deposition of metallic lithium. The present invention can mitigate this susceptibility while retaining the advantages, so that the use of graphite is particularly preferred.

The volume-average particle size is 50 μm or less in the case of carbon materials such as graphite, for example. Typical values may preferably be between 0.01 and 20 μm, more preferably between 0.1 and 10 μm.

Alloy formers are elements such as Si, Sn, Zn or P, preferably Si, which can reversibly form alloys with lithium. Alloying provides a stoichiometrically significantly higher lithium uptake capacity than the carbon based intercalation materials. Consequently, alloy formers are advantageous in terms of energy density. However, this high absorption capacity also leads to large volume differences between the lithiated and the delithiated state, which during the cyclization can lead to mechanical stresses on the electrode structure and to damage and detachment of the SEI. Again, the GC coating according to the invention can mitigate this risk and stabilize the SEI. With regard to the risk of mechanical stresses due to the volume change, in the case of alloying agents, smaller particle sizes of 20 μm or less are preferred, preferably 0.01 to 10 μm, in particular 0.05 to 5 μm.

One way to overcome the large volume change in cyclization is to use composite materials of an alloying agent and a softer carbon material, such as graphite or soft carbon, which is believed to act as a buffer. Such composite materials, for example Si / C, can likewise be used according to the invention. In the case of such composite materials, the particle size of the agglomerated composite particle may be the same as that of pure carbon-active materials. The particle size of the primary particles of the alloying agent contained in the composite particles, however, is significantly smaller.

Conversion materials such as those of the type MO, M ₃ O ₄ or MM ' ₂ O ₄ , wherein M and M' are each a transition metal, such as FeO, Fe ₃ O ₄ and FeMn ₂ O ₄ , or SnO ₂ are also contemplated , In such a conversion material, lithiation involves a conversion reaction in which the metal is reduced, optionally to zero oxidation state, and lithium oxide is formed. The stoichiometric lithium content is again greater than that of the intercalation materials, which allows for high energy densities but also results in large volume differences between lithiated and delithiated phases. In addition, the conversion reaction is generally associated with major phase and crystal structure conversions and concomitant atom rearrangements, resulting in slower kinetics.

For this reason, the particle size of the conversion active material is preferably small, for example, 20 μm or less, preferably 0.01 to 10 μm, especially 0.05 to 5 μm.

Similar to the alloying agents, it may also be beneficial for the conversion active materials
To use composite materials, in particular with a carbon-active material such as graphite, graphene or soft carbon, to improve the reaction kinetics and the electrical conductivity and to absorb the mechanical stresses caused by the structural transformations of the conversion material. Again, the particle size of the agglomerated composite particle may preferably be in the same range as pure carbon-active materials; the primary particle size of the conversion material particles contained in the composite material, however, is preferably much smaller.

GC-coating

According to the invention, the active material is coated with a layer of glassy carbon (Glassy Carbon, GC).

GC is generally produced by pyrolysis or carbonization of polymers. The coating according to the invention can be obtained, for example, by applying a polymer coating to the active material as precursor, followed by pyrolysis. Suitable processes for the preparation of vitreous carbon coatings are, for example, in US 3,854,979 and US 4,816,338 described.

US 3,854,979 concerns the application of GC layers to medical implants made of carbon or ceramic. The process disclosed therein involves heating and partially pyrolyzing a precursor polymer material selected from halogenated polymers such as PVC and natural organic materials such as petroleum pitch to obtain a pitch-like material having the approximate formula C _n H _n , mixing the pitch-like material an aromatic solvent to obtain a slurry, coating the substrate with the slurry, and firing the substrate. US 4,816,338 describes a similar process especially for the coating of graphite.

The carbonizable precursor polymers according to the invention are not particularly limited. On the one hand thermosetting resins are suitable, for example based on aromatic compounds such as phenolic resins such as phenol / formaldehyde resins, resins based on furfuryl alcohol or the like. Furthermore, vinyl or acrylic polymers such as e.g. Polyacrylonitrile or polyvinyl chloride can be used.

The active material according to the invention can be obtained by first coating the particles of the active material with the precursor polymer, for example by impregnation with a polymer solution or a slurry, optionally followed by further crosslinking of the polymer layer. Subsequently, the thus coated active material is optionally dried and degassed, and finally carbonized to form a GC layer.

The carbonization can be carried out in several stages, for example in a first stage at about 500 to 800 ° C to eliminate hydrogen, water or halogen, and a second stage at about 1000 to 1400 ° C to those formed in the first stage, typically to further condense largely aromatic hydrocarbon structures. The temperature and the time of carbonization may be suitably selected depending on the kind of the precursor polymer and the active material. The maximum temperature may optionally be limited by the stability and melting point of the active material. The carbonization can be carried out under reduced pressure and / or inert gas atmosphere.

The coating according to the invention provides a very good protective effect even at low layer thicknesses in the nm range. In view of high energy density and good kinetics, therefore, the layer thickness is preferably 1000 nm or less, more preferably 500 nm or less, especially 5 to 250 nm, more preferably 10 to 100 nm. Layer thicknesses in this range can be determined by suitably selecting the concentration of the precursor polymer or the quantitative ratio of active material to precursor polymer can be adjusted.

The layer thickness may be calculated from the specific surface area of the active material and the applied coating amount, or it may be e.g. by secondary ion mass spectrometry (TOF-SIMS), X-ray photoelectron spectroscopy (XPS)) or Focused Ion Beam (FIB) microscopy.

Electrodes and cells

Apart from the use of the GC-coated active material, the anode according to the invention largely corresponds to a corresponding conventional anode.

The preparation can be carried out for example by a known slurry coating method. In this case, a slurry of the active material, optionally in combination with a binder such as vinylidene fluoride (PVdF), applied in a suitable solvent to a current collector and dried. If necessary, a conductivity additive such as conductive carbon black may be added to improve electrical conductivity. The current collector is typically copper foil.

In order to improve the ion exchange with the electrolyte and to provide buffer volume for intercepting the volume change of the active material upon cyclization, the electrode typically has a porous structure. The remaining porosity of the electrode in the maximum lithiated state, for example, be 10-20%.

The anode according to the invention can be used in a manner known per se in lithium-ion cells with nonaqueous electrolyte. The GC coating of the active material gives a very stable SEI with good passivation effect, and low overvoltages and high exchange rates for Li ⁺ can be achieved. In addition, these advantages are not associated with excessive compromises in terms of electrolyte composition. Since GC effectively inhibits the co-intercalation of PC, it is possible according to the invention to use electrolytes with a comparatively high PC concentration in order to achieve high conductive salt concentrations and at the same time good low-temperature properties, while keeping the risk of degradation low.

In particular, according to the invention, pure PC can also be used as the electrolyte solvent, which simplifies the production of electrolytes. EC is not required, but can optionally up to 10 % By weight, based on the total weight of the solvent, of EC. For certain applications, for example for use at low temperatures, optionally up to 25% by mass of linear carbonates such as DMC, DEC or EMC may be added. Furthermore, small amounts of up to 5% by weight, preferably up to 3% by weight, of further additives may also be present. In particular, SEI film formers such as vinyl ethylene carbonate (VEC), vinylene carbonate (VC) or fluoroethylene carbonate (FEC), which polymerize upon formation to promote SEI formation, may be added as additives. As conductive salt, for example, LiPF ₆ can be used in a concentration of about 1.0 to 1.2 mol / l.

Examples

Reference Example 1

In order to verify the different properties of glassy carbon (GC) and graphite, first reference tests were carried out with pure GC compared to graphite powder.

To this was added GC powder (available from Sigma-Aldrich, spherical with 2-12 μm diameter, Gaussian size distribution with measured maximum at 6.5 pm) or graphite powder (MAG-D20, available from Hitachi Chemical) with 2 wt% CMC / SBR as a binder and 2 wt% super-C64 carbon (Timcal) as a conductivity additive dispersed in demineralized water to make a slurry. The slurry was applied to copper foil ( 10 μm thick) in an amount of about 7 mg / cm ² , based on the solid substance, pre-dried and calendered to a porosity of about 35%. Subsequently, test electrodes were punched out and in a vacuum oven 12 Hours dried at 120 ° C.

The electrodes were grown in laboratory half cells (Swagelok type T cells with Cellgard 2325 PP / PE separator) introduced. Lithium foil was used as the counterelectrode and the electrolyte used was EC: EMC 3: 7 with 2% by weight of VC and 1 M of LiPF ₆ . The formation and cyclization was performed with a laboratory cell tester (Maccor series 4200 ) carried out. For this purpose, the electrodes were lithiated or delithiated with a current of 0.1C.

1 shows the result for GC. As can be seen from the curves, GC has a comparatively low capacitance density of about 110 mAh / g and a relatively heavily load-dependent potential of 0.4 V on average compared to Li. The curves are close together, which results in low overpotential and thus high reaction kinetics displays.

By comparison, graphite has, as in 2 On the other hand, the distance between charge and discharge curve is greater than GC, which is a higher Indicates overvoltage and a slower reaction kinetics.

Reference Example 2

In order to investigate the long-term stability of GC, the GC electrode described in Reference Example 1 after the formation (FIG. 4 Cycles at 0.1C) undergo cyclization with a current of 1C. 3 shows as a result the capacity as a function of the number of cycles. It can be seen that the capacity loss after 400 cycles is only about 1.7%. This suggests a lifetime (80% SOH) of over 4000 cycles, which is extremely high.

These results show that GC is very stable in cyclization and shows little degradation phenomena.

Since the SEI represents the main reason for the aging of anode half-cells, it can be concluded that GC has a very stable SEI.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• US 3854979 [0032, 0033]
• US 4816338 [0032, 0033]

Claims (8)

A coated negative electrode active material of a lithium ion cell, comprising: an anode active material, a coating of glassy carbon (GC). Coated active material according to Claim 1 wherein the anode active material comprises a carbon active material selected from graphite, hard carbon and soft carbon, an alloy former selected from Si, Sn, Zn and P, a conversion material selected from FeO, Fe ₃ O ₄ and FeMn ₂ O ₄ and SnO ₂ , or a composite material of one or more of the aforementioned active materials. Coated active material according to Claim 2 wherein the anode active material is selected from carbon active materials and composite active materials made of carbon active materials with an alloy former or a conversion active material, and the volume average diameter is between 0.01 and 20 μm. Coated active material according to any one of Claims 1 to 3 , wherein the layer thickness of the GC coating is 5 to 1000 nm. Coated active material according to any one of Claims 1 to 4 wherein the GC coating is obtainable by applying a precursor layer of a carbonizable polymer to the anode active material followed by carbonization. A negative electrode for a lithium ion cell comprising the active material according to any one of Claims 1 to 5 and a current collector. Lithium-ion cell with non-aqueous liquid electrolyte, the negative electrode according to Claim 6 includes. Lithium ion cell according to Claim 7 in which the liquid electrolyte comprises a conducting salt and a solvent, the solvent comprising the following constituents, in each case based on the total weight of the solvent: - up to 10% of ethylene carbonate; up to 25% of linear carbonates selected from diethyl carbonate, dimethyl carbonate, methyl ethyl carbonate and a mixture thereof; up to 5% of additives to promote SEI formation selected from vinyl ethylene carbonate, vinylene carbonate, fluoroethylene carbonate and a mixture thereof; and - the balance of propylene carbonate.

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