DE102011012552A1 - Lithium-ion battery - Google Patents

Abstract

A lithium-ion accumulator is specified, with at least one cell in which at least one positive electrode, one negative electrode, a separator and an electrolyte are accommodated, and with at least one glass-based material that is in contact with the electrolyte under fundamentally Retention of cell function is leachable. In this way, substances can be released which are particularly suitable for HF gettering.

Description

The invention relates to a rechargeable lithium-ion secondary battery, having at least one cell in which at least one positive electrode, a negative electrode, a separator and an electrolyte are accommodated.

Lithium-ion batteries have at least one cell in which a positive and a negative electrode (anode / cathode), a separator which prevents the electrical short circuit, and an electrolyte are accommodated. The electrolyte is usually a liquid electrolyte, more rarely a polymer electrolyte that facilitates Li-ion migration.

Lithium-ion cells are basically advantageous energy storage with high life, low self-discharge and high specific storage capacity with an operating window under normal conditions of about -20 ° C to + 85 ° C.

Future applications of lithium-ion batteries require an improvement in the accumulators in terms of safety, cost and weight. The latter is particularly necessary with regard to increasing the specific energy or power density.

Also, the life of the battery in both the cyclization and in terms of general life (calendar life) plays an important role. The life-impairing effects are, among many others, for example, the increasing formation of a surface layer on the anode (SEI layer), with the consequence of increasing increasing the internal resistance.

Furthermore, the degradation of cell materials or components by the formation of HF is a significant mechanism that interferes with the lifetime. The formation of HF depends in particular on the water content in the cell, the water reacting with the fluorine-containing electrolyte salt. Moisture can occur via the cell production process or through uncontrolled diffusion of moisture through the cell housing. In addition to damage in the form of HF formed, fluoride can also lead to the formation of poorly soluble LiF. By the precipitation of LiF, the lithium from the electrolyte is no longer available for the charge transport in the cell, which leads to the minimization of the conductivity and thus also of the cell performance.

Exemplary reactions or reaction equilibria of the conducting salt are: LiPF ₆ ↔ LiF ↓ + PF ₅ PF ₅ + H ₂ O ↔ 2HF + POF ₃

In general, additives are added to the liquid electrolyte in order to prevent these and other interfering cell chemical processes. For example, low temperature additives, SEI formation enhancers or controllers, flame retardant additives, wetting additives, anionic receptors, overcharge additives, water / acid scavengers, additives for smooth Li deposition, etc. are added. The additives are in practice exclusively organic, aromatic or organometallic compounds or molecules which often contain halogens or sulfur groups. In addition, additives with organic components are partially toxic - both in handling during production, as well as in uncontrolled application. They are also expensive and highly flammable or at least only very little temperature-resistant or easily soluble. An example of this is heptamethyldisilizane, an additive for deactivating hydrogen fluoride (HF), which already dissolves at 85 ° C. as a fluorinated salt ( J. of Power Sources 189 (2009) 685-688 ).

In a few cases, oxidic additives such. As TiO ₂ , called, however, could not prevail in the application due to their electrochemical instability.

Another disadvantage of the current additive solutions is the lack of flexibility with regard to the integration of the additives mentioned. In the form of a liquid component or alternatively in the form of a solid which is completely soluble in the electrolyte, it is not possible if necessary to offer the effect more effectively locally and thus "on site". An example of this is the use of a processable because integratable solid state, functional additives to the local protection of the RF sensitive cathode material LiMn ₂ O ₄ (LMOs) prior to HF. As in S.-T. Myung et al., "Improvement of cycling performance of Li1.1Mn1.9O4 at 60 ° C by NiO addition for Li-ion secondary batteries", Electrochimica Acta 51 (2006) 2912-2919 , described, for this purpose, NiO is added. This can optionally be integrated into the composite by coating the electrode particles or the entire electrode or by admixing the cathode material. Disadvantageously, the HF-stable coating material is transition metal-containing and thus cost-driving.

Out Y.-K. Sun et al., "Electrochemical performance of nano-sized ZnO-coated LiNi0.5Mn1.5O4 spinel as 5V materials at elevated temperatures", Electrochemistry Communications 4 (2002) 344-348 For example, the addition of ZnO is known as a coating material on the cathode for the purpose of RF gettering. In contrast to a pure inert protective coating, ZnO obviously acts as a functional additive, ie it reacts with the HF present.

From the EP 0 903 798 A1 Furthermore, the addition of BF _{3 is known} as a reaction additive for improving the electrolyte chemistry. In the EP 1 567 452 B1 or the DE 69700312 T2 the addition of B ₂ O _{3 is described} as an additive to a rechargeable Li-ion cell. The addition increases the lifespan in terms of preservation of assets, but the background remains unclear. The integration of the B ₂ O ₃ can take place either by adding to the liquid electrolyte or by integration into anode or cathode or separation composite. The material can therefore be integrated at all locations of the cell, provided that it is only exposed to the electrolyte. A reference to the gettering of HF is not established. The disadvantage of using pure boron oxide is the introduction of significant amounts of water, which is adhesively bonded to the surface. This hygroscopy must be minimized by appropriate, costly drying steps otherwise the life of the cell is significantly reduced.

Alternatively, LMO can also be coated with a glass of the type Li ₂ OB ₂ O ₃ (LBO) (cf. C. Li et al., Cathode materials modified by surface coating for lithium ion batteries, Electrochimica Acta 51 (2006) 3872-3883 ). The glass has the advantages of good wettability, low melt viscosity, good ionic conductivity, and protection against reactive attack by the liquid electrolyte.

The solutions described here are so passivating protection solutions of batteries and no solutions for harmlessness of HF z. B. in the vicinity of the cathode material. Accordingly, if the grain is completely surrounded, the coating material must be necessarily ion-conductive.

Against this background, the invention has for its object to provide an improved lithium-ion battery, which has a cost-effective, non-toxic, temperature-stable additive to improve performance. Preferably, it should also be possible to render HF harmless.

This object is achieved by a lithium ion secondary battery having at least one cell in which at least a positive electrode, a negative electrode, a separator and an electrolyte are accommodated, and at least one glass-based material in contact with the electrolyte can be leached under the fundamental retention of cell function.

The object of the invention is completely solved in this way.

According to the invention, the addition of the glass-based material leachable in contact with the electrolyte enables the release of active substances into electrolytes, whereby the cell performance can be increased. For the purposes of this application, a glass-based material is understood to mean either a glass or a glass ceramic, ie a glass-crystallite structure produced by a melting process and a controlled temperature treatment, either during the cooling of the melt or in a downstream process step. Also, a production of a glass or a glass ceramic by a sol-gel process is conceivable.

The use of a glass-based material is an inorganic additive that is non-toxic, temperature-stable and long-term stable. In addition, a very cost effective solution can be achieved. The material can be tailored to the particular application in order to deliver certain products in the electrolyte.

By "leachable" is meant in this context that the material in contact with the electrolyte has a certain mass loss per unit time. In this case, the leaching rate in contact with the electrolyte may initially be at least about 1 μg / h, in particular 10 μg / h, or at least 25 μg / h. Such leaching already occurs during normal operation of the cell, i. H. under normal operating conditions (ambient temperatures) even without current delivery.

In principle, the leaching process can be designed so that the composition of the mass-depleted material does not change as a result of the leaching. In such a case, it is a congruent leaching of the material.

However, if the composition of the solid changes as a result of the leaching or if the leached substance does not correspond in its chemical composition to that of the starting material, this is referred to below as incongruent leaching or selective leaching. This means that one or more components are preferably dissolved out of the material.

Due to the selective leaching of the material, in this case components can preferably enter the electrolyte which perform special functions in cell chemistry. By selectively leaching the material, the functional efficiency can be increased or tailored.

Depending on the material used, the leaching rate may remain approximately constant over a defined period of time or may change over time, especially with time.

According to a further embodiment of the invention, the material in contact with the electrolyte releases a getter substance for gettering of HF. By "gettering" is to be understood that unwanted or even harmful substances are removed from the system and thus rendered ineffective or harmless. However, the cell remains functional. Also, the arrangement may in particular be such that the material binds fluorine to form species inactive for the electrolyte.

So it does not necessarily have a getter substance released from the material first. It is also possible to bind fluorine directly with the glass substances, forming inactive species or substances for the electrolyte.

In this way, the unfavorable for cell performance fluorine is bound and converted into harmless species.

At this point is on the Japanese patent application JP 2005-11614 directed. Herein, the immobilization of Li by reaction with a SiO ₂ -containing glass in the case of excessive generation of HF, z. B. at too high operating temperatures described. In the case described, the separator consists of 50 to 90 wt .-% of glass, which consists of 40 to 90 wt .-% of SiO ₂ , ie predominantly of SiO ₂ . In the case of excessive release of HF, SiO ₂ reacts directly with HF, resulting in the poorly soluble Li ₂ SiF ₆ . The electrochemical cell activity is thus irreversibly contained, ie the cell becomes inoperative. In addition, the glass provided is not a "leachable" glass in the sense of this application, since the glass according to the JP 2005-11614 in normal operation has no mass loss.

According to a further embodiment of the invention, the material exposes, in contact with the electrolyte, a substance which contains boron, in particular oxygen-bound boron is liberated.

It has been shown that the release of boron is particularly suitable for the gettering of HF or the presence of boron in the glass favors the binding of harmful fluorine.

According to a development of this embodiment, the material sets in contact with the electrolyte B ₂ O ₃ , which is then complexed as a cell-compatible boron-fluorine species present in the electrolyte. The initial leaching rate in contact with the electrolyte may be about at least 3 μg / h, especially at least 5 μg / h.

It has been found that the release of B ₂ O ₃ at such a rate enables complete gettering of HF and thus ensures a significant improvement in cell performance.

According to a further development of this embodiment, the material releases into contact with the electrolyte Al ₂ O ₃ . Here, the initial leaching rate in contact with the electrolyte may be at least about 1 μg / h, in particular 2.5 μg / h.

According to a further embodiment of the invention, the material has phasesentmischte area. In this case, the phase-separated regions are preferably leachable. A glass ceramic can also be used, ie a glass-crystallite structure produced by a melting process and a controlled temperature treatment. In this case, the crystallites are preferably leachable.

As a result, the leaching of the material is simplified or can be made more selective. The use of phase-separated glass or phase-demixed glass-ceramic makes it possible to achieve a particularly good adaptation to a wide variety of requirements. By means of suitable T-t profiles during and / or after the cooling of the glass, the constituents labile with respect to the electrolyte attack can be particularly "applied" in the solid state.

The leachable material according to the invention can in principle be integrated in any way in the cell.

For example, the material can be accommodated in the cell as a powdery additive. It may be integrated in the region of a separator, for example as a coating on a polymer membrane, be it exclusively or together with other inorganic or organic particles. It may also be integrated as part of a filler-composite mixture. It can also be integrated as a coating of an electrode. The mixing in of particles into the electrode material as well as co-coating on an arrester foil is also conceivable. Also conceivable is the coating of a housing surrounding the cell. Other variants are conceivable.

Thus, the material may also be part of a separator membrane, be part of a separator nonwoven or a separator composite. A "nonwoven" in this context is understood as a fiber waffle. By "membrane" is meant porous polymer components made by wet or dry manufacturing processes. Separator composites are intimate mixtures of polymer and filler which are applied as a slurry to a makeshift sheet and cured to form pores.

In a preferred embodiment of the invention remains after the leaching of the material a resistant to the electrolyte, temperature-resistant residual phase, in particular a residual glass phase.

It may be here z. B. is a porous and thermally stable residual phase, which remains as a material for a heat-resistant coating, infiltration or as admixture in a composite.

Thus, the remaining phase may be, for example, a very light, porous, skeletal structure. Thus, this structure, as long as it is bound to a carrier such as a binder or the like, will continue to be on the spot. The material or the residual phase remaining after the leaching can thus be approximately part of a separator, an electrode, a housing coating or the like.

Inventive materials preferably have a certain reactivity towards solvent constituents of the electrolyte or towards the Li-conducting salts present therein. Solvent components may be carbonated by way of example, such as ethylene carbonates, dimethyl carbonates or diethyl carbonates.

According to a further embodiment of the invention, the material contains at least 1% by weight of B ₂ O ₃ , preferably more than 3, particularly preferably more than 4.5% by weight of B ₂ O ₃

By the release of B ₂ O ₃ from the material, an HF gettering can preferably be achieved.

Preferably, the material contains at least SiO _{2 in} addition to boron.

A possible material which is suitable for the invention has, for example, at least the following constituents (in% by weight based on oxide): SiO ₂ 45-60, B ₂ O ₃ 5-30, Al ₂ O ₃ 5-30.

Furthermore, the material z. B. 20-30 wt .-% of BaO. Furthermore, the material may further contain 0 to 10% by weight of ZrO ₂ , in particular at least 0.1% by weight, in particular at least 1% by weight, of ZrO ₂ .

A material suitable according to the invention has in particular the following composition (in% by weight based on oxide): SiO ₂ 48-58, B ₂ O ₃ 7-13, Al ₂ O ₃ 7-13, BaO 22-28 ZrO ₂ 0.1-8.

The material may in principle contain alkali oxides but, apart from incidental impurities and apart from Li ₂ O, it is preferably free of alkali oxides. It may therefore contain Li ₂ O, but preferably no Na ₂ O, K ₂ O, Rb ₂ O, Cs ₂ O. Random impurity in this context means, depending on the raw materials used, a random impurity which is less than 0 , 5 wt .-%, in particular less than 0.1 wt .-%, in particular less than 0.05 wt .-% or even less than 0.01 wt .-% is.

Otherwise, there would be a risk of reducing the life of the cell due to the irreversible incorporation of Na or K, Rb or Cs in the crystal structure of the electrode materials.

It is understood that the features of the invention to be explained below and those to be explained below can be used not only in the particular combination indicated, but also in other combinations or in isolation of the invention, without departing from the scope of the invention.

The invention will be explained in more detail with reference to Ausführungsbesispielen with reference to the drawings. Show:

1 a schematic representation of a lithium-ion battery according to the invention;

2 cyclic voltammetric measurements on a material according to the invention in comparison to comparative examples;

3 NMR measurements on a material according to the invention in comparison to comparative examples and

4 a polished sample of a phase-separated material (AB3), with clearly recognizable areas of the spinodal segregated Li-borate.

1 shows a schematic representation of an exemplary lithium-ion battery, the total with 10 is designated. The lithium ion accumulator 10 has a housing 18 with two electrode feedthroughs 12 on. The electrode feedthroughs 12 are with a first electrode 14 , which consists of Cu and coated with anode material, or with a second electrode 16 connected, which may be coated with a cathode material Al-Ableiterfolie. Exemplary anode materials are carbonaceous phases such as hard carbon, graphite or Li titanate, exemplary cathode materials layer oxides such as LiCoO ₂ , spinels such as LiMn ₂ O ₄ or olivine phases such as LiFePO ₄ or their mixed crystal phases with other transition metal cations or mixtures thereof.

Between the electrodes 14 . 16 there is a separator 22 , which may be a polymer membrane or polymer nonwoven that is (co-) coated or (co-) infiltrated with glass particles. In the present case, the lithium-ion battery has only a single cell, which of a housing 18 is enclosed and with electrolytic fluid 20 is filled. According to the invention, a leachable glass is now integrated into the cell, which, for example, as a coating on at least one of the electrodes 14 . 16 or on the separator 22 can be done. Submixing of particles into the electrode material as well as co-coating of an arrester foil is a conceivable form of integration. In the present case is only with the numeral 24 a coating by the leachable glass according to the invention on the inner surface of the housing 18 indicated. With such a design results in a particularly large glass surface and thus high efficiency. As already indicated above, any other forms of integration are possible for the glass according to the invention.

Various embodiments (AB) in comparison with comparative examples (VB) are summarized in Table 1.

example 1

As the first embodiment, a glass (AB4) having the following composition was used (in% by weight based on oxide): SiO ₂ 55 B ₂ O ₃ 10 Al ₂ O ₃ 10 BaO 25th

To check the HF gettering of a material in lithium-ion cells, electrochemical investigation methods can be used. In the process, current-voltage curves are recorded which indicate a reacting species (eg HF) in the form of peaks and overall higher current values. In the field of lithium-ion cells, cyclic voltammetry is usually used as the electrochemical method. This cycle is driven, which detect the back and forth reaction of a species. In the present case, the cycles were started at 3 V, went to 0.05 V and back to 3 V. From the applied voltage results in the system, a current that is measured.

In order to be able to carry out cyclovoltammetric measurements of materials in the vicinity of lithium-ion cells, these materials must be brought into suitable form in contact with suitable electrolytes. In the present example, an electrolyte based on LiPF _{6 was} used.

For the electrochemical measurements usually a three-electrode arrangement is used. It requires a working electrode (with the material to be tested), a counter electrode made of lithium and a reference electrode made of lithium. To prepare the working electrode, a mixture of the material to be investigated, a conductive carbon black and a suitable binder (eg PVDF) is first introduced into a suitable solvent (eg NMP). For the mixture of material, Leitruß and binder can z. B. a mixing ratio of 80: 12: 8 are used. For the ratio of mixture to solvent, a ratio of 25:75 can be used. The liquid mixture is applied to a suitable material (eg copper) (eg by doctoring). The electrode is then dried and installed with Li reference electrode and Li counter electrode in a suitable measuring cell. Due to the use of lithium and LiPF ₆ for the construction of the measuring cell, it is necessary to work under protective gas in a glovebox. After completion of the measuring cell, the three electrodes are connected to a suitable potentiostat and the current-voltage curves are recorded. In this case, HF appears in the form of peaks (between 2.0 and 3.0 V) and increased currents in this range.

In 2 Two examples of traces are shown showing that RF gettering occurs with the use of appropriate materials. The following figure shows the current-voltage curves of two different working electrodes. If the working electrode is only coated with soot and binder (without glass or alternative material) (dashed line), the largest currents and pronounced peaks can be seen. If, on the other hand, a suitable glass AB4 is brought onto the working electrode together with soot and binder (solid line), the currents are much lower and no pronounced peaks can be detected. This shows the successful RF gettering through the glass.

By means of NMR spectroscopy, the chemical resonance properties of atomic nuclei can be used to characterize their chemical environment so that molecular structures can be identified and compounds identified. Furthermore, compounds can be quantified. As part of a storage test in the liquid electrolyte of a lithium-ion battery (here 1 M LiPF ₆ in EC: DMC = 1: 1 (m / m) for 30 days at 60 ° C) with the addition of 1000 mg / kg of water assume that hydrolysis of the LiPF ₆ produces free hydrogen fluoride (HF). The HF formation is strongly supported by the addition of Waser. If, within the scope of this stability test, the material claimed in the abovementioned glass AB4 is stored in the moistened electrolyte for a relatively long time, the contents of the dissolved fluorine-containing compounds can be determined by ¹⁹ F-NMR, giving concentration-time profiles of the various fluorine -Species. Despite the addition of water induced hydrolysis of LiPF ₆ , which leads to significant HF formation in the absence of glass B, no free fluoride ions (indicator of HF) were found after 30 days. This demonstrates the properties of HF gettering.

This property is not common to all glassy materials, but it suffice only glasses of certain compositions, which are preferably at least boron-containing. The results of time-dependent NMR measurements are in 3 shown. On the ordinate, the content of the fluoride in the electrolyte (here the relative proportion of total dissolved fluorine species) is plotted as a measure of the HF formation. It turns out that in the presence of glass AB4 no HF is released.

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

• EP 0903798 A1 [0012]
• EP 1567452 B1 [0012]
• DE 69700312 T2 [0012]
• JP 2005-11614 [0028, 0028]

Cited non-patent literature

• J. of Power Sources 189 (2009) 685-688 [0008]
• S.-T. Myung et al., "Improvement of cycling performance of Li1.1Mn1.9O4 at 60 ° C by NiO addition for Li-ion secondary batteries", Electrochimica Acta 51 (2006) 2912-2919 [0010]
• Y.-K. Sun et al., "Electrochemical performance of nano-sized ZnO-coated LiNi0.5Mn1.5O4 spinel as 5V materials at elevated temperatures", Electrochemistry Communications 4 (2002) 344-348 [0011]
• C. Li et al., "Cathode modified by surface coating for lithium ion batteries", Electrochimica Acta 51 (2006) 3872-3883 [0013]

Claims (24)

A lithium-ion secondary battery, comprising at least one cell, in which at least one positive electrode, a negative electrode, a separator and an electrolyte are accommodated, with at least one glass-based material which is leachable in contact with the electrolyte while basically maintaining the cell function , A lithium-ion secondary battery according to claim 1, wherein the material in contact with the electrolyte has a leaching rate of at least 1 μg / h, in particular at least 10 μg / h or at least 25 μg / h. A lithium-ion secondary battery according to claim 2, wherein the leaching rate changes with time, in particular decreases with time. A lithium-ion secondary battery according to any one of the preceding claims, wherein the composition of the material is changed by the leaching. A lithium-ion secondary battery according to any one of the preceding claims, wherein the material in contact with the electrolyte releases a gettering substance for gettering of HF. A lithium-ion secondary battery according to any one of the preceding claims, wherein the material binds fluorine to form species which are inactive for the electrolyte. A lithium ion secondary battery according to any one of the preceding claims, wherein the material in contact with the electrolyte releases a substance containing boron, in particular releasing oxygenated boron. Lithium ion accumulator according to one of the preceding claims, in which the material in contact with the electrolyte has a mass loss of B ₂ O ₃ of at least 3, in particular at least 5 micrograms / hour. A lithium ion secondary battery according to any one of the preceding claims, wherein the material comprises phase demixed regions. Lithium-ion accumulator according to claim 9, wherein preferably the Phasenentmischten areas are leachable. Lithium-ion accumulator according to one of the preceding claims, wherein the material is a glass or a glass ceramic. Lithium-ion accumulator according to claim 11, wherein the material is a glass ceramic and the crystallites are preferably leachable. Lithium-ion accumulator according to one of the preceding claims, wherein the material is integrated as a powdery additive in the cell, or is integrated in the region of a separator, or integrated as part of a filler-composite mixture, or integrated as a coating of an electrode is, or is coated as a mixture together with electrode material on a current collector, or is integrated as a coating on a housing enclosing the cell. Lithium ion accumulator according to one of the preceding claims, in which the material is part of a separator membrane, a separator nonwoven or a separator composite. Lithium-ion accumulator according to one of the preceding claims, in which remains after the leaching of the material a resistant to the electrolyte, temperature-resistant residual phase, in particular residual glass phase. A lithium-ion secondary battery according to claim 16, wherein the leached residual phase is porous and thermally stable for use as a material for a heat-resistant coating, infiltration or as an admixture in a composite. Lithium-ion secondary battery according to one of the preceding claims, wherein the material contains at least 1 wt .-%, preferably at least 3, more preferably 4.5 wt .-% of B ₂ O ₃ . Lithium-ion accumulator according to one of the preceding claims, in which the glass contains at least SiO _{2 in} addition to boron. Lithium ion accumulator according to one of the preceding claims, in which the material contains at least the following components (in% by weight based on oxide): SiO ₂ 45-60 B ₂ O ₃ 5-30 Al ₂ O ₃ 5-30. The lithium ion secondary battery according to claim 19, wherein the material further contains 20-30% by weight of BaO. Lithium-ion accumulator according to one of claims 17-20, wherein the material further contains 0-10 wt .-% of ZrO ₂ , in particular at least 0.1 wt .-%, in particular at least 1 wt .-% ZrO ₂ . A lithium-ion secondary battery according to claim 19, wherein the material has the following composition (in% by weight based on oxide): SiO ₂ 48-58 B ₂ O ₃ 7-13 Al ₂ O ₃ 7-13 BaO 22-28 ZrO ₂ 0.1-8. A lithium-ion secondary battery according to any one of the preceding claims, wherein the material is free of alkali oxides except for incidental impurities other than Li ₂ O. Lithium ion accumulator according to one of the preceding claims, in which the material in contact with the electrolyte has a mass loss of Al ₂ O ₃ of at least 1, in particular 2.5 micrograms / hour.

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