Safety measure for large-format, high-energy-density lithium-ion cells

Abstract

Li-ion battery (200, 300) for storing electrical energy, wherein - the accumulator (200, 300) comprises a cathode (110), an anode (120) and a separator (103) arranged between the cathode (110) and the anode (120); and - the cathode (110) comprises a mixture of an oxide cathode material and a cathode material that is inert with respect to oxygen release; wherein - the cathode (110) has a first layer (102) comprising oxide cathode material; - the first layer (102) comprises a multitude of core-shell cathode elements (310, 320); - the multitude of core-shell cathode elements (310, 320) each comprise a core (301) and a shell (302) enclosing the core (301); - the density of oxide cathode material in the mantle (302) is lower than the density of oxide cathode material in the core (301); - the cathode (110) has a second layer (202) arranged between the first layer (102) and the separator (103); and - the second layer (202) contains inert cathode material and no oxide cathode material.

Description

[0001] The invention relates to accumulators, e.g., lithium-ion accumulators. In particular, the invention relates to accumulators with a high degree of safety with regard to thermal reactions and with a high energy density.

[0002] The trend towards using lithium-ion batteries with ever-increasing energy densities is leading to the increased use of oxide cathode materials. In cases of misuse or material failure, such as of the battery's separator, these batteries can reach temperatures so high that the oxide cathode materials release oxygen. This situation can be simulated, for example, by driving a nail into the battery. The oxygen reacts exothermically with the organic electrolyte in the battery (also known as the cell), potentially releasing gases. This typically results in high-energy-density lithium-ion batteries failing the required safety tests for approval, as any harmful effects of the batteries on their environment are unacceptable.

[0003] US 2006 / 0099495 A1 describes a cathode with different layers of different active materials. US 2013 / 0183579 A1 describes a cathode with a material having a core surrounded by a shell. US 2005 / 0196673 A1 describes a cathode with a mixture of different materials. WO 2013 / 129182 A1 describes a cathode with a mixture of different materials.

[0004] This document addresses the aforementioned technical problems. In particular, it deals with the technical challenge of providing a high-energy-density accumulator with a high degree of safety, especially with regard to thermal reactions.

[0005] The problem is solved by the independent claim. Advantageous embodiments are described, among other things, in the dependent claims.

[0006] According to one aspect, a lithium-ion battery for storing electrical energy is described. The battery comprises a cathode, an anode, and a separator located between the cathode and anode. The cathode consists of a mixture of an oxide cathode material and a cathode material that is inert with respect to oxygen release. The addition of an inert cathode material ensures that, in the event of separator failure, the heat generated within the battery is insufficient to raise its temperature to a critical level for oxygen release. This increases the battery's safety.

[0007] The inert cathode material can exhibit a relatively high heat capacity. In particular, the inert cathode material can have a higher heat capacity than the oxide cathode material. By adding an inert cathode material with a high heat capacity, the temperature rise in the event of separator failure can be reduced, thus further increasing the safety level of the accumulator.

[0008] The inert cathode material can have an electrical energy storage capacity greater than a predefined threshold. For example, the specific capacity of the inert cathode material can be greater than or equal to 150 mAh / g. By using an inert cathode material with such a high electrical energy storage capacity, batteries with high energy density can be provided. In particular, this allows for the development of batteries with high energy density and a high degree of safety.

[0009] The inert cathode material can, in particular, comprise lithium iron phosphate and / or a material with an olivine structure. The oxide cathode material can comprise a transition metal oxide. Both the inert cathode material and the oxide cathode material can comprise a wide variety of different materials.

[0010] The inert cathode material can be arranged such that a diffusion path (e.g., an average diffusion path) between the separator and the oxide cathode material is longer than in a cathode comprising only the oxide cathode material. Alternatively or additionally, the inert cathode material can be arranged at least partially between the separator and the oxide cathode material. Alternatively or additionally, the inert cathode material can be arranged such that the accumulator has no direct contact between the separator and the oxide cathode material. These measures can reduce the reaction rate with respect to oxygen release. In other words, the above measures allow for a further increase in the safety level of the accumulator.

[0011] In particular, the cathode has a first layer comprising oxide cathode material. Furthermore, the cathode has a second layer located between the first layer and the separator. This second layer consists of inert cathode material and does not contain oxide cathode material. The use of a second layer of inert cathode material allows for longer oxygen diffusion paths, thereby reducing the reaction rates with respect to oxygen release.

[0012] The first layer of the cathode comprises a multitude of core-shell cathode elements (e.g., spherical cathode elements). Each core-shell cathode element consists of a core and a shell enclosing the core. The density of oxide cathode material in the shell is lower than the density of oxide cathode material in the core. Specifically, the shell can consist (or may consist entirely) of inert cathode material, while the core may not contain any inert cathode material. This allows the oxide cathode material to be separated from the separator (and thus reduces reaction rates). Furthermore, the oxide cathode material can be broken down into individual particles, separated by inert cathode material. This reduces the risk of a snowball effect during oxygen release.

[0013] The cathode elements can be arranged in layers, and the cathode can have multiple layers of core-shell cathode elements that separate the separator from a cathode current collector (e.g., an aluminum foil). This allows for the efficient construction of a cathode with high energy density and a high degree of safety.

[0014] The invention will now be described in more detail using exemplary embodiments. Fig. 1 a block diagram of an example accumulator; Fig. 2 a block diagram of an accumulator with an exemplary layer structure of a cathode; Fig. 3a Exemplary core-shell cathode elements with defined layer transition (left) and graded layer transition (right); and Fig. 3b a block diagram of an accumulator with an exemplary cathode with core-shell cathode elements.

[0015] As stated at the beginning, this document deals with accumulators, in particular lithium-ion accumulators, which provide the highest possible energy density while simultaneously meeting high safety requirements with regard to thermal reactions.

[0016] The cause of the problem described in the introduction regarding gas release lies in the high energy density of the cells. When the energy stored in the cell is converted into heat, a critical temperature is exceeded due to this high energy density, above which the oxide cathode materials release oxygen. This creates a conflict between the requirement for high energy density on the one hand and the requirement to meet high safety standards on the other. In the automotive sector, this leads to a conflict between the requirement for a long driving range for electric vehicles on the one hand and high safety requirements for the vehicle on the other. Above a certain energy density, battery systems with oxide cathode materials typically do not meet the safety requirements placed on electrical storage devices.Consequently, the energy density of the battery systems, and thus the range of the vehicles, cannot be increased further.

[0017] Fig. Figure 1 shows a block diagram of an exemplary (non-inventive) accumulator 100, in particular a lithium-ion accumulator. The accumulator 100 comprises a positive electrode, i.e., a cathode, 110 and a negative electrode, i.e., an anode, 120, which are separated from each other by a separator 103. The cathode 110 typically comprises a cathode arrester 101, which, for example, comprises aluminum, in particular an aluminum foil, and the anode 120 typically comprises an anode arrester 106, which, for example, comprises copper, in particular a copper foil.

[0018] The cathode 110 further comprises an ion source, in particular a source of lithium ions (Li₂). +For example, the cathode 110 can comprise a layer 102 of LiCoO2 (lithium cobalt dioxide), LiNiO2 (lithium nickel dioxide), or LiMnO2 (lithium manganese dioxide). In particular, the cathode 110 can comprise a layer 102 containing lithium ions and transition metal ions. The anode 120 can comprise a layer or host lattice 105 for the uptake of lithium, in particular graphite.

[0019] During charging of the accumulator 100, lithium ions migrate from the cathode 110 to the anode 120. To enable the ion flow between the cathode 110 and the anode 120 (and vice versa), the accumulator 100, i.e., in particular the separator 103, contains an electrolyte 104. The electrolyte 104 can, for example, consist of salts in anhydrous solvents. At the anode 120, a host lattice 105 accepts one electron for each incorporated lithium ion. The lithium ion is thus intercalated by the host lattice 105 of the anode 120.

[0020] During discharge, the host grid 105 of the anode 120 releases one electron per lithium ion, which flows to the cathode 110 via an external circuit. Simultaneously, an equal number of lithium ions migrate through the electrolyte 104 from the anode 120 to the cathode 110. At the cathode, the highly ionized transition metal ions, present in the charged state of the battery 110, accept the electrons supplied via the external circuit.

[0021] As explained above, damage to the accumulator 100 can lead to a thermal reaction. In particular, water entering the accumulator 100, overcharging, and / or damage to the separator 103 can result in significant heat generation and the release of oxygen. This problem is further exacerbated at high energy densities of the accumulator 100.

[0022] This document proposes a solution to this problem by adding an additional energy-storing material to the cathode 110. This material is inert with respect to oxygen release and optionally has a high heat capacity. The addition of this inactive material ensures that the heat generated, for example, in the event of a failure of the separator 103, is insufficient to raise the cell temperature above the critical temperature for the onset of oxygen release. Furthermore, the dilution of the oxide cathode material reduces the amount of gases that can be released and the rate at which these gases are released. The addition of this inactive material thus significantly facilitates compliance with the safety requirements for the accumulator 110.

[0023] The material used for cathode material dilution, which is inert to oxygen release, can itself be a cathode material or a mixture of several cathode materials capable of storing electrical energy. In this case, the materials contribute to further increasing the energy density. The combination of an oxide cathode material and one or more inert cathode materials is particularly beneficial when the oxide cathode material has a higher energy density than the inert cathode material, resulting in a high-energy-density, high-safety accumulator. For example, the inert cathode material can be derived from a range of materials with an olivine structure. Lithium iron phosphate (LiFePO4) is particularly suitable for use as an inert cathode material.

[0024] As explained above, typical high-energy-density cathode materials are transition metal oxides. Table 1 specifically lists lithium nickel cobalt aluminum oxide (NCA) and nickel manganese cobalt oxide (111-NMC). These oxide cathode materials exhibit high energy density but also a relatively high tendency to release clean matter. The specific capacity C in mAh / g (milliampere-hours per gram) and a typical rated operating voltage U are given for these materials. n The voltage is given in V (volts) for accumulator cells with carbon anodes. From this, the usable capacities C in Ah (ampere-hours) can be derived for a cathode of 100 with 200g or 220g of oxide cathode material. Table 1 Material C [mAh / g] Dn [V] C [Ah] @ m Kath = C [Ah] @ m Kath = 200 g 220 g LiFePO4 150 3,30 27,3 30,0 NCA 175 3,60 31,9 35,0 111-NMC 155 3,65 28,2 31,0

[0025] Furthermore, Table 1 shows the capacitances C and the nominal voltage U. nfor the cathode material lithium iron phosphate, which is inert with respect to oxygen release. The values ​​given in Table 1 include formation losses in capacity (typically 9%).

[0026] As described in this document, the cathode 110 comprises a mixture of an oxide material, which has a high energy density but typically also a relatively high tendency to release oxygen, and an inert cathode material, which has a lower energy density than the oxide material but also a lower tendency or no tendency to release oxygen. Such a cathode 110 with a mixture of one or more oxide materials and one or more inert cathode materials can be referred to as a blended cathode.

[0027] Table 2 shows the capacities C and energy contents E in Wh (watt-hours) of blended cathodes comprising a mixture of lithium iron phosphate (LFP) and lithium nickel cobalt aluminum oxide (NCA). The values ​​are given for 200 g and 220 g of cathode material, respectively. The values ​​were calculated using volume fractions, assuming a constant total volume, i.e., a constant layer thickness. The specific material and layer density differences of the individual cathode materials, as well as the formation losses, were also taken into account. Table 2 Anteil LFP [Vol-%] C [Ah] @ m Kath = C [Ah] @ m Kath = E brutto [Wh] @ m Kath = E brutto [Wh] @ m Kath = 200 g 220 g 200 g 220 g 0 31,9 35,0 115 126 10 30,0 33,0 108 118 20 28,2 31,0 101 111 30 26,4 29,0 94 103 50 22,7 25,0 80 88

[0028] Table 2 shows that a blended cathode with 200 g of cathode material containing 10 vol% LFP and 90 vol% NCA results in only a 6% reduction in energy storage capacity (from 115 Wh to 108 Wh) due to the storage capacity of LFP. A version with 30 vol% LFP and 70 vol% NCA results in an 18.3% reduction in energy storage capacity (from 115 Wh to 94 Wh). On the other hand, the addition of LFP increases the safety of the battery.

[0029] It should be noted that Table 2 illustrates blended cathodes using the example of a mixture of LFP and NCA. Other cathode materials with high energy density, such as nickel-rich lithium nickel manganese cobalt oxides (so-called Ni-rich NMCs), can also be blended with LFP or another oxygen-inert cathode material to achieve the effect of high energy density and high safety.

[0030] This document therefore proposes diluting highly reactive cathode materials with inert, i.e., non-oxygen-releasing, cathode active materials. This results in an increase in the critical cell winding temperature while maintaining a high energy density.

[0031] A particularly significant increase in the safety of a battery can be achieved through a special arrangement of the cathode materials of the blended cathode. Specifically, by arranging the different materials in one or more layers, the oxygen diffusion pathways to the electrolyte-filled separator 103 can be extended. This then leads to a slowing of the reaction rate in the event of battery damage, and thus to a further increase in battery safety.

[0032] Fig. Figure 2 shows a block diagram of an exemplary accumulator 200. The accumulator 200 comprises a blended cathode 110, which has a first layer 102 and a second layer 202. The first layer 102 comprises an oxide material (e.g., NCA), and the second layer 202 comprises an inert cathode material (e.g., LFP). The second layer 202 is arranged between the first layer 102 and the separator 103, thus shielding the oxide material of the first layer 102 from the electrolyte 104 and the separator 103.

[0033] The oxide materials and the inert cathode materials are arranged in a core-shell configuration. The inert cathode material encloses an oxide material to form a core-shell cathode element. The blended cathode 110 features a multitude of such core-shell cathode elements. The arrangement of a blended cathode 110 with a multitude of core-shell cathode elements leads to an extension of the oxygen diffusion paths to the electrolyte-filled separator 103 and thus to a slowing of the reaction rate. Furthermore, the use of core-shell cathode elements leads to an extension of the diffusion path between the individual reactive cathode particles (e.g., between individual NCA particles). This reduces the risk of a snowball effect in oxygen release in the event of damage to the accumulator.

[0034] Fig. Figure 3a shows the exemplary structure of core-shell cathode elements 310 and 320. These elements comprise a core 301 and a shell 302 enclosing the core 301. The core 301 comprises reactive cathode material, in particular an oxide material such as NCA. The shell 302 comprises, with respect to oxygen release, inert cathode material such as LFP. The core-shell cathode element 310 has a defined transition between the core 301 and the shell 302, while the cathode element 320 has a graded or continuous transition between the core 301 and the shell 302.

[0035] Fig. Figure 3b shows a block diagram of an exemplary (non-inventive) accumulator 300 with a cathode layer 312 comprising a plurality of cathode elements 310, 320. The cathode layer 312 can, in particular, comprise a plurality of layers of cathode elements 310, 320.

[0036] The measures described in this document allow the energy density of a battery to be increased beyond a value that would otherwise be considered critical without compromising safety requirements. In other words, the measures described in this document make it possible to provide a battery with high energy density and a high degree of safety.

[0037] The present invention is not limited to the embodiments shown. In particular, it should be noted that the description and the figures are intended only to illustrate the principle of the proposed methods, devices, and systems.

Claims

[1] Li-ion accumulator (200, 300) for storing electrical energy, wherein - the accumulator (200, 300) comprises a cathode (110), an anode (120) and a separator (103) arranged between the cathode (110) and the anode (120); and - the cathode (110) comprises a mixture of an oxide cathode material and a cathode material that is inert with respect to oxygen release; wherein - the cathode (110) has a first layer (102) comprising oxide cathode material; - the first layer (102) comprises a plurality of core-shell cathode elements (310, 320); - the plurality of core-shell cathode elements (310, 320) each comprise a core (301) and a shell (302) enclosing the core (301); - a density of oxide cathode material in the jacket (302) is lower than a density of oxide cathode material in the core (301); - the cathode (110) has a second layer (202) arranged between the first layer (102) and the separator (103); and - the second layer (202) comprises inert cathode material and no oxide cathode material. [2] Li-ion battery (200, 300) according to claim 1, wherein the inert cathode material is arranged such that a diffusion path between the separator (103) and the oxide cathode material is longer than in a cathode comprising only the oxide cathode material. [3] Li-ion accumulator (200, 300) according to any preceding claim, wherein - the inert cathode material is arranged at least partially between the separator (103) and the oxide cathode material; and / or - the inert cathode material is arranged such that the accumulator (200, 300) has no direct contact between the separator (103) and the oxide cathode material. [4] Li-ion accumulator (200, 300) according to one of the preceding claims, wherein - the sheath (302) comprises inert cathode material; and - the core (301) does not comprise any inert cathode material. [5] Li-ion battery (200, 300) according to one of the preceding claims, wherein the cathode (110) comprises a plurality of layers of core-shell cathode elements (310, 320) separating the separator (103) from a conductor (101) of the cathode (110). [6] Li-ion battery (200, 300) according to any preceding claim, wherein the inert cathode material has a higher heat capacity than the oxide cathode material. [7] Li-ion battery (200, 300) according to any preceding claim, wherein the inert cathode material has a capacity for storing electrical energy that is greater than a predefined threshold value. [8] Li-ion accumulator (200, 300) according to any preceding claim, wherein - the inert cathode material comprises lithium iron phosphate and / or a material with an olivine structure; and / or - the oxide cathode material comprises a transition metal oxide.

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