Electrochemical cell

Abstract

The invention relates to an electrochemical cell (2) having a cell housing (4) and at least one cell element (6) accommodated in the cell housing (4), the cell element (6) having a cathode layer (8), wherein the cathode layer (8) has pure lithium nickel dioxide as an active material, and wherein a pressurizing device (14) is provided by means of which an increased pressure can be applied to the cathode layer (8) at least temporarily.

Description

FIELD OF THE INVENTION

[0001] The present invention relates to an electrochemical cell (or electrochemical accumulator), in particular an electrochemical cell for a motor vehicle, having a cell housing and at least one electrochemical cell unit received in the cell housing, the electrochemical cell unit having at least one cathode layer. The invention also relates to a method for operating such a cell (or accumulator). BACKGROUND OF THE INVENTION

[0002] Electrically or electromechanically driven or drivable motor vehicles, such as electric or hybrid vehicles, generally include an electric motor by means of which one or two axles can be driven. To provide electrical energy, the electric motor is usually connected to a (high-voltage) battery inside the vehicle as an electrical energy storage device.

[0003] In particular, an electrochemical cell is to be understood here and hereinafter in particular as a so-called secondary battery (secondary cell) of a motor vehicle. In such a (secondary) vehicle battery, the consumed chemical energy can be regenerated by an electrical (charging) process. A vehicle battery of this type is designed, for example, as an electrochemical accumulator, in particular a lithium-ion accumulator. To generate or provide an operating voltage, such a vehicle battery usually has a cell housing together with at least one cell unit received in the cell housing.

[0004] The cell unit is designed, for example, as an electrochemical (thin) layer unit. The thin layer unit has a layered structure having a cathode layer (cathode), an anode layer (anode) and a separator layer (separator) introduced therebetween. These components are penetrated, for example, by a liquid electrolyte (electrolyte solution), which establishes an ionic conduction connection or charge balance of the components.

[0005] As the active material of the positive electrode layer, for example, a nickel-rich layered positive electrode material for a lithium-ion battery can be used, for example, lithium nickel oxide with the chemical formula LiNiO 2 which has a relatively high energy density and low manufacturing cost. However, in this case, during battery operation, redox processes (reduction-oxidation processes) occur in the cell unit, which leads to an aging mechanism that ages the cathode material. Depending on the temperature and pressure in the cell unit, these aging processes can also occur spontaneously when the battery is not operating.

[0006] For example, during the so-called delithiation process, nickel is thermochemically driven and continuously migrates to the lithium plane. This means that oxygen (O) is reduced during the charging process, resulting in the formation of gaseous molecular oxygen (O 2 ) and the formation of lithium oxide with a Li 2 O composition. When the proportion of nickel in the lithium plane is high, the layered structure with the R-3m space group can be transformed into a rock-salt, cubic Li with the Fm-3m space group1-x Ni 1+x O 2 structures (where x can be up to 0.4). Due to the higher surface activity, these interrelated processes mainly occur on the cathode surface.

[0007] For the application in vehicle batteries, this results in that, on the one hand, nickel-rich cathode materials lead to a longer cruising range, but on the other hand, they have a shortened service life and low safety. Especially in the field of electric vehicles (where a particularly long cruising range must be achieved with a sufficient service life and high safety), it shows that it is necessary to optimize the stability of nickel-rich cathode materials without significantly reducing their energy density.

[0008] To improve the stability of the battery and thus increase the service life and safety of the battery, LiNiO 2 cathode materials are mostly changed chemically, especially by doping and / or coating with various transition metals such as cobalt (Co), manganese (Mn) or tungsten (W), and morphologically, especially by micro primary and secondary particles and / or grain boundary geometries. As LiNiO 2 a supplement or alternative to the structural optimization of, the charging and discharging processes can be adjusted so that the threats to the service life and safety are reduced. This is achieved by a lower charging voltage, a lower C-rate and by thermal management (heat dissipation, i.e., energy loss). Here, the C-rate should be understood in particular as the charging (or discharging current) of the battery based on its capacity (C).

[0009] The conjunction "and / or" is understood in this way here and hereinafter, such that the features connected by this conjunction can be formed jointly or can also be formed as alternatives to each other.

[0010] LiNiO 2 changes in the chemical composition and the micron and nanostructure of cathode materials always lead to a reduction in the theoretical capacity and the electrochemical potential, and thus to a reduction in the energy density of the battery cell. For example, a cathode layer made of Li(Ni x Co y Mn z )O 2 , i.e., the so-called NMC cathode, has higher electrochemical stability and thermal stability than LiNiO 2 . Therefore, although the NMC cathode has a higher available discharge capacity after long-term use compared to the LiNiO 2 cathode, they have a lower discharge capacity in the first (charge / discharge) cycle. Compared with the overall (bulk) or massive LiNiO 2 , the microstructure modification makes the interfacial diffusion of lithium ions easier, so that more capacity can be obtained in a short time. Here, the disadvantage is the exacerbated irreversible interfacial process (O 2The formation of gas, the phase transition from a layered to a local, cubic, electrochemically inert rock salt phase). In addition, more complex, hitherto non-standardized manufacturing processes and the higher manufacturing costs of complexly doped, coated, and microstructured structures must be considered, especially for industrial purposes. In terms of process optimization, when reducing the charging voltage, C-rate, and temperature, the achievable electrochemical energy and power density that contribute to thermochemical stability decrease.

[0011] The above solutions (doping, coating, microstructuring) can also be combined with each other, for example. For example, manganese- and cobalt-doped LNO particles can be coated and arranged targeted at the microscale such that the maximum lithium diffusivity can be achieved locally and globally, thereby achieving high performance and energy density. In addition, the charging and discharging processes can also be carried out at a limited C-rate and temperature. Therefore, a compromise can be found between service life, range, and performance. Summary of the Invention

[0012] The object of the present invention is to provide a particularly suitable electrochemical cell. In particular, it should be possible to simply and effectively use pure lithium nickel dioxide as the cathode material. The object of the present invention also lies in proposing a particularly suitable method for operating such a cell.

[0013] According to the present invention, the object is achieved in terms of the cell by means of an electrochemical cell, and in terms of the method by means of a method for operating the above-mentioned electrochemical cell. Advantageous improvements and developments are the subject of this application. The advantages and designs cited with respect to the cell can also be applied to the method, and vice versa.

[0014] The present invention is based on the following recognition: by reversing the electrochemically induced aging mechanism, pure lithium nickel dioxide (LiNiO 2 ) can be used as the cathode material for high-energy and high-performance applications. Since these mechanisms are irreversible under constant thermodynamic conditions (temperature, pressure, state of charge), at least one of these three state variables must be changed during or after the aging mechanism such that its gradient drives self-healing. Ab initio simulations based on quantum mechanics (density functional theory, DFT) show that the LiNiO 2 bulk, i.e., the volume LiNiO 2 , is always more stable than a system with reaction products (Li 1-x Ni 1+x O, Li 2 O, O 2 ), especially at low temperatures. Only when the cathode surface proportion is high enough will the reaction products become more stable due to the very high surface stability of Li 2 O, so that an exothermic aging reaction will also occur during the charging (delithiation) process of the cell.

[0015] In the case where the influence of the surface on the energy balance is negligible, metallic lithium in the cathode, for example as dendrites, is necessary for the aging reaction to occur. With the help of a generator charging the battery cell, only a part of the lithium ions together with the electrons from the transition metal (Ni 3+ to Ni 4+ ) reach the anode at high C-rates, while some lithium ions and electrons do not leave the cathode in time, causing lithium metal to form at the surface and interfaces. The cathode is heated by the ohmic dissipation heat generated during the lithium metal formation process. At the surface, oxygen and lithium combine and form a more stable Li 2 O surface, while other oxygen atoms (O) contribute to the formation of gas. Due to buoyancy and the hydrodynamic conditions of the electrolyte and other unit cell components, the gaseous, heated (molecular) oxygen (O 2 ) is lost.

[0016] The loss of oxygen is one of the most important reasons for the irreversibility of the reaction. According to the present invention, an increase in the static internal pressure of the battery cell is thus provided, which increases the gas pressure. The pressure generated on the battery cell causes an increase in the mechanical compressive stress within the cathode, and as a result, oxygen molecules (O 2 ) more frequently impact the aged LNO surface due to their reduced volume in this situation. Ideally, the thermal energy dissipated during the aging process should be retained in the system, i.e., the aging process should proceed adiabatically rather than isothermally with heat dissipation. When compressive stress is applied, the chemical potential of O 2 increases while its volume decreases. Due to the already high adiabatic temperature (resulting in a faster reaction rate), the higher hydrostatic pressure (resulting in a higher molecular collision frequency and thus a faster reaction), and the increased energy level of the aging products (resulting in a spontaneous reaction), the aging reaction can proceed in the reverse direction, enabling NiO, Li 2 O, and O 2 to reform LiNiO 2 again, and self-healing of the cathode layer is achieved.

[0017] The electrochemical cell according to the present invention is, for example, part of a vehicle battery of a motor vehicle, in particular an electric or drivable motor vehicle, such as an electric or hybrid vehicle. The battery can be designed, for example, as a battery module or a battery cell module. The battery is in particular designed as a lithium-ion battery.

[0018] The battery herein has in particular a pressure-sealed battery housing and at least one (electrochemical) battery cell accommodated in the battery housing, the battery cell having at least one cathode layer. The battery housing is designed as, for example, a cylindrical, in particular cylindrical, pressure vessel. The battery housing is made of, for example, aluminum or steel material.

[0019] The battery cell is designed, for example, as a prismatic or cylindrical cell, or as a pouch cell. The battery cell is designed, for example, as a (thin) layer cell having a cathode layer and an anode layer and a separator layer arranged therebetween.

[0020] According to the invention, the cathode layer has pure lithium nickel oxide (LiNiO 2 , LNO) as the active material. In other words, only pure LiNiO 2 is used as the cathode material. "Pure lithium nickel oxide" or "pure LiNiO 2 " should be understood herein and hereinafter in particular to mean lithium nickel oxide having a high purity, for example at least 95%, especially at least 98%, preferably at least 99%.

[0021] According to the invention, a pressurizing device is also provided, which is suitable and configured to generate or effect an increased pressure on the cathode layer, if necessary, at least temporarily. This means that the pressurizing device generates an increased pressure or overpressure, which acts on the battery cell or at least causes an increase in the pressure in the battery cell. Thereby, a particularly suitable battery is achieved, which allows self-healing of the cathode layer.

[0022] Compared with the active materials used hitherto (such as NMC, NCA, LFP...), the active material having a self-healing or renewable surface consists of pure LiNiO 2 . Therefore, traditional standardized methods can be used in manufacturing. Since pure LNO can be used as the active material, a higher energy density is achieved for the battery, that is, a longer cruising range and a sustainable and renewable service life.

[0023] The battery according to the invention thus has technically and economically cost-beneficial and more environmentally friendly manufacturing costs, because pure LiNiO 2 is used, that is, it contains no cobalt and has low CO 2 emissions in synthesis. Compared with doped and coated positive electrode materials, the battery has a higher reversible capacity and energy density, and through self-healing, an increasingly extended service life is achieved. Due to the presence of a higher surface area ratio (or portion) for lithium insertion and extraction, the battery also has an improved C-rate (fast charging capacity) through small particles.

[0024] The mechanical pressure that is typically used in a battery to ensure electrical contact between adjacent components has no effect on the liquids / gases present within the battery housing, and thus the mechanical pressure does not cause the desired repair mechanism. Accordingly, in a suitable embodiment, the battery has a pressurizing device by which, if desired, an increased hydraulic internal pressure or hydraulic overpressure can be generated within the battery housing at least temporarily. In other words, a pressurizing device for temporarily increasing or establishing pressure is provided inside the battery housing. Such a hydraulic pressure can be thermodynamically and kinetically favorable for the repair mechanism in the cathode layer.

[0025] The dimensions of the battery housing are preferably determined in such a way that it can safely control the regeneration process of the electrodes, in particular the cathode supporting at least one battery cell, by applying a hydraulic overpressure, and at the same time control thermal runaway for a predetermined time, such as five minutes.

[0026] In a preferred embodiment, the increased internal pressure is a static pressure. In other words, the pressurizing device is provided and configured to increase the static pressure inside the battery housing. Preferably, a hydrostatic or aerostatic overpressure is generated by the pressurizing device. This can provide reliable support for the regeneration or self-healing process.

[0027] In a conceivable embodiment, at least one battery cell is designed to be shell-less, i.e., open. This means that the layer system of the battery cell, in particular the cathode layer, is arranged freely or exposed within the battery housing. Accordingly, there is no fluid or pressure-sealed partition within the battery housing. This enables the increased internal pressure or overpressure inside the battery housing to act directly or immediately on the cathode layer.

[0028] The pressure increase in the battery cell according to the invention is caused by a pressurizing device, for example pneumatically (pressure membrane or pressure bag) or hydraulically using a piston pump (changing the volume by inserting a bolt) or piezoelectrically or by an actuator with a shape memory alloy (FGL actuator) or an electrolyte pump. For passive solutions for generating pressure, no external energy supply from a pump is required. In addition, due to the closed structure, maintenance costs can be eliminated.

[0029] In a possible improvement, the pressurizing device has a valve, especially in the battery housing. This means that the housing wall of the battery housing is penetrated by the valve. To increase the internal pressure, a fluid, especially a gas, can be conveyed into the interior of the battery housing through the valve. For example, the battery housing can be pressurized with pure oxygen through the valve, and the pure oxygen can be directly used for the regeneration process of the electrodes in the integrated (open-columnar) battery cells. As an alternative, the battery housing can be pressurized, for example, with pure nitrogen, which increases the pressure in the enclosed built-in pouch cells and at the same time serves as a fire protection gas. The fluid is conveyed into the battery housing here, for example, from a connectable gas cylinder through the pressure inside the gas cylinder or by means of a pump. The gas cylinder and / or the pump are arranged, for example, in the charging station, so that an overpressure required for self-healing can be generated after the battery is charged.

[0030] In a further or other aspect of the invention, there is provided an inflatable bag of a pressurizing device arranged inside the battery housing. The bag can be filled here with a gaseous or liquid fluid as a working substance, whereby its volume increases or expands. With the help of a (air) bag pre-mounted or integrated in the battery housing, ambient air at a certain pressure can be conveyed into the bag inside the battery housing by means of a pump. The bag inside the battery housing expands, i.e., its volume increases, thereby reducing the remaining volume inside the housing and thus increasing the internal pressure. This generates a hydraulic or pneumatic pressure on the battery cells, such that the pressure of oxygen (O 2 ) also increases due to the mechanical compressive stress generated inside the cathode.

[0031] As a technical device for temporarily increasing the internal pressure of the battery cells, a working substance can be preferably used, the physical state of which can change from liquid to gaseous when heat is generated during the charging process. Thus, the limited volume in the battery housing leads to an increase in the pressure of the working substance, which activates the regeneration process in the cathode layer.

[0032] As an alternative, an external tensioning frame can also be used as a pressurizing device outside the battery cells, which can temporarily tension the pouch cells from the outside by means of tie rods and piezoelectric actuators.

[0033] The method according to the invention is intended for the operation of the above-described electrochemical battery, is suitable for it, and is designed for it.

[0034] According to this method, the battery is charged or discharged with electrical energy during the charge-discharge process to a predetermined or stored state of charge. The term "charge-discharge process" here and hereinafter particularly refers to the charging process in which the state of charge increases and the discharging process in which the state of charge decreases or reduces. The state of charge (SoC) of the battery describes the capacity of the battery that is still available relative to its nominal value. The state of charge is usually given here as a percentage relative to the fully charged state of the battery. In other words, a state of charge of 100% means that the battery is fully charged (fully charged state), while a state of charge of 0% means that the battery is completely discharged (discharged state). In the fully charged state, there is basically no lithium in the cathode or cathode layer, where the cathode or cathode layer is basically completely lithiated in the discharged state.

[0035] After the (discharge) charging process, i.e., after the lithium ions are re-embedded in the cathode, an increased pressure is generated or induced on the battery cell within a predetermined period of time by means of a pressurizing device, in particular by a hydrostatic or pneumatic overpressure in the battery housing.

[0036] After the said period of time, the pressure drops to normal pressure (operating pressure). This means that the pressure is reduced to the pressure value that existed before or during the (discharge) charging process. The temporary or transient overpressure during this period supports the regeneration and self-healing processes in the cathode layer, so that at the end of this process, the cathode layer again has stoichiometrically pure lithium nickel dioxide.

[0037] For example, according to the requirements of the customer, the regeneration can be run only to a certain point. Then the temperature and pressure can be set back to the standard or operating conditions by heat dissipation and mechanical decompression. If the material cannot withstand high pressure and the safety of the entire battery and the electric vehicle is threatened, these recovery processes can be carried out off-site, i.e., only with the cathode module.

[0038] In a suitable embodiment of the said method, the battery or at least one battery cell is charged during the charge-discharge process to a state of charge between 30% and 15%, especially between 30% and 20%, preferably 20%. In other words, the battery is charged / discharged to a state that is not completely (0%) discharged. For a completely discharged battery, i.e., 0% SoC, the healing reaction energy is most favorable, but when the state of charge is below about 15%, further aging mechanisms occur in the cathode layer. The battery or at least one battery cell is preferably charged to a state of charge of 20%, so that there is the most favorable healing reaction energy, but the unwanted aging mechanisms are avoided.

[0039] In a purposeful improvement, the cathode layer is heated within a period of time, i.e., during the pressure increase, for example, by the ohmic dissipation energy generated during discharge or by conduction heating using external heat. This accelerates the regeneration and self-healing processes of the cathode layer.

[0040] In an advantageous embodiment, the cathode layer is heated during the heating process to a temperature between 50 °C (degrees Celsius) and 70 °C, in particular between 55 °C and 65 °C, preferably heated to 60 °C. This temperature range is adjusted here according to the (liquid) electrolyte of at least one battery cell and according to the separator or separator layer. This means that, as far as possible, no unwanted reactions or phase changes of the electrolyte and / or the separator occur within this temperature range.

[0041] In a preferred embodiment, heating is carried out without additional components by discharging the "empty" battery without electrical work, thereby directly generating ohmic resistance heat only in the cathode layer.

[0042] If the surface ratio is too high (i.e., smaller cathode particles) such that LiNiO 2 ages spontaneously (i.e., not only during the delithiation process), regeneration can also occur in the above manner under compressive stress. If the temperature is not sufficient and the battery has been discharged and cannot be electrically heated, heat can be generated by an external heat source.

[0043] In a further or other aspect of the present invention, in order to support the regeneration process of the electrode, the pressure, in particular the internal hydrostatic pressure in the battery housing, is increased to a maximum of 25 bar, in particular to a pressure range between 5 bar and 10 bar during this period. The pressure is preferably increased to a maximum of 10 bar. In this way, a particularly suitable pressure is exerted on the cathode layer, which supports the self-healing and regeneration of LiNiO 2

[0044] In a possible further development, the pressurizing device is coupled to an external conveying device in order to increase the pressure, and during operation, the external conveying device conveys fluid into the battery housing. For example, a fixed pump connected to a bag is provided in a workshop, for example, to generate pressure, and the pump can be connected through a valve of the pressurizing device. In particular, the pump can be arranged in a charging station or an external unit specifically designed for this purpose. The pump is connected to the motor vehicle and is connected to the battery or the bag of the pressurizing device contained therein through a pipeline system. Then, the pressure is applied after the actual charge and discharge process, so that the battery heals. Therefore, there is no need to carry a device / pump in the motor vehicle. Description of the Drawings

[0045] The exemplary embodiments of the present invention will be explained in more detail below with reference to the drawings. In the drawings:

[0046] Figure 1 An electrochemical cell with battery cells is schematically shown;

[0047] Figures 2 to 6 The aging and regeneration processes in the cathode layer of the battery cell during the process of the method according to the present invention are schematically shown continuously;

[0048] ​Figure 7 shows a temperature - pressure diagram of the motive gas efficiency relative to standard conditions; and

[0049] Figure 8 shows a temperature - pressure diagram of the total reaction efficiency relative to standard conditions for an activation energy of 100 kJ / mol.

[0050] In all the figures, corresponding components and dimensions are always provided with the same reference numerals. Detailed Description of the Invention

[0051] Figure 1 The electrochemical cell 2 is shown in a schematic and greatly simplified representation. The cell 2 has a pressure - sealed cell housing 4, in which an open, i.e., shell - less, cell unit 6 is accommodated. The cell unit 6 has a cathode layer 8 and an anode layer 10 and a separator layer 12 arranged therebetween. The cell 2 has a pressurizing device 14 with an inflatable bag 16 and a valve 18. A pump or gas cylinder can be connected to the pressurizing device 14 via the valve 18, by means of which the bag 16 can be inflated. During the inflation process, the bag 16 increases the internal hydraulic pressure inside the cell housing 4, thereby increasing the pressure on the cathode layer 8. The cathode layer 8 is made of pure lithium nickel dioxide.

[0052] The following will refer to Figures 2 to 6 explain in more detail the method for operating the cell 2 according to the present invention. Figures 2 to 6 shows the process in the cathode layer 8 during the process of the method, where the illustration is schematic and simplified and represents the extreme cases of phase changes. Here, the process during the charging and discharging of the cell unit 6 is generally explained with the help of Figures 2 to 4 to explain.

[0053] Figure 2 shows the cathode layer 8 together with the adjacent current conductor 20 made of aluminum and together with the anode layer 10 at an initial or operating temperature T of 25 °C A . The isolation layer 12 is not shown.

[0054] The cathode layer 8 is shown as a simplified atomic crystal structure, where the horizontal lines of the cathode layer 8 correspond to different monolayers of the layered crystal structure, i.e., the spatial position of the elements relative to the surface. In pure lithium nickel dioxide, lithium has a monovalent positive charge (Li + ), and nickel has a trivalent positive charge (Ni 3+ ), and oxygen has a divalent negative charge (O 2 2- ). The layer positions are marked with their respective chemical symbols in Figure 2 . The upper or more upper layer is in the region of the cathode surface 22, where with increasing depth or distance from the surface, the lower or deeper layer takes on the values in the volume or body 24 of the cathode layer 8.

[0055] In Figure 2 , the battery 2 or the battery cell 6 is in a low state of charge and is charged with electrical energy to a predetermined or stored state of charge, preferably to 20%. This means that the battery 2 is supplied with electrical energy E, for example, by a generator 0 . The chemical reaction is expressed as:

[0056]

[0057] During the charging process (delithiation), nickel releases an electron e - , such that nickel has a tetravalent positive charge (Ni 4+ ). The electron e - combines with lithium on the surface, where thermal energy ΔE is generated due to ohmic heat dissipation 12 .

[0058] Below the lower dashed line of the surface 22, the electron e - does not directly combine with Li at the surface + , but flows through the electrical conductor and reduces Li only at the anode layer 10 + (i.e., combines with Li + ). Above the dashed line, the lithium ions Li + and the electrons e - remain in the cathode layer 8

[0059] The resulting heat flow 26 from the cathode surface 22 to the body structure is shown by an arrow in Figure 3 . As a result, the cathode layer 8 has an elevated temperature T B , where the temperature T at the surface B,2 is higher than the temperature T in the body 24 B,1 . When the surface area ratio is high, i.e., when there are small active particles, Figure 3 the process shown in

[0060] Then, an adiabatic or isothermal (depending on the cooling capacity) phase change occurs in the cathode layer 8, which results in heat generation ΔE due to the exothermic phase and gas formation 23 :

[0061]

[0062] During the phase change, NiO and Li 2 O and molecular oxygen O discharged in the form of gas 2 are formed at the cathode surface 22

[0063] The heat generation ΔE 23 causes an elevated temperature T of the cathode surface 22 C,2 , and an elevated temperature T in the body 24 C,1 . The temperature TC,2 Greater than temperature T B,2 , where the temperature T C,1 Greater than temperature T B,1 , and where the temperature T C,2 Greater than temperature T C,1 . This results in a heat flux 28 from the surface to the bulk. By generating heat ΔE 23 Enthalpy h is provided (emitted) to oxygen 0 , such that the formed molecular oxygen (O 2 ) can escape at the cathode surface 22.

[0064] This means that during charging, oxygen (O) is reduced, resulting in the formation of gaseous molecular oxygen (O 2 ) and the formation of lithium oxide with a Li 2 O composition. Only a portion of the lithium ions enter the anode 10 at a high C-rate together with electrons (e 3+ to Ni 4+ ) from the transition metal (Ni - ), while some lithium ions and electrons do not leave the cathode layer 8 in time, thus forming lithium metal at the surface and interface. The cathode layer 8 is heated by the ohmic heat dissipation ΔE 12 generated during the formation of lithium metal. A stable Li 2 O surface is formed at the cathode surface 22, while other oxygen atoms (O) contribute to gas formation.

[0065] According to the present invention, in the Figure 5 method steps shown, the hydraulic pressure on the cathode layer 8 increases over a predetermined period of time. For example, a hydrostatic or aerostatic overpressure is generated in the battery housing 4, which acts as a compressive stress W 34 on the cathode layer 8, and this compressive stress supports the regeneration process at the cathode surface 22:

[0066]

[0067] The cathode layer 8 is heated during this process, which means that heat input Q 34 occurs. For example, ohmic heat generation is used for heating or heat input Q 34 . For this purpose, lithiation is carried out by discharging the empty battery without any external electrical work. Due to the elevated temperature T D,1 in the volume of the cathode layer 8 and the elevated temperature T D,2 of the cathode surface 22, the regeneration process is accelerated. In addition, when the state of charge is low, the bulk phase change is reduced because as x increases, Li x NiO 2 becomes more stable. The temperatures T D,1 and T D,2 are approximately the same, but greater than temperature TC,2 。

[0068] Heat input Q 34 is preferably such that the temperature T D,1 and T D,2 is about 60 °C. At an appropriate level, the compressive stress W 34 is at most 10 bar.

[0069] The compressive stress W 34 increases the free enthalpy h of the substances from the aging reaction s , such that gaseous oxygen (O 2 ) can re-enter the cathode material.

[0070] Finally, i.e., after this time period or at the end of the method, the compressive stress W 34 is reduced to the original value, and the cathode layer 8 is cooled to the initial temperature T of, for example, 25 °C A ( Figure 6 ), thereby regenerating the original lithium nickel dioxide layer:

[0071]

[0072] The reversibility of the aging mechanism mainly depends on the collision frequency of oxygen molecules with the surface of the LNO particles and the rate of the chemical regeneration process. This will be explained in more detail below with reference to Figure 7 and 8 .

[0073] The state equation of an ideal gas is:

[0074] pV = Nk B T

[0075] where p is the pressure, V is the volume, N is the number of molecules, T is the temperature, and k B is the Boltzmann constant. Here, the average velocity c of the gas molecules in one spatial direction is:

[0076]

[0077] where m is the mass of the gas molecule. From this, the average collision frequency ν of oxygen molecules per unit area A of the LNO cathode surface is obtained:

[0078]

[0079] The ratio of the collision frequency at temperature T and pressure p to the collision frequency under standard conditions (T 0 = 25 °C = 298.15 K, p 0 = 1 atmosphere) is hereinafter referred to as the kinetic gas efficiency η g :

[0080]

[0081] Figure 7 Shows the dynamic gas efficiency η in the pressure range from 1 to 10 atmospheres (physical atmospheres) and in the temperature range from -100 °C to 100 °C g temperature - pressure diagram.

[0082] The reaction rate Γ can be approximated by the Arrhenius equation because the activation energy E is high enough due to solid Li 1-δ Ni 1+δ and Li 2 O:

[0083]

[0084] The ratio of the reaction rate to the standard conditions is as follows:

[0085]

[0086] Therefore, the effective efficiency or rate of the reaction is:

[0087]

[0088] Figure 7 Shows the total reaction efficiency N for an activation energy of 100 kJ / mol (kilojoules per mole) in the pressure range from 1 to 10 atmospheres and in the temperature range from -100 °C to 100 °C 效率 temperature - pressure diagram.

[0089] The claimed invention is not limited to the above - described exemplary embodiments. On the contrary, those skilled in the art can also derive other variants of the present invention from them without departing from the subject matter of the claimed invention within the scope of the disclosed claims. In particular, all the individual features described in the various exemplary embodiments can be combined in other ways within the scope of the disclosed claims without departing from the subject matter of the claimed invention.

[0090] Reference numerals

[0091] 2 Battery

[0092] 4 Battery housing

[0093] 6 Battery cell

[0094] 8 Cathode layer

[0095] 10 Anode layer

[0096] 12 Separator layer

[0097] 14 Pressurizing device

[0098] 16 bags

[0099] 18 valves

[0100] 20 current conductors

[0101] 22 cathode surfaces

[0102] 24 bodies

[0103] 26 heat fluxes

[0104] 28 heat fluxes

Claims

1. An electrochemical cell (2) having a cell housing (4) and at least one cell unit (6) received in the cell housing (4), the cell unit (6) having at least one cathode layer (8), the electrochemical cell (2) further having a pressurizing device (14) by means of which an increased pressure can be applied to the cathode layer (8) at least temporarily, - wherein the cathode layer (8) has lithium nickel dioxide with a purity of at least 95% as the active material, and - wherein, the cell (2) is charged or discharged to a predetermined state of charge during the charge and discharge process using electrical energy, and the pressurizing device (14) generates an increased pressure on the cathode layer (8) during a period of time after the charge and discharge process, and the increased pressure supports the regeneration and self-healing processes in the cathode layer (8).

2. The cell (2) according to claim 1, characterized in that, the pressurizing device (14) is arranged and configured to generate an increased static internal pressure within the cell housing (4).

3. The cell (2) according to claim 1, characterized in that, the cell unit (6) is designed without a housing.

4. The cell (2) according to claim 1, characterized in that, the pressurizing device (14) has a valve (18) in the cell housing (4) through which a fluid can be conveyed into the interior of the cell housing (4).

5. The cell (2) according to claim 1, characterized in that, the pressurizing device (14) has an inflatable bag (16) inside the cell housing (4).

6. A method for operating an electrochemical cell (2) according to any one of claims 1 to 5, - wherein the cell (2) is charged or discharged to a predetermined state of charge during the charge and discharge process using electrical energy, - wherein an increased pressure is generated on the cathode layer (8) by the pressurizing device (14) during a period of time after the charge and discharge process, and - wherein the pressure is reduced to atmospheric pressure after the period of time.

7. The method according to claim 6, characterized in that, the cell (2) is charged or discharged to a state of charge between 30% and 15%.

8. The method according to claim 6, characterized in that, the cell (2) is charged or discharged to a state of charge between 30% and 20%.

9. The method according to claim 6, characterized in that, the cell (2) is charged or discharged to a state of charge of 20%.

10. The method according to claim 6, characterized in that, the cathode layer (8) is heated during the period of time.

11. The method according to claim 10, characterized in that, during the period of time, the cathode layer (8) is heated to a temperature between 50 °C and 70 °C.

12. The method according to claim 10, characterized in that, during the period of time, the cathode layer (8) is heated to a temperature between 55 °C and 65 °C.

13. The method according to claim 10, characterized in that, during the period of time, the cathode layer (8) is heated to a temperature of 60 °C.

14. The method according to claim 10, wherein, the heating is achieved by discharging the battery without power operation.

15. The method according to claim 6, wherein, during the time period, the pressure is increased to a maximum of 25 bar.

16. The method according to claim 6, wherein, during the time period, the pressure is increased to 5 to 10 bar.

17. The method according to claim 6, wherein, the pressurizing device (14) is coupled to an external conveying device to increase the pressure, and during operation, the external conveying device conveys fluid into the battery housing.

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