DE102017005595A1 - Method and device for charging a rechargeable electrochemical energy storage cell - Google Patents

Abstract

The invention relates to a method for charging a rechargeable electrochemical energy storage cell (6) with an anode (1) and a cathode (3), wherein an electrical charging current (I) is fed into the energy storage cell (6) during the charging process, wherein the electrical charging current (I) is controlled such that the cell voltage (U _cell (t)) is less than or equal to the concentration-dependent electrochemical potential of the cathode based on the electrochemical potential of the metal reference electrode (φ + / 0 (x _pos , t) vs. Me / Me ^{n +} ) is to avoid an electrochemical reaction at the anode (1). The present invention also relates to a device (4) for carrying out the method.

Description

The present invention relates to a method and a device for charging a rechargeable electrochemical energy storage cell.

In motor vehicles with a (partial) electric powertrain (such as hybrid, plug-in hybrid and purely electrically powered vehicles) are currently almost exclusively lithium-ion battery cells as rechargeable electrochemical energy storage cells (hereinafter often referred to as battery cells) for the battery packs used, which serve as electrical energy storage for the drive train.

Lithium-ion battery cells are known to comprise a negative electrode (anode), a positive electrode (cathode), a lithium ion permeable separator disposed between the anode and cathode, and a liquid electrolyte. The anode and the cathode are each further electrically connected to a current conductor.

The anode and the cathode of a lithium-ion battery cell each consist of or contain active material. During a charging process, lithium ions intercalate into the active anode material; during the discharge process, the lithium ions deintercalate again. In the case of the active cathode material, the opposite action takes place. Such electrodes are often referred to as intercalation electrodes.

For the anode, graphite is mainly used as the active material in the current state of the art, the cathode is often a mixed oxide.

One reason for using graphite as the anode material is that the electrochemical potential of charged graphite is very low compared to metallic lithium. Thus, the voltage difference between the positive and negative electrodes is close to the energetic optimum of a metallic lithium anode. However, the low potential difference of lithiated graphite to metallic lithium has the disadvantage that when positive charging power is supplied, precipitation of metallic lithium on the surface of the graphite particles can occur, the so-called lithium plating.

Lithium plating leads to increased degradation of the battery cell (if the deposited metallic lithium reacts with the electrolyte or loses electrical contact, it is no longer available for further charging and discharging, only if the metallic lithium is still electrically connected , it can continue to intercalate or oxidize again during discharge) and it is also discussed in the literature an impact on the safety of the battery cell. For these reasons, lithium plating must be avoided when charging a lithium-ion battery cell.

From the DE 10 2016 007 479 A1 a method and a device is known to charge a battery cell with a high charging current in the shortest possible time, without resulting in lithium plating. In the method according to the cited publication, the charging current is calculated from a charging current intensity map as a function of the (local) state of charge and the (local) temperature of the battery cell. The charging current strength map is determined in advance experimentally or simulatively. As an application, a full charge was chosen.

During charging from the lowest state of charge approved by the system (vehicle / battery) to the highest system-approved state of charge with the maximum charge current determined in accordance with this disclosure, the lithium ion concentration gradients in the active material and in the electrolyte become greatest. For this reason, the full charge of a battery cell from the lowest to the highest state of charge is the most critical application.

In motor vehicles with a (partial) electric drive train, the battery is not only charged via the mains, but it is also in driving braking energy in the form of positive power fed back into the battery (so-called recuperation). The battery cells are thus briefly recharged while driving, usually after a discharge phase (positive acceleration and holding the speed) (during a braking operation or a negative acceleration).

For such loading operations that is from the DE 10 2016 007 479 A1 known method not optimal.

Against this background, it is an object of the present invention to provide a method and a device, with which a rechargeable, electrochemical energy storage cell can be charged with a maximum, not leading to lithium plating performance, both in driving and in network operation.

These objects are achieved by the method according to claim 1 and the device according to claim 9. Advantageous developments of the method and the device are the subject of the dependent claims.

According to the invention, a method for charging a rechargeable electrochemical energy storage cell (battery cell) having an anode and a cathode is proposed, wherein an electrical charging current (I) is fed into the energy storage cell during the charging process. The method is characterized in that the electrical charging current (I) is regulated by means of a cell model such that the cell voltage (U _cell (t)) is less than or equal to the (metal ion) concentration-dependent electrochemical potential of the cathode based on the electrochemical potential of the metal reference electrode (φ + / 0 (x _pos , t) vs. Me / Me ^{n +} ) is to avoid an electrochemical reaction at the anode.

The method is characterized in particular by the fact that a rechargeable electrochemical energy storage cell (battery cell) with a maximum, not to increased damage by deposition of metallic lithium on the surface of the particles of the anode (lithium plating) leading load in driving and mains operation loaded can be, without this, a calculation of the anode potential is required.

In the initially mentioned method according to the published patent application DE 10 2016 007 479 A1 For determining the maximum charging power at which no deposition of metallic lithium still occurs on the anode, it is necessary to calculate the anode potential (for instance by means of a control unit in a motor vehicle). However, according to the inventors' knowledge, an exact, accurate calculation of the anode potential by means of a physico-electrochemical cell model, which takes place in a motor vehicle, can currently not be implemented in a series product either technically or economically or only with considerable effort. This disadvantage is overcome by the present invention.

As already mentioned above, before charging a battery of a motor vehicle by means of recuperation usually a (short-term) discharge takes place. This previous discharge causes the concentration of lithium ions on the surface of the graphite particles to be less than the average lithium ion concentration of the particles. The charging current of a recuperative charging pulse can thus be selected higher than the charging current intensity map according to the published patent application DE 10 2016 007 479 A1 for the same (local) state of charge and the same (local) temperature.

This also applies to charging operations that are started at a higher than the lowest state of charge, ie in a partially discharged state of the battery cell. Again, the maximum charging current not leading to lithium plating is higher than the charging current intensity map from the publication because of the lower lithium ion concentration at the surface of the graphite particles and the lower concentration gradient in the electrolyte at the corresponding state of charge DE 10 2016 007 479 A1 for the same (local) state of charge and the same (local) temperature.

There are no particular restrictions with regard to the cell model used or usable, and it is possible, for example, for known electrical, physical-electrochemical models, models simplified in complexity, for example by a mathematical model order reduction (MOR), by reducing the simulated particle number, be implemented by an image of the physical-electrochemical cell model to electrical components, through the use of knowledge-based methods, such as artificial neural networks, support vector methods and fuzzy systems, or by similar, related methods to U _ch ( t) to calculate or to determine.

For the calculation of the voltage quantities by the cell model, a suitable (digital) computing device is to be provided with (a) memory device (s) in order to provide the parameters necessary for the respective model used.

According to a first advantageous development of the method, the electrical charging current (I) is regulated by means of a cell model such that the cell voltage (U _cell (t)) is less than or equal to the sum (U _ch (t)) which is formed from the concentration-dependent one electrochemical potential of the cathode based on the electrochemical potential of the metal reference electrode (φ + / 0 (x _pos , t) vs. Me / Me ^{n +} ) and at least one other addend, which represents an overvoltage, preferably the reaction overvoltage at the cathode (η ⁺ (x _pos , t)) and / or the electrolyte overvoltage (η ₁ (t)).

According to a second advantageous development of the method, for the calculation of the voltage values by the cell model (in addition to the cell current) as an input variable is a vector with at least one measured temperature (T) of the rechargeable electrochemical energy storage cell, optionally with temperature measured at different points of the rechargeable electrochemical energy storage cell (T) used.

For example. In the case of a large-sized lithium-ion battery cell, the temperature (T) is not homogeneous due to cell design and / or cooling, but there are warmer and colder ones Areas. Due to the strongly temperature-dependent electrical and ionic conductivities of the materials installed in a battery cell, it follows that the current density in a battery cell is likewise not homogeneous. This results in different states of charge within the battery cell. The method should therefore be carried out in an advantageous manner taking into account the local charge states and the current density distribution.

It is likewise advantageous if an overvoltage or overvoltages by an offset value are taken into account by the cell model.

Since the models described so far require a lot of computing power and have a high memory requirement, the method can advantageously be carried out using at least one simplifying model approach, for example when carrying out the method in a motor vehicle. Simplifying model approaches are known to the person skilled in the art and are encompassed by the present invention.

Furthermore, it is advantageous if the charging current is determined according to the inventive method disclosed above or one of its advantageous developments and additionally an empirically or simulatively determined parameter is subtracted from the cell voltage determined by the cell model in order to take into account a model error of the cell model.

It is also advantageous if in the used (possibly simplifying) cell model a tracking of the model parameters on the aging of the rechargeable electrochemical energy storage cell is made.

• – das Verfahren mit einer wiederaufladbaren elektrochemischen Energiespeicherzelle durchgeführt wird, die nur teilweise entladen ist;
• – das Verfahren mit einer Lithium-Ionen-Energiespeicherzelle durchgeführt wird;
• – das Verfahren mit einer Lithium-Ionen-Energiespeicherzelle mit einer Graphit-Anode durchgeführt wird;
• – die Strom-, Temperatur- und Ladezustandsverteilung innerhalb der wiederaufladbaren elektrochemischen Energiespeicherzelle berücksichtigt werden;
• – die Alterung der wiederaufladbaren elektrochemischen Energiespeicherzelle durch eine Anpassung der alterungsabhängigen Modellparameter über die Betriebsdauer der wiederaufladbaren elektrochemischen Energiespeicherzelle nachgeführt wird;
• – das Verfahren mit einer wiederaufladbaren Batterie mit parallel verschalteten, wiederaufladbaren elektrochemischen Energiespeicherzellen oder mit einer Reihenschaltung von einer oder mehreren parallel verschalteten wiederaufladbaren elektrochemischen Energiespeicherzellen durchgeführt wird, insbesondere mit einer Batterie mit Lithium-Ionen-Energiespeicherzellen, bevorzugt mit einer Batterie mit Lithium-Ionen-Energiespeicherzellen, die jeweils eine Graphit-Anode enthalten.

Further advantageous embodiments of the method encompassed by the present application include

• The method is carried out with a rechargeable electrochemical energy storage cell which is only partially discharged;
• - The method is carried out with a lithium-ion energy storage cell;
• - The method is carried out with a lithium-ion energy storage cell with a graphite anode;
• - the current, temperature and charge state distribution within the rechargeable electrochemical energy storage cell are taken into account;
• - The aging of the rechargeable electrochemical energy storage cell is tracked by an adaptation of the age-dependent model parameters over the service life of the rechargeable electrochemical energy storage cell;
• The method is carried out with a rechargeable battery with parallel connected, rechargeable electrochemical energy storage cells or with a series circuit of one or more parallel connected rechargeable electrochemical energy storage cells, in particular with a battery with lithium ion energy storage cells, preferably with a battery with lithium ion Energy storage cells, each containing a graphite anode.

Also included in the present invention is an apparatus for charging a rechargeable electrochemical energy storage cell having an anode and a cathode, the apparatus being characterized in that it is designed to regulate the charging current (I) by means of a cell model in such a way that the cell voltage (U _cell (t)) less than or equal to the concentration-dependent electrochemical potential of the cathode based on the electrochemical potential of the metal reference electrode (φ + / 0 (x _pos , t) vs. Me / Me ^{n +} ) is to avoid an electrochemical reaction at the anode.

The device can also be set up in an advantageous manner to be able to carry out one of the advantageous developments of the method.

Further advantages, features and details of the invention will become apparent from the following description and from the drawing. The features and feature combinations mentioned above in the description as well as the features and feature combinations mentioned below in the description of the figures and / or in the figures alone can be used not only in the respectively specified combination but also in other combinations or in isolation, without the scope of To leave invention.

Showing:

1 a one-dimensional model of a battery cell; and

2 a schematic representation of an exemplary device (an exemplary control loop) for carrying out the method according to the invention or one of its advantageous developments.

In order to describe the solution according to the invention, a simplified, one-dimensional view of a battery cell is assumed below. The function of the invention But solution is not limited by this reduced approach.

As in 1 is shown, there is the simplified model of the battery cell 6 from an anode 1 with the length L _an , a separator 2 the length L _Sep and a cathode 3 the length L _cat . The current collector at the anode 1 and the cathode 3 are assumed to be ideally conductive and are omitted in this approach. The two electrodes 1 . 3 are made of porous active material, so it can be anywhere x in the electrodes 1 . 3 and at any time t in the solid state the potential φ _s (x, t) and in the electrolyte the potential φ _l (x, t) is given. In the area of the separator 2 only the potential in the electrolyte φ _l (x, t) exists.

All sizes in the anode 1 are marked for a better overview with a superscript minus (eg. φ - / s (x, t) ) and all sizes in the cathode 3 with a superscript plus (eg φ + / s (x, t) ). Furthermore, overvoltages on cover layers such. B. the Solid Electrolyte Interphase (SEI) of the anode 1 and the influence of electrochemical double layers neglected. These simplifications have no influence on the validity and feasibility of the solution according to the invention.

Lithium plating occurs when the potential at the interface surface graphite particles and electrolyte falls below 0 V versus Li / Li ⁺ . In order to avoid lithium plating, it is necessary for x in the area of the anode 1 be valid: φ _An (x, t): = φ - / s (x, t) - φ - / l (x, t) ≥ 0 V vs.. Li / Li ⁺

The deposition of metallic lithium on the surface of the graphite particles typically takes place first on the surface of the separator-near graphite particles due to the high electrical conductivity of the active material and the comparatively low transport speed of the lithium ions in the electrolyte by diffusion and migration. Therefore: x _crit = L _An

And thus for the criterion to avoid lithium plating: φ _An ( _xcrit , t) = φ - / s ( _xcrit , t) - φ - / l ( _xcrit , t) ≥ 0 V vs.. Li / Li ⁺

For the cell voltage applied to the terminals of the battery cell 6 can be measured, the following applies: U _cell (t) = φ + / s (x _pos , t) - φ - / s (0, t) with x _pos = L _An + L _Sep + L _cat

The cathode potential at the cathode particle and electrolyte interface at position x _{pos is} calculated as: φ _Kat (x _pos , t) ≔ ≔ - / s (x _pos , t) - ≔ + / l (x _pos , t) = (φ + / 0 (x _pos , t) + η ⁺ (x _pos , t )

Here is ≔ + / 0 the potential of the cathode material versus Li / Li ⁺ as a function of the concentration of lithium ions on the particle surface and η ⁺ the reaction overvoltage for the charge transfer. This is at the cathode 3 positive in case of a charge of the battery cell 6 ,

In the following, an electrolyte overvoltage η _{l is defined} for further use: η _l (t) ≔ ≔ + / l (x _pos , t) - ≔ - / l (x _crit , t)

With these parameters, the basic equation for the inventive solution for avoiding lithium plating can now be derived: ≔ _{An (crit} x, t) = ≔ - / s _(crit x, t) - φ - / l (x _crit, t) ≥ 0 V vs. Li / Li ⁺ ⇔ ⇔ - / s (x _crit , t) ≥ φ - / l (x _crit , t) = φ + / l (x _pos , t) - η _l (t) ⇔ ⇔ - / s (x _crit , t) ≥ φ + / s (x _pos , t) - φ _Kat (x _pos , t) - η _l (t) ⇔ ⇔ - / s (x _crit , t) - ⇔ - / s (0, t ) ≥ U _cell (t) - φ _Kat (x _pos , t) - η _l (t) ⇔ φ - / s (x _crit , t) - φ - / s (0, t) ≥ U _cell (t) - (φ + / 0 (x _pos , t) - η ⁺ (x _pos , t) - η _l (t)

Since the electrical conductivity of the anode material is high, the ohmic voltage drop across the active material of the anode 1 small, positive in the loading direction and can therefore be estimated as advantageous for the inventive solution down to zero: 0 V ≤ φ - / s (x _crit , t) - φ - / s (0, t) << 1 mV ⇒ φ - / s (x _crit , t) - φ - / s (0, t) ≈ 0 V

It follows that to avoid lithium plating: U _cell (t) ≤ (φ + / 0 (x _pos , t) + η ⁺ (x _pos , t) + η _l (t) =: U _ch (t)

The cell voltage of the battery cell 6 must therefore be controlled by a corresponding locations of the charging current so that it is less than or equal to the concentration-dependent electrochemical potential of the cathode material based on the electrochemical potential of the metal reference electrode, advantageously less than or equal to the concentration-dependent electrochemical potential of the cathode material based on the electrochemical potential of Metal reference electrode plus at least one overvoltage, in the example of the reaction overvoltage at the cathode illustrated here 3 and / or the electrolyte overvoltage, thereby depositing metallic lithium on the surface of the anode 1 is avoided. The size (s) or the respective sum of the variables, hereinafter referred to as U _ch , must be calculated continuously (for example by means of a control device of a motor vehicle).

In comparison to the direct calculation of the anode potential φ _An (x, t) (for instance by means of the mentioned control device), it is advantageous that, when the quantities to be calculated (U _{ch, calculated} <U _{ch, is} ) are underestimated _, the control of the cell voltage is up U _cell (t) = U _{ch, calculated} (t) leads to a lower charging current and thus lithium plating is excluded. This is especially helpful if the aging of the battery cell 6 can not be tracked. An aging of a battery cell 6 tends to lead to higher overvoltages, so that the method according to the present invention even with aging, ie during the entire life of the battery cell 6 applicable or remains valid.

Another advantage especially with lithium-ion cells with a negative porous graphite electrode is that the modeling of the diffusion in the graphite particles is much more complex than with the mixed oxides used on the cathode side. Thus, can φ + / 0 (x _pos , t) be calculated with the existing model approaches with a better accuracy than the concentration-dependent electrochemical potential of the anode φ - / 0 (x, t) , which is needed to calculate the anode potential φ _An (x, t).

2 schematically shows an exemplary device 4 (a control loop 4 ) for carrying out the method according to the invention, such as, for example, in a motor vehicle for the charging of a battery cell 6 is usable.

In order to prevent other damage mechanisms in addition to lithium plating, a maximum cell voltage U _{max is} specified by the manufacturer during a charge of the battery cell 6 may not be exceeded. For this reason, in the vehicle, the minimum (min) between this upper limit voltage U _max and the cell model 7 calculated size U _ch (t) are formed. The result is a maximum target voltage. The difference to the actual cell voltage is the input variable for a controller 5 , which has as output a maximum charging current I _max as a manipulated variable. This value can be reduced by a further minimum formation (min), since the maximum charging current I _max, for example, must be limited in addition to lithium plating in order to avoid other damage mechanisms. Also, the desired torque of the drive for speed reduction, which is about to be converted by the energy management system of the motor vehicle into a charging current, or limit the maximum charging current of the charger or the charging station. The result is a maximum charging current with which the battery cell 6 can be charged without damaging lithium plating.

The method and the device 4 according to the present invention are also in a direct parallel connection of multiple battery cells 6 applicable. In the cell model 7 In this case, only the changed thermal and electrical boundary conditions have to be adapted. The calculated maximum voltage U _ch (t) then applies to the entire composite of parallel-connected battery cells 6 , Next it is necessary that the regulator 5 on the higher capacity and lower internal resistance of the parallel connection of battery cells 6 is set.

Be several individual battery cells 6 or several series-connected battery cell units connected in series, must for each battery cell unit separately a maximum voltage U _ch (t) by a cell model 7 , as described above. The regulator 5 must then ensure that no battery cell unit exceeds the maximum voltage due to an excessively high charging current.

The method according to the invention and the device according to the invention 4 and its or their advantageous developments are in all battery cells 6 with an intercalation electrode, are stored in the ions during charging, applicable.

Also, the inventive method and apparatus of the invention 4 and its or their advantageous developments in the automotive field, in stationary storage, in the field of "Consumer Electronics", in the field of model making or other areas on or usable, in which rechargeable electrochemical energy storage cells (accumulators) are used.

LIST OF REFERENCE NUMBERS

1

anode

2

separator

3

cathode

4

contraption

5

regulator

6

battery cell

7

cell model

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

• DE 102016007479 A1 [0008, 0011, 0016, 0017, 0018]

Claims (10)

Method for charging a rechargeable electrochemical energy storage cell ( 6 ) with an anode ( 1 ) and a cathode ( 3 ), wherein during the charging process, an electrical charging current (I) in the energy storage cell ( 6 ) is fed, characterized in that the electrical charging current (I) is controlled by means of a cell model such that the cell voltage (U _cell (t)) is less than or equal to the concentration-dependent electrochemical potential of the cathode based on the electrochemical potential of the metal reference electrode (φ + / 0 (x _pos , t) vs. Me / Me ^{n +} ) is an electrochemical reaction at the anode ( 1 ) to avoid. A method according to claim 1, characterized in that the electrical charging current (I) is controlled by means of a cell model such that the cell voltage (U _cell (t)) is less than or equal to the sum (U _ch (t)) that is formed from the concentration-dependent electrochemical potential of the cathode based on the electrochemical potential of the metal reference electrode (φ + / 0 (x _pos , t) vs. Me / Me ^{n +} ) and at least one other addend, which represents an overvoltage, preferably the reaction overvoltage at the cathode (η ⁺ (x _pos , t)) and / or the electrolyte overvoltage (η _l (t)). Method according to claim 1 or 2, characterized in that as an input to the cell model ( 7 ) a vector having at least one measured temperature (T) of the rechargeable electrochemical energy storage cell ( 6 ), optionally with at different locations of the rechargeable electrochemical energy storage cell ( 6 ) measured temperature (T) is used. Method according to claim 2 or 3, characterized in that by the cell model ( 7 ) an overvoltage or overvoltages are taken into account by an offset value. Method according to one of the preceding claims, characterized in that the determination of the concentration-dependent electrochemical potential of the cathode based on the electrochemical potential of the metal reference electrode (φ + / 0 (x _pos , t) vs. Me / Me ^{n +} ) or the sum (U _ch (t)) using at least one simplifying model approach. Method according to one of the preceding claims, characterized in that the charging current is determined according to one of claims 1 to 5 and additionally an empirically or simulatively determined parameter is subtracted from the cell voltage determined by the cell model to account for a model error of the cell model. Method according to one of the preceding claims, characterized in that in the cell model used ( 7 ) a tracking of the model parameters via the aging of the rechargeable electrochemical energy storage cell ( 6 ) is made. Method according to one of the preceding claims, characterized in that it - with a rechargeable electrochemical energy storage cell ( 6 ) which is only partially unloaded; - it with a lithium-ion energy storage cell ( 6 ) is carried out; - it with a lithium-ion energy storage cell ( 6 ) with a graphite anode ( 1 ) is carried out; The current, temperature and charge state distribution within the rechargeable electrochemical energy storage cell ( 6 ) are taken into account; - the aging of the rechargeable electrochemical energy storage cell ( 6 ) by adapting the age-dependent model parameters over the operating life of the rechargeable electrochemical energy storage cell ( 6 ) is taken into account; - it with a rechargeable battery with parallel connected, rechargeable electrochemical energy storage cells ( 6 ) or with a series connection of one or more parallel-connected rechargeable electrochemical energy storage cells ( 6 ), in particular with a battery having lithium-ion energy storage cells ( 6 ), preferably with a battery having lithium-ion energy storage cells ( 6 ), each having a graphite anode ( 1 ) contain. Contraption ( 4 ) for charging a rechargeable electrochemical energy storage cell ( 6 ) with an anode ( 1 ) and a cathode ( 3 ), characterized in that the device ( 4 ) is adapted to control the charging current (I) by means of a cell model such that the cell voltage (U _cell (t)) is less than or equal to the concentration-dependent electrochemical potential of the cathode based on the electrochemical potential of the metal reference electrode (φ + / 0 (x _pos , t) vs. Me / Me ^{n +} ) is an electrochemical reaction at the anode ( 1 ) to avoid. Contraption ( 4 ) according to claim 9, characterized in that it is adapted to carry out a method according to one of claims 2 to 8.

Images