PRIMARY ALKALI METAL CELLS WITH GEMINAL DINITRILE ADDITIVES

Abstract

The invention belongs to a primary cell. Said primary cell contains at least one anode, with an alkali metal as active anode material, at least one cathode with an active cathode material and an electrolyte. Additionally the electrolyte comprises at least one additive whereby it is in accordance with the inventive idea that the at least one additive is a nonionic or ionic compound having at least one geminal dinitrile moiety and is selected from the group consisting of aliphatic heterocycles, compounds of the formula

##STR00001##

wherein M.sup.y+ denotes a counterion with a valence of y, and wherein R, R and R are substituents with an aliphatic or aliphatic heterocyclic backbone.

Claims

1. A primary cell comprising at least one anode, with an alkali metal as active anode material, at least one cathode with an active cathode material and an electrolyte, wherein the electrolyte comprises at least one additive, wherein the at least one additive is a nonionic or ionic compound having at least one geminal dinitrile moiety and is selected from the group consisting of aliphatic heterocycles, compounds of the formula ##STR00010## compounds of the formula ##STR00011## or compounds of the formula ##STR00012## wherein M.sup.y+ denotes a counterion with a valence of y, and wherein R, R and R are substituents with an aliphatic or aliphatic heterocyclic backbone.

2. A primary cell according to claim 1, wherein the nonionic compounds having at least one geminal dinitrile moiety are selected from the group consisting of ##STR00013## ##STR00014## wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, and R.sub.6, independently of one another, denote hydrogen, unsubstituted, mono- or multi-substituted alkyl, alkenyl, cycloalkyl, thioether, heterocyclic, aryl and/or heteroaryl substituents.

3. A primary cell according to claim 1, wherein the ionic compounds having at least one geminal dinitrile moiety are selected from the group consisting of ##STR00015## ##STR00016## wherein R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, and R.sub.6, independently of one another, denote hydrogen, unsubstituted, mono- or multi-substituted alkyl, alkenyl, cycloalkyl, thioether, heterocyclic, aryl and/or heteroaryl substituents.

4. A primary cell according to claim 2, wherein the substituents of mono- or multi-substituted R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, and R.sub.6 moieties, independently of one another, are selected from the group comprising alkyl, fluoroalkyl, alkoxy, carbonyl, carboxyl, thiol, thioalkoxide, aryl, ether, thioether, nitro, cyano, amino, azido, amidino, hydrazino, hydrazono, carbamoyl, sulfo, sulfamoyl, sulfonylamino, alkyl-aminosulfonyl, alkylsulfonylamino moieties, and/or halogens, preferably halogens, fluoroalkyl and/or cyano moieties.

5. A primary cell according to claim 1, wherein y=1 or 2, m=1 or 2 and the counterion M.sup.y+ is selected from the group comprising hydroxonium, nitrosonium, ammonium, alkali metal ions, metal ions of valence y, organic ions of valence y and/or an organometallic cations of valence y for the ionic geminal dinitrile additive.

6. A primary cell according to claim 1, wherein the at least one additive has a concentration of 0.0005 mol/l to 0.5 mol/l, preferably 0.005 mol/l to 0.4 mol/l, most preferably 0.05 mol/l to 0.3 mol/l in the electrolyte.

7. A primary cell according to claim 1, wherein the alkali metal used as active anode material is metallic sodium, a sodium alloy, metallic lithium or a lithium alloy, preferably lithium or a lithium alloy.

8. A primary cell according to claim 1, wherein the active cathode material is a solid material comprising a metal, a metal oxide, a mixed metal oxide, a metal sulfide, a metal fluoride, carbonaceous compounds or mixtures thereof, preferably comprising MnO.sub.2, silver vanadium oxide (SVO), copper silver vanadium oxide (CSVO), V.sub.2O.sub.2, TiS.sub.2, CuO.sub.2, Cu.sub.2S, FeS, FeS.sub.2, CF.sub.x, Ag.sub.2O, Ag.sub.2O.sub.2, CuF, Ag.sub.2CrO.sub.4, CuO, copper vanadium oxide or a mixture thereof, particularly preferably comprising MnO.sub.2.

9. A primary cell according to claim 1, wherein the primary cell is a lithium metal battery comprising at least one anode with lithium as active anode material and at least one cathode with MnO.sub.2 as active cathode material.

10. Use of a primary cell according to claim 1 as an implantable battery or battery in a medical device, particularly an implantable medical device.

11. A medical device, particularly an implantable medical device, comprising a primary cell according to claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0053] In the figures:

[0054] FIG. 1 is a graph showing an illustrative pulse discharge curve of a primary cell as according to example 1.

[0055] FIG. 2 is a graph showing an illustrative pulse discharge curve of a primary cell as according to example 2.

[0056] FIG. 3 is a graph showing an illustrative pulse discharge curve of a primary cell as according to example 3.

[0057] FIG. 4 is a graph showing an illustrative pulse discharge curve of a primary cell as according to comparative example C1.

[0058] FIG. 5 is a graph showing the total amounts of deposited lithium on the inner lid surface, the inner housing surface and contact components of cells 1 to 20.

[0059] FIG. 6 is a graph showing a comparison of the cyclic voltammograms of electrolyte 1 with and without tetracyanoethylene as additive.

[0060] FIG. 7A is a graph showing an illustrative first pulse discharge of an electrochemical cell without voltage delay.

[0061] FIG. 7B is a graph showing an illustrative second pulse discharge of an electrochemical cell without voltage delay.

[0062] FIG. 8A is a graph showing an illustrative first pulse discharge of an electrochemical cell with voltage delay.

[0063] FIG. 8B as a graph showing an illustrative second pulse discharge of an electrochemical cell with voltage delay.

[0064] FIG. 9A is a graph showing the development of the 1 kHz-impedance of a primary cell as according to example 2.

[0065] FIG. 9B is a graph showing the development of the kHz-impedance of a primary cell as according to comparative example C1.

[0066] FIG. 10A is a graph showing an illustrative pulse discharge of a primary cell as according to example 2 after a 30-day recovery phase.

[0067] FIG. 10B is a graph showing an illustrative pulse discharge of a primary cell as according to comparative example C1 after a 30-day recovery phase.

[0068] FIG. 11A is a graph showing an illustrative cyclic voltammogram the standard electrolyte with 0.01 M tetracyanoethylene.

[0069] FIG. 11B is a graph showing a comparison of the cyclic voltammograms of the standard electrolyte with and without tetracyanoethylene as additive in presence of Mn.sup.2+ ions.

[0070] FIG. 12 is a graph showing a comparison of the cyclic voltammogram's of the standard electrolyte with and without tetracyanoethylene as additive in presence of Fe.sup.2+ ions.

DETAILED DESCRIPTION

[0071] The following examples illustrate the current invention, but the present invention is not limited by and to these examples. All examples are shown in table 1.

[0072] Primary lithium metal MnO.sub.2 cells are used as model system to determine the effect of the geminal dinitriles on the lithium deposition as well as a voltage delay. The cathode comprises MnO.sub.2 as active cathode material, mixed with graphite (3 wt % of total composition) and carbon black (2 wt % of total composition) as conductive additives as well as polytetrafluoroethylene (3 wt % of total composition) as binder. The anode comprises metallic lithium and the electrolyte comprises LiClO.sub.4 (1 mol/l) in a mixture of 1,2-dimethoxyethan, ethylencarbonate and propylencarbonate (ratio 4:4:2 (v/v)). Further the geminal dinitriles tert-butylmalononitrile (TBMN), tetracyanoethylene (TCNE) and acetylmalononitrile (AMN) were added in a concentration of 0.02 mol/l to the electrolyte (see Examples 1, 2 and 3 in table below. Primary lithium metal MnO.sub.2 cells without a geminal dinitrile additive are used as comparative examples C1. Multiple cells (see cells 1 to 20) of each example outlined above were analyzed to ensure reproducibility of the results.

TABLE-US-00001 TABLE 1 Examples with different electrolytes. Example cell no. electrolyte Example cell 1 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethan, ethylencarbonate and 1 propylencarbonate (4:4:2) + TBMN (0.02M) cell 2 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethan, ethylencarbonate and propylencarbonate (4:4:2) + TBMN (0.02M) cell 3 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethan, ethylencarbonate and propylencarbonate (4:4:2) + TBMN (0.02M) cell 4 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethan, ethylencarbonate and propylencarbonate (4:4:2) + TBMN (0.02M) cell 5 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethan, ethylencarbonate and propylencarbonate (4:4:2) + TBMN (0.02M) Example cell 6 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethan, ethylencarbonate and 2 propylencarbonate (4:4:2) + TCNE (0.02M) cell 7 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethan, ethylencarbonate and propylencarbonate (4:4:2) + TCNE (0.02M) cell 8 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethan, ethylencarbonate and propylencarbonate (4:4:2) + TCNE (0.02M) cell 9 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethan, ethylencarbonate and propylencarbonate (4:4:2) + TCNE (0.02M) cell 10 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethan, ethylencarbonate and propylencarbonate (4:4:2) + TCNE (0.02M) Example cell 11 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethan, ethylencarbonate and 3 propylencarbonate (4:4:2) + AMN (0.02M) cell 12 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethan, ethylencarbonate and propylencarbonate (4:4:2) + AMN (0.02M) cell 13 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethan, ethylencarbonate and propylencarbonate (4:4:2) + AMN (0.02M) cell 14 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethan, ethylencarbonate and propylencarbonate (4:4:2) + AMN (0.02M) cell 15 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethane, ethylencarbonate and propylencarbonate (4:4:2) + AMN (0.02M) Comparative cell 16 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethane, ethylencarbonate and example propylencarbonate (4:4:2) C1 cell 17 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethane, ethylencarbonate and propylencarbonate (4:4:2) cell 18 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethane, ethylencarbonate and propylencarbonate (4:4:2) cell 19 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethane, ethylencarbonate and propylencarbonate (4:4:2) cell 20 LiClO.sub.4 (1 mol/l) in 1,2-dimethoxyethane, ethylencarbonate and propylencarbonate (4:4:2)

[0073] Herein, the term standard electrolyte is used for a 1 mol/l solution of LiClO.sub.4 in a mixture of 1,2-dimethoxyethan, ethylencarbonate and propylencarbonate (4:4:2 (v/v).

[0074] In order to provoke the deposition lithium on the anode surface as well the inner surfaces of the metallic passive parts, cells 1 to 25 were subjected to a 16-day test until cutoff voltage of 1.5 V has been reached in order to simulate usage in an ICD under accelerated conditions. The daily pulse trains consisted of 25 pulses with a duration of 10 seconds, a current density of 39 mA/cm.sup.2 and a voltage of up to 1.5 V. The resting period in-between two pulses was 15 seconds and the testing temperature was set to 37? C. to mimic a human's body temperature. Upon finishing the testing period, the cells were opened and the inner surfaces, in particular the inner surface of the lid, were analyzed for lithium depositions using inductively coupled plasma atomic emission spectroscopy (ICP-OES).

[0075] The graphs depicted in FIGS. 1 to 4 show the pulse discharge curves of primary cells as according to examples 1, 2, 3 and comparative example C1. The cells according to examples 1, 2 and 3 exhibit a similar behavior during testing period.

[0076] The initial voltage is about 2.25 V, which subsequently rises to reach a maximum of approximately 2.35 to 2.4 V after around 600 to 700 mAh. This initial increase may be attributed to structure changes of the cathode active material during the cell discharge, particularly for the use of MnO.sub.2, mostly a change from Pyrolusite structure to Spinel Structure is observed.

[0077] After reaching its maximum the measured voltage drops only slowly initially, thus creating a plateau between 0 and 1200 mAh. During this plateau phase the cell's output remains effectively constant. After the voltage falls below the initial voltage, the drop in voltage subsequently accelerates until the cutoff voltage of 1.5 V is reached at around 1800 mAh, which denoted the end of life of the respective cell.

[0078] It becomes immediately obvious, that the presence of a geminal dinitrile as according to the present invention reproducibly leads to a near-identical behavior of each individual cell, i.e., the individual discharge curves only differ by a small margin.

[0079] A comparison of examples 1 to 3 to the cells lacking a geminal dinitrile, i.e., cells 16 to 20 of comparative example C1, shows, that despite having a slightly higher initial voltage (approximately 2.37 V as compared to 2.25 V) and higher maximum voltage (approximately 2.45 V at 500 mAh) the end of the desired plateau is already reached at about 1000 mAh.

[0080] Moreover, two cells exhibited a sharp drop in voltage immediately after the end of the plateau phase, thus reaching the cutoff voltage of 1.5 V prematurely at 1400 mAh. This observation is attributed to an excessive deposition of metallic lithium resulting in a bridging of the anode and another metallic component of the cell. Consequently, the cell gets drained due to an internal short-circuit.

[0081] FIG. 5 shows the total amounts of deposited lithium on the inner lid surface, the inner housing surface and contact components of cells 1 to 20. The beneficial effect of the geminal dinitrile additive becomes immediately obvious. Whereas the primary cells featuring a geminal dinitrile exhibit hardly any lithium deposition even after the testing period, the determined amount of lithium ranged between 950 to 1200 ?g for comparative cells 16 to 20, thus exceeding cells as according to the present invention by up to three orders of magnitude.

[0082] The influence of the geminal dinitrile additive was further investigated by cyclovoltammetry (CV). CV is a common method to study redox and follow-up reactions taking place in a cell. Thereby, valuable information regarding the reactions themselves, possible depositions and the effect of additives in the electrolyte can be obtained. CV is a potentiodynamic method, whereby the potential of the working electrode is ramped linearly versus time. Once the desired potential is reached, the working electrode's potential is ramped in the opposite direction to return to the initial potential, thus leading to a triangular potential-time function. The current at the working electrode is plotted versus the applied voltage to give the cyclic voltammogram.

[0083] FIG. 6 shows a comparison of the standard electrolyte with and without TCNE (0.01 mol/l). The CV setup comprises a platinum working electrode, a platinum counter electrode and a lithium reference electrode. The scan rate was set to 50 mV/s.

[0084] The cyclic voltammogram of the standard electrolyte shows two distinct peaks, whereby the reduction of Li.sup.+ ions to metallic lithium is observed in a potential range between 0 V and ?0.6 V (vs. Li/Li.sup.+). This corresponds to the deposition of lithium. The back scan shows the oxidation of the metallic lithium to Li.sup.+ ions in a potential range between 0V and 0.6 V (vs Li/Li.sup.+), corresponding to a dissolution of lithium in the electrolyte. The reduction and oxidation reactions are as follows:

Reduction: Li.sup.++e.sup.?.fwdarw.Li.sup.0

Oxidation: Li.sup.0.fwdarw.Li.sup.++e.sup.?

[0085] In contrast to the two distinct peaks in the cyclic voltammogram of the standard electrolyte, the addition of TCNE in a concentration of 0.01 mol/l shows two reduction peaks at E.sub.red1=2.34 V and E.sub.red2=1.41 V, indicating the reduction of the TCNE additive on the electrode surface. The lack of corresponding oxidation peaks suggests that these reactions are irreversible, forming a stable SEI layer. Moreover, the reduction reactions of the TCNE prevents the subsequent reduction of Li.sup.+ ions and thus deposition of metallic lithium on the inner surfaces. This is confirmed by the absence of the redox peaks observed for the standard electrolyte without the geminal dinitrile additive.

[0086] Lithium MnO.sub.2 cells are based on the intercalation of Li.sup.+ ions into the MnO.sub.2 lattice. The underlying redox reactions are as follows:

Anode: Li.sup.0.fwdarw.Li.sup.++e.sup.?

Cathode: Mn.sup.IVO.sub.2+Li.sup.++e.sup.?.fwdarw.LiMn.sup.IIIO.sub.2

Total: Li.sup.0+MnO.sub.2.fwdarw.LiMnO.sub.2

[0087] Whereas the overall reaction is irreversible, the dissolution of manganese ions under certain conditions and their subsequent deposition on the anode surface results in the formation of a high-resistance surface layer leading to an increase of the impedance and a voltage delay, respectively.

[0088] FIGS. 7A and 7B show an illustrative first and second pulse discharge of an electrochemical cell without voltage delay. For both pulses the cell potential decreases throughout the pulse duration, reaching a minimum at the end of a pulse. Moreover, the minimum potential of the first pulse is higher than the minimum potentials of subsequent pulses in a pulse train.

[0089] However, if a cell exhibits a voltage delay, the leading-edge potential of the first pulse forms a minimum potential, which is lower than the end potential of the first pulse. This behavior reflects the increased cell impedance due to the formation of high-resistance surface layers on the electrodes, thereby limiting the effectiveness and in the worst case the proper functioning of the cell and the supplied electronic device. FIGS. 8A and 8B show an illustrative first and second pulse discharge of an electrochemical cell exhibiting voltage delay.

[0090] In order to provoke the deposition of manganese on the anode surface, three cells as described, for example 1, and for comparative example C1, respectively, were discharged to 90% DOD. This was achieved under pulse discharge conditions with pulse trains consisting of four pulses with a duration 10 seconds each, a current density of 33 mA/cm.sup.2, a 10 second resting period in between two pulses, and a 30-minute period in between two pulse trains.

[0091] Subsequently, a 30-day recovery period followed, in which the cells were further discharged with a load of just 100 k?. Thereby, the manganese dissolved during the pulse discharge could form a high-resistance layer on the surface.

[0092] FIGS. 9A and 9B show the development of the impedance measured of cells according to example 1 and comparative example C1, respectively. The data shown was obtained by impedance measurement at 1 KHz. Whereas the impedance of the cells in presence of TCNE remains essentially constant (approximately 0.16?) over the 30-day recovery period, the impedance of C1 cells doubled to nearly 0.3?. This observation already pronounces the excellent long-term stability of the primary cells comprising a geminal dinitriles as according to the present invention.

[0093] The pulse discharge curves of the 90% DOD cells according to example 1 and comparative example C1, respectively, after the 30-day recovery period are shown in FIGS. 10A and 10B.

[0094] Whereas the former exhibit the desired monotone potential decrease during the pulse duration, the latter clearly show a voltage delay, indicating the formation of a high-resistance anode surface layer. Moreover, the individual C1 cells significantly differ in their discharge behavior. In contrast, the curves of the individual example 1 cells only differ by a small margin, showing that the desired effect of geminal dinitrile additive is achieved reliably and reproducible.

[0095] In order to gain more insight on the manganese deposition reaction, CV measurements of a) the standard electrolyte with 0.01 mol/l TNCE (see FIG. 11A), the standard electrolyte with 0.01 mol/l MnClO.sub.4 (see FIG. 11B) and the standard electrolyte with 0.01 mol/l TCNE and 0.01 mol/l MnClO.sub.4 (see FIG. 11B) were conducted. MnClO.sub.4 was used to mimic the behavior of Mn.sup.II+ ions dissolved from the active cathode material.

[0096] Mn(II)ClO.sub.4 has been used since during the Li/Mn(IV)O.sub.2 cell discharge, Mn(IV) is reduced to Mn(III) and in some part of the cathode at lower voltages Mn(III) can be reduced to Mn(II) (Mn(II).sub.2O.sub.3 is the soluble version of manganese dioxide).

[0097] The CV setup comprises a platinum working electrode, a platinum counter electrode and a lithium reference electrode. The scan rate was set to 50 mV/s.

[0098] The cyclic voltammogram of the standard electrolyte with TCNE as additive exhibits two reduction peaks at E.sub.Red1=2.985 V and E.sub.Red2=2.265 V (vs Li/Li.sup.+) and two corresponding oxidation peaks at E.sub.Ox1=2.410 V and E.sub.Ox2=3.080 V (vs Li/Li.sup.+). The redox behavior of TCNE is therefore reversible.

[0099] In contrast, a reversible behavior is not observed for the standard electrolyte comprising Mn.sup.II+ ions. The irreversible reduction peak at 1.6 V (vs Li/Li.sup.+) indicates the reduction and deposition of manganese on the electrode surface.

[0100] In presence of both Mn.sup.II ions and a geminal dinitrile additive the redox behavior of the system is considerably altered. Whereas the absence of an oxidation peak still indicates an irreversible reaction, the reduction peak is shifted from 1.6 V to a significantly higher potential of 2.625 V (vs Li/Li.sup.+). This is a clear indication, that reduction reaction and the resulting reduction products have been modified. A possible explanation for this observation may be the formation of TCNE-Mn complexes in solution. Apparently, a subsequent reduction of said complexes does not lead to elemental manganese. Thus, the formation of a high-resistance anode layer is prevented.

[0101] This observation is not limited to Mn.sup.II ions. FIG. 12 shows a comparison of the cyclic voltammograms of standard electrolyte with and without TCNE as additive in presence of Fe.sup.II ions. Fe.sup.II ions were added in form of Fe(ClO.sub.4).sub.2 (c=0.01 mol/l) to simulate the dissolution of Fe.sup.II ions from a cathode comprising Fe.sup.II as active cathode material. The CV were measured under the same conditions as outlined above.

[0102] The reduction of Fe.sup.II leads to a deposition of elemental iron on the electrode surface in absence of a geminal dinitrile additive. This is indicated by an irreversible reduction peak at a potential of 1.975 V (vs. Li/Li.sup.+). The presence of TCNE shifts the irreversible reduction peak to a potential of 2.54 V, again indicating an alteration of reduction reaction and the resulting reduction species.

[0103] These results clearly show that geminal dinitriles as according to the present invention effectively prevent the deposition of metallic lithium on the inner surfaces as well as the formation of a high-resistance anodic layers.

[0104] It will be apparent to those skilled in the art that numerous modifications and variations of the described examples and embodiments are possible in light of the above teachings of the disclosure. The disclosed examples and embodiments are presented for purposes of illustration only. Other alternate embodiments may include some or all of the features disclosed herein. Therefore, it is the intent to cover all such modifications and alternate embodiments as may come within the true scope of this invention, which is to be given the full breadth thereof. Additionally, the disclosure of a range of values is a disclosure of every numerical value within that range, including the end points.