SEMI-INTERPENETRATING POLYMER NETWORKS BASED ON POLYCARBONATES AS SEPARATORS FOR USE IN ALKALI-METAL BATTERIES

Abstract

A solid electrolyte for an alkali metal solid state battery, the solid electrolyte comprising a mixture of two different alkali metal conducting salts and a semi-interpenetrating network (sIPN) of a crosslinked and a non-crosslinked polymer, wherein the semi-interpenetrating network is greater than or equal to 50 wt.-% and less than or equal to 80 wt.-% of a non-crosslinked polymer selected from the group consisting of polyethylene oxide (PEO), polycarbonate (PC), polycaprolactone (PCL), chain end modified derivatives of these polymers or mixtures of at least two components thereof; and greater than or equal to 10 wt.-% and less than or equal to 50 wt.-% of a polycarbonate of crosslinkable polyalkyl carbonate monomers having a carbon number greater than or equal to 2 and less than or equal to 15 based on the single monomer as the crosslinked polymer.

Claims

1. Solid electrolyte for an alkali metal solid state battery, wherein the solid electrolyte comprises a mixture of two different alkali metal conducting salts and a semi-interpenetrating network (sIPN) of a crosslinked and a non-crosslinked polymer, wherein the semi-interpenetrating network comprises: greater than or equal to 50 wt.-% and less than or equal to 80 wt.-% of a non-crosslinked polymer selected from the group consisting of polyethylene oxide (PEO), polycarbonate (PC), polycaprolactone (PCL), chain end modified derivatives of these polymers, or mixtures of at least two components thereof; and greater than or equal to 10 wt.-% and less than or equal to 50 wt.-% of a polycarbonate of crosslinkable polyalkyl carbonate monomers having a carbon number greater than or equal to 2 and less than or equal to 15 based on the single monomer as the crosslinked polymer, wherein the single polyalkyl carbonate monomer may be substituted or unsubstituted and comprises two crosslinkable groups selected from the group consisting of acrylic, methacrylic, epoxy, vinyl, isocyanide or mixtures of two different groups thereof

2. The solid electrolyte according to claim 1, wherein the weight fraction of crosslinked to uncrosslinked polymer in the sIPN is greater than or equal to 20 wt.-% and less than or equal to 40 wt.-%.

3. The solid electrolyte according to claim 1, wherein the molecular weight of the polyalkyl carbonate monomers is greater than or equal to 100 g/mol and less than or equal to 3500 g/mol.

4. The solid electrolyte according to claim 1, wherein the polyalkyl carbonate monomers are selected from the group consisting of straight-chain or branched, substituted or unsubstituted polyethylene, polymethylene, polypropylene, polybutylene, polyhexylene carbonates or mixtures of at least two components thereof.

5. The solid electrolyte according to claim 1, wherein the polyalkyl carbonate monomers each carry two identical functional groups and the functional group is a methacryl group.

6. The solid electrolyte according to claim 1, wherein the mixture of two different alkali metal conducting salts comprises at least the salts alkali (fluorosulfonyl) (trifluoromethanesulfonyl)imide (FTFSI) and alkali bis(trifluoromethanesulfonyl)imide) (TFSI).

7. The solid electrolyte according to claim 6, wherein the weight ratio of alkali (fluorosulfonyl) (trifluoromethanesulfonyl)imide (FTFSI) to the weight sum of the components of sIPN and the further conducting salt, expressed as the weight of alkali FTFSI divided by the sum of the weights of sIPN and further conducting salt, is greater than or equal to 0.005 and less than or equal to 0.1.

8. The solid electrolyte according to claim 1, wherein the solid electrolyte is a solid electrolyte for a Li-solid battery.

9. Alkali metal battery comprising an anode, a cathode, and a solid electrolyte disposed between the anode and the cathode, wherein the solid electrolyte is a solid electrolyte according to claim 1.

10. The battery according to claim 9, wherein the battery is a Li metal battery and the battery comprises at least one high current or high voltage electrode.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0037] These and other aspects of the invention will be apparent from and elucidated with reference to the figures and examples described hereinafter, wherein even individual features disclosed in the figures and the examples and in the disclosure as a whole can constitute an aspect of the present invention alone or in combination, wherein additionally, features of different embodiments can be carried over from one embodiment to another embodiment without leaving the scope of the present invention.

[0038] In the drawings:

[0039] FIGS. 1 shows the result of a Li plating/stripping experiment of a cell assembly not according to an embodiment as a function of time;

[0040] FIG. 2 shows the result of galvanostatic cycling of a cell assembly not according to an embodiment as a function of time;

[0041] FIG. 3 shows the result of a Li plating/stripping test once of a cell assembly not according to an embodiment and once of a cell assembly according to an embodiment with a dual salt electrolyte as a function of time;

[0042] FIG. 4 shows the result of galvanostatic cycling of a cell assembly not according to an embodiment and a cell assembly according to an embodiment with a dual-salt electrolyte as a function of time; and

[0043] FIG. 5 shows the result of a mechanical stability test, once of a cell structure not according to an embodiment and once of a cell structure according to an embodiment, as a function of the pressure deflection.

DETAILED DESCRIPTION

EXAMPLS

I. Preparation of the Solid Electrolytes

[0044] An sIPN for a Li battery is produced.

I.a. Synthesis of the Polycarbonate Network Former

[0045] The synthesis of the polycarbonate network former is carried out under inert gas. 10 g poly(l,6-hexanediol)carbonate diol (Mw=1000 g/mol) are dissolved in dry dichloromethane (100 mL). Approximately 0.5 g magnesium sulfate is added to dry the polycarbonate and the mixture is stirred overnight. The mixture is filtrated to remove the magnesium sulfate. DMAP (4-(dimethylamino)pyridine) (0.001 mol % per terminal hydroxyl group), and triethylamine (2 equivalents based on terminal hydroxyl groups) are then added. With stirring and cooling to 0° C., methacryloyl chloride (1.2 equivalents based on terminal hydroxyl groups) is carefully added. The reaction mixture is stirred at room temperature for three days. The crude product is washed 5 times with 2M aqueous HCl solution (5×50 mL) to extract from the organic phase the polar reactants and by-products of the reaction. A separatory funnel is used for phase separation. The organic phase was dried over magnesium sulfate and the solvent was removed under reduced pressure. The product is dried under vacuum at RT for several days. The dried product is stored under inert gas.

I.b. Preparation of an SIPN according to the Invention

[0046] The conducting salt combination in the molar ratio of 13 parts Li-TFSI (0.289 g) to 1 part Li-FTFSI (0.018 g) is dissolved together with polycarbonate (poly(l,6-hexanediol) carbonate dimethacrylate) (0.125 g) and the radical initiator AIBN (azobisisobutyronitrile) (0.018 g) in 3 mL acetonitrile or THF as solvent and then the PEO powder (0.5 g) is added. The mixture with a conducting salt to polymer ratio of 1 to 3 is stirred for several hours and, after complete homogenization, can be applied to a Mylar film by film casting in basically any thickness. The solvent is evaporated in a fume hood, the polymer film produced is polymerized at 70° C. under nitrogen flow for one hour and then dried overnight under vacuum. Possible thicknesses for the solid electrolyte range from greater than or equal to 1 μm to less than or equal to 500 μm.

I.c. Production of a Battery

[0047] For use in lithium metal battery cells, a round piece of polymer film 200 μm high and 17 mm in diameter is die-cut and used analogously to a separator between lithium metal electrode and positive electrode consisting of 91 wt % LiNi.sub.0.6Mn.sub.0.2Co.sub.0.2O.sub.2, 4 wt.-% carbon black and wt.-% PVdF. Lithium metal battery cells prepared in this way were tested at 60° C.

[0048] The electrochemical behavior of battery assemblies according to an embodiment and those not according to an embodiment is shown in FIGS. 1 to 5. It shows the [0049] FIG. 1 the result of a Li plating/stripping experiment of a cell assembly not according to an embodiment as a function of time; [0050] FIG. 2 the result of galvanostatic cycling of a cell assembly not according to an embodiment as a function of time; [0051] FIG. 3 the result of a Li plating/stripping test once of a cell assembly not according to an embodiment and once of a cell assembly according to an embodiment with a dual salt electrolyte as a function of time; [0052] FIG. 4 the result of galvanostatic cycling of a cell assembly not according to an embodiment and a cell assembly according to an embodiment with a dual-salt electrolyte as a function of time; [0053] FIG. 5 the result of a mechanical stability test, once of a cell structure not according to an embodiment and once of a cell structure according to an embodiment, as a function of the pressure deflection.

[0054] FIG. 1 shows the voltage response of a Li ∥ Li cell with an s-IPN without the use of two different electrolytes. The two Li electrodes are used alternately for one hour each as positive and negative electrode under the influence of a constant current of 50 μA/cm.sup.2, whereby Li is alternately transported from one side to the other through the electrolyte. The cell shows cell failure after a relatively short time of 100 h, which is due to a short circuit.

[0055] FIG. 2 shows the voltage behavior of a conventional galvanostatic cycling of a Li ∥ NMC622 cell with a polymer electrolyte of PEO and polycarbonate, but with only one conducting salt (Li-TFSI) in a concentration of 30 wt.-% conducting salt to the total weight of the s-IPNs. The weight ratio of PEO to polycarbonate here is 1 to 4. This cell also shows a time-dependent error, evident from the noise in the voltage curve.

[0056] FIG. 3 shows the voltage curve of a Li plating/stripping experiment as a function of time, once for a cell setup according to an embodiment and once for a cell setup not according to an embodiment. The same Li ∥ Li cell assembly was run with the same s-IPN but once, according to an embodiment, with Li-FTFSI/Li-TFSI as “dual salt” electrolyte and once with only Li-TFSI as electrolyte. It can be clearly seen that the dual salt approach with Li-FTFSI and Li-TFSI runs significantly longer and without failure. Without being bound by theory, it is suspected that the Li electrode is stabilized by the combination of the s-IPN with Li-FTFSI and Li-TFSI.

[0057] FIG. 4 shows the result of galvanostatic cycling of a Li ∥ NMC622 cell with different compositions of the solid electrolytes. If PEO and Li-TFSI alone are used (PEO.sub.12LiTFSI), the battery cell already exhibits a defect at the beginning of the cyclization, presumably caused by a short circuit. The addition of another lead salt only to PEO as the sole polymer (LiFTFSI+PEO.sub.12LiTFSI) shows no significant improvement in electrical behavior and the cell fails after a short time. The addition of carbonate to Li-FTFSI in PEO.sub.12LiTFSI without formation of a semi-interpenetrating network, i.e. without crosslinking of the individual polycarbonate monomers, also shows no improvement in electrical behavior. Only the combination of two different electrolytes (Li-FTFSI and TFSI) and the formation of a semi-interpenetrating network of PEO and crosslinked polycarbonate, on the other hand, shows error-free cycling over the entire measurement period.

[0058] FIG. 5 shows a mechanical stability test of a solid electrolyte according to an embodiment and one not according to an embodiment. By using solid electrolytes according to an embodiment with an sIPN of crosslinked polycarbonates and non-crosslinked PEO, a significant improvement in compressive strength is shown compared to only a PEO network. The compressive strength was determined using a compressibility test rig, in which a polymer sample of 2 mm height and 18 mm diameter is compressed between two stainless steel plates under a constant feed rate of 20 μm/min and the force required to achieve this is measured. This improved compressive strength, together with the dual salt approach, could be the reason for the improved Li compatibility and error-free cycling of Li ∥ NMC622 cells.

[0059] All the features and advantages, including structural details, spatial arrangements and method steps, which follow from the claims, the description and the drawing can be fundamental to the invention both on their own and in different combinations. It is to be understood that the foregoing is a description of one or more preferred exemplary embodiments of the invention. The invention is not limited to the particular embodiment(s) disclosed herein, but rather is defined solely by the claims below. Furthermore, the statements contained in the foregoing description relate to particular embodiments and are not to be construed as limitations on the scope of the invention or on the definition of terms used in the claims, except where a term or phrase is expressly defined above. Various other embodiments and various changes and modifications to the disclosed embodiment(s) will become apparent to those skilled in the art. All such other embodiments, changes, and modifications are intended to come within the scope of the appended claims.

[0060] As used in this specification and claims, the terms “for example,” “for instance,” “such as,” and “like,” and the verbs “comprising,” “having,” “including,” and their other verb forms, when used in conjunction with a listing of one or more components or other items, are each to be construed as open-ended, meaning that the listing is not to be considered as excluding other, additional components or items. Other terms are to be construed using their broadest reasonable meaning unless they are used in a context that requires a different interpretation.