Safe Battery Solvents

Abstract

An ion transporting solvent for use with batteries can be improved by simultaneously shortening a phosphazene compound's pendent groups, eliminating most or all of the distal ion carriers, and randomizing the solvent molecules so as to intentionally disrupt symmetry to the maximum degree possible. The combination of these strategies dramatically improves battery performance to the point where the performance recorded is comparable to batteries using conventional organic solvents.

Claims

1. An electrolyte solvent, comprising: a cyclic phosphazene compound comprising: a cyclic backbone of alternating phosphorus atoms and nitrogen atoms and two associated pendent chemical chains bound to each of said phosphorus atoms, wherein said associated pendent chemical chains are randomized and comprise no more than ten skeletal atoms; at least one of said associated pendent chemical chains comprises no more than four skeletal atoms; each of said skeletal atoms chosen from the group consisting of carbon, oxygen, and sulfur; and each of said skeletal atoms bound directly to said phosphorous atoms chosen from the group consisting of oxygen and sulfur.

2. The electrolyte solvent of claim 1, further comprising an electrolyte salt.

3. The electrolyte solvent of claim 2, wherein the electrolyte salt is added in an amount sufficient to saturate the cyclic phosphazene compound.

4. The electrolyte solvent of claim 2, wherein said electrolyte salt is a lithium salt.

5. The electrolyte solvent of claim 1, further comprising a plurality of compatible carbonate solvent molecules.

6. The electrolyte solvent of claim 5, wherein said compatible carbonate solvent molecules are added in an amount comprising between about 1% and about 99.95% of the total chemical solvent composition.

7. The electrolyte solvent of claim 1, further wherein at least one of said associated pendent chemical chains comprises no more than three skeletal atoms.

8. The electrolyte solvent of claim 1, further wherein at least one of said associated pendent chemical chains comprises no more than two skeletal atoms.

9. The electrolyte solvent of claim 1, further wherein at least one of said associated pendent chemical chains comprises between two and four skeletal atoms.

10. The electrolyte solvent of claim 1, further wherein no more than three of said skeletal atoms in at least one of said associated pendent chemical chains are selected from the group consisting of oxygen and sulfur.

11. An electrolyte solvent, comprising: a cyclic phosphazene compound comprising: a cyclic backbone of alternating phosphorus atoms and nitrogen atoms and two associated pendent chemical chains bound to each of said phosphorus atoms, wherein said associated pendent chemical chains are randomized and comprise between zero and three distal ion carriers; and at least one of said associated pendent chemical chains comprises zero distal ion carriers.

12. The electrolyte solvent of claim 11, wherein each of said distal ion carriers are chosen from the group consisting of oxygen and sulfur.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0016] FIG. 1 is a table listing seven representative formulations of a compound suitable for use as a battery solvent.

[0017] FIG. 2 is a table showing the representative formulations having experienced a dramatic reduction in viscosity, particularly when saturated with lithium salt.

[0018] FIG. 3 shows that in the representative compounds, the solubility of the lithium salts did not drop as would have been expected under the teachings of the prior art.

[0019] FIG. 4 shows a specific example formulation according to the invention, comprising a plurality of reactions demonstrating the method of the claimed invention.

DETAILED DESCRIPTION

[0020] The present invention overcomes the deficiencies in the prior art by simultaneously shortening the pendent groups, eliminating most or all of the distal ion carriers, and randomizing the solvent molecules so as to intentionally disrupt symmetry to the maximum degree possible. The combination of these strategies dramatically improves battery performance to the point where the performance recorded is comparable to batteries using conventional organic solvents. The invention centers upon the improvement of the compound taught by the prior art, namely hexa-MEEP-T. In total, seven representative formulations were developed that improved upon hexa-MEEP-T as a battery solvent, though those of skill in the art will appreciate that many others are possible and will still fall within the scope of this disclosure. The formulations presented are described in FIG. 1.

[0021] As shown in FIG. 2, in contrast to the prior art, particularly hexa-MEEP-T, the new formulations experience a dramatic reduction in viscosity, particularly when saturated with a lithium salt, typically LiPF.sub.6. As shown in FIG. 3, the solubility of the lithium salts did not drop nearly as precipitously as was expected from the teachings of the prior art. This is postulated to be due to the direct association of the phosphazene nitrogen with the lithium ion, especially in the smallest systems where the nitrogen centers are the most sterically exposed.

[0022] A further aspect of the invention builds upon the concepts of pendent group randomization to reduce symmetry. While differing pendent arms may be incorporated into a single formulation, the performance can be further improved by physically admixing two or more phosphazene formulations to produce a blended formulation. In a further embodiment, a percentage of compatible carbonate solvent molecules are incorporated to aid in the disruption of solvent self-association and transient solvent-ion-solvent agglomerations already known to reduce performance. The phosphazene composition of the blend may range, for example, from about 0.05% to about 99%. Even a small percentage of phosphazene or blended carbonate phosphazene results in a significantly improved safety performance.

[0023] It was indeed counter-intuitive to one skilled in the art that the removal of ion carriers that are critical for facile ion mobility would in fact forge improvements in phosphazene liquid systems. Also, molecular symmetry or lack thereof was not previously known to have a meaningful effect on the performance of these solvent systems. Lastly, it was unanticipated that exposure of the phosphazene skeleton could keep lithium salt levels high enough to h practical with a significant fraction of the long pendent groups containing high numbers of distal ion carriers removed.

Example Formulation

[0024] To produce the new formulations, in one embodiment, an organic aprotic solvent, such as 1,4-dioxane, is mixed with an alkali metal or alkali metal hydride to form a reactive alkoxide from its corresponding alcohol as shown in Reaction 1 in FIG. 4. While not particularly described, the same principles enumerated herein apply to thioalkoxides. A solution of perchlorophosphazene is added to the reactive alkoxide, and the compound self-assembles, forming a phosphazene compound with a by-product of sodium chloride as shown in Reaction 3a in FIG. 4. Where two or more pendent groups are to be incorporated into the same formulation, the alkoxides and/or thioalkoxides are formed in separate reaction vessels, as shown in Reaction 1 and Reaction 2 in FIG. 4.

[0025] Then, the perchlorophosphazene solution is added to the minor component solution, as shown in Reaction 3a of FIG. 4. After attachment of the minor pendent arms is complete, an excess of the major component is added to the reaction, and the synthesis is allowed to go to completion as shown in Reaction 3b of FIG. 4, thereby resulting in the final desired product.

[0026] After the solvent is removed, the resultant product is isolated and purified via extraction with basic water. The product is then dried in a vacuum/argon oven for many hours and transferred in a sealed container to an argon glovebox.

[0027] The foregoing specification is provided for illustrative purposes only, and is not intended to describe all possible aspects of the present invention. Moreover, while the invention has been shown and described in detail with respect to several exemplary embodiments, those of ordinary skill in the art will appreciate that minor changes to the description, and various other modifications, omissions, and additions may also be made without departing from the spirit of scope thereof. It is envisioned that multiple combinations of phosphazene compounds, incorporating various lengths of pendent arms can be created with similar results.