Patent Application
20220367874
The present invention relates to the manufacture of lithium metal rechargeable batteries using polymeric solid state electrolytes. The resultant batteries are safer and have increased cycle life compared to lithium metal batteries manufactured by conventional methods.
Lithium ion batteries (LIBs) dominate the lithium battery market. LIBs contain no metallic lithium present as such. The negative electrode comprises a carbon host for neutral lithium which is contained therein. In the electrolyte and in the positive electrode lithium is present only as the ion. Such batteries are attractive for their high energy density compared to that of other rechargeable batteries and for their ability to operate over multiple charge/discharge cycles. In lithium metal batteries (LMBs) by contrast the negative electrode comprises metallic lithium, just as in lead-acid batteries the negative electrode comprises metallic lead. During discharge of an LMB, lithium metal dissociates to form lithium ions and electrons. The lithium ions migrate through the electrolyte to the positive electrode. The electrons flow through an external circuit where they power a device. As the LMB recharges, lithium ions are reduced back to lithium metal as electrons flow back into the negative electrode. Because LMBs have intrinsically higher capacity than LIBs, they are the preferred technology for primary batteries. Moreover, since LMBs can be manufactured in the fully charged state, they do not require the lengthy formation process needed for LIBs However, poor cycle life, volumetric expansion, and safety concerns relating to shorts resulting from dendrite formation and the potential for violent combustion of the flammable organic electrolytes used in LMBs have limited their practical use as rechargeable batteries.
Lithium metal batteries using sulfur as the positive electrode offer higher specific capacity than the lithium intercalation compounds that currently dominate the market. However, complex polysulfide species produced upon the reduction of elemental sulfur are soluble in the organic electrolytes typically used in lithium batteries, resulting in reduced cycle life due to the “polysulfide shuttle” effect.
A novel rechargeable lithium metal battery and methods to produce the same are needed to improve the cycle life and enhance the safety profile of rechargeable lithium metal batteries, in particular lithium metal batteries using elemental sulfur in the positive electrode.
In accordance with embodiments of the invention, a rechargeable lithium metal battery is disclosed which includes a negative electrode, the negative electrode having a conductive substrate coated with a layer of lithium metal, the layer of lithium metal having an inner face and an outer face, the inner face contacting the conductive substrate. The disclosed rechargeable lithium metal battery further includes a positive electrode. In such embodiments, a lithium ion conductive copolymer functional as a solid electrolyte coats the outer face of the lithium metal on the negative electrode, the lithium ion conductive copolymer having microphase separated first domains and second domains, each domain above its respective glass transition temperature, Tg, the first domains formed from lithium ion solvating segments and providing continuous conductive pathways for the transport of lithium ions and the second domains formed from second segments immiscible with the first segments, the copolymer being selected from the group consisting of a block copolymer and a graft copolymer.
In such embodiments, the solid electrolyte is disposed between the negative electrode and the positive electrode, and is in direct physical contact with both the layer of lithium metal and the cathode. The embodied rechargeable lithium metal battery further includes a lithium salt dispersed within the solid electrolyte. In such embodiments the lithium metal battery is configured to interact with an external circuit so that during discharge the layer of lithium metal decreases in thickness, and the copolymer coating conforms its shape to continue to cover the thinning layer of lithium metal, and to accommodate any volume changes that may occur at the positive electrode. In such embodiments, the lithium metal battery is further configured to interact with the external circuit so that during electrolytic recharging voltage applied across the external circuit causes the layer of lithium metal to grow in thickness, and the copolymer coating to adjust shape to continue to cover the growing layer of lithium metal, and to accommodate any volume changes that may occur at the positive electrode.
In some embodiments, the positive electrode of the rechargeable lithium metal battery includes elemental sulfur. In some embodiments, the lithium ion solvating segments comprise poly(oxyethylene)n side chains, where n is an integer between 4 and 20. In some embodiments, the copolymer is a block copolymer. In other embodiments, the copolymer is a graft copolymer.
In some embodiments of the invention, a process is disclosed for manufacturing a lithium metal electrode coated with a lithium ion conductive copolymer, the process including the steps of:
(1) Preparing a coating solution of a lithium salt and a graft or block copolymer in a cosolvent, the copolymer having first segments and second segments, each segment above its respective glass transition temperature, Tg, the first segments formed from lithium ion solvating groups and the second segments being immiscible with the first segments, wherein each segment of the block or graft copolymer is separately soluble in the cosolvent.
(2) Coating a first conductive substrate with the coating solution.
(3) Evaporating the cosolvent from the coated conductive substrate so that the first conductive substrate is coated with a first layer of the lithium ion conductive copolymer, the lithium ion conductive copolymer forming microphase separated first domains and second domains, the first domains formed from the first segments and providing continuous conductive pathways for transport of lithium ions and the second domains formed from the second segments.
(4) Configuring an electrolytic cell with an anode.
(5) Configuring the copolymer coated first conductive substrate as a cathode in the electrolytic cell, the electrolytic cell containing a lithium salt solution interposed between the anode and the copolymer coated first conductive substrate.
(6) Applying a voltage across the first conductive substrate and the anode, causing a first layer of lithium metal to deposit on the surface of the first conductive substrate, sandwiched between the first conductive substrate and the first layer of lithium ion conductive copolymer coating, the first layer of lithium ion conductive copolymer coating adjusting shape to continue to cover the growing layer of lithium metal, thereby forming the lithium metal electrode coated with the first layer of lithium ion conductive copolymer.
In some embodiments, a lithium metal electrode is disclosed that is prepared according to these steps. In some embodiments, during the manufacturing process the contents of the electrolytic cell are covered by a blanketing atmosphere, the blanketing atmosphere having no more than 10 ppm of lithium reactive components on a molar basis.
In some embodiments of the process, the anode of the electrodeposition cell is prepared by the additional steps of depositing a second layer of lithium metal on a second conductive substrate coating the second layer of lithium metal with the coating solution evaporating the cosolvent from the coated second layer of lithium metal so that the second layer of lithium metal is coated with a second layer of lithium ion conductive copolymer, the lithium ion conductive copolymer forming microphase separated first domains and second domains, each domain above its respective glass transition temperature, Tg, the first domains formed from the first segments and providing continuous conductive pathways for the transport of lithium ions and the second domains formed from the second segments, thereby obtaining the anode comprising the second layer of lithium metal sandwiched between the second conductive substrate and the second layer of lithium ion conductive copolymer.
In some embodiments, a lithium metal electrode is disclosed that is prepared according to these additional steps.
In some embodiments, a lithium metal electrode is disclosed that is coated with a lithium ion conductive copolymer that is a block copolymer. In some embodiments, a lithium metal electrode is disclosed that is coated with a lithium ion conductive copolymer that is a graft copolymer.
In some embodiments, the lithium ion conductive copolymer has segments with poly(oxyethylene)n side chains, where n is an integer between 4 and 20. In some such embodiments, the lithium ion conductive copolymer further has segments of poly(alkyl methacrylate). In the copolymer each segment is above its respective glass transition temperature, Tg.
In some embodiments, the lithium conductive copolymer is a graft copolymer with main chain segments including poly(oxyethylene)n side chains, where n is an integer between 4 and 20, and branch segments including poly(dimethyl siloxane).
In some embodiments, the lithium ion conductive copolymer is poly[(oxyethylene)9 methacrylate]-b-poly(butyl methacrylate) (POEM-b-PBMA). In some such embodiments, the ratio of POEM to PBMA is between 55:45 and 70:30 on a molar basis. In some embodiments, the lithium ion conductive copolymer is poly[(oxyethylene)9 methacrylate]-g-poly(dimethyl siloxane).
In some embodiments, a process is disclosed for manufacturing a lithium metal electrode that includes the steps of:
Inserting a first conductive substrate as a cathode in an electrolytic cell.
Inserting a second conductive substrate coated with lithium metal as an anode in the electrolytic cell.
Providing a lithium ion conducting copolymer separating and surrounding the first conductive substrate and the anode, the lithium ion conductive copolymer being a graft or block copolymer with first segments and second segments, the first segments formed from lithium ion solvating groups and the second segments being immiscible with the first segments.
Applying a voltage across the conductive substrate and the anode, causing lithium metal to deposit on the surface of the first conductive substrate, the lithium ion conductive copolymer adjusting shape to cover a growing layer of lithium metal on the first conductive substrate, and a thinning layer of lithium metal on the second conductive substrate, thereby forming the lithium metal electrode comprising the first conductive substrate and the lithium metal coating the first conductive substrate, wherein the lithium metal on the first conductive substrate is more pure than the lithium metal on the second conductive substrate.
According to some embodiments of the invention, a rechargeable lithium metal battery is disclosed that includes a positive electrode and a negative electrode, the negative electrode having a layer of lithium metal coated with a layer of lithium ion conductive copolymer, wherein the lithium ion conductive copolymer is disposed between the negative electrode and the positive electrode, and is in direct physical contact with both the layer of lithium metal and the positive electrode. According to such embodiments, the lithium metal battery is configured so that during discharge the layer of lithium metal decreases in thickness, and the copolymer coating conforms its shape to continue to cover the thinning layer of lithium metal. Further, according to such embodiments, the lithium metal battery is configured so that during electrolytic recharging the layer of lithium metal grows in thickness, and the copolymer coating conforms its shape to continue to cover the growing layer of lithium metal.
The foregoing features of embodiments will be more readily understood by reference to the following detailed description, taken with reference to the accompanying drawings, in which:
Definitions. As used in this description and the accompanying claims, the following terms shall have the meanings indicated, unless the context otherwise requires:
A “solid electrolyte” is solid material at room temperature which allows ion transport between electrodes of an electrolytic or galvanic cell.
A “block copolymer” is a polymer with blocks made up of one monomer alternating with blocks of another monomer along a linear polymer strand.
A “graft copolymer” is a polymer which has a backbone strand made up of one type of monomer and branches of a second monomer.
A “segment” is a block for a block copolymer and a side chain or backbone for a graft copolymer.
“Microphase separation” of a block or graft copolymers occurs when polymer segments segregate into domains according to their monomeric units.
A “cosolvent” for different monomers is a solvent capable of dissolving each of the different segments of a block or graft copolymer.
A “common solvent” is identical with a “cosolvent.”
A “negative electrode” functions as an anode in a galvanic cell and as a cathode in an electrolytic cell.
A “positive electrode” functions as a cathode in a galvanic cell and as an anode in an electrolytic cell.
The tendency for lithium metal batteries to form dendrites can lead to electrical shorting. The common use of flammable organic electrolytes for such batteries exacerbates the potential of such shorts to lead to fires and explosions. Solid electrolytes have potential for eliminating these safety concerns by reducing dendrite formation and by avoiding the use of flammable organic electrolytes.
The ideal solid electrolyte has the ion transport properties of a liquid, the ability to preferentially transport desired ionic species, while blocking the transport of undesirable species. The ideal solid electrolyte has low flammability, and a resistance to dendrite formation. The ideal solid electrolyte has the mechanical properties of a solid, but can undergo molecular rearrangements to grow, to shrink, and to accommodate volume changes associated with positive and negative electrodes while still maintaining physical contact with both positive and negative electrodes.
Lithium sulfur (Li—S) batteries using sulfur as the positive electrode offer higher specific capacity than the lithium intercalation compounds that currently dominate the market. However, complex polysulfide species produced upon the reduction of elemental sulfur dissolve in the organic electrolytes typically used in lithium batteries, resulting in reduced cycle life due to the “polysulfide shuttle” effect.
Consequently, another desirable feature of a solid electrolyte for lithium metal batteries is the ability to block the “polysulfide shuttle” between the positive and negative electrodes that reduces battery performance and cycle life of Li—S batteries.
As illustrated in
Block copolymers with blocks of immiscible groups and graft copolymers with immiscible backbone and side-chain segments as embodied in this application provide a solid electrolyte with the ion transport properties of a liquid, and with the ability to preferentially transport desired ionic species, while blocking the transport of undesirable species. The thus embodied solid electrolyte has low flammability, and a resistance to dendrite formation. The thus embodied solid electrolyte has the mechanical properties of a solid, but can undergo molecular rearrangements to grow, to shrink, and to accommodate volume changes associated with positive and negative electrodes while still maintaining physical contact with both positive and negative electrodes.
Consequently, block copolymers with blocks of immiscible groups and graft copolymers with immiscible backbone and side-chain segments as embodied in this application provide a solid electrolyte technology for lithium metal batteries in general and Li—S batteries in particular, promising improved safety and performance, longer battery life, and a solution to the “polysulfide shuttle” problem. In short, block copolymers and graft copolymers as embodied in this application provide the key features of an ideal solid electrolyte for lithium metal batteries.
A block or graft copolymer as embodied in this application has one or more “A” segments of more hydrophilic lithium salt solvating polymers interspersed with one or more “B” segments of more hydrophobic polymers. All segments are above their respective glass transition temperatures, Tg. Material incorporating such a block or graft copolymer will microphase separate into locally segregated nanoscale domains of “A” and “B” segments. The resultant ordering of segments in turn confers conformational rigidity to the material even though all of the constituents are segmentally liquid. For suitable A:B ratios, the A segments form continuous lithium ion solvating channels. For lithium ion solvating segments having suitably high local chain mobility, high lithium conductivity allows the directed flow of lithium ions through the copolymer upon application of an electric field.
Dissolving the block or graft copolymer and a lithium salt in a suitable common solvent (cosolvent) that is capable of dissolving both A and B segments allows ready processing of the polymer with solvated lithium ions by conventional coating methods. For example, electrodes can be directly coated with a lithium ion conductive block or graft copolymer electrolyte by dipping the electrode in a solution of lithium salt and copolymer dissolved in cosolvent, and allowing the cosolvent to evaporate. Such an electrode can then be directly used in a battery or electrolytic cell. In this manner, as described below, lithium metal electrodes can be coated with lithium ion conducting block or graft copolymer solid electrolytes for use in solid state batteries.
Suitable copolymers can be di-block copolymers (AB), tri-block copolymers (ABA or BAB), or higher multiblock polymers with alternating A and B blocks. All blocks are above their respective glass transition temperatures, Tg. Likewise suitable are graft copolymers with backbone A monomers and side-chain B monomers, or back-bone B monomers and side-chain A monomers. In some embodiments, the A segments incorporate short poly(oxyethylene)n side chains, where n, the number of oxyethylene groups in the side chain ranges from 4 to 20, preferably between 7 and 11. In some embodiments n is equal to nine. In some embodiments the poly(oxyethylene)n side chains are incorporated by polymerization of poly(oxyethylene)n methacrylate monomers. In a preferred embodiment, the A segments are synthesized by polymerization of poly(oxyethylene)9 methacrylate monomers.
In some embodiments, the B segments have alkyl side chains having from 3 to 6 carbons. In some embodiments, the B segments are synthesized from a poly(alkyl methacrylate). In some embodiments, the poly(alkyl methacrylate) is chosen from the group consisting of poly(propyl methacrylate), poly(butyl methacrylate), poly(pentyl methacrylate), and poly(hexyl methacrylate). In a preferred embodiment, the poly(alkyl methacrylate) is poly(butyl methacrylate).
In some embodiments the “A” segments incorporate a mixture of neutral and anionic groups. In some such embodiments, the anionic groups are configured in order to minimize coordination of the anionic groups with lithium cations.
In a particularly preferred embodiment, the copolymer is the di-block copolymer poly[(oxyethylene)9 methacrylate]-b-poly(butyl methacrylate) (POEM-b-PBMA).
In some embodiments, the block copolymers are synthesized by living anionic polymerization. In some embodiments, the block copolymers are synthesized by atom transfer radical polymerization (ATRP).
In some embodiments, the copolymer is a graft copolymer with a hydrophilic backbone of “A” segments that are lithium salt solvating and hydrophobic side-chains of “B” segments made up of hydrophobic polymers. Each segment is above its respective glass transition temperature, Tg.
In a preferred embodiment, the copolymer is a graft copolymer with backbone “A” segments incorporating short poly(oxyethylene)n side chains, where n, the number of oxyethylene groups in the side chain ranges from 4 to 20, preferably between 7 and 11. In some embodiments, n is equal to nine. In some embodiments, the poly(oxyethylene)n side chains are incorporated by polymerization of poly(oxyethylene)n methacrylate monomers. In a preferred embodiment, the A segments are synthesized by polymerization of poly(oxyethylene)9 methacrylate monomers.
In some embodiments, the polymer is a graft copolymer with side chain “B” segments incorporating poly(dimethyl siloxane) (PDMS). In a preferred embodiment, the graft copolymer is incorporated into a poly(oxyethylene)n methacrylate backbone by random copolymerization of poly(dimethyl siloxane) monomethacrylate macromonomer (PDMSMA) with poly(oxyethylene)n methacrylate monomers to form a graft copolymer of type POEM-g-PDMS. In preferred embodiments, poly(oxyethylene)9 methacrylate monomers are reacted to form the POEM-g-PDMS copolymer.
In some embodiments, the “A” backbone includes additional monomers. In some embodiments the additional monomers are anionic. In an embodiment, poly(oxyethylene)9 methacrylate monomers are copolymerized with methacrylate monomers (MAA) and with PDMSMA to form poly(oxyethylene)9-ran-MAA-g-PDMS. In a preferred embodiment, the carboxylic acid groups of this polymer are reacted with BF3 to give anionic boron trifluoride esters, which have a reduced tendency to complex lithium ions when compared with MAA carboxylate groups.
As summarized by the manufacturing steps shown in
The steps of this embodiment are as follows: First, prepare a solution of lithium ion salt and block or graft copolymer in a cosolvent capable of dissolving both A and B segments of the copolymer 2. Second, coat an electrically conductive substrate with lithium salt doped copolymer by dipping the substrate in the lithium salt and copolymer solution 4. Third, evaporate the cosolvent to leave the electrolytically conductive substrate coated with lithium ion conductive copolymer 6. Next, insert the lithium ion conductive copolymer-coated conductive substrate as a cathode in an electrolytic cell, the electrolytic cell including an anode and a lithium salt solution 8. Then, apply voltage across the anode and the substrate, acting as a cathode, causing electrons to flow from the anode through an external circuit to the conductive substrate, causing lithium ions to be pulled through the copolymer coating, to be reduced at the substrate surface, thereby electrolytically plating lithium metal onto the surface 10. As lithium metal plates, the polymer chains of the copolymer coating undergo a molecular rearrangement, allowing the copolymer coating to continue to cover the growing layer of lithium metal, resulting in a final product for which the substrate is coated with a layer of lithium metal, and the layer of lithium metal is in turn coated with a layer of copolymer solid electrolyte. In the final step, the conductive substrate layered with lithium metal and a copolymer solid electrolyte is inserted as the combined lithium metal negative electrode and solid electrolyte in a lithium metal battery 12.
In the embodiment shown in
In preferred embodiments, the lithium metal in the copolymer coated lithium metal electrode 116 is ultrapure, having no more than five ppm of non-metallic elements by mass. In some embodiments, the lithium metal in the copolymer coated lithium metal electrode 116 includes no more than one ppm of non-metallic elements by mass. In some embodiments the lithium coated conductive substrate 117 is manufactured by methods described in U.S. patent application Ser. Nos. 17/006,048 and 17/006,073, both of which were filed Aug. 28, 2020 and are incorporated by reference herein in their entirety.
In preferred embodiments, the conductive substrate is selected from the group consisting of copper, aluminum, graphite coated copper, and nickel. In some embodiments, the copolymer is (POEM-b-PBMA). In some embodiments, the ratio of POEM to PBMA is greater than 50:50 on a molar basis. In preferred embodiments, the ratio of POEM to PBMA is between 55:45 and 70:30 on a molar basis. In a preferred embodiment, the cosolvent is tetrahydrofuran (THF).
An embodiment of an electrolytic cell 105 for electroplating the electrically conductive substrate 110 with a layer of lithium metal 150 sandwiched between the conductive substrate 110 and the copolymer coating 160 is shown in
In some embodiments, the electrolytic cell 105 is configured as a flow chamber, with an entrance port 170 and an exit port 180 allowing lithium salt solution 140 to enter the electrolytic cell 105 to provide a renewable supply of lithium ions for electroplating. In some embodiments, the electrolytic cell is completely blanketed with a blanketing atmosphere 124, the blanketing atmosphere being substantially free of lithium reactive components. In a preferred embodiment, the blanketing atmosphere includes no more than 10 ppm of lithium reactive components on a molar basis. In a preferred embodiment, the blanketing atmosphere includes no more than 5 ppm of lithium reactive components on a molar basis. In a preferred embodiment, the blanketing atmosphere includes no more than 10 ppm of nitrogen on a per molar basis. In a preferred embodiment, the blanketing atmosphere includes no more than 5 ppm of nitrogen on a per molar basis. In a preferred embodiment, the blanketing atmosphere includes no more than 1 ppm of nitrogen on a per molar basis. In a preferred environment, the blanketing atmosphere comprises argon with a purity of greater than 99.998 weight percent. In a preferred embodiment the blanketing atmosphere 124 and the electrolytic cell 105 are enclosed in a gas-impermeable container 500.
As shown in
As shown in
In the embodiment of
An advantage of the embodiments of
The copolymer coated lithium metal electrode 116, prepared by electrolytic or other methods, can be inserted directly into a rechargeable lithium battery, shown in cross-section in
In the battery embodied in
In the battery embodied in
In preferred embodiments of the batteries of
In some embodiments the rechargeable batteries of
Li—S batteries constructed in the manner of
The embodiments of the invention described above are intended to be merely exemplary; numerous variations and modifications will be apparent to those skilled in the art. All such variations and modifications are intended to be within the scope of the present invention as defined in any appended claims.
| Number | Date | Country | |
|---|---|---|---|
| 63187688 | May 2021 | US |
