Edge-functionalized hexagonal boron nitride and composites of the same

Abstract

The present invention discloses edge-functionalized hexagonal boron nitride (hBN) materials. The edge-functionalization is realized through ring- forming Lewis acid-base reactions between the reactive edges of hBN and a suitable Lewis acid / base. Typically, five- and six-membered rings form, such as cyclic carbamates in the arm-chair edge or rings due to chelate formation between the edge boron atoms and bi- or multidentate Lewis bases. The resulting functionalized hBN (FBN) may be used as a filler in polymer composites to enhance the mechanical properties, thermal resistance and ionic conductivity of the composite.

Description

Edge-functionalized Hexagonal Boron Nitride and Composites of the Same DESCRIPTION

[0001] The present application takes full benefit of US provisional patent application 63 / 754,608 filed on February 6, 2025, entitled “Edge-functionalized Hexagonal Boron Nitride as a Solid Ion Conductor” by Karoly Nemeth, to the extent allowed by law.BRIEF DESCRIPTION OF THE DRAWINGS

[0002] Figure 1 is a schematic representation of the chemical bonds in the armchair edge of hBN chelated at a boron atom by an oxalate ion of lithium oxalate. Before the chelation, the armchair edge may be represented by a resonance structure that indicates a polarization such that the boron atoms take +1 and the N atoms -1 charge. The chelation favors a positive charge on the boron atom before the chelation happens. After the chelation, the two negative charge of the oxalate ion results in a net negative charge on the chelated boron as well.

[0003] Figure 2 is a schematic representation of the chemical bonds in the armchair edge of hBN chelated at a boron atom by a malonate ion ( OOCCH2COO ) from lithium-malonate (Li2[OOCCH2COO]).

[0004] Figure 3 is a schematic representation of the chemical bonds in the armchair edge of hBN chelated at a boron atom by a trifluoroacetate ion (F3CCOO ) from lithium-trifluoroacetate (F3CCOOLi) without carbamate bond formation.

[0005] Figure 4 is a schematic representation of the chemical bonds in the armchair edge of hBN functionalized by a trifluoroacetate ion(F3CCOO ) from lithium-trifluoroacetate (F3CCOOLi) whereby a carbamate bond forms.

[0006] Figure 5 is a schematic representation of the chemical bonds in the armchair edge of hBN chelated at a boron atom by a polyacrylate ion (CHCH2COO )nfrom lithium-polyacrylate (CHCH2COOLi)n.

[0007] Figure 6 is a schematic representation of the chemical bonds in the zigzag edge of hBN after the reaction with lithium-carbonate (Li2CO3).

[0008] Figure 7 is a schematic representation of the chemical bonds in the zigzag edge of hBN after the reaction with lithium-nitrate (LiNO3).

[0009] Figure 8 is a schematic representation of the chemical bonds in the zigzag edge of hBN after the reaction with lithium-sulfate Li2SO4.

[0010] Figure 9 is a schematic representation of the chemical bonds in the zigzag edge of hBN after the reaction with lithium-bis(fluorosulfonyl)imide (F2LiNO4S2, LiFSI).

[0011] Figure 10 is a schematic representation of the chemical bonds in the zigzag edge of hBN after the reaction with lithium bis(trifluoromethanesulfonyl)imide (LiC2F6NO4S2, LiTFSI).

[0012] Figure 11 is a schematic representation of the chemical bonds in the armchair edge of hBN after the reaction with polyacrylonitrile (PAN).BACKGROUND OF THE INVENTION

[0013] Functionalized hexagonal boron nitride (hBN) was proposed for use as electroactive species in cathodes of batteries as well as component of the anode in K. Nemeth: "Functionalized Boron Nitride Material as Electroactive Species in Electrochemical Energy Storage Devices”, WO / 2015 / 006161 , US 10693137 B2 and US11552296B2. One possible synthesis route for the effective functionalization of hBN via radical anion functionalization of hBN was described in K. Nemeth: "Radical Anion Functionalization of Two-Dimensional Materials”, WO / 2020 / 180680, US 11453596 B2 and CN 1142216B. A specific example of such radical anion functionalization happens during the mechanochemical reaction of hexagonal boron nitride with lithium oxalate (Li2C2O4). In this case, the oxalate anions (as a Lewis base) react with the edge boron atoms (Lewis acid site) of hBN and form a chelate complex (see Fig 1 ). Upon further milling or heating, the oxalate C-C bond in this complex splits up to form radical species which further react with the hBN sheet, typically binding to nearby nitrogen sites. This reaction sequence exploits the proposed reaction sequence in WO / 2020 / 180680 . The resulting material may have good ionic conductivity for Li or other cations, depending on the type of the cation in the oxalate salt reactant. The above mentioned WO / 2020 / 180680 also proposed the use of suchfunctionalized hBN for protective coating on lithium metal anodes. The first experimental realization of Li2C2O4functionalized hBN was published in Sivaviswa Radhakrishnan: "Functionalized 2D Materials as Enablers of High Energy and High Power Energy Storage Devices." Master's thesis, Illinois Institute of Technology, 2023.SUMMARY OF THE INVENTION

[0014] The freshly cleaved edges of hexagonal boron nitride (hBN) are very reactive as they contain dangling bonds. The present invention utilizes this reactivity for the production of functionalized hBN (FBN). The FBN species may further be utilized as fillers in polymer composites to enhance their mechanical properties, thermal stability and ionic conductivity.DETAILED DESCRIPTION OF THE INVENTION

[0015] The edge-functionalization is realized through ring-forming Lewis acidbase reactions between the reactive edges of hBN and the suitable Lewis acid / base functionalization agents. Typically, five- and six-membered rings form during the reaction. These include cyclic carbamates in the arm-chair edge or rings due to chelate formation between the edge boron and bi / multi-dentate Lewis bases. Chelate forming agents include oxalate, malonate, trifluoroacetate and polyacrylic acid anions, typically as provided in the form of their respective salts. Some other anions, such as carbonates, nitrates, sulfates, phosphates,thiophosphates, bis(fluorosulfonyl)imide and bis(trifluoromethanesulfonyl)imide, prefer to react with the zig-zag edge and form six-membered rings with hBN. Examples of such FBNs are shown in Figs 1-11.

[0016] A preferable means of carrying out the reaction is mechano-chemical, such as high energy ball milling. Such a milling creates an abundance of reactive edges in hBN which is necessary for the consumption of the functionalization agent by hBN. It further allows for the physical proximity of the reagents. The milling is typically carried out till the functionalization agent is completely consumed as indicated by X-ray diffraction (XRD) and spectroscopic means. The formation of the ring structures can be pointed out using NMR and IR spectroscopy, besides other types of spectroscopies.

[0017] Oxalate salts are one example of suitable functionalization agents. The oxalate anion easily chelates the edge boron atoms forming a five membered ring (Fig 1), analogously to the well known electrolyte salt LiBOB. Esters of oxalic acid and oxalyl halides are also capable to functionalize hBN in a similar fashion.

[0018] Salts of malonic acid (CF^COOH^), such as lithium malonate (CH2(COOLi)2) are examples of a bidentate ligand forming a six-membered ring with the chelated boron atom (Fig 2). The malonic acid may also be used in its substituted form (CRIR2(COOH)2) where Ri and R2are arbitrary functional groups, such as alkyl groups or hydrogen.

[0019] Other malonic acid derivatives, such as malonitrile (CF^CN^) or malonamide (CH2(CONH2)2) are also suitable bidentate ligands for the chelationof the edge boron atoms of hBN or for forming rings with the armchair edge of hBN. Another suitable functionalization agent is polyacrylonitrile (Fig 11) or any polymer having cyano (-CN) groups.

[0020] Cyanide (CN_), cyanate (OCN ) or tiocyanate (SCN ) salts are also suitable functionalization agents forming rings with the armchair edge of hBN.

[0021] In case the functionalization agent does not contain a cation for which an ionic conductor application or similar is sought after, the cation should be added by doping. The doping may be carried out via dry milling the chelated hBN with the respective metal (Li, Na, K, etc) or in suitable solution, such as liquid ammonia solution of the respective metal or by electrochemical means. The doping may also be carried out on FBNs which already contain cations, in order to increase their concentration of cations.

[0022] Some FBNs allow for reversible redox-reactions and can also be applied as cathode active species thereby serving both as ionic conductors and electroactive species. An example of such FBNs is LiSCN functionalized hBN.

[0023] Bidentate ligands with mixed oxygen and halide sites as Lewis bases with binding ability to edge borons, such as salts of trifluoroacetic acid are another example of suitable functionalization agents, see Figs 3 and 4 .

[0024] Any salt of carboxylic acids, such as for example lithium acetate, is a suitable functionalization agent as the carboxyl functional group can form covalent bonds with the edge of hBN, as shown in Fig 4.

[0025] Nitrates, nitrites, carbonates, phosphates, tiophosphates, sulfides, sulfates, sulfites, pyrosulfates and acetylides are also reactive with the edge ofhBN (primarily with the zig-zag edge) and functionalize it by ring formation during high energy ball milling due to the contact of strong Lewis acid and Lewis base sites between hBN and these salts, see Figs (6-8).

[0026] Salts of bis(fluorosulfonyl)imide (F2NO4S2; LiFSI) and bis(trifluoromethanesulfonyl)imide (C2F6NO4S2', LiTFSI) also have the ability to form rings with the edge of hBN, especially with the zig-zag edge (Figs 9 and 10).

[0027] Polymers having carboxyl functional groups and the salts of such polymers are another example of suitable functionalization agents of hBN. For example, the lithium, sodium and magnesium salts of polyacrylic acid are such functionalization agents, see Figure 5.

[0028] Polymers having functional groups which can form salts, such as organosulfates (-OSO3‘) and organonosulfites (-OSO2'), are fully capable of functionalizing the edge of hBN during high energy ball milling similarly to the above mentioned sulfates and sulfites.

[0029] One important application field of the above mentioned functionalized hBN materials is in polymer composites or in general composite materials. In these applications, the functionalized hBN is typically a filler in a polymer matrix.

[0030] The polymer composite with the functionalized hBN filler may contain additional salts and small solvent molecules to enhance the ionic conductivity of said polymer composite. The small solvent molecules play the role of a plasticizer enhancing the mechanical properties and the ionic conductivity of the composite. Examples of plasticizers can be found in US20240332640A1 entitled“Solid electrolyte membrane including cyan-based polymer electrolyte and battery including the same’’ by Sungjin Cho and Jongsoo Cho.

[0031] Such functionalized boron nitrides and their polymer composites can be applied as protective coatings / separators on lithium anode materials, particularly lithium metal anodes in batteries, to suppress the growth of dendrites and enable the safe, long cycle life operation of batteries. Furthermore, they can be applied as general solid ion conductors in electrochemical energy storage devices.

[0032] One embodiment of such FBN-composite polymer electrolyte can be synthesized by dissolving polyacrylonitrile (PAN) in dimethyl-formamide (DMF) and further dissolving lithium perchlorate (l_iCIO4) in the solution. The molar amount of LiC IO4 should be in the range of 0.4 and 0.6 relative to the molar amount of CH2CHCN repeat units of PAN present in the solution. Next, the FBN material should be added in the solution so that its mass will be 3-10 % of the combined mass of the resulting PAN+LiCIO4+FBN mixture not counting the amount of DMF. After a homogeneous slurry is formed, it should be cast on a substrate and dried, until the amount of residual DMF decreases to about 10 % of the total mass of the composite. This recipe follows a similar one from Chen, H. C., F. J. Lin, and C. C. Chen. "Polyacrylonitrile electrolytes 1. A novel high- conductivity composite polymer electrolyte based on PAN, LiC IO4and a-AI2O3." Solid State Ionics 150 (2002): 327-335. The main difference is that FBN is used for filler, instead of AI2O3.

[0033] In another embodiment, instead of pure PAN, a mixture of PAN and poly(ethylene oxide) (PEO) may be used following the above recipe, such as in the ratio of 80:20 wt% respectively.

[0034] In a third embodiment, FBN and PAN are mixed in a 9:1 mass ratio in a DMF solution / slurry, respectively. The slurry is cast to form a film on a substrate and dried. The resulting composite film may be coated on the surface of a lithium metal foil by mechanical rolling, such as in US Patents 12040493B2 and 12095096B2, both entitled “Lithium Metal Anodes and Method Of Making Same” by Sungjin Cho and Jongsoo Cho (2024) or in Wang, Zhiyu, Si Qin, Fangfang Chen, Shasha Chen, Dan Liu, Degang Jiang, Peng Zhang et al. "Interfacial Modification of Lithium Metal Anode by Boron Nitride Nanosheets." ACS nano 18, no. 4 (2024): 3531-3541.

[0035] In a forth embodiment, FBN and fully lithiated polyacrylic acid (LiPAA) are mixed in a DMF solution. The amount of LiPAA may be between 10 and 20 wt% of the mixture. The slurry is cast on a substrate and dried. It may be used as a lone standing separator film or it may be coated on the surface of a lithium metal foil by mechanical rolling.

[0036] Suitable polymers to combine with FBN for the formation of effective composites include polyacrylonitrile (PAN), polyacrylic acid (PAA) and its salts, poly(ethylene oxide) (PEO), polyvinylidene difluoride (PVDF), poly(terafluoro ethylene) (PTFE), polyacryl amide, polyvinyl alcohol (PVA), polyvinylamine, polyethylenimine (PEI), and their co-polymers.

[0037] Suitable electrolyte salts for the proposed polymer composites of FBN include LiPF6, LiBF4, Li-bis(oxalato)borate, l_iCIO4, LiFSI and LiTFSI.

[0038] The above described FBN materials are also capable to provide a porous matrix for the storage of metallic lithium. Such Li-FBN composites may be produced by adding FBN to the melt of of lithium, or by pressing FBN materials into the surface of lithium metal foil by mechanical rolling.

[0039] The molar ratio of the polymer, FBN electrolyte salt and plasticizer may vary according to the required specifications of the actual application. The above described embodiments give a guideline for separator applications. In other applications, such as hard / fireproof coating of surfaces, the relative amount of FBN in the composite may increase substantially and may be at least 50 wt% of the composite.

[0040] It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention and the appended claims are intended to cover such modifications and arrangements.

[0041] All publications and patent documents cited in this application are incorporated by reference in their entirety for all purposes to the same extent as if each individual publication or patent document were so individually denoted.

Claims

CLAIMS1. A synthesis method for the covalent functionalization of hexagonal boron nitride (hBN) through a mechanochemical reaction between hBN and a functionalization agent containing one of a Lewis base and Lewis acid and the functionalization agent chelates the edge boron atoms of hBN or forms other types of rings on the molecular level in the edge of hBN.

2. The synthesis method of Claim 1 in which the functionalization agent is one of a salt of oxalic acid, an ester of oxalic acid and oxalyl halide.

3. The synthesis method of Claim 1 in which the functionalization agent is one of malonic acid CRIR2(COOH)2 and its salts and esters where Ri and R2are arbitrary functional groups including alkyl groups and hydrogen.

4. The synthesis method of Claim 1 in which the functionalization agent is one of a salt and ester of one of acetic acid and halide substituted acetic acid.

5. The synthesis method of Claim 1 in which the functionalization agent is a polymer containing carboxyl groups.

6. The synthesis method of Claim 1 in which the functionalization agent is a polymer containing cyano groups.

7. The synthesis method of Claim 1 in which the functionalization agent is one of a cyanide, cyanate, tiocyanate, carbonate, nitrate, nitrite, phosphate, thiophosphate, sulfate, sulfite and pyrosulfate.

8. The synthesis method of Claim 1 in which the functionalization agent is one of a salt of bis(fluorosulfonyl)imide (F2NO4S2; LiFSI) and bis(trifluoromethanesulfonyl)imide (C2F6NO4S2', LiTFSI).

9. A new composition of matter containing functionalized hBN (FBN) materials from Claims 1-8 as a filler in a solid polymer matrix.

10. Anew composition of matter containing functionalized hBN (FBN) materials from Claims 1-8 as a filler in a solid polymer matrix also containing plasticizers and dissolved electrolyte salts of one of LiPF6, LiBF4, Li-bis(oxalato)borate, LiCIO4, LiFSI and LiTFSI.

11. The new composition of matter in Claims 9-10 where the polymer is one of polyacrylonitrile (PAN), polyacrylic acid (PAA) and its salts, poly(ethylene oxide) (PEO), polyvinylidene difluoride (PVDF), poly(terafluoro ethylene) (PTFE), polyacryl amide, polyvinyl alcohol (PVA), polyvinylamine, polyethylenimine (PEI), and their co-polymers.

12. A composite formed by dispersing FBN materials from Claims 1-8 in molten lithium and forming sheets of it.

13. A composite formed by pressing FBN materials from Claims 1-8 in the surface of lithium metal sheets by mechanical rolling.

14. The application of the composite materials of Claims 9-11 as one of anode coating and separator in lithium metal batteries.

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