Metastable vanadium oxide cathode materials for rechargeable magnesium battery

Abstract

A magnesium electrochemical cell having a positive electrode containing as an active ingredient, a material of formula [V.sub.2O.sub.5].sub.c [M.sub.aO.sub.b].sub.d and/or a material of formula [V.sub.2O.sub.5].sub.c[M.sub.aO.sub.b].sub.d[MgX.sub.e].sub.g in a metastable structural and morphological phase is provided. In the formulas M is an element selected from the group consisting of P, B, Si, Ge and Mo; and X is O or a halide.

Claims

1. A cathode for a magnesium battery comprising: a current collector; and an active material of formula (I):
[V.sub.2O.sub.5].sub.c[M.sub.aO.sub.b].sub.d  (I) wherein M is an element selected from the group consisting of P, B, Si, Ge and Mo, a is an integer of from 1 to 2, b is an integer of from 1 to 5, c is from 35 to 80 mol %, d is from 20 to 65 mol %, and the active material of formula (I) is a metastable structural and morphological phase between an amorphous phase and a crystalline phase.

2. The cathode according to claim 1, wherein a content of the V.sub.2O.sub.5 is from 50 to 80 mol %.

3. The cathode according to claim 1, wherein a content of the V.sub.2O.sub.5 is from 70 to 80 mol %.

4. The cathode according to claim 1, rein a content of the V.sub.2O.sub.5 is 75 mol %.

5. The cathode according to claim 1, wherein M.sub.aO.sub.b is at least one material selected from the group consisting of P.sub.2O.sub.5, B.sub.2O.sub.3, SiO.sub.2, GeO.sub.2 and MoO.sub.3.

6. The cathode according to claim 1, wherein M.sub.aO.sub.b is P.sub.2O.sub.5.

7. A magnesium battery comprising: an anode; a cathode; and an electrolyte; wherein the cathode comprises: an active material of formula (I):
[V.sub.2O.sub.5].sub.c[M.sub.aO.sub.b].sub.d  (I) wherein M is an element selected from the group consisting of P, B, Si, Ge and Mo, a is an integer of from 1 to 2, b is an integer of from 1 to 5, c is from 35 to 80 mol %, d is from 20 to 65 mol %, and the active material of formula (I) is a metastable structural and morphological phase between an amorphous phase and a crystalline phase.

8. The magnesium battery according to claim 7, wherein a content of the V.sub.2O.sub.5 in the compound of formula (I) is from 50 to 80 mol %.

9. The magnesium battery according to claim 7, wherein a content of the V.sub.2O.sub.5 in the compound of formula (I) is from 70 to 80 mol %.

10. The magnesium battery according to claim 7, wherein M.sub.aO.sub.b content of the V.sub.2O.sub.5 in the compound of formula (I) is 75 mol %.

11. The magnesium battery according to claim 7, wherein M.sub.aO.sub.b is at least one material selected from the group consisting of P.sub.2O.sub.5, B.sub.2O.sub.3, SiO.sub.2, GeO.sub.2 and MoO.sub.3.

12. The magnesium battery according to claim 7, wherein M.sub.aO.sub.b is P.sub.2O.sub.5.

13. A cathode for a magnesium battery comprising: a current collector; and an active material of formula (II):
[V.sub.2O.sub.5].sub.c[M.sub.aO.sub.b].sub.d[MgX.sub.e].sub.g  (II) wherein M is an element selected from the group consisting of P, B, Si, Ge and Mo, X is O, F, Cl, Br, or I, a is an integer of from 1 to 2, b is an integer of from 1 to 5, c is from 35 to 80 mol %, g is from greater than 0 to 25 mol %, e is 1 when X is O, e is 2 when X is F, Cl, Br, or I, and the sum of c, d and g is substantially 100%, and the active material of formula (II) is a metastable structural and morphological phase between an amorphous phase and a crystalline phase.

14. The cathode according to claim 13, wherein a content of the V.sub.2O.sub.5 is from 50 to 80 mol %.

15. The cathode according to claim 13, wherein M.sub.aO.sub.b is at least one material selected from the group consisting of P.sub.2O.sub.5, B.sub.2O.sub.3, SiO.sub.2, GeO.sub.2 and MoO.sub.3.

16. The cathode according to claim 13, wherein M.sub.aO.sub.b is P.sub.2O.sub.5.

17. A magnesium battery comprising: an anode; a cathode; and an electrolyte; wherein the cathode comprises: an active material of formula (II):
[V.sub.2O.sub.5].sub.c[M.sub.aO.sub.b].sub.d[MgX.sub.e].sub.g  (II) wherein M is an element selected from the group consisting of P, B, Si, Ge and Mo, X is O, F, CI, Br, or I, a is an integer of from 1 to 2, b is an integer of from 1 to 5, c is from 35 to 80 mol %, g is from greater than 0 to 25 mol %, e is 1 when X is O, e is 2 When X is F, Cl, Br, or I, and the sum of c, d and g is substantially 100% and the active material of formula (II) is a metastable structural and morphological phase between an amorphous phase and a crystalline phase.

18. The magnesium battery according to claim 17, wherein a content of the V.sub.2O.sub.5 in the compound of formula (II) is from 50 to 80 mol %.

19. The magnesium battery according to claim 17, wherein a content of the V.sub.2O.sub.5 in the compound of formula (II) is from 70 to 80 mol %.

20. The magnesium battery according to claim 17, wherein M.sub.aO.sub.b is at least one material selected from the group consisting of P.sub.2O.sub.5, B.sub.2O.sub.3, SiO.sub.2, GeO.sub.2 and MoO.sub.3.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the differential scanning calorimetry curves for crystalline V.sub.2O.sub.5 and amorphous V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio).

(2) FIG. 2 shows a CV performance comparison of crystalline V.sub.2O.sub.5, amorphous V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio), metastable V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio) according to one embodiment of the present invention, and thermodynamically stable crystalline phase V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio).

(3) FIG. 3 shows XRD spectra of crystalline V.sub.2O.sub.5, amorphous V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio), metastable V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio) according to one embodiment of the present invention, and thermodynamically stable crystalline phase V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio).

(4) FIGS. 4A, 4B and 4C show scanning electron micrographs (SEM) of amorphous V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio) (4A), metastable V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio) (4B) according to one embodiment of the present invention, and thermodynamically stable crystalline phase V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio).

(5) FIG. 5 is a schematic diagram of a magnesium battery according to one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

(6) The present inventors are conducting a wide scale study and evaluation of materials which may function as cathode active materials for a magnesium secondary battery. The object of this study is to discover cathode active materials which are readily available, safe and comparatively easy to handle in a production environment and which provide a magnesium battery having high capacity and high working potential.

(7) Throughout this description all ranges described include all values and sub-ranges therein, unless otherwise specified. Additionally, the indefinite article “a” or “an” carries the meaning of “one or more” throughout the description, unless otherwise specified.

(8) As described in U.S. patent application Ser. No. 14/978,635, filed Nov. 13, 2014, the inventors discovered that amorphous compositions of vanadium oxide are capable of magnesium insertion and extraction and that such material when formulated into a cathode allows for the production of a magnesium battery having high capacity and working potential. In ongoing studies of such amorphous materials, the inventors have learned that upon heat treatment the amorphous V.sub.2O.sub.5 materials undergo structural and morphological change at temperatures above the glass transition temperature of the system wherein prior to formation of a thermodynamically stable crystalline state, a structural and morphological form which the inventors identify as the metastable state is obtained. The transition temperatures for each of these structural and morphological transitions may be seen on differential scanning calorimetry (DSC) analysis of the material.

(9) For example as shown in FIG. 1 the DSC scan for amorphous V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio) exhibits a first maximum identified as P1 and a second maximum identified as P2. The inventors have learned that at temperatures of P2 and higher, a thermodynamically stable crystal structure is obtained. However, when the amorphous system is heated only to a temperature from P1 to a temperature less than the onset of the P2 maximum, a metastable morphological form is obtained and unexpectedly, the inventors have discovered that V.sub.2O.sub.5 of this metastable structural and morphological phase is employed as a cathode active material, significantly improved battery performance may be obtained in comparison to cathodic materials based on the amorphous phase and/or the thermodynamically stable crystalline phase.

(10) Thus, in the first embodiment, the present invention provides a cathode for a magnesium battery comprising: a current collector; and an active material of formula (I):
[V.sub.2O.sub.5].sub.c[M.sub.aO.sub.b].sub.d  (I)
wherein M is an element selected from the group consisting of P, B, Si, Ge and Mo, a is an integer of from 1 to 2, b is an integer of from 1 to 5, c is from 35 to 80 mol %, d is from 20 to 65 mol %, and the active material of formula (I) is a metastable structural and morphological phase between an amorphous phase and a crystalline phase.

(11) The inventors have surprisingly discovered that when amorphous V.sub.2O.sub.5 materials are prepared and heat treated at a temperature from P1 to a temperature less than the onset of the P2 maximum for that system, a metastable structural and morphological form is obtained which can provide a cathode active material capable of a 3V class redox reaction.

(12) As shown in Table 1, the values of the glass transition temperature (T.sub.g), metastable phase transition temperature (P1) and thermodynamically stable phase transition temperature (P2) vary according to the chemical composition of the amorphous V.sub.2O.sub.5.

(13) Amorphorization of the V.sub.2O.sub.5 may be conducted employing quenching and ball milling methods which are conventionally known. Addition of glass forming agents containing at least one of P.sub.2O.sub.5, B.sub.2O.sub.3, SiO.sub.2, GeO.sub.2 and MoO.sub.3 to the V.sub.2O.sub.5 during the preparation and by careful monitoring of the formation conditions, provides a substantially amorphous material. According to the present invention, the description “substantially amorphous” means that the material when analyzed by XRD does not show any crystalline peaks.

(14) In a further embodiment, a magnesium halide or magnesium oxide may be added to the V.sub.2O.sub.5/M.sub.aO.sub.b mixture to form a composite mix of formula (II):
[V.sub.2O.sub.5].sub.c[M.sub.aO.sub.b].sub.d[MgX.sub.e].sub.g  (II) wherein M is an element selected from the group consisting of P, B, Si, Ge and Mo, X is O, F, Cl, Br, or I, a is an integer of from 1 to 2, b is an integer of from 1 to 5, c is from 35 to 80 mol %, g is from greater than 0 to 25 mol %, e is 1 when X is O, e is 2 when X is F, Cl, Br, or I, and the sum of c, d and g is substantially 100%, and the active material of formula (II) is a metastable structural and morphological phase between an amorphous phase and a crystalline phase. According to the invention, the description “substantially 100%” means that at least 98 mol % of the mixture is due to the components of formula (II).

(15) The relative mol % content of V.sub.2O.sub.5 in the material of formula (I) or of formula (II) affects the performance of a magnesium cell containing the material as a cathode active ingredient. Thus in one embodiment commercially available V.sub.2O.sub.5 having a minimum purity of 98%, preferably, a minimum purity of 99% and most preferably, a minimum purity of 99.5% may be physically mixed with a glass forming agent and optionally, magnesium oxide or a magnesium halide in a selected mole % ratio. The physical mixture may then be co-comminuted in any conventional milling apparatus such as a ball mill until an XRD spectrum of the milled composite mixture is devoid of peaks associated with a crystalline material.

(16) In another embodiment, the physical mixture of the V.sub.2O.sub.5, glass forming agent and optional magnesium oxide or magnesium halide is heated in an appropriate furnace or oven and quenched by dropping into water or by pressing between two plates or rollers. The amorphous solid solution obtained may then be pulverized.

(17) In either case, the obtained amorphous phase material is heat treated or annealed at a temperature above the T.sub.g from P1 to a temperature less than the onset of the P2 maximum for that amorphous material to obtain the metastable structural and morphological form.

(18) The annealing time may be from 0.5 to 24 hours and the optimum time and temperature to obtain the metastable phase may be determined by DSC analysis as known to one of ordinary skill in the art.

(19) The annealing may be conducted under an inert gas or in ambient air. In one embodiment, the annealing is conducted in ambient air.

(20) Although the grain size of the pulverulent material is not limited, in a preferred embodiment, the grain size is 10 μm or less, more preferably 5 μm or less and most preferably 1 μm or less.

(21) To prepare the cathode the metastable [V.sub.2O.sub.5].sub.c[M.sub.aO.sub.b].sub.d of formula (I) and/or metastable material [V.sub.2O.sub.5].sub.c[M.sub.aO.sub.b].sub.d[MgX.sub.e].sub.g of formula (II) may be mixed with a binder. The binder material is not particularly limited and any binder recognized by one of skill in the art as suitable may be employed. Suitable binders may include polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), and polyimide. Polytetrafluorethylene may be employed in one preferred embodiment.

(22) In an embodiment of the invention the metastable [V.sub.2O.sub.5].sub.c[M.sub.aO.sub.b].sub.d and/or [V.sub.2O.sub.5][M.sub.aO.sub.b].sub.d[MgX.sub.e].sub.g may be mixed with a carbonaceous material such as graphite, carbon nanotubes or carbon black.

(23) The amount of binder and carbonaceous material in the cathode composition may be no greater than 50% by weight, preferably no greater than 30% by weight and more preferably, no greater than 10% by weight.

(24) In a further embodiment the present invention provides a magnesium battery comprising: an anode; a cathode; and an electrolyte; wherein the cathode comprises: an active material of formula (I):
[V.sub.2O.sub.5].sub.c[M.sub.aO.sub.b].sub.d  (I) wherein M is an element selected from the group consisting of P, B, Si, Ge and Mo, a is an integer of from 1 to 2, b is an integer of from 1 to 5, c is from 35 to 80 mol %, d is from 20 to 65 mol %, and the active material of formula (I) is a metastable structural and morphological phase between an amorphous phase and a crystalline phase.

(25) In a still further embodiment the present invention provides a magnesium battery comprising: an anode; a cathode; and an electrolyte; wherein the cathode comprises: an active material of formula (II):
[V.sub.2O.sub.5].sub.c[M.sub.aO.sub.b].sub.d[MgX.sub.e].sub.g  (II) wherein M is an element selected from the group consisting of P, B, Si, Ge and Mo, X is O, F, Cl, Br, or I, a is an integer of from 1 to 2, b is an integer of from 1 to 5, c is from 35 to 80 mol %, g is from greater than 0 to 25 mol %, e is 1 when X is 0, e is 2 when X is F, Cl, Br, or I, and the sum of c, d and g is substantially 100% and the active material of formula (II) is a metastable structural and morphological phase between an amorphous phase and a crystalline phase.

(26) Construction of the cathode is described above.

(27) The anode of the magnesium battery may be any anode suitable for a magnesium battery, including an anode of magnesium metal or a composition containing magnesium metal, such as Mg.sub.3Bi.sub.2. The anode active material may further include an electrically conductive material and a binder. Examples of electrically conducting materials include carbon particles, such as carbon black. Example binders include various polymers, such as PVDF, PTFE, SBR, and polyimide.

(28) An electrolyte layer is disposed between the anode and cathode and may include a separator which helps maintain electrical isolation between the positive and negative electrodes. A separator may include fibers, particles, web, porous sheet, or other form of material configured to reduce the risk of physical contact and/or short circuit between the electrodes. The separator may be a unitary element, or may include a plurality of discrete spacer elements such as particles or fibers. The electrolyte layer may include a separator infused with an electrolyte solution. In some examples, for example using a polymer electrolyte, the separator may be omitted.

(29) The electrolyte layer may include a non-aqueous solvent, such as an organic solvent, and a salt of the active ion, for example a magnesium salt. Magnesium ions provided by the magnesium salt interact electrolytically with the active material(s). An electrolyte may be an electrolyte including or otherwise providing magnesium ions, such as a non-aqueous or aprotic electrolyte including a magnesium salt. The electrolyte may include an organic solvent. Magnesium ions may be present as a salt or complex of magnesium, or as any appropriate form.

(30) An electrolyte may include other compounds, for example additives to enhance ionic conductivity, and may in some examples include acidic or basic compounds as additives. An electrolyte may be a liquid, gel, or solid. An electrolyte may be a polymer electrolyte, for example including a plasticized polymer, and may have a polymer infused with or otherwise including magnesium ions. In some examples, an electrolyte may include a molten salt. In one aspect, the electrolyte may include phenyl magnesium chloride (PhMgCl.sup.+) aluminum trichloride (AlCl.sub.3.sup.−) in tetrahydrofuran (THF) or magnesium bis(trifluoromethanesulfonyl)imide [Mg(TFSI).sub.2] in acetonitrile (ACN). In a preferred embodiment, the electrolyte may be Mg(TFSI).sub.2 in ACN.

(31) The cathode active material may be present as a sheet, ribbon, particles, or other physical form. An electrode containing the cathode active material may be supported by a current collector.

(32) A current collector may include a metal or other electrically conducting sheet on which the electrode is supported. The current collector may be formed of carbon, carbon paper, carbon cloth or a metal or noble metal mesh or foil.

(33) FIG. 4 shows an example of one configuration of a rechargeable magnesium cell 5. The cell 5 includes a positive electrode 10 including the metastable [V.sub.2O.sub.5].sub.c[M.sub.aO.sub.b].sub.d and/or [V.sub.2O.sub.5].sub.c[M.sub.aO.sub.b].sub.d[MgX.sub.e].sub.g material according to the invention as the cathode active material, an electrolyte layer 12, a negative electrode 14, a cathode current collector 16, a negative electrode housing 18, a positive electrode housing 20 including an inert layer 21, and a sealing gasket 22. The electrolyte layer 12 may include a separator soaked in electrolyte solution, and the positive electrode 10 may be supported by the cathode current collector 16. In this example, the negative electrode 14 includes an active material of magnesium metal.

(34) Having generally described this invention, a further understanding can be obtained by reference to certain specific examples which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified.

Examples

(35) Test materials as listed in Table 1, were prepared by ball milling of the V.sub.2O.sub.5 and P.sub.2O.sub.5 materials under a rotation speed of 370 rpm for 20 h in Ar atmosphere. The balls and pot used for ball milling synthesis were constructed of ZrO2. After ball milling, the samples were annealed to metastable and thermodynamically stable phases respectively in an atmosphere of ambient air.

(36) TABLE-US-00001 V:P Tg (° C.) P1 (° C.) P2 (° C.) 8515 234.67 265.24 423.05 8020 236.07 287.02 390.84 7525 244.45 313.32 391.43 7030 256.76 409.42 440.45

(37) FIG. 3 shows the XRD patterns of thermodynamically stable crystalline V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio), amorphous V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio), metastable V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio) and crystalline V.sub.2O.sub.5. After preparing the amorphous material as described above, the metastable phase V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio) was obtained by annealing for 5 hours at 325° C. and the crystalline V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio) was obtained by annealing for 5 hours at 450° C.

(38) FIG. 4 shows the SEM images of thermodynamically stable crystalline V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio), amorphous V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio) and metastable V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio). After preparing the amorphous material as described above, the metastable phase V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio) was obtained by annealing for 5 hours at 325° C. and the crystalline V.sub.2O.sub.5:P.sub.2O.sub.5 (75:25 mol ratio) was obtained by annealing for 5 hours at 450° C.

(39) Cyclic voltammograms of each of the prepared samples were obtained by using Ag reference electrode. In each case the working electrode was composed of the prepared active material, acetylene black and PVDF binder with a weight ratio of 50:25:25 on stainless steel mesh. Mg metal was used as counter electrode, and then either Mg(TFSI).sub.2 or Mg(ClO.sub.4).sub.2 were used as a salt of Mg electrolyte coupled with battery grade acetonitrile solution. The Ag reference solution consisted of 0.1M AgNO.sub.3 and 0.01M TBAP as supporting salt in acetonitrile solution. The scanning rate was 0.1 mV/sec and the operating temperature was 25° C. under Ar atmosphere. The Cyclic voltammograms are shown in FIG. 2. As indicated in the curve a higher oxidation peak height was obtained with the metastable phase.

(40) Numerous modifications and variations on the present invention are possible in light of the above description and examples. It is therefore to be understood that within the scope of the following Claims, the invention may be practiced otherwise than as specifically described herein. Any such embodiments are intended to be within the scope of the present invention.