VOPO.SUB.4 .cathode for sodium ion batteries

Abstract

An electrode comprising a space group Pna2.sub.1 VOPO.sub.4 lattice, capable of electrochemical insertion and release of alkali metal ions, e.g., sodium ions. The VOPO.sub.4 lattice may be formed by solid phase synthesis of KVOPO.sub.4, milled with carbon particles to increase conductivity. A method of forming an electrode is provided, comprising milling a mixture of ammonium metavanadate, ammonium phosphate monobasic, and potassium carbonate; heating the milled mixture to a reaction temperature, and holding the reaction temperature until a solid phase synthesis of KVOPO.sub.4 occurs; milling the KVOPO.sub.4 together with conductive particles to form a conductive mixture of fine particles; and adding binder material to form a conductive cathode. A sodium ion battery is provided having a conductive NaVOPO.sub.4 cathode derived by replacement of potassium in KVOPO.sub.4, a sodium ion donor anode, and a sodium ion transport electrolyte. The VOPO.sub.4, preferably has a volume greater than 90 .sup.3 per VOPO.sub.4.

Claims

1. A method of forming an electrode, comprising: milling a mixture of ammonium metavanadate, ammonium phosphate monobasic, and potassium carbonate; solid phase synthesizing KVOPO.sub.4 by heating the milled mixture to a reaction temperature; milling the KVOPO.sub.4 together with conductive particles to form a conductive mixture of fine particles; and replacing potassium ions with sodium ions to form NaVOPO.sub.4.

2. The method according to claim 1, wherein said heating is at a temperature of between 600 C. and 800 C. for about 10 hours.

3. The method according to claim 1, wherein the conductive particles comprise carbon particles.

4. The method according to claim 1, further comprising adding a binder material selected from one or more of the group consisting of a poly (vinylidene fluoride), a polytetrafluoroethylene, a styrene butadiene rubber, and a polyimide.

5. The method according to claim 1, wherein the NaVOPO.sub.4 has a lattice having a crystalline structure having orthorhombic symmetry with space group Pna2.sub.1.

6. The method according to claim 1, wherein further comprises an insoluble conductive additive.

7. The method according to claim 1, wherein the conductive particles comprise conductive carbon.

8. The method according to claim 1, further comprising: providing a sodium donor anode and a sodium ion transport electrolyte to form a sodium ion battery; and operating the sodium ion battery with a discharge voltage curve comprising a first voltage plateau region comprising 3.8 V, and a second voltage plateau region comprising 2 V, the electrode having a capacity of at least C=133 mAhg.sup.1.

9. The method according to claim 1, wherein the conductive mixture, after replacement of the potassium ions with sodium ions comprises NaVOPO.sub.4 particles having particle size of 200 nm and conductive particles having a particle size of 2 m.

10. The method according to claim 1, further comprising a poly(vinylidene fluoride) binder.

11. The method according to claim 1, wherein the electrode has a capacity of at least C=133 mAhg.sup.1.

12. The method according to claim 1, wherein the NaVOPO.sub.4 comprises a lattice having a volume greater than 90 .sup.3 per VOPO.sub.4.

13. A method of forming an NaVOPO.sub.4 electrode, comprising a VOPO.sub.4 lattice, the VOPO.sub.4 lattice having a volume greater than 90 .sup.3 per VOPO.sub.4; and a portion of the VOPO.sub.4 lattice having two sodium ions per VOPO.sub.4, the method comprising: forming a mixture of ammonium metavanadate, ammonium phosphate monobasic, and potassium carbonate; synthesizing KVOPO.sub.4 in a solid phase by heating the mixture; combining the KVOPO.sub.4 with conductive particles to form a conductive mixture; and replacing at least a portion of potassium ions with sodium ions to form a NaVOPO.sub.4.

14. The method according to claim 13, wherein the NaVOPO.sub.4 has orthorhombic symmetry has space group Pna2.sub.1.

15. The method according to claim 13, wherein the KVOPO.sub.4 is milled together with the conductive particles comprising carbon particles, before the replacement of at least a portion of potassium with sodium.

16. The method according to claim 13, further comprising forming a sodium ion battery by providing a sodium battery anode and a sodium ion transport electrolyte in combination with the electrode, the sodium ion battery having a capacity of at least C=133 mAhg.sup.1, and a discharge voltage curve comprising a first voltage plateau region comprising 3.8 V, and a second voltage plateau region comprising 2 V.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1A shows an X-ray diffraction (XRD) pattern and Rietveld refinement of as-synthesized KVOPO.sub.4.

(2) FIG. 1B shows a scanning electron microscope (SEM) image of ball milled KVOPO.sub.4 cathode material.

(3) FIG. 2A shows the two intersecting six side tunnels along an a axis and a b axis, representing tunnel 1: Along the a axis, the tunnel is formed of four VO.sub.6 octahedra (1,3,4,6) and two PO.sub.4 tetrahedra (2,5).

(4) FIG. 2B shows the [VO.sub.3].sub. chain, representing tunnel 2: Along the b axis, the tunnel is formed of three VO.sub.6 octahedra (1,3,5) and three PO.sub.4 tetrahedra (2,4,6).

(5) FIG. 2C shows the coordinates of vanadium cation.

(6) FIG. 2D shows two types of VO.sub.6 tetrahedra.

(7) FIG. 3A shows galvanostatic charge/discharge profiles of KVOPO.sub.4 cathode.

(8) FIG. 3B shows specific discharge capacities of KVOPO.sub.4 cathode as a function of cycle number.

(9) FIG. 4A shows GITT capacity-voltage profiles of the KVOPO.sub.4 cathode.

(10) FIG. 4B shows GITT time-voltage profiles of the KVOPO.sub.4 cathode.

(11) FIGS. 5A-5D show ex situ XRD patterns at different states of charge/discharge.

(12) FIGS. 6A-6D show structural illustrations of the monoclinic NaVOPO.sub.4 polymorph consisting of VO.sub.6 octahedra (blue), PO.sub.4 tetrahedra (dark green) and Na atoms (white).

(13) FIGS. 7A-7D show (7A) CV profiles (scan rate at 0.1 mV s1); (7B) charge-discharge curves (current density of 5 mA g1) at room temperature of (b) non-ball-milled NaVOPO.sub.4; (7C) ball-milled NaVOPO.sub.4 and (7D) cycling performance of ball-milled NaVOPO.sub.4 at a current density of 10 mA g.sup.1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Example 1

(14) Synthesis of KVOPO.sub.4 Cathode Material

(15) 1.17 g of ammonium metavanadate, 1.15 g of ammonium phosphate monobasic and 0.69 g of potassium carbonate were uniformly mixed by 4 hours planetary ball milling in the presence of 20 mL acetone. The obtained powders were completely dried in air at room temperature, which were used as precursor for the following solid state reaction, conducted under an argon atmosphere.

(16) The dry powders were pressed into pellets at a pressure of 3 tons for 2 mins, and each pellet has a typical weight of 200 mg. Five such pellets were used for one batch solid state synthesis. The pellets were heated to 700 C. with a heating rate of 5 C. per min, maintained at 700 C. for 10 hours, and then cooled down to room temperature at cooling rate of 5 C. per min.

(17) Reddish brown powders were obtained after the solid state reaction process. The typical yield is 700 mg for each batch.

(18) 400 mg of KVOPO.sub.4 reddish brown powder and 114 mg of carbon black were mixed using a mortar and pestle (i.e., a weight ratio of 7:2). The mixture was high energy ball milled for 12 mins. (This time may be extended to, e.g., 36 min. as may be desired). Black color powders were obtained after the ball milling. The typical yield is 450 mg for each batch.

(19) Material Characterization

(20) The X-ray diffraction data was collected by a Scintag XDS2000 diffractometer equipped with Cu K sealed tube. XRD data Rietveld refinement was performed using the GSAS/EXPGUI package. The SEM image was collected by Zeiss Supra-55 field emission scanning electron microscope, and is shown in FIG. 1 and Table 2.

(21) TABLE-US-00002 TABLE 2 XRD powder diffraction and Rietveld refinement results for KVOPO.sub.4 powder sample. Symmetry Orthorhombic Space group Pna2.sub.1 Lattice parameters a = 12.7671(7) , b = 6.3726(2) , c = 10.5163(0) , V= 855.6(1) .sup.3 R.sub.wp (%) 4.82 x.sup.2 3.423

(22) Basically, the compound was completely indexed with space group of Pna2.sub.1 with orthorhombic symmetry. The cell dimension parameters (a=12.7671(7) , b=6.3726(2) , c=10.5163(0) , V=855.6(1) .sup.3) is much larger comparing to the lithium and sodium counterparts. This should be due to the much larger size of potassium ions (2.76 vs. 2.04 ). The as-prepared KVOPO.sub.4 power was ball milled together with super P (weight ratio=7:2) in order to decrease the particle size of KVOPO.sub.4 and wrap the smaller particles with amorphous carbons. FIG. 1B displays an SEM image of the ball milled material. The secondary particle size is around 2 m, and the primary particle size is around 200 nm.

(23) The remaining VOPO.sub.4 framework of the KVOPO.sub.4 compound is assembled by corner-sharing VO.sub.6 octahedra and PO.sub.4 tetrahedra. The whole VOPO.sub.4 structure can provide two intersecting six side tunnels for the following Na intercalation. FIG. 2 shows the two types of tunnels along the a and b axis respectively. The tunnel along the a axis is formed of rings of two PO.sub.4 tetrahedra (marked as 2, 5) and four VO.sub.6 octahedra (marked as 1, 3, 4, 6). The tunnel along the b axis is formed of rings of three PO.sub.4 tetrahedra (marked as 2, 4, 6) and three VO.sub.6 octahedra (marked as 1, 3, 5). Due to the intersection of the two tunnels, the effective diffusion pathways for Na ions could be two or even three dimensional instead of one dimensional, which is more benefitting for the kinetics of cathode.

(24) TABLE-US-00003 TABLE 3 Atomic coordinates for KVOPO4. X y z V(1) 0.123949 0.006684 0.255227 V(2) 0.247143 0.272448 0.507111 P(1) 0.182754 0.507027 0.235075 P(2) 0.007772 0.178751 0.493906 K(1) 0.382056 0.772807 0.435412 K(2) 0.397591 0.203688 0.162091 O(1) 0.105733 0.304952 0.196314 O(2) 0.107539 0.309492 0.269465 O(3) 0.002507 0.015824 0.369038 O(4) 0.015493 0.034588 0.100065 O(5) 0.232475 0.036421 0.109097 O(6) 0.219554 0.021262 0.355514 O(7) 0.397020 0.200980 0.458928 O(8) 0.249809 0.018958 0.629969 O(9) 0.086298 0.304742 0.502290 O(10) 0.260082 0.470928 0.341831

(25) TABLE-US-00004 TABLE 4 V-O bonds distance () for KVOPO.sub.4. Bond CN R () V(1) V-O.sub.short 1 1.697(11) V-O.sub.eq 4 1.833(15)-2.187(10) V-O.sub.long 1 2.370(11) V(2) V-O.sub.short 1 1.881(12) V-O.sub.eq 4 1.889(13)-2.134(10) V-O.sub.long 1 2.313(13)

(26) The atomic coordinate values were listed in Table 3, there are two different potassium, two different vanadium (V(1), V(2)) and two different phosphorus. As shown in FIG. 4a, V(1) and V(2) alternate with each other forming the infinite [VO.sub.3].sub. chain. The potassium ions sitting in the two kinds of tunnels also differ from each other in term of local coordination environment. In the VO.sub.6 octahedra, the six oxygen atoms link with the central vanadium atom by one short, one long and four equatorial VO bonds. The bond length for V(1) and V(2) were listed in Table 4. It is worthwhile to mention that the different coordinates of vanadium ions could provide different local environments around the Na sites.

(27) Electrochemical Tests

(28) 200 mg of ball milled KVOPO.sub.4/Carbon composite was mixed with 22.2 mg poly(vinylidene fluoride) (PVDF) together with 500 L N-Methyl-2-pyrrolidone to form a uniform viscous slurry. The slurry was casted on to aluminum foil using doctor blade. After drying, circular electrodes with area of 1.2 cm.sup.2 were punched from the foil with 2-4 mg of active material on each circular electrode. The electrode was immersed in a 1 M solution of sodium hexafluorophosphate in propylene carbonate. A sheet of sodium, 1.24 cm in diameter, served as the anode. 30 L liquid electrolyte was used in each half cell (i.e., 2325-type coin cells). All manipulations were performed in a helium environment.

(29) The initial open circuit voltage of the cell was around 3 volts. The cells were tested using a VMP2 mutichannel potentiostat (Biologic).

(30) The electrochemical performance of KVOPO.sub.4 as cathode was tested in a half cell configuration with sodium metal as both counter and reference electrode. The pristine electrode was first galvanostatically charged to a high cut-off voltage of 4.7 V vs. Na/Na.sup.+ in order to furthest remove the potassium ion from the structure and oxidize V.sup.4+ to V.sup.5+. The current density used was C/50 (C=133 mAhg.sup.1). According to the charge profile, there should be side reaction of the electrolyte involved in the very high voltage region above 4.5 V. Since normal propylene carbonate electrolyte was used here, this side reaction above 4.8 V vs. Li/Li.sup.+ is expected and acceptable. The subsequent discharge process should insert sodium into the electrode.

(31) As shown in FIG. 3A, discharge capacity of 158 mAhg.sup.1 was obtained in the first discharge (i.e., 2.sup.nd cycle) at current density of C/50. After discharging the electrode was charged back to cut-off voltage of 4.5 V vs. Na/Na.sup.+ to remove sodium again. In the following cycles, the electrode swung between 1.3 and 4.5 V vs. Na/Na.sup.+. Apparently, the electrode exhibited two major plateau regions within the voltage window. The higher voltage plateau region was centered at ca. 3.8 V and the low voltage plateau region was centered at ca. 2 V.

(32) From a thermodynamic point of view, the higher voltage region should be related to the V.sup.5+/V.sup.4+ redox couple and the lower voltage region should be related to the V.sup.4+/V.sup.3+ redox couple. Based on the specific discharge capacity of 158 mAhg.sup.1 (i.e., exceeding the theoretical value derived from one Na), the KVOPO.sub.4 should be a two-electron cathode which should have theoretical capacity of 266 mAhg.sup.1. This multi-electron characteristic is rarely observed for sodium ion cathode, which is greatly helpful for solving the intrinsic low energy issue of sodium-based systems.

(33) There is hysteresis observed in the charge/discharge profile, which is most likely due to the potential coexistence of the two redox couples. Both the high and low voltage regions exhibited additional substructure, i.e., there are slope changes along the sloppy plateau regions. These sub-plateaus indicated the multiple sodium storage sites existing in the structure and there is no preference for sodium ions to enter any specific site. This sodium site multiplicity is closely related to the different local coordinate environments of vanadium, which has been detailed discussed in the crystal structure section.

(34) As shown in FIGS. 3A and 3B, the KVOPO.sub.4 cathode was reversible cycled over 25 cycles. The discharge capacity keeps increase during cycling with maximum value of 181 mAhg.sup.1, which is 68% of the theoretical capacity based on two sodium storage. The increasing discharge capacity indicated a continuously activation of the cathode, which mainly resulted from more potassium extraction from the structure. The sodium storage capability of KVOPO.sub.4 could be further improved by more deeply removing potassium from the structure to empty more sodium intercalation sites.

(35) The properties of various cathode materials for use in sodium ion batteries are shown in Table 5, in comparison to KVOPO.sub.4.

(36) TABLE-US-00005 TABLE 5 Properties of cathode materials. Layered Olivine oxide Tunnel (e.g., Na NASICONS Fluorophosphates Fluorides (e.g., oxide (e.g., Fe Pyrophosphates (e.g., (e.g., (e.g., (e.g., KVOPO.sub.4 NaMnO.sub.2) Na.sub.0.44MnO.sub.2) Mn.sub.0.5PO.sub.4) Na.sub.2FeP.sub.2O.sub.7) NaV(PO)) NaVPO.sub.4F) FeF) Capacity 180 ~185 ~140 ~93 ~90 ~140 ~120 ~125 (mAhg.sup.1) Energy 442.5 ~470 ~400 ~280 ~300 ~330 ~400 ~350 density (Whkg.sup.1) Voltage 1.5-4.3 2.0-3.8 2-3.8 2-4 2-4.5 1.2-3.5 3-4.5 1.5-4 window (V) Safety stable up to stable up to stable up to stable up to stable up to stable up to stable up to stable up to (thermal 600 C. ~300 C. ~300 C. ~600 C. ~600 C. ~450 C. ~500 C. ~320 C. stability) Materials Depending on the element the specific cathode material contains. costs Mfg Largely depending on the synthesis strategy and elements contained for a specific costs cathode material.

(37) The reaction kinetics of KVOPO.sub.4 cathode was investigated by GITT in FIGS. 4A and 4B. The electrode shows very small hysteresis during the charging in the low voltage region (i.e., V.sup.3+/V.sup.4+ transition) and discharging in the high voltage region (i.e., V.sup.5+/V.sup.4+ transition). According to the voltage versus time chart in FIG. 4B, the overpotential during these two processes is only 2.8 mV during discharge and 32 mV during charge, respectively. The small polarization indicated the relatively fast kinetics of electrochemical reaction of the cathode. The overpotential has significantly increased when the electrode was charged into the high voltage region or discharged into the low voltage region, which is expected due to the higher energy barrier for the sodium ion bulk diffusion in the related voltage regions. If all the open circuit voltage point in the GITT curve was linked, the formed OCV curve is a sloping shape within the whole voltage region without any pronounced flat plateau. This sloping OCV curve indicated the solid solution behavior during the sodium ion intercalation/extraction.

(38) The crystallographic evolution of KVOPO.sub.4 cathode during charge/discharge was investigated by ex situ XRD. The electrodes were galvanostatically sodiated/desodiated to different cut-off voltages at C/50 and then tested by XRD. The patterns were shown in FIGS. 5A-5D. For all the XRD patterns at different voltage states, there is no addition diffraction peaks observed. The absence of new peaks indicated the absence of additional new phase during the whole voltage window. As shown in the highlighted regions, some of the peak positions (e.g., (200), (201), (110)) displayed continuously shift in one direction during discharging and shifted back during charge. Some of the well-resolved peaks (e.g., (221), (022)) in the pristine material has merged into one broad peak during some states of charge/discharge, indicating they could shift towards different directions. These new peak absence and peaks shift are clear indications of a single-phase reaction mechanism of the electrochemical reaction. Otherwise, emergence and growth of second phase peaks would happen if a two-phase reaction involved.

(39) FIGS. 6A-6D show structural illustrations of the monoclinic NaVOPO.sub.4 polymorph consisting of VO.sub.6 octahedra (blue), PO.sub.4 tetrahedra (dark green) and Na atoms (white). These are comparable to the structural illustrations shown in FIGS. 2A-2D for KVOPO.sub.4.

(40) FIGS. 7A-7D show (7A) CV profiles (scan rate at 0.1 mV s1); (7B) charge-discharge curves (current density of 5 mA g1) at room temperature of (b) non-ball-milled NaVOPO.sub.4; (7C) ball-milled NaVOPO.sub.4 and (7D) cycling performance of ball-milled NaVOPO.sub.4 at a current density of 10 mA g.sup.1. This shows the much lower capacity of the NaVOPO.sub.4 as compared to the KVOPO.sub.4 material according to the present invention.

(41) 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. Where a numerical limit or range is stated, the endpoints are included. Also, all values and subranges within a numerical limit or range are specifically included as if explicitly written out.

(42) The above description is presented to enable a person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the preferred embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, this invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein. In this regard, certain embodiments within the invention may not show every benefit of the invention, considered broadly.