Tin Alloy Sheets as Negative Electrodes for Non-Aqueous Li and Na-ion Batteries

Abstract

This invention relates to materials for the negative electrode in non-aqueous rechargeable alkali-ion batteries in free-standing form. In particular, this invention relates to the use of metal ribbon that is produced by melt spinning directly as a battery electrode. The invention also relates to a method producing a highly dispersed, multiphase composite material in a single step, as well as a way to generate porosity while maintaining the ‘binder-free’ and ‘additive-free’ characterization of the electrode.

Claims

1. An electrode comprised of a composite ribbon of Sn.

2. An electrode as claimed in claim 1 comprising Sn phase with Cu6Sn5 inclusions below 1 micron or Al inclusions below 250 nm.

3. An electrode as claimed in claim 1 further comprising an element selected from the group consisting of Cu and Al.

4. An electrode according to claim 2 wherein the electrode is free of binder and conductive additives.

5. An electrode as claimed in claim 2 comprising an alkali metal ion battery anode.

6. An electrode as claimed in claim 5 wherein the Al inclusions is up to 50 atomic %.

7. An electrode as claimed in claim 6 wherein the porosity is equal to the volume fraction of Al in the original alloy ribbon.

8. An electrode as claimed in claim 7 wherein the porosity is between 0 and 38% for 0 and 50 at % Al, respectively.

9. An alkali metal ion battery comprising a porous ribbon of Sn subjected to leaching of a second element selected from the group consisting of Cu and Al.

10. A method of producing a battery electrode comprising the steps of: a) spinning a metal ribbon on a melt spinning apparatus; to produce a ribbon between 20 and 45 micrometer thickness.

11. The method as claimed in claim 10 further comprising: a) setting the spin speed between a surface velocity of between 20 to 50 m/s; b) selecting a rotating copper wheel with a crucible; c) selecting Sn and Cu or Sn and Al as metal in the crucible; d) melting the mixture of c) in the crucible; e) setting the gap between a bottom of the crucible and the copper rotating wheel between 0.1 and 0.5 mm when the molten material is ejected; f) selecting the overpressure on top of the melt used to expel the molten material from the crucible onto the copper wheel between 0.2 and 0.4 atmospheres;

12. The method of claim 10 wherein a composite from the group comprising Sn—Al and Sn—Cu is produced.

13. The method as claimed in claim 10 followed by a cooling step to ambient temperatures.

14. The method as claimed in claim 10 further including dissolving Al in 1 M (aq) KOH solution resulting in a porous Sn structure

15. The method as claimed in claim 10 wherein; a) the spin speed is selected between a surface velocity of between of 28.27 to 42.4 m/s; b) the gap between a bottom of the crucible and the copper rotating wheel is selected between 0.25 and 0.4 mm when the molten material is ejected; and c) the overpressure on top of the melt used to expel the molten material from the crucible onto the copper wheel between 0.2 and 0.4 atmospheres;

16. The method as claimed in claim 11 comprising: a) cold rolling the Sn—Al ribbon to densify the Sn—Al ribbon.

17. The method as claimed in claim 13 comprising a) alkaline etching of the Al using potassium hydroxide solution to introduce pores into the Sn ribbon.

18. The method as claimed in claim 17 wherein the ribbon is used as a battery anode in the form of a sheet, rather than powder.

Description

DESCRIPTION OF THE DRAWINGS

[0015] FIG. 1 is a photo of the melt spinning apparatus.

[0016] FIG. 2a is a schematic overview of a Prior Art commercial electrode preparation processes; as compared to FIG. 2b, which is a schematic flow chart that shows an overview of the invention described herein.

[0017] FIG. 3 is a schematic drawing of a coin cell that was used for electrochemical testing. Half-cell configuration is depicted using a Na or Li metal negative electrode.

[0018] FIG. 4a is an SEM micrograph and FIGS. 4b and 4c are EDX elemental mappings of Sn and Cu of a Cu30Sn70 ribbon melt spun at a wheel rotation frequency of 30 Hz, gap between the wheel and crucible of 0.25 mm and ejection pressure differential of 0.2 bar.

[0019] FIGS. 5a, 5b, and 5c show SEM micrographs of (from top to bottom) Sn75Al25, Sn65Al35 and Sn50Al50 ribbons melt spun at a wheel rotation frequency of 30 Hz, a gap between the wheel and the crucible of 0.25 mm and an ejection pressure differential of 0.2 bar

[0020] FIGS. 6a, 6b, and 6c show the same ribbons as FIGS. 5a, 5b, and 5c in the same order, after dissolving all the Al in 1 M (aq) KOH solution resulting in a porous Sn structure

[0021] FIG. 7a shows the voltage profiles of a Cu30Sn70 ribbon cycled in SIB half-cell configuration using 1 M NaPF6 in dimethoxyethane (DME) as the electrolyte. The dashed line demarcates the capacity of a typical hard carbon material. Capacity and Coulombic efficiency (CE) as a function of cycle number are shown in FIG. 7b. CE is consistently above 99%.

[0022] FIGS. 8a, 8b, and 8c show SEM micrographs of solid Sn, composite Sn75Al25 and etched, porous Sn75Al25 after 200 cycles as a SIB anode at the same capacity showing progressively less fracturing going from solid elemental Sn to porous Sn.

[0023] FIG. 9a shows the voltage profiles in the first cycle of as-made Sn75Al25 and porous etched Sn75A125 cycled in a LIB half cell in 1 M LiPF6 in a 1:1 by volume ethylene carbonate:diethyl Carbonate (EC:DEC) electrolyte containing 10 volume % fluoroethylene carbonate (FEC) Coulombic efficiency (CE) as a function of cycle number are shown in FIG. 9b. Efficiency in the early cycles is much better for the porous material. CE is consistently above 99.5%.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The description herein utilizes nomenclature found in texts. During discharge the positive is a cathode, the negative is an anode. During charge the positive is an anode, the negative is a cathode. Texts describing battery anodes or cathodes imply considering the case of a discharge.

[0025] In addition, the starting materials that are used in the invention to be described utilize materials available in the market place that may have impurities or trace amounts of other materials. For example, in one embodiment the Sn powder described herein has a purity of at least 99.5%. The Al is at least 99.7% pure.

Explanation of Terms

[0026] LIB: Lithium ion battery
SIB: Sodium ion battery
Anode: Negative electrode in full-cell configuration
Cathode: Positive electrode in full-cell configuration
Half-cell: Battery containing the active material, subject of the present invention, as the positive electrode and Li or Na metal as the negative electrode
Full cell: Battery containing the materials described herein as the negative electrode and lithium or sodium transition metal oxides as the positive electrode
Coin cell: Most commonly used battery format in laboratory research mAh/g (milli-Ampere-hours-per-gram): Unit of capacity of an active electrode material. 1 mAh is equal to 3.6 Coulombs
Coulombic efficiency: Amount of Li extracted/Amount of Li inserted into an electrode

[0027] FIG. 1 is a photo of the meltspinning apparatus showing the copper wheel. In one embodiment a boron nitride crucible filled with the starting materials (Sn and Cu or Al) is positioned in the middle of the RF induction coil that is visible above the copper wheel. When a temperature of at least 200 degrees above the liquidus, as read from a phase diagram, is reached, the crucible is lowered to a position 0.25-0.4 mm above the wheel and the molten material is ejected onto the rapidly spinning copper wheel, producing a long ribbon. Also shown is a long length of unbroken ribbon that is formed by the melt spinning process.

[0028] FIG. 2a is a schematic overview of a prior art commercial electrode preparation processes; while FIG. 2b is a schematic flow chart that shows an overview of the invention described herein. One advantage of using the process described herein lies in the fact that, all the powder mixing, coating and drying steps, which are highly time- and energy-consuming, can be circumvented.

[0029] FIG. 3 is a schematic drawing of the coin cell that was used for electrochemical testing. A cross section or half-cell configuration is depicted using a Na or Li metal negative electrode. The alkali metal foil is separated from the working electrode by a porous polypropylene (Celgard™) separator. The porous Sn or composite Sn—Al or Sn—Cu anode is contacting the positive electrode cup. The top surface of the Sn-based anode and both sides of the separator are wetted with electrolyte before the cell is crimped shut. The strips are cut into 1 cm.sup.2 squares with scissors or 1 cm diameter discs using a die cutter to fit inside the ‘2032’ (20 mm diameter, 3.2 mm height) coin cells (see FIG. 3) that were used for electrochemical cycle life testing.

[0030] FIG. 4a is a scanning electron microscope (SEM) micrograph and FIGS. 4b and 4c are energy-dispersive X-ray spectroscopy (EDX) elemental mappings of Sn and Cu respectively of a Cu30Sn70 ribbon meltspun at a wheel rotation frequency of 30 Hz, gap between the wheel and crucible of 0.25 mm and ejection pressure differential of 0.2 bar. The scalebar is 1 micrometer in all three micrographs. The composition is given in atomic percent.

[0031] FIGS. 5a, 5b, and 5c are SEM micrographs of (from top to bottom) Sn75Al25, Sn65Al35 and Sn50Al50 respectively of ribbons melt spun at a wheel rotation frequency of 30 Hz, gap between the wheel and crucible of 0.25 mm and ejection pressure differential of 0.2 bar. The compositions are given as atomic percentages. The aluminum particles are all smaller than 250 nm.

[0032] FIGS. 6a, 6b, and 6c show the same ribbons as in FIGS. 5a, 5b, and 5c respectively in the same order, after dissolving all the Al in 1 M KOH solution resulting in a porous Sn structure. The interconnected Sn filaments are visible in the micrographs as well as the voids left behind after the Al is dissolved.

[0033] FIG. 7a shows the voltage profiles of a Cu30Sn70 ribbon cycled in SIB half-cell configuration using 1 M NaPF6 in dimethoxyethane (DME) as the electrolyte. The dashed line in FIG. 7b demarcates the capacity of a typical hard carbon material at 250 mAh/g. The Cu30Sn70 ribbon cycles at more than twice that capacity; 560 mAh/g. Capacity and Coulombic Efficiency (CE) as a function of cycle number are shown on the right. CE is consistently above 99%

[0034] More particularly FIG. 7 shows that a 1 cm.sup.2 Cu30Sn70 anode cut directly from an as-produced melt spun ribbon can be stably cycled at more than twice the capacity of hard carbon, which is a typically used anode material in sodium-ion batteries.

[0035] FIGS. 8a, 8b, and 8c show SEM micrographs of solid Sn (FIG. 8a), composite Sn—Al (FIG. 8b) and etched, porous Sn—Al (FIG. 8c) after 200 cycles as a sodium-ion battery anode at the same capacity showing progressively less fracturing going from solid elemental Sn to porous Sn, emphasizing the beneficial effect of the composites and porous anode material, described herein respectively, over a dense material. More particularly FIG. 8 shows the beneficial effects of using composite or porous Sn. The amount of fracturing diminishes continuously moving from a pure, solid (i.e. no pores) Sn ribbon in FIG. 8a, to a composite Sn—Al ribbon used directly as it is produced by the melt spinning process and cut to size (FIG. 8b) and a porous Sn ribbon produced by dissolving the Al from a Sn—Al composite ribbon (FIG. 8c). This emphasizes the beneficial effects of using composite and porous anode material, respectively.

[0036] FIG. 9a shows the voltage profiles in the first cycle of as-made Sn—Al composite and porous etched Sn—Al composite cycled in a LIB half cell in 1 M LiPF6 in a 1:1 by volume ethylene carbonate:diethyl carbonate (EC:DEC) electrolyte containing 10 volume % fluoroethylene carbonate (FEC). Coulombic Efficiency (CE) as a function of cycle number are shown on the bottom in FIG. 9b. Coulombic Efficiency in the early cycles is much better for the porous material, which extracts 78% of the inserted Li in the first cycle vs. 45% for the composite. CE is consistently above 99.5% for both after the 3.sup.rd cycle. Furthermore FIG. 9 shows that in a lithium-ion coin cell, the porous Sn has better reversibility in the first cycle compared to the composite ribbon; 78% vs. 45%, again emphasizing the advantages of generating the internal porous structure in the Sn. Both cells can cycle at close-to-100% efficiency for at least 50 cycles.

[0037] The present invention uses the thin metal ribbon that is produced by the melt spinning process directly as a battery electrode, without the need for powder processing. Using a spin speed of 30-45 Hz, corresponding to a surface velocity of 28.27 to 42.4 m/s. The gap between the bottom of the crucible and the rotating copper wheel was set to between 0.25 and 0.4 mm at the moment the molten material was ejected. The overpressure on top of the melt used to expel the molten material from the crucible and onto the copper wheel was between 0.2 and 0.4 atmosphere. Ribbons between 20 and 45 micrometer thickness and 1 cm wide are produced in this manner. Thanks to the extremely high cooling rates achieved in a melt spinning process, the secondary phase, when there is one, is very finely dispersed throughout the material. Composite Sn—Cu and Sn—Al ribbons can be produced in this way where the Cu6Sn5 (bronze) and Al metal inclusions have maximum dimensions below 1 micron and below 250 nm, respectively. Preferably, the Al content in Sn—Al is between 0 and 50 atomic %. After optional densification of the Sn—Al ribbon by cold rolling to improve mechanical strength during the leaching step, alkaline leaching of the Al using potassium hydroxide solution introduces pores into the Sn ribbon that can then still be used as a battery anode in the form of a sheet, rather than powder. The porosity is equal to the volume fraction of Al in the original alloy ribbon; between 0 and 38% for 0 and 50 at % Al, respectively.