Use of novel compounds as negative electrode active material in a sodium-ion battery

Abstract

Precursor compounds of sodium alloy(s), for use as negative electrode active material in a sodium-ion battery, as well as to a negative electrode have the precursor compound of sodium alloy(s), as well as a sodium-ion battery having a negative electrode of this kind.

Claims

1. A sodium-ion battery comprising: at least one negative electrode in contact with a current collector; and at least one positive electrode in contact with a current collector, wherein said electrodes are separated from one another by an electrolyte, wherein said negative electrode comprises as a negative active material, a precursor compound of sodium alloy(s) responding to the following formula:
M.sub.nE1.sub.xE2(I) in which: M is a transition metal selected from the group consisting of Co, Cu, Ni and Fe; E1 and E2 are elements selected from the group consisting of In, Bi, Ge, Sn, Sb and P; the values of n and x are such that the compound of formula (I) is electronically neutral; n=0 and x0; or n0 and x=0; with the following conditions: when n=0 and x0, E1 and E2 are different from one another and are selected from the group consisting of In, Bi, Ge, Sn, Sb and P, with x such that 0.1x2; it being understood that when x=1 and E1 (respectively E2) is Sn, E2 (respectively E1) is different from Sb; when n0 and x=0, E2 is selected from the group consisting of In, Bi, Ge, Sn, Sb and P, with n such that 0.1n3; it being understood that when n=2 and M is Cu, E2 is different from Sb and when n=6/5 and M is Cu, E2 is different from Sn, wherein the precursor compound of sodium alloy(s) responds to the following formula(Ib-1): Bi.sub.xSb (Ib-1) in which: the value of x is such that the compound of formula (Ib-1) is electronically neutral; x is such that 0.1x<1.

2. The sodium-ion battery according to claim 1, wherein the precursor compound of sodium alloy(s) responds to the following formula (Ia):
M.sub.nE1.sub.xP(Ia) in which: M is a transition metal selected from the group consisting of Co, Cu, Ni and Fe; E1 is an element selected from the group consisting of In, Bi, Ge, Sn and Sb; the values of n and x are such that the compound of formula (Ia) is electronically neutral; n=0 and x0; or n0 and x=0; with the following conditions: when n=0 and x 0, x is such that 0.1x1; when n0 and x=0, n is such that 0.1x3.

3. The sodium-ion battery according to claim 1, wherein the precursor compound of sodium alloy(s) responds to the following formula (Ib):
M.sub.nE1.sub.xSb(Ib) in which: M is a transition metal selected from the group consisting of Co, Cu, Ni and Fe; E1 is an element selected from the group consisting of In, Bi, Ge, Sn and P; the values of n and x are such that the compound of formula (Ib) is electronically neutral; n=0 and x0; or n0 and x=0; with the following conditions: when n=0 and x0, x is such that 0.1x1; when n0 and x=0, n is such that 0.1x3.

4. The sodium-ion battery according to claim 1, wherein the precursor compound of sodium alloy(s) is SnGe.

5. The sodium-ion battery according to claim 1, wherein the negative electrode comprises: (i) 45 to 75% of said precursor compound of sodium alloy(s) responding to the formula (I), (ii) 0 to 30% of a binder, and (iii) 0 to 30% of an agent conferring electron conductivity, the percentages being expressed by weight relative to the total weight of the negative electrode.

6. The sodium-ion battery according to claim 1, wherein the negative electrode comprises: (i) 55 to 75% of said precursor compound of sodium alloy(s) responding to the formula (I), (ii) 10 to 25% of a binder, and (iii) 15 to 25% of an agent conferring electron conductivity, the percentages being expressed by weight relative to the total weight of the negative electrode.

7. The sodium-ion battery according to claim 5, wherein the negative electrode comprises: (i) 60 to 67% of the precursor compound of sodium alloy(s) responding to said formula (Ia), (ii) 12 to 20% of a binder, and (iii) 18 to 25% of an agent conferring electron conductivity, the percentages being expressed by weight relative to the total weight of the negative electrode.

8. The sodium-ion battery according to claim 5, wherein the binder is selected from the group consisting of carboxymethylcellulose (CMC), polyvinylidene fluoride (PVDF), styrene/butadiene copolymer (SBR) and mixtures thereof.

9. The sodium-ion battery according to claim 5, wherein the agent conferring electron conductivity is selected from the group consisting of carbon black, vapour grown carbon fibres, carbon nanotubes, carbon SP and mixtures thereof.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) FIG. 1 is a graph from example 3 showing the NiP.sub.3 anode has a specific capacity of about 1050 mAh/g and is stable for at least 20 cycles;

(2) FIG. 2 is a graph from example 3 showing the CoSb.sub.3 anode has a specific capacity of about 440 mAh/g and is stable for at least 30 cycles;

(3) FIG. 3 is a graph from example 3 showing the FeSb.sub.2 anode has a specific capacity of about 450 mAh/g and is stable for at least 40 cycles;

(4) FIG. 4 is a graph from example 3 showing the Bi.sub.0.22Sb.sub.0.78 anode has a specific capacity of about 550 mAh/g and is stable for at least 40 cycles.

(5) FIG. 5-11 are graphs from example 4 showing respectively the change in potential E (in volts vs Na.sup.+/Na) as a function of the equivalent of the number X of Na.sup.+ ions inserted during at least the first two charge/discharge cycles for the negative electrodes;

(6) FIG. 12a is a graph from example 4 showing the specific capacity (in mAh.Math.g.sup.1) as a function of the number of cycles with a current regime of C/5 for an electrode prepared as described in example 1

(7) FIG. 12b is a graph from example 4 showing the change in potential E (in volts vs Na.sup.+/Na) as a function of the equivalent of the number X of Na.sup.+ ions inserted during at least the first two charge/discharge cycles;

(8) FIG. 13 is a graph from example 6 showing the CoP.sub.3 anode has a specific capacity of about 210-230 mAh/g and is stable for at least 30 cycles;

(9) FIG. 14 is a graph from example 6 showing the CuP.sub.2 anode has a specific capacity of about 700 mAh/g and is stable for at least 20 cycles;

(10) FIGS. 15 and 16 are graphs from example 6 showing respectively the change in potential E (in volts vs Na.sup.+/Na) as a function of the equivalent of the number X of Na.sup.+ ions inserted during at least the first two charge/discharge cycles for the negative electrodes of the invention.

DETAILED DESCRIPTION

(11) The present invention is illustrated by the following examples, but it is not limited to these.

EXAMPLES

(12) The raw materials used in the examples are listed below: Tin Sn powder, Aldrich, particle size of 10 m, purity>99%, Antimony Sb powder, Alfa-Aesar, purity 99.5%, 325 mesh, Cobalt Co powder, Aldrich, purity>99.99%, 100 mesh, Nickel Ni powder, Acros Organics, purity 99.9%, 325 mesh, Iron powder, Alfa-Aesar, purity 99%, 200 mesh, Indium In powder, Aldrich, purity 99.99%, Red phosphorus powder, Aldrich, purity 99%, Bismuth powder, Fluka, purity 99.9%, Copper Cu powder, Alpha Aesar, purity 99.9%, 100-325 mesh Carboxymethylcellulose (CMC) with weight-average molecular weight of 250000 g.Math.mol.sup.1 and with degree of substitution of 0.7, Ethylene carbonate (EC), fluoroethylene carbonate (FEC), vinylidene carbonate (VC), propylene carbonate (PC) and dimethyl carbonate (DMC), battery grade, Sodium perchlorate NaClO.sub.4Addrich, purity 98%, Carbon black (NC), Y50A, surface area (BET method)=66 m.sup.2/g, Vapour grown carbon fibres (VGCF-S), Showa Denko, surface area (BET method)=35 m.sup.2/g, diameter of fibres 100 nm.
All the materials were used as received from the manufacturers.

Example 1

Preparation of the Negative Electrode Active Materials of Formula (I) Used According to the Invention

(13) When the active material of the invention comprises only the element E2 (x=0 and n=0), it was prepared by fast grinding of element E2 as received from the manufacturers.

(14) When the active material of the invention comprises a mixture of powders of a transition metal M and of an element E2 (M/E2) or a mixture of powders of an element E1 and of an element E2 (E1/E2), it was prepared either by grinding said mixture of powders (mechanosynthesis), or by treatment of said mixture of powders under vacuum and at high temperature in a sealed tube (high-temperature synthesis).

(15) 1.1 Preparation of the Negative Electrode Active Materials by Grinding

(16) When the active materials were prepared by grinding, said grinding was carried out in a planetary ball mill sold under the trade name Retsch PM 100 comprising 6 stainless steel bails each of 3 grams and 1 cm diameter. This grinding mill functions by centrifugal motion of the balls, at a speed of up to 600 rev/min.

(17) Under the effect of the centrifugal forces generated by the rotation, the motion of the balls crushes the powders to be ground against the inside wall of the container (which has a volume of 50 cm.sup.3). Grinding is then essentially effected by pressure and friction. The combination of the forces of impact and forces of friction thus created guarantees a high and very efficient degree of grinding of the planetary ball mills. Moreover, the duration of grinding depends on the energy developed by the grinding mill and the amount of powder to be ground.

(18) A powder of transition metal M (respectively a powder of element E1) and a powder of element E2 were introduced in stoichiometric proportions into the grinding mill as described above to obtain 1 to 2 grams of powder of active material of formula (I).

(19) The grinding speed was 500 rev/min and grinding was carried out at room temperature without supply of heat.

(20) In order to avoid an excessive increase in temperature, grinding was carried out in several sequences, separated by pauses for cooling the grinding container and the powders that it contains. For this purpose, a series of sequences 10 minutes grinding/10 minutes pause was employed. The grinding time (including pauses) for obtaining the active material of formula (I) was from 2 to 72 hours using a ratio weight of balls/total weight of powder to be ground in the range from 9 to 18.

(21) The volume occupied by the mixture of powders to be ground was less than of the volume of the grinding container.

(22) After grinding as described above, a step of thermal treatment in a sealed tube, under vacuum, at a temperature in the range from 400 C. to 800 C., for 4 to 14 days was in certain cases carried out in order to form the compounds of formula (I) as such, i.e. in the form of alloys.

(23) 1.2 Preparation of the Negative Electrode Active Materials by High-temperature Synthesis

(24) When preparation of the active materials used according to the invention was carried out by high-temperature synthesis, a mixture of powders of a transition metal M and of an element E2 (M/E2) or a mixture of powders of an element E1 and of an element E2 (E1/E2) was put in a sealed tube and treated under vacuum, at a temperature in the range from 400 C. to 800 C. for a time in the range from 4 to 14 days in order to form the compounds of formula (I) as such, i.e. in the form of alloys.

(25) Table 1 below shows the active material used according to the invention, the type of preparation employed if required (grinding with optionally thermal treatment or treatment in a sealed tube at high temperature), and the parameters of temperature and time used in said preparation.

(26) TABLE-US-00001 TABLE 1 Parameters of temperature Active substance Type of preparation and/or time SnP.sub.3 Grinding 60 hours NiP.sub.3 Sealed tube 700 C., 4 days CuP.sub.2 Grinding 24 hours CoP.sub.3 Grinding 24 hours CoSb.sub.3 Grinding then 48 hours; Sealed tube 750 C., 7 days NiSb.sub.2 Sealed tube 610 C., 12 days FeSb.sub.2 Sealed tube 600 C., 5 days Bi.sub.0.22Sb.sub.0.78 (x = 0.28) Grinding 24 hours Bi.sub.0.44Sb.sub.0.56 (x = 0.79) Grinding 24 hours GeSn Grinding 24 hours In Grinding 2 hours Bi Grinding 2 hours

Example 2

Preparation of Negative Electrodes Comprising an Active Material of Formula (I) According to the Invention

(27) Electrodes comprising an active material of formula (I) were prepared as follows:

(28) An aqueous suspension comprising water, 300 mg of active material of formula (I) (for CoSb.sub.3, NiSb.sub.2, FeSb.sub.2, Bi.sub.0.22Sb.sub.0.78, Bi.sub.0.44Sb.sub.0.56, GeSn, In and Bi) or 200 mg of active material of formula (I) (for SnP.sub.3 and NiP.sub.3), carboxymethylcellulose (CMC), carbon black (NC) and optionally vapour grown carbon fibres (VGCF-S) was homogenized for 1 hour at room temperature, in a planetary mixer sold under the trade name Fritsch Pulverisette 7.

(29) Table 2 below shows the amounts by weight of each of the compounds (in %) and the volume of water (in ml) used for formulation of each of the electrodes A1-A10 according to the invention.

(30) TABLE-US-00002 TABLE 2 Active substance CMC NC VGCF-S Water Electrode Type % % % % ml A1 SnP.sub.3 63 16 21 0 1.2 A2 NiP.sub.3 63 16 21 0 1.2 A3 CoSb.sub.3 70 12 18 0 1.3 A4 NiSb.sub.2 70 12 9 9 1.1 A5 FeSb.sub.2 70 12 9 9 1.35 A6 Bi.sub.0.22Sb.sub.0.78 70 12 9 9 1.3 A7 Bi.sub.0.44Sb.sub.0.56 70 12 9 9 1.3 A8 GeSn 70 12 9 9 1.3 A9 In 70 12 18 0 1.3 A10 Bi 70 12 18 0 1.3

(31) After said homogenization step, some milliliters of the aqueous suspension were applied by the doctor blade method on a copper sheet with thickness of 20 m used as a current collector, and the assembly obtained was dried at room temperature for 12 hours, then at 100 C. under vacuum for 2 hours to obtain an assembly of negative electrode according to the invention+current collector, said negative electrode according to the invention having a thickness of about 10 to 30 m. The thickness of the negative electrode was measured by scanning electron microscopy with a microscope with a field effect detector of secondary and backscattered electrons sold under the name Hitachi S4800.

Example 3

Electrochemical Performance of the Negative Electrodes Comprising an Active Material of Formula (I) According to the Invention

(32) 3.1 Measurements of the Specific Capacity

(33) Half-cell electrochemical tests in a cell of the button type were carried out for each of the electrodes A1 to A10 as prepared above in example 2, using a sodium sheet as counter-electrode, and as electrolyte a solution of sodium perchlorate (NaClO.sub.4 1 mol/l) in PC and 5% FEC (volume ratio PC/FEC=95/5), a separator of the Whatman glass fibre type, and each of the negative electrodes assembled with a current collector as obtained in example 2 above.

(34) The cell of the button type underwent cycles of charging (C)-discharging (D) in different conditions C/n (n being the number of moles of sodium per mole of active material of formula (I) and per hour), between 0.02 V and 1.5 V (0.02 V and 2 V for Bi.sub.xSb).

(35) The measurements of specific capacities for the negative electrodes of the invention comprising as active material NiP.sub.3, CoSb.sub.3, FeSb.sub.2 and Bi.sub.0.22Sb.sub.0.78 are presented respectively in the appended FIGS. 1, 2, 3 and 4, in which the specific capacity (in mAh.Math.g.sup.1) is a function of the number of cycles with a current regime of C for NiP.sub.3, C/2 for CoSb.sub.3 and FeSb.sub.2, and C/5 for Bi.sub.0.22Sb.sub.0.78. In these figures, the curves with the tilled triangles correspond to the measurements taken during charging and the curves with the empty triangles correspond to the measurements taken during discharge, the calculation being effected relative to the weight of electrode active material of formula (I).

(36) According to FIG. 1, the NiP.sub.3 anode has a specific capacity of about 1050 mAh/g and is stable for at least 20 cycles.

(37) According to FIG. 2, the CoSb.sub.3 anode has a specific capacity of about 440 mAh/g and is stable for at least 30 cycles.

(38) According to FIG. 3, the FeSb.sub.2 anode has a specific capacity of about 450 mAh/g and is stable for at least 40 cycles.

(39) According to FIG. 4, the Bi.sub.0.22Sb.sub.0.78 anode has a specific capacity of about 550 mAh4.5, and is stable for at least 40 cycles.

(40) These results show that the negative electrodes according to the invention have excellent cycling stability both in charging and in discharging.

(41) The measurements of the reversible specific capacities (1st cycle) depending on the regime used, of the reversible volume capacities (1st cycle), of the theoretical specific capacities and of the coulombic efficiencies of the negative electrodes according to the invention are listed in Table 3 below. The coulombic efficiencies presented are those measured beyond the first cycle and while the capacity is stable.

(42) TABLE-US-00003 TABLE 3 Reversible Theo- Reversible specific retical volume capacity specific capacity Coulombic Elec- Active 1st cycle capacity 1st cycle efficiency trode substance mAh/g at C/n mAh/cm.sup.3 % A1 SnP.sub.3 866 at C/15 1140 3680 95 A2 NiP.sub.3 1050 at C 1590 4600 99 A3 CoSb.sub.3 440 at C/2 569 3420 97 A4 NiSb.sub.2 440 at C/30 532 3476 92 A5 FeSb.sub.2 450 at C/2 537 3690 98 A6 Bi.sub.0.22Sb.sub.0.78 550 at C/5 570 4200 96 A7 Bi.sub.0.44Sb.sub.0.56 525 at C/5 502 3795 91 A8 GeSn 500 at C/15 1050 3300 97 A9 In 265 at C/10 350 1934 89 A10 Bi 464 at C/10 385 4547 85

(43) According to the results obtained above, we may conclude that the electrodes according to the invention have good electrochemical properties.

(44) 3.2 Analysis of Sodium Insertion According to the Cycling Curves

(45) The appended FIGS. 5, 6, 7, 8, 9, 10 and 11 show respectively the change in potential E (in volts vs Na.sup.+/Na) as a function of the equivalent of the number X of Na.sup.+ ions inserted during at least the first two charge/discharge cycles for the negative electrodes of the invention comprising as active material NiP.sub.3, SnP.sub.3, CoSb.sub.3, FeSb.sub.2, in, Bi and Bi.sub.0.22Sb.sub.0.78 with respective current regimes of C/10, C/15, C/2, C/2, C/10, C/10 and C/5. Cycling was performed between 2.5 V (or 1.5 V) and 0 V vs Na.sup.+/Na.sup.0 , with exchange of the Na.sup.+ ion per period of C/n (corresponding to 1 mole of Na exchanged, inserted or extracted in n hour(s)).

(46) FIGS. 5 to 11 show that the negative electrodes according to the invention have good electrochemical performance, which is surprising bearing in mind that passage from Li.sup.+ to Na.sup.+ is not favourable, neither for ion diffusion, nor for electron conduction since Li loses its electron more easily than Na), nor for the volume expansion generated.

(47) Thus, the performance of the materials cannot be simply deduced from that obtained in an Li-ion battery since it depends on many parameters such as the volume expansion, bond rupture/formation taking place during the various phase transitions, electronic charge transfers, etc.

Comparative Example 4

(48) The appended FIG. 12a shows the specific capacity (in mAh.Math.g.sup.1) as a function of the number of cycles with a current regime of C/5 for an electrode prepared as described in example 1 with an active material BiSb (x=1), said electrode therefore not being according to the invention. In this figure, the curve with the filled triangles corresponds to the measurements taken during charging and the curve with the empty triangles corresponds to the measurements taken during discharge, the calculation being performed relative to the weight of active material.

(49) The appended FIG. 12b shows the change in potential E (in volts vs Na.sup.+/Na) as a function of the equivalent of the number X of Na.sup.+ ions inserted during at least the first two charge/discharge cycles for the negative electrode not according to the invention comprising BiSb as active material with a current regime of C/5.

(50) FIGS. 12a and 12b show that the negative electrode not according to the invention has very poor cycling stability after 10 cycles.

Example 5

Preparation of Other Negative Electrodes Comprising an Active Material of Formula (I) According to the Invention

(51) Electrodes comprising an active material of formula I were prepared as follows:

(52) An aqueous suspension comprising water, 300.7 mg of CoP.sub.3 active material, or 301.4 mg of CuP.sub.2 active material, carboxymethylcellulose (CMC), carbon black (NC) and optionally vapour grown carbon fibres (VGCF-S) was homogenized for 1 hour at room temperature, in a planetary mixer sold under the trade name Fritsch Pulverisette 7.

(53) Table 4 below shows the amounts by weight of each of the compounds (in %) and the volume of water (in ml) used for formulation of each of the electrodes A11 and A12 according to the invention.

(54) TABLE-US-00004 TABLE 4 Active substance CMC NC VGCF-S Water Electrode Type % % % % ml A11 CoP.sub.3 62.9 16 10.5 10.6 1.2 A12 CuP.sub.2 70 12 18 0 0.95

(55) After said homogenization step, some milliliters of the aqueous suspension were applied by the doctor blade method on a copper sheet with a thickness of 20 m used as current collector, and the assembly obtained was dried at room temperature for 12 hours, then at 100 C. under vacuum for 2 hours to obtain an assembly of current collector+negative electrode according to the invention, said negative electrode according to the invention having a thickness of about 10 to 30 m. Said thickness of the negative electrode was measured by scanning electron microscopy with a microscope with a field effect detector of secondary and backscattered electrons sold under the name Hitachi S4800.

Example 6

Electrochemical Performance of the Negative Electrodes as Prepared in Example 5 and Comprising an Active Material of Formula (I) According to the Invention

(56) 6.1 Measurements of Specific Capacity

(57) Half-cell electrochemical tests in a cell of the button type were performed for each of the electrodes A11 and A12 as prepared above in example 5, using a sodium sheet as counter-electrode, and as electrolyte a solution of sodium perchlorate (NaClO.sub.4 1 mol/l) in PC and 5% FEC (volume ratio PC/FEC=95/5), a separator of the Whatman glass fibre type, and each of the negative electrodes assembled with a current collector as obtained in example 5 above.

(58) The cell of the button type underwent cycles of charging (C)-discharging (D) in different conditions C/n (n being the number of moles of sodium per mole of active material of formula (I) and per hour), between 0.02 V and 1.5 V.

(59) The measurements of specific capacities for the negative electrodes of the invention comprising CoP.sub.3 and CuP.sub.2 as active material are presented respectively in the appended FIGS. 13 and 14, in which the specific capacity (in mAh.Math.g.sup.1) is a function of the number of cycles with a current regime of C/20 (squares), then C/10 (diamonds), then C/5 (hexagons), then C/2 (stars) for CoP.sub.3, and of C (diamonds) for CuP.sub.2. In these figures, the curves with the filled squares or the filled diamonds or the filled hexagons or the filled stars correspond to the measurements taken during discharge and the curves with the empty squares or the empty diamonds or the empty hexagons or the empty stars correspond to the measurements taken during charging, the calculation being performed relative to the weight of electrode active material of formula (f).

(60) According to FIG. 13, the CoP.sub.3 anode has a specific capacity of about 210-230 mAh/g and is stable for at least 30 cycles.

(61) According to FIG. 14, the CuP.sub.2 anode has a specific capacity of about 700 mAh/g and is stable for at least 20 cycles.

(62) The measurements of the reversible specific capacities (1st cycle) depending on the regime used, of the reversible volume capacities (1st cycle), of the theoretical specific capacities and of the coulombic efficiencies of the negative electrodes according to the invention are listed in Table 5 below. The coulombic efficiencies presented are those measured beyond the first cycle and while the capacity is stable.

(63) TABLE-US-00005 TABLE 5 Reversible Reversible specific Theoretical volume Active capacity specific capacity Coulombic sub- 1st cycle capacity 1st cycle efficiency Electrode stance mAh/g at C/n mAh/cm.sup.3 % A11 CoP.sub.3 220 C/20 1590 968 98% A12 CuP.sub.2 743.03 C 1280 3195 99%

(64) According to the results obtained above, we may conclude that the electrodes according to the invention have good electrochemical properties.

(65) 6.2 Analysis of Sodium Insertion According to the Cycling Curves

(66) The appended FIGS. 15 and 16 show respectively the change in potential E (in volts vs Na.sup.+/Na) as a function of the equivalent of the number X of Na.sup.+ ions inserted during at least the first two charge/discharge cycles for the negative electrodes of the invention comprising CoP.sub.3 as active material with current regimes of C/20, then C/10, then C/5 and then C/2, and CuP.sub.2 as active material with current regimes of C/10 and then C. Cycling was carried out between 1.5 V and 0 V vs Na.sup.+/Na.sup.0 , with exchange of the Na.sup.+ ion per period of C/n (corresponding to 1 mole of Na exchanged, inserted or extracted in n hour(s)).

(67) FIGS. 15 and 16 show that the negative electrodes according to the invention have good electrochemical performance, which is surprising bearing in mind that passage from Li.sup.+ to Na.sup.+ is not favourable, neither for ion diffusion, nor for electron conduction (since Li loses its electron more easily than Na), nor for the volume expansion generated.

(68) Thus, the performance of the materials cannot be simply deduced from that obtained in an Li-ion battery since it depends on many parameters such as volume expansion, bond rupture/formation taking place during the various phase transitions, electronic charge transfers, etc.