Positive electrode active material for sodium-ion batteries
10544050 · 2020-01-28
Assignee
Inventors
Cpc classification
C01G45/1228
CHEMISTRY; METALLURGY
C01P2002/76
CHEMISTRY; METALLURGY
H01M4/525
ELECTRICITY
C01G53/50
CHEMISTRY; METALLURGY
C01P2002/77
CHEMISTRY; METALLURGY
H01M4/505
ELECTRICITY
H01M4/131
ELECTRICITY
Y02E60/10
GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
C01P2002/72
CHEMISTRY; METALLURGY
H01M10/054
ELECTRICITY
International classification
C01G45/12
CHEMISTRY; METALLURGY
H01M4/505
ELECTRICITY
H01M4/525
ELECTRICITY
H01M4/131
ELECTRICITY
H01M10/054
ELECTRICITY
Abstract
A positive electrode active material for a sodium-ion secondary battery contains a compound of formula Na.sub.xMn.sub.1-y-zM.sub.yM.sub.zO.sub.2 or its hydrate. A cathode can contain the active material and rechargeable sodium-ion battery can contain such a cathode.
Claims
1. A positive electrode active material for a sodium-ion secondary battery, said positive electrode active material comprising a compound of formula (I):
Na.sub.xMn.sub.1-y-zM.sub.yM.sub.zO.sub.2(I) or a hydrated form thereof, wherein: M is one or more elements selected from Ti, V, Cr, Zr, and Al; wherein Ti is selected from Ti.sup.3+ and Ti.sup.4+; V is selected from V.sup.2+, V.sup.3+, V.sup.4+, V.sup.5+; Cr is selected from Cr.sup.2+, Cr.sup.3+ and Cr.sup.6+; Zr is Zr.sup.4+; and Al is Al.sup.3+; M is one or more elements selected from Fe, Ni and Zn; wherein Fe is selected from Fe.sup.2+ and Fe.sup.3+; Ni is Ni.sup.3+; and Zn is Zn.sup.2+; the oxidation state of Mn ranges from 3.sup.+ to 4.sup.+; 0.6<x1.0; 0<y0.4; 0.05z<0.2; provided that: 0.05<(y+z)0.4; and wherein the compound of formula (I) is single phase and has a pristine P2-layered structure.
2. The positive electrode active material according to claim 1, wherein the index y is defined as 0.05y0.2.
3. The positive electrode active material according to claim 1, wherein the index y is defined as 0.05y0.1.
4. The positive electrode active material according to claim 1, wherein the index z is defined as 0.05z0.1.
5. The positive electrode active material according to claim 1, wherein the sum of indexes (y+z) is defined as 0.1(y+z)0.2.
6. The positive electrode active material according to claim 1, wherein the index y is 0.1.
7. The positive electrode active material according to claim 1, wherein the index z is 0.1.
8. The positive electrode active material according to claim 1, wherein M is selected from Ti, and Al, wherein Ti is selected from Ti.sup.3+ and Ti.sup.4+, and Al is Al.sup.3+.
9. The positive electrode active material according to claim 1, wherein M is selected from Fe and Zn, wherein Fe is selected from Fe.sup.2+ and Fe.sup.3+ and Zn is Zn.sup.2+.
10. The positive electrode active material according to claim 1, wherein the compound of formula (I) is selected from Na.sub.2/3Mn.sub.0.8Ti.sub.0.1Fe.sub.0.1O.sub.2; Na.sub.2/3Mn.sub.0.8Al.sub.0.1Fe.sub.0.1O.sub.2; Na.sub.2/3Mn.sub.0.8Ti.sub.0.1Zn.sub.0.1O.sub.2; Na.sub.2/3Mn.sub.0.8Al.sub.0.1Zn.sub.0.1O.sub.2 and Na.sub.2/3Mn.sub.0.9Ti.sub.0.05Fe.sub.0.05O.sub.2.
11. A cathode for a rechargeable sodium-ion secondary battery, said cathode comprising a positive electrode active material, wherein the positive electrode active material comprises a compound of formula (I):
Na.sub.xMn.sub.1-y-zM.sub.yM.sub.zO.sub.2(I) or a hydrated form thereof, wherein: M is one or more elements selected from Ti, V, Cr, Zr, and Al; wherein Ti is selected from Ti.sup.3+ and Ti.sup.4+; V is selected from V.sup.2+, V.sup.3+, V.sup.4+, V.sup.5+; Cr is selected from Cr.sup.2+, Cr.sup.3+ and Cr.sup.6+; Zr is Zr.sup.4+; and Al is Al.sup.3+; M is one or more elements selected from Fe, Ni and Zn; wherein Fe is selected from Fe.sup.2+ and Fe.sup.3+; Ni is Ni.sup.3+; and Zn is Zn.sup.2+; the oxidation state of Mn ranges from 3.sup.+ to 4.sup.+; 0.6<x1.0; 0<y0.4; 0.05z<0.2; provided that: 0.05<(y+z)0.4; and wherein the compound of formula (I) is single phase and has a pristine P2-layered strcture.
12. The cathode according to claim 11, which comprises a current collector made of an aluminum or copper foil covered by a film, said film comprising the positive electrode active material, a conductive additive and a binder, wherein the positive electrode active material comprises a compound of formula (I):
Na.sub.xMn.sub.1-y-zM.sub.yM.sub.zO.sub.2(I) or a hydrated form thereof, wherein: M is one or more elements selected from Ti, V, Cr, Zr, and Al; wherein Ti is selected from Ti.sup.3+ and Ti.sup.4+; V is selected from V.sup.2+, V.sup.3+, V.sup.4+, V.sup.5+; Cr is selected from Cr.sup.2+, Cr.sup.3+ and Cr.sup.6+; Zr is Zr.sup.4+; and Al is Al.sup.3+; M is one or more elements selected from Fe, Ni and Zn; wherein Fe is selected from Fe.sup.2+ and Fe.sup.3+; Ni is Ni.sup.3+; and Zn is Zn.sup.2+; the oxidation state of Mn ranges from 3.sup.+ to 4.sup.+; 0.6<x1.0; 0<y0.4; 0.05z<0.2; provided that: 0.05<(y+z)0.4; and wherein the compound of formula (I) is single phase and has a pristine P2-layered structure.
13. A rechargeable sodium-ion secondary battery comprising the cathode as defined in claim 11.
14. A method of storing energy, the method comprising: providing an energy storage device, wherein: the energy storage device comprising a rechargeable sodium-ion secondary battery comprising a cathode, said cathode comprising a current collector made of an aluminum or copper foil covered by a film, said film comprising the positive electrode active material, a conductive additive and a binder, wherein the positive electrode active material comprises a compound of formula (I):
Na.sub.xMn.sub.1-y-zM.sub.yM.sub.zO.sub.2(I) or a hydrated form thereof, wherein: M is one or more elements selected from Ti, V, Cr, Zr, and Al; wherein Ti is selected from Ti.sup.3+ and Ti.sup.4+; V is selected from V.sup.2+, V.sup.3+, V.sup.4+, V.sup.5+; Cr is selected from Cr.sup.2+, Cr.sup.3+ and Cr.sup.6+; Zr is Zr.sup.4+; and Al is Al.sup.3+; M is one or more elements selected from Fe, Ni and Zn; wherein Fe is selected from Fe.sup.2+ and Fe.sup.3+; Ni is Ni.sup.3+; and Zn is Zn.sub.2+; the oxidation state of Mn ranges from 3.sup.+ to 4.sup.+; 0.6<x1.0; 0<y0.4; 0.05z<0.2; provided that: 0.05<(y+z)0.4; and wherein the compound of formula (I) is single phase and has a pristine P2-layered; and storing in the energy storage device.
Description
BRIEF DESCRIPTION OF THE FIGURES
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DETAILED DESCRIPTION OF THE INVENTION
(12) As stated above, the first aspect of the present invention refers to a positive electrode active material for a sodium-ion secondary battery, said positive electrode active material comprising a compound of formula (I):
Na.sub.xMn.sub.1-y-zM.sub.yM.sub.zO.sub.2(I)
or a hydrated form thereof,
wherein: M is one or more elements selected from Ti, V, Cr, Zr, Al and Cu; wherein Ti is selected from Ti.sup.3+ and Ti.sup.4+; V is selected from V.sup.2+, V.sup.3+, V.sup.4+, V.sup.5+; Cr is selected from Cr.sup.2+, Cr.sup.3+ and Cr.sup.6+; Zr is Zr.sup.4+, Al is Al.sup.3+ and Cu is Cu.sup.2+. M is one or more elements selected from Fe, Ni and Zn; wherein Fe is selected from Fe.sup.2+ and Fe.sup.3+; Ni is Ni.sup.3+ and Zn is Zn.sup.2+; the oxidation state of Mn ranges from 3.sup.+ to 4.sup.+; 0.6<x1.0; 0<y0.4; 0.05z<0.2;
provided that: 0.05<(y+z)0.4.
(13) The compound of formula (I) comprised in the positive electrode active material of the present invention present a crystal structure which belongs to the so-called bronze phases. There are different crystal phases for sodium transition metal oxides depending on exact sodium to metal stoichiometry and preparation conditions.
(14) In a particular embodiment, the compound of formula (I) presents a layer bronze phase selected from P2, P3 and O.sub.3.
(15) The P2, P3, O3 nomenclature is a simple way to classify layered bronze phases. The letter stands for the sodium environment (P for prismatic, O for octahedral) and the number stands for the number of metal layers within the prismatic or octahedral unit cell. Another way to describe the P2 and O3 phases is to refer to the oxygen positions within a hexagonal stacking. The oxygen stacking in P2 is ABBAAB with metal located on c and sodium partially located on prismatic sites x=(a,b) and y=(b,c), resulting in a stacking sequence (AcB x BcA y AcB . . . ). In the O3-phase the stacking is ABCABC . . . .
(16) In a preferred embodiment of the present invention, the compound of formula (I) is single phase and has a P2 or O3 crystal structure, also known as pristine P2- or O3 layered crystal structure.
(17) In a particular embodiment, in the compound of formula (I) the sum of indexes (y+z) is defined as 0.05<(y+z)0.2.
(18) In a particular embodiment, in the compound of formula (I), the index y is defined as 0.05y0.2. Preferably the index y is defined as 0.05y0.1, more preferably the index y is 0.1 or 0.05, even more preferably is 0.1.
(19) In another particular embodiment, in the compound of formula (I), the index z is defined as 0.05z0.1, provided that 0.05<(y+z)0.4. Preferably the index z is 0.1 or 0.05, even more preferably is 0.1.
(20) In another particular embodiment, the index y is defined as 0.05y0.1 and the index z is defined as 0.05y0.1. In another preferred embodiment y is 0.1 and z is 0.1 or y is 0.05 and z is 0.05.
(21) In another particular embodiment, M is selected from Ti, Al and Cu, wherein Ti is selected from Ti.sup.3+ and Ti.sup.4+, Al is Al.sup.3+ and Cu is Cu.sup.2+. Preferably, M is Ti, wherein Ti is selected from Ti.sup.3+ and Ti.sub.4+.
(22) In another particular embodiment, M is selected from Fe and Zn, wherein Fe is selected from Fe.sup.2+ and Fe.sup.3+ and Zn is Zn.sup.2+. Preferably, M is Fe, wherein Fe is Fe.sup.2+ or Fe.sup.3+.
(23) In a preferred embodiment, M is Ti and M is Fe, wherein Ti is selected from Ti.sup.3+ and Ti.sup.4+ and Fe is selected from Fe.sup.2+ and Fe.sup.3+.
(24) In another particular embodiment, the compound of formula (I) is selected from Na.sub.2/3Mn.sub.0.8Ti.sub.0.1Fe.sub.0.1O.sub.2, Na.sub.2/3Mn.sub.0.8Cu.sub.0.1Fe.sub.0.1O.sub.2; Na.sub.2/3Mn.sub.0.8Al.sub.0.1Fe.sub.0.1O.sub.2; Na.sub.2/3Mn.sub.0.8Ti.sub.0.1Zn.sub.0.1O.sub.2; Na.sub.2/3Mn.sub.0.8Cu.sub.0.1Zn.sub.0.1O.sub.2; Na.sub.2/3Mn.sub.0.8Al.sub.0.1Zn.sub.0.1O.sub.2 and Na.sub.2/3Mn.sub.0.9Ti.sub.0.05Fe.sub.0.05O.sub.2, preferably is Na.sub.2/3Mn.sub.0.8Ti.sub.0.1Fe.sub.0.1O.sub.2 or Na.sub.2/3Mn.sub.0.9Ti.sub.0.05Fe.sub.0.05O.sub.2, even more preferably is Na.sub.2/3Mn.sub.0.8Ti.sub.0.1Fe.sub.0.1O.sub.2.
(25) In a particular embodiment, the compound of formula (I) is in a hydrated form. By the term hydrated form should be understood the compound of formula (I) having H.sub.2O molecules and/or H.sup.+ co-intercalated between the layers of its crystal structure. For example, when the compound of formula (I) has a pristine P2-layer crystal structure, the hydrated form corresponds to the compound of formula (I) having H.sub.2O molecules and/or H.sup.+ inserted into the spacing between the layers of the pristine crystal structure.
(26) In a particular embodiment, the hydrated form of the compound of formula (I) can be represented as follows:
Na.sub.x(H.sub.2O).sub.aH.sub.bMn.sub.1-y-zM.sub.yM.sub.zO.sub.2
wherein: M, M, x, y and z are as defined above, 0a0.1 and 0b0.2
and wherein H.sub.2O and H.sup.+ are co-intercalated between the layers of the crystal structure of the compound of formula (I).
(27) The compound of formula (I) may be prepared by many different processes known for those skilled in the art. For example, the process may be a simple solid state reaction using a sodium precursor (typically Na.sub.2CO.sub.3), a manganese precursor (Mn.sub.2O.sub.3, MnO.sub.2, MnCO.sub.3, MnOOH, etc.) and precursors of transition metals Ti, V, Cr, Fe, Ni, Zr, Al, Cu and Zn (transition metal oxides such as Fe.sub.2O.sub.3, FeO, TiO.sub.2, ZrO.sub.2, ZnO, CuO, Al.sub.2O.sub.3, etc.). The corresponding amounts of the Na, Mn and transition metals precursors are mixed and then heated in a tube furnace or a chamber furnace using either a flowing inert atmosphere (e.g., argon or nitrogen) or an ambient air atmosphere until reaction product is formed. The sintering temperature should be high enough to allow a complete reaction and formation of crystals, but not too high to avoid excessive sintering. In a preferred embodiment, the sintering temperature ranges from 400 to 1200 C., more preferably from 700 to 1000 C.
(28) In a particular embodiment, the mixture of the different precursors is subjected to a freeze step as a previous step of sintering. In this particular case, the mixture can be frozen by directly pouring liquid nitrogen into the mixing container and then dried in a freeze drier prior to be subjected to the sintering step.
(29) For the synthesis of the hydrated form of the compound of formula (I), this compound in its powder form can be simply dispersed in water, filtered and washed with water.
(30) In a particular embodiment, the compound of formula (I) delivered a second charge/discharge capacity of 146.57/144.16 mAh/g within the voltage range 4.0-2.0 V at C/10 which implies the utilization of approximately 0.60 Na considering full capacity of 245.19 mAh/g. The average voltage and energy density of the compound of formula (I) are calculated to be 2.77 V and 399.32 mWh/g vs. Na, respectively.
(31) In another particular embodiment, the hydrated compound of formula (I) delivered a second charge/discharge capacity of 130.40/129.22 mAh/g, which slightly increases to reach the maximum over 134 mAh/g. Slight increase in reversible capacity during the first cycles is probably due to redistribution and stabilization of Na.sup.+ and H.sup.+ in the interlayer spacing.
(32) In another particular embodiment, the compound of formula (I) exhibits an excellent capacity retention by maintaining 95.09% of discharge capacity at 50.sup.th cycle comparing to second cycle (only 0.1% capacity degradation on each cycle) along with over 98% Coulombic efficiency which points out the improved electrochemical properties of the active material of the invention.
(33) In another particular embodiment, the hydrated compound of formula (I) exhibits even better performance by retaining over 99% of reversible capacity after initial increase in specific capacity.
(34) In another particular embodiment, the compound of formula (I) exhibits a remarkable rate capability by maintaining 99.40 mAh/g discharge capacity at 1C which is 68.95% of C/10, and such high rate capability indicates a fast de/intercalation kinetic of pristine electrode.
(35) In a further embodiment, the compound of formula (I) exhibits an exceptional stability by maintaining 87.17 mAh/g at 300.sup.th cycle, which means 87.70% capacity retention from the second cycle (0.041% capacity degradation on each cycle). Furthermore, Coulombic efficiency is near 100% for all cycles, while the polarization (18.93-8.18 mV) is almost identical to that of C/10.
(36) The electrochemical process is based on Mn.sup.3+/Mn.sup.4+ redox process because the transition metals represented by M are inactive at the tested voltage range. During the discharge process, the Mn should be reduced to 3+ oxidation state, whereas the concentration of Mn.sup.4+ increases during the charge process.
(37) Comparing to compounds having larger substitutions of various transition metals into Mn containing layered oxides which have demonstrated either to attain structural stability at an expense of specific capacity or to have a high specific capacity at an expense of structural stability, the active material used in the present invention attains vastly improved structural stability while retaining high specific capacity at low level of substitution, which illustrates the possibility of full utilization of maximum energy density with long term stability for layered oxide Na-ion battery cathode.
(38) Thus, the positive electrode active material of the present invention can be applied to a cathode of a Na-ion battery. Thus, the invention also refers to a cathode for a Na-ion battery comprising the positive electrode active material as defined above.
(39) A typical cathode comprises a current collector which can be made of an aluminum or copper foil covered by a film containing the positive electrode active material. In a particular embodiment, said film also comprises a conductive additive and a binder.
(40) Examples of conductive additives include carbon black or acetylene black. The addition of a conductive additive provides an excellent charge and discharge cycle characteristics and a high energy density can be obtained.
(41) The content of the conductive additive in the cathode is preferably not higher than 40 mass % and not lower to 5 mass % of cathode in order to achieve the required charge and discharge cycle characteristics and a high energy density.
(42) Additionally, the binder is added to securely adhere the positive electrode active material contained in the cathode and the conductive additive to each other. Without particularly limited, polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), polyethylene-propylene-diene where the diene content is 10% or lower known as EPDM are preferred.
(43) The content of binder in the cathode is preferably not higher than 40 mass % and not lower than 1 mass % in order to achieve a good adhesion between the active material and the conductive additive.
(44) The cathode can be prepared by solvent-casting a slurry comprising the active material, the conductive additive, the binder and a solvent. Examples of solvents include an organic solvent, such as acetone or N-methyl-2-pyrrolidone (NMP). The slurry is then casted onto the aluminum or cooper foil and a free-standing electrode film is formed as the solvent evaporates. The electrode is then dried further at about 80-100 C. Alternatively, the slurry containing the active material, the conductive additive, the binder and the solvent can be casted onto a glass and, subsequently the current collector be applied.
(45) Another aspect of the present invention refers to a sodium-ion battery comprising a cathode as defined above.
(46) In a particular embodiment, said sodium-ion battery further comprises a negative electrode (anode) and an electrolyte. Thus, in this particular embodiment, the sodium-ion battery comprises a cathode comprising the positive electrode active material of the invention as defined above, an anode and an electrolyte.
(47) In a particular embodiment, the electrolyte is a solid sodium ion-conductive ceramic electrolyte separating both positive electrode (cathode) and negative electrode (anode).
(48) Suitable materials for the solid electrolyte may include -alumina, -alumina and -alumina. Other exemplary electrolyte materials include silicophosphates, such as NASICON (Na.sub.3-xZr.sub.2Si.sub.2+xP.sub.1+xO.sub.12) wherein 1x2, and glassy ceramics, such as alkali-silicon sulfide glasses.
(49) In a preferred embodiment, the solid sodium-ion conductive ceramic electrolyte includes beta-alumina or NASICON, more preferably, the solid electrolyte includes beta-alumina.
(50) The solid sodium-ion conductive ceramic electrolyte can be sized and shaped to have a cross-sectional profile that is square, polygonal, circular, or clover leaf, to provide a maximum surface area for sodium ion transport, and can have a width to length ratio that is greater than about 1:10 along a vertical axis.
(51) The solid electrolyte may be stabilized by the addition of small amounts of a dopant. The dopant may include one or more oxides selected from lithia, magnesia, zirconia, zinc oxide, and yttria.
(52) These stabilizers may be used alone or in combination or even combined with other materials. In a particular embodiment, the solid electrolyte comprises beta-alumina and may include one or more dopants.
(53) In another particular embodiment, the anode of the battery comprises, as the main component, solid metallic sodium or an alloy comprising solid metallic sodium. The content of sodium in the anode is higher than 50 mass %.
(54) The anode may further comprise additives which include an oxygen scavenger. Suitable metal oxygen scavengers include one or more of manganese, vanadium, zirconium, aluminum or titanium.
(55) In a particular embodiment, the cathode, the anode and the electrolyte are incorporated in a battery case. This battery case includes electrode terminals which are extended from the inside to the outside of the battery case. The negative electrode current collector is in electrical communication with the anode, and the positive electrode current collector is in electrical communication with the cathode. Suitable materials for the anode current collector may include aluminium, tungsten, titanium, nickel, copper, molybdenum, and combinations of two or more thereof. Other suitable materials for the anode current collector may include carbon. The positive electrode current collector may be a wire, paddle or mesh formed from aluminium, palladium, gold, nickel, copper, carbon or titanium. The current collector may be plated or clad.
(56) As will be appreciated by those skilled in the art, the sodium-ion battery of the present invention may be implemented in a variety of configurations and designs. For example, it can take a planar configuration or a tubular configuration in which the different components are contained inside a tubular assembly or housing to form the battery.
(57) The sodium-ion battery of the invention can be used as an energy storage device, being rechargeable over a plurality of charge-discharge cycles. The energy storage device can be employed in a variety of applications, and the plurality of cycles for recharge is dependent on factors such as, charge and discharge current, depth of discharge, cell voltage limits, and the like.
EXAMPLES
Example 1
Synthesis of Pristine P2-phase Powder Materials
(58) The following pristine P2-phase powder materials were synthesized: Na.sub.2/3Mn.sub.0.8Fe.sub.0.1Ti.sub.0.1O.sub.2, and its hydrated form. Na.sub.2/3Mn.sub.0.8Zn.sub.0.1Al.sub.0.1O.sub.2 Na.sub.2/3Mn.sub.0.8Zn.sub.0.1Cu.sub.0.1O.sub.2 Na.sub.2/3Mn.sub.0.8Zn.sub.0.1Ti.sub.0.1O.sub.2 Na.sub.2/3Mn.sub.0.8Fe.sub.0.1Cu.sub.0.1O.sub.2 Na.sub.2/3Mn.sub.0.8Fe.sub.0.1Al.sub.0.1O.sub.2 Na.sub.2/3Mn.sub.0.9Fe.sub.0.05Ti.sub.0.05O.sub.2.
(59) The different materials were synthesized by ceramic method. Thus, stoichiometric amounts of the reagents (Na.sub.2CO.sub.3, Mn.sub.2O.sub.3, Fe.sub.2O.sub.3, TiO.sub.2, ZnO, CuO, Al.sub.2O.sub.3) in powdered form were physically mixed in a speed mixer in aqueous medium for 20 minutes at 1000 rpm. The mixture was quickly frozen by directly pouring liquid nitrogen into the mixing container and then dried in a freeze drier for 2 days. A pellet with a diameter of 20 mm and thickness of 3 mm was obtained and annealed at 1000 C. for 6 hours under normal atmospheric conditions and, upon free cooling, the pellet was quickly transferred to a glove box in order to avoid moisture contact.
(60) Then, the powders were immediately frozen using liquid nitrogen, and dried using a freeze drier for 1 day. Dried powders were transferred to an Ar-filled glove box.
(61) For the synthesis of the hydrated form of pristine P2-phase Na.sub.2/3Mn.sub.0.8Ti.sub.0.1Fe.sub.0.1O.sub.2 (moisture exposure experiment), 100 mg of pristine P2-phase Na.sub.2/3Mn.sub.0.8Ti.sub.0.1Fe.sub.0.1O.sub.2 powder was dispersed in 10 mL of distilled and deionized water for 24 hours. The powder was filtered and washed with 100 mL of distilled and deionized water. Then, the powder was immediately frozen using liquid nitrogen, and dried using a freeze drier for 1 day. Dried powder was transferred to an Ar-filled glove box.
(62) The different materials were characterized by X-ray diffraction. XRD pattern of powder samples was collected using Bruker D8 Advanced with Cu K radiation of =1.5418 within the 2 range of 15-80. All samples were mounted onto an atmosphere protective XRD sample holder with Kapton film cover.
(63) Pure materials were obtained in all the studied compositions.
(64) P2-phase layered oxide is characterized by oxide layer stacking of AABBAA where Na.sup.+ occupies trigonal prismatic sites and shares either entirely edge or entirely face with adjacent oxide layers as can be seen in
(65) The XRD pattern matching of the different pristine powder materials shows a good fit between Bragg positions and observed peaks without any noticeable impurity peaks, and all crystallized in the P6.sub.3/mmc space group being cell parameters in good agreement with the reported values for Na.sub.xMnO.sub.2, a=b=2.8603 c=11.153 .
(66)
(67) Cell parameters are listed in Table 1:
(68) TABLE-US-00001 TABLE 1 Cell parameters and oxidation state of manganese of the different materials synthesized. Cell parameters Oxidation Sample a = b () c () state of Mn Na.sub.2/3Mn.sub.0.8Fe.sub.0.1Ti.sub.0.1O.sub.2 2.9143(2) 11.184(1) Na.sub.2/3Mn.sub.0.8Zn.sub.0.1Al.sub.0.1O.sub.2 2.8772(5) 11.179(3) 3.55 Na.sub.2/3Mn.sub.0.8Zn.sub.0.1Cu.sub.0.1O.sub.2 2.9253(7) 11.241(3) 3.67 Na.sub.2/3Mn.sub.0.8Zn.sub.0.1Ti.sub.0.1O.sub.2 2.9055(3) 11.196(2) 3.42 Na.sub.2/3Mn.sub.0.8Fe.sub.0.1Cu.sub.0.1O.sub.2 2.8806(2) 11.174(1) 3.67 Na.sub.2/3Mn.sub.0.8Fe.sub.0.1Al.sub.0.1O.sub.2 2.8723(5) 11.197(2) 3.42 Na.sub.2/3Mn.sub.0.9Fe.sub.0.05Ti.sub.0.05O.sub.2 2.8934(5) 11.175(1) 3.32
(69) A deviation in a-parameter comparing unsubstituted compound is due to the introduction of larger ionic radii Ti.sup.4+, Cu.sup.2+ or Al.sup.3+, into the oxide layer. By unsubstituted compounds are understood those containing Na and Mn without any other metallic element partially substituting Mn.
(70) For the case of hydrated samples, moisture exposure is known to cause Na.sup.+/H.sup.+ exchange to form protonated phase, and further exposure to moisture promotes the formation of hydrated phase where H.sub.2O molecules occupy vacancy interlayer spacing. The XRD pattern of hydrated state, refined using hexagonal P6.sub.3/mmc symmetry by assuming both protonated and hydrated phase (2=25.3), resulted also a good fit with observed pattern without any impurities. Slight decrease in a-parameter and increase in c-parameter are characteristics of protonated phase, and even larger c-parameter is observed for hydrated phase because large water molecules occupy interlayer space.
(71)
Example 2
Preparation of Electrodes Comprising Pristine P2-phase Powder Materials as Active Material and Cell Assemblies Containing Said Electrodes
(72) Slurry electrodes were prepared by mixing the pristine P2-phase powder material prepared according to example 1, super carbon C.sub.65 and PVDF (polyvinylidene difluoride) in a mass ratio of 80:10:10 in NMP (N-methyl-2-pyrrolidone) followed by vigorous stirring for 2 hours. The slurry then was casted onto an aluminum current collector sheet using a mini coater. The laminate was vacuum-dried at 120 C. for 12 hours. Circular electrodes were punched out and pressed at 5 tons prior to battery assembly. Na half-cells were constructed with 1.0 M NaPF.sub.6 in EC (ethylene carbonate):PC (propylene carbonate):FEC (fluoroethylene carbonate) in 49:49:2 ratio and metallic Na as anode in a 2032 coin cell in an Ar-filled glove box.
Example 3
Electrochemical Tests for the Electrode Containing Na2/3Mn0.8Fe0.1Ti0.1O2 as Active Material
(73) The galvanostatic electrochemical testing was performed with BioLogic VMP3 within the voltage range of 4.0-2.0 V. All the C-rates are calculated based on full theoretical specific capacity of 245.19 mAh/g of 1 Na de-intercalation.
(74) Galvanostatic charge/discharge profile of pristine P2-phase Na.sub.2/3Mn.sub.0.8Ti.sub.0.1Fe.sub.0.1O.sub.2 from the second to 50.sup.th cycle is shown in
(75) The hydrated electrode delivered a second charge/discharge capacity of 130.40/129.22 mAh/g (
(76) The average voltage and energy density of pristine electrode were calculated to be 2.77 V and 399.32 mWh/g vs. Na, respectively, and this energy density approaches to that of LiMnO.sub.2.
(77) Within the given voltage range, the charge/discharge curves of both electrodes (non-hydrated and hydrated) are smooth comparing to unsubstituted counterpart, without any apparent plateau being indicative of improved structural stability. Furthermore, Fe is electrochemically inactive during de/intercalation process because Mn to Fe redox transition plateau does not appear. As can be seen in
(78) The hydrated electrode exhibited even better performance by retaining over 99% of reversible capacity after initial increase in specific capacity.
(79) The electrochemical process is solely based on Mn.sup.3+/Mn.sup.4+ redox process because Ti.sup.4+ is electrochemically inactive at the tested voltage range and Fe.sup.3+ plateau does not appear up to 4.0 V. During the discharge process, the Mn should be reduced to 3+ oxidation state where high spin Mn.sup.3+ induces strong Jahn-Teller distorsion, being this effect opposite during the charge process due to increasing concentration of high spin Mn.sub.4+. However, and compared to unsubstituted compounds, the partially substituted Na.sub.2/3Mn.sub.0.8Fe.sub.0.1Ti.sub.0.1O.sub.2 compound exhibits much enhanced electrochemical performances including smoothness of profile and excellent capacity retention.
(80) Furthermore, comparing to compounds having larger substitutions of various transition metals into Mn containing layered oxides which have demonstrated either to attain structural stability at an expense of specific capacity or to have a high specific capacity at an expense of structural stability, the active material used in the present invention attains vastly improved structural stability while retaining high specific capacity at low level of substitution, which illustrates the possibility of full utilization of maximum energy density with long term stability for layered oxide Na-ion battery cathode.
(81)
(82) As can also be seen in
(83) As can be seen in
(84)
Example 4
Electrochemical Tests for the Electrodes Containing Na2/3Mn0.8Zn0.1Al0.1O2; Na2/3Mn0.8Zn0.1Cu0.1O2; Na2/3Mn0.8Zn0.1Ti0.1O2; Na2/3Mn0.8Fe0.1Cu0.1O2; Na2/3Mn0.8Fe0.1Al0.1O2 or Na2/3 Mn0.9Fe0.05Ti0.05O2
(85) The galvanostatic electrochemical testing was performed with BioLogic VMP3 within the voltage range of 4.0-2.0 V. Two different protocols were used; the first one is the cycling stability test at 1C for 40 cycles and the second one the rate capability test (5 cycles at C/10, C/5, 1C, 5C, 10C and 50C).
(86) 4.1. Cycling Stability Test at 1C
(87) The electrochemical performance of the layered oxides at 1C after 100 cycles can be observed in
(88) A remarkable good stability at 1C was found for the four presented examples. In the case of ZnCu and ZnTi combinations; 76.53 mAh/g and 75.78 mAh/g charge capacity values were found respectively after 100 cycles at 1C; which represent 83.8% and 83.7% of the initial capacity at 1C (2.sup.nd cycle). It has been therefore proved that these electrochemically inactive elements act as junctions holding the layered structures and improving their electrochemical performance at high C-rates.
(89) In the other two cases (c) and (d), although the electrochemical performance of the materials was still good, a slight capacity drop in the first 25 cycles from 90.33 mAh/g and 88.95 mAh/g (second charge capacity) respectively to 78.86 mAh/g and 72.82 mAh/g could be found. These values remain almost constant until the 100.sup.th cycle.
(90) The different capacity values upon cycling (1.sup.st, 2.sup.nd, 25.sup.th and 100.sup.th cycle) can be found in table 2.
(91) TABLE-US-00002 TABLE 2 Electrochemical capacity values of some layered oxides in the 1.sup.st, 2.sup.nd, 25.sup.th and 100.sup.th cycle. 1.sup.st 2.sup.nd 25.sup.th 100.sup.th Compound charge/disch charge/disch charge/disch charge/disch Na.sub.2/3Mn.sub.0.8Zn.sub.0.1Cu.sub.0.1O.sub.2 59.49/89.53 91.24/89.26 90.30/87.86 76.53/75.47 Na.sub.2/3Mn.sub.0.8Zn.sub.0.1Ti.sub.0.1O.sub.2 50.71/89.69 90.43/88.90 84.35/83.50 75.78/75.56 Na.sub.2/3Mn.sub.0.8Fe.sub.0.1Cu.sub.0.1O.sub.2 54.48/91.02 90.33/88.68 78.86/78.23 69.45/69.79 Na.sub.2/3Mn.sub.0.8Fe.sub.0.1Al.sub.0.1O.sub.2 23.43/89.89 88.95/86.32 72.82/72.21 62.47/61.55
Rate Capability Test.
(92)
(93) A normal diminution of the capacity values when the current is increased can be observed in all the cases, but it has to be noticed that when the C/10 rate is recovered in the 30.sup.th cycle the capacity values are recovered as they were in the 5.sup.th cycle (see Table 4).
(94) The Coulombic efficiency in all the materials is very close to 100% even at very high currents such as 50C (although capacity values are reaching neglectful values at these high current values). The presence of higher oxidation state for Manganese in a) and c) materials (see Table 2) can be translated in a better Coulombic efficiency in the last 30.sup.th-40.sup.th cycles compared to the other two materials. Less Jahn-Teller effect due to the presence of more Mn.sup.4+ can increase the stability of the layer structure. It can be specifically noticed when the material after being pushed to very high currents can be somehow relaxed to C/10.
(95) TABLE-US-00003 TABLE 3 Electrochemical capacity values of some examples in the 1.sup.st, 2.sup.nd, 25.sup.th and 100.sup.th cycle. C/10- 2.sup.nd C/5- 6.sup.th 1 C- 11.sup.th 5 C- 16.sup.th Compound charge/disch charge/disch charge/disch charge/disch Na.sub.2/3Mn.sub.0.8Zn.sub.0.1Cu.sub.0.1O.sub.2 121.63/117.87 122.92/108.74 108.17/75.35 63.20/31.36 Na.sub.2/3Mn.sub.0.8Zn.sub.0.1Ti.sub.0.1O.sub.2 152.24/147.68 146.17/125.62 123.91/75.49 31.54/27.02 Na.sub.2/3Mn.sub.0.8Fe.sub.0.1Cu.sub.0.1O.sub.2 142.59/141.67 141.27/120.38 117.00/74.11 73.78/41.76 Na.sub.2/3Mn.sub.0.8Fe.sub.0.1Al.sub.0.1O.sub.2 123.25/117.44 114.51/100.42 93.39/58.91 56.59/30.53 10 C- 21.sup.th 50 C- 26.sup.th C/10- 31.sup.h Compound charge/disch charge/disch charge/disch Na.sub.2/3Mn.sub.0.8Zn.sub.0.1Cu.sub.0.1O.sub.2 25.42/11.43 0.24/0.02 44.29/116.88 Na.sub.2/3Mn.sub.0.8Zn.sub.0.1Ti.sub.0.1O.sub.2 27.02/10.11 0.01/0.01 32.07/135.98 Na.sub.2/3Mn.sub.0.8Fe.sub.0.1Cu.sub.0.1O.sub.2 43.77/28.97 6.97/0.01 50.59/127.43 Na.sub.2/3Mn.sub.0.8Fe.sub.0.1Al.sub.0.1O.sub.2 32.65/22.02 12.14/2.36 35.19/107.19
(96) One example that has to be mentioned is Na.sub.2/3Mn.sub.0.9Fe.sub.0.05Ti.sub.0.05O.sub.2 where the effect of a small amount of substituents 0.05 Fe and 0.05 Ti per f.u. can be noticed in the stability of the material when it is charged and discharged at 1C and even at higher C-rates.
(97)
(98) When a rate capability test was applied to the material (see