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  • C01G53/42—Complex oxides containing nickel and at least one other metal element containing alkali metals, e.g. LiNiO2
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  • C01G53/50—Complex oxides containing nickel and at least one other metal element containing alkali metals, e.g. LiNiO2 containing manganese of the type (MnO2)n-, e.g. Li(NixMn1-x)O2 or Li(MyNixMn1-x-y)O2
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  • H01M4/36—Selection of substances as active materials, active masses, active liquids
  • H01M4/48—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides
  • H01M4/50—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese
  • H01M4/505—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of manganese of mixed oxides or hydroxides containing manganese for inserting or intercalating light metals, e.g. LiMn2O4 or LiMn2OxFy
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  • H01M4/52—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron
  • H01M4/525—Selection of substances as active materials, active masses, active liquids of inorganic oxides or hydroxides of nickel, cobalt or iron of mixed oxides or hydroxides containing iron, cobalt or nickel for inserting or intercalating light metals, e.g. LiNiO2, LiCoO2 or LiCoOxFy
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  • H01M2004/026—Electrodes composed of, or comprising, active material characterised by the polarity
  • H01M2004/028—Positive electrodes
• • Y—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
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  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/10—Energy storage using batteries

Definitions

• the present invention relates to sodium-containing transition metal oxides with Mn-rich phase for its use as cathode materials for a rechargeable sodium battery and other energy storage devices.
• Li-ion battery (LIB)
• LIB Li-ion battery
• NAB Na-ion battery
• a-NaFe0 2 exhibits high, flat voltage profile during the cycling but undergoes an irreversible structural change above approximately 3.6 V which causes a rapid capacity degradation after only a few cycles [Yabuuchi, N. et al, J. Mater. Chem., A, 2014, 40, 16851-16855].
• sodium-ion secondary batteries comprising a positive electrode active material which includes sodium and lithium Na a LibM x 0 2 , wherein M is Mn, Fe, Co, Ni or a combination of two of them [US2007/0218361 ; US2010/0248040 and US2011/0200879].
• phosphates like NaMno.5Feo.5PO4, or fluorite-phosphates like NaVP0 4 F or Na 2 FeP0 4 F.
• fluorophosphates exhibit certain difficulties in their chemical preparation and have a low gravimetric capacity and their use in Na-ion batteries is quite limited.
• the authors of the present invention have developed a sodium-containing layer oxide with Mn-rich phase to be used as a cathode material for Na-ion batteries. This particular material has shown to be stable, even against water, and has a long term performance as well as a high rate capability.
• the material of the invention delivers at C/10 a specific capacity around 145 mAh g, and also exhibits stable cycling by maintaining more than 98% of discharge capacity for 50 cycles and more than 87% of discharge capacity for 300 cycles.
• a 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 x Mm_ y _ z M y M ' z 0 2 (I) or a hydrated form thereof,
• M is one or more elements selected from Ti, V, Cr, Zr, Al and Cu; wherein Ti is selected from Ti 3+ and Ti 4+ ; V is selected from V 2+ , V 3+ , V 4+ , V 5+ ; Cr is selected from Cr 2+ , Cr 3+ and Cr 6+ ; Zr is Zr 4+ , Al is Al 3+ and Cu is Cu 2+ .
• M ' is one or more elements selected from Fe, Ni and Zn; wherein Fe is selected from Fe 2+ and Fe 3+ ; Ni is Ni 3+ and Zn is Zn 4+ ;
• the oxidation state of Mn ranges from 3 + to 4 + ;
• the positive electrode active material of 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.
• a second aspect of the present invention refers to a cathode for a rechargeable sodium- ion secondary battery, said cathode comprises the positive electrode active material as defined above.
• An additional aspect of the invention relates to a rechargeable sodium-ion secondary battery comprising the cathode as defined above.
• Another aspect of the present inventions relates to the use of the rechargeable sodium-ion battery as defined above as energy storage device.
• FIGURES Figure 1 XRD pattern matching of pristine Na 2/3 Mno. 8 Tio.iFeo.i0 2 powder with hexagonal P6 3 /mmc symmetry. Inset is a structural illustration of typical P2-phase layered oxide.
• Figure 3 SEM micrographs of a) pristine P2-phase Na 2/3 Mno. 8 Tio.iFeo.i0 2 powder; b) moisture exposed powder.
• Figure 4. Charge/discharge profile of: (a) pristine P2-phase Na 2 /3Mno.8Tio.iFeo.i0 2 electrode; (b) hydrated electrode.
• Figure 8 (a) Charge/discharge profile of pristine P2-phase Na 2 /3Mno.8Tio.iFeo.i0 2 electrode; (b) Plot of capacity retentions and Coulombic efficiency as a function of cycle number within the voltage range 4.0-2.0 V at 1C for 300 cycles.
• Figure 9 Cycling stability performance of (a) Na 2/3 Mno.8Zno.iCuo.i0 2 ; (b) Na 2 /3Mn 0 .8Zno.iTi 0 .i0 2 ; (c) Na 2 /3Mno.8Feo.iCuo.i0 2 ; (d) Na 2 /3Mno.8Feo.iAlo.i0 2 , at 1C in the 2-4 V voltage range.
• Rate capability performance of (a) Na 2 /3Mno.8Zno.iCuo.i0 2 ; (b) Na 2 /3Mn 0 .8Zno.iTi 0 .i0 2 ; (c) Na 2 /3Mno.8Feo.iCuo.i0 2 ; (d) Na 2 /3Mn 0 .8Feo.iAl 0 .i0 2 , in the 2- 4 V voltage range.
• Figure 11 Electrochemical performance of Na 2 /3Mno.9Feo.o5Tio.o50 2 (a) at 1C and (b) under a rate capability test.
• 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):
• M is one or more elements selected from Ti, V, Cr, Zr, Al and Cu; wherein Ti is selected from Ti 3+ and Ti 4+ ; V is selected from V 2+ , V 3+ , V 4+ , V 5+ ; Cr is selected from Cr 2+ , Cr 3+ and Cr 6+ ; Zr is Zr 4+ , Al is Al 3+ and Cu is Cu 2+ .
• M ' is one or more elements selected from Fe, Ni and Zn; wherein Fe is selected from Fe 2+ and Fe 3+ ; Ni is Ni 3+ and Zn is Zn 4+ ;
• the oxidation state of Mn ranges from 3 + to 4 + ;
• 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.
• the compound of formula (I) presents a layer bronze phase selected from P2, P3 and 03.
• the P2, P3, 03 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 03 phases is to refer to the oxygen positions within a hexagonal stacking.
• the stacking is ABC ABC ...
• the compound of formula (I) is single phase and has a P2 or 03 crystal structure, also known as pristine P2-or 03 layered crystal structure.
• the sum of indexes (y + z) is defined as 0.05 ⁇ (y+z) ⁇ 0.2.
• the index "y” is defined as 0.05 ⁇ y ⁇ 0.2.
• the index "y” is defined as 0.05 ⁇ y ⁇ 0.1 , more preferably the index "y” is 0.1 or 0.05, even more preferably is 0.1.
• the index "z” is defined as 0.05 ⁇ z ⁇ 0.1, provided that 0.05 ⁇ (y + z) ⁇ 0.4.
• the index "z” is 0.1 or 0.05, even more preferably is 0.1.
• the index "y” is defined as 0.05 ⁇ y ⁇ 0.1 and the index "z” is defined as 0.05 ⁇ y ⁇ 0.1. In another preferred embodiment "y” is 0.1 and “z” is 0.1 or “y” is 0.05 and “z” is 0.05.
• M is selected from Ti, Al and Cu, wherein Ti is selected from Ti 3+ and Ti 4+ , Al is Al 3+ and Cu is Cu 2+ . Preferably, M is Ti, wherein Ti is selected from Ti 3+ and Ti 4+ .
• M' is selected from Fe and Zn, wherein Fe is selected from Fe and Fe and Zn is Zn .
• M' is Fe, wherein Fe is Fe or Fe .
• M is Ti and M' is Fe, wherein Ti is selected from Ti 3+ and Ti 4+ and Fe is selected from Fe 2+ and Fe 3+ .
• the compound of formula (I) is selected from Na 2 /3Mn 0 . 8 Ti 0 .iFeo.l02, Na 2 /3Mno.8Cuo.iFeo.i0 2 ; Na 2 /3Mno.8Alo.iFeo.i0 2 ;
• Na 2 /3Mno.8Tio.iZno.i02; Na 2 /3Mno.8Alo.iZno.i0 2 and Na2/3Mno.9Tio.o5Feo.o502 preferably is Na 2 /3Mno. 8 Tio.iFeo.i02 or NaMMno.9Tio.05Feo.05Ch, even more preferably is Na 2 /3Mno.8Tio.iFeo.i0 2 .
• the compound of formula (I) is in a hydrated form.
• hydrated form should be understood the compound of formula (I) having H 2 0 molecules and/or H + co-intercalated between the layers of its crystal structure.
• the hydrated form corresponds to the compound of formula (I) having H 2 0 molecules and/or H + inserted into the spacing between the layers of the pristine crystal structure.
• the hydrated form of the compound of formula (I) can be represented as fo Hows :
• M, M', x, y and z are as defined above, 0 ⁇ a ⁇ 0.1 and 0 ⁇ b ⁇ 0.2 and wherein H 2 0 and H + are co-intercalated between the layers of the crystal structure of the compound of formula (I).
• the compound of formula (I) may be prepared by many different processes known for those skilled in the art.
• the process may be a simple solid state reaction using a sodium precursor (typically Na 2 CC" 3 ), a manganese precursor (Mn 2 0 3 , Mn0 2 , MnCCh, MnOOH, etc.) and precursors of transition metals Ti, V, Cr, Fe, Ni, Zr, Al, Cu and Zn (transition metal oxides such as Fe 2 C" 3 , FeO, Ti0 2 , Zr0 2 , ZnO, CuO, Al 2 0 3 , etc.).
• a sodium precursor typically Na 2 CC" 3
• transition metal oxides such as Fe 2 C" 3 , FeO, Ti0 2 , Zr0 2 , ZnO, CuO, Al 2 0 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.
• a flowing inert atmosphere e.g., argon or nitrogen
• 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.
• the mixture of the different precursors is subjected to a freeze step as a previous step of sintering.
• 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.
• this compound in its powder form can be simply dispersed in water, filtered and washed with water.
• 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.
• 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 + and H + in the interlayer spacing.
• the compound of formula (I) exhibits an excellent capacity retention by maintaining 95.09% of discharge capacity at 50 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.
• the hydrated compound of formula (I) exhibits even better performance by retaining over 99% of reversible capacity after initial increase in specific capacity.
• 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.
• the compound of formula (I) exhibits an exceptional stability by maintaining 87.17 mAh/g at 300 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.
• the electrochemical process is based on Mn 3+ /Mn 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 4+ increases during the charge process.
• 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.
• the positive electrode active material of the present invention can be applied to a cathode of a Na-ion battery.
• the invention also refers to a cathode for a Na-ion battery comprising the positive electrode active material as defined above.
• 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.
• said film also comprises a conductive additive and a binder.
• 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.
• 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.
• the binder is added to securely adhere the positive electrode active material contained in the cathode and the conductive additive to each other.
• PTFE polytetrafluoroethylene
• PVDF polyvinylidene fluoride
• EPDM polyethylene-propylene-diene where the diene content is 10% or lower known as EPDM are preferred.
• 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.
• the cathode can be prepared by solvent-casting a slurry comprising the active material, the conductive additive, the binder and a solvent.
• solvents include an organic solvent, such as acetone or N-methyl-2-pyrrolidone (NMP).
• NMP N-methyl-2-pyrrolidone
• 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.
• 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.
• Another aspect of the present invention refers to a sodium-ion battery comprising a cathode as defined above.
• said sodium-ion battery further comprises a negative electrode (anode) and an electrolyte.
• the sodium- ion battery comprises a cathode comprising the positive electrode active material of the invention as defined above, an anode and an electrolyte.
• the electrolyte is a solid sodium ion-conductive ceramic electrolyte separating both positive electrode (cathode) and negative electrode (anode).
• Suitable materials for the solid electrolyte may include ⁇ -alumina, ⁇ ' -alumina and ⁇ "- alumina.
• Other exemplary electrolyte materials include silicophosphates, such as NASICON (Na3_ x Zr 2 Si2 +x Pi +x Oi2) wherein -l ⁇ x ⁇ 2, and glassy ceramics, such as alkali- silicon sulfide glasses.
• the solid sodium-ion conductive ceramic electrolyte includes beta"-alumina or NASICON, more preferably, the solid electrolyte includes beta"-alumina.
• 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.
• 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. These stabilizers may be used alone or in combination or even combined with other materials.
• the solid electrolyte comprises beta"-alumina and may include one or more dopants.
• 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 %.
• 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.
• 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
• 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.
• the sodium- ion battery of the present invention may be implemented in a variety of configurations and designs.
• 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.
• 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.
• the different materials were synthesized by ceramic method.
• stoichiometric amounts of the reagents Na 2 C03, Mn 2 03, Fe 2 03, Ti0 2 , ZnO, CuO, AI2O3
• 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.
• 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.
• P2-phase layered oxide is characterized by oxide layer stacking of AABBAA where Na + occupies trigonal prismatic sites and shares either entirely edge or entirely face with adjacent oxide layers as can be seen in figure 1.
• Figure 1 depicts the X-ray diffractogram of the Na 2/3 Mno. 8 Tio.iFeo.i0 2 sample
• figure 2 shows the X-ray diffractogram of (a) Na 2/3 Mno. 8 Zno.iAl 0 .i0 2 ; (b) Na2/3Mno. 8 Zno.iCuo.i02; (c) (d) Na2/3Mno. 8 Fe 0 .iCuo.i02; (e) Na2/3Mno.8Feo.iAlo.i02; and (f) samples.
• Cell parameters are listed in Table 1 :
• a deviation in a-parameter comparing unsubstituted compound is due to the introduction of larger ionic radii Ti 4+ , Cu 2+ or Al 3+ , into the oxide layer.
• unsubstituted compounds are understood those containing Na and Mn without any other metallic element partially substituting Mn.
• Figure 3 shows SEM micrographs of pristine P2-phase Na 2 / 3 Mno. 8 Feo.iTio.i0 2 and its hydrated form.
• Example 2 Preparation of electrodes comprising pristine P2-phase powder materials as active material and cell assemblies containing said electrodes
• Slurry electrodes were prepared by mixing the pristine P2-phase powder material prepared according to example 1, super carbon C 6 5 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.
• PVDF polyvinylidene difluoride
• Na half-cells were constructed with 1.0 M NaPF 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.
• EC ethylene carbonate
• PC propylene carbonate
• FEC fluoroethylene carbonate
• 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.
• Galvanostatic charge/discharge profile of pristine P2-phase Na 2 / 3 Mno.8Tio.iFeo.i0 2 from the second to 50 th cycle is shown in figure 4(a).
• the pristine electrode 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 hydrated electrode delivered a second charge/discharge capacity of 130.40/129.22 mAh/g (figure 4(b)), 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 + and H + in the interlayer spacing. It should be noted that fully sodiated phase could not be obtained during the discharge down to 2.0
• the average voltage and energy density of pristine electrode were calculated to be 2.77
• the charge/discharge curves of both electrodes are smooth comparing to unsubstituted counterpart, without any apparent plateau being indicative of improved structural stability.
• Fe is electrochemically inactive during de/intercalation process because Mn to Fe redox transition plateau does not appear.
• the pristine electrode exhibits an excellent capacity retention by maintaining 95.09% of discharge capacity at 50 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.
• the hydrated electrode exhibited even better performance by retaining over 99% of reversible capacity after initial increase in specific capacity.
• the electrochemical process is solely based on Mn 3+ /Mn 4+ redox process because Ti 4+ is electrochemically inactive at the tested voltage range and Fe 3+ plateau does not appear up to 4.0 V.
• the Mn should be reduced to 3+ oxidation state where high spin Mn 3+ induces strong Jahn-Teller distorsion, being this effect opposite during the charge process due to increasing concentration of high spin Mn 4+ .
• the partially substituted Na 2/3 Mno. 8 Feo.iTio.i0 2 compound exhibits much enhanced electrochemical performances including smoothness of profile and excellent capacity retention.
• 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.
• Figure 6(a) and 6(b) are cyclic voltammogram of pristine and hydrated phase at given voltage. No peaks, other than that at 2.51 V, relating to redox couple or phase change are observed for pristine electrode, which is in good agreement with smooth charge/discharge profile without plateau behavior. Cyclic voltammogram of hydrated phase is somewhat noisy during the first few cycles, but becomes smooth later on. This observation is reflected on the capacity retention plot where a slight increase in reversible capacity occurs during the first cycles. No peaks relating to redox couple or phase changes were observed.
• the pristine electrode 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.
• Such high rate capability is unique among the Na layered oxide considering the bulkiness of the particle, and further increase of rate capability could be possible by reducing the size of carbon coating.
• the hydrated state electrode exhibited slightly lower rate capability specifically at 5 C or higher rate, which is attributed to a reduced kinetic by interlay er proton and water molecule, acting as diffusion block.
• Figure 8 shows the cycling profile and capacity retention plot of pristine electrode at 1C for 300 cycles.
• the exceptional stability is achieved by maintaining 87.17 mAh/g at 300 th cycle, which means 87.70% capacity retention from the second cycle (0.041% capacity degradation on each cycle).
• Coulombic efficiency is near 100% for all cyles, while the polarization (18.93-8.18 mV) is almost identical to that of C/10.
• 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, IOC and 50C). 4.1. Cycling stability test at 1C
• Fig. 9 The electrochemical performance of the layered oxides at 1C after 100 cycles can be observed in Fig. 9 for (a) Na2/3Mno.8Zno.iCuo.i0 2 ; (b) Na 2 /3Mno.8Zno.iTio.i0 2 ; (c) Na2/3Mno.8Feo.iCuo.i02 and (d) Na 2 /3Mno.8Feo.iAlo.i0 2 .
• Figure 10 shows the electrochemical performance of the above mentioned materials at different C-rates up to 50C. 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 th cycle the capacity values are recovered as they were in the 5 th cycle (see Table 4) ⁇
• Fig 11(a) shows the extremely good capacity values of Na 2 /3Mno. 9 Feo.o5 io.o502 at a voltage range where Ti 4+ and Fe 4+ are electrochemically inactive.
• Capacity values of 63.86mAh/g and 115.45mAh/g in the 1 st charge /discharge cycle have been found, and they seem to be constant in the 25 th cycle (107.03/103.77mAh/g). After 100 cycles the 76.5% of the initial capacity is maintained.

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• 2016
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