WO2017073058A1 - High Energy Density Metal Ion Cathode - Google Patents

High Energy Density Metal Ion Cathode Download PDF

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WO2017073058A1
WO2017073058A1 PCT/JP2016/004711 JP2016004711W WO2017073058A1 WO 2017073058 A1 WO2017073058 A1 WO 2017073058A1 JP 2016004711 W JP2016004711 W JP 2016004711W WO 2017073058 A1 WO2017073058 A1 WO 2017073058A1
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oxygen
electrode
constituent
metal
cathode
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PCT/JP2016/004711
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French (fr)
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Emma Kendrick
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Sharp Kabushiki Kaisha
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Definitions

  • the present invention relates to a novel hybrid metal ion and oxygen battery, and to an electrode therefor.
  • Novel engineered electrodes of the invention comprise a metal ion intercalation material, an oxygen capturing material and an oxygen production material.
  • the invention also relates to the use of these electrodes, for example in rechargeable batteries and other energy storage devices.
  • Lithium-ion battery technology has received a lot of attention in recent years and provides the preferred portable battery for most electronic devices in use today.
  • lithium is not a cheap or an abundant metal to source and is considered too expensive for use in large scale applications.
  • sodium-ion battery technology is still in its relative infancy but is seen as advantageous; as sodium is much more abundant than lithium and some researchers predict this will provide a cheaper and more durable way to store energy into the future, particularly for large scale applications such as storing energy on the electrical grid. Nevertheless a lot of work has yet to be done before sodium-ion batteries are a commercial reality. Alternatively potassium ion batteries may be used.
  • Both lithium ion and sodium ion batteries are reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism.
  • anode negative electrode
  • cathode positive electrode
  • electrolyte material both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism.
  • Na + (or Li + ) ions de-intercalate from the cathode and insert into the anode.
  • charge balancing electrons pass from the cathode through the external circuit containing the charge and into the anode of the battery. During discharge the same process occurs but in the opposite direction.
  • Metal air batteries contain an alkali metal anode such as lithium or sodium and a gas diffusion electrode (cathode), which are separated by an electrolyte. Metal ions are oxidised by the air at the cathode to form an oxide or peroxide during the charging of the battery. Upon discharge the oxide or peroxide reduces back to the constituent gas (oxygen) and the metal. In all these systems the cells must be partially exposed to oxygen gas or air and the cell therefore forms an “open” system in which oxygen may transfer between the cell and its surrounding environment.
  • alkali metal anode such as lithium or sodium and a gas diffusion electrode (cathode)
  • cathode gas diffusion electrode
  • Gas diffusion electrodes are porous and have bifunctional actions.
  • Alkali-metal-air batteries must enable the reduction of the atmospheric oxygen to oxide or peroxide ions in the course of discharging (4M + O 2 ⁇ 2M 2 O),and the oxidation of the oxide or peroxide ions to oxygen in the course of charging (2M 2 O ⁇ 4M+O 2 ),.
  • These gas diffusion electrodes are typically composed of a finely divided carbon or electronically conductive medium and may contain an oxygen reduction or evolution catalyst.
  • US 2014/0295272 uses an oxygen capturing material within the cathode of a lithium air battery. This is a solid mixed metal oxide material which does not decompose within the voltage limits of the cell operation. The material is based upon a YMnO 3 type structure.
  • US 8658319B proposes an oxygen air battery which has an internal oxygen loop and is closed towards the air.
  • the cell contains a first electrode and a second electrode that includes a metal material M.
  • the cell further comprises an oxygen storage material (OSM), which may be adjacent to the first electrode, in intimate contact with the first electrode, or in an external storage device and able to communicate oxygen to or from the first electrode.
  • OSM oxygen storage material
  • JP5383954B1 also proposes the inclusion of an oxygen storage material in conjunction with a conducting polymer.
  • EP 2 580 802 B proposes a higher temperature liquid alkali metal anode air-battery to be operated at temperatures above room temperature. This also includes an air cathode comprised of an oxygen air catalyst.
  • high capacity materials can be obtained by preconditioning the materials electrochemically in an electrochemical cell, this produces oxygen gas and the oxygen is removed from the cell.
  • the oxygen is provided externally to the cell, either during its working, or during the cell construction.
  • An aspect of the present invention provides an electrode having a conductive matrix and a plurality of materials disposed in the conductive matrix, wherein the electrode comprises: a metal intercalation constituent that, upon charging of the electrode, de-intercalates metal ions; an oxygen-producing constituent that, upon initial charging of the electrode, produces electrochemically active oxygen; and an oxygen capture constituent for capturing and storing oxygen produced by the oxygen producing constituent.
  • Another aspect of the present invention provides a method of operating an electrochemical device, the electrochemical device comprising an anode, a cathode and an electrolyte, the cathode comprising a metal intercalation constituent, an oxygen-producing constituent and an oxygen capture constituent, the method comprising: in a formation charge of the electrochemical device, charging the cathode such that the metal intercalation constituent de-intercalates metal ions, the formation charging comprising charging the cathode to a voltage sufficient to initiate at least partial decomposition of the oxygen-producing constituent thereby to produce electrochemically active oxygen; discharging the electrochemical device such that at least some of the de-intercalated metal ions combine with oxygen produced by decomposition of the oxygen-producing constituent to form a metal oxide compound; and in a subsequent charge of the electrochemical device, charging the cathode such that the metal oxide compound disassociates to form metal ions and an oxygen species.
  • Figure 1 is depicts a schematic representation of a reversible metal-ion cell
  • Figure 2 is depicts a schematic representation of a composite cathode electrode of the invention
  • Figure 3 is depicts the first charge voltage curves for a sodium ion intercalation material NaNi 0.33 Mn 0.33 Ti 0.17 Mg 0.17 O 2
  • Figure 4 is depicts a schematic representation of the first charge and discharge voltage curves for a sodium cathode material as described in this one embodiment of the invention for a sodium ion battery.
  • An embodiment of this invention relates to a metal-ion and metal air cell which incorporates an electrode in accordance with an embodiment of the present invention and which may be repeatedly charged and discharged, to store energy upon charge and produce energy during the discharge.
  • the cell is comprised of an anode 5 made either of a metal (M), metal alloy (MX) or a metal intercalation, or a combination; a hybrid metal ion intercalation and air cathode 3 according to an embodiment the present invention, and an ionically conducting electrolyte medium 4 sandwiched between the anode and cathode.
  • the cathode electrode in accordance with an embodiment of the present invention is a highly engineered electrode which may be comprised of an intercalation material and an oxygen producing material, an oxygen storage material and an oxygen catalyst within an electronically conductive matrix.
  • An embodiment in accordance with an embodiment of this invention is a cathode electrode ( Figure 2) which is comprised of a composite electrode comprising a metal ion intercalation material (9) in an electronically conductive matrix.
  • An oxygen diffusion electrode is typically composed of a porous carbon conductive matrix and oxygen catalyst.
  • the conductive matrix is formed by a combination of a binder 12, for example a polymeric binder, and a conductive additive 7.
  • the electrode is additionally comprised of an oxygen capture/storage material (11), and optionally an oxygen catalyst (8).
  • the electrode further comprises an oxygen producing material - in the embodiment of figure 2 the metal ion intercalation material (9) also acts as the oxygen producing material but in principle the metal ion intercalation material and the oxygen producing material may be different materials.
  • the conductive additive, catalyst, metal intercalation material, oxygen producing material, oxygen capture material are coated onto a current collector (10) and adhered together and to the current collector using the binder (12).
  • the metal ion intercalation material (9) may alternatively act as the oxygen capture/storage material or as the catalyst.
  • the alkali metal ions are removed from the intercalation cathode to the anode, and are deposited within the anode; on further charge, to a second charging voltage greater than the first charging voltage, oxygen is produced at the cathode.
  • the oxygen is captured by the oxygen capture/storage material and is available for further electrochemical reactions.
  • the alkali metal Upon discharge of the cell, the alkali metal is shuttled back from the anode through the electrolyte and intercalates back into the intercalation material, in addition the metal ions combine with the oxygen stored in the oxygen capture/storage material in the cathode to form a metal oxide or peroxide.
  • intercalation materials do not produce oxygen if they are cycled in a cell between stable voltage limits, however upon overcharge some of these materials may decompose to produce oxygen.
  • Examples of materials in which over charging produces oxygen gas can be found within the literature. Typically this is detrimental to the performance of the cell and can lead to heat production and thermal runaway.
  • the inventor has realised that if the oxygen produced during the further charging after the initial charge is captured in an oxygen electrode, the redox of the oxygen can be utilised in the discharge capacity of an electrochemical energy storage device.
  • An embodiment of this invention includes a layered oxide material which may also have a dual purpose of being an intercalation material and an oxygen redox catalyst.
  • these materials include Na 7/6 Ni 1/4 Mn 7/12 O 2 , or Na 1 Li 1/6 Ni 1/4 Mn 4/12 TiO 2 .
  • non intercalation materials can be added into an electrode to produce the oxygen upon initial charge.
  • materials which produce oxygen upon high voltages are Li 2 MnO 3 , Li 2 RuO 3 , Li 2 ReO 3 , Li 2 IrO 3 , and Li 2 PtO 3, Na 2 MnO 3 , Na 2 SnO 3 Na 2 RuO 3 , Na 2 ReO 3 , Na 2 IrO 3 , and Na 2 PtO 3 and alkali metal oxides and peroxides such as Na 2 O, Na 2 O 2 , Li 2 O 2 and Li 2 O.
  • transition metal peroxides such as MgO 2 , NiO 2 , ZnO 2 , Ba/Sr/BaO 2 , Sodium perborate, may also be added into the cathode.
  • the disclosure also contains a secondary intercalation cathode constituent which reversibly intercalates and de-intercalates sodium or another alkali metal, and a constituent which produces oxygen upon formation charge.
  • a secondary intercalation cathode constituent which reversibly intercalates and de-intercalates sodium or another alkali metal, and a constituent which produces oxygen upon formation charge.
  • a single material may act as both constituents, or the two constituents may be provided by different materials.
  • the secondary intercalation cathode material is oxygen deficient after charging, since during the 1 st charge both sodium and oxygen may be extracted.
  • the electronically conductive matrix may be comprised of materials such as acetylene black, carbon fibres, carbon nano tubes or titanium carbide.
  • the oxygen capturing material may be comprised of a porous high surface area material such as any of the following, frame-work type materials such as zeolitic materials, metal organic framework materials (MOF’s) or clathrates.
  • Porous materials such as aerogel type substances with nanometer and micrometer porosities, porous carbons, porous carbon gels, carbon nanotubes or porous metals.
  • MOF’s may include HKUST-1 (copper benzene-1,3,5-tricarboxylate, Cu-BTC) and NU-125.
  • the cathode may include an oxygen redox catalyst.
  • the redox catalyst may be comprised of a spinel, perovskite or pyrochlore type structure, or materials such as LaNiO 3 , LaMnO 3 , MnO x , ⁇ -MnO 2 ⁇ -MnOOH (manganite), Fe 2 O 3 , Fe 3 O 4 , CuO, CoFe 2 O 4 and Co 3 O 4 , Pt and Au, MnO 2 , Ag, Co 3 O 4 , La 2 O 3 , LaNiO 3 , NiCo 2 O 4 and LaMnO 3 .
  • FIG. 1 depicts a schematic representation of a reversible metal-ion cell.
  • the Cathode (3) is coated onto a cathode current collector (2), usually aluminium or carbon coated aluminium.
  • An anode (5) is disposed on an anode current collector (6), the cathode is separated from the anode (5) by a separator (4), which is usually a polyethylene or polypropylene macroporous separator, and electrolyte.
  • the cathode current collector 2, the cathode 3, the electrolyte/separator 4, the anode 5 and the anode current collector 6 are contained in a cell container 1.
  • the cathode current collector 2 and anode current collector 5 are electrically connected to terminal (not shown) on the exterior of the cell container 1.
  • the electrolyte material(s) may be any conventional or known material(s) and may comprise either aqueous electrolyte(s) or non-aqueous electrolyte(s) or mixtures thereof.
  • the electrolyte medium may include at least one of an ionic liquid or a polymer electrolyte such as polyethylene oxide (PEO), a block or co polymer such as polyethylene oxide-co-propylene oxide) acrylate.
  • the polymer may be plasticised with a solvent such as propylene carbonate, dimethyl sulfoxide, ethylene glycol, triethylamine, DMF (dimethylformamide), DMSO (dimethyl sulphoxide), polyethoxide ether, poly ethylene succinate, aprotic organic solvents, solid electrolyte such as garnets, nasicons, lisicons, beta alumina.
  • a solvent such as propylene carbonate, dimethyl sulfoxide, ethylene glycol, triethylamine, DMF (dimethylformamide), DMSO (dimethyl sulphoxide), polyethoxide ether, poly ethylene succinate, aprotic organic solvents, solid electrolyte such as garnets, nasicons, lisicons, beta alumina.
  • Common solvents for sodium ion and lithium ion batteries include propylene carbonate, ethylene carbonate, diethylene carbonate, dimethyl carbon
  • electrolyte solvents advantageously contain an alkali metal conducting salt with a weakly bound cation such as perchlorate ClO 4 - , PF 6 - , triflate (CF 3 SO 3 ) - , bis(oxalato) borate (BC 4 O 8 - , BOB) or imide/TFSI (N(SO 2 CF 3 ) 2 ).
  • a weakly bound cation such as perchlorate ClO 4 - , PF 6 - , triflate (CF 3 SO 3 ) - , bis(oxalato) borate (BC 4 O 8 - , BOB) or imide/TFSI (N(SO 2 CF 3 ) 2 ).
  • Ionic liquid electrolytes may be comprised of one or more of the following salts 1-ethyl- 3-methylimidazolium bis(trifluoromethylsulfonyl)imide; 1-ethyl-3-methylimidazolium tetrafluoroborate; 1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide; 1-butyl-3-methylimidazolium tetrafluoroborate; 1-hexyl-3-methylimidazolium; bis(trifluoromethylsulfonyl)imide; 1-hexyl-3-methylimidazolium tetrafluoroborate; 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide; 1-butyl-2,3-dimethylimidazolium tetrafluoroborate; N-octylpyridinium tetrafluoroborate; N-
  • Ionic liquids included in electrolyte medium may comprise cations of the pyridine and pyrrollidinium group such as: methyl-1-propyl pyrrolidinium [MPPyrro] + , 1-methyl-1-butyl pyrrolidinium [MBPyrro] + , 1-methyl-1-propyl piperidinium [MPPip] + , 1-methyl-1-butyl piperidinium [MBPip] + , 1-methyl-1-octylpyrrolidinium [MOPyrro] + and 1-methyl-1- octylpiperidinium [MOPip] + .
  • the Anode (5) is coated onto an Anode current collector (6), the current collector is usually a copper current collector for lithium systems, but aluminium, carbon coated aluminium or copper may be used for other alkali metal systems.
  • the anode can be graphite for lithium ion systems, or hard carbon for sodium ion systems, and may include partially or wholly a small alkali metal content such as lithium or sodium metal.
  • the anode may also be composed of an alloy metal such those which contain Sn or Sb, Si or Ge.
  • This arrangement can be used as a single cell, as described here, or stacked.
  • the cell stack is placed into a cell container, either an aluminium can, or a laminated aluminium pouch and sealed.
  • the electrolyte may contain redox mediators to aid the oxygen reduction and oxidation reaction.
  • ethyl viologen (EtV 2+ /EtV + )
  • tetrathiafulvalene tetrathiafulvalene
  • ferrocene and its derivatives
  • Prussian Blue and Prussian white and other metal hexacyanoferrates Prussian Blue and Prussian white and other metal hexacyanoferrates
  • quinones and methyl viologen 2'-bipyridyl and 1,10-phenanthroline complexes of iron.
  • an embodiment of the present invention provides an energy storage device that utilises an electrode comprising the active materials described above, and particularly an energy storage device for use as one or more of the following: a sodium and/or lithium and/or potassium ion cell; a sodium and/or lithium and/or potassium metal cell; a non-aqueous electrolyte sodium and/or potassium ion; an aqueous electrolyte sodium and/or lithium and/or potassium ion cell.
  • Electrodes according to an embodiment of the present invention are suitable for use in many different applications, for example energy storage devices, rechargeable batteries, electrochemical devices and electrochromic devices.
  • the cell is rechargeable.
  • Figure 3 depicts the first charge voltage curves for a sodium ion intercalation material NaNi 0.33 Mn 0.33 Ti 0.17 Mg 0.17 O 2 .
  • the cathode in this example is made from the active material (the sodium ion intercalation material), carbon black and a PVDF binder.
  • This material has an active redox centre of Nickel Ni2+ ⁇ Ni4+. This active redox centre corresponds to 0.67 sodium ions being removed from the layered oxide structure.
  • Figure 3 shows that the initial charge is greater than 0.7 Na+ ⁇ Na, and the top plateaux at 4.1V corresponds to a second redox centre.
  • the 2 nd redox Upon discharge, the 2 nd redox does not appear to be very reversible and there is a larger difference in between the 1 st charge and 1 st discharge due to this poor reversibility, leading to a high first cycle loss with this material.
  • the differential capacity plot shows the reversible redox voltages, and the high voltage redox at 4.1V at charge does not appear upon discharge. Although the oxygen is being produced by this cathode material the reversibility is poor.
  • a composite electrode in accordance with an embodiment of the present invention provides an improved performance compared to prior art electrodes.
  • a composite electrode in accordance with an embodiment of the invention is made from a lithium intercalation material, binder, conductive additive, oxygen producing material, oxygen capturing material and an oxygen catalyst as described in Figure 2.
  • an electrochemical device such as a cell
  • the intercalation compound initially releases the alkali metal cations and undergoes oxidation, for example as in eq (2) below.
  • This initial release of alkali metal cations occurs as the cathode is charged to an initial voltage LiMO 2 ⁇ xLi + +xe - +Li 1-x MO 2 (2)
  • the lithium intercalation material Upon further charging in the initial charging cycle - that is, charging to a voltage greater than the initial voltage, the lithium intercalation material further de-intercalates lithium and decomposes in part to produce electrochemically active oxygen.
  • the decomposition upon further charge may be according to: Li x MO 2 - 4 ⁇ e- ⁇ Li x-4 ⁇ MO 2-2 ⁇ + 4 ⁇ Li + + ⁇ O 2 (3)
  • equation (3) indicates that oxygen molecules (O 2 ) are produced
  • charging the device may not result in the production of gaseous oxygen in the device.
  • the produced oxygen may react with the transition metals and the catalyst to form a non gaseous oxygen species such as a transition metal peroxide or superoxide.
  • a gaseous or non gaseous oxygen species the oxygen is trapped in the electrode and can be utilised in further discharging and charging of the electrode.
  • lithium ions Upon discharge, lithium ions reintercalate into the intercalation material which, as shown in equation (3) is now oxygen deficient, for example according to: xLi + +xe - + Li 1-x MO 2-2 ⁇ ⁇ LiMO 2-2 ⁇ (4) Also, lithium that was de-intercalated upon charging can also react with the oxygen that was trapped in the electrode during charging to form a lithium oxygen compound such as lithium oxide, for example according to Eq (5).
  • the process of formation charging may require only the first charge of the device, but in some case the formation may require multiple charge-discharge cycles - for example may take up to 10 charge-discharge cycles.
  • the second charge of the device will result in a mixture of the reactions of equations (2) and (3) and the reactions of equations (6) and (7), as will any further charges required in the formation process.
  • further charges of the device cause reactions to occur according to equations (6) and (7), and this is also true of any subsequent charge of the electrode - and the reverse reactions occur on discharge.
  • equations (2) to (7) do not show the only possible reactions upon charge and discharge.
  • the products upon discharge are not limited to oxides as shown in equation (5), but could be/include peroxides and/or superoxides.
  • the constituents of a cell having an electrode in accordance with an embodiment of the present invention are preferably provided in a sealed container. While the provision of the oxygen-capture material in the electrode will prevent most oxygen that is generated during the first charge from leaving the cell, providing the cell in a sealed container means that the cell forms a closed system so that all oxygen generated during the first charge is retained in the cell and is available to be utilised in subsequent discharging and charge of the cell.
  • a “closed system” is meant that no transfer of material, in particular of oxygen, occurs between the cell and its surrounding environment, although heat and/or work may be transferred.
  • Figure 4 is a representation of the first charge and discharge voltage curves for a sodium cathode as described in this one embodiment of the invention for a sodium ion battery.
  • the nominal active cathode material is NaNi 1/4 Mn 7/12 Na 3/24 Li 1/24 O 2 , this is mixed with a conductive carbon black, and an oxygen capturing material - in this example Zeolite A. In this case the intercalation material and the oxygen producing material act as the catalyst.
  • the redox active centre in the cathode material is nickel and the number of mole of sodium removal per formula unit corresponds to 0.5Na+ ⁇ Na as the nickel changes oxidation state from Ni 2 + to Ni 4 + .
  • the corresponding number of sodium removal in the first charge is 0.7Na per formula unit, significantly higher than corresponds to the nickel redox.
  • the oxygen is produced at high voltage, and the active material in this case is acting partly as a catalyst which improves the reversibility of this oxygen redox centre.
  • the oxygen redox plateaux is reversible and there is very little first cycle loss in capacity from this composite cathode material.
  • This example shows the improved reversibility of the oxygen redox with a catalyst and an oxygen capturing material.
  • the oxygen was produced in situ by the sodium intercalation material, and the intercalation capacity is observed in addition to the oxygen redox.
  • a cathode electrode is made by adding a lithium ion cathode material Li[Ni 0.2 Li 0.2 Mn 0.6 ]O 2 , to Timcal C65 carbon black, and ball milled for 20 minutes at 400rpm in a sealed zirconia container.
  • a lithium ion cathode material Li[Ni 0.2 Li 0.2 Mn 0.6 ]O 2 Li[Ni 0.2 Li 0.2 Mn 0.6 ]O 2
  • an oxygen capturing material Zeolite A and catalyst Pt and a binder PVDF was added.
  • the ratio of the electrode formulation is 73:10:5:2:10 (this ratio is by weight, as are all other electrode formulation ratios given below), active material, carbon black, zeolite, catalyst and binder.
  • the conductive carbon used in the slurry was Super P C65, manufactured by Timcal.
  • the binder used in the slurry was polyvinylidene fluoride (PVDF) ( Kynar HSV900), manufactured by Arkema.
  • the solvent used in the slurry was N-Methyl-2-pyrrolidone (NMP), Anhydrous, manufactured by Sigma.
  • a composite electrode slurry was prepared by weighing the active, catalysts, oxygen capturing agents and conductive materials into a container, to which a binder solution was then added, The composite cathode slurry is then mixed using a planetary centrifugal mixer (Thinky mixer) for 10 minutes at 1600rpm to obtain a homogeneous mix.
  • Thinky mixer planetary centrifugal mixer
  • each electrode film contained the following components, expressed in percent by weight: 73% active material, 10% C65 carbon, 5% Zeolite A (Aldrich), 2% Platinum on Carbon black (Aldrich) and 7% Kynar binder.
  • this ratio can be varied (e.g., by adjusting the amounts of the components in the slurry) to optimize the electrode properties such as, adhesion, resistivity and porosity and performance.
  • An electrode of a sodium layered oxide material of the formula Na 7/6 Mn 7/12 Ni 1/4 O 2 is made in a similar method to Example1.
  • the active material Na 7/6 Mn 7/12 Ni 1/4 O 2 is first mixed with carbon black C65 from Timcal and an oxygen capturing material Nu-125 a metal organic framework material (MOF).
  • the active material Na 7/6 Mn 7/12 Ni 1/4 O 2 also acts as a catalyst for the oxygen redox reaction.
  • Na 7/6 Mn 7/12 Ni 1/4 O 2 is mixed with a carbon black such as Timal C65, the mixture is then sonicated using an ultrasonic probe for 2 minutes before transferring to a planetary centrifugal mixer for mixing at 1100Rpm for 5 minutes.
  • the electrode was then coated onto a carbon coated aluminium foil current collector (Showa Denko), using the doctor blade method and dried as in Example 1.
  • the final ratio of the electrode coating was Na 7/6 Mn 7/12 Ni 1/4 O 2 active material, Carbon Black C65, Nu-125, PVDF 80:10:5:5.
  • the oxygen for the reversible electrochemical reaction is produced in the first charge of a cell by the breakdown of the intercalation cathode material.
  • the catalyst aids the reversible oxygen reduction reaction, and the oxygen capturing agent prevents the oxygen leaving the cell.
  • Active carbons, and carbon blacks can also be used as oxygen capturing agents and are typically used in fuel cell applications.
  • a lithium ion battery electrode is made with a lithium layered oxide material, NMC (lithium nickel manganese cobalt oxide), and an oxygen producing material Li 2 MnO 3 , to these materials an oxygen capturing material Zeolite NaX and a conductive additive (carbon black - C65) are added.
  • NMC lithium nickel manganese cobalt oxide
  • Li 2 MnO 3 oxygen producing material
  • NMC NMC-Li 2 MNO 3 :ZeoliteX:C65:PVDF 65:5:5:10:5.
  • a sodium ion battery electrode is material using a sodium ion layered oxide intercalation material, NaNi 1/2 Mn 1/3 Mg 1/6 Ti 1/6 O 2 ; an oxygen producing material, Na 2 SnO 3 ; an oxygen capturing material, activated carbon - Norit; a ruthenium metal catalyst; and a conductive additive titanium carbide.
  • the electrode was made the same way as example 3, with the final ratios in the composite electrode as NaNi 1/2 Mn 1/3 Mg 1/6 Ti 1/6 O 2 :Na 2 SnO 3 :Norit:Ru:TiC:PVDF 77:5:5:2:5:6.
  • Example of a typical cell testing method Electrochemical cells of the example electrodes can be tested in an electrochemical cell.
  • a typical electrochemical cell is described in Figure 1.
  • a glass fiber separator is interposed between the positive and negative electrodes forming the electrochemical test cell.
  • a suitable glass fibre separator is a Whatman grade GF/A separator.
  • a porous polypropylene or a porous polyethylene separator wetted by the electrolyte is interposed between the positive and negative electrodes forming the electrochemical test cell.
  • a suitable porous polypropylene separator is Celgard 2400. GFA was used in the cell manufacture.
  • the electrochemical cell is cycled using a constant current density charge and discharge between pre-set voltage limits as deemed appropriate for the material under test. In most cases a low constant current charge is performed at 5-10mA/g, the upper voltage limit is between 3.0V and 5.0 V depending upon the electrolyte and the material.
  • sodium ions are extracted from the cathode and migrate to the anode.
  • the reverse process occurs.
  • the alkali metal ions are removed from the intercalation material and in addition oxide ions are oxidised from the oxygen producing material, and then the resulting oxygen species, oxygen gas, peroxide or superoxide is captured in the electrode.
  • the catalyst aids the reversibility of the oxygen redox.
  • the oxygen species is reduced.
  • the alkali metal cations are reintercalated into the intercalation material.
  • a first aspect of the present invention provides an electrode having a conductive matrix and a plurality of materials disposed in the conductive matrix, wherein the electrode comprises: a metal intercalation constituent that, upon charging of the electrode, de-intercalates metal ions; an oxygen-producing constituent that, upon initial charging of the electrode, produces electrochemically active oxygen; and an oxygen capture constituent for capturing and storing oxygen produced by the oxygen producing constituent.
  • intercalation is the reversible inclusion or insertion of a molecule or ion into a compound.
  • an “intercalation material” is a material that exhibits intercalation and can reversibly act as a host to guest molecules or ions.
  • a “metal intercalation material” is an intercalation material that hosts a metal.
  • An electrode in accordance with an embodiment of the invention is able to generate and store (for future use) oxygen owing the presence of the oxygen-producing constituent (which is not itself oxygen but is a constituent that, upon charging generates oxygen for example by a decomposition or partial decomposition of the oxygen-producing constituent) thereby avoiding the need to provide an external source of oxygen.
  • the oxygen-producing constituent which is not itself oxygen but is a constituent that, upon charging generates oxygen for example by a decomposition or partial decomposition of the oxygen-producing constituent
  • the electrochemically active oxygen may be stored by a number of possible mechanisms, such as: - oxygen gas may be generated upon charging, and captured and stored by the oxygen capture constituent (for example in pores or in a material structure); - in some embodiments the electrode further comprises a catalyst, and oxygen gas generated upon charging may react with the catalyst to form an intermediate; - oxygen generated on charging may react with an electrode component, with the reaction product remaining in the electrode so that the oxygen may be recovered - for example oxygen generated on charging may attach itself to a transition metal compound present in the electrode, forming a transition metal peroxide or superoxide; - if the oxygen capture constituent is an oxygen deficient material, oxygen generated on charging may be captured by entering into the crystal lattice (perovskite oxygen capturing).
  • perovskite oxygen capturing perovskite oxygen capturing
  • This one embodiment of the invention utilises an oxygen by product formed electrochemically within a cell and an intercalation cathode to increase the energy density of a battery.
  • the electrode may further comprise an oxygen reduction/oxidation catalyst.
  • a single material may comprise the metal intercalation constituent and the oxygen reduction/oxidation catalyst.
  • a single material may comprise the metal intercalation constituent and the oxygen-producing constituent.
  • a single material may comprise the metal intercalation constituent and the oxygen-capturing (oxygen capture) constituent.
  • a transition metal oxide may act as a metal intercalation constituent, and may also be able to store oxygen as mentioned above.
  • the metal intercalation constituent may de-intercalate metal ions upon charging of the electrode to a first voltage and the oxygen producing constituent may produce oxygen molecules upon charging of the electrode to a second voltage higher than the first voltage.
  • the metal intercalation constituent may de-intercalate, upon charging of the electrode, alkali metal ions.
  • the metal intercalation constituent may be a layered-oxide material.
  • the conductive matrix may be permeable to oxygen.
  • the oxygen-producing constituent may comprise material that contains a transition metal.
  • a second aspect of the invention provides an electrochemical device comprising an anode, a cathode and an electrolyte, wherein the cathode is an electrode of the first aspect.
  • the electrolyte may be a liquid electrolyte.
  • the electrolyte may contain a redox mediator.
  • the electrochemical device may comprise one of: a sodium and/or potassium ion hybrid cell; a sodium and/or potassium metal hybrid cell; a non-aqueous electrolyte sodium and/or potassium ion cell; and an aqueous electrolyte sodium and/or lithium and/or potassium ion cell.
  • the electrochemical device may comprise a rechargeable battery.
  • a “cell” is the smallest unit possible, consisting essentially of an anode, a cathode and an electrolyte.
  • a “battery” may comprise a single cell, or it may comprise multiple cells.
  • the anode, the cathode and the electrolyte may be disposed in a sealed container.
  • a third aspect of the present invention provides a method of operating an electrochemical device, the electrochemical device comprising an anode, a cathode and an electrolyte, the cathode comprising a metal intercalation constituent, an oxygen-producing constituent and an oxygen capture constituent, the method comprising: in a formation charge of the electrochemical device, charging the cathode such that the metal intercalation constituent de-intercalates metal ions, the formation charging comprising charging the cathode to a voltage sufficient to initiate at least partial decomposition of the oxygen-producing constituent thereby to produce electrochemically active oxygen; discharging the electrochemical device such that at least some of the de-intercalated metal ions combine with oxygen produced by decomposition of the oxygen-producing constituent to form a metal oxide compound; and in a subsequent charge of the electrochemical device, charging the cathode such that the metal oxide compound disassociates to form metal ions and an oxygen species.
  • metal oxide compound as used in this statement of the third aspect is intended to include one or more of a metal oxide, metal peroxides and metal superoxide.
  • an embodiment of the invention comprises the features hereinafter fully described and identified in the claims.
  • the foregoing description and the annexed drawings set forth detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of an embodiment of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the foregoing detailed description of an embodiment of the invention when considered in conjunction with the drawings.
  • This disclosure incorporates an oxygen producing material with into a intercalation cathode, and the use of the electrode in an electrochemical device.
  • Electrodes according to an embodiment of the present invention are suitable for use in many different applications, energy storage devices, rechargeable cells and/or batteries, electrochemical devices and electrochromic devices.
  • the electrodes according to an embodiment of the invention are used in conjunction with a counter electrode and one or more electrolyte materials.
  • the electrolyte materials may be any conventional or known materials and may comprise of either aqueous electrolyte or non-aqueous electrolytes or a mixture therof.

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Abstract

An electrode has a conductive matrix and a plurality of materials disposed in the conductive matrix (7,12), such that the electrode comprise a metal intercalation constituent (9) that, upon charging of the electrode, de-intercalates metal ions; an oxygen-producing constituent that, upon initial charging of the electrode, produces electrochemically active oxygen; and an oxygen capture constituent (11)for capturing and storing the oxygen produced by the oxygen producing constituent.An electrode in accordance with an embodiment of the invention is able, upon charging, to generate and store oxygen. The oxygen is available to be utilised in subsequent discharging and charge of the cell.

Description

High Energy Density Metal Ion Cathode
The present invention relates to a novel hybrid metal ion and oxygen battery, and to an electrode therefor. Novel engineered electrodes of the invention comprise a metal ion intercalation material, an oxygen capturing material and an oxygen production material. The invention also relates to the use of these electrodes, for example in rechargeable batteries and other energy storage devices.
Lithium-ion battery technology has received a lot of attention in recent years and provides the preferred portable battery for most electronic devices in use today. However, lithium is not a cheap or an abundant metal to source and is considered too expensive for use in large scale applications. By contrast sodium-ion battery technology is still in its relative infancy but is seen as advantageous; as sodium is much more abundant than lithium and some researchers predict this will provide a cheaper and more durable way to store energy into the future, particularly for large scale applications such as storing energy on the electrical grid. Nevertheless a lot of work has yet to be done before sodium-ion batteries are a commercial reality. Alternatively potassium ion batteries may be used.
Both lithium ion and sodium ion batteries are reusable secondary batteries that comprise an anode (negative electrode), a cathode (positive electrode) and an electrolyte material, both are capable of storing energy, and they both charge and discharge via a similar reaction mechanism. When a sodium-ion (or lithium-ion battery) is charging, Na+ (or Li+) ions de-intercalate from the cathode and insert into the anode. Meanwhile charge balancing electrons pass from the cathode through the external circuit containing the charge and into the anode of the battery. During discharge the same process occurs but in the opposite direction.
The energy density of conventional lithium ion and sodium ion batteries which possess a carbon anode and a cathode based on metal oxides is limited. Recently there has been a resurgence in the investigation of metal anodes to improve the energy density and batteries such as in lithium-sulfur and lithium-air batteries.
Metal air batteries contain an alkali metal anode such as lithium or sodium and a gas diffusion electrode (cathode), which are separated by an electrolyte. Metal ions are oxidised by the air at the cathode to form an oxide or peroxide during the charging of the battery. Upon discharge the oxide or peroxide reduces back to the constituent gas (oxygen) and the metal. In all these systems the cells must be partially exposed to oxygen gas or air and the cell therefore forms an “open” system in which oxygen may transfer between the cell and its surrounding environment. For example the following reactions may occur at the cathode and anode respectively:
Cathode: Li+ + e- +O2 → LiO2; Li+ + e- +LiO2 →Li2O2
Anode: Li+ + e-→Li.
In order for the oxygen gas reactions to take place, engineered gas diffusion electrodes are required. Gas diffusion electrodes are porous and have bifunctional actions. Alkali-metal-air batteries must enable the reduction of the atmospheric oxygen to oxide or peroxide ions in the course of discharging (4M + O2 → 2M2O),and the oxidation of the oxide or peroxide ions to oxygen in the course of charging (2M2O →4M+O2),. These gas diffusion electrodes are typically composed of a finely divided carbon or electronically conductive medium and may contain an oxygen reduction or evolution catalyst.
Peled et al., J. Power Sources 196 (2011 ) 6835-6840, compare lithium-air batteries with sodium-air batteries. They propose to replace metallic lithium anode by liquid sodium and to operate the sodium-oxygen cell above the melting point of sodium (97.8 °C).
There are references to a non-aqueous metal-air battery system in which contain an oxygen storage material. US 2014/0295272 uses an oxygen capturing material within the cathode of a lithium air battery. This is a solid mixed metal oxide material which does not decompose within the voltage limits of the cell operation. The material is based upon a YMnO3 type structure. US 8658319B proposes an oxygen air battery which has an internal oxygen loop and is closed towards the air. The cell contains a first electrode and a second electrode that includes a metal material M. The cell further comprises an oxygen storage material (OSM), which may be adjacent to the first electrode, in intimate contact with the first electrode, or in an external storage device and able to communicate oxygen to or from the first electrode. During discharge of the cell the first electrode functions as a cathode and the second electrode functions as an anode. When the cell is charged, oxygen is released at the first electrode, and is stored in the OSM. JP5383954B1 also proposes the inclusion of an oxygen storage material in conjunction with a conducting polymer. EP 2 580 802 B proposes a higher temperature liquid alkali metal anode air-battery to be operated at temperatures above room temperature. This also includes an air cathode comprised of an oxygen air catalyst. In cases of the lithium excess materials xLiMO2.(1-x)Li2M′O3 (patent US 6677082 B2) high capacity materials can be obtained by preconditioning the materials electrochemically in an electrochemical cell, this produces oxygen gas and the oxygen is removed from the cell.
In all the above examples of an alkali metal air battery the oxygen is provided externally to the cell, either during its working, or during the cell construction.
An aspect of the present invention provides an electrode having a conductive matrix and a plurality of materials disposed in the conductive matrix, wherein the electrode comprises: a metal intercalation constituent that, upon charging of the electrode, de-intercalates metal ions; an oxygen-producing constituent that, upon initial charging of the electrode, produces electrochemically active oxygen; and an oxygen capture constituent for capturing and storing oxygen produced by the oxygen producing constituent.
Another aspect of the present invention provides a method of operating an electrochemical device, the electrochemical device comprising an anode, a cathode and an electrolyte, the cathode comprising a metal intercalation constituent, an oxygen-producing constituent and an oxygen capture constituent, the method comprising:
in a formation charge of the electrochemical device, charging the cathode such that the metal intercalation constituent de-intercalates metal ions, the formation charging comprising charging the cathode to a voltage sufficient to initiate at least partial decomposition of the oxygen-producing constituent thereby to produce electrochemically active oxygen;
discharging the electrochemical device such that at least some of the de-intercalated metal ions combine with oxygen produced by decomposition of the oxygen-producing constituent to form a metal oxide compound; and
in a subsequent charge of the electrochemical device, charging the cathode such that the metal oxide compound disassociates to form metal ions and an oxygen species.
Figure 1 is depicts a schematic representation of a reversible metal-ion cell Figure 2 is depicts a schematic representation of a composite cathode electrode of the invention Figure 3 is depicts the first charge voltage curves for a sodium ion intercalation material NaNi0.33Mn0.33Ti0.17Mg0.17O2 Figure 4 is depicts a schematic representation of the first charge and discharge voltage curves for a sodium cathode material as described in this one embodiment of the invention for a sodium ion battery.
An embodiment of this invention relates to a metal-ion and metal air cell which incorporates an electrode in accordance with an embodiment of the present invention and which may be repeatedly charged and discharged, to store energy upon charge and produce energy during the discharge. As described in Figure 1, the cell is comprised of an anode 5 made either of a metal (M), metal alloy (MX) or a metal intercalation, or a combination; a hybrid metal ion intercalation and air cathode 3 according to an embodiment the present invention, and an ionically conducting electrolyte medium 4 sandwiched between the anode and cathode.
The cathode electrode in accordance with an embodiment of the present invention is a highly engineered electrode which may be comprised of an intercalation material and an oxygen producing material, an oxygen storage material and an oxygen catalyst within an electronically conductive matrix.
An embodiment in accordance with an embodiment of this invention is a cathode electrode (Figure 2) which is comprised of a composite electrode comprising a metal ion intercalation material (9) in an electronically conductive matrix. An oxygen diffusion electrode is typically composed of a porous carbon conductive matrix and oxygen catalyst. In this example the conductive matrix is formed by a combination of a binder 12, for example a polymeric binder, and a conductive additive 7. The electrode is additionally comprised of an oxygen capture/storage material (11), and optionally an oxygen catalyst (8). The electrode further comprises an oxygen producing material - in the embodiment of figure 2 the metal ion intercalation material (9) also acts as the oxygen producing material but in principle the metal ion intercalation material and the oxygen producing material may be different materials. The conductive additive, catalyst, metal intercalation material, oxygen producing material, oxygen capture material are coated onto a current collector (10) and adhered together and to the current collector using the binder (12). (In other embodiments, the metal ion intercalation material (9) may alternatively act as the oxygen capture/storage material or as the catalyst.)
During initial charge of this cell to a first charging voltage the alkali metal ions are removed from the intercalation cathode to the anode, and are deposited within the anode; on further charge, to a second charging voltage greater than the first charging voltage, oxygen is produced at the cathode. The oxygen is captured by the oxygen capture/storage material and is available for further electrochemical reactions. Upon discharge of the cell, the alkali metal is shuttled back from the anode through the electrolyte and intercalates back into the intercalation material, in addition the metal ions combine with the oxygen stored in the oxygen capture/storage material in the cathode to form a metal oxide or peroxide.
Typically intercalation materials do not produce oxygen if they are cycled in a cell between stable voltage limits, however upon overcharge some of these materials may decompose to produce oxygen. Examples of materials in which over charging produces oxygen gas can be found within the literature. Typically this is detrimental to the performance of the cell and can lead to heat production and thermal runaway. However, the inventor has realised that if the oxygen produced during the further charging after the initial charge is captured in an oxygen electrode, the redox of the oxygen can be utilised in the discharge capacity of an electrochemical energy storage device. Examples of other oxygen producing materials are Li2MnO3.LiMO2 (M= combination of transition metals) Na1+xMO2.
An embodiment of this invention includes a layered oxide material which may also have a dual purpose of being an intercalation material and an oxygen redox catalyst. Examples of these materials include Na7/6Ni1/4Mn7/12O2, or Na1Li1/6Ni1/4Mn4/12TiO2.
In some cases non intercalation materials can be added into an electrode to produce the oxygen upon initial charge. Examples of materials which produce oxygen upon high voltages are Li2MnO3, Li2RuO3, Li2ReO3, Li2IrO3, and Li2PtO3, Na2MnO3, Na2SnO3 Na2RuO3, Na2ReO3, Na2IrO3, and Na2PtO3 and alkali metal oxides and peroxides such as Na2O, Na2O2, Li2O2 and Li2O. Additionally or alternatively transition metal peroxides, such as MgO2, NiO2, ZnO2, Ba/Sr/BaO2, Sodium perborate, may also be added into the cathode.
Viewed from another aspect, the disclosure also contains a secondary intercalation cathode constituent which reversibly intercalates and de-intercalates sodium or another alkali metal, and a constituent which produces oxygen upon formation charge. A single material may act as both constituents, or the two constituents may be provided by different materials.
In one embodiment the secondary intercalation cathode material is oxygen deficient after charging, since during the 1st charge both sodium and oxygen may be extracted.
The electronically conductive matrix may be comprised of materials such as acetylene black, carbon fibres, carbon nano tubes or titanium carbide.
The oxygen capturing material may be comprised of a porous high surface area material such as any of the following, frame-work type materials such as zeolitic materials, metal organic framework materials (MOF’s) or clathrates. Porous materials such as aerogel type substances with nanometer and micrometer porosities, porous carbons, porous carbon gels, carbon nanotubes or porous metals. MOF’s may include HKUST-1 (copper benzene-1,3,5-tricarboxylate, Cu-BTC) and NU-125.
The cathode may include an oxygen redox catalyst. The redox catalyst may be comprised of a spinel, perovskite or pyrochlore type structure, or materials such as LaNiO3, LaMnO3, MnOx,α-MnO2 γ-MnOOH (manganite), Fe2O3, Fe3O4, CuO, CoFe2O4 and Co3O4, Pt and Au, MnO2, Ag, Co3O4, La2O3, LaNiO3, NiCo2O4 and LaMnO3.
Figure 1 depicts a schematic representation of a reversible metal-ion cell. The Cathode (3) is coated onto a cathode current collector (2), usually aluminium or carbon coated aluminium. An anode (5) is disposed on an anode current collector (6), the cathode is separated from the anode (5) by a separator (4), which is usually a polyethylene or polypropylene macroporous separator, and electrolyte. The cathode current collector 2, the cathode 3, the electrolyte/separator 4, the anode 5 and the anode current collector 6 are contained in a cell container 1. The cathode current collector 2 and anode current collector 5 are electrically connected to terminal (not shown) on the exterior of the cell container 1.
The electrolyte material(s) may be any conventional or known material(s) and may comprise either aqueous electrolyte(s) or non-aqueous electrolyte(s) or mixtures thereof. The electrolyte medium may include at least one of an ionic liquid or a polymer electrolyte such as polyethylene oxide (PEO), a block or co polymer such as polyethylene oxide-co-propylene oxide) acrylate. In some embodiments the polymer may be plasticised with a solvent such as propylene carbonate, dimethyl sulfoxide, ethylene glycol, triethylamine, DMF (dimethylformamide), DMSO (dimethyl sulphoxide), polyethoxide ether, poly ethylene succinate, aprotic organic solvents, solid electrolyte such as garnets, nasicons, lisicons, beta alumina. Common solvents for sodium ion and lithium ion batteries include propylene carbonate, ethylene carbonate, diethylene carbonate, dimethyl carbonates. These electrolyte solvents advantageously contain an alkali metal conducting salt with a weakly bound cation such as perchlorate ClO4 -, PF6 -, triflate (CF3SO3)-, bis(oxalato) borate (BC4O8 -, BOB) or imide/TFSI (N(SO2CF3)2).
Ionic liquid electrolytes may be comprised of one or more of the following salts 1-ethyl-
3-methylimidazolium bis(trifluoromethylsulfonyl)imide; 1-ethyl-3-methylimidazolium tetrafluoroborate; 1-butyl-3-methylimidazoliumbis(trifluoromethylsulfonyl)imide; 1-butyl-3-methylimidazolium tetrafluoroborate; 1-hexyl-3-methylimidazolium;
bis(trifluoromethylsulfonyl)imide; 1-hexyl-3-methylimidazolium tetrafluoroborate; 1-butyl-2,3-dimethylimidazolium bis(trifluoromethylsulfonyl)imide; 1-butyl-2,3-dimethylimidazolium tetrafluoroborate; N-octylpyridinium tetrafluoroborate; N-butyl-4-methylpyridinium tetrafluoroborate; and N-butyl-4-methylpyridinium hexafluorophosphate. Ionic liquids included in electrolyte medium may comprise cations of the pyridine and pyrrollidinium group such as: methyl-1-propyl pyrrolidinium [MPPyrro]+, 1-methyl-1-butyl pyrrolidinium [MBPyrro]+, 1-methyl-1-propyl piperidinium [MPPip]+, 1-methyl-1-butyl piperidinium [MBPip]+, 1-methyl-1-octylpyrrolidinium [MOPyrro]+ and 1-methyl-1- octylpiperidinium [MOPip]+.
The Anode (5) is coated onto an Anode current collector (6), the current collector is usually a copper current collector for lithium systems, but aluminium, carbon coated aluminium or copper may be used for other alkali metal systems. The anode can be graphite for lithium ion systems, or hard carbon for sodium ion systems, and may include partially or wholly a small alkali metal content such as lithium or sodium metal. The anode may also be composed of an alloy metal such those which contain Sn or Sb, Si or Ge.
This arrangement can be used as a single cell, as described here, or stacked. The cell stack is placed into a cell container, either an aluminium can, or a laminated aluminium pouch and sealed.
In another embodiment the electrolyte may contain redox mediators to aid the oxygen reduction and oxidation reaction. ethyl viologen (EtV2+/EtV+), tetrathiafulvalene, ferrocene and its derivatives, Prussian Blue and Prussian white and other metal hexacyanoferrates, quinones and methyl viologen, 2'-bipyridyl and 1,10-phenanthroline complexes of iron.
In another embodiment, an embodiment of the present invention provides an energy storage device that utilises an electrode comprising the active materials described above, and particularly an energy storage device for use as one or more of the following: a sodium and/or lithium and/or potassium ion cell; a sodium and/or lithium and/or potassium metal cell; a non-aqueous electrolyte sodium and/or potassium ion; an aqueous electrolyte sodium and/or lithium and/or potassium ion cell.
Electrodes according to an embodiment of the present invention are suitable for use in many different applications, for example energy storage devices, rechargeable batteries, electrochemical devices and electrochromic devices. Typically, the cell is rechargeable.
Figure 3: depicts the first charge voltage curves for a sodium ion intercalation material NaNi0.33Mn0.33Ti0.17Mg0.17O2. The cathode in this example is made from the active material (the sodium ion intercalation material), carbon black and a PVDF binder. This material has an active redox centre of Nickel Ni2+→Ni4+. This active redox centre corresponds to 0.67 sodium ions being removed from the layered oxide structure. Figure 3 shows that the initial charge is greater than 0.7 Na+→Na, and the top plateaux at 4.1V corresponds to a second redox centre. Upon discharge, the 2nd redox does not appear to be very reversible and there is a larger difference in between the 1st charge and 1st discharge due to this poor reversibility, leading to a high first cycle loss with this material. The differential capacity plot shows the reversible redox voltages, and the high voltage redox at 4.1V at charge does not appear upon discharge. Although the oxygen is being produced by this cathode material the reversibility is poor.
An electrode in accordance with an embodiment of the present invention provides an improved performance compared to prior art electrodes. In one example, a composite electrode in accordance with an embodiment of the invention is made from a lithium intercalation material, binder, conductive additive, oxygen producing material, oxygen capturing material and an oxygen catalyst as described in Figure 2. When an electrochemical device, such as a cell, that incorporates the electrode is charged for the first time (this may be considered a “formation charge” of the device, for reasons that will become clear below), the intercalation compound initially releases the alkali metal cations and undergoes oxidation, for example as in eq (2) below. This initial release of alkali metal cations occurs as the cathode is charged to an initial voltage
LiMO2 →xLi++xe-+Li1-xMO2 (2)
Upon further charging in the initial charging cycle - that is, charging to a voltage greater than the initial voltage, the lithium intercalation material further de-intercalates lithium and decomposes in part to produce electrochemically active oxygen. In a case where the initial charge occurs as given in equation (2), the decomposition upon further charge may be according to:
LixMO2 - 4δe-→Lix-4δMO2-2δ + 4δLi+ + δO2 (3)
It should be noted that, while equation (3) indicates that oxygen molecules (O2) are produced, charging the device may not result in the production of gaseous oxygen in the device. The produced oxygen may react with the transition metals and the catalyst to form a non gaseous oxygen species such as a transition metal peroxide or superoxide. In both the case of a gaseous or non gaseous oxygen species the oxygen is trapped in the electrode and can be utilised in further discharging and charging of the electrode.
Upon discharge, lithium ions reintercalate into the intercalation material which, as shown in equation (3) is now oxygen deficient, for example according to:
xLi++xe-+ Li1-xMO2-2δ →LiMO2-2δ (4)
Also, lithium that was de-intercalated upon charging can also react with the oxygen that was trapped in the electrode during charging to form a lithium oxygen compound such as lithium oxide, for example according to Eq (5).
2xLi++2xe-+O2→2LixO (5)
Once the formation charging is completed, if the device is charged for a subsequent time, the lithium oxide forms lithium and an oxygen species and the lithium intercalation material de-intercalates lithium.
2LixO→2xLi++2xe-+ O2 (6)
LiMO2-2δ→xLi++ xe-+ Li1-xMO2-2δ (7)
On subsequent discharge the reverse reactions to those shown in equations (6) and (7) occur. If the device is then charged again the reactions shown in equations (6) and (7) occur, and so on.
In a practical device the process of formation charging may require only the first charge of the device, but in some case the formation may require multiple charge-discharge cycles - for example may take up to 10 charge-discharge cycles. Where the formation requires multiple charge-discharge cycles, the second charge of the device will result in a mixture of the reactions of equations (2) and (3) and the reactions of equations (6) and (7), as will any further charges required in the formation process. Once the formation process is complete, further charges of the device cause reactions to occur according to equations (6) and (7), and this is also true of any subsequent charge of the electrode - and the reverse reactions occur on discharge.
It should be understood that equations (2) to (7) do not show the only possible reactions upon charge and discharge. For example, the products upon discharge are not limited to oxides as shown in equation (5), but could be/include peroxides and/or superoxides.
The constituents of a cell having an electrode in accordance with an embodiment of the present invention are preferably provided in a sealed container. While the provision of the oxygen-capture material in the electrode will prevent most oxygen that is generated during the first charge from leaving the cell, providing the cell in a sealed container means that the cell forms a closed system so that all oxygen generated during the first charge is retained in the cell and is available to be utilised in subsequent discharging and charge of the cell. By a “closed system” is meant that no transfer of material, in particular of oxygen, occurs between the cell and its surrounding environment, although heat and/or work may be transferred.
Figure 4 is a representation of the first charge and discharge voltage curves for a sodium cathode as described in this one embodiment of the invention for a sodium ion battery. The nominal active cathode material is NaNi1/4Mn7/12Na3/24Li1/24O2, this is mixed with a conductive carbon black, and an oxygen capturing material - in this example Zeolite A. In this case the intercalation material and the oxygen producing material act as the catalyst. The redox active centre in the cathode material is nickel and the number of mole of sodium removal per formula unit corresponds to 0.5Na+→Na as the nickel changes oxidation state from Ni2 +to Ni4 +. In figure 4 the corresponding number of sodium removal in the first charge is 0.7Na per formula unit, significantly higher than corresponds to the nickel redox. The oxygen is produced at high voltage, and the active material in this case is acting partly as a catalyst which improves the reversibility of this oxygen redox centre. Upon discharge the oxygen redox plateaux is reversible and there is very little first cycle loss in capacity from this composite cathode material. This example shows the improved reversibility of the oxygen redox with a catalyst and an oxygen capturing material. The oxygen was produced in situ by the sodium intercalation material, and the intercalation capacity is observed in addition to the oxygen redox.
Examples of alternative electrodes are given below.
A cathode electrode is made by adding a lithium ion cathode material Li[Ni0.2Li0.2Mn0.6]O2, to Timcal C65 carbon black, and ball milled for 20 minutes at 400rpm in a sealed zirconia container. To this powder an oxygen capturing material, Zeolite A and catalyst Pt and a binder PVDF was added. The ratio of the electrode formulation is 73:10:5:2:10 (this ratio is by weight, as are all other electrode formulation ratios given below), active material, carbon black, zeolite, catalyst and binder. The conductive carbon used in the slurry was Super P C65, manufactured by Timcal. The binder used in the slurry was polyvinylidene fluoride (PVDF) ( Kynar HSV900), manufactured by Arkema. The solvent used in the slurry was N-Methyl-2-pyrrolidone (NMP), Anhydrous, manufactured by Sigma. A composite electrode slurry was prepared by weighing the active, catalysts, oxygen capturing agents and conductive materials into a container, to which a binder solution was then added, The composite cathode slurry is then mixed using a planetary centrifugal mixer (Thinky mixer) for 10 minutes at 1600rpm to obtain a homogeneous mix. The slurry is then doctor blade coated onto an aluminium current collector, and the NMP evaporated using an IR lamp leaving an even homogeneous coating. The formed cast electrode was then dried under Vacuum at about 80 - 120°C for about 4 hours. As formed, each electrode film contained the following components, expressed in percent by weight: 73% active material, 10% C65 carbon, 5% Zeolite A (Aldrich), 2% Platinum on Carbon black (Aldrich) and 7% Kynar binder. Optionally, this ratio can be varied (e.g., by adjusting the amounts of the components in the slurry) to optimize the electrode properties such as, adhesion, resistivity and porosity and performance.
An electrode of a sodium layered oxide material of the formula Na7/6Mn7/12Ni1/4O2, is made in a similar method to Example1. The active material Na7/6Mn7/12Ni1/4O2 is first mixed with carbon black C65 from Timcal and an oxygen capturing material Nu-125 a metal organic framework material (MOF). In this example the active material Na7/6Mn7/12Ni1/4O2 also acts as a catalyst for the oxygen redox reaction. Na7/6Mn7/12Ni1/4O2 is mixed with a carbon black such as Timal C65, the mixture is then sonicated using an ultrasonic probe for 2 minutes before transferring to a planetary centrifugal mixer for mixing at 1100Rpm for 5 minutes. The electrode was then coated onto a carbon coated aluminium foil current collector (Showa Denko), using the doctor blade method and dried as in Example 1. The final ratio of the electrode coating was Na7/6Mn7/12Ni1/4O2 active material, Carbon Black C65, Nu-125, PVDF 80:10:5:5.
In examples 1 and 2 the oxygen for the reversible electrochemical reaction is produced in the first charge of a cell by the breakdown of the intercalation cathode material. The catalyst aids the reversible oxygen reduction reaction, and the oxygen capturing agent prevents the oxygen leaving the cell. Active carbons, and carbon blacks can also be used as oxygen capturing agents and are typically used in fuel cell applications.
A lithium ion battery electrode is made with a lithium layered oxide material, NMC (lithium nickel manganese cobalt oxide), and an oxygen producing material Li2MnO3, to these materials an oxygen capturing material Zeolite NaX and a conductive additive (carbon black - C65) are added.
A commercial grade of NMC is first ball milled with Li2MnO3, carbon black and the zeolite NaX for 1hr at 300rpm. Composite electrode slurry is made with the powder mix by adding in a PVDF solution (4% HSV900 in NMP (N-Methyl-2-pyrrolidone)) and mixing in a planetary centrifugal mixer for 15 minutes at 1000 rpm. The slurry is then coated onto a carbon coated aluminium current collector foil using the doctor blade technique and dried as detailed in example 1. The final ratio of the composition of the electrode coating is NMC:Li2MNO3:ZeoliteX:C65:PVDF 65:5:5:10:5.
A sodium ion battery electrode is material using a sodium ion layered oxide intercalation material, NaNi1/2Mn1/3Mg1/6Ti1/6O2; an oxygen producing material, Na2SnO3; an oxygen capturing material, activated carbon - Norit; a ruthenium metal catalyst; and a conductive additive titanium carbide. The electrode was made the same way as example 3, with the final ratios in the composite electrode as NaNi1/2Mn1/3Mg1/6Ti1/6O2:Na2SnO3:Norit:Ru:TiC:PVDF 77:5:5:2:5:6.
Example of a typical cell testing method
Electrochemical cells of the example electrodes can be tested in an electrochemical cell. A typical electrochemical cell is described in Figure 1. In some embodiments, a glass fiber separator is interposed between the positive and negative electrodes forming the electrochemical test cell. One example of a suitable glass fibre separator is a Whatman grade GF/A separator. In other embodiments, a porous polypropylene or a porous polyethylene separator wetted by the electrolyte is interposed between the positive and negative electrodes forming the electrochemical test cell. One example of a suitable porous polypropylene separator is Celgard 2400. GFA was used in the cell manufacture.
The electrochemical cell is cycled using a constant current density charge and discharge between pre-set voltage limits as deemed appropriate for the material under test. In most cases a low constant current charge is performed at 5-10mA/g, the upper voltage limit is between 3.0V and 5.0 V depending upon the electrolyte and the material. On charge, sodium ions are extracted from the cathode and migrate to the anode. On discharge the reverse process occurs. In the first charge the alkali metal ions are removed from the intercalation material and in addition oxide ions are oxidised from the oxygen producing material, and then the resulting oxygen species, oxygen gas, peroxide or superoxide is captured in the electrode. The catalyst aids the reversibility of the oxygen redox. Upon discharge the oxygen species is reduced. In addition to the oxygen species be reduced the alkali metal cations are reintercalated into the intercalation material.
(Overview)
A first aspect of the present invention provides an electrode having a conductive matrix and a plurality of materials disposed in the conductive matrix, wherein the electrode comprises: a metal intercalation constituent that, upon charging of the electrode, de-intercalates metal ions; an oxygen-producing constituent that, upon initial charging of the electrode, produces electrochemically active oxygen; and an oxygen capture constituent for capturing and storing oxygen produced by the oxygen producing constituent.
As is known, “intercalation” is the reversible inclusion or insertion of a molecule or ion into a compound. As used herein, an “intercalation material” is a material that exhibits intercalation and can reversibly act as a host to guest molecules or ions. As used herein a “metal intercalation material” is an intercalation material that hosts a metal.
In all the above-described examples of a conventional alkali-metal air cell oxygen is provided externally to the cell during operation of the cell or during cell construction. An electrode in accordance with an embodiment of the invention, in contrast, is able to generate and store (for future use) oxygen owing the presence of the oxygen-producing constituent (which is not itself oxygen but is a constituent that, upon charging generates oxygen for example by a decomposition or partial decomposition of the oxygen-producing constituent) thereby avoiding the need to provide an external source of oxygen. The electrochemically active oxygen may be stored by a number of possible mechanisms, such as:
- oxygen gas may be generated upon charging, and captured and stored by the oxygen capture constituent (for example in pores or in a material structure);
- in some embodiments the electrode further comprises a catalyst, and oxygen gas generated upon charging may react with the catalyst to form an intermediate;
- oxygen generated on charging may react with an electrode component, with the reaction product remaining in the electrode so that the oxygen may be recovered - for example oxygen generated on charging may attach itself to a transition metal compound present in the electrode, forming a transition metal peroxide or superoxide;
- if the oxygen capture constituent is an oxygen deficient material, oxygen generated on charging may be captured by entering into the crystal lattice (perovskite oxygen capturing).
To the inventors knowledge there is no device which utilises the benefits of a metal ion battery system and a metal air or oxygen system in a sealed cell with an oxygen capturing agent and a catalyst. This one embodiment of the invention utilises an oxygen by product formed electrochemically within a cell and an intercalation cathode to increase the energy density of a battery.
The electrode may further comprise an oxygen reduction/oxidation catalyst.
A single material may comprise the metal intercalation constituent and the oxygen reduction/oxidation catalyst.
A single material may comprise the metal intercalation constituent and the oxygen-producing constituent.
A single material may comprise the metal intercalation constituent and the oxygen-capturing (oxygen capture) constituent. For example, a transition metal oxide may act as a metal intercalation constituent, and may also be able to store oxygen as mentioned above.
The metal intercalation constituent may de-intercalate metal ions upon charging of the electrode to a first voltage and the oxygen producing constituent may produce oxygen molecules upon charging of the electrode to a second voltage higher than the first voltage.
The metal intercalation constituent may de-intercalate, upon charging of the electrode, alkali metal ions.
The metal intercalation constituent may be a layered-oxide material.
The conductive matrix may be permeable to oxygen.
The oxygen-producing constituent may comprise material that contains a transition metal.
A second aspect of the invention provides an electrochemical device comprising an anode, a cathode and an electrolyte, wherein the cathode is an electrode of the first aspect.
The electrolyte may be a liquid electrolyte.
The electrolyte may contain a redox mediator.
The electrochemical device may comprise one of: a sodium and/or potassium ion hybrid cell; a sodium and/or potassium metal hybrid cell; a non-aqueous electrolyte sodium and/or potassium ion cell; and an aqueous electrolyte sodium and/or lithium and/or potassium ion cell.
The electrochemical device may comprise a rechargeable battery. (As used herein, a “cell” is the smallest unit possible, consisting essentially of an anode, a cathode and an electrolyte. A “battery” may comprise a single cell, or it may comprise multiple cells.)
The anode, the cathode and the electrolyte may be disposed in a sealed container.
A third aspect of the present invention provides a method of operating an electrochemical device, the electrochemical device comprising an anode, a cathode and an electrolyte, the cathode comprising a metal intercalation constituent, an oxygen-producing constituent and an oxygen capture constituent, the method comprising:
in a formation charge of the electrochemical device, charging the cathode such that the metal intercalation constituent de-intercalates metal ions, the formation charging comprising charging the cathode to a voltage sufficient to initiate at least partial decomposition of the oxygen-producing constituent thereby to produce electrochemically active oxygen;
discharging the electrochemical device such that at least some of the de-intercalated metal ions combine with oxygen produced by decomposition of the oxygen-producing constituent to form a metal oxide compound; and
in a subsequent charge of the electrochemical device, charging the cathode such that the metal oxide compound disassociates to form metal ions and an oxygen species.
The term “metal oxide compound” as used in this statement of the third aspect is intended to include one or more of a metal oxide, metal peroxides and metal superoxide.
To the accomplishment of the foregoing and related ends, an embodiment of the invention comprises the features hereinafter fully described and identified in the claims. The foregoing description and the annexed drawings set forth detail certain illustrative embodiments of the invention. These embodiments are indicative, however, of but a few of the various ways in which the principles of an embodiment of the invention may be employed. Other objects, advantages and novel features of the invention will become apparent from the foregoing detailed description of an embodiment of the invention when considered in conjunction with the drawings.
This disclosure incorporates an oxygen producing material with into a intercalation cathode, and the use of the electrode in an electrochemical device.
This Nonprovisional application claims priority under 35 U.S.C. § 119 on Patent Application No. 1519231.3 filed in Great Britain on October 30, 2015, the entire contents of which are hereby incorporated by reference.
Electrodes according to an embodiment of the present invention are suitable for use in many different applications, energy storage devices, rechargeable cells and/or batteries, electrochemical devices and electrochromic devices. Advantageously the electrodes according to an embodiment of the invention are used in conjunction with a counter electrode and one or more electrolyte materials. The electrolyte materials may be any conventional or known materials and may comprise of either aqueous electrolyte or non-aqueous electrolytes or a mixture therof.

Claims (17)

  1. An electrode having a conductive matrix and a plurality of materials disposed in the conductive matrix, wherein the electrode comprises:
    a metal intercalation constituent that, upon charging of the electrode, de-intercalates metal ions;
    an oxygen-producing constituent that, upon initial charging of the electrode, produces electrochemically active oxygen; and
    an oxygen capture constituent for capturing and storing the oxygen produced by the oxygen producing constituent.
  2. An electrode as claimed in claim 1 wherein the electrode further comprises an oxygen reduction/oxidation catalyst.
  3. An electrode as claimed in claim 2 wherein a single material comprises the metal intercalation constituent and the oxygen reduction/oxidation catalyst.
  4. An electrode as claimed in claim 1 or 2 wherein a single material comprises the metal intercalation constituent and the oxygen-producing constituent.
  5. An electrode as claimed in claim 1 or 2 wherein a single material comprises the metal intercalation constituent and the oxygen capture constituent.
  6. An electrode as claimed in any preceding claim wherein the metal intercalation constituent de-intercalates metal ions upon charging of the electrode to a first voltage and the oxygen producing constituent produces oxygen molecules upon charging of the electrode to a second voltage higher than the first voltage.
  7. An electrode as claimed in any preceding claim wherein the metal intercalation constituent de-intercalates, upon charging of the electrode, alkali metal ions.
  8. An electrode as claimed in any preceding claim wherein the metal intercalation constituent is a layered-oxide material.
  9. An electrode as claimed in any preceding claim, wherein the conductive matrix is permeable to oxygen.
  10. An electrode as claimed in any preceding claim wherein the oxygen-producing constituent comprises a transition metal-containing material.
  11. An electrochemical device comprising an anode, a cathode and an electrolyte, wherein the cathode is an electrode as defined in any one of claims 1 to 10.
  12. An electrochemical device as claimed in claim 11 wherein the electrolyte is a liquid electrolyte.
  13. An electrochemical device as claimed in claim 11 or 12 wherein the electrolyte contains a redox mediator.
  14. An electrochemical device as claimed in claim 11, 12 or 13, and comprising one of: a sodium and/or potassium ion hybrid cell; a sodium and/or potassium metal hybrid cell; a non-aqueous electrolyte sodium and/or potassium ion cell; and an aqueous electrolyte sodium and/or lithium and/or potassium ion cell.
  15. An electrochemical device as claimed in any one of claims 11 to 14, and comprising a rechargeable battery.
  16. An electrochemical device as claimed in any one of claims 11 to 15 wherein the anode, the cathode and the electrolyte are disposed in a sealed container.
  17. A method of operating an electrochemical device, the electrochemical device comprising an anode, a cathode and an electrolyte, the cathode comprising a metal intercalation constituent, an oxygen-producing constituent and an oxygen capture constituent, the method comprising:
    in a formation charge of the electrochemical device, charging the cathode such that the metal intercalation constituent de-intercalates metal ions, the formation charging comprising charging the cathode to a voltage sufficient to initiate at least partial decomposition of the oxygen-producing constituent thereby to produce electrochemically active oxygen;
    discharging the electrochemical device such that at least some of the de-intercalated metal ions combine with oxygen produced by decomposition of the oxygen-producing constituent to form a metal oxide compound; and
    in a subsequent charge of the electrochemical device, charging the cathode such that the metal oxide compound disassociates to form metal ions and an oxygen species.
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