AQUEOUS ELECTROCHEMICAL DEVICES AND PREPARATION METHOD THEREOF

Abstract

The disclosure relates to an aqueous electrochemical device comprising a negative electrode, a positive electrode, a separator and an aqueous electrolyte having an alkaline pH, wherein onto the positive electrode is disposed at least one layer of nanoparticles capable of being used to form a local hydronium ion rich environment at the positive electrode during operation of the device, and/or the capacity ratio between the negative electrode and the positive electrode is less than 1 so as to substantially avoid production of oxygen at the positive electrode. The electrochemical device may find particular use in large-scale energy storage.

Claims

1. An aqueous electrochemical device comprising a negative electrode, a positive electrode, a separator and an aqueous electrolyte having an alkaline pH, wherein the positive electrode has disposed thereon at least one layer of nanoparticles capable of forming a local hydronium ion rich environment at the positive electrode during operation of the device, and/or the capacity ratio between the negative electrode and the positive electrode is less than 1 so as to substantially avoid production of oxygen at the positive electrode.

2. A method of fabricating an aqueous electrochemical device according to claim 1, wherein the method includes applying onto the positive electrode at least one layer of nanoparticles capable of being used to form a local hydronium ion rich environment at the positive electrode during operation of the device, and/or making the capacity ratio between the negative electrode and the positive electrode less than 1 so as to substantially avoid production of oxygen at the positive electrode.

3. The device according to claim 1, wherein the aqueous battery is an aqueous metal-ion battery.

4. The device according to claim 3, wherein the aqueous battery is an aqueous lithium-ion battery, an aqueous sodium-ion battery, or an aqueous potassium-ion battery.

5. The device according to claim 1, wherein the nanoparticles are made from a support and any one selected from the group consisting of Ni, Pt, Fe, Co, Pd, Cu and combinations thereof.

6. The device according to claim 5, wherein the support within the nanoparticles is selected from the group consisting of carbon black, carbon nanotubes, graphite, graphitised carbon black, graphene, reduced graphene oxide and combinations thereof.

7. The device according to claim 1, wherein the nanoparticles are selected from the group consisting of Ni/C, Pt/C, Fe/C, Co/C, Pd/C, Cu/C, PtNi/C, PtFe/C, PtCo/C, PtCu/C, PdNi/C, Ni/rGO, Pt/rGO, Fe/rGO, Co/rGO, Pd/rGO, Cu/rGO, PtNi/rGO and PdNi/rGO nanoparticles.

8. The device according to claim 7, wherein the nanoparticles are Ni/C and/or Co/C nanoparticles with a Ni and/or Co loading of about 1% by weight to about 40% by weight.

9. The device according to claim 1, wherein the average particle size of the nanoparticles ranges from about 1 nm to about 100 nm.

10. The device according to claim 1, wherein the at least one layer of nanoparticles has a thickness of about 5 m to 100 m.

11. The device according to claim 1, wherein the pH of the aqueous electrolyte is about 9 to about 13.

12. The device according to claim 1, wherein the aqueous electrochemical device is an aqueous sodium ion battery, the aqueous electrolyte having an alkaline pH comprises a salt as the electrolyte which is selected from sodium perchlorate (NaClO.sub.4), sodium trifluoromethanesulfonate (NaCF.sub.3SO.sub.3), sodium nitrate (NaNO.sub.3), sodium chloride (NaCl), sodium sulfate (Na.sub.2SO.sub.4), sodium acetate (CH.sub.3COONa), sodium carbonate (Na.sub.2CO.sub.3), sodium hexafluorophosphate (NaPF.sub.6), and combinations thereof.

13. The device according to claim 12, wherein the aqueous electrolyte having an alkaline pH is a saturated aqueous solution of sodium perchlorate.

14. The device according to claim 1, wherein the aqueous electrochemical device is an aqueous sodium ion battery, the positive electrode comprises a positive electrode material which is selected from the group consisting of Na.sub.xFe.sub.yMn.sub.1-y[Fe(CN).sub.6].sub.w.Math.zH.sub.2O (1x2, 0.8y1, 0.8w1, 0.5z2), Na.sub.2Mn.sub.xFe.sub.1-xFe(CN).sub.6 (0.8x1), Na.sub.2Mn.sub.xNi.sub.1-xFe(CN).sub.6 (0.8x1), Na.sub.2Mn.sub.xCo.sub.1-xFe(CN).sub.6 (0.8x1.0), Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3, Na.sub.0.44MnO.sub.2, Na.sub.2NiFe(CN).sub.6, Na.sub.2CuFe(CN).sub.6, Na.sub.2NiMn(CN).sub.6, Na.sub.3V.sub.2(PO).sub.4, NaMnO.sub.2, Na.sub.0.66[Mn.sub.0.66Ti.sub.0.34]O.sub.2, and Na.sub.2Zn.sub.3[Fe(CN).sub.6].sub.2, Na.sub.3MnTi(PO.sub.4).sub.3 and Na.sub.4Fe.sub.3(PO.sub.4).sub.2(P.sub.2O.sub.7).

15. The device according to claim 1, wherein the aqueous electrochemical device is an aqueous sodium ion battery, the negative electrode comprises a negative electrode material which is selected from the group consisting of NaTi.sub.2(PO.sub.4).sub.3, Na.sub.3MnTi(PO.sub.4).sub.3, NaTiOPO.sub.4, Na.sub.2VTi(PO.sub.4).sub.3, Na.sub.3V.sub.2(PO.sub.4).sub.3, TiSe.sub.2, TiS.sub.2, hard carbon and perylenetetracarboxylic diimide.

16. The device according to claim 1, wherein the capacity ratio between the negative electrode and the positive electrode is about 0.56:1 to about 0.95:1.

17. The device according to claim 16, wherein the capacity ratio between the negative electrode and the positive electrode is about 0.62:1.

18. The device according to claim 16, wherein the capacity ratio between the negative electrode and the positive electrode is about 0.75:1.

19. A positive electrode for an aqueous electrochemical device, which has disposed thereon at least one layer of nanoparticles capable of forming a local hydronium ion rich environment at the positive electrode during operation of the device.

20. The positive electrode according to claim 19, wherein the nanoparticles are selected from the group consisting of Ni/C, Pt/C, Fe/C, Co/C, Pd/C, Cu/C, PtNi/C, PtFe/C, PtCo/C, PtCu/C, PdNi/C, Ni/rGO, Pt/rGO, Fe/rGO, Co/rGO, Pd/rGO, Cu/rGO, PtNi/rGO and PdNi/rGO nanoparticles.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0025] Non-limiting embodiments of the present disclosure will be discussed with reference to the accompanying figures wherein:

[0026] FIG. 1 shows (a) an X-ray powder diffraction (XRD) spectrum of Ni/C, and (b) a transmission electron microscopy (TEM) image of Ni/C (PDF #04-0850).

[0027] FIG. 2 shows a linear sweep voltammetry (LSV) curve after the surface treatment of the positive electrode: Cal: 0.000174 m/pix: 11:01 2022 Jul. 27; Camera: NANOSPRT15, Exposure: 400 (ms)4 std. frames, Gain: 1, Bin: 1: Gamma: 1.00, No sharpening, Normal Contrast.

[0028] FIG. 3 depicts (a) discharge capacity of batteries at different rates, and (b) Coulombic efficiency of batteries at different rates.

[0029] FIG. 4 depicts electrochemical performance of the NMF/NTP full cell at voltage range of 0.5-2.2 V. (a) Rate capability at various current rates and the corresponding Coulombic efficiency of the NMF/NTP full cell using local microenvironment (LME) at room temperature. (b) Comparison of average voltage at various current rates of the NMF/NTP full cell in different system at room temperature. (c) Cycling performance of NMF/NTP full cell in different systems at current rate of 1 C at room temperature. (d) Cycling performance of NMF/NTP full cell in different systems at a current rate of 0.5 C and at 30 C. (e) Long-term cycling performance of the NMF/NTP full cell using local environment at a current rate of 10 C and at room temperature (cathode mass loading: 20.45 mg.Math. cm.sup.2, anode mass loading: 19.45 mg.Math.cm.sup.2, the mass of negative electrode material comprised by the negative electrode and the mass of positive electrode material comprised by the positive electrode is at a ratio of about 0.95:1).

[0030] FIG. 5 depicts cycling performance (a) discharge capacity, and (b) coulombic efficiency of NMF/NTP full batteries at 0.5 C.

[0031] FIG. 6 shows a comparison of reported sodium aqueous batteries with the batteries according to the present disclosure.

[0032] FIG. 7 shows a comparison of the batteries according to the present disclosure with previously reported batteries in terms of cost and electrochemical performance. (a) Comparison of the total cost for the full battery with reported aqueous Li, Na and K-ion full batteries (the prices are based on the sigma in Australia, Table 1). (b) Comparison of lifespan and energy density for our works with reported aqueous Na and K-ion full batteries. (c) Comparison of batteries according to the present disclosure with commercial batteries as quantified in Table 2.

[0033] FIG. 8 shows a safety test of ASIB pouch cell using the surface treatment of the positive electrode. (a) Output voltage of pouch cell. (b) Picture of blue lights powered by two ASIB pouch cells. (c) Picture of cut pouch cells immersed in water to power blue lights. (d) Charge-discharge curves of 32 mAh ASIB pouch cell. (e) Picture of electric fan powered by ASIB pouch cell. (f) Charge curves of ASIB pouch cell before and after being cut and immersed in water. (g-i) A cut pouch cell after being recharged powers the temperature hygrometer in water over 10 h.

[0034] FIG. 9 depicts the generation of a local environment. (a) In-situ surface-enhanced IR spectra of C at different potentials. (b) In-situ surface-enhanced IR spectra of Ni/C at different potentials. (c) operando differential electrochemical mass spectrometry (DEMS) results to evaluate the H.sub.2 and O.sub.2 evolution during NMF/NTP battery cycling at the voltage range of 0.5 V to 2.2 V at 0.5 C. (d) Scanning electron microscope cross-section image of Ni/C coated NMF. (c) Schematic illustration of the water reduction mechanism on the electrode surface with pure carbon and Ni/C in the alkaline electrolyte.

[0035] FIG. 10 shows in-situ Fourier-transform infrared spectroscopy (FTIR) for C and Ni/C in neutral electrolytes.

[0036] FIG. 11 shows the investigation of the reaction mechanism and in situ Ni substitution. The charge-discharge curves of the NMF/NTP electrodes in (a) neutral electrolyte, (b) alkaline electrolyte, (c) alkaline electrolyte with the surface treatment of the positive electrode strategy. (d) TEM image of NMF electrode after being cycled in neutral electrolyte, alkaline electrolyte and alkaline electrolyte with the surface treatment of the positive electrode strategy. (c) Energy-dispersive X-ray spectroscopy (EDS) spectra taken from the NMF electrodes after being cycled in neutral electrolyte, alkaline electrolyte and alkaline electrolyte with the surface treatment of the positive electrode strategy. (f) Raman spectra of the NMF electrodes after being cycled in neutral electrolyte, alkaline electrolyte and alkaline electrolyte with the surface treatment of the positive electrode strategy.

[0037] FIG. 12 depicts electrochemical performance of the NMF/NTP full cell at voltage range of 0.5-2.2 V. (a). (c) and (e) cycling performance of NMF/NTP full cell with a layer of Pd/C, Cu/C or Co/C nanoparticles on the positive electrode at current rate of 1 C at 25 C. (b), (d) and (f) charge/discharge curves of NMF/NTP full cell with a layer of Pd/C (b), Cu/C (d) or Co/C (f) nanoparticles on the positive electrode.

[0038] FIG. 13 depicts cycling performance of NMF/NTP full cell with a NTP/NMF ratio of 1:1 and of a NMF/NTP full cell with a NTP/NMF ratio of 0.75:1.

[0039] FIG. 14 depicts cycling performance of NMF/NTP full cell with a NTP/NMF ratio of 1:1 and of a NMF/NTP full cell with a NTP/NMF ratio of 0.62:1.

[0040] FIG. 15 depicts cycling performance of NMF/NTP full cell with a NTP/NMF ratio of 1:1 and of a NMF/NTP full cell with a NTP/NMF/ratio of 0.56:1.

DESCRIPTION OF EMBODIMENTS

[0041] The term electrochemical device used herein refers to a device that can convert chemical energy into electrical energy through an electrochemical reaction.

[0042] The term aqueous electrolyte used herein generally refers to a water-based electrolyte solution. However, this does not exclude the possibility of presence of an amount of organic co-solvent (such as dimethyl carbonate (DMC) and acetonitrile) that would not have adverse impact on forming a local hydronium ion rich environment at the positive electrode with the aid of at least one layer of nanoparticles disposed onto the positive electrode.

[0043] The term water-in-salt electrolyte used herein refers to a highly concentrated electrolyte solution wherein the dissolved salt molecules greatly outnumber water molecules (salt/solvent ratio>1 by volume or weight) and there are barely enough water molecules available to form the classical primary solvation.

[0044] The term negative electrode material used herein refers to an active material for the negative electrode of the electrochemical device. The term positive electrode material used herein refers to an active material for the positive electrode of the electrochemical device.

[0045] The phrase hydronium ion rich used herein means that H.sub.3O.sup.+ ions accumulate at the surface of a positive electrode, which may be evidenced by the asymmetric OH stretching modes of H.sub.3O.sup.+ at 2020 cm.sup.1 as well as the umbrella vibration of H.sub.3O.sup.+ at 1230 cm.sup.1 via in-situ IR. It will be appreciated that a hydronium ion rich environment at the positive electrode results in a local acidic environment at the electrode.

[0046] The term capacity used in relation to an electrode refers to the total amount of electricity generated due to an electrochemical reaction at an electrode. It may be determined by the usable amount (e.g. mass) of active material of an electrode that participates in the redox reactions.

[0047] The term capacity ratio between the negative electrode and the positive electrode used herein is also known and referred to in the art as the N/P capacity ratio.

[0048] The disclosure arises from the inventors' research into stabilisation of aqueous electrochemical devices. It has been surprisingly found that forming a local hydronium ion rich environment at a positive electrode (cathode) in an alkaline (or high pH) electrolyte during operation of the device can suppress oxygen production at the positive electrode while the alkalinity of the electrolyte is helpful in retarding hydrogen production at the negative electrode (anode). In this way, the electrochemical stability window (ESW) of an aqueous electrolyte can be expanded and the stability of an aqueous electrochemical device is improved. By way of example in a case where an Mn-rich Prussian Blue Analogue (PBA) such as Na:MnFe(CN).sub.6 was used as the positive electrode material and at least one layer of Ni-based nanoparticles was disposed on the positive electrode, the edges of Ni-based nanoparticles prompted the dissociation of water and large amounts of H* were produced. However, under a positive potential. H* is hard to adsorb onto the surface of the positive electrode, and instead. H ions are still bonded to nearby water molecules but not to the surface of the catalyst. Thus, hydronium ions (H.sub.3O.sup.+) accumulated at the surface of the positive electrode to form a local acidic environment. This H.sub.3O.sup.+ rich environment may effectively restrain the OH in the bulk electrolyte from contacting the positive electrode, so that the OER at the positive electrode is suppressed. The H.sub.3O.sup.+ rich environment may also retard the OH species adsorbing onto the surface of the positive electrode, thereby weakening the dissolution of Mn and stabilising the positive electrode. During charging, oxidisation of Ni-based nanoparticles in the layer(s) has been found to promote in-situ substitution of Ni.sup.2+ for Mn which then further enhances the stability of the aqueous electrochemical device. It has also been surprisingly found that making the capacity ratio between the negative electrode and the positive electrode (i.e. the N/P capacity ratio) less than 1 allows the voltage of the electrochemical device to be altered to a voltage range at which hydrogen is more likely produced and production of oxygen at the positive electrode is substantially avoided. This cathode sacrifice strategy combined with an alkaline pH (or high pH) electrolyte, which assists in suppressing production of hydrogen at the negative electrode, may significantly improve the stability of aqueous electrochemical devices.

[0049] Accordingly, disclosed herein is an aqueous electrochemical device comprising a negative electrode, a positive electrode, a separator, and an aqueous electrolyte having an alkaline pH. There is at least one layer of nanoparticles disposed onto the positive electrode which are capable of being used to form a local hydronium ion rich environment at the positive electrode during operation of the device. In addition or alternatively, the capacity ratio between the negative electrode and the positive electrode is selected to be less than 1 so as to substantially avoid production of oxygen at the positive electrode.

[0050] The aqueous electrochemical device disclosed herein may be in the form of a battery or a cell. For the purpose of illustration, aqueous batteries may include aqueous magnesium-ion batteries (AMIBs), aqueous aluminium-ion batteries (AAIBs), and aqueous alkali metal-ion batteries such as aqueous lithium-ion batteries (ALIBs), aqueous potassium-ion batteries (AKIBs) and aqueous sodium-ion batteries (ASIBs). In some embodiments, the aqueous electrochemical device may be ALIBs as they tend to have a high energy density. In some other embodiments, aqueous sodium-ion batteries (ASIBs) may be preferable because of an abundance of raw materials, safety and low costs.

[0051] As explained above, the electrochemical stability window of aqueous batteries is as narrow as 1.23V, which restricts the selection of a negative electrode material and a positive electrode material due to the occurrence of hydrogen and/or oxygen production reactions. Ideally, the redox potentials of electrodes should lie in between the hydrogen and oxygen production potentials to avoid the electrolysis of water.

[0052] Generally, negative electrode materials and positive electrode materials for aqueous batteries that are known in the art can be used for the present disclosure. Examples of the negative electrode material for aqueous lithium-ion batteries include conductive additives. LTO (lithium titanate), surface-functionalized silicon, and high-performance powdered graphene. A lithium metal oxide compound, such as lithium cobalt dioxide LiCoO.sub.2, lithium nickel dioxide LiNiO.sub.2, lithium manganese dioxide LiMnO.sub.2, lithium manganese oxide LiMn.sub.2O.sub.4, lithium nickel manganese oxide Li.sub.1.0Ni.sub.0.5Mn.sub.1.5O.sub.4, lithium nickel manganese cobalt oxide LiNi.sub.0.33Mn.sub.0.33Co.sub.0.33O.sub.2, or high energy lithium nickel manganese cobalt oxide Li.sub.1.2Ni.sub.0.176Mn.sub.0.524Co.sub.0.100O.sub.2, is normally used as the positive electrode material. Consideration may also be given to FeS.sub.2 and lithium ion phosphate, etc. If needed, elemental doping and coatings can be applied to modify the electrode materials.

[0053] Turning to aqueous sodium-ion batteries, the negative electrode material may be selected from NaTi.sub.2(PO.sub.4).sub.3 (NTP). Na.sub.3MnTi(PO.sub.4).sub.3, NaTiOPO.sub.4, Na.sub.2VTi(PO.sub.4).sub.3, Na.sub.3V.sub.2(PO.sub.4).sub.3, TiSe.sub.2, TiS.sub.2, hard carbon and perylenetetracarboxylic diimide. In some embodiments, the negative electrode material is NaTi.sub.2(PO.sub.4).sub.3. Among the positive electrode materials that can be used to fabricate aqueous sodium ion batteries disclosed herein. Prussian Blue Analogues (PBA) are promising because of their excellent redox properties and relatively high standard potential. For ASIBs. PBA may have the general formula Na.sub.xP[R(CN).sub.6].sub.1-y.Math.wH.sub.2O where P and R are transition metals such as Mn, Ni and Fe, and y is the number of [R(CN).sub.6] vacancies. The cage-like structure exhibits wide channels, allowing for insertion of a wide range of intercalation ions. PBA can be prepared from abundant and non-toxic elements by simple and low-cost co-precipitation synthesis of a metal salt and a hexacyanoferrate complex. In this regard. Na.sub.xFe.sub.yMn.sub.1-y[Fe(CN).sub.6].sub.w.Math.zH.sub.2O (1x2.0.8y1.0.8w1.0.5z2), Na.sub.2NiFe(CN).sub.6, Na.sub.2Mn.sub.xFe.sub.1-xFe(CN).sub.6 (0.8x1) such as Na.sub.2MnFe(CN).sub.6 (NMF), Na.sub.2Mn.sub.xNi.sub.1-xFe(CN).sub.6 (0.8x1). Na.sub.2Zn.sub.3[Fe(CN).sub.6].sub.2, Na.sub.2CuFe(CN).sub.6 and/or Na.sub.2NiMn(CN).sub.6 may be chosen as the positive electrode material. Other examples of the positive electrode material that can be useful for the aqueous sodium ion batteries disclosed herein include, but are not limited to. Na.sub.0.44MnO.sub.2, Na.sub.3V.sub.2(PO).sub.4, NaMnO.sub.2, Na.sub.0.66[Mn.sub.0.66Ti.sub.0.34]O.sub.2, Na.sub.3MnTi(PO.sub.4).sub.3, Na.sub.2Mn.sub.xCo.sub.1-xFe(CN).sub.6 (0.8x1.0), Na.sub.3V.sub.2(PO.sub.4).sub.2F.sub.3, and Na.sub.4Fe.sub.3(PO.sub.4).sub.2(P.sub.2O.sub.7).

[0054] In order to suppress oxygen production at the positive electrode, the capacity ratio between the negative electrode and the positive electrode may be chosen to be less than 1. In some cases, such as in a NMF/NTP full cell, this can be achieved through making the mass ratio between the negative electrode material of the negative electrode and positive electrode material of the positive electrode in the range of about 0.56:1 to about 0.95:1, for example, about 0.62:1 and about 0.75:1. When the mass ratio is reduced to about 0.62:1, the electrochemical device disclosed herein may experience substantially no capacity fading after 160 cycles in 1 C at 25 C. This strategy is to improve the mass of the positive electrode and make it surpass the mass of the negative electrode to alter the voltage of the electrochemical device to a voltage at which only H.sub.2 is produced and generation of O.sub.2 is avoided. Furthermore, improving the alkalinity of the electrolyte can also help suppress the H.sub.2 production. Then desirable stability may be achieved with the electrochemical device.

[0055] The positive electrode and the negative electrode can be fabricated by any method known in the art. For instance, an electrode can be prepared by compressing a mixture of an active material, a support material (such as carbon black) and a binder (such as polytetrafluoroethylene) against a stainless steel grid or a titanium (Ti) mesh. Alternatively, an electrode can be fabricated by applying a coating slurry onto a metallic foil (e.g. titanium (Ti), copper (Cu) and aluminium (Al)) or a carbon paper wherein the coating slurry contains an organic solvent, an active material, conducting particles and a binder.

[0056] For the electrochemical device disclosed herein, at least one layer of nanoparticles is disposed onto the positive electrode. The at least one layer of nanoparticles is capable of being used to form a local hydronium ion rich environment and thereby suppressing oxygen production at the positive electrode during operation of the device. Without being bound by any theory, it is believed that nanoparticles such as Ni-based nanoparticles can promote water dissociation and, as a result, large amounts of H.sup.+ and OH.sup. are produced in the at least one layer of nanoparticles due to the water dissociation. Then the strong interaction between Ni and OH helps to confine OH to the surface of the nanoparticle layer rather than escape to the surrounding electrolyte. However. H.sup.+ has a poor interaction with Ni nanoparticles and they will tend to bond with the nearby water molecules to form H.sub.3O.sup.+ around the nanoparticles layer, which leads to a local H.sub.3O.sup.+ rich environment.

[0057] The nanoparticles used herein may be based on Ni, Pt, Fe, Co, Pd and/or Cu and may further contain a support. Examples of the support within the nanoparticles include, but are not limited to, carbon black, carbon nanotubes, graphite, graphitised carbon black, graphene, reduced graphene oxide (rGO) and combinations thereof. To form a layer of these nanoparticles, it is possible to use a membrane substance such as Nafion-Na. Nafion perfluorosulfonic acid (PFSA) membranes are based on a PFSA/polytetrafluoroethylene (PTFE) copolymer and have low ion transport resistance. Nafion products are commercially available from Chemours (formerly DuPont), Delaware, United States and in the types of Nafion 117. Nafion 115, Nafion 212, Nafion 211, etc.

[0058] In some embodiments, the nanoparticles used herein may include Ni/C, Pt/C, Fe/C, Co/C, Pd/C, Cu/C, PtNi/C, PtFe/C, PtCo/C, PtCu/C, PdNi/C, Ni/rGO, Pt/rGO, Fe/rGO, Co/rGO, Cu/rGO, Pd/rGO, PtNi/rGO and PdNi/rGO nanoparticles. In some specific embodiments, the nanoparticles are selected from the group consisting of Ni/C, Fe/C, Co/C and Cu/C nanoparticles. Taking the Ni/C nanoparticles as an example, they can be nanoparticles with a Ni loading of about 1% by weight to about 40% by weight, for example, about 5% by weight, 10% by weight, 15% by weight, 20% by weight. 25% by weight, 30% by weight, 35% by weight, 40% by weight. It may be preferable that the nanoparticles are Ni/C nanoparticles and/or Co/C nanoparticles, and that the Ni loading and/or the Co loading is about 1% by weight to about 40% by weight, for example about 20% by weight. Many of the nanoparticles mentioned herein are commercially available from Fuel Cell Store, Texas, United States of America. Non-limiting examples are 10% Nickel on Vulcan, 20% Nickel on Vulcan, 40% Nickel on Vulcan, 10% Iron on Vulcan, 10% Cobalt on Vulcan, and 40% Platinum Nickel (1:1 ratio) on Vulcan.

[0059] It is possible for the nanoparticles to have an average particle size ranging from about 1 nm to about 100 nm. In some embodiments, the average particle size of the nanoparticles ranges from 40 nm to 60 nm. The average particle size may be determined by means of, for example, transmission electron microscopy (TEM). See FIG. 1 (b).

[0060] The at least one layer of nanoparticles may have a thickness of about 5 m to about 100 m. If it is too thick, the cost will increase and the capability of ion transportation will be compromised. If it is too thin, the at least one layer of nanoparticles might not be sufficient to form a local hydronium ion rich environment at the positive electrode. Measurement of the thickness can be performed by use of a spectrometer.

[0061] The nanoparticles described above can be disposed onto the positive electrode by solution casting. For example, nanoparticles, a membrane substance and a solvent are combined to prepare a solution, which is then cast onto the positive electrode. After the solvent is removed, the positive electrode will be coated with a layer of the nanoparticles.

[0062] The negative electrode and the positive electrode are connected to each other by an aqueous electrolyte. The aqueous electrolyte for the electrochemical device disclosed herein is required to have an alkaline pH. It is believed that increasing the pH is helpful to effectively suppress hydrogen production at the anode. For the purpose of the present disclosure, the pH of the aqueous electrolyte may be chosen to be about 9 to about 13, for example about 9, 9.5, 10, 10.5, 11, 11.5, 12, 12.5 and 13. In some embodiments, the pH of the aqueous electrolyte is about 12 to about 13. The aqueous electrolyte can be adjusted to a desirable pH by use of a suitable alkaline, for example, NaOH and KOH.

[0063] The electrolyte plays a key role in transporting positive ions between the positive electrode and the negative electrode. In choosing an electrolyte for the aqueous electrolyte, at least the following factors may be considered: (i) chemical inertness: (ii) wide liquidus range and thermal stability: (iii) wide electrochemical stability window: (iv) high ionic and no electronic conductivity: (v) interphase properties; and (vi) availability. In some circumstances, it may be desirable for the electrolyte disclosed herein to be modified by introducing corrosion inhibitors or complexing agents in order to make the electrolyte less corrosive. It is also possible to introduce an additive into the aqueous electrolyte to optimise electrochemical performance in the electrochemical device disclosed herein. For the purpose of illustration, the electrolyte to be used for ALIBs may include LiPF.sub.6, LiClO.sub.4. LiAsF.sub.6, lithium bis(fluorosulfonyl)imide (LiFSI), lithium bis(trifluoromethylsulfonyl)imide (LiTFSI), LiCF.sub.3SO.sub.3 and combinations thereof. The electrolyte to be used for ASIBs may include sodium perchlorate (NaClO.sub.4), sodium trifluoromethanesulfonate (NaCF.sub.3SO.sub.3), sodium bis(fluorosulfonyl)imide (NaFSI), sodium bis(trifluoromethylsulfonyl)imide (NaTFSI), sodium nitrate (NaNO.sub.3), sodium sulphate (NaSO.sub.4), sodium chloride (NaCl), sodium acetate (CH.sub.3COONa), sodium carbonate (Na.sub.2CO.sub.3), sodium hexafluorophosphate (NaPF.sub.6) and combinations thereof. NaClO.sub.4 may be a preferred electrolyte salt for a low-cost, high-voltage sodium aqueous electrolyte with a wide electrochemical stability window.

[0064] In the case that the aqueous electrochemical device is an aqueous sodium ion battery, the aqueous electrolyte having an alkaline pH may be a water-in-salt electrolyte solution. It is believed that the formation of a solid electrolyte interphase layer with a high salt concentration on the electrode surface can prevent water reduction, thus positively contributing to a wide electrochemical stability window. Preferably, the salt is selected from sodium perchlorate (NaClO.sub.4), sodium trifluoromethanesulfonate (NaCF.sub.3SO.sub.3), sodium nitrate (NaNO.sub.3), sodium chloride (NaCl), sodium sulfate (Na.sub.2SO.sub.4), sodium acetate (CH.sub.3COONa), sodium carbonate (Na.sub.2CO.sub.3), sodium hexafluorophosphate (NaPF.sub.6) and combinations thereof. More preferably, the water-in-salt electrolyte solution is a saturated aqueous solution of sodium perchlorate. That is, the concentration of sodium perchlorate in the water-in-salt electrolyte is about 17 mol/kg at 25 C. which is the highest among the other common sodium salts such as CH.sub.3COONa: 5.7 mol/kg: NaCl: 6.1 mol/kg. NaNO.sub.3: 10.3 mol/kg.

[0065] In fabricating the electrochemical device disclosed herein, other components such as a separator, a binder, a conductive agent and a current collector may be employed. A separator serves to provide a barrier with no electrical conductivity between the negative electrode (anode) and the positive electrode (cathode) while allowing ion transport from one electrode to the other electrode. The separator is expected to retain chemical stability in the electrolyte while also having a high affinity for the electrolyte. Non-limiting examples of the separator include glass fibre separators, polyolefin separators and nonwoven separators. When powdered materials are used for the electrodes, a binder may be added in the electrodes to bring various components together and provide consistent mixing of electrode components so as to allow the electrodes to conduct the requisite amount of electrons and guarantee electronic contact during cycling of the electrochemical device. Non-limiting examples of the binder include polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF) and carboxymethyl cellulose (CMC). A conductive agent may be used to enhance conductivity of an electrode, examples of which include, but are not limited to, carbon black, Ketjen black, graphene, conductive nano carbon fiber (VGCF), carbon nanotubes (CNTs), and multi-walled carbon nanotubes (MWCNTs). A current collector is a bridging component that collects electrical current generated at the electrodes and connects with external circuits. It could have great influence on capacity, rate capability and long-term stability of the electrochemical device. Non-limiting examples of the current collector include aluminium (Al) foil, copper (Cu) foil, Titanium (Ti) mesh, stainless steel mesh, carbon-coated aluminium, carbonaceous materials.

[0066] The aqueous electrochemical device disclosed herein may be advantageous in many aspects and especially achieve significant improvement in electrochemical performance and stability. Therefore, the aqueous electrochemical device disclosed herein is very promising to satisfy the stringent requirements about electrochemical performance, stability, cost effectiveness and safety.

[0067] In particular, the aqueous electrochemical device may exhibit an energy density of about 82 Wh kg.sup.1 at 0.5 C or even at least about 90 Wh kg.sup.1 at 0.5 C. Energy density is the measure of how much energy the electrochemical device contains in proportion to its weight. The energy density can be calculated using the formula: Average Battery Discharge Voltage (V) x Battery Discharge Specific Capacity (mAh)/Total Weight of Electrodes (g)=Specific Energy or Energy Density (Wh/kg).

[0068] In addition, or alternatively, the aqueous electrochemical device may have a cycling life over 14,000 cycles at 10 C (1 C=118 mA/g). In some instances, the aqueous electrochemical device may have a cycling life of up to 200 cycles at 1 C, for example 200 cycles at 1 C, 150 cycles at 1 C and 100 cycles at 1 C. The cycling life is the number of charge and discharge cycles that the electrochemical device can complete before losing performance. The voltage range is 0.5 to 2.2 V, and the temperature is 25 C. The batteries are first charged to 2.2 V and then discharged to 0.5 V at 25 C.

[0069] In addition, or alternatively, the aqueous electrochemical device may show a capacity retention of 86% at 0.5 C after 200 cycles at 30 C. (Voltage range: 0.5 V to 2.2 V). Moreover, the aqueous electrochemical device may demonstrate a high capacity of 32 mAh and superior stability under harsh conditions.

[0070] In the case of a Na.sub.2MnFe(CN).sub.6/NaTi.sub.2(PO.sub.4).sub.3 pouch cell with a similar electrode loading of about 20 mg cm.sup.2, the aqueous electrochemical device may exhibit an average Coulombic efficiency of 99% and retains 85% capacity after 1,000 cycles at 1 C. In the case of a 50 mAh Na.sub.2MnFe(CN).sub.6/NaTi.sub.2 (PO.sub.4).sub.3 pouch cell with an electrode loading over 30 mg.Math.cm.sup.2, the aqueous electrochemical device may demonstrate a capacity retention of nearly 100% after 200 cycles at 300 mA g-1 at 25 C.

[0071] On this basis, a method of fabricating an aqueous electrochemical device comprising a negative electrode, a positive electrode, a separator, and an aqueous electrolyte having an alkaline pH has been developed. The method includes applying onto the positive electrode at least one layer of nanoparticles capable of forming a local hydronium ion rich environment at the positive electrode during operation of the device, and/or making the capacity ratio between the negative electrode and the positive electrode less than 1 (i.e. the N/P capacity ratio<1) so as to substantially avoid production of oxygen at the positive electrode. Methods of fabricating an electrochemical device, such as a battery, are known in the art and can be adapted to the present disclosure. The at least one layer of nanoparticles that is used to form a local hydronium ion rich environment at the positive electrode can be properly selected and applied onto the positive electrode with reference to the detailed description herein and the Examples.

[0072] Also disclosed herein is a positive electrode for an aqueous electrochemical device, which has disposed thereon at least one layer of nanoparticles capable of forming a local hydronium ion rich environment at the positive electrode during operation of the device. The nanoparticles, the at least one layer of nanoparticles, the positive electrode, and the electrochemical device may be those described herein above.

[0073] Also disclosed herein is a method of preparing a positive electrode for an aqueous electrochemical device, wherein the method includes applying onto the positive electrode at least one layer of nanoparticles capable of forming a local hydronium ion rich environment at the positive electrode during operation of the device. The nanoparticles, the at least one layer of nanoparticles, the positive electrode, and the electrochemical device may be those described herein above.

[0074] Surface treatment of the positive electrode by the at least one layer of nanoparticles can induce a local hydronium ion rich environment at the positive electrode in an aqueous electrolyte with an alkaline pH. It has been found by the inventors that this surface treatment may be used to suppress oxygen production at the positive electrode and stabilise the positive electrode. In the case of a Mn-rich Prussian Blue Analogue (PBA) such as Na.sub.2MnFe(CN).sub.6 as a positive electrode material and at least one layer of Ni-based nanoparticles disposed onto the positive electrode, it has been found by the inventors that formation of a local hydronium ion rich environment at the positive electrode in an alkaline (or high pH) electrolyte can also facilitate in-situ substitution of Ni.sup.2+ for Mn and thereby enhance stability of an aqueous electrochemical device. On this basis, significant improvement in electrochemical performance and stability of the aqueous electrochemical device can be achieved. In addition, or alternatively, the capacity ratio between the negative electrode and the positive electrode can be selected to be less than 1 so as to substantially avoid production of oxygen at the positive electrode. This cathode sacrifice strategy combined with an alkaline pH (or high pH) electrolyte which assists in suppressing production of hydrogen at the negative electrode may also contribute to improvement in electrochemical performance and stability of the aqueous electrochemical device. It is expected that the aqueous electrochemical device disclosed herein may find particular use in large-scale energy storage.

EXAMPLES

Experiments in Relation to Application of at Least One Layer of Nanoparticles

Preparation of Materials

[0075] Na.sub.2MnFe(CN).sub.6 was synthesized by a co-precipitation method.sup.17. 5 mmol Na Fe(CN).sub.6 (Sigma-aldrich) and 15 g NaCl (Sigma-aldrich) were dissolved into 100 mL deionized water to form solution A. 5 mmol MnCl.sub.2 (Sigma-aldrich) was dissolved into 50 mL deionized water to form solution B. Then, solution B was slowly (over about 20 minutes) dropped into solution A with stirring, and then stirring was continued for 2 h. The solid phase was obtained by centrifuging the prepared solution and washing three times with 30 mL of deionized water. Then, the solid phase was dried and ground into a powder, and dried in a vacuum oven at 110 C. for 24 hours before use.

[0076] NaTi.sub.2(PO.sub.4).sub.3/C was synthesized via a sol-gel method.sup.17. Typically, 2.5 mmol CH.sub.3COONa.Math.3H.sub.2O (Sigma-aldrich) and 7.5 mmol NH.sub.4H.sub.2PO.sub.4 (Sigma-aldrich) were dissolved into 100 mL deionized water to form solution C. 0.4 g polyvinylpyrrolidone (Sigma-aldrich) and 5 mmol Ti(CH.sub.3CH.sub.2CH.sub.2CH.sub.2O).sub.4 (TCI) (Sigma-aldrich) were dissolved in 50 mL anhydrous ethanol to form solution D. Next, solution D was poured into solution C quickly with rigorous stirring, and the resulting mixed solution was stirred continuously for 3 hours and was evaporated to remove the solvent at 80 C. in order to prepare the precursor. The obtained precursor was ground and calcined at 800 C. for 12 hours in an argon flow to obtain the NTP/C composite. The carbon content of the NTP/C composite was 5%.

[0077] Nafion-Na was prepared by the following method: the purchased Nafion 115 (DuPont, D520, 5 wt %) was neutralized to pH=7 by 0.01 M NaOH solution drop by drop. Subsequently, the solution was ion exchanged in distilled water for 12 h. Finally, the product was collected after removing the solvent at 60 C.

Preparation of Na.sub.2MnFe(CN).sub.6 (NMF) Positive Electrode and NaTi.sub.2(PO.sub.4).sub.3 (NTP) Negative Electrode

[0078] The positive electrode using NMF was prepared by mechanically mixing 80 wt % NMF, 10 wt % SuperP carbon black, and 10 wt % polytetrafluoroethylene (PTFE) binder dispersed in ethanol solvent. Then the mixture was pressed on a Ti-mesh at a pressure of 6 MPa and dried at 70 C. for 2 h. The NTP negative electrode was prepared by the same procedure with 80 wt % NTP, 10 wt % SuperP carbon black, and 10 wt % PTFE. The mass loading of electrodes is 20 mg/cm.sup.2. The N/P is 1.05-1.

Preparation of Aqueous Electrolyte

[0079] 41.6 g NaClO.sub.4 was dissolved in 20 mL water to obtain 17 M NaClO.sub.4. An alkaline electrolyte is obtained by adding a desired amount (0.1 mL, 0.2 mL, 0.4 mL) of 1 mol/L NaOH water solution into 30 mL of 17 m NaClO.sub.4. The pH of the electrolyte is 12.75. To eliminate the influence of concentration changes, the same amount (0.1 mL, 0.2 mL, 0.4 mL) of pure water was added into 17 M NaClO.sub.4 to obtain a neutral electrolyte.

Application of a Layer of Nanoparticles onto the Positive Electrode

[0080] A solution of nanoparticles was prepared as follows: 0.1 g Nafion-Na was dissolved in 0.45 g N, N-Dimethylformamide (DMF) and 0.9 g isopropanol mixed solution at 60 C.; then 0.025 g Ni/C (with a 20% Ni loading, purchased from Fuel Cell Store, Texas, United States of America) was added into the above solution and stirred for 0.5 hours and sonicated for 0.5 hours. The above procedures were repeated three times to obtain an even mixture. Then, 10 L of the solution was dropped on the surface of positive electrode discs. After removing the solvent at room temperature, the positive electrode discs were coated with a layer of the Ni-based nanoparticles. The particles-size of Ni-based nanoparticles is about 50 nm, and the thickness of the nanoparticles layer is about 5 m.

Assembly of a Coin Cell

[0081] The negative electrode was placed on a smaller cell cap. A glass fiber separator was disposed onto the negative electrode as centered as possible, and a desired amount of electrolyte was dropped onto the separator. A positive electrode was placed on top of the separator, with the cast nanoparticle layer facing the negative electrode. The positive electrode was centered as much as possible with the negative electrode to avoid uneven current densities. A stainless steel mesh and a spring were placed in order. A larger cap was placed on top and pressed to seal.

Assembly of a Pouch Cell

[0082] A pouch cell was assembled by using a stacking machine, the glass fibre separator was placed between the electrodes, forming a stack that was inserted in the pouch. The sides of the pouch were joined together by heat sealing, leaving one side open. An electrolyte filling system was then used to add a liquid electrolyte into the cell. Then the cell was sealed using a vacuum sealing machine, and the pouch cell assembly was complete.

Results

[0083] It is shown by FIG. 1 and FIG. 2 that after applying a layer of 20% Ni/C nanoparticles to create a local hydronium ion rich environment at the positive electrode, the onset of OER has been pushed from 1 V to 1.2 V.

[0084] Application of a layer of Ni/C nanoparticles to create a local hydronium ion rich environment at the positive electrode in sodium aqueous batteries has unexpectedly improved the electrochemical performance of NMF/NTP cells. As shown in FIG. 3. NMF/NTP full cells cycled in neutral and alkaline electrolyte displayed a very poor rate performance, as well as the low Coulombic efficiency at low rate (less than 80% for neutral electrolyte and less than 85% for alkaline electrolyte at 0.5 C) and low capacity at high rate (less than 40 mAh g.sup.1). In sharp contrast, after applying a layer of Ni/C nanoparticles at the positive electrode, the battery delivered a reversible capacity of 118, 117, 100 and 88 mAh g.sup.1 at current density of 0.5 C. 1 C. 5 C and 10 C respectively. (FIG. 4a). The battery with a layer of Ni/C nanoparticles at the positive electrode displayed an impressive stability during high rate. Besides, the gradually decreased discharge average voltage (DAV) of the batteries cycled in neutral and alkaline electrolyte also indicated the instable nature of aqueous batteries, which would compromise the energy density during both cycling and storage (FIG. 4b). However, the surface treatment of the positive electrode can effectively stabilise DVA at low rate and also guarantee the battery a high DVA (1.2 V) at high rate of 10 C. Then, the cycling performance of the batteries was examined at a low rate of 0.5 C. in FIG. 5. The capacity of batteries using the surface treatment of the positive electrode was much higher than other systems. More importantly, the batteries cycled in neutral electrolyte showed a pretty low Coulombic efficiency which was lower than 80% and also gradually decreased due to severe side reactions. After adding NaOH in the electrolyte to increase the pH, the Coulombic efficiency increased to 85% due to suppression of HER. However, after applying a layer of Ni/C nanoparticles at the positive electrode, the Coulombic efficiency greatly increased to over 96%. The battery with the surface treatment of the positive electrode can also achieve an improved performance at 1 C with no obvious capacity fading (FIG. 4c). More importantly, the battery with the surface treatment of the positive electrode can stably cycle under a harsh environment of 30 C. with a capacity retention of 86% at 0.5 C. after 200 cycles (FIG. 4d), which exceeds most previous reported aqueous batteries.sup.18-19. Most importantly, the battery with the surface treatment of the positive electrode achieved an unprecedented long-cycling life of over 14000 cycles at 10 C as well as a favorable capacity retention of 56% with high electrode loadings (20 mg cm.sup.1. FIG. 4c).

[0085] The current work was compared with previous reported works. As seen in FIG. 6 and Table 1, among recently reported sodium aqueous batteries, the battery according to the present disclosure possesses the highest electrode loading and energy density, the longest lifespan, as well as the lowest costs. Even if the average voltage is limited due to limitations of the electrodes, it still can reach a high value of 1.4 V. Compared with recently reported aqueous Li, Na, K batteries with favorable stability using expensive F-contained salts, our work, which uses cheap NaClO.sub.4 single water solution, produced undeniable advantages. As seen in FIG. 7a (the prices of salts are based on the data from the sigma in Australia and the costs of solvents are overlooked), the costs of regular WIS electrolytes are very expensive, such as 21 m bis(trifluoromethane) sulfonimide lithium salt (LiTFSI).sup.2, 21 m KOTF.sup.19 and 9 M NaOTF.sup.20. Some researchers have introduced a large number of organic solvents to reduce the use of salts as well as the costs, but these caused safety problems.sup.21-23. In sharp contrast, by using a single water solvent NaClO.sub.4 solution with the surface treatment strategy, the electrochemical device of the present disclosure can achieve good electrochemical performance at very low cost (at least 40 times less than regular WIS strategy based on F-contained salts). Due to the relatively limited storage and high price of Li, aqueous Li ion batteries may not be suitable for large-scale energy storage. Thus, the inventors compared the recently reported aqueous Na and K batteries in relation to energy density and lifespan (FIG. 7b). Even with limitations by the electrodes, the energy density of the ASIB of the present disclosure is slightly lower than one previously reported work. However, the lifespan of the battery according to the present disclosure (14000 cycles) is twice that of the battery at second place (6500 cycles). The ASIB of the present disclosure is a promising candidate for practical application in large-scale energy storage. Then, the inventors compared this ASIB with other electrochemical storage systems. As can be seen in FIG. 7c and Table 2, even if the energy density of the ASIB of the present disclosure is lower than Li-ion and NiMn batteries, it demonstrates remarkable advantages in respect of abundance of the key elements, safety, environmental friendliness over all other batteries and has an ultra-long lifespan, which makes it a promising candidate for large-scale energy storage.

TABLE-US-00001 TABLE 1 Comparison of recently reported sodium aqueous batteries Average Energy Cathode Electrolyte cost voltage Cycling density loading (AUD based on the System (V) life (Wh kg.sup.1) (mg/cm.sup.2) price in sigma) Ni/C coated NMF/NTP 1.4 13000 82 20.45 1040.6 Na.sub.3V.sub.2(PO.sub.4).sub.3//NTP.sup.3 ~1.1 100 ~64 5 Over 15665.8 Na.sub.0.66Mn.sub.0.66Ti.sub.0.44O.sub.2//NTP.sup.4 1 1200 31 16124.0 Na.sub.1.88Mn[Fe(CN).sub.6].sub.0.971.35H.sub.2O//NaTiOPO.sub.4.sup.5 1.74 800 71 Over 15665.8 NaMnHCF//KMnHCC.sup.6 1.7 100 58 1.7 330461.6

TABLE-US-00002 TABLE 2 Comparison of our works with commercial batteries Abundance of key Energy elements in the density Environment Cost- System Earth's crust Lifespan (Wh kg.sup.1) Safety friendliness efficacy Ni/C coated Fe (41000) 15000 ~50 High High High NMF/NTP Mn (950) Na (23000) Ti (5600) Ni (80) Lead-acid.sup.7,8 Pb (14) 300-1000 25-40 High Low High NiMH.sup.24 Ni (80) 200-1200 50-85 Medium Medium Medium La (32) Li-ion Li (20) 160-2000 160 Low Medium Medium (LFP/graphite).sup.16 Supercapacitor.sup.24 C (480) 5000-10.sup.5 5-10 High High Low Flow battery V (120) 5000-10.sup.4 10-25 High Low Medium (Vanadium redox battery).sup.7,8

[0086] Safety is one of the key parameters to evaluate the effectiveness of a battery system suitable for large-scale energy storage. Thus, the inventors assembled an NMF/NTP cell to conduct tests under very harsh environments. As seen in FIG. 8a, the output voltage of the pouch cell is 1.775 V, which is much higher than normal sodium aqueous batteries. Benefiting from this high voltage, two cells can power the blue lights (the lower voltage requirement of blue light is 3 V). More intriguingly, the pouch cell displayed incomparable stability in the cut experiment (FIG. 8c). The pouch cell can be cut and immersed in water without its function of powering the lights being compromised. The high capacity of the pouch cells also showed a strong performance of powering an electric fan before or after being cut and immersed in water (FIG. 8e). Besides, the pouch cell can be recharged to 2.2 V after being cut and powered a fan in water, displaying an outstanding stability (FIG. 8f). Most importantly, the recharged cut pouch cell can continuously power a hygrometer in water over 10 hours (FIGS. 8g-h). This means the batteries can withstand electrolyte leaks in high humidity environments (even in the water) without causing serious damage to the whole system while maintaining the ability to power an electric equipment, which leads to a great improvement in the safety of large-scale energy storage and multiple applications in underwater electrical equipment.

DISCUSSIONS

[0087] The unprecedented electrochemical performance of electrochemical devices of the present disclosure can be attributed to the local hydronium ion rich environment created at the positive electrode. In order to test the assumptions, in-situ IR was used to verify the generation of H.sub.3O.sup.+ (FIGS. 9a and 9b). To rule out the influence from the electrolyte and nanoparticles per se, reference spectra (with no potential applied) were taken as background for each group of tests. Carbon black (C) was taken as a control group to eliminate the influence of the polymer support and carbon per se. As expected, there is no obvious change in the spectra of C even the voltage raised to 1.3 V, indicating the C and polymer support cannot create a local hydronium ion rich environment. In contrast, for the Ni/C in the alkaline electrolyte, new peaks showed up with an applied potential higher than 0.6 V. The peaks at 2218 cm.sup.1 and 1810 cm.sup.1 appeared, which could be attributed to the two asymmetric OH stretching modes of

[00001]$H_3{O}^{+}(v_{H_3{O}^{+}}^{a^1}\mathrm{and}{v}_{H_3{O}^{+}}^{a^1},$

FIG. 9a). Moreover, the peak of the resonance state of the asymmetric OH stretching modes of H.sub.3O.sup.+ at 2020 cm.sup.1

[00002]$\left(v_{H_3{O}^{+}}^{a^1+a^2}\right),$

as well as the distinct and isolated peak at 1230 cm.sup.1 corresponding to the umbrella vibration

[00003]$H_3{O}^{+}\left(v_{H_3{O}^{+}}^u\right),$

also appeared in the spectrum. To conclude, the in-situ IR has provided clear evidence for the generation of H.sub.3O.sup.+.

[0088] The Ni/C coating layer (FIG. 9d) induces a gap between the coating layer and the cathode layer, which can accommodate the H.sub.3O.sup.+ and separate it from the bulk alkaline electrolyte. Ni nanoparticles can promote water dissociation, which has been proved in previous catalysis studies.sup.25,26. As a result, large amounts of H.sup.+ and OH.sup. are produced around this layer due to the water dissociation, as illustrated in FIG. 9c. The strong interaction between Ni and OH.sup. helps to confine OH.sup. on the surface of the Ni nanoparticles which makes it difficult to escape to the surrounding solution. However. H.sup.+ has poor interaction with Ni nanoparticles in an alkaline medium and will bond with nearby water molecules to form H.sub.3O.sup.+. These H.sub.3O.sup.+ ions exposed to the bulk alkaline electrolyte will be easily neutralized by excess OH. In contrast, due to the blocking effect of the coating layer. H.sub.3O.sup.+ ions will accumulate underneath the layer, leading to a H.sub.3O.sup.+-rich environment on the cathode surface, which in turn suppresses OER during the battery operation.

[0089] The high pH of electrolyte not only causes the OER of water, but also intensifies the issue of cathode dissolution. Prussian Blue Analogues (PBAs) are promising cathode materials for sodium batteries due to their environmental friendliness and facile intercalation/deintercalation mechanism.sup.27,28. However, hydroxide anions can interact with N-coordinated metal atoms and then rupture the PBAs.sup.29,30. Besides, some OH species will adsorb at a cathode with operating potentials close to the OER, further promoting detrimental side reactions.sup.31. Moreover, this problem is aggravated in Mn-rich PBAs like NMF due to Mn dissolution driven by the disproportionation reaction of Mn.sup.3+ and Jahn-Teller (JT) distortion.sup.32. As shown in FIGS. 11a and 11b, the second plateau is missing in the charge-discharge curves of the NMF/NTP electrodes in the neutral electrolyte and alkaline electrolyte due to the above-mentioned reasons. However, for the batteries after application of the surface treatment to the positive electrode, the second plateau is ultra-stable even after 40 cycles (FIG. 11c). It is believed that there are two major contributions to the ultra-stability of the PBA cathode in alkaline electrolyte. First, as mentioned above, a local hydronium ion rich environment at the cathode was created by the surface treatment of the positive electrode. Then, the H.sub.3O.sup.+ rich surface layer prevents the OH species from adsorbing onto the surface of the cathode, therefore reducing dissolution of Mn. Secondly, according to literature.sup.33, the capacity and cycling performance can be improved for Mn-based electrodes if electrochemically active cations (such as Ni.sup.2+ and Co.sup.2+) are substituted for the Mn. Thus, it is believed that this unusual stability of NMF also can be attributed to the in-situ substituted Ni.sup.2+ for Mn due to the oxidation of Ni particles in the nanoparticle layer during the charging process. In this regard. Raman spectroscopy was applied to verify our assumption (FIG. 11d). Peaks in the range of 20502200 cm.sup.1 in Raman, which were assigned to the CN.sup. groups, indicate that the transition-metal ions bonded to the CN groups exhibit different valence states.sup.34. In Raman spectra of NMF electrodes cycled in neutral and alkaline electrolyte, there are three peaks at 2137 cm.sup.1 and 2158 cm.sup.1 belonging to Fe.sup.2+NCMn.sup.2+ and Fe.sup.2+NCMn.sup.3+ vibrations respectively. In comparison, the Raman spectrum of the positive electrode with the surface treatment cycled in alkaline electrolyte presented two shifted peaks at 2130 cm.sup.1 and 2150 cm.sup.1 respectively. In addition, a new weak peak attributing to Fe.sup.2+NCNi.sup.2+ is observed at 2163 cm.sup.1. All of these changes suggest that Ni was being introduced into the structure of NMF after it was cycled in alkaline electrolyte with local microenvironment (LME). Then TEM and EDS spectra were also used to verify the existence of Ni in the cathode. As seen in FIG. 11e, the crystal structure of NMF is well preserved after applying the layer of nanoparticles, but the crystal structure is destroyed in neutral electrolyte and alkaline electrolyte. EDS of cycled NMF cathode also indicates the existence of the Ni peak at 0.82 keV (FIG. 11f).

[0090] In summary, it is demonstrated that the surface treatment strategy disclosed herein can greatly improve the stability of aqueous electrolyte as well as the Mn-based cathode without compromising the low cost and environmental friendliness of sodium aqueous batteries. This strategy can enable an ultra-long lifespan and high energy density sodium aqueous batteries, while maintaining cost effectiveness, environment friendliness and toleration of low-temperature. More importantly, pouch cells using the surface treatment of the positive electrode strategy can achieve an unprecedented stability even after being cut and immersed in water. This represents a significant advancement in the design of aqueous batteries, both in concept and demonstration, and sets a new performance standard that promises to yield battery systems that exceed previous energy density and practical application limitations while reducing or eliminating the need for changing the battery industry infrastructure, have high compatibility with current commercial Li-ion and Na-ion battery systems, and do not need the expensive moisture-free process and the safety management required for flammable electrolytes. It is believed that the batteries based on this strategy could promote the application of aqueous batteries in large-scale energy storage and underwater equipment due to the abundance of the raw materials used, low cost, environmental friendliness, long lifespan, ultra-high stability, safety in water environment, and favourable energy density.

[0091] In addition to Ni, other metal nanoparticles were also explored in the alkaline ASIB system, including Pd, Cu, and Co (FIG. 12). Co nanoparticles also can effectively stabilise the NMF/NTP cells in alkaline electrolyte. This indicates the applicability of creating a local environment to put the high-performance alkaline ASIBs into practice.

Experiments in Relation to Cathode Sacrifice Strategy

Preparation of Na.sub.2MnFe(CN).sub.6 (NMF) Positive Electrode and NaTi.sub.2(PO.sub.4).sub.3 (NTP) Negative Electrode

[0092] The NMF and the NTP were prepared as stated above except that no nanoparticles layer was applied onto the surface of the positive electrode

Preparation of Aqueous Electrolyte

[0093] The aqueous electrolyte was prepared as stated above.

Results and Discussions

[0094] In some circumstances, the mass ratio between negative electrode and positive electrode can be reduced to less than 1, so as to improve the stability of batteries. When the mass ratio of NTP/NMF is 1:1, without presence of the nanoparticle layer, the batteries exhibit rapid capacity fading at 1 C in 25 C. However, if reducing the mass ratio between NTP and NMF to 0.75:1, the stability of batteries can be greatly improved, and the batteries achieved a capacity retention of 90% at 1 C in 25 C. (FIG. 13). When reducing the mass ratio between NTP and NMF to 0.62:1, the stability of battery was further improved (FIG. 14). When the mass ratio was reduced to 0.56:1, the battery maintained 90% capacity at high rate of 10 C (compared with capacity in 1 C, FIG. 15). It also maintained a nearly 100% capacity retention at 10 C after 1600 cycles at 25 C. (FIG. 15).

[0095] The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that such prior art forms part of the common general knowledge.

[0096] It will be understood that the terms comprise and include and any of their derivatives (e.g. comprises, comprising, includes, including) as used in this specification, and the claims that follow, is to be taken to be inclusive of features to which the term refers, and is not meant to exclude the presence of any additional features unless otherwise stated or implied.

[0097] In some cases, a single embodiment may, for succinctness and/or to assist in understanding the scope of the disclosure, combine multiple features. It is to be understood that in such a case, these multiple features may be provided separately (in separate embodiments), or in any other suitable combination. Alternatively, where separate features are described in separate embodiments, these separate features may be combined into a single embodiment unless otherwise stated or implied. This also applies to the claims which can be recombined in any combination. That is a claim may be amended to include a feature defined in any other claim. Further a phrase referring to at least one of a list of items refers to any combination of those items, including single members. As an example, at least one of: a, b, or c is intended to cover: a, b, c, a-b, a-c, b-c, and a-b-c.

[0098] It will be appreciated by those skilled in the art that the disclosure is not restricted in its use to the particular application or applications described. Neither is the present disclosure restricted in its preferred embodiment with regard to the particular elements and/or features described or depicted herein. It will be appreciated that the disclosure is not limited to the embodiment or embodiments disclosed, but is capable of numerous rearrangements, modifications and substitutions without departing from the scope as set forth and defined by the following claims.

REFERENCES

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