Claims:Claims

We claim:

1. A Zinc-Manganese dioxide cell comprising:
at least one anode comprising Zinc;
at least one cathode comprising hollandite alpha-Manganese dioxide; and
an electrolyte comprising Zinc sulphate and Manganese sulphate.

2. The Zinc-Manganese dioxide cell as claimed in claim 1, wherein the concentration of Manganese sulphate is in the range of 0.1M to 0.2M.

3. The Zinc-Manganese dioxide cell as claimed in claim 1, wherein the concentration of Zinc sulphate is in the range of 1M to 2M.

4. The Zinc-Manganese dioxide cell as claimed in claim 1, wherein pH of said electrolyte is maintained in the range of 3 to 6.

5. The Zinc-Manganese dioxide cell as claimed in claim 1, wherein said anode is high density Zinc foil.

6. The Zinc-Manganese dioxide cell as claimed in claim 1, wherein said hollandite alpha-Manganese dioxide is Bismuth-modified alpha-Manganese dioxide.

7. The Zinc-Manganese dioxide cell as claimed in claim 1, wherein said cathode further comprises a current collector.

8. The Zinc-Manganese dioxide cell as claimed in claim 7, wherein said current collector is flexible graphite foil.
9. The Zinc-Manganese dioxide cell as claimed in claim 1, wherein said hollandite alpha-Manganese dioxide is prepared by a method comprising:
treating Potassium permanganate with at least one reducing agent selected from a group consisting of Ethylene glycol and Polyethylene glycol, wherein molar ratio of Potassium permanganate and said reducing agent is 0.1:0.5; and stirring to obtain a black precipitate.

10. The Zinc-Manganese dioxide cell as claimed in claim 1, wherein said hollandite alpha-Manganese dioxide is prepared by a method comprising:
treating Potassium permanganate with Manganese sulphate in aqueous medium, wherein molar ratio of Manganese sulphate and Potassium permanganate is 1.4:1.6; and stirring to obtain a dark brown precipitate.

11. The Zinc-Manganese dioxide cell as claimed in claim 1, wherein said cell is capable of operating at a pH in the range of 3 to 6.

12. A method of constructing a Zinc-Manganese dioxide cell, comprising:
providing an electrolyte comprising Zinc sulphate and Manganese sulphate;
providing at least one anode comprising Zinc by immersing said anode into said electrolyte;
providing at least one cathode comprising hollandite alpha manganese dioxide and current collector by immersing said cathode into said electrolyte;
providing at least one electrically insulative separator between said anode and cathode; and
forming an electrical connection between said anode and cathode.

13. The method as claimed in claim 12, wherein said cell is capable of operating at a pH in the range of 3 to 6.

14. A method for producing modified alpha-Manganese dioxide electrode, said method comprising:
preparing a modified alpha-Manganese dioxide ink by adding at least one solvent to a mixture comprising hollandite alpha-Manganese dioxide, bismuth oxide, graphite particles and polyvinylidene fluoride, wherein said solvent is selected from a group consisting of n-methyl-2-pyrrolidone, triethyl phosphate, trimethyl phosphate, trimethyl urea, dimethyl formamide, dimethyl acetamide, hexamethyl phosphor amide, dimethyl sulfoxide and propylene carbonate; and
coating a current collector with said ink.

15. The method as claimed in claim 14, wherein said solvent is n-methyl-2-pyrrolidone.

16. The method as claimed in claim 14, wherein said graphite particle is expanded graphite.

17. The method as claimed in claim 14, wherein the amount of hollandite alpha-Manganese dioxide is in the range of 45 to 90 wt %; bismuth oxide is in the range of 1 to 20 wt %; graphite particles is in the range of 0 to 40 wt %; and polyvinylidene fluoride is in the range of 0 to 20 wt %.

18. The method as claimed in claim 14, wherein said hollandite alpha-Manganese dioxide is prepared by a method comprising:
treating Potassium permanganate with at least one reducing agent selected from a group consisting of Ethylene glycol and Polyethylene glycol; and
stirring to obtain a black precipitate.

19. The method as claimed in claim 18, wherein said stirring is continued for a period in the range of 20 to 30 minutes after completely adding said reducing agent.

20. The method as claimed in claim 18, wherein molar ratio of Potassium permanganate and said reducing agent is 0.1:0.5.

21. The method as claimed in claim 18, further comprising filtering said black precipitate; washing said precipitate with at least one solution selected from a group consisting of deionized water and ethanol; and drying.

22. The method as claimed in claim 14, wherein the hollandite alpha-Manganese dioxide is prepared by a method comprising
treating Potassium permanganate and Manganese sulphate in aqueous medium; and
stirring to obtain a dark brown precipitate.

23. The method as claimed in claim 22, wherein molar ratio of Manganese sulphate and Potassium permanganate is 1.4:1.6.

24. The method as claimed in claim 22, further comprising filtering said dark brown precipitate; washing said precipitate with at least one solution selected from a group consisting of deionized water and ethanol; and drying.

25. A modified alpha-Manganese dioxide electrode produced by a method claimed in claim 14.

26. A modified alpha-Manganese dioxide electrode produced by a method claimed in claim 18.

27. A modified alpha-Manganese dioxide electrode produced by a method claimed in claim 22.

28. A method for producing modified alpha-Manganese dioxide ink, said method comprising:
treating potassium permanganate with at least one reducing agent selected from a group consisting of Ethylene glycol and Polyethylene glycol to obtain a precipitate;
filtering the obtained precipitate;
washing the precipitate with at least one solution selected from a group consisting of deionized water and ethanol; and
adding a suitable amount of at least one solvent to a mixture comprising said precipitate, bismuth oxide, graphite particles, and polyvinylidene fluoride, wherein said solvent is selected from a group consisting of n-methyl-2-pyrrolidone, triethyl phosphate, trimethyl phosphate, trimethyl urea, dimethyl formamide, dimethyl acetamide, hexamethyl phosphor amide, dimethyl sulfoxide and propylene carbonate.

29. The method as claimed in claim 28, wherein molar ratio of Potassium permanganate and said reducing agent is 0.1:0.5.

30. The method as claimed in claim 28, wherein the amount of said hollandite alpha-Manganese dioxide is in the range of 45 to 90 wt %; bismuth oxide is in the range of 1 to 20 wt %; graphite particles is in the range of 0 to 40 wt %; and polyvinylidene fluoride is in the range of 0 to 20 wt %, of the total mixture.

31. A method for producing modified alpha-Manganese dioxide ink, said method comprising:
treating potassium permanganate with Manganese sulphate in aqueous medium to obtain a precipitate;
filtering said precipitate;
washing said precipitate with at least one solution selected from a group consisting of deionized water and ethanol; and
adding a suitable amount of at least one solvent to a mixture comprising said precipitate; bismuth oxide; graphite particles; and polyvinylidene fluoride, wherein said solvent is selected from a group consisting of n-methyl-2-pyrrolidone, triethyl phosphate, trimethyl phosphate, trimethyl urea, dimethyl formamide, dimethyl acetamide, hexamethyl phosphor amide, dimethyl sulfoxide and propylene carbonate.

32. The method as claimed in claim 31, wherein molar ratio of Manganese sulphate and Potassium permanganate is 1.4:1.6.

33. The method as claimed in claim 31, wherein the amount of said hollandite alpha-Manganese dioxide is in the range of 45 to 90 wt %; bismuth oxide is in the range of 1 to 20 wt %; graphite particles is in the range of 0 to 40 wt %; and polyvinylidene fluoride is in the range of 0 to 20 wt %, of the total mixture.

34. A method for producing alpha-Manganese dioxide, said method comprising:
treating potassium permanganate with at least one reagent selected from a group consisting of Manganese sulphate, Ethylene glycol and Polyethylene glycol to obtain a precipitate;
filtering the obtained precipitate; and
washing said precipitate with at least one solution selected from a group consisting of deionized water and ethanol; and drying to obtain a powder.

35. The method as claimed in claim 34, wherein said reagent is Manganese sulphate, wherein molar ratio of Manganese sulphate and Potassium permanganate is 1.4:1.6.

36. The method as claimed in claim 34, wherein said reagent is Ethylene glycol, wherein molar ratio of Potassium permanganate and Ethylene glycol is 0.1:0.5.

37. The method as claimed in claim 34, wherein said reagent is Polyethylene glycol, wherein molar ratio of Potassium permanganate and Polyethylene glycol is 0.1:0.5.

Dated this __th day of __________ 2020

Ajai Kumar Jain
For Godrej & Boyce Manufacturing Company Ltd

To,
The Controller of Patents
The Patent Office, Mumbai , Description:TECHNICAL FIELD
[001] The present invention relates to electrochemical cells, and more particularly to rechargeable Zinc-Manganese dioxide cells.
BACKGROUND
[002] Electrochemical cells, utilizing oxidation-reduction reactions to create a potential difference across electrodes, are of various types. The basic electrochemical cells using metals such as Zn, Lead, Lithium, Nickel, etc. as electrodes and their salt solutions as electrolytes, have been improvised numerously to meet new standards. The generally used cells include Zn/air cells, Lead-acid batteries, Nickel-cadmium, nickel metal hydride, Nickel iron batteries, etc. (1-2)
[003] The lead-acid batteries developed in the mid-1800s are one of the oldest and extensively used batteries commercially. They are inexpensive, simple and yet capable of high discharge rates, which make them a popular choice. However, these batteries are known to have limited full discharge cycles, low energy density, and are unfriendly to the environment.
[004] Subsequently developed, Lithium and Nickel based batteries offer better cycle count, high energy density and longer shelf life. However, the drawbacks of Nickel batteries include memory effect, relatively high self-discharge and toxicity due to cadmium. Lithium batteries, on the other hand, although better as compared to Nickel batteries in terms of memory effect and self-discharge, pose handling issues such as transportation and storage, and are also expensive.
[005] Further research and development in the field led to the emergence of, Zinc batteries, a cost effective and environmentally safe alternative for the then existing battery technologies. Although, the concept of Zinc batteries as such is relatively old, re-chargeable zinc batteries using Zinc-air technology were manufactured in late 1990s. New technologies are continuously being investigated wherein the old Zinc batteries are being resurrected by various complexation and intercalation techniques.
[006] Nickel-Zinc, Zinc-air and Zinc-Manganese dioxide batteries due to their improved specific capacity have been considered commercially more viable as compared to lithium or lead-acid batteries. Recent developments are directed towards improvising re-chargeability of Zn-MnO2 batteries. Developments to improve the cycling capacity of Zinc batteries by treatment of Zinc with metals such as antimony, tin, etc., are being made. Further, doping of Manganese dioxide with calcium, magnesium, etc., have also been performed in attempts to yield superior cycle performance. The use of gamma-Manganese di oxide (?-MnO2) has been reported in various patents and publications (3-6). In practice, however, they display a slow but steady capacity fade. Alpha-Manganese dioxide (a-MnO2) has a structure which facilitates high intercalation and de-intercalation of Zinc ions into the electrode and is considered a potential candidate for use as cathode in Zn-MnO2 systems (7). However, there is always a need for better systems that are capable of improving re-chargeability and overall efficiency of Zinc batteries.
OBJECTS
[007] The principal object of the embodiments disclosed herein is to provide an electrochemical cell having high reversibility and efficiency.
[008] A second object of the embodiments disclosed herein is to provide a rechargeable Zinc-Manganese dioxide battery.
[009] Another object of the embodiments disclosed herein is to provide a bismuth-modified hollandite a-MnO2 cathode for use in Zinc-Manganese dioxide cell.
[0010] Yet another object of the embodiments disclosed herein is to provide a Zinc-Manganese dioxide battery having high efficiency.
[0011] Another object of the embodiments disclosed herein is to provide a method for preparing hollandite a-MnO2 by one step process.
[0012] Further, another object of the embodiments disclosed herein is to provide a method for producing a modified hollandite a-MnO2 electrode for use in Zinc-Manganese dioxide cell.
[0013] These and other objects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.

BRIEF DESCRIPTION OF FIGURES
[0014] The embodiments disclosed herein are illustrated in the accompanying drawings, throughout which like reference letters indicate corresponding parts in the various figures. The embodiments herein will be better understood from the following description with reference to the drawings, in which:
[0015] Fig. 1 is a schematic representation of Electrochemical cell, according to the various embodiments disclosed herein;
[0016] Fig. 2 is the Powder X-ray diffraction (XRD) pattern of a-MnO2 obtained by one-step ethylene glycol or polyethylene glycol reduction of KMnO4 (also referred to as Route-a), according to an embodiment disclosed herein;
[0017] Fig. 3 is the Powder X-ray diffraction pattern of a-MnO2 obtained by one-step redox reaction of KMnO4 and MnSO4 (also referred to as Route-b), according to an embodiment disclosed herein;
[0018] Fig. 4 depicts the galvanostatic Charge-Discharge pattern for a-MnO2 cathode vs. Hg/Hg2SO4, K2SO4 reference electrode at C/10 rate, wherein a-MnO2 is synthesized by Route-a, according to various embodiments disclosed herein;
[0019] Fig. 5 depicts the Charge-Discharge pattern for the cell with a-MnO2 electrode at C/10 rate in the potential window between 1.0 V and 1.8 V, wherein a-MnO2 is synthesized by Route-a, according to various embodiments disclosed herein;
[0020] Fig. 6 depicts the Galvanostatic Charge-Discharge data for a-MnO2 electrode at C/10 rate in the potential window between 1.0 V and 1.8 V, wherein a-MnO2 is synthesized by Route-b, according to various embodiments disclosed herein;
[0021] Fig. 7 depicts the Specific Discharge Capacity and coulombic efficiency versus cycle number, cycling data for Electrochemical cell at C/10 rate in the potential window between 1.0 V and 1.8 V, wherein a-MnO2 is synthesized by Route-a, according to various embodiments disclosed herein; and
[0022] Fig. 8 depicts the Specific Discharge Capacity and coulombic efficiency versus cycle number, cycling data for Zn - a-MnO2 electrode at C/10 rate in potential window between 1.0 V and 1.8 V wherein a-MnO2 is synthesized by Route-b, according to various embodiments disclosed herein.

DETAILED DESCRIPTION
[0023] The embodiments herein and the various features and advantageous details thereof are explained more fully with reference to the non-limiting embodiments that are illustrated in the accompanying drawings and detailed in the following description. Descriptions of well-known components and processing techniques are omitted so as not to unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.
[0024] The embodiments herein achieve an electrochemical cell having zinc (Zn) and manganese dioxide (MnO2) electrodes. The disclosed electrochemical cell is a Zinc-Manganese dioxide (Zn-MnO2) cell which includes hollandite alpha Manganese dioxide (a-MnO2) as cathode and Zinc as anode. The hollandite a-MnO2 electrode used in the various embodiments herein is bismuth-modified hollandite a-MnO2. Accordingly, disclosed are embodiments of a method for producing bismuth-modified hollandite a-MnO2 electrodes. Further, the various embodiments disclosed herein also include methods for preparing hollandite a-MnO2. The hollandite a-MnO2 may be obtained by single step reduction reactions involving reduction of potassium permanganate. The disclosed Zinc-Manganese dioxide cell are highly reversible, having low or no dendritic growth, and have improved efficiency.

Electrochemical cell
[0025] The embodiments herein disclose electrochemical cell having improved columbic and energy efficiency. The term “Electrochemical cell” disclosed in the various embodiments herein refer to systems that are capable of converting chemical energy to electrical energy and/or vice versa. Typically, electrochemical cells comprise two or more electrodes immersed in at least one electrolyte which is the ionic conductor. The electrochemical cell disclosed in the various embodiments herein is Zn-MnO2 cell. The disclosed Zn-MnO2 cell is such that it is capable of operating in mildly acidic pH, in the range of 3 to 6. In an embodiment, the Zn-MnO2 cell is capable of operating at a pH of 6. The various embodiments of the Zn-MnO2 cell disclosed herein are capable of achieving reduced or no dendritic growth.
[0026] The embodiments of the electrochemical cell disclosed herein includes zinc and alpha manganese dioxide electrodes. In an embodiment, the Zn-MnO2 cell comprises at least one anode comprising Zinc, at least one cathode comprising a-MnO2, and an electrolyte comprising Zinc sulphate and Manganese sulphate.
[0027] A method of constructing a Zinc-Manganese dioxide cell is also disclosed in the various embodiments herein. In an embodiment, the method comprises providing an electrolyte comprising Zinc sulphate and Manganese sulphate; providing at least one anode comprising Zinc by immersing said anode into said electrolyte; providing at least one cathode comprising hollandite alpha manganese dioxide by immersing said cathode into said electrolyte; providing at least one electrically insulative separator between said anode and cathode; and forming an electrical connection between said anode and cathode. In an embodiment, the method comprises coating the hollandite alpha manganese dioxide on the surface of a current collector prior to its immersion into the electrochemical cell. In an embodiment, the electrically insulative separator is wrapped around the anode and cathode prior to its immersion into the electrochemical cell. In an embodiment, the Zinc-Manganese dioxide cell is constructed such that it is capable of operating at a pH in the range of 3 to 6. In an embodiment, the Zinc-Manganese dioxide cell is capable of operating at a pH of 6. The electrical connection between the anode and the cathode may be formed by methods generally known in the field.
Anode
[0028] The anode according to the various embodiments herein comprises primarily of Zinc or Zinc particles. In an embodiment, the electrochemical cell comprises Zinc metal as anode. The zinc particles include zinc-based particles such as zinc dust, zinc fines, etc of various sizes and shapes that are generally known in the field. The zinc-based particles may be of pure zinc or zinc alloys. The anode may also be, but not limited to, zinc in the form of sheets, rods, mesh or foils. In an embodiment, the anode is high density zinc foil of suitable dimension (e.g.: Zinc foil having dimension of 30 mm X 30 mm X 0.4 mm). The anode may further comprise of current collectors in ways that are generally known in the field. Any suitable current collectors that are generally known in the field may be used. It would be apparent to a person skilled in the art that various other Zinc based electrodes that are generally known in the field may also be used without departing from the scope of the claimed embodiments.
Cathode
[0029] The cathode, according to the various embodiments herein, comprises manganese dioxide. The manganese dioxide used in the preparation of the electrode, according to the various embodiments herein, is alpha (a) polymorph of Manganese dioxide having hollandite-type structures which is also referred to as hollandite alpha manganese dioxide or hollandite a-MnO2. In some embodiments, the hollandite a-MnO2 is treated with bismuth oxide to obtain bismuth-modified a-MnO2 which is used as cathode. In an embodiment, the Zn-MnO2 cell comprises bismuth-modified hollandite a-MnO2 as cathode. The modified hollandite a-MnO2 obtained may be used to coat a current collector. In an embodiment, the cathode instrumental in the various embodiments herein further comprises a current collector. In an embodiment, the cathode comprises at least one current collector having a coating of bismuth-modified hollandite a-MnO2.
[0030] The current collector that may be used in the various embodiments herein may include any current collectors generally known to be used in electrochemical cells, such as, but not limited to, Carbon, Graphite, Stainless steel and Nickel. The current collector may be in any suitable form such as, but not limited to, sheet, foam, mesh, etc, and may be used in ways that are known in the field. In an embodiment, the Zn-MnO2 cell includes a current collector comprising conductive carbon. In an embodiment, the current collector is flexible graphite sheet. One example of flexible graphite sheet is Grafoil® (30 mm X 30 mm X 0.32 mm).
[0031] The electrolyte includes any ionic conductor disposed between the anode and the cathode, according to the various embodiments herein. The electrolyte may be selected from the group consisting of aqueous, acidic and alkaline electrolytes. In an embodiment, the electrolyte is aqueous electrolyte comprising salts of Zinc and Manganese. The electrolyte includes Zinc and Manganese salts such as, but not limited to, triflates, chlorides, sulphates, nitrates, etc. The electrolyte may be in any suitable form such as, but not limited to, liquid form, solid form and gel form. The suitable form of the electrolyte depends on the application of the disclosed cell, and may be achieved by methods generally known in the art. The electrolyte may further include various combinations of the generally known electrolytes.
[0032] In an embodiment, the electrolyte is an aqueous electrolyte comprising sulphates of Zinc and/or Manganese. In an embodiment, the cell includes an electrolyte comprising Zinc sulphate (ZnSO4). In another embodiment, the cell includes an electrolyte comprising Manganese sulphate (MnSO4). In an embodiment, the cell includes an electrolyte comprising aqueous solution of ZnSO4 and MnSO4. The concentration of ZnSO4 may be in the range of 1M to 2M, and concentration of MnSO4 may be in the range of 0.1M to 0.2M. In an embodiment, the electrolyte is an aqueous solution comprising ZnSO4 of 2M concentration and MnSO4 of 0.1M concentration. The electrolyte may have pH such that it is suitable for use in Zn-MnO2 cell. In an embodiment, pH of the electrolyte is maintained in the range of 3 to 6. In an embodiment, pH of the electrolyte is maintained at 6.
[0033] The embodiments of electrochemical cell disclosed herein may further include one or more reference electrodes which may be used to monitor the potentials of cathode and anode during cell charge-discharge cycles. In an embodiment, the cell comprises mercury / mercury sulphate (Hg/Hg2SO4) and saturated potassium sulphate (K2SO4) as the reference electrode. Alternatively, any reference electrode that is generally known to be used in Zinc-Manganese dioxide cell systems may be used.
[0034] The electrochemical cell may further include one or more suitable separators positioned between the anode and cathode. Any suitable electrically insulative separator generally known in the field may be used in various embodiments described herein. The separator may be chosen such that it has sufficient chemical, electrochemical and mechanical stability. In an embodiment, the cell comprises polyolefin nonwoven membrane separators. The separators in the various embodiments may be positioned in between the anode and the cathode such that efficient physical and electrical separation between the electrodes is achieved. In an embodiment, the electrodes are wrapped and thermally sealed with polyolefin nonwoven membranes, such as, but not limited to, FS 2192 SG (by Freudenberg Vliesstoffe KG, Germany).
[0035] The various embodiments of electrochemical cell disclosed herein may be assembled into any type of cell packaging or configuration that is generally known in the field such as, but not limited to, cylindrical cell, prismatic cell and pouch cell. In an embodiment, the electrochemical cell is assembled into prismatic cell configuration.
[0036] Referring now to the drawings, and more particularly to Fig. 1, where similar reference characters denote corresponding features consistently throughout the figures. Fig. 1 is a schematic representation of electrochemical cell (100). The cell (100) comprises Manganese dioxide cathode (102), Zinc anode (104), and electrolyte (106). The electrolyte (106) comprises a solution of Zinc sulphate and Manganese sulphate, according to the various embodiments herein. The cell (100) further comprises a reference electrode (108) for monitoring the potentials of cathode (102) and anode (104). The reference electrode (108) is Hg/Hg2SO4 and saturated K2SO4. The cathode (102) includes a current collector, i.e. Grafoil® (30 mm × 30 mm X 0.30 mm), having a coating of bismuth-modified hollandite a-MnO2.
Hollandite alpha manganese dioxide
[0037] The hollandite a-MnO2 used in the various embodiments of electrochemical cell disclosed herein may be prepared by reduction of potassium permanganate by treating it with at least one reagent according to the methods disclosed herein. The obtained hollandite a-MnO2 may then be used to produce conductive ink. Conductive ink may be achieved by methods generally known in the field. In an embodiment, the method of preparing a-MnO2 (also referred to as Route-a) comprises treating potassium permanganate (KMnO4) with at least one reducing agent in aqueous medium to obtain a precipitate. In an embodiment, the reducing agent is selected from a group consisting of ethylene glycol and polyethylene glycol. In an embodiment, the method (Route-a) comprises mixing KMnO4 with deionized water to obtain a solution; and adding ethylene glycol drop wise to said solution while stirring to obtain a black precipitate. In another embodiment, the method (Route-a) comprises mixing KMnO4 with deionized water to obtain a solution; and adding polyethylene glycol drop wise to said solution while stirring to obtain a black precipitate. The obtained black precipitate is hollandite a-MnO2 which may further be filtered and rinsed. In an embodiment, the method further comprises filtering to separate said precipitate, washing said precipitate with deionized water and/or ethanol, and drying at a temperature of about 50-150o Celsius. The drying may be performed to remove moisture, and until a dry powder is obtained. In an embodiment, drying is performed at a temperature of 70o Celsius.
[0038] The molar ratio of potassium permanganate and reducing agent, according to various embodiments herein, is 0.1:0.5. In an embodiment, the concentration of KMnO4 is 22 mmol, and the concentration of ethylene glycol or polyethylene glycol is 90 mmol. The quantities of KMnO4 and ethylene glycol or polyethylene glycol are scalable and may be altered as per requirement. In an embodiment, the quantity of KMnO4 is 3.47 g, and the quantity of ethylene glycol or polyethylene glycol is 5 ml. The ethylene glycol or polyethylene glycol may be added while constantly stirring the solution. The stirring may be continued for a period of about 20 to 40 minutes after addition of ethylene glycol or polyethylene glycol is complete. In an embodiment, stirring is performed by a mechanical stirrer. In an embodiment, the stirring is continued for a period of about 30 minutes after addition of ethylene glycol or polyethylene glycol is complete.
[0039] In another embodiment, the method (also referred to as Route-b) comprises treating KMnO4 with Manganese sulphate (MnSO4). The method (Route-b) comprises reacting stoichiometric quantities of MnSO4 and KMnO4 in aqueous medium. In an embodiment, the method comprises mixing KMnO4 and MnSO4.H2O while constantly stirring to obtain a dark brown precipitate. The ratio of MnSO4H2O and KMnO4, according to various embodiments herein, is 1.4:1.6. The quantities of KMnO4 and MnSO4 are scalable and may be altered as per requirement. In an embodiment, the concentration of KMnO4 is 0.1M, and MnSO4.H2O is 0.15M. The quantities of KMnO4 and MnSO4.H2O may be suited to requirement. In an embodiment, the quantity of each of KMnO4 and MnSO4.H2O is 10 mL.
[0040] The obtained dark brown precipitate is hollandite a-MnO2 which may further be filtered and rinsed. In an embodiment, the method further comprises filtering to separate said black precipitate, washing said precipitate with deionized water and/or ethanol, and drying at a temperature of about 50-150o Celsius. The drying may be performed to remove moisture, and until a dry powder is obtained. In an embodiment, drying is performed at a temperature of 70o Celsius. The disclosed molar ratios, conditions, concentrations and quantities of KMnO4 and other reagents are presented herein only by way of illustration of best suited parameters and are not to be construed as limiting the inventive concept. While the parameters presented are critical for optimal results, it would be apparent to a person skilled in the art that minor modification to such parameters may be performed/practiced without departing from the scope of the claimed embodiments.
[0041] Further embodiments provide a method for producing modified alpha-Manganese dioxide ink. In an embodiment, the method for producing modified alpha-Manganese dioxide ink comprises treating Potassium permanganate with Manganese sulphate in aqueous medium to obtain a precipitate; filtering the obtained precipitate; washing said precipitate with at least one solution selected from a group consisting of deionized water and ethanol; and adding a suitable amount of at least one solvent to a mixture comprising said precipitate, Bismuth oxide, graphite particles, and polyvinylidene fluoride to obtain modified alpha-Manganese dioxide ink.
[0042] In another embodiment, the method for producing modified alpha-Manganese dioxide ink comprises treating potassium permanganate with at least one reducing agent selected from the group consisting of Ethylene glycol and Polyethylene glycol to obtain a precipitate; filtering the obtained precipitate; washing said precipitate with at least one solution selected from a group consisting of deionized water and ethanol; and adding a suitable amount of at least one solvent to a mixture comprising said precipitate, Bismuth oxide, graphite particles and polyvinylidene fluoride to obtain modified alpha-Manganese dioxide ink.
[0043] The solvent, in the embodiments herein is at least one solvent selected from a group consisting of n-methyl-2-pyrrolidone, triethyl phosphate, trimethyl phosphate, trimethyl urea, dimethyl formamide, dimethyl acetamide, hexamethyl phosphor amide, dimethyl sulfoxide and propylene carbonate. In an embodiment, the solvent is n-methyl-2-pyrrolidone. The suitable amount of solvent may be an amount which is enough to form a paste or ink-like consistency such that it is capable of being coated onto a surface. The suitable amount may depend on the quantity of ink that is to be prepared. In an embodiment, the suitable amount is a few drops. It would be apparent to a person skilled in the art that the electrochemical cell disclosed in the various embodiments herein may, to some extent, also be achieved by using, alternatively or additionally, a-MnO2 obtained by other methods of modification and/or preparations as well.
Cathode Synthesis
[0044] The a-MnO2 cathode used in the various embodiments herein is bismuth-modified hollandite a-MnO2 cathode produced by treating hollandite a-MnO2 with Bismuth oxide (Bi2O3). In an embodiment, the method for producing bismuth-modified hollandite a-MnO2 cathode comprises
preparing modified alpha-Manganese dioxide ink by adding a suitable amount of at least one solvent to a mixture comprising hollandite alpha-Manganese dioxide, Bismuth oxide, graphite particles and polyvinylidene fluoride, wherein said solvent is selected from a group consisting of n-methyl-2-pyrrolidone, triethyl phosphate, trimethyl phosphate, trimethyl urea, dimethyl formamide, dimethyl acetamide, hexamethyl phosphor amide, dimethyl sulfoxide and propylene carbonate; and
coating a current collector with said ink.
[0045] The hollandite a-MnO2 used in the preparation of cathode may be produced by various embodiments disclosed herein. The amount of hollandite a-MnO2, Bismuth oxide, graphite particles and polyvinylidene fluoride (PVDF) used in the embodiments herein may vary as per requirement. In one embodiment, the amount of alpha-Manganese dioxide is in the range of 45 to 90 wt %; Bismuth oxide is in the range of 1 to 20 wt %; graphite particles is in the range of 0 to 40 wt %; and polyvinylidene fluoride is in the range of 0 to 20 wt %, of the total mixture.
[0046] In an embodiment, the method comprises adding a solvent to a mixture comprising 60 wt % of hollandite a-MnO2, 10 wt % of Bismuth oxide (Bi2O3), 20 wt % of expanded graphite and 10 wt % of polyvinylidene fluoride (PVDF).
[0047] In an embodiment, the solvent is n-methyl-2-pyrrolidone (NMP). In an embodiment, the solvent is added in an amount suitable to obtain an ink. In another embodiment the solvent is added dropwise until an ink is obtained. Any known solvents of PVDF may be used such as, but not limited to, triethyl phosphate, trimethyl phosphate, trimethyl urea, dimethyl formamide, dimethyl acetamide, hexamethyl phosphor amide, dimethyl sulfoxide, propylene carbonate, etc.
[0048] In an embodiment, the method comprises of adding a few drops of n-methyl-2-pyrrolidone (NMP) to a mixture of a-MnO2, Bismuth oxide (Bi2O3), graphite particles and polyvinylidene fluoride (PVDF). In an embodiment, the graphite particles used is expanded graphite. Various types of conductive graphite may be used such as, but not limited to, TIMREX® BNB90, TIMREX, Primary Synthetic Graphite (all types), TIMREX Natural Flake Graphite (all types), TIMREX MB, MK, MX, KC, B, LB Grades (examples, KS15, KS44, KC44, MB15, MB25, MK15, MK25, MK44, MX15, MX25, LB family) TIMREX Dispersions: ENASCO 150G, 210G, 250G, 260G, 350G, 150P 250P: SUPER P. SUPER P Li, carbon black (examples include Ketjenblack EC-300J, Ketjenblack EC-600JD, Ketjenblack EC-600JD powder), acetylene black, carbon nanotubes (single or multi-walled), graphene, graphyne, graphene oxide etc. Prior to coating, the current collector may be subjected to mechanical cleaning with zero-emery paper; and washing with water and acetone. The collector may then be dried by exposure to air before coating.
[0049] In an embodiment, coating of current collector may be performed by spraying the modified a-MnO2 ink on either side of the collector sheet. In another embodiment, coating of current collector may be performed by dipping the collector sheet into the modified a-MnO2 ink. Alternatively, other coating methods that are generally known in the field may also be used.
[0050] Drying of the coated current collector may be performed by exposure to a temperature in the range of 60-120 degree Celsius. In an embodiment, the drying is performed in a hot air oven at a temperature of 100 degree Celsius. Drying is performed to fix the coating of modified a-MnO2 on to the collector surface, and to obtain a coating which is dry and free of moisture.
[0051] It will be apparent to a person skilled in the art that various modifications to the methods and materials disclosed herein may be performed without departing from the scope of the claimed embodiments.
[0052] The invention is further described by reference to the following examples by way of illustration only and should not be construed to limit the scope of the present invention. It will be apparent to those skilled in the art that many modifications, both to materials and methods, may be practiced without departing from the scope of the claimed embodiments.
Example 1: Preparation of a-MnO2 by Route-a (reducing Potassium permanganate (KMnO4) by reacting with ethylene glycol or polyethylene glycol)
[0053] 3.47 g (conc. 22 mmol) KMnO4 was dissolved in 200 ml deionized (DI) water and 5 ml (conc. 90 mmol) ethylene glycol or polyethylene glycol was added drop wise to it under constant mechanical stirring. After complete addition of ethylene glycol or polyethylene glycol, the reaction mixture was further stirred for about 30 min. The black precipitate formed was filtered, copiously washed with deionized water followed by ethanol and dried in an air oven at 70 °C. Powder X-ray diffraction study of the obtained a-MnO2 powder was performed. Fig. 2 shows the Powder X-ray diffraction (XRD) pattern of the obtained a-MnO2. The XRD pattern consists of three broad peaks at 12.8o, 37.5o and 65.9o suggesting a poorly crystalline nature of the precipitate.
Example 2: Preparation of a-MnO2 by Route-b (reacting KMnO4 and MnSO4)
[0054] 10 mL of 0.1M KMnO4 solution was mixed with 10 mL of 0.15M MnSO4.H2O solution under continuous mechanical stirring. A dark-brown precipitate was obtained which was filtered and washed copiously with deionized water. The precipitate was dried at 70 °C in an air oven. Powder X-ray diffraction study of the obtained a-MnO2 powder was performed. Fig. 3 shows the Powder X-ray diffraction (XRD) pattern of the obtained a-MnO2. The XRD pattern consists of three broad peaks at 12.8o, 37.5o and 65.9o suggesting a poorly crystalline nature of the precipitate.
[0055] All chemicals that were used were of analytical grade and were used as received. KMnO4 was procured from Merck, MnSO4.H2O; Ethylene glycol or polyethylene glycol were procured from S.D. Fine Chemicals Ltd.; and Ethanol was procured from Changshu Hongsheng Fine Chemical Co., Ltd. All solutions were prepared in deionized (DI) water.
Example 3: Preparation of a-MnO2 cathode
[0056] MnO2 cathodes were fabricated by mixing 60 wt % of MnO2 (MnO2 obtained in Example 2 and 3 were used), 10 wt % of Bismuth oxide (Bi2O3), 20 wt % of Expanded graphite and 10 wt % of polyvinylidene fluoride (PVDF). To this admix, a few drops of n-methyl-2-pyrrolidone (NMP) were added to form ink. High-density Grafoil® (30 mm X 30 mm X 0.30 mm) was used as the current collector subsequent to its mechanical cleaning with a zero-emery paper. It was then washed with water, rinsed with acetone and dried by exposure to air. The ink was coated on both sides of Grafoil® and dried by exposure to hot air at a temperature of about 100oC.
[0057] Polarization profiles for cathodes having a-MnO2 prepared by Route-a and Route-b were obtained. Fig. 4 shows the galvanostatic charge-discharge profile for a-MnO2 (obtained from Route-a, Example 1) cathode vs. Hg/Hg2SO4, K2SO4 reference electrode at C/10 rate. Fig. 5 shows the charge-discharge curves for the cell with a-MnO2 electrode (wherein a-MnO2 was obtained from Route-a, Example 1) at C/10 rate in the potential window between 1.0 V and 1.8 V. Fig. 6 shows the galvanostatic charge-discharge data for a-MnO2 electrode (wherein a-MnO2 was obtained from Route-b, Example 2) at C/10 rate in the potential window between 1.0 V and 1.8 V.
Example 4: Construction of Electrochemical cell
[0058] The MnO2 cathodes (obtained in Example 3) and high-density zinc foil (30 mm X 30 mm X 0.4 mm) were used in constructing the Electrochemical cell disclosed in the embodiments herein. Two Zn-foil anode were placed on either side of one MnO2 cathode. Hg/Hg2SO4, K2SO4 (sat’d) was used as reference electrode. A solution of 2M ZnSO4 and 0.1M MnSO4 was used as the electrolyte. In construction of a cell, the Zinc foil anode and MnO2 cathode were wrapped and thermally sealed with, non-woven separator, FS 2192 SG (by Freudenberg Vliesstoffe KG, Germany). Zn/MnO2, cell, as depicted in Fig. 1, was assembled into a prismatic box cell configuration.
[0059] Analysis of electrochemical performance of the cells having a-MnO2 cathodes prepared from Routes-a and b were performed. The cycling performance of the cells with their coulombic efficiencies at ambient temperature was measured. Coulombic efficiency of Zn-MnO2 cells reported here is about 90 % at C/10 rate, which is much akin to the coulombic efficiency of 98 % reported for Lithium cells at the same rate. Energy efficiency of the Zn-MnO2 cells, reported here ranges between 85 - 90 %, which is also close to the values of energy efficiencies between 90 - 98 % reported for Lithium cells at varying C-rates. It is noteworthy that both the coulombic and energy efficiency values for Zn-MnO2 cells are higher than the respective values for Lead-acid cells.
[0060] Fig. 7 depicts the specific discharge capacity and coulombic efficiency versus cycle number charge-discharge cycling data for Electrochemical cell at C/10 rate in the potential window between 1.0 V and 1.8 V, wherein the a-MnO2 was synthesized by Route-a. Fig. 8 depicts the specific discharge capacity and coulombic efficiency versus cycle number charge-discharge cycling data for Zn - a-MnO2 electrode at C/10 rate in potential window between 1.0 V and 1.8 V, wherein the a-MnO2 is synthesized by Route-b. Based on the studies, it was concluded that the cells are positive (cathode) limited with specific capacities of the cells with a-MnO2 cathodes synthesized by both Route-a and Route-b being ~ 200 mAh/g.
[0061] The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.