WO2011044644A1

WO2011044644A1 - Battery

Info

Publication number: WO2011044644A1
Application number: PCT/AU2010/001385
Authority: WO
Inventors: Manickam Minakshi Sundaram
Original assignee: Murdoch University
Priority date: 2009-10-16
Filing date: 2010-10-18
Publication date: 2011-04-21

Abstract

The present invention relates to a battery. More particularly, the present invention relates to an aqueous lithium hydroxide electrolyte secondary battery employing alkaline-earth oxide additives to a manganese dioxide cathode.

Description

Battery

Field of the Invention

The present invention relates to a battery. More particularly, the present invention relates; to an aqueous lithium hydroxide electrolyte secondary battery employing alkaline-earth oxide additives to a manganese dioxide cathode. The object of the present invention is to achieve higher rechargeable capacity of -manganese dioxide batteries.

Background Art

Alkaline batteries are commonly used as energy sources. The manganese dioxide (MnCb) battery has continuously been improved starting with the earliest zinc-manganese dry battery, then Zn-MnO₂ primary battery and culminating with the current commercially available alkaline zinc-MnO₂ battery.

Small primary batteries employing MnQ₂ have been commercially available for more than a century. Among them, the alkaline Zn-Mn0₂ cell provides a major advance on most conventional battery types in portable modern electronic devices such as digital cameras, MP3 players and high-tech toys.

This battery type is still in high demand in the consumer market because it is mercury-free, provides high rate capability and the cost is significantly lower than for the dominant rechargeable (secondary) lithium-ion battery. Indeed, the zinc- manganese dioxide (Zn-MnO₂) battery Is the most commonly known primary battery which dominates the primary battery market segment (D. Linden, in Handbook of Batteries, 2^nd ed., Ed. By D. Linden, McGraw-Hill, New York, 1995, Chapters 6 and 7). The primary reasons for using Mn0₂ as a cathode material are its low cost, low toxicity and high availability compared with competing battery materials such as Co and Ni. Also, from a thermodynamic point of view, MnO₂ is the most stable form of tetravalent manganese to retain oxygen at standard temperature and oxygen pressure, whereas cobalt and nickel are thermally unstable. Therefore, MnO₂-based cathodes are attractive for energy storage applications ranging from alkaline to lithium batteries and even to supercapacitors. Despite significant advances in the development and commercialization of new battery systems, primary alkaline Zn-MnC>2 batteries still dominate the consumer battery market. The most common electrolytes used in aqueous Zn-MnOj batteries is a KOH electrolyte. Despite a number of reported attempts to develop rechargeable Zn-Mn02 batteries, there still remains a need for a commercially acceptable rechargeable Zn-Mn0₂ battery. Many efforts have been made to achieve better rechargeability. Manganese dioxides (Mn0₂) are recognised as promising cathode materials, delivering good voltage with respect to a Zinc anode in batteries using aqueous potassium hydroxide (KOH) electrolytes, (Y. F. Fao, N. Gupta and H. S. Wroblowa, J. Electroanal. Chem 223 (1987) 107). The charge storage mechanism in this type of battery is based on proton intercalation into Mn0₂ crystal lattice and correspondingly Mn⁴⁺ becomes Mn³⁺ to balance the charge. However, the life of the Ζη-MnO₂ cell is limited to a few tens of cycles, even when cycled under the one electron level capacity (K. Kordesch, J. Gsellmann, M. Peri, K. Tomantschger and R. Chemelli, Electrochim.Acta 26 (1981) 1495). Attempts to discharge the cell beyond this capacity lead to total irreversibility. This could be due to a loss of contact between ΜηΟ^ particles (D. Boden, C.S. Venuto, D. Wisler and R.B. Wylie, J. Electrochem. Soc. 114 (1967) 415).

Until the 1980's, batteries using a Mn0₂ cathode and KOH as the electrolyte were effectively non-rechargeable. The development of practical rechargeable alkaline Mn0₂ cells was attempted in the mid 1980's. Those works focussed on Mn0₂ cathode in KOH electrolyte; examining the influence of doping with lead or bismuth ions. Since then, studies such as Bi₂0₃ (A.M. Kannan, S. Bhavaraju, F. Prado, M. Manivel Raja and A. Manthiram, J. Electrochem. Soc. 149 (2002) p. : A483) and Pb0₂ (B. Sajdl, K. Micka and P. Krtil, Electrochim. Acta 40 (1995), p.

2005) were rigorously investigated. It is found that manganese dioxide electrodes doped with these additives undergo structural changes during repeated cycles that results in poor rechargeable capacity. This is due to dissolution of Mn oxides in the KOH electrolyte allowing the formation of non-rechargeable products like Mn(OH)₂, Μn2.03 and ΜΠ3Ο4. Hence Zn-Mn0₂ remains a primary battery.

However, in recent years, the inventor has investigated the use of an aqueous Lithium Hydroxide (LiOH) electrolyte in a Zn-Mn0₂ cell (M. Minakshi, P. Singh, T. B. Issa, S. Thurgate and R. DeMarco, J. Power Sources, 130 (2004) 254; M. Minakshi, P. Singh and D.R.G. Mitchell, Electrochim. Acta, 52 (2007) 7007). This research has demonstrated that lithium intercalation occurs into the Mn0₂ crystal lattice during the discharge cycle and found the process is reversible.

In order to enhance the overall performance of this aqueous secondary battery, the inventor has investigated various additives such as TiS₂ (M. Minakshi, D. R. G. Mitchell and P. Singh, Electrochim. Acta 52 (2007) p. 3294), TiB₂ (M. Minakshi, D. R. G. Mitchell and K. Prince, Solid State Ionics 179 (2008) p. 355). Bi₂0₃ (M. Minakshi and D. R. G. Mitchell, Electrochim. Acta 53 (2008) p. 6323) and Ce0₂ (M. Minakshi, D. R. G. Mitchell, M. L. Carter, D. Appadoo and N. Kalaiselvi, Electrochim. Acta 54 (2009), p. 3244) which are physically added to the Mn0₂ cathode during electrode preparation. These additives enhanced the lithium intercalation while suppressing the formation of non-rechargeable products like Mn₃0₄ and MnOOH that is also observed in KOH electrolyte. However, the cycleability of the battery deteriorated after hundreds of cycles and hence is not promising as a potential candidate for commercial applications.

The preceding discussion of the background art is intended to facilitate an understanding of the present invention only. The discussion is not an acknowledgement or admission that any of the material referred to is or was part of the common general knowledge as at the priority date of any of the clairhs. Disclosure of the Invention

Throughout, this specification, unless the context requires otherwise, the term "charging" of the battery refers to lithium extraction from the Mn0₂ cathode and the term "discharging" refers to lithium intercalation into the Mn0₂ crystal lattice.

Throughout this specification, unless the context requires otherwise, the term cathode includes the electrode that accepts electrons and cations (referred to as Li ⁺ when concerning aqueous LiOH Ζn-ΜnO₂ batteries) when the battery is discharging.

Throughout this specification, unless the context requires otherwise, the term anode includes the electrode that generates electrons and captures cations (referred to as Zn⁺ when concerning aqueous LiOH Zn-Mn0₂ batteries) when the battery is discharging.

Throughout this specification, unless the context requires otherwise, potentials are expressed relative to the anode.

The pH is measured in the conventional way using a pH-meter calibrated using commercial calibration solutions.

Throughout the specification and claims, unless the context requires otherwise, the word "comprise" or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated Integer or group of integers but not the exclusion of any other integer or group of integers. In one aspect, the invention comprises an aqueous electrochemical battery comprising a manganese dioxide cathode, an anode, and a lithium containing electrolyte, Wherein the cathode further comprises at least one alkaline-earth oxide.

In another aspect, the invention comprises an aqueous electrochemical battery comprising a manganese dioxide cathode, a zinc anode, and a lithium hydroxide electrolyte, wherein the cathode further comprises at least one alkaline-earth oxide.

The inventor has previously demonstrated that zinc-manganese dioxide batteries become rechargeable while using LiOH as electrolyte (M. Minakshi, P. Singh, T. B. Issa, S. Thurgate and R. DeMarco, J. Power Sources, 130 (2004) 254; M. Minakshi, P. Singh and D.R.G. Mitchell, Electrochim. Acta, 52 (2007) 7007). The mechanism involved is lithium insertion/extraction into/from the MnO₂ lattice. The formation of a lithium carbonate layer from a LiOH electrolyte acts as a barrier for protons while permitting lithium ion insertion in aqueous solutions forming lithium intercalated manganese dioxide (UxMn0₂) upon discharge. However, the cycling ability of this cell is limited to around 40 cycles after which the capacity diminishes. This is due to the formation of non-rechargeable products like MnOOH, Mn₂0₃ and Mn₃0₄. Further to this, in an attempt to improve the overall performance of this aqueous rechargeable cell they have investigated various additives like TiS₂ (M. Minakshi, D. R. G. Mitchell and P. Singh, Electrochim. Acta, 52 (2007) 3294), TiB₂ (M. Minakshi, D. R. G. Mitchell and K. Prince, Solid State Ionics, 179 (2008) 355), Bi₂0₃ (M. Minakshi and D. R. G. Mitchell, Electrochim. Acta, 53 (2008) 6323) and Ce0₂ (M. Minakshi, D. R. G. Mitchell, M. L. Carter. D. Appadoo and N. Kalaiselvi, Electochim. Acta 54 (2009), p. 3244) made to the Mn0₂ cathode. These additives significantly improve the discharge performance of Mn0₂ battery by stabilizing the Mn0₂ crystal lattice. This enhances the amount of lithium intercalated into the host Mn0₂ structure.

The inventor has also previously demonstrated that discharge-charge cycle of Zn- Mn0₂ . alkaline cell containing KOH electrolyte shows that the discharge mechanism involves a K⁺ ion insertion into the Mn0₂ lattice and this process is not fully reversible, hence KOH battery remained a primary battery (M. Minakshi, J. Electroanalytical Chem., 616 (2008) 99-106). The mechanism of K⁺ ion insertion was demonstrated by this inventor for the first time, otherwise it is commonly ' believed in the literature that alkaline cells are based on the insertion of protons (H⁺) and the process is irreversible (J. McBreen, J. power Sources 5 (1975) 525.K. Kordesch, M. Weissenbacher, J. Power Sources 51 (1994) 61 ; D. Im, A. Manthiram, B. Coffey, J. Eiectrochem. Soc. 150 (2003) 1651.)

The Inventor has now discovered that adding alkaline earth oxides to the manganese dioxide cathode affects the rechargeability of Mn0₂/LiOH batteries, both in terms of discharge capacity and life cycle. Furthermore and without wishing to be bound by theory, it is believed that the mechanism by which alkaline earth oxides affect the rechargeability of the Mn0₂/LiOH batteries of the present invention differs from that disclosed in the inventors previous work. The inventor found that the alkaline-earth additives containing bivalent cations (Mg²* or Ba²⁺) appear to exhibit high reversibility and good rate ability but the observed mechanism for the alkaline earth additives is different from that of TiS₂, TIB₂, Bi₂0₃ or Ce0₂ additives. It is also considered likely that the addition of a mixture of additives (for instance T1S2 + MgO) into Mn0₂ could enhance the discharge capacity approaching equivalent to one electron capacity of Μηθί, which is 308 mAh/g. In addition, other additives suitable for use in accordance with the invention will be known to persons skilled in the art. The person skilled in the art will test the suitability of the appropriate additives in accordance with methods published in the art.

The manganese dioxide cathode of the present invention may comprise more than one alkaline earth oxide.

In one form of the invention, the alkaline-earth oxide is selected from the group consisting of: calcium oxide, barium oxide, and magnesium oxide.

In one form of the invention, the alkaline-earth oxide is present in the manganese dioxide cathode at a concentration of between 0.5-5 weight percent relative to the weight of manganese dioxide. In one form of the invention, the alkaline-earth oxide is present in the manganese dioxide cathode at a concentration of between 1-5 weight percent relative to the weight of manganese dioxide present. In one form of the invention, the alkaline-earth oxide is present in the manganese dioxide cathode at a concentration of between 2-5 weight percent relative to the weight of manganese dioxide present. In one form of the invention, the alkaline-earth oxide is present in the manganese dioxide cathode at a concentration of about 2 weight percent relative to the weight of manganese dioxide present.

In a preferred form of the invention, where the alkaline-earth oxide is barium oxide, the barium oxide is present in the cathode at a concentration of between 0.5-5 weight percent relative to the weight of manganese dioxide present- Preferably still, where the alkaline-earth oxide is barium oxide, the barium oxide is present in the cathode at a concentration of between 1-5 weight percent relative to the weight of manganese dioxide present. Preferably still, where the alkaline- earth oxide Is barium oxide, the barium oxide is present in the cathode at a concentration of between 2-5 weight percent relative to the weight of manganese dioxide present. In a highly preferred form of the invention, where the alkaline- earth oxide is barium oxide, the barium oxide is present in the cathode at a concentration of about 2 weight percent relative to the weight of manganese dioxide present.

As will be , readily apparent from the following non-limiting examples, the inventor has discovered that the addition of small quantities of barium oxide to the manganese dioxide cathode significantly enhances the discharge capacity of the battery (or in an alternative expression, it significantly increases energy density).

In a preferred form of the invention, where the alkaline-earth oxide is magnesium oxide, the magnesium oxide is present in the cathode at a concentration of between 0.5-5 weight percent relative to the weight of manganese dioxide present. Preferably still, where the alkaline-earth oxide is magnesium oxide, the magnesium oxide is present in the cathode at a concentration of between 1-5 weight percent relative to the weight of manganese dioxide present. Preferably still, where the alkaline-earth oxide is magnesium oxide, the magnesium oxide is present in the cathode at a concentration of between 1-2 weight percent relative to the weight of manganese dioxide present. In a highly preferred form of the invention, where the alkaline-earth oxide is magnesium oxide, the magnesium oxide is present in the cathode at a concentration of about 2 weight percent relative to the weight of manganese dioxide present. As will be readily apparent from the following non-limiting examples, the inventor has discovered that the addition of small quantities of magnesium oxide to the manganese dioxide cathode, while not enhancing the discharge capacity of the battery to the extent of barium oxide, however produces a battery where the discharge capacity is better stabilised through multiple discharge/charge cycles. In an alternative expression, the inventor has discovered that the addition of small quantities of magnesium oxide to the manganese dioxide cathode, while not Increasing energy density of the battery to the extent of barium oxide, however, produces a battery with improved energy density.

In one form of the invention, the manganese dioxide cathode comprises conductive particles to enhance the conductivity of the cathode. Suitable conductive particles include carbon particles. The manganese dioxide cathode may further comprise a binder. For example, the cathode may further comprise acetylene black and polyvinylidene difluoride. r Other suitable binders include cellulose, other polymers and elastomers.

Other conductive particles and binders suitable for use in accordance with the invention will be known to persons skilled in the art. The person skilled in the art will test the suitability of the appropriate conductive particle and/or binder in accordance with methods published in the art.

The manganese dioxide cathode may further comprise additives in addition to the alkaline earth oxides to improve the discharge performance of the manganese dioxide cathode by stabilising the manganese dioxide cathode crystal structure. These additional additives include at least one additive selected from the group consisting of: TiS₂ TiB₂, Bi₂0₃ and Ce0₂.

In a specific form of the invention, the manganese dioxide cathode comprises approximately 65-74.5 weight percent of manganese dioxide. In a specific form of the invention, the manganese dioxide cathode comprises approximately 15 weight percent of acetylene black.

In a specific form of the invention, the manganese dioxide cathode comprises approximately 10 weight percent of polyvinylidene difluoride.

In a specific form of the invention, the manganese dioxide cathode comprises approximately 0.5-5 weight percent of alkaline-earth oxide selected from the group consisting of: calcium oxide, barium oxide, and magnesium oxide

In a specific form of the invention, the manganese dioxide cathode comprises approximately 0,01-5 weight percent of an additional additive selected from the group consisting of: TiS2TiB₂, Bi2O₃ and CeO₂. In one highly specific form, the manganese dioxide cathode comprises 70 weight percent manganese dioxide, 15 weight percent acetylene black, 10 weight percent polyvinylidene difluoride, 4.5 weight percent alkaline-earth oxide, and 0.5 weight percent of an additional additive as disclosed herein relative to the weight of the manganese dioxide cathode. In one form of the invention, the manganese dioxide of the manganese dioxide cathode has a predominantly gamma-type structure. In one form of the invention, the manganese dioxide of the manganese dioxide cathode has an intergrowth structure of gamma and beta type.

In one form of the invention, the battery comprises an electrode structure wherein the electrodes are separated by a separator which is adapted to allow the flow of ions between the electrodes but separating the electrodes to avoid internal short circuitS-

In one form of the invention, the electrolyte solution is dispersed on the cathode. In another form of the invention, the electrolyte solution is dispersed throughout the battery. In another form of the invention, the electrolyte solution is dispersed on the separator.

In one form of the invention, the battery has an average closed circuit voltage of 1.7V at low rates of discharge.

In one form of the invention, the cathode has a specific discharge capacity of greater than 150mAh/g at low-rate (e.g., C/30 or 10 mA/g of active cathode material) to a 1.0 V and 1.9V cut-off while discharge and charge respectively.

In one preferred embodiment, the battery has a discharge performance of 50- 100% of the unit capacity of the Mn02. In another embodiment, the battery has a discharge performance of 55% of the unit capacity of the Mn02. In another embodiment, the battery has a discharge performance of 65% of the unit capacity of the Mn02. In another embodiment, the .battery has a discharge performance of 70 - 85% of the unit capacity of the Mn02. For example, the discharge performance is 310 mAh/g. Iri another embodiment, the battery has a discharge performance of 85-100% of the unit capacity of the Mn02.

In one preferred embodiment, the battery has a rechargeability performance of between 50-100% of the prior discharge performance. In another embodiment, the battery has a rechargeability performance of 50% of the prior discharge performance. In another embodiment, the battery has a rechargeability oerformance of 80-85% of the Drior discharae Derformance. In one another embodiment, the battery has a rechargeability performance of between 95% of the prior discharge performance. In one another embodiment, the battery has a rechargeability performance of 100% of the prior discharge performance. Τηβ· term "prior discharge performance" used herein refers to the performance of the discharge occurring immediately before the recharge. It does not refer to the original discharge made during the first discharge of the battery's life.

In another embodiment, the battery has a storage capacity performance of 190 - 265 mAh/g.

In one form of the invention, the lithium hydroxide electrolyte has a pH of between 9 to 14. Preferably the pH is between 10 and 14. Alternatively, the preferred pH is between 10 and 11. In one example, the preferred pH is 10.5.

In one form of the invention, the battery is characterised by being a rechargeable battery.

In one form of the invention, the battery Is labelled with instructions that the battery is rechargeable.

In one form of the invention, the battery is any existing battery whether commercialised or not.

In one'form the battery is not restricted by application but can be applied to a wide range of devices requiring energy storage. For example, the form of the invention can be adapted to be applied to electric vehicles, stationary power generation storage devices, all portable appliances including power tools, digital cameras, toys, mp3 players, computers and mobile phones.

In one form of the invention, the battery is of a class selected from the group: AAA, AA, C, D, 9-Volt, Lantern, AAAA, ¹/₂ AA, A23, CR123A, CR2, CR-V3, Duplex, F, J, N, 4.5 Volt, No. 6 and Sub-C

In one form of the invention, the battery is of a class selected from the group: CR636-2, CR736-2, CR927. CR1025, CR1216, CR1220, CR1225, CR1616, CR1620, CR1632, CR2012, CR2016, CR2025, CR2032, CR2330, CR2354, CR2450, CR2477, CR3032, SR41 , SR43, SR44, SR45, SR48, SR54, SR55, SR57, SR58, SR59, SR60, SR63, SR66 and SR69.

In one form of the invention, the anode comprises a planar zinc anode to complete the electric circuit. Other anode material or combinations of material suitable for use in accordance with the invention will be known to persons skilled in the art, including other formations of zinc metal, and manganese dioxide with or without additives. The persons skilled in the art will test the suitability of the appropriate material in accordance with methods published in the art.

In one embodiment, the battery of the present invention may be manufactured by a method comprising the following steps:

Producing a cathode, a separator, and an anode; and

Contacting said separator with an electrolyte.

In a preferred form, the electrolyte is a liquid aqueous electrolyte.

In a further embodiment, the battery of the present invention may be

manufactured by a method comprising the following steps:

Producing a cathode, a separator, and an anode; and

Impregnating said separator with an electrolyte.

In a preferred form, the electrolyte is a liquid aqueous electrolyte.

In accordance with a further aspect of the invention, there is provided a method of improving the discharge performance of an aqueous electrochemical battery comprising a cathode, an anode, and a lithium containing electrolyte, wherein the cathode comprises manganese dioxide, wherein the anode comprises zinc, and wherein said method comprises adding to the manganese dioxide cathode an alkaline-earth oxide additive. The improved discharged performance may be determined by the galvanostatic method. In accordance with a further aspect of the invention, there is provided a method of improving the rechargeability performance of an aqueous electrochemical battery comprising a cathode, an anode, and a lithium containing electrolyte, wherein the cathode comprises manganese dioxide, wherein the anode comprises zinc and wherein said method comprises adding to the cathode an alkaline-earth oxide additive.

The rechargeability performance may be determined by the galvanostatic method. Brief Description of the Drawings

The present invention will now be described with reference to a number of experimental examples and the following Figures, in which:

Figure 1 is a comparison of the first discharge profiles of Mn0₂ (manganese dioxide) in the presence of small amounts of additives (weight percent is indicated in the profiles). The Zn|LiOH|Mn02 cell consists of Zn anode, Mn0₂ cathode using saturated- aqueous LiOH containing Imol.L^-1 of ZnS0₄ as the electrolyte. The cathode pellets were prepared by mixing 70-

72 wt % Mn0₂ consisting of 0 and 2 wt % CaO or MgO or BaO respectively with 15 wt % acetylene black and 10 wt % poly (vinylidene difluoride) binder.

Figure 2 is a comparison of the first discharge profiles of Mn0₂ (manganese dioxide) in the presence of small amounts of BaO additives

(weight percent is indicated in the profiles).

Figure 3 is a comparison of the first discharge profiles of Mn0₂ (manganese dioxide) in the presence of small amounts of MgO additives (weight percent is indicated in the profiles). Figure 4 shows first discharge-charge behavior of an alkaline earth oxide

(MgO or BaO) modified (2 wt%) Mn0₂ cathode, illustrating the rechargeability of Mn0₂ samples. Figure 5 shows multiple discharge-charge behavior of an unmodified Mn0₂ cathode (0 wt.% additive). The capacity of the cell decreases on cycling - cycle numbers shown.

Figure 6 shows multiple discharge-charge behavior of a BaO modified (2 wt.%) Mn0₂ cathode, illustrating the cyclability of Mn0₂ samples. The striking behavior here is the retention of capacity on the initial cycles.

Figure 7 shows multiple discharge-charge behavior, of a BaO modified (2 wt. %) Mn0₂ cathode-illustrating the cyclability of Mn0₂ samples. The capacity fades much more quickly on further cycling Figure 8 is a XRD pattern of the MnO₂ in the presence of small amounts of alkaline earth oxide additives (a) 0 wt% (b) MgO 2 wt% and (c) BaO 2 wt%.

Figure 9 is a XRD pattern of the discharged Mn0₂ in the presence of small amounts of alkaline earth oxide additives (a) 0 wt% (b) MgO 2 wt% and (c) BaO 2 wt%. Figure 10 is a XRD pattern of the discharged MnO₂ after multiple cycles in the presence of 2wt % of alkaline earth oxide additives (a) MgO and (b) BaO.

Figures 11(a) to (d) are TEM images and its selected area diffraction patterns (SADP) of the additive free MnO₂ cathode after discharge, (a-b) and (c-d) shows various regions of Mn rich area in the cathode. Diffraction patterns in Figures 11 (b & d) shows two different patterns. 11b shows nice bright spots without any diffused pattern. 1 1d shows with irregularities.

Figures 12(a) to (d) are TEM images and its selected area diffraction patterns (SADP) of the MgO (2 wt. %) modified MnO2 cathode, (a-b) shows after first discharge and (od) shows after multiple cycles. Diffraction patterns in figures 12 (b & d) are.identical implying the structure is versatile for reversibility.

Figures 13(a) to (d) are TEM images and its selected area diffraction patterns (SADP) of the BaO (2 wt. %) modified MnO₂ cathode, (a-b) shows after first discharge and (c-d) shows after multiple cycles. Diffraction patterns, in figures 13 (b & d) are^' not identical implying the structure is not reversible.

Figure 14 comprises a TEM image (top) of the BaO (2 wt. %) modified Mn(¼ discharged after multiple cycles in LiOH solution. Bright field image (top) showing clustered Zn area (sp 7) and Zn associated with Mn0₂ areas (sp 8-11 ) and its corresponding EDS spectra (bottom) of locations sp7 and sp9 (term "sp" refers to spectra). Elements are shown in the figure. It is evidenced that there is a formation of zincate ions in the BaO added MnO₂ cathode.

Figure 15 comprises a TEM image (left) of the BaO (2 wt. %) modified Mn0₂ discharged in LiOH solution. Bright field image (left) of clustered Barium rich area (sp 6) and its corresponding EDS spectra (right) of location sp6. Elements are shown in the figure. It is evidenced that barium (Ba) is associated with sulphate (S) which means barium oxide (BaO) is reduced to barium sulphate - that allows the formation of zincate ions near the cathode.

Figure 16 comprises a TEM image (left) of the MgO (2 wt. %.) modified Mn0₂ discharged in LiOH solution. Bright field image (left) showing clustered manganese rich area (sp 1) and its corresponding EDS spectra

(right) of location sp1. Elements are shown in the figure. There is no . evidence of the formation of zincate ions or sulphate ions see in the EDS spectra. Hence MgO blocks the zincate ions into Mn0₂ cathode.

The description of the examples should not be understood to limit the preceding description of the invention.

Best Mode(s) for Carrying Out the Invention

The scope and application of the present invention, and preferred methods for carrying out the invention!, will ^{now be} described with reference to a series of non- limiting examples and experiments. Examples

Specifically, the discharge characteristics of manganese dioxide ( MnO₂) cathode in the presence of small amounts of at least one of the alkaline-earth oxides such as calcium oxide (CaO), barium oxide (BaO) or magnesium oxide (MgO) additive was investigated and compared with additive free MnO₂ cathode.

In these experiments, the EMD (electrolytic manganese dioxide; γ-Μη0₂) type (IBA sample 32) material used in this work was purchased from the Kerr McGee Chemical Corporation. Alkaline earth oxides i.e. calcium oxide (CaO), magnesium oxide (MgO) and barium oxide . (BaO) were obtained from Aldrich chemical company. An aqueous electrochemical battery comprises a cathode, an anode, a separator between the anode and cathode, an electrolyte is used. The cathode comprises Mn0₂ active material, the anode is zinc, and the electrolyte is an aqueous lithium hydroxide solution. The cathode has been prepared from γ- Mn0₂ by physical admixture of at least one of the alkaline-earth oxides such as calcium oxide (CaO), barium oxide (BaO) or magnesium oxide (MgO). A cathode structure for an aqueous secondary battery comprises 70-75 wt% γ-Μη0₂, 15 wt% acetylene black as conductive powder, 10 wt% poly (vinylidene difluoride) (PVDF, Sigma-Aldrich) as binder and 0-5 wt% alkaline-earth oxide additive material uniformly mixed in a mortar and pestle and pressed to form a disk-like pellet. The pellet is 8 mm diameter and 0 5 mm thickness. The electrolyte was a saturated solution of lithium hydroxide (LiOH) with a pH equivalent to 10.5.

The morphology and interplanar spacings of the products formed before and after discharge were characterized by transmission electron microscopy (TEM), associated with energy dispersive spectra (EDS) using a JEOL 201 OF TEM model operated at 200kV. TEM specimens were prepared by grinding a small fragment scraped from the pressed pellet under methanol and dispersing on a holey carbon support film. Specimens were examined at liquid nitrogen temperature in a cooling stage, to reduce beam damage and contamination effects. For X-ray analysis a Siemens D500 X-ray diffracto meter 5635 using Co-Koc radiation was used. The voltage and current were 30 kV and 40 mA. The scan rate was 1 degree per minute. Two theta values were recorded between 20 and 60 degrees. As can be seen in Figure 1 , the incorporation of BaO or MgO additives into Mn0₂ was found to result in significantly higher discharge capacity cpmpared with the battery using CaO additive or additive free Mn0₂. In this work, small amounts of BaO or MgO (1 , 2 and 5 wt.%) as additives have been ground together (by physical admixture) with Mn0₂ and investigated their electrochemical behaviour.

Figures 2 and 3 compare the discharge performance of the Mn0₂ containing small amounts of BaO or MgO (1 , 2 and 5 wt.%). It is found that the presence of BaO or MgO to < 2 wt. % improves the discharge capacity significantly to 265 and 200 mAh/g respectively of MnO₂. However; increasing the dopant content of BaO or MgO above this amount causes a decrease in its discharge capacity to 220 and 115 mAh/g. This demonstrates that different alka|ine-earth oxides have different influences on the electrochemical behavior and microstructurai development of the Mn0₂ cathode. It is also found that the 1 wt% additives of either BaO or MgO are not ideal for longer life cycles. The discharge-charge cycle of the 2wt% BaO or MgO modified material are shown in Figure 4. The modified cells appear to be fully reversible on a charge cycling. The behaviour on the subsequent discharge cycle appears quite different from that observed with respective additives. Unlike the additive free material (Figure 5), which showed a 30% decrease in capacity after the 25th cycle, the capacity of the MgO modified material changed very little between cycles 1 and 5 (Figure 6), resulting in a 20% fading after the 25^th cycle. The BaO additive (Figure -7) enhances the initial discharge capacity of the cell but, the fade in capacity is very strong while compared with the battery using MgO additive (Figure 6) or additive freeMnO₂ (Figure 5). Capacity fading is usually accompanied by an increase in the internal impedance of the battery during cycling (C. Wang, X. Ma, J. Cheng, J. Sun and Y. Zhou, J. Solid State Electrochem., 1 1 (2007) 361; D. Zhang, B.S. Haran, A. Durairajan, R.E. White, Y. Podrazhansky and B.N. Popov, J. Power Sources 91 (2000) 122) but the presence of MgO additive clearly suppressed this. The enhancement in the performance of the modified cell In terms of discharge capacity looks similar to that observed for a range of other additives (TiB₂ [M. Minakshi, D. R. G. Mitchell and P. Singh, Electrochim. Acta, 52 (2007) 3294]; TIS₂ [M. Mlnakshl, D. R. G. Mitchell and K. Prince, Solid State Ionics, 179 (2008) 355], Bi₂0₃ [M. Minakshi and D. R. G. Mitchell, Electrochim. Acta, 53 (2008) 6323]), in that the formation of non-electroactive forms of manganese are suppressed. But, strikingly in the alkaline earth oxide modified cathodes, the XRD patterns of the discharged product still show that these non-electroactive forms of manganese are present.

Figures 8 to 10 show x-ray diffraction patterns of the alkaline earth oxide modified cathode before and after discharge, and after multiple cycles respectively. The before discharge material (Figure 8) shows the characteristic peaks of Mn0₂ and BaO as quoted in the JCPDS database [Joint Committee on Powder Diffraction Standards (JCPDS) International Center for Diffraction File PDF Database sets 1- 49 (2000), Pennsylvania: Card Number 44-0143; Joint Committee on Powder Diffraction Standards (JCPDS) International Center for Diffraction File PDF Database sets 1 -49 (2000), Pennsylvania: Card Number 26-177]. The main Bragg reflection corresponding to graphite (acetylene black) is seen at 2e=30°(C). The alkaline earth oxide modified discharged cathode (Figures 9b and c) shows the emergence of new peaks (ojand●). The original peaks of Mn0₂ (+) are replaced by those of a number of new phases whereas the peak corresponding to C (graphite) is almost unchanged. As indexed in the XRD pattern these new reflections are in good agreement with those reported for the following materials: Mn₂0₃ φ, MnOOH (o) and Li₀.gMnO₂ (♦) i.e. lithium intercalated Mn0₂ [Joint Committee on Powder Diffraction Standards (JCPDS) International Center for Diffraction File PDF Database sets 1-49 (2000), Pennsylvania: Card Number 43- 1455; Joint Committee on Powder Diffraction Standards (JCPDS) International Center for Diffraction File PDF Database sets 1-49 (2000), Pennsylvania: Card Number 34-0394; Joint Committee on Powder Diffraction Standards (JCPDS) International Center for Diffraction File PDF Database sets 1-49 (2000), Pennsylvania: Card Number 44-0143]. This shows that during discharge manganese is, in part, reduced to various oxyhydroxides and lithium is also intercalated into the Mn0₂ structure to form Li_xMn0₂. The peaks corresponding to the new phases in the BaO modified discharged cathode (Fig. 9c) is higher in intensity indicating that the manganese is reduced to a larger extent, as evidenced in the discharge behaviour (Fig. 2). The XRD pattern for the multiple cycles of the MgO modified cathode material (Fig. 10a) was quite similar to that of the initial discharged cycle (Fig. 9b) indicating that the structure is reversible. The Li_xMn0₂ peaks were very similar suggesting that lithium insertion mechanism is reversible in the electrochemical process. However, peaks corresponding to manganese oxides and hydroxides i.e. non-rechargeable products were increased in peak intensity on multiple cycles indicating this causes the fade in capacity. These secondary compounds are well known to be a poorly electro-active. Interestingly, for the multiple cycles of the BaO modified Mn0₂ cathode (Fig. 10b) a major reflections corresponding to Li_xMn0₂ (·) and MnOOH (o) are not present. However, a reflection corresponding to Μn₃O4 (+) can be observed. The absence of Mn³⁺ in the XRD pattern suggests that the BaO addition does prevent the dissolution of Mn³⁺ from the solid Mn0₂. Hence, to gain insight into the role of these additives in Mn0₂ and its mechanism, morphology and lattice imaging of the discharged cathode material were extensively investigated by TEM and its associated techniques.

The TEM image of the unmodified (additive free) discharged Mn0₂ cathode is shown in Fig.11 a & c. The bright field image showed the Mn0₂ to be crystalline and diffusive. The selected area diffraction (SADP) (Figs, 11 b & d) on these areas showed a crystalline like pattern with bright spots (Fig 11 b) and another region to . be quite diffuse (Fig 11 d), supporting the images. Hence, the resultant discharged product consists of mixed' phases like Li_xMn0₂ and manganese oxides and hydroxides. The TEM images of the MgO modified Mn0₂ particles after multiple cycles (Fig 12c) were similar to those in the initial discharged state (Fig. 12a). The selected area diffraction (SADP - Figs 12 b & d) yielded spacings consistent with lithium Intercalated Mn0₂. This supports the XRD results that the product obtained upon discharge is reversible. Bright field imaging of the BaO modified Mn0₂ particles after the first discharge cycle showed them to be highly crystalline (Fig. 13a). However, the selected area diffraction of the first discharged material (Fig. 13b) and after multiple discharged (Fig. 13d) highlighted the differences between the patterns for the two phases. The Mn0₂ was poorly crystalline, resulting in diffuse diffraction features after several cycles (Fig. 13c). This indicates that the structure is not reversible, hence the loss in discharge capacity. Now the question arises, why MgO is reversible but not BaO additive? In order to elucidate the role of barium oxide in the LiOH electrolyte, the discharged BaO modified MnO₂ cathode was characterised using TEM with EDS technique. Bright field TEM images of the BaO modified Mn0₂ cathode after the multiple discharge cycle in LiOH electrolyte are shown in Figure 14. The general morphology of the Mn0₂ clump is similar to the original material but individual crystals appear to be like spongy. The corresponding EDS spectra of this spongy region (Spectra "Sp" 7 & 9) show regions of polycrystalline zinc oxide in the vicinity of the Mn0₂ cathode. EDS spectra recorded from the Zn region associated with Mn0₂ invariably contained small amounts of zincate ions which is from the electrolyte or anodic region. In another region of labelled (Sp6) a barium rich area in the discharged material, in Fig. 15, it is evidenced through EDS analysis that barium is associated with sulphate which means barium oxide is reduced to barium sulphate in the LiOH electrolyte. This allows the formation of zincate ions near the cathode. Hence, in the BaO modified cell not ail of the MnO₂ changes to Li_xMn0₂ and manganese oxyhydroxide (MnOOH) while discharge as observed in MgO additive but a part of it produces as a byproduct double oxide of manganese and zinc such as hetaerolite (ZnO.Mn₂0₃). Such a byproduct of discharge reaction lowers electron conductivity of MnO₂ electrode causing an increase in its internal resistance while hindering intercalation mechanism thereby causing an increase in concentration polarization. However, the bright field imaging of the MgO modified Mn0₂ cell after multiple discharge (Fig. 16) in the region Sp1 , does not show any evidence of the formation of zincate ions or sulphate ions through the EDS analysis. This suggests MgO additive blocks the zincate ions into the region ofMnO₂ cathode thereby the formation of byproduct double oxide of manganese and zinc are prevented. This enhances the cycle life of the battery. The addition of alkaline earth oxides causes a better utilization of manganese dioxide cathode; however MgO behaves differently from BaO in terms of cycleability. As the experiments described clearly indicate, the effect of alkaline earth oxide additions to Mn0₂ enhances the performance of the Zn-Mn0₂ battery. The addition of 2 wt. % MgO or BaO additives to Mn0₂ resulted in a significant higher discharge capacity. Increasing the additive levels to 5 wt. % causes a decrease in cell capacity. MgO modified manganese dioxide cathode had been shown to exhibit good cyclability. The MgO-doped cathode retained more than 70% of its initial capacity after 25 cycles. The discharged product of the MgO-doped Mn0₂ characterized by TEM showed that MgO helps to reduce the formation of manganese and zinc such as hetaerolite products while improving the reversibility of the lithium intercalation-de intercalation process. The barium oxide additive reduced to barium sulphate in the LiOH electrolyte which allowed the formation of zincate ions in the vicinity of the ΜnΟ₂ cathode. This deteriorated the host MnO≤ structure resulting in a loss in capacity.

Claims

The Claims Defining the Invention is as Follows:

1. An aqueous electrochemical battery comprising a manganese cathode, an anode, and a lithium containing electrolyte, wherein the cathode further comprises at least one alkaline-earth oxide.

2. An aqueous electrochemical battery comprising a manganese dioxide cathode, a zinc anode, and a lithium hydroxide electrolyte, wherein the cathode further comprises at least one alkaline-earth oxide,

3. A method for manufacturing an aqueous electrochemical battery.

4. A method of improving the discharge performance of an aqueous electrochemical battery.

5. A method of improving the rechargeability performance of an aqueous electrochemical battery.