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A new sandwich cathode design having a second cathode active material of a relatively high energy density but of a relatively low rate capability sandwiched between two current collectors and with a first cathode active material having a relatively low energy density but of a relatively high rate ca

A new sandwich cathode design having a second cathode active material of a relatively high energy density but of a relatively low rate capability sandwiched between two current collectors and with a first cathode active material having a relatively low energy density but of a relatively high rate capability in contact with the opposite sides of the two current collectors, is described. The present cathode design is useful for powering an implantable medical device requiring a high rate discharge application.

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A new sandwich cathode design having a second cathode active material of a relatively high energy density but of a relatively low rate capability sandwiched between two current collectors and with a first cathode active material having a relatively low energy density but of a relatively high rate ca

A new sandwich cathode design having a second cathode active material of a relatively high energy density but of a relatively low rate capability sandwiched between two current collectors and with a first cathode active material having a relatively low energy density but of a relatively high rate capability in contact with the opposite sides of the two current collectors, is described. The present cathode design is useful for powering an implantable medical device requiring a high rate discharge application. >2; the materials of (a) being present in the electrode structure in a total amount corresponding to 20-98% by weight of the complete electrode structure; and the orthorhombic LiMnO2of (b) being present in the electrode structure in an amount corresponding to 1-79% by weight of the complete electrode structure, with the proviso that the orthorhombic LiMnO2in the positive electrode structure has a higher first charge specific capacity than any material of (a) in the positive electrode structure. 4. A rechargeable electrochemical cell according to claim 3, wherein the orthorhombic LiMnO2is characterised by peaks of full width at half maximum of less than 0.2° at 2θ-values of 25.0°, 39.4° and 45.2° upon XRD analysis using CuKα. 5. A rechargeable electrochemical cell according to claim 3, wherein the orthorhombic LiMnO2is prepared at a temperature above 600° C. 6. A rechargeable electrochemical cell according to claim 3, wherein the orthorhombic LiMnO2has a mean particle size in the range 20-40μ. 7. A rechargeable electrochemical cell according to claim 3, wherein the orthorhombic LiMnO2is characterised by peaks of full width at half maximum of at least 0.25° at 2θ-values of 25.0°, 39.4° and 45.2° upon XRD analysis using CuKα. 8. A rechargeable electrochemical cell according to claim 3, wherein the orthorhombic LiMnO2is prepared at a temperature up to 600° C. 9. A rechargeable electrochemical cell according to claim 3, wherein the orthorhombic LiMnO2has a mean particle size in the range 5-15μ. 10. A rechargeable electrochemical cell comprising a negative electrode, an electrolyte and a positive electrode, the positive electrode structure comprising: (a) one or more materials selected from the group consisting of LiMn2O4,LiCoO2,LiNiO2,LiNixCo1-xO2where 02; the materials of (a) being present in the electrode structure in a total amount corresponding to 20-98% by weight of the complete electrode structure; and the α-NaFeO2of (b) being present in the electrode structure in an amount corresponding to 1-79% by weight of the complete electrode structure, with the proviso that the amount of active sodium originally present in the positive electrode is lower than the amount of lithium originally present in the electrolyte phase, and with the further proviso that the α-NaFeO2has a higher first charge specific capacity than any material of (a) in the positive electrode structure. 11. A rechargeable electrochemical cell comprising a negative electrode, an electrolyte and a positive electrode, the positive electrode structure comprising: (a) one or more materials selected from the group consisting of LiMn2O4,LiCoO2,LiNiO2,LiNixCo1-xO2where 0xNayM(II)O1-1/2(x+y), where x≥0, y≥0 and x+y≤2, and where M(II) is a transition metal in oxidation state +2, selected from the group consisting of Mn, Co, Ni and Fe; the materials of (a) being present in the electrode structure in a total amount corresponding to 20-98% by weight of the complete electrode structure; and the lithium/sodium compounds of the formula LixNayM(II)O1+1/2(x+y), where x≥0, y≥0 and x+y≤2, and where M(II) is a transition metal in oxidation state +2, selected from the group consisting of Mn, Co, Ni and Fe of (b) being present in the electrode structure in a total amount corresponding to 1-79% by weight of the complete electrode structure, with the proviso that the amount of active sodium originally present in the positive electrode is lower than the amount of lithi um originally present in the electrolyte phase and with the further proviso that any material of (b) in the positive electrode structure has a higher first charge specific capacity than any material of (a) in the positive electrode structure. 12. An electrochemical cell according to claim 1, in which the negative electrode comprises a metal capable of alloying with lithium, or a metal oxide which can react with lithium to form the corresponding metal capable of alloying with lithium, or a carbon structure capable of intercalating lithium. 13. An electrochemical cell according to claim 12, in which the negative electrode consists of a coke or a carbon black. 14. An electrochemical cell according to claim 13, in which the materials selected from (a) and the materials selected from (b) are present in the electrode structure in a total amount corresponding to 50-98% and 1-49% by weight of the complete electrode structure, respectively, with the proviso that in the case of the materials of (b) including any of α-NaMnO2,β-NaMnO2, α-NaFeO2and LixNayM(II)O1+1/2(x+y), the amount of active sodium originally present in the positive electrode is lower than the amount of lithium originally present in the electrolyte phase. 15. An electrochemical cell according to claim 12, in which the negative electrode consists of a graphite. 16. An electrochemical cell according to claim 15, in which the materials selected from (a) and the materials selected from (b) are present in the electrode structure in a total amount corresponding to 50-98% and 1-49% by weight of the complete electrode structure, respectively, with the proviso that in the case of the materials of (b) including any of α-NaMnO2,β-NaMnO2,α-NaFeO2and LixNayM(II)O1+1/2(x+y), the amount of active sodium originally present in the positive electrode is lower than the amount of lithium originally present in the electrolyte phase. 17. An electrochemical cell according to claim 1, in which the electrolyte comprises one or more non-aqueous solvent selected from the group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, γ-valerolactone, γ-butyrolactone and one or more salts selected from the group consisting of LiCF3SO3,LiAsF6,LiBF4,LiPF6and LiClO4.