Positive Electrode Active Material, Method for Manufacturing Positive Electrode Active Material, and Secondary Battery

Abstract

A positive electrode active material which can improve cycle characteristics of a secondary battery is provided. Two kinds of regions are provided in a superficial portion of a positive electrode active material such as lithium cobaltate which has a layered rock-salt crystal structure. The inner region is a non-stoichiometric compound containing a transition metal such as titanium, and the outer region is a compound of representative elements such as magnesium oxide. The two kinds of regions each have a rock-salt crystal structure. The inner layered rock-salt crystal structure and the two kinds of regions in the superficial portion are topotaxy; thus, a change of the crystal structure of the positive electrode active material generated by charging and discharging can be effectively suppressed. In addition, since the outer coating layer in contact with an electrolyte solution is the compound of representative elements which is chemically stable, the secondary battery having excellent cycle characteristics can be obtained.

1 . A method for forming a lithium-ion secondary battery comprising an electrolyte solution, a negative electrode, and a positive electrode comprising a positive electrode active material and a decomposition product of the electrolyte solution on a surface of the positive electrode active material, the method comprising the steps of:
 forming a composite oxide comprising lithium, cobalt, magnesium, and fluorine;   coating the composite oxide with a material comprising titanium; and   heating the composite oxide coated with the material comprising titanium at a temperature higher than or equal to 500° C. and lower than or equal to 1200° C. to form the positive electrode active material,   wherein magnesium and fluorine are segregated on a surface of the composite oxide at the heating,   wherein the positive electrode active material comprises a layered rock-salt crystal structure inside the positive electrode active material,   wherein the positive electrode active material comprises a rock-salt crystal structure in a superficial portion of the positive electrode active material,   wherein the positive electrode active material comprises lithium cobaltate inside the positive electrode active material,   wherein the positive electrode active material comprises cobalt, magnesium, fluorine, and oxygen in the superficial portion of the positive electrode active material,   wherein hexafluorophosphate is used as an electrolyte in the electrolyte solution, and   wherein a solution in which ethylene carbonate, diethyl carbonate, and vinylene carbonate are mixed is used as the electrolyte solution.   
     
     
         2 . A method for forming a lithium-ion secondary battery comprising an electrolyte solution, a negative electrode, and a positive electrode comprising a positive electrode active material and a decomposition product of the electrolyte solution on a surface of the positive electrode active material, the method comprising the steps of:
 forming a composite oxide comprising lithium, cobalt, magnesium, and fluorine;   coating the composite oxide with a material comprising titanium; and   heating the composite oxide coated with the material comprising titanium at a temperature higher than or equal to 500° C. and lower than or equal to 1200° C. to form the positive electrode active material,   wherein magnesium and fluorine are segregated on a surface of the composite oxide at the heating,   wherein the positive electrode active material comprises a layered rock-salt crystal structure inside the positive electrode active material,   wherein the positive electrode active material comprises a rock-salt crystal structure in a superficial portion of the positive electrode active material,   wherein the positive electrode active material comprises lithium cobaltate inside the positive electrode active material,   wherein the positive electrode active material comprises cobalt, magnesium, fluorine, and oxygen in the superficial portion of the positive electrode active material, and   wherein a solution in which vinylene carbonate is mixed is used as the electrolyte solution.   
     
     
         3 . A method for forming a lithium-ion secondary battery comprising an electrolyte solution, a negative electrode, and a positive electrode comprising a positive electrode active material and a decomposition product of the electrolyte solution on a surface of the positive electrode active material, the method comprising the steps of:
 forming a composite oxide comprising lithium, cobalt, magnesium, and fluorine;   coating the composite oxide with a material comprising titanium; and   heating the composite oxide coated with the material comprising titanium at a temperature higher than or equal to 500° C. and lower than or equal to 1200° C. to form the positive electrode active material,   wherein magnesium and fluorine are segregated on a surface of the composite oxide at the heating,   wherein the positive electrode active material comprises a layered rock-salt crystal structure inside the positive electrode active material,   wherein the positive electrode active material comprises a rock-salt crystal structure in a superficial portion of the positive electrode active material,   wherein the positive electrode active material comprises lithium cobaltate inside the positive electrode active material, and   wherein the positive electrode active material comprises cobalt, magnesium, fluorine, and oxygen in the superficial portion of the positive electrode active material.   
     
     
         4 . The method for forming a lithium-ion secondary battery, according to  claim 1 ,
 wherein the positive electrode active material comprises cobalt oxide having a rock-salt crystal structure and magnesium oxide having a rock-salt crystal structure in the superficial portion of the positive electrode active material.   
     
     
         5 . The method for forming a lithium-ion secondary battery, according to  claim 1 ,
 wherein a source of lithium, a source of cobalt, a source of magnesium, and a source of fluorine are starting materials of the composite oxide comprising lithium, cobalt, magnesium and fluorine.   
     
     
         6 . The method for forming a lithium-ion secondary battery, according to  claim 1 ,
 wherein the electrolyte solution comprises ethylene carbonate and diethyl carbonate at a volume ratio of 3:7 and vinylene carbonate at a 2 weight %.   
     
     
         7 . The method for forming a lithium-ion secondary battery, according to  claim 1 ,
 wherein the heating is performed at a temperature higher than or equal to 800° C. and lower than or equal to 1000° C.   
     
     
         8 . The method for forming a lithium-ion secondary battery, according to  claim 1 ,
 wherein titanium is diffused into an inside of the composite oxide comprising lithium, cobalt, magnesium, and fluorine at the heating.   
     
     
         9 . The method for forming a lithium-ion secondary battery, according to  claim 1 ,
 wherein the superficial portion of the positive electrode active material comprising the rock-salt crystal structure comprises titanium.   
     
     
         10 . The method for forming a lithium-ion secondary battery, according to  claim 1 ,
 wherein a region comprising titanium overlaps with a region comprising magnesium in EDX line analysis of the positive electrode active material.   
     
     
         11 . The method for forming a lithium-ion secondary battery, according to  claim 5 ,
 wherein an amount of fluorine contained in the source of fluorine is 1.0 time to 4 times (atomic ratio).   
     
     
         12 . The method for forming a lithium-ion secondary battery, according to  claim 1 ,
 wherein a crystal orientation of the layered rock-salt crystal structure inside the positive electrode active material and a crystal orientation of the rock-salt crystal structure in the superficial portion of the positive electrode active material are substantially aligned with each other.   
     
     
         13 . The method for forming a lithium-ion secondary battery, according to  claim 2 ,
 wherein the positive electrode active material comprises cobalt oxide a rock-salt crystal structure and magnesium oxide a rock-salt crystal structure in the superficial portion of the positive electrode active material.   
     
     
         14 . The method for forming a lithium-ion secondary battery, according to  claim 2 ,
 wherein a source of lithium, a source of cobalt, a source of magnesium, and a source of fluorine are starting materials of the composite oxide comprising lithium, cobalt, magnesium and fluorine.   
     
     
         15 . The method for forming a lithium-ion secondary battery, according to  claim 2 ,
 wherein the electrolyte solution comprises ethylene carbonate and diethyl carbonate at a volume ratio of 3:7 and vinylene carbonate at a 2 weight %.   
     
     
         16 . The method for forming a lithium-ion secondary battery, according to  claim 2 ,
 wherein the heating is performed at a temperature higher than or equal to 800° C. and lower than or equal to 1000° C.   
     
     
         17 . The method for forming a lithium-ion secondary battery, according to  claim 2 ,
 wherein titanium is diffused into an inside of the composite oxide comprising lithium, cobalt, magnesium, and fluorine at the heating.   
     
     
         18 . The method for forming a lithium-ion secondary battery, according to  claim 2 ,
 wherein the superficial portion of the positive electrode active material comprising the rock-salt crystal structure comprises titanium.   
     
     
         19 . The method for forming a lithium-ion secondary battery, according to  claim 2 ,
 wherein a region comprising titanium overlaps with a region comprising magnesium in EDX line analysis of the positive electrode active material.   
     
     
         20 . The method for forming a lithium-ion secondary battery, according to  claim 14 ,
 wherein an amount of fluorine contained in the source of fluorine is 1.0 time to 4 times (atomic ratio).   
     
     
         21 . The method for forming a lithium-ion secondary battery, according to  claim 2 ,
 wherein a crystal orientation of the layered rock-salt crystal structure inside the positive electrode active material and a crystal orientation of the rock-salt crystal structure in the superficial portion of the positive electrode active material are substantially aligned with each other.   
     
     
         22 . The method for forming a lithium-ion secondary battery, according to  claim 3 ,
 wherein the positive electrode active material comprises cobalt oxide a rock-salt crystal structure and magnesium oxide a rock-salt crystal structure in the superficial portion of the positive electrode active material.   
     
     
         23 . The method for forming a lithium-ion secondary battery, according to  claim 3 ,
 wherein a source of lithium, a source of cobalt, a source of magnesium, and a source of fluorine are starting materials of the composite oxide comprising lithium, cobalt, magnesium and fluorine.   
     
     
         24 . The method for forming a lithium-ion secondary battery, according to  claim 3 ,
 wherein the electrolyte solution comprises ethylene carbonate and diethyl carbonate at a volume ratio of 3:7 and vinylene carbonate at a 2 weight %.   
     
     
         25 . The method for forming a lithium-ion secondary battery, according to  claim 3 ,
 wherein the heating is performed at a temperature higher than or equal to 800° C. and lower than or equal to 1000° C.   
     
     
         26 . The method for forming a lithium-ion secondary battery, according to  claim 3 ,
 wherein titanium is diffused into an inside of the composite oxide comprising lithium, cobalt, magnesium, and fluorine at the heating.   
     
     
         27 . The method for forming a lithium-ion secondary battery, according to  claim 3 ,
 wherein the superficial portion of the positive electrode active material comprising the rock-salt crystal structure comprises titanium.   
     
     
         28 . The method for forming a lithium-ion secondary battery, according to  claim 3 ,
 wherein a region comprising titanium overlaps with a region comprising magnesium in EDX line analysis of the positive electrode active material.   
     
     
         29 . The method for forming a lithium-ion secondary battery, according to  claim 23 ,
 wherein an amount of fluorine contained in the source of fluorine is 1.0 time to 4 times (atomic ratio).   
     
     
         30 . The method for forming a lithium-ion secondary battery, according to  claim 3 ,
 wherein a crystal orientation of the layered rock-salt crystal structure inside the positive electrode active material and a crystal orientation of the rock-salt crystal structure in the superficial portion of the positive electrode active material are substantially aligned with each other.