Patent Publication Number: US-2022223858-A1

Title: Cathode active material

Description:
STATEMENT REGARDING GOVERNMENT SPONSORED RESEARCH OR DEVELOPMENT 
     The inventive concept is derived from research conducted as part of personal basic research (Ministry of Science and ICT) (R&amp;D) (Project Identification Number: 1711111832, Research Management Institution: National Research Foundation of Korea, Research Project Title: Search and research on new electrode material based on complementary complex method and anion oxidation/reduction, Host Institution: Korea University Industry-University Cooperation Foundation, Research Period: Mar. 1, 2020˜Feb. 28, 2021, Contribution Rate: ½) of the Ministry of Science and ICT. 
     In addition, the inventive concept is derived from research conducted as part of Future Material Discovery (R&amp;D) (Project Identification Number: 1711121529, Research Management Institution: National Research Foundation of Korea, Research Project Title: Development of 3D cathode for metal-air battery with maximized reaction interface based on digital twin technology, Host Institution: Korea University Industry-University Cooperation Foundation, Research Period: Aug. 31, 2020˜Dec. 31, 2024, Contribution Rate: ½) of the Ministry of Science and ICT. 
     There is no property interest of the Korean government in any aspect of this invention. 
    
    
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     A claim for priority under 35 U.S.C. § 119 is made to Korean Patent Application No. 10-2021-0002501 filed on Jan. 8, 2021, in the Korean Intellectual Property Office, the entire contents of which are hereby incorporated by reference. 
     BACKGROUND 
     Embodiments of the inventive concept described herein relate to a cathode active material. 
     Batteries are devices that stores electrical energy in a form of chemical energy. Among them, a secondary battery has reversible charging/discharging characteristics, and includes an electrode, an electrolyte, and a separator. 
     Among them, the electrode that mainly contributes to an electrochemical characteristic of the secondary battery is composed of an anode and a cathode, and contains an active material for electrochemical activity. 
     Among various types of the secondary batteries, a lithium secondary battery containing a lithium-based material as a cathode active material has high energy density, no memory effect, and low self-discharge, so that the lithium secondary battery is widely utilized as an energy storage device for a portable electronic device. 
     However, as devices, such as an electric vehicle, requiring various electrochemical characteristics such as an energy density, a lifespan, and the like higher than those of existing device requiring energy storage appear, research on an electrode active material superior to LiCoO 2  and carbon-based graphite, which were commercialized as cathode and anode active materials, is being actively conducted. 
     SUMMARY 
     Embodiments of the inventive concept provide a cathode active material that exhibits excellent capacity and lifespan characteristics by making a structural change that appears during charging/discharging to be reversible. 
     In one example, the technical problems to be achieved in the inventive concept are not limited to the technical problems mentioned above. Other technical problems not mentioned will be clearly understood by those of ordinary skill in the art to which the inventive concept belongs from a following description. 
     According to an exemplary embodiment, a cathode active material contains crystal water and a manganese-based metal oxide, the cathode active material includes a first crystal phase having a two-dimensional crystal structure and a second crystal phase having a three-dimensional crystal structure, the first crystal phase is a thermodynamically stable two-dimensional layered crystal structure where a unit lattice structure composed of the manganese-based metal oxide, and having manganese filling octahedral sites of a unit lattice where manganese (Mn) and oxygen (O) are bonded to each other is two-dimensionally extended, the second crystal phase is a thermodynamically metastable three-dimensional crystal structure where, as some manganese in the manganese-based metal oxide forming the layered structure is chemically bonded to oxygen in the crystal water, the unit lattice structure having manganese filling the octahedral sites of the unit lattice and a unit lattice structure having manganese filling tetrahedral sites are mixed, the two-dimensional crystal structure includes two or more unit layers composed of a manganese oxide, and contains at least one metal element disposed between the two or more unit layers and selected from a group of metals other than manganese, and a dopant is doped between the two or more unit layers. 
     The cathode active material may be expressed by a following [Chemical Formula 1]. 
       Me x D o   y MnO 2   .z H 2 O  [Chemical Formula 1]
 
     in Chemical Formula 1, the Me is at least one metal element selected from a group consisting of an alkali metal element and an alkaline earth metal element, 
     the dopant (Do) is at least one metal element selected from a group consisting of the alkaline earth metal element, a transition metal element, and a post-transition metal element, and 
     0.23≤x≤0.01≤y≤1, and 0.01≤z≤0.5. 
     The Me may be at least one element selected from Li, Na, and K. 
     The dopant may be at least one element selected from Al, Mg, Cu, Zn, and Fe. 
     The Me and the dopant may be composed of different elements. 
     According to an exemplary embodiment, a method for preparing a cathode active material includes a first step of obtaining a first crystal phase having a two-dimensional crystal structure containing crystal water and including two or more unit layers composed of a manganese oxide, a second step of heat-treating a manganese-based oxide obtained in the first step to adjust a content of the crystal water to obtain a cathode active material of the manganese-based oxide including both a first crystal phase having a two-dimensional crystal structure and a second crystal phase having a three-dimensional crystal structure, and a third step of doping a dopant between the two or more unit layers. 
     The third step may be performed by charging and discharging the cathode active material in an electrolyte where the dopant is added. 
     The third step may be performed by ion exchange after mixing a dopant precursor solution in the first step. 
     The third step may be performed by chemical synthesis after mixing the manganese-based oxide including both the first crystal phase and the second crystal phase having the three-dimensional crystal structure with a precursor containing an electrochemically active element and the dopant in the second step. 
     The cathode active material may have an initial capacity in a range from 250 to 285 mAh/g when charging/discharging is performed with a current density of 0.1 C in a voltage range from 4.8 V to 1.5 V through a reversible phase transition stabilized by the dopant with high oxygen affinity of the metal (Me) manganese oxide containing the dopant, and a capacity reduction rate of the cathode active material after 100 times of the charging/discharging may be higher than 0% and equal to or lower than 25%. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The above and other objects and features will become apparent from the following description with reference to the following figures, wherein like reference numerals refer to like parts throughout the various figures unless otherwise specified, and wherein: 
         FIG. 1  is an exemplary diagram showing two different crystal structures in a cathode active material according to an embodiment of the inventive concept; 
         FIGS. 2 and 3  are exemplary views showing a crystal structure change during charging/discharging of a cathode active material according to an embodiment of the inventive concept,  FIG. 2  shows a charging state, and  FIG. 3  shows a discharging state; 
         FIG. 4  is a view showing a secondary battery containing a cathode active material according to an embodiment of the inventive concept; 
         FIG. 5  is a graph showing cycle charging/discharging characteristics of a secondary battery containing a cathode active material according to Example of the inventive concept; 
         FIG. 6  is a graph showing cycle charging/discharging characteristics of secondary batteries containing cathode active materials according to Comparative Example; 
         FIG. 7  is a graph showing cycle capacity characteristics of secondary batteries containing cathode active materials according to Example of the inventive concept and Comparative Example; and 
         FIG. 8  shows an ICP (Inductively Coupled Plasma) analysis result of a cathode active material according to Example of the inventive concept. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, embodiments of the inventive concept will be described in more detail with reference to the accompanying drawings. The embodiments of the inventive concept may be modified in various forms, and the scope of the inventive concept should not be construed as being limited to the following embodiments. The present embodiment is provided to more completely describe the inventive concept to those of ordinary skill in the art. Accordingly, shapes of elements in the drawings are exaggerated to emphasize a clearer description. 
     Components of the inventive concept for clarifying solutions to the problems to be solved by the inventive concept will be described in detail with reference to the accompanying drawings based on preferred embodiments of the inventive concept. In assigning reference numerals to the components of the drawings, the same reference numbers are given to the same components even when the components are in different drawings. It should be noted in advance that components of other drawings may be cited when necessary in the description of the corresponding drawing. 
     Among various kinds of cathode active materials, a layered structure-based material is being actively studied in recent years because the layered structure-based material is able to achieve a high energy density required in a field such as an electric vehicle with a high theoretical capacity and an average voltage thereof. However, the high theoretical capacity, which is an advantage of the layered structure-based material, has been considered an unattainable goal due to an irreversible structural change or the like that occurs when an amount of desorption of alkali ions increases. An existing solution is to replace a transition metal, which is a center of oxidation/reduction, or to delay the irreversible structural change through doping of a metal element with high affinity for oxygen. 
     In general, the layered structure-based cathode active material is composed of an alkali metal (Li, Na, K, and the like) layer as a charge transfer medium and a transition metal oxide layer. In this connection, when the alkali metal in the transition metal oxide layer escapes at a rate equal to or higher than a certain rate, a layered structure is not able to be maintained thermodynamically, and thus, the structure is changed. The structure formed as such is thermodynamically more stable than the existing layered structure, so that the structure has irreversibility of not being able to be returned to the existing layered structure. 
     Such irreversible structural change is not able to be completely prevented even with the above solution. Therefore it has been considered impossible to fully utilize the high theoretical capacity of the layered structure-based material. 
     The inventive concept is to induce a reversible structural change using crystal water inserted into a manganese-based metal oxide layer to realize an electrochemical characteristic close to the theoretical capacity of the layered structure-based material and maximize charging/discharging reversibility. 
     In the following, with reference to  FIGS. 1 to 3  together, a cathode active material, which is prepared according to an embodiment of the inventive concept, exhibits a two-dimensional layered crystal structure, and is lithium-birnessite, will be described. 
       FIG. 1  is an exemplary diagram showing two different crystal structures in a cathode active material according to an embodiment of the inventive concept.  FIGS. 2 and 3  are exemplary views showing a crystal structure change during charging/discharging of a cathode active material according to an embodiment of the inventive concept,  FIG. 2  shows a charging state, and  FIG. 3  shows a discharging state. 
     The cathode active material according to an embodiment of the inventive concept is a manganese-based metal oxide containing the crystal water, and is able to exhibit a reversible phase transition of returning to an original structure during discharging as a structural change to a metastable phase instead of a stable phase is induced by the crystal water properly contained in the crystal structure before the irreversible structural change occurs completely. 
     The corresponding cathode active material may contain the crystal water and the manganese-based metal oxide. As shown in  FIG. 1 , the manganese-based metal oxide may have a two-dimensional crystal structure (a dotted square portion in  FIG. 1 ) in which a unit lattice structure in which manganese (Mn) and oxygen (O) are bonded to each other is extended two-dimensionally, and may have a three-dimensional crystal structure (a dotted circle portion in  FIG. 1 ) as manganese (Mn) in the manganese-based metal oxide is bonded with oxygen (O) in the crystal water. 
     Hereinafter, in one embodiment, for convenience, a phase having the two-dimensional crystal structure is referred to as a first crystal phase, and a phase having the three-dimensional crystal structure is referred to as a second crystal phase. 
     First, the first crystal phase has the two-dimensional crystal structure (a layered crystal structure) composed of the manganese-based metal oxide. In one embodiment, in the unit lattice of the two-dimensional crystal structure, the manganese fills octahedral sites of the unit lattice. 
     The two-dimensional crystal structure may again include two or more unit layers made of a manganese oxide, and may contain at least one metal element (Me) disposed between the two or more unit layers and selected from a group of metals other than the manganese. 
     The metal element (Me) other than the manganese may be repeatedly inserted into or desorbed from a space between the two or more unit layers during a charging/discharging process to repeatedly oxidize/reduce the manganese (Mn). Specifically, in the discharging process of the cathode active material according to an embodiment, the metal element (Me) other than the manganese may be inserted into the space between the unit layers. In addition, in the charging process of the cathode active material, the metal element (Me) other than the manganese may be desorbed from the space between the unit layers. 
     That is, the metal element (Me) other than the manganese serves to induce electrochemical activity of the manganese (Mn) while going in and out of the space between the two or more unit layers. 
     In one embodiment, examples of the metal element (Me) other than the manganese may include an alkali metal element, an alkaline earth metal element, or a combination thereof. However, the metal element (Me) other than the manganese according to an embodiment may not be necessarily limited thereto, and may include other typical elements or transition elements that may contribute to the electrochemical activity of the manganese (Mn) in the space between the two or more unit layers. 
     In addition, at least one dopant (e.g., Mg, Al, and Zn) may be doped and positioned in the space between the two or more unit layers. 
     In this connection, the dopant may be composed of a metal element having excellent oxygen affinity. 
     The dopant is positioned between the two or more unit layers and contributes to structural stabilization of the manganese (Mn) oxide. 
     In one embodiment, the crystal water may be further disposed between the two or more unit layers. That is, the crystal water may be disposed between the two or more unit layers together with the metal element (Me) other than the manganese. 
     In one example, the crystal water may exist between the two or more unit layers. Unlike the metal elements (Me) other than the manganese, the crystal water may remain in the space between the two or more unit layers without being desorbed. Accordingly, even when the metal element (Me) other than the manganese is repeatedly inserted into or desorbed from the space between the two or more unit layers, an arrangement of the unit layers may be continuously maintained without collapsing. 
     In one example, the crystal water according to an embodiment may form the chemical bonding with the manganese (Mn) as described above. In one embodiment, the chemical bonding between the crystal water and the manganese may proceed reversibly based on the charging/discharging. Specifically, in the charging process of the cathode active material according to an embodiment, bonding between the manganese (Mn) and oxygen constituting the unit layer is broken, and the metal element (Me) other than the manganese is desorbed out of the unit layer. In this connection, the manganese (Mn) whose bonding is broken is bonded with the oxygen in the crystal water to form the three-dimensional crystal structure. 
     On the other hand, in the discharging process of the cathode active material, the bonding between the oxygen of the crystal water and the manganese (Mn) is broken, and the manganese (Mn) whose bonding is broken is recombined with the oxygen constituting the unit layer. 
     That is, in one embodiment, the second crystal structure forming reaction by the bonding between the oxygen in the crystal water and the manganese and the first crystal structure forming reaction by the bonding between the oxygen constituting the unit layer and the manganese (Mn) may be in a mutually reversible relationship. 
     The phenomenon in which the formation reactions of the first crystal structure and the second crystal structure are mutually reversible as described above is resulted from a difference in thermodynamic stability between the bonding between the oxygen in the crystal water and the manganese (Mn) and the bonding between the oxygen constituting the unit layer and the manganese (Mn). 
     In one embodiment, the first crystal phase formed through the bonding between the oxygen constituting the unit layer and the manganese (Mn) is a thermodynamically stable phase. However, the second crystal phase formed through the bonding between the oxygen in the crystal water and the manganese (Mn) is a thermodynamically metastable phase. 
     Accordingly, in the cathode active material according to an embodiment, the first crystal phase, which is the stable phase, may be changed to the second crystal phase, which is the metastable phase, by external energy in a charging state, and the second crystal phase, which is the metastable phase, may be changed to the first crystal phase, which is the stable phase in a discharging state. 
     In one embodiment, the second crystal phase has the three-dimensional crystal structure as described above, and is the thermodynamically metastable phase. 
     When the second crystal phase is included within the above range in an initial state of the cathode active material according to an embodiment, even when the number (cycles) of charging/discharging of the cathode active material is increased, the reversible phase transition relationship between the first crystal phase and the second crystal phase may be maintained. 
     The second crystal phase according to an embodiment may have the three-dimensional crystal structure as described above. In a unit lattice of the three-dimensional crystal structure, the manganese may fill octahedral sites and tetrahedral sites of the unit lattice. That is, the number of bonding with the oxygen of the manganese in the unit lattice of the two-dimensional crystal structure and the manganese in the unit lattice of the three-dimensional crystal structure may be different. 
     In one embodiment, an arrangement relationship of the manganese and the oxygen in the unit lattice of the three-dimensional crystal structure is similar to that of a spinel crystal structure having the manganese as a central element. However, unlike the general spinel crystal structure having 6 oxygen atoms bonded to one manganese (Mn) atom, in the three-dimensional crystal structure according to an embodiment, 4 oxygen atoms may be bonded to one manganese (Mn) or 6 oxygen atoms may be bonded to one manganese (Mn) in the unit lattice. 
     The general spinel crystal structure is thermodynamically stable and exhibits irreversible characteristic. When the layered structure undergoes a phase transition to the thermodynamically stable spinel crystal structure during the charging, a content of the spinel crystal structure gradually increases by repeated charging/discharging. Because the spinel crystal structure has a lower theoretical capacity compared to the layered structure, a capacity and a lifespan of the cathode active material may be significantly reduced. 
     Therefore, a general cathode active material for a secondary battery not only should be adjusted not to contain a material having such a spinel crystal structure, but also should be adjusted such that the material having the spinel crystal structure is not formed in the repeated charging/discharging states. 
     In the general cathode active material for the secondary battery, the spinel crystal structure is formed in the process of charging to high voltage, so that the charging/discharging is performed by lowering the voltage to prevent the formation of the spinel crystal structure. As the charging/discharging driving is performed by lowering the voltage as such, it was inevitable to exhibit a lower capacity than the theoretical capacity of the cathode active material. 
     However, in the cathode active material according to an embodiment, because the first crystal phase and the second crystal phase have the mutually reversible phase transition relationship, even when the charging/discharging is repeated at the high voltage, excellent capacity characteristics close to the theoretical capacity and excellent lifespan characteristics may be exhibited at the same time through the reversible phase transition that forms the second crystal phase, which is the thermodynamically metastable phase, by bonding the manganese with the oxygen in the crystal water. 
     In one example, the first crystal phase and the second crystal phase may be mixed in the cathode active material according to an embodiment, as shown in  FIG. 1 . However, because the first crystal phase and the second crystal phase have different dimensions, even when the first crystal phase and the second crystal phase are mixed in the cathode active material, an overall microstructure may be different depending on contents of the first crystal phase and the second crystal phase. 
     That is, when one region of the cathode active material is selected, a microstructure in the corresponding region exhibits a microstructure corresponding to a crystal phase having a higher content among the first crystal phase and the second crystal phase. 
     The cathode active material is synthesized through oxidation and reduction of a precursor containing Mn (II) or Mn (VII). Various methods such as microwave, hydrothermal, and room temperature synthesis are used, and an oxidation/reduction agent, a hydrogen concentration index, and a chelating agent the required are required in each synthesis environment are used. 
     In addition, referring to  FIGS. 2 and 3 , the cathode active material according to  FIG. 1  is in the initial state without being charged/discharged. In the cathode active material according to an embodiment, as the charging and the discharging are performed once, the internal crystal structure is sequentially changed in an order of  FIGS. 1, 2, and 3 . 
     In addition, when the charging/discharging is repeated for the cathode active material whose discharging has been completed, the cathode active material becomes to exhibit the internal crystal structures in  FIGS. 2 and 3  repeatedly. 
     Referring to  FIG. 2  first, in the charging state of the cathode active material according to an embodiment, the first crystal phase is reduced and the second crystal phase is formed. Specifically, in the charging state of the cathode active material, at least some of the manganese (Mn) in the manganese-based metal oxide migrates from the two-dimensional layered crystal structure and becomes somewhat thermodynamically unstable compared to the two-dimensional layered crystal structure, and a reaction in which the migrated manganese (Mn) is combined with the oxygen (O) in the crystal water predominantly proceeds. Therefore, as the charging proceeds, the content of the second crystal phase in the cathode active material gradually increases. 
     On the other hand, referring to  FIG. 3 , in the discharging state of the cathode active material according to an embodiment, the second crystal phase is reduced and the first crystal phase is formed. Specifically, in the discharging state of the cathode active material, a reaction in which the bonding between the manganese (Mn) in the manganese-based metal oxide and the oxygen (O) in the crystal water is broken predominantly proceeds. As the migrated manganese (Mn) becomes to be thermodynamically stable again, the migrated manganese (Mn) is bonded with the oxygen constituting the unit layer to form the two-dimensional layered crystal structure. Therefore, as the discharging progresses, the content of the first crystal phase in the cathode active material gradually increases. 
     In addition, in the discharging state, the dopant is doped in the space between the two or more unit layers to stabilize the structure. 
     As such, in the cathode active material according to an embodiment, the first crystal phase, which is the thermodynamically stable phase, and the second crystal phase, which is the thermodynamically metastable phase, have the mutually reversible phase transition relationship, so that even when the charging/discharging is repeated, the excellent capacity characteristics close to the theoretical capacity and the excellent lifespan characteristics may be realized at the same time. 
     Hereinafter, a secondary battery containing a cathode active material according to an embodiment of the inventive concept will be described with reference to  FIG. 4 . 
       FIG. 4  is a view showing a secondary battery containing a cathode active material according to an embodiment of the inventive concept. 
     A secondary battery  21  according to  FIG. 4  includes a cathode  23  containing the cathode active material according to an embodiment, an anode  22 , and a separator  24  containing an electrolyte. 
     The cathode  23  and the anode  22  are manufactured by coating and drying a composition for preparing a cathode active material layer and a composition for preparing a cathode active material layer on a current collector, respectively. 
     The composition for preparing the cathode active material is prepared by mixing the cathode active material, a conductive agent, a binder, and a solvent with each other. As the cathode active material, the cathode active material according to the above-described embodiment is used. 
     In one example, the secondary battery  21  according to  FIG. 4  merely relates to the secondary battery structure containing the cathode active material according to the inventive concept, and the inventive concept is not limited thereto. A specific implementation detail will be described later through a following implementation process. 
     Example 
     Electrochemical Doping 
     Electrolyte Preparation: For the electrochemical doping, an electrolyte to which a dopant (another element with high oxygen affinity such as Mg, Al, Cu, Zn, or the like) is added is prepared. First, 1M LiPF 6  in EC/DEC electrolyte in which lithium salt (LiPF 6 ) of 1 molar concentration is dissolved in an EC/DEC electrolyte with different carbonate-based non-aqueous electrolytes, EC (ethylene carbonate) and DEC (diethylene carbonate) being mixed in a volume ratio of 1:1 is prepared. A dopant salt [Do·(TFSI) 2 , Do·bis(trifluoromethanesulfonyl)imide] corresponding to 1 weight ratio (1 wt %) of the corresponding electrolyte is added and stirred for 24 hours. The prepared electrolyte is an electrolyte (1 wt % Do·TFSI added 1M LiPF 6  in EC/DEC) containing the dopant salt. 
     Electrochemical Doping: When the charging/discharging is performed at a current density of 0.1 C in a voltage range from 1.5 V to 4.8 V after manufacturing a 2032 coin type Li half cell in a glove box using the previously prepared dopant salt-added electrolyte, at a point from which the alkali metal element (Me), which plays a role of transporting the charge in the cathode active material during the charging, escaped, the dopant existing in the electrolyte together with the alkali metal element is inserted into the transition metal oxide layer during the discharging. 
     The corresponding process is shown in  FIG. 1 . When the dopant (Mg) is inserted into the layer by such electrochemical doping, because of the high oxygen affinity of the dopant (Mg), the formation of the first crystal phase becomes easier, and as a result, the reversible phase transition is facilitated, thereby maximizing the charging/discharging reversibility of the corresponding cathode active material. 
     In one example, herein, the dopant may be defined as an element to be doped, and the dopant may not be limited to Mg, Al, Cu, or Zn and may be another metal element having the high oxygen affinity. 
     In one example, the dopant may be a metal different from the above-described metal element (Me). 
     Doping Using Chemical Ion Exchange 
     Sodium Manganese Oxide Synthesis Method 1: A sodium manganese oxide is obtained by adding 2.5 ml of NaMnO 4  as a manganese precursor to 40 ml of distilled water and performing the hydrothermal synthesis or the microwave synthesis at 220° C. for 6 hours. 
     After washing the obtained sodium manganese oxide with the distilled water three times, residual moisture remaining on a surface is removed by drying the sodium manganese oxide at about 60° C. under vacuum for 24 hours. 
     Sodium Manganese Oxide Synthesis Method 2: 8 mmol of MnSO 4 ·H 2 O as the manganese precursor is put into 25 ml of the distilled water together with 8 mmol of EDTA (Ethylenediamine tetraacetic acid) disodium salt and mixed. 1.2M NaOH aqueous solution is added to the mixed solution at a rate of 3 mL for each minute. After stirring the mixed solution for about 1 hour, the manganese-based oxide is obtained. After washing the obtained material with the distilled water three times, the residual moisture remaining on the surface is removed by drying the obtained material under the vacuum at about 60° C. for 24 hours. 
     Doping Using Chemical Ion Exchange: 0.1 g of the sodium manganese oxide synthesized using the above synthesis method is put into 10 ml of an electrolyte (1 wt % Do·TFSI added 1M LiPF 6  in EC/DEC) to which the dopant salt is added, and stirred for 24 hours to obtain a doped manganese-based oxide. Thereafter, after washing the obtained material with the distilled water three times, the residual moisture remaining on the surface is removed by drying the obtained material under the vacuum at about 60° C. for 24 hours. 
     Direct Synthesis 
     In addition to the electrochemical doping and the doping through the chemical ion exchange, the manganese precursor, a precursor containing an electrochemically active element (Li, Na, K, and the like) that is inserted/desorbed during the charging/discharging, and an aqueous solution containing a doping element (an electrolyte to which the dopant salt is added or an aqueous solution of the dopant salt) are simultaneous reacted to prepare a doped layered manganese oxide by the hydrothermal synthesis. Thereafter, after washing the obtained material with the distilled water, the residual moisture remaining on the surface is removed by drying the obtained material at 60° C. for 24 hours. 
     The cathode active material of Example prepared through the above process has a composition expressed as Li 0.43 Do 0.07 MnO 2 ·0.30H 2 O. 
     Electrode and Battery Manufacturing 
     Electrode Manufacturing 
     The composition for preparing the cathode active material is prepared by mixing the cathode active material, the conductive agent, the binder, and the solvent with each other. As the cathode active material, the cathode active material according to the above-described embodiment is used. 
     The binder, which is a component that assists in bonding of the active material and the conductive agent and bonding to the current collector, is added in an amount of 1 to 50 parts by weight based on 100 parts by weight of a total weight of the cathode active material. Non-limiting examples of such binder may include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene ether polymer (EPDM), sulfonated EPDM, styrene butadiene rubber, fluororubber, various copolymers, and the like. A content thereof is 2 to 5 parts by weight based on 100 parts by weight of the total weight of the cathode active material. When the binder content is within the above range, a binding force of the active material layer to the current collector is good. 
     The conductive agent is not particularly limited as long as it has conductivity without causing a chemical change in the battery. For example, graphite such as natural graphite, artificial graphite, and the like, a carbon-based material such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, summer black, and the like, a conductive fiber such as a carbon fiber, a metal fiber, and the like, a carbon fluoride, a metal powder such as an aluminum powder, a nickel powder, and the like, a conductive whiskey such as a zinc oxide, a potassium titanate, and the like, a conductive metal oxide such as a titanium oxide and the like, a conductive material such as a polyphenylene derivative and the like may be used. 
     A content of the conductive agent is 1 to 10 parts by weight based on 100 parts by weight of the total weight of the cathode active material. When the content of the conductive agent is within the above range, a conductivity characteristic of the finally obtained electrode is excellent. 
     As a non-limiting example of the solvent, N-methylpyrrolidone and the like are used. 
     A content of the solvent is 1 to 10 parts by weight based on 100 parts by weight of the cathode active material. When the content of solvent is within the above range, a work for forming the active material layer becomes easy. 
     The cathode current collector has a thickness in a range from 3 to 500 μm, and is not particularly limited as long as it has high conductivity without causing the chemical change in the battery. For example, stainless steel, aluminum, nickel, titanium, heat-treated carbon, or aluminum or stainless steel subjected to a surface treatment with carbon, nickel, titanium, silver, and the like may be used. The current collector may increase an adhesion of the cathode active material by forming fine irregularities on a surface thereof, and may be in various forms, such as a film, a sheet, a foil, a net, a porous body, a foam, a nonwoven body, and the like. 
     In the present experiment, to manufacture the cathode, on an Al current collector, the cathode active material: the conductive agent: the binder are mixed with each other in a weight ratio of 8:1:1, and subjected to a heat treatment in a vacuum oven at 70° C. for 5 hours. 
     Battery Manufacturing 
     The 2032 coin type Li half cell is manufactured in the glove box. 
     The electrolyte according to an embodiment may be a non-aqueous electrolyte. In one embodiment, the non-aqueous electrolyte may be a metal salt-containing non-aqueous electrolyte. 
     The metal salt-containing non-aqueous electrolyte is composed of the non-aqueous electrolyte and the metal salt. As the non-aqueous electrolyte, a non-aqueous electrolyte, an organic solid electrolyte, an inorganic solid electrolyte, and the like are used. As a metal constituting the metal salt, at least one element selected from the alkali metal and the alkaline earth metal may be used. In one embodiment, the metal salt may be one selected from a lithium salt, a sodium salt, a magnesium salt, a potassium salt, and a calcium salt. In one embodiment, the metal salt may be one selected from the lithium salt and the sodium salt. 
     As the non-aqueous electrolyte, as non-limiting examples, N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, N,N-dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, a dioxolane derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazole ridinone, a propylene carbonate derivative, a tetrahydrofuran derivative, and an aprotic organic solvent such as ether, methyl pyropionate, and ethyl propionate may be used. 
     As the organic solid electrolyte, as non-limiting examples, a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphoric acid ester polymer, polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, and the like may be used. 
     As the inorganic solid electrolyte, as non-limiting examples, Li 3 N, LiI, Li 5 NI 2 , Li 3 N—LiI—LiOH, LiSiO 4 , LiSiO 4 —LiI—LiOH, Li 2 SiS 3 , Li 4 SiO 4 , Li 4 SiO 4 —LiI—LiOH, Li 3 PO 4 —Li 2 S—SiS 2 , and the like may be used. 
     When the lithium salt is used as the metal salt, as a material that is good to dissolve in the non-aqueous electrolyte, as non-limiting examples, LiCl, LiBr, LiI, LiClO 4 , LiBF 4 , LiBioClio, LiPF 6 , LiCF 3 SO 3 , LiCF 3 CO 2 , LiAsF 6 , LiSbF 6 , LiAlCl 4 , CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2 ) 2 NLi, lithium chloroborate, lithium lower aliphatic carboxylate, lithium tetraphenyl borate, imide, and the like may be used. 
     In Example, 1 wt % of an Mg(TFSI) 2  additive was mixed with the 1M LiPF 6  in EC/DEC electrolyte for insertion of Mg having the high oxygen affinity as the dopant and used for a test. 
     In Comparative Example, the 1M LiPF 6  in EC/DEC electrolyte without the dopant was used for the test. 
     Hereinafter, with reference to  FIGS. 5 to 7 , characteristics of the cathode active material and the secondary battery containing the cathode active material according to Example of the inventive concept will be described. 
     Analysis Result 
     Evaluation 1: Comparison of Electrochemical Characteristics of Cathode Active Materials 
     After performing the charging/discharging at the current density of 0.1 C in the voltage range from 1.5 V to 4.8 V for manufactured half cells of Example and Comparative Example, results of the 1st, 2nd, 10th, 30th, 50th, 70th, and 100th times are shown in  FIGS. 5 and 6 , respectively. 
       FIG. 5  is a graph showing charging/discharging characteristics of the 1st, 2nd, 10th, 30th, 50th, 70th, and 100th times of a secondary battery containing a cathode active material according to Example. 
       FIG. 6  is a graph showing charging/discharging characteristics of 1st, 2nd, 10th, 30th, 50th, 70th, and 100th times of secondary battery containing a cathode active material according to Comparative Example. 
     Referring to  FIGS. 5 and 6 , both the cathode active materials exhibited capacities of about 74 (Example) to 80 (Comparative Example) mAh/g when first charging was performed. In one example, it may be seen that the cathode active material according to Example exhibits a capacity of about 275 mAh/g, and the cathode active material according to Comparative Example exhibits a capacity of about 277 mAh/g when performing first discharging. However, when viewing the charging/discharging graphs after the 1st time, it may be seen that Example exhibits higher capacity than Comparative Example. 
     In addition, referring to  FIG. 7 , based on the 100th time, Example exhibits a discharging capacity of about 207 mAh/g, and Comparative Example exhibits a discharging capacity of about 132 mAh/g, which respectively represent capacity retention rates of 75% and 47% compared to the 1st time. 
     That is, it may be seen through  FIGS. 5 to 7  that the cathode active material according to Example in which the dopant (Mg) having the high oxygen affinity is inserted exhibits a higher capacity retention rate compared to Comparative Example based on the maximized reversibility. 
     By the addition of the dopant, the dopant having the excellent oxygen affinity is doped between the two or more unit layers in the discharging state, thereby structurally stabilizing the space between the two or more unit layers. 
     Evaluation 2: Dopant Amount Analysis 
       FIG. 8  is a result of analyzing an active material prepared by electrochemical doping according to Example of the inventive concept through ICP (Inductively Coupled Plasma). The ICP analysis is a method for ionizing a sample using plasma to analyze a concentration of ions contained in the sample. Through the analysis, it was identified that 0.7 mol % of Mg was inserted into the active material prepared through the above-described process. 
     The above detailed description exemplifies the inventive concept. In addition, the above-mentioned content is to describe a preferred embodiment of the inventive concept, and the inventive concept is able to be used in various other combinations, modifications, and environments. That is, changes or modifications are possible within the scope of the concept of the inventive concept disclosed herein, the scope equivalent to the written disclosure, and/or within the scope of skill or knowledge in the art. The written embodiment is to describe the best state for implementing the technical idea of the inventive concept, and various changes thereof required in specific application fields and uses of the inventive concept are also possible. Therefore, the above detailed description of the inventive concept is not intended to limit the inventive concept to the disclosed embodiment. The appended claims should also be construed to include other embodiments as well. 
     According to embodiments of the inventive concept, it is possible to exhibit the excellent capacity and lifespan characteristics by making the structural change that appears during the charging/discharging to be reversible. 
     In one example, effects that may be obtained from the inventive concept are not limited to the above-mentioned effects. Other effects not mentioned will be clearly understood by those of ordinary skill in the art to which the inventive concept belongs from a following description. 
     While the inventive concept has been described with reference to exemplary embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the inventive concept. Therefore, it should be understood that the above embodiments are not limiting, but illustrative.