Patent Publication Number: US-2013252107-A1

Title: Positive electrode active material for lithium ion secondary battery and lithium ion secondary battery including positive electrode active material

Description:
BACKGROUND 
     1. Field 
     Embodiments relate to a positive electrode active material for a lithium ion secondary battery and a lithium ion secondary battery including the same, and more particularly, to a positive electrode active material for a lithium ion secondary battery including layer-structured lithium metal oxide and one or more selected from the group consisting of spinel-structured lithium metal oxides and olivine-structured lithium metal oxides, and a lithium ion secondary battery including the positive electrode active material. 
     2. Description of the Related Art 
     Recently, in line with the requirements for miniaturization and lightness of portable electronic devices such as mobile phones, notebooks, and personal digital assistants (PDAs), improvements in characteristics, such as high capacity, long lifetime, and high safety, of lithium (Li) ion secondary batteries used as power sources of these electronic devices are required. Also, interests in automotive electrification have grown and a lithium ion secondary battery as a power source of electric vehicles has emerged as a powerful alternative. 
     Currently, an industrially most used positive electrode active material in lithium ion secondary batteries is layer-structured LiCoO 2 . The layer-structured LiCoO 2  has a high market share because lifetime characteristics are excellent and manufacturing is easy. However, high price of cobalt (Co) used as a raw material of transition metal may serve as a cause of increasing the price of secondary battery. 
     Recently, layer-structured LiCoO 2  becomes replaced by Li(NiCoMn)O 2  (NCM)-based materials having a low cobalt content and high capacity, as an application range of lithium ion secondary battery is extended from small electronic devices to electric vehicles and power storage. 
     A spinel-structured positive electrode active material and an olivine-structured positive electrode active material may have a relatively stable structure in comparison to that of a layer-structured positive electrode active material. However, the spinel-structured and olivine-structured positive electrode active materials may have capacities lower than those of NCM-based materials. 
     SUMMARY 
     An aspect of the present invention provides a positive electrode active material having improved stability by using a high-capacity positive electrode active material, xLiMO 2 ·(1−x)Li 2 MnO 3  lithium metal oxide, and a high-stability positive electrode active material, an olivine-structured lithium metal oxide and/or a spinel-structured lithium metal oxide, and a lithium ion secondary battery including the positive electrode active material. 
     According to at least one of embodiments, a positive electrode active material for a lithium ion secondary battery includes layer-structured lithium metal oxide expressed by the following Chemical Formula (1); and at least one selected from the group consisting of spinel-structured lithium metal oxides and olivine-structured lithium metal oxides, 
         x LiMO 2 ·(1 −x )Li 2 MnO 3   (1)
 
     where M is one or more selected from the group consisting of nickel (Ni), cobalt (Co), and manganese (Mn), and 0&lt;x&lt;1. 
     The spinel-structured lithium metal oxide may be LiMn 2 O 4 . 
     The olivine-structured lithium metal oxide may be one or more selected from the group consisting of LiFePO 4 , LiMnPO 4 , and LiFe x Mn (1−x) PO 4  (0&lt;x&lt;1). 
     A total content of the spinel-structured lithium metal oxide and the olivine-structured lithium metal oxide may be in a range of 10 wt % to 30 wt % of the entire positive electrode active material. 
     According to another embodiment, a positive electrode includes the positive electrode active material of the present disclosure. 
     According to another embodiment, a lithium ion secondary battery includes the positive electrode including the positive electrode active material of the present disclosure, an electrolyte, and a negative electrode. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain principles of the present disclosure. In the drawings: 
         FIG. 1  is scanning electron microscope (SEM) images of lithium metal oxides of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Korean Patent Application No. 10-2012-0029508 filed on Mar. 22, 2012, in the Korean Intellectual Property Office, and entitled: “Positive Electrode Active Material for Lithium Ion Secondary Battery and Lithium Ion Secondary Battery Including the Same” is incorporated by reference herein in its entirety. 
     Embodiments relate to a positive electrode active material for a lithium ion secondary battery including layer-structured lithium metal oxide expressed by the following Chemical Formula 1 and at least one selected from the group consisting of spinel-structured lithium metal oxides and olivine-structured lithium metal oxides. 
         x LiMO 2 ·(1 −x )Li 2 MnO 3   (1)
 
     (where M is one or more selected from the group consisting of nickel (Ni), cobalt (Co), and manganese (Mn), and 0&lt;x&lt;1.) 
     Lithium metal oxide is mainly used for a positive electrode active material as a positive electrode material of a lithium ion secondary battery, and may be broadly classified as a layer type, a spinel type, and an olivine type according to the structure thereof. Layer-structured oxide may be expressed by a chemical formula of LiMO 2  (M=Co, Ni, Mn, etc.) and has a form, in which two MO 2  layers exist in a single crystal structure and lithium ions exist between each MO 2  layer. The layer-type positive electrode active material has a limitation in that electrode capacity may decrease because the structure thereof may be changed according to the extraction of lithium ions. 
     In the present disclosure, Li 2 MnO 3  is used for stabilizing the foregoing layer structure. Since Mn in Li 2 MnO 3  exists as a stable tetravalent cation and has a high activation barrier for diffusion, Mn contributes to stabilize the layer structure. 
     Since the lithium metal oxide expressed by Chemical Formula 1 of the present disclosure has a sufficient amount of lithium, the lithium metal oxide may exhibit high capacity and may be used at a high voltage as compared to a typical lithium nickel cobalt manganese oxide (NCM)-based positive electrode active material, and thus, a highly safe lithium ion secondary battery may be prepared. 
     The spinel-structured lithium metal oxide may have a composition of LiM 2 O 4  (M=titanium (Ti), vanadium (V), Mn, or Ni) and have a cubic crystal structure. Since the spinel-structured lithium metal oxide has a three-dimensional crystal structure, a movement path of lithium ions is short and ionic conductivity is high, and since there is no destruction of the overall structure thereof during the extraction of lithium ions, the spinel-structured lithium metal oxide is very stable. In the embodiments, LiM 2 O 4  may be used as the spinel-structured lithium metal oxide. The reason for this is that manganese is more abundant and environmentally friendly than cobalt and has excellent thermal stability for a positive electrode active material in relation to the safety of lithium ion secondary battery. 
     Since the olivine-structured lithium metal oxide has a very stable structure, there is virtually no decrease in capacity and chemical stability thereof is also high. In the embodiments, one or more selected from the group consisting of LiFePO 4 , LiMnPO 4 , and LiFe x Mn (1−x) PO 4  (0&lt;x&lt;1) may be used as the olivine-structured lithium metal oxide. 
     Therefore, the positive electrode active material of the present disclosure may improve lifetime characteristics of lithium ion secondary battery and provide a positive electrode active material having improved stability by using high-stability spinel-structured lithium metal oxide and/or olivine-structured lithium metal oxide as well as high-capacity xLiMO 2 ·(1−x)Li 2 MnO 3  lithium metal oxide. 
     In the embodiments, a total content of the spinel-structured lithium metal oxide and the olivine-structured lithium metal oxide may be in a range of 10 wt % to 30 wt % of the entire positive electrode active material. In the case that the total content thereof is less than 10 wt % or greater than 30 wt %, stability and lifetime characteristics may decrease. 
     The positive electrode active material according to the embodiments may be obtained through a heat treatment after uniformly mixing lithium metal oxide powder of Chemical Formula 1 with spinel-structured lithium metal oxide powder and/or olivine-structured lithium metal oxide powder. The mixing may be performed by using various methods. When the mixing is performed by using a Thinky Mixer, the mixing is performed at 1000 rpm to 2000 rpm for 1 minute and the method is repeated for three times. Also, the mixing may be performed for about 30 minutes to one hour by using a mortar and a ball mill may be used as another method. 
     The embodiment provides a lithium ion secondary battery including a positive electrode including the positive electrode active material of the present disclosure, a negative electrode, and an electrolyte. 
     The lithium ion secondary battery may be prepared by introducing the electrolyte between the positive electrode and the negative electrode according to a typical method well known in the art. 
     An electrode used in a lithium ion secondary battery is typically prepared in such a manner that an active material, a binder, and a conductive agent are mixed with a solvent to form a slurry, and an electrode collector is coated with the slurry, dried, and pressed. 
     In the lithium ion secondary battery of the present disclosure, natural graphite, artificial graphite, carbon fibers, cokes, carbon black, carbon nanotubes, fullerenes, active carbon, lithium metal, or a lithium alloy may be used as a negative electrode active material, but the present disclosure is not limited thereto. 
     The binder functions to bond the active material and the conductive agent to be adhered to the electrode collector. A binder typically used in a lithium ion secondary battery, such as polyvinylidene fluoride, polypropylene, carboxymethyl cellulose, starch, hydroxypropyl cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, an ethylene-propylene-diene polymer (EPDM), polyvinyl alcohol, a styrene-butadiene rubber, and a fluoro rubber, may be used. 
     The conductive agent is not particularly limited so long as it does not cause chemical changes in the battery as well as having conductivity. For example, artificial graphite, natural graphite, Denka black, acetylene black, Ketjen black, channel black, lamp black, thermal black, conductive fibers such as carbon fibers or metal fibers, conductive metal oxides such as titanium oxide, and metal powders such as aluminum powder and nickel powder, may be used. 
     The electrolyte acts as a medium for transferring lithium ions in the positive electrode and the negative electrode, and an electrolyte having a lithium salt dissolved in an organic solvent may be used. Examples of the organic solvent may be ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, butylene carbonate, and acetonitrile, and the organic solvent may be used alone or in combination thereof. The lithium salt acts as a source of lithium ions and for example, a lithium salt typically used in an electrolyte of a lithium ion secondary battery, such as LiPF 6 , LiBF 4 , LiSbF 6 , LiAsF 6 , LiClO 4 , LiN(C 2 F 5 SO 2 ) 2 , LiN(CF 3 SO 2 ) 2 , CF 3 SO 3 Li, and LiC(CF 3 SO 2 ) 3 , may be used. 
     The lithium ion secondary battery of the present disclosure may prevent a short circuit between two electrodes by including a separator between the positive electrode and the negative electrode. A separator typically used in a lithium ion secondary battery, for example, a single olefin, such as polyethylene (PE) and polypropylene (PP), or an olefin composite, polyamide (PA), poly(acrylonitrile) (PAN), poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO), poly(ethylene glycol)diacrylate (PEGA), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), and polyvinyl chloride (PVC), may be used. 
     Hereinafter, the present disclosure will be described in more detail according to the following examples. However, the present disclosure is not limited thereto. 
     EXAMPLES 
     Example 1 
     10 wt % of LiFePO 4  and 90 wt % of 0.17Li 2 MnO 3 -0.83LiNi 0.58 Co 0.20 Mn 0.23 O 2  were weighed and ball milled at 100 rpm. 
     Example 2 
     20 wt % of LiFePO 4  and 80 wt % of 0.17Li 2 MnO 3 -0.83LiNi 0.58 Co 0.20 Mn 0.23 O 2  were weighed and ball milled at 100 rpm. 
     Example 3 
     30 wt % of LiFePO 4  and 70 wt % of 0.17Li 2 MnO 3 -0.83LiNi 0.58 Co 0.20 Mn 0.23 O 2  were weighed and ball milled at 100 rpm. 
     Example 4 
     10 wt % of LiMn 2 O 4  and 90 wt % of 0.17Li 2 MnO 3 -0.83LiNi 0.58 Co 0.20 Mn 0.23 O 2  were weighed and mixed by using a Thinky Mixer at 1000 rpm to 2000 rpm for 1 minute, and the mixing was repeated for three times. 
     Example 5 
     20 wt % of LiMn 2 O 4  and 80 wt % of 0.17Li 2 MnO 3 -0.83LiNi 0.58 Co 0.20 Mn 0.23 O 2  were weighed and mixed by using a Thinky Mixer at 1000 rpm to 2000 rpm for 1 minute, and the mixing was repeated for three times. 
     Example 6 
     10 wt % of LiFePO 4 , 20 wt % of LiMn 2 O 4  and 70 wt % of 0.17Li 2 MnO 3 -0.83LiNi 0.58 Co 0.20 Mn 0.23 O 2  were weighed and mixed by using a Thinky Mixer at 1000 rpm to 2000 rpm for 1 minute, and the mixing was repeated for three times. 
     Example 7 
     20 wt % of LiFePO 4 , 10 wt % of LiMn 2 O 4  and 70 wt % of 0.17Li 2 MnO 3 -0.83LiNi 0.58 Co 0.20 Mn 0.23 O 2  were weighed and mixed by using a Thinky Mixer at 1000 rpm to 2000 rpm for 1 minute, and the mixing was repeated for three times. 
     Experimental Example 1 
     Each positive electrode active material obtained from the forgoing Examples, a Denka black conductive agent, and a polyvinylidene fluoride (PVDF) binder were mixed at a weight ratio of 92:4:4 in N-methyl-2-pyrrolidone (NMP) to form slurries. The slurries were casted on thin aluminum foils, and the thin aluminum foils were dried in an oven at 80° C. for 1 hour and in a vacuum oven at 120° C. for 2 hours, and then pressed to prepare positive electrodes. Each coin cell was prepared by using lithium metal as a negative electrode and a 1.3M LiPF 6  ethylene carbonate (EC)/dimethylene carbonate (DMC)/EC (=5:3:2) solution as an electrolyte. 
     The batteries thus prepared were charged to 4.5 V, and then voltages of the batteries in a fully charged state were measured and the batteries were disassembled within a short period of time so as not to allow the voltages to be decreased to 4.3 V or less. Positive electrodes of the disassembled batteries were cleaned with the electrolyte and dried in an oven at 100° C. for 1 hour, and then positive electrode active materials were collected. 
     The collected positive electrode active materials and electrolytes were put in cans for differential scanning calorimetery (DSC) and assembled, and thermal stabilities were then measured. The results thereof are presented in Table 1. 
     Experimental Example 2 
     50 cycles of charge and discharge were repeated for batteries prepared in the same manner as Experimental Example 1 to measure capacity retention ratios. The results thereof are presented in Table 1. 
     Comparative Example 
     0.8Li 2 MnO 3 -0.2LiNiCoMnO 2  was used as a positive electrode active material to measure for a DSC calorific value and a capacity retention ratio in the same manner as described in the foregoing Experimental Examples. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                 Discharge capacity 
               
               
                   
                   
                 0.17Li 2 MnO 3 — 
                   
                 DSC calorific 
                 retention ratio 
               
               
                 Category 
                 LiFePO 4   
                 0.83LiNi 0.58 Co 0.20 Mn 0.23 O 2   
                 LiMn 2 O 4   
                 value (J/g) 
                 after 50 cycles 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 10 
                 90 
                 — 
                 1000 
                 90% 
               
               
                 Example 2 
                 20 
                 80 
                 — 
                 970 
                 93% 
               
               
                 Example 3 
                 30 
                 70 
                 — 
                 900 
                 97% 
               
               
                 Example 4 
                 — 
                 90 
                 10 
                 1200 
                 80% 
               
               
                 Example 5 
                 — 
                 80 
                 20 
                 1150 
                 80% 
               
               
                 Example 6 
                 10 
                 70 
                 20 
                 1100 
                 83% 
               
               
                 Example 7 
                 20 
                 70 
                 10 
                 1050 
                 88% 
               
               
                 Comparative 
                 — 
                 100 
                 — 
                 1244 
                 76% 
               
               
                 Example 
               
               
                   
               
            
           
         
       
     
     A positive electrode active material according to the present disclosure may provide a high-capacity and stable lithium ion secondary battery having improved thermal stability and capacity retention ratio. 
     Exemplary embodiments have been disclosed herein, and although specific terms are employed, they are used and are to be interpreted in a generic and descriptive sense only and not for purpose of limitation. Accordingly, it will be understood by those of ordinary skill in the art that various changes in form and details may be made without departing from the spirit and scope of the present disclosure as set forth in the following claims.