Abstract:
A solid-electrolyte battery is provided that includes a LiNbO 3  film serving as a buffer layer between a positive-electrode active material and a solid electrolyte and has a sufficiently low electrical resistance. The solid-electrolyte battery includes a positive-electrode layer, a negative-electrode layer, and a solid-electrolyte layer that conducts lithium ions between the electrode layers, wherein a buffer layer that is a LiNbO 3  film is disposed between a positive-electrode active material and a solid electrolyte, and a composition ratio (Li/Nb) of Li to Nb in the LiNbO 3  film satisfies 0.93≦Li/Nb≦0.98. The buffer layer may be disposed between the positive-electrode layer and the solid-electrolyte layer or on the surface of a particle of the positive-electrode active material. The buffer layer may have a thickness of 2 nm to 1 μm.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a solid-electrolyte battery including a positive-electrode layer, a negative-electrode layer, and a solid-electrolyte layer that conducts lithium ions between the electrode layers, in particular, to a solid-electrolyte battery including a buffer layer between a positive-electrode active material and a solid electrolyte. 
       BACKGROUND ART 
       [0002]    In recent years, solid-electrolyte batteries including a positive-electrode layer, a negative-electrode layer, and a solid-electrolyte layer that mediates conduction of lithium ions between the two layers have been developed as power supplies for small portable electronic devices such as cellular phones and notebook computers. 
         [0003]    Use of a solid-electrolyte layer can overcome disadvantages caused by existing electrolytic solutions composed of organic solvents, for example, safety problems caused by leakage of electrolytic solutions and heat-resistance problems caused by evaporation of organic electrolytic solutions at high temperatures that are higher than the boiling points of the solutions. 
         [0004]    However, in such a solid-electrolyte battery, a layer (depletion layer) in which lithium ions are depleted in a region of the solid electrolyte, the region being close to the positive-electrode layer, is formed as a resistive layer and this layer increases the electrical resistance, which is problematic (Non Patent Literature 1). 
         [0005]    To overcome this problem, the inventors of the present invention disclosed that, by forming a buffer layer composed of a lithium-ion-conductive oxide such as LiNbO 3  between the positive-electrode layer and the solid-electrolyte layer, the formation of the resistive layer is suppressed to decrease the electrical resistance (Japanese Patent Application No. 2007-235885). 
         [0006]    Another method of suppressing the formation of the resistive layer has been proposed in which a buffer layer composed of a lithium-ion-conductive oxide such as LiNbO 3  is formed such that the buffer layer covers the surface of a positive-electrode active material and the positive-electrode active material is not in contact with the solid electrolyte (Patent Literature 1). 
       CITATION LIST 
     Non Patent Literature 
       [0000]    
       
         NPL 1: Advanced Materials 2006. 18, 2226-2229 
       
     
       Patent Literature 
       [0000]    
       
         PTL 1: Domestic Re-publication of PCT International Publication for Patent Application No. WO2007/004590 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0009]    However, even in such solid-electrolyte batteries, the electrical resistance is not sufficiently decreased. 
         [0010]    Accordingly, an object of the present invention is to provide a solid-electrolyte battery that includes a LiNbO 3  film serving as a buffer layer between a positive-electrode active material and a solid electrolyte and has a sufficiently low electrical resistance. 
       Solution to Problem 
       [0011]    The inventors of the present invention first performed various experiments and studies on the reason why the electrical resistance is not sufficiently decreased in a solid-electrolyte battery including a LiNbO 3  film serving as a buffer layer. As a result, the inventors have found that the Li component of the LiNbO 3  film, which is an amorphous unstable film, partially reacts with the air to form Li 2 CO 3 ; the Li 2 CO 3  layer, which does not let electricity pass therethrough, serves as a highly resistive layer; as a result, the effective area contributing to the battery operation decreases and the internal resistance of the battery cannot be sufficiently decreased. 
         [0012]    The inventors of the present invention presumed that the Li 2 CO 3  layer is formed because the Li content of the LiNbO 3  film is high. The inventors further considered that the LiNbO 3  film is an amorphous film and is stable even when it does not satisfy the stoichiometric ratio; accordingly, when the Li content is decreased, that is, the stoichiometric ratio of Li to Nb (composition ratio Li/Nb) in the LiNbO 3  film is decreased, the formation of the Li 2 CO 3  layer can be suppressed. Thus, the inventors further performed experiments in which they varied the composition ratio Li/Nb of the LiNbO 3  film. 
         [0013]    As a result, it has been found that the formation of Li 2 CO 3  is suppressed in a LiNbO 3  film having a composition ratio Li/Nb of 0.98 or less. 
         [0014]    That is, it has been found that the Li 2 CO 3  layer is formed in existing techniques because the LiNbO 3  film is formed so as to have a composition ratio Li/Nb of 1.0. 
         [0015]    However, it has also been found that, when a LiNbO 3  film has an excessively small composition ratio Li/Nb, specifically, less than 0.93, the Nb content becomes excessively high; the excess Nb forms a Nb oxide layer formed of NbO in the LiNbO 3  film; the Nb oxide layer causes a decrease in the electric conductivity of the formed LiNbO 3  film and serves as a resistive layer. 
         [0016]    In summary, the finding has been obtained that, when a LiNbO 3  film has a composition ratio Li/Nb satisfying 0.93≦Li/Nb≦0.98, the formation of a Li 2 CO 3  layer and Nb oxide layers that serve as resistive layers can be suppressed and the electrical resistance can be sufficiently decreased. 
         [0017]    The present invention is based on the finding and provides 
         [0018]    a solid-electrolyte battery including a positive-electrode layer, a negative-electrode layer, and a solid-electrolyte layer that conducts lithium ions between the electrode layers, wherein 
         [0019]    a buffer layer that is a LiNbO 3  film is disposed between a positive-electrode active material and a solid electrolyte, and 
         [0020]    a composition ratio (Li/Nb) of Li to Nb in the LiNbO 3  film satisfies 0.93≦Li/Nb≦0.98. 
         [0021]    As described above, when a LiNbO 3  film has a composition ratio (Li/Nb) of Li to Nb satisfying 0.93≦Li/Nb≦0.98, the formation of a Li 2 CO 3  layer and Nb oxide layers that serve as resistive layers can be suppressed. Accordingly, in a solid-electrolyte battery having such a buffer layer, the effective area contributing to the battery operation does not decrease. Thus, solid-electrolyte batteries whose electrical resistance (internal resistance) is sufficiently decreased can be stably provided. 
         [0022]    The positive-electrode layer of such a solid-electrolyte battery may be a thin-film layer formed by vapor deposition or a compacted-powder layer formed by compacting powder. 
         [0023]    In the former case of the thin-film layer, the buffer layer is formed as an intermediate layer between the positive-electrode layer and the solid-electrolyte layer. The buffer layer thus inhibits the contact between the positive-electrode layer and the solid-electrolyte layer, that is, the contact between the positive-electrode active material of the positive-electrode layer and the solid electrolyte, to thereby suppress the formation of resistive layers. 
         [0024]    In the latter case of the compacted-powder layer, because the interface resistance between the particles is generally high and the positive-electrode active material alone does not provide sufficiently high ion conductivity, a powder mixture prepared by mixing a positive-electrode active-material powder and a solid-electrolyte powder is used as a raw material powder. For this reason, the buffer layers are formed on the surfaces of the particles of the positive-electrode active-material powder. As a result, the contact between the positive-electrode active-material powder and the solid-electrolyte powder is inhibited and the formation of resistive layers is suppressed. 
         [0025]    As described above, the present invention may be applied to the case of the thin-film layer and the case of the compacted-powder layer. In both of the cases, a buffer layer that is a LiNbO 3  film is disposed between a positive-electrode active material and a solid electrolyte to inhibit the contact between the positive-electrode active material and the solid electrolyte and to suppress the formation of resistive layers. 
         [0026]    In summary, in the solid-electrolyte battery, 
         [0027]    the buffer layer may be disposed between the positive-electrode layer and the solid-electrolyte layer. 
         [0028]    Alternatively, in the solid-electrolyte battery, 
         [0029]    the buffer layer may be disposed on a surface of a particle of the positive-electrode active material. 
         [0030]    The buffer layer may be formed by a vapor-phase method such as a laser ablation method or a sputtering method or by a liquid-phase method such as a sol-gel method. The composition ratio of Li to Nb is controlled in the vapor-phase method by controlling the composition of the target. The composition ratio of Li to Nb is controlled in the liquid-phase method by controlling the composition of the solution. 
         [0031]    The inventors of the present invention further performed experiments and studies on a preferred thickness of the LiNbO 3  film obtained above that has a composition ratio Li/Nb satisfying 0.93≦Li/Nb≦0.98 in the case of using the LiNbO 3  film as a buffer layer between a solid electrolyte and a positive-electrode active material. As a result, the inventors have reached a conclusion that a buffer layer having a thickness of less than 2 nm does not sufficiently exhibit its function, whereas a buffer layer having a thickness of more than 1 μm results in a battery having a large thickness, which is not preferable; accordingly, a thickness of 2 nm to 1 μm is preferable. 
         [0032]    In summary, in the solid-electrolyte battery, 
         [0033]    the buffer layer may have a thickness of 2 nm to 1 μm. 
         [0034]    When the buffer layer is formed so as to have a thickness of 2 nm to 1 μm, the buffer layer can sufficiently exhibit its function and a solid-electrolyte battery having a small thickness can be provided. 
       Advantageous Effects of Invention 
       [0035]    According to the present invention, a solid-electrolyte battery can be provided that includes a LiNbO 3  film serving as a buffer layer between a solid electrolyte and a positive-electrode active material and has a sufficiently low electrical resistance. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0036]      FIG. 1  is a schematic view illustrating the sectional configuration of a solid-electrolyte battery according to an embodiment of the present invention. 
           [0037]      FIG. 2  is a schematic view illustrating the sectional configuration of a solid-electrolyte battery according to another embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0038]    Hereinafter, the present invention will be described with reference to embodiments. However, the present invention is not limited to the embodiments below. Various modifications can be made to the following embodiments within the scope identical to the present invention and the scope of its equivalence. 
       EXAMPLES 
     [1] Examples in which Buffer Layer is Formed Between Positive-Electrode Layer and Solid-Electrolyte Layer 
       [0039]    Examples in which an intermediate layer serving as a buffer layer is formed between a positive-electrode layer and a solid-electrolyte layer will be first described. 
       Examples 1 to 3 
     1. Production of Solid-Electrolyte Batteries 
       [0040]    Solid-electrolyte batteries illustrated in  FIG. 1  were produced by a procedure described below.  FIG. 1  is a schematic view illustrating the sectional configuration of a solid-electrolyte battery according to an embodiment of the present invention. In  FIG. 1 , the reference sign  1  denotes a positive electrode; the reference sign  2  denotes an intermediate layer; the reference sign  3  denotes a solid-electrolyte layer; and the reference sign  4  denotes a negative electrode. 
         [0000]    (1) Formation of positive electrode 
         [0041]    A LiCoO 2  layer having a thickness of 5μm was formed on a surface of a steel use stainless (SUS)  316 L substrate having a thickness of 0.5 mm by a pulsed laser deposition (PLD) method. Thus, a positive electrode was prepared. 
         [0000]    (2) Formation of intermediate layers 
         [0042]    Three positive electrodes were prepared in this manner. LiNbO 3  layers having a thickness of 0.01 μm were formed on the surfaces of the positive electrodes by the PLD method with LiNbO 3  targets having Li/Nb ratios of 1.0, 1.2, and 1.4; the LiNbO 3  layers were then annealed at 400° C. for 0.5 hours; and the resultant intermediate layers were respectively defined as Examples 1, 2, and 3. 
       (3) Formation of Solid-Electrolyte Layers 
       [0043]    A solid-electrolyte layer composed of Li 2 S—P 2 S 5  and having a thickness of 10 μm was formed by the PLD method on the surface of each of the intermediate layers of Examples 1 to 3. 
       (4) Formation of Negative Electrodes 
       [0044]    A negative electrode composed of Li metal and having a thickness of 1 μm was formed by a vacuum deposition method on the surface of each of the solid-electrolyte layers of Examples 1 to 3. Thus, solid-electrolyte batteries were produced. 
       Comparative Examples 1 and 2 
       [0045]    Solid-electrolyte batteries were produced in the same manner as in Examples 1 to 3 except that intermediate layers were formed in the following manner. 
       Formation of Intermediate Layers 
       [0046]    Intermediate layers were formed in the same manner as in Examples except that LiNbO 3  targets having Li/Nb ratios of 0.95 and 1.6 were used; the intermediate layers were respectively defined as Comparative examples 1 and 2. 
       2. Evaluations of Intermediate Layers and Solid-Electrolyte Batteries 
     (1) Measurement of Li/Nb Ratios of Intermediate Layers 
       [0047]    i. Measurement Method 
         [0048]    The Li/Nb ratios of the intermediate layers were measured by inductively coupled plasma (ICP) composition analysis. Specifically, a reference including a thick LiNbO 3  film (having a known Li/Nb) was prepared. The reference, Examples 1 to 3, and Comparative examples 1 and 2 were measured by the ICP composition analysis. The Li/Nb ratios of Examples 1 to 3 and Comparative examples 1 and 2 were determined on the basis of the measurement results obtained by the ICP composition analysis. 
         [0000]    ii. Measurement Results 
         [0049]    The Li/Nb ratios of Examples 1 to 3, that is, x/y in a chemical formula Li x Nb y O 3-z  were respectively 0.93, 0.96, and 0.98 (0≦z≦0.75); x/y of Comparative examples 1 and 2 were respectively 0.91 and 1.00 (z=0.45 and 0). These Li/Nb ratios (x/y) are described in Table I. 
       (2) Evaluation of Solid-Electrolyte Batteries 
       [0050]    i. Evaluation Method
 
a. Assembly of Cells for Characteristic Evaluation
 
         [0051]    The produced solid-electrolyte batteries were built in coin-shaped cells to provide cells for characteristic evaluation. 
         [0000]    b. Measurement of Internal Resistance 
         [0052]    A characteristic of the solid-electrolyte batteries was evaluated on the basis of the magnitude of internal resistance. Specifically, a charge-discharge cycle test (temperature: 25° C.) was performed in which a cutoff voltage was 3 to 4.2 V and a current density was 0.05 mA/cm 2 ; and the internal resistance of each battery was determined on the basis of a voltage drop for 60 seconds after the initiation of discharge. 
         [0000]    ii. Evaluation Results 
         [0053]    Evaluation results of Examples 1 to 3 and Comparative examples 1 and 2 are described in Table I. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE I 
               
               
                   
                   
               
               
                   
                 Li/Nb ratio 
                 Internal resistance 
               
               
                   
                 (x/y) 
                 (Ω cm 2 ) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Example 1 
                 0.93 
                 120 
               
               
                   
                 Example 2 
                 0.96 
                 60 
               
               
                   
                 Example 3 
                 0.98 
                 100 
               
               
                   
                 Comparative example 1 
                 0.91 
                 600 
               
               
                   
                 Comparative example 2 
                 1.00 
                 300 
               
               
                   
                   
               
             
          
         
       
     
         [0054]    Table I indicates that, by making the Li/Nb ratio of LiNbO 3  of an intermediate layer be 0.93 to 0.98, a solid-electrolyte battery having a low internal resistance can be produced. 
       [2] Examples in which Buffer Layers are Formed on Surfaces of Positive-Electrode Active-Material Particles 
       [0055]    Examples in which a positive-electrode layer is formed of positive-electrode active-material particles having a Li x Nb y O 3-z  film serving as a buffer layer and a solid-electrolyte powder, and a solid-electrolyte layer is formed on the surface of the positive-electrode layer will be subsequently described. 
       Examples 4 to 6 
     1. Production of Solid-Electrolyte Batteries 
       [0056]    Solid-electrolyte batteries illustrated in  FIG. 2  were produced by a procedure described below.  FIG. 2  is a schematic view illustrating the sectional configuration of a solid-electrolyte battery according to the present embodiment of the present invention. In  FIG. 2 , the reference sign  1  denotes a positive electrode; the reference sign la denotes a positive-electrode active-material particle; the reference sign  2   a  denotes a buffer layer; the reference sign  3  denotes a solid-electrolyte layer; and the reference sign  4  denotes a negative electrode. 
       (1) Preparation of Positive-Electrode Mixtures 
       [0057]    i. Formation of Buffer Layers 
         [0058]    Ethoxylithium (LiOC 2 H 5 ) and pentaethoxyniobium (Nb(OC 2 H 5 ) 5 ) were mixed with molar ratios ([LiOC 2 H 5 ]/[Nb(OC 2 H 5 ) 5 ]) of 0.93, 0.96, and 0.98 and dissolved in ethanol. Each of the resultant solutions was sprayed onto the surfaces of the LiCoO 2  particles  1   a  having an average size of 10 μm. The LiCoO 2  particles  1   a  were then left at rest in the air so that ethanol was removed and hydrolysis was caused with moisture in the air. The LiCoO 2  particles  1   a  were then heated at 300° C. for 30 minutes to form, on the surfaces thereof, amorphous Li x Nb y O 3-z  films having a thickness of 0.01 μm (10 nm), that is, the buffer layers  2   a.    
         [0000]    ii. Preparation Of Solid-Electrolyte Powder 
         [0059]    A Li 2 S powder and a P 2 S 5  powder were mixed with a mass ratio of 5:6. The mixture was further ground and mixed with a mortar and the reaction between Li 2 S and P 2 S 5  was subsequently caused with a planetary ball mill apparatus by a mechanical milling method. The resultant powder was then heated at 210° C. for an hour to prepare a crystalline sulfide solid-electrolyte powder composed of Li 2 S—P 2 S 5 . 
         [0000]    iii. Preparation of Positive-Electrode Mixtures 
         [0060]    The LiCoO 2  particles having such a Li x Nb y O 3-z  film and the solid-electrolyte powder were mixed in a weight ratio of 7:3 with a mortar to prepare a positive-electrode mixture. 
       (2) Production of Solid-Electrolyte Batteries 
       [0061]    i. Formation of Positive-Electrode Layer and Solid-Electrolyte Layer 
         [0062]    A cylindrical resin container having an internal diameter of 10 mm was charged with 10 mg of such a positive-electrode mixture and then 50 mg of the solid-electrolyte powder. The charged materials were compacted with a hydraulic press employing a stainless-steel piston under a pressure of 500 MPa to form a positive-electrode layer and a solid-electrolyte layer. 
         [0000]    ii. Formation of Negative Electrode 
         [0063]    The piston on the solid-electrolyte layer was then withdrawn and an indium (In) foil having a thickness of 300 μm and a lithium (Li) foil having a thickness of 250 μm were placed on the solid-electrolyte layer. The piston was used again to compact the foils under a pressure of 100 MPa to form a negative electrode. Thus, solid-electrolyte batteries were produced. 
       Comparative Examples 3 and 4 
       [0064]    Solid-electrolyte batteries were produced in the same manner as in Examples 4 to 6 except that buffer layers were formed in the following manner. 
         [0065]    Intermediate layers were formed in the same manner as in Examples 4 to 6 except that LiOC 2 H 5  and Nb(OC 2 H 5 ) 5  were mixed with molar ratios ([LiOC 2 H 5 ]/[Nb(OC 2 H 5 ) 5 ]) of 0.91 and 1.00 and dissolved in ethanol; and the intermediate layers were respectively defined as Comparative examples 3 and 4. 
       2. Evaluations of Buffer Layers and Solid-Electrolyte Batteries 
     (1) Measurement of Li/Nb Ratios of Buffer Layers 
       [0066]    The Li/Nb ratios (x/y) of the thus-formed buffer layers  2   a  were measured by the same measurement method as in Examples 1 to 3. The results indicate that the Li/Nb ratios of Examples 4 to 6 and Comparative examples 3 and 4 were the same as the [LiOC 2 H 5 ]/[Nb(OC 2 H 5 ) 5 ] of the corresponding ethanol solutions, 0.93, 0.96, 0.98, 0.91, and 1.00, respectively. These Li/Nb ratios are described in Table II. 
       (2) Evaluation of Solid-Electrolyte Batteries 
       [0067]    The internal resistance of the batteries was measured and the batteries were evaluated on the basis of the magnitude of the internal resistance. 
         [0000]    i. Measurement Method of Internal Resistance 
         [0068]    Each battery was charged with a current density of 0.05 mA/cm2 and a cutoff voltage of 4.2 V and the internal resistance was then measured by a complex impedance method. 
         [0000]    ii. Evaluation Results 
         [0069]    Evaluation results of Examples 4 to 6 and Comparative examples 3 and 4 are summarized in Table II. 
         [0000]    
       
         
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE II 
               
               
                   
                   
               
               
                   
                 Li/Nb ratio 
                 Internal resistance 
               
               
                   
                 (x/y) 
                 (Ω cm 2 ) 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Example 4 
                 0.93 
                 300 
               
               
                   
                 Example 5 
                 0.96 
                 200 
               
               
                   
                 Example 6 
                 0.98 
                 250 
               
               
                   
                 Comparative example 3 
                 0.91 
                 1000 
               
               
                   
                 Comparative example 4 
                 1.00 
                 600 
               
               
                   
                   
               
             
          
         
       
     
         [0070]    Table II indicates that, in the case of forming buffer layers on the surfaces of positive-electrode active-material particles, by making the Li/Nb ratio of LiNbO 3  of the buffer layers be 0.93 to 0.98, a solid-electrolyte battery having a low internal resistance can also be produced. 
       REFERENCE SIGNS LIST 
       [0000]    
       
         
           
               1  positive electrode 
               1   a  positive-electrode active-material particle 
               2  intermediate layer 
               2   a  buffer layer 
               3  solid-electrolyte layer 
               4  negative electrode