Patent Publication Number: US-2016233543-A1

Title: Solid electrolyte, all-solid-state secondary battery using the same, and manufacturing methods thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application filed under 35 U.S.C. 111(a) claiming benefit under 35 U.S.C. 120 and 365(c) of PCT International Application No. PCT/JP2013/078809 filed on Oct. 24, 2013, the contents of which are incorporated herein by reference in its entirety. 
    
    
     FIELD 
     The disclosures herein relate to a solid electrolyte and a manufacturing method thereof, as well as to an all-solid-state secondary battery using the solid electrolyte and a manufacturing method thereof. 
     BACKGROUND 
     Energy harvesting to convert micro-energy captured from sunlight, vibrations, body temperatures of humans and animals, etc. into electric energy is being researched and developed. The accumulated electric energy is used for sensor systems, radio transmission power, and other systems using electricity. In energy harvesting, safe and reliable secondary batteries that are operable in any environment on the earth are desired. Liquid batteries which are typically used today have some drawbacks. The cathode active material is degraded upon repetition of battery cycles and the battery capacity decreases. Dendrite-based short-circuiting of a battery is also a concern because the electrolyte organic solvent possibly catches fire. Liquid electrolyte secondary batteries are unsatisfactory in reliability and safety when used in energy harvesting devices intended to operate over a time span of a decade or more. In view of these circumstances, an all-solid-state secondary battery using a solid material for its electrolyte is attracting attention. All-solid-state secondary batteries are unlikely to cause liquid leakage or catch fire, while being superior in cycle characteristics. 
     As a material of a solid electrolyte of an all-solid-state secondary battery, lithium phosphorous oxynitride (LiPON) is generally used because of its high ionic conductivity. However, pinholes and cracks are easily produced in LiPON based amorphous solid electrolytes, and it is difficult to form a uniform and defectless film. With defects existing in a solid electrolyte, the anode material may reach the cathode (i.e., the positive electrode) through the defects and short-circuit the battery. In this case, the operations of the battery become instable and the production yield decreases. 
     To solve this problem, a technique for blocking the pinholes and cracks inherently existing in the LiPON based amorphous solid electrolyte by placing lithium carbonate (Li 2 CO 3 ) on the cathode surface is proposed. See, for example, Japanese Patent Application Laid-open Publication No. 2009-272050. 
     Meanwhile, a single crystal ionic conductor acquired by crystalizing alumina (Al 2 O 3 ) in lithium phosphate flux is known. Seem for example, S. Poisson, et al. “Crystal Structure and Cation Transport Properties of the layered Monodiphosphates: Li 9 M 3  (P 2 O 7 ) 3 (PO 4 ) 2  (M=Al, Ga, Cr, Fe)”, Journal of Solid State Chemistry 138, 32-40 (1998). However, in order to use the ionic conductive crystal acquired by this technique in a thin-film all-solid-state secondary battery, additional processes including powdering the crystal, disposing the powder on the cathode, and sintering are necessary. During these processes, ionic conductive film is polycrystallized and cations leak out from the crystal grain boundary. It is difficult to expect appropriate charge and discharge cycles. 
     Accordingly, it is desired to provide a solid electrolyte and an all-solid-state secondary battery using the solid electrolyte that achieve stable battery operations. 
     SUMMARY 
     According to an aspect described in the disclosure, a solid electrolyte is an amorphous solid electrolyte that contains of lithium, aluminum, phosphorus, and oxygen. 
     In another aspect described in the disclosure, an all-solid-state secondary battery using a solid electrolyte has a positive electrode, a negative electrode, and an amorphous solid electrolyte positioned between the positive electrode and the negative electrode, the amorphous solid electrolyte containing lithium, aluminum, phosphorus, and oxygen. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive to the invention as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1A  is a schematic diagram of an all-solid-state secondary battery using an amorphous solid electrolyte according to the embodiment; 
         FIG. 1B  is a cross-sectional transmission electron microscopy (TEM) image of the all-solid-state secondary battery of  FIG. 1A ; 
         FIG. 2  is an X-ray diffraction diagram representing crystallization of the solid electrolyte of the embodiment at different temperatures of heat treatment; 
         FIG. 3  is a diagram of infrared absorption spectra of the solid electrolyte of the embodiment at different temperatures of heat treatment; 
         FIG. 4  is a diagram illustrating the crystalline state of the solid electrolyte along with heat treating temperature; 
         FIG. 5  illustrates the infrared absorption spectrum of the amorphous solid electrolyte compared with that of a crystalline solid electrolyte; 
         FIG. 6  is a chart illustrating a discharge curve and a charging curve; 
         FIG. 7  is a schematic diagram illustrating the structure of a sample fabricated for measurement of battery characteristics; 
         FIG. 8  is a chart illustrating repetition characteristics of the all-solid-state secondary battery using the solid electrolyte of the embodiment, compared with that of an all-solid state secondary battery using conventional lithium phosphorous oxynitride (LiPON); and 
         FIG. 9  is a chart illustrating manufacturing yield of the all-solid-state secondary battery using the solid electrolyte of the embodiment, compared with that of the all-solid state secondary battery using conventional lithium phosphorous oxynitride (LiPON). 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1A  is a schematic diagram of a sample  1  with a solid electrolyte film  19  according to the embodiment, and  FIG. 1B  is a cross-sectional transmission electron microscopy (TEM) image of the sample. In the embodiment, lithium aluminum phosphate (LiAlPO) based amorphous solid electrolyte is provided. 
     The inventors found that a uniform solid electrolyte film  19  with fewer defects can be obtained by forming a LiAlPO based amorphous film, in place of a LiPON based solid electrolyte material. It is also found that an all-solid-state secondary battery using the solid electrolyte film  19  of the LiAlPO based amorphous film is superior in repetition characteristics and capable of stable battery operations. 
     In  FIG. 1A , the sample  1  has a LiAlPO based amorphous solid electrolyte film  19  between LiCoO 2  film  17  and lithium metal film  21 .  FIG. 1B  is a TEM image of the as-deposited films of the sample  1  in which the solid electrolyte film  19  with a thickness of 1000 nm is deposited over the LiCoO 2  film  17 . 
     The LiAlPO based amorphous solid state electrolyte film  19  is formed by applying alternating current power of 140 W to a target with composition of Li 9 Al 3  (P 2 O 7 ) 3 (PO 4 ) 2  and generating plasma in an argon atmosphere at a pressure of 0.5 Pa. The substrate temperature during the film deposition is 27° C. 
     As understood from  FIG. 1B , cracks are produced in the LiCoO 2  film  17 , which film is a thin crystalline layer, in a direction perpendicular to the substrate  11 . In contrast, a uniform solid electrolyte film  19  is formed without pinholes or cracks observed according to the embodiment. It is evaluated that the void fraction of the solid electrolyte film  19  of the sample  1  is 0%. 
     When performing heat treatment on the sample  1  at different temperatures, the amorphous solid electrolyte film  19  is crystallized. Along with the crystallization, pinholes or cracks are generated in the solid electrolyte film  19 . To confirm this, an experiment was conducted to determine the crystallization temperature by applying heat treatment to the LiAlPO based solid electrolyte film  19  at various temperatures. 
       FIG. 2  illustrates measurement results of X-ray powder diffraction when applying heat treatment to the solid electrolyte film  19  in argon atmosphere at 300° C., 450° C., and 600° C. The measurement conditions are tube voltage of 40 kV and tube current of 30 mA. The 20 range of measurement is from 5° to 70° (5°≦2θ≦70°) and continuous scan is carried out at a scan rate of 1.2 degrees per minute (1.2°/min) to acquire the diffraction patterns. 
     In the figure, spectrum A represents an X-ray diffraction pattern of the as-deposited solid electrolyte film  19 , spectrum B represents an X-ray diffraction pattern of the heat-treated solid electrolyte film  19  at 300° C., spectrum C represents an X-ray diffraction pattern of the heat-treated solid electrolyte film  19  at 450° C., and spectrum D represents an X-ray diffraction pattern of the heat-treated solid electrolyte film  19  at 600° C. Spectrum E represents the X-ray diffraction pattern of single crystal Li 9 Al 3 (P 2 O 7 ) 3 (PO 4 ) 2  disclosed in S. Poisson, et al., noted above, which spectrum is presented for reference. 
     In the diffraction pattern A of the as-deposited film and the diffraction pattern B of the 300-degrees heat-treated film, only diffraction peaks associated with the crystal planes of the substrate  11  are observed. This indicates that the solid electrolyte film  19  is amorphous (uncrystallized). In the diffraction pattern C of the 450° C. heat-treated film, peaks associated with polycrystals produced during crystallization of the LiAlPO based solid electrolyte film  19  are observed as indicated by the dark circles. In the diffraction pattern D of the 600° C. heat-treated film, peaks associated with the crystallization increase. 
     From  FIG. 2 , it is understood that the solid electrolyte film  19  is crystallized when heat treatment is performed at a temperature 450° C. or higher. 
       FIG. 3  illustrates infrared absorption spectra acquired by Fourier transform infrared (FT-IR) spectroscopy, which technique is another method for confirming crystallization associated with heat treatment. As equipment for measurement, Nicolet™ 8700 manufactured by Thermo Fisher Scientific Inc., is used and unpolarized IR light is irradiated at an incident angle of 80 degrees to the reflective optics. Interference patterns measured using a glow lamp as a light source, a Michelson interferometer, and a triglycine sulfate (TGS) detector are Fourier transformed. The wavenumber resolution of the equipment is 4 cm −1  and the cumulative number of scans is 64. 
     In the figure, pattern A represents the spectrum of the as-deposited film, pattern B represents the spectrum of the 300° C. heat treated film, pattern C represents the 450° C. heat treated film, and pattern D represents the 600° C. heat treated film. As a reference, spectrum E of the solid electrolyte of LiAlPO-based single crystal grown at a substrate temperature of 700° C. is presented. 
     Main peaks indicate vibration of P—O bonds are observed at or near 600 cm −1  of wavenumber (on the horizontal axis). Comparing the observations of the full width at half maximum (FWHM) of the peak of each spectrum, the as-deposited film (Spectrum A) and the 300° C. heat treated film (Spectrum B) exhibit relatively wide FWHM. In contrast, the spectrum C of the 450° C. heat treated film exhibits a narrow FWHM with an abrupt change. This observation result indicates that the LiAlPO-based solid electrolyte film is in the crystalline state at 450° C. 
       FIG. 4  is a chart to estimate the temperature at which the LiAlPO-based solid electrolyte changes from the amorphous state to the crystalline state. Based upon the measurement result of  FIG. 3 , the FWHM [cm −1 ] of the main peak is plotted as a function of temperature, and interpolation is made between measured points. The FWHM value abruptly decreases beyond 400° C., falling to 80 cm −1  or below. At heat treatment temperature of 450° C. or higher, the FWHM becomes equal to or less than 60 cm −1 . From the estimation, it is assumed that the LiAlPO based solid electrolyte stays in the amorphous state at heat treatment temperature of 400° C. or below. 
     Based upon  FIG. 3  and  FIG. 4 , it may be defined for the LiAlPO based solid electrolyte that the state with the FWHM value of the main peak indicating the vibration of the P—O bonds at or near 600 cm −1  of FT-IR spectrum equal to or greater than 100 cm −1  is amorphous. Besides, according to  FIG. 2 , the state in which no diffraction peak except for that from the substrate is observed in the XRD pattern may be defined as “amorphous”. 
       FIG. 5  is a chart illustrating the infrared absorption spectrum of the LiAlPO based amorphous solid electrolyte, compared with the IR absorption spectrum of the LiAlPO based crystalline solid electrolyte of the same composition. The upper spectrum is one from the crystalline solid electrolyte and the lower spectrum is one from the amorphous solid electrolyte in the chart. 
     The LiAlPO based amorphous solid electrolyte contains phosphorus oxoacids PO 4  and P 2 O 7 , similarly to the LiAlPO based crystalline solid electrolyte. The PO 4  peak appearing near (on the left side of) 1200 cm −1  in the crystalline solid electrolyte spectrum is due to P—O bond stretching vibration in PO 4  molecules. The other PO 4  peak appearing near (on the right side of) 600 cm-1 in this spectrum is due to P—O—P bond angle bending vibration in PO 4  molecules. The same things apply to the spectra of  FIG. 3 . When observing and comparing the change in FWHM, either peak may be employed. For example, focusing on the peaks near 600 cm −1 , the crystalline solid electrolyte exhibits a sharp peak with narrow FWHM, while the spectrum of the amorphous solid electrolyte changes gently with very wide FWHM. 
     The crystalline solid electrolyte also exhibits a P 2 O 7  peak near 800 cm −1  with narrow FWHM, while no peak is observed in the amorphous solid electrolyte spectrum near this wavenumber. 
     From  FIG. 3  through  FIG. 5 , the LiAlPO based amorphous solid electrolyte film  19  of the embodiment contains uncrystallized phosphorus oxoacids PO 4  and P 2 O 7  and has a uniform film structure with fewer pinholes or cracks. When applying the amorphous solid electrolyte film  19  to an all-solid-state secondary battery, the battery operations become stable. 
     Because the solid electrolyte film  19  of the embodiment is an oxide, it is not immediately hydrolyzed in the air unlike the conventional nitride based LiPON solid electrolyte film. The solid electrolyte film  19  of the embodiment also has an advantageous effect of ease in handling during the fabrication process of batteries. 
     By applying heat treatment at or below 400° C., more preferably at or below 300° C., to the solid electrolyte of the embodiment, film defects such as voids can be reduced while maintaining the amorphous state. 
       FIG. 6  illustrates a charging curve and a discharge curve of an all-solid-state secondary battery using the solid electrolyte film  19  of the embodiment. To measure the charge and discharge characteristics, a sample of the all-solid-state secondary battery  2  illustrated in  FIG. 7  is fabricated. The all-solid-state secondary battery  2  has a layered structure over a substrate  11  on which silicon oxide film  12  is formed, the layered structure including a current collector  15 , a positive electrode  17 , a solid electrolyte film  19 , and a metal film  21  deposited in this order. The metal film  21  serves as a negative electrode and a current collector. 
     To fabricate the sample, the current collector  15  is formed over the substrate  11  covered with the silicon oxide film  12  with a thickness of 100 nm, by depositing a titanium (Ti) film  13  with a thickness of 30 nm and a platinum (Pt) film  14  with a thickness of 170 nm in this order. 
     Then, LiCoO 2  film  17 , which is a cathode active material, is formed over the current collector  15 . The film deposition is performed using a LiCoO 2  alloy target, supplying argon (Ar) gas and oxygen (O 2 ) gas at volume ratio 3:1, under the conditions of power 600 W, gas pressure 0.5 Pa, and substrate temperature 250° C. The deposition time is determined depending on the thickness of the positive electrode (liCoO 2  film)  17 , which thickness is, for example, 100 nm to 1000 nm. After the film deposition, annealing for crystallization is performed in an oxygen atmosphere at 600° C. for 5 to 10 minutes to produce the positive electrode  17 . 
     Then, the LiAlPO based amorphous solid electrolyte film  19  is formed over the positive electrode  17 . The solid electrolyte film  19  with a thickness of 150 nm to 1000 nm is formed using Li 9 Al 3  (P 2 O 7 ) 3 (PO 4 ) 2  sintered body as a target under the conditions of alternating current power of 70 W to 140 W, in an argon (Ar) atmosphere at a gas pressure of 0.2 to 0.5 Pa, a substrate temperature of room temperature (ambient temperature without applying heat or cooling) and deposition time of 3 to 55 hours. The thinner the solid electrolyte film  19  is, the lower the electric resistance is. However, if the solid electrolyte film  19  is too thin, a short circuit is likely to occur. The thickness of the solid electrolyte film  19  is selected appropriately in the above-stated range. Annealing may be applied to the solid electrolyte film  19  at or below 400° C., more preferably, at or below 300° C., as necessary. 
     Then, a lithium metal film with a thickness of 2 μm is formed as the metal film  21  by vacuum deposition, and the all-solid-state secondary battery  2  is obtained. 
     Using the fabricated all-solid-state secondary battery  2 , a charge and discharge evaluation test is performed. The all-solid-state secondary battery  2  is placed under the environment of temperature 50° C. and dew point temperature −46° C., and charge and discharge test is carried out at a current density of 48 pA/cm 2  with electric voltage of 4.2 V and 2.0 V. As the test result, the charge/discharge curve of  FIG. 6  is acquired. Because the solid electrolyte film  19  of the all-solid-state secondary battery  2  is a uniform amorphous film with fewer pinholes or cracks as illustrated in  FIG. 1B , satisfactory characteristics can be obtained. 
     For comparison, an all-solid-state secondary battery with a solid electrolyte having been heat-treated at 450° C. after the film deposition is fabricated and charge and discharge test is performed under the same conditions. In this comparison test, the battery does not work and a charge/discharge curve cannot be acquired. This may be explained by an over discharge having occurred due to cracks generated from the crystal grain boundaries along with the crystallization. 
       FIG. 8  illustrates charge and discharge repetition (cycle) characteristics of the all-solid-state secondary battery  2  of  FIG. 7 . The repetition characteristics of the secondary battery using the LiAlPO based amorphous solid electrolyte film  19  are represented by white circles. As a comparison, the repetition characteristics of an all-solid-state secondary battery using the conventional LiPON based amorphous solid electrolyte are represented by dark circles. 
     The all-solid-state secondary battery  2  using the LiAlPO based amorphous solid electrolyte film  19  exhibits satisfactory capacity retention rate up to 27 cycles. It is understood that this battery operates in a stable manner. In contrast, in the all-solid-state secondary battery using the conventional LiPON based amorphous solid electrolyte, an internal short circuit has occurred due to growth of pinholes and the discharge capacity abruptly decreases. After the 5th cycle, no capacity is measured. 
     Concerning the all-solid-state secondary battery using the LiAlPO based polycrystalline solid electrolyte having undergone 450° C. heat treatment after the film deposition, a repetition characteristics test cannot be performed because the battery does not work from the beginning as has been explained in connection with  FIG. 6 . 
       FIG. 9  illustrates manufacturing yield [o] of the battery using the LiAlPO based amorphous solid electrolyte film  19  according to the embodiment, compared with that of a battery using the conventional LiPON based solid electrolyte. When using the solid electrolyte film  19  of the embodiment, the manufacturing yield reaches 80% because fewer defects are generated and because hydrolysis hardly occurs. In contrast, with the conventional solid electrolyte, the manufacturing yield stays at 50% because pinholes and cracks are likely to be generated and hydrolysis easily occurs. 
     In conclusion, the solid electrolyte film  19  of the embodiment is a uniform amorphous film with fewer pinholes and cracks. The all-solid-state secondary battery using this solid electrolyte film  19  performs stable charge and discharge operations. 
     The solid electrolyte of the invention can be applied to all-solid-state secondary batteries used in energy harvesting devices. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of superiority or inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.