Patent Application
20230420734
The present invention relates to a solid electrolyte and a battery.
In recent years, with the development of portable equipment such as personal computers and mobile phones, demand for batteries serving as power sources of such portable equipment is increasing considerably. The batteries used for such purposes conventionally use organic electrolyte solutions that contain electrolytes dissolving in inflammable organic solvents, as media for transporting ions. The batteries containing the organic electrolyte solutions may cause safety concerns.
In order to resolve the concerns, the development of all solid-state batteries using solid electrolytes, instead of organic electrolyte solutions, is advancing to ensure intrinsic safety. Since the solid electrolytes are incombustible materials, it is possible to achieve lithium (Li) ion batteries with a high degree of safety.
Among the solid electrolytes, those like sulfide-based ones that are made of materials that react with moisture and generate hydrogen sulfide gas are widely known. Meanwhile, although oxide-based solid electrolytes that do not generate gas such as hydrogen sulfide is also being developed widely, materials for oxide-based solid electrolytes have lower lithium ion conductivity than materials for sulfide-based solid electrolytes, and thus it is difficult to improve battery output (or draw a large current). In view of this, “Li+-Ion Conduction of Li3AlF6 Mechanically Milled with LiCl” by R. Miyazaki and H. Maekawa, ECS Electrochemistry Letters, 2012, the first volume, the sixth issue, pp. A87-A89 (Document 1) discloses development of solid electrolytes having improved lithium ion conductivity through a mechanical milling process conducted on Li3AlF6 and LiCl.
According to FIG. 1 of Document 1 mentioned above, the solid electrolytes have higher lithium ion conductivity with an increase in temperature, but the lithium ion conductivity decreases significantly at temperatures above 120° C. There is thus demand for solid electrolytes having high lithium ion conductivity and high-temperature stability.
The present invention is intended for a solid electrolyte, and it is an object of the present invention to provide a solid electrolyte having high lithium ion conductivity and high-temperature stability.
A solid electrolyte according to a preferable embodiment of the present invention is composed primarily of a component expressed by a composition formula of Lia+dMbXcAeOf by using values a to f that are greater than 0, where M is an element serving as a trivalent cation, X is a halogen element, and A is a sulfur element or a phosphorus element, wherein 0.8c≤(a+3b)≤1.2c and 1.6f≤(d+n×e)≤2.4f are satisfied, where when A is a sulfur element, n is 4 or 6, and when A is a phosphorus element, n is 5.
According to the present invention, it is possible to provide a solid electrolyte having high lithium ion conductivity and high-temperature stability.
Preferably, 0<e/(c+e)≤0.5 is satisfied in the composition formula.
Preferably, 0.1a≤3b≤10a is satisfied in the composition formula.
Preferably, in the composition formula, 0.8d≤2e≤1.2d is satisfied when A is a sulfur element, and 0.8d≤e≤1.2d is satisfied when A is a phosphorus element.
Preferably, in the composition formula, A is a sulfur element and n is 6. Preferably, in the composition formula, M is an aluminum element and X is a fluorine element.
Preferably, in measurement conducted on the solid electrolyte by an X-ray diffractometer using a CuKα ray, a half-value width of a peak detected within a range of 2θ from 20° to 25° is greater than or equal to 0.4°.
The present invention is also intended for a method of producing a solid electrolyte. The method of producing a solid electrolyte according to a preferable embodiment of the present invention includes a) preparing a compound expressed by a composition formula of LiaMbXc and a compound expressed by a composition formula of LidAeOf by using values a to f that are greater than 0, where M is an element serving as a trivalent cation, X is a halogen element, A is a sulfur element or a phosphorus element, the values a to f satisfy 0.8c≤(a+3b)≤1.2c and 1.6f≤(d+n×e)≤2.4f, and when A is a sulfur element, n is 4 or 6, and when A is a phosphorus element, n is 5, and b) subjecting LiaMbXc and LidAeOf to a mechanical milling process. In the operation b), a ratio of the amount of substance of LidAeOf to a total of the amount of substance of LiaMbXc and the amount of substance of LidAeOf is lower than or equal to 85%.
The method of producing a solid electrolyte according to another preferable embodiment of the present invention includes a) preparing a first compound that is LiX, a second compound that is MX3, and a third compound composed of Li, A, and O, where X is a halogen element, M is an element serving as a trivalent cation, and A is a sulfur element or a phosphorus element, and b) subjecting the first compound, the second compound, and the third compound to a mechanical milling process. In the operation a), 0.8c≤(a+3b)≤1.2c and 1.6f≤(d+n×e)≤2.4f are satisfied when a ratio of the amounts of substance of Li, M, and X in a mixture of the first compound and the second compound and the amounts of substance of Li, A, and O in the third compound is expressed as a:b:c:d:e:f by using values a to f that are greater than 0, where when A is a sulfur element, n is 4 or 6, and when A is a phosphorus element, n is 5, and 0<e/(c+e)≤0.5 is further satisfied.
The present invention is also intended for a battery. The battery according to a preferable embodiment of the present invention includes a positive electrode, a negative electrode, and an electrolyte layer provided between the positive electrode and the negative electrode. At least one of the positive electrode, the negative electrode, or the electrolyte layer contains the solid electrolyte described above.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.
The positive active material of the positive electrode layer 112 may preferably contain a lithium complex oxide. A preferable positive active material may be a lithium complex oxide having a layered rock-salt structure and may, for example, be NCM (Li(Ni, Co, Mn)O2). The positive active material may also be any other lithium complex oxide and may, for example, be NCA (Li(Ni, Co, Al)O2) or LCO (LiCoO2) having a layered rock-salt structure, LNMO (LiNi0.5Mn1.5O4) having a spinel structure, or LFP (LiFePO4) having an olivine structure. In addition to the positive active material, the positive electrode layer 112 further includes a solid electrolyte, which will be described later, and an electron conductive agent (e.g., carbon black). The positive electrode layer 112 according to the present embodiment is obtained by integrating these substances by pressurization or heating.
Examples of the negative active material of the negative electrode layer 122 include compounds such as LTO (Li4Ti5O12), NTO (Nb2TiO7), titanium oxide (TiO2), graphite, and silicon monoxide (SiO). In addition to the negative active material, the negative electrode layer 122 includes a solid electrolyte, which will be described later. The negative electrode layer 122 may further include an electron conductive agent (e.g., carbon black). The negative electrode layer 122 according to the present embodiment is obtained by integrating these substances by pressurization or heating.
The configurations and materials of the positive electrode 11 and the negative electrode 12 of the all solid-state secondary battery 1 are not limited to the examples described above, and may be any of various configurations and materials.
The solid electrolyte layer 13 may be a solid electrolyte according to the present invention (hereinafter, also referred to as the “present solid electrolyte”), or may include the present solid electrolyte. The present solid electrolyte is a lithium (Li) ion conductive material. For example, the present solid electrolyte may be a mixture (or synthetic substance) of an Li3MX6 compound that is composed of a lithium element (Li), an element (M) serving as a trivalent cation, and a halogen element (X), and inorganic salt that contains a lithium element and a sulfur (S) or phosphorus (P) element. As will be described later, the present solid electrolyte is expressed by a composition formula of Lia+dMbXcAeOf by using values a to f that are greater than 0, where M is an element serving as a trivalent cation, X is a halogen element, and A is a sulfur element or a phosphorus element, in which 0.8c≤(a+3b)≤1.2c and 1.6f≤(d+n×e)≤2.4f are satisfied, where when A is a sulfur element, n is 4 or 6, and when A is a phosphorus element, n is 5. Preferably, it is also satisfied that, in measurement conducted by an X-ray diffractometer using a CuKα ray, the half-value width of a peak detected within the range of 2θ from 20° to 25° is greater than or equal to 0.4°. The present solid electrolyte has high lithium ion conductivity and high-temperature stability. Besides, the present solid electrolyte is incombustible and chemically stable and does not generate hydrogen sulfide gas. Thus, it is possible to provide the all solid-state secondary battery 1 with intrinsic safety.
A preferable example of the present solid electrolyte may be a mixture of Li3AlF6 (hereinafter, also referred to as “LAF”) composed of a lithium element, an aluminum element, and a fluorine element, and either lithium sulfate (Li2SO4) (hereinafter, also referred to as “LPO”) or lithium metaphosphate (LiPO3) (hereinafter, also referred to as “LSO”) that is inorganic salt containing a lithium element and a sulfur or phosphorus element. The present solid electrolyte containing LAF and either LSO or LPO has higher lithium ion conductivity than a simple substance of LAF. The reason for this is not clear, but one conceivable reason is that an interface layer having high lithium ion conductivity is formed at the interface of particles between LAF and LSO or LPO by the mechanical milling process.
Document 1 above, i.e., “Li+-Ion Conduction of Li3AlF6 Mechanically Milled with LiCl,” by R. Miyazaki and H. Maekawa (ECS Electrochemistry Letters, 2012, the first volume, the sixth issue, pp. A87-A89), discloses the solid electrolyte obtained by subjecting LAF and LiCl to a mechanical milling process. As shown in FIG. 1 of Document 1 above, the solid electrolyte has higher lithium ion conductivity than a single substance of LAF. Besides, the lithium ion conductivity increases with an increase in temperature. However, the lithium ion conductivity decreases significantly at temperatures above 120° C. The reason for this is not clear, but one conceivable reason is that crystallization of LiCl progresses in the range of temperatures above 120° C. As a result, it becomes difficult to stably ensure lithium ion conductivity of higher than or equal to 1.0×10−4 S/cm (siemens per centimeter) at high temperatures.
In contrast to this, the present solid electrolyte maintains high lithium ion conductivity even in the range of temperatures higher than 120° C. and, for example, at 150° C. That is, the present solid electrolyte achieves high-temperature stability. The reason for this is not always clear, but one conceivable reason is that, since the inorganic salt used in the production of the present solid electrolyte contains a sulfur or phosphorus element, the progress of crystallization of the inorganic salt is hindered within the range of temperatures above 120° C. The inorganic salt may preferably be LSO (Li2SO4) or LPO (LiPO3). This hinders the progress of crystallization of the inorganic salt within the range of temperatures above 120° C. and more reliably allows the solid electrolyte to maintain high lithium ion conductivity.
Then, the LAF powder and the LSO powder are mixed and thereafter subjected to a mechanical milling process (step S12). In one example of the mechanical milling process, a planetary ball mill may be used. The planetary ball mill is capable of generating very high impact energy because a stage with a pot placed thereon revolves while the pot rotates on its axis. The mechanical milling process may use any other type of pulverizer. Through the mechanical milling process described above, powder of the present solid electrolyte is obtained for use in the positive electrode layer 112, the negative electrode layer 122, or the solid electrolyte layer 13. In the present example of processing, the mechanical milling process is conducted at ordinary temperatures, but conditions therefor such as temperature may be appropriately changed. Step S12 is a step of subjecting LiaMbXc and LidAeOf to the mechanical milling process.
In step S12, it is preferable that the ratio of the amount of substance of LSO to the total of the amount of substance of LAF and the amount of substance of LSO is higher than 0% and lower than or equal to 85%. In this case, it is possible to more reliably make the lithium ion conductivity of the present solid electrolyte higher than the lithium ion conductivity of a single substance of LAF at both ambient and high temperatures (e.g., at 150° C.). As will be shown in experiments described later, the present solid electrolyte containing LAF and LSO has lithium ion conductivity of higher than or equal to 1.0×10−4 S/cm at 150° C. when the ratio of the amount of substance of LSO is in the range of 5% to 85%. In order to further increase the lithium ion conductivity of the present solid electrolyte, it is preferable that the ratio of the amount of substance of LSO is in the range of 20% to 70%. The same applies to the case of using LPO, instead of LSO.
Next, experiments on the solid electrolyte will be described. The experiments described below were conducted in a dry room or in a glove box with a dew point of less than or equal to −40° C.
Commercial lithium fluoride (LiF) powder and commercial aluminum fluoride (AlF3) powder were prepared as raw materials. These raw materials were weighed and mixed so that the ratio of LiF and AlF3 (the ratio of the amounts of substance) became 3:1. A resultant mixture was subjected to heat treatment at 900° C. and thereafter pulverized into small particles to obtain Li3AlF6 (i.e., LAF) powder. In this way, solid electrolyte powder that contains only the LAF powder was obtained.
In addition to the LAF powder described above, commercial Li2SO4 (i.e., LSO) powder was prepared. These raw materials were weighed so that the ratio of LAF and LSO (the ratio of the amounts of substance) became 90:10, and were subjected to a mechanical milling process using a planetary ball mill to obtain solid electrolyte powder that contains LAF and LSO.
Solid electrolyte powder was obtained by performing the same processing as in Experiment 2, except that the ratio of LAF and LSO ratio became 80:20 (the ratio of the amounts of substance).
Solid electrolyte powder was obtained by performing the same processing as in Experiment 2, except that the ratio of LAF and LSO (the ratio of the amounts of substance) became 70:30.
Solid electrolyte powder was obtained by performing the same processing as in Experiment 2, except that the ratio of LAF and LSO (the ratio of the amounts of substance) became 50:50.
Solid electrolyte powder was obtained by performing the same processing as in Experiment 2, except that the ratio of LAF and LSO (the ratio of the amounts of substance) became 20:80.
In addition to the LAF powder described above, commercial LiPO3 (i.e., LPO) powder was prepared. These raw materials were weighed so that the ratio of LAF and LPO ratio (the ratio of the amounts of substance) became 70:30, and were subjected to a mechanical milling process using a planetary ball mill to obtain solid electrolyte powder that contains LAF and LPO.
Measurement of Lithium Ion Conductivity
The solid electrolyte powder was introduced into a mold that includes a sleeve made of resin and upper and lower punches made of stainless steel (SUS), and was subjected to uniaxial press molding by pressurization at 150 MPa. The upper and lower punches were connected to conductors, and lithium ion conductivity was calculated by impedance measurement conducted at temperatures ranging from ambient temperature to 150° C.
X-ray Powder Diffraction Measurement
The solid electrolyte powder was analyzed by an X-ray diffractometer (XRD) using a CuKα ray to obtain an X-ray diffraction pattern. The obtained X-ray diffraction pattern was fitted with a pseudo-Voigt peak function (Type 1) using data analysis software, Origin Pro 2020b, to calculate the half-value width of a peak.
Table 1 shows the compositions of the solid electrolytes, the lithium ion conductivities of the solid electrolytes at 150° C., and some of the half-value widths of peaks of maximum intensities within the range of 20° to 25° obtained by X-ray diffractometry according to Experiments 1 to 7. In Table 1, Experiments 2 to 7 correspond to examples according to the present invention, and Experiment 1 corresponds to a comparative example.
The solid electrolyte powder containing LSO according to each of Experiments 2 to 6 and the solid electrolyte powder containing LPO according to Experiment 7 had higher lithium ion conductivity at 150° C. than the solid electrolyte powder containing neither LSO nor LPO according to Experiment 1. To be more specific, the lithium ion conductivity at 150° C. increased with an increase in the ratio of the amount of substance of LSO within a range in which the ratio of the amount of substance of LSO to the total of the amount of substance of LAF and the amount of substance of LSO was relatively low (e.g., within the range of 50% or less). The lithium ion conductivity decreased when the ratio of the amount of substance of LSO became excessively high. However, even in Experiment 6 in which the ratio of the amount of substance of LSO was 80%, the lithium ion conductivity was 1.1×10−4 S/cm. Thus, it is conceivable that, if the ratio of the amount of substance of LSO is higher than 0% and lower than or equal to 85%, it is possible to ensure higher lithium ion conductivity than the solid electrolyte powder according to Experiment 1.
According to Experiment 2 in which the ratio of the amount of substance of LSO was 10%, the lithium ion conductivity at 150° C. was 1.2×10−4 S/cm. Thus, it is conceivable that, if the ratio of the amount of substance of LSO is in the range of 5% to 85% (i.e., the molar ratio of LAF and LSO is in the range of 95:5 to 15:85, it is possible to ensure the lithium ion conductivity of approximately 1.0×10−4 S/cm or higher. In order to further increase the lithium ion conductivity (e.g., to approximately 2.0×10−4 S/cm or higher), it is preferable that the ratio of the amount of substance of LSO is in the range of 20% to 70%. According to Experiments 2 to 6 and Experiment 7, the solid electrolyte powder maintained lithium ion conductivity of higher than or equal to 1.0×10−4 S/cm even after retained at 150° C. for some time. In this way, the solid electrolyte powder according to the examples of the present invention has high-temperature stability.
According to Experiments 2 to 5, the solid electrolyte powder exhibited an XRD pattern with a very broad peak, in which the half-value width of a peak of maximum intensity detected within the range of 20° to 25° was in the range of 0.77° to 1.67°.
Preparation of All Solid-State Battery
Positive active material powder that was NCM, solid electrolyte powder, and electron conductive agent powder were weighed and mixed to obtain ready mixed powder of a positive electrode. Also, negative active material powder that was graphite, solid electrolyte powder, and electron conductive agent powder were weighed and mixed to obtain ready mixed powder of a negative electrode. The solid electrolyte powder was introduced into a mold including a sleeve made of resin and upper and lower punches made of SUS, and was subjected to uniaxial press molding by pressurization at 150 MPa. The ready mixed powder of the positive electrode was introduced on the pressed solid electrolyte powder and integrated together by pressurization at 150 MPa. The ready mixed powder of the negative electrode was introduced on the opposite side of the pressed solid electrolyte powder to the positive electrode and integrated together by pressurization at 150 MPa. In this way, a battery that included a positive electrode layer, a solid electrolyte layer, and a negative electrode layer was prepared.
Charge/Discharge Test
The battery described above was placed stationary in a constant-temperature bath kept at 150° C. in order to conduct a charge/discharge test. Charge referred to constant-current charge with a current density of 0.4 mA/cm2 and a cut-off voltage of 4.15V, and discharge referred to constant-current discharge with a current density of 0.4 mA/cm2 and a cut-off voltage of 2.0V. A battery using the solid electrolyte powder according to Experiment 1 (comparative example) had a discharge capacity of less than 10 mAh/g-NCM (less than 6% of a theoretical capacity). A battery using the solid electrolyte powder according to Experiment 5 (example according to the present invention) had a discharge capacity of 47 mAh/g-NCM (approximately 30% of the theoretical capacity), which was approximately five times of that in the comparative example.
Although, in the present embodiment described above, the present solid electrolyte is obtained by subjecting Li3AlF6 and the inorganic salt, which is either Li2SO4 or LiPO3, to a mechanical milling process, the present solid electrolyte may be produced by mixing a compound that contains a lithium element (Li), an element (M) serving as a trivalent cation, and a halogen element (X) with arbitrary inorganic salt that contains a lithium element and a sulfur or phosphorus element. In this case, the compound described above can be expressed by a composition formula of LiaMbXc, where M is an aluminum (Al) element or a different element serving as a trivalent cation (typically, a metallic element), X is a fluorine (F) element or a different halogen element, and a to c are values greater than 0. Here, Li, M, and X are respectively a univalent cation, a trivalent cation, and a univalent anion, and the compound described above is expressed by Li3MX6. In the composition formula of Li3MX6, a+3b=c and a=3b are satisfied. Other examples of the compound described above include, besides Li3AlF6, Li3YF6, Li3LaF6, Li3GaF6, Li3YCl6, Li3YBr6, Li3InCl6, and Li3LaI6.
Similarly, the inorganic salt described above can be expressed by a composition formula of LidAeOf, where A is a sulfur or phosphorus element, and d to f are values greater than 0. When A is a sulfur element, Li, A, and O are respectively an univalent cation, a tetravalent or hexavalent cation, and a bivalent anion, and therefore d+4e=2f and d+6e=2f need to be satisfied. When A is a phosphorus element, Li, A, and O are respectively a univalent cation, a pentad cation, and a bivalent anion, and therefore d+5e=2f needs to be satisfied. In other words, d+n×e=2f needs to be satisfied, where when A is a sulfur element, n is 4 or 6, and when A is a phosphorus element, n is 5. Other examples of the inorganic salt described above include, besides Li2SO4 or LiPO3, lithium sulfite (Li2SO3) and lithium orthophosphate (Li3PO4).
As described above, when the compound containing a lithium element (Li), an element (M) serving as a trivalent cation, and a halogen element (X) is expressed by LiaMbXc and the inorganic salt containing a lithium element and a sulfur or phosphorus element is expressed by LidAeOf, by using the values a to f that are greater than 0, the present solid electrolyte obtained by mixing the compound and the inorganic salt is expressed by a composition formula of Lia+dMbXcAeOf. Moreover, the conditions previously described, i.e., a+3b=c, a=3b, and d+n×e=2f, need to be satisfied, where when A is a sulfur element, n is 4 or 6, and when A is a phosphorus element, n is 5. With consideration given to factors such as errors of measurement, 0.8c≤(a+3b)≤1.2c, 0.8a≤3b≤1.2a, and 1.6f≤(d+n×e)≤2.4f may be satisfied. Preferably, 0.9c≤(a+3b)≤1.1c, 0.9a≤3b≤1.1a, and 1.8f≤(d+n×e)≤2.2f may be satisfied. Accordingly, the present solid electrolyte expressed by the composition formula of Lia+dMbXcAeOf has high lithium ion conductivity and high-temperature stability. As will be described later, in the case where the compound expressed by LiaMbXc is not generated, there is no need to satisfy 0.8a≤3b≤1.2a.
As described previously, it is possible to more reliably make the lithium ion conductivity of the present solid electrolyte higher than that of the single substance of LAF if the ratio of the amount of substance of LSO to the total of the amount of substance of LAF and the amount of substance of LSO is lower than or equal to 85%. This relationship is considered to also apply to any solid electrolyte that is generated from an arbitrary compound and arbitrary inorganic salt and expressed by a composition formula of Lia+dMbXcAeOf. When the ratio of the amount of substance of the aforementioned inorganic salt (e.g., LSO) is 85%, e/(c+e) becomes 0.49 in the composition formula of Lia+dMbXcAeOf described above. Therefore, if the composition formula of Lia+dMbXcAeOf satisfies 0<e/(c+e)≤0.5, it is possible to more reliably make the lithium ion conductivity of the present solid electrolyte higher than that of a simple substance of LiaMbXc. As described previously, a to f are values greater than 0, and the solid electrolyte contains LidAeOf.
In order to further increase the lithium ion conductivity by setting the ratio of the amount of substance of the aforementioned inorganic salt in the range of 5% to 85%, it is preferable that the composition formula of Lia+dMbXcAeOf satisfies 0.01≤e/(c+e)≤0.5. In order to yet further increase the lithium ion conductivity by setting the ratio of the amount of substance of the aforementioned inorganic salt in the range of 20 to 70%, it is preferable that the composition formula of Lia+dMbXcAeOf satisfies 0.04≤e/(c+e)≤0.28.
Preferably, LidAeOf may be Li2SO4 or LiPO3 (i.e., LSO or LPO). In this case, it is possible to more reliably maintain high lithium ion conductivity at high temperatures. When LidAeOf is LSO, A is a sulfur element, and d=2e is satisfied in the composition formula of Lia+dMbXcAeOf. With consideration given to factors such as errors of measurement, 0.8d≤2e≤1.2d may be satisfied, and preferably 0.9d≤2e≤1.1d may be satisfied. When LidAeOf is LPO, A is a phosphorus element, and d=e is satisfied in the composition formula of Lia+dMbXcAeOf. With consideration given to factors such as errors of measurement, 0.8d≤e≤1.2d may be satisfied, and preferably 0.9d≤e≤1.1d may be satisfied. Also, LiaMbXc may preferably be Li3AlF6 (i.e., LAF). In other words, in the composition formula of Lia+dMbXcAeOf, M is an aluminum element and X is a fluorine element. In this case, it is possible to more reliably achieve high lithium ion conductivity.
As described previously, the composition formula of the present solid electrolyte is expressed as Lia+dMbXcAeOf. For example, this composition formula is expressed as Lia+dAlbFcSeOf in the case of mixing LAF and LSO. This composition formula is also expressed as Lia+dAlbFcPeOf in the case of mixing LAF and LPO. To identify whether an unknown solid electrolyte is the present solid electrolyte, the unknown solid electrolyte is subjected to chemical analysis to identify whether constituent elements of the unknown solid electrolyte are Li, M, X, A, and O, where M is an element serving as a trivalent cation, X is a halogen element, and A is a sulfur or phosphorus element. When the constituent elements are Li, M, X, A, and O and if the values a to f of the molar ratio of the constituent elements are all greater than 0, and 0.8c≤(a+3b)≤1.2c and 1.6f≤(d+n×e)≤2.4f are satisfied, where when A is a sulfur element, n is 4 or 6, and when A is a phosphorus element, n is 5, the unknown solid electrolyte can be said as the present solid electrolyte. When 0<e/(c+e)≤0.5 is satisfied, it can be said that the ratio of the amount of substance of the aforementioned inorganic salt to the total of the amount of substance of the aforementioned compound (e.g., LAF) and the amount of substance of the aforementioned inorganic salt (e.g., LSO) is lower than or equal to 85%.
For example, Li, Al, S, and P may be quantitated by an ICP emission spectral analyzer, and F may be quantitated by ion chromatography. Oxygen (O) may be quantitated by oxygen/nitrogen gas analysis. When the present solid electrolyte contains an element M other than Al and an element X other than F, measurement methods that are capable of quantitating the elements M and X may be appropriately selected.
The above example of processing describes the case in which, after Li3AlF6 (i.e., LAF) is generated by mixing LiF and AlF3 so that the ratio of the amounts of substance becomes 3:1, Li3AlF6 and Li2SO4 (or LiPO3) are subjected to a mechanical milling process. Alternatively, LiF, AlF3, and Li2SO4 (or LiPO3) may be subjected to a mechanical milling process without generating Li3AlF6.
At this time, the ratio of the amounts of substance of LiF and AlF3 may be other than 3:1. The ratio of the amount of substance of LiF to the total of the amounts of substance of LiF, AlF3, and Li2SO4 may, for example, be higher than or equal to 10% and lower than or equal to 80% and preferably higher than or equal to 30% and lower than or equal to 70%. The ratio of the amount of substance of AlF3 may, for example, be higher than or equal to 5% and lower than or equal to 60% and preferably higher than or equal to 10% and lower than or equal to 50%. The ratio of the amount of substance of Li2SO4 may, for example, be higher than or equal to 2% and lower than or equal to 50% and preferably higher than or equal to 10% and lower than or equal to 40%. Through the processing described above, it is possible to obtain powder of the present solid electrolyte that has high lithium ion conductivity and high-temperature stability. The same applies the case of using LiPO3, instead of Li2SO4.
Here, the ratio of the amounts of substance of Li, Al, and F in a mixture obtained by mixing only LiF and AlF3 prepared in step S21 is expressed as a:b:c, where a to c are values greater than 0. In this case, a+3b=c is satisfied because Li, Al, and F are respectively an univalent cation, a trivalent cation, and an univalent anion. With consideration given to factors such as errors of measurement, 0.8c≤(a+3b)≤1.2c is satisfied. Meanwhile, unlike in the case where Li3AlF6 is generated in advance, the ratio of the amounts of substance of LiF and AlF3 is not limited to 3:1, and therefore a=3b is not always satisfied. In other words, even if 0.8a≤3b≤1.2a is not satisfied, it is possible to obtain the present solid electrolyte that has high lithium ion conductivity and high-temperature stability.
Next, experiments in producing the present solid electrolyte by mixing LiF, AlF3, and Li2SO4 will be described with reference to Table 2. In Table 2, Experiments 8 to 17 correspond to examples according to the present invention.
Commercial LiF powder, commercial AlF3 powder, and commercial Li2SO4 powder were prepared. These raw materials were weighed so that the ratio of LiF, AlF3, and Li2SO4 (the ratio of the amounts of substance) became 60:20:20, and were subjected to a mechanical milling process using a planetary ball mill to obtain solid electrolyte powder that contained LiF, AlF3, and Li2SO4. Thereafter, lithium ion conductivity was measured in the same manner as described above in “Measurement of Lithium Ion Conductivity.” According to Experiment 8, lithium ion conductivity at 150° C. was 4.1×10−4 S/cm and higher than the ion conductivity (5.0×10−5 S/cm) according to Experiment 1, which was a comparative example. The ratio of the amounts of substance of LiF, AlF3, and Li2SO4 according to Experiment 8 was the same as that according to Experiment 5 described previously, and Experiment 8 differed from Experiment 5 in that Li3AlF6 was not generated in advance.
According to Experiments 9 to 17, the solid electrolyte powder was obtained by performing the same processing as in Experiment 8, except that LiF, AlF3, and Li2SO4 were weighed so that the ratio of the amounts of substance became as shown in Table 2. Thereafter, lithium ion conductivity was measured. According to Experiments 9 to 17, although the ratio of the amounts of substance of LiF and AlF3 was different from 3:1, the lithium ion conductivity at 150° C. was higher than in Experiment 1.
As described above, even if a=3b is not satisfied when the ratio of the amounts of substance of Li, Al, and F in the mixture of LiF and AlF3 is expressed as a:b:c (i.e., the ratio of the amounts of substance of LiF and AlF3 is different from 3:1), it is possible to achieve a solid electrolyte that has high lithium ion conductivity. In actuality, the solid electrolyte powder according to Experiments 8 to 17 maintained high lithium ion conductivity even after retained at 150° C. for some time, and achieved high-temperature stability.
Next, preferable ranges of values such as 3b/a will be described. According to Experiments 8 to 17, the ratio of the amounts of substance of Li, Al, and F in a mixture of LiF and AlF3 and the ratio of the amounts of substance of Li, S, and O in Li2SO4, among LiF, AlF3, and Li2SO4 used in the production of the solid electrolyte, were expressed as a:b:c:d:e:f According to Experiments 1 to 6, the ratio of the amounts of substance of Li, Al, and F in Li3AlF6 and the ratio of the amounts of substance of Li, S, and O in Li2SO4, which were used in the production of the solid electrolyte, were expressed as a:b:c:d:e:f Here, a to f are values greater than 0.
In the production of the solid electrolyte in
When the ratio of the amounts of substance of Li, M, and X in the mixture of the first and second compounds and the amounts of substance of Li, A, and O in the third compound is expressed as a:b:c:d:e:f by using values a to f that are greater than 0, the mixture satisfies 0.8c≤(a+3b)≤1.2c as described previously. The third compound satisfies 1.6f≤(d+n×e)≤2.4f as described previously, where when A is a sulfur element, n is 4 or 6, and when A is a phosphorus element, n is 5. Other examples of the first compound of LiX include, besides LiF, LiCl and LiBr. Other examples of the second compound of MX3 include, besides AlF3, YF3, LaF3, and GaF3. Other examples of the third compound of LidAeOf include, besides Li2SO4 and LiPO3, lithium sulfite (Li2SO3) and lithium orthophosphate (Li3PO4).
In the production of the solid electrolyte shown in
The present solid electrolyte, the method of producing the present solid electrolyte, and the battery may be modified in various ways.
The present solid electrolyte may contain impurities and contain a component expressed by the composition formula of Lia+dMbXcAeOf as its principal component. The principal component refers to a component with a highest mass ratio among the components of the solid electrolyte. The mass ratio of the principal component of the solid electrolyte may be preferably higher than or equal to 80% by mass and more preferably higher than or equal to 90% by mass. The present solid electrolyte may be mixed with other substances (which may include Li) and used as an electrolyte material. The mass ratio of the present solid electrolyte in the electrolyte material may be preferably higher than or equal to 50% by mass, more preferably higher than or equal to 60% by mass, and yet more preferably higher than or equal to 70% by mass.
In the X-ray diffraction pattern obtained from the present solid electrolyte, the half-value width of a peak detected within the range of a diffraction angle 2θ from 20° to 25° may be less than 0.4°.
The solid electrolyte layer 13 may contain substances other than the present solid electrolyte. The present solid electrolyte used in the all solid-state secondary battery 1 does not necessarily have to be contained in all of the positive electrode 11, the negative electrode 12, and the solid electrolyte layer 13, and may be contained in at least one of the positive electrode 11, the negative electrode 12, or the solid electrolyte layer 13. The present solid electrolyte may be used in batteries other than all solid-state secondary batteries, and may be used for purposes other than batteries. The production of the present solid electrolyte may be conducted by a process other than the mechanical milling process.
The configurations of the above-described preferred embodiment and variations may be appropriately combined as long as there are no mutual inconsistencies.
While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2021-057224 | Mar 2021 | JP | national |
The present application is a continuation application of International Application No. PCT/JP2022/14852 filed on Mar. 28, 2022, which claims priority to Japanese Patent Application No. 2021-057224 filed on Mar. 30, 2021. The contents of these applications are incorporated herein by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP2022/014852 | Mar 2022 | US |
| Child | 18461757 | US |
