JP7680973B2 - Oxide-based solid electrolytes and all-solid-state lithium-ion batteries - Google Patents

Description

The present invention relates to an oxide-based solid electrolyte and an all-solid-state lithium-ion battery.

With the rapid spread of information-related devices and communication devices such as personal computers, video cameras, and mobile phones in recent years, the development of batteries to be used as power sources for these devices has become important. Among these batteries, lithium-ion batteries have attracted attention because of their high energy density. In addition, there is a demand for improved energy density and battery characteristics for lithium secondary batteries used in large-scale applications such as vehicle-mounted power sources and load leveling.

LISICON-type oxide-based solid electrolytes are solid materials that conduct Li ions, and are expected to be the solid electrolytes for next-generation all-solid-state batteries. In addition, by replacing them with ions of different valences, it is possible to introduce Li deficiencies or excess Li, which has the advantage that materials can be created in combination with various elements.

In addition, to improve the characteristics of all-solid-state lithium-ion batteries, solid electrolytes with high ionic conductivity are required. Non-Patent Document 1 discloses a technology aimed at improving ionic conductivity in LISICON-type oxide-based solid electrolytes.

Guowei zhao et al., Enhancing Fast Lithium Ion Conduction in Li4GeO4-Li3PO4 Solid Electrolytes, ACS Appl. Energy Mater., 2, 2019, 6608-6615

Non-Patent Document 1 describes a technology aimed at improving the ionic conductivity of oxide-based solid electrolytes by substituting various cations _M into _{Li3.75Ge0.75P0.25O4} . The generalized composition formula is Li3.75 _{±y ( Ge0.75P0.25} ) _{1 - xMxO4} , where Mg2 ⁺ , B3+, Al3 ⁺ , ^{Ga3+ ,} or V5 ₊ is selected _as M. It describes that in a system where ^{V5+ is} selected, the highest ionic conductivity is shown when x=0.3, and is about 5.1× ^10-5 S/cm at 25°C.

Thus, even the LISICON-type solid electrolytes that have been reported to date, which have the highest ionic conductivity, have ionic conductivity that is one to two orders of magnitude lower than other oxide-based solid electrolyte materials such as garnet-type materials, and further improvement in ionic conductivity is desired for application in all-solid-state batteries.

The present invention has been made to solve the above problems, and aims to provide an oxide-based solid electrolyte with good ionic conductivity and an all-solid-state lithium-ion battery using the same.

The present invention, which was completed based on the above findings, provides _{an oxide} -based solid electrolyte represented by the composition formula _{LiαGexVyMzO4} (wherein _3.30 ≦α≦3.80, 0.35≦x≦0.60, 0.20≦y≦0.50, and 0.03≦z≦0.25, and M is Fe or Tl) _.

In another embodiment of the oxide-based solid electrolyte of the present invention, in the formula, 0.30≦y≦0.45.

In yet another embodiment of the oxide-based solid electrolyte of the present invention, in the above formula, 0.05≦z≦0.15.

In yet another embodiment, the present invention is an all-solid-state lithium-ion battery including a solid electrolyte layer made of an oxide-based solid electrolyte according to an embodiment of the present invention, a positive electrode layer, and a negative electrode layer.

The present invention provides an oxide-based solid electrolyte with good ionic conductivity, and an all-solid-state lithium-ion battery using the same.

FIG. 1 is a schematic diagram of an all-solid-state lithium-ion battery according to an embodiment of the present invention.

Next, the embodiments for carrying out the present invention will be described in detail with reference to the drawings. It should be understood that the present invention is not limited to the following embodiments, and that appropriate design changes, improvements, etc. may be made based on the ordinary knowledge of those skilled in the art without departing from the spirit of the present invention.

(Oxide solid electrolyte)
The oxide-based solid electrolyte of this embodiment is represented by the composition formula _{LiαGexVyMzO4} (wherein 3.30≦α≦ _3.80 , 0.35≦x≦0.60, 0.20≦y≦0.50, and 0.03≦z≦ _0.25 , and M is Fe _{or Tl} ). The oxide-based solid electrolyte of this embodiment has a composition in which a group ₁₃ element M (Fe or _Tl ) is substituted for a base material of _LiαGexVyO4 , which is a LISICON-type solid electrolyte _. With this configuration, the number of elements constituting the oxide-based solid electrolyte is increased, and the activation energy is reduced, so that an oxide-based solid electrolyte having good ion conductivity can be obtained.
In the present invention, the oxide-based solid electrolyte refers to a solid electrolyte having, in its skeleton, an oxoacid ion in which an oxygen atom is coordinately bonded to a central element, as a counter anion to a lithium ion.

In the oxide-based solid electrolyte of this embodiment, if α is less than 3.30 in the above composition formula, the carrier concentration is low, which may cause a problem of reduced ion conductivity. Furthermore, if α is more than 3.80, there is a risk of a problem of single-phase synthesis being difficult. In the oxide-based solid electrolyte of this embodiment, it is preferable that 3.30≦α≦3.75 in the above composition formula.

In the oxide-based solid electrolyte of this embodiment, if x is less than 0.35 in the composition formula, _{a Li4GeO4} phase with low ionic conductivity may be generated. If x is more than 0.60, the crystal lattice becomes large, the distance between Li sites becomes long, and the ionic conductivity may decrease. In the oxide-based solid electrolyte of this embodiment, it is preferable that 0.35≦x≦0.57 in the composition formula.

In the oxide-based solid electrolyte of the present embodiment, if y is less than 0.20 in the composition formula, the effect of reducing activation energy by element substitution is weak, so the effect of improving ion conductivity may be small. Also, if y is more than 0.50, impurity phases such as Li ₄ VO ₄ may be generated, and ion conductivity may decrease. In the oxide-based solid electrolyte of the present embodiment, it is preferable that 0.30≦y≦0.45 in the composition formula.

In the oxide-based solid electrolyte of this embodiment, in the above composition formula, M is Fe or Tl. When M is Fe or Tl, it can be substituted for the base material, and there is an effect of improving ionic conductivity.

In the oxide-based solid electrolyte of this embodiment, if z exceeds 0.25 in the above composition formula, an impurity phase such _{as Li5MO4} is generated, and the ion conductivity may decrease. In the oxide-based solid electrolyte of this embodiment, it is preferable that 0.05≦z≦0.15 in the above composition formula.

The average particle size of the oxide-based solid electrolyte according to the embodiment of the present invention is not particularly limited, but may be 0.01 to 100 μm, 0.1 to 100 μm, or 0.1 to 50 μm.

The ionic conductivity of the oxide-based solid electrolyte according to the embodiment of the present invention is preferably 9.5×10 ⁻⁵ S/cm or more, more preferably 1.0×10 ⁻⁴ S/cm or more, and even more preferably 1.0×10 ⁻³ S/cm or more at 30° C. The ionic conductivity of the oxide-based solid electrolyte can be measured, for example, by the following method.
First, 0.2 g of the oxide-based solid electrolyte powder is pressed under a pressure of 550 MPa to form a plate-like provisional shape, and then sintered at 800°C for 12 hours to obtain a molded body. A pellet with a diameter of 10 mm is prepared by applying silver paste to both sides of the molded body. Using the pellet, AC impedance measurements from 20 Hz to 50 MHz are performed at 30°C with an applied voltage of 100 mV. The AC impedance measurements can be performed using an E4990A manufactured by Toyo Corporation, which has been subjected to open-short correction. The Li ion migration resistance is obtained by analyzing the arcs seen in the part of the Cole-Cole plot obtained by the AC impedance measurement at frequencies of 30 kHz or more. Next, the ion conductivity is obtained from the Li ion migration resistance and the thickness and area of the solid electrolyte part of the pellet used for the measurement based on the following formula.
Ion conductivity=thickness of solid electrolyte part of pellet/[(Li ion migration resistance)×(pellet area)]

(Method for producing oxide-based solid electrolyte)
Next, a method for producing an oxide-based solid electrolyte according to an embodiment of the present invention will be described.
First, raw materials are weighed out so as to obtain a predetermined composition in a glove box in an inert gas atmosphere such _as argon gas _or nitrogen gas. Examples of the raw materials used here include _LiOH.H2O , _GeO2 , _V2O5 , _Fe2O3 , _{and Tl2O3} .

Next, mix in a mortar or other container for 5 to 30 minutes to produce a mixed powder. At this time, it is preferable to mix for a time such that the average particle size of the mixed powder is 5 to 40 μm.

Next, the mixed powder is placed in an alumina sagger and fired at 600 to 1000° C. for 1 to 20 hours to produce an oxide _- based solid electrolyte according to an embodiment of the present invention, which is represented by the composition formula _{LiαGexVyMzO4 (} wherein _3.30 ≦ _α ≦3.80, 0.35≦x≦0.60, 0.20≦y≦0.50, and 0.03≦z≦0.25, and M is Fe or Tl).

(All-solid-state lithium-ion battery)
A solid electrolyte layer is formed by the oxide-based solid electrolyte according to the embodiment of the present invention, and an all-solid-state lithium ion battery including the solid electrolyte layer, a positive electrode layer, and a negative electrode layer can be produced. The positive electrode layer and the negative electrode layer constituting the all-solid-state lithium ion battery according to the embodiment of the present invention are not particularly limited and can be formed of known materials and can have a known configuration as shown in FIG.

The positive electrode layer of the lithium ion battery is a layer of a positive electrode mixture made by mixing a known positive electrode active material for lithium ion batteries with the oxide-based solid electrolyte according to the embodiment of the present invention or another oxide-based solid electrolyte.

The positive electrode mixture may further contain a conductive assistant. As the conductive assistant, a carbon material, a metal material, or a mixture thereof may be used. The conductive assistant may contain at least one element selected from the group consisting of, for example, carbon, nickel, copper, aluminum, indium, silver, cobalt, magnesium, lithium, chromium, gold, ruthenium, platinum, beryllium, iridium, molybdenum, niobium, osmium, rhodium, tungsten, and zinc. The conductive assistant is preferably a highly conductive carbon element, a metal element, mixture, or compound containing carbon, nickel, copper, silver, cobalt, magnesium, lithium, ruthenium, gold, platinum, niobium, osmium, or rhodium. As the carbon material, for example, carbon black such as Ketjen black, acetylene black, denka black, thermal black, and channel black, graphite, carbon fiber, activated carbon, etc. may be used.

The average thickness of the positive electrode layer of the lithium ion battery is not particularly limited and can be designed appropriately according to the purpose. The average thickness of the positive electrode layer of the lithium ion battery may be, for example, 1 μm to 100 μm, or 1 μm to 10 μm.

The method for forming the positive electrode layer of the lithium ion battery is not particularly limited and can be appropriately selected depending on the purpose. Examples of the method for forming the positive electrode layer of the lithium ion battery include sputtering using a target material of the positive electrode active material, or compression molding of the positive electrode active material.

The negative electrode layer of the lithium ion battery may be a layer of a known negative electrode active material for lithium ion batteries. The negative electrode layer may also be a layer of a negative electrode composite material formed by mixing a known negative electrode active material for lithium ion batteries with the oxide-based solid electrolyte according to the embodiment of the present invention or another oxide-based solid electrolyte.

The negative electrode layer may contain a conductive additive, as in the positive electrode layer. The conductive additive may be the same material as that described for the positive electrode layer. As the negative electrode active material, for example, a carbon material, specifically, artificial graphite, graphite carbon fiber, resin-sintered carbon, pyrolytic vapor-grown carbon, coke, mesocarbon microbeads (MCMB), furfuryl alcohol resin-sintered carbon, polyacene, pitch-based carbon fiber, vapor-grown carbon fiber, natural graphite, non-graphitizable carbon, or a mixture thereof may be used. As the negative electrode material, for example, a metal such as metallic lithium, metallic indium, metallic aluminum, or metallic silicon itself, or an alloy in combination with other elements or compounds may be used.

The average thickness of the negative electrode layer of the lithium ion battery is not particularly limited and can be appropriately selected depending on the purpose. The average thickness of the negative electrode layer of the lithium ion battery may be, for example, 1 μm to 100 μm, or 1 μm to 10 μm.

The method for forming the negative electrode layer of the lithium ion battery is not particularly limited and can be appropriately selected depending on the purpose. Examples of the method for forming the negative electrode layer of the lithium ion battery include sputtering using a target material of the negative electrode active material, compression molding of the negative electrode active material, and vapor deposition of the negative electrode active material.

The average thickness of the solid electrolyte layer of the lithium ion battery formed by the oxide-based solid electrolyte according to the embodiment of the present invention is not particularly limited and can be designed appropriately according to the purpose. The average thickness of the solid electrolyte layer of the lithium ion battery may be, for example, 50 μm to 500 μm, or 50 μm to 100 μm.

The method for forming the solid electrolyte layer of the lithium ion battery is not particularly limited and can be appropriately selected depending on the purpose. Examples of the method for forming the solid electrolyte layer of the lithium ion battery include sputtering using a target material for the solid electrolyte, and compression molding of the solid electrolyte.

Other components constituting the lithium-ion battery are not particularly limited and can be selected appropriately depending on the purpose. Examples include a positive electrode collector, a negative electrode collector, and a battery case.

The size and structure of the positive electrode current collector are not particularly limited and can be appropriately selected depending on the purpose.
Examples of the material for the positive electrode current collector include die steel, stainless steel, aluminum, aluminum alloys, titanium alloys, copper, gold, and nickel.
The positive electrode current collector may be in the form of, for example, a foil, a plate, or a mesh.
The average thickness of the positive electrode current collector may be, for example, 10 μm to 500 μm, or 50 μm to 100 μm.

The size and structure of the negative electrode current collector are not particularly limited and can be appropriately selected depending on the purpose.
Examples of the material for the negative electrode current collector include die steel, gold, indium, nickel, copper, and stainless steel.
The negative electrode current collector may be in the form of, for example, a foil, a plate, or a mesh.
The average thickness of the negative electrode current collector may be, for example, 10 μm to 500 μm, or 50 μm to 100 μm.

The battery case is not particularly limited and can be appropriately selected depending on the purpose, and examples thereof include known laminate films that can be used in conventional all-solid-state batteries, etc. Examples of the laminate film include a resin laminate film and a film in which a metal is vapor-deposited on a resin laminate film.
The shape of the battery is not particularly limited and can be appropriately selected depending on the purpose. Examples of the shape of the battery include cylindrical, square, button, coin, and flat types.

The following examples are provided to better understand the present invention and its advantages, but the present invention is not limited to these examples.

Example 1
In a glove box with an argon atmosphere, the raw materials were _weighed so that the raw material composition was _{Li3.60Ge0.50V0.45Fe0.05 ,} and mixed in a mortar for 15 minutes to prepare a mixed powder. Next, about 1 g of the mixed powder was placed _on an alumina boat and fired at 800°C for 12 _hours to obtain an oxide _- based solid electrolyte having a _{composition of Li3.50Ge0.48V0.45Fe0.05O4} .

Example 2
The same procedure as in Example 1 was repeated, except that the raw material composition was Li _3.70 Ge _0.50 V _0.40 Fe _0.10.

Example 3
The same procedure as in Example 1 was carried out, except that the raw material composition was Li _3.80 Ge _0.50 V _0.35 Fe _0.15.

Example 4
The same procedure as in Example 1 was repeated except that the raw material composition was Li _4.00 Ge _0.50 V _0.25 Fe _0.25.

Example 5
The same procedure as in Example 1 was carried out, except that the raw material composition was Li _3.77 Ge _0.55 V _0.39 Fe _0.06.

Example 6
The same procedure as in Example 1 was repeated, except that the raw material composition was Li _3.90 Ge _0.60 V _0.30 Fe _0.10.

(Example 7)
The same procedure as in Example 1 was repeated, except that the raw material composition was Li _3.70 Ge _0.50 V _0.40 Tl _0.10.

(Comparative Example 1)
The same procedure as in Example 1 was carried out, except that the raw material composition was Li _3.52 Ge _0.50 V _0.49 Fe _0.01.

(Comparative Example 2)
The same procedure as in Example 1 was repeated except that the raw material composition was Li _3.50 Ge _0.50 V _0.50.

<Composition analysis>
0.5 g of each oxide-based solid electrolyte sample (powder) obtained in Examples 1 to 7 and Comparative Examples 1 and 2 was weighed out and dissolved in various acids, and then the composition was analyzed using an inductively coupled plasma optical emission spectrometer (ICP-OES) "PS7800" manufactured by Hitachi High-Tech Corporation.

<Ionic Conductivity>
0.2 g of each oxide-based solid electrolyte powder obtained in Examples 1 to 7 and Comparative Examples 1 to 2 was pressed at a pressure of 550 MPa to form a plate-like provisional shape, and then sintered at 800°C for 12 hours to obtain a molded body. A pellet with a diameter of 10 mm was produced by applying silver paste to both sides of the molded body. Using the pellet, AC impedance measurement from 20 Hz to 50 MHz was performed at 30°C with an applied voltage of 100 mV using an E4990A manufactured by Toyo Corporation, which was subjected to open-short correction. The arc seen in the part of the Cole-Cole plot obtained by the AC impedance measurement at frequencies of 30 kHz or more was analyzed to obtain the Li ion migration resistance. Next, based on the following formula, the ion conductivity was obtained from the Li ion migration resistance and the thickness and area of the solid electrolyte part of the pellet used for the measurement.
Ion conductivity=thickness of solid electrolyte part of pellet/[(Li ion migration resistance)×(pellet area)]
The above production conditions and test results are shown in Table 1.

(Evaluation Results)
The oxide-based solid electrolytes of Examples 1 to ₇ all had a composition represented by the formula _{LiαGexVyMzO4} (wherein _3.30 ≦α≦3.80, 0.35≦x≦0.60, 0.20≦y≦0.50, and 0.03≦z≦0.25, and _M is Fe or Tl), but Comparative Examples 1 and ₂ did not have this composition.
Therefore, the oxide-based solid electrolytes according to Examples 1 to 7 had better ionic conductivity than those not having the above composition in Comparative Examples 1 and 2. Therefore, an improvement in the battery capacity of the all-solid-state lithium ion battery using the solid electrolytes according to Examples 1 to 7 can be expected.

Claims (4)

Composition formula: _{LiαGexVyMzO4} (wherein, _3.30 ≦α≦ _3.80 , _0.35 ≦x≦0.60, 0.20≦y≦0.50, and 0.03≦z≦0.25; and M is Fe or _Tl .)
An oxide-based solid electrolyte represented by the formula: The oxide-based solid electrolyte according to claim 1, wherein 0.30≦y≦0.45 in the formula. The oxide-based solid electrolyte according to claim 1 or 2, wherein 0.05≦z≦0.15 in the formula. An all-solid-state lithium ion battery comprising a solid electrolyte layer made of the oxide-based solid electrolyte according to any one of claims 1 to 3, a positive electrode layer, and a negative electrode layer.