JP6660662B2 - Lithium ion secondary battery - Google Patents

Description

  The present invention relates to a lithium ion secondary battery.

  Conventionally, as a lithium ion secondary battery, a sulfide excellent in lithium ion conductivity is used as an inorganic solid electrolyte, and electrodes (positive electrode and negative electrode) formed on both surfaces of the solid electrolyte layer are joined to each electrode. A configuration including a current collector is known.

  In a lithium ion secondary battery, graphite capable of inserting and extracting lithium and exhibiting a large capacity is mainly used as a negative electrode active material. Patent Literature 1 discloses a lithium secondary battery that has improved electric conductivity by using a polymer electrolyte as a solid electrolyte and fixing transition metal particles that do not alloy with lithium on the surface of a negative electrode active material made of a carbon material. Has been described.

  Patent Literature 2 describes that graphite is coated with graphite for coating, pitch, or the like, and is used as a negative electrode of a lithium ion secondary battery. Further, Patent Document 3 describes an inorganic all-solid-state secondary battery in which a negative electrode active material made of graphite is coated with amorphous carbon in order to reduce the lithium ion conduction resistance of the negative electrode.

  Patent Document 4 discloses that in order to provide an inorganic solid electrolyte secondary battery having good battery characteristics, a positive electrode, a negative electrode, and an inorganic solid electrolyte are provided, and the positive electrode includes a positive electrode active material layer and a positive electrode current collector. The negative electrode is composed of a negative electrode active material layer and a negative electrode current collector layer, and the positive electrode current collector layer and the negative electrode current collector layer are conductive metal oxide layers; Discloses that a substance whose operating potential of the negative electrode is more noble than 1.0 V with respect to the potential of metallic lithium was used.

  On the other hand, in an inorganic all-solid-state secondary battery using a sulfide-based solid electrolyte, in order to grasp the charge / discharge state, as in Patent Document 5, a lithium-ion secondary battery and a lithium-ion secondary battery were stacked. The pressure sensor includes a pressure sensor, and a holding member that sandwiches the lithium ion secondary battery and the pressure sensor from both sides in a direction in which they are stacked. When the lithium secondary battery expands and contracts due to charge and discharge, the force applied to the pressure sensor changes accordingly, and the change in volume of the lithium ion secondary battery is detected as the output value of the pressure sensor. I have.

  In the system for estimating the state of a secondary battery of Patent Document 6, each secondary battery cell includes a positive electrode and a negative electrode stored in a housing. And the temperature sensor is arrange | positioned between the restraint plates, and is integrally restrained with the secondary battery cell. The temperature sensor is arranged so as to detect the temperature of the positive electrode and the negative electrode for at least one secondary battery cell. The ECU (Electronic Control Unit) considers that the volume change characteristic of the electrode changes depending on the temperature, and based on the pressure detection value from the pressure sensor and the temperature detection value from the temperature sensor, the state of charge of the secondary battery ( SOC (State of Charge) is calculated.

  The overvoltage detection device of Patent Document 7 is laminated in a thickness direction and inserted into a laminated outer battery composed of a lithium ion secondary battery and a laminated body of the laminated outer battery in a laminated surface between adjacent laminated outer batteries. Pressure sensor. Then, it is determined whether or not an overvoltage has occurred in the laminated exterior battery by measuring the surface pressure (contact pressure) acting on the lamination surface and determining whether or not the detected contact pressure exceeds a threshold value.

  Patent Literature 8 discloses a solid-state battery, a storage case for storing the solid-state battery, and a load provided on the storage case for the purpose of being able to accurately grasp the state of charge in a battery unit including the solid-state battery. A battery unit including a sensor and a holding member for holding the storage case and the load sensor is disclosed.

  Further, in Patent Document 9, the relationship between the potential and the capacity of the positive electrode at the time of discharge is such that lithium nickel oxide and lithium iron phosphate reversibly occlude lithium, and these are combined. There is described a lithium ion secondary battery in which a discharge end voltage can be set so that a negative electrode potential at the end of discharge becomes a potential that does not cause deterioration of silicon oxide from a voltage.

JP 2008-300488 A JP-A-11-310405 JP 2012-049001 A JP 2006-107812 A JP 2005-285647 A JP-A-2006-12761 JP 2006-269345 A JP 2010-73544 A JP 2013-65553 A

  However, in the conventional lithium ion secondary battery, when graphite is used as the negative electrode active material, the difference between the oxidation-reduction potential of lithium and the oxidation-reduction potential of the graphite-lithium intercalation compound is small, so that lithium dendrite easily precipitates. However, it was difficult to charge at a high rate. Also, in many lithium ion secondary batteries, the charge / discharge state was grasped by changes in physical properties such as pressure and temperature of the battery, so that there was a problem that the device configuration of the lithium ion secondary battery became complicated. . Further, in the lithium secondary battery described in Patent Literature 6, although the potential of the negative electrode can be predicted from the relationship between the discharge potential at the positive electrode and the battery capacity, the state of the negative electrode cannot be directly grasped.

  Therefore, a first object of the present invention is to provide a lithium ion secondary battery capable of suppressing dendrite precipitation and realizing high charging characteristics regardless of the negative electrode active material. A second object of the present invention is to provide a lithium ion secondary battery capable of directly grasping the charge / discharge state of a negative electrode without being based on information from a pressure sensor or the like, while suppressing the precipitation of dendrite and realizing high charging characteristics. The purpose is to provide.

  In order to achieve the above object, the present invention provides a lithium ion secondary battery including a positive electrode layer, a negative electrode layer, and a solid electrolyte layer sandwiched between the positive electrode layer and the negative electrode layer, wherein the negative electrode layer At least a portion of the surface of the negative electrode active material is coated with a metal or a compound of the metal that forms a lithium compound by an electrochemical reaction with lithium ions, and the reaction potential of the electrochemical reaction is the lithium ion of the negative electrode active material. A lithium ion secondary battery, which is more noble than the reaction potential in the reaction with

  According to a second aspect of the present invention, there is provided a discharge control method for a lithium ion secondary battery, comprising the steps of: monitoring a discharge curve of a negative electrode; And a step of terminating the discharge based on the detection of the inflection point.

  According to a third aspect of the present invention, there is provided a discharge control method for a lithium ion secondary battery, comprising the steps of: monitoring a discharge curve of a negative electrode; And a step of determining deterioration of the negative electrode based on the detected inflection point. 4. A method for determining battery characteristics of a lithium ion secondary battery, comprising the steps of:

According to the present invention, a metal capable of forming a lithium compound at a noble potential than the oxidation-reduction potential of the negative electrode active material and lithium is coated, coated, or attached to the surface of the negative electrode active material, whereby the metal is formed. By absorbing lithium into the negative electrode active material via the lithium compound, dendrite deposition can be suppressed and high chargeability can be realized. If a metal capable of forming a lithium compound at a potential more noble than the compound of the negative electrode active material and lithium is, for example, bismuth and the negative electrode active material is graphite, the occlusion process of lithium into the negative electrode involves the reaction between bismuth and lithium. A solid solution alloy or BiLi, BiLi ₃ intermetallic compound is once generated, through which lithium is absorbed into graphite.

  At this time, bismuth forms a lithium compound at a potential that is more noble than the compound of the negative electrode active material and lithium, so even if the oxidation-reduction potential of lithium and graphite is similar, it suppresses dendrite precipitation. High chargeability can be exhibited.

  Thus, in the present invention, the “electrochemical reaction with lithium ions” is, for example, an intercalation reaction of lithium ions with graphite. Further, according to the present invention, by coating a soft metal such as bismuth on the graphite surface, the adhesion between the negative electrode active materials is improved by a consolidation step such as a roll press, which leads to a decrease in interface resistance and a reduction in cycle characteristics. Can also be improved.

  The discharge characteristics of graphite are discharge at a voltage that is almost flat from the beginning of discharge to the end of discharge at a nearly flat voltage, and the voltage drops sharply at the end of discharge. Incidentally, in the case of hard carbon, the voltage uniformly drops to the discharge end voltage. Hard carbon can directly and accurately determine the battery capacity by measuring the voltage, but has the disadvantage that the battery voltage is not stable. Since the voltage change is small in graphite, it is difficult to know the capacity of the battery from the battery voltage, but it is possible to maintain a relatively stable high voltage until the end of discharge. However, in the case of graphite, since the voltage drops sharply at the end of discharge, it is difficult to measure the battery capacity from the voltage in the first place, and there is a possibility that the battery will be in an overdischarged state. Conventionally, the lower limit voltage of the negative electrode has been predicted from the voltage change of the positive electrode, but the state of the negative electrode cannot be directly detected.

  According to the above-described electrochemical reaction of the present invention, lithium starts to be released from the bismuth-lithium compound immediately before overdischarge, so that in the process of sharp voltage drop at the end of discharge, the timing at which the degree of voltage drop weakens is reduced. Occurs. In the discharge curve, this timing appears as an inflection point, so measure the change in the battery voltage to detect the inflection point and stop the subsequent discharge so that the overdischarge state does not occur. Can be. Furthermore, since the position of the inflection point also changes due to the cycle deterioration of the negative electrode, the life of the negative electrode of the lithium ion secondary battery can be evaluated and determined based on the detected inflection point.

  ADVANTAGE OF THE INVENTION According to the lithium ion secondary battery of this invention, regardless of the negative electrode active material, it can suppress precipitation of dendrite and can provide the lithium secondary battery which can implement | achieve a high charge characteristic. Furthermore, it is possible to provide a lithium ion secondary battery capable of directly grasping the state of the negative electrode without relying on information from a pressure sensor or the like while realizing high charging characteristics by suppressing precipitation of dendrite.

1 is a sectional view of a lithium ion secondary battery according to an embodiment of the present invention. It is an electron micrograph of a negative electrode active material. 5 is a charging curve of the lithium ion secondary battery (Example 1) according to the embodiment of the present invention. 4 is a discharge curve of the lithium ion secondary battery (Example 1) according to the embodiment of the present invention. 5 is a discharge curve of the lithium ion secondary battery (Example 2) according to the embodiment of the present invention.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. In this specification and the drawings, components having substantially the same function and configuration are denoted by the same reference numerals, and redundant description is omitted.

  The configuration of the solid state battery will be described based on FIG. The solid-state battery includes a positive electrode layer 1, an electrolyte layer 2, and a negative electrode layer 3.

<Positive electrode layer>
The positive electrode layer 1 is composed of a sulfide-based solid electrolyte, a positive electrode active material, and a positive electrode layer conductive material. The surface of the positive electrode active material is coated with a coating layer made of, for example, Li ₂ O—ZrO ₂ . The all-solid-state lithium ion secondary battery using the sulfide-based solid electrolyte has a problem that the interface resistance increases due to a reaction at the interface between the positive electrode active material and the solid electrolyte, and the output of the battery decreases. However, since the surface of the positive electrode active material is coated with the coating layer made of Li ₂ O—ZrO ₂ , the coating layer prevents direct contact between the solid electrolyte particles included in the solid electrolyte layer and the positive electrode active material. Therefore, it is difficult to generate a resistance component at the interface between the positive electrode active material and the solid electrolyte. In addition, when the surface of the positive electrode active material is coated with Li ₂ O—ZrO ₂ , a decrease in the lithium-in concentration at the interface between the positive electrode active material and the solid electrolyte is suppressed, and further, lithium ions can move. Since a path can be formed, it is also possible to suppress an increase in resistance at the interface between the positive electrode active material and the solid electrolyte. Therefore, the rate characteristics and the cycle characteristics are excellent.

Since Li ₂ O—ZrO ₂ shown here is chemically stable, if the surface of the positive electrode active material is covered with Li ₂ O—ZrO ₂ , the positive electrode active material and the solid electrolyte may come into direct contact. Therefore, the reaction at the interface between the positive electrode active material and the solid electrolyte can be suppressed, and the generation of a resistance component can be suppressed. Note that the positive electrode active material only needs to have at least a part of its surface covered with a coating layer, and when the entire surface of the positive electrode active material is covered with the coating layer, the surface of the positive electrode active material is partially covered. May be coated with a layer.

“Coating” in the present embodiment means that the state where Li ₂ O—ZrO ₂ is arranged on the surface of the particles of the positive electrode active material in a form in which Li ₂ O—ZrO ₂ does not flow is maintained. Further, the coating layer covering the particle surface of the positive electrode active material has lithium ion conductivity and can maintain a layered form that does not flow even when it comes into contact with the positive electrode active material or the solid electrolyte.

  The positive electrode active material contained in the positive electrode layer 1 is not particularly limited as long as it can reversibly occlude and release lithium ions. For example, lithium cobaltate, lithium nickelate, nickel lithium cobaltate, nickel nickel Examples include lithium cobalt aluminum oxide, lithium nickel cobalt manganate, lithium manganate, lithium iron phosphate, nickel sulfide, copper sulfide, sulfur, iron oxide, and vanadium oxide. These positive electrode active materials may be used alone or in combination of two or more.

The positive electrode active material is preferably a lithium transition metal oxide having a layered rock-salt structure among the examples of the positive electrode active material described above. As the lithium transition metal oxide having a layered rock-salt structure, _{such, Li 1-x-y-} z Ni x Co y Al z O 2 or _{Li 1-x-y-z} Ni x Co y Mn z O 2 ( There is a ternary system represented by 0 <x <1, 0 <y <1, 0 <z <1, and x + y + z <1).

  Examples of the conductive material for the positive electrode layer include graphite, carbon black, acetylene black, Ketjen black, carbon fiber, and metal powder.

<Electrolyte layer>
The electrolyte layer 2 is composed of a sulfide-based solid electrolyte. The sulfide-based solid electrolyte is a sulfide solid electrolyte including at least lithium (Li) as a first component and phosphorus (P) and sulfur (S) as a second component. Such a sulfide-based solid electrolyte is obtained, for example, by heating Li ₂ S and P ₂ S ₅ to a melting temperature or higher, melting and mixing them at a predetermined ratio, holding the mixture for a predetermined time, and then rapidly cooling. (Melt quenching method). Alternatively, Li ₂ SP ₂ S ₅ can be obtained by a mechanical treatment such as a mechanical milling method.

The ion conductivity can be improved by making the obtained amorphous body crystalline by heat treatment. When the solid electrolyte is a sulfide-based solid electrolyte made of Li ₂ SP ₂ S ₅ , the lithium ion conductivity of the amorphous body is 10 ⁻⁴ Scm ⁻¹ . On the other hand, the lithium ion conductivity of the crystalline material is 10 ⁻³ Scm ⁻¹ .

  As the inorganic solid electrolyte, a known inorganic solid electrolyte that can be used for a lithium ion secondary battery can be appropriately used. Examples of such an inorganic solid electrolyte include, besides a sulfide-based inorganic solid electrolyte, an oxide-based inorganic solid electrolyte, a phosphate-based inorganic solid electrolyte, and the like.

<Negative electrode layer>
The negative electrode layer 3 includes graphite as a sulfide-based solid electrolyte and a negative electrode active material, for example, artificial graphite, natural graphite, a mixture of artificial graphite and natural graphite, and natural graphite coated with artificial graphite. Alternatively, amorphous carbon, such as hard carbon, may be used. The negative electrode active material is not particularly limited as long as it can store and release lithium, and may be graphite, tin, a silicon material, or a composite material of a tin-silicon material and a carbon material.

  The element coated on the surface of the negative electrode active material may be any metal capable of forming a lithium compound at a nobleer potential than a compound of the negative electrode active material and lithium, and is preferably Al (aluminum) or Si (silicon). ), Ti (titanium), Zr (zirconium), Nb (niobium), Ge (germanium), Ga (gallium), Ag (silver), In (indium), Sn (tin), Sb (antimony), Bi (bismuth) At least one metal selected from the group consisting of: an alloy of two or more metals; a compound of one or more elements and lithium obtained from the group; or one or more compounds obtained from the group It is a metal oxide. As the metal, bismuth and antimony are preferable.

  As the negative electrode active material, graphite dried in vacuum at 80 ° C. for 24 hours is prepared. Bismuth, which is one of the elements forming a lithium compound at a potential higher than the oxidation-reduction potential of lithium than a graphite-lithium intercalation compound, is coated on the graphite surface. Bismuth is soft and can be firmly adhered to the graphite surface by dry particle compounding (collision of bismuth particles on the graphite surface). In addition, there is an advantage that the electrode is easily deformed when the electrodes are compacted in the final pressurizing step of battery production. Fine particles of bismuth (particle diameter 0.7 μm) as a coating element were uniformly fixed on the surface of graphite (average particle diameter 10 μm) at a ratio of 1 wt% with respect to graphite by a dry particle compounding method. Dry particle compounding is a method of rotating a specially shaped rotor at a high speed of 40 m / s or more in a horizontal cylindrical mixing vessel to apply the impact, compression, and shear forces uniformly to individual particles. This is a method of immobilizing fine particles on mother particles. Bismuth coating of graphite was performed using a commercially available dry particle composite apparatus. As another example of the metal coating, a method such as plating, vapor deposition, or sputtering may be used. The coating is preferably formed thinly over the entire surface of the negative electrode active material so that the reaction proceeds promptly and uniformly. On the other hand, the coating amount is preferably small (for example, 0.1 to 5 wt%) from the viewpoint of the weight energy density. In addition, it is important that the interface between the negative electrode active material and the coating layer comes into more firm contact, so that the coating metal does not easily come off the graphite surface. For this reason, it is preferable to perform a long-time treatment at a high-speed rotation in a dry-type compounding device.

<Production method of lithium ion secondary battery>
Although the configuration of the all-solid-state lithium-ion secondary battery according to the preferred embodiment of the present invention has been described above, a method of manufacturing a lithium-ion secondary battery having the configuration will be described. The lithium ion secondary battery is obtained by forming the positive electrode layer 1, the negative electrode layer 3, and the solid electrolyte layer 2, and then laminating these layers. Hereinafter, each step will be described in detail.

<Preparation of positive electrode layer 1>
The method for producing the positive electrode layer 1 is as follows. For example, a mixture of the positive electrode active material and the sulfide-based solid electrolyte, the surface of which is coated with aLi ₂ O—ZrO ₂ (0.1 ≦ a ≦ 2.0), a conductive additive, and a binder such as an organic solvent are used. The slurry or paste was added to a solvent to form a slurry or paste, and the obtained slurry or paste was applied to a current collector using a doctor blade or the like, dried, and then compacted with a rolling roll or the like to form the positive electrode layer 1. Obtainable. Examples of the current collector that can be used at this time include SUS, aluminum, nickel, iron, titanium, and carbon, and among them, aluminum is preferable. The powder mixture of the positive electrode active material, the surface of which was coated with aLi ₂ O—ZrO ₂ , the sulfide-based solid electrolyte, and the conductive additive was compacted into pellets without using the current collector and the binder. To form a positive electrode layer.

<Preparation of negative electrode layer 3>
The method for producing the negative electrode layer 3 is as follows. For example, the negative electrode active material and sulfide-based solid electrolyte coated with bismuth, a mixture of a conductive additive and a binder are added to a solvent such as an organic solvent to form a slurry or paste, and the obtained slurry or paste is formed. The negative electrode layer 3 can be obtained by applying the powder to the current collector using a doctor blade or the like, drying it, and then compacting it with a rolling roll or the like. The current collector that can be used at this time includes, for example, SUS, copper, nickel, and carbon. Among them, nickel is preferable. Note that, without using the current collector and the binder, the powder mixture of the negative electrode active material and the sulfide-based solid electrolyte, the surface of which was coated with bismuth, and a powder mixture of a conductive auxiliary were compacted into a pellet shape to form a negative electrode layer. Good.

<Preparation of electrolyte layer 2>
The method for producing the solid electrolyte layer 2 is as follows. As a method for producing a sulfide-based solid electrolyte used as a solid electrolyte, there are the above-mentioned melt quenching method and mechanical milling method. In the case of the melt quenching method, a mixture of Li ₂ S and P ₂ S ₅ in a predetermined amount and formed into a pellet is reacted at a predetermined reaction temperature in a vacuum, and then rapidly cooled to obtain a sulfide-based material. A solid electrolyte can be obtained. The reaction temperature at this time is preferably from 400C to 1000C, more preferably from 800C to 900C. Further, the reaction time is preferably 0.1 hour to 12 hours, more preferably 1 hour to 12 hours. Further, the quenching temperature of the reactant is usually 10 ° C. or lower, preferably 0 ° C. or lower, and the cooling rate is usually about 1 to 10,000 K / sec, preferably 1 to 1000 K / sec.

In the case of using the mechanical milling method, a sulfide-based solid electrolyte can be obtained by mixing a predetermined amount of Li ₂ S and P ₂ S ₅ and reacting the mixture for a predetermined time by the mechanical milling method. The mechanical milling method using the above raw materials has an advantage that the reaction can be performed at room temperature. According to the mechanical milling method, since a solid electrolyte can be produced at room temperature, thermal decomposition of a raw material does not occur, and a solid electrolyte having a charged composition can be obtained. The rotation speed and rotation time of the mechanical milling method are not particularly limited, but the higher the rotation speed, the higher the solid electrolyte generation speed, and the longer the rotation time, the higher the conversion rate of the raw material to the solid electrolyte. Thereafter, the obtained solid electrolyte is heat-treated at a predetermined temperature, and then pulverized to obtain a particulate solid electrolyte.

  The solid electrolyte layer 3 can be produced by forming a film of the particulate solid electrolyte thus obtained by using a known film forming method such as an aerosol deposition method, a sputtering method, and a thermal spraying method. Alternatively, a method in which a solution in which a solid electrolyte is mixed with a solvent or a binder is applied, and then the solvent is removed to form a film may be used. Alternatively, by pressing the solid electrolyte itself or an electrolyte in which the solid electrolyte is mixed with a binder or a support (a material or a compound for reinforcing the strength of the solid electrolyte layer 3 or preventing a short circuit of the solid electrolyte itself). It can also be formed into a film.

<Lamination and joining of each layer>
The positive electrode layer 1, the solid electrolyte layer 2, and the negative electrode layer 3 obtained as described above are stacked via the solid electrolyte layer 2 and pressed to manufacture the lithium ion secondary battery according to the present embodiment. be able to. Further, by heating at the time of pressing, the adhesion within each layer and the interface between the layers can be further strengthened.

[Battery evaluation]
(Example 1)
The positive electrode active material was prepared according to the following procedure. Lithium methoxide and zirconium (IV) propoxide were mixed in a mixed solution of ethanol, ethyl acetoacetate and water for 30 minutes. Then, to this mixed solution, the _{Li 1-x-y-z} Ni x Co y Al z O 2 as the positive electrode active _material, and the coating amount is 0.5 mol% of _{aLi 2 O-ZrO 2 (a} = 1) The mixture was heated to 40 ° C., and the solvent was evaporated to dryness while stirring. At this time, ultrasonic waves were applied to the mixed solution. Further, the precursor of Li ₂ O—ZrO ₂ supported on the surface of the positive electrode active material is calcined at 300 ° C. for 2 hours while blowing oxygen, and the surface is coated with 0.5 mol% of Li ₂ O—ZrO _2. Li _1-x-y-z Ni _x Co _y Al _z O ₂ was obtained.

The negative electrode active material was prepared according to the following procedure. Graphite dried under vacuum at 80 ° C. for 24 hours was prepared. Bismuth, which is one of the elements forming a lithium compound at a potential higher than the oxidation-reduction potential of lithium than the graphite-lithium intercalation compound, was coated on the graphite surface. For coating, fine particles of bismuth (particle diameter: 0.7 μm) as a coating element were uniformly fixed on the surface of graphite (average particle diameter: 10 μm) at a ratio of 1 wt% to graphite by a dry particle composite method. The processing conditions in the dry compounding device were 5000 rpm for 30 minutes. FIG. 2A is an electron microscope photograph (magnification: 10,000 times) showing the surface morphology of graphite coated with bismuth, and FIG. 2B is an electron microscope showing the surface morphology of graphite not coated with bismuth. It is a photograph (magnification 10,000 times). On the other hand, a material prepared by subjecting a solid electrolyte Li ₂ SP ₂ S ₅ (80 to 20 mol%) to mechanical milling was prepared.

A mixture of the surface-coated positive electrode and negative electrode obtained as described above, a solid electrolyte, and a vapor-grown carbon fiber (VGCF) as a conductive agent at a ratio of 60/35/5% by mass was mixed with the positive electrode, respectively. As an agent and a negative electrode mixture, 15 mg was placed via 70 mg of a solid electrolyte, and in that state, a pressure was applied at a pressure of 3 t / cm ² to produce a pellet, which was used as a test cell.

The test cell was prepared as described above, in a thermostat at 25 ° C., by Toyo System SeiTakashi discharge evaluation apparatus TOSCAT-3100 the current density at 0.05mA ^/ cm ² ~0.20mA ^/ cm 2 The charge was evaluated by changing the charge. As a comparative example, charge / discharge evaluation was performed on a solid-state battery using a negative electrode active material not coated with bismuth. FIG. 3 shows a charging curve. As can be seen from FIG. 3, in the solid state battery in which the negative electrode active material was covered with bismuth, the chargeability was significantly improved since the battery was charged to a high value capacity.

(Example 2)
The prepared battery was charged and discharged at a current density of 0.05 mA / cm ² in a constant temperature bath at 25 ° C., and the initial battery capacity was confirmed. Next, the battery after 100 cycles was discharged at a current density of 0.05 mA / cm ² under capacity regulation. FIG. 4 shows a discharge curve of the negative electrode. In a battery in which the negative electrode active material is coated with bismuth or the like, an inflection point appears in the middle of the discharge curve in order to form a lithium compound at a potential more noble than the oxidation-reduction potential of the negative electrode active material and lithium, No inflection point appeared in a battery in which the negative electrode active material was not coated with bismuth. In the former solid battery, since an inflection point appears, overdischarging can be suppressed by stopping the discharge at the time when the inflection point occurs. On the other hand, in the latter solid battery, since no inflection point appears, it is difficult to stop discharging immediately before the lower limit voltage. In a lithium ion secondary battery, since the lower limit voltage fluctuates due to cycle deterioration, it is difficult to stop discharging immediately before the lower limit voltage. When the negative electrode active material is covered with bismuth or the like, the inflection point moves in the same manner as the correlation curve between the number of cycles and the capacity by repeating the charge / discharge cycle. Therefore, the deterioration of the negative electrode can be determined based on the inflection point. As described above, by covering the negative electrode active material with bismuth or the like, overdischarge can be suppressed, and furthermore, deterioration of the negative electrode can be determined. Therefore, bismuth or the like is a marker for various management of lithium ion secondary batteries. It may be classified as an element.

  Furthermore, by coating the negative electrode active material with bismuth or the like, the remaining battery level can be accurately grasped. Conventionally, on the premise that the battery does not deteriorate, the operation lower limit voltage for stopping the device is determined to be, for example, 3 V, and a battery remaining amount table is performed. That is, the cut voltage is set to be somewhat higher in consideration of a voltage error of a circuit included in the device or other hardware. As a result, when the discharge voltage drops due to deterioration of the battery and an increase in the internal impedance, the device is stopped even though there is enough remaining battery power, so that unused capacity is generated. By coating the negative electrode active material with bismuth, etc., the remaining battery capacity can be accurately grasped from the inflection point even when the battery capacity is degraded. Can be used.

(Example 3)
Graphite (negative electrode) was prepared in which the amount of bismuth coating was changed according to the above-described steps. The coating amount was 1, 3, 10 wt% without coating. FIG. 5 is an initial discharge curve (lithium insertion into graphite) using a lithium-indium alloy as a counter electrode. The battery was charged at a current density of 0.03 mA / cm ² in a thermostat at 25 ° C. The relationship between the initial battery capacity and the voltage according to the bismuth coverage was confirmed as shown in FIG. The relationship between the amount of bismuth added and the amount of charge was clarified by daringly omitting the solid electrolyte and evaluating the battery under conditions that are strict for the negative electrode to accept lithium. According to FIG. 5, it can be seen that the larger the amount of bismuth coating, the higher the battery capacity.

  As described above, the preferred embodiments of the present invention have been described in detail with reference to the accompanying drawings, but the present invention is not limited to such examples. It is apparent that those skilled in the art to which the present invention pertains can conceive various changes or modifications within the scope of the technical idea described in the claims. Of course, these also belong to the technical scope of the present invention.

1 positive electrode layer 2 electrolyte tank 3 negative electrode layer

Claims (2)

A positive electrode layer;
A negative electrode layer;
A solid electrolyte layer sandwiched between the positive electrode layer and the negative electrode layer,
A lithium ion secondary battery comprising:
At least a part of the surface of each of the negative electrode active material particles of the negative electrode layer is coated with a metal that forms a lithium compound by an electrochemical reaction with lithium ions, and the reaction potential of the electrochemical reaction is determined by the lithium ion of the negative electrode active material. Is more noble than the reaction potential in the reaction with
The negative active material is graphite, Ri intercalation reaction der of the the negative electrode active reaction with the lithium ions of the lithium-ion substance graphite,
A lithium ion secondary battery, wherein the metal is bismuth .   The lithium ion secondary battery according to claim 1, wherein the solid electrolyte layer is made of a sulfide solid electrolyte containing Li (lithium), P (phosphorus), and S (sulfur).