JP2014220139A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve input/output characteristics of a nonaqueous electrolyte secondary battery.SOLUTION: A nonaqueous electrolyte secondary battery of the present invention includes a positive electrode, a negative electrode having a negative electrode mixture layer 25, and a nonaqueous electrolyte. The negative electrode mixture layer 25 includes: an electrode layer 20 in contact with a negative electrode collector 10 and containing a negative electrode active material 21; and a coat layer 30 in contact with the surface of the electrode layer 20, the surface being on a side of the nonaqueous electrolyte. The coat layer 30 contains lithium titanium oxide (LTO particle 32). The positive electrode has a positive electrode active material containing tungsten. The electrode layer 20 dose not contain the LTO particle 32, or has a content of lithium titanium oxide less than the coat layer.

Description

  The present invention relates to a non-aqueous electrolyte secondary battery.

  One of the nonaqueous electrolyte secondary batteries is a lithium ion secondary battery. A lithium ion secondary battery is a secondary battery that can be charged and discharged by moving lithium ions in a non-aqueous electrolyte between a positive electrode and a negative electrode that occlude and release lithium ions.

  Patent Document 1 discloses a lithium ion secondary battery having tungsten as a positive electrode active material. The positive electrode active material contains an additive composed of tungsten trioxide. Tungsten trioxide acts as an additive that prevents aggregation of primary particles of the positive electrode active material.

JP 2012-074334 A

  The lithium ion secondary battery described in Patent Document 1 has excellent output characteristics and high duty cycle characteristics. However, tungsten is eluted from the positive electrode surface by repeated use of the battery. Tungsten in the positive electrode elutes into the non-aqueous electrolyte as ions. Tungsten may be reduced and deposited on the negative electrode active material in the negative electrode mixture layer to increase the resistance of the negative electrode.

  An object of the present invention is to improve input / output characteristics of a non-aqueous electrolyte secondary battery. An object of the present invention is to further improve the input / output characteristics of a non-aqueous electrolyte secondary battery having tungsten as a positive electrode active material.

  The non-aqueous electrolyte secondary battery of the present invention includes a positive electrode, a negative electrode having a negative electrode mixture layer, and a non-aqueous electrolyte, the positive electrode has a positive electrode active material containing tungsten, and the negative electrode mixture layer includes: An electrode layer formed on a negative electrode current collector and containing a negative electrode active material; and a coat layer formed on a surface of the electrode layer opposite to the negative electrode current collector, wherein the coat layer is lithium titanium Contains oxides.

  It is preferable that the electrode layer does not contain lithium titanium oxide, or the content of lithium titanium oxide is smaller than that of the coating layer. The coat layer preferably further contains a conductive carbon material.

  The conductive carbon material preferably has a total weight percentage content with respect to the coat layer larger than that of the lithium titanium oxide contained in the coat layer. The coating layer preferably contains 0.1 to 5.0 parts by weight of the lithium titanium oxide with respect to 100 parts by weight of the conductive carbon material.

  The conductive carbon material is preferably graphite. The coat layer is preferably located in the outermost layer of the negative electrode mixture layer, and particularly preferably in contact with the non-aqueous electrolyte. The negative electrode active material of the electrode layer preferably contains graphite.

  The non-aqueous electrolyte secondary battery is preferably formed by performing initial charging with a current of preferably 0.33 C or less, particularly preferably 0.02 C or less. The non-aqueous electrolyte secondary battery is preferably applied with a current pulse after the initial charging and before discharging.

  The positive electrode preferably has a positive electrode active material to which tungstate is added. The tungstate is preferably sodium tungstate. The amount of sodium tungstate added is preferably 0.5 mol% or less with respect to the positive electrode active material.

  The present invention improves the input / output characteristics of a lithium ion secondary battery having tungsten as the positive electrode active material.

It is sectional drawing of the negative electrode of the lithium ion secondary battery concerning embodiment. It is sectional drawing of the negative electrode of the conventional lithium ion secondary battery.

<Structure>
A nonaqueous electrolyte secondary battery (hereinafter also referred to as a battery) according to an embodiment of the present invention is a lithium ion secondary battery including a positive electrode containing tungsten, a negative electrode, and a nonaqueous electrolyte. Since the positive electrode containing tungsten has a small electric resistance, a battery including such a positive electrode has a high output.

  As shown in FIG. 1, the negative electrode 50 of the battery of this embodiment includes a negative electrode mixture layer 25 and a current collector 10. The negative electrode mixture layer 25 has an electrode layer 20 and a coat layer 30. The electrode layer 20 is in contact with the negative electrode current collector and contains a negative electrode active material. The coat layer 30 coats the surface of the negative electrode mixture layer on the non-aqueous electrolyte side. That is, the coat layer 30 is in contact with the surface of the electrode layer 20 opposite to the surface in contact with the current collector 10.

The coat layer 30 has a lithium titanium oxide (also known as lithium titanate, hereinafter sometimes referred to as LTO) represented by the chemical formula Li ₄ Ti ₅ O ₁₂ . The lithium titanium oxide is dispersed in the coating layer 30 as LTO particles 32 containing the lithium titanium oxide. The LTO particles 32 may be composed only of lithium titanium oxide.

  Through repeated use of the battery, tungsten is eluted from the positive electrode surface. Tungsten elutes into the non-aqueous electrolyte, for example, as tungstate ions. As shown in FIG. 1, the eluted tungsten 40 may be reduced and deposited on the negative electrode active material in the negative electrode mixture layer to increase the resistance of the negative electrode.

  Since the coat layer 30 is located on the non-aqueous electrolyte side of the negative electrode, the LTO particles 32 efficiently adsorb tungsten and prevent the tungsten 40 from entering the electrode layer. Further, the LTO particles 32 do not significantly increase the negative electrode resistance. For this reason, the battery of this embodiment is excellent in the initial input / output characteristics, and the input / output characteristics are not easily deteriorated even by repeated use.

<Positive electrode active material>
The positive electrode active material is a material capable of inserting and extracting lithium. Such a material is preferably a ternary material containing lithium ions, transition metal ions, and counter anions. Examples of such ternary materials are lithium cobaltate (LiCoO ₂ ), lithium manganate (LiMn ₂ O ₄ ), or lithium nickelate (LiNiO ₂ ).

  Another example of such a ternary material is lithium nickel manganate or nickel cobalt lithium manganate, which is a mixture of the above substances. From the viewpoint of electromotive force, energy density, and charge / discharge efficiency, the mixture is preferably lithium nickel cobalt manganate.

The composition of such a mixture may be LiNi _1/3 Co _1/3 Mn _1/3 O ₂ . The ratio of Ni: Co: Mn can be selected as appropriate. In such a case, particularly excellent electromotive force, energy density, and charge / discharge efficiency can be obtained.

<Formation of positive electrode active material particles>
The active material particles are preferably formed by spray drying or crystallization. In the crystallization method, the transition metal sulfate hydrate is used as a starting material, and the reaction layer is neutralized to produce nuclei and grow particles. Thereafter, the resultant is filtered, and the obtained powder is dried to obtain particles of NiCoMn hydroxide.

  In the spray drying method, equivalent particles can be obtained by adjusting the solution concentration and temperature. Both methods have the common point of co-precipitation in the same solution. From the viewpoint of production efficiency, the crystallization method is preferable.

  The obtained particles are preferably mixed with lithium carbonate and fired at 800 to 1000 ° C. The firing temperature may be 800-900 ° C or 900-1000 ° C. After firing, it is finely crushed and sieved to obtain an oxide with a desired particle size.

<Additive to positive electrode active material>
The positive electrode active material preferably contains an additive. The additive preferably includes tungsten. The additive is preferably tungsten trioxide (WO ₃ ) or tungstate. The tungstate is preferably sodium tungstate (Na ₂ WO ₄ ) from the viewpoint that it is easily dispersed when dissolved and heat-treated.

  When sodium tungstate is added to the positive electrode active material, the addition amount is preferably less than 1.1 mol%, particularly preferably 0.5 mol% or less, based on the positive electrode active material. Such an added amount of sodium tungstate suppresses an increase in electric resistance and a decrease in battery capacity after repeated use.

  A positive electrode active material containing tungsten has a low electric resistance. For this reason, in this embodiment, the battery has a high output and the loss of power during charging is small. Further, the positive electrode active material may further contain 0.5% by weight of Zr as an additive.

<Physical properties of positive electrode active material>
The particle size of the positive electrode active material is preferably adjusted to 1 to 10 μm depending on the temperature during crystallization. The density of the particles is preferably 2.0 to 2.2 g / cm ³ . The N ₂ BET specific surface area of the particles is preferably 0.3 to 2.0 m ² / g.

<Conductive agent>
Examples of the conductive material are carbon black such as acetylene black (AB) and ketjen black (registered trademark), and graphite (graphite).

<Binder>
Examples of the binder are polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), polytetrafluoroethylene (PTFE), carboxymethyl cellulose (CMC) and the like.

<Solvent for positive electrode>
When the positive electrode binder is PVDF, the positive electrode solvent is preferably an organic solvent. As the organic solvent, an aprotic polar solvent is preferable, and NMP (N-methyl-2-pyrrolidone) is particularly preferable. The said solvent can disperse | distribute the said positive electrode material rapidly in a positive mix as a dispersing agent.

<Preparation of positive electrode>
A positive electrode active material, a conductive material, and a binder can be simultaneously added to a solvent and kneaded to prepare a positive electrode mixture. The kneading is preferably performed by adding the above materials to a solvent and then stirring and mixing with a planetary mixer for 2 hours. The shear rate at the time of stirring is preferably 35 rpm.

  A positive electrode of a lithium ion secondary battery can be manufactured by applying the positive electrode mixture thus prepared to a positive electrode current collector and drying it. As the positive electrode current collector, aluminum or an alloy containing aluminum as a main component can be used.

<Negative electrode active material>
The negative electrode of the lithium ion secondary battery has a negative electrode active material. The negative electrode active material is a material capable of inserting and extracting lithium. Since the negative electrode active material has conductivity and has a high reversible capacity, it is a powdery carbon material made of graphite or the like, or amorphous carbon-coated natural graphite in which natural graphite is coated with amorphous carbon Is preferred.

<Negative electrode layer>
A graphite negative electrode is applied to form an electrode layer 20 on the current collector 10. When forming the electrode layer 20, it is preferable to knead the negative electrode active material, the solvent, the binder, and the thickener, and apply the dried negative electrode mixture to the negative electrode current collector and dry it. The electrode layer preferably does not contain LTO or has a lower LTO content than the coating layer 30 in order to prevent a decrease in negative electrode capacity per unit volume.

  The binder is not particularly limited. When SBR is used as the binder, water is preferable as the solvent. The thickener is not particularly limited. An example of a thickener is CMC. The binder and thickener are dispersed and located in the gap 23 between the negative electrode active materials 21. As the negative electrode current collector, for example, copper, nickel, or an alloy thereof is preferable.

<Negative coat layer>
From the viewpoint of efficiently adsorbing tungsten and preventing tungsten 40 from entering the electrode layer, the coat layer 30 is preferably located in the outermost layer of the negative electrode mixture layer 25 as shown in FIG. It is particularly preferable to contact the non-aqueous electrolyte.

  The coating layer is preferably a single layer so as not to inhibit the diffusion of lithium ions. For the same reason, the coat layer preferably contains a conductive carbon material such as graphite, amorphous carbon, or carbon nanotube. The amorphous carbon is preferably carbon black such as acetylene black (AB) and ketjen black (registered trademark).

  Moreover, it is preferable that this electroconductive carbon material has a total weight percentage content with respect to a coating layer larger than LTO which a coating layer contains. With this configuration, lithium diffusion in the mixed layer at the interface between the electrode layer and the coat layer is promoted, so that the resistance of the negative electrode is reduced.

  As shown in FIG. 1, the coat layer 30 contains a negative electrode active material 31 containing graphite. For this reason, since the coat layer in contact with the non-aqueous electrolyte also functions as an electrode, the capacity of the battery increases. The binder and thickener are the same as those of the electrode layer. The negative electrode active material, LTO, binder, and thickener are preferably added to water and kneaded. The kneaded negative electrode mixture is preferably applied to a negative electrode current collector and dried.

  LTO is preferably spherical particles having an average particle diameter D50 of 0.5 to 2.0 μm. When the LTO particles are mixed, the ratio relative to 100 parts by weight of graphite is preferably 0.1 to 5.0 parts by weight, and particularly preferably 3 parts by weight or more. As shown in FIG. 1, the binder, the thickener, and the LTO particles 32 are dispersed and located in the gap 33 between the negative electrode active materials 31.

<Conductive agent>
The electrode layer and the coat layer may have a conductive agent. Examples of the conductive material include acetylene black (AB) and carbon nanotube (CNT).

<Non-aqueous electrolyte>
The non-aqueous electrolyte is preferably a composition containing a non-aqueous solvent and a supporting salt. Here, examples of the non-aqueous solvent are selected from the group consisting of a fluorine-based solvent, propylene carbonate (PC), ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), and the like. One or two or more kinds of materials.

<Supporting salt>
Examples of the supporting salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiI, etc. Or one or more lithium compounds (lithium salts) selected from From the viewpoint of improving battery voltage and durability, LiPF ₆ is preferable.

<Separator>
In this embodiment, the lithium ion secondary battery includes a separator. Preferred examples of the separator include porous polymer films such as porous polyethylene film (PE), porous polypropylene film (PP), porous polyolefin film, and porous polyvinyl chloride film, or lithium ion or ion conductive polymer. The electrolyte membrane is used alone or in combination. From the viewpoint of increasing battery output, the separator is preferably a three-layer coated separator in which PE is sandwiched between two upper and lower layers of PP.

<Battery assembly>
Hereinafter, a lithium ion secondary battery provided with a wound electrode body will be described as an example. The member becomes a laminate by a step of laminating a long positive electrode sheet (positive electrode) and a long negative electrode sheet (negative electrode) through a long separator. The laminate is wound into a wound body. Furthermore, a winding body turns into a flat winding body by the process of crushing a winding body from a side surface direction.

  Here, in the positive electrode sheet, both surfaces of the foil-shaped positive electrode current collector hold a positive electrode mixture layer containing a positive electrode active material. As in the positive electrode sheet, both surfaces of the foil-like negative electrode current collector hold a negative electrode mixture layer containing a negative electrode active material in the negative electrode sheet.

  A well-known thing can be utilized as a container of a lithium ion secondary battery. Examples of the battery type include a cylindrical type, a coin type, a square type, and a film type (laminate type). The selection of the battery container should be matched to the desired battery type.

  In the case of a rectangular shape, it is preferable to include a flat rectangular parallelepiped container body having an open upper end and a lid that closes the opening. Examples of the material constituting the container are metal materials such as aluminum and steel.

  Another example of the battery container is a container formed by molding a resin material such as polyphenylene sulfide resin (PPS) or polyimide resin. The upper surface (that is, the lid) of the container has a positive terminal that is electrically connected to the positive electrode of the wound electrode body and a negative electrode terminal that is electrically connected to the negative electrode of the wound electrode body.

  And the part which the positive electrode sheet and negative electrode sheet of the both ends of the winding electrode body exposed becomes a positive electrode lead terminal and a negative electrode lead terminal. Such an exposed part is a part without the positive electrode mixture layer and the negative electrode mixture layer. Such lead terminals are electrically connected to the positive terminal and the negative terminal, respectively.

  The battery container accommodates the wound body. The lid seals the opening of the container body. Thereafter, the non-aqueous electrolyte is injected into the battery container from the liquid injection hole of the lid. The sealing cap closes the liquid injection hole.

<Conditioning process>
The conditioning process is a process of repeating charging and discharging of the lithium ion secondary battery a predetermined number of times. Such processing has conditions of a predetermined charging rate, discharging rate, and set voltage for charging / discharging. Such conditions can be appropriately set according to the battery specifications.

  It is preferable to perform the initial charge with a current of 0.33 C or less. The initial charging may be stopped when the voltage rise reaches a peak. The current for performing the initial charge may be 0.02 C or less. By such initial charging, LTO specifically adsorbs tungsten.

  For this reason, since LTO suppresses the increase in resistance of the negative electrode 50 (depot suppression), the battery has high input / output characteristics in the initial use. Furthermore, the input / output characteristics of the battery are improved by applying a current pulse after the initial charge and before the discharge.

  The battery capacity may be confirmed after conditioning. For example, after charging to 4.1V with a constant current of 1 / 3C, after charging with a constant voltage of 4.1V, discharging to 3.0V with a constant current of 1 / 3C, By discharging at a constant voltage of 3.0 V, the battery capacity can be measured from the sum of the current values.

<Description of effects>
In a battery having tungsten, when tungsten is added to the positive electrode active material, the resistance of the positive electrode is reduced, so that the output of the battery is improved. However, tungsten diffuses from the positive electrode into the electrolyte.

  As shown in FIG. 2, in the comparative battery having no coating layer, tungsten 40 penetrates through the gap 23 into the electrode layer 20 of the negative electrode 60. Tungsten 40 is reduced and deposited on the surface of the negative electrode active material 21 made of graphite.

  At this time, tungsten deposited on the negative electrode increases the deactivation reaction of lithium. The deactivation reaction is a reaction that prevents lithium ions from returning to the non-aqueous electrolyte while being incorporated into the graphite. The deactivation reaction greatly increases the electrical resistance of the negative electrode 60.

  In the present embodiment, as shown in FIG. 1, the coating layer 30 containing the LTO particles 32 coats the surface of the negative electrode mixture layer 25. When the LTO particles 32 receive a weak current, the LTO particles 32 attract metal ions in the vicinity of the negative electrode and reduce them. For this reason, the LTO particles 32 prevent the tungsten 40 diffused in the electrolyte from being deposited on the negative electrode 50.

  In the conventional conditioning method, the current application amount of the first charge is about 1 / 3C. However, in this embodiment, the current application amount of the first charge is smaller than 1 / 3C. In addition, a pulse current that is not in the conventional conditioning method is applied after the initial charge. For this reason, since LTO adsorb | sucks tungsten, deposition suppression works and the resistance increase of a negative electrode is suppressed. Therefore, the high input / output characteristics of the battery are maintained.

<Deformation of Embodiment, etc.>
The battery of this embodiment can be mounted on a power machine such as an electric vehicle (EV) or a plug-in hybrid vehicle (PHV) and used as an operating power source. In addition, this invention is not limited to the said embodiment, It is possible to change suitably in the range which does not deviate from the meaning.

The positive electrode active material may be formed by spray drying after adding tungsten trioxide to a ternary material. For example, Li ₂ CO ₃ , Ni (OH) ₂ , MnO ₂ , CoOOH, B ₂ O ₃ , WO ₃ are mixed, pure water is added to this, and after stirring, wet grinding, spray drying, firing, lithium transition A metal oxide can be obtained.

  In the above embodiment, LTO is dispersed in the coating layer, but LTO may cover the negative electrode active material of the coating layer. For example, LTO may coat graphite in the coating layer. For the coating, a wet coating method or a dry coating method can be used.

  In wet coating, a dispersion or suspension in which LTO is dispersed or a solution of LTO is sprayed onto the surface of the electrode layer. Alternatively, the surface of the electrode layer is impregnated with such a dispersion, suspension, or solution. Thereafter, the electrode layer is dried. In the wet coating, the surface of the negative electrode active material can be uniformly coated with the coating material.

  In dry coating, a negative electrode mixture layer can be used as a core part, and a coating material can be used as a shell part to coat by mechanical means. The coating material may be LTO particles. In dry coating, shearing force, impact force, or compressive force is generated by mechanical means. For this reason, it is possible from simple mixing to coating by mechanical adjustment.

  In the said embodiment, although the coating layer containing LTO is one layer, you may further provide the other coating layer with same or different content of LTO with respect to this. Moreover, in the said embodiment, LTO is contained in the coating layer of the negative electrode, However, LTO may be contained in the separator or the positive electrode in the modification of the embodiment.

  When mixing the LTO particles in the electrode layer, the ratio of LTO to graphite is preferably smaller than the coat layer, and particularly preferably less than 3 parts by weight.

[Example 1]
<Positive electrode active material>
Nickel sulfate, cobalt sulfate, and manganese sulfate were dissolved in water so that the molar ratio of Ni: Co: Mn was appropriately selected and these sulfuric acid hydrates were saturated to prepare a mixed aqueous solution. Furthermore, 0.5 wt% of zirconium sulfate (Zr (SO ₄ ) ₂ ) was added to NiCoMn.

Further, Na ₂ WO ₄ was added to NiCoMn. In Table 1, the added amount of tungsten (mol%) represents the added amount of Na ₂ WO ₄ in the positive electrode active material. The positive electrode active material according to Example 1 was formed by adding 0.5 mol% of Na ₂ WO ₄ .

By supplying the mixed aqueous solution, a NaOH aqueous solution having a predetermined concentration, and aqueous ammonia at a constant rate into the reaction vessel, the reaction solution is controlled to a predetermined pH and NH ₄ ⁺ concentration, while the NiCoMn composite is mixed from the reaction solution Hydroxide nuclei were crystallized (nucleation stage).

  When a predetermined time had elapsed from the start of the supply of the mixed aqueous solution, the supply of the NaOH aqueous solution was stopped. The mixed aqueous solution and aqueous ammonia were continuously supplied at a constant rate. After the pH of the reaction solution decreased to a predetermined value, the supply of the NaOH aqueous solution was resumed.

While controlling the reaction solution to a predetermined pH and a predetermined NH ₄ ⁺ concentration, the operation of supplying the mixed aqueous solution, the NaOH aqueous solution and the ammonia water was continued for 4 hours to grow NiCoMn composite hydroxide particles (particle growth stage) . The product was then removed from the reaction vessel by cake filtration, washed with water and dried. In this way, composite hydroxide particles were obtained.

The composite hydroxide particles were heat-treated at 150 ° C. for 12 hours in an air atmosphere. Next, Li ₂ CO ₃ as the lithium source and the composite hydroxide particles are combined into the number of moles of lithium (M _Li ) and the total number of moles of Ni, Co and Mn constituting the composite hydroxide (M _Me ). And the mixture (M _Li : M _Me ) was 1.15: 1.

  This mixture was fired at 760 ° C. for 4 hours (first firing stage), and then fired at 950 ° C. for 10 hours (second firing stage). Thereafter, the fired product was crushed and sieved. In this way, an active material particle sample having a composition composed of a suitably selected Ni: Co: Mn ratio was obtained.

The size of the particles was adjusted to 7 μm depending on the temperature during crystallization. The density of the particles was 2.1 g / cm ³ . The N ₂ BET specific surface area of the particles was 0.5 m ² / g.

<Preparation of positive electrode>
The positive electrode active material, the conductive material, and the binder were simultaneously added to the solvent and kneaded. Kneading was performed by adding the above materials to the solvent and then stirring and mixing for 2 hours with a planetary mixer. The shear rate during stirring was 35 rpm. The positive electrode mixture thus produced was applied to a positive electrode current collector made of aluminum and dried to obtain a positive electrode.

<Negative electrode layer>
To 100% by weight of natural graphite, 0.7% by weight of SBR, 0.7% by weight of CMC, and water were added and mixed to prepare a negative electrode mixture paste. As shown in FIG. 1, this paste was applied onto a current collector 10 made of a copper foil and further dried to form an electrode layer 20. The coating weight per unit area of the electrode layer 20 was 18 mg / cm ² .

<Negative coat layer>
As shown in FIG. 1, a coat layer 30 was formed on the surface of the electrode plate on the electrode layer 20 side. 3 wt% LTO, 2 wt% SBR, 2 wt% CMC, and water were added to 100 wt% graphite and mixed to prepare a negative electrode mixture paste. The average particle diameter D50 of LTO was 1.0 μm.

  The electrode plate further provided with the coat layer 30 was rolled. The electrode plate including the current collector 10, the electrode layer 20, and the coat layer 30 was cut to a width of 80.9 mm to form a hoop of a negative electrode plate. One side of the hoop in the width direction had a current collector exposed portion with a width of 20 mm.

<Nonaqueous electrolytes, etc.>
LiPF ₆ was dissolved in a non-aqueous solvent as a non-aqueous electrolyte. The non-aqueous solvent of this example is a mixture of EC, DMC, and EMC at a volume ratio of 30/40/30.

<Battery assembly>
The separator was a three-layer coated separator in which PE was sandwiched between two layers of PP. A positive electrode, a separator, a negative electrode, and a separator were laminated in this order and wound to produce a 18650 type battery cell.

<Conditioning>
Conditioning was performed at 60 ° C. At the first charge, the battery was charged slowly to 4.1 V with a weak current of 0.02C (1 / 50C). Next, a pulse current was applied to the battery at 4.1 V while maintaining the charged state of the battery. The pulse current was repeatedly energized for 10 seconds and paused for 20 seconds. The pulse current value was 1 / 10C. The number of pulses was 10.

  The battery was discharged at a constant current of 1/3 C to 3.0 V with a pause of 10 minutes, and further paused for 10 minutes. In addition, after charging to 4.1V at a constant current of 1 / 3C, pause for 10 minutes, then discharge to 3.0V at a constant current of 1 / 3C and then pause for 10 minutes twice. It was.

[Example 2]
A battery was produced in the same manner as in Example 1 except that the amount of current applied during the initial charge was 0.33 C (1/3 C).

[Example 3]
A battery was fabricated in the same manner as in Example 3 except that Na ₂ WO ₄ was not added to the ternary material of the positive electrode active material.

[Example 4]
A battery was fabricated in the same manner as in Example 1 except that the amount of current applied during the initial charge was 1.0 C and 1.1 mol% of Na ₂ WO ₄ was added to the ternary material of the positive electrode active material.
[Verification of effect]

<Durability test>
An endurance test was conducted by simulating the general usage conditions of the battery. The batteries of Examples 1 to 4 after conditioning were charged at 60 ° C. to a constant current of 4.1 V at a charge rate of 2 C, and then a 10-minute rest period was taken.

  Furthermore, it discharged until it became 3.0V with the discharge rate of 2C, and the discharge capacity at this time was made into the initial stage battery capacity. After taking a rest time of 10 minutes, it was charged and discharged again in the same process as above. A total of 500 cycles of charge and discharge were repeated, and the discharge capacity at the 500th cycle was defined as the battery capacity after the test.

<Rate increase rate>
After conditioning, the batteries of Examples 1 to 4 charged to SOC 60% were discharged at a temperature of 25 ° C. for 10 seconds. The discharge current rate was 10 C, and the voltage after discharging was measured. The initial IV resistance was calculated from the current rate and voltage. It measured similarly after the endurance test, and calculated IV resistance after the test. The resistance increase rate (times) was determined using the following formula.

Resistance increase rate (times) = Post-test IV resistance / initial IV resistance

  As shown in Table 1, in Example 1, the rate of increase in resistance was lower than in Examples 2 to 4. From this, it was shown that durability against repeated use of the battery is improved by making the current applied at the time of the initial charge weak.

  Further, in Example 2, the rate of increase in resistance was lower than that in Example 3. From this, it was shown that durability against repeated use of the battery is improved by adding tungsten to the positive electrode active material.

<Capacity maintenance rate>
The capacity retention rate (%) was determined using the following formula.

Capacity maintenance rate (%) = (battery capacity after test / initial battery capacity) × 100

  As shown in Table 1, in Example 1, the capacity retention rate was higher than in Examples 2 to 4. From this, it was shown that durability against repeated use of the battery is improved by making the current applied at the time of the initial charge weak.

  In Example 2, the capacity retention rate was higher than that in Example 3. From this, it was shown that durability against repeated use of the battery is improved by adding tungsten to the positive electrode active material.

DESCRIPTION OF SYMBOLS 10 Current collector 20 Electrode layer 21 Negative electrode active material 25 Negative electrode mixture layer 30 Coat layer 31 Negative electrode active material 32 LTO particle 40 Tungsten 50 Negative electrode

Claims (15)

A positive electrode, a negative electrode having a negative electrode mixture layer, and a non-aqueous electrolyte;
The positive electrode has a positive electrode active material containing tungsten,
The negative electrode mixture layer has an electrode layer formed on a negative electrode current collector and containing a negative electrode active material, and a coat layer formed on the surface of the electrode layer opposite to the negative electrode current collector. ,
The said coating layer is a non-aqueous electrolyte secondary battery containing a lithium titanium oxide.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the electrode layer does not contain lithium titanium oxide, or contains less lithium titanium oxide than the coat layer.   The non-aqueous electrolyte secondary battery according to claim 2, wherein the coat layer further contains a conductive carbon material.   The non-aqueous electrolyte secondary battery according to claim 3, wherein the conductive carbon material has a total weight percentage content with respect to the coat layer larger than that of the lithium titanium oxide contained in the coat layer.   The non-aqueous electrolyte secondary battery according to claim 4, wherein the coat layer contains 0.1 to 5.0 parts by weight of the lithium titanium oxide with respect to 100 parts by weight of the conductive carbon material.   The non-aqueous electrolyte secondary battery according to claim 3, wherein the conductive carbon material is graphite.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the coat layer is located in an outermost layer of the negative electrode mixture layer.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the coat layer is in contact with the non-aqueous electrolyte.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the negative electrode active material of the electrode layer contains graphite.   The nonaqueous electrolyte secondary battery according to any one of claims 1 to 9, wherein the first charge is performed at a current of 0.33 C or less.   The nonaqueous electrolyte secondary battery according to any one of claims 1 to 9, wherein the first charge is performed at a current of 0.02 C or less.   The nonaqueous electrolyte secondary battery according to claim 10 or 11, wherein a current pulse is applied after the initial charge and before discharging.   The non-aqueous electrolyte secondary battery according to claim 1, wherein the positive electrode has a positive electrode active material obtained by adding tungstate.   The non-aqueous electrolyte secondary battery according to claim 13, wherein the tungstate is sodium tungstate.   The nonaqueous electrolyte secondary battery according to claim 14, wherein an addition amount of the sodium tungstate is 0.5 mol% or less with respect to the positive electrode active material.

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