JP2022158172A - Manufacturing method of non-aqueous electrolyte secondary battery - Google Patents

Abstract

To provide means for further improving cycle durability in a non-aqueous electrolyte secondary battery using a positive electrode active material having a layered rock salt structure.SOLUTION: In a manufacturing method of a non-aqueous electrolyte secondary battery including a power generation elements including a positive electrode having a positive electrode active material layer containing a positive electrode active material having a layered rock salt structure, a negative electrode, and an electrolyte layer containing an electrolytic solution, the electrolytic solution contains a non-aqueous solvent and lithium ions having a concentration of 50% or more of the lithium ion concentration corresponding to the saturated concentration of the lithium salt at 25°C, and conditioning is performed in the voltage range in which the power generation element has an upper limit voltage of 4.0 to 4.5 V.SELECTED DRAWING: None

Description

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

2. Description of the Related Art In recent years, various electric vehicles are expected to spread toward solving environmental and energy problems. Development of a secondary battery is earnestly carried out as an in-vehicle power source such as a power source for driving a motor, which holds the key to popularization of these electric vehicles. As a secondary battery, attention is focused on a non-aqueous electrolyte secondary battery that can be expected to have high energy density and high output.

As a positive electrode material for a non-aqueous electrolyte secondary battery, a positive electrode active material having a layered rock salt structure such as a lithium-transition metal composite oxide having a high capacity is used.

On the other hand, for non-aqueous electrolyte secondary batteries, in order to improve the charge-discharge cycle characteristics, it is proposed to perform the initial charge and discharge under predetermined conditions, and to perform aging treatment and conditioning treatment after the initial charge and discharge. It is For example, Patent Document 1 discloses that a battery assembly including a positive electrode, a negative electrode, an electrolytic solution, and a case containing them is subjected to initial charge and discharge, then subjected to aging treatment, and then to a predetermined voltage. It is described that a conditioning treatment of charging and then discharging is performed.

JP 2015-153484 A

However, as a result of investigation by the present inventors, it has been found that even a battery using the technology described in Patent Document 1 may have poor cycle durability under charging/discharging conditions with a high cell voltage. found.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide means for further improving the cycle durability of a non-aqueous electrolyte secondary battery using a positive electrode active material having a layered rock salt structure.

The present inventors have made intensive studies to solve the above problems. In the process, the decrease in cycle durability under high cell voltage charging/discharging conditions causes structural damage to the positive electrode active material under high voltage, elution of transition metals from the surface of the positive electrode active material, and loss of electrolyte on the surface of the positive electrode active material. It was found that decomposition was the cause. Based on this knowledge, the present inventors found that a film derived from a lithium salt, which is an electrolyte, is uniformly formed on the surface of the positive electrode active material by performing conditioning under predetermined conditions. The uniform formation of this coating prevents structural damage to the positive electrode active material, elution of transition metals from the surface of the positive electrode active material, and electrolyte solution on the surface of the positive electrode active material even under high cell voltage charging/discharging conditions. The inventors have found that it is possible to suppress the deterioration of the cycle durability, and have completed the present invention.

That is, one embodiment of the present invention provides a non-aqueous power generating element including a positive electrode including a positive electrode active material layer containing a positive electrode active material having a layered rock salt structure, a negative electrode, and an electrolyte layer including an electrolytic solution. In the method for producing an electrolyte secondary battery, the electrolytic solution contains a non-aqueous solvent and lithium ions at a concentration of 50% or more of the lithium ion concentration corresponding to the saturated concentration of the lithium salt at 25 ° C. (i.e. , a concentrated electrolyte), and conditioning the power generation element in a voltage range in which the upper limit voltage is 4.0 to 4.5V.

Further, one embodiment of the present invention is a positive electrode for a non-aqueous electrolyte secondary battery including a positive electrode active material layer containing a positive electrode active material having a layered rock salt structure and an electrolytic solution, wherein the electrolytic solution is a non-aqueous solvent and lithium ions at a concentration of 50% or more of the lithium ion concentration corresponding to the saturated concentration of the lithium salt at 25 ° C., the surface of the positive electrode active material containing an element derived from the lithium salt, and the thickness Provided is a positive electrode for a non-aqueous electrolyte secondary battery, which has a coating layer with a thickness of 9 to 15 nm and a coverage of 85% or more.

According to the present invention, in a non-aqueous electrolyte secondary battery using a positive electrode active material having a layered rock salt structure, conditioning is performed in a concentrated electrolytic solution in a predetermined voltage range, so that the surface of the positive electrode active material is coated with an electrolyte. A coating layer derived from a certain lithium salt is uniformly formed. As a result, structural damage to the positive electrode active material, elution of transition metals from the surface of the positive electrode active material, and decomposition of the electrolyte on the surface of the positive electrode active material are less likely to proceed even under high-voltage charge/discharge conditions. Therefore, in a non-aqueous electrolyte secondary battery that uses a combination of a positive electrode active material having a layered rock salt structure and a concentrated electrolyte, it is possible to operate stably even at high voltages and further improve cycle durability. be.

FIG. 1 is a schematic cross-sectional view schematically showing the overall structure of a flat (laminated) non-bipolar (internal parallel connection type) secondary battery (laminated secondary battery) that is an embodiment of the present invention. is. FIG. 2 is a cross-sectional view schematically showing a bipolar secondary battery that is another embodiment of the present invention.

One embodiment of the present invention provides a non-aqueous electrolyte having a power generation element including a positive electrode including a positive electrode active material layer containing a positive electrode active material having a layered rock salt structure, a negative electrode, and an electrolyte layer including an electrolytic solution. A method for producing a secondary battery, wherein the electrolyte contains a non-aqueous solvent and lithium ions having a concentration of 50% or more of the lithium ion concentration corresponding to the saturated concentration of the lithium salt at 25 ° C., and the power generation element and performing conditioning in a voltage range in which the upper limit voltage is 4.0 to 4.5V.

According to the method for manufacturing a non-aqueous electrolyte secondary battery according to the present embodiment, it is possible to further improve cycle durability in a non-aqueous electrolyte secondary battery using a positive electrode active material having a layered rock salt structure. .

Further, one embodiment of the present invention is a positive electrode for a non-aqueous electrolyte secondary battery including a positive electrode active material layer containing a positive electrode active material having a layered rock salt structure and an electrolytic solution, wherein the electrolytic solution is a non-aqueous solvent and lithium ions at a concentration of 50% or more of the lithium ion concentration corresponding to the saturated concentration of the lithium salt at 25 ° C., the surface of the positive electrode active material containing an element derived from the lithium salt, and the thickness is 9 to 15 nm and the coverage is 85% or more.

According to the positive electrode for a nonaqueous electrolyte secondary battery according to the present embodiment, cycle durability can be further improved in a nonaqueous electrolyte secondary battery using a positive electrode active material having a layered rock salt structure.

Hereinafter, the embodiments of the present embodiment described above will be described with reference to the drawings, but the technical scope of the present invention should be determined based on the description of the claims and is not limited to the following embodiments. Note that the dimensional ratios in the drawings are exaggerated for convenience of explanation, and may differ from the actual ratios. In this specification, "X to Y" indicating a range means "X or more and Y or less". Unless otherwise specified, operations and measurements of physical properties are performed at room temperature (20 to 25° C.) and relative humidity of 40 to 50% RH.

FIG. 1 schematically shows the overall structure of a flat (stacked) non-bipolar (internal parallel connection type) secondary battery (hereinafter also simply referred to as a “stacked secondary battery”) that is an embodiment of the present invention. 1 is a schematic cross-sectional view schematically represented. FIG.

As shown in FIG. 1, the laminated secondary battery 10a of the present embodiment has a structure in which a substantially rectangular power generation element 21 in which charge/discharge reactions actually progress is sealed inside a laminate film 29. As shown in FIG. Here, the power generation element 21 includes a positive electrode in which the positive electrode active material layer 13 is arranged on both sides of the positive electrode current collector 11 ′, an electrolyte layer 17 made of a separator containing an electrolytic solution, and both sides of the negative electrode current collector 11 ″. Specifically, one positive electrode active material layer 13 and the negative electrode active material layer 15 adjacent thereto are laminated, and the electrolyte layer 17 A negative electrode, an electrolyte layer, and a positive electrode are laminated in this order so as to face each other with the electrodes interposed therebetween.

As a result, the adjacent positive electrode, electrolyte layer, and negative electrode form one single cell layer 19 . Therefore, it can be said that the stacked secondary battery 10a shown in FIG. 1 has a configuration in which a plurality of cell layers 19 are stacked and electrically connected in parallel. The positive electrode current collectors of the outermost layers located on both outermost layers of the power generating element 21 have the positive electrode active material layer 13 only on one side, but the active material layer may be provided on both sides. . That is, a current collector having active material layers on both sides may be used as it is as the current collector for the outermost layer, instead of using a current collector exclusively for the outermost layer having an active material layer on only one side. In addition, by reversing the arrangement of the positive electrode and the negative electrode from FIG. 1, the outermost negative electrode current collector is positioned on both outermost layers of the power generation element 21, A negative electrode active material layer may be arranged on both sides.

A positive electrode current collector 25 and a negative electrode current collector 27 electrically connected to each electrode (positive electrode and negative electrode) are attached to the positive electrode current collector 11 ′ and the negative electrode current collector 11 ″, respectively. It has a structure in which it is sandwiched and led out to the outside of the laminate film 29. The positive electrode current collector plate 25 and the negative electrode current collector plate 27 are connected to a positive electrode terminal lead and a negative electrode terminal lead (not shown), respectively, as necessary. ) to the positive electrode current collector 11′ and the negative electrode current collector 11″ of each electrode by ultrasonic welding, resistance welding, or the like.

FIG. 2 is a cross-sectional view schematically showing a bipolar secondary battery that is another embodiment of the present invention. A bipolar secondary battery 10b shown in FIG. 2 has a structure in which a substantially rectangular power generation element 21 in which charge/discharge reactions actually progress is sealed inside a laminate film 29 that is a battery exterior body. In this specification, a bipolar lithium ion secondary battery may be simply referred to as a "bipolar secondary battery", and an electrode for a bipolar lithium ion secondary battery may be simply referred to as a "bipolar electrode".

As shown in FIG. 2, in the power generation element 21 of the bipolar secondary battery 10b of the present embodiment, the positive electrode active material layer 13 electrically coupled to one surface of the current collector 11 is formed. It has a plurality of bipolar electrodes 23 with a negative electrode active material layer 15 electrically coupled to the opposite surface. Each bipolar electrode 23 is laminated via the electrolyte layer 17 to form the power generating element 21 . At this time, the positive electrode active material layer 13 of one bipolar electrode 23 and the negative electrode active material layer 15 of the other bipolar electrode 23 adjacent to the one bipolar electrode 23 face each other with the electrolyte layer 17 interposed therebetween. Each bipolar electrode 23 and electrolyte layer 17 are alternately stacked. That is, the electrolyte layer 17 is sandwiched between the positive electrode active material layer 13 of one bipolar electrode 23 and the negative electrode active material layer 15 of the other bipolar electrode 23 adjacent to the one bipolar electrode 23 . ing.

Adjacent positive electrode active material layer 13 , electrolyte layer 17 , and negative electrode active material layer 15 constitute one cell layer 19 . Therefore, it can be said that the bipolar secondary battery 10b has a structure in which the cell layers 19 are stacked. A sealing portion (insulating layer) 31 is arranged on the outer peripheral portion of the cell layer 19 . This prevents a liquid junction due to leakage of the electrolyte from the electrolyte layer 17, contact between adjacent current collectors 11 in the battery, and slight unevenness at the end of the cell layer 19 in the power generation element 21. This prevents short circuits caused by The positive electrode active material layer 13 is formed only on one side of the outermost current collector 11 a on the positive electrode side located in the outermost layer of the power generating element 21 . In addition, the negative electrode active material layer 15 is formed only on one side of the outermost current collector 11b on the negative electrode side located in the outermost layer of the power generating element 21 .

Furthermore, in the bipolar secondary battery 10b shown in FIG. 2, a positive electrode current collector plate (positive electrode tab) 25 is arranged adjacent to the outermost layer current collector 11a on the positive electrode side, and this is extended to form a battery outer package. It is led out from the laminate film 29 . On the other hand, a negative electrode current collector plate (negative electrode tab) 27 is arranged adjacent to the outermost layer current collector 11 b on the negative electrode side, and is similarly extended to lead out from the laminate film 29 .

Note that the number of times the cell layers 19 are stacked is adjusted according to the desired voltage. Further, in the bipolar secondary battery 10b, the number of stacking of the cell layers 19 may be reduced if sufficient output can be secured even if the thickness of the battery is made as thin as possible. In the bipolar secondary battery 10b as well, in order to prevent external shocks and environmental deterioration during use, the power generation element 21 is enclosed under reduced pressure in a laminate film 29, which is a battery outer body, and a positive electrode collector plate 25 and a negative electrode collector are enclosed. A structure in which the electric plate 27 is taken out of the laminate film 29 is preferable.

Main constituent members of the positive electrode for a non-aqueous electrolyte secondary battery according to this embodiment will be described below. A positive electrode for a non-aqueous electrolyte secondary battery according to this embodiment includes a current collector and a positive electrode active material layer. In addition, the positive electrode active material layer essentially contains a positive electrode active material having a layered rock salt structure and an electrolytic solution, and if necessary, further contains other additives such as a conductive aid, a conductive member, and a binder. And the electrolytic solution contains a non-aqueous solvent and a high-concentration electrolyte (here, a lithium salt).

[Current collector]
The current collector has a function of mediating transfer of electrons from the electrode active material layer. There are no particular restrictions on the material that constitutes the current collector. As the constituent material of the current collector, for example, a metal or a conductive resin can be used.

Specifically, examples of metals include aluminum, nickel, iron, stainless steel, titanium, and copper. In addition to these, a clad material of nickel and aluminum, a clad material of copper and aluminum, or the like may be used. Alternatively, a foil in which a metal surface is coated with aluminum may be used. Among them, aluminum, stainless steel, copper, and nickel are preferable from the viewpoint of electronic conductivity, battery operating potential, adhesion of the negative electrode active material to the current collector by sputtering, and the like.

As the latter conductive resin, a resin obtained by adding a conductive filler to a non-conductive polymeric material as necessary can be used.

Examples of non-conductive polymer materials include polyethylene (PE; high-density polyethylene (HDPE), low-density polyethylene (LDPE), etc.), polypropylene (PP), polyethylene terephthalate (PET), polyethernitrile (PEN), and polyimide. (PI), polyamideimide (PAI), polyamide (PA), polytetrafluoroethylene (PTFE), styrene-butadiene rubber (SBR), polyacrylonitrile (PAN), polymethylacrylate (PMA), polymethylmethacrylate (PMMA) , polyvinyl chloride (PVC), polyvinylidene fluoride (PVDF), or polystyrene (PS).

Conductive fillers may be added to the conductive polymer material or non-conductive polymer material as necessary. In the case where the resin that serves as the base material of the current collector consists only of a non-conductive polymer, a conductive filler is essential.

The amount of the conductive filler added is not particularly limited as long as it is an amount that can impart sufficient conductivity to the current collector, and is generally 5 to 80% by mass with respect to 100% by mass of the total mass of the current collector. is.

The current collector may have a single-layer structure made of a single material, or may have a laminated structure in which layers made of these materials are appropriately combined. From the viewpoint of reducing the weight of the current collector, it is preferable to include at least a conductive resin layer made of a resin having conductivity. Moreover, from the viewpoint of blocking movement of lithium ions between the single cell layers, a metal layer may be provided on a part of the current collector.

[Positive electrode active material layer]
The positive electrode active material layer contains a positive electrode active material that can release ions such as lithium ions during charging and can occlude ions such as lithium ions during discharging. Specifically, as described above, the positive electrode active material layer includes a positive electrode active material having a layered rock salt structure. A positive electrode active material having a layered rock salt structure generally has a high capacity. Therefore, the battery capacity of the non-aqueous electrolyte secondary battery can be improved by using the positive electrode active material having the layered rock salt structure.

(Positive electrode active material having a layered rock salt structure)
Examples of positive electrode active materials having a layered rock salt structure include LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , Li(Ni—Mn—Co)O ₂ , Li(Ni—Co—Al)O ₂ and transition metals thereof. Examples include lithium-transition metal composite oxides such as those partially substituted with other elements. In some cases, two or more positive electrode active materials may be used together. More preferably, a composite oxide containing lithium and nickel is used, and more preferably Li(Ni--Mn--Co) O ₂ and those in which a portion of these transition metals are replaced with other elements (hereinafter referred to as Also simply referred to as "NMC composite oxide") or Li (Ni-Co-Al) O ₂ and those in which a part of these transition metals are replaced by other elements (hereinafter simply referred to as "NCA composite oxide" ) is used, and NMC composite oxide is particularly preferably used. NMC composite oxides and NCA composite oxides have a layered crystal structure in which lithium atomic layers and transition metal atomic layers are alternately stacked via oxygen atomic layers, and one Li atom per transition metal M atom is The amount of Li that can be contained and taken out is twice that of the spinel-based lithium manganese oxide, that is, the supply capacity is doubled, and a high capacity can be obtained.

NMC composite oxides and NCA composite oxides also include composite oxides in which a portion of the transition metal element is replaced with another metal element, as described above. Other elements in that case include Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, Fe, B, Ga, In, Si, Mo, Y, Sn, V, and Cu. , Ag, Zn, etc., preferably Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, Cr, more preferably Ti, Zr, P, Al, Mg, Cr, more preferably Ti, Zr, Al, Mg, or Cr from the viewpoint of improving cycle characteristics. However, other metal elements that can substitute for the transition metal elements of the NCA composite oxide are those other than Al.

Since the NMC composite oxide has a high theoretical discharge capacity, it is preferably represented by the general formula (1): _{LiaNibMncCodMxO2 (} wherein a, _b , _c , _d , _x satisfies 0.9≦a≦1.2, 0<b<1, 0<c≦0.5, 0<d≦0.5, 0≦x≦0.3, and b+c+d+x=1. at least one element selected from Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr, and Cr). Here, a represents the atomic ratio of Li, b represents the atomic ratio of Ni, c represents the atomic ratio of Mn, d represents the atomic ratio of Co, and x represents the atomic ratio of M. represents From the viewpoint of cycle characteristics, it is preferable that 0.4≦b≦0.92 in general formula (1). The composition of each element can be measured, for example, by plasma (ICP) emission spectrometry.

Nickel (Ni), cobalt (Co) and manganese (Mn) are generally known to contribute to capacity and output characteristics from the viewpoint of improving material purity and electronic conductivity. Ti or the like partially replaces the transition metal in the crystal lattice. From the viewpoint of cycle characteristics, part of the transition element may be substituted with another metal element. Since at least one selected from the group consisting of Ti, Zr, Nb, W, P, Al, Mg, V, Ca, Sr and Cr dissolves in a solid solution, the crystal structure is stabilized. It is considered that the battery capacity can be prevented from decreasing even if the cycle is repeated, and excellent cycle characteristics can be realized.

As a more preferred embodiment, in general formula (1), b, c and d are 0.44≦b≦0.92, 0.05≦c≦0.31, and 0.03≦d≦0.26. is preferable from the viewpoint of improving the balance between capacity and life characteristics. For example, LiNi _0.5 Mn _0.3 Co _0.2 O ₂ is LiCoO ₂ , LiMn ₂ O ₄ , LiNi _1/3 Mn _1/3 Co _1/3 O ₂ etc. which have been proven in general consumer batteries. Compared to , it has the advantage of being able to produce a compact and high-capacity battery because the capacity per unit weight is large and the energy density can be improved, and it is also preferable from the viewpoint of cruising distance. _{LiNi0.8Co0.1Al0.1O2 , LiNi0.8Mn0.1Co0.1O2 , and LiNi0.88Mn0.06Co0.88Mn0.06Co0 . _ _ _ _ 06 O2} is more preferred. On the other hand, LiNi _0.5 Mn _0.3 Co _0.2 O ₂ has life characteristics as excellent as LiNi _1/3 Mn _1/3 Co _1/3 O ₂ .

In the positive electrode for a non-aqueous electrolyte secondary battery according to the present embodiment, the positive electrode active material is a positive electrode active material other than a positive electrode active material having a layered rock salt structure (for example, lithium-transition metal phosphate compound, lithium-transition metal sulfate compounds, etc.). However, in this embodiment, the content of the positive electrode active material having a layered rock salt structure in 100% by mass of the positive electrode active material is, for example, 50% by mass or more, preferably more than 50% by mass, and more preferably 70% by mass. % or more, more preferably 80 mass % or more, still more preferably 90 mass % or more, particularly preferably 95 mass % or more, most preferably 100 mass %.

In the positive electrode for a non-aqueous electrolyte secondary battery according to the present embodiment, a coating layer containing an element derived from a lithium salt and having a thickness of 9 to 15 nm and a coverage of 85% or more is formed on the surface of the positive electrode active material. have. The uniform formation of a film derived from the electrolyte on the surface of the positive electrode active material promotes structural damage to the positive electrode active material, elution of the transition metal from the surface of the positive electrode active material, and decomposition of the electrolyte on the surface of the positive electrode active material. become difficult. Therefore, it can operate stably even at a high voltage, and the durability of the battery can be improved.

The thickness and coverage of the coating layer can be measured by the method described in the section of Examples described later. In addition, the thickness of the above coating layer means the average value of the thickness measured by the method described in the Examples.

In addition, the maximum and minimum values of the thickness of the coating layer are within a range of ±10 nm with respect to the average thickness (the absolute value of the difference between the maximum value and the average value and the difference between the minimum value and the average value). The absolute value of each difference is 10 nm or less), more preferably within the range of ±7 nm, and even more preferably within the range of ±5 nm. The effects of the present invention can be obtained more remarkably by this form.

A method for forming a coating layer having a predetermined thickness and coverage is not particularly limited. As an example, as described in the manufacturing method of a non-aqueous electrolyte secondary battery described later, a positive electrode including a positive electrode active material layer containing a positive electrode active material having a layered rock salt structure, a negative electrode, and an electrolyte including a concentrated electrolyte A method of conditioning a power generation element including a layer in a voltage range in which the upper limit voltage is 4.0 to 4.5V.

In the positive electrode for a non-aqueous electrolyte secondary battery according to this embodiment, the positive electrode active material preferably has a roughness factor of 3 or less. Here, the "roughness factor" is a parameter that serves as an index of the smoothness of the surface of the positive electrode active material. It is calculated as a value of the ratio of "BET specific surface area" to "geometric specific surface area" (= BET specific surface area/geometric specific surface area). According to the non-aqueous electrolyte secondary battery using the positive electrode for a non-aqueous electrolyte secondary battery according to the present embodiment, the roughness factor of the positive electrode active material is controlled to be 3 or less, so that the positive electrode active material having a layered rock salt structure There is an advantage that deterioration of cycle durability can be suppressed even when charging/discharging is performed under conditions of high cell voltage using the substance and a concentrated electrolytic solution described later. The roughness factor value is more preferably 2.7 or less, still more preferably 2.4 or less, even more preferably 2.3 or less, and still more preferably 2.2 or less. On the other hand, the lower limit of the roughness factor is 1.0 or more. In addition, when the positive electrode active material is a mixture of two or more kinds, first, the BET specific surface area and the geometric specific surface area of each positive electrode active material constituting the mixture are measured by the above method, and the obtained values are The BET specific surface area and the geometric specific surface area of the mixture are calculated by summing the mixture ratios. Then, the roughness factor is calculated from these calculated values by the same method as described above.

When the surface of the positive electrode active material particles is highly smooth, a more uniform coating layer is formed on the surface, which is more effective in improving cycle durability.

The technique for controlling the roughness factor of the positive electrode active material to be 3 or less is not particularly limited, and conventionally known knowledge capable of controlling the smoothness of the surface of the positive electrode active material particles can be appropriately referred to. For example, as an example of a technique for producing an NMC composite oxide, first, (1) the precursor _{( NixCoyMnz} )( _OH ) ₂ is synthesized by a coprecipitation method. Specifically, an aqueous solution containing raw materials such as nickel sulfate, cobalt sulfate, manganese sulfate, sodium hydroxide, and ammonium hydroxide in a desired stoichiometric ratio is stirred for a predetermined time. After separating the precipitate by filtration, it is dried at a temperature of about 80° C. for about 8 to 15 hours to obtain a precursor. Next, (2) the obtained precursor is calcined at a temperature of about 600 to 800° C. for about 3 to 7 hours to obtain Ni _x Co _y Mn _z O ₂ . Then, (3) a lithium salt (for example, lithium carbonate) is added to the obtained oxide so as to have a stoichiometric ratio of 1 to 1.2 times, and the mixture is pulverized and mixed with a ball mill. (4) After firing at a temperature of about 600 to 800° C. for about 3 to 5 hours in the air or in an oxygen atmosphere, firing is further performed at a temperature of about 750 to 1000° C. for about 2 to 12 hours. Finally, (5) the fired powder is washed with water to remove the remaining lithium salt, and then dried at a temperature of about 80° C. for about 10 to 15 hours to obtain an NMC composite oxide. can. In addition, (6) if necessary, after pulverizing the sample with a ball mill, it may be refired at 500 to 1000° C. for 3 to 5 hours in the air or in an oxygen atmosphere. At this time, in order to control the value of the roughness factor of the NMC composite oxide, (2) firing temperature and time, (3) degree of pulverization, (4) firing time and temperature, (6) pulverization By adjusting the degree of , the size of the primary particles can be controlled. Further, by performing the re-baking in (6) at a high temperature for a long time, the surface after pulverization becomes smoother and the roughness factor can be lowered.

Although the average particle size of the positive electrode active material contained in the positive electrode active material layer is not particularly limited, it is preferably 1 to 10 μm, more preferably 1.5 to 6 μm, and even more preferably 1.5 to 6 μm from the viewpoint of increasing output. 2 to 5 μm. In addition, as the value of the average particle size of the positive electrode active material, the value measured by the method described in the section of Examples described later is adopted.

From the viewpoint of further improving the cycle durability, in the positive electrode for a non-aqueous electrolyte secondary battery according to the present embodiment, the crystallite size of the positive electrode active material is preferably 1 μm or more. Here, whether or not the crystallite size of the positive electrode active material is 1 μm or more can be determined by the method described in the section of Examples described later. If it is possible to measure the crystallite size more accurately, the determination may be made from the crystallite size thus measured. For the same reason as described above, the ratio of the number of particles composed of a single crystallite to the particles constituting the positive electrode active material is preferably 10% or more, more preferably 20% or more, and even more preferably. is 30% or more.

In addition, the crystallite diameter of the positive electrode active material is controlled to a large value such as 1 μm or more, or the number ratio of particles composed of a single crystallite in the particles constituting the positive electrode active material is controlled within the above preferable range. As a method to do, patent documents such as Patent No. 6574222 and Patent No. 5702289, Solid State Ionics, Volume 345, February 2020, 115200, Journal of The Electrochemical Society, 165 (5) A1038-A1048) non-patent documents such as 5 (2018) The method disclosed in can be referred to.

The content of the positive electrode active material in the positive electrode active material layer is not particularly limited. more preferred.

In the positive electrode for a non-aqueous electrolyte secondary battery according to this embodiment, the positive electrode active material layer contains an electrolytic solution in addition to the positive electrode active material having the layered rock salt structure, as described above.

The electrolytic solution (liquid electrolyte) functions as a carrier for lithium ions. The electrolytic solution (liquid electrolyte) forming the electrolytic solution layer has a form in which a lithium salt is dissolved in a non-aqueous solvent (organic solvent), and contains lithium ions derived from the lithium salt. Non-aqueous solvents used include, for example, dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), ethyl methyl carbonate (EMC), methyl propionate (MP), methyl acetate (MA), formic acid Methyl (MF), 4-methyldioxolane (4MeDOL), dioxolane (DOL), 2-methyltetrahydrofuran (2MeTHF), tetrahydrofuran (THF), dimethoxyethane (DME), propylene carbonate (PC), butylene carbonate (BC), dimethyl sulfoxide (DMSO), γ-butyrolactone (GBL), and the like. Among them, the non-aqueous solvent is preferably a chain carbonate, more preferably diethyl carbonate (DEC), ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC ), more preferably selected from ethyl methyl carbonate (EMC) and dimethyl carbonate (DMC).

Lithium salts include Li(FSO ₂ ) ₂ N (lithium bis(fluorosulfonyl)imide; LiFSI), Li(C ₂ F ₅ SO ₂ ) ₂ N, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , LiCF _{3 SO3} and the like. Among them, the lithium salt preferably contains an imide group-containing anion as a counter anion for the lithium ion, more preferably Li(FSO ₂ ) ₂ N (LiFSI), from the viewpoint of battery output and charge-discharge cycle characteristics. .

The electrolytic solution may further contain additives other than the components described above. Specific examples of such compounds include ethylene carbonate, vinylene carbonate, methylvinylene carbonate, dimethylvinylene carbonate, phenylvinylene carbonate, diphenylvinylene carbonate, ethylvinylene carbonate, diethylvinylene carbonate, vinylethylene carbonate, 1,2- Divinylethylene carbonate, 1-methyl-1-vinylethylene carbonate, 1-methyl-2-vinylethylene carbonate, 1-ethyl-1-vinylethylene carbonate, 1-ethyl-2-vinylethylene carbonate, vinyl vinylene carbonate, allyl ethylene carbonate, vinyloxymethylethylene carbonate, allyloxymethylethylene carbonate, acryloxymethylethylene carbonate, methacryloxymethylethylene carbonate, ethynylethylene carbonate, propargylethylene carbonate, ethynyloxymethylethylene carbonate, propargyloxyethylene carbonate, methyleneethylene carbonate, 1 , 1-dimethyl-2-methylene ethylene carbonate and the like. These additives may be used alone or in combination of two or more. In addition, when the additive is used in the electrolytic solution, the amount used can be appropriately adjusted.

Note that the non-aqueous electrolyte secondary battery of this embodiment has excellent durability even when operated at a high voltage. Therefore, it is not necessary to add known additives (for example, vinylene carbonate, diphenyl ether, N-methylpyrrole, etc.) that form a positive electrode interfacial protective film to the electrolytic solution in order to stably operate the battery at high voltage. As a result, the cost can be reduced, which is preferable.

The non-aqueous electrolyte secondary battery according to the present embodiment is characterized in that it contains a high concentration of lithium salt as an electrolyte (that is, it is a concentrated electrolytic solution). That is, the positive electrode for a non-aqueous electrolyte secondary battery according to the present embodiment is characterized by using a high-concentration electrolytic solution. Specifically, the electrolyte contained in the positive electrode active material layer constituting the positive electrode for a non-aqueous electrolyte secondary battery according to the present embodiment has a concentration of 50% or more of the lithium ion concentration corresponding to the saturated concentration of lithium salt at 25 ° C. of lithium ions. The saturated concentration of the lithium salt at 25° C. can be measured by using the time point at which the lithium salt is dissolved in the non-aqueous solvent at 25° C. and precipitated when the lithium salt is no longer dissolved. Further, the electrolytic solution preferably contains lithium ions at a concentration of 53% or more of the lithium ion concentration corresponding to the saturated concentration of the lithium salt at 25°C. Lithium ions with a concentration of 56% or more of the lithium ion concentration corresponding to the concentration, more preferably the electrolyte contains lithium ions with a concentration of 60% or more of the lithium ion concentration corresponding to the lithium ion saturation concentration at 25 ° C. contains Although the upper limit of this concentration is not particularly limited, it is usually 95% or less, preferably 90% or less, and more preferably 85% or less.

In this embodiment, the concentration of the lithium salt in the electrolytic solution is preferably 3 mol/L or more, more preferably 3.5 mol, from the viewpoint of improving battery characteristics such as rate characteristics and cycle durability of the battery. /L or more, more preferably 4 mol/L or more. On the other hand, the upper limit of the concentration of the lithium salt in the electrolytic solution is not particularly limited, but is usually 6 mol/L or less.

The positive electrode active material layer contains the positive electrode active material as described above, and if necessary, further contains other additives such as a conductive aid, a conductive member, and a binder.

A conductive aid is an additive compounded to improve the conductivity of the active material layer. Examples of conductive aids include carbon materials such as carbon black such as acetylene black. When the positive electrode active material layer contains a conductive aid, an electronic network is effectively formed inside the positive electrode active material layer, which can contribute to improvement of the output characteristics of the battery.

The conductive member has a function of forming an electron conduction path in the positive electrode active material layer. In particular, at least part of the conductive member forms a conductive path that electrically connects the first main surface in contact with the electrolyte layer side of the positive electrode active material layer to the second main surface in contact with the current collector side. is preferred. By having such a form, the electron migration resistance in the thickness direction in the positive electrode active material layer is further reduced. As a result, the output characteristics of the battery at a high rate can be further improved. At least part of the conductive member forms a conductive path that electrically connects the first main surface in contact with the electrolyte layer side of the positive electrode active material layer to the second main surface in contact with the current collector side. Whether or not it can be confirmed by observing the cross section of the positive electrode active material layer using an SEM or an optical microscope.

The conductive member is preferably a conductive fiber having a fibrous form. Specifically, carbon fibers such as PAN-based carbon fibers and pitch-based carbon fibers, conductive fibers made by uniformly dispersing highly conductive metals and graphite in synthetic fibers, and metals such as stainless steel fibers. conductive fibers obtained by coating the surface of organic fibers with a metal, conductive fibers obtained by coating the surfaces of organic fibers with a resin containing a conductive substance, and the like. Among them, carbon fiber is preferable because of its excellent conductivity and light weight.

The optional component binder used for the positive electrode active material layer is not particularly limited, but examples include polybutylene terephthalate, polyethylene terephthalate, and polyvinylidene fluoride (PVDF) (including compounds in which hydrogen atoms are replaced with other halogen elements). ), polyethylene, polypropylene, polymethylpentene, polybutene, polyethernitrile, polytetrafluoroethylene, polyacrylonitrile, polyimide, polyamide, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), ethylene- Thermoplastic polymers such as propylene-diene copolymers, styrene-butadiene-styrene block copolymers and hydrogenated products thereof, styrene-isoprene-styrene block copolymers and hydrogenated products thereof, tetrafluoroethylene-hexafluoropropylene Copolymer (FEP), tetrafluoroethylene/perfluoroalkyl vinyl ether copolymer (PFA), ethylene/tetrafluoroethylene copolymer (ETFE), polychlorotrifluoroethylene (PCTFE), ethylene/chlorotrifluoroethylene copolymer Polymer (ECTFE), fluororesin such as polyvinyl fluoride (PVF), vinylidene fluoride-hexafluoropropylene fluororubber (VDF-HFP fluororubber), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene fluororubber (VDF-HFP-TFE fluororubber), vinylidene fluoride-pentafluoropropylene fluororubber (VDF-PFP fluororubber), vinylidene fluoride-pentafluoropropylene-tetrafluoroethylene fluororubber (VDF-PFP-TFE fluororubber), vinylidene fluoride-perfluoromethyl vinyl ether-tetrafluoroethylene fluororubber (VDF-PFMVE-TFE fluororubber), vinylidene fluoride-chlorotrifluoroethylene fluororubber (VDF-CTFE fluororubber) and vinylidene fluoride-based fluororubbers, epoxy resins, and the like. Among them, polyimide, styrene-butadiene rubber, carboxymethylcellulose, polypropylene, polytetrafluoroethylene, polyacrylonitrile, and polyamide are more preferable.

Moreover, in this embodiment, the compounding ratio of other additives such as a conductive aid, a conductive member, and a binder that can be contained in the positive electrode active material layer is not particularly limited. Their compounding ratio can be adjusted by appropriately referring to known knowledge about non-aqueous electrolyte secondary batteries.

In the positive electrode for a non-aqueous electrolyte secondary battery of this embodiment, the thickness of the positive electrode active material layer is not particularly limited, and conventionally known knowledge about batteries can be appropriately referred to. For example, the thickness of the positive electrode active material layer is usually about 1 to 1000 μm, preferably 20 to 800 μm, more preferably 30 to 500 μm, still more preferably 40 to 200 μm. As the thickness of the positive electrode active material layer increases, it becomes possible to hold the positive electrode active material for exhibiting a sufficient capacity (energy density). On the other hand, the smaller the thickness of the positive electrode active material layer, the better the discharge rate characteristics.

A positive electrode for a non-aqueous electrolyte secondary battery according to this embodiment is used as a constituent member of a non-aqueous electrolyte secondary battery. That is, according to another aspect of the present invention, there is provided a non-aqueous electrolyte secondary battery including a power generation element having the positive electrode for a non-aqueous electrolyte secondary battery according to one aspect of the present invention described above. Components other than the positive electrode of the non-aqueous electrolyte secondary battery will be briefly described below.

[Negative electrode active material layer]
The negative electrode active material layer includes a negative electrode active material that can release ions such as lithium ions during discharge and can occlude ions such as lithium ions during charge. The types of negative electrode active materials are not particularly limited, but include carbon materials, metal oxides and metal active materials. Examples of carbon materials include natural graphite, artificial graphite, mesocarbon microbeads (MCMB), highly oriented graphite (HOPG), hard carbon, soft carbon, and the like. Moreover, as a metal _oxide , _Nb2O5 , _{Li4Ti5O12 etc.} are _mentioned , for example. Furthermore, a silicon-based negative electrode active material or a tin-based negative electrode active material may be used. Here, silicon and tin belong to Group 14 elements and are known to be negative electrode active materials capable of greatly improving the capacity of non-aqueous electrolyte secondary batteries. Since these simple substances can occlude and release a large number of charge carriers (lithium ions, etc.) per unit volume (mass), they become high-capacity negative electrode active materials. Here, it is preferable to use simple Si as the silicon-based negative electrode active material. Similarly, it is also preferable to use a silicon oxide such as SiO _x (0.3≦x≦1.6) disproportionated into two phases of Si phase and silicon oxide phase. At this time, the range of x is more preferably 0.5≦x≦1.5, and more preferably 0.7≦x≦1.2. Furthermore, an alloy containing silicon (silicon-containing alloy-based negative electrode active material) may be used. On the other hand, examples of negative electrode active materials containing a tin element (tin-based negative electrode active materials) include Sn alone, tin alloys (Cu—Sn alloys, Co—Sn alloys), amorphous tin oxides, tin silicon oxides, and the like. Among these, SnB _0.4 P _0.6 O _3.1 is exemplified as an amorphous tin oxide. SnSiO ₃ is exemplified as tin silicon oxide. A metal containing lithium may also be used as the negative electrode active material. Such a negative electrode active material is not particularly limited as long as it is an active material containing lithium, and examples thereof include metallic lithium and lithium-containing alloys. Lithium-containing alloys include, for example, alloys of Li and at least one of In, Al, Si and Sn. In some cases, two or more kinds of negative electrode active materials may be used together. Needless to say, negative electrode active materials other than those described above may be used. The present invention exhibits particularly excellent effects when the expansion and contraction of the negative electrode active material during charging and discharging are large. From this point of view and high capacity, the negative electrode active material preferably contains a carbon material, metallic lithium, a silicon-based negative electrode active material, or a tin-based negative electrode active material.

Examples of the shape of the negative electrode active material include particulate (spherical, fibrous), thin film, and the like. When the negative electrode active material is in the form of particles, the average particle size (D50) thereof is, for example, preferably in the range of 1 nm to 100 μm, more preferably in the range of 10 nm to 50 μm, still more preferably 100 nm to 100 μm. It is in the range of 20 μm, particularly preferably in the range of 1 to 20 μm. In addition, in this specification, the value of the average particle size (D50) of the active material can be measured by a laser diffraction scattering method.

The content of the negative electrode active material in the negative electrode active material layer is not particularly limited. more preferred.

In addition, the negative electrode active material layer further contains an electrolytic solution in the same manner as described above for the positive electrode active material layer, and if necessary, further contains other additives such as a conductive aid, a conductive member, and a binder.

In the nonaqueous electrolyte secondary battery of the present embodiment, the thickness of the negative electrode active material layer is not particularly limited, and conventionally known knowledge about batteries can be appropriately referred to. For example, the thickness of the negative electrode active material layer is usually about 1 to 1000 μm, preferably 10 to 800 μm, more preferably 15 to 600 μm, still more preferably 20 to 200 μm. As the thickness of the negative electrode active material layer increases, it becomes possible to hold the negative electrode active material for exhibiting a sufficient capacity (energy density). On the other hand, the smaller the thickness of the negative electrode active material layer, the better the discharge rate characteristics.

[Electrolyte layer]
The electrolyte layer of the nonaqueous electrolyte secondary battery according to the present embodiment preferably has a structure in which a separator is impregnated with an electrolytic solution. The specific configuration and preferred embodiments of the electrolytic solution (liquid electrolyte) are as described above in the section on the positive electrode, and thus detailed description thereof is omitted here.

In the non-aqueous electrolyte secondary battery according to this embodiment, a separator may be used for the electrolyte layer. The separator has a function of retaining the electrolyte to ensure lithium ion conductivity between the positive electrode and the negative electrode, and a function of a partition wall between the positive electrode and the negative electrode.

Examples of the form of the separator include porous sheet separators and non-woven fabric separators made of a polymer or fiber that absorbs and retains the electrolytic solution.

As a porous sheet separator made of polymer or fiber, for example, a microporous (microporous membrane) can be used. Specific forms of the porous sheet made of the polymer or fiber include, for example, polyolefins such as polyethylene (PE) and polypropylene (PP); structural laminates, etc.), hydrocarbon resins such as polyimide, aramid, and polyvinylidene fluoride-hexafluoropropylene (PVDF-HFP), and microporous (microporous membrane) separators made of glass fibers.

The thickness of the microporous (microporous membrane) separator cannot be defined uniquely because it varies depending on the intended use. As an example, in applications such as secondary batteries for driving motors such as electric vehicles (EV), hybrid electric vehicles (HEV), and fuel cell vehicles (FCV), the single layer or multiple layers should be 4 to 60 μm. is desirable. The fine pore size of the microporous (microporous membrane) separator is desirably 1 μm or less at maximum (usually, the pore size is about several tens of nanometers).

As the non-woven fabric separator, conventionally known materials such as cotton, rayon, acetate, nylon, polyester; polyolefins such as PP and PE; polyimide and aramid are used singly or in combination. The thickness of the non-woven fabric separator may be the same as that of the electrolyte layer, preferably 5 to 200 μm, particularly preferably 10 to 100 μm.

The separator is preferably a separator in which a heat-resistant insulating layer is laminated on a porous substrate (separator with a heat-resistant insulating layer). A heat-resistant insulating layer is a ceramic layer containing inorganic particles and a binder. As the separator with a heat-resistant insulating layer, one having high heat resistance with a melting point or thermal softening point of 150° C. or higher, preferably 200° C. or higher is used. By having the heat-resistant insulating layer, the internal stress of the separator, which increases when the temperature rises, is relieved, so that the effect of suppressing thermal shrinkage can be obtained. As a result, it is possible to prevent the induction of short-circuiting between the electrodes of the battery, so that the battery structure is such that the deterioration in performance due to the temperature rise is less likely to occur. Moreover, the presence of the heat-resistant insulating layer improves the mechanical strength of the separator with the heat-resistant insulating layer, and the separator is less likely to break. Furthermore, due to the effect of suppressing heat shrinkage and high mechanical strength, the separator is less likely to curl during the battery manufacturing process.

The inorganic particles in the heat-resistant insulating layer contribute to the mechanical strength of the heat-resistant insulating layer and the effect of suppressing thermal shrinkage. Materials used as inorganic particles are not particularly limited. For example, silicon, aluminum,
Zirconium, titanium oxides _{( SiO2} , _{Al2O3 ,} ZrO2, _TiO2 ), hydroxides, and nitrides, and composites thereof. These inorganic particles may be derived from mineral resources such as boehmite, zeolite, apatite, kaolin, mullite, spinel, olivine, and mica, or may be artificially produced. In addition, these inorganic particles may be used alone or in combination of two or more. Among these, silica (SiO ₂ ) or alumina (Al ₂ O ₃ ) is preferably used from the viewpoint of cost, and alumina (Al ₂ O ₃ ) is more preferably used.

The basis weight of the heat-resistant particles is not particularly limited, but is preferably 5 to 15 g/m ² . Within this range, sufficient ionic conductivity is obtained and heat resistance strength is maintained, which is preferable.

The binder in the heat-resistant insulating layer has a role of bonding the inorganic particles to each other and bonding the inorganic particles to the resin porous substrate layer. The binder stably forms the heat-resistant insulating layer and prevents separation between the porous substrate layer and the heat-resistant insulating layer.

The binder used for the heat-resistant insulating layer is not particularly limited, and examples thereof include carboxymethylcellulose (CMC), polyacrylonitrile, cellulose, ethylene-vinyl acetate copolymer, polyvinyl chloride, styrene-butadiene rubber (SBR), and isoprene rubber. , butadiene rubber, polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), polyvinyl fluoride (PVF), methyl acrylate, and other compounds can be used as binders. Among these, it is preferable to use carboxymethyl cellulose (CMC), methyl acrylate, or polyvinylidene fluoride (PVDF). These compounds may be used alone or in combination of two or more.

The content of the binder in the heat-resistant insulating layer is preferably 2 to 20% by mass with respect to 100% by mass of the heat-resistant insulating layer. When the content of the binder is 2% by mass or more, the peel strength between the heat-resistant insulating layer and the porous substrate layer can be increased, and the vibration resistance of the separator can be improved. On the other hand, when the content of the binder is 20% by mass or less, the gaps between the inorganic particles are appropriately maintained, so sufficient lithium ion conductivity can be ensured.

The heat shrinkage rate of the separator with a heat-resistant insulating layer is preferably 10% or less in both MD and TD after holding for 1 hour under conditions of 150° C. and 2 gf/cm ² . By using such a highly heat-resistant material, the shrinkage of the separator can be effectively prevented even when the heat value increases and the internal temperature of the battery reaches 150°C. As a result, it is possible to prevent the induction of short-circuiting between the electrodes of the battery, so that the battery structure is such that the deterioration in performance due to the temperature rise is less likely to occur.

[Positive collector plate and negative collector plate]
The material constituting the current collectors (25, 27) is not particularly limited, and known highly conductive materials conventionally used as current collectors for lithium ion secondary batteries can be used. Metal materials such as aluminum, copper, titanium, nickel, stainless steel (SUS), and alloys thereof are preferable as the constituent material of the current collector plate. From the viewpoint of light weight, corrosion resistance and high conductivity, aluminum and copper are more preferred, and aluminum is particularly preferred. The same material or different materials may be used for the positive collector plate 25 and the negative collector plate 27 .

[Seal part]
The sealing portion is a member unique to bipolar secondary batteries (series stacked batteries) and has a function of preventing leakage of electrolyte from the electrolyte layer. In addition, it is also possible to prevent contact between adjacent current collectors in the battery and short circuits due to slight irregularities in the ends of the laminated electrodes.

The material constituting the sealing portion is not particularly limited, but polyolefin resins such as polyethylene and polypropylene, epoxy resins, rubbers, polyimides, and the like can be used. Among these, polyolefin resins are preferably used from the viewpoint of corrosion resistance, chemical resistance, film formability, economy, and the like.

[Positive lead and negative lead]
Although not shown, the current collector and the current collector plate may be electrically connected via a positive lead or a negative lead. Materials used in known lithium-ion secondary batteries can also be employed as the constituent materials of the positive and negative electrode leads. In addition, the parts taken out from the exterior should be heat-shrunk with heat-resistant insulation so that they do not come into contact with peripheral equipment or wiring and cause electric leakage and affect the product (for example, automobile parts, especially electronic equipment, etc.). Covering with a tube or the like is preferred.

[Battery exterior body]
As the battery exterior body, a known metal can case can be used, and as shown in FIGS. 1 and 2, a bag-like case using a laminate film 29 containing aluminum that can cover the power generation element is used. can be The laminated film may be, for example, a laminated film having a three-layer structure in which PP, aluminum and nylon are laminated in this order, but is not limited thereto. A laminate film is desirable from the viewpoint that it is excellent in high power output and cooling performance and can be suitably used for batteries for large equipment for EV and HEV. Moreover, since the group pressure applied to the power generating element from the outside can be easily adjusted, the outer package is more preferably a laminate film containing aluminum.

The non-aqueous electrolyte secondary battery according to the present embodiment has a structure in which a plurality of cell layers are connected in parallel, and thus has a high capacity and excellent cycle durability. Therefore, the stacked secondary battery according to this embodiment is suitably used as a power source for driving EVs and HEVs.

In particular, the non-aqueous electrolyte secondary battery according to the present embodiment, despite using a positive electrode active material having a layered rock salt structure and a concentrated electrolyte, exhibits poor cycle durability even under high cell voltage conditions. It has the advantage of being able to improve Therefore, the non-aqueous electrolyte secondary battery according to one aspect of the present invention exhibits an operating voltage of preferably 4.4 V or higher, more preferably 4.5 V or higher, and even more preferably 4.6 V or higher as a cell voltage. . In other words, according to yet another aspect of the present invention, the cell voltage of the present invention is preferably 4.4 V or higher, more preferably 4.5 V or higher, and still more preferably 4.6 V or higher, so as to undergo an operating voltage of 4.6 V or higher. A method of operating a non-aqueous electrolyte secondary battery according to one aspect is also provided. If the battery can be operated under such a high cell voltage condition, it is possible to fully extract the high capacity originally possessed by the positive electrode active material having a layered rock salt structure, which is very suitable. be.

[Method for producing non-aqueous electrolyte secondary battery]
In the method for manufacturing a non-aqueous electrolyte secondary battery according to the present embodiment, the positive electrode containing the positive electrode active material having a layered rock salt structure and the power generating element having the above concentrated electrolyte have an upper limit voltage of 4.0 to 4.5 V. There is no particular limitation as long as it includes a step of conditioning within a voltage range. In the present invention, the conditioning means maintaining the power generating element in the voltage range of 4.0 to 4.5 V continuously for 30 minutes or longer.

As an example, first, electrodes (a positive electrode and a negative electrode) are produced. As a method for producing an electrode, an electrode active material slurry is prepared, the electrode active material slurry is applied to the surface of a current collector, dried, and then pressed to produce an electrode active material layer. is mentioned. Then, the obtained positive electrode and negative electrode are laminated with a separator interposed therebetween, and placed inside a pack made of an aluminum laminate film or the like, which is an exterior body. After that, by injecting an electrolytic solution and sealing in a vacuum, the power generating element enclosed in the exterior body can be prepared.

The conditioning is performed in a voltage range with an upper limit voltage of 4.0 to 4.5V. If the voltage is less than 4.0 V, a uniform film having a predetermined thickness may not be formed on the surface of the positive electrode active material, failing to obtain a sufficient effect of improving cycle durability. If it exceeds 4.5 V, a uniform film may not be formed on the surface of the positive electrode active material. In addition, decomposition of the electrolytic solution and oxidative decomposition of the eluted active material film itself tend to occur. As a result, the effect of sufficient cycle durability improvement may not be obtained. Preferably, the conditioning is performed in a voltage range in which the upper limit voltage is 4.0-4.3V.

Conditions for the conditioning are not particularly limited. For example, 2 to 30 charge/discharge cycles including a voltage range in which the upper limit voltage is 4.0 to 4.5V can be applied to the power generation element. By setting the number of cycles to 2 to 30, it is possible to prevent the coating from becoming too thick or uneven. In addition, the decomposition reaction of the film itself is less likely to occur. Therefore, the effects of the present invention can be obtained more remarkably. Preferably, charge/discharge cycles are performed in a voltage range in which the upper limit voltage is 4.0 to 4.3V. More preferably, the number of cycles is 2-10 cycles. At this time, the time to maintain the voltage range of 4.0 to 4.5 V includes at least one step of maintaining the power generation element in the voltage range of 4.0 to 4.5 V continuously for 30 minutes or more. is not particularly limited.

In a preferred embodiment, said conditioning comprises holding at a voltage range of 4.0-4.5V for 1-30 hours. By setting the holding time to 1 to 30 hours, it is possible to prevent the coating from becoming too thick or uneven. In addition, the decomposition reaction of the film itself is less likely to occur. Therefore, the effects of the present invention can be obtained more remarkably. More preferably, the holding time is 2-30 hours, more preferably 2-10 hours.

Although the temperature conditions for conditioning are not particularly limited, they are, for example, 20 to 30.degree.

[Assembled battery]
An assembled battery is configured by connecting a plurality of batteries. More specifically, at least two batteries are used and connected in series or in parallel, or both. By connecting in series or in parallel, it becomes possible to freely adjust the capacity and voltage.

A plurality of batteries can be connected in series or in parallel to form a small detachable assembled battery. A plurality of these detachable small battery packs are further connected in series or in parallel to form a large-capacity, large-capacity battery suitable for vehicle drive power sources and auxiliary power sources that require high volumetric energy density and high volumetric output density. It is also possible to form assembled batteries (battery modules, battery packs, etc.) with output. How many batteries are connected to make an assembled battery, and how many stages of small assembled batteries are stacked to make a large-capacity assembled battery, depends on the battery capacity of the vehicle (electric vehicle) in which it is installed. It should be decided according to the output.

[vehicle]
A battery or an assembled battery formed by combining a plurality of these batteries can be mounted on the vehicle. In the present invention, since a long-life battery with excellent long-term reliability can be configured, a plug-in hybrid electric vehicle with a long EV driving range and an electric vehicle with a long driving range per charge can be configured by mounting such a battery. Batteries or assembled batteries made by combining a plurality of these, for example, in the case of automobiles, hybrid vehicles, fuel cell vehicles, electric vehicles (all four-wheeled vehicles (commercial vehicles such as passenger cars, trucks, buses, light vehicles, etc.) , two-wheeled vehicles (motorcycles) and three-wheeled vehicles) will provide a long-lasting and highly reliable automobile. However, the application is not limited to automobiles, for example, it can be applied to various power sources for other vehicles, such as moving bodies such as trains, and power sources for installation such as uninterruptible power supplies. It can also be used as

EXAMPLES Hereinafter, the present invention will be described in more detail using examples and comparative examples, but the present invention is not limited to the examples below.

<<Preparation of positive electrode active material>>
As a positive electrode active material, the following two commercial products were purchased and prepared.

Positive electrode active material (1): LiNi _0.88 Mn _0.06 Co _0.06 O ₂ , average particle diameter 4.4 μm, crystallite diameter 1 μm or more, roughness factor 3.0, particles consisting of a single crystallite is 30% or more Positive electrode active material (2): LiNi _0.80 Mn _0.10 Co _0.10 O ₂ , average particle size 10.2 μm, crystallite size less than 1 μm, roughness factor 4.3, single The number ratio of particles consisting of one crystallite is less than 10%.

The average particle size and roughness factor of each positive electrode active material were measured by the following methods.

[Measurement method of roughness factor]
First, the powder of the positive electrode active material was observed using a scanning electron microscope (SEM). , and the arithmetic average value of the particle diameters of the particles observed in several tens of fields was calculated as the average particle diameter. Whether or not the crystallite diameter was 1 μm or more was judged from the SEM image.

Next, from the value of the average particle diameter calculated above, the particle volume [m ³ ] and the particle surface area [m ² ] of the spherical particles having the average particle diameter as the particle diameter are calculated, and the obtained particle volume was multiplied by the specific gravity of the active material (here, 4.78 [g/cm ³ ]) to calculate the particle weight [g] of the spherical particles. Then, the "geometric specific surface area [m ² /g] calculated from the average particle diameter" was calculated by dividing the particle surface area calculated above by the particle weight calculated above.

On the other hand, according to JIS Z8830: 2013 (ISO 9277: 2010) "Method for measuring specific surface area of powder (solid) by gas adsorption", measurement was performed using nitrogen gas as an adsorption gas by a static volumetric method. The BET specific surface area [m ² /g] was calculated by analyzing by the point method. Then, the roughness factor was calculated as the ratio of the "BET specific surface area" to the "geometric specific surface area calculated from the average particle diameter" calculated above (=BET specific surface area/geometric specific surface area).

《Production of batteries》
[Example 1]
<Preparation of electrolytic solution>
Li(FSO ₂ ) ₂ N (lithium bis(fluorosulfonyl)imide; LiFSI), which is a lithium salt, was dissolved in dimethyl carbonate (DMC), which is a non-aqueous solvent, at a concentration of 4 mol/L (4 M), and Example electrolytes were prepared. The amount of dissolved lithium salt in the electrolytic solution was 73% when the saturated dissolved amount (here, 5.5 M) was 100%.

<Preparation of positive electrode>
A solid content of 95% by mass of the positive electrode active material (2) prepared above, 3% by mass of conductive carbon black as a conductive aid, and 2% by mass of polyvinylidene fluoride (PVDF) as a binder was prepared. An appropriate amount of N-methyl-2-pyrrolidone (NMP), which is a slurry viscosity adjusting solvent, was added to the solid content and mixed to prepare a cathode active material slurry. Next, the resulting positive electrode active material slurry was applied to one side of an aluminum foil (20 μm thick) as a current collector using a doctor blade, and dried on a hot plate at 80° C. for 1 hour. After that, the porosity of the positive electrode active material layer was controlled to 25% by pressing the obtained laminate using a roll press. Then, it was placed in a vacuum dryer and dried at 130° C. for 8 hours under vacuum conditions to prepare a positive electrode (thickness: 80 μm) of this example.

<Production of negative electrode>
A solid content consisting of 95.5% by mass of graphite (average particle size: 20 μm) as a negative electrode active material, 0.5% by mass of carbon black as a conductive aid, and 4% by mass of polyvinylidene fluoride (PVDF) as a binder. prepared. An appropriate amount of N-methyl-2-pyrrolidone (NMP), which is a slurry viscosity adjusting solvent, was added to the solid content and mixed to prepare a negative electrode active material slurry. Next, the obtained negative electrode active material slurry was applied to one side of a copper foil (20 μm thick) as a current collector, dried and pressed in the same manner as described above, and the negative electrode of this example (thickness: 60 μm).

<Preparation of test cell>
(Cell assembly)
The positive electrode obtained above was cut into a size of 12.4 cm ² and the negative electrode was cut into a size of 13.8 cm ² . Then, an aluminum terminal was welded to the current collector (aluminum foil) of the positive electrode. On the other hand, a nickel terminal was welded to the current collector (copper foil) of the negative electrode.

Next, a separator (Celgard (registered trademark) 3501, polypropylene (PP), thickness 25 μm, manufactured by Celgard) was inserted between the electrode active material layers of the positive electrode and the negative electrode to form a laminate. This laminate is sandwiched between heat-sealable aluminum laminate films (thickness 150 μm) that are the exterior body, and after injecting the electrolytic solution prepared above, the interior of the exterior body is evacuated to a vacuum using a vacuum sealer. After releasing and returning to atmospheric pressure, the pressure is reduced to 99.7% again and sealed, so that the positive electrode active material layer and the negative electrode active material layer are laminated so as to face each other with the separator interposed. A pouch-type lithium ion battery (test cell) having elements was fabricated.

(conditioning)
In order to apply a uniform pressure to the electrode reaction surface, the electrode part of the cell was sandwiched between rubber plates, further sandwiched between flat aluminum plates, and fixed with bolts.

At 25°C, 1C = 220 mAh/g and the charge-discharge cycle was performed at a rate of 0.1C. At this time, the cell voltage was set to 2.5 V as the lower limit voltage and 4.3 V as the upper limit voltage, and 10 cycles were performed. Charging was carried out in CC-CV mode and discharging in CC mode. At this time, rest times between charging and starting of discharging and after discharging and starting of charging were each set to 1 hour (rest time between charging and discharging: 1 hour).

<Evaluation of test cell (measurement of cycle durability)>
A charge-discharge cycle durability test was performed on the test cell prepared and conditioned as described above. Specifically, 100 cycles of charging and discharging were performed at a rate of 0.33 C and a cell voltage range of 2.5 to 4.6 V. Then, the ratio of the discharge capacity at the 100th cycle to the discharge capacity at the 1st cycle was calculated as the capacity retention rate [%]. The results are shown in Table 1 below.

<Observation of the coating layer on the surface of the positive electrode active material>
For the test cell prepared and conditioned as described above, the positive electrode is thinned with a focused ion beam processing device (Nova200 NanoLab manufactured by FEI), and its cross section is observed and composition analyzed by TEM-DEX (JEM-F200 manufactured by JEOL). carried out. The thickness of the coating layer covering the surface of the positive electrode active material was obtained from cross-sectional images of the particles in several fields of view. A site where F and S were detected by EDX was defined as a site covered with the coating layer. The thickness of the coating layer was measured at 20 points on the portion covered with the coating layer, and the average, maximum and minimum values were obtained. The coverage rate was obtained by distinguishing between the portion covered with the coating layer and the exposed portion, and from the ratio thereof. The results are shown in Table 1 below. In Table 1, the thickness of the coating layer shows the average value and the maximum and minimum values as ± values with respect to the average value.

[Examples 2 to 17, Comparative Examples 1 to 6]
A pouch-type lithium ion battery (test cell) of this example was produced in the same manner as in Example 1 described above, except that the electrolyte solution, positive electrode active material, and conditioning conditions were changed as shown in Table 1 below. . The obtained battery was subjected to a cycle durability test, and separately, the coating layer on the surface of the positive electrode active material was observed. The results are shown in Table 1 below.

Conditioning was performed at 25° C. with either the following cycle or hold:
Cycle: 1C = 220 mAh/g with conditioning at a rate of 0.1C. At this time, the lower limit voltage was set to 2.5V and the upper limit voltage was set to 4.0 to 4.6V as the cell voltage, and the test was performed a predetermined number of times (1 to 50 cycles). Charging was carried out in CC-CV mode and discharging in CC mode (1 hour rest time between charging and discharging):
Holding: 1C = 220 mAh/g, conditioning at a rate of 0.1C. After reaching 4.3 V during the initial charge, the voltage was maintained for a predetermined time (1 to 50 hours). Thereafter, discharge was performed in CC mode to a lower limit voltage of 2.5V.

The electrolytic solutions of Comparative Examples 1 and 2 consisted of an equal volume mixture of ethylene carbonate (EC) and diethyl carbonate (DEC), which are non-aqueous solvents, and vinylene carbonate (VC), which is an additive. Electrolytic solutions of Comparative Examples 1 and 2 were prepared by dissolving at a concentration of 0.5 mass % with respect to mass % and dissolving lithium salt LiPF ₆ at a concentration of 1 mol/L (1 M). The amount of dissolved lithium salt in the electrolytic solution was 33% when the saturated dissolved amount of lithium salt (here, 3.0 M) was taken as 100%.

The electrolytic solution of Comparative Example 3 is an equal volume mixture of ethylene carbonate (EC) and ethyl methyl carbonate (EMC), which are nonaqueous solvents, and vinylene carbonate (VC), which is an additive, to 100% by mass of the mixture. Lithium salt Li(FSO ₂ ) ₂ N (lithium bis(fluorosulfonyl)imide; LiFSI) was dissolved at a concentration of 2 mol/L (2M), and the present A comparative electrolytic solution was prepared. The amount of dissolved lithium salt in the electrolytic solution was 38% when the saturated dissolved amount of lithium salt (here, 5.2 M) was taken as 100%.

From the results shown in Table 1, in a battery using a positive electrode active material having a layered rock salt structure and using a concentrated electrolyte, an example in which conditioning was performed in a voltage range in which the upper limit voltage was 4.0 to 4.5 V. It can be seen that 1 to 17 are excellent in cycle durability.

In addition, in the batteries of Examples 1 and 5, the average thickness of the surface of the positive electrode active material was within the range of 9 to 15 nm, and a coating layer derived from the electrolyte was formed, and the coverage was 85. % or more. The uniform formation of a film derived from the electrolyte on the surface of the positive electrode active material promotes structural damage to the positive electrode active material, elution of the transition metal from the surface of the positive electrode active material, and decomposition of the electrolyte on the surface of the positive electrode active material. It is thought that it will become difficult. As a result, it is believed that the battery operates stably even at high voltages and the durability of the battery is improved.

Among them, Examples 1 to 10, in which charge/discharge cycles were performed 2 to 30 times in a predetermined voltage range, exhibited higher durability than Example 16, in which 50 cycles were performed. Further, Examples 11 to 15, in which the retention time at the predetermined voltage was 1 to 30 hours, were higher in durability than Example 17, in which the retention time was 50 hours.

On the other hand, in Comparative Examples 1 to 3 using low-concentration electrolytes, no improvement in cycle durability was observed even when conditioning was performed in the voltage range of the present invention. Also, a uniform coating layer of sufficient thickness was not formed.

Also, in Comparative Examples 4 and 5 in which the concentrated electrolyte solution was used without conditioning, and in Comparative Example 6 in which the voltage during conditioning exceeded 4.5 V, the durability was insufficient and the coating layer was uneven. rice field.

10a stacked secondary battery,
10b bipolar secondary battery 11 current collector,
11a outermost current collector on the positive electrode side,
11b outermost current collector on the negative electrode side,
11′ positive electrode current collector 11″ negative electrode current collector 13 positive electrode active material layer,
15 negative electrode active material layer,
17 electrolyte layer,
19 cell layer,
21 power generation element,
23 bipolar electrodes,
25 positive collector plate (positive tab),
27 negative electrode current collector (negative electrode tab),
29 laminate film,
31 seal.

Claims (14)

A method for producing a non-aqueous electrolyte secondary battery having a power generation element including a positive electrode including a positive electrode active material layer containing a positive electrode active material having a layered rock salt structure, a negative electrode, and an electrolyte layer including an electrolytic solution. hand,
The electrolytic solution contains a non-aqueous solvent and lithium ions at a concentration of 50% or more of the lithium ion concentration corresponding to the saturated concentration of the lithium salt at 25 ° C.,
A method, comprising conditioning the power generation element in a voltage range in which the upper limit voltage is 4.0 to 4.5V. 2. The method according to claim 1, wherein said conditioning includes performing 2 to 30 charge/discharge cycles in a voltage range with an upper limit voltage of 4.0 to 4.5V. 2. The method of claim 1, wherein said conditioning comprises holding in the range of 4.0-4.5V for 1-30 hours. A method according to any one of claims 1 to 3, wherein the lithium salt comprises an imide group-containing anion as a counter anion to the lithium ion. The positive electrode active material according to any one of claims 1 to 4, wherein the roughness factor defined as the ratio of the BET specific surface area to the geometric specific surface area calculated from the average particle size is 3 or less. the method of. The method according to any one of claims 1 to 5, wherein the positive electrode active material has a crystallite diameter of 1 µm or more. 7. The method according to any one of claims 1 to 6, wherein the number ratio of particles composed of a single crystallite to the particles constituting the positive electrode active material is 10% or more. A positive electrode for a non-aqueous electrolyte secondary battery comprising a positive electrode active material layer having a layered rock salt structure and a positive electrode active material layer containing an electrolytic solution,
The electrolytic solution contains a non-aqueous solvent and lithium ions at a concentration of 50% or more of the lithium ion concentration corresponding to the saturated concentration of the lithium salt at 25 ° C.,
A positive electrode for a non-aqueous electrolyte secondary battery, comprising a coating layer containing an element derived from the lithium salt, having a thickness of 9 to 15 nm and a coverage of 85% or more, on the surface of the positive electrode active material. 9. The non-aqueous electrolyte secondary battery according to claim 8, wherein the positive electrode active material has a roughness factor of 3 or less, which is defined as the ratio of the BET specific surface area to the geometric specific surface area calculated from the average particle size. positive electrode. 10. The positive electrode for a non-aqueous electrolyte secondary battery in accordance with claim 8, wherein said positive electrode active material has a crystallite diameter of 1 μm or more. The positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 8 to 10, wherein the number ratio of particles composed of a single crystallite to the particles constituting said positive electrode active material is 10% or more. 12. Any one of claims 8 to 11, wherein the positive electrode active material comprises Li(Ni-Mn-Co)O ₂ or an NMC composite oxide having a composition in which a portion of these transition metals are replaced with other elements. 2. The positive electrode for a non-aqueous electrolyte secondary battery according to item 1. The positive electrode for a non-aqueous electrolyte secondary battery according to any one of claims 8 to 12, wherein the lithium salt contains an imide group-containing anion as a counter anion for the lithium ion. A nonaqueous electrolyte secondary battery comprising a power generation element having the positive electrode for a nonaqueous electrolyte secondary battery according to any one of claims 8 to 13.

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