JP2019087312A - Cathode active material, cathode and lithium secondary battery - Google Patents

Abstract

To raise a low-temperature output of a battery.SOLUTION: A lithium secondary battery 10 comprises: a cathode sheet 13 having a cathode active material 12 formed on a current collector 11; an anode sheet 18 having an anode active material 17 formed on a surface of a current collector 14; a separator 19 provided between the cathode sheet 13 and the anode sheet 18; and a nonaqueous electrolyte 20 which fills between the cathode sheet 13 and the anode sheet 18. The lithium secondary battery 10 includes the cathode active material 12. As to the cathode active material, the value of dQ/dV [AhgV] representing the ratio of a difference between a capacity Q when a capacity per composite oxide in charge/discharge at a rate of 0.1 C under a 20°C-temperature environment with an evaluation cell including a working electrode including a composite oxide having a spinel structure including Li, Mn and M (where M represents one or more of Co and Ni) and a counter electrode of lithium metal is q [mAhg] and a capacity when the capacity is q+5 [mAhg] to a difference of a voltage V satisfies -3<dQ/dV<3 in a whole electric potential range used in the charge/discharge.SELECTED DRAWING: Figure 1

Description

  The invention disclosed herein relates to a positive electrode active material, a positive electrode and a lithium secondary battery.

Conventionally, it is known to use a spinel complex oxide containing Li, Mn and a transition metal as a positive electrode active material of a secondary battery. For example, in Patent Document 1, Li _{1 + x} Mn _2-yz Ni _y M _z O _4-d (wherein, x, y, z, d are −0.3 <x <0.3, 0 <y <0.5, 0 <z <0.3, -0.2 <d <0.2 is satisfied, and M is a transition metal other than Ni, Mn, Cr, Fe, Co and Cu. ing. In this positive electrode active material, the voltage difference between the two peaks obtained from the dQ / dV curve is 30 mV or more in a voltage region of 4.5 V or more based on lithium. In such a thing, it is supposed that the cycle characteristic is more excellent by the crystal structure in which the order of the arrangement | positioning to the crystallographic separate site of Mn ion and Ni ion is low. Further, for example, in Patent Document 2, and LiNi _0.5 Mn _1.5 O ₄ high potential positive electrode active material open circuit voltage of lithium metal reference is more than 4.3V, such as an inorganic phosphoric acid compounds such as Li ₃ PO ₄ The positive electrode active material to contain is disclosed. In this positive electrode active material, it is said that the inorganic phosphoric acid compound can simultaneously suppress the capacity deterioration due to the elution of the transition metal from the positive electrode active material and the suppression of the resistance increase by the phosphate film.

International Publication No. 2015/174225 brochure JP, 2016-62644, A

  By the way, for example, in a vehicle-mounted battery or the like, high output characteristics are required to be maintained even when used at a low temperature of about -30.degree. However, in the patent documents 1 and 2 mentioned above, it is not examined about a low temperature output characteristic, but to raise a low temperature output more was desired.

  The invention disclosed herein has been made to solve such problems, and has as its main object to further increase the low-temperature output of the battery.

In order to achieve the above-mentioned object, the present inventors are a composite oxide having a spinel structure including Li, Mn and M (wherein M is one or more of Co and Ni), and the composite The capacity per complex oxide is q [mAhg ^{-1 in} charge and discharge at a rate of 0.1C in a temperature environment of 20 ° C using an evaluation cell equipped with a working electrode with oxide and a counter electrode of lithium metal. The value of dQ / dV [Ahg ^-1 V ^-1 ], which indicates the ratio of the difference of the capacity Q to the difference of the voltage V between the time of q + 5 [mAhg ^-1 ] and the time of q + 5 [mAhg ^-1 ] When a composite oxide satisfying -3 <dQ / dV <3 in the entire range was used as a positive electrode active material, it was found that the low temperature output of the battery was further enhanced, and the invention disclosed in the present specification was completed.

That is, the positive electrode active material of the present disclosure is
A composite oxide having a spinel structure including Li, Mn and M (wherein M is one or more of Co and Ni),
In charge and discharge at a rate of 0.1 C in a temperature environment of 20 ° C. using an evaluation cell provided with a working electrode provided with the above-described composite oxide and a lithium metal counter electrode, the capacity per composite oxide is q [ mAhg ^-1] between time and q + 5 of [mAhg ^-1] when the capacity Q shows the ratio differential voltage V of the difference, dQ / dV [Ahg ^-1 V to be calculated from the following equation (1) ^- The value of ¹ ] satisfies -3 <dQ / dV <3 in the whole range of the potential range used for charge and discharge.
dQ / dV = (Q (q + 5) -Q (q)) / (V (q + 5) -V (q)) Formula (1)

  The positive electrode of the present disclosure contains the above-described positive electrode active material.

The lithium secondary battery of the present disclosure is
A positive electrode containing the above-described positive electrode active material,
A negative electrode containing a negative electrode active material,
An ion conducting medium interposed between the positive electrode and the negative electrode for conducting lithium ions;
Is provided.

  The invention disclosed herein can further enhance the low temperature output of the battery. The reason why such effects can be obtained is presumed as follows. For example, although the positive electrode active material of the present disclosure has a low peak satisfying -3 <dQ / dV <3 in the dQ / dV curve in which the voltage V is taken along the horizontal axis and the aforementioned dQ / dV is taken along the vertical axis, In this case, it is considered that the single phase complex oxide is charged and discharged by continuously changing the composition, that is, charged and discharged in a single phase reaction. On the other hand, in the case of sharp peaks such that dQ / dV is dQ / dV <-3 or 3 <dQ / dV, the composite oxide of two phases is charged / discharged by changing the ratio, that is, It is considered that charge and discharge are caused by two-phase coexistence reaction and the like. And in two phase coexistence reaction etc., although phase boundary diffusion of lithium ion is considered to be rate-limiting of reaction, the speed of phase boundary diffusion is slower than the speed of lithium ion transport. On the other hand, in the one-phase reaction, there is no such phase boundary diffusion, and since it becomes rate-limiting of lithium ion transport faster than the phase boundary diffusion, it is considered that the output can be increased.

FIG. 2 is a schematic view showing an example of a lithium secondary battery 10; Charge-discharge curve of the bipolar evaluation cell of Experimental example 1. Charge-discharge curve of the bipolar evaluation cell of Experimental example 2. Charge-discharge curve of the bipolar evaluation cell of Experimental example 3. Charge-discharge curve of the bipolar evaluation cell of Experimental example 4. DQ / dV curve of the bipolar evaluation cell of Experimental example 1. FIG. DQ / dV curve of the bipolar evaluation cell of Experimental example 2. FIG. DQ / dV curve of the bipolar evaluation cell of Experimental example 3. FIG. DQ / dV curve of the bipolar evaluation cell of Experimental example 4;

The positive electrode active material of the present disclosure is a composite oxide having a spinel structure including Li, Mn and M (wherein M is one or more of Co and Ni). This positive electrode active material is a composite oxide in charge and discharge at a rate of 0.1 C in a temperature environment of 20 ° C. using an evaluation cell provided with a working electrode provided with the above composite oxide and a counter electrode of lithium metal. DQ / dV calculated from the following equation (1), which indicates the ratio of the voltage V to the difference of the capacity Q when the capacity of the terminal is q [mAhg ^-1 ] and q + 5 [mAhg ^-1 ] The value of [Ahg ⁻¹ V ⁻¹ ] satisfies −3 <dQ / dV <3 in the entire range of the potential range used for charge and discharge. In addition, dQ / dV shows a positive value at the time of charge, and shows a negative value at the time of discharge. The potential range used for charge and discharge can be, for example, a potential range of 3 V or more based on lithium, more specifically, a potential range of 3 V or more and 5 V or less.
dQ / dV = (Q (q + 5) -Q (q)) / (V (q + 5) -V (q)) Formula (1)

  In the dQ / dV curve, sharp peaks for which dQ / dV does not satisfy -3 <dQ / dV <3 are charged / discharged by changing the ratio of the composite oxide of two phases, that is, two-phase coexistence reaction In many cases, it is confirmed by an active material that charges and discharges. On the other hand, as shown by the positive electrode active material of the present disclosure, a small peak with dQ / dV satisfying -3 <dQ / dV <3 indicates that the single-phase composite oxide continuously changes the composition to charge and discharge. In other words, it is often confirmed with an active material that charges and discharges in a one-phase reaction. The relationship between the dQ / dV curve and the type of reaction is described, for example, in T. Ohzuku, A. Ueda, J. Electrochem. Soc., 144, 2780-2785.

  The positive electrode active material preferably has a lattice constant of 8.20 Å or more. If the lattice constant is 8.20 Å or more, the low temperature output characteristics can be further enhanced even when M is Ni. The upper limit of the lattice constant is not particularly limited, but may be, for example, 8.26 Å or less.

  The positive electrode active material has a value of MOL (Li) / MOL (Me) which is a ratio of the number of moles of lithium MOL (Li) to the total number of moles of transition metal element Me (Me is M and Mn) MOL (Me) It is preferable that it is 0.90 or more and 1.00 or less, and it is more preferable that it is 0.95 or more and 0.99 or less. In such a thing, a low temperature output characteristic can be raised more. In addition, in the positive electrode active material, the value of MOL (M) / MOL (Mn), which is the ratio of the number of moles of M M (M) of M to the number of moles M L (Mn), is 3/17 to 7/13. Is preferable, and 1/3 is more preferable.

The positive electrode active material has a basic composition formula Li _x M _y Mn _2-y O _z (where x, y and z are 0.9 ≦ x ≦ 1, 0 <y ≦ 1, 3.7 ≦ z ≦ 4. It may be expressed by satisfying 0). Among these, x preferably satisfies 0.95 ≦ x ≦ 0.99, and z preferably satisfies 3.80 ≦ z ≦ 3.85. In such a case, even when M is Ni, the low temperature output characteristics can be further enhanced. In addition, y preferably satisfies 0.3 ≦ y ≦ 0.7, and more preferably 0.45 ≦ y ≦ 0.55. The term "basic composition formula" is intended to mean that part of the elements may be replaced with another element (for example, Al or Mg). The positive electrode active material may be, for example, LiCoMnO ₄ , Li _0.95 Ni _0.5 Mn _1.5 O _3.80 , or the like.

  The composition of the positive electrode active material can be identified as follows. First, composition analysis is performed on the positive electrode active material using inductively coupled plasma emission spectroscopy (ICP-AES) to determine the amount of lithium and the amount of transition metal in the positive electrode active material. Next, the average oxidation number of the transition metal element Me is determined by redox titration using iodine. From the amount of lithium thus determined, the amount of transition metal, the average oxidation number of the transition metal, and the oxidation number 1 of lithium, the amount of oxygen is determined so as to balance the charge.

The redox titration using iodine is described below. When the positive electrode active material is dissolved in an acidic aqueous solution and mixed with a potassium iodide solution, nickel, cobalt, and manganese higher than the divalent in the positive electrode active material are reduced to Ni ²⁺ , Co ²⁺ , and Mn ²⁺ . while being, reduced amount with an equal volume of I ^- is oxidized I ₂ generates. The generated I ₂ amount is identified by titration with sodium thiosulfate standard solution of starch solution as an indicator _{(Na 2 S 2 O 3)} . The titration should be performed in the following order.
(1) Put a ball filter into a 200 mL Erlenmeyer flask and replace nitrogen gas with 3 L / min for 5 minutes.
(2) Add 10 mL of KI (100 g / L) solution and 20 mL of HCl (a mixture of hydrochloric acid [reagent grade] and distilled water in a volume ratio of 1: 1) in an Erlenmeyer flask.
(3) Weigh out 100 mg of the positive electrode active material at ± 0.1 mg.
(4) The positive electrode active material is transferred into an Erlenmeyer flask, sealed, and dissolved by heating at 70 ° C. while stirring with a rotor.
(5) After cooling the Erlenmeyer flask with running water, add 80 mL of distilled water from which residual oxygen has been removed.
(6) Titrate quickly with a 0.05 mol / L sodium thiosulfate solution until the solution turns brown to pale yellow.
(7) Add 0.5 mL of starch solution to make it purple.
(8) Subsequently, titration is performed with 0.05 mol / L sodium thiosulfate, and the point where purple color disappears is used as an end point, and the average oxidation number of transition metal ions is calculated based on the following formulas (2) to (4).
^{Me n + + (n-2} ) I - → Me 2+ +1/2 (n-2) I 2 ··· formula (2)
_{I 2 + 2S 2 O 3 →} 2I - + S 4 O 6 2- ··· formula (3)
Average oxidation number = I ₂ (mol) / Me (mol) + 2 ... Formula (4)

  The surface of the positive electrode active material may be coated with lithium phosphate. In such a thing, low temperature output can be raised more. Although the coating amount of lithium phosphate is not particularly limited, it is more preferable to coat the surface of the positive electrode active material with lithium phosphate in the range where the amount of phosphorus is 5 mol% or more and 40 mol% or less based on the number of moles of the positive electrode active material. .

  Next, a method of manufacturing a positive electrode active material will be described. The manufacturing method may include (1) a raw material preparation step and (2) a baking step. In addition, the raw material prepared beforehand may be prepared and a raw material preparation process may be abbreviate | omitted. Also, the lithium phosphate coating step may be omitted. Moreover, the positive electrode active material of this indication is not limited to what was obtained by such a manufacturing method.

(1) Raw material preparation process At this process, the raw material of a positive electrode active material is prepared. As the raw material, the substance to be added and the amount thereof may be selected according to the desired composition of the positive electrode active material described above. The raw material composition is, for example, a basic composition formula Li _x M _y Mn _2-y O _z (where M is one or more of Co and Ni, x, y and z are 0.9 ≦ x ≦ 1, Li, M and Mn may be mixed so as to obtain a positive electrode active material of 0 <y ≦ 1, 3.7 ≦ z ≦ 4.0. At this time, the value of Mol (Li) / Mol (Me), which is the ratio of the number of moles of lithium Mol (Li) in the raw material to the total number of moles Mol (Me) of the transition metal element Me (Me is M and Mn). It is preferable to mix so that it may be 0.90 or more and 1.00 or less, and it is more preferable to mix so that it may be 0.95 or more and 0.99 or less. In addition, the value of Mol (M) / Mol (Mn), which is the ratio of the number of moles of M (M) to the number of moles of Mn (M) in the raw material, is 3/17 to 7/13. Preferably, 1/3 is more preferred. The raw material of the transition metal is preferably synthesized by coprecipitation method. This is because metallic elements can be uniformly mixed at the atomic level, and more preferable performance can be obtained. In the coprecipitation method, a precursor in which transition metal ions are allowed to coexist in one particle may be prepared and mixed with a lithium salt. When obtaining a precursor in which metal ions are uniformly distributed by coprecipitation method, it is preferable to remove dissolved oxygen by bubbling an inert gas into an aqueous solution. The precursor and the lithium salt may be hydroxides, carbonates, citrates or the like, and sparingly soluble salts in which elements are uniformly mixed at the atomic level are preferable. With complexing agents, it is also possible to make higher density precursors. The raw material of the precursor may be any material that can form a precursor using a basic aqueous solution, but metal salts having high solubility are preferable. For example, nickel sulfate, nickel nitrate, nickel acetate, nickel hydroxide, nickel carbonate, basic nickel carbonate and the like can be used as a nickel source. As a cobalt source, cobalt sulfate, cobalt nitrate, cobalt acetate, cobalt hydroxide, cobalt carbonate, basic cobalt carbonate and the like can be used. Further, as a manganese source, manganese sulfate, manganese nitrate, manganese acetate, manganese oxide, manganese carbonate or the like can be used. As a lithium salt to be mixed with the precursor, lithium hydroxide, lithium carbonate, lithium nitrate, lithium acetate or the like can be used.

(2) Firing Step In this step, the raw material obtained above is fired. In the firing treatment, the raw material is fired at a temperature range of 700 ° C. to 1100 ° C., preferably 700 ° C. to 900 ° C., more preferably 750 ° C. to 850 ° C. When the firing is performed at 900 ° C. or less, there is almost no loss of Li due to the firing, and the raw material Mol (Li) / Mol (Me) is maintained after the firing. The firing time may be, for example, in the range of 5 hours to 24 hours. The firing atmosphere may be an oxidizing atmosphere such as an air atmosphere or an inert atmosphere such as an argon atmosphere, but an inert atmosphere is preferable. If firing is performed in an inert atmosphere, it is considered that oxygen deficiency can be generated in the crystal structure even at a low temperature such as 900 ° C. or less. In addition, it is considered that when firing is performed in an inert atmosphere, the average oxidation number of transition metal ions can be set to a more preferable range, and the lattice constant can be set to a more preferable range. Through such processing, the positive electrode active material of the present disclosure can be obtained.

  Next, the positive electrode of the present disclosure will be described. This positive electrode contains the positive electrode active material described above. This positive electrode is prepared, for example, by mixing a positive electrode active material, a conductive material, and a binder, adding a suitable solvent to form a paste-like positive electrode mixture, and coating and drying it on the surface of the current collector. It may be compressed to increase the electrode density. The positive electrode active material is the positive electrode active material described above. The conductive material is not particularly limited as long as it is an electron conductive material that does not adversely affect the cell performance of the positive electrode, and, for example, graphite such as natural graphite (flaky graphite, scaly graphite) or artificial graphite, acetylene black, carbon black It is possible to use one or a mixture of two or more of ketjen black, carbon whisker, needle coke, carbon fiber, metal (copper, nickel, aluminum, silver, gold and the like). Among these, carbon black and acetylene black are preferable as the conductive material from the viewpoint of electron conductivity and coatability. The binder plays a role of holding the active material particles and the conductive material particles, and is, for example, a fluorine-containing resin such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), fluororubber, or polypropylene. A thermoplastic resin such as polyethylene, ethylene propylene diene monomer (EPDM) rubber, sulfonated EPDM rubber, natural butyl rubber (NBR) or the like can be used alone or as a mixture of two or more. In addition, an aqueous dispersion of a cellulose based or styrene butadiene rubber (SBR), which is an aqueous binder, can also be used. As the current collector, aluminum, titanium, stainless steel, nickel, iron, calcined carbon, conductive polymer, conductive glass, etc., aluminum, copper, etc. for the purpose of improving adhesion, conductivity and oxidation resistance The surface of the above may be treated with carbon, nickel, titanium, silver or the like. The thickness of the current collector is, for example, 1 to 500 μm.

  Next, the lithium secondary battery of the present disclosure will be described. The lithium secondary battery includes a positive electrode containing the above-described positive electrode active material which occludes and releases lithium ions, a negative electrode containing the negative electrode active material which occludes and releases lithium ions, and the positive electrode and the negative electrode. And an ion conducting medium for conducting lithium ions. In this lithium secondary battery, the positive electrode contains the above-described positive electrode active material, and for example, the above-described positive electrode can be used.

  The negative electrode is, for example, a mixture of a negative electrode active material and a binder, and an appropriate solvent added to form a paste-like negative electrode mixture, which is coated and dried on the surface of the current collector, and the electrode density is adjusted if necessary. It may be compressed to increase. Examples of negative electrode active materials include inorganic compounds such as lithium, lithium alloys and tin compounds, carbonaceous materials capable of absorbing and desorbing lithium ions, lithium titanium composite oxides, conductive polymers and the like, among which carbon Quality materials are preferable in terms of safety. The carbonaceous material is not particularly limited, and examples thereof include cokes, glassy carbons, graphites, non-graphitizable carbons, pyrolytic carbons, carbon fibers and the like. Among them, graphites such as artificial graphite and natural graphite have an operating potential close to that of metal lithium and can be charged and discharged at a high operating voltage, and when self-discharge is used when lithium salt is used as an electrolyte salt, And since irreversible capacity at the time of charge can be decreased, it is desirable. As the conductive material, the binder, the solvent, and the like used for the negative electrode, those exemplified for the positive electrode can be used. In the current collector of the negative electrode, in addition to copper, nickel, stainless steel, titanium, aluminum, calcined carbon, conductive polymer, conductive glass, Al-Cd alloy, etc., adhesion, conductivity and reduction resistance are improved. For the purpose, for example, one obtained by treating the surface of copper or the like with carbon, nickel, titanium, silver or the like can be used. For these, it is also possible to oxidize the surface. The shape of the current collector can be the same as that of the positive electrode.

  The ion conductive medium may be a non-aqueous electrolyte containing a lithium-containing support salt and a non-aqueous solvent. The non-aqueous solvent includes carbonates, esters, ethers, nitriles, furans, sulfolanes and dioxolanes, and these can be used alone or in combination. Specifically, as carbonates, cyclic carbonates such as ethylene carbonate, propylene carbonate, vinylene carbonate, butylene carbonate, chloroethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, ethyl-n-butyl carbonate, methyl-t -Linear carbonates such as -butyl carbonate, di-i-propyl carbonate, t-butyl-i-propyl carbonate, cyclic esters such as γ-butyl lactone and γ-valerolactone, methyl formate, methyl acetate, ethyl acetate, Chain esters such as methyl butyrate; ethers such as dimethoxyethane, ethoxymethoxyethane and diethoxyethane; nitriles such as acetonitrile and benzonitrile; Examples include furans such as orchid, methyltetrahydrofuran and the like, sulfolanes such as sulfolane and tetramethyl sulfolane, and dioxolanes such as 1,3-dioxolane and methyldioxolane. Among these, a combination of cyclic carbonates and linear carbonates is preferred. According to this combination, not only the cycle characteristics representing the battery characteristics in repetition of charge and discharge are excellent, but also the viscosity of the electrolytic solution, the electric capacity of the obtained battery, the battery output and the like can be balanced. it can.

The supporting salt is, for example, LiPF ₆ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiSbF ₆ , LiSiF ₆ , LiSiF ₆ , LiAlF ₄ , LiSCN, LiClO ₄ LiCl, LiF, LiBr, LiI, LiAlCl _{4 and the} like. Among them, inorganic salts such as LiPF ₆ , LiBF ₄ , LiAsF ₆ and LiClO ₄ , and organic salts such as LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ and LiC (CF ₃ SO ₂ ) ₃ It is preferred from the viewpoint of the electrical properties to use one or more selected salts in combination. The concentration of the supporting salt in the ion conductive medium is preferably 0.1 mol / L to 5 mol / L, and more preferably 0.5 mol / L to 2 mol / L. When the concentration at which the supporting salt is dissolved is 0.1 mol / L or more, a sufficient current density can be obtained, and when the concentration is 5 mol / L or less, the electrolytic solution can be made more stable.

  Also, instead of the liquid ion conducting medium, a solid ion conducting polymer can be used as the ion conducting medium. As the ion conductive polymer, for example, a polymer gel composed of a polymer such as acrylonitrile, ethylene oxide, propylene oxide, methyl methacrylate, vinyl acetate, vinyl pyrrolidone, vinylidene fluoride and the like and a support salt can be used. Furthermore, the ion conductive polymer and the non-aqueous electrolyte can be used in combination. In addition to the ion conductive polymer, an inorganic solid electrolyte, a mixed material of an organic polymer electrolyte and an inorganic solid electrolyte, or an inorganic solid powder bound with an organic binder can be used as the ion conductive medium.

  The lithium secondary battery may include a separator between the positive electrode and the negative electrode. The separator is not particularly limited as long as it has a composition that can withstand the use range of the lithium secondary battery, but, for example, polymer nonwoven fabrics such as polypropylene nonwoven fabrics and polyphenylene sulfide nonwoven fabrics, and thin fine particles of olefin resins such as polyethylene and polypropylene A porous membrane is mentioned. These may be used alone or in combination of two or more.

The shape of the lithium secondary battery is not particularly limited, and examples thereof include coin type, button type, sheet type, laminated type, cylindrical type, flat type, and square type. In addition, the present invention may be applied to a large one used for an electric car or the like. The lithium secondary battery may be applied to portable terminals, portable electronic devices, compact power storage devices, electric vehicles, hybrid electric vehicles and the like. FIG. 1 is a schematic view showing an example of the lithium secondary battery 10 of the present disclosure. The lithium secondary battery 10 includes a positive electrode sheet 13 having a positive electrode active material 12 formed on a current collector 11, a negative electrode sheet 18 having a negative electrode active material 17 formed on the surface of a current collector 14, a positive electrode sheet 13 and a negative electrode sheet. And a non-aqueous electrolytic solution 20 filled between the positive electrode sheet 13 and the negative electrode sheet 18. In this lithium secondary battery 10, the separator 19 is sandwiched between the positive electrode sheet 13 and the negative electrode sheet 18, and these are wound and inserted into the cylindrical case 22, and the positive electrode terminal 24 connected to the positive electrode sheet 13 and the negative electrode sheet It is formed by arranging the connected negative electrode terminal 26. The lithium secondary battery 10 is a composite oxide having a spinel structure including Li, Mn and M (where M is one or more of Co and Ni), and a working electrode comprising the composite oxide Charge and discharge at a rate of 0.1 C in a temperature environment of 20 ° C. using an evaluation cell equipped with a metal and a counter electrode of lithium metal, and the capacity per complex oxide is q [mAh g ⁻¹ ] and q + 5 [mAh g ^{The value of} dQ / dV [Ahg ^-1 V ^-1 ], which indicates the ratio of the difference in capacitance Q to the difference in voltage V from that of ^-1 , is -3 <dQ in the entire range of potentials used for charging and discharging The positive electrode active material 12 satisfying / dV <3 is provided.

  In the positive electrode active material, the positive electrode, and the lithium secondary battery described in detail above, the output characteristics at low temperatures can be further enhanced. The reason why such effects can be obtained is presumed as follows. For example, since this positive electrode active material charges and discharges in a single phase reaction (for example, in a potential region of 3 V or more based on lithium used as a positive electrode active material) in the entire range of potential range used for charge and discharge Unlike the two-phase co-reaction where diffusion is the rate-limiting reaction, it is thought that there is no phase boundary diffusion. For this reason, it is assumed that lithium ion transport faster than phase boundary diffusion becomes reaction rate-limiting and the output increases.

  Further, in the positive electrode active material of the present disclosure, when the surface is coated with lithium phosphate, the low temperature output characteristics can be further enhanced. The reason why such an effect is obtained is presumed to be because, for example, the intrasolid transport of lithium ions and the electrochemical reaction on the surface of active material particles are promoted by the presence of lithium phosphate. In particular, in the case of the positive electrode active material of the present disclosure which is supposed to be charged / discharged in a one-phase reaction, the lattice constant changes continuously as a whole during charge / discharge. It is guessed to be remarkable. In the positive electrode active material that is charged and discharged in the two-phase coexistence reaction, the ratio of the large phase to the small phase of the lattice constant changes during charge and discharge, so that the volume fluctuation is large and the film is likely to be distorted. It is presumed that the substance is less likely to cause distortion in the film.

  It is needless to say that the present disclosure is not limited to the above-described embodiment at all, and may be implemented in various aspects within the technical scope of the present disclosure.

  Below, the example which produced the lithium secondary battery of this indication concretely is demonstrated as an Example. Experimental Examples 3 to 9 correspond to Examples, and Experimental Examples 1, 2 and 10 correspond to Comparative Examples.

(Synthesis of positive electrode active material)
[Experimental Example 1]
(1) Raw material preparation step In the ion-exchanged water from which dissolved oxygen has been removed by bubbling an inert gas in advance, nickel sulfate and manganese sulfate each have a molar ratio of Ni and Mn of 0.25: 0.75. The mixed aqueous solution was adjusted so that the total molar concentration of these metal elements was 2 mol / L. On the other hand, 2 mol / L sodium hydroxide aqueous solution and 0.352 mol / L ammonia water were respectively adjusted using ion exchange water from which dissolved oxygen was removed similarly. When ion-exchanged water from which dissolved oxygen has been removed is placed in a reaction tank set to a tank temperature of 50 ° C and stirred at 800 rpm, a sodium hydroxide aqueous solution is added dropwise thereto and the liquid temperature is based on 25 ° C The pH was adjusted to 12. A mixed aqueous solution, an aqueous solution of sodium hydroxide and aqueous ammonia were added to the reaction vessel while controlling the pH to 12 to obtain a composite hydroxide of the coprecipitated product. The aqueous solution of sodium hydroxide alone was appropriately added to keep the pH at 12, and stirring was continued for 2 hours. Thereafter, the composite hydroxide was allowed to grow in particles by resting at 60 ° C. for 12 hours. After completion of the reaction, the composite hydroxide was filtered, washed with water and taken out, and dried overnight in an oven at 120 ° C. to obtain a powder sample of the composite hydroxide. The obtained composite hydroxide powder (precursor) and lithium hydroxide powder (lithium salt) were mixed so that the value of Mol (Li) / Mol (Me) would be 1.05. The mixed powder was pressure-molded at a pressure of 6 MPa into pellets having a diameter of 2 cm and a thickness of about 5 mm.

(2) Firing step The pellet is heated at a temperature of 1000 ° C. at 10 ° C./min in an electric furnace in an air atmosphere, fired at 1000 ° C. for 12 hours, allowed to naturally cool, and cooled down to 700 ° C. Annealed for time. After firing, the heater was turned off and allowed to cool naturally. After about 8 hours, it was confirmed that the temperature in the furnace became 100 ° C. or less, and the pellet was taken out. Thus, a highly crystalline positive electrode active material was synthesized. This was used as the positive electrode active material of Experimental Example 1.

[Experimental Example 2]
In the firing step of Experimental Example 1, the temperature of the pellet was raised at 10 ° C./min to a temperature of 1000 ° C. in an electric furnace under an argon atmosphere, fired at 1000 ° C. for 12 hours, and naturally cooled to synthesize a positive electrode active material. The other conditions were the same as in Experimental Example 1.

[Experimental Example 3]
In the raw material preparation step of Experimental example 1, cobalt sulfate is used instead of nickel sulfate, and cobalt sulfate and manganese sulfate are ion-exchanged water so that each element of Co and Mn has a molar ratio of 0.5: 0.5. It was dissolved in The other conditions were the same as in Experimental Example 1.

[Experimental Example 4]
In the raw material preparation step of Experimental example 1, the precursor and the lithium salt were mixed so that the value of Mol (Li) / Mol (Me) would be 0.95. Further, in the firing step, the temperature of the pellet was raised at 10 ° C./min to a temperature of 850 ° C. in an electric furnace under an argon atmosphere, and firing was carried out at 850 ° C. for 16 hours, followed by natural cooling to synthesize a positive electrode active material. The other conditions were the same as in Experimental Example 1.

[Experimental Example 5]
In the raw material preparation step of Experimental example 4, the precursor and the lithium salt were mixed so that the value of Mol (Li) / Mol (Me) would be 0.97. In the firing step, the firing conditions were set to 800 ° C. for 12 hours. The other conditions were the same as in Experimental Example 4.

[Experimental Example 6]
In the raw material preparation step of Experimental example 4, the precursor and the lithium salt were mixed such that the value of Mol (Li) / Mol (Me) was 0.99. In the firing step, the firing conditions were set to 750 ° C. for 12 hours. The other conditions were the same as in Experimental Example 4.

[Experimental Example 7]
Lithium phosphate was coated on the positive electrode active material of Experimental Example 4 as follows. 0.48 g of lithium hydroxide, 0.44 g of NH ₄ H ₂ PO ₄ in 1 L of ion-exchanged water containing 10 g of a positive electrode active material (the pellet is crushed), a molar ratio of 3 to 1 As it was dissolved. The aqueous solution was allowed to stir at 50 ° C. for 24 hours, dried by a rotary evaporator, and then heat-treated at 450 ° C. in air. The amount of phosphorus on the surface of the positive electrode active material was quantified by ICP-AES. The amount of lithium phosphate coated was 6 mol% with respect to the number of moles of the positive electrode active material.

[Experimental Example 8]
In the lithium phosphate-coating step of Experimental Example 7, 1.79 g of lithium hydroxide and 1.64 g of NH ₄ H ₂ PO ₄ were used. The other conditions were the same as in Experimental Example 7. The coating amount of lithium phosphate was 19 mol% with respect to the number of moles of the positive electrode active material.

[Experimental Example 9]
In the lithium phosphate-coating step of Experimental Example 7, 2.76 g of lithium hydroxide and 2.52 g of NH ₄ H ₂ PO ₄ were used. The other conditions were the same as in Experimental Example 7. The coating amount of lithium phosphate was 39 mol% with respect to the number of moles of the positive electrode active material.

[Experimental Example 10]
Lithium phosphate was coated on the positive electrode active material of Experimental Example 1 as in Experimental Example 8. The coating amount of lithium phosphate was 18 mol% with respect to the number of moles of the positive electrode active material.

(Identification of chemical composition of positive electrode active material)
The chemical composition of the positive electrode active material was identified as follows. First, composition analysis was performed on the positive electrode active material using ICP-AES to determine the amount of lithium and the amount of transition metal in the positive electrode active material. Next, the average oxidation number of the transition metal element Me was determined by redox titration using iodine. From the amount of lithium, the amount of transition metal, the average oxidation number of transition metals, and the oxidation number of lithium 1 thus determined, the amount of oxygen was determined so as to balance the charge.

The redox titration using iodine was performed in the following order.
(1) A ball filter was placed in a 200 mL Erlenmeyer flask, and nitrogen gas was replaced with 3 L / min for 5 minutes.
(2) 20 mL of 10 mL of KI (100 g / L) solution and 20 mL of HCl (a mixture of hydrochloric acid (reagent grade) and distilled water in a volume ratio of 1: 1) were added to an Erlenmeyer flask.
(3) 100 mg of the positive electrode active material was weighed at ± 0.1 mg.
(4) The positive electrode active material was transferred into an Erlenmeyer flask, sealed, and dissolved by heating at 70 ° C. while stirring with a rotor.
(5) After cooling the Erlenmeyer flask with running water, 80 mL of distilled water from which residual oxygen was removed was added.
(6) The solution was quickly titrated with 0.05 mol / L sodium thiosulfate until it turned brown to pale yellow.
(7) 0.5 mL of starch solution was added to make it purple.
(8) Subsequently, titration was performed with 0.05 mol / L sodium thiosulfate, and the point at which purple color disappeared disappeared was an end point, and the average oxidation number of transition metal ions was calculated based on the following formulas (2) to (4).
^{Me n + + (n-2} ) I - → Me 2+ +1/2 (n-2) I 2 ··· formula (2)
_{I 2 + 2S 2 O 3 →} 2I - + S 4 O 6 2- ··· formula (3)
Average oxidation number = I ₂ (mol) / Me (mol) + 2 ... Formula (4)

(X-ray diffraction measurement)
Powder X-ray diffraction measurement was performed on the obtained positive electrode active material. The measurement was performed using an X-ray diffractometer (Ultima IV, Rigaku) using CuKα radiation (wavelength 1.54051 Å) as radiation. Measurement was performed using a single crystal monochromator of graphite and setting an applied voltage to 40 kV and a current of 40 mA for monochromating X-rays. Also, the measurement was performed at a scanning speed of 5 ° / min, and recording was performed in an angle range of 10 ° to 100 ° (2θ). As a result of analysis using structural analysis software (Rietan-FP), it was found that the positive electrode active material is a cubic crystal of space group Fd3m. Further, it was indexed with cubic crystals of space group Fd 3 m, and the lattice constant was calculated by the least squares method to be 8.168 Å.

(Production of coated electrode)
A positive electrode mixture containing 85% by mass of the obtained positive electrode active material, 10% by mass of carbon black as a conductive material, and 5% by mass of polyvinylidene fluoride as a binder, and an appropriate amount of N-methyl-2-pyrrolidone as a dispersing agent A slurry-like mixture was obtained by adding and dispersing. The slurry-like mixture was uniformly applied to a 15 μm thick aluminum foil current collector, and dried by heating to prepare a coated sheet. Thereafter, the coated sheet was passed through a roll press to densify it to produce a coated electrode.

(Preparation of bipolar evaluation cell)
The coated electrode produced by the above method was punched into an area of 2.05 cm ² to prepare a disk-shaped electrode. Using this electrode as a working electrode and a lithium metal foil (thickness 300 μm) as a counter electrode, a bipolar separator was made to sandwich a polyethylene separator impregnated with a non-aqueous electrolyte between both electrodes, to prepare a bipolar evaluation cell. Dissolve LiPF ₆ at a concentration of 1 M in a mixed solvent in which ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) are mixed in a volume ratio of 30/40/30 in the non-aqueous electrolytic solution. Used.

(Charge and discharge test)
Using the bipolar evaluation cell, a charge / discharge test was performed at a rate of 0.1 C in a range of 5.0 to 3.0 V under a temperature environment of 20 ° C. Then, the value of dQ / dV was calculated based on the following (1) at a capacity interval of 5 [mAh g ⁻¹ ] per composite oxide. Then, the maximum value (dQ / dV) max and the minimum value (dQ / dV) min of dQ / dV were determined. In addition, dQ / dV showed a positive value at the time of charge, and showed a negative value at the time of discharge.
dQ / dV = (Q (q + 5) -Q (q)) / (V (q + 5) -V (q)) Formula (1)

(Preparation of lithium ion secondary battery)
The coated electrode produced by the said method was cut out in the shape of 120 mm width x 100 mm length, and it was set as the positive electrode sheet. On the other hand, graphite was used as the negative electrode active material, 95% by mass of the active material, and 5% by mass of polyvinylidene fluoride as the binder were mixed to form a slurry-like mixture in the same manner as the positive electrode. The slurry-like mixture was uniformly applied to a 10 μm thick copper foil current collector, and dried by heating to prepare a coated sheet. Thereafter, the coated sheet was passed through a roll press to densify it, and cut into a shape of 122 mm wide × 102 mm long to obtain a negative electrode sheet. The obtained positive electrode sheet and the negative electrode sheet were opposed to each other with a 25 μm-thick polyethylene separator interposed therebetween to prepare a laminated electrode body. The electrode body was sealed in an aluminum laminate type bag, impregnated with a non-aqueous electrolyte, and sealed to prepare a lithium secondary battery. Dissolve LiPF ₆ at a concentration of 1 M in a mixed solvent in which ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) are mixed in a volume ratio of 30/40/30 in the non-aqueous electrolytic solution. Used.

(Low temperature output characteristics test)
After adjusting to 70% (SOC = 70%) of the battery capacity at -30 ° C using the above lithium ion secondary battery, current was applied at various current values, and the battery voltage after 2 seconds was measured. The flowed current and voltage were linearly interpolated, the current value when the voltage after 2 seconds became 3.0 V was determined, and the output power was determined from the product of the current and the voltage.

(Experimental result)
The charge / discharge curves of Experimental Examples 1 to 4 are shown in FIGS. 2 to 5 and the dQ / dV curves are shown on FIGS. The chemical composition, lattice constant, (dQ / dV) max, (dQ / dV) min, and low-temperature output characteristics are shown in Table 1 for Experimental Examples 1 to 10. In Table 1, the low-temperature output characteristic is a value of the output power normalized with the value of the output power of Experimental Example 1 as one.

  In Experimental Example 1, as shown in FIG. 6, a sharp peak not satisfying -3 <dQ / dV <3 which is characteristic of two-phase coexistence reaction is confirmed, and two-phase coexistence reaction in the whole potential range charged and discharged. It was guessed that the battery was charged and discharged. The lattice constant of the positive electrode active material was 8.168 Å, and the lattice constant was less than 8.20 Å. In the experimental example 1, the low temperature output characteristics were particularly low.

  In Experimental Example 2, as shown in FIG. 7, although a relatively broad peak satisfying -3 <dQ / dV <3 characteristic of one-phase reaction was confirmed around 4.65V, around 4.75V. A sharp peak characteristic of a two-phase coexistence reaction not satisfying this relationship was confirmed in. That is, it was speculated that this material was charged and discharged in a one-phase reaction on the low potential side, but was charged and discharged in a two-phase coexistence reaction on the high potential side. The lattice constant of the positive electrode active material was 8.192 Å, which was less than 8.20 Å. The low temperature output characteristics of Experimental Example 2 were similar to the results of Experimental Example 1.

  In Experimental Example 3, as shown in FIG. 8, a relatively broad peak satisfying -3 <dQ / dV <3 characteristic of one-phase reaction was confirmed. That is, it was inferred that this material was charged and discharged in a one-phase reaction in the entire potential range of charge and discharge. In Experimental Example 3, although the lattice constant was less than 8.20 Å, relatively good low temperature output characteristics were exhibited.

  In Experimental Example 4, as shown in FIG. 9, a broad peak satisfying -3 <dQ / dV <3 characteristic of one-phase reaction was confirmed. The same charge-discharge curve and dQ / dV curve as in Experimental Example 4 were obtained in Experimental Examples 5 and 6 as well. That is, it was inferred that these materials were charged and discharged in a one-phase reaction in the entire potential range of charge and discharge. Furthermore, in Experimental Examples 4 to 6, the lattice constant exceeded 8.20 Å, and showed good low temperature output characteristics.

  In Experimental Examples 7 to 9 in which lithium phosphate was coated on the surface, the low-temperature output characteristics were improved as compared with Experimental Examples 4 to 6 of the same chemical composition. From this, it was found that low temperature output characteristics can be further enhanced by covering lithium phosphate with a positive electrode active material that satisfies −3 <dQ / dV <3 and has a lattice constant of 8.20 Å or more. In Experimental Examples 7 to 9, the charge / discharge curve and the dQ / dV curve were similar to those of Experimental Example 4.

  In Experimental Example 10 in which lithium phosphate was coated on the surface, the low temperature output characteristics were improved more than Experimental Example 1 of the same chemical composition. However, the low temperature output characteristics were lower than those of Experimental Examples 3 to 9. From this, when lithium phosphate was coated on the positive electrode active material to be charged and discharged in the two-phase coexistence reaction, it was found that although the performance was slightly improved, the performance improvement effect as in Experimental Examples 7 to 9 was not obtained. .

From the above, it is a complex oxide having a spinel structure including Li, Mn and M, and in the dQ / dV curve, dQ / dV [Ahg ⁻¹ V ⁻¹ ] in the entire range of potentials used for charge and discharge. It was found that, in a battery using a positive electrode active material having a value of -3 <dQ / dV <3, the output characteristics at low temperature can be further enhanced. Moreover, it was found that when the surface of such a positive electrode active material is coated with lithium phosphate, the low temperature output characteristics can be further enhanced. It is needless to say that the present disclosure is not limited to the above-described embodiment at all, and may be implemented in various modes within the technical scope of the present disclosure.

  The present disclosure is applicable, for example, to the field of battery industry.

  DESCRIPTION OF SYMBOLS 10 lithium secondary battery, 11 current collector, 12 positive electrode active material, 13 positive electrode sheet, 14 current collector, 17 negative electrode active material, 18 negative electrode sheet, 19 separator, 20 non-aqueous electrolyte solution, 22 cylindrical case, 24 positive electrode terminal , 26 negative terminal.

Claims (6)

A composite oxide having a spinel structure including Li, Mn and M (wherein M is one or more of Co and Ni),
In charge and discharge at a rate of 0.1 C in a temperature environment of 20 ° C. using an evaluation cell provided with a working electrode provided with the composite oxide and a counter electrode of lithium metal, the capacity per composite oxide is q [ mAhg ^-1] between time and q + 5 of [mAhg ^-1] when the capacity Q shows the ratio differential voltage V of the difference, dQ / dV [Ahg ^-1 V to be calculated from the following equation (1) ^{- 1} ] satisfies -3 <dQ / dV <3 in the whole range of the potential range used for charge and discharge,
Positive electrode active material.
dQ / dV = (Q (q + 5) -Q (q)) / (V (q + 5) -V (q)) Formula (1)   The positive electrode active material according to claim 1, wherein the lattice constant is 8.20 Å or more. Basic composition formula Li _x M _y Mn _2-y O _z (however, x, y, z satisfy 0.9 ≦ x ≦ 1, 0 <y ≦ 1, 3.7 ≦ z ≦ 4.0) The positive electrode active material of Claim 1 or 2 represented by these.   The positive electrode active material according to any one of claims 1 to 3, wherein lithium phosphate is coated on the surface.   The positive electrode containing the positive electrode active material of any one of Claims 1-4. The positive electrode containing the positive electrode active material of any one of Claims 1-4,
A negative electrode containing a negative electrode active material,
An ion conducting medium interposed between the positive electrode and the negative electrode for conducting lithium ions;
Equipped with a lithium secondary battery.