JP2022108812A - Manufacturing method of positive electrode active material - Google Patents

Abstract

To provide a manufacturing method of a positive electrode active material with good rate characteristics.SOLUTION: A manufacturing method of a positive electrode active material having an O2 type structure includes: a preparation step of preparing a transition metal oxide having a P2 type structure and containing K (potassium); and an ion exchange step of ion-exchanging the K ions contained in the transition metal oxide with Li ions while leaving a part of the K ions.SELECTED DRAWING: Figure 1

Description

The present disclosure relates to a method for manufacturing a positive electrode active material.

With the miniaturization of personal computers, video cameras, mobile phones, etc., lithium secondary batteries have been put to practical use and widely used as power sources for information-related devices and communication devices because of their high energy density. It has become popular. On the other hand, in the field of automobiles as well, the development of electric vehicles is urgent due to environmental and resource problems, and lithium secondary batteries are being studied as power sources for these electric vehicles.

Various oxides are known as positive electrode active materials for batteries. Hitherto, a layered compound having an O3 type structure has been used as a positive electrode active material. A layered compound having an O3 type structure may change its crystal structure under high potential conditions (for example, 4.4 V or higher). As a result, there is a problem that the capacity retention ratio decreases when the charge/discharge cycle is repeated under high potential conditions.

Against this background, for example, as disclosed in Patent Documents 1 and 2, a Na-doped precursor having a P2-type structure is ion-exchanged with Na ions and Li ions to form an O2-type structure. A method for synthesizing a layered positive electrode active material having

JP 2014-186937 A JP 2010-92824 A

From the viewpoint of improving the performance of batteries, positive electrode active materials with good rate characteristics are desired. The present disclosure has been made in view of the above circumstances, and a main object thereof is to provide a method for producing a positive electrode active material with good rate characteristics.

In order to solve the above problems, the present disclosure provides a method for producing a positive electrode active material having an O2 type structure, comprising preparing a transition metal oxide having a P2 type structure and containing K (potassium). and an ion exchange step of ion-exchanging the K ions contained in the transition metal oxide into Li ions while allowing some of the K ions to remain.

According to the present disclosure, a positive electrode active material with good rate characteristics is obtained by using a transition metal oxide containing K (potassium) as a precursor and performing ion exchange so that part of the K ions remain. be able to.

The present disclosure has the effect of being able to provide a positive electrode active material with good rate characteristics.

4 is a flow chart showing an example of a method for producing a positive electrode active material according to the present disclosure; FIG. 4 is a schematic diagram showing an effect of enlarging the interlayer distance by residual K ions; 4 shows the result of X-ray diffraction measurement for the positive electrode active material produced in Example 1. FIG. 4 shows the results of X-ray diffraction measurement for positive electrode active materials produced in Examples 1 and 2 and Comparative Examples 1 and 2. FIG. 1 is a schematic cross-sectional view showing an example of a lithium ion battery using a positive electrode active material manufactured in accordance with the present disclosure; FIG.

Hereinafter, the method for manufacturing the positive electrode active material in the present disclosure will be described in detail.

FIG. 1 is a flow chart showing an example of a method for producing a positive electrode active material according to the present disclosure. In FIG. 1, first, a transition metal oxide having a P2 type structure and containing K (potassium) is prepared as a precursor (preparation step). Next, while K ions contained in the transition metal oxide are ion-exchanged for Li ions, part of the K ions are left to obtain a positive electrode active material (ion exchange step).

According to the present disclosure, a positive electrode active material with good rate characteristics by ion-exchanging K ions contained in a transition metal oxide containing K (potassium) for Li ions while leaving a part of K ions. can be obtained. This is because when the K ions contained in the transition metal oxide are ion-exchanged for Li ions, K (potassium), which has a large ionic radius, is partially left in the Li layer, so that the interlayer distance around Li is large O2 type It is presumed that this is because the positive electrode active material has a structure and the deinsertion/insertion of Li ions is facilitated.

On the other hand, in a conventional process for synthesizing a positive electrode active material having an O2 type structure, attempts have been made to introduce Li into all the Li sites of the Li layer in order to increase the charge/discharge capacity. However, in such an O structure in which the entire Li layer is occupied by Li, the interlayer distance around Li is narrow, and resistance to deinsertion of Li cannot be deinserted in a sufficient amount. In particular, the charge/discharge capacity at high rates tends to decrease.

In addition, the P2-type K-containing transition metal oxide has a wider interlayer distance than the P2-type Na-containing transition metal oxide, and is unstable and difficult to synthesize. was used to produce a positive electrode active material having an O2 type structure. F. Tournadre, et al., J. Solid State Chem. 177 (2004) 2803-2809. discusses the structure of layered cathode active materials with O2 type structures. It is described that ion exchange between Na ions and Li ions yields a layered positive electrode active material having an O2 type structure with a small amount of Na remaining, but it is described that there is no effect on the interlayer distance. .

The present inventor has discovered the conditions for synthesizing a transition metal oxide having a P2 type structure and containing K (potassium), and while ion-exchanging the K ions and Li ions, the K ions remain between the layers. The inventors have found that a layered positive electrode active material having an O2 type structure with a large interlayer distance can be synthesized by allowing the layers to be separated. Furthermore, the present inventor synthesized a K-containing transition metal oxide (precursor) having a P2-type structure and containing Li in the transition metal layer, and ion-exchanged K ions and Li ions to obtain a transition of Li. It has been found that a layered positive electrode active material having an O2 type structure in which K exists also in the metal layer and remains in the Li layer can be obtained, and capacity reduction due to introduction of K can be suppressed.

The method for producing the positive electrode active material in the present disclosure will be further described below.

1. Preparing Step The preparing step in the present disclosure is a step of preparing a transition metal oxide having a P2 type structure and containing K (potassium). The P2-type structure belongs to the space group P6 ₃ /mmc, has two kinds of oxide layers with different oxygen positions in the unit cell, and is a crystal structure in which potassium ions occupy prismatic sites. .

A transition metal oxide can be prepared, for example, by synthesizing as follows. First, a Mn source, a Ni source, and a Co source (any one or two elements can be omitted if necessary) are mixed in a ratio to give a desired composition, and precipitated using a base. Then, a K source is added to the precipitated powder at a ratio that gives a desired composition, and sintering is performed. At this time, M sources such as Li, Al, Fe, Mg, Ca, Ti, Cr, Cu, Zn, Nb and Mo may be mixed so as to obtain a desired composition. In addition, preliminary firing may be performed before firing. Thereby, a transition metal oxide, which is a K-doped precursor, can be obtained.

Examples of the Mn source, Ni source, and Co source include nitrates, sulfates, hydroxides, and carbonates containing these metal elements. These may be hydrates. Bases used for precipitation include, for example, sodium carbonate and sodium hydroxide. These may be used as an aqueous solution. Furthermore, an aqueous ammonia solution may be added for basicity adjustment. K sources include, for example, potassium carbonate, potassium oxide, potassium nitrate, and potassium hydroxide. The firing temperature is, for example, 700° C. or higher and 1100° C. or lower. Firing temperatures that are too low may not provide sufficient K doping, and firing temperatures that are too high may result in the formation of O3-type structures instead of P2-type structures. The firing temperature may be 800° C. or higher and 1000° C. or lower. Moreover, when pre-firing is performed, the pre-firing is preferably performed at a temperature equal to or lower than the main firing temperature, for example, around 600°C. The firing atmosphere may be in the air or under a gas flow containing oxygen.

The transition metal oxide preferably has a P2-type structure as a main phase. “Having a P2-type structure as a main phase” means that one of the peaks belonging to the P2-type structure corresponds to the peak with the highest diffraction intensity observed by X-ray diffraction (XRD) measurement. The transition metal oxide may be a single-phase material with a P2-type structure. Also, the transition metal oxide does not have to have an O3 type structure. "Not having O3 type structure" means that no peak belonging to O3 type structure is observed by XRD measurement.

_The composition of the _{transition metal oxide is} not _particularly limited. 0≦y≦1, 0≦z≦1, 0<x+y+z≦1, r satisfies 0.5≦p≦1, Me is Li, Al, Fe, Mg, Ca, Ti, Cr, Cu, at least one of Zn, Nb and Mo). x may be 0 or greater than 0. y may be 0 or greater than 0. z may be 0 or greater than 0. Also, x+y+z may be 1 or may be smaller than 1. The composition of the transition metal oxide can be confirmed by, for example, ICP. Moreover, as will be described later, the transition metal oxide preferably contains Li.

2. Ion Exchange Step The ion exchange step in the present disclosure is a step of ion-exchanging the K ions contained in the transition metal oxide for Li ions while allowing some of the K ions to remain. In the ion exchange step, the ion exchange reaction between the transition metal oxide and the Li ion source is used to replace the K ions contained in the transition metal oxide with Li ions.

In the present disclosure, while K ions contained in the transition metal oxide are ion-exchanged for Li ions, some of the K ions remain. This allows K ions to remain in the Li layer as shown in FIG. The presence of this residual K ion increases the interlayer distance.

Examples of Li ion sources for ion exchange include lithium salts such as lithium chloride, lithium bromide, lithium iodide, and lithium nitrate. Two or more lithium salts may be used as the Li ion source. Especially when a mixture of lithium chloride and lithium nitrate is used, the melting point of the mixture can be lowered. Also, the ratio of lithium chloride to the total of lithium chloride and lithium nitrate is, for example, 5 mol % or more and 80 mol % or less, and may be 10 mol % or more and 70 mol % or less.

As a method for advancing the ion exchange reaction, for example, a method of adding a lithium salt to the transition metal oxide and heating the transition metal oxide to hold the transition metal oxide in a molten salt bed of the lithium salt. is mentioned. The heating temperature should be at least the melting point of the salt or higher. For example, when a mixture of lithium chloride and lithium nitrate is used as the salt, the heating temperature is preferably 400° C. or lower. This is because if the temperature is higher than 400° C., there is a possibility that the O2 type structure will change to the O3 type structure, which is a stable phase, in about one hour. It is preferably 350° C. or less. In addition, the lower limit is preferably 250° C. considering the time required for ion exchange.

The amount of Li ion source used is not particularly limited. The amount of Li contained in the Li ion source is, for example, 1.1 times or more, may be 3 times or more, or 5 times or more in molar ratio with respect to the amount of K contained in the transition metal oxide. may On the other hand, the molar ratio of the Li amount is, for example, 15 times or less, and may be 12 times or less.

In the ion exchange step, part of the K ions contained in the transition metal oxide are replaced with Li ions. Above all, it is preferable that the positive electrode active material after ion exchange contains 1 at % or more and 20 at % or less of K with respect to Li. The composition after ion exchange can be measured by ICP or the like.

3. Cathode Active Material The cathode active material in the present disclosure has an O2 type structure. In the O2 type structure, Li occupies octahedral sites in the oxide, and there are two types of oxide layers (layers containing oxygen and transition metals) with different oxygen positions in the unit cell. It is a crystal structure that

It can be confirmed by XRD measurement that the positive electrode active material has an O2 type structure. The positive electrode active material in the present disclosure preferably has an O2 type structure as a main phase. “Having an O2 structure as a main phase” means that one of the peaks belonging to the O2 structure corresponds to the peak with the highest diffraction intensity observed by X-ray diffraction (XRD) measurement. The positive electrode active material may be a single-phase material with an O2 type structure.

The positive electrode active material in the present disclosure does not have to have an O3 type structure. The O3 type structure means a structure in which Li occupies octahedral sites in the oxide and three types of oxide layers exist in which the oxygen positions are different in the unit cell. "Not having O3 type structure" means that no peak belonging to O3 type structure is observed by XRD measurement. On the other hand, the positive electrode active material may have an O3 type structure. In the XRD measurement using CuKα rays, when the peak intensity derived from the 002 plane of the O2 type structure is I ₀₀₂ and the peak intensity derived from the 003 plane of the O3 type structure is I ₀₀₃ , I ₀₀₃ /I ₀₀₂ is , for example, 0.3 or less, and may be 0.1 or less.

The positive electrode active material in the present disclosure usually contains K. The amount of K with respect to the amount of Li (amount of K/amount of Li) is preferably 1 at % or more. When K is less than 1 at %, the distance between adjacent K reaches about 3 nm (equivalent to 10 Li sites). The difference in ionic radius between Li and K is 0.062 nm. Considering that the interlayer distance is reduced to the level corresponding to Li at the position where K does not exist, the layers above and below the Li layer are distorted. The strain amount is approximately 4% when K is 1 at %. In general, the amount of strain that can be deformed without dislocations is considered to be about 3% in the case of oxide materials, so when K is less than 1 at%, the interlayer distance can be sufficiently reduced by strain. end up On the other hand, when K is 1 at % or more, the effect of increasing the interlayer distance is obtained in the form of creating a "space" in a range that cannot be dealt with due to strain. Therefore, when K is less than 1 at %, the effect of enlarging the interlayer distance due to the introduction of K cannot be sufficiently obtained. The amount of K with respect to the amount of Li may be 2 at % or more, or may be 3 at % or more.

Also, the amount of K with respect to the amount of Li is preferably 20 at % or less. When K is more than 20 at %, the amount of Li in the Li layer is reduced by that amount (more than 20 at %) compared to when K is not present. Therefore, the charge/discharge capacity at a high rate is lower than the maximum capacity in the absence of K, and the amount of reduction is greater than 20%. Since this is a value that can be obtained even in high-rate charge/discharge when K does not exist, the effect of the presence of K cannot be sufficiently obtained.

The positive electrode active material in the present disclosure may or may not have a P2-type structure derived from a transition metal oxide. In the XRD measurement using CuKα rays, when the peak intensity derived from the 002 plane of the O2 type structure is I ₀₀₂ and the peak intensity derived from the 002 plane of the P2 type structure is I ₀₀₂ ', I ₀₀₂ '/I ₀₀₂ is, for example, 0.3 or less, and may be 0.1 or less.

_{The composition} of the _{positive electrode active material is} not particularly limited. , 0≦y≦1, 0≦z≦1, 0<x+y+z≦1, p satisfies 0.5≦p≦0.99, q satisfies 0.01≦q≦0.2, Me is at least one of Li, Al, Fe, Mg, Ca, Ti, Cr, Cu, Zn, Nb and Mo). x may be 0 or greater than 0. y may be 0 or greater than 0. z may be 0 or greater than 0. Also, x+y+z may be 1 or may be smaller than 1. The composition of the positive electrode active material can be confirmed by, for example, ICP.

A positive electrode active material having an O2-type structure is characterized in that Li can be deinserted and deinserted with good cyclic performance even when the transition metal layer is partially replaced with Li (Benoit Mortemard de Boisse, et al., J. Electrochem. Soc 165 (2018) A3630.). Therefore, in the present disclosure, the capacity decrease due to K introduction can be compensated for by introducing Li into the transition metal layer. Since Li in the transition metal layer cannot be intercalated/deintercalated at a high rate, it does not contribute to the capacity during high-rate charge/discharge, and only the capacity is improved during low-rate charge/discharge. Therefore, it is preferable to appropriately control the K residual amount in the positive electrode active material and the Li amount in the transition metal layer.

The shape of the positive electrode active material is, for example, particulate. The average particle size (D ₅₀ ) of the positive electrode active material is, for example, 1 nm or more, and may be 10 nm or more. The average particle size (D ₅₀ ) of the positive electrode active material is, for example, 100 μm or less, and may be 30 μm or less.

As described above, the positive electrode active material in the present disclosure has a large interlayer distance around Li. The interlayer distance varies greatly depending on the composition of the transition metal layer and the amount of Li, but as a result of measurement by the θ-2θ method using X-ray diffraction, the measurement results for materials with the same composition and the same structure that do not contain K except for K. , the lattice constant in the c-axis direction is preferably elongated by 0.15 nm or more and 0.24 nm or less. The maximum value is the value when all the Li layers are replaced with K, and the minimum value is the value when the amount of K is 1% and the K positions of different layers are arranged so that the strains are canceled. value.

4. Batteries Cathode active materials made according to the present disclosure are used in batteries, particularly lithium-ion batteries. FIG. 5 is a schematic cross-sectional view showing an example of a lithium ion battery using a positive electrode active material manufactured in the present disclosure. Lithium-ion battery 10 includes positive electrode layer 1 using the positive electrode active material manufactured in the present disclosure, negative electrode layer 2 , and electrolyte layer 3 disposed between positive electrode layer 1 and negative electrode layer 2 . The electrolyte layer 3 may be a layer containing an electrolytic solution, or a layer containing a solid electrolyte (especially an inorganic solid electrolyte). The former corresponds to a liquid battery, and the latter corresponds to an all-solid battery. In addition, the lithium ion battery 10 normally has a positive electrode current collector 4 on the surface of the positive electrode layer 1 opposite to the electrolyte layer 3 , and a negative electrode current collector on the surface of the negative electrode layer 2 opposite to the electrolyte layer 3 . has a body 5;

According to the present disclosure, a lithium ion battery with good rate characteristics can be obtained by using the positive electrode active material produced by the above-described method for producing a positive electrode active material. The lithium ion battery may be a primary battery or a secondary battery, but is preferably a secondary battery. This is because they can be repeatedly charged and discharged, and are useful, for example, as batteries for vehicles. Moreover, examples of the shape of the lithium ion battery include coin type, laminated type, cylindrical type, and rectangular type.

Note that the present disclosure is not limited to the above embodiments. The above embodiment is an example, and any device that has substantially the same configuration as the technical idea described in the claims of the present disclosure and produces the same effect is the present invention. It is included in the technical scope of the disclosure.

(Example 1)
Mn(NO ₃ ) 2.6H ₂ O, Ni(NO ₃ ) _{2.6H 2} O and Co(NO ₃ ) _{2.6H 2} O are used as raw materials, and the molar ratio of Mn, Ni and Co is 5: ₂ :3. It was dissolved in pure water so that Together, a Na ₂ CO ₃ solution with a concentration of 12 wt % was prepared, and these two solutions were simultaneously titrated into a beaker. At this time, the titration rate was controlled so that the pH was 7.0 or more and less than 7.1. After the titration was completed, the mixed solution was stirred at 50° C. and 300 rpm for 24 hours. The resulting reaction product was washed with pure water and centrifuged to separate only the precipitated powder. The obtained powder was dried at 120° C. for 48 hours and then pulverized in an agate mortar.

K ₂ CO ₃ was added to the obtained powder so as to have a composition ratio of K _0.7 [Mn _0.5 Ni _0.2 Co _0.3 ]O ₂ and mixed. The mixed powder was pressed with a load of 2 tons by a cold isostatic pressing method to prepare pellets. The obtained pellets were pre-fired at 600° C. for 6 hours in the atmosphere, and then fired at 900° C. for 24 hours to obtain a K-doped precursor (transition metal oxide having a P2 type structure and containing K ) was synthesized.

LiNO ₃ and LiCl were mixed at a molar ratio of 50:50 and weighed so that the molar ratio of the amount of Li contained in the mixed powder to the amount of K contained in the K-doped precursor was 10 times. The K-doped precursor and the LiNO ₃ ·LiCl mixed powder were mixed, and ion exchange was performed in air at 350° C. for 1 hour. After the ion exchange, water was added to dissolve the salt, and the mixture was washed with water to obtain a positive electrode active material.

85 g of this positive electrode active material (powder after ball milling) was added to 125 mL of a solvent n-methylpyrrolidone solution in which 5 g of polyvinylidene fluoride (PVDF) as a binder was dissolved. 10 g was added. After that, they were kneaded until they were uniformly mixed to prepare a paste. This paste was applied to one side of an Al current collector having a thickness of 15 μm at a basis weight of 6 mg/cm ² and dried to obtain an electrode. After that, this electrode was pressed to a paste thickness of 45 μm and a paste density of 2.4 g/cm ³ . Finally, this electrode was cut to obtain a positive electrode having a diameter of 16 mm. On the other hand, a negative electrode was obtained by cutting the Li foil into a size of φ19 mm.

A CR2032 type coin cell was produced using the obtained positive electrode and negative electrode. In addition, a PP porous separator is used as the separator, and the electrolyte is a mixture of EC (ethylene carbonate) and DMC (dimethyl carbonate) at a volume ratio of 3:7, and the supporting salt is lithium hexafluorophosphate (LiPF ₆ ) dissolved at a concentration of 1 mol/L was used.

(Example 2)
Mn(NO ₃ ) _{2.6H 2 O, Ni(NO 3 ) so} that the composition of the precursor before ion exchange is K _0.7 [Li _0.12 Mn _0.71 Ni _0.17 ]O ₂ A positive electrode active material and a coin cell were obtained in the same manner as in Example 1, except that ₂ ·6H ₂ O, Li ₂ CO ₃ and K ₂ CO ₃ were used to synthesize the precursor.

(Comparative example 1)
Mn(NO ₃ ) _{2.6H 2 O, Ni(NO 3 ) so} that the composition of the precursor before ion exchange is Na _0.7 [Mn _0.5 Ni _0.2 Co _0.3 ]O ₂ A positive electrode active material and a coin cell were obtained in the same manner as in Example 1, except that ₂ ·6H ₂ O, Co(NO ₃ ) ₂ ·6H ₂ O, and Na ₂ CO ₃ were used to synthesize the precursors.

(Comparative example 2)
Mn(NO ₃ ) _{2.6H 2 O, Ni(NO 3 ) so} that the composition of the precursor before ion exchange is Na _0.7 [Li _0.12 Mn _0.71 Ni _0.17 ]O ₂ A positive electrode active material and a coin cell were obtained in the same manner as in Example 1, except that ₂ ·6H ₂ O, Li ₂ CO ₃ and Na ₂ CO ₃ were used to synthesize the precursors.

(X-ray diffraction measurement)
The positive electrode active material produced in Example 1 was subjected to X-ray diffraction measurement. As a result, as shown in FIG. 3, it was confirmed that a single phase with an O2 type structure was obtained. FIG. 4 shows an enlarged view of the 002 peak among the results of X-ray diffraction measurement of the positive electrode active materials produced in Examples 1 and 2 and Comparative Examples 1 and 2. As shown in FIG. In Examples 1 and 2, the peaks were shifted to the lower angle side compared to Comparative Examples 1 and 2, respectively, and it was confirmed that the lattice expanded in the c-axis direction. Regarding the positive electrode active materials prepared in Examples 1 and 2 and Comparative Examples 1 and 2, the lattice constant obtained from the X-ray diffraction results and the composition of K to Li obtained from ICP-AES (K amount/Li amount) are shown in Table 1. are summarized in

As shown in Table 1, it was confirmed that in the positive electrode active materials produced in Examples 1 and 2, the interlayer distance was increased due to K remaining after the ion exchange.

(Charging and discharging test)
A charging/discharging test was performed on the coin cells produced in Examples 1 and 2 and Comparative Examples 1 and 2. Specifically, the battery was charged at 0.1 C to 4.8 V, then discharged at 0.1 C to 2.0 V, and this cycle was performed for 5 cycles. Table 2 shows the results of the initial discharge capacity (0.1C). Next, Table 2 summarizes the discharge capacity ratio for charging and discharging at 0.1C when charging and discharging at each C rate (0.2C, 0.5C, 1C, 2C, 5C and 10C). In the charge/discharge test at each C rate, 5 cycles of charging to 4.8 V and then discharging to 2.0 V were performed, and the discharge capacity at each C rate was the first of the 5 cycles. Using.

As shown in Table 2, the positive electrode active materials produced in Examples 1 and 2 have better rate characteristics than the positive electrode active materials produced in Comparative Examples 1 and 2 due to K remaining. was confirmed.

DESCRIPTION OF SYMBOLS 1... Positive electrode layer 2... Negative electrode layer 3... Electrolyte layer 4... Positive electrode collector 5... Negative electrode collector 10... Lithium ion battery

Claims (1)

A method for producing a positive electrode active material having an O2 type structure, comprising:
A preparation step of preparing a transition metal oxide having a P2 type structure and containing K (potassium);
and an ion exchange step of ion-exchanging K ions contained in the transition metal oxide into Li ions while allowing some of the K ions to remain.

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