JP2020102301A - Evaluation method for substituent element of cathode active material for lithium ion secondary battery - Google Patents

Abstract

To provide an evaluation method for a substituent element of a cathode active material for lithium ion secondary battery capable of efficiently evaluating a degree of influence to be exerted upon a cycle characteristic in a case where a substituted cathode active material is used for a lithium ion secondary battery, regarding the substituent element of the cathode active material.SOLUTION: The present invention relates to an evaluation method for a substituent element of a cathode active material for lithium ion secondary battery including: a selection step of selecting a candidate element to partially substitute elements of a cathode active material having a layered structure, a calculation step of calculating a change of an inter-layer distance in a case where Li is inserted or desorbed between layers, in accordance with first principle calculation using a van der Waals density function, in the cathode active material of which the element is partially substituted by the candidate element; a repetition step of repeatedly implementing the selection step and the calculation step while changing the kind of the candidate element to be selected in the selection step; and an evaluation step of arraying the candidate elements used for the calculation step in an order from a candidate element with the least change of the inter-layer distance calculated in the calculated step, and evaluating that an effect of improving a cycle characteristic is high, in the order.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method for evaluating a substitutional element of a positive electrode active material for a lithium ion secondary battery.

Since lithium-ion secondary batteries have high voltage and high capacity, they have become popular as secondary batteries for notebook PCs and mobile phones that require high output and miniaturization, and as in-vehicle batteries for hybrid vehicles and electric vehicles. ing.

The lithium-ion secondary battery is composed of, for example, a positive electrode, a negative electrode, an electrolyte, and the like, and LiCoO ₂ , LiNiO _2, or the like is used as the positive electrode active material (positive electrode material) of the positive electrode. Among them, LiNiO ₂ is a promising material because it has a high energy density and is cheaper than Co.

However, the positive electrode active material used in the lithium-ion secondary battery is generally unstable, and the cycle characteristics of the lithium-ion secondary battery, that is, the capacity retention rate of the battery when repeatedly charged and discharged is sufficient. Was not high, and it was required to improve cycle characteristics.

As a method of improving the stability of the positive electrode active material and improving the cycle characteristics of a lithium ion secondary battery using the positive electrode active material, it is conceivable to replace the element of the positive electrode active material with another element.

For example, in Patent Document 1, a lithium-containing composite oxide represented by the formula 1: Li _x Ni _1-yz-v-w Co _y Al _z M ¹ _v M ² _w O ₂ is included, and the element in the formula 1 is included. M ¹ is at least one selected from the group consisting of Mn, Ti, Y, Nb, Mo and W, and the element M ^{2 in} Formula 1 is selected from the group consisting of Mg, Ca, Sr and Ba. And the element M ² contains at least Mg and Ca, and the formula 1 is 0.97≦x≦1.1, 0.05≦y≦0.35, 0. 005≦z≦0.1, 0.0001≦v≦0.05, and 0.0001≦w≦0.05 are satisfied, and in the composite oxide, primary particles are aggregated to form secondary particles. The average particle size of primary particles of the composite oxide is 0.1 μm or more and 3 μm or less, and the average particle size of secondary particles of the composite oxide is 8 μm or more and 20 μm or less. A positive electrode active material for a liquid secondary battery is disclosed, and an example in which Ni is replaced with another element is shown.

JP, 2006-310181, A

Tsutomu Ohzuku et al., "Electrochemistry and Structural Chemistry of LiNiO2 (R3m) for 4 Volt Secondary Lithium Cells", Journal of The Electrochemical Society, 1993, Vol.140, No.7, PP.1862-1870.

However, it takes several months to evaluate the cycle characteristics of a lithium ion secondary battery, and if the repeated synthesis and evaluation of the positive electrode active material substituted for a plurality of candidate elements is repeated experimentally, it takes a huge amount of time for development. ·There will be a cost. Therefore, regarding the substitution element of the positive electrode active material for the lithium ion secondary battery, the degree of influence on the cycle characteristics when the substituted positive electrode active material for the lithium ion secondary battery is used for the lithium ion secondary battery is efficiently determined. There has been a demand for a method of evaluating a substitution element of a positive electrode active material for a lithium-ion secondary battery that can be evaluated.

Therefore, in view of the problems of the above-mentioned conventional technology, in one aspect of the present invention, the substituted positive electrode active material for a lithium ion secondary battery is used for a lithium ion secondary battery as a substitution element of the positive electrode active material for a lithium ion secondary battery. It is an object of the present invention to provide a method for evaluating a substitutional element of a positive electrode active material for a lithium ion secondary battery, which can efficiently evaluate the degree of influence on cycle characteristics.

According to an aspect of the present invention to solve the above problems,
A selection step of selecting a candidate element which is a candidate element for substituting a part of the elements of the positive electrode active material for a lithium ion secondary battery having a layered structure,
In the positive electrode active material for a lithium-ion secondary battery in which a part of the elements is replaced by the candidate element, the change in the interlayer distance when lithium is inserted or deintercalated between layers is measured by using the van der Waals density functional. The calculation process of calculating by the first-principles calculation,
A repeating step of changing the type of the candidate element selected in the selecting step and repeating the selecting step and the calculating step;
Lithium ion ion having an evaluation step of arranging the candidate elements that have been subjected to the calculation step in order from the one having the smallest change in the interlayer distance calculated in the calculation step, and evaluating that the effect of enhancing cycle characteristics is high in that order. Provided is a method for evaluating a substitution element of a positive electrode active material for a secondary battery.

According to one aspect of the present invention, regarding the substitution element of the positive electrode active material for a lithium ion secondary battery, the effect on the cycle characteristics when the substituted positive electrode active material for a lithium ion secondary battery is used for a lithium ion secondary battery. It is possible to provide a method for evaluating a substitution element of a positive electrode active material for a lithium ion secondary battery, which can efficiently evaluate the degree.

FIG. 6 is a diagram showing a difference in calculation result of a change in interlayer distance due to Li desorption due to a difference in functional calculated in Reference Example 1.

Hereinafter, modes for carrying out the present invention will be described, but the present invention is not limited to the following embodiments, and various modifications and changes to the following embodiments without departing from the scope of the present invention. Substitutions can be added.

The method for evaluating a substituting element of a positive electrode active material for a lithium ion secondary battery according to this embodiment can include the following steps.

A selection step of selecting a candidate element that is a candidate element for substituting a part of the elements of the positive electrode active material for a lithium ion secondary battery having a layered structure.
In a positive electrode active material for a lithium-ion secondary battery in which part of the elements was replaced with a candidate element, the change in interlayer distance when lithium was inserted or deintercalated between layers was measured using van der Waals density functional A calculation process that uses one-principles calculation
Iterative process in which the selection process and the calculation process are repeatedly performed by changing the types of candidate elements selected in the selection process.

An evaluation step of arranging candidate elements that have been subjected to the calculation step in order from the one having the smallest change in interlayer distance calculated in the calculation step, and evaluating that the effect of enhancing the cycle characteristics is high in that order.

When the substituted positive electrode active material is used in a lithium ion secondary battery, the inventors of the present invention have used a substituted element of a positive electrode active material for a lithium ion secondary battery (hereinafter, also simply referred to as “positive electrode active material”). The inventors have made earnest studies on a method for evaluating a substitution element of a positive electrode active material for a lithium ion secondary battery, which can efficiently evaluate the degree of influence on the cycle characteristics.

As described above, enormous amount of time and cost are required to experimentally search and evaluate a substitution element. Therefore, regarding the substitution element of the positive electrode active material, as a method of efficiently evaluating the effect of the substituted positive electrode active material on the cycle characteristics when used in a lithium ion secondary battery, and efficiently searching for the substitution element, The inventors of the invention have focused on a method that uses first-principles calculations.

In order to evaluate the effect of the substituting element on the cycle characteristics using the first principle calculation, it is necessary to understand the factors affecting the cycle characteristics. However, until now, such factors have not been sufficiently examined.

According to the study of the inventors of the present invention, the positive electrode active material contained in the lithium ion secondary battery which is repeatedly charged and discharged and has a reduced capacity has cracks and cracks which particles did not have before the start of use. I was able to confirm that.

In a lithium ion secondary battery using a positive electrode active material having a layered structure, charging and discharging are performed by repeatedly inserting lithium into and removing lithium from the layers. Therefore, it is understood that the cracks and the like generated in the positive electrode active material are caused by the expansion and contraction of the positive electrode active material accompanying the insertion and desorption of lithium. Then, according to Non-Patent Document 1, in Li _1-x NiO ₂ which is a positive electrode active material having a layered structure, charging and discharging are performed and the value of x is changed to insert and release lithium, and It has been reported that the distance changes.

Therefore, the cracks and the like that were generated in the positive electrode active material after the above repeated charge and discharge were caused by the change in the interlayer distance due to the repeated insertion and desorption of lithium between the layers of the positive electrode active material along with the charge and discharge. It is thought to have occurred.

From the above-mentioned examination results, it was found that, with respect to the positive electrode active material substituted with a substitution element, a substitution element capable of suppressing a change in interlayer distance when lithium is inserted or desorbed can be evaluated as a substitution element having a high effect of enhancing cycle characteristics. It was

However, in the first-principles calculation generally used conventionally, it has not been possible to accurately evaluate the interlayer distance of the positive electrode active material having a layered structure.

In the first-principles calculation based on the density functional theory, a PBE (Perew-Burke-Ernzehof) functional, which is a kind of functional by the generalized gradient approximation, is widely used. This is because in many cases, the PBE functional can accurately calculate the crystal structure.

However, for example, in the positive electrode active material having a layered structure, the charge around the layered structure is almost zero when Li (lithium) is desorbed, and the Van der Waals force mainly acts between layers. And the van der Waals force is not correctly incorporated in the PBE functional. For this reason, the distance between the layers of the positive electrode active material could not be accurately evaluated especially in the state where Li was desorbed.

As a result of further study, the first-principles calculation was performed using the van der Waals density functional with the van der Waals force added as a correction term, which was not used in the first-principles calculation for the positive electrode active material having a layered structure. It has been found that by performing the principle calculation, it is possible to accurately evaluate the inter-layer distance when Li is inserted and desorbed in the positive electrode active material having a layered structure. Then, the first-principles calculation using the van der Waals density functional is used to efficiently evaluate the degree of influence of the substitution element on the cycle characteristics when the substituted positive electrode active material is used in a lithium ion secondary battery. The inventors have found out what can be done and have completed the present invention.

Hereinafter, the method for evaluating the substitution element of the positive electrode active material for a lithium ion secondary battery of the present embodiment will be described for each step.
(Selection process)
In the selection step, a candidate element that is a candidate element that replaces a part of the element of the positive electrode active material having a layered structure can be selected.

At this time, the type of candidate element to be selected is not particularly limited, and can be arbitrarily selected.

Note that the positive electrode active material having a layered structure to be replaced is not particularly limited, and a positive electrode active material having a layered structure and various layered structures capable of inserting and removing Li between layers is used. You can For example, as the positive electrode active material, LiNiO ₂ (lithium nickel oxide), LiCoO ₂ (lithium _{cobaltate), LiNi 1-t Co t} O 2 (0 <t <1), LiNi 1-x-y Co x Al y O _2, and _LiNi 1/3 _Mn 1/3 can be one selected from _Co 1/3 _{O 2.}
(Calculation process)
In the calculation step, in the positive electrode active material in which a part of the element is replaced by the candidate element selected in the selection step, the change in the interlayer distance when lithium is inserted or deintercalated between the layers is calculated by the van der Waals density functional. It can be calculated by the first-principles calculation used.

The element in the positive electrode active material to be replaced by the candidate element is not particularly limited and can be arbitrarily selected. As the positive electrode active material, and LiNiO ₂ as described _{above, LiCoO 2, LiNi 1-t} Co t O 2, LiNi 1-x-y Co x Al y O 2, LiNi 1/3 Mn 1/3 Co 1/3 When O ₂ or the like is selected, it can be an element to be replaced with Ni, Co, or the like which is a transition metal. The degree of substitution of the element to be substituted is not particularly limited, and may be an arbitrary substitution ratio determined in advance.

Then, for example, the van der Waals density functional was used for the positive electrode active material in which a part of the elements was replaced with a candidate element and the positive electrode active material in which at least a part of Li of the replaced positive electrode active material was desorbed. The distance between layers can be calculated by the first principle calculation. From the obtained calculation result, it is possible to calculate the change in the interlayer distance when Li is inserted or deintercalated between layers in the positive electrode active material in which a part of the elements is replaced by the candidate element.

The degree of insertion or desorption of Li at the time of calculation is not particularly limited, and may be a predetermined insertion amount or desorption amount. However, it is preferable that Li be inserted or desorbed so that the change in the interlayer distance is sufficiently large, and when desorbing Li, for example, it is preferable to desorb Li by 75% or more. Note that, since Li can be desorbed entirely in the calculation, the desorption amount of Li is preferably 100% or less, and more preferably 99% or less.

The term "interlayer" as used herein means a portion in which Li is arranged when Li is inserted. The Van der Waals density functional is a functional in which a correction term is added to the energy functional by the generalized density gradient approximation so that the Van der Waals force can be evaluated.
(Repeat process)
In the repeating step, the selecting step and the calculating step can be repeatedly performed in the same manner as the selecting step and the calculating step described above except that the kind of the candidate element selected in the selecting step is changed.

That is, it is possible to select a candidate element that is different from the element that has already been subjected to the calculation in the selection step and use the candidate element to carry out the calculation step.

In the calculation step, it is preferable to perform the calculation under the same conditions as in the calculation step previously performed, except that different candidate elements are used.
(Evaluation process)
In the evaluation step, it is possible to evaluate that the candidate elements provided in the calculation step are arranged in order from the one having the smallest change in the interlayer distance calculated in the calculation step, and the effect of enhancing the cycle characteristics is high in that order.

As described above, according to the study by the inventors of the present invention, a substitution element capable of suppressing a change in interlayer distance when Li is inserted or desorbed between layers is a substitution element having a high effect of enhancing cycle characteristics. Can be evaluated as Therefore, as described above, when the candidate elements subjected to the calculation step are arranged in order from the one with the smallest change in the interlayer distance calculated in the calculation step, the cycle characteristics are arranged in that order, that is, from the one with the smallest change in the interlayer distance. It can be evaluated as an element having a high enhancing effect. That is, the arrangement of the above candidate elements is arranged in the order of the degree of influence on the cycle characteristics when the substituted positive electrode active material is used for a lithium ion secondary battery, for the substitution elements of the positive electrode active material used for evaluation. Becomes

The evaluation method in the evaluation step is not limited to the above method. For example, the calculation process described above is performed under the same conditions for the unsubstituted positive electrode active material, and the change in the interlayer distance is suppressed more than the change in the interlayer distance when Li is inserted or desorbed in the unsubstituted positive electrode active material. All possible candidate elements can be evaluated as elements having a high effect of improving the cycle characteristics. Further, only the candidate elements that can suppress the change in the interlayer distance than the change in the interlayer distance when Li is inserted or desorbed in the unsubstituted positive electrode active material can be arranged in the evaluation step described above.

According to the method for evaluating a substituting element of the positive electrode active material for a lithium ion secondary battery of the present embodiment described above, when the substituting positive electrode active material is used in the lithium ion secondary battery for the substituting element of the positive electrode active material. The degree of influence on the cycle characteristics of can be efficiently evaluated by the first-principles calculation.

It is also possible to select a substituting element based on the evaluation result of the substituting element evaluation method for the positive electrode active material for a lithium-ion secondary battery according to the present embodiment, and then select an optimal substitution amount with the substituting element. Also in this case, the optimum replacement amount can be evaluated by performing the above-described selection process, calculation process, repeating process, and evaluation process for selecting the replacement amount.

However, in the case of selecting the substitution amount, in the selection step, the substitution amount (substitution ratio) for substituting a part of the elements of the positive electrode active material having a layered structure with the selected substitution element can be arbitrarily selected. In the calculation step, in the positive electrode active material in which a part of the element is replaced by the replacement element with the replacement amount selected in the selection step, the change in the interlayer distance when lithium is inserted into or removed from the interlayer is van der Waals. It can be calculated by the first-principles calculation using a density functional. In the repeating step, the selecting step and the calculating step can be repeated by changing the substitution amount selected in the selecting step. Then, in the evaluation step, it is possible to evaluate that the replacement ratios are arranged in the descending order of the change in the interlayer distance calculated in the calculation step, and the effect of enhancing the cycle characteristics is high in that order.

In addition, when distinguishing from the above-mentioned steps for evaluating the element to be replaced, the selection steps for the substitution amount selection, the calculation step for the substitution amount selection, the repetition step for the substitution amount selection, the evaluation step for the substitution amount selection, etc. Each step can also be called. Further, when the selection step is repeatedly performed in the repeating step, it is preferable to change the substitution amount at a constant rate from the viewpoint of comprehensively evaluating the substitution amount.

Hereinafter, the present invention will be described more specifically with reference to Examples. However, the present invention is not limited to the following examples.
[Reference Example 1]
In order to confirm the difference in the calculation result of the interlayer distance due to the difference in the functional, the change in the interlayer distance d due to the desorption of Li was examined for LiNiO ₂ between the PBE functional and the van der Waals density functional. I calculated. The interlayer distance d means the distance between the layers in which Li was inserted.

The calculation is performed using VASP (Vienna Ab initio Simulation Package), which is a plane wave basis first-principle calculation software, in the category of Density Functional Theory (DFT), PBE functional, or van der Waals density functional. Was performed using. The cutoff energy of the plane wave base was 650 eV.

The calculation result is shown in FIG. In FIG. 1, the horizontal axis indicates the degree of desorption of Li, and the larger the value on the horizontal axis, the larger _{x of} Li _1-x NiO ₂ means that Li is desorbed. ..

As shown in FIG. 1, when the van der Waals density functional is used, the inter-layer distance initially becomes slightly longer as the Li desorption amount increases, but when the Li desorption amount further increases, It was confirmed that the distance would be greatly shortened. It was confirmed that this result matches the result of Non-Patent Document 1.
[Example 1]
The following procedure was evaluated substitution elements of _LiNi 0.84 _{Co 0.16} O ₂ is the cathode active material for a lithium ion secondary battery having a layered structure.
(Selection process)
In the selection step, a candidate element, which is a candidate element for substituting a part of the Ni element of LiNi _0.84 Co _0.16 O ₂ , which is a positive electrode active material for a lithium ion secondary battery having a layered structure, was selected. Specifically, Mg was selected.
(Calculation process)
In the calculation step, in the positive electrode active material for a lithium ion secondary battery in which a part of the elements was replaced by a candidate element, when the lithium was desorbed from the layers, the change in the interlayer distance was calculated by the van der Waals density functional. It was calculated by the first principle calculation used.

Note that a part of Ni was replaced with a candidate element so that the general formula after replacement with the candidate element was LiNi _0.75 Co _0.16 Mg _0.09 O _2, and the lithium desorption amount was 0%. , 75% and the interlayer distance was calculated.

The calculation was performed in the same manner as in Reference Example 1 except that the van der Waals density functional was used for the positive electrode active material represented by the above general formula.
(Repeat process)
In the repeating step, the selection step and the calculation step described above were performed in the same manner except that the candidate element selected in the selection step was changed from Mg to any of Co, Na, Al, and Mn.
(Evaluation process)
The change in the interlayer distance calculated by the above calculation step has the results shown in Table 1 below.

上記算出工程において算出した層間距離の変化が小さいものから順に算出工程に供した候補元素を配列したところ、Ｍｇ、Ｃｏ、Ｎａ、Ａｌ、Ｍｎの順となり、その順にサイクル特性を高める効果が高いと評価することができた。

When the candidate elements subjected to the calculation step are arranged in order from the one having the smallest change in the interlayer distance calculated in the above calculation step, the order is Mg, Co, Na, Al, and Mn, and the effect of enhancing the cycle characteristics is high in that order. I was able to evaluate it.

Claims (2)

A selection step of selecting a candidate element which is a candidate element for substituting a part of the elements of the positive electrode active material for a lithium ion secondary battery having a layered structure,
In the positive electrode active material for a lithium-ion secondary battery in which a part of the elements is replaced by the candidate element, the change in the interlayer distance when lithium is inserted or deintercalated between layers is measured by using the van der Waals density functional. The calculation process of calculating by the first-principles calculation,
A repeating step of changing the type of the candidate element selected in the selecting step and repeating the selecting step and the calculating step;
Lithium ion ion having an evaluation step of arranging the candidate elements that have been subjected to the calculation step in order from the one having the smallest change in the interlayer distance calculated in the calculation step, and evaluating that the effect of enhancing cycle characteristics is high in that order. A method for evaluating a substitution element of a positive electrode active material for a secondary battery. The lithium ion secondary battery positive electrode active material for _{the, LiNiO 2, LiCoO 2, LiNi} 1-t Co t O 2 (0 <t <1), LiNi 1-x-y Co x Al y O 2, and LiNi _{1 The} method for evaluating a substituting element of a positive electrode active material for a lithium ion secondary battery according to claim 1, which is one kind selected from _/3 Mn _1/3 Co _1/3 O ₂ .

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