JP5598726B2 - Lithium secondary battery - Google Patents

Description

  The present invention relates to a lithium secondary battery using a lithium transition metal oxide as a positive electrode active material and a positive electrode active material for the battery.

  Lithium secondary batteries are becoming increasingly important as on-vehicle power supplies or personal computers and portable terminals. In particular, a lithium ion secondary battery that is lightweight and obtains a high energy density is expected to be suitable as a high-output power source mounted on a vehicle. As a typical example of a positive electrode active material used for a lithium secondary battery, a composite oxide containing lithium (Li) and at least one transition metal element (hereinafter also referred to as lithium transition metal oxide) can be given. Patent document 1 is mentioned as a technical document regarding a lithium secondary battery.

JP 2006-310181 A

  When a lithium secondary battery is used in a wide SOC (state of charge) width, the amount of energy that can be effectively utilized by taking out the unit volume or unit mass of the battery can be increased. This is particularly significant in, for example, a vehicle-mounted battery (for example, a vehicle drive power source) that requires high output and high energy density. However, since the output (for example, output at a low temperature) of a lithium secondary battery generally decreases as the SOC decreases, the output value that can be secured over the entire SOC width is reduced simply by widening the SOC width. End up. Further, when a battery is used with a wider SOC width, deterioration due to charge / discharge cycles (for example, reduction in capacity) tends to increase. In particular, the battery is likely to deteriorate in a charge / discharge cycle at a high temperature and a large current as required in a vehicle drive power supply.

  One object of the present invention is to provide a lithium secondary battery that is excellent in output in a low SOC region and in which deterioration due to charge / discharge cycles is suppressed. Another related object is to provide a method for producing such a positive electrode active material for a lithium secondary battery.

According to the present invention, a lithium secondary battery including a positive electrode and a negative electrode is provided. The positive electrode has a positive electrode active material. The positive electrode active material takes the form of secondary particles in which primary particles of a lithium transition metal oxide having a layered structure are collected. The positive electrode active material includes at least one of nickel (Ni), cobalt (Co), and manganese (Mn). The positive electrode active material further contains tungsten (W), calcium (Ca), and magnesium (Mg). W contained in the positive electrode active material is biased to the surface of the primary particles (which can also be grasped as a grain boundary). In the positive electrode active material, the total amount of Ni, Co, and Mn contained in the positive electrode active material is 100 mol%, the number of moles of _W m contained in the positive electrode active material m _W (mol%), the number of moles of _Ca m _Ca (Mol%) and the number of moles of _Mg m _Mg (mol%) satisfy the following formula.
2 ≦ (m _W / m _Ca ) ≦ 50
( _{M W} + m _Ca ) ≧ 0.26
(M _Mg / m _Ca ) ≧ 1.5
A lithium secondary battery including such a positive electrode active material has an improvement in output at a low SOC (in other words, an increase in SOC width at which a desired output is obtained) and a charge / discharge cycle (particularly, a charge / discharge cycle at a high temperature). It is possible to realize improvement of durability at the same time. Therefore, the lithium secondary battery can be appropriately used with a wider SOC width.

  In the present specification, the “lithium secondary battery” refers to a secondary battery that uses lithium ions as electrolyte ions and is charged / discharged by the movement of charges accompanying the lithium ions between the positive and negative electrodes. A battery generally called a lithium ion secondary battery is a typical example included in the lithium secondary battery in this specification. Further, in this specification, the “active material” can reversibly occlude and release (typically insertion and desorption) chemical species (that is, lithium ions here) that serve as charge carriers in the secondary battery. A substance. Further, in this specification, “SOC” refers to a state of charge of a battery based on a voltage range in which the battery is normally used unless otherwise specified. For example, in a lithium ion secondary battery including a lithium transition metal oxide having a layered structure, a rated capacity (typically measured under conditions of a terminal voltage of 4.1 V (upper limit voltage) to 3.0 V (lower limit voltage)) Refers to the state of charge based on the rated capacity specified under the same conditions as the rated capacity measurement of the evaluation test battery described later.

  In a preferred embodiment, Ca contained in the positive electrode active material is biased to the surface of the primary particles. Thus, according to the aspect in which both W and Ca are present (distributed) biased to the surface of the primary particles, the grain boundary strength can be effectively improved by the interaction between W and Ca. It is possible to better suppress deterioration due to cycles.

  In another preferred embodiment, Mg contained in the positive electrode active material is present in the entire interior of the primary particles. According to such an embodiment, the synergistic effect of W present on the surface of the primary particles of the positive electrode active material and Mg present in the entire interior of the primary particles allows for lower SOC in addition to suppressing deterioration due to the charge / discharge cycle. A battery excellent in output in the region can be realized.

The number m _W (mol%) of _W contained in the positive electrode active material is 0.05 ≦ m _W ≦ 1.0, where the total amount of Ni, Co and Mn contained in the positive electrode active material is 100 mol%. It is preferable. As a result, it is possible to achieve both an improvement in output in the low SOC region and suppression of deterioration due to charge / discharge cycles at a higher level.

  In a preferred embodiment, W and Ca contained in the positive electrode active material are compounded with a lithium transition metal oxide. According to this aspect, since the grain boundary strength can be more effectively improved using the W and Ca, deterioration due to the charge / discharge cycle can be more effectively suppressed. According to the embodiment in which both W and Ca are present on the surface of the primary particles in a biased manner, the above-described effect due to the compounding of W and Ca can be exhibited particularly well.

  In a preferred aspect of the technology disclosed herein, the lithium transition metal oxide is an oxide containing at least Ni as a constituent metal element. For example, a lithium transition metal oxide containing all of Ni, Co, and Mn as constituent metal elements (hereinafter also referred to as “LiNiCoMn oxide”) is preferable. According to the positive electrode active material having such a composition, a higher performance lithium secondary battery can be realized.

According to the present invention, the positive electrode active material for a lithium secondary battery is in the form of secondary particles in which primary particles of a lithium transition metal oxide having a layered structure are gathered, and among Ni, Co, and Mn A method for producing a positive electrode active material containing at least one kind and further containing W, Ca, and Mg is provided. The method includes preparing an aqueous solution (typically an acidic aqueous solution) Aq _A containing at least one of Ni, Co, and Mn and Mg and Ca. In addition, the method includes preparing an aqueous solution Aq _C containing W. Then, the aqueous solution Aq _A and the aqueous solution Aq _C are mixed under alkaline conditions to precipitate a hydroxide containing at least one of the Ni, Co, and Mn, Mg, Ca, and W. including. This method typically further comprises mixing the hydroxide and a lithium compound. In addition, the method may include firing the mixture to form the lithium transition metal oxide.

Thus, in the positive electrode active material manufacturing method disclosed herein, an aqueous solution Aq _A containing at least one of Ni, Co, and Mn, Mg and Ca, and an aqueous solution Aq _C containing W are separated into separate aqueous solutions. Preparing and mixing the aqueous solution Aq _A and the aqueous solution Aq _C under alkaline conditions (that is, under conditions where the pH exceeds 7), and containing at least one of Ni, Co and Mn, Mg, Ca and W A hydroxide (hereinafter also referred to as “precursor hydroxide”) is precipitated. Then, this precursor hydroxide is mixed with a lithium compound (Li source) and fired. Such a method is suitable as a method for producing a positive electrode active material which is in the form of secondary particles in which primary particles of lithium transition metal oxide are gathered and W is present on the surface of the primary particles. Therefore, the above method includes the production of any positive electrode active material disclosed herein, the production of a positive electrode for a lithium secondary battery provided with the positive electrode active material, the production of a lithium secondary battery provided with the positive electrode active material, etc. It can be preferably applied.

  The precursor hydroxide is preferably precipitated while maintaining the pH at 11 to 14 (for example, pH 11.5 to 12.5, typically around pH 12). According to the positive electrode active material obtained using such a precursor hydroxide, a lithium secondary battery with higher performance (for example, superior in output in the low SOC region and / or durability against charge / discharge cycles) is realized. Can be done. The pH value in this specification refers to a pH value based on a liquid temperature of 25 ° C. unless otherwise specified.

In carrying out the mixing under the alkaline condition, an alkaline aqueous solution is prepared separately from the aqueous solution Aq _A and the aqueous solution Aq _C , and the alkaline condition is maintained using the alkaline aqueous solution (for example, the pH is set to 11 to 11). 14). As the alkaline aqueous solution, an aqueous solution containing at least ammonia can be preferably used. As a preferable aspect of the technology disclosed herein, an aspect in which a mixed solution of ammonia water and sodium hydroxide aqueous solution is used as the alkaline aqueous solution. Another preferred embodiment is a mode in which two or more types of alkaline aqueous solutions (for example, aqueous ammonia and aqueous sodium hydroxide) are used separately (for example, each alkaline aqueous solution is supplied to the reaction tank independently). . You may combine these aspects.

  According to the present invention, a positive electrode active material produced by any of the methods disclosed herein is provided. Further, according to the present invention, there is provided a positive electrode for a lithium secondary battery provided with the positive electrode active material disclosed herein (which can be a positive electrode active material produced by any of the methods disclosed herein). . Furthermore, a lithium secondary battery including such a positive electrode is provided.

  As described above, the lithium secondary battery disclosed herein (typically, a lithium ion secondary battery) can obtain a good output even in a low SOC region and has excellent durability against charge / discharge cycles. In a wider SOC range, it can be suitably used as a power source or the like. Therefore, as another aspect of the present invention, for example, as shown in FIG. 13, any of the lithium secondary batteries 100 disclosed herein (a form of a battery pack in which a plurality of batteries are typically connected in series) There is provided a vehicle 1 provided with In particular, a vehicle (for example, an automobile) including such a lithium secondary battery as a power source (typically, a power source of a hybrid vehicle or an electric vehicle) is preferable. In addition, according to the present invention, a lithium secondary battery 100 for a vehicle driving power source is provided.

It is a flowchart which shows the outline of the positive electrode active material manufacturing method which concerns on one Embodiment. It is a perspective view which shows typically one structural example of a lithium secondary battery. It is the III-III sectional view taken on the line of FIG. It is a TEM image of the positive electrode active material which concerns on one Embodiment. It is an image which shows distribution of W in the positive electrode active material shown in FIG. It is an image which shows distribution of Ca in the positive electrode active material shown in FIG. It is an image which shows distribution of Mg in the positive electrode active material shown in FIG. It is a part of TOF-SIMS (time-of-flight secondary ion mass spectrometry) positive spectrum of Sample 1. It is a part of TOF-SIMS positive spectrum of Sample 1. It is a part of TOF-SIMS positive spectrum of Sample 1. It is a part of TOF-SIMS negative spectrum of sample 1. It is a part of TOF-SIMS negative spectrum of sample 1. It is a side view which shows typically the vehicle (automobile) carrying a lithium secondary battery.

  Hereinafter, preferred embodiments of the present invention will be described. Note that matters other than matters specifically mentioned in the present specification and necessary for the implementation of the present invention can be grasped as design matters of those skilled in the art based on the prior art in this field. The present invention can be carried out based on the contents disclosed in this specification and common technical knowledge in the field. Further, in the following drawings, members / parts having the same action are described with the same reference numerals, and overlapping descriptions may be omitted or simplified.

≪Positive electrode active material≫
The positive electrode active material in the technology disclosed herein is in the form of secondary particles in which primary particles of lithium transition metal oxide are collected. The lithium transition metal oxide is typically a lithium oxide having a layered (typically rock salt) crystal structure, and includes at least one of Ni, Co, and Mn. The technology disclosed herein is applied to a positive electrode for a lithium secondary battery provided with such a positive electrode active material, and various lithium secondary batteries (typically lithium ion secondary batteries) having the positive electrode as a constituent element. Can be done.

  The total content of Ni, Co, and Mn in the positive electrode active material is, for example, 85 mol% or more (preferably 90 mol% or more, preferably 100 mol%), with the total amount of metal elements other than lithium contained in the positive electrode active material being 100 mol%. Typically 95 mol% or more). A positive electrode active material containing at least Ni is preferable. For example, a positive electrode containing 10 mol% or more (more preferably 25 mol% or more) of Ni with the total amount of metal elements other than lithium contained in the positive electrode active material being 100 mol%. An active material is preferred.

  As a preferable example of the lithium transition metal oxide in the technology disclosed herein, a lithium oxide containing at least Li, Ni, Co, and Mn (that is, LiNiCoMn oxide) can be given. For example, assuming that the total amount (based on the number of atoms) of Ni, Co and Mn is 1, the amounts of Ni, Co and Mn all exceed 0 and not more than 0.7 (for example, more than 0.1 and not more than 0.6) LiNiCoMn oxide that is typically more than 0.3 and 0.5 or less can be preferably used. The first element of Ni, Co, and Mn (the element that is contained most on the basis of the number of atoms) may be any of Ni, Co, and Mn. In a preferred embodiment, the first element is Ni. In another preferred embodiment, the amounts of Ni, Co and Mn (based on the number of atoms) are approximately the same. Such a ternary lithium transition metal oxide is preferable because it exhibits excellent thermal stability as a positive electrode active material.

≪W distribution≫
The positive electrode active material contains at least one of Ni, Co, and Mn, and further contains W, Mg, and Ca. One feature of the technology disclosed herein is that W exists in a biased manner on the surface of the primary particles in the positive electrode active material. As a result, it is possible to effectively improve the output (particularly low temperature output) in the low SOC region and realize a lithium secondary battery excellent in durability against charge / discharge cycles.

  Here, W is “biased on the surface of the primary particles” means that W is present (distributed) concentrated on the surface (grain boundaries) of the primary particles compared to the inside of the primary particles. means. Therefore, it does not mean only an aspect in which W exists only at the grain boundary (in other words, does not exist at all inside the primary particle). The presence of W on the surface of primary particles is, for example, the distribution of W using energy dispersive X-ray spectroscopy (EDX) for active material particles (secondary particles). In the mapping result, it can be understood that W is concentrated on the grain boundary (the amount of W present per area at the grain boundary is larger than that in the primary particle). (See FIG. 5). The position of the grain boundary (primary particle surface) can be grasped by, for example, observation of the cross section with a transmission electron microscope (TEM). A TEM with EDX may be preferably used.

≪Ca distribution≫
In the technique disclosed herein, Ca contained in the positive electrode active material is preferably present (distributed) on the surface of the primary particles in the same manner as W. This makes it possible to realize a lithium secondary battery that is more excellent in durability against charge / discharge cycles. The fact that Ca is biased to the surface of the primary particles can be grasped, for example, by mapping Ca distribution using EDX in the same manner as W (see FIG. 6). The degree of bias of Ca to the primary particle surface may be the same as or different from W.

≪Mg distribution≫
In the technology disclosed herein, Mg contained in the positive electrode active material is preferably present in the entire interior of the primary particles. Here, “Mg is present in the entire interior of the primary particle” means that Mg is present (distributed) in the whole positive electrode active material without showing a noticeable bias (preferably substantially uniformly). means. Therefore, in contrast to the W distribution, there is no bias toward the primary particle surface in Mg. The fact that there is no bias in the distribution of Mg can be recognized, for example, by analyzing the line of active material particles (secondary particles) with EDX and not concentrating them at positions corresponding to the grain boundaries. It can also be grasped by mapping the Mg distribution in the same manner as in the case of W and no concentration at the grain boundary is observed (see FIG. 7). In a preferred embodiment, the result of the line analysis is substantially uniform throughout the interior of the primary particles (eg, throughout the active material particles).

  The positive electrode active material contains at least one of Ni, Co, and Mn (typically, it is included as a constituent metal element of the lithium transition metal oxide). Preferably, it is present substantially uniformly).

In the positive electrode active material in the technology disclosed herein, at least one (preferably both) of W and Ca is in a state where at least a part thereof is compounded with the lithium transition metal oxide in the positive electrode active material (that is, It is preferable that it is contained not in a mixture with lithium transition metal oxides but in a state in which a lithium compound is actually formed. By such compounding, durability against charge / discharge cycles can be further improved. The compounding can be confirmed through, for example, the TOF-SIMS spectrum of the positive electrode active material. When W and Ca are sufficiently compounded, they typically correspond to LiCaWO ₄ ⁺ , Li ₃ CaW ₂ O ₈ O ⁺ , CaWO ₄ ⁻ , LiCaW ₂ O ₈ ^{− and the} like in the TOF-SIMS spectrum. A peak is observed. As other peaks, for example, the above-mentioned compounding can be confirmed also by the fact that a peak corresponding to LiCaO ⁺ is particularly remarkable. A positive electrode active material in which W and Ca are both distributed to grain boundaries and at least a part of them is compounded with a transition metal oxide is preferable.

  The positive electrode active material can contain one or more metal elements in addition to the metal elements described above (that is, at least one of Ni, Co, and Mn, Li, W, Ca, and Mg). . Such a metal element can be, for example, one or more elements selected from Al, Cr, Fe, V, Nb, Mo, Ti, Cu, Zn, Ga, In, Sn, La, Ce, and Na. . Each distribution of such an arbitrary metal element is not particularly limited. For example, it may be present on the surface of the primary particle in a biased manner, or may exist in the entire interior of the primary particle. Such an arbitrary metal element can bring about an effect of reducing the reaction resistance of the battery, for example. The content of each arbitrary metal element (in the case where two or more kinds are included, each content) is, for example, the content of each arbitrary metal element is 1 mol% or less of the total amount of all metal elements other than Li (typically May be less than 1 mol%), and is usually preferably 0.1 mol% or less (typically less than 0.1 mol%). When two or more kinds of optional metal elements are included, the total amount of these optional metal elements should be 2 mol% or less (typically less than 2 mol%) of the total amount of all metal elements other than Li. Usually, it is preferably 0.2 mol% or less (typically less than 0.2 mol%). Alternatively, it does not substantially contain any metal element other than Li, Ni, Co, Mn, W, Ca, and Mg. (This means that such an optional metal element is at least not intentionally included. It may be allowed to be included inevitably or unavoidably.) Lithium transition metal oxides may also be used.

The positive electrode active material may be, for example, a material whose composition excluding W, Mg, and Ca (referred to as an average composition of the entire positive electrode active material) is represented by the following formula (I).
_{Li 1 + m Ni p Co q} Mn r M 1 s O 2 (I)
In the above formula (I), m may be 0 ≦ m ≦ 0.2 (for example, 0.05 ≦ x ≦ 0.2). P in the above formula may be 0.1 <p ≦ 1 (for example, 0.3 <p <0.9, preferably 0.3 <p <0.6). q may be 0 ≦ q ≦ 0.5 (for example, 0.1 <q <0.4, preferably 0.3 <q <0.6). r may be 0 ≦ r ≦ 0.5 (eg, 0.1 <r <0.4, preferably 0.3 <r <0.6). However, p + q + r ≦ 1 (typically 0.8 ≦ p + q + r ≦ 1, for example 0.9 ≦ p + q + r ≦ 1, M ¹ is Al, Cr, Fe, V, Ti, Mo, Cu, Zn, Ga , In, Sn, La, Ce and Na may be one or two or more selected from s, 0 may satisfy ≦ s ≦ 0.05, and s may be substantially 0 (that is, M ¹ substantially Oxide which does not contain in particular.

  A positive electrode active material having an average composition in which predetermined amounts of W, Ca, and Mg are added to the composition represented by the above formula (I) is preferable. In addition, the said Formula (I) points out the composition remove | excluding W, Ca, and Mg from the composition of the positive electrode active material at the time of battery construction (in other words, composition of the positive electrode active material used for manufacture of a battery). This composition is generally the same as the composition at the time of complete discharge of the battery.

In the technology disclosed herein, the positive electrode active material has a total amount of Ni, Co and Mn contained in the positive electrode active material of 100 mol%, and the number of moles W of W contained in the positive electrode active material m _W (mol%), It is preferable that the relationship between the number of moles of _Ca m _Ca (mol%) and the number of moles of _Mg m _Mg (mol%) satisfy the following formulas (III) to (V).
2 ≦ (m _W / m _Ca ) ≦ 50 (III)
( _{M W} + m _Ca ) ≧ 0.26 (IV)
(M _Mg / m _Ca ) ≧ 1.5 (V)

When m _W / m _Ca is less than 2, the output at low SOC (for example, low SOC output at low temperature as in the low SOC-30 ° C. output described later) tends to be low. When m _W / m _Ca is too larger than 50, durability against charge / discharge cycles (for example, durability against charge / discharge cycles at a high temperature and a large current as in the 4C-CC capacity retention rate described later) tends to decrease. . In a preferred embodiment, m _W / m _Ca is 2 to 40 (for example, 2 to 35).

When m _W + m _Ca is less than 0.26 mol%, durability (for example, capacity retention rate) with respect to the charge / discharge cycle tends to decrease. In particular, the deterioration with respect to the charge / discharge cycle at a high temperature and a large current tends to be large like a 4C-CC capacity retention rate described later. Although the upper limit of m _W + m _Ca is not particularly limited, m _W + m _Ca is usually about 3.0 mol% or less (typically 2.0 mol% or less, for example, 1. 5 mol% or less) is appropriate. For example, a positive electrode active material satisfying 0.28 mol% ≦ (m _W + m _Ca ) ≦ 1.20 mol% is preferable.

If m _Mg / m _Ca is too smaller than 1.5, the output at low SOC (for example, low SOC output at a low temperature of about −30 ° C.) tends to be low. The upper limit of m _Mg / m _Ca is not particularly limited, but considering the balance with m _W / m _Ca and m _W + m _Ca and the initial capacity and cost, m _Mg / m _Ca is usually about 50 or less (typical Is suitably 20 or less, for example 10 or less.

  In the technology disclosed herein, the content of W in the positive electrode active material is, for example, more than 0 mol% and not more than 3 mol%, where the total amount of Ni, Co and Mn contained in the positive electrode active material is 100 mol%. (Typically 0.01 mol% or more and 3 mol% or less). If the W content is too small, the battery performance improvement effect (for example, the effect of improving the output in the low SOC region, the effect of reducing the reaction resistance, etc.) with respect to the positive electrode active material having a composition not containing W is hardly exhibited. It can be. In addition, when the amount of W is too large, the effect of improving the battery performance with respect to the composition containing no W may not be sufficiently exhibited, or the battery performance may be deteriorated. In a preferred embodiment, the W content is 0.05 mol% or more and 2.0 mol% or less (for example, 0.1 mol% or more and 1.0 mol% or less). In the technique disclosed here, W is concentrated at a position suitable for performing a desired function (specifically, the surface of the primary particle), so even a smaller amount of W is sufficient. A significant performance improvement effect can be exhibited. Therefore, adverse effects (traversal) that can occur with the use of W can be better suppressed. It is also advantageous from the viewpoint of reducing the resource risk of battery materials. The content of W can be measured according to JIS K 0116, for example, by ICP (Inductively Coupled Plasma) emission analysis.

  The content of Ca in the positive electrode active material is, for example, more than 0 mol% and not more than 3 mol% (typically 0.005) with the total amount of Ni, Co, and Mn contained in the positive electrode active material being 100 mol%. Mol% or more and 2 mol% or less). When there is too little content of Ca, the effect which improves the durability with respect to a charging / discharging cycle may become difficult to fully exhibit. If the amount of Ca is too large, the initial capacity may tend to decrease. In a preferred embodiment, the Ca content is 0.01 mol% or more and 1.0 mol% or less (eg, 0.01 mol% or more and 0.5 mol% or less, typically 0.01 mol% or more and 0.25 mol or less). Mol% or less). According to an embodiment in which Ca is concentrated on the grain boundary (the surface of the primary particles), a sufficient performance improvement effect can be exhibited even with a smaller amount of Ca, which is preferable. The Ca content can be measured by, for example, ICP emission analysis.

  The content of Mg in the positive electrode active material is, for example, more than 0 mol% and not more than 3 mol% (typically 0.005), with the total amount of Ni, Co, and Mn contained in the positive electrode active material being 100 mol%. Mol% or more and 2 mol% or less). If the content of Mg is too small, the battery performance improvement effect (for example, the effect of improving the output in the low SOC region) with respect to the positive electrode active material having a composition not containing Mg may not be sufficiently exhibited. If the Mg content is too high, the initial capacity may tend to decrease. In a preferred embodiment, the Mg content is 0.05 mol% or more and 2.0 mol% or less (for example, 0.05 mol% or more and 1.0 mol% or less). The Mg content can be measured by, for example, ICP emission analysis.

  In carrying out the technology disclosed herein, it is not necessary to clarify the reason why both the output in the low SOC region and the durability against the charge / discharge cycle are improved according to the battery using the positive electrode active material having the above configuration, for example, The following inferences are possible. That is, as one method for improving the output in the low SOC region, it is conceivable to reduce the discharge depth of the positive electrode in the low SOC region. The shallow depth of discharge of the positive electrode means that the amount of Li ions that can be accepted by the positive electrode active material in a predetermined SOC is large with respect to the SOC of the battery (a large margin for accepting Li ions). When the amount of Li ions that can be accepted increases, the mobility (diffusibility) of Li in the solid of the positive electrode active material tends to increase. Therefore, it is expected that the output in the low SOC region (particularly, the low temperature output that is easily affected by the diffusibility of Li) can be improved by reducing the discharge depth of the positive electrode in the low SOC region.

  In order to reduce the depth of discharge of the positive electrode in the low SOC region, it is effective to increase the initial charge / discharge efficiency of the positive electrode (in other words, reduce the irreversible capacity). In the positive electrode active material disclosed herein, W present on the surface (particle interface) of the primary particles helps to improve the initial charge / discharge efficiency by, for example, contributing to charge / discharge of the battery due to its own valence change. obtain. This is considered to improve the output in the low SOC region. The W can also exert an effect of reducing the reaction resistance of the positive electrode active material. Mg contained in the positive electrode active material can help improve the initial charge / discharge efficiency by stabilizing the crystal structure change due to charge / discharge. In the positive electrode active material in which Mg is present in the entire interior of the primary particles (typically substantially uniformly distributed), the above-described stabilization effect can be exhibited better. Thus, by arranging W and Mg in suitable positions in the positive electrode active material, the output in the low SOC region is effectively improved in the battery using the positive electrode active material. Conceivable.

  W present on the surface of the primary particles can also help improve the grain boundary strength. In the case where W is compounded with a lithium transition metal oxide, the effect of improving the grain boundary strength can be exhibited particularly well. When the grain boundary strength increases, the strength of the secondary particles (positive electrode active material) is improved, thereby suppressing the deterioration of the positive electrode active material associated with the charge / discharge cycle (for example, withstanding the expansion and contraction associated with charge / discharge and the initial It is presumed that the particle structure of the Ca contained in the positive electrode active material can also contribute to an improvement in battery performance through an improvement in the grain boundary strength of the positive electrode active material. In the case where Ca is compounded with a lithium transition metal oxide, the effect of improving the grain boundary strength can be better exhibited. It is preferable that Ca is present on the surface of the primary particles so that a high grain boundary strength improving effect can be exhibited by a smaller amount of Ca.

≪Method for producing positive electrode active material≫
As a method for producing such a positive electrode active material, a method capable of preparing the active material as a final product can be appropriately employed. Hereinafter, an example of a positive electrode active material in which the lithium transition metal oxide is mainly a layered structure oxide (LiNiCoMn oxide) containing all of Ni, Co and Mn will be described. Although the embodiment will be described in more detail, it is not intended to limit the application target of the technology disclosed herein to such a positive electrode active material.

As shown in FIG. 1, the positive electrode active material manufacturing method according to the present embodiment prepares an aqueous solution (typically acidic, that is, an aqueous solution having a pH of less than 7) Aq _A containing Ni, Co, Mn, Ca and Mg. (Step S110). This aqueous solution Aq _A is typically a composition that is substantially free of W. The amount ratio of each metal element contained in the aqueous solution Aq _A can be appropriately set according to the composition of the positive electrode active material that is the object. For example, the molar ratio of Ni, Co, and Mn can be approximately the same as the molar ratio of these elements in the positive electrode active material. The ratio of the total mass of Ni, Co, and Mn contained in the aqueous solution Aq _A to the mass of each of Ca and Mg can be approximately the same as the mass ratio of the positive electrode active material. The aqueous solution Aq _A may be one type of aqueous solution containing all of Ni, Co, Mn, Ca, and Mg, or may be two or more types of aqueous solutions having different compositions. For example, two types of aqueous solutions Aq _A1 containing only Ni, Co, Mn and Ca as metal elements and aqueous solutions Aq _A2 containing only Mg as metal elements may be used as the Aq _A. Usually, from the viewpoint of avoiding complication of the manufacturing apparatus and easy control of the manufacturing conditions, an embodiment using one kind of aqueous solution Aq _A containing all of Ni, Co, Mn, Ca and Mg is preferably adopted. obtain.

≪Aqueous solution Aq _A ≫
The aqueous solution Aq _A can be prepared, for example, by dissolving a predetermined amount of each of an appropriate Ni compound, Co compound, Mn compound, Ca compound, and Mg compound in an aqueous solvent. As these metal compounds, salts of each metal (namely, Ni salt, Co salt, Mn salt, Ca salt and Mg salt) can be preferably used. The order in which these metal salts are added to the aqueous solvent is not particularly limited. Moreover, you may prepare by mixing the aqueous solution of each salt. Or you may mix the aqueous solution of Ca salt and Mg salt with the aqueous solution containing Ni salt, Co salt, and Mn salt. The anions in these metal salts (Ni salt, Co salt, Mn salt, Ca salt, Mg salt) may be selected so that the salt has a desired water solubility. For example, it may be sulfate ion, nitrate ion, chloride ion, carbonate ion, hydroxide ion, and the like. That is, the metal salt may be Ni, Co, Mn, Ca, Mg sulfate, nitrate, hydrochloride, carbonate, hydroxide, and the like. All or part of these metal salt anions may be the same or different from each other. Each of these salts may be a solvate such as a hydrate. FIG. 1 shows an example in which sulfates of respective metals are used. The concentration of the aqueous solution Aq _A is preferably such that the total of all transition metals (Ni, Co, Mn) is about 1.0 to 2.2 mol / L.

≪Aqueous solution Aq _C (W aqueous solution) ≫
The positive electrode active material manufacturing method according to this embodiment also includes preparing an aqueous solution Aq _C containing W (hereinafter also referred to as “W aqueous solution”) (step S120). This W aqueous solution typically does not substantially contain Ni, Co, Mn, Ca and Mg (which means that these metal elements are at least not intentionally contained and mixed as an inevitable impurity). Is acceptable.) A composition. For example, a W aqueous solution containing substantially only W as a metal element can be preferably used. The aqueous W solution can be prepared by dissolving a predetermined amount of a W compound in an aqueous solvent, similar to the aqueous solution Aq _A described above. As such a W compound, for example, various W salts can be used. In a preferred embodiment, a salt of tungstic acid (an oxo acid having W as a central element) is used. The cation in the W salt can be selected so that the salt is water-soluble, and may be, for example, ammonium ion, sodium ion, potassium ion, or the like. An example of a W salt that can be preferably used is ammonium paratungstate (5 (NH ₄ ) ₂ O · 12WO ₃ ) (FIG. 1). The W salt may be a solvate such as a hydrate. The concentration of the aqueous W solution is preferably about 0.01 to 1 mol / L based on the W element.

The aqueous solvent used for the preparation of the aqueous solution Aq _A and the aqueous W solution is typically water. Depending on the solubility of each salt metal compound used, water containing a reagent (acid, base, etc.) that improves solubility may be used.

≪Alkaline aqueous solution≫
The method according to this aspect may further include providing an alkaline aqueous solution (step S130). This alkaline aqueous solution is an aqueous solution in which an alkaline agent (a compound having an action of tilting the liquid property toward the alkali side) is dissolved in an aqueous solvent. As the alkali agent, any of strong bases (for example, alkali metal hydroxides such as sodium hydroxide and potassium hydroxide) and weak bases (ammonia, amine, etc.) can be used. Alkaline aqueous solution Aq _B containing at least ammonia is preferred. In a preferred embodiment of the technology disclosed herein, an alkaline aqueous solution containing both a weak base and a strong base is used. For example, an alkaline aqueous solution containing ammonia and sodium hydroxide can be preferably used. A plurality of alkaline aqueous solutions having different compositions (for example, aqueous ammonia and aqueous sodium hydroxide solution) may be used. Typically, Ni, Co, Mn, Ca, Mg and W are not substantially contained. (It means that these metal elements are not at least intentionally contained, and it is allowed to be mixed as an inevitable impurity etc.) It is a composition.

≪Crystal crystallization of precursor hydroxide≫
_A precursor hydroxide containing Ni, Co, Mn, Ca, Mg and W is precipitated by mixing the aqueous solution Aq _A and the aqueous W solution under alkaline (preferably pH 11 to 14) conditions ( Crystallization) (step S140). Therefore, the aqueous solution Aq _A is mixed with W after being neutralized. For example, an alkaline aqueous solution having an initial pH of 11 to 14 (typically 11.5 to 12.5, for example, about 12.0) is prepared in the reaction tank (step S142), while maintaining the initial pH, Aqueous solution Aq _A and aqueous W solution Aq _C are supplied to the reaction vessel at an appropriate rate and mixed with stirring. At this time, in order to maintain the initial pH, an alkaline aqueous solution may be additionally supplied to the reaction vessel as necessary (step S144). The precipitated precursor hydroxide is preferably washed and filtered, dried after crystallization, and prepared into particles having a desired particle size (step S150).

Thus, by preparing Ni, Co, Mn, Ca and Mg, and W in separate aqueous solutions and mixing them under alkaline conditions (typically under pH 11 or higher), Precursor hydroxide (typically suitable for the production of a positive electrode active material in which W is present on the surface of primary particles (typically, a positive electrode active material in which Mg is present throughout the primary particles) Particulate) can be produced. As described above, the aqueous solution Aq _A containing Ni, Co, Mn, Ca and Mg is neutralized and then mixed with W, so that the hydroxide containing Ni, Co, Mn, Ca and Mg is first used. It is thought that W starts to precipitate and W easily precipitates by contacting the precipitate.

  While the precipitation reaction of the precursor hydroxide (the formation reaction of the hydroxide) proceeds, the temperature of the reaction solution can be controlled in the range of 20 ° C. to 60 ° C. (for example, 30 ° C. to 50 ° C.). preferable. Moreover, it is preferable to adjust the pH of the reaction solution to 11 to 14 (typically 11.5 to 12.5, for example, about 12.0). In the embodiment using an alkaline aqueous solution containing ammonia, it is preferable to adjust the ammonia concentration in the reaction solution to 3 to 25 g / L. The time for continuing the precipitation reaction of the precursor hydroxide can be appropriately set according to the particle diameter (typically average particle diameter) of the target positive electrode active material. As a tendency, in order to obtain a positive electrode active material having a larger particle diameter, it is preferable to increase the reaction time.

  The positive electrode active material manufacturing method according to this aspect may include mixing the precursor hydroxide and the Li compound (step S160). As the Li compound, an oxide containing Li may be used, and a compound that can be converted into an oxide by heating (Li carbonate, nitrate, sulfate, oxalate, hydroxide, ammonium salt, sodium salt, etc.) May be used. Examples of preferred Li compounds include lithium carbonate and lithium hydroxide. Such Li compounds can be used alone or in combination of two or more. Mixing of the precursor hydroxide and the Li compound may be performed by either wet mixing or dry mixing. From the viewpoint of simplicity and cost, dry mixing is preferred. The mixing ratio of the precursor hydroxide and the Li compound can be determined such that the molar ratio of Li to Ni, Co, and Mn in the target positive electrode active material is realized. For example, the precursor hydroxide and the Li compound may be mixed so that the molar ratio of Li to Ni, Co, and Mn is approximately the same as the molar ratio in the positive electrode active material.

  Then, the mixture is fired to produce a lithium transition metal oxide (step S170). The firing temperature is preferably in the range of about 700 to 1000 ° C. Firing may be performed at the same temperature at a time, or may be performed stepwise at different temperatures. The firing time can be appropriately selected. For example, it may be baked at about 800 to 1000 ° C. for about 2 to 24 hours, or after baking at about 700 to 800 ° C. for about 1 to 12 hours and then at about 800 to 1000 ° C. for about 2 to 24 hours. Also good. In order to obtain a higher output, it is preferable to set the firing temperature in the range of 850 ° C. to 980 ° C. (more preferably 850 ° C. to 950 ° C.). Such firing conditions can be preferably employed in the production of a positive electrode active material used in a lithium secondary battery for applications where emphasis is placed on enhancing output performance, such as a hybrid vehicle. Moreover, in order to make the range of SOC that can exhibit a desired output wider, it is preferable that the firing temperature is in the range of 900 ° C to 1000 ° C. Such firing conditions can be preferably employed in the production of a positive electrode active material used for a lithium secondary battery for applications in which it is important to increase the amount of electricity that can be taken out, such as an electric vehicle.

  Preferably, after the firing step, the fired product is crushed and sieved as necessary to adjust the particle size of the positive electrode active material. In this way, it is possible to obtain a positive electrode active material that is in the form of secondary particles in which primary particles of lithium transition metal oxide are gathered and W is present on the surface of the primary particles. In a preferred embodiment, Ca contained in the positive electrode active material is also present on the surface (grain boundary) of the primary particles. On the other hand, Mg contained in the positive electrode active material is preferably present substantially uniformly throughout the interior of the primary particles. According to such a manufacturing method, typically, as seen in a TOF-SIMS spectrum of Sample 1 described later, a positive electrode active material containing a part of W and at least a part of Ca in a compounded state with Li. Is obtained. One of the reasons why a positive electrode active material in which W and Ca are both distributed to grain boundaries is obtained by the above production method is the contribution of such compounding (that is, formation of a compound containing Li, W and Ca). Inferred.

The positive electrode active material in the technology disclosed herein may have an average particle size of the secondary particles of about 1 μm to 50 μm. Usually, a positive electrode active material having an average particle size of about 2 μm to 20 μm (typically 3 μm to 10 μm, for example, about 3 μm to 8 μm) is preferable. In the present specification, the “average particle diameter” means a median diameter (50% volume average particle diameter; below), which can be derived from a particle size distribution measured based on a particle size distribution measuring apparatus based on a laser scattering / diffraction method, unless otherwise specified. "D50"). The specific surface area of the positive electrode active material is preferably in the range of approximately 0.5 to 1.9 m ² / g.

  The average particle diameter of the primary particles constituting the positive electrode active material is at least 5 (for example, 5 to 5) using an electron microscope (both transmission type (TEM) and scanning type (SEM) can be used). It can be grasped by measuring the diameter (longest difference length) with respect to a certain direction of primary particles (about 10) and averaging them. Usually, a positive electrode active material in which the average particle diameter of the primary particles is 0.1 μm to 1.0 μm (for example, 0.2 μm to 0.7 μm) is preferable.

  According to the present invention, a positive electrode having any of the positive electrode active materials disclosed herein is provided. Moreover, the lithium ion secondary battery provided with the said positive electrode is provided. An embodiment of such a lithium ion secondary battery will be described in detail by taking as an example a lithium ion secondary battery having a configuration in which a wound electrode body and a non-aqueous electrolyte are accommodated in a flat rectangular battery case. However, it is not intended to limit the embodiments of the technology disclosed herein.

≪Lithium ion secondary battery≫
In the lithium ion secondary battery according to an embodiment of the technology disclosed herein, for example, as shown in FIGS. 2 and 3, the wound electrode body 20 includes the non-aqueous electrolyte 90 and the electrode body 20. It has the structure accommodated in the flat box-shaped battery case 10 corresponding to a shape. The opening 12 of the case 10 is closed with a lid 14. The lid body 14 is provided with a positive terminal 38 and a negative terminal 48 for external connection so that a part of the terminals protrudes outward from the lid body 14. The lithium ion secondary battery 100 having such a configuration is provided in the lid body 14 after the electrode body 20 is accommodated inside the opening portion 12 of the case 10 and the lid body 14 is attached to the opening portion 12 of the case 10, for example. It can be constructed by injecting an electrolytic solution 90 from a prepared electrolytic solution injection hole (not shown) and then closing the injection hole.

  The electrode body 20 has a positive electrode sheet 30 in which a positive electrode mixture layer 34 containing a positive electrode active material is held by a long sheet-like positive electrode current collector 32 and a negative electrode mixture layer 44 containing a negative electrode active material in a long sheet form. The negative electrode sheet 40 held on the negative electrode current collector 42 is overlapped and wound, and the obtained wound body is pressed from the side surface direction and flared to form a flat shape. Typically, an insulating layer that prevents direct contact between the positive electrode mixture layer 34 and the negative electrode mixture layer 44 is disposed between the positive electrode mixture layer 34 and the negative electrode mixture layer 44. In a preferred embodiment, two long sheet-like separators 50 are used as the insulating layer. For example, the electrode body 20 is configured by winding these separators 50 together with the positive electrode sheet 30 and the negative electrode sheet 40. The insulating layer may be coated on one or both surfaces of the positive electrode mixture layer 34 and the negative electrode mixture layer 44.

  The positive electrode sheet 30 is formed such that the positive electrode mixture layer 34 is not provided at one end along the longitudinal direction, and the positive electrode current collector 32 is exposed. Similarly, the negative electrode sheet 40 is formed so that the negative electrode mixture layer 44 is not provided at one end along the longitudinal direction, and the negative electrode current collector 42 is exposed. A positive electrode terminal 38 is joined to the exposed end portion of the positive electrode current collector 32, and a negative electrode terminal 48 is joined to the exposed end portion of the negative electrode current collector 42. The positive and negative electrode terminals 38 and 48 and the positive and negative electrode current collectors 32 and 42 can be joined by, for example, ultrasonic welding, resistance welding, or the like.

  The positive electrode sheet 30 is a paste-like or slurry-like composition in which any of the positive electrode active materials disclosed herein is dispersed in an appropriate solvent together with a conductive material, a binder (binder) and the like used as necessary. The positive electrode mixture composition can be preferably prepared by applying the positive electrode current collector composition 32 to the positive electrode current collector 32 and drying the composition. As the solvent, any of an aqueous solvent and an organic solvent can be used. From the viewpoint of highly preventing the W contained in the positive electrode active material from eluting into the solvent, it is preferable to use an organic solvent (for example, N-methyl-2-pyrrolidone (NMP)) as the solvent.

  As the conductive material, a conductive powder material such as carbon powder or carbon fiber is preferably used. As the carbon powder, various carbon blacks such as acetylene black, furnace black, ketjen black, and graphite powder are preferable. A conductive material can be used alone or in combination of two or more.

  Examples of the binder include carboxymethyl cellulose (CMC; typically sodium salt), polyvinyl alcohol (PVA), polytetrafluoroethylene (PTFE), styrene butadiene rubber (SBR), polyvinylidene fluoride (PVDF), and the like. It is done. Such a binder can be used individually by 1 type or in combination of 2 or more types as appropriate. Such a binder can also function as a thickener for the positive electrode mixture composition.

  The proportion of the positive electrode active material in the entire positive electrode mixture layer is suitably about 50% by mass or more (typically 50 to 95% by mass), and usually about 70 to 95% by mass. preferable. When the conductive material is used, the ratio of the conductive material in the entire positive electrode mixture layer can be, for example, about 2 to 20% by mass, and is usually preferably about 2 to 15% by mass. When using a binder, the ratio of the binder to the whole positive electrode mixture layer can be, for example, about 0.5 to 10% by mass, and usually about 1 to 5% by mass is appropriate.

  As the positive electrode current collector 32, a conductive member made of a highly conductive metal is preferably used. For example, aluminum or an alloy containing aluminum as a main component can be used. The shape of the positive electrode current collector 32 may vary depending on the shape of the lithium ion secondary battery, and is not particularly limited, and may be various forms such as a rod shape, a plate shape, a sheet shape, a foil shape, and a mesh shape. . In the lithium ion secondary battery 100 including the wound electrode body 20 as in the present embodiment, an aluminum sheet (aluminum foil) having a thickness of about 10 μm to 30 μm can be preferably used as the positive electrode current collector 32.

The positive electrode mixture composition applied to the positive electrode current collector 32 can be dried under heating as necessary. After drying, the whole may be pressed as necessary. The mass of the positive electrode mixture layer 34 provided per unit area of the positive electrode current collector 32 (the total mass on both sides in the configuration having the positive electrode mixture layer 34 on both surfaces of the positive electrode current collector 32) is, for example, 5 to 40 mg / cm. ² (typically 5 to 20 mg / cm ² ) is appropriate. The density of the positive electrode mixture layer 34 can be, for example, about 1.0 to 3.0 g / cm ³ (typically 1.5 to 3.0 g / cm ³ ).

  The negative electrode sheet 40 provides, for example, a negative electrode current collector 42 with a paste or slurry-like composition (negative electrode mixture composition) in which a negative electrode active material is dispersed in an appropriate solvent together with a binder used as necessary. And it can produce preferably by drying this composition.

  As the negative electrode active material, one type or two or more types of materials conventionally used for lithium ion secondary batteries can be used without any particular limitation. A carbon material is mentioned as a suitable negative electrode active material. Particulate carbon materials (carbon particles) having a graphite structure (layered structure) at least partially are preferable. Any carbon material of a so-called graphitic material (graphite), non-graphitizable carbon material (hard carbon), easily graphitized carbon material (soft carbon), or a combination of these materials is preferably used. obtain. Among these, graphite particles such as natural graphite can be preferably used. Carbon particles in which amorphous (amorphous) carbon is added to the surface of graphite may be used. The ratio of the negative electrode active material in the entire negative electrode mixture layer is not particularly limited, but it is usually appropriately about 50% by mass or more, preferably about 90 to 99% by mass (for example, about 95 to 99% by mass). It is.

  As a binder, the thing similar to the positive electrode mentioned above can be used individually or in combination of 2 or more types. What is necessary is just to select the addition amount of a binder suitably according to the kind and quantity of a negative electrode active material, for example, can be about 1-5 mass% of the whole negative mix layer.

  As the negative electrode current collector 42, a conductive member made of a metal having good conductivity is preferably used. For example, copper or an alloy containing copper as a main component can be used. Further, the shape of the negative electrode current collector 42 can be in various forms like the positive electrode current collector 32. In the lithium ion secondary battery 100 including the wound electrode body 20 as in the present embodiment, a copper sheet (copper foil) having a thickness of about 5 μm to 30 μm can be preferably used as the negative electrode current collector 42.

Drying of the positive electrode mixture composition applied to the negative electrode current collector 42 can be performed under heating as necessary. After drying, the whole may be pressed as necessary. The mass (total mass of both surfaces) of the negative electrode mixture layer 44 provided per unit area of the negative electrode current collector 42 is, for example, about 3 to 30 mg / cm ² (typically 3 to 15 mg / cm ² ). Is appropriate. The density of the negative electrode mixture layer 44 can be, for example, about 0.8 to 2.0 g / cm ³ (typically 1.0 to 2.0 g / cm ³ ).

  As the separator 50 interposed between the positive electrode sheet 30 and the negative electrode sheet 40, the same separator as a general separator in the field can be used without particular limitation. For example, a porous sheet made of a resin such as polyethylene (PE), polypropylene (PP), polyester, cellulose, or polyamide, a nonwoven fabric, or the like can be used. Preferable examples include a single layer or multilayer structure porous sheet (microporous resin sheet) mainly composed of one or more kinds of polyolefin resins. For example, a PE sheet, a PP sheet, a sheet having a three-layer structure (PP / PE / PP structure) in which PP layers are laminated on both sides of the PE layer, and the like can be suitably used. The thickness of the separator is preferably set within a range of approximately 10 μm to 40 μm, for example. The separator in the technique disclosed herein may have a configuration in which a porous heat-resistant layer is provided on one side or both sides (typically, one side) of the porous sheet or the nonwoven fabric. Such a porous heat-resistant layer can be, for example, a layer containing an inorganic material (inorganic fillers such as alumina particles can be preferably employed) and a binder.

  As the nonaqueous electrolyte solution 90, a solution containing an electrolyte (supporting salt) in a nonaqueous solvent (organic solvent) is used. As said non-aqueous solvent, the organic solvent used for the electrolyte solution of a general lithium ion secondary battery can be used, selecting suitably 1 type, or 2 or more types. Examples of particularly preferred non-aqueous solvents include carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), vinylene carbonate (VC), and propylene carbonate (PC). The For example, a mixed solvent containing EC, EMC, and DMC at a volume ratio of 3: 3: 4 can be preferably used.

As the electrolyte, one or more lithium salts used as an electrolyte in a general lithium ion secondary battery can be appropriately selected and used. Examples of the lithium salt include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li (CF ₃ SO ₂ ) ₂ N, LiCF ₃ SO _{3 and the} like. A particularly preferred example is LiPF ₆ . The nonaqueous electrolytic solution 90 is preferably prepared so that, for example, the electrolyte concentration is within a range of 0.7 to 1.3 mol / L (typically 1.0 to 1.2 mol / L).

The nonaqueous electrolytic solution 90 may contain an optional additive as necessary as long as the object of the present invention is not significantly impaired. Such additives include one or more purposes such as, for example, improvement in output performance of the battery 100, improvement in storage stability (suppression of capacity reduction during storage, etc.), improvement in cycle characteristics, improvement in initial charge / discharge efficiency, and the like. Can be used in Examples of preferable additives include fluorophosphate (preferably difluorophosphate, for example, lithium difluorophosphate represented by LiPO ₂ F ₂ ), lithium bisoxalate borate (LiBOB), and the like. The concentration of each additive in the nonaqueous electrolytic solution 90 is usually suitably 0.20 mol / L or less (typically 0.005 to 0.20 mol / L), for example, 0.10 mol / L. Or less (typically 0.01 to 0.10 mol / L). As a preferable embodiment, a nonaqueous electrolytic solution 90 containing both LiPO ₂ F ₂ and LiBOB at a concentration of 0.01 to 0.05 mol / L (for example, 0.025 mol / L, respectively) can be given.

  Several examples relating to the present invention will be described below, but the present invention is not intended to be limited to the specific examples.

≪Preparation of positive electrode active material sample≫
(Sample 1)
About half the volume of water was placed in a reaction vessel equipped with a stirrer and a nitrogen introduction tube, and heated to 40 ° C. with stirring. After replacing the reaction tank with nitrogen, a 25% (mass basis) aqueous sodium hydroxide solution and 25% while maintaining the space in the reaction tank in a non-oxidizing atmosphere having an oxygen concentration of about 2.0% under a nitrogen stream (Mass basis) Aqueous aqueous solution (NH ₃ · NaOH aqueous solution) having a pH of 12.0 based on a liquid temperature of 25 ° C. and an ammonia concentration of 15 g / L based on a liquid temperature of 25 ° C. by adding appropriate amounts of ammonia water. Was prepared.

Nickel sulfate (NiSO ₄ ), cobalt sulfate (CoSO ₄ ), and manganese sulfate (MnSO ₄ ) have a metal element molar ratio (Ni: Co: Mn) of 0.34: 0.33: 0.33, and these metals The total concentration of was dissolved in water so that it would be 1.8 mol / L. This aqueous solution was further mixed with calcium sulfate (CaSO ₄ ) and magnesium sulfate (MgSO ₄ ) to prepare an aqueous solution Aq _A containing Ni, Co, Mn, Ca and Mg.

Ammonium paratungstate (5 (NH ₄ ) ₂ O · 12WO ₃ ) was dissolved in water to prepare an aqueous solution Aq _C (W aqueous solution) having a tungsten (W) concentration of 0.05 mol / L.

The aqueous solution Aq _A obtained above, the aqueous solution Aq _C , the 25% aqueous sodium hydroxide solution, and the 25% aqueous ammonia were added to the alkaline aqueous solution in the reaction vessel, and the pH of the reaction solution was maintained at 12.0. The mixture was added and mixed while maintaining the ammonia concentration in the liquid phase at 15 g / L. The pH and ammonia concentration were adjusted by adjusting the supply rate of each solution to the reaction tank.

The precipitated product is separated, washed with water and dried, and a precursor having a molar ratio of Ni: Co: Mn: W: Ca: Mg of 34: 33: 33: 0.5: 0.213: 0.326 Body hydroxide was obtained. The average composition of this hydroxide (hydroxide particles) is Ni _0.34 Co _0.33 Mn _0.33 W _0.005 Ca _0.00213 Mg _0.00326 (OH) _{2 + α} (0 ≦ α ≦ 0. 5). The hydroxide was held in an air atmosphere at a temperature of 150 ° C. for 12 hours.

Next, lithium carbonate is weighed so that the total molar number of Ni, Co and Mn contained in the hydroxide is M _T , and the molar ratio of lithium to M _T (Li / M _T ) is 1.15. And mixed with the hydroxide. The obtained mixture was fired at 800 ° C. to 950 ° C. for 5 to 20 hours in air having an oxygen (O ₂ ) concentration of 21 vol%. The fired product is pulverized and sieved, and a positive electrode whose average composition is represented by Li _1.15 Ni _0.34 Co _0.33 Mn _0.33 W _0.005 Ca _0.00213 Mg _0.00326 O ₂ An active material sample 1 was obtained. The Ca and Mg contents were measured by ICP emission spectroscopic analysis.

(Samples 2 to 21)
The concentration of Ca and Mg in the aqueous solution Aq _A and the concentration of W in the aqueous W solution are set so that the contents of Ca, Mg and W (m _W , m _Ca and m _Mg ) are the values shown in Tables 1 and 2, respectively Adjusted. About other points, it carried out similarly to preparation of the positive electrode active material sample 1, and produced the positive electrode active material samples 2-21.
Each of these positive electrode active material samples 1 to 21 was adjusted so that the average particle diameter (D50) was in the range of 3 μm to 8 μm and the specific surface area was in the range of 0.5 to 1.9 m ² / g.

≪TEM observation≫
When the positive electrode active material samples 1 to 21 obtained above were observed with a TEM, it was confirmed that all of them were in the form of secondary particles in which a plurality of primary particles were collected. The average particle diameter of the primary particles obtained from the TEM image was in the range of about 0.1 μm to 0.6 μm. The average particle diameter of the primary particles was obtained by measuring the diameter (longest difference length) in a certain direction for about 10 primary particles and arithmetically averaging them. One of the TEM images of Sample 1 is shown in FIG. This TEM image was obtained under the conditions of an acceleration voltage of 200 kV using a transmission electron microscope manufactured by JEOL Ltd., model “JEM-2100F”. A white line in the figure is added on the image to make the figure easy to see, and indicates a position corresponding to a grain boundary (boundary between primary particles).

≪W and Mg distribution≫
The positive electrode active material sample 1 was subjected to EDX analysis in the same field of view as in FIG. 4, and the distribution of W was mapped. The result is shown in FIG. In FIG. 5, the portion where a larger amount of W is detected is displayed brighter. As is clear by comparing the brightly displayed portion in FIG. 5 with the position of the grain boundary shown in FIG. 4, W is present in the sample 1 with a bias toward the grain boundary. When the distribution of W was similarly mapped for Samples 2 to 16 and Samples 21 to 21, although there was a difference in the degree of brightness depending on the _W content (m _W ), W was biased toward the grain boundaries. It was confirmed that it existed.

Further, the positive electrode active material sample 1 was subjected to EDX analysis in the same field of view as in FIG. 4 to map the Ca distribution. The result is shown in FIG. A portion where a large amount of Ca is detected is displayed brighter. As is clear by comparing FIG. 6 and FIG. 4, it was confirmed that Ca is present at a grain boundary. Similarly, the Ca distribution of Samples 2 to 21 is mapped, and although there is a difference in brightness depending on the _Ca content (m _Ca ), Ca is present in a biased manner at the grain boundaries. Was confirmed.

  Further, the positive electrode active material sample 1 was subjected to EDX analysis in the same field of view as in FIG. 4 to map the Mg distribution. The result is shown in FIG. A portion where a large amount of Mg is detected is displayed brighter. As is clear by comparing FIG. 7 and FIG. 4, it was confirmed that Mg was present substantially uniformly in sample 1 (the brightness was not biased). When the distribution of Mg was similarly mapped for samples 2 to 21, it was confirmed that Mg was present without any bias.

≪TOF-SIMS analysis≫
About positive electrode active material samples 1-16 and samples 18-21, using a TOF-SIMS device (manufactured by ION-TOF, model “TOF.SIMS 5”), time-of-flight primary ion mass spectrometry is performed to obtain a spectrum. It was. In each spectrum, the presence or absence of peaks of LiCaO ⁺ , LiCaWO ₄ ⁺ , Li ₃ CaW ₂ O ₈ O ⁺ , CaWO ₄ ⁻ , and LiCaW ₂ O ₈ ⁻ was confirmed. Among these spectra, some of the positive and negative spectra according to Sample 1 are shown in FIGS. The measurement conditions for TOF-SIMS were as follows.
Primary ion: Bi ₃ ²⁺
Primary ion energy: 25 kV
Pulse width: 5.1 ns
Punching: Yes Charge neutralization: No Measurement vacuum: 4 × 10 ⁻⁷ Pa (3 × 10 ⁻⁹ Torr)
Secondary ion polarity: positive, negative Mass range (m / z): 0-1500μ
Cluster size: 200μm
Number of scans: 16
Number of pixels: 256 pixels Post acceleration: 10 kV

≪Production of evaluation battery≫
Using each of the positive electrode active material samples 1 to 21, a lithium ion secondary battery (evaluation battery) 100 having a structure shown in FIGS. 2 and 3 was produced. Hereinafter, these batteries may be referred to as batteries 1 to 21 in association with the positive electrode active material samples 1 to 21 used.

The positive electrode active material sample, acetylene black (AB) as a conductive material, and PVDF as a binder are mixed with NMP so that the mass ratio thereof is 90: 8: 2, A composition was prepared. This composition was applied to both surfaces of an aluminum foil (positive electrode current collector) 32 having a thickness of 15 μm so that the mass (weight per unit area) after drying was 11.8 mg / cm ^{2 in} total on both surfaces. After drying, the density of the positive electrode mixture layer 34 was adjusted to 2.1 g / cm ³ by pressing with a rolling press. Thus, the positive electrode sheet 30 was produced.

As the negative electrode active material, carbon particles having a structure in which amorphous carbon was coated on the surface of graphite particles were used. More specifically, natural graphite powder and pitch are mixed and the pitch is adhered to the surface of the graphite powder (the mass ratio of natural graphite powder: pitch is 96: 4), and 1000 under an inert atmosphere. After baking for 10 hours at a temperature of 1 ° C. to 1300 ° C., it was sieved to obtain a negative electrode active material having an average particle size (D50) of 8 to 11 μm and a specific surface area of 3.5 to 5.5 m ² / g. This negative electrode active material, CMC, and SBR were mixed with ion-exchanged water so that the mass ratio thereof was 98.6: 0.7: 0.7 to prepare a slurry composition. This composition was applied to both surfaces of a copper foil (negative electrode current collector) 42 having a thickness of 10 μm so that the total mass (weight per unit area) after drying was 7.5 mg / cm ² . After drying, the density of the negative electrode mixture layer 44 was adjusted to 1.0 to 1.2 g / cm ³ by pressing with a rolling press. Thus, the negative electrode sheet 40 was produced.

  The positive electrode sheet 30 and the negative electrode sheet 40 were wound together with two porous polyethylene sheets 50 (thickness 20 μm), and formed into a flat shape to produce an electrode body 20. The positive electrode terminal 38 and the negative electrode terminal 48 were attached to the lid body 14, and these terminals 38 and 48 were welded to the positive electrode current collector 32 and the negative electrode current collector 42 exposed at the end of the electrode body 20, respectively. The electrode body 20 thus connected to the lid body 14 was accommodated in the opening 10 of the case 10 and the lid body 14 was laser welded to the opening 12 of the case 10.

A nonaqueous electrolytic solution 90 was injected from an electrolytic solution injection hole (not shown) provided in the lid 14. As the non-aqueous electrolyte 90, a solution containing LiPF ₆ as a supporting salt at a concentration of 1.1 mol / L (1.1 M) in a mixed solvent of EC, EMC, and DMC in a volume ratio of 3: 3: 4. It was used. Then, the said injection hole was plugged and the lithium ion secondary battery 100 for evaluation was constructed | assembled. In this battery 100, the facing capacity ratio calculated from the charge capacity of the positive electrode and the charge capacity of the negative electrode is adjusted to 1.5 to 1.9. The capacity of the battery 100 is approximately 4 Ah.

  Next, a description will be given of the conditioning process and rated capacity measurement performed for the evaluation test battery constructed as described above.

<< conditioning >>
The conditioning process was performed according to the following procedures 1-2.
[Procedure 1] Voltage between terminals at a constant current of 1 C (1 C means a current value that discharges a fully charged battery to a discharge end voltage in 1 hour, sometimes referred to as a discharge time rate). Is charged (CC charge) until it reaches 4.1V, and then rests for 5 minutes.
[Procedure 2] After Procedure 1, charge at a constant voltage for 1.5 hours (CV charge) and rest for 5 minutes.

≪Measurement of rated capacity≫
The rated capacity of the evaluation test battery was measured according to the following procedures 1 to 3 in the voltage range of 3.0 V to 4.1 V at a temperature of 25 ° C. for the evaluation test battery after the conditioning step.
[Procedure 1] After reaching 3.0 V by 1 C constant current discharge, discharge at a constant voltage for 2 hours, and then rest for 10 seconds.
[Procedure 2] After reaching 4.1 V by constant current charging at 1 C, charging is performed for 2.5 hours by constant voltage charging, and then paused for 10 seconds.
[Procedure 3] After reaching 3.0 V by constant current discharge of 0.5 C, discharge at constant voltage discharge for 2 hours, and then stop for 10 seconds.
The discharge capacity (CCCV discharge capacity) in the discharge from the constant current discharge to the constant voltage discharge in the procedure 3 is defined as the rated capacity.

≪Measurement of output at low SOC and -30 ℃ ≫
The output at −30 ° C. of the evaluation battery adjusted to low SOC was measured by the following procedures 1 to 5.
[Procedure 1; SOC adjustment] The battery after the conditioning process and the rated capacity measurement is charged from 3 V to SOC 25% (CC charge) at a constant current of 1 C in a temperature environment of normal temperature (here, 25 ° C.), and then The battery was charged at a constant voltage for 2.5 hours (CV charge).
[Procedure 2; Hold at −30 ° C.] The battery after Procedure 1 was held in a thermostatic bath at −30 ° C. for 6 hours.
[Procedure 3; constant watt discharge] The battery after the procedure 2 is discharged at a constant watt (W) in a temperature environment of −30 ° C., and the time from the start of discharge until the voltage becomes 2.0 V (discharge cut voltage). Measure the number.
[Procedure 4; Repetition]: The above procedure 1 to 3 is repeated with the discharge output (constant watt discharge electric energy) in the constant watt discharge of procedure 3 varied between 80 W and 200 W. More specifically, while increasing the discharge output in the constant watt discharge of the procedure 3 by 10 W, the first 80 W, the second 90 W, the third 100 W,... Until the wattage reaches 200 W. repeat.
[Procedure 5: Calculation of output value]: Approximation of a plot in which the horizontal axis represents the number of seconds up to a voltage of 2.0 V measured in each constant watt discharge in procedure 4, and the vertical axis represents the constant watt discharge output at that time From the curve, an output value (low SOC · −30 ° C. output) when the number of seconds to the voltage of 2.0 V is 2 seconds is obtained.

  Here, steps 1 to 3 were repeated while increasing the constant watt discharge output from 80 W to 200 W in step 4 in step 4, but the measurement conditions for the low SOC · −30 ° C. output are not limited to this. The watt discharge output may be increased from 80 W by a certain wattage different from the above (for example, 5 W or 15 W), or from 200 W by a certain wattage (for example, 5 W, 10 W, or It may be lowered by 15W).

  The low SOC · −30 ° C. output indicates an output that can be exhibited by the evaluation battery even when left at a very low temperature environment of −30 ° C. for a predetermined time. It shows that the higher the output value (wattage), the higher the output of the evaluation battery can be under such severe use conditions.

≪2C-CCCV capacity maintenance ratio≫
The battery after the conditioning process and the rated capacity measurement was adjusted to SOC 100%, and CC discharge was performed at a temperature of 25 ° C. until the voltage between both terminals reached 3 V at a rate of 1 C. ) Discharged. After a 10-minute pause, CC charging was performed at a rate of 1 C until the voltage between both terminals reached 4.1 V, and then CV charging was performed for 2.5 hours at the same voltage. After 10 minutes rest, CC discharge is performed at a rate of 0.2 C until the voltage between both terminals becomes 3 V, then CV discharge is performed at the same voltage for 2 hours, and the total discharge capacity measured at the time of this CCCV discharge is the initial 2C-CCCV The capacity.
The battery after the initial 2C-CCCV capacity measurement was adjusted to 100% SOC, and subjected to charge and discharge of 1000 cycles between 0% and 100% SOC at a temperature of 60 ° C. One cycle was an operation of performing CC discharge until the voltage reached 3V at a rate of 2C, and then an operation of performing CC charge until the voltage reached 4.1V at a rate of 2C. With respect to the battery after completion of 1000 cycles, the discharge capacity (2C-CCCV capacity after endurance) was measured in the same manner as in the initial discharge capacity measurement.
The percentage of 2C-CCCV capacity after endurance with respect to the initial 2C-CCCV capacity (that is, (2C-CCCV capacity after endurance) / (initial 2C-CCCV capacity) × 100 (%)) is obtained as the 2C-CCCV capacity maintenance rate. It was.

≪4C-CC capacity maintenance rate≫
The battery after the conditioning process and the rated capacity measurement was adjusted to SOC 100% and subjected to charge / discharge of 1000 cycles at a temperature of 60 ° C. One cycle was an operation of performing CC discharge until the voltage reached 3 V at a rate of 4 C, and then an operation of performing CC charging until the voltage reached 4.1 V at a rate of 4 C. When 1000 cycles were completed, CC discharge was performed until the voltage became 3 V at a rate of 4 C, and the discharge capacity at this time was measured. The percentage of the discharge capacity (4C-CC capacity after endurance) after the end of the cycle with respect to the 4C-CC discharge capacity (initial 4C-CC capacity) of the first cycle was determined as the 4C-CC capacity maintenance rate.

  Tables 1 and 2 show the values of the low SOC · −30 ° C. output, the 2C-CCCV capacity retention rate, and the 4C-CC capacity retention rate obtained above. In these tables, the contents of W, Ca, and Mg in the positive electrode active material sample of the battery according to each example are shown together.

As shown in these tables, W is unevenly present on the surface of the primary particles, m _W is 0.05 mol% or more and 1.0 mol% or less, and m _W / m _Ca is 2 or more and 50 or less. _{M W} + m _Ca is 0.26 mol% or more (more specifically, 0.26 mol% or more and 1.5 mol% or less), and m _Mg / m _Ca is 1.5 or more (more specifically, The batteries according to Samples 1 to 12 that are 1.5 or more and 10 or less have a high low SOC output at low temperatures, and not only the 2C-CCCV capacity maintenance ratio but also the 4C-CC capacity maintenance ratio (that is, high temperature and large capacity). The capacity retention rate with respect to charge / discharge cycles with current was also excellent.

Compared with the batteries according to Samples 1 to 12, the battery according to Sample 17 containing no W clearly had a low SOC · −30 ° C. output and a 4C-CC capacity retention rate. In the battery according to sample 18 having a low W content (m _W is less than 0.05 mol%), the low SOC · −30 ° C. output is improved as compared with sample 17, but it reaches the range of samples 1-12. In addition, the 4C-CC capacity maintenance rate was also low. m _W / _{m Ca} is low SOC · -30 ° C. output of a battery according to excessively samples 13-16 and _m W _{/ m Ca} is too small sample 19 small, although higher than the sample 17, as compared with the samples 1-12 Obviously it was low. In the battery according to the sample 20 in which m _W / m _Ca is too large and the sample 21 in which m _W + m _Ca is too small, the 4C-CC capacity retention rate is significantly reduced.

In addition, as shown in FIGS. 8 to 12, the TOF-SIMS spectrum according to Sample 1 includes LiCaO ⁺ , LiCaWO ₄ ⁺ , Li ₃ CaW ₂ O ₈ O ⁺ , CaWO ₄ ⁻ , and LiCaW ₂ O ₈ ⁻ . A peak (portion surrounded by a broken line in each figure) was confirmed at the position of the corresponding molecular weight. From these results, it was confirmed that Ca and W contained in Sample 1 were compounded with the lithium transition metal oxide.

  As mentioned above, although this invention was demonstrated in detail, the said embodiment is only an illustration and what changed and modified the above-mentioned specific example is included in the invention disclosed here.

DESCRIPTION OF SYMBOLS 1 Vehicle 10 Battery case 12 Opening part 14 Cover body 20 Winding electrode body 30 Positive electrode sheet (positive electrode)
32 Positive electrode current collector 34 Positive electrode mixture layer 38 Positive electrode terminal 40 Negative electrode sheet (negative electrode)
42 Negative electrode current collector 44 Negative electrode mixture layer 48 Negative electrode terminal 50 Separator 90 Non-aqueous electrolyte 100 Lithium ion secondary battery (lithium secondary battery)

Claims (11)

A lithium secondary battery comprising a positive electrode and a negative electrode,
The positive electrode has a positive electrode active material in the form of secondary particles in which primary particles of a lithium transition metal oxide having a layered structure are collected,
The positive electrode active material includes at least one of Ni, Co, and Mn, and further includes W, Ca, and Mg,
W contained in the positive electrode active material is biased to the surface of the primary particles,
When the total amount of Ni, Co and Mn contained in the positive electrode active material is 100 mol%, the number of moles W of W contained in the positive electrode active material m _W (mol%), the number of moles of _Ca m _Ca (mol%), and The relationship between the number of moles of Mg m _Mg (mol%) is given by the following formula:
2 ≦ (m _W / m _Ca ) ≦ 50;
( _{M W} + m _Ca ) ≧ 0.26; and (m _Mg / m _Ca ) ≧ 1.5;
Satisfying the lithium secondary battery.   The lithium secondary battery according to claim 1, wherein Ca contained in the positive electrode active material is biased to a surface of the primary particles.   3. The lithium secondary battery according to claim 1, wherein Mg contained in the positive electrode active material is present in the entire interior of the primary particles. When the total amount of Ni, Co and Mn contained in the positive electrode active material is 100 mol%, the number of moles m _W (mol%) of _W contained in the positive electrode active material is 0.05 ≦ m _W ≦ 1.0. The lithium secondary battery according to any one of claims 1 to 3.   The lithium secondary battery according to any one of claims 1 to 4, wherein W and Ca contained in the positive electrode active material are compounded with a lithium transition metal oxide.   The lithium secondary battery according to any one of claims 1 to 5, wherein the lithium transition metal oxide is an oxide containing all of Ni, Co, and Mn.   The lithium secondary battery according to any one of claims 1 to 6, which is used as a drive power source for a vehicle. A method for producing a positive electrode active material according to claim 1, comprising :
Preparing an aqueous solution Aq _A containing at least one of Ni, Co and Mn and Mg and Ca;
Preparing an aqueous solution Aq _C containing W;
Mixing the aqueous solution Aq _A and the aqueous solution Aq _C under alkaline conditions to precipitate a hydroxide containing at least one of the Ni, Co and Mn, Mg, Ca and W;
Mixing the hydroxide and lithium compound; and
Firing the mixture to produce the lithium transition metal oxide;
A method for producing a positive electrode active material.   The method according to claim 8, wherein the precipitation of the hydroxide is performed while maintaining the pH at 11 to 14.   10. The method according to claim 8, wherein the hydroxide is deposited while maintaining the alkaline condition using an alkaline aqueous solution containing at least ammonia.   A lithium secondary battery provided with the positive electrode active material manufactured by the method as described in any one of Claims 8 to 10.