JP7074697B2 - Positive electrode material for lithium secondary batteries - Google Patents

Description

The present invention relates to a positive electrode material for a lithium secondary battery.

In recent years, lithium secondary batteries have been suitably used for portable power sources such as personal computers and mobile terminals, and vehicle drive power sources for electric vehicles (EV), hybrid vehicles (HV), plug-in hybrid vehicles (PHV) and the like. There is.

With the widespread use of lithium secondary batteries, further improvement in performance is desired. Generally, a positive electrode active material capable of storing and releasing lithium ions is used for the positive electrode of a lithium secondary battery. In order to improve the performance of a lithium secondary battery, there is known a technique of using a positive electrode material in which the surface of positive electrode active material particles is coated with a lithium compound containing boron (see, for example, Patent Documents 1 and 2).

Japanese Unexamined Patent Publication No. 2016-33854 Japanese Unexamined Patent Publication No. 2013-37950

As a result of diligent studies by the present inventor, it has been found that there is room for improvement in the resistance reduction of the lithium secondary battery using the positive electrode material of the prior art. It was also found that there is room for improvement in the high temperature cycle characteristics of the lithium secondary battery using the positive electrode material of the prior art. Furthermore, it has been found that the positive electrode material of the prior art has room for improvement in moisture resistance.

Therefore, an object of the present invention is to provide a positive electrode material which has high moisture resistance, can reduce the resistance of a lithium secondary battery, and can improve the high temperature cycle characteristics of a lithium secondary battery.

The positive electrode material of the lithium secondary battery disclosed herein includes positive electrode active material particles, Li ₃ BO ₃ on the surface of the positive electrode active material particles, and Li B (OH) ₄ on the surface of the positive electrode active material particles. include.
According to such a configuration, the moisture resistance is high, the resistance of the lithium secondary battery can be reduced, and the high temperature cycle characteristics of the lithium secondary battery (particularly, the capacity deterioration resistance to repeated charging and discharging at high temperature). A positive electrode material that can improve the above is provided.

In a preferred embodiment of the positive electrode material of the lithium secondary battery disclosed herein, Li ₃ BO ₃ and Li B (OH) ₄ are in contact with each other.
With such a configuration, the effect of improving moisture resistance, the effect of reducing battery resistance, and the effect of improving high temperature cycle characteristics are particularly high.
In a preferred embodiment of the positive electrode material of the lithium secondary battery disclosed herein, the amount of Li ₃ BO ₃ is 0.01% by mass or more and 10% by mass or less with respect to the positive electrode active material particles.
With such a configuration, the effect of improving moisture resistance, the effect of reducing battery resistance, and the effect of improving high temperature cycle characteristics are particularly high.
In a preferred embodiment of the positive electrode material of the lithium secondary battery disclosed herein, the amount of LiB (OH) ₄ is 0.001% by mass or more and 2% by mass or less with respect to the positive electrode active material particles.
With such a configuration, the effect of improving moisture resistance, the effect of reducing battery resistance, and the effect of improving high temperature cycle characteristics are particularly high.

It is a schematic cross-sectional view of an example of the positive electrode material which concerns on one Embodiment of this invention. It is sectional drawing which shows typically the structure of the lithium secondary battery constructed by using the positive electrode material which concerns on one Embodiment of this invention. It is a schematic diagram which shows the structure of the winding electrode body of the lithium secondary battery constructed by using the positive electrode material which concerns on one Embodiment of this invention.

Hereinafter, embodiments according to the present invention will be described. It should be noted that matters other than those specifically mentioned in the present specification and necessary for carrying out the present invention (for example, a general configuration of a positive electrode material of a lithium secondary battery which does not characterize the present invention) are described. It can be grasped as a design matter of a person skilled in the art based on the prior art in the field. The present invention can be carried out based on the contents disclosed in the present specification and the common general technical knowledge in the art.

In the present specification, the "secondary battery" generally refers to a power storage device capable of being repeatedly charged and discharged, and is a term including a so-called storage battery and a power storage element such as an electric double layer capacitor.
Further, in the present specification, the "lithium secondary battery" refers to a secondary battery that uses lithium ions as a charge carrier and realizes charge / discharge by the transfer of charges accompanying the lithium ions between the positive and negative electrodes.

The positive electrode material of the lithium secondary battery according to the present embodiment includes positive electrode active material particles, Li ₃ BO ₃ on the surface of the positive electrode active material particles, and Li B (OH) ₄ on the surface of the positive electrode active material particles. include. That is, two types of boron-containing lithium compounds, Li ₃ BO ₃ and Li B (OH) ₄ , coexist on the surface of the positive electrode active material particles.

As the positive electrode active material contained in the positive electrode material according to the present embodiment, a known positive electrode active material used in a lithium secondary battery may be used. Specifically, for example, a lithium composite oxide, a lithium transition metal phosphoric acid compound, or the like can be used. The crystal structure of the positive electrode active material is not particularly limited, and may be a layered structure, a spinel structure, an olivine structure, or the like.

As the lithium composite oxide, a lithium transition metal composite oxide containing at least one of Ni, Co, and Mn is preferable as the transition metal element, and specific examples thereof include a lithium nickel-based composite oxide and a lithium cobalt-based composite oxide. Examples thereof include composite oxides, lithium manganese-based composite oxides, lithium nickel-manganese-based composite oxides, lithium nickel-cobalt manganese-based composite oxides, lithium nickel-cobalt-aluminum-based composite oxides, and lithium iron-nickel-manganese-based composite oxides.

In the present specification, the "lithium-nickel-cobalt-manganese-based composite oxide" is an oxide containing Li, Ni, Co, Mn, and O as constituent elements, and one or more of the other oxides. It is a term that also includes oxides containing typical elements. Examples of such additive elements include transition metal elements such as Mg, Ca, Al, Ti, V, Cr, Si, Y, Zr, Nb, Mo, Hf, Ta, W, Na, Fe, Zn and Sn. And typical metal elements. Further, the additive element may be a metalloid element such as B, C, Si, P or a non-metal element such as S, F, Cl, Br, I. This means that the above-mentioned lithium nickel-based composite oxide, lithium cobalt-based composite oxide, lithium manganese-based composite oxide, lithium nickel-manganese-based composite oxide, lithium nickel-cobalt-aluminum-based composite oxide, and lithium iron-nickel-manganese-based composite The same applies to oxides and the like.

The lithium nickel cobalt manganese-based composite oxide preferably has a composition represented by the following formula (I).
Li _{1 + x} Ni _y Co _z Mn _(1-yz) M _α O _2-β Q _β (I)
In formula (I), x, y, z, α, and β are 0 ≦ x ≦ 0.7, 0.1 <y <0.9, 0.1 <z <0.4, 0 ≦ α ≦. Satisfy 0.1, 0 ≦ β ≦ 0.5. M is at least one element selected from the group consisting of Zr, Mo, W, Mg, Ca, Na, Fe, Cr, Zn, Si, Sn, and Al. Q is at least one element selected from the group consisting of F, Cl and Br. From the viewpoint of energy density and thermal stability, it is preferable that y and z satisfy 0.3 ≦ y ≦ 0.5 and 0.20 ≦ z <0.4, respectively.

Examples of the lithium transition metal phosphoric acid compound include lithium iron phosphate (LiFePO ₄ ), lithium manganese phosphate (LiMnPO ₄ ), lithium manganese iron phosphate, and the like.

The shape of the positive electrode active material particles is not particularly limited, and may be spherical, plate-shaped, needle-shaped, indefinite-shaped, or the like. Further, the positive electrode active material particles may be in the form of secondary particles in which primary particles are aggregated, or may be in the form of hollow particles.

The average particle size (D50) of the positive electrode active material particles is not particularly limited, but is, for example, 0.05 μm or more and 20 μm or less, preferably 0.5 μm or more and 15 μm or less, and more preferably 3 μm or more and 15 μm or less. ..
The average particle size (D50) of the positive electrode active material particles can be obtained, for example, by a laser diffraction / scattering method or the like.

The positive electrode material according to the present embodiment has two types of boron-containing lithium compounds, Li ₃ BO ₃ and Li B (OH) ₄ , on the surface of the positive electrode active material particles.
The positive electrode material according to this embodiment typically has a Li ₃ BO ₃ coating (particularly a Li ₃ BO ₃ crystal) and a LiB (OH) ₄ coating (particularly a LiB (OH) ₄ crystal). Are scattered on the surface of the positive electrode active material particles, respectively. That is, on the surface of the positive electrode material according to the present embodiment, the coating of Li ₃ BO ₃ and the coating of Li B (OH) ₄ each exist in an island shape.
In the positive electrode material according to the present embodiment, Li ₃ BO ₃ and LiB (OH) ₄ may form one layer in which they coexist, and the layer may completely cover the positive electrode active material. However, from the viewpoint of battery characteristics, Li ₃ BO ₃ and LiB (OH) ₄ are scattered on the surface of the positive electrode active material particles as described above, and Li ₃ BO ₃ and LiB (OH) ₄ are positive electrode active. It is preferred that the material particles are partially covered.

An example of the positive electrode material according to this embodiment is shown in FIG. FIG. 1 is a schematic cross-sectional view of the positive electrode material 10 according to the example. As shown, Li ₃ BO ₃ 14 and LiB (OH) ₄ 16 are interspersed on the surface of the positive electrode active material particles 12. Therefore, the Li ₃ BO ₃ 14 and the LiB (OH) ₄ 16 partially cover the surface of the positive electrode active material particles 12. Some Li ₃ BO ₃ 14 and some LiB (OH) ₄ 16 are in contact.

The presence of two types of boron-containing lithium compounds, Li ₃ BO ₃ and Li B (OH) ₄ , on the surface of the positive electrode active material particles reduces the resistance of the lithium secondary battery using the positive electrode material according to the present embodiment. Will be done. In addition, the high temperature cycle characteristics of the lithium secondary battery (particularly, capacity deterioration resistance to repeated charging and discharging at high temperatures) are improved. Further, the moisture resistance of the positive electrode material according to the present embodiment is improved. The reason for this is inferred as follows.

According to the studies by the present inventors, it was confirmed that the solvation energy of Li ions is small in the present embodiment. From this, the structure of the solvated Li ion present between Li ₃ BO ₃ and LiB (OH) ₄ is specifically changed, resulting in an increase in the number of Li carriers present on the surface of the positive electrode active material. it is conceivable that. Therefore, it is considered that the battery resistance is reduced by this.
In addition, the structure of the solvated Li ions present between Li ₃ BO ₃ and LiB (OH) ₄ changes specifically, resulting in a barrier layer in which Li ions are preferentially arranged on the surface of the positive electrode active material. It is thought to be formed. Therefore, since Li ions are preferentially present in the vicinity of the positive electrode active material, it is possible to suppress the approach of the solvent or anionic species that causes the side reaction to the positive electrode active material, thereby suppressing the side reaction. It is considered that the high temperature cycle characteristics are improved.
Further, in general, when the positive electrode active material is left unattended in an atmospheric environment, Li is desorbed from the positive electrode active material due to the reaction between the positive electrode active material and the moisture in the air, which causes a decrease in battery capacity. However, in this embodiment, it is considered that the Li ₃ BO ₃ and LiB (OH) ₄ present on the surface of the positive electrode active material repel the moisture in the air due to their electrostatic repulsive force. As a result, it is considered that the frequency of contact between the positive electrode active material and the moisture in the air is reduced, and the decrease in battery capacity can be suppressed.

In the present embodiment, it is preferable that the Li ₃ BO ₃ and the Li B (OH) ₄ present on the surface of the positive electrode active material are in contact with each other. At this time, the battery resistance reducing effect, the high temperature cycle characteristic improving effect, and the moisture resistance improving effect are further enhanced. Regarding further improvement of the battery reduction effect, when Li ₃ BO ₃ and Li B (OH) ₄ come into contact with each other, Li ions become rich at these interfaces, and the conductivity of Li ions is improved in this portion. It is considered that this is because the supply of Li ions to the positive electrode active material is promoted.
It suffices that at least a part of Li ₃ BO ₃ and Li B (OH) ₄ are in contact with each other.
The contact between Li ₃ BO ₃ and LiB (OH) ₄ can be confirmed by observing the cross section of the positive electrode material with a scanning transmission electron microscope (STEM).

There is no particular limitation on the amount of Li ₃ BO ₃ present on the surface of the positive electrode active material. Since the effect of reducing battery resistance, the effect of improving high temperature cycle characteristics, and the effect of improving moisture resistance are particularly high, the amount of Li ₃ BO ₃ is preferably 0.01% by mass or more and 10% by mass or less with respect to the positive electrode active material particles. It is more preferably 0.01% by mass or more and 5% by mass or less.

The amount of LiB (OH) ₄ present on the surface of the positive electrode active material is not particularly limited. Since the effect of reducing battery resistance, the effect of improving high temperature cycle characteristics, and the effect of improving moisture resistance are particularly high, the amount of LiB (OH) ₄ is preferably 0.001% by mass or more and 2% by mass with respect to the positive electrode active material particles. It is more preferably 0.01% by mass or more and 2% by mass or less.

The positive electrode material according to the present embodiment is produced by subjecting a mixture of positive electrode active material particles, Li ₃ BO ₃ , and LiB (OH) ₄ to a mechanochemical treatment using, for example, a known mechanochemical device. be able to. At this time, by increasing the applied mechanical energy, the Li ₃ BO ₃ and the Li B (OH) ₄ can be brought into contact with each other on the surface of the positive electrode active material particles.

The positive electrode material according to the present embodiment is for a lithium secondary battery, and a lithium secondary battery can be constructed using the positive electrode material according to the present embodiment according to a known method. Therefore, a specific configuration example of the lithium secondary battery provided with the positive electrode material according to the present embodiment will be described below with reference to the drawings. The lithium secondary battery is not limited to the examples described below. In the following drawings, members / parts having the same function are described with the same reference numerals. Further, the dimensional relations (length, width, thickness, etc.) in each drawing do not reflect the actual dimensional relations.

The lithium secondary battery 100 shown in FIG. 2 is constructed by housing a flat wound electrode body 20 and a non-aqueous electrolyte (not shown) in a flat square battery case (that is, an outer container) 30. It is a sealed battery. The battery case 30 is provided with a positive electrode terminal 42 and a negative electrode terminal 44 for external connection, and a thin-walled safety valve 36 set to release the internal pressure when the internal pressure of the battery case 30 rises above a predetermined level. Has been done. The positive and negative electrodes 42 and 44 are electrically connected to the positive and negative current collector plates 42a and 44a, respectively. As the material of the battery case 30, for example, a lightweight metal material having good thermal conductivity such as aluminum is used.

As shown in FIGS. 2 and 3, in the wound electrode body 20, the positive electrode active material layer 54 is formed along the longitudinal direction on one side or both sides (here, both sides) of the long positive electrode current collector 52. The positive electrode sheet 50 and the negative electrode sheet 60 in which the negative electrode active material layer 64 is formed along the longitudinal direction on one side or both sides (here, both sides) of the long negative electrode current collector 62 are two long sheets. It is overlapped with each other via the separator sheet 70 and wound in the longitudinal direction. The positive electrode active material layer non-formed portion 52a (that is, the positive electrode) formed so as to protrude outward from both ends of the winding electrode body 20 in the winding axis direction (meaning the sheet width direction orthogonal to the longitudinal direction). A portion where the positive electrode current collector 52 is exposed without forming the active material layer 54) and a portion 62a where the negative electrode active material layer is not formed (that is, a portion where the negative electrode current collector 62 is exposed without forming the negative electrode active material layer 64). ) Are joined to a positive electrode current collector plate 42a and a negative electrode current collector plate 44a, respectively.

Examples of the positive electrode current collector 52 constituting the positive electrode sheet 50 include aluminum foil and the like. The positive electrode active material layer 54 contains the positive electrode material according to the above-described embodiment, which is a material containing the positive electrode active material. Further, the positive electrode active material layer 54 may further contain a conductive material, a binder and the like. As the conductive material, for example, carbon black such as acetylene black (AB) or other carbon material (graphite or the like) can be preferably used. As the binder, for example, polyvinylidene fluoride (PVDF) or the like can be used.

Examples of the negative electrode current collector 62 constituting the negative electrode sheet 60 include copper foil and the like. The negative electrode active material layer 64 contains a negative electrode active material. As the negative electrode active material, a carbon material such as graphite, hard carbon, or soft carbon can be used. The negative electrode active material layer 64 may further contain a binder, a thickener and the like. As the binder, for example, styrene butadiene rubber (SBR) or the like can be used. As the thickener, for example, carboxymethyl cellulose (CMC) or the like can be used.

As the separator 70, various porous sheets similar to those conventionally used for lithium secondary batteries can be used, and examples thereof are porous resins made of resins such as polyethylene (PE) and polypropylene (PP). The sheet is mentioned. The porous resin sheet may have a single-layer structure or a multi-layer structure having two or more layers (for example, a three-layer structure in which PP layers are laminated on both sides of a PE layer). The separator 70 may include a heat resistant layer (HRL).

As the non-aqueous electrolyte, the same as the conventional lithium secondary battery can be used, and typically, an organic solvent (non-aqueous solvent) containing a supporting salt can be used. As the non-aqueous solvent, aprotic solvents such as carbonates, esters and ethers can be used. Among them, carbonates such as ethylene carbonate (EC), diethyl carbonate (DEC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC) and the like can be preferably adopted. Alternatively, a fluorinated solvent such as a fluorinated carbonate such as monofluoroethylene carbonate (MFEC), difluoroethylene carbonate (DFEC), monofluoromethyldifluoromethyl carbonate (F-DMC) and trifluorodimethylcarbonate (TFDMC) is preferably used. be able to. As such a non-aqueous solvent, one kind may be used alone, or two or more kinds may be used in combination as appropriate. As the supporting salt, for example, a lithium salt such as LiPF ₆ , LiBF ₄ , or lithium bis (fluorosulfonyl) imide (LiFSI) can be preferably used. The concentration of the supporting salt is preferably 0.7 mol / L or more and 1.3 mol / L or less.
The non-aqueous electrolyte contains various components other than the above-mentioned non-aqueous solvent and supporting salt, such as a gas generator, a film-forming agent, a dispersant, and a thickener, as long as the effects of the present invention are not significantly impaired. May contain additives.

The lithium secondary battery 100 can be used for various purposes. Suitable applications include drive power supplies mounted on vehicles such as plug-in hybrid vehicles (PHVs), hybrid vehicles (HVs), and electric vehicles (EVs). The lithium secondary battery 100 can also be used in the form of an assembled battery in which a plurality of lithium secondary batteries 100 are electrically connected.

As described above, a square lithium secondary battery provided with a flat wound electrode body has been described as an example. However, the positive electrode material according to the present embodiment can be used for other types of lithium secondary batteries according to a known method. For example, a lithium secondary battery including a laminated electrode body can be constructed by using the positive electrode material according to the present embodiment. Further, a cylindrical lithium secondary battery, a laminated lithium secondary battery, or the like can also be constructed by using the positive electrode material according to the present embodiment. Further, an all-solid-state lithium secondary battery can also be constructed by using the positive electrode material according to the present embodiment.

Hereinafter, examples relating to the present invention will be described, but the present invention is not intended to be limited to those shown in such examples.

<Preparation of positive electrode material 1>
[Example 1]
First, LiNi _1/3 Co _1/3 Mn _{1/3 O2} particles having a layered structure as positive electrode active material particles were prepared according to a conventional method.
Specifically, sulfates of Ni, Co, and Mn were each dissolved in water so that the molar ratio of Ni, Co, and Mn was 1: 1: 1. By adding NaOH to neutralize it, a composite hydroxide containing Ni, Co, and Mn, which are precursors of the positive electrode active material, was precipitated. The obtained composite hydroxide and lithium carbonate were mixed so that their molar ratio was 1: 1. The mixture was calcined at 800 ° C. for 15 hours to give LiNi _1/3 Co _1/3 Mn _{1/3 O2} particles having a layered structure. The average particle size (D50) of the LiNi _1/3 Co _1/3 Mn _{1/3 O2} particles was measured by the laser diffraction / scattering method and found to be 10 μm.
Next, the LiNi _1/3 Co _1/3 Mn _1/3 O ₂ particles having a layered structure were charged into the mechanochemical apparatus together with the Li ₃ BO ₃ powder and the LiB (OH) ₄ powder. At this time, the amount of Li ₃ BO ₃ powder is 1% by mass with respect to the LiNi _1/3 Co _1/3 Mn _1/3 O ₂ particles, and the amount of LiB (OH) ₄ powder is also LiNi _1/3 Co. It was set to 1% by mass with respect to _1/3 Mn _{1/3 O2} particles. These were treated with mechanochemicals at 6000 rpm for 30 minutes to obtain the positive electrode material of Example 1.

[Comparative Example 1]
The LiNi _1/3 Co _1/3 Mn _{1/3 O2} particles having a layered structure produced in Example 1 were used as they were as the positive electrode material of Comparative Example 1.

[Comparative Example 2]
The positive electrode material of Comparative Example 2 was obtained by the same method as in Example 1 except that the LiNi _1/3 Co _1/3 Mn _1/3 O ₂ particles were treated with Li ₃ BO ₃ powder only and mechanochemically.

[Comparative Example 3]
The positive electrode material of Comparative Example 3 was obtained by the same method as in Example 1 except that LiNi _1/3 Co _1/3 Mn _1/3 O ₂ particles were treated with LiB (OH) ₄ powder only and mechanochemically. ..

[Example 2]
The positive electrode material of Example 2 was obtained by the same method as in Example 1 except that the mechanochemical treatment was carried out at 8000 rpm for 30 minutes.

[Examples 3 to 14]
The amount of Li ₃ BO ₃ powder for LiNi _1/3 Co _1/3 Mn _1/3 O ₂ particles and the amount of LiB (OH) ₄ powder for LiNi _1/3 Co _1/3 Mn _1/3 O ₂ particles. The positive electrode materials of Examples 3 to 14 were obtained by the same method as in Example 1 except that the values shown in Table 1 were used and the mechanochemical treatment was performed at 8000 rpm for 30 minutes.

<Observation of covering condition>
The cross section of the positive electrode material of each example was observed by STEM. As a result, it was confirmed that the surface of the LiNi _1/3 Co _1/3 Mn _1/3 O ₂ particles was interspersed with the Li ₃ BO ₃ coating and the LiB (OH) ₄ coating. Further, in the positive electrode material of Example 1, the coating of Li ₃ BO ₃ and the coating of LiB (OH) ₄ are not in contact with each other, and in the positive electrode materials of Examples 2 to 14, the coating of Li ₃ BO ₃ and LiB are not in contact with each other. It was confirmed that the coatings of (OH) ₄ were in contact.

<Manufacturing of lithium secondary battery for evaluation>
The positive electrode material produced above, acetylene black (AB) as a conductive material, and polyvinylidene fluoride (PVDF) as a binder are mixed with N-methylpyrrolidone in a mass ratio of positive electrode material: AB: PVDF = 80: 8: 2. A slurry for forming a positive electrode active material layer having a solid content concentration of 56% by mass was prepared by mixing in (NMP) using a planetary mixer. This slurry was applied to both sides of the aluminum foil using a die coater, dried, and then pressed to prepare a positive electrode sheet.
Further, natural graphite (C) as a negative electrode active material, styrene butadiene rubber (SBR) as a binder, and carboxymethyl cellulose (CMC) as a thickener are used in C: SBR: CMC = 98: 1: 1. A slurry for forming a negative electrode active material layer was prepared by mixing in ion-exchanged water by mass ratio. This slurry was applied to both sides of the copper foil, dried, and then pressed to prepare a negative electrode sheet.
In addition, two separator sheets (porous polyolefin sheets) were prepared.
The prepared positive electrode sheet, the negative electrode sheet, and the two prepared separator sheets were superposed and wound to prepare a wound electrode body. Electrode terminals were attached to the positive electrode sheet and the negative electrode sheet of the manufactured wound electrode body by welding, respectively, and these were housed in a battery case having a liquid injection port.
Subsequently, a non-aqueous electrolytic solution was injected from the injection port of the battery case, and the injection port was hermetically sealed. The non-aqueous electrolytic solution contains LiPF ₆ as a supporting salt in a mixed solvent containing ethylene carbonate (EC), ethylmethyl carbonate (EMC) and dimethyl carbonate (DMC) in a volume ratio of 1: 1: 1. The one dissolved at a concentration of 1.0 mol / L was used.

<Activation and initial volume measurement>
Each evaluation lithium secondary battery produced above was placed in an environment of 25 ° C. The activation (initial charge) is a constant current-constant voltage method, and after constant current charging of each evaluation lithium secondary battery to 4.2V with a current value of 1 / 3C, the current value becomes 1 / 50C. It was charged at a constant voltage until it became fully charged. Then, each evaluation lithium secondary battery was constantly discharged to 3.0 V at a current value of 1 / 3C. Then, the discharge capacity at this time was measured to obtain the initial capacity.

<Battery resistance measurement>
Each activated lithium secondary battery for evaluation was prepared to a voltage of 3.70 V (open circuit voltage) and then placed in an environment of −5 ° C. Each of the evaluation lithium secondary batteries was discharged at a current value of 20 C for 8 seconds. The voltage drop amount ΔV at this time was acquired, and the battery resistance was calculated using the current value and ΔV. When the resistance of the evaluation lithium secondary battery using the positive electrode material of Comparative Example 1 is 1.00, the ratio of the resistance of the evaluation lithium secondary battery using the positive electrode material of other Comparative Examples and Examples is I asked. The results are shown in Table 1.

<High temperature cycle characterization>
Each activated lithium secondary battery for evaluation was placed in an environment of 60 ° C., and charging / discharging with constant current charging up to 4.2 V at 10 C and constant current discharge up to 3.3 V at 10 C was repeated for 200 cycles. .. The discharge capacity at the 200th cycle was determined by the same method as the initial capacity. As an index of the high temperature cycle characteristic, the capacity retention rate (%) was obtained from (discharge capacity / initial capacity at the 200th charge / discharge cycle) × 100. The results are shown in Table 1.

<Moisture resistance evaluation>
A positive electrode sheet was prepared by the same procedure as above, and the positive electrode sheet was stored in a constant temperature bath at 25 ° C. and 100% humidity for 12 hours. Using this positive electrode sheet, a lithium secondary battery for evaluation was produced by the same method as described above. Each evaluation lithium secondary battery was placed in an environment of 25 ° C. Each evaluation lithium secondary battery was charged with a constant current of 4.2 V at a current value of 1 / 3C, and then charged with a constant voltage until the current value became 1 / 50C to bring it into a fully charged state. Then, each evaluation lithium secondary battery was constantly discharged to 3.0 V at a current value of 1 / 3C. Then, the discharge capacity at this time was measured. When the discharge capacity of the evaluation lithium secondary battery using the positive electrode material of Comparative Example 1 is 1.00, the discharge capacity of the evaluation lithium secondary battery using the positive electrode materials of other Comparative Examples and Examples. The ratio was calculated. The results are shown in Table 1.

From the comparison between Example 1 and Comparative Examples 1 to 3, by allowing Li ₃ BO ₃ and Li B (OH) ₄ to coexist on the surface of the positive electrode active material, the battery resistance is reduced, and the high temperature cycle characteristics and moisture resistance are improved. It can be seen that it is improving. Here, the effect of having Li ₃ BO ₃ on the surface of the positive electrode active material can be grasped from the comparison between Comparative Example 1 and Comparative Example 2, and the surface of the positive electrode active material can be understood from the comparison between Comparative Example 1 and Comparative Example 3. The effect of having LiB (OH) ₄ present on the surface can be grasped. In Comparative Example 3 in which LiB (OH) ₄ was present alone on the surface of the positive electrode active material, the resistance was higher in Example 1 than in Comparative Example 1 and Comparative Example 2, although the battery resistance was increased. It has become significantly smaller. In addition, the improvement effect of high temperature cycle characteristics and moisture resistance when Li ₃ BO ₃ and LiB (OH) ₄ are used together is the improvement effect obtained by the single use of Li ₃ BO ₃ and the single use of LiB (OH) ₄ . It is larger than the sum of the improvement effects obtained by. From this, it can be seen that a synergistic effect can be obtained by allowing Li ₃ BO ₃ and Li B (OH) ₄ to coexist on the surface of the positive electrode active material.
In Example 2 in which the conditions of the mechanochemical treatment were changed, the Li ₃ BO ₃ and the Li B (OH) ₄ were in contact with each other. In this Example 2, further low resistance can be achieved as compared with Example 1, and the high temperature cycle characteristics and moisture resistance are further improved.
From the results of Examples 2 to 8, it can be seen that even if the amount of Li ₃ BO ₃ is changed, the effect of reducing the resistance, the effect of improving the high temperature cycle characteristics, and the effect of improving the moisture resistance can be obtained. It can be said that the amount of Li ₃ BO ₃ having a particularly high effect is in the range of 0.01% by mass or more and 10% by mass or less.
From the results of Examples 2 and 9 to 14, it can be seen that even if the amount of LiB (OH) ₄ is changed, the resistance reducing effect, the high temperature cycle characteristic improving effect, and the moisture resistance improving effect can be obtained. It can be said that the amount of LiB (OH) ₄ , which is particularly effective, is in the range of 0.001% by mass or more and 2% by mass or less.

<Preparation of positive electrode material 2>
[Comparative Examples 4 to 9]
The positive electrode active material having the composition shown in Table 2 was prepared and used as it was as the positive electrode material of Comparative Examples 4 to 9.

[Examples 15 to 20]
The positive electrode active material having the composition shown in Table 2 was prepared and charged into the mechanochemical apparatus together with the Li ₃ BO ₃ powder and the LiB (OH) ₄ powder. At this time, the amount of the Li ₃ BO ₃ powder was 1% by mass with respect to the positive electrode active material, and the amount of the LiB (OH) ₄ powder was also 1% by mass with respect to the positive electrode active material. These were treated with mechanochemicals at 8000 rpm for 30 minutes to obtain positive electrode materials of Examples 15 to 20.
The cross sections of the positive electrode materials of Examples 15 to 20 were observed by STEM. As a result, it was confirmed that the surface of the positive electrode active material particles was interspersed with the coatings of Li ₃ BO ₃ and LiB (OH) ₄ . Further, it was confirmed that the coating of Li ₃ BO ₃ and the coating of LiB (OH) ₄ were in contact with each other.

<Evaluation of positive electrode material>
Battery resistance measurement, high temperature cycle characteristic evaluation (capacity retention rate measurement), and moisture resistance evaluation were performed on the positive electrode materials of each of the prepared Examples and Comparative Examples by the same procedure as described above.
Regarding the battery resistance, the resistance ratio of each example was determined based on the value of the battery using the positive electrode material of the comparative example using the positive electrode active material having the same composition (1.00) (for example, Example). 15 was based on Comparative Example 4, and Example 20 was based on Comparative Example 9).
For the evaluation of moisture resistance, the ratio of the discharge capacities of each example was determined based on the value of the comparative example using the positive electrode active material having the same composition (1.00).
These results are shown in Table 2.

By comparing Comparative Examples 4 to 9 and Examples 15 to 20, respectively, resistance reduction effect, high temperature cycle characteristic improvement effect, and moisture resistance improvement effect are obtained regardless of the composition and crystal structure of the positive electrode active material. You can see that.

From the above, according to the positive electrode material of the lithium secondary battery according to the present embodiment, the moisture resistance can be improved, the resistance of the lithium secondary battery can be reduced, and the high temperature of the lithium secondary battery can be reduced. It can be seen that the cycle characteristics can be improved.

Although specific examples of the present invention have been described in detail above, these are merely examples and do not limit the scope of the claims. The techniques described in the claims include various modifications and modifications of the specific examples exemplified above.

10 Positive electrode material 12 Positive electrode active material particles 14 Li ₃ BO ₃
16 LiB (OH) ₄
20 Winding electrode body 30 Battery case 36 Safety valve 42 Positive electrode terminal 42a Positive electrode current collector plate 44 Negative electrode terminal 44a Negative electrode current collector plate 50 Positive electrode sheet (positive electrode)
52 Positive electrode current collector 52a Positive electrode active material layer non-formed portion 54 Positive electrode active material layer 60 Negative electrode sheet (negative electrode)
62 Negative electrode current collector 62a Negative electrode active material layer non-formed portion 64 Negative electrode active material layer 70 Separator sheet (separator)
100 lithium secondary battery

Claims (4)

Positive electrode active material particles and
On the surface of the positive electrode active material particles, Li ₃ BO ₃ and
On the surface of the positive electrode active material particles, LiB (OH) ₄ and
Positive electrode material for lithium secondary batteries, including. The positive electrode material for a lithium secondary battery according to claim 1, wherein the Li ₃ BO ₃ and the LiB (OH) ₄ are in contact with each other. The positive electrode material for a lithium secondary battery according to claim 1 or 2, wherein the amount of Li ₃ BO ₃ is 0.01% by mass or more and 10% by mass or less with respect to the positive electrode active material particles. The positive electrode of the lithium secondary battery according to any one of claims 1 to 3, wherein the amount of LiB (OH) ₄ is 0.001% by mass or more and 2% by mass or less with respect to the positive electrode active material particles. material.

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