Positive electrode active material for secondary battery, and secondary battery

Abstract

This positive electrode active material for a secondary battery contains a lithium metal composite oxide having a crystal structure that can belong to space group Fm-3m. The lithium metal composite oxide contains Li and a positive element M different from Li. The positive element M contains at least Mn and Co, and 50 at% or more of the positive element M is Mn.

Description

Positive electrode active material for secondary battery and secondary battery CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This disclosure claims the benefit of priority to Japanese Patent Application No. 2023-170654, filed on September 29, 2023, in the Japan Patent Office, the entire contents of which are incorporated herein by reference.

[0002] The present disclosure relates to a positive electrode active material for a secondary battery and a secondary battery.

[0003] Secondary batteries, especially lithium-ion secondary batteries, have high power output and high energy density, and are expected to be used in small consumer applications, power storage devices, and electric vehicles. The cathode active material of lithium-ion secondary batteries is a composite oxide of lithium and a transition metal (e.g., cobalt). Replacing part of the cobalt with nickel can increase the battery capacity.

[0004] On the other hand, in recent years, in response to the demand for high energy density, the rock salt structure Li １＋ｘ Mn １－ｘ O ２ Li-excess lithium metal composite oxides based on the above have been attracting attention.

[0005] Patent Document 1 describes a compound having a crystal structure belonging to the space group Fm-3m and having the composition formula Li １＋ｘ Nb ｙ Me ｚ A ｐ O ２ (Me is a transition metal including Fe and / or Mn, 0<x<1, 0<y<0.5, 0.25≦z<1, A is an element other than Nb and Me, 0≦p≦0.2, provided that Li １＋ｐ Fe １－ｑ Nb ｑ O ２ and excluding those in which 0.15<p≦0.3 and 0<q≦0.3), a positive electrode active material containing a lithium transition metal composite oxide represented by the formula (2) is disclosed.

[0006] Patent No. 6197029

[0007] In Patent Document 1, high capacity is possible by controlling the composition (i.e., by adding Nb), but the effect of improving capacity is insufficient, and there is still room for improvement.

[0008] In view of the above, one aspect of the present disclosure relates to a positive electrode active material for a secondary battery, comprising a lithium metal composite oxide having a crystal structure assignable to space group Fm-3m, the lithium metal composite oxide comprising Li and an electropositive element M different from Li, the electropositive element M comprising at least Mn and Co, and 50 atomic % or more of the electropositive element M being Mn.

[0009] Another aspect of the present disclosure relates to a secondary battery including a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode includes the above-described positive electrode active material for a secondary battery.

[0010] According to the present disclosure, a secondary battery with a high energy density can be realized. The novel features of the present invention are set forth in the appended claims, but the present invention, both in terms of structure and content, together with other objects and features of the present invention, will be better understood from the following detailed description taken in conjunction with the drawings.

[0011] 1 is a schematic perspective view of a secondary battery according to an embodiment of the present disclosure, with a portion cut away;

[0012] The following describes embodiments of the present disclosure using examples, but the present disclosure is not limited to the examples described below. In the following description, specific numerical values ​​and materials may be exemplified, but other numerical values ​​and materials may be applied as long as the effects of the present disclosure are obtained. In this specification, the term "numerical value A to numerical value B" includes numerical value A and numerical value B and can be read as "numerical value A or more and numerical value B or less." In the following description, when lower and upper limits for specific physical properties or conditions are exemplified, any of the exemplified lower limits and any of the exemplified upper limits can be arbitrarily combined, as long as the lower limit is not equal to or greater than the upper limit. When multiple materials are exemplified, one of the materials may be selected and used alone, or two or more materials may be used in combination.

[0013] The present disclosure also encompasses combinations of two or more features arbitrarily selected from the appended claims, i.e., two or more features arbitrarily selected from the appended claims can be combined unless a technical contradiction arises.

[0014] In the following description, the term "comprises (or includes)" encompasses "comprises (or includes)", "consists essentially of", and "consists of".

[0015] The secondary battery includes at least non-aqueous electrolyte secondary batteries such as lithium ion batteries and lithium metal secondary batteries, and all-solid-state batteries using a solid electrolyte.

[0016] A positive electrode active material for a secondary battery according to an embodiment of the present disclosure includes a lithium metal composite oxide (hereinafter also referred to as "lithium metal composite oxide (Fm)") having a crystal structure that can be assigned to the space group Fm-3m. The lithium metal composite oxide (Fm) has, for example, a crystal structure based on a rock salt structure that belongs to the space group Fm-3m, and has a crystal structure similar to the rock salt structure represented by NaCl. In such a crystal structure, oxygen atoms are arranged at the anion sites, and Li atoms and metal atoms other than Li can be irregularly arranged at the cation sites.

[0017] The lithium metal composite oxide (Fm) may be an oxide having a rock salt structure based on a composite oxide of Li and Mn. The lithium metal composite oxide (Fm) may be an oxide having a rock salt structure based on a composite oxide of Li and Mn. １＋ｘ Mn １－ｘ O ２ The lithium-excess state refers to a state in which the number of Li atoms in the lithium metal composite oxide (Fm) is greater than the total number of atoms of metal elements other than Li.

[0018] The crystal structure of the lithium metal composite oxide (Fm) can be identified from an X-ray diffraction pattern measured using, for example, a powder X-ray diffractometer (for example, an X-ray diffractometer manufactured by Rigaku Corporation).

[0019] The lithium metal composite oxide (Fm) according to one embodiment of the present disclosure contains Li and an electropositive element M different from Li. The total amount of all metal elements contained in the lithium metal composite oxide (Fm) corresponds to the sum of the total amount of Li and the total amount of the electropositive element M contained in the lithium metal composite oxide (Fm). The electropositive element M contains at least Mn and Co. Mn is a main component of the electropositive element M, and 50 atomic % or more of the electropositive element M is Mn.

[0020] The ratio (mLi / mMn) of the number of Li atoms mLi contained in the lithium metal composite oxide (Fm) to the number of Mn atoms mMn contained in the lithium metal composite oxide (Fm) preferably satisfies 1.7≦mLi / mM≦2.5, and more preferably satisfies 1.8≦mLi / mM≦2.3.

[0021] The electropositive element M may further include an element Mm in addition to Mn and Co. The element Mm is an electropositive element other than Li, Mn, and Co. Note that the electropositive element is an element that can form a cation other than hydrogen, and is typically a metal element (including so-called metalloid elements).

[0022] Assuming that the number of Mn atoms contained in the lithium metal composite oxide (Fm) is mMn, the number of Co atoms contained in the lithium metal composite oxide (Fm) is mCo, and the number of atoms of positive elements Mm other than Li, Mn, and Co contained in the lithium metal composite oxide (Fm) is mMm, the ratio of mMn / (mCo+mMm) is, for example, 1 or more and 15 or less, preferably 1 or more and 12 or less, may be greater than 1 and 12 or less, or may be 2 or more and 12 or less.

[0023] Co can stabilize the crystal structure based on a rock salt structure belonging to the space group Fm-3m and has the effect of increasing the capacity of the lithium metal composite oxide (Fm). The coexistence of Mn and Co significantly improves charge / discharge efficiency and enables the achievement of high energy density. Although the reason for this is unclear, trivalent Co has many vacant orbitals in the eg orbital in a low spin state, which can be used for hopping electrons in the eg orbital of trivalent Mn, possibly improving electronic conductivity at the micro level. When the lithium metal composite oxide (Fm) contains Co, the ratio (mMn / mCo) of the number of Mn atoms contained in the lithium metal composite oxide, mMn, to the number of Co atoms contained in the lithium metal composite oxide, mCo, is, for example, 5 to 65, or may be 5.5 to 60, or may be 10 to 50.

[0024] The element Mm may include, for example, at least one element selected from the group consisting of Ti, Fe, Ge, Si, Ga, Ni, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Gd, Sm, Eu, Yb, Dy, Al, and Er. Among these, the element Mm is preferably at least one element selected from the group consisting of Ti, Mg, Fe, Nb, and Ni, in terms of ease of forming a rock-salt crystal structure, and it is preferable that 50 atomic % or more, further 80 atomic % or more, or 100% of Mm is at least one element selected from the group consisting of Ti, Mg, and Fe.

[0025] Among the elements Mm, Ti is particularly preferred, and Ti may account for 50 atomic % or more, further 80 atomic % or more, or even 100% of Mm. Ti can stabilize the crystal structure based on a rock salt structure belonging to the space group Fm-3m, and has the effect of increasing the capacity of the lithium metal composite oxide (Fm).

[0026] The coexistence of Mn, Co, and Ti significantly improves the charge-discharge efficiency and enables the achievement of high energy density. Although the reason for this is not clear, one possible reason is that Ti has an empty d-orbital in the lithium metal composite oxide. ４＋ Co exists in the form of Co with an empty eg orbital４＋ In this case, the highly symmetric rock salt structure is likely to be stable, and the rock salt structure may remain stable even after repeated charging and discharging.

[0027] The ratio (mMn / mTi) of the number of Mn atoms mMn contained in the lithium metal composite oxide (Fm) to the number of Ti atoms mTi contained in the lithium metal composite oxide (Fm) may be 2 or more, 3 or more, 4 or more, and preferably 5 or more. Moreover, mMn / mTi may be 70 or less, 30 or less, 15 or less, and desirably 7 or less. The range of the mMn / mTi ratio may be, for example, 2 to 30, 2 to 15, or 2 to 7.

[0028] Furthermore, the ratio (mTi / mCo) of the number of Ti atoms mTi contained in the lithium metal composite oxide (Fm) to the number of Co atoms mCo contained in the lithium metal composite oxide (Fm) is preferably 1 to 3.

[0029] By controlling the ratio (mLi / mM) of the number of Li atoms contained in the lithium metal composite oxide (Fm) to the number of atoms mM of the electropositive element M contained in the lithium metal composite oxide (Fm), it is possible to increase the capacity of the lithium metal composite oxide (Fm). Specifically, when 1.2≦mLi / mM≦2.0 is satisfied, high capacity can be obtained. Furthermore, it is preferable that 1.2≦mLi / mM≦1.8 is satisfied, more preferably that 1.3≦mLi / mM≦1.7 is satisfied, and even more preferably that 1.4≦mLi / mM≦1.6 is satisfied.

[0030] The lithium metal composite oxide (Fm) may contain fluorine (F). Fluorine can substitute oxygen atoms at the anion site in the crystal structure. This stabilizes the Li-excess state and results in a higher capacity. Furthermore, the substitution of fluorine atoms increases the average discharge potential.

[0031] In lithium metal composite oxides (Fm), the arrangement of Li at the cation sites is irregular and the bonding state of Li is varied, resulting in a wide voltage distribution associated with Li release. This can make it difficult to utilize the low-potential tail of the voltage distribution as capacitance. However, the introduction of fluorine atoms shifts the voltage distribution associated with Li release toward the high-potential side, making it easier to utilize the tail as capacitance. This further increases the available capacity.

[0032] The lithium metal composite oxide (Fm) is, for example, a compound represented by the composition formula Li ａ Mn ｂ Co ｃ Mm ｄ O ｅ F ｆ In this case, it is preferable to satisfy the following conditions: 1≦a≦1.4, 0.5≦b≦0.9, 0<c≦0.2, 0<d≦0.2, 0≦e≦0.67, and 1.7≦e+f≦2. e+f is often 2 or less, and may be 1.94 or less, 1.9 or less, or 1.8 or less. 0<c / b≦1 may be satisfied.

[0033] Composition formula Li ａ Mn ｂ Co ｃ Mm ｄ O ｅ F ｆ may satisfy 0.05≦c≦0.1 and 0.15≦(c+d) / b≦1. Furthermore, 50 atomic % or more, further 80 atomic % or more, or even 100% of Mm may be Ti.

[0034] As shown in the above composition formula, some of the oxygen atoms in the anion site may be substituted with fluorine atoms. This stabilizes the Li-excess state (a > 1) and provides high capacity. Furthermore, as described above, the average discharge potential increases, further increasing the available capacity.

[0035] To achieve improved charge-discharge efficiency and high energy density, in addition to the above conditions, the crystal of the lithium metal composite oxide (Fm) preferably satisfies at least one of the following conditions (A) and (B): Both conditions (A) and (B) may be satisfied.

[0036] Condition (A): The crystallite size of the lithium metal composite oxide (Fm) is in the range of 1 nm to 200 nm.

[0037] Condition (B): In an X-ray diffraction (XRD) profile of the lithium metal composite oxide (Fm) using CuKα radiation, the half-width of the diffraction peak attributable to the (200) plane is in the range of 0.1° to 2.5° based on 2θ.

[0038] When condition (A) is satisfied, the crystallite size of the lithium metal composite oxide (Fm) may be 100 nm or less, or may be 80 nm or less. Furthermore, the crystallite size of the lithium metal composite oxide is preferably 4 nm or more, and may be 5 nm or more, or 6 nm or more. The crystallite size of the lithium metal composite oxide may be 4 nm to 200 nm, 5 nm to 200 nm, 4 nm to 80 nm, or 5 nm to 80 nm. The upper and lower limits of the crystallite size can be combined arbitrarily.

[0039] The lithium metal composite oxide (Fm) may contain two or more particle groups having different crystallite sizes within the range of 1 nm to 200 nm. That is, the volume-based particle size distribution of the crystallite sizes may have multiple peaks. In this case, it is preferable to contain particle groups having crystallite sizes within the range of at least 10 nm to 200 nm. However, each particle group is a component that accounts for 10% or more, or even 30% or more, of the total volume of the lithium metal composite oxide (Fm). Furthermore, the sum of the total volumes of the particle groups accounts for 50% or more of the total volume of the lithium metal composite oxide (Fm).

[0040] When the lithium metal composite oxide contains two or more types of particle groups having different crystallite sizes, it may contain a first particle group having a first crystallite size d1 in the range of at least 10 nm to 200 nm and a second particle group having a second crystallite size d2 that is 30% or less, or even 20% or less, of d1.

[0041] The first particle group appears as a sharp peak in the volume-based particle size distribution of crystallite size. The first crystallite size d1 may be 100 nm or less, or 50 nm or less. The first crystallite size d1 may be, for example, 20 nm to 100 nm, or 20 nm to 50 nm.

[0042] The second particle group appears as a broad peak in the volume-based particle size distribution of crystallite size. The second crystallite size d2 may be 3 nm or more, or 4 nm or more. The second crystallite size d2 may be, for example, 3 nm to 10 nm, or 4 nm to 7 nm.

[0043] In general, lithium metal composite oxides have large crystallite sizes and often have improved capacity when the crystal structure is sufficiently developed. On the other hand, lithium metal composite oxides (Fm) with small crystallite sizes of 200 nm or less are thought to have significantly increased capacity due to the increased grain boundary area, which promotes the release and absorption of lithium ions.

[0044] The crystallite size can be calculated based on the Scherrer formula from the half-width of the diffraction peak attributable to the (200) plane in the X-ray diffraction (XRD) profile of the lithium metal composite oxide (Fm) using CuKα radiation. In the X-ray diffraction (XRD) profile of the lithium metal composite oxide (Fm) using CuKα radiation, the half-width of the diffraction peak attributable to the (200) plane may be, for example, in the range of 0.1° to 2.5° based on 2θ. The half-width of the sharp peak may be in the range of 0.2° to 0.5°. The half-width of the broad peak may be in the range of 2.0° to 2.2°. An example of preferred conditions for XRD measurement is shown below. Gonioradius: 150 mm Divergence slit (DS): 1.25°

[0045] When condition (B) is satisfied and the volume-based particle size distribution of the crystallite size has one main peak, in the X-ray diffraction (XRD) profile of the lithium metal composite oxide (Fm) using CuKα radiation, the half-width of the diffraction peak attributable to the (200) plane may be 1.0° or less, 0.8° or less, or 0.5° or less, based on 2θ. The half-width of the diffraction peak may be 0.2° or more, or 0.3° or more, based on 2θ. The half-width of the diffraction peak attributable to the (200) plane may be 0.2° to 1.0°, 0.3° to 0.8°, or 0.3° to 0.5°, based on 2θ. The upper and lower limits of the half-width can be arbitrarily combined.

[0046] A lithium metal composite oxide (Fm) satisfying at least one of the conditions (A) and (B) can be obtained by synthesizing a raw material mixture by a method of calcining at a temperature of, for example, 700°C to 1300°C (calcination method) and pulverizing the calcined product. After sufficient crystallite growth by the calcination method, the crystallite size can be controlled to 1 nm to 200 nm by appropriate pulverization. In one aspect, a lithium metal composite oxide (Fm) satisfying at least one of the conditions (A) and (B) may be referred to as a lithium metal composite oxide (Fm) synthesized by a calcination method.

[0047] It is believed that a crystal structure similar to a rock salt structure belonging to the Li-rich space group Fm-3m will have a greater effect on improving capacity by reducing the crystallite size to 200 nm or less than a crystal structure that clearly belongs to the space group Fm-3m.

[0048] The crystal lattice of the lithium metal composite oxide (Fm) preferably has a lattice constant a of 4.09 Å or more and 4.16 Å or less. In this case, a significant improvement in capacity can be achieved. Although the detailed reason for this is unknown, it is presumed that when the lattice constant a is within the above range, the crystal structure is easily stabilized, and electron tunneling (charge transfer reaction) accompanying lithium ion migration between the electrolyte and the active material is easily promoted. The lattice constant a may be 4.10 Å or more and 4.15 Å or less.

[0049] The lattice constant a, which indicates the length of the crystal lattice of the lithium metal composite oxide in the a-axis direction, is determined using a desktop X-ray diffractometer "MiniFlex" manufactured by Rigaku Corporation and integrated powder X-ray analysis software "PDXL." In the X-ray diffraction measurement using the X-ray diffractometer, the X-ray source is CuKα radiation, and the measurement range of 2θ is 10° to 100°.

[0050] Note that the lattice constant a determined using the integrated powder X-ray analysis software "PDXL" is followed by a number in parentheses, which indicates an error to the third decimal place. For example, 4.115(2) Å means that the lattice constant a is 4.113 Å or more and 4.117 Å or less. In the present disclosure, a lattice constant a of 4.09 Å or more and 4.16 Å or less means that the lattice constant a is within the range of 4.09 Å or more and 4.16 Å or less within the above error range.

[0051] The lithium metal composite oxide (Fm) is preferably obtained by calcining a mixture of raw materials of elements constituting the lithium metal composite oxide (Fm). By calcining the raw material mixture, the lithium metal composite oxide (Fm) can be efficiently synthesized, even when mass-producing the lithium metal composite oxide (Fm). Calcination promotes the growth of crystals similar to the rock salt structure belonging to the space group Fm-3m, resulting in a lithium metal composite oxide with a large crystallite size. The particles of the lithium metal composite oxide (Fm) synthesized by calcination are often hard and lumpy, but by subjecting them to a pulverization treatment, the crystallite size can be controlled to a range of 1 nm to 200 nm.

[0052] When subjected to a pulverization treatment, the lithium metal composite oxide (Fm) may contain two or more particle groups having different crystallite sizes within the range of 1 nm to 200 nm. Furthermore, the lithium metal composite oxide (Fm) may contain particle groups having crystallite sizes at least within the range of 10 nm to 200 nm. That is, the volume-based particle size distribution of the crystallite size has a sharp peak and a broad peak, with the sharp peak being attributed to a first particle group having a first crystallite size d1 within the range of 10 nm to 200 nm, and the broad peak being attributed to a second particle group having a smaller second crystallite size d2. The lithium metal composite oxide (Fm) is highly hard, making it difficult to pulverize it to achieve uniform crystallite sizes, and is therefore likely to have such a particle size distribution. In one aspect, a lithium metal composite oxide (Fm) having a volume-based particle size distribution of crystallite size with a sharp peak and a broad peak may be referred to as a lithium metal composite oxide (Fm) synthesized by a calcination method.

[0053] Currently, several examples have been reported in which lithium metal composite oxides (Fm) have been synthesized by applying high shear force to a raw material mixture using a stirring device such as a ball mill at the laboratory level without calcining the raw material mixture. However, lithium metal composite oxides (Fm) synthesized without calcining the raw material mixture have small crystallite sizes, and it is difficult to achieve crystallite sizes of 3 nm or more or 5 nm or more. Furthermore, it is not possible to achieve a sharp peak and a broad peak in the volume-based particle size distribution of the crystallite size.

[0054] <Method for producing lithium metal composite oxide (Fm)> The raw materials of the elements constituting the lithium metal composite oxide include oxides of Mn, oxides of Co, oxides of Ti, oxides of element Mm, oxides of lithium, lithium salts, lithium manganese oxide (LiMnO ２ ), lithium titanate, Li ２ O, Co ２ O ３ , TiO ２ , Mn ２ O ３The fluorine source may be lithium fluoride (LiF). The types and mixing ratios of the raw materials may be appropriately selected depending on the desired composition described above.

[0055] The atmosphere for firing the raw material mixture may vary depending on the desired composition of the lithium metal composite oxide (Fm) and the types of raw materials, but may be, for example, an inert atmosphere (e.g., an Ar atmosphere) or an oxidizing atmosphere (e.g., air or in the presence of oxygen). It is desirable to circulate the atmospheric gas. The raw material mixture may be fired while stirring the raw material mixture. For example, the raw material mixture may be fired while stirring using a firing furnace equipped with a fluidized bed.

[0056] The firing temperature of the raw material mixture may vary depending on the composition of the lithium metal composite oxide (Fm) to be obtained and the types of raw materials, but may be, for example, 700°C or higher, and is preferably 900°C or higher and 1300°C or lower.

[0057] When the lithium metal composite oxide (Fm) obtained by firing is in the form of aggregated particles, the aggregated particles may be pulverized using a stirring device capable of applying a large shear force to particles, such as a ball mill or a bead mill.

[0058] Next, a secondary battery according to an embodiment of the present disclosure will be described in detail. The secondary battery includes, for example, a positive electrode, a negative electrode, an electrolyte, and a separator as described below.

[0059] [Positive Electrode] The positive electrode comprises a positive electrode current collector and a positive electrode mixture layer formed on the surface of the positive electrode current collector and containing a positive electrode active material. The above-mentioned positive electrode for a secondary battery is used as the positive electrode. The positive electrode mixture layer can be formed, for example, by applying a positive electrode slurry, in which a positive electrode mixture containing a positive electrode active material, a binder, etc. is dispersed in a dispersion medium, to the surface of the positive electrode current collector and drying it. The dried coating may be rolled as necessary. The positive electrode mixture layer may be formed on one surface or both surfaces of the positive electrode current collector.

[0060] The positive electrode mixture layer contains a positive electrode active material as an essential component, and may contain, as optional components, a binder, a thickener, a conductive agent, a positive electrode additive, etc. Known materials can be used as the binder, thickener, and conductive agent.

[0061] The positive electrode active material includes the aforementioned lithium metal composite oxide (Fm) having a crystal structure similar to a rock salt structure belonging to the space group Fm-3m. The lithium metal composite oxide (Fm) is, for example, a secondary particle formed by aggregation of multiple primary particles. The particle size of the primary particles is generally 0.01 μm to 1 μm. The average particle size of the secondary particles of the lithium metal composite oxide (Fm) is, for example, 10 μm or less, and may be 1 μm or less, preferably 0.05 μm to 10 μm or 0.05 μm to 1 μm. Here, the average particle size of the lithium metal composite oxide (Fm) refers to the median diameter (D50) at which the cumulative frequency is 50% in the volume-based particle size distribution, and is measured using a laser diffraction particle size distribution analyzer. The diameter (D90) at which the cumulative frequency is 90% in the volume-based particle size distribution of the lithium metal composite oxide (Fm) may be 1 μm to 10 μm. The diameter (D10) at which the cumulative frequency is 10% in the volume-based particle size distribution of the lithium metal composite oxide (Fm) may be 0.03 μm or less.

[0062] The BET surface area of ​​the composite oxide is 0.01 m ２ / g to 15m ２ It is preferable that the range is / g.

[0063] The content of elements constituting the composite oxide can be measured using an inductively coupled plasma atomic emission spectrometer (ICP-AES), an electron probe microanalyzer (EPMA), an energy dispersive X-ray analyzer (EDX), or the like.

[0064] As the positive electrode active material, the lithium metal composite oxide (Fm) may be mixed with other known lithium metal oxides other than the lithium metal composite oxide (Fm). Examples of other lithium metal oxides include Li a CoO2, Li a NiO2, Li a MnO2, Li a Co b Ni 1-b O2, Li a Co b M 1-b O c , Li a Ni 1-bM b O c , Li a Mn2O4, Li a Mn 2-b M b O 4、 LiMePO 4、 Examples of lithium transition metal composite oxides include Li2MePO4F. Here, M is at least one selected from the group consisting of Na, Mg, Sc, Y, Mn, Fe, Co, Ni, Cu, Zn, Al, Cr, Pb, Sb, and B. Me contains at least a transition element (e.g., at least one selected from the group consisting of Mn, Fe, Co, and Ni). Here, 0≦a≦1.2, 0≦b≦0.9, and 2.0≦c≦2.3. The value a, which indicates the molar ratio of lithium, increases or decreases with charge and discharge.

[0065] The shape and thickness of the positive electrode current collector can be selected from the shape and range corresponding to those of the negative electrode current collector. Examples of the material of the positive electrode current collector include stainless steel, aluminum, aluminum alloy, and titanium.

[0066] [Negative Electrode] The negative electrode may include, for example, a negative electrode current collector and a negative electrode active material layer formed on the surface of the negative electrode current collector. The negative electrode active material layer can be formed, for example, by applying a negative electrode slurry, in which a negative electrode mixture containing a negative electrode active material, a binder, etc. is dispersed in a dispersion medium, to the surface of the negative electrode current collector and drying the slurry. The dried coating may be rolled as necessary. In other words, the negative electrode active material may be a mixture layer. Alternatively, a lithium metal foil or a lithium alloy foil may be attached to the negative electrode current collector. The negative electrode active material layer may be formed on one surface or both surfaces of the negative electrode current collector.

[0067] The negative electrode active material layer contains a negative electrode active material as an essential component, and may contain optional components such as a binder, a conductive agent, a thickener, etc. Known materials can be used as the binder, conductive agent, and thickener.

[0068] The negative electrode active material includes a material that electrochemically absorbs and releases lithium ions, lithium metal, and / or a lithium alloy. Examples of materials that electrochemically absorb and release lithium ions include carbon materials and alloy-based materials. Examples of carbon materials include graphite, easily graphitized carbon (soft carbon), and hardly graphitized carbon (hard carbon). Of these, graphite is preferred because of its excellent charge / discharge stability and low irreversible capacity. Examples of alloy-based materials include those containing at least one metal that can form an alloy with lithium, such as silicon, tin, silicon alloys, tin alloys, and silicon compounds. Silicon oxides and tin oxides formed by bonding these with oxygen may also be used.

[0069] Examples of alloy-based materials containing silicon include a lithium ion conductive phase and a silicon composite material in which silicon particles are dispersed in the lithium ion conductive phase. Examples of lithium ion conductive phases that can be used include a silicon oxide phase, a silicate phase, and / or a carbon phase. The silicon oxide phase may be primarily composed of silicon dioxide (e.g., 95 to 100% by mass). Among these, composite materials composed of a silicate phase and silicon particles dispersed in the silicate phase are preferred because of their high capacity and low irreversible capacity.

[0070] The silicate phase may contain, for example, at least one element selected from the group consisting of Group 1 and Group 2 elements of the long periodic table. Examples of Group 1 elements of the long periodic table and Group 2 elements of the long periodic table include lithium (Li), potassium (K), sodium (Na), magnesium (Mg), calcium (Ca), strontium (Sr), and barium (Ba). Other elements may include aluminum (Al), boron (B), lanthanum (La), phosphorus (P), zirconium (Zr), and titanium (Ti). Among these, a silicate phase containing lithium (hereinafter also referred to as a lithium silicate phase) is preferred due to its small irreversible capacity and high initial charge / discharge efficiency.

[0071] The lithium silicate phase may be an oxide phase containing lithium (Li), silicon (Si), and oxygen (O), and may contain other elements. The atomic ratio of O to Si in the lithium silicate phase, O / Si, is, for example, greater than 2 and less than 4. Preferably, O / Si is greater than 2 and less than 3. The atomic ratio of Li to Si in the lithium silicate phase, Li / Si, is, for example, greater than 0 and less than 4. The lithium silicate phase has the formula: Li ２ｚ SiO ２＋ｚ (0<z<2). Preferably, z satisfies the relationship 0<z<1, and more preferably z=1 / 2. Examples of elements other than Li, Si, and O that can be contained in the lithium silicate phase include iron (Fe), chromium (Cr), nickel (Ni), manganese (Mn), copper (Cu), molybdenum (Mo), zinc (Zn), and aluminum (Al).

[0072] The carbon phase may be composed of, for example, amorphous carbon with low crystallinity (i.e., amorphous carbon). The amorphous carbon may be, for example, hard carbon, soft carbon, or other.

[0073] The negative electrode current collector may be a non-porous conductive substrate (such as a metal foil) or a porous conductive substrate (such as a mesh, net, or punched sheet). Examples of the material for the negative electrode current collector include stainless steel, nickel, nickel alloys, copper, and copper alloys.

[0074] [Electrolyte] The electrolyte may be a liquid electrolyte (electrolytic solution), a gel electrolyte, or a solid electrolyte. The liquid electrolyte is, for example, an electrolytic solution containing a non-aqueous solvent and a salt dissolved in the non-aqueous solvent. The concentration of the salt in the electrolytic solution is, for example, 0.5 mol / L or more and 2 mol / L or less. The electrolytic solution may contain known additives.

[0075] The gel electrolyte contains a salt and a matrix polymer, or a salt, a non-aqueous solvent, and a matrix polymer. The matrix polymer is, for example, a polymer material that absorbs the non-aqueous solvent and gels. Examples of the polymer material include fluororesin, acrylic resin, polyether resin, and polyethylene oxide.

[0076] As the solid electrolyte, for example, a material known in all-solid-state lithium ion secondary batteries (for example, oxide-based solid electrolytes, sulfide-based solid electrolytes, halide-based solid electrolytes, etc.) can be used.

[0077] For example, a liquid non-aqueous electrolyte is prepared by dissolving a salt in a non-aqueous solvent. The salt is an electrolyte salt that ionizes in the electrolyte, and may include, for example, a lithium salt. The electrolyte may contain various additives. The electrolyte is usually used in its liquid state, but its fluidity may be limited by a gelling agent or the like.

[0078] Examples of non-aqueous solvents that can be used include cyclic carbonates, chain carbonates, cyclic carboxylic acid esters, and chain carboxylic acid esters. Examples of cyclic carbonates include propylene carbonate (PC), ethylene carbonate (EC), and vinylene carbonate (VC). Examples of chain carbonates include diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC). Examples of cyclic carboxylic acid esters include γ-butyrolactone (GBL) and γ-valerolactone (GVL). Examples of chain carboxylic acid esters include methyl acetate, ethyl acetate, propyl acetate, methyl propionate (MP), and ethyl propionate (EP). One type of non-aqueous solvent may be used alone, or two or more types may be used in combination.

[0079] Other examples of the non-aqueous solvent include cyclic ethers, chain ethers, nitriles such as acetonitrile, and amides such as dimethylformamide.

[0080] Examples of cyclic ethers include 1,3-dioxolane, 4-methyl-1,3-dioxolane, tetrahydrofuran, 2-methyltetrahydrofuran, propylene oxide, 1,2-butylene oxide, 1,3-dioxane, 1,4-dioxane, 1,3,5-trioxane, furan, 2-methylfuran, 1,8-cineole, and crown ethers.

[0081] Examples of chain ethers include 1,2-dimethoxyethane, dimethyl ether, diethyl ether, dipropyl ether, diisopropyl ether, dibutyl ether, dihexyl ether, ethyl vinyl ether, butyl vinyl ether, methyl phenyl ether, ethyl phenyl ether, butyl phenyl ether, pentyl phenyl ether, methoxytoluene, benzyl ethyl ether, diphenyl ether, dibenzyl ether, o-dimethoxybenzene, 1,2-diethoxyethane, 1,2-dibutoxyethane, diethylene glycol dimethyl ether, diethylene glycol diethyl ether, diethylene glycol dibutyl ether, 1,1-dimethoxymethane, 1,1-diethoxyethane, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether.

[0082] These solvents may be fluorinated solvents in which some of the hydrogen atoms are substituted with fluorine atoms, such as fluoroethylene carbonate (FEC).

[0083] Examples of lithium salts include lithium salts of chlorine-containing acids (LiClO4, LiAlCl4, LiB 10 Cl 10 etc.), lithium salts of fluorine-containing acids (LiPF6, LiPF ２ O ２, LiBF4, LiSbF6, LiAsF6, LiCF3SO3, LiCF3CO2, etc.), lithium salts of fluorine-containing acid imides (LiN(FSO2)2, LiN(CF3SO2)2, LiN(CF3SO2)(CF4F9SO2), LiN(CF5SO2)2, etc.), lithium halides (LiCl, LiBr, LiI, etc.), etc., can be used. One type of lithium salt may be used alone, or two or more types may be used in combination.

[0084] The concentration of the lithium salt in the electrolyte may be 1 mol / L or more and 2 mol / L or less, or 1 mol / L or more and 1.5 mol / L or less. By controlling the lithium salt concentration within the above range, an electrolyte having excellent ionic conductivity and appropriate viscosity can be obtained. However, the lithium salt concentration is not limited to the above.

[0085] The electrolyte may contain other known additives, such as 1,3-propane sultone, methylbenzenesulfonate, cyclohexylbenzene, biphenyl, diphenyl ether, and fluorobenzene.

[0086] [Separator] A separator is interposed between the positive electrode and the negative electrode. The separator has high ion permeability and adequate mechanical strength and insulating properties. The separator can be made of a microporous thin film, woven fabric, nonwoven fabric, or the like. The separator is preferably made of polyolefin such as polypropylene or polyethylene.

[0087] An example of the structure of a secondary battery is a structure in which an electrode group formed by winding a positive electrode and a negative electrode with a separator interposed therebetween and a non-aqueous electrolyte are housed in an outer casing. Alternatively, instead of a wound electrode group, other types of electrode groups may be used, such as a stacked electrode group formed by stacking a positive electrode and a negative electrode with a separator interposed therebetween. The secondary battery may be in any shape, such as a cylindrical shape, a prismatic shape, a coin shape, a button shape, or a laminate shape.

[0088] FIG. 1 is a schematic perspective view, partially cut away, of a prismatic secondary battery according to an embodiment of the present disclosure. The battery includes a bottomed prismatic battery case 4, an electrode group 1, and a nonaqueous electrolyte (not shown) housed within the battery case 4. The electrode group 1 includes a long, strip-shaped negative electrode, a long, strip-shaped positive electrode, and a separator interposed therebetween. The negative electrode current collector is electrically connected to a negative electrode terminal 6 provided on a sealing plate 5 via a negative electrode lead 3. The negative electrode terminal 6 is insulated from the sealing plate 5 by a resin gasket 7. The positive electrode current collector is electrically connected to the back surface of the sealing plate 5 via a positive electrode lead 2. That is, the positive electrode is electrically connected to the battery case 4, which also serves as the positive electrode terminal. The periphery of the sealing plate 5 fits into the open end of the battery case 4, and the fitting is laser-welded. The sealing plate 5 has an injection hole for the nonaqueous electrolyte, which is closed with a seal plug 8 after injection.

[0089] The secondary battery may have a cylindrical, coin, or button shape with a metal battery case, or may be a laminated battery with a battery case made of a laminate sheet that is a laminate of a barrier layer and a resin sheet. In the present disclosure, the type, shape, etc. of the secondary battery are not particularly limited.

[0090] (Additional Notes) The above description discloses the following technologies. (Technology 1) A positive electrode active material for a secondary battery, comprising a lithium metal composite oxide having a crystal structure assignable to space group Fm-3m, the lithium metal composite oxide comprising Li and an electropositive element M different from Li, the electropositive element M comprising at least Mn and Co, and 50 atomic % or more of the electropositive element M being Mn. (Technology 2) The positive electrode active material for a secondary battery according to Technology 1, wherein a ratio (mMn / mCo) of the number of Mn atoms contained in the lithium metal composite oxide to the number of Co atoms contained in the lithium metal composite oxide is 5 to 65. (Technology 3) The positive element M further contains an element Mm, and the Mm contains at least one element selected from the group consisting of Ti, Fe, Ge, Si, Ga, Ni, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Gd, Sm, Eu, Yb, Dy, Al, and Er. (Technology 4) The positive electrode active material for secondary batteries according to Technology 3, in which 50 atomic % or more of the Mm is at least one element selected from the group consisting of Ti, Mg, and Fe. (Technology 5) The positive electrode active material for secondary batteries according to Technology 3 or 4, in which 50 atomic % or more of the Mm is Ti. (Technology 6) The positive electrode active material for a secondary battery according to any one of Technologies 1 to 5, wherein the ratio (mTi / mCo) of the number of Ti atoms mTi contained in the lithium metal composite oxide to the number of Co atoms mCo contained in the lithium metal composite oxide is 1 to 3. (Technology 7) The lithium metal composite oxide is a positive electrode active material for a secondary battery according to any one of Technologies 1 to 5, wherein the ratio (mTi / mCo) of the number of Ti atoms mTi contained in the lithium metal composite oxide to the number of Co atoms mCo contained in the lithium metal composite oxide is 1 to 3. ａ Mn ｂ Co ｃ Mm ｄ O ｅ F ｆand satisfying 1≦a≦1.4, 0.5≦b≦0.9, 0<c≦0.2, 0<d≦0.2, 0≦e≦0.67, and 1.7≦e+f≦2. (Technology 8) The positive electrode active material for a secondary battery according to any one of Techniques 3 to 6, which satisfies 0.05≦c≦0.1 and 0.15≦(c+d) / b≦1, and 50 atomic % or more of the Mm is Ti. (Technology 9) The positive electrode active material for a secondary battery according to any one of Techniques 1 to 8, wherein the crystallite size of the lithium metal composite oxide is in the range of 1 nm to 200 nm, or the half width at half maximum of the diffraction peak assigned to the (200) plane in an X-ray diffraction (XRD) profile of the lithium metal composite oxide using CuKα radiation is in the range of 0.1° to 2.5° based on 2θ. (Technology 10) The positive electrode active material for secondary batteries according to Technology 9, wherein the crystallite size is 100 nm or less. (Technology 11) The positive electrode active material for secondary batteries according to Technology 9 or 10, wherein the crystallite size is 80 nm or less. (Technology 12) The positive electrode active material for secondary batteries according to any one of Technology 9 to 11, wherein the crystallite size of the lithium metal composite oxide is 3 nm or more. (Technology 13) A secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode comprises the positive electrode active material for secondary batteries according to any one of Technology 1 to 12.

[0091] Hereinafter, the present disclosure will be specifically described based on examples and comparative examples, but the present disclosure is not limited to the following examples.

[0092] Examples 1 to 12 [Fabrication of Positive Electrode] Manganese oxide (Mn ２ O ３ ), lithium carbonate (Li ２ CO ３ ), titanium oxide (TiO ２ ), lithium fluoride (LiF) and cobalt oxide (CoO, Co ３ O ４ ) or LiCoO ２ These were mixed to obtain the compositions shown in Tables 1 and 2. The mixture was fired at 950° C. for 10 hours to obtain a lithium metal composite oxide (Fm).

[0093] The fired lithium metal composite oxide (Fm) was pulverized under various conditions. The conditions varied were the rotation speed (here, 150 to 500 rpm) and the treatment time. As an example, in the case of the lithium metal composite oxide (Fm) of A1, the sintered lithium metal composite oxide (Fm) was placed in a planetary ball (Fritsch Premium-Line P7, rotation speed: 300 rpm, container: 45 mL, ball: φ3 mm Zr ball) and treated in a dry air atmosphere at room temperature for 12 hours (24 cycles of 0.5 hour operation followed by a 5-minute break), thereby obtaining a powder of the lithium metal composite oxide (Fm).

[0094] The X-ray diffraction pattern of the obtained lithium metal composite oxide (Fm) was measured and analyzed using a powder X-ray diffractometer using CuKα radiation. From the number and peak positions of the XRD peaks, it was confirmed that a lithium metal composite oxide (Fm) having a rock-salt-type crystal structure belonging to the space group Fm-3m was formed. Furthermore, in the X-ray diffraction pattern, sharp diffraction peaks and broad diffraction peaks were observed, which belong to the (200) plane of the lithium metal composite oxide (Fm). The half-width of the sharp peak was 0.2° to 0.5°, and the half-width of the broad peak was 2.0° to 2.2°. The crystallite size was calculated from the half-width based on the Scherrer formula, and was 20 to 65 nm for the sharp peak and 3 to 9 nm for the broad peak. Furthermore, the lattice constant a of the lithium metal composite oxide (Fm) calculated by the method described above was 4.09 to 4.16 Å.

[0095] The resulting lithium metal composite oxide (Fm), acetylene black, and polyvinylidene fluoride were mixed in a solids mass ratio of 7:2:1, and N-methyl-2-pyrrolidone (NMP) was used as a dispersion medium to prepare a positive electrode slurry. The positive electrode slurry was then applied to a positive electrode current collector made of aluminum foil, the coating was dried and compressed, and then cut to a predetermined electrode size to obtain a positive electrode.

[0096] [Preparation of Electrolyte] A non-aqueous electrolyte was prepared by adding LiPF6 as a lithium salt to a mixed solvent prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) at a predetermined volume ratio.

[0097] [Preparation of Test Cell] A test cell was prepared using the above-described positive electrode and a negative electrode counter electrode made of lithium metal foil. The above-described positive electrode and negative electrode counter electrode were arranged opposite each other with a separator interposed therebetween to form an electrode assembly, and the electrode assembly was housed in a coin-shaped outer can. After injecting an electrolyte into the outer can, the outer can was sealed to obtain a coin-shaped test secondary battery.

[0098] [Evaluation] (Initial Discharge Capacity) The secondary battery was charged at a constant current of 0.1 C to a battery voltage of 4.95 V under room temperature conditions, and then charged at a constant voltage of 4.95 V until the current reached 0.01 C, and the charge capacity was determined. After a 20-minute pause, the battery was discharged at a constant current of 0.1 C to a battery voltage of 2.5 V, and the discharge capacity was measured. The discharge capacity per mass of the lithium metal composite oxide (Fm) is shown in the table. Measurements were made up to three times in the same manner, and the average value was calculated. The results are shown in Tables 1 and 2.

[0099] Comparative Examples 1 to 10 Manganese oxide (Mn ２ O ３ ), lithium carbonate (Li ２ CO ３ ), titanium oxide (TiO ２ ) and an oxide of metal M1 (M1 = Mg, Ca, Zn, Fe, Cr, Al, Ni, Zr, Nb, Si) were mixed in the composition shown in Table 1. The mixture was fired at 950°C for 10 hours to obtain a lithium metal composite oxide (Fm). A secondary battery was produced and evaluated in the same manner as in Example 1 except for the above. The results are shown in Table 1.

[0100]

[0101] As can be seen from Table 1, when the lithium metal composite oxide (Fm) contains Co, the initial discharge capacity is significantly improved compared to when other metal elements are contained.

[0102]

[0103] Example 13 and Comparative Examples 13 to 14 Manganese oxide (Mn ２ O ３ ), lithium carbonate (Li ２ CO ３ ) and an oxide of metal M2 (M2 = Co, Mg, Fe) were mixed in a charge composition that would give the composition shown in Table 3. The mixture was fired at 950°C for 10 hours to obtain a lithium metal composite oxide (Fm). A secondary battery was fabricated and evaluated in the same manner as in Example 1, except for the above. The results are shown in Table 3.

[0104]

[0105] As can be seen from Table 3, when the lithium metal composite oxide (Fm) contains Co, the initial discharge capacity is significantly improved compared to when other metal elements are contained.

[0106] Examples 14 to 18 Manganese oxide (Mn ２ O ３ ), lithium carbonate (Li ２ CO ３ ), lithium fluoride (LiF) and cobalt oxide (CoO, Co ３ O ４ ) or LiCoO ２ The resulting mixture was weighed out to obtain the composition shown in Table 4, placed in a planetary ball (Fritsch Premium-Line P7, rotation speed: 600 rpm, container: 45 mL, ball: φ3 mm Zr ball), and treated in an Ar atmosphere at room temperature for 35 hours (35 cycles of 1 hour operation followed by a 10-minute break), thereby obtaining a powder of lithium metal composite oxide (Fm). Thereafter, a secondary battery was fabricated and evaluated in the same manner as in Example 1. The results are shown in Table 4.

[0107]

[0108] The positive electrode active material for a secondary battery according to the present disclosure can provide a secondary battery with high energy density. The secondary battery according to the present disclosure is useful as a main power source for mobile communication devices, portable electronic devices, and the like. Although the present invention has been described with reference to presently preferred embodiments, such disclosure should not be construed as limiting. Various modifications and alterations will no doubt become apparent to those skilled in the art to which the present invention pertains upon reading the above disclosure. Accordingly, the appended claims should be construed to include all modifications and alterations without departing from the true spirit and scope of the present invention.

[0109] 1: electrode group, 2: positive electrode lead, 3: negative electrode lead, 4: battery case, 5: sealing plate, 6: negative electrode terminal, 7: gasket, 8: sealing plug

Claims

1. A positive electrode active material for a secondary battery, comprising a lithium metal composite oxide having a crystal structure that can be assigned to space group Fm-3m, the lithium metal composite oxide comprising Li and an electropositive element M different from Li, the electropositive element M comprising at least Mn and Co, and 50 atomic % or more of the electropositive element M being Mn.

2. The positive electrode active material for a secondary battery according to claim 1, wherein the ratio (mMn / mCo) of the number of Mn atoms mMn contained in the lithium metal composite oxide to the number of Co atoms mCo contained in the lithium metal composite oxide is 5 to 65.

3. The positive electrode active material for a secondary battery according to claim 1, wherein the positive element M further contains an element Mm, and the Mm contains at least one element selected from the group consisting of Ti, Fe, Ge, Si, Ga, Ni, Sn, Cu, Nb, Mo, Bi, V, Cr, Y, Zr, Zn, Na, K, Ca, Mg, Pt, Au, Ag, Ru, Ta, W, La, Ce, Pr, Gd, Sm, Eu, Yb, Dy, Al, and Er.

4. The positive electrode active material for a secondary battery according to claim 3, wherein 50 atomic % or more of said Mm is at least one element selected from the group consisting of Ti, Mg and Fe.

5. The positive electrode active material for a secondary battery according to claim 4, wherein 50 atomic % or more of said Mm is Ti.

6. The positive electrode active material for a secondary battery according to claim 5, wherein the ratio (mTi / mCo) of the number of Ti atoms mTi contained in the lithium metal composite oxide to the number of Co atoms mCo contained in the lithium metal composite oxide is 1 to 3.

7. The lithium metal composite oxide has the composition formula Li ａ Mn ｂ Co ｃ Mm ｄ O ｅ F ｆ 7. The positive electrode active material for a secondary battery according to claim 6, which satisfies the following conditions: 1≦a≦1.4, 0.5≦b≦0.9, 0<c≦0.2, 0<d≦0.2, 0≦e≦0.67, and 1.7≦e+f≦2.

8. The positive electrode active material for a secondary battery according to claim 7, wherein 0.05≦c≦0.1 and 0.15≦(c+d) / b≦1 are satisfied, and 50 atomic % or more of the Mm is Ti.

9. The positive electrode active material for a secondary battery according to claim 1, wherein the crystallite size of the lithium metal composite oxide is in the range of 1 nm to 200 nm, or the half-width of a diffraction peak belonging to a (200) plane in an X-ray diffraction (XRD) profile of the lithium metal composite oxide using CuKα radiation is in the range of 0.1° to 2.5° based on 2θ.

10. The positive electrode active material for a secondary battery according to claim 9, wherein the crystallite size is 100 nm or less.

11. The positive electrode active material for a secondary battery according to claim 9, wherein the crystallite size is 80 nm or less.

12. The positive electrode active material for a secondary battery according to claim 9, wherein the crystallite size of the lithium metal composite oxide is 3 nm or more.

13. A secondary battery comprising a positive electrode, a negative electrode, an electrolyte, and a separator interposed between the positive electrode and the negative electrode, wherein the positive electrode contains the positive electrode active material for secondary batteries according to any one of claims 1 to 12.

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