Negative plate and lithium ion secondary battery

Abstract

The invention discloses a negative plate and a lithium ion secondary battery. The negative plate comprises a negative active layer, the negative active layer comprises a negative active material and an inorganic oxide solid electrolyte, the negative active material comprises graphite and silicon carbon, and by taking the mass of the negative active layer as a reference, the silicon element content A is more than or equal to 1% and less than or equal to 10%; the content B of a first characteristic element in the inorganic oxide solid electrolyte is greater than or equal to 0.1% and less than or equal to 0.5%, the first characteristic element comprises at least two of Al, Ti, La, Zr and Ta, and the particle size D1 of the inorganic oxide solid electrolyte is 0.5-3 [mu] m. By adding a proper amount of inorganic oxide solid electrolyte with a specific particle size, lithium ions are conducted along the inorganic oxide solid electrolyte under a low-temperature condition, a conduction path of partial electrolyte is replaced, and the defect of poor low-temperature discharge performance of the battery is improved.

Description

Technical Field

[0001] This application relates to the technical field of lithium-ion batteries, and particularly to a negative electrode sheet and a lithium-ion secondary battery. Background Art

[0002] In the technical field of lithium-ion secondary batteries, the poor discharge performance at low temperatures has always been one of the key factors restricting its application scope. Especially for lithium-ion secondary batteries using graphite-doped silicon negative electrodes, although they have a high energy density and can provide more electrical energy, due to the poor conductivity of silicon-carbon materials, which is not conducive to the diffusion of lithium ions, the discharge performance of the battery deteriorates more severely at low temperatures. Summary of the Invention

[0003] In view of this, this application provides a negative electrode sheet and a lithium-ion secondary battery, aiming to solve the problem of poor low-temperature discharge performance of existing lithium-ion secondary batteries to a certain extent.

[0004] According to an embodiment of this application, in a first aspect, a negative electrode sheet is provided, which includes a negative electrode current collector and a negative electrode active layer disposed on at least one surface in the thickness direction of the negative electrode current collector; the negative electrode active layer includes a negative electrode active material, the negative electrode active material includes a graphite material and a silicon-carbon material, and the content A of silicon element based on the mass of the negative electrode active layer satisfies 1% ≤ A ≤ 10%;

[0005] The negative electrode active layer further includes an inorganic oxide solid electrolyte, and the particle size D1 of the inorganic oxide solid electrolyte is 0.5 μm - 3 μm, and D1 is the maximum diameter of the inorganic oxide solid electrolyte observed at a magnification of 5K in SEM;

[0006] The inorganic oxide solid electrolyte includes a first characteristic element, the first characteristic element includes at least two of Al, Ti, La, Zr, Ta; based on the mass of the negative electrode active layer, the content of the first characteristic element is denoted as B, and satisfies 0.1% ≤ B ≤ 0.8%.

[0007] In some optional embodiments, the particle size D1 of the inorganic oxide solid electrolyte is 1 μm - 2 μm.

[0008] In some optional embodiments, A and B satisfy: 0.05 ≤ B / A ≤ 0.5.

[0009] In some optional embodiments, the inorganic oxide solid electrolyte includes at least one of lithium titanium aluminum phosphate, lithium lanthanum zirconium tantalum oxide, and lithium lanthanum titanium oxide.

[0010] In some optional embodiments, based on the mass of the negative electrode active layer, the content of the inorganic oxide solid electrolyte is 0.05% - 1%.

[0011] In some alternative embodiments, the graphite material includes artificial graphite, the artificial graphite includes graphite single particles and graphite secondary particles, the median particle size D4 of the graphite single particles is 5 μm - 12 μm, and the median particle size D5 of the graphite secondary particles is 10 μm - 20 μm.

[0012] In some alternative embodiments, the median particle size D6 of the silicon-carbon material is 3 μm - 15 μm.

[0013] In some alternative embodiments, the content of silicon element in the silicon-carbon material is 35% - 70%.

[0014] In some alternative embodiments, the silicon-carbon material includes a porous carbon substrate and silicon particles dispersed inside the pores of the porous carbon substrate.

[0015] In some alternative embodiments, the negative electrode active layer further includes a conductive agent, the conductive agent includes conductive carbon black, the particle size D3 of the conductive carbon black is 0.02 μm - 2 μm, and D3 is the average diameter of 10 conductive carbon black particles observed at an SEM magnification of 5K; the particle size D2 of the inorganic oxide solid electrolyte is 0.8 μm - 3 μm, and D2 is the average diameter of 10 inorganic oxide solid electrolyte particles observed at an SEM magnification of 5K; D3 < D2 is satisfied.

[0016] In some alternative embodiments, D2 is 1 μm - 2 μm.

[0017] In some alternative embodiments, D3 is 0.1 μm - 1 μm.

[0018] In some alternative embodiments, the conductive agent further includes single-walled carbon nanotubes, and the tube diameter of the single-walled carbon nanotubes is 1 nm - 50 nm.

[0019] In some alternative embodiments, the negative electrode active layer further includes an adhesive, the adhesive includes a fibrous adhesive, and the fibrous adhesive includes a fibrillatable resin.

[0020] In some alternative embodiments, based on the mass of the negative electrode active layer, the content of the fibrillatable resin is 0.1% - 20%.

[0021] Further, based on the mass of the negative electrode active layer, the content of the fibrillatable resin is 1% - 5%.

[0022] In some alternative embodiments, the fibrillatable resin includes at least one of polytetrafluoroethylene (PTFE) and PTFE derivatives.

[0023] According to an embodiment of the present application, in a second aspect, a lithium-ion secondary battery is provided, which includes a positive electrode sheet, a separator, and the negative electrode sheet described in the first aspect.

[0024] In some alternative embodiments, the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer provided on at least one surface in the thickness direction of the positive electrode current collector. The positive electrode active layer includes a positive electrode active material and a lithium supplement material. The lithium supplement material includes a second characteristic element, and the second characteristic element includes at least one of Ni, Mo, Nb, Ti, Zr, Fe, Y, Mn, Cu, Mg, and Zn; based on the mass of the positive electrode active layer, the content of the second characteristic element is denoted as N, and 1000 ppm ≤ N ≤ 10000 ppm is satisfied.

[0025] In some alternative embodiments, it includes Li5FeO4 and / or a substance with the chemical formula Li 2±δ Ni α M β O2, where 0 ≤ δ ≤ 0.5, 0 ≤ α ≤ 1, 0 ≤ β ≤ 1, and M includes at least one of Mo, Nb, Ti, Zr, Fe, Y, Mn, Cu, Mg, and Zn.

[0026] In some alternative embodiments, N and B satisfy: 0.125 ≤ N / B ≤ 10, where B is the content of the first characteristic element in the negative electrode active layer, and 0.1% ≤ B ≤ 0.8%.

[0027] In some alternative embodiments, 0.25 ≤ N / B ≤ 3.5.

[0028] In some alternative embodiments, the positive electrode active layer further includes at least one of lithium cobaltate and ternary material. The chemical formula of the lithium cobaltate is Li x Co y E z O2, where 0.8 ≤ x ≤ 1.1, 0.85 ≤ y ≤ 1, 0 ≤ z ≤ 0.15, and E includes at least one of Al, Mg, Ti, Y, La, Ga, Ge, Sn, Si, Zr, Ca, Sb, In, Ni, and Mn. The chemical formula of the ternary material is Li 1+a Ni b Co c L d O2, where, -0.05 ≤ a ≤ 0.2, 0.6 ≤ b < 1, 0 ≤ c ≤ 0.2, 0 ≤ d ≤ 0.4, and L includes at least one of Mn, Al, Mg, Zr, Ti, W, and B.

[0029] The technical solution of the present application has the following advantages:

[0030] The negative electrode sheet provided by the present application includes a negative electrode current collector and a negative electrode active layer provided on at least one surface in the thickness direction of the negative electrode current collector; the negative electrode active layer includes a negative electrode active material, the negative electrode active material includes a graphite material and a silicon-carbon material, and the content A of silicon element based on the mass of the negative electrode active layer satisfies 1% ≤ A ≤ 10%; the negative electrode active layer further includes an inorganic oxide solid electrolyte, and the particle size D1 of the inorganic oxide solid electrolyte is 0.5 μm - 3 μm; the inorganic oxide solid electrolyte includes a first characteristic element, and the first characteristic element includes at least two of Al, Ti, La, Zr, and Ta; based on the mass of the negative electrode active layer, the content of the first characteristic element is denoted as B, and satisfies 0.1% ≤ B ≤ 0.8%. It is found in the research of the present application that the ionic conductivity of the inorganic oxide solid electrolyte does not decrease significantly with the decrease of temperature. By adding an appropriate amount of inorganic oxide solid electrolyte with a specific particle size to the negative electrode active layer, active ions can conduct along the inorganic oxide solid electrolyte under extremely low temperature conditions, thereby replacing part of the conduction path of the electrolyte, ensuring that the battery can discharge normally under low temperature conditions, and improving the defect of poor low-temperature discharge performance of the battery.

[0031] Additional aspects and advantages of embodiments of the present application will be described and shown in part in the following description, or will be explained through the implementation of embodiments of the present application. Detailed implementation manners

[0032] The following embodiments are provided to better further understand the present application. They are not limited to the best implementation manners, and do not limit the content and protection scope of the present application. Any product that is the same as or similar to the present application obtained by anyone under the inspiration of the present application or by combining the features of the present application with other prior art features falls within the protection scope of the present application.

[0033] It should be noted that in the description of the present application, the terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance. In addition, the technical features involved in different embodiments of the present application described below can be combined with each other as long as they do not conflict with each other.

[0034] First of all, it should be noted that for lithium-ion secondary batteries, a low-temperature environment will cause an increase in the viscosity of the electrolyte and a decrease in conductivity. Moreover, as the ambient temperature decreases, the conductivity of the electrolyte of the lithium-ion secondary battery drops sharply after 0 °C, making the diffusion rate of lithium ions in the positive and negative electrode materials significantly slow down, resulting in a significant reduction in the discharge capacity and discharge efficiency of the battery.

[0035] To solve the above problems existing in the related art, according to the first aspect of the present application, a negative electrode sheet is provided, which includes a negative electrode current collector and a negative electrode active layer provided on at least one surface in the thickness direction of the negative electrode current collector; the negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes a graphite material and a silicon-carbon material. The content A of silicon element based on the mass of the negative electrode active layer satisfies 1% ≤ A ≤ 10%. As an example, it can be 1%, 3%, 5%, 8%, 10%, etc. or within the range composed of any two of the above values;

[0036] The negative electrode active layer further includes an inorganic oxide solid electrolyte. The particle size D1 of the inorganic oxide solid electrolyte is 0.5 μm - 3 μm, and D1 is the maximum diameter of the inorganic oxide solid electrolyte observed at a magnification of 5K in SEM; the inorganic oxide solid electrolyte includes a first characteristic element, and the first characteristic element includes at least two of Al, Ti, La, Zr, and Ta; based on the mass of the negative electrode active layer, the content of the first characteristic element is denoted as B, which satisfies 0.1% ≤ B ≤ 0.8%.

[0037] The present application's research finds that, compared with the graphite material, the silicon-carbon material has poor electrical conductivity, resulting in that the graphite-doped silicon negative electrode is more unfavorable for the diffusion of lithium ions in a low-temperature environment; while the ionic conductivity of the inorganic oxide solid electrolyte does not decrease significantly with the decrease of temperature. Adding an appropriate amount of inorganic oxide solid electrolyte with a specific particle size to the negative electrode active layer enables active ions to conduct along the inorganic oxide solid electrolyte under extremely low-temperature conditions, thereby replacing part of the conduction path of the electrolyte and ensuring that the battery can discharge normally under low-temperature conditions, improving the defect of poor low-temperature discharge performance of the battery.

[0038] It can be understood that if the value of the content B of the first characteristic element in the negative electrode active layer is too low, it means that the content of the inorganic oxide solid electrolyte is too small, and the effect on improving the low-temperature discharge performance of the battery is not obvious; if the value of B is too high, it will affect the discharge capacity of the battery in a normal-temperature environment, and further affect the energy density. Moreover, the inorganic oxide solid electrolyte is prone to gas generation during storage in a high-temperature and high-voltage environment, affecting the safety performance of the battery. Therefore, reasonably regulating the content of the first characteristic element in the negative electrode active layer within the range of 0.1% - 0.8% can improve the low-temperature discharge performance of the battery without losing energy density and high-temperature safety. Exemplarily, the content of the first characteristic element in the negative electrode active layer can be 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, etc. or within the range composed of any two of the above values.

[0039] Meanwhile, the research of this application finds that the particle size D1 of the inorganic oxide solid electrolyte also has an important influence on the improvement effect of the low-temperature discharge performance of the battery. If the particle size of the inorganic oxide solid electrolyte particles is too large, the ability to conduct lithium ions will decrease. On the contrary, if the particle size is too small, the particles are prone to agglomeration, and neither can improve the low-temperature discharge performance of the battery. Therefore, by reasonably controlling the particle size D1 of the inorganic oxide solid electrolyte particles within the range of 0.5 μm - 3 μm, especially 1 μm - 2 μm, the low-temperature discharge performance of the battery can be effectively improved. Exemplarily, the particle size D1 of the inorganic oxide solid electrolyte can be 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, etc. or within the range composed of any two of the above values.

[0040] It should be noted that the test method for the particle size D1 of the inorganic oxide solid electrolyte includes: disassembling and taking out the negative electrode sheet after discharging the lithium-ion secondary battery to 0% SOC, soaking it in dimethyl carbonate (DMC) solvent for 12 h and then rinsing it with DMC to remove the lithium salt attached to the negative electrode sheet, using an argon ion milling machine CP to laser cut the negative electrode sheet, observing it at a magnification of 5K in a scanning electron microscope (SEM), and defining the maximum diameter of the inorganic oxide solid electrolyte particles in the field of view as the particle size D1.

[0041] Due to the poor conductivity of the silicon-carbon material itself, the low-temperature environment is more unfavorable for the diffusion of lithium ions in the silicon-doped graphite negative electrode. And as the temperature decreases, the ionic conductivity of the electrolyte drops significantly, resulting in the inability of the silicon-doped system battery to discharge normally in extremely low-temperature weather (such as when the temperature is -20°C and below). The ionic conductivity of the inorganic oxide solid electrolyte does not decrease significantly with the decrease of temperature. By adding an appropriate amount of inorganic oxide solid electrolyte to the negative electrode active layer and reasonably regulating the ratio B / A of the content of the first characteristic element to the content of silicon element in the negative electrode active layer within the range of 0.05 - 0.5, it can further ensure the conduction of lithium ions along the inorganic oxide solid electrolyte under extremely low-temperature conditions, ensure that the battery can discharge normally under low-temperature conditions, and thus ensure that the silicon-doped negative electrode battery system has the advantages of both high energy density and good low-temperature discharge performance.

[0042] Exemplarily, the value of B / A can be 0.05, 0.1, 0.2, 0.3, 0.4, 0.5, etc. or within the range composed of any two of the above values.

[0043] Exemplarily, the inorganic oxide solid electrolyte includes at least one of lithium aluminum titanium phosphate (abbreviated as LATP), lithium lanthanum zirconium tantalum oxide (abbreviated as LLZTO), and lithium lanthanum titanium oxide (abbreviated as LLTO). The chemical formula of the lithium aluminum titanium phosphate is Li 1+ u Al u Ti 2-u(PO4)3, 0 < u ≤ 0.5, the chemical formula of the lithium lanthanum zirconium tantalum oxide is Li 7-r La3Zr 2-r Ta r O 12 , 0 ≤ r ≤ 0.5, the chemical formula of the lithium lanthanum titanium oxide is Li 3v La 2 / 3-v TiO3, 0 < v ≤ 0.2.

[0044] In some embodiments, based on the mass of the negative electrode active layer, the content of the inorganic oxide solid electrolyte is 0.05% - 1%. For example, it can be 0.05%, 0.1%, 0.3%, 0.5%, 0.8%, 1%, etc. or within the range composed of any two of the above values. If the content of the inorganic oxide solid electrolyte is too low, the effect on improving the low-temperature discharge performance of the battery is not obvious; however, if the content of the inorganic oxide solid electrolyte is too high, it will affect the discharge capacity of the battery in the normal temperature environment, thereby affecting the energy density, and the inorganic oxide solid electrolyte is also prone to gas generation during storage in a high-temperature and high-voltage environment, affecting the safety performance of the battery. Therefore, controlling the content of the inorganic oxide solid electrolyte in the negative electrode active layer within the range of 0.05% - 1% can effectively improve the low-temperature discharge performance of the battery without losing energy density and high-temperature safety.

[0045] The higher the silicon content in the graphite-doped silicon negative electrode, the more beneficial it is to improve the energy density of the battery, but it is not conducive to the diffusion of lithium ions in the negative electrode under low-temperature conditions. Therefore, in some embodiments, the content of silicon element in the silicon-carbon material is 35% - 70%. Exemplarily, for example, it can be 35%, 45%, 55%, 65%, 70%, etc. or within the range composed of any two of the above values.

[0046] In order to balance the contradiction between the energy density and the low-temperature discharge performance of the high-silicon negative electrode battery, by adding an inorganic oxide solid electrolyte to the negative electrode active layer, lithium ions can be conducted along the inorganic oxide solid electrolyte, replacing part of the conduction path of the electrolyte, ensuring that the battery can be normally discharged under low-temperature conditions, thereby improving the low-temperature discharge performance while ensuring the high energy density of the battery.

[0047] In some embodiments, the silicon-carbon material includes a porous carbon substrate and silicon particles dispersed inside the pores of the porous carbon substrate, and the porous carbon substrate includes a porous carbon skeleton and a carbon coating layer covering at least part of the surface of the porous carbon skeleton.

[0048] The median particle size D6 of the silicon-carbon material is 3 μm - 15 μm. Using a silicon-carbon material with a smaller particle size is beneficial to improving the conductivity of the negative electrode, enhancing the low-temperature performance, and the particle size of the silicon-carbon material is relatively close to that of the inorganic oxide solid electrolyte, which can further improve the lithium-ion transport ability in the negative electrode under low-temperature conditions, thereby effectively improving the low-temperature discharge performance of the battery.

[0049] The median particle size D6 of the silicon-carbon material is obtained by laser particle size analyzer or SEM test. As an example, the median particle size D6 of the silicon-carbon material can be 3 μm, 5 μm, 7 μm, 9 μm, 11 μm, 13 μm, 15 μm, etc. or within the range composed of any two of the above values.

[0050] Graphite materials have the advantages of good conductivity, high specific capacity, low cost, etc., and are the mainstream materials for the negative electrode active materials of current lithium-ion secondary batteries. In some embodiments, the graphite material includes artificial graphite, and the artificial graphite includes graphite single particles and graphite secondary particles. The median particle size D4 of the graphite single particles is 5 μm - 12 μm, and D4 can be, for example, 5 μm, 7 μm, 9 μm, 10 μm, 12 μm, etc. or within the range composed of any two of the above values; the median particle size D5 of the graphite secondary particles is 10 μm - 20 μm. Exemplarily, D5 can be 10 μm, 12 μm, 14 μm, 16 μm, 18 μm, 20 μm, etc. or within the range composed of any two of the above values.

[0051] The median particle size D4 of the graphite single particles and the median particle size D5 of the graphite secondary particles can both be obtained by laser particle size analyzer or SEM test.

[0052] Compared with graphite single particles, graphite secondary particles have poor ion conductivity but excellent orientation, so they have the advantages of low expansion and high compaction. Using the particle size grading of the above-mentioned graphite secondary particles and graphite single particles can well alleviate the expansion of the silicon-doped negative electrode while ensuring the high capacity of the battery. By combining artificial graphite with inorganic oxide solid electrolyte, it is beneficial to improve the ion conductivity of the negative electrode under low-temperature conditions and improve the low-temperature discharge performance of the battery, so as to better adapt to the silicon-doped carbon battery system.

[0053] It can be understood that graphite single particles are primary particles, while graphite secondary particles are formed by the aggregation of graphite primary particles.

[0054] In some embodiments, the negative electrode active layer further includes a conductive agent, the conductive agent includes conductive carbon black, and the particle size D3 of the conductive carbon black is 0.02 μm - 2 μm. D3 is the average diameter of 10 conductive carbon black particles observed at a magnification of 5K in SEM; Exemplarily, the particle size D3 of the conductive carbon black can be 0.02 μm, 0.06 μm, 0.1 μm, 0.5 μm, 1 μm, 1.5 μm, 2 μm, etc. or within the range formed by any two of the above values. Adding a conductive agent to the negative electrode active layer can form a conductive network, thereby enhancing the conductivity between the negative electrode active material particles.

[0055] The particle size D2 of the inorganic oxide solid electrolyte is 0.8 μm - 3 μm. D2 is the average diameter of 10 inorganic oxide solid electrolyte particles observed at a magnification of 5K in SEM; Exemplarily, the particle size D2 of the inorganic oxide solid electrolyte can be 0.8 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, etc. or within the range formed by any two of the above values.

[0056] By controlling the particle size D2 of the inorganic oxide solid electrolyte to be larger than the particle size D3 of the conductive agent, on the one hand, it can prevent the aggregation of inorganic oxide solid electrolyte particles and conductive agent particles, which affects the dispersion uniformity and further affects the exertion of their respective conductivity. On the other hand, it is beneficial to the direct contact between the inorganic oxide solid electrolyte particles, thereby forming a conductive network, further enhancing the conductivity of the negative electrode active layer, increasing the diffusion rate of lithium ions in the negative electrode sheet under low-temperature conditions, and improving the low-temperature discharge performance.

[0057] It should be noted that the particle size D2 of the inorganic oxide solid electrolyte is the average diameter of 10 inorganic oxide solid electrolyte particles observed at a magnification of 5K in SEM. The test method for D2 includes: disassembling and taking out the negative electrode sheet after discharging the lithium-ion secondary battery to 0% SOC, soaking it in dimethyl carbonate (DMC) solvent for 12h and then rinsing it with DMC to remove the lithium salt attached to the negative electrode sheet, using an argon ion milling machine CP to laser cut the negative electrode sheet, observing it at a magnification of 5K in a scanning electron microscope (SEM), and defining the average diameter of 10 randomly selected inorganic oxide solid electrolyte particles in the field of view as the particle size D2.

[0058] Similarly, the particle size D3 of the conductive carbon black is the average diameter of 10 conductive carbon black particles observed at a magnification of 5K in SEM. The test method for D3 includes: disassembling and removing the negative electrode sheet after discharging the lithium-ion secondary battery to 0% SOC, soaking it in dimethyl carbonate (DMC) solvent for 12 hours and then rinsing it with DMC to remove the lithium salt attached to the negative electrode sheet, using an argon ion milling instrument CP to laser cut the negative electrode sheet, observing it at a magnification of 5K in a scanning electron microscope (SEM), and defining the average diameter of 10 randomly selected conductive carbon black particles in the field of view as the particle size D3.

[0059] In other embodiments, the conductive agent may further include single-walled carbon nanotubes, and the tube diameter of the single-walled carbon nanotubes is 1 nm - 50 nm. Exemplarily, the tube diameter of the single-walled carbon nanotubes can be 1 nm, 5 nm, 10 nm, 20 nm, 30 nm, 40 nm, 50 nm, etc. or within the range composed of any two of the above values.

[0060] Due to the unique structure of single-walled carbon nanotubes, they have better electrical conductivity and lithium intercalation performance compared to conductive carbon black. By adding single-walled carbon nanotubes with a tube diameter of 1 nm - 50 nm to the negative electrode active layer, a stable three-dimensional conductive network can be formed between silicon-carbon and graphite particles. This network can not only adapt to the volume change of silicon but also maintain the continuity of the electron conduction path during charge and discharge, thereby forming an efficient electron conduction path between the negative electrode active materials. That is, the mixture of single-walled carbon nanotubes and silicon-carbon particles can significantly improve the electron conduction ability of silicon-carbon and reduce the internal resistance. If the tube diameter of the single-walled carbon nanotubes is too small, the single-walled carbon nanotubes are prone to agglomeration, making it difficult to form a conductive network, which is not conducive to the transmission of electrons to the current collector and also not conducive to reducing the internal resistance, affecting the low-temperature discharge performance of the battery; while if the tube diameter of the single-walled carbon nanotubes is too large, it will affect the crystallinity of the single-walled carbon nanotubes, resulting in unsatisfactory electrical conductivity, strength, and toughness, and thus being not conducive to the low-temperature discharge performance and high-temperature safety of the battery.

[0061] In some embodiments, the negative electrode active layer further includes a binder, the binder includes a fibrous binder, and the fibrous binder includes a fibrillatable resin. Exemplarily, the fibrillatable resin includes at least one of polytetrafluoroethylene (PTFE) and PTFE derivatives.

[0062] It should be noted that due to the small surface binding energy of PTFE, it is difficult for fillers to adhere to the surface of PTFE. Therefore, the related art uses physical and chemical methods to modify the surface of PTFE, thereby generating reaction sites on the surface of PTFE and increasing the surface roughness of PTFE. Common modification methods such as plasma treatment method, electron irradiation treatment method, coupling agent treatment method, etc. are all prior arts and will not be elaborated here. The present application can select PTFE derivatives obtained by appropriate modification methods according to needs.

[0063] Due to the low van der Waals force of the fibrillatable resin, the packing is loose, and under the action of external shear force, it will change from an aggregate to fibrils, that is, fibrillization, to form a fibrillated elastic layer (corresponding to the negative electrode active layer in this application) containing an elastic network structure. After that, the elastic layer is calendered and roll-pressed by a multi-stage calender, and then thermally compounded with the coated current collector to obtain a finished electrode sheet. Using the fibrillatable resin as the binder, a network structure is formed by the interlacing of the fine filaments of the binder fibrils, so that the negative electrode active material and the conductive agent are placed in the network structure, thereby forming a uniform and regular pore structure, which is more conducive to the performance of the battery.

[0064] Based on the mass of the negative electrode active layer, the content of the fibrillatable resin is 0.1%-20%, for example, it can be 0.1%, 1%, 2.5%, 5%, 10%, 15%, 20%, etc. or within the range composed of any two of the above values. If the content of the fibrillatable resin is too low, it is not conducive to constructing a network structure, and the uniform loading of the negative electrode active material and the conductive agent cannot be achieved. If the content of the fibrillatable resin is too high, it will cause the electrode sheet to be too brittle and the compaction density to be low, thereby affecting the energy density of the battery.

[0065] The production of the electrode sheet is the primary link in the manufacturing of the battery cell and has a decisive impact on the basic performance of the battery cell. The production process of the electrode sheet can be divided into a wet process and a dry process. Among them, the wet process is the currently commonly used method for producing the electrode sheet. The traditional wet process is to mix the active material, the conductive agent, and the binder in a solvent, and coat it on the surface of the current collector through a slit coating die head as required, and then roll-press and slit it to obtain the electrode sheet. Since the wet process requires evaporation and recovery of the solvent, the energy consumption is high, and at the same time, the solvent is highly toxic, which is not conducive to the health of personnel and environmental protection. Therefore, it is imperative to deeply explore the dry production process of the electrode sheet.

[0066] In some embodiments of the present application, the negative electrode sheet is prepared by a dry process, including the following steps:

[0067] Dry-mix the negative electrode active material, the inorganic oxide solid electrolyte, the conductive agent, and the fibrillatable resin and then fibrillate to obtain a negative electrode active layer; roll-press the negative electrode active layer onto at least one surface in the thickness direction of the negative electrode current collector to obtain the negative electrode sheet.

[0068] Based on the mass of the negative electrode active layer, the content of the fibrillatable resin is 0.1%-20%, for example, it can be 0.1%, 1%, 2.5%, 5%, 10%, 15%, 20%, etc. or within the range composed of any two of the above values.

[0069] The conductive agent includes conductive carbon black, and may further include at least one of carbon nanotubes, conductive graphite, graphene, and carbon fiber. Based on the mass of the negative electrode active layer, the content of the conductive agent is 0.1%-20%, for example, it can be 0.1%, 1%, 2.5%, 5%, 10%, 15%, 20%, etc., or within the range composed of any two of the above values.

[0070] Based on the mass of the negative electrode active layer, the content of the inorganic oxide solid electrolyte is 0.1%-1%, for example, it can be 0.1%, 0.3%, 0.5%, 0.8%, 1%, etc., or within the range composed of any two of the above values. The chemical structure of the inorganic oxide solid electrolyte is as described above and will not be elaborated here.

[0071] The negative electrode active material includes a graphite material and a silicon-carbon material. Based on the mass of the negative electrode active layer, the content of the negative electrode active material is 59%-99.7%, for example, it can be 59%, 69%, 79%, 89%, 94%, 97.9%, etc., or within the range composed of any two of the above values.

[0072] According to the second aspect of the present application, a lithium-ion secondary battery is provided, including a positive electrode sheet, a separator, and the negative electrode sheet described in the first aspect.

[0073] The lithium-ion secondary battery provided by the present application can still maintain good discharge performance in a low-temperature environment, and thus can meet the usage requirements of electronic products in cold regions.

[0074] It should be noted that during the first charging process of a lithium-ion secondary battery, a solid electrolyte interface (SEI) film will be formed on the surface of the negative electrode sheet, thereby consuming a large amount of lithium ions, resulting in a significant reduction in the capacity of lithium ions. The lithium ions deintercalated from the negative electrode during the first discharge are much less than the lithium ions deintercalated from the positive electrode during charging, leading to a reduction in the Coulomb efficiency of the battery. At the same time, during subsequent charge and discharge processes, due to the large side reactions of silicon particles in the negative electrode sheet, the SEI film on its surface will repeatedly break and repair, continuously consuming a large amount of lithium ions, directly affecting the cycle life and energy density of the battery.

[0075] To eliminate the loss of this part of irreversible capacity, in some embodiments of the present application, the positive electrode sheet includes a positive electrode current collector and a positive electrode active layer disposed on at least one surface in the thickness direction of the positive electrode current collector; the positive electrode active layer includes a lithium supplement material, the lithium supplement material includes a second characteristic element, and the second characteristic element includes at least one of Ni, Mo, Nb, Ti, Zr, Fe, Y, Mn, Cu, Mg, Zn; based on the mass of the positive electrode active layer, the content of the second characteristic element is denoted as N, and 1000 ppm ≤ N ≤ 10000 ppm is satisfied. Exemplarily, the content N of the second characteristic element can be, for example, 1000 ppm, 2000 ppm, 3000 ppm, 4000 ppm, 5000 ppm, 6000 ppm, 7000 ppm, 8000 ppm, 9000 ppm, 10000 ppm, etc. or within the range composed of any two of the above values.

[0076] By adding an appropriate amount of lithium supplement material to the positive electrode active layer, the lithium ions missing in the negative electrode can be compensated. These lithium ions have a high transmission rate under the conduction of the inorganic oxide solid electrolyte and will not affect the conduction of lithium ions in the negative electrode active layer even in a low-temperature environment. Therefore, the low-temperature discharge performance of the battery can be improved while taking into account the energy density and cycle life of the battery.

[0077] Exemplarily, the lithium supplement material includes Li5FeO4 and / or a substance with the chemical formula Li 2±δ Ni α M β O2, 0 ≤ δ ≤ 0.5, 0 ≤ α ≤ 1, 0 ≤ β ≤ 1, and M includes at least one of Mo, Nb, Ti, Zr, Fe, Y, Mn, Cu, Mg, Zn.

[0078] Furthermore, in some embodiments, by reasonably regulating the content N of the second characteristic element in the positive electrode active layer and the content B of the first characteristic element in the negative electrode active layer to satisfy 0.125 ≤ N / B ≤ 10, on the one hand, the content of active lithium in the positive electrode can be increased, and the reaction activity of the positive electrode material can be improved, so that the battery can still effectively perform charge and discharge reactions in a low-temperature environment, reducing the polarization phenomenon. On the other hand, the lithium ions missing in the negative electrode can be compensated. These lithium ions have a high transmission rate under the conduction of the inorganic oxide solid electrolyte and will not affect the conduction of lithium ions in the negative electrode active layer even in a low-temperature environment. Thus, the low-temperature discharge performance of the battery can be improved while taking into account the energy density and cycle life of the battery.

[0079] Exemplarily, the value of N / B can be 0.125, 0.25, 0.5, 1.0, 2.5, 3.5, 5, 7.5, 10, etc. or within the range composed of any two of the above values.

[0080] In some embodiments, the positive electrode active layer further includes at least one of lithium cobaltate and ternary material. The chemical formula of lithium cobaltate is Li x Co y E z O2, where 0.8 ≤ x ≤ 1.1, 0.85 ≤ y ≤ 1, 0 ≤ z ≤ 0.15, and E includes at least one of Al, Mg, Ti, Y, La, Ga, Ge, Sn, Si, Zr, Ca, Sb, In, Ni, and Mn. The chemical formula of the ternary material is Li 1+ a Ni b Co c L d O2, where -0.05 ≤ a ≤ 0.2, 0.6 ≤ b < 1, 0 ≤ c ≤ 0.2, 0 ≤ d ≤ 0.4, and L includes at least one of Mn, Al, Mg, Zr, Ti, W, and B.

[0081] It can be understood that both lithium cobaltate and ternary material are currently the mainstream commercial positive electrode active materials. The negative electrode sheet of the present application is used in combination with the positive electrode sheet containing the above active materials, and has a wide range of application scenarios, especially not limited by too low ambient temperature.

[0082] The following further describes the present application in detail with specific embodiments. These embodiments should not be construed as limiting the scope claimed by the present application. For those in the embodiments and comparative examples where specific experimental steps or conditions are not indicated, the operations or conditions of the conventional experimental steps described in the literature in the art can be followed. For the reagents or instruments whose manufacturers are not indicated, they are all conventional reagent products that can be obtained through commercial purchase. In all the embodiments and comparative examples of the present application, the unit % represents mass percentage.

[0083] Example 1

[0084] This example provides a method for preparing a lithium-ion secondary battery, including the following steps:

[0085] The first step: Preparation of the positive electrode sheet

[0086] Lithium cobaltate doped with a lithium supplement material, conductive carbon black (SP), and polyvinylidene fluoride (PVDF) are mixed in a mass ratio of 97:1:2, and N-methylpyrrolidone (NMP) is added and stirred evenly to prepare a positive electrode slurry, where the lithium supplement material (chemical formula Li2NiO2) accounts for 0.45% of the weight of lithium cobaltate;

[0087] The positive electrode slurry is coated on both the front and back surfaces of the aluminum foil, and after baking and rolling, a positive electrode sheet with a thickness of 100 μm is obtained.

[0088] The second step: Preparation of the negative electrode sheet

[0089] The negative electrode active material (i.e., silicon carbide and artificial graphite, and silicon carbide accounts for 10% of the total weight of the negative electrode active material), solid electrolyte Li 1.3 Al 0.3 Ti 1.7 (PO4)3, conductive agent (the mass ratio of conductive carbon black to single-walled carbon nanotubes is 1:1), and fibrillatable resin PTFE are dry-mixed in a mass ratio of 97:0.5:1.25:1.25, and then formed into filaments by supersonic gas injection in a jet mill. Subsequently, the filaments are interlaced to form an elastic network structure. The negative electrode active material, electrolyte, and conductive agent particles are placed in the network structure to obtain a fibrillated elastic layer. The elastic layer is calendered and roll-pressed using a multi-stage calender, and then thermally compounded with a copper foil current collector coated with glue to produce a negative electrode sheet with a thickness of 110 μm. A slot with a fixed size is provided at a certain position of the negative electrode sheet, and a copper-plated nickel tab is laser-welded into this slot.

[0090] Step 3: The positive and negative electrode sheets are slit, sliced, and wound with a separator to obtain a core, and then the lithium-ion secondary battery is obtained through encapsulation, baking, liquid injection, formation, second sealing, sorting, and OCV.

[0091] Among them, the electrolyte is a commercially available conventional electrolyte, and the lithium salt is LiFP6; the separator includes a polyethylene-based film (thickness of 4 μm), a polymethyl methacrylate adhesive layer (thickness of 0.5 μm) on one surface of the base film, a ceramic layer (thickness of 2 μm) on the other surface of the base film, and a polyvinylidene fluoride and polymethyl methacrylate adhesive layer (thickness of 2.5 μm) on the surface of the ceramic layer facing away from the base film.

[0092] The preparation methods of Examples 2-21 and Comparative Examples 1-5 are basically the same as those of Example 1. The differences are shown in Tables 1 and 2. Among them, D1 and D2 are the particle sizes of the solid electrolyte, D3 is the particle size of the conductive carbon black, D4 is the median particle size of the graphite single particles, D5 is the median particle size of the graphite secondary particles, D6 is the median particle size of the silicon carbide material, D7 is the tube diameter of the single-walled carbon nanotubes, B is the content of the first characteristic element in the negative electrode active layer (i.e., the sum of the contents of Ti and Al), A is the silicon content in the negative electrode active layer, and N is the content of the second characteristic element Ni in the positive electrode active layer. " / " indicates that this item does not exist.

[0093] Table 1

[0094]

[0095] Table 2

[0096]

[0097]

[0098] Example 22

[0099] The difference from Example 1 is that the mass ratio of the negative electrode active material, solid electrolyte, conductive agent and fibrillatable resin is 96.65:0.85:1.25:1.25. Among them, the chemical formula of the solid electrolyte is Li 6.5 La3Zr 1.5 Ta 0.5 O 12 , the content B of the first characteristic element (i.e., the sum of the contents of La, Zr, and Ta) in the negative electrode active layer is 0.7%, then B / A = 0.156; the content N of the second characteristic element Ni in the positive electrode active layer is 3000 ppm, then N / B = 0.429.

[0100] Example 23

[0101] The difference from Example 1 is that the mass ratio of the negative electrode active material, solid electrolyte, conductive agent and fibrillatable resin is 96.73:0.77:1.25:1.25. Among them, the chemical formula of the solid electrolyte is Li 0.3 La 0.57 TiO3, the content B of the first characteristic element (i.e., the sum of the contents of La and Ti) in the negative electrode active layer is 0.6%, then B / A = 0.133; the diameter of the single-walled carbon nanotubes in the conductive agent is 1 nm.

[0102] The content N of the second characteristic element Ni in the positive electrode active layer is 7000 ppm, then N / B = 1.167.

[0103] Example 24

[0104] The difference from Example 1 is only that: the mass ratio of the negative electrode active material, solid electrolyte, conductive agent and fibrillatable resin is 97.42:0.08:1.25:1.25. Among them, the content B of the first characteristic element in the negative electrode active layer is 0.25%, then B / A = 0.056; the lithium supplement material is Li5FeO4, and the content N of the second characteristic element Fe in the positive electrode active layer is 5500 ppm, then N / B = 2.2.

[0105] Example 25

[0106] The difference from Example 1 is only that: only conductive carbon black is used as the conductive agent in the negative electrode active layer, and single-walled carbon nanotubes are not included.

[0107] Test Example

[0108] 1. Element content test

[0109] After discharging the lithium-ion secondary battery to 0% SOC, disassemble it to remove the electrode sheets. Immerse the electrode sheets in dimethyl carbonate (DMC) solvent for 12 h, then rinse them with DMC to remove the lithium salts attached to the electrode sheets. After preparing the electrode sheets by argon ion polishing, perform elemental analysis using an energy dispersive spectrometer (EDS) coupled with a field emission scanning electron microscope (FESEM) at a magnification of 5K to obtain the content of the elements to be measured.

[0110] 2. Median Particle Size Test

[0111] Test the particle size distribution range of the material using a laser particle size analyzer or SEM.

[0112] 3. -20°C Low-Temperature Discharge Performance Test

[0113] Charge the lithium-ion secondary battery at a constant current and constant voltage of 0.2C to the upper limit voltage at room temperature, cut off at 0.02C, and after standing for 5 min, discharge at 0.2C to 3.0V, and record the discharge capacity C1. Then charge the battery at a constant current and constant voltage of 0.2C to the upper limit voltage at room temperature again, cut off at 0.02C to full charge. Put the fully charged battery into a -20°C constant temperature chamber or incubator, and after standing for 2 h, discharge at 0.2C to 3.0V, and record the discharge capacity C2. C2 / C1 is the 0.2C low-temperature discharge capacity retention rate at -20°C.

[0114] 4. Volume Energy Density (ED) Test

[0115] The volume energy density ED = E / V, where E is the discharge energy of the battery. The test method is as follows: Charge the lithium-ion secondary battery at a current of 0.2C to the upper limit voltage, then charge at a constant voltage until the current drops to 0.02C, and then discharge at a current of 0.2C to 3.0V. The discharged energy is E. V is the volume of the lithium-ion secondary battery. By measuring the thickness, width, and length of the battery and calculating the product of the three, the volume of the battery is obtained, and E / V is the volume energy density obtained by the test.

[0116] 5. 85°C High-Temperature Storage for 6 h Swelling Rate Test

[0117] Charge the battery cell at a constant current and constant voltage of 0.2C to the upper limit voltage at room temperature, cut off at 0.02C. Use a battery thickness gauge to set a pressure of 600 g to measure the first thickness h1. Then place the battery cell in a condition of 85°C ± 2°C for 6 h, take out the battery cell immediately, and measure the thickness h2. h2 / h1 is the thickness swelling change rate.

[0118] The above test results are shown in Table 3.

[0119] Table 3

[0120]

[0121]

[0122] As can be seen from Table 1 - Table 3, compared with Comparative Example 1 (without inorganic oxide solid electrolyte), the battery energy density of Examples 1 - 25 remains unchanged, and the low - temperature discharge performance of the batteries has been improved to varying degrees. This shows that by adding an appropriate amount of inorganic oxide solid electrolyte with a specific particle size to the negative electrode active layer, the low - temperature discharge performance of the battery can be improved without sacrificing the battery energy density.

[0123] It is worth mentioning that compared with Example 1, in Example 12, due to the relatively low content of silicon element in its negative electrode active layer (A = 1%), the battery energy density is significantly lower and the battery thickness expansion is smaller. At the same time, due to the relatively high content of the first characteristic element in its negative electrode active layer (B = 0.8%), that is, the value of B / A is larger, it can effectively improve the low - temperature discharge performance of the battery, making the battery of Example 12 exhibit a lower energy density, thickness expansion rate and a higher low - temperature discharge capacity. On the contrary, in Example 13, the content of silicon element in the negative electrode active layer is relatively high while the content of the first characteristic element is relatively low, making the battery of Example 13 exhibit a relatively high energy density, thickness expansion rate and a lower low - temperature discharge capacity.

[0124] Compared with Example 1, in Comparative Example 2, the content of the first characteristic element in the negative electrode active layer is too low, so the improvement effect on the low - temperature discharge performance of the battery is not obvious. On the contrary, in Comparative Example 3, the content of the first characteristic element in the negative electrode active layer is too high, which can significantly improve the low - temperature discharge performance of the battery, but at the same time, it will also cause the battery to easily generate gas and have a large expansion rate during high - temperature storage. In Comparative Example 4, the particle size D1 of the solid electrolyte is too large, and in Comparative Example 5, the particle size D1 of the solid electrolyte is too small, both of which are not conducive to the improvement of the low - temperature discharge performance of the battery.

[0125] It should be noted that the lithium - ion secondary battery system formed by replacing lithium cobaltate in the above - mentioned examples and comparative examples with ternary materials also shows the above experimental rules, further verifying that adding an appropriate amount of inorganic oxide solid electrolyte with a specific particle size to the negative electrode active layer can improve the low - temperature discharge performance of lithium - ion secondary batteries without sacrificing the battery energy density.

[0126] Obviously, the above - mentioned examples are only for clear illustration and not a limitation of the implementation methods. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all implementation methods here. And the obvious changes or modifications derived therefrom are still within the protection scope of this application.

Claims

1. A negative electrode sheet, comprising a negative electrode current collector and a negative electrode active layer provided on at least one surface in the thickness direction of the negative electrode current collector; characterized in that, The negative electrode active layer includes a negative electrode active material, and the negative electrode active material includes a graphite material and a silicon-carbon material. The content A of silicon element, based on the mass of the negative electrode active layer, satisfies 1% ≤ A ≤ 10%; The negative electrode active layer further includes an inorganic oxide solid electrolyte. The particle size D1 of the inorganic oxide solid electrolyte is 0.5 μm - 3 μm, and D1 is the maximum diameter of the inorganic oxide solid electrolyte observed at a magnification of 5K in SEM; The inorganic oxide solid electrolyte includes a first characteristic element, and the first characteristic element includes at least two of Al, Ti, La, Zr, and Ta. The content of the first characteristic element, based on the mass of the negative electrode active layer, is denoted as B and satisfies 0.1% ≤ B ≤ 0.8%.

2. The negative electrode sheet according to claim 1, wherein, The particle size D1 of the inorganic oxide solid electrolyte is 1 μm - 2 μm; Preferably, A and B satisfy: 0.05 ≤ B / A ≤ 0.

5.

3. The negative electrode sheet according to claim 1 or 2, characterized in that, The inorganic oxide solid electrolyte includes at least one of lithium aluminum titanium phosphate, lithium lanthanum zirconium tantalum oxide, and lithium lanthanum titanium oxide; And / or, based on the mass of the negative electrode active layer, the content of the inorganic oxide solid electrolyte is 0.05% - 1%.

4. The negative electrode sheet according to claim 1, wherein The graphite material includes artificial graphite, and the artificial graphite includes graphite single particles and graphite secondary particles. The median particle size D4 of the graphite single particles is 5 μm - 12 μm, and the median particle size D5 of the graphite secondary particles is 10 μm - 20 μm; And / or, the median particle size D6 of the silicon-carbon material is 3 μm - 15 μm; And / or, the content of silicon element in the silicon-carbon material is 35% - 70%; And / or, the silicon-carbon material includes a porous carbon substrate and silicon particles dispersed inside the pores of the porous carbon substrate.

5. The negative electrode sheet according to claim 1, 2 or 4, characterized in that The negative electrode active layer further includes a conductive agent, and the conductive agent includes conductive carbon black. The particle size D3 of the conductive carbon black is 0.02 μm - 2 μm, and D3 is the average diameter of 10 conductive carbon black particles observed at a magnification of 5K in SEM; The particle size D2 of the inorganic oxide solid electrolyte is 0.8 μm - 3 μm, and D2 is the average diameter of 10 inorganic oxide solid electrolyte particles observed at a magnification of 5K in SEM; and D3 < D2; Preferably, D2 is 1 μm - 2 μm; Preferably, D3 is 0.1 μm - 1 μm; Preferably, the conductive agent further includes single-walled carbon nanotubes, and the tube diameter of the single-walled carbon nanotubes is 1 nm - 50 nm.

6. The negative electrode sheet according to claim 1, 2 or 4, characterized in that, The negative electrode active layer further includes a binder, and the binder includes a fibrous binder, and the fibrous binder includes a fibrillatable resin; Preferably, based on the mass of the negative electrode active layer, the content of the fibrillatable resin is 0.1% - 20%, preferably 1% - 5%; And / or, the fibrillatable resin includes at least one of PTFE and its derivatives.

7. A lithium ion secondary battery, characterized in that, It includes a positive electrode sheet, a separator, and the negative electrode sheet according to any one of claims 1 - 6.

8. The lithium ion secondary battery according to claim 7, wherein, The positive electrode sheet includes a positive electrode current collector and a positive electrode active layer provided on at least one surface in the thickness direction of the positive electrode current collector. The positive electrode active layer includes a positive electrode active material and a lithium supplement material. The lithium supplement material includes a second characteristic element, and the second characteristic element includes at least one of Ni, Mo, Nb, Ti, Zr, Fe, Y, Mn, Cu, Mg, and Zn. Based on the mass of the positive electrode active layer, the content of the second characteristic element is denoted as N, satisfying 1000 ppm ≤ N ≤ 10000 ppm; Preferably, the lithium supplement material includes Li5FeO4 and / or a substance with the chemical formula Li 2±δ Ni α M β O2, where 0 ≤ δ ≤ 0.5, 0 ≤ α ≤ 1, 0 ≤ β ≤ 1, and M includes at least one of Mo, Nb, Ti, Zr, Fe, Y, Mn, Cu, Mg, Zn.

9. The lithium ion secondary battery according to claim 8, wherein, N and B satisfy: 0.125 ≤ N / B ≤ 10, where B is the content of the first characteristic element in the negative electrode active layer, and 0.1% ≤ B ≤ 0.8%; Preferably, 0.25 ≤ N / B ≤ 3.

5.

10. The lithium ion secondary battery according to claim 8 or 9, characterized in that, The positive electrode active layer further includes at least one of lithium cobaltate and ternary material; The chemical formula of the lithium cobaltate is Li x Co y E z O2, where 0.8 ≤ x ≤ 1.1, 0.85 ≤ y ≤ 1, 0 ≤ z ≤ 0.15, and E includes at least one of Al, Mg, Ti, Y, La, Ga, Ge, Sn, Si, Zr, Ca, Sb, In, Ni, and Mn; The chemical formula of the ternary material is Li 1+a Ni b Co c L d O2, where -0.05 ≤ a ≤ 0.2, 0.6 ≤ b < 1, 0 ≤ c ≤ 0.2, 0 ≤ d ≤ 0.4, and L includes at least one of Mn, Al, Mg, Zr, Ti, W, and B.

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