Secondary battery and electric device

Abstract

The invention provides a secondary battery and a power utilization device, and belongs to the technical field of secondary batteries, the secondary battery comprises a first electrode assembly and a second electrode assembly which are connected in parallel, the first electrode assembly comprises a first negative electrode material, the first negative electrode material comprises graphite, the second electrode assembly comprises a second negative electrode material, and the second negative electrode material comprises graphite. And the second negative electrode material comprises graphite and a silicon-based material. According to the secondary battery disclosed by the invention, by utilizing the characteristic that the first electrode assembly and the second electrode assembly are connected in parallel and share voltage and shunt, the charging and discharging current of the second electrode assembly under the graphite intercalation and de-intercalation lithium potential can be reduced, and the probability that a silicon-based material in the second electrode assembly participates in the charging and discharging process under the graphite intercalation and de-intercalation lithium potential due to the driving of overlarge current is reduced; and the inertia of the silicon-based material under the working condition of shallow DOD is increased, so that the battery meets the requirement of high endurance under the working condition of deep discharge and the requirement of long service life under the daily working condition.

Description

Technical Field

[0001] The present invention relates to the technical field of secondary batteries, and in particular to a secondary battery and an electrical device. Background Art

[0002] With the widespread use of electronic products and the popularity of electric vehicles, people are placing higher demands on batteries. Energy density is a key indicator of battery life. To address range anxiety, silicon-based materials are currently commonly introduced into the battery negative electrode, leveraging their higher gram capacity to further increase the energy density of lithium-ion batteries. However, during the alloying process with lithium ions, silicon-based materials undergo significant volume expansion, which can easily cause particles in the active material layer of the negative electrode sheet to break and fall off, leading to rapid degradation of the battery's cycle capacity.

[0003] Therefore, it is necessary to design a secondary battery and an electrical device to improve the above problems. Summary of the Invention

[0004] In view of the above shortcomings of the prior art, the present invention provides a secondary battery and an electrical device to improve the problem of cycle capacity attenuation during use of the existing secondary battery due to the introduction of silicon-based materials into the negative electrode.

[0005] To achieve the above-mentioned objectives and other related objectives, the present invention provides a secondary battery, which includes a first electrode assembly and a second electrode assembly connected in parallel, wherein the first electrode assembly includes a first negative electrode material, which includes graphite, and the second electrode assembly includes a second negative electrode material, which includes graphite and a silicon-based material.

[0006] In an example of the present invention, the capacity of the second electrode assembly accounts for 30% to 60% of the total capacity of the secondary battery, and the mass content of the silicon-based material in the second negative electrode material is 5% to 30%.

[0007] In an example of the present invention, when the secondary battery is at the graphite deintercalation lithium potential, the charge and discharge current of the second electrode assembly is smaller than the charge and discharge current of the first electrode assembly.

[0008] In an example of the present invention, the secondary battery includes N second electrode assemblies, where N≥2.

[0009] In an example of the present invention, the sum of the capacities of the N second electrode assemblies accounts for A% of the total capacity of the secondary battery, and the capacity of any one of the N second electrode assemblies accounts for A / 2N% to 2A / N% of the total capacity of the secondary battery; the mass of the silicon-based material accounts for B% of the total mass of the second negative electrode material of the secondary battery; in each of the second electrode assemblies, the mass content of the silicon-based material in the second negative electrode material is 0.1% to 2B%.

[0010] In one example of the present invention, the mass of the second negative electrode material in each of the second electrode assemblies is used as a benchmark for the mass content of the silicon-based material in the corresponding second electrode assembly, and the mass content of the silicon-based material in the N second electrode assemblies is gradient distributed, and the gradient value of the gradient distribution is greater than or equal to 1.5.

[0011] In an example of the present invention, the first electrode assembly and the second electrode assembly are stacked or arranged in parallel; and / or, a thickness ratio of the second electrode assembly to the first electrode assembly is 1 / 3 to 1.

[0012] In one example of the present invention, the first electrode assembly includes a first negative electrode sheet, the second electrode assembly includes a second negative electrode sheet, the second negative electrode sheet includes a second negative electrode collector, a first active material layer and a second active material layer, the first active material layer and the second active material layer are arranged in sequence on the second negative electrode collector along the thickness direction, one of the first active material layer and the second active material layer contains the graphite, and the other of the first active material layer and the second active material layer contains the silicon-based material.

[0013] In an example of the present invention, the first electrode assembly includes a first negative electrode plate, the second electrode assembly includes a second negative electrode plate, the second negative electrode plate includes a negative electrode current collector, a first active material layer and a second active material layer, the first active material layer and the second active material layer are sequentially arranged on the second negative electrode current collector along the thickness direction, the first active material layer and the second active material layer both include the graphite and the silicon-based material, and the mass content of the silicon-based material in the first active material layer is higher than the mass content of the silicon-based material in the second active material layer.

[0014] In an example of the present invention, the silicon-based material includes at least one of silicon, silicon monoxide, and a silicon-carbon composite material.

[0015] The present invention further provides an electrical device, comprising the secondary battery described in any one of the above examples.

[0016] The present invention provides a secondary battery comprising a first electrode assembly and a second electrode assembly arranged in parallel. The negative electrode of the first electrode assembly comprises only a graphite material, while the negative electrode of the second electrode assembly comprises both a graphite material and a silicon-based material. By utilizing the common voltage and current shunting characteristics of the first and second electrode assemblies connected in parallel, the secondary battery can reduce the charge and discharge current of the second electrode assembly at a potential where lithium is deintercalated from the graphite. This reduces the probability of the silicon-based material in the second electrode assembly being involved in the charge and discharge process due to excessive current at this potential, and increases the inertness of the silicon-based material under shallow DOD conditions. This allows the battery to meet both high endurance requirements under deep discharge conditions and long life requirements under everyday operating conditions. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other embodiments can be obtained based on these drawings without paying any creative work.

[0018] Figure 1 1 is a front view of the structure of a secondary battery in one embodiment of the present invention.

[0019] Figure 2 This is a left side view of the structure of a secondary battery in one embodiment of the present invention.

[0020] Figure 3 Schematic diagram of the structure of the second electrode assembly before winding or lamination in one embodiment of the present invention.

[0021] Figure 4 Schematic diagram of the structure of the second negative electrode plate in one embodiment of the present invention.

[0022] Component number description:

[0023] 1. Secondary battery; 100. Housing; 110. Positive electrode terminal; 120. Negative electrode terminal; 200. First electrode assembly; 300. Second electrode assembly; 310. Second positive electrode sheet; 320. Second separator; 330. Second negative electrode sheet; 331. Second negative electrode current collector; 332. Second negative electrode active material layer; 3321. First active material layer; 3322. Second active material layer. DETAILED DESCRIPTION

[0024] The following describes the embodiments of the present invention through specific examples. Those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. The present invention can also be implemented or applied through other different specific embodiments, and the details in this specification can also be modified or changed based on different viewpoints and applications without departing from the spirit of the present invention. It should be noted that the following examples and the features in the examples can be combined with each other unless they conflict. It should also be understood that the terms used in the examples of the present invention are for the purpose of describing specific embodiments, not for the purpose of limiting the scope of protection of the present invention. The test methods for which specific conditions are not specified in the following examples are generally carried out under conventional conditions or under the conditions recommended by the manufacturers.

[0025] It should be noted that the terms such as "upper", "lower", "left", "right", "middle" and "one" cited in this specification are only for the convenience of description and are not used to limit the scope of implementation of the present invention. Changes or adjustments to their relative relationships should be regarded as the scope of implementation of the present invention without substantially changing the technical content.

[0026] The negative electrode material is one of the key materials that determines the performance of secondary batteries, and directly affects the energy density, cycle life, rate performance and safety performance of the battery. Currently, secondary batteries usually introduce silicon-based materials into the negative electrode containing graphite to further improve its charge and discharge capacity. Due to the lithium insertion and deintercalation potential of silicon-based materials (~0.4V vs Li / Li + ) is higher than the lithium insertion and deintercalation potential of graphite (0.1-0.2 V vs Li / Li + ), based on the above-mentioned negative electrode material characteristics, the secondary battery can mainly use silicon-based materials to deintercalate lithium in a low SOC state (State of Charge), and mainly use graphite materials to deintercalate lithium in a high SOC state.

[0027] Correspondingly, users' demand for high battery capacity is mainly reflected in deep discharge scenarios with a wide depth of discharge (DOD), such as long-distance operation of new energy vehicles. However, the vast majority of users' usage scenarios are not wide DOD conditions of deep charge and discharge (for example, corresponding to 0% to 100% SOC of the battery), but shallow DOD conditions of short-range charge and discharge (for example, corresponding to 20-30% to 100% SOC of the battery), such as medium- and short-range daily operation of new energy vehicles. Therefore, in theory, batteries with a mixed negative electrode of graphite and silicon-based materials can call on high-capacity silicon-based materials to participate in discharge under wide DOD conditions to meet the high capacity requirements during deep discharge, and avoid calling on silicon-based materials with serious expansion problems to participate in charge and discharge under daily shallow DOD conditions, thereby slowing down the deterioration of the battery's cycle capacity due to frequent daily use, and thus maintaining the battery's long life requirements under shallow DOD conditions.

[0028] However, the inventors have discovered that in existing batteries that incorporate a mixture of graphite and silicon-based materials into the negative electrode of a single-electrode assembly, the rapid lithium deintercalation process of the graphite material within the negative electrode at high current rates can easily generate a solid-phase concentration gradient within a local region within the negative electrode active material layer. This causes the silicon-based material in these local regions to participate in the charge and discharge process even at high SOC conditions (e.g., above 30% SOC). Because the secondary batteries provided by existing solutions cannot prevent a large amount of silicon-based material from participating in the battery charge and discharge process under daily shallow DOD conditions, and the silicon-based material participating in shallow DOD charging and discharging can easily cause the negative electrode active layer to fracture and peel due to its own expansion problems, the battery can experience rapid capacity decay even when used only under shallow DOD conditions, making it impossible for the battery to achieve its theoretically high capacity performance in actual applications.

[0029] Based on this, the present invention provides a secondary battery comprising a first electrode assembly and a second electrode assembly connected in parallel, wherein only a graphite material is introduced into the negative electrode of the first electrode assembly, and a graphite material and a silicon-based material are simultaneously introduced into the negative electrode of the second electrode assembly. This secondary battery utilizes the common voltage and current shunting characteristics of the first and second electrode assemblies connected in parallel to reduce the charge and discharge current of the second electrode assembly under shallow DOD conditions, lowering the probability of the silicon-based material in the second electrode assembly participating in the charge and discharge process at the graphite deintercalation potential, and increasing the inertness of the silicon-based material under shallow DOD conditions. This allows the battery to meet both the high endurance requirements under deep discharge conditions (i.e., wide DOD conditions) and the long life requirements under everyday conditions (i.e., shallow DOD conditions).

[0030] In one aspect, the present invention provides a secondary battery. The secondary battery in the embodiments of the present invention is also called a rechargeable battery or storage battery, which refers to a battery that can be recharged to activate the active material after discharge and continue to be used.

[0031] Typically, a secondary battery includes an electrode assembly, which includes a positive electrode sheet, a negative electrode sheet, an electrolyte, a separator, and corresponding connecting accessories and circuits. The positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector. The positive electrode active material layer includes a positive electrode material, a conductive agent, and a binder. The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode active material layer includes a negative electrode material, a conductive agent, a thickener, and a binder. In a secondary battery, the positive and negative electrode materials can intercalate and deintercalate lithium ions to achieve energy storage and release. The electrolyte is the carrier for lithium ions to be transmitted between the positive and negative electrode sheets. The separator is permeable to lithium ions but is non-conductive, thereby separating the positive and negative electrode sheets to prevent short circuits. It should be noted that the type of positive electrode material used in the positive electrode plate can be any positive electrode material known in the art, for example, it can be one or more of lithium cobalt oxide (LCO), lithium nickel oxide (LNO), lithium manganese oxide (LMO), lithium nickel manganese oxide (LNMO), lithium nickel cobalt manganese oxide (NCM), lithium nickel cobalt aluminum oxide (NCA), lithium-rich manganese-based oxide (LRMO) and lithium-containing phosphate; lithium-containing phosphate includes but is not limited to lithium iron manganese phosphate, lithium iron phosphate, lithium manganese phosphate, etc.

[0032] like Figures 1 to 3 As shown, the secondary battery 1 provided by the present invention includes a first electrode assembly 200 and a second electrode assembly 300 arranged in parallel. The first electrode assembly 200 includes a first positive electrode sheet, a first negative electrode sheet, and a first separator positioned between the first positive and first negative electrode sheets. The first negative electrode sheet includes a first negative electrode material, which includes graphite. The second electrode assembly 300 includes a second positive electrode sheet 310, a second negative electrode sheet 330, and a second separator 320 positioned between the second positive and second negative electrode sheets 310 and 330. The second negative electrode sheet 330 includes a second negative electrode material, which includes graphite and a silicon-based material. It should be noted that in the present invention, the type of silicon-based material in the second electrode assembly 300 is not limited. In one embodiment, the silicon-based material can be selected from at least one of silicon, silicon monoxide, and a silicon-carbon composite material. Alternatively, the silicon-based material can be a silicon-carbon composite material or silicon monoxide.

[0033] The secondary battery utilizes the characteristics of the first electrode assembly and the second electrode assembly being connected in parallel and shunting the common voltage to enable the graphite materials in the first electrode assembly and the second electrode assembly to participate in the battery's charge and discharge process simultaneously when driven by the same potential (i.e., shallow DOD conditions). The shunting characteristics can reduce the charge and discharge current of the second electrode assembly at the graphite deintercalation potential while retaining the high rate performance of the battery, slow down the rate of graphite deintercalation of lithium in the second electrode assembly, inhibit the solid phase concentration gradient formed in the negative electrode of the second electrode assembly, and reduce the probability of the silicon-based material in the second electrode assembly participating in the charge and discharge process at the graphite deintercalation potential, thereby maintaining the inertness of the silicon-based material at the graphite deintercalation potential, avoiding more silicon-based materials from participating in the charge and discharge process under shallow DOD conditions, thereby extending the cycle life of the battery under shallow DOD conditions, so that the battery can meet both the high endurance requirements under deep discharge conditions (i.e., wide DOD conditions) and the long life requirements under daily conditions (i.e., shallow DOD conditions).

[0034] At the same time, the secondary battery uses a first electrode assembly and a second electrode assembly to isolate part of the graphite material in the battery from the silicon-based material, thereby avoiding the expansion of the silicon-based material in the second electrode assembly from negatively affecting the negative electrode of the first electrode assembly, slowing down the attenuation process of the overall capacity of the battery, and further improving the cycle life of the battery.

[0035] In some embodiments, the capacity of the second electrode assembly involved in charge and discharge accounts for 30% to 60% of the total capacity of the secondary battery. For example, the capacity of the second electrode assembly in the secondary battery can account for 30%, 35%, 40%, 45%, 50%, 55% or 60%. In the second electrode assembly, the mass content of the silicon-based material in the second negative electrode material is any value between 5% and 30%, for example, it can be 5%, 7%, 10%, 13%, 15%, 17%, 20%, 23%, 25%, 27% or 30%. When the capacity proportion of the second electrode assembly in the secondary battery and the mass proportion of the silicon-based material in the second negative electrode material are within the above ranges, it can ensure that the capacity of the graphite material involved in charge and discharge occupies most of the power range of the shallow DOD operating condition (such as 30% to 100% SOC), thereby reducing the content of the silicon-based material involved in the charge and discharge of the shallow DOD operating condition while increasing the total capacity of the battery by using the silicon-based material.

[0036] In some embodiments, when the secondary battery is at the lithium deintercalation potential of the graphite material, the charge and discharge current of the second electrode assembly is less than the charge and discharge current of the first electrode assembly. This allows the graphite material in the first electrode assembly to rapidly deintercalate lithium during the charge and discharge process of the secondary battery under shallow DOD conditions (e.g., 20-30% to 100% SOC of the battery), thereby primarily providing the majority of the current during the battery charge and discharge process. At the same time, the reduced conductivity of the second electrode assembly caused by the mixing of the silicon-based material is utilized to allow the second electrode assembly to participate in charge and discharge with a smaller current, thereby slowing down the lithium deintercalation rate of the graphite material in the second electrode assembly and avoiding the formation of a solid phase concentration gradient in the negative electrode of the second electrode assembly that matches the lithium deintercalation potential of the silicon-based material in a local area. This minimizes the amount of silicon-based material involved in the charge and discharge process under shallow DOD conditions, effectively extending the battery's service life under normal operating conditions. For example, when the capacity ratio of the second electrode assembly in the secondary battery and the mass ratio of the silicon-based material in the second negative electrode material are within the above ranges, the proportion of the capacity of the silicon-based material involved in the shallow DOD condition relative to the total capacity of the silicon-based material is less than 10%.

[0037] Furthermore, it should be noted that the number of first electrode assemblies and second electrode assemblies in a secondary battery is not limited. For example, the number of first electrode assemblies and second electrode assemblies can be one each; or the number of second electrode assemblies can be one and the number of first electrode assemblies can be multiple; or the number of first electrode assemblies can be one and the number of second electrode assemblies can be multiple. Alternatively, in one example, the number of first electrode assemblies in a secondary battery is one and the number of second electrode assemblies can be multiple, such as two or three.

[0038] In some embodiments, the secondary battery includes N second electrode assemblies, and N ≥ 2; by dispersing the silicon-based material into the N second electrode assemblies, the secondary battery can further reduce the content of the silicon-based material in the second electrode assembly participating in shallow DOD charging and discharging conditions, inhibit the volume expansion of the silicon-based material in each second electrode assembly, and thus extend the service life of the battery under daily operating conditions.

[0039] In some embodiments, the sum of the capacities of N second electrode assemblies involved in charging and discharging accounts for A% of the total capacity of the secondary battery, such as the capacity percentage A% of the N second electrode assemblies can be any value between 30% and 60%; the capacity of any one of the N second electrode assemblies accounts for A / 2N% to 2A / N% of the total capacity of the secondary battery; for example, the secondary battery includes two second electrode assemblies, and the total capacity of the two second electrode assemblies in the secondary battery accounts for 50%, wherein the capacity of one second electrode assembly in the secondary battery accounts for 30%, and the capacity of the other second electrode assembly in the secondary battery accounts for 20%.

[0040] The mass percentage of the silicon-based material in the total mass of the second negative electrode material of the secondary battery is B%. For example, the mass percentage of the silicon-based material in the second negative electrode material of the secondary battery, B%, can be any value between 5% and 30%. The total mass of the second negative electrode material in the secondary battery is the sum of the masses of the second negative electrode materials in N second electrode assemblies. At the same time, in each second electrode assembly, the mass content of the silicon-based material in the second negative electrode material is any value between 0.1% and 2B%. For example, the secondary battery includes two second electrode assemblies, and the mass of the silicon-based material accounts for 10% of the total mass of the second negative electrode material in the secondary battery, wherein in one second electrode assembly, the mass content of the silicon-based material in the second negative electrode material is 15%, and in the other second electrode assembly, the mass content of the silicon-based material in the second negative electrode material is 5%.

[0041] In some embodiments, the mass of the second negative electrode material in each second electrode assembly is used as a comparison benchmark for the mass content of the silicon-based material in the corresponding second electrode assembly, that is, the mass content of the silicon-based material in the second electrode assembly described in this embodiment corresponds to the mass ratio of the silicon-based material in the second electrode assembly relative to the mass ratio of the second negative electrode material in the second electrode assembly. In a secondary battery, the mass content of the silicon-based material in the N second electrode assemblies is distributed in a gradient, and the gradient value of the gradient distribution is greater than or equal to 1.5. For example, the mass content of the silicon-based material in the N second electrode assemblies is B1%, B2%, ..., B N %, and meet B N %>B N-1 %>…>B1%;The mass content gradient of the silicon-based material in the N second electrode assemblies is greater than or equal to 1.5, that is, B N % / B N-1 %≥1.5. Among them, the secondary battery distributes the silicon-based material gradiently in N second electrode assemblies, which can further divert the charge and discharge current of the battery under the graphite deintercalation lithium potential (i.e., shallow DOD conditions), and adjust the charge and discharge current of different second electrode assemblies under the graphite deintercalation lithium potential by introducing different silicon-based material contents. Specifically, the second electrode assembly equipped with less silicon-based material can participate in the charge and discharge with a larger current under the shallow DOD conditions, and the second electrode assembly equipped with more silicon-based material can participate in the charge and discharge with a smaller current under the shallow DOD conditions. The above-mentioned gradient distribution method of silicon-based materials can make the second electrode assembly with a greater risk of performance deterioration under shallow DOD conditions only loaded with less silicon-based material, while the second electrode assembly with a lower risk of performance deterioration is equipped with more silicon-based material, thereby further reducing the total content of silicon-based materials participating in the shallow DOD condition charge and discharge in the secondary battery.

[0042] like Figure 1 and Figure 2As shown, in some embodiments, the secondary battery 1 includes a shell 100, which has a accommodating cavity and a positive terminal 110 and a negative terminal 120. The first electrode assembly 200 and the second electrode assembly 300 are assembled into the accommodating cavity of the shell 100, and the first electrode assembly 200 and the second electrode assembly 300 are electrically connected in parallel in the accommodating cavity. For example, the positive electrode tabs of the first electrode assembly 200 and the second electrode assembly 300 are connected in parallel to the positive terminal 110 of the shell 100, and the negative electrode tabs of the first electrode assembly 200 and the second electrode assembly 300 are connected in parallel to the negative terminal 120 of the shell 100.

[0043] It should be noted that the assembly method of the first electrode assembly 200 and the second electrode assembly 300 in the secondary battery 1 may not be limited. For example, in some embodiments, the first electrode assembly 200 and the second electrode assembly 300 may be stacked in the height direction in the shell 100, or may be arranged in parallel in the length or width direction in the shell 100.

[0044] In addition, in some embodiments, in the secondary battery, the thickness ratio of the second electrode assembly to the first electrode assembly can be any value between 1 / 3 and 1, for example, 1 / 3, 2 / 5, 1 / 2, 3 / 5, 2 / 3, 3 / 4, 4 / 5 or 1.

[0045] like Figure 3 and Figure 4As shown, the second negative electrode sheet 330 in the second electrode assembly 300 includes a second negative electrode current collector 331 and a second negative electrode active material layer 332. The second negative electrode active material layer 332 is disposed on at least one side of the second negative electrode current collector 331. In some embodiments, the graphite material and the silicon-based material are uniformly distributed in the second negative electrode active material layer 332. In other embodiments, the second negative electrode active material layer 332 includes a first active material layer 3321 and a second active material layer 3322. The first active material layer 3321 and the second active material layer 3322 are sequentially disposed on the second negative electrode current collector 331 along the thickness direction. One of the first active material layer 3321 and the second active material layer 3322 includes only a graphite material, and the other of the first active material layer 3321 and the second active material layer 3322 includes only a silicon-based material. For example, the first active material layer 3321 includes a silicon-based material, and the second active material layer 3322 includes a graphite material. In some further embodiments, the second negative electrode active material layer 332 includes a first active material layer 3321 and a second active material layer 3322, and the first active material layer 3321 and the second active material layer 3322 are sequentially arranged on the second negative electrode current collector 331 along the thickness direction. The first active material layer 3321 and the second active material layer 3322 both include graphite material and silicon-based material, and the mass content of the silicon-based material in the first active material layer 3321 is higher than the mass content of the silicon-based material in the second active material layer 3322.

[0046] The composition and preparation method of the secondary battery provided by the present invention can be carried out using conventional methods in the art. For example, the specific preparation process of the secondary battery is as follows:

[0047] Preparation of the first and second positive electrode sheets: The positive electrode material, positive electrode conductive agent, and positive electrode binder are mixed in a weight ratio of (90 to 99): (1 to 10): (1 to 10), optionally in a weight ratio of 97:2:1, and N-methylpyrrolidone (NMP) is added as a solvent and thoroughly stirred to obtain a positive electrode slurry. The positive electrode slurry is evenly coated on a positive electrode current collector aluminum foil, which is then dried at room temperature and then transferred to an oven for drying. The first and second positive electrode sheets are then cold pressed and cut. Among them, the positive electrode conductive agent can be selected from: at least one of conductive materials such as carbon black (Super P), acetylene black, carbon nanotubes (CNT), graphene, nanofibers (VGCF), for example, the positive electrode conductive agent is SP and CNT, and the mass ratio of SP to CNT is 2:1; the positive electrode binder can be selected from: at least one of polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE), for example, the binder can be PVDF.

[0048] Preparation of the first negative electrode sheet: Using graphite material as the first negative electrode material, the first negative electrode material, negative electrode conductive agent, and negative electrode binder are mixed in a mass ratio of (90 to 99): (1 to 10): (1 to 10), and the weight ratio can optionally be 96:3:1, deionized water is added, and the mixture is fully stirred to obtain a negative electrode slurry; the negative electrode slurry is evenly coated on both sides of the first negative electrode current collector copper foil; after drying at room temperature, it is transferred to an oven for drying, and then cold pressed, cut, and other processes are performed to obtain the first negative electrode sheet.

[0049] Preparation of the second negative electrode sheet: using graphite material and silicon-based material as the second negative electrode material, the mass ratio of graphite material and silicon-based material in the second negative electrode material is (70-95):(5-30); mixing the second negative electrode material, negative electrode conductive agent, and negative electrode binder in a mass ratio of (90 to 99):(1 to 10):(1 to 10), optionally the weight ratio can be 96:3:1, adding deionized water, and stirring and mixing thoroughly to obtain a negative electrode slurry; uniformly coating the negative electrode slurry on both sides of the second negative electrode current collector copper foil; after drying at room temperature, transferring to an oven for drying, and then undergoing cold pressing, slitting and other processes to obtain the second negative electrode sheet.

[0050] Among them, the negative electrode conductive agent can be selected from at least one conductive material such as carbon black (Super P), acetylene black, carbon nanotubes (CNT), graphene, nanofibers (VGCF); the negative electrode binder is selected from at least one binding material such as polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), styrene-butadiene rubber (SBR); the negative electrode thickener is selected from carboxymethyl cellulose, which can be sodium carboxymethyl cellulose (CMC-Na) or lithium carboxymethyl cellulose (CMC-Li).

[0051] Preparation of the first diaphragm and the second diaphragm: the diaphragm is made of PE or PP porous membrane, and PP / PE / PP porous membrane can be selected, with a thickness of 9μm to 18μm, for example, 9μm, 12μm, 16μm or 18μm; the air permeability is 180s / 100mL to 380s / 100mL, for example, 180s / 100mL, 280s / 100mL or 380s / 100mL; the porosity is 30% to 50%, for example, 30%, 40% or 50%.

[0052] Electrolyte: The electrolyte in this application can be selected from conventional types of electrolytes in the field. For example, ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are mixed in a volume ratio of 20:20:60 to obtain an organic solvent, and then the fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0053] Assembling a secondary battery: stacking the prepared first positive electrode sheet, first separator, and first negative electrode sheet in sequence, and pressing them tightly by hot pressing to obtain a first electrode assembly; stacking the prepared second positive electrode sheet, second separator, and second negative electrode sheet in sequence, and pressing them tightly by hot pressing to obtain a second electrode assembly; packaging, standing, and forming the first electrode assembly and the second electrode assembly to finally prepare a battery (i.e., a lithium-ion secondary battery).

[0054] The specific steps and conditions for electrolyte formation are as follows: after the electrolyte is injected, maintain a hot press environment of 0.1MPa, charge at 0.02C for 17 minutes at 45°C in a static state, stand for 5 minutes and then charge to 0.3Ah at 0.02C, then cut off the air bag and vacuum seal, and stand at room temperature for 48 hours to complete the electrolyte formation.

[0055] On the other hand, the present invention provides an electric device, which includes the secondary battery described in any of the above embodiments, and the secondary battery can be used as a power source for the electric device, and can also be used as an energy storage unit for the electric device. The electric device can be a vehicle, a mobile phone, a portable device, a laptop computer, a ship, a spacecraft, an electric toy, an electric tool, etc. The vehicle can be a fuel vehicle, a gas vehicle or a new energy vehicle, and the new energy vehicle can be a pure electric vehicle, a hybrid vehicle or an extended-range vehicle, etc.; the spacecraft includes an airplane, a rocket, a space shuttle and a spacecraft, etc.; the electric toy includes a fixed or mobile electric toy, such as a game console, an electric car toy, an electric ship toy and an electric airplane toy, etc.; the electric tool includes a metal cutting electric tool, a grinding electric tool, an assembly electric tool and a railway electric tool, such as an electric drill, an electric grinder, an electric wrench, an electric screwdriver, an electric hammer, an impact drill, a concrete vibrator and an electric planer, etc. The embodiments of the present application do not impose any special restrictions on the above-mentioned electric devices.

[0056] The technical solutions of the present invention are described in detail below through several specific examples and comparative examples. Unless otherwise specified, the raw materials and reagents used in the following examples are commercially available or can be prepared by conventional methods in the art.

[0057] Example 1

[0058] This embodiment provides a secondary battery, which includes a first electrode assembly and a second electrode assembly, wherein the capacity of the first electrode assembly in the secondary battery accounts for 45%, the capacity of the second electrode assembly in the secondary battery accounts for 55%, and the CB value (Cell Balance) of the first electrode assembly and the second electrode assembly remains the same. The first negative electrode material in the first electrode assembly includes a graphite material, and the second negative electrode material in the second electrode assembly includes a graphite material and a silicon-based material. The mass ratio of the graphite material to the silicon-based material in the second negative electrode material is 85:15. In addition, in the secondary battery, the capacity of the silicon-based material involved in charging and discharging accounts for 24% of the total capacity of the secondary battery, and the capacity of the graphite material involved in charging and discharging accounts for 76% of the total capacity of the secondary battery.

[0059] The preparation process of the secondary battery is as follows:

[0060] S1, preparation of the first positive electrode sheet and the second positive electrode sheet: the positive electrode material NCM811 (LiNi 0.8 Co 0.1 Mn 0.1 O2), a positive electrode conductive agent SP, and a positive electrode binder PVDF are mixed in a mass ratio of 97:2:1, a solvent N-methylpyrrolidone (NMP) is added, and the mixture is thoroughly stirred to obtain a positive electrode slurry; the mixture is stirred under the action of a vacuum stirrer until the system becomes uniform and transparent to obtain a positive electrode slurry; the positive electrode slurry is evenly coated on a 13 μm thick positive electrode current collector aluminum foil, transferred to an oven for drying, and then cold pressed and cut to obtain a first positive electrode sheet and a second positive electrode sheet.

[0061] S2. Preparation of the first negative electrode sheet: Using graphite material as the first negative electrode material, the first negative electrode material, negative electrode conductive agent acetylene black and negative electrode binder styrene butadiene rubber (SBR) are mixed in a mass ratio of 96:3:1, deionized water is added, and the mixture is fully stirred to obtain a negative electrode slurry; the negative electrode slurry is evenly coated on both sides of a 6μm thick copper foil of the first negative electrode current collector, transferred to an oven for drying, and then cold pressed and cut to obtain the first negative electrode sheet.

[0062] S3. Preparation of the second negative electrode sheet: Graphite material and silicon-based material are used as the second negative electrode material, the mass ratio of graphite material and silicon-based material in the second negative electrode material is 85:15, the graphite material is artificial graphite, and the silicon-based material uses a carbon-silicon composite material; the second negative electrode material, the negative electrode conductive agent acetylene black and the negative electrode binder styrene-butadiene rubber (SBR) are mixed in a mass ratio of 96:3:1, deionized water is added, and the mixture is fully stirred to obtain a negative electrode slurry; the negative electrode slurry is evenly coated on both sides of the 6μm thick copper foil of the first negative electrode current collector, transferred to an oven for drying, and then cold pressed and cut to obtain the second negative electrode sheet.

[0063] S4. Preparation of the first and second diaphragms: A polyethylene (PE) diaphragm with a thickness of 10 μm was selected.

[0064] S5. Preparation of electrolyte: Ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are mixed in a volume ratio of 20:20:60 to obtain an organic solvent, and then fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0065] S6. Assembling a secondary battery: stacking the prepared first positive electrode sheet, first separator, and first negative electrode sheet in sequence, with the first separator positioned between the first positive electrode sheet and the first negative electrode sheet to serve as an isolation, and pressing them tightly through hot pressing to obtain a first electrode assembly; stacking the prepared second positive electrode sheet, second separator, and second negative electrode sheet in sequence, with the second separator positioned between the second positive electrode sheet and the second negative electrode sheet to serve as an isolation, and pressing them tightly through hot pressing to obtain a second electrode assembly; encapsulating the first electrode assembly and the second electrode assembly into an aluminum-plastic film, injecting an electrolyte and then sealing, and finally preparing a lithium-ion secondary battery after standing, hot and cold pressing, formation, clamping, capacity division and other processes.

[0066] Example 2

[0067] This embodiment provides a secondary battery with the same system as that of Example 1. The difference between this embodiment and Example 1 is that the capacity proportion of the first electrode assembly in the secondary battery is adjusted to 70%, the capacity proportion of the second electrode assembly in the secondary battery is adjusted to 30%, and the mass proportion of the silicon-based material in the second electrode assembly relative to the second negative electrode material is adjusted to 5%.

[0068] Example 3

[0069] This embodiment provides a secondary battery having the same system as that of Example 2. The difference between this embodiment and Example 2 is that the mass ratio of the silicon-based material to the second negative electrode material in the second electrode assembly is adjusted to 30%.

[0070] Example 4

[0071] This embodiment provides a secondary battery with the same system as that of Example 1. The difference between this embodiment and Example 1 is that the capacity proportion of the first electrode assembly in the secondary battery is adjusted to 40%, the capacity proportion of the second electrode assembly in the secondary battery is adjusted to 60%, and the mass proportion of the silicon-based material in the second electrode assembly relative to the second negative electrode material is adjusted to 5%.

[0072] Example 5

[0073] This embodiment provides a secondary battery having the same system as that of embodiment 4. The difference between this embodiment and embodiment 4 is that the mass ratio of the silicon-based material to the second negative electrode material in the second electrode assembly is adjusted to 30%.

[0074] Example 6

[0075] This embodiment provides a secondary battery having the same system as that of embodiment 1. The difference between this embodiment and embodiment 1 is that silicon 2 oxide is used as the silicon-based material in the second negative electrode material.

[0076] Comparative Example 1

[0077] This comparative example provides a secondary battery having a single electrode assembly. The negative electrode material contained in the negative electrode plate of the secondary battery includes a silicon-based material and a graphite material. The silicon-based material is the same carbon-silicon composite material as in Example 1. The capacity of the silicon-based material involved in charge and discharge accounts for 24% of the total capacity of the secondary battery. The preparation process of the secondary battery is as follows:

[0078] S1. Preparation of positive electrode sheet: Prepare the positive electrode material NCM811 (LiNi 0.8 Co 0.1 Mn 0.1 O2), a positive electrode conductive agent SP, and a positive electrode binder PVDF are mixed in a mass ratio of 97:2:1, a solvent N-methylpyrrolidone (NMP) is added, and the mixture is thoroughly stirred to obtain a positive electrode slurry; the mixture is stirred under the action of a vacuum stirrer until the system becomes uniform and transparent to obtain a positive electrode slurry; the positive electrode slurry is evenly coated on a 13 μm thick positive electrode current collector aluminum foil, transferred to an oven for drying, and then cold pressed and cut to obtain a positive electrode sheet.

[0079] S2. Preparation of negative electrode sheets: Graphite material and silicon-based material are used as negative electrode materials, the mass ratio of graphite material and silicon-based material in the negative electrode material is 93:7, the graphite material is artificial graphite, and the silicon-based material uses a carbon-silicon composite material; the negative electrode material, the negative electrode conductive agent acetylene black and the negative electrode binder styrene-butadiene rubber (SBR) are mixed in a mass ratio of 96:3:1, deionized water is added, and the mixture is fully stirred to obtain a negative electrode slurry; the negative electrode slurry is evenly coated on both sides of the 6μm thick copper foil of the first negative electrode current collector, transferred to an oven for drying, and then cold pressed and cut to obtain a negative electrode sheet.

[0080] S3. Diaphragm preparation: A 10 μm thick polyethylene (PE) diaphragm was selected.

[0081] S4. Preparation of electrolyte: Ethylene carbonate (EC), ethyl methyl carbonate (EMC) and diethyl carbonate (DEC) are mixed in a volume ratio of 20:20:60 to obtain an organic solvent, and then fully dried lithium salt LiPF6 is dissolved in the mixed organic solvent to prepare an electrolyte with a concentration of 1 mol / L.

[0082] S5. Assemble secondary batteries: stack the prepared positive electrode sheets, separators, and negative electrode sheets in sequence, and press them tightly through hot pressing to obtain an electrode assembly; encapsulate the electrode assembly and the second electrode assembly into an aluminum-plastic film, inject the electrolyte and seal it, and finally prepare a lithium-ion secondary battery after standing, hot and cold pressing, formation, clamping, capacity division and other processes.

[0083] Comparative Example 2

[0084] This comparative example provides a secondary battery having the same system as that of comparative example 1. The difference between this comparative example and comparative example 1 is that silicon oxide is used as the silicon-based material in the negative electrode material.

[0085] The secondary batteries prepared in Examples 1 to 6 and Comparative Examples 1 to 2 were subjected to cycling performance tests under shallow DOD conditions to verify the effectiveness of the present invention in improving the service life of secondary batteries under shallow DOD conditions. The test results are shown in Table 1, and the testing method is as follows:

[0086] Battery cycle life test under shallow DOD conditions: At room temperature of 25°C, in the power test range of 20% to 100% SOC, the battery is charged and discharged at a current rate of 1C / 1C until the capacity is less than or equal to 80% of the initial capacity, and the number of cycles is recorded.

[0087] Table 1: Performance test results of batteries prepared in Examples 1 to 6 and Comparative Examples 1 to 2

[0088]

[0089] By comparing the test results of Examples 1 to 6 and Comparative Examples 1 to 2, it can be seen that compared to the secondary battery in which the silicon-based material and the graphite material are evenly placed at the negative electrode of a single electrode assembly in the comparative example, the embodiment of the present invention isolates the silicon-based material and part of the graphite material and places them in the first electrode assembly and the second electrode assembly, and utilizes the characteristics of the first electrode assembly and the second electrode assembly being connected in parallel and common-pressure shunting, thereby reducing the probability of the silicon-based material in the second electrode assembly participating in the charging and discharging process under shallow DOD conditions, increasing the inertness of the silicon-based material under shallow DOD conditions, and thereby improving the cycle life of the secondary battery under shallow DOD conditions, and meeting the high endurance requirements under wide DOD conditions.

[0090] The above embodiments are merely illustrative of the principles and effects of the present invention and are not intended to limit the present invention. Anyone skilled in the art may modify or alter the above embodiments without departing from the spirit and scope of the present invention. Therefore, all equivalent modifications or alterations made by one of ordinary skill in the art without departing from the spirit and technical principles disclosed herein are intended to be covered by the claims of the present invention.

Claims

1. A secondary battery, characterized in that: The invention comprises a first electrode assembly and a second electrode assembly connected in parallel, wherein the first electrode assembly comprises a first negative electrode material comprising graphite, and the second electrode assembly comprises a second negative electrode material comprising graphite and a silicon-based material.

2. The secondary battery according to claim 1, wherein The capacity of the second electrode assembly accounts for 30% to 60% of the total capacity of the secondary battery, and the mass content of the silicon-based material in the second negative electrode material is 5% to 30%.

3. The secondary battery according to claim 1 or 2, characterized in that The secondary battery includes N second electrode assemblies, where N≥2.

4. The secondary battery according to claim 3, wherein The sum of the capacities of the N second electrode assemblies accounts for A% of the total capacity of the secondary battery, and the capacity of any one of the N second electrode assemblies accounts for A / 2N% to 2A / N% of the total capacity of the secondary battery; the mass of the silicon-based material accounts for B% of the total mass of the second negative electrode material of the secondary battery; in each of the second electrode assemblies, the mass content of the silicon-based material in the second negative electrode material is 0.1% to 2B%.

5. The secondary battery according to claim 3, wherein The mass of the second negative electrode material in each second electrode assembly is used as the benchmark for the mass content of the silicon-based material in the corresponding second electrode assembly. The mass content of the silicon-based material in the N second electrode assemblies is distributed in a gradient, and the gradient value of the gradient distribution is greater than or equal to 1.

5.

6. The secondary battery according to claim 1, wherein The first electrode assembly and the second electrode assembly are stacked or arranged in parallel; and / or a thickness ratio of the second electrode assembly to the first electrode assembly is 1 / 3 to 1.

7. The secondary battery according to claim 1, wherein The first electrode assembly includes a first negative electrode sheet, the second electrode assembly includes a second negative electrode sheet, the second negative electrode sheet includes a second negative electrode collector, a first active material layer and a second active material layer, the first active material layer and the second active material layer are arranged in sequence on the second negative electrode collector along the thickness direction, one of the first active material layer and the second active material layer contains the graphite, and the other of the first active material layer and the second active material layer contains the silicon-based material.

8. The secondary battery according to claim 1, wherein The first electrode assembly includes a first negative electrode plate, the second electrode assembly includes a second negative electrode plate, the second negative electrode plate includes a negative electrode current collector, a first active material layer and a second active material layer, the first active material layer and the second active material layer are sequentially arranged on the second negative electrode current collector along the thickness direction, the first active material layer and the second active material layer both include the graphite and the silicon-based material, and the mass content of the silicon-based material in the first active material layer is higher than the mass content of the silicon-based material in the second active material layer.

9. The secondary battery according to claim 1, wherein The silicon-based material includes at least one of silicon, silicon monoxide, and a silicon-carbon composite material.

10. An electrical device, characterized in that: The electric device includes the secondary battery according to any one of claims 1 to 9.

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