Lithium-ion battery, battery pack, and electric device

Abstract

The present invention relates to a lithium-ion battery, a battery pack, and an electric device. The lithium-ion battery comprises: a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein the separator is arranged between the positive electrode sheet and the negative electrode sheet to form an electrode assembly, and the electrolyte at least infiltrates part of the electrode assembly; the electrolyte comprises a lithium salt and a solvent, the solvent comprising a linear solvent and a cyclic solvent; the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer; and the negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer. The lithium-ion battery satisfies: 0.46≤(10×D / (CWA+CWC))×Wb / Wa≤3.8, wherein CWC is 0.13 mg / mm2 to 0.26 mg / mm2; CWA is 0.065 mg / mm2 to 0.13 mg / mm2; D is the lithium-ion transport number of the electrolyte; Wb is the mass fraction of the lithium salt in the electrolyte, with the unit thereof being %; and Wa is the mass fraction of the linear solvent in the electrolyte, with the unit thereof being %. Moreover, the lithium-ion battery has improved energy density, energy efficiency and dynamic performance.

Description

Lithium-ion battery, battery pack, and power-consuming device

[0001] This application claims priority to the Chinese patent application filed with the China Patent Office on September 21, 2023, with application number 2023112233393 and application name “A lithium-ion battery, battery pack and electrical device”, the entire contents of which are incorporated by reference into this application. Technical Field

[0002] The present application relates to the field of battery technology, and in particular to a lithium-ion battery, a battery pack, and an electrical device thereof. Background Art

[0003] In lithium-ion batteries, in order to improve the battery's energy density and capacity, the coating weight of the active material layer on the positive and negative electrode sheets per unit area can be increased. However, the increase in the coating weight of the active material layer on the positive and negative electrode sheets per unit area will hinder the transmission of lithium ions between the positive and negative electrode sheets, thereby worsening the energy efficiency of the battery cell. Although increasing the lithium salt concentration in the electrolyte can increase the battery's energy efficiency, the increase in lithium salt concentration is accompanied by an increase in the viscosity of the electrolyte. High-viscosity electrolytes will worsen the battery's energy efficiency and kinetic performance. As a result, the battery's energy density, energy efficiency, and kinetic performance are mutually exclusive, making it difficult to improve them all at the same time.

[0004] Summary of the Invention

[0005] The embodiments of the present application disclose a lithium-ion battery, a battery pack, and an electrical device thereof, which solve the problem in the prior art that the energy density, energy efficiency, and dynamic performance of batteries are mutually restricted and difficult to improve simultaneously.

[0006] In a first aspect, an embodiment of the present application provides a lithium-ion battery, comprising: a positive electrode sheet, a negative electrode sheet, an electrolyte, and a separator, wherein the separator is disposed between the positive electrode sheet and the negative electrode sheet to form an electrode assembly, and the electrolyte at least partially soaks the electrode assembly;

[0007] The electrolyte comprises a lithium salt and a solvent, the solvent comprises a linear solvent and a ring-shaped solvent, the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer, and the negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer;

[0008] The lithium-ion battery meets the following requirements:

[0009] 0.46≤(10×D / (CWA+CWC))×Wb / Wa≤3.8;

[0010] Wherein, the CWC is the coating weight per unit area of ​​the positive electrode active material layer on the positive electrode sheet, in mg / mm 2 ;

[0011] The CWA is the coating weight per unit area of ​​the negative electrode active material layer on the negative electrode sheet, expressed in mg / mm 2 ;

[0012] D is the lithium ion transference number of the electrolyte;

[0013] Wb is the mass fraction of the lithium salt in the electrolyte, in %;

[0014] Wa is the mass fraction of the linear solvent in the electrolyte, in %;

[0015] Wherein, the CWC is 0.13 mg / mm 2 ~0.26mg / mm 2 ;

[0016] The CWA is 0.065 mg / mm 2 ~0.13mg / mm 2 .

[0017] In a second aspect, an embodiment of the present application provides a battery pack, comprising a box and a lithium-ion battery as described in the first aspect placed in the box.

[0018] In a third aspect, an embodiment of the present application provides an electrical device, which includes an electrical device body and a lithium-ion battery as described in the first aspect disposed in the electrical device body.

[0019] Compared with the prior art, the present invention has the following advantages:

[0020] An embodiment of the present application provides a lithium-ion battery. By exploring the relationship between the coating weight CWC of the positive electrode active material layer per unit area on the positive electrode sheet, the coating weight CWA of the negative electrode active material layer per unit area on the negative electrode sheet, the lithium ion migration number D in the electrolyte, the mass fraction Wb of the lithium salt in the electrolyte, and the mass fraction Wa of the linear solvent in the electrolyte, it is found that when the above parameters satisfy the relationship 0.46≤(10×D / (CWA+CWC))×Wb / Wa≤3.8, not only can the advantage of the high coating weight of the positive and negative electrode sheets per unit area be utilized, but also the disadvantage of reducing the battery's kinetic performance and energy efficiency can be avoided. This is because when the contents of CWA and CWC are high, the energy density of the battery increases, but the transmission of lithium ions is hindered and the kinetic performance is poor. At this time, the lithium ion concentration and the lithium ion migration number are increased by increasing Wb. However, when Wb is too high, the lithium ion migration number will decrease due to the increase in electrolyte viscosity. Therefore, it is necessary to adjust the Wa content to adjust the viscosity of the electrolyte, thereby increasing the migration ability of lithium ions and improving the energy efficiency and kinetic performance of the battery.

[0021] That is to say, when the above relationship is satisfied, the high coating weight of the positive and negative electrode sheets per unit area can be used to improve the battery energy density and capacity, while at the same time, by adjusting the mass fraction of lithium salt in the electrolyte, wherein the mass fraction of lithium salt is related to the lithium ion migration number and the content of linear solvent, that is, the synergistic combination of these three can improve the lithium ion transmission capacity, improve the energy efficiency and kinetic performance of the battery, thereby achieving the purpose of simultaneously improving the battery energy density, energy efficiency and kinetic performance. DETAILED DESCRIPTION

[0022] The technical solution provided in this application will be further described below with reference to embodiments.

[0023] With the rapid development of new energy technologies, the demand for the capacity, lifespan and energy density of lithium-ion batteries is getting higher and higher. In order to increase the energy density of the battery, the coating weight of the active material layer per unit area on the corresponding electrode can be increased. For example, the coating weight of the active material layer per unit area of ​​the positive electrode is 0.18 mg / mm 2 ~0.20mg / mm 2 The coating weight of the active material layer per unit area of ​​the negative electrode is 0.08 mg / mm 2 ~0.10mg / mm 2 .

[0024] However, the increase in the coating weight of the active material layer per unit area means an increase in the thickness of the active material layer of the positive electrode sheet and the negative electrode sheet, which affects the porosity of the active material layer, and further affects the diffusion degree of the electrolyte in the positive and negative electrode sheets, deteriorating the electrochemical performance of the battery; the increase in thickness increases the impedance of the positive and negative electrode sheets, increases the migration impedance of lithium ions, and affects the kinetic performance of the battery.

[0025] In order to reduce the migration resistance of lithium ions by increasing the coating weight of the active material layer per unit area of ​​the positive and negative electrode sheets, the lithium salt concentration in the electrolyte is adjusted to increase the migration number of lithium ions, thereby improving the migration ability of lithium ions between the positive and negative electrodes and improving the battery's kinetic performance. However, as the lithium salt concentration increases, the viscosity of the electrolyte also increases. High-viscosity electrolytes increase the resistance to lithium ion shuttle movement, deteriorating the battery's kinetic performance, leading to increased battery polarization and poor battery cycle performance.

[0026] In order to solve the above problems, the embodiments of the present application optimize the electrical performance of lithium-ion batteries by exploring the coating weight of the active material layer in the positive and negative electrode sheets and its relationship with parameters such as the lithium salt concentration in the electrolyte and the electrolyte solvent, so that the battery has a high energy density through the coating amount while also having good energy efficiency and kinetic performance.

[0027] In a first aspect, an embodiment of the present application provides a lithium-ion battery:

[0028] The lithium-ion battery comprises: a positive electrode sheet, a negative electrode sheet, an electrolyte and a diaphragm, wherein the diaphragm is arranged between the positive electrode sheet and the negative electrode sheet to form an electrode assembly, and the electrolyte at least partially soaks the electrode assembly;

[0029] The electrolyte comprises a lithium salt and a solvent, the solvent comprises a linear solvent and a ring-shaped solvent, the positive electrode sheet comprises a positive electrode current collector and a positive electrode active material layer, and the negative electrode sheet comprises a negative electrode current collector and a negative electrode active material layer;

[0030] The lithium-ion battery meets the following requirements:

[0031] 0.46≤(10×D / (CWA+CWC))×Wb / Wa≤3.8;

[0032] Wherein, the CWC is the coating weight per unit area of ​​the positive electrode active material layer on the positive electrode sheet, in mg / mm 2 ;

[0033] The CWA is the coating weight per unit area of ​​the negative electrode active material layer on the negative electrode sheet, expressed in mg / mm 2 ;

[0034] D is the lithium ion transference number of the electrolyte;

[0035] Wb is the mass fraction of the lithium salt in the electrolyte, in %;

[0036] Wa is the mass fraction of the linear solvent in the electrolyte, in %;

[0037] Wherein, the CWC is 0.13 mg / mm 2 ~0.26mg / mm 2 ;

[0038] The CWA is 0.065 mg / mm 2 ~0.13mg / mm 2 .

[0039] The present application increases the capacity and energy density of the battery by increasing the coating weight of the positive and negative electrode sheets per unit area of ​​the positive electrode active material layer and the negative electrode active material layer, respectively. As for the impact of the increase in coating weight on the lithium ion migration number and battery kinetic performance, it is improved by exploring the specific relationship between the above coating weight and the lithium salt concentration and the mass fraction of the linear solvent in the electrolyte. Specifically, increasing the lithium salt concentration can increase the migration number of lithium ions, but if the lithium salt concentration is high, the viscosity of the electrolyte increases, reducing the lithium ion migration number in the battery, worsening the battery's energy efficiency and cycle problems. Therefore, in order to simultaneously improve the battery's energy density and kinetic performance, the present application improves the viscosity of the electrolyte, utilizing the characteristic of the linear solvent having a lower viscosity to improve the viscosity of the electrolyte, promote the transmission of lithium ions, increase the lithium ion migration number of the battery, and thereby improve the battery's kinetic performance, so that the battery's energy density and kinetics are simultaneously improved.

[0040] Therefore, based on the premise that the coating weight of the active material layer per unit area of ​​the positive and negative electrode sheets is relatively high, this application explores the specific relationship between the coating weight per unit area of ​​the positive electrode active material layer on the positive electrode sheet CWC, the coating weight per unit area of ​​the negative electrode active material layer on the negative electrode sheet CWA, the lithium ion migration number D, the mass fraction of the lithium salt in the electrolyte Wb, and the mass fraction of the linear solvent in the electrolyte Wa. The relationship of 0.46≤(10×D / (CWA+CWC))×Wb / Wa≤3.8 is obtained. The study found that when this relationship holds, the lithium-ion battery has higher kinetic performance and energy density.

[0041] The relationship proposed in this application shows that when CWC and CWA are within the above range, the battery has a higher energy density. At this time, if CWC, CWA, D, Wb, and Wa satisfy the above relationship, not only can the advantage of high coating weight of positive and negative electrode sheets per unit area be utilized, but also the kinetic performance and energy efficiency of the battery can be improved by changing the lithium salt concentration and electrolyte viscosity. When CWC and CWA are large, the porosity of the positive and negative electrode sheets is limited, the wettability of the electrolyte is poor, and the migration of lithium ions is greatly hindered, affecting the electrochemical performance of the battery. In this case, the lithium ion transference number can be improved by increasing the Wb content to increase the lithium salt concentration of the battery. However, the increase in Wb means an increase in the viscosity of the electrolyte, which hinders the transfer of lithium ions between the positive and negative electrode sheets. In this case, the viscosity of the electrolyte can be improved by increasing the Wa content, further improving the lithium ion transference number and improving the kinetic performance of the battery. In other words, when the above relationship holds, the lithium ion battery has a higher energy density and kinetic performance.

[0042] For example, 10×D / (CWA+CWC))×Wb / Wa is 0.46, 0.55, 0.60, 0.75, 0.85, 0.95, 1.5, 2.5, 3.5, 3.8, etc. CWC is 0.13 mg / mm 2 ~0.26mg / mm 2 Including any point value within this range, for example, CWC can be 0.13mg / mm 2 , 0.15mg / mm 2 , 0.18mg / mm 2 , 0.20mg / mm 2 , 0.22mg / mm 2 , 0.24mg / mm 2 , 0.26mg / mm 2 etc. CWA can be 0.065mg / mm 2 , 0.07mg / mm 2 , 0.08mg / mm 2 , 0.09mg / mm 2 , 0.10mg / mm 2 , 0.12mg / mm 2 , 0.13mg / mm 2 wait.

[0043] It can be understood that the coating weight per unit area of ​​the positive electrode active material layer on the positive electrode sheet refers to the coating weight per unit area of ​​the coating slurry of the positive electrode active material layer on the positive electrode sheet, and the coating weight per unit area of ​​the negative electrode active material layer on the negative electrode sheet refers to the coating weight per unit area of ​​the coating slurry of the negative electrode active material layer on the negative electrode sheet.

[0044] Furthermore, Wb / Wa satisfies: 0.15 <Wb / Wa≤0.25。

[0045] When Wb / Wa is within the above range, lithium ion migration is optimized, which in turn benefits the battery's dynamic performance. This not only avoids the situation where, when Wb / Wa is high, the electrolyte lithium salt concentration is high and the content of linear solvent is low, resulting in a large number of lithium ions, but also hinders lithium ion migration and battery dynamic performance due to increased electrolyte viscosity; it also avoids the situation where, when Wb / Wa is too low, the electrolyte lithium salt concentration is low, the number of lithium ions that can be transferred is limited, and the electrochemical performance of the battery is affected. For example, Wb / Wa is 0.155, 0.16, 0.17, 0.18, 0.19, 0.20, 0.22, 0.23, 0.24, 0.25, etc.

[0046] Furthermore, the Wb is 8% to 20%.

[0047] When Wb is within this range, the number of lithium ions available increases, the lithium ion transference number increases, and the battery exhibits higher kinetic performance. This not only avoids the low lithium ion transference number and poor battery kinetic performance that occurs when the lithium salt concentration is low, but also prevents the increased viscosity of the electrolyte when the lithium salt concentration is too high, which increases the transport barrier of lithium ions and affects the battery's kinetic performance. Exemplary values ​​for Wb are 8%, 9%, 12%, 13%, 14%, 16%, 18%, 19%, 20%, and so on.

[0048] Furthermore, the lithium salt includes at least one of lithium hexafluorophosphate, lithium difluorosulfonyl imide, lithium difluorooxalatoborate, and lithium bisoxalatoborate.

[0049] Lithium salts contribute lithium ions to the electrolyte. These types of lithium salts exhibit good solubility and conductivity in non-aqueous solvents. They can synergize with carbonate solvents to form a stable passivation film on the surface of the negative electrode, resulting in high stability and, in turn, improved electrochemical performance of lithium-ion batteries. Examples of lithium salts include lithium hexafluorophosphate, lithium bis(oxalatoborate), lithium difluorooxalatoborate, lithium hexafluorophosphate and lithium difluorosulfonyl imide, lithium hexafluorophosphate, lithium difluorosulfonyl imide, and lithium difluorooxalatoborate.

[0050] Furthermore, Wa is 20% to 80%.

[0051] Linear solvents have the characteristics of poor conductivity, low viscosity, and low melting point. When Wa is within the above range, the viscosity of the electrolyte is good, the lithium ion transmission capacity is high, and the battery's dynamic performance is good. This can avoid the poor conductivity of the electrolyte and the poor stability of the interfacial passivation film of the negative electrode when Wa is too high; it can also avoid the high viscosity of the electrolyte and the obstruction of lithium ion transmission when Wa is too low, which affects the battery's dynamic performance. Exemplarily, Wa is 20%, 25%, 30%, 35%, 40%, 50%, 60%, 70%, 75%, 80%, etc.

[0052] Furthermore, the linear solvent includes at least one of diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate, ethyl acetate, ethyl propionate, propyl propionate, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and ethylene glycol dimethyl ether; and / or,

[0053] The cyclic solvent includes at least one of ethylene carbonate and propylene carbonate.

[0054] Organic solvents in electrolytes dissolve lithium salts, promote lithium ion transport, participate in the formation of the interfacial passivation film on the negative electrode, and exhibit high stability. Among these, linear solvents have low melting points, low viscosity, and a low dielectric constant, which can alter the viscosity of the electrolyte. Annular solvents have high dielectric constants and high ionic conductivity, participating in the formation of the interfacial passivation film on the negative electrode. By coordinating the use of linear and cyclic solvents, the electrolyte achieves higher conductivity, enhanced lithium ion transport capabilities, and participation in the formation of a stable interfacial passivation film on the negative electrode, thereby improving the electrochemical performance of the battery.

[0055] Furthermore, the D is 0.2 to 0.6.

[0056] The lithium ion migration number is an important indicator for measuring the effective migration of lithium ions in lithium-ion batteries. When the lithium ion migration number is within this range, on the one hand, lithium ions can be effectively deintercalated and intercalated in the positive and negative electrodes, reducing the contact between lithium ions and the electrolyte, reducing the occurrence of interfacial side reactions, and improving the service life of the battery. On the other hand, lithium ions are important cations involved in transport migration. A high lithium ion migration number indicates a low anion migration number, which is beneficial to reducing concentration polarization during charge and discharge, reducing the resistance of interfacial transmission, and improving the electrochemical performance of the battery. When the lithium ion migration number is lower than this range, the transmission of lithium ions is limited, the migration of anions increases, the interfacial transmission resistance of the battery increases, and the effect of improving the electrochemical performance of the battery is limited. For example, D is 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, etc.

[0057] Furthermore, the D and the CWC satisfy: 3.84<10×D / CWC<46.15; and / or,

[0058] The D and the CWA satisfy: 7.69<10×D / CWA<92.3.

[0059] It should be noted that 10×D / CWC represents the numerical ratio of the lithium ion migration number to the coating weight per unit area of ​​the positive electrode active material layer on the positive electrode sheet; 10×D / CWA represents the numerical ratio of the lithium ion migration number to the coating weight per unit area of ​​the negative electrode active material layer on the negative electrode sheet.

[0060] When the above relationship holds, lithium-ion batteries have high energy density and energy efficiency. This not only avoids the situation when CWC and CWA are too high, resulting in thicker coating weights on the positive and negative electrodes, hindered lithium ion transport, a lower lithium ion migration number, and poor battery kinetic performance; it also avoids the situation when CWC and CWA are too small, resulting in better lithium ion transport but lower battery energy density and a more difficult coating process to control. For example, 10×D / CWC is 3.85, 5, 10, 15, 20, 25, 30, 35, 40, 45, 46.10, etc.; 10×D / CWA is 7.7, 8, 10, 15, 20, 30, 45, 55, 65, 75, 85, 90, 92.20, etc.

[0061] In a second aspect, an embodiment of the present application provides a battery pack, comprising a box and a lithium-ion battery as described in the first aspect placed in the box.

[0062] Multiple lithium-ion batteries are placed in series or in parallel in a battery pack box, and the battery pack has a protective effect on the lithium-ion batteries.

[0063] In a third aspect, an embodiment of the present application provides an electrical device, which includes an electrical device body and a lithium-ion battery as described in the first aspect disposed in the electrical device body.

[0064] The lithium-ion battery is used to provide power to an electrical device. The electrical device includes a positive electrode and a negative electrode. The negative electrode of the lithium-ion battery is connected to the negative electrode of the device, and the positive electrode of the lithium-ion battery is connected to the positive electrode of the device.

[0065] The technical solution of the present application will be further explained below in conjunction with more specific embodiments and experimental test results.

[0066] Example 1

[0067] A lithium-ion battery comprises: a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, wherein the separator is arranged between the positive electrode sheet and the negative electrode sheet to form an electrode assembly, and the electrolyte at least partially infiltrates the electrode assembly;

[0068] The electrolyte includes a lithium salt and a solvent, the solvent includes a linear solvent and a ring solvent, the positive electrode sheet includes a positive electrode current collector and a positive electrode active material layer, and the negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer;

[0069] The lithium-ion battery meets the following requirements:

[0070] CWC is 0.165 mg / mm 2 , CWA is 0.079 mg / mm 2 , D is 0.25, Wb is 11.25%, and Wa is 63.66%;

[0071] Wherein, CWC is the coating weight per unit area of ​​the positive electrode active material layer on the positive electrode sheet, in mg / mm 2 ;

[0072] CWA is the coating weight of the negative electrode active material layer per unit area on the negative electrode sheet, expressed in mg / mm 2 ;

[0073] D is the lithium ion migration number of the electrolyte;

[0074] Wb is the mass fraction of lithium hexafluorophosphate in the electrolyte, in %;

[0075] Wa is the mass fraction of ethyl methyl carbonate (EMC) and dimethyl carbonate in the electrolyte, in %.

[0076] This embodiment also provides a method for preparing the above-mentioned lithium-ion battery, which comprises the following steps:

[0077] Preparation of positive electrode sheet: The positive electrode active material lithium iron phosphate, conductive carbon black (SP), and adhesive polyvinylidene fluoride (PVDF) are dispersed in the solvent N-methylpyrrolidone (NMP) in a mass ratio of 97:2:1 and mixed evenly to obtain a positive electrode slurry. The positive electrode slurry is coated on the positive electrode current collector aluminum foil, where the CWC is 0.165 mg / mm 2 , i.e. 254 / 1540.25mg / mm 2 After drying, cold pressing, slitting and cutting, the positive electrode sheet is obtained.

[0078] Preparation of negative electrode sheet: Disperse the negative electrode active material artificial graphite, conductive carbon SP, thickener CMC and binder SBR in deionized water at a mass ratio of 96.5:0.5:1:2 and mix them evenly to obtain negative electrode slurry. Apply the negative electrode slurry on the negative electrode current collector copper foil. The CWA is 0.079 mg / mm 2 , i.e. 122 / 1540.25mg / mm 2 After drying, cold pressing, slitting and cutting, the negative electrode sheet is obtained.

[0079] Preparation of the electrolyte: In an argon atmosphere glove box with a moisture content of ≤1ppm, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in rows, with the mass fractions of the three in the electrolyte being 21.59%, 31.83%, and 31.83%, respectively. EMC and DMC were linear solvents, and the sum of their mass ratios, Wa, was 63.66%. Dry electrolyte lithium salt, lithium hexafluorophosphate, was then dissolved in the solvent, with its mass fraction, Wb, in the electrolyte being 11.25%. The lithium salt was stirred until completely dissolved, and vinylene carbonate and fluoroethylene carbonate were added and mixed thoroughly to obtain an electrolyte, wherein the mass fraction of vinylene carbonate in the electrolyte was 2%, and the mass fraction of fluoroethylene carbonate in the electrolyte was 1.5%.

[0080] Preparation of diaphragm: 16 micron polyethylene film was selected as the diaphragm.

[0081] Battery assembly: The prepared positive electrode sheet, separator, and negative electrode sheet are stacked in order, so that the separator is in the middle of the positive electrode sheet and the negative electrode sheet to separate the positive electrode sheet and the negative electrode sheet. After winding, the electrode assembly is formed. After welding the tabs, the electrode assembly is assembled into the outer packaging. After injecting the prepared electrolyte, the electrode assembly is packaged, allowed to stand, formed, shaped, and capacity tested, and finally a lithium-ion battery is prepared.

[0082] Determination of lithium ion migration number D: Symmetrical batteries assembled with different electrolytes are used to test the lithium ion migration number using the constant potential polarization method, which requires the use of both AC impedance and DC polarization methods.

[0083] 1) Li / Li symmetric battery assembly

[0084] In an argon atmosphere glove box with a moisture content of ≤1ppm, a button battery with a model number of CR2032 was assembled using metal lithium sheets with a diameter of 16 microns as the positive and negative electrodes, a polyethylene film with a diameter of 19 microns as the separator, and an electrolyte containing the above-mentioned linear solvent with a mass fraction of Wa and a lithium salt with a mass fraction of Wb.

[0085] 2) Constant potential polarization method to test the lithium ion migration number of button battery

[0086] The assembled button battery is first subjected to an AC impedance test (EIS) on an electrochemical workstation. The EIS test frequency range is 100,000-0.01HZ, and the measured resistance is R0. A DC polarization test is then performed, applying a small and constant potential difference ΔV (generally around 10mV) to the battery, applying a constant voltage for 2000S, and recording the change in current over time. In the initial state, all ions that can migrate in the battery have an impact on charge transfer, and at this time I0 (initial current) is the largest. As polarization proceeds, a stable ion concentration gradient is gradually formed inside the battery, anion migration is suppressed, and the current of the battery system is contributed by cations (i.e., lithium ions), and the current Iss (steady-state current) at this time is recorded. The battery is then subjected to an EIS test again, and the test frequency range is also 100,000-0.01HZ, and the measured resistance is Rss. At this time, the lithium ion migration number can be calculated using the following formula:

[0087] 3) Lithium-ion battery DC resistance (DCR) test

[0088] The prepared soft-pack battery (rated capacity 3.2Ah) was tested in a charge-discharge test cabinet. The battery cell was activated for two cycles in a 45°C oven. The charge and discharge process was as follows: 1C constant current and constant voltage charging to 3.65V, and 1C constant current and constant voltage discharge to 2.5V.

[0089] Place the battery cell in a 25℃ oven and let it stand. After the surface temperature of the battery cell reaches 25℃, charge and discharge the battery cell for 3 cycles to fix the capacity. The charge and discharge steps are 0.5C constant current and constant voltage charging to 3.65V, and 0.5C discharge to 2.5V. The initial capacity of the battery cell is C0.

[0090] Perform DCR test on the battery cell at different state of charge (SOC): first, charge the battery to 3.65V at a constant current and constant voltage of 0.5C, let it stand for 10 minutes, then discharge it at a current of 0.1C0 for 1 hour, record the voltage as V1 after standing for 1 hour, then discharge it at 1.07C0 for 30 seconds, record the voltage as V2, then the DCR resistance of the battery at 90% SOC is (V2-V1)× 1000 / C0; discharge at 0.1C0 for 3h, let it stand for 1h and record the voltage as V3, then discharge at 1.07C for 30s and record the voltage as V4, then the DCR resistance of the battery at 60% SOC state is (V4-V3)×1000 / C0; then discharge at 0.1C0 for 3h, let it stand for 1h and record the voltage as V5, then discharge at 1.07C0 for 30s and record the voltage as V6, then the DCR resistance of the battery at 30% SOC state is (V4-V3)×1000 / C0.

[0091] Compared with Example 1, Examples 2 to 11 differ in that the mass fractions of CWC, CWA, D, Wb, Wa, and the cyclic solvent are different, as shown in Table 1.

[0092] Comparative Examples 1 to 5 are different from Example 1 in that the mass fractions of CWC, CWA, D, Wb, Wa, and the cyclic solvent are different, as shown in Table 1.

[0093] The parameter change details of Examples 1 to 11 and Comparative Examples 1 to 5 are shown in Table 1.

[0094] Table 1 Summary of components and parameters of Examples and Comparative Examples

[0095] The lithium-ion battery obtained in the above example was subjected to charge and discharge cycle testing on a charge-discharge instrument at test temperatures of 25°C and 45°C. The battery was tested at a current of 0.2C (rated capacity 3.2Ah). Each test cycle consisted of charging at 0.5C constant current and constant voltage to 3.65V, a cutoff current of 0.02C, and then discharging at 0.2C constant current to 2.5V. The battery's DCR performance was also tested at different SOC states to further evaluate its dynamic performance.

[0096] The capacity retention rate is calculated as follows: capacity retention rate after the 800th cycle = (discharge capacity after the 800th cycle / discharge capacity in the first cycle) × 100%.

[0097] Energy efficiency is calculated as follows: battery energy efficiency after the 800th cycle = (discharge energy after the 800th cycle / charge energy after the 800th cycle) × 100%.

[0098] Table 2 Battery performance test data

[0099] Combining Tables 1 and 2, it can be seen that the capacity retention and energy efficiency of Comparative Examples 1 to 5 are lower than those of Examples 1 to 11. Under different SOC conditions, the DCR of Comparative Examples 1 to 5 is higher than that of Examples 1 to 11. The smaller the DCR value, the lower the impedance in the battery, which also reflects that the battery has better dynamic performance. The above data shows that since Comparative Examples 1 to 5 do not satisfy the relationship of 0.46≤(10×D / (CWA+CWC))×Wb / Wa≤3.8, it means that when this relationship holds, the lithium-ion battery can not only take advantage of the higher energy density brought by the high coating weight of the positive and negative electrode sheets per unit area, but also, when the lithium salt concentration is high, by adjusting the amount of linear solvent, the lithium ion transfer number of the battery is increased, thereby optimizing the electrochemical performance of the battery.

[0100] Analysis of Comparative Example 4 and Examples 3 to 11 shows that when CWA and CWC are similar, the values ​​of Wa and Wb in Comparative Example 4 are higher and the content of the cyclic solvent is lower. In this case, the capacity retention rate and energy efficiency of the battery are more severely degraded than those of Examples 3 to 11. This is because the cyclic solvent participates in the formation of the interfacial passivation film on the negative electrode plate, resulting in poor stability of the interfacial passivation film, which affects the electrochemical performance of the battery.

[0101] Analysis of Comparative Example 5 and Example 5 shows that the electrochemical performance of Comparative Example 5 is lower than that of Example 5. This is because when CMC and CMD are small, the content of active material in the electrode is low, which reduces the compaction density of the battery, reduces the reversible capacity and voltage of the battery, and thus the electrochemical performance of the battery is poor.

[0102] Analysis of Example 4 and Example 7 shows that the capacity retention rate and energy efficiency of Example 4 are better than those of Example 7, and the DCR value of Example 4 is lower than that of Example 7, indicating that when the coating weights of the positive and negative electrode sheets are consistent, if the value of Wb is increased, the value of Wa needs to be further increased to improve the viscosity of the electrolyte, thereby increasing the migration number of lithium ions, improving the kinetic performance of the battery, and improving the electrochemical performance of the battery.

[0103] Analysis of the two sets of data of Example 8, Example 9 and Comparative Example 3 and Example 10 shows that the electrochemical performance of Example 9 is better than that of Example 8, and the electrochemical performance of Example 10 is better than that of Comparative Example 3, indicating that when the values ​​of CMC, CMD, and Wa are the same, as Wb increases, the number of lithium ions provided increases, the number of lithium ion migration is higher, and the battery kinetic performance is better. However, if the Wb value is too high, its electrochemical performance is negatively affected. This is because when the lithium salt concentration is too high, the viscosity of the battery increases, which is not conducive to the transmission of lithium ions and affects the battery kinetic performance.

[0104] Analysis of Example 10 and Example 11 shows that the capacity retention rate and energy efficiency of Example 11 are better than those of Example 10, and the DCR of Example 11 is lower than that of Example 10. This shows that when the values ​​of CMC, CMD, and Wb are the same, as Wa increases, the viscosity of the electrolyte is improved, which is beneficial to the migration of lithium ions and further beneficial to the improvement of the electrochemical performance of the battery.

[0105] In summary, performance tests show that adjusting CWA and CWC can improve the energy density of the battery, and regulating Wb and Wa can improve the kinetic performance of the lithium-ion battery. That is to say, when the relationship 0.46≤(10×D / (CWA+CWC))×Wb / Wa≤3.8 holds true, not only can the advantage of the high coating weight of the positive and negative electrodes per unit area be brought into play to improve the energy density of the battery, but the kinetic performance and energy efficiency of the battery can also be improved by changing the lithium salt concentration and electrolyte viscosity.

[0106] The above is a detailed introduction to the lithium-ion batteries, battery packs and electrical devices disclosed in the embodiments of the present application. Specific examples are used herein to illustrate the principles and implementation methods of the present application. The description of the above embodiments is only used to help understand the lithium-ion batteries, battery packs and electrical devices: At the same time, for those skilled in the art, based on the ideas of the present application, there will be changes in the specific implementation methods and application scopes. In summary, the contents of this specification should not be understood as limiting the present application.

Claims

1. A lithium ion battery, characterized in that: The lithium-ion battery comprises: a positive electrode sheet, a negative electrode sheet, an electrolyte and a diaphragm, wherein the diaphragm is arranged between the positive electrode sheet and the negative electrode sheet to form an electrode assembly, and the electrolyte at least partially soaks the electrode assembly; The electrolyte includes a lithium salt and a solvent, the solvent includes a linear solvent and a ring-shaped solvent, the positive electrode plate includes a positive electrode current collector and a positive electrode active material layer, and the negative electrode plate includes a negative electrode current collector and a negative electrode active material layer; The lithium-ion battery meets the following requirements: 0.46≤(10×D / (CWA+CWC))×Wb / Wa≤3.8; Wherein, the CWC is the coating weight per unit area of ​​the positive electrode active material layer on the positive electrode sheet, in mg / mm 2 ; The CWA is the coating weight per unit area of ​​the negative electrode active material layer on the negative electrode sheet, expressed in mg / mm 2 ; The D is the lithium ion migration number of the electrolyte; Wb is the mass fraction of the lithium salt in the electrolyte, in %; Wa is the mass fraction of the linear solvent in the electrolyte, in %; Wherein, the CWC is 0.13 mg / mm 2 ~0.26mg / mm 2 ; The CWA is 0.065 mg / mm 2 ~0.13mg / mm 2 .

2. The lithium-ion battery according to claim 1, characterized in that The Wb / Wa satisfies: 0.15 <Wb / Wa≤0.25。 3. The lithium-ion battery according to claim 2, characterized in that: The Wb is 8% to 20%.

4. The lithium-ion battery according to claim 2, characterized in that: The lithium salt includes at least one of lithium hexafluorophosphate, lithium difluorosulfonyl imide, lithium difluorooxalatoborate, and lithium bis(oxalatoborate).

5. The lithium-ion battery according to claim 2, characterized in that: The Wa is 20% to 80%.

6. The lithium-ion battery according to claim 2, characterized in that: The linear solvent includes at least one of diethyl carbonate, ethyl methyl carbonate, dimethyl carbonate, ethyl acetate, ethyl propionate, propyl propionate, 1,1,2,2-tetrafluoroethyl-2,2,3,3-tetrafluoropropyl ether, and ethylene glycol dimethyl ether; and / or, The cyclic solvent includes at least one of ethylene carbonate and propylene carbonate.

7. The lithium-ion battery according to claim 1, characterized in that: The D is 0.2 to 0.

6.

8. The lithium-ion battery according to claim 1, characterized in that The D and the CWC satisfy: 3.84<10×D / CWC<46.15; and / or, The D and the CWA satisfy: 7.69<10×D / CWA<92.

3.

9. A battery pack, characterized in that: The battery pack comprises a box and a lithium-ion battery as claimed in any one of claims 1 to 8 placed in the box.

10. An electrical device, characterized in that: The electrical device comprises an electrical device body and a lithium-ion battery as claimed in any one of claims 1 to 8 disposed in the electrical device body.

Images