A lithium-ion battery, battery pack and power device

Abstract

This invention relates to the field of battery technology, specifically to a lithium-ion battery, a battery pack, and an electrical device. The lithium-ion battery includes: a positive electrode, a negative electrode, an electrolyte, and a separator. The separator is disposed between the positive and negative electrode to form an electrode assembly. The electrolyte at least partially wets the electrode assembly. The electrolyte includes a lithium salt and a solvent, the solvent including linear and cyclic solvents. The positive electrode includes a positive current collector and a positive active material layer. The negative electrode includes a negative current collector and a negative active material layer. The lithium-ion battery satisfies the following conditions: 0.46 ≤ (10 × D / (CWA + CWC)) × Wb / Wa ≤ 3.8; CWC is 0.13 mg / mm³. 2 ~0.26mg / mm 2 CWA was 0.065 mg / mm. 2 ~0.13mg / mm 2 Lithium-ion batteries simultaneously improve energy density, energy efficiency, and kinetic performance.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and in particular to a lithium-ion battery, a battery pack, and an electrical device thereof. Background Technology

[0002] In lithium-ion batteries, to improve energy density and capacity, the coating weight of the active material layer on the positive and negative electrodes per unit area can be increased. However, increasing the coating weight of the active material layer on the positive and negative electrodes per unit area hinders the transport of lithium ions between the electrodes, thus deteriorating the cell's energy efficiency. Although increasing the lithium salt concentration in the electrolyte can increase battery energy efficiency, this increase is accompanied by an increase in electrolyte viscosity. High-viscosity electrolytes deteriorate battery energy efficiency and kinetic performance. Therefore, battery energy density, energy efficiency, and kinetic performance are mutually constrained and difficult to improve simultaneously. Summary of the Invention

[0003] This application discloses a lithium-ion battery, a battery pack, and an electrical device thereof, which solves the problem in the prior art that the energy density, energy efficiency, and dynamic performance of batteries are mutually constrained and difficult to improve simultaneously.

[0004] In a first aspect, embodiments of this application provide a lithium-ion battery, the lithium-ion battery comprising: a positive electrode, a negative electrode, an electrolyte, and a separator, the separator being disposed between the positive electrode and the negative electrode to form an electrode assembly, and the electrolyte at least partially wetting the electrode assembly;

[0005] The electrolyte includes lithium salt and solvent, the solvent includes linear solvent and cyclic solvent, the positive electrode includes a positive current collector and a positive active material layer, and the negative electrode includes a negative current collector and a negative active material layer.

[0006] The lithium-ion battery satisfies:

[0007] 0.46≤(10×D / (CWA+CWC))×Wb / Wa≤3.8;

[0008] Wherein, CWC is the coating weight per unit area of ​​the positive electrode active material layer on the positive electrode sheet, in mg / mm². 2 ;

[0009] The CWA refers to the coating weight per unit area of ​​the negative electrode active material layer on the negative electrode sheet, expressed in mg / mm². 2 ;

[0010] D is the lithium-ion transport number of the electrolyte;

[0011] Wb is the mass fraction of the lithium salt in the electrolyte, expressed as %;

[0012] Wa is the mass fraction of the linear solvent in the electrolyte, expressed in %;

[0013] Wherein, the CWC is 0.13 mg / mm 2 ~0.26mg / mm 2 ;

[0014] The CWA was 0.065 mg / mm. 2 ~0.13mg / mm 2 .

[0015] Furthermore, the Wb / Wa ratio satisfies: 0.15 <Wb / Wa≤0.25。

[0016] Furthermore, the Wb is 8% to 20%.

[0017] Furthermore, the lithium salt includes at least one of lithium hexafluorophosphate, lithium difluorosulfonylimide, lithium difluorooxalate borate, and lithium bis(oxalate borate).

[0018] Furthermore, the Wa content is 20% to 80%.

[0019] Further, the linear solvent includes at least one selected from diethyl carbonate, methyl ethyl 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,

[0020] The cyclic solvent includes at least one of ethylene carbonate and propylene carbonate.

[0021] Furthermore, D is 0.2 to 0.6.

[0022] Furthermore, the relationship between D and CWC satisfies: 3.84 < 10 × D / CWC < 46.15; and / or,

[0023] The condition D and CWA satisfy: 7.69 < 10 × D / CWA < 92.3.

[0024] Secondly, embodiments of this application provide a battery pack, the battery pack including a housing and a lithium-ion battery as described in the first aspect disposed in the housing.

[0025] Thirdly, embodiments of this application provide an electrical device, the electrical device including an electrical device body and a lithium-ion battery as described in the first aspect disposed on the electrical device body.

[0026] Compared with the prior art, the beneficial effects of this application are as follows:

[0027] This application provides a lithium-ion battery. By exploring the relationship between the coating weight per unit area of ​​the positive electrode active material layer (CWC), the coating weight per unit area of ​​the negative electrode active material layer (CWA), the lithium ion transport number (D) in the electrolyte, the mass fraction of lithium salt in the electrolyte (Wb), and the mass fraction of linear solvent in the electrolyte (Wa), it was 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 high coating weight per unit area of ​​the positive and negative electrodes be utilized, but the disadvantage of reduced battery dynamic performance and energy efficiency can also be avoided. This is because when the content of CWA and CWC is high, although the energy density of the battery increases, the transport of lithium ions is hindered, resulting in poor kinetic performance. At this time, the lithium ion concentration and lithium ion transference number can be increased by increasing Wb. However, when Wb is too high, the lithium ion transference number will decrease due to the increase in electrolyte viscosity. Therefore, it is necessary to adjust the content of Wa 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.

[0028] In other words, when the above relationship is satisfied, the battery's energy density and capacity can be improved by leveraging the high coating weight of the positive and negative electrode sheets per unit area. At the same time, by adjusting the mass fraction of lithium salt in the electrolyte, which is related to the lithium-ion transport number and the content of linear solvent, the synergistic adjustment of these three factors can improve the lithium-ion transport capability, thereby enhancing the battery's energy efficiency and kinetic performance. This achieves the goal of simultaneously improving the battery's energy density, energy efficiency, and kinetic performance. Detailed Implementation

[0029] The technical solutions provided in this application will be further described below with reference to the embodiments.

[0030] With the rapid development of new energy technologies, the demand for capacity, lifespan, and energy density of lithium-ion batteries is increasing. To increase battery energy density, 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 on the positive electrode can be 0.18 mg / mm². 2 ~0.20mg / mm 2 The coating weight of the active material layer per unit area of ​​the negative electrode is 0.08 mg / mm². 2 ~0.10mg / mm 2 .

[0031] 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 and negative electrode sheets, which affects the porosity of the active material layer and thus affects the diffusion degree of the electrolyte on the positive and negative electrode sheets, deteriorating the electrochemical performance of the battery. The increase in thickness also 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.

[0032] To reduce lithium-ion migration resistance by increasing the coating weight of the active material layers on the positive and negative electrodes per unit area, the lithium salt concentration in the electrolyte is adjusted to increase the lithium-ion transference number, thereby improving the migration ability of lithium-ions at the positive and negative electrodes and enhancing the battery's kinetic performance. However, as the lithium salt concentration increases, the viscosity of the electrolyte also increases. High-viscosity electrolytes increase lithium-ion shuttle resistance, deteriorate the battery's kinetic performance, lead to increased battery polarization, and worsen the battery's cycle performance.

[0033] To address the aforementioned issues, this application's embodiments explore the relationship between the coating weight of the active material layers in the positive and negative electrode sheets and parameters such as the lithium salt concentration and electrolyte solvent in the electrolyte. This optimizes the electrical performance of lithium-ion batteries, enabling them to achieve high energy density through coating amount while also exhibiting good energy efficiency and kinetic performance.

[0034] Firstly, embodiments of this application provide a lithium-ion battery:

[0035] The lithium-ion battery includes: a positive electrode, a negative electrode, an electrolyte, and a separator. The separator is disposed between the positive electrode and the negative electrode to form an electrode assembly. The electrolyte at least partially wets the electrode assembly.

[0036] The electrolyte includes lithium salt and solvent, the solvent includes linear solvent and cyclic solvent, the positive electrode includes a positive current collector and a positive active material layer, and the negative electrode includes a negative current collector and a negative active material layer.

[0037] The lithium-ion battery satisfies:

[0038] 0.46≤(10×D / (CWA+CWC))×Wb / Wa≤3.8;

[0039] Wherein, CWC is the coating weight per unit area of ​​the positive electrode active material layer on the positive electrode sheet, in mg / mm². 2 ;

[0040] The CWA refers to the coating weight per unit area of ​​the negative electrode active material layer on the negative electrode sheet, expressed in mg / mm². 2 ;

[0041] D is the lithium-ion transport number of the electrolyte;

[0042] Wb is the mass fraction of the lithium salt in the electrolyte, expressed as %;

[0043] Wa is the mass fraction of the linear solvent in the electrolyte, expressed in %;

[0044] Wherein, the CWC is 0.13 mg / mm 2 ~0.26mg / mm 2 ;

[0045] The CWA was 0.065 mg / mm. 2 ~0.13mg / mm 2 .

[0046] This application increases the battery's capacity and energy density by increasing the coating weight of the positive and negative active material layers per unit area on the positive and negative electrode sheets, respectively. The impact of this increased coating weight on lithium-ion transport number and battery kinetic performance is addressed by exploring the specific relationship between the coating weight and the lithium salt concentration and the mass fraction of the linear solvent in the electrolyte. Specifically, increasing the lithium salt concentration increases the lithium-ion transport number; however, a high lithium salt concentration increases the electrolyte viscosity, reducing the lithium-ion transport number and worsening battery efficiency and cycle life. Therefore, to simultaneously improve battery energy density and kinetic performance, this application improves the electrolyte viscosity by utilizing the low viscosity of the linear solvent. This improves the electrolyte viscosity, promotes lithium-ion transport, increases the lithium-ion transport number, and thus enhances battery kinetic performance, resulting in simultaneous improvements in both energy density and kinetics.

[0047] Therefore, based on the premise that the coating weight of the active material layer per unit area on the positive and negative electrode sheets is relatively high, this application explores the specific relationships between several parameters, including 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 transport number D, the mass fraction of lithium salt in the electrolyte Wb, and the mass fraction of linear solvent in the electrolyte Wa. The relationship 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 high kinetic performance and energy density.

[0048] The relationship proposed in this application indicates that when CWC and CWA are within the aforementioned ranges, the battery exhibits a high energy density. In this case, if CWC, CWA, D, Wb, and Wa satisfy the above relationship, not only can the advantage of high coating weight per unit area of ​​the positive and negative electrode sheets be utilized, but the battery's kinetic performance and energy efficiency can also be improved by altering 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 lithium-ion migration is significantly hindered, affecting the battery's electrochemical performance. In this case, increasing the Wb content increases the lithium salt concentration to improve the lithium-ion transference number. However, increased Wb means increased electrolyte viscosity, hindering lithium-ion transport between the positive and negative electrode sheets. Therefore, increasing the Wa content improves the electrolyte viscosity, further improving the lithium-ion transference number and enhancing the battery's kinetic performance. In other words, when the above relationship holds true, the lithium-ion battery exhibits high energy density and kinetic performance.

[0049] 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 / mm². 2 ~0.26mg / mm 2 This includes any point within that range, for example, CWC could be 0.13 mg / mm. 2 0.15 mg / mm 2 0.18 mg / mm 2 0.20 mg / mm 2 0.22 mg / mm 2 0.24 mg / mm 2 0.26 mg / mm 2 Etc. CWA can be 0.065 mg / mm. 2 0.07 mg / mm 2 0.08 mg / mm 2 0.09 mg / mm 2 0.10 mg / mm 2 0.12 mg / mm 2 0.13 mg / mm 2 wait.

[0050] It is understandable that the coating weight per unit area of ​​the positive electrode active material layer on the positive electrode sheet refers to the coating weight 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 of the coating slurry of the negative electrode active material layer on the negative electrode sheet.

[0051] Furthermore, Wb / Wa satisfies: 0.15 <Wb / Wa≤0.25。

[0052] When Wb / Wa is within the above range, the migration of lithium ions is optimal, which is beneficial to the kinetic performance of the battery. This avoids the situation where a high Wb / Wa ratio results in a high concentration of lithium salt in the electrolyte and a low content of linear solvent, leading to a large number of lithium ions but also increased electrolyte viscosity, which hinders lithium ion migration and impairs battery kinetic performance. Conversely, a low Wb / Wa ratio results in a low concentration of lithium salt in the electrolyte, limiting the number of lithium ions that can be transported and affecting the battery's electrochemical performance. Examples of Wb / Wa ratios include 0.155, 0.16, 0.17, 0.18, 0.19, 0.20, 0.22, 0.23, 0.24, and 0.25.

[0053] Furthermore, the Wb is 8% to 20%.

[0054] When Wb is within this range, the number of lithium ions available increases, the lithium ion transport number increases, and the battery exhibits higher kinetic performance. This not only avoids the poor kinetic performance of the battery due to low lithium ion transport numbers at low lithium salt concentrations, but also avoids the increased viscosity of the electrolyte, which increases lithium ion transport resistance and affects battery kinetic performance, when the lithium salt concentration is too high. Examples of Wb values ​​include 8%, 9%, 12%, 13%, 14%, 16%, 18%, 19%, and 20%.

[0055] Furthermore, the lithium salt includes at least one of lithium hexafluorophosphate, lithium difluorosulfonylimide, lithium difluorooxalate borate, and lithium bis(oxalate borate).

[0056] Lithium salts are the providers of lithium ions in the electrolyte. The types of lithium salts mentioned above exhibit good solubility and conductivity in non-aqueous solvents, and can synergistically form a stable passivation film on the surface of the negative electrode with carbonate solvents, exhibiting high stability, which is beneficial to improving the electrochemical performance of lithium-ion batteries. Furthermore, lithium salts can be lithium hexafluorophosphate, lithium bis(oxalato)borate, lithium difluorooxalatoborate, lithium hexafluorophosphate and lithium difluorosulfonylimide, lithium hexafluorophosphate, lithium difluorosulfonylimide and lithium difluorooxalatoborate, etc.

[0057] Furthermore, Wa ranges from 20% to 80%.

[0058] Linear solvents are characterized by poor conductivity, low viscosity, and low melting point. When Wa is within the above-mentioned range, the electrolyte viscosity is good, the lithium-ion transport capacity is high, and the battery kinetic performance is good. This avoids the following problems: when Wa is too high, the electrolyte conductivity is poor, and the interfacial passivation film stability of the negative electrode is poor; it also avoids the following problems: when Wa is too low, the electrolyte viscosity is high, and the lithium-ion transport is hindered, affecting the battery kinetic performance. For example, Wa is 20%, 25%, 30%, 35%, 40%, 50%, 60%, 70%, 75%, 80%, etc.

[0059] Further, the linear solvent includes at least one selected from diethyl carbonate, methyl ethyl 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,

[0060] The cyclic solvent includes at least one of ethylene carbonate and propylene carbonate.

[0061] Organic solvents in electrolytes possess the characteristics of dissolving lithium salts, promoting lithium-ion transport, participating in the formation of the interfacial passivation film on the negative electrode, and exhibiting high stability. Linear solvents, in particular, have low melting points, low viscosity, and low dielectric constants, which can alter the viscosity of the electrolyte. Cyclic solvents, on the other hand, have high dielectric constants and high ionic conductivity, participating in the formation of the interfacial passivation film on the negative electrode. By using linear and cyclic solvents in a coordinated manner, the electrolyte achieves higher conductivity, higher lithium-ion transport capacity, and participates in the formation of a stable interfacial passivation film on the negative electrode, thereby contributing to the improvement of the battery's electrochemical performance.

[0062] Furthermore, D is 0.2 to 0.6.

[0063] The lithium-ion transference number (LTL) is an important indicator of the effective migration of lithium ions in a lithium-ion battery. When the LTL is within this range, lithium ions can effectively intercalate and deintercalate on both the positive and negative electrodes, reducing contact between lithium ions and the electrolyte, decreasing interfacial side reactions, and improving battery life. Furthermore, as a crucial cation involved in transport, a high LTL indicates a low anion transference number, which helps reduce concentration polarization during charging and discharging, decreases interfacial resistance, and improves the battery's electrochemical performance. When the LTL is below this range, lithium ion transport is limited, anion migration increases, the interfacial resistance increases, and the improvement in electrochemical performance is limited. For example, D can be 0.2, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, 0.55, 0.6, etc.

[0064] Furthermore, the relationship between D and CWC satisfies: 3.84 < 10 × D / CWC < 46.15; and / or,

[0065] The condition D and CWA satisfy: 7.69 < 10 × D / CWA < 92.3.

[0066] It is important to note that 10×D / CWC represents the ratio of 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 ratio of lithium-ion migration number to the coating weight per unit area of ​​the negative electrode active material layer on the negative electrode sheet.

[0067] When the above relationship holds true, lithium-ion batteries exhibit high energy density and energy efficiency. This not only avoids the situation where excessively high CWC and CWA result in thicker coatings on the positive and negative electrodes, hindering lithium-ion transport, lower lithium-ion transport numbers, and poor battery kinetic performance; it also avoids the situation where excessively low CWC and CWA, while resulting in good lithium-ion transport capabilities, leads to lower battery energy density and makes coating processes difficult to control. For example, 10×D / CWC can be 3.85, 5, 10, 15, 20, 25, 30, 35, 40, 45, 46.10, etc.; and 10×D / CWA can be 7.7, 8, 10, 15, 20, 30, 45, 55, 65, 75, 85, 90, 92.20, etc.

[0068] Secondly, embodiments of this application provide a battery pack, the battery pack including a housing and a lithium-ion battery as described in the first aspect disposed in the housing.

[0069] Multiple lithium-ion batteries are connected in series or parallel and placed in the battery pack housing, which protects the lithium-ion batteries.

[0070] Thirdly, embodiments of this application provide an electrical device, the electrical device including an electrical device body and a lithium-ion battery as described in the first aspect disposed on the electrical device body.

[0071] Lithium-ion batteries are used to provide power to electrical devices, which include 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.

[0072] The technical solution of this application will be further explained below with reference to more specific embodiments and experimental test results.

[0073] Example 1

[0074] A lithium-ion battery includes: a positive electrode, a negative electrode, an electrolyte, and a separator. The separator is disposed between the positive electrode and the negative electrode to form an electrode assembly, and the electrolyte at least partially wets the electrode assembly.

[0075] The electrolyte includes lithium salt and solvent, the solvent includes linear solvent and cyclic solvent, the positive electrode includes positive current collector and positive active material layer, and the negative electrode includes negative current collector and negative active material layer.

[0076] The lithium-ion battery satisfies:

[0077] CWC was 0.165 mg / mm. 2 CWA was 0.079 mg / mm 2 D was 0.25%, Wb was 11.25%, and Wa was 63.66%.

[0078] Wherein, CWC is the coating weight per unit area of ​​the positive electrode active material layer on the positive electrode sheet, expressed in mg / mm². 2 ;

[0079] CWA is the coating weight per unit area of ​​the negative electrode active material layer on the negative electrode sheet, expressed in mg / mm². 2 ;

[0080] D is the lithium-ion transference number of the electrolyte;

[0081] Wb is the mass fraction of lithium hexafluorophosphate in the electrolyte, expressed in %;

[0082] Wa represents the mass fraction of ethyl methyl carbonate (EMC) and dimethyl carbonate in the electrolyte, expressed as a percentage.

[0083] This embodiment also provides a method for preparing the above-mentioned lithium-ion battery, which includes the following steps:

[0084] Preparation of the positive electrode sheet: Lithium iron phosphate (LiFePO4), conductive carbon black (SP), and polyvinylidene fluoride (PVDF) binder are mixed uniformly in a solvent N-methylpyrrolidone (NMP) at a mass ratio of 97:2:1 to obtain a positive electrode slurry. The positive electrode slurry is then coated onto a positive electrode current collector aluminum foil, wherein the CWC is 0.165 mg / mm². 2 That is, 254 / 1540.25mg / mm 2 After drying, cold pressing, slitting, and cutting, the positive electrode sheet is obtained.

[0085] Preparation of the negative electrode sheet: Artificial graphite (anode active material), conductive carbon (SP), thickener (CMC), and binder (SBR) were dispersed in deionized water at a mass ratio of 96.5:0.5:1:2 and mixed evenly to obtain a negative electrode slurry. The negative electrode slurry was then coated onto the negative electrode current collector copper foil, with a CWA of 0.079 mg / mm². 2 That is, 122 / 1540.25 mg / mm 2 After drying, cold pressing, slitting, and cutting, negative electrode sheets are obtained.

[0086] Electrolyte preparation: In an argon-atmosphere glove box with a moisture content ≤1ppm, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in rows, with mass fractions of 21.59%, 31.83%, and 31.83%, respectively. EMC and DMC were linear solvents, and their combined mass ratio (Wa) was 63.66%. Then, dried lithium hexafluorophosphate was dissolved in the solvent, with a mass fraction (Wb) of 11.25%. The lithium salt was stirred until completely dissolved and homogeneous. Ethylene carbonate and fluoroethylene carbonate were then added and mixed thoroughly to obtain the electrolyte, with ethyleneene carbonate comprising 2% and fluoroethylene carbonate comprising 1.5% of the electrolyte.

[0087] Preparation of the diaphragm: A 16-micron polyethylene film was selected as the diaphragm.

[0088] Battery assembly: The prepared positive electrode, separator, and negative electrode are stacked in sequence, with the separator in the middle of the positive and negative electrode to separate them. After winding, they are assembled into an electrode assembly. After welding the tabs, the electrode assembly is assembled into an outer packaging. After injecting the prepared electrolyte, the electrode assembly is encapsulated, left to stand, formed, shaped, and capacity tested to finally prepare a lithium-ion battery.

[0089] Determination of lithium-ion transport number D: The lithium-ion transport number is tested by constant potential polarization method using symmetrical cells assembled with different electrolytes. It is necessary to use both AC impedance and DC polarization methods simultaneously.

[0090] 1) Li / Li symmetric cell assembly

[0091] In an argon atmosphere glove box with a moisture content ≤1ppm, a 16-micrometer diameter lithium metal sheet was used as the positive and negative electrodes, and a 19-micrometer diameter polyethylene film was used as the separator. An electrolyte containing the above-mentioned linear solvent with a mass fraction of Wa and a lithium salt with a mass fraction of Wb was used to assemble a CR2032 coin cell.

[0092] 2) Testing the lithium-ion transference number of coin cells using the potentiostatic polarization method

[0093] The assembled coin cell was first subjected to an electrochemical impedance spectroscopy (EIS) test on an electrochemical workstation. The EIS test frequency range was 100,000–0.01 Hz, and the measured resistance was R0. Then, a DC polarization test was performed, applying a small and constant potential difference ΔV (typically around 10 mV) to the battery for 2000 seconds, while recording the current change over time. Initially, all migratable ions in the battery affect charge transport, and I0 (initial current) is at its maximum. As polarization progresses, a stable ion concentration gradient gradually forms inside the battery, suppressing anion migration. The current in the battery system is contributed by cations (i.e., lithium ions), and the current Iss (steady-state current) is recorded at this point. The battery was then subjected to another EIS test, again within the 100,000–0.01 Hz frequency range, and the measured resistance was Rss. The lithium ion transference number can then be calculated using the following formula:

[0094]

[0095] 3) Lithium-ion battery DC resistance (DCR) test

[0096] The prepared pouch battery (rated capacity 3.2Ah) was tested on a charge-discharge test cabinet. The battery cell was placed in a 45℃ oven for two cycles of activation. The charge-discharge steps were: constant current and constant voltage charging at 1C to 3.65V, and constant current discharging at 1C to 2.5V.

[0097] Place the battery cell in a 25°C oven and let it stand until the surface temperature of the battery cell reaches 25°C. Then, charge and discharge the battery cell for 3 cycles to establish the initial capacity. The charging and discharging steps are: 0.5C constant current and constant voltage charging to 3.65V, and 0.5C discharging to 2.5V. The initial capacity of the battery cell is C0.

[0098] DCR tests were conducted on the battery cells under different states of charge (SOC): First, the cells were fully charged to 3.65V at a constant current and constant voltage rate of 0.5C. After resting for 10 minutes, they were discharged at a 0.1C0 current for 1 hour. After resting for 1 hour, the voltage was recorded as V1. Then, the cells were discharged at a 1.07C0 current for 30 seconds, and the voltage was recorded as V2. The DCR value of the battery at 90% SOC is (V2-V1)×1000 / C0; the DCR value of the cells at a 0.1C0 current for 3 hours is V2. After standing for 1 hour, the voltage was recorded as V3. Then, after discharging at 1.07C for 30 seconds, the voltage was recorded as V4. Therefore, the DCR resistance of the battery at 60% SOC is (V4-V3)×1000 / C0. Next, the battery was discharged at 0.1C0 current for 3 hours. After standing for 1 hour, the voltage was recorded as V5. After discharging at 1.07C0 for 30 seconds, the voltage was recorded as V6. Therefore, the DCR resistance of the battery at 30% SOC is (V4-V3)×1000 / C0.

[0099] Compared with Example 1, Examples 2 to 11 differ in the mass fractions of CWC, CWA, D, Wb, Wa, and the cyclic solvent, as detailed in Table 1.

[0100] The difference between Comparative Examples 1 to 5 and Example 1 lies in the different mass fractions of CWC, CWA, D, Wb, Wa, and the cyclic solvent, as detailed in Table 1.

[0101] The parameter changes for Examples 1 to 11 and Comparative Examples 1 to 5 are detailed in Table 1.

[0102] Table 1 Summary of components and parameters for the examples and comparative examples

[0103]

[0104]

[0105] The lithium-ion batteries obtained in the above embodiments were subjected to charge-discharge cycle tests on a charge-discharge apparatus. The test temperatures included 25°C and 45°C. The batteries were tested at a current of 0.2C (rated capacity 3.2Ah). Each test cycle consisted of: constant current and constant voltage charging at 0.5C to 3.65V, with a cutoff current of 0.02C, followed by constant current discharge at 0.2C to 2.5V. The DCR performance of the batteries under different SOC states was also tested to further evaluate their kinetic performance.

[0106] The capacity retention rate is calculated as follows: Capacity retention rate after 800 cycles = (Discharge capacity after 800 cycles / Discharge capacity in the first cycle) × 100%.

[0107] Energy efficiency is calculated using the formula: Battery energy efficiency after 800 cycles = (Discharge energy after 800 cycles / Charge energy after 800 cycles) × 100%.

[0108] Table 2 Battery performance test data

[0109]

[0110]

[0111] As shown in Tables 1 and 2, 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 the better kinetic performance of the battery. The above data indicate that since Comparative Examples 1 to 5 do not satisfy the relationship 0.46≤(10×D / (CWA+CWC))×Wb / Wa≤3.8, it means that when this relationship holds, lithium-ion batteries can not only take advantage of the high energy density brought about by the high coating weight of positive and negative electrode sheets per unit area, but also, when the lithium salt concentration is high, the lithium-ion transference number of the battery can be increased by adjusting the amount of linear solvent, thereby optimizing the electrochemical performance of the battery.

[0112] 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 cyclic solvent is lower. At this time, the capacity retention rate and energy efficiency of the battery are significantly reduced compared with Examples 3 to 11. This is because the cyclic solvent participates in the formation of the interface passivation film of the negative electrode sheet. Therefore, the stability of the interface passivation film is poor, which affects the electrochemical performance of the battery.

[0113] 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 smaller, the content of active material in the electrode is lower, which reduces the compaction density of the battery, reduces the reversible capacity and voltage of the battery, and thus results in poorer electrochemical performance of the battery.

[0114] Analysis of Examples 4 and 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. This indicates that when the coating weight of the positive and negative electrode sheets is the same, if the Wb value is increased, the Wa value needs to be further increased to improve the viscosity of the electrolyte, thereby increasing the lithium ion transference number, improving the kinetic performance of the battery, and improving the electrochemical performance of the battery.

[0115] Analysis of the data from Examples 8 and 9, and Comparative Examples 3 and 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. This indicates that when the values ​​of CMC, CMD, and Wa are the same, as Wb increases, the number of lithium ions provided increases, the lithium ion transport number is higher, and the battery's kinetic performance is better. However, if the value of Wb is too high, its electrochemical performance will be negatively affected. This is because when the lithium salt concentration is too high, the battery viscosity increases, which is not conducive to the transport of lithium ions and affects the battery's kinetic performance.

[0116] Analysis of Examples 10 and 11 shows that the capacity retention and energy efficiency of Example 11 are better than those of Example 10, while the DCR of Example 11 is lower than that of Example 10. This indicates that when the values ​​of CMC, CMD, and Wb are the same, the viscosity of the electrolyte is improved as Wa increases, which is beneficial to the migration of lithium ions and thus to the improvement of the electrochemical performance of the battery.

[0117] In summary, performance tests show that adjusting CWA and CWC can improve the energy density of the battery, and controlling 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, it can not only take advantage of the high coating weight of the positive and negative electrode sheets per unit area to improve the energy density of the battery, but also improve the kinetic performance and energy efficiency of the battery by changing the lithium salt concentration and electrolyte viscosity.

[0118] The lithium-ion battery, battery pack, and power device disclosed in the embodiments of this application have been described in detail above. Specific examples have been used to illustrate the principles and implementation methods of this application. The descriptions of the embodiments above are only for the purpose of helping to understand the lithium-ion battery, battery pack, and power device. At the same time, for those skilled in the art, there will be changes in the specific implementation methods and application scope based on the ideas of this invention. Therefore, the content of this specification should not be construed as a limitation of this invention.

Claims

1. A lithium-ion battery, characterized in that, The lithium-ion battery includes: a positive electrode, a negative electrode, an electrolyte, and a separator. The separator is disposed between the positive electrode and the negative electrode to form an electrode assembly. The electrolyte at least partially wets the electrode assembly. The electrolyte includes lithium salt and solvent, the solvent includes linear solvent and cyclic solvent, the positive electrode includes a positive current collector and a positive active material layer, and the negative electrode includes a negative current collector and a negative active material layer. The lithium-ion battery satisfies: 0.46≤(10×D / (CWA+CWC))×Wb / Wa≤3.8; Wherein, CWC is the coating weight per unit area of ​​the positive electrode active material layer on the positive electrode sheet, in mg / mm². 2 ; The CWA refers to the coating weight per unit area of ​​the negative electrode active material layer on the negative electrode sheet, expressed in mg / mm². 2 ; D is the lithium-ion transport number of the electrolyte; Wb is the mass fraction of the lithium salt in the electrolyte, expressed as %; Wa is the mass fraction of the linear solvent in the electrolyte, expressed in %; Wherein, the CWC is 0.13 mg / mm 2 ~0.26mg / mm 2 ; The CWA was 0.065 mg / mm. 2 ~0.13mg / mm 2 .

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

4. The lithium-ion battery according to claim 2, characterized in that, The lithium salt includes at least one of lithium hexafluorophosphate, lithium difluorosulfonylimide, lithium difluorooxalate borate, and lithium dioxalate borate.

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

6. The lithium-ion battery according to claim 2, characterized in that, The linear solvent comprises at least one of diethyl carbonate, methyl ethyl 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 value of D is 0.2 to 0.

6.

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

3.

9. A battery pack, characterized in that, The battery pack includes a housing and a lithium-ion battery as described in any one of claims 1 to 8 disposed within the housing.

10. An electrical appliance, characterized in that, The electrical device includes a device body and a lithium-ion battery as described in any one of claims 1 to 8 disposed in the device body.

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