Battery

Abstract

The invention discloses a battery. The battery comprises a shell and a battery cell, the battery cell comprises a negative plate, the negative plate comprises a negative current collector and a negative active layer, and the negative active layer comprises a silicon-based negative active material; the negative plate comprises a single-sided negative plate part and a double-sided negative plate part; a negative active layer is arranged on one side of the single-side negative plate part; negative electrode active layers are arranged on two sides of the double-sided negative electrode plate part; wherein the Li / Si ratio of the unit area of the negative electrode active layer of the single-sided negative electrode plate part is A, the Li / Si ratio of the unit area of the negative electrode active layer of any side of the double-sided negative electrode plate part is B, and A and B meet the condition that A is smaller than B. According to the invention, the bending deformation of the single-sided negative plate part can be effectively inhibited, the initial coulombic efficiency is improved, and the cycle life of the battery is prolonged.

Description

Technical Field

[0001] This invention relates to the field of battery technology, and more specifically to a lithium-ion battery. Background Technology

[0002] With the rapid development of new energy technologies, batteries are being used more and more widely. Batteries are now being used in electric vehicles, electronic devices, and energy storage devices. Currently, higher requirements are being placed on the energy density and cycle life of batteries.

[0003] Because silicon-based materials have a higher theoretical specific capacity than graphite and a richer lithium intercalation pathway, lithium ions can be intercalated from the four sides of silicon-based materials, while graphite can only intercalate between layers. Therefore, using silicon-based materials can improve the energy density and fast charging performance of batteries. However, silicon-based materials have large volume changes, are prone to damage during cycling, and have many side reactions when in contact with the electrolyte. New SEI films (solid electrolyte films) are constantly generated on the surface of the negative electrode, resulting in low initial coulombic efficiency and poor cycle performance of the battery. Summary of the Invention

[0004] This invention provides a battery to solve the problems of low initial coulombic efficiency and poor cycle performance of batteries containing silicon-based materials.

[0005] This invention provides a battery, comprising: a casing and a battery cell disposed within the casing; the battery cell includes a positive electrode, a negative electrode, and a separator located between the positive electrode and the negative electrode; the negative electrode includes a negative current collector and a negative active layer located on the surface of the negative current collector, the negative active layer comprising a silicon-based negative active material; along the thickness direction of the battery cell, the negative electrode includes a single-sided negative electrode portion and a double-sided negative electrode portion; the negative active layer is disposed on the side of the negative current collector of the single-sided negative electrode portion near the center of the battery cell along the thickness direction; the negative active layer is disposed on both sides of the negative current collector of the double-sided negative electrode portion along the thickness direction; wherein, the Li / Si ratio per unit area of ​​the negative active layer of the single-sided negative electrode portion is A, and the Li / Si ratio per unit area of ​​the negative active layer of either side of the double-sided negative electrode portion is B, wherein A and B satisfy: A is less than B.

[0006] In some embodiments, the double-sided negative electrode portion is the outermost negative electrode portion located away from the center of the cell and on the outermost side of the cell; the double-sided negative electrode portion is the inner negative electrode portion located close to the center of the cell; and / or, A and B satisfy the following: the difference between B and A is 0.05≤BA≤2, preferably 0.05≤BA≤1.1.

[0007] In some embodiments, A satisfies: 0 ≤ A ≤ 1.95, and B satisfies: 0 < B ≤ 3.

[0008] In some embodiments, the thickness of the negative current collector of the outermost negative electrode sheet is h μm, and A and h satisfy: 0.01≤A / h≤0.2; and / or, the thickness h μm of the negative current collector of the outermost negative electrode sheet satisfies: 10≤h≤20.

[0009] In some embodiments, the thickness h of the negative current collector of the outermost negative electrode sheet is greater than the thickness T of the negative current collector of the inner negative electrode sheet; and / or, 1 < h / T ≤ 4; and / or, the thickness T μm of the negative current collector of the inner negative electrode sheet satisfies: 5 ≤ T ≤ 12.

[0010] In some embodiments, the negative electrode active layer comprises at least one of silicon-oxygen composite material and silicon-carbon composite material.

[0011] In some embodiments, the internal resistance of the negative active layer of the outermost negative electrode sheet is less than the internal resistance of the negative active layer of the inner negative electrode sheet; and / or, the internal resistance of the negative active layer of the outermost negative electrode sheet is in the range of 5Ω to 70Ω; and / or, the internal resistance of the negative active layer of the inner negative electrode sheet is in the range of 8Ω to 90Ω.

[0012] In some embodiments, the peeling force between the negative active layer of the inner negative electrode sheet and the negative current collector of the inner negative electrode sheet is greater than the peeling force between the negative active layer of the outermost negative electrode sheet and the negative current collector of the outermost negative electrode sheet.

[0013] In some embodiments, the positive electrode sheet includes a positive current collector and positive active layers disposed on both sides of the positive current collector along the thickness direction; the positive electrode sheet includes a first positive electrode sheet adjacent to the outermost negative electrode sheet and a second positive electrode sheet located on the inner side of the first positive electrode sheet near the center of the cell; the Li / Co ratio of a single layer of the positive active layer of the first positive electrode sheet is C, and the Li / Co ratio of a single layer of the positive active layer of the second positive electrode sheet is D, wherein C and D satisfy: C < D.

[0014] In some embodiments, the negative electrode active layer comprises trimethyl phosphate, and the mass content of trimethyl phosphate is less than 1% based on the total mass of the negative electrode active layer; and / or, the battery further comprises an electrolyte disposed within the casing, the electrolyte comprising trimethyl phosphate, and the mass content of trimethyl phosphate is less than 1% based on the total mass of the electrolyte; and / or, the negative electrode active layer comprises hydrofluoroether, and the mass content of hydrofluoroether is less than 0.5% based on the total mass of the negative electrode active layer; and / or, the battery further comprises an electrolyte disposed within the casing, the electrolyte comprising hydrofluoroether, and the mass content of hydrofluoroether is less than 0.5% based on the total mass of the electrolyte.

[0015] In some embodiments, at least one of the single-sided negative electrode portion and the double-sided negative electrode portion is a lithium-filled negative electrode.

[0016] Beneficial effects: Silicon-based anode materials can improve the energy density and rate performance of batteries. During the first charge and discharge of a battery, the electrolyte reacts with the silicon-based material in the active layer of the anode, forming a lithium-containing SEI film on the surface of the active layer. The lithium in the lithium-containing SEI film comes from the cathode material. The lithium ions consumed in forming the SEI film cannot return to the cathode to participate in charge and discharge in subsequent cycles, resulting in low initial coulombic efficiency and capacity loss. During battery cycling, the silicon-based material undergoes repeated volume changes, which can break through the formed SEI film and expose new silicon surfaces. These new silicon surfaces then come into contact with the electrolyte and continuously consume lithium from the cathode, constantly forming new SEI films. The thickening of the SEI film reduces the lithium ion transport rate, exacerbates the capacity loss of the battery, and thus reduces the battery cycle life. Pre-lithiation of the negative electrode can replenish lithium in the negative electrode, making up for the lithium consumed in the formation of the SEI film on the negative electrode, thereby improving the battery's initial coulombic efficiency (the ratio of the battery's initial discharge capacity to its initial charge capacity). However, pre-lithiated negative electrode sheets undergo expansion when lithium ions are embedded in the silicon-based negative electrode active material. For negative electrode sheets with a negative electrode active layer on only one side, since the other side lacks a negative electrode active layer for support, excessive lithium ions added during pre-lithiation can cause severe lithium intercalation expansion, leading to electrode curling and bending deformation. Even with methods to flatten the negative electrode sheet with a negative electrode active layer on only one side, it is still impossible to completely eliminate the internal expansion stress generated by excessive lithium ion embedding during pre-lithiation. As a result, after charge-discharge cycles, the volume change of the silicon-based material on one side, coupled with the internal expansion force accumulated in the negative electrode active layer after pre-lithiation and lithium intercalation, makes it easier for the single-sided negative electrode sheet to detach from the inner electrode sheet, thereby reducing battery cycle life and causing lithium plating problems. Because double-sided negative electrode sheets have negative electrode active layers on both sides, the negative electrode active layer on the other side binds them during the pre-lithiation process, dissipating the expansion stress generated inside during pre-lithiation. When the negative electrode active layers on both sides of the inner negative electrode sheet are pre-lithlated, they can also cancel each other out, suppressing the problem of the inner negative electrode sheet curling to one side. Moreover, the inner negative electrode sheet is located on the inside of the battery and is tightly pressed together with other positive electrode sheets and the outermost negative electrode sheet. Even if some expansion stress remains inside during pre-lithiation, it will not produce a large bending deformation problem during subsequent battery cycling.

[0017] Therefore, by adjusting the lithium-silicon ratio of the single-sided negative electrode portion and the double-sided negative electrode portion, making the Li / Si ratio per unit area of ​​the negative electrode active layer in the single-sided negative electrode portion smaller than that in the double-sided negative electrode portion, the initial coulombic efficiency can be improved, while also reducing the bending deformation of the outermost negative electrode and improving the battery cycle life.

[0018] If the single-sided negative electrode section is not pre-lithiated, or if the pre-lithiation amount is controlled to be excessive, the lithium-silicon ratio (Li / Si) per unit area of ​​the single-sided negative electrode section will be at a low level. When the Li / Si ratio per unit area of ​​the single-sided negative electrode section is at a low level, the internal expansion force is less, and the volume change is smaller during subsequent battery cycles. This can improve the bending deformation problem of the single-sided negative electrode section, make the structure of the single-sided negative electrode section more stable, and reduce the problem of active material pulverization and shedding. When the double-sided negative electrode section is pre-lithiated, by controlling the amount of pre-lithiated lithium, the Li / Si ratio of the double-sided negative electrode active layer of the double-sided negative electrode section is larger, which can provide higher capacity, compensate for the lithium lost by the SEI film, improve the first efficiency of the battery and the overall energy density of the battery. In addition, a larger Li / Si ratio is usually accompanied by higher ionic conductivity, which helps the ion transport efficiency between the double-sided negative electrode section with active layers on both sides and the positive electrode, thus improving the battery cycle performance. Attached Figure Description

[0019] To more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0020] Figure 1 This is a schematic diagram of the battery structure according to an embodiment of the present invention; Figure 2 This is a cross-sectional view of the battery cell according to an embodiment of the present invention.

[0021] Explanation of reference numerals in the attached figures: 10-Shell; 20-Cell; 21-Positive electrode; 21a-First positive electrode; 21b-Second positive electrode; 211-Positive current collector; 212-Positive active layer; 22-Negative electrode; 22a-Outermost negative electrode; 22b-Inner negative electrode; 221-Negative current collector; 222-Negative active layer; 23-Separator; Z-Thickness direction. Detailed Implementation

[0022] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0023] The following is combined with Figure 1 , Figure 2 The embodiments of the present invention will be described in detail below.

[0024] This application discloses a battery, including a casing 10 and a battery cell 20 disposed within the casing 10; the battery cell 20 includes a positive electrode 21, a negative electrode 22, and a separator 23 located between the positive electrode 21 and the negative electrode 22; the negative electrode 22 includes a negative electrode current collector 221 and a negative electrode active layer 222 located on the surface of the negative electrode current collector 221, the negative electrode active layer 222 including a silicon-based negative electrode active material; along the thickness direction Z of the battery cell 20, the negative electrode 22 includes a single-sided negative electrode portion and a double-sided negative electrode portion; the single-sided... The negative electrode current collector 221 of the negative electrode sheet is provided with the negative electrode active layer 222 on the side of the negative electrode current collector 221 along the thickness direction Z near the center of the cell 20; the negative electrode active layer 222 is provided on both sides of the negative electrode current collector 221 along the thickness direction Z of the double-sided negative electrode sheet; wherein, the Li / Si ratio of the negative electrode active layer 222 per unit area of ​​the single-sided negative electrode sheet is A, and the Li / Si ratio of the negative electrode active layer 222 per unit area of ​​either side of the double-sided negative electrode sheet is B, and A and B satisfy: A is less than B.

[0025] Silicon-based anode active materials are anode active materials containing silicon. Due to their high theoretical specific capacity, they can significantly improve the energy density of batteries. Silicon-based anode active materials include at least one of silicon-oxygen composite materials and silicon-carbon composite materials.

[0026] The single-sided negative electrode section is a negative electrode 22 with a negative electrode active layer 222 coated on only one side. The double-sided negative electrode section is a negative electrode 22 with a negative electrode active layer 222 coated on both sides.

[0027] When the battery cell 20 has a laminated structure, the single-sided negative electrode portion is the outermost negative electrode 22a, which is furthest from the center of the battery cell 20 and located on the outermost side of the battery cell 20. The double-sided negative electrode portion is the innermost negative electrode 22b, which is closer to the center of the battery cell 20. The outermost negative electrode 22a refers to the negative electrode 22 located on the outermost side of the battery cell 20 and furthest from the center of the battery cell 20 along the thickness direction Z of the battery cell 20. The innermost negative electrode 22b refers to the negative electrode 22 located inside the battery cell 20 and closer to the center of the battery cell 20.

[0028] The battery cell 20 of the present invention is not limited to a stacked structure, but can also be a wound structure. The wound structure battery cell 20 also has a single-sided negative electrode portion coated with a negative electrode active layer 222 on only one side. If it is a wound structure, it is not necessary to limit the single-sided negative electrode portion of the wound structure battery cell 20 to be located on the outermost surface of the battery cell 20.

[0029] The Li / Si ratio refers to the ratio of the number of lithium atoms to the number of silicon atoms in the negative electrode active layer 222 per unit area.

[0030] The smaller the Li / Si ratio per unit area of ​​the single-sided negative electrode, the less expansion force accumulates inside it. During subsequent battery cycles, its volume change is smaller, which can improve the bending deformation problem of the single-sided negative electrode, make the structure of the single-sided negative electrode more stable, and reduce the problem of active material pulverization and shedding. The double-sided negative electrode active layer 222 of the double-sided negative electrode has a larger Li / Si ratio, which can provide higher capacity, compensate for the lithium loss of the formed SEI film, improve the first efficiency of the battery and the overall energy density of the battery. In addition, a larger Li / Si ratio means that more lithium ions can be inserted and extracted, which helps the ion transport efficiency between the double-sided negative electrode and the positive electrode 21 where active layers are set on both sides, and improves the battery cycle performance.

[0031] In some embodiments, A and B satisfy the following condition: the difference between B and A is in the range of 0.05≤BA≤2, preferably 0.05≤BA≤1.1.

[0032] When the Li / Si ratio in the active layer of the electrode is different, their internal resistances differ. A significant difference in impedance leads to uneven current distribution, increasing local polarization and causing localized lithium plating. This also degrades the battery's cycle performance. Furthermore, if the difference between B and A is too large, the risk of lithium plating on the outermost negative electrode 22a increases; if the difference is too small, the risk of curling and deformation on the outermost negative electrode 22a increases. Therefore, by limiting the difference between B and A within the aforementioned range, sufficient pre-lithiation compensation for the outermost negative electrode 22a can be ensured, while reducing the risk of deformation, thus guaranteeing the battery's cycle performance. Meanwhile, the inner negative electrode 22b can achieve more complete pre-lithiation to compensate for lithium loss during the first cycle without generating excessive internal stress or adverse side reactions, thereby improving the battery's structural stability and electrochemical performance.

[0033] At least one of the single-sided negative electrode portion and the double-sided negative electrode portion mentioned above is a lithium-filled negative electrode.

[0034] It should be noted that pre-lithiation of the negative electrode sheet to achieve lithium replenishment can be achieved through methods including but not limited to electrochemical lithium replenishment, lithium band lithium replenishment, and adding lithium replenishing agents to the negative electrode active layer.

[0035] In some embodiments, the Li / Si ratio A satisfies: 0 ≤ A ≤ 1.95, and can be any value or a value between any two of 0, 0.1, 0.2, 0.5, 0.8, 1, 1.3, 1.6, 1.8, and 1.95. The Li / Si ratio B satisfies: 0 < B ≤ 3, and can be any value or a value between any two of 0.1, 1, 1.2, 1.5, 1.6, 1.8, 1.9, 2.2, 2.4, and 3.

[0036] It should be noted that the internal resistance of the active layer of different negative electrode sheets 22 can be tested using the four-probe method of a powder resistivity tester, referring to GB / T 45324—2025.

[0037] For the outermost single-sided negative electrode section, since the negative electrode current collector 221 has a negative electrode active layer 222 on the side closer to the center of the cell 20 along the thickness direction Z, while the other side directly faces the shell 10, the expansion stress generated inside during pre-lithiation is difficult to release or offset. Therefore, limiting the Li / Si ratio A of the negative electrode active layer 222 per unit area to a low range of 0 to 1.95 can effectively avoid the problem of excessive expansion of the active layer due to excessive lithium ion insertion during pre-lithiation, which would cause the electrode to bend and deform outward and cause the active material to pulverize and fall off, thus deteriorating the battery cycle and inducing lithium plating in the battery.

[0038] Meanwhile, for the inner double-sided negative electrode portion, since the negative electrode current collector 221 has negative electrode active layers 222 on both sides along the thickness direction Z, this double-sided active layer structure can provide mutual binding and support during lithium-ion insertion, effectively dissipating or offsetting the expansion stress generated after internal lithium insertion. Therefore, by limiting the Li / Si ratio B of the negative electrode active layer 222 per unit area on either side to a relatively high range of greater than 0 and less than or equal to 3, it can ensure that there are more lithium ions inside the inner battery to form a more stable and dense SEI film, and significantly improve the battery's initial coulombic efficiency and cycle performance.

[0039] In some embodiments, the thickness of the negative current collector 221 of the outermost negative electrode 22a is h μm, and the Li / Si ratio A and the thickness h satisfy: 0.01 ≤ A / h ≤ 0.2. A / h can be any one of 0.01, 0.05, 0.07, 0.1, 0.11, 0.12, 0.15, 0.16, 0.18, 0.2 or any value between two of them.

[0040] In some embodiments, the thickness h μm of the negative electrode current collector 221 satisfies: 10 ≤ h ≤ 20. h can be any one of 10, 12, 13, 15, 16, 18, 20 or any combination thereof.

[0041] By setting an appropriate current collector thickness h, a certain amount of physical support and mechanical strength can be provided for the outermost negative electrode 22a to resist the stress generated by the expansion of lithium intercalation in the silicon-based material, thereby reducing the risk of warping deformation of the outermost negative electrode 22a. Simultaneously, controlling the Li / Si ratio A to the current collector thickness h within the range of 0.05 to 0.12 ensures a balance between the amount of lithium ion intercalation and the mechanical support force provided by the current collector during pre-lithiation, further reducing the risk of warping deformation of the outermost negative electrode 22a.

[0042] When the A / h ratio is too small, it means that the Li / Si content is relatively low or the current collector thickness is relatively large. Although this can effectively suppress deformation, it may lead to poor pre-lithiation or reduce the battery's energy density due to excessive current collector thickness. When the A / h ratio is too large, it means that the Li / Si content is relatively high or the current collector thickness is relatively small. The current collector may not be able to effectively resist excessive expansion stress, which may cause deformation or even breakage of the outermost negative electrode 22a, as well as pulverization and shedding of the active material layer in the outermost negative electrode 22a, resulting in deterioration of battery cycle performance and increased risk of lithium plating. Therefore, by controlling the A / h ratio within the range of 0.01 ≤ A / h ≤ 0.2, it is possible to ensure the mechanical support strength of the current collector, prevent electrode curling and deformation, and also take into account the battery's energy density and cycle stability.

[0043] In some embodiments, the thickness h of the negative current collector 221 of the outermost negative electrode 22a is greater than the thickness T of the negative current collector 221 of the inner negative electrode 22b. And / or, 1 < h / T ≤ 4.

[0044] In some embodiments, the thickness T μm of the negative current collector 221 of the inner negative electrode 22b satisfies: 5≤T≤12.

[0045] The thickness h of the negative electrode current collector 221 of the outermost negative electrode 22a is greater than the thickness T of the negative electrode current collector 221 of the inner negative electrode 22b. This ensures that the negative electrode current collector 221 of the outermost negative electrode 22a has sufficient resistance to the stress generated by the expansion of lithium intercalation in the silicon-based material. If the thickness T of the negative electrode current collector 221 of the inner negative electrode 22b meets the above range, the overall thickness of the battery can be reduced, and the volumetric energy density and gravimetric energy density of the battery will be reduced. By controlling 1 < h / T ≤ 4, while ensuring that the outermost single-sided negative electrode 22 of the battery is free from deformation risk, the overall thickness of the battery is avoided from being too thick, which would lead to a reduction in the volumetric energy density and gravimetric energy density of the battery.

[0046] In some embodiments, the internal resistance of the negative electrode active layer 222 of the outermost negative electrode 22a is less than the internal resistance of the negative electrode active layer 222 of the inner negative electrode 22b; and / or, the internal resistance R1 of the negative electrode active layer 222 of the outermost negative electrode 22a is in the range of 5Ω to 70Ω; and / or, the internal resistance R2 of the negative electrode active layer 222 of the inner negative electrode 22b is in the range of 8Ω to 90Ω.

[0047] The Li / Si ratio A per unit area of ​​the negative electrode active layer 222 of the outermost negative electrode 22a is less than the Li / Si ratio B per unit area of ​​the negative electrode active layer 222 on either side of the inner negative electrode 22b. This means that the inner negative electrode 22b contains more lithium ions. During cycling, lithium can participate in the formation of a thicker and denser SEI film, reducing side reactions during cycling. However, increasing the thickness of the SEI film reduces the migration rate of lithium ions between the electrodes. By making the internal resistance of the negative electrode active layer 222 of the outermost negative electrode 22a less than that of the inner negative electrode active layer 222 of the inner negative electrode 22b, the uniform and rapid insertion of lithium ions into the outermost negative electrode 22a can be further promoted. Lower internal resistance helps reduce local overpotential and lithium ion accumulation on the electrode surface, thereby effectively suppressing the formation of lithium dendrites and local overheating. Meanwhile, since the inner negative electrode 22b has negative electrode active layers 222 on both sides and a relatively high Li / Si ratio (B), the internal expansion stress can be dissipated to a certain extent by the mutual binding of the two active layers. The higher internal resistance of the inner negative electrode 22b can moderately adjust the transport rate of lithium ions in the inner negative electrode 22b, avoid unstable reactions caused by excessively rapid lithium ion insertion, and help form a denser and more stable solid electrolyte interface (SEI) film, thereby reducing the continuous consumption of lithium ions in subsequent cycles, thus ensuring the battery capacity and cycle performance.

[0048] In some embodiments, an electrochemical lithium replenishment method is used to replenish lithium on the negative electrode 22. The negative electrode 22 and the lithium replenishment sheet form a circuit in a lithium replenishment cell containing electrolyte. By applying different currents and controlling the current magnitude or the lithium replenishment time, the amount of lithium replenishment on the outermost negative electrode 22a and the inner negative electrode 22b can be controlled. The amount of lithium replenishment affects the internal resistance of the outermost negative electrode 22a and the inner negative electrode 22b, causing a difference in their internal resistance. Generally, the greater the amount of lithium replenishment, the greater the internal resistance. Reasonably setting the resistance of different negative electrodes helps to better utilize battery energy and improve battery cycle performance. On the other hand, it also allows for the control of the conductive agent content in the outermost negative electrode 22a and the inner negative electrode 22b, thereby controlling the internal resistance of the outermost negative electrode 22a and the internal resistance of the negative electrode active layer 222 of the inner negative electrode 22b, achieving the aforementioned effects.

[0049] In some embodiments, the peel force F1 between the negative electrode active layer 222 of the inner negative electrode 22b and the negative electrode current collector 221 of the inner negative electrode 22b is greater than the peel force F2 between the negative electrode active layer 222 of the outermost negative electrode 22a and the negative electrode current collector 221 of the outermost negative electrode 22a. It should be noted that the peel force F1 between the negative electrode active layer 222 of the inner negative electrode 22b and the negative electrode current collector 221 can be adjusted to achieve a greater peel force F2 between the negative electrode active layer 222 of the inner negative electrode 22b and the negative electrode current collector 221 of the outermost negative electrode 22a. For example, the adhesive content in the negative electrode active layer 222 of the inner negative electrode 22b can be greater than the adhesive content in the negative electrode active layer 222 of the outermost negative electrode 22a.

[0050] The peeling force between the negative active layer 222 of the inner negative electrode 22b and the negative current collector 221 of the inner negative electrode 22b is greater than the peeling force between the negative active layer 222 of the outermost negative electrode 22a and the negative current collector 221 of the outermost negative electrode 22a. This is partly because the outermost negative electrode 22a, having only a negative active layer 222 on one side, is more prone to bending deformation during pre-lithiation and cycling. By appropriately reducing the peeling force of the outermost negative electrode 22a, the positive active layer 212 on the outermost negative electrode 22a can be allowed to bend to a certain extent. The expansion stress is released, which can alleviate the curling phenomenon of the outermost negative electrode 22a. On the other hand, after the negative active layer 222 on both sides of the inner negative electrode 22b is pre-lithiated, the active layer will have a certain expansion rebound. By setting the peeling force between the negative active layer 222 of the inner negative electrode 22b and the negative current collector 221 of the inner negative electrode 22b to be larger, it is beneficial to reduce the electrode rebound of the negative active layer 222 of the inner negative electrode 22b during the battery charging and discharging process, thereby improving the energy density of the battery and improving the cycle stability of the battery.

[0051] In some embodiments, the positive electrode 21 includes a positive current collector 211 and positive active layers 212 disposed on both sides of the positive current collector 211 along the thickness direction Z; the positive electrode 21 includes a first positive electrode 21a adjacent to the outermost negative electrode 22a and a second positive electrode 21b located on the inner side of the first positive electrode 21a near the center of the cell 20, the second positive electrode 21b being disposed adjacent to the inner negative electrode 22b; the Li / Co ratio of the single-sided positive active layer 212 of the first positive electrode 21a is C, and the Li / Co ratio of the single-sided positive active layer 212 of the second positive electrode 21b is D, C and D satisfy: C < D.

[0052] A lower Li / Co ratio results in less lithium ions being released from the first positive electrode 21a during charging and discharging. This reduces the amount of lithium ions embedded in the adjacent outermost negative electrode 22a during cycling, decreasing the expansion stress generated on the single-sided negative electrode 22a during cycling. This, in turn, suppresses the curling deformation of the outermost negative electrode 22a and improves the battery's cycle performance. Furthermore, since the outermost negative electrode 22a has an active layer on only one side, its local current density may be relatively high. If there is an excessive supply of lithium ions, lithium ions can easily deposit on the negative electrode surface. By reducing the Li / Co ratio of the first positive electrode 21a, the lithium ion flux flowing to the outermost negative electrode 22a can be reduced, thereby balancing the local lithium ion flux and effectively preventing lithium deposition. Meanwhile, since the inner negative electrode 22b has a double-sided active layer and a relatively high Li / Si ratio B, and its structure is relatively stable, it can better withstand lithium intercalation expansion. Therefore, the adjacent second positive electrode 21b adopts a higher Li / Co ratio to provide sufficient lithium ions, ensuring that the inner negative electrode 22b can fully utilize its capacity and improve the overall battery's first coulombic efficiency and energy density.

[0053] On the one hand, because the amount of lithium replenished to the outermost negative electrode 22a is less than that to the inner negative electrode 22b during pre-lithiation of the electrodes, and the outermost negative electrode 22a is consumed during the initial charging of the SEI layer, the number of lithium ions that escape from the outermost negative electrode 22a into the first positive electrode 21a during charging and discharging is less than the number of lithium ions that escape from the inner negative electrode 22b into the second positive electrode 21b, resulting in C < D. On the other hand, the content of cobalt-containing positive electrodes in the first and second positive electrodes 21a can be adjusted according to the amount of lithium replenishment to achieve the above effect.

[0054] In some embodiments, the negative electrode active layer 222 includes trimethyl phosphate (TMP), and the mass content of trimethyl phosphate M1 is less than 1% based on the total mass of the negative electrode active layer 222; and / or, the battery also includes an electrolyte disposed within the housing 10, the electrolyte including trimethyl phosphate, and the mass content of trimethyl phosphate M2 is less than 1% based on the total mass of the electrolyte.

[0055] When batteries are subjected to abuse conditions such as high temperatures, TMP decomposes upon heating, generating phosphorus-containing free radicals (such as PO·). These free radicals can capture high-energy hydrogen free radicals (H·) and hydroxyl free radicals (HO·) that maintain the chain reaction during combustion, transforming them into inert substances and effectively interrupting the battery's violent combustion. However, the phosphate ester groups in the TMP molecular structure have poor electrochemical stability at low potentials and will preferentially undergo reduction decomposition over conventional carbonate solvents (such as EC) in the electrolyte. Its decomposition products cannot form stable and dense SEI film components such as Li2CO3 and LiF. Instead, they may generate metastable organophosphorus compounds, resulting in a loose and unstable SEI film that cannot effectively suppress the cracking of silicon-based materials in the negative electrode 22 due to volume changes. If the negative electrode active layer 222 also includes graphite material, the TMP decomposition process will also trigger the co-intercalation of solvated lithium ions between graphite layers, causing the graphite structure to expand, peel off, and break down, reducing the battery's cycle performance. Therefore, it is essential to ensure that its content is within an appropriate range to ensure battery cycle performance while improving the battery's furnace temperature safety performance. Excessive TMP content reduces conductivity, increases internal resistance, and disrupts the formation of the SEI film on the negative electrode surface, leading to battery capacity decay and reduced cycle performance. If the negative electrode active layer 222 contains TMP, the temperature of the lithium metal embedded in the negative electrode active layer 222 will rise and release a large amount of heat after pre-lithiation of the negative electrode sheet 22. The presence of TMP can reduce the temperature of the negative electrode sheet 22 after lithium replenishment. The high temperature will accelerate the oxidation of the copper foil of the negative electrode current collector 221 in the air, forming a thicker copper oxide layer, increasing contact resistance, and resulting in poor battery cycle performance.

[0056] In some embodiments, the negative electrode active layer 222 includes hydrofluoroether (HFE), and the mass content of hydrofluoroether N1 is less than 0.5% based on the total mass of the negative electrode active layer 222; and / or, the battery also includes an electrolyte disposed within the housing 10, the electrolyte including hydrofluoroether, and the mass content of hydrofluoroether N2 is less than 0.5% based on the total mass of the electrolyte.

[0057] The interface stability is indirectly improved by using the negative electrode active layer 222 and / or the electrolyte, which includes hydrofluoroether (HFE). HFE forms a liquid film on the surface of the negative electrode 22, preventing direct contact between air and the negative electrode 22. This avoids capacity loss caused by the oxidation of active lithium in the negative electrode active layer 222 during or after pre-lithiation, thus preventing the battery's initial efficiency and cycle life from being compromised. The mass content of HFE should not be too high, as excessive content may weaken the interaction between Li+ and anions, thereby reducing ionic conductivity and lithium-ion transference number, and affecting the battery's cycle performance.

[0058] Trimethyl phosphate (TMP) and hydrofluoroether (HFE) test methods: Disassemble cell 20, remove negative electrode 22, soak it in acetonitrile for 12 hours, and then take the solution for GC analysis, or squeeze out the electrolyte and perform GC analysis directly.

[0059] This invention also provides a method for preparing a negative electrode sheet 22, comprising: providing a negative electrode sheet 22 containing a negative electrode active material, wherein the negative electrode sheet 22 includes a single-sided negative electrode sheet portion and a double-sided negative electrode sheet portion; performing lithium replenishment on the single-sided negative electrode sheet portion and the double-sided negative electrode sheet portion, wherein the lithium replenishment includes one or more of electrochemical lithium replenishment, lithium band lithium replenishment, adding a lithium replenishing agent to the negative electrode active material, and adding a solid electrolyte to the negative electrode active material.

[0060] Example 1: Preparation of the battery I. Preparation of positive electrode 21 Lithium cobalt oxide (LCO), polyvinylidene fluoride (PVDF), conductive carbon (SuperP), and carbon nanotubes (CNT) were dissolved in N-methylpyrrolidone (NMP) at a mass ratio of 97.5:1:1:0.5 and stirred evenly to form a positive electrode slurry. This slurry was then uniformly coated onto both sides of the positive electrode current collector 211 (aluminum foil) along its thickness direction Z. After drying and rolling, the positive electrode sheet 21 was formed.

[0061] II. Preparation of negative electrode 22 Inner negative electrode 22b: Silicon-carbon composite material, graphite, conductive carbon black, polyacrylic acid, sodium carboxymethyl cellulose and styrene-butadiene rubber are mixed in a mass ratio of 55:42:0.5:1.2:0.4:0.9, and then mixed evenly with deionized water to form a negative electrode slurry. This slurry is then uniformly coated on both sides of the negative electrode current collector 221 (a copper foil with a thickness of 5μm) along its thickness direction Z to form a negative electrode active layer 222. After drying and rolling, the inner negative electrode 22b is formed.

[0062] Outermost negative electrode 22a: Silicon-carbon composite material, graphite, conductive carbon black, polyacrylic acid, sodium carboxymethyl cellulose and styrene-butadiene rubber are mixed in a mass ratio of 55:42:0.5:1.2:0.4:0.9, and then mixed evenly with deionized water to form a negative electrode slurry. This slurry is then uniformly coated on one side of the negative electrode current collector 221 (a copper foil with a thickness of 10 μm) along its thickness direction Z to form a negative electrode active layer 222. After drying and rolling, the outermost negative electrode 22a is formed.

[0063] III. Preparation of Lithium-Supplemented Electrodes An electrochemical lithium replenishment method was used to pre-lithiate both the outermost and inner lithium replenishment negative electrode sheets to achieve lithium replenishment of the negative electrode sheets.

[0064] Outermost lithium-replenishing negative electrode: In an environment with a dew point of -45°C, the single-sided negative electrode 22 is conveyed by a conveyor to a specific position in the lithium replenishment cell of the electrochemical lithium replenishment machine. The lithium replenishment cell is equipped with a lithium replenishment sheet corresponding to the single-sided negative electrode 22. The lithium replenishment sheet is a lithium metal sheet or a lithium replenishment sheet containing a lithium metal salt (e.g., a lithium replenishment sheet coated with lithium cobalt oxide on an aluminum foil). In this embodiment, an aluminum foil coated with lithium cobalt oxide is used as the lithium replenishment sheet. A current of 0.5A is applied to replenish the outermost negative electrode 22a for 5 minutes. After the lithium replenishment is completed, the outermost lithium-replenishing negative electrode 22 is obtained.

[0065] Inner lithium-replenishing negative electrode sheet: Under an environment with a dew point of -45°C, the inner negative electrode sheet 22b is conveyed to a specific position in the lithium replenishment cell of the electrochemical lithium replenishment machine using a winding machine. The lithium replenishment cell is equipped with a lithium replenishment sheet corresponding to the inner negative electrode sheet 22b. In this embodiment, aluminum foil coated with lithium cobalt oxide is used as the lithium replenishment sheet. A current of 2.5A is applied to replenish the innermost negative electrode sheet 22b for 5 minutes. After the lithium replenishment is completed, the inner lithium-replenishing negative electrode sheet 22 is obtained.

[0066] IV. Preparation of Electrolyte Using ethylene carbonate (EC), propyl propionate (PP), and dimethyl carbonate (DMC) in a mass ratio of 2:3:5 as solvents, the electrolyte, based on its total volume, also includes 20% 2,2-difluoroethyl acetate, 15% fluoroethylene carbonate, 5% nitrile additives (adiponitrile and 1,3,6-hexanetrionitrile in a 1:1 ratio), 3% 1,3-propanesulfonate lactone, 12% lithium hexafluorophosphate (LiPF6) as the lithium salt, 0.8% trimethyl phosphate, and the hydrofluoroether (N1) having a mass content of less than 0.2%.

[0067] V. Battery Assembly The positive electrode 21, separator 23, and lithium-added negative electrode 22 are stacked in sequence, with the separator acting as a separator between the positive and negative electrode 22. The outermost layer is a single-sided negative electrode 22 with the copper foil side facing outward. The bare cell 20 is placed in a steel shell, and the electrolyte is injected into the bare cell 20. After vacuum sealing, settling, formation (0.1C constant current charging to 4% SOC, then 0.2C constant current charging to 10% SOC), shaping, and capacity testing, a steel-shell lithium-ion battery is obtained.

[0068] The batteries of each embodiment are processed according to Tables 1-1 to 1-3 for other parameters. For specific parameter differences, please refer to the tables, which will not be described in detail here.

[0069] It should be noted that the electrolyte in Example 7 did not contain trimethyl phosphate (TMP), and the mass content of fluoroethylene carbonate was 15.8%.

[0070] The lithium replenishment times in the examples are as follows: Example 2: outermost negative electrode 22a: 0.5 min, inner negative electrode 22b: 5 min; Example 3: outermost negative electrode 22a: no lithium replenishment, inner negative electrode 22b: 0.2 min; Example 4: outermost negative electrode 22a: 0.1 min, inner negative electrode 22b: 10 min; Example 5: outermost negative electrode 22a: 4.5 min, inner negative electrode 22b: 16 min; the lithium replenishment times for Examples 6-10 are the same as in Example 1; the lithium replenishment time for Example 11 is the same as in Example 5.

[0071] Table 1-1

[0072] Table 1-2

[0073] Table 1-3

[0074] Li / Si ratio test method: Disassemble a fully charged battery, take a small negative electrode piece (e.g., a 1mm x 1mm negative electrode piece 22), clean it with dimethyl carbonate (DMC), and then use ICP (inductively coupled plasma atomic emission spectrometry) to test the Li / Si ratio of the negative electrode active layer 222 in the taken negative electrode piece 22. Multiple (e.g., 20) of the above negative electrode pieces can be tested and the average value is taken.

[0075] Li / Co ratio test method: Disassemble a fully charged battery, take a small positive electrode piece (e.g., a 1mm x 1mm positive electrode piece 21), clean it with dimethyl carbonate (DMC), and then use ICP (inductively coupled plasma atomic emission spectrometry) to test the Li / Co ratio. Multiple (e.g., 20) of the above positive electrode pieces can be tested and the average value is taken.

[0076] The initial coulombic efficiency test method is as follows: The lithium-ion battery is placed on the Blue Battery charge and discharge test cabinet for charge and discharge cycle test. The test conditions are 30℃, 0.05C / 0.05C charge and discharge, and the start and end voltage of charge and discharge is 3.0-4.30V. The charging capacity and discharge capacity of the battery in the first cycle are recorded. The measured charging capacity is divided by the discharge capacity and the charge and discharge amount are compared to obtain the battery's initial efficiency.

[0077] VED test method: In an incubator at 25°C, charge at a constant current of 1C0.5C until the voltage reaches 4.4V, then charge at a constant voltage of 4.4V until the current reaches 0.05C, and then discharge at a constant current of 0.51C until the voltage reaches 3.0V. The obtained discharge capacity is the battery capacity. Use a thickness gauge and a 2.5D microscope to measure the dimensions of the battery cell 20 to obtain the volume of the battery cell 20; VED = battery capacity * voltage / battery volume.

[0078] Cycle 400T capacity retention rate test method: Perform cycle tests on the batteries prepared in the examples and comparative examples. The specific test method is as follows: Charge at 0.5C to 4.45V, charge with constant current and constant voltage until cutoff at 0.025C, discharge at 0.1C to 3.0V. Record the capacity after one cycle as the initial capacity. Cycle 400 times and measure the discharge capacity of the 400th cycle. Divide the capacity after 400 cycles by the initial capacity to obtain the capacity retention rate after 400 cycles and record it in Tables 1-1 to 1-3.

[0079] Lithium plating situation after 400 cycles: Disassemble the fully charged battery after the cycle test. The normal negative electrode 22 is golden yellow. Measure the area of the grayish-black, purple, and silver-white regions (the grayish-black, purple, and silver-white regions are regarded as lithium plating) on the surface of the negative electrode 22, and then divide it by the total area of the negative electrode 22 to obtain the lithium plating area. Among them, if the lithium plating area is less than 10%, it is regarded as slight lithium plating.

[0080] 130°C oven temperature test: Test a fully charged battery (i.e., charge to 4.45V and charge to cutoff at a current of 0.02C) in an environment at 130°C for 1h. If it does not explode or catch fire, it is regarded as qualified. 10P / 10T means that among the 10 tested batteries, 10 are qualified.

[0081] "Capacity drop" in Comparative Example 1 and Comparative Example 2: It means that after the battery is cycled to a certain number of cycles (less than 400 cycles), its capacity is less than half of the initial capacity, which is regarded as capacity drop occurring.

[0082] As can be seen from the above table: When A is less than B in Examples 1-11, there is no capacity drop situation, and they have a high capacity retention rate; when A is greater than or equal to B in Comparative Example 1 and Comparative Example 2, the battery capacity drops.

[0083] Although the embodiments of the present invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the present invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A battery, characterized in that, include: Housing (10) and battery cell (20) disposed within the housing; The cell (20) includes a positive electrode (21), a negative electrode (22) and a separator (23) located between the positive electrode and the negative electrode. The negative electrode (22) includes a negative current collector (221) and a negative active layer (222) located on the surface of the negative current collector. The negative active layer includes a silicon-based negative active material. Along the thickness direction of the battery cell, the negative electrode sheet (22) includes a single-sided negative electrode sheet portion and a double-sided negative electrode sheet portion; The negative electrode current collector (221) of the single-sided negative electrode sheet has a negative electrode active layer (222) on the side of the current collector (221) near the center of the cell along the thickness direction (Z); the negative electrode active layer (222) is provided on both sides of the negative electrode current collector (221) of the double-sided negative electrode sheet along the thickness direction (Z). Wherein, the Li / Si ratio per unit area of ​​the negative electrode active layer (222) of the single-sided negative electrode sheet is A, and the Li / Si ratio per unit area of ​​the negative electrode active layer (222) on either side of the double-sided negative electrode sheet is B, wherein A and B satisfy: A is less than B.

2. The battery according to claim 1, characterized in that, The condition A satisfies: 0 ≤ A ≤ 1.95, and the condition B satisfies: 0 < B ≤ 3.

3. The battery according to claim 1, characterized in that, The double-sided negative electrode portion is the outermost negative electrode (22a) of the negative electrode (22) located away from the center of the cell and on the outermost side of the cell; the double-sided negative electrode portion is the inner negative electrode (22b) of the negative electrode (22) located close to the inner side of the center of the cell; and / or, The condition A and B satisfy the following: the difference between B and A is in the range of 0.05≤BA≤2, preferably 0.05≤BA≤1.

1.

4. The battery according to claim 3, characterized in that, The thickness of the negative current collector (221) of the outermost negative electrode sheet (22a) is h μm, and A and h satisfy: 0.01≤A / h≤0.2; and / or, the thickness h μm of the negative current collector (221) of the outermost negative electrode sheet (22a) satisfies: 10≤h≤20.

5. The battery according to claim 3, characterized in that, The thickness h of the negative current collector (221) of the outermost negative electrode (22a) is greater than the thickness T of the negative current collector (221) of the inner negative electrode (22b); and / or, 1 < h / T ≤ 4; and / or, the thickness T μm of the negative current collector (221) of the inner negative electrode (22b) satisfies: 5 ≤ T ≤ 12.

6. The battery according to any one of claims 3-5, characterized in that, The internal resistance of the negative active layer (222) of the outermost negative electrode (22a) is less than the internal resistance of the negative active layer (222) of the inner negative electrode (22b); and / or, the internal resistance R1 of the negative active layer of the outermost negative electrode is in the range of 5Ω~70Ω; and / or, the internal resistance R2 of the negative active layer of the inner negative electrode is in the range of 8Ω~90Ω.

7. The battery according to any one of claims 3-5, characterized in that, The peeling force between the negative active layer of the inner negative electrode (22b) and the negative current collector of the inner negative electrode is greater than the peeling force between the negative active layer of the outermost negative electrode (22a) and the negative current collector of the outermost negative electrode.

8. The battery according to any one of claims 3-5, characterized in that, The positive electrode (21) includes a positive current collector (211) and a positive active layer (212) disposed on both sides of the positive current collector along the thickness direction; the positive electrode (21) includes a first positive electrode (21a) adjacent to the outermost negative electrode (22a) and a second positive electrode (21b) located on the inner side of the first positive electrode (21a) near the center of the cell, and the second positive electrode is disposed adjacent to the inner negative electrode along the thickness direction of the cell; The Li / Co ratio of the positive active layer on one side of the first positive electrode (21a) is C, and the Li / Co ratio of the positive active layer on one side of the second positive electrode (21b) is D, wherein C and D satisfy: C < D.

9. The battery according to any one of claims 3-5, characterized in that, The negative electrode active layer (222) comprises trimethyl phosphate, and the mass content of the trimethyl phosphate, M1, is less than 1% based on the total mass of the negative electrode active layer; and / or, the battery further comprises an electrolyte disposed within the casing, the electrolyte comprising trimethyl phosphate, and the mass content of the trimethyl phosphate, M2, is less than 1% based on the total mass of the electrolyte; and / or, The negative electrode active layer (222) includes hydrofluoroether, and the mass content of the hydrofluoroether N1 is less than 0.5% based on the total mass of the negative electrode active layer; and / or, the battery further includes an electrolyte disposed in the casing, the electrolyte including hydrofluoroether, and the mass content of the hydrofluoroether N2 is less than 0.5% based on the total mass of the electrolyte.

10. The battery according to claim 1, characterized in that, include: At least one of the single-sided negative electrode portion and the double-sided negative electrode portion is a lithium-filled negative electrode.

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