Pole piece with differentiated gradient structure, battery cell and battery

Abstract

This invention discloses an electrode sheet, a bare cell, and a battery with a differentiated gradient structure, belonging to the field of lithium-ion battery electrode technology. The electrode sheet includes a current collector and active material layers disposed on its upper and lower surfaces. Along the thickness direction of the electrode sheet, from the current collector towards one or both sides of the separator, the compaction density of the active material layers decreases and the porosity increases, forming a differentiated gradient structure with the current collector as the boundary. This invention significantly reduces battery internal resistance, improves initial charge-discharge efficiency and rate performance, and enhances electrode structural stability and cycle durability. When used to prepare bare cells or batteries, the initial charge-discharge efficiency can reach 95.6%, the 2C rate discharge capacity retention rate can reach 94.2%, and the capacity retention rate after 1000 cycles can reach 91.5%. This invention is applicable to the preparation of lithium-ion battery electrodes.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery electrode technology, specifically an electrode, cell, and battery with a differentiated gradient structure. Background Technology

[0002] Lithium-ion batteries are widely used in new energy vehicles, energy storage devices, and digital electronics due to their advantages such as high energy density, long cycle life, and no memory effect. As the core energy storage and mass transfer conductive structure of the battery, the electrode's internal pore structure and compaction density directly determine the lithium-ion diffusion rate and electron conduction efficiency.

[0003] In the rolling process of lithium-ion battery electrode manufacturing, the rolling pressure acts on the outer surface of the active material layer and is transmitted inward along the thickness direction. Since the active material layer itself is a porous system of particle packing, the pressure decreases layer by layer during transmission, resulting in a significant difference in compaction gradient along the thickness direction within the active material layer on one side: the surface area directly subjected to the rolling pressure has sufficient particle rearrangement, high compaction, and low porosity; while the bottom layer area, far from the rolling surface and close to the current collector, is affected by pressure attenuation, resulting in insufficient compaction, inadequate contact between particles and between particles and the current collector, and a relatively loose structure with higher porosity. Thus, an asymmetric structural defect, characterized by a dense surface and a loose bottom, is formed within the active material layer on one side.

[0004] This "dense on top, sparse on the bottom" structure first leads to an imbalance in the ion transport and electron conduction paths within the electrode, creating a dual bottleneck that restricts the overall battery performance. Regarding ion transport, the excessively dense surface of the active material layer hinders electrolyte wetting, severely blocking the diffusion channels for lithium ions to enter the active material layer from the separator side. This forces a longer lithium ion migration path, significantly reducing the transport kinetic rate and directly causing a decline in the battery's rate charge / discharge performance, failing to meet the demands of fast charging and high-power applications for rapid lithium ion insertion / extraction. Regarding electron conduction, the loose structure of the bottom layer of the active material results in insufficient contact area and unstable contact between active material particles and between the active material and the current collector. This leads to poor continuity of the electron conduction pathway within the electrode, significantly increasing contact resistance and severely impacting the battery's voltage stability and energy efficiency during charge and discharge.

[0005] Secondly, under long-term battery cycling conditions, the aforementioned "dense on top, sparse on the bottom" active material layer experiences repeated expansion and contraction of the active material during charge and discharge, causing stress to continuously accumulate in the loose bottom layer. Due to the weak bonding force between bottom particles and insufficient adhesion strength at the interface with the current collector, microcracks easily develop in this area and gradually expand, eventually leading to irreversible structural failures such as powdering, shedding, and even interlayer delamination of the active material. Such failures not only directly cause active material deactivation and capacity decay but may also lead to safety hazards such as localized lithium plating, restricting the battery's long cycle life and reliability.

[0006] Furthermore, lithium-ion battery electrodes need to simultaneously meet two contradictory structural requirements: the separator side requires high porosity to ensure rapid lithium-ion insertion and extraction, while the current collector side requires high compaction density to ensure rapid electron extraction and maintain good interfacial contact. However, the inherent mechanical characteristic of decreasing pressure from top to bottom in traditional roll forming inevitably creates a reverse gradient structure of "dense on top and sparse on the bottom" for the active materials, making the separator side denser and the current collector side more porous. This structural characteristic directly contradicts the functional requirements. This structural contradiction has become a key technical bottleneck limiting further improvements in the initial coulombic efficiency, rate performance, and long-cycle stability of power batteries. Summary of the Invention

[0007] The purpose of this invention is to provide an electrode, bare cell, and battery with a differentiated gradient structure to solve technical problems in the prior art such as obstructed ion transport on the electrode surface, poor electron conduction on the current collector side, easy powder shedding and material loss during cycling, short battery cycle life, and poor rate performance.

[0008] This invention constructs an electrode with a differentiated upper sparse and lower dense structure on one or both sides of the current collector, so that the electrode surface maintains reasonable porosity to ensure rapid lithium ion migration, while the side near the current collector has excellent particle contact density and conductive pathway, thereby achieving a bidirectional synergistic improvement in ion transport efficiency and electronic conductivity.

[0009] Meanwhile, this invention can effectively balance the overall mechanical structure and deformation stress distribution of the electrode, alleviate interlayer peeling and powder shedding failure caused by the expansion and contraction of the active material during charging and discharging, and significantly improve the stability of the electrode structure and the cycle durability of the battery, thereby meeting the industrial mass production and high-end application requirements of high-rate, long-life, and highly consistent lithium-ion batteries.

[0010] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0011] On the one hand, an electrode with a differentiated gradient structure is provided, including a current collector and active material layers respectively disposed on the upper and lower surfaces of the current collector. Along the thickness direction of the electrode, from the current collector to one or both sides of the diaphragm side, the compaction density of the active material layers is distributed in a decreasing manner, and the porosity is distributed in an increasing manner, forming a differentiated gradient structure that changes from the surface of the current collector to the diaphragm side.

[0012] Among them, a high-density, low-porosity, dense conductive electron structure is formed on one or both sides of the current collector to construct an efficient electron transport channel; a low-density, high-porosity, loose and porous ion-conducting structure is formed on the separator side to construct an efficient lithium-ion transport channel, thereby achieving a matching balance between electron conduction and lithium-ion migration inside the electrode.

[0013] Preferably, the active material layer mainly comprises an active material, a binder, and a conductive agent; wherein, the active material of the positive electrode is one or more of lithium cobalt oxide, ternary materials, lithium manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, and sodium ion positive electrode materials; and the active material of the negative electrode is one or more of graphite, hard carbon, soft carbon, silicon carbide, lithium titanate, and metal oxides.

[0014] As a limitation of the present invention, the decreasing distribution is a gradually decreasing distribution or a step-decreasing distribution;

[0015] The increasing distribution is either a gradually increasing distribution or a step-like increasing distribution.

[0016] Among them, the gradual decreasing distribution or the gradual increasing distribution makes the parameters of the entire thickness range change linearly and smoothly without structural abrupt changes, while the step decreasing distribution or the step increasing distribution is a segmented step gradient change, which divides the electrode thickness equally into at least two layers, with uniform parameters inside a single layer and gradient differences between layers.

[0017] As another limitation of the present invention, the decreasing distribution is a gradually decreasing distribution;

[0018] When the electrode is a positive electrode, the difference between the compaction density on the current collector side and the compaction density on the separator side of the active material layer is 0.1–0.5 g / cm³. 3 ;

[0019] When the electrode is a negative electrode, the difference between the compaction density on the current collector side and the compaction density on the separator side of the active material layer is 0.05–0.3 g / cm³. 3 .

[0020] To ensure the effectiveness of the gradient structure, the lower bound of the parameter difference guarantees the differentiated transmission effect of the gradient structure.

[0021] As a further limitation of the present invention, the compaction density and porosity of the active material layer satisfy the following relationship:

[0022] ε = 1 - ρ / ρ z

[0023] Where ε is the local porosity of the active material layer, ρ is the local compaction density of the active material layer, and ρ z This represents the true density of the active material.

[0024] The decreasing distribution is a stepped decreasing distribution; N stepped regions with different compaction densities are sequentially set from the current collector side to the diaphragm side in the single-sided or double-sided active material layer, where N is an integer and N≥2;

[0025] Using i as the step number, from the current collector side to the diaphragm side, the numbers are i = 1, 2, ..., N;

[0026] When the electrode is a positive electrode, the compaction density ρ of the i-th step region v,i + = ρ v + +Δ + ·Ki;

[0027] Where, ρ v + It is the average compaction density of the positive electrode active material layer;

[0028] Δ + Δ represents the change in compaction density between adjacent steps of the positive electrode. + =0.2 / N;

[0029] Ki is the coefficient corresponding to the step number i, and the rules for the value of Ki are as follows:

[0030] With M as an intermediate parameter, M= ,in, The floor symbol;

[0031] When N is odd, starting from i=1,2,…,N, Ki takes the values: M, M-1,…,1,0, -1, …, -M;

[0032] When N is even, Ki takes the following values ​​from i=1,2,…,N: M, M-1, …, 1, -1, …, -M.

[0033] That is, Ki satisfies:

[0034] If i ≤ M, then Ki = M - i + 1;

[0035] If N is odd and i = M+1, then Ki = 0;

[0036] If i > M+(N mod 2), let j represent the intermediate variable, j = iM-(N mod 2), then Ki = -j.

[0037] As a further limitation of the present invention, when the electrode is a negative electrode, the negative electrode compaction density ρ of the i-th step is... v,i - = ρ v - + Δ - ·Ki;

[0038] Where, ρ v - The average compaction density of the negative electrode in a single-sided active material layer;

[0039] Δ - Δ represents the change in compaction density of the adjacent steps to the negative pole. - =0.15 / N.

[0040] As a further limitation of the present invention, the active material layers on the upper and lower surfaces of the current collector are symmetrical about the current collector, wherein the stepped regions corresponding to the serial numbers have the same or different compaction densities.

[0041] The gradient structure of this invention adapts to the lithium-ion insertion and extraction transport path, which can effectively reduce the polarization effect of the electrode during charging and discharging, alleviate the springback deformation during electrode processing and cycling, improve the uniformity of electrolyte wetting, and significantly increase the lithium-ion diffusion rate inside the electrode.

[0042] In a second aspect, the present invention also provides a bare battery cell, comprising a multilayer separator and alternating positive and negative electrode sheets, wherein the positive and / or negative electrode sheets are electrode sheets with differentiated gradient structures as described in any of the above.

[0043] In the active material layer of the electrode, the compaction density on the side near the separator is lower than the compaction density on the side near the current collector, and the electrodes are stacked with the lower compaction density side facing the adjacent separator.

[0044] When assembling bare cells, the separator side of the electrode is arranged facing the separator, and the current collector side is arranged away from the separator, so as to match the functional zone transmission characteristics of the electrode and ensure that the mass transfer and conduction process of the cell as a whole is coordinated and consistent.

[0045] Preferably, both the positive and negative electrode sheets adopt a symmetrical differentiated structure with a sparse upper layer and a dense lower layer, and their compaction and porosity parameters are matched to each other to eliminate problems caused by transmission impedance mismatch inside the cell.

[0046] Thirdly, the present invention also provides a battery comprising a casing, an electrolyte, and the aforementioned bare cell.

[0047] As a limitation of the present invention, the bare battery cell is encapsulated inside the housing, and the electrolyte impregnates and covers the electrode plates and diaphragm pores of the bare battery cell.

[0048] By adopting the above technical solution, the technical progress achieved by this invention compared with the prior art is as follows:

[0049] This invention overcomes the dual bottleneck of ion transport obstruction and poor electron conduction caused by the traditional rolling process, which constructs a differential gradient structure electrode with decreasing compaction density and increasing porosity towards both sides of the separator, centered on the current collector. This allows the high-compact, low-porosity region on the current collector side to ensure rapid electron conduction, while the low-compact, high-porosity region on the separator side ensures rapid lithium-ion migration, thus accommodating both transport requirements within the same electrode.

[0050] (1) In terms of reducing the internal resistance of the battery, the current collector side adopts a high compaction density and low porosity structure, which increases the contact area between active material particles and between particles and the current collector, improves the continuity of the conductive network, and reduces the electron transport impedance inside the electrode. The separator side adopts a high porosity and low compaction density structure, which allows the electrolyte to wet the active material layer more fully, shortens the migration path of lithium ions inside the electrode, and reduces the transport resistance. The test results show that the battery prepared using this symmetrical gradient structure electrode can reduce its internal resistance to 10.1 mΩ, which is more than 30% lower than the 14.5 mΩ of the battery prepared using the traditional top-dense and bottom-sparse structure electrode.

[0051] (2) In terms of improving the first charge and discharge efficiency of the battery, the good electron conduction capability on the current collector side and the rapid ion diffusion capability on the separator side work together to reduce polarization caused by the mismatch of transmission rates during charge and discharge, and the irreversible capacity loss is suppressed. Test data show that the battery using the electrode of this invention can achieve a first charge and discharge efficiency of up to 95.6%, which is significantly improved compared with the 92.1% of the traditional homogeneous structure electrode.

[0052] (3) In terms of improving battery rate performance, the high porosity region on the separator side provides sufficient diffusion channels for rapid lithium-ion insertion and extraction, and can maintain a high ion transport flux even under high current charge and discharge conditions, thus alleviating concentration polarization. The battery using the electrode of this invention can achieve a maximum 2C rate discharge capacity retention of 94.2%, which is significantly improved compared to 85.3% for traditional electrode structures, making the battery more adaptable to fast charging and high-power applications.

[0053] (4) In terms of extending battery cycle life, the dense structure on the current collector side provides a more stable support for the active material layer. The internal stress generated by the volume expansion and contraction of the active material during charging and discharging is buffered, the initiation and propagation of microcracks are reduced, and problems such as active material powdering, shedding, and interlayer delamination are effectively suppressed. The results of 1000 cycles show that the battery capacity retention rate using the electrode of this invention can reach up to 91.5%, and the electrode powder shedding rate is as low as 0.08%, while the capacity retention rate of the traditional electrode structure is 76.2% and the powder shedding rate is 0.45%, showing a significant improvement in cycle durability and structural stability.

[0054] (5) Regarding the synergistic effect of the dual gradient of positive and negative electrodes, when both the positive and negative electrodes adopt differentiated gradient structures, the compaction density gradient and porosity gradient of the positive and negative electrodes work together, reducing the transmission impedance difference between the positive and negative electrodes inside the cell, lowering the local overpotential, reducing the risk of lithium plating, and achieving the optimal overall battery performance. Example data shows that the battery prepared using the dual gradient scheme has better internal resistance, initial efficiency, 2C rate retention, and 1000-cycle retention than the scheme using only a single-sided electrode with a gradient structure.

[0055] (6) In terms of process adaptability and mass production feasibility, the differentiated gradient structure of the present invention provides two implementation forms: gradual gradient and step gradient. The continuous linear gradient structure has better electrochemical performance, while the segmented step gradient structure is easier to cooperate with existing industrial rolling production lines. It can be selected according to the performance requirements and mass production conditions of different application scenarios. There is no need to make large-scale modifications to the existing lamination or winding assembly process. The process is simple and suitable for large-scale industrial production. Attached Figure Description

[0056] Figure 1 The diagram shown is a microstructure of the positive electrode with a continuous linear gradient structure in Embodiment 1 of the present invention.

[0057] Figure 2 The image shows SEM images of different positions on the cross-section of the positive electrode with a continuous linear gradient structure in Embodiment 1 of the present invention;

[0058] Figure 3 The diagram shown is a microstructure of the positive electrode segmented stepped gradient structure electrode in Embodiment 2 of the present invention.

[0059] Figure 4 The image shown is a SEM image of different positions on the cross-section of the positive electrode segmented stepped gradient structure in Embodiment 2 of the present invention.

[0060] In the figure: 1-current collector; 2-upper side of active material layer; 3-lower side of active material layer; 4-SEM of the separator side of the electrode cross-section; 5-SEM of the middle layer of the electrode cross-section; 6-SEM of the current collector side of the electrode cross-section. Detailed Implementation

[0061] The present invention will be further described in detail below through specific embodiments. It should be understood that the described embodiments are only for explaining the present invention and do not limit the present invention.

[0062] Unless otherwise specified, the experimental methods used in the following examples are conventional methods in the art. Unless otherwise specified, the materials and reagents used in the following examples are commercially available.

[0063] The specific embodiments and comparative examples of this invention are all based on conventional lithium-ion battery stacking production processes. The positive electrode active material used is lithium iron phosphate (LFP), with a true density of 3.55 g / cm³. 3 The negative electrode active material is graphite, with an actual density of 2.2 g / cm³. 3 All samples used the same electrolyte, diaphragm, and current collector. To ensure the comparability of the test results, all samples were kept consistent in terms of main material formulation, total coating thickness, baking process, liquid injection volume, chemical composition and compatibility process, and test environment temperature (25℃). The only difference was in the compaction structure and parameters of the electrode after roller pressing, so as to eliminate the interference of irrelevant variables on the test results.

[0064] All prepared cells are square stacked lithium batteries, with a total coating thickness of 200 μm for the positive electrode and 170 μm for the negative electrode, and a standard capacity of 2 Ah. Performance test indicators include: internal resistance, first charge / discharge efficiency at 1C rate, capacity retention rate at 2C rate, capacity retention rate after 1000 cycles, and electrode powder shedding rate.

[0065] Example 1

[0066] This embodiment discloses a positive electrode with a continuous linear gradient structure.

[0067] The positive electrode includes a current collector and active material layers disposed on the upper and lower surfaces of the current collector. Along the thickness direction of the electrode, from the current collector towards the two diaphragm sides, the compaction density of the active material layers gradually decreases, while the porosity gradually increases, forming a bidirectional symmetrical gradient structure centered on the current collector. The compaction density and porosity satisfy ε = 1 - ρ / ρ z The relationship is given by ε, where ε is the local porosity of the active material layer, ρ is the local compaction density of the active material layer, and ρ z This represents the true density of the active material.

[0068] Along the thickness direction from the current collector side to the diaphragm side, the parameters change smoothly and gradually without structural abrupt changes. Specifically, the compaction density linearly decreases from 2.6 g / cm³ on the current collector side to 2.2 g / cm³ on the diaphragm side, with a difference of 0.4 g / cm³ between the two sides; the porosity linearly increases from 27% on the current collector side to 38% on the diaphragm side, with a difference of 11% between the two sides. A highly dense conductive region is formed on the current collector side, while a highly ion-permeable region is formed on the diaphragm side.

[0069] A schematic diagram of the microstructure of the fabricated positive electrode with a continuous linear gradient structure is shown below. Figure 1 SEM images of different locations on the electrode cross-section, such as Figure 2 It can be seen that the density change of the active material in different regions after compaction is as follows: along the thickness direction of the electrode, from the current collector to the diaphragm side on both sides, the compaction density of the active material layer gradually decreases, while the porosity gradually increases.

[0070] This positive electrode sheet is used in conjunction with a conventional homogeneous negative electrode to prepare bare cells and finished batteries. During assembly, the electrodes are stacked with the low-compact density side facing the adjacent separator.

[0071] Example 2

[0072] This embodiment discloses a positive LFP segmented stepped gradient structure electrode.

[0073] The positive electrode includes a current collector and active material layers disposed on the upper and lower surfaces of the current collector. Along the thickness direction of the electrode, from the current collector to the two diaphragm sides, the compaction density of the active material layers decreases in a stepwise manner, while the porosity increases in a stepwise manner, forming a gradient structure that is bidirectionally symmetrical with the current collector as the center.

[0074] Two stepped regions (N=2) with different compaction densities are sequentially arranged from the current collector side to the diaphragm side in the single-sided active material layer; that is, the first stepped region and the second stepped region are arranged sequentially from the current collector side to the diaphragm side. The active material layers on the upper and lower surfaces of the current collector are symmetrical about the current collector, and the stepped regions corresponding to the numbered sequences have the same compaction density. From top to bottom, they are: upper diaphragm, upper second stepped region, upper first stepped region, current collector, lower first stepped region, lower second stepped region, and lower diaphragm. The compaction density of the upper and lower second stepped regions is the same, and the compaction density of the upper and lower first stepped regions is the same, but the compaction density of the first stepped region is higher than that of the second stepped region.

[0075] The change in compaction density Δ between adjacent positive steps + =0.2 / N=0.1g / cm 3 The average compaction density ρ of the positive electrode v + =2.5 / cm 3 .

[0076] The compaction density ρ of the i-th step region v,i + = ρ v + +Δ + ·Ki;

[0077] At this point, N=2 is an even number, and the intermediate parameter M= =1;

[0078] The rule for the value of Ki is as follows: from i=1 to 2, Ki takes the values ​​M, M-1, …, 1, -1, …, -M in sequence;

[0079] In this embodiment, M=1, therefore K1=1, K2=-1;

[0080] The compaction density ρ of the first-step region v,1 + = 2.5 + 0.1 × 1 = 2.6 g / cm³ 3 ;

[0081] The compaction density of the second-step region is ρ v,2 + = 2.5 + 0.1 × (-1) = 2.4 g / cm³.

[0082] The difference in compaction density between the current collector side and the diaphragm side is 0.2 g / cm³.

[0083] The porosity of each step is calculated according to ε = 1-ρ / ρ z It was determined that the porosity of the first step region was 27%, and the porosity of the second step region was 32%, with a difference of 5%.

[0084] A schematic diagram of the microstructure of the prepared positive electrode segmented stepped gradient structure is shown below. Figure 3 SEM images of different locations on the electrode cross-section, such as Figure 4 It can be seen that the density change of the active material in different regions after compaction is as follows: along the thickness direction of the electrode, from the current collector to the diaphragm side on both sides, the compaction density of the active material layer decreases in a stepwise manner, while the porosity increases in a stepwise manner.

[0085] This segmented gradient positive electrode is used in conjunction with a conventional homogeneous negative electrode to prepare bare cells and finished batteries. During assembly, the electrodes are stacked with the low-compact density side facing the adjacent separator.

[0086] Example 3

[0087] This embodiment discloses a negative electrode with a continuous linear gradient structure.

[0088] The negative electrode includes a current collector and active material layers disposed on the upper and lower surfaces of the current collector. Along the thickness direction of the electrode, from the current collector to the two diaphragm sides, the compaction density of the active material layers gradually decreases, while the porosity gradually increases, forming a bidirectional symmetrical gradient structure centered on the current collector.

[0089] Along the thickness direction from the current collector side to the diaphragm side, the parameters change smoothly and gradually without structural abrupt changes; specifically, the compaction density starts from 1.65 g / cm³. 3 The current collector side decreased linearly to 1.4 g / cm³. 3 The difference in compaction density between the diaphragm side and the current collector side is 0.25 g / cm³. 3 The porosity increases linearly from 25% on the current collector side to 36% on the separator side, with a porosity difference of 11% between the current collector side and the separator side. The current collector side constructs a dense conductive framework, while the high-porosity structure on the separator side is adapted for rapid lithium-ion insertion and extraction.

[0090] This gradient negative electrode is used in conjunction with a conventional homogeneous positive electrode to prepare bare cells and finished batteries. During assembly, the electrodes are stacked with the low-compact density side facing the adjacent separator.

[0091] Example 4

[0092] This embodiment discloses a negative electrode with a segmented stepped gradient structure.

[0093] The negative electrode includes a current collector and active material layers disposed on the upper and lower surfaces of the current collector. Along the thickness direction of the electrode, from the current collector to the two diaphragm sides, the compaction density of the active material layers decreases in a stepwise manner, while the porosity increases in a stepwise manner, forming a bidirectional symmetrical gradient structure centered on the current collector.

[0094] The single-sided active material layer has two stepped regions with different compaction densities arranged sequentially from the current collector side to the diaphragm side; that is, the first stepped region and the second stepped region from the current collector side to the diaphragm side. The active material layers on the upper and lower surfaces of the current collector are symmetrical about the current collector, and the stepped regions corresponding to the numbered sequences have the same compaction density. From top to bottom, they are: upper diaphragm, upper second stepped region, upper first stepped region, current collector, lower first stepped region, lower second stepped region, and lower diaphragm. The compaction density of the upper and lower second stepped regions is the same, and the compaction density of the upper and lower first stepped regions is the same, but the compaction density of the first stepped region is higher than that of the second stepped region.

[0095] The change in compaction density Δ of adjacent steps of the negative electrode - =0.15 / 2=0.075g / cm 3 The average compaction density ρ of the negative electrode v - =1.65g / cm3 .

[0096] The compaction density ρ of the i-th step region v,i - =ρ v - +Δ - ·Ki.

[0097] At this point, N=2 is an even number, and the intermediate parameter M= =1.

[0098] The rule for the value of Ki is: from i=1 to 2, Ki takes the values ​​1 and -1 respectively.

[0099] In this embodiment, K1=1, K2=-1.

[0100] The compaction density ρ of the first-step region v,1 - = 1.65 + 0.075 × 1 = 1.725 g / cm³ 3 ;

[0101] The compaction density ρ of the second-step region v,2 - = 1.65 + 0.075 × (-1) = 1.575 g / cm³ 3 .

[0102] The difference in compaction density between the first-step region on the current collector side and the second-step region on the diaphragm side is 0.15 g / cm³. 3 .

[0103] The porosity of each step is calculated according to ε = 1-ρ / ρ z It is determined that the porosity of the first step region is 22%, and the porosity of the second step region is 28%.

[0104] This gradient negative electrode is used in conjunction with a conventional homogeneous positive electrode to prepare bare cells and finished batteries. During assembly, the electrodes are stacked with the low-compact density side facing the adjacent separator.

[0105] Example 5

[0106] This embodiment discloses a synchronous continuous linear gradient structure for positive and negative poles.

[0107] The positive electrode uses the same continuous linear gradient parameters as in Example 1: compaction density from 2.6 g / cm³. 3 The current collector side decreased linearly to 2.2 g / cm³. 3 On the diaphragm side, the difference between the two is 0.4 g / cm. 3The porosity linearly increases from 27% on the current collector side to 38% on the separator side, with a difference of 11%. The negative electrode uses the same continuous linear gradient parameters as in Example 3: compaction density from 1.65 g / cm³... 3 The current collector side decreased linearly to 1.4 g / cm³. 3 On the diaphragm side, the difference between the two is 0.25 g / cm. 3 The porosity increases linearly from 25% on the current collector side to 36% on the diaphragm side, with a difference of 11%.

[0108] Both the positive and negative electrode sheets are alternately stacked with the low compaction density side facing the adjacent separator to prepare bare cells and finished batteries.

[0109] Example 6

[0110] This embodiment discloses a segmented stepped gradient structure for synchronous positive and negative electrodes.

[0111] The positive electrode uses the same segmented stepped gradient parameters as in Example 2: the compaction density on the current collector side of the first step region is 2.6 g / cm³. 3 The porosity is 27%, and the compaction density on the diaphragm side of the second-step region is 2.4 g / cm³. 3 The porosity is 32%. The negative electrode uses the same segmented stepped gradient parameters as in Example 4: the compaction density of the current collector side in the first step region is 1.725 g / cm³. 3 The porosity is 22%, and the compaction density on the diaphragm side of the second-step region is 1.575 g / cm³. 3 The porosity is 28%, and the compaction density difference is 0.15 g / cm³. 3 .

[0112] Both the positive and negative electrode sheets are alternately stacked with the low compaction density side facing the adjacent separator to prepare bare cells and finished batteries.

[0113] Comparative Example 1

[0114] Conventional homogeneous positive electrode sheets were prepared using existing traditional roll pressing technology. The sheet exhibits uniform compaction density and porosity without gradient along its thickness direction, but suffers from the inherent structural defect of a denser top and looser bottom characteristic of traditional processes. The overall compaction density is 2.4 g / cm³. 3 The porosity is 25%. This electrode has insufficient porosity on the diaphragm side, which hinders ion transport, and relatively insufficient compactness and poor electronic conductivity on the current collector side, which is a common traditional structure in the industry.

[0115] Bare cells and finished batteries were prepared using the same conventional negative electrode, separator, and electrolyte as in Examples 1 and 2.

[0116] Comparative Example 2

[0117] Conventional homogeneous negative electrode sheets were prepared using existing traditional roll forming processes. These sheets exhibit uniform compaction density and porosity without gradients along their thickness direction, exhibiting the inherent structural defect of a denser top and looser bottom characteristic of traditional processes. The overall compaction density is 1.6 g / cm³. 3 The porosity is 27%.

[0118] Bare cells and finished batteries are prepared by combining conventional homogeneous cathodes.

[0119] Example 7

[0120] This embodiment prepares a battery comprising a casing, an electrolyte, and a bare cell. The bare cell is encapsulated inside the casing, and the electrolyte impregnates and coats the electrode plates and separator pores of the bare cell.

[0121] The specific method for manufacturing the battery is as follows:

[0122] According to the electrode configurations described in each embodiment and comparative example, corresponding positive and negative electrodes were taken, Celgard 2500 separators were selected, and a 1.2 mol / L LiPF6 EC / DEC volume ratio 1:1 solution was used as the electrolyte. A 2Ah square soft-pack bare cell was assembled using conventional stacking technology, and then packaged to complete the battery fabrication. The batteries obtained in Examples 1 to 6 are designated as batteries E1 to E6, respectively, and the batteries obtained in Comparative Examples 1 to 2 are designated as batteries C1 to C2, respectively.

[0123] Effect Experiment Example

[0124] The internal resistance, first charge-discharge efficiency, 2C rate discharge capacity retention rate, 1000-cycle capacity retention rate, and electrode powder shedding rate of batteries E1 to E6 and batteries C1 to C2 prepared in Example 7 were tested, and the results are shown in Table 1.

[0125] Table 1 Comparison of core performance results of each group of samples

[0126]

[0127] As shown in Table 1, compared with Comparative Example 1 and Comparative Example 2, the batteries of Examples 1 to 8 show significant improvements in internal resistance, initial charge-discharge efficiency, 2C rate discharge capacity retention, 1000-cycle capacity retention, and electrode powder shedding rate.

[0128] Among them, Example 5, employing a dual-gradient synergistic continuous linear gradient scheme for both positive and negative electrodes, exhibited the best performance, with an internal resistance of 10.1 mΩ, an initial charge-discharge efficiency of 95.6%, a 2C rate discharge capacity retention rate of 94.2%, a 1000-cycle capacity retention rate of 91.5%, and an electrode powder shedding rate of 0.08%. Example 6, employing a dual-gradient synergistic segmented stepped gradient scheme for both positive and negative electrodes, ensured electrochemical performance while also considering industrial mass production suitability, with an internal resistance of 10.4 mΩ, an initial charge-discharge efficiency of 95.2%, a 2C rate discharge capacity retention rate of 93.5%, a 1000-cycle capacity retention rate of 90.8%, and an electrode powder shedding rate of 0.10%.

[0129] Example 8

[0130] This embodiment discloses a positive electrode with a segmented stepped gradient structure, which differs from Embodiment 2 in that the active material layer on one side is provided with three stepped regions.

[0131] The positive electrode includes a current collector and active material layers disposed on the upper and lower surfaces of the current collector. Along the thickness direction of the electrode, from the current collector to the two diaphragm sides, the compaction density of the active material layers decreases in a stepwise manner, while the porosity increases in a stepwise manner, forming a gradient structure that is bidirectionally symmetrical with the current collector as the center.

[0132] The single-sided active material layer has three stepped regions with different compaction densities arranged sequentially from the current collector side to the diaphragm side: the first stepped region, the second stepped region, and the third stepped region. The active material layers on the upper and lower surfaces of the current collector are symmetrical about the current collector, and the corresponding stepped regions have the same compaction density: from top to bottom, they are the upper diaphragm, the upper third stepped region, the upper second stepped region, the upper first stepped region, the current collector, the lower first stepped region, the lower second stepped region, the lower third stepped region, and the lower diaphragm. The corresponding stepped regions on both the upper and lower sides have the same compaction density, and from the current collector side to the diaphragm side, the first stepped region has the highest compaction density, the second stepped region is in the middle, and the third stepped region has the lowest.

[0133] N=3 is an odd number, so we take the intermediate parameter M= =1, N mod 2=1.

[0134] The change in compacted density of the adjacent steps at the positive electrode is Δ+ = 0.2 / N = 0.2 / 3 ≈ 0.0667 g / cm³. 3 The average compaction density ρ of the positive electrode v + =2.5g / cm 3 .

[0135] The compaction density ρ of the i-th step region v,i+ =ρ v + +Δ + ·Ki.

[0136] The rules for choosing the value of Ki are as follows:

[0137] i=1≤M=1: K1 = M-i+1 = 1-1+1 = 1;

[0138] N is odd and i = M + 1 = 2: K² = 0;

[0139] i=3>M+(N mod 2)=1+1=2, j=iM-(N mod 2)=3-1-1=1, K3 = -j = -1.

[0140] That is, from i=1 to 3, Ki takes the values ​​1, 0, and -1 respectively.

[0141] The compaction density ρ of the first-step region v,1 + = 2.5 + 0.0667 × 1 = 2.5667 g / cm³ 3 ;

[0142] The compaction density ρ of the second-step region v,2 + = 2.5 + 0.0667 × 0 = 2.5 g / cm³ 3 ;

[0143] The compaction density ρ of the third-step region v,3 + = 2.5+0.0667×(-1) = 2.4333g / cm 3 .

[0144] The compaction density of the three stepped regions is distributed in a symmetrical arithmetic sequence with the second stepped region as the center, decreasing step by step from the collector side to the diaphragm side.

[0145] The porosity of each step is calculated according to ε = 1 - ρ / ρ z Sure.

[0146] This segmented gradient positive electrode is used in conjunction with a conventional homogeneous negative electrode to prepare bare cells and finished batteries. During assembly, the electrodes are stacked with the low-compact density side facing the adjacent separator.

[0147] This embodiment discloses a negative electrode with a segmented stepped gradient structure, which differs from Embodiment 4 in that the active material layer on one side is provided with three stepped regions.

[0148] The negative electrode includes a current collector and active material layers disposed on the upper and lower surfaces of the current collector. Along the thickness direction of the electrode, from the current collector to the two diaphragm sides, the compaction density of the active material layers decreases in a stepwise manner, while the porosity increases in a stepwise manner, forming a bidirectional symmetrical gradient structure centered on the current collector.

[0149] The single-sided active material layer has three stepped regions with different compaction densities arranged sequentially from the current collector side to the diaphragm side: the first stepped region, the second stepped region, and the third stepped region. The active material layers on the upper and lower surfaces of the current collector are symmetrical about the current collector, and the corresponding stepped regions have the same compaction density: from top to bottom, they are the upper diaphragm, the upper third stepped region, the upper second stepped region, the upper first stepped region, the current collector, the lower first stepped region, the lower second stepped region, the lower third stepped region, and the lower diaphragm. The corresponding stepped regions on both the upper and lower sides have the same compaction density, and from the current collector side to the diaphragm side, the first stepped region has the highest compaction density, the second stepped region is in the middle, and the third stepped region has the lowest.

[0150] N=3 is an odd number, so we take the intermediate parameter M= =1, N mod 2=1.

[0151] The change in compacted density of the adjacent steps of the negative electrode is Δ-=0.15 / N=0.05g / cm³. 3 The average compaction density ρ of the negative electrode v - =1.6g / cm 3 .

[0152] The compaction density ρ of the i-th step region v,i - = ρ v - +Δ - ·Ki.

[0153] The rules for choosing the value of Ki are as follows:

[0154] i=1≤M=1: K1 = M-i+1 = 1-1+1 = 1;

[0155] N is odd and i = M + 1 = 2: K² = 0;

[0156] i=3>M+(N mod 2)=1+1=2, j=iM-(N mod 2)=3-1-1=1, K3 = -j = -1.

[0157] That is, from i=1 to 3, Ki takes the values ​​1, 0, and -1 respectively.

[0158] The compaction density ρ of the first-step regionv,1 - = 1.6 + 0.05 × 1 = 1.65 g / cm³ 3 ;

[0159] The compaction density ρ of the second-step region v,2 - = 1.6 + 0.05 × 0 = 1.6 g / cm³ 3 ;

[0160] The compaction density ρ of the third-step region v,3 - = 1.6 + 0.05 × (-1) = 1.55 g / cm³ 3 .

[0161] The compaction density of the three stepped regions is distributed in a symmetrical arithmetic sequence with the second stepped region as the center, decreasing step by step from the collector side to the diaphragm side.

[0162] The porosity of each step is calculated according to ε = 1 - ρ / ρ z Sure.

[0163] This segmented gradient negative electrode is used in conjunction with a conventional homogeneous positive electrode to prepare bare cells and finished batteries. During assembly, the electrodes are stacked with the low-compact density side facing the adjacent separator.

[0164] In other embodiments, the main components of the active material layer include active materials, binders, and conductive agents; wherein, the active material of the positive electrode is one or more selected from lithium cobalt oxide, ternary materials, lithium manganese oxide, lithium iron phosphate, lithium manganese iron phosphate, and sodium-ion positive electrode materials; and the active material of the negative electrode is one or more selected from graphite, hard carbon, soft carbon, silicon carbide, lithium titanate, and metal oxides. The difference in compaction density between the current collector side and the separator side of the positive electrode sheet can also be adjusted according to actual performance requirements. For example, the difference in compaction density of the positive electrode sheet can also be 0.1 g / cm³. 3 0.2g / cm 3 0.3g / cm 3 or 0.5g / cm 3 The compaction density difference of the negative electrode sheet can also be 0.05 g / cm³. 3 0.10 g / cm 3 0.15g / cm 3 0.20g / cm 3 Or 0.30g / cm 3 When the difference is small, the gradient change tends to be gentle, and the overall uniformity of the electrode is higher; when the difference is large, the functional zoning of the current collector side and the diaphragm side is more significant, and the differentiated optimization effect of electron transport and ion transport is more prominent. The adjustment of the above difference range can be achieved by changing the rolling process parameters, and all fall within the protection scope of the claims of this invention.

[0165] In the above-described segmented stepped gradient structure embodiments, N=2 and N=3 are used as examples. In other embodiments, the total number of steps N in the single-sided active material layer can also be 4, 5, 6, 7, 8, 9, or 10. As N increases, the change in compaction density Δ between adjacent steps decreases accordingly, the parameter transition between each step is smoother, and the overall gradient distribution is closer to a continuous linear change. The value of N can be selected according to the cell performance requirements and process precision. When N is smaller, the process is simpler and the adaptability is better; when N is larger, the gradient transition is smoother and the electrochemical performance is better.

[0166] In other embodiments, an electrode with a differential gradient structure includes a current collector and active material layers disposed on the upper and lower surfaces of the current collector. Along the thickness direction of the electrode, from the current collector to one side of the separator, the compaction density of the active material layers decreases and the porosity increases, forming a differential gradient structure that changes from the surface of the current collector to the separator side. From the current collector to the other side of the separator, the compaction density of the active material layers increases.

[0167] In this invention, the gradient changes in compaction density and porosity are implemented in two forms: a gradually decreasing distribution and a step-decreasing distribution. The advantage of a continuous linear gradient structure lies in the smooth change of parameters along the thickness direction, the absence of abrupt interface changes, a more continuous transition in ion transport and electronic conduction, and optimal electrochemical performance. The advantage of a segmented step-gradient structure is its simpler manufacturing process; the parameters within a single layer are uniform, and gradient differences can be formed simply by controlling the pressure of the rollers in different layers. This makes it more suitable for the segmented roller pressing process of existing industrial roller pressing lines, resulting in higher mass production feasibility. In practical applications, the appropriate variation mode can be selected based on the cell performance targets and production line conditions.

[0168] Based on the disclosure of this invention, those skilled in the art can make equivalent modifications or adaptive adjustments to similar solutions within the scope of the claims. If the modified or adjusted solution includes all the technical features described in the claims, it falls within the protection scope of this invention. It should be specifically noted that the materials involved in the embodiments are all commercially available products or prepared using methods known in the art; the content of these embodiments is not intended to limit the scope of protection of the claims.

Claims

1. A pole piece having a differentiated gradient structure, comprising a current collector and active material layers respectively arranged on the upper and lower surfaces of the current collector, characterized in that, Along the thickness direction of the electrode, from the current collector to one or both sides of the diaphragm side, the compaction density of the active material layer decreases and the porosity increases, forming a differentiated gradient structure that changes from the surface of the current collector to the diaphragm side.

2. The electrode with a differentiated gradient structure according to claim 1, characterized in that, The decreasing distribution is a gradually decreasing distribution or a step-decreasing distribution; The increasing distribution is either a gradually increasing distribution or a step-like increasing distribution.

3. The pole piece having a differentiated gradient structure according to claim 2, characterized in that, The decreasing distribution is a gradually decreasing distribution; When the electrode sheet is a positive electrode sheet, the difference between the compaction density on the side of the current collector and the compaction density on the side of the separator in the active material layer is 0.1 to 0.5 g / cm 3 ; When the electrode sheet is a negative electrode sheet, the difference between the compaction density on the side of the current collector and the compaction density on the side of the separator in the active material layer is 0.05 to 0.3 g / cm 3 .

4. The pole piece having a differentiated gradient structure according to claim 3, wherein, The compaction density and porosity of the active material layer satisfy the following relationship: ε = 1 - p / p z where ε is the local porosity of the active material layer, p is the local compacted density of the active material layer, p z is the true density of the active material.

5. The pole piece having a differentiated gradient structure according to claim 2, wherein, The decreasing distribution is a stepped decreasing distribution; N stepped regions with different compaction densities are sequentially set from the current collector side to the diaphragm side in the single-sided or double-sided active material layer, where N is an integer and N≥2; Using i as the step number, from the current collector side to the diaphragm side, the numbers are i = 1, 2, ..., N; When the pole piece is a positive pole piece, the compaction density p of the i th step region v,i + = p v + + Δ + · Ki; Where, ρ v + It is the average compaction density of the positive electrode active material layer; Δ + Δ represents the change in compaction density between adjacent steps of the positive electrode. + =0.2 / N; Ki is the coefficient corresponding to the step number i, and the rules for the value of Ki are as follows: With M as an intermediate parameter, M= ,in, The floor symbol; When N is odd, starting from i=1,2,…,N, Ki takes the values: M, M-1,…,1,0, -1, …, -M; When N is even, Ki takes the following values ​​from i=1,2,…,N: M, M-1, …, 1, -1, …, -M.

6. An electrode with a differentiated gradient structure according to claim 5, characterized in that, When the electrode is a negative electrode, the negative electrode compaction density ρ of the i-th step v,i - = ρ v - + Δ - ·Ki; Where, ρ v - The average compaction density of the negative electrode in a single-sided active material layer; Δ - The amount of change in the compaction density of the negative electrode adjacent step, Δ - = 0.15 / N.

7. An electrode with a differentiated gradient structure according to claim 5 or 6, characterized in that, The stepped regions with corresponding serial numbers in the active material layers on the upper and lower surfaces of the current collector have the same or different compaction densities.

8. A bare cell characterized by, It includes a multilayer diaphragm and alternating positive and negative electrode sheets, wherein the positive and / or negative electrode sheets are electrode sheets with differentiated gradient structures as described in any one of claims 1 to 7; In the active material layer of the electrode, the compaction density on the side near the separator is lower than the compaction density on the side near the current collector, and the electrodes are stacked with the lower compaction density side facing the adjacent separator.

9. A battery, characterized by It includes a casing, an electrolyte, and the bare battery cell as described in claim 8.

10. The battery of claim 9, wherein, The bare battery cell is encapsulated inside the housing, and the electrolyte impregnates and coats the electrode plates and diaphragm pores of the bare battery cell.

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