CN116470003A - Pre-lithiated negative electrode piece and lithium ion battery - Google Patents

Abstract

The invention discloses a pre-lithiated negative electrode sheet and a lithium ion battery, comprising a lithium powder layer coated on the surface of the negative electrode sheet; it meets the following requirements: 0.1≤(D1×W1)/(D2×ε×W2×Ks×10⁹)≤20; wherein, D1 is the particle size D50 of the lithium powder particles, the unit is μm; W1 is the coating amount of lithium powder particles per unit area, which only contains lithium powder particles, and the unit is g/m²; D2 is the particle size D50 of the negative electrode material, and the unit is μm; ε is the porosity of the negative electrode sheet; W2 is the coating amount of the negative electrode material per unit area, and the unit is g/m²; Ks is the lithium ion diffusion coefficient of the negative electrode material, and the unit is m²/s.

Description

A kind of pre-lithiated negative pole piece and lithium ion battery

technical field

The invention relates to the technical field of a lithium ion battery, in particular to a pre-lithiated negative pole piece and a lithium ion battery.

Background technique

The statements herein merely provide background information related to the present invention and do not necessarily constitute prior art.

With the use of lithium-ion batteries in electric vehicles, smart grids, distributed energy storage and other scenarios, people have put forward higher requirements for the cycle life of lithium-ion batteries. However, during the first charge and discharge process of lithium-ion batteries, severe irreversible capacity loss is common, mainly because the formation of SEI film on the surface of the negative electrode will consume a large amount of active lithium. At present, the most widely used graphite material has an irreversible lithium loss of more than 6% for the first time, and for silicon-based and tin-based alloy anodes with high specific capacity, the first irreversible lithium loss is even as high as 10% to 20%. Pre-lithiation compensates for the first capacity loss of the battery by storing lithium ions in the electrode in advance, which can effectively improve the capacity and cycle stability of the battery. Therefore, pre-lithiation technology is considered to be an effective solution to the loss of lithium in the negative electrode.

The current mainstream pre-lithiation schemes are mainly divided into positive electrode pre-lithiation and negative electrode pre-lithiation. Among them, the positive electrode pre-lithiation mainly uses lithium-rich materials or binary lithium compounds as pre-lithiation additives, but these materials generally have disadvantages such as poor stability, high-pressure decomposition, and low lithiation efficiency. Negative electrode pre-lithiation mainly includes metal lithium physical mixing pre-lithiation, self-discharge lithiation, chemical pre-lithiation, electrochemical lithiation and other pre-lithiation methods, among which the research on metal lithium physical mixing pre-lithiation technology is more extensive. Although the lithiation efficiency of metal lithium pre-lithiation is higher than that of positive electrode pre-lithiation additives, metal lithium cannot be completely embedded in the negative electrode material during the pre-lithiation process, and a passivation layer will be formed on the surface of the unembedded part, which will lose electronic conductivity and become "dead lithium", causing serious safety hazards.

Contents of the invention

Aiming at the deficiencies in the prior art, the object of the present invention is to provide a pre-lithiated negative pole piece and a lithium ion battery.

In order to achieve the above object, the present invention is achieved through the following technical solutions:

In a first aspect, the present invention provides a pre-lithiated negative electrode sheet, including a lithium powder layer coated on the surface of the negative electrode sheet; it meets the following requirements:

0.1≤(D1×W1)/(D2×ε×W2×Ks×10 ⁹ )≤20;

Wherein, D1 is the particle size D50 of the lithium powder particles, in μm;

W1 is the coating amount of lithium powder particles per unit area, which only contains lithium powder particles, and the unit is g/m ² ;

D2 is the particle size D50 of the negative electrode material, in μm;

ε is the porosity of the negative pole piece;

W2 is the coating amount of negative electrode material per unit area, and the unit is g/m ² ;

Ks is the lithium ion diffusion coefficient of the negative electrode material, and the unit is m ² /s.

The particle size of the lithium powder will affect the conversion efficiency of the lithium source. After liquid injection, due to the potential difference between the lithium metal and the negative electrode material, the electrons in the lithium metal are embedded in the negative electrode material under the potential, and the lithium ions are extracted from the lithium metal and migrate to the negative electrode material through the electrolyte to achieve charge balance. Finally, the negative electrode material is pre-lithiated. The larger the particle size of the lithium powder particles, the lower the efficiency of converting metallic lithium into lithium ions during the pre-lithiation process. A layer of SEI film will be formed on the surface of unconverted metallic lithium, which loses the electron transfer function and remains on the surface of the negative electrode material, thereby affecting the safety performance of the battery.

The coating amount W1 of lithium powder particles and the coating amount W2 of the negative electrode material jointly affect the proportion of active lithium in the negative electrode material after pre-lithiation. The higher the proportion of active lithium, the more obvious the improvement in cycle performance. During the pre-lithiation process, when the coating amount of the negative electrode is constant, increasing the coating amount W1 of metal lithium can increase the amount of active lithium inside the lithium-ion cell after the formation, thereby improving the cycle performance of the cell. If the coating amount is too small, the cycle performance will not be significantly improved; however, if the coating amount is too large, lithium ions cannot be embedded in the negative electrode material in time during pre-lithiation, which will lead to lithium precipitation and affect the safety performance of the cell. Therefore, choosing an appropriate amount of lithium powder coating can balance the cycle performance and safety performance of the battery.

ε represents the porosity of the negative electrode sheet. If the porosity is too high, the electronic impedance of the negative electrode material will increase, which will affect the rate and cycle performance of the battery. Too low porosity will reduce the electronic impedance of the negative electrode, but will reduce the ion transport rate, thereby affecting the pre-lithiation efficiency. Therefore, a suitable porosity can balance the electron transport and ion transport in the pre-lithiation process, and improve the conversion efficiency of lithium powder particles.

Ks is the lithium ion diffusion coefficient of the negative electrode material. The larger the Ks, the more favorable it is for the deintercalation of lithium ions. The smaller the Ks, the less favorable it is for the deintercalation of lithium ions, and the higher the risk of lithium deposition on the surface of the negative electrode.

Therefore, the particle size of lithium powder particles, the amount of lithium supplement per unit area, the particle size of the negative electrode material, the amount of negative electrode coating per unit area, the porosity of the negative electrode material layer, and the diffusion coefficient of the negative electrode material all affect the conversion efficiency of metal lithium during pre-lithiation, and have a significant impact on the cycle performance and interface conditions of the battery.

Therefore, in the pre-lithiated negative electrode sheet designed by the present invention, the above-mentioned several parameters are considered comprehensively, and when the relational formula of 0.1≤(D1×W1)/(D2×ε×W2×Ks×10 ⁹ )≤20 is satisfied, the conversion efficiency of the pre-lithiated lithium-ion battery can be optimally matched, the cycle performance of the lithium-ion battery is significantly improved, and at the same time, it has better safety.

在本发明的一些实施例中，当预锂化极片满足0.1≤(D1×W1)/(D2×ε×W2×Ks×10 ⁹ )＜1时，意味着补锂量W1较少及锂粉颗粒D1较小，使得参与预锂化过程的活性锂减少，产生“死锂”的概率减少，但对循环性能的改善也不明显；当预锂化极片满足15＜(D1×W1)/(D2×ε×W2×Ks×10 ⁹ )≤20时，意味着补锂量W1增加及锂粉颗粒D1较大，二者的增加会显著改善电芯的循环性能，同时产生“死锂”的风险也会加大，降低电芯的安全性能。 Therefore, it is very important to select the appropriate design parameters of the pre-lithiated negative electrode for the safety and cycle performance of the battery.

In some embodiments, the pre-lithiated negative electrode sheet meets the following requirements:

1≤(D1×W1)/(D2×ε×W2×Ks×10 ⁹ )≤15.

More preferably: 1≤(D1×W1)/(D2×ε×W2×Ks×10 ⁹ )≤5.

In some embodiments, the particle size D50 of the lithium powder in the lithium powder layer is 3-60 μm. Lithium powder with a smaller particle size is more difficult to produce and practically apply. Lithium powder smaller than 3 μm has higher activity and is more dangerous during production.

Preferably, the coating amount per unit area of the lithium powder layer is 1-30 g/m ² .

In some embodiments, the particle size D50 of the active material of the negative electrode sheet is 1-25 μm. Negative electrode particles within this range are beneficial to increase the contact points between the two, improve the conversion efficiency of lithium in the pre-lithiation process, and prevent the formation of "dead lithium".

Preferably, the area coating amount of the active material of the negative electrode sheet ranges from 60 to 120 g/m ² .

In some embodiments, the porosity of the negative electrode sheet is 10%-45%.

In some embodiments, the lithium ion diffusion coefficient of the negative electrode sheet is 10 ⁻¹³ to 10 ⁻¹² m ² /s.

In some embodiments, the active material of the negative electrode sheet is selected from artificial graphite, natural graphite, activated carbon, silicon carbon material, hard carbon, soft carbon, mesocarbon microspheres, lithium titanate or a combination thereof.

Lithium powder particles include any coatable lithium powder particle-containing slurries.

In a second aspect, the present invention provides a lithium ion battery, the negative pole piece of which is the pre-lithiated negative pole piece.

In some embodiments, the lithium-ion battery includes a pre-lithiated negative pole piece, a positive pole piece, a separator, and an electrolyte;

The positive electrode sheet active material is selected from one or a combination of layered positive electrode active materials, spinel-type positive electrode active materials, olivine-type positive electrode active materials, and metal sulfides.

The beneficial effects obtained by one or more embodiments of the present invention are as follows:

The negative electrode pre-lithiated pole piece of the present invention satisfies 0.1≤(D1×W1)/(D2×ε×W2×Ks×10 by regulating the relationship between the physical property parameters of the surface lithium powder particle layer and the active material layer⁹) ≤ 20, the contact points between the lithium powder particles and the negative electrode active material particles are increased, and the metal lithium source can be rapidly oxidized and embedded in the negative electrode material after the lithium powder particles are pre-lithiated.

Description of drawings

The accompanying drawings constituting a part of the present invention are used to provide a further understanding of the present invention, and the schematic embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations to the present invention.

Fig. 1 is a structural diagram of a lithium-replenishing pole piece for a negative electrode of a lithium-ion battery according to the present invention.

Detailed ways

It should be noted that the following detailed description is exemplary and intended to provide further explanation of the present invention. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

An embodiment of the present invention provides a lithium ion battery, comprising the pre-lithiated negative electrode sheet, positive electrode sheet, separator, and electrolyte. The above-mentioned positive electrode material, conductive carbon black, and binder are added to the solvent in a certain proportion to disperse to obtain a positive electrode slurry, and then the positive electrode sheet is obtained through coating, rolling and other processes; the above-mentioned negative electrode material, conductive carbon black, binder, and dispersant are added to the solvent according to a certain ratio to disperse to obtain the negative electrode slurry, and then the negative electrode sheet is obtained through coating, rolling and other processes, and then the lithium powder particles are coated on the surface of the negative electrode sheet to obtain a pre-lithiated negative electrode sheet; Negative pole pieces and separators are assembled into batteries by winding or stacking, and then lithium-ion batteries are obtained after liquid injection, chemical formation, and capacity separation.

The present invention will be further described below in conjunction with embodiment.

Example 1

A lithium ion battery comprises a positive pole piece, a negative pole piece, a separator between the positive pole piece and the negative pole piece, and an electrolyte. Among them, the positive electrode sheet is coated with lithium iron phosphate as the positive active material on the aluminum foil to obtain the positive electrode sheet; the negative electrode sheet is coated on the copper foil with a D50 of 19 μm and a lithium ion diffusion ^coefficient Ks of 6*10 ^-13 m ² /s. The powder particles are coated on the surface of the negative electrode sheet, and the coating surface density is 2g/m ² to obtain a pre-lithiated negative electrode sheet; and then assembled to obtain a lithium ion battery.

Example 2

The negative pole tablets use D50 to 12 μm, and the lithium -ion diffusion coefficient KS is 9*10 ^-13 m ² /s graphite as the negative active material to apply it to the copper foil. The single surface density is 64g /m ² , and the thickened thickness of the pore rate is 25.4 %. The polar tablet surface, the single surface density is 3g/m ² .

Other conditions and parameters are exactly the same as those in Embodiment 1, and will not be repeated here.

Example 3

The negative pole sheet uses the D50 to 8 μm, and the lithium -ion diffusion coefficient KS is 10*10 ^-13 m ² /s graphite as the negative active material on the copper foil. The single surface density is 64g /m ² , and the thickened thickness of the pore rate is 25.4 %. The polar tablet surface, the single surface density is 5g/m ² .

Other conditions and parameters are exactly the same as those in Embodiment 1, and will not be repeated here.

Example 4

The negative pole sheet uses the D50 to be 25 μm, and the lithium -ion diffusion coefficient KS is 6*10 ^-13 m ² /s graphite as a negative active material on the copper foil. The single surface density is 64g /m ² , and the thickened thickness of the pore rate is 22.4 %. To the surface of the negative pole, the single surface density is 5g/m ² .

Other conditions and parameters are exactly the same as those in Embodiment 1, and will not be repeated here.

Comparative example 1

The negative electrode sheet uses graphite with a D50 of 19 μm and a lithium ion diffusion coefficient Ks of 6*10 ^-13 m ² /s as the negative electrode active material to be coated on the copper foil, with a surface density of 64g/m ² on one side of the coating, and a negative electrode sheet with a porosity of 25.4% obtained by controlling the rolling thickness; then, lithium powder particles with a D50 of 20 μm are coated on the surface of the negative electrode sheet, and the density on one side of the coating is 0.1g/m ² .

Other conditions and parameters are exactly the same as those in Embodiment 1, and will not be repeated here.

Comparative example 2

Negative electrode sheet uses artificial graphite with a D50 of 14 μm and a lithium ion diffusion coefficient Ks of 2*10 ^-13 m ² /s as the negative active material, and coats it on the copper foil with a surface density of 64 g/m ² , and obtains a negative electrode sheet with a porosity of 25.4% by controlling the rolling thickness; then, apply lithium powder particles with a D50 of 30 μm on the surface of the negative electrode sheet, and the coating surface density is 5 g/m ² .

Other conditions and parameters are exactly the same as those in Embodiment 1, and will not be repeated here.

Comparative example 3

Negative electrode sheet uses artificial graphite with a D50 of 16 μm and a lithium ion diffusion coefficient Ks of 1*10 ^-13 m ² /s as the negative electrode active material, and coats it on the copper foil with a surface density of 64g/m ² , and obtains a negative electrode sheet with a porosity of 36.7% by controlling the rolling thickness; then apply lithium powder particles with a D50 of 30 μm on the surface of the negative electrode sheet, and the coating surface density is 3g/m ² .

Other conditions and parameters are exactly the same as those in Embodiment 1, and will not be repeated here.

Performance Testing:

Porosity test method

Cut out an appropriate amount of pole piece, and record the mass of the pole piece as M ₀ ; measure the volume V of the pole piece; place the pole piece in a container, which is provided with hexadecane, soak the pole piece completely, and soak for a certain period of time; take out the pole piece, place it on filter paper, wipe it to a constant weight, and measure the mass M ₁ of the pole piece; according to the formula, ε=(M ₁ −M ₀ )/ρ/V×100%, calculate the porosity ε of the pole piece.

Wherein, the cut pole piece is a cuboid pole piece, the volume of the pole piece V=length×width×thickness, the thickness is the thickness of the pole piece minus the thickness of the foil; the hexadecane is analytically pure, and the p is the density of the hexadecane at room temperature;

Ks test method

For the reversible system controlled by the diffusion step, the chemical diffusion coefficient at room temperature was measured by cyclic voltammetry, and the negative electrode active material powder was assembled into a button battery, and the buttons were all in a completely delithiated state; cyclic voltammetry (C-V) scanning, scanning speed 0.1mv/s; and calculated according to the following formula.

Ip＝2.69×10 ⁵ n ^3/2 AKs ^1/2 v ^1/ 2ΔCo (2)

Where Ip is the magnitude of the peak current, n is the number of electrons participating in the reaction, A is the electrode area immersed in the solution, Ks is the diffusion coefficient of Li in the electrode, υ is the scan rate, and ΔCo is the change of Li concentration before and after the reaction.

Confirmation of the interface effect: Discharge the obtained lithium-ion battery at a rate of 0.5C to the discharge cut-off voltage, and after standing for 30 minutes, charge it at a constant current and constant voltage at a rate of 1C to the charge cut-off voltage, then disassemble the negative electrode sheet and observe the residual floating lithium on the surface of the negative electrode sheet. Among them, if the area of floating lithium remaining on the surface of the negative electrode is less than or equal to 0%, it is considered that the interface is good; if the area of floating lithium remaining on the surface of the negative electrode is less than 5%, it is considered as slight lithium precipitation;

Cycle test: Cycle test is carried out in accordance with the standard cycle life test requirements of GB/T 31484-2015 "Cycle Life Requirements and Test Methods for Traction Batteries for Electric Vehicles".

Specifically, the relevant parameters of Examples 1-4 and Comparative Examples 1-3 and the performance test results under the same conditions are shown in Table 1 below. Wherein, relational expression 1 refers to the calculated value of (D1×W1)/(D2×ε×W2×Ks*10 ⁹ ).

Table 1 embodiment and comparative example phase property contrast

It can be seen from Table 1 that the parameters of the negative electrode active material and the parameters of the lithium powder particle layer in Example 1 are satisfied, which can make the pre-lithiation effect of the metal lithium powder particles better (the pre-lithiation effect can be comprehensively judged by cycle performance and interface state, and there are many factors that affect the relationship, and the impact on performance is not linear), to ensure that the negative electrode is fully charged. The utilization rate of metal lithium is improved, and the cycle performance of the lithium-ion battery is increased, and the safety performance of the battery after pre-lithiation is also improved.

Compared with Example 1, Example 2 reduces the particle size of the negative electrode material, improves the utilization rate of lithium in the pre-lithiation process, and thus improves the cycle performance.

In Example 3, compared with Example 1, the particle diameters of the negative electrode particles and the lithium powder particles are reduced at the same time, which accelerates the conversion efficiency of lithium metal and is beneficial to the improvement of the cycle performance of the lithium ion battery.

Compared with Example 1, Comparative Example 1 reduces the coating amount of lithium powder particles and increases the coating amount of the negative electrode. Although the conversion time of lithium metal can be reduced and the negative electrode interface is guaranteed to be good, the available active lithium is also reduced, which cannot effectively improve the cycle performance of the lithium-ion battery.

In Comparative Example 2, compared with Example 1, the particle size of the lithium powder increases, the coating amount of the lithium powder increases, the contact points between the lithium powder particles and the negative electrode particles decrease, and the diffusion coefficient of the negative electrode is low. After the liquid injection, the metal lithium dissolves, an SEI film is formed on the surface of the lithium powder particles, and the electronic path between the lithium powder particles and the negative electrode particles is disconnected to form "dead lithium", resulting in a low utilization rate of the lithium metal source, resulting in lithium precipitation.

In Comparative Example 3, compared with Example 1, the porosity of the negative electrode sheet is increased and the diffusion coefficient is low, which is not conducive to the rapid intercalation of lithium ions. It is easy to cause serious lithium precipitation during pre-lithiation, which seriously reduces the safety performance of the battery cell.

It can be seen that the smaller the lithium powder particles and the negative electrode particles are, the more contact points between them will be increased, the conversion efficiency of lithium in the pre-lithiation process will be improved, and it is not easy to form "dead lithium". An increase in the coating amount of lithium powder particles is beneficial to improve the cycle performance of lithium-ion batteries, but if the lithium powder particles cannot be transformed in time during the pre-lithiation process, "dead lithium" will be formed instead, which will affect the cycle performance and safety performance of the battery. At the same time, the porosity and diffusion coefficient of the negative electrode sheet also affect the pre-lithiation. The larger the porosity, the fewer contact points between the negative electrode particles and the lower the diffusion coefficient, which is not conducive to the rapid insertion of lithium ions, thus affecting the cycle performance and safety performance of the battery.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. For those skilled in the art, the present invention may have various modifications and changes. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (10)

1. A pre-lithiated negative electrode sheet, characterized in that: comprise a lithium powder layer coated on the surface of the negative electrode sheet; it meets the following requirements: 0.1≤(D1×W1)/(D2×ε×W2×Ks×10 ⁹ )≤20; Wherein, D1 is the particle size D50 of the lithium powder particles, in μm; W1 is the coating amount of lithium powder particles per unit area, which only contains lithium powder particles, and the unit is g/m ² ; D2 is the particle size D50 of the negative electrode material, in μm; ε is the porosity of the negative pole piece; W2 is the coating amount of negative electrode material per unit area, and the unit is g/m ² ; Ks is the lithium ion diffusion coefficient of the negative electrode material, and the unit is m ² /s. 2. The pre-lithiated negative electrode sheet according to claim 1, wherein the pre-lithiated negative electrode sheet meets the following requirements: 1≤(D1×W1)/(D2×ε×W2×Ks×10 ⁹ )≤15; preferably 1≤(D1×W1)/(D2×ε×W2×Ks×10 ⁹ )≤5. 3. The pre-lithiated negative electrode sheet according to claim 1 or 2, characterized in that: the particle size D50 of the lithium powder in the lithium powder layer is 3-60 μm. 4. The pre-lithiated negative electrode sheet according to claim 1 or 2, characterized in that the coating amount per unit area of the lithium powder layer is 1-30 g/m ² . 5. The pre-lithiated negative electrode sheet according to claim 1 or 2, characterized in that: the particle size D50 of the active material of the negative electrode sheet is 1-25 μm. 6 . The pre-lithiated negative electrode sheet according to claim 1 or 2 , characterized in that the area coating amount of the active material of the negative electrode sheet is 60-120 g/m ² . 7. The pre-lithiated negative electrode sheet according to claim 1 or 2, characterized in that: the porosity of the negative electrode sheet is 10%-45%. 8. The pre-lithiated negative electrode sheet according to claim 1 or 2, characterized in that: the lithium ion diffusion coefficient of the negative electrode sheet is 10 ⁻¹³ to 10 ⁻¹² m ² /s. 9. according to claim 1 and 2 described pre-lithiation negative pole pieces, it is characterized in that: the active material of negative pole piece is selected from one or its combination in artificial graphite, natural graphite, activated carbon, silicon carbon material, hard carbon, soft carbon, mesocarbon microspheres, lithium titanate. 10. A lithium ion battery, characterized in that: its negative pole piece is the pre-lithiated negative pole piece.