CN113471561A

CN113471561A - Method for determining activation process of lithium ion battery

Info

Publication number: CN113471561A
Application number: CN202110741562.1A
Authority: CN
Inventors: 余大强; 肖艳; 何浩; 张富军; 张盟; 耿铁成; 范世军
Original assignee: Sichuan Changhong Battery Co ltd
Current assignee: Sichuan Changhong Battery Co ltd
Priority date: 2021-07-01
Filing date: 2021-07-01
Publication date: 2021-10-01
Anticipated expiration: 2041-07-01
Also published as: CN113471561B

Abstract

The invention relates to the technical field of battery activation, and provides a method for determining an activation process of a lithium ion battery in order to improve the efficiency of the activation process of the lithium ion battery containing silicon-based materials, which comprises the following steps: step A, preparing a simulated battery sample and obtaining an SOC-OCV curve and an SOC-expansion rate curve of the simulated battery sample; b, determining a target SOC interval according to the SOC-expansion rate curve; step C, determining a target OCV interval and a charging cut-off voltage corresponding to the target SOC interval according to the SOC-OCV curve; and D, performing battery activation by adopting a corresponding activation strategy according to the target OCV interval and the charge cut-off voltage. By adopting the method, the activation condition which is beneficial to the formation of the SEI film can be adopted in the target SOC interval more pertinently, and the repeated activation test of the lithium ion battery is avoided.

Description

Method for determining activation process of lithium ion battery

Technical Field

The invention relates to the technical field of battery activation, in particular to a method for determining an activation process of a lithium ion battery containing a silicon-based material cathode.

Background

After the lithium ion battery is assembled, the lithium ion battery needs to be activated. The activation is a first charging process of the battery after the liquid injection standing. A dense Solid Electrolyte Interface (SEI) film can be formed on the surface of the negative electrode by activation. The SEI film has a property of allowing lithium ions to freely pass through but insulating electrons. The SEI film can prevent further electrochemical reactions from occurring on the surface of the negative electrode, avoid the continuous consumption of lithium ions, and reduce the generation of various byproducts. The SEI film formation condition can affect the impedance of the negative electrode end and the subsequent electrochemical reaction, and also affect the consumption of active materials, thereby being particularly important for various performances of the battery. At the same time, the activation allows the gas generated by the electrochemical reaction during the initial charging to be removed.

Existing anode materials are classified into carbon materials and non-carbon materials, wherein graphite-based carbon materials are the most common anode materials, and non-carbon materials mainly include titanium-based materials and silicon-based materials. At present, because of higher gram capacity of silicon-based materials, the lithium ion battery containing the silicon-based materials has the obvious advantage of more appropriate potential, and the materials are environment-friendly and rich in resources and are a better choice for pursuing a lithium ion battery cathode material with higher energy density at present.

However, the significant drawbacks of silicon-based materials are: has a higher expansion rate than the carbon material during the activation process. Due to the high expansion rate, the material can continuously expose new interfaces during activation, so that new SEI films are continuously generated, dense SEI films cannot be formed, and finally, the electrochemical performance, particularly the cycle performance and the like of the battery are poor.

The lithium intercalation reaction mechanism of the silicon-containing based material is different from that of the conventionally used graphite based material. At present, theoretical understanding on the activation of the lithium ion battery containing the silicon-based material is not yet available. In developing a lithium ion battery containing a silicon-based material, it is a common practice to set activation parameters for activation based on experience, and then adjust the activation based on performance measurements of the lithium ion battery. However, since silicon-based materials have relatively high expansion rates, the performance differences of silicon-based materials from different sources, even from different batches, may cause large fluctuations in the performance of the battery. These results in that the activation of the lithium ion batteries containing silicon-based materials still has blindness, and the activation conditions of the lithium ion batteries containing silicon-based materials of different specifications and different batches need to be searched through repeated tests. This causes a long time and a large amount of resources to be consumed for adjusting the process parameters for activating the lithium ion battery containing the silicon-based material, and the development cycle of a new battery is also lengthened.

Disclosure of Invention

In order to improve the efficiency of the lithium ion battery activation process containing the silicon-based material, the invention provides a method for determining the lithium ion battery activation process.

The technical scheme adopted by the invention for solving the problems is as follows:

a method of determining a lithium ion battery activation process, comprising:

step A, preparing a simulated battery sample and obtaining an SOC-OCV curve and an SOC-expansion rate curve of the simulated battery sample;

b, determining a target SOC interval according to the SOC-expansion rate curve;

step C, determining a target OCV interval and a charging cut-off voltage corresponding to the target SOC interval according to the SOC-OCV curve;

and D, performing battery activation by adopting a corresponding activation strategy according to the target OCV interval and the charge cut-off voltage.

Further, the step B is to select an SOC interval with the interval expansion index I being more than or equal to 0.4 as the target SOC interval.

Further, the interval expansion index I is calculated in the following manner: where Pm and Pn are the start point expansion rate and the end point expansion rate corresponding to the section, respectively, and Sm and Sn are the start point SOC value and the end point SOC value corresponding to the section, respectively.

Further, the determination of the target SOC interval is performed in an SOC interval in which lithium intercalation from silicon is predominant in the electrochemical reaction of the negative electrode.

Further, the difference between the starting SOC and the ending SOC corresponding to the target SOC interval is greater than or equal to 5% and less than or equal to 30%.

Further, the corresponding activation strategy is to charge the voltage to Vm by adopting a first conventional charging rate, charge the voltage from Vm to Vn by adopting the first charging rate, then discharge to Vm by adopting a discharging rate, charge to Vn by adopting a second charging rate, and finally charge the voltage to a charging cut-off voltage by adopting the second conventional charging rate; vm and Vn are respectively the starting voltage and the ending voltage on the SOC-OCV curve corresponding to the target SOC interval.

Further, the first charge rate and the second charge rate are both in the range of 0.1C to 0.5C, and the discharge rate is in the range of 0.1C to 0.5C.

Further, the first and second charge rates are in a range of 0.1C to 0.2C, and the discharge rate is in a range of 0.2C to 0.4C.

Further, the first charging rate is equal to the second charging rate, and the discharging rate is 1.5-2.5 times of the first charging rate or the second charging rate.

Further, when there are a plurality of target SOC intervals, the corresponding activation strategy is applied to all the target SOC intervals.

Compared with the prior art, the invention has the beneficial effects that: the method determines a target SOC interval by obtaining a negative electrode expansion characteristic curve of the lithium ion battery to be activated in advance, and ensures that a proper activation process can be determined by determining the target SOC interval so as to obtain good battery performance through the process by adopting an activation condition which is favorable for SEI film formation in the target SOC interval in a more targeted manner in the subsequent steps; the repeated activation test of the lithium ion battery is avoided.

Drawings

FIG. 1 is a flow chart of a method of determining a lithium ion battery activation process according to the present invention;

FIG. 2 is a SOC-OCV curve and a SOC-expansion ratio curve corresponding to the example;

FIG. 3 is a graph comparing the cycle life curves of the examples;

reference numerals: A. the cycle life curves for example 2A, B and 2B, C and 2C, and D and comparative example 1.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is further described in detail with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention.

The method of the present invention is applicable to any form of lithium ion battery, including but not limited to ternary lithium ion batteries; the lithium ion battery is also not limited to any shape, for example, a cylindrical battery, a prismatic battery, a pouch battery, and the like. The lithium ion battery positive electrode material may be Lithium Cobaltate (LCO), lithium iron phosphate (LFP), lithium manganate, ternary material lithium Nickel Cobalt Manganese (NCM), and lithium Nickel Cobalt Aluminate (NCA), but is not limited thereto.

The method of the invention aims at that the cathode material of the lithium ion battery contains silicon-based materials. The silicon-based material refers to a silicon or silicon oxide material. The negative electrode material containing the silicon-based material is generally referred to as a silicon-carbon composite (Si/C) and/or a silicon oxide-carbon composite (SiOx/C). The current collector of the lithium ion battery may be a metal foil, such as a copper foil or an aluminum foil. The electrolyte of the lithium ion battery is not particularly limited, and can be prepared from a proper organic solvent and a proper electrolyte according to a certain proportion, and can also be purchased commercially.

The separator of the lithium ion battery generally employs a polyolefin porous membrane having high strength and being made into a thin film, and commonly used separators include a polypropylene and polyethylene microporous separator, a copolymer of propylene and ethylene, a polyethylene homopolymer film, and the like.

In this application, "SOC" is an english abbreviation for State of charge (State off charge), expressed in percent. "OCV" is an English abbreviation for Open Circuit Voltage (Open Circuit Voltage) in volts (V). The negative electrode expansion rate refers to the change rate of the thickness of the negative electrode piece relative to the thickness before measurement along with the increase of the SOC when the simulated battery sample is initially charged.

As shown in fig. 1, a method of determining an activation process for a lithium ion battery includes:

and step A, preparing a simulated battery sample by adopting a positive plate, a negative plate, a diaphragm and electrolyte which are the same as those of the lithium ion battery, obtaining an SOC-OCV curve of the simulated battery sample, and initially charging the simulated battery sample to obtain an SOC-expansion rate curve of the negative electrode.

The invention adopts an electrochemical reaction visualization Confocal System (namely the electrochemical reaction visualization Confocal System) to quantitatively measure the thickness change of a negative pole piece in real time to obtain an SOC-expansion rate curve. The present invention is not particularly limited to the measurement device for the SOC-expansion ratio curve of the negative electrode, and any device capable of quantitatively obtaining the change value of the thickness of the negative electrode with charging may be used in the present invention.

Corresponding simulated battery samples can be prepared for detection according to the requirements of specific equipment. The simulated battery sample is made into the size required by detection by adopting a positive plate, a negative plate, a diaphragm and electrolyte which are the same as those of the lithium ion battery to be activated. For example, the size of the simulated cell sample should be compatible with the mold of the electrochemical reaction visualization confocal system apparatus used for detection.

In order to obtain the SOC-expansion rate curve of the negative electrode, the above-described simulated battery sample was initially charged with a relatively small charge rate. The rate of initial charging may be 0.005-0.02C, for example, 0.01C, 0.015C, 0.02C, and the like. The SOC-expansion rate curve with clear change of the expansion rate of the negative electrode can be obtained by adopting small-rate charging, if the charging rate is too large, the expansion rate of the negative electrode in the whole detection process is increased too fast, and the difficulty of identifying the area with the too fast expansion of the silicon material from the curve is increased.

For the SOC-OCV curve, any method known to those skilled in the art may be used to obtain the SOC-OCV curve, for example, a battery test cabinet may be used to perform a charge and discharge test on the battery in a certain voltage interval to obtain a capacity and voltage curve of the battery. It can also be acquired by using an apparatus such as an electrochemical reaction visualization confocal system at the same time as the acquisition of the SOC-expansion ratio curve of the negative electrode.

And B, determining a target SOC interval according to the SOC-expansion rate curve. The interval expansion index I of the target SOC interval needs to meet the condition that I is more than or equal to 0.4. The section expansion index is a ratio of a change value of the negative electrode expansion rate to a change value of the SOC in a specific SOC section from the SOC-expansion rate graph of the negative electrode, and indicates a degree of the negative electrode expansion in the specific SOC section. The section expansion index may be calculated as I ═ p-Pm)/(Sn-Sm), where Pm and Pn are the start-point expansion rate and the end-point expansion rate, respectively, corresponding to the target SOC section, and Sm and Sn are the start-point SOC value and the end-point SOC value, respectively, of the target SOC section.

The determination of the target SOC interval can be performed in different ways as long as a suitable curve segment with an interval expansion index I of 0.4 or more can be determined.

The present invention provides two ways to determine the target SOC interval: (1) firstly, a section with a curve which is approximately linearly changed is selected from the SOC-expansion rate curve, then the section expansion index I of the SOC section corresponding to the section is calculated according to the I ═ Pm)/(Sn-Sm), and when the I is larger than or equal to 0.4, the SOC section corresponding to the section is determined as the target SOC section.

(2) Firstly, selecting a section between two adjacent platform areas with expansion rates which are substantially unchanged along with the increase of the SOC in an SOC-expansion rate curve, then calculating a section expansion index I of an SOC section corresponding to the section according to the ratio of I to (Pn-Pm)/(Sn-Sm), and determining the SOC section corresponding to the section as a target SOC section when the I is larger than or equal to 0.4. The specific manner in which the target SOC interval is determined may be determined according to the obtained specific SOC-expansion ratio curve, and is not limited herein.

When I is not less than 0.4, it is considered that the negative electrode expands more rapidly in the charge region, which affects the SEI film formation. Therefore, the corresponding SOC interval when the cathode expands faster can be accurately obtained according to the SOC-expansion rate curve, so that the activation process condition which is favorable for forming a compact SEI film is pertinently adopted in the interval, and the waste of time and cost caused by determining a better activation process through blind repeated tests in the conventional method is avoided.

During charging, the electrochemical reaction in the negative electrode containing the silicon-based material includes a reaction portion mainly composed of silicon intercalation and a reaction portion mainly composed of carbon intercalation. Among these, the reaction mainly involving silicon intercalation generally occurs in the first half of the charging reaction, i.e., generally corresponding to an SOC of 0% to about 50%, while the reaction mainly involving carbon intercalation generally occurs in the second half of the charging reaction, i.e., generally corresponding to an SOC of about 50% to 100%.

Since the high expansion rate characteristic of the silicon-based material is a key factor influencing the formation of the SEI film, the determination of the target SOC interval may be performed only in the SOC interval mainly based on the electrochemical reaction of the negative electrode, for example, the target SOC interval may be determined in the interval of 0% to 50% of the SOC value, but is not limited to the specific interval according to different lithium ion batteries. The specific electrochemical reaction mainly involving lithium intercalation with silicon corresponds to which part of the SOC interval, and those skilled in the art can determine the SOC interval according to the composition of the negative electrode active material of the lithium ion battery to be activated and the obtained negative electrode SOC-expansion rate curve in combination with experience.

In addition, in order to avoid the activation process from being too complicated due to the determination of a large number of target SOC intervals, a section having an SOC difference value of 5% or more and 30% or less may be selected to determine whether the section is the target SOC interval. The interval with the too small SOC difference value can be ignored, or a plurality of adjacent small intervals are combined into a larger interval to be considered, for example, the interval with the SOC difference value more than or equal to 5%, 6%, 7%, 8%, 9%, even more than 10% can be selected; an excessively large SOC interval may result in the finally determined activation process being too rough to obtain optimum performance such as cycle characteristics, and therefore a section having an SOC difference of 30%, 25%, 20%, or 15% or less may be selected.

The number of the finally determined target SOC intervals may be one or more according to the actual situation.

And step C, determining a target OCV interval and a charging cut-off voltage corresponding to the target SOC interval according to the SOC-OCV curve. After the target SOC interval is determined, at least one target SOC interval is selected, an OCV interval corresponding to the SOC interval, namely the target OCV interval is found by combining an SOC-OCV curve, and a starting point OCV value Vm and an end point OCV value Vn in the interval are determined.

According to the SOC value corresponding to the condition that the final expansion rate is not increased in the SOC-expansion rate curve, determining a corresponding OCV value V in the SOC-OCV curve_Cut-offAs a charge cut-off voltage for the lithium ion battery activation process. The final cut-off voltage determined according to the SOC-expansion rate curve can enable the activation effect to be better.

The corresponding activation strategy is to adopt a first conventional charging multiplying factor to charge the voltage to Vm, adopt the first charging multiplying factor to charge the voltage from Vm to Vn, then adopt a discharging multiplying factor to discharge to Vm, adopt a second charging multiplying factor to charge to Vn, and finally adopt the second conventional charging multiplying factor to charge the voltage to a charging cut-off voltage; the first charging rate and the second charging rate are in a range from 0.1C to 0.5C, and the first charging rate and the second charging rate may be respectively and independently 0.1C, 0.2C, 0.3C, 0.4C, 0.5C, or the like, that is, they may be equal or unequal.

For the discharge, 0.1C to 0.5C may be used, and for example, the discharge rate may be 0.1C, 0.2C, 0.3C, 0.4C, 0.5C, but is not limited thereto as long as it is within the above range.

Preferably, the first charge rate and the second charge rate are in the range of 0.1C to 0.2C, and the discharge rate is in the range of 0.2C to 0.4C. Further, the first charging rate is equal to the second charging rate, and the discharging rate is 1.5-2.5 times, such as 2 times, of the first charging rate or the second charging rate.

In the interval from Vm to Vn, the tendency of the negative electrode to swell is delayed by charging-discharging-charging at a higher rate than by directly charging from Vm to Vn at a higher rate, which contributes to the formation of a dense SEI film.

And when the target SOC intervals are multiple, adopting corresponding activation strategies for all the target SOC intervals. Corresponding activation strategies are adopted for all target SOC intervals, so that the negative electrode does not expand too fast in the whole activation process, the SEI film is ensured to be formed better, and the better battery cycle performance is obtained. However, it should be noted that the charge cut-off voltage is charged only in the last interval, and Vn in the previous interval is used as the initial charge voltage in the next interval, so that the continuous charge is realized.

If the finally determined target SOC intervals are too many, in order to consider the time efficiency problem of activation, some target SOC intervals may be selected to be ignored, for example, an interval in which the interval expansion index I is very close to 0.4 (e.g., 0.40 to 0.42), or an interval in which the interval SOC difference is small (e.g., 5% to 7%), or two or three adjacent smaller SOC intervals are combined into one large SOC interval to determine a corresponding strategy.

An empirical activation process may be employed for portions outside of the selected target SOC window, particularly for SOC windows where the electrochemical reaction of the negative electrode is dominated by carbon intercalation. For example, a conventional activation process for a lithium ion battery using a carbon material as a negative electrode, or an already-established activation process for a lithium ion battery having a similar negative electrode composition may be employed. Such an activation process may be exemplified by first charging with a current of a small rate and then charging with a current of a larger rate. For example, in the present invention, an activation strategy with a large constant current charge rate (0.1C to 0.5C) can be used for a region with an SOC of 50% to 100%.

The scheme of the invention is illustrated by comparative experiments below:

comparative example 1

This example is an empirically determined activation process.

Firstly, preparing a lithium ion battery containing a negative electrode of a silicon-based material, wherein:

negative electrode: the negative electrode coating material is prepared by mixing active substances, conductive carbon black SP, single-walled carbon nanotubes SWCNTs, styrene butadiene rubber SBR and sodium carboxymethylcellulose CMC in a proportion of 96.4% by weight of graphite, and the compacted density of the rolled negative electrode sheet is 1.65g/cm 3. And the negative coating material is coated on the negative foil copper foil on two sides.

And (3) positive electrode: the anode foil is a metal aluminum foil with the specification of 0.012 mm multiplied by 190 mm; the coating material of the positive electrode is as follows: the ternary nickel-cobalt-manganese NCM positive electrode material, the carbon nano tubes CNTs, the conductive carbon black SP and the polyvinylidene fluoride PVDF are prepared according to the weight ratio of 96.2:1.2:1.6: 1.0. Wherein, the positive coating material is coated on the positive foil aluminum foil on both sides.

A diaphragm: the gauge was 12 μm 88mm septum.

Electrolyte solution: the concentration of LiPF6 is 1.1mol/L, the solvent is diethyl carbonate DEC, ethylene carbonate EC, ethyl methyl carbonate EMC, and the electrolyte contains additive: vinylene carbonate VC, vinyl sulfate DTD and fluoroethylene carbonate FEC.

Then, activating the lithium ion battery containing the negative electrode of the silicon-based material according to empirical activation conditions: the empirical activation conditions are shown in Table 1 below, the cell holder pressure is 500 + -5 kgf, the cell temperature is 45 + -1 deg.C during activation, and the time for each sleep step is set to 5 min.

Table 1 empirical activation conditions for lithium ion batteries containing negative electrodes of silicon based materials:

working steps	Mode of operation	Multiplying power (Current)	Voltage limiting
				1	Dormancy	/	/
2	Constant current charging	0.02C	2.5V
				3	Dormancy	/	/
4	Constant current charging	0.05C	3.5V
				5	Dormancy	/	/
6	Constant current charging	0.10C	3.7V
				7	Dormancy	/	/
8	Constant current charging	0.20C	4.0V
				9	Dormancy	/	/

Comparative example 2

This example identifies the activation process according to the method of the invention.

First, an electrochemical reaction visualization confocal system in-situ test was performed for obtaining a negative electrode expansion rate and OCV versus SOC variation of the battery prepared in comparative example 1. Wherein, the used electrochemical reaction visualization confocal system equipment is provided by Japanese LaserTac company, and the model is as follows: B320.

specifically, the same positive electrode plate, negative electrode plate, separator, and electrolyte as those of the battery prepared in example 1 were used to form a battery cell suitable for the observation requirements of the electrochemical reaction visualization confocal system, and in-situ charging observation was performed using the electrochemical reaction visualization confocal system apparatus. Wherein, the charging rate of the in-situ charging is 0.01C, the cut-off voltage is 4.2V, and a relation curve of the SOC-expansion rate curve and the SOC-OCV curve is obtained, as shown in fig. 2.

Example 2A, target interval S1 (12%) to S2 (20%):

first, as shown in fig. 2, in the SOC-expansion ratio curve, there are a section corresponding to SOC S1 (12%) to S2 (20%), a plateau region corresponding to SOC 0% to 12% and a plateau region corresponding to SOC 20% to 24% before and after the section, respectively.

The SOC is in the range of S1 (12%) to S2 (20%), corresponding to expansion ratios of P1 (1.1%) to P2 (4.8%). The interval inflation I index within the interval S1 to S2 is calculated as follows:

I＝(P2-P1)/(S2-S1)＝(4.8％-1.1％)/(20％-12％)＝0.46。

the expansion index of the interval from S1 to S2 is greater than 0.4, and therefore, S1 (12%) to S2 (20%) are determined as the target SOC interval.

In the target SOC ranges S1 (12%) to S2 (20%), the corresponding OCVs were determined to be V1(3.43V) to V2(3.49V) in conjunction with the SOC-OCV curves. Therefore, the following activation strategy was adopted in the interval in which the OCV was V1(3.43V) to V2 (3.49V).

First, the battery is charged from an initial state to V1(3.43V) with a first normal charge rate current (0.1C), then the voltage is charged from V1 to V2(3.49V) with the first charge rate (0.1C for the first charge rate), and then discharged to V1(3.43V) with a larger current of 0.2C rate, and after sleep, the battery is charged to V2(3.49V) with a second charge rate (0.1C for the second charge rate). That is, the steps of charging, discharging, and recharging are performed once in the voltage range corresponding to the section. Among them, the purpose of adding the discharge-recharge step is to retard the expansion tendency of the already expanded negative electrode. And the adoption of larger multiplying power for charging and discharging can shorten the activation time while obtaining a better SEI film.

Because the final expansion rate in the SOC-expansion rate curve is not increased when the SOC is 92 percent, the corresponding OCV is determined as the cut-off voltage Vcut-off of 4.08V according to the SOC-OCV curve, and the cut-off voltage Vcut-off is used as the charge cut-off voltage of the lithium ion battery activation process.

After the activation in the target SOC interval, the voltage is charged to the charge cut-off voltage vbutt (4.08V) with the second conventional charge rate, which is 0.2C, according to the process parameters of the conventional activation.

Specific activation parameters are shown in Table 2 below, the cell holder pressure was 500 + -5 kgf, the cell temperature was 45 + -1 deg.C during activation, and the time for each sleep step was set to 5 min.

TABLE 2 activation conditions of lithium ion battery comprising negative electrode of silicon-based material

Example 2B, target intervals S3 (24%) to S4 (44%)

As shown in fig. 2, in the SOC-expansion ratio curve, there is also a substantially linear section, wherein the SOC of the section is from S3 (24%) to S4 (44%). The two judgment methods can be used independently or in combination when the section selection is carried out.

The SOC is in the range of S3 (24%) to S4 (44%), corresponding to expansion ratios of P3 (5.0%) to P4 (13.8%). The interval expansion index I in the interval S3-S4 is calculated as follows:

I＝(P4-P3)/(S4-S3)＝(13.8％-5.0％)/(44％-24％)＝0.44。

the expansion index of the interval from S3 to S4 is greater than 0.4, and therefore, S3 (24%) to S4 (44%) are determined as the target SOC interval.

In the target SOC ranges S3 (24%) to S4 (44%), the corresponding OCVs were determined to be V3(3.53V) to V4(3.64V) in conjunction with the SOC-OCV curves.

Therefore, a suitable activation process is employed in the region of OCV from V3(3.53V) to V4 (3.64V). Specifically, the battery is charged from the initial state with a normal charge rate current of 0.1C rate, the cutoff voltage is V3(3.53V), after sleep, the battery is continuously charged to V4(3.64V) with a current of 0.1C rate, and then discharged to V3(3.53V) with a larger current of 0.2C rate, and then charged to V4(3.64V) again with a current of 0.1C rate. That is, the steps of charging, discharging, and recharging are performed once in the voltage range corresponding to the section. Cut-off voltage V at this time_Cut-offStill 4.08V.

After the target SOC interval, the process parameters of conventional activation, i.e. charging at a rate of 0.2C, with a charge cut-off voltage V, are used_Cut-offIt was 4.08V.

Specific activation parameters are shown in Table 3 below, the cell holder pressure was 500 + -5 kgf, the cell temperature was 45 + -1 deg.C during activation, and the time for each sleep step was set to 5 min.

TABLE 3 activation conditions for lithium ion battery comprising negative electrode of silicon-based material (II)

Example 2C, target intervals were S1 (12%) to S2 (20%) and S3 (24%) to S4 (44%):

in the present embodiment, the same activation strategies as in embodiments 2A and 2B were performed, respectively, within the voltage ranges corresponding to the two target SOC intervals determined in embodiments 2A and 2B above as S1 through S2 and S3 through S4.

Specific activation conditions are shown in Table 4 below, and the cell holder pressure was 500. + -. 5kgf and the cell temperature was 45. + -. 1 ℃ in the activation process, and further, the time for each sleep step was set to 5 min.

TABLE 4 activation conditions (III) for lithium ion battery comprising negative electrode of silicon-based material

Working steps	Mode of operation	Multiplying power (Current)	Voltage limiting
				1	Dormancy	/	/
2	Constant current charging	0.1C	3.43V
				3	Dormancy	/	/
4	Constant current charging	0.1C	3.49V
				5	Dormancy	/	/
6	Constant current discharge	0.2C	3.43V
				7	Dormancy	/	/
8	Constant current charging	0.1C	3.49V
				9	Dormancy	/	/
10	Constant current charging	0.1C	3.53V
				11	Dormancy	/	/
12	Constant current charging	0.1C	3.64V
				13	Dormancy	/	/
14	Constant current discharge	0.2C	3.53V
				15	Dormancy	/	/
16	Constant current charging	0.1C	3.64V
				17	Dormancy	/	/
18	Constant current charging	0.2C	4.08V
				19	Dormancy	/	/

And (3) testing the performance of the activated lithium ion battery:

the activated lithium ion batteries of comparative examples 1 and 2 were subjected to performance testing: the results of the capacity retention rate variation curve of each battery after 500 cycles of charge-discharge cycles at 25 ℃ are shown in FIG. 3.

As can be seen from fig. 3, the capacity retention rate of the lithium ion battery of comparative example 1 activated according to the empirical process after 500 charge-discharge cycles was 84%; the capacity retention of the battery activated according to method (one) determined in example 2A was 90%; the capacity retention of the battery activated by method (two) as determined in example 2B was 88%. While the capacity retention of the battery activated by the method (iii) determined in example 2C was 94%.

For a lithium battery containing the negative electrode of the silicon-based material, according to the expansion property of the battery negative electrode, namely an SOC-expansion rate curve chart, which is measured in advance, and by combining the SOC-OCV curve chart, for a target OCV interval corresponding to at least one target SOC interval with an interval expansion index of more than or equal to 0.4, an activation process capable of delaying the expansion of the negative electrode is determined in the target OCV interval, so that the activation process capable of obtaining a proper SEI film to further improve the electrochemical performance (such as the cycle performance) of the battery can be directly determined, repeated experiments are not needed, resources are saved, and the time for determining the proper activation process and the development period of a new battery are shortened.

Claims

1. A method of determining an activation process for a lithium ion battery, comprising:

b, determining a target SOC interval according to the SOC-expansion rate curve;

2. The method for determining the activation process of the lithium ion battery according to claim 1, wherein the step B is to select an SOC interval with an interval expansion index I greater than or equal to 0.4 as the target SOC interval.

3. The method of claim 2, wherein the interval expansion index I is calculated by: where Pm and Pn are the start point expansion rate and the end point expansion rate corresponding to the section, respectively, and Sm and Sn are the start point SOC value and the end point SOC value corresponding to the section, respectively.

4. The method of claim 1, wherein the target SOC interval is determined in an SOC interval in which negative electrode electrochemical reaction is dominated by silicon intercalation.

5. The method of claim 1, wherein the difference between the starting SOC and the ending SOC corresponding to the target SOC interval is greater than or equal to 5% and less than or equal to 30%.

6. The method of claim 1, wherein the corresponding activation strategy is to charge the voltage to Vm using a first conventional charge rate, charge the voltage from Vm to Vn using the first charge rate, then discharge to Vm using a discharge rate, then charge to Vn using a second charge rate, and finally charge the voltage to a charge cutoff voltage using the second conventional charge rate; vm and Vn are respectively the starting voltage and the ending voltage on the SOC-OCV curve corresponding to the target SOC interval.

7. The method of claim 6, wherein the first charge rate and the second charge rate are both in a range of 0.1C to 0.5C, and the discharge rate is in a range of 0.1C to 0.5C.

8. The method of claim 7, wherein the first and second charge rates are in a range of 0.1C to 0.2C, and the discharge rate is in a range of 0.2C to 0.4C.

9. The method of claim 8, wherein the first charge rate is equal to the second charge rate, and the discharge rate is 1.5-2.5 times the first charge rate or the second charge rate.

10. The method for determining the activation process of the lithium ion battery according to any one of claims 1 to 9, wherein when a plurality of target SOC intervals are provided, corresponding activation strategies are adopted for all the target SOC intervals.