CN113471561B

CN113471561B - Method for determining activation process of lithium ion battery

Info

Publication number: CN113471561B
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: 2023-06-27
Anticipated expiration: 2041-07-01
Also published as: CN113471561A

Abstract

The invention relates to the technical field of battery activation, and provides a method for determining a lithium ion battery activation process in order to improve the efficiency of the lithium ion battery activation process of a silicon-containing material, which comprises the following steps: step A, preparing a simulated battery sample and acquiring an SOC-OCV curve and an SOC-expansion rate curve of the simulated battery sample; step B, determining a target SOC interval according to the SOC-expansion rate curve; step C, determining a target OCV section corresponding to the target SOC section and a charging cut-off voltage according to the SOC-OCV curve; and D, activating the battery by adopting a corresponding activation strategy according to the target OCV interval and the charging cut-off voltage. By adopting the mode, the activation condition favorable for SEI film formation 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 a lithium ion battery activation process of a negative electrode containing a silicon-based material.

Background

After the lithium ion battery is assembled, the lithium ion battery needs to be activated. The activation is to charge the battery after filling and standing for the first time. A dense solid electrolyte interface (solid electrolyte interphase, SEI) film can be formed on the surface of the anode by activation. The SEI film has a property of allowing lithium ions to freely pass therethrough, but insulating electrons. The SEI film can prevent the surface of the negative electrode from further generating electrochemical reaction, avoid continuous consumption of lithium ions, and reduce the generation of various byproducts. The SEI film forming condition can influence the impedance of a negative electrode end and the subsequent electrochemical reaction, and also influence the consumption of active substances, so that the SEI film forming condition is particularly important for various performances of a battery. Meanwhile, the activation allows the gas generated by the electrochemical reaction during the primary charging to be removed.

Existing negative electrode materials are classified into carbon materials and non-carbon materials, wherein graphite-based carbon materials are the most common negative electrode materials, and non-carbon materials mainly include titanium-based materials and silicon-based materials. At present, the silicon-based material has the remarkable advantages of more proper potential due to higher gram capacity, and the lithium ion battery containing the silicon-based material is environment-friendly and rich in resources, so that the material is a better choice for the lithium ion battery cathode material pursuing higher energy density at present.

However, the significant drawbacks of silicon-based materials are: has higher expansion rate relative to carbon materials during the activation process. Due to the high expansion rate, the material is continuously exposed to a new interface in the activation process, so that a new SEI film is continuously generated, a compact SEI film cannot be formed, and the electrochemical performance, particularly the cycle performance and the like of the finally obtained battery are poor.

The lithium intercalation mechanism of silicon-containing based materials is different from that of conventionally used graphite-based materials. At present, there is no theoretical knowledge about the activation of lithium ion batteries containing silicon-based materials. In developing lithium ion batteries containing silicon-based materials, it is common practice to empirically set activation parameters for activation, and then adjust the activation based on performance measurements of the lithium ion battery. However, since silicon-based materials have a relatively high expansion ratio, the difference in performance of silicon-based materials from different sources, even different batches, may cause large fluctuations in battery performance. These all lead to blindness in the current activation of the lithium ion batteries with silicon-based materials, and the activation conditions of the lithium ion batteries with silicon-based materials of different specifications and different batches are required to be searched through trial and error. This results in long time consuming and resource intensive process parameter adjustments for lithium ion battery activation of silicon-based materials, and also in extended development cycles for new batteries.

Disclosure of Invention

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

The invention solves the problems by adopting the following technical scheme:

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

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

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

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

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

Further, the step B specifically includes selecting an SOC section with a section expansion index I greater than or equal to 0.4 as a target SOC section.

Further, the interval expansion index I is calculated by the following steps: i= (Pn-Pm)/(Sn-Sm), where Pm and Pn are the start expansion rate and the end expansion rate, respectively, corresponding to the section, and Sm and Sn are the start SOC value and the end SOC value, respectively, corresponding to the section.

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

Further, the difference between the start SOC and the end 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, discharge the voltage to Vm by adopting a discharging rate, charge the voltage to Vn by adopting a second charging rate, and charge the voltage to a charging cut-off voltage by adopting the second conventional charging rate; and 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 and second charging magnifications are each in the range of 0.1C to 0.5C, and the discharging magnifications are in the range of 0.1C to 0.5C.

Further, the first and second charging magnifications are in the range of 0.1C to 0.2C, and the discharging magnifications are in the range of 0.2C to 0.4C.

Further, the first charge rate is equal to the second charge rate, and the discharge rate is 1.5 to 2.5 times as high as the first charge rate or the second charge rate.

Further, when the number of the target SOC intervals is multiple, a corresponding activation strategy is adopted for all the target SOC intervals.

Compared with the prior art, the invention has the following beneficial effects: according to the method, the target SOC interval is determined by obtaining the negative electrode expansion characteristic curve of the lithium ion battery to be activated in advance, and the activation condition favorable for SEI film formation is adopted in the target SOC interval more pertinently in the subsequent step by determining the target SOC interval, so that a proper activation process can be ensured to be determined, and good battery performance can be obtained through the process; the repeated activation test with 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 corresponding SOC-OCV curve and SOC-expansion curve for an example;

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

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

Detailed Description

The present invention will be described in further detail with reference to the following examples in order to make the objects, technical solutions and advantages of the present invention more apparent. It should be understood that the specific embodiments described herein are for purposes of illustration only and are not intended to limit the scope of 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 cathode material may be Lithium Cobalt Oxide (LCO), lithium iron phosphate (LFP), lithium manganate, ternary materials lithium Nickel Cobalt Manganate (NCM) and lithium Nickel Cobalt Aluminate (NCA), but is not limited thereto.

The negative electrode material of the lithium ion battery comprises a silicon-based material. Silicon-based materials refer to silicon or silicon oxide materials. The negative electrode material of the silicon-containing based material is generally referred to as a silicon-carbon composite material (Si/C) and/or a silicon oxide-carbon composite material (SiOx/C). The current collector of the lithium ion battery may be a metal foil, such as copper foil or aluminum foil. The electrolyte of the lithium ion battery is not particularly limited, and may be prepared from a suitable organic solvent and a suitable electrolyte in a certain ratio, or may be commercially available.

The separator of the lithium ion battery generally adopts a polyolefin porous film with high strength and thin film, and common separators include polypropylene and polyethylene microporous separators, and propylene-ethylene copolymer and polyethylene homopolymer films.

In the present application, "SOC" is an english abbreviation of State of charge (State OfCharge), expressed as a percentage. "OCV" is an English abbreviation for open circuit voltage (Open Circuit Voltage), in volts (V). The expansion rate of the negative electrode refers to the change rate of the thickness of the negative electrode plate 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 for determining an activation process of a lithium ion battery includes:

and A, preparing a simulated battery sample by adopting the positive plate, the negative plate, the diaphragm and the electrolyte which are the same as those of the lithium ion battery, acquiring an SOC-OCV curve of the simulated battery sample, and carrying out initial charging on the simulated battery sample to acquire an SOC-expansion rate curve of the negative electrode.

The invention adopts an electrochemical reaction visual confocal system (Electro-Chemical reaction visualizing Confocal System, namely an electrochemical reaction visual confocal system) to quantitatively measure the thickness change of the negative pole piece in real time to obtain an SOC-expansion rate curve. The measuring apparatus of the SOC-expansion ratio curve of the anode is not particularly limited in the present invention, as long as the apparatus capable of quantitatively obtaining the variation value of the anode thickness with charge can be used in the present invention.

Corresponding simulated battery samples may be prepared for detection, depending on the requirements of the particular device. The simulated battery sample is made into the size required by detection by adopting the same positive plate, negative plate, diaphragm and electrolyte as the lithium ion battery to be activated. For example, the size of the simulated battery sample should be compatible with the mold of the electrochemical reaction visualization confocal system apparatus used for detection.

To obtain the SOC-expansion ratio curve of the negative electrode, the above-described simulated battery sample was initially charged with a relatively small charge rate. The initial charge rate may be 0.005-0.02C, for example 0.01C, 0.015C, 0.02C, etc. The adoption of small-rate charging is favorable for obtaining an SOC-expansion rate curve with clear change of the expansion rate of the negative electrode, and if the charging rate is too large, the expansion rate of the negative electrode can be increased too fast in the whole detection process, so that the difficulty in identifying the area with 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-discharge test on the battery in a certain voltage interval to obtain a capacity and voltage curve of the battery. The SOC-expansion curve of the negative electrode may also be acquired at the same time using, for example, an electrochemical reaction visualization confocal system apparatus.

And B, determining a target SOC section according to the SOC-expansion rate curve. The interval expansion index I of the target SOC interval needs to meet that I is more than or equal to 0.4. The interval expansion index is a ratio of a change value of the anode expansion rate to a change value of the SOC in a specific SOC interval based on an SOC-expansion rate graph of the anode, and is used to indicate the degree of anode expansion in the specific SOC interval. The interval expansion index may be calculated as i= (Pn-Pm)/(Sn-Sm), where Pm and Pn are the start expansion rate and the end expansion rate corresponding to the target SOC interval, respectively, and Sm and Sn are the start SOC value and the end SOC value of the target SOC interval, respectively.

The determination of the target SOC interval can be performed in different ways, as long as a suitable curve section with the interval expansion index I not less than 0.4 can be determined.

The present invention provides two ways of determining a target SOC interval: (1) Firstly, selecting a section with a curve approximately linearly changing from an SOC-expansion rate curve, then calculating a section expansion index I of an SOC section corresponding to the section according to I= (Pn-Pm)/(Sn-Sm), and determining the SOC section corresponding to the section as a target SOC section when I is more than or equal to 0.4.

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

When I is not less than 0.4, it is considered that the expansion of the negative electrode is faster in this charging interval, and the SEI film formation condition is affected. Therefore, the corresponding SOC interval when the anode expands fast can be accurately known according to the SOC-expansion rate curve, so that the activation process conditions favorable for forming a compact SEI film are adopted in the interval in a targeted manner, and the time and cost waste caused by determining the preferred activation process through blind repeated experiments in the conventional method is avoided.

In the charging process, the electrochemical reaction in the negative electrode containing the silicon-based material comprises a reaction part mainly containing silicon intercalation lithium and a reaction part mainly containing carbon intercalation lithium. The silicon-intercalation reaction generally occurs in the first half of the charge reaction, i.e., generally corresponding to an SOC of 0% to about 50%, while the carbon-intercalation reaction generally occurs in the second half of the charge 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 affecting 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 negative electrode electrochemical reaction, for example, the target SOC interval may be determined in the interval of 0% to 50% of the SOC value, but the present invention is not limited to this specific interval depending on the lithium ion battery. The specific portion of the SOC interval to which the electrochemical reaction based on the lithium intercalation of silicon corresponds can be determined by those skilled in the art based on the composition of the negative active material of the lithium ion battery to be activated and the obtained negative SOC-expansion ratio curve in combination with experience.

In addition, in order to avoid the activation process being too complicated due to the determination of a large number of target SOC intervals, a section having an SOC difference of 5% or more and 30% or less may be selected to determine whether the section is the target SOC interval. For the interval with the excessively small SOC difference, the interval can be ignored, or a plurality of adjacent cells are combined into a larger interval to be considered, for example, the interval with the SOC difference being more than or equal to 5%, 6%, 7%, 8%, 9% and even more than or equal to 10% can be selected; an excessively large SOC interval may cause the finally determined activation process to be too rough to obtain optimal performance such as cycle characteristics, and thus segments having SOC differences 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 C, determining a target OCV section corresponding to the target SOC section according to the SOC-OCV curve. After determining the target SOC section, selecting at least one target SOC section, combining the SOC-OCV curve, finding out an OCV section corresponding to the SOC section, namely, the target OCV section, and determining a starting point OCV value Vm and an ending point OCV value Vn in the section.

Determining a corresponding OCV value V in an SOC-OCV curve according to a corresponding SOC value when the final expansion rate is not increased in the SOC-expansion rate curve _Cut-off And the charge cut-off voltage is used as the lithium ion battery activation process. The final cut-off voltage determined according to the SOC-expansion ratio curve can make the activation effect better.

The corresponding activation strategy is to charge the voltage to Vm by adopting a first conventional charging rate, charge the voltage to Vn from Vm by adopting the first charging rate, discharge the voltage to Vm by adopting a discharge rate, charge the voltage to Vn by adopting a second charging rate, and charge the voltage to a charge cut-off voltage by adopting the second conventional charging rate; wherein the first charging magnification and the second charging magnification are in the range of 0.1C to 0.5C, and the first charging magnification and the second charging magnification may each be independently 0.1C, 0.2C, 0.3C, 0.4C, 0.5C, or the like, i.e., they may be equal or unequal.

For the discharge, for example, 0.1C to 0.5C may be used, and the discharge magnification 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-described range.

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

In the region Vm to Vn, the charging-discharging-charging method with a higher rate is used to delay the expansion tendency of the negative electrode compared with the charging directly from Vm to Vn with a higher rate, thereby contributing to the formation of a dense SEI film.

When the number of the target SOC intervals is multiple, corresponding activation strategies are adopted for all the target SOC intervals. And the corresponding activation strategy is adopted for all the target SOC intervals, so that the negative electrode does not generate excessive rapid expansion in the whole activation process, thereby ensuring that an SEI film is better formed and obtaining better battery cycle performance. It should be noted that the charging cutoff voltage is charged only when the last section is charged, and Vn of the preceding section is taken as the initial charging voltage of the next section, thereby realizing continuous charging.

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 with an interval expansion index I very close to 0.4 (e.g., 0.40-0.42), or an interval with a smaller interval SOC difference (e.g., 5% -7%), or two or three adjacent smaller SOC intervals may be combined into one large SOC interval to determine the corresponding strategy.

For portions other than the selected target SOC interval, particularly for SOC intervals in which the negative electrode electrochemical reaction is based on carbon intercalation of lithium, an empirical activation process may be employed. For example, a conventional activation process of a lithium ion battery using a carbon material as a negative electrode, or a well-established activation process of a lithium ion battery having a similar negative electrode composition may be employed. Such an activation process may be exemplified by charging with a small-rate current and then charging with a larger-rate current. For example, in the present invention, for a section with an SOC of 50% to 100%, an activation strategy with a larger constant current charging rate (0.1C-0.5C) may be employed.

The scheme of the invention is illustrated by comparative experiments:

comparative example 1

This example is an empirically determined activation process.

First, a lithium ion battery of a negative electrode containing a silicon-based material is prepared, wherein:

and (3) a negative electrode: the artificial graphite and SiOx/C mixture, the negative electrode foil is copper foil with the specification of 0.008 multiplied by 196mm, the negative electrode coating material is prepared by mixing active substances, conductive carbon black SP, single-wall carbon nano tube SWCNTs, styrene butadiene rubber SBR and sodium carboxymethylcellulose CMC according to the graphite weight ratio of 96.4%, and the compacted density of the negative electrode sheet after rolling is 1.65g/cm < 3 >. Wherein, the negative electrode coating material is coated on the copper foil of the negative electrode foil material on two sides.

And (3) a positive electrode: the positive electrode foil is a metal aluminum foil with the specification of 0.012 multiplied by 190 mm; the positive electrode coating material is as follows: the ternary nickel cobalt manganese NCM positive electrode material, the carbon nano tube 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 electrode coating material is coated on the positive electrode foil aluminum foil on two sides.

A diaphragm: a separator with a specification of 12 μm x 88 mm.

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

Then, according to the empirical activation conditions, activating the lithium ion battery of the negative electrode of the silicon-containing material: empirical activation conditions are shown in Table 1 below, the activation process battery clamp pressure was 500.+ -. 5kgf, the battery temperature was 45.+ -. 1 ℃ and, furthermore, the time for each resting step was set to 5min.

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

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

Comparative example 2

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

First, an electrochemical reaction visualization confocal system in-situ test was performed for obtaining a relationship curve of the anode expansion rate and OCV as a function of SOC for the battery prepared in comparative example 1. Wherein, the used electrochemical reaction visualization confocal system equipment is provided by Japan LaserTac company, model: B320.

specifically, the same positive electrode plate, negative electrode plate, diaphragm and electrolyte as those of the battery prepared in the example 1 are used for forming a battery unit which is suitable for the observation requirement of an electrochemical reaction visual confocal system, and the electrochemical reaction visual confocal system equipment is used for in-situ charging observation. Wherein, the charging rate of in-situ charging is 0.01C, the cut-off voltage is 4.2V, and a relation curve of the SOC-expansion ratio 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 is one segment corresponding to the SOC of S1 (12%) to S2 (20%), and before and after this segment, there are plateau regions corresponding to the SOC of 0% to 12%, and plateau regions corresponding to the SOC of 20% to 24%, respectively.

SOC ranges from S1 (12%) to S2 (20%), and the corresponding expansion ratios range from P1 (1.1%) to P2 (4.8%). The section expansion I index in the section S1 to S2 is calculated as follows:

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

since the section expansion index in the section S1 to S2 is calculated to be larger than 0.4, the section S1 (12%) to S2 (20%) are determined as the target SOC section.

In the target SOC intervals S1 (12%) to S2 (20%), the corresponding OCVs are determined as V1 (3.43V) to V2 (3.49V) in conjunction with the SOC-OCV curve. Therefore, the following activation strategy was adopted in the interval of OCV from V1 (3.43V) to V2 (3.49V).

First, the voltage is charged from V1 to V2 (3.49V) by using a first normal charging rate current (0.1C) from an initial state to V1 (3.43V), then the voltage is charged from V1 to V2 (3.49V) by using a first charging rate (the first charging rate is 0.1C), then the voltage is discharged to V1 (3.43V) by using a larger current with 0.2C rate, and after dormancy, the voltage is charged to V2 (3.49V) by using a current with a second charging rate (the second charging rate is also 0.1C). 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 anode. And by adopting larger multiplying power for charging and discharging, the activation time can be shortened while a better SEI film is obtained.

Since the final expansion rate in the SOC-expansion rate curve is not increased any more at 92% of the SOC, according to the SOC-OCV curve, the corresponding OCV is determined to be cut-off voltage Vs which are 4.08V, and the cut-off voltage Vs are used as the charging cut-off voltage of the lithium ion battery activation process.

After the target SOC interval is activated, the voltage is charged to the charge cutoff voltage vblock (4.08V) with a second conventional charge rate, which is 0.2C, according to the process parameters of conventional activation.

Specific activation parameters are shown in Table 2 below, the activation process cell clamp pressure was 500.+ -. 5kgf, the cell temperature was 45.+ -. 1 ℃ and the time for each resting step was set at 5min.

TABLE 2 activation conditions for lithium ion batteries for negative electrodes containing silicon-based materials (I)

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

As shown in fig. 2, in the SOC-expansion ratio curve, there is also a segment that varies substantially linearly, where the segment corresponds to SOCs 3 (24%) to S4 (44%). The two judging modes can be used alone or in combination when the section selection is performed.

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

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

the section expansion index in the section S3 to S4 is calculated to be larger than 0.4, and therefore, S3 (24%) to S4 (44%) are determined as the target SOC section.

In the target SOC intervals S3 (24%) to S4 (44%), the corresponding OCVs are determined as V3 (3.53V) to V4 (3.64V) in conjunction with the SOC-OCV curve.

Therefore, a suitable activation process is employed in the interval of OCV V from V3 (3.53V) to V4 (3.64V). Specifically, the charging is performed from an initial state with a normal charging rate current of 0.1C rate, the cut-off voltage is V3 (3.53V), after dormancy, the charging is continued with a current of 0.1C rate to V4 (3.64V), then the charging is performed with a larger current of 0.2C rate to V3 (3.53V), and then the charging is performed again with a current of 0.1C rate to V4 (3.64V). 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-off Still 4.08V.

After the target SOC interval, the charge cut-off voltage V is charged by using the conventional activated process parameters, namely, charging at a rate of 0.2C _Cut-off 4.08V.

Specific activation parameters are shown in Table 3 below, the activation process cell clamp pressure was 500.+ -. 5kgf, the cell temperature was 45.+ -. 1 ℃ and, furthermore, the time for each resting step was set at 5min.

TABLE 3 activation conditions of lithium ion batteries for negative electrodes containing silicon-based materials (II)

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

in this embodiment, the same activation strategies as in embodiments 2A and 2B are respectively performed in the voltage ranges corresponding to the two target SOC sections determined in embodiments 2A and 2B above as S1 to S2 and S3 to S4.

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

TABLE 4 activation conditions for lithium ion batteries of negative electrodes containing silicon-based materials (III)

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

Testing the performance of the activated lithium ion battery:

performance testing was performed on the activated lithium ion batteries of comparative examples 1 and 2: the capacity retention rate change curve of each battery was obtained by conducting 500 cycles of charge-discharge at 25 c, and the results 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, which was activated according to an empirical process, was 84% after 500 charge-discharge cycles; the capacity retention of the battery activated by the method (one) determined in example 2A was 90%; the capacity retention of the battery activated in the method (two) determined in example 2B was 88%. Whereas the capacity retention of the battery activated in the method (three) determined in example 2C was 94%.

For a lithium battery of a negative electrode of a silicon-containing base material, according to the pre-determined expansion property of the negative electrode of the battery, namely an SOC-expansion rate curve chart, and 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 adopted in the target OCV interval, so that an activation process suitable for forming an SEI film and further improving the electrochemical performance (such as the cycle performance) of the battery can be directly determined, and experiments are not required to be repeated, thereby saving resources and shortening the time for determining the suitable activation process and the development period of a new battery.

Claims

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

step B, determining a target SOC interval according to the SOC-expansion rate curve: the step B specifically comprises the steps of selecting an SOC interval with an interval expansion index I more than or equal to 0.4 as a target SOC interval; the calculation mode of the interval expansion index I is as follows: i= (Pn-Pm)/(Sn-Sm), where Pm and Pn are the start expansion rate and the end expansion rate corresponding to the section, respectively, and Sm and Sn are the start SOC value and the end SOC value corresponding to the section, respectively; determining a target SOC section in the SOC section mainly taking the cathode electrochemical reaction as silicon intercalation;

step D, activating the battery by adopting a corresponding activation strategy according to the target OCV interval and the charging cut-off voltage; 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, discharge the voltage to Vm by adopting a discharge rate, charge the voltage to Vn by adopting a second charging rate, and charge the voltage to a charge cut-off voltage by adopting the second conventional charging rate; the Vm and the Vn are respectively the initial voltage and the termination voltage on the SOC-OCV curve corresponding to the target SOC interval; the first and second charging rates are both in the range of 0.1C to 0.5C, and the discharging rate is in the range of 0.1C to 0.5C; the first regular charging rate takes 0.1C and the second regular charging rate takes 0.2C.

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

3. The method of claim 1, wherein the first and second charge rates are in the range of 0.1C to 0.2C and the discharge rate is in the range of 0.2C to 0.4C.

4. The method of claim 3, 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.

5. The method for determining a lithium ion battery activation process according to any one of claims 1 to 4, wherein when the number of target SOC intervals is plural, a corresponding activation strategy is adopted for all the target SOC intervals.