Disclosure of Invention
The invention aims to provide a method for rapidly evaluating the stability of a high-voltage lithium ion battery system aiming at the technical defects in the prior art.
Therefore, the invention provides a method for rapidly evaluating the stability of a high-voltage lithium ion battery system, which comprises the following steps:
step S1, under a preset temperature environment, carrying out constant current charging on the soft package lithium ion battery at a preset constant current I1 until the soft package lithium ion battery is charged to a preset high voltage U1, then keeping the preset high voltage U1 unchanged, and continuously carrying out floating charging for a preset time;
step S2, detecting the magnitude of the floating charging current in the floating charging process, and simultaneously, performing a thickness expansion test on the soft package lithium ion battery after the floating charging process is completed to obtain the thickness expansion rate of the soft package lithium ion battery after the charging process is completed;
step S3, when the floating charging current in the floating charging process is gradually reduced, the final floating charging current stable value can be reduced to be less than or equal to the preset current stable value, and the thickness expansion rate of the soft package lithium ion battery after charging is less than the preset expansion rate, the cycle performance of the lithium ion battery system is judged to be stable.
Preferably, in step S1, the preset temperature is in the range of 20 to 55 ℃;
in step S1, the value range of the current I1 is greater than 0 and less than 1C;
in step S1, a high voltage U1, greater than or equal to 4.5V, is preset;
in step S1, the preset duration has a value range of: 0.1 to 10 hours.
Preferably, in step S1, the value range of the current I1 is 0.01 to 0.2C;
in step S1, a high voltage U1 of 4.6V is preset;
in step S1, the preset time period is 10 hours.
Preferably, in step S3, the preset current stabilization value is less than or equal to 0.001C and the preset expansion ratio is less than 20%.
Preferably, in step S3, after the preset high voltage U1 is 4.6V and the preset high voltage U1 is kept constant for 10 hours of floating charge, if the final floating charge current stability value is less than or equal to 0.001C and the thickness expansion rate of the soft package lithium ion battery after the charging is completed is less than 20%, it is determined that the cycle performance of the lithium ion battery system is stable.
Compared with the prior art, the method for rapidly evaluating the stability of the high-voltage lithium ion battery system has the advantages that the design is scientific, and due to the limitation of the button half battery and the long-period defect of the full battery cycle performance test, the method is based on the full battery system, can simply and effectively screen out the high-voltage system with stable cycle by using the test method of the gas expansion rate after floating charge and charging, and has great practical significance.
In the present invention, the stability condition of the electrolyte (positive electrode material) is defined based on the same kind of positive electrode (electrolyte).
Detailed Description
In order that those skilled in the art will better understand the technical solution of the present invention, the following detailed description of the present invention is provided in conjunction with the accompanying drawings and embodiments.
Referring to fig. 1 to 3, the present invention provides a method for rapidly evaluating the stability of a high voltage lithium ion battery system, wherein the positive active material of the lithium ion battery is a lithium-rich manganese-based oxide, and the lithium ion battery system particularly refers to a lithium-rich manganese-based (Li) battery system1.2Ni0.13Co0.13Mn0.54O2) Positive electrode material-electrolyte; the method is suitable for soft package lithium ion batteries;
the method comprises the following steps:
step S1, under a preset temperature environment, constant current charging is carried out on the soft package lithium ion battery (namely a battery finished product) with a preset constant current I1 until the battery is charged to a preset high voltage U1 (not less than 4.5V), then the preset high voltage U1 is kept unchanged, and constant voltage charging (namely floating charging) is continuously carried out for a preset time length;
in the invention, in particular implementation, the floating charging mode specifically comprises: the constant voltage U1 is kept charged, and the termination current is not set, but the preset time is used as a charging cut-off mode, and the current drops slowly to be stable after rapidly reducing in the early period within a certain time. For example, after charging to 4.6V at a constant current of 0.1C (i.e. the predetermined high voltage U1), the mode is changed to 4.6V constant voltage charging, and the step conditions are added: and stopping charging when the charging is fully performed for 10 hours. In general, the current level first decreases from 0.1C to 0.025C within 1 hour, gradually decreases to 0.001C, and the level of the stable value depends on the high voltage stability of the system, and the more quickly the current decreases to the stable time and the lower the stable value, no sudden change such as a sudden current increase occurs, indicating that the side reaction of consuming active lithium ions under the high voltage of the system is small, and this is one of the criteria for determining the high voltage stability.
Step S2, detecting the magnitude of the floating charging current in the floating charging process, and simultaneously, performing a thickness expansion test on the soft package lithium ion battery after the floating charging process is completed to obtain the thickness expansion rate of the soft package lithium ion battery after the charging process is completed;
step S3, when the magnitude of the float charging current in the float charging process is gradually decreased, and the final float charging current stability value can be decreased to be less than or equal to the preset current stability value, and the thickness expansion rate of the soft package lithium ion battery after the charging (i.e. the increase rate of the thickness of the battery after the charging and the battery before the charging) is less than the preset expansion rate, determining that the cycle performance of the lithium ion battery system is stable (i.e. meets the preset cycle life requirement, specifically, the cycle life of the battery system with a cycle capacity attenuation value of 80% can be not less than 500 cycle lives).
In the present invention, in a specific implementation, the negative active material of the lithium ion battery includes, but is not limited to, at least one or more of silicon and graphite, and the lithium ion battery is of a soft pack type.
In the invention, in step S1, the preset temperature is in the range of 20 to 55 ℃;
in step S1, the value range of the current I1 is greater than 0 and less than 1C, preferably 0.01 to 0.2C;
in step S1, the high voltage U1 is preset to be not lower than 4.5V, i.e., greater than or equal to 4.5V, preferably 4.6V.
In step S1, the preset duration has a value range of: 0.1 to 10 hours, preferably 10 hours.
In particular, the higher the preset high voltage U1 is, the shorter the charging time should be, and for the comprehensive consideration of safety and efficiency, the method of the present invention suggests that the preset high voltage U1 is 4.6V, and charging time is 10 hours as a reference standard.
When the lithium-manganese-based electrolyte is charged in the voltage range (namely not less than 4.5V), lithium ions are continuously extracted, lattice oxygen is released along with the lithium ions, the electrolyte is oxidized, meanwhile, the electrolyte is continuously under high working voltage, the electrolyte is easy to oxidize and decompose, and in addition, the lithium-manganese-based electrolyte (Li) is rich in lithium1.2Ni0.13Co0.13Mn0.54O2) The positive electrode material-electrolyte interface film is continuously reconstructed and repaired, and the three reactions are gas generation and mainly contain carbon dioxide.
It should be noted that the stability of the material structure and the interface determines the release of active oxygen and the degree of the side reaction of the interface oxidation, and the high-pressure resistance of the electrolyte solvent and the additive and the film forming stability determine the degree of the electrochemical oxidation, so that the gas generation degree can calibrate the stability of the material and the electrolyte in the high-voltage lithium-rich manganese-based system.
In step S3, if the preset current stability value is less than or equal to (i.e., not higher than) 0.001C and the preset expansion rate is less than 20%, the cycle performance of the lithium ion battery system is determined to be stable (i.e., the battery system meets the preset cycle life requirement), and it can be determined that the battery system has a cycle life of more than 500 times, and the expansion rate is lower. Indicating that the more stable the cycling performance.
For the present invention, in step S3, specifically, after the preset high voltage U1 is 4.6V and the preset high voltage U1 is kept unchanged for 10 hours of float charging, if the final float charging current stability value is less than or equal to (i.e., not higher than) 0.001C and the thickness expansion rate of the soft package lithium ion battery after the charging is completed is less than 20%, it is determined that the cycle performance of the lithium ion battery system is stable. Therefore, the invention can obtain a stable system, the cycle can reach more than or equal to 500 circles, and no obvious gas production occurs.
It should be noted that, the method provided by the invention is mainly applied to screening of a lithium-rich manganese-based battery system, the technical method of the invention is simple and efficient in process level, and the verification efficiency of the battery system can be obviously improved, so that the process of industrial application of a high-voltage system is promoted.
In order to more clearly understand the technical solution of the present invention, the technical solution of the present invention is described below by specific examples.
Example 1.
Using a lithium-rich manganese base (Li)1.2Ni0.13Co0.13Mn0.54O2) Matching the material A with a graphite negative electrode to perform the procedures of pole piece preparation, pole group assembly, liquid injection (comprising three electrolyte systems of EL1, EL2 and EL 3), formation and the like, and finishing the manufacture of the 2Ah soft package lithium ion batteryDo this.
Based on the method, firstly, a finished product lithium ion battery is charged to 4.6V at the room temperature of 25 ℃ by constant current of 200mA, and then floating charging is carried out for 10 hours by keeping the constant voltage of 4.6V;
and then, comparing the change of the floating charging current in the charging process, simultaneously carrying out a thickness expansion test on the soft package battery after the charging is finished, and determining the stability of the system through the change of the floating charging current and the change of the thickness.
Fig. 2a is a current-time change curve of different electrolyte systems of the lithium-rich manganese-based battery in the floating charge process, and as can be seen from fig. 2a, the current of the three electrolyte systems in the graph is gradually reduced to be constant within 10 hours, and no extremely unstable phenomenon such as sudden increase occurs, and the lower the stable current is, the lower the reaction degree of the system consuming active lithium ions is, the more stable the reaction is.
Fig. 2b is a bar graph of the thickness expansion after float charging for different electrolyte systems of a lithium-rich manganese-based battery. The figure shows that gas generation expansion of three electrolytic liquids in different degrees occurs after floating charge is carried out for 10 hours, and the higher the gas generation expansion rate is, the electrolyte is easy to oxidize and side reaction is easy to occur between the electrolyte and the interface of a positive electrode material under high voltage, so that gas precipitation is unstable under high voltage.
The results shown in fig. 2a show that, from the beginning of the float charging process, the current of each system is rapidly reduced to 0.05C, i.e. 10mA, within 3 hours, and the current is gradually reduced until 10 hours, and the current can be stabilized at about 0.001C, i.e. 2 mA. In particular, the stable current ratio of the EL2 and EL3 electrolyte systems is lower, which indicates that the system has less side reactions of lithium ion consumption, less polarization, more advantageous impedance and the like. Referring to fig. 2b, the thickness expansion rates of the three systems are 35%, 20% and 80%, respectively, which collectively indicates that the EL2 system can achieve high stability, and secondly the EL1 system, while the EL3 system produces a large amount of gas, which is the least stable.
FIG. 3 is a 25 ℃ cycle performance test curve of different electrolyte systems of the lithium-rich manganese-based battery. Shows the law of capacity retention rate of the batteries of three systems after 0.5C cycle test (namely a conventional test method). The longer the cycle life, the slower the decrease in the cycle discharge capacity retention rate, indicating that the system has higher electrochemical stability.
As can be seen from fig. 3, after the EL2 system is charged and discharged 500 times, the capacity retention rate decreases to 80% slowly, and the capacities of EL1 and EL3 decay to 80% respectively have lifetimes of about 320 times and 120 times, which indicates that the system has the highest electrochemical stability in the same period comparison, which is consistent with the rules of the float charging result reflected in fig. 2a and 2b, and indicates that the method related to the patent has accurate prediction capability.
Example 2.
Using a lithium-rich manganese base (Li)1.2Ni0.13Co0.13Mn0.54O2) And matching the materials B and C with a graphite negative electrode to perform the procedures of pole piece preparation, pole group assembly, liquid injection (EL2 electrolyte), formation and the like, and finishing the manufacture of the about 2Ah soft package battery.
Based on the method, firstly, a finished product lithium ion battery is charged to 4.6V at the room temperature of 25 ℃ by constant current of 200mA, and then floating charging is carried out for 10 hours under the condition of keeping the constant voltage of 4.6V;
and then, comparing the change of the floating charging current in the charging process, simultaneously carrying out a thickness expansion test on the soft package battery after the charging is finished, and determining the stability of the system through the change of the current and the thickness.
Under the same electrolyte system, the active material will cause the difference of system stability. In the embodiment, the lithium-rich manganese-based active material prepared by two different processes is subjected to float charging under high voltage, the current can be stabilized at 0.001C within 10 hours, the gas yield of the lithium-rich manganese-based material B is obviously higher than that of the lithium-rich manganese-based material C, the expansion rates of the lithium-rich manganese-based material B and the lithium-rich manganese-based material C are respectively 40% and 25%, and the stability of the process material of the lithium-rich manganese-based material C is higher than that of the lithium-rich manganese-based material B.
Similarly, the cycle life of the lithium-rich manganese-based material B-EL2 system is verified by the cycle period of the full-cell at normal temperature of 0.5C: the capacity retention rate of 80% is corresponded to 240 times, and the cycle life of the lithium-rich manganese-based material C-EL2 system is as follows: the capacity retention rate of 80% is obtained after 420 times of charge evaluation, and the result rule is consistent with that of the floating charge evaluation method.
Example 3.
Based on the method of the invention, a lithium-rich manganese base (Li) is adopted1.2Ni0.13Co0.13Mn0.54O2) And matching the materials B and C with a graphite negative electrode to perform the procedures of pole piece preparation, pole group assembly, liquid injection (EL2 electrolyte), formation and the like, and finishing the manufacture of the about 2Ah soft package battery.
Firstly, charging a finished product lithium ion battery to a voltage of 4.7V at a constant current of 200mA at a room temperature of 25 ℃, and then maintaining the constant voltage of 4.7V for float charging for 5 hours;
and then, comparing the change of the floating charging current in the charging process, and simultaneously carrying out a thickness expansion test on the soft package battery after the charging is finished.
The floating charging is carried out under the voltage of 4.7V, the gas production rate is accelerated in the process, the expansion rate of the battery exceeds 100 percent after 5 hours, and is respectively 180 percent and 115 percent, although the rule is consistent with that under the condition of 4.6V, and the screening can be finished in a shorter time, but in view of comprehensive consideration of safety and the like, the implementation of the method by increasing the voltage is not recommended.
Since the buckling performance cannot be used as a reference in a lithium-rich manganese-based special system, the electrolyte (reference example 1) and the cathode material (reference example 2) with good performance can be evaluated and screened in a very short time based on a soft-package full battery.
It should be noted that the above evaluation techniques for the stability of the lithium-rich manganese-based battery system with reference to the examples are illustrative and not restrictive, and therefore, changes and modifications without departing from the general concept of the present invention shall fall within the protection scope of the present invention.
Based on the technical scheme, the method can effectively screen the electrolyte (anode) system which is more stable under high voltage within 10 hours at the longest under the condition of designating the anode (electrolyte) system, the evaluation result is completely consistent with the rule of the result which is invalid after the cycle period, and meanwhile, the implementation method is simple, convenient and feasible, the test period is obviously shortened, and the test efficiency is improved.
In conclusion, compared with the prior art, the method for rapidly evaluating the stability of the high-voltage lithium ion battery system provided by the invention has the advantages that the design is scientific, and due to the limitation of the button half battery and the long-period defect of the full battery cycle performance test, the method is based on the full battery system, the high-voltage system with stable cycle can be simply and effectively screened out by using the test method of the gas expansion rate after floating charge and charging, and the practical significance is great.
In the present invention, the stability condition of the electrolyte (positive electrode material) is defined based on the same kind of positive electrode (electrolyte).
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.