The cycle life of lithium ion battery is an important performance index. No matter the research of anode material or anode material, the cycle performance test of battery assembled by application materials should be carried out in the laboratory. In this paper, the experimental instruments and methods are explained in detail.
Buckle battery charge and discharge modes include constant current charge, constant voltage charge, constant discharge, constant resistance discharge, hybrid charge and discharge, step and other modes. Constant current charge (CC), constant current-constant voltage charge (CC-CV), constant voltage charge (CV) and constant discharge current (DC) are often used to test and analyze the charging and discharging behavior of batteries in the laboratory, while the step charge and discharge mode is mostly used to test the DC internal resistance, polarization and diffusion impedance. Considering the influence of the content of the active material and the size of the electrode on the test current, the current density often appears in the form of constant current charging, such as mA/g(current per unit mass of the active substance) and mA/cm2(current per unit area of the electrode plate). The size of the charge and discharge current is usually expressed by the charge and discharge rate, that is: charge and discharge rate (C)= charge and discharge current (mA)/ rated capacity (mA˙ H), such as the rated capacity of 1000 mA˙h to charge and discharge current of 500 mA, the charge and discharge rate is 0.5C. At present, the industry standard QCT/ 743-2006, which has been released and used for lithium ion batteries for electric vehicles, indicates that the universal charge and discharge current of lithium ion is C/3, so the charge and discharge behavior test containing C/3 often appears in the charge and discharge test of lithium ion batteries in laboratories.
There are three forms of rate performance test, including constant current and constant voltage charging at the same rate and constant discharge test at different rates, to characterize and evaluate the performance of lithium ion battery at different discharge rates; Or using the same rate of constant discharge, and constant current charging test at different rates, to characterize the battery charging performance at different rates; And charge and discharge with the same rate of charge and discharge test. The charge and discharge rates commonly used are 0.02C, 0.05c, 0.1c, C/3, 0.5C, 1 C, 2 C, 3 C, 5 C and 10 C, etc.
When testing the performance of the cycle of the battery, mainly to determine the battery charging and discharging mode, cycle to battery capacity decline to a specified value (usually 80% of the rated capacity), when the battery charge and discharge times, experienced by contrast or same weekly cycle after the residual capacity of battery, this characterization test cell cycle performance. In addition, the test environment of the battery has a certain influence on its charge-discharge performance.
The instruments and methods used in charge and discharge tests will be described in detail below.
1. Introduction of experimental instruments
Lithium battery charge and discharge test commonly used constant current, constant voltage and constant exile electric mode, record the process of testing data, such as time, voltage and current by analyzing the change of the data in the process of the capacity of battery or materials in the characterization, the coulomb efficiency, and battery charge and discharge platform internal parameters such as electrochemical performance parameters.
At present, the battery test systems used by relevant institutions at home and abroad include the battery test system of Arbin Company, The battery test system of Xinwei Company, the battery test system of Landian Company and the battery test system of MACCOR Company, as shown in Table 1. In addition, the Battery test system of Beit and Bitrode is mainly used for the test and analysis of large-capacity batteries, battery packs and other devices. Some electrochemical workstations also have the function of buckle lithium battery electrochemical performance test, but due to channel design, functional design and other reasons, they are mostly used for cyclic voltammetry test analysis, impedance test and short time charge and discharge test of the cell. Electrochemical workstation instrument manufacturers include Autolab, Solartron, VMP3, Princeton, Zahner(IM6), Shanghai Chenhua, etc.
Explosion-proof boxes and thermostats are often used in laboratory tests of lithium batteries (Figure 1). Laboratory battery explosion-proof box is used for large-capacity battery testing, in the study of buckle battery some special performance test will also be used, such as high rate, high temperature performance test. The temperature of the laboratory thermostat is mostly 25 ℃, and the accuracy of the temperature difference between the actual temperature and the set temperature is not more than 1 ℃. In the high and low temperature performance test of the battery, the lowest temperature can reach 70 ℃, the highest temperature can reach 150 ℃. Considering that the thermostat with a wide temperature range is expensive and the application is more concentrated, it is suggested to set different temperatures for centralized testing of multiple thermoboxes, that is, to test the normal temperature and high and low temperature performance respectively by assembling multi-button batteries with the same verification material. The common temperatures of laboratory tests are 25 ℃, 55 ℃ and 80 ℃(Figure 2). When choosing a thermostat, try to use a thermostat specially for battery testing. This kind of thermostat contains a professional insulation port for connecting battery test wires. When connecting the battery to the test fixture, use insulating tweezers and place the battery in an explosion-proof box or a constant temperature box. Set the test temperature and start the battery test program when the temperature reaches the set temperature. It is recommended to label the test information during the test
2. Routine experiment process of charge and discharge test
Install the test battery on the test instrument and place it in the test environment of (25±1)℃. Set the following procedures: stand for 10 min; Charge at constant current of 1.0C to 4.2V, and then charge at constant voltage until the current drops to 0.05c and the charging stops; Let stand for 5 min. Then a constant current of 1.0C was applied to 3.0V. Repeat the above charging and discharging steps 5 to 10 times.
The above test parameters are conventional full battery test parameters. The voltage range of the general positive material/lithium metal button battery is 3.0 ~ 4.3V, and the voltage range of the negative material/lithium metal button battery is 0.005 ~ 1.0V. Special high voltage cathode materials (such as high voltage lithium cobalt acid, spinel lithium nickel manganese acid, lithium rich manganese oxide and other materials) or other cathode materials (such as lithium iron phosphate material) can be adjusted according to the characteristics of electrode material and electrolyte, solid electrolyte withstand oxidation voltage, other parameters remain unchanged. The anode material/metal lithium button battery and the lithium-free positive material (such as MnO2, etc.)/ metal lithium button battery are first discharged to the lowest voltage window during the test, and then charged. It should be noted that the current test range of anode materials in many articles is 0.005 ~ 3.0V, and in the full battery test process, the voltage range corresponding to the negative half battery test is generally not more than 1.0V, for example, for graphite or silica-based anode materials, the available voltage range is 0.005 ~ 0.8V, For lithium titanate anode material, the available voltage range is 1.2 ~ 1.9 V. Therefore, for the high capacity and high first coulomb efficiency obtained in a wide voltage range in some articles, they cannot be used in the full battery, and have little practical significance. For soft carbon or hard carbon anode materials, or the composite metal lithium anode materials currently under development, the discharge cut-off voltage can be lower, such as 0 mV or even 50 mV, the specific situation needs to be analyzed. It is recommended that the half-cell test control voltage range of most anode materials is 0.005 ~ 1.0V. Beyond this voltage range, special statements should be made in the statement of results and the description of application prospects to avoid exaggerating the results.
When testing the actual capacity of battery materials, charge and discharge at a small rate should be used as far as possible to reduce the capacity error caused by polarization and get the true capacity of the battery. Generally, 0.1c is selected for testing.
Operators should wear insulating gloves, masks and protective glasses when loading and unloading buckle-type batteries on the test instruments. Because there are many test channels, it is necessary to mark the test battery and test channel with special labels, and paste eye-catching labels in front of the relevant instruments to prevent others from misoperation.
3 rate charge and discharge test routine experiment process
Rate charge and discharge test is generally divided into three forms, the same rate charge different rate discharge, different rate charge the same rate discharge and different rate charge and discharge test. This section uses the button battery test with the charging and discharging voltage window of 3.0 V to 4.2 V as an example.
The battery was connected to the test instrument and placed in a steady-state environment for 5 min. Discharge at 0.5C current to 3.0V, stand for 10 min, charge at 0.5C constant current to 4.2V, at 4.2V constant voltage to the current drops to 0.05c cutoff, and then perform rate charge and discharge tests in different forms.
The experimental process of charging at the same rate and discharging at different rates was as follows: discharging at different rates to 3.0V after standing for 5 min, and recording the discharge capacity; charging at 0.5C constant current to 4.2V after standing for 10 min, and the current dropped to 0.05C at 4.2V constant voltage. The experimental process of charging at different rates and discharging at the same rate was as follows: after standing for 5 min, constant discharging at 0.5C rate to 3.0V, standing for 10 minutes, and then charging at different rates to 4.2V at 4.2V constant voltage to the current dropping to 0.05c.
The experimental process of charging and discharging with different rates was as follows: after standing for 5 min, constant current was charged to 3.0V at different rates; after standing for 10 min, constant current was charged to 4.2V at the same rate (current); at 4.2V, constant voltage to current decreased to 0.05c.
According to the test form, change different rates to repeat one of the above experimental process, charge and discharge rates from low to high (generally 0.2C, 0.5C, 1C, 2C and 5C higher rates). It is recommended to charge and discharge the same rate for 5 ~ 10 times.
4 charge and discharge cycle test routine experimental process
When testing the cycling of the battery, you can increase the number of cycles on the basis of the charge and discharge test (2 sections) above, and compare the capacity retention rate after the same cycle number. Or repeat the charge-discharge cycle. When the discharge capacity is lower than 80% of the initial discharge capacity for two consecutive times, determine the cycle number at this time.
5. Routine experiment process of high and low temperature test
In the high and low temperature performance test of lithium ion battery, the high temperature performance test is generally set to 45 ° C, 55 ° C, 80 ° C or higher, and the low temperature performance test is generally set to 0 ° C, -10 ° C, -20 ° C, -30 ° C or -40 ° C. The testing process is the same as 2, 3 and 4 sections. The test data need to be compared with the data at room temperature, so the charge and discharge test at room temperature should be performed before the high and low temperature test (that is, two test sections). For discharge efficiency test, it is recommended to charge cc-CV mode at room temperature (25±1)℃ to 100% SOC, and stand for 30 min at different temperatures before constant discharge (DC).
