The charging and discharging modes of button battery include constant current charging, constant voltage charging, constant discharge, constant resistance discharge, hybrid charging and discharging, and step charging and discharging modes. Constant-current charging (CC), constant-current-constant-voltage charging (CC-CV), constant-voltage charging (CV) and constant-current discharge (DC) are often used to test and analyze the charging and discharging behavior of batteries in laboratories, while step charging and discharging mode is mostly used to test the DC internal resistance, polarization and diffusion impedance. Considering the influence of active material content and electrode plate size on test current, constant-current charging is often in the form of current density, such as mA/g (current per unit of active material mass), mA/cm2 (current per unit of electrode plate area). The charge and discharge current is usually expressed by the charge and discharge ratio, that is, the charge and discharge ratio (C) = the charge and discharge current (mA)/rated capacity (mA·h). If the rated capacity is 1000 mA·h, the charge and discharge ratio is 0.5C. At present, the industry standard QCT/ 743-2006 for lithium-ion batteries used in electric vehicles has been published and pointed out that the general charging and discharging current of lithium ions is C/3, so the charging and discharging behavior test containing C/3 is often used in the laboratory lithium-ion battery charging and discharging test.

           

                There are three types of magnification performance test, including constant current and constant voltage charging at the same magnification rate and constant discharge current testing at different magnification rates to characterize and evaluate the performance of lithium ion batteries at different discharge rates. Or use the same magnification rate for constant discharge, and different magnification constant-current charging test, characterizing the battery charging performance at different magnifications; And charge and discharge test with the same rate. The commonly used charging and discharging ratios are 0.02 C, 0.05 C, 0.1 C, C/ 3,0.5 C, 1 C, 2 C, 3 C, 5 C and 10 C, etc.

When testing the battery cycle performance, it is mainly necessary to determine the charging and discharging mode of the battery, cycle until the battery capacity drops to a specified value (usually 80% of the rated capacity), the number of charging and discharging times of the battery, or compare the remaining capacity of the battery after the same cycle, to characterize the battery cycle performance. In addition, the test environment of the battery has certain influence on its charge-discharge performance.

The instruments and methods used for charge and discharge testing are described in detail below.

        

• 1. Introduction of experimental instruments
  

The charge and discharge test of lithium battery generally adopts constant-current - constant-voltage charging and constant-current discharge mode, and records the test time, voltage and current and other data in the process. By analyzing the changes of the data in the process, the electrochemical performance parameters such as the capacity of the battery or material, coulomb efficiency, charging and discharging platform and the changes of the internal parameters of the battery are characterized.

At present, the battery test systems used by relevant units at home and abroad include the battery test system of Arbin Company, the battery test system of Xinwei Company, the battery test system of Blue Power Company and the battery test system of MACCOR Company, etc., as shown in Table 1. In addition, Bette battery test system and Bitrode battery test system are mainly used for testing and analysis of large-capacity batteries, battery packs and other devices. Some electrochemical workstations can also test the electrochemical performance of button lithium batteries, but due to channel design, function design and other reasons, they are mostly used for cyclic voltammetry analysis, impedance test and short time charge and discharge test of cell.Electrochemical workstation instrument manufacturers include Autolab, Solartron, VMP3, Princeton, Zahner (IM6), Shanghai Chenhua, etc.

In laboratory lithium battery testing process, explosion-proof box and thermostat are often used (Figure 1). The battery explosion-proof box used in the laboratory is mostly used for the test of large-capacity batteries. It will also be used in the study of some special performance tests of buckle batteries, such as high rate and high temperature performance tests. Laboratory thermostat temperature control is mostly 25 ℃, and the temperature difference accuracy between the actual temperature and the set temperature is not more than 1 ℃. In the battery's high and low temperature performance test, the lowest temperature can reach 70 ℃, the highest temperature can reach 150 ℃. Considering that incubators with a wide temperature range are more expensive and have more concentrated applications, it is recommended to set multiple incubators at different temperatures for centralized testing, that is, to assemble multiple buckle batteries with the same verification material to test normal temperature and high and low temperature performance respectively. Common laboratory testing temperatures are 25 ℃, 55 ℃ and 80 ℃ (Figure 2). When choosing the thermostat, try to use the thermostat specially used for battery testing, such thermostat contains professional insulation for connecting the battery test wire. When the battery is connected to the test fixture, it is necessary to use insulated tweezers, and the test battery should be neatly placed in the explosion-proof box or thermostatic box, set the test temperature, and start the battery test procedure when the temperature reaches the set temperature. It is recommended to label the test information during the test process (Figure 3).

• 2. General experimental procedures of charge and discharge test
  

• Install the test battery on the test instrument and place it in a test environment of (25±1) ℃. Set the following procedures: Let stand for 10 minutes; Charge at 1.0C constant current to 4.2V, and then charge at constant voltage until the current drops to 0.05C, and stop charging; Let stand for 5 min; Then a constant current of 1.0C is applied to 3.0V. Repeat the preceding steps 5 to 10 times.
  

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• The above test parameters are conventional full-battery test parameters. Generally, the voltage range of positive material/lithium metal buckle battery is 3.0 ~ 4.3V, and the voltage range of negative material/lithium metal buckle battery is 0.005 ~ 1.0V. Special high-voltage cathode materials (such as high-voltage lithium cobalt, lithium spinel manganese, lithium manganese rich basic oxide materials) or other cathode materials (such as lithium iron phosphate materials) can be adjusted according to the characteristics of electrode materials and electrolyte, solid electrolyte tolerance oxidation voltage, other parameters remain unchanged. Negative material/metal Li-button batteries and non-lithium positive material (e.g. MnO2)/metal Li-button batteries are first discharged to the lowest voltage window and then charged. It should be noted that the current testing range of negative electrode materials in many articles is 0.005 ~ 3.0V, while in the full cell testing process, the voltage range that can be used for the negative half cell test is actually no more than 1.0V. For example, for graphite or silicon-based negative electrode materials, the voltage range available is 0.005 ~ 0.8V. For lithium titanate, a negative material, the voltage range available is 1.2 to 1.9 V.Therefore, the high capacity and high first coulomb efficiency obtained in a wide voltage range in some articles cannot be used in the whole battery, so the practical significance is not great. For soft carbon or hard carbon anode materials, or currently under development of composite metal lithium anode materials, discharge cut-off voltage can be lower, such as 0 mV or even 50 mV, specific circumstances need specific analysis. It is suggested that the test control voltage range of most anode materials is 0.005 ~ 1.0V. If the voltage range is beyond this, special statements should be made in the statement of results and the description of application prospects, so as not to exaggerate the results.
  

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• When testing the actual capacity of battery materials, the charge and discharge should be carried out with a small ratio as far as possible to reduce the capacity error caused by polarization and obtain the real capacity of the battery. Generally, 0.1C ratio is selected for testing.

• Operators should wear insulation gloves, masks and protective glasses when loading and unloading buckle batteries on the test equipment; Due to the large number of test channels, special marks shall be made on test batteries and channels, and eye-catching labels shall be affixed in front of relevant instruments to prevent others from misoperation.

  
  

• Routine experimental procedure of 3 - rate charge and discharge test

• Rate charge and discharge test is generally divided into three forms: same rate charge with different rate discharge, different rate charge with the same rate discharge and different rate charge and discharge test. The following uses the button battery test whose charge-discharge voltage window is 3.0 to 4.2 V as an example.

• The battery is 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, stop at 4.2V constant voltage until the current drops to 0.05C, and then carry out rate charge and discharge test in different forms.

• The experimental flow of discharge with the same rate of charge and different rate of discharge is as follows: after standing for 5 min, discharge at different rate to 3.0V, and record the discharge capacity. After standing for 10 min, charge at 0.5C constant current to 4.2V, and stop at 4.2V constant voltage until the current drops to 0.05C. The experimental process of charging at different rates and discharging at the same rate is as follows: after standing for 5 min, constant discharge at 0.5C rate to 3.0V, standing for 10 minutes, and then constant current charging at different rates to 4.2V, constant voltage at 4.2V until the current drops to 0.05C.

• The experimental flow of charge and discharge at different rates is as follows: after standing for 5 min, constant charge at different rates to 3.0V, standing for 10 min, and then constant current charge at the same rate (current) to 4.2V, and stop at constant voltage at 4.2V until current drops to 0.05C.

• According to the test form, change different magnifications and repeat one of the above experimental procedures, charging and discharging magnifications from low to high (generally higher magnifications such as 0.2C, 0.5C, 1C, 2C and 5C). 5 ~ 10 cycles of charge and discharge with the same rate are recommended.

• 4. General experimental procedures of charge and discharge cycle test

• When testing the circularity of the battery, the number of cycles can be increased on the basis of the above charging and discharging test (2 sections), and the capacity retention rate after the same number of cycles can be compared. Or repeated charge and 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 experimental procedures of high and low temperature test

• In the high and low temperature performance test of lithium ion batteries, the high temperature performance test is generally set at 45 ℃, 55 ℃, 80 ℃ or higher, and the low temperature performance test is generally set at 0 ℃, -10 ℃, -20 ℃, -30 ℃ or -40 ℃. The test procedure is the same as in Section 2, 3 and 4. The test data need to be compared with the data at room temperature, so the charge and discharge test at room temperature should be conducted before the high and low temperature test (Section 2 test content). In the discharge efficiency test, it is recommended to use constant current-constant voltage (CC-CV) mode at room temperature (25±1) ℃ to charge to 100% SOC. After standing for 30 min at different temperatures, constant discharge (DC) is carried out.