The cycle life of lithium-ion batteries is an important performance index. Whether the study of cathode or anode materials, the cycle performance of batteries assembled with applied materials needs to be tested in the laboratory, and the experimental instruments and methods are explained in detail in this paper.
Button battery charging and discharging modes include constant current charging, constant voltage charging, constant current discharging, constant resistance discharging, hybrid charging and discharging, and step charging and discharging in different modes. Constant current charging (CC), constant current-constant voltage charging (CC-CV), constant voltage charging (CV), and constant current discharging (DC) are often used in the laboratory to test and analyze the battery charging and discharging behavior, while the step charging and discharging modes are mostly used for testing the DC internal resistance, polarization, and diffusion impedance performance. Considering the content of active materials and the influence of the pole piece size on the test current, the constant current charging is often in the form of current density, such as mA/g (current per unit active material mass), mA/cm2 (current per unit pole piece area). The size of charge/discharge current is often expressed in terms of charge/discharge rate, i.e.: charge/discharge rate (C) = charge/discharge current (mA) / rated capacity (mA˙h), such as the rated capacity of 1000 mA˙h battery charged and discharged at a current of 500 mA, the charge/discharge rate is 0.5 C. The current industry standard QCT/743 for lithium-ion batteries for electric vehicles has been released for use In 2006, it is stated that the charge/discharge current for lithium-ion batteries is C/3, so the charge/discharge behavior test with C/3 is also often found in the charge/discharge test of lithium-ion batteries in the laboratory.
There are three forms of multiplier performance testing, including the same multiplier constant-current constant-voltage charging and different multiplier constant-current discharge test to characterize and evaluate the performance of lithium-ion batteries at different discharge multipliers; or the same multiplier constant-current discharge and different multiplier constant-current charging test to characterize the charging performance of the battery at different multipliers; and charge and discharge using the same multiplier for charge and discharge testing. The charge/discharge multipliers often used 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.
To test the cycle performance of the battery, it is necessary to determine the charge and discharge mode of the battery, cycle until the battery capacity drops to a specified value (usually 80% of the rated capacity), the number of times the battery has been charged and discharged, or compare the remaining capacity of the battery after cycling the same number of cycles, so as to characterize the cycle performance of the test battery. In addition, the test environment of the battery has a certain influence on its charge and discharge performance.
The following section will introduce in detail the instruments and methods used for charge/discharge testing.
1 Introduction of experimental instruments
The charge/discharge test of lithium battery generally adopts constant current-constant voltage charging and constant current discharging modes, records the test time, voltage and current data in the process, and characterizes the electrochemical performance parameters such as capacity, coulombic efficiency, charge/discharge platform and changes in internal parameters of the battery by analyzing the changes in the data in the process.
At this stage, the battery test systems used by relevant units at home and abroad include Arbin's battery test system, Xinwei's battery test system, Blue Power's series of battery test systems and MACCOR's battery test system, etc. See Table 1. Some electrochemical workstations also have button lithium battery electrochemical performance test function, but due to the channel design, functional design and other reasons, mostly used for battery cyclic voltammetry test analysis, impedance testing and short time charge and discharge test, electrochemical workstation instrument manufacturers including Autolab, Solartron, VMP3, Princeton, Zahner (IM6), Shanghai Chenhua, etc. Shanghai Chenhua, etc.
In the process of laboratory lithium battery testing, explosion-proof boxes and thermostats are often used (Figure 1). Laboratory battery explosion-proof boxes are mostly used for high-capacity battery testing, and also used in the study of some special performance testing of button batteries, such as high multiplier, high-temperature performance testing. Laboratory thermostat temperature control more than 25 ℃, and the actual temperature and the set temperature between the accuracy of the temperature difference does not exceed 1 ℃. In the battery high and low temperature performance testing, the lowest temperature can reach 70 ℃, the highest temperature up to 150 ℃. Considering the wide temperature range of the thermostat is more expensive, and the application is more concentrated, so it is recommended that multiple thermostats set different temperatures for centralized testing, that is, the same verification material to assemble multiple buckle batteries are tested at room temperature and high and low temperature performance, laboratory testing common temperature of 25 ℃, 55 ℃ and 80 ℃ (Figure 2). In the choice of thermostat, try to use a special thermostat for battery testing, such thermostats contain professional insulation insulation port for connecting the battery test leads. When the battery is connected to the test fixture, insulated tweezers should be used, and the test battery should be placed neatly in the explosion-proof box or thermostat, the test temperature should be set, and the battery test procedure should be started after the temperature reaches the set temperature.
