Test method for open circuit voltage of lithium batteries

　　The balance potential test process of electrode materials is as follows: electrode materials are prepared into electrode sheets, and button half cells are assembled with lithium metal. The open circuit voltage of button half cells under different SOC conditions is measured, and the mathematical expression of the open circuit voltage curve is determined by polynomial or Gaussian fitting. The open circuit voltage test methods mainly include:

　　(1) The basic principle of galvanostatic intermittent titration technique (GITT) is to apply a constant current to the measuring system in a specific environment and cut off the current after it lasts for a period of time. Observe the change of the system potential with time in the applied current section and the voltage that reaches equilibrium after relaxation (i.e. open circuit voltage). GITT test examples are as follows: (i) Charge at C/50 until the voltage reaches the upper limit, such as 4.2 V; (ii) Allow to stand for 2 hours; (iii) Discharge at 1C for 6min, and record the discharge capacity; (iv) Hold for 15min and record the voltage; (v) Repeat steps (iii) and (iv) for 9 times; (vi) Discharge at C/50 until the voltage reaches the lower limit, such as 3.0 V; (vii) Normalize the capacity voltage curves recorded in steps (iii) and (iv) to form a SOC voltage curve, and fit the mathematical expression of the open circuit voltage curve.

　　(2) Low current charge discharge curve: charge and discharge at a very low magnification (such as 0.01C) current at constant current, set the upper and lower limits of voltage, and obtain the battery low current charge discharge curve. The point with the same amount of electricity is taken as the starting point of the curve. Average the voltage in the charge discharge curve, normalize the abscissa of the curve according to the theoretical capacity, and then use the curve fitting to obtain the open circuit voltage curve

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