Abstract: With the wide application of lithium batteries in the fields of power and energy storage, the lifetime of lithium batteries has become an important shackle limiting their development. Considering the fact that the battery in storage state will also undergo performance degradation and affect the lifetime, it is of great significance to study the impedance change during the self-discharge process of the battery. This study aids in characterizing internal electrochemical reactions and structural phase transitions, which are essential for enhancing battery lifespan models. This paper analyses the internal causes of battery capacity degradation through the capacity increment curve. Meanwhile, on the basis of the variation of the impedance spectrum of the battery in the self-discharge process under different influencing factors, this paper analyses the theoretical principle by using the relaxation time distribution method. Finally, on the basis of the principle derivation of the battery self-discharge aging process, this paper summarizes the variation of battery capacity degradation with time, and combines the experimental data to establish the battery capacity degradation model under different influencing factors. The results show that the internal resistance of the solid electrolyte interface (SEI) of the battery increases during the storage process, and the corresponding increase in impedance is greater when the storage temperature of the battery is higher and the initial state of charge (SOC) is larger. The main reasons for battery aging during self-discharge are the consumption of recyclable active lithium ions and the growth of SEI. Meanwhile, this paper deduces that the capacity loss of the battery is approximately 0.5 power relationship with time, and uses the experimental data to fit to obtain the capacity change model of the battery under the influence of different initial SOC and different storage temperatures, which provides a further reference for the prediction of lithium battery life model.