Lithium evolution is the main failure problem faced by lithium-ion batteries during low temperature charging. Due to the low diffusion rate of lithium ions in the solid phase of graphite particles and in the liquid phase of the electrolyte at low temperatures, the negative polarization increases during charging and lithium evolution occurs, and the precipitated lithium metal is divided into reversible lithium directly involved in the battery discharge reaction and irreversible lithium generated by the reaction with the electrolyte (" dead lithium "). The results show that "dead lithium" directly causes the loss of active lithium, and may also cause the loss of active substances and the increase of impedance. With the increase of the number of cycles, the minimum potential of negative charge increases, the SOC range of charge and discharge Narrows, the lithium analysis is inhibited, and the attenuation of cycle capacity becomes relatively stable. It is found that the decay mechanism of lithium evolution for the cycle capacity is different with different temperature. The precipitation of lithium metal at lower temperature produces more "dead lithium", resulting in more loss of active lithium and more decay of cycle capacity. The study of cycling performance at 5 ℃ showed that the capacity diving occurred after 20 weeks of cycling, which was attributed to the formation of deposits on the surface of the negative graphite electrode caused by trace lithium evolution, which filled the pores on the surface of the electrode and blocked the diffusion of lithium ions inside the electrode in the liquid phase.
The lithium evolution and dissolution behavior of lithium iron phosphate battery during charge and discharge at low temperature were studied. The morphology change, element distribution and surface composition of the negative electrode of the disassembled battery after low temperature charge and discharge were analyzed. The charge and discharge performance and cycle performance of the battery after low temperature charge and discharge were investigated, as well as the influence mechanism of irreversible lithium evolution.
The main conclusions are as follows:
1. At low temperature, lithium evolution reaction occurs in the negative electrode of lithium iron phosphate battery during charging, lithium metal precipitation during shelving is not back embedded in graphite, and electrochemical dissolution reaction occurs during discharge. Based on the Li-dissolution behavior of the battery at low temperature, the reversible Li-evolution capacity, irreversible Li-evolution capacity and total Li-evolution capacity are calculated. The results show that the proportion of irreversible Li-evolution capacity is higher when the total Li-evolution capacity is larger, and the proportion of irreversible Li-evolution capacity is higher in the lower temperature range.
2. After disassembly of the low-temperature charge-discharge battery, it was found that the negative electrode morphology did not change significantly at 5 ℃, but oxygen-containing substances were distributed in the surface, surface and inner layers, mainly in the pores between graphite particles; At -8 ℃ and -12 ℃, the surface of the negative electrode was covered with oxygen-containing compounds, and the morphology and element distribution of the inner layer of the electrode were basically unchanged. The analysis shows that a slight lithium evolution reaction occurs in all regions of the negative electrode when charging at 5 ℃, while the lithium evolution reaction mainly occurs on the negative electrode surface when charging at -12 ℃.
3. The charge and discharge capacity of the battery decreases after low temperature charge and discharge, and the capacity decreases more significantly with the decrease of charge and discharge temperature; After charging and discharging at 5 ℃, the capacity decay of the battery is faster than that of the original battery at 0.5 C cycle, and the battery at lower temperature is better than that of the original battery at 0.5 C cycle. After low temperature charge and discharge, the decrease of battery capacity is mainly due to the loss of active lithium. After lower temperature charge and discharge, the loss of active lithium is more serious, the minimum lithium impingement potential of the negative graphite of the battery increases, the range of impingement lithium is narrowed, and the cycle performance is better. After charging and discharging at 5 ℃, the cycle capacity of the battery decreases faster, because the non-dissoluble lithium changes the element distribution, pore structure and surface composition of the negative electrode, and the stability of the SEI film is poor and the polarization increases greatly during the cycle process.
