关键词: 锂电池, 开路电压, 界面, 黑斑, 析锂

Abstract:

In this paper, the influencing factors and causes of low initial open circuit voltage, the correlation between low initial open circuit voltage and interface with black spots, and its relationship with battery performance were discussed. The factors affecting the initial open circuit voltage were mainly verified from five aspects : type & stability of electrolyte, standing time, electrode potential of positive & negative electrode, moisture of battery after baking, and impurity content of the negative electrode. At the same time, the interface under 100% state of charge and battery performance of the batteries with low initial open circuit voltage were studied. The results show that : 1) For the regenerated material system, affected by the stability of the electrolyte, especially the electrolyte containing additive such as tris ( trimethylsilyl ) phosphate (TMSP), the content of free acid in the electrolyte increases greatly during the high temperature standing process, resulting in the phenomenon of low initial open circuit voltage. The free acid of the electrolyte containing ethylene sulfate (DTD) additive is the second, and the room temperature electrolyte is the most stable during high temperature standing process. 2 ) The initial open circuit voltage of the battery is basically stable above 24 h, and the change trend is affected by the material system. 3) The electrode potential of the positive and negative electrodes in different electrolytes is displayed. The initial open circuit voltage mainly depends on the difference of the negative electrode potential; 4 ) The higher the moisture of battery after baking, the higher the probability of low initial open circuit voltage; 5 ) The phenomenon of low initial open circuit voltage mainly occurs in the material system of the regenerated anode and is associated with increase of Cu cation in electrolyte. The positive electrode and negative electrode of the battery are immersed in the electrolyte after injection. The electrode potential of the positive electrode is high and the electrode potential of the negative electrode is low. It is speculated that the high content of Cu impurity in the regenerated anode material is dissolved under the HF in the electrolyte to produce Cu cation. The newly dissolved Cu ions will lead to the increase of the electrode potential of the negative electrode continuously and the decrease of the potential difference between the positive electrode and the negative electrode, resulting in the phenomenon of low initial open circuit voltage. 6 )There is a central black spot at the interface of battery under 100% state of charge with low initial open-circuit voltage, and the black spot lithium deposition increases with the increase of the charging current. The SEM and EDS characterization of the interface black spot showed that the Cu element in the black spot area was significantly higher than that in the normal area, which was consistent with the increase of Cu impurity content in the electrolyte with lower initial open circuit voltage. Therefore, in the production, the stability and consistency of the initial open circuit voltage in battery should be improved from the aspects of controlling the Cu impurity content of the regenerated graphite, improving the water consistency, and optimizing the stability of the electrolyte, so as to improve the performance and competitiveness of lithium iron phosphate batteries.

Key words: Li-ion battery, open-circuit voltage, interface, black spot, lithium precipitation