Ultrasonic For The Prediction Of SoC Of Lithium-ion Batteries And Its Decoupling

Lithium-ion batteries（LIBs）have been widely used in many fields,including electronic devices,new energy vehicles and energy storage systems due to their high energy density,superior cycling performance and environmental friendliness.In practical applications,to extend the cycling life of LIBs and avoid safety accident,the state of charge（SoC）needs in to be measured accurately to prevent LIBs from over-charging/over-discharging.However,the SoC of LIBs cannot be measured directly owing to the complex structure of LIBs.The SoC can be predicted only by using related characteristic calculation formula based on some external parameters,e.g.voltage,current,temperature,internal resistance,etc..These methods are unavailable for practical industrial application because of the low accuracy.Therefore,this paper introduces a method to acquire the SoC of LIBs using ultrasonic technique with good directivity and penetration,which can measure the SoC of batteries quickly and accurately.Firstly,the relationship between ultrasonic transmission signals and SoC during charging/discharging process is investigated based on the Li Fe PO₄|Graphite full cell.It is founded that the change of intensity of ultrasonic transmission signals increases with SoC increasing.To discover the relationship between ultrasonic reflection signals and SoC,the contribution of cathode and anode to ultrasonic signals of full cells is decoupled.By assembling LFP|Li and graphite|Li（AG|Li）half cells,the relationship is studied based on the ultrasonic reflection signals.The result shows that with the SoC changing,the tendency of reflection intensity change of LFP|Li and AG|Li half cells is the same.When Li-ions are extracted,the ultrasonic intensity decreases.Moreover,the changing rate of LFP|Li half cells is lower than that of AG|Li ones.To further discover the reason of ultrasonic characteristic changing with SoC,it is investigated by adopting different current densities and compassion densities for LFP|Li and AG|Li half cells.For LFP|Li half cells,the change rates at different current densities are almost the same.Different current densities result in different internal stress,so the internal stress is not the major factor on relationship between ultrasonic intensity and SoC.On the contrary,the results of LFP|Li half cells with different compaction densities illustrates that the changing rates are different for different cells,and it is largest for the cells with lowest compaction density.Compaction density shows influence on the volume change and change of particle packing mode resulting from Li insertion,which plays an important role on the changing rate.Moreover,the results of AG|Li half cells with different current densities and compaction densities are consistent with those of LFP|Li half cells.As a result,the volume change and change of particle packing mode rather than internal stress is a main factor affecting the relationship between ultrasonic intensity and SoC.