Study On Failure Regularity Of LiNi_0.5Mn_0.3Co_0.2O₂/graphite 18650 Battery Pack

Lithium-ion batteries are widely used in electric vehicle field due to their high energy density and long life.Single-cell lithium-ion batteries must be connected in series or in parallel to meet the voltage and capacity requirements of electric vehicles.Even if the manufacturing conditions are the same,there are still differences such as voltage,ohmic internal resistance,and discharge capacity in the same batch of batteries manufactured.Due to these differences between the cells,the electrochemical performance of the battery pack will decrease.Based on cell inconsistency.The failure of LiNi_0.5Mn_0.3Co_0.2O₂/graphite 18650 single cell was studied.Then,the failure regularity of the series-parallel battery pack was studied by quantifying the internal resistance difference and the capacity difference of the single cells.Finally,the optimum mixing scheme was put forward.The specific research contents and conclusions are as follows:?1?Based on the failure of single cells,this paper studied the failure mechanism of single cells under different rates,different SOC states and different DOD states.The results showed that when the battery was operated at a large magnification,the migration rate of lithium ions was quick,which caused the electrolyte to oxidize and decompose on the surface of the negative electrode,causing micro short circuit of the battery,increasing the internal resistance of the battery,and ultimately causing the battery capacity to decay or even be permanent damage.When the SOC of the battery exceeded 100%,the positive and negative materials were destabilized,and the internal resistance,temperature and voltage of the battery were increased due to the reaction of the unstable electrode with the electrolyte,resulting in battery damage.When the DOD of the battery exceeded 100%,the copper was oxidized and dissolved,so that copper ions were deposited on the positive electrode,accelerating the battery decay.?2?In this paper,by quantifying parallel battery packs with different internal resistance differences and different capacity differences,the main reason for the failure of parallel battery packs is that the current distribution of the single cells is not uniform,which leads to accelerated attenuation of the parallel battery packs.The battery pack exhibited good cycle performance when the internal resistance of the battery pack was within 30%and the capacity difference was within 20%.The equivalent circuit modeling method was used to evaluate the SOC of the parallel battery pack.The feasibility of the parallel battery model was verified by the experimental data of the parallel battery pack.?3?By quantifying the series battery packs with different capacity differences,the main reason for the failure of the series battery pack is that the voltage distribution of the single cells is uneven,resulting in overcharging and overdischarging of the single cells,and ultimately causing damage to the battery pack.In the battery matching process,when the single cells with different capacity were matched,the batteries with small capacity difference were connected in series.When the single cells with inconsistent internal resistance were matched,the batteries with tiny internal resistance were connected in parallel,which was beneficial to extend battery life.