Experimental study on charging energy efficiency of lithium-ion battery under different charging stress

Accurate measurement of the energy efficiency of lithium-ion batteries is critical to the development of efficient charging strategies. Energy efficiency is discussed in published work from the perspective of cell design, more than that, the insufficient probe of stresses influencing the energy efficiency of commercialized batteries while the conclusions coupled with the effect of battery temperature rise have limited engineering guidance value. Few papers specify test profiles for energy efficiency baseline. This paper designs a charging energy efficiency (CEE) test profile to present an offline map of baseline value for commercial ternary lithium-ion batteries. The time-frequency domain impedance characteristics are analyzed based on the distribution of relaxation times, providing a mechanistic explanation of the CEE evolution. The contributions of temperature, capacity-rate (C-rate), and state of charge (SOC) to the CEE are quantified by global sensitivity analysis and range statistics. The findings show that the proposed profile decouples the effect of temperature rise under forced convection conditions, and has an error of less than 0.04 % for 3 cycles compared to the FreedomCAR guidance of at least 10 cycles. The CEE-SOC curve shape is similar to "M", which is attributed to the impedance curve shape of "W", breaking the previous inherent experience of "lower impedance in the middle SOC". The median of the Map is 0.964, concomitantly the outlier points are concentrated below 0.887 at 10 °C. The global sensitivity analysis carried out by the Sobol method and the range statistical indicates that the importance ranked is C-rate > Temperature > SOC, with the range statistical of each stress being 11.8 %, 10.223 %, and 7.51 %, respectively, as well as the total-order Sobol indexes being 0.56, 0.439, and 0.066, which is beneficial for optimizing the operating SOC interval of lithium-ion batteries and guiding the development of fast charging strategies.