Unveiling the Origins of “Memory Effects” in Thick Electrodes through 2D XRF-XANES Mapping and Modeling

Abstract

Making electrodes much thicker than the current commercial lithium-ion batteries is a promising approach to further increase the energy density of rechargeable batteries by reducing the weight/volume fraction of inactive components at the battery cell level. However, thick electrodes suffer inferior rate performance as non-uniform reaction develops within the electrode due to the increased ionic and electronic transport distances, which reduces the capacity utilization. In this work, we demonstrate yet another challenge faced by the application of thick electrodes (>100 µm), that is, they exhibit pronounced “memory effect” that is not seen in their thin electrode counterpart. While “memory effect” was initially observed in nickel-cadmium and nickel metal-hybrid batteries, which experience capacity reduction upon repeated shallow discharging, here we use the terminology to refer to the phenomenon that the (dis)charging performance of a lithium-ion battery depends on its cycling history. An example is illustrated in Figure 1, which shows the 2C discharge capacity of a 150μm-thick LiFePO4 electrode in a half cell when it starts from an initial state of charge (SoC) at 50%. Our experiment reveals that the discharge capacity has a strong dependence on how the electrode was first brought to the initial state. When it is charged to 50% SoC at 1.5C, the subsequent discharge capacity is 137% larger compared to when it is discharged to the same SoC at 1.5C. Similar phenomenon is also observed in another mainstream cathode NMC622. We show that such memory effect is caused by the reaction polarization present in the thick electrodes. To this end, 2D X-ray fluorescence (XRF) spectroscopy and Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) were employed to characterize the reaction distribution within the electrodes, and its correlation to the (dis)charging kinetics and OCV relaxation were investigated by electrochemical testing and pseudo-2D battery simulation. Our study highlights the issues created by the memory effect on the capacity prediction and SoC determination, which need to be addressed to develop a better battery management system for thick-electrode-based battery cells and packs. Figure 1