Distinguishing Li+ Charge Transfer Kinetics at NCA/Electrolyte and Graphite/Electrolyte Interfaces, and NCA/Electrolyte and LFP/Electrolyte Interfaces in Li-Ion Cells

In examining the Li + charge transfer kinetics at the graphite anode and the lithium nickel cobalt aluminum oxide, LiNi0.80Co0.15Al0.05O2 (NCA), cathode in a full cell, we found that the activation energy, Ea, for the charge transfer at the graphite/electrolyte interface is about 68 kJ/mol, which is consistent with recently reported values. However, the Ea for the charge transfer at the NCA/electrolyte interface is about 50 kJ/mol, which is lower than at the graphite anode. With desolvation as the predominate step for limiting the kinetics and both electrodes subjected to the same electrolyte, the difference in Ea suggests that it is greatly influenced with respect to the nature of the electrode materials and their associated SEIs. This is further confirmed by the examination of Li + charge transfer at the LiFePO4 (LFP)/electrolyte and the graphite/electrolyte interfaces using a LFP/graphite In developing high power Li-ion batteries, reducing resistance that limitsthechargeanddischargeratesisimportanttoimprovethepower capability of Li-ion cells. Various resistances existing in cells such as contact resistance between the current collector of the electrodes and the cell container and electrolyte resistance can be reduced through engineering and theuseof a moreconductive electrolyte, respectively. Forthesameelectrodematerial,theuseofthinnerelectrodeswillresult in lower resistance cells as the length of the electrodes is increased when packaging them in the same size of cells such as 18650. This is simply due to the fact that the resistance is proportional to the thickness of the electrodes and inversely proportional to the area of theelectrodes.Thereductionoftheparticlesizeoftheactivematerials can increase the number of electrochemical reaction sites for Li + and reduce the time to utilize the active materials. However, the Li + chargetransferresistance,Rct,theresistancethatLi + encounterswhen moving from a solvated ionic state in the electrolyte solution crossing the electrode-electrolyte interface and inserting into the electrodes is one critical source of resistance that requires further understanding and reduction. During the charge process, for a state-of-the-art Li-ion battery, an electron leaves the lithium metal oxide cathode via an external cir- cuit and moves to the graphite anode. To retain the charge neutrality of the cathode, Li + is released from the cathode moving across the cathode SEI and entering the electrolyte. The Li + , which is solvated by the solvent molecules in electrolyte, needs to be desolvated before moving across the graphite anode SEI, inserts into the graphite pro- viding charge neutrality by compensating or accepting the electron coming from the cathode through the external circuit. This process is reversed during discharge. The Li + charge transfer process, described in this paper, involves the desolvation of the solvated Li + in the liq- uid electrolyte, crossing of Li + through the SEI layer formed at the electrolyte/electrode interface and the acceptance of an electron from the external circuit while inserting into the intercalation type of elec- trode materials. The resistance resulting from this process is Rct.Ifthe charge transfer across the interface is a thermally activated process, Rct follows the relationship, 1