Effects of Surface-Film Formation on the Electrochemical Characteristics of LiMn[sub 2]O[sub 4] Cathodes of Lithium Ion Batteries

Abstract

LiMn2normalO4 cathodes of Li ion batteries tend to develop native surface films in carbonate-based electrolytes. By combining dc voltammetry and ac electrochemical impedance spectroscopy, we study how these films react with an electrolyte of LiBF4 in ethylene and diethyl carbonates. We demonstrate that such reactions can affect the measurement of the characteristic electrochemical parameters of the cathode, namely, the intercalation capacitance, the initial capacity loss, and the solid-state diffusion coefficient of Li+ . A generalized framework for data analysis, based on the considerations of electrode equivalent circuits, is used to combine the results of the dc and ac measurements. The Li+ conducting film on the cathode introduces an ohmic drop at the electrode surface. Oxidative dissolution of the electrode’s native surface film occurs during the first charge cycle. The potential- and cycle-dependent effects of the associated faradaic reactions have been examined under voltage-controlled charge/discharge conditions at different rates.

Electrochemical features of ball-milled lithium manganate spinel for rapid-charge cathodes of lithium ion batteries

“…The mechanism of this CSF reaction has been proposed previously, suggesting that the CSF, composed primarily of a Li-EC complex, oxidizes with simultaneous releases of C 3 H 4 O 3 and Li + in the electrolyte [15]. The values of R r measured here for the BM-LMO sample is about an order of magnitude larger than those measured previously under similar conditions using a cathode of micron-sized LMO particles [2].…”

“…1a. Based on earlier reported observations [21][22][23], this peak can be associated with Faradic reactions of the native cathode surface films (CSFs) [2,10]. This current most likely comes from irreversible anodic oxidation of Li-EC complexes contained in the surface films formed on LMO.…”

“…Compared with Fig. 5 Ragone plots of specific energy (ε s ) vs. specific power (P s ), recorded for a all-micrometric and b BM-LMO cathodes using galvanostatic charge (1-C rate) and discharge (variable rates between 1 C and 50 C) experiments performed in LMO|Li cells earlier reported results for all-micrometric LMO [2], the overall impedance of BM-LMO obtained here is substantially reduced, and indicates more efficient Li + transport in the latter case. Because the EIS measurements correspond to equilibrium conditions, the data collected during the charge and discharge cycles are mutually similar.…”

“…A "doctor's blade" was used to coat the Au film with the LMO composite paste [2]. The resulting electrodes were dried in a vacuum oven for 24 h at 100°C.…”

“…Because intercalation/de-intercalation occurs primarily as "surface" (rather than bulk) reactions in such cases, the Li-host nanoparticles experience reduced volumetric stress of lattice deformation; this serves to increase both the cycle-life and the coulombic efficiency of the battery. However, while the increased specific area improves the rate capability and cyclability of electrode particles, this also makes the electrode more susceptible to segregation and/or modification by Faradaic reactions of electrolytes/impurities [2].…”