Impedance measurements of lithium-ion batteries are a powerful tool to investigate the electrolyte/electrode interface. To separate the contributions of anode and cathode to the full-cell impedance, a reference electrode is required. However, if the reference electrode is placed inappropriately, the impedance response can easily be biased and lead to erroneous conclusions. In this study, we present a novel micro-reference electrode for Swagelok-type T-cells which is suitable for long-term impedance and reference potential measurements. The reference electrode consists of a thin insulated gold wire, which is placed centrally between cathode and anode and is in-situ electrochemically alloyed with lithium. The resulting lithium-gold alloy reference electrode shows remarkable stability (>500 h) even during cycling or at elevated temperatures (40 • C). The accuracy of impedance measurements with this novel reference electrode is carefully validated. Further, we investigate the effect of different vinylene carbonate (VC) contents in the electrolyte on the charge transfer resistance of LFP/graphite full cells and demonstrate that the ratio of VC to active material, rather than the VC concentration, determines the impedance of the anode SEI.

The quality of metal oxide-based battery active materials is compromised by surface contamination from storage and handling at ambient conditions. We present a detailed analysis of the true nature and the quantity of the surface contaminants on two different cathode active materials, the widely used LiNi1/3Co1/3Mn1/3O2 (NCM111) and the Ni-rich LiNi0.8Co0.1Mn0.1O2 (NCM811). We process these materials in three distinct conditions “wet” (excessive exposure to moisture), “dry” (standard drying of as-received materials), and “calcined” (heat-treatment of cathode powders). Surface contaminants are then quantified by thermogravimetric analysis coupled with mass spectrometry (TGA-MS), and their reactivity with an ethylene carbonate-based electrolyte is evaluated using on-line mass spectrometry (OMS). We demonstrate that not only the commonly assumed LiOH and Li2CO3 residues account for NCM performance deterioration upon storage in moisture and CO2 containing atmosphere, but also basic transition metal hydroxides/carbonates formed on the material surface. Eventually, we showcase a thermal treatment that removes these transition metal based surface contaminants and leads to superior cycling stability.

A key for the interpretation of porous lithium ion battery electrode impedance spectra is a meaningful and physically motivated equivalent-circuit model. In this work we present a novel approach, utilizing a general transmission line equivalent-circuit model to exemplarily analyze the impedance of a porous high-voltage LiNi 0.5 Mn 1.5 O 4 (LNMO) cathode. It is based on a LNMO/graphite full-cell setup equipped with a gold wire micro-reference electrode (GWRE) to obtain impedance spectra in both, non-blocking conditions at a potential of 4.4 V cell voltage and in blocking configuration achieved at 4.9 V cell voltage. A simultaneous fitting of both spectra enables the deconvolution of physical effects to quantify over the course of 85 cycles at 40 • C: a) the true charge transfer resistance (R CT ), b) the pore resistance (R Pore ), and c) the contact resistance (R Cont. ). We demonstrate that the charge transfer resistance would be overestimated significantly, if the spectra are fitted with a conventionally used simplified R/Q equivalent-circuit compared to our full transmission line analysis. Advanced analysis techniques for lithium ion batteries are a key requirement to deconvolute the complex interplay between the aging mechanisms occurring at the anode and the cathode. In principle, this can be accomplished by electrochemical impedance spectroscopy (EIS), if the individual contributions of anode and cathode to the overall cell impedance can be determined, and if this EIS response can be fitted unambiguously to physically motivated equivalent-circuit models. In general, the measured cell and/or electrode impedances are usually fitted with a serial connection of an ohmic resistor (R), with a parallel circuit of a resistor and a capacitor (C), commonly referred to as R/C element and often also modified to an R/Q element (Q representing a constant-phase element), as well as with a Warburg element (W). [1][2][3][4][5] Recently, more elaborate equivalentcircuits using a transmission line model are getting more and more attention. [6][7][8] In order to independently obtain the impedance of anode and cathode, there are two possible options: i) the assembly of symmetric cells as shown by Chen et al. 9 or Petibon et al., 10 where coin cells out of two anodes (impedance of the negative electrode) or two cathodes (impedance of the positive electrode) are assembled in a glove box or dry-room from two (aged) full-cells at a specified state-of-charge (SOC); ii) the use of three-electrode setups consisting of a working electrode (WE), a counter electrode (CE) and a reference electrode (RE), which allows to individually determine the impedance of the anode and the cathode of a lithium ion battery full-cell. The latter is a more convenient approach, as individual impedance spectra can be recorded continuously during battery cycling, so that anode and cathode impedance can be monitored during cycle-life studies on a fullcell instead of obtaining only one set of anode and cathode impedance spectra after disassembly of a full-cell...

Silicon-graphite electrodes usually experience an increase in cycling performance by the addition of graphite, however, the relation of the silicon/graphite ratio and the aging mechanisms of the individual electrode and electrolyte compounds still requires a more fundamental understanding. In this study, we present a comprehensive approach to understand and quantify the degradation phenomena in silicon-graphite electrodes with silicon contents between 20-60 wt%. By evaluating the cycling performance and total irreversible capacity of silicon-graphite electrodes vs. capacitively oversized LiFePO 4 electrodes in presence of a fluoroethylene carbonate (FEC)-containing electrolyte, we demonstrate that the aging of silicon-based electrodes can be distinguished into two distinct phenomena, which we describe as silicon particle degradation and electrode degradation. Cross-sectional scanning electron microscopy (SEM) images and a detailed analysis of the electrode polarization upon cycling complement our discussion. Further, we deploy post-mortem 19 F-NMR spectroscopy to (i) quantify to loss of moles of FEC in the electrolyte and correlate this with the amount of charge that was exchanged by the silicon-graphite electrodes, (ii) estimate the pore volume of the silicon-graphite electrodes that is occupied by FEC decomposition products, and (iii) Silicon-based electrodes are very promising candidates to enable the next generation of Li-ion batteries with energy densities on the cell level beyond 350 Wh kg −1 . 1,2 In contrast to conventional intercalation anode materials, such as graphite (LiC 6 , 372 mAh g −1 , 890 Ah L −1 ), the specific capacity of silicon alloy electrodes is significantly higher (Li 15 Si 4 , 3579 mAh g −1 , 2194 Ah L −1 ). 3 Nonetheless, commercialization of silicon-based electrodes is still hampered because of two major challenges: 4 (i) Large volume expansions up to 280% upon repeated (de-)lithiation of silicon particles deteriorate the electrode integrity, thus causing isolation of active material. [5][6][7] The formerly reported pulverization of micron-sized silicon particles due to mechanical stress upon repeated volume expansion has been partially solved by using nanometer-sized particles. However, reduction of the silicon particle size also leads to inferior electronic conduction due to more numerous interparticle contacts, and higher solid-electrolyte-interphase (SEI) losses due to the larger relative surface area. [8][9][10] (ii) Continuous side reactions at the silicon/electrolyte interface caused by repeated volume expansion and contraction result in ongoing electrolyte decomposition and in a gradual loss of active lithium. 8In the course of this, SEI-forming additives in the electrolyte, e.g., FEC, are depleted, which was shown to result in a significant increase in cell polarization and a concomitant rapid capacity drop. 8,11 Various strategies have been proposed to overcome the detrimental effects associated with the volume expansion during (de-)lithiation of silicon and to reduce conc...

Washing is a commonly used method to remove surface impurities of cathode materials for lithium-ion batteries. However, a clear mechanistic understanding of the washing process is missing in the literature. In this study, we will investigate the effect of washing and subsequent drying of nickel-rich NCM cathodes (85% nickel) with respect to gassing and impedance of the washed cathodes. By on-line electrochemical mass spectrometry (OEMS), we will show a drastic reduction of the O 2 release above 80% SOC for the NCM washed with deionized water, suggesting the formation of an oxygen-depleted surface layer on the NCM particle surface. The modification of the surface can be confirmed by a strong impedance buildup of cathodes composed of washed NCM (using a microreference electrode in a full-cell), revealing that the impedance increases strongly with increasing drying temperature after washing. Last, we will propose a comprehensive mechanism on the processes occurring during the washing/drying process of nickel-rich NCM materials and identify the drying temperature after washing as the dominant factor influencing the surface properties.