Investigation of Reaction Kinetics in Carbon Anode with Nano-Fillers through Impedance Spectroscopy

Abstract

Lithium-ion batteries have been very successful in consumer electronics as viable and efficient power sources for many small and portable devices such as mobiles, laptop, watches, camcorders, PDAs, cameras and other communication devices [1-2]. However, poor electronic conduction in electrodes due to an unstable and fragile conducting network of traditional conducting additives is still the limitation for large scale commercialization of this energy storage technology in automotives (such as EVs, HEVs, PHEVs, etc.) where high power density is desired frequently [2, 3]. In addition, poor contact due to large size of active material and sluggish kinetics makes it difficult to achieve their full theoretical capacities at a practical level [4]. The conducting network in an electrode influences the performance of charge transfer kinetics at the electrode-electrolyte interface and ion migration through the solid electrolyte interface (SEI) layer as suggested by an earlier study [5]. In graphitic anode, when an ion of lithium is inserted into the material, the crystal structure of the active material changes with the state of charge (SOC), and the resulting changes affect the intercalation/deintercalation reactions [6].In this work, we investigate the kinetics of lithium-ion transfer for mesocarbon mocrobeads (MCMB) as active material with nano-conducting fillers, namely carbon black (CB) and multiwalled carbon nanotubes (MWCNT), by carrying out electrochemical impedance spectroscopy (EIS) measurements at different states-of-charge (SOC) and temperatures for CR2016 coin half-cells. The working electrodes were fabricated with 88% MCMB, 4% conducting additives and 8% PVDF coated on copper foil. It has recently been demonstrated that a synergistic improvement in electrical and electrochemical performance occurs with partial substitution of CB by CNT [7]. By recording impedance spectra at different temperatures and SOCs for electrodes having different conducting additives (CB, CNT or CB+CNT), we analyse the impact of conducting nano-fillers upon the kinetics of the anode reactions and the dependency of these on the SOC. The impedance spectra were measured at the set SOC with an AC perturbation of amplitude 5mV in the range 30mHz to 200kHz. The conductivity of interfacial charge transfer (1/Rct) obeys the Arrhenius equation, 1/Rct=A0exp(Ect/RT). The symbols A0, Ect, R and T stand for frequency factor, activation energy, gas constant and absolute temperature. A typical EIS spectrum of MCMB electrode in a lithium-ion half-cell and the respective resistances deconvoluted using an equivalent circuit is demonstrated in Fig. 1(a). Impedance spectra plotted at different temperatures for MCMB electrode having 4% CNT as conducting additives at OCV of 0.17V vs. Li/Li+ are illustrated in Fig. 1(b) and the corresponding Arrhenius plot of interfacial conductivity (1/Rct) obtained from impedance plot is shown in Fig. 1(c). The activation energy of charge transfer as a function of SOC and open-circuit potential is illustrated in Fig. 1(d). The lithium-ion staging process in MCMB electrode is highlighted by vertical lines as shown in Fig. 1(d). From Fig. 1(d), the variation in activation energy can be attributed to the insertion/deinsertion of lithium ions into carbon and the chemical nature of the solid-electrolyte interphase (SEI) layer deposited on the surface of the active material. The activation energy associated with Rct tends to decrease with the potential and vary with different conducting nano-fillers. This indicates that the staging phenomenon and SEI composition affect the charge transfer process. It is observed that the activation energy in stage-I is relatively high than the other stages which is attributed to the large structural change associated with lithium-ion insertion/deinsertion in this stage. For comparative analysis, the activation energy for the interfacial Li-ion transfer in electrodes having conducting additives CB, and CB+CNT will be evaluate. Moreover, Arrhenius plot for the Li-ion transport conductivity through the SEI layer and the corresponding activation energy at different SOC will also be discuss in order to better understand the reaction kinetics in MCMB carbon electrode with different conducting nano-fillers.