Kinetics of Lithium - Ion Transfer at Carbon-Electrolyte Interface in Presence of Conducting Nano-Fillers
Authors: Ahamad Salahuddin , Gupta Amit 
Published: February 2020

Besides highly successful in portable electronic devices, Li-ion batteries are regarded as key technology for power sources for next generation HEVs, PHEVs and EVs. However, batteries in transportation vehicles are required to deliver sufficiently high power with longer lifetime and better safety. Graphitic carbon is the most successful and commonly used candidate for negative electrodes in Li-ion batteries. However, this material is known to have moderate rate capability due to slow kinetics [1, 2]. It has also been proposed that it is not only graphite which limits high rate performance, rather other parameters such as binder amount, electrolyte solvents and conducting salts are also likely responsible [3-5]. The surface charge-transfer kinetics in graphite/electrolyte interface studied by Xu [3] has shown that the activation energy for interfacial Li-ion transfer with LiPF6 in EC:DMC (1:1) as electrolyte was 64 kJ mol-1 which is high compared to solid phase transport [6] and liquid electrolyte transport of Li-ion [7]. This large activation energy is attributed to desolvation of ions on SEI layer [6].

In order to achieve good performance in terms of realizing higher power density, the interface charge transfer kinetic must improve. In this work, we investigate the kinetics of Li-ion transfer for mesocarbon mocrobeads (MCMB) carbon active material with nano-conducting fillers such as carbon black (CB) and multiwalled carbon nanotubes (MWCNTs) in a CR2016 type coin. It has earlier been shown that a synergistic improvement in electrical and electrochemical performance occurs with partial substitution of CB by CNT [8]. In this work, we examine the effect of conductive fillers on charge-transfer resistance, Rct, for various temperatures. A thermal chamber with temperature controller as shown in Figure 1 was designed and fabricated to conduct experiments with reproducible EIS results in a wide temperature range. The working electrodes were fabricated with 88% MCMB, 4% conducting additives (CB, CNT or CB+CNT) and 8% PVDF coated on copper foil. The steps followed for fabricating the electrodes and cells have been described in detail elsewhere [8]. Before each impedance test the cells were cycled at 0.2C-rate for five cycles between 2.0 V to 0.005 V to ensure interfacial film formation, subsequent to which the cells were stabilized at 0.2 V by keeping them at rest for three hours after discharging them with 0.05C-rate to 0.2V. The EIS spectra were measured at OCP of 0.2 V with an AC perturbation of amplitude 5mV in the range 30mHz to 200kHz. The conductivity of interfacial Li-ion transfer (1/Rct) obeys the Arrhenius equation, 1/Rct=A0exp(Ea/RT). The symbols A0, Ea, R and T stand for frequency factor, activation energy, gas constant and absolute temperature. In this study, the activation energy is used to quantify the kinetics of interfacial Li-ion transfer as its value indicates essential kinetics for solid/electrolyte interface process without the effects of temperature, surface area and ion activity.

Figure 2 shows the Arrhenius plot of interfacial conductivity (1/Rct) obtained from impedance plot for different conducting fillers at various temperatures. The activation energies evaluated were 56.2 kJmol-1, 53.6 kJmol-1 and 49.6 kJmol-1 for electrodes containing conducting nano-fillers namely CB, CNT and hybrid CB+CNT (1:3), respectively. The lower activation energy or improved kinetic performance of electrode containing CNT nano-fillers is attributed to the ease of ion transfer to active material surface from bulk electrolyte through SEI layer. Further improvement of kinetics for hybrid conducting fillers (CB+CNT) electrodes is attributed to the formation of dense, thin and ionically conductive surface film that can pave the way for rapid migration of Li-ion and faster electron transfer for interfacial reaction.

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Figure 1