Frank E. Little

Electrochemical impedance study of initial lithium ion intercalation into graphite powders

A Johnson Matthey 287 (JM 287) graphite powder disks sandwiched between two nickel screens was used as working electrodes in 1.0 M lithium hexafluorophosphate in mixed carbonate ester electrolyte. Their intrinsic impedance and lithium intercalation kinetics were determined using several different electrochemical impedance spectroscopy (EIS) protocols. The thermal stability of the electrolyte was similarly studied using a floating palladium wire working electrode. Studies at 25°C and 65°C show that a first high frequency depressed semicircle, a second depressed semicircle, and a sloping line in the low frequency range are respectively related to the initial formation of a solid electrolyte interface (SEI) film, the charge-transfer reaction, and lithium diffusion in graphite. The intrinsic resistance is in series with the reaction impedance and effectively increases the latter. The charge-transfer semicircle at the open-circuit potential moves to the low frequency range, whereas the semicircle associated with the freshly formed SEI film moves to high frequency range as lithium insertion proceeds. The passivating SEI film formed at 25°C slows down co-insertion of the electrolyte into graphite when the electrode is cycled at 65°C, which increases the cycle life of graphite at elevated temperature.

Electrochemical study on nano-Sn, Li4.4Sn and AlSi0.1 powders used as secondary lithium battery anodes

It is believed that particle cracking resulting from phase transformation is responsible for the poor cycle performance of lithium alloy anodes. Pulverization effects may be reduced by using, (i) smaller active particles; (ii) active particle composites with different potentials for the onset of lithium alloy formation; and (iii) expanded alloys which have undergone a major increase during initial charging. Three alloys of the above types (nano-Sn, AlSi0.1 and Li4.4Sn) were studied by electrochemical impedance spectroscopy (EIS) to determine their electrochemical kinetics and intrinsic resistance during initial lithium insertion-extraction. The electrodes were prepared by sandwiching a disk of active powder between two nickel screens, so that the contact resistance may be determined by EIS and from a d.c. voltage difference across the electrode (trans-electrode voltage). A large increase in contact resistance was found during lithium discharge (extraction) from nano-LixSn and LixAlSi0.1 alloys, compared with the small increase during the initial charge. This result suggest that the matrix materials should have a small coefficient of elasticity to give low stress on expansion of the active alloy, together with a large elastic deformation to compensate for volume reduction. This is contrary to generally accepted argument that the matrix should have a high ductility. EIS results for measurement of intrinsic resistance and reaction kinetics during initial lithium insertion into nano-Sn and AlSi0.1 alloys show that both solid electrolyte interphase (SEI) films formed on particle surfaces, together with particle pulverization, are responsible for the high contact resistance. The electrochemical kinetics of both lithium charge and discharge are controlled by contact resistance at high states of charge. # 2001 Elsevier Science B.V. All rights reserved.

Charge–discharge stability of graphite anodes for lithium-ion batteries

Abstract A graphite powder disk sandwiched between two nickel screens was used as a lithium-insertion working electrode. Electrochemical impedance spectroscopy (EIS), galvanostatic intermittent titration (GIT) using pulsed microcurrent, and in-situ intrinsic resistance measurements were used for the evaluation of kinetics and intrinsic (i.e. physical) resistance changes during charge–discharge cycling from room temperature to elevated temperatures. The investigation of the thermal stability of the electrolyte at elevated temperature used an EIS study of a palladium electrode in the electrolyte. EIS measurements for electrochemical reaction and intrinsic resistances of a graphite electrode show that the first high-frequency depressed semicircle is due to the ‘solid electrolyte interphase’ (SEI) film, although it is also influenced by the electrode contact impedance. The growth of the SEI film on the MCMB 10-28 graphite electrode surface with cycling, results in a decline in kinetic rate and a corresponding increase in contact resistance giving rapid capacity fade. The high stability of the capacity of JM 287 electrodes is due to the slow increase in SEI film thickness on their surfaces. Although new SEI films were formed on the originals at elevated temperature, the kinetics were still more rapid than at room temperature in the initial cycling.

In situ investigation of electrochemical lithium intercalation into graphite powder

Abstract The reaction kinetics for the electrochemical insertion–extraction of lithium into and out of graphite and the electrode intrinsic resistance have been studied using galvanostatic intermittent titration with applied microcurrent pulses (GIT), electrochemical impedance spectroscopy (EIS) and in situ intrinsic (i.e. physical) resistance measurements. The formation of a so-called solid electrolyte interphase (SEI) or ‘passivation layer’ on the graphite surface at about +0.8 V versus Li+/Li increases the total intrinsic resistance within the electrode on the first cycle. The gradual growth (i.e. change in volume) of the SEI film during repeated lithium insertion–extraction results in a continued increase in intrinsic resistance. The good agreement between EIS and in situ intrinsic resistance measurements indicates that the latter is a simple and powerful method for investigating the change in this parameter for graphite electrodes. GIT showed a gradual increase in kinetic rate in an initial single-phase region, followed by a decrease in a two-phase transformation region, which was in turn followed by an increase in kinetic rate in a second single-phase region. This is explained by a shrinking unreacted core model. The controlling step of lithium insertion–extraction during the two-phase transformation region was investigated by EIS and GIT.

Characteristics of lithium-ion-conducting composite polymer-glass secondary cell electrolytes

A family of lithium-ion-conducting composite polymer-glass electrolytes containing the glass composition 14Li2O–9Al2O3–38TiO2– 39P2O5 (abbreviated as (LiAlTiP)xOy) with high ionic conductivity, an excellent electrochemical stability range, and high compatibility with lithium insertion anodes is described. An optimized composition has a room temperature conductivity of 1:7 � 10 � 4 Sc m � 1 ,a n Li þ transference number of 0.39, and an electrochemical stability window to þ5.1 V versus Li/Li þ . It also has good interfacial stability under both open-circuit and lithium metal plating–stripping conditions and provides good shelf-life. # 2002 Elsevier Science B.V. All rights reserved.

Low-Temperature Characterization of Lithium-Ion Carbon Anodes via Microperturbation Measurement

The low-temperature performance limits of Johnson Matthey ~JM! 287 graphite and mesocarbon microbead ~MCMB! 6-10 coke were investigated using galvanostatic intermittent titration ~GITT! and electrochemical impedance spectroscopy. The poor lowtemperature (230°C) performance of graphite insertion anodes results from a low lithium insertion capacity because polarization or overpotential is higher than the stage transformation plateau potential. This results in a shorter plateau potential region containing the lithium-rich stages, e.g. ,L i 0.33C6 ,L i 0.5C6 , and Li1C6 . Overall, there is an incomplete transformation from Li-poor to Li-rich stages when the cutoff potential is limited to 0.0 V ~vs. Li/Li

Improvement in electrochemical properties of nano-tin-polyaniline lithium-ion composite anodes by control of electrode microstructure

Four different types of nano-tin-polyaniline (nano-Sn-PAni) lithium-ion composite anode microstructures have been examined to investigate the relationship between this parameter and anode characteristics. Scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS) show that the electrochemical properties of nano-tin composite electrodes can be significantly affected by microstructure variation. To simultaneously obtain high capacity and long cycle life, the active materials should be encased in a polymer matrix to accommodate volume changes during cycling, and porosity is required to offer low interfacial lithium insertion/extraction impedance. The polymer matrix should have a high binding strength to prevent the anode cracking.

Electrochemical study of the SnO2 lithium-insertion anode using microperturbation techniques

Abstract A disk pressed from commercial SnO 2 powder, sandwiched between two nickel screen current collectors, was used as a lithium-ion secondary anode. Its electrochemical lithium insertion–extraction behavior was investigated by galvanostatic charge–discharge and galvanostatic intermittent titration (GITT) using a microcurrent on one current collector. The trans-electrode voltage was measured to monitor the transmissive resistance across the SnO 2 electrode during the discharge–charge process. Special electrochemical impedance spectroscopy (EIS) protocols were used to investigate the kinetic and transmissive impedances during initial lithium insertion. Protocol B or C EIS, described in the text, give the local transmissive impedance near the operating current collector, while Protocol B′ or C′ give the local transmissive impedance near the other current collector. The use of special EIS protocols showed that the inner transmissive impedance near the operating current collector side is higher than that near the other current collector.

Composite doped emeraldine–polyethylene oxide-bonded lithium-ion nano-tin anodes with electronic–ionic mixed conduction

Mixed-conducting lithium-ion doped emeraldine polyaniline (PAni)–polyethylene oxide (PEO) blends have been developed to achieve an optimal electronic–ionic conductivity balance in nano-tin composite anodes. Electrochemical evaluation was performed on the anodes with differing electrode preparation procedures, doping methods and PEO contents. Results indicate that both good electronic and ionic conductivity in the binder are required for rapid lithium insertion/extraction and low polarization. This doped PAni–PEO polymer blend is an attractive binder for high capacity composite anodes with low polarization.

Characterization of Metal Hydride Electrodes via Microperturbation and In Situ Intrinsic Resistance Measurement

Microperturbation methods, including galvanostatic intermittent titration using applied microcurrent pulses (GIT), small amplitud e cyclic voltammetry, and electrochemical impedance spectroscopy (EIS), together with in situ intrinsic (i.e., physical) resistance measurements, have been applied to investigate charge/discharge kinetics of a LaNi 4.4Sn0.25 metal hydride (MH) electrode as a function of cycling. The results show that electrode capacity loss was caused by reduced ability to absorb hydrogen. High frequency semicircles in the Nyquist plots may be attributed to hydrogen transition between the adsorbed and the absorbed states, and are unrelated to contact resistance between the current collector and the hydride particles, although this showed contact resistance bet ween alloy particles and current collector. This contradicts a generally accepted interpretation of EIS data. It was difficult to ob tain hydrogen diffusion resistance from EIS results when the potential change in the electrode charge-discharge plateau is smaller than t he EIS voltage perturbation, because long charge/discharge times in the low frequency range change the state of discharge. Alternat ing current EIS is more suitable than alternating voltage studies to investigate hydrogen diffusion kinetics in MH alloys with a flat potential plateau. Similarly, it is also difficult to obtain hydrogen diffusion resistance from small amplitude cyclic voltammet ry because of the size of the charge/discharge capacitance. However, the total reaction resistance may be obtained from GIT.