Chemical Physics Letters - Lithium Battery Research

1 FRONTIERS ARTICLED irectin situmeasurements of Li transport in Li-ion Battery negative electrodesStephen J. Harrisa,*, Adam Timmonsa, Daniel R. Bakera, Charles MonroebaElectrochemical Energy Research Lab, General Motors R&D Center, Mail Code 480-102-000, Warren, MI 48090, United StatesbDepartment of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, United Statesarticle infoArticle history:Received 14 November 2009In final form 10 December 2009 Available online 22 December 2009abstractWe describe the first directin situmeasurements of Li transport in an operating cell. Motion of the lith-iation front in the graphite electrode suggests that transport could be controlled by liquid-phase diffu-sion. The electrochemical (current voltage) data are successfully modeled with a diffusion equationthat contains no material or microstructural information.

2 The model is only qualitatively successful inpredicting observed Li transport rate data, suggesting that microstructural information is required andthat the actual process is more complex than simply diffusion. The technique can provide data for study-ing Li plating and Li dendrite growth, both of which can cause Battery degradation. 2009 Elsevier All rights IntroductionBecause of their high energy density and long cycle life, Li-ionbatteries are used today in many practical devices including cellphones and laptop computers, and they are now being contem-plated for mass-produced hybrid and electric vehicles[1]. A typicalLi-ion Battery is shown schematically inFig. 1a and as an SEM inFig. 1b. As the Battery is charged and discharged, Li, originally pres-ent in the electrolyte and in the positive electrode*, chemically re-acts with the negative electrode, inserting or intercalating into thebulk material.

3 This lithiation process changes the chemistry of theelectrode particles, so the properties of the Li-ion Battery dependcritically on the Chemical nature of the electrode material. Since,by definition, Li is thermodynamically more stable in the positiveelectrode, a power supply is required to detach Li from the positiveelectrode (usually a transition metal oxide or phosphate) to formLi+ions and then to push them into the negative electrode* (almostalways a form of graphite or carbon) for charging, as illustrated inFig. 1a. Because Lithium reacts with practically everything, thenumber of potential Lithium -ion Battery electrode materials and,therefore, the number of potential Lithium -ion Battery types is al-most batteries are generally analyzed using the macro-homo-geneous porous electrode model developed by Newman and co-workers[2,3].

4 The model consists of equations for: (1) electroniccharge balance in the solid phase (Ohm s law); (2) electrolytecharge and mass balance for Li+using concentrated electrolyte the-ory; (3) diffusion of Lithium in the electrode particles (Fick s law);(4) Butler Volmer* charge transfer kinetics at the electrolyte-solidphase boundary; (5) and associated boundary conditions. Themodel requires as input no microstructural information beyondparticle radius, electrode thickness, and electrode porosity. Other-wise, it assumes that the microstructure can be described as an iso-tropic, homogeneous, 1-dimensional porous material made upfrom monodisperse non-porous isotropic spherical particles thatare small compared to the electrode course, none of these assumptions and approximations canbe truly correct.

5 For example, significant inhomogeneity in theelectrodes and in the state of lithiation within an electrode thatshould be at equilibrium has been observed[4 6]. The chargetransfer step is modeled as a single global Chemical reaction inwhich Li+ions in the electrolyte solution de-solvate, transportthrough a 1 10 nm thick solid electrolyte interphase (SEI) layer[7 12]consisting of various degradation products, and react withthe electrode material. Remarkably little is known about the de-tailed chemistry of the Butler Volmer step[12 14], even thoughit is involved in many proposed degradation mechanisms[11,15,16]. Diffusion of Lithium in the solid phase active particlesis treated with a shrinking core diffusion model[17,18]althoughits validity is at best uncertain for many commonly used elec-trodes, and although it has been shown to be invalid for at leastone material[19].

6 Properties of the conductive carbon and binder,while of considerable importance to Battery performance, are ab-sorbed into other the years since the model appeared, a number of papers,some from Newman s group, have examined the effects of relaxingsome of the microstructural assumptions of the original model. Forexample, Darling and Newman[20] analyzed the effects of multi-ple particle sizes, Yi and Sastry[21] considered particles with ellip-soidal shapes, and Santhanagopalan et al.[22] and Yi et al.[23]looked at extending the model to higher dimensions. These andother efforts notwithstanding, the original macro-homogenousmodel performs very well and is still widely and successfully used0009-2614/$ - see front matter 2009 Elsevier All rights *Corresponding author.

7 Address: Electrochemical Energy Research Lab, GeneralMotors R&D Center, Mail Code 480-102-000, 30500 Mound Rd., Warren, MI 48090-9055, United States. Fax: +1 586 986 Harris). Chemical Physics Letters 485 (2010) 265 274 Contents lists available atScienceDirectChemical Physics Lettersjournal homepage: [24]for optimizing electrode parameters such as thickness andporosity. It is, in fact, the basis for COMSOL s commercial Li-ionbattery the other hand, the ability to predict cell degradation re-mains a challenge because so many unaccounted for and seem-ingly unrelated micro-scale degradation mechanisms have beenidentified or postulated[4,21,25 39]. Experimental measurementsdescribing local chemistry, details of the microstructure and trans-port, and an understanding of how these factors evolve are re-quired in order to sort out the issues involved with present, analysis of specific degradation mechanisms cansometimes offer explanations for experimentally observed degra-dation[22,29,40,41], but without additional experimental dataand associated theoretical analysis, cause-and-effect relationshipsbetween observation and degradation pathway can be difficult todemonstrate.

8 For example, a widely invoked degradation mecha-nism is loss of internal electrical connectivity. The loss of connec-tivity has been directly observed by Kostecki and McLarnon[4],and they attributed it to the movement of conductive carbon ( car-bon retreat ), reducing electron transport within the electrode. Butloss of internal electrical connectivity has also been attributed toparticle fracture[36,38,42], to precipitation of thick surface films[30,35], to gas generation[43], to loss of contact between activematerial and the current collector[44] or between the current col-lector and the cell housing[45], and to degradation of the binder[46]. As a result, there has not appeared to be any experimentalor modeling strategy that elucidates degradation as a general phe-nomenon.

9 Because a lower degradation rate translates directly intolower-cost batteries, the ability to predict, mitigate, and deal withdegradation by understanding fundamental Chemical and materialproperties is critical if batteries for transportation are to becomeeconomically an ideal Li-ion Battery , the only process that should occur atthe mesoscale (smaller than an electrode, larger than a molecule) istransport of Lithium ions through the electrolyte and in the activeparticles, accompanied by reversible reactions of Lithium at appro-priate locations within the electrodes. All of the seemingly dispa-rate mechanisms of Battery degradation lead in some way toinefficiency or irreversibility of these fundamental transport andassociated Chemical processes. The present work is predicated onthe notion that a general study of degradation can begin with mea-surements of Li transport and insertion into porous measurements could then guide researchers towards otherexperiments and models that provide fundamental knowledge ofdegradation.

10 With this goal in mind, we provide herein situtime-dependent Li spatial maps and transport rate measurementsat the ExperimentalCharging and discharging experiments were carried out in anoptical half-cell*.Fig. 2shows the optical half cell as seen fromthe side (schematic) and from above (photograph). The cell wasassembled in a glove box under an Ar atmosphere (<1 ppm oxygenand water), since even N2reacts with Li. A brushed piece of Li foilacted as the negative electrode, while a porous graphite electrodecut from an LR1865AH 18650* laptop Battery made by TianjinLishen Battery Co. served as the positive electrode. The electrodematerial coated both sides of a copper current collector, as canbe seen for a different electrode inFig. , electrodes are stacked (or wound) facing eachother with a separator keeping them apart so they do not short,as shown inFig.