Article pubs.acs.org/JPCC
Uneven Film Formation across Depth of Porous Graphite Electrodes in Cycled Commercial Li-Ion Batteries § ́ Matilda Klett,† Pontus Svens,†,‡ Carl Tengstedt,‡ Antoine Seyeux,§ Jolanta Swiatowska, † ,† Göran Lindbergh, and Rakel Wreland Lindström* †
Department of Chemical Engineering and Technology, Applied Electrochemistry, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden ‡ Scania CV AB, SE-151 87 Södertälje, Sweden § PSL Research University, Chimie ParisTech - CNRS, Institut de Recherche de Chimie Paris, 75005 Paris, France S Supporting Information *
ABSTRACT: A critical aging mechanism in lithium-ion batteries is the decomposition of the electrolyte at the negative electrode forming a solid electrolyte interphase (SEI) layer that increases impedance and consumes cyclable lithium. In contrast to the typical nanometer SEI layer generally discussed, this paper reports on the formation of a micrometer thick film on top of and within the upper part of a porous graphite electrode in a deep-cycled commercial cylindrical LiFePO4/graphite cell. Morphological, chemical, and electrochemical characterizations were performed by means of cross-sectional electron microscopy in combination with energy dispersive X-ray spectroscopy and focused ion-beam milling, time-of-flight secondary ion mass spectrometry, and electrochemical impedance spectroscopy (EIS) to evaluate the properties and impact of the uneven film. It is shown that the film is enriched in P−O and carbonate species but is otherwise similar in composition to the thin SEI formed on a calendar-aged electrode and clogs the pores in the electrode closest to the separator. Performance evaluation by physics-based EIS modeling supports a local porosity decrease, impeding the effective electrolyte transport in the electrode. The local variation of electrode properties implies that current distribution in the porous electrode under these cycling conditions causes inefficient material utilization and sustained uneven electrode degradation.
1. INTRODUCTION Graphite is commonly used as the negative electrode material in commercial Li-ion battery cells intended for both consumer electronic products and electrified vehicles.1,2 One major cause for aging in this type of cells is capacity fade from lithiumconsuming side reactions on the graphite surface due to reductive decomposition of the organic electrolyte at low potentials.3−6 The rate of these side reactions decreases during the first formation cycles as the reaction products form a passive surface film, a so-called solid electrolyte interphase (SEI), which in an ideal case would prevent further electrolyte reduction. However, gradual capacity fade related to loss of cyclable lithium during battery cycling is frequently reported in literature, showing that continuous film growth occurs during cycling. The composition of the SEI during and after formation has been studied extensively, for example, in refs 6−16, and depends on the composition of the electrolyte,7,13,14 surface activity,7,9,15 and the cycling conditions7,16 including temperature. The SEI film growth is considered to be a combined result of electrochemical and chemical reactions; the initial reduction of carbonate solvents can produce solid products directly or reactive nonradical or radical intermediates that © 2014 American Chemical Society
further react with electrolyte species and subsequently form solid polymerized products and salts. On the scale of porous electrodes, the SEI is often considered a rather evenly formed film on all graphite particles and of nanometer scale thickness. Recently, however, we reported observations of anomalous film growth on graphite electrode samples obtained from the inner parts of a commercial cylindrical LiFePO4/graphite cell that had been subjected to 3.75 C rate constant-current-, wide state-of-charge (SOC)-cycling during approximately 2.5 months.17 The graphite electrode samples were entirely covered by a thick deposited film, which was associated with a severe loss of cyclable lithium. Samples from the same area also exhibited high impedance, as observed by electrochemical impedance spectroscopy (EIS) measurements. However, no in-depth analysis was presented in that study. A similarly thick film on the porous graphite electrode in a LiMn2O4−Li(Ni0.5Mn0.3Co0.2)O2/graphite commercial cell as a result of cycle-aging was also recently reported.18 Received: September 24, 2014 Revised: December 3, 2014 Published: December 3, 2014 90
dx.doi.org/10.1021/jp509665e | J. Phys. Chem. C 2015, 119, 90−100
The Journal of Physical Chemistry C
Article
To further evaluate the properties of the thick film, this research paper presents detailed chemical and morphological material characterization of the cycled graphite electrode, with the purpose of evaluating the extent, composition, and implications of the surface film (SEI layer) on electrode properties. The cycled electrode is compared to a calendar-aged one, obtained from another commercial cell of the same type stored at floating potential. Surface and cross-sectional analyses of morphological changes and elemental compositions are performed by means of scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) on manually ripped as well as on polished, focused ion beam (FIB) milled cross sections. In addition, ion depth profiling using time-offlight secondary ion mass spectrometry (ToF-SIMS) is performed to analyze the chemical modifications on the electrode surface from cycling compared to calendar aging. The big advantage of ToF-SIMS as a surface sensitive technique is a local ion detection (i.e., lithium ions and species characteristic of the products of electrolyte decomposition) with a high sensitivity and a very high in-depth resolution (∼1 nm). TofSIMS has previously been used for investigating SEI layers on carbon surfaces.9,15 For depth profiling, however, this surface ionic spectrometry technique is often used to study interfaces on planar thin films.19−21 Nevertheless, it has also been used to compare the SEI composition on commercial porous graphite electrodes subjected to different aging conditions.11 Lithium-consuming film growth during aging can affect the power capability of a cell as well as the capacity, if the deposited film impacts, for example, local resistances, porosity, or available surface area22−26 of the electrodes. It is relevant to identify the specific electrode properties that are modified during aging when, for example, incorporating aging parameters in predictive models. Therefore, in addition to the material characterization, this study evaluates the type of limitation on the electrode performance caused by the increased film formation using physics-based impedance modeling. The distinction of bottlenecks for electrode performance is essential to better understand power fade related to film formation and capacity loss. The graphite electrodes under investigation in this paper are obtained from commercial cells. This renders some uncertainties regarding electrolyte composition and material properties when evaluating the surface film. However, the study of realistic systems, regarding cell design (larger cylindrical cells and porous electrodes) and cycling conditions, is important when targeting aging phenomena in specific applications. The cells evaluated here are part of projects targeted at aging in heavyduty hybrid electric vehicles.17,27 Detailed physical and electrochemical studies of aging phenomena in these cells can highlight the relevance of different degradation mechanisms observed in laboratory model systems.
approximately 45% SOC during the test period. At end-of-life (EOL) the capacities were 34% and 99% of the initial capacity for the cycle-aged and stored cells, respectively. The cells were opened at discharged state in a glovebox under argon atmosphere (H2O and O2 < 1 ppm). Samples of the graphite negative electrode were taken from the middle part of the electrode tape, both regarding length and width, and subsequently rinsed in dimethyl carbonate (DMC) (Merck) before performing electrochemical and material characterization. Samples of 18 mm diameter intended for electrochemical measurements were stripped of material from one side of the double-coated electrode using N-methyl-2-pyrrolidone (NMP) (Merck) solvent. All sample preparations and cell assemblies were performed under argon atmosphere. 2.2. Capacity and EIS Measurements. Low-rate capacity and EIS measurements were performed on the harvested electrodes in a three-electrode setup28 as previously described,22 using lithium foil as reference and counter electrode, 1 M LiPF6 in 1:1 ethylene carbonate (EC):diethylene carbonate (DEC) (Merck, LP40), and three glass-wool separators (Whatman GF/A). The galvanostatic discharge capacity was measured between 1.5 and 0.002 V using a current of 0.3 mA, corresponding approximately to a C/13 rate. EIS measurements were performed at an open circuit potential of 127 mV vs Li/Li+, corresponding to roughly 36% lithiation degree arriving from the lithiated state following the low rate capacity measurements. The impedance was measured at frequencies between 10 kHz and 5 mHz using a 5 mV rms sinusoidal potential perturbation. Electrochemical measurements were performed using a Gamry PCI4/750 galvanostat/ potentiostat. 2.3. SEM and EDX Measurements. SEM images were obtained using a Zeiss Sigma VP with an acceleration voltage of 3−20 kV and a vacuum of
