Understanding Lithium Inventory Loss and Sudden Performance

Article pubs.acs.org/JPCC

Understanding Lithium Inventory Loss and Sudden Performance Fade in Cylindrical Cells during Cycling with Deep-Discharge Steps E. Sarasketa-Zabala,*,† F. Aguesse,‡ I. Villarreal,† L. M. Rodriguez-Martinez,† C. M. López,‡ and P. Kubiak‡ †

IK4-Ikerlan, Energy Business Unit, Arabako Teknologi Parkea , Juan de La Cierva 1, E-01510 Miñao, Spain CIC EnergiGUNE, Arabako Teknologi Parkea, Albert Einstein 48, E-01510 Miñao, Spain

J. Phys. Chem. C 2015.119:896-906. Downloaded from pubs.acs.org by TULANE UNIV on 01/09/19. For personal use only.

‡

ABSTRACT: The cycling performance fade of LFP-based Li-ion cylindrical batteries is evaluated under maximum cycling voltage amplitude. Diagnostic evaluation of the aging mechanisms included in situ electrochemical measurements and ex situ destructive physicochemical and electrochemical analyses of cell components. SEM, EDS, XRD, and electrochemical measurements of harvested electrodes confirmed that the primary cell performance degradation modes are loss of active lithium inventory (LLI) and loss of active material (LAM) related to graphite electrode. Aging phenomena were associated with the progressive decomposition of the electrolyte. Cell capacity loss was concluded to be dominated by SEI layer growth, which also led to a sharp power loss together with localized lithium plating on the negative electrode surface upon prolonged cycling. The graphite surface was polymerized and inactivated in localized central parts of the jelly-roll, leading to large cavities as a result of metallic lithium and electrolyte reactions. No degradation of the structure or performance of the LFP positive electrode was detected. In this paper, aging processes are examined in the overall context of cell performance fade during accelerated cycling operation.

1. INTRODUCTION The state of development of lithium-ion (Li-ion) batteries has met global battery market specifications in various sectors: portable and consumer electronic devices, vehicle and other means of transport traction, stationary and grid (e.g., load leveling) applications. They are the most promising rechargeable technology due to their high specific energy and power. However, the lifetime and safety of this technology are still a barrier for large-scale potential applications. Automotive and most of the stationary applications require 10−20 years of calendar life,1 which is still beyond the actual life of available Liion batteries. Hence, it is necessary to understand internal resistance rise and capacity fade over calendar and cycle life and to identify the factors affecting the lifetime. Various types of Liion batteries can be found on the market, and their characteristics depend strongly on the active materials combination and also cell design and manufacturing. In this sense, the determination of the degradation mechanisms and their correlation with cell performance fade is critical for working on cell concepts that can meet target lifetimes and also for establishing precise algorithms for long-term performance predictions based on short-term test data. The performance loss of Li-ion batteries is due to a combination of loss of lithium inventory (LLI), loading mismatching between electrodes, decrease of electrode active area, loss of active material (LAM), and severe drop of conductivity.2 LLI was reported to be the primary aging mechanism for commercial Li-ion cells cycled at room and elevated temperatures3−5 and even when stored at elevated temperatures.3,6,7 LLI in batteries with graphite anode mainly © 2014 American Chemical Society

arises from the formation of the solid electrolyte interphase (SEI) layer due to side reactions of the electrolyte at the graphite surface that result in electrolyte decomposition. LAM is a secondary aging effect, which is mainly a result of structural damage and material loss related to, for instance, metal dissolution. Other typical aging effects can also be found, such as binder decomposition, current collector corrosion, separator melting and corrosion, etc. The evaluation of degradation mechanisms hence involves the examination of physical, chemical, and structural changes in the cell and cell components that resulted from cell operation. Electrochemical techniques, such as electrochemical impedance spectroscopy (EIS) or incremental capacity (IC) and differential voltage (DV) analyses, are valuable for characterizing degradation mechanisms, but for distinguishing aging processes occurring on the different components, post-mortem analysis is crucial. Several research groups have carried out significant postmortem analysis of commercial and prototype Li-ion cells aiming at identifying the aging processes.3,5,7−40 This paper evaluates the performance loss of a commercial LiFePO4 (LFP)/graphite 26650-type cell with 2.3 Ah nominal capacity, specially designed for high-power applications. LFP cathode has been considered as a promising candidate for Liion large scale applications because of its excellent chemical and thermal stability, meaning safe performance, low cost, high specific power, and high cycling capability.41−49 LFP-based Received: October 6, 2014 Revised: December 12, 2014 Published: December 12, 2014 896

DOI: 10.1021/jp510071d J. Phys. Chem. C 2015, 119, 896−906

The Journal of Physical Chemistry C

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CC−CV, mode) and discharging (CC modes) the cells at nominal conditions defined by the manufacturer (cut-off in CC mode). Also, cells were fully discharged at C/5 in order to check close-to-equilibrium open circuit voltage (OCV) characteristics over time. The C/5 C-rate was chosen, as it enabled the observation of phase changes of the graphite on the voltage response of the cell. A pulse power test was performed to assess actual IR: Current pulses at cell maximum acceptable charge and discharge C-rates were carried out over cell entire SOC range. Additionally, impedance evolution was evaluated at different SOCs at the beginning of life (BOL) and end of test (EOT) using the electrochemical impedance spectroscopy (EIS) technique in galvanostatic mode using frequency range from 0.1 Hz to 6.5 kHz. BOL tests included initial formation cycles (cell conditioning) carried out following the same procedure as for the nominal capacity test. Digatron power supply with test circuits and data acquisition (BTS-600 software) and both CTS and Prebatem climatic chambers were used for battery cycling and testing. EIS measurements were carried out using a Digatron EISmeter impedance spectroscope. 2.2. Diagnostics of Cell Performance Fading by Means of Post-mortem Analysis. Pristine, conditioned, and BOL cells were first opened to characterize the purchasing cell condition, effect of formation cycles, and aging initial state (control experiments). Two specimens for each condition were examined in order to check the reproducibility of cell physicochemical characteristics and diagnostic results. Aged cells at different SOH (aging conditions detailed in section 2.1.) were opened and analyzed in order to evaluate the evolution of cell degradation due to 100% DOD continuous cycling. After discharging the cylindrical cells at 1C and 25 °C up to the cut-off voltage established by the manufacturer (safety reasons), they were opened and disassembled in a dry, Aratmosphere glovebox (H2O and O2 levels below 1 ppm), avoiding any short-circuit and components damage. Cell design was first analyzed to observe any possible manufacturing issue. Cell assembly was unrolled, and each component (positive and negative electrode and separators) was visually inspected. Mass and dimensions of the different components were assessed (electrodes and separators thickness were measured using a micrometer) in order to gain an insight regarding the cell performance (e.g., analysis of active masses proportion in the cell). Electrodes and separators were stored separately in sealed plastic bags for further measurements. Two distinct areas of the jelly-roll height were selected for harvested electrodes characterization, as depicted in Figure 1a: the middle and the external area of the electrodes height (0.5 cm from the edge), named as side. Additionally, the inhomogeneities at the aging effects along the jelly roll were checked, as cell design can have an influence on internal temperature, current, and SOC distribution.56,57 For that reason, different samples were collected along the whole electrode length from the inner to the outer part of the jelly roll for further physicochemical and electrochemical analyses. It was decided not to wash the samples before the different examination steps to prevent changes in the surface structure and to avoid any changes in the electrode performance due to the rinsing solvent polarity.15,39 The double-sided electrode samples collected from the cylindrical configuration were prepared before electrochemical measurements. The coating on one side of the electrode laminate was removed with N-methyl-2-pyrrolidone (NMP) solvent to expose the metallic current collector and reduce the

large cells post-mortem analyses are found in the literature.3,5,9,19,20,29 A two-stage degradation process has been previously reported for LFP/graphite Li-ion cells.50 Graphite anodes are known to be likely to suffer active area reduction (LAM).3,5,9,19,35 Analyzing the crystalline microstructure, surface, and morphology of graphite electrode is thus of great importance, as they directly influence the cell performance. Liu et al. concluded that LFP cathodes are not normally damaged during cell operation. On the contrary, other authors51−53 demonstrated coarsening of particles and porosity decrease at the LFP electrode upon cycling. This way, special focus was put on the analysis of cycling performance of this competitive technology. This paper specifically investigates the impact of deep charge/discharge on cell performance, since cycling in a wide state of charge (SOC) range has a profound effect on aging.19,54,55 100% depth of discharge (DOD) cycling regime (constant current, CC, mode) was chosen as a basis for the comparison of cell degradation processes with realistic cycling patterns. Likewise, cylindrical cells were under test because this is a very common cell design in the market for different applications. Some authors5,7,8,10,17,18,23,26,28,32,36,37,39 already worked on cylindrical cells post-mortem analysis with different purposes. Most of these publications refer to either confirming or further understanding aging mechanisms of cylindrical LiNixCo1−xO2,8,10,17 LCO,23,26,32,36,37 and NCA18,23,28,39 batteries under calendar and/or cycling aging conditions. Only a couple of publications were found for LFP-based cells,5,7 which covers a wide range of cycling conditions. The objective of the present work is to identify the main degradation mechanisms of LFP cyclindrical cells that are induced during accelerated 100% DOD cycling tests and to also identify the failures that cause cell capacity and power fade. The inhomogeneity and complexity of aging processes make necessary assessing detailed cell performance information. The same extreme discharge condition has been analyzed at different state of health (SOH) of reproducible cells. This approach allows a progressive identification of critical aging processes and their possible correlation with changes in performance behavior. A complete analysis of the impact of large DOD stress factor on LFP/ graphite cylindrical cells performance is presented in this paper, using a combination of nondestructive diagnostic techniques and post-mortem characterization of cell components.

2. EXPERIMENTAL SECTION 2.1. Cell Cycling, Evaluation of State of Health (SOH), and Preliminary Diagnostics. Cycle aging of a LFP/graphite cylindrical cell was analyzed under ca. 100% DOD. Cells were cycled at constant current (CC) mode between charge and discharge cut-off voltages defined by the manufacturer (3.65−2 V). Baseline current rate (C-rate), temperature, and voltage cycling conditions were cut-off, 30 °C, and 50% middle SOC, respectively. Two cells from the same manufacturing batch were aged under the same conditions in order to check the repeatability of the tests. One test was carried out until the actual nominal capacity was ca. 80% of the initial value. The other cell was stopped at ca. midlife so that cell components could be further characterized in between aging conditions. The electrochemical performance of the cells was characterized by monitoring the evolution of capacity and internal resistance (IR) aging metrics. Intermittent electrical parameters identification tests (EPIT) were conducted at room temperature (25 °C). The actual nominal capacity (Qactual) was measured by fully charging (constant current−constant voltage, 897



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Figure 1. Electrode samples harvesting. Photograph of a cathode electrode indicating the regions (middle and sides of the electrode width) analyzed along the jelly-roll.

internal resistance in the coin cell configuration. Coin cells were assembled from punched harvested electrodes (2016-type from Hohsen Corp.) using lithium−metal symmetrically placed as counter electrode, 16 mm diameter separators collected from the pristine cells, and commercial 1 M LiPF6 in 1:1 EC/DMC fresh electrolyte from UBE. The entire process was conducted in an Ar atmosphere, avoiding any exposure to ambient atmosphere. Galvanostatic measurements were carried out in a Maccor battery tester at room temperature. Half-cells with an LFP electrode were cycled at C/6 between 4.2 and 2.8 V, whereas graphite electrodes were tested between 0.01 and 1 V at C/2. The stability upon cycling and the cell operating SOC window were analyzed. FEI 200 Quanta field emission gun-scanning electron microscope (FEG-SEM) with energy dispersive X-ray spectroscopy (EDS) analysis was used for the harvested electrodes and separators examination. Microstructures were studied at low vacuum mode at 15 kV, and EDS analyses were carried out using EDAX Genesis software. A thin layer of carbon was previously deposited by sputtering onto the separator samples to avoid electronic charging. Samples in the glovebox were taken to the microscope in a hermetically sealed vacuum container in order to avoid contact of the sample with air as much as possible. Diagnostic evaluation of active materials included structural analysis by X-ray diffraction (XRD). A Bruker-D8 diffractometer mounted with a Cu Kα radiation source (accelerating power of 30 kV × 50 mA) was used in the Bragg−Brentano geometry mode. The diffraction patterns were recorded from 15° to 80° in two-theta values. Both negative and positive harvested electrode samples were examined, which were prepared inside the glovebox using a special airtight XRD holder covered with Kapton tape.

Figure 2. SOH as a function of (a) capacity (Q) and (b) internal resistance (IR) due to continuous cycling (FEC: full equivalent cycles).

shows clearly that the capacity loss tendency changed upon cycling. After ca. 2500 FEC it was no longer linear but parabolic, as is the typical aging behavior of Li-ion technology according to the literature.61 This means that the dominant degradation mode was not the same over the whole lifetime of the battery. This phenomenon was also detectable in the IR data, as shown in Figure 2b. The trend of IR increase changed clearly at the same time the evolution of capacity loss did. There was no clear IR change at the beginning, but a slight progressive diminution of IR as observed, in agreement with other studies with the same cell configuration.58−60 Evolution of the impedance was also investigated. Figure 3 shows impedance spectra at different SOH measured at galvanostatic mode between 0.1 Hz and 6.5 kHz frequency

3. RESULTS AND DISCUSSION 3.1. Characterization of Cell Performance. Commercial cylindrical 2.3 Ah LFP/graphite cells were continuously cycled between cut-off maximum and minimum voltages at 1C, 30 °C, and 50% middle SOC in order to evaluate cycle-life performance at large cycling amplitudes (impact factor). Figure 2 depicts capacity loss and IR increase as a function of full equivalent cycles (FEC). These results show that the aging of the investigated LFP cells was primarily evidenced by capacity fade rather than resistance increase, as also reported by other authors.5,58,59 Modeling of this cycling behavior was published in ref 60 in combination with other aging conditions. The analysis of the shape of the capacity loss curve in Figure 2a

Figure 3. Impedance spectra measured over different SOH at 25 °C and (a) 30% SOC and (b) 70% SOC. 100% DOD (1C and 30 °C) tests. 898



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range at 25 °C and 30% and 70% SOC. Even though the semicircle resistance at medium frequency changed slightly at different EIS measuring SOCs (Figures 3a and 3b), it can be considered negligible and assumed that the influence of SOC on impedance is of minor impact, in agreement with direct current (DC) IR measurements published in ref 60. The highfrequency intercept with the real axis represents the resistance related to the electrolyte (considering that external leads and connections are negligible). This interception of the impedance spectra shifted toward larger resistance values upon cycling, which indicates degradation of electrolyte that leads to growth of SEI layer. Cell ohmic resistance increases during cycling. Therefore, it can be deduced that the lithium transport from one electrode to another is the limiting factor during cycling. This effect may be related to high cell voltages (cycling was carried out between cell maximum charge and cut-off discharge voltages). Higher SOC results in low anode potential, where the electrolyte suffers from thermodynamic instability.62 At this condition most solvents undergo a decomposition reaction that induces SEI layer formation. 3.2. Aging Mechanisms Identification. In order to understand cell impedance change at high frequency and the reasons for the observed evolution in capacity loss, close-toequilibrium OCV measurements (full discharges at C/5) were analyzed. Voltage plateaus of the cells were examined from incremental capacity (IC) and differential voltage (DV) curves, which are shown in Figures 4a and 4b, respectively. Aging

LiC6 into C) coupled with the voltage plateau of LFP (which corresponds to FePO4−LiFePO4 phase transformation) are observed.5,65 However, for the studied LFP-based cell, five regions within graphite intercalation−deintercalation window coupled with LFP potential plateau were distinguished, as also reported by Dubarry et al. on other LFP cells.4 The five LixC6 transformation phases in equilibrium are revealed as five IC peaks (numbered as 1 to 5 in Figure 4a) and five valleys in DV curves (Figure 4b). Safari and Delacourt58 reported four staging phenomena for the same cell configuration (they did not take into account the staging transition marked as 4 in Figure 4a). From IC signatures (Figure 4a), it was deduced that active lithium loss (LLI) was apparently the main degradation mode. The size of the peak numbered as 1, around 3.2 V, reduced significantly upon cycling. This was also manifested in the respective first valley on DV curves (Figure 4b), for which the corresponding normalized capacity reduced in ca. 5.6% after 3063 FEC. Mechanical stress during cycling might have a larger influence in the loss of exchangeable lithium observed, when compared to the storage effect. The observed electrode structure alterations are probably related to this effect. All peaks/valleys in IC (dQ/dV)/DV (dV/dQ) curves somehow faded after prolonged cycling even though the first valley in DV curves (peak 1 in IC curves) did not completely disappear. It was therefore concluded that the amount of active areas decreases (LAM) specially at the graphite anode, which means that the graphite could not be lithiated to the same level as it had been initially.63 All DV peaks except the last one moved toward higher SOC values. The last DV peak barely shifted, with negligible changes in the last valley corresponding to ca. 7.8% of the normalized capacity. None of the valleys disappeared at all (even though they changed gradually), so the graphite staging process could apparently be completed upon cycling. These observations agree with the mainly reported two-stage degradation process of LFP/graphite Liion cells.50 Regarding cell impedance changes, from cell voltage profiles derivatives analysis it was not possible to find out the effect causing impedance change: IC (Figure 4) peaks were approximately positioned at the same voltage values during nearly the entire cycling test. In this sense, post-mortem analysis of these cells may provide further insights into improving the understanding of cell performance deterioration. 4.3. Cell Components Post-mortem Analysis. 4.3.1. Visual and Optical Inspection of Cell Components. 4.3.1.1. Visual Inspection. The first step after cell disassembling was the initial visual inspection of the unravelled components. LFP active material was strongly adhered onto the aluminum current collector, and no macroscopic changes were identified. The graphite electrode was not delaminated in any of the cases, but its appearance had changed (surface color distribution), as shown in the images of Figure 5. A gold-colored line in the middle of the negative electrodes collected on pristine, conditioned, and BOL (Figure 5a−c) cells was observed. It is assumed that remaining lithium was present on the graphite electrode even after cell discharge up to the cut-off voltage. After continuous cycling, the gold line turned into a blue coloration (Figures 5d and 5e-1). The blue color is ascribed to organic decomposition products because graphite lithiated phases are reported in colors ranging from gold to various shades of orange and red.19 Moreover, localized areas containing plated metallic lithium (clear bright spots) were detected in the cell at 79.6% SOH (Figure 5e-2). In all cases, the negative electrode sides presented the gray color of graphite

Figure 4. IC (dQ/dV) (a) and DV (dV/dQ) (b) curves, obtained from C/5 discharge voltage profiles, from initial state (BOL) to end of ca. 20% capacity loss degradation (after 3063 FEC at 79.6% SOH) as a function FEC.

mechanisms were evaluated by analyzing the changes in the peaks with an increasing number of cycles, as the area under the peaks in IC curves and distance between the peaks in DV curves represent the capacity involved in the related phase transformation reaction.63,64 These curves are mainly representative of the graphite electrode in LFP-based cells, since LFP electrode shows voltage profiles with a single voltage plateau that yields a featureless differentiation curve. However, graphite shows different plateaus on charge−discharge voltage curves. Typically, three-transformation processes of the graphite (from 899



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Figure 5. Initial photographs of the negative electrode: (a) pristine cell, (b) conditioned cell, (c) BOL cell, (d) two areas along the length of the electrode of the cell cycled for 1521 FEC (93.03% SOH), (e-1) separator of the cycled cell for 3276 FEC (79.6% SOH), and (e-2) cell cycled for 3276 FEC (79.6% SOH). Cycling tests: 100% DOD, 1C, and 30 °C.

Figure 6. Secondary electron SEM images of the negative electrode: pristine cell, conditioned cell, BOL cell, cell cycled for 1521 FEC (93.03% SOH), and cell cycled for 3276 FEC (79.6% SOH). Cycling tests: 100% DOD, 1C, and 30 °C. The scale is the same for all images.

indicating less electrode degradation, as also reported by Klett et al.19 These areas apparently remained unchanged upon cycling. Lastly, no apparent physical damage of the separators was detected, despite the coloration change and an electrolyte brownish stain in the middle along the whole length of the jelly roll, as shown in Figure 5e-2 for the cell at 79.6% SOH. At this stage, only a few drops of liquid electrolyte were collected. 4.3.1.2. Microscopy Analyses. a. Anode. Backscattered SEM images from different negative electrodes areas, as specified in Figure 1 within the Experimental Section, are shown in Figure 6. No microstructure changes were observed between pristine, conditioned, and BOL graphite electrodes, nor on the sides of any of the electrodes, whereas the middle area was altered for cycled electrodes. The morphology of graphite flakes did not change in any case. Apparently the external areas barely took part or did only partially in the electrochemical processes, and the negative electrode degradation was concentrated in the middle area, probably as a consequence of inhomogeneous current distribution in the system, although temperature increase in the core of the jelly-roll and system pressure could also have contributed to it.19 SEM images of selected representative zones of graphite electrodes middle areas are presented in Figure 7. The different images (Figure 7a−d) indicate different degradation processes that were induced within the same electrode upon cycling. On the surface of graphite sample cycled for 1521 FEC (Figure

7a,b), two aging phenomena were revealed: partial covering of graphite rough particles, looking like drops (Figure 7b), indicating SEI formation, and cavities appearance (Figure 7 a,c) that were not observed in pristine, conditioned, and BOL negative electrodes. In Figure 7a, the cavities (indicated with red arrows) may indicate an early stage of negative electrode delamination that results in loss of cohesion between graphite particles, which may have affected the electrical conductivity. The cavities of up to 10−50 μm in Figure 7c (area indicated with the blue arrow) where observed in areas where metallic lithium was visually detected (Figure 5e-2). They may be generated as a consequence of gas evolution and the polymerization of the electrolyte on the surface of the plated lithium. Figure 7d indicates that in localized areas the resistive surface layer got thicker and denser upon cycling (compare with Figure 7b). Hence, the cavities altering the surface layer most probably originate from two different mechanisms: negative electrode delamination (Figure 7a) and gas formation due to electrolyte decomposition (Figure 7c).66,67 Several design parameters dictate the susceptibility to lithium plating: (i) the nature of electrolyte crucially affects on lithium intercalation kinetics,68 and (ii) graphite is prone to it due to the proximity its reversible potential to that of lithium.68,69 Strenuous charge conditions implying large exchange currents and low temperatures lead to such a phenomenon that radically deteriorates cell performance. Cells were fully charged at cut-off 900



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Figure 7. Secondary electron SEM images of graphite electrodes surface (different areas of interest) corresponding to cells cycles for 1521 FEC (a, b) and 3276 FEC (c, d). Some of the cavities of the same kind indicated with the same color arrows.

thousands of times, which may have reduced lithium intercalation kinetics at the anode and caused its polarization. Charge transfer resistance increase, due to the progressive growth of the passivating layer, may have led to metallic lithium deposition in preference to lithium intercalation. The plated lithium is largely reversible and reoxidizes during the subsequent discharge at potentials about 100 mV higher than the lithium deintercalation potentials.68 If given sufficient time, it can also intercalate into graphite.68 Unfortunately, it does not disappear entirely. No metallic lithium was observed in the harvested electrode analyzed after 1521 FEC (Figure 7a), where localized and reversible lithium plating may also have occurred. After 3276 FEC, however, there was nonreversible metallic lithium that was largely spread (Figure 5e-1). Its reaction with electrolyte degraded graphite surface as observed before (Figure 7d), which may have largely increased the internal resistance of the cell, in agreement with reference performance tests (EPIT) results at the EOT (Figure 2). On the other hand, isolated, nonactive lithium on a graphite surface may have contributed significantly to the enhanced capacity loss after ca. 2500 FEC. Hence, these lithium plating effects may explain the observed cell performance loss (Figure 2) and the dominant degradation mechanism change (LLI) diagnosed from DV and IC curves (Figure 4). Close-to-equilibrium OCV measurements did not however enable the detection of any metallic lithium. b. Cathode. SEM images of harvested positive electrodes are presented in Figure 8. They do not show distinctive change of

Figure 8. Backscattered electron SEM images of the positive LFP electrode: (a) pristine cell and (b) cell cycled at 100% DOD during 3276 FEC (79.6% SOH). The scale is the same for both images.

LFP active material microstructure even after more than 3000 cycles and large cell impedance change. Elemental analysis by EDS indicated that there was no measurable variation of Fe content in the positive electrode. No Fe traces were detected in the negative electrode either. It was therefore observed, in agreement with other authors,5,19 that the studied LFP cathode is robust and stable upon continuous cycling at large DOD, confirming the large cycling capability of this positive electrode chemistry. c. Separator. Figure 9 shows the microstructure of the separator, demonstrating that even after more than 3000 cycles there were still unclogged pores on its surface. The lithium ions transfer between the two electrodes was therefore possible. The separators did not show any apparent macroscopic defect 901



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Figure 9. SEM images of the separator extracted from the pristine cell (a), the cell cycled at 100% DOD during 1521 FEC (b), and the cell cycled at 100% DOD during 3276 FEC (c). The scale is the same for all images.

observations are in good correlation with the changes of morphology at the surface of the graphite observed in Figure 7 by SEM. Consequently, it seems that a secondary SEI layer is formed after cell prolonged cycling. The absence of new peaks in the XRD of harvested graphite samples cycled for 1521 FEC and 3276 FEC indicates that the surface layer formed is not crystalline, but it influences the intensity of the representative peaks of the graphite. b. Cathode: LFP Evaluation. XRD patterns of harvested positive electrodes from pristine and the most aged cells are presented in Figure 11. The crystal structure of the electrode

either. However, Figure 9c shows agglomeration after prolonged cycling. EDS analysis of unwashed separator samples reveal presence of F and P elements on the surface (14.3 and 2.32 at. % of F and P, respectively) that may come from electrolyte salts. Hence, they may indicate (i) the presence of free electrolyte, or (ii) electrolyte crystallization on separator surface, or (iii) electrolyte decomposition products formation. Despite it, the pores were only partially clogged, and it was rejected the possible contribution of the separator to the resistance increase observed over cycling (Figures 2b and 3). 4.3.2. Structural Characterization. a. Anode: Graphite Evaluation. XRD analyses of graphite negative electrode samples extracted from the center of the harvested electrodes are shown in Figure 10. The most intense peaks correspond to

Figure 11. XRD patterns of the LFP positive electrode of pristine cell and cell cycled at 100% DOD after 3276 FEC (79.6% SOH). Figure 10. X-ray diffraction patterns of the harvested negative electrodes from pristine and BOL cells compared with the ones from 93.0% and 79.6% SOH cells.

was identified as olivine-type LFP in the Pnma symmetry. The main crystallographic phase of the LFP remained unchanged regardless of the cell degradation, but small diffraction peaks at 18.1° and 38.7° appeared (indicated with arrows in Figure 11), which reveals a small presence of unlithiated FePO4 phase70 after more than 3000 FEC. 4.3.3. Electrochemistry of Harvested Electrodes. Galvanostatic half-cells cycling of the negative and positive harvested electrodes using lithium metal as a counter electrode are shown in Figure 12. Half-cells were cycled repeatedly in order to evaluate the cycling capability and the reproducibility of the results. Figure 12 shows the first five cycles for the selected electrode samples. Figure 12a shows cycling profiles of harvested graphite electrode samples of the pristine cell and the cell cycled for 3276 FEC, the performance of which depended distinctly on the selected electrode area from the original aged cell. Graphite electrode polarization increased with aging. Samples harvested from the middle area of the highly aged electrode showed a

the diffraction lines of graphite and copper (current collector). Additional less intense peaks mainly located between 20° and 40° in two-theta angle are also present. These peaks may be attributed to the crystallization of organic electrolyte salts, such as LiPF6, and additives (unwashed electrodes were examined). However, (002) the characteristic peak of the graphite was not consequently shifted, as can be observed in the inset of Figure 10. This indicates that the amount of lithium within the graphite layers after the delithiation process is similar at the different aging stages.15 Hence, at first, no additional trapped lithium within the graphite structure seemed to have led to lithium loss. However, even though the peak position change is slight, its intensity decreased upon cycling. This reduction of the diffracted X-ray signal may be attributed to the growth of an amorphous layer on the top of graphite flakes. These 902



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process may have somehow influenced harvested electrode samples properties, although they were all treated in the same way. Cycling performance of harvested LFP electrode is shown in Figure 12b. The voltage profiles, voltage plateau, and the specific capacity were similar for all studied samples (pristine, BOL, and aged cells, after 1521 and 3276 FEC), so the voltage cycling profiles of a sample from the middle area of the LFP electrode harvested from the most aged cell are shown as an example. On the other hand, as presented in the inset of Figure 12b, the discharge specific capacity did not change regardless of the aging rate or the sampling electrode area. Hence, electrochemical properties of the LFP electrode were not altered upon cycling. All in all, it was concluded that the graphite electrode was responsible for both the sharp increase of the full cell internal resistance, observed after more than 3000 cycles (Figure 13),

Figure 12. Galvanostatic half-cells cycling of (a) negative graphite and (b) positive LFP (middle area, as an example) harvested electrodes from pristine cell, BOL cell, cell cycled for 1521 FEC (93.03% SOH), and cell cycled for 3276 FEC (79.6% SOH). Cycling tests: 100% DOD, 1C, and 30 °C. Figure 13. Positive and negative electrodes matching from capacity measurements on electrodes harvested from (a) pristine cell, (b) cell cycled for 1521 FEC (93.03% SOH), and (c) cell cycled for 3276 FEC (79.6% SOH). Cycling tests: 100% DOD, 1C, and 30 °C.

capacity loss of more than 95%, whereas on the side areas the capacity decreased by 60% on average. The lithium intercalation−deintercalation process in the graphite could therefore hardly be performed in the middle area, and its efficiency was reduced appreciably on the sides. Hence, the passivating layer formed on top of the graphite surface seemed to substantially limit lithium transport from the electrolyte into the graphite. As observed by electron microscopy and already shown in Figure 6 (3276 FEC), the SEI layer was more homogeneous and thicker (covering all particles) on the middle of the electrode than on the sides. The inset in Figure 12a compares the specific capacity of the graphite from pristine, BOL and aged cells (after 1521 and 3276 FEC). For the first two, just the values corresponding to the middle electrode area are shown, as the differences between different areas were not as appreciable as for the aged cells, which proportionally showed the same specific capacity differences between the electrode areas. Full cell electrochemical measurements showed large capacity decline for the anode, but data in Figure 12a overstep reasonable values according to the theoretical specific capacity of the half-cells electrode samples. Hence, cell disassembling, electrodes handling, and half-cells assembly

and the overall cell capacity fade. It was demonstrated that the LFP cathode is a robust material that presents good cycling capability with no structural degradation after over 3000 cycles. There was therefore a mismatch effect between the positive and negative electrodes, and the degree of lithiation of the LFP electrode decreased significantly with cell aging regardless of the good performance of the LFP electrode: from ca. 90% to ca.79% after 1521 FEC and to ca. 70% after 3276 FEC. These results are in good agreement with the unlithiated olivine structure detected from XRD patterns. In consequence, the usable SOC range of the LFP electrode and, to the same extent, full cell SOC operation window were reduced upon cycling, as shown in Figure 13 for the harvested electrodes middle areas from different fresh and aged electrodes. The cell SOC operation window reduced by 23% after 1521 FEC and by 54% after 3276 FEC. 903



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the rolling electrode (center area). This effect reduces lithium intercalation kinetics at the negative electrode and provoked irreversible metallic lithium deposition, resulting in additional loss of active lithium (LLI) and uneven active material loss (LAM) due to isolated electrode surface. LLI was not due to the lithium intercalation process within the anode (lithium trapping within the host structure of the graphite), but its main cause factor was the side reactions that decomposed the electrolyte on the anode side. Hence, even though initially LLI was the main degradation mechanism, cell performance fade (especially capacity loss) was afterward caused by a combination of LLI, reduced active electrode area (LAM), and cell impedance increase (stepwise electrolyte degradation). Post-mortem observations are in good agreement with the main aging processes deduced from in situ electrochemical examinations. Hence, cell components physicochemical evaluation was valuable for validating degradation modes for an adequate aging predictive model development. This analysis also evidenced the heterogeneity of the complex aging processes inside the cell and showed that it is not possible to set apart the observed mechanisms in order to evaluate their particular dependence on operating conditions. Besides, the observed uneven aging of the cell components is an important issue for cylindrical cells, taking into account that the inefficient use of active materials, on the one hand, requires the overdimensioning of the battery system and, on the other, influences the operating strategies both to meet end-user functional specifications.

The degree of lithiation in the harvested positive LFP electrodes, both in middle and side areas, were assessed from the first half-cell cycle (Figure 12b) and are summarized in Table 1. The amount of intercalated lithium into the positive Table 1. Degree of Lithiation in the Positive LFP Electrode Assessed from the First Cycle of the Galvanostatic Cycling in the Half-Cell Configuration (All Values in %)

side middle

pristine

BOL

1521 FEC (93.03% SOH)

3276 FEC (79.6% SOH)

89−91 89−91

90−93 90−93

92−95 ca. 79

90−94 ca. 70

electrode was constant for the pristine, conditioned, and BOL cells (91−92% degree of intercalation on average), indicating that cells formation was completed. These measurements are comparable as all the cells were fully discharged at the same rate before the post-mortem analysis. The slight differences in the measurements may be either related to reversible capacity loss (self-discharge) due to cell storage at open circuit (OC) before their use and consequent reactivation of the chemistry or also to experimental errors. The lithium intercalation however decreased significantly with aging on the middle area (ca. 70% degree of intercalation after 3276 FEC), indicating that SEI protective layer grew progressively in this area of the graphite electrode. This uneven degree of lithiation was a sustained effect, and not a temporal effect of, for instance, temperature distribution within the cell due to cycling, as cell full discharging previous to the cell disassembly was carried out after a significant period at open circuit and room temperature.19 The pronounced difference on the degree of lithiation between the LFP electrode areas was related to an effect of aging state mismatch between positive and negative electrodes and charge imbalance of the negative electrode. It was not a consequence of inhomogeneous degradation of the LFP electrode itself, in view of the good performance of the harvested LFP electrodes in half-cell measurements (Figure 12b). Therefore, it was once again confirmed that the growth of SEI layer on the graphite surface middle area reduced the active graphite anode surface and that, as a consequence, led to uneven battery utilization. Further analyses are ongoing to determine the properties of the SEI layer.

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AUTHOR INFORMATION

Corresponding Author

*Tel +34 945 297 032; fax +34 945 296 926; e-mail [email protected] (E.S.-Z.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This investigation work was financially supported by ETORTEK (Energigune’12 − I+D+i en almacenamiento de ́ energiá electroquimica y térmica, y en energiá marina, IE12335) Strategic Program of the Basque Government.

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5. SUMMARY AND CONCLUSIONS The present work is aimed at investigating the cycle-life performance of a high power LFP/graphite cylindrical cell and the induced aging effects under constant current cycling conditions in a large SOC window. From electrochemical nondestructive tests results, it was deduced that loss of lithium inventory (LLI) and loss of active material (LAM) of graphite negative electrode were the main degradation mechanisms responsible for cell performance fade. Post-mortem analysis provided conclusive physical evidence that the main cause of aging was the nonuniform decomposition of the electrolyte and the resulting evolution of the SEI layer growth due to deposition of decomposition products over the graphite surface. A nonhomogeneous layer was formed progressively consuming active lithium, which also led to loss of conductivity (electrolyte solvents consumption) as the resistance at the interception of the real axis was increased over cycling (indicating electrolyte oxidation). After prolonged cycling lithium ions transport process seemed to be blocked in the middle along the length of

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published to the Web on December 26, 2014, with mistakes in the headings in section 4.3 and other minor text errors. These were fixed in the version published to the Web on December 31, 2014.

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