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A Multistage Mechanism of Lithium Intercalation into Graphite Anodes in Presence of the Solid Electrolyte Interface Franz Dinkelacker, Philipp Marzak, Jeongsik Yun, Yunchang Liang, and Aliaksandr S. Bandarenka ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18738 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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ACS Applied Materials & Interfaces

A Multistage Mechanism of Lithium Intercalation into Graphite Anodes in Presence of the Solid Electrolyte Interface

Franz Dinkelacker‡,1, Philipp Marzak‡,1,2, Jeongsik Yun1,2, Yunchang Liang1, Aliaksandr S. Bandarenka*,1,2

1-

Physik-Department ECS, Technische Universität München, James-Franck-Straße 1, 85748 Garching, Germany 2-

Nanosystems Initiative Munich (NIM), Schellingstraße 4, 80799 Munich, Germany

Keywords: Lithium-ion batteries, Solid electrolyte interface, Electrochemical impedance spectroscopy, Graphite anodes, Three-stage mechanism, SEI thickness

Corresponding Author *E-mail: [email protected] (A.S. Bandarenka) Author Contributions ‡These authors contributed equally.

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ABSTRACT

So-called solid electrolyte interface (SEI) in a lithium ion battery largely determines the performance of the whole system. However, it is one of the least understood objects in these types of batteries. SEIs are formed during the initial charge discharge cycles, prevent the organic electrolytes from further decomposition and at the same time govern lithium intercalation into the graphite anodes. In this work, we use electrochemical impedance spectroscopy and atomic force microscopy (AFM) to investigate the properties of SEI film and electrified “graphite/SEI/electrolyte interface”. We reveal a multistage mechanism of lithium intercalation and de-intercalation in the case of graphite anodes covered by SEI. Based on this mechanism, we propose a relatively simple model, which perfectly explains the impedance response of the “graphite/SEI/electrolyte” interface at different temperatures and states of charge (SOCs). From the whole data obtained in this work, suggest that not only Li+ but also negatively charged species, such as anions from the electrolyte or functional groups of the SEI, likely interact with the surface of the graphite anode.

1. INTRODUCTION

After the successful commercialization of Li-ion batteries, the demand for these batteries has been continuously increasing due to their applications in numerous electric devices. Furthermore, with growing concerns about environment and sustainability of resources, traditional energy sources have been being replaced with renewable energy ones, and the regulation on automobile industries has been tightened up as years go by. With the wider propagation of the electric vehicles, the demand for Li-ion batteries is forecasted to progressively increase.1


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Rechargeable Li-ion batteries were firstly commercialized using oxide cathodes and graphite anodes by Sony in 1991. 2 , 3 , 4 After more than two decades of research, many advanced cathode materials have been proposed. However, graphite still occupies the firm position as the main anode material in Li-ion batteries. Graphite shows a reasonable specific capacity, flat potential profile, low discharge potential close to Li/Li+, and it is relatively abundant. However, its performance is largely affected by the growth of so-called solid electrolyte interface (SEI) on the electrode surface. Therefore, there has been a large number of studies to understand the role of SEI layers in the interfacial charge and mass transfer between graphite and organic electrolytes.5,6,7,8 Still, SEI remains perhaps the least understood part in Li-ion batteries. Recently, we utilized analysis of electrochemical impedance spectroscopy (EIS) data to reveal the nature of the mechanisms of interfacial charge and mass transfer during (de-)intercalation in various battery systems and identified, for example, that these are significantly influenced by electrolyte composition. 9 , 10 , 11 , 12 , 13 Due to the dissimilar time constants of different electrochemical processes, EIS may directly provide valuable information about battery systems in-situ and in-operando. 14,15,16,17,18,19,20,21,22,23,24,25 While there have been numerous studies of graphite electrodes using EIS, fitting of the impedance spectra is often missing due to the difficulty of their interpretation and analysis.26,27,28,29,30,31,32 In this work, we reveal a multistage mechanism of lithium intercalation and deintercalation in the case of graphite electrodes covered by SEI. Based on this mechanism, we propose a relatively simple impedance model, which perfectly explains the EIS response of the graphite/SEI-electrolyte interface at different temperatures and states of charge (SOC). Using this model we propose that not only Li+ but also negatively charged species, such as functional groups of SEI or anions, can likely interact with the surface of the anode.



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2. EXPERIMENTAL

The battery cells were assembled using both commercially manufactured nickel cobalt manganese (NCM) dioxide cathodes and graphite anodes with a 260 μm glass fiber separator. The cathode material contained a ratio of 6:2:2 (LiNi0.6Co0.2Mn0.2O2) where 4 % of a conductive agent and 2 % of binder were added. The graphite anode consisted of 96.7 % graphite, 1 % conductive additive and 2.3 % binder. Both electrode materials were compressed for higher volumetric energy density (so-called calendaring), with a resulting porosity of ~30 % . The thickness of the NCM-electrode was ~49 μm and the graphite electrode was ~44 μm; both electrodes had a geometrical surface area of ~2.54 cm2 . A commercial EL-CELL PAT-Core cell (Figure 1A) was used for electrochemical measurements. The PAT-core cell was assembled using a sleeve equipped with a built-in ringshaped Li-reference electrode allowing reliable impedance measurements using a three electrode configuration. The electrolyte was composed of a 1 M LiPF6 solution in ethylene carbonate (EC)/ethyl methyl carbonate (EMC) 3:7 (v/v) with 2 wt% of vinyl carbonate. To ensure low cell resistance, a defined force is applied to the "sandwich" using a spring. The scheme of this assembly is shown as a sketch in Figure 1B.


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Capacity / mAh/g Figure 1. (A) Experimental PAT-Core cell from EL-CELL. (B) Sketch of an assembled cell including housing and cell components. (C) A typical potential curve for the graphite electrodes at C/10, where the three intercalation plateaus associated with different occupancy states of the graphene layers are marked as states I, II and III respectively.

After assembly, SEI formation was performed for two cycles at a C/10 rate. The potential curve of graphite is shown in Figure 1C. The cycling followed a so-called “cc-cv protocol” of the battery between 3.0 V and 4.2 V. The cv-step was limited by the current falling below C/40. A capacity determination cycle was performed at C/2, where the capacity was calculated from the discharge step. Subsequently, the cell was charged and again discharged to the desired SOC. All battery tests (full charge/discharge, cycling and capacity measurements) were performed using a multichannel “Material Development System (MDS)” (BaSyTec) or “Cell Test System (CTS)” (BaSyTec). Impedance spectroscopy was performed using an impedance 5 ACS Paragon Plus Environment


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analyzer “IM6ex” (Zahner-Elektrik). The spectra were collected in the potentiostatic mode in the frequency range between 100 kHz and 10 mHz. The excitation amplitude was set to 10 mV. To validate the measured impedance data, Kramers-Kronig checks were performed. In order to secure stable measurement conditions, all impedance measurements were performed in a temperature chamber with thermoelectric Peltier technology “KT 53” (Binder) ensuring temperature fluctuations not greater than 0.1 °C. For exemplary determination of the dielectric constant of a SEI, highly-doped silicon wafer electrodes with surface area of ~1.13 cm2 and HOPG were used. The wafer electrodes were assembled in a half-cell configuration with a lithium counter electrode using the Swagelok cell design with the 260 μm glass fiber separator. As an electrolyte, 1 M LiPF6 in fluoroethylene carbonate (FEC)/ethyl methyl carbonate (EMC) 3:7 (v/v) with 2 wt% of vinyl carbonate was used. Again, a Li metal point reference electrode enabled three electrode impedance measurements. The passivating SEI film was formed onto the wafer surface with a controlled voltage profile. The open circuit potential (OCP) of the cell was ~1.2 V vs. Li/Li+ after assembly and was maintained for 4 h as a waiting period. The following potential profile consisted of a ramp down from the OCP to 200 mV vs. Li/Li+ with a scan rate of 15mV/s and a holding period of 20 h at 200 mV vs. Li/Li+. The chosen potential assured no incipient lithiation of the silicon wafer which takes place at potentials smaller than 150 mV vs. Li/Li+. SEI formation on HOPG was performed in a three electrode setup using a HOPG as a working electrode and Li-metal as reference and counter electrodes, respectively. 33 The electrolyte containing a 1 M LiPF6 solution in ethylene carbonate (EC)/ethyl methyl carbonate (EMC) 3:7 (v/v) with 2 wt% of vinyl carbonate. After the assembly, the OCP of the cell exhibited ~ 2.5 V vs. Li/Li and the cell was maintained for 4 h as a waiting period. Subsequently, a potential profile consisted of a ramp down from the OCP to 100 mV vs. Li/Li+ with a scan rate of 15 mV/s and a holding period of 20 h at 100 mV vs. Li/Li+. After 6 ACS Paragon Plus Environment

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the SEI formation, the electrode potential was applied to 2.0 V vs. Li/Li+. for 1 h to fully delithiate the HOPG for better SEI thickness measurements. For SEI thickness determination, the cells were opened inside the glove box and rinsed carefully using fluoroethylene carbonate (FEC)/ethyl methyl carbonate (EMC) 3:7 (v/v) without conducting salt or additives. This step is necessary to remove excess of the conducting salt from the surface after evaporation of the solvents. Subsequently, half of the film in the area not in contact with the separator was removed using a razor blade. Previous to this procedure the razor was applied to a fresh silicon wafer and thickness profiling revealed no residual marks at the surface. This is important to avoid confusion of steps in the film thickness with steps on the wafer surface due to previous manipulation. The fresh surface exhibited excellent flatness with height deviations of 5 to 10 nm. After the preparation, the silicon wafers were transferred to the thickness measurement instruments under a protective argon atmosphere to avoid degradation of the SEI. After the transfer, layer thickness measurements were conducted using atomic force microscopy (AFM) and a profilometer in ambient air. A multimode EC-STM/AFM instrument (Veeco VI) operated in tapping mode (AFM tips BRUKER RTESP-300) was used for AFM measurements. Further data analysis was performed using the software package “WSxM v5.0 Develop 8.3” (WSxM solutions34). The profilometer “DekTak 6M” (Veeco) was operated in “valley and hills” mode to measure the height of thin films using a stylus with 1 mg weight.

3. RESULTS AND DISCUSSION

For the investigation of interfacial processes at the graphite anodes, impedance measurements were performed under varying operating conditions. Impedance spectra were recorded at 5 °C, 10 °C, 20 °C, 24 °C and 30 °C, and at different SOCs between 0 % and



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100 % at 24 °C. A typical experimental impedance spectrum of the investigated graphite anodes is shown in Figure 2A. It consists of a curvature along the high frequency region (HFR, 100 kHz to 1 kHz, approximation is given in red) and the mid-frequency region (MFR, 1 kHz to 1 Hz, approximation is given in blue). This curvature is divided into one semi-circle in the HFR and one in the MFR, which can be clearly observed at low temperatures (see Figure 2B). The inflection point between MFR and low frequency region (LFR, 1 Hz to 10 mHz, green area) is followed by a linear slope towards the very low frequencies. As shown in Figure 2B, x-intercepts of the impedance spectra in the HFR, which are related to Ohmic losses, such as the electrolyte uncompensated resistance, contacts of active materials and current collector, as well as the diameters of semicircles in the MFR that are correlated to the charge transfer resistance tend to increase as temperature decreases.


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Figure 2. (A) Nyquist plot of an impedance spectrum of a graphite anode at 50 % SOC and 24 °C. Prominent frequencies 100 kHz (red), 1 kHz (blue), 1 Hz and 0.01 kHz (green) and ranges are highlighted. The colored lines are introduced to visually extrapolate the shape of the part of the spectra in the respective frequency ranges. (B,C) Impedance spectra of the grapite anodes for (B) varying temperatures at 50 % SOC and (C) varying SOCs at 24 °C.

This fact might be interconnected with the reduced mobility of ions in the electrolyte at lower temperatures, additionally slowing down the kinetics of the otherwise thermally 9 ACS Paragon Plus Environment


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activated Li-intercalation and de-intercalation. On the contrary, the slopes of the impedance spectra in the low frequency region appear to be unchanged irrespectively from the temperature. As shown in Figure 2C, characteristic loops can be observed in the transition between MFR and LFR at low SOCs (< 20 %). These loops are not experimental artifacts35 (as they are highly reproducible and Kramers-Kronig checks do not indicate any issues), but likely a direct indicator of the presence of multistage processes9,10,11,12,13, 36 in Li+ (de)intercalation from and into graphite. It is important to differentiate between the occupancy states of the graphene layers (“I”, “II” and “III” in Figure 1C) that occur depending on the state of charge of the anode, and the stages of interfacial charge and mass transport that all together govern Li+ (de-)intercalation at the same distinct states of charge (“1”, “2” and “3” in the following). The generally proposed mechanism of Li+ de-intercalation and intercalation in the case of graphite is given by: LiC6 ⇌ C6 + Li+ + e− For simplicity, the equation is denoted for integer-valued electron transfer. This scheme describes Li-intercalation with a very simple one-stage process in which solvent and other electrolyte components are not considered at all. However, this simplified scheme cannot account for the complexity of the spectra shown in Figure 2. A general mechanism comprising of at least three quasi-reversible interconnected stages involving specific adsorption of anions in the electrolyte has recently been proposed by Yun et al.9 It was also found to be applicable to a range of intercalation materials including Li-ion, Na-ion, K-ion cathode and anode materials in aqueous and organic electrolytes in absence of SEI.10 All of the previously investigated systems demonstrated characteristic impedance responses with specific highly reproducible loops at some potentials or SOCs. One should clarify here that due to the properties of the impedance spectra, the electrochemical reaction mechanism can only be identified with the corresponding fitting to a physical model.


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In presence of SEI, the specific shape of the impedance spectra at some SOCs (Figure 2B,C) indeed suggests the existence of a rather complex (de-)intercalation mechanism for the graphite-based anode materials. The mechanism can be described with a Faradaic reaction involving at least three steps with adsorbed species. General impedance analysis of the electrochemical reactions of those types are well described in the literature36 being associated with an equivalent electric circuit (EEC) illustrated in Figure 3A,9 where the elements in the dashed square can formally have positive or negative values (see Supporting Information, Figures S1-S5). Similar to the analysis and interpretation of impedance spectra by Yun et al.9 and Ventosa et al.10, the following three-stage scheme can be applied to the graphite anodes under EIS probing (for simplicity only the de-intercalation process is explained here, although these three stages are quasi-reversibly interconnected): 1) “oxidation” with very fast kinetics due to the good electronic conductivity of graphite; this can also be described as a short-time excess of a positive charge at the electrode: LiC6 ⇌ [LiC6]+ + e−

(1)

2) specific adsorption of negatively charged species A- (e.g. PF6- or negatively charged functional groups of the SEI7,

37

) including Faradaic charge transfer for temporary

compensation of the excess positive electrode charge due to relatively slow de-intercalation of Li-ions: [LiC6]+ + A− ⇌ [LiC6]A

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3) Li-ions leave the graphite together with desorption of the negatively charged species A-: [LiC6]A ⇌ C6 + Li+ + A−

(3)

Unlike the case of intercalation processes in cathodes, the intercalation mechanism of Li-ions into graphite needs further consideration due to SEI layer formation. To describe the SEI formation with an EEC model, a “classical” modification of the EEC should be 11 ACS Paragon Plus Environment


introduced with a parallel connection of RC elements. A typical parallel combination of the R and C elements is normally sufficient in the impedance analysis to account for the capacitive and active response of organic coatings. This results in the modified equivalent circuit depicted in Figure 3B with the modification highlighted in the red dashed square. This new EEC was used to fit the SOC- and temperature-dependent data of the graphite anode. The spectra exhibiting the typical loops as well as spectra without loops can be fitted well, which results in the normalized root-mean-square deviation below 2 % (see Figure 3C,D). (A)

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Figure 3. (A) EEC model for a Faradaic reaction mechanism involving adsorbed species9 and implying at least three quasi-reversibly interconnected stages. It involves an uncompensated resistance Ru, a Zdl corresponding to the constant phase element behaviour of the double layer, the interfacial charge transfer resistance Rct as well as additional elements Ra,1, Ca,1, Ra,2 and Ca,2 mainly related to specific adsorption/desorption (see Supporting Information). (B) Modified EEC for the graphite-based anodes. The red dashed lines highlight the CSEI and RSEI elements belonging to the capacitance and resistance of SEI, the green dashed lines highlight the part belonging to overall (de-)intercalation reaction, and the blue dashed line mark elements mainly related to specific adsorption/desorption. (C) Nyquist plot of the impedance spectrum of graphite at 0 % SOC and 24 °C (open circles). The data is featuring a loop at the transition from MFR to LFR and was fitted (line) using the model from (B). (D) 3D Nyquist representation of the measured impedance data (dots) and the respective fittings (lines) at various SOC steps.


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The exact values of the fitting parameters can be found in the Supporting Information. Values of CSEI (SEI capacitance) and Rct (charge transfer resistance) are furthermore plotted as a function of temperature and SOC in Figure 4. These results confirm the validity of the threestage mechanism (Equations 1-3) also for graphite-based anode materials in presence of SEI. Moreover, in order to allow for this process to be possible, the SEI must either be permeable not only for Li+ but also for the PF6– anions (which e.g. directly penetrate the SEI or travel along cracks and channels possibly present in the SEI), or its negatively charged functional groups7,37 are involved in the (de-)intercalation mechanism. Otherwise, the specific adsorption, which includes Faradaic charge transfer, would not be possible due to the electronically insulating properties of the SEI. This assumption is important and needs further investigation. Figure 4 shows the dependence of SEI-capacitance CSEI and charge transfer resistance Rct on temperature and SOC. (A)

(B)

Figure 4. Area specific SEI-related capacitance, CSEI, and area specific charge transfer resistance, Rct, as a function of (A) varying temperatures at 50 % SOC with Arrhenius fit and (B) varying SOCs at 24 °C.

In order to intercalate into graphite, Li+ ions first need to strip their solvation shell off before penetrating the SEI which causes a resistance. This leads to a charge accumulation which is compensated on the surface of the electrode causing a capacitance CSEI across the SEI. This 13 ACS Paragon Plus Environment


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capacitance is dependent on the temperature (see Figure 4A), which can be explained with the thermal activity of the charge carriers. A high mobility of the charge carriers results in smaller accumulation lowering the capacity. As displayed in Figure 4B, CSEI appears not to have obvious dependence on the SOC. This is in accordance with the physical interpretation of the impedance model: the characteristics of the SEI do not change upon lithiation. At the SEIelectrode interface, a Faradaic process takes place, where graphite is either “oxidized” or “reduced”. Accordingly, an interfacial charge transfer takes place at the anode, which is described by the resistance Rct. It is highly dependent on the temperature as shown in Figure 4A and demonstrates almost ideal Arrhenius behavior. In dependence on SOC, however, Rct appears to exhibit only minor changes. To determine the dielectric constant of a SEI from impedance data as an example, a system consisting of a silicon wafer electrode was used: conductivity measurements on an electrode with a well-defined surface area and forming SEIs on a flat surface simplify data analysis. The potential profile and the resulting current characterizing film deposition are shown in Figure 5A. The measured currents were in a range of μA; it is, however, apparent that no current flows during the waiting period followed by a peak at the ramp of -4 μA. During the phase of constant potential of 200 mV vs. Li/Li+, a smaller current of -1 μA flows. To investigate whether this current is associated with the formation of a film, impedance measurements were conducted before and after the voltage profile experiment. The resulting impedance spectra are shown in Figure 5B. They exhibit a pronounced change from almost entirely capacitive behavior at 1500 mV vs. Li/Li+ to a combination of a curvature in the high frequency region and the characteristic capacitive behavior at 200 mV vs. Li/Li+ manifesting the formation of the SEI. The thickness measurements were conducted using AFM and profilometry in the area where the film was partially removed to capture the film edge for subsequent thickness evaluation, as described in the experimental section. 14 ACS Paragon Plus Environment

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The AFM picture shown in Figure 5C corresponds to the profilometer results marked in red in Figure 5D. It helps to verify that the measured step height is not a local unevenness but a 3D step. For the profilometer and AFM data, the thickness was evaluated by subtracting the mean of the lower edges from the mean of the upper edges for the step in the marked area of Figure 5D and AFM picture in Figure 5C. Both methods give the resulting film thickness of ~23.7 nm which is plausible according to Dahn et al.38 who investigated SEI on a-Si. Just as well, SEI on the HOPG, a system even closer to the graphite anodes investigated in this research, was investigated by means of AFM (see Figure 5E). A representative 2D profile of SEI layers and HOPG is shown in Figure 5F. The thickness profile resulted in the average thickness of SEI on HOPG as ~ 39 nm, which is in a good agreement with the thickness values in the literatures. For example, Domi et al. measured 47 – 77 nm using LiClO4 in EC/DEC39 or Deng et al. measured ~10 nm using LiTFSI in EC/DEC40. With an additional knowledge of the SEI thickness and determination of the SEI’s capacitance by means of standard impedance fitting one can calculate the dielectric constant of the SEI formed in the electrolyte in-situ following the same formation protocol. According to the expression of a plate capacitor the dielectric constant can be derived from:

εr = (CSEI/A) · (dSEI/ε0) where CSEI denotes the film capacitance, A is the surface area, dSEI is the SEI thickness and ε0 is the vacuum permittivity. Using the impedance data for the capacitance and the measured film thickness, the resulting dielectric constant of the SEI layer in the electrolyte was calculated to be ~57 for the Si substrate and ~93 for the HOPG substrate at 24°C. This value is plausible when compared to literature values of liquid EC/EMC which are in the range of 5 to 100 depending on the weight fraction41. The value for the dielectric constant can now be used to derive SEI thickness from capacity parameters determined by analysis of impedance data using the model described above. 16 ACS Paragon Plus Environment

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CONCLUSIONS

In this work, we propose a mechanism of intercalation of Li-ions into graphite electrode in the presence of SEI layers based on data obtained using electrochemical impedance spectroscopy and atomic force microscopy. Impedance studies of graphite electrodes were conducted at different temperatures and at different states of charge. The results from the impedance studies revealed that the charge transfer resistance and the SEI capacitance tend to increase at lower temperatures. This can be explained with the reduced mobility of ions in the electrolyte at lower temperatures. The analysis of EIS data also suggests that not only Li-ions are involved in the intercalation processes, but also negatively charged species, e.g. PF6anions from the electrolyte or negatively charged functional groups from the SEI present at the SEI-anode-interface. Estimation of the dielectric constant of exemplary SEI-layers on flat Si electrodes was performed by means of EIS analysis; the value of the dielectric constant was estimated to be ~57 and ~93 at 24°C for Si and HOPG substrates, respectively. One can apply this method in order to retrieved values for the dielectric constant and such estimate the thickness of SEI layers in subsequent experiments in many comparable systems. This gives rise to a powerful tool for estimating the thickness of SEI layers in-situ and even in-operando.

ASSOCIATED CONTENT The Supporting Information is available free of charge on the ACS Publications website at DOI: Fitting and Interpretation of the “Loop-Shaped” Impedance Spectra, the EEC; EEC parameters for Figure 3C and 3D; Parameters for Calculation of the dielectric constant of SEI layers.



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ACKNOWLEDGEMENTS

This work was supported by Wacker Chemie AG. We gratefully acknowledge the cooperation and especially thank Dr. Stefan Haufe for the fruitful discussion and his extensive contributions. Jeongsik Yun is thankful for the financial support from Nagelschneider Stiftung.

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