The electrochemical behavior of micro-particulate silicon anodes in

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The electrochemical behavior of micro-particulate silicon anodes in ether-based electrolytes: Why does LiNO3 affect negatively? Jing Guo, Ahmad Omar, Anna Urbanski, Steffen Oswald, Petra Uhlmann, and Lars Giebeler ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00590 • Publication Date (Web): 15 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019

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ACS Applied Energy Materials

The electrochemical behavior of micro-particulate silicon anodes in ether-based electrolytes: Why does LiNO3 affect negatively? Jing Guo,a,* Ahmad Omar,a Anna Urbanski,b Steffen Oswald,a Petra Uhlmann,b,

c

Lars

Giebelera,* a

b

Leibniz Institute for Solid State and Materials Research Dresden (IFW), Helmholtzstr. 20, 01069 Dresden, Germany

Leibniz-Institut für Polymerforschung (IPF) Dresden, Hohe Str. 6, 01069 Dresden, Germany c

University of Nebraska-Lincoln, Department of Chemistry, Hamilton Hall, 639 North 12th Street, Lincoln, NE 68588, USA

Keywords: silicon anode, ether-based electrolytes, LiNO3, SEI, surface chemistry Abstract Silicon has great potential to be applied as an alternative anode in lithium-ion batteries. However, the electrochemical performance of silicon anodes in ether-based electrolytes, which is essential for better understanding the electrochemistry of sulfur-silicon full cells, has not been fully investigated. In this work, the effect of ether-based electrolytes on the cycling performance and surface chemistry of silicon electrodes containing micro-sized particles was systematically studied. In LiNO3-containing ether-based electrolyte, silicon electrode showed the fastest capacity fading and the shortest cycle life. Electrochemical impedance spectroscopy (EIS), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) were employed to examine the solid electrolyte interphase (SEI) properties of silicon micro-particles. An unfavorable SEI is responsible for poor cycling performance in ether-based electrolytes. 1 ACS Paragon Plus Environment

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Our results shed new light on the reason for short cycle life of most of the reported sulfur/silicon full cells.

Introduction Lithium-ion batteries (LIBs) based on intercalation electrochemistry (LiNi1-x-yMnxCoyO2, NMC as cathode and graphite, C as anode) have been successfully and widely used for portable electronic devices. However, the emerging market of electric vehicles and hybrid electric vehicles has led to greatly increasing demands for the development of battery systems with larger energy density, higher power density and improved safety. Electrode materials with large capacities are desired to be combined together to build high energy-density full cells. Silicon and sulfur, both have significantly high theoretical specific capacities (3579 mAh g-1Li15Si4 and 1672 mAh g-1S, respectively) and low cost, as well as are environmentally benign.1-2 Additionally, a sulfur/silicon full cell (SSFC) is correspondingly characterized by a higher theoretical energy density (1982 Wh kg-1) compared to commercial NMC/C LIBs (605 Wh kg1),

which makes this battery system highly promising, e.g., for long-distance electric vehicles.3

Unfortunately, both silicon and sulfur have their own challenges, where the most important to name are huge volume changes during cycling and low initial Coulombic efficiency of silicon, as well as severe shuttle effect of formed polysulfides. A large number of studies have been done to address these issues.4-7 In order to establish SSFC as a viable alternative, there is a need to have a synergistic understanding and development of their operation and performance. Two main challenges have to be addressed when a full cell is designed: i) lack of lithium; ii) individually different electrolyte systems. Due to lack of lithium in both silicon anode and sulfur cathode, at least one electrode needs to be prelithiated before being assembled into full cells, which further involves additional issues. More importantly, for silicon anodes, the electrolytes normally used are commercial carbonate-based non-aqueous solutions, in particular, mixtures of ethylene carbonate (EC), dimethyl carbonate (DMC) and/or ethylmethyl carbonate (EMC), 2 ACS Paragon Plus Environment

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containing lithium hexafluorophosphate (LiPF6) salt.8 On the other hand, for sulfur cathodes, ether-based electrolytes with 1,3-dioxolane (DOL), 1,2-dimethoxyethane (DME) as solvents and bis(trifluoromethane)sulfone-amide (LiTFSI) as the conducting salt, are commonly used. Unfortunately, carbonate-based electrolytes are not compatible with the chemistry of typical sulfur/carbon composite electrodes due to the decomposition of carbonate solvents induced by polysulfides formed during cycling.9 Therefore, ether-based electrolytes, ionic liquids, and solid electrolytes, instead of carbonate-based electrolytes, need to be explored for SSFCs.

A large number of studies related to SSFC have been done, such as battery architecture design,3, 10

micro-/nano-structured silicon,11-13 varied shuttle effect inhibition strategies,14-15 prelithiation

methods of silicon16 and the use of ionic liquids17 etc. Nonetheless, only a rather short cycle life has been achieved for most of the reported works and very few of them are focused on the effect of electrolytes. A few related investigations, especially with regard to the electrolytes, have been undertaken in Li-Si half cells, in order to better understand the individual reactions and formed components and to design better SSFC configurations. Aurbach and co-workers reported silicon nanowire anodes with superior cycling performance in DOL-based electrolytes compared to standard alkyl carbonate electrolytes at 60 °C.18 The electrolyte containing LiNO3 as additive was found to significantly improve the cell performance, which was attributed to the thin, elastic and uniform solid electrolyte interphase (SEI) film. Our group also reported that the best performance for nanocrystalline silicon inside carbon shells was observed with a LiNO3-containing ether-based electrolyte.19

Before we proceed, it is pertinent to briefly discuss an important point, why microparticle silicon should be treated as an interesting material in relevant applications, given the larger stresses. In the case of silicon, a small crystallite size (99.99%, dried at 100 °C under vacuum) were used without further purification. Galvanostatic cycling was carried out at 250 mA gSi-1 (≈ 1/14 C, 1 C= 357.9 mA g-1) in the voltage range of 0.01 - 1.2 V (vs. Li/Li+) with a BaSyTec cell test system. The dQ/dV data were recorded with an EC-Lab (BioLogic) potentiostat. Electrochemical impedance spectroscopy (EIS) measurements were carried out using the EC-lab at the open circuit voltage vs. Li/Li+ with an amplitude of 5 mV in the frequency range of 10 mHz - 1 MHz. All measurements were conducted at 25 °C as ensured by a temperature-controlled chamber. Specific capacity was calculated based on the mass of silicon of each electrode individually. To ensure the reproducibility, at least three cells were tested under the same conditions and used for capacity calculation as shown in Figure S1 of the

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Supporting Information. The Coulombic efficiency is calculated as the delithiation capacity divided by the lithiation capacity.

Microscopy and Spectroscopy measurements

In order to do the morphology characterization, coin cells after cycling were first disassembled in the glove box. Scanning electron microscopy (SEM) was performed with a Gemini 1530 from Zeiss/LEO. The samples for cross-section observation were prepared by cutting the electrodes with scissors. All the samples for SEM were sputtered with chromium for 20 s (Leica EM ACE600, Leica Microsystems) in order to reduce the static electric charging effect. For Xray photoelectron spectroscopy (XPS) measurements, the cycled electrodes were rinsed and washed with DMC or DME twice and then left to dry at room temperature overnight inside the glove box. The measurements were carried out using a Physical Electronics PHI 5600 CI system with Mg Kα radiation (350 W) at a pass energy of 29 eV and a step size of 0.1 eV. A PHI 04110 vessel (Physical Electronics) was used to prevent the sample from exposure to air. The binding energy scale of the spectrometer was calibrated with metal foils of Au for the binding energy (BE) of Au 4f7/2 at 84.0 eV and Cu foil for the BE of Cu 2p3/2 at 932.7 eV. All spectra were recorded in an energy range of 0 – 1000 eV with a hemispherical analyzer. The binding energies of the spectra were calibrated according to LiF F 1s (685.5 eV). Elemental concentrations from the XPS were calculated using standard single-element sensitivity factors. The core level signals were fitted with a Gaussian function using a nonlinear Shirley-type background.

Results and Discussion

The typical galvanostatic cycling performance of the silicon micro-particles (SiMPs) vs. Li/Li+ in three different electrolytes (ether-based DOL/DME and DOL/DME + LiNO3; carbonate6 ACS Paragon Plus Environment

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based LP30) is shown in Figure 1. All cells with the SiMP anodes start at high specific capacities in the first cycle up to 3750 mAh g-1. In the subsequent cycles, a fast capacity drop is displayed for the cells with ether-based electrolytes while the capacity of the cell with the carbonate-based one continuously decreases. In the 10th cycle, the cells with LP30, DOL/DME and DOL/DME + LiNO3 electrolytes deliver specific capacities of 1988 mAh g-1, 1088 mAh g-1 and 196 mAh g-1, respectively. In comparison to the carbonate-based LP30 electrolyte, worse cycling stability was shown in ether-based electrolytes and especially the fastest capacity fading was observed with the addition of LiNO3. The performance of cells with LP30 and DOL/DME electrolytes is rather similar after 40 cycles. Furthermore, Li-Si cell with DOL/DME + LiNO3 electrolyte has the highest initial lithiation capacity but the lowest initial Coulombic efficiency (CE), as shown in the inset(s) of Figure 1. This indicates different irreversible capacities for Li-Si cells using different electrolytes primarily due to the formation of different SEI for each electrolyte.

With regard to the CE, only the comparison in the first ten cycles is reasonable as the cell with DOL/DME + LiNO3 electrolyte just delivers 1/20th of the initial capacity after approximately ten cycles. For the cell with the LiNO3-containing electrolyte, the CE of the 3rd cycle reaches only 70%, even lower than the initial CE of the other two cells. Such low CE, even in the 2nd and 3rd cycles, prove that the SEI of the SiMP anode with the LiNO3-containing electrolyte is significantly unstable and likely cracks most easily among the three electrolytes. The CE in the case of LiNO3 does increase with continued cycling as the SEI film stability is improved. Nevertheless, the cell degrades very quickly in between the first ten cycles and SEI formation never reaches even close to a steady state. The cell is effectively dead after 20 cycles, and the CE values can be ignored, although they seem rather stable. In contrast, the cell with the LiNO3free ether-based electrolyte is characterized by a higher and more stable CE of around 92% at the 3rd cycle, but it is still lower than the one with LP30 (96% at the 3rd cycle). 7 ACS Paragon Plus Environment


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Figure 1. Galvanostatic cycling of the Li/Si cells with different electrolytes between 1.2 V and 0.01 V at a current density of 250 mA gSi-1. (a) Specific discharging (lithiation) capacities vs. cycle number; (b) the corresponding Coulombic efficiencies. (Insets are initial lithiation capacities and initial Coulombic efficiencies. The specific delithiation capacities are shown in Figure S2).

Figure 2 displays the discharging/charging profiles and the differential capacity (dQ/dV) plots of the Li-Si cells for the initial three cycles with the individual electrolytes. Differential capacity (dQ/dV) plots allow a better understanding and comparison of the reaction processes during cycling as shown in Figure 2b, 2d and 2e. The cells with ether-based electrolytes show a noticeable 0.02 V higher initial lithiation potential but the same delithiation potential (0.43 V), compared to the one with LP30 (Figure 2a). The sharp lithiation peaks in the dQ/dV plots indicate the alloying of crystalline Si while the sharp delithiation peaks originate from the dealloying process of LixSi.21-22 In the case of the LiNO3-containing electrolyte, a small plateau around 1.6 V is observed which corresponds to the reduction of LiNO3,23 as shown in the insets of Figure 2a,b. Based on the discharging profile, the reduction of the LiNO3 additive contributes to an irreversible capacity of approximately 80 mAh g-1. In the 2nd cycle (Figure 2c,d), all three cells show three lithiation plateaus at 0.25 V, 0.095 V and 0.05 V (0.037 V for LP30) and 8 ACS Paragon Plus Environment

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similar delithiation plateaus at 0.44 V, which has also been observed in another work.24 The 3rd cycle is similar to the 2nd one, except that the lithiation plateau at 0.25 V of the Li-Si cell with LiNO3 electrolyte becomes hardly visible.

Figure 2. Voltage profiles and dQ/dV plots of Li-Si cells with different electrolytes for the initial three cycles at a current density of 250 mA gSi-1. (a, b) the 1st cycle (the insets show the reduction 9 ACS Paragon Plus Environment


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of LiNO3); (c, d) the 2nd cycle; (e, f) the 3rd cycle. The dQ/dV plots are only shown between 0.01 V and 0.6 V since no peaks are observed beyond 0.6 V. The plots of the 1st cycle in the whole voltage range are shown in Figure S3.

In order to understand the evolution of internal resistances, electrochemical impedance spectroscopy (EIS) measurements were carried out for the cells with all three electrolytes. Figures 3a and 3b show the Nyquist plots of the Li-Si cells before cycling and after ten cycles. Only one semicircle appears in the measurements before cycling, corresponding to a Faradaic charge transfer resistance (RCT) contribution that reflects the kinetics of the electrode reactions.25 In contrast, two semicircles are observed after ten cycles as shown in the Nyquist plots in Figure 3b. The one at higher frequency range is attributed to SEI film resistance (RSEI) and the other one is RCT. The EIS spectra are fitted with the equivalent circuits (Figure 3c) and the corresponding resistances are obtained (Figure 3d). The fitted plots are shown in Figure S4. RCT after cycling is much smaller than the values before cycling for all three electrolytes, which is attributed to the amorphization of crystalline silicon after lithiation/delithiation and a more favorable kinetics of amorphous silicon.26 Particularly, the impedance of the LP30-containing cell is dominated by RCT while RSEI dominates the impedance of the cells with the ether-based electrolytes. This observation highlights the fact that the total electrode reaction kinetics in different electrolyte systems is limited by different processes. The much larger RSEI of the SiMP electrode is found in the ether-based electrolytes, especially with LiNO3. Therefore, it implies either the formation of a much thicker SEI layer or a lower ionic conductivity of SEI, and is most likely a combination of both. The thick SEI layer is expected, since the SEI is continuously (re-)formed due to the generation of fresh, SEI-uncovered surfaces on the silicon particles, as understood from the low CE values.


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Figure 3. Experimental Nyquist plots of the Li-Si cells with different electrolytes (a) before cycling and (b) after 10 cycles in the delithiated state. (c) Equivalent circuits used for fitting. (d) Summary of RCT and RSEI values. Note: lines in (a) and (b) are not fitted plots.

The electrolyte composition indeed affects SEI formation, but can also in turn affect the structural integrity of the electrode. Especially in the case of silicon, where a large volume change is involved, a rigid and fragile SEI can further intensify cracking and subsequent pulverization, resulting in a fast capacity fade due to a quick contact loss between the newly formed smaller particles. Therefore, detailed ex situ characterization of the surface and crosssection morphology of the SiMP electrodes with different electrolytes after cycling were performed by SEM. Figure 4 shows the cross-section images of SiMP electrode before cycling 11 ACS Paragon Plus Environment


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and after ten cycles with the three different electrolytes. SiMP electrodes cycled with etherbased electrolytes present more severe pulverization and a higher porosity compared to the electrode used in the LP30-containing cell. In fact, even after only one cycle, the difference in morphology and thickness among the three SiMP electrodes is pronounced (Figure S5). The SiMP electrodes´ surface morphology is shown in Figures S6 and S7, where a strong surface roughness is observed for the cells with the ether-based electrolytes. Additionally, it was difficult to get clear surface images of the electrodes cycled with ether-based electrolytes at lower magnification due to noticeable charging of these samples even after chromium sputtering. This behavior suggests a higher void fraction due to increased delamination as well as a more insulating nature of the formed SEI resulting in a less electrically conducting electrode, which further highlights the reason for the fast capacity fade. The large difference in morphology confirms a more fragile SEI film for ether-based electrolytes which undergoes sustained cracking as well as grows continuously during lithiation/delithiation. Thus, a thicker SEI layer is formed, which is consistent with the EIS data. This results in a higher amount of silicon particles losing contact with at least the conducting carbon but most probably also the current collector, finally leading to a fast capacity fading.


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Figure 4. SEM secondary electron images of silicon electrode cross-section (a) before cycling; and after 10 cycles with (b) LP30, (c) DOL/DME, and (d) DOL/DME + LiNO3.

XPS was conducted to examine the surface film components of SiMP electrodes cycled with the three electrolytes. The XP spectra are shown in Figure 5. In the C 1s spectra, carbon black, hydrocarbon species, alkoxy carbon, ether-related carbon species, and carbonate species are detected for all electrolytes, as they are identified as common components of SEI films in liquid organic electrolytes.27-28 R-CHF-OCO2-R´ groups were found on the electrode with LP30, although not commonly observed, but these groups were reported elsewhere as well.29 -CF3 group, which is part of the reduction products from LiTFSI, was only found for the DOL/DME electrolyte, indicating less decomposition of LiTFSI in the LiNO3-containing electrolyte. The suppression of the LiTFSI decomposition by LiNO3 is further corroborated by the much lower atomic concentration of F and S for the electrodes cycled in the LiNO3-containing electrolyte (Table S1).19



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As visualized by the F 1s spectra (Figure 5), LiF is the common component for the SEI of all three electrodes. The SEI film with LP30 also contains LixPFy, which is the decomposition product of LiPF6.30 An additional strong signal at a high binding energy of ~ 689 eV is attributed to organic fluorine species and was exclusively detected for the cells with the ether-based electrolytes.18 The peak area concentrations of LiF and organic fluorine species are 53% and 47% for the electrode with DOL/DME electrolyte, respectively, and are 56% and 44% for the electrode with DOL/DME + LiNO3 electrolyte, respectively. This indicates that in the etherbased electrolytes, fluorinated components in the SEI contain considerable amounts of organic fluorine species, almost as much as LiF. The difference of the F 1s spectra seems to be responsible for their individual SEI properties and the variation in cycling stability.


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Figure 5. High resolution XP spectra of electrodes cycled with different electrolytes (R denotes alkyl chain): (a) C 1s, (b) F 1s, (c) N 1s and (d) S 2p. N 1s and S 2p are only shown for the ether-based electrolytes as LP30 does not contain any N/S species. For all graphs, the black curves give the measured data and the red dotted curves are the sum of the fits.

As the LiTFSI salt contains N and S, N 1s and S 2p spectra were measured for the electrodes cycled with ether-based electrolytes. In the N 1s spectra, the peaks around 399.8 eV and 398 eV are ascribed to TFSI-

31

and Li3N 32, respectively. The N 1s and S 2p spectra for the two

ether-based electrolytes show similar components but differ in one small peak of R-NO2 as found in N 1s spectra of electrode with LiNO3, and the observation of S-S bridging species, which was only observed in the S 2p spectrum for the electrode cycled without LiNO3. In the absence of LiNO3, as the decomposition of LiTFSI is pronounced, it may result in a reduction of the S-containing species to elemental sulfur.

The elemental atomic concentrations obtained from XPS also provide an important observation. Much less silicon was detected on the electrode surfaces for the LiNO3-free (0.16 at.%) and LiNO3-containing (0.11 at.%) ether-based electrolytes, compared to the case of LP30 electrolyte (0.79 at.%, seen in Table S1). Considering the non-uniform thickness of the SEI layer and the probing depth of the photoelectrons of 2-5 nm, the thinnest SEI layer in the etherbased electrolytes is thicker than or close to the probing depth, leading to the negligible amount of silicon that can be detected on the electrode surface. In contrast, the thinner area of the SEI in LP30 is less than the probing depth, which explains the higher amount of detected silicon on the surface. These observations confirm that the overall SEI thickness in the ether-based electrolytes is larger. They are also consistent with the less pronounced C 1s signals of carbon black for the electrodes cycled in the ether-based electrolytes.



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Our results on micro-particulate silicon used in combination with LiNO3 containing electrolytes are rather contradictory to what has been reported in literature. However, in the work on pristine silicon nanowires by Aurbach et al.,18 SEI components of silicon nanowires in carbonate-based and ether-based electrolytes are rather similar to our work, but the cycling performances are totally different. They observed that the LiNO3-containing electrolyte performs even better than the LP30 electrolyte, which is in direct contrast to our results. Our group also reported recently that the nano-sized silicon showed the best performance with the same LiNO3-containing electrolyte,19 but an influence of the carbon coating on the silicon, e.g. on the formation of a stable SEI on the carbon instead of the silicon or a partial stabilization by the carbon to the silicon towards expansion/contraction stresses, cannot be fully excluded in this work.

Our work implies that the variety of SEI components is not the only reason for dissimilar properties of the SEI. The structure and morphology of the SEI film may also play a significant role for the SEI properties and finally the cycling performance.33-36 Decomposition of different electrolytes may involve dissimilar reaction pathways, which shall lead to a varying distribution of the components in the SEI and therewith influence SEI properties, even though, they may have similar SEI compositions in the end. Moreover, the preferred reactions at the silicon surface may also be determined by the surface chemistry of that particular system. Surface chemistry is a rather broad term, and can involve various aspects: surface energy due to size effects and actual morphology as well as state of the surface due to processing which can involve surface defect density, thickness of oxide layer, presence of adsorbed molecules or covalently bonded ligands etc. Each of the different aspects may affect the SEI formation as well as the reaction pathways at different voltages. For example, silicon anodes with and without native oxide layers have totally different SEI compositions, structures and properties. The absence of native oxide layer on silicon surface leads to a thicker SEI due to larger amounts of LiOx components in the SEI.37 16 ACS Paragon Plus Environment

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In our case, for silicon microparticles, the SEI compositions are rather similar in ether-based electrolyte with and without LiNO3, but the resultant performance noticeably differs. The composition of the SEI in SiMPs is also comparable to what has been reported for Si-NPs/NWs but with significant differences in properties which can be due to many factors. The different dimensionality would lead to different surface energies due to size, ordering or particle/crystallite shape effects.38-39 Moreover, significantly different synthesis routes are employed in order to prepare different morphologies. As a result, the silicon surface chemistries are markedly different which likely changes the SEI properties as well. It must be mentioned that with regards to commerically available silicon nanoparticles, additional groups/surfactants are commonly present on the surface of silicon nanoparticles to avoid agglomeration. Moreover, the Si-NPs from different sources may contain different compounds and each would have to be individually tested to establish the performance with different electrolytes. Absence of such a coating would be better to establish properties for industrial applications, but may pose other challenges during storage, slurry making and electrode preparation.

Since the LiNO3-containing electrolyte is rather promising for SSFC, we also made an additional study with alternate cycling profiles in order to improve the cycling performance. The cycling stability of Li-Si cells in the LiNO3-containing electrolyte was tested with different discharge cut-off voltages and different initial lithiation capacities. The idea was that cut-off potential/lithiation would limit the volume expansion, in turn limiting the stress generated inside the active material, thereby improving the cycling performance through reduction of cracking and delamination. In the first cycling profile, three cells were cycled where the 1st lithiation capacity was limited to 33% of the complete initial lithiation capacity (taken as 3750 mAh g-1; as observed previously in Figure 1a) and for the subsequent cycles, the lithiation potential was limited to 0.07 V, 0.12 V and 0.17 V, respectively. These discharge cut-off voltage values were chosen since cycling stability of Si anodes could be improved using these 17 ACS Paragon Plus Environment


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cut-off voltages, as reported in literature.22, 40 The cycling performance is shown in Figure 6a, where a current rate of 250 mA gSi-1 was taken for all cells. The cell still failed quickly when the discharge cut-off voltage was increased from 0.01 V to 0.07 V. The fast capacity fade was avoided when the lithiation potential is limited to 0.12 V. The cycling stability was found to be highly improved with a lithiation potential of 0.17 V.

Figure 6. Galvanostatic cycling tests of Li/Si cells in DOL/DME + LiNO3 with limited discharge cut-off voltage at current density of 250 mA gSi-1. (a) Lithiation until 33% of a full initial lithiation to different discharge cut-off voltages; (b) lithiation with a constant discharge cut-off voltage (0.17 V) until 33%, 66% and a full initial lithiation (100%).

In an additional profile, three cells were cycled where the 1st lithiation capacity was limited to 33%, 67% and 100% to the complete initial lithiation capacity (taken as 3750 mAh g-1; as observed previously in Figure 1a) respectively, and for subsequent cycles, a constant discharge cut-off voltage of 0.17 V was set. Similar to previous profile, a current rate of 250 mA gSi-1 was chosen for all cells. As shown in Figure 6b, the cycling performance was only improved significantly by limiting an initial lithiation to 33% and afterwards lithiating till 0.17 V, in which case a specific capacity of 250 mAh gSi-1 can be obtained after 50 cycles. Indeed,


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potential/capacity limitation can help to improve the cycling performance of Si micro-particle anodes with LiNO3-containing ether-based electrolyte.

In general, through potential limitation, cycling efficiency will improve as less stress is induced inside the Si particles, but the specific capacities obtained will decrease, due to lower lithiation. In addition, as we are indirectly limiting expansion of Si, the stress on SEI is also reduced. in turn lowering the extent of SEI cracking and subsequent formation of new SEI. We have shown that SiMP anodes with LiNO3 containing ether electrolyte demonstrated extremely short cycle life under normal cycling conditions. The rather significant improvement in cyclability just by potential limitation, further supports the fact that the SEI formed, in the initial stages as well as in subsequent cycles, is indeed mechanically unstable and was the underlying cause of the problem.

It must be mentioned that voltage-dependent SEI morphology has also been reported, and possibly plays an important role in pulverization of the electrode active material and cycling performance.36 It was also observed in the same work that the SEI formed below 0.1 V consisted of a composite of different kinds of mainly inorganic particles and deposits and is less polymeric.This may have a strong impact of the fragility of the SEI although a more polymeric and therewith flexible SEI is formed at higher voltages, the Si expansion leads to cracking and exposure of fresh surfaces at low voltages, where a more inorganic SEI may be formed yielding higher stress on the particles e.g. as observed for graphite.41. Moreover, SEI cracking and reformation in successive cycles mostly occurs at low voltages, leading to constantly changing the SEI composition and structure.

The cracking in silicon is due to the tensile and compressive stresses inside and probably onto the particle during the alloying reaction. Even an elastic physical coating, let alone a mechanically weak SEI, cannot prevent the particle cracking. However, depending on the SEI, 19 ACS Paragon Plus Environment


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it can contribute in the cracking process especially in the case of large particles. As the large particles start to crack during the initial phase of lithiation, new surface is exposed which is quickly covered with SEI. It has been reported that a more inorganic and hard SEI is formed on the Si-near surface 42-43, especially at lower voltages.36 Moreover, such a hard SEI inside a crack may act as splitting wedges for the expanding particle, thereby accelerating the cracking process, as shown in the scheme given in Figure 7. Another important aspect is found in the constant cracking and formation of new SEI that would lead to a continuous modification of the overall SEI supporting the cracking issue or even aggravate it. Apart from capacity fade due to contact loss and delamination, it may also be detrimental to Li diffusion and kinetics due to constantly changing interfaces.36 On the other hand, a relatively more stable or an elastic SEI can accommodate itself better to the volume change and have better structural integrity, minimizing exposure of fresh silicon surface, and new SEI formation. This possibility would in turn lower Li ion consumption as well as minimize large modifications in the overall morphology of the SEI with cycling, leading to better efficiency and cycle life.

Figure 7: Schematic of SEI formation during silicon cracking process, which as a splitting wedge upon further lithiation leading to accelerated crack propagation.


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Thus, the drastic capacity fade in SiMPs with LiNO3 additive can be understood as a result of the increasing fragile SEI being formed, which further acts to accelerate the cracking process and the resultant contact loss and delamination. Although, in the case of microparticles, the volume change of silicon will always be the dominating factor towards stress generation, the intrinsic stresses, and problems resulting from that, can be significantly enhanced by the nature of (electro)chemically formed SEI that is present and/or is incorporated in successive cycles.

The effect of above mentioned scenario is expected to be minimal in the case of nanoparticles. For SiNPs, the volume expansion and corresponding stress on the SEI is much lower, and the cracking would be a very gradual long term process, due to stress accumulation. The stress in nanoparticles maybe a bit intense if the SEI is predominantly inorganic, but for microparticles, that would not be a major contributing factor, as the volume change of silicon will always dominate the possible effect of any kind of surface-(electro)chemically formed SEI.

Conclusions In conclusion, micro-sized silicon shows poor electrochemical performance with ether-based electrolytes in half cells, especially with LiNO3 as the additive, due to the rather unfortunate property of the SEI film. However, our results are contradictory to some reported works, although the surface film components analyzed by XPS are similar. The main reason is ascribed to different physical properties of the SEI film on the silicon in the different electrolytes due to dissimilar reaction pathways. The contrasting results with regards to nanoparticulate Si anode as reported earlier18-19 is likely due to the surface chemistry of Si materials. Thus, silicon with dissimilar morphologies, nano-/micro- structures and surface chemistries behaves differently in the same type of ether-based electrolytes. Moreover, we have also tried to develop a detailed understanding of the drastic capacity fade in silicon microparticles with LiNO3 additive.



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Our work rather highlights an important aspect which may be often overlooked but can act as a significant bottleneck towards electrochemical performance. Even a small amount of electrolyte additive, normally used for sulfur electrodes, can have a damaging effect when used in full cell configuration with silicon. Therefore, for constructing SSFC batteries, apart from common issues such as the polysulfides shuttle, irreversible reaction between polysulfides and silicon, or silicon volume changes, the reaction chemistry with ether-based electrolytes may also be a major reason for their short cycle life. More efforts should be made in investigating the effects of different electrolytes and additives on SEI formation of various silicon anode materials. Apart from improving existing systems, alternate electrolyte additives for etherbased electrolytes should be developed to possibly bring together a synergistic effect towards an efficient, robust SEI. We hope that our results will stimulate the research community to focus on these issues.

Acknowledgements The authors acknowledge the financial support from the Federal Ministry of Education and Research (BMBF) under grant no. 03X4637 (WING-Center: Battery – Mobile in Saxony (BamoSa)). Dr. Qingsong Wang in Institute of Nanotechnology, Karlsruhe Institute of Technology is acknowledged for his helpful discussions.

Supporting information This material is available free of charge via the Internet at http://pubs.acs.org. Cycling performance data with delithiation capacities. SEM images of the electrodes before cycling, after 1 cycle and 10 cycles. The fitted EIS Nyquist plots of the Li-Si cells after cycling. The elemental atomic concentrations table on the electrode surface after cycling determined by XPS.


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Corresponding authors *E-mail:

[email protected]

*E-mail:

[email protected]

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Graphical abstract


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Figure 1. Galvanostatic cycling of the Li/Si cells with different electrolytes between 1.2 V and 0.01 V at a current density of 250 mA gSi-1. (a) Specific discharging (lithiation) capacities vs. cycle number; (b) the corresponding Coulombic efficiencies. (Insets are initial lithiation capacities and initial Coulombic efficiencies. The specific delithiation capacities are shown in Figure S2). 226x90mm (150 x 150 DPI)

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Figure 2. Voltage profiles and dQ/dV plots of Li-Si cells with different electrolytes for the initial three cycles at a current density of 250 mA gSi-1. (a, b) the 1st cycle (the insets show the reduction of LiNO3); (c, d) the 2nd cycle; (e, f) the 3rd cycle. The dQ/dV plots are only shown between 0.01 V and 0.6 V since no peaks are observed beyond 0.6 V. The plots of the 1st cycle in the whole voltage range are shown in Figure S2. 162x188mm (150 x 150 DPI)


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Figure 3. Experimental Nyquist plots of the Li-Si cells with different electrolytes (a) before cycling and (b) after 10 cycles in the delithiated state. (c) Equivalent circuits used for fitting. (d) Summary of RCT and RSEI values. Note: lines in (a) and (b) are not fitted plots. 175x148mm (150 x 150 DPI)



Figure 4. SEM secondary electron images of silicon electrode cross-section (a) before cycling; and after 10 cycles with (b) LP30, (c) DOL/DME, and (d) DOL/DME + LiNO3. 77x89mm (150 x 150 DPI)


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Figure 5. High resolution XP spectra of electrodes cycled with different electrolytes (R denotes alkyl chain): (a) C 1s, (b) F 1s, (c) N 1s and (d) S 2p. N 1s and S 2p are only shown for the ether-based electrolytes as LP30 does not contain any N/S species. For all graphs, the black curves give the measured data and the red dotted curves are the sum of the fits. 160x127mm (220 x 220 DPI)



Figure 6. Galvanostatic cycling tests of Li/Si cells in DOL/DME + LiNO3 with limited discharge cut-off voltage at current density of 250 mA gSi-1. (a) Lithiation until 33% of a full initial lithiation to different discharge cut-off voltages; (b) lithiation with a constant discharge cut-off voltage (0.17 V) until 33%, 66% and a full initial lithiation (100%). 212x83mm (150 x 150 DPI)


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Figure 7: Schematic of SEI formation during silicon cracking process, which as a splitting wedge upon further lithiation leading to accelerated crack propagation. 80x59mm (150 x 150 DPI)


The electrochemical behavior of micro-particulate silicon anodes in

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