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Article  |  Open Access  |  11 Mar 2024

Superior compatibility of silicon nanowire anodes in ionic liquid electrolytes

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Energy Mater 2024;4:400017.
10.20517/energymater.2023.84 |  © The Author(s) 2024.
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Abstract

Silicon nanowire anodes were investigated in lithium-metal cells using different electrolyte formulations based on 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide and N-trimethyl-N-butyl-ammonium bis(fluoro sulfonyl)imide ionic liquids. The lithium insertion process in the silicon anode was analyzed by cyclic voltammetry measurements, performed at different scan rates and for prolonged cycles, combined with impedance spectroscopy analysis. A galvanostatic charge-discharge cycling test was performed to analyze the electrochemical performances using different types of ionic liquids. A study of the Solid Electrolyte Interphase layer on the silicon nanowire electrode surface was carried out through X-ray photoelectron spectroscopy. In general, the silicon anodes in 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide-based electrolytes show very good reversibility, reproducibility, and efficiency in the lithiation process, even at high scan rates, and exhibit a reversible capacity exceeding 1,000 mA h g-1 after 2,000 charge-discharge cycles, corresponding to 46% of the initial value.

Keywords

Silicon anodes, ionic liquids, solid electrolyte interphase, XPS analysis, lithium-ion batteries

INTRODUCTION

Lithium-ion batteries (LIBs) have revolutionized the field of energy storage and are widely used in a wide variety of applications, including electric vehicles (EVs). These batteries are known for their high energy density, lightweight design, and long cycle life, making them an ideal choice for powering EVs[1-4]. The thriving EV markets have created a pressing demand for the advancement of high-energy-density storage devices aiming to achieve an energy density of 500 W h kg-1 or higher[5]. However, it is important to note that while LIBs have made significant strides in the EV industry, further research and development are underway to enhance their performance, sustainability, and safety[1-4]. For example, extremely fast overcharging could promote lithium (Li) plating, resulting in detrimental effects on battery performance and safety. Moreover, lithium metal can lead to the growth of dendrites, which are conductive filaments capable of penetrating into the separator and causing short circuits, thermal runaway, and even battery fires or explosions. This risk is particularly relevant in EVs, where large numbers of lithium batteries are densely packed together[4-6].

At the system level, several strategies can be used to prevent the spread of thermal runaways, including the construction of fire-resistant casings, the use of high thermal resistance separators, and the setting of thermal barriers between the cells.

At the material level, ensuring the thermal stability of the cell components, such as cathodes, anodes, and electrolytes, can significantly enhance the intrinsic safety of the LIBs. For the cathode, a protective coating can prevent the extended reaction at the cathode-electrolyte interphase, minimizing the risk of side reactions or degradation[5-7]. From the anode side, replacing the lithium metal anode with an alternative material could be an important step for improving battery safety. In this regard, silicon (Si) has gained attention due to its high capacity for energy storage (4,200 mA h g-1 for Li22Si5), high abundance (second most abundant element in the Earth’s crust), and intermediate discharging potential (about 0.3-0.4 V vs. Li+/Li0) which can help in preventing Li dendrite formation. However, for accommodating 4.4 Li atoms, which correspond to the Li22Si5 alloy, the Si structure is exposed to detrimental stress induced by a large volume change (about 400%), which can lead to rapid degradation of the electrode and pulverization under prolonged cycling[8-10].

In 2008, Chan et al. demonstrated that the nanostructures, particularly nanowires (NWs), can hold a large strain without developing fractures and cracks on the NW surface, securing a good electrical contact between the current collector and the tip of the NW[11]. In addition, the large volume expansion in the Si anode strongly affects the stability of the so-called solid electrolyte interphase (SEI)[8]. The SEI layer forms at the negative electrode during the initial charging process and consists of a complex heterogeneous and structurally disordered passivation layer containing the organic and inorganic species coming from the decomposition of the electrolyte. It acts as a protective barrier, blocking further reactions between the electrolyte and the anode while allowing the transport of Li-ions[12-15]. For the Si-based anodes, the cyclic expansion and contraction of the active material expose the SEI layer to continuous cracks, forming a “fresh” Si surface that interacts with the electrolyte, creating a new SEI layer[16]. This dynamic, uncontrolled, and non-uniform formation of the layer can negatively affect the safety and long-term stability of the cell.

Ensuring a mechanically robust and chemically stable SEI layer can be facilitated by using functional additives in the electrolyte, such as vinylene carbonate (VC) and fluoroethylene carbonate (FEC)[17-19]. Although the SEI layer is essential for the safety and stability of the cell, the understanding of this nature and behavior remains a significant challenge. The nanometric dimension of this layer (10-50 nm thick) makes its characterization challenging and easily biased by artifacts. For example, incomplete electrode post-mortem cleaning procedures (washing and drying) can leave electrolyte/salt residues, whereas many SEI constituents are particularly sensitive to humidity and air pollution. As an example, ROLi and ROCO2Li can react with ambient CO2 to induce ex-situ SEI evolution to form large amounts of Li2CO3. The SEI formation is affected by multiple factors: current rates, temperature, voltages, electrolyte composition, and concentration. Recently, the use of complementary techniques, including predictive computational models such as machine learning, has been demonstrated successfully to gain insights into the formation and progression of the SEI layer. For the characterization of the surface of the SEI layer, the most common surface analysis techniques are atomic force microscopy (AFM), secondary ion mass spectroscopy (SIMS), and X-ray photoelectron spectroscopy (XPS). Additional information on the morphology of the layer can be obtained using scanning electron microscopy (SEM) and transmission electron microscopy (TEM)[12,13,16,19,20].

Turning back to the safety concerns related to electrolytes, the most radical way to remove this risk is the substitution of flammable organic carbonate-based electrolytes with intrinsically non-flammable ionic liquid (IL) electrolytes. ILs also have the advantages of wide electrochemical stability windows and excellent thermal stability. They consist of salts that are liquid at room temperature because of their loosely connected cation and anion asymmetric structure. Typically, IL electrolytes are formed by quaternary ammonium cations such as imidazolium, pyridinium, pyrrolidinium, piperidinium, and ammonium, coupled with anions (with low Lewis basicity) such as tetrafluoroborate (BF4-), hexafluorophosphate (PF6-), bis(trifluoromethylsulfonyl)imide (TFSI-), and bis(fluorosulphonyl)imide (FSI-)[16,19,21,22]. It has been demonstrated that, particularly for the ILs formed by quaternary ammonium cations and TFSI or FSI anions, they are cathodically stable on Si electrodes[23,24]. Notably, FSI-based ILs revealed better cycling behavior and higher capacity retention as compared to the TFSI-based ones in the Si-based electrode, probably due to the formation of a more stable SEI layer[22,25]. The inorganic products coming from the decomposition of FSI-based IL electrolytes (such as LiOH and Li2O) on the Si anode can promote Li-ion diffusion and boost the cycling performance[22].

In this scenario, this work focused on investigating the lithiation process in Sn-seeded Si NW (hereafter Sn-Si NW) anodes with different binary IL formulations (0.2LiTFSI-0.8EMITFSI (LiTFSI: lithium bis(trifluoromethylsulfonyl)imide; EMITFSI: 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide), 0.2LiTFSI-0.8EMIFSI (EMIFSI: 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)imide), and 0.2LiFSI (LiFSI: lithium bis(fluorosulfonyl)imide)-0.8EMIFSI) with LiTFSI and LiFSI salts. The effect of the FSI and TFSI anions was examined through cyclic voltammetry (CV) and impedance analysis. The modulation of the Li-ion diffusivity across the SEI layer, on which the rate performance of Sn-Si NW anodes depends, was evaluated by analyzing the Li-ion diffusion coefficient. The role of the anions and cations in the cell performance in galvanostatic tests was decoupled by comparing different IL electrolyte formulations (0.2LiFSI-0.8 N-trimethyl-N-butyl-ammonium bis(fluoro sulfonyl)imide (N1114FSI)). Moreover, the effect of the FEC additive was studied for the LiTFSI-EMIFSI electrolyte. Finally, the SEI layer composition was analyzed (ex-situ XPS) for the Sn/Si NW electrode after a different number of charge-discharge cycles.

EXPERIMENTAL

The EMITFSI, EMIFSI, and N1114FSI were synthesized and purified according to an eco-friendly route reported elsewhere[26-28]. The ILs were investigated as electrolyte components in (binary) mixtures with LiTFSI (3M, > 99.9 wt.%, battery grade) and LiFSI (Solvionic, 99.9 wt.%) in the Li:IL = 1:4 mole ratio. The 0.2LiTFSI:0.8EMITFSI, 0.2LiTFSI:0.8EMIFSI, 0.2LiFSI:0.8EMIFSI, and 0.2LiFSI:0.8N1114FSI electrolyte formulations were considered for electrochemical characterization in Si-based electrodes. The FEC (Solvionic, > 99 wt.%) was used as an organic additive for optimizing the electrolyte/silicon interface. It was used in the mole fraction from 0.01 to 0.04 to define the optimal content for the electrolyte formulations. Table 1 summarizes the weight composition of the investigated electrolyte formulations. The 1M Lithium hexafluorophosphate (LiPF6) in Ethylene Carbonate: Dimethyl Carbonate (EC:DC = 1:1 vol.%) with 3 wt.% FEC solution was provided by Solvionic and used for comparison purposes.

Table 1

IL electrolyte formulations investigated with Si anodes

Mole fractionLithium saltMole fractionILMole fractionAdditive
0.20LiTFSI0.80EMITFSI--
0.20LiTFSI0.80EMIFSI--
0.20LiTFSI0.79EMIFSI0.01FEC
0.20LiTFSI0.78EMIFSI0.02FEC
0.20LiTFSI0.73EMIFSI0.07FEC
0.20LiTFSI0.80EMIFSI+ 3 wt.%FEC
0.20LiFSI0.80EMIFSI--
0.20LiFSI0.80N1114FSI--

Copper silicide (Cu15Si4) NWs were utilized as nanostructured hosts for amorphous silicon (a-Si) deposition; the procedure was already described in detail elsewhere[29,30]. The a-Si anodes have a Si mass loading ranging from 0.1 to 0.25 mg cm-2, corresponding (accounting for a Si theoretical specific capacity equal to 4,200 mA h g-1) to a capacity from 0.42 to 1.05 mA cm-2. The tin-seeded silicon NW (Sn-Si NW) anodes were synthesized in a solvent-vapor system following a method already reported in the literature[31]. The Sn/Si NW anodes present a mass loading between 0.1 and 0.25 mg cm-2, corresponding (accounting for nominal capacity values equal to 2,000 and 994 mA h g-1 for Si and Sn, respectively) to an overall capacity from 0.13 to 0.54 mA h cm-2.

The materials used in this work were dried, stored, and handled in an Ar-filled dry box (Jacomex, Dagneux, France, O2, and H2O level < 1 ppm).

Electrochemical measurements

The electrochemical measurements were carried out on lithium metal cells using CR2032 coin cells. The lithium metal disk (500 μ thickness, 10 mm diameter) was purchased by Linyi Gelon LIB Co and used as the counter electrode. The a-Si and/or Sn-Si NW anodes were used as working electrodes. The electrodes were separated using two disks of glass fiber (16 mm diameter) separators (Whatman TM, Maidstone, UK). The cells were kept under vacuum for 30 min to allow complete loading of the IL electrolyte into the electrode and the separator.

A preliminary evaluation of the electrochemical processes taking place in Li/Si cells was run in a-Si anodes through cyclic voltammetric (CV) analysis paired with potentiostatic electrochemical impedance spectroscopy (PEIS) measurements. The following electrolyte formulations were used: 0.2LiTFSI:0.8EMITFSI, 0.2LiTFSI:0.8EMIFSI, and 0.2LiFSI:0.8EMIFSI. The CV tests were performed between 0.01 to 2 V vs. Li+/Li° at increasing scan rates, i.e., from 0.05 to 1 mV s-1 (four consecutive CV cycles were run at each selected scan rate), followed by 500 cycles at constant scan rate of 1 mV s-1. The PEIS was carried out on the fresh cells and at the end of each increasing scan rate CV family in the 10 kHz-0.1 Hz frequency range with 10 mV amplitude voltage.

Galvanostatic charge-discharge (GC) measurements were carried out on Li/Sn-Si NW cells using 0.2LiTFSI:0.8EMIFSI, 0.2LiFSI:0.8EMIFSI, and 0.2LiFSI:0.8N1114FSI as the electrolytes. The cells were cycled between 0.01 and 2 V vs. Li+/Li° at current rates from 0.1C to 10C.

All the electrochemical tests were carried out in a climate chamber at 20 °C using a Biologic (Seyssinet-Pariset, France) multichannel battery cycler.

Interfacial characterization

XPS was carried out to get information on electrochemical passivation layer (SEI) formation on the Sn-Si NW surface. XPS analysis was performed under an ultra-high vacuum (~10-9 mbar) using a Kratos AXIS ULTRA spectrometer with a monochromatic Al Kα X-ray radiation source (hν = 1,486.6 eV). Pass energies of 160 and 20 eV were used for survey spectra and narrow regions, respectively. The C 1s line at 284.8 eV was used as a charge reference. These spectroscopic studies were performed on the Sn-Si NW surface after electrochemical tests at different states of charge in lithium salt-IL electrolytes: (i) pristine material; (ii) 1 cycle @0.1C; and (iii) 1 cycle @0.1C + 10 cycles @1C. After the cell dismantling, the Si electrodes were rinsed with Tetrahydrofuran (Sigma Aldrich, anhydrous, ≥ 99.9%) at least three times to remove the residual electrolyte, vacuum-dried at room temperature, and then transferred directly to the XPS analysis chamber.

RESULTS AND DISCUSSION

Several electrolyte formulations based on different ILs in combination with the LiTFSI salt have been previously synthesized and characterized in terms of ion transport properties and electrochemical stability[32]. Results summarized in Table 2 show that the EMIFSI-, EMITFSI-, and N1114FSI-based electrolytes are rather promising as electrolyte components in LIB systems, and therefore, they were selected for the electrochemical tests with Si anodes.

Table 2

Summary of ionic conductivity (error bar: 10%) and anodic breakdown voltage (error bar: 0.01 mV) values (vs. the Li+/Li0 redox couple), determined by CV tests of different ionic liquid electrolytes with the LiTFSI salt (mole fraction equal to 0.2)

IL sampleConductivity/S cm-1Voltage (1st scan)/VVoltage (2nd scan)/V
-10 °C20 °C50 °C10 μA cm-220 μA cm-210 μA cm-220 μA cm-2
EMIFSI1.5 × 10-36.0 × 10-31.3 × 10-24.314.684.504.91
EMITFSI3.6 × 10-41.4 × 10-33.5 × 10-34.344.954.664.94
N1114FSI1.6 × 10-41.3 × 10-33.9 × 10-34.234.934.75> 5.00
N1114TFSI5.0 × 10-83.9 × 10-41.9 × 10-34.87> 5.00> 5.00> 5.00
N1113TFSI< 1 × 10-94.1 × 10-53.1 × 10-3----
N111(2O1)TFSI< 1 × 10-92.9 × 10-63.8 × 10-3----
N122(2O1)TFSI4.8 × 10-55.6 × 10-41.9 × 10-44.484.904.84> 5.00

Lithium intercalation process

The reversibility of the Li+ storage process was studied on a-Si anodes in the 0.2LiTFSI-0.8EMITFSI, 0.2LiTFSI-0.8EMIFSI, and 0.2LiFSI-0.8EMIFSI electrolytes through CV tests at different scan rates and for prolonged cycles. The results are displayed in Figure 1. The current value is normalized with respect to the Si electrode mass loading (A g-1).

Superior compatibility of silicon nanowire anodes in ionic liquid electrolytes

Figure 1. Cyclic voltammetry of Li/Si cells containing 0.2LiTFSI-0.8EMITFSI (A), 0.2LiTFSI-0.8EMIFSI (B), and 0.2LiFSI-0.8EMIFSI (C) electrolytes run at different scan rates and for prolonged cycles with a scan rate of 1 mV s-1carried out on 0.2LiTFSI-0.8EMIFSI (D)- and 0.2LiFSI-0.8EMIFSI (E)-based cells. T = 20 °C.

A broad feature located around 1.2 V vs. Li+/Li was observed during the first cathodic scans in all three cases [Figure 1A-C], which relates to the passivation layer (SEI) formation onto the Si anodes[23,33]. This peak (magnified in Supplementary Figure 1) disappears in the subsequent cycles, suggesting stabilization of the electrode/electrolyte interface. The Li/Si cell with 0.2LiTFSI-0.8EMITFSI shows the peaks relative to the lithiation of a-Si in the first cathodic scan, but no evident peaks can be noticed in the following cycles; this suggests non-optimal SEI formation (instead progressive lithiation of the amorphized Si anode)[23,33], highlighting for large irreversible capacity and, consequently, low initial coulombic efficiency and indicating poor reversibility of the lithium dealloying. The progressive replacement of the TFSI anion with the FSI one in the IL [Figure 1B] and in both IL and lithium salt [Figure 1C] results in very beneficial effect on the cycling behavior. The lithiation of Si initially causes the formation of the amorphous phase LixSi, which turns into a crystalline phase below 0.06 V vs. Li+/Li[24], corresponding to two well-defined peaks, located at 0.15 and 0.05 V vs. Li+/Li, as can be observed in Figure 1B and C. In the reverse scan, two similar features around 0.3 and 0.5 V vs. Li+/Li arise from the two-phase reaction from crystalline Li15Si4 to amorphous LixSi (x ≈ 2), followed by the formation of the delithiated amorphous Si[24,34]. This indicates good reversibility of the delithiation process during the anodic scan, i.e., leading to much higher stored and delivered capacity, possibly due to an optimal SEI growth onto the Si electrode, given by the much better film-forming ability of the FSI anion with respect to the TFSI one[23,24]. The complete overlap of the voltammograms, even at high scan rates and for prolonged cycles [Figure 1D and E], suggests good reversibility and high coulombic efficiency of the alloying/dealloying process during cycles in the 0.2LiTFSI-0.8EMIFSI and 0.2LiFSI-0.8EMIFSI electrolytes. The increase of the scan rate results in progressive enhancement of the current flow through the cell, leading to more pronounced and defined current vs. voltage features in both verses, with only a moderate shift of the maximum value due to diffusive processes that take place into the electrolyte, according to the Randles-Sevcik equation[35,36]. At 0.2 mV s-1, two distinguished profiles are still observed, both in the cathodic (Li+ alloying) and anodic (Li+ dealloying) verse, whereas at faster scan rates, they are not split. Larger values of specific current associated with the electrochemical processes, resulting in higher capacity involved in the (de)lithiation process, were observed in the 0.2LiFSI-0.8EMIFSI cells with respect to the 0.2LiTFSI-0.8EMIFSI ones. Prolonged CVs were run only on 0.2LiTFSI-0.8EMIFSI and 0.2LiFSI-0.8EMIFSI cells after CV tests displayed in Figure 1B and C to investigate the reversibility of the Li+ alloying process under hard operative conditions, i.e., 500 cycles at 1 mV s-1. The results, as illustrated in Figure 1D and E, show two defined features around 0.25 and 0.1 V (cathodic scan) and around 0.6 V (anodic scan). A very good reversibility of the lithium alloying process is observed even after 500 cycles, witnessed by the good correspondence between the cathodic and anodic features. A remarkable reproducibility of the CV traces is recorded for the first 200 cycles, whereas progressive, even modest, decrease of the CV profile is observed up to the 500th cycle. To summarize, the 0.2LiFSI-0.8EMIFSI electrolyte exhibits comparable voltammetric features to those of 0.2LiTFSI-0.8EMIFSI; however, it shows evidence of higher capacity decay (i.e., more depleted CV profile) after prolonged CV tests (> 200 cycles).

The mobility of Li-ions at the electrolyte/silicon interface directly depends on the redox reaction rate according to the Randles-Sevcik equation[35,36] and can be modulated by the SEI layer formed onto the silicon particles, which can enhance the rate performances and reduce the lithium trapping[37]. Therefore, the Li-ion diffusion coefficient DLi+ (cm2 s-1) was calculated for a-Si anode cycled into 0.2LiTFSI-0.8EMIFSI and 0.2LiFSI-0.8EMIFSI electrolytes, evaluating the CV tests recorded at different scan rate [Figure 1B and C] described in the previous paragraph. The DLi+ was calculated based on the Randles-Sevcik equation (Equation 1)[38,39] for a semi-infinite diffusion of Li-ions:

$$ \begin{equation} \begin{aligned} Ip=0.4463~n~FAC~\left(n FVD_{Li^{+}}/RT\right)^{1/2} \end{aligned} \end{equation} $$

where the Ip is the peak current, and R, F, and T are the gas constant, Faraday constant, and temperature (K), respectively. A (cm2) is the active surface area of the a-Si anode, n is the number of electrons in the reaction, C (mol cm-3) is the Li+-ion concentration in the electrolyte, and ν (V s-1) is the scan rate. The relationship of the cathodic and anodic current peaks (Ic and Ia) with the square root of the scan rate is reported in Figure 2. The current peaks in anodic An and cathodic Cn scan, taken into consideration for the evaluation of DLi+, are displayed in Supplementary Figure 2. The slope of the linear fit [Supplementary Figure 3] between Ip and ν0.5 was used to estimate the DLi+[40-42]. Both systems show a linear behavior, meaning the diffusion-controlled reaction[43] and a peak current ratio (Ipa/Ipc) ≈ 1, suggesting reversible lithiation process and no parasitic reaction associated with the electron transfer[39]. The DLi+ was found to be equal to (9.6 ± 0.5) × 10-10 cm2 s-1 for 0.2LiTFSI-0.8EMIFSI and (2.5 ± 0.1) × 10-10 cm2 s-1 for 0.2LiFSI-0.8EMIFSI electrolytes, comparable with the results reported in the literature for LixSi phases in organic electrolytes and obtained through different techniques[44,45]. The error related to the D values is equal to 5%.

Superior compatibility of silicon nanowire anodes in ionic liquid electrolytes

Figure 2. Linear fits for the anodic and cathodic peak currents versus square root of the scanning rates of a-Si electrodes using 0.2LiTFSI-0.8EMIFSI and 0.2LiFSI-0.8EMIFSI electrolytes. T = 20 °C.

Impedance measurements

Impedance measurements were conducted after each CV family test run at different scan rates [Figure 1] and reported in Supplementary Figure 4 as Nyquist plots. The impedance responses display a semicircle pattern at medium-high frequencies and a straight line at low frequencies, in agreement with other studies on similar anode materials in IL electrolytes[43,46]. The high-frequency intercept with the real axis corresponds to the bulk resistance (Rbulk) of the IL[47-49], and the semicircle diameter gives the overall interfacial resistance Rint[47-49].Figure 3 plots the dependence of Rbulk (A) and Rint (B), determined from the impedance responses [Supplementary Figure 4] taken [in open circuit voltage (OCV) conditions] after each CV family of Figure 1, as a function of the scan rate. The resistance values summarized in Table 3 were evaluated through the equivalent circuit models depicted in Supplementary Figure 5, where Qdl is the constant phase element taking into account the double layer capacitance, and Cl is the electrode limit capacitance[47-49]. Rbulk values equal to 1.7, 1.6, and 1.3 Ω cm were detected (at 20 °C) for the 0.2LiTFSI-0.8EMITFSI, 0.2LiTFSI-0.8EMIFSI, and 0.2LiFSI-0.8EMIFSI cells, respectively, i.e., close to those reported in previous work and summarized in Table 2[32]. Significantly, no change of Rbulk is observed during the CV tests, confirming the good electrochemical stability of the IL-based electrolytes and the negligible lithium depletion from the electrolyte. However, the replacement of the TFSI anion with FSI results in a sharp decrease of Rint. For instance, more than 200 Ω cm2 are recorded for the 0.2LiTFSI-0.8EMITFSI cells [Supplementary Figure 4A], whereas only 40 and 20 Ω cm2 are approximately exhibited by those with 0.2LiTFSI-0.8EMIFSI [Supplementary Figure 4B] and 0.2LiFSI-0.8EMIFSI[Supplementary Figure 4C], respectively. These results, in very good agreement with the voltammetry data of Figure 1, indicate large decrease in interfacial resistance accompanying the transformation of the original discrete a-Si into a stable porous network (nano-ligaments)[23,33]. Once more, it highlighted the beneficial effect of the FSI anion at the interface with the Si electrode, playing a key role in the SEI composition and, therefore, the active material morphology. For instance, a suitable SEI layer leads to a larger surface area offered for the alloying process by the a-Si network and to the accommodation of more facile charge transfer at the electrode/electrolyte interface. Conversely, the TFSI anion is not able to promote the growing-up of suitable SEI, leading to poor Li+ alloying and, therefore, low performance. This issue is in good agreement with both the CV results of Figure 1. The impedance response recorded after 500 cycles (shown in Supplementary Figure 4D and E) exhibits similar shape with respect to that of the Nyquist plots of the first family cycles [Supplementary Figure 4B and C]. No practical change of Rbulk is observed, whereas a more slightly depressed semicircle is exhibited by the 0.2LiFSI-0.8EMIFSI cells, nevertheless demonstrating the electrochemical reliability of the 0.2LiTFSI-0.8EMIFSI and 0.2LiFSI-0.8EMIFSI electrolytes. An overall interfacial resistance lower than 100 Ω cm2 is recorded in both electrolyte formulations. To summarize, the a-Si anodes seem to show comparable behavior in both 0.2LiTFSI-0.8EMIFSI and 0.2LiFSI-0.8EMIFSI.

Superior compatibility of silicon nanowire anodes in ionic liquid electrolytes

Figure 3. Evolution of the electrolyte Rbulk (A) and overall interfacial Rint (B) resistance during CV tests run at different scan rates for Li/a-Si cells in 0.2LiTFSI-0.8EMIFSI and 0.2LiFSI-0.8EMIFSI. T = 20 °C.

Table 3

Rbulk (Ω cm) and Rint (Ω cm2) parameters, associated with Li/a-Si cells containing the 0.2LiTFSI-0.8EMITFSI, 0.2LiTFSI-0.8EMIFSI, and 0.2LiFSI-0.8EMIFSI electrolytes, determined in OCV conditions after CV tests run at increasing scan rates. T = 20 °C

Scan rate/mVs -1Pristine0.050.10.20.51
0.2LiTFSI-0.8EMITFSIRbulk1.7 ± 0.23.2 ± 0.35.8 ± 0.65.8 ± 0.66.3 ± 0.66.1 ± 0.6
Rint-200 ± 20230 ± 20250 ± 30250 ± 30250 ± 30
0.2LiTFSI-0.8EMIFSIRbulk1.6 ± 0.20.9 ± 0.11.5 ± 0.21.6 ± 0.21.7 ± 0.21.6 ± 0.2
Rint-65 ± 639 ± 437 ± 433 ± 335 ± 3
0.2LiFSI-0.8EMIFSIRbulk1.3 ± 0.10.8 ± 0.11.3 ± 0.11.3 ± 0.11.3 ± 0.11.3 ± 0.1
Rint-30 ± 318 ± 218 ± 218 ± 218 ± 2

Cycling behavior

The CV investigation on a-Si anodes has shown how EMIFSI is a more appropriate IL solvent compared to EMITFSI for Si electrodes. Therefore, this IL was used as the electrolyte component for further investigations on Sn-Si NW anodes. The N1114FSI IL was employed for comparison purposes. The performances in Li-metal cells of Sn-Si NW anodes were estimated by galvanostatic cycling measurements. Preliminary examinations [Supplementary Figure 6] have evidenced good and reproducible cycling behavior for a Si:Sn weight ratio above 2.6:1. As a consequence, only anode samples satisfying this parameter were subjected to cycling tests.

Effect of anion and cation

The voltage-capacity profile of the 1st charge-discharge cycle for Sn-Si NW electrodes in EMIFSI and N1114FSI electrolytes is reported in Figure 4A. A series of features, between 1.2 and 0.2 V, can be noticed during the first discharge curve at 0.1C, likely due to the IL electrolyte degradation leading to SEI layer formation onto the electrode surface[23,33]. These features are followed by two plateaus located around 0.25 and 0.15 V, ascribable to the lithiation process. The value of the initial coulombic efficiency (η) and the nominal capacities (Q) delivered during the rate capability tests on Li/Sn-Si NW cells are reported in Table 4. The 0.2LiFSI-0.8EMIFSI cell shows the lowest initial irreversible capacity (Qirr), amounting to 28%, with respect to 0.2LiTFSI-0.8EMIFSI and 0.2LiFSI-0.8N1114FSI cells, which show Qirr equal to 55% and 35%, respectively, but quickly leveling to 99% in all cases, as can be seen in Figure 4A. A stable capacity value is observed in all electrochemical formulations up to 100 cycles, even at very high rates (10C). Higher capacity values were delivered by the 0.2LiFSI-0.8EMIFSI cells at different scan rates, with a gain of ~550 mA h g-1 up to 1C and of ~800 mA h g-1 at higher current rates (> 2C) with respect to the other electrolyte systems investigated. The LiFSI-EMIFSI cell can deliver above 50% (1,300 mA h g-1) of the nominal capacity at 10 C; meanwhile, the 0.2LiTFSI-0.8EMIFSI and 0.2LiFSI-0.8N1114FSI cells deliver less than 29% and 16%, respectively, of their nominal capacity at the same current rate. The 0.2LiTFSI-0.8EMIFSI and 0.2LiFSI-0.8N1114FSI electrolyte formulations show very similar capacity values which, at high rates (> 1C), slightly decay for the N1114FSI cells, likely due to the faster ion transport properties of the EMIFSI IL with respect to N114FSI[32]. The best performances of the full-FSI cell configuration, in agreement with the CV results, are attributable to the better film-forming ability of the FSI anion with respect to TFSI[23,24]. Conversely, the N1114FSI IL displays lower compatibility towards Sn-Si NW anodes than EMIFSI. Supplementary Figure 7 displays the voltage vs. capacity profile of Sn-Si NW anodes in different electrolyte formulations at increasing scan rates. The voltage of Si alloying plateaus is distributed around 0.1 V, and the dealloying plateaus are at about 0.4 V, as reported in the literature[34,50,51]. The presence of Sn in the working electrode is evidenced by the two less-defined plateaus in the potential range of 0.45-0.6V[Supplementary Figure 7], attributable to the two-phase alloying reaction that involved the formation of the dominant phases Li2Sn5 and LiSn[34,51]. The increase of the current rate from 0.1C to 5C does not result in significant qualitative change of the voltage profile shape, but it does lead to an increase of lithiation/de-lithiation plateau slope attributed (and to an increase in ohmic drop) to diffusive phenomena within the IL electrolyte, especially at high rates. The rate capability tests [Figure 4B] were followed by prolonged tests run at 1C [Figure 4C] and have shown a capacity recovery up to the starting value at 1C. Very interesting capacity retention, despite a decay observed after 100 cycles, is generally exhibited by these electrolyte formulations; i.e., 1,005 (0.2LiFSI-0.8EMIFSI cells), 608 (0.2LiTFSI-0.8EMIFSI), and 509 (0.2LiFSI-0.8N114FSI) mA h g-1 are still delivered after 2,000 consecutive charge-discharge cycles (1C) at 100% of DoD, corresponding to 46% (capacity fading equal to 0.027 per cycle), 37% (0.031% per cycle), and 33% (0.034% per cycle) of the initial capacity discharged at 1C. The remarkably enhanced performances of the 0.2LiFSI-0.8EMIFSI electrolyte, comparable to or overcoming those observed in alkyl carbonate and other IL electrolytes[23,33], are once more likely ascribed to the better film-forming ability of the FSI anion, in combination with the transport properties of the EMI cation. The reversible (charge) capacity vs. current density dependence of the Li/Sn-Si NW Sn cells [Figure 4D] evidences a lower rate region, where the capacity is limited by the diffusion phenomena taking place within the Si electrode[52,53], distinguished by the higher rate one characterized by a more pronounced slope of the capacity vs. current density curve due to diffusive phenomena occurring in the IL electrolyte[52,53]. The lower rate region limit is around 0.3 mA cm-2 for the 0.2LiTFSI-0.8EMIFSI and 0.2LiFSI-0.8N114FSI electrolytes: above this current rate, the electrochemical behavior of the Li/Si cells is governed by diffusive phenomena. Interestingly, the lower rate region extends up to 1 mA cm-2 in 0.2LiFSI-0.8EMIFSI cells, indicating that diffusion processes become predominant in this electrolyte at much higher current density with respect to that observed in 0.2LiTFSI-0.8EMIFSI and 0.2LiFSI-0.8N114FSI. To summarize, the 0.2LiFSI-0.8EMIFSI electrolyte exhibits, among the investigated IL formulations, the best cyclic behavior in Sn-Si NW anodes.

Superior compatibility of silicon nanowire anodes in ionic liquid electrolytes

Figure 4. 1st discharge-charge curves (A), cycling performance at different rates (B and C), and reversible capacity vs. current density dependence (D) of Li/Si cells in 0.2LiTFSI-0.8EMIFSI, 0.2LiFSI-0.8EMIFSI, and 0.2LiFSI-0.8N1114FSI electrolytes. The cycling behavior of Li/Si cells in organic electrolytes (1M LiPF6 in EC:DMC 1:1 v:v 3 wt.% FEC) is reported for comparison purposes. T = 20 °C.

Table 4

Initial Coulombic efficiency (η) and reversible capacity, delivered at different scan rates, of Li/Sn-Si NW cells in different electrolyte formulations

Electrolyte mixture1st cycle η/%Q/mA h g-1
0.1C0.2C0.5C1C2C5C10C1C100th
0.2LiTFSI-0.8EMIFSI451,9951,9181,7741,6111,3639045861,622
0.2LiFSI-0.8EMIFSI722,5222,4422,3172,1752,0061,7201,3132,027
0.2LiFSI-0.8 N1114FSI651,9981,9051,7151,5121,2267314141,494

Effect of FEC organic additive content

The Sn-Si NW anodes have also been studied in the presence of FEC organic additives to investigate a possible improvement of the electrolyte/electrode interface[17]. The GC tests, as reported in Figure 5, were carried out in LiTFSI-EMIFSI electrolytes containing different FEC molar fractions (summarized in Table 1). The first discharge profile at 0.1C [Figure 5A] exhibits the features already discussed in the previous paragraph; however, the progressively increasing FEC content is seen to enhance the features located in the 1.7-0.2 V range and ascribable to irreversible electrolyte decomposition. In particular, at FEC content equal to 3 wt.%, a very wide plateau is observed around 0.4 V, leading to much larger irreversible capacity. The reversible nominal capacity, the initial coulombic efficiency (i.e., from 45% to 55%), and the cycling behavior (even at 10C) up to about 100 cycles are seen slightly increasing (i.e., from 2,000 to about 2,100 mA h g-1) after incorporation of a FEC mole fraction equal to 0.01, which exhibits at 1C and 10C a delivered capacity of 1,800 and 750 mA h g-1 (vs. 1,650 and 450 mA h g-1 of the FEC-free cells), respectively. Further increases of the FEC up to 3 wt.% content lead to decreased performance. The nominal capacity and the coulombic efficiency fall down to 1,500 mA h g-1 and 14%, respectively. Therefore, the presence of a FEC mole fraction > 0.01 in IL electrolytes seems to support larger consumption of electrolytes and/or hinder the formation of an optimal SEI onto the Sn-Si NW surface. As reported in the literature[54,55], the uncoordinated FEC can passivate the anode surface by forming LiF, which can undergo the formation of HF in the presence of small amount of water, causing the dissolution of the SEI on the Si anode surface. However, more prolonged discharge-charge tests (C) reveal progressive decay in performance. For instance, after 1,000 cycles, the 0.01 FEC cells exhibit lower capacities (950 mA h g-1) than those delivered by the FEC-free ones (1,200 mA h g-1), even approaching the values (900 mA h g-1) shown by the 0.07 FEC and 3 wt.% FEC cells (i.e., no practical difference is seen for FEC mole fractions exceeding 0.07). This means an increase in capacity fading from 0.03% (FEC-free cells) to 0.05% per cycle.

Superior compatibility of silicon nanowire anodes in ionic liquid electrolytes

Figure 5. 1st discharge-charge curves (A) and cycling performance at different rates (B and C) of Li/Sn-Si NW cells in LiTFSI-EMIFSI electrolytes with different FEC contents. T = 20 °C.

XPS analysis on Sn-Si NW anode surface

To get insight into the SEI layer composition formed on the Sn-Si NW anodes, ex-situ XPS analysis was carried out on pristine electrodes after the first charge-discharge cycle and several cycles (reported in Supplementary Figure 8). In order to avoid the binding energy (BE) shifts associated with charging phenomena and to gain good accuracy of the peak assignments, the XPS spectra were collected in the delithiated state. The atomic concentrations of the post-mortem Sn-Si NW anodes are summarized in Table 5 (the pristine electrode is reported for comparison). The SEI layer of the Si electrodes cycled in FEC-based and 0.2LiTFSI-0.8EMIFSI electrolytes exhibits high concentration of fluorine and a bit lower oxygen concentration in conjunction with higher silicon concentration onto the anode surface after several cycles, suggesting thinner passive layer onto a Si electrode, as previously reported in literature[56,57]. Otherwise, the Si anodes in 0.2LiFSI-0.8EMIFSI and 0.2LiFSI-0.8N1114FSI show high sulfur concentrations, which depend on the degradation products of the LiFSI salt[55].

Table 5

Surface composition (atom percentage) of pristine and cycled Sn-Si NW electrodes

O 1sC 1sN 1sF 1sS 2pSi 2pLi 1sP 2p
Pristine14.856.3---28.0--
0.2LiTFSI-0.8EMITFSI1st cycle31.233.01.54.01.12.226.6-
10th cycle27.932.43.98.22.40.923.8
0.2LiFSI-0.8EMIFSI1st cycle31.133.13.42.05.20.924.0-
10th cycle31.531.73.72.15.51.324.3
0.2LiFSI-0.8 N1114FSI1st cycle30.030.83.83.87.32.021.9-
10th cycle31.229.73.63.16.70.924.6
0.2LiFSI-0.8EMIFSI
+ 3 wt.% FEC
1st cycle28.536.74.44.64.01.620.0-
10th cycle30.733.02.85.62.60.524.6-
1M LiPF6 in EC:DMC 1:1 v:v + 3 wt.% FEC1st cycle31.629.6-5.90.31.230.80.1
10th cycle29.530.6-6.80.21.431.30.1

High-resolution XPS spectra of C 1s, Si 2p, F 1s, Li 1s, S 2p, O 1s, and N 1s, collected for each sample after the first charge-discharge cycles, are displayed in Supplementary Figures 9 and 10, whereas those taken after several cycles are given in Figures 5 and 6. The Si 2p spectra of pristine Sn-Si NW anodes exhibit a narrow feature at 99.8 eV corresponding to the bulk silicon, and the presence of silicon oxides is observed at 100.5 and 102.5 eV[56,58], also confirmed by the O 1s spectra at 532.4 eV[56,58].

Superior compatibility of silicon nanowire anodes in ionic liquid electrolytes

Figure 6. High-resolution C 1s, Si 2p, F 1s, and Li 1s core level spectra obtained on the Sn-Si NW anodes after consecutive lithiation/delithiation steps in different electrolyte formulations (see legend).

The N 1s XPS region of the Sn-Si NW [Figure 6] electrodes, cycled in IL electrolytes, displayed three well-separated peaks: the first two are related to the cation (-C-N-, N+), i.e., EMI+ (401.5 eV) and N+1114(402.6 eV)[59,60], and the anion, which appears around 400 eV (C-N, N-SO) for both TFSI- and FSI-[55,61]. The presence of the IL signals is probably due to the caught IL in the porous structure of the Si anodes[62]. The last peak at 398.5 eV is ascribable to Li3N/LiCxHyN or -N=C- double bond in a polymeric network[63-66]. These compounds are associated with the reduction of the electrolyte, which gave place to graphitic carbon[62], evidenced in C 1s spectra [Figure 6] with the sp2 graphitic carbon feature at 284.8 eV. The relative atomic concentration increases upon cycles for the all-TFSI-based electrolyte, decreases for LiFSI-N1114FSI and LiFSI-EMIFSI + 3 wt.% FEC, and finally remains unchanged in LiFSI-EMIFSI, suggesting the growing-up of a stable Li3N/LiCxHyN polymeric network during the first cycle.

The high-resolution XPS spectra of C 1s [Figure 6] display four main features centered around 284.8, 286.1, 288.5, and 289.5 eV in all cases, associated with C-C/C=C[55,67], C-O/C-N[64,68,69], O-C=O/C-F, and $$\mathrm{CO}_{3}^{2-}$$[55,67,70] species, respectively. The C-C/C=C, C-O/C-N, and CF groups arise from the decomposition of IL (i.e., EMI+, N1114+, TFSI-, and FSI-). For Si-based electrodes cycled in LiTFSI-EMITFSI, the C 1s spectra show a very small peak at 293 eV associated with the CF3 group[59,61] derived by the TFSI anion, also confirmed by the well-pronounced peak at 688.6 eV in the F 1s XPS spectra [Figure 6]. The $$\mathrm{CO}_{3}^{2-}$$ can be assigned to Li-OCOOC2H5 and/or Li2CO3 arising from FEC degradation[59] but also from salt decomposition, as was demonstrated in other works[63,71]. The presence of this compound was also detected in the Li 1s spectra at 54.7 eV. This peak is more prominent in FEC-based electrolytes and in LiTFSI-EMITFSI. The area ratio of C286.1/C288.5 does not substantially change for LiFSI-EMIFSI and organic electrolytes during cycling, indicating the growth-up of stable SEI during the first cycle. The C 1s spectra of the Sn-Si NW cycled in organic electrolytes reveal quite similar features to those detected in ILs, as already reported in the previous paper[62].

The deconvolution of O 1s spectra [Figure 7] highlights the presence of carbonate species, already observed in the C 1s spectra and evidenced around 532 eV[55,72]. Lithium silicate species (i.e., LixSiOy), related to the lithiation of Si oxide[56,58,73], can be attributed to the peak at 531 eV, which are visible also in the Si 2p spectra at 100.5 eV[55,72]. As described in detail elsewhere[74-76], the broad peak at 533.6 eV, ascribable to O-C=O species, depends on solvent and/or additive polymerization, and it was also observed in the C 1s spectra at 286.1 eV. In all electrolytes, the Si surface is rich in carbonate species, as confirmed by the features in the C 1s and O 1s spectra [Figures 6 and 7].

Superior compatibility of silicon nanowire anodes in ionic liquid electrolytes

Figure 7. High-resolution S 2p, O 1s, and N 1s core level spectra obtained on pristine Sn-Si NW anodes and after consecutive lithiation/delithiation steps in different electrolyte formulations (see legend).

A large LiF peak at 684.8 eV[55,77] can be observed in the F 1s spectra, in agreement with the Li 1s spectra at 55.6 eV[55,77], due to decomposition of salt (LiTFSI, LiFSI, and LiPF6) by electrochemical reduction of S-F bonds[58] and, during cycling, it can be observed that TFSI-based and FEC-based electrolytes exhibit a higher amount of LiF. As described by Piper et al., FSI- and TFSI- are subject to a quite different reduction process, where the FSI- rapidly releases F-, forming LiF and SO2, which suggests the formation of SEI rich in small inorganic compounds[60]. Conversely, TFSI- can form different products, such as -SO2CF3. The -SO2 species can also be observed at 168.8 eV in the S 2p spectra, where results are more prominent in the FSI-based systems. The fast release of F- and SO2 may be correlated to the high cycling performance shown by Si-Si NW anodes in LiFSI-EMIFSI electrolytes. This speculation derived from modeling studies performed on the FEC system[78], demonstrating its ability to rapidly release F- (to form LiF) during its decomposition. The SiOF signal at 686.8 eV corresponds to the Si substrate, indicating the thickness of the SEI layer: in organic electrolytes, it might be covered by the LiPF6 signal, resulting from incomplete removal during the electrode rinsing and/or from intermediate decomposition products with similar BE to LiPF6[76,79]. This feature is more pronounced in LiFSI-EMIFSI and organic (1M LiPF6 in EC:DMC 1:1 v:v + 3 wt.% FEC) electrolytes, indicating thinner SEI on the Si surface. The presence of residual lithium salts (LiTFSI, LiFSI, and LiPF6) and/or their incomplete decomposition products is detected around 688 eV[58,77]. In the case of LiTFSI, it is completely overlapped by the CF3 signal at 688.6 eV, and the SiOF is not visible.

The S 2p deconvolution [Figure 7] results in a pretty complex spectrum for the Si anode cycled in IL-based electrolytes. It can be divided into three main components. The first one was represented by the Sp3/2 peak, which confirms the presence of residual LiTFSI and/or LiFSI salt at 168.8 and 169.1 eV[77], respectively. However, the second one encloses several features, in the range between 169.8 and 168 eV, due to degradation products of imide-based salts that might be assigned to S=O bonds[77,80]. The last weak component, at around 167 eV, is also related to the reduction of Li-imide salt, but this process can also be induced by the X-ray beam[55,76]. The s 2p spectra display very weak peaks, probably due to residual traces of electrolytes.

The Si 2p spectra show the peak related to the bulk silicon at ~98 and ~99 eV[55,56], which, in the case of LiFSI-EMIFSI, tends to increase, meaning a reduction of the SEI layer thickness and in agreement with the results from impedance measurements discussed in Paragraph 3.2. A signal at 100.5 eV can be attributed to LixSiOy, as already previously described, and the last peak at 102.8 eV is associated with the SiO2 species.

The XPS results suggest a bilayer SEI onto the Si surface, i.e., an outer part composed of organic compounds within a polymeric network that can provide flexibility and good mechanical stability against the volume changes during the Si cycling, and an inner layer dominated by inorganic compounds such as Li3N and LiF, delivering high Li-ion conductivity, and a graphitic network that provides electronic conductivity[62,63,71]. The different cycling performances can be due to the different densities and porosity of the SEI on the Si surface. The SEI layer onto the Si anodes cycled in LiFSI-EMIFSI and organic electrolytes may be rich in stable LiF and -Si-F compounds which, thanks to their high bonding energy, do not decompose during cycling and promote stable interface with the electrolyte[81,82]. Although, in the other case, the surface layer can contain less stable compounds, such as metastable and less dense, linear alkyl carbonates [-Si-OCH2CH2OCO2Li, -Si-CH2CH2OCO2Li, R(OCO2Li)2] and, due to their low bonding energy, can continue to decompose upon cycling, thus feeding irreversible redox reactions that promote silicon dendritization and pulverization, as has been shown by Bongiorno et al. The low density of the SEI layer allows the transition of the electrolyte during the Li+ insertion, which causes the breakup of the -Si-Si- network bond, giving rise to porous structures and cracking[83].

CONCLUSIONS

The lithiation process of Sn-Si NW anodes in different IL electrolyte formulations was investigated through electrochemical and XPS measurements. High reversibility and reproducibility of the Li+-allowing process are observed for 0.2LiTFSI-0.8EMIFSI and 0.2LiFSI-0.8EMIFSI electrolyte systems through CV analysis carried out at increasing scan rates and for prolonged cycles. Conversely, the full-TFSI electrolytes do not show evident features related to the electrochemical processes, meaning poor reversibility of the lithiation process. The full-FSI cells display higher specific current even after prolonged CV tests (> 200 cycles), indicating larger charge amount involved in the lithiation process.

The Li+ diffusion coefficient (DLi+), evaluated using the Randles-Savcik equation, is found to be equal to (9.6 ± 0.5) × 10-10 cm2 s-1 and (2.5 ± 0.1) × 10-10 cm2 s-1 in the 0.2LiTFSI-0.8EMIFSI and 0.2LiFSI-0.8EMIFSI electrolytes, respectively. These results are consistent with those reported in the literature for organic electrolytes.

The interfacial properties of Li/Si cells were investigated by impedance spectroscopy analysis combined with CV tests. A stable bulk resistance is recorded during CV tests, confirming the good electrochemical stability of the IL-based electrolytes. However, marked improvement in interfacial resistance is observed after replacing the TFSI anion with the FSI. These results, in agreement with the CV tests, are ascribable to the good film-forming ability of the FSI anion.

A reversible capacity exceeding 1,000 mA h g-1 is delivered after 2,000 charge-discharge cycles at 1C, corresponding to 46% of the initial value. LiFSI-based electrolytes are seen to behave better compared to those containing LiTFSI. The electrode performances of Sn-Si NW anodes are seen to increase with the Si:Sn weight ratio: for instance, when passing from a Si:Sn ratio of 2.17 to 2.68, 2,400 mA h g-1 (about 89% of the nominal capacity = 2,700 mA h g-1) is still delivered at 1C, and 1,000 mA h g-1 is even exhibited at 10C (4 mA cm-2). These represent some of the best, if not the best, results observed till now for Si anodes in IL electrolytes. A moderate increase in capacity is observed in the presence of modest contents (0.01 as the mole fraction) of FEC; further addition of FEC, however, does result in performance decay.

A SEI layer, externally composed of a polymeric network and a graphitic network inside, on the Sn-Si NW anode surface was revealed by XPS analysis. The supposed high density of the SEI layer, which hinders the crack formation on the Si surface in the case of 0.2LiFSI-0.8EMIFSI and organic (1M LiPF6 in EC:DMC 1:1 v:v + 3 wt.% FEC) electrolytes, can support for the better cycling behavior.

To summarize, the 0.2LiFSI-0.8EMIFSI electrolyte has shown very high compatibility and excellent battery behavior in lithium cells with Sn-Si NW anodes, and therefore, it can be considered a very appealing electrolyte as an alternative to the commercial organic ones for realizing safer and more reliable, highly performant LIB systems.

DECLARATIONS

Acknowledgments

The authors acknowledge the support by the European Union's Horizon 2020 Research and Innovation Program under grant agreement No. 814464 (Si-DRIVE Project).

Authors’ contributions

Investigation, methodology, validation, formal analysis, writing - original draft: Maresca G

Formal analysis and material support: Sankaran A, Santa Maria LJ, Ottaviani M, Fantini S

Supervision, writing- reviewing and editing: Brutti S, Ryan KM

Conceptualization, validation, supervision, writing- reviewing and editing: Appetecchi GB

Availability of data and materials

The authors should declare where the data supporting their findings can be found. Data can be deposited into data repositories or published as Supplementary Material in the journal.

Financial support and sponsorship

This project has received funding from the European Union’s Horizon 2020 research and innovation program under grant agreement No. 814464.

Conflicts of interest

All authors declared that there are no conflicts of interest.

Ethical Approval and Consent to Participate

Not applicable.

Consent for Publication

Not applicable.

Copyright

© The Author(s) 2024.

Supplementary Materials

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Superior compatibility of silicon nanowire anodes in ionic liquid electrolytes
Giovanna MarescaGiovanna  Maresca, ... Giovanni Battista AppetecchiGiovanni Battista  Appetecchi

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Maresca, G.; Sankaran A.; Santa Maria L. J.; Ottaviani M.; Fantini S.; Ryan K. M.; Brutti S.; Appetecchi G. B. Superior compatibility of silicon nanowire anodes in ionic liquid electrolytes. Energy. Mater. 2024, 4, 400017. http://dx.doi.org/10.20517/energymater.2023.84

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