Forming Robust and Highly Li‐Ion Conductive Interfaces in High‐Performance Lithium Metal Batteries Using Chloroethylene Carbonate Additive

Developing high‐rate Li metal batteries (LMBs) is challenging because of dendrite growth and irreversible parasitic reactions of the Li metal. Herein, a robust and highly ion‐conductive solid electrolyte interphase (SEI) layer is designed on a Li metal anode and a Ni‐rich layered cathode by incorporating chloroethylene carbonate (ClEC) as an additive in fluoroethylene carbonate‐based electrolytes. ClEC induces the formation of LiCl, which facilitates Li‐ion diffusion in the robust LiF‐rich SEI layer, thereby improving the cycle stability of the Li metal anode and suppressing microcracking of the Ni‐rich layered cathode, especially during charging and discharging at high current densities. By using the newly developed combination of electrolyte solution, an LMB featuring the Li[Ni0.78Co0.1Mn0.12]O2 cathode (2.3 mAh cm−2, 0.1 C) affords a superior capacity retention of 80.2% over 400 cycles at high charge and discharge current densities of 2.0 C (3.6 mA cm−2) and 5.0 C (9.0 mA cm−2). This study provides insights into the use of ClEC as an electrolyte additive and highlights the importance of constructing robust and highly ion‐conductive interfaces on both Li metal anodes and Ni‐rich cathodes for high‐performance LMBs.


Introduction
[3] Batteries with high energy densities are crucial for enhancing EV driving range per charge.Furthermore, continuous efforts to expand transportation from densely populated ground areas to airspaces have necessitated the development of lighter batteries with higher power densities. [4,5]raphite, generally used as an anode material in commercial Li-ion batteries (LIBs), is electrochemically stable but has a relatively low theoretical capacity (380 mAh g À1 ).Replacing graphite with Li metal as an anode material can lead to remarkable improvements in the energy density of batteries.[8] However, the stable operation of Li metal batteries (LMBs) is compromised by dendrite growth and the highly reductive nature of Li metal.Li metal anodes can easily trigger short circuits, potentially leading to fires, and continuously corrode cell components through reactions with the electrolyte, with consequent formation of dead Li and an increase in the resistance of the battery. [9,10]he cycling behavior of Li metal anodes can vary significantly depending on the composition of the solid electrolyte interphase (SEI) layer formed on the Li metal surface. [11,12]The SEI layer on Li metal surface formed through the reductive decompositions of electrolytes. [13,14]A robust and stable SEI layer can inhibit continuous irreversible side reactions on Li metal; therefore, various studies have been conducted on durable SEI layer formation. [15,16]LiF has been highlighted as a key component of SEI layers because of its low electrical conductivity, mechanical robustness, and excellent electrochemical and chemical stability. [17,18]Our group developed a Li-metal-stabilizing electrolyte called EF31D, with a high content of fluoroethylene carbonate (FEC) solvent and Li difluoro(oxalato)borate (LiDFOB) salt as an additive; the EF31D electrolyte enables the formation of a LiF-rich SEI layer on the Li metal surface, significantly improving the cycling stability of LMBs. [19,20]23] In this study, we design a robust and highly ion-conductive SEI layer on both a Li metal anode and a Ni-rich layered cathode by incorporating chloroethylene carbonate (ClEC) as an electrolyte additive into our previously developed FEC-based electrolyte. [19]ClEC is a form of FEC in which F is substituted with Cl, and it forms Cl compounds in both the anode and cathode via the same mechanism as the formation of LiF from FEC. [24][25][26] LiCl, which has Li-ion diffusion barrier that is half as low as that of LiF but is still sufficiently robust, [27][28][29] facilitates Li-ion transport in the SEI and cathode-electrolyte interphase (CEI) layers.ClEC initially attracted attention as a solvent for SEI layer formation in early LIB research, along with FEC. [24]However, research on ClECs was not actively pursued after it was reported that Cl species derived from ClECs cause shuttle phenomena. [30,31]erein, using a small amount of the ClEC additive (0.5 wt%) in conjunction with the EF31D electrolyte effectively enhances the capacity retention of LMBs, affording a high Coulombic efficiency of 99.67%.Additionally, the stability of the proposed electrolyte is validated using graphite full cells, with the electrolyte effectively utilized in pouch-type high-energy LMBs featuring Ni-rich cathodes (with a Ni content of 90%).

Results and Discussion
To investigate the role of ClEC in the growth of Li metal, the morphology of the deposits was observed in real-time using a homemade in situ cell equipped with an optical window (Figure S1, Supporting Information).The baseline electrolyte consisted of 1 M LiPF 6 and 0.05 M LiDFOB dissolved in EMC:FEC (3:1 by vol.) developed by our group, [19] while the ClEC electrolyte contained 0.5 wt% ClEC as an additive in the baseline electrolyte.Li metal was electrochemically deposited on the Li metal electrode at a current density of 18 mA cm À2 , with the morphology of the deposited Li metal observed through a window using high-resolution digital optical microscopy.Figure 1a,b shows the morphology of Li metal deposited in the baseline and ClEC electrolytes, respectively, at 4 min intervals during electrochemical deposition.In the ClEC electrolyte, the individual deposited Li metal particles were larger, while the Li deposited after 12 min had a lower final thickness, i.e., denser growth of Li metal was observed in the ClEC electrolyte than in the baseline electrolyte, further confirmed by surface and crosssectional images of Li metal films deposited on Cu foil in 2032 coin-type cells.Li metal was electrochemically deposited onto Cu foil at a current density of 4 mA cm À2 for 1 h in a Li||Cu cell.As shown in Figure 1c,d, Li metal deposited in the baseline electrolyte exhibited a noodle-like morphology and was loosely packed, with a film thickness of 61.2 μm, whereas Li metal deposited in the ClEC electrolyte formed thicker particles and a denser film with a thickness of 46.7 μm.The composition of the SEI layer of the deposited Li films was investigated using X-ray photoelectron spectroscopy (XPS).The Cl 2p XPS profiles (Figure 1e,f ) revealed the appearance of a 2p 3/2 peak of LiCl at the binding energy of 198.5 eV in the case of the ClEC electrolyte. [26]Figure S2, Supporting Information, demonstrates the presence of LiF in both the baseline electrolytes with and without ClEC, which is a feature of EF31D as a baseline electrolyte. [19]o evaluate the cycling stability of Li metal in the cell with the ClEC electrolyte, a Li||Li symmetric cell was galvanostatically cycled at a current density of 2.0 mA cm À2 for 2 h (4 mAh cm À2 ) (Figure 2a).Before cycling, the cell was precycled twice at a lower current density of 0.4 mA cm À2 for 10 h and at 1.0 mA cm À2 for 4 h.As shown in Figure 2b, a lower nucleation overpotential (50.3 mV) was observed for the first Li electrodeposition in the cell with the ClEC electrolyte than in the cell with the baseline electrolyte (61.8 mV), indicating a lower energy barrier for Li metal nucleation in the ClEC electrolyte. [32]During the cycling, the voltage curves displayed a reduced overpotential in the cell with the ClEC electrolyte (Figure 2c).Although the cell with the baseline electrolyte showed a rapid voltage spike after 150 h, the cell with the ClEC electrolyte exhibited a longer lifetime of up to 200 h.Electrochemical impedance spectroscopy (EIS) further supported the effect of ClECs in lowering the resistance of the Li anode.The impedance of the Li||Li symmetric cell was measured after the 10th, 20th, and 30th cycles.With the ClEC electrolyte, the overall EIS profile showed small curves, and the increase in the size of the curve was less pronounced when the ClEC electrolyte was used.Specifically, when the observed profiles were fitted using the equivalent circuit shown in Figure S3, Supporting Information, and interpreted based on each resistance value, both the impedances from the SEI layer on the Li metal (R SEI ) and the charge-transfer reaction (R ct ) increased significantly in the baseline electrolyte compared with the ClEC electrolyte.Referring to previous reports, both the R SEI and R ct increased simultaneously due to the formation of dead Li on the Li anode surface during repeated Li deposition and dissolution. [33,34]The EIS results were found to be correlated to the changes in the morphology of Li metal deposited using the ClEC additive, which played a role in suppressing the formation of dead Li, resulting in a smaller increase in the resistance of the cells with the ClEC electrolyte during repeated cycling.When Li||Li symmetric cells were operated at a higher current density (3.0 mA cm À2 ) under mild operating conditions employing a relatively low capacity of 1.5 mAh cm À2 and flooded electrolyte, the improvement in the cycling stability was notable in the case with the ClEC additive (Figure 2d).The high ionic conductivity of the formed SEI layer lowers the charge-transfer resistance and induces an even flux of Li-ion throughout the Li metal surface.This not only induces uniform Li growth, but also lowers the local current density, deriving the Li metal anode to grow more densely and nondendritically.When ClEC is included in the electrolyte solution, the LiCl with high ionic conductivity formed by ClEC induces denser and spherical Li growth.In addition, as a result of the formation of the conductive SEI layer by the ClEC, Li||Li symmetric cell exhibited the enhanced cycling stability and lowered overpotential during the Li deposition and dissolution.
To investigate the effects of ClEC on the performance of LMBs using high-energy, Ni-rich cathodes, a Li[Ni 0.78 Co 0.1 Mn 0.12 ]O 2 cathode with a full concentration gradient of transition metals (FCG78) was employed as the cathode material.Applying a concentration gradient in Ni-rich cathodes is a field-proven strategy for achieving the potential capacity of Ni-rich cathodes while improving cycling stability.[37]  95.6%, 93.0%, 89.9%, 85.1%, 82.4%, 75.8%, 61.3%, and 48.9% relative to the values at 0.1 C discharge capacity at each C-rate.In contrast, with the base electrolyte, rate capabilities of 97.8%, 94.3%, 91.2%, 87.8%, 82.6%, 79.1%, 69.4%, 50.7%, and 38.6% were obtained.These results demonstrate that Li diffusion in the SEI layer, which determines the rate capability, is facilitated by the ClEC additive, inducing the formation of highly Li-ion-conductive LiCl.
The LMBs were cycled at 2.0 C (3.6 mA cm À2 ) for charging and 5.0 C (9.0 mA cm À2 ) for discharging (full charge in 30 min and discharge in 12 min) to evaluate the cycling performance of Li||FCG78 cells with the baseline electrolyte and ClEC electrolyte under fast charging and discharging conditions.Adding ClEC enabled stable and ultrafast charging and discharging of the Li||FCG78 cell, as shown in Figure 3c.With the ClEC additive, the cycling stability of the Li||FCG78 cell increased to 80.2% over 400 cycles (compared to 71.3% for the base electrolyte (Figure S4a, Supporting Information)), with a high Coulombic efficiency of 99.67%.To further compare the long-term cycling performance and stability of the ClEC electrolyte, pouch-type bicells with a graphite anode were fabricated and cycled for 1500 cycles (Figure 3d,e, and Figure S4b, Supporting Information).Although the charge and discharge capacities and Coulombic efficiencies in the initial cycle at 0.1 C were similar with both electrolytes, when using the ClEC additive, a higher maximum capacity was achieved during the activation for initial 50 cycles.Subsequently, the graphite||FCG78 cell with the ClEC additive in the electrolyte exhibited higher capacities at the same current density compared to the base electrolyte, maintaining an excellent capacity retention of 94.4% over 1500 cycles.The cells with the ClEC electrolyte demonstrated better electrochemical performance, reflected by the superior rate capability and improved cycling stability, highlighting the beneficial effects of the ClEC additive.
Stabilizing both the Ni-rich cathode and the Li metal anode is crucial to ensure the stable operation of Li||Ni-rich cathode cells with high energy densities.As degradation of the Ni-rich cathode primarily occurs at the CEI owing to the deleterious reaction between unstable Ni 4þ and the electrolyte, the rate of degradation is significantly influenced by the area exposed to the electrolyte. [1,3]In Ni-rich cathodes, the strain resulting from abrupt anisotropic changes in the lattice during the H2-H3 phase transition leads to the formation of severe microcracks within the cathode particles. [38,39]These microcracks provide pathways for the electrolyte to penetrate the interior of the particles, increasing the surface area exposed to electrolyte attack.This expanded surface area accelerates capacity fading in Ni-rich cathodes because both the outer and inner regions of the particles are susceptible to electrolyte attack.Consequent surface degradation leads to the formation of NiO-like impurity phases, further compromising the cathode performance. [40]To assess the effect of the ClEC additive on the cathode, the cycled cathodes were recovered from the Li||FCG78 cells, as shown in Figure 3c.As shown in Figure 4a, residual microcracks were observed in the cathodes of the LMBs with and without the ClEC additive after 400 cycles; however, some differences were observed in the extent of crack formation.With the baseline electrolyte, the microcracks were more prominent within the cathode, and the areal fraction of the microcracks was higher than that when the ClEC additive was added (Figure 4b).Recent studies have shown that fast cycling of batteries with Ni-rich cathodes induces increased heterogeneity in the distribution of Li within the particles and compromises the structural integrity of the cathode, resulting in rapid capacity fading. [41,42]In the fast charging-discharging process (charging at 2.0 C and discharging at 5.0 C), heterogeneity in the Li concentration and corresponding structural differences among the primary particles lead to increased strain in specific regions and accumulation of irreversible damage.However, when ClEC was used in the electrolyte, fewer cracks were observed, indicating that less strain was induced in the particles, thus demonstrating that the primary particles constituting the cathode underwent more homogeneous lattice volume changes, suppressing strain accumulation in specific regions.As demonstrated by XPS analysis of the CEI (Figure 4c), the presence of LiCl, which exhibits high ionic conductivity, can facilitate the (de)intercalation of Li þ ions within the CEI, which allowed most of the primary particles to undergo more homogeneous lattice changes and suppressed strain accumulation.Time-of-flight secondary-ion mass spectrometry (TOF-SIMS) was used to further investigate the CEI of the FCG78 cathode after cycling (Figure 4d); more chloride-based species were observed on the surface of the cathode cycled with ClEC, consistent with the XPS results.The ClEC additive created more chloride-based species within the SEI as well as within the CEI, resulting in higher ionic conductivity at both interfaces.Furthermore, fewer PO 2 À moieties were detected on the surface of the cathode cycled with the ClEC additive than on that cycled with the base electrolyte.As PO 2 À species are byproducts of electrolyte decomposition, the TOF-SIMS data confirm the suppression of electrolyte decomposition in the Cl-rich CEI.The ClEC additive induced the formation of a high-ion-conductivity CEI, which maintained the structural integrity of the cathode and suppressed electrolyte decomposition, resulting in improved rate capability and cycling stability (Figure 3c).In recent years, the industry has adopted the strategy of increasing the nickel content in the cathode to over 90% to maximize the energy density of LIBs.Thus, to further enhance the energy density of the LMBs, the FCG78 cathode was replaced with a concentration-gradient-type Li[Ni 0.90 Co 0.05 Mn 0.05 ]O 2 cathode with a protective LiF coating (denoted as F-CG90), with the loading of the cathode further increased to 20 mg cm À2 .Introducing a LiF coating on the concentration gradient Ni-rich cathode suppressed side reactions at the cathode-electrolyte interface, enabling stable cycling of the Ni-rich cathodes with more than 90% Ni. [22,43] LMBs featuring F-CG90 with the baseline electrolyte and ClEC electrolyte (E/C ratio: 50 μL mAh À1 ) were fabricated and cycled at a charging-discharging rate of 0.5 C (Figure 5a).The Li||F-CG90 cell with the ClEC additive in the electrolyte maintained 88.1% of its initial capacity after 200 cycles (84.2% for the baseline electrolyte).The improved cycling stability compared to that of the baseline electrolyte demonstrates the effectiveness of the ClEC additive in stabilizing the CEI, even if the Ni composition in the cathode increases, thus enabling high-energy-density LMBs with extended cycle life.A pouch-type LMB was fabricated with a cathode loading level of 22 mg cm À2 , an E/C ratio of 5 μL mAh À1 , and a Li metal anode thickness of 100 μm (Figure 5b) to further demonstrate the practical feasibility of the proposed LMB.To enhance the cycling stability of the pouch-type high-energy LMB, an additional external pressure of 1200 kPa was applied to the cell. [43]The pouchtype LMB exhibited a high areal capacity of 5.0 mAh cm À2 (227.2 mAh g À1 ) at 0.1 C and 4.7 mAh cm À2 (212.7 mAh g À1 ) at 0.5 C.After 250 cycles of charging and discharging at 0.5 C, the cell retained 81.8% of its initial capacity.These results are of great significance as they verify the commercial viability of the additive and demonstrate the feasibility of long-lasting and high-energy-density LMBs for real applications, such as EVs.

Conclusion
The effectiveness of ClEC as an electrolyte additive in high-rate LMBs was investigated.Even in small amounts, the ClEC additive helped modulate the deposition process at the Li metal anode, promoting dense growth with a lower surface area and alleviating the formation of dead Li, which enables the easy (de)intercalation of Li-ion in the cathode, inducing uniform reactions even at high current densities and suppressing cathode cracking and surface degradation.The effectiveness of ClEC was confirmed by various electrochemical cycling tests.Although studies on ClEC were previously discontinued due to the shuttling phenomenon of the Cl species, we demonstrated the possibility of utilizing ClEC as an effective electrolyte additive for forming highly conductive SEI and CEI layers through combination with EF31D.We believe this study provides an important foundation to stimulate research to reconsider ClEC as a functional electrolyte additive.
Coin Cell and Pouch Cell Fabrication: Coin cells were assembled using 2032 cell parts (Hoshen) in an Ar-filled glove box.For the Li||Cu deposition test, Li (Honjo Metal; thickness, 200 μm) was punched into a disc with a diameter of 14 mm; a 16 mm Cu disc was also prepared.A PE separator (Celgard 2320, Celgard) with a diameter of 19 mm was used for the deposition test; however, a boehmite-coated PE separator (Enerever, PE thickness of 11 μm with 3 μm coating on both sides) was used for all other electrochemical tests.For the symmetric coin cell test, Li was punched into discs with diameters of 14 and 16 mm.The cathode was punched onto a disc with a diameter of 10 mm, while a Li metal disc with a 16 mm diameter was used for the half-coin cell test.For fabrication of the pouch-type cell, Li metal (Honjo Metal; thickness: 100 μm; width: 31 mm; height: 51 mm) was attached to a Cu foil (FUKUDA Metal; thickness: 8 μm; width: 31 mm; height: 51 mm), and the cathode (width: 30 mm; height: 50 mm) was cast on carbon-coated Al foil.
Electrochemical Measurements: A battery testing system (TOSCAT-3100, Toyo System Co.) was used to perform galvanostatic electrochemical cycling tests.In situ optical microscopy (OM) observations were performed with a homemade in situ cell equipped with an optical window.In the Ar-filled glove box, in situ cell was fabricated.Li (400 μm) was attached to one side of each of the two rods.The two rods, which will be used as counter and working electrodes, respectively, were equipped into the main body.An internal space of 5 mm between the two rods was filled with the electrolyte (baseline electrolyte or ClEC electrolyte) and quartz window cover the top surface.To avoid ambient air exposure, the two rods and window were fixed to the main body using O-rings.Then, in situ cell was connected to the galvanostat and Li was electrochemically deposited from the counter Li electrode to the working Li electrode for 12 min at a current density of 18 mA cm À2 .During the electrodeposition, internal space was observed through the window using high-resolution digital OM equipment (VHX-7100, KEYENCE).EIS analysis was performed using a potentiostat (VMP3, Bio-Logic) in the range of 1 MHz to 1 mHz; data were acquired every ten cycles using a symmetric cell.
Characterizations: Before characterization, the cathode and anode were washed with dimethyl carbonate (DMC, 99.5%, Daejung) after being purified with molecular sieves.The cathode was polished using a crosssectional polisher (CP, IB-19520CCP, JEOL) to compare the microcracks after 400 cycles.ImageJ software was used to calculate the area fraction of the microcracks.The particle morphology of the cathode and thickness of the deposited anode was observed using scanning electron microscopy (SEM, Verios G4UC, FEI).ToF-SIMS was performed using a mass spectrometer (TOF.SIMS5, IONTOF) with a 30 keV Bi þ primary ion gun and 500 eV Cs þ sputter gun.Depth profiling was performed on an area of 400 Â 400 μm 2 in the negative mode.The chemical compositions of the deposited Li metal and the cycled FCG78 cathode were characterized using XPS (K-alpha Plus, Thermo Scientific).

Figure 1 .
Figure 1.Comparative analysis of morphology and components of Li metal using baseline electrolyte and ClEC electrolyte.Morphology of Li deposited in a) baseline electrolyte and b) ClEC electrolyte, observed using homemade in situ cell equipped with window.Li metal was electrochemically deposited for 0, 4, 8, and 12 min at current density of 18 mA cm À2 .The white and red vertical lines illustrate the bottom part where Li metal begins to be deposited and top part of the deposited Li, respectively.Surface and cross-sectional SEM images of Li metal film deposited on Cu foil in Li||Cu cell (4 mA cm À2 , 4 mAh cm À2 ) with c) baseline electrolyte and d) ClEC electrolyte.XPS Cl 2p profile of Li metal film deposited with e) baseline electrolyte and f ) ClEC electrolyte.

Figure 2 .
Figure 2. Results of Li||Li symmetric cell test and EIS test using baseline electrolyte and ClEC electrolyte.a) Electrochemical performance of Li||Li symmetric cell (2.0 mA cm À2 , 4.0 mAh cm À2 ) using limited amount of electrolyte (40 μL).b) Voltage profiles for the first cycle.c) Voltage profiles during the cycle test from 100 to 104 h.d) Electrochemical performance of Li||Li symmetric cell (3.0 mA cm À2 , 1.5 mAh cm À2 ) with excess electrolyte.e) EIS data acquired every ten cycles with limited electrolyte using (e) baseline electrolyte and f ) ClEC electrolyte.g) Numerical value of SEI resistance and charge transfer resistance from EIS test.

Figure 3 .
Figure 3. Improved rate capability and cycling performance of Li||FCG78 cells using baseline electrolyte and ClEC electrolyte.a) Voltage profile and b) discharge rate test results.c) Cycling performance at fast charge and discharge rate of 2.0 and 5.0 C. c) Pouch-type cell test using graphite anode.

Figure 4 .
Figure 4. Postmortem analysis of FCG78 cathodes after fast charge and discharge cycling test in discharged state after 400 cycles.a) Cross-sectional CP-SEM image.b) Box-whisker plot for comparison of microcrack area ratio of cross-sectional cathode.c) XPS results.d) 3D rendering images and depth profiling of TOF-SIMS.

Figure 5 .
Figure 5. Cycling performance of LMBs featuring high-loading F-CG90 cathode.Voltage profile (fifth cycle) and cycling data of a) coin cell with cathode loading level of 20 mg cm À2 and b) pouch-type cell with cathode loading level of 22 mg cm À2 .External pressure of 1200 kPa was applied to operate the pouch-type cell.Inset is the digital photograph of the fabricated pouch-type cell.