RbF as a Dendrite-inhibiting Additive in Lithium ... - ACS Publications

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RbF as a Dendrite-inhibiting Additive in Lithium Metal Batteries Shaopeng Li, Shan Fang, Hui Dou, and Xiaogang Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2019 Downloaded from http://pubs.acs.org on May 26, 2019

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ACS Applied Materials & Interfaces

RbF as a Dendrite-inhibiting Additive in Lithium Metal Batteries Shaopeng Li, Shan Fang, Hui Dou, Xiaogang Zhang* Jiangsu Key Laboratory of Electrochemical Energy Storage Technologies, College of Material Science and Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, 210016, P. R. China. *Corresponding author’s e-mail: [email protected] ABSTRACT：Lithium metal is considered to be one of the most potential anode material on account of its high theoretical specific capacity and lowermost electrochemical potential. Nevertheless, the critical challenges of dendrites growth and low Coulomb efficiency (CE) of the lithium metal anode during the cycling prevent the commercial application of the lithium metal battery. Herein, Rubidium fluoride (RbF) as additives are utilized to inhibit the lithium dendrites growth. Benefit by Rb+ stronger electropositive property, it can adsorb on the surface of protuberances during the lithium deposition process, form electrostatic shielding field of the surface, and guides the uniform deposition of Li+. The CE can maintain above 90% with the additives of RbF in the carbonate electrolyte. Besides that, RbF improves the cycle life and decreases the polarization, a stable cycling performance of more than 1000 hours has been obtained and the overvoltage is controlled below 100 mV in the cycling test of Li||Li symmetrical

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cells at 0.5 mA cm-2. The additive of RbF offers a feasible way for the development of Lithium metal batteries (LMBs) in future. KEYWORDS: electrolyte additive, RbF, electrostatic shielding, dendrite inhibition, lithium metal battery 1. INTRODUCTION

Since the first battery was invented in the 1800s, the requirement for an electrochemical energy storage system with high-energy density has never stopped. Over the past few decades, lithium-ion batteries (LIBs) have experienced rapid development and have been successfully commercialized1,2. However, the capacity of LIBs are approaching the theoretical limit of cathode/anode materials while the requirement of high-energy-density energy storage system is ever-growing3-5. Lithium metal has excellent superiorities as the anode in LMBs, because lithium metal has the unparalleled superior theoretical specific capacity (3.86 Ah g-1) as well as the lowest electrochemical potential (–3.04 V), furthermore, lithium metal is the lightest alkali metal with a density of 0.54 g cm-3. These factors give metallic lithium the largest gravimetric energy densities which was considered to be the ideal anode for rechargeable batteries in the future6-10. However, lithium metal batteries are not yet commercially viable which is impeded by its safety issues arising from the uncontrolled growth of lithium dendrites and the inefficient CE in the process of


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lithium plating and stripping repeatedly. The thermodynamically feature of Li metal is unstable in most organic electrolytes, it can react spontaneously with electrolyte to form solid electrolyte interphase (SEI) layer11,12. Additionally, the plating/stripping processes of lithium associated with infinite volume changes, leading to the mechanical deformation of the brittle SEI layer. Thus, the SEI layer consecutively breaks and regenerates during the process of charging and discharging, besides, the Li dendrites are grown preferentially at the spot where the current density is locally enhanced. In this case the Li surface area will increase remarkably with the growth of Li dendrites, which in turn increases the side reaction between the organic electrolyte and lithium anode, decreasing the CE of Li during plating/stripping processes. Finally, the battery degradation and failure because of the consumption of electrolyte couple with severe corrosion of Li metal anode13. Over the past few decades, a variety of strategies have been investigated to suppress the uncontrolled growth of Li dendrites in the LMBs. From a nanoscale interface engineering perspective, various surface layers have been proposed to block dendrites growth. For example, using magnetron sputtering14-16, ALD17-19, Langmuir-Blodgett physical transplantation20,21 or other method to cover the surface of lithium anode with carbon film22 or LiF-rich artificial layer23. Another way is using the three-dimensional current collector to adjust the surface electric



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field in order to alter the original nucleation of lithium deposition, such as 3D porous copper24-26, copper nanowire mesh27 and submicrometer skeleton of copper28,29. Besides that, highly infiltrated coating separators are introduced to reduce the inhomogeneous distribution of Li+

30,31.

All of these methods are

improving the performance to a certain degree. Based on the fundamental self-reinforcing action of the Li dendrites growth, a further effective method should be investigated. As mentioned above, in the conventional LiPF6 carbonate electrolyte, lithium is thermodynamically unstable, and the formed SEI layer is primary composed of ROLi, lithium carbonate, and lithium fluoride which is structurally porous, brittle, partially soluble, and may break easily when the volume changes during cycling32. In this regard, the establishment of a stable and dense SEI layer seems to be very critical for stable Li anode. In order to modify the SEI layer of the Li anode, various additives are used, such as FEC (Fluoroethylene carbonate)33, traces of water34, carbonates, carboxylates35, LiNO336 and LiCl37 etc. The results show that the modified SEI layer is dense and uniform, which can effectively inhibit lithium dendrites and obtain high CE. In this regard, the formation of SEI with considerable accordance and stabilization is one of the most effective and feasible ways to inhibit the growth of lithium dendrites as a result of its superior efficient and lower cost, especially in the industrial production of energy storage equipment.


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Recently, alkali metal ion having a lower reduction potential has been reported as a hopeful additive in LMBs to enhance the CE38. Illuminated by these works, herein, we report the discovery of the prominent effect of RbF on the morphology and reversibility of Li deposition in a conventional 1 M LiPF6 with ethylene carbonate (EC): dimethyl carbonate (DMC) (1:1 by volume) based electrolyte. At very low concentration, we observe that the Rb+ with more negative reduction potential in carbonates, which could adequately transform the interfacial electric field, resulting in bulk Li nuclei (rather than dendritic) and greatly promoted CE, and the cycle life. 2. EXPERIMENTAL SECTION

Electrolyte preparation and battery assembly: The commercially RbF was ordered from Aladdin Reagent (Shanghai) Co., Ltd. The lithium salt of 1.0 M LiPF6 was dissolved in EC and DMC (1:1 by Volume) solution which was get from DodoChem, named as the normal electrolyte in the following description. Different quantity of RbF were dissolved into the normal electrolyte in a glovebox filled with high purity argon gas (the contents of oxygen and water were limited below 0.1 ppm), the molar fraction of RbF additive is set to 0.01 M, 0.05 M, 0.1 M (We tested the solubility of RBF in EC/DMC, when the concentration was 0.4M, RbF can’t dissolved completely, some solid RbF appeared at the bottom of the glass bottle as shown in Support Information Figure S1). The normal electrolyte



without adding RbF was used as a control sample. The coin cells of CR2016 type were used and commercial separator (Celgard 2400) was employed. To standardize the experiment, each coin cell was added 60 μL electrolytes with or without RbF. Electrochemical test: The plating/stripping experiment of lithium metal was carried out utilizing a Land 2001A button battery test system (Wuhan LAND Electronic co. LTD), More specifically, the current densities were controlled at 0.5 and 1.0 mA cm-2, respectively, and the specific capacities were fixed at 1.0 mAh cm-2. Electrochemical impedance spectroscopy (EIS) was performed by ZIVE SP2 electrochemical workstation (WonATech Co., Ltd), with a frequency range of 1×105 Hz to 1×10-2 Hz and disturbance amplitude of 10 mV. The exchange current density and CV curve were tested on electrochemical instrumentation (CH Instruments Ins). In situ optical microscope real-time observation device: The real-time observation device of in situ optical microscope is composed of an optical microscope, CCD camera, transparent battery and electrochemical workstation, which was used to characterize the process of lithium plating. We devised a transparent battery using a quartz container and a visual window is reserved in advance. The internal body of the battery is mainly composed of a sandwich structure with Copper foil, electrolyte and lithium metal. The transparent battery


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was fixed on the carrier of the optical microscope with the section of copper foil facing the optical microscope lens for in-situ observation. The deposition of lithium on the copper foil was recorded at 0.1 mA cm-2 Characterization: Li was deposited on the foil of copper (Cu) substrates (Φ12 millimeter) use the electrolyte with or without RbF at the predesigned current densities. After charging and discharging cycle, the electrode was cleaned with DMC to wipe off the remanent lithium salts and electrolyte. Finally, the electrode was dried in a vacuum environment. The surface morphologies and the thickness of the deposited Li on the foil of copper were observed by scanning electron microscopy (SEM) making use of LYRA3 TESCAN scanning electron microscope and an accelerating voltage of 5 keV and 15 keV. 3. RESULTS AND DISCUSSION

As we know, the tip of an electrical conductor often exhibits a stronger electric field. When some fine protrusions appear on the surface of the substrate, lithium metal preferentially deposits on the tip, resulting in dendrites growth. According to the Nernst equation, the effective reduction potential of low



concentration Rb+ is less than Li+ 4, so Rb+ can be attached to the prominent position as shown in Figure 1. Therefore, Rb+ will be preferentially adsorbed on the surface of the synapse than Li to inhibit the growth of dendrites. Figure 1. (a) Schematic illustrating the growth process of lithium dendrites. (b) Schematic illustrating RbF inhibits the growth of lithium dendrites.

In order to study the effect of additives on the long-term stability of lithium metal plating/stripping behavior, we assembled Li||Cu cells. The CE of the Li||Cu cells were further tested at 0.5 mA cm-2 and 1.0 mA cm-2 with 1 mAh cm-2 plating/stripping separately. From the Figure 2a we can see the electrolyte with RbF exhibits superior cycle performance at 0.5 mA cm-2. The CE of the normal electrolyte reached ~92% for the first 10 cycles, and then gradually decreased. At about 60 cycles, the CE decreased significantly. After 87 cycles, the CE of the normal electrolyte had dropped to 71.8%, while the CE of the cells in the electrolyte with 0.05 M RbF was very stable, maintaining above 90% even after 120 cycles. From Figure 2b we get similar results when the current density was further increased to 1.0 mA cm-2, the CE is dramatically reduced in the normal electrolyte, the initial CE is only 56.3%, and keep bellowing 85% in the subsequent cycle. After 60 cycles, the CE is significantly reduced. While the electrolyte with RbF can still achieve a stable cycle life with high CE at high


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current densities, the CE at all three different concentrations of RbF is greater than 80% after 100 cycles, obviously outperformed the normal electrolyte over extended cycling, enhancing the Li cycling efficiency and stability. This is attributed to the Rb+ can be adsorbed on the protruding sites of the carrier. The field strength at the tip is lowered to effectively inhibit the growth of lithium dendrites, thereby preventing direct contact between the lithium dendrites and the electrolyte, and reducing the side reaction consumption of the lithium metal and the electrolyte. The concentrations of RbF at 0.05M show the best CE for 0.5 mA cm-2 and 1mA cm-2, therefore, we choose to focus the remainder of our studies on electrolyte with 0.05M RbF additives.

Figure 2 ． Coulomb efficiency of Li||Cu cells in different electrolytes (a) the



current density is 0.5 mA cm-2 (b) the current density is 1 mA cm-2 with 1 mAh cm-2 deposit capacity at 25 °C.

The Li plating/stripping curves with different electrolyte was shown in Figure 3, it’s apparently observed that the electrolyte with 0.05 M RbF shows a smaller voltage hysteresis than the normal electrolyte at 0.5 mA cm-2. After 50 cycles, the electrolyte with 0.05M RbF had a voltage hysteresis of 58 mV, which is much lower than the normal electrolyte with a high voltage hysteresis of 92 mV. When the current density increases, the voltage hysteresis of the electrolyte added with RbF is 86 mV, while the voltage hysteresis of the normal electrolyte rises to 156 mV, this is because Rb+ inhibits the continuous generation of lithium dendrites and stabilize lithium metal anode.


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Figure 3. After 50 cycles, the charge-discharge curve of Li||Cu cells in normal electrolyte and electrolyte with 0.05 M RbF at (a) (b) 0.5 mA cm-2 and (c) (d) 1 mA cm-2 with 1 mAh cm-2 deposit capacity. For purpose of researching the effect of RbF on the on the dynamical stability of the electrolyte interface and long-period cycle stability of lithium negative electrode, Li||Li symmetrical cells with different electrolytes were assembled to test the effect of RbF on the overpotential performance. It can be seen from Figure 4 that the overpotential of the symmetrical cells in the normal electrolyte is about 80 mV, and the overpotential in the electrolyte with RbF is about 60 mV, which shows excellent cycle stability compared with the normal electrolyte. A



large and irreversible voltage drop at 450 h in the normal electrolyte, this is caused by the lithium dendrite piercing the separator. While the cells in the electrolyte with 0.05 M RbF reveals intensive cycling life and the potential remains stable even after 1000 h. It is indicated that Rb+ can stabilize the SEI layer, improving the cycle stability of the battery.

Figure 4. Effects of RbF on long cycle Li||Li symmetrical batteries at 0.5 mA cm-2 with 1mAh cm-2 plating/stripping at 25 °C (a) without RbF (b) with RbF. The effect of electrolyte additives on lithium metal electrochemical performance is mainly reflected in cycle life, CE, and morphology of lithium deposition, the fundamental reason is that the additive changes the dynamic parameters of lithium deposition. In order to quantify the difference in reaction dynamic of lithium anodes caused by electrolyte additives, the Li|Li|Li


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three-electrode cells were assembled to measure the exchange current density of the electrode in the different electrolyte. Exchange current density is a pivotal dynamic parameter in the electrochemical reaction process, which is used to evaluate the electron loss and electron repulsion ability of an electrode, or to reflect the characteristic parameter of the electrochemical reaction. The higher the exchange current density, the easier the electrode reaction is. Figure 5 shows the Tafel curve measured in the normal electrolyte and the electrolyte with 0.05 M RbF. The linear parts of the cathode and anode polarization curves were extrapolated respectively, and the intersection point is Log j0 value of the electrode reaction. It can be seen that the exchange current density is 0.23 mA cm-2 in the electrolyte with 0.05M RbF. Its two times greater than that in normal electrolytes which is 0.12 mA cm-2. The additives of RbF significantly enhance the exchange current

density

of

electrode

systems.

This

indicates

that

the

lithium

plating/stripping process has greater reversibility in the electrolyte with RbF. Demonstrating that the lithium metal electrode is less susceptible to polarization, which is beneficial to stabilize the formation of the SEI layer and inhibit the growth of lithium dendrites. Li|Li|Li three-electrode cells measures the exchange current density of lithium ion on the surface of lithium anode39, we also assembled Li|Li|Cu three-electrode cells to measures the exchange current density of lithium ion on the surface of copper foil40. It can be seen that the exchange current density of the



three-electrode system in the normal electrolyte is 1.58×10-4 mA cm-2, while in the electrolyte with RbF is 5.26×10-4 mA cm-2 as shown in Support Information Figure S2. Cyclic voltammetry (CV) test was carried on Li||Cu cells with different electrolyte. The scanning voltage range was set to -0.2 to 3 V, at the same time, we control the scanning rate to be 1 mV s-1. The test results are shown in Figure 5(c, d). The anodic peak potential of the normal electrolyte is 0.17 V, the cathodic peak potential is -0.2 V, the voltage difference between the anodic peak and the cathodic peak is 0.37 V. In the electrolyte with 0.05 M RbF, the anodic peak potential decrease to 0.12 V, the negative shift of the peak potential indicates the polarization decrease, demonstrating that the RbF does decrease the uneven distribution of the electric field caused by the growth of dendrites on lithium negative electrode and play a role in reducing polarization. The CV test of the other two electrolytes with different concentrations of Rb+ was added, as shown in Support Information Figure S3. In the electrolyte with 0.01 M RbF and 0.1 M RbF, the anodic peak potential increase to 0.16 V and 0.13V, it shows that when the concentration of RbF is too high or too low, the electrostatic shielding effect gradually fails.


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Figure 5. Tafel curves of the three-electrode system in normal electrolyte (a) and electrolyte with 0.05 M RbF electrolyte (b). Li||Cu cells Cyclic Voltammetry curves in (c) normal electrolyte and in (d) electrolyte with 0.05 M RbF. To elucidate the feasible cause for superior CE and more outstanding stability of the Li||Cu cells with RbF, after diﬀerent cycles we test the electrochemical impedances of the cells moreover (Figure 6). Nyquist plots generally consist of a semi-circular arc and a nearly linear line which were showed in the high-frequency and low-frequency region respectively. The semi-circular arc is mainly composed of the charge transfer resistance (Rct) and the SEI layer resistance (RSEI) of the surface of the lithium anode. The linear portion is mainly the diffusion resistance (Warburg resistance) of lithium ions.



Obviously, with the additive of RbF in the normal electrolyte the transfer resistance (Rct) of the electrode has been reduced. During the cycling, the cells containing 0.05 M RbF has great stable SEI layer. As shown in Figure 6 after 20 cycles, the Rct of the electrode in the normal electrolyte was 292.7 Ω, which is almost three times of the values in modified electrolyte. After 60 cycles, the SEI layer impedance circulating in the normal electrolyte showed significant fluctuate, because of the continuous cracking/regeneration of the SEI layer during continuous plating/stripping of lithium metal. On the contrary, benefit from the RbF additives, the electrode has a stable SEI layer formation during cycling. These result further confirmed that the Rb+ inhibits the growth of lithium dendrites and promoting the electrochemical kinetics. Therefore, the outstanding stable SEI layer formed by RbF makes a great contribution to the superior CE and more outstanding stability.


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Figure 6 Nyquist plots and its circuit simulation diagram of the Li|| Cu cells (a) (c) without RbF (b) (d) with 0.05 M RbF after selected cycles. To further investigate the effect of RbF on the composition of the SEI film on the surface of the lithium anode, the Li, F, P, O, C, and Rb elements on the surface of the lithium anode after 10 cycles were analyzed by XPS. The peak at 685.9 eV in the Figure corresponds to LixPOyFz and is mainly derived from the decomposition of LiPF641. The peak at 684.5 eV corresponds to LiF42. Due to the introduction of F anion, the content of F in the surface of the lithium anode was improved, indicating that more LiF was formed at the interface. At the same time, the increase in the content of LixPOyFz in SEI film, which is also beneficial to improve the coulombic efficiency and cycle stability of the



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lithium anode43. On the other hand, the test results also show no evidence of Rb in the deposited Li film as shown in Support Information Figure S4. In addition, the growth of lithium dendrite was viewed through in-situ optical microscopy

by

a

self-design

transparent

quartz

cell.

Tested

by

the

chronopotentiometry method. the minimum potential was set as -1 V and the cathode current was 0.1 mA, the deposition time was 0.5 h. As shown in the blue box of Figure 7, we can see some fine and sharp lithium dendrites are grown when the deposition time is 300s in the normal electrolyte (marked by the red arrow). As the deposition time increases to 1500s, on the surface of the originally flat copper foil, there are many protrusions with different sizes has been observed, which is due to lithium dendrites formation. This is because the lithium dendrites have higher electric field strength at the tip end, thereby attracting Li+ to continuously deposit on the tip. In the electrolyte with 0.05 M RbF (the green box), it can be observed that as the deposition time increases, the lithium dendrites at the tip do not continue to grow, revealing the electrostatic shielding of Rb+ and uniform Li deposition.


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Figure 7 In-situ optical microscopy investigates the dendrites growth at 0.1mA of cathodic current on the Copper substrate in electrolyte without RbF (the blue box) and with 0.05 M RbF (the green box). To further exploring the modification of RbF additive for the lithium anode, we characterized the deposition morphology of Li electrode after recycling. Numerous pine-needle-like lithium dendrites were clearly observed on the surface in the normal electrolyte for 1st deposition as shown in Figure 8 (a, b). With the extension of the cycle period, the surface of the deposited lithium metal is jagged and accompanied by many sharp lithium dendrites, the surface is loose with plenty of “dead” Li as shown in Figure 8 (e, f, i, g). Since the growth of lithium dendrites is self-reinforcing, this loose packing structure can exacerbate



the inhomogeneous distribution of the electric field, thereby further causing ununiformed deposition of lithium. On the contrary, the deposition of lithium in the electrolyte with 0.05 M RbF presents massive Li particles without Li dendrites (Figure 8 c, d). Compared with the needle-like dendrites, the morphology of granular lithium deposition has two advantages: 1) the massive Li motes have a larger dimension, which can guard against needle-like dendrites from penetrating through the separator; 2) the large Li particles has a small specific surface area, thus the notorious secondary reaction among the lithium metal with electrolyte can be effectively suppressed. When the number of deposition cycles increases, the interface morphology of the lithium metal is gel-like, the contact between the lithium deposits is tighter, and the surface is flatter and tighter, which is helpful to form a steadier interface and the electrode structure. Because there is no lithium deposition of the fibrous structure, so it is unable to pierce the separator, thereby extending the cycle stability and lifetime of the lithium anode.


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Figure 8 SEM images of lithium deposition morphology in Li||Cu cells at 0.5 mA cm-2, 1mAh cm-2 plating/stripping for 1st deposition (a, b) without RbF (c, d) with 0.05 M RbF; and for 10th deposition (e, f) without RbF (g, h) with 0.05 M RbF; and for 50th deposition (i, j) without RbF (k, l) with 0.05 M RbF. Figure 9 shows the cross-section SEM of the deposition of lithium metal. It can be found that as the cycle number increases, the lithium metal in the normal electrolyte gradually becomes powdered, the deposition thickness also increases, which is consist with Figure 8, the deposition of lithium metal without RbF reveals loose and porous structure (Figure 8 e, f, i, j). When RbF is added to the electrolyte, the stability of lithium deposition is significantly improved, the deposition thickness is thinner, and a dense packing structure is formed,



indicating that RbF can indeed inhibit the growth of dendrites, and finally, improving the CE values during repeated Li plating/stripping.

Figure 9 Cross-section SEM images of lithium deposition morphology in Li||Cu cells at 0.5 mA cm-2 with 1 mAh cm-2 deposit capacity in (a) normal electrolyte (b) 0.05 M RbF electrolyte after 10 cycles, (c) without RbF (d) with 0.05 M RbF after 50 cycles. 4. CONCLUSIONS

In order to solve the problems of capacity and lifetime decay caused by lithium dendrites growth, dendritic fracture and formation of dead lithium during


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the cycle of LMBs, RbF is an effective additive, in particular, inhibiting the disreputable growth of dendrites, thereby protecting the metal lithium anode. Speciﬁcally, Rb+ can be preferentially adsorbed on the synaptic surface than lithium to inhibit the growth of dendrites. In the Li||Cu cells test, the cells reveal better average CE of 90.2% with 0.05 M RbF when compared to 83.0% for that of the normal electrolyte. In the cycle test of symmetrical cells, RbF improves the cycle life and show lower polarization degree. More attractively in-situ real-time observation of optical microscopy and SEM images are more directly to show that RbF can help lithium anode to form a smooth surface and modify the morphology of deposition. The results show that RbF has high perspective for application in rechargeable LMBs, but its practical utilizations still need more evaluations and investigations, especially in the combination of lithium anode and cathode. Besides, we also need to identify the effect of additive on anode materials so as to obtain superior stability and high efficiency, which is needed for stabilizing Li anode and the development of LMBs. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX. Digital photo of the solubility of RBF in EC/DMC, Tafel date, CV data, XPS data



of Li||Cu batteries. Video1: In normal electrolytes, the lithium deposits show a porous, irregular morphology (MP4) Video2: In electrolytes with 0.05 M RbF, the lithium deposits show a compact and uniform morphology (MP4) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (X.G.Zhang)

ORCID Author Contributions These authors contributed equally. Notes ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (U1802256，51372116, 51672128, 21773118, 51802154,21875107), Key Research and Development Project of Jiangsu Province (BE2018122), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). REFERENCES


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