Guided Lithium Metal Deposition and Improved Lithium Coulombic

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Guided Lithium Metal Deposition and Improved Lithium Coulombic Efficiency through Synergistic Effects of LiAsF6 and Cyclic Carbonate Additives Xiaodi Ren, Yaohui Zhang, Mark H Engelhard, Qiuyan Li, Ji-Guang Zhang, and Wu Xu ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b00982 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 19, 2017

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Guided Lithium Metal Deposition and Improved Lithium Coulombic Efficiency through Synergistic Effects of LiAsF6 and Cyclic Carbonate Additives Xiaodi Ren†,§, Yaohui Zhang†,#,§, Mark H. Engelhard‡, Qiuyan Li†, Ji-Guang Zhang†,*and Wu Xu†,* †

Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland,

Washington 99354, USA ‡

Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, 3335

Innovation Boulevard, Richland, Washington 99354, USA *

Corresponding author: E-mail: [email protected] (W.X.), [email protected] (J.-G.Z.)

#

Present Addresses: Department of Physics, Harbin Institute of Technology, Harbin,

Heilongjiang 150001, China §

These authors contributed equally to this work

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Abstract Spatial and morphology control over lithium (Li) metal nucleation/growth, as well as improving Li Coulombic efficiency (CE) are of the most challenging issues for rechargeable Li metal batteries. Here, we report that LiAsF6 and cyclic carbonate additives such as vinylene carbonate (VC) or fluoroethylene carbonate (FEC) can work synergistically to address these challenges. It is revealed that LiAsF6 can be reduced to LixAs alloy and LiF, which can act as nano-sized seeds for Li growth and form a robust solid electrolyte interface layer. The addition of VC or FEC not only enables the uniform distribution of LixAs seeds, but also improves the SEI layer flexibility. As a result, highly compact, uniform and dendrite-free Li film with vertically aligned columns structure can be obtained with increased Li CE, and the Li metal batteries using the electrolyte with both LiAsF6 and cyclic carbonate additives can have improved cycle life.

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Lithium (Li) metal has long been regarded as an ideal anode material for electrochemical energy storage systems because of its ultrahigh theoretical specific capacity (3,860 mAh g-1), the lowest electrochemical potential (-3.040 V vs. standard hydrogen electrode) and low density (0.534 g cm-3).1-4 The pursuit on Li metal batteries (LMBs) dated back four decades ago and has been greatly intensified in recent years with the needs for high energy density batteries. Nevertheless, the large-scale implementation of LMBs still faces enormous challenges due to the well-known dendritic Li growth issue and the low Li Coulombic efficiency (CE) during plating/stripping cycles.5-6 In particular, dendritic Li growth or mossy Li formation could significantly increase side reactions with the electrolyte to generate enlarged surface areas and promote the generation of electrically isolated or “dead” Li, leading to very poor CEs. More importantly, the potential of the penetration through the battery separator by Li dendrites and the resulting short-circuit could lead to serious safety hazardous. Until recently, there are few studies on mitigating irregular Li dendrite growth by guided Li nucleation and growth. Cui and co-workers discovered that Li deposition can preferably take place on the gold (Au) nanoparticles (NPs) decorated on the inner-wall of hollow carbon spheres.7 LixAu alloy formed acts as seeds for Li nucleation and enables Li deposition spatial control. Hu et al. also used silver (Ag) NPs dispersed in 3D carbon matrix as seeds to guide Li deposition through LixAg alloy formation and enables stable Li anode cycling.8 It shows that guided Li deposition is a very important and promising approach for enabling LMBs. On the other hand, the composition of the solid electrolyte interface layer generated from the spontaneous reactions between Li metal and the electrolyte (e.g. ROCO2Li, ROLi, Li2CO3, LiF etc. as in conventional LiPF6-carbonate electrolytes), has a major influence on the Li growth behavior.9-10 Li metal dendrite prefers to grow where the interface layer is more ionically


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conductive but mechanically soft. Significant research progress has been made on improving the interface stability, especially by enriching LiF component, which has one of the highest shear modulus (55.1 GPa) among various inorganic compounds.11 Archer and co-workers reported that blending 30 mol% LiF into liquid electrolytes could enable stable cycling of Li metal anodes.12 A recent theoretical calculation shows LiF has a high Li+ surface diffusion rate, which can promote the uniform growth of Li deposits.13 Our group has found that the formation of LiF-rich interface layers prior to Li deposition can be induced by CsPF6 additive or HF in-situ generated through LiPF6 hydrolysis. In both cases, self-aligned Li metal columns or nanorods can grow uniformly without protrusion through the interface layer. Nevertheless, the obtained Li metal cycling CE was poor because the rigid interface layer alone cannot accommodate the volume changes during Li platting/stripping cycles.14-16 Herein, we report a facile but very effective method that could lead to a guided Li nucleation and growth, a robust LiF-enriched interface layer and a much improved Li CE by using dual additives of LiAsF6 and cyclic carbonate compounds such as vinylene carbonate (VC) or fluoroethylene carbonate (FEC) in the conventional electrolyte, 1 M LiPF6 in propylene carbonate (PC). We discovered that a small amount of LiAsF6 can be reduced to form LixAs alloy and LiF, inducing the seeded-growth of Li metal under a strong interface layer. Furthermore, it is shown that VC or FEC additive not only improve the Li metal CE by enhancing the flexibility of interface, but also enable the initial uniform nucleation of LixAs seeds. With the synergistic of LiAsF6 and VC or FEC, great improvements of Li deposition uniformity, Li metal CE and cycle life of LMBs can be achieved.


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Figure 1. SEM images of electrodeposited Li films on Cu substrates in different electrolytes: (ab) baseline (1 M LiPF6-PC), (c-d) with VC additive, (e-f) with FEC additive, (g-h) with LiAsF6 additive, (i-j) with LiAsF6 + VC additives and (k-l) with LiAsF6 + FEC additives (0.1 mA cm-2, 1.5 mAh cm-2). Both top-views (a, c, e, g, i, k) and cross-sections (b, d, f, h, j, l) are shown.

The morphologies of electrochemical deposited Li films were first characterized in 1 M LiPF6-PC (baseline) electrolyte with and without additives. As shown in Figure 1a-b, the topview and cross-section view SEM images of deposited Li metal show rough and porous structures, indicating the uncontrolled growth of Li metal in the baseline electrolyte. When just 2 wt% VC is added to the baseline electrolyte, the deposited Li metal is less irregular and no apparent dendritic growth is observed (Fig. 1c-d). However, Li metal grows into loosely-packed particles with bulky heads. The addition of FEC additive generates a Li film with large particles (diameter 0.8~2 micron) densely packed on the surface (Fig. 1e-f), showing a better uniformity


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than the Li film in the electrolyte with VC additive. In addition, when 2 wt% of LiAsF6 additive is added, the deposited Li film is packed with particles with diameter around 0.5-2 µm, showing no obvious porosity (Figure 1g). Interestingly, numerous self-aligned columns vertical to the Cu substrate are seen from the cross-section of the Li film (Figure 1h). This structure is very similar to the morphologies obtained with the addition of CsPF6 or trace amount of H2O in the baseline electrolyte, despite of the less consistency of the length as well as the diameter of the columns.1516

More importantly, significant Li morphology changes are observed when the combinations of

LiAsF6 and VC or FEC additives are employed. As shown in top-view and cross-section SEI images in Figure 1i-l, highly compact and smooth Li films with uniformly aligned columns can be plated with the LiAsF6 + VC and LiAsF6 + FEC additives combinations. Compared to the Li film deposited with LiAsF6 additive alone (Figure 1g-h), the addition of VC or FEC greatly improves the film uniformity, implying a highly controlled Li metal growth behavior. Furthermore, the consistency of the Li column diameters was significantly higher, especially with FEC additive. The majority of the Li columns have a diameter close to 500 nm.


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Figure 2. (a) The comparison of Li metal CEs measured in different electrolytes. (b) Cycling performance of Li||NMC cells in different electrolytes.

With the proof that a small amount of LiAsF6 and VC or FEC additives can lead a controlled Li deposition morphology, the influence of these additives on Li metal CE, which is another critical factor of electrolytes for LMBs, was studied. A modified Aurbach’s method was employed to evaluate the Li CE with repeated platting/stripping cycles (details described in supporting information). A representative voltage and current density profile is shown in Figure S1, and the measured CEs are shown in Figure 2a. In agreement with the uncontrolled Li


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deposition morphology, the baseline electrolyte (1 M LiPF6-PC) exhibits a Li CE of only 73.2%, due to the excessive side reactions between the porously deposited Li and the electrolyte components. The addition of LiAsF6 could only increase the Li CE by about 4% in spite of the greatly decreased Li porosity. Similar effect was also observed in our previous study of Cs+ additive, where the Li CE was just 76.6% with the formation of similar packed Li columns.14 It suggests that although the LiF-rich SEI layer is effective in smoothing Li deposition and preventing dendrite growth because of its superior mechanical strength, its rigidity makes it unable to accommodate the volume changes during repeated plating/stripping processes. Therefore, continuous consumption of Li metal cannot be effectively prevented due to the side reactions between Li and electrolyte. On the other hand, it is found that adding 2 wt% VC or 2 wt% FEC into the baseline electrolyte could significantly improve the Li CE to 94.1% and 93.7%, respectively. As reported before, VC and FEC are able to undergo reductive polymerization, which induces the generation of polycarbonate species in the SEI layer.17-18 It is likely that these polymeric species could make the SEI layer more resilient against volume changes. As a result, the improved Li CEs were obtained through the suppression of side reactions with the electrolyte. Importantly, the combination of LiAsF6 additive with VC or FEC could further increase the Li CE to 96.7% and 96.4%. It shows that the synergistic effects between LiAsF6 additive and VC or FEC can not only effectively control the Li metal deposition morphology, but also improve the Li CE. The reason of the different Li deposition morphology and Li CE with FEC from those with VC is likely due to the C-F bond in FEC. It is likely that the C-F bond in the FEC molecule, which is shown to be labile in contact with Li metal, can incorporate more LiF into the solid electrolyte interface layer to further enhance its strength.19-20


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This could explain the more uniform Li deposits obtained with FEC additive compared to VC additive. The synergistic effects between LiAsF6 and VC as well as LiAsF6 and FEC can be further demonstrated by comparing the cycling performance of Li||LiNi1/3Mn1/3Co1/3O2 (Li||NMC) cells using electrolytes with different additives. A piece of Li foil (250 µm thick) was sandwiched with a NMC cathode (1.5 mAh cm-2) for cycling tests. As shown in Figure 2b, the cell capacity continuously decreases upon cycling before fast fading at ~120 cycles in the LiPF6-PC baseline electrolyte and the voltage hysteresis (Figure S2) also quickly increases. It agrees well with the results above that Li CE is low in the baseline electrolyte, which results in fast Li anode consumption and a large amount of side product accumulation. With the addition of LiAsF6 additive, the cell cycling stability can be improved, which could be attributed to the dense Li deposition morphology promoted by LiAsF6. The enhancement of the interface layer flexibility with VC addition results in apparently improved Li metal anode stability as well. Nevertheless, the potential instability of FEC in electrolytes would release HF21, which could react with Li metal causing further Li metal consumption thus slightly lower Li CE (93.7%) as well as corrode the NMC cathode thus affecting its stability as indicated by the low initial cell CE (Figure S3) and the limited cycle life. The combinations of LiAsF6 + VC and LiAsF6 + FEC additives both show improved cycling stability compared to those using single additive and the best cell cycling performance is achieved with the combination of LiAsF6 and VC, realizing a capacity retention of ~95% after 250 cycles. Because of the similarity between VC and FEC in terms of the polymerization pattern, the exploration of the synergistic effects between LiAsF6 and VC or FEC in the following sections will focus on the use of LiAsF6 with VC to avoid the interference from the additional HF generated from FEC.


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Figure 3. (a) LSV curves from OCV to 0.1 V in different electrolytes at a scan rate of 5 mV s-1. (b) Wide scan XPS spectra of the SEI layers formed at 0.1 V in different electrolytes. (c) Atomic ratios of elements in the SEI layers formed at 0.1 V from XPS results. (d) The XPS spectra of Li 1s, As 3d and F 2s of the SEI layers formed in different electrolytes.

The electrochemical processes prior to Li metal deposition, which we believe are critical for subsequent Li growth morphology, were studied to understand the underlying mechanism of the homogeneous Li growth using the LiAsF6 and VC dual additives. Figure 3a shows the current-voltage profiles of linear sweep voltammetry (LSV) scans in electrolytes with and


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without additives. All three electrolytes have common reduction peaks located at about 2.7 V, 2.0 V and 0.5~0.7 V, which can be ascribed to the reduction processes of HF,16 PF6- anions,15 and PC solvent,15-16 respectively, as observed in the 1 M LiPF6-PC based electrolytes with trace amount of water or CsPF6 additive. The two electrolytes containing LiAsF6 additive show an extra and strong reductive peak at ~1.05 V (Figure 3a), which should be related to the reduction of LiAsF6. The components of the solid electrolyte interface layers formed at 0.1 V (vs. Li/Li+, the same reference in the following context) in the three electrolytes were characterized by widerange XPS scans (Figure 3b) and the quantification results of the interface layers

are

summarized in Figure 3c. The baseline electrolyte shows enrichment in C (60.7%, atomic ratio) and O (20.7%, atomic ratio) elements. When 2 wt% LiAsF6 is added, the interface layer composition shows a major change, with C and O atomic ratio decreased to 12.7% and 10.0%, while F ratio is increased to 21.6%. This is also suggested that the reduction process between 1.0 and 1.2 V is mainly from the reaction of LiAsF6 instead of PC molecules. No apparent changes in XPS spectra can be seen when adding another 2 wt% VC, except slight increased C and O atomic ratio from VC reduction. It suggests VC addition won’t change the working mechanism of LiAsF6 additive. To have a better understanding on the reduction process of LiAsF6, the LSV scan was carried out in 1 M LiAsF6-PC electrolyte (Figure S4) and the SEI layer composition change at different potentials was characterized by XPS. In 1 M LiAsF6-PC, there is a similar sharp reduction peak starting at 1.2 V, confirming this peak is actually from LiAsF6 reduction. This agrees with the previous result that LiAsF6 reduction takes place around this potential region.22 Here, the reduction pathway of LiAsF6 can be clearly explained by the XPS results of the SEI


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layers formed at 1.0 V and 0.1 V. As shown in Figure S5, the As 3d peak shifts to lower binding energies upon reduction. At 1.0 V, the As 3d peak at 49.0 eV from AsF6- splits into two peaks at 44.4 eV and 41.6 eV. In contrary to the previous hypothesis that LiAsF6 is reduced to AsF3, no evidence of AsF3 formation (As 3d, 47.1 eV23) can be found on XPS. Instead, the 41.6 eV peak is assigned to As0 according to the previous literature24, while the 44.4 eV broad peak is likely to be from a complex of arsenic oxides (probably mostly As2O325) generated from the side reactions with PC molecules in the reduction process. This reduction process of LiAsF6 is accompanied with the formation of LiF, as evidenced from the Li 1s peak at 55.7 eV and the F 2s peak at 29.6 eV. Further scan to 0.1 V would converge almost all As species into a single peak at 38.4 eV. This negative shift of As bonding energy suggests the formation of LixAs alloy phase (possibly Li3As phase according to the previous study26) during the reduction process. Accordingly, a new Li 1s peak at a lower binding energy of 53.9 eV appears because of the alloy phase. The XPS changes discussed above are very similar to those obtained with the baseline electrolyte with LiAsF6 additive. The XPS spectra of the SEI layers formed at 0.1 V with 2 wt% LiAsF6 are shown in Figure 3d, the Li 1s peaks at 55.7 eV (LiF) and 53.9 eV (LixAs), the As 3d peak at 38.5 eV (LixAs) as well as the F 2s peak at 29.6 eV (LiF) all agree very well with the changes found in the 1 M LiAsF6-PC electrolyte. Therefore, the reduction process of the LiAsF6 additive could be summarized as follows: LiAsF6 + Li+ + e- → As0 + LiF

(1a)

LiAsF6 + Li+ + e- + PC → AsxOy + LiF + side products

(1b)

AsxOy + Li+ + e- → As0 + Li2O As0 + Li+ + e- → LixAs

(2) (3)


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Figure 4. SEM images of the SEI layers on Cu substrates after LSV to 0 V and held at 0 V for 60 s in electrolytes (a) 1 M LiPF6-PC, (b) 1 M LiPF6-PC + LiAsF6, and (c, d) 1 M LiPF6-PC + LiAsF6 + VC). (e, f) SEM images of Li deposition morphologies after 10 min at 0.1 mA cm-2 in 1 M LiPF6-PC + LiAsF6 + VC.

As a result of these reactions, the solid electrolyte interface layer on Cu substrate is enriched in LiF with embedded LixAs phase. In addition, a very recent study by Nazar group shows the formation of Li3As alloy can help stabilize the Li metal-electrolyte interface.27 It is likely that the LixAs and LiF formed through LiAsF6 reduction could have similar effect on Li metal anode. This interface composition should be beneficial for improving the uniformity of the subsequent Li metal growth for the reasons discussed above. However, it proves to be challenging to obtain uniform interface spatial distribution with LiAsF6 additive alone. The initial interface spatial distribution patterns in different electrolytes were examined after the Cu


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electrodes (SEM of pristine Cu shown in Figure S6) were scanned to 0 V at a rate of 5 mV s-1 and then held at 0 V for 60 s. As shown in Figure 4a, the interface layer formed in the baseline electrolyte is non-evenly distributed. The darker regions are likely to have thinner and more conductive interface layer, while the lighter regions have charge accumulation due to the thicker interface layer. This could explain why the subsequent Li deposition morphology is highly irregular (Figure 1a-b). When only LiAsF6 additive was used, the initial interface layer uniformity doesn’t have apparent improvement (Figure 4b). Thicker interface layer seems to preferentially grow along the textured lines of the Cu substrate, which may have more defects due to strain effect. Although the LiF and LixAs species from LiAsF6 reduction could help mitigate this difference in subsequent Li growth, the variations in surface reactivity could lead to variations in Li growth rate at different regions (Figure 1g-h). However, this behavior could be significantly changed by further adding VC additive. Figure 4c and 4d clearly indicate the greatly improved spatial control over nucleation, where numerous nano-sized seeds were distributed very uniformly on the Cu substrate. These seeds would grow to ~100-200 nm after 10 min Li deposition at 0.1 mA cm-2 and cover the whole Cu surface nicely (Figure 4e, 4f and Figure S7). This indicates that the addition of VC is critical for eliminating the surface reactivity difference during the initial nucleation process. In previous studies, the reduction potential of VC on graphite was reported to be ~2.5-2.6 V.17, 28 It is likely that the similar reduction of VC on Cu could passivate the defect sites and facilitate uniform LiAsF6 reduction in the subsequent process. However, due to the complicated nature of the species in the electrolyte and the relatively low Cu surface area, the reduction process of VC cannot be clearly distinguished from the LSV curve. More importantly, although the polymeric SEI layer formed from VC reduction alone is less robust and could not lead to uniform Li deposition (Figure 1c-d), the incorporation


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of polymeric components into the SEI layer is of great importance for accommodating the volume changes during Li plating/stripping and achieving high Li CE. Therefore, the synergistic effect between LiAsF6 and VC, which enables both uniform Li deposition and high Li CE, is highlighted.

In summary, we reported the morphology control and guided Li nucleation/growth as well as the improvement of Li CE by the synergistic effect between LiAsF6 and cyclic carbonate additives (VC or FEC). It was proved for the first time that LiAsF6 can be reduced to form LixAs alloy phase and LiF, which can enable the seeded growth of Li metal and the formation of a stable solid electrolyte interface layer simultaneously. Initial surface passivation of the Cu substrate by VC or FEC reduction is critical for the uniformity of the subsequent LiAsF6 reduction reactions. Highly compact, uniform and dendrite-free Li film with self-aligned Li columns can thus be obtained with the combination of LiAsF6 and VC (or FEC) additives. Greatly improved Li CE was realized by controlling spatial Li nucleation/growth and incorporating polymeric components into the robust and rigid LiF-enriched interface layer, which also results in much elongated cycle life of Li||NMC cells. The better understanding of Li nucleation spatial control and the interaction between different electrolyte components shed light on how to eliminate Li dendrites and stabilize Li anode for long-life battery cycling. However, it should be noted that when Li deposition current density was increased above 0.5 mA cm-2, protrusion and even dendritic of Li growth was found (Figure S8), which indicates that the interface layer has a limited capability to suppress Li dendrite under high current density conditions. Further work is still needed to improve the mechanical and ionic conduction properties of the solid electrolyte interface layer for high power applications.


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Acknowledgements This work was supported by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Vehicle Technologies, the Advanced Battery Materials Research (BMR) programs of the U.S. Department of Energy (DOE) under contract no. DE-AC02-05CH11231, subcontract no. 18769. Y.Z. was grateful for the support from the National Science Foundation of China (21103037). The SEM and XPS were conducted in the William R. Wiley Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at PNNL. PNNL is operated by Battelle for the DOE under Contract DE-AC05-76RLO1830. Supporting Information Available: Detailed description of experimental methods, Li CE test protocol scheme, the voltage profiles and cell CEs of Li||NMC cells during cycling in different electrolytes, LSV curve in 1 M LiAsF6-PC electrolyte, XPS spectra of SEI layers formed at different potentials in 1 M LiAsF6-PC electrolyte, the SEM image of pristine Cu substrate, a measurement of typical Li deposition particle sizes and SEM images of Li deposition morphologies with LiAsF6 and VC additives at different current densities.

References (1) Aurbach, D.; Zinigrad, E.; Cohen, Y.; Teller, H. A short review of failure mechanisms of lithium metal and lithiated graphite anodes in liquid electrolyte solutions. Solid State Ionics 2002, 148 (3-4), 405-416. (2) Zheng, G.; Lee, S. W.; Liang, Z.; Lee, H. W.; Yan, K.; Yao, H.; Wang, H.; Li, W.; Chu, S.; Cui, Y. Interconnected hollow carbon nanospheres for stable lithium metal anodes. Nat. Nanotech 2014, 9 (8), 618-623.


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Guided Lithium Metal Deposition and Improved Lithium Coulombic

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