Guided Lithium Metal Deposition and Improved ... - ACS Publications

Nov 16, 2017 - fluoroethylene carbonate (FEC) can work synergistically to address these challenges. It is revealed that LiAsF6 can be reduced to LixAs...
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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, United States Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Washington 99354, United States

‡

S Supporting Information *

ABSTRACT: Spatial and morphology control over lithium (Li) metal nucleation and growth, as well as improving Li Coulombic efficiency (CE), are among 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 act as nanosized 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 flexibility of the solid electrolyte interface layer. As a result, highly compact, uniform, and dendrite-free Li film with vertically aligned column 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.

L

dispersed in a three-dimensional carbon matrix as seeds to guide Li deposition through LixAg alloy formation, which enables stable Li anode cycling.8 These results show 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 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 moduli (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

ithium (Li) metal has long been regarded as an ideal anode material for electrochemical energy storage systems because of its ultrahigh theoretical specific capacity (3860 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) dates back four decades 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 penetration of Li dendrites through the battery separator and the resulting short-circuit could lead to serious safety hazardous. Until recently, there have been 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 gold (Au) nanoparticles (NPs) decorated on the inner-wall of hollow carbon spheres.7 The LixAu alloy formed acts as seeds for Li nucleation and enables Li deposition spatial control. Hu et al. also used silver (Ag) NPs © 2017 American Chemical Society

Received: October 9, 2017 Accepted: November 16, 2017 Published: November 17, 2017 14

DOI: 10.1021/acsenergylett.7b00982 ACS Energy Lett. 2018, 3, 14−19

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Figure 1. SEM images of electrodeposited Li films on Cu substrates in different electrolytes: (a, b) 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.

baseline electrolyte, despite the lower consistency of the length as well as the diameter of the columns.15,16 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-sectional Li 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 additive 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. With the proof that a small amount of LiAsF6 and VC or FEC additives can lead to 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 plating−stripping cycles (details described in the 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 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 increase the Li CE by only 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 This result suggests that although the LiF-rich solid electrolyte interface 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. In addition, the surface area of the well aligned Li columns is still high. Therefore, continuous consumption of Li metal cannot be effectively prevented because of 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

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 plating−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 LiFenriched 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 improves the Li metal CE by enhancing the flexibility of interface but also enables the initial uniform nucleation of LixAs seeds. With the synergistic effects of LiAsF6 and VC or FEC, great improvements of Li deposition uniformity, Li metal CE, and cycle life of LMBs can be achieved. The morphologies of electrochemically 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-sectional 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 (Figure 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 μm) densely packed on the surface (Figure 1e,f), showing a better uniformity 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 15

DOI: 10.1021/acsenergylett.7b00982 ACS Energy Lett. 2018, 3, 14−19

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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%, respectively. These results show 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 for 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 interface layer to further enhance its strength.19,20 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 performances of Li∥LiNi1/3Mn1/3Co1/3O2 (Li∥NMC) cells using electrolytes with different additives. A piece of Li foil (250 μm thick) was paired 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. This 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

Figure 2. (a) Comparison of Li metal CEs measured in different electrolytes. (b) Cycling performances of Li∥NMC cells with different electrolytes.

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 interface layer.17,18 It is likely that these polymeric species could make the surface layer more resilient against volume changes. As a result, the

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 interface layers formed at 0.1 V in different electrolytes. (c) Atomic ratios of elements in the interface layers formed at 0.1 V from wide scan XPS results. (d) Narrow scan XPS spectra of Li 1s, As 3d, and F 2s of the interface layers formed in different electrolytes. 16

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literature,24 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 by 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 from the two LiPF6−PC-based electrolytes with LiAsF6 additive. The narrow scan XPS spectra of the interface 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:

improved Li metal anode stability as well. Nevertheless, the potential instability of FEC in electrolytes would release HF,21 which could react with Li metal causing further Li metal consumption, thus slightly lowering Li CE (93.7%) as well as corroding the NMC cathode and 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. 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 without additives. All three electrolytes have common reduction peaks located at about 2.7, 2.0, 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 a 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 X-ray photoelectron spectroscopy (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%, respectively, while F ratio is increased to 21.6%. This 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 slightly increased C and O atomic ratio from VC reduction. This suggests VC addition will not 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 interface 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 interface layers formed at 1.0 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 and 41.6 eV. In contrast 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

LiAsF6 + Li+ + e− → As0 + LiF

(1a)

LiAsF6 + Li+ + e− + PC → Asx Oy + LiF + side products (1b) +

−

0

Asx Oy + Li + e → As + Li 2O

(2)

As0 + Li+ + e− → LixAs

(3)

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 the 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, obtaining uniform interface spatial distribution with LiAsF6 additive alone proves to be challenging. The initial interface spatial distribution patterns in different electrolytes were examined after the Cu 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 nonevenly 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 does not 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,d clearly indicates the greatly improved spatial control over nucleation, where numerous nanosized seeds were distributed very uniformly on the Cu substrate. These seeds would grow to 17

DOI: 10.1021/acsenergylett.7b00982 ACS Energy Lett. 2018, 3, 14−19

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should be noted that when Li deposition current density was increased above 0.5 mA cm−2, protrusion and even dendritic Li growth was found (Figure S8), which indicates that such an interface layer has a limited capability to suppress Li dendrite under high current density conditions. Further work is needed to improve the mechanical and the ionic conductive properties of the solid electrolyte interface layer for high-power applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b00982. Detailed description of experimental methods, Li CE test protocol scheme, 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 interface layers formed at different potentials in 1 M LiAsF6−PC electrolyte, SEM image of pristine Cu substrate, typical Li deposition particle sizes, and SEM images of Li deposition morphologies with LiAsF6 and VC additives at different current densities (PDF)

Figure 4. SEM images of the interface 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.

∼100−200 nm after 10 min of Li deposition at 0.1 mA cm−2 and cover the whole Cu surface uniformly (Figure 4e,f 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, because of 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 interface layer formed from VC reduction alone is less robust and could not lead to uniform Li deposition (Figure 1c,d), the incorporation of polymeric components into the interface 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 and growth as well as 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 the Li anode for long-life battery cycling. However, it

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Ji-Guang Zhang: 0000-0001-7343-4609 Wu Xu: 0000-0002-2685-8684 Present Address #

Y.Z.: Department of Physics, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China. Author Contributions §

X.R. and Y.Z. contributed equally to this work

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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. is grateful for the support from the National Science Foundation of China (21103037). The SEM and XPS measurements 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 DEAC05-76RLO1830.

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