Lanthanum Nitrate As Electrolyte Additive To Stabilize the Surface

Mar 16, 2016 - Matthew J. Lacey , Viking Österlund , Andreas Bergfelt , Fabian Jeschull , Tim Bowden , Daniel Brandell. ChemSusChem 2017 10 (13), 275...
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Lanthanum Nitrate As Electrolyte Additive To Stabilize the Surface Morphology of Lithium Anode for Lithium−Sulfur Battery Sheng Liu,† Guo-Ran Li,† and Xue-Ping Gao*,†,‡ †

Institute of New Energy Material Chemistry, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), School of Materials Science and Engineering, Nankai University, Tianjin 300350, China ‡ National Institute for Advanced Materials, Tianjin Key Laboratory of Metal and Molecule Based Material Chemistry, Nankai University, Tianjin 300350, China ABSTRACT: Lithium−sulfur (Li−S) battery is regarded as one of the most promising candidates beyond conventional lithium ion batteries. However, the instability of the metallic lithium anode during lithium electrochemical dissolution/deposition is still a major barrier for the practical application of Li−S battery. In this work, lanthanum nitrate, as electrolyte additive, is introduced into Li−S battery to stabilize the surface of lithium anode. By introducing lanthanum nitrate into electrolyte, a composite passivation film of lanthanum/lithium sulfides can be formed on metallic lithium anode, which is beneficial to decrease the reducibility of metallic lithium and slow down the electrochemical dissolution/deposition reaction on lithium anode for stabilizing the surface morphology of metallic Li anode in lithium−sulfur battery. Meanwhile, the cycle stability of the fabricated Li−S cell is improved by introducing lanthanum nitrate into electrolyte. Apparently, lanthanum nitrate is an effective additive for the protection of lithium anode and the cycling stability of Li−S battery. KEYWORDS: lithium−sulfur battery, electrolyte additive, lanthanum nitrate, surface morphology, lithium anode

1. INTRODUCTION With the growing demand for high-energy rechargeable batteries to power energy storage systems and unmanned aerial vehicles (UAV), lithium−sulfur battery has been considered as one of the most promising candidates beyond conventional lithium ion batteries.1−3 However, there are still many challenges for the practical application of Li−S battery. The main issues for sulfur cathode are related to the low conductivities of elemental sulfur and the final discharge products of Li2S/Li2S2. Meanwhile, the “shuttle effect” induced by the parasitic reaction between soluble intermediate lithium polysulfides and metallic lithium anode can lead to the loss of sulfur active material, resulting in the poor cycle stability of Li− S cells.1,4 In order to solve these problems for a better utility of sulfur cathode, continuous efforts have been conducted to confine soluble polysulfide species in the cathode. Effective methods are employed by confining sulfur into unique carbon materials that have different sized pores,5−7 interconnected structures or function groups.8,9 Polymer coating on S/C composites has also been proposed to restrict the dissolution of polysulfides.10−12 Recently, some metal oxides such as Ti4O7 and MnO 2 have been verified to anchor polysulfides effectively.13,14 The performance of sulfur cathodes is improved significantly by employing these materials as mentioned above. However, the stability of the metallic Li anode in the Li−S battery system remains a huge challenge, which is attracting more and more attentions recently.15 © XXXX American Chemical Society

As the most promising anode, metallic lithium with the high theoretical capacity of 3860 mA h g−1 has been investigated for many years. The formation of lithium dendrites during lithium electrochemical dissolution/deposition can lead to serious safety issues, which is regarded as one of the main obstacles for the practical applications of the metallic Li-based batteries.16 Interestingly, in the Li−S battery system with ether-based electrolyte, the soluble polysulfides in the electrolyte can react with lithium dendrites, and lithium dendrites are rarely identified.17 Therefore, with the wide application of etherbased electrolyte, the concern about lithium dendrites has been alleviated greatly. However, the continued growth and pulverization of the solid electrolyte interface (SEI) on the surface of lithium anode will result in the loss of lithium active material and the degradation of lithium anode during cycles, leading to the failure of Li−S cells finally.18 In addition, it is believed that the polysulfides can penetrate the passivation layer and react with the fresh metallic lithium anode to form some sulfur species that are unavailable in the following cycles, leading to fast capacity decay of the Li−S cell. Therefore, the problem of the degradation of lithium anode remains unsolved in the Li−S system. It is of high significance to stabilize the Received: December 15, 2015 Accepted: March 16, 2016

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DOI: 10.1021/acsami.5b12231 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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drying at 70 °C overnight. The cathode were punched into 1.13 cm−2 circular disks with the average sulfur mass loading of about 0.9 mg cm−2. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI, Acros, 1 M) in 1,2-dimethoxyethane (DME, J&K) and 1,3-dioxolane (DOL, J&K) (1:1, V/V) was used as the basic composition of the electrolyte. In order to suppress the shuttle effect, LiNO3 (2 wt %, J&K) was added into the basic electrolyte to form the common electrolyte. The experimental electrolyte with La3+ additive is obtained by adding lanthanum nitrate (2% in weight, J&K) to the common electrolyte. It is noted here that the usage of nitrate anions is preferred to prevent the possible interference by other anions, because the foreign anions could influence the release of metal cations in a specific solvent. The La(NO3)3 used as additive was prepared by careful heating of La(NO3)3·6H2O at 180 °C under vacuum to remove crystal water. 2.2. Electrochemical Measurements. The testing Li−S coin cells (CR2032) were assembled in an argon-filled glovebox, with metallic lithium foil anode, microporous polypropylene film (Celgard 2400) separator and the as-prepared sulfur cathode. Electrolytes with and without lanthanum nitrate were employed, respectively, to evaluate the effects of La(NO3)3 as additive. The galvanostatic discharge/charge measurements were conducted at 0.2 C between 1.8−2.6 V on a LAND-CT2001A battery testing system (Wuhan Jinnuo, China). Specific capacity values were calculated based on the mass of sulfur. CV test was performed on a Solartron SI 1287 electrochemical station with a scanning rate of 0.1 mV s1−. All the electrochemical tests were performed at ambient temperature. 2.3. Characterization of the Lithium Surface before and after Cycling. The Li−S coin cells after 100 discharge−charge cycles were disassembled in an argon-filled glovebox. The cycled anodes were retrieved by rinsing with dimethoxymethane (DME) thoroughly and drying in the glovebox. Morphology and elemental mapping were examined by scanning electron microscopy and energy-dispersive Xray spectroscopy (SEM/EDS, Zeiss Supra 55VP). Surface chemistry of the lithium metals before cycling and after 100 cycles in La(NO3)3 contained electrolyte were analyzed by X-ray photoelectron spectroscopy (XPS) in PHI-5000 VersaProbe with Mg Ka radiation of 1253.6 eV. A fresh Li foil was dipped into the experimental electrolyte with 2% lanthanum nitrate for 20 s and dried in Ar-filled glovebox for XPS characterization before cycling.

surface morphology of metallic lithium anode for improving the performance of Li−S battery. Compared with the SEI layer formed on the graphite anode in conventional lithium ion batteries, the SEI layer on the surface of lithium anode in the Li−S battery is more complicated due to the influence of soluble polysulfides in the ether-based electrolyte. The introduction of soluble sulfur species into electrolytes gives a new chance to revisit the previous ways for protecting metallic lithium anode in Li−S battery. Recent strategies to stabilize the metallic lithium anode in Li−S batteries include changing fluorinated ethers solvents,19,20 using lithium salt with high concentration,21 employing binary lithium salts,22,23 introducing functional additives into electrolyte,24,25 coating or in situ forming protecting layer on lithium anode,26−28 and mechanical surface modification of metallic lithium.29 These approaches have proven to be effective for the stabilization of lithium anode; however, building a stable interface on metallic lithium anode still remains unsolved. Introducing functional additives into electrolytes is the most facile way to modify the interface of metallic lithium for stabilizing lithium anode. In particular, the nitrate anion is demonstrated to be effective to reduce the shuttle effect in Li−S battery. However, the only nitrate anion in electrolyte cannot effectively stabilize the surface morphology of lithium anode due to the progressive consumption of the nitrate anion with the breakdown of the passivation film, as well as the regeneration of new passivation film during cycling.30 Metal cations as electrolyte additives can form metal or alloy layers on the lithium surface to improve the stability of lithium anode.16,31 However, the effects of metal cations as electrolyte additives in Li−S system still need to be further explored. In particular, the reducibility of metallic lithium is too strong, and electrochemical dissolution/deposition reaction on lithium anode is too fast, leading to a dramatic change of the surface morphology of lithium anode. Recently, Zu and Manthiram verify that copper acetate can be used as an effective electrolyte additive by forming CuS/Cu2S film on lithium surface, which is beneficial for the stabilization of lithium anode and the improvement of the cycle performance for Li−S cell.32 The effect of metal cations on the stabilization of lithium anode should be stressed in the polysulfide-rich environment. To our knowledge, rare earth metal ionic compounds have not yet been reported as electrolyte additives in Li−S battery until now. In this study, a facile strategy for stabilizing the surface morphology of Li anode by adding lanthanum nitrate (La(NO3)3) into the electrolyte is illustrated for the first time. The effect of adding La(NO3)3 into ether-based electrolyte on the surface morphology of metallic Li anode as well as the electrochemical performance of the fabricated Li−S cell is investigated.

3. RESULTS AND DISCUSSION To evaluate the effect of La(NO3)3 on the electrochemical performance of the Li−S cell, the sulfur/multiwalled carbon nanotubes/carbon black composite is used as cathode active material to fabricate the Li−S coin cells. SEM images show that multiwalled carbon nanotubes and carbon black are blended uniformly in the sulfur-based composite (Figure 1). The multiwalled carbon nanotubes are used here to improve the electronic conductivity of the sulfur cathode. However, the

2. EXPERIMENTAL SECTION 2.1. Preparations of Sulfur Cathode and Electrolytes. S/C composites were prepared by melt-diffusion approach. Specifically, porous carbon (PBX51, Cabot Corporation), multiwalled carbon nanotubes (0.5−2 μm, Nanjing XF Nano Inc.) and sublimed sulfur (Alfa Aesar, 99.5%) were mixed in the weight ratio of 2:1:7 by hand milling, and then the mixture was transferred into a sealed Teflon vessel filled with Ar gas and heat treated at 155 °C for 12 h, resulting sulfur/multiwalled carbon nanotubes/carbon black composites at last. Sulfur cathode was prepared by casting the N-methyl-2-pyrrolidone (NMP, J&K) slurry containing S/C composites, carbon black (Super P) and polyvinylidene fluoride (PVdF, Bamo, Tianjin) binder (7:2:1 by weight) onto a carbon-coated Al foil current collector and then

Figure 1. SEM image of the as-prepared sulfur/multiwalled carbon nanotubes/carbon black composites. B

DOI: 10.1021/acsami.5b12231 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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first cycle. The second and long potential plateau is located at 2.07 V (vs Li/Li+), indicating the formation of a passive film by parasitic reactions among La(NO3)3, polysulfides and Li anode. Meanwhile, the discharge capacity of the cell using common electrolyte decreases quickly from the initial capacity of 1067 mAh g−1 to 782 mAh g−1 in the 10th cycle, showing a fast decay. As for the experimental cell with La(NO3)3 in electrolyte, a conditioning process is observed, as shown in Figure 2b. The long potential plateau has recovered to 2.10 V in the 10th cycle, and the discharge capacity increases slowly from 862 mAh g−1 in the 1st cycle to 912 mAh g−1 in the 10th cycle. It means that the activation process is required in the electrolyte with La(NO3)3 additive due to the formation of the passive film on Li anode surface with cycling.33 To identify the redox reactions in the Li−S cell using La(NO3)3 as electrolyte additive, cyclic voltammograms (CVs) were conducted, and the results are shown in Figure 3. In the initial cathodic process, two obvious cathodic peaks are observed at around 2.24 and 1.98 V (vs Li/Li+), respectively. The two peaks could be assigned to the reduction of sulfur to high ordered intermediate lithium polysulfides and the further reduction of lithium polysulfides to insoluble Li2S2/Li2S. While the two obvious peaks are well-defined, an additional weak peak is also observed at 2.48 V (vs Li/Li+), which is marked in Figure 3a and enlarged in Figure 3b. However, this peak at 2.48 V (vs Li/Li+) disappears in the following cycle. On the basis of the hard and soft acids and bases theory, polysulfide anion (Sn2−) is a soft base, while La3+ cation is a less hard acid compared with Li ion. Therefore, in the experimental electrolyte with La3+, the combination of La3+ cations and polysulfide anions to form lanthanum polysulfides (La2(Sn)3) is more stable and preferred than lithium polysulfides.34 Once the first cathodic process commences, polysulfide anions would be immediately formed on the cathode. Lanthanum polysulfides can be easily formed with the existing lanthanum cations, since lanthanum polysulfides are more stable than lithium polysulfides under the conditions. The additional reduction at 2.48 V (vs Li/Li+) in the first scanning is attributed to the reduction of elemental sulfur to La2(Sn)3, where the combination of La3+ and Sn2− anion leads to a rise in the reduction potential. The normal reduction of elemental sulfur to Li2Sn at 2.24 V does not initiate until the La3+ ions are entirely combined into La2(Sn)3. It is

sulfur has not yet penetrated into the carbon pores completely, while some bulk sulfur still can be found stacking on the surface of the composites. The charge/discharge profiles of the Li−S cells in the electrolytes with and without La(NO3)3 are shown in Figure 2.

Figure 2. Typical charge/discharge profiles of the Li−S cells using (a) common electrolyte without lanthanum nitrate and (b) experimental electrolyte with lanthanum nitrate as additive.

A typical discharge curve can be observed in the first cycle for the cell in the common electrolyte without La(NO3)3, as shown in Figure 2a. There are two potential plateaus at 2.3 and 2.1 V (vs Li/Li+) in the discharge process, corresponding to the reduction from elemental sulfur to high-order lithium polysulfides and the sequential reduction to low order Li2S2/ Li2S, respectively. As compared to that of the cell using common electrolyte, the experimental cell in the electrolyte with La(NO3)3 shows a lower discharge potential plateau in the

Figure 3. (a) CVs in the first and second cycles of the Li−S cell using experimental electrolyte with lanthanum nitrate as additive at a scanning rate of 0.1 mV s1− and (b) an enlarged view of the selected potential range in the cathodic process. C

DOI: 10.1021/acsami.5b12231 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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To further investigate the effect of La(NO3)3 as electrolyte additive on the surface and cross-section morphologies of Li anode, SEM images are provided for Li anode after cycling in the electrolyte with and without La(NO3)3 (Figure 5). After

worth mentioning here that the CV test as shown in Figure 3 is the cathode character, and the distribution of elemental La on the lithium anode is investigated in the latter discussion. Figure 4 shows the cycle performance of the Li−S cells using electrolytes with and without lanthanum nitrate as additive at

Figure 4. Cycle performance of the Li−S cells in common electrolyte with LiNO3, experimental electrolyte with LiNO3 and La(NO3)3, and electrolyte with only La(NO3)3 as additive at 0.2 C rate.

Figure 5. SEM images of the surface and cross-section morphology of Li anode after 100 charge/discharge cycles in common electrolyte (a, c), and experimental electrolyte with lanthanum nitrate as additive (b, d).

the current rate of 0.2 C. It is clear that the cell using the common electrolyte suffers a fast decay from the beginning. The discharge capacity decreased from the initial 1067 to 782 mAh g−1 in the 10th cycle. The capacity retention is about 41.5% after 100 cycles. Generally, the fast capacity decay relates to the loss of the active sulfur, which arises from the irreversible depositing of Li2S2/Li2S and LixSOy on both sulfur cathode and Li anode. As for the experimental cell in the electrolyte with LiNO3 and La(NO3)3, on the contrary, an activation process is observed during the initial 10 cycles. The discharge capacity reaches the maximum value of 912 mAh g−1 in the 10th cycle, and then begins to decrease slowly. After 100 cycles, the discharge capacity drops to 553 mAh g−1 with the capacity retention of 64.2%. Apparently, the introduction of La(NO3)3 into electrolyte is effective for improving the cycle stability of sulfur cathode. It is well-known that the soluble polysulfides species in electrolyte can react with Li anode, and the formed products deposit on the surface of Li anode in the forms of LixSOy and Li2S2/Li2S, resulting in the loss of active sulfur.17 Even more, the resulting passivation film on Li anode is usually unstable due to the strong reducibility of metallic Li. Therefore, the passivation film is easily destroyed and reformed with cycling, further leading to the capacity fading of the cell. In the experimental cell with LiNO3 and La(NO3)3 in the electrolyte, the improved cycling stability is obtained, indicating the possible formation of more stable passive film of lanthanum sulfides on the surface of Li anode. The main reason for the capacity difference is due to the activation process that means the reforming of the passivation film on the Li surface. After several cycles of activation, the cycle performance of the Li−S cell in electrolyte with only La(NO3)3 as additive is similar to that of the experimental cell in electrolyte with mixed LiNO3/ La(NO3)3, indicating the difference of the cycle stability between the Li−S cells with LiNO3/La(NO3)3 is mainly due to introduction of La cations. Each of the three Li−S cells shows a high Coulombic efficiency, illustrating that the shuttle effect is mitigated effectively by the nitrate anion from either LiNO3 or La(NO3)3. However, the introduction of nitrate anion cannot effectively improve the surface morphology and reduce the degradation of lithium anode.

100 cycles, the surface of Li anode cycled in the common electrolyte is loose and mossy as shown in Figure 5a, which is attributed to the repeated cracking of passive film and reforming of new film on Li anode due to the fast dissolution/deposition reaction. The sulfur species, such as LixSOy and Li2S2/Li2S involved in the passive film, are partly responsible for the loss of active sulfur and capacity decay. Some other loss of active sulfur species are sacrificed on cathode, separator and even in electrolyte. After cycling in the experimental electrolyte with La(NO3)3, the Li surface is much more smooth with only a few of cracks as shown in Figure 5b. The smooth morphology is attributed to the formation of the stable passive film on Li anode by introducing La(NO3)3 into the electrolyte, which is also beneficial to decrease the strong reducibility of metallic Li to a certain extent, and ensure the relatively homogeneous dissolution/deposition processes on Li anode. SEM cross-section morphology of the Li anodes after 100 charge/discharge cycles is shown in Figure 5c and 5d. The thickness of the passivation layer on the surface of Li anode cycled in the common electrolyte is as high as 68 μm, while the thickness of the passivation layer is only about 24 μm for the Li anode cycled in the experimental electrolyte with La(NO3)3. The restrictive growth of the passivation film is originated from the reduced reducibility of metallic Li, meaning that the depletion of metallic lithium as well as sulfur species on the SEI film is depressed to some extent. More importantly, the smooth and compact surface morphology accompanied by the restrictive growth of the passivation film are effective to suppress the pulverization of Li anode, which is highly desired in Li metal-based secondary batteries. In order to determine the distribution of elemental lanthanum on the surface of Li anode, EDS mapping test was performed for the Li anode after cycling in the Li−S cell in the electrolyte with La(NO3)3, the results are shown in Figure 6. In the selected region, fluorine is uniformly distributed. The signal of F is believed to arise from LiF, which is a typical product resulted from the reaction between LiTFSI and metallic Li. D

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lies in 0.661 V (vs Li/Li+), theoretically, metallic La would be formed on the Li surface. To testify the assume, a fresh lithium foil was dipped into the experimental electrolyte with 2% La(NO3)3 and dried in Ar-filled glovebox for XPS characterization. As shown in Figure 7a, the signal of metallic La (835.9 eV) in XPS spectra indicates that the La3+ can be reduced by metallic lithium to form metallic La on the Li surface. However, there is no apparent signal of metallic La in XPS spectra after 100 cycles. La 3d5/2 core level on the Li surface after 100 cycles is shown in Figure 7b. The peaks at 832.4 and 838.0 eV can be attributed to the orbit of La 3d5/2 and its satellite peak of La2S3, respectively. Although the signal of La is weak due to the low content on the Li surface, the double peak structure of the spin−orbit split can still be observed, which is generally agreed with the reported literature.35 Based on the above results, we speculate a mechanism for the formation of La2S3 on the Li surface. The La3+ ions are reduced by metallic Li to metallic La and the formed La subsequently reacts with polysulfide anions to form La2S3. Compared with the signal of La, the core level of S 2p is very rich in peaks. The deconvolution of the broad peaks of S 2p to specific peaks reflects the chemical state change of the sulfur element. The peak assignments are indicated in Figure 7c. Here, the peak at 169.0 eV is attributed to the product of Li salt (LiTFSI) decomposition. The peak at 167.0 eV can be assigned to LixSOy, which is the product of oxidation of sulfur by LiNO3 in the electrolyte.17,36 The peaks within 165−158 eV are attributed to sulfides. Specifically, the peak with the maximum area at 161.8 eV reflects the formation of La2S3/Li2S2 on the Li anode surface, while Li2S is responsible for the peak at 159.8 eV.17 The contrast between the compressed Li2S peaks and the dominant La2S3/Li2S2 peaks shows an obvious effect on the formation of the composite film on the Li surface by introducing La(NO)3 into electrolyte. The formation process of the composite film with La2S3 is demonstrated by a significant activation process during cycling as shown in Figure 4, which leads to the capacity difference between the cells with and without La3+ in the electrolytes. As illustrated in Figure 8, after introducing La(NO3)3 into electrolyte, La3+ can be reduced immediately by Li to form metallic La that subsequently reacts with polysulfide anions to form La2S3 on the Li surface. The formation of La2S3 along with the deposition of Li2S2/Li2S and LixSOy fabricates the composite passivation film on the surface of Li anode. Because of the inactive nature of La2S3, the strong reducibility of

Figure 6. SEM image and EDS maps of Li anode surface after 100 charge/discharge cycles in experimental electrolyte with lanthanum nitrate as additive.

Sulfur species on the Li surface is predictable for the well understood depositions of Li2S2/Li2S and LixSOy. As for the lanthanum element, although the signal of La is weak, but still can be observed with nonuniform dispersion in the entire region. It is noted here, in some region as marked by arrows in the S and La mapping images, the distribution of the enhanced La signal matches well with that of sulfur, indicating the favorable combination of La and S elements on the Li surface. Here it is easily deduced that lanthanum element exists as lanthanum sulfides (La2S3), which are stable enough in the polysulfide-rich electrolyte. Therefore, the lanthanum sulfides are preferentially formed on the Li surface in the polysulfiderich electrolyte. Meanwhile, the rich distribution of sulfur as compared with lanthanum illustrated that lithium sulfides also deposited together with lanthanum sulfides, forming stable and passive composite film on Li anode. The formation of lanthanum sulfides and lithium sulfides is also verified in the following characterization of the surface chemistry on Li anode. In order to deeply understand the surface chemistry of Li anode, X-ray photoelectron spectroscopy (XPS) is characterized to examine the existing states of elemental lanthanum and sulfur on the Li surface. The standard potential for La/La3+

Figure 7. La 3d5/2 XPS spectra of the Li foil surface after dipping into the experimental electrolyte with 2% La(NO3)3 and drying in an Ar-filled glovebox (a). XPS spectra of the Li electrode after 100 cycles in the electrolyte with La(NO3)3 as additive: (b) La 3d5/2 and (c) S 2p core levels. E

DOI: 10.1021/acsami.5b12231 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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stable and passive composite film on the surface of Li anode, leading to the smooth morphology of Li surface and enhanced cycling stability of the fabricated Li−S cell during cycling. More importantly, this work indicates the potential benefits by revisiting metal cations as electrolyte additive in Li−S battery system.

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AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected]. Tel: +86-22-23500876. Fax: +86-22-23500876.

Figure 8. Schematic illustration of the formed passive composite film on the surface of Li anode by adding La(NO3)3 into the electrolyte.

Notes

The authors declare no competing financial interest.

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metallic Li beneath the composite film could be decreased to a certain extent. Meanwhile, Li ions can diffuse via Li2S2/Li2S and LixSOy layer in the composite passivation film in charge/ discharge processes. The insoluble lanthanum sulfides existing in the layer are helpful to form composite passivation film as Li ion conducting phase due to the good ionic conductivity of LixLa2S3.37 Similarly, the ionic conductivity of sulfide-based solid electrolyte can be also improved by incorporating lanthanum sulfides.38 Therefore, the formation of the stable and passive composite film is beneficial to slow down the dissolution/deposition reaction on Li anode for stabilizing the surface morphology of metallic Li anode and improving the cycle stability of Li−S cell. It is worth mentioning that the existing state of metal cations, particularly for the divalent and multivalent cations, in the ether-based electrolyte for Li−S cell may be different from that in conventional Li-based batteries. A strong Lewis acid of multivalent cations can catalyze DOL ring-opening polymerization, especially for the addition of Al3+ cations with small ionic radius as Lewis acid into the electrolyte. However, the electrolyte with La3+ cations is stable, indicating that the Lewis acidity of La3+ cations with large ionic radius is not strong enough to catalyze the ring-opening polymerization of DOL due to the small ion charge/ion radius (e−/Å) for La3+ cations as compared with Al3+ cations.39 Meanwhile, metal cations tend to form inactive metal sulfides in the polysulfide-rich environment in Li−S cell, depositing on the surface of the electrodes. It is also noted that insoluble lanthanum sulfides should be formed on both cathode and anode. The formation process of lanthanum sulfides arising from metallic La and polysulfides on the anode side can result in the slow activation as shown in Figure 4, while the composite passivation film with insoluble lanthanum sulfides on the surface of Li anode is beneficial to stabilize the surface morphology of Li anode. Similarly, the formation of the CuS/Cu2S passivation film was also observed on Li surface in electrolyte with copper acetate as additive.32 Thus, the modified interface is highly anticipated for the improvement of the performance of Li−S cell. From this view of point, the effect of metal cations as electrolyte additive on both the surface morphology of Li anode and the electrochemical performance of Li−S cell need to be stressed in the future work.

ACKNOWLEDGMENTS Financial support from the 973 Program (2015CB251100) and NSFC (51502145, 21573114, and 21421001) of China are gratefully acknowledged.

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REFERENCES

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4. CONCLUSIONS In summary, lanthanum nitrate is first introduced into the electrolyte as additive for Li−S battery in this work. The lanthanum element exists as lanthanum sulfides after activation on the surface of lithium anode. The resulting lanthanum sulfides are incorporated into Li2S2/Li2S and LixSOy to form the F

DOI: 10.1021/acsami.5b12231 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

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DOI: 10.1021/acsami.5b12231 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX