Letter pubs.acs.org/JPCL
Activated Li2S as a High-Performance Cathode for Rechargeable Lithium−Sulfur Batteries Chenxi Zu, Michael Klein, and Arumugam Manthiram* Materials Science and Engineering Program & Texas Materials Institute, The University of Texas at Austin, 1 University Station C2200, Austin, Texas 78712, United States S Supporting Information *
ABSTRACT: Lithium−sulfur (Li−S) batteries with a high theoretical energy density of ∼2500 Wh kg−1 are considered as one promising rechargeable battery chemistry for next-generation energy storage. However, lithium−metal anode degradation remains a persistent problem causing safety concerns for Li−S batteries, hindering their practical utility. One possible strategy to circumvent the aforementioned problems is to use alternative, high-capacity, lithium-free anodes (e.g., Si, Sn, carbon) and a Li2S cathode. However, a large potential barrier was identified on the initial charge of insulating bulk Li2S particles, limiting the cell performance. In this work, the bulk Li2S particles were effectively activated with an electrolyte containing P2S5, resulting in a lowered initial charging voltage plateau. This permits the direct use of commercially available bulk Li2S particles as a high-capacity cathode for room-temperature, rechargeable Li−S batteries, significantly lowering the manufacturing cost of Li−S cells. SECTION: Energy Conversion and Storage; Energy and Charge Transport
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commercially available bulk Li2S particles for their direct use in room-temperature rechargeable Li−S batteries without applying a high charging cut-off voltage. It is found that the electrolyte with the P2S5 additive effectively enhances the electrochemical activity of Li2S. The activated Li2S cathode exhibits a greatly lowered initial charging voltage plateau, indicating effective oxidation of Li2S to polysulfides.25 Furthermore, the activation correlates to the formation of intermediate sulfur- and phosphorus-containing species on the surface of Li2S, assisting the surface charge transfer. Figure 1a compares the initial charging voltage profiles of bulk Li2S particles pretreated in tetraethylene glycol dimethyl ether (TEGDME) without P2S5 (defined as pristine Li2S) and with P2S5 (defined as P2S5-activated Li2S). In contrast with the pristine Li2S, the P2S5-activated Li2S cathodes demonstrate lower initial charging voltage plateaus. Furthermore, the lowest charging voltage plateau appears when the molar ratio between Li2S and P2S5 is 7:1, resulting in an activation of ∼80% of the theoretical capacity of Li2S below 3.0 V at C/10 (1 C = 1166 mAh g (Li2S)−1). Considering that Li2S or P2S5 alone lacks electrochemical activity before charging above 3.0 V (see the voltage profiles of the pristine Li2S and P2S5 in the first cycle in Figure S1 Supporting Information), we believe the accessible charge capacity below 3.0 V is attributed to the interaction between P2S5 and Li2S, leading to Li2S activation. However, further increase in the concentration of P2S5 in the electrolyte (when the molar ratio between Li2S and P2S5 is 5:1 or 3:1)
ithium−sulfur (Li−S) batteries, with a high theoretical specific energy of ∼2500 Wh kg−1, are considered to be a promising alternative to lithium−ion (Li−ion) batteries as the next generation of rechargeable batteries.1−6 Despite significant progress in recent years,7−18 degradation of lithium−metal anode remains a major challenge for Li−S batteries, hindering their practical utility.19−21 Although metallic lithium has a high theoretical capacity of ∼3860 mAh g−1, it suffers from dendrite formation and parasitic side reactions with the organic electrolyte and dissolved polysulfide species, resulting in instabilities and safety concerns for Li−S batteries. One possible strategy to circumvent the aforementioned problems is to use alternative, high-capacity, lithium-free anodes (e.g., Si-, Sn-, or carbon-based anodes) coupled to a lithium-containing sulfur-based cathode. In this context, Li2S with a theoretical capacity of ∼1166 mAh g−1, has attracted much attention as a prospective cathode material. However, a large potential barrier (∼1 V) was identified on the initial charge of insulating bulk Li2S particles, which was believed to relate to the nucleation of a new polysulfide phase.22 The magnitude of this overpotential is likely influenced by kinetic factors such as the electronic conductivity of Li2S, lithium-ion diffusivity in Li2S, and charge transfer at the surface of Li2S particles.22 The large voltage barrier can be overcome by applying a high initial charging cutoff voltage of ∼4.0 V,22 but it causes instabilities in the commonly used ether-based electrolytes and deteriorates the electrochemical performance.23 It has recently been reported that P2S5 reacts with Li2S in ether-based electrolytes.24 In this work, we systematically investigate the interaction between P2S5 and Li2S and identify the possibility of using P2S5 as an electrolyte additive to activate © 2014 American Chemical Society
Received: October 3, 2014 Accepted: October 31, 2014 Published: October 31, 2014 3986
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Figure 1. (a) Initial charging voltage profiles and (b) electrochemical impedance spectra of the pristine Li2S- and P2S5-activated Li2S cathodes when the molar ratio between Li2S and P2S5 is 10:1, 7:1, 5:1, and 3:1.
produces a gradual increase in the charging voltage plateau, likely due to the increased cell resistance. The increase in cell resistance is also evidenced by electrochemical impedance spectroscopy (EIS). The impedance measurement was conducted before charging. In Figure 1b, the cell resistance is constant with P2S5 addition until the molar ratio between Li2S and P2S5 is 7:1, and the cell exhibits its lowest resistance, indicating an accelerated charge-transfer process,26 consistent with its initial charging voltage plateau exhibiting the most effective activation. It is noted that despite a lack of significant decrease in cell resistance when the molar ratio between Li2S and P2S5 is 10:1, there is still a dramatic decrease in the activation overpotential (Figure 1a), demonstrating that the effect of P2S5 addition is not limited to its influence on the electronic or ionic conductivity of the cell. The cell resistance grows with further addition of P2S5 (when the molar ratio between Li2S and P2S5 is 5:1 or 3:1), indicating increased resistance from passivation layer formation, inhibiting the charge-transfer process or increased viscosity of the electrolyte, limiting ionic conductivity. The high-frequency region of the impedance profile is shown in Figure S2 (Supporting Information). Scanning electron microscopy (SEM) characterization was conducted to examine the morphology variation of the activated Li2S, with typical particle morphologies shown in Figure 2. Pristine Li2S appeared as irregular bulk particles with a rough surface (Figure S3 Supporting Information). In Figure 2a, a spherical morphology was observed for Li2S after suspending in TEGDME with P2S5 (molar ratio of Li2S:P2S5 is 10:1). With increasing concentration of P2S5 (when the molar ratio between Li2S and P2S5 is 7:1), the spherical morphology is retained, but some dark spots are observed on the surface of the Li2S particles (Figure 2b). In addition, when the molar ratio between Li2S and P2S5 is 5:1, a core−shell-like morphology appeared (Figure 2c). EDS elemental mapping confirms the core−shell contrast and demonstrates the existence of a sulfur-rich core, as seen in the EDS line scan in Figure S4 (Supporting Information). In addition, Figure 2d shows the scanning transmission electron microscopy (STEM) image of one selected Li2S particle at this concentration, where morphological details can be identified. Moreover, lowmagnification SEM images showing Li2S particle distribution at different P2S5 concentrations were analyzed by ImageJ and are shown in Figure S5 (Supporting Information). It can be seen that the Li2S particles exhibited a small increase in average particle size with slight addition of P2S5 to the electrolyte. It is obvious that the typical Li2S particles are micron-sized when the molar ratio between Li2S and P2S5 is more than 5:1. Obvious decrease in Li2S particle size occurs when the molar
Figure 2. SEM images of the P2S5-activated Li2S particles when the molar ratio between Li2S and P2S5 is (a) 10:1, (b) 7:1, and (c) 5:1. (d) STEM image of a single Li2S particle at the composition of panel c.
ratio of Li2S:P2S5 reaches 3:1, which is the stoichiometric ratio for the reaction to form Li3PS4.27 The corresponding SEM image is presented in Figure S6 (Supporting Information). To verify the structural change in the bulk core of the Li2S cathode with the gradual addition of P2S5, we performed X-ray diffraction (XRD) characterization, with the results shown in Figure 3a. It can be seen from Figure 3a that when the molar ratio between Li2S and P2S5 is 7:1, the P2S5-acivated Li2S samples exhibit characteristic Li2S reflection peaks at 2θ = 27, 31, 45, 53, 56, 65, 72, and 74°.28 The broad amorphous peak at low angles is from the Kapton film used to protect the samples. The activated Li2S exhibits large particle size and crystalline structure, in contrast with the electrochemically obtained Li2S after the first discharge, which possesses an amorphous XRD pattern (Figure S7 Supporting Information). The characteristic peaks of crystalline Li2S are retained when the molar ratio between Li2S and P2S5 is increased to 5:1. With increasing P2S5 addition into the electrolyte (when the molar ratio between Li2S and P2S5 is 3:1), the activated Li2S exhibits an amorphous XRD pattern, with only one obvious peak at 26° related to the carbon substrate underneath.29 This indicates a major structural change in the bulk core of the Li2S cathode. To further understand the activation effect, we investigated surface properties of the pristine P2S5 and the P2S5-activated Li2S cathodes by X-ray photoelectron spectroscopy (XPS), with the results shown in Figure 3b,c. In Figure 3b, when the molar 3987
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Figure 3. (a) XRD patterns of the P2S5-activated Li2S when the molar ratio between Li2S and P2S5 is 7:1, 5:1, and 3:1. XPS S 2p (b) and P 2p (c) region spectra of P2S5 and the P2S5-activated Li2S when the molar ratio between Li2S and P2S5 is 10:1, 7:1, 5:1, and 3:1.
Figure 4. (a) CV profiles, (b) discharge/charge voltage profiles (C/10, 1C = 1166 mAh g (Li2S)−1), and (c) cycling performance of the P2S5activated Li2S (C/10). CE refers to Coulombic efficiency.
species and not simply the deposition of P2S5. This is confirmed by the P 2p signal (Figure 3c). In contrast with the P 2p 3/2 peak in P2S5 that is centered at 133.9 eV, in the activated Li2S samples with Li2S:P2S5 ratios of 10:1, 7:1, and 5:1, the P 2p 3/2 peak is centered at 131.9 eV. With increasing P 2 S 5 concentration (i.e., the molar ratio of Li2S:P2S5 is 3:1), the S 2p signal exhibits new intensity above 163 eV, and the P 2p signal is greatly altered with multiple peaks in the deconvolution. This suggests a complete change in the particles’ surface composition, in agreement with the structural changes observed by XRD and SEM. The existence of sulfurand phosphorus-containing intermediate species at the Li2S surface in electrolytes containing P2S5 is further evidenced by Raman spectroscopy, as shown in Figure S8 (Supporting
ratio between Li2S and P2S5 is 10:1, the S 2p region spectra can be deconvolved into three signals, with S 2p 3/2 peaks centered at 159.8, 161.0, and 162.6 eV. The 159.8 eV peak is characteristic of Li2S, indicating it is still present at the particle surface at this composition.30,31 The 159.8 eV Li2S peak is still present when the Li2S:P2S5 ratio is 7:1 but disappears when the ratio reaches 5:1, indicating a change in surface composition. The peaks at 161.0 and 162.6 eV are consistent with the binding energies associated with sulfur−sulfur bonds, as in polysulfide species, as well as sulfur−phosphorus bonds.32 The S 2p spectra for P2S5 contain S 2p 3/2 peaks centered at 162.1 and 163.5 eV; the lack of any higher binding energy peaks above 163 eV in the P2S5-activated Li2S surface indicates the formation of a new sulfur- and phosphorus-containing surface 3988
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Coulombic efficiency of the pristine Li2S cathode far exceeded 100% in the first few cycles, it appears that the dominant capacity loss mechanism in the pristine sample is the material dissolving into the electrolyte on charge and becoming electrochemically inaccessible. Problematic polysulfide dissolution and migration are not as serious in the activated Li2S, considering its superior capacity retention. This is further confirmed by the SEM/EDS image of the lithium−metal surface after 100 cycles in Figure S9 (Supporting Information). It can be seen that the sulfur-containing species exist on lithium−metal surface; however, pristine lithium is also present. The controlled capacity loss could additionally be the result of surface species formed (e.g., Li3PS4) acting as an anion barrier, limiting polysulfide dissolution, particularly in the initial cycles. Moreover, Li2S is more easily converted to polysulfide species during charge because of the enhanced charge transfer resulting from P2S5 activation. The absence of insulating bulk Li2S particles after charge results in better cathode conductivity; therefore, more homogeneous Li2S deposition can be expected during discharge, ensuring good electrochemical contact between the redeposited Li2S and the conductive matrix. The redeposited Li2S is electrochemically accessible, which is beneficial to capacity retention; cathode morphology after the 100th charge is shown in Figure S10 in the Supporting Information).39−41 In summary, we have demonstrated in this work that commercially available bulk Li2S particles can be effectively activated by an electrolyte containing P2S5 additive, resulting in a room-temperature, high-capacity cathode without the initial charging barrier. The interaction between P2S5 and Li2S results in reduced cell resistance and enhanced surface oxidation of Li2S to polysulfides, significantly lowering the initial charging voltage plateau. Morphological, structural, and compositional changes in Li2S are observed with the gradual addition of P2S5 into the conventional TEGDME electrolyte. The most effective electrochemical activation occurs when the molar ratio between Li2S and P2S5 is 7:1, before a thick solid electrolyte layer forms on the surface of Li2S. Because the core structure of the P2S5activated, micron-sized Li2S is retained when the most effective activation occurs, the activation is mainly a surface effect. Sulfur- and phosphorus-containing surface species are identified, which may serves as active sites facilitating the surface charge transfer and Li2S oxidation. Reversible capacities of ∼800 mAh g (Li2S)−1 are obtained. This facile activation enables the direct use of commercially available bulk Li2S particles as high-capacity cathode materials for rechargeable Li−S batteries, without intricate synthesis or application of a high charging cut-off voltage that deteriorates the electrolyte stability and safety of Li−S batteries. This significantly decreases the manufacturing cost of Li−S batteries with a Li2S cathode. We believe this strategy is of great significance for the safe and effective use of Li2S as a cathode material and is a promising step toward the low-cost fabrication of metallic lithium-free Li−S batteries.
Information). In contrast with pristine Li2S showing one major peak at ∼375 cm−1, which corresponds to Li−S bond stretching, a weak shoulder peak at ∼425 cm−1 corresponding to the stretching vibration of the P−S bonds in PS43− ion (Li3PS4) is identified in the P2S5-activated Li2S (with molar ratio of 7:1 between Li2S and P2S5).33,34 It is worth mentioning that Li3PS4 has good chemical stability and is regularly used as a solid electrolyte material for all-solid-state lithium batteries. Additionally, nanoporous β-Li3SP4 particles synthesized by a wet chemical method have been reported to have three orders of magnitude higher ionic conductivity than that of crystalline Li3PS4 obtained through the conventional method.35−37 Considering the SEM and XRD results evidencing the retained core structure of Li2S with the slight addition of P2S5 and the XPS and Raman results showing the formation of intermediate surface species, we believe that these sulfur- and phosphoruscontaining surface species accelerate surface charge transport and thus Li2S oxidation, thereby resulting in a greatly decreased cell resistance and lowered charging voltage plateau. This is in agreement with previous work that surface charge transfer dominates the Li2S activation.22 Detailed results of the surface composition analysis are shown in Table S1 in the Supporting Information. Electrochemical performance of the Li2S cathode in electrolyte with P2S5 was examined with CR2032 coin cells, with the results shown in Figure 4 for the cell where the molar ratio between Li2S and P2S5 is 7:1. Figure 4a presents the cyclic voltammogram (CV) profiles, revealing one anodic shoulder peak at ∼2.3 V and a main peak at 2.6 V in the first charge from the open-circuit voltage (OCV) to 4.0 V. The shoulder peak at ∼2.3 V, which is likely related to the existence of intermediate sulfur- and phosphorus−containing surface species, disappears in the second charge, and only one major peak at ∼2.46 V exists corresponding to the oxidation of Li2S to polysulfides or sulfur. The anodic peak split into dual peaks at ∼2.43 and ∼2.53 V in the following charge (third charge) corresponding to, respectively, the transitions from Li2S to lower order polysulfides (e.g., Li2S4) and from lower order polysulfides to higher order polysulfides/sulfur.38 Because the major redox reactions are proceeding below 3.0 V, cell performances with the P2S5-activated Li2S cathodes were evaluated between 1.8 and 3.0 V at a rate of C/10. Figure 4b demonstrates the discharge−charge voltage profiles in the first two cycles. One main plateau at ∼2.5 V related to the transition from Li2S to higher order polysulfides/sulfur is identified in the first charge cycle, in agreement with the CV profile. During discharge, typical sulfur redox reactions are identified, where the higher voltage plateaus at ∼2.4 and 2.3 V together with the ensuing sloping region correlate to transitions from higher order polysulfides/sulfur to Li2S4, and the lower voltage plateau at ∼2.0 V corresponds to transition from Li2S4 to Li2S/Li2S2.38 Discharge voltage profiles overlap between the first two cycles, suggesting the high reversibility of the cathode materials between 1.8 and 3.0 V. Cycling performance of the activated Li2S cathode is shown in Figure 4c. Reversible discharge capacities of ∼800 mAh g (Li2S)−1 are obtained and the capacity retention is as high as 83% after 80 cycles. Moreover, Coulombic efficiency remains near 100% with cycling. This is in contrast with the pristine Li2S that delivers limited reactivity and inferior capacity due to the incomplete oxidation reaction below 3.0 V. The incomplete oxidation results in insulating bulk Li2S particles remaining in the cathode after charge, decreasing the cathode conductivity. Furthermore, considering that the
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ASSOCIATED CONTENT
S Supporting Information *
Experimental methods, supplemental electrochemical performance, SEM images, XRD results, Raman spectra, and XPS results. This material is available free of charge via the Internet at http://pubs.acs.org. 3989
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award Number DE-SC0005397. C.Z. acknowledges Kang Yu for his assistance in image processing. M.K. acknowledges the support of the National Science Foundation Graduate Research Fellowship Program under grant number DGE-1110007.
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