Anode-free, Lean-Electrolyte Lithium-Sulfur Batteries Enabled by Tellurium-Stabilized Lithium Deposition

•The cycling efficiency of Li-metal anode determines the cycle life of Li-S batteries•7-fold improvement in cyclability is seen by introducing Te at the cathode•In situ generated lithium thiotellurate-based SEI stabilizes Li plating and stripping•The unique SEI improves longevity of high-energy, lean-electrolyte pouch cells In order to maximize the specific energy of lithium-sulfur (Li-S) batteries, the cell must be operated without any excess Li. In such a Li-limited system, the loss of Li inventory is the primary determinant of cycle life. The introduction of tellurium (Te) as an additive in Li-S batteries engenders a unique tellurized and sulfide-rich solid-electrolyte interphase (SEI) comprising lithium thiotellurate (Li2TeS3) on the Li surface. This type of SEI stabilizes Li deposition and ensures deposition of dense Li layers at the anode. This prevents electrolyte decomposition, curtails Li loss, and hence extends cycle life. The anode-free full-cell configuration provides a reliable and robust framework for evaluating the dynamics of Li deposition in conjunction with various cathode systems. Additionally, a class of ternary sulfides similar to Li2TeS3 can be explored as artificial SEI layers to enable the stable operation of energy-dense, anode-free Li batteries. For realizing practically viable lithium-sulfur (Li-S) batteries, it is imperative to stabilize Li deposition and improve cyclability while reducing excess Li and electrolyte. We have discovered that introducing tellurium (Te) into the Li-S system as a cathode additive significantly improves the reversibility of Li plating and stripping by forming a tellurized and sulfide-rich solid-electrolyte interphase (SEI) layer on the Li surface. A remarkable improvement in cyclability is demonstrated in anode-free full cells with limited Li inventory and large-area Li-S pouch cells under lean electrolyte conditions. Tellurium reacts with polysulfides to generate soluble polytellurosulfides that migrate to the anode side and form stabilizing lithium thiotellurate and lithium telluride in situ as SEI components. A significant reduction in electrolyte decomposition on the Li surface is also engendered. This work demonstrates Te inclusion as a viable strategy for stabilizing Li deposition and establishes a robust evaluation framework for preserving electrochemical performance under limited Li and limited electrolyte conditions. For realizing practically viable lithium-sulfur (Li-S) batteries, it is imperative to stabilize Li deposition and improve cyclability while reducing excess Li and electrolyte. We have discovered that introducing tellurium (Te) into the Li-S system as a cathode additive significantly improves the reversibility of Li plating and stripping by forming a tellurized and sulfide-rich solid-electrolyte interphase (SEI) layer on the Li surface. A remarkable improvement in cyclability is demonstrated in anode-free full cells with limited Li inventory and large-area Li-S pouch cells under lean electrolyte conditions. Tellurium reacts with polysulfides to generate soluble polytellurosulfides that migrate to the anode side and form stabilizing lithium thiotellurate and lithium telluride in situ as SEI components. A significant reduction in electrolyte decomposition on the Li surface is also engendered. This work demonstrates Te inclusion as a viable strategy for stabilizing Li deposition and establishes a robust evaluation framework for preserving electrochemical performance under limited Li and limited electrolyte conditions. The lithium-sulfur (Li-S) couple holds tremendous potential for enabling the next generation of high-energy density rechargeable batteries, combining the large gravimetric capacities of S (1,675 mA h g−1) and Li (3,861 mA h g−1).1Manthiram A. Chung S.H. Zu C. 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Alkali-metal anodes: From lab to market.Joule. 2019; 3: 2334-2363Abstract Full Text Full Text PDF Scopus (172) Google Scholar In Li-S batteries, the presence of soluble and highly reactive polysulfide intermediates in the ether-based electrolyte acutely impacts Li deposition and renders its characteristics fundamentally distinct from other systems.17Zhao C.-Z. Cheng X.-B. Zhang R. Peng H.-J. Huang J.-Q. Ran R. Huang Z.-H. Wei F. Zhang Q. Li2S5-based ternary-salt electrolyte for robust lithium metal anode.Energy Storage Mater. 2016; 3: 77-84Crossref Scopus (213) Google Scholar,18Cheng X.-B. Huang J.-Q. Zhang Q. Review—li metal anode in working lithium-sulfur batteries.J. Electrochem. Soc. 2018; 165: A6058-A6072Crossref Scopus (197) Google Scholar This necessitates bold new strategies toward improving Li deposition in Li-S batteries that account for their unique chemistry and ensure compatibility with polysulfide species. Stabilized Li deposition will also help mitigate substantially the safety concerns associated with Li dendrites causing internal shorting and catastrophic failure of the battery. In this work, we demonstrate that introducing elemental tellurium (Te0) as an additive in the sulfur or Li2S cathode leads to a dramatic improvement in the reversibility of the Li-metal anode. Both anode-free full cells (limited Li) and large-area pouch cells (limited electrolyte) show significant improvement in cyclability. At the cathode, Te0 is oxidized and incorporated by the generated polysulfides (Li2Sn) to form soluble polytellurosulfide species (Li2TexSy). The migration of the formed Li2TexSy to the anode side and their reduction on the deposited Li help form a novel bilayer SEI structure in situ, consisting of Li2TeS3 and Li2Te. Compared with Li2S, which is the corresponding SEI component in a control system, the Te-containing SEI species confer considerable advantages for stabilizing Li deposition. 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The elimination of free Li metal and excess Li inventory leads to a significant enhancement in energy density and alleviates many safety concerns. More importantly, it allows electrochemical performance to be constrained entirely by the efficiency of Li plating and stripping. Capacity fade can now be used to model Li inventory loss rates (LILR) (Figure S1). This makes anode-free full cells excellent templates for realistic evaluation of cyclability in Li-metal batteries. The rapid capacity fade of the anode-free Ni||Li2S full cell (0% excess Li) compared with the Li||Li2S half-cell (3,300% excess Li) at C/5 (1 mA cm−2) clearly demonstrates the irretrievable loss of Li inventory with cycling and the impact of a limited Li inventory on electrochemical performance (Figure 1B). The LILR is calculated to be 2.02% per cycle. X-ray photoelectron spectroscopy (XPS) analysis of the deposited Li sheds light on the changes in SEI layer composition accompanying the loss of Li inventory (Figure 1C). It reveals a transition from reduced-sulfur (Li2S2 and Li2S, between 159 and 165 eV) to oxidized-sulfur (SO32−, SO42−, between 165 and 171 eV) species with cycling. The initial formation of reduced-sulfur species is attributed to polysulfide (Li2Sn) decomposition and is known to intrinsically stabilize Li deposition in Li-S batteries.18Cheng X.-B. Huang J.-Q. Zhang Q. Review—li metal anode in working lithium-sulfur batteries.J. Electrochem. Soc. 2018; 165: A6058-A6072Crossref Scopus (197) Google Scholar,29Nanda S. Gupta A. Manthiram A. A lithium–sulfur cell based on reversible lithium deposition from a Li2S cathode host onto a hostless-anode substrate.Adv. Energy Mater. 2018; 8: 1801556Crossref Scopus (59) Google Scholar,30Li W. Yao H. Yan K. Zheng G. Liang Z. Chiang Y.M. Cui Y. The synergetic effect of lithium polysulfide and lithium nitrate to prevent lithium dendrite growth.Nat. Commun. 2015; 6: 7436Crossref PubMed Scopus (1102) Google Scholar It is particularly effective at low current densities, as can be inferred from the much smaller LILR (0.38% per cycle) at C/10 rate (0.5 mA cm−2) (Figure S1). However, this intrinsic stabilization due to the presence of Li2S and Li2S2 is not adequate to enable high-efficiency plating and stripping of Li at the elevated current density of 1 mA cm−2 (C/5). The implied inhomogeneous deposition of Li also engenders severe side reactions with the electrolyte. This was confirmed by XPS, which shows the replacement of Li2S and Li2S2 by electrolyte salt (lithium bis(trifluoromethanesulfonyl)imide, or LiTFSI) decomposition products with cycling. These side reactions not only consume available Li, but the electrolyte decomposition products can prevent electronic and ionic access to the trapped metallic or “dead” Li. This leads to further degradation and eventual irreversible depletion of Li inventory.30Li W. Yao H. Yan K. Zheng G. Liang Z. Chiang Y.M. Cui Y. The synergetic effect of lithium polysulfide and lithium nitrate to prevent lithium dendrite growth.Nat. Commun. 2015; 6: 7436Crossref PubMed Scopus (1102) Google Scholar,31Cheng X.-B. Yan C. Peng H.-J. Huang J.-Q. Yang S.-T. Zhang Q. Sulfurized solid electrolyte interphases with a rapid Li+ diffusion on dendrite-free Li metal anodes.Energy Storage Mater. 2018; 10: 199-205Crossref Scopus (189) Google Scholar We hypothesize that modifying the reduced-S species and engineering a stable sulfide-rich SEI layer, such that the reversibility of Li deposition and resilience to electrolyte decomposition is improved when compared with Li2S and Li2S2, could be the key to extending the cyclability of Li-metal anodes in Li-S batteries at practical current rates. One potential pathway to engineering a stable sulfide-rich SEI layer is to replace the binary sulfide species (Li2S and Li2S2) with ternary sulfide species of the general formula LiaXbSc, where X is a high-oxidation state cation of an element less electronegative than S. By appropriately choosing element X, the properties of the reduced-S SEI components could be modified toward stabilizing Li deposition. One particularly attractive candidate element for X is Te. Tellurium and S share a similar chemistry as Group 16 chalcogens and form ether-soluble catenated compounds,32Devillanova F.A. Du Mont W.-W. Handbook of Chalcogen Chemistry: New Perspectives in Sulfur, Selenium and Tellurium.First Edition. Royal Society of Chemistry, 2013Google Scholar potentially enabling facile incorporation of Te into the sulfide-rich Li SEI. Moreover, Te has a lower electronegativity than S and forms more polarizable ions due to its larger size and enhanced shielding effect, potentially enabling higher Li+-ion conductivity of the tellurized sulfide-rich interphases on Li surface.33Lang P.F. Smith B.C. Ionic radii for group 1 and group 2 halide, hydride, fluoride, oxide, sulfide, selenide and telluride crystals.Dalton Trans. 2010; 39: 7786-7791Crossref PubMed Scopus (64) Google Scholar,34Chen S. Xie D. Liu G. Mwizerwa J.P. Zhang Q. Zhao Y. Xu X. Yao X. Sulfide solid electrolytes for all-solid-state lithium batteries: structure, conductivity, stability and application.Energy Storage Mater. 2018; 14: 58-74Crossref Scopus (277) Google Scholar A possible approach to forming the Te-containing reduced-S SEI components is substituting Te for some of the S atoms in the polysulfide (Li2Sn) chain and allowing their reduction on the Li surface. In attempting to synthesize Te-substituted polysulfide species, we discovered that Te0 spontaneously reacts with ethereal solutions of polysulfides to form soluble polytellurosulfide (Li2TexSy) species. Figure 2A shows the change in color from yellow to red when a 0.02 M Li2S6 solution in 1,3-dioxolane and 1,2-dimethoxyethane (DOL/DME) is reacted with excess Te0 powder undisturbed at room temperature overnight. UV-vis spectra of the two solutions confirm the reaction of polysulfides (Figure S2). The residue left after drying the supernatant from the red-colored [Li2S6 + Te] solution on an inert silicon substrate was analyzed by time-of-flight secondary-ion mass spectrometry (ToF-SIMS). Strong signals for TeS− and TeS2− molecular fragments (separated by 32 amu) were observed, with the appropriate isotopic ratios for 126Te, 128Te, and 130Te (Figure 2B),35Lee D.C. Halliday A.N. Precise determinations of the isotopic compositions and atomic weights of molybdenum, tellurium, tin and tungsten using ICP magnetic sector multiple collector mass spectrometry.Int. J. Mass Spectrom. Ion Process. 1995; 146–147: 35-46Crossref Scopus (127) Google Scholar confirming the formation of polytellurosulfide species (Li2TexSy). XPS analysis of the dried [Li2S6 + Te] residue showed that Te is present in the +4-oxidation state and S is present in the −1 or −2 oxidation states in the dissolved polytellurosulfide species (Figure 2C). When compared with S 2p signals for a similarly prepared dried [Li2S6] residue and Te 3d signal for commercial Te powder (Figure S3), it can be inferred that Te0 is oxidized by Li2S6, which itself is reduced, during the formation of Li2TexSy species. The positive shift in binding energy from Te0 to Te+4 is balanced by a negative shift in binding energy from S0 to S−1 and S−2, which confirms the redox reaction between polysulfides and Te. The [Li2S6 + Te] solution was drop-cast onto and allowed to react with a Li-metal foil, which was then washed with a blank solvent. The red-colored reaction product on the surface of the Li foil was subsequently analyzed by XPS. It revealed a single peak for Te in the +4 oxidation state, and a single peak for S in the −2 oxidation state (Figure 2C). Comparing with Figure 2B, it can be seen that the Te atoms remain unchanged at Te+4, but all the S atoms on the surface are fully reduced to S−2. This indicates an overall partial reduction of the soluble Li2TexSy polytellurosulfides to form an insoluble Te-containing sulfide species on the reducing surface of Li metal. Elemental ratios obtained from the quantification of Te 3d and S 2p signals (Figure S4) and the presence of a single peak for S indicate the presence of Li2TeS3. Subsequently, X-ray diffraction measurements on the Li-metal foil reacted with the [Li2S6 + Te] solution confirmed the formation of Li2TeS3 (Figure 2E). These results allow us to formulate a straightforward strategy for incorporating Te in situ into the sulfide-rich Li SEI and evaluate its impact on Li deposition in Li-S batteries. Elemental Te is added to the Li2S cathode in a 1:10 molar ratio (hereafter designated as Li2S + 0.1Te) and investigated in the anode-free Ni||Li2S full-cell configuration. It is anticipated that polysulfides generated from Li2S cathode would react with Te0 additive, forming polytellurosulfides that migrate to the anode side and form Li2TeS3 on the deposited Li. The proposed mechanism is illustrated in Figure 2F. We hypothesize that the formation of such Te-containing reduced-S SEI components could stabilize Li deposition and improve performance under the limited Li conditions of anode-free full cells. The effect of Te additive on electrochemical performance of anode-free Ni||Li2S full cells at C/5 rate (∼1 mA cm−2) is seen in Figure 3A. Both Li2S and Te stay inert through the cathode slurry preparation (Figure S5). The standard electrolyte used in all cases is 1 M LiTFSI + 0.1 M LiNO3 in DOL/DME (1:1 vol), unless otherwise stated. The Ni||Li2S cell with no additive shows rapid capacity fade, with 50% of its peak capacity (629 mA h g−1 = 2.49 mA h cm−2) lost within 34 cycles. This corresponds to poor efficiency of Li plating and stripping and rapid loss of Li inventory. In contrast, the addition of Te leads to a dramatic improvement in the cyclability of the anode-free full cell. The Ni||(Li2S + 0.1Te) full cell retains over 50% of its peak capacity (620 mA h g−1 = 2.45 mA h cm−2) for 240 cycles, which corresponds to a 7-fold enhancement in cycle life. The LILR shows a 7-fold reduction, from 2.02% to 0.28% per cycle. In other words, with an initial Li plating capacity of nearly 2.5 mA h cm−2 and an applied current density of 1 mA cm−2, the round-trip efficiency of plating and stripping Li can be improved from 97.98% to 99.72% with the introduction of Te in Li-S batteries. The first-cycle charging step is conducted at C/20 rate (∼0.25 mA cm−2), though the overall cycling performance does not depend on the initial cycling conditions (Figure S6). Thus, the introduction of Te as a cathode additive leads to a significant stabilization of Li deposition. The Ni||Li2S control cell displays Coulombic efficiency (CE) values with a geometric mean of 97.02% between the 2nd and 34th cycles (at 50% capacity retention), before cell failure leads to arbitrary changes in the CE values. In contrast, the Ni||(Li2S + 0.1Te) full cell displays stable and consistent CE values with a geometric mean of 96.45% between the 2nd and 240th cycles (at 50% capacity retention). The CE during the first formation cycle was not considered for this calculation. The observed CE values fall far short of what would be expected, given the observed capacity retention trends. For instance, the round-trip Li plating and stripping efficiency is 97.98% for the Ni||Li2S control cell and 99.72% for the Ni||(Li2S + 0.1Te) full cell. This discrepancy can be explained as the result of soluble redox mediators, in this case polysulfides or polytellurosulfides, that shuttle between the cathode and the anode and effectively behave as an “internal short.”29Nanda S. Gupta A. Manthiram A. A lithium–sulfur cell based on reversible lithium deposition from a Li2S cathode host onto a hostless-anode substrate.Adv. Energy Mater. 2018; 8: 1801556Crossref Scopus (59) Google Scholar,36Li S. Jiang M. Xie Y. Xu H. Jia J. Li J. Developing high-performance lithium metal anode in liquid electrolytes: challenges and progress.Adv. Mater. 2018; 30: e1706375Crossref PubMed Scopus (291) Google Scholar Hence, the observed values of CE overestimate the actual LILRs with cycling and are not a useful predictor for capacity fade in anode-free full cells employing Li2S cathodes. Figure S7 shows the electrochemical performance of Li||Li2S and Li||(Li2S + 0.1 Te) half cells at C/5 rate. Both demonstrate similar capacity retention as a function of cycle number. Since the cyclability of half cells depends entirely on the cathode, it can be concluded that a simple mechanical addition of 0.1 Te to the Li2S cathode has no effect on its electrochemical performance. Hence, the excellent cyclability observed with anode-free Ni||(Li2S + 0.1Te) full cells can be attributed entirely to the stabilization of Li deposition with the addition of Te. The extent of this stabilization is demonstrated in Figure 3B, where the Ni||(Li2S + 0.1Te) full cell (0% Li excess) shows nearly identical electrochemical performance as the Li||(Li2S + 0.1Te) half cell (3,300% Li excess). Despite the complete elimination of excess Li, the anode-free full cell with Te additive maintains its cyclability due to the improved reversibility of Li plating and stripping. Thus, the addition of Te makes the Li-S system remarkably resilient toward a restriction in Li supply. Figure 3C shows the charge and discharge profiles of the anode-free full cells between 2nd and 50th cycles (see Figure S8 for first-cycle voltage curves). Compared to the control cell without additive, which shows rapid capacity fade and increasing overpotentials with cycling, the Ni||(Li2S + 0.1Te) cell exhibits limited capacity fade and low, stable overpotentials. From the literature and Figure S9, we expect Te to be electrochemically inactive in the operating voltage range (2.8–1.8 V).37He J. Chen Y. Lv W. Wen K. Wang Z. Zhang W. Li Y. Qin W. He W. Three-dimensional hierarchical reduced graphene oxide/tellurium nanowires: a high-performance freestanding cathode for Li-Te batteries.ACS Nano. 2016; 10: 8837-8842Crossref PubMed Scopus (169) Google Scholar,38Liu Y. Wang J. Xu Y. Zhu Y. Bigio D. Wang C. Lithium-tellurium batteries based on tellurium/porous carbon composite.J. Mater. Chem. A. 2014; 2: 12201-12207Crossref Google Scholar However, the Ni||(Li2S + 0.1Te) cell shows two new plateaus in the voltage profiles, which decrease in magnitude until disappearing in the 6th cycle. This is corroborated by two additional peaks that appear in the cyclic voltammograms at 2.55 V (anodic scan) and 2.45 V (cathodic scan) (Figures 3C and S10), suggesting that Te is indeed electrochemically active in the Li-S system. This can be attributed to the redox reaction of Te with long-chain polysulfide intermediates, which was discussed in the previous section. The disappearance of the plateaus after the first few cycles indicates that the entirety of the added Te has been consumed by reaction with polysulfides. The successful stabilization of Li deposition with the introduction of Te as a cathode additive in the Li-S system begs the question of how it impacts the composition of the SEI layer formed on Li surface. The deposited Li in the anode-free Ni||(Li2S + 0.1Te) full-cell after 30 cycles was analyzed by XPS (Figure 4A). It reveals the presence of Te in +4 oxidation state, primarily bonded to S at 574.5 eV. Simultaneously, S is found in −2 oxidation state, primarily bonded to Te at 160.5 eV. The S to Te ratio, obtained from quantification of Te+4 (-S) and S−2(-Te) signals, is calculated to be 2.98 (Figure S11), identifying the Te-containing reduced-S species as Li2TeS3. Here, Te+4(-S) means Te+4 is bonded to S and S−2(-Te) means S−2 is bonded to Te. It should be emphasized that during cell assembly, Te is only present as Te0 in the cathode. The transfer of Te species from cathode to anode with cycling occurs due to the formation of soluble polytellurosulfides (Li2TexSy), as evidenced in Figure 3D (See Figure S9 for supporting discussion). Thus, the mechanism proposed in Figure 2F is successfully demonstrated, and the formation of a