New Insights on Graphite Anode Stability in Rechargeable Batteries: Li Ion Coordination Structures Prevail over Solid Electrolyte Interphases Jun Ming,†,‡ Zhen Cao,†,‡ Wandi Wahyudi,†,‡ Mengliu Li,† Pushpendra Kumar,† Yingqiang Wu,† Jang-Yeon Hwang,§ Mohamed Nejib Hedhili,† Luigi Cavallo,† Yang-Kook Sun,*,§ and Lain-Jong Li*,† †
Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia § Department of Energy Engineering, Hanyang University, Seoul 133-791, Republic of Korea S Supporting Information *
ABSTRACT: Graphite anodes are not stable in most noncarbonate solvents (e.g., ether, sulfoxide, sulfone) upon Li ion intercalation, known as an urgent issue in present Li ions and next-generation Li−S and Li−O2 batteries for storage of Li ions within the anode for safety features. The solid electrolyte interphase (SEI) is commonly believed to be decisive for stabilizing the graphite anode. However, here we find that the solvation structure of the Li ions, determined by the electrolyte composition including lithium salts, solvents, and additives, plays a more dominant role than SEI in graphite anode stability. The Li ion intercalation desired for battery operation competes with the undesired Li+−solvent co-insertion, leading to graphite exfoliation. The increase in organic lithium salt LiN(SO2CF3)2 concentration or, more effectively, the addition of LiNO3 lowers the interaction strength between Li+ and solvents, suppressing the graphite exfoliation caused by Li+−solvent co-insertion. Our findings refresh the knowledge of the well-known SEI for graphite stability in metal ion batteries and also provide new guidelines for electrolyte systems to achieve reliable and safe Li−S full batteries.
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calation of Li+ ions without accompanying solvent molecules is critical. Such a target can be achieved via increasing the LiN(SO2CF3)2 (LiTFSI) concentration, where the Li+−solvent interaction is weakened and the Li+ ions can be dissociated from solvents and inserted into graphite. Moreover, adding inorganic salts containing NO3− more efficiently weakens the Li+−solvent interaction without the necessity to use highly concentrated LiTFSI solutions. This report provides new interpretation for a graphite failure mechanism; therefore, new guidelines are proposed for designing electrolytes suitable for next-generation lithium ion as well as other kinds of metal ion (e.g., Na+, K+, Ca2+, Mg2+, Al3+) batteries.19 Controversial Issue for Graphite Stability. The discovered controversial issue for the stability of graphite anodes in lithium
raphite exhibits reversible storage capability of Li ions (Li+), and it has been predominantly adopted as an anode in commercial Li ion batteries because it is much safer than a lithium metal.1−3 However, only very few carbonate-based solvents allow reversible Li+ intercalation in graphite, while other solvents often result in graphite exfoliation.4−6 Resolving the poor compatibility of graphite with diverse electrolytes is still an urgent and great challenge to storage of Li ions in high energy density lithium−sulfur (Li−S) and Li−O2 battery technologies7 because the Li metal anode exhibits serious safety concerns for practical applications.8−10 Recently, the use of superconcentrated electrolytes in various solvents has been shown to improve cathode performance11−14 and enhance the graphite stability for Li+ intercalation.15−18 The improved structural stability of graphite is widely perceived as protection from the solid electrolyte interface (SEI) formed on graphite surfaces.15−18 However, our experiments reveal that the SEI on graphite does not guarantee reversible Li+ intercalation in ether-based solvents. Instead, selective inter© 2018 American Chemical Society
Received: November 26, 2017 Accepted: January 4, 2018 Published: January 4, 2018 335
DOI: 10.1021/acsenergylett.7b01177 ACS Energy Lett. 2018, 3, 335−340
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Cite This: ACS Energy Lett. 2018, 3, 335−340
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ACS Energy Letters
Figure 1. Revisiting the SEI role for graphite anode stability in various electrolytes. (a) Schematic view of the discovered controversial issue for the stability of a graphite anode in a Li ion battery. The SEI protected graphite obtained in the carbonate-based electrolyte (b) was reexamined in ether-based electrolyte (c): it can be easily exfoliated in diluted lithium salt concentrations (black), but it is stable in concentrated ones (red). The tested condition was further overturned: the SEI protected graphite in concentrated ether-based electrolyte (d) was reassembled in different electrolytes; (e) the graphite can be exfoliated in dilute ether-based electrolyte (black), but it is stable in carbonatebased electrolyte (e.g., 1.0 M LiPF6 in EC/DMC, red). The ether-based electrolyte of 1.0 M LiTFSI/0.4 M LiNO3 in a DOL/DME = 1/1 volume ratio is abbreviated as 1.0M/0.4M, which is the same for others, such as 2.5M/0.4M.
(obtained in EC/DMC = 1/1 electrolyte, Figure 1b) is reassembled in one of our designed electrolytes (e.g., 2.5 M LiTFSI/0.4 M LiNO3 in DOL/DME = 1/1 volume ratio; abbreviated as 2.5M/0.4M), a stable cycle performance is observed without any obvious electrolyte decomposition (Figure 1c). These results suggest that (i) the SEI formed in carbonate electrolyte may not efficiently stop the graphite exfoliation and, more importantly, (ii) increasing lithium salt concentration inhibits the failure of graphite anodes. Next, we examine the property of the SEI formed in etherbased electrolyte 2.5M/0.4 M (Figure S1−S3), where the shoulder in the first discharge curve of a graphite anode in Figure 1d indicates the formation of SEI with the electrolyte decomposition26 and the battery exhibits good cycle performance. This SEI-coated graphite anode also shows stable cycle performance even when it is reassembled in a carbonate-based electrolyte (Figures 1e and S4). However, such a SEI-protected graphite anode does not sustain cycle performance once it is reassembled in a lower concentration of electrolyte 1.0M/0.4 M (Figure 1e), agreeing well with Figure 1c. Various types of carbonate- and ether-based electrolytes have been further examined (Figures S5−S7). Particularly, a known SEI film-
ion batteries is presented in Figure 1a. The roles of the SEI on graphite electrodes, formed in different electrolytes, are revisited by examining the voltage vs capacity profiles of graphite/Li half batteries (Figure 1b−e). In a commercial carbonate-based electrolyte (1.0 M LiPF6 in ethyl carbonate (EC)/dimethyl carbonate (DMC) = 1/1 volume ratio), the formation of SEI on graphite is shown in the first discharge process,20 as indicated in Figure 1b. The resulting SEI can protect the graphite anode from the second cycle, giving smooth and stable charge−discharge profiles.21 The stabilized graphite anode protected with SEI is reassembled into another ether-based electrolyte (1.0 M LiTFSI, 0.4 M LiNO3 in dioxolane (DOL)/dimethoxyethane (DME) = 1/1 volume ratio; abbreviated as 1.0M/0.4M) typically used for Li−S batteries.22−25 Pronounced electrolyte decomposition occurs immediately in the first discharge process followed by an obvious capacity decrease with cycling (Figure 1c). The extended capacity in the first discharge process (larger than the theoretical graphite capacity of 372 mAh g−1) is attributed to electrolyte degradation and the intercalation of Li+−solvent clusters (which results in graphite exfoliation), which shall be discussed later. If the same SEI-coated graphite anode 336
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Figure 2. Effects of electrolyte compositions on graphite anode capacity and stability. (a) Comparative voltage vs capacity profiles of the graphite anode in ether-based electrolytes with different lithium salt concentrations in the first cycle. (b,c) XRD, TEM, and SAED patterns of the cycled graphite anodes in electrolytes of (b) 2.5M/0.4 M and (c) 1.0M/0.4 M, respectively. (d) Typical charge−discharge curves of the graphite anodes in ether-based electrolytes with different LiTFSI and/or LiNO3 concentrations. (e) Effect of the LiNO3 concentration on the reversible capacity of graphite. Reversible capacity = Ccharge − (Cdischarge − Ccharge) = 2Ccharge − Cdischarge (where Ccharge and Cdischarge are the charge and discharge capacities of graphite).
forming additive vinylene carbonate27 is added on purpose, but it still does not provide sufficient protection for graphite anodes in a low-concentration electrolyte 1.0M/0.4 M (Figure S7). All of these observations indicate that the graphite surface protection offered by SEI is limited and the electrolyte structure seems more dominant. Lithium Salts Effects for Intercalation Chemistry of Graphite. To explore the failure mechanism of graphite anodes, we monitor the structural change of graphite through the ex situ Xray diffraction pattern (XRD) while tracing their charge/ discharge curves in ether-based electrolytes. The graphite anode cycled in 2.5M/0.4 M displays ideal charge/discharge profiles, whereas the graphite operated in typical 1.0M/0.4 M or 2.5M/ 0 M (2.5 M LiTFSI without LiNO3) shows obvious tailing expanding from 230 to around 800 mAh g−1 in the first discharge process (Figure 2a), and the capacity decays fast thereafter (Figure S8). The back and forth shift of the (002) peak in Figure 2b, which originated from the graphene stacking, demonstrates that Li+ can be reversibly inserted into and flee out of the graphite interlayer in 2.5M/0.4M. However, the graphitic stacking is destroyed and not recoverable after the first discharge process when a lower electrolyte concentration (1.0M/0.4M) is adopted (Figure 2c). The high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) in the insets of Figure 2b,c also corroborate the results. The graphite failure is associated with exfoliation (Figure S9). It has been well accepted that graphite can be exfoliated with the co-intercalation of solvents and Li+ ions (i.e., Li+−solvent) into graphite,27 a process similar to that of intentional electrochemical exfoliation of graphite for producing graphene flakes.28
Interplay of Lithium Salts and Solvents. To identify the critical factors determining the graphite anode stability in ether, various compositions of LiTFSI-based electrolytes are examined. The charge/discharge profiles in Figure 2d show that the LiTFSI salt concentration significantly affects the graphite stability. In general, a higher salt concentration can inhibit the graphite exfoliation. Note that the decreased graphite performances at 10 M in Figure 2d are ascribed to the high viscosity of electrolyte when the salt concentration is further increased from 7.5 to 10 M, in which the lithium diffusion is very difficult and then gives rise to inferior performances (Figure S10). Second, adding additional anions, such as NO3−, can efficiently decrease the critical value of the LiTFSI concentration required for stable cycling of graphite (Figures 2e and S10). Third, different solvents (DOL only, DME only, or DME/DOL mixture) can lead to significantly different battery performance. We show in Figures S11 and S12 that the graphite stability in DOL is superior to that in a the DME/DOL mixture, whereas graphite is least stable in DME. Thus, various salt concentrations are required to stabilize graphite in different solvents. Because all of these factors, including LiTFSI, LiNO3, and solvents, govern the structures of the electrolytes and affect the intercalation behaviors of Li+, we move the focus to the exploration of Li+ solvation structures in different electrolytes. Raman Spectrum for Li Ion Coordination. The Raman S−N−S bending frequency in TFSI− can be shifted depending on the interaction with Li+ ions. For example, its Raman profile can be decomposed into a few peaks associated with different aggregation states including “free ion” (FI) (737 cm−1), “loose ion pair” (LIP) (741 cm−1), “intimate ion pair” (IIP) (745 cm−1), and “aggregated ion pair” (AIP) (747 cm−1) 337
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ACS Energy Letters (Figure S13).15,29 The S−N−S bending modes of TFSI− ions exhibit different populations in various electrolyte systems (Figure 3a), indicating remarkably different interaction between
during the dynamic solvation process. After the addition of NO3− anions, the RDF of Li+ to the oxygen of TFSI− suggests that NO3− can replace the TFSI− and therefore change the solvation structure of the Li+ centered clusters, as seen in the snapshot displayed in Figure 3e. We also observe another simulated structure in which solvent molecules are replaced by the NO3− anions, as shown in Figure 3f, indicating weakening of the Li+−solvent interaction. Note that it is not necessary to keep the clusters charge-neutral, especially when NO3− anions are added into the electrolytes. The snapshots in Figure 3e,f clearly reveal that the NO3− anions in the structure present negatively charged regions, capable of further interacting with more adjacent Li+ ions to form larger aggregates, which further weakens the Li+−solvent interaction (Figure S14). This effect can be further characterized by the heterogeneous order parameter (HOP)30 (see details in the Supporting Information). In general, larger order parameters, correlated with stronger Li+−solvent interactions, are obtained for the systems with lower salt concentrations. The order parameter is 3.6 for 1.0M/0.4 M in DOL/DME, while it decreases to 1.8 by increasing the salt concentration to 2.5M/0.4 M in DOL/DME, indicating a weaker Li+−solvent interaction. Besides, for the electrolyte 1.0M/0.4M, this parameter is 3.6, which is lower than the 4.7 for the electrolyte 2.5M/0.0M, suggesting that the role of NO3− in altering the Li+-solvation structure is pronounced. These calculation results are consistent with the experiments (Figure 2a) where the 2.5M/0.4 M electrolyte results in the most reliable electrochemical performance. Although we may oversimplify the real situation into fivecoordinated local clusters centered on Li+ and long-range structural correlation originating from the Coulomb interaction, the consistency of simulations and experiments supports our arguments that the Li+-solvation structure dominates graphite anode stability. Principles for Designing Electrolyte and Safer Li−S Battery Application. In general, weaker Li+−solvent interaction can be achieved through increasing salt concentration or, equivalently, using the solvent system with a lower solubility such as a DOLdominated system or adding specific salts (i.e., LiNO3 or NaNO3). On the basis of this discovery, a series of electrolytes with different lithium salts and solvent components are examined in detail, and the trends of these experimental results including the Raman and electrochemical results fully support the proposed mechanism and the transferability of our discovery (Figure S15). Thereafter, the commonly used 1.0M/0.4 M and the selected electrolytes (e.g., 2.5M/0.4 M and 1.5M/Sat. in DOL/DME) are applied in Li−S full batteries for further examination. Figure 4a shows the configuration of the full battery composed of a commonly used S cathode, electrolyte (separator), and Li+-intercalated graphite anode. Compared with the 1.0M/0.4 M electrolyte, much better battery characteristics in rate capabilities (Figures 4b and S16) can be obtained in the selected electrolytes. Figure 4c shows that an impressive Coulombic efficiency of around 100% is obtained for the full battery using 2.5M/0.4M, higher than the 97.5% in the half battery (vs lithium metal). Besides, the commonly used electrolyte of 1.0M/0.4 M always leads to low capacity and poor cycle performance due to the irreversible Li+ storage capability. However, the Li−S battery using the designed electrolyte such as 2.5M/0.4 M can cycle beyond 200 cycles with an initial capacity of 1200 mAh g−1 at a rate of 0.1C (Figure 4d). An average capacity of 700 mAh g−1 is achieved, and the energy density can be as high as 2000 Wh/kg
Figure 3. Coordination structures of lithium salts and solvents. (a) Raman spectra of S−N−S bending motions for TFSI− in electrolytes using (I) DME, (II) DOL, and (III−VI) DOL/DME as solvents. (b) RDF of Li+ to the oxygen of DME and DOL. (c) RDF of Li+ to the oxygen of TFSI− in the electrolytes with (red) and without (black) NO3−. (d) Schematic view of the first solvation shell of Li+ in different solvents, where DME is the dominant solvent component. NO3− anions can replace the (e) TFSI− or (f) solvent molecules in the first solvation shell. The red-colored shell indicates the negative surface areas of this cluster. Clusters containing the NO3− anion(s) present more significant negative areas, which aer capable of interacting with more adjacent lithium cations and form aggregated structures.
TFSI− with Li+ ions: (i) with a relatively low salt concentration, the TFSI− is classified as FI or LIP, whereas the population shifts to LIP and IIP with the increment of salt concentrations; (ii) compared with DOL, DME has better capability in dissolving the lithium salts, leading to more populations in FI; (iii) NO3− can dramatically affect the Li+−TFSI− interaction and change the population toward more aggregated states. Simulations of Li Ion Coordination Structures. Both counterions and solvent molecules can locally form solvation shells surrounding the central Li+, and detailed structures can be obtained through the density functional theory (DFT) molecular dynamics (MD) simulations. Because of the small diameter of the Li+ ion, it usually forms a five-coodinated structure in electrolytes.5 The radial distribution function (RDF) of Li+ to the oxygen in DOL/DME without adding NO3− concludes that DME is the dominant solvent component within the first solvation shell (Figure 3b), with a few snapshots of the simulated structures shown in Figure 3d. The RDF of Li+ to the oxygen of TFSI− indicates strong involvement of TFSI− to the solvation of Li+ (black line in Figure 3c). It means a high appearance frequency of TFSI− in the first solvation shell of Li+ 338
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Figure 4. Principles for designing an electrolyte and safer lithium ion sulfur battery. (a) Schematic drawing of a Li−S full battery, consisting of 70 wt % sulfur−C cathode, different electrolytes, and Li+-intercalated graphite anodes. (b) Comparative rate capabilities of the Li−S full battery using various electrolytes. Comparative (c) Coulombic efficiency and (d) cycle performances of half and full batteries using lithium and a lithium-intercalated graphite anode in the specified electrolytes at 0.1C.
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(calculated by the weight of sulfur). Besides, a high capacity of around 920 mAh g−1 with good cycle ability is further demonstrated at a relatively high rate of 0.25C (Figure S17). Note that, although the storage of Li ion within graphite may reduce the practical energy density of the battery, its safety issues and cycle ability are significantly superior to those of the half battery using lithium as the anode. Also, the safer Li−S full battery application herein is only one of the cases to further prove the transferability of our presented principles in designing electrolyte for a graphite anode. In addition to the stabilization and safety features of graphite, the superior performance of Li−S battery in high-concentration electrolytes is also relevant to the aggregated Li+−solvent structures, which can efficiently slow down the dissolution/migration of polysulfide and thus avoid the self-discharge. We believe that the performances of Li−S batteries can be further improved after combining those strategies reported in Li−S batteries, such as the insertion of a carbon interlayer and/or modifying the separators.31−35 In summary, we demonstrate that the coordination structures of Li+ ions is more critical than the commonly believed SEI in stabilizing the graphite for reversible Li+ (de)intercalation. Weakening the Li+−solvent interaction through an increase in salt concentration (or, equivalently, a decrease in solvent solubility) and the addition of other anions such as NO3− can significantly hinder the co-intercalation of Li+−solvent, resulting in better graphite stability. This study discovers the mechanism of enhancing the stability of the widely used graphite anodes, offering new insights for the construction of next-generation lithium ion batteries (e.g., Li−S and Li−O2) with desired safer features for storage of lithium and reliable performance.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.7b01177. Methods, electrochemical performances of graphite in different kinds of electrolyte, characterizations of electrolyte, and Li−S battery performances (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (L.-J.L.). *E-mail:
[email protected] (Y.-K.S.). ORCID
Jun Ming: 0000-0001-9561-5718 Luigi Cavallo: 0000-0002-1398-338X Yang-Kook Sun: 0000-0002-0117-0170 Lain-Jong Li: 0000-0002-4059-7783 Author Contributions ‡
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. J. M., Z.C., and W.W. contributed equally. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The research was supported by KAUST. The simulations were performed on the KAUST supercomputer. 339
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