New Insights of Graphite Anode Stability in Rechargeable Batteries: Li

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New Insights of Graphite Anode Stability in Rechargeable Batteries: LiIon Coordination Structures Prevail over Solid Electrolyte Interphases Jun Ming, Zhen Cao, Wandi Wahyudi, Mengliu Li, Pushpendra Kumar, Yingqiang Wu, Jang-Yeon Hwang, Mohamed N. Hedhili, Luigi Cavallo, Yang-Kook Sun, and Lain-Jong Li ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01177 • Publication Date (Web): 04 Jan 2018 Downloaded from http://pubs.acs.org on January 4, 2018

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ACS Energy Letters

New Insights of 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,§* 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

*To whom correspondence should be addressed: [email protected]; [email protected].

1 ACS Paragon Plus Environment

ACS Energy Letters

ABSTRACT: Graphite anodes are not stable in most non-carbonate solvents (e.g. ether, sulfoxide, sulfone) upon Li ion intercalation, known as an urgent issue in present Li-ion, and next-generation Li-S and Li-O2 batteries to storage Li ions within anode for safety features. The solid electrolyte interphase (SEI) is commonly believed 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 is competed with the undesired Li+-solvent co-insertion leading to graphite exfoliation. The increase in organic lithium salt LiN(SO2CF3)2 concentration or more effectively by adding 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 wellknown SEI for graphite stability in metal-ion batteries and also provide new guidelines for electrolyte systems to achieve reliable and safe Li-S full battery.

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Li+ Graphite stability as Li+ intercalation: SEI or (?) Li+ coordination in electrolyte


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ACS Energy Letters

Graphite exhibits reversible storage capability of Li ion (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 Li ions in high energy density lithium-sulfur (Li-S) and Li-O2 battery technologies,7 since 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 the 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 the reversible Li+ intercalation in ether-based solvents. Instead, selective intercalation of Li+ ions without accompanied solvent molecules is critical. Such 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 graphite failure mechanism; therefore, new guidelines are proposed for designing electrolytes suitable for next generation lithium ion as well as other kind 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 anode in lithium ion batteries is presented in Figure 1a. The roles of the SEI on graphite electrodes, formed in different electrolytes, are re-visited through examining the voltage


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vs. capacity profiles of graphite/Li half batteries (Figure 1b-e). In a commercial carbonate-based electrolyte (1.0M LiPF6 in ethyl carbonate (EC)/dimethyl carbonate (DMC) = 1/1 volume ratio), the formation of SEI on graphite is shown in the 1st discharge process20 as indicated in Figure 1b. The resulted SEI can protect the graphite anode from the 2nd cycle, giving smooth and stable charge-discharge profiles.21 The stabilized graphite anode protected with SEI was re-assembled into

another

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0.4M

LiNO3

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dioxolane

(DOL)/dimethoxyethane (DME) = 1/1 volume ratio; abbreviated as 1.0M/0.4M) typically used for Li-S battery.22-25 Pronounced electrolyte decomposition occurs immediately in the 1st discharge process followed by an obvious capacity decrease as cycling (Figure 1c). The extended capacity in the 1st discharged process (larger than the theoretical graphite capacity 372 mAh g-1) is attributed to the electrolyte degradation and the intercalation of Li+-solvent clusters (where results in graphite exfoliation), which shall be discussed later. If the same SEI-coated graphite anode (obtained in EC/DMC=1/1 electrolyte, Figure 1b) was re-assembled in one of our designed electrolytes (e.g., 2.5M LiTFSI/0.4M 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 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 ether-based electrolyte 2.5M/0.4M (Figure S1-S3), where the shoulder in the 1st 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 it is re-assembled in carbonate-based electrolyte (Figure 1e, Figure S4). However, such SEI


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protected graphite anode does not sustain cycle performance once it is re-assembled in a lower concentration of electrolyte 1.0M/0.4M (Figure 1e), agreeing well with Figure 1c. Various types of carbonate and ether-based electrolytes have been further examined (Figure S5-S7). Particularly, a known SEI film-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/0.4M (Figure S7). All 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 X-ray diffraction pattern (XRD) while tracing their charge/discharge curves in ether-based electrolytes. The graphite anode cycled in 2.5M/0.4M displays ideal charge/discharge profiles, whereas the graphite operated in typical 1.0M/0.4M or 2.5M/0M (2.5M LiTFSI without LiNO3) shows obvious tailing expanding from 230 to around 800 mAh g-1 in the 1st discharge process (Figure 2a) and the capacity decays fast thereafter (Figure S8). The back and forth shift of the (002) peak in Figure 2b, 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 1st 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 insets of Figures 2b & 2c also corroborate the results. The graphite failure is associated with the exfoliation (Figure S9). It has been well accepted that graphite can be exfoliated with the cointercalation of solvents and Li+ ions (i.e., Li+-solvent) into graphite,27 which process is similar to those intentional electrochemical exfoliation of graphite for producing graphene flakes.28



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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 10M in Figure 2d is ascribed to the high viscosity of electrolyte when the salt concentration was further increased from 7.5M to 10 M, in which the lithium diffusion is very difficult and then give rise to inferior performances (Figure S10). Secondly, adding additional anions, such as NO3-, can efficiently decrease the critical value of the LiTFSI concentration required for stable cycling of graphite (Figure 2e, Figure S10). Thirdly, different solvents (DOL only, DME only, or DME/DOL mixture) can lead to significantly different battery performance. We show in Figures S11-S12 that the graphite stability in DOL is superior to that in a DME/DOL mixture whereas the graphite is least stable in DME. Thus, various salt concentrations are required to stabilize graphite in different solvents. Since all 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 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) (Figure S13).15, 29 The S-N-S bending modes of TFSIions exhibit different populations in various electrolyte systems (Figure 3a), indicating remarkably different interaction between TFSI- with Li+ ions: i) with a relative low salt


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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 towards more aggregated states. Simulations of Li-Ion Coordination Structures. Both counter ions and solvent molecules can locally form solvation shells surrounding the central Li+ ion and the detailed structures can be obtained through the density functional theory (DFT) molecular dynamics (MD) simulations. Because of the small diameter of Li+ ion, it usually forms a 5-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), where few snapshots of the simulated structures are 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+ 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, where the snapshot is displayed in Figure 3e. We also observe another simulated structure that solvent molecules are replaced by the NO3- anions as shown in Figure 3f, indicating the weakening of 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-3f 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).



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This effect can be further characterized by the heterogeneous order parameter (HOP)30 (see details in Supporting Information). In general, larger order parameters, correlate to stronger Li+-solvent interactions, are obtained for the systems with lower salt concentrations. The order parameter is 3.6 for 1.0M/0.4M in DOL/DME, while it decreases to 1.8 by increasing the salt concentration to 2.5M/0.4M 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 role of NO3- in altering Li+-solvation structure is pronounced. These calculation results are consistent with the experiments (Figure 2a) that the 2.5M/0.4M electrolyte results in the most reliable electrochemical performance. Although we may oversimplify the real situation into 5-coordinated local clusters centered on Li+ and long-range structural correlation originated from the Coulomb interaction, the consistency of simulations and experiments supports our arguments that 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 DOL-dominated system, or adding specific salts (i.e., LiNO3 or NaNO3). Based on this discovery, a series of electrolytes with different lithium salts and solvent components are examined in details, and the trend 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/0.4M and the selected electrolytes (e.g., 2.5M/0.4M 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 a Li+-


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ACS Energy Letters

intercalated graphite anode. Compared with the 1.0M/0.4M electrolyte, much better battery characteristics in rate capabilities (Figure 4b, Figure S16) can be obtained in the selected electrolytes. Figure 4c shows that the impressive coulombic efficiency around 100% is obtained for the full battery using 2.5M/0.4M, higher than the 97.5% in half battery (vs. Lithium metal). Besides, the commonly used electrolyte of 1.0M/0.4M always leads to low capacity and poor cycle performance due to the irreversible Li+ storage capability. While the Li-S battery using the designed electrolyte such as 2.5M/0.4M can cycle beyond 200 cycles with an initial capacity of 1200 mAh g-1 at the rate of 0.1 C (Figure 4d). Average capacity of 700 mAh g-1 is achieved, and the energy density can be as high as 2000 Wh/kg (calculated by the weight of sulfur). Besides, a high capacity around 920 mAh g-1 with a 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 battery, its safety issues and cycle ability are significantly superior to that of the half battery using lithium as anode. And 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 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 battery can be further improved after combing those strategies reported in Li-S batteries, such as the insertion of carbon interlayer and/or modifying the separators.31-34 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 the increase in salt concentration (or equivalently



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decrease in solvent solubility) and the addition of other anions such as NO3- can significantly hinder the co-intercalation of Li+-solvent, resulting in the 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 to storage lithium and reliable performance.


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Current Collector

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Li+ Graphite stability as Li+ intercalation: SEI or (?) Li+ coordination in electrolyte

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ACS Energy Letters

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Figure 1. Re-visit SEI role for graphite anode stability in various electrolytes. (a) Schematic view of the discovered controversial issue for the stability of graphite anode in Li ion battery. The SEI protected graphite obtained in carbonate based electrolyte (b) was re-examined in ether based electrolyte (c): it can be easily exfoliated in diluted lithium salt concentration (black), but it is stable in concentrated one (red). The tested condition was further overturned: the SEI protected graphite in concentrated ether-based electrolyte (d) was re-assembled in different electrolytes: (e) the graphite can be exfoliated in dilute ether-based electrolyte (black), but it is stable in carbonate based electrolyte (e.g., 1.0M LiPF6 in EC/DMC, red). The ether-based electrolyte of 1.0M LiTFSI/0.4M LiNO3 in DOL/DME=1/1 volume ratio is abbreviated as 1.0M/0.4M, which description is the same for others, such as 2.5M/0.4M.


ACS Energy Letters

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Figure 2. Effects of electrolyte compositions on graphite anode capacity and stability. (a) Comparative voltage vs. capacity profiles of graphite anode in ether-based electrolytes with different lithium salt concentrations in the 1st cycle. (b, c) XRD, TEM and SAED patterns of the cycled graphite anodes in the electrolyte of (b) 2.5M/0.4M and (c) 1.0M/0.4M 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) = 2*Ccharge – Cdischarge (where the Ccharge and Cdischarge are the charge and discharge capacity of graphite).


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Figure 3. Coordination structures of lithium salts and solvents. (a) Raman spectrum 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 TFSIin the electrolytes with (red) and without (black) NO3-. (d) Schematic view of the first solvation shell of Li+ in different solvents, where the DME is the dominant solvent component. (e) NO3anions can replace the TFSI- or (f) solvent molecules in the first solvation shell. The red-colorshell indicates the negative surface areas of this cluster. Clusters contain NO3- anion(s) present more significant negative areas, which is capable to interact with more adjacent lithium cations and form aggregated structures. 13 ACS Paragon Plus Environment

ACS Energy Letters

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Figure 4. Principles for designing 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 Li-S full battery using various electrolytes. Comparative (c) coulombic efficiency and (d) cycle performances of half and full battery using lithium and lithium intercalated graphite anode in the specified electrolytes at 0.1C.


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ASSOCIATED CONTENT Supporting Information. Methods, electrochemical performances of graphite in different kind of electrolyte, characterizations of electrolyte and Li-S battery performances. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (L.-J.L.). *E-mail: [email protected] (Y.-K.S.). 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. ACKNOWLEDGMENT The research was supported by KAUST. The simulations were performed on KAUST supercomputer.



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REFERENCES (1) Dahn, J. R.; Zheng, T.; Liu, Y. H.; Xue, J. S. Mechanisms for Lithium Insertion in Carbonaceous Materials. Science 1995, 270, 590-593. (2) Nayak, P. K.; Penki, T. R.; Markovsky, B.; Aurbach, D. Electrochemical Performance of Liand Mn-Rich Cathodes in Full Cells with Prelithiated Graphite Negative Electrodes. ACS Energy Lett. 2017, 2, 544-548. (3) Wood, K. N.; Noked, M.; Dasgupta, N. P. Lithium Metal Anodes: Toward an Improved Understanding of Coupled Morphological, Electrochemical, and Mechanical Behavior. ACS Energy Lett. 2017, 2, 664-672. (4) Abe, T.; Kawabata, N.; Mizutani, Y.; Inaba, M.; Ogumi, Z. Correlation between Cointercalation of Solvents and Electrochemical Intercalation of Lithium into Graphite in Propylene Carbonate Solution. J. Electrochem. Soc. 2003, 150, A257-A261. (5) Abe, T.; Fukuda, H.; Iriyama, Y.; Ogumi, Z. Solvated Li-Ion Transfer at Interface Between Graphite and Electrolyte. J. Electrochem. Soc. 2004, 151, A1120. (6) Xu, K. Electrolytes and Interphases in Li-Ion Batteries and Beyond. Chem. Rev. 2014, 114, 11503-11618. (7) Bruce, P. G.; Freunberger, S. A.; Hardwick, L. J.; Tarascon, J. M. Li-O2 and Li-S batteries with High Energy Storage. Nat. Mater. 2011, 11, 19-29. (8) Tarascon, J. M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359-367. (9) Harry, K. J.; Hallinan, D. T.; Parkinson, D. Y.; MacDowell, A. A.; Balsara, N. P. Detection of Subsurface Structures Underneath Dendrites Formed on Cycled Lithium Metal Electrodes. Nat. Mater. 2014, 13, 69-73.


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