Redox Species-Based Electrolytes for Advanced Rechargeable

Aug 15, 2016 - Scalable Approach To Construct Free-Standing and Flexible Carbon Networks for Lithium–Sulfur Battery. Mengliu Li , Wandi Wahyudi , Pu...
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Redox Species-Based Electrolytes for Advanced Rechargeable Lithium Ion Batteries Jun Ming,*,† Mengliu Li,† Pushpendra Kumar, Ang-Yu Lu, Wandi Wahyudi, and Lain-Jong Li* Physical Sciences and Engineering Division, King Abdullah University of Science and Technology, Thuwal 23955-6900, Kingdom of Saudi Arabia S Supporting Information *

ABSTRACT: Seeking high-capacity cathodes has become an intensive effort in lithium ion battery research; however, the low energy density still remains a major issue for sustainable handheld devices and vehicles. Herein, we present a new strategy of integrating a redox species-based electrolyte in batteries to boost their performance. Taking the olivine LiFePO4-based battery as an example, the incorporation of redox species (i.e., polysulfide of Li2S8) in the electrolyte results in much lower polarization and superior stability, where the dissociated Li+/Sx2− can significantly speed up the lithium diffusion. More importantly, the presence of the S82−/S2− redox reaction further contributes extra capacity, making a completely new LiFePO4/Li2Sx hybrid battery with a high energy density of 1124 Wh kgcathode−1 and a capacity of 442 mAh gcathode−1. The marriage of appropriate redox species in an electrolyte for a rechargeable battery is an efficient and scalable approach for obtaining higher energy density storage devices.

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developed, which can deliver a high effective capacity of 442 mAh gcathode−1, and it is much higher than the ∼160 mAh g−1 of the Li/LFP battery using commercial electrolyte. In addition to the extra capacity, the polysulfide-based electrolyte also significantly enhances the cycle life and lowers the polarization of the LFP in the (dis-)charge process. The stability and rate capabilities of the LIB using proposed polysulfide electrolytes are also overwhelmingly better than those using commercial electrolytes. A model is proposed for expounding the reason for the lower polarization. Hybrid Lithium Battery. Figure 1a schematically illustrates the structure of the proposed hybrid battery, where the olivine structured LFP and Li (or lithiated graphite) are used as the cathode and anode. The electrolyte is composed of 1 M bis(trifluoromethane) sulfonimide lithium salt (LiTFSI) in 1,3dioxolane (DOL)/1,2-dimethoxyethane (DME) (v/v, 1/1), 0.4 M LiNO3, and 0.05 M Li2S8. Note that to prepare the electrolyte containing 0.05 M Li2S8, stoichiometric ratios of lithium metal pieces and sulfur powder were reacted in a solvent of DOL/DME (1:1) first. Upon completion of polysulfide formation, other salts including LiTFSI and LiNO3 were then added. The separators used in the study were coated with multiwalled carbon nanotubes (MWCNTs), where the MWCNT layer is the main host for the redox

ince the commercialization of rechargeable lithium ion batteries (LIBs) in 1991, several eminent cathodes including layered lithium metal oxide (i.e., LiMO2, M = Co, Ni, Mn, etc.),1−3 spinel structured LiM2O4 (M = Mn, NixMn1−x, etc.),4−6 and olivine lithium metal phosphate (LiMPO4, M = Fe, Co, Mn, etc.)7−9 have dominated the energy market, ranging from portable electric devices and grid storage to automotive propulsion. However, insufficient cathode capacity less than 200 mAh g−1 (e.g., < 145 mAh g−1 of commercialized LiCoO2), much lower than the counterpart of the carbon anode (e.g., ∼372 mAh g−1 of natural graphite), has always been the bottleneck in high-power or long-life device applications.10−12 Although huge efforts based on solid-state chemistry have been paid for synthesizing new cathodes in LIBs such as gradient nickel-rich13,14 and lithium-rich compounds15−17 with a higher capacity, the durable cycle life and stable voltage plateau still warrant further investigation.18,19 Instead of searching for higher-capacity cathodes,20−23 herein we present an alternative concept of introducing a redox species-based electrolyte for boosting up LIB performance. A newly designed polysulfide (Li2S8)-based electrolyte is applied in a LIB system for the first time, and it is able to lower the polarization and improve the cycle stability of LiFePO4 (LFP)based LIBs. Furthermore, it can contribute an additional capacity as high as ∼290 mAh gcathode−1 with a fine voltage plateau at around 2.0 V because of the redox reaction of Li2Sx/ Li2S (6 ≤ x ≤ 8). As a result, a completely new hybrid battery with the combined advantages of a Li/LFP and Li−S system is © XXXX American Chemical Society

Received: July 13, 2016 Accepted: August 14, 2016

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DOI: 10.1021/acsenergylett.6b00274 ACS Energy Lett. 2016, 1, 529−534

Letter

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ACS Energy Letters

Figure 1. Schematic of a hybrid rechargeable battery and electrochemical performances. (a) The configuration of the hybrid battery consists of a LFP cathode, a MWCNT-modified separator, and an anode of lithium metal (or lithiated graphite), in which an electrolyte of 1.0 M LiTFSI in DOL/DME (v/v, 1/1), 0.4 M LiNO3 containing 0.05 M Li2S8 is used. (b) Typical voltage vs capacity profiles of a hybrid battery cycled at 0.6 C. (c) Cycle performance and (d) rate capability of the LFP-based batteries using three different electrolytes.

Figure 2. Analysis of the overpotential for LFP-based batteries using different electrolytes. Voltage vs capacity profiles of (a) the electrolyte with Li2S8, (b) the commercial LIB electrolyte, and (c) the electrolyte without Li2S8 at rates of 0.25, 0.6, and 1.2 C, charged to 3.6 V. (d) Difference of the charge/discharge overpotential using different electrolytes. (e,f) Voltage vs capacity profiles of Li2S8-based (solid line) and commercial electrolytes (dot line) at high rates of 2.5, 6, and 12 C, charged to 4.0 V.

LFP (LiFePO4 → Li1−xFePO4 + xLi+ + e−, x ≤ 1). In the subsequent discharge process, the Li+ first inserts into the crystalline channel of FePO4 at 3.45 V, and then the sulfur species in the electrolyte reacts with the Li+ to form Li2Sx (1 ≤ x ≤ 2) on the cathode side (Figure S2), finally giving rise to a total capacity of 370 mAh gcathode−1 at 0.6 C and 442 mAh gcathode−1 at 0.25 C (Figure S3a). The voltage vs capacity profiles for the second cycle and onward suggest that the hybrid battery has the combined advantages of Li/LFP and a Li−S

reaction of Li2Sx/Li2S (6 ≤ x ≤ 8) in the (dis-)charge process (Figure S1). Figure 1b shows the charge/discharge profiles for the hybrid battery at a rate of 0.6 C. During the first charge, the initial plateau at 2.45 V corresponds to oxidation of Li2S8 (Li2S8 → S8 + 2Li+ + 2e−, 209.4 mAh g−1).24−26 On the basis of the charge capacity of 107 mAh gsulfur−1 associated with this plateau, we estimate that about half of the Li2S8 was oxidized. The following charging platform at around 3.5 V indicates the deinsertion of Li ions (Li+) from the crystalline channels of 530

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respectively, which are much better than 0.74 Li+ (vs 260 mV), 0.54 Li+ (vs 599 mV), and 0.09 Li+ (vs 1600 mV) of the LFP battery with commercial LIB electrolytes. The lower overpotential was further confirmed by cyclic voltammetry (CV) analysis, where the polarization and charge potentials of the hybrid battery are significantly lower than those in commercial LIBs, particularly with increasing the scan rate (Figure 3a−c). In detail, the polarization of deinsertion/

battery system (Figure S4). If we consider only the weight of the LFP cathode (treating Li2S8 as an additive in electrolyte), the energy density of this hybrid lithium battery can be as high as 1124 Wh kg−1, which is over 2 times higher than 513 Wh kg−1 of pristine LFP (Figure S3b) and ∼536 Wh kg−1 of commercialized LiCoO2 in theory. Even considering the mass of Li2S8 (i.e., 1.755 mg calculated from the 0.05 M Li2S8 in 130 μL of electrolyte) in each cell, the energy density of the hybrid battery is still as high as 896 Wh kg−1. Furthermore, another significance of the present battery system is the improvement in the volume energy density as compared to traditional LIBs. Different from the conventional electrolyte used for LIBs, the same volume of Li2S8-based electrolyte used here can largely enhance the overall capacity. To understand the effect clearly, we have compared the Li2S8-based electrolyte with other two electrolytes, (i) commercial LIB electrolyte (1 M LiPF6 in ethylene carbonate/dimethyl carbonate (EC/DMC)) and (ii) Li−S battery electrolyte without Li2S8 (1 M LiTFSI in DOL/ DME with 0.4 M LiNO3).27 The superior capacity of 370 mAh g−1 using a Li2S8-based electrolyte is much higher than 150 and 6 mAh g−1 obtained using commercial Li ion and Li−S battery electrolytes (Figure 1c). The durable performance and high Coulombic efficiency of the battery over 100 cycles well confirms the stability of the Li2S8-based electrolyte. Furthermore, the hybrid battery demonstrates high rate capabilities of 437, 381, 339, 288, 122, and 41 mAh g−1 at rates of 0.25, 0.6, 1.2, 2.5, 6, and 12 C, respectively, and it can recover to 340 mAh g−1 at 0.6 C (Figures 1d and S3a). These values are superior to 155, 151, 137, 3.3, 1.8, and 0.7 mAh g−1 of the battery using a commercial LIB electrolyte (Figure S3c,d). Even with the increasing charging cutoff voltage at 4.0 V, higher rate capacities of 234 and 161 mAh g−1 were attained at high rates of 6 and 12 C for the hybrid battery, which are much better than 92 and 14 mAh g−1 of the LFP battery using a commercial LIB electrolyte (Figure S3e). Note that the battery using a Li−S battery electrolyte without Li2S8 always exhibits the worst performance among these three, and it needs activation at higher voltages because of the larger polarization (see below). Therefore, the presence of redox species with lithium polysulfides significantly contributes to the battery performance enhancement. Electrochemical Analysis. In addition to aforementioned advantages, additional merit of the battery using a Li2S8-based electrolyte is the lower polarization for the (dis-)charge of LFP. Figure 2a−c demonstrates the charge/discharge profiles of the batteries carried out in different electrolytes. At the typical rate of 0.6 C, the hybrid battery has the lowest overpotentail of 98.1 mV, which is much lower than 115.2 and 214.1 mV of commerical Li ion and Li−S battery electrolytes. The difference of polarization becomes larger as the rate increases, under which the hybrid battery always shows a lower charge potential and higher discharge voltage at all variable rates, thereby giving rise to the lowest polarization among the three batteries using different electrolytes (Figure 2d). The advantages of Li2S8based electrolyte over the other two are more obvious at a higher rate (Figure 2e,f). For example, most lithium ions (that is, 0.66 Li+ vs 112 mAh g−1) in LFP can be extracted using the Li2S8-based electrolyte at a high rate of 2.5 C (cutoff voltage at 3.6 V), but it is impossible in commercial LIB electrolyte (only ∼3 mAh g−1 at a 3.6 V cutoff voltage; see details in Figure S5). Considerable lithium ions of 0.88 Li+, 0.75 Li+, and 0.58 Li+ can be deinserted/inserted in Li2S8-based electrolytes with lower polarizations of 148, 286, and 540 mV at 2.5, 6, and 12 C,

Figure 3. Electrochemical analysis. Comparative CV of the LFP lithium battery using Li2S8-based electrolyte and commercial electrolyte scanning from (a) 0.075 to (b) 0.1 to (c) 0.25 mV s−1. Plots of the normalized peak current (ip) with the square root of the scan rate (υ1/2) for (d) Li2S8-based electrolyte and (e) commercial LIB electrolyte. The insets of (d) and (e) are detailed descriptions of linear fitted results in the form of y = a + bx. Electrochemical impedance spectroscopy of batteries using different electrolytes, (f) Li2S8-based electrolyte, (g) commercial electrolyte, and (h) an electrolyte without Li2S8 as cycling.

insertion of lithium ions for LFP (i.e., Fe2+/Fe3+ redox reaction in LFP) in Li2S8-based electrolytes is only 347.9, 364.9, and 465.9 mV, which is much lower than 420.5, 439.0, and 555.7 mV of batteries using commercial LIB electrolytes at scan rates of 0.075, 0.1, and 0.25 mV s−1, respectively. The corresponding charge potentials of 3.61, 3.61, and 3.65 V are also largely reduced from the 3.68, 3.69, and 3.79 V of commercial LIB batteries. Another distinction of the battery using a commercial LIB electrolyte is the anodic peak at around 3.25 V, which is very broad and even tails to 2.5 V at a high scan rate (pink area below 3.25 V in Figure 3a−c). By contrast, the peak of the hybrid battery using Li2S8-based electrolyte is very sharp. It 531

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soluble Li2Sx (x ≥ 6), can be formed at the end of each charge (i.e., Li2S → 1/xLi2Sx + 2Li+ + 2e−, almost end at 2.8 V),30,31 under which it is dissociated (i.e., 1/xLi2Sx ↔ (2 + 2/x)Li+ + Sx2−, x ≥ 6), and then can provide abundant free Li+ and Sx2− in the electrolyte. Compared to the large dendritic anion TFSI (C2F6NO4S2−1), the size of Sx2− is much smaller; hence, the extracted Li+ can be quickly transported to the electrolyte likely due to smaller steric hindrance. (ii) With increasing charge voltage to ∼3.5 V, the LFP cathode is positively charged, and the lithium ions can be extracted from its crystalline structure (i.e., LiFePO4 → xLi+ + Li1−xFePO4 + xe−). At this time, the dissociated/soluble Sx2− (i.e., negative state) can gather around the LFP surface (i.e., δ+, positive state) by electrostatic interaction, and then, the extracted Li+ can be quickly transported to electrolytes in the presence of Sx2− (Figure 4b). The fast transfer of lithium ions may accelerate and make the extraction of lithium ions from LFP easier. (iii) Inversely for the discharge process at 3.45 V, the FePO4 electrode is negatively discharged (that is, δ−, negative state, enrichment of Li+ on the surface), and the Li+ can be inserted into the FePO4 reversely (i.e., FePO4 + xLi+ + xe− → LixFePO4) (inset of Figure 4a). In this way, the amount of inserted Li+ into FePO4 can be quickly supplemented by Sx2−, which can transfer Li+ from the electrolyte (Figure 4c). Thus, the lithium ion concentration gradient around LixFePO4 can be quickly compensated, facilitating the faster insertion rate. Therefore, faster lithium diffusion and lower electrical impedance give rise to lower polarization and better performance. Full Battery Performance. For the practical utilization of Li2S8based electrolyte, the electrochemical behavior of the hybrid LIB versus graphite (full battery) was further studied (Figure 5a). The full battery delivers 330 mAh g−1 at 0.6 C with fine charge−discharge curves; particularly, it can stabilize the battery performance over 500 cycles with the final capacity at around 145 mAh g−1. For the LFP/graphite full battery in commercial LIB electrolyte, a low capacity of 140 mAh g−1 is obtained, and it decays fast to 80 mAh g−1 and finally remains at around 50 mAh g−1 (Figure 5b). The stabilization effect of Li2S8-based electrolyte should be ascribed to the ether-based electrolyte, which can get a fine utilization of lithium,32,33 which is also consistent with the stabilized EIS results in the half battery (Figure 3f). This is the first time applying the polysulfide/etherbased electrolyte in a LIB for a higher capacity and stability. The capacity contribution of polysulfide (Li2Sx) can persist for several hundred cycles, and the electrolyte can stabilize the battery performance better with much lower polarization (Figure 5c,d). Although the slow decay of redox species exits (which may result from the migration and deposition of Li2Sx on the surface of the anode), it can be well addressed by many other approaches such as the intercalated layer as reported in Li−S/Mg−S batteries,34−39 and the impressive positive effect for lower polarization and higher stability is always demonstrated except the capacity contribution (Figure 5c,d). Finally, we believe that the intermediate discharge voltage from the Li2Sx can actually help to extend the working hour for LFP even after the full discharge of LFP. In particular, the voltage of a typical LFP/graphite battery always drops quickly at the end of discharge without any detectable sign. Thus, the redox species can also act as a cushion to maintain the voltage and avoid instant power shut-off in devices. Furthermore, the reproducible cycle performance over several hundred cycles and robust rate capacity at 0.25−12 C fully confirm the availability and

indicates a faster insertion rate of Li+ into LFP using Li2S8based electrolytes. To further understand the phenomena, the lithium diffusion constant (D) is extracted by the Randles− Sevcik equation28,29 i p = 2.69 × 105n3/2AD1/2 Cυ1/2

where ip indicates the peak current, n is the number of electrons in reaction, A is the electrode area, υ is the scanning rate, and C is the variation of lithium ion concentration in the electrolyte. The plot of the normalized peak current (ip) with the square root of the scan rate (υ1/2) is displayed in Figure 3d,e. The values of the diffusion constant are extracted as 9.75 × 10−6 and 7.97 × 10−6 cm2 s−1 for cathodic/anodic current peaks of the LFP cathode with the Li2S8-based electrolyte, which is much faster than that using commercial LIB electrolyte (3.04 × 10−6 and 3.21 × 10−7 cm2 s−1). The impedance spectra were also measured to examine the kinetic process (Figure 3f−h). Two salient features of the hybrid battery are confirmed, (i) higher stability and (ii) lower resistance. The hybrid battery using Li2S8-based electrolyte can achieve a stable state faster in initial cycles and keep a stabilized and lower resistance (i.e., 8 Ω) (Figure 3f). The results are significantly better than that in commercial electrolyte (i.e., 10 Ω) and the Li−S electrolyte (i.e., 80 Ω), which still has a wide fluctuation (Figure 3g,h). Clearly, the additive of Li2S8 into the Li−S battery electrolyte has significant effects on the final LIB performance, apart from the capacity contribution. Reaction Behaviors and Mechanism. The role of Li2S8 can be described in our presented model based on the typical voltage vs capacity profile (Figure 4a): (i) First, an ionic compound,

Figure 4. Reaction behaviors of the hybrid battery and schematic mechanism of the polysulfide as a (dis-)charge process. (a) Typical voltage vs capacity profile of the hybrid battery at 0.25 C, in which the Li2Sx and LFP come part in the redox reactions step-by-step. Light orange and blue arrows in (a) demonstrate the charge and discharge routes. The lithium ions (Li+) and polysulfide (Sx2−) can gather around LFP particles in turn as the (dis-)charged state is varied. The kinetic model of (b) Li1−xFePO4 (i.e., deinsert Li+ from LFP) and (c) LixFePO4 (i.e., insert Li+ into FePO4) in the charged and discharged states, in which the Sx2− and Li+ interact with the cathode particles by electrostatic interactions. The gradient color of the particles illustrates the variation of structure accompanying the deinsertion/insertion of lithium ions. The gray column represents the current collectors. 532

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Figure 5. Electrochemical performances of the full battery. (a) Typical voltage vs capacity profiles and (b) cycle performance of hybrid LFP/ graphite with Li2S8-based electrolyte and a normal LFP/graphite LIB in the initial 500 cycles under 0.6 C. Comparative (c) charge and (d) discharge profile of the full and half battery using different kinds of electrolytes at the 200th cycle. (e) Rate, cycle ability, and (f) typical voltage vs capacity profiles of the LFP/graphite full battery in Li2S8-based electrolyte from 0.25 to 12 C.

Notes

stability of this strategy for boosting up the performances of this hybrid LIB (Figure 5e,f). In summary, a novel concept of introducing redox species in electrolyte is proposed for the first time to enhance the performances of LIBs, and the addition of polysulfide improving the energy capacity and reducing the polarization of LFP-based battery is demonstrated. A completely new hybrid LFP/Li2Sx lithium battery with an extremely high energy density of 1124 Wh kgcathode−1 (442 mAh gcathode−1), robust rate capacities, and durable cycle performance was developed. Furthermore, the success of a LFP/Li2Sx full LIB with higher capacity, better stability, and more durable life over 500 cycles in a Li2S8-based electrolyte confirmed their prospective applications in commercialization. This strategy is expected to be applicable to other battery systems but also likely stimulates research discovering other kinds of reactive species with redox characteristics and/or additives in electrolyte for higher capacity and stability.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research was supported by KAUST.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.6b00274. Experimentsl section; digital photo and SEM images of the MWCNT-modified separator (Figure S1); SEM images, elemental mapping and EDX spectroscopy of the cycled electrode and separator (Figure S2); electrochemical performances of batteries (Figure S3); reaction route of the hybrid battery (Figure S4); and comparative electrochemical performances of batteries using Li2S8based and commercial LIB electrolyte (Figure S5) (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.M.). *E-mail: [email protected] (L.-J.L.). Author Contributions †

J.M. and M.L. contributed equally. 533

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DOI: 10.1021/acsenergylett.6b00274 ACS Energy Lett. 2016, 1, 529−534