An Ultrahigh Capacity Graphite/Li2S Battery with Holey‐Li2S Nanoarchitectures

Abstract The pairing of high‐capacity Li2S cathode (1166 mAh g−1) and lithium‐free anode (LFA) provides an unparalleled potential in developing safe and energy‐dense next‐generation secondary batteries. However, the low utilization of the Li2S cathode and the lack of electrolytes compatible to both electrodes are impeding the development. Here, a novel graphite/Li2S battery system, which features a self‐assembled, holey‐Li2S nanoarchitecture and a stable solid electrolyte interface (SEI) on the graphite electrode, is reported. The holey structure on Li2S is beneficial in decomposing Li2S at the first charging process due to the enhanced Li ion extraction and transfer from the Li2S to the electrolyte. In addition, the concentrated dioxolane (DOL)‐rich electrolyte designed lowers the irreversible capacity loss for SEI formation. By using the combined strategies, the graphite/holey‐Li2S battery delivers an ultrahigh discharge capacity of 810 mAh g−1 at 0.1 C (based on the mass of Li2S) and of 714 mAh g−1 at 0.2 C. Moreover, it exhibits a reversible capacity of 300 mAh g−1 after a record lifecycle of 600 cycles at 1 C. These results suggest the great potential of the designed LFA/holey‐Li2S batteries for practical use.


DOI: 10.1002/advs.201800139
(LFA), e.g., graphite, [14][15][16][17][18] tin (Sn), [19] silicon (Si), [20][21][22][23][24] and metal oxides, [25,26] which are more stable and safer than Li metal electrodes. Moreover, Li 2 S-based electrodes are advantageous in maintaining their structural integrity because the as-prepared Li 2 S electrodes are at their maximal volume. As an LFA for Li 2 S-based batteries, graphite can provide a higher cycling stability than conversion-type Sn and Si anodes due to its smaller volume expansion (9-13%) upon lithiation. These incentives have motivated the development of graphite/Li 2 S batteries in recent years. [14][15][16][17][18] However, two intractable barriers are impeding the progress. These are 1) the high potential barrier against Li 2 S oxidation during the first charge step and 2) the large irreversible capacity caused by the formation of SEI on the graphite surfaces. [15,18] These barriers are responsible for the low specific capacities of graphite/Li 2 S batteries. Because of its low electronic and ionic conductivity, bulk Li 2 S powder shows a high potential barrier when activating Li 2 S cathode-based batteries. The high potential barrier represents the difficulty in extracting lithium ion from Li 2 S, [27] which limits the depth of charging, thereby causing the low Li 2 S utilization. To address this issue, the size reduction of Li 2 S powders [27][28][29][30][31][32] and the fabrication of various Li 2 S composites [33][34][35][36][37][38][39][40] from Li 2 S powder have been widely reported. However, the use of commercial Li 2 S powder does not reduce the cost of Li 2 S cathodes for consumer-oriented batteries. The low-cost production is highly important for energy storage systems and electric vehicle applications where battery cost reduction is a key driver for their successful implementation. As a cost-effective route, the fabrication of Li 2 S cathodes by a carbothermal conversion of Li 2 SO 4 has been reported recently, [41][42][43][44][45][46][47][48][49] which also provides a chance to scale the fabrication of Li 2 S electrodes. However, due to the high conversion temperature required, most of the Li 2 SO 4 -converted Li 2 S cathodes have a high potential barrier upon their activation. Therefore, to achieve a high Li 2 S utilization of Li 2 S cathodes with an intrinsically high potential barrier is a great challenge to advance the graphite/Li 2 S batteries.
On the other hand, electrolyte design is another critical issue for achieving high-capacity graphite/Li 2 S batteries. Previously, the electrolytes containing 1 m bistrifluoromethanesulfonimide lithium salt (LiTFSI) in dioxolane (DOL)/dimethoxymethane (DME) and LiNO 3 additives were reported for graphite/Li 2 S batteries. [18] However, such ether-based electrolytes are known to decompose above 3.5 V, [50] which is unsuitable to Li 2 S cathodes with a high potential barrier. As a means to improve the oxidative stability of the ether-based electrolyte, a highly concentrated electrolyte (5 m LiTFSI in DME) was suggested for lithiated graphite/sulfur batteries. [51] The use of highly concentrated electrolyte is also beneficial in reducing the polysulfide shuttle [52][53][54][55] and the irreversible lithium loss for SEI formation on the graphite surface, [51,[56][57][58] both of which can enhance the Coulombic efficiency (CE) of the battery. In spite of these advantages, the highly concentrated electrolytes usually have high viscosity and low ion conductivity, [58,59] which can result in the poor electrolyte wetting of porous electrodes and low-rate capability, respectively. Therefore, the careful tuning of lithium salt concentration and solvent composition is needed.
Against the backdrop, we report a novel holey-Li 2 S nanoarchitecture fabricated by a facile, low-cost, and solid-state carbothermal reaction of Li 2 SO 4 , and a high-performance graphite/ Li 2 S battery with the holey-Li 2 S-based cathode as well as a conventional graphite electrode, and a concentrated DOL-rich electrolyte. The unique holey nanostructure, which can expand the Li 2 S/electrolyte interface, facilitates the oxidation of Li 2 S during the initial activation process. A 3 m LiTFSI DOL-rich electrolyte was rationally designed that considered the balance among ionic conductivity, oxidation stability, and SEI formation on the graphite anode. In addition, due to the use of the graphite anode instead of the Li metal anode, problematic polysulfide shuttle can be eliminated accordingly. The combined approach results in a graphite/holey-Li 2 S battery that has an ultrahigh initial discharge capacity of 810 mAh g −1 at 0.1 C and the long lifecycle over 600 cycles at a 1 C rate. These performances are far superior to those of the previous studies on graphite/Li 2 S batteries (Table S1, Supporting Information) and even better than conventional lithium ion batteries in terms of specific energy (Table S2, Supporting Information). This suggests that the graphite/Li 2 S battery with holey-Li 2 S nanoarchitectures and concentrated DOLrich electrolyte is highly promising for practical applications.
The novel Li 2 S cathode consisting of micrometer-sized Li 2 S particles with a hole (holey-Li 2 S) and carbon nanotube (CNT) network is fabricated from a low-cost commercial Li 2 SO 4 ·H 2 O via a facile two-step method (see the Experimental Section for the detailed fabrication process). First, plate-shaped Li 2 SO 4 particles embedded in a CNT network (plate-Li 2 SO 4 / CNT) (Figure 1, left) are obtained by the precipitation of the Li 2 SO 4 ·H 2 O aqueous solution in CNT containing ethanol solution and subsequent filtration. Second, the as-prepared plate-Li 2 SO 4 /CNT electrodes are converted to holey-Li 2 S/CNT electrodes (Figure 1, middle) via a carbothermal reduction reaction under N 2 gas at 700 °C for 3 h. The resulting holey-Li 2 S/CNT electrode is freestanding, and thus, can be used as a cathode without an additional binder. The pristine holey-Li 2 S particles embedded in the CNT network are oxidized to higher-order sulfur species, and these are redistributed in the CNT network during the initial charge process (Figure 1, right). More interestingly, with the conversion of these holey-Li 2 S particles, micrometer-sized pores can be formed accordingly in the electrodes, which, in the subsequent discharge/charge process, could enhance the lithium ion transport and improve the rate capability. The formation of such a plate structure can possibly profit from the use of negatively charged poly(acrylic) acid (PAA) as a surfactant, which has a strong affinity with Li ion and thus favors the formation of Li 2 SO 4 plates. The use of neutral poly(vinyl pyrrolidone) instead of PAA led to the formation of few micrometer-long strip-shaped Li 2 SO 4 ( Figure S3, Supporting Information). [60] Complete conversion from Li 2 SO 4 to Li 2 S during the carbothermal conversion is confirmed by the XRD pattern of the as-converted Li 2 S electrode ( Figure S4, Supporting Information), which shows that the diffraction peaks perfectly match with those of the cubic Li 2 S phase (JCPDS card No. . Interestingly, as shown in the SEM image of the asconverted Li 2 S electrode (Figure 2e), the plateshaped Li 2 SO 4 particle was transformed to a doughnut-shaped Li 2 S particle with a hole (holey-Li 2 S) during the carbothermal reduction process. This is different in shape from the shapes of the previously reported Li 2 S particles derived from Li 2 SO 4 compounds. [51][52][53][54][55][56][57][58][59] A magnified SEM image ( Figure 2f Figure 2f). It should be noted that the holey structure is beneficial in improving the lithium ion transfer between the electrolyte and Li 2 S particle due to the expanded interface. The inscrutable structural change from plate to doughnut can be understood as a self-assembly of the plates with the consumption of the near-by CNT matrix (Figure 2i). As indicated in the carbothermal reduction reaction equation, [43] Li 2 SO 4 + 2 C → Li 2 S + 2 CO 2 , the CNTs adjacent to the Li 2 SO 4 plates (left) are first consumed and the skin of the Li 2 SO 4 plates is converted to Li 2 S, preventing the direct contact between the surrounding CNTs and inner Li 2 SO 4 . However, the reaction between the generated CO 2 and CNT (CO 2 + 2 C → 2 CO) produces CO, [61] and the carbothermal reduction of the inner Li 2 SO can occur due to the strong reducing power of CO (Li 2 SO 4 + 4 CO → Li 2 S + 4 CO 2 ). The carbothermal conversion accompanies the compaction of the plates, and due to the absence of the nearby CNTs, the resulting Li 2 S plates become closer and eventually merge into a holey structure (right). The absence of CNT inside the hole suggests that the removal of CNT drives the assembly of the Li 2 S plates. The jointing of two plates observed for the holey-Li 2 S particle ( Figure S5, Supporting Information) further supports the self-assembled process of Li 2 S plates.
For comparison, a Li 2 S particulate without any holes was prepared by further heating the above holey-Li 2 S nanoarchitectures at 1000 °C for 3 h. Since the applied temperature is higher than the melting point of Li 2 S (938 °C), the holey structure was disrupted and a nonholey Li 2 S particulate was obtained, as shown in the SEM image and XRD pattern of the nonholey Li 2 S (solid-Li 2 S) ( Figures S6 and S7, Supporting Information).
To achieve high performance graphite/Li 2 S batteries, the selection of liquid electrolytes presents a challenge. In this work, we paid attention to 3 m LiTFSI DOL/DME electrolytes by considering the balance between high oxidation stability, high ionic conductivity, and the compatibility with Li 2 S cathode and graphite anode. As shown in Figure 3a, the 3 m LiTFSI DOL/DME electrolytes with different DOL/DME volume ratios (DOL/DME = 100/0, 85/15, 75/25, 50/50, and 0/100) showed higher oxidation stabilities compared with 1 m LiTFSI in DOL/ DME = 50/50 with 0.2 m LiNO 3 , which is conventionally used for Li 2 S or sulfur batteries. Although the oxidative stability might be further improved by increasing the LiTFSI concentration, there is a significant loss in ionic conductivity as previously reported. [59] and due to a solubility limit of LiTFSI salt at room temperature, the concentrations over 3 m could not be achieved ( Figure S8, Supporting Information).
In order to check the compatibility of the 3 m TFSI DOL/ DME electrolytes with graphite anode, the CEs during the 0.1 C rate cycling were measured for the graphite/Li batteries with the 3 m LiTFSI DOL/DME electrolytes. As shown in Figure 3b, the DOL/DME = 100/0, 85/15, and 0/100 electrolyte have an initial CE of 82.2%, 82.2%, and 82.6%, respectively. These values are much higher than those of the DOL/DME  reached over 99% after a few cycles, indicating the formed SEI films are quite stable. For the DOL/DME = 50/50 electrolytes, CEs gradually increased with the cycle, suggesting a gradual coverage of the SEI film on the graphite anode with the cycle. The DOL/DME = 0/100 electrolyte exhibited a fast CE fade with the cycle, which indicates that the SEI layer formed by 3 m TFSI in DME is not dense enough to prevent the cointercalation of the lithium ion and DME molecular into graphite. [62] The low irreversible capacity loss for the DOL-rich electrolytes can be associated with the formation of thin and uniform polymeric layer on graphite in DOL-based electrolytes. The advantage of the DOL-rich electrolytes is also supported by the good cycling stabilities for the various electrolytes (Figure 3c). For the DOLrich electrolytes (DOL/DME = 100/0, 85/15, and 75/25), highly stable cycling performances with discharge capacities higher than 350 mA g −1 were obtained over 40 cycles, which could be ascribed to a compact and uniform SEI layer derived from the DOL solvent [63] and an electrochemically stable SEI layer. [64] On the other hand, the DOL/DME = 0/100 electrolyte showed a fast capacity fade (Figure 3c, blue) due to the cointercalation. A more stable cycling performance was observed for the DOL/ DME = 50/50 electrolyte, meaning that the introduction of DOL solvent improves the SEI layer. However, the discharge capacities were quite low (≈220 mAh g −1 ), which can be attributed to the large irreversible capacity at the first charging step. Therefore, the DOL-rich electrolytes (DOL/DME = 100/0, 85/15, and 75/25) are more suitable for the graphite anode.
The compatibilities with Li 2 S cathode for the three DOLrich electrolytes were assessed with Li/holey-Li 2 S batteries. As shown in Figure S10 in the Supporting Information, the first discharge capacity was 773, 880, 792, and 870 mAh g −1 for DOL/DME = 100/0, 85/15, and 75/25 and 50/50, respectively. The discharge voltage plateau was 2.05, 2.10, 2.10, and 2.10 V for DOL/DME = 100/0, 85/15, 75/25, and 50/50 respectively. However, the initial potential barrier of Li 2 S cathode increases with the increase of DME content in the electrolyte and, when the content of DME arrives at 100%, the initial charge/discharge process quickly completed with a very low charge/discharge capacity ( Figure S10e, Supporting Information). These results indicate that a certain amount of DME is needed to attain the compatibility with the Li 2 S cathode, as previously observed for Li/sulfur batteries. [65,66] As compared (Table S1, Supporting Information), the ionic conductivity was the highest for DOL/DME = 85/15 among the three 3 m electrolytes and the lower concentrated electrolytes. Therefore, taking the above results into consideration, the 3 m LiTFSI DOL/DME = 85/15 electrolyte was selected for the graphite/holey-Li 2 S battery. Figure 4a compares the initial charge/discharge curves at the 0.1 C rate for the graphite/holey-Li 2 S and graphite/solid-Li 2 S batteries. The holey-Li 2 S cathode showed a lower initial potential barrier than the solid-Li 2 S electrode, which means that the holey structure facilitates the lithium extraction from Li 2 S. The charging capacity for the first charging with a cut-off voltage of 3.8 V was 1166 mAh g −1 for the graphite/holey-Li 2 S and 750 mAh g −1 for the graphite/solid-Li 2 S. It clearly shows that, by introducing the submicrometer scale hole to the Li 2 S particle, the charging overpotential for the Li 2 S oxidation can be considerably lowered and a higher depth of charging can be obtained.
The cycling performances of the graphite/holey-Li 2 S battery and graphite/solid-Li 2 S battery at 0.2 C after the initial activation were investigated (Figure 4b). The graphite/holey-Li 2 S battery delivered a discharge specific capacity of 712 mAh g −1 at the first cycle and 583 mAh g −1 at 100 cycles, which is the highest value among the graphite/Li 2 S batteries ever reported (Table S2, Supporting Information). The graphite/holey-Li 2 S battery showed a lower capacity fade rate (0.184%/cycle) than the Li metal/holey-Li 2 S battery (0.414%/cycle) ( Figure S11, Supporting Information), demonstrating the benefit of using graphite instead of Li metal in terms of cycling stability. The CEs of the graphite/holey-Li 2 S battery were maintained above 99% during the whole cycles, which is contrasted by the CEs around 98% for the Li /holey-Li 2 S battery ( Figure S8, Supporting Information). It indicates that the polysulfide shuttle can be suppressed in the graphite/holely-Li 2 S battery. To further clarify the polysulfide shuttle issue, the lithium metal and graphite electrode of the Li 2 S batteries were analyzed by X-ray photoelectron spectroscopy (XPS) after the initial charge. As shown ( Figure S12, Supporting Information), the peaks from the insoluble Li 2 S/Li 2 S 2 were clearly detected for the lithium metal surface electrode, while these peaks were unseen for the graphite surface, ensuring the prevention of polysulfide shuttle in the graphite/Li 2 S battery. The discharge capacity of the graphite/solid-Li 2 S battery gradually increased from 237 to 450 mAh g −1 during the first 30 cycles, followed by a mild capacity fade. The initial capacity increase in the early cycles suggests that the unactivated Li 2 S particles gradually decomposed during the cycle. In spite of the additional Li 2 S activation, Adv. Sci. 2018, 5, 1800139 the maximum discharge capacity was still far lower than that of the holey-Li 2 S electrode.
The rate capability of the holey-Li 2 S electrode was evaluated by investigating the discharge capacities for five cycles at a discharge rate with a successively increasing discharge rate as 0.1, 0.2, 0.3, 0.4 0.5, 0.6, 0.8, and 1 C and returning to 0.1 C. As shown in Figure 4d, the averaged discharge capacity was 760, 749, 734, 717, 700, 677, 632, and 570 mAh g −1 for the discharge rate of 0.1, 0.2, 0.3, 0.4 0.5, 0.6, 0.8, and 1 C, respectively. When the discharge current density returned to 0.1 C, a high discharge capacity of 664 mAh g −1 was recovered. The capacity retention from the C rate increase from 0.1 to 1 C was 75%. In addition, the graphite/holey-Li 2 S battery showed a high discharge potential plateau of 1.99 V at 0.1 C and 1.68 V at 1 C (Figure 4e), indicating a high power and energy density for the holey-Li 2 S/CNT electrode. The exceptionally excellent rate capacity can be attributed to the multiscale porosity of the activated holey-Li 2 S electrode. After the initial activation, micrometer-sized pores were generated with the decomposition of the holey-Li 2 S particles ( Figure S13, Supporting Information), which will be described in a later section. For sulfur cathodes, the efficacy of the multiscale porosity with micrometer and submicrometer pores in enhancing the rate capability was previously demonstrated. [67] The superiority of the graphite/holey-Li 2 S battery can be further supported by a comparison of battery energy densities between the graphite/LiCoO 2 and graphite/holey-Li 2 S batteries ( Figure 4e; Table S3, Supporting Information). The areal capacities of the two cathodes were controlled to be identical for fair comparison. As marked in Figure 4e, the energy density (based on the total mass of cathode and anode) at a current density of 56 mA g −1 (based on whole cathode mass) is 270 Wh kg −1 for the graphite/holey-Li 2 S battery and 206 Wh kg −1 for the graphite/LiCoO 2 battery, respectively. The comparison indicates that the holey-Li 2 S cathodes can exceed conventional metal oxide cathode in terms of energy density. Figure 4f shows an extended cycling stability test at 1 C for the graphite/holey-Li 2 S battery. When the current density was increased to 1 C after the initial activation at 0.1 C for the cycling, the discharge capacity at the first cycle was as low as 195 mAh g −1 . This is because the redistribution of the sulfur species over the CNT network was not fully proceeded during the initial activation. However, the discharge capacity gradually increased up to a maximum value of 400 mAh g −1 after 70 cycles, which was probably due to a gradual redistribution. To our interest, the discharge capacity of 300 mAh g −1 was maintained at 600 cycles with a high CE of 99%, which firmly demonstrates the merit of the graphite/holey-Li 2 S battery in terms of discharge capacity and cycling stability.
According to the initial electrochemical reaction of Li 2 S cathode:Li 2 S → S + 2Li + + 2e − , the original Li 2 S is converted into lithium polysulfides and sulfur upon the initial activation process. Accordingly, the electric energy is stored in the battery  system, which can be used in the discharge process. Therefore, the initial activation process is highly critical to the electrochemical performances of Li 2 S batteries. To further understand the structural and electrochemical changes during the initial activation for the holey-Li 2 S and solid-Li 2 S cathodes, SEM, XRD, and electrochemical impedance spectroscopy (EIS) analysis were conducted for the two electrodes, and the results are compared in Figure 5. After the initial activation, the holey-Li 2 S particles completely disappeared ( Figure 5a); however, some portion of the solid-Li 2 S particles remained in the CNT matrix as shown in the SEM images taken after the initial activation (Figure 5b). The XRD pattern of the holey-Li 2 S electrode after the initial activation did not show any peaks from crystalline Li 2 S (Figure 5c), indicating that the crystalline Li 2 S is completely decomposed (more easily decomposed for amorphous Li 2 S during charging [68] ) and the charged sulfur products are in their amorphous state. However, the XRD patterns from Li 2 S were clearly seen (Figure 5d) after the initial activation process for the solid-Li 2 S electrode, indicating the presence of undecomposed, residual Li 2 S. As shown in Figure 5e, the impedances of the two electrodes were nearly identical before the initial activation. However, after the initial activation, the impedances of the two batteries became very different (Figure 5f). For the holey-Li 2 S cathode, the semicircles were significantly reduced, indicating a faster charge transfer reaction after the activation. In contrast, for the solid Li 2 S cathode, two large semicircles and a long low frequency tail appeared after the activation. The small semicircle for the activated holey-Li 2 S cathode is in good agreement with the formation of amorphous charged sulfur species and effective redistribution of these species over the CNT matrix with the multiscale porosity. The appearance of the two semicircles may reflect the existence of decomposed and undecomposed regions in the CNT matrix. The EIS results indicate that the holey-Li 2 S structure is quite effective in achieving a high Li 2 S utilization and constructing high-performance LFA/Li 2 S batteries.

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In summary, a novel holey-Li 2 S nanoarchitecture was achieved by the newly invented carbothermal conversion process, which features the formation of plate-shaped Li 2 SO 4 particles and the self-assembly of the plates during the carbon thermal conversion from Li 2 SO 4 to Li 2 S. In order to reduce the electrolyte decomposition in the Li 2 S cathode at high potentials and the irreversible capacity loss of graphite anode, 3 m LiTFSI in DOL-rich DOL/DME electrolytes were newly designed. With the combination of the holey-Li 2 S cathode and the concentrated, DOL-rich electrolyte, the resulting graphite/holey-Li 2 S battery provided a record high specific capacity of 810 mAh g −1 at 0.1 C and exhibited excellent cycling stability over 600 cycles at 1 C rate. The systematic variations of the Li 2 S structure, electrolyte composition, and anode material (graphite and Li metal) indicate that the high performances of the graphite/holey-Li 2 S battery can be attributed to the three cooperative contributions; 1) The holey-Li 2 S nanostructure which facilitates the decomposition of Li 2 S particles during the initial activation, 2) the formation of a stable SEI layer on graphite with the electrolyte, and 3) the prevention of the polysulfide shuttle due to the use of the graphite anode. We believe that the novel holey-Li 2 S nanoarchitectures and the electrolyte design can boost the development of high-energy LFA/Li 2 S batteries for practical applications.
In a typical experiment, the freestanding holey-Li 2 S/CNT electrodes were fabricated by the following two steps. First, the sandwich-typed plate-Li 2 SO 4 /CNT electrodes were prepared by a precipitation method. Detailedly, an 80 mg of Li 2 SO 4 ·H 2 O powder was dissolved into a 5 mL of deionized water upon stirring to form a transparent Li 2 SO 4 solution. At the same time, a 25 mg of CNT and a 100 mg of PAA were in turn added to a 50 mL of absolute ethanol to form a uniformly dispersed CNT suspension via a sonication of 30 min. Then, another CNT suspension was prepared by the same treatment (Recipe: 10 mg CNT, 20 mg PVP, and 20 mL absolute ethanol). After that, the suspension containing a 25 mg of CNT mixed with a more 50 mL of ethanol (total volume: 100 mL) and the prepared Li 2 SO 4 aqueous solution were soaked in an iced water bath for 30 min upon stirring. Followed this step, the icy Li 2 SO 4 solution was transferred to a syringe and was injected slowly to the icy CNT suspension upon stirring to obtain a uniform Li 2 SO 4 /CNT suspension. Finally, the Li 2 SO 4 /CNT suspension was used to fabricate a sandwich-typed Li 2 SO 4 /CNT film by a vacuum filtration (The unused CNT suspension was evenly divided into two parts and was filtrated to as a bottom and an upper CNT layer, respectively. The as-prepared Li 2 SO 4 /CNT suspension was filtrated into the two CNT layers). The as-fabricated sandwich-typed film was peeled off and dried, and was punched into disks with a diameter of 12 mm for further drying at room temperature for overnight. Second, the holey-Li 2 S/CNT electrodes were obtained by a carbothermal reaction. Operationally, the as-prepared Li 2 SO 4 /CNT electrodes were put into the tube furnace under a flowing N 2 at 700 °C for 3 h and were converted into final holey-Li 2 S/CNT electrodes. The solid-Li 2 S/ CNT electrodes were obtained via a further heat treatment of holey-Li 2 S/CNT at 1000 °C for 3 h. The Li 2 S content and Li 2 S area loading in the two electrodes according to the mass change of before and after dissolution of Li 2 S into ethanol and deionized water are around 48 wt% and 2.0-2.25 mg cm −2 , respectively.

Microstructure Characterization:
The crystalline phase structures of all the converted electrodes were characterized by XRD (Smart lab). The morphology and structure of the electrodes were characterized by SEM (S4800) and TEM (Tecnai F30 ST). XPS characterization was carried out on an X-ray photoelectron spectroscopy (Kα). EIS data were collected in a frequency range of 1 MHz to 10 Hz using an alternating current (AC) impedance analyzer with amplitude of 10 mV. The electrochemical measurements were carried out on a battery cycler (TOSCAT-3000U) at 25 °C.
Electrochemical Characterization: Electrochemical performances of the electrodes were evaluated by using the assembled button-type batteries. The holey-Li 2 S/CNT and solid-Li 2 S/CNT electrodes were used as a working electrode and lithium metal foil (half batteries) or graphite electrode (full batteries) (provided by Samsumg Company) were used as a counter electrode. Celgard 2400 and 3 m-LiTFSI in DOL/DME (=85/15) were used as a separator and an electrolyte, respectively. All the batteries were assembled in an argon-filled glove box (H 2 O and O 2 content: <1 ppm). For charge/discharge behavior at a constant current density, the Li/Li 2 S batteries were first charge to 4.0 V then discharged to 1.5 V at a rate of 0.1 C (1 C = 1166 mA g −1 ). After that, the battery was cycled at a potential range from 1.5 to 3.0 V at a rate of 0.2 C. For the graphite/ Li 2 S full batteries (the capacity ratio of graphite anode and Li 2 S cathode is around 1.05-1.1:1), the batteries were first charged to 3.8 V and then discharged to 1.0 V at a rate of 0.1 C. Subsequently, the batteries were cycled at a rate of 0.2 C/1 C with a potential range from 1.0 to 3.0 V. The rate capabilities of the graphite/holey-Li 2 S battery were evaluated in a successive manner by varying the charge/discharge current density as 0.1, 0.2, 0.3, 0.4 0.5, 0.6, 0.8, and 1 C, and finally went back to 0.1 C, respectively.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.