Enhanced Lithium Storage Capacity of a Tetralithium 1,2,4,5

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Enhanced lithium storage capacity of a tetralithium 1,2,4,5benzenetetracarboxylate (LiC HO) salt through crystal structure transformation 4

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Handi Setiadi Cahyadi, Wendy William, Deepak Verma, Sang Kyu Kwak, and Jaehoon Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b03323 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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ACS Applied Materials & Interfaces

Enhanced Lithium Storage Capacity of A Tetralithium 1,2,4,5Benzenetetracarboxylate (Li4C10H2O8) Salt through Crystal Structure Transformation Handi Setiadi Cahyadi,a, ¤ Wendy William,a, ¤ Deepak Verma,a,b Sang Kyu Kwak,c,* and Jaehoon Kima,b,d,*

a

SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University,

2066, Seobu-Ro, Jangan-Gu, Suwon, Gyeong Gi-Do 16419, Republic of Korea b

School of Mechanical Engineering, Sungkyunkwan University,

2066, Seobu-Ro, Jangan-Gu, Suwon, Gyeong Gi-Do 16419, Republic of Korea c

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology,

50 Unist-gil, Ulsan 44919, Korea d

School of Chemical Engineering, Sungkyunkwan University,

2066, Seobu-Ro, Jangan-Gu, Suwon, Gyeong Gi-Do 16419, Republic of Korea

*

To whom correspondence should be addressed: e-mail: [email protected] (Prof. Jaehoon Kim),

[email protected] (Prof. Sang Kyu Kwak) ¤

Equally contributed as first authors

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Abstract Because of their low price, design flexibility, and sustainability, organic-based electrode materials are considered one of most promising next-generation alternatives to inorganic materials in Li-ion batteries. However, a clear understanding of the changes in the molecular crystal structure during Li-ion insertion/extraction and its relationship to excess capacity (over theoretical capacity) is still lacking. Herein, the tetralithium 1,2,4,5-benzenetetracarboxylate (Li4C10H2O8, Li4BTC) salt was prepared using a simple ion-exchange reaction at room temperature and under solvothermal conditions (100 °C). The solvothermally synthesised salt (Li4BTC-S) exhibited a well-ordered nanosheet morphology, while the room-temperature salt (Li4BTC-R) was comprised of irregularly shaped particles. During the cycling of Li4BTC-S, molecular rearrangement occurred to reduce the stress caused by repeated Li-ion insertion/extraction, resulting in a change in the crystal structure from triclinic to monoclinic and an increased free volume. This contributed to an increase in the reversible capacity to 1016 mAh g-1 during the initial 25 cycles at 0.1 A g-1, and, finally, the capacity stabilised at ca. 600 mAh g-1 after 100 cycles, which is much higher than its theoretical capacity (234 mAh g-1). Compared to Li4BTC-R, Li4BTC-S delivered a higher reversible capacity of 190 mAh g-1 at a high current density of 2 A g-1 with an excellent long-term cyclability of up to 1000 cycles, which was attributed to the straight free volume columns and the low charge transfer limitation.

Keywords: Organic electrode materials, lithium-ion batteries, Li4C10H2O8, solvothermal, density fluctuation theory, excess capacity

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Introduction Rechargeable lithium-ion batteries (LIBs) are promising power sources for large-scale energy storage applications, such as electric vehicles and energy storage systems, because of their high energy densities, high power densities, and safety.1-3 To meet the demands of the rapidly growing market for large-scale energy storage applications, many efforts have been devoted to developing alternative, safer, and cheaper electrode materials with higher energy and power densities and better cycling stabilities as compared to the currently used materials. Even though inorganic and carbon-based materials (e.g. LiCoO2, LiFePO4, LiNiyCoxMnzO2, graphite, and Li4Ti5O12) are the current electrode materials for commercial LIBs, the low theoretical capacities, high costs of starting materials and synthesis, and safety issues caused by Li metal plating (in the case of graphite) make them unsuitable for use in largescale applications.4 Recently, organic electrode materials such as organic free radical compounds, organosulfur compounds, and organic carbonyl compounds have drawn considerable attention as potential alternatives to the traditional transition metal-based inorganic and carbon-based materials because of their high capacities, low molecular weights, low starting material and synthetic costs, structural tunability, and sustainability.5-7 Much progress has been made to enhance the electrochemical performance of organic electrode materials for use as cathodes in LIBs by controlling their molecular structures.6-7 However, compared to the number of organic cathode materials, relatively few organic compounds have been proposed as potential, low-voltage anode materials. In 2009, Armand et al., first reported that a conjugated dicarboxylate, dilithium terephthalate (Li2C8H4O4) salt, which has low solubility in carbonyl-based electrolytes, can deliver a reversible capacity of 234 mAh g-1 at a current density of 15 mA g-1 after 50 cycles with a flat voltage plateau at 0.8 V (hereafter vs Li/Li+).8 Since this seminal work, various approaches have been developed to enhance the electrochemical performance of low-voltage organic electrode materials for potential anode applications. For example, to enhance the 3



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high rate performance of the Li2C8H4O4 salt, N-doped carbon was coated on the surface of porous Li2C8H4O4 particles;9 at a high current density of 301 mA g-1, a high reversible capacity of 121 mAh g-1 was obtained. When the Li2C8H4O4 salt was cycled at a low potential below 0.7 V, an excess reversible capacity of ca. 800 mAh g-1 was observed, which stabilised at ca. 600 mAh g-1.10 This was attributed to the additional attachment of Li to the aromatic core and the degradation of the crystalline structure. The cation exchange from Li to Ca in the terephthalate molecule (Ca2C8H4O4 salt) decreased the metal dissolution in the electrolyte, resulting in a high reversible capacity of 399 mAh g-1 at 0.05 C after 40 cycles.11 To further increase the energy density, lithium 4,4'-tolane-dicarboxylate (Li2C16H8O4), which has a large conjugated structure than that of Li2C8H4O4, was synthesised12; a reversible capacity of 200 mAh g-1 with a voltage plateau at 0.65 V was obtained. An organic tetralithium salt of 2,5dihydroxyterephthalic acid (Li4C8H2O6) with a nanosheet morphology exhibited a reversible capacity of 232 mAh g-1 after 50 cycles at 24.1 mA g-1 with a voltage plateau at 0.8 V.13 Bulk Li4C8H2O6 exhibited lower rate performance compared to the Li4C8H2O6 nanosheets because of its inherently low electronic conductivity. In this study, we demonstrate that tetralithium 1,2,4,5-benzenetetracarboxylate (Li4BTC, Li4C6H2(CO2H)4, Li4C10H6O8) exhibits remarkable electrochemical performance when used as an anode in LIBs. The symmetric aromatic system with π-conjugation in the core unit of the Li4BTC molecule increased the electrode kinetics and stability during the charge–discharge process because the core aromatic unit provides effective electronic delocalisation in the molecule.8, 14 To date, organicbased electrode materials for LIBs have been prepared through simple ion-exchange reactions under room temperature conditions.8, 10, 13-14 On the other hand, hydrothermal and solvothermal synthesis methods have been applied to control the morphologies of various types of inorganic-based electrode materials including LiFePO4,15-18 manganese fluoride,19 Li4Ti5O12,20-23 TiO2,24 and ZnO25. Herein, we introduce the relationship between morphology and electrochemical performance of Li4BTC 4


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synthesised using a solvothermal route and a conventional room temperature route. When tested as an anode material, the solvothermally synthesised Li4BTC was found to have a capacity of up to 1016 mAh g-1 during the first 25 cycles, which stabilised at ca. 600 mAh g-1 after 100 cycles. To understand the abnormal increase in capacity during cycling, the crystalline structure of Li4BTC was modelled using density functional theory Rietveld refinement. Total free volume of crystalline structure of the Li4BTC was further analysed using Connolly volume calculation.

Experimental Materials 1,2,4,5-Benzenetetracarboxylic acid (BTC, C6H2(CO2H)4, purity ≥96%) and lithium methoxide (CH3OLi, purity ≥98%) were purchased from Alfa Aesar (USA). High-performance liquid chromatography (HPLC) grade methanol (purity ≥99.9%) was purchased from Honeywell, Burdick & Jackson (USA). Distilled and deionised (DDI) water was prepared using an AQUAMaxTM-Basic 363 water purification system equipped with a 0.22-µm filter (Young Lin instrument Co., Ltd. South Korea). Poly(vinylidenedifluoride) (PVDF, Kureha Chem. Co., Japan), carbon black (Denka Co., Ltd., Japan), and 1-methyl-2-pyrrolidinone (NMP, purity ≥98%, Alfa Aesar) were used as received. Synthesis of the Li4BTC salt The Li4BTC salt was synthesised using an ion-exchange reaction, as shown in Scheme 1. First, 1.016 g of BTC (4 mmol) and 0.608 g of CH3OLi (16 mmol) were dissolved in 50 mL methanol separately. The CH3OLi solution was added into the BTC solution dropwise under vigorous stirring at room temperature. After 24 h stirring, the white precipitate that had formed in the mixed solution was collected by filtration and washed several times with ethanol. The product was then dried at 80 °C for 12 h in a vacuum oven. The dried sample was designated Li4BTC-R. The solvothermally synthesised Li4BTC was prepared by introducing the BTC and CH3OLi solutions into a Teflon-lined reactor with 5



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an inner volume of 150 mL at 25 °C. The solvothermal reaction was carried out at 100 °C for 24 h with stirring. The pressure of the reactor reached 0.33 MPa at 100 °C. The Li4BTC salt was washed using the same method as that was used for Li4BTC-R. The solvothermally-synthesised Li4BTC was designated Li4BTC-S. Characterisation The crystalline structures of the samples were analysed using a D/Max-2500 V/PC Rigaku X-ray diffractometer (XRD, Japan) with Cu Kα radiation generated at 40 kV and 50 mA. The diffraction patterns were measured in the 2θ range of 10 to 90°. The thermal decomposition behaviours of the samples were analysed using a Q50 thermogravimetric analyser (TGA, TA Instruments, USA) at temperatures ranging from 30 to 800 °C at a heating rate of 10 °C min-1 and an air flow rate of 60 mL min-1. The Brunauer–Emmett–Teller (BET) surface area was measured using a Belsorp-mini II apparatus (BEL Inc., Japan). The morphology of the samples was observed using a JEOL JSM-7500F field-emission-scanning electron microscope (FE-SEM, JEOL, USA). The functional groups of the samples were characterised using a NICOLET iS10 Fourier transform infrared (FT-IR) spectrometer (Thermo Electron Co. USA). The dissolution of the Li4BTC salt was measured via inductively coupled plasma mass spectrometry (ICP-MS) (Varian Inc., USA). Electrochemical property measurements The electrochemical measurements were conducted in a 2032-type coin-cell configuration assembled in an argon-filled glove box with the concentrations of moisture and oxygen below 0.1 ppm. A mixture of active material, carbon black, and PVDF with a weight ratio of 60:30:10 was blended in NMP to prepare a slurry of the active materials. The slurry was cast uniformly on a Cu foil using a doctor blade, followed by drying in a vacuum oven at 80 °C for 24 h. The dried electrode film was then punched into 14-mm-diameter discs (area, 1.54 cm2, and active material loading ~1.2 mg cm-2) and weighed. The 6


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test cells were fabricated with a composite anode and lithium metal as the counter electrode, which was separated by a Celgard®2500 microporous membrane (Celgard LLC, USA). The electrolyte was LiPF6 (1 M) dissolved in ethylene carbonate (EC)/dimethyl carbonate (DMC)/ethylmethyl carbonate (EMC) mixed solvent (volume ratio of EC/DMC/EMC = 1:1:1). Prior to electrochemical performance test, the samples were stored in a desiccator at room temperature to suppress self-discharging. The galvanostatic discharge–charge performances were monitored using a model WBCS 3000 battery testing system (WonA Tech Corp., South Korea) in the voltage range of 0.005–3.0 V at room temperature. Cyclic voltammetry (CV) curves were obtained using a model ZIVE MP1 potentiostatic analyser (WonATech Corp.) at room temperature at a scan rate of 0.5 mV s-1 between 0 and 3.0 V for the initial five charge– discharge cycles. Electrochemical impedance spectroscopy (EIS) tests were performed using a ZIVE MP1 impedance analyser (WonATech Corp.) over a 100 kHz to 0.01 Hz range. Computational details Density functional theory (DFT) calculations were carried out to determine the most stable structure of an isolated Li4BTC molecule using the DMol3 program.26-27 The generalised gradient approximation (GGA) with the Perdew–Burke–Ernzerhof (PBE) functional28 was employed to describe the exchangecorrelation energy of electrons. Double numerical basis sets with polarisation functions (DNP) under a 3.5 level were used. In addition, the semi-empirical dispersion correction method proposed by Tkatchenko and Scheffler29 was applied to correct for van der Waals dispersion effects. The convergence criteria for the geometry optimisation were set with an energy tolerance of 1.0 × 10-5 Ha, a maximum force tolerance of 0.002 Ha Å-1, and a maximum displacement of 0.005 Å. The crystal structure was determined from powder diffraction data using the reflex plus module of the material studio (MS) software. The overall procedure was carried out in four steps i.e. indexing, Pawley refinement, structure solution, and Rietveld refinement. During the indexing, experimentally 7



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obtained powder diffraction data were indexed using TREOR9030 program in the MS bundle to generate a unit cell. The accurate lattice constants and the cell parameters were determined by the Pawley refinement.31 The (weighted Rietveld parameter) value obtained after the refinement was used to establish the agreement between the calculated and the experimental powder patterns, confirming the accuracy of the crystal class and the cell parameters. The optimized structure of the Li4BTC with distinct structural variables such as torsions, angles and distances was imported into the refined unit cell and motion groups were assigned. The structure was obtained using the reflex powder solve module32 that involved the Monte-Carlo simulated annealing procedure. Ten cycles of simulated annealing were selected with each cycle involving 30,000,000 steps. The similarity between the experimental and the calculated diffraction patterns was confirmed by the values. The optimised structure solution was used as the initial structural model for Rietveld refinement33 to obtain a final crystal structure solution and a final value, which was calculated using eq 1.34

= ∑ ( , − , ) / ∑ ( , ).

(1)

In eq 1, is the weight function, , is the simulated model, and , is the observed intensity values obtained from experiment. On average, the values were in the range of 8.0–9.0%, indicating good agreement between the simulated structures and the experimental results. The obtained crystal structure was then used to estimate the Connolly volume, an empty volume spatially defined by rolling a solvent sphere around the van der Waals energy surface, thus yielding an accessible void in which Li ions could reside with a high probability.35 The resulting Connolly volume was compared with the increase of capacity between different cycle of Li4BTC.

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Results and discussion The FT-IR spectra of BTC, Li4BTC-S, and Li4BTC-R are shown in Figure 1. After the salt formation, the peaks at 3060–2550 cm-1, associated with the–OH stretching vibrations in carboxylic acids and aromatic –CH present in the BTC molecule, disappeared almost completely. In addition, the asymmetric and symmetric -C=O- stretching vibration at 1720 and 1440 cm-1, respectively, which are characteristic of the carboxylic acid groups in BTC, disappeared completely after the salt formation, while the peaks at 1583 and 1404 cm-1 appeared, which correspond to the asymmetric and symmetric COO- stretching vibration. In addition, the strong peaks at 1273/1119 and 926 cm-1 in the BTC spectrum, which can be attributed to the stretching vibrations of the C-O groups attached to the aromatic ring and out-of-plane OH bending, respectively, disappeared after salt formation; in contrast, O-Li out-of-plane bending peaks at 534 cm-1 were observed in the Li4BTC-S and Li4BTC-R samples. These results confirmed that the ion exchange reaction between BTC and CH3OLi had been successfully carried out, forming Li4BTC salts under both room temperature and solvothermal conditions. A low intensity peak at 1070 cm-1, which can be assigned to methanol C-O vibrations, was observed in the Li4BTC-R sample, suggesting the incomplete removal of the methanol from the roomtemperature synthesised salt.13 The bands at 3421 and 3530 cm-1 in the Li4BTC samples correspond to ν–OH vibrations of coordinated water molecules, which can be caused by the water adsorption when the sample are exposed to air.36-37 These band decreased in the Li4BTC-S and Li4BTC-R samples, indicating the loss of the loss of coordinated water after the salt formation to some extent. To examine the decomposition stability of Li4BTC-R and Li4BTC-S, TGA was carried out under air flow conditions, and the results are shown in Figure 2. Li4BTC-R exhibited a gradual weight loss of approximately 3.6 wt% at temperatures in the range 70–150 °C, which could be caused by the evaporation of residual methanol and any other volatile species that could be trapped in the Li4BTC-R sample. On the other hand, Li4BTC-S exhibited almost no weight loss at temperatures less than 500 °C, 9



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indicating the complete removal of methanol in the solvothermally synthesised, anhydrous Li4BTC salt. As shown in Figure S1, the TGA and differential thermogravimetric (DTG) profiles of BTC indicate a slight weight loss at 70 °C, which was caused by the evaporation of water and other volatile species, and a significant weight loss at 300 °C, which was caused by the total oxidation of the organic molecules into CO2 and H2O. The major decomposition temperature of BTC was much lower than those of Li4BTC samples (ca. 500 °C, Figure 2b). Therefore, the addition of Li to the carboxylic acid group in BTC enhanced the thermal stability of the Li4BTC salt significantly, which is beneficial for developing a highly stable and safe anode for LIBs. The residual weight remaining at 600 °C (~ 55 wt%) could be caused by the formation of Li2CO3 after the decomposition of the organic components in the Li4BTC salt ( () +

()

→ 2

()

+ () + (!) ). If 100 g Li4BTC salt

is completely combusted, 53.24 g Li2CO3 remains, which is very similar to the residual weight in the TGA profiles. This confirms the molecular formula of the Li4BTC salt. The gradual weight loss on increasing the temperature to 700 °C can be attributed to the decomposition of Li2CO3 to Li2O and CO2.38-39 The solvothermally synthesised Li4BTC-S exhibited different morphological and textural properties compared to those of Li4BTC-R, as shown in Figure 3. Li4BTC-R consists of irregularly shaped, primary particles with sizes of 100–500 nm that were heavily aggregated to form large, secondary particles with sizes of 0.5–3 µm. In contrast, Li4BTC-S was composed of well-ordered, uniform nanosheets with a thickness of ca. 100 nm that were well-stacked. The low-magnification SEM image shows that the rod-like bundles of the nanosheet with lengths of 10–15 µm are well-separated. The unique stacking behaviour could result in well-developed pores between the nanosheets. The textural properties and pore size distribution of Li4BTC-S and Li4BTC-R were examined using N2 adsorption-desorption isotherms, as shown in Figures 3c–d and listed in Table 1. The BET surface area 10


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of Li4BTC-S was approximately two times larger (10.9 m2 g-1) than that of Li4BTC-R (4.3 m2 g-1). The larger degree of hysteresis between the N2 adsorption and desorption isotherms of Li4BTC-S compared to Li4BTC-R suggests a larger pore volume in the former, as shown in the inset figures and listed in Table 1. In addition, the well-ordered uniform nanosheet structure of Li4BTC-S resulted in a higher tap density of 0.48 g cm-3 compared to that of Li4BTC-R (0.43 g cm-3). The crystalline structures of Li4BTC-R and Li4BTC-S were examined using XRD, and the patterns were simulated using Rietveld refinement, as shown in Figure 4. The structural and atomic coordinate parameters of Li4BTC-R and Li4BTC-S are listed in Tables S1 and S2. Both Li4BTC-R and Li4BTC-S show main Bragg peaks at 2θ = 10.1°, 16.4°, 20.4°, 22.3°, 25.5°, 30.9°, and 39.2° which were assigned to the (-101), (310), (410), (-112), (112), (-522), and (241) planes of the Li4BTC salt. The simulated crystalline structures are shown in the inset of the figures and indicate that the Li4BTC molecules were assembled in an A-B-A-B stacking mode and each Li4BTC was bound by π-π interactions between the aromatic groups. This A-B-A-B stacking leads to the formation of a layered structure with a small vacant volume in between the layers. This small vacant volume could provide lithiation sites for Li-ion during insertion/deinsertion process. In addition, a close inspection of the simulated structures in Figure S2 revealed that the Li atoms occupy the lateral layer spaces between the BTC molecules via interactions with multiple BTC molecules. These multiple interactions between Li atoms and BTC molecules could hold the layered structure together tightly, resulting in high decomposition stability, as observed in the TGA profiles. Because of the molecular self-assembly and the tight binding, the Li4BTC salt was barely soluble in the common solvents (water, methanol, and electrolyte) used for the seven-day immersion test (Figure S3 and Table S3). In the XRD patterns, the peaks of the assynthesised Li4BTC-S are stronger and narrower, especially those corresponding to the (310), (410), and (112) planes, than those of Li4BTC-R, indicating the Li4BTC-S salt exhibits higher crystallinity.

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This result agrees well with the SEM images (Figures 3a–b), which show that Li4BTC-S exhibited a well-ordered nanosheet structure compared to that of Li4BTC-R. Figures 5a and 5b show the CVs of the Li4BTC-R and Li4BTC-S electrodes collected at between 0 and 3.0 V at a scan rate of 0.5 mV s-1. During the initial cathodic scan (Li insertion), the Li4BTC-R electrode exhibited an irreversible peak at 0.66 V, which was caused by solid electrolyte interface (SEI) formation and irreversible Li-ion trapping.40 The redox pair at 1.11/1.57 V observed during the initial cathodic/anodic scans, which shifted in the low voltage direction of 0.97/1.51 V during the subsequent scans, is attributed to the enolisation of the carboxylates (two Li-ion redox reaction, see Scheme 1) in the Li4BTC-R salt. The peak shifting could be caused by the change in microscopic structure of Li4BTC salt during cycling as discussed in the previous section, which could result in the change in electronic and ionic conductivities. The low voltage redox pair at 0.01/0.22 V is caused by the insertion of Li ions into the carbon black that was used as one of the ingredients in the slurry preparation. The CV profile of the carbon black exhibited a similar low voltage redox pair at 0.01/0.25 V (Figure S4a). As shown in Figure 5b, the Li4BTC-S electrode shows a similar CV profile as the Li4BTC-R electrode with a slight difference in the redox peak positions. The slight difference in the redox peak positions between Li4BTC-R and Li4BTC-S might come from the difference in their crystallinity and morphology, which would change in resistance for Li ion diffusion. The subsequent cycles yielded CV profiles that are very similar, suggesting that Li-ion insertion/extraction in Li4BTC has good electrochemical reversibility.41 The galvanostatic charge–discharge properties of the Li4BTC-R and the Li4BTC-S electrodes were examined at a current density of 0.1 A g-1. Figure 5c shows the charge–discharge voltage profiles of the first and second cycles. The voltage plateaus at ca. 0.72 V during the discharge process (Li insertion) and ca. 1.75 V during the charging process (Li extraction) can be attributed to the reversible Li-ion uptake in the carboxylates, which agrees well with the CV results. The initial discharge capacity of 12


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Li4BTC-S is larger (349 mAh g-1) than that of Li4BTC-R (291 mAh g-1). In the subsequent cycle, the discharge capacities of Li4BTC-R and Li4BTC-S decreased to 109 and 148 mAh g-1, respectively, because of the formation of a SEI during the first Li-ion insertion. To understand the reaction mechanism of Li4BTC-S during the initial discharge process, ex situ powder XRD measurements were conducted, and the results are shown in Figure 5d. Although the peaks associated with the (310), (-112), and (112) planes were maintained during the first Li insertion process, the intensities of the peaks measured from the same loading of active material decreased significantly when cycled to ca. 0.5 V. This implies that the crystalline structure of the Li4BTC electrode changed during the charge–discharge process. The changes in the crystalline structure during cycling will be discussed more in detail in the following section. To investigate any possibility of chemical structure changes during the cycling, ex situ FT-IR spectra of the electrodes were measured, as shown in Figure S5. After the 1st discharge– charge process and 50 cycles at 0.1 A g-1, the peaks associated with the asymmetric and symmetric COO- stretching vibrations and the aromatic C=C vibrations were maintained. This suggests that the chemical structure of the Li4BTC salt was maintained during the Li insertion/extraction process. The cycling performances of the Li4BTC-R and the Li4BTC-S electrodes at 0.1 A g-1 are shown in Figure 6a. The discharge–charge profiles at different cycles are shown in Figure S6. After the 2nd cycle, the reversible capacities of Li4BTC-S rapidly increased to 1016 mAh g-1 over 25 cycles, then slowly decreasing and stabilising to approximately 600 mAh g-1 over the subsequent 100 cycles. In the case of Li4BTC-R, the reversible capacity gradually increased to 748 mAh g-1 during the first 45 cycles and stabilised to approximately 600 mAh g-1 after 100 cycles. The maximum reversible capacities and the stable capacities of the Li4BTC salts are much higher than the theoretically estimated capacity of Li4BTC (201 mAh g-1 based on the uptake of two Li ions per BTC molecule by the enolisation of carboxylates, Scheme 1). The contribution of carbon black, which was used to increase the conductivity of the composite electrode, to the reversible capacity was very small (100 mAh g-1 × 30 wt% 13



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carbon black ÷ 60 wt% Li4BTC = 50 mAh g-1). Therefore, additional Li uptake sites, other than the formation of –COOLi and the carbon black, must be present in the Li4BTC electrode. To gain an insight into the reasons for the abnormal increase in the capacity over the theoretical limit during cycling, ex situ XRD analyses of the Li4BTC-R and Li4BTC-S electrodes at the 1st, 10th and 50th cycles of the fully charged state (3.0 V) were conducted, as shown in Figures S7a–f. The XRD patterns were simulated using a Rietveld refinement produced from the DFT crystal model, and the results are listed in Tables S4–S9. Because the chemical structure of the Li4BTC salt was maintained during cycling (Figure S5), the Li4BTC molecule was used in the simulation. Plausible differences in the Li lithiation sites caused by the structure change in the Li4BTC crystal during cycling were further investigated using Connolly volume calculations, which allowed us to calculate the empty spaces in the structure that could accommodate Li ions, and the results are shown in Figures 6b–g. During the first cycle (point AS and AR in Figure 6a), the Li4BTC-R and Li4BTC-S salts have a triclinic crystal structure, forming layers between the molecular assemblies (Figures 6b and e). This implies that the Li4BTC electrode has well-developed lithiation sites located in the spaces between the lateral planes of the Li4BTC molecules. The structure of the Li4BTC-S salt has a larger spacing between each molecule of Li4BTC compared to that of the Li4BTC-R salt (Figures 6b and e). In addition, the total free volume of Li4BTC-S was about 20% larger than that of Li4BTC-R. Thus, it can be expected that Li4BTC-S would have facile solid-state lithiation sites for Li ions to penetrate the Li4BTC-S structure compared to Li4BTC-R. During the first discharge and charge cycle, the total free volume of Li4BTC-R and Li4BTC-S decreased to 60% and 57%, respectively, of the electrodes prior to cycling. The change in the XRD intensity could be related to the shrinking size of the crystal of Li4BTC-R and Li4BTC-S (Figures S7a–b). This causes the capacity loss from 350 to 130 mAh g-1 and from 290 to 85 mAh g-1 for the Li4BTC-R and Li4BTC-S electrodes, respectively. After the 10th cycle (point BS and BR in Figure 6a), the Li4BTC-R electrode maintained the triclinic lattice crystal structure with a slight 14


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increase in the Connolly volume (Figure 6c). On the other hand, for the Li4BTC-S electrode, a significant increase in the lattice parameters on cycling was observed, but the triclinic system was maintained (Figure 6f); the Li4BTC-S electrode has a free volume around three times larger than that at the 1st cycle. This is because a new lithiation sites is formed during cycling, which is connected in the diagonal direction with respect to the lateral plane of the Li4BTC molecules. The significant increase in the free volume during the cycling of Li4BTC-S agrees well with the more than triple increase in capacity from 130 (point AS) to 403 mAh g-1 (point BS). In the case of the Li4BTC-R electrode, the reversible capacity increased almost doubled, from 85 (point AR) to 151 mAh g-1 (point BR). After the 50th cycle (point CS and CR in Figure 6a), the crystal structure of the Li4BTC-R electrode changed from triclinic to hexagonal, while the crystal structure of the Li4BTC-S electrode changed from triclinic to monoclinic. This change in the crystal system of both samples altered the lithiation sites dramatically. A schematic representation of the crystal structure change during cycling is shown in Scheme 2. In the case of Li4BTC-S, a tubular cavity formed between the rearranged Li4BTC-S molecules after the 10th cycle, and this volume then increased. In addition, the cavities connected to form long diagonal tubular cavities as the distance between the Li4BTC-S molecules increased at the end of the 50th cycle. This formed a straight lithiation sites, which is beneficial for the electrochemical performance under high charge–discharge conditions. In the case of Li4BTC-R, the Li4BTC molecules did not experience any significant increase in the lateral plane distance during the 10 cycles, as observed in the Li4BTC-S electrode. The zig-zag arrangement of the Li4BTC-R molecules was maintained after 50 cycles, resulting in the lithiation sites also having a zig-zag shape. Because of this significant molecular rearrangement, the free volume of the Li4BTC-R electrode increased to around nine times at point CR compared to the free volume at point AR. Similarly, the free volume of the Li4BTC-S electrode increased to around six times at point CS compared to that at point AS. The increase in free volume is in a good agreement with the increase in the reversible capacity from 85 (point AR) to 745 mAh g-1 (point 15



Page 16 of 41

CR) in the case of the Li4BTC-R electrode and from 130 (point AS) to 790 mAh g-1 (point CS) in the case of the Li4BTC-S electrode. After stable reversible capacities had been achieved at 0.1 A g-1 over 100 cycles (Figure 6a), both electrodes were progressively charged and discharged from 0.1 to 8 A g-1 for 70 cycles and again at 0.1 A g-1 for 30 cycles to investigate the high-rate performance. The discharge–charge profiles at different current densities are shown in Figure S8. As shown in Figure 7a, in the low current density regimes of 0.1 to 0.25 A g-1, the reversible capacities of the Li4BTC-S electrode were slightly larger than those of the Li4BTC-R electrode, whereas, as the current density increased to 1 to 8 A g-1, the Li4BTC-S electrode exhibited significantly larger reversible capacities than those of the Li4BTC-R electrode. This indicates that the monoclinic crystal structure of the Li4BTC-S electrode, which contains straight free volume columns between the molecular assemblies, has higher Li-ion diffusion kinetics compared to the hexagonal crystal structure of the Li4BTC-R electrode, which contains a zig-zag shaped free volume. When returning to 0.1 A g-1 after 30 cycles of fast charging and discharging at 2 to 8 A g-1, the Li4BTC-S electrode exhibited an initial capacity loss of 8.3% (from 590 to 541 mAh g-1), while the Li4BTC-R electrode exhibited a higher loss of its initial capacity of 16.0% (from 575 to 483 mAh g-1). This suggests the better structural integrity of the solvothermally synthesised sample compared to the room-temperature synthesised sample. To investigate the long-term cyclability, the previously tested Li4BTC-S electrode (Figures 6a and 7a) was further cycled at 0.5 A g-1, and the results are shown in Figure 7b. Excellent cycling performance with no capacity decay was observed. During the 1000 cycles, the reversible capacity was increased from 340 to 420 mAh g-1 with almost 100% coulombic efficiency. The capacity after the 1000 cycles was higher than that at the same current density in Figure 7a (310 mAh g-1). The increase in capacity could be caused by the microscopic volume expansion during the long-term cycling.

16


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To understand the effect of solvothermal synthesis on the Li-ion transport kinetics further, EIS analysis of Li4BTC-S and Li4BTC-R was performed at 3.0 V prior to cycling after the 1st, 10th, and 50th charge–discharge processes. Nyquist plots of Li4BTC-R and Li4BTC-S are shown in Figure 8a and Figure 8b, respectively. Before cycling, the Nyquist plots are composed of two parts: a depressed semicircle in the high-to-middle frequency region, which corresponds to the charge transfer associated with the Li-ion redox reaction at the surface of the active material, and a linear sloping line at the lowfrequency region, which reflects the solid-state Li-ion diffusion within the electrode. After the 1st cycle, an additional semicircle was observed in the high-frequency region, which is related to the formation of the SEI on the electrode surface. The data were fitted using an equivalent circuit model with electrolyte solution resistance (Re), SEI resistance (RSEI), and charge transfer resistance (Rct), and the Warburg impedance caused by solid-state Li-ion diffusion into the Li4BTC phase (Zw) as parameters, and the results are listed in Table 2. Overall, the resistance values of Li4BTC-S are consistently lower than those of Li4BTC-R, which could be one of the reasons for the better high-rate performance of the Li4BTC-S electrode. The RSEI and Rct values of the Li4BTC-R and Li4BTC-S electrodes decreased with cycling. It is not clear what is causing this, but re-dissolution of SEI layer formed during the first discharge-charge cycle into the electrolyte phase and the structural rearrangement of the Li4BTC molecules during cycling that favors the charge transfer kinetics could be responsible for the decrease in RSEI and Rct. As shown in Figure 7b, the Li4BTC-S electrode exhibited an extremely high cycling stability for up to 1000 cycles. To further investigate the cycling stability of the Li4BTC electrode, the possible dissolution of the Li4BTC molecule after the cycling was tested. After 20 cycles of the Li4BTC-S electrode at 0.1 A g-1, the fully discharged (at 0.005 V) and charged (at 3.0 V) electrodes were immersed in the electrolyte solution for seven days, and the Li-ion concentration in the electrolyte was measured using ICP-MS. As listed in Table S3, a negligible number of Li ions was detected in the 17



Page 18 of 41

electrolyte solution, which is very similar to the Li4BTC-S electrode before cycling. This indicates that the crystalline structural change during the cycling did not have a significant effect on the dissolution stability of the electrode. Lastly, the Li4BTC-R and Li4BTC-S electrodes prior to cycling and after 20 cycles at 0.1 A g-1 were observed using SEM to examine possible morphological changes of the Li4BTC composite electrode during the Li-ion insertion/extraction (Figures 9). The surface and the cross-sectional morphologies of the Li4BTC electrode before cycling and after the 20th cycle did not change significantly. In the case of alloying and conversion materials, the continuous pulverisation of the active material phase during cycling leads to the formation of thick SEI layers and huge volume expansion caused by the alloying and conversion reactions, which results in a loss of electrical contact and the degradation of composite electrodes.42-43 On the other hand, the microscopic volume expansion of the Li4BTC salt was accommodated in the composite electrode geometry, so that the macroscopic morphological change to the Li4BTC composite electrode was negligible. This could be the reason for the excellent cycling stability of the Li4BTC-S electrode. Therefore, the extremely stable cyclability, the high reversible capacity, and the high thermal and chemical stability make Li4BTC a promising electrode material for next-generation LIB anodes.

Conclusions In summary, we have successfully demonstrated a promising solvothermal method to synthesize lithium 1,2,4,5-benzenetetracarboxylic acid (Li4BTC) nanosheets with enhanced electrochemical performance. The solvothermally synthesised Li4BTC-S salt exhibited a well-ordered, uniform nanosheet morphology with a thickness of ca. 100 nm. In contrast, the room-temperature-synthesised Li4BTC-R salt was composed of irregularly shaped particles. During cycling, the reversible capacity of the Li4BTC electrode increased beyond the theoretical limit (201 mAh g-1) and then stabilised at ca. 18


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600 mAh g-1. Based on the experimental and computational analyses, the abnormal increase in capacity of Li4BTC-S was attributed to the changes in its crystalline structure, from triclinic to monoclinic, and consequent increased free volume, all of which occurred without changes to the chemical structure. In addition, the chemical and structural integrity of the Li4BTC-S composite electrode resulted in a high reversible capacity at a high current density (420 mAh g-1 at 0.5 A g-1) and an excellent long-term cyclability of up to 1000 cycles without a capacity loss. Therefore, we believe that the solvothermally synthesised Li4BTC-S is a promising electrode material for next-generation large-scale LIB applications.

Supporting Information TGA and DTG profiles of BTC, Structural parameters and atomic coordinates of the Li4BTC-S and the Li4BTC-R powders, Layered structures of Li4BTC-R and Li4BTC-S, electrode dissolution test, cyclic voltammograms and charge–discharge capacity of the carbon black electrode, FT-IR spectra of the Li4BTC-R and the Li4BTC-S electrodes, and Rietveld refinements. This material is available free of charge via the Internet at http://pubs.acs.org.

Conflicts of interest There are no conflicts to declare.

Acknowledgements This research was supported by a National Research Foundation of Korea (NRF) grant provided by the Korean government (MSIP) (No. 2016R1A2B3008800). Additional support provided by an NRF grant funded by the Korean Government (MSIP) (No. 2016R1A2B3008800 and 2015H1D3A1066544) is also appreciated.

19



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References 1.

Goodenough, J. B.; Park, K.-S., The Li-Ion Rechargeable Battery: A Perspective. J. Am. Chem.

Soc. 2013, 135, 1167-1176. 2.

Goodenough, J. B., Electrochemical Energy Storage in a Sustainable Modern Society. Energy

Environ. Sci. 2014, 7, 14-18. 3.

Armand, M.; Tarascon, J. M., Building Better Batteries. Nature 2008, 451, 652-657.

4.

Ji, L.; Lin, Z.; Alcoutlabi, M.; Zhang, X., Recent Developments in Nanostructured Anode

Materials for Rechargeable Lithium-Ion Batteries. Energy Environ. Sci. 2011, 4, 2682-2699. 5.

Poizot, P.; Dolhem, F., Clean Energy New Deal for a Sustainable World: From Non-CO2

Generating Energy Sources to Greener Electrochemical Storage Devices. Energy Environ. Sci. 2011, 4, 2003-2019. 6.

Liang, Y.; Tao, Z.; Chen, J., Organic Electrode Materials for Rechargeable Lithium Batteries.

Adv. Energy Mater. 2012, 2, 742-769. 7.

Häupler, B.; Wild, A.; Schubert, U. S., Carbonyls: Powerful Organic Materials for Secondary

Batteries. Adv. Energy Mater. 2015, 5, 1402034-n/a. 8.

Armand, M.; Grugeon, S.; Vezin, H.; Laruelle, S.; Ribiere, P.; Poizot, P.; Tarascon, J. M.,

Conjugated Dicarboxylate Anodes for Li-Ion Batteries. Nat. Mater. 2009, 8, 120-125. 9.

Zhang, H.; Deng, Q.; Zhou, A.; Liu, X.; Li, J., Porous Li2C8H4O4 Coated with N-Doped

Carbon by Using CVD as an Anode Material for Li-Ion Batteries. J. Mater. Chem. A 2014, 2, 56965702. 10.

Lee, H. H.; Park, Y.; Shin, K.-H.; Lee, K. T.; Hong, S. Y., Abnormal Excess Capacity of

Conjugated Dicarboxylates in Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2014, 6, 1911819126.

20


Page 21 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11.


Abdolhosseinzadeh, S.; Asgharzadeh, H.; Seop Kim, H., Fast and Fully-Scalable Synthesis of

Reduced Graphene Oxide. Sci. Rep. 2015, 5, 10160. 12.

Walker, W.; Grugeon, S.; Vezin, H.; Laruelle, S.; Armand, M.; Wudl, F.; Tarascon, J.-M.,

Electrochemical Characterization of Lithium 4,4[Prime or Minute]-Tolane-Dicarboxylate for Use as a Negative Electrode in Li-Ion Batteries. J. Mater. Chem. 2011, 21, 1615-1620. 13.

Wang, S.; Wang, L.; Zhang, K.; Zhu, Z.; Tao, Z.; Chen, J., Organic Li4C8H2O6 Nanosheets for

Lithium-Ion Batteries. Nano Lett. 2013, 13, 4404-4409. 14.

Walker, W.; Grugeon, S.; Vezin, H.; Laruelle, S.; Armand, M.; Tarascon, J. M.; Wudl, F., The

Effect of Length and Cis/Trans Relationship of Conjugated Pathway on Secondary Battery Performance in Organolithium Electrodes. Electrochem. Commun. 2010, 12, 1348-1351. 15.

Yang, H.; Wu, X.-L.; Cao, M.-H.; Guo, Y.-G., Solvothermal Synthesis of Lifepo4

Hierarchically Dumbbell-Like Microstructures by Nanoplate Self-Assembly and Their Application as a Cathode Material in Lithium-Ion Batteries. J. Phys. Chem. C 2009, 113, 3345-3351. 16.

Saravanan, K.; Balaya, P.; Reddy, M. V.; Chowdari, B. V. R.; Vittal, J. J., Morphology

Controlled Synthesis of LiFePO4/C Nanoplates for Li-Ion Batteries. Energy Environ. Sci. 2010, 3, 457463. 17.

Hwang, J.; Kong, K. C.; Chang, W.; Jo, E.; Nam, K.; Kim, J., New Liquid Carbon Dioxide

Based Strategy for High Energy/Power Density Lifepo4. Nano Energy 2017, 36, 398-410. 18.

Hong, S.-A.; Kim, S. J.; Chung, K. Y.; Chun, M.-S.; Lee, B. G.; Kim, J., Continuous Synthesis

of Lithium Iron Phosphate (LiFePO4) Nanoparticles in Supercritical Water: Effect of Mixing Tee. J. Supercrit. Fluids 2013, 73, 70-79. 19.

Rui, K.; Wen, Z.; Lu, Y.; Jin, J.; Shen, C., One-Step Solvothermal Synthesis of Nanostructured

Manganese Fluoride as an Anode for Rechargeable Lithium-Ion Batteries and Insights into the Conversion Mechanism. Adv. Energy Mater. 2015, 5, 1401716-n/a. 21



20.

Page 22 of 41

Nugroho, A.; Yoon, D.; Chung, K. Y.; Kim, J., Synthesis of Li4Ti5O12/Carbon

Nanocomposites in Supercritical Methanol for Anode in Li-Ion Batteries: Effect of Surface Modifiers. J. Supercrit. Fluids 2015, 101, 72-80. 21.

Nugroho, A.; Kim, S. J.; Chang, W.; Chung, K. Y.; Kim, J., Facile Synthesis of Hierarchical

Mesoporous Li4Ti5O12 Microspheres in Supercritical Methanol. J. Power Sources 2013, 244, 164-169. 22.

Nugroho, A.; Kim, S. J.; Chung, K. Y.; Kim, J., Synthesis of Li4Ti5O12 in Supercritical Water

for Li-Ion Batteries: Reaction Mechanism and High-Rate Performance. Electrochim. Acta 2012, 78, 623-632. 23.

Nugroho, A.; Chung, K. Y.; Kim, J., A Facile Supercritical Alcohol Route for Synthesizing

Carbon Coated Hierarchically Mesoporous Li4Ti5O12 Microspheres. J. Phys. Chem. C 2013, 118, 183-193. 24.

Permana, A. D. C.; Nugroho, A.; Chung, K. Y.; Chang, W.; Kim, J., Template-Free Synthesis

of Hierarchical Porous Anatase TiO2 Microspheres with Carbon Coating and Their Electrochemical Properties. Chem. Eng. J. 2014, 241, 216-227. 25.

Kim, J.; Hong, S.-A.; Yoo, J., Continuous Synthesis of Hierarchical Porous Zno Microspheres

in Supercritical Methanol and Their Enhanced Electrochemical Performance in Lithium Ion Batteries. Chem. Eng. J. 2015, 266, 179-188. 26.

Delley, B., An All‐Electron Numerical Method for Solving the Local Density Functional for

Polyatomic Molecules. J. Chem. Phys. 1990, 92, 508-517. 27.

Delley, B., Fast Calculation of Electrostatics in Crystals and Large Molecules. J. Phys. Chem.

1996, 100, 6107-6110. 28.

Perdew, J. P.; Burke, K.; Ernzerhof, M., Generalized Gradient Approximation Made Simple.

Phys. Rev. Lett. 1996, 77, 3865-3868.

22


Page 23 of 41 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

29.


Tkatchenko, A.; Scheffler, M., Accurate Molecular Van Der Waals Interactions from Ground-

State Electron Density and Free-Atom Reference Data. Phys. Rev. Lett. 2009, 102, 073005. 30.

Werner, P.-E.; Eriksson, L.; Westdahl, M., Treor, a Semi-Exhaustive Trial-and-Error Powder

Indexing Program for All Symmetries. J. Appl. Crystallogr. 1985, 18, 367-370. 31.

Pawley, G., Unit-Cell Refinement from Powder Diffraction Scans. J. Appl. Crystallogr. 1981,

14, 357-361. 32.

Engel, G.; Wilke, S.; König, O.; Harris, K.; Leusen, F., Powdersolve–a Complete Package for

Crystal Structure Solution from Powder Diffraction Patterns. J. Appl. Crystallogr. 1999, 32, 1169-1179. 33.

Rietveld, H., A Profile Refinement Method for Nuclear and Magnetic Structures. J. Appl.

Crystallogr. 1969, 2, 65-71. 34.

Young, R. A.; Wiles, D. B., Profile Shape Functions in Rietveld Refinements. J. Appl.

Crystallogr. 1982, 15, 430-438. 35.

Connolly, M. L., Analytical Molecular Surface Calculation. J. Appl. Crystallogr. 1983, 16, 548-

558. 36.

Rzączyńska, Z.; Ostasz, A.; Sikorska-Iwan, M.; Głuchowska, H.; Olszewska, E.; Pikus, S.,

Synthesis and Characterization of Metal Polycarboxylates Constructed from Lanthanides(Iii) and 1,2,4,5-Benzenetetracarboxylic Acid. J. Therm. Anal. Calorim. 2006, 84, 575-579. 37.

Li, X.; Wang, Y.; Ma, Z.; Zhang, R.; Zhao, J., Synthesis and Characterization of Two 3-D

Polymeric Lanthanide Complexes Constructed from 1,2,4,5-Benzenetetracarboxylic Acid. J. Coord. Chem. 2010, 63, 1029-1037. 38.

Curini, R.; D'Ascenzo, G.; De Robertis, A.; De Stefano, C.; Sammartano, S., Studies on

Polyfunctional O-Ligands. Solubility and Thermal Stability of 1,2,4,5-Benzenetetracarboxylate Complexes with Alkali and Alkaline Earth Metal Ions. Thermochim. Acta 1990, 173, 25-41.

23



39.

Beyer, H.; Meini, S.; Tsiouvaras, N.; Piana, M.; Gasteiger, H. A., Thermal and Electrochemical

Decomposition of Lithium Peroxide in Non-Catalyzed Carbon Cathodes for Li-Air Batteries. Phys. Chem. Chem. Phys. 2013, 15, 11025-11037. 40.

Verma, P.; Maire, P.; Novák, P., A Review of the Features and Analyses of the Solid

Electrolyte Interphase in Li-Ion Batteries. Electrochim. Acta 2010, 55, 6332-6341. 41.

Appetecchi, G. B.; Croce, F.; Marassi, R.; Panero, S.; Ronci, F.; Savo, G.; Scrosati, B., Novel

Types of Lithium-Ion Polymer Electrolyte Batteries. Solid State Ionics 2001, 143, 73-81. 42.

Cook, J. B.; Lin, T. C.; Detsi, E.; Weker, J. N.; Tolbert, S. H., Using X-Ray Microscopy to

Understand How Nanoporous Materials Can Be Used to Reduce the Large Volume Change in Alloy Anodes. Nano Lett. 2017, 17, 870-877. 43.

Page 24 of 41

Oumellal, Y.; Delpuech, N.; Mazouzi, D.; Dupre, N.; Gaubicher, J.; Moreau, P.; Soudan, P.;

Lestriez, B.; Guyomard, D., The Failure Mechanism of Nano-Sized Si-Based Negative Electrodes for Lithium Ion Batteries. J. Mater. Chem. 2011, 21, 6201-6208.

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Figure captions Scheme 1. Formation of the Li4BTC salt and insertion/de-insertion of Li ions during the charge– discharge process Figure 1. FT-IR spectra of BTC, Li4BTC-R, and Li4BTC-S. Figure 2. (a) TGA and (b) DTG profiles of Li4BTC-R and Li4BTC-S. Figure 3. (a) and (b) SEM images and (c) and (d) N2 adsorption-desorption isotherms of Li4BTC-R and Li4BTC-S Figure 4. Rietveld refinement of XRD patterns of (a) Li4BTC-R and (b) Li4BTC-S powders; the experimental powder XRD patterns (red + marks), calculated powder XRD patterns (green line), and difference profiles (blue line). Inset: Structure of the powder sample taken from its XRD pattern (Li = purple, O = red, C = grey, and H = white) Figure 5. Cyclic voltammograms of (a) Li4BTC-R and (b) Li4BTC-S, charge–discharge profile at a current density of 0.1 A g-1 of (c) Li4BTC-R and Li4BTC-S, and ex situ XRD patterns of (d) Li4BTC-S during at different state of charges. Figure 6. Electrochemical performance of (a) Li4BTC-R and Li4BTC-S electrodes and cyclability at a current density of 100 mA g-1 and crystalline structure of the Li4BTC modelled from its XRD pattern (intercalated Li = blue, O = red, C = grey, and H = white) viewed along the b-axis at (b, c) 1st cycle, (d, e) 10th cycles, and (f, g) 50 cycles Scheme 2. Illustration of structural changes during the cycling of (a) Li4BTC-S and (b) Li4BTC-R Figure 7. (a) High-rate performance of Li4BTC-R and Li4BTC-S at current density from 0.1 to 8 A g-1 and (b) long-term cycling performance of Li4BTC-S at 0.5 A g-1 over 1000 cycles Figure 8. EIS spectra for (a) Li4BTC-R and (b) Li4BTC-S electrodes at different cycles, and equivalent circuit models of (c) before and (d) after cycling. 25



Figure 9. SEM images of Li4BTC-R electrode surface and cross section area (a, c) before cycling and (b, d) after 20 cycles, and the Li4BTC-S electrode surface and cross-section (e, f) before cycling and (g, h) after 20 cycles.

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Table 1 Summary of BET results of Li4BTC-R and Li4BTC-S. Samples

SBETa (m2 g-1)

Vmb (cm3 g-1)

DBJHc (nm)

Tap density (g cm-3)

Li4BTC-R

4.3

0.98

119.5

0.43

Li4BTC-S

10.9

2.51

140.0

0.48

a

BET surface areas calculated in the range of relative pressure (p/po) = 0.05–0.35, b Total pore volume measured at p/po = 0.99, c Calculated from the desorption branch of the N2 adsorption-desorption isotherms.

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Table 2 Summary of EIS parameters for Li4BTC-R and Li4BTC-S. Samples

Li4BTC-R

Li4BTC-S

Cycle

Rea(Ω)

RSEIb(Ω)

Rctc(Ω)

Before cycling

2.7

-

106.5

1st

5.6

183.5

407.8

10th

3.8

76.5

35.9

50th

3.4

35.0

19.5

Before cycling

3.6

-

178.7

1st

6.5

125.4

206.9

10th

5.2

45.9

22.6

50th

4.7

30.7

17.6

a

Re: electrolyte solution resistance. RSEI: SEI resistance. c Rct: charge transfer resistance. b

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O

OH

O

O OH HO

HO

O

OLi O

Discharge OLi +2e-, 2Li+

+4e-, 4Li+ Ion exchange LiO

O

LiO

OLi O

-2e-, 2Li+ Charge O LiO

O

Scheme 1

29


OLi LiO O LiO

OLi


Figure 1

30


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Figure 2

31



Figure 3

32


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Figure 4

33



Figure 5

34


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Figure 6

35



Scheme 2

36


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Figure 7

37



Figure 8

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Figure 9

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Table of Content (TOC)

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Increased reversible capacity of an organic electrode by changing its structure 338x190mm (96 x 96 DPI)


Enhanced Lithium Storage Capacity of a Tetralithium 1,2,4,5

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