Composite Polymer Electrolytes Encompassing Metal Organic Frame

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Composite Polymer Electrolytes Encompassing Metal Organic Frame Works: a New Strategy for All-solid-state Lithium Batteries Natarajan Angulakshmi, Rathinam Senthil Kumar, M. Anbu Kulandainathan, and Arul Manuel Stephan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp506464v • Publication Date (Web): 30 Sep 2014 Downloaded from http://pubs.acs.org on October 6, 2014

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The Journal of Physical Chemistry

Composite Polymer Electrolytes Encompassing Metal Organic Frame Works: a New Strategy for All-solid-state Lithium Batteries N. Angulakshmi, R. Senthil Kumar, M. Anbu Kulandainathan, A. Manuel Stephan* Central Electrochemical Research Institute (CSIR- CECRI), Karaikudi 630 006, India

ABSTRACT Magnesium-benzene tricarboxylate metal organic frame work (Mg-BTC MOF)- incorporated composite polymer electrolytes (CPE) composed of poly(ethylene oxide) (PEO) and lithium bistrifluoromethane sulfonylimide (LiTFSI) were prepared by a simple hot-press technique. The incorporation of Mg-BTC MOF in the polymeric matrix has significantly enhanced the ionic conductivity of CPE up to two order magnitudes even at 0 °C. It also improved the thermal stability, compatibility and elongation-at-break of the polymeric membrane. The allsolid-state-lithium polymer cell composed of Li/CPE/LiFePO4 has delivered a stable discharge capacity of 110 mAh g-1 at 70 °C with a current rate of 1-C rate, which is higher than that of those reported earlier. The appealing properties such as high ionic conductivity, better compatibility and stable cycling qualify this membrane as electrolyte for all-solid-state lithium batteries for elevated temperature applications. Key words: Lithium-polymer batteries; composite polymer electrolytes; compatibility; thermal stability; ionic conductivity; charge-discharge studies. *e-mail: [email protected] or [email protected] Fax: +91 4565 227779

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Introduction The escalating demand for energy due to fluctuation in the oil prices and global warming have prompted researchers to find alternative energy resources across the world.1 Fuel cells, supercapacitors and lithium batteries are identified as strong contenders to fulfil the future energy requirements.2The unique properties, such as high single cell voltage, energy density, nomemory effect and long cycle life have qualified lithium-ion batteries as ultimate power source for portable electronic gadgets such as laptop computers, mobile phones etc.,3. The state-of-art lithium-ion batteries are composed of a carbonaceous anode (LiC6) and a lithium transition metal oxide cathode separated by a polyolefin membrane soaked in a non-aqueous liquid electrolyte. However, this system suffers from poor safety problems due to the presence of volatile and flammable organic solvents that present in the electrolyte solution. In this juncture, identification of alternative electrolyte materials is urgently needed.4 Recent studies reveal, that all solid-state lithium polymer configuration can offer reliable batteries for electric vehicle applications with improved safety aspects. The systems with solid polymer electrolytes have several advantages that include high energy density, better safety, flexible geometry and freedom from electrolyte leakage.5 In order to achieve the goal, numerous polymer hosts have been synthesized and examined.5,6 Undoubtedly, the greatest attention has been paid to PEO-based electrolytes because of their appealing advantages, such as low cost, good chemical stability and safety.7 Unfortunately, the applications of dry polymer electrolytes systems, (PEO-LiX) are hampered due to their low ionic conductivity of the order of 10-6 S cm-1 at ambient temperature. Also the low lithium transference number (0.2 to 0.3) that arises due to concentration polarization also reduces the rate capability 2


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of the battery system.3 Interestingly, these drawbacks have been successfully overcome by the addition of nanoparticulate ceramic fillers such as Al2O3, SiO2, TiO2 etc.,6. The addition of inert fillers stabilizes the amorphous phase of the polymer host and thereby promotes its electrochemical property and mechanical integrity7-11. The degree of enhancement, however, mainly depends on the nature of the ceramic filler and its surface states12. It has also been demonstrated that nano-sized particles with Lewis-acidic surface properties received much attention in order to increase ionic conductivity and their large surface-to-volume ratio stabilize the electrolyte/lithium interface13. Interestingly, the metal organic framework with organic functional groups as filler provides hybrid properties which promotes miscibility with PEO and thus enhances the ionic conductivity and interfacial properties14. These unique properties have prompted our attention to employ metal organic frameworks as filler for a solid polymer electrolyte. In a decade time, metal organic frame work has attracted the attention of many researcher15,16. MOF’s are widely used in catalysis, sensors, gas storage, purification, separation and sequestration17. Very recently, it has also been employed in dye sensitized solar cells (DSSC) in order to enhance the electrochemical properties by us18. Even though, numerous articles appear on ceramic fillers-laden composite polymer electrolytes, studies on polymeric membranes encompassing MOFs are very scanty. In order to identify the polymer electrolytes with maximum ionic conductivity CPE’s have been prepared for different concentrations of Mg-BTC MOF and lithium salt, LiTFSI. Generally, the LiTFSI salt is prone to corrode the aluminum current collector19. However, the corrosion of aluminium foil can be prevented when the concentration of lithium salt is above 10 wt.%.20 In the present study, composite polymeric membrane has been prepared with a maximum concentration of lithium salt (10 wt.%) in order to obtain membrane with high ionic conductivity 3



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and to avoid the corrosion of aluminum current collector. Additionally, the LiTFSI salt itself acts as a plasticizing agent.21 Therefore, Mg-BTC MOF was synthesized and employed as filler in a PEO+LiTFSI matrix and was thoroughly analyzed by physico-chemical and electrochemical points of view. Experimental Procedure Preparation of composite polymer electrolytes Mg-BTC MOF was synthesized and characterized as reported earlier by us.18 (See supplementary information DOI: 10.1039/c3ta12135f). PEO (Mol. wt. 3x105, Aldrich, USA) and lithium bistrifluorosulfonylimide, LiTFSI (E.Merck, Germany), were dried under vacuum for 48h at 50 °C and 100 °C, respectively. Mg-BTC MOF was also dried under vacuum at 50 °C for 5 days before use. CPEs were prepared by dispersing appropriate amounts of Mg-BTC MOF in PEO-LiN(CF3SO2)2 (as displayed in Table-1) in acetonitrile and the resultant precursors were hot-pressed into films with an average thickness of 30-50 µ was described elsewhere.22,23 This method offered uniform, flexible and mechanically robust polymeric membranes. The hotpressed membranes were stored in a vacuum oven at 50 °C for 24 hours for further electrochemical characterization.

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Table 1 Compositions of PEO, Mg-BTC MOF and LiTFSI Sample

PEO

Mg-BTC (MOF)

LiTFSI

(wt. %)

(wt. %)

(wt. %)

S1

95

0

5

S2

93

2

5

S3

85

10

5

S4

80

10

10

S5

75

10

15

Electrochemical Characterizations The ionic conductivity of the membranes sandwiched between two stainless steel blocking electrodes (1 cm2 area) was measured by an electrochemical impedance analyzer (IM6-Bio Analytical Systems, Germany) for various temperatures (0, 15, 30, 40, 50, 60 and 70 °C) between the frequency ranges from 50 mHz and 100 kHz. The compatibility of CPE with lithium metal anode was investigated by measuring the time dependence of the impedance for the cell composed of Li/CPE/Li under open-circuit potential at 70 °C. The lithium transference number was calculated using equation (1) as proposed by Vincent and co-workers.24 =

..............(1)

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The Li/CPE/Li cell was polarized by a dc pulse of 10 mV. The values of initial (I0) and steady state (Iss) current, flowing through the cell during the polarization were measured, while R0 and Rss respectively represents the resistance values before and after the perturbation of the system. In order to obtain the values of R0 and Rss impedance spectra were measured before and after perturbation. The thermogravimmetric (TG-DTA) analysis was made in the temperature ranges between 20-300 °C while the differential scanning calorimetry (DSC) as performed between 100 and 100 °C at a heating rate of 10 °C min-1. Nanostrctured LiFePO4/C cathode material was synthesized by a mild hydrothermal procedure as described by Meligrana and co-workers.25,26 The composite cathode was prepared by bladecoating a slurry of 70 wt.% active material (LiFePO4/C) with 20 wt.% conductive acetylene black and 10 wt.% poly(vinylidene fluoride) (SolvaySolef 6020) binder in NMP on an aluminium foil. The average thickness of the coated films was around 70 µm. The cell assembly was done in an argon-filled glove box (MBraun, Germany) that contained < 1ppm oxygen and moisture. The cells were cycled between 2.5 and 4.0 V in a multi-channel battery tester (Arbin, USA) at 70 °C for different current rates as reported earlier.25,26 .

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Results and Discussion Thermal analyses Differential scanning calorimmetry profiles of the samples S1 (Without Mg-BTC MOF) and S5 (with 10 wt.% Mg-BTC MOF) are displayed in Fig. 1 between the temperatures -100 °C and 100 °C. The sample S1 showed a glass transition temperature (Tg) at -52 °C with a melting temperature of 59 °C while the Mg-BTC MOF-incorporated polymeric membrane (sample S5) showed a heat variation at higher temperature of about 3°C (i.e. -49 °C) and a slightly higher melting point, Tm. The addition of Mg-BTC MOF as filler has a minimum effect on the value of Tg which is attributed to two counteracting phenomena, namely the amorphization of PEO by lithium salt and the interaction between MOF/PEO which may increase the Tg value. The increase in Tg value of Mg-BTC MOF-added membrabe is ascribed to interactions between Lewis acidic site of MOF and the oxygen of ethylene oxide (-EO-) moiety, similar to hydrogen bonding. This resulting physical-cross linking of –EO chains, restricts segmental movement of PEO-chains, which manifests as an increased value of ‘Tg’.

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Fig.1 DSC thermograms of PEO, PEO+LiTFSI and PEO+LiTFSI+Mg-BTCMOF.

The thermogravimmetric curves of samples S1 (without Mg-BTC MOF) and S5 (with 10 wt.% of Mg-BTC MOF) are depicted in Fig. 2. While heating composite polymer electrolytes a lot of changes are induced in the system and finally leaves inert residues. As seen in Figure 1, the observed weight loss of approximately 3% around 50 °C is ascribed to the removal of moisture contents absorbed at the time of handling the sample. The irreversible decomposition begins around 190 °C for the sample S1 (Mg-BTC MOF –free sample)27 which implies that the sample is thermally stable up to 190 °C. Apparently, the irreversible decomposition of CPE (sample S5) commences at 290 °C. The increased thermal stability of Mg-BTC MOF-laden composite electrolytes is due to the intercalation/exfoliation of the polymeric matrix with Mg-BTC MOF, filler particles, which, provides a strong obstruction effect to the polymeric membrane from 8


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thermal degradation to some extent. This indicates the fact that the sample, PEO+LiTFSI+MgBTC MOF is thermally stable up to a temperature of 290 °C in a nitrogen atmosphere.28-30

Fig.2 TG-DTA thermograms of PEO+LiTFSI (sample S1) and PEO+LiTFSI+Mg-BTC MOF (sample S5).

Tensile Strength The stress- strain traces of sample S1 (without Mg-BTC MOF) and S5 (with 10 wt.% of MgBTC MOF) are displayed in Fig. 3. The tensile strength of sample S1 is 3.15 Mpa with an elongation-at-break value of 63%. Upon addition of 10% of Mg-BTC MOF in the PEO+LiTFSI complexes the elongation-at-break is increased to 117% with a loss in the mechanical strength 9



(1.25 MPa). This reduction in mechanical strength arises from the plasticization of PEO matrix by MOF. A similar trend was reported by Fan and Maier where the authors analysed the mechanical properties of succinonitrile–added PEO+LiTFSI complexes.31

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PEO+LiTFSI PEO+LiTFSI+Mg-BTC MOF

Stress (MPa)

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3

2

1

0 0

20

40

60

80

100

120

140

Strain (%)

Fig. 3 Stress vs. Strain behaviour of PEO+LiTFSI and PEO+LiTFSI+ Mg-BTC MOF at 25 ˚C.

Ionic conductivity In order to employ the polymer electrolytes in all-solid-state lithium polymer batteries, the ionic conductivity of the polymeric membranes was measured for the polymeric samples prepared with various concentrations of PEO, LiTFSI and Mg-BTC MOF as depicted in Table 1. Fig. 4 10


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illustrates the variation of ionic conductivity as a function of 1/T(K) for the CPEs having different amounts of Mg-BTC MOF and lithium salt. It can be seen that the ionic conductivity increases with the increase of Mg-BTC MOF content (samples S1 to S5) and for content of lithium salt up to 10 wt.%. The ionic conductivity varies from 10-8 to 10-4 Scm-1 for sample S1 (PEO+LiTFSI) between 20 and 70 °C. An increase in ionic conductivity of one order magnitude is observed upon addition of (2 wt.%) Mg-BTC MOF in the PEO matrix. However, when the content of both Mg-BTC MOF and lithium salt are increased to 10 wt.% (sample S5), the ionic conductivity is outstandingly increased by more than two orders of magnitude even at 20 °C. The activation energy Ea has been calculated for the sample S1 and S5 as 1.3 keV and 0.6 keV respectively. The ionic conductivity is not a linear function of the filler concentration as normally seen in the composite polymer electrolytes. At low concentration levels the dilution effect which, tends to depress the conductivity, is effectively opposed by the specific interactions of the ceramic surfaces that promotes fast ion transport. Apparently, an increase in ionic conductivity is observed in both cases. When the concentration of the filler is increased, on the other hand, the dilution effect predominates and the conductivity decreases. Thus, the maximum conductivity is generally achieved only in the concentration region 8–10 wt. %22,30,32. The Lewis acid groups of the added inert filler (Mg-BTC MOF) will compete with the lithium cations (which is also Lewis acidic) for the formation of complexes with the alkoxide of PEO chains, and the anions of the lithium salt.32 This results in structural modifications of the filler surfaces, due to the specific actions of the polar surface groups of the inorganic filler. The Lewis acid– base interaction centres on the electrolytic species thus lowering, the ionic coupling and promoting the salt dissolution by the formation of a sort of “ion-filler complex”.

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In the present study, Mg-BTC MOF was added as filler. It has a Lewis acid centre, which can react with the anions of the lithium salt and leads to a reduction in the crystallinity of the polymer host.33 However, the Mg-BTC MOF reacts with lithium salt to yield a conducting matrix. Therefore, the contribution of the Mg-BTC MOF-lithium salt matrix to the overall conductivity of the composite polymer electrolyte must be accounted for. It must be noted that the conductivity of the composite polymer electrolyte without the lithium salt was 9.8x10-8 Scm-1 at 30˚C (not shown in the Figure). Very recently, Zheng et al34 enhanced the performance of lithium-sulfur batteries by the incorporation of MOF in the cathode material by significantly slowing down the migration of polysulfides and is attributed to the interaction between Lewis acidic nickel II- MOF and polysulfides base. The ionic conduction mechanism is schematically represented in Figure 5. This could be the reason for a remarkable increase in the ionic conductivity.33, 35

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Fig.4. The variation of ionic conductivity as a function of inverse temperature (1000/T) for various concentrations of LiTFSI.

Fig. 5. Schematic representation of ionic conduction in the composite polymer electrolyte.

Lithium transference number The lithium- ion transference number, Lit+, is a crucial factor which guarantees the performance and rate capability of lithium batteries for high power applications such as hybrid electric vehicles.36 As mentioned in the experimental section the lithium transference number was calculated using the equation (1).24 Fig.6a and b show the chronoamperometric curve of sample S1(without Mg-BTC MOF) and S5 and inset shows the Nyquist plots before and after perturbation respectively. Apparently, both curves (before and after perturbation) overlap, suggesting little difference between the initial (R0) and the final (Rss) resistances of the two Li 13



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interphases, which further confirms the stability of the lithium metal electrode with the Mg-BTC MOF incorporated CPE34.The value of Lit+ has been calculated as 0.4 which is sufficient for battery applications.

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Fig. 6. Chronoamperometric curve of Cu/CPE/Li cell after a perturbation of 10mV dc pulse. 6(a) Cell with sample S1 and 6(b) Cell with sample S5 Inset: impedance spectrum of the cell before and after the dc polarization at 70 ˚C respectively.

Compatibility It is well known that, irrespective of the electrolytes used, the anode of lithium batteries system is always covered by a passivating layer called solid electrolyte interface (SEI).37 This SEI layer plays a pivotal role in determining shelf life, power capability, low temperature performance and cycle life. The composition of SEI layer is very complicated which contains both inorganic and organic ingredients which are formed due to the salt degradation and organic compounds. The thickness of the SEI layer is not constant throughout cycling or storage time. However, the thickness of the SEI layer may vary from a fewA° to ten or hundreds of A°. The average thickness of the SEI can be roughly estimated by impedance spectroscopy.22,39 In the case of 15



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polymer electrolyte systems, a resistive layer covers the lithium metal electrode and the resistance of this layer grows with time, possibly reaching values more than 10 kΩ cm−2. 40 In the present study in order to ascertain the interfacial stability of CPE with lithium metal electrodes a symmetric cell composed of Li/CPE/Li was assembled and its interfacial resistance values are measured as a function of time. Fig. 7 illustrates the variation of interfacial resistance, Ri, as a function of time for the Li/ CPE (with sample S5/sample S1)/Li symmetric cells at 70 °C. The values of interfacial resistances can be measured from the Cole–Cole impedance plots in which the large semi-circles represent a parallel combination of resistance (Rfilm) and capacitance associated with the passivation film on the lithium metal anode.41 The intercept of the large semicircle at high frequency on the Z-axis is mostly associated with the interfacial resistance “Ri” of the system. The appearance of a small semicircle is due to the charge transfer resistance in parallel with the double layer capacitance. It is seen from the figure 7 that the CPE containing the Mg-BTC MOF filler is definitively more compatible than the polymer electrolyte (sample S1) prepared by PEO and LiTFSI. As clearly evident, for the sample S5 the resistance values remain more or less same even after 300 h. This is attributed to the morphological changes of passivated film with time which finally acquire a non-compact, possibly porous structure.42 Further, it is seen from the figure that interfacial resistance ‘Ri’ of the composite polymer electrolytes has been significanly reduced upon the incorporation of the inert filler (much lower than S1 sample).

Generally inert particles, depending upon their volume fraction, may tend to minimize the area of the lithium electrode exposed to polymers containing -O- and -OH species, thus reducing the passivation process. It is also clear that lower sized particles for a similar volume fraction of the ceramic phase which might impart better performance as compared to larger sized particles due 16


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to their wider coverage by high surface area.43,44 The formation of an insulated layer of Mg-BTC MOF particles at the electrode surface is probably due to higher volume fraction of Mg-BTC MOF phase. This insulating layer will impede further electrode reactions. In the present system the CPE with 10 wt. % of Mg-BTC MOF exhibited better compatibility with lithium metal anode. In addition to the above factors the improved interfacial properties can also be ascribed to the porous filler’s scavenging ability to trace solvent impurities.45 It prevents these impurities from accumulating at the lithium/CPE interface to form a passivating layer.

Interfacial Resistance (Ohms)

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1200 1000

S1 800 600

S5 400 200 0 0

2

4

6

8

10

Time (Day)

Fig. 7. Variation of interfacial resistance (Ri) as a function of time for the symmetric cells

comprising Li/CPE/Li cells at 70°C.

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Charge- Discharge studies

Fig.8 depicts the variation of discharge capacity as a function of cycle number for the cell Li/CPE/LiFePO4 at 70 ˚C. The inset figure illustrates the cycling behavior of the cell at 1-C rate. In the present study LiFePO4 has been chosen because of its appealing properties such as nontoxicity, thermal stability and environment friendly. It has been identified as the ultimate choice of cathode material for nanocomposite polymer electrolyte system which shows a flat operating voltage of 3.45 V vs. Li.43 The cell exhibits a typical cycling profile in the 3.0- 3.8 V window vs. Li at 1-C rate with well defined plateaus at about 3.48 V vs. Li upon charge (de-lithiation) and about 3.37 V vs. Li upon discharge. The cycling behaviour of the cell with sample S1 is shown in Figure 8(a). Although the cell exhibited a discharge capacity of 110 mAh g-1 during its first cycle, the discharge capacity rapidly reduced in the subsequent cycles with less than 50% columbic efficiency. This poor cycling performance has been attributed to low ionic conductivity of the CPE membrane (sample S1 without Mg-BTC MOF). Figure 8 (b) demonstrates the cycling profile of the LiFePO4/CPE/Li cell with sample S5. The cell delivered an initial discharge capacity of 120 mAh g-1 at C/20-rate and 115 mAh g-1at C/10-rate without much fade in capacity. At 1-C rate the cell delivered 100 mAh g-1 with 98% columbic efficiency. The cell can also deliver a specific capacity of 25 mAh g-1 even at 5C-rate. An abrupt reduction in capacity was observed at 5C-rate. The reduction in the discharge capacity at higher current regime is a typical characteristic of LiFePO4 material which is attributed to its low electronic conductivity and limited diffusion of Li+- ion into its structure that causes electrode polarization.25,26,46 Further, the declining discharge capacity at higher C-rates may be due to the solid electrolyte interface (SEI) film formation with electrolyte 18


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decomposition.44 Recent study also revealed that the increase in interfacial resistance value which originates from parameters related to the electrodedesign such as thickness and density can cause capacity fading at higher rates.47It is also obvious from the figure that the cell restores its specific capacity again at 1C-rate from its 36th cycle indicating that the structural stability of the cathode material is retained.

Fig. 8(a) Cycle number vs. Specific capacity of the cell composed of Li/CPE (sample S1)/LiFePO4 at 70 ˚C.

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Fig.8(b) Cycle number vs. Specific capacity of the cell comprising Li/CPE (sample S5)/LiFePO4 at 70 ˚C. Inset: cycling profile of the cell.

Conclusions Mg-BTC MOF has been successfully incorporated in a PEO+LiTFSI complexes. The incorporation of Mg-BTC MOF has significantly improved the physical and electrochemical properties of the system. Although, the results discussed here, are preliminary, they demonstrate the viability of employing the Mg-BTC MOF-laden composite for all-solid state polymer electrolyte batteries designed for use at elevated temperatures.

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Subramanian,V.; Luo, C,; Manuel Stephan, A.; Nahm, K. S.; Thomas, S. Supercapacitors from Activated Carbon Derived from Banana Fibers. J. Phys. Chem. C, 2007, 111, 75277538.

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Tarascon, J. -M.; Armand, Issues and Challenges Facing Rechargeable Lithium Batteries. Nature, 2001, 414, 359-367.

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Table of the Contents (TOC) Graphics

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Composite Polymer Electrolytes Encompassing Metal Organic Frame

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