Skip to main content
NCBI home page
ACS Omega logoLink to ACS Omega
. 2020 Apr 1;5(14):7885–7894. doi: 10.1021/acsomega.9b04133

Microporous Metal–Organic Framework (MOF)-Based Composite Polymer Electrolyte (CPE) Mitigating Lithium Dendrite Formation in All-Solid-State-Lithium Batteries

N Angulakshmi †, Yingke Zhou †,*, Shruti Suriyakumar ‡, R Baby Dhanalakshmi ‡, M Satishrajan ‡, Subbiah Alwarappan ‡, Mohamed H Alkordi §,*, A Manuel Stephan ‡,*
PMCID: PMC7160835  PMID: 32309697

Abstract

graphic file with name ao9b04133_0003.jpg

Poly(ethylene oxide) (PEO)-based composite polymer electrolytes (CPEs) containing the amine-functionalized, zirconium-based metal–organic framework @silica (UiO-66-NH2@SiO2) and lithium, LiN(CF3SO2)2 salt (LiTFSI) are prepared using a simple hot press method. The electrochemical properties such as compatibility of the electrolyte with the Li metal anode, Li transference number, and ionic conductivity are investigated for the different systems containing different relative concentrations of the additives. The incorporation of UiO-66-NH2@SiO2 in the PEO-LiTFSI matrix not only enhanced ionic conductivity by one order of magnitude but also offered better compatibility and suppressed the formation of lithium dendrites appreciably. X-ray photoelectron spectroscopy studies on post-cycled materials revealed the formation of lithium alkoxide (RO–Li) on the cathode and Li2O on the anode. The coin cell (2032-type) consisting of LiFePO4/CPE/Li with UiO-66-NH2@SiO2 as filler provided a discharge capacity of 151 mA h g–1 at 0.1 C-rate at 60 °C, measurably higher than control experiments utilizing SiO2 and UiO-66-NH2. The notable enhancement of electrochemical properties when incorporating the UiO-66-NH2@SiO2 at the CPE was attributed to formation of more uniform ion conduction pockets and channels within the PEO matrix, facilitated by the presence of the microporous UiO-66-NH2@SiO2. The enhanced distribution of microporous channels, where Li ions are assumed to percolate through within the matrix, is assumed to desirably reduce formation of Li dendrites by increasing diffusion channels and therefore reducing crystallization and growth of dendrites at the electrode surface.

Introduction

The recent advancement in lithium-ion batteries utilizing a transition metal oxide cathode, carbonaceous anode, and a nonaqueous electrolyte enabled wide-scale applications in portable electronic devices despite certain existing safety issues that necessitate further research.1,2An essential battery component is the electrolyte, where solid electrolytes demonstrate unique properties including no leakage of electrolytes, suppression of dendrite formation, and better safety profile. Therefore, solid electrolytes have been identified as an alternative to nonaqueous liquid electrolytes. Compared to inorganic solid electrolytes, solid polymer electrolytes are flexible, and thus, batteries containing polymer electrolytes can be made of desirable geometry, deriving strong impetus for novel polymeric electrolytes for application in all-solid-state batteries.

Numerous polymers including poly(methyl methacrylate), poly(acrylonitrile), and poly(vinyl chloride) among others have been explored as hosts for solid polymer electrolytes.3 Unfortunately, polymers tend to recrystallize at room temperature leading to poor lithium-ion conductivity and thus hamper applicability of solid electrolytes comprising such polymers in ambient-temperature applications. Several strategies have been employed toward the enhancement of ionic conductivity of polymer electrolytes. Among various polymeric hosts, poly(ethylene oxide) (PEO) has been widely investigated as solid electrolytes in batteries in the form of a composite polymer electrolyte (CPE) due to its benefits such as high salt content, affordable cost, mechanical strength, and good corrosion resistance. Generally, CPEs are prepared by the incorporation of lithium salt (LiX) and inert fillers like SiO2 and TiO2 within a matrix of selected polymer. The addition of fillers generally reduces the crystalline phase of the polymeric host, which promotes the segmental motion of the polymeric chains and thus enhances the ionic conductivity. In addition to metal oxides, biomolecules such as chitin,4 cellulose,5,6 starch,7 etc. have also been explored as filler to promote the electrochemical properties of composite polymer electrolytes. The microporosity and hybrid composition of the metal–organic frameworks (MOFs), which are crystalline solids, stimulated considerable attention in current research toward their utilization to meet out the current demanding applications. The MOFs composed of hybrid organic–inorganic molecules, constructed under moderate conditions, enabled control over their properties, tailored after specific targeted applications. Such applications include heterogeneous catalysis8 and drug delivery9 as well as hydrogen storage and separation.10 Very recently, their composition and structural properties have enabled applications in toxic ion removal11 and also as a separator material in lithium–sulfur batteries.12−14

The present work utilizes one of the most chemically stable MOFs, the amine-functionalized zirconium-based MOF (UiO-66-NH2) anchored on silica (SiO2) microparticles (UiO-66-NH2@SiO2) as the microporous functional additive in CPE using PEO as the polymer matrix. The UiO-66-NH2@SiO2 was selected due to the easier handling of the hybrid material where the challenge in handling the commonly isolated nanocrystalline UiO-66-NH2 is overcome by chemically anchoring them on the surface of silica microparticles through in situ seeding and growth. The UiO-66-NH2@SiO2 was selected due to its several desirable attributes including pronounced microporosity, chemical stability, and the presence of abundant chemical functionalities decorating its cages (hydroxyl and amine groups). The utilization of the MOF as filler added to the PEO is rationalized due to the microporosity of the MOF, with its cages and channels that can provide efficient diffusion channels for Li ions as compared to other commonly utilized, nonporous metal oxide additives including TiO2 or SiO2 previously reported elsewhere.

In the present work, LiTFSI has been added as a conducting species for lithium ions. Generally, LiTFSI is prone to corrode an aluminum current collector.15 According to Matsumoto et al.,16 the corrosion of aluminum foil can be suppressed by increasing the content of LiTFSI to above 10 wt %. In the present work, therefore, the concentration of LiTFSI was increased up to 10 wt % in order to obtain composite polymer electrolytes with maximum ionic conductivity and a corrosion-free aluminum current collector.

Results and Discussion

Ionic Conductivity and Lithium Transference Number

The ionic conductivity of the CPE for various concentrations of filler, SiO2/UiO-66-NH2/UiO-66-NH2@SiO2 is depicted in Figure S4. Invariably, the ionic conductivity increases with the increase of temperature for all compositions. The filler-free solid polymer electrolyte comprising PEO + LiTFSI exhibited ionic conductivity in the order of 10–8 to 10–6 S cm–1 between 0 and 70 °C. The CPE with SiO2 showed a lithium-ion conduction almost one order of magnitude higher (8.1 × 10–6S cm–1) at 60 °C than the filler-free PEO electrolyte (7.6 × 10–7 S cm–1) at 60 °C. In a similar way, the ionic conductivity of the CPE containing UiO-66-NH2 and UiO-66-NH2@SiO2 showed an appreciable increase in ionic conductivity, ranging from 10–6 to 10–4 and 10–6 to 10–5 S cm–1 between 0 and 70 °C, respectively. The increase in ionic conductivity of filler—added CPE—can be attributed to the local structural deformation of the PEO polymer around the solid particles into an amorphous region4,17 as well as to the presence of diffusion channels within the microporous MOF. The accessibility of the MOF crust built on the SiO2 microparticles by different small molecules was recently established by one of the authors.18 This latter effect is rather evident comparing the much enhanced Li-ion conductivity within the CPE containing similar weight of the SiO2. Further support of this hypothesis is the demonstrated midway value of Li conductivity of the CPE containing the UiO-66-NH2@SiO2 (Figure1a) as compared to higher conductivity for the CPE containing the UiO-66-NH2 (nanocrystallites) or the one containing only the SiO2 microparticles. The reduced Li-ion conductivity for the CPE containing UiO-66-NH2@SiO2 as compared to the CPE containing the UiO-66-NH2 (similar mass concentration) can be attributed to the lower abundance of microporous diffusion channels available for Li-ion conduction in the latter (normalized by mass of solids).

Figure 1.

Figure 1

Open in a new tab

(a) Variation of ionic conductivity as a function of inverse temperature for samples S1 and S4 with different fillers. (b) 7Li and (c) 19F NMR analysis of sample S1 and S4 with fillers SiO2, UiO-66-NH2, and UiO-66-NH2@SiO2. (d) Schematic representation of Li-ion migration and electrode/electrolyte interfaces of all-solid-state-lithium batteries.

As described by Roman19 and Qiao et al.,20 the conductivity of CPEs can be enhanced by optimizing nanoscale filler or improving polymer conductivity or their interface conductivity. In the present work, the high-surface-area nanofiller, UiO-66-NH2@SiO2, would result in large volume fraction of the interface and thus enhance the ionic conductivity (Figure 1a).The activation energy for the filler-free, SiO2, UiO-66-NH2, and UiO-66-NH2@SiO2-added polymer electrolytes has been calculated as 0.08, 0.05, 0.07, and 0.02 eV, respectively.

In the present work, the microporous nature of the UiO-66-NH2@SiO2 and the presence of multiple polar sites (Zr–carboxylate clusters and −NH2 groups) facilitated ionic diffusion within the CPE. The nonporous, aprotic, and weak ionic characteristics of PEO do not provide a good matrix for the solid-state ion transport electrolyte despite its favorable mechanical characteristics. The synergy between microporous functional filler and the PEO is therefore realized by the efficient compounding within the developed CPE. Both 7Li and 19F NMR were recorded to analyze the mobility of ionic species. The broadening of NMR spectral line widths confirms the interaction between the polymeric matrix and the filler (Figure 1b), decreasing crystallinity of the PEO and therefore increasing the ion diffusion pockets/channels within the CPE.21 The line narrowing of 7Li and the chemical shift in 19F indicate (Figure 1c) that both cations and anions are mobile in the polymer electrolyte, which confirms the no ion pairing or ion aggregation within the system.22 The proposed ionic conduction mechanism in the CPE containing the UiO-66-NH2@SiO2 is schematically illustrated in Figure 1d.

The Lit+ is a key parameter that determines the cycling and rate capability of lithium batteries especially for applications in electric tools and electric vehicles.23,24 The lithium transference number was estimated using eq 1. Figure 2a illustrates the chronoamperometric (CA) plot of sample S1 (without filler) and S4 (PEO+LiTFSI+UiO-66-NH2@SiO2) and the inset displays the Cole–Cole before and after perturbation. Apparently, the overlapping Nyquist curves ascertain the lithium metal electrode stability with filler-incorporated composite polymer electrolytes.25 The CA curve of PEO+LiTFSI+UiO-66-NH2/UiO-66-NH2@SiO2 samples and their respective Nyquist plots are shown in Figure S5. The values of Lit+ were measured as 0.2, 0.41, 0.48, and 0.68, respectively, for filler-free, SiO2, UiO-66-NH2, and UiO-66-NH2@SiO2-added polymer electrolytes. While comparing values of Lit+ for different electrolytes, the composite polymer electrolyte added with UiO-66-NH2@SiO2 offered a maximum value of 0.68, which is appreciable for battery applications.24,25 The highest value of transference number is attributed to added micropores of MOF filler to the PEO with its cages and channels that provide diffusion channels for Li ions effectively.

Figure 2.

Figure 2

Open in a new tab

(a) Chronoamperometric curves of Li/CPE/Li symmetric cells (CPE with UiO-66-NH2@SiO2) at 60 °C. (b) Variation of interfacial resistance Ri vs. time for the Li/CPE/Li symmetric cells at 60 °C for samples S1 and S4 with different fillers. (c) Lithium stripping/plating analysis with Li/CPE/Li symmetric cells at 60 °C with a current density of 0.1 mA cm–2 under 60 min charging/discharging.

Compatibility and Dendrite Studies

The SEI has a critical role in shaping not only the cycle life and rate capability but also the low-temperature performance and shelf life. It is proven that the lithium metal anode is always covered by this SEI layer irrespective of the electrolytes used.26 The constituents of the SEI is highly complex as it constitutes both organic and inorganic compounds, which are made due to the degradation of electrolyte salt and organic molecules. In the case of polymer electrolyte systems, the resistance of this layer nurtures with the time, almost reaching values as high as 10 kΩ cm–2.27

In the present work, symmetric cells comprising Li/CPE/Li were fabricated, and their Ri values have been estimated as a function of storage time in order to ascertain the constancy of the interface of the CPE with different fillers against the lithium electrode. Figure 2b depicts the interfacial resistance, Ri, variation with time for the Li/CPE (with sample S4/S1)/Li cells at 60 °C. The Ri value can be measured from the Nyquist plots. The plot has large semicircles that indicate a combination of capacitance (Cdl) and resistance (Rfilm) connected parallel corresponding to the thin layer covering the lithium surface.28,29 The interfacial resistance Ri of the system is reflected in the intercept on the z-axis of the large semicircle in the higher frequency region. The Rct in parallel with the Cdl is visible explicitly from the existence of a small semicircle. From Figure 2b, the value of Ri is found to increase along with time for the filler-free and SiO2-added polymeric membranes. On the contrary, it increases and then remains stable after 196 h for the UiO-66-NH2- and UiO-66-NH2@SiO2-added polymer electrolytes. It is also seen that the CPE containing filler (sample S4) exhibits lower Ri than the filler-free polymer electrolyte (sample S1). The appreciable reduction in the values of Ri is ascribed to the morphological changes of the passivated film with time, which finally attains a noncompact and seemingly porous structure.30

The inert particles tend to minimize the area of the lithium electrode exposed to polymer containing −O– and −OH species depending on their volume fraction, thus reducing the passivation. Moreover, the presence of diffusion channels within the CPE matrix increases the overall flux of Li ions reaching the electrode surface in more uniform fashion, as compared to random conduction channels that form within the CPE in the absence of the microporous additives.

The formation of more percolation passage for Li ions through incorporation of UiO-66-NH2@SiO2 particles, leading to more uniform ion transport and utilization at the solid–electrode interface, is presumably contributing significantly to the observed overall enhancement in electrochemical performance here. In this work, the CPE with 10 wt % UiO-66-NH2@SiO2 offered lower Ri values and thereby improved compatibility with lithium.

The conventional nonaqueous electrolyte cannot be used with the lithium metal anode due to the formation of dendrites on the lithium surface during charging and discharging and subsequent formation of an irreversible reaction that occurs on both electrolyte and lithium metal surface.31 According to Monroe and Newman,32 the lithium dendrite problem can be tackled by introducing solid electrolytes with a modulus value of 6 MPa between the electrodes. In order to quantify the effect of composite polymer electrolytes on the lifetime of lithium metal-based batteries, Li/CPE/Li cells were assembled and lithium plating/stripping electrochemical cycling measurements were performed. Figure 2c shows the lithium stripping/plating analysis with Li/CPE/Li symmetric cells at 60 °C with a current density of 0.1 mA cm–2 under 60 min charging/discharging.

It is obvious from Figure 2c that the CPE comprising SiO2 exposed a sharp potential drop after 3.4 h of cycling as a consequence of dendrite crossover. The time-dependent voltage profile of the Li/CPE/Li cell containing UiO-66-NH2 showed a slight decrease in voltage after 3.5 h and the membrane is stable up to 9 h. Although a similar drop was observed after 17 h in UiO-66-NH2@SiO2-laden membrane, the cell was stable even after 120 h, which indicates that the added UiO-66-NH2@SiO2 resists dendrite crossover during long-term cycling.17,33 The initial cell voltages, +0.05 and −0.058 mV, of composite polymer electrolytes with SiO2 filler were increased to +0.06 and −0.062 mV. On the contrary, the initial cell voltages (+0.18 and −0.10 for UiO-66-NH2-added) (+0.24 and −0.107 mV for UiO-66-NH2@SiO2-added) have been reduced to +0.066 and −0.062 and 0.064 and −0.063 mV, respectively.

According to Bieker et al.,34 the increase in the value of cell voltage as a function of time is a direct indication of dendrite growth. The reduction in the value of overpotential indicates the effective suppression of dendrites with a stable SEI forming in the UiO-66-NH2- and UiO-66-NH2@SiO2-added polymer electrolytes as evidenced in Figure 2c. As demonstrated by Goodenough and co-workers,17 a solid electrolyte with a conduction band above the Fermi energy of metallic lithium can efficiently mitigate the formation of a passivation of a solid–electrolyte interface (SEI) layer on the lithium metal anode. This can be realized only by all-solid-state-lithium batteries by replacing the nonaqueous liquid electrolyte with solid electrolytes, where the dendrites can be effectively blocked from reaching the cathode from the anode. In the present case, the composite polymer electrolyte with its effective mechanical strength suppresses the lithium dendrites from making contact with the cathode. Among the composite polymer electrolytes analyzed, sample S4 was found to be optimal in terms of ionic conductivity and therefore was used for further XPS and charge–discharge studies.

Charge–Discharge Studies of LiFePO4/CPE/Li Cells at 60 °C

The cycling curves of the LiFePO4/CPE/Li cells at 60 °C in the voltage window of 2.5–4.2 V are shown in Figure 3a. Among the prominent cathode materials such as LiCoO2, LiMnO2, and LiCo0.8Ni0.2O2, in the present work, lithium iron phosphate has been employed due to its unique attributes such as nontoxicity, better thermal stability, low cost, and environmentally benign nature. Moreover, it is recognized as the best-rated cathode material for the solid polymer electrolyte systems with a flat operating cell voltage of 3.45 V vs. Li+/Li.35

Figure 3.

Figure 3

Open in a new tab

(a) Cycling profile at a 0.1 C-rate and (b) long-term cycling performance at 0.1, 1, 2, and 5 C-rates with Coulombic efficiency for the LiFePO4/CPE/Li cells at 60 °C.

Typically, LiFePO4 exhibits a typical charge–discharge profile in the 3.0–3.8 V potential window vs. Li+/Li with a defined plateau at around 3.48 V vs. Li upon charging and about 3.37 V vs. Li during discharge. It is seen from Figure 3a that the flat and reversible voltage plateaus for the LiFePO4/Li cell with CPE containing SiO2 indicate a small polarization. However, the cells with CPE containing UiO-66 and UiO@SiO2 show larger overpotential than the cell with CPE containing SiO2 on its first cycle. Besides the cycling stability, a good rate capability is also a key parameter for practical application of lithium batteries. Figure 3b displays the long-term cycling performance at 0.1, 1, 2, and 5 C-rates along with Coulombic efficiency for the LiFePO4/CPE/Li cells during charge/discharge processes between 2.5 and 4.2 V. The charge–discharge behavior of the cell with the filler-free sample, S1, is shown in Figure S6. Although the cell exhibited a discharge capacity of 110 mA h g–1 during its first cycle, a dramatic reduction in discharge capacity was found in the following cycles with Coulombic efficiency less than 50%. This reduced cycling is ascribed to poor ionic conductivity of the CPE membrane.

The LiFePO4/CPE/Li cell with SiO2 delivered a discharge capacity of 119, 64, 55, and 9 mA h g–1 at 0.1, 1, 2, and 5 C-rates, respectively. The Coulombic efficiency is higher than 99.6% at a current density of 2 and 5 C. On the other hand, theLiFePO4/CPE/Li cell with UiO-66-NH2 and UiO-66-NH2@SiO2 composite polymer electrolytes delivered a discharge capacity of 127, 87, 72, and 25 and 138, 96, 76, and 37 mA h g–1 at 0.1, 1, 2, and 5 C-rates on their first cycle, respectively. Among the LiFePO4/CPE/Li cells, the cell with UiO-66-NH2@SiO2 composite polymer electrolytes delivered the highest discharge capacity and stable cycling. The cell cycles with the typical voltage profiles provide a high fraction (75%) of its theoretical capacity.

The cycling performance is appealing because only negligible reduction in discharge capacity is seen in the initial test and the Coulombic efficiency following the first cycles and is attributed to the rearrangements in the structure of the electrode that take place during cycling. A drastic decline in capacity was observed at the 5 C-rate. This is a typical characteristic of the LiFePO4 material at the high-current regime and is due to low electronic conductivity and restricted Li ion diffusion into its crystal structure, which results in polarization of the electrode.35,36 Furthermore, the lower discharging capacity at high C-rates is because of the SEI.37 Studies also show that the increase in Ri values, which generally arises from parameters related to the electrode architecture such as thickness and density causes capacity fading at higher rates. It is seen from Figure 3 that the cell restores its original capacity again at the 0.1 C-rate, indicating that the cathode material maintains its structural stability from its 95th cycle.

Postcycling XPS analysis of LiFePO4/CPE/Li Interfaces

In order to further probe the composition of the SEI, XPS has been used as a tool to analyze the Li/CPE and LiFePO4/CPE interfaces separately. The role of the solid–electrolyte interface (SEI) on the anode side and the solid-permeable interface (SPI) on the cathode side is considered to determine the capacity retention and safety of lithium batteries.38Figure 4a,b shows the XPS data from interface layers formed in Li/CPE and LiFePO4/CPE, respectively. It is seen from Figure 4a that a small amount of LiF was detected at 685 eV in F 1s and can be ascribed to the formation of LiF originated from the decomposed salt or binder.

Figure 4.

Figure 4

Open in a new tab

Postcycling XPS analysis of (a) anode/CPE interface and (b) cathode/CPE.

Furthermore, the F 1s peak at 688.9 eV, the S 2p peak at 168.2 eV, and the N 1s peak at 399.7 eV are assigned to the decomposed LiTFSI salt. The S 2p peaks at 169.7 and 163.7 eV are due to SO2 and Li2S–SO3, respectively. The peaks observed at 531.6 eV (O 1s) and 290 eV (C 1s) are indicative of the formation of carbonate species such as Li2CO3 at the lithium composite polymer electrolyte interface. The peak at 398.7 eV (N 1s) is assigned to Li3N interfacial species. Also, the signal observed at 688.2 eV is due to the added Kynar binder. The cycled lithium metal surface further indicates the presence of carbonate species such as Li2CO3 as appeared in the O 1s peak at 531 eV and the C 1s peak at 289.3 eV. The sulfur species residue found in the solid polymer electrolyte is evidenced from the 166.8 eV arising from the S 2p peak.

Similarly, in Figure 4b, the presence of ether oxygen of PEO and oxygen from salt anions is confirmed by the C 1s peak at 286.6 eV and the O 1s peak at 533 eV. Furthermore, the 688.9 eV peak arising from the F 1s spectra, and the 168.9 eV peak from the S 2p spectra are attributed to the decomposed products of added LiTFSI salt. The presence of the solid–electrolyte interface on LiTFSI-based lithium-ion batteries is confirmed by the 167.5 eV peak, which arises from the sulfur species. The C 1s peak at 285 eV is originated from hydrocarbons. The presence of ROLi compounds at the LiFePO4/CPE interface is confirmed by the O 1s peak at 530.6 eV.

The FTIR spectra of the composite polymer electrolyte containing UiO-66-NH2@SiO2, the lithium surface, composite LiFePO4 upon cycling, and pristine LiFePO4 are shown in Figure 5a–d. As evident from Figure 5d, the CPE containing UiO-66-NH2@SiO2 exhibited a stretching at 485 cm–1, confirming the presence of the Zr–O bond (γZr–O). Further peaks at 770 and 950 cm–1 are ascribed to C–H stretching (γC–H) and Si–O bending (δSi–O), respectively. The appearance of peaks at 1050 and 1256 cm–1 is attributed to Si–O bending (γSi–O) and C–N stretching (γC–N), respectively. The peak at 1502 cm–1 is assigned to C=C stretching (γC=C), while that at 1518 cm–1 is due to C=N stretching (γC=N). The peaks at 1575 and 1627 cm–1 represent C=O stretching (γC=O) and N–H bending (δN–H) modes, respectively.39,40 On the other hand, the FTIR spectra of the lithium surface upon cycling UiO-66-NH2@SiO2 have all the peaks with the same intensity. However, it also contains one additional peak at 700 cm–1 due to PO42– (Figure 5c), thereby confirming the dissociation of the phosphate group from the lithium iron phosphate upon cycling. Figure 5a shows the FTIR peaks of pristine LiFePO4. The peaks, which appear in the wavenumbers 300–600 cm–1, are referred to bending modes, while the peaks between 600 and 1300 cm–1 represent stretching vibrations. The two well-resolved doublets at 468–500 cm–1 are assigned to the asymmetric (γ2–γ4) bending modes of P–O bonds. The intramolecular stretching (γ1 and γ3) motions of each PO43– anion is correlated to those of the other PO43– ions in the unit cell, which produces peaks that are observed at 943, 1068, and 1140 cm–1. The bands in the region of 643–633 cm–1 can be assigned to the bending modes (γ4) of phosphate anions of the LiFePO4 pristine cathode.41

Figure 5.

Figure 5

Open in a new tab

FTIR spectra of (a) pristine LiFePO4, (b) composite LiFePO4/CPE-UiO-66-NH2@SiO2 interface upon cycling, (c) lithium/CPE-UiO-66-NH2@SiO2 interface, and (d) composite polymer electrolyte containing UiO-66-NH2@SiO2.

The bands at 506 and 470 cm–1 might be Li+ ion “cage modes”, which has been earlier confirmed by isotopic substitution studies on LiMPO4 (M = Fe, Mg, and Ni) as discussed by Burba and Frech.42 In addition, it is evident from Figure 5a,b that the band around 650 cm–1 and the γ3 bands around 1137 and 1068 cm–1 in pristine LiFePO4 slightly shift to higher frequencies upon cycling, which is attributed to the extraction of Li+ ions from LiFePO4 to produce FePO4, resulting in dramatic changes in γ1 and γ3 frequencies.41

Conclusions

The UiO-66-NH2 and UiO-66-NH2@SiO2 were successfully synthesized and added to PEO+LiTFSI to formulate a novel CPE for all-solid-state Li-ion batteries. The addition of UiO-66-NH2@SiO2 has apparently enhanced the physical as well as electrochemical properties of the CPE. The results presented here suggest an important role of the MOF@SiO2, providing uniform and highly dispersed Li-ion percolation passage within the CPE, in turn providing more homogeneous Li-ion flux between the two electrodes and enhanced electrode–solid electrolyte interfacial properties. The observed enhancement in performance strongly demonstrates the potential for such a novel MOF-containing CPE to establish better interfacial contact between electrodes and the electrolyte, the efficient ionic conduction in the CPE, and the favorable interfacial Li-ion transport across the electrodes through the electrolyte. The excellent chemical and thermal stability of UiO-66-NH2 MOF suggests a larger potential for its utilization as additive for all-solid-state lithium-polymer cells for reliable application at elevated temperature.

Experimental Section

Synthesis of UiO-66-NH2@SiO2

The MOF, UiO-66-NH2@SiO2,was synthesized as reported by us earlier.43 Briefly, a mixture of 2-aminoterephthalic acid (135.86 mg, 0.75 mmol) and silica (100 mg) was mixed in a scintillation vial and was subsequently sonicated in 10 mL of DMF for 5 min. This solution was mixed with a separately prepared solution of ZrCl4 (125.8 mg, 0.54 mmol) in 5 mL of dimethyl formamide and 1 mL of 37% HCl. The vial was finally capped, and the mixture was stirred for 12 h at 80 °C, filtered and washed with ACN, and then exchanged in heated ACN at 80 °C under autogenous pressure for 2 h. The powder was filtered and dried in an isothermal oven at 80 °C for 2 h. The yield was 230 mg of UiO-66-NH2@SiO2.

Preparation of Composite Polymer Electrolytes (CPEs)

CPEs were prepared for different PEO, filler, and lithium salt, namely, lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) ratios by a hot-press method as described earlier.4,44 Appropriate amounts (Table 1) of PEO and LiTFSI were carefully dissolved in anhydrous acetonitrile and allowed stirring for 8 h. Finally, the filler SiO2/UiO-66-NH2/UiO-66-NH2@SiO2 was dispersed in the polymeric solution, which was cast as a film and pressed. The thickness of the membrane was 50–60 μm in average. For further characterizations, the CPE was stored in a glovebox.

Table 1. Amount of Polymer, Li Salt, and Metal–Organic Framework Filler in the Various Composite Polymer Electrolytes.

sample PEO (wt %) MOF filler (wt %) LiTFSI (wt %)
S1 95 5 0
S2 93 2 5
S3 85 5 5
S4 80 10 10
S5 75 15 10
Open in a new tab

Materials Characterization

The surface of the composite polymeric film was probed using a scanning electron microscope (TESCAN, VEGA3, Germany). A tensile machine (Tinius Olsen, Netherlands) was employed to probe the mechanical strength of the CPE with and without fillers. During measurement, the constant crosshead speed was set to 10 mm min–1. The sample size was 20 mm (W) × 50 mm (L). The thermal stability of pure PEO, PEO+LiTFSI (sample S1), and SiO2/UiO-66-NH2@SiO2-added membranes was determined by thermogravimetric analysis (Netzsch, Germany) in a N2 atmosphere between RT and 600 °C, while differential scanning calorimetry (DSC) was carried out between −100 and 100 °C, both at a ramp rate of 10 °C min–1. In order to measure the ionic conductivity at different temperatures (0 to 60 °C), electrochemical impedance analysis was carried out (Biologic, France) in the frequency range of 1 MHz to 100 mHz for the membranes placed between two blocking electrodes. In an attempt to analyze the interfacial properties, symmetric nonblocking cells, Li/CPE/Li cells, were also assembled and the influence of time on the impedance was measured after extended storage under open-circuit voltage at 60 °C. A potentiostatic polarization method was employed as proposed by Bruce et al.45

graphic file with name ao9b04133_m001.jpg 1

where Iss and I0 represent the final and initial steady-state current, respectively. The R0 and Rss resistance values were obtained from EIS measurements before and after perturbation with 10 mV DC voltage.

Additionally, the dendrite growth in the lithium metal anode was probed using symmetrical coin cells of lithium electrodes with the CPE consisting of various fillers. The cycling performance of the symmetrical cells was tested using a battery test system (VSP3 Biologic, France) with a constant current density of 0.1 mA cm–2, each cycle consisting of 1 h charge and 1 h discharge at 60 °C.46,47

The X-ray photoelectron spectroscopy (XPS) studies of the composite cathode and lithium surface upon charge–discharge were conducted on a Thermo Scientific ESCA 2-250-XI. The solid-state nuclear magnetic resonance (NMR) for 7Li and 19F analysis of the composite polymer electrolytes was carried out using a Bruker model AVANCE III HD equipped with a rotor maintained at 16,000 rpm.

Composite cathode preparation was carried out by coating a slurry comprising 80 wt % lithium iron phosphate (LiFePO4), 10 wt % poly(vinylidene fluoride) (PVdF), and 10 wt % conducting carbon (Super P) in N-methyl-2-pyrrolidone (NMP) on a smooth aluminum foil doctor blade technique and was dried in an vacuum oven at 110 °C.16 The active material loading was 20 mg cm–2 and the density of the electrode is 1.53 g cm–3. The CPE membrane was sandwiched between the composite cathode and lithium metal (Foote Minerals, USA) anode. A coin cell of 2032-type with a configuration of LiFePO4/CPE/Li metal was assembled in a glovebox filled with inert argon gas (M Braun, Germany). Constant current charge–discharge profiles were obtained between 2.5 and 4.2 V using a cycle life tester (Arbin, USA).48,49

Acknowledgments

We acknowledge the funding from Egyptian Academy for Scientific Research and Technology (ASRT), Zewail City of Science and Technology (CMS-MA), and the Alexander von Humboldt Foundation (MA). The authors gratefully acknowledge the National Natural Science Foundation of China (no. 51974209) and the Natural Science Foundation of Hubei Province of China (nos. 2013CFA021, 2017 CFB 401, and 2018CFA022).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b04133.

  • (Figure S1) Surface morphology of S1 and S4 with fillers SiO2/UiO-66-NH2/UiO-66-NH2@SiO2. (Figure S2a) Differential scanning calorimetry profiles of pure PEO, sample S1 (filler-free), and S5 (with 10 wt % UiO-66-NH2/UiO-66-NH2@SiO2). (Figure S2b) Thermogravimetric traces of samples S1 and S4. (Figure S2c) Shrinkage studies of (i) S1 and S4 with fillers, (ii) SiO2, (iii) UiO-66-NH2, and (iv) UiO-66-NH2@SiO2 at 150 °C for 3 h. (Figure S3) Stress–strain curves of sample S1 and sample S4 with different fillers. (Figure S4) Ionic conductivity studies of sample (a) S1–S4 with fillers, (a)SiO2, (b) UiO-66-NH2, and (c) UiO-66-NH2@SiO2 between 0 and 70 °C. (Figure S5) Chronoamperometric curve of Li/ (a) SiO2 and (b) UiO-66-NH2-based CPE/Li cells with Nyquist plots before and after 10 mV DC perturbation. (Figure S6) Discharge capacity vs. cycle number of the LiFePO4/PE (S1)/Li cell with sample S1 at 60 °C. (PDF)

The authors declare no competing financial interest.

Supplementary Material

ao9b04133_si_001.pdf (750.1KB, pdf)

References

  1. Bruce P. G.; Scrosati B.; Tarascon J.-M. Nanomaterials for Rechargeable Lithium Batteries. Angew. Chem., Int. Ed. 2008, 47, 2930–2946. 10.1002/anie.200702505. [DOI] [PubMed] [Google Scholar]
  2. Yoo H. D.; Markevich E.; Salitra G.; Sharon D.; Aurbach D. On the Challenge of Developing Advanced Technologies for Electrochemical Energy Storage and Conversion. Mater. Today 2014, 17, 110–121. 10.1016/j.mattod.2014.02.014. [DOI] [Google Scholar]
  3. Manuel Stephan A.; Nahm K. S. Review on Composite Polymer Electrolytes for Lithium Batteries. Polymer 2006, 47, 5952–5964. 10.1016/j.polymer.2006.05.069. [DOI] [Google Scholar]
  4. Manuel Stephan A.; Prem Kumar T.; Anbu Kulandainathan M.; Angu Lakshmi N. Chitin incorporated Poly(ethylene oxide)-based nanocomposite electrolytes for lithium batteries. J. Phys. Chem. B 2009, 113, 1963–1971. 10.1021/jp808640j. [DOI] [PubMed] [Google Scholar]
  5. Wang S.; Zhang L.; Zeng Q.; Liu X.; Lai W.-Y.; Zhang L. Cellulose microcrystals with brush-like architectures as flexible all-solid-state polymer electrolyte for lithium-ion battery. ACS Sustainable Chem. Eng. 2020, 8, 3200–3207. 10.1021/acssuschemeng.9b06658. [DOI] [Google Scholar]
  6. Asghar M. R.; Anwar M. T.; Rasheed T.; Naveed A.; Yan X.; Zhang J. Lithium salt doped Poly(Vinylidene Fluoride)/cellulose acetate composite gel electrolyte membrane for lithium ion battery. IOP Conf. Ser.: Mater. Sci. Eng. 2019, 654, 012017 10.1088/1757-899X/654/1/012017. [DOI] [Google Scholar]
  7. Xie M.; Li L.; Zhang Y.; Du J.; Li Y.; Shan Y.; Zheng H. Mastering high ion conducting of room-temperature all-solid state lithium-ion batteries via safe phthaloyl starch-Poly(vinylidene fluoride)-based polymer electrolyte. Ionics 2020, 1109–1117. 10.1007/s11581-019-03309-6. [DOI] [Google Scholar]
  8. Zhang C.-L.; Lu B.-R.; Cao F.-H.; Wu Z.-Y.; Zhang W.; Cong H.-P.; Yu S.-H. Electrospun Metal-Organic Framework Nanoparticle Fibers and Their Derived Electrocatalysts for Oxygen Reduction Reaction. Nano Energy 2019, 55, 226–233. 10.1016/j.nanoen.2018.10.029. [DOI] [Google Scholar]
  9. Cai H.; Huang Y.-L.; Li D. Biological Metal–Organic Frameworks: Structures, Host–Guest Chemistry and Bio-Applications. Coord. Chem. Rev. 2019, 378, 207–221. 10.1016/j.ccr.2017.12.003. [DOI] [Google Scholar]
  10. Ma S.; Zhang M.; Nie J.; Tan J.; Yang B.; Song S. Design of Double-Component Metal–Organic Framework Air Filters with PM2.5 Capture, Gas Adsorption and Antibacterial Capacities. Carbohydr. Polym. 2019, 203, 415–422. 10.1016/j.carbpol.2018.09.039. [DOI] [PubMed] [Google Scholar]
  11. Barea E.; Montoro C.; Navarro J. A. R. Toxic Gas Removal–Metal–Organic Frameworks for the Capture and Degradation of Toxic Gases and Vapours. Chem. Soc. Rev. 2014, 43, 5419–5430. 10.1039/C3CS60475F. [DOI] [PubMed] [Google Scholar]
  12. Bai S.; Liu X.; Zhu K.; Wu S.; Zhou H. Metal-Organic Framework-Based Separator for Lithium-Sulfur Batteries. Nat. Energy 2016, 1, 16094. 10.1038/nenergy.2016.94. [DOI] [Google Scholar]
  13. Bai S.; Zhu K.; Wu S.; Wang Y.; Yi J.; Ishida M.; Zhou H. A Long-Life Lithium–Sulphur Battery by Integrating Zinc–Organic Framework Based Separator. J. Mater.Chem. A 2016, 4, 16812–16817. 10.1039/C6TA07337A. [DOI] [Google Scholar]
  14. Suriyakumar S.; Kanagaraj M.; Kathiresan M.; Angulakshmi N.; Thomas S.; Stephan A. M. Metal-Organic Frameworks Based Membrane as a Permselective Separator for Lithium-Sulfur Batteries. Electrochim. Acta 2018, 265, 151–159. 10.1016/j.electacta.2018.01.155. [DOI] [Google Scholar]
  15. Xu K. Nonaqueous Liquid Electrolytes for Lithium-Based Rechargeable Batteries. Chem. Rev. 2004, 104, 4303–4418. 10.1021/cr030203g. [DOI] [PubMed] [Google Scholar]
  16. Matsumoto K.; Inoue K.; Nakahara K.; Yuge R.; Noguchi T.; Utsugi K. Suppression of Aluminum Corrosion by Using High Concentration LiTFSI Electrolyte. J. Power Sources 2013, 231, 234–238. 10.1016/j.jpowsour.2012.12.028. [DOI] [Google Scholar]
  17. Zhou W.; Gao H.; Goodenough J. B. Low-Cost Hollow Mesoporous Polymer Spheres and All-Solid-State Lithium, Sodium Batteries. Adv. Energy Mater. 2016, 6, 1–8. 10.1002/aenm.201501802. [DOI] [Google Scholar]
  18. El-Mehalmey W. A.; Ibrahim A. H.; Abugable A. A.; Hassan M. H.; Haikal R. R.; Karakalos S. G.; Zaki O.; Alkordi M. H. Metal–organic framework@silica as a stationary phase sorbent for rapid and cost-effective removal of hexavalent chromium. J. Mater. Chem. A 2018, 6, 2742. 10.1039/C7TA08281A. [DOI] [Google Scholar]
  19. Roman H. E. A Continuum Percolation Model for Dispersed Ionic Conductors. J. Phys.: Condens. Matter 1990, 2, 3909–3917. 10.1088/0953-8984/2/17/002. [DOI] [Google Scholar]
  20. Qiao R.; Deng H.; Putz K. W.; Brinson L. C. Effect of Particle Agglomeration and Interphase on the Glass Transition Temperature of Polymer Nanocomposites. J. Polym. Sci., Part B: Polym. Phys. 2011, 49, 740–748. 10.1002/polb.22236. [DOI] [Google Scholar]
  21. Abbrent S.; Greenbaum S. Recent progress in NMR spectroscopy of polymer electrolytes for lithium batteries. Curr. Opin. Colloid Interface Sci. 2013, 18, 228–244. 10.1016/j.cocis.2013.03.008. [DOI] [Google Scholar]
  22. Forsyth M.; Macfarlane D. R.; Meakin P.; Smith M. E.; Bastow T. J. An nmr investigation on ionic structure and mobility in plasticized solid polymer electrolytes. Electrochim. Acta 1995, 40, 2343–2347. 10.1016/0013-4686(95)00190-P. [DOI] [Google Scholar]
  23. Fernicola A.; Croce F.; Scrosati B.; Watanabe T.; Ohno H. LiTFSI-BEPyTFSI as an Improved Ionic Liquid Electrolyte for Rechargeable Lithium Batteries. J. Power Sources 2007, 174, 342–348. 10.1016/j.jpowsour.2007.09.013. [DOI] [Google Scholar]
  24. Zheng J.; Tian J.; Wu D.; Gu M.; Xu W.; Wang C.; Gao F.; Engelhard M. H.; Zhang J.-G.; Liu J.; et al. Lewis Acid–Base Interactions between Polysulfides and Metal Organic Framework in Lithium Sulfur Batteries. Nano Lett. 2014, 14, 2345–2352. 10.1021/nl404721h. [DOI] [PubMed] [Google Scholar]
  25. Li Q.; Takeda Y.; Imanish N.; Yang J.; Sun H. Y.; Yamamoto O. Cycling Performances and Interfacial Properties of a Li/PEO-Li(CF3SO2)2N-Ceramic Filler/LiNi0.8Co0.2O2 Cell. J. Power Sources 2001, 97-98, 795–797. 10.1016/S0378-7753(01)00610-3. [DOI] [Google Scholar]
  26. Peled E.; Golodnitsky D.; Ardel G. Advanced Model for Solid Electrolyte Interphase Electrodes in Liquid and Polymer Electrolytes. J. Electrochem. Soc. 1997, 144, L208. 10.1149/1.1837858. [DOI] [Google Scholar]
  27. Aurbach D.; Weissman I.; Yamin H.; Elster E. The Correlation Between Charge/Discharge Rates and Morphology, Surface Chemistry, and Performance of Li Electrodes and the Connection to Cycle Life of Practical Batteries. J. Electrochem. Soc. 1998, 145, 1421. 10.1149/1.1838498. [DOI] [Google Scholar]
  28. Wang L.; Byon H. R. N-Methyl-N-Propylpiperidinium Bis(Trifluoromethanesulfonyl) Imide-Based Organic Electrolyte for High Performance Lithium-Sulfur Batteries. J. Power Sources 2013, 236, 207–214. 10.1016/j.jpowsour.2013.02.068. [DOI] [Google Scholar]
  29. Jiang Z.; Carroll B.; Abraham K. M. Studies of Some Poly(Vinylidene Fluoride) Electrolytes. Electrochim. Acta 1997, 42, 2667–2677. 10.1016/S0013-4686(97)00005-4. [DOI] [Google Scholar]
  30. Appetecchi G. B.; Croce F.; Scrosati B. Kinetics and Stability of the Lithium Electrode in Poly(Methylmethacrylate)-Based Gel Electrolytes. Electrochim.Acta 1995, 40, 991–997. 10.1016/0013-4686(94)00345-2. [DOI] [Google Scholar]
  31. Tarascon J.-M.; Armand M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359–367. 10.1038/35104644. [DOI] [PubMed] [Google Scholar]
  32. Monroe C.; Newman J. The Impact of Elastic Deformation on Deposition Kinetics at Lithium/Polymer Interfaces. J. Electrochem. Soc. 2005, 152, A396. 10.1149/1.1850854. [DOI] [Google Scholar]
  33. Pan Q.; Smith D. M.; Qi H.; Wang S.; Li C. Y. Hybrid Electrolytes with Controlled Network Structures for Lithium Metal Batteries. Adv. Mater. 2015, 27, 5995–6001. 10.1002/adma.201502059. [DOI] [PubMed] [Google Scholar]
  34. Bieker G.; Winter M.; Bieker P. Electrochemical in Situ Investigations of SEI and Dendrite Formation on the Lithium Metal Anode. Phys. Chem. Chem. Phys. 2015, 17, 8670–8679. 10.1039/C4CP05865H. [DOI] [PubMed] [Google Scholar]
  35. Bodoardo S.; Gerbaldi C.; Meligrana G.; Tuel A.; Enzo S.; Penazzi N. Optimisation of Some Parameters for the Preparation of Nanostructured LiFePO4/C Cathode. Ionics 2009, 15, 19–26. 10.1007/s11581-008-0259-3. [DOI] [Google Scholar]
  36. Meligrana G.; Gerbaldi C.; Tuel A.; Bodoardo S.; Penazzi N. Hydrothermal Synthesis of High Surface LiFePO4 Powders as Cathode for Li-Ion Cells. J. Power Sources 2006, 160, 516–522. 10.1016/j.jpowsour.2005.12.067. [DOI] [Google Scholar]
  37. Ghosh A.; Wang C.; Kofinas P. Block Copolymer Solid Battery Electrolyte with High Li-Ion Transference Number. J. Electrochem. Soc. 2010, 157, A846. 10.1149/1.3428710. [DOI] [Google Scholar]
  38. Sun B.; Xu C.; Mindemark J.; Gustafsson T.; Edström K.; Brandell D. At the Polymer Electrolyte Interfaces: The Role of the Polymer Host in Interphase Layer Formation in Li-Batteries. J. Mater. Chem. A 2015, 3, 13994–14000. 10.1039/C5TA02485D. [DOI] [Google Scholar]
  39. Bunge M. A.; Davis A. B.; West K. N.; West C. W.; Grant Glover T. Synthesis and Characterization of UiO-66-NH2 Metal–Organic Framework Cotton Composite Textiles. Ind. Eng. Chem. Res. 2018, 57, 9151–9161. 10.1021/acs.iecr.8b01010. [DOI] [Google Scholar]
  40. Abid H. R.; Shang J.; Ang H.-M.; Wang S. Amino-Functionalized Zr-MOF Nanoparticles for Adsorption of CO2and CH4. Int. J. Smart Nano Mater. 2013, 4, 72–82. 10.1080/19475411.2012.688773. [DOI] [Google Scholar]
  41. Rangappa D.; Sone K.; Kudo T.; Honma I. Directed Growth of Nanoarchitectured LiFePO4 Electrode by Solvothermal Synthesis and Their Cathode Properties. J. Power Sources 2010, 195, 6167–6171. 10.1016/j.jpowsour.2009.11.095. [DOI] [Google Scholar]
  42. Burba C. M.; Frech R. Vibrational Spectroscopic Investigation of Structurally-Related LiFePO4, NaFePO4, and FePO4 compounds. Spectrochim. Acta, Part A 2006, 65, 44–50. 10.1016/j.saa.2005.09.025. [DOI] [PubMed] [Google Scholar]
  43. Suriyakumar S.; Stephan A. M.; Angulakshmi N.; Hassan M. H.; Alkordi M. H. Metal–organic framework@SiO2 as permselective separator for lithium–sulfur batteries. J. Mater. Chem. A 2018, 14623–14632. 10.1039/C8TA02259C. [DOI] [Google Scholar]
  44. Croce F.; Appetecchi G. B.; Persi L.; Scrosati B. Nanocomposite Polymer Electrolytes for Lithium Batteries. Nature 1998, 394, 456–458. 10.1038/28818. [DOI] [Google Scholar]
  45. Evans J.; Vincent C. A.; Bruce P. G. Electrochemical Measurement of Transference Numbers in Polymer Electrolytes. Polymer 1987, 28, 2324–2328. 10.1016/0032-3861(87)90394-6. [DOI] [Google Scholar]
  46. Angulakshmi N.; Nahm K. S.; Nair J. R.; Gerbaldi C.; Bongiovanni R.; Penazzi N.; Stephan A. M. Cycling Profile of MgAl2O4-Incorporated Composite Electrolytes Composed of PEO and LiPF6 for Lithium Polymer Batteries. Electrochim. Acta 2013, 90, 179–185. 10.1016/j.electacta.2012.12.003. [DOI] [Google Scholar]
  47. Stephan A. M.; Nahm K. S.; Prem Kumar T.; Anbu Kulandainathan M.; Ravi G.; Wilson J. Nanofiller Incorporated Poly(Vinylidene Fluoride–Hexafluoropropylene)(PVdF–HFP) Composite Electrolytes for Lithium Batteries. J. Power Sources 2006, 159, 1316–1321. 10.1016/j.jpowsour.2005.11.055. [DOI] [Google Scholar]
  48. Lin D.; Liu W.; Liu Y.; Lee H. R.; Hsu P.-C.; Liu K.; Cui Y. High Ionic Conductivity of Composite Solid Polymer Electrolyte via In Situ Synthesis of Monodispersed SiO2 Nanospheres in Poly(Ethylene Oxide). Nano Lett. 2016, 16, 459–465. 10.1021/acs.nanolett.5b04117. [DOI] [PubMed] [Google Scholar]
  49. Capiglia C.; Saito Y.; Yamamoto H.; Kageyama H.; Mustarelli P. Transport Properties and Microstructure of Gel Polymer Electrolytes. Electrochim. Acta 2000, 45, 1341–1345. 10.1016/S0013-4686(99)00342-4. [DOI] [Google Scholar]

Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

ao9b04133_si_001.pdf (750.1KB, pdf)

Articles from ACS Omega are provided here courtesy of American Chemical Society

RESOURCES