Letter pubs.acs.org/NanoLett
Nanostructured Conductive Polymer Gels as a General Framework Material To Improve Electrochemical Performance of Cathode Materials in Li-Ion Batteries Ye Shi,† Xingyi Zhou,† Jun Zhang,† Andrea M. Bruck,‡ Andrew C. Bond,† Amy C. Marschilok,‡,§ Kenneth J. Takeuchi,‡,§ Esther S. Takeuchi,*,‡,§,⊥ and Guihua Yu*,†
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†
Materials Science and Engineering Program and Department of Mechanical Engineering, The University of Texas at Austin, Austin, Texas 78712, United States ‡ Department of Chemistry and §Department of Materials Science and Chemical Engineering, Stony Brook University, Stony Brook, New York 11794, United States ⊥ Energy Sciences Directorate, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *
ABSTRACT: Controlling architecture of electrode composites is of particular importance to optimize both electronic and ionic conduction within the entire electrode and improve the dispersion of active particles, thus achieving the best energy delivery from a battery. Electrodes based on conventional binder systems that consist of carbon additives and nonconductive binder polymers suffer from aggregation of particles and poor physical connections, leading to decreased effective electronic and ionic conductivities. Here we developed a three-dimensional (3D) nanostructured hybrid inorganic-gel framework electrode by in situ polymerization of conductive polymer gel onto commercial lithium iron phosphate particles. This framework electrode exhibits greatly improved rate and cyclic performance because the highly conductive and hierarchically porous network of the hybrid gel framework promotes both electronic and ionic transport. In addition, both inorganic and organic components are uniformly distributed within the electrode because the polymer coating prevents active particles from aggregation, enabling full access to each particle. The robust framework further provides mechanical strength to support active electrode materials and improves the long-term electrochemical stability. The multifunctional conductive gel framework can be generalized for other high-capacity inorganic electrode materials to enable high-performance lithium ion batteries. KEYWORDS: Lithium ion battery, conductive polymer, gel framework, lithium iron phosphate, energy storage, electrochemistry
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electrode components, especially when nano/micro-sized materials are involved.5 In principle, the intrinsic energy capacity is determined by the charge transfer reactions that occur when a Li ion encounters an electron at an active site within the electrode.4 To maximize energy use, pathways within the electrode for both electrons and ions must be low-resistance and continuous, reaching the entire volume of the battery.6 However, in conventional electrodes, conductive additive carbon particles are randomly distributed and tend to aggregate during electrochemical reactions, thus poor contacts of electronic connections may occur.7,8 To avoid these negative effects, the amount of binder needs to be improved, which will lead to
ithium ion batteries have been dominating the market of consumer electronics as power sources for decades owing to their advantageous features of high energy density, high efficiency, lightweight and portability.1,2 Now the growing demand for large-scale energy storage such as power sources for electric vehicles and stationary energy storage requires researchers to further improve the performance of lithium ion batteries in various aspects including energy density, rate capability, cyclic stability, and safety.3 However, in conventional battery cathodes active particles such as lithium iron phosphate (LFP) are connected by the binder system that consists of conductive additives (usually carbon nanoparticles) and nonconductive binder polymer. This binder system often becomes the bottleneck for development of higher-performance lithium ion batteries mainly due to two important issues. The first is the difficulty to achieve both high electronic and ionic conductivity4 and the other is the poor dispersion of © 2017 American Chemical Society
Received: December 16, 2016 Revised: February 7, 2017 Published: February 13, 2017 1906
DOI: 10.1021/acs.nanolett.6b05227 Nano Lett. 2017, 17, 1906−1914
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Nano Letters
Figure 1. (A) Schematic of synthetic and structural features of C-LFP/C-PPy hybrid gel framework. (B) SEM images of C-LFP/C-PPy hybrid gel framework with different magnification. The yellow arrows indicate pores with different sizes. (C) SEM images of control electrode with different magnification. The blue arrows indicate PVDF domains. (D) STEM images of C-LFP/C-PPy hybrid gel framework with different magnification. (E) High-resolution TEM image of C-LFP particle coated by C-PPy layer in hybrid gel framework. (F) EDX mapping of C-LFP/C-PPy hybrid gel framework.
decreased porosity and impeded ion conduction.4,9 The distribution of both active particles and binder materials is another leading factor that impacts the performance of battery electrodes.10−12 In an ideal situation, the active particles should be uniformly distributed and thus every particle is electronically and ionically “wired” to the current collector and electrolyte. However, in conventional electrodes, inorganic particles lack mechanical binding forces and may not be highly compatible with polymer binder, resulting in aggregation of both inorganic and organic components, which hinders electron and ion transport within the electrode.6,13,14 In the case that active particles possess a large size distribution such as commercialized cathode materials, the problem will become even more
serious because the small particles tend to be adsorbed on the surface of large particles, forming large size aggregates.12 Different strategies have been proposed to improve the electron and ion transport within battery electrodes. For example, three-dimensional (3D) network structured carbon materials have been introduced as a conducting framework matrix to connect the active particles to the current collector. The network may also create porosity to facilitate the ion diffusion. Ruoff et al.15 prepared a cathode by loading lithium iron phosphate on highly conductive ultrathin graphite foam. The cathode achieved a higher rate capability and specific capacity simultaneously, owing to the conductive and 3D graphitic structure of graphite foam. However, the employment 1907
DOI: 10.1021/acs.nanolett.6b05227 Nano Lett. 2017, 17, 1906−1914
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Figure 2. (A) XPS full spectrum of C-LFP/C-PPy hybrid gel framework. (B,C) High-resolution spectra of N 1s and Fe 2p3/2 signals from the hybrid gel framework. (D,E) Raman and FTIR spectra of C-LFP/C-PPy hybrid gel framework.
conductive polymer gels with a 3D networked structure that may be useful as multifunctional binder systems for lithium ion batteries.21,22 The conductive polymer chains can be polymerized in situ with electrode materials and cross-linked by molecules with multiple functional groups, resulting in a polymeric network connecting all active particles (Figure 1A). The hybrid inorganic−organic framework may provide structure-derived mechanical robustness and continuous pathways for electron transport owing to conductive nature and high doping level of conductive polymers.23 The hierarchical pores formed within the network and solvent compatibility of gel also facilitate the uptake of electrolytes, as well as the ion diffusion within the electrode.24,25 In addition, because the gel framework is in situ synthesized, the conductive polymer layer can be uniformly coated on every active particle, thus preventing aggregation of both inorganic and organic components and achieving full access to each particle.19 This gel binder serves multiple functionalities in the electrode, thus improving the loading ratio of electrode materials as well as the energy density of battery. In this study, we demonstrated nanostructured conductive polymer gels as a general framework material to enable much higher performance of conventional inorganic cathode materials for Li-ion batteries. We used copper(II) phthalocyanine
of 3D carbon materials usually leads to low mass loading of active materials and may cause the aggregation of particles on the carbon framework surfaces. Another example is that conductive or conjugated polymers have been adopted as binder for a battery electrode with potentially dual functionalities of adhesive and conductive nature.16,17 Liu et al.8,18 developed a promising conductive polymer binder with tailored electronic conductivity and mechanical binding force. Researchers also found that conjugated polymers could improve the dispersion of active particles within an electrode due to the surface coating. Reichmanis et al.19 wrapped P3HT polymer on the surface of nanomaterials and significantly enhanced material dispersity over the composite electrode. However, using only conductive polymers could result in low ionic transport property because the pores within the electrode are filled with soft polymers. Another recent example is the in situ formed conductive coating for some special cathode materials, such as Ag2VP2O8, which covers the surfaces of each particle.20 Ideally, a binder system that can combine the advantageous features of 3D network structured materials and conductive polymers may be a promising candidate for advanced battery electrodes. Inspired by the chemical and structural features of gel materials from nature, we designed and synthesized a series of 1908
DOI: 10.1021/acs.nanolett.6b05227 Nano Lett. 2017, 17, 1906−1914
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Figure 3. TEM images (A,B) as-prepared C-LFP control electrode and (C,D) C-LFP/C-PPy electrode. Panels B and D are false color images where organic areas are shown in red and inorganic areas are in blue.
crystalline and covered by an amorphous layer, which can be attributed to conductive polymer coating. Energy dispersive Xray (EDX) analysis was further conducted to demonstrate this uniform coating (Figure 1F). While Fe element can be only detected from the restricted area with C-LFP particle, N element coming from the backbone of PPy and S element from CuPcTs molecules can be found from the whole area covered with the gel framework. These results demonstrate that the conductive gel framework is built up due to the cross-linking effect through CuPcTs and active particles are indeed coated and interconnected by the gel framework. The composition of the hybrid gel framework was examined by X-ray powder diffraction (XRD) and thermogravimetric analysis (TGA). As shown in Figure S3, sharp and strong characteristic peaks that can be attributed to LFP crystals are observed in the XRD pattern of hybrid gel framework, indicating that the crystal structure of C-LFP is well maintained during in situ synthesis, despite a strong oxidant APS is being used.27,28 The TGA test was conducted to evaluate the weight ratio of C-LFP particles in the hybrid gel framework (Figure S4). PPy is completely removed before the temperature reaches 450 °C, thus the weight ratio of C-LFP particles is calculated to be ∼85%. This mass ratio of active materials in our hybrid gel framework is comparable to those in commercialized batteries. It is worthy to mention that we also synthesized C-LFP/C-PPy hybrid gels with different C-LFP ratios from ∼80% to ∼90%. In this study, we mainly focused on 85% sample because it shows the best rate performance (Figure S5). The chemical structures of C-LFP/C-PPy framework were further investigated by X-ray photoelectron spectroscopy (XPS), Raman, and Fourier Transform infrared spectroscopy (FTIR). The full XPS spectra shown in Figure 2A indicate the existence of oxygen, nitrogen, and carbon. Figure 2B,C show the high-resolution spectra of N 1s and Fe 2p3/2 signals from the hybrid gel framework. The N 1s peaks shown in Figure 2B can be divided into two parts. The two peaks at higher binding energy can be attributed to two kinds of N atoms in CuPcTs molecules under different chemical environments: CNC and CN(Cu)C.29 The N 1s spectra from PPy show chemical splitting: the peak centered at 399.5 eV can be
tetrasulfonate salts (CuPcTs) cross-linked polypyrrole (C-PPy) as a multifunctional framework and commercial lithium iron phosphate (C-LFP) particles as model cathode materials.26 PPy gel framework shows high electronic conductivity and decent mechanical strength when compared to other conductive polymer gels, such as polyaniline and poly(3,4-ethylenedioxythiophene)−poly(styrenesulfonate) (PEDOT:PSS). In a typical synthesis, certain amounts of pyrrole monomers, CuPcTs cross-linkers, and C-LFP particles were dispersed in deionized water (DI water). An oxidant (ammonium persulfate, APS) was added into the mixture to initialize the polymerization and the gelation could be observed within 1 min (Figure S1). Each CuPcTs molecule with four functional groups can interact with more than one PPy chain through pronating reactions and hydrogen bonding, thus cross-linking the conductive polymer into gel framework.22 The CuPcTs doped PPy shows a high electrical conductivity of 7.8 S cm−1 while PPy without doping shows an electrical conductivity of only 0.07 S cm−1.26 Because of the in situ polymerization, CLFP particles were uniformly coated by conductive polymer and embedded in the gel framework, resulting in a 3D nanostructured electrode. The structure of gel framework can be well maintained during electrode fabrication. The morphology and structure of resulted electrode were examined by scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM). The SEM images in Figure 1B show that C-LFP particles are interconnected with each other by conductive polymers, forming an integrated hybrid gel framework with 3D networked structure. The framework also enables hierarchical pores within the electrode that can facilitate ion diffusion. As a comparison, control electrode fabricated by conventional method (C-LFP/SuperP/ PVDF = 85:10:5) was also examined, showing low porosity and random distribution of organic domains (Figure 1C). The STEM images in Figure 1D show that C-LFP particles are wrapped by a layer of conductive polymer with thickness of 5 to 10 nm and interconnected with each other through the organic framework. The polymer coating is also confirmed by transmission electron microscopy (TEM) imaging (Figure S2). Figure 1E clearly shows that the C-LFP particle is highly 1909
DOI: 10.1021/acs.nanolett.6b05227 Nano Lett. 2017, 17, 1906−1914
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Figure 4. (A) Rate properties of different electrodes. (B) Voltage profiles of C-LFP/C-PPy hybrid gel framework at different charge/discharge rates. (C,D) Nyquist curves and the Warburg plots of different electrodes before and after first cycle. (E) Cyclic voltammetry profiles of C-LFP/C-PPy hybrid gel framework at different scanning rates. (F) The average cathodic and anodic diffusion coefficients of C-LFP/C-PPy hybrid gel framework were calculated from the linear relationship between peak current and the square root of scanning rate.
attributed to nitrogen that is least influenced by CuPcTs anions and the shoulder shifted by 1.3 eV can be attributed to the nitrogen that electrostatically interacts with CuPcTs.30 The peak area ratio of electrostatically unscreened to screened nitrogen atoms is 1:1, indicating a high doping level of C-PPy. Because the conductivity of conductive polymer highly depends on the doping states, the high doping level confirmed in XPS results indicates that our C-PPy based gel framework exhibits high electronic conductivity. The Fe 2p peak shown in Figure 2C can be deconvoluted to FeII, which is corresponding to signals from LFP and can confirm the existence of C-LFP in the hybrid gel framework. In the Raman spectra shown in Figure 2D, the characteristic bands observed at 1380 and 1577 cm−1 can be attributed to the ring stretching mode and the CC backbone stretching of PPy, respectively, which demonstrates that CuPcTs doped PPy in the hybrid gel framework maintains its highly conjugated structure.31 Also, intramolecular stretching vibrations of PO43− anions are recorded at 586, 990, and 1040 cm−1, which indicates the good crystallization of the LFP units.32 In Figure 2E, FTIR spectra clearly show the peaks at 1045 and 973 cm−1, which can be attributed to stretching of NH and PO.33,34 This further confirms the existence of PPy and C-LFP in the hybrid gel framework. The distribution of different components within the hybrid gel framework was examined by TEM. The dispersion of both active particles and binder materials is demonstrated to be a
leading factor that impacts the performance of battery electrode, especially when nanosized materials are involved.5 In conventional electrodes, both inorganic particles and polymers tend to agglomerate during battery electrode processing and operation, thus hindering homogeneous current distribution over the electrode. Modeling studies revealed that the size of the parent particle (crystallite) and the size of the aggregate must be considered to describe battery performance.5 To demonstrate the uniform distribution of both C-LFP particles and conductive polymers in C-LFP/C-PPy hybrid gel framework, we sectioned the electrodes using ultramicrotone and compared them to control sample through TEM observation. The TEM images of the fully constructed, asprepared electrodes show considerable differences in the distribution of the organic constituents and the C-LFP particles when prepared conventionally as compared to the method including the PPy gel framework. In Figure 3A, micron length (light gray) regions of organic material are presented with clear distinction between the organic content and the C-LFP particles. This is further highlighted in the false-colored images in Figure 3B. The control electrode shows many more regions of isolated C-LFP particles (blue) and organic domains (red) compared to the C-LFP/C-PPy hybrid gel framework. These inorganic particle aggregates hinder the electron and ion transport from the surface of aggregates to their central areas and the nonconductive polymer domains impede the transport within whole electrode. The images obtained from the C-LFP/ 1910
DOI: 10.1021/acs.nanolett.6b05227 Nano Lett. 2017, 17, 1906−1914
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Figure 5. (A,B) Cyclic performance of different electrodes at 1C and 20C, respectively. (C,D) SEM images of control electrode and C-LFP/C-PPy hybrid gel after cycling, respectively. (E) EDX mapping of C-LFP/C-PPy hybrid gel electrode after cycling.
the electrode prepared by conventional binders. The compatibility between solvents and ingredients is an important factor affecting the dispersion. It has been demonstrated that slurries comprising LFP powder, carbon black, and polymeric binder in solvent N-methyl-2-pyrrolidone (NMP) without any surfactant usually result in poor dispersion due to the hydrophilic nature of LFP particles.35 A control experiment was also conducted to demonstrate improved dispersion of LFP in water. It showed that the LFP particles were still well
C-PPy electrode, shown in Figure 3C, indicate the electrode has a much more uniform distribution of C-LFP particles and organic component. The false color images of the C-LFP/CPPy electrode (Figure 3D) show a PPy network that is homogeneously dispersed with the PPy regions less than 200 nm as shown in Figure S6. These images illustrate that the electrode preparation using the PPy gel framework provides a well dispersed, homogeneous C-LFP particle and conductive gel distribution within the electrode and stands in contrast to 1911
DOI: 10.1021/acs.nanolett.6b05227 Nano Lett. 2017, 17, 1906−1914
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hybrid gel framework shows much lower impedance when compared to control sample. The Warburg coefficients for CLFP/C-PPy hybrid gel framework and control sample are 91 and 237 Ω s−1/2, respectively, indicating much improved ionic conductivity in the 3D gel framework (Figure 4D). We further used Randles-Sevcik equation to calculate the lithium ion diffusion coefficients of C-LFP/C-PPy hybrid gel framework during charge/discharge28
dispersed in water compared to depositing in NMP after standing for 2 h (Figure S7). The electrochemical properties of C-LFP/C-PPy hybrid gel framework were first investigated by cyclic voltammetry (CV) test. As shown in Figure S8A, CV profile of the gel framework at a fixed scan rate of 0.3 mV s−1 shows symmetric and sharp anodic and cathodic peaks and the potential interval between two peaks is ∼250 mV, demonstrating the high redox kinetics of the hybrid gel framework.28 It is worth mentioning that the potential window was confined to 2.5−4 V because we found that side reactions between conductive polymer gel and electrolyte occurred when the voltage was above 4 V, which consumed electrolyte and led to low Coulombic efficiency.36 Within this potential window, the C-LFP/C-PPy hybrid gel framework can deliver a specific capacity of ∼150 mAh g−1 at 1C (1C = 170 mA g−1). The capacity contribution of PPy gel was estimated by conducting CV tests on electrode based on pure C-PPy. According to the CV result shown in Figure S8B, the specific capacity of pure C-PPy in the potential range of 2.5−4 V is calculated to be ∼15 mAh g−1. Considering that the mass ratio of gel framework is only 15% in the total electrode, the capacity contribution of polymer gel in the total electrode is negligible. We further examined the rate characteristics of C-LFP/CPPy hybrid gel framework by charging/discharging it from 0.2C up to 30C and the results are shown in Figure 4A,B. At each current density, C-LFP/C-PPy hybrid gel framework shows much higher capacity than control sample. Especially at the high charge/discharge rate of 30C, the gel framework can maintain a capacity of ∼60 mAh g−1 while the control sample only delivers a capacity less than 30 mAh g−1. When the current density is back to 0.5C again, the capacity of gel framework recovered to ∼150 mAh g−1. These results demonstrate much improved rate capability of C-LFP/C-PPy hybrid gel framework. We further compare our hybrid gel framework with other C-LFP based electrodes using 3D structured carbon materials or conductive polymers as conductive networks and the results show that the rate performance of C-PPy gel framework based electrode is among the best values (Table S1). To investigate the origin of significantly enhanced rate capability of C-LFP/C-PPy hybrid gel framework, electrochemical impedance spectra (EIS) studies were conducted on both C-LFP/C-PPy hybrid gel framework electrodes and control samples (Figure 4C). A Randles circuit was used to fit the results with Rs as the ohmic resistance from the system, Q as the constant phase element of the double layer capacity, Rct as the charge transfer resistance and a specific electrochemical element (Warburg element) of diffusion W. It is found that these two electrodes exhibit greatly decreased impedance after first cycle’s charge/discharge, mainly due to the activation effect.37 By examining the semicircles of EIS curves at high frequency for both hybrid gel framework and control sample after first cycle, charge transfer resistances of 35.4 and 44.8 Ω, respectively are found (Table S2). This result implied that electrons can be well transferred within the conductive gel framework and encountered with Li ions at active sites to enable charge transfer reactions. Impedance in low frequency (10 mHz to 10 Hz) can be attributed to ion diffusion resistance in the electrode. For the as-assembled cells, C-LFP/C-PPy hybrid gel framework exhibited higher impedance than control sample because the interfaces between polymer and active particles are not activated. However, after being charged/discharged for one cycle, the C-LFP/C-PPy
Ipc = 2.69 × 105n3/2AD1/2 Cυ1/2
where Ipc is the peak-current (A), n is the number of electrons in the charge-transfer step (for LiFePO4, n = 1), A is the surface area of electrode (cm2), D is the Li ion diffusion coefficient in LiFePO4 at 298 K (cm2 s−1), C is the molar concentration of Li ions in LiFePO4 (2.28 × 10−2 mol cm−3), and υ is the scan rate (V s−1). Cyclic voltammetry profiles of C-LFP/C-PPy hybrid gel framework at different scanning rates are shown in Figure 4E and the peak current is in linear response to the square root of scanning rate (Figure 4F). The average cathodic and anodic diffusion coefficients of the C-LFP/C-PPy hybrid gel framework are calculated to be 1.42 × 10−12 and 8.34 × 10−13 cm2 s−1, respectively, which are about four times higher than those of control sample. The control sample shows average cathodic and anodic diffusion coefficients of 3.49 × 10−13 and 1.91 × 10−13 cm2 s−1, respectively (Figure S9). The results from EIS and CV studies demonstrate that the hybrid gel framework facilitates the diffusion of lithium ions in the whole electrode, which contributes to the improved rate performance of hybrid gel framework. The hierarchically porous structure of gel framework can improve the uptake of electrolyte and the uniform polymer coating with high solvent compatibility may lower the ion transfer resistance. The cyclic stability of C-LFP/C-PPy hybrid gel framework was also investigated at both low and high current densities. Figure 5A shows the cyclic performance of C-LFP/C-PPy hybrid gel framework and control sample at 1C. The C-LFP/CPPy hybrid gel framework delivers an initial discharge capacity of 140 mAh g−1 and achieved capacity retention of 75% after 500 cycles while the control sample exhibits an initial discharge capacity of 102 mAh g−1 and 55.4% capacity retention after 500 cycles’ charge/discharge. When the current density is increased to 20C, C-LFP/C-PPy hybrid gel framework also shows excellent cyclic stability. As shown in Figure 5B, the gel framework can maintain a discharge capacity of ∼80 mAh g−1 for over 1000 cycles. At 20C, the capacity is less than 40 mAh g−1 for control samples. Notably except for the first cycle, the Coulombic efficiency of C-LFP/C-PPy hybrid gel framework stays near 100% during the whole cyclic tests at 1C and 20C, demonstrating its excellent reversibility. EIS studies were conducted on the hybrid gel framework after cycling (Figure S10). The charge transfer resistance remains almost the same and the Warburg coefficient increases a little, demonstrating well maintained electronic and ionic transport properties. The excellent cyclic stability of C-LFP/C-PPy hybrid gel framework indicates the doping states and electrochemical properties of gel framework are stable during electrochemical reactions, mainly due to strong interactions between CuPcTs cross-linkers and PPy polymeric chains, as well as between conductive polymers and active particles. The highly conductive network of C-PPy gel maintains the effective transport of electrons within the framework and the porous structure ensures efficient ion 1912
DOI: 10.1021/acs.nanolett.6b05227 Nano Lett. 2017, 17, 1906−1914
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diffusion from the electrolyte to active particles, thus contributing to improved stability of gel framework. We also examined the microstructure and morphology of CLFP/C-PPy hybrid gel framework after cycling and compared to those of control sample. As shown in Figure 5C, the porosity of control sample decreases due to the aggregation of carbon additives and deformation of binder polymers. However, for the C-LFP/C-PPy hybrid gel framework, the porous structure is maintained and active particles are still interconnected with each other through polymer framework after 500 cycles’ charge/discharge, indicating decent mechanical robustness of the gel framework. Although traditional conductive polymers show poor flexibility due to their rigid backbones caused by highly conjugated ring structure, nanostructured PPy gel framework exhibits structure derived elastic modulus to sustain the mechanical change and maintain the 3D nanostructure during electrochemical reactions.38 We further conducted EDX mapping on the cycled gel framework (Figure 5E). The STEM image showed that the polymer layer was still coated on the LFP particles. And the Cu element can be detected from the whole gel framework, demonstrating that the core/shell structure of hybrid gel and cross-linked network structure of C-PPy are preserved. Raman spectra were also measured for cycled hybrid gel, which supported its excellent stability (Figure S11). The conductive polymer layer demonstrates its excellent chemical/physical stability owing to the strong interactions between polymeric chains and active particles, as well as the mechanical flexibility of gel framework.39 In summary, we demonstrated and promising conductive gel framework material via molecular cross-linking as the binder system for battery electrodes. The conductive polymer gel is in situ polymerized onto commercial LFP particles used as a model cathode material, forming a hybrid inorganic−organic gel framework with 3D networked structure. The hybrid framework electrodes achieve both high electronic and ionic conductivity because the continuous network can promote electron transport owing to conjugated polymer chains and high doping level and the hierarchically porous structure facilitates the ion diffusion. Meanwhile in situ uniform polymer coating on the surfaces helps prevent inorganic LFP particles from aggregation and promotes the electron and ion transport. The chemical stability and decent mechanical robustness of conductive gel framework contribute to the cyclic stability of the battery electrode. As a result, the hybrid gel framework electrodes exhibit excellent rate and cyclic performance. To further improve the performance of hybrid gel, the active particles need to be engineered for uniform size distribution and controlled crystal structure and new cross-linking molecules can be adopted to improve the electrochemical properties of polymer gel. This type of multifunctional conductive gel frameworks can be generalized for other highcapacity inorganic electrode materials for high-performance lithium ion batteries.
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Letter
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Esther S. Takeuchi: 0000-0001-8518-1047 Guihua Yu: 0000-0002-3253-0749 Author Contributions
Y.S. and X. Z. contributed equally to this work. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors acknowledge the Center for Mesoscale Transport Properties, an Energy Frontier Research Center from the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under award DE-SC0012673 for financial support on electrode synthesis, electrochemistry, and cross-sectional analysis. They acknowledge Dr. Jarvis at Texas Material Institute for her assistance in TEM test. They also acknowledge the Transmission Electron Microscopy Facility in the Central Microscopy Imaging Center (C-MIC) at Stony Brook University, Stony Brook, New York for their contribution towards the TEM preparation and data collection. A.M.B. acknowledges support from the National Science Foundation Graduate Research Fellowship Program under Grant 1109408.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.6b05227. Detailed experimental procedures and supplementary characterization (PDF) 1913
DOI: 10.1021/acs.nanolett.6b05227 Nano Lett. 2017, 17, 1906−1914
Letter
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DOI: 10.1021/acs.nanolett.6b05227 Nano Lett. 2017, 17, 1906−1914
