Polyethylenimine-Mediated Electrostatic Assembly of MnO2 Nanorods

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Polyethyleneimine-Mediated Electrostatic Assembly of MnO2 Nanorods on Graphene Oxides for Use as Anodes in Lithium-Ion Batteries Changju Chae, Ki Woong Kim, Young Jun Yun, Daehee Lee, Jooho Moon, Youngmin Choi, Sun Sook Lee, Sungho Choi, and Sunho Jeong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b01931 • Publication Date (Web): 15 Apr 2016 Downloaded from http://pubs.acs.org on April 16, 2016

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ACS Applied Materials & Interfaces

Polyethyleneimine-Mediated

Electrostatic

Assembly

of

MnO2

Nanorods on Graphene Oxides for Use as Anodes in Lithium-Ion Batteries

Changju Chae,a Ki Woong Kim,a Young Jun Yun,a Daehee Lee,b Jooho Moon,b Youngmin Choi,a Sun Sook Lee,a,* Sungho Choi,a,* Sunho Jeonga,*

a

Division of Advanced Materials, Korea Research Institute of Chemical Technology

(KRICT), 141 Kajeongro, Daejeon 305-600, Republic of Korea. b

Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro

Seodaemun-gu, Seou1 120-749, Republic of Korea.

KEYWORDS: polyethyleneimine, electrostatic, assembly, MnO2, graphene oxide, battery

ACS Paragon Plus Environment


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ABSTRACT In recent years, the development of electrochemically active materials with excellent lithium storage capacity has attracted tremendous attention for application in highperformance lithium-ion batteries. MnO2-based composite materials have been recognized one of promising candidates owing to their high theoretical capacity and cost-effectiveness. In this study, a previously-unrecognized chemical method is proposed to induce intra-stacked assembly from MnO2 nanorods and graphene oxide (GO), which is incorporated as an electrically conductive medium and a structural template, through polyethyleneimine (PEI)derived electrostatic modulation between both constituent materials. It is revealed that PEI, a cationic polyelectrolyte, is capable of effectively forming hierarchical, 2-dimensional MnO2RGO composites, enabling highly reversible capacities of 880, 770, 630, and 460 mAh/g at current densities of 0.1, 1, 3, and 5 A/g, respectively. The role of PEI in electrostaticallyassembled composite materials is clarified through electrochemical impedance spectroscopybased comparative analysis.


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1. INTRODUCTION Rechargeable lithium ion batteries have been attracting intense interest for application in various portable electronic devices as well as pure electric/hybrid electric vehicle applications.1 For anode materials, a variety of candidates have been suggested as alternatives to their conventional graphite-based counterparts to improve the performance of batteries, based on their different electrochemical mechanisms, including alloying and conversion reaction.2 Among them, transition metal oxides, in which the conversion reaction mechanism is predominantly applied, have gained tremendous attention as viable alternatives for high-density energy-storage systems, owing to their high theoretical capacities over that (372 mAh/g) of graphite.1-3 In particular, manganese-incorporating oxides (MnOx) have theoretical capacities over 700 mAh/g (even over 1200 mAh/g for manganese dioxide, MnO2), with advantages of abundance, cost effectiveness, and environmental friendliness.4-12 However, in general, they suffer from significant degradation of electrochemical performance by repeated volumetric deformation during prolonged cycling, which is one of inherent drawbacks of transition metal oxide-based anodes, along with suppressed capacities under the conditions of high current density due to the limited electrical conductivity of the oxides themselves. In addressing these issues, based on manganese dioxides with the highest theoretical capacity (1230 mAh/g), various chemical/physical approaches have been exploited by hybridizing with carbon materials (carbon nanotubes and graphene-related materials). These hybrid materials are capable of accommodating structural deformation and exhibiting excellent electrical conductivity. It has been reported that morphological control at the nanoscale could resolve a fading problem in capacities to some extent,4,5and nanostructured MnO2-carbon hierarchical composites have opened up another opportunity for anodic



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materials with high capacity, cycling stability, and high rate capability.6-12 To date, the in-situ synthesis methodology has been widely suggested; the direct growth of nanostructured MnO2 phase has been induced on three-dimensionally porous graphene, carbon nanohorns, and carbon nanotubes, providing the fundamental potential in evolving uniformly-intermixed composite materials.6-8 However, these in-situ synthesized composite materials have shown low capacities below 500 mAh/g under the condition of high current density around 2 A/g, even when a variety of carbons with different atomic arrangements and morphological structures at the nanoscale have been incorporated. As an effort to realize the well-engineered heterogeneous interfacial growth of MnO2 phase on graphene, a conjugated polymer derived from 3,4-ethylenedioxythiophene was also introduced to adjust the molecular mismatch at an interface, reducing the activation energy to trigger the heterogeneous nucleation of MnO2 phase. However, the capacities of 930 and 698 mAh/g were reported only at current densities of 0.1 and 0.4 A/g, respectively.9 Recently, ex-situ hybridization between MnO2 and carbons has been considered as a viable alternative methodology, as the pre-synthesized, nanostructured MnO2 phase with a high crystallinity can be chemically stacked on top of graphene oxides,10,11 without the ambiguous consideration of how well-designed interfacial growth is achievable between oxides and carbons with different surficial properties; it has enabled the achievement of reversible capacities over 900, 700, and 400 mAh/g at current densities of 0.1, 1.2, and 6.1 A/g, respectively.11 In contrast, for physically stacked MnO2/graphene composite materials prepared by sequential vacuum filtration, a relatively poor electrochemical performance was reported with the capacity of 208 mAh/g at a current density of 1.6 A/g.12 In this study, we have developed an extremely simple, environmental-friendly, facile method to chemically hybridize rod-shaped, nanosized MnO2 and graphene oxide (GO) in an


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aqueous medium, by introducing polyethyleneimine (PEI) as an interfacial chemical moiety for instant electrostatic assembly between MnO2 and GO. The PEI, which acts as a cationic polyelectrolyte in an aqueous medium, allows for the formation of hierarchical composites composed of MnO2 and GO. When the resulting composites are employed as an anodic material for lithium-ion batteries, they enable the achievement of reversible capacities of 880, 770, 630, and 460 mAh/g at current densities of 0.1, 1, 3 and 5 A/g, respectively. Along with the demonstration of improved electrochemical performance, the critical impact of PEImediated assembly in electrochemical reactions is also revealed through comparative studies with corresponding materials prepared without polyethyleneimine and previously-reported MnO2-based composite materials.

2. METHOD 2.1 Synthesis of MnO2 nanorods. MnO2 nanorods were synthesized using a microwaveassisted hydrothermal method. 1.537 g of MnSO4·H2O (Sigma Aldrich, 99%) and 0.958 g of KMnO4 (Sigma Aldrich, 99%) were dissolved in 50 ml of deionized water and vigorously stirred at room temperature for 1 h. Then, the precursor solution was transferred to a microwave reactor (Model MARS6, CEM corp., USA) and heated at 150 oC for 10 min using 400 W power. After the autoclave cooled down to room temperature, the product was thoroughly washed in ethanol to remove residual by-products. The nanorods were collected by centrifugation with repeated washing in deionized water. The resulting MnO2 nanorods were re-dispersed in distilled water and stored in air. 2.2 Synthesis of graphene oxides. Graphene oxides (GOs) were prepared by the subsequent reaction of oxidation/exfoliation, following a modified Hummer’s method. A mixture of natural graphite flakes (Sigma Aldrich, 5.0 g) and NaNO3 (3.75 g) was inserted in a 2L-round



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bottom flask containing H2SO4 (95%, 375 mL) with stirring in an ice bath. KMnO4 (22.5 g) was added slowly, while the reaction temperature was kept below 20 °C. Then, the flask was placed in an oil bath at 30 oC, and it was removed from the oil bath two days later. After air cooling, 5 wt% H2SO4 (700 ml) was slowly added to the flask, and stirring was maintained for 2 h. Next, 30 wt% H2O2 (15 ml) was slowly added. The color of the suspension changed from dark brown to yellow, and the stirring was maintianed for 2 h. The as-obtained graphite oxide was purified with distilled water several times by centrifugation. GO sheets were exfoliated from graphite oxide by ultra-sonication. The products were re-dispersed in distilled water and stored in air. 2.3 Preparation of MnO2-PEI-RGO composites. The synthesized MnO2 nanorods were collected by centrifugation from aqueous dispersed solutions, and the fraction of water in the precipitates was measured to be 84%. The 1.25 g of MnO2 wet precipitate was dispersed in 8 g of deionized water by sonication. In our study, to prevent the formation of highly stacked aggregates, un-dried MnO2 nanorods and graphene oxides were used in experiments. Next, 50 mg of graphene oxide was also dispersed in 8 g of deionized water with a corresponding sonication process. Then, both aqueous solutions were mixed by sonication, and in the case of MnO2-PEI-RGO composite materials, aqueous polyethyleneimine (PEI, Aldrich, molecular weight: 1,300) solution (0.3 g of PEI was dissolved in 2.7 g of deionized water) was added with stirring at room temperature. The resulting composite materials were collected by centrifugation, followed by a vacuum drying at 80 oC for 12 h and a thermal annealing for reducing graphene oxides at 200 oC for 1 h under Ar atmosphere with a flow rate of 100 sccm. 2.4 Fabrication of anodes for lithium-ion batteries. Electrochemical tests were conducted using CR2032 coin cells with Li metal as a reference electrode. The working electrodes were prepared by casting a paste onto a copper foil current collector. The pastes were composed of


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active materials, Super-P carbon black, polyvinylidene fluoride (PVDF, Kureha KF-1100) as a binder, and NMP as a solvent (active material : Super-P : PVDF = 8 : 1 : 1 in weight). All working electrodes were pressed and vacuum dried at 120 oC for 12 h. A Celgard 2400 was used as a separator and 1M LiPF6 in ethylene carbonate/diethyl carbonate (EC/DEC) mixture (1:1 v/v) was used as an electrolyte. The cells were assembled in an Ar-filled glove box. The galvanostatic charge-discharge profile, cycling performance, and rate performance were investigated in the voltage range of 0.01-3.0 V vs. Li+/Li, using battery testing equipment (TOSCAT-3100, Toyo Co. Ltd). 2.5 Characterization. The size and shape of synthesized MnO2 nanorods and the morphologies of composite materials were observed by scanning electron microscopy (SEM, JSM-6700, JEOL). The crystal structures were analyzed using an X-ray diffractometer (XRD, D/MAX-2200V, Rigaku), and the chemical structural analysis was performed by X-ray photoelectron spectroscopy (XPS, K-Alpha, Thermo Fisher Scientific). The thermal decomposition behaviors of the composite materials were monitored by thermal gravimetric analysis (TGA, SDT2960, TA Instruments). Electrochemical impedance spectra (EIS) analyses were conducted using a potentiostat (1287A, Solartron) and a frequency analyzer (1260, Solartron) with a four-probe configuration. The coin cells with electrode area of 1.767 cm2 (d=15 mm) underwent initial discharge/charge cycles at a current density of 100 mA/g and were subsequently discharged to the plateau voltage of 0.42 V. All cells were equilibrated under open-circuit condition for 1 h prior to the EIS measurements, which were conducted at frequencies ranging from 300 kHz to 0.01 Hz and and AC amplitude of 10 mV.



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3. RESULTS AND DISCUSSION The schematic diagram shows the procedure to prepare the well-engineered composite materials from pre-formed MnO2 nanorods and graphene oxide (Scheme 1). The MnO2 nanorods were synthesized by a microwave-assisted hydrothermal reaction, and GO was prepared using a modified Hummers method. As shown in Figure 1a, the synthesized MnO2 is rod-shaped with a diameter of 30 nm and a high aspect ratio of 56.6. It is observed that the resulting MnO2 nanorods are free from aggregates, showing an individual nanostructured morphology. The X-ray diffraction result (Figure 1b) revealed that the MnO2 nanorod is composed of phase-pure, layered birnessite-type manganese dioxide, consisting of two-dimensional monolayers of edge-shared MnO6 octahedra; it is well known that such an open structure for MnO2 could promote a high lithium ion battery capacity. As shown in Figure 1c, the resulting MnO2 nanorods and graphene oxide exhibited a negative zeta potential over -20 mV in an aqueous medium with a neutral pH, allowing good dispersion stability without additional procedures. The isoelectric point (IEP) in pH, at which the surface zeta potential is lost toward zero, is known to be 4-5 for MnO213. This corresponds well to the measured IEP value, 4.3, for the MnO2 nanorods synthesized in this study. The graphene oxides have a high negative zeta potential over a wide pH range (even under acidic environment) due to their surface functional groups.14 As can be expected, the electrostatic attractive interaction could not be involved in both materials with negative surface charges; after extracting a solvent medium, DI-water, in an aqueous mixture of MnO2 and GO, the physically mixed composites were only obtainable with an inhomogeneous morphology due to the absence of chemical interactions between them. For synthesizing the composite materials with the characteristic benefits from both MnO2 phase with a high electrochemical activity and graphene oxide as a stress-releasing,


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highly conductive layer, they should be assembled with a reinforced interfacial contact behavior. Polyethyleneimine (PEI) is a representative cationic polyelectrolyte, and the evolution of positive surface charge along its polymeric chain is adjustable depending on the pH of its surroundings. PEI has a high positive zeta potential in an acidic aqueous medium due to a protonation reaction, and its surface charge can be preserved to some extent even in a slightly basic aqueous medium. In fact, by the simple addition of an aqueous PEI solution into a MnO2 nanorod or GO suspending aqueous solution at a neutral pH, the surface charge of both the MnO2 nanorod and GO was significantly shifted even over 20 mV. Prior to measurement of the surface zeta potentials, the excessive PEI was washed out by a centrifugation. The zeta potentials of PEI-treated MnO2 nanorod and GO were measured to be 22.7 and 39.3 mV at a neutral pH, respectively. Correspondingly, as shown in Figure 1d, the IEP value moved from 4.3 to 8.3 for the MnO2 nanorods, and it was measured to be 9.9 for graphene oxide. GO has no isoelectric point on a non-treated bare surface. This indicates that electrostatic assembly between negatively surface charged MnO2 nanorods and GO, can be induced via simple PEI-mediated chemical approaches without the involvement of a complicated synthetic scheme and surface modification. For homogenously-assembled composite materials, an aqueous PEI solution was added to a pre-mixed aqueous solution including MnO2 nanorods and GO with a simple agitation, followed by centrifugation and conventional drying procedures to obtain the dried powders. Note that in this chemicallyderived synthetic scheme, only an extremely benign solvent, water, was used at a neutral pH, without the involvement of highly acidic and basic chemicals for adjusting pH, and a simple mixing process without complicated post-treatments was employed to form homogenous composites. Subsequently, the prepared composite materials were annealed at a temperature as low as 200 oC under an inert atmosphere to reduce the GO while preventing the thermal decomposition of the PEI itself. As shown in the X-ray diffraction results (Figure S1), after



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annealing at 200 oC, the crystalline phase of MnO2 was preserved well, along with the partial reduction of graphene oxides, represented by the 2-theta value of 22.1o.15 Figure 2a-b shows the SEM images for PEI-involved and PEI-free MnO2-RGO composites. As seen in Figure 2a, one-dimensional MnO2 nanorods are positioned softly on top of flattened RGO surfaces. The characteristic morphology was also confirmed by TEM images along with a compositional analysis (Figure S2). In general, under a conventional drying process, other than a freeze-drying process, graphene oxides with a high surface-tovolume ratio are entangled with the generation of undesirably stacked morphologies, losing their characteristic two-dimensional properties.16 However, in our scheme, prior to drying, the MnO2 nanorods are first anchored on the surface of GOs by PEI-mediated electrostatic attraction, preventing the GOs from moving freely in contact with other GOs when the solvents are extracted between neighboring GO sheets. The population density of MnO2 nanorods on GOs can be determined by steric hindrance due to the rotational movement of one-dimensional rods in a liquid phase, as the assembly of MnO2 nanorods would occur instantly upon the adjustment of surface charges by positively charged PEIs. Taking into account the fact that the MnO2-GO assemblies are derived by a simple agitation process, it is speculated that a large number of MnO2 nanorods are stacked uniformly without the formation of aggregates on the surfaces of graphene oxides. In contrast, in composites prepared without PEI, GOs were hardly observable and MnO2 nanorods were significantly aggregated (Figure 2b). The atomic ratio of C/Mn was measured to be 0.07 by energydispersive X-ray spectroscopy analysis. In aqueous medium, under a high centrifugal force, GOs were barely precipitated, whereas MnO2 easily settled down because of the high density of the volumetric MnO2 rods. Then, MnO2 nanorods are aggregated while the solvent is extracted, as the spatial displacement between neighboring nanorods is not preserved. These


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distinctively different morphologies represent the critical role of the electrostatic mediator, PEI, in assembling both MnO2 nanorods and GOs. The effective inclusion of graphene oxides by an intact chemical interaction was also confirmed by the thermogravimetric analysis (TGA) results. As shown in Figure S3, the PEIfree MnO2-RGO composite underwent a weight loss of 6.4% during annealing at elevated temperatures in air. This loss is ascribed to the evaporation of adsorbed water and the further thermal reduction of RGOs. For a PEI-mediated MnO2-RGO composite, a weight loss of 30.7% was measured, and the difference in weight loss for both composite materials is attributed to the amount of PEI adsorbed to both MnO2 and RGO as well as the amount of incorporated RGOs. To precisely measure the composition of MnO2-PEI-RGO, we carried out elemental analysis (EA). As for the TGA result for MnO2-PEI-RGO composite, the composition of MnO2 phase was measured to be 69.3 wt%, and the weight ratio of nitrogen to carbon was determined to be 0.21 by EA; thus, assuming that the amount of nitrogen present in thermally-driven RGO and the relative weight fraction of hydrogen in PEI are negligible, it can be calculated that the weight ratio of PEI to RGO is 5.3/25.4 and the composition of MnO2 : RGO is 73.2 : 26.8 in weight, which is well-corresponding to the weight ratio, 8/2, of MnO2 nanorod to graphene oxide in a synthetic batch. In addition to morphological properties, to elucidate the influence of the PEI layer, which covers the surface of GOs, on the chemical structure of graphene oxides, we analyzed C1s X-ray photoelectron spectroscopy (XPS) spectra for pristine dried-GO, thermallyannealed reduced graphene oxides (RGO), and thermally-annealed MnO2-PEI-RGO composite (Figure 2c). According to the areal integration-based semi-quantitative analysis for the sub-peak at 284.5 eV, representing the C-C chemical bond,17 the fractions were measured to be 0.38, 0.64, and 0.62 for dried-GO, thermally-annealed RGO, and thermally-annealed



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MnO2-PEI-RGO composite, respectively. This implies that even with the presence of a PEI layer overlying RGOs, the thermal reduction of graphene oxides themselves is not significantly suppressed. Rather, the sub-peak, detected to be π-π*18 at 290.75 eV was absent for the thermally-annealed MnO2-PEI-RGO composite, while it was present for both the dried-GO and thermally-annealed RGO. This is attributable to the absence of inter-planar stacking betwen neighboring graphene oxide sheets in the MnO2-PEI-RGO composite, which is indicative of effective PEI-mediated assembly of MnO2 nanorods on the surface of GOs. MnO2 nanorod, MnO2-RGO, and MnO2-PEI-RGO electrodes were tested for use as anodes in Li-ion batteries. Electrochemical measurements were carried out based on a halfcell configuration. Figure 3a shows the cycling performance for three different anodes. As expected, MnO2 nanorod-based electrodes suffered from significant degradation in capacity with a dramatic drop even in the first charging process; this indicates that the formation of MnO2 phase in a nanostructured rod shape with a high aspect ratio does not allow acceptable cycling stability. For the PEI-free composite (RGO-MnO2) electrode, a similar performance trend was observed due to the absence of a sufficient amount of partially-reduced graphene oxide. Interestingly, the MnO2-PEI-RGO electrodes exhibited a much more improved cycling stability with a high capacity reaching 880 mAh/g at a current density of 100 mA/g. As shown in Figure S5, for PEI-free MnO2-RGO composites, the SEI layers were grown fully on the surface of MnO2 rods, because of the lack of stress-releasing carbon moieties. The internal stress accumulated by a significant volume expansion/contraction generates the formation of cracked surface, on which another SEI layer is formed, resulting in the further consumption of Li ions in electrolyte and the demolishment of electrical junctions between active materials and current collectors. Note that when the composite materials are prepared without the incorporation of PEI, the MnO2-RGO composites are almost free of graphene


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oxides. In contrast, for the case of PEI-involved MnO2-RGO composites, it was clearly observed that the SEI layer is formed stably on top of MnO2 nanorods, maintaining the morphological structure of 1-dimensiaonl active materials and their isolated residence on graphene oxides. This is, in general, caused by the presence of deformable, 2-dimensional carbon sheets acting as a stress-accommodating layer. As for the capacity unit gram, the weight of active materials was calculated together with carbons (graphene oxides). Thus, when it comes to the true weight of MnO2 phase, the obtained capacity value approaches 1202 mAh/g, which is similar to the theoretical capacity of MnO2 phase. The stable cycling performance was also observable in the voltage profiles of MnO2-PEI-RGO electrodes, with an unchanged plateau during a repeated cycling test (Figure 3b). We also evaluated the rate capability of electrodes employing MnO2 nanorod, MnO2RGO, and MnO2-PEI-RGO composites (Figure 4a). The voltage profiles of the tested cells are shown in Figure S4. For the MnO2 nanorod and PEI-free MnO2-RGO based electrodes, at a current density over 500 mA/g, the cells were almost inactive, losing their electrochemical performance. In contrast, the MnO2-PEI-RGO electrodes still showed relatively high capacities even at a current density of 5000 mA/g (880, 770, 630, and 460 mAh/g at current densities of 100, 1000, 3000 and 5000 mA/g), and the electrochemical performance recovered well after operation at a current density of 5000 mA/g. As shown in Figure 4b, the normalized capacity variation at current densities ranging from 100 to 3000 mA/g was merely 0.67, which, to the best our knowledge, is superior to previously reported MnO2-based anodic materials with capacities over 600 mAh/g at a current density around 100 mA/g. The composition of MnO2 phase in composites reported to date was 32-70 wt%8-11; thus, it is clearly believed that both of high capacity and high rate capability, obtained in our study, results from the inclusion of sufficient amount of MnO2 phase and the well-designed



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nanostructure based on an efficient interfacial mediator, PEI. Figure 4c shows the cycle performance of MnO2-PEI-RGO electrodes at a current density of 100 and 3000 mA/g, with a stable operation during a prolonged cycling test even under a high current density. The voltage profiles at a current density of 3000 mA/g are shown in Figure S6. In the initial cycles, stepwise increments in current density were applied, and the current density of 3000 mA/g was applied over 10 cycles. It was clearly observed that a high capacity around 600 mAh/g was stably maintained even when the current density was as high as 3000 mA/g. To date, the ex-situ assembly of MnO2 phase with carbons for stable lithium-ion storage has been accomplished through electrostatic stacking with carbons.10,11 For this scheme, the surface charge modulation of MnO2 phase is activated in two ways, namely, carbon doping into nanostructured MnO2 phase through an additional in-situ microwave reaction with graphene11 and the adsorption of poly(diallyldimethylammonium) (PDDA) chloride on the surface of porous MnO2 phase directly grown graphene nanoribbons10. The latter is based on surface functionalization using a cationic polyelectrolyte, similar to our study using the PEI as a interfacial layer; however, this method has not achieved an excellent rate capability (below 500 and 300 mAh/g at current densities of 0.4 and 1 A/g, respectively), even showing the capacity of 890 mAh/g at a current density of 0.1 A/g. It also requires complicated synthetic method to form graphene nanoribbons and in-situ synthesis of porous MnO2 phase on them. Maintaining a high capacity under conditions of high current densities is of paramount practical importance, as it enables a rapid charging process in practical applications. The rate capability is predominantly determined by the effectiveness of charge transport from active materials toward current collectors. In a chemical method of electrostatic assembly using polyelectrolytes, the polyelectrolyte acts as a blocking layer against efficient charge transfer at an interface between electrochemically active oxides and


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conductive carbons. This is supported by the fact that polyelectrolyte-free electrostatic assembly between MnO2 nanophase and GOs enabled capacities around 1000 and 400 mAh/g at current densities of 123 and 6160 mA/g, respectively.11 However, this method requires complicated in-situ carbon-doping synthesis to alter the surface charge of MnO2 phase; the inversed positive surface charge is as low as 8.5 mV, and a plausible explanation for the role of carbon doping needs to be further studied. In contrast, the PEI introduced in this study has been widely incorporated as an electron transporting layer in organic-based solar cells, improving the electron mobility and lowering the series resistance in an overall cell by effectively collecting electrons from photo-active layers generating electron-hole pairs.19-21 It is speculated that even for electrochemistry-involved active materials, the PEI interfacial layer allows the enhanced capability of transporting charge carriers, resulting in improved electrochemical capacity and rate capability, while providing a facile synthetic pathway to electrostatically hybridize both MnO2 phase and graphene oxide. Electrochemical impedance spectroscopy (EIS) was performed to elucidate the role of the PEI interfacial layer and the electrochemical reaction kinetics in the MnO2-PEI-RGO composite electrode. The EIS measurement was conducted at the plateau potential of 0.42 V, at which the conversion reaction of MnO2 phase takes place predominantly, to precisely characterize the electrochemical reaction kinetics. The reliable EIS spectra for the cells with bare MnO2 and RGO-MnO2 electrodes were not obtainable with much larger polarizations at all frequencies than that of the MnO2-PEI-RGO cell owing to the time-dependent degradation in a static condition. The Nyquist plot of the MnO2-PEI-RGO cell was deconvoluted as shown in Figure 5a, with a Randle-type equivalent circuit model depicted in the plot. The equivalent circuit model describes the electrochemical reaction steps, including Li ion migration through SEI layers, a charge transfer reaction, and Li ion diffusion kinetics



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throughout the active materials (Warburg element). The polarization resistance for Li ion migration through solid-electrolyte interface (SEI) layers (RSEI) was measured to be 5.9 Ω, while that of charge transfer reaction (Rct) was 12.4 Ω. These values were much lower than those of previously reported MnO2-based cells that employ ex-situ-assembled composite electrodes (Figure 5b). If an electrode is not capable of accommodating the internal mechanical stress accumulated by repeated volumetric change of MnO2 phase during chargedischarge reactions, the gradual growth of SEI layers takes place. The volumetric fraction of MnO2 phase participating in electrochemical reactions is subsequently diminished with partial collapse of electrical contact with the current collector. This plays a critical role in significantly increasing the polarization resistance; thus, it is strongly believed that structural stabilization is effectively achievable through chemically-reinforced contact formation in PEI-mediated assembly between highly flexible graphene oxide and electrochemically-active MnO2 phase. Interestingly, taking into consideration PDDA-assisted electrostatic assembly for graphene and MnO2 phase10, the value of RSEI was comparable with that of our MnO2-PEIRGO cell, but the MnO2-PEI-RGO cell had a much lower Rct value by a factor of 3.6; this implies that the PEI-mediated electrostatic assembly enables further improvement of charge transfer kinetics in electrochemical reactions. In well-engineered carbon-oxide composite electrodes with hierarchical nanostructures, the ohmic resistance should be extremely low due to the facile interpenetration of liquid electrolyte and the formation of electrical contact with the current collector; thus, the rate capability can be determined by Li diffusion into the solid active materials as well as the degree of charge transfer between conductive carbon and oxide phase. As long as a nanoscale oxide phase is employed as a predominant electrochemicallyactive constituent, the charge transfer reaction in a heterogeneous interface would be a


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dominant factor determining the performance under the operation condition of high current density. This is in line with the fact that the MnO2-PEI-RGO electrode proposed in this study showed excellent rate capability performance, with the role of the PEI layer as an electron transport-favorable interfacial layer. Note that, in this study, the MnO2-PEI-RGO electrode was derived by an extremely simple, easily-scalable, wet assembly method in an aqueous medium. The Rct value of 12.4 Ω is quite comparable to that of 5 Ω of heterogeneously-grown iron-graphene-carbon nanotube composites22 with a firm electrical junction at a heterogeneous interface. It should be noted that the cells with a diameter of 15 mm, which is the largest of CR2032 coin cells, was used in this study for the EIS analysis. Thus, it is highly conceivable that the approach of PEI-mediated assembly between carbons and metal oxides can provide the new means of chemically hybridizing constituent materials for Li-ion battery electrodes, and the electrochemical performance would be further improved through the further optimization of the constituent materials and structural morphologies.

4. CONCLUSIONS We have demonstrated the effective synthesis of PEI-derived MnO2-GO assemblies for high-performance anodes of lithium-ion batteries. It was clearly demonstrated that cationic polyeletrolyte PEI adjusts the surface zeta potential of both MnO2 nanorods and graphene oxides, with the facile formation of intra-stacked, 2-dimensional MnO2-based composite materials, enabling the achievement of highly reversible capacities of 880 mAh/g at a current density of 0.1 A/g. In addition, the combinatorial role of the PEI interfacial layer and conductive reduced graphene oxide allowed the excellent rate performance of 460 mAh/g, even at a current density as high as 5 A/g, and a normalized capacity variation of 0.67 at current densities ranging from 100 to 3000 mA/g. The high reversible capacity and rate



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capability was verified by EIS-based electrochemical analysis with the suggestion of values, 5.9 Ω in RSEI and 12.4 Ω in Rct.

ASSOCIATED CONTENT Supporting Information XRD and TGA results for composite materials, and voltage profiles for electrodes tested in this study. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *

E-mail: [email protected] (S. S. Lee)

*

E-mail: [email protected] (S. Choi)

*

E-mail: [email protected] (S. Jeong)

Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS This research was supported by Global Research Lab. (GRL) program of the National Research Foundation (NRF) funded by Ministry of Science, ICT (Information and Communication Technologies) and Future Planning (NRF-2015K1A1A2029679), and partially supported by grants from the National Research Foundation of Korea funded by Korean government (MSIP) (2012R1A3A2026417).


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REFERENCES 1. Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Challenges in the Development of Advanced Li-Ion Batteries: A Review. Energy Environ. Sci. 2011, 4, 32433262. 2. Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. R. Metal Oxides and Oxysalts as Anode Materials for Li Ion Batteries. Chem. Rev. 2013, 113, 5364-5457. 3. Chae, C.; Kim, K. W.; Kim, S. J.; Lee, D.; Jo, Y.; Yun, Y. J.; Moon, J.; Choi, Y.; Lee, S. S.; Choi, S.; Jeong, S. 3D Intra-Stacked CoO/Carbon Nanocomposites Welded by Ag Nanoparticles for High-Capacity, Reversible Lithium Storage. Nanoscale 2015, 7, 1036810376. 4. Chen, J.; Wang, Y.; He, X.; Xu, S.; Fang, M.; Zhao, X.; Shang, Y. Electrochemical Properties of MnO2 Nanorods as Anode Materials for Lithium Ion Batteries. Electrochim. Acta 2014, 142, 152-156. 5. Feng, L.; Xuan, Z.; Zhao, H.; Bai, Y,; Guo, J.; Su, C.-W.; Chen, X. MnO2 Prepared by Hydrothermal Method and Electrochemical Performance as Anode for Lithium-Ion Battery. Nanoscale Res. Lett. 2014, 9, 290-297. 6. Li, Y.; Zhang, Q.; Zhu, J.; Wei, X.-L.; Shen. P. K. An Extremely Stable MnO2 Anode Incorporated with 3D Porous Graphene-Like Networks for Lithium-Ion Batteries. J. Mater. Chem. A 2014, 2, 3163-3168. 7. Lai, H.; Li, J.; Chen, Z.; Hung, Z. Carbon Nanohorns as a High-Performance Carrier for MnO2 Anode in Lithium-Ion Batteries. ACS Appl. Mater. Interfaces 2012, 4, 2325−2328. 8. Xia, H.; Lai, M.; Lu, L. Nanoflaky MnO2/Carbon Nanotube Nanocomposites as Anode



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

Materials for Lithium-Ion Batteries. J. Mater. Chem. 2010, 20, 6896-6902. 9. Guo, C. X.; Wang, M.; Chen, T.; Lou, X. W.; Li, C. M. A Hierarchically Nanostructured Composite of MnO2/Conjugated Polymer/Graphene for High-Performance Lithium Ion Batteries. Adv. Energy Mater. 2011, 1, 736–741. 10. Li, L.; Raji, A.-R. O.; Tour, J. M. Graphene-Wrapped MnO2–Graphene Nanoribbons as Anode Materials for High-Performance Lithium Ion Batteries. Adv. Mater. 2013, 25, 6298– 6302. 11. Kim, S. J.; Yun, Y. J.; Kim, K. W.; Chae, C.; Jeong, S.; Kang, Y.; Choi, S.-Y.; Lee, S. S.; Choi, S. Superior Lithium Storage Performance Using Sequentially Stacked MnO2/Reduced Graphene Oxide Composite Electrodes. ChemSusChem 2015, 8, 1484-1491. 12. Yu, A.; Park, H. W.; Davies, A.; Higgins, D. C.; Chen, Z.; Xiao, X. Free-Standing LayerBy-Layer Hybrid Thin Film of Graphene-MnO2 Nanotube as Anode for Lithium Ion Batteries. J. Phys. Chem. Lett. 2011, 2, 1855–1860. 13. Rahaman, M. N. Ceramic Processing; CRC Press, Taylor & Francis Group, 2007; pp 150153. 14. Li, D.; Muller, M. B.; Gilje, S.; Kaner, R. B.; Wallace, G. G. Processable Aqueous Dispersions of Graphene Nanosheets. Nat. Nanotechnol. 2008, 3, 101-105. 15. Park, S.; An, J.; Potts, J. R.; Velamakanni, A.; Murali, S.; Ruoff, R. S. HydrazineReduction of Graphite- and Graphene Oxide. Carbon 2011, 49, 3019-3023. 16. Ham, H.; Khai, T. V.; Park, N.-H.; So, D. S.; Lee, J.-W.; Na, H. G.; Kwon, Y. J.; Cho, H. Y.; Kim, H. W. Freeze-Drying-Induced Changes in the Properties of Graphene Oxides. Nanotechnology 2014, 25, 235601-235608.


Page 20 of 28

Page 21 of 28

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


17. Becerril, H. A.; Mao, J.; Liu, Z.; Stoltenberg, R. M.; Bao, Z.; Chen, Y. Evaluation of Solution-Processed Reduced Graphene Oxide Films as Transparent Conductors. ACS nano 2008, 2, 463-470. 18. Teng, C.-C.; Ma, C.-C. M.; Lu, C.-H.; Y, S.-Y.; Lee, S.-H.; Hsiao, M.-C.; Yen, M.-Y.; Chiou, K.-C.; Lee, T.-M. Thermal Conductivity and Structure of Non-Covalent Functionalized Graphene/Epoxy Composites. Carbon 2011, 49, 5107-5116. 19. Li, K.; Zhen, H.; Niu, L.; Fang, X.; Zhang, Y.; Guo, R.; Yu, Y.; Yan, F.; Li, H.; Zheng, Z. Full-Solution Processed Flexible Organic Solar Cells Using Low-Cost Printable Copper Electrodes. Adv. Mater. 2014, 26, 7271-7278. 20. Yang, D.; Fu, P.; Zhang, F.; Wang, N.; Zhang, J.; Li, C. High Efficiency Inverted Polymer Solar Cells with Room-Temperature Titanium Oxide/Polyethylenimine Films as Electron Transport Layers. J. Mater. Chem. A 2014, 2, 17281-17285. 21. Khan, T. M.; Zhou, Y.; Dindar, A.; Shim, J. W.; Fuentes-Hernandz, C.; Kippelen, B. Organic

Photovoltaic

Cells

with

Stable

Top

Metal

Electrodes

Modified

with

Polyethylenimine. ACS Appl. Mater. Interfaces 2014, 6, 6202-6207. 22. Lee, S.-H.; Sridhar, V.; Jung, J.-H.; Karthikeyan, K.; Lee, Y.-S.; Mukherjee, R.; Koratkar, N.; Oh, I.-K. Graphene-Nanotube-Iron Hierarchical Nanostructure as Lithium Ion Battery Anode. ACS nano 2013, 7, 4242-4251.



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Scheme 1. Schematic diagram showing the sequential procedure to synthesize MnO2-PEIRGO composite material.


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Figure 1. (a) SEM images and (b) XRD result for synthesized MnO2 nanorods; (c) pHdependent variation of zeta potential and (d) isoelectric points for pristine graphene oxide, pristine MnO2, PEI-adsorbed graphene oxide, and PEI-adsorbed MnO2.



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Figure 2. SEM images of (a) MnO2-PEI-RGO and (b) MnO2-RGO composite electrode material; (c) XPS C1s spectra for pristine graphene oxide, thermally(200oC)-reduced graphene oxide, MnO2-PEI-RGO composite material.


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-1

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Figure 3. (a) Cycling performance and (b) voltage profiles for electrodes employing MnO2PEI-RGO, MnO2-RGO, MnO2 nanorod composite materials.



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in-situ synthesis Ref.4; MnO2 nanorod

100

Ref.8; CNT-MnO2 nanoflake Ref.9; MnO2/PEDOT/G

ex-situ synthesis

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MnO2/PEI/RGO(this study) Ref.10; GNR-MnO2/PDDA/G Ref.11; G-MnO2/GO

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Current density (mA/g) Figure 4. (a) Rate performance for electrodes employing MnO2 nanorod, MnO2-RGO, MnO2-PEI-RGO composite materials, (b) rate performance comparison with previouslyreported MnO2-based anodes for lithium-ion batteries, (c) cycling performance at a current density as high as 3 A/g for electrode employing MnO2-PEI-RGO composite material. In figure 4b, GNR, G, CNT, and PDDA represent graphene nanoribbon, graphene, carbon nanotube, and poly(diallyldimethylammonium), respectively.


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Figure 5. EIS response of the MnO2-PEI-RGO cell; (a) Nyquist plot of the MnO2-PEI-RGO cell and (b) comparison in polarization resistance with previously-reported MnO2-based anodes for lithium-ion batteries. In Figure 5b, orange and blue columns represent the polarization resistance for charge transfer reaction (Rct) and that for Li ion migration through SEI layers (RSEI), respectively.



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Table of Contents


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Polyethylenimine-Mediated Electrostatic Assembly of MnO2 Nanorods

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