Graphene Foam

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Electrostatically assembled magnetite nanoparticles/ graphene foam as a binder-free anode for lithium ion battery Ning Zhang, Xiaohui Yan, Yuan Huang, Jia Li, Jianmin Ma, and Dickon H.L. Ng Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b01519 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 3, 2017

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Electrostatically Assembled Magnetite Nanoparticles/Graphene Foam as a Binder-Free Anode for Lithium Ion Battery Ning Zhang, † Xiaohui Yan, † Yuan Huang, † Jia Li, † Jianmin Ma, *, ‡ and Dickon H.L. Ng *, † †

Department of Physics, The Chinese University of Hong Kong, Hong Kong, China;

‡

School of Physics and Electronics, Hunan University, Changsha, China

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Keywords: Fe3O4; Graphene Foam; Electrostatic assembly; Electrochemical properties; Lithium ion battery Abstract 10

Lithium ion batteries (LIBs) are promising candidates for energy storage, with the development of novel anode materials. In this work, we report the fabrication of Fe3O4 nanoparticles/graphene foam via electrostatic assembly and directly utilize it as a binder-free anode for LIBs. Owing to the integrated effect of the well-dispersed Fe3O4 nanoparticles and the conductive graphene foam network, such composite exhibited remarkable electrochemical performances. It delivered a large reversible

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specific capacity reaching to ~1198 mAh g-1 at a current density of 100 mA g-1, a good rate capacity, and an excellent cyclic stability over 400 cycles. This work demonstrated a facile methodology to design and construct high-performance anode materials for LIBs. Introduction There is an urgent need to develop sustainable energy storage systems with high performance and

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low cost. 1-8 Lithium ion batteries (LIBs) have been attracted attention because of their long cycle life, ACS Paragon Plus Environment

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high energy density, and good safety.

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Intensive efforts have been made to construct novel LIBs'

anodes using the transition metal oxides (TMOx, M= Fe, Co, Mo, Ni, etc). Attributed to conversion reactions during charge/discharge processes, the TMOx anodes have ultra-high theoretical capacities (>600 mAh g-1) when compared to the commercial graphite-type anode material (372 mAh g-1). 6-15 In 5

particular, the magnetite (Fe3O4)-based materials are found to have a superior theoretical capacity (926 mAh g-1), excellent electronic conductivity (2×104 S m-1) and environmental friendliness.

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However, Fe3O4-based materials are often subjected to large volume variation caused by the lithium insertion (lithiation) and extraction (delithiation) processes,

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leading to the cracking or

pulverization of the electrodes and hence poor cycling stability. 20-24 10

In order to overcome this challenge, a feasible strategy is to hybridize nano sized Fe3O4 of varied morphologies, e.g., nanoparticles,

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nanowires,

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materials such as porous carbon, 21, carbon aerogel,

nanorods,

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and nanospheres,

carbon nanotube,

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and graphene.

with buffer 23-28

Scaling

down Fe3O4 particles to nanoscale can minimize their volume variation and optimize the lithiation and delithiation processes; while the carbon buffer materials can act as the matrix to release the mechanical 15

stress, prevent aggregation of Fe3O4 during the cycling process, and enhance the conductivity of the anode. Among various carbon buffer materials, three-dimensional (3-D) graphene foam (GF) has the advantages of light weight, interconnected structure, unique mechanical properties, high electrical conductivity, and large specific surface area.

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Depositing Fe3O4 nanostructures onto the GF would

generate a binder-free LIB anode of excellent electrochemical performance. Several approaches were 20

reported for the depositing process. Hu et al. utilized a supercritical carbon dioxide (scCO2) strategy to anchor layers of Fe3O4 nanoparticles onto the GF surface. The as-obtained Fe3O4@GF-scCO2 based anode demonstrated a high discharge capacity and good cycling stability,

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Luo et al. presented an

atomic layer deposited approach to graft Fe3O4 nanostructures on the GF. The composite delivered a fast discharge/charge capacity with good rate performance. 31 Nevertheless, these fabrication processes

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were complicated and required the specific experimental conditions, which greatly impeded their commercial applications. In this work, we reported a facile electrostatic assembly approach for the fabrication of Fe3O4 nanoparticles (Fe3O4 NPs) on GF (Fe3O4 NPs/GF) for anode material, and the obtained Fe3O4 NPs/GF 5

composite manifests excellent electrochemical properties. The excellent electrochemical performance of this composite can be attributed to the advantages of its hierarchical structure. On the one hand, the well-dispersed Fe3O4 NPs with nanostructure delivers a higher capacity by conversion reactions. On the other hand, the GF matrix provides a stable and conductive framework to enhance the cyclic property and capability for the free-standing anode composite. Additionally, the Fe3O4 NPs can be

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easily attached to the surface of GF to form binder-free anode by a novel one-step approach, without any complicated procedures. More importantly, this facile synthesize approach could lead to a large scale production, and it would be further applied to other high-performance energy storage materials. Experimental Section Preparation of Fe3O4 NPs Suspension. FeCl2· 6H2O, FeCl3· 3H2O, and NaOH were purchased

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from Sigma-Aldrich (USA). Fe3O4 NPs were prepared by a co-precipitation method. In a typical synthesis, a mixed solution of ferrous and ferric ions in the molar ratio of 1:2 was prepared by dissolving FeCl2·6H2O and FeCl3· 3H2O at 65°C in an ultra high purity N2 environment. A 0.5M NaOH solution was then added to the FeCl2-FeCl3 mixture until a black suspension of Fe3O4 NPs was observed. To obtain Fe3O4 NPs with the positively-charged surface, the 30 mL of the as-

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prepared suspension was allowed to react with the 5 mL of 1 mg mL-1 CTAB (cetyltrimethyl ammonium bromide) surfactant solution. The above mixture was then transferred to a Teflon-lined stainless-steel autoclave and maintained at 120 °C in an oven for 2 hours. Fabrication of Fe3O4 NPs/GF Composite. The GF used in this work was purchased from the 6 Carbon technology Co., Ltd (China). The pure GF obtained via chemical etching away of Ni by using ACS Paragon Plus Environment

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a 2 M FeCl3 aqueous solution at 60oC for 1 hour. After washed with deionized (DI) water and ethanol, the etched GF was cut into 10×10×1.5 mm3 pieces before immersed into the as-prepared Fe3O4 NPs solution. The positively-charged Fe3O4 NPs would be easily attracted to the GF with the negativelycharged surface via electrostatic interaction. Before completion, the as-synthesized Fe3O4 NPs/GF 5

hierarchical composite was dried at 60oC for 6 hours in vacuum. Structural Characterization. The X-ray diffraction (XRD) analysis was performed with RU300 (SmartLab, Rigaku) using a Cu Kα radiation (λ = 0.1540598 nm). Raman scattering analysis was conducted by a RM-1000 Micro-Raman spectrometer (RM-1000, Renishaw Co., Ltd.) with a 10 mW helium neon laser at 514 nm. Zeta potential measurements were taken by a Nano ZSP (Malvern

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Zetasizer). X-ray photoelectron spectroscopy (XPS) was collected with Al Kα radiation on a PHI Model 5802 (calibrated with C1s at 284.8 eV) to perform an elemental analysis. The morphologies and elemental analyses were characterized by a Quanta 200, field-emission scanning electron microscope (FE-SEM, FEI). Detailed nanostructures were examined by a Tecnai F20 transmission electron microscopy (TEM) operating at 200 kV.

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Electrochemical Characterization. The evaluation of the electrochemical properties of the Fe3O4 NPs/GF composite was carried out by the standard CR2032 coin-type cells. The assembly was done in an Argon-filled glove box. The as-prepared Fe3O4 NPs/GF was directly performed as a working electrode without using any binders or conducting agents. Lithium foil (1.5 mm thickness) was used as the counter electrode. The liquid electrolyte consisted of a solution of 1.0M LiPF6 in

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the mixture of ethylene carbonate (EC) and diethyl carbonate (DMC) 1 : 1 by volume (Novolyte Co.). The cyclic voltammetry (CV) measurement was conducted with the electrochemical working station CHI760 (Shanghai CH Instrument Co., Ltd.) at the scanning rate of 0.1 mV s-1 between a voltage range of 0.01 and 3.00 V. The galvanostatic charging/discharging (GCD) and cycling


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stability performances were evaluated by a multichannel battery test system CT2001A (Wuhan Kingnuo Electronic Co., Ltd.) at different current densities. Results and Discussion The overall fabrication processes for Fe3O4 NPs/GF composite is shown in Figure. 1a. The GF 5

with 3-D network structure was chosen as the matrix. The matrix not only can provide sufficient sites to attract modified Fe3O4 NPs, it also possesses excellent conductivity, extremely light and flexible characteristics.

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As shown in Figure. 1a, after the Ni skeleton was etched away, the

remaining pure GF matrix was immersed into the positively-charged Fe3O4 NPs suspension. Figure. 1b illustrates the results of zeta potential measurement of the Fe3O4 NPs and the GF matrix. The 10

positively-charged of Fe3O4 NPs shows a peak at the 42.3mV (red curve). In contrast, the asprepared GF matrix exhibits a peak in the negative zeta potential regime (black curve). Our result is consistent with some previous works.

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As a consequence, the self-assembly process between

the Fe3O4 NPs and GF matrix can be easily triggered via electrostatic interactions under neutral conditions. The as-synthesized Fe3O4 NPs/GF can be directly utilized as a binder-free anode for 15

the LIBs.

Figure. 1. (a) Schematic illustration of the fabrication processes of Fe3O4 NPs/GF composite. (b) Zeta potential for pure GF and Fe3O4 NPs. The Raman spectra of pure GF and the as-prepared Fe3O4 NPs/GF are presented in Figure. 2a. The 20

black curve in the bottom shows three typical peaks of the pure GF, illustrating the characteristic G ACS Paragon Plus Environment

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(~1580 cm-1), G* (~2550 cm-1) and 2D (~2720 cm-1) bands. The relatively weak Raman intensity of the 2D band comparing with G band suggests that the obtained GF structure consists of multilayer graphene sheets, which makes it suitable to serve as a matrix for loading electrochemical active materials. 5

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After the electrostatic assembly of Fe3O4 NPs on the surface of GF, two additional

peaks appear on the red curve of Fe3O4 NPs/GF. The Raman A1g mode centered at 667 cm-1 corresponds to the mode of the Fe3O4 NPs. The D band peak at 1350 cm-1 of Fe3O4 NPs/GF corresponds to the disorder of the graphene. As a number of disorder increases, the Raman D band become more intensive.37-39 Such D band of Fe3O4 NPs/GF shows more obviously when compared with pure GF, indicating the new active sites on the surface of GF were induced by the assembly of

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Fe3O4 NPs. The relatively small D band peak of Fe3O4 NPs/GF composite illustrates a less disorder in GF structure, further confirming that the Fe3O4 NPs are far less aggressive than other deposition methods of Fe3O4 nanostructures such as ozone plasma treatments and ALD deposition.

30-33, 37-39

The

less aggressive of Fe3O4 NPs on the surface of GF would prevent the volume change of Fe3O4, making the as-synthesized Fe3O4 NPs/GF composite appropriate for being an anode material for LIBs. 15

The crystalline structure of the Fe3O4 NPs/GF composite was characterized by X-ray diffraction (XRD), and the result is shown in Figure. 2b. Two typical peaks centered at 26.5o and 54.6o can be assigned to the (002) and (004) of graphitic carbon (JCPDS card of 75-1621). The strong peak of (002) indicates a high crystalline structure of the GF matrix, which is favorable for the electron transfer and ion diffusion. 30-33 The other peaks at (220), (311), (400), (511) and (440) can be observed in the XRD

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pattern, reflecting the magnetite structure of the decorated Fe3O4 NPs (JCPDS card 19-0629).

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No

additional diffraction peaks appear in the XRD patterns, suggesting the purity of our synthesis approach. Thermogravimetric analysis (TGA) was performed to evaluate the composition of Fe3O4 NPs/GF composite with the heating rate of 5 oC min-1 from room temperature to 800 oC in the air, and the result is shown in Figure. 2c. It can be found that the carbon component in Fe3O4 NPs/GF


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composite was oxidized to CO2. The Fe3O4 component was oxidized to Fe2O3, shown claim was supported by the XRD pattern in the insert of Figure. 2c. Lastly, from the TGA result, it was found that the weight ratio of Fe3O4 was 76% of the Fe3O4 NPs/GF composite.

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Figure. 2. (a) Raman spectra of pure GF and Fe3O4 NPs/GF composite. (b) XRD pattern of Fe3O4 NPs/GF composite. (c) Thermogravimetric analysis (TGA) curve of Fe3O4 NPs/GF composite. Insert of (c) presents the XRD pattern of Fe2O3, which was derived from Fe3O4 in the Fe3O4 NPs/GF composite after TGA test. X-ray photoelectron spectroscopy (XPS) measurement of the as-synthesized Fe3O4 NPs/GF

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composite was performed in the range from 200 to 800 eV, and the results are illustrated in Figure. 3. The presence of iron (Fe-2p), oxygen (O-1s), and carbon (C-1s) peaks in the survey spectra of Figure. 3a, arising from the Fe3O4 NPs and GF matrix. In the high-resolution Fe-2p spectrum in Figure. 3b, the presence of Fe2+ and Fe3+ are assigned to the Fe 2p1/2 and 2p3/2 states of Fe3O4 NPs, respectively. Both of the Fe 2p1/2 and 2p3/2 states can be resolved into two peaks by peak fitting. The first typical peak

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located at the binding energy of 724.1 eV in Fe 2p1/2 and the second typical peak located at a binding energy of 710.7 eV in Fe 2p3/2 component correspond to the Fe2+. The other two peaks centered at 725.7 and 713.3 eV in Fe 2p1/2 and Fe 2p3/2 attribute to Fe3+ configuration.

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located at 730.4 and 718.6 eV correspond to the Fe shakeup satellite peaks.

In addition, two peaks

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Figure. 3c illustrates

the XPS spectrum of the O-1s state. It can be found that the strong peak located at a binding energy of


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530.3 eV attributes to the anionic oxygen in Fe3O4, and other two peaks at 531.7 and 532.3 eV are attributing to the chemisorbed oxygen from the surface -OH groups. 30-32 The XPS spectrum for the C1s state is shown in Figure. 3d, and it can be resolved into three peaks. The strong peak at 284.8 eV corresponds to the C=C and C-C bond of sp2-hybridized carbon for the graphene sheets, and the peak 5

at 286.4 and 288.6 eV are assigned to the C-O bond and C=O band for the Fe3O4 NPs/GF, respectively. 30-32

Figure. 3. (a) XPS spectrum of the Fe3O4 NPs/GF composite. High-resolution XPS spectra of (b) Fe2p state, (c) O-1s state, and (d) C-1s state.

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In order to investigate the morphology of the as-synthesized Fe3O4 NPs/GF composite, we performed the SEM, TEM, and high-resolution TEM (HR-TEM) characterizations. Figure. 4a and 4b show the low and high magnification SEM images of pure GF after the removal of the nickel backbone. It is clear that continuous 3-D interconnected foam-like GF (Figure. 4a) with an extremely freestanding smooth graphene sheets are well-maintained (Figure. 4b). Such network structure would offer ACS Paragon Plus Environment

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the durability and affordability sites to be attracted by active materials. After electrostatic assembly of the Fe3O4 NPs on the surface of GF, the morphology of the obtained Fe3O4 NPs/GF composite is shown in Figure. 4c and 4d. Rather than the smooth surface of original pure GF, the obtained Fe3O4 NPs/GF composite has a roughness surface and there has a uniform distribution of Fe3O4 NPs. And the 5

insert of Figure. 4c shows the photograph of the free-standing Fe3O4 NPs/GF composite.

Figure. 4. (a), (b) SEM images of the pure GF. (c), (d) SEM images of Fe3O4 NPs/GF composite, at different magnification. Insert of (c) shows the photograph of Fe3O4 NPs/GF composite. The detailed morphology and crystalline structure of Fe3O4 NPs/GF composite are shown in 10

Figure. 5a-b. From the TEM images, we can identify that all of the NPs are uniformly distributed and well adhered on the surface of graphene sheets. The dimensions of Fe3O4 NPs are around 20-30 nm, which is consisted with particle diameter distribution shown in the insert of Figure. 5a. The HRTEM image inserted in Figure. 5b of Fe3O4 NPs/GF composite clearly reveals that the Fe3O4 NPs are


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highly crystallized and perfectly be attracted on the GF. The lattice fringes with the d-spacing of 0.25 nm, corresponds to (311) plane of Fe3O4.

Figure. 5. (a), (b) TEM images of the Fe3O4 NPs/GF composite. Insert of (a) presents the Fe3O4 5

NPs suspension size distribution, and insert of (b) shows the HR-TEM image of an individual Fe3O4 nanoparticle. In order to evaluate the electrochemical performances of the as-synthesized Fe3O4 NPs/GF composite, the coin-type cells were assembled by using the Fe3O4 NPs/GF directly as a binder-free working electrode, and a lithium foil as the counter electrode. Cyclic voltammetry (CV) measurement

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was first employed to investigate the charge storage behavior of the Fe3O4 NPs/GF anode. Figure. 6a shows the first three CV cycles of the Fe3O4 NPs/GF composite anode, which was tested at the scan rate of 0.1 mV s-1. In the first cycle, two typical well-defined reduction peaks can be found at 0.78 V and 0.51 V (vs Li+ /Li), corresponding to the two steps lithium side reaction of the Fe3O4 anode, respectively. In step 1, it is the structure transition induced by lithium ion intercalation into crystalline

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of Fe3O4 [Fe3O4 + xLi+ + xe- → LixFe3O4]. In step 2, it is the further reduction of LixFe3O4 to Fe (0) by conversion reaction [LixFe3O4 + (8 - x) Li+ + (8 - x) e- → 4Li2O + 3 Fe]. These processes in good agreement with the previous studies.

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The subsequent cycles onward exhibit in similar shape, that

are quite different from the first cycle. Such difference reflects the irreversible capacity loss of LIBs in


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the first cycle, mainly due to the lithium intercalation at a low voltage below 1.0 V causes the inevitable formation of a stable solid electrolyte interphase (SEI) layer, or the decomposition of the electrolyte. During the subsequent reproducible cycles, the electrochemical reduction/oxidation (Fe3O4 ↔ Fe0) reactions of lithium insertion/extraction are highly reversible. It can be shown in CV curves 5

(2nd and 3rd cycle), the cathodic lithium insertions (lithiation) mainly occur at 1.48 V and 0.75 V, and the anodic lithium extractions (delithiation) occur at 1.68V and 1.85 V, corresponding to the Fe0 to Fe2+ and Fe2+ to Fe3+, respectively. It should also be noted that, apart from the pairs of reaction peaks for Fe3O4, the CV curves also have an additional pair of redox peaks, which located at 0.13 and 0.25 V, corresponding to the lithiation and delithiation of GF, respectively. 30-32

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The galvanostatic charge/discharge voltage profiles for the 1st, 2nd, 3rd, 100th and 400th cycle were tested at a current density of 100 mA g-1 over a potential window of 0.01 to 3.0 V at room temperature, and the results are shown in Figure. 6b. The Fe3O4 NPs/GF anode delivers a high initial discharge capacity of 1350 mAh g-1 and the corresponding charge capacity of the first cycle is 938 mAh g-1. The low Coulombic efficiency of 69% and irreversible capacity loss for the first

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discharge/charge cycle should be attributed to the formation of SEI layer that mentioned above. Such behavior often occurred in transitional metal oxide-based anodes. 6-14 Moreover, in agreement with the CV curve in Figure. 6a, the discharge curve of the first cycle in Figure. 6b has two typical voltage plateaus at ~0.78 V and ~0.51 V, respectively. However, the discharge voltage plateaus in the first cycle are different from subsequent cycles, which further indicate the irreversible reactions take place

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in the first cycle. In particular, there is no obvious shape change in the discharge curves from 100th cycle to 400th cycle, indicating the long-term cycling stability of the Fe3O4 NPs/GF anode. To evaluate the rate performance of the Fe3O4 NPs/GF composite anode, we had conducted the galvanostatic charge/discharge measurement of the as-synthesized anode at different current densities from 100 mA g-1 to 10 A g-1. Additionally, in order to elaborate the advantages of the electrostatic


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assembly of Fe3O4 NPs on GF, we also took a measurement for the controlled experiment of pure GF. As illustrated in Figure. 6c, the Fe3O4 NPs/GF composite anode exhibits a better rate performance than pure GF electrode. Moreover, the high capacity of 1020 mAh g-1 can be obtained after charge/discharge cycles when the current density returns back from 10 A g-1 to 100 mA g-1. Such 5

phenomenon indicates the excellent reversibility and good rate stability of the Fe3O4 NPs/GF composite anode. Same as the rate performance, the as-synthesized Fe3O4 NPs/GF composite and pure GF had also been subjected to further test the durability with the cycling stability measurements at the current density of 100 mA g-1, and the result can be seen in Figure. 6d. The specific capacity of the Fe3O4

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NPs/GF composite anode experienced a slight variation and remained to a high capacity of 1198 mAh g-1 after 400 cycles with a high Coulombic efficiency of 99.9%. It is worth noting that the discharge capacity of Fe3O4 NPs/GF composite shows ups and downs at first 200 cycles, and continues up to 400 cycles. The variation of the discharge capacity can result from the re-distribution of the Fe3O4 NPs on the surface of GF during charge/discharge processes. The re-distribution of Fe3O4 NPs would optimize

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the contact of GF surface and further result in the slight variation of the discharge capacity of the composite.

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The discharge capacity of Fe3O4 NPs/GF composite is much higher than the pure GF

anode of approximately ~300 mAh g-1. The enhanced electrochemical performance of the Fe3O4 NPs/GF anode is mainly due to the presence of the nano sized Fe3O4 particles and the special structure design of anode. These claims can be illustrated by the following evidence. The positively-charged 20

Fe3O4 NPs are in the form of interconnected nanocrystallites that are homogeneously distributed on the surface of GF with a close physical contact. Moreover, the network structured GF prevents direct contact of the Fe3O4 NPs and thereby minimizes the aggregation of the NPs during charge/discharge processes. Such hierarchical structure could effectively enhance the electron/ion transport, and avoid the volume variation effect of Fe3O4.

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In addition, the capacity of Fe3O4


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NPs/GF composite anode (1198 mAh g-1) is higher than that of theoretical capacity value for Fe3O4 (926 mAh g-1) and pure GF (300 mAh g-1) after 400 cycles. Such increment in capacity should result from the more active sites at metal-Li2O phase boundaries appear with the increasing of cycling process, which can localize and store the additional interfacial Li-ion and electrons; and 5

the induced SEI layer provides extra Li-ion storage, leading to a significant higher lithium capacity.10-21

Figure. 6. (a) CV curves of the Fe3O4 NPs/GF composite anode at a potential range of 0.01 to 3.0 V (vs Li+/Li) at the scan rate of 0.1 mV s-1. (b) Charge/discharge curves of the Fe3O4 NPs/GF composite 10

anode at the current density of 100 mA g-1. (c) Rate performances of the Fe3O4 NPs/GF composite anode and pure GF anode at various current densities. (d) Cycling stability of the Fe3O4 NPs/GF and pure GF anodes at 100 mA g-1, and the right y-axial shows the Coulombic efficiency of Fe3O4 NPs/GF composite. ACS Paragon Plus Environment

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Conclusion We have successfully developed a facile synthetic route to fabricate positively-charged Fe3O4 NPs on the surface of 3-D GF via electrostatically assembly. The as-synthesized Fe3O4 NPs/GF composite can be directly employed as the binder-free anode for LIBs. With this design, the intact 5

contact between Fe3O4 NPs and GF prevents the aggregation of Fe3O4 NPs and ensures the efficient lithium ion and electron transportation. Moreover, the nanosized Fe3O4 NPs and GF matrix can effectively alleviate the volume variations during cycling. The Fe3O4 NPs/GF anode exhibited remarkable electrochemical properties, it delivered a high capacity of 1198 mAh g-1 after 400 cycles at the current density of 100 mA g-1, accompanied with good rate capability and high

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Coulombic efficiency. This facile and straightforward synthesis route presented herein could be further extended to a wide range of functional materials in energy storage system applications. Corresponding authors.

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*

E-mail: [email protected] (Prof. Dickon H. L. Ng)

*

E-mail: [email protected] (Prof. Jianmin Ma)

Acknowledgements This work was supported by the Direct Grand (Project Code: 3132731) from the Faculty of Science, The Chinese University of Hong Kong. References Section [1] Cao, X.; Shi, Y.; Shi, W.; Lu, G.; Huang, X.; Yan, Q.; Zhang, Q.; Zhang, H. Preparation of Novel

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3D Graphene Networks for Supercapacitor Applications. Small 2011, 7, 3163-3168. [2] 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, 3243.


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Graphene Foam

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