Graphene nanosheets (GNS) with attached MnOx nanoparticles are studied in regard to their structure and morphology. The relationship between the lithium storage performances and GNS contents as well as manganese valency was investigated. Experimental results showed that the specimen with 44 wt% GNS and high content of MnO delivered high reversible capacity (over twice of that in graphitic carbon anode), good cycling stability (0.8% fading per cycle), and high rate capability (67% at the 800 mA/g), which are dramatically better than pure Mn3O4. The improvement is attributed to the presence of GNS which provides continuous networks for fast electronic conduction and mechanical flexibility for accommodating the large volume change. The MnOx/GNS hybrid material has the added advantages over pure GNS, benefiting from its lithium storage potential of around 0.5 V which not only ensures high rate capability but also reduces the risk of metallic lithium formation with its safety hazard.
Nanoparticles of transition metal oxides (MOx, where M is Co, Fe, Ni, or Cu) have recently received much attention as alternative anode materials for Li-ion batteries. MOx can deliver over twice the gravimetric capacity and six times volumetric capacity in comparison with graphitic carbon anode [
Manganese oxide is an attractive anode candidate for its low cost and environmental friendliness. The fact that one mole of MnO reacts with 2 moles of Li corresponds to a maximum reversible lithium storage capacity of 756 mAh/g. Mn3O4 increases the theoretical value to 936 mAh/g. However, a capacity less than 300 mAh/g was constantly reported in the past using pure Mn3O4 micropowders [
Complied information MnOx and GNS/MnOx composites in published literatures including their prepared conditions, manganese component phases, GNS contents, and electrochemical performances.
Sample | Morphology | Basic synthesis info | 1st |
Rate capability |
Reference |
---|---|---|---|---|---|
MnO2 | MnAc + (NH4)2S2O7, 140°C 2 h to form amorphous MnO2, 500°C 10 h | 1146/627, 55% | [ | ||
Mn2O3 | rodlike 100–150 nm in diameter, 1-2 |
ibid amorphous MnO2, 350°C 10 h | 1156/547, 47% | No info | [ |
Mn3O4 | ibid amorphous MnO2, 280°C 3 h in H2/Ar, 700°C 2 h in Ar | 1265/528, 42% | [ | ||
MnO | ibid amorphous MnO2, 400°C 10 h in H2/Ar, 700°C 2 h in Ar | 1728/488, 28% | [ | ||
| |||||
MnO | agglomerate |
MnO-L (commercial) |
1270/690, 53% |
300 at 800 | [ |
MnO | Sheets, |
MnO-S (ball milled) |
1240/750, 61% |
300 at 800 | [ |
Mn3O4 | spongelike |
Mn(CH3COO)2 + NH4OH, 300°C 5 h | 1327/869, 65% | 520 at 500 | [ |
Mn3O4/GNS | 20–30 nm | GO + MnCl2·2H2O + KMnO4 + NH4OH, |
1789/1100, 62% | 400 at 1000 | [ |
Mn3O4/GNS | 30–50 nm | GO + Mn(CH3COO)2 + NaOH, hydrothermal 180°C for 12 h, GNS 15 wt% | 1100/750, 68% | 610 at 200 | [ |
MnO/GNS | 30–50 nm | ibid, then 400°C 2 h in H2/N2, GNS 12 wt% | 1320/820, 62% | 200 at 600 | [ |
Mn3O4/GNS | 10–20 nm | GO + Mn(CH3COO)2, hydrolysis, hydrothermal at 180°C for 10 h, 10 wt% RGO | 1320/850, 64% | 600 at 800 | [ |
MnO2/GNS | 70–80 nm |
GO + MnO2 nanotube hydrothermal from KMnO4, layer by layer assembly | 1232/686, 55.7% | 300 at 800 | [ |
GNS powders were synthesized using the traditional Hummers approach. Specifically, 1 gram graphite and 0.5 gram sodium nitrate were firstly mixed in 70 mL concentrated sulfuric acid. Then 3 gram potassium permanganate was gradually added to the mixture and stirred for 5 hrs at room temperature. Afterwards, hydrogen peroxide was added to this mixture until the mixture turned into bright yellow color. This mixture was then rinsed thoroughly until the pH value was close to 7. After filtered and dried, the fine powders were heat treated at 250°C for 6 hrs in air. All the precursor chemicals were purchased from Aldrich.
The GNS/MnOx hybrid materials were chemically synthesized followed by appropriate thermal treatment. Initially, 22 mg as-prepared GNS were ultrasonicated in water for 3 hrs. Then 78 mg manganese acetate (MnAc) dissolved in water was gradually added to the GNS suspension solution and continuously stirred for 2 hrs. Then ammonium hydroxide and hydrazine were sequentially added and mixture was stirred for 3 hrs at 100°C. The product was filtered and dried at 150°C for 3 hrs. The as-prepared powder is named as GNS/MnOx-1. GNS/MnOx-1 powders were then subjected to thermal treatment at 400°C for 12 hrs in air or in 5% H2/Ar atmosphere, which are referred to as GNS/MnOx-2 and GNS/MnOx-3, respectively. The thermal treatment in air resulted in the changes of GNS content. Thermal treatment under the reduction environment altered the manganese valency state in the hybrid materials.
Bruker D8 X-ray diffractometer (XRD) was used to identify the crystal structure of the manganese component in the hybrids. JEOL scanning electron microscope (SEM) was used to visualize the morphologies. The weight loss of the powders after the thermal treatment and energy dispersive spectroscopy (EDX) were used to determine the carbon content.
The active anode powders were mixed with polyvinylidene fluoride (PVDF) binder in the weight ratios of 90 : 10 in N-methylpyrrolidone (NMP) to form a viscous slurry. The slurry was uniformly coated on a Cu foil. The electrode sheets were dried at 120°C for 12 hrs under vacuum. Swagelok cells were assembled in a glove box which controlled the moisture and oxygen levels less than 0.5 ppm. The electrolyte was 1 M LiPF6 dissolved in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DEC) at 1 : 1 volumetric ratio. Li metal foil was used as the counter electrode. The cells were galvanostatically discharged and charged at the preset current densities within the cutoff voltage window of 0.01–3.0 V on a battery testing station (Land CT). Electrochemical impedance spectra (EIS) of the GNS/MnOx electrodes were obtained by applying a sine-wave signal with an amplitude of 10 mV in the frequency range of 1 MHz to 0.1 Hz on Camry electrochemical analysis system at the preset capacity interval after relaxation for 2 hrs.
Figure
XRD profiles of the GNS/MnOx nanocomposites in comparison with precursors GNS and GNS+MnAc.
The carbon and manganese content was determined using EDX microelemental composition analysis attached to the SEM instrument. To ensure the results consistency and relative accuracy, five different regions with different area sizes were chosen on each specimen compositional analyses. The average carbon contents in the three specimens were 44 wt%, 20 wt%, and 42 wt%, respectively.
Although the GNS/MnOx-1 was prepared from GNS and MnAc, XRD analyses confirmed no existence of MnAc and all the Mn component had been transformed into Mn3O4. Accordingly, from the weight of the precursors, the GNS composition in the GNS/MnOx-1 was calculated to be 44 wt%, consistent with the EDX analyses. When the GNS/MnOx-1 was thermally treated in air at 400°C, the specimen lost 30% of the total weight. Control experiment at the same condition on pure GNS verified the occurrence of the GNS combustion. Further, since Mn3O4 neither decomposed nor evaporated at 400°C, all the weight loss originated from the GNS combustion into CO2. Accordingly, the GNS content in the GNS/MnOx-2 was determined to be 20 wt%. In contrast, heat treatment under the H2/Ar reduction/inert atmosphere mainly resulted in reduction of Mn3O4 into MnO but insignificant combustion of GNS. Based on the 4.5 wt% weight loss of GNS/MnOx-1 under the experimental condition (400°C for 12 hrs), it was calculated that the MnO to Mn3O4 ratio in the GNS/MnOx-3 was 61 : 39, corroborated well with XRD semiquantitative analysis. Table
Summary of GNS and three GNS-MnOx specimens including their prepared conditions, manganese component phases, GNS contents, and electrochemical performances.
Sample | GNS | GNS/MnOx-1 | GNS/MnOx-2 | GNS/MnOx-3 |
---|---|---|---|---|
Synthesis info | As-prepared | As-prepared | 400°C 12 h |
400°C 12 h |
GNS content | 100% | 44% | 20% | 42% |
MnOx phase | N/A | Mn3O4 | Mn3O4 | Mn3O4 + MnO (40 : 60) |
1st capacity | 1248/843 | 1430/850 | 1433/578 | 1279/838 |
1st efficiency | 68% | 59% | 40% | 65% |
2nd capacity | 810 | 806 | 494 | 772 |
10th capacity | 790 | 619 | 233 | 719 |
Capacity fading | 0.3% | 2.6% | 5.9% | 0.8% |
SEM imaging was used to visualize the particle morphological evolution from GNS to GNS/MnOx hybrids. GNS exhibited thin wrinkled flakes suggesting high surface area (see Figures
SEM images of GNS and GNS/MnOx specimens at the low magnification (left column) and high magnification (right column). (a)-(b) GNS; (c)-(d) GNS/MnOx-1; (e)-(f) GNS/MnOx-2; (g)-(h) MnAc400 (MnAc thermally decomposed at 400°C).
After the GNS/MnOx-1 was subjected to 400°C sintering in air, the flaky structure transformed into the loose fluffy agglomerates in some areas, revealed from the circled region in the image of GNS/MnOx-2 (Figure
Figures
The 1st, 2nd, 5th, and 10th discharge-charge profiles of (a) GNS; (b) GNS/MnOx-1; (c) GNS/MnOx-2; (d) GNS/MnOx-3. (e) Cycling performance of GNS/MnOx nanocomposites in comparison with pure GNS and Mn3O4 (from [
When Mn3O4 nanoparticles are anchored on the GNS, significant changes are observed in the profiling shapes (see Figures
The remaining high-voltage slope was the contribution from the irreversible SEI reaction on the surface of GNS and manganese oxide particles as well as the phase transformation from LiMn3O4 to MnO as follows:
Since the reactions (
The capacity around the 0.4 V plateau and below in the GNS/MnOx hybrid anodes reflected the displacement reaction between Li and manganese oxide [
According to previous ex-situ XRD or Raman analyses [
Figures
The discharge/charge profiles of the four samples obtained at different discharge rate from 50 mA/g to 800 mA/g are presented in Figures
Discharge-charge profiles of (a) GNS; (b) GNS/MnOx-1; (c) GNS/MnOx-2; (d) GNS/MnOx-3. The discharge rates increase from 25 mA/g to 800 mA/g. The charge rate is fixed to 50 mA/g. (e) Capacity as a function of the discharge rate.
Capacity loss at increasing current rate roots from the electrode kinetics and overpotential induced loss. The charge transfer impedances can be derived from electrochemical impedance spectroscopy analyses. As can be seen from Figures
EIS Nyquist plots of (a) the cells consisting of different working electrode material at the open circuit voltage; (b) the cells consisting of different working electrode material at the fully discharge condition.
Comparing the EIS spectra of GNS and GNS/MnOx-3, the values of the charge transfer impedances were almost the same, which excluded the difference of electrode kinetics. The difference in rate capability can be interpreted from their different discharge profiling. From the GNS discharge profile, it can be seen that major capacity is delivered at the potential region less than 0.3 V. Since the cutoff potential was preset to 10 mV to avoid metallic lithium formation, any polarization caused by increasing discharge current is equivalent to shift the cutoff voltage upwards. Consequently, the capacity will be ineffectively used due to up-shifting cutoff baseline. For instance, when the current rate increased from 50 mA/g to 800 mA/g, the overvoltage was around 300 mV (see Figure
In this paper, structure, morphology, and lithium storage performances in terms of first Coulombic efficiency, cycling stability, and rate capability are characterized in GNS/MnOx hybrids with different GNS contents and manganese valency. It is experimentally verified that GNS/MnOx with high content of GNS and MnO delivered the better performances. Nanoparticle MnOx anchored on the surface of GNS layers can be provided with continuous electrical paths from GNS to ensure fast electronic conduction. Further, GNS’ mechanical flexibility is capable of mitigating the large volume change caused by the manganese oxide displacement reaction. GNS/MnOx hybrids consisting of GNS 42 wt%, Mn3O4 23 wt%, and MnO 35 wt% have a high reversibility capacity (up to 838 mAh/g) with a high Coulombic efficiency (65%), good cycling stability (0.8% fading per cycle), and high rate capability (67% at the 800 mA/g). Its lithium storage potential centered around 0.5 V versus Li which is beneficial for the high rate capability and can also reduce the risk of metallic lithium formation and safety hazard.