CV Performance of ZnO-Carbon Nanocomposite Based On ZIF-8 as Lithium-Ion Battery Anode

Compositing between metal oxide and carbon is strategy to improve the performance of lithium-ion battery anode. This is due to the high theoretical capacity of metal oxides, carbon acts as a lithium ion accelerator and supports the structural stability of metal oxides. The advantages of the ZnO-Carbon nanocomposite anode based on ZIF-8 are its large surface area and porosity. The method used is non-solvothermal through precipitation at room temperature. After obtaining ZIF-8 powder, then it was carbonized at temperatures 600°C. The characterization used were SEM, EDS and TEM. The samples also have a porous character with a nano-sized pore radius. Cyclic Voltammetry (CV) was carried out using 3-electrode system. The CV test showed 3 reduction peaks. The first peak indicated the conversion reaction, the second peak indicated the alloying reaction, and the third peak indicated a solid electrolyte interface (SEI) reaction. The highest diffusion coefficient is alloying reaction with 3.65 x 10−8 cm2/s.


Introduction
Energy security become main attention of every nation in worldwide.Alternative energy should be implemented to substitute conventional fossil fuel energy.There are several reasons why fossil fuel energy should be substitute, fossil fuel energy belongs to unrenewable energy, continuous consumption of this energy resources causes energy scarcity so fossil fuel energy inevitably will be running out 1 .At the other side fossil fuel energy contributes to the increasing of carbon emission which linearly affect global warming and climate change.Some renewable energy resources have been utilized to solve this concern, such as solar, wind, ocean energy, etc 2 .Unfortunately, these renewable energy supply intermittent electricity.So, energy storage is needed.Energy storage is also needed in electric vehicle as replacement of combustion engine vehicle which require fossil fuel.The next few years, production of electric vehicle increase dramatically.Application of renewable energy and electric vehicle demands high expectation of energy storage performance, such as lithium-ion battery [3][4] .Nanotechnology and nanomaterials facilitate effort to manufacturing high performance lithium-ion battery materials, so lithium-ion battery have high capacity, high stability, and high conductivity 5 .Particularly for anode, metal oxide provides high theoretical capacity, such as ZnO.Resources of ZnO is abundant and ZnO is environmentally friendliness 1 .But this is contradictory with its stability and its conductivity.Metal oxide have low stability because of expansion volume by material degradation during lithiation and delithiation.So, metal oxide must be composited with other materials to overcome this problem, such as carbon.Carbon has high stability affected by insertion mechanism during lithiation and de-lithiation, while metal oxide use conversion mechanism, but carbon has low theoretical capacity.In nanocomposite, carbon play a role as mechanical support and conductive agent to metal oxide, because structural damage during conversion reaction 7 .Therefore, nanocomposite between metal oxide and carbon result high capacity, high stability, and high conductivity.Recent popular method used to synthesize nanocomposite materials was MOF templates.MOF (Metal Organic Frameworks) contain of metal precursor and organic ligand.MOF has unique characteristic, such as high porosity, high surface area, and tunable morphology 8 .After carbonization, metal precursor transforms into metal oxide and organic ligand transforms into carbon.These characteristic influence active sites of nanocomposite materials which beneficial for lithium transport.In this research we utilize ZIF-8 (Zeolite Imidazole Frameworks) as template

ZIF-8 Preparation
Typically, 0.510 g of Zn(NO3)2.6H2Owas added into solution consist of 20 mL of methanol and 5 mL of DMF, the solution was labeled as solution A. At different container, 0.766 g of 2-methylimidazole added into solution consist of 20 mL of methanol and 5 mL of DMF, the solution was labeled as solution B. These solution were stirred for 30 minutes at room temperature.Next, the solution B was added to the solution A and stirred for 5 minutes at room temperature.The mixing solution was kept at room temperature for 24 hours.White precipitated was collected after 3 cycles of centrifugation and washed with methanol.Finally, the precipitated was dried at 60 °C for 12 hours and ZIF-8 was collected.

Conversion of ZIF-8 to porous ZnO-C composite
To obtain the ZnO-C composite powder, the as synthesized ZIF-8 was placed on ceramic boat and calcined in tube furnace at three different temperatures 600 °C with heating rate of 2 °C/min for 2 hours under N2 atmosphere.The calcined samples were labeled as ZnO-C 600.

Materials Characterization
Morphology of the all samples were analyzed using scanning electron microscope (SEM, JEOL JSM-IT300) completed with energy dispersive spectrometer (EDS) and transmittance electron microscope (TEM: Tecnai G20 S-Twin).

Electrode Preparation and Electrochemical Performance
Slurry formation of all samples was fabricated by mixing CMC: Super-P: active materials with the ratio of 40:5:3 wt%.400 mg of CMC and 50 mg of super P carbon was mixed directly.The mixture was then added into 0.8 mL of NMP. 30 mg of active materials (samples) and 0.2 mL of NMP were added into 300 mg of slurry.Electrode slurry was then coated onto copper foil .Electrochemical properties were obtained by performing cyclic voltammetry (CV) using three electrode configuration.Lithium foil was used as counter electrode and reference electrode.1 M of LiPF6 in EC/DEC (1:1 v/v) was used as electrolyte.CV measurement was performed in glovebox using Palmsense4 electrochemical workstation with potential window of 0-3 V and scan rate of 0.0002 mV/s.

Result and Discussion
SEM characterization showed the morphology of ZIF-8 (a).It was seen as nanoparticle with radius around 200 nm.After carbonization to 600°C, polyhedral morphology was created, effect of heat treatment.Homogeneous morphology was created.It indicated that pure phase of ZnO was formed.From line measurement, it showed that the size range of polyhedral morphology was from 52 nm -94 nm.It approved the nanosize morphology of ZnO-C 600.To observe specific morphology of ZnO-C, we used TEM.   1 show the composition of element in ZnO-C 600.Carbon composite was formed with 44.28% carbon element percentage.TEM image in figure 2 of lattice fringe (a) from ZnO-C 600 indicated the phase of ZnO, because it is impossible to find carbon graphitic crystall at 600°C, karimi research presented that graphitic carbon from MOF derived materials was initially formed at 800°C 9 .TEM images of ZnO-C 600 also confirmed the dodecahedron morphology (b) of composite as standard morphology of ZIF-8.In order to investigate lithium storage mechanism 600.Cyclic voltammetry measurement was conducted at 0.0002 V/s with range 0-3 V as seen in Fig. 6.There were 3 peaks obtained as shown in fig 5.

ZnO
Theoretically reduction reaction of ZnO-C was described as below 10 ZnO + 2Li + + 2e -↔ Zn + Li2O (Conversion) Zn + xLi + + xe -↔ LixZn, (x≤1) (Alloying) First reduction occurred when ZnO was reduced from Zn 2+ to Zn 0 .Second reduction occurred when Zn was reduced from Zn 0 to Zn -1 .These 2 peaks occurred simultaneously.From CV anodic profile there were 3 peaks (2.3V, 1.8V, 1.4V).We concluded that first peak (2.3V) was conversion reaction.Simultaneously after first peak, the second peak (1.8V) was alloying reaction.The third peak (1.4V) was predicted as solid electrolyte interface (SEI) reaction, effect of side reactions between active materials and carbonate or floride in electrolyte 11 .Oxidation occurred once at 2.9V with small peak current 0.05 mA.It indicated dealloying reaction.The small peak showed irreversible reaction after solid electrolyte formation.Solid electrolyte interface caused difficulty of ion diffusion.diffusion mechanism, where lithium ion was entered the porous morphology of carbon shell from composite at first peak to reduct ZnO into Zn.Lithium ion decelerated while entered porous carbon shell, then lithium ion accelerated reduction of Zn to LiZn at second peak after inserted into core of composite.Alloying reaction simultaneously was occured after conversion reaction.So morphology affected the ionic diffusion.Finally, lithium ion left the composite at oxidation.Ionic diffusion was decreased because effect of solid electrolyte interface (SEI) formation.

Conclusion
In summary, ZnO-C nanoporous composite was succesfully synthesized by non-solvothermal method followed by heat treatment (carbonization).Carbonization temperature influenced the crystall phase of ZnO and formation of Carbon.When ZnO-C was exhibited as anode of lithium-ion battery, there was reaction beetwen lithium ion and ZnO-C.The highest ionic diffusion of ZnO-C 600 was alloying reaction.Ionic diffusion was affected by morphology of ZnO-C composite.The synthesis method which performed in this research can be extended to get other performance of ZnO-C such as initial capacity, rate capability and cycling performance.

Figure 2 .
Figure 2. TEM images of ZnO-C 600, a) lattice fringe of ZnO, b-c) Dodecahedron morphology structure of ZnO-C

Table 1 .
Carbon content in composite and other content

Table 2 .
show the ionic diffusion of ZnO-C 600.It showed the highest diffusion of alloying reaction at second peak.The diffusion was increase from conversion reaction at first peak.It exhibited the ionic