Few layered graphene and its composites as anode materials in Li-ion batteries and EMI shielding materials

Abstract

In this thesis, few layered graphene (FLG) and its composites are synthesized using easy and novel synthesis routes. The synthesized composites are tested as anode materials in Li ion batteries and as materials for electromagnetic interference (EMI) shielding. Two novel graphene based composites namely Co2Mo3O8/reduced graphene oxide (Co2Mo3O8/rGO) and Germanium/reduced graphene oxide (Ge/rGO) composites are synthesized and tested as anode materials in Li ion batteries while two other novel composites namely few layered graphene (FLG)/poly vinyl alcohol (PVA) and graphene wrapped magnesium oxide (G@MgO)/PVA composites are synthesized and tested as EMI shielding materials. Co2Mo3O8/rGO composite is constituted with hierarchical and submicron sized Co2Mo3O8 hexagonal nanoplatelets (50 nm thick) entrenched in between thin graphene layers. When cycled in the voltage range of 0.005-3.0 V at a current density of 60 mA/g, Co2Mo3O8/rGO composite delivered an excellent reversible specific capacity of ~954 mA h/g that corresponds to 82% capacity retention at the end of the 60th cycle which is higher than the theoretical capacity of both Co2Mo3O8 and graphene. Co2Mo3O8/rGO composite also showed excellent rate capability. Electrochemical conversion and electrochemical adsorption and desorption are found to be the primary reasons for Li storage in Co2Mo3O8/rGO composite. Excellent synergy between Co2Mo3O8 and rGO in the composite is attributed for the observed high specific capacity, long cycle life and good rate capability. When Ge/rGO composite was cycled in the voltage range of 0.005-3.0 V at a current density of 160 mA/g, it showed an excellent reversible capacity of ~539 mA h/g that corresponds to 98% capacity retention at the end of the 100th cycle. Ge/rGO composite also showed high reversible capacity retention, good rate capability, and excellent cycle life. Alloying, de-alloying and electrochemical adsorption and desorption type reaction mechanism are the primary reasons for lithium storage in germanium and graphene, respectively. In the case of EMI shielding materials, 1 mm thick FLG/PVA composite sheets containing 0.2 and 0.6 wt.% of FLG as filler are prepared using a simple casting process. A maximum electromagnetic interference (EMI) shielding effectiveness (SE) of ~19.5 dB (in the X-band, 8.2–12.4 GHz) was obtained in case of the FLG/PVA composite with 0.6 wt.% of FLG. Absorption is found to be the dominant mechanism for EMI shielding. The high EMI SE is attributed to the network-like features formed by FLG in PVA matrix. In another work, temperature (30-120 °C) dependent complex permittivity of 1 mm thick G@MgO/PVA_Coag (0.4, 0.6, 0.8, 1.0, 3.0, 10.0, and 15.0wt.%) and G@MgO/PVA_ac (0.4, and 3.0wt.% as-casted) composite samples in low frequency (20 Hz - 2 MHz), and high frequency (i.e., 8.2-12.4 GHz)ranges are measured. In both the frequency ranges, G@MgO/PVA_Coag composites performed superior dielectric properties than PVA, and G@MgO/PVA_ac composites. A strong interfacial polarization (from M” vs. log (f) plots) is observed in coagulated, and as-casted G@MgO/PVA composites. It is noticed from the calculated activation energies that conduction is the dominating mechanism for energy transfer in both the composites, however, it is predominant in G@MgO/PVA_Coag composites than in G@MgO/PVA_ac composites. In the X-band, EMI SE of the G@MgO/PVA_Coag composites is greater than that of the G@MgO/PVA_ac composites at a particular weight fraction of G@MgO. At 30 and 120 °C, the average EMI SE values of ~14.2 and ~18.9 dB, respectively, are measured for 10.0 G@MgO/PVA_Coag while 15.0G@MgO/PVA_Coag composite showed EMI SE of ~20.6 and ~27.5 dB, respectively. In the case of G@MgO/PVA_Coag and G@MgO/PVA_ac composites too absorption is found to be the dominating mechanism for EMI SE.