Graphene Based Cathode Materials For The Application In Li-S Batteries

Lithium-sulfur battery is a promising electrochemical device for future energy conversion and storage. Its theoretical capacity is1675mAh g-1,much higher than that of conventional lithium-ion battery. However, it suffers from rapid capacity decay and low energy efficiency. In this thesis, we are mainly use graphene into the cathode materials to inhibit the polysulfides shuttle and increase the conductivity, hence improve the electrochemical preformance of lithium sulfur battery. Our work in this thesis can be divided into three parts:(1) A porous three-dimensional nitrogen-doped graphene (3D-NG) was introduced as an interconnected framework for sulfur in lithium-sulfur battery using a solution react method. The3D-NG-sulfur composite (3D-NGS) has a high sulfur content of87.6wt.%and sulfur was wrapped very well by graphene. The as-designed3D-NGS composite exhibits excellent rate capability and cyclability. The discharge specific capacity is792mAh g-1after145cycles at a current density of600mA g-1and the capacity fading rate is0.05%per cycle. Even at a high rate of1500mA g-1, the composite still shows a good cycle performance with a capacity of671mAh g-1after200cycles. The outstanding electrochemical performance can be attributed to the flexible porous3D structure and N-doping in graphene. The flexible3D-NG can provide a conductive framework for electron transport and alleviate the volume effect during cycling; N-doping can facilitate the penetration of Li ions across the graphene and restrain sulfur due to the strong chemical bonding between S and nearby N atoms.(2) A macroporous free-standing nano-sulfur/graphene (S-rGO) paper is introduced directly as an electrode for lithium-sulfur battery. The S-rGO paper is synthesized via a facile freeze drying route followed by low-temperature heat treatment. The flexible S-rGO paper not only provides a conductive framework for electron transport but also alleviates volume effect during cycling. The as-designed S-rGO paper exhibits excellent rate capability and cyclability. The specific discharge capacity is800mAh g-1after200cycles at a current density of300mA g-1and the capacity fading rate is only0.035%per cycle. Even at a high current density of1500mA g-1, it still shows a good performance. We ascribe the high performance of the S-rGO paper to stable macroporous structure and strong interaction between sulfur nanoparticles and graphene.(3)In order to forbid the use of lithium metal as the anode in lithium sulfur battery, we developed lithium sulfide as a cathode. Lithium sulfide is a promising cathode material for Li-S batteries with high capacity (theoretically1166mAh g-1), and can be paired with non-lithium-metal anodes to avoid potential safety issues. However, the cycle life of coarse Li2S particles suffers from poor electronic conductivity and polysulfide shuttling. Li2S is not stable in air. Here we develop a flexible slurryless Li2S/reduced graphene oxide cathode paper (Li2S/rGO paper) by simple drop-coating. The Li2S/rGO paper can be directly used as a free-standing and binder-free cathode without metal substrate, which leads to significant weight savings. It shows excellent rate capability (up to7C with discharge capacity of597mAh g"1) and cycle life in coin cell tests, due to the high electron conductivity, flexibility and strong solvent absorbency of rGO paper. The Li2S particles that precipitate out of the solvent on rGO have diameters25-50nm, in contrast to the3-5μm coarse Li2S particles without rGO.