| Lithium-ion batteries (LIBs), in the last decades, have occupied the power source market for portable electronic devices and incoming hybrid electric vehicles (HEVs) and electric vehicles (EVs) because of their advantages of light weight, high energy density and stable performance over long-term cycles. Meanwhile, the expanded renewable energy and smart grid markets raise the demand for higher safety, higher power density and lower cost energy storage systems. However, the rarity and uneven distribution of lithium resources significantly limited the further extension of LIBs. Therefore, great growth of research interest in alternative energy storage technologies, for example, sodium-ion batteries (SIBs), has been witnessed in the last few years. Compared to LIBs, SIBs is one of the most promising candidates for grid-scale energy storage due to the abundance and wide availability of sodium resources,420 times more than lithium resources. In the past few years, people have focused on developing cathode materials for SIBs, which has made great progress by learning from the well-established LIB technology. However, developing high-performance anodes for SIBs is still a challenge. The commercial anode, graphite, shows a very limited capacity for SIBs, which is attributed to the mismatch between graphite and the large Na+ ions. Many efforts are underway to search anodes for SIBs. Among various anode choices, like alloys, metal oxides, organic compounds, carbon-based material is still the most attractive one due to its high capacity, low potential, and low cost. However, carbon based anodes still face some main barriers like low initial Coulombic efficiency, poor rate and cycling performance.In the first part, we report for the first time that rapid reduction process is much more effective to achieve superior performance for SIB anode based on reduced graphene oxide (rGO). Compared with slow reduction process which has been widely done for rGO, rapid reduction leads to:(1) larger layer-layer distance allowing Na+ ions access; (2) better porous structure with hierarchical pores for Na+ ions access; (3) fast reduction process to save energy, time, and cost. We first used Xenon lamp exposure to reduce graphene oxide within 2 seconds in air. This rapid rGO has a large layer spacing of 0.367 nm, resulting in a high Na-ion storage capacity of around 400 mAh g-1 at a current density of 25 mA g-1, an outstanding rate performance of around 200 mAh g-1 at 250 mA g-1. Moreover, the as-obtained rapid rGO anode can be cycled for 750 cycles at 250 mA g-1 with capacity loss less than 0.025% per cycle. Furthermore, we also demonstrate that rapid thermal shock reduction of GO can obtain a much higher specific capacity than slowly thermal reduced GO. The extraordinary properties of rapid rGO show great promise as a high performance, cost-effective anode material for SIBs.In the second part, we used TEMPO-oxidized nanofibrillated cellulose (NFC) as a green dispersant to disperse MoS2 and prepared MoS2/C film by thermal treating the MoS2/NFC film. During ultrasonic exfoliation process, NFC automatically attached to the MoS2 flakes through the interaction between the hydrophobic sites of NFC and the hydrophobic layer plane of the flakes as well as the hydrogen bonding between the hydroxyl groups of the NFC on MoS2 flakes defective edges. Both the electrostatic repulsive forces generated from charged carboxyl groups on NFC and the steric hindrance caused by NFC helped to stabilize the exfoliated MoS2 nano flakes. After thermal treating MoS2/NFC film, NFC was carbonized at high temperature and formed an in-situ carbon coating layer on the MoS2 surface. When evaluated as anode for sodium ion batteries, MoS2/C film exhibits remarkable electrochemical performances. It delivers a capacity of 337 mAh g-1 at 40 mA g’1 and a good rate capacity of 211 mAh g-1 at a high current density of 1A g-1, indicating a good anode for sodium ion batteries.In the third part, we report wood fiber derived carbon paper and carbon film as anodes for SIBs. We discovered that the surface area of wood fiber derived hard carbon showed a dramatic decrease to 126 m2 g-1 with TEMPO treatment compared to 586 m2 g-1 without treatment. We reveal that TEMPO treatment can swell, corrupt, and even partially unzip the cell wall of the fiber. The treated fiber with a fewer hollow structure and larger fine fraction leads to a paper with higher packing density, and thus results in a low surface area carbon paper after carbonization. As expected, when evaluated as anode for SIBs, the low surface area carbon paper exhibits a much higher initial Coulombic efficiency (72%) compared to carbon derived directly from natural wood fiber (28%). Moreover, the low surface area hard carbon also displays high capacity of 246 mAh g-1 and stable cycling performance of 196 mAh g-1 at 100 mA g-1 after 200 cycles, suggesting a great anode for SIBs. We then carbonized the nature made ordered cellulose nanocrystals obtained from wood fibers into conductive carbon film with increased short-range ordered carbon at a relatively low processing temperature of 1000 ℃. Graphitic structure with an interlayer spacing of 0.39 nm and mesoporous structure endue fast Na ion intercalation and cluster formation. Thus, this more conductive carbon film shows excellent performance in SIB anode, i.e. a high capacity of 340 mAh g-1 at a current density of 100 mA g-1 and stable cycling over 400 cycles. This study sheds light on obtaining high quality carbon from the natural bio resource with good crystalline structure.In the fourth part, we present a novel 3D structured porous carbon with low tortuosity derived from natural wood directly. This low tortuosity porous carbon itself has a regular and well-defined channel system which can enhance the electrolyte transport within electrode. Good ionic conductivity is maintained even the thickness of electrode reaches up to lmm with high mass loading. Benefit from the unique construction, ultra-thick electrode with high mass loading of 55mg cm-2 can be achieved, delivering an areal capacity up to 13.6 mAh cm-2 at 0.55 mA cm-2. The full cell demonstration with our porous carbon as anode and Na3V2(PO4)3 as cathode shows a stable cycling performance with Coulombic efficiency nearly 100% during cycling. The excellent electrochemical properties of this 3D porous carbon indicates a promising use in the emerging SIB technology. |
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