We report electrical characterization of monolayer molybdenum disulfide (MoS 2 ) devices using a thin layer of polymer electrolyte consisting of poly(ethylene oxide) (PEO) and lithium perchlorate (LiClO 4 ) as both a contact-barrier reducer and channel mobility booster. We find that bare MoS 2 devices (without polymer electrolyte) fabricated on Si/SiO 2 have low channel mobility and large contact resistance, both of which severely limit the field-effect mobility of the devices. A thin layer of PEO/ LiClO 4 deposited on top of the devices not only substantially reduces the contact resistance but also boost the channel mobility, leading up to three-orders-ofmagnitude enhancement of the field-effect mobility of the device. When the polymer electrolyte is used as a gate medium, the MoS 2 field-effect transistors exhibit excellent device characteristics such as a near ideal subthreshold swing and an on/off ratio of 10 6 as a result of the strong gatechannel coupling.

With the rapid growth of the volume of spent Li-ion batteries (LIBs), recycling of spent LIBs has attracted significant attention in recent years for future sustainability. In particular, there remains a great need for the development of a scalable and environmentfriendly separation process to recycle valuable cathode active materials from spent LIBs and electrode scraps. In this work, froth flotation technique was adopted to separate cathode active materials from a mixture of cathode and anode materials. To evaluate whether the recovered cathode materials maintain their functional integrity after the developed separation process, a variety of electrochemical analyses have been conducted systematically. The present work demonstrated that froth flotation process with kerosene enhanced separability of mixed electrode materials and the recycled cathode materials almost preserved their original electrochemical reactivity. Cycle performance (up to 200 cycles) and rate capability (up to 1 C) of the recycled cathodes were comparable to those of a pristine cathode. However, the higher polarization observed in the recycled cathodes was identified as a key challenge, and it needs to be addressed further. This work provides valuable insights into further development of a scalable froth flotation-based recycling process which can be implemented in a direct recycling process.

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A simple one-stage solution-based method was developed to produce graphene nanoribbons by sonicating graphite powder in organic solutions with polymer surfactant. The graphene nanoribbons were deposited on silicon substrate, and characterized by Raman spectroscopy and atomic force microscopy. Single-layer and few-layer graphene nanoribbons with a width ranging from sub-10 nm to tens of nm and length ranging from hundreds of nm to 1 µm were routinely observed. Electrical transport properties of individual graphene nanoribbons were measured in both the back-gate and polymer-electrolyte top-gate configurations. The mobility of the graphene nanoribbons was found to be over an order of magnitude higher when measured in the latter than in the former configuration (without the polymer electrolyte), which can be attributed to the screening of the charged impurities by the counter-ions in the polymer electrolyte. This finding suggests that the charge transport in these solution-produced graphene nanoribbons is largely limited by charged impurity scattering.

Abstract

The increasing demand for Li-ion batteries (LIBs) in hybrid and electric vehicles had led to a significant increase in the volume of new and end-of-life LIBs. For this reason, recycling of spent LIBs has attracted significant attention in recent years for future sustainability. Different from existing recycling methods such as pyrometallurgical and hydrometallurgical methods, direct recycling method recovers cathode and anode active materials directly in reusable forms at a low cost. This process consists of two steps: 1) liberation and separation, and 2) re-functionalization (or re-lithiation). Many previous studies have been focusing on the second step only. To successfully implement the direct recycling process, development of a scalable and environmentally friendly separation process for battery active materials while preserving their functional integrity is necessary. To date, no studies have been conducted to evaluate the technical feasibility of such a scalable separation process for the direct recycling method. In this work, a water-based recycling process was developed to recover cathode active materials from LIBs. In this recycling process, froth flotation technique was used to separate cathode active materials from a mixture of cathode and anode materials. A variety of electrochemical analyses of the recycled cathode active materials were systematically conducted to evaluate technical feasibility and understand current challenges of this recycling process. The present research demonstrated that the use of kerosene as the collector in the froth flotation process improved the purity of produced cathode active materials, and the recycled cathode materials preserved their original electrochemical reactivity. Cycle performance (up to 200 cycles) and rate capability (up to 1C) of the recycled cathodes were comparable to those of a pristine cathode. However, the reversible capacity of the recycled cathodes was slightly lower than that of a pristine cathode because of cell polarization. The polarization caused by electrode wettability and surface impurities on the recycled cathodes was identified as a key challenge that needs to be addressed further. This work will provide valuable insights into further development of a froth flotation-based recycling process which can be implemented in the direct recycling process.

Abstract:We have prepared nano-structured In-doped (1 mol %) LiFePO 4 /C samples by sol-gel method followed by a selective high temperature (600 and 700 • C) annealing in a reducing environment of flowing Ar/H 2 atmosphere. The crystal structure, particle size, morphology, and magnetic properties of nano-composites were characterized by X-ray diffraction (XRD), scanning electron microsopy (SEM), transmission electron microscopy (TEM), and 57 Fe Mössbauer spectroscopy. The Rietveld refinement of XRD patterns of the nano-composites were indexed to the olivine crystal structure of LiFePO 4 with space group Pnma, showing minor impurities of Fe 2 P and Li 3 PO 4 due to decomposition of LiFePO 4 . We found that the doping of In in LiFePO 4 /C nanocomposites affects the amount of decomposed products, when compared to the un-doped ones treated under similar conditions. An optimum amount of Fe 2 P present in the In-doped samples enhances the electronic conductivity to achieve a much improved electrochemical performance. The galvanostatic charge/discharge curves show a significant improvement in the electrochemical performance of 700 • C annealed In-doped-LiFePO 4 /C sample with a discharge capacity of 142 mAh·g −1 at 1 C rate, better rate capability (~128 mAh·g −1 at 10 C rate,~75% of the theoretical capacity) and excellent cyclic stability (96% retention after 250 cycles) compared to other samples. This enhancement in electrochemical performance is consistent with the results of our electrochemical impedance spectroscopy measurements showing decreased charge-transfer resistance and high exchange current density.