Mun Sek Kim

Enhanced Li–S Batteries Using Amine-Functionalized Carbon Nanotubes in the Cathode

The rechargeable lithium-sulfur (Li-S) battery is an attractive platform for high-energy, low-cost electrochemical energy storage. Practical Li-S cells are limited by several fundamental issues, including the low conductivity of sulfur and its reduction compounds with Li and the dissolution of long-chain lithium polysulfides (LiPS) into the electrolyte. We report on an approach that allows high-performance sulfur-carbon cathodes to be designed based on tethering polyethylenimine (PEI) polymers bearing large numbers of amine groups in every molecular unit to hydroxyl- and carboxyl-functionalized multiwall carbon nanotubes. Significantly, for the first time we show by means of direct dissolution kinetics measurements that the incorporation of CNT-PEI hybrids in a sulfur cathode stabilizes the cathode by both kinetic and thermodynamic processes. Composite sulfur cathodes based the CNT-PEI hybrids display high capacity at both low and high current rates, with capacity retention rates exceeding 90%. The attractive electrochemical performance of the materials is shown by means of DFT calculations and physical analysis to originate from three principal sources: (i) specific and strong interaction between sulfur species and amine groups in PEI; (ii) an interconnected conductive CNT substrate; and (iii) the combination of physical and thermal sequestration of LiPS provided by the CNT=PEI composite.

Fabricating multifunctional nanoparticle membranes by a fast layer-by-layer Langmuir–Blodgett process: application in lithium–sulfur batteries

The Langmuir–Blodgett technique is a powerful and widely used method for preparing coatings of amphiphilic molecules at air/water interfaces with thickness control down to a single molecule. Here, we report two new LB techniques designed to create ordered, multifunctional nanoparticle films. The methods utilize Marangoni stresses produced by surfactants at a fluid/solid/gas interface and self-assembly of nanoparticles to facilitate rapid creation of ultrathin films of carbon, metal-oxide nanoparticles, polymers, and combinations of these materials on any non-reactive support in a layer-by-layer configuration. Using polyolefin separators in lithium sulfur electrochemical cells as an example, we illustrate how the method can be used to create structured membranes for regulating mass and charge transport. We further show that a layered MWCNT/SiO2/MWCNT nanomaterial created in a clip-like configuration, with gravimetric areal coverage of ∼130 μg cm−2 and a thickness of ∼3 μm, efficiently intercept and reutilize dissolved lithium polysulfides for improving electrochemical performances of lithium sulfur batteries.

Determination of optimal arm and hand configurations for grasping objects by humanoid robots and avatars

This paper presents an algorithm that computes arm motion and hand grasp configuration to reach and grasp an object by a humanoid robot. Although grasping an object is a relatively easy task for humans, this task needs to take into account many constraints including arm joint limits, stability of grasp, and the possibility of collisions between the robot and objects in the environment, The presented algorithm finds the optimal arm and hand configuration to grasp an object without enumerating all possible configurations by employing heuristics to guide the search. Efficiency is gained by evaluating different constraints in increasing order of complexity so as to eliminate infeasible grasp configurations with minimal computation. Computed grasp configurations are such that arm joints are far from their limits, and they are close to grasps used by humans. Our algorithm will be an important module for humanoid robots and avatars in virtual reality systems.

Complete motion planner for time-varying environments

This paper presents a complete motion planner in three-dimensional changing environment. The robot is either a manipulator or a rigid object that translates and rotates, while obstacles are polyhedra translating and rotating along known trajectories. The motion planner is based on the SANDROS search strategy, which uses a hierarchical, multi-resolution representation of the product space of the time and the configuration space. A solution is found by generating a candidate sequence of subgoals and subsequently computing collision-free path along the sequence. For changing environments it is the first planner for non-point robots that is efficient and guarantees a solution. This planner can control the trade-offs between the computation resource and algorithmic completeness/solution path quality, and thus can fully utilize the available computing power. This algorithm is useful for navigation of mobile robots and submarines, or motion planning of multiple robots by regarding other robots as moving obstacles.