Transmission electron microscopy analysis of some transition metal compounds for energy storage and conversion

In Situ Observation of the Effect of Accelerating Voltage on Electron Beam Damage of Layered Cathode Materials for Lithium-Ion Batteries

Electron beam damage from transmission electron microscopy of layered lithium transition-metal oxides is a threshold phenomenon that depends on the electron beam energy, which we demonstrate in this study by varying the accelerating voltage of a scanning transmission electron microscope. The electron beam irradiation experiment shows that Ni in LiNiO₂ has much lower threshold energy for displacement than Co in LiCoO₂, which is supported by DFT calculations predicting that Ni has lower migration energy. The transition-metal ions are reduced from the oxidation state of +3 to +2 during migration from their original positions to the lithium sites, and Ni is more easily reduced than Co because of its electronic configuration. In addition, the high-energy electron beam induces oxygen release, which is another symptom of degradation of materials that occurs more strongly in Ni-containing materials with ion displacement.

A 3D conductive network of porous carbon nanoparticles interconnected with carbon nanotubes as the sulfur host for long cycle life lithium-sulfur batteries

Constructing an interlinked three-dimensional conductive carbon structure as a sulfur host is considered to be an effective strategy for suppressing the capacity decay over long-term cycling and improving the rate performance of lithium-sulfur (Li-S) batteries, because it can not only facilitate rapid electronic and ionic transportation in the cathode, but also be conducive to confine lithium polysulfide (LiPS) dissolution and shuttling. In this report, we designed a novel 3D conductive network structure (CNTs/Co-NC), which is composed of Co-NC (cobalt embedded in an N-doped porous carbon composite) derived from ZIF-67 polyhedra and inserted carbon nanotubes (CNTs), and applied it as a sulfur host for Li-S batteries. The CNT/Co-NC network structure is firstly prepared via the in situ nucleation of small ZIF-67 crystals on the surface of CNTs and eventually grown into CNT/ZIF-67 hybrid materials; after subsequent carbonization and infiltration of sulfur procedures, the S@CNT/Co-NC cathode is obtained. Li-S batteries based on the S@CNT/Co-NC cathode show an improved rate capability of 772.6 mA h g^-1 at the 2 C rate, enhanced long cycling stability under a large current density with a low capacity decay rate of ∼0.067% per cycle at the 0.5 C rate after 500 cycles and ∼0.072% per cycle at the 1 C rate after 700 cycles and an excellent coulombic efficiency of about 95% up to 500 cycles at 0.5 C and 91% up to 700 cycles at 1 C. The superior performance of S@CNTs/Co-NC should be ascribed to the rapid charge transfer, excellent electron conductivity, improved adsorption capability for LiPSs and enhanced redox kinetics of this 3D conductive network structure.

Superhierarchical Cobalt-Embedded Nitrogen-Doped Porous Carbon Nanosheets as Two-in-One Hosts for High-Performance Lithium-Sulfur Batteries

Lithium-sulfur (Li-S) batteries, based on the redox reaction between elemental sulfur and lithium metal, have attracted great interest because of their inherently high theoretical energy density. However, the severe polysulfide shuttle effect and sluggish reaction kinetics in sulfur cathodes, as well as dendrite growth in lithium-metal anodes are great obstacles for their practical application. Herein, a two-in-one approach with superhierarchical cobalt-embedded nitrogen-doped porous carbon nanosheets (Co/N-PCNSs) as stable hosts for both elemental sulfur and metallic lithium to improve their performance simultaneously is reported. Experimental and theoretical results reveal that stable Co nanoparticles, elaborately encapsulated by N-doped graphitic carbon, can work synergistically with N heteroatoms to reserve the soluble polysulfides and promote the redox reaction kinetics of sulfur cathodes. Moreover, the high-surface-area pore structure and the Co-enhanced lithiophilic N heteroatoms in Co/N-PCNSs can regulate metallic lithium plating and successfully suppress lithium dendrite growth in the anodes. As a result, a full lithium-sulfur cell constructed with Co/N-PCNSs as two-in-one hosts demonstrates excellent capacity retention with stable Coulombic efficiency.