Jongchan Song

Directly grown Co3O4 nanowire arrays on Ni-foam: structural effects of carbon-free and binder-free cathodes for lithium–oxygen batteries

The problem of carbon and binder decomposition, degrading the performance levels of the cathodes used in lithium–oxygen (Li–O2) batteries, remains unsolved. For this reason, using carbon and binder-free cathodes may be an ideal approach to remedy this problem. Here, we have developed a carbon free- and binder-free cathode for Li–O2 batteries based on vertically grown Co3O4 nanowire (NW) arrays on Ni-foam and demonstrated the suppression of this type of decomposition. The highly organized texture and high catalytic activity of the cathode provide high capacity with a reduced overvoltage. Interestingly, the charge voltage profile changed with the discharge rate, which is associated with the variation in the phase and local distribution of the discharge products. At a low discharge rate, a morphology resembling the pointed tip of a brush was observed, indicating that the crystalline discharge products were concentrated on the skin of the cathode, causing the deformation of the NW array. At a high discharge rate, a more uniform distribution of the quasi-amorphous discharge products was favored, resulting in a relatively stable voltage profile with cycling. These findings suggest the importance of the electrode structure and discharge product distribution during the design of carbon- and binder-free cathodes.

Defect-free, size-tunable graphene for high-performance lithium ion battery.

The scalable preparation of graphene in control of its structure would significantly improve its commercial viability. Despite intense research in this area, the size control of defect-free graphene (df-G) without any trace of oxidation or structural damage remains a key challenge. Here, we propose a new scalable route for generating df-G with a controllable size of submicron to micron through sequential insertion of potassium and pyridine at low temperature. Structural and chemical analyses confirm that the df-G perfectly preserves the intrinsic properties of graphene. The Co3O4 (<50 nm) wrapped by ∼ 10.5 μm(2) df-G has unprecedented capacity, rate capability, and cycling stability with capacities as high as 1050 mAh g(-1) at 500 mA g(-1) and 900 mAh g(-1) at 1000 mA g(-1) even after 200 cycles, which suggests enticing potential for the use in high performance lithium ion batteries.

Electrospun Three-Dimensional Mesoporous Silicon Nanofibers as an Anode Material for High-Performance Lithium Secondary Batteries

Mesoporous silicon nanofibers (m-SiNFs) have been fabricated using a simple and scalable method via electrospinning and reduction with magnesium. The prepared m-SiNFs have a unique structure in which clusters of the primary Si nanoparticles interconnect to form a secondary three-dimensional mesoporous structure. Although only a few nanosized primary Si particles lead to faster electronic and Li(+) ion diffusion compared to tens of nanosized Si, the secondary nanofiber structure (a few micrometers in length) results in the uniform distribution of the nanoparticles, allowing for the easy fabrication of electrodes. Moreover, these m-SiNFs exhibit impressive electrochemical characteristics when used as the anode materials in lithium ion batteries (LIBs). These include a high reversible capacity of 2846.7 mAh g(-1) at a current density of 0.1 A g(-1), a stable capacity retention of 89.4% at a 1 C rate (2 A g(-1)) for 100 cycles, and a rate capability of 1214.0 mAh g(-1) (at 18 C rate for a discharge time of ∼3 min).

Ionomer-Liquid Electrolyte Hybrid Ionic Conductor for High Cycling Stability of Lithium Metal Electrodes

The inhomogeneous Li electrodeposition of lithium metal electrode has been a major impediment to the realization of rechargeable lithium metal batteries. Although single ion conducting ionomers can induce more homogeneous Li electrodeposition by preventing Li+ depletion at Li surface, currently available materials do not allow room-temperature operation due to their low room temperature conductivities. In the paper, we report that a highly conductive ionomer/liquid electrolyte hybrid layer tightly laminated on Li metal electrode can realize stable Li electrodeposition at high current densities up to 10 mA cm−2 and permit room-temperature operation of corresponding Li metal batteries with low polarizations. The hybrid layer is fabricated by laminating few micron-thick Nafion layer on Li metal electrode followed by soaking 1 M LiPF6 EC/DEC (1/1) electrolyte. The Li/Li symmetric cell with the hybrid layer stably operates at a high current density of 10 mA cm−2 for more than 2000 h, which corresponds to more than five-fold enhancement compared with bare Li metal electrode. Also, the prototype Li/LiCoO2 battery with the hybrid layer offers cycling stability more than 350 cycles. These results demonstrate that the hybrid strategy successfully combines the advantages of bi-ionic liquid electrolyte (fast Li+ transport) and single ionic ionomer (prevention of Li+ depletion).

Structural modulation of lithium metal-electrolyte interface with three-dimensional metallic interlayer for high-performance lithium metal batteries

The use of lithium (Li) metal anodes has been reconsidered because of the necessity for a higher energy density in secondary batteries. However, Li metal anodes suffer from ‘dead’ Li formation and surface deactivation which consequently form a porous layer of redundant Li aggregates. In this work, a fibrous metal felt (FMF) as a three-dimensional conductive interlayer was introduced between the separator and the Li metal anode to improve the reversibility of the Li metal anode. The FMF can facilitate charge transfer in the porous layer, rendering it electrochemically more active. In addition, the FMF acted as a robust scaffold to accommodate Li deposits compactly in its interstitial sites. The FMF-integrated Li metal (FMF/Li) electrode operated with a small polarisation even at a current density of 10 mA cm−2, and it exhibited a seven times longer cycle-life than that of an FMF-free Li electrode in a symmetric cell configuration. A Li metal battery (LMB) using the FMF/Li electrode and a LiFePO4 electrode exhibited a two-fold increase in cycling stability compared with that of a bare Li metal electrode, demonstrating the practical effectiveness of this approach for high performance LMBs.

Polysulfide rejection layer from alpha-lipoic acid for high performance lithium–sulfur battery

The polysulfide shuttle has been an impediment to the development of lithium–sulfur batteries with high capacity and cycling stability. Here, we report a new strategy to remedy the problem that uses alpha-lipoic acid (ALA) as an electrolyte additive to form a polysulfide rejection layer on the cathode surface via the electrochemical and chemical polymerization of ALA and a stable solid electrolyte interface (SEI) layer on the Li metal anode during the first discharge. The poly(ALA) layer formed in situ effectively prevents the polysulfide shuttle and consequently enhances the discharge capacity and cycling stability, owing to the Donnan potential developed between the polysulfide-concentrated cathode and the fixed negative charge-concentrated poly(ALA) layer. Also, the SEI layer additionally prevents the chemical reaction of the polysulfide and Li metal anode. The approach, based on the double effect, encompasses a new scientific strategy and provides a practical methodology for high performance lithium–sulfur batteries.

Perfluorinated Ionomer-Enveloped Sulfur Cathodes for Lithium–Sulfur Batteries

Nafion is known to suppress the polysulfide (PS) shuttle effect, a major obstacle to achieving high capacity and long cycle life for lithium-sulfur batteries. However, elaborate control of the layers configuration is required for high performance. In this regard, we designed a Nafion-enveloped sulfur cathode, where the Nafion layer is formed on the skin of the cathode, covering its surface and edge while not restricting the porosity. Discharge capacity and efficiency were enhanced with the enveloping configuration, demonstrating suppression of shuttle. The edge protection exhibited better cycling stability than an edge-open configuration. In the absence of the Nafion envelope, charged sulfur concentrated on the top region of the cathode because of the relatively lower PS concentration at the cathode surface. Surprisingly, for the Nafion-enveloped cathode, sulfur was evenly distributed along the cathode, indicating that the configuration imparts a uniform PS concentration within the cathode.

Perfluorosulfonic acid-functionalized Pt/graphene as a high-performance oxygen reduction reaction catalyst for proton exchange membrane fuel cells

Platinum nanoparticles (Pt NPs) on carbon black (CB) have been used as catalysts for the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells for a while. However, this catalyst has suffered from aggregation and dissolution of Pt NPs as well as CB dissolution. In this study, we resolve those issues by developing perfluorosulfonic acid (PFSA)-functionalized Pt/graphene as a high-performance ORR catalyst. The noncovalently bonded PFSA remarkably decreases the dissolution and aggregation of Pt NPs. Moreover, unlike typical NP functionalization with other capping agents, PFSA is a proton conductor and thus efficiently develops a triple-phase boundary. These advantageous features are reflected in the improved cell performance in electrochemical active surface area, catalytic activity, and long-term durability, compared to those of the commercial Pt/C catalysts and graphene-based catalysts with no such treatment.

Glucosamine-derived encapsulation of silicon nanoparticles for high-performance lithium ion batteries

The use of nitrogen-doped carbon (NC) layers has proved effective for enhancing the cycling stability of nanostructured silicon (Si) anodes of lithium ion batteries. It has also motivated further exploration of cost- and performance-effective synthetic routes. In this regard, we propose glucosamine-derived encapsulation of Si nanoparticles (NPs), which features the use of inexpensive glucosamine as a N-containing carbon source, and conventional solution-coating and carbonization processes. With this method, a 5 nm-thick, uniform and defect-free NC layer, with pyridinic and pyrrolic nitrogen, was successfully created on Si NPs. The NC–Si anode derived from glucosamine exhibited a reversible capacity of 1775 mA h g−1 at a current density of 2000 mA g−1 after 100 cycles, and 69% capacity retention with a 20-fold increase in the current rate (from 200 mA g−1 to 4000 mA g−1). Electrochemical and spectroscopic analyses suggest the formation of a more stable solid electrolyte interface (SEI) layer of lower resistance, higher homogeneity, and higher LiF content after N-doping. Therefore, this is a cost-effective approach for enhancing the performance of Si anodes.