Soo Yeon Lim

Extremely stable cycling of ultra-thin V2O5 nanowire–graphene electrodes for lithium rechargeable battery cathodes

Vanadium pentoxide (V2O5) has received considerable attention as a lithium battery cathode because its specific capacity (>250 mA h g−1) is higher than those (<170 mA h g−1) of most commercial cathode materials. Despite this conspicuous advantage, V2O5 has suffered from limited cycle life, typically below a couple of hundred cycles due to the agglomeration of its particles. Once V2O5 particles are agglomerated, the insulating phases continuously expand to an extent that ionic and electronic conduction is severely deteriorated, leading to the significant capacity decay. In this study, in order to overcome the agglomeration issue, the electrodes were uniquely designed such that ultrathin V2O5 nanowires were uniformly incorporated into graphene paper. In this composite structure, the dispersion of V2O5 nanowires was preserved in a robust manner, and, as a result, enabled substantially improved cycle life: decent specific capacities were preserved over 100000 cycles, which are 2–3 orders of magnitude larger than those of typical battery materials.

Anomalous Manganese Activation of a Pyrophosphate Cathode in Sodium Ion Batteries: A Combined Experimental and Theoretical Study

Sodium ion batteries (SIBs) have many advantages such as the low price and abundance of sodium raw materials that are suitable for large-scale energy storage applications. Herein, we report an Mn-based pyrophosphate, Na(2)MnP(2)O(7), as a new SIB cathode material. Unlike most Mn-based cathode materials, which suffer severely from sluggish kinetics, Na(2)MnP(2)O(7) exhibits good electrochemical activity at ~3.8 V vs Na/Na(+) with a reversible capacity of 90 mAh g(-1) at room temperature. It also shows an excellent cycling and rate performance: 96% capacity retention after 30 cycles and 70% capacity retention at a c-rate increase from 0.05C to 1C. These electrochemical activities of the Mn-containing cathode material even at room temperature with relatively large particle sizes are remarkable considering an almost complete inactivity of the Li counterpart, Li(2)MnP(2)O(7). Using first-principles calculations, we find that the significantly enhanced kinetics of Na(2)MnP(2)O(7) is mainly due to the locally flexible accommodation of Jahn-Teller distortions aided by the corner-sharing crystal structure in triclinic Na(2)MnP(2)O(7). By contrast, in monoclinic Li(2)MnP(2)O(7), the edge-sharing geometry causes multiple bonds to be broken and formed during charging reaction with a large degree of atomic rearrangements. We expect that the similar computational strategy to analyze the atomic rearrangements can be used to predict the kinetics behavior when exploring new cathode candidates.

Role of intermediate phase for stable cycling of Na7V4(P2O7)4PO4 in sodium ion battery

Significance Utilizing low-cost materials, sodium ion batteries (SIBs) are beginning to attract considerable attention, particularly for large-scale utility grid applications. However, electrochemical performance of most SIB active materials is still insufficient for various practical applications. In the current study, we discovered a vanadium-based ortho-diphosphate, Na7V4(P2O7)4PO4, or VODP, that holds exceptional electrochemical properties represented by well-defined high voltage profiles at 3.88 V (vs. Na/Na+) and substantial capacity retention over 1,000 cycles. A theoretical analysis suggests that an intermediate phase encountered during phase transformation of VODP is crucial for better kinetics during battery operations, which can be expanded as a general principle in understanding diverse battery materials. Sodium ion batteries offer promising opportunities in emerging utility grid applications because of the low cost of raw materials, yet low energy density and limited cycle life remain critical drawbacks in their electrochemical operations. Herein, we report a vanadium-based ortho-diphosphate, Na7V4(P2O7)4PO4, or VODP, that significantly reduces all these drawbacks. Indeed, VODP exhibits single-valued voltage plateaus at 3.88 V vs. Na/Na+ while retaining substantial capacity (>78%) over 1,000 cycles. Electronic structure calculations reveal that the remarkable single plateau and cycle life originate from an intermediate phase (a very shallow voltage step) that is similar both in the energy level and lattice parameters to those of fully intercalated and deintercalated states. We propose a theoretical scheme in which the reaction barrier that arises from lattice mismatches can be evaluated by using a simple energetic consideration, suggesting that the presence of intermediate phases is beneficial for cell kinetics by buffering the differences in lattice parameters between initial and final phases. We expect these insights into the role of intermediate phases found for VODP hold in general and thus provide a helpful guideline in the further understanding and design of battery materials.

Combined CO2-philicity and Ordered Mesoporosity for Highly Selective CO2 Capture at High Temperatures

Various dry sorbents have been lately introduced as promising media to capture carbon dioxide (CO2). However, it is still desirable to further improve their performance in diverse aspects, and high temperature selectivity of CO2 over other gases is clearly one of them. Here, we report a co-assembly approach to turn nonporous melamine resin to a highly ordered mesoporous polymeric network (space group: Im3̅m) containing high nitrogen content (∼18 at%). This mesoporous network shows anomalously increasing CO2/N2 selectivity with temperature rise, with the selectivity at 323 K reaching 117 (Henry method). This selectivity behavior is attributed to a combined effect of the high nitrogen content allowing for high binding affinity with CO2 and well-defined mesopores (2.5-2.9 nm) accelerating release of N2 with temperature rise. The given orthogonal approach suggests a new direction in designing dry sorbents with excellent selectivities at high temperatures.

Site-Specific Transition Metal Occupation in Multicomponent Pyrophosphate for Improved Electrochemical and Thermal Properties in Lithium Battery Cathodes: A Combined Experimental and Theoretical Study

As an attempt to develop lithium ion batteries with excellent performance, which is desirable for a variety of applications including mobile electronics, electrical vehicles, and utility grids, the battery community has continuously pursued cathode materials that function at higher potentials with efficient kinetics for lithium insertion and extraction. By employing both experimental and theoretical tools, herein we report multicomponent pyrophosphate (Li(2)MP(2)O(7), M = Fe(1/3)Mn(1/3)Co(1/3)) cathode materials with novel and advantageous properties as compared to the single-component analogues and other multicomponent polyanions. Li(2)Fe(1/3)Mn(1/3)Co(1/3)P(2)O(7) is formed on the basis of a solid solution among the three individual transition-metal-based pyrophosphates. The unique crystal structure of pyrophosphate and the first principles calculations show that different transition metals have a tendency to preferentially occupy either octahedral or pyramidal sites, and this site-specific transition metal occupation leads to significant improvements in various battery properties: a single-phase mode for Li insertion/extraction, improved cell potentials for Fe(2+)/Fe(3+) (raised by 0.18 eV) and Co(2+)/Co(3+) (lowered by 0.26 eV), and increased activity for Mn(2+)/Mn(3+) with significantly reduced overpotential. We reveal that the favorable energy of transition metal mixing and the sequential redox reaction for each TM element with a sufficient redox gap is the underlying physical reason for the preferential single-phase mode of Li intercalation/deintercalation reaction in pyrophosphate, a general concept that can be applied to other multicomponent systems. Furthermore, an extremely small volume change of ~0.7% between the fully charged and discharged states and the significantly enhanced thermal stability are observed for the present material, the effects unseen in previous multicomponent battery materials.

Modified graphite and graphene electrodes for high-performance lithium ion hybrid capacitors

Lithium ion capacitors (LICs) have recently received considerable attention as a new class of energy storage system because they possess the combined advantages of lithium ion batteries and supercapacitors. LICs typically consist of activated carbon cathodes and pre-lithiated graphite anodes. Despite the promising electrochemical performance, most LICs still hold room for further improvement in terms of power density, which is largely related to the limited (de)intercalation kinetics of graphite. In an attempt to address these limited kinetics, we have developed a simple treatment to modify the morphology and surface characteristics of graphite engaging hydrogen peroxide. The treatment increases the exposed edge planes and generates more stable solid-electrolyte-interphase layers, which facilitate substantially improved power and cycling performance of the graphite anodes. Especially when integrated with a urea-reduced graphene cathode, the modified graphite-based LIC exhibits significantly higher energy and power densities compared to those of the pristine graphite-based and other reported counterparts.

A Carbon Nanotubes-Silicon Nanoparticles Network for High Performance Lithium Rechargeable Battery Anodes

ABSTRACT: As an effort to address the chronic capacity fading of Si anodes and thus achieve their robustcycling performance, herein, we develop a unique electrode in which silicon nanoparticles areembedded in the carbon nanotubes network. Utilizing robust contacts between silicon nanoparticlesand carbon nanotubes, the composite electrodes exhibit excellent electrochemical performance :95.5% capacity retention after 140 cycles as well as rate capability such that at the C-rate increasefrom 0.1C to 1C to 10C, the specific capacities of 850, 698, and 312 mAh/g are obtained, respec-tively. The present investigation suggests a useful design principle for silicon as well as other highcapacity alloying electrodes that undergo large volume expansions during battery operations.Keywords : Silicon, Carbon nanotube, Chemical vapor deposition, Lithium ion battery, Anode. Received August 22, 2012 : Accepted September 30, 2012 1. IntroductionAs the demand on lithium ion batteries (LIBs) withhigh energy densities is rapidly increasing to enableemerging large-scale applications such as electricalvehicles and stationary grid storage systems,