Won-Kyung Shin

Effective Suppression of Dendritic Lithium Growth Using an Ultrathin Coating of Nitrogen and Sulfur Codoped Graphene Nanosheets on Polymer Separator for Lithium Metal Batteries

The enhanced stability of lithium metal is vital to the development of high energy density lithium batteries due to its higher specific capacity and low redox potential. Herein, we demonstrate that nitrogen and sulfur codoped graphene (NSG) nanosheets coated on a polyethylene separator stabilized the lithium electrode in lithium metal batteries by effectively suppressing dendrite growth and maintaining a uniform ionic flux on the metal surface. The ultrathin layer of NSG nanosheets also improved the dimensional stability of the polymer separator at elevated temperatures. In addition, the enhanced interfacial interaction between the NSG-coated separator and lithium metal via electrostatic attraction released the surface tension of lithium metal and suppressed the initiation of dendrite growth on lithium metal. As a result, the electrochemical performance of a lithium metal cell composed of a LiNi0.8Co0.15Al0.05O2 positive electrode with an NSG-coated separator was remarkably improved as compared to the cell with an uncoated polyethylene separator.

Improvement of the Cycling Performance of LiNi0.6Co0.2Mn0.2O2 Cathode Active Materials by a Dual-Conductive Polymer Coating

LiNi0.6Co0.2Mn0.2O2 cathode materials were surface-modified by coating with a dual conductive poly(3,4-ethylenedioxythiophene)-co-poly(ethylene glycol) (PEDOT-co-PEG) copolymer, and their resulting electrochemical properties were investigated. The surface-modified LiNi0.6Co0.2Mn0.2O2 cathode material exhibited a high discharge capacity and good high rate performance due to enhanced transport of Li(+) ions as well as electrons. The presence of a protective conducting polymer layer formed on the cathode also suppressed the growth of a resistive layer and inhibited the dissolution of transition metals from the active cathode materials, which resulted in more stable cycling characteristics than the pristine LiNi0.6Co0.2Mn0.2O2 cathode material at 55 (o)C.

Improvement of the Cycling Performance and Thermal Stability of Lithium-Ion Cells by Double-Layer Coating of Cathode Materials with Al2O3 Nanoparticles and Conductive Polymer

We demonstrate the effectiveness of dual-layer coating of cathode active materials for improving the cycling performance and thermal stability of lithium-ion cells. Layered nickel-rich LiNi0.6Co0.2Mn0.2O2 cathode material was synthesized and double-layer coated with alumina nanoparticles and poly(3,4-ethylenedioxythiophene)-co-poly(ethylene glycol). The lithium-ion cells assembled with a graphite negative electrode and a double-layer-coated LiNi0.6Co0.2Mn0.2O2 positive electrode exhibited high discharge capacity, good cycling stability, and improved rate capability. The protective double layer formed on the surface of LiNi0.6Co0.2Mn0.2O2 materials effectively inhibited the dissolution of Ni, Co, and Mn metals from cathode active materials and improved thermal stability by suppressing direct contact between electrolyte solution and delithiated Li(1-x)Ni0.6Co0.2Mn0.2O2 materials. This effective design strategy can be adopted to enhance the cycling performance and thermal stability of other layered nickel-rich cathode materials used in lithium-ion batteries.

Study on the cycling performance of LiNi0.5Mn1.5O4 electrodes modified by reactive SiO2 nanoparticles

We demonstrate a facile approach to improve the cycling stability of spinel LiNi0.5Mn1.5O4 materials by their surface modification. The cross-linked composite polymer electrolyte layer was formed on the surface of LiNi0.5Mn1.5O4 by radical polymerization between diethylene glycol diacrylate and SiO2 nanoparticles with reactive vinyl groups. The protective composite polymer layer formed on the LiNi0.5Mn1.5O4 materials suppressed the irreversible decomposition of the electrolyte at high voltages and reduced the dissolution of transition metals from the charged LiNi0.5Mn1.5O4 electrode into the electrolyte at elevated temperature, which resulted in more stable cycling characteristics than the pristine LiNi0.5Mn1.5O4 electrode.

Cross-linked Composite Gel Polymer Electrolyte using Mesoporous Methacrylate-Functionalized SiO2 Nanoparticles for Lithium-Ion Polymer Batteries.

Liquid electrolytes composed of lithium salt in a mixture of organic solvents have been widely used for lithium-ion batteries. However, the high flammability of the organic solvents can lead to thermal runaway and explosions if the system is accidentally subjected to a short circuit or experiences local overheating. In this work, a cross-linked composite gel polymer electrolyte was prepared and applied to lithium-ion polymer cells as a safer and more reliable electrolyte. Mesoporous SiO2 nanoparticles containing reactive methacrylate groups as cross-linking sites were synthesized and dispersed into the fibrous polyacrylonitrile membrane. They directly reacted with gel electrolyte precursors containing tri(ethylene glycol) diacrylate, resulting in the formation of a cross-linked composite gel polymer electrolyte with high ionic conductivity and favorable interfacial characteristics. The mesoporous SiO2 particles also served as HF scavengers to reduce the HF content in the electrolyte at high temperature. As a result, the cycling performance of the lithium-ion polymer cells with cross-linked composite gel polymer electrolytes employing methacrylate-functionalized mesoporous SiO2 nanoparticles was remarkably improved at elevated temperatures.

Cycling performance of lithium-ion polymer cells assembled with a cross-linked composite polymer electrolyte using a fibrous polyacrylonitrile membrane and vinyl-functionalized SiO2 nanoparticles

Vinyl-functionalized SiO2 nanoparticles were synthesized and uniformly dispersed on the surface of a fibrous polyacrylonitrile (PAN) membrane for use as cross-linking sites. A composite polymer electrolyte was prepared by in situ cross-linking between vinyl-functionalized SiO2 particles on the PAN membrane and the electrolyte precursor containing tri(ethylene glycol) diacrylate. The cross-linked composite polymer electrolyte effectively encapsulated the electrolyte solution without leakage. It exhibited good thermal stability as well as favorable interfacial characteristics toward electrodes. Lithium-ion polymer cells composed of a graphite negative electrode and a LiNi0.8Co0.15Al0.05O2 positive electrode were assembled with the in situ cross-linked composite polymer electrolyte. The cells with cross-linked composite polymer electrolytes using the fibrous PAN membrane and vinyl-functionalized SiO2 particles exhibited high discharge capacity and good capacity retention at both ambient temperature and elevated temperature.

High performance composite polymer electrolytes for lithium-ion polymer cells composed of a graphite negative electrode and LiFePO4 positive electrode

Core–shell structured SiO2 particles with different core diameters were synthesized by radical polymerization of 4-styrenesulfonic acid sodium salt with vinyl-functionalized SiO2 core particles and were used as Li+ ion-conducting fillers in composite polymer electrolytes. Composite polymer electrolytes prepared with core–shell SiO2 particles exhibited high ionic conductivity exceeding 10−3 S cm−1 at room temperature and good mechanical properties, allowing the preparation of a free-standing film with a thickness of 30 μm. Lithium-ion polymer cells composed of graphite negative electrode, composite polymer electrolyte and LiFePO4 positive electrode were assembled, and their cycling performance was evaluated. Cells assembled with a composite polymer electrolyte containing core–shell SiO2 particles with a core diameter of 250 nm exhibited good cycling performance in terms of discharge capacity, capacity retention and rate capability.

Electrochemical Characterization of Electric Double Layer Capacitors Assembled with Pyrrolidinium-Based Ionic Liquid Electrolytes

We present the electrochemical performance of electric double layer capacitors (EDLCs) assembled with pyrrolidinium (Pyr)-based ionic liquid electrolytes at 55 C. Cations with various alkyl chain lengths were employed in Pyr-based ionic liquids to investigate the effect of cation structure on the cycling stability of EDLCs. The EDLCs exhibited initial specific capacitances ranging from 122.4 to 131.6 F g− based on activated carbon material at 55 C. Cycling data and XPS results demonstrate that Pyr-based ionic liquid with longer alkyl chain is more effective for enhancing the cycling stability of EDLC by suppressing the reductive decomposition of pyrrolidinium cations during cycling at high temperatures.

Trifluoropropyltrimethoxysilane as an Electrolyte Additive to Enhance the Cycling Performances of Lithium-Ion Cells

본 연구에서는 불소계 실란을 첨가제로 사용하여 전해액의 열화 반응을 억제함으로써 리튬이온전지의 싸이클 특성을 향상시키고자 하였다. 첨가제로 사용된 trifluoropropyltrimethoxysilane은 리튬염과 카보네이트계 유기 용매로 이루어진 액체 전해질보다 전기화학적 산화, 환원 분해반응이먼저 일어나 음극 및 양극 표면에서 안정적인 고체전해질계면 (solid electrolyte interphase,SEI) 막을 형성하며, 5 wt.%의 첨가제를 포함하는 경우 가장 우수한 전기화학적 특성을 나타내었다. SEM 및 XPS 분석을 통해 전극 표면에 생성된 피막의 화학 성분을 분석하였으며, 이들결과로부터 새로운 SEI 형성 첨가제로서 불소계 실란의 가능성을 확인하였다.Abstract: In this study, we tried to improve the cycling performance of lithium-ion batteries bysuppressing decomposition of the electrolyte solution containing fluorsilane-based additive. Triflu-oropropyltrimethoxysilane was electrochemically oxidized and reduced prior to the decompositionof the liquid electrolyte composed of lithium salt and carbonate-based organic solvent. Thus, thestable solid electrolyte interphase (SEI) layer on both negative electrode and positive electrode wasformed, and it was confirmed that the cycling performance of lithium-ion batteries assembled withelectrolyte solution containing 5 wt.% trifluoropropyltrimethoxysilane was the mostly enhanced. Theproducts formed on electrodes were analyzed by the SEM and XPS analysis, and it was demon-strated that trifluoropropyltrimethoxysilane can be one of the promising SEI-forming additives.Keywords : Trifluoropropyltrimethoxysilane, Solid electrolyte interphase, SEI-forming additive, Liq-uid electrolyte, Lithium-ion battery