Jung-Gu Han

Recent advances in the electrolytes for interfacial stability of high-voltage cathodes in lithium-ion batteries

Advanced electrolytes with unique functions such as in situ formation of a stable artificial solid electrolyte interphase (SEI) layer on the anode and the cathode, and the improvement in oxidation stability of the electrolyte have recently gained recognition as a promising means for highly reliable lithium-ion batteries with high energy density. In this review, we describe several challenges for the cathode (spinel lithium manganese oxide (LMO), lithium cobalt oxide (LCO), lithium nickel cobalt manganese oxide (NCM), spinel lithium manganese nickel oxide (LNMO), and lithium-rich layered oxide (Li-rich cathode))-electrolyte interfaces and highlight the recent progress in the use of oxidative additives and high-voltage solvents in high-performance cells.

A multifunctional phosphite-containing electrolyte for 5 V-class LiNi0.5Mn1.5O4 cathodes with superior electrochemical performance

We report a highly promising organophosphorus compound with an organic substituent, tris(trimethylsilyl)phosphite (TMSP), to improve the electrochemical performance of 5 V-class LiNi0.5Mn1.5O4 cathode materials. Our investigation reveals that TMSP alleviates the decomposition of LiPF6 by hydrolysis, effectively eliminates HF promoting Mn/Ni dissolution from the cathode, and forms a protective layer on the cathode surface against severe electrolyte decomposition at high voltages. Remarkable improvements in the cycling stability and rate capability of high voltage cathodes were achieved in the TMSP-containing electrolyte. After 100 cycles at 60 °C, the discharge capacity retention was 73% in the baseline electrolyte, whereas the TMSP-added electrolyte maintained 90% of its initial discharge capacity. In addition, the LiNi0.5Mn1.5O4 cathode with TMSP delivers a superior discharge capacity of 105 mA h g−1 at a high rate of 3 C and an excellent capacity retention of 81% with a high coulombic efficiency of over 99.6% is exhibited for a graphite/LiNi0.5Mn1.5O4 full cell after 100 cycles at 30 °C.

Tunable and Robust Phosphite-Derived Surface Film to Protect Lithium-Rich Cathodes in Lithium-Ion Batteries

A thin, uniform, and highly stable protective layer tailored using tris(trimethylsilyl) phosphite (TMSP) with a high tendency to donate electrons is formed on the Li-rich layered cathode, Li1.17Ni0.17Mn0.5Co0.17O2. This approach inhibits severe electrolyte decomposition at high operating voltages during cycling and dramatically improves the interfacial stability of the cathode. The TMSP additive in the LiPF6-based electrolyte is found to preferentially eliminate HF, which promotes the dissolution of metal ions from the cathode. Our investigation revealed that the TMSP-derived surface layer can overcome the significant capacity fading of the Li-rich cathode by structural instability ascribed to an irreversible phase transformation from layered to spinel-like structures. Moreover, the superior rate capability of the Li-rich cathode is achieved because the TMSP-originated surface layer allows facile charge transport at high C rates for the lithiation process.

Foldable Electrode Architectures Based on Silver-Nanowire-Wound or Carbon-Nanotube-Webbed Micrometer-Scale Fibers of Polyethylene Terephthalate Mats for Flexible Lithium-Ion Batteries

A crumply and highly flexible lithium-ion battery is realized by using microfiber mat electrodes in which the microfibers are wound or webbed with conductive nanowires. This electrode architecture guarantees extraordinary mechanical durability without any increase in resistance after folding 1000 times. Its areal energy density is easily controllable by the number of folded stacks of a piece of the electrode mat. Deformable lithium-ion batteries of lithium iron phosphate as cathode and lithium titanium oxide as anode at high areal capacity (3.2 mAh cm-2 ) are successfully operated without structural failure and performance loss, even after repeated crumpling and folding during charging and discharging.

Unsymmetrical fluorinated malonatoborate as an amphoteric additive for high-energy-density lithium-ion batteries

High-capacity Si-embedded anodes and Li-rich cathodes are considered key compartments for post lithium-ion batteries with high energy densities. However, the significant volume changes of Si and the irreversible phase transformation of Li-rich cathodes prevent their practical application. Here we report lithium fluoromalonato(difluoro)borate (LiFMDFB) as an unusual dual-function additive to resolve these structural instability issues of the electrodes. This molecularly engineered borate additive protects the Li-rich cathode by generating a stable cathode electrolyte interphase (CEI) while simultaneously tuning the fluoroethylene carbonate (FEC)-oriented solid electrolyte interphase (SEI) on the Si–graphite composite (SGC) anode. The complementary electrolyte design utilizing both LiFMDFB and FEC exhibited an improved capacity retention of 85%, a high Coulombic efficiency of ∼99.5%, and an excellent energy density of ∼400 W h kg−1 in Li-rich/SGC full cells at a practical mass loading after 100 cycles. This dual-function additive approach offers a way to develop electrolyte additives to build a more favorable SEI for high-capacity electrodes.

Understanding voltage decay in lithium-excess layered cathode materials through oxygen-centred structural arrangement

Lithium-excess 3d-transition-metal layered oxides (Li1+xNiyCozMn1−x−y−zO2, >250 mAh g−1) suffer from severe voltage decay upon cycling, which decreases energy density and hinders further research and development. Nevertheless, the lack of understanding on chemical and structural uniqueness of the material prevents the interpretation of internal degradation chemistry. Here, we discover a fundamental reason of the voltage decay phenomenon by comparing ordered and cation-disordered materials with a combination of X-ray absorption spectroscopy and transmission electron microscopy studies. The cation arrangement determines the transition metal-oxygen covalency and structural reversibility related to voltage decay. The identification of structural arrangement with de-lithiated oxygen-centred octahedron and interactions between octahedrons affecting the oxygen stability and transition metal mobility of layered oxide provides the insight into the degradation chemistry of cathode materials and a way to develop high-energy density electrodes.There is growing interest in the fundamental understanding of the voltage decay mechanism in Li-excess layered cathode materials. Here, the authors report a multilateral and macroscopic analysis that considers interaction between oxygen and atomic arrangement of Li1+xNiyCozMn1−x−y−zO2.

Zinc‐Reduced Mesoporous TiOx Li‐Ion Battery Anodes with Exceptional Rate Capability and Cycling Stability

We demonstrate a unique synthetic route for oxygen-deficient mesoporous TiOx by a redox-transmetalation process by using Zn metal as the reducing agent. The as-obtained materials have significantly enhanced electronic conductivity; 20 times higher than that of as-synthesized TiO2 material. Moreover, electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT) measurements are performed to validate the low charge carrier resistance of the oxygen-deficient TiOx . The resulting oxygen-deficient TiOx battery anode exhibits a high reversible capacity (∼180 mA h g-1 at a discharge/charge rate of 1 C/1 C after 400 cycles) and an excellent rate capability (∼90 mA h g-1 even at a rate of 10 C). Also, the full cell, which is coupled with a LiCoO2 cathode material, exhibits an outstanding rate capability (>75 mA h g-1 at a rate of 3.0 C) and maintains a reversible capacity of over 100 mA h g-1 at a discharge/charge of 1 C/1 C for 300 cycles.