讲座题目1：协同设计高压锂离子电池正极材料
Title：Synergistic design principles of cathode materials based on various complexation methods toward high voltage Li ion battery
报告人：Yong-Mook Kang（Professor, Korea University, Korea）
时间及地点：2020年10月31日8:00-12:00
报告地点：https://meeting.tencent.com/s/3hVGrPPf6E6i会议ID：914 739 381
Abstract:Lithium ion battery has been rapidly developed for the applications in portable devices, electric vehicles (EV), and energy storage system (ESS) due to their high energy density and durable cycle life. From the practical viewpoint of electrode, high voltage charging and fast kinetics are essential to realize not only high power density but also high energy density for EV or ESS. However, it is quite challenging to improve both properties at the same time, especially for cathode materials. Thus, no matter what we develop for new cathode material, there have been severe limitations. Therefore, we devised a typical but synergistic way to fabricate a superb hybrid material in which the drawbacks of one material are completely covered by the advantages of the other material.
In this synergistic way, we could successfully enhance the kinetic properties of Li3V2PO4 that is one of the representative high voltage cathode materials for Li ion battery, and utilize a conventional cathode material, LiCoO2, above 4.3 V. The detailed contents are as follows.
Phosphate (PO4)3--based materials have been attracting tremendous interests due to their competitive energy density and remarkable thermal stability. Among these phosphates, monoclinic Li3V2(PO4)3 (LVP) was proposed as a highly promising cathode material, owing to its high operating voltage and very high theoretical capacity of 197 mAh g-1 when all three lithium ions are extracted/inserted between 3.0 and 4.8 V. In particular, its three-dimensional structure framework consisting of slightly distorted VO6 octahedral and PO4 tetrahedral sharing oxygen vertex provides large interstitial space that makes lithium ions capable of moving fast inside the structure [2]. However, just like the other metal phosphates, it shows a limited electron conductivity (10-8 - 10-9 S cm-1), which makes LVP to be difficult for practical applications [2]. Here, we present a sagacious design of LVP/carbon nanofiber with a distinctive morphology where LVP nanoparticles are anchored on the surface of carbon nanofiber. In general carbon-coated LVP 1D materials, there has been a problem that the carbon coating layer additionally interferes with lithium ion diffusion into LVP bulk. However, in our structure, LVP particles can directly contact with electrolytes leading to an improved lithium ion conductivity. Furthermore, carbon nanofiber can provide fast electron transport along its 1D pathway. As a result, it was demonstrated that this unique structure is favorable for simultaneously maximizing Li ion conductivity and electronic conductivity for great electrochemical improvement of LVP materials.
LiCoO2 has the hexagonal α-NaFeO2 phase consisting of the layered rock-salt structure with the order of Li+ and Co3+ on alternating (111) planes in its cubic structure. When a Li/Li1-xCoxO2 cell is typically charged within limited composition range (0 < x < 0.5, 4.2v), it shows reasonably good capacity retention. however, the discharge capacity under the cut-off voltage of 4.2 v is around only 140 mah/g, which is much lower than the theoretical value (274 mah/g) of licoo2. unfortunately, the practical use of licoo2 has been limited because its stability could be rapidly deteriorated at potentials higher than 4.2 v. some research groups have reported that the poor cycling performance above 4.2 v is caused by structural instability induced by a phase transition from hexagonal phase to a monoclinic phase, which accompanies a volume change of ~2.6 % along the c-axis. to overcome the above problems, many researchers have developed lots of surface modifications that can improve the structural stability of licoo2. in this study, we tried to control the interlayer distance variation (lattice parameter c) though the substitution of phosphorus for li+ sites by a phosphidation process. as a result, the unwanted phase transition could be suppressed by the existence of po4 framework formed on the surface of licoo2 dramatically improving the cycleability and rate capability of licoo2 even above 4.5 v. using this approach, we could easily change the surface o2-framework of licoo2 to po4-framework. consequently, phosphidated licoo2 came to retain very high structural stability and a stable surface film was formed in contact with electrolyte during charging/discharging. phosphidated licoo2 exhibited greater bulk and surface stability compared to pristine licoo2 even in high voltage range, suggesting that this methodology will be also promising for other high-voltage cathode materials in lithium ion batteries.
讲座题目2：合理设计锰基层状氧化物钠离子电池正极材料
Title：Rational Design of Manganese Based Layered Oxides for Sodium Ion Batteries
报告人：Yong-Mook Kang（Professor, Korea University, Korea）
时间及地点：2020年10月31日13:00-17:00
报告地点：https://meeting.tencent.com/s/3hVGrPPf6E6i会议ID：914 739 381
Abstract: Phase transformation of layered structure into spinel structure has been detrimental for most of layered oxide cathodes.1Even if a lot of efforts have been made to relieve this highly irreversible phase transformation, there have been few successful results. However, we firstly observed the possibility to make this irreversible phase transformation extremely reversible by utilizing Na-birnessite (NaxMnO2•yH2O; Na-bir) as a basic structural unit, which has distinctive layered structure containing crystal water. Herein, the crystal water in the structure contributes to generating metastable spinel-like phase, which is the key factor for making this unusual reversibility happen.2The reversible structural rearrangement between layered and spinel-like phases during electrochemical reaction could activate new cation sites and enhance ion diffusion with higher structural stability. This unprecedented reversible phase transformation between spinel and layered structure was deeply analyzed via combinedex situsoft and hard X-ray absorption spectroscopy (XAS) analysis within situX-ray diffraction (XRD). Fundamental mechanism on this reversible phase transformation was theoretically elucidated and confirmed by kinetic investigation using first-principle calculation. These results provide deep insight into novel class of intercalating materials which can deal with highly reversible framework changes, and thus it can pave an innovative way for the development of cathode materials for next-generation rechargeable batteries.
Biography of Speaker:Yong-Mook Kang completed his B.S. (1999), M.S. (2001), and Ph.D. (2004) in Korea Advanced Institute of Science and Technology. He has been a senior researcher in Samsung SDI Co., LTD and a professor at Department of Energy and Materials Engineering in Dongguk University. He is now a full professor at Department of Materials Science and Engineering in Korea University. His research area covers electrode or catalyst materials for Li rechargeable batteries and various post Li batteries, such as Li-air battery, Na rechargeable battery and so on. To date, he has co-authored more than 150 refereed journal articles, more than 50 domestic or international patents, several articles in books or proceedings, and a textbook of nano-science and electrochemical devices. For his research achievements in energy conversion & storage materials, he was appointed as a TWAS(Academy of Science for Developing Worlds) Young Affiliate for the first time in South Korea, and awarded the International Collaboration Award of Australian Research Council-2010. From 2015, he has been appointed as a RSC (Royal Society of Chemistry) fellow & representative of Korea. From 2020, he has been appointed as a member of Y(Young)-KAST(The Korean Academy of Science and Technology).
