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Materials Synthesis

Research Topics
Fig. 1. Schema of layered structures P2 and O3, where purple octahedra – 3d-metal polyhedrons, yellow octahedra and prisms – Na polyhedrons
Fig. 2. Na3Ni2BiO6 honeycomb structure, where Bi blue octahedra are surrounded by Ni yellow octahedra. Na ions are situated between 3d-metal layers.
Fig 3. SEM images of the commercial TiO2 anantase (a) and self-prepared TiO2(B) (b)
Fig 3. SEM images of the commercial TiO2 anantase (a) and self-prepared TiO2(B) (b)
Fig 4. SEM images of obtained V2O5 oxides with different morphology
Fig. 5 - Schematic of a Li-ion capacitor based on bi-material electrodes

In the Material Synthesis group, we are developing novel advanced functional materials for lithium and post-lithium ion energy storage systems

Beside the affirmed Li-ion batteries (LIBs), the research community is looking to other appealing technologies which can decrease the cost and safety issues of LIBs.
Na, K and Mg are more abundant than Li in the earth crust and they can represent a valid solution to replace the existing energy storage technologies.
In our group we focus on novel materials for Na-ion batteries, hybrid Li+/Mg2+ batteries, K-ion batteries and hybrid capacitors.



Cathode materials for Na-ion batteries

In the recent past, sodium-ion batteries (NIBs) have attracted tremendous attention from the battery community, particularly due to the economic and abundant nature of Na over Li. Due to the overall low cost nature of NIBs, they could be employed as post-lithium energy storage systems particularly for large-scale energy storage applications.
However, many efforts are still required for developing novel electrode materials and improving existing ones to optimize the NIBs technology to further make it suitable for commercialization. As the capacity delivered by a cell is mostly limited by the positive electrode in a NIB, it is essential to focus on developing and improving the cathode materials. Layered oxide cathode materials are particularly attractive due to their good rate capabilities and specific capacities. The layered oxide cathode materials exhibit different structural features such as Na+/Vacancy ordering, or honeycomb ordering, depending on the nature of the cations present and their composition. In our research we are dealing with:

• compositions which are isostructural to layered P2-type Na2/3Fe1/2Mn1/2O2 and O3-type NaFe1/2Mn1/2O2 [Fig.1]. The 3d-metal cation substitution is applied to obtain new compositions like Na2/3Ni1/6Cu1/6Mn2/3O2 and Na2/3Ni1/3Mn1/2Ti1/6O2. Substituted compositions show improved electrochemical performance with better cycling stability.
• O3 honeycomb-ordered compositions Na3M2BiO6 and Na3M2SbO6 with M = Ni, Cr, Cu, Co, etc. [Fig.2]. It is interesting to explore the possibility of substitution of Bi5+ with smaller and lighter Nb5+, V5+ ions in this type of structure. Also combination of two-valence ions can change the ordering in the 3d-metal plains and hence amend the electrochemistry. Materials with high ordering display better electrochemical performance.



Cathode and anode materials for Li+/Mg2+ hybrid batteries

Mg-ion batteries (MIBs) recently have attracted extensive attentions worldwide because of their notable advantages, such as Mg dendritic-free, abundant sources of Magnesium and easy handling compared with Li-ion batteries. However, the sluggish Mg-ion diffusion in electrode materials and the lack of suitable cathode materials and electrolytes hinder the development and the commercialization of magnesium-ion batteries. This problem can be overcome by using the principle of the hybrid batteries, where the cathode represents a lithium-insertion host, the anode is metallic magnesium and the electrolyte contains both Li+/Mg2+ (hybrid electrolyte). During the electrochemical processes, Li+ intercalation/de-intercalation dominates in the cathode side and only Mg deposition/dissolution occurs on the anode side. In this hybrid system, the usage of Mg anode reduces the risk of internal short-circuit caused by dendrites. Many Li+ host materials, such as MoS2, Li4Ti5O12, TiS2, and Mo6S8 have been used in Li/Mg-ion hybrid batteries (LMIBs), which showed much improved electrochemical performance compared to conventional MIBs.

Some of the materials under investigation in our group are:
• different modification of TiO2 [Fig.3], as electrode materials for hybrid Li+/Mg2+ battery, characterization of structure and electrochemical mechanism.

• V2O5 synthesized with different nanoparticle morphology, using hydrothermal method with various precursors [Fig.4]. We are making correlations between particle shape and electrochemical performance. In addition, we perform doping of V2O5 with M cations having different oxidation states MxV2-xO5 (M=Cu, Ni, Co, Fe, Ti, Mn) x=0.02, 0.04, 0.06, x=0.1, 0.2.



Materials for K-ion batteries

Potassium batteries have recently drawn interest as novel rechargeable batteries. Some properties are very appealing for using K instead of Na and Li. One advantage is the low standard potential of K/K+ (even lower than lithium in organic electrolytes) which can offer an improvement in energy. Moreover, the weaker desolvation energy of K+ compared to the other two metal ions relays in a faster transport number and in an increased kinetic, thus allowing to achieve high power (promising for supercapacitor applications). Regarding the electrode materials for K-ion intercalation/deintercalation, some progresses have been reached on the anode side. Graphite can be potassiated and de-potassiated at relatively high C-rate, being very promising for capacitor applications. However, respect to the cathode side, only very few studies have been reported in organic electrolytes. In our group we are studying various structures as intercalation guests for K-ions. Different structures having large interstitial channels and voids able to accommodate the large K-ion are currently under development.



Hybrid and asymmetric supercapacitors

To meet growing demands for electric automotive and regenerative energy storage applications, researchers all over the world have sought to increase the power density of batteries and energy density of electrochemical capacitors. Hybridizing battery capacitor electrodes can overcome the energy density limitation of the conventional electrochemical capacitors because they employ both the system of a battery-like (redox) and a capacitor-like (double-layer) electrode, producing a larger working voltage and capacitance. Our goal is to hybridize materials at molecular level by incorporating the battery-like material with a carbonaceous capacitor-type material. [Fig.5]

Group Members
Name Title Phone E-Mail
M.Sc. +49 721 608 41445 georg boschExh7∂kit edu
Dr. +49 6151 16 21026 cordula braunRsz1∂kit edu
Dr. +49 721 608 41445 qiang fuOgz8∂kit edu
M.Sc. +49 721 608 41445 chengping liKmp6∂kit edu
+49 721 608 41448 marcus mayerEtb6∂kit edu
M.Sc. +49 721 608 41445 kristina pfeiferWca9∂kit edu
Dr. +49 721 608 41915 angelina sarapulovaCsm3∂kit edu
M.Sc. +49 721 608 41445 zijian zhao2Jjv7∂partner kit edu