Roland Jung

Transition metal dissolution and deposition in Li-ion batteries investigated by operando X-ray absorption spectroscopy

In Li-ion batteries the dissolution of transition metals from the cathode and their subsequent deposition on the anode are known to contribute to capacity fading. In this study, we investigate these processes using an NMC cathode and a graphite anode under operating conditions using X-ray absorption spectroscopy. The experiments are carried out in an operando cell, which allows both the time/voltage and spatially resolved determination of metal concentration and oxidation state of transition metal deposits on the graphite electrode. NMC shows a strong increase of the metal dissolution rate, if the upper cut off potential exceeds 4.6 V. Under operating conditions, the oxidation state of manganese, cobalt and nickel are found to be always +2 both on lithiated and delithiated graphite. In contrast, manganese is found to be present in the metallic state on lithiated graphite in the ex situ analysis, thus highlighting the importance of the operando characterization.

Chemical versus Electrochemical Electrolyte Oxidation on NMC111, NMC622, NMC811, LNMO, and Conductive Carbon

We compare the stability of alkyl carbonate electrolyte on NMC111, -622, and -811, LNMO, and conductive carbon electrodes. We prove that CO2 and CO evolution onset potentials depend on the electrode material and increase in the order NMC811 < NMC111 ≈ NMC622 < conductive carbon ≈ LNMO, which we rationalize by two fundamentally different oxidation mechanisms, the chemical and the electrochemical electrolyte oxidation. Additionally, in contrast to the widespread understanding that transition metals in cathode active materials catalyze the electrolyte oxidation, we will prove that such a catalytic effect on the electrochemical electrolyte oxidation does not exist.

A wet-chemical route for macroporous inverse opal Ge anodes for lithium ion batteries with high capacity retention

Germanium holds great potential as an anode material for lithium ion batteries due to its high specific capacity and its favorable properties such as good lithium ion diffusivity and electronic conductivity. However, the high cost of germanium and large volume changes during cycling, which lead to a rapid capacity fading for bulk Ge materials, demand for nanostructured thin film devices. Herein we report the preparation and electrochemical properties of thin films of porous, inverse opal structured Ge anodes obtained via a simple, up-scalable wet-chemical route utilizing [Ge9]4− Zintl ions. In the absence of conductive additives, they show high initial capacities of >1300 mA h g−1 and promisingly high coulombic efficiencies of up to 99.3% and deliver over 73% of their initial capacity after 100 cycles when cycled vs. metallic lithium. In contrast to many other porous structured Ge electrodes, they show very little to almost no capacity fading after an initial drop, which makes them promising candidates for long life applications.