Li-ion transport in Li-ion and solid state batteries,  direct observations from Neutron Depth Profiling and solid state Nuclear Magnetic Resonance

Marnix Wagemaker

Department of Radiation Science and Technology, Delft University of Technology

Abstract: Li-ion transport is vital in current and future batteries utilizing Li-ions as charge carriers, however hard to monitor on a microscopic level. Here we will present our recent progress in the use of Neutron Depth Profiling and Nuclear Magnetic Resonance.

Through the capture reaction of neutrons with 6Li Neutron Depth Profiling (NDP) is able to detect the Li-ion distribution in batteries with a resolution down to 5 to 50 nanometer [1]. This makes it possible to monitor the Li-ion transport in working Li-ion batteries. Results for several Li-ion battery electrode materials will be presented, giving insight in transport limitations and relaxation effects that determine the performance of Li-ion batteries.

Figure 1. Top: NDP principle. Bottom: Example experiment. During delithiation and lithiation of LiFePO4 electrodes the smaller particles transform first, illustrating how the Li-ion transport can be monitored with NDP.

Solid state NMR is used to unravel the Li-ion charge transport in solid state batteries utilizing argyrodite solid electrolytes, Li6PS5H (H=Cl,Br) [2], in combination with Li2S cathodes. The poor rate and cycle performance of solid state batteries is typically ascribed to a high internal resistance for Li-ion transfer over the electrode-electrolyte interfaces. The origin and quantification of the interfacial resistance has been difficult to ascertain experimentally and is expected to depend both on the electrode-electrolyte combination and preparation route. Exchange NMR is used to quantify the exchange current density between the solid electrolyte and electrode material for different cathode preparation steps and after cycling. This gives direct insight in how the interfacial resistance depends on preparation conditions and cycling, one of the major bottlenecks for all solid state batteries. 

Figure 2. Top: 7Li NMR of Li6PS5Br-Li2S cathode mixtures distinguishing the resonances of the Li6PS5Br solid electrolyte and the Li2S cathode. Bottom: 7Li NMR Exchange experiment measuring the spontaneous Li-ion exchange between the Li6PS5Br solid electrolyte and the Li2S cathode quantifying the exchange current density.

Biography: Marnix Wagemaker completed his MSc in Applied Physics at the Delft University of Technology in 1996. After working as  development engineer in computer aided techniques he completed his PhD at the Delft University of Technology on neutron, X-ray and NMR research of Li-ion battery materials. This was followed by a visiting scholarship at the Department of Materials Science and Engineering of the Massachusetts Institute of Technology (MIT) on DFT modeling and a national Post-Doc VENI Grant (2004) entitled “Lithium-ion dynamics in electrode materials” at the Delft University of Technology and a national VIDI Grant (2007) entitled “Storage and dynamics of Li-ions in nano-sized electrode materials. In 2012 he received an ERC-StG grant on “Hunting for high performance energy storage in batteries” and became associated professor at the Delft University of Technology. In 2017 he became head of the section Storage of Electrochemical Energy, the battery research group at the Delft University of Technology.

The research of Marnix Wagemaker aims at fundamental understanding and improvement of electrochemical energy storage processes in batteries. During the last five years a red line is the development and application of relatively new (operando) experimental approaches including Neutron Depth Profiling, solid state NMR and microbeam diffraction, in many cases combined with (ab-initio) simulations. Current research focusses on the fundamental processes in next generation Li-ion/Na-ion, solid state and Li-air batteries.