Lithiation of Silicon Electrodes Investigated by In-Operando Neutron Reflectometry

Kinetic processes and interface phenomena at electrodes during charging and discharging cycles play a key role for optimization of lithium-ion batteries (e.g. for charging times and power density). In order to study lithium insertion/de-insertion into electrodes and the underlying mechanisms, we applied neutron reflectometry as a non-destructive analysis technique. This method allows to study lithiation in-operando during cell operation. As negative electrode material, silicon is used a promising candidate for future Li-ion battery applications due to its high theoretical specific capacity of about 4 Ah/g.

The experiments are done using a three electrode electrochemical cell setup with a layered nanometer-sized amorphous silicon anode. The working electrode is deposited by magnetron sputtering on a quartz block covered by a current collector. Counter and reference electrode are made of pure lithium. A liquid electrolyte is used together with a housing made of high density polyethylene.

The experiments allow to monitor the modification of Li content and the corresponding volume expansion/contraction of the electrode during lithiation on the nanometer scale. The measurements indicate that during galvanostatic charging Li incorporation takes place in form of a process, where a moving Li_xSi phase boundary plays an important role. Possible lithiation mechanisms are discussed. Neutron reflectometry measurements during cyclic voltammetry allow to identify regions (U/I characteristics) where considerable lithium incorporation takes place and where not. For comparison, experiments on the lithiation of massive crystalline silicon blocks are shown.

B. Jerliu et al. Phys. Chem. Chem. Phys., 15, 7777, (2013)