In Operando Neutron Reflectometry Measurements Demonstrate Structural Stability in Thin Film Silicon Anodes for Lithium Ion Batteries

Despite its incredibly high theoretical capacity (~4200 mA-h/g), implementation of Si electrodes as high-capacity Li-battery anodes is hindered by poor mechanical durability.  Si lithiation is accompanied by very high mechanical strain values, with nearly 370% volume expansion at maximum capacity, which typically leads to fracture and pulverization of the material and eventual battery failure.  While numerous strategies have been employed to improve the durability of Si anodes, including nano-structured anode architectures, composite anode materials, and protective coatings, efforts have yet to identify a Si anode design that is mechanically robust and maintains a stable anode capacity.  At least part of the difficulty is due to a lack of fundamental insight into the correlation between structure, mechanical properties, and function for Si anodes, owing to a paucity of in situ techniques to definitively observe them.   

In this presentation, we report the structural and composition changes in an amorphous thin-film silicon anode with a protective Al sub-oxide (AlO_x) capping layer, using in operando neutron reflectivity (NR) and electronic impedance spectroscopy (EIS) measurements. NR measurements were carried out in situ on an operating Li half-cell at various charge states over six shallow charge-discharge cycles.  These measurements quantify the correlation between mesoscopic strain and the degree of lithiation in a 10-nm thick amorphous silicon thin-film anode, which expands and contracts with repeated cycling (Figure 1). 

Results demonstrate the mechanical durability of both the Si thin-film anode and the protective AlO_x layer, which maintain their low surface roughness and film integrity over the course of the measurements.  While the macroscopic strain of the Si shows significant non-linearity with Li content after several shallow charge-discharge cycles, in agreement with previous reports¹, careful inclusion of a-Si microstructural effects (via direct measurement of the a-Si porosity) reveal that the volumetric strain for the solid portion of a-Si thin films varies linearly with lithium content (Figure 2).  Results demonstrate that the porosity is eliminated during the expansion that accompanies lithiation, and is then recovered during subsequent de-lithiation.  Furthermore, it is shown here that the delithiated a-Si porosity does not vary significantly over the course of the measurements – ranging from 10.0 [5.4 – 28.0]% in the as-deposited state to 13.1 [10.02 – 15.73]% after 6 shallow charge and discharge cycles – demonstrating the structural reproducibility of the thin-film anode.

References: 

1.  Chevrier, V. L. and Dahn, J. R., J. Electrochem. Soc. 2009, 156 (6), A454-A458.