In-Situ Nanomechanical Measurement of High-Capacity Battery Materials Using MEMS Cantilevers

This paper investigates a bilayer cantilever for a sensitive and quantitative measurement of the mechanical stress of high-capacity battery materials during electrochemical cycles. Silicon is a promising candidate for anode materials thanks to its high specific capacity (4200 mAh/g). One of the main challenges for silicon anode is the large volume expansion (~400%), causing material degradation and pulverization of the electrode [1-2]. In-situ characterization of stress evolution is critical because it enables the development of nanostructures and nanocomposites in order to enhance mechanical integrity [3-4].

 As shown in Fig 1, the bilayer cantilever consists of an active material and copper. Copper is used as a mechanical structure because of its good mechanical properties as well as its popular use as a current collector in anode. During lithiation, where lithium ions are inserted in silicon, the anode is subjected to compressive stress and the cantilever curls up. Similarly, the anode experiences tensile stress during delithiation and the cantilever curls down. The bending of a bilayer cantilever is described by a Timoshenko beam theory. The active material (250nm α-Si) was used as a working electrode, and faced lithium metal (as a counter and a reference electrode). A separator was placed between two electrodes in order to prevent an electrically short circuit. Electrolytes (1 mol LiPF₆in ethylene carbonate and dimethyl carbonate with 1:1 ratio) were filled in the chamber. A white light interferometry was used to determine the deformation of the cantilever during electrochemical cycles that was controlled by a potentiostat. Figure 2 shows the voltage vs. time and a corresponding stress profile during the first five cycles. Interestingly, during the first lithiation, the stress did not increase rapidly when the voltage was larger than 300mV, indicating the formation of solid-electrolyte interfaces (SEI). SEI was a result of the decomposition of electrolytes. When the voltage was smaller than 300mV, compressive stress reached -0.3GPa. Then the compressive stress slightly decreased at a lower voltage, probably due to the material cracking. The tensile stress appeared during delithiation. As we observed, the anode material experienced a dynamic change of stress during the first five cycles. Figure 3 shows the surface morphology of silicon anode before and after cycling. Silicon was initially uniformly coated, but many cracks were generated during electrochemical cycles due to mechanical stress.

 

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