Investigation and Improvement of the Pressure Control in Si Anode Based Solid-State Batteries

Abstract

Solid state batteries (SSBs) are promising next generation propulsion for electric vehicles due to their improved safety, high gravimetric and volumetric energy densities, and increased reliability. Silicon is one of the limited choices for the anode material for the SSBs due to its high applicable capacity, low operating potential and natural abundance. However, significant challenges remain for the commercialization of Si anode based SSBs, including the insufficient operational and calendar life, requirement of high stack pressure for operation, and inadequate fast charge capability.Almost each of the technical challenges associated with Si anode is entangled with the volume change of Si during charging and discharging. Si possesses high lithium storage capacity because it can form various LixSiy compounds upon lithiation. For example, when 3.75 moles of Li+ is added to 1 mole of Si, the compound Li15Si4 is formed, which gives a capacity of 3578 mAh/gSi. What accompanies this is a considerable volume expansion (e.g., 280% increase for Li15Si4), resulting from the reorganization of the compound crystals. Assuming an anode that initially composes of perfectly compacted Si particles, this volume expansion translates to a 56% increase in the thickness of the Si anode. In a practical SSB, the real thickness variation in the Si anode is expected to be smaller because of the architecture design of the anode. Nonetheless, this volume change in Si creates many problems, including the Si particle pulverization, delamination of electrode from the current collector and electrolyte separator, loss of electronic and ion conductions in the electrode, among others. To maintain the electric contact in Si anode based SSBs, high stack pressures, from several MPa to several tens of MPa, are normally applied.In this work, we carefully measure the real-time stack pressure in a specially designed setup. The stress-strain characteristics of Si anode based SSBs during operation, including the stack pressure spike and the degradation of the average stack pressure, is studied and correlated with the Si anode structure changes. Furthermore, different measures, including external and internal ones, are explored to improve the cell pressure control. The external measures include selecting an appropriate spring and using a mechanical buffer layer, while the internal measure refers to an elastic current collector design. As a result, a better SSB pressure control is obtained. More importantly, a reduction of the stack pressure significantly below 10 MPa without compromising the cell performance is achieved.