Shedding X-ray Light on the Interfacial Electrochemistry of Silicon Anodes for Li-Ion Batteries

Electrochemical alloying reactions of group IV elements, such as Si, Ge, or Sn, with lithium provide a promising route to next-generation anode materials for lithium-ion batteries (LIBs) due to their high volumetric and gravimetric capacities. However, commercialization of these anodes is still sparse owing to quick capacity fading and limited Coulombic efficiency, which arise from large volume expansion leading to particle cracking and subsequent electrochemical inactivity. As a result, the solid electrolyte interphase (SEI), originating in the decomposition of the electrolyte upon battery operation outside the electrolyte's thermodynamic stability window, grows uncontrollably. While a large number of mitigation strategies have been developed, an improved nanometer level fundamental understanding of the (de)lithiation process and SEI formation, growth, and evolution is necessary to overcome these challenges. Toward this end, many experimental and theoretical approaches have been utilized but still provide an incomplete picture. This is due to the difficulty of investigating buried interfaces and interphases of lithiation products and thin SEI layers (nanometer-scale) in situ and with the desired nanometer accuracy. In this Account, we illustrate the utilization of in situ X-ray reflectivity (XRR) to provide nanometer-scale insights on the SEI nucleation, growth, and evolution, and well as the (de)lithiation process of Si electrodes. XRR is a nondestructive and surface-and interface-sensitive technique that allows for in situ investigations during battery operation under realistic electrochemical conditions. Insight into the system is provided via the surface-normal density profile, which is interpreted in terms of thickness, density, and roughness of individual surface layers, allowing monitoring of the interfacial morphology and chemistry evolution, through which the SEI growth and Si (de)lithiation process can be resolved. We utilized a model battery anode consisting of a native oxide terminated single crystalline Si wafer in half cell configuration with standard electrolyte in a specifically designed in situ XRR electrochemical cell. We have resolved the nucleation and formation process of the inner inorganic SEI and have observed two well-defined inorganic SEI layers on Si anodes: a bottom-SEI layer (adjacent to the electrode) formed via the lithiation of the native oxide and a top-SEI layer mainly consisting of the electrolyte decomposition product, LiF. This SEI layer grows during lithiation and contracts during delithiation. Further, our results show that the lithiation of crystalline Si (c-Si) is a layer-by-layer, reaction-limited, two-phase process with a well-defined phase boundary between LixSi lithiation product and c-Si; in contrast, the delithiation of LixSi and the lithiation of amorphous Si (a-Si) are reaction-limited, single-phase processes. Moreover, we resolved the influences of current density and the Si crystallographic orientation of the reaction interface on the (de)lithiation process. The implications of our findings are discussed with regard to battery performance.