Uncovering Unique Interfacial Properties in Different Lithium Fluoride Phases: A First-Principles Prediction
Authors: Basu Swastik , Hwang Gyeong S. 
Published: July 2022

Electrolytes are susceptible to reductive decomposition on the surface of negative electrodes leading to the formation and growth of solid-electrolyte interphase (SEI) layer [1]. Stable SEI can be beneficial as a protective layer, given that it can provide insulation to electron transport from the anode to electrolyte, prevent solvent molecules from reaching the anode, and at the same time allow transport of Li+ [2]. Thus, the SEI can contribute critically to the safety and operability of lithium-ion batteries (LIBs), but its functionality heavily depends on the conditions under which it gets synthesized [3]. This can have a significant effect on battery performance. For instance, nucleation and growth of lithium dendrites during cycling is known to result in continuous electrolyte degradation, destruction of the SEI, agglomeration of so-called “dead lithium”, and short-circuiting of the battery [4]. This greatly affects next-generation electric vehicles (EVs), for which all-solid-state lithium-metal batteries (ASSLMBs) have garnered significant attention due to their superior energy storage capacity and safety over LIBs [5]. However, the considerable interfacial impedance originating from poor physical contact and/or parasitic reactions at the Li/SSE interface hinders the development of ASSLMBs. Alternative approaches towards achieving a stable protective interface have been pursued: like use of electrolyte additives to manipulate the constituents and compositions of SEI or designing artificial protective layers to improve performance. For instance, admixture of optimum amounts of fluorine-rich additives like fluoroethylene carbonate (FEC) to traditional carbonate electrolytes (like EC/DMC) has been seen to generate a robust LiF-rich SEI. However, the intrinsic role of LiF remains a topic of uncertainty. Conventional understanding would posit that dominance of LiF leaves the SEI susceptible to poor lithium-ion transport properties. Previously, theoretical works reported significantly lower ionic conductivities in crystalline LiF when compared to other common inorganic SEI materials like Li2O and Li2CO3. Underwhelming transport properties pose serious rate limitations in its effectiveness as a serious candidate for interfacial protection in ASSLMBs. To potentially overcome such bottleneck, this study systematically investigates different phases of LiF: their structures, stabilities and interfacial properties are described using first principles calculations. Careful analysis of the structural models reveals that unlike the widely studied rock salt ordered counterpart, certain phases of LiF exhibit excellent Li+ transport properties with a high predicted diffusivity at room temperature. Mechanically too, improved interfacial qualities are demonstrated with increased flexibility and fracture resistance, opening up important avenues for structural and compositional stability over cell cycling while maintaining its desirable electron-blocking characteristics. However, it is also important to take a step back and note that the rock salt phase is the most energetically stable among LiF phases, which thereby exhibit a propensity for phase transformation under ambient conditions. To overcome this, a strategy of incorporating hetero dopants as impurities to stabilize the host matrix is discussed. By increasing the dopant concentration up to an optimum amount, relative thermodynamic stability of the interface-friendly phases of LiF is achieved. Our examination of the structure reveals unique lithium-dopant interactions which help sustain such LiF phases in the host matrix. The combination of excellent Li-ion transport properties and electron blocking ability makes such LiF-rich composites an excellent candidate for use as an interfacial protective layer that can effectively suppress electrolyte decomposition and Li dendrite propagation, while simultaneously improving the contact and compatibility of the electrode/electrolyte interface. These unique and exceptional traits make them materials of great promise for protecting critical interfaces in ASSLMBs and LIBs.
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