Silicon Nitride Anodes for Li-Ion Batteries: Understanding the Origin of Enhanced Cycle Stability over Silicon with Operando AFM

Silicon is highly coveted as the potential anode for next generation batteries, owing to its high specific capacity (3579 mAh g -1 , Li 3.75 Si), high abundance and low cost. However, the large volume expansion of silicon during lithiation (up to 300 %) poses inherent challenges, namely cracking/fracturing of the anode which constitutes to drastic capacity fade, thereby greatly limiting the long-term cyclability and ultimately leads to terminal cell failure mechanisms such as delamination [1]. A promising alternative is the use of conversion alloying materials such as silicon nitride (SiN x ). During the initial lithiation (first cycle), these materials irreversibly convert into a mixture of matrix (Li x Si y N z ) and the redox active electrode (Li x Si). In subsequent cycles, the improved cycle performance and lifetime is attributed to the matrix helping buffer the volume expansion of the Si particles, thus mitigating against the aforementioned degradation pathways and ultimately extending the cycle performance lifetime. Currently, insights into the silicon nitride alloying conversion reaction are limited. There remains debate in the literature around whether the matrix is redox active, and uncertainty over the topographical distribution of the anode, specifically whether discrete domains of silicon and matrix are generated in situ , or the silicon nitride remains as amorphous solid solution of silicon and lithiated nitridosilicates, where only the short-range chemical environment resembles a given phase [2-4]. At time of writing there have been no operando observations of the reaction. In this presentation, we address this by studying pulse laser deposited (PLD) SiN x thin films with operando electrochemical atomic force microscopy (EC-AFM). This technique (performed in a glovebox under inert Ar conditions) enables insights into the morphological, mechanical, chemical, and physical properties of battery materials when they evolve under electrochemical control. Crucially, operando measurements enable real time detection of the morphological changes of the SiN x anode during the formation cycle, in conjunction with Young’s Modulus mapping enabling phase-identification of the matrix and silicon components. First, we report thorough electrochemical characterisation of SiN x films with varying composition (x), demonstrating performance equivalent to the best-in-class. Then, EC-AFM studies are communicated; comparisons are made between SiN x and Si films to e