Tuning C-F Bonding of Graphite Fluoride by Applying High Pressure: Experimental and Theoretical Study C. Cavallari, S. Radescu, M. Dubois, N. Batisse, H. Diaf, and V. Pischedda* Cite This: J. Phys. Chem. C 2020, 124, 24747-24755 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: C 2 F graphite fluoride has a unique mutable structure when subjected to external applied pressure. In the present study, we examine C 2 F using X-ray Raman scattering (XRS) up to 6.5 GPa coupled with theoretical simulations. Using the XRS technique, we follow the in situ high-pressure evolution of the energy loss corresponding to the C and F K-edge. Significant variations occur at 2.9 GPa, remain up to 6.5 GPa, and persist at ambient conditions after decompression. The permanent changes are related to an increased planarity of the graphitic layers and a modulation of the fluorine configuration. The all-electron density of a C 2 F-sp 3 slab obtained from the DFT simulation and the quantum theory of atoms in molecules reveals the appearance of bond-critical points between in-plane and out-of-plane F-F, suggesting an increasingly ionic character of the structure under biaxial isotropic strain. Pressure can be used as an alternative to chemical synthesis for tuning C-F bonding and metastabilizing new intercalated fluorine compounds with potentially improved electrochemical properties. 1. INTRODUCTION Graphite fluorides, C x F, are used in a wide range of applications such as solid lubricants or cathode materials in high energy-density primary lithium batteries. 1-3 The highly reversible capacity of Na/CF x batteries, recently reported, 4 makes graphite fluorides promising cathode materials for future rechargeable sodium batteries. The unique character of these compounds stems from the combination of anisotropic structural and electronic properties and the versatility of the C-F bonding. This can be either covalent or ionic, depending on the (i) synthesis conditions (fluorination temperature, fluorination time, gas flow rate, and fluorination agents), (ii) the fluorine content (C-F bond evolves from semi-ionic to covalent, increasing the fluorine content), and (iii) the curvature of the carbon lattice. 5 Graphite fluoride C 2 F is a covalent compound prepared using molecular fluorine F 2 gas, at 350-380°C. 6 It is considered an electronic insulator because the carbon skeleton consists of trans-linked cyclohexane chairs with sp 3 bonding. 7,8 However, the synthesized C 2 F compounds have heterogeneous structures with a highly fluorinated surface and a more graphitic core 9,10 because the diffusion of fluorine is limited in the bulk of the material. Available samples with C 2 F stoichiometry can be better described with a mixed sp 2 -sp 3 character where graphene layers are intercalated among the sp 3 C and C-F structural domains. 8,11 The presence of a stacking sequence of non-fluorinated graphitic carbon is important for improving some properties, such as the electron flux when C x F is used as an electrode in primary batteries or high-performing solid lubricants. 1,3,12-14 Moreover, when non-fluorinated carbon atoms are present in the neighborhood of the C-F bond, hyperconjugation occurs, and the C-F covalence is weakened. 15-17 The electrons involved in the C-F covalent bonds are partly delocalized by the presence of out-of-plane p z orbitals from the graphitic planes. An extremely interesting route to further modulate sp 2 -sp 3 hybridization and C-F bonding and related properties is the external application of high pressure. Applying pressure to intercalated layered materials can induce mobility of the intercalated ions causing staging, in-plane decoration, and stacking-sequence changes. 18-21 The performance of many technologies such as Li- and Na- ion batteries is dependent upon the capability of layered materials to reversibly intercalate ions. In Li-ion batteries, during lithiation and delithiation, internal pressure is applied to layered carbon electrodes, and staging transformations are observed. 22 Moreover, stacking-order changes during the electrochemical cycling of Li- and Na-ion battery materials Received: July 27, 2020 Revised: October 12, 2020 Published: October 29, 2020 Article pubs.acs.org/JPCC © 2020 American Chemical Society 24747 https://dx.doi.org/10.1021/acs.jpcc.0c06860 J. Phys. Chem. C 2020, 124, 24747-24755 Downloaded via UNIV CLAUDE BERNARD LYON 1 on December 11, 2020 at 14:59:45 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.