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J. Phys. Energy 3 (2021) 031503 N Tapia-Ruiz et al 2. Anode materials 2.1. Hard carbon Heather Au, Hande Alptekin, Maria Crespo Ribadeneyra and Maria-Magdalena Titirici Department of Chemical Engineering, Imperial College London, London SW7 2AZ, United Kingdom Status Hard carbon materials are the most popular choice for NIB anodes. They are produced from oxygen-rich precursors which cannot be converted into graphite, no matter how high the carbonisation temperature. Hard carbons consist of randomly oriented graphitic domains possessing a higher interlayer spacing than graphite (i.e. >0.34 nm) connected by disordered carbon regions with different curvatures. In between these disordered and more ordered domains, hard carbons have ‘closed’ pores, meaning they cannot be probed by gas sorption. The larger the size of the graphitic crystallites, the larger the pore size in between. In addition to curvatures and edge sites, hard carbons also contain the remaining heteroatoms (mainly oxygenated groups). Such a complex mixture of crystalline and disordered domains with defects allows sodium diffusion pathways and sodium storage sites. The first structure of ‘graphitisable’ vs. ‘non-graphitisable’ carbons was described by Rosalind Franklin in a seminal paper entitled ‘The interpretation of diffuse x-ray diagrams of carbon’ published in Acta Crystallographica in 1950 (figure 9(a)) [67]. Interestingly, until today, the exact structure of hard carbon has remained unknown, as this depends on the precursor and carbonisation conditions, which result in materials with varying interlayer spacing, crystallite size, pore domains and edge termination. Such different structures store Na ions in very different ways. Hence, it is no surprise that there is much debate in the literature about the mechanism of Na insertion into hard carbons. Figure 9(b) provides a typical Na insertion load curve (sodiation curve in a half-cell built with hard carbon vs. metallic Na) showing a typical sloping region between 1 V and 0.1 V and a plateau region below 0.1 V. The first theory by Stevens and Dahn using the ‘house of cards’ model for hard carbon attributed the slope to Na insertion into expanded graphitic domains, and the plateau to pore filling by Na (figure 10) [68]. Yet the situation is more complex than this, as hard carbons can exhibit different features depending on the choice of precursor, pre-treatment, and carbonisation conditions and temperature. Current and future challenges The biggest challenge for hard carbons is to improve their storage capacity to match or even overtake that of graphite in LIBs (>372 mAh g−1). The practical measured capacities for hard carbons today (mostly in coin cells vs. metallic Na) vary from 200 to 450 mAh g−1; most of the values reported in the literature average around 300 mAh g−1. While the theoretical energy density of the commercial graphite used in LIBs can be calculated using the LiC6 formula, for Na in hard carbons, the situation is far more complicated, due to the heterogenous nature of hard carbons and the unknown storage mechanism, which depend on many structural parameters. Therefore, all the structural and morphological features of hard carbons must be clearly determined using multiple characterisation techniques applied in concert to accurately determine the exact features, such as the interlayer spacing (XRD), defects (Raman, positron annihilation spectroscopy), functional groups (XPS), the nature of ordered vs. disordered domains and the interconnectivity between the two (x-ray pair distribution function), opened vs. closed pores and pore sizes (SAXS, SANS, gas adsorption). All these features must be precisely correlated with the electrochemical performance, which should be accurately determined in half cells, but also using three-electrode configurations [71] to better understand the fundamental electrochemistry happening at the working anode. Finally, operando characterisation techniques should be employed to understand the structural and morphological changes occurring during the intercalation of Na ions into hard carbons. Some of these techniques involve the use of XRD/SAXS to determine the change in the interlayer spacing as proof of Na intercalation [68, 72, 73], operando XPS [74] to determine the interaction of Na with different functional groups, in situ electrochemical TEM to observe the interaction of Na with defects [75], operando Raman spectroscopy to understand the changes in graphitic/amorphous structure upon (de)sodiation [76], operando NMR [77–79], MRI [80] and EPR [81] to understand the Na chemical state/deposition inside pores at less than 0.1 V. Another major challenge in the development of high-performance hard carbon anodes is to understand the solid–electrolyte interface [82, 83] and how the anode’s structural features, in combination with the electrolyte of choice, affect the thickness and, more importantly, the ionic conductivity of the SEI. To probe this, we recommend operando AFM [84] as well as electrochemical impedance spectroscopy coupled with quartz crystal microbalance measurements [85], corroborated by electrochemical mass spectroscopy [86] and operando FTIR [87]. To understand macroscale phenomena related to the anode volume change, operando x-ray computed tomography [88] and electrochemical dilatometry are good options [89, 90]. Once the fundamentals are understood, both in an individual electrode as well as during its operation, these could be used to build 22PDF Image | 2021 roadmap for sodium-ion batteries
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