Reaction mechanism and structural evolution of fluorographite cathodes in Li/Na/K batteries
Authors: Ding Zhengping , Yang Chen , Jiang Cheng , Chen Shulin , Qu Ke , Ma Xiumei , Zhang Jingmin , Lu Jing , Wei Weifeng , Gao Peng , Wang Liping 
Published: July 2020

Abstract
Fluorographites (CFx) are potential cathode materials for alkaline metal primary batteries with ultrahigh energy densities. To elucidate the reaction mechanism and structural evolution of CFx cathodes, we combine in situ transmission electron microscopy and ab initio calculations and discover a two-phase mechanism upon Li/Na/K ion insertion. Amorphous LiF (crystalline NaF and KF nanoparticles) are generated uniformly within the amorphous carbon matrix, which retains an unchanged volume during the discharge process. The diffusivity for K/Na/Li ion migration within the CFx is approximately 232.9 nm2/s, 479.7 nm2/s, and 2133.0 nm2/s, respectively, which is comparable to the diffusivity of Li/Na/K ions in liquid-state cells. During charging, amorphous LiF and crystal NaF decompose into alkali metal and F2. Our results reveal no substantial volume change and good ion diffusion kinetics, demonstrating the applicability of all-solid-state M/CFx (M = Li/Na/K) primary batteries. This new understanding may promote the design and development of better CFx-based batteries.While lithium ion batteries (LIBs) are widely applied in portable electronics and electric vehicles, cheaper, more advanced battery systems with higher energy and power densities, and improved safety performance remain in demand.1-5 Among the materials employed in cathode systems, conversion-type materials, such as halides, chalcogenides, and oxides, demonstrate high specific capacities, offering lithium-metal batteries with high energy densities.4,6,7 For example, fluorinated graphite (CFx, x = 1) cathodes exhibit a high theoretical energy density and power density of 3725 Wh kg-1 and 9313 Wh L-1, respectively 6, which can be used in Li primary cells. 8 The practical energy density in Li/CF1.0 cells reaches 2600 Wh kg-1, which is much higher than that in Li/I2, Li/SOCl2, Li/MnO2, Li/Ag2CrO4, and Li/CuS systems.9,10 CFx also delivers a discharge specific capacity of 1061 mAh/g (1439 Wh kg-1) with a discharge plateau of 2.4 V11,12 in a Na/CFx system, and 749 mAh g-1 (1869 Wh kg-1) with a discharge voltage plateau of 3.0 V in a K/CFx system13, which makes CFx applicable for multiple alkali ion batteries. Moreover, the irreversible conversion of CFx into LiF and carbon during discharge, due to the high dissociation energy of LiF (6.1 eV), means the decomposition of LiF by charging alone is not possible.9,10,12 Yazami et al. first demonstrated the reversible electrochemical reaction of CFx with lithium in F ion batteries with a reversible capacity of ~120 mAh g-1.14 Later, Liu et al. reported a reversible Na/CFx battery with a specific capacity of 786 mAh g-1.11 However, these batteries suffer from poor rate performance at low temperatures15, initial voltage delay during the discharge process8 and large heat generation at high discharge rates, which limit their application in harsh environments.9,16 Therefore, a detailed study of the reaction mechanisms is urgently required for the further optimization. Various studies have been conducted to understand these battery systems using different techniques.8,16-20 For example, in situ XRD results suggested a simultaneous formation of a CF(x-y)-Li+ intermediate phase and crystal LiF15,18, while other studies reported a formation of amorphous LiF followed by recrystallization to crystalline LiF.17,19 The crystalline LiF is generated in an orientation that relates to the absorption energy of the solvents on the LiF surface.16 The reversibility and reaction mechanism of CFx in Na/CFx batteries was studied using softX-ray absorption spectroscopy (SXAS) and nuclear magnetic resonance (NMR) techniques, which revealed reversible conversions between CFx and NaF.12 The liquid electrolyte was reported to act as an ion conductor and solution medium to dissolve and aggregate alkali fluoride crystals in a M–CFx (M = Li, Na, and K) system, resulting in large crystalline alkali fluorides.13 Despite intensive efforts focusing on the reaction mechanism, the structural evolution and reaction pathway of CFx in Li/Na/K batteries remains unclear. In this study, we use in situ transmission electron microscopy (TEM) with high spatial/temporal resolution to probe the phase transformation, intermediate phase, and volume change in real time (Figure 1a).21,22 We find that a two-phase reaction occurs during alkali ion intercalation, and the diffusivity of K/Na/Li ion intercalation in CFx is approximately 232.9 nm2/s, 479.7 nm2/s, and 2133.0 nm2/s, respectively. In situ electron diffraction patterns show the formation and even distribution of crystalline KF and NaF nanoparticles and amorphous LiF in the amorphous carbon matrix, leading to no volume change. Upon the insertion of K with a large ionic radius, the interlayer spacing of CF increases, and while only subtle changes are observed during Na/Li ion insertion, both are confirmed through density functional theory (DFT) calculations. Moreover, we find both amorphous LiF and crystalline NaF decompose into alkali metal and F2 during the charging process under vacuum. The results show no volume change and good ion diffusion kinetics during ion intercalation, suggesting that the all-solid-state M/CFx (M = Li/Na/K) primary batteries have broad applicability.