Energetics of Phase Transformation Mechanisms in Li-CF_x Batteries

Abstract

Li-CF x batteries have the potential to successfully electrify segments of aviation due to their high specific energy of over 800 Wh/kg. However, the rechargeability of this chemistry is hindered due to the formation of LiF. Previously, we showed that a lithium-intercalated CF x is most likely to form during discharge due to favorable kinetics. In this work, we propose three mechanisms by which the lithium-intercalated CF x could transform to yield LiF and graphite, the thermodynamically stable products. We use density functional theory calculations to compute the energetics of the mechanisms and estimate the regimes in which the mechanisms could occur feasibly. This work could lead to targeted synthesis strategies to suppress the decomposition mechanisms, paving the way for a high specific energy rechargeable Li-CF x battery. The mechanisms described herein may be more broadly applied to investigate phase transformations in two-dimensional heterostructures and other layered materials.

Research progress in fluorinated carbon sources and the discharge mechanism for Li/CFxprimary batteries

“…The discharging process may be through the diffusion of lithium ions into the CF x body to form LiCF ternary compounds. The OCV of forming ternary LiCF is calculated by DFT as 4.27 ± 0.14 V, which is very close to the theoretical OCV = 4.5 V. The article suggests that the inhibition of the conversion of LiCF to LiF and graphite is crucial if the rechargeability of Li/CF x batteries is to be achieved, and Krishnamurthy et al 7 used DFT calculations to propose three nucleation-growth mechanisms for the phase transition of LiCF to generate LiF and carbon. Atomic diffusion was calculated to be the main mechanism by which this transition occurs, and may be assisted by the lithium fluoride layer in the small particles of LiCF aided by coordinated sliding; the mechanism is described in Fig.…”

“…On the basis of the GIC intermediate layer view, Read et al 22 in 2009 proposed a core-shell model to supplement the explanation of the voltage delay of the discharge process, the CF x phase always remains within the core and gradually shrinks into a solid core with the discharge process, the GIC intermediate-carbon-LiF formation of the nuclear shell then increases with the depth of discharge (DOD), the composition of the generator shell changes with the proceeding of eqn (7) generating carbon, LiF, and solvent S. Due to the increase in the volume of the nal product, the discharge process is accompanied by signicant cathode volume expansion, which leads to the conclusion that volume expansion is prone to occur in liquid electrolyte systems. Fig.…”

“…The coordinated sliding mechanism is only applicable to small LiCF particles with a critical grain size of 137 nm. These works done by Krishnamurthy 7,133 pave the way for the development of high specific energy rechargeable Li/CF x cells by proposing a rational mechanism to generate the stable intermediate ternary compound Li x CF, thus achieving the rechargeability of the Li/CF x battery.…”

“…Although the first report of the synthesis of CF x dates back to 1934, 6 it was not until the 1970s that this substance was employed as a cathode material for Li/CF x batteries. 7,8 The commercial manufacture of Li/CF x batteries with energy densities up to 560 Wh kg −1 began in 1975 thanks to Matsushita Electric Corporation (Japan). 9 The cathode material of commercial Li/CF x batteries is a fluorinated carbon material, and the negative electrode is lithium metal.…”