Solid-State NMR Studies of Chemically Lithiated CF

Fluorinated aggregated nanocarbon with high discharge voltage as cathode materials for alkali-metal primary batteries

Due to its exceptionally high theoretical energy density, fluorinated carbon has been recognized as a strong contender for the cathode material in lithium primary batteries particularly valued in aerospace and related industries. However, CF x cathode with high F/C ratio, which enables higher energy density, often suffer from inadequate rate capability and are unable to satisfy escalating demand. Furthermore, their intrinsic low discharge voltage imposes constraints on their applicability. In this study, a novel and high F/C ratio fluorinated carbon nanomaterials (FNC) enriched with semi-ionic C-F bonds is synthesized at a lower fluorination temperature, using aggregated nanocarbon as the precursor. The increased presence semi-ionic C-F bonds of the FNC enhances conductivity, thereby ameliorating ohmic polarization effects during initial discharge. In addition, the spherical shape and aggregated configuration of FNC facilitate the diffusion of Li+ to abundant active sites through continuous paths. Consequently, the FNC exhibits high discharge voltage of 3.15 V at 0.01C and superior rate capability in lithium primary batteries. At a high rate of 20C, power density of 33,694 W kg-1 and energy density of 1,250 Wh kg-1 are achieved. Moreover, FNC also demonstrates notable electrochemical performance in sodium/potassium-CF x primary batteries. This new-type alkali-metal/CF x primary batteries exhibit outstanding rate capability, rendering them with vast potential in high-power applications.

Boosting the Electrochemical Performance of Primary and Secondary Lithium Batteries with Mn-Doped All-Fluoride Cathodes

Transition metal fluorides are potentially high specific energy cathode materials of next-generation lithium batteries, and strategies to address their low conductivity typically involve a large amount of carbon coating, which reduces the specific energy of the electrode. In this study, MnyFe1-yF3@CFx was generated by the all-fluoride strategy, converting most of the carbon in MnyFe1-yF3@C into electrochemical active CFx through a controllable NF3 gas phase fluorination method, while still retaining a tightly bound graphite layer to provide initial conductivity, which greatly improved the energy density of the composite. This synergistic effect of nonfluorinated residual carbon (∼11%) and Mn doping ensures the electrochemical kinetics of the composite. The loading mass of the active substance had been increased to 86%. The theoretical and actual discharge capacity of MnyFe1-yF3@CFx composite was up to 765 mAh g-1 (pure FeF3 is 712 mAh g-1) and 728 mAh g-1, respectively. The discharge capacity at the high-voltage (3.0 V) platform was more than three times higher than that of the non-Mn-doped composite (FeF3@CFx).

Elucidating the Role of Electrochemically Formed LiF in Discharge and Aging of Li-CFx Batteries

Fifty years after its introduction, the lithium-carbon monofluoride (Li-CFx) battery still has the highest cell-level specific energy demonstrated in a practical cell format. However, few studies have analyzed how the main electrochemical discharge product, LiF, evolves during the discharge and cell rest periods. To fill this gap in understanding, we investigated molecular-level and interfacial changes in CFx electrodes upon the discharge and aging of Li-CFx cells, revealing the role of LiF beyond that of a simple discharge product. We reveal that electrochemically formed LiF deposits on the surface of the CFx electrode and subsequently partially disperses into the electrolyte to form a colloidal suspension during cell aging, as determined from galvanostatic electrochemical impedance spectroscopy (EIS), solid-state 19F nuclear magnetic resonance (NMR), dynamic light scattering (DLS), and operando optical light microscopy measurements. Electrochemical LiF formation and LiF dispersion into the electrolyte are distinct competing rate processes that each affect the cell impedance differently. Using knowledge of LiF dispersion and saturation, an in-line EIS method was developed to compute the depth of discharge of CFx cells beyond coulomb counting. Solid-state 19F NMR measurements quantitatively revealed how LiF and CF moieties evolved with discharge. Covalent CF bonds react first, followed by a combination of covalent and ionic CF bonds. Quantitively correlating NMR and electrochemical measurements reveals not only how LiF formation affects cell impedance but also that CF bonds with the most ionic character remain unreacted, which limits realization of the full theoretical specific capacity of the CFx electrode. The results reveal new insights into the electrochemical discharge mechanism of Li-CFx cells and the unique role of LiF in cell discharge and aging, which suggest pretreatment strategies and methods to improve and measure the performance of Li-CFx batteries.

Modes of Disorder in Poly(carbon monofluoride)

Poly(carbon monofluoride), or (CF)_n, is a layered fluorinated graphite material consisting of nanosized platelets. Here, we present experimental multidimensional solid-state NMR spectra of (CF)_n, supported by density functional theory (DFT) calculations of NMR parameters, which overhauls our understanding of structure and bonding in the material by elucidating many ways in which disorder manifests. We observe strong ¹⁹F NMR signals conventionally assigned to elongated or "semi-ionic" C-F bonds and find that these signals are in fact due to domains where the framework locally adopts boat-like cyclohexane conformations. We calculate that C-F bonds are weakened but are not elongated by this conformational disorder. Exchange NMR suggests that conformational disorder avoids platelet edges. We also use a new J-resolved NMR method for disordered solids, which provides molecular-level resolution of highly fluorinated edge states. The strings of consecutive difluoromethylene groups at edges are relatively mobile. Topologically distinct edge features, including zigzag edges, crenellated edges, and coves, are resolved in our samples by solid-state NMR. Disorder should be controllable in a manner dependent on synthesis, affording new opportunities for tuning the properties of graphite fluorides.

First-principles study of the adsorption behaviors of Li atoms and LiF on the CFx (x = 1.0, 0.9, 0.8, 0.5, ∼0.0) surface

Based on first principles calculation, the adsorption properties of Li atoms and LiF molecules on the fluorographene (CF_x) surface with different F/C ratios (x = 1.0, 0.9, 0.8, 0.5 and ∼0.0) have been studied in the present work. The calculated binding energy of Li and CF_x is greater than 2.29 eV under different F/C ratios, indicating that the battery has the potential to maintain a high discharge platform during the whole discharge process. But the adsorption energies of LiF on a CF_x layer for different F/C ratios are 0.12–1.04 eV, which means LiF is not easy to desorb from a CF_x surface even at room temperature. It will stay on the surface for a long time and affect the subsequent discharge. Current calculations also show the structure of the CF_x-skeleton will change greatly during the reaction, when there are many unsaturated carbon atoms on the CF_x surface, such as at x = 0.8 and 0.5. Moreover, the discharge voltage is strongly dependent on the discharge site. After discharge, the CF_x-skeleton may continue to relax and release a lot of heat energy.

Based on first principles calculation, the adsorption properties of Li atoms and LiF molecules on the fluorographene (CF_x) surface with different F/C ratio (x = 1.0, 0.9, 0.8, 0.5 and ∼0.0) have been studied in the present work.

High-Power-Density, High-Energy-Density Fluorinated Graphene for Primary Lithium Batteries

Li/CF_x is one of the highest-energy-density primary batteries; however, poor rate capability hinders its practical applications in high-power devices. Here we report a preparation of fluorinated graphene (GF_x) with superior performance through a direct gas fluorination method. We find that the so-called “semi-ionic” C-F bond content in all C-F bonds presents a more critical impact on rate performance of the GF_x in comparison with sp² C content in the GF_x, morphology, structure, and specific surface area of the materials. The rate capability remains excellent before the semi-ionic C-F bond proportion in the GF_x decreases. Thus, by optimizing semi-ionic C-F content in our GF_x, we obtain the optimal x of 0.8, with which the GF_0.8 exhibits a very high energy density of 1,073 Wh kg⁻¹ and an excellent power density of 21,460 W kg⁻¹ at a high current density of 10 A g⁻¹. More importantly, our approach opens a new avenue to obtain fluorinated carbon with high energy densities without compromising high power densities.

Energetics of defects on graphene through fluorination

Functionalized graphene sheets (FGSs) comprise a unique member of the carbon family, demonstrating excellent electrical conductivity and mechanical strength. However, the detailed chemical composition of this material is still unclear. Herein, we take advantage of the fluorination process to semiquantitatively probe the defects and functional groups on graphene surface. Functionalized graphene sheets are used as substrate for low-temperature (<150 °C) direct fluorination. The fluorine content has been modified to investigate the formation mechanism of different functional groups such as C-F, CF2, O-CF2 and (C=O)F during fluorination. The detailed structure and chemical bonds are simulated by density functional theory (DFT) and quantified experimentally by nuclear magnetic resonance (NMR). The electrochemical properties of fluorinated graphene are also discussed extending the use of graphene from fundamental research to practical applications.

Improving the energy density and power density of CFx by mechanical milling: a primary lithium battery electrode

The effect of high energy ball milling on the electrochemical performance of graphite fluoride (CFx) was investigated. A significant improvement was observed in both energy density and power density. The volumetric energy density was increased up to a factor of 3 with ball milled materials compared with pristine materials. The gravimetric energy density was increased up to a factor of 2, depending on the discharge rates. At 6C the ball milled material still delivered 40% of its nominal capacity, whereas the pristine material did not exhibit any capacity any more. We achieved the power density of 9860 W/kg with a gravimetric energy density of 800 Wh/kg for the optimized material.

Solid-State Nuclear Magnetic Resonance Studies of Electrochemically Discharged CF(x)

Electrochemical studies of three types of CF(x) (F - Fiber based, C - Petroleum coke based, G - Graphite based) have demonstrated different electrochemical performances types in previous work, with fiber based CF(x) delivering superior performance over those based on petroleum coke and graphite. (13)C and (19)F MAS (Magic Angle Spinning) NMR techniques are employed to identify the atomic/molecular structural factors that might account for differences in electrochemical performance among the different types of CF(x). Small quantitative variations of covalent CF and LiF are noted as a function of discharge and sp(3) bonded carbons are detected in discharged F type of CF(x).