As a result, it extended the cycle life up to 86% capacity retention after 3000 cycles for the DIB cells with the natural graphite with the LiF-rich skin as the cathode with lithium metal anode (NG@LiF||Li). There was also negligible capacity drop when the NG@LiF||Li cell was charged even at 30C. Furthermore, the double graphite battery (DGB) cell with NG@LiF cathode showed 97% capacity retention up to 400 cycles.
Mechanism of graphite intercalation compounds. a) Schematic representation of meaning. grade numbers for intercalated graphite. Natural graphite with an artificial layer/coating of LiF CEI (NG@LiF). electrode compared to the NG@LIF electrode with optical images of the electrodes. b and c) Top view by scanning electron microscopy (SEM). e) High-resolution transmission electron microscopy (HRTEM) image of a single NG@LIF particle. Electrochemical properties of DIB cells (NG@LiF||Li and NG||Li). voltage profiles at different charge rates followed by a fixed discharge rate at 0.5C. d) Performance retention along cycles at 1C. e).
The linear sweep voltammetry (LSV) curves of Super P/PVDF electrodes with LiF skin. e) Three-dimensional distribution of LiF2, Li2CO3 and PO2F2 on NG@LiF and NG electrodes after 200 cycles by secondary ion mass spectroscopy (TOF-SIMS). Volume expansion of NG versus NG@LiF on electrodes. a and b) HR-TEM images of. The graphite interlayer spacings (d002) were indicated. a) XRD patterns of bare NG@LiF and NG electrode.
Introduction
LIBs were designed using the potential gap between the cathode and the anode as an electromotive force to move the lithium ions in the electrolyte (Figure 3).6-7 The lithium ions in the electrolyte act as a carrier of cations that combine with electrons on the electrode. came from the connected external circuit. Conventional LIBs consist of lithium metal oxides as the cathode material and graphite or lithium alloy material as the anode material. A suitable cathode material has been proposed since the LiCoO2 (LCO) layered cathode material was proposed due to its excellent mobility of Li+ ions during the redox reaction.8 In recent decades, a variety of inorganic lithium metal oxide materials have been investigated as promising. cathode candidate in terms of high energy density, speed capability, safety and cost.9 However, they all suffered from fluctuations in transition metal prices due to the rapid increase in demand for batteries and the collapse of the global supply chain.
One of the main obstacles hindering the growth of the battery application market is the vulnerable basic resource supply, especially for transition metal elements. Li+/Li or VLi) was negative well away from the lithium deinsertion reaction potential of lithium metal oxides (>3.5 VLi). Lithium ions released from lithium metal oxide cathode are intercalated to graphite anode during charging.21-22.
In addition to the cation intercalation, on the other hand, the anion intercalation in graphite has been reported, which is the electrochemical oxidative intercalation of anions characterized by highly positive reaction potential around 5 VLi.23-24 Dual ion batteries (DIBs) have been introduced using of the anion intercalation reaction (Cf. The merits of DIBs over LIBs include (1) low cost cathode materials (graphite versus lithiated transition metal oxides), (2) higher operating voltage and (3) better rate capability.29-30 The theoretical capacity of however, the anion intercalation in graphite was limited: less than 124 mAh g-1 for the PF6- The graphite cathode suffered from the large volume expansion, the solvent co-intercalation followed by graphene layer exfoliation and the unwanted electrolyte decomposition.32-35 The importance of the solid-electrolyte interphase (SEI) layer on LIB anodes has been repeatedly emphasized.36-37 The SEI layer is electrochemically generated by reductive decomposition of electrolyte molecules within LIB cells.
The graphite cathode containing the SEI layer showed an improved reversibility of anion intercalation/deintercalation.41 In addition, the cathode with mesocarbon microbeads (MCMBs) coated by Li4Ti5O12 (LTO) layer with sulfolane (SL) additive demonstrated a large increase in oxidizing ability of electrolysis. and cycle life of DIB cells due to the protective layer of the cathode surface.42 The LTO layer as the artificial cathode electrolyte layer (CEI) played an important role of. Lithium fluoride (LiF) has been known to be the inorganic component of the SEI layer useful for LIB anode reactions due to its affinity for lithium and mechanical strength.43-45 The LiF artificial layer was used in the lithium metal anode (1 ) to provide mechanical forces high enough to suppress lithium metal dendrite growth, (2) to regulate the flow of lithium ions for homogeneous coating of lithium metal, and (3) to protect the fresh front of lithium metal deposit for improved chemical stability.46-49 Also, LiF component worked for anion intercalation in DIB as well as cation intercalation in LIB. The main component of the SEI-containing graphite cathode for the aforementioned DIBs was found to be LiF in X-ray photoelectron spectra (XPS).41 However, this strategy of electrochemically generated LiF-enriched SEI layer was not practical feasible because the graphite electrode was transferred from the generating graphite SEI || lithium metal cell in DIB cell.
Inspired by LiF artificial layer on lithium metal49, in this work, we chemically deposited LiF on graphite electrode as artificial CEI layer to improve anion intercalation electrochemistry in terms of its reversibility and kinetics. The LiF skin/deposition bond significantly suppressed the volume change experienced by the graphite cathode during anion intercalation and de-intercalation, in addition to the conventional practical roles of the CEI layer, such as facilitating anion intercalation kinetics and suppressing electrolyte decomposition.
Result and Discussion
Therefore, the discharge capacity of the NG cells at high rates dropped to less than half of the capacity at low rates. On the other hand, the ohmic polarization of the NG@LiF||Li DIB cell was not as severe as that of the NG||Li. 86% of the anion intercalation capacity of the NG@LiF||Li cell (~ 74 mAh g-1) was charged even at the ultrafast charging rate, 30 C.
On the contrary, the capacity retention of the NG@LiF cell during cycles was excellent: 86% capacity retention even after 3000 cycles with 94% columbic efficiency (CE). This improved anion intercalation kinetics achieved by the LiF-rich skin and deposit supported the extremely superior rate capability of the DIBs. The RCT behavior of the anion intercalation during charging was similar to lithium ion intercalation up to 2L steps.58.
However, lithium ion intercalation showed a dramatic increase at the end of the intercalation process. Therefore, we expected that the LiF-rich skin/sediment suppressed at least one of the deterioration processes and furthermore limited the graphite structural change (e.g., increase in interlayer spacing) fortunately to an optimized level. Therefore, anion transport pathways were secured in the presence of LiF-rich skin/coating.
The resonance frequency of the NG@LiF-loaded resonator suddenly decreased for 1.5 minutes just after the potentiostatic stimulus application. Both scenarios are acceptable if the strong affinity of the LiF layer to anions is available. The CEI composites were analyzed by X-ray photoelectron spectroscopy to investigate the effects of the LiF-rich skin on the electrolyte dissolution.66-68 LiF was dominant in the NG@LiF while it was not detected in the bare NG (F 1s and Li 1s spectra in Figure 18a to d).
However, the amount of electrolyte decomposition product in NG@LiF was estimated to be much smaller than that of NG. The three-dimensional spatial distributions of the CEI layer components generated after 200 cycles were presented (Figure 18e). More importantly, much weaker signals of Li2CO3- and PO2F2- species were detected in the presence of LiF-rich CEI skin.
The d002 of graphite (measured from TEM images) increased to 0.352 nm after cycling, while a d002 of 0.337 nm was obtained in the presence of the LiF skin.
Conclusion
Experimental Section
- Electrode preparation
- Cell preparation
- Electrochemical characterization
- Material characterization
- Calculation of rate constant
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