Morphological characterization of Li metal with cLC-CNC skin after single-sided Li plating for 72 h: (c) cross-section and (d) surface (pristine Li (inset)) SEM images. 33 Effect of cLC-CNC skin on Li plating/stripping. a) Cross-sectional SEM image of pristine Li metal on Cu foil.
Overview of Rechargeable Battery Systems
Principles of Lithium-Ion Battery (LIB)
Needs for Battery Separators
Requirements for Battery Separators
Current Battery Separators
Therefore, to satisfy the requirements of energy storage systems, advanced electrode is getting more attention for exceptional energy density. The energy density of the LMB has been predicted to reach the high gravimetric and volumetric energy density (900-1900 Wh L-1 and 400-1000 Wh kg-1), leading to longer driving distances in excess of 400 miles per hour. single charging and solves the problem limitation of LIB.2.
Issues of Lithium-Metal Battery (LMB)
Principles of Lithium-Sulfur Battery (LSB)
Issues of Lithium-Sulfur Battery (LSB)
Approaches
Separators for Lithium-Sulfur battery
- Carbon-based material-coated separators
The separation approaches for LMB can be divided into 1) mechanical stabilization: mechanical suppression of Li dendrites, 2) physical stabilization: homogeneous Li ion flux through well-ordered porous structure and 3) chemical stabilization: uniform Li deposition through functionality. To guide the stabilization of Li-ion flux, the well-ordered porous structure must be fixed. COMSOL simulation of Li-ion flux for the (c) bare separator and (d) nanochannel membrane with e) cell geometry (Y. Cui et al., J. Am. Chem.
In addition to controlling physical and mechanical properties, the chemical properties by means of functionality can also play a critical role in the stabilization of Li metal anode.
Introduction
The spider web separator consists of three functional sandwich-type nanomats (top and bottom layers = multi-walled carbon nanotubes (MWCNT)-wrapped polyetherimide (PEI) nanomats, middle layer = PVIm[TFSI]/poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) nanomat) on top of a porous polyethylene (PE) separator. The three nanomat layers of the spider web separator are fabricated via two nozzle-based, dual electrospinning (or simultaneous electrospinning/electrospraying) processes. Meanwhile, the top and bottom nanomat layers (PEI/MWCNT) act as an upper current collector (the top nanomat: adjacent to the sulfur cathodes) to stimulate the electrochemical redox reaction of the sulfur cathodes and a blocking layer (the bottom nanomat: opposite layer). the PE separator) to respectively prevent crossover of the polysulfides to the Li metal anodes.
Taking advantage of the structural uniqueness and chemical functionalities that have never been reported in Li-S battery separators, the spiderweb separator significantly suppresses the shuttle effect while ensuring easy Li-ion transport, ultimately providing an exceptional improvement in electrochemical performance (especially charge/discharge) capacity storage with cycling) of Li-S batteries.
Experimental Section
Synthesis of PVIm[TFSI]
Fabrication of PVIm[TFSI]/PVdF-HFP-based nanomat (VF nanomat)
Fabrication of spiderweb separator
Thin CLC-CNC (1 µm in thickness) was deposited on top of the Li foil. We investigated the effects of the above-prepared cLC-CNC skin on the stabilization of Li metal electrodes. The difference in height values was verified by measuring the ionic conductivity of the cLC-CNC skin (Figure 31d).
cLC-CNC (0 h) had the lowest ionic conductivity due to the longer tortuosity caused by the shorter pitch, which hinders ion transport leading to voltage rise (inset of Fig. 31e). Meanwhile, the structural stability of the cLC-CNC skin (cLC-CNC@Li) during the cycling test was investigated by atomic force microscopy (AFM) analysis. 33 Effect of cLC-CNC coating on lithium coating/removal. a) SEM image of a cross section of pristine Li metal on a Cu foil.
We investigated the potential application of cLC-CNC@Li for practical Li-metal solid cells. Moreover, even under more severe conditions (excess capacity of Li metal over the stable cycle capacity of NCM) was observed for the cLC-CNC@Li anode (capacity retention = 80% after 100 cycles) (Fig. 36b), highlighting the beneficial effects of the cLC-CNC cladding on the electrochemical efficiency of Li metal full cells.
Result and discussion
Characterizations of PVIm[TFSI] for anion-exchange mechanism
Characterizations of VF nanomat for anion-exchange mechanism
A forged Li-S cell (sulfur cathode/Li metal anode, 1 M LiTFSI in DOL/DME = 1/1 (v/v) with 2 wt% LiNO3 additive) was assembled with a VF-PE separator. Soluble long-chain polysulfides are believed to be trapped by the VF-PE separator through an anion exchange reaction with TFSI-anions PVIm[TFSI]. Under this condition, a concentration gradient of long-chain polysulfides is established between the VF-PE separator and the bulk liquid electrolyte.
In comparison, a significant improvement in the initial Coulombic efficiency was observed for the VF-PE separator.
Basic membrane properties of spiderweb separator
Before using the spider web separator on Li-S cells, its basic membrane characteristics were investigated. As described earlier, all layers of the spider web separator have a well-developed porous morphology, which contributes to its high ionic conductivity (= 1.17 mS cm-1) after being filled with liquid electrolytes (Fig. 19a). This superior polysulfide capture capability of the spider web separator was further verified by monitoring the variation of the open circuit voltage (OCV) of the cells as a function of elapsed time (Figure 19d).
This result shows that the spider web separator effectively solved the self-discharge problem of Li-S cells, possibly due to its excellent polysulfide capturing ability. 19Basic membrane characteristics of the spider web separator. a) Nyquist plot of the spider web separator filled with liquid electrolyte.
Electrochemical performance of spiderweb separator
21(a) Cycle performance (charge/discharge current density = 0.5 C/0.5 C) (surface sulfur charge = 4.5 mg cm-2) assembled with spider web separator. b) SEM images of the spider web separator after 200 charge/discharge cycles on the top nanomat (PEI/MWCNT) adjacent to the sulfur cathode. This beneficial role of the spider web separator was confirmed by measuring the change in charge transfer resistance (RCT) of the cells as a function of DOD (Figure 22). A post-mortem analysis of the cycled Li-S cells was performed to further elucidate the effects of the spider web separator.
The surface of the Li metal anode after the cycling test was investigated by measuring the amount of deposited polysulfides.
Conclusion
Here, we present a cholesteric liquid crystalline (cLC) cellulose nanocrystalline (CNC) skin for sustainable Li metal electrodes as a natural material strategy. The cLC-CNC coating is deposited on a thin Li metal electrode (20 µm) which is desirable for high energy density full cells. Li-metal full cells (consisting of cLC-CNC-coated Li-metal electrodes and high-performance LiNi0.8Co0.1Mn0.1O2 (NCM811) cathodes) exhibit stable charge/discharge cycling performance while maintaining their volumetric energy density.
To the best of our knowledge, this is the first study to report the cLC-CNC skin as a new concept of ion-conductive protective layer for Li metal electrodes.
Experimental Section
Fabrication of cLC-CNC@Li
COMSOL simulation of Li-ion flux through the cLC-CNC
This beneficial effect of the cLC-CNC (fabricated via EISA) was theoretically elucidated by simulating its Li-ion flux. In sharp contrast, the cLC-CNC (fabricated via EISA) showed the uniform Li-ion flux through the cLC-structured porous channels (Figure 29c). Intriguingly, the cLC-CNC (sonication time = 1 hour) showed the best cyclability than the other samples (Figure 31e).
To confirm the superior cycling performance of the cLC-CNC@Li, we examined cross-sectional SEM images of the Li metal anodes after the cycling tests (after 160 cycles).
Result and Discussion
Characterizations of cLC-CNC Skin
Neither structural disruption nor delamination of the cLC-CNC was observed after tape testing (Figure 26b), revealing the mechanical stability and good adhesion of cLC-CNC to the Li metal. 26 (a) Schematic illustration showing the transfer of the cLC-CNC thin film to a Li metal foil using a roll pressing process at room temperature. 27 (a) Schematic illustration showing the transfer of the cLC-CNC thin film to a Li metal foil using a roll pressing process at room temperature. b) Photos of the cLC-CNC@Li before/after the tapping test.
This simulation result is very consistent with the above described experimental results of Li/Li symmetrical cell test (shown in Figure 27b).
Suppressing Li dendrite growth by cLC-CNC skin
The beneficial effects of the cLC-CNC skin were verified by examining the morphological changes of the PE separator after the cycling test. 35 Photographs and SEM images of the PE separators facing the Li metal anodes after the cycling test (460 h) of symmetrical cells: (a) pristine Li vs. The importance of the cLC-CNC skin was further highlighted by comparing these results with previous reports on Li protective layers.
Notably, the thickness change of the Li metal during charging/discharging was suppressed in the cLC-CNC@Li, resulting in the superior retention of the cell's volumetric energy density.
Pitch variation-driven control of the nanoporous structure of cLC-CNC skin
Li plating/stripping behavior of cLC-CNC@Li
Electrochemical performance of cLC-CNC@Li
Most of the previous studies on Li protective layers did not pay serious attention to the change in the thickness of Li metal electrodes during the charge/discharge cycle (Table 3). 36 Potential use of cLC-CNC@Li for practical full metal Li cells. a) Cycling performance of Li metal full cell (composed of cLC-CNC@Li (vs. pristine Li) and NCM811 cathode (area capacitance. ΔT) of Li metals and storage of volumetric energy density of of the cell after 160 cycles (pristine Li vs cLC-CNC@Li) e) Schematic illustration depicting the favorable roles of the cLC-CNC skin as a multifunctional protective layer for Li metal anodes in full cells.
3 Summary of the previous studies on Li protective layers for Li metal full cells, focusing on thickness change (ΔT) of Li metals before/after cycling test, electrolyte chemistry, N/P ratio and cycling performance.
Conclusion
In anode side, to stabilize the Li metal anode, the functional paper separator based on. The nanocellulose for cathode (N-C) was synthesized with a functional group as thiol group (- SH) to chelate the heavy metal ions from cathode, resulting in suppression of heavy metal ion transport to Li metal anode. We investigated the potential use of the functional paper two-layer separator (N-A@N-C separator) for practical Li-metal full cells.
43Cycling performance of a Li-metal full cell (consisting of a paper cathode and a two-layer separator (compared to pristine Li), N/P ratio = 3.5) at charge/discharge current densities of 1.0 C/1.0 C.
Result and Discussion
Characterizations of nanocellulose for anode (N-A)
The nanocellulose for anode (N-A) was synthesized with functional group as quaternary ammonium group to trap the anions in electrolyte, resulting in high Li ion transfer number. The nitrogen of the quaternary ammonium group was also observed by the results of FT-IR and XPS.
Characterizations of nanocellulose for cathode (N-C)
Functional performance of N-A separator
Functional performance of N-C separator
Electrochemical performance of bi-layer separator (N-A@N-C separator)
Jung*, “Thin and flexible solid electrolyte membranes with ultra-high thermal stability derived from solution-processable Li-argyrodites for all-solid Li-ion batteries.”ACS Energy Lett. Lee*, “Polysulfide breathable/dual conductive heterolayered battery separation membranes based on 0D/1D mixed nanomaterial composite mats” Nano Lett. Mendez, “Recent Advances in Membrane Separations for Lithium-Ion Battery Applications: From Porous Membranes to Solid Electrolytes.” Energy Storage Mater.
