【Background】
Normally, during the charging process of a lithium metal battery (LMB), the desolubilized Li+ captures electrons on the surface of the negative electrode and is reduced to lithium metal, while the desoluble solvent molecules and anions are reduced at the same time to form a solid electrolyte interface layer (SEI). In the above process, the desolubilization of Li+ and the composition of SEI are the main factors affecting the transport kinetics of Li+ and the deposition morphology of lithium metal. The high Li+ desolvation energy barrier limits the fast charging capability of the battery, and the inhomogeneous SEI layer promotes the growth of lithium dendrites, which ultimately leads to reduced coulombic efficiency and shorter battery life, thus hindering the practical application of LMB. Therefore, designing an interface layer that can promote Li+ desolvation and phobic solvent is an important strategy to achieve LMB fast charging and lithium dendrite inhibition.
【Job Introduction】
Recently, Professor Lu Shushen and Postdoctoral Fellow Dai Yao of Sun Yat-sen University synthesized a material for CS grafting GO by reducing graphene oxide (GO) with chondroitin sulfate (CS) with abundant polar functional groups, and applied it to the surface of copper foil to obtain an improved current collector (CrG-Cu). The results show that, on the one hand, there are abundant polar functional groups distributed on the CrG film, which can effectively promote the detachment of Li+ from the solvent structure and alienate the solvent molecules, thus forming a LiF-rich SEI layer. On the other hand, the introduction of graphene oxide (GO) improves the mechanical stability of CrG thin films and prevents the penetration of lithium dendrites. This strategy ensures uniform deposition of lithium metal, effectively inhibits the growth of lithium dendrites, and significantly improves the reversibility of lithium metal deposition/stripping. Our results show that the CrG-Cu anode can achieve 500 (150) reversible deposition/stripping cycles at a current density of 1 mA cm-2 (2 mA cm-2) and a surface capacity of 1 mAh cm-2 (2 mAh cm-2). It is worth noting that the whole cell consisting of Li@CrG-Cu and lithium iron phosphate (LFP) cathode (negative electrode/positive capacity (N/P) ratio of 2.87) is capable of stable operation for 500 cycles at 1 C at a cathode mass load of 10 mg cm-2. The article was published in the international authoritative journal Nano Letters. Qu Zongtao, a doctoral student at Sun Yat-sen University, is the first author of this paper.
Figure 1. Schematic diagram of the mechanism of action of the CrG interfacial layer.
【Content Description】
1. Characterization of CrG materials
In order to achieve the rapid desolvation of Li+ and the formation of stable SEI, a thin film material grafted with chondroitin sulfate grafted graphene oxide was designed, and its abundant polar functional groups could effectively desolute Li+ quickly, and the solvent molecules were alienated to form LiF-rich SEI, and the introduction of graphene oxide further improved the mechanical properties of the film and met the mechanical stability during long-term cycling. Experimental characterization proves the above point. Firstly, XRD, FT-IR and XPS characterization showed that CS was successfully grafted to GO, and there were a large number of polar functional groups on CrG. SEM characterization showed that CrG formed a uniform film with a thickness of about 2 μm on the surface of the copper foil, and finally the CrG-Cu modified current collector was obtained. The electrolyte immersion experiment also preliminarily proved that CrG can exist stably in the electrolyte environment for a long time, which provides a prerequisite for its application in batteries. Similarly, the electrolyte has a lower contact angle on the CrG-Cu surface, which also provides suitable conditions for the deposition/stripping of lithium metal.
图2.(a、b)CrG-Cu集流体的横截面SEM图和元素分布图。 (c) CrG-Cu集流体的SEM图。 (d) GO、CS和CrG的XRD图谱。 (e) GO、CS和CrG的FT-IR图谱。 (f) GO、CS和CrG的C 1s XPS。 (g) CrG的力-距离(F-D)曲线和相应的拟合杨氏模量。 (h) CrG-Cu和GO-Cu在醚类电解液中的浸泡测试。 (i)醚类电解质在裸Cu和CrG-Cu集流体上的接触角。
2. Effect of CrG interface layer on lithium metal deposition behavior
The deposition behavior of lithium metal on bare Cu and CrG-Cu was observed by light microscopy, and for the bare Cu anode, obvious small-sized dendritic protrusions appeared after 10 minutes of deposition, and the dendritic protrusions increased continuously in the following 20 minutes. Eventually, the dendritic protrusions turn irregular mossy after 30 minutes. On the CrG-Cu anode, the formation of lithium dendrites was almost observable within 30 minutes. On the contrary, lithium metal is uniformly deposited on the CrG layer, and its thickness is constantly increasing. In addition, the finite element simulation was combined to confirm that the CrG layer contributes to the rapid desolvation of Li+, and reduces the concentration polarization at the interface and the uniform distribution of Li+.
Figure 3 (a-d) is the optical micrograph of the Li//Cu cell using bare Cu and CrG-Cu and the corresponding constant current discharge curves, respectively. The current density is 5 mA cm-2. (e, f) Li+ concentration curves and lithium metal deposition topography of bare Cu and CrG-Cu simulated by finite element simulation.
3. Electrochemical properties of CrG interface layer applied to lithium metal half-cells
CrG can significantly improve the electrochemical performance of half-cells. As shown in Figure 4a, for the bare Cu anode, the Coulombic Efficiency (CE) drops dramatically after 90 cycles at a current density of 1 mA cm-2 and an area capacity of 1 mAh cm-2. When using CrG-Cu as the negative electrode, it can maintain a high CE of up to 98.4% over 500 cycles. When a high current density (2 mA cm-2) is applied, the CrG-Cu anode still has excellent cycling stability and can be cycled 480 times, while the bare Cu anode loses its reversibility after only 50 cycles. In addition, in order to evaluate the effect of CrG-Cu on stable lithium metal electrodes, electrochemical impedance spectroscopy (EIS) was used to detect the resistance changes of CrG-Cu and bare Cu anodes. CrG-Cu has a higher charge transfer resistance compared to the bare Cu negative electrode (Rct = 29.7 Ω vs 4.9 Ω). After 50 cycles, the CrG layer effectively guided the stable deposition of lithium metal and formed a stable SEI, with slight differences in Rct (37.4 Ω) and RSEI (10.6 Ω) compared to the initial values, while Rct (123.2 Ω) and RSEI (50.6 Ω) were significantly higher for the bare copper anode. These results suggest that the CrG-Cu anode is able to improve the diffusion of Li+ by forming a stable SEI. When using a 6 mAh cm-2 lithium source in a symmetrical battery, the CrG-Cu electrode is able to operate stably for 1200 hours at 1 mA cm-2 and 1 mAh cm-2. In terms of rate performance, CrG-Cu has lower overpotential than bare Cu in the range of 0.5 to 6 mA cm-2 current densities. All the results fully demonstrate the excellent performance of the CrG-Cu anode in the half-cell, which is attributed to the function of the CrG layer to regulate the Li+ flux.
Figure 4.(a) Plated/stripped CE in Li/Cu cells using bare Cu and CrG-Cu current collectors at current densities and areal capacities of 1 mA cm-2 and 1 mAh cm-2, 2 mA cm-2 and 1 mAh cm-2, 4 mA cm-2 and 1 mAh cm-2, and 2 mA cm-2 and 2 mAh cm-2, respectively. (b) The 30th constant current charge-discharge curve of Li//Cu and Li//CrG-Cu cells cycled at 1 mA cm-2 and 1 mAh cm-2. Inset: A partial enlarged view of some curves. (c) Comparison of different resistances of Li//Cu cells containing CrG-Cu and bare Cu after 1 and 50 cycles at 1 mA cm-2 and 1 mAh cm-2. RCT and RSEI denote charge transfer resistance and SEI resistance, respectively. (d, e) Voltage curves of symmetrical cells with Li@Cu electrodes or Li@CrG-Cu electrodes at 1 mA cm-2 and 1 mAh cm-2, and at various current densities with a fixed capacity of 1 mAh cm-2.
4. Functional characterization of CrG interfacial layer desolvation
SEM further demonstrated that CrG can guide the dense and uniform deposition of Li metal. In order to explore the role of functional groups in regulating ion transport characteristics, the molecular interaction between CrG and ether electrolytes was studied by NMR. Figure 5d shows the NMR signal of 7Li in an ether electrolyte. After the introduction of CrG into the ether electrolyte, the chemical shift of Li+ shifted downward, indicating that the introduction of CrG weakened the coordination between the solvent and Li+. Density functional theory (DFT) calculations further investigated the effect of CS fragments on the desolvation behavior of Li+. After the introduction of CS, the length of Li-O bond in Li-DME increased from 1.90 Å to 2.12 Å, which proved that CS has the function of accelerating Li+ desolvation and effectively alienating solvent molecules. In addition, the energy interaction of the calculated Li+ with the C═O site (-1.02 eV) and the C-O-C site (-2.70 eV) anchored on the CS was significantly stronger than that at the DOL site (-0.52 eV) and the DME site (-0.53 eV), indicating that Li+ preferred to desolvate from the solvent cluster and be adsorbed by CS. The experimental and calculated results fully demonstrate that CrG is beneficial to Li+ desolvation. XPS was used to analyze the surface composition of CrG-Cu and bare Cu anode after 30 cycles. The results show that a LiF-rich SEI layer is generated on the surface of CrG-Cu. The LiF-rich layer provides more channels for the rapid transfer of Li+. This may be due to the fact that CrG promotes the desolvation of Li+ and alienates the solvent molecules, causing TFSI- to aggregate on the anode surface and be reduced to form LiF-rich SEI.
Figure 5.(a,b) SEM plot of lithium deposition morphology from 0.1 mAh cm-2 to 6 mAh cm-2 on bare Cu and CrG-Cu at a current density of 1 mA cm-2. (c) Histogram of lithium particle size at 0.1 mAh cm-2. (d) Comparison of NMR spectra of 7Li of CrG dispersed in ether electrolytes with NMR spectra of blank ether electrolytes. (e) The bond length between Li and the solvent oxygen atom in the optimization model. (f) The chemical coordination of the simulated adsorption energy between CS and the ether electrolyte, and the optimized geometry of the terminal. (h, g) High-resolution XPS spectra of Li 1s and F 1s after 30 lithium depositions on bare Cu and CrG-Cu.
5. CrG interface layer improves the performance of Li//LFP whole battery
The CrG interface is applied to the whole battery, and the cycle performance and rate performance of the whole battery are improved. First, after 200 cycles, the Li@CrG-Cu//LFP cell had an overpotential of 425 mV, which was significantly lower than the 559 mV of the Li@Cu//LEP cell (Figure 6b). Secondly, from 0.1 to 2 C, the specific capacity of the Li@CrG-Li@Cu//LFP battery changed from 150 mAh g-1 to 76 mAh g-1, respectively. In contrast, the capacity of a Li@Cu//LFP battery is almost zero at 2 C (Figure 6c). After 50 cycles, the surface morphology of the CrG-Cu anode remained smooth and dense (Fig. 6e); Conversely, the bare Cu surface is uneven and a large amount of "dead lithium" appears (Fig. 6d), demonstrating that the CrG layer plays a crucial role in stabilizing the anode interface over time. The EIS of bare copper is significantly greater than that of CrG-Cu (Figure 6f), which also indicates that naturally occurring SEI has high resistance to electrons and ions. Figure 5g illustrates the cyclic performance test at 1 C. For Li@Cu//LFP batteries, the capacity decays rapidly after 290 cycles, and the coulombic efficiency drops dramatically. The Li@CrG-Cu//LFP battery can still maintain 80% of the initial capacity after 470 cycles, and the corresponding coulombic efficiency is maintained at 99.9%.
图6. (a) Li@CrG-Cu//LFP电池示意图。 (b) Li@Cu//LFP和Li@CrG-Cu//LFP电池在倍率为1 C时的恒流充放电曲线;(c)Li@Cu//LFP和Li@CrG-Cu//LFP电池在0.1 C至3 C倍率下的性能;(d, e)Li@Cu和Li@CrG-Cu阳极在50个循环后的SEM图。 (f)50次循环后Li@Cu//LFP和Li@CrG-Cu//LFP电池的奈奎斯特图。 (g)当LFP质量为10 mg cm-2时,Li@Cu//LFP和Li@CrG-Cu//LFP电池的循环性能。 (h) Li@CrG-Cu//LFP电池和先进的Li//LFP电池的N/P比和循环寿命的比较。
【Conclusion】
In conclusion, CrG was synthesized by CS reduction GO, which can be used as a multifunctional interface layer for lithium metal batteries. The CrG-Cu current collector can effectively promote the detachment of Li+ from the solvent structure and alienate the solvent molecules, thus forming a LiF-rich SEI layer. The deposition uniformity of lithium metal and the enhancement of kinetic properties were realized. After 500 cycles in the half-cell, the CrG-Cu electrode has a deposition/stripping coulombic efficiency of up to 98.4%. When the Li@CrG-Cu electrode and LFP cathode form a full cell with N/P of 2.87, 80% of the initial capacity can be maintained after 470 cycles with a coulombic efficiency of 99.9%. The strategy of desolving the functional interface layer proposed in this paper is helpful for the commercial application of LMB and provides a new paradigm for the design of anode interface for Na, K and Ca metal batteries.
Zongtao Qu, Kaixuan Chen, Wenkang Wang, Yao Dai*, Xia Lu, and Shu-Shen Lyu*, Interfacial Layers with Desolvation Function Induced Stable Deposition of Lithium Metal for Long-Cycling Lithium Metal Batteries, Nano Lett. 2024.
https://doi.org/10.1021/acs.nanolett.4c01738
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