First author: Prayag Biswal
Corresponding author: Lynden A. Archer
Corresponding unit: Cornell University, USA
Research shows that the interface formed on the reactive metal electrodeposit is determined by the spatial changes in its chemical composition and transport characteristics. This improves the stability and reversibility of batteries using reactive metals as the negative electrode. Previous research (Nat. Energy (2022) https://doi.org/10.1038/s41560-022-01055-0) has shown that SEI dissolution during cycling will accelerate the degradation of battery performance and reduce the solvation capacity of SEI components. As well as promoting the formation of insoluble SEI components, it can minimize SEI dissolution. Just imagine: Is the SEI formed in situ during the cycle beneficial to stabilizing the interface? Wouldn't it be better to have such "SEI dissolution" if selective "removal" of unwanted SEI components could be achieved?
Here, Professor Lynden A. Archer of Cornell University in the United States and others studied the addition of carbonate and fluorinated electrolytes with ether as additives in the early stages of lithium metal anode deposition and the later stages of deep cycling. Effects on the physical and chemical properties and reversibility of lithium metal. This paper studies the physicochemical properties and electrochemical reversibility of metallic lithium in carbonate and fluorinated electrolytes with or without the addition of diglyme (G2).
The specific composition of electrolyte is: 1M LiPF6 EC:DMC (50:50), 1M LiPF6 EC:DMC:G2 (45:45:10), 1M LiPF6 FEC and 1M LiPF6 FEC:G2 (90:10) . It is crucial to achieve high reversibility by regulating and maintaining the surface energy of the SEI, which can be achieved through the SEI fluorine-rich mechanism of FEC and the cleaning effect of G2. At the same time, the spatial and chemical characteristics of lithium deposition were studied by combining scanning electron microscopy (SEM), energy dispersive X-ray (EDX), XPS and electrochemical impedance spectroscopy (EIS). The results show that the fluorine-rich SEI is spatially homogeneous and has good uniform lithium deposition kinetics in the early stages of deposition. In addition, a moderate proportion of G2 can dissolve unwanted carbon-containing compounds and preserve the original fluorine-rich SEI for later cycles, enabling the lithium metal anode to have higher reversibility, longer cycle life and obvious The non-dendritic morphology of . Cycling studies in cells composed of thin metallic lithium anodes (50 μm) and commercial nickel cobalt manganese oxide (NCM) cathodes show that fluorinated electrolytes containing G2 additives achieve high Coulombic efficiency (CE) and stable Long cycle performance.
Related research results were published on Cell Reports Physical Science under the title "A reaction-dissolution strategy for designing solid electrolyte interphases with stable energetics for lithium metal anodes".
[Research Background]
In liquid electrolytes, spatial changes in the chemical composition and transport properties of the solid electrolyte interface phase (SEI) formed on lithium metal are the reasons why lithium is deposited in an irregular, non-planar morphology. Meanwhile, the origin of these spatial changes is believed to be the side reactions between lithium and electrolyte components (solvents, salts, and additives), thus forming a series of inorganic and organic products (i.e., Li2CO3, LiF, LiOH, Li2O, LiH, ROLi and ROCO2Li oligomers). The reason is that chemically non-uniform SEI means that ion transport and mechanics are different at different locations on the lithium, resulting in uneven lithium growth. This process is exacerbated by repeated charge/discharge processes, causing mechanical loss of active metal and Chemical loss of electrolyte components and reduced electrochemical reversibility of lithium metal battery .
A growing number of studies claim that selectively changing the chemical and transport properties of the SEI by adding certain additives to the liquid electrolyte is an effective method to achieve highly reversible lithium deposition in the liquid electrolyte. At the same time, transport at this interface is significantly enhanced when LiX (X=BrClF) is the only component of the SEI.However, in traditional ester electrolytes, the improvement of the deposition reversibility of lithium for long-term and deep cycles is limited, and there are three reasons why deep cycling cannot be achieved in electrolytes rich in halide .
First , conventional ester electrolytes, which serve as carrier media rich in halide additives, decompose into carbon-containing compounds (Li2CO3, ROCO2Li, etc.) at the reduction potential, where Li+ is reduced to form lithium at the negative electrode during battery charging. followed by , and ultimately the carbonate compound that dominates the SEI composition fundamentally alters the interfacial ion transport properties, negating any benefit of the fluorinated component, and pure Li2CO3 enriches the energy barrier for in-plane Li adatom diffusion of the SEI About five times higher than pure LiF SEI. Likewise, the ion diffusion characteristics of SEI are also hindered by the presence of Li2CO3. In the long term, repeated deposition/stripping cycles of the negative electrode produce large amounts of these compounds at the SEI, which negates any beneficial effects of the halide additive in the early stages. and finally , mitigating negative reactions and retaining the halide-rich SEI during repeated deposition/stripping cycles of lithium anodes provide a strategy to achieve high electrochemical reversibility for deep cycling.
One way to alleviate mesophase side reactions is to replace the traditional ester electrolyte with perfluorinated electrolyte. The latter method is to use fluoroethylene carbonate (FEC) as an electrolyte additive. FEC regulates the solvation sheath of lithium ions to promote preferential reduction, thereby passivating the negative electrode by forming a fluorine-rich SEI layer. Although FEC can stabilize the SEI, achieving the elimination of any side reactions and unwanted by-products during hundreds of charge/discharge cycles at the anode is a necessary condition for progress toward practically relevant lithium metal batteries. Specifically, FEC is a fluorinated carbonate with a similar structure to traditional ester solvents ( ethylene carbonate ) and is expected to produce by-products such as carbon-containing compounds (Li2CO3, ROCO2Li, etc.), which means that during lithium deposition The beneficial fluorine-rich interface formed in the early stages is easily overwhelmed by carbonate .
[Core content]
1. Physical and chemical characterization of original SEI
In order to study the influence of fluorinated electrolyte (FEC) and diglyme additive (G2) on the spatial uniformity of the interface, SEI was studied based on different electrolytes. nature. The lower capacity loss observed during SEI formation in fluorinated electrolytes indicates fewer side reactions and faster electron passivation relative to carbonate electrolytes. Meanwhile, no change in areal capacity was observed after adding G2 additive, indicating that G2 is inert to decomposition in SEI formation. SEM results show that adding G2 to the ester electrolyte can correct the incomplete SEI. The SEI has less spatial contrast in morphology and more fluorine-rich areas. This is likely due to the carbon-rich precipitation at the interface of G2. Due to the solubilization effect of the substance (Figure 1C) . In addition, by analyzing the deposition pattern of non-reactive metals (such as Cu), it is possible to compare the ion transport characteristics of the SEI spatial region. Therefore, based on the uniform deposition of Cu on the formed SEI, it is confirmed that the fluorine-rich region has lower ion transmission energy barriers, thereby enabling metals to be transported and reduced through them, and enhanced surface coverage through fluorine-rich regions can achieve uniform and easy nucleation of metals.
2. Physical and chemical characterization of redeposited lithium
Under the conditions of current density of 1 mA/cm2 and capacity of 0.05 mAh/cm2, the SEI-covered substrate is subjected to galvanostatic deposition of metal lithium again in the corresponding electrolyte, the voltage is - The capacity curve is shown in Figure 1B and the ex-situ SEM image is shown in Figure 1C. The results show that the overpotentials of batteries based on fluorinated electrolytes are lower than those of carbonate electrolytes, and the addition of G2 further reduces the potential . The electrolyte with the addition of G2 enables a more spatially uniform SEI and subsequently provides more nucleation sites for incoming lithium ions, thereby forming a greater number of nuclei across the entire surface to achieve the same electrodeposition capacity .The effect of this spatial uniformity on the lithium deposition morphology is more obvious in the case of fluorinated electrolyte (FEC, FEC:G2).
Figure 1. Effects of different electrolytes on the morphology and uniformity of early SEI and deposition of polished SS.
To determine the root cause of the observed phenomena, XPS analysis of the surface chemistry of the SEI formed during the early stages of lithium deposition was performed to observe changes in the chemical composition of the SEI. The results show that fluorinated electrolyte (FEC) decomposes into LiF and oxygen-free ethylene species (C-C), which can polymerize at the reducing negative electrode potential to form fluorinated SEI rich in polyene (-C-C-) networks. as a G2 additive for polyether (C-C-O) dissolves oxygen-rich carbonaceous species (such as Li2CO3 and ethylene oxide [C-C-O]-rich species) but does not affect ethylene species in the SEI (C-C), increasing the fluorine and ethylene content of SEI . Therefore, the SEI formed in the FEC:G2 electrolyte has the highest enrichment in fluorine and ethylene bonds among the electrolyte chemistries studied.
Figure 2. Interface chemistry for lithium deposition at low volumes.
Meanwhile, EIS and CV tests on deposited lithium formed on SS at low capacity (0.05 mAh/cm2) revealed the transport of lithium ions in the SEI, as shown in Figure 3. The SEI and charge transfer (CT) impedance of fluorinated electrolytes (FEC) are lower than those of carbonate electrolytes (EC:DMC). Adding G2 additives to these electrolytes will further reduce the resistance, forming the lowest resistance in FEC:G2. Impedance interface. Furthermore, the ion diffusivity (Ds) of the SEI determined by the Warburg zone is shown in Figure 3C, with Ds increasing 4-fold throughout the electrolyte chemistry. A fast scan CV performed to probe the reaction kinetics of SEI showed a similar exchange current density trend for the electrolyte (Fig. 3B,C). These measurements are consistent with the observed nucleation overpotential, as well as the decrease in SEI and CT impedance. The higher surface diffusivity indicates that the two-dimensional migration of lithium ions in fluorinated SEI is relatively easy, thus promoting more diffusion/delocalization deposition .
This analysis therefore provides insights into the role of surface uniformity, chemistry, and energetics in uniform deposition of low-capacity lithium. However, it remains to be determined whether the surface energetics, chemistry, and uniform morphology observed at low capacities translate to similar morphologies at later stages of higher capacity charge/discharge cycles relevant for practical lithium metal battery (LMBs) applications. Appearance and energetics. In this context, this paper studies the morphology, chemical composition, and surface energy of deposited lithium after multiple cycling/stripping cycles (Figures 4 and 5), and further studies the electrochemical reversibility (Figure 6). Finally, the electrochemical performance of full cells containing thin lithium metal (~50 μm) and NCM cathodes was evaluated (Figure 7).
Figure 3. Interfacial transport and reaction kinetics of deposited lithium formed on SS with different electrolytes at low capacity.
3. Physicochemical characterization of lithium deposition after cycling
The stainless steel substrate (SS) after 100 cycles at a current density of 1 mA/cm2 and a capacity of 1 mAh/cm2 was analyzed. The results showed that had a good performance in EC:DMC , the deposited lithium exhibits entangled linear mossy dendrites, while the morphology of EC:DMC:G2 is relatively smooth, and FEC:G2 has a more uniform morphology. In other words, the entangled linear deposits observed in EC:DMC bear no resemblance to those observed in earlier deposits, whereas the smooth morphology observed in FEC:G2 retains the morphology observed at lower volumes. The spatial uniformity of the deposition morphology is achieved. The chemical composition of the SEI after 100 cycles was detected by XPS measurement patterns and high-resolution C1s and Li1s spectra: similar to the low-capacity case, the F:C and F:O ratios increased by approximately 200%, and the ethylene-C-C-/CH bonds increased (from 32.12% to 57.46%) and ethylene oxide C-C-O (from 40.22% to 28.76%) and Li2CO3 (from 27.67% to 13.78%) (EC:DMC to FEC:G2) decreased.Notably, the LiF:Li2CO3 ratio increases by approximately 700% across the electrolyte spectrum, indicating that the chemical composition of the fluorine-rich polyene SEI formed at lower capacities is retained during cycling of deposited lithium, G2 The cleaning effect on oxygen-rich carbonaceous substances at the interface lasts throughout the cycle life, further protecting the fluorine-rich interface.
Figure 4. Effect of electrolyte on morphology and interfacial chemistry of lithium deposition on SS substrate.
studied the battery impedance based on EIS testing. At 0.05 mAh/cm2 and after 100 cycles, the Ea of SEI and CT kinetics were similar to those of the base electrolyte. Fluorine-rich polyene SEI (FEC:G2) has higher Ds and exchange currents and is chemically preserved through deposition/stripping cycles, showing similar activation energy across the entire volume. The impact of G2 on clean interfaces is important for retention The kinetics of lithium metal cycle life are critical.
Figure 5. Effect of electrolyte chemistry on the interfacial activation energy of lithium deposited on SS substrate.
4. Reversibility and cyclicity of deposited lithium
Under the influence of different electrolytes, the electrochemical reversibility and cyclicity of deposited lithium are shown in Figure 6. CE tests performed in an electrochemical cell (Li||SS) with a current density of 1 mA/cm2 and a capacity of 1 mAh/cm2 confirmed the previous inference, with CE following EC:DMC (66.7 %) EC:DMC:G2 (81.2%) FEC (96.5%) FEC:G2 (98.8%) of the order, especially the fluorinated electrolyte with added G2 (FEC:G2) showed 97 in its up to 200 cycles %-99.5% CE, in contrast to the oscillating/sharply declining CE of other electrolytes. Although CE is widely used to evaluate the electrochemical reversibility of metals in asymmetric batteries, the information obtained from it on lithium metal dendrite growth and its impact on cycle life is rather limited. In other words, different tests are needed to evaluate the interaction of the lithium anode with the electrolyte-separator during cycling. As shown in Figures 6B and 6C, first the G2 additive increases the cycle life (short circuit time) of the base electrolyte by nearly 2 times ; in addition, the cycle life of the entire electrolyte will increase. Secondly, compared with the basic electrolyte, the cyclic voltage window of the electrolyte added with G2 is lower, and the voltage window of the electrolyte is reduced. The voltage window is also arched (U-shaped) . It is speculated that the arch is due to: ( 1) The over potential deposited during the initial cycle phase is higher due to the ion transport resistance provided by the existing natural SEI on the lithium anode; (2) followed by the platform, most likely due to the passage of newly formed SEI ions Transport is easy; (3) Overpotential subsequently increases at the end of cycle life due to increased resistance to ion transport, a thickened SEI formed by degradation products of the electrolyte and dead lithium from repeated deposition/stripping the result of. The lowest overpotential and longest voltage platform displayed by FEC:G2 electrolyte proves the efficacy of the retained fluorine-rich SEI in promoting long-lasting ion transport. The modified SEI can reduce the polarization of lithium metal and increase uniform deposition. , alleviate the chemical and morphological instability of the lithium metal-electrolyte interface, and improve the reversibility and cycle life of the lithium metal anode.
Figure 6. Effect of different electrolytes on the reversibility and cyclability of lithium deposition.
5. Performance of LMBs
The suitability of the electrolyte in LMBs was evaluated through voltammetry, float test and full cell electrochemical cycle test, as shown in Figure 7. After float tests, the electrolytes were subjected to increasing potential in Li||NCM cells for long periods of time to study the evolution of the current (leakage current), where all electrolytes showed oxidation stability below 4.5 V. At the same time, full battery evaluation was performed based on the pairing of a thin Li (50 μm) anode and an NCM 622 cathode with an area capacity of 2 mAh/cm2. The battery was cycled in CC-CV charge mode and CC discharge mode at 0.5 C.Figure 7 summarizes the CE and discharge capacity retention curves. FEC:G2 electrolyte has the highest average CE after 200 cycles: 99.85% (5-200 cycles) and 74% discharge capacity retention (vs. 5th cycle). This difference may be due to the presence of G2 at the cathode-electrolyte interface ( CEI) caused by the carbonate-rich environment, further research is needed to explain the impact of such ether additives on the LMB transition metal cathode . Overall, the electrochemical performance of LMB is significantly improved by using G2-rich fluorinated electrolyte.
Figure 7. Oxidation stability and full cell electrochemical performance tests.
[Conclusion Outlook]
In summary, this paper studies the morphology, chemical composition, interface energetics and electrochemical reversibility of lithium anodes in the early and late stages of lithium anode deposition using carbonate and fluorinated electrolytes with or without ether. The experiments reveal the role of SEI's surface energetics (transport reaction kinetics) at different stages. The results show that the addition of an ether-based additive (G2) has a beneficial "cleaning" effect on lithium deposition and is critical to maintaining the surface energetics (transport reaction kinetics), chemistry and uniform morphology observed during the initial stages of deposition. Designing and maintaining interfaces with enhanced transport-reaction kinetics is a feasible solution to stabilize the deposition of active metals such as lithium. In addition, systematic research on different proportions of electrolytes in lithium metal anodes is needed to realize and optimize the "reaction-dissolution" strategy of lithium metal anodes. Among them, further research will involve understanding and eliminating key issues such as dendrite nucleation and growth, which are crucial for designing reactive metal batteries.
[Literature information]
Prayag Biswal, Joshua Rodrigues, Atsu Kludze, Yue Deng, Qing Zhao, Jiefu Yin, Lynden A. Archer*, A reaction-dissolution strategy for designing solid electrolyte interphases with stable energetics for lithium metal anodes, 2022, Cell Reports Physical Science, https://doi.org/10.1016/j.xcrp.2022.100948
