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## Creator

[Yushen Wang](https://orcid.org/0000-0001-6461-0607), [Hidenori Noguchi](https://orcid.org/0000-0001-9643-1689)

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[Revealing the Enhancement of Li Plating/Stripping Efficiency in the TEGDME-based Low-concentration Electrolytes for Anode-free Lithium Metal Batteries](https://mdr.nims.go.jp/datasets/6c757e5e-6e46-43dd-9e98-00400f572af6)

## Fulltext

Revealing the Enhancement of Li Plating/Stripping Efficiency in the TEGDME-based Low-concentration Electrolytes for Anode-free Lithium Metal BatteriesYushen Wang1,2 and Hidenori Noguchi1,2*1Graduate School of Science and Engineering, Hokkaido University, Sapporo 065-8628, Japan.2Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science, Tsukuba 305-0044, Japan.AbstractAnode-free lithium metal batteries (AFLMBs) have attracted great attention owing to higher energy density than conventional lithium metal batteries. Unfortunately, AFLMBs still suffer from poor Coulombic efficiencies (CEs) due to severe dendrite growth and unstable solid-electrolyte interface (SEI) at the anode. Therefore, we explored the effect of concentration and LiNO3 additive on SEI layer and on Li plating/stripping efficiency using low-concentration tetraethylene glycol dimethyl ether (TEGDME)-based electrolyte in an AFLMB anode half-cell configuration. It was found that the formation of protective Li2O-based SEI layers when blocking the oxidative subsequent SEI formation (called “OSS”) above 2.2 V vs. Li/Li+ improved Li plating/stripping stability, while LiNO3 additive suppressed the oxidation of Li2O into Li2O2 and sustained the existence of Li2O without blocking OSS, therefore improving CE performance. Although increasing concentration (from 0.4 M to 2.0 M) did not function well, the 2.0 M electrolyte with LiNO3 additive shifted the dominant SEI species from Li2O to LiF, further enhancing CE performance.1. IntroductionLi metal batteries (LMBs) have been considered as promising alternative to Li-ion batteries, as the replacement of conventional graphite anode with Li metal anode can greatly enhance the energy density owing to the low redox potential (-3.04 V vs. SHE) and high theoretical capacity (3860 mAh/g) of Li metal.1,2 However, the most fundamental obstacle that hinders the practical application of LMBs lies in the highly reactive nature of Li metal, which leads to the occurrence of parasitic reactions and dendrites growth during charge-discharge, resulting in the irreversible loss of both Li metal and electrolyte. To compensate the loss in Li reservoir, LMBs require the usage of excess amount of Li metal in the anode side, which however increases the weight and volume of batteries, the cost, as well as the risk during battery operation.3 Under this circumstance, anode-free Li-metal batteries (AFLMBs) attract growing attentions to be an alternative of LMBs.3–6 In the configuration of AFLMB, the anode active material is absent in the as-fabricated state, and a bare anode current collector (e.g. Cu) is used as a substrate for Li deposition to create a Li anode after initial charging. It is worth noting that in AFLMBs, the negative to positive electrode capacity (N/P) ratio is 0 and 1 after cell assembly and after initial charging, respectively, meaning that all Li resources in the anode come from the cathode during initial charging. Therefore, the use of Li metal can be fully minimized, rendering greatly enhanced energy density and safety compared with LMBs. Furthermore, based on the above properties, the performance of AFLMB is exclusively dependent on Li plating/stripping efficiency, making the analysis of capacity loss more practical.7 Despite the above advantages, the research bottleneck in AFLMB mainly originates from the immense volume expansion and quick capacity drop during initial state.8 In particular, due to the limited Li inventory (low N/P) in AFLMBs, Li plating on the anode usually takes place less evenly,9 hence the mechanical breakage of solid electrolyte interphase (SEI) in the stripping and further plating, which often occurs in LMBs, can be more easily found in AFLMBs. Depending on different electrochemical stages, the SEI can be classified into the “native-SEI (or, N-SEI)” prior to applying bias voltage,10 the “pre-SEI” prior to initial Li plating, and the “subsequent-SEI (or, s-SEI)” constructing after the breakage of pre-SEI.11 An ideal SEI layer should facilitate rapid Li+ conduction while electrically insulated to suppress electrolyte decomposition, and currently the most important effort in designing the SEI layer should be the modification of electrolytes.There are diverse strategies for modifying electrolytes applied in AFLMBs. In general, the selection of solvent is confined to carbonate-based or ether-based solvents, or the “co-solvent”. Compared with carbonate-based electrolytes, ether-based electrolytes have better Li metal compatibility (i.e. less dendrite formation), which ensures a higher Li plating and stripping efficiency. However, the oxidation stability (> 4 V) of ether-based electrolytes usually cannot surpass carbonate-electrolytes.12 As an ether-based solvent, tetraethylene glycol dimethyl ether (TEGDME) can tolerate high potential to ensure a wide operating window.13 Moreover, considering the unstable nature of TEGDME solvent-based electrolyte in the existence of Li metal,14,15 TEGDME was selected in this study in view of studying the performance degradation and enhancement in a broad potential range for AFLMB. Meanwhile, the modification of Li salts in the electrolyte features a broad range of categories, such as dual-salt electrolytes,16 high-concentration electrolytes (HCEs),4,17 and localized high-concentration electrolytes (LHCEs).18 Typically, the composition of the SEI layer is highly dependent on the concentration of electrolyte which is related to the solvation structure of Li+.19 For low-concentration electrolytes (<3 M), free solvent dominated solvation structure (i.e., more interaction between Li+ and solvent molecules) lead to the formation of fragile organic-rich SEI layers, while HCEs (>3 M) can usually assure an anion-involved solvation structure, giving rise to the formation of more rigid inorganic-rich SEI and the enhancement of anti-oxidation ability.20 However, HCEs generally possess higher viscosity and lower conductivity compared to low-concentration electrolyte, and the use of the higher amount of Li salt in HCEs not only reduces the energy density but also raises the cost since Li salt accounts for more than half of the total cost in electrolytes.21,22 Therefore, using low-concentration electrolytes while ensuring their high-voltage stability is considered as a promising electrolyte modification approach in the future. Another common technique in the electrolyte modification is using electrolyte additives. Among them, LiNO3 has been studied by many groups as a powerful additive applied in LMBs,15,23–25 and can be also applied in AFLMBs.26 It can generate the N-rich components in the inner SEI layer including Li3N, LiNxOy, LiNO2, R-NO2, most of which are known as good Li+ conductors that can promote efficient cycling of Li plating/stripping.15,23,25 Furthermore, it was reported that adding LiNO3 as an additive can regulate the morphology of plated Li from dendritic Li to spherical Li to suppress the undesired reacting area between Li and the free solvent.26 In the case of using LiTFSI with the LiNO3 additive, the coordination of TFSI– anions with Li+ can be strengthened, and the decomposition of TFSI– anions can be facilitated to generate low electronic conductive components such as LiF or Li2CO3.24In the present study, the effects of electrolyte concentration and LiNO3 additive on SEI layer structure and Li plating/stripping efficiency were investigated using the Li||Cu half-cell in the TEGDME-based low-concentration electrolytes (0.4 M and 2.0 M) dissolving the LiTFSI salt. In total four kinds of electrolytes were prepared prior to measurements. For convenience, these electrolytes are hereafter abbreviated as 0.4LT, 0.4LTLN, 2.0LT, and 2.0LTLN, where LT means LiTFSI and LN means 0.1 M LiNO3 as an additive. Then the properties of formed SEI layers during electrochemical cycling were elucidated via their abilities in Li-ion/electron conduction, chemical compositions, and surface morphology.2. Results and discussion2.1. Identification of Electrochemical Reactions during CyclingThe first- and second-cycle CV curves measured in 0.4LT were shown in Figure 1a, in which all observed peaks were named by using abbreviated symbols. During the first cycle without (hereafter, w/o) Li plating/stripping, three redox peak couples were observed, corresponding to the reduction and oxidation of electrolyte (REL/OEL),24,27,28 the reduction and oxidation of copper oxides (CuxO) remaining on the Cu substrate before measurement (RCO/OCO),29 and the reduction and oxidation reactions related to pre-SEI formation (RPS/(OPS+OPS')).10,29,30 However, when Li plating/stripping was involved, a strong oxidative current started to flow from 2.2 V, which was related to s-SEI layer construction process occurring on the surface of unstripped Li metal. This process was named as “OSS” which paired with the correspondent reduction process RSS of the next cycle,29 whereas this redox couple was only partially reversible since the peak RSS was apparently weaker. From the second cycle, one can observe another redox couple relevant to s-SEI construction (RSS'/(OSS'+OSS'')) involved in the commonly accepted potential area where the SEI layer usually forms.30 Consequently, it is evident that there are two sets of s-SEI related redox reactions in this electrolyte: the OSS/RSS and the RSS'/(OSS'+OSS''), while the former occurs at a higher potential. Notably, the abovementioned redox couples were observed in all tested electrolytes in this study, except for artificially blocking OSS by scanning up to 2.2 V which presented only the RSS'/(OSS'+OSS'') couple from the second cycle, as summarized in Figure 1b.Figure 1. (a) Enlarged view of the first- and second-cycle CV curves measured in 0.4LT; (b) Enlarged view of the first- and second-cycle CV curves measured in 0.4LT, 0.4LT w/o OSS, 0.4LTLN, 2.0LT and 2.0LTLN with Li plating/stripping. The prime symbols ' and '' on the capitals indicate that the corresponding peaks locate at a lower potential than those without the prime symbol. The arrows represent the direction of scanning. Potential range with Li plating/stripping: -0.5 ~ 3.2 V, and potential range w/o Li plating/stripping: 0.2 ~ 3.2 V. Scan rate: 10 mV/s.2.2. Comparison of CE PerformancesFigure 2a shows the calculated CE values up to 50 cycles in different electrolytes based on their CV curves shown in Figure S1 by taking the influence of concentration, LiNO3 additive and reaction OSS into consideration. Overall, increasing concentration from 0.4 M to 2.0 M enhanced the CE values. The CE in 2.0LT displayed a 4-fold increase at the beginning compared with 0.4LT, while both showed a gradual decrease afterwards, revealing a very unstable Li plating/stripping process during cycling. In the meantime, blocking reaction OSS by scanning up to 2.2 V can effectively stabilize the CE performance in prolonged cycles in both 0.4LT and 2.0LT. This indicates that reaction OSS must be one of the major causes for the CE degradation, which may promote unstable SEI layer formation. Furthermore, adding 0.1 M LiNO3 as an additive in either 0.4LT or 2.0LT can not only enhance but also stabilize the CE performance. Interestingly, this remains true when adding LiNO3 based on blocking OSS, while the opposite operation (blocking OSS based on adding LiNO3) did not show any positive effect. Therefore, it is concluded that adding LiNO3 could significantly compensate for the degradation caused by the reaction OSS, apart from its effect on enhancing the CE values. Besides, the current densities of Li plating/stripping in 0.4LT w/o OSS, 0.4LTLN, and 2.0LTLN were lower than those in 0.4LT and 2.0LT (Figure S1), which could result in a smoother and denser Li morphology.102.3. Li-ion / Electronic Conductivity Studies of The SEI LayerSince the CE performances should be deeply affected by the stability of electrolytes which can induce corresponding electrochemical reactions as cycling proceeds, a detailed analysis of the CV peaks was further conducted within the range of 0 to 3.2 V throughout the cycling process. Figure 2b exhibits the potential and current variations of the peak OSS, RSS' and REL during cycling obtained from the 50-cycle CV results measured in 0.4LT, 0.4LT w/o OSS, 0.4LTLN, 2.0LT and 2.0LTLN (Figure S2), which implies the reconstruction of the SEI layer. Peak OSS and RSS', belonging to the two aforementioned s-SEI related redox reactions, were selected for discussions together with peak REL. Here, the OSS and RSS' currents can be regarded as the indicator of the ionic conduction ability of the s-SEI layer (i.e. the larger or smaller currents represent the higher or lower efficiency of Li+ transport through the s-SEI layer), and the REL currents reflect the electronic conductivity of s-SEI which influences the reaction kinetics between electrolyte and plated Li. For 0.4LT, the largely increased REL currents with a severe polarization, together with the decreased OSS and RSS' currents demonstrate the formation of thick s-SEI layers with poor Li+ conduction and rapid electronic tunneling. However, blocking reaction OSS remarkably reduced the electronic conductivity to suppress the decomposition of electrolyte to a certain extent, whereas the Li+ conductivity still decreased as the RSS' currents almost exhausted within 10 cycles. This could explain why CE became more stable but was not enhanced when blocking reaction OSS in 0.4LT. By increasing concentration to 2.0LT, the Li+ conductivity substantially increased as the OSS and RSS' currents were much larger than those in the 0.4LT after several cycles, but the electrolyte decomposition still proceeded as indicated by the increased REL current. In the case of both 0.4LTLN and 2.0LTLN, one can observe a cathodic peak at 1.70 V belonging to the reduction of NO3– from the second cycle (marked by the triangle symbol in Figure S2), a similar value as that reported by Fu et al. (1.66 V in the first cycle),24 while it was not observed in the first cycle in this study probably due to the overlap from the strong REL peak. This demonstrates that LiNO3 will be reduced prior to the reduction of LiTFSI, which is in agreements with Fu et al.’s report. Notably, it was found that adding 0.1 M LiNO3 not only increased Li+ conductivity but also greatly suppressed the electrolyte decomposition, as supported by the peak current density comparison between the electrolytes with and w/o LiNO3 additive (Figure 2b). However, it should be mentioned that the REL currents in 2.0LTLN gradually increased during cycling and surpassed the REL currents in 0.4LTLN. This might be due to the decomposition of LiNO3, which caused a decrease of LiNO3:LiTFSI ratio rendering the weakened ability to suppress further electrolyte decomposition. Figure 2. (a) 50-cycle CE performances in 0.4LT, 0.4LTLN, 2.0LT and 2.0LTLN (with and w/o the occurrence of OSS); (b) Cycling dependence of the peak potential and peak current density of OSS, RSS' and REL obtained from CV measured in 0.4LT, 0.4LT w/o OSS, 0.4LTLN, 2.0LT and 2.0LTLN.2.4. Composition and Morphology Studies of The SEI layerThe first-cycle QCM test was performed in 0.4LT, 0.4LTLN, 2.0LT and 2.0LTLN, by which the real-time change of mass and surface roughness of the SEI layer were obtained. Prior to the first Li plating, the increase of mass on the Cu surface measured was induced by electrolyte reduction, CuxO reduction, and the pre-SEI formation (Figure 3a). The average MPE (mass change per mol e−) values were then calculated from the slope of the plot with Δm as a function of the passed charge, showing that the deposits in 2.0LTLN were the heaviest among four electrolytes (Figure 3b). The relatively small MPE values in the REL+RCO area could be attributed to: (1) the generation of soluble products or predominant inorganic deposits from TFSI− reduction,31,32 and (2) the reduction of CuxO causing a decrease of deposit mass. Then, pre-SEI formation (RPS) reactions occurred based on the products during REL+RCO reactions. Considering the formation of potential pre-SEI species in the LiTFSI/TEGDME based electrolytes, e.g. Li2O (MPE = 15), LiF (MPE = 26) and Li2CO3 (MPE = 37),33,34 as well as the correspondent reductions among them (for example, Li2CO3 reducing to Li2O, MPE = 7),35 it can be inferred that the major species in the pre-SEI layer was likely to be LiF and Li2O for all electrolytes, while the relative ratio of LiF was higher for 2.0LTLN than that for other 3 electrolytes. Figure 3c shows the first-cycle CV curves as well as potential dependent Δm and ΔR (the change of resonant resistance, which is positively correlated with the change of surface roughness)36 measured in the four electrolytes. It can be found that the surface layer in the 2.0 M electrolytes displayed an elevated roughness before electrochemical measurement compared with 0.4 M electrolytes mainly due to the increased density and viscosity by increasing the concentration,37 whereas the effect of the N-SEI layer should not be ignored. Markedly, both Δm and ΔR increased to various extents after the initial potential scan for all electrolytes, while the overall increase of mass was smaller in the electrolytes with LiNO3 additive. Moreover, the considerably reduced ΔR during Li plating/stripping for the electrolytes with LiNO3 additive can be regarded as a greatly mitigated roughening process.The MPE values during the initial Li plating/stripping in these electrolytes were also calculated for the prediction of possible dominant SEI components generated in this process, which were exhibited in the Δm vs. charge profile (Figure 3d). Here, multiple MPE values were confirmed during Li plating reflecting complex electrochemical processes, where an extra SEI layer was formed presumably due to the reaction of plated Li metal with electrolytes as verified by MPE values much larger than 7 (ideally, it can be regarded as Li plating/stripping exclusively taking place when MPE ≈ 7, i.e., 5 to 9).38 The major species of the SEI layer are more likely to be the organic Li salts (MPE around 50 ~ 80) from decomposed TEGDME solvent since these electrolytes are low-concentrated (< 3 M) and free-solvent dominated.33 It should be noted that the total charge consumed during Li plating should include the charge from associated electrolyte reactions in case of the MPE value larger than 7, thus the actual MPE related to electrolyte reactions is normally larger than the observed one which is dependent on the ratio of the charge from electrolyte reactions. Nevertheless, in this study the electrolytes with LiNO3 additive presented smaller MPE values during Li plating, demonstrating the possibility of more inorganic species generated with the help of LiNO3 (Figure 3d). During Li stripping, the extra mass loss except for the stripped Li was observed for 0.4LT and 2.0LT probably due to the concurrent oxidation of SEI species and/or the crack of SEI layer. In contrast, the MPE closer to 7 for 0.4LTLN and 2.0LTLN indicates a relatively protective inorganic-based SEI was formed during Li plating, with which the reaction of Li metal with electrolyte can be efficiently suppressed.Figure 3. (a) The cathodic process prior to initial Li plating and the simultaneous mass change measured in 0.4LT, 0.4LTLN, 2.0LT and 2.0LTLN; (b) Calculated MPE values during the reaction REL+RCO and reaction RPS, respectively, in above four electrolytes; (c) First-cycle CV curves as well as the potential dependence of Δm and ΔR measured in 0.4LT, 0.4LTLN, 2.0LT and 2.0LTLN. Scan rate: 10 mV/s. The arrows represent the direction of scanning; (d) Calculated MPE values during the initial Li plating/stripping in above four electrolytes. The vertical dash-dotted lines function as borderlines to separate Li plating region and Li stripping region; The gray dashed lines were manually added as a reference showing only Li plating and stripping (MPE = 7).39 The analyses on the cycling-dependent MPE and ΔR were conducted for the 10th and 20th cycles. As shown in Figure 4a, the MPE value of -7 during Li stripping markedly deviated from 7 in the 20th cycle for 0.4LT, showing the simultaneous increase of surface mass originated from more undesired side reactions. Upon increasing the concentration to 2.0LT, the continuously decreased mass except for that contributed by stripped Li may imply the mechanical disconnection of unreactive Li and/or SEI components from the surface, giving rise to a substantial decrease of surface mass from the 10th to the 20th cycle. These behaviors also reflected on the prominent rising of surface roughness in 0.4LT and 2.0LT (Figure 4b). In contrast, by either blocking OSS or adding LiNO3 to 0.4LT, a more stable Li plating/stripping process was achieved as indicated by their MPE close to 7. Moreover, reaction OSS kept on increasing the surface roughness upon cycling, while blocking OSS was not as effective as adding LiNO3 in suppressing the increased roughness. The above discussions were evidenced by the SEM and optical images of electrode surface after cycling among different electrolytes (Figure S3). The whisker-like lithium accumulates in 0.4LT, whereas some film-like products with a relatively smooth surface morphology can be found when blocking OSS. However, a dark grey deposit indicating the formation a thick layer on the Cu surface was visually observed regardless of OSS process.40,41 With the help of LiNO3 additive, smoother morphology and the silver white color of Li deposits proves the formation of denser and dendrite-free SEI layers, and further blocking reaction OSS in 0.4LTLN did not hugely change the surface morphology. Although both blocking OSS and adding LiNO3 additive can result in a better Li stripping reversibility, the fact of lower CE values in 0.4LT compared with those in 0.4LTLN should be associated with the inferior Li+ conductivity as well as higher surface roughness. However, for 2.0LTLN cycled to the 10th and 20th cycles, the increase of surface roughness was not effectively suppressed (Figure 4b), and the MPE value of 1 or 2 during the Li stripping indicates the simultaneous occurrence of electrolyte decomposition in spite of the best CE performance among the tested electrolytes (Figure 4a). This agrees well with the CV results where the REL currents in 2.0LTLN gradually increased upon cycling (see Figure 2b).Figure 4. (a) Calculated MPE values during Li plating/stripping in the 10th and 20th cycles measured in 0.4LT, 0.4LT w/o OSS, 0.4LTLN, 2.0LT and 2.0LTLN. The negative MPE indicates the loss (or growth) of mass during Li plating (or stripping). The vertical dash-dotted lines function as borderlines to separate Li plating region and Li stripping region; (b) Potential-dependent ΔR in the10th and 20th cycles measured in 0.4LT, 0.4LT w/o OSS, 0.4LTLN, 2.0LT and 2.0LTLN.To better understand the detailed variation of SEI compositions at different stages of cycling, the SEI layer formed in 0.4LT, 0.4LT w/o OSS, 0.4LTLN and 2.0LTLN after 50-time cycling were characterized using in-depth XPS analysis by Ar+ sputtering. Figure 5 shows the Li 1s, N 1s, O 1s, S 2p, F 1s, and C 1s XPS spectra of the SEI layers formed at different periods of cycles on the Cu substrate in these electrolytes. In either O 1s or C 1s spectra, two peaks were observed in all electrolytes: the one corresponding to the –C–O–C– bonding of poly(ethylene oxide) ((CH2–CH2–O)n-) or ROCO2Li from the decomposition of TEGDME molecules, and the other corresponding to Li2CO3.15,42 Typically, the LiNO3 decomposition products, such as Li3N and LiNxOy that reportedly enable high Li+ conductivity,15,23 were confirmed for 0.4LTLN and 2.0LTLN (Figure 5a, b). Moreover, the spectra intensities displayed a descending order of 0.4LT > 0.4LTLN > 2.0LTLN, showing an improving stability of s-SEI layers to suppress electrolyte decomposition. In addition, a down-shift of binding energy from the O 1s and C 1s spectra was found for the two 0.4LT electrolytes upon cycling, which was due to the existence of polar decomposed species (such as carbonates, which could involve in further degradation process) on the electrode surface that caused the formation of an electric potential gradient at the interface.43 However, the downshift of binding energy was not observed for 0.4LTLN and 2.0LTLN during cycling, instead the decrease of inorganic constituent (e.g. Li2CO3) and the increase of organic constituent (e.g. ROCO2Li) could be the proof of the formation of heterogeneous SEI layer structure.42,44 Furthermore, Li2O was detected (Figure 5a, c) for 0.4LT w/o OSS, which as discussed above could generate from the SEI layer related reactions including the reactions between plated Li metal and electrolyte. However, Li2O was absent in the O 1s spectra for 0.4LT (Figure 5c). Instead, lithium peroxide (Li2O2) was detected,45 suggesting that Li2O could be oxidized to Li2O2 during OSS in 0.4LT (the co-existence of Li2O and Li2O2 detected in the Li 1s spectra may indicate that this oxidation is an incomplete process).29 Interestingly, Li2O was detected for 0.4LTLN at the occurrence of OSS but was hardly found for 2.0LTLN. It was previously reported that the decomposition of LiNO3 could contribute to the formation of Li2O together with Li3N.23 The formed insoluble Li2O at anode side contributes to construct the SEI layer.46 However, the existence of Li2O2 cannot be excluded since it is difficult to unambiguously distinguish Li2O2 and Li2O in the spectra due to their close binding energy. To this end, in situ SERS measurement was conducted before and after reaction OSS (Figure S4) to clarify whether the oxidation of Li2O occurred or not via OSS in 0.4LTLN and 2.0LTLN. It should be mentioned that the SERS band of Li2O (near 530 cm-1)47 cannot be confirmed due to the broad peak from Cu substrate at 500~600 cm-1. However, in both 0.4LT and 2.0LT, lithium superoxide (LiO2) and Li2O2 were detected after reaction OSS.48–50 This indicates that the SEI components Li2O was oxidized to Li2O2 and insoluble LiO2 on the electrode surface during reaction OSS when using low-donor-number TEGDME as solvent.51–53 However, neither Li2O2 nor LiO2 was confirmed for 0.4LTLN and 2.0LTLN regardless of reaction OSS, which implies that on one hand the LiNO3 additive strongly suppressed the oxidation of Li2O via reaction OSS in 0.4LTLN, on the other hand Li2O was not the major SEI component in 2.0LTLN since both Li2O and its oxidation products were hardly found. The function of LiNO3 additive in suppressing the oxidation of Li2O was further proved by a more careful analysis of the OSS process in 0.4LT and 0.4LTLN via QCM. When repeating the QCM test with the focus on the OSS process in 0.4LT, a sudden change of MPE around 2.7 V was found in all three tests, which was highly likely to be caused by the oxidation of Li2O (Figure S5a),54,55 irrespective of the distinct MPE values among these tests probably due to the complicated physical and chemical phenomena caused by SEI crack and Li dendrite during OSS. In contrast, the sudden change of MPE was not observed in 0.4LTLN, meaning that the oxidation of Li2O hardly occurred during OSS (Figure S5b). Sharon et al. reported that Li2O2 can be oxidized by the reaction product (NO2) originated from LiNO3 which can act as a redox mediator in the Li-oxygen battery.56 However, this process was thought to occur at the cathode side above 3.6 V vs. Li/Li+. Therefore, the possibility of LiNO3 further oxidizing Li2O2 that generated from Li2O oxidation during OSS can be excluded in our study. It is worth noting that the possible reaction showing the oxidation of Li2O to Li2O2 is considered as: 2Li2O – 2e– → Li2O2 + 2Li+      (reaction 1)The correspondent MPE of reaction 1 equals to -7. Assuming that the change of MPE was mostly due to Li2O oxidation, then the ratio of the charge released by electrochemical Li2O oxidation over the total charges during OSS can be calculated. As a result, similar ratios (see Table S1) were confirmed in all tests using 0.4LT, further proving the above assumption. This demonstrates that the oxidation of Li2O was the major OSS reaction that occupied 80 ~ 90% charges of the total charges, which can be effectively suppressed by LiNO3.Since we have demonstrated that the addition of LiNO3 can effectively inhibit Li2O oxidation, it was expected that the total charges transferred during OSS process should be less for the electrolytes with LiNO3 than for the electrolytes w/o LiNO3. However, if we look more carefully at Figure S5, the transferred charges from 2.6 V to 3.2 V for 0.4LT and for 0.4LTLN were very close to each other (5 ~ 6 mC/cm2), meaning that when LiNO3 was added, some extra electrochemical oxidations occurred during OSS that compensated for the reduced charge from Li2O oxidation. Note that the XPS spectra for 0.4LTLN exhibit three major differences from 0.4LT regarding the species (Figure 5): 1. The disappearance of Li2O2; 2. The generation of nitrogen-containing species including Li3N, LiNO2 and a series of LiNxOy components; 3. The disappearance of lithium carbide (Li2C2) during cycling. Since Li2C2 was only related to the deposited Li metal which will be discussed in next section, the only difference created by LiNO3 during OSS process (except for the suppression of Li2O oxidation) was the electrochemical oxidation of specific nitrogen-containing species (equivalent to peak a in Figure S6). The possible reactions can be expressed as: LixNOy – e– → Li(x-1)NOy + Li+ (x>1, y≥0)        (reaction 2)As an example, the oxidation of Li3N to LiNOx (x = 2, 3) can possibly occur, whose reverse reaction occurs around 1.7 V during negative scanning (equivalent to peak a'' in Figure S6).24,57 Considering the fact that the extra reactions instead of Li2O oxidation occurred in the OSS process for 0.4LTLN, such reactions are therefore highly possible to be related to reaction 2. However, the detailed reaction scheme is still unclear and require a further study.The comparison among the four electrolytes regarding the S 2p spectra (Figure 5d) leads to the conclusion that the oxidation of short-chain lithium polysulfides (e.g. Li2S/Li2S2), which were from the reduction of LiTFSI, towards Li2SO4 occurred via OSS.29 This could be related to the degradation of CE performance in 0.4LT.58 It also suggests that LiNO3 additive cannot prevent the oxidation of these short-chain lithium polysulfides which were from TFSI– decomposition, whereas the lack of these species did not degrade the CE performance in the existence of LiNO3 additive, showing the importance of the presence of Li2O rather than Li2S/Li2S2. Notably, the peak intensity of LiF in 0.4LTLN is much weaker than that in 0.4LT (Figure 5e). This observation may, on the contrary, imply that the very-low CE in 0.4LT strongly correlates with the severe decomposition of LiTFSI (thus, more LiF). It was previously reported that LiNO3 additive can facilitate the decomposition of LiTFSI to generate more LiF.24 The different results in our study might be because that we used low-concentration electrolyte while they adopted concentrated electrolyte (3.25 M). Meanwhile, the atomic concentration percentage of the s-SEI composition demonstrates a higher F ratio for 2.0LTLN compared with that for 0.4LTLN (Figure 5g). This also demonstrates that LiF was the major SEI component for 2.0LTLN which played a decisive role in enhancing and stabilizing the CE performance, in line with the above discussions on the pre-SEI species. For the more dilute 0.4 M electrolytes, however, the existence of Li2O (and the existence of short-chain lithium polysulfides when blocking OSS) was rather regarded as the major contributor than LiF to the improved Li plating/stripping efficiency.29Figure 5. (a) Li 1s, (b) N 1s, (c) O 1s, (d) S 2p, (e) F 1s, (f) C 1s depth profiling XPS spectra of the SEI layers for 0.4LT, 0.4LT w/o OSS, 0.4LTLN and 2.0LTLN at different etching times; (g) The atomic concentration percentage of the s-SEI composition after different etching times cycled in 0.4LT, 0.4LT w/o OSS, 0.4LTLN and 2.0LTLN.2.5. Studies of Li Inventory LossAs shown in Figure 5f, Li2C2 was only found in the inner s-SEI layer measured in 0.4LT with the occurrence of OSS, whereas it was observed throughout all s-SEI layers when blocking OSS. On the other hand, it was not detected at any depth of the s-SEI layers formed in 0.4LTLN and 2.0LTLN. It was reported that the formation of Li2C2 is prone to occur on the massively plated Li metal surface.59 These differences therefore indicate that with the occurrence of OSS a poor reversibility of Li plating/stripping will lead to the accumulation of Li2C2, and ultimately a severe loss of Li inventory fundamentally prohibit the further formation of Li2C2. In sharp contrast, a greatly improved reversibility of the dissolution of formed Li2C2 together with an enhanced Li stripping efficiency can be achieved by the LiNO3 additive. The in situ SERS also proves that Li2C2 is the major component of the formed s-SEI layers, which was detected around 1853 cm-1 on the plated Li metal surface (Figure S7), similar as Schmitz et al.’s report.59 During the initial 10 cycles for 0.4LT, the amount of deposited Li2C2 drastically decreased even at -0.5 V where Li was deeply plated, showing the severe loss of Li inventory. Similar tendency was also observed in 2.0LT, while for 0.4LT w/o OSS, 0.4LTLN and 2.0LTLN, the intensity of Li2C2 band almost did not change during cycling, meaning that a more stable SEI layer was formed to suppress Li loss in reacting with electrolyte and reconstructing s-SEI layers. These results were well in accordance with the observed C 1s spectral behaviors. Meanwhile, the depth-dependent atomic concentration percentage also demonstrates the Li inventory change during cycling, as shown in Figure 5g. In the case of 0.4LT, the apparent drop of Li content in the outer SEI layer proves the severe decomposition of electrolyte associated by Li exhaustion. By contrast, Li deposits were more evenly distributed across the s-SEI layers during cycling by blocking OSS, and a homogeneously distribution of Li was almost achieved by the help of LiNO3 additive.2.6. Proposed Model of Major SEI Components with Morphology Variation via OSSIn our previous study, a model showing the possible major SEI components at each stage of the first CV cycle was given in the dilute LiTFSI-TEGDME electrolyte.29 The N-SEI layer contains species such as LiF, Li2CO3 and Cu oxides.10 After electrolyte reductive decomposition, some inorganic species like Li2O mainly constituted the pre-SEI layer. During reaction OSS, these species were thought to be oxidized to construct the s-SEI layer. In this study, a new model was proposed based on above results in order to show the variation of major species via OSS in more details, with the emphasis on the effect of LiNO3 (Figure 6). The major SEI components in the 0.4LT electrolyte prior to reaction OSS include Li2O and Li2Sx. When adding LiNO3, nitrogen-containing species appear to be another major species. When increasing the concentration to 2.0 M, LiF replaces Li2Ox and becomes the major species. In the case of w/o LiNO3 additive, Li dendrite growth proceeds more heavily via OSS due to the change of major SEI components (Li2O → Li2O2, Li2Sx → Li2SOx), resulting in the roughening of electrode surface. By adding LiNO3 additive, the problem can be solved through the prevention of Li2O oxidation, and the resultant electrode surface after OSS will retain rather lower roughness.Figure 6. The proposed model of the change of major SEI components via OSS under the influence of electrolyte concentration and LiNO3 additive in the LiTFSI/TEGDME electrolytes.3. ConclusionsIn conclusion, by adopting the AFLMB anode half-cell (Li||Cu) configuration, this study provided a viewpoint in evaluating the positive and negative impacts of the electrochemical reactions in the potential range of 0 ~ 3.2 V vs. Li/Li+, with an emphasis on the effect of “pre-SEI” layer and “s-SEI” layer modified by concentration and the LiNO3 additive. Specifically, two low-concentration (0.4 M and 2.0 M) LiTFSI/TEGDME electrolytes with and w/o LiNO3 additive were investigated and the performance of Li plating/stripping under different modification conditions were discussed in detail. It was found that purely increasing salt concentration from 0.4 M to 2.0 M was not beneficial to improve the stability of s-SEI layers on the surface under a low-concentration condition due to the unavoidable electrolyte decomposition, while either blocking the reaction OSS at above 2.2 V or adding 0.1 M LiNO3 as an additive could result in the formation of Li2O-based SEI layers with smaller surface roughness (i.e., less amount of dendrite) by blocking its oxidation to Li2O2, suppressing the decomposition of electrolyte as well as enabling a more stable and efficient Li plating/stripping. Meanwhile, the effect of increasing concentration (to 2.0 M) in the existence of 0.1 M LiNO3 additive shifted the dominant SEI species from Li2O to LiF, further assuring the enhanced Li plating/stripping performance. However, in consideration of the gradual decomposition of LiNO3 additive during cycling, the optimization of the LiNO3:LiTFSI ratio should be further considered to achieve better performances based on the concept of low-concentration. In addition, other countermeasures targeted on up-regulating the composition and morphology caused by reaction OSS will be investigated in the future. These results suggest that adding LiNO3 as an additive could be of great help in sustaining the SEI layer stability when the operating potential range of AFLMBs is considered to be broadened. In addition, this study provides a promising strategy to optimize the compositional and morphological conditions that could be degraded by reaction OSS and similar reactions occurring in a higher potential region.ASSOCIATED CONTENTSupporting InformationThe Supporting Information is available free of charge athttps://pubs.acs.org/doi/AUTHOR INFORMATIONCorresponding AuthorHidenori Noguchi − Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-8628, Japan; Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan; orcid.org/0000-0001-9643-168AuthorsYushen Wang − Graduate School of Chemical Sciences and Engineering, Hokkaido University, Sapporo 060-8628, Japan; Center for Green Research on Energy and Environmental Materials, National Institute for Materials Science (NIMS), Tsukuba 305-0044, Japan; orcid.org/0000-0001-6461-0607AUTHOR CONTRIBUTIONH.N.: Provided the conception and design of the study.W.Y.: Acquired data analysis.H.N., W.Y.: Interpreted the results.W.Y.: Wrote the draft of the manuscript.H.N.: Provided the study materials, laboratory samples, instrumentation, computing resources, and other analysis tools.H.N.: Acquired the financial support.H.N., W.Y.: Analyzed and interpreted the data, and revised the manuscript critically for important intellectual content.H.N.: Managed and coordinated responsibility for the research activity planning and execution.CONFLICTS OF INTERESTThe authors declare no competing financial interest.ACKNOWLEDGMENTSThe authors gratefully acknowledge the NIMS Battery Research Platform.REFERENCES1 X.-B. 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Passerini and M. Winter, SEI investigations on copper electrodes after lithium plating with Raman spectroscopy and mass spectrometry, J. Power Sources, 2013, 233, 110–114.2image3.pngimage4.tiffimage5.tiffimage6.emfCurrent densityw/o LiNO30.4 м   Li2O   Li2SxOSSLi2O2Li2SOx▼LiLi2.2 V{   major speciesw/ LiNO30.4 мLi2OLi2SOxLi(x-1)NOy▼▼Li2.2 VOSS   Li2O   Li2SxLixNOy{    major speciesPotential vs. Li/Li+     LiF   Li2SxLixNOyw/ LiNO32.0 мLiFLi2SOxLi(x-1)NOy▼Li2.2 VOSS{   major speciesimage1.pngimage2.png