Revealing and Overcoming Unfavorable Electrochemical Behaviors of Thick LiNbO3-Coated NCM523 for All-Solid-State Lithium Batteries

All-solid-state batteries (ASSBs) are very promising for next-generation energy storage technologies owing to several key advantages including higher power density, better thermal and electrochemical stability, and improved safety for electric vehicles. In this work, bulk-type ASSB cells were prepared with 0–25 nm thick LiNbO3 coatings, and their electrochemical behaviors at different upper cutoff voltages (upper cutoff potential ≥ 4.40 V) were systematically compared. A thicker coating caused three unfavorable electrochemical behaviors in the first three cycles: (1) a higher overpotential, (2) sluggish discharge kinetics, and (3) capacity fading. The measured electronic conductivity decreased drastically with increasing coating thickness, suggesting that this may have caused behaviors (1–3). To overcome this, a carbon additive was used to improve electronic transport in the composite cathode and successfully suppressed the aforementioned behaviors. Our findings indicate that the combination of a thick LiNbO3 coating on NCM523 and carbon additive can achieve synergistic effects to improve both the electrochemical properties and durability of ASSB cells.

][6][7] ASSBs with high energy densities are in high demand to achieve a long driving range.][10] Using higher voltages (⩾4.40 V vs Li/Li + ) is an effective way to further enhance the energy density of ASSBs. 11,12However, several challenges must be addressed to realize next-generation ASSB architectures.4][15][16] The poor interface stability is caused by the narrow thermodynamic electrochemical stability window of sulfide-based SEs (1.7-2.3V vs Li/Li + ) and the tendency of CAMs to oxidize the sulfide electrolyte upon physical contact, in particular at high charging potentials. 17,18These interfacial reactions decompose SEs into undesirable passivated products with poor ionic conductivity.Furthermore, they form resistive spacecharge layers and induce elemental diffusion from both CAMs and SEs, leading to a constant increase in the SE/CAM interfacial resistance.The reversible interfacial reaction causes a high initial capacity loss and low Coulombic efficiency.
0][21][22][23] Those studies demonstrated that surface engineering is pivotal for improving the electrochemical performance of ASSBs.In our previous research, we developed a new technique to produce LiNbO 3 coating layers with different uniformities and thicknesses.Our results showed that (1) a thick (average thickness ⩾9 nm) and uniform coating layer could significantly enhance the cycling performance of a full-cell ASSB in the potential range of 3.0-4.35V (vs graphite) and (2) the uniformity and thickness of the coating layer can change the relative severity of interfacial stability by three different breakdown mechanisms. 24However, because our previous study focused on enhancing the cycle performance of full-cell ASSB, additives such as vapor-grown carbon fibers (VGCF) and binder were added to the composite cathode to maximize the battery performance.Meanwhile, understanding is lacking on the individual effects from the additives and thick LiNbO 3 coating layers on the chargedischarge behaviors of ASSB cells.In addition, since the study used a lower upper cutoff voltage, electrochemical behaviors of the ASSB cells and side reactions at the SE/CAM interface when cycled at higher voltages (upper cutting potential ⩾4.40 V vs Li/Li + ) to obtain higher energy density, remain to be clarified.
Two opposite effects may take place when these cells are cycled at higher voltages.On one hand, a thick LiNbO 3 coating might impede Li + mobility and thus increase charge transfer resistance at the interface, resulting in poor charge-discharge behaviors. 25On the other hand, a thick LiNbO 3 coating might help stabilize the SE/CAM interface, minimizing the decomposition reaction and enhancing the overall battery performance. 24It is necessary to understand which effect is dominant and (if applicable) under which conditions.7][28] Nevertheless, there is very limited knowledge on the combined effects of applying a thick LiNbO 3 coating and adding conductive carbon to the composite cathode of ASSB cells.
To reveal any possible performance bottleneck of ASSB cells incorporating NCM523 with thick LiNbO 3 coating and their use at a higher upper cutoff voltage, it is necessary to closely examine the fundamental charge-discharge properties of materials in these cells.To eliminate the influence of additives, in the current study chargedischarge curves were recorded without conductive carbon or binders in the composite cathodes.The method proposed by Siroma et al. 29 was used to quantitatively evaluate the ionic and electronic conductivities of NCM523 cathodes at different LiNbO 3 coating thicknesses (average: 0-25 nm).The conductivities were used to interpret the electrochemical properties of the samples.Morino et al. reported that float charging tests, in which cells are continuously charged and soaked at higher voltages and temperatures, are useful for testing the durability of ASSBs (i.e., degree of interfacial degradation at high voltage). 30,31Here, similar float charging tests were carried out to reveal the degree of degradation z E-mail: bong.shunkai@libtec.or.jpECS Advances, 2024 3 020503 at the SE/CAM interface and the synergistic effects from the thick LiNbO 3 coating and conductive carbon additive.
Our results demonstrate that insufficient electronic conductivity is likely the main cause of the bottleneck in ASSB cells that use thick LiNbO 3 -coated NCM523 as CAM (namely high overpotential, sluggish discharge kinetics, and capacity fading).However, this bottleneck can be overcome by adding an appropriate amount of carbon additive (vapor-grown carbon fibers, VGCF) to the composite cathode.The combination of thick LiNbO 3 coating and VGCF synergistically improves the charge-discharge kinetic behavior and durability of the ASSB cells.The results provide valuable information about the effect of thick LiNbO 3 coatings on the electrochemical properties of NCM523 and ways to optimize the battery performance of ASSBs.
Since this study aims to investigate the direct influence of LiNbO 3 coating layer thickness on the charge-discharge behavior of ASSB cells, we adopted two types of coatings that were used in our previous study, namely Nb-1 (average thickness: 1-3 nm) and Nb-3 (average thickness: 9-18 nm), 24 and denoted them here as LNb-3 and LNb-9, respectively.In addition, since LNb-9 might still not be thick enough for studying the charge-discharge behavior of thick LiNbO 3 coatings, thicker coating samples, LNb-12 and LNb-15, were fabricated using the coating method reported in our previous study.
Specifically, LNb-12 and LNb-15 were fabricated using the same NCM523 powder.To prepare the LiNbO 3 -coated layers, Li-Nb double alkoxide was dissolved in anhydrous ethanol and used as the precursor. 21After adding bare NCM523 particles to the precursor solution, the mixture was applied through a high-pressure gas-flow fluidized coating machine (JD-1, Kawata) to fabricate LiNbO 3 -coated layers with average thicknesses of 12-22 (LNb-12)  and 15-25 nm (LNb-15).The thickness of the coated layers was adjusted by varying the number of coatings as described in the literature. 24All the samples were then heated under oxygen for 3 h at 300 °C to obtain LiNbO 3 -coated layers in an amorphous state.
The uniformity and morphology of the coated layers were investigated using a scanning electron microscope attached to an Auger electron spectrometer (SEM-AES, PHI710, Ulvac-Phi) and a field-emission scanning electron microscope (FE-SEM, JSM-7900F, JEOL) with an energy-dispersive X-ray (EDX) detector (Ultim Max, Oxford Instruments) at an accelerating voltage of 15 keV.The thicknesses of the LiNbO 3 -coated layers were evaluated using highresolution transmission electron microscopy (HR-TEM), and EDX elemental mapping was performed using a TEM apparatus (JEM-ARM200F, JEOL).The relationship between the amount of LiNbO 3 on the NCM523 surface and the coating thickness was investigated by X-ray fluorescence (XRF, ZSX Primus III NEXT, Rigaku).
Cell assembly.-BareNCM523 or LiNbO 3 -coated NCM523 particles and argyrodite-type SE at a weight ratio of 70:30 were manually mixed in an agate mortar for 10 min to prepare the composite positive electrode.Bulk-type ASSB cells were then constructed as follows.First, a pellet of the separator layer was formed by pressing SE (127.3 mg) at 98 MPa in a ceramic cylinder with a cross-sectional area of 1 cm 2 .Subsequently, the cathode mixture (17.7 mg) was uniformly spread on one side of the separator layer, before pressing it with the SE separator layer at 588 MPa.Finally, the In-Li alloy was pressed at 98 MPa onto the other side of the separator layer to provide a stable anode potential of 0.62 V (vs Li/Li + ).The sandwiched pellet cell was held between two stainless steel (SS) rods that served as current collectors for the positive and negative electrodes.The cell was attached horizontally using three bolts and placed in a gas-closed container for electrochemical measurements under a pressure of 200 MPa.The carbon-based conductive additive (VGCF) was added to bare NCM523, LNb-3, −9, and −15 at a weight ratio of NCM523:SE:VGCF = 71:26:3, in order to verify the effect of electronic conductivity on the cell's electrochemical behavior.The ratio of VGCF (3 wt%) is considered as appropriate to provide enough electronic pathways in the composite cathodes based on the study by Walther et al. 28 The cells were first charged and discharged for three cycles between 3.0-4.25,3.0-4.40,and 3.0-4.55V (vs Li/Li + ) under a constant current (CC) density of 200 μA cm -2 (C/10) and held at a constant potential (CP) with a cutoff current of 20 μA cm -2 (C/100) at 25 °C.The cycles were applied using a charge-discharge measuring device (580, Scribner) in a thermostatic chamber (SU-222, Espec).AC impedance measurements of the pellets were carried out for all cells using an impedance analyzer (VSP-300, Biologic) in the frequency range of 10 -2 -10 6 Hz with an applied voltage of 10 mV at 4.40 V (vs Li/Li + ) after three charge-discharge cycles.
Ionic and electronic conductivity measurement of composite cathodes.-Toinvestigate the charge transport in the composite cathodes, their electronic and ionic conductivities were measured.The LiNbO 3 -coated NCM523 particles and argyrodite-type SE were mixed at 50:50 v/v (i.e., 70:30 w/w) in a mortar, and 100 mg of the mixture was pressed at 588 MPa into a pellet with an area of 1 cm 2 .The pellet was placed between two SS plates as current collectors.The pelletized cells were attached horizontally using three bolts and placed in an airtight container.The electronic and ionic conductivities of the cells were measured via the AC impedance technique at 25 °C, using the method proposed by Shiroma et al. 29 AC impedance measurements were performed using an impedance analyzer (VSP-300, Biologic) in the frequency range from 10 -2 to 10 6 Hz at an applied voltage of 10 mV.
High-voltage float charging test.-For the high-voltage durability test, all cells were first charged and discharged for three cycles between 3.0-4.55V (vs Li/Li + ) under a constant current (CC) density of 200 μA cm -2 (C/10) and held at a constant potential (CP) with a cutoff current of 20 μA cm -2 (C/100) at 25 °C.AC impedance measurements were then carried out for all cells using an impedance analyzer (VSP-300, Biologic) in the frequency range of 10 -2 -10 6 Hz at an applied voltage of 10 mV at 4.40 V (vs Li/Li + ) after three charge-discharge cycles.In the subsequent high-voltage float charging test, the cells were charged at a constant current of 200 μA cm −2 up to a voltage of 4.55 V (vs Li/Li + ) and then held at 4.55 V (vs Li/Li + ) for 72 h at 80 °C.Afterwards, all cells were charged and discharged between 3.0-4.55V (vs Li/Li + ) at 25 °C and AC impedance measurements were also performed again under the same conditions.

Results and Discussion
Characterization of the uniformity and thickness of LiNbO 3 coating layers.-SEM-AESand SEM-EDS analyses were conducted to evaluate the overall uniformity of the coating layers.Figure 1a shows the SEM-AES elemental mapping results for Nb and Ni for LNb-3, −9, −12, and −15.Because the maximum detection depth of SEM-AES is approximately 5 nm, the color of elemental mapping could be used to estimate the approximate thickness of the coated layer.The red, yellow, and green colors denote the uncoated area, coated layer with a thickness of approximately 5 nm, and coated layer thicker than 5 nm, respectively.As ECS Advances, 2024 3 020503 reported in our previous study, LNb-3 has the thinnest coating layer, and many areas on it have barely any coating. 24In contrast, LNb-9, −12, and −15 are fully covered with homogenous coating layers thicker than 5 nm.Since SEM-AES has difficulty evaluating coating layers thicker than 9 nm, SEM-EDX elemental mapping was also employed to provide a deeper detection depth (about a few μm). Figure 1b shows the SEM-EDX elemental mapping results of Nb and Mn for all the samples.The Nb concentration clearly increased with coating thickness on the NCM523 particle surface, particularly when the coating was thicker (e.g., LNb-9, −12, and −15).
Cross-sectional TEM measurements were performed to gain insight into the surface layers and verify the average thickness of the LiNbO 3 -coated layers.Figure 1c shows the cross-sectional TEM images and TEM-EDX elemental mapping images of Nb. Figure 1d shows the high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) images for LNb-3, −9, and −15.In Fig. 1c, the thickness of the coated layers increased from LNb-3 to −9 and −15, with the average coating thicknesses of approximately 1-3, 9-18, and 15-25 nm, respectively.Regarding the uniformity of the coating layer, it is obvious that the NCM523 particles in LNb-3 still displayed some uncoated areas, while those in LNb-9 and −15 were fully covered by a thick and uniformly coated layer.To further investigate the nature of the LiNbO 3 -coated layers, an enlarged region of the surface of NCM523 particles is shown in Fig. 1d.No crystalline lattice was observed in the LiNbO 3 -coated layers, confirming that all the samples were covered with a LiNbO 3 coating layer in the amorphous state.
The ratio of Nb to (Ni + Co + Mn) (at%), which represents the amount of LiNbO 3 on the NCM523 surface of each sample, was obtained from XRF measurements.The results in Fig. 1e confirm that the amount of LiNbO 3 on the NCM523 surface increased with increasing coating thickness, which agrees well with the SEM-EDX results in Fig. 1b.The thickness and uniformity of the coated layers of each sample observed by cross-sectional TEM were also in good agreement with the SEM-AES and SEM-EDX mapping results.They revealed that the surface engineering techniques used in this study successfully produced LiNbO 3 -coated layers with different thicknesses and uniformities.
Electrochemical behavior of composite cathodes with LiNbO 3 -coated layer of different thickness.-Tofurther verify the direct influence of the electronic and ionic characteristics of the different cathodes (bare NCM523, LNb-3, −9, −12, and −15) on the performance of assembled ASSBs cycled in different voltage windows, the cells were charged/discharged at a C/10 rate and room temperature.Figure 2a shows the charge-discharge profiles of the first three cycles, with an upper cutoff potential of 4.25 V (for bare NCM523, LNb-3, and −15.LNb-9 and −12 were not measured because of insufficient sample) or 4.40 and 4.55 V (vs Li/Li + ) (for bare NCM523 LNb-3, −9, −12, and −15).In the first cycle, all LiNbO 3 -coated samples showed higher mean Coulombic efficiencies (⩾90%) than that of the bare NCM523 (⩽87%), indicating that the LiNbO 3 coating lowered the irreversibility.This confirms that the LiNbO 3 -coated layer acted as a protective layer to suppress the degradation of SE.
Next, we compared the mean Coulombic efficiency in the first cycle among the four LiNbO 3 -coated samples cycled at the upper cutoff potentials of 4.25 and 4.40 V (vs Li/Li + ).The mean Coulombic efficiency shows an increasing trend with the coating thickness: the most thinly coated sample (LNb-3) gave the lowest value of 90%-91%, while the thickest one (LNb-15) gave the highest value of 93%.This tendency implies that a thick LiNbO 3 coating (⩾9 nm) can further stabilize the interface by suppressing the SE decomposition and inhibiting side chemical reactions at the SE/ CAM interface, because the irreversibility of the initial specific capacity usually reflects the degree of side reactions at the interface. 17,28However, three unfavorable electrochemical behaviors were observed for LNb-9, −12, and −15 at both upper cutoff voltages: (1) overpotential during charging (Li + de-intercalation reaction), (2) sluggish discharge kinetics, and (3) capacity fading.For example, at the upper cutoff voltages of 4.25 and 4.40 V (vs Li/Li + ) in Fig. 2a, LNb-3 displays higher 3rd cycle specific discharge capacities (162 and 186 mAh g -1 ) than those of LNb-15 (151 and 170 mAh g -1 ).We tentatively attribute these unfavorable charge-discharge behaviors to two (overlapping) effects: (i) degradation caused by SE oxidation or chemical reaction at the SE/CAM interface, and (ii) poor electronic or Li + transport owing to the thick LiNbO 3 coating.However, because the mean Columbic efficiency of the initial cycle increased with the LiNbO 3 coating thickness, we concluded that (ii) is more likely the main reason than (i).
When cycled at the highest upper cutoff potential (4.55 V vs Li/Li + ), the bare NCM523 cell showed the lowest initial Columbic efficiency of 84% compared to the same cell cycled at 4.25 or 4.40 V (vs Li/Li + ).Moreover, the three unfavorable electrochemical behaviors (1-3) mentioned above became more pronounced at 4.55 V (vs Li/Li + ).These phenomena can be attributed to severe interfacial instability caused by the higher tendency of bare NCM523 to oxidize SE when the upper cutoff voltage rises to 4.55 V (vs Li/Li + ).More specifically, as reported in our previous study, 24 because the electronic conductivity of bare NCM523 is higher than that of the LiNbO 3 -coated samples, the higher electronic conductivity at the CAM/SE interface is likely to cause severe SE degradation of bare NCM523 when the upper cutoff voltage rises to 4.55 V (vs Li/Li + ).This severe SE degradation could also be caused by the higher tendency of SE oxidation, elemental interdiffusion, and/or electrochemical/chemical reactions at the interface.Consequently, more passivated degradation products are produced at the CAM/SE interface as the upper cutoff voltage increases, leading to greater interfacial resistance and lower Li + ion diffusion in the cathode composite.This results in more pronounced unfavorable electrochemical behaviors (1-3) compared with those of the cells that cycled at 4.25 or 4.40 V (vs Li/Li + ).
Among the LiNbO 3 -coated samples, LNb-3 and −9 showed the highest 3rd cycle discharge specific capacities (200 and 194 mAh g -1 , respectively), revealing that the protective effects of the LiNbO 3 layer remained strong at the higher upper cutoff voltage to suppress side reactions at the interface.However, when the coating layer was thicker than 9 nm, lower specific discharge capacities were observed for LNb-12 (184 mAh g -1 ) and LNb-15 (176 mAh g -1 ).In addition, the unfavorable electrochemical behaviors (1-3) were observed in all LiNbO 3 -coated samples, especially in LNb-12 and −15 at a cutoff voltage of 4.55 V (vs Li/Li + ).We assumed that the larger amount of passivated products caused by interfacial side reactions and the lack of electronic or ionic charge transport in the composite electrodes may increase the interface resistance, which brings forth significant changes in the chargedischarge profiles for all samples when the cutoff voltage rises to 4.55 V (vs Li/Li + ).However, because samples with a LiNbO 3 layer thinner than 12 nm (i.e., LNb-3 and −9) showed the highest discharge capacity among others, which indicates a stable interface, it is possible that the influence from ionic or electronic transport might be less pronounced when the LiNbO 3 coating is thinner than 9 nm.
Figure 2b shows the corresponding differential capacity (dQ/dV) curves as a function of cell potential for the first three cycles at the upper cutoff potentials of 4.25, 4.40, and 4.55 V (vs Li/Li + ).All curves have similar shapes, and the pair of peaks at approximately 3.73 V are attributed to the phase transition from hexagonal (H1) to monoclinic (M).To further verify the sample polarization, the peak shift between the 1st and 3rd cycle during charging (ΔV) is also plotted in Fig. 2b.The peaks of bare NCM523 and LNb-3 showed almost no shift when the upper cutoff voltage was raised to 4.25 or 4.40 V (vs Li/Li + ), while those of composite cathodes with thicker coating displayed larger ΔV values at higher cutoff voltages.For example, at the upper cutoff voltage of 4.55 V (vs Li/Li + ) LNb-3 showed the smallest ΔV (0.04 V) while LNb-15 showed the largest ΔV (0.25 V).These results correspond well with the chargedischarge profiles in Fig. 2a, revealing that polarization increased with both the coating thickness and the upper cutoff voltage.
Bare NCM523 exhibited a much smaller ΔV at the upper cutoff voltage of 4.55 V (vs Li/Li + ) compared to LNb-15 (0.06 and 0.25 V, respectively).Since bare NCM523 is assumed to generate the most amount of passivated interfacial decomposition products (with the SE and CAM particles in direct contact at the SE/CAM interface), we expected this sample to show the strongest polarization, i.e., the largest ΔV value.However, the opposite results were observed, with ECS Advances, 2024 3 020503 the largest ΔV occurring in the sample with the thickest LiNbO 3 coating (LNb-15).This result suggests that SE degradation caused by side reactions at the SE/CAM interface may have less effect on the polarization of samples with thick LiNbO 3 coatings compared with bare NCM523.For example, we assumed that the ΔV values of LNb-15 would be similar when cycled at different upper cutoff voltages, because the coating thickness remained the same.However, the ΔV value actually increased significantly at an upper cutoff voltage of 4.55 V (vs Li/Li + ) compared to that at 4.25 or 4.40 V (vs Li/Li + ).This result suggests that, besides insufficient electronic or ionic charge transfer owing to the thick LiNbO 3 coating, other factors may contribute to the remarkably increased polarization of LNb-15 when cycled at 4.55 V (vs Li/Li + ).Possible factors include volume expansion and contraction of the CAM during charging and discharging, which would lead to poor interfacial contact between the CAM and SE. 32o verify the cell resistance of the samples, electrochemical impedance spectroscopy (EIS) analysis was performed at 4.40 V (vs Li/Li + ) for all cells after cycling between 3.0-4.40and 3.0-4.55V (vs Li/Li + ). Figure 3a shows the equivalent circuit and Fig. 3b shows the representative Nyquist plots of bare NCM523 and LNb-15 after cycling between 3.0-4.55V (vs Li/Li + ).As shown in Fig. 3b, the Nyquist plot of bare NCM523 contains two semicircles in the low (10 2.8 -10 -0.5 Hz) and high (10 6 -10 2.8 Hz) frequency regions, which represent the charge transfer resistance (R ct ) and surface film resistance (R sf ), respectively.The short intercept in the highfrequency region is assigned to ohmic resistance (R s ) derived from the SE separator.For LNb-15, R sf and R ct were determined in accordance with the low and high frequency regions of bare NCM523.
Figure 3c displays the Nyquist plots of all samples after cycling between 3.0-4.40and 3.0-4.55V (vs Li/Li + ).For each cell, the cell resistance increased from 4.40 to 4.55 V (vs Li/Li + ), which is in agreement with the degree of polarization shown in Fig. 2b. Figure 3d shows the ratios of R sf and R ct between the LiNbO 3 -coated samples and bare NCM523 at the upper cutoff potential of 4.55 V (vs Li/Li + ).Clearly, R sf increased remarkably with increasing coating thickness, with LNb-15 exhibiting an R sf value 4.5 times higher than that of bare NCM523.This result implies that thick LiNbO 3 coatings may hinder ion or electron transport at the interface, leading to a high cell resistance.In contrast, the LiNbO 3 -coated samples exhibited much smaller R ct values than that of bare NCM523, suggesting that side reactions at the interface were significantly suppressed by the introduced LiNbO 3 coating regardless of its thickness.
The total resistance (R sf + R ct ) and its increase ratio (ΔR) were calculated for all cells cycled at 3.0-4.55 to 3.0-4.40V (vs Li/Li + ) using the following equation where the subscripts "a" and "b" refer to the values measured at 3.0-4.55and 3.0-4.40V (vs Li/Li + ), respectively.The results are plotted in Fig. 3e.Corresponding to Fig. 3c, the same cell always has a higher total resistance when cycled at 3.0-4.55V (vs Li/Li + ) compared to when cycled at 3.0-4.40V (vs Li/Li + ).Meanwhile, bare NCM523 showed the highest ΔR value of 3.7, whereas all LiNbO 3 -coated samples only exhibited small variations in their total resistance (ΔR ≈ 1.2-1.4).This implies that the additional passivated products at the interface of bare NCM523 with no surface engineering can cause a significant increase in cell resistance (mainly the R ct component, as shown in Fig. 3b).
Overall, these observations suggest that the unfavorable chargedischarge behaviors (1-3) shown in Fig. 2a are more likely due to the second hypothesized reason, namely a thick LiNbO 3 coating retards electronic or ionic charge transport in the composite cathodes, resulting in a higher cell resistance.

Ionic and electronic charge transfer in composite cathodes.-
To study the mechanisms behind the unfavorable charge-discharge behaviors and verify the reasons for the higher R sf for thick LiNbO 3 coatings, it is necessary to directly measure the ionic and electronic conductivities of the composite cathodes.Figure 4a shows the electron-electron connection formed between the SS electrodes used to measure the ionic and electronic conductivities of bare NCM523 and LNb-3, −9, −12, and −15. Figure 4b shows the Nyquist plots of the composite cathodes consisting of NCM523 particles and SE in the same ratio (50:50 v/v or 70:30 w/w) used for the charge-discharge electrochemical measurement in Fig. 2. Two depressed semicircles were observed.Siroma et al. reported that the "teardrop" shape at low frequencies is due to ionic and electronic resistances from a transmission line model. 29The high-and lowfrequency limits of the teardrop shape correspond to σ ion = r ion r e L/(r ion +r e ) and σ e = r e L; where r ion , r e , and L are the ionic resistance, electronic resistance, and length of the composite, respectively.Using this model, we determined the electronic and ionic conductivities for all samples from the AC polarization measurements and summarized the results in Table I. Figure 4c plots the electronic and ionic conductivities at different LiNbO 3 coating thicknesses.Interestingly, all samples had higher ionic conductivity than electronic conductivity, suggesting that the unfavorable electrochemical behaviors are more likely associated with the electronic conductivity.Compared to bare NCM523 or LNb-3 with thin coating, the ionic conductivities of other composite electrodes showed only a slight decrease with increasing coating thickness.Because amorphous LiNbO 3 is among the best coating materials owing to its relatively high Li + conductivity, we thought that the ionic path of the composites was not obstructed by the thick coatings. 23n contrast, Table I shows that the electronic conductivity of the composites decreased drastically with an increase in coating thickness, with the lowest value observed in LNb-15 (3.7 × 10 -7 S cm -1 , two orders of magnitude lower than that of bare NCM523 at 4.9 × 10 -5 S cm -1 ).These results confirm that thick LiNbO 3 coatings lower the electron conductivity in the composite cathode, which is most likely the cause of unfavorable charge-discharge behaviors (1-3) observed in thick LiNbO 3 coatings (Fig. 2a).
Tackling the bottleneck of thick LiNbO 3 coating by adding carbon conductive.-Manyresearchers have reported that because VGCF is composed of sub-micrometer-long fibers, it more readily forms a continuous path for electronic conduction within the composite electrode than other carbon additives such as acetylene black. 27,28Therefore, adding VGCF to ASSB cells is the key to lower the resistance of composite electrodes with low electronic conductivity.To verify and overcome the bottleneck associated with thick LiNbO 3 coatings, we employed VGCF as a conductive additive in the LNb-15 composite electrode.
Figures 5a and 5b show the charge and discharge curves in the 1st and 3rd cycles of LNb-15 composite cathodes with and without VGCF at an upper cutoff voltage of 4.55 V (vs Li/Li + ).Remarkably, the unfavorable electrochemical behaviors (1-3) were completely eliminated in the cell with added VGCF.The overpotential was significantly suppressed, and the specific capacity was greatly increased.These results verify that the low electronic conductivity causes the bottleneck when using thick LiNbO 3 coatings, and it can be tackled by adding an electronically conductive additive (VGCF) to the cell.Figures 5c and 5d show the corresponding dQ/dV curves as a function of the cell potential for the 1st and 3rd cycles of LNb-15 with and without VGCF.The addition of VGCF caused the hexagonal (H1) to monoclinic (M) phase transition peaks to shift to lower potentials (3.78 to 3.75 V for the 1st cycle and 4.07 to 3.75 V for the 3rd cycle), while the peak at 3.75 V (vs Li/Li + ) in the charging curve remained unshifted between the 1st and 3rd cycles.Therefore, cell polarization was significantly suppressed by the addition of VGCF as a conductive additive, which is expected to lead to a much lower cell resistance.
Figures 5e and 5f show the cell resistance measured by EIS for all cells after three cycles at 4.40 V (vs Li/Li + ).The addition of VGCF decreased cell resistance remarkably by a factor of approximately 28.5 compared to cells without VGCF (485 to 17 Ω cm).Simultaneously, the unfavorable electrochemical behaviors (1-3) in Fig. 2a completely vanished; Figs.5a and 5b demonstrate the effective suppression of overpotential, electrochemically active charge-discharge kinetics, and an increase in capacity.
Figures 6a and 6b show the SEM image and corresponding SEM-EDX elemental mapping image of C, S, and Ni of the LNb-15 composite cathode after mixing in an agate mortar for 10 min.The SEM image (Fig. 6a) demonstrates that the sub-micrometer-long ECS Advances, 2024 3 020503 VGCFs were distributed homogeneously in the mixture.In addition, the elemental mapping image (Fig. 6b) revealed that the VGCF bridged all the CAM particles, which is believed to provide sufficient electronic pathways among the CAM particles.Because the cells were fabricated under a high pressure of 588 MPa, it is also possible that some of the VGCFs might have pierced through the thick layer of electronically insulating LiNbO 3 and embedded on the CAM surface during cell assembly.
Schematic illustrations of electron migration in the composite cathodes without and with VGCF are shown in Figs.7a and 7b, respectively.In the cell without VGCF, we suppose that the electronic transport channel between CAM particles could be blocked by the electronically insulating SE.This reduction in electronic channel in the composite cathode leads to a lower electronic conductivity and a higher cell resistance. 27,28In contrast, adding VGCF to the cell results in a more effective electronic transport network, because the carbon fibers bridge the CAM particles electronically over a wider space (Fig. 6b).The results are improved electronic path within the composite cathode, a higher electronic conductivity, and a lower cell resistance during the charging-discharging process.In conclusion, our results proved that the addition of VGCF to thickly coated samples can overcome the bottleneck due to the thick coating and enhance the cell's electrochemical performance.
7][28] Therefore, it is important to explore the durability of ASSB cells incorporating both thick LiNbO 3 -coated NCM and VGCF.To this end, float charging tests were conducted on bare NCM523, LNb-3, −9, and −15 cells at an upper cutoff voltage of 4.55 V (vs Li/Li + ) by charging the cells continuously at a constant potential of 4.55 V (vs Li/Li + ) for 72 h at an elevated temperature of 80 °C.The temperature of 80 °C was used because it enables a clearer evaluation of the durability performance of the samples.Specifically, because more kinetic energy is provided to the reactant at the SE/CAM interface at higher temperatures, the chemical and electrochemical reactions rates increase.The capacity retention rate (%) was calculated using the following equation: CC capacity after float charging CC capacity of 3rd cycle before float charging 100 where CC refers to the constant current.
Figure 8a shows the charge-discharge profiles before and after the float charging test, as well as the CC capacity retention rate after the float charging test.Bare NCM523 without surface engineering showed the lowest capacity retention rate (0.1%) compared to the LiNbO 3 -coated samples, revealing extremely severe side effects.This cell also failed to function after the float charging test.Among the LiNbO 3 -coated samples, their capacity retention rate increased with the thickness of the coating layer, with LNb-3, −9, and −15 exhibiting a capacity retention of 68%, 91%, and 93%, respectively.These results reveal that a thicker LiNbO 3 coating layer can greatly suppress the interfacial side reactions even when the composite cathodes contained 3 wt% VGCF.It is also noteworthy that LNb-15 showed a high capacity retention rate of 93%, suggesting that an even thicker LiNbO 3 coating layer might provide better durability.
To measure the cell resistance before and after the float charging test, EIS analysis was performed at 4.40 V (vs Li/Li + ) after three cycles.EIS analysis was performed at 4.40 V (vs Li/Li + ) rather than at 4.55 V (vs Li/Li + ) because of the significant increase in resistance   shows the smallest increase ratio in cell resistance and this resistance decreases at a higher LiNbO 3 coating thickness, it seems that a thick LiNbO 3 coating provides a stable interface with less interfacial side reactions and passivated decomposition products compared to the case of a thin coating.The floating results are in accordance with the trends of capacity retention rate during cycling reported in our previous study, 24 meaning that a thicker and more uniform LiNbO 3 coating layer enhances the cyclability.
To provide a thorough understanding and to formulate a hypothesis regarding the increase of cell resistance after floating, Fig. 8c schematically illustrates the possible SE degradation area due to the float charging test within the bare NCM523, LNb-3, −9 and −15 composite cathodes at the interface of SE toward the VGCF, current collector, and CAM particles.Bare NCM523 shows the largest resistance after floating.Therefore, it is reasonable to assume that the high electronic conductivity of uncoated CAM particles caused the formation of the largest area of SE degradation products, inhibiting Li + ion diffusion in this area and yielding the lowest capacity retention rates after float charging test.All the LiNbO 3 -coated samples contained the same amount of VGCF, hence we presume similar degradation area at the SE/VGCF and SE/current collector interfaces in these samples.However, the observed decrease in cell resistance for thicker coatings after floating implies that the degradation area at the CAM/SE interface could be reduced due to a lower electronic conductivity and suppression of side reactions, thereby enhancing Li + ion diffusion.Consequently, the smaller degradation area leads to a better capacity retention rate.Walther et al. purposed that a combination of X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS) can be used to detect and confirm the formation of degradation products in the composite cathodes. 28To verify our assumption regarding the degradation area within the composite cathodes, further comprehensive analysis should be carried out on the degradation products at the interface of composite cathodes.
Overall, the present study further confirms that the bottleneck associated with thick LiNbO 3 coatings can be tackled by adding an appropriate amount of VGCF, which improves both the chargedischarge behavior and durability compared to bare NCM523 or thin LiNbO 3 coatings (e.g., LNb-3).However, it is also important to point out that the different types of conductive additives and the mixing ratio of cathode materials/SEs/carbon additives might affect the durability, because the balance between ionic and electronic charge transfer through the composite cathodes can change accordingly.In addition, because of the tradeoff between coating thickness and rate performance, it is difficult to determine the thickness that maximizes battery performance.Nevertheless, it is important to confirm that combining VGCF and a thick LiNbO 3 coating that fully covers the NCM523 particles in the composite cathodes can synergistically optimize the electrochemical performance of ASSBs.

Conclusions
Our previous study revealed that thick and uniform LiNbO 3 coatings on NCM523 particles can improve the electrochemical performance of bulk-type ASSB full cells when cycling between 3.0-4.35V (vs graphite).To reveal any possible performance bottleneck of ASSB cells incorporating NCM523 with thick LiNbO 3 coating and their use at a higher upper cutoff voltage, the  ECS Advances, 2024 3 020503 fundamental electrochemical behaviors of the bulk-type ASSBs cells were explored.Unfavorable electrochemical behaviors of (1) overpotential during charging (Li + de-intercalation reaction), (2) sluggish discharge kinetic behavior, and (3) capacity fading was observed in the ASSBs cells when no electronic conductive additive was added to the composite cathode.To explore the reasons for such behaviors (1-3), we further examined the ionic and electronic transfer in cells with LiNbO 3 -coated layers at different thicknesses.The results showed that the possible cause of this bottleneck is the insufficient electronic conductivity in the presence of a thick and uniform LiNbO 3 coating, and this issue could be overcome by incorporating an electronically conductive additive (VGCF) in the composite electrode.Specifically, an appropriate amount of VGCF led to a significantly lower resistivity and the unfavorable electrochemical behaviors (1-3) were eliminated.
Regarding VGCF, its high electronic conductivity may accelerate the degradation of SEs at high voltages, which would reduce the durability of the ASSB cells.To examine this possibility, float charging tests were conducted on cells at different LiNbO 3 coating thicknesses.Thicker LiNbO 3 coatings still led to higher capacity retention rates, whereas the thickest coating (15-25 nm) yielded a capacity retention rate of 93%.Moreover, results from the float charging tests also demonstrated that the charge-discharge behaviors and interfacial stability can be synergistically improved by adding VGCF to cells incorporating thick LiNbO 3 coatings.
Overall, we demonstrated an effective interface engineering design with conductive coating strategies that can achieve uniform and amorphous LiNbO 3 -coated layers of various thicknesses.Our findings provide insights on how adjusting the thickness of LiNbO 3 coating can directly influence the electrochemical properties of ASSBs, especially when cycled at high voltages (upper cutting potential ⩾4.40 V vs Li/Li + ).In addition, our findings reveal that the combination of a thick LiNbO 3 coating on NCM523 and a carbon additive can achieve synergistic effects to improve both the electrochemical properties and durability of ASSB cells.The obtained results are vital for determining how the coating thickness affects its protective performance at higher voltages, suggesting better ways for interface design in future ASSBs.

Figure 1 .
Figure 1.(a) AES elemental mapping images of Ni and Nb and (b) SEM-EDX elemental mapping images of Mn and Nb for LNb-3, −9, −12, and −15.(c) Cross-sectional TEM images and TEM-EDX mapping images of Nb (the void at the lower right part of the LNb-9 images is caused by detachment of the coating layer while thinning down the thin section during FIB milling) and (d) HAADF-STEM images for LNb-3, −9, and −15, (e) Ratio of Nb to (Ni + Co + Mn) (at %) obtained from XRF measurements for LNb-3, −9, −12, and −15.(The results for LNb-3 and −9 in Figs.1a, 1c, and 1d are adapted with permission from Ref. 24).

Figure 2 .
Figure 2. (a) Capacity curves and (b) differential capacity (dQ/dV) curves of bulk-type ASSB cells for bare NCM523, LNb-3, −9, −12, and −15.The upper cutoff potential was 4.25, 4.40, and 4.55 V (vs Li + /Li) at a rate of 0.1 CC, 0.01 CV.The initial Columbic efficiency and 3rd cycle capacity of each sample are indicated in panel (a), and the peak shift between the 1st and 3rd cycles (ΔV) during charging is indicated in panel (b).

Figure
Figure 8b shows the Nyquist plots of all cells before and after the float charging test.LNb-15 clearly displays the smallest semicircle in the impedance spectra after floating (27 Ω), whereas the other samples showed much larger ones (36, 90, and 20369 Ω for LNb-9, LNb-3 and bare NCM523, respectively).Black texts in Fig. 8b also label the increase ratio after floating, which is approximately 1.4 for LNb-15 and much higher for the other samples (3, 4.5, and 81 for LNb-9, LNb-3, and bare NCM523, respectively).Because LNb-15

Figure 5 .
Figure 5. Capacity curves and differential capacity (dQ/dV) curves for (a), (c) the 1st cycle and (b, d) the 3rd cycle of LNb-15 with and without VGCF, measured at an upper cutoff potential of 4.55 V (vs Li/Li + ) and a current rate of 0.1 CC, 0.01 CV.(e) Nyquist plot and (f) an enlarged part of the Nyquist plot of LNb-15 with and without VGCF after 3 cycles.The addition of VGCF led to a higher capacity and a lower resistance.

Figure 6 .
Figure 6.(a) SEM image and (b) SEM-EDX elemental mapping image of C, S, and Ni for the composite cathode mixture of VGCF, argyrodite-type SE, and LNb-15.

Figure 7 .
Figure 7. Schematic illustrations of electron migration in composite cathodes (a) without and (b) with VGCF.

Figure 8 .
Figure 8.(a) Capacity curves and (b) Nyquist plots of bulk-type ASSB cells with bare NCM523 and LNb-3, −9, and −15 composite cathodes with VGCF at 25 °C, before and after the float charging test.(c) Schematic illustration showing the possible SE degradation area within the composite cathodes at the interface of SE toward VGCF, current collector, and CAM in the cases of bare NCM523, LNb-3, −9, and −15 after the float charging test.