Ji-Hoon
Han
ab,
Do Kyung
Kim
c,
Young Joo
Lee
cd,
Young-Su
Lee
*a,
Kyung-Woo
Yi
b and
Young Whan
Cho
*a
aEnergy Materials Research Center, Korea Institute of Science and Technology (KIST), Seoul 02792, Republic of Korea. E-mail: oze@kist.re.kr
bDepartment of Materials Science and Engineering, Seoul National University, Seoul, 08826, Republic of Korea
cWestern Seoul Center, Korea Basic Science Institute, Seoul 03759, Republic of Korea
dDepartment of chemistry, Chung-Ang University, Seoul 06974, Republic of Korea
First published on 6th November 2023
Solid electrolyte is a crucial component of all-solid-state batteries, with sulphide solid electrolytes such as lithium argyrodite being closest to commercialization due to their high ionic conductivity and formability. In this study, borohydride/halide dual-substituted argyrodite-type electrolytes, Li7−α−βPS6−α−β(BH4)αXβ (X = Cl, Br, I; α + β ≤ 1.8), have been synthesized using a two-step ball-milling method without post-annealing. Among the various compositions, Li5.35PS4.35(BH4)1.15Cl0.5 exhibits the highest ionic conductivity of 16.4 mS cm−1 at 25 °C when cold-pressed, which further improves to 26.1 mS cm−1 after low temperature sintering. The enhanced conductivity can be attributed to the increased number of Li vacancies resulting from increased BH4 and halide occupancy and site disorder. Li symmetric cells with Li5.35PS4.35(BH4)1.15Cl0.5 demonstrate stable Li plating and stripping cycling for over 2,000 hours at 1 mA cm−2, along with a high critical current density of 2.1 mA cm−2. An all-solid-state battery prepared using Li5.35PS4.35(BH4)1.15Cl0.5 as the electrolyte and pure Li as the anode exhibits an initial coulombic efficiency of 86.4%. Although these electrolytes have limited thermal stability, it shows a wide compositional range while maintaining high ionic conductivity.
New conceptsIn this study, we have achieved one of the highest ionic conductivities among argyrodite-type electrolytes reported so far by simultaneously substituting halide and BH4 for sulphur via room temperature mechanochemical reaction without a post-heating process. XRD and solid-state NMR spectroscopy suggest that this high ionic conductivity is due to disordered occupancy of BH4 and halide at the 4a and 4d sites in the argyrodite structure. In previous studies, the amount of substitution of BH4 was very small or only BH4 was substituted alone, but this is the first case in which high conductivity was obtained by simultaneously substituting BH4 and halide, with the total amount of substitution up to 1.8. These electrolytes exhibit a very wide compositional range and high tolerance to second phase impurities while maintaining the high ionic conductivity. It also shows good compatibility with Li metal from 0 to 5 V. The high critical current density resulting from the high ionic conductivity, along with the wide composition range and good electrochemical stability, makes these electrolytes very promising for application in all-solid-state lithium rechargeable batteries. |
Attempts to substitute S2− with BH4− have been rarely reported and their ionic conductivity was rather low.12–15 Yamauchi et al.14 first reported a glass solid electrolyte with a composition of (100 − x)(0.75Li2S–0.25P2S5)·xLiBH4. In their subsequent studies, they increased the LiBH4 content and obtained a conductivity of 1.8 mS cm−1 at 25 °C.15 The structure was identified as an argyrodite with the chemical formula Li7−xPS6−xXx, where X is BH4−. Dao et al. synthesized an argyrodite in which BH4− anions partially replaced Cl− or I− and obtained a conductivity of 0.4 and 0.76 mS cm−1, respectively.13,16 Wang et al. reported on BH4−-substituted Li6PS5Cl electrolytes with a low conductivity of 0.12 mS cm−1 at room temperature.12 They heated the sample to 550 °C, and it is likely that BH4− did not substitute for X− but decomposed. Furthermore, they ball milled all the precursors together. LiBH4 and P2S5 were reported17 to react with each other and our thermodynamic calculations have indicated that LiBH4 and P2S5 react to form compounds such as H2S, Li2S and B2S3. In fact, when we ball-milled LiBH4 and P2S5 together, the pressure inside the milling container abruptly increased due to the generation of very toxic H2S gas, and a rapid exothermic reaction was also observed.
Sun et al. recently synthesized Li5.91PS4.91(BH4)1.09 and reported a conductivity of 4.8 mS cm−1.18 Very recently, we have synthesized electrolytes by ball milling LiBH4 and Li3PS4 and obtained electrolytes with a high ionic conductivity of up to 11 mS cm−1 at 25 °C.19 We also demonstrated that the changes in conductivity can be attributed to local structural variations caused by adopting different ball milling conditions. Inappropriate ball milling conditions can lead to the formation of non-stoichiometric thiophosphate units such as P2S74− and P2S64−, which negatively impacts the ionic conductivity.19
In the present study, we have synthesized mixed pseudohalide/halide-substituted Li-argyrodite-type electrolytes with a composition of Li7−α−βPS6−α−β(BH4)αXβ (X = Cl, Br, I; 0 ≤ α + β ≤ 1.8) using a two-step ball-milling method without post-annealing. The as-synthesized electrolytes exhibit very high ionic conductivity up to 16.4 mS cm−1 at room temperature. To explain the reason for the high conductivity, the site occupancies of the anionic species were analysed using Rietveld refinement of the XRD data. Solid-state magic angle spinning (MAS) NMR spectroscopy has also been used to analyse the motion of Li ions and the local environments at each crystallographic site. The Li5.35PS4.35(BH4)1.15Cl0.5 electrolyte shows promising electrochemical properties, including high initial coulombic efficiency (ICE), high critical current density (CCD), and good anodic and cathodic stability.
Fig. 1b presents XRD patterns of commercial Li6PS5Cl and Li3PS4 + xLiBH4 + 0.5LiCl (1.5 ≤ x ≤ 2.5). Excess LiBH4 is required because LiBH4 is partially decomposed during ball milling. As the content of LiBH4 in the sample increased, the main peak position shifted to a lower angle. This is because the size of the BH4− anion is much larger than that of the Cl− or S2− anion and the lattice parameter increases with an increasing amount of BH4−. Table S6 (ESI†) displays the sizes of the halides and BH4− anions and the volume per formula unit for their respective Li compounds.21 The phase appearing in the XRD pattern was determined to be an argyrodite-type phase, and the diffraction peak shifted to a lower angle with increasing LiBH4, indicating that BH4− occupied 4a and/or 4d sites in the argyrodite structure.15,18,19
The lattice parameters and occupancy of each atomic site of the identified argyrodite-type phase were obtained by Rietveld refinement.22 All the parameters were based on the data of Li6PS5Br, which has similar lattice parameters,23,24 and we introduced a few constraints to extract the occupancies of each ionic species. First, for Li, because of the poor quality of the XRD data, it was not possible to refine the commonly known lithium sites such as T5, T2 in 48h and T5a in 24g.4,23,24 Therefore, all the lithium was assumed to be located at the T5 site for 48h. The results are presented in Table 1 and Tables S8–S11, S13–S17 in the ESI.† The T5 site for 48h, as shown in Table 1, and the corresponding position presented in the ESI† were reported based on the neutron diffraction data.23,24
Atom | Site | x | y | z | Occupancy |
---|---|---|---|---|---|
Space group, F3m; lattice parameter, a = 9.9636(8) Å; Rwp = 3.70%, Rexp = 2.00%, Rp = 2.76%. | |||||
Li | 48h (T5) | 0.3139 | 0.0219 | 0.6861 | 0.446(19) |
P | 4b | 0 | 0 | 0.5 | 1 |
S | 16e | 0.1189(2) | −0.1189(2) | 0.6189(2) | 1 |
S | 4d | 0.25 | 0.25 | 0.75 | 0.215(14) |
B | 4d | 0.25 | 0.25 | 0.75 | 0.535(14) |
Cl | 4d | 0.25 | 0.25 | 0.75 | 0.25 |
S | 4a | 0 | 0 | 0 | 0.136(13) |
B | 4a | 0 | 0 | 0 | 0.614(13) |
Cl | 4a | 0 | 0 | 0 | 0.25 |
H | 4d | 0.3135 | 0.1865 | 0.8135 | 0.267(7) |
H | 4d | 0.3135 | 0.3135 | 0.8135 | 0.267(7) |
H | 4a | 0.0635 | −0.0635 | 0.0635 | 0.307(6) |
H | 4a | 0.0635 | 0.0635 | 0.0635 | 0.307(6) |
Again, the quality of the XRD data do not allow us to independently fit the occupancies of the three different anionic species at the 4a and 4d sites. Moreover, the atomic scattering factors of S2− and Cl− are similar. Therefore, we assumed that all 0.5 Cl− occupies 4a and 4d sites evenly (see the ESI† for the details on the assumption).
Applying the aforementioned constraints, we performed the Rietveld refinement and the key features are summarized in Fig. 1c and d. The BH4− occupancy at the 4a and 4d sites and the lattice parameter of the argyrodite phase increase as the amount of LiBH4 increases. The sample with the highest ionic conductivity (x = 2) has BH4− occupancies at the 4a and 4d sites of 0.61 and 0.54, respectively (Table 1). This result indicates that BH4− and Cl− substituted 1.15 (α) and 0.5 (β) of the two non-bridging S2−, respectively. Therefore, the chemical composition of the argyrodite-type phase can be given as Li5.35PS4.35(BH4)1.15Cl0.5.
In most halide-substituted argyrodites, the Wyckoff 16e site is solely occupied by S2− forming PS43− tetrahedra, whereas the 4a and 4d sites are occupied by S2− and the halide. It has been reported that ordering, S2− at the 4d site and I− at the 4a site, whereas disordering is favoured for Cl−/Br− substitution.8,26 Interestingly, disordering of anions on 4a/4d sites has been suggested to invoke high conductivity in halide-substituted argyrodites.26 As anion mixing on 4a/4d sites results in variation of the coordination environment of P and 31P MAS NMR is sensitive to the local structure, valuable information about anion disorder can be obtained using 31P MAS NMR. Deiseroth et al. reported that a single sharp 31P signal was observed for ordered I−-substituted argyrodites, whereas broad overlapping signals were detected for disordered Cl−- and Br−-substituted argyrodites.8 Feng et al.6 demonstrated that multiple discrete signals were seen for Li6−xPS5−xCl1+x with varying intensity ratio as a function of Cl− substitution content and they assigned each signal to the local P structure surrounded by (S2−)3(Cl−)1, (S2−)2(Cl−)2, (S2−)1(Cl−)3, and (Cl−)4 in their 2nd coordination shell. 31P MAS NMR spectra of Li3PS4 + xLiBH4 + 0.5LiCl (1.5 ≤ x ≤ 2.5) display a broad Gaussian-like signal at higher frequency than that of the structural analogues Li6PS5Cl and Li7PS6 (Fig. 2b). The unresolved broad signals suggest that S2−, Cl−, and BH4− are randomly distributed over 4a/4d sites and that PS43− tetrahedra are located at highly disordered local environments, which is consistent with the XRD refinement results (Table 1).
The 11B MAS NMR spectra of Li3PS4 + xLiBH4 + 0.5LiCl (1.5 ≤ x ≤ 2.5) show a broad dominant signal at a similar frequency to that of LiBH4, indicating that boron is indeed incorporated into the argyrodite structure as BH4. Very weak signals are also present at −2 to 15 ppm and −30 to −20 ppm, which can be assigned to the borates and unknown impurity with no bonded hydrogen, respectively. The amounts of these impurities are negligibly small. This assignment was confirmed by a 11B{1H} cross-polarization experiment (see Fig. S3, ESI†). In contrast to the highly symmetric 11B NMR signal of LiBH4, which is consistent with the highly symmetric environment of BH4− tetrahedra, the 11B NMR signals of Li3PS4 + xLiBH4 + 0.5LiCl (1.5 ≤ x ≤ 2.5) are asymmetric. There are two possible explanations for the asymmetric line shape. First, the BH4− tetrahedra at 4a/4d sites in argyrodite structures could be distorted, and the effect of the non-zero quadrupolar interaction leads to the line broadening. As 11B (I = 3/2) are quadrupolar nuclei, the characteristic line shape of the central transition influenced by the second-order quadrupolar interaction will appear if boron resides in non-symmetric environments. However, simulation with various quadrupolar coupling constants and asymmetry parameters could not reproduce our spectral pattern. Second, multiple signals resulting from BH4− at different Wyckoff sites could overlap, leading to an asymmetric line shape. This explanation is more plausible considering the Rietveld refinement results, which indicate the distribution of BH4− at both 4a and 4d sites. The 11B NMR signals of Li3PS4 + xLiBH4 + 0.5LiCl (1.5 ≤ x ≤ 2.5) could be deconvoluted into two components at −43 and −44 ppm with different intensity ratios among samples (Fig. 2d–f). In particular, for the sample with x = 2.0, the intensity ratio of the signals at −43 ppm vs. −44 ppm is 46:54, which is consistent with the occupancy results from the Rietveld refinement (4d site:4a site = 47:53, Table 1). Thus, we tentatively assign the 11B NMR signal at −43 and −44 ppm to BH4− residing at 4d and 4a sites, respectively. Our results are not consistent with previous reports, where two clearly resolved signals for substituted halides were observed, i.e., one sharp and one broad signal. For Cl-substituted argyrodite, a very sharp and a broad 35Cl NMR signal were assigned to 4d and 4a sites, respectively.6 However, Deiseroth et al. reported that a broad 35Cl signal was seen for Cl-substituted argyrodite, whereas a very narrow and a broad 79Br NMR signal were observed for Br-substituted argyrodite.8 These two signals have been assigned to ordered and disordered domains of the material. It is likely that boron in BH4 is dominantly affected by the 1st coordination environment, which is symmetric tetrahedra, and the effect from the 2nd coordination environment is not significant. Thus, boron at 4a and 4d sites gives rise to 11B NMR signals at a similar frequency.
(1) |
As the amount of substituted BH4− increases, the conductivity reaches a maximum when x = 2 and starts to decrease when x becomes higher than 2. There are three possible explanations for this behaviour. First, as mentioned earlier, the XRD and solid-state NMR results show that free LiCl was observed when x is higher than 2.25. The same was observed when the molar ratio of LiX (X = Cl, Br, I) to Li3PS4 was 0.75 or higher. As LiX (X = Cl, Br, I) have very low ionic conductivity, their existence would certainly lower the conductivity. However, only a small amount of LiCl remains for Li3PS4 + 2.25LiBH4 + 0.5LiCl (1.4 wt%) and Li3PS4 + 2.5LiBH4 + 0.5LiCl (2.2 wt%). Therefore, the drop in conductivity due to the remaining LiCl is likely to be small.
Second, the decrease in conductivity can be related to the difference in distribution between the 4a and 4d sites of the substituted anion. For Li6PS5Cl and Li6PS5I, it is well known that the difference in conductivity is mainly due to the disorderly distribution of Cl− over the 4a and 4d sites, whereas I− only occupy the 4a site.26–28 According to the Rietveld refinement results in Fig. 3d, as the amount of LiBH4 increases, more BH4− occupy the 4a site, which may cause a drop in conductivity. This result is consistent with the NMR spectra as shown in Fig. 2e and f. Therefore, it is suggested that the difference in distribution of BH4− between the 4a and 4d sites is also responsible for the decrease in conductivity despite the increase in the total amount of substitution.
The third explanation for the decrease in conductivity is that an unknown phase begins to appear in samples containing higher than 2.25 mol of LiBH4. Fig. 1d shows that the amount of total substitution exceeds 1.7 when x is higher than 2.25. As reported in previous studies,29–31 when the total amount of halide substitution exceeds 1.7, the ionic conductivity decreases due to the existence of a secondary phase. In addition, the estimated composition of Li5.35PS4.35(BH4)1.15Cl0.5 indicates that four moles of S2− in β-Li3PS4 is not sufficient to provide the necessary amount of S2−. In fact, the estimated compositions from the Rietveld refinement deviate from the Li3PS4–LiX line in the tentative ternary phase diagram, as illustrated in Fig. S4 (ESI†). Therefore, the second phase has inevitably to form and the existence of this unknown phase is thought to be one of the main causes of the drop in ionic conductivity. Because this unknown phase could not be analysed by XRD or NMR, further study is needed to identify it.
We also compared the conductivity of Li3PS4 + 2LiBH4 (without LiCl) with that of Li3PS4 + 1.5LiBH4 + 0.5LiCl with a similar amount of BH4− + Cl− substitution. For Li3PS4 + 2LiBH4, the BH4− occupancy at the 4a and 4d sites is 0.79 and 0.62, respectively (Table S18, ESI†), giving the composition of Li5.59PS4.59(BH4)1.41. For Li3PS4+ 1.5LiBH4 + 0.5LiCl, the BH4− occupancy of the 4a and 4d sites is 0.37 and 0.46, respectively (Table S8, ESI†), giving the composition of Li5.67PS4.67(BH4)0.83Cl0.5. Although the total amount of substitution is slightly less, the conductivity of Li3PS4+ 1.5LiBH4 + 0.5LiCl is higher (10.1 mS cm−1) than that of Li3PS4 + 2LiBH4 (8.4 mS cm−1). Upon appropriate synthesis, the argyrodite-type electrolytes containing both BH4− and halide anions tend to have higher conductivity than those solely substituted with only one type of X (X = Cl, Br, I, BH4).
In general, Li-argyrodite shows an increasing trend of ionic conductivity with a higher number of substituted halides (see Table S19, ESI†). Nevertheless, the conductivity of cold-pressed Li-argyrodite is generally limited to a maximum of around 9 mS cm−1, even with the maximum halide substitution.6,25,30,32–34 In the case of BH4-substituted argyrodite, when cold-pressed, it exhibits an ionic conductivity of up to 11 mS cm−1 at room temperature. When a halide is simultaneously substituted with BH4−, it reaches a maximum conductivity of 16.4 mS cm−1. In addition, Li5.3PS4.3ClBr0.7 has been reported to have a maximum ionic conductivity of 24 mS cm−1 after high temperature sintering.10 In the present study, Li5.35PS4.35(BH4)1.15Cl0.5 has a higher conductivity of 26.1 mS cm−1 after warm pressing (see Fig. S7, ESI†). Several theoretical studies18,19,35,36 have been conducted to elucidate the possible mechanisms behind the enhanced ionic conductivity of BH4-substituted argyrodite. Fang et al.35 demonstrated through molecular dynamics (MD) simulations and density functional theory (DFT) calculations that BH4− substituted argyrodite can exhibit up to 177 mS cm−1 of ionic conductivity at room temperature. This is possible due to the paddle-wheel effect induced by the rotation of BH4− clusters, promoting the motion of Li ions. It is reported that this paddle-wheel effect also exists in other electrolytes containing PS43− anions.36 Moreover, a recent study has shown that the high ionic conductivity in Na3OBH4, which has an antiperovskite structure with BH4− substitution, is also attributed to the rotation of BH4− clusters.37 Very recently, we have also reported that,19 through DFT calculations, BH4− anions in the argyrodite structure at 4a and 4d sites could exhibit a higher degree of disorder, leading to increased ionic conductivity and that the interaction between Li+ and S2−/PS43− could be weakened, resulting in a more uniform Li+ distribution.
However, Sun et al.18 recently presented results from ab initio molecular dynamics (AIMD) simulations, suggesting that the paddle-wheel effect arising from the BH4− cluster is not correlated with the motion of Li ions in the argyrodite structure. The high conductivity observed in BH4−-substituted argyrodite is attributed to the weak interaction between Li+ and BH4−, resulting in a weaker anchoring effect.18 In summary, while there is strong consensus regarding the positive effect of BH4− substitution on ionic conductivity, a unified explanation of why BH4− substitution enhances conductivity to a greater extent than simple halide substitution has not yet been established.
To investigate the thermal stability of our samples at much higher temperatures, two annealing conditions were applied for Li5.35PS4.35(BH4)1.15Cl0.5: annealing at 200 °C for 4 h followed by 300 °C for 1 h and annealing at 300 °C for 12 h followed by 550 °C for 2 h. The XRD and EIS results are presented in Fig. S6 (ESI†). When the sample was annealed at 300 °C for 1 h, the total resistance increased by about 15 times, and the conductivity was measured to be 1.2 mS cm−1 at room temperature. The XRD data show a notable shift of the peaks towards higher angles, due to a contraction in the lattice parameter of the argyrodite phase in the absence of BH4−. The Rietveld refinement results of the XRD data support this explanation, as the occupancy of BH4− converged toward zero (Tables S21 and S23, ESI†). For the sample annealed at 550 °C for 2 h, its colour turned black, and the conductivity could not be measured. In the XRD pattern, only the peaks corresponding to Li2S and LiCl were detected, indicating that the argyrodite structure was completely destroyed. As mentioned before, LiBH4 decomposes at high temperatures, which may be the reason why the conductivities of some heat-treated samples at high temperatures were quite low.12,16 These results suggest that post-annealing at high temperatures is not suitable for the argyrodites containing BH4−.
As sintering can significantly enhance the ionic conductivity mainly due to the reduction in grain-boundary resistance,38–40 we performed warm pressing for the pellets with the composition Li3PS4 + xLiBH4 + 0.5LiCl (1.5 ≤ x ≤ 2.5). As argyrodite containing BH4− decomposes above 160 °C, we pressed the pellets at 120 °C for 2 h. After cooling to the room temperature, the ionic conductivity was measured again and significantly increased up to 26.1 mS cm−1 (Fig. S7, ESI†). To determine whether compositional or chemical changes occurred during this heat treatment, the conductivity was measured again by cold pressing after finely grinding the warm pressed pellets with a mortar and pestle inside a glove box, and the conductivity returned to the original value before warm pressing. Based on these results, we believe that the main cause of significant increase in ionic conductivity of the warm pressed electrolyte is mainly due to the reduction of grain boundary resistance through the annealing effect, but not from the change in chemistry after thermal treatment.
Critical current density (CCD) was measured to evaluate the dendrite suppression capability of synthesized electrolyte. CCD represents the maximum current density that a lithium battery can endure through cycling without cell failure or shorting caused by lithium dendrite growth. Electrolytes with an argyrodite structure are generally known to have relatively low (0.1–1 mA cm−2, see Table S24, ESI†) CCD values.41–50 CCD is influenced by various factors, such as temperature, thickness, and stack pressure, leading to variations in its value depending on the measurement conditions.51,52 There are two different CCD testing methods: the time-constant mode, in which the cycling period is kept constant while gradually increasing the areal capacity, and the capacity-constant mode, in which areal capacity is fixed while gradually decreasing the cycling period.53 In this study, both methods were employed for more accurate measurements.
Fig. S9a (ESI†) presents the CCD profile of cold-pressed Li5.35PS4.35(BH4)1.15Cl0.5 measured in the time-constant mode, obtained by setting the fabrication pressure at 356 MPa and the stack pressure of 7 N m using a torque wrench. The sudden voltage drop observed at a current density of 2.1 mA cm−2 indicates a short circuit due to dendrite growth within the solid electrolyte, confirming the high CCD value. On the other hand, the CCD value measured in the capacity constant mode under the same conditions recorded 2.5 mA cm−2, as shown in Fig. S9b (ESI†). It is well-established that materials with elevated ionic conductivity tend to achieve a uniform current density distribution, thereby enhancing dendrite suppression capabilities.51 Consequently, the higher CCD at 2.1/2.5 mA cm−2 aligns with the high ionic conductivity of the electrolyte.
An all-solid-state cell was constructed using Li5.35PS4.35(BH4)1.15Cl0.5 as the electrolyte, a lithium-metal foil anode, and a LiNbO3-coated LiNi0.8Co0.1Mn0.1O2 cathode. The main aim was to investigate the stability between the electrolyte and the cathode. CV measurements were conducted by varying the cut-off voltage from 4.3 up to 4.7 V (vs. Li+/Li), while scanning between 2.6 and a maximum 4.7 V (vs. Li+/Li) at a scanning rate of 0.1 mV s−1. As shown in Fig. S10a (ESI†), all cyclic voltammograms exhibit three distinct pairs of anodic and cathodic peaks. These reversible peaks appear at 3.73, 4.00, and 4.21 V, as well as at 3.71, 3.97, and 4.15 V. These peaks correspond to the phase transitions of NCM 811, a well-known cathode material with transitions from hexagonal to monoclinic (H1 to M), monoclinic to hexagonal (M to H2), and hexagonal to hexagonal (H2 to H3) phases.54–56 Up to a cut-off voltage of 4.5 V, the CV curve demonstrates only slight variations, suggesting that the electrolyte does not induce irreversible reactions or decomposition with the cathode material within this range. However, beyond 4.6 V, undesirable side reactions begin to appear, leading to a shift in the oxidation peak towards higher potentials over a wider voltage range. This indicates a potential promotion of solid electrolyte decomposition, especially at voltages beyond 4.6 V. To check whether these results are primarily due to increased voltage or just a result of cycling, CV measurements were repeated for 10 cycles at a scanning rate of 0.1 mV s−1, with a fixed cut-off voltage of 4.3 V (Fig. S10b, ESI†). Despite undergoing 10 cycles up to 4.3 V, notable changes were limited to a slight increase in overpotential. This result underscores the electrochemical stability of the electrolyte on side reactions at elevated voltages against the LiNbO3-coated LiNi0.8Co0.1Mn0.1O2 cathode.
It is well known that LiBH4 exhibits good chemical compatibility with lithium metal.57 As we confirm that no significant change was observed in the CV measurement, Li metal was used as the anode. The cell was cycled at different C-rates between 2.7 and 4.3 V (versus Li+/Li) at 30 °C. The capacity was calculated based on the mass loading of the cathode active materials. Fig. 4c presents the voltage profiles at various C-rates. At 0.05C, the initial charge and discharge capacities were 217.1 and 187.4 mA h g−1, respectively, and the initial coulombic efficiency was 86.4%. The irreversible capacity is believed to result from contact loss in the charge/discharge cycles and the formation of an interfacial layer between the electrolyte and electrodes. This phenomenon is well-known and has been observed in many ASSB studies.58,59Fig. 4d illustrates the rate performance of the cell. It performed quite well even at 5C (69.5 mA h g−1), indicating that the solid electrolyte functions stably at high C-rates. After 25 cycles, with increasing C-rates up to 5C, the discharge capacity at 0.05C was recovered up to 179.5 mA h g−1, with a capacity retention of 95.8%. This result indicates that the electrolyte performs well even at a high C-rate without decomposition. The cycle performance with the Li–In anode at 0.5C is shown in Fig. S14 (ESI†). The charge and discharge capacity were well maintained up to 100 cycles. The discharge capacity after 100 cycles at 0.5C was 121.46 mA h g−1, 72.6% of the initial discharge capacity.
Footnote |
† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3mh01450a |
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