SPM analysis for composite positive electrodes. SPM measurements were conducted on composite positive electrodes consisting of NMC and Li3PS4 glass SEs before and after the initial charge test. Figure S1 (Supplementary information) shows the initial charge curve of the all-solid-state cell, which was charged to 4.4 V (vs. Li+/Li) at 25 °C at a current density of 0.13 mA cm–2; it exhibited an initial charge capacity of 164 mAh g–1. Before SPM measurements, we conducted ion milling on the cells to prepare flat samples. Subsequently, KPFM, C-AFM, SSRM, and SEM-EDX were conducted sequentially on the composite positive electrodes (Figure 1). An air-protected sample holder was used to transfer samples between the ion milling apparatus, SPM, and SEM. The negative electrode side was placed in contact with an insulating tape on the sample holder (details of the measurement conditions are included in the Methods section). Figure S2 shows an SEM image of the composite positive electrode after ion milling. Then SPM and SEM-EDX measurements were conducted for the same areas on the electrodes near the SE layer. The potential, current, and resistance distributions were measured by KPFM, C-AFM, and SSRM, respectively, in the composite positive electrodes. In the setup used for both C-AFM and SSRM, a bias voltage of –2 V was applied on the sample holder. In general, the SSRM technique results in a wide measurement range of 7 orders of magnitude, thereby enabling the analysis of composite materials exhibiting large differences in resistance. Meanwhile, by using sweeping bias voltage, C-AFM yields the I-V characteristics of the composite electrodes and SEs. We investigated I-V characteristics of the SE separator layer by C-AFM and confirmed that the bias voltage of –2 V did not induce SE decomposition (Figure S3; see details in the Methods section). Finally, a bias voltage of –2 V was found to be suitable for detecting small currents (10 pA) and high resistances (109 Ω) in composite electrodes.
Figures 2 and 3 show the SEM image, EDX mappings, and SSRM, C-AFM, and KPFM images of the composite electrode before cell operation and after the initial charge test, respectively. S and Ni could be detected in the EDX maps of SE and NMC, respectively. Furthermore, all SSRM, C-AFM, and KPFM images overlapped both the NMC and SE areas, indicating that all measurements could be carried out successfully in the same area. The SSRM, C-AFM, and KPFM images of the electrode captured before the charge test indicated minimal differences between individual NMC particles (Figure 2(d)–(f)). In the SSRM image, it can be observed that the resistance at the center of the NMC particles and SE/NMC interfaces was lower than that in other NMC areas. However, the range of these values was 1.8–2.2 × 109 Ω, indicating that all areas in the composite electrodes showed a similar resistance before charging. In contrast, the resistance of some delithiated NMC particles varied from that of other NMC particles in the charged sample (Figure 3(d)). Most of the NMC particles contacted each other sufficiently and exhibited a resistance of 107 Ω, while some NMC particles ‘point-contacted’ with other particles as shown in the SEM image (Figure 3(a)) with a yellow broken line; these particles exhibited a higher resistance of 109 Ω. Such higher resistance may be attributed to poor electronic conduction due to inadequate contact among these NMC particles. In the C-AFM image (Figure 3(e)), the contrast of these NMC particles and SE was similar, indicating that NMC particles in minimal contact with other particles exhibited lower electronic conductivity due to their higher resistance. Meanwhile, the resistance of SE remained constant at 109 Ω after charging.
The contact potential difference (VCPD) between the tip and positive electrode was measured by KPFM. As shown in Figure 1, the negative electrode side was opened and the positive electrode was in electrical contact with the sample holder. To compare KPFM images before and after the charge test, VCPD was converted into VCPD’ by adding the open circuit voltage (OCV), as described in the Methods section. Hereafter, we shall discuss KPFM results in terms of VCPD’. Figures 2(f) and 3(f) depict the observed VCPD’ values. In the composite electrodes before charging, the VCPD’ values of NMC and SE were 2.06–2.12 and 2.22 V, respectively; these values increased to 3.11–3.29 and 2.76 V, respectively, after charging. When compared to SE, NMC exhibited a larger difference in VCPD’ (1.1 V) before and after charging. Although VCPD’ does not correspond with cell voltage quantitatively, we considered that KPFM measurements can be used to evaluate potential changes qualitatively as the VCPD’ of NMC increased after charging in response to an increase in the cell voltage. In the present study, we discuss the potential distribution in each electrode before and after the charge test.
I-V characteristics of electrodes measured by C-AFM. The I-V characteristics of NMC particles and the SEs were compared using C-AFM (Figure 4). Figure 4(b) shows the I-V curves of NMC ((a1) and (a2)) and SE ((a3) and (a4)) from the C-AFM image before charging (Figure 4(a)). The I-V curves of active electrode materials yield information about their electrical properties at the measuring point21,22. Although this experimental technique is typically applied to thin-film electrodes, we assumed that it could be applied to bulk-type ASSLBs in order to investigate the local charge-discharge properties of a single electrode active material particle in the presence of SE. While the current in the SE was ~0 A, the NMC particles displayed different I-V curves before charging owing to their different charge-discharge reactivities at each single point; NMC (a1) responded with a higher current than (a2). As shown in Figure 4(b), I-V characteristics are locally different even before charging. In the composite electrodes after charging, NMC particles exhibiting higher resistance ((c1) and (c2)) show completely different I-V curves compared to those exhibiting lower resistance ((c3) and (c4)). The latter show higher current responses due to their lower resistances, suggesting that charge-discharge reactions occurred easily at these NMC particles. Investigation of the I-V characteristics of electrodes by C-AFM can help us understand the local reactivities in charge-discharge reactions.
Potential and resistance distributions of individual NMC particles measured by KPFM and SSRM. We evaluated the electrical properties of individual NMC particles. We selected 14 NMC particles from the KPFM and SSRM images as shown in Figures 5(a)–(d) and evaluated the average VCPD’ and resistance of each particle; the obtained results are shown in Figure 5(e). Before charging, the VCPD’ and resistance values of all the NMC particles were ~2.1 V and 2.1 × 109 Ω, respectively. After charging, the value of VCPD’ increased to 3.1–3.3 V, suggesting the occurrence of delithiation in all NMC. The resistance of most of the NMC particles decreased to ~107–108 Ω, indicating an increase in their electronic conductivity. This behavior corresponded with our previous observations that the electronic conductivity of composite positive electrodes increased by 1–2 orders of magnitude after charging5. However, the resistances of three NMC particles, numbered 03, 08, and 10, were 109 Ω even after charging. As described earlier, these particles had minimal contact with other NMC particles. It can be inferred from the VCPD’ results that delithiation occurred in each NMC particle in the charged electrode because VCPD’ of observed NMC particles increased after charging. However, the local electronic conductivity was not uniform as some NMC particles showed higher resistance after charging. This phenomenon was prominent in the case of NMC particles in ‘point’ contact with other particles without any conductive additive. At high current densities, it is likely that inhomogeneous electronic conduction degrades battery performance due to current concentration. Increasing the amount of electrode active materials5 and tailoring their sizes23,24 can improve the electronic conductivity of carbon-free composite electrodes. However, electrode utilization is limited in the case of no or insufficient quantity of conductive additives. Moreover, some reports state that all-solid-state cells degrade in the presence of carbon25, which necessitates the optimization of these composite electrodes. Information on local electronic conduction provided by SSRM makes it a powerful tool to optimize electrode design. Our previous studies using Raman imaging indicated that aggregated electrode active materials show a low SOC as they contain only a small number of ionic conduction paths9. In contrast, in the present study, SSRM showed that minimal contact between NMC particles resulted in a low local electronic conductivity.
Subsequently, we analyzed the resistance and VCPD’ distribution in the cross-sectional direction. The NMC particles are numbered from the current collector (CC) side toward the SE layer as shown in Figures 5(a)–(d). Figure 5(f) shows the variation in the resistance of the NMC particles before and after the charge test. As described earlier, all the NMC particles in the measurement zone exhibited a similar resistance before charging. At the end of the charge test, the resistance of most of the NMC particles decreased except for the particles numbered 03, 08, and 10, which did not have much contact with other NMC particles. Figure 5(f) indicates that there was no resistance gradient in the cross-sectional direction. However, the VCPD’ distribution was different from that corresponding to resistance of NMC particles. The VCPD’ values of all the NMC particles before and after the charge test are plotted in Figures S4(a) and (b), respectively. Before charging, there was no gradient in the VCPD’; in contrast, after charging, it increased gradually from the SE layer side to the CC side except for particles 03 and 08, which exhibited a higher resistance. The difference in VCPD’ values on the SE layer and CC sides was ~0.2 V (except for particles 03 and 08), indicating a potential gradient in the cross-sectional direction after the charge test. In our previous studies using Raman imaging and optical microscopy, we observed inhomogeneous SOC distributions in composite electrodes after charging9,10,12,26,27. The results showed that charge-discharge reactions proceeded preferentially from the SE-layer side because the rate-determining step was related to Li+-ion conduction, indicating that the potential of NMC increased from the SE-layer side. In contrast, the results in Figure S4(b) indicate that NMC particles on the SE-layer side exhibit a lower potential than those on the CC side. In this study, the measurement area was focused on 15 μm2 near the SE layer of the 50 μm thick electrode layer (Figure S2). Therefore, KPFM analysis indicates a local potential gradient. To further investigate the presence of a potential gradient across the entire composite electrode, in situ and wide-range measurements are required. Figure S4(b) indicates that a potential gradient remained after the charge test in the all-solid-state cells. Moreover, Tanida et al. reported that after relaxation in charged cells with an organic liquid electrolyte, the SOC of LiCoO2 electrodes became uniform with a local potential difference as the driving force28. The behavior of all-solid-state cells is different from that of conventional batteries with an organic liquid electrolyte, possibly because the former contains fewer conductive networks19,23. From Figures 5 and S4, it could be inferred that the inhomogeneous resistance and potential distributions are likely to be dependent on the percolation of NMC particles and their distance from the SE layer, respectively.