In situ IR spectroscopy during oxidation process of cobalt Prussian blue analogues

Cobalt Prussian blue analogues (Co-PBA; NaxCo[Fe(CN)6]y), consisting of cyano-bridged transition metal network, –Fe–CN–Co–NC–Fe–, are promising cathode materials for Na-ion secondary batteries. In the oxidation process, oxidization of Fe and/or Co are compensated by Na+ deintercalation. Here, we investigated the oxidization process of three Co-PBAs by means of in situ infrared absorption (IR) spectroscopy. With use of an empirical rule of the frequencies of the CN− stretching mode in ferrocyanide ([FeII(CN)6]4−) and ferricyanide ([FeIII(CN)6]3−), the oxidation processes of Co-PBAs were determined against the Fe concentration (y) and temperature (T). We will discuss the interrelation between the oxidation processes and Fe concentration (y).

www.nature.com/scientificreports/ In this paper, we will demonstrate that the in situ IR spectroscopy is a sensitive tool to determine the type of the oxidization process. We investigated the oxidization process of three Co-PBAs with different Fe concentration (y = 0.71, 0.81, and 0.90) and temperature (T = 293 and 330 K). With use of the empirical rule of the CNstretching mode, the oxidation processes of Co-PBA were classified into the M II -type or M III -type without ambiguity. Figure 2a shows in situ IR spectra of the NCF90 film measured at 293 K. The horizontal arrows indicate the empirical region for the CNstretching modes in ferrocyanide ([Fe II (CN) 6 ] 4-) and ferricyanide ([Fe III (CN) 6 ] 3-), respectively. Figure 2b shows the corresponding charge curve of the NCF90 film against x at 1.6 C. The charge curve shows two plateaus at around 3.5 and 4.0 V vs. Na/Na + , indicating two step oxidation process. In the  6 ] y . We call the upper and lower processes as M II -type and M IIItype, respectively. www.nature.com/scientificreports/ lower-lying plateau near 3.5 V, single CNstretching mode is observed in the range from 2080 cm -1 to 2120 cm -1 . The empirical rule of the CNstretching mode (horizontal arrows) indicates that the Fe remains divalent in this plateau. Then, the oxidization process in the lower-lying plateau is carried out by the valence change of Co from Co 2+ to Co 3+ . Reflecting the valence change of Co, the CNstretching mode shows blue shift with decrease in x. In the higher-lying plateau, an additional CNstretching mode appears at around 2200 cm -1 . The spectral weight transfers from lower-lying band at 2120 cm -1 to the higher-lying band at 2200 cm -1 with decrease in x.

Results
The empirical rule of the CNstretching mode indicates that the additional mode at 2200 cm -1 is due to ferricyanide. In short, the redox site in the higher-lying plateau is Fe, because the divalent and trivalent Fe coexist. Thus, the in situ IR spectroscopy classifies the oxidation process of NCF90 into the M III -type. The classification is consistent with the literature 8,15 .  Figure 3b shows the corresponding charge curve of the NCF71 film against x at 1.9 C. The charge curve shows two plateaus at around 3.5 and 4.0 V vs. Na/Na + , indicating two step oxidation process. In the lower-lying plateau, two CNstretching modes are observed in the range at around 2090 cm -1 and 2155 cm -1 . The spectral weight transfers from lower-lying band at 2090 cm -1 to the higher-lying band at 2155 cm -1 with decrease in x. The empirical rule indicates that the former and latter mode are ascribed to the ferrocyanide and ferricyanide, respectively. In short, the redox site in the lower-lying plateau is Fe, because the divalent and trivalent Fe coexist. In the higher-lying plateau, two CNstretching modes are observed in the range at around 2155 cm -1 and 2185 cm -1 . The empirical rule indicates that the Fe is trivalent in this plateau. Then, the oxidization process in the higher-lying plateau is carried out by the valence change of Co from Co 2+ to Co 3+ . With decrease in x, the spectral weight transfers from the lower-lying to higher-lying bands. Therefore, the lower-lying (higher-lying) band can be ascribed to the Fe III -CN-Co II (Fe III -CN-Co III ) mode. Thus, the in situ IR spectroscopy classifies the oxidation process of NCF71 into the M II -type. The classification is consistent with the literature 23 . Figure 4a shows in situ IR spectra of the Na 1.24 Co[Fe(CN) 6 ] 0.81 (NCF81) film measured at 293 K. Figure 4b shows the corresponding charge curve of the NCF81 film against x at 2.0 C. The charge curve shows two plateaus at around 3.5 and 3.8 V vs. Na/Na + , indicating two step oxidation process. In the lower-lying plateau near 3.5 V, single CNstretching mode is observed in the range from 2085 cm -1 to 2125 cm -1 . The empirical rule indicates that the Fe is divalent in this plateau. Then, the oxidization process in the lower-lying plateau is carried out by www.nature.com/scientificreports/ the valence change of Co from Co 2+ to Co 3+ . Reflecting the valence change of Co, the CNstretching mode shows blue shift with decrease in x. In the higher-lying plateau, an additional CNstretching mode appears at around 2200 cm -1 . The spectral weight transfers from lower-lying band at 2125 cm -1 to the higher-lying band at 2200 cm -1 with decrease in x. The empirical rule indicates that the additional mode at 2200 cm -1 is due to ferricyanide. In short, the redox site in the higher-lying plateau is Fe, because the divalent and trivalent Fe coexists. Thus, the in situ IR spectroscopy classifies the oxidation process of NCF81 at 293 K into the M III -type. Figure 5a shows in situ IR spectra of the NCF81 film at 330 K. We note that the x-dependent spectral changes of the NCF81 film are qualitatively different between at 293 K (Fig. 4a) and at 330 K (Fig. 5a). Figure 5b shows the corresponding charge curve of the NCF81 film at 1.4 C. The charge curve shows two plateaus at around 3.6 and 3.8 V vs. Na/Na + , indicating two step oxidation process. In the lower-lying plateau near 3.6 V, two CNstretching modes are observed in the range at around 2090 cm -1 and 2155 cm -1 . The spectral weight transfers from lowerlying band at 2090 cm -1 to the higher-lying band at 2155 cm -1 with decrease in x. The empirical rule indicates that the former and latter mode are ascribed to the ferrocyanide and ferricyanide, respectively. In short, the redox site in the lower-lying plateau is Fe, because the divalent and trivalent Fe coexists. In the higher-lying plateau near 3.8 V, two CNstretching modes are observed in the range at around 2155 cm -1 and 2195 cm -1 . The empirical rule indicates that the Fe is trivalent in this plateau. Then, the oxidization process in the higher-lying plateau is carried out by the valence change of Co from Co 2+ to Co 3+ . With decrease in x, the spectral weight transfers from the lower-lying to higher-lying band. Therefore, the lower-lying (higher-lying) band can be ascribed to the Fe III -CN-Co II (Fe III -CN-Co III ) mode. Thus, the in situ IR spectroscopy classifies the oxidation process of NCF81 at 330 K into the M II -type.

Discussion
We investigated the redox process of four (y, T) points, i.e.,   33,34 . The electronic configuration is LS Co 3+ and LS Fe 2+ in the LS phase, while it is HS Co 2+ and LS Fe 3+ in the HS phase. The phase transition accompanies cooperative charge transfer from Fe 2+ to Co 3+ as well as the expansion of the unit cell volume. The volume expansion is ascribed to the larger ionic radius of HS Co 2+ . We consider that the relative stability of the two phases in the intermediate state determines the type of the oxidization process; the oxidization process is M III -type if the LS phase is stable, while the oxidization process is M II -type if the HS phase is stable. The HS state, and hence, the M II -type process, is stabilized with decrease in y, because the weaker ligand field from the octahedrally coordinated [Fe(CN) 6 ] stabilizes the HS Co 2+ configuration. This argument well explains why the M II -type reaction appears when the T is raised, or y is lowered.

Summary
We demonstrated that the in situ IR spectroscopy is a sensitive tool to determine the type of the oxidization process. The oxidation processes were classified into the M II -type or M III -type against Fe concentration (y) and temperature; The y = 0.71 (at 293 K) and 0.81 (at 330 K) compounds are classified into M II -type while the y = 0.81 (at 293 K) and 0.90 (at 293 K) compounds are classified into M III -type. Especially, the y = 0.81 shows thermal switching of the oxidation process from the M III -type (293 K) to M II -type (at 330 K). Such a thermal switching is advantageous for thermal energy harvesting, as demonstrated by Shibata et al. 35 in the tertiary battery with use of phase transition.   Table 2. In this process, the reduction reaction ([Fe(CN) 6 ] 3-+ e -→ [Fe(CN) 6 ] 4-) triggers the deposition of PBAs. Therefore, Fe, Co and Ni in the as-grown films are divalent. Among the three Co-PBA film, NCF90 film shows the highest discharge capacity (135 mAh/g) 8 . In addition, the NCF90 film exhibits excellent rate properties (discharge capacity at 60C is 90% of the OCV value) 8 .  www.nature.com/scientificreports/ The X-ray diffraction patterns of the NNF68, NCF71, NCF81, and NCF90 films at 293 K were shown in Figure S1. The X-ray source was the Cu Kα line. NNF68, NCF71, and NCF81 show face-centered cubic (fcc) ( Fm In situ infrared spectroscopy. The in situ IR spectra were measured with the use of an infrared microscopy system (JASCO IRT-3000) equipped with a Fourier transform infrared spectrometer (JASCO FT/IR-660 Plus) in the 400-7000 cm -1 region with 2 cm -1 resolution. The transmitted light was focused on a cooled HgCdTe detector. The Na concentrations (x) were electrochemically controlled in the oxidation (charging) process with a potentiostat (Bio-logic VSP multichannel potentiostat). x in NCF90, NCF81, and NCF71 films were evaluated from the extracted charge under the assumption that x = 0.0 in the fully oxidized state. In situ IR spectra of NCF71, NCF81, and NCF90 at 293 K were measured in the first oxidation process films. The spectra of NCF81 at 330 K were measured in the second oxidation process films after the measurement at 293 K. We confirmed that the spectral profiles returned to the initial ones after the reduction process. Each spectrum was recorded in every 90 s during the oxidization process.

Methods
Optical electrochemical cell. Figure 6 schematically shows the optical electrochemical cell used for the in situ IR measurements. The cathode, anode and electrolyte were the Co-PBA and NNF68 film grown on the ITO glass substrates, and 17 mol/kg NaClO 4 aqueous solution. To measure the IR spectra of the Co-PBA film, the center of NNF68 film has hollow. The anode and cathode sandwich a separator whose thickness was 25 μm. The voltage between the anode and cathode was controlled so that a constant current (75 μA/cm 2 ) would flow between the electrodes. The voltage control range was from − 0.05 V to 1.0 V vs. NNF68. The cell temperature was controlled by a silicone rubber heater. IR absorptions due to the ITO glass substrates and the electrolyte were subtracted as backgrounds.
We chose the NNF68 film as anode, because the film is stable in aqueous electrolyte 37 . The anode NNF68 were pre-oxidized to 3.45 V vs. Na/Na + . The active area of anode (213 mm 2 ) was set to be much larger than that of cathode (4 mm 2 ) so that x, and hence, the potential of anode can be regarded as constant, as schematically shown in Fig. S2. Therefore, the voltage vs. Na/Na + . can be obtained by adding 3.45 V to the voltage vs. NNF68 in the optical cell. In the in situ IR experiment, the current density was determined so that the charge rate was 1 C (= 75 μA/cm 2 ), assuming that the film thickness was 1 μm. After the measurement, the film thickness was determined with use of the profilometer. Mass was evacuated from the thickness and area (= 4 mm 2 ). Mass of the NCF90, NCF81, and NCF71 films were 5.8 μg, 6.6 μg, and 6.7 μg, respectively. The actual charge rate of NCF90 at 293 K, NCF81 at 293 K, NCF81 at 330 K, and NCF71 at 293 K were 1.6 C, 2.0 C, 1.4 C, and 1.9 C, respectively. www.nature.com/scientificreports/