New In-Situ Characterization Technique of Active Materials in Batteries: Electrochemical Acoustic Emission Method

Active materials, reversible reactants at the positive and negative electrodes, in secondary batteries like lithium ion batteries and nickel-metalhydride (Ni-MH) batteries etc. expand and shrink their volume in charging and discharging processes. For example, in Ni-MH batteries, hydrogen storage alloy negative electrodes initially form hydrogen atoms (Had) on the alloy surface during the charging process (eq. 1).


Introduction
Active materials, reversible reactants at the positive and negative electrodes, in secondary batteries like lithium ion batteries and nickel-metalhydride (Ni-MH) batteries etc. expand and shrink their volume in charging and discharging processes. For example, in Ni-MH batteries, hydrogen storage alloy negative electrodes initially form hydrogen atoms (H ad ) on the alloy surface during the charging process (eq. 1).
The resultant hydrogen atoms are absorbed into the alloy particles (eq. 2) while in the final stage of the charging process hydrogen evolution also occurs on the alloy surface (eq. 3).
H ad --> H ab (2) 2H ad --> H 2 Here, H ab i s a h y dr og e n at om a b s or b e d i nt o a lloy particles. When hydrogen atoms are absorbed into the alloy particles, their lattice volume expands. When the alloys absorb hydrogen atoms more and more, they eventually crack due to excessive strain. Consequently, fresh alloy surfaces appear, leading to improvement in reactivity for hydrogen absorption and discharge capacity, but the progress of the cracking causes the degradation of the alloys, leading to the decrease in discharge capacity. Therefore, characterization of the cracking, i.e. when and how the alloys crack, is important for the design of new alloys with higher p e r f o r m a n c e . H o w e v e r , t h u s f a r , c r a c k i n g o f t h e a l l o y s d u r i n g c h a r g i n g h a s o n l y b e e n confirmed ex situ from scanning electron microscopy of the alloy surface.
Some in situ characterization techniques for the change in lattice volume of hydrogen storage alloys in charging have been developed. For example, in situ X-ray and neutron diffractometry clarified the relationship between lattice volume and state of charge, and phase transformation during charging in detail. On the other hand, in situ scanning tunneling microscopy demonstrated reversible volume expansion of the secondary phase in a V-Ti-Ni-based multiphase alloy. However, these techniques are so skilled to get satisfactory data that simpler in situ techniques are desirable.
The acoustic emission (AE) method is a nondestructive and sensitive technique for detecting elastic waves generated by the cracking of materials. In addition, the AE method is a real time in situ analytical technique, and it's a low-priced method compared to X-ray and neutron diffractometry and STM. Physical phenomena such as corrosion, oxide formation, gas evolution, thin film rupture, cracking etc. can be discriminated by the AE method. In this chapter, I would like to introduce some examples in which the AE method was applied to the direct analysis of physical phenomena linked to electrochemical processes occurring at some hydrogen storage alloy negative electrodes during charging, including cracking of alloy particles and hydrogen evolution.

Experimental details
Ingots of MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 (Mm: Mischmetal) and TiCr 0.3 V 1.8 Ni 0.3 alloys were prepared by an arc melting method in an Ar atmosphere. The mischmetal is a mixture of rare earth elements (La, Ce, Pr, Nd etc.). Each alloy ingot was ground into powder, and sieved into powders with particle sizes of 106-125 m and less than 25 m for the MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 alloy and 75-106 m and 25-53 m for the TiCr 0.3 V 1.8 Ni 0.3 alloy. Each alloy powder was mixed with Cu powder with a mass ratio of 1:3, and the mixture was subjected to a pressure of 200 MPa at room temperature for 1 min to yield pellets for a negative electrode.
A typical experimental cell assembly for AE monitoring is shown in Fig. 1. The pellet-type negative electrode, a sulfonated-polypropylene separator and a Ni(OH) 2 /NiOOH positive electrode were stacked as shown in Fig. 1. A nickel sheet was used as a current collector for the negative electrode and protected the AE sensor from a 6M KOH aqueous solution which was used as the electrolyte. Before fabricating the cell assembly, the separator was thoroughly soaked in a 6M KOH aqueous solution. An Hg/HgO electrode was used as the reference electrode. www.intechopen.com The MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 electrodes were charged at 100 mA g(alloy) -1 for 3 h and discharged at 50 mA g(alloy) -1 to -0.65 V vs. Hg/HgO, while the TiV 1.8 Cr 0.3 Ni 0.3 electrodes were charged at 100 mA g(alloy) -1 for 6 h and discharged at 50 mA g(alloy) -1 to -0.75 V vs. Hg/HgO. The unit "g(alloy)" means mass of alloy. After every charging the circuit was opened for 10 min. A setup for AE monitoring is shown in Fig. 2. An automatic AE monitoring system (NF Electronic Instruments, 7600/0710) was used for AE monitoring with an AE transducer (NF electronic Instruments, AE-900S-WB) and a preamplifier (30 dB gain for MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 and 40 dB gain for TiV 1.8 Cr 0.3 Ni 0.3 ). The elastic waves generated by various phenomena such as cracking were detected by the AE transducer and transformed into AE signals. The AE signals were amplified by the preamplifier and stored in the automatic AE monitoring system. The stored data were displayed as time histories, power spectra and AE waveforms of the AE signals. The power spectra and AE waveforms express the distribution of frequencies for the AE signals and duration of the AE signals, respectively. All experiments were carried out at room temperature.

Acoustic emission for hydrogen evolution on a copper electrode
During charging, hydrogen evolution occurs on the surface of the hydrogen storage alloy negative electrodes as a side reaction of the hydrogen absorption into the alloy. Not only the cracking of alloy particles but also the release of hydrogen bubbles on the electrode surface can be detected as AE signals. Figure 3 shows typical AE waveforms and power spectra for hydrogen evolution on a Cu electrode at -1.0 and -1.1 V vs. Hg/HgO. The AE waveforms at both applied potentials had long durations of over 0.15 ms. The AE waveform at -1.1 V had a larger maximum amplitude and longer duration than that at -1.0 V, suggesting hydrogen evolution at -1.1 V was more vigorous. As for power spectra, both had a relatively narrow frequency distribution of approximately 0.6 MHz and maximum amplitude at about 0.1 MHz. The AE waveforms and power spectra in Fig. 3 did not change with time. Sawai et al. have also obtained an AE waveform and power spectrum for hydrogen evolution when a sodium hydrogencarbonate aqueous solution was neutralized by dilute sulfuric acid. The present data for electrochemical hydrogen evolution were quite similar to their data. Therefore, the electrochemical hydrogen evolution is characterized by the AE waveform with a long duration of over 0.15 ms and a power spectrum with a relatively narrow frequency distribution of around 0.6 MHz and the maximum amplitude at about 0.1 MHz. . AE waveforms and power spectra observed for the initial 1 h were quite different from those identified as the hydrogen evolution. In the former, AE waveforms have a sharp spike with duration shorter than 0.1 ms and larger amplitude, and power spectra have wide frequency distribution, particularly some peaks at frequencies more than 0.6 MHz, which is not characteristic of hydrogen evolution. The AE waveforms and power spectra, however, changed to those due to hydrogen evolution between 1 h and 2 h after start of the charging process. During the first half of the 1st charging process, a phenomenon other than hydrogen evolution, probably cracking of individual alloy particles, must have occurred. From the comparison of scanning electron micrographs (see Fig. 7) for the surface of www.intechopen.com MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 particles before and after the 1st charge-discharge cycle, a number of large and small cracks were observed on the alloy surface after the charge-discharge cycle. Therefore, it is inferred that the alloy particles cracked in the first half of the charging process. Thus the cracking of alloy particles is characterized by a burst-type AE waveform with duration shorter than 0.1 ms and large amplitude, and a power spectrum with wide frequency distribution, particularly with some peaks at frequencies more than 0.6 MHz.
In the 2nd charging process, AE waveforms and power spectra for the MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 electrode measured every 1 h are summarized in Fig. 5. The AE waveforms after 1 h and 2 h had a sharp spike with duration shorter than 0.1 ms, but their amplitude was much smaller than that in the 1st charging process. In addition, power spectra after 1 h and 2h had wide frequency distribution. These results clearly exhibit that the cracking of alloy particles occurs at 1 h and 2 h after start of the 2nd charging process. On the other hand, the AE waveforms and power spectra at the beginning and 3 h suggested hydrogen evolution. Thus, in the 2nd charging process the cracking occurred preferentially after 1 h and 2 h after start of charging while hydrogen evolution proceeded throughout charging, but it was not vigorous compared to the 1st charging process.  Figure 6 shows initial activation behavior for a MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 electrode and time history of frequency of AE signals and time course of electrode potential in the 1st, 2nd or 6th charging process. In each time history, the frequency of AE signals is defined as the number of AE signals with amplitude over a threshold of 0.5 V for one minute. In the initial activation behavior, discharge capacity was ca. 140 mAh g(alloy) -1 at 1st cycle, and then it increased with cycle number and approximately reached a constant value (ca. 220 mAh g(alloy) -1 ) at the 4th cycle, indicating that the initial activation was completed for four charge-discharge cycles.
www.intechopen.com In the first half of the 1st charging process, a large number of AE signals were observed. This comes from the cracking based on lattice expansion due to hydrogen absorption, which has also been confirmed by the AE wave forms and power spectra in the same period (Fig. 4). The electrode potential reached ca. -1.1 V immediately after start of charging and then it hardly changed during the charging. This suggests that hydrogen evolution proceeds throughout the charging. Figure 7 shows scanning electron micrographs of the MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 electrode surface before charging and after the 1st, 2nd and 6th cycles. As described above, a number of large and small cracks appeared on the alloy surface after the 1st charge-discharge cycle, supporting a large number of AE signals observed in Fig. 6. In the 2nd charging process, the frequency of AE signals greatly decreased compared to the 1st charging process as shown in Fig. 6, and the scanning electron micrograph after 2nd charging showed that smaller cracks increased compared to after 1st charging. These results indicate that the fine cracking of alloy particles occur in the 2nd charging process. After 6th cycle, the number of AE signals significantly decreased and discharge capacity showed a constant value as shown in Fig. 6. A lot of finer cracks were observed after 6th cycle as shown in Fig. 7. Thus, in situ analysis by the AE technique confirms the well-known conclusion that during initial activation the MmNi3.6Mn0.4Al0.3Co0.7 particles frequently cracks and the increase in the cracks with cycle number leads to the increase in discharge capacity. The pattern of cracking during charging may change with size of alloy particles. The cracking behavior of small alloy particles was also characterized by the AE technique. Figure 8 shows initial activation behavior for a MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 (less than 25 m) electrode and time history of frequency of AE signals and time course of electrode potential in the 1st, 2nd or 6th charging process. In this case, discharge capacity was ca. 220 mAh g(alloy) -1 even at the 1st cycle, and then it was almost constant. This indicates that the small alloy particles do not have to be activated. In the time history at the 1st cycle, there were very few AE signals compared to the time history for the larger alloy particles (see Fig. 6), suggesting that the cracking of the small alloy particles poorly occurred probably because the lattice stress of the small particles www.intechopen.com with lattice expansion was not so large as larger particles. The similar results were observed in the 2nd and 6th cycles. Figure 9 shows scanning electron micrographs of the MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 (less than 25 m) electrode surface before charging and after 1st, 2nd and 6th cycles. There were very few cracks even after the 1st cycle, which proves poor cracking of the small ally particles. Thus the small particles with sizes of less than 25 m are hard to crack even though they absorb a great amount of hydrogen in charging.  www.intechopen.com Figure 10 shows AE waveforms and power spectra for the MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 (less than 25 m) negative electrode measured every 1 h during the 1st charging process. AE waveforms and power spectra at 1 h after start of charging suggested the cracking of particles, but otherwise hydrogen evolution was dominant. Moreover, in the 1st charging process electrode potential reached ca. -1.1 V immediately after start of charging and then it hardly changed during the charging (Fig. 8), suggesting that hydrogen evolution proceeded throughout the charging. Time history of frequency of AE signals and electrode potential in the 1st, 2nd and 6th charging processes and initial activation behavior for the TiV 1.8 Cr 0.3 Ni 0.3 (75-106 m) electrode were summarized in Fig. 11. In the 1st charging process, a large number of AE signals were observed for initial 2 h. Since electrode potential was ca. -1.1 V in this term, hydrogen evolution is also expected to proceed.  Fig. 13 show AE waveforms and power spectra for the TiV 1.8 Cr 0.3 Ni 0.3 (75-106 m) electrode measured every 1 h during the 1st charging process. Both the AE waveform and power spectrum at 1 h exhibit that hydrogen evolution proceeds, supporting the inference from Fig. 11. In further charging, the AE waveform and power spectrum due to www.intechopen.com the cracking were mainly observed, but judging from the time history of the frequency of AE signals, the cracking was sporadic. The TiV 1.8 Cr 0.3 Ni 0.3 electrode began to crack later than the MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 electrode, suggesting that the former was more patient for cracking than the latter. The maximum amplitude of AE waveforms was just ca. one forth of that for the MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 electrode, suggesting that relatively small cracks were formed on the TiV 1.8 Cr 0.3 Ni 0.3 alloy surface. Figure 14 shows scanning electron micrographs of the TiV 1.8 Cr 0.3 Ni 0.3 (75-106 m) electrode surface before charging and after the 1st, 2nd and 6th cycles. The scanning electron micrograph after 1st cycle exhibits that a few and relatively small cracks were observed on the TiV 1.8 Cr 0.3 Ni 0.3 particles, which agreed with the suggestion from the AE measurements. In Fig. 11, frequency of AE signals decreased greatly with repeating charge-discharge cycles. The AE waveforms (Fig. 15) and power spectra (Fig. 16) in the 2nd charging process showed that the cracking based on hydrogen absorption occurred from the beginning. As can be seen from charge-discharge cycle performance in Fig. 11, discharge capacity greatly increases for the first 2 cycles due to initial activation. In the initial activation the surface oxides inhibiting hydrogen absorption into alloy seemed to be removed by sporadic cracking of alloy particles during the 1st cycle to expose fresh surface, leading to hydrogen absorption from the beginning of the 2nd charging and the increase in discharge capacity.
Moreover, Fig. 14      www.intechopen.com Figure 17 shows initial activation behavior for a TiV 1.8 Cr 0.3 Ni 0.3 (25-53 m) electrode and time history of frequency of AE signals and time course of electrode potential in the 1st, 2nd or 6th charging process. From Fig. 11 and Fig. 17, charge-discharge cycle performance is independent of particle size of the alloy. As for time history, in the 1st charging process, a large number of AE signals were observed for initial 2 h, which is similar to the alloy particles with sizes of 75-106 m. The frequency of AE signals significantly decreased with repeating charge-discharge cycling. Electrode potential in this term, however, was -1.0 V which was more positive compared to the TiV 1.8 Cr 0.3 Ni 0.3 (75-106 m) electrode. In this case hydrogen evolution is expected to proceed but it seems to be poor. This is also suggested from Fig. 18 and Fig. 19. Both AE waveforms and power spectra in the 1st charging process show hydrogen evolution occurs for initial 2 h, and then cracking also occurs. But the term for cracking was shorter than that for the TiV 1.8 Cr 0.3 Ni 0.3 (75-106 m) electrode, suggesting that the cracking was poorer for smaller alloy particles. Figure 20 shows scanning electron micrographs of the TiV 1.8 Cr 0.3 Ni 0.3 (25-53 m) electrode surface before charging and after the 1st, 2nd and 6th cycles. The scanning electron micrograph after 1st cycle exhibits that very few cracks were observed on the TiV 1.8 Cr 0.3 Ni 0.3 particles, indicating that the number of cracks decreased with particle size. The number of cracks slightly increased with cycle number. www.intechopen.com

Conclusion
In charging of hydrogen storage alloy negative electrodes, MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 and TiV 1.8 Cr 0.3 Ni 0.3 electrodes, cracking based on hydrogen absorption into alloy and hydrogen evolution were clearly discriminated using an AE technique in terms of waveforms, power spectra and time history of the AE signals. The MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 (106-125 m) particles cracked intensively in the first half of the 1st charging process and were broken down with many large and small cracks during initial activation. In contrast, the MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 particles (less than 25 m) had high discharge capacity even in the 1st charging process, but it poorly cracked, suggesting that the particle size was enough small to diffuse throughout the particles without cracking. As for the TiCr 0.3 V 1.8 Ni 0.3 particles, irrespective of particle size the cracking sporadically occurred after hydrogen evolution in the 1st charging process and the AE signals for the cracking were very weak compared to those for the MmNi 3.6 Mn 0.4 Al 0.3 Co 0.7 particles, indicating that the TiCr 0.3 V 1.8 Ni 0.3 particles were hard to crack. Moreover, the smaller particles (25-53 m) were more resistant to cracking than the larger ones (75-106 m).
Lithium ion battery (LIB) is the primary candidate for plug-in hybrid electric vehicles and electric vehicles because of their high energy and power density compared to Ni-MH battery. At both electrodes in LIB, lithium intercalation/deintercalation reactions occur. Recently, to improve energy and power density more and more, researches on new negative electrode active materials with high capacity like Si, Sn etc. have been actively performed. Such active material particles are easy to crack with repeated lithium intercalation/deintercalation, leading to capacity loss in a LIB or short lifetime. So the in situ real analysis of stress and cracking are important for clarifying the mechanism of capacity degradation, and the electrochemical AE technique must be a powerful tool.