Corrosion of Positive Current Collector in Li||Sb-Pb Liquid Metal Battery for Grid-Scale Energy Storage

As part of a comprehensive effort to develop a low-cost, grid-scale electrochemical energy storage device with low material cost with long cycle life, corrosion of the positive current collector (PCC) in a Li||Sb-Pb liquid metal battery (LMB) was investigated. A LMB consists of a low-density liquid negative electrode, an intermediate-density molten salt electrolyte, and a high-density liquid positive electrode, which self-segregate into three distinct liquid layers due to their immiscibility and density differences ^[1]. During discharge of the Li||Sb-Pb system, a liquid Li negative electrode is oxidized to form Li⁺ which is conducted through the electrolyte to a liquid Sb-Pb positive electrode where the Li⁺ is electrochemically reduced to a neutral Li and alloys with the Sb-Pb. This process is reversed on charging. The Li||Sb-Pb cells operate with high round trip efficiency (> 98 %) and wide capability of current density (> 1 A cm^-2) at 450°C ^[1]; however, there is concern about the high reactivities of active materials and their potential for corrosion of cell components. Herein, we focused on the PCC. The Li||Sb-Pb cell configured with graphite PCC exhibited no corrosion and small decay of capacity, but in order to reduce costs of materials and large-scale manufacturing, in this study, commonly available steels or stainless steels were selected for the investigation due to their low cost, affordable machinability, and flexibility.

We began with an evaluation of the corrosion of the steels by pure liquid Sb-Pb alloy representing the fully charged state of the Li||Sb-Pb cell. To the best of our knowledge, there has been no systematic investigation of the corrosion of metals in liquid Sb-Pb alloy; accordingly, thermodynamic evaluation of metals composing steels and stainless steels, such as iron (Fe), nickel (Ni), and chromium (Cr), equilibrated with liquid Sb-Pb alloy was the first step. Based on survays of phase diagrams, it was confimed that many kinds of metals dissolve in liquid Sb, Pb, and their alloys, and Fe, Ni, and Cr form intermetallic compounds with antimony. To evaluate the formation of intermetallics, static corrosion testing of pure Fe, Ni, and Cr was carried out. Test coupons were immersed in the liquid Sb-Pb alloys at constant temperature (450 ± 0.5 °C) and under Ar atmosphere (O₂< 0.1 ppm).

The cross-sections at the interfaces between coupons and Sb-Pb alloys after 500 h immersion are shown in Fig. 1. The layers of Fe-Sb, Ni-Sb, and Cr-Sb intermetallic alloys were observed in Fig. 1. Thicknesses of the layers were arranged as Ni > Fe > Cr. In order to elucidate the effects of concentration of Fe, Ni, and Cr in steel and stainless steels, a low-carbon steel (1018), a FeNiCr-based stainless steel (SS301), and a FeCr-based stainless steel (SS430) were selected for the investigation.

Static corrosion tests of 1018, SS301, and SS430 in the liquid Sb-Pb alloy were performed and cross sections of coupons after 500 h immersion are shown in Fig. 2. On 1018 and SS301, layers of Fe-Sb alloy and Fe-Ni-Cr-Sb alloy were formed, respectively, in contrast SS430 showed no layer, which may be due to the formation of a thin oxide layer. One of the typical methods to mitigate corrosion of stainless steels in liquid Bi-Pb alloys is controlling the oxygen concentration in the liquid metal to form oxide protective layers on the surface of the stainless steels ^[2].

To confirm the corrosion of the steels and stainless steels in an operating Li||Sb-Pb cells, test coupons placed as PCCs in the cells which were charged/discharged for 500 h are analyzed (Fig. 3). Obvious recession without the Fe-Sb layer was confirmed on 1018. Although the Fe-Cr-Ni-Sb layer was formed on SS301, some recession occurred. SS430 shows thin uniform Fe-Cr-Sb layer and no recession. During discharge, Li mainly alloys with Sb in the positive electrode to form Li-Sb-Pb alloys with low Pb concentration, and then the Li dealloys from the Li-Sb-Pb alloys on charging. These processes may destroy the layer of Fe-Sb and Fe-Cr-Ni-Sb alloys. Furthermore, Li dramatically reduces the partial pressure of oxygen in the system, and then oxide layers on the steel or stainless steel may not be stable. However, it was suggested in this study that Fe-Cr-Sb alloy is a stable intermetallic compounds during the charge/discharge processes. Therefore, SS430 is a candidate PCC for Li||Sb-Pb LMB.

[1] H. Kim, et al., Chem. Rev., (2012). [2] J. Zhang, et al., J. Nucl. Mater., 373, 351–377 (2008).

The authors are grateful for the financial support of the USDOE ARPA-E, TOTAL, Marubun Research Promotion Foundation, and Murata Overseas Scholarship Foundation.