Investigation of manufacturing parameters for NaCl–Ni granule type cathodes used in low temperature NaSICON sodium-metal chloride batteries

Highlights

• The granule-type cathodes for Zebra battery were prepared by a roll compaction method.

• The roll speed is critical to produce sound granules with good metal connectivity.

• Polygon-shape granules enable favorable battery performances.

• We have proved the stable performance of low temperature Zebra battery with NaSICON.

Abstract

In this work, the optimum manufacturing condition was investigated for the granule-type cathodes used in low temperature sodium-metal chloride batteries using NaSICON (Na_1+xZr₂Si_xP_3-xO₁₂) solid electrolytes. The granule-type cathodes with different shapes, pore sizes and tap densities were obtained by controlling operation parameters of granulator such as roll speed and pressure. From standard cell tests at 195 °C using four granule-type cathodes, the optimum roll speed (1.5 rpm) was sorted to produce stable granule-type cathodes for efficient electrochemical performance. From the characterization of granules and the battery tests, it was revealed that the polygon-type granules (A, C) produced at an adequate roll speed (1.5 rpm) enable favorable battery performances even at high current density of 50 mA/cm² for 200 cycles. The main advantages of A and C granule-type cathodes seem to come from the stable metal connection among the granules that attribute to the favorable morphology (polygon) and homogenous green compacts. The relatively more efficient cathode kinetics were observed for the A sample manufactured at higher roll pressure (10 MPa) compared with the C sample (1.2 MPa), which supported the importance of roll pressure in optimizing the microscopic properties inside the granules (pore size distribution, green density, metal connectivity, etc.) for positive cathode kinetics.

Introduction

Sodium-metal chloride battery (Zebra battery) is a promising candidate for grid energy storage and electric vehicle applications due to its high energy density, high durability and zero self-discharge [1], [2], [3], [4], [5], [6]. Although the Zebra battery is already pre-commercialized by FIAMM SoNick and General Electric, many researchers are still investigating advanced technologies for enhanced performance, long-term durability and cost competiveness [7], [8], [9], [10].

The low temperature operation (below 200 °C) of Zebra battery is fairly attractive to obtain longer life cycle and low-cost construction materials (sealant, metal parts) [8], [11]. To get appropriate battery performances at low temperature below 200 °C, significant efforts should be focused on reducing internal resistance contributed by the solid electrolyte, negative/positive electrodes and metal current collector parts. In particular, NaSICON (Na_1+xZr₂Si_xP_3-xO₁₂) materials have attracted significant interests as a promising solid electrolyte for low temperature sodium rechargeable batteries due to its high ionic conductivity of 0.15–0.2 S/cm⁻¹ at 200 °C which are comparable to that β”-Alumina at 260 °C [3]. In addition to the preliminary reports that reveal the high stability of NaSICON in molten sodium and molten salt electrolyte [12], [13], [14], the researchers recently reported the low temperature Zebra battery using NaSICON which are successfully operational at 195 °C [15].

To get further improvement in cell performance, thorough investigations are necessary in cathode development to get an optimum composition, microstructure and geometry. Zebra battery is typically fabricated in a discharge state and the initial cathode material is composed of transition metal chlorides and excess metals. There are a few literature reports regarding cathode development which deal with the composition control [4], [16], [17], [18] and manufacturing conditions [4], [19], [20], [21]. Although cathode materials for Zebra batteries can be prepared by various processes to have diverse shapes and microstructures, the major commercial players have focused on developing granule-type cathodes due to their advantages in manufacturing simplicity and cost [4], [8], [16], [19], [20].

Roll compaction is a proven dry-granulation process in pharmaceutical industry for producing fine powders in order to improve the fluidity and homogeneity of powder mixture, so that it can be handled easily [22]. In Zebra battery industry, this roll compaction seems to be advantageous in manufacturing high capacity cells by simply filling them in the cathode volume through a small hole. Furthermore, another benefit of the granule-type cathodes is fabrication of Zebra cells with diverse shapes regardless of cell design due to its high fluidity. However, no publications and patents were reported so far to explain the manufacturing processes of granule-type cathodes used in Zebra battery.

In this work, the researchers have studied to find out key parameters to manufacture high-quality cathode-type granules using a dry roll compaction method that enables the optimal electrochemical performance of Zebra battery at 195 °C. The roll compaction conditions were controlled as key influential parameters which determine the size, shape and pore structure of the granules. The optimum processing condition for the granule-type cathodes was discussed considering electrochemical performance in unique cell tests with a self-developed Zebra cell configuration with NaSICON solid electrolyte membrane. The long-term performances of the cells using stable granule-type cathodes were evaluated with the consideration of cathode characteristics and NaSICON stability.