Enhancing formability of NiS2 cathode by SiO2 doping for high-performance thermal batteries

Abstract

As a cathode material for thermal batteries, NiS2 has a high theoretical capacity but low thermal stability. Besides, the poor formability of NiS2 powders also restricts the cathode performance of thermal batteries. In this paper, the novel NiS2/SiO2 composite material was developed by high temperature vulcanization to improve the thermal stability formability of NiS2. The good filling and lubrication of spherical SiO2 can improve the thermal conductivity of NiS2 electrode. The discharge test shows that the NiS2/SiO2 cathode has a stable discharge voltage at a current density of 200 mA/cm2, and the activation time is shortened by nearly 20% compared with the NiS2 cathode. In addition, due to the favorable thermal insulation protection of SiO2, the initial decomposition temperature of NiS2 is increased by 30 ℃ after the addition of SiO2. The incorporation of SiO2 not only effectively improves the thermal stability and electrochemical properties of NiS2, but also improves the cold pressing forming performance of the NiS2 powder. Therefore, the novel NiS2/SiO2 composite material is more suitable for thermal batteries with high stability and fast response, which is of great significance for improving the maneuverability and quality reliability of weapons and equipment.

1. Introduction

The thermal battery is a primary reserve power activated by an internal pyrotechnic source [1], using high-temperature molten salt as the electrolyte [2]. Due to its high output power, long storage life, rapid activation, and strong environmental adaptability [3], thermal battery has emerged as one of the remarkable chemical power sources for sophisticated weapons. The demands for thermal batteries in conventional weapons such as anti-aircraft guns and guided bombs are also increasing rapidly. The development of modern warfare not only puts forward the electrical properties of thermal batteries, such as high power and rapid activation, but also pays attention to higher requirement for the safety, stability, and reliability of thermal batteries.

Li-B alloy has been widely employed as a high-performance thermal battery anode material [4], primarily composed of LiB skeleton and high free Li load. The cathode has become one of the major factors restricting technological advances in the thermal batteries [5]. Currently, the cathode materials of thermal batteries mainly include FeS2, CoS2, oxides [6], halides [7, 8], etc. However, FeS2 has poor thermal stability and can decompose rapidly during activation, resulting in voltage spikes. The relatively high electrolyte solubility of FeS2 in molten salts typically results in lower active material utilization, especially at low current densities. CoS2 has some defects, such as expensive price, unstable properties and low discharge voltage (about 2.0 V) [9 −11]. Oxide cathode materials generally have considerable internal resistance and weak conductivity [12, 13]. Due to the short discharge duration, halides have not been widely used [14].

Against this background, NiS2 has attracted the attention of researchers, mainly due to its high thermal stability and discharge-specific capacity [15, 16]. However, the poor formability of NiS2 powder leads to the decrease of cathode quality, resulting in defects such as large internal resistance and long activation time of thermal battery. Therefore, improving the forming performance, thermal stability, and high temperature ionic conductivity of NiS2 powder has become a significant development trend of NiS2 cathode materials. SiO2 itself has the characteristics of high melting point, low thermal conductivity and high stability [17]. The inclusion of SiO2 embedded on the surface of the active material can play a role in thermal insulation and protection [18]. Alternatively, the SiO2 after high temperature heat treatment has high mechanical hardness [19], which may further break the sulfide during the mixing process to improve the particle size uniformity of the powder material. In the process of powder tabletting, SiO2 can act as a filler and a flow promoter [20], filling the gap between the active material particles.