Article category: Full Paper

Subcategory: thermal battery, primary battery, reserve battery

Low-volatile binder enables thermal shock-resistant thin-film cathodes for thermal batteries

Yong Xie*, Xu Zhang, Liangping Dong, Yong Cao, Xiaojiang Liu, Yixiu Cui, Chao Wang*, Yanhua Cui*, Xuyong Feng, Hongfa Xiang, and Long Qie

Dr. Y. Xie, Dr. X. Zhang, Dr. L. P. Dong, Dr. Y. Cao¹, Prof. X. J. Liu, Prof. Y. X. Cui, Dr. C. Wang, Prof. Y. H. Cui

Laboratory of Electrochemical Power Sources, Institute of Electronic Engineering, China Academy of Engineering Physics, Mianyang, Sichuan 621000, China E-mail:yongxie@caep.cn (Y. Xie);wangchao_1988924@126.com (C. Wang);cuiyanhua@netease.com (Y. H. Cui)

Prof. X. Y. Feng, Prof. H. F. Xiang School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009, Anhui, China

Prof. L. Qie

State Key Laboratory of Material Processing and Die & Mold Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan, Hubei 430074, China

Keywords: thermal battery, thin-film cathode, low-volatile binder, gas production, high-power

Manufacturing thin-film components is crucial for achieving high-efficiency and high-power thermal batteries (TBs). However, developing binders with low gas production at the operating temperature range of TBs (400−550 °C) has proven to be a significant challenge. Here we report the use of acrylic acid derivative terpolymer (LA136D) as a low-volatile binder for thin-film cathode fabrication and studied the chain scission and chemical bond-breaking mechanisms in pyrolysis. It is shown LA136D defers to random-chain scission and cross-linking chain scission mechanisms, which gifts it with a low proportion of volatile products (ψ , ψ=39.2wt%) at even up to 550 °C, well below those of the conventional PVDF (77.6wt%) and SBR (99.2wt%) binders. Surprisingly, LA136D contributes to constructing a thermal shock-resistant cathode due to the step-by-step bond-breaking process. This is beneficial for the overall performance of TBs. In a 130 s pulse discharging test, the thin-film cathodes exhibited a remarkable 440% reduction in polarization and 300% enhancement in the utilization efficiency of cathode materials, while with just a slight increase of 0.05 MPa in gas pressure compared with traditional “thick-film” cathode. Our work highlights the potential of LA136D as a low-volatile binder for thin-film cathodes and shows the feasibility of manufacturing high-efficiency and high-power TBs through polymer molecule engineering.

1. Introduction

High-temperature thermally activated reserve batteries (also known as thermal batteries or TBs) are primary (non-rechargeable), single-use reserve power sources characterized by long shelf-life (>20 years), high-power capability (pulse current reaches 1 A cm⁻²), and excellent environment adaptability (−60 to 90 °C), and widely used in modern weaponry and aerospace, such as missiles, artillery, ejector seats, and helicopter turbine.^[1-3]Next-generation of weaponry and aerospace equipment calls for TBs with higher power capability, reduced volume, and the ability to accommodate unconventional volume geometries, proposing an urgent need for innovative technologies of electrode preparation.^[4-6] The conventional manufacturing process for TBs, referred to as pressed-pellet, is limited to the production of thick (>250 μm) circular-shaped electrodes for it works directly by mechanical pressing rather than binder usage to form the electrodes.^[7,8] For most applications where the TBs’ working time is short, such as artillery fuze batteries and aircraft ejection seat batteries, the overall operation time does not exceed 100 seconds and the required thickness of electrochemically active materials does not exceed 100 μm.^[9]Electrodes produced using the pressed-pellet approach exhibit low utilization efficiency of active materials and possess excessive volume and weight compared to the mission requirements. Moreover, the thick electrodes increase the transport distance of ions and electrons, resulting in a high polarization and limited power capability of TBs. Furthermore, the restriction to circular-shaped electrodes hinders the shape-accommodation capability of TBs, which is especially crucial for space utilization in minimal environments.^[6,10-12]

Slurry-coating, which is extensively used in fabricating electrodes for commercial secondary lithium-ion batteries,^[13,14]is capable of continuously producing thin sheets with a thickness of 10–500 μm and shows great potential to break through pressed-pellet limitations.^[7,15,16] Nevertheless, one drawback of slurry-coating is the introduction of binders to the electrodes. These binders tend to thermally decompose and generate gases at the operating temperature of TBs (400 to 550 °C). This gas production within the hermetically sealed TBs can raise the internal gas pressure, posing serious risks such as deformation, cracking, or even explosion, which endanger surrounding electronics and explosive components. To prevent such catastrophic events, it is crucial to investigate the gas production resulting from binder decomposition and the ability of batteries to withstand increased gas pressure. However, current research primarily focuses on the adhesive strength of binders and their corresponding electrochemical performance, paying limited attention to gas production.^[17-19] Binders with low gas production at the operating temperature of TBs are the key to the success of slurry-coating. In real TBs stacks (including single-cells, pyrotechnic, ignition strips, insulation layers, battery case, and other functional components), once the maximum internal temperature is determined, the overall internal gas pressure (P) of TBs is decided by the initial gas content (P_o) and the gases produced by binder decomposition (P_b). P_o depends on the total pore volume and gas species present in the battery stacks, typically falling within a constant range for typical TBs products. While on the other hand, P_b is closely associated with the proportion of gas released by the binder during thermal decomposition (which can be qualitatively described by the proportion of mass reduction (ψ )) and the binder content in the electrode (χ ). The relationship governing P_b can be expressed as:

\(P_{b}=\frac{\text{χψ}}{M}\frac{\text{RT}}{V}\) (1)

In equation (1), M signifies the average molecular weight of the gases, while R, T, and V represent the ideal gas constant, the internal temperature of TBs, and the volume of TBs respectively. Typically, pyrolysis of binders yields small molecules as gas products, and the value of M shows minimal variation across different binders. By equation (1), the key point in reducing P lies in the development of binders characterized by low ψ and low χ .

Previous works have studied the PVDF as a binder to fabricate thin-film cathodes for TBs.^[12,20] The PVDF-based cathodes have shown superior power capability compared to pressed-pellet cathodes and can be shaped into irregular forms. However, PVDF tends to produce volatile small molecules with high weight loss (ψ =77.6wt% at 550 °C) during thermal decomposition, which is not desirable for reducing the P of the TBs.^[21] Inorganic ceramic binders, such as glassy silicate (SiO₂/NaO),^[22]Poly(imide-co-siloxane) (PIS),^[17] and Na₂SiO₃,^[12] have attracted a lot of attention in recent years due to their higher thermal decomposition onset temperature (T₀ , 450°C<T₀ <550°C) and lowerψ compared to polymer binders. However, these ceramic binders are often having poor adhesion strength. Generally, the content of these electrochemical-inactive and electronic-insulation ceramic binders in the electrodes (χ ) reaches 5wt %−10wt %,^[12,22] significantly dilutes the electrodes’ total capacity and power capability. To the best of our knowledge, there haven’t been any reported binders that have both low ψ and do not compromise the electrochemical performance of thermal batteries (low χ ). Moreover, previous studies on binders mainly focused on the performance of thermal battery single-cells, neglecting the investigation of performance in real hermetically sealed thermal battery stacks.^[19,20] Since gases released by the binder’s thermal decomposition would dissipate in an open environment (the discharge tests of thermal battery single-cells are often conducted in an open environment), the negative impact on the internal gas pressure of thermal batteries cannot be detected in single-cell setups. Therefore, it would be more meaningful to study the binder’s performance in real hermetically sealed thermal batteries to better understand its effects.