Figure 1. The gas production of electrodes with different binders. (a) Low-volatile electrode fabricated by acrylic acid derivative terpolymer binder (LA136D), the proportion of gases produced in acrylic acid derivative terpolymer thermal decomposition is only 39.2wt % at 550 °C; (b) Medium-volatile electrode fabricated by poly(vinylidene fluoride) binder, the proportion of gases produced in poly(vinylidene fluoride) thermal decomposition is 77.6wt % at 550 °C; (c) High-volatile electrode fabricated by styrene butadiene rubber binder, the proportion of gases produced in styrene-butadiene rubber thermal decomposition is 99.2wt % at 550 °C.
According to polymer materials science,[23,24] theψ and χ values of organic polymers are tightly related to their molecule structure and molecular weight. Chemical bonds with high bonding energy are typically used to create thermal-resistant polymers. For example, −C≡N (bond energy: 887 KJ mol−1) and −CONH2 (C=O, bond energy: 799 KJ mol−1) are more stable at high temperatures compared to C−C (346 KJ mol−1) and C−H (411 KJ mol−1). Also, the ψ value of organic polymers is determined by the chain-scission mechanisms in pyrolysis. Polymers that undergo random-chain scission and cross-linking mechanisms in pyrolysis tend to have lower ψ values. On the other hand, polymers with end-chain scission mechanisms usually result in higher ψ values because they tend to involve the breaking off of a small unit or group at the end of the polymer chain, and generating a larger number of small volatile fragments. Among the commonly used polymers, poly(methyl acrylate) exhibits random-chain scission behavior and polyacrylonitrile follows cross-linking mechanisms. Both of these are desirable to construct low ψ binders for thin-film cathodes in TBs. The presence of highly polar groups like −C≡N and C=O in polyacrylonitrile and poly(methyl acrylate) also contributes to achieving a strong adhesion strength by forming covalent bonds with the electrode materials. Which is beneficial to reduce the content of the binder in the electrodes (χ ). In addition, polymers with higher molecular weight are also beneficial for enhancing thermal stability and bonding strength. Therefore, polyacrylonitrile or poly(methyl acrylate) multi-element polymer with a high molecule weight may be a good choice to fabricate thin-film cathodes of TBs.
In this work, we first studied the gas pressure durability of TBs with a typical size of Φ 83×83mm by using the COMSOL simulation platform and giving out a quantitative value that will deform TBs, which provides an instruction to develop binders with low gas production. Secondly, we reported acrylic acid derivative terpolymer (LA136D) with ultra-high molecule weight (>100,000) as a low-volatile binder to fabricate thin-film cathode for TBs (as shown in Figure 1 (a) ), which shows both low gas-production and high adhesion capability. The performance of LA136D is verified in both thermal battery single-cells and stacks. Owing to the presence of polyacrylonitrile and poly (methyl acrylate), the ψ of LA136D is only 39.2wt % at even up to 550°C, well below the value of PVDF (77.6wt %) and SBR (99.2wt% ), as shown in Figure 1 (b) and (c) . In our experiments, the content of LA136D in the electrode can be reduced to as low as 1wt % while maintaining excellent mechanical properties. Importantly, we found the rate of volatiles released in LA136D pyrolysis is step-by-step (slowly) due to the reasonable collocation of thermal stable (C≡N, C=O) and hypo-thermal stable groups (C−C, C−H), this is truly beneficial to maintain the mechanical integrity of the cathode in the high thermal shock. Especially, in a 130s pulse discharging test, LA136D thin-film cathode stacks indicate a 77% reduction in polarization and 300% enhancement in cathode materials utilization efficiency, while with only ~0.05 MPa gas pressure increase compare with traditional pressed-pellet “thick-film” cathode.
2. Results and discussions
2.1. The gas pressure durability simulation of thermal battery
In this work, to reach the TBs stainless steel shell gas pressure durability value, a digital model of a thermal battery with 40×single cells is constructed in the COMSOL simulation platform. The detailed structure and parameters of the TBs model are shown in Figure 2 (a) . Figure S1 and Figure 2 (b) show the simulation of the Von Mises Stress brought to a thermal battery stainless steel shell under different internal gas pressure. Generally, the highest temperature of the stainless-steel shell surface would not exceed 100°C even if the thermal battery’s internal temperature is high (the detail value is according to the thermal battery design requirement). The yield strength of stainless steel at 100 °C is about 230 MPa.[25]From Figure 2 (b) we can see as long as the total internal gas pressure (P) in the thermal battery does not exceed 0.3 MPa, the Von Mises Stress brought to the stainless-steel shell would not exceed 220 MPa (even at the corners of column TBs, where are known to have severe stress concentration), and the stainless-steel would not go to experience any distortion. According to equation (1), the gases in TBs include two parts: the gases originally in the pores of TBs and produced by the thermal decomposition of materials in later periods. Commonly, the porosity of real TBs products is 30vol %~40vol %, herein, we assume the porosity of TBs to be 35vol %. According to the size of the TBs model in this work, the Po of original gases is calculated to be 0.149 MPa at 550 °C based on Ideal Gas Law (PV=nRT). Therefore, there is 0.151 MPa space for binder decomposition (Pb), as shown in Figure 2 (c) . That is to say, as long as the Pb is below 0.151 MPa, TBs would not be going to any distortion.