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.