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\begin{document}

\title{Low-volatile binder enables thermal shock-resistant thin-film cathodes
for thermal batteries}



\author[1]{Yong Xie}%
\author[1]{Liang Dong}%
\author[1]{Xu Zhang}%
\author[1]{Yong Cao}%
\author[1]{Yan Cui}%
\author[1]{Xiao Liu}%
\author[1]{Yi Cui}%
\author[1]{Chao Wang}%
\author[2]{Hongfa Xiang}%
\author[2]{Xuyong Feng}%
\author[3]{Long Qie}%
\affil[1]{China Academy of Engineering Physics Institute of Electronic Engineering}%
\affil[2]{Hefei University of Technology}%
\affil[3]{Huazhong University of Science and Technology School of Material Science and Engineering}%


\vspace{-1em}



  
  \date{June 11, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
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 \selectlanguage{ngerman}°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 (\selectlanguage{greek}ψ, ψ\selectlanguage{english}=39.2wt\%) at even up to 550 \selectlanguage{ngerman}°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.%
\end{abstract}\selectlanguage{ngerman}%



\sloppy


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. Cao\textsuperscript{1},
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:\emph{yongxie@caep.cn (Y.
Xie)};\emph{wangchao\_1988924@126.com (C.
Wang)};\emph{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 \selectlanguage{ngerman}°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 (\selectlanguage{greek}\emph{ψ} , ψ\selectlanguage{english}=39.2wt\%) at even up to 550 \selectlanguage{ngerman}°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 (\textgreater{}20
years), high-power capability (pulse current reaches 1 A
cm\selectlanguage{english}\textsuperscript{-2}), and excellent environment adaptability (\selectlanguage{english}-60 to
90 degC), and widely used in modern weaponry and aerospace, such as
missiles, artillery, ejector seats, and helicopter
turbine.\textsuperscript{{[}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.\textsuperscript{{[}4-6{]}} The conventional manufacturing
process for TBs, referred to as pressed-pellet, is limited to the
production of thick (\textgreater{}250 \selectlanguage{greek}μ\selectlanguage{english}m) circular-shaped electrodes
for it works directly by mechanical pressing rather than binder usage to
form the electrodes.\textsuperscript{{[}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
\selectlanguage{greek}μ\selectlanguage{english}m.\textsuperscript{{[}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.\textsuperscript{{[}6,10-12{]}}

Slurry-coating, which is extensively used in fabricating electrodes for
commercial secondary lithium-ion
batteries,\textsuperscript{{[}13,14{]}}is capable of continuously
producing thin sheets with a thickness of 10--500 \selectlanguage{greek}μ\selectlanguage{english}m and shows great
potential to break through pressed-pellet
limitations.\textsuperscript{{[}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 \selectlanguage{ngerman}°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.\textsuperscript{{[}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\textsubscript{o}) and the gases produced by
binder decomposition (P\textsubscript{b}). P\textsubscript{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\textsubscript{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 (\selectlanguage{greek}\emph{ψ} \selectlanguage{english})) and the binder content in the electrode
(\selectlanguage{greek}\emph{χ} \selectlanguage{english}). The relationship governing P\textsubscript{b} can be
expressed as:

\(P_{b}=\selectlanguage{greek}\selectlanguage{english}\frac{\text{χψ}}{M}\frac{\text{RT}}{V}\) (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 \selectlanguage{greek}\emph{ψ} \selectlanguage{english}and low \selectlanguage{greek}\emph{χ} .

\selectlanguage{english}Previous works have studied the PVDF as a binder to fabricate thin-film
cathodes for TBs.\textsuperscript{{[}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 (\selectlanguage{greek}\emph{ψ} \selectlanguage{english}=77.6wt\% at
550 \selectlanguage{ngerman}°C) during thermal decomposition, which is not desirable for
reducing the P of the TBs.\textsuperscript{{[}21{]}} Inorganic ceramic
binders, such as glassy silicate
(SiO\textsubscript{2}/NaO),\textsuperscript{{[}22{]}}Poly(imide-co-siloxane)
(PIS),\textsuperscript{{[}17{]}} and
Na\textsubscript{2}SiO\textsubscript{3},\textsuperscript{{[}12{]}} have
attracted a lot of attention in recent years due to their higher thermal
decomposition onset temperature (\emph{T\textsubscript{0}} ,
450°C\textless{}\emph{T\textsubscript{0}} \textless{}550°C) and
lower\selectlanguage{greek}\emph{ψ} \selectlanguage{english}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 (\selectlanguage{greek}\emph{χ} \selectlanguage{english}) reaches 5\emph{wt} \%-10\emph{wt}
\%,\textsuperscript{{[}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 \selectlanguage{greek}\emph{ψ} \selectlanguage{english}and do not
compromise the electrochemical performance of thermal batteries (low
\selectlanguage{greek}\emph{χ} \selectlanguage{english}). 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.\textsuperscript{{[}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.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{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
39.2\emph{wt} \% at 550 \selectlanguage{ngerman}°C; (b) Medium-volatile electrode fabricated by
poly(vinylidene fluoride) binder, the proportion of gases produced in
poly(vinylidene fluoride) thermal decomposition is 77.6\emph{wt} \% 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 99.2\emph{wt} \% at 550 °C.

According to polymer materials science,\textsuperscript{{[}23,24{]}}
the\selectlanguage{greek}\emph{ψ} \selectlanguage{english}and \selectlanguage{greek}χ \selectlanguage{english}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\textsuperscript{-1}) and
-CONH\textsubscript{2} (C=O, bond energy: 799 KJ
mol\textsuperscript{-1}) are more stable at high temperatures compared
to C-C (346 KJ mol\textsuperscript{-1}) and C-H (411 KJ
mol\textsuperscript{-1}). Also, the \selectlanguage{greek}\emph{ψ} \selectlanguage{english}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 \selectlanguage{greek}ψ \selectlanguage{english}values. On the other hand, polymers with
end-chain scission mechanisms usually result in higher \selectlanguage{greek}ψ \selectlanguage{english}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 \selectlanguage{greek}\emph{ψ} \selectlanguage{english}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 (\selectlanguage{greek}\emph{χ} \selectlanguage{english}). 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 \selectlanguage{greek}\emph{Φ} \selectlanguage{english}83\selectlanguage{ngerman}×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 (\selectlanguage{english}>100,000) as a low-volatile binder to fabricate
thin-film cathode for TBs (as shown in \textbf{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 \selectlanguage{greek}\emph{ψ} \selectlanguage{english}of LA136D is only 39.2\emph{wt} \% at even up to
550\selectlanguage{ngerman}°C, well below the value of PVDF (77.6\emph{wt} \%) and SBR
(99.2\emph{wt\%} ), as shown in \textbf{Figure 1 (b)} and \textbf{(c)} .
In our experiments, the content of LA136D in the electrode can be
reduced to as low as 1\emph{wt} \% 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\selectlanguage{english}[?]N, C=O) and hypo-thermal
stable groups (C\selectlanguage{english}-C, C\selectlanguage{english}-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
\textasciitilde{}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 40xsingle
cells is constructed in the COMSOL simulation platform. The detailed
structure and parameters of the TBs model are shown in \textbf{Figure 2
(a)} . \textbf{Figure S1} and \textbf{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 100degC
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 degC is about 230
MPa.\textsuperscript{{[}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 30\emph{vol}
\%\textasciitilde{}40\emph{vol} \%, herein, we assume the porosity of
TBs to be 35\emph{vol} \%. According to the size of the TBs model in
this work, the P\textsubscript{o} of original gases is calculated to be
0.149 MPa at 550 degC based on Ideal Gas Law (PV=nRT). Therefore, there is
0.151 MPa space for binder decomposition (P\textsubscript{b}), as shown
in \textbf{Figure 2 (c)} . That is to say, as long as the
P\textsubscript{b} is below 0.151 MPa, TBs would not be going to any
distortion.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 2. The simulation of gas pressure resistance of the
thermal battery.} (a) The schematics (2D) of the internal configuration
of \selectlanguage{greek}\emph{Φ} \selectlanguage{english}83\selectlanguage{ngerman}×83mm column thermal battery with 40 single-cells in the
series; (b) the simulation of the Von Mises Stress (MPa) brings to the
thermal battery stainless-steel shell under 0.3 MPa internal gas
pressure, according to the simulation, the highest stress brings to
stainless steel is about 220 MPa; (c) the relationships between thermal
battery internal gas pressure (aroused by the thermal decomposition of
binder), binder content in the cathode, and the proportion of volatiles
generated in binder thermal decomposition at 550 °C.

Commonly, the volatile products in polymer binder pyrolysis are small
molecules which with a similar average molecule weight. Here we
presumably set an M of 25 according to the volatile products of PVDF and
SBR binder in pyrolysis, as shown in Figure 1 (b) and
(c).\textsuperscript{{[}26,27{]}} Combined with equation (1), we can
reach the quantitative relationship between P\textsubscript{b},\selectlanguage{greek}\emph{χ,}
\selectlanguage{english}and \selectlanguage{greek}\emph{ψ} , \selectlanguage{english}as shown in Figure 2 (c). From Figure 2 (c), to ensure
the P\textsubscript{b} lies in the security zones, the binders with high
\selectlanguage{greek}\emph{ψ} \selectlanguage{english}should have low \selectlanguage{greek}\emph{χ} , \selectlanguage{english}\emph{e.g.,} a binder with a
\selectlanguage{greek}\emph{ψ} \selectlanguage{english}of 80\emph{wt} \% the \selectlanguage{greek}\emph{χ} \selectlanguage{english}in the cathode should not exceed
1.2\emph{wt} \%. In contrast, the binders with low \selectlanguage{greek}\emph{ψ} \selectlanguage{english}have a
wilder binder content space for choosing. \emph{e.g.} binder with
a\selectlanguage{greek}\emph{ψ} \selectlanguage{english}of 40\emph{wt} \% the binder content limit is 2.3\emph{wt} \%.
From the simulation, we can get a guideline for selecting and designing
binders for thin-film TBs.

\textbf{2.2. The thermal properties of acrylic acid derivative
terpolymer}

\textbf{Figure 3 (a)} shows the TG curves of LA136D, PVDF, and SBR, the
test is carried out in an N\textsubscript{2} atmosphere and the
calefactive rate is 10 \selectlanguage{ngerman}°C min\textsuperscript{--1}. From Figure 3 (a),
all the binders show thermal decomposition. The thermal decomposition
onset temperature (\emph{T\textsubscript{o}} ) is about 260°C, 270°C,
and 400°C respectively for SBR, LA136D, and PVDF. The
lower\emph{T\textsubscript{o}} of SBR and LA136D are reasonable because
of the presence of C\selectlanguage{english}-C and C\selectlanguage{english}-N bonds in the molecule structure which
with an inferior bonding energy of 347 KJ mol\selectlanguage{english}\textsuperscript{-1} and
305 KJ mol\selectlanguage{english}\textsuperscript{-1} respectively. The
higher\emph{T\textsubscript{o}} of PVDF is attributed to the high
electronic negativity of fluorine atoms that affects the excursion of an
electronic cloud of C\selectlanguage{english}-C and leads to the increase of thermal stability.
In contrast to \emph{T\textsubscript{o}} , the \selectlanguage{greek}\emph{ψ} \selectlanguage{english}of the binders
is more important in real hermetically sealed TBs. It directly relates
to the P of TBs. Figure 3 (a) shows LA136D having the lowest \selectlanguage{greek}\emph{ψ} \selectlanguage{english}of
39.2\emph{wt} \%, well below the value of PVDF (77.6\emph{wt} \%), and
SBR (99.2\emph{wt} \%). The difference of \selectlanguage{greek}\emph{ψ} \selectlanguage{english}in the three binders
can be interpreted by two aspects: 1) chain session mechanisms, and 2)
chemical bonding energy.\textsuperscript{{[}28{]}} According to polymer
science, polyacrylonitrile and poly (methyl acrylate) parts defer to the
cross-linking and random-chain scission mechanisms respectively in the
pyrolysis, while PVDF and SBR defer to the random-chain scission
mechanisms.\textsuperscript{{[}23,24{]}} Polymers with random-chain
scission and cross-linking mechanisms often lead to a low \selectlanguage{greek}\emph{ψ} \selectlanguage{english}since
the less small volatile fragments produced, while polymers with
end-chain scission mechanism usually lead to high \selectlanguage{greek}\emph{ψ} \selectlanguage{english}due to it
often involves the breaking of a small unit or group at the end of the
chain, which results in many small volatiles fragments. Thus, the
polyacrylonitrile and poly (methyl acrylate) parts in LA136D help it
with a low \selectlanguage{greek}\emph{ψ.} \selectlanguage{english}On the other hand, chemical bonds with high energy
are more stable at high temperatures. The double bonds (the bond energy
of C=O is 799 KJ mol\textsuperscript{-1}) and triple bonds (the bond
energy of C[?]N is 887 KJ mol\textsuperscript{-1}) in LA136D have higher
bonding energy than C-C (347 KJ mol\textsuperscript{-1}) and C-H (411 KJ
mol\textsuperscript{-1}), which mainly presence in PVDF and SBR, also
contributes to reducing volatile fragments at high temperature. The FTIR
analysis of the non-volatile residues after 550 \selectlanguage{ngerman}°C treatment of LA136D
shows the presence of C\selectlanguage{english}-O and C\selectlanguage{english}-N groups (\textbf{Figure 3 (c)} ). It
verifies that polyacrylonitrile and poly (methyl acrylate) is the key to
the low \selectlanguage{greek}\emph{ψ} \selectlanguage{english}of LA136D. In addition, LA136D shows a low thermal
decomposition rate in comparison with PVDF and SBR, as shown
in\textbf{Figure 3 (b)} . The slow mass reduction of LA136D as the
temperature increase indicates a step-by-step decomposition of the
molecule groups. This is due to the reasonable setting of thermal
hypo-stable groups (C-N, C-C, C-H) and thermal stable groups (C[?]N, C=O)
which have a different \emph{T\textsubscript{o}} . The low thermal
decomposition rate means a low released rate of gas fragments in binder
thermal decomposition, this is especially important to maintain
electrode mechanical integrity in TBs operation as will be proved in the
next part.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 3. The thermal properties of acrylic acid derivative
terpolymer.} (a) The thermal gravimetric (TG) curves of LA136D, PVDF,
and SBR, the proportions of volatiles generated in the three binders
thermal decomposition are 39.2\emph{wt} \%, 77.6\emph{wt} \% and
99.2\emph{wt} \% respectively; (b) the differential coefficient of the
TG curves in (a), it is shown LA136D has the lowest volatiles generation
rate in the thermal decomposition; (c) the Fourier-transform infrared
spectroscopy (FTIR) results of LA136D before and after 550 \selectlanguage{ngerman}°C treatment,
it is shown there are some C\selectlanguage{english}-N and C\selectlanguage{english}-O groups in the solid residues,
which indicates C\selectlanguage{english}[?]N and C=O are beneficial to reach a low volatile
proportion in thermal decomposition; (d) the in-situ analysis of the
gaseous fragments produced in LA136D thermal decomposition by using
TG-FTIR; (e) The proposed thermal decomposition process of LA136D at the
high temperatures.

To further study the thermal decomposition process of LA136D, we studied
the gas products of LA136D by TG-FTIR. The result is shown
in\textbf{Figure 3 (d)} . It is shown that few NH\textsubscript{3} occur
at about 230 degC and spurts out after 300 degC, indicating the breaking of
C\selectlanguage{english}-H (possibly the ends of the main chain) and C\selectlanguage{english}-NH\textsubscript{2}. At
the same time, the CO\textsubscript{2} generates at nearly 300 degC,
indicates the breaking of R\selectlanguage{english}-COOLi (the occurrence of CO\textsubscript{2}
before 300 degC is due to the background of TG-FTIR). The
CH\textsubscript{4} and other C\selectlanguage{english}-H volatiles occur after 400degC, referring
to the break of the \selectlanguage{english}-C\selectlanguage{english}-C\selectlanguage{english}- chain, this can also be verified by the
differential curve of LA136D in Figure 3 (b) whereas the rate of thermal
decomposition reaches peak value. Based on the discussions above, the
thermal decomposition process and the gas products of LA136D can be
described in\textbf{Figure 3 (e)} . When heating the LA136D,
RCO\selectlanguage{english}-NH\textsubscript{2,}and R\selectlanguage{english}-COOLi begin to break at nearly 230 degC and
300 degC accompanying the generation of NH\textsubscript{3} and
CO\textsubscript{2} gaseous respectively. The RCO\selectlanguage{english}-NH\textsubscript{2}
breaking rate reaches a peak value at 320 degC. As the temperature keeps
increasing to 400 degC, the main chain \selectlanguage{english}-C\selectlanguage{english}-C\selectlanguage{english}- begins to break and
CH\textsubscript{4} or other C\selectlanguage{english}-H gaseous fragments are produced. The
break of R\selectlanguage{english}-COOLi goes with the whole heating process.

\textbf{2.3. The adhesion strength of acrylic acid derivative
terpolymer}

Due to most of the binders being non-electrochemical active and
non-electronic conductivity, it is desirable to develop binders with
high adhesion capability to reduce the addition content (\selectlanguage{greek}\emph{χ} \selectlanguage{english}) in
the electrode to reach high electrochemical performance. As regards TBs,
this is more important because the gas production of binders in thermal
decomposition at TBs operation temperature may lead to the break of the
battery bulk. LA136D is known for its excellent adhesion strength and is
used in silicon and sulfur electrode manufacturing in secondary
batteries.\textsuperscript{{[}29,30{]}} In contrast to the traditional
PVDF binder, which interacts with electrode materials via weak Van der
Waals forces, LA136D interacts with electrode materials via covalent
bonds.\textsuperscript{{[}31{]}} The large amounts of high polarity
groups (-C[?]N, -COONH\textsubscript{2}, and -COOLi) in the side chain of
LA136D act as the adsorption points to bond the electrode materials and
current collector. In this work, we fabricated TBs thin film electrode
by using LA136D as the binder and studied the electrode adhesion
capability through an electrode peel-off test. \textbf{Figure 4
(a)}shows the preparation process of slurry coating and \textbf{Figure 4
(b)} is the prepared thin film cathode. \textbf{Figure 4 (c)} is the
schematics of the electrode peel-off test process. \textbf{Figure 4
(d)}shows the peel-off force used to peel the different electrodes. For
comparison, we also studied the electrode adhesion ability fabricated by
PVDF and SBR. The content of the different binders in the electrode is
kept to the same (1\emph{wt} \%), and the electrode thickness is
controlled at 100 \selectlanguage{greek}μ\selectlanguage{english}m and with similar tap density (\textasciitilde{}3.5
mg cm\textsuperscript{-3}). From Figure 4 (d), LA136D shows the highest
peel-off force of 140 N, SBR shows a medium peel-off force of 98 N, and
PVDF shows the lowest peel-off force of 60 N. The peel-off test
indicates LA136D is capable to fabricate a mechanically robust electrode
even at a low content. \textbf{Figure 4 (e)} is the digital pictures of
electrodes after the peeling test. It can be seen that LA136D electrodes
have the best surface integrity. This result is consistent with the
peel-off force.

Traditional pressed-pellet technology is hard to produce electrodes with
non-circular shapes and restricts the shape-accommodation capability of
TBs.\textsuperscript{{[}32{]}} Herein, it is shown that LA136D thin film
electrode can be cut into various shapes randomly, as shown
in\textbf{Figure 4 (f)} . In addition, the LA136D electrode also shows
excellent handling properties (\textbf{Figure 4(g)} ). The electrode
materials do not drop even at a high crinkle angle. It is worth noting
that the content of LA136D used in the electrode is 1\emph{wt} \%. The
``sticky'' property of LA136D can be observed visibly in \textbf{Figure
4 (h)} , it is consistent with the high bonding capability of LA136D
that few LA136D can bond to electrode materials efficiently through the
high polarity groups.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 4. The characterization of} \textbf{adhesion strength of
acrylic acid derivative terpolymer.} (a) The schematics of the
fabricating process of slurry coating; (b) the digital picture of
Fe\textsubscript{0.5}Co\textsubscript{0.5}S\textsubscript{2} thin film
cathode prepared by LA136D; (c) the schematics of electrode peel-off
test; (d) the peel-off force of electrodes with a different binder, the
content of the binders in the electrodes are 1\emph{wt} \%, the
electrode thickness is about 100 \selectlanguage{greek}μ\selectlanguage{english}m, the tape density of different
electrodes is controlled at 3.5 mg cm\textsuperscript{-3}; (e) the
digital pictures of electrodes after peel-off test; (f) the digital
picture of LA136D thin film electrode after being cut into different
shapes, it is shown that the electrode maintains excellent mechanical
integrity and without any powder materials drop after cut; (g) the
demonstration of the mechanical properties of LA136D thin film
electrode, LA136D electrode shows no electrode powder materials drop
even at a high curvature; (h) the SEM picture of LA136D electrode, it
indicates that small dots of LA136D can bond the electrode powders
efficiently.

\textbf{2.4. The study of the electrochemical performance of thin-film
thermal batteries}

The low \selectlanguage{greek}\emph{χ} \selectlanguage{english}and low \selectlanguage{greek}\emph{ψ} \selectlanguage{english}of LA136D contribute to manufacturing
thin film cathodes with low volatility. Herein, we studied the LA136D
thin film electrode electrochemical performance in both thermal battery
single cells and stacks and verified the volatility of the
electrode.\textbf{Figure 5 (a)} is the schematics of equipment used for
thermal battery single-cell discharge. The single cell consists of a
cathode, separator, and anode set between the two-heating plate. The
temperature used to heat the single cell is 500 \selectlanguage{ngerman}°C and the stress is 20
N cm\selectlanguage{english}\textsuperscript{-2}. \textbf{Figure 5 (b)} are the discharge curves
of single cells assembled by LA136D thin film cathode and traditional
pellet cathode. It is shown that LA136D thin film cathode has a
discharge capacity of 1360 As g\selectlanguage{english}\textsuperscript{-2} at 0.2 A
cm\selectlanguage{english}\textsuperscript{-2}. In contrast, the value is 1170 As
g\selectlanguage{english}\textsuperscript{-2} for the pellet cathode. In addition, LA136D thin
film cathode shows smaller polarization (\textasciitilde{}0.5 V) in
comparison with the pellet electrode. Combined with the discharge
capacity and voltage platform, LA136D thin film cathode shows a 22\%
energy density increase. \textbf{Figure 5 (c)} shows the pulse
properties of single cells with different cathodes. It indicates that
the internal resistance of the LA136D cell is only 17 m\selectlanguage{greek}Ω \selectlanguage{english}upon 1 A
cm\textsuperscript{-2} pulse test (The internal resistance is calculated
according to
R=U\textsubscript{2}-U\textsubscript{1}/I\textsubscript{2}-I\textsubscript{1}).
In contrast, the internal resistance of the pellet cell is 75 m\selectlanguage{greek}Ω, \selectlanguage{english}which
is about four times higher than the LA136D cell. The superior discharge
capacity and pulse performance indicate the superiority of the LA136D
thin-film cathode to the traditional pressed-pellet thick cathode and
prove that the LA136D binder is compatible with TBs cathode and molten
electrolyte materials.

Furthermore, to verify the volatiles that produced in the thermal
decomposition of LA136D do not bring safety risks to TBs, we studied the
performance of the LA136D thin film cathode in the real hermetically
sealed TBs stacks. \textbf{Figure 5 (d)} is the schematics of thermal
battery stacks assembled in this study. The batteries include 40
single-cells in the series and 41 pyrotechnic pellets, the pyrotechnic
pellets are positioned between cells and the two ends of the stacks. The
thickness of the LA136D thin film cathode used in assembling thermal
battery is 200 \selectlanguage{greek}μ\selectlanguage{english}m (100 \selectlanguage{greek}μ\selectlanguage{english}m electrode+100\selectlanguage{greek}μ\selectlanguage{english}m current collector). For
comparison, a 300 \selectlanguage{greek}μ\selectlanguage{english}m+100 \selectlanguage{greek}μ\selectlanguage{english}m pellet cathode is also used to assemble the
thermal battery stack. The diameter of both electrodes is 60 mm. The
thermal design of the two thermal batteries is kept the same, namely,
the highest temperature of the two thermal batteries is the same. The
height of the thin film thermal battery and pellet thermal battery stack
is 45.4 cm and 60.6 cm respectively. Thin-film thermal battery reduces
about 25\% height in comparison with pellet thermal battery. The height
difference directly affects the activation time of the thermal battery.
The activation time is extraordinarily important for military and
emergency power sources because it directly relates to the response time
in emergency accidents. \textbf{Figure 5 (e)} is the activation voltage
curves of thermal batteries. The thin film thermal battery spends about
395 mS to reach 80 V working voltage, which is 38\% faster than the
pellet thermal battery. This means LA136D thin film cathode is more
suitable to construct rapidly-activation thermal
batteries.\textbf{Figure 5 (f)} are the 130 s pulse test results of the
different thermal batteries. Consistent with single-cells, LA136D thin
film cathode shows superior discharging capacity at big current density
which indicates LA136D thin film is beneficial to construct high power
thermal battery. When the TBs have a working time of approximately 100
seconds, the utilization efficiency of cathode materials of the
thin-film cathode is three times higher than that of the pressed-pellet
cathode.\textbf{Figure 5 (g)} is the schematics of the pressure and
temperature test of the TBs. To reach the real-time temperature and
pressure, a temperature and pressure sensor is put into the thermal
battery. The temperature sensor is put on the stack's side surface and
the pressure sensor is put at the ends of the stacks. To examine the
thermal battery in a rigorous state, thermal batteries are open circuits
after activation without load.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure 5. The electrochemical performance of thermal batteries
with LA136D thin film electrode.} (a) The schematics of the equipment
used for thermal batteries single-cell discharge, the heating
temperature is 500 \selectlanguage{ngerman}°C and the compress is controlled at 20 N
cm\selectlanguage{english}\textsuperscript{-2}; (b) the comparison of the discharge specific
capacity and energy density between LA136D thin film cathode and pellet
cathode in single-cell configuration, the thickness of electrochemical
materials is 100 \selectlanguage{greek}μ\selectlanguage{english}m and 300 \selectlanguage{greek}μ\selectlanguage{english}m respectivly for thin-film and pellet
cathodes, the diameter of the cathode is 60 mm, the cathode materials
and molten electrolyte used in this experiment are
Fe\textsubscript{0.5}Co\textsubscript{0.5}S\textsubscript{2} and
LiCl-KCl; (c) the comparison of pulse discharge performance of thermal
batteries with LA136D thin film cathode and pellet cathode; (d) the
comparision of the stacks height of thermal batteries with LA136D thin
film cathode and pellet cathode; (e) the comparison of activation time
of thermal batteries with different cathodes; (f) the comparison of
pulse discharge performance of thermal battery stacks with different
cathodes; (g) the illustration of the equipment and method used to
detect the internal temperature and pressure of the thermal batteries,
the results are shown in (h) and (i) respectively.

\textbf{Figure 5 (h)} and \textbf{(i)} are the temperature and pressure
curves of thermal batteries after activation. Due to the smaller height,
the temperature increase in the thin film thermal battery is faster than
the pellet. The P is directly related to temperature and the amount of
gases in binder thermal decomposition. The pressure increase in the
thin-film thermal battery is faster than the pellet due to the faster
rate of temperature increase. The gases produced by binder decomposition
also contribute to the fast increase of gas pressure in thin-film
thermal batteries. The highest relative pressure after activation is
0.16 MPa and 0.21 MPa respectively for the thin-film and pellet thermal
batteries which indicates LA136D only produces 0.05 MPa gases in the
thermal battery. The internal gas pressure of the LA136D thermal battery
is less than the safety threshold (0.3 MPa) which we simulate in the
previous, indicating that the LA136D does not lead to any distortion of
TBs bulk. Therefore, we have shown LA136D will not bring safety risks to
the thermal battery stacks and will not detriment the electrochemical
performance due to the minim addition.

Excepting for the gas pressure increment, the gas released in binder
thermal decomposition may deteriorate the electrode integrity, as shown
in \textbf{Figures 6 (a)} and \textbf{(b)} . A binder with a low gas
production rate in thermal decomposition is beneficial to maintain the
mechanical integrity electrode while a binder with a high gas production
rate is bad for the electrode's mechanical integrity. To verify the
mechanical integrity of the LA136D thin film electrode in TBs operation,
we disassembled the thermal battery stack after discharge.\textbf{Figure
6 (c)} is the digital picture of the LA136D thin film cathode after
discharge. It can be seen that the thin film electrode maintains well
integrity even after discharging at high temperatures, and without any
powdered materials drop. The cross-section of the discharged single cell
also shows a regular morphology (\textbf{Figure 6 (d)} ). These results
prove that LA136 is capable to maintain mechanical integrity in TBs
operation. In contrast, a thin-film cathode prepared by PVDF binder
shows many cracks after discharge.

\textbf{Figure 6 (e)} is the comparison of the reported binder used in
fabricating thermal battery thin film cathodes in the aspects of\selectlanguage{greek}\emph{χ}
\selectlanguage{english}and \selectlanguage{greek}\emph{ψ} . \selectlanguage{english}It can be found LA136D have the lowest \selectlanguage{greek}\emph{χ}\selectlanguage{english}and
\selectlanguage{greek}\emph{ψ} . \selectlanguage{english}Such properties enable LA136D to fabricate thin-film cathodes
with low volatility and high electrochemical performance.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 6. The illustration of mechanical integrity of LA136D
thin-film cathodes at high temperatures.} (a) and (b) are the schematics
of slurry-coated cathodes experienced in the high-temperature
environment, binder with a low gas production rate in thermal
decomposition is beneficial to maintain the mechanical integrity in TBs
operation (a), while a binder with a high gas production rate always
leads to electrode cracks (b); (c) the digital picture of LA136D thin
film cathode after discharging, it shows an integrity electrode surface;
(d) the cross-section of LA136D single-cell after discharging; (e) the
comparison of \selectlanguage{greek}\emph{ψ} \selectlanguage{english}and \selectlanguage{greek}\emph{χ} \selectlanguage{english}of binder in the recent
slurry-coating works.

\textbf{3. Conclusions}

We fabricated a 100 \selectlanguage{greek}μ\selectlanguage{english}m thin-film cathode for TBs by using an acrylic
acid derivative terpolymer (LA136D) binder and studied the performance
in both single-cells and stacks. It is found the \selectlanguage{greek}\emph{ψ} \selectlanguage{english}of LA136D is
relatively low (39.2\emph{wt} \%). And due to the high adhesion
strength, LA136D can fabricate thin-film, non-circular, and mechanical
robust electrodes at a content of 1\emph{wt} \%, which contributes to
only 0.05 MPa gas pressure increase in real hermetically sealed thermal
battery stacks. The single-cell and stacks electrochemical performance
tests are shown LA136D is well compatible with the cathode and molten
salt materials, and the LA136D thin-film cathode exhibits superior
electrochemical performance to the traditional pressed-pellet cathode.
Especially, in a 130s pulse discharging test, the TBs it is indicated
that LA136D thin-film cathode leads to a remarkable 440\% reduction in
polarization and 300\% enhancement in the utilization efficiency of
cathode materials, while with only \textasciitilde{}0.05 MPa gas
pressure increment compare with the traditional pressed-pellet cathode.
Our work proves the feasibility of LA136D in fabricating safe thin-film
cathodes for high-efficiency and high-power TBs and contributes to
arousing the attention to gas production of binders in real thermal
battery stacks.

\textbf{4. Experimental Section}

\emph{Materials and electrode preparation:} the acrylic acid derivative
terpolymer (LA136D, Chengdu Indigo, China), polyvinylidene fluoride
(PVDF) binder (Arkema, HSV900), butadiene-styrene rubber (SBR) emulsion
(JSR, 104A),
Fe\textsubscript{0.5}Co\textsubscript{0.5}S\textsubscript{2}, LiCl-KCl
molten electrolyte and LiB alloy were commercially available and used as
received. The LA136D-based cathode slurry was prepared by mixing LA136D
and Fe\textsubscript{0.5}Co\textsubscript{0.5}S\textsubscript{2} cathode
materials in a planetary centrifugal mixer and stirred for 1.5 h, the
mass ratio of LA136D and
Fe\textsubscript{0.5}Co\textsubscript{0.5}S\textsubscript{2} is
controlled at 1:99.The same procedure and material ratio are applied in
PVDF and SBR cathode slurry. The slurry was coated on graphite paper
current collector (100 \selectlanguage{greek}μ\selectlanguage{english}m) by blade casting. The electrode mass loading
was controlled at 35 mg cm\textsuperscript{-2} by adjusting the casting
thickness and solid content of the slurry. The thin film electrode
thickness after the calendar is about 100 \selectlanguage{greek}μ\selectlanguage{english}m. The contrast pellet
electrode was prepared by pressed-pellet technology with a thickness of
400 \selectlanguage{greek}μ\selectlanguage{english}m (including 100 \selectlanguage{greek}μ\selectlanguage{english}m graphite paper current collector).

\emph{Binder and thin film electrode characterization:} the thermal
properties testing was conducted on a thermal analysis system with a
heating rate of 10 \selectlanguage{ngerman}°C min\selectlanguage{english}\textsuperscript{-1} from 30 degC to 1000 degC in
N\textsubscript{2}. The Fourier Transform Infrared (FTIR) spectroscopy
test of LA136D before and after 550 degC treatment is conducted by using
Attenuated Total Reflection (ATR) model. The gaseous products of LA136D
were conducted on TG-FTIR with a heating rate of 10 degC
min\selectlanguage{english}\textsuperscript{-1} from 30 degC to 600 degC in N\textsubscript{2}. The
electrode peeling test was conducted. The microstructure morphology was
observed by SEM.

\emph{Electrochemical performance characterization:} the thermal battery
single cells were assembled by using a pellet cathode and thin film
cathode respectively. The cathode material is
Fe\textsubscript{0.5}Co\textsubscript{0.5}S\textsubscript{2}, the molten
electrolyte is LiCl-KCl, and the anode material is LiB alloy. The
compressed pressure used for cell construction is about 0.5 t
cm\selectlanguage{english}\textsuperscript{-2}. The single cells discharge test was conducted in
an Ar\textsubscript{2} atmosphere environment, with a temperature of 500
degC and with a compress of 20 N cm\selectlanguage{english}\textsuperscript{-2}. The discharge was
conducted by a LAND battery test system (Wuhan LAND, China). The current
used for single cells capacity characterization is controlled at 0.2 A
cm\selectlanguage{english}\textsuperscript{-2} and the cut-off voltage is 1.6 V. The pulse
discharge was conducted with a consistent current density of 0.1 A
cm\selectlanguage{english}\textsuperscript{-2} and a pulse current density of 1 A
cm\selectlanguage{english}\textsuperscript{-2}. The thermal battery stacks was assembled in a
dry air room and a pulse discharge pattern of 0.05 A
cm\selectlanguage{english}\textsuperscript{-2} (15s)-0.2 A cm\selectlanguage{english}\textsuperscript{-2}(5s)-0.05 A
cm\selectlanguage{english}\textsuperscript{-2}(5s)-0.5 A cm\selectlanguage{english}\textsuperscript{-2}(5s)-0.05 A
cm\selectlanguage{english}\textsuperscript{-2} (5s)-0.2 A cm\selectlanguage{english}\textsuperscript{-2}(5s).

\emph{Thermal battery inner temperature and pressure
characterization:}the column thermal battery stainless steel shell was
pre-processed with two holes on the ends for signal transfer. The
temperature sensor was set on the thermal battery stack surface and can
work normally at 600 degC. The pressure sensor was put at the ends of the
column thermal battery, outside the insulation layer. The holes were
sealed by a high-temperature ceramic binder after the battery assembly.

\textbf{Supporting Information}

Supporting Information is available from the Wiley Online Library or the
author.

\textbf{Acknowledgment}

The authors are grateful for the support from the China Academy of
Engineering Physics and the National Natural Science Foundation of China
(NSFC-No. 52102319, 12104422, U1930208).

\textbf{Conflict of Interest}

The authors declare no conflict of interest.

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Supporting Information

\textbf{Low-volatile binder for safety slurry-coated thin-film cathodes
of high-efficiency and high-power thermal batteries}

Yong Xie\textsuperscript{1*}, Xu Zhang\textsuperscript{1}, Liangping
Dong\textsuperscript{1}, Yong Cao\textsuperscript{1}, Xiaojiang
Liu\textsuperscript{1}, Yixiu Cui\textsuperscript{1}, Chao
Wang\textsuperscript{1*}, Yanhua Cui\textsuperscript{1*}, Xuyong
Feng\textsuperscript{2}, Hongfa Xiang\textsuperscript{2}, and Long
Qie\textsuperscript{3}

\textsuperscript{1}Laboratory of Electrochemical Power Sources,
Institute of Electronic Engineering, China Academy of Engineering
Physics, Mianyang, Sichuan 621000, China

\textsuperscript{2}School of Materials Science and Engineering, Hefei
University of Technology, Hefei 230009, Anhui, China

\textsuperscript{3}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

\textsuperscript{*}Corresponding authors

Email address: yongxie@caep.cn\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{Figure S1.} The simulation of the Von Mises Stress brought to a
thermal battery stainless steel shell under different internal gas
pressure.

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