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 (Po) and the gases produced by
binder decomposition (Pb). Po 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, Pb 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 Pb 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.