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• Self-discharge kinetics behavior of Li-Si anode is investigated by theoretical model and experiment. • Effective diffusion coefficients of soluble lithium in porous electrode are determined to verify the availability of model. • Effects of temperature, discharge current and electrode thickness on self-discharge kinetics behavior are discussed. • The three factors are coupled in the form of arrhenius equation by multivariate nonlinear regression. Self-discharge has significant influence on the service life of thermal batteries. The kinetics process of self-discharge depends on both the structure evolution and intrinsic property of electrodes. Herein, the kinetics model to quantify the self-discharge degree of the Li-Si anode is constructed according to the structure of thermal cells. Typical operation and preparation parameters, i.e. temperature, discharge current and electrode thickness, are evaluated based on the specially designed combined-discharge scheme. Then the effective diffusion coefficients (D eff) of soluble lithium (Li0) that are affected by the three variables are calculated and fitted. The calculation results consequently confirm self-discharge process of the Li-Si anode can be described by the derived equations exactly, and then the process is demonstrated to be diffusion-controlled. Further, D eff increase at the higher temperature and larger discharge current, while decrease with the increased electrode thickness, which is analyzed by scanning electron microscope and X-ray microcomputed tomography. The entire effects on D eff can be coupled in one fitting expression based on Arrhenius equation, and the apparent activation energies decrease under the larger discharge current and smaller electrode thickness. Finally, the additional experiments were conducted to verify the accuracy of the fitting expression. [Display omitted] [ABSTRACT FROM AUTHOR]

• A system-level thermal-electrochemical model of thermal batteries during activation stage is built. • Fuse strip has important effects on the activation process of thermal batteries. • The difference of internal resistance between the unactivated and activated state can reach 12 orders of magnitude. As the primary reserve batteries widely used in military domain, the intricate thermal-electrochemical behaviors and tiny time scale of the activation process make the development of thermal batteries a pain-stacking work, as lacking of the precise model to guide the design process. In this study, a system-level thermal-electrochemical model for evaluating the main technical indexes of LiSi/FeS 2 thermal batteries during activation stage is built, which comprehensively considers the main physical and chemical processes such as combustion, heat transfer, phase change, ion transport, voltage variation and so on. The activation characteristics of two types of 5-cell thermal batteries are predicted by this model, and the activation voltages of the two batteries are tested to validate the model. The model possesses the high precision with error less than 5 % by comparing the simulation results of activation voltage with that of experiment results. Specifically, the model reveals that the instantaneous temperature of pyrotechnic pellet after combustion is around 1570 K, and the electrolyte becomes molten and its internal resistance sharply decreases to mΩ level when the temperature reaches its melting point. The variation process of activation voltage is mainly dominated by the geometry size of cell stack and the thickness ratio of pyrotechnic pellet to single cell, besides, the activation time can be considered to be the peak value time of activation voltage. Fuse strip has important effects on the activation process of thermal batteries, especially in the rapid increase period of activation voltage, and the heat transfer is the time-limited step of the activation process. The difference of internal resistance between the unactivated state and activated state of thermal batteries can reach 12 orders of magnitude. These results indicate that our model can perform well in evaluating the activation process of thermal batteries and guiding the optimal design. [ABSTRACT FROM AUTHOR]

[Display omitted] • The combined electrochemical and transport kinetic parameters of CoS 2 /LiCl-LiBr-KBr system at the varied high temperature were systematically studied. • The effects of electrochemical and transport behavior on the impedance characteristic of CoS 2 /LiCl-LiBr-KBr were explored by proposing a equivalent-circuit model. • The results of this research is basis for the design and optimization of the new thermal battery cell. Systematic researches of ionic transport and electrochemical characteristics have been seldom implemented for the porous CoS 2 electrode in ternary LiCl-LiBr-KBr, which are ideal and main form for advanced thermal batteries. In the work, we systematically measured the combined electrochemical and transport kinetic parameters of CoS 2 /LiCl-LiBr-KBr system at the varied high temperature. Specifically, we firstly explored the discharge mechanism of CoS 2 /molten LiCl-LiBr-KBr using galvanostatic discharge experiment and X-ray diffraction (XRD). For the ionic transport performance, the diffusion coefficient of active Li ions was calculated from cyclic voltammograms (CV) data, and the ionic conductivity of LiCl-LiBr-KBr and electric conductivity of CoS 2 and generated Co 9 S 8 were also measured. Then, the interfacial electrochemical characteristics (exchange current density and transfer coefficients) were comparatively studied by the electrochemical impedance spectroscopy (EIS) and linear sweep voltammograms (LSV) method. Based on the above-mentioned results, we further investigated the comprehensive effects of these factors on the electrochemical and transport behavior by proposing a equivalent-circuit model of the porous CoS 2 electrode soaked in LiCl-LiBr-KBr molten salts. The results of this research is basis for the design and optimization of the new thermal battery cell, and will contribute to a knowledge-based development of the advanced TBs in the future. [ABSTRACT FROM AUTHOR]

[Display omitted] • A route is proposed to load nanoparticles and electrolyte into ceramic paper separator. • The proposed route is efficient and safe, which can be promoted to used in large scales. • Electrochemical performance of thermal batteries can be obviously improved with ceramic paper as separator. Composites consisted of ceramic paper, nanoparticles as well as molten salt electrolyte, are developed as the new electrolyte separators to substitute the traditional ones in thermal batteries, due to their flexible manufacture and good exhibition in electrochemical performance. However, the existed methods for fabricating the composites by roughly dip-coating with aqueous solutions, are suffering from drawbacks such as inefficiency, low consistence, or high temperature risk. Therefore, a new approach based on a melting-absorption-diffusion process, by using precursors without solvent, is developed. Firstly, the salts are accurately weighted. Then they melt on surfaces of ceramic papers and diffuse inside. After being treated at appropriate temperature for hours, high quality electrolyte separators based on ceramic papers are achieved. Compared to the traditional approaches, the new method can obviously increase and accurately control the amount of loaded nanoparticles and molten salts for each time. The achieved electrolyte separators also exhibit good electrochemical performance in single cells of thermal batteries compared with the traditional ones. With advantages like easy operation, high efficiency, and improved performance, this melting and in situ loading method holds broad prospect in preparing electrolyte separators at large scale. [ABSTRACT FROM AUTHOR]

In this paper, single-phase pyrite with different particle sizes is prepared by hydrothermal method with the reactants of NaS, S, and FeSO. Compared with natural pyrite powder, synthesized pyrite nanocubes with small particle sizes and fine distribution have lower resistance, higher specific capacity and power density during the discharging process. The enlarging contacting surface area of pyrite with electrolytes will reduce the concentration polarization caused by the discharging current and improve the utilization efficiency of the pyrite materials. When pulse currents are loaded, compared with natural pyrite single-cell thermal battery, synthetic ones have smaller voltage drops and higher power density during the pulse discharging process. Influences of particle sizes on the thermal stability of the pyrite powders have also been investigated. [ABSTRACT FROM AUTHOR]