The Reasearch On Nickel-containing Cathode Materials In Thermal Batteries

Thermal batteries are non-rechargeable and thermal-activated batteries with high specific energy in harsh environment,which are used primarily for military weapon and space exploration.At present,nickel compound(NiCl2、NiS2)are identified as perfect cathode materials of high specific energy and long-life thermal batteries.However,low conductivity and some weakness of high-discharge environment result in poor discharge performances which are far below theoretical values.Aiming at the above-mentioned problems of nickel compound cathode materials,we introduced nanocrystallization and carbon coating into the modification of nickel compound cathode materials,which improve the conductivity and thermal stability.We systematically studied the effects of nanocrystallization and carbon coating which enhance discharge performances in high-current discharge mechanism.The carbon coated NiCl2 composite materials were prepared by high temperature carbonization of stearic acid.The partial graphitized carbon which coats NiCl2 surface improved the conductivity and specific surface area.The results showed that carbon could remit the infiltration between NiCl2 and electrolyte,limit discharge products to migrate into electrolyte,decrease self-discharge of NiCl2 cathodes.The LiB anode and all Li-electrolyte(9.6 wt.%LiF-22 wt.%LiCl-68.4 wt.%LiBr m.p.=436℃)were used for testing.The specific energies of 1016 Wh kg-1 and 641 Wh kg-1 were obtained in 0.1 A cm-2 and 0.5 A cm-2,respectively.High specific energy and short discharge time make NiCl2/C be only applicable to high-power thermal batteries.The nano-NiS2 cathode materials with high specific capacity had been synthesized via milling-low temperature sintering.The precursor(Ni3S2)were prepared by ball-milling base on the raw materials of carbonyl nickel powders and sublimed sulfur.Then,the nano-NiS2 with 86 nm were synthesized when the precursor was sintered in 400℃ for 1 h.The nanocrystallization not only increased the specific surface area and conductivity,but also improved the utilization rate of NiS2 by accelerating the reaction rate of intermediate phase in Li/NiS2 thermal batteries.The nano-NiS2 cathodes show excellent discharge performances(500℃-0.5 V).The specific capacity of 794 mAh g-1 and specific energy of 1263 Wh kg-1 were obtained in 0.1 A cm-2.Accordingly 654 mAh g-1 and 950 Wh kg-1 were obtained in 0.5 A cm-2,which exceed that of micron-NiS2 cathodes.The biggest defect of nanocrystallization is that it reduces the thermal stability of NiS2,resulted in poor discharge performances in higher temperatures(>550℃).The hierarchical carbon coated nano-NiS2 no-shell pomegranate structure was constructed by high temperature decomposition of stearic acid.The hierarchical carbon structure enhanced thermal stable of nano-NiS2.The initial decomposition temperature increased from 400℃ to 590℃ after this hierarchical carbon coating.The hierarchical carbon coated nano-NiS2 cathodes revealed excellent discharge performances in thermal battery at different discharge temperatures.Specifically with 0.1 A cm-2 at 500℃,the specific capacity and energy reaches to 610 mAh g’1 and 1082 Wh kg-1,respectively,at an operating cut off of 1.4 V.The NiS2/C cathodes keep excellent performances even at higher discharge temperatures(700℃).The hierarchical carbon structure not only keeps nano-effects and conductivity of nano-NiS2 cathodes,but also has multiple protective effects:1)keeps the stability of cathodes structure in high temperatures,prevents the collapse of cathode because of chemical reaction and electrolyte flow;2)inhibits the dissolution and shuttling of product Ni,enhances the electrolyte stability.The carbon coated NiS2/NiCl2 composite materials were synthesized by the high temperature decomposition of stearic acid.The best discharge performances(550℃)of C@NiS2/NiCl2 cathodes were higher than that of NiS2/C cathodes(500℃).Furthermore,their highest discharge curve was 2.7 V.The specific capacity of 1132 mAh g-1 and energy of 1907 Wh kg-1 were obtained under this discharge condition(0.1 A cm-2-550℃-0.5 V).The analysis of electrode after discharge showed that the carbon coated NiS2/NiCl2 cathodes with better structural stability were more suitable for high-power thermal batteries.The nanocrystallization and carbon coating were used for enhancing thermal stability and conductivity of nickel compound cathode materials in high temperatures.The NiCl2/C cathode materials with high specific capacity and NiS2/C cathode materials with high specific energy were successfully obtained in the paper.The paper revealed the discharge mechanism of nickel compound cathode materials in high temperature;which made the nanocrystallization and carbon coating be applied to other cathode materials modification of thermal batteries;and also established the foundation which the nickel compound are industrially used in thermal batteries.