Two-dimensional
(2D) nanomaterials possessing a unique sheet structure, compared to
correlative bulk materials, exhibit excellent properties, especially
in the energy storage and energy conversion field. In this case, NiCl2 nanosheets with thicknesses of 2–8 nm are first prepared
by a simple chemical vapor deposition method. For the Li–B/LiF–LiCl–LiBr/NiCl2 thermal battery, the specific energy of NiCl2 nanosheets
increases from 510 W h kg–1 (NiCl2 rods)
to 616 W h kg–1 at an operation temperature of 500
°C and a current density of 0.2 A cm–2. The
2D morphology and large numbers of defects not only improve the redox
reaction rates and the lithium storage capacity, but also enhance
the adsorption capacity with the flake-like binder MgO, which prolong
the discharge time by suppressing the discharge product diffusion
to the electrolyte. These results indicate that NiCl2 nanosheets
have a great possibility to become a desirable candidate of cathode
materials for assisting in the development of high energy output and
provide a new way to restrain the immersion between the electrode
and electrolyte.
Nanocrystallization can shorten the Li+ transport distance, resulting in the enhancement of electrochemical activity for cathode materials. However, nanocathode materials tend to be thermally unstable, further leading to poor electrochemical performance of a battery system. This disadvantage can be especially detrimental for thermal batteries because they are often operated at high temperatures (≥450 °C). Herein, the decomposition character of NiS2 at 500 °C is investigated. The decomposition temperatures of NiS2 are found to decrease from 510 to 350 °C with the grain size decreasing to 39 nm, due to the dramatically increased surface energy. The decomposition product is confirmed to be NiS, evidenced by a high‐temperature X‐ray diffractometer. The useful mass of the cathode will reduce once the discharging temperature is higher than 500 °C. Namely, although the small grain size shorthens the Li+ transport distance, the discharge performance of the NiS2 cathode may also decrease due to its inferior thermal stability. For the Li‐B/LiF–LiCl–LiBr/NiS2 system, the NiS2 with the grain size of 70 nm shows the highest specific capacity of 831 mAh g−1 under the discharging temperature of 500 °C, with the cut‐off voltage of 0.5 V, compared with other grain sizes from 39 to 112 nm.
Garnet-type Ta-doped Li7La3Zr2O12 (LLZTO) solid electrolyte has been widely investigated for secondary Li ionic or metal batteries at ambient temperature. Because of the increasing ionic conductivity of LLZTO with temperature, we applied the LLZTO solid electrolyte to thermal battery working at 550℃. The LLZTO presents ultrahigh specific energy as the discharge specific energy and specific power is 605 W h/kg and 2.74 kW/kg at 100 mA/cm2 with a cut-off voltage of 1.8 V, respectively. This is larger than the LiF–LiCl-LiBr electrolyte which is commonly used in thermal battery with a specific energy of 514 W h/kg. The internal resistance of the single cell reaches 0.65 Ω, but the specific energy remains at about 400 W h/kg as the current density increases to 400 mA/cm2. We report the application of LLZTO in thermal battery with high specific energy, large current, and high voltage discharge for the first time, broadening the application range of solid electrolytes.
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