Multifunctional thermochromic materials integrated optical, ferroelectric, and/or magnetic properties have been long envisioned and played an invaluable role in signal processing and non-contact optical information storage in smart windows. However, poor materials stability, together with high discoloration temperature, absence of ferroelectric, or magnetic behaviors, impedes most thermochromic materials for practical applications. Here, the thermochromic behavior of all-inorganic lead-free ferroelectric Sn 2 P 2 S 6 (SPS) is extensively studied, which exhibits a reversible color change between light orange and red around the ferroelectric-paraelectric phase transition temperature (∼ 67 °C). In situ powder X-ray diffraction (PXRD) and Raman spectroscopy indicate the change of P-S and Sn-S bond distance, and thus result in the modification of (P 2 S 6 ) octahedral geometry and the coordination environment of Sn 2+ ions along with the evolution of thermochromism (TCM). Additionally, the first-principles calculations reveal that, the Sn 5 s 2 stereochemically active lone pair electrons evoke negative lattice expansion comparable to that of the known ZrW 2 O 8 at warming and is responsible for the reversible TCM of SPS. SPS also possesses high near-infrared reflectance ( R > 80 %, 1100-2500 nm). The integration of ferroelectricity, high near-infrared reflectance, and TCM below 100 °C renders potential applications of the titled compound in multifunctional optical smart windows.
Pseudocapacitive Na + storage based on the fast absorption/desorption kinetics is a new alternative to realize the high rate capability, which rely on increasing the surface area and creating surface vacancies. This means low Na + capacity because of low dielectric constant for the reported pseudocapacitive Na + storage materials. According to the capacitance formula, we find increasing the dielectric constant is a new path to improve the Na + capacity. This dielectricity (other than surface engineering) based pseudocapacitance can be called more real pseudocapacitance. The superparaelectric 2-dimensional (2D) Sn 2 P 2 S 6 has much higher dielectric constant than the bulk counterpart. The identical polaron direction triggered superparaelectricity and high dielectric constant of 2D Sn 2 P 2 S 6 are well verified. And 2D assembly between the 2D Sn 2 P 2 S 6 and graphene construct the compact network for conductivity of electron and Na + . As results, the 2D Sn 2 P 2 S 6 battery based on more real pseudocapacitance delivers alloy-like capacity and long cycle stability.
Sodium ion batteries (SIB) are considered promising alternative candidates for lithium ion batteries (LIB) because of the wide availability and low cost of sodium, therefore the development of alternative sodium storage materials with comparable performance to LIB is urgently desired. The sodium ions with larger sizes resist intercalation or alloying because of slow reaction kinetics. Most pseudocapacitive sodium storage materials are based on subtle nanomaterial engineering, which is difficult for large-scale production. Here, ferroelectric Sn 2 P 2 S 6 with layered nanostructure is developed as sodium ion storage material. The ferroelectricity-enhanced pseudocapacitance of sodium ion in the interlayer spacing makes the electrochemical reaction easier and faster, endowing the Sn 2 P 2 S 6 electrode with excellent rate capability and cycle stability. Furthermore, the facile solid state reaction synthesis and common electrode fabrication make the Sn 2 P 2 S 6 that becomes a promising anode material of SIB.
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