Novel Sub‐5 nm Layered Niobium Phosphate Nanosheets for High‐Voltage, Cation‐Intercalation Typed Electrochemical Energy Storage in Wearable Pseudocapacitors

Abstract

A series of advanced progress has been made toward flexible supercapacitor design in recent years, including micro-SCs based on serpentine interconnections, [8] fiber-shaped SCs, [9] and the devices based on new crumpled-graphene papers, [10] tricot weave, [11] hierarchically buckled sheah-core fibers. [12] Nevertheless, the practical application of WSSPs is still limited by their low energy density. [13] According to the equation of energy density E = 1/2 C V 2 , for a given supercapacitor (SC), the energy density highly depends on the specific capacitance (C) and operating voltage window (V). [13] Given by this equation, the energy density is proportional to the square of operating voltage window. In general, the electrochemical stability window of most energystorage materials is less than 0.8 V in aqueous electrolytes due to the water splitting at 1.23 V, and high-voltage materials is highly desired to enhance the energy density. [14] Most recently, layered polyanionic phosphates, such as VOPO 4 , Na 3 V 2 (PO 4 ) 3 , NaVOPO 4 , KVPO 4 F, and KVOPO 4 , have been considered as promising electrode materials for energy storage because of their high thermal and structural stability. [15] More importantly, it has been theoretically predicted a much higher potential (≈1.0 V in aqueous electrolyte) in layered phosphates than in most of the traditional pseudocapacitive materials due to the enhanced iconicity of metaloxygen bond with the existence of [PO 4 ] 3− group. [16][17][18][19] Meanwhile, niobium (Nb) is an important transition metal element with rich oxidation states (Nb 3+ , Nb 4+ , and Nb 5+ ) for potential electrochemical energy storage. A unique Li + -intercalation pseudocapacitance has been found in T-Nb 2 O 5 (40 µm thick) electrode, which holds a great perspective on the development of electrochemical electrodes. [20,21] Especially, the pseudocapacitance occurs in the T-Nb 2 O 5 bulk rather than the surface or near-surface of the electrode, resulting high specific capacitances and energy density. [20] These interesting results inspire us to consider whether Nb atoms could be intercalated into the layered polyanionic phosphate to form layered niobium phosphate as high-performance electrode materials for WSSPs.

N/O co-enriched graphene hydrogels as high-performance electrodes for aqueous symmetric supercapacitors

“…[5][6][7] Supercapacitors have proved to be the hot spot of research in the eld of energy storage devices due to their characteristics of high current charge and discharge, exceptional power density and long service life. [8][9][10] The electrochemical properties of supercapacitors are primarily determined by their electrode materials. Carbon materials have always been the most widely used electrode materials for supercapacitors due to their high specic surface area and excellent physicochemical stability.…”

“… 5–7 Supercapacitors have proved to be the hot spot of research in the field of energy storage devices due to their characteristics of high current charge and discharge, exceptional power density and long service life. 8–10 …”

Unveiling the Interlayer Spacing Dependence of Zn-Ion Storage Performance in Layered Vanadium Phosphates

“…[7][8][9] Remarkably, taking advantage of the enhanced ionicity of V-O bonds with the existence of PO 4 3-, layered VOPO 4 • 2H 2 O exhibites a much higher discharge plateau (1.1 -1.2 V) than other V-based cathode materials. [10][11][12] It should be emphasized that energy density is quite dependent on the discharge plateau of cathode, and such a merit of layered VOPO 4 • 2H 2 O is highly desirable to achieve high energy density of ZIBs. 10 Unfortunately, the specific capacity, rate capability of VOPO 4 with a rather poor cyclstability.…”

“…[10][11][12] It should be emphasized that energy density is quite dependent on the discharge plateau of cathode, and such a merit of layered VOPO 4 • 2H 2 O is highly desirable to achieve high energy density of ZIBs. 10 Unfortunately, the specific capacity, rate capability of VOPO 4 with a rather poor cyclstability. Although the dissolution / decomposion issue has been addressed by PO 4 3addition and high salt concentration of the aqueous electrolyte to the shift the decomposion equilibrium and prevent dissolution 9 , the as-reported capacity of VOPO 4 •2H 2 O cathode is still less 4 than170 mAh• g -1 , which is much smaller than its theoretical specific capacity (>300 mAh• g -1 ).…”

“…[10][11][12] It should be emphasized that energy density is quite dependent on the discharge plateau of cathode, and such a merit of layered VOPO 4 • 2H 2 O is highly desirable to achieve high energy density of ZIBs. 10 Recent progress on lithium-/ sodium-/ magnesium-ion batteries unveils the interlayer spacing of layered materials as natural two-dimensional (2D) ion transport / diffusion channels. [13][14][15][16] In principle, increasing the interlayer spacing of layered structure facilitates the ion transport by creating a lower energy barrier.…”