Improved Electrochemical Performance of NaVOPO₄Positive Electrodes at Elevated Temperature in an Ionic Liquid Electrolyte

Abstract

Sodium secondary batteries operating in a wide temperature range are attractive as large-scale energy storage devices, and ionic liquid electrolytes are suitable for this purpose. In this study, NaVOPO 4 has been investigated as positive electrode material for Na secondary batteries, and its electrochemical performance has been examined in the Na[FSA]-[C 3 C 1 pyrr] [FSA] ionic liquid (C 3 C 1 pyrr = N-methyl-N-propylpyrrolidinium and FSA = bis(fluorosulfonyl)amide) at 298 and 363 K. The NaVOPO 4 electrode exhibits a reversible capacity of 60 and 101 mAh g -1 at 298 and 363 K, respectively. Acceptably good rate capability is achieved at 363 K, as 76% of the maximum capacity is maintained at 5 C rate. Cyclability tests prove good reversibility of the material, in which 74% of the initial specific capacity maintains over 300 cycles at 363 K. XRD measurements reveal that the charge-discharge process of NaVOPO 4 involves a single-phase reaction. Galvanostatic intermittent titration technique (GITT) analysis highlights a 3-5-fold increase of the apparent Na chemical diffusion coefficient in NaVOPO 4 upon increasing the temperature from 298 to 363 K, which is reflected in the superior electrochemical performance at 363 K than at 298 K.Sodium secondary batteries have attracted considerable attention as alternatives or complements to the prevailing Li-ion batteries, owing to their virtually unlimited supply, low material cost, and large worldwide availability. 1-3 Research on Na secondary batteries has continued to gain momentum from the systematic extrapolation of the well-established knowledge regarding lithium ion batteries. The larger ionic radii of Na + (1.02 Å) than Li + (0.76 Å) 3 was formerly deemed to frustrate the reversible insertion and fast transport of sodium within rigid inorganic hosts. 4 However, a recent computational study suggested that the diffusion of Na can be faster than that of Li in certain crystal structures. 5 The milder Lewis acidity of Na + than Li + has also been shown to lead to a commonly smaller desolvation energy in polar solvents. 6-8 Since the desolvation process of alkali ions highly influences their kinetics of insertion at the electrolyte interface, 9 the relatively low desolvation energy coupled with the facile bulk diffusion may open up new appealing possibilities for high-power Na secondary batteries. 3 One of the main issues for Na secondary battery systems is their inferior energy densities compared to the Li-based ones, 3,5,10 which is caused by (1) the larger mass (22.99 g mol -1 for Na and 6.94 g mol -1 for Li) and (2) the higher redox potential (-2.71 V vs. SHE for Na + /Na and -3.04 V vs. SHE for Li + /Li). 10 In order to counteract these intrinsic limitations, positive electrode materials that enable the realization of high capacity and high operating voltages are essential for Na secondary batteries. In this respect, vanadium-based phosphates are gaining increasing prominence. 10-18 Relatively high operating potentials (vs. Na + /Na) were achieved for Na 3 V 2 (PO 4 ) ...

Advances in sodium secondary batteries utilizing ionic liquid electrolytes

“…Reversible capacities of 60 and 101 mA h g À1 were obtained at 25 and 90 1C, respectively. 303 3.1.2 Transition-metal oxides. LIB usage has been promoted by the intensive utilisation of transition-metal oxides such as Li x MO 2 (M = Co, Ni, Mn).…”

A Fully Sodiated NaVOPO4 with Layered Structure for High-Voltage and Long-Lifespan Sodium-Ion Batteries

“…In the quest for new cathode materials, a number of V-based phosphates, such as Na 3 V 2 (PO 4 ) 3 , 27,28 NaVOPO 4 , 29,30 and Na 3 V 2 (PO 4 ) 2 O x F 3À2x , [31][32][33] have attracted considerable attention because of their high redox potentials and stable reversible capacities. However, almost all the V-based phosphates and fluorophosphates have tunnel structures in which Na ions are zigzagged to transport through the lattice frameworks, leading to slow kinetics.…”

CuP2/C Composite Negative Electrodes for Sodium Secondary Batteries Operating at Room‐to‐Intermediate Temperatures Utilizing Ionic Liquid Electrolyte

“…Activation of maricite‐type NaFePO 4 by intermediate‐temperature operation was also a typical example, where IL drew intrinsic performance of an electrode material by fully utilizing thermal and electrochemical stability of IL electrolytes . Performance of Na secondary batteries is enhanced by operation at intermediate temperatures when using IL electrolytes, and their application may target the use of waste heat from devices such as electric vehicles and stationary power sources ,. High performance from phosphorous‐based materials was achieved in recent works using the Na[FSA]‐[C 3 C 1 pyrr][FSA] (C 3 C 1 pyrr= N ‐methyl‐ N ‐propylpyrrolidinium cation) IL system (Sn 4 P 3 and red phosphorus).…”

“…[25] Performance of Na secondary batteries is enhanced by operation at intermediate temperatures when using IL electrolytes, and their application may target the use of waste heat from devices such as electric vehicles and stationary power sources. [26,27] High performance from phosphorous-based materials was achieved in recent works using the Na[FSA]-[C 3 C 1 pyrr] 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 [FSA] (C 3 C 1 pyrr = N-methyl-N-propylpyrrolidinium cation) IL system [21] (Sn 4 P 3 [28] and red phosphorus [29] ). In this work, we report the electrochemical performance of a CuP 2 /C negative electrode in sodium secondary batteries with an ionic liquid electrolyte at room to intermediate temperatures.…”