An Overview of Mixed Polyanionic Cathode Materials for Sodium‐Ion Batteries

Abstract

to battery driven (electric) mobility. While a multibillion-dollar industry has come up catering various needs, the manifold consumption of Li-based resources has led to steep price rise and concerns over future geo-political tension due to its nonuniform geographic distribution. To combat this imminent issue, various alternative battery chemistries (termed as "Beyond Li-ion batteries") are widely pursued, particularly to replace Li-ion batteries in applications unrestricted by gravimetric and/or volumetric energy density. In this case, sodium-ion batteries (NIBs) are touted as the emerging futuristic battery chemistry owing to the widespread availability of sodium-based resources and well-understood electrochemical operation involving the Na + charge carriers. [1][2][3][4][5] In the quest to build robust sodiumion batteries, various 2D layered transition metal oxides and 3D polyanionic framework materials have been unraveled. While the oxide cathodes can deliver high theoretical (and reversible) capacity, they suffer from lower redox potential owing to strong covalence nature. [6] It is more so acute in case of Na-ion batteries considering the higher potential of Na/Na + (−2.71 V vs normal hydrogen electrode (NHE)) vis-à-vis Li/Li + (−3.03 V vs NHE). This issue can be circumvented by implementing polyanionic cathode materials with tunable crystal structure, robust framework providing safe operation and higher redox potential due to inductive effect. [7,8] Following the inductive effect principle, plethora of cathodes with polyanionic units [(XO 4 ) m n− : X = B, P, Si, S, W, Mo, As, Ti, V, etc.] have been discovered both for Li-ion and Na-ion batteries. [9,10] Moving a step further with polyanionic chemistry, off late, materials discovery has been realized by using different types of polyanion units. These subclass of materials are known as "mixed polyanionic" cathode materials, which can be designed by simultaneously having i) polyanionic units [(XO 4 ) m n− , X = P, S, V, etc.] along with other single anions [Y − = F − /OH − /O 2− /N 3− ], ii) different structural units of same polyanion units [(XO m ) (X 2 O m ), e.g., PO 4 -P 2 O 7 , BO 3 -B 2 O 5 , SO 4 -S 2 O 3 , etc.], and iii) two different oxyanionic [(XO 4 ) m n− ] units (e.g., PO 4 -SO 4 , PO 4 -NO 3 , PO 4 -CO 3 , etc.) as illustrated in Figure 1. These polyanionic combinations can lead to rich structural diversity and multi ple electron redox activity leading to robust electrochemical "Building better batteries" remains an ongoing process to cater diverse energy demands starting from small-scale consumer electronics to large-scale automobiles and grid storage. While Li-ion batteries have carried this burden over the last three decades, the ever-growing and highly diverse applications (based on size, energy-density, and stationary vs mobile usages) have led to an era of "beyond lithium-ion batteries." In this postlithium-battery era, sodium-ion batteries (NIBs) have emerged as a pragmatic option particularly for large-scale applications. They attract...

Advances in sodium secondary batteries utilizing ionic liquid electrolytes

“…87,[275][276][277] Polyanionic compounds usually exhibit high thermal stabilities (owing to the strong X-O covalent bonds) and strong resistance to natural oxidation but low electronic conductivities. 271,[278][279][280] However, poor electronic conductivity results in poor electrochemical performance, but it can be overcome by surface modifications, carbon coating, and elevation of operating temperature using IL electrolytes, as reported in recent studies.…”

A square channel vanadium phosphite framework as a high voltage cathode for Li- and Na-ion batteries

“…Using this strategy, various electroactive compounds have been synthesized using a variety of polyanions, 11 such as, phosphates, [11][12][13][14] pyrophosphates, 15,16 sulfates, 17 borates, 18 silicates, 19,20 as well as a range of mixed polyanions such as carbono-phosphates, 21,22 phosphate-pyrophosphates, 23,24 phosphatonitrate, 25 boro-phosphates 26 and so on. 27 Polyanions in combination with another electronegative element such as fluoride have been successfully used to further boost the insertion voltage through an inductive effect. 28,29 Polyanion-based electrodes are also an excellent host for Na-ion batteries because of the availability of appropriate channel dimension/volume for the size of the Na-ion.…”

Effect of Mixed Li+/Na+-ion Electrolyte on Electrochemical Performance of Na4Fe3(PO4)2P2O7 in Hybrid Batteries

“…However, a number of sodium intercalation electrodes is much less due to a higher ionic radius of the Na + ions. Special attention is paid to the polyanionic-type cathode materials because of good thermal stability due to strong covalent bonds in polyanions, which imply improved safety characteristics of the batteries [2]. Despite a larger ionic radius of the Na + ions (1.02 Å) compared to the Li + ions (0.76 Å), fast kinetics with Na + as a charge carrier could nevertheless be expected, since (i) sodium forms longer Na-O bonds, providing more open framework structure; (ii) it is characterized by less polarizing ability than lithium; (iii) weaker Lewis acidity of the Na + cations reduces solvation energy, hence facilitating their transfer through the electrode/electrolyte interface [3].…”