Li-ion batteries have no doubt seen unprecedented commercial success over the last 3 decades. But, the ever-growing consumption of lithium resources for Li-ion batteries has led to concern over their paucity and high cost. This has led to extensive search for alternative battery chemistries with stress on elemental abundance, economy, non-toxicity and uniform geographical distribution of alkali resources. In this ‘’beyond Li-ion scenario’, sodium-ion batteries have emerged as a winner with suites of oxide and polyanionic materials reported with robust electrochemical performance. Moving beyond sodium-ion batteries, divalent Mg-ion, Ca-ion and Zn-ion batteries are being increasingly pursued as well as monovalent K-ion batteries. Several K-based insertion systems have been reported albeit with moderate electrochemical performance.

In our quest for K-ion batteries, one approach to design K-ion based insertion system is to employ already known Na-based insertion compounds as starting materials. Their desodiated derivatives have large voids, which can reversibly host K-ion. Using this route, we have employed Na_xCoO₂ layered oxide as a starting compound to reversibly intercalate K⁺ ion. As per our recent report (Chem. Commun. 53, 8588, 2017), reversible electrochemical potassium-ion intercalation in P2-type Na_xCoO₂ was observed for the first time. Hexagonal Na_0.84CoO₂ platelets prepared by combustion synthesis were found to work as an efficient host for K⁺ intercalation. They deliver a high reversible capacity of 82 mA h g^-1, good rate capability and excellent cycling performance. This performance is better than K-based metal oxides e.g. K_0.44CoO₂. We will describe the electrochemical performance of few sodium metal oxides (Na_xCoO₂, Na_xMn_0.5Co_0.5O₂) as cathode for K-ion intercalation.

Following the oxide systems, we have also extended our effort to sodium-based polyanionic compounds. First, we have employed a 3 V Na₂FePO₄F fluorophosphate insertion system. Upon first desodiation, the resulting NaFePO₄F composition worked as a 2.8 V insertion host for reversible K⁺ insertion. Involving a two-step flat voltage profile, it delivered a reversible capacity exceeding 80 mA h.g^-1. Further, we have investigated the mixed polyanionic Na₄Fe₃(PO₄)₂P₂O₇ system for possible K-insertion. Combustion synthesis prepared Na₄Fe₃(PO₄)₂P₂O₇ was employed in K-half cell architecture. Upon electrochemical cycling, three Na⁺ were replaced by K⁺ to form NaK₃Fe₃(PO₄)₂P₂O₇, which works as a 3 V cathode for K-ion batteries. As shown in Figure 1, a step-wise voltage profiles were observed with as many as six cathodic/ anodic plateaus. It signals at gradual structural ordering during (de)potassiation. It delivers a reversible capacity of ~100 mA h.g^-1 with excellent cycling stability. Using experimental and computational analyses, we will describe the structural, diffusional and electrochemical properties of these Na-containing PO₄-based polyanionic hosts for efficient K-insertion.

Finally, we will demonstrate the fabrication of all-solid-state K-ion micro-batteries using 100-300 nm thin-films grown by pulsed laser deposition (PLD) method (as shown in Fig. 1). Various oxide and polyanionic systems were used as target to assemble thin-film micro-batteries and their electrochemical performance was tested in K-half cell architecture. The PLD growth and resulting performance of K-ion thin film micro-batteries will be shown.

Figure 1: (Left, center) Electrochemical performance of Na₄Fe₃(PO₄)₂P₂O₇ insertion host in potassium half-cell assembly. (Right)Instrumentation for pulsed laser deposition (PLD) used to grown thin film micro-batteries.