Manifold consumption of lithium resources for Li-ion batteries has led to concern over their paucity and high cost. It has triggered global research on alternative battery chemistries with stress on elemental abundance, economy, non-toxicity and uniform geographical distribution of alkali resources. It has paved way for sodium-ion batteries, where driven by economy and sustainability, suites of oxide and polyanionic materials have been reported as efficient insertion materials. Moving beyond sodium-ion batteries, divalent alkali-based 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.

One approach to design K-ion based insertion system is to use already existing Na-based insertion compounds as host. Their desodiated derivatives can be effective used for reversible K-ion intercalation. 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 made by solution 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.

Encouraged by the study on oxide, we extended our effort to polyanionic compositions. First, we tested Na₂FePO₄F fluorophosphate originally reported as a 3 V cathode for Na-ion batteries. 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. Following, we studied the complex mixed polyanionic Na₄Fe₃(PO₄)₂P₂O₇ system for possible K-insertion. This compound is reported as a 3.1 V sodium battery material. 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 yields a reversible capacity of ~100 mA h.g^-1 with excellent cycling stability. Using suites of characterization techniques, we will present the details of material synthesis, structural and electrochemical analyses of these polyanionic hosts for K-insertion.

Eventually, we will demonstrate the fabrication of all-solid-state K-ion micro-batteries using thin-films grown on stainless steel substrates. After optimizing several parameters, pulsed laser deposition (PLD) method was used to grown 100-300 nm thin films of oxides and polyanionic systems (as shown in Fig. 1). They were used 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 batteries will be demonstrated.

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.