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We report the synthesis, crystal structure, thermal response, and electrochemical behavior of the Prussian blue analogue (PBA) K2Cu[Fe(CN)6]. From a structural perspective, this is the most complex PBA yet characterized: its triclinic crystal structure results from an interplay of cooperative Jahn–Teller order, octahedral tilts, and a collective “slide” distortion involving K-ion displacements. These different distortions give rise to two crystallographically distinct K-ion channels with different mobilities. Variable-temperature X-ray powder diffraction measurements show that K-ion slides are the lowest-energy distortion mechanism at play, as they are the only distortion to be switched off with increasing temperature. Electrochemically, the material operates as a K-ion cathode with a high operating voltage and an improved initial capacity relative to higher-vacancy PBA alternatives. On charging, K+ions are selectively removed from a single K-ion channel type, and the slide distortions are again switched on and off accordingly. We discuss the functional importance of various aspects of structural complexity in this system, placing our discussion in the context of other related PBAs.

Layered sodium metal oxide Na2Mn3O7 and mixed polyanion Na4Fe3(PO4)2P2O7 cathode materials were prepared by facile solid-state route and solution combustion synthesis respectively. The Na2Mn3O7 compound crystallized in triclinic structure with space group P-1, whereas Na4Fe3(PO4)2P2O7 assumed an orthorhombic structure with space group Pn21a. Here, both these sodium-based materials are presented as intercalation hosts for K-ion batteries for the first time. An initial discharge capacity of 152 mAh g-1 was realized with a nominal voltage of 2.1 V vs K/K+ for Na2Mn3O7. The reversible K-ion (de) intercalation in Na4Fe3(PO4)2P2O7 was observed with high capacity of 116 mAh g-1 with average potential of 3.2 V vs K/K+ with fast rate capability and good cycling stability. These sodium-based materials act as robust insertion hosts for secondary potassium-ion batteries.

Electrochemical intercalation of sodium and potassium-ion in NaxCoO2 and NaxFeO2 was successfully studied in this work. The Na0.84CoO2 and NaxFeO2 were synthesized by solution combustion synthesis technique using urea as fuel. The as-prepared Na0.84CoO2 assumed a hexagonal structure with P63/mmc space group built from slabs of edge-sharing CoO6 octahedra accommodating Na in prismatic sites. Hexagonal platelet-like morphology was observed by SEM and TEM micrography. Firstly, the Na-ion intercalation in P2-type Na0.84CoO2 layered material was examined. The P2-type Na0.84CoO2 exhibited a reversible capacity of 148 mA h g-1 in Na-ion cell. Following, the K-ion (de)intercalation behavior of the material was investigated for the first time. The reversible K-ion (de)intercalation was observed with the high capacity of 71 mA h g-1 at C/10 current rate. Na0.84CoO2 cathode act as a suitable host for K+ intercalation delivering robust cycling, rate capability and Coulombic efficiency. In contrast, NaxFeO2 was found to be electrochemically inactive for K-ion insertion.

Economic and sustainable (ecological) energy storage forms a major pillar of the global energy sector. Bifunctional electrocatalysts, based on oxygen electrolysis, play a key role in the development of rechargeable metal–air batteries. Pursuing precious metal-free economic catalysts, here, we report K2CoP2O7pyrophosphate as a robust cathode for secondary zinc–air batteries with efficient oxygen evolution and oxygen reduction (OER||ORR) activity. Prepared by autocombustion, nanoscale K2CoP2O7exhibited excellent oxygen reduction and evolution reactions among all phosphate-based electrocatalysts. In particular, the OER activity surpassed that of commercial RuO2with low overpotential (0.27 V). First-principles calculations revealed that the bifunctional activity is rooted in the Co active site with the CoO5local coordination in the most stable (110) surface. This nanostructured (tetragonal) pyrophosphate can be harnessed as an economic bifunctional catalyst for zinc–air batteries.

The transition from fossil fuels to carbon-free forms of renewable energy has become a spotlight with the revolutionary emergence of efficient electrochemical energy storage systems. It enables us to realize electric mobility empowered by Li-ion battery technology. Nevertheless, for the past three decades, the development of battery technology has been very sluggish, and it warrants new strategies to meet the growing demand for high energy density. In this spirit, we are working to develop versatile battery cathodes, which can be used for electrochemical and electrocatalytic applications.

Electrochemical intercalation of sodium and potassium-ion in NaxCoO2and NaxFeO2 was successfully studied in this work. The Na0.84CoO2and NaxFeO2 were synthesized by solution combustion synthesis technique using urea as fuel. The as-prepared Na0.84CoO2assumed a hexagonal structure with P63/mmcspace group built from slabs of edge-sharing CoO6octahedra accommodating Na in prismatic sites. Hexagonal platelet-like morphology was observed by SEM and TEM micrography. Firstly, the Na-ion intercalation in P2-type Na0.84CoO2layered material was examined. The P2-type Na0.84CoO2exhibited a reversible capacity of 148 mA h g-1in Na-ion cell. Following, the K-ion (de)intercalation behavior of the material was investigated for the first time. The reversible K-ion (de)intercalation was observed with the high capacity of 71 mA h g-1at C/10 current rate. Na0.84CoO2cathode act as a suitable host for K+intercalation delivering robust cycling, rate capability and Coulombic efficiency. In contrast, NaxFeO2was found to be electrochemically inactive for K-ion insertion.