Followed by decades of successful efforts in developing cathode materials for high specific capacity lithium-ion batteries, currently the attention is on developing a high-voltage battery (>5 V vs Li/Li+) with an aim to increase the energy density for their many fold advantages over conventional <4 V batteries. Among the various cathode materials, phosphate polyanion materials (LiMPO4, where M is a single metal or a combination of metals) showed promising candidacy given their high electrochemical potential (4.8–5 V vs Li/Li+), long cycle stability, low cost, and achieved specific capacity (∼165 mAh·g–1) near to its theoretical limit (170 mAh·g–1). In this review, factors affecting the electrochemical potential of the cathode materials are reviewed and discussed. Techniques to improve the electrical and ionic conductivities of phosphate polyanion cathodes, namely, surface coating, particle size reduction, doping, and morphology engineering, are also discussed. A processing–property correlation in phosphate polyanion materials is also undertaken to understand relative merits and drawbacks of diverse processing techniques to deliver a material with targeted functionality. Strategies required for high-voltage phosphate polyanion cathode materials are envisioned, which are expected to deliver lithium-ion battery cathodes with higher working potential and gravimetric specific capacity.

Discovering revolutionary cathode materials for lithium ion batteries (LIBs) with a high power/energy density to confront the existing energy, economic, and ecologic crises is one of the persistent challenges in materials research. [1] The most promising olivine phosphate LiMPO 4 electrode materials (M = Fe, Mn, or Co) [2] have already attained maximum capacity through the development of new synthetic methods, such as downsizing, [3] conductive wiring, [3b, 4] and doping. [5] Recently, hierarchical olivine-based materials produced by soft synthetic strategies have been shown to have higher bulk density, superior performance at high rates, and better cycling stability. [6] Besides commercialization of olivine phosphates, current research is also focused on enhancing the performance of LIBs for high-current applications by developing new electrode materials. Many alternative candidates, such as Li 3 M 2 (PO 4 ) 3 , [7] Li 2 MSiO 4 , [8] LiMSO 4 F, [9] LiMBO 3 [1d, 10] (M = Fe, V, Mn, Co, etc.) LiVOPO 4 , [11] LiVPO 4 F, [12] and Li 2 FeP 2 O 7 [13] have been pursued as cathode materials. Further to these inorganic materials, new organic [14] and metal-organic framework [15] (MOF) materials have also shown promise as electrode materials for LIBs. The major challenges ahead in the utilization of MOFs as electrode materials for LIBs are the high molecular weight, low density, difficulty of synthesizing large quantities, and challenging isolation of robust MOFs with redox-active metal centers, compared to inorganic materials. [15a,c] These drawbacks may delay their utilization and commercialization as cathode materials.For this reason, it is worthwhile to investigate energy storage in an entirely new class of phosphate-based hybrid materials. [16] Metal organic-phosphate open frameworks (MOPOF) are hybrid materials with multidimensional architectures constructed from transition-metal phosphates crosslinked by simple organic linkers, which in turn can encapsulate a diverse range of alkali ions (Li + , Na + , and K + ) between the layers.Though MOPOFs are well known for their structural diversity and for conventional industrial applications, such as redox reactions, catalysis, gas storage, and MRI contrast agents, these materials have not been investigated as energy storage materials. [17] The presence of mixed organic oxalate and inorganic phosphate anions is expected to enhance the redox properties of the transition-metal ions, provide robustness to the material, and reduce the synthesis temperature required, along with the versatility offered by the organic ligands. Furthermore, the synthetic simplicity of MOPOFs with redox-active metal centers and the possible two-dimensional migration pathways for M + ions (deduced from careful analysis of the packing patterns of the reported crystal structures) motivated us to investigate these MOPOFs as cathode materials for LIBs. Herein, we disclose for the first time the lithium storage performance of MOPOFs K 2.5 [(VO) 2 (HPO 4 ) 1.5 (PO 4 ) 0.5 (C 2 O 4 )], 1, Na 2 [(VO...

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