Vying for newer sodium-ion chemistry for rechargeable batteries, Na2FeP2O7 pyrophosphate has been recently unveiled as a 3 V high-rate cathode. In addition to its low cost and promising electrochemical performance, here we demonstrate Na2FeP2O7 as a safe cathode with high thermal stability. Chemical/electrochemical desodiation of this insertion compound has led to the discovery of a new polymorph of NaFeP2O7. High-temperature analyses of the desodiated state NaFeP2O7 show an irreversible phase transition from triclinic (P1̅) to the ground state monoclinic (P21/c) polymorph above 560 °C. It demonstrates high thermal stability, with no thermal decomposition and/or oxygen evolution until 600 °C, the upper limit of the present investigation. This high operational stability is rooted in the stable pyrophosphate (P2O7)4– anion, which offers better safety than other phosphate-based cathodes. It establishes Na2FeP2O7 as a safe cathode candidate for large-scale economic sodium-ion battery applications.

Rechargeable aqueous Zn/MnO2 batteries are very attractive large‐scale energy storage technologies, but still suffer from limited cycle life and low capacity. Here the novel adoption of a near‐neutral acetate‐based electrolyte (pH ≈ 6) is presented to promote the two‐electron Mn4+/Mn2+ redox reaction and simultaneously enable a stable Zn anode. The acetate anion triggers a highly reversible MnO2/Mn2+ reaction, which ensures high capacity and avoids the issue of structural collapse of MnO2. Meanwhile, the anode‐friendly electrolyte enables a dendrite‐free Zn anode with outstanding stability and high plating/stripping Coulombic efficiency (99.8%). Hence, a high capacity of 556 mA h g−1, a lifetime of 4000 cycles without decay, and excellent rate capability up to 70 mA cm−2 are demonstated in this new near‐neutral aqueous Zn/MnO2 battery by simply manipulating the salt anion in the electrolyte. The acetate anion not only modifies the surface properties of MnO2 cathode but also creates a highly compatible environment for the Zn anode. This work provides a new opportunity for developing high‐performance Zn/MnO2 and other aqueous batteries based on the salt anion chemistry.

The crystallographic and magnetic phase diagram of the nϭ2 layered manganite La 2Ϫ2x Sr 1ϩ2x Mn 2 O 7 in the region xу0.5 has been studied using temperature-dependent neutron powder diffraction. The magnetic phase diagram reveals a progression of ordered magnetic structures generally paralleling that of three-dimensional ͑3D͒ perovskites with similar electronic doping: A (0.5рxр0.66)→C (0.75рxр0.90)→G (0.90рxр1.0). However, the quasi-2D structure amplifies this progression to expose features of manganite physics uniquely accessible in the layered systems: ͑i͒ a ''frustrated'' region between the A and C regimes where no long-range magnetic order is observed; ͑ii͒ magnetic polytypism arising from weak interbilayer magnetic exchange in the type-C regime; and ͑iii͒ a tetragonal-to-orthorhombic phase transition whose temperature evolution directly measures ordering of d 3y 2 Ϫr 2 orbitals in the a-b plane. This orbital-ordering transition is precursory to type-C magnetic ordering, where ferromagnetic rods lie parallel to the b axis. These observations support the notion that e g orbital polarization is the driving force behind magnetic spin ordering. Finally, in the crossover region between type-C and type-G states, we see some evidence for the development of local type-C clusters embedded in a type-G framework, directly addressing proposals of similar short-range magnetic ordering in highly doped La 1Ϫx Ca x MnO 3 perovskites.

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The magnetic structure and properties of polycrystalline NaFePO4 polymorphs, maricite and triphylite, both derived from the olivine structure type, have been investigated using magnetic susceptibility, heat capacity, and low-temperature neutron powder diffraction. These NaFePO4 polymorphs assume orthorhombic frameworks (space group No. 62, Pnma), built from FeO6 octahedral and PO4 tetrahedral units having corner-sharing and edge-sharing arrangements. Both polymorphs demonstrate antiferromagnetic ordering below 13 K for maricite and 50 K for triphylite. The magnetic structure and properties are discussed considering super- and supersuperexchange interactions in comparison to those of triphylite-LiFePO4.