The safety, affordability, and impressive electrochemical performance of many Zn-ion batteries (ZIBs) has recently triggered an overwhelming literature surge. As is typical for a new area, initial enthusiasm and high expectations have now been replaced by a more measured period of research that reaches deep into the underlying factors controlling electrochemical properties. Rather than battery metrics, this review focuses on fundamental aspects of the chemistry of ZIBs that are the least understood and on which there has been progress over the last few years. We provide guidance for future research regarding (1) the significant challenge of proton/Zn 2+ co-intercalation in aqueous media, (2) limitations to conversion chemistry that often accompanies ZIB electrochemistry, (3) positive aspects of facile Zn 2+ (de)intercalation in nonaqueous electrolytes and organic cathode materials, (4) the desolvation penalty at electrode-electrolyte interfaces, (5) solutions for controlling Zn dendritic growth, and (6) suggested electrochemistry protocols for the field.

The provision of efficient electron and ion transport is a critical issue in an exciting new group of materials based on lithium metal phosphates that are important as cathodes for lithium-ion batteries. Much interest centres on olivine-type LiFePO(4), the most prominent member of this family. Whereas the one-dimensional lithium-ion mobility in this framework is high, the electronically insulating phosphate groups that benefit the voltage also isolate the redox centres within the lattice. The pristine compound is a very poor conductor (sigma approximately 10(-9) S cm(-1)), thus limiting its electrochemical response. One approach to overcome this is to include conductive phases, increasing its capacity to near-theoretical values. There have also been attempts to alter the inherent conductivity of the lattice by doping it with a supervalent ion. Compositions were reported to be black p-type semiconductors with conductivities of approximately 10(-2) S cm(-1) arising from minority Fe(3+) hole carriers. Our results for doped (and undoped) LiMPO(4) (M = Fe, Ni) show that a percolating nano-network of metal-rich phosphides are responsible for the enhanced conductivity. We believe our demonstration of non-carbonaceous-network grain-boundary conduction to be the first in these materials, and that it holds promise for other insulating phosphates.

Impressive capacity improvements can be obtained by wrapping insulating crystallites of Li3V2(PO4)3 within a conductive carbon web. The single crystal analysis (see Figure) and electrochemical characteristics of Li3V2(PO4)3 are reported. X‐ray diffraction analysis of the single phases formed on Li extraction shows that the framework is maintained with a little loss of crystallinity; on re‐insertion of Li, the Li3.0V2(PO4)3 framework is fully recrystallized.

Na super ion conductor (NaSICON), Na 1+n Zr 2 Si n P 3-n O 12 is considered one of the most promising solid electrolytes; however, the underlying mechanism governing ion transport is still not fully understood. Here, the existence of a previously unreported Na5 site in monoclinic Na 3 Zr 2 Si 2 PO 12 is unveiled. It is revealed that Na + -ions tend to migrate in a correlated mechanism, as suggested by a much lower energy barrier compared to the single-ion migration barrier. Furthermore, computational work uncovers the origin of the improved conductivity in the NaSICON structure, that is, the enhanced correlated migration induced by increasing the Na + -ion concentration. Systematic impedance studies on doped NaSICON materials bolster this finding. Significant improvements in both the bulk and total ion conductivity (e.g., σ bulk = 4.0 mS cm −1 , σ total = 2.4 mS cm −1 at 25 °C) are achieved by increasing the Na content from 3.0 to 3.30-3.55 mol formula unit −1 . These improvements stem from the enhanced correlated migration invoked by the increased Coulombic repulsions when more Na + -ions populate the structure rather than solely from the increased mobile ion carrier concentration. The studies also verify a strategy to enhance ion conductivity, namely, pushing the cations into high energy sites to therefore lower the energy barrier for cation migration.

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