The status of room‐temperature potassium‐ion batteries is reviewed in light of recent concerns regarding the rising cost of lithium and the fact that room‐temperature sodium‐ion batteries have yet to be commercialised thus far. Initial reports of potassium‐ion cells appear promising given the infancy of the research area. This review presents not only an overview of the current potassium‐ion battery literature, but also attempts to provide context by describing previous developments in lithium‐ion and sodium‐ion batteries and the electrochemical reaction mechanisms discovered thus far. Perspectives and directions on the techniques available to characterize newly developed battery materials are also provided based on our experience and knowledge from the literature. It is hoped that through this review, the potential of potassium‐ion batteries as a competitive energy‐storage technology will be realised, and the accessibility and available knowledge of the techniques required to develop the technology will be made apparent.

Sodium-ion batteries are considered as a favorable alternative to the widely used lithium-ion batteries for applications such as grid-scale energy storage. However, to meet the energy density and reliability that is necessary, electrodes that are structurally stable and well characterized during electrochemical cycling need to be developed. Here, we report on how the applied discharge current rate influences the structural evolution of Na 0.67 Mn 0.8 Mg 0.2 O 2 electrode materials. A combination of ex situ and in situ X-ray diffraction (XRD) data were used to probe the structural transitions at the discharged state and during charge/discharge. Ex situ data shows a two-phase electrode at the discharged state comprised of phases that adopt Cmcm and P6 3 /mmc symmetries at the 100 mA/g rate but a predominantly P6 3 /mmc electrode at 200 and 400 mA/g rates. In situ synchrotron XRD data at 100 mA/g shows a solely P6 3 /mmc electrode when 12 mA/g charge and 100 mA/g discharge is used even though ex situ XRD data shows the presence of both Cmcm and P6 3 / mmc phases. The in situ data allows the Na site occupancy evolution to be determined as well as the rate of lattice expansion and contraction. Electrochemically, lower applied discharge currents, e.g., 100 mA/g, produce better capacity than higher applied currents, e.g., 400 mA/g, and this is related in part to the quantity of the Cmcm phase that is formed near the discharged state during a two-phase reaction (via ex situ measurements), with lower rates producing more of this Cmcm phase. Thus, producing more Cmcm phase allows access to higher capacities while higher rates show a lower utilization of the cathode during discharge as less (if any) Cmcm phase is formed. Therefore, this work shows how structural transitions can depend on the electrochemically applied current which has significant ramifications on how sodium-ion batteries, and batteries in general, are analyzed for performance during operation. 50 cathode materials for Na-ion batteries due to their high 51 reversible capacity, and for M = Fe and Mn, cost and safety. 2−4 52 These materials share many common features with their Li-53 counterparts despite the larger size of Na. Sodium layered 54 oxides are typically identified using Delmas' notation 5 P2, P3, 55 O3, etc., where P and O indicate the Na sites (P = trigonal 56 prismatic and O = octahedral) and the number relates to the 57 transition metal layers within the unit cell. From this 58 classification, numerous studies have been reported for P2-59 type Na y MO 2 materials demonstrating their higher capacities, 60 diffusion rate, and better cyclability than that of the O3 61 structure. 6 62 In 1999, Paulsen and Dahn reported an exhaustive study on 63 P2-sodium manganese oxide compounds 7 after reports from

Cathodes that feature a layered structure are attractive reversible sodium hosts for ambient temperature sodium-ion batteries which may meet the demands for large-scale energy storage devices. However, crystallographic data on these electrodes are limited to equilibrium or quasi-equilibrium information.Here we report the current-dependent structural evolution of the P2-Na 2/3 Fe 2/3 Mn 1/3 O 2 electrode during charge/discharge at different current rates. The structural evolution is highly dependent on the current rate used, e.g., there is significant disorder in the layered structure near the charged state at slower rates and following the cessation of high-current rate cycling. At moderate and high rates this disordered structure does not appear. In addition, at the slower rates the disordered structure persists during subsequent discharge. In all rates examined, we show the presence of an additional two-phase region that has not been observed before, where both phases maintain P6 3 /mmc symmetry but with varying sodium contents. Notably, most of the charge at each current rate is transferred via P2 (P6 3 / mmc) phases with varying sodium contents. This illustrates that the high-rate performance of these electrodes is in part due to the preservation of the P2 structure and the disordered phases appear predominantly at lower rates. Such current-dependent structural information is critical to understand how electrodes function in batteries which can be used to develop optimised charge/discharge routines and better materials.

The development of new insertion electrodes in sodium-ion batteries requires an in-depth understanding of the relationship between electrochemical performance and the structural evolution during cycling. To date in situ synchrotron X-ray and neutron diffraction methods appear to be the only probes of in situ electrode evolution at high rates, a critical condition for battery development. Here, the structural evolution of the recently synthesized O3-phase of Na 2/3 Fe 2/3 Mn 1/3 O 2 is reported under relatively high current rates. The evolution of the phases, their lattice parameters, and phase fractions, and the sodium content in the crystal structure as a function of the charge/discharge process are shown. It is found that the O3-phase persists throughout the charge/discharge cycle but undergoes a series of two-phase and solid-solution transitions subtly modifying the sodium content and atomic positions but keeping the overall space-group symmetry (structural motif ). In addition, for the fi rst time, evidence of a structurally characterized region is shown that undergoes two-phase and solid-solution phase transitions simultaneously. The Mn/Fe-O bond lengths, c lattice parameter evolution, and the distance between the Mn/FeO 6 layers are shown to concertedly change in a favorable manner for Na + insertion/extraction. The exceptional electrochemical performance of this electrode can be related in part to the electrode maintaining the O3-phase throughout the charge/discharge process.

Mn-rich layered oxides of P2 Na2/3Mn0.8Fe0.1Ti0.1O2 have been shown to exhibit a remarkably stable electrochemical performance even after exposure to moisture for extended periods of time.

The development of electrodes for ambient temperature sodium-ion batteries requires the study of new materials and the understanding of how crystal structure influences properties. In this study, we investigate where sodium locates in two Prussian blue analogues, Fe[Fe(CN)6]1-x·yH2O and FeCo(CN)6. The evolution of the sodium site occupancies, lattice and volume is shown during charge-discharge using in situ synchrotron X-ray powder diffraction data. Sodium insertion is found to occur in these electrodes during cell construction and therefore Fe[Fe(CN)6]1-x·yH2O and FeCo(CN)6 can be used as positive electrodes. NazFeFe(CN)6 electrodes feature higher reversible capacities relative to NazFeCo(CN)6 electrodes which can be associated with a combination of structural factors, for example, a major sodium-containing phase, ∼Na0.5FeFe(CN)6 with sodium locating either at the x = y = z = 0.25 or x = y = 0.25 and z = 0.227(11) sites and an electrochemically inactive sodium-free Fe[Fe(CN)6]1-x·yH2O phase. This study demonstrates that key questions about electrode performance and attributes in sodium-ion batteries can be addressed using time-resolved in situ synchrotron X-ray diffraction studies.

The development of new insertion electrodes requires an in-depth understanding of the structure–function relationships in order to rationally develop better electrodes. Sodium layered oxides such as P2 Na2/3Fe1/2Mn1/2O2 and Na2/3Fe2/3Mn1/3O2 are particularly interesting due to their performance, price, and low toxicity. Using in situ synchrotron X-ray diffraction during electrochemical cycling of P2 Na2/3Fe0.4Mn0.6O2, changes in the phase composition, lattice parameters, and critically sodium content within the crystal structure are determined. Approaching the charged state, there is an increase in the interlayer distance, brought about via a subtle two-phase region that maintains the structure type as P2. Interestingly, this appears to stabilize the P2 structure in the charged state and inhibits the formation of the highly disordered and typically unfavorable “Z” or OP4 phases up to 4.2 V at 20 mA/g. At the discharged state, at least three phases are present, including P′2 and two subtly different P2 phases. This structural evolution and these parameters are critically assessed in light of data from other compositions in the P2 Na2/3Fe1–y Mn y O2 system. This study represents a method for performance optimization by tuning the Fe:Mn ratio.

The performance of graphene, and a few selected derivatives, was investigated as a negative electrode material in sodium‐ and lithium‐ion batteries. Hydrogenated graphene shows significant improvement in battery performance compared with as‐prepared graphene, with reversible capacities of 488 mA h g−1 for lithium‐ion batteries after 50 cycles and 491 mA h g−1 for sodium‐ion batteries after 20 cycles. Notably, high rates of 1 A g−1 for graphene and 5 A g−1 for hydrogenated graphene indicate higher capacities in sodium‐ion batteries than in lithium‐ion batteries. Alternatively, nickel‐nanoparticle‐decorated graphene performed relatively poorly in lithium‐ion batteries. However, in sodium‐ion batteries they showed the highest reversible capacities of all studied batteries and graphene derivatives, with 826 mA h g−1 after 25 cycles with ≈97 % coulombic efficiency. Overall, minor modifications to graphene can dramatically improve electrochemical performance in both lithium‐ion and sodium‐ion batteries.