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.

All-solid-state rechargeable lithium-ion batteries (AS-LIBs) are attractive power sources for electrochemical applications due to their potentiality in improving safety and stability over conventional batteries with liquid electrolytes. Finding a solid electrolyte with high ionic conductivity and compatibility with other battery components is a key factor in raising the performance of AS-LIBs. In this work, we prepare argyrodite-type Li 6 PS 5 X (X = Cl, Br, I) using mechanical milling followed by annealing. X-ray diffraction characterization reveals the formation and growth of crystalline Li 6 PS 5 X in all cases. Ionic conductivity of the order of 7×10 −4 S cm −1 in Li 6 PS 5 Cl and Li 6 PS 5 Br renders these phases suitable for AS-LIBs. Joint structure refinements using high-resolution neutron and laboratory X-ray diffraction provide insight into the influence of disorder on the fast ionic conductivity. Besides the disorder in the lithium distribution, it is the disorder in the S 2− /Cl − or S 2− /Br − distribution that we find to promote ion mobility, whereas the large I − cannot be exchanged for S 2− and the resulting more ordered Li 6 PS 5 I exhibits only a moderate conductivity. Li + ion migration pathways in the crystalline compounds are modelled using the bond valence approach to interpret the differences between argyrodites containing different halide ions.

High-performance Mn-rich P2-phase Na2/3Mn0.8Fe0.1Ti0.1O2 is synthesized by a ceramic method, and its stable electrochemical performance is demonstrated. 23Na solid-state NMR confirms the substitution of Ti4+ ions in the transition metal oxide layer and very fast Na+ mobility in the interlayer space. The pristine electrode delivers a second charge/discharge capacity of 146.57/144.16 mA·h·g–1 and retains 95.09% of discharge capacity at the 50th cycle within the voltage range 4.0–2.0 V at C/10. At 1C, the reversible specific capacity still reaches 99.40 mA·h·g–1, and capacity retention of 87.70% is achieved from second to 300th cycle. In addition, the moisture-exposed electrode reaches reversible capacities of more than 130 and 80 mA·h·g–1 for C/10 and 1C, respectively, with excellent capacity retention. The correlation between overall electrochemical performance of both electrodes and crystal structural characteristics are investigated by neutron powder diffraction. The stability of pristine electrode’s crystallographic structure during the charge/discharge process has been investigated by in situ X-ray diffraction, where only a solid solution reaction occurs within the given voltage range except for a small biphasic mechanism occurring at or below 2.2 V during the discharge process. The relatively small substitution (20%) at the transition metal site leads to stable electrochemical performance, which is in part derived from the structural stability during electrochemical cycling. Therefore, the small cosubstitution (e.g., with Ti and Fe) route suggests a possible new scope for the design of sodium-ion battery electrodes that are suitable for long-term cycling.

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Lithium-ion batteries power many portable devices and in the future are likely to play a significant role in sustainable-energy systems for transportation and the electrical grid. LiFePO(4) is a candidate cathode material for second-generation lithium-ion batteries, bringing a high rate capability to this technology. LiFePO(4) functions as a cathode where delithiation occurs via either a solid-solution or a two-phase mechanism, the pathway taken being influenced by sample preparation and electrochemical conditions. The details of the delithiation pathway and the relationship between the two-phase and solid-solution reactions remain controversial. Here we report, using real-time in situ neutron powder diffraction, the simultaneous occurrence of solid-solution and two-phase reactions after deep discharge in nonequilibrium conditions. This work is an example of the experimental investigation of nonequilibrium states in a commercially available LiFePO(4) cathode and reveals the concurrent occurrence of and transition between the solid-solution and two-phase reactions.

Sodium-ion batteries are the next-generation in battery technology; however, their commercial development is hampered by electrode performance. The P2-type Na 2/3 (Fe 1/2 Mn 1/2 )O 2 with a hexagonal structure and P6 3 /mmc space group is considered a candidate sodium-ion battery cathode material due to its high capacity (~ 190 mAh.g -1 ) and energy density (~ 520 mWh.g -1 ), which are comparable to the commercial LiFePO 4 and LiMn 2 O 4 lithium-ion battery cathodes, with previously-unexplained poor cycling performance being the major barrier to its commercial application. We use operando synchrotron X-ray powder diffraction to understand the origins of the capacity fade of the Na 2/3 (Fe 1/2 Mn 1/2 )O 2 material during cycling over the relatively-wide 1.5 -4.2 V (vs. Na) window. We found a complex phase-evolution, involving transitions from P6 3 /mmc (P2-type at the open-circuit voltage) -P6 3 (OP4-type when fully-charged) -P6 3 /mmc (P2-type at 3.4 -2.0 V) -Cmcm (P2-type at 2.0 -1.5 V) symmetry structures during the desodiation and sodiation of the Na 2/3 (Fe 1/2 Mn 1/2 )O 2 cathode. The associated large cell-volume changes with the multiple two-phase reactions are likely to be responsible for the poor cycling performance, clearly suggesting to a 2.0 -4.0 V window of operation as a strategy to improve cycling performance. We demonstrated here that the P2-type Na 2/3 (Fe 1/2 Mn 1/2 )O 2 cathode is able to deliver ~25% better cycling performance with the strategic operation window. This significant improvement in cycling performance implies that by characterizing the phase evolution and reaction mechanisms during battery function we are able to propose these modifications to the conditions of battery use that improve performance, highlighting the importance of the interplay between structure and electrochemistry.

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A detailed investigation on the effects of Mg substitution (0 ≤ x ≤ 0.2) in high voltage P2-Na2/3Ni1/3−xMgxMn2/3O2 cathode materials for Na-ion batteries.