James C. Pramudita

A comprehensive picture of the current rate dependence of the structural evolution of P2-Na2/3Fe2/3Mn1/3O2

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-Na2/3Fe2/3Mn1/3O2 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 P63/mmc symmetry but with varying sodium contents. Notably, most of the charge at each current rate is transferred via P2 (P63/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.

Moisture exposed layered oxide electrodes as Na-ion battery cathodes

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. Here, a detailed investigation of the electrochemical performance of pristine, protonated, and hydrated electrodes is reported. Neutron powder diffraction and 23Na NMR are employed in order to correlate the overall electrochemical performance of each electrode with that of the as-synthesized crystal structure. The effects of proton and water (or OH) moieties on the Na+ layers are discussed based on the electrochemical performance of each phase. The complete structural evolution of the protonated and pristine P2 Na2/3Mn0.8Fe0.1Ti0.1O2 electrodes during charge/discharge is determined via in situ synchrotron X-ray diffraction. The protonated phase at the potential cut-offs (1.5–4.2 and 2–4 V) and the applied currents used shows a predominantly solid-solution reaction with little evidence of a secondary phase while the pristine phase shows the formation of secondary phases and typically better electrochemical capacities. Therefore, the formation of the secondary phase, in part, enhances capacity in this system. Thus moisture exposure (and subsequent treatment) of generally P2 electrodes can lead to significantly different structural evolution during charge/discharge reactions and hence observed capacities.

Potassium-ion intercalation in graphite within a potassium-ion battery examined using in situ X-ray diffraction

Graphite has been widely used as a negative electrode material in lithium-ion batteries, and recently it has attracted attention for its use in potassium-ion batteries. In this study, the first in situ X-ray diffraction characterisation of a K/graphite electrochemical cell is performed. Various graphite intercalation compounds are found, including the stage three KC 36 and stage one KC 8 compounds , along with the disappearance of the graphite during the potassiation process. These results show new insights on the non-equilibrium states of potassium-ion intercalation into graphite in K/graphite electrochemical cells.

Decorated and Modified Graphenes as Electrodes in Na and Li-Ion Batteries

Nowadays, rechargeable Li-ion batteries represent the state of the art for the power supply in technological devices. However, the wide-scale implementation of this technology, for example in the automotive field or for large stationary applications, could raise issues, i.e. concerning the limited lithium mineral reserves. The investigation of alternatives to lithium is hence highly desirable, although it requires the identification of new materials suitable as components for new batteries, displaying similar or possibly even better performances with respect to the current systems. Here we show that electrodes based on graphene derivatives are able not only to support the insertion of Li+, but also of Na+ ions, with high capacity and stability upon cycling, leading to the development of novel Na-ion batteries.