Insights into Electrochemical Sodium Metal Deposition as Probed with <i>in Situ</i> ²³Na NMR

Sodium batteries have seen a resurgence of interest from researchers in recent years, owing to numerous favorable properties including cost and abundance. Here we examine the feasibility of studying this battery chemistry with <i>in situ</i> NMR, focusing on Na metal anodes. Quantification of the NMR signal indicates that Na metal deposits with a morphology associated with an extremely high surface area, the deposits continually accumulating, even in the case of galvanostatic cycling. Two regimes for the electrochemical cycling of Na metal are apparent that have implications for the use of Na anodes: at low currents, the Na deposits are partially removed on reversing the current, while at high currents, there is essentially no removal of the deposits in the initial stages. At longer times, high currents show a significantly greater accumulation of deposits during cycling, again indicating a much lower efficiency of removal of these structures when the current is reversed

In Situ, Real-Time Visualization of Electrochemistry Using Magnetic Resonance Imaging

The drive to develop better electrochemical energy storage devices requires the development of not only new materials, but also better understanding of the underpinning chemical and dynamical processes within such devices during operation, for which new analytical techniques are required. Currently, there are few techniques that can probe local composition and transport in the electrolyte during battery operation. In this paper, we report a novel application of magnetic resonance imaging (MRI) for probing electrochemical processes in a model electrochemical cell. Using MRI, the transport and zinc and oxygen electrochemistry in an alkaline electrolyte, typical of that found in zinc-air batteries, are investigated. Magnetic resonance relaxation maps of the electrolyte are used to visualize the chemical composition and electrochemical processes occurring during discharge in this model metal-air battery. Such experiments will be useful in the development of new energy storage/conversion devices, as well as other electrochemical technologies

Mg_<i>x</i>Mn_2–<i>x</i>B₂O₅ Pyroborates (2/3 ≤ <i>x</i> ≤ 4/3): High Capacity and High Rate Cathodes for Li-Ion Batteries

MgMnB₂O₅, Mg_2/3Mn_4/3B₂O₅, and Mg_4/3Mn_2/3B₂O₅ pyroborates have been prepared via a ceramic method. When charging MgMnB₂O₅ vs Li, all of the Mg²⁺ can be removed, and with subsequent cycles, 1.4 Li ions, corresponding to a capacity of 250 mAhg^–1, can be reversibly intercalated. This is achieved at a C/25 rate with 99.6% Coulombic efficiency. Significant capacity is retained at high rates with 97 mAhg^–1 at a rate of 2C. Continuous cycling at moderate rates gradually improves performance leading to insertion of 1.8 Li, 314 mAhg^–1 with a specific energy of 802 Whkg^–1, after 1000 cycles at C/5. Ex situ X-ray and neutron diffraction demonstrate the retention of the pyroborate structure on cycling vs Li and a small volume change (1%) between the fully lithiated and delithiated structures. Mg_2/3Mn_4/3B₂O₅ and Mg_4/3Mn_2/3B₂O₅ are also shown to reversibly intercalate Li at 17.8 and 188.6 mAhg^–1, respectively, with Mn ions likely blocking Mg/Li transport in the Mg_2/3Mn_4/3B₂O₅ material. The electrochemical ion-exchange of polyanion materials with labile Mg ions could prove to be a route to high energy density Li-ion cathodes

Unraveling the Complex Delithiation and Lithiation Mechanisms of the High Capacity Cathode Material V₆O₁₃

V₆O₁₃ is a promising Li-ion battery cathode material for use in the high temperature oil field environment. The material exhibits a high capacity, and the voltage profile contains several plateaus associated with a series of complex structural transformations, which are not fully understood. The underlying mechanisms are central to understanding and improving the performance of V₆O₁₃-based rechargeable batteries. In this study, we present <i>in situ</i> X-ray diffraction data that highlight an asymmetric six-step discharge and five-step charge process, due to a phase that is only formed on discharge. The Li_<i>x</i>V₆O₁₃ unit cell expands sequentially in <i>c</i>, <i>b</i>, and <i>a</i> directions during discharge and reversibly contracts back during charge. The process is associated with change of Li ion positions as well as charge ordering in Li_<i>x</i>V₆O₁₃. Density functional theory calculations give further insight into the electronic structures and preferred Li positions in the different structures formed upon cycling, particularly at high lithium contents, where no prior structural data are available. The results shed light into the high specific capacity of V₆O₁₃ and are likely to aid in the development of this material for use as a cathode for secondary lithium batteries