![]() ![]() Here we report for the first time, T 2 contrast 23Na MR images of metallic Na electrodes in a pristine state and after short-circuiting. The achieved spatial contrast supplies greater information to the images and allows one to comment on the dynamics or local site symmetry of individual pixels. DENDRITE LABELED SERIESSpecifically, with a time-incremented series of MRI experiments, spin-spin ( T 2) relaxation can be measured at each spatial element of the object and an MRI image based on relaxation rather than spin density can be produced. 15 However, these larger properties can be utilised to achieve greater image contrast. 14 Likewise, 23Na MRI is considered more challenging than 7Li, as it has a significant chemical shift, greater quadrupole moment, and lower sensitivity than 7Li. Therefore, work in this field has focussed on the narrower linewidths of 7Li and its distribution within the solid-electrolyte. Conventional 1H MRI is not possible in ASSBs due to the lack of protons in the system and if the system did contain protonated groups these would have broad intrinsic linewidths (kHz) which would dephase during the application of gradients. ![]() Imaging solid-state electrolytes is technically more difficult than their solution counterparts as the NMR linewidths are substantially broader, this causes the T 2/ T 2 * to be short and hence the signal to dephase during the application of the imaging gradients. However, NMR relaxometry measurements (spin-lattice relaxation T 1, spin-spin relaxation T 2) have been vital in understanding ion dynamics in a range of battery systems. 12 Direct T 2 contrast MRI on any battery material ( 1H, 6/7Li and 23Na) has never been investigated. 9 In-situ 23Na nuclear magnetic resonance (NMR) during electro-deposition of Na, shows that reversible high-surface-area mossy and/or dendritic structures can be observed and attributed to a nucleation mechanism. shows that in-operando 23Na MRI and MRS studies on sodium cells with organic liquid electrolytes were able to determine the sodium speciation upon galvanostatic cycling. 6 Both 1H and 7Li MRI and magnetic resonance spectroscopy (MRS) have been utilised to probe dendrites in liquid electrolyte electrochemical cells, 7 and 7Li chemical shift imaging has explored Li microstructural growth in ASSBs. Magnetic resonance imaging (MRI) can provide non-destructive, isotope specific, structural, time-resolved, and quantifiable multi-dimensional information. Imaging such dendrites is essential to understand their growth and to develop mechanisms to prevent their formation. These growths have been categorized into four discrete morphologies straight, branching, spalling, and diffuse. 3 Dendritic growths in ASSB systems have different morphologies to their solution counterparts, which can be correlated to the electrochemistry. 1, 2 One of the greatest barriers to the progress of ASSBs is the formation of dendrites (filaments of alkali metal) on charging that penetrate the ceramic leading to a short-circuit and cell failure. 1 These advantages are protracted when coupled with sodium anodes which allow the use of aluminium current collectors (whereas more-expensive Cu is required for Li), sodium also has a significantly higher natural abundance (2.36 % abundance in the earth's crust) compared to that of conventionally used lithium (<0.002 %) and, therefore, offers more security against a volatile Li market. The use of solid-state electrolytes has numerous advantages over the conventional organic electrolytes such as the ability to use metal anodes, removal of volatile and flammable electrolyte organics, and they open up the possibility of Li-Air and Li-Sulphur cathodes (which have higher volumetric density). All-solid-state batteries (ASSB) with a ceramic electrolyte and an alkali metal anode could deliver a step-change in energy storage and safety. ![]()
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