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Solid-State NMR Studies of Lithium Ion Dynamics Across Materials Classes

Lithium sodium sulfate (LiNaSO4) was studied by several groups using solid-state NMR methods to elucidate the complex ion dynamics. LiNaSO4 is a superionic conductor which has a quasi-liquid cationic sublattice above 788 K. But both the cations start to diffuse well below this temperature. The activation energies for this motion estimated from 7Li and 23Na SLR experiments were 0.70 and 1.20 eV, respectively. T1 relaxation was observed faster in trigonal LiNaSO4 than in monoclinic Li2SO4, indicating a faster motion in the former case at 520–775 K. For LiNaSO4 the activation energies obtained from the LT flanks of the SLR rate peaks were 0.91 (7Li) and 1.07 eV (23Na). With a wider temperature range, Massiot et al. obtained activation energies of 0.86 (7Li) and 1.54 eV (23Na) from SLR rates below phase transition. In a recent study a two-stepped 7Li MN curve was observed for Li2SO4 in the temperature window of 298–788 K. A two-time dipolar correlation function was introduced to characterize the fluctuations in the dipolar interaction when two independent cationic sublattices diffuse simultaneously. The LT step with an activation energy of 0.70 eV was attributed to the local motional process of Li+ ion and the HT step (1.33 eV) to the Na+-hopping process.

 

The process of making chromic acid begins with the alkaline roasting of chrome ore in a rotary kiln. Air is used to oxidize the trivalent chromium in the ore to the hexavalent state in the form of sodium chromate. This is leached with water to yield a concentrated solution of sodium chromate. Half of its sodium content can be removed inexpensively by acidification with carbon dioxide under pressure. While carbon dioxide is a rather weak acid, the process is driven by the low solubility of the sodium bicarbonate formed. The sodium bicarbonate removed by filtration and is converted to sodium carbonate by heating. The sodium carbonate is fed back into the rotary kiln, closing one half of the alkalinity loop.

 

At this point the electrochemical route departs from the older technology, acidification using sulfuric acid. The sodium dichromate solution is fed into the anode compartment of an electrolytic cell. The electric current forces sodium ions through the membrane into the cathode compartment where they could form sodium hydroxide just as in the electrolysis of NaCl.

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