Electrolytic production of battery-grade lithium hydroxide could remove 60% of the industry’s carbon footprint while also eliminating certain chemical waste types, a team of researchers at the Eindhoven University of Technology, the Netherlands, found. They carried out a comparative study between conventional and electrochemical lithium processing technologies. The most promising was the process of rapid electric heating of spodumene ore to produce lithium chloride which is then converted directly to lithium hydroxide by electrolysis.
Lithium hydroxide (LiOH) is used in the manufacturing of nickel-rich battery cathodes. It is currently produced from spodumen through a long chain of chemical transformations. The ore has to be extracted, degraded and heated at above 1,000°C, then converted to lithium sulfate using sulfuric acid. The next phase is treatment with sodium carbonate to produce lithium carbonate, which then reacts with calcium hydroxide to produce lithium hydroxide. The whole process consumes around 39.3 kWh of electricity per kilogram of LiOH with CO2 emissions up to 16.9 kg. It also produces around 2.95 kg of sodium sulfate and 4.18 kg of calcium carbonate per kilogram of LiOH.
The research team suggested shortening the production chain by applying the chloralkali electrolysis technology which is already broadly used in manufacturing industry to produce sodium hydroxide from sodium chloride. In this particular case the precursor substance is lithium chloride (LiCl). Electric current causes lithium ions to move through a specially designed membrane to the other part of the electrolyzer where LiOH solution is generated, while chlorine and hydrogen are produced at the electrodes. Direct conversion of LiCl to lithium hydroxide eliminates lithium carbonate as an intermediary product. Neither does it use sodium carbonate and calcium hydroxide with their associated waste.
For spodumen processing, the team combined electrolysis with the Flash Joule Heating technology whereby the material is subject to ultra-fast electric heating. Lithium chloride is thus obtained directly from spodumen without intermediate lithium sulfate production. After purification LiCl is fed into the electrolyzer and converted to lithium hydroxide. The estimated power consumption throughout the production chain is 33.9 kWh per kilogram of LiOH compared to 39.3 kWh with the conventional process, approximately a 14% reduction.
In addition, chlorine generated in the electrolyzer can be reused at the spodumen processing stage to produce LiCl. Chlorine thus stays within a closed loop reducing the need for additional reactive chemicals and avoiding production of calcium sulfate. The other by-product, hydrogen, can be used elsewhere at the plant or sold as industrial gas.
Electrolysis can also be used when producing lithium from natural brines. In this case the conventional technology so far consumes relatively less energy – around 4.8 kWh per kilogram of LiOH against 6.7 kWh taken up by chloralkali electrolysis. However, the electrochemical option requires neither sodium carbonate nor calcium hydroxide and produces no calcium carbonate. When using a low-carbon power source its carbon footprint can be around 1.38 kg of CO2 per kilogram of LiOH. If the share of coal power is relatively high the environmental advantage of electrolysis is diminished or nullified.
The electrolyzer membrane still poses the major technical challenge. When moving through the membrane lithium ions carry water molecules causing it to bulge and partially lose its capacity to hold hydroxide ions. As a result, some ions move in the opposite direction thus reducing the LiOH production process efficiency: according to the experimental data employed by the authors, it never rose above 73% at the current density of 250 mА/cm². Therefore, commercial application of this technology would require more resilient and selective membranes. It also calls for pilot testing of the whole process chain, including closed loop chlorine recycling.
