Raw Material Supply for Lithium-Ion Batteries in the Circular Economy

The energy transition relies on the development of technologies that make it possible to produce energy in a sustainable manner from resources such as wind, sun, potential energy, etc. The energy produced as part of the energy transition is intermittent, and it is therefore necessary to be able to store it. Electric mobility can significantly contribute to reducing the impacts of human activity on the environment and the climate, since it contributes to mitigating greenhouse gas emissions. Lithium-ion batteries (LIBs) are at the heart of energy storage for stationary applications or electric mobility (electric vehicles). They are now widely used in phones, laptops, etc., and their increasing use in electric vehicles is indisputable. Along with the development of Gigafactories for LIB production, the improvement of the performance and safety of LIBs, or even the optimal management of the supply of resources necessary for their manufacture, the development of LIB recycling is challenging and should not be overlooked.

A circular economy aims to change the paradigm compared to the so-called linear economy by limiting the waste of resources and the environmental impact, while increasing the efficiency at all stages of product life, including the design, production, use and end of life. It is a complex approach that involves many players, many regions and many interfaces. The value chain leading to the production of LIBs will therefore be effective providing that each step is mastered in a holistic manner, since each action on one part of the value chain will act on the other steps.

The challenges for the development of an industrial sector for LIB production are therefore not limited to the technological aspects related to the efficient and sustainable manufacturing of LIBs by building Gigafactories but it is also of great importance to recycle LIBs in order to reduce environmental impacts, safety risks and raw material supply risks. The growth of the electric vehicle (EV) market and the global need for intermittent energy source storage leads to exponential growth of the LIB market. However, battery production is energy-consuming and causes severe environmental impacts. The critical material dependence of some countries, and ethical and environmental issues accentuate the need of a fair supply. Therefore, recycling becomes essential from the perspective of significant LIB and EV market development. End-of-life LIBs are a serious industrial issue in all parts of the recycling value chain, because of the high environmental and economic process costs. Batteries are complex systems (comprising polymer, metals and plastics). In today’s society, recycling LIBs represents a technological challenge with the development of efficient, safe, low cost and low environmental impact processes to selectively recover high-purity metal salts of cobalt, lithium, nickel and manganese.

Figure 1 shows the main technical challenges to be addressed in each stage of the processes for the recovery of metals from spent LIBs:

Lithium-ion batteries (LIBs) are expected to dominate the market for e-mobility and stationary energy storage in the next decade [1]. This will result in a large amount of waste from both LIB production and spent LIBs [2]. Today, a common LIB cathode material in use is LiNi_1/3Mn_1/3Co_1/3O₂, also called NMC111 [3,4]. In the future, this material is expected to be replaced by materials with higher Ni content, such as NMC811 [4].

The raw metal supply relies on efficient recycling processes. Recycling of LIBs minimizes raw material shortage and environmental and human health concerns [5]. At least 22.2 t of primary high-grade ore is required to generate the market value equivalent of 1 t of end-of-life LIBs. Despite the uncertainty facing the EU supply of critical materials and the European Directive specifying battery collection, recycling rates are low.

Several processes have been developed to recycle LIBs. The first step after collection is a deep discharge of the batteries. Two short-circuit-based discharge methods are widely employed [6]:

• The resistor-based method that consumes the residual electric energy.
• The salt-solution-immersion discharge, which immerses the LIB in ionically conducting salt water to form a short circuit and thus effectively induces the electrochemical discharge of the LIB.

After deep discharge, the batteries are treated mechanically followed by hydrometallurgical processing where the black mass is usually first digested in sulfuric acid, followed by the precipitation of impurities, separation and purification of metals by liquid–liquid extraction and ion exchange, and finally crystallization and precipitation to obtain metal salts.

It is possible to remove the main part of iron and aluminum (considered as impurities) from the leach liquid by precipitation as hydroxides by adding, e.g., sodium hydroxide up to a pH of about 5 [7,8]. A challenge in the precipitation of impurities is the losses of cobalt and nickel due to co-precipitation because of local supersaturation during dosing.

After the impurities have been removed, liquid–liquid extraction can be used to separate copper, lithium, nickel, manganese, and cobalt into separate fractions [8,9,10]. Copper (II) from the anode collector can be extracted by using extractants of the Acorga or the LIX series at very low pH, DEHPA can be used to extract manganese (II) at pH 2.5, and Cyanex 272 can efficiently extract and separate cobalt (II) and nickel (II) and pH 4.3 and 6, respectively [11,12]. Most of the solvent extraction processes using these commercial extractants have been developed to extract metals from acidic sulfate media, as the leaching of cathode materials is usually based on the use of sulfuric acid in the presence of hydrogen peroxide. However, the use of other leaching reagents such as carboxylic acid or hydrochloric acid could be an alternative route, which could pave the way for new solvent extraction routes with new extractant molecules exhibiting better extraction efficiency and better selectivity [13,14,15,16]. In particular, special attention must be paid to reduce the co-extraction of impurities in order to produce battery-grade salt while reducing the operating cost. For instance, there is a need to identify new extractants to efficiently separate cobalt (II) and manganese (II) or to improve the aluminum management in hydrometallurgical processes (particularly when the recycling process concerns batteries containing cathodes with a high aluminum content such as lithium nickel–cobalt–aluminum oxide electrodes, NCA). Other less conventional processes based on the use of supercritical fluids could also be an interesting alternative to classical liquid–liquid extraction. More attention could also be paid to the development of technologies to recover lithium from leach solutions in order to reduce lithium losses throughout the process.

After separation and purification, evaporative crystallization and cooling crystallization can be used to obtain Ni, Co and Mn in the form of sulfate hydrate crystals [8,9,10], whereas lithium can be recovered as lithium carbonate or lithium hydroxide. The salts can be used to produce new battery cathode materials if the purity is high enough. There are also other uses for these salts [17,18]. An alternative method to the evaporative crystallization of metal sulfate salts may be to recover the salts by eutectic freeze crystallization. This technique is based on cooling a concentrated aqueous salt stream down to the eutectic point where salt crystals and ice crystals form simultaneously. The lower energy consumption, the high purity of the salts, the possibility to recover pure water and low corrosion at low temperatures make eutectic freeze crystallization a promising alternative compared to evaporative crystallization to produce pure metal sulfate salts.

The metal sulfate salts obtained from the recycling process are used in the synthesis of NMC cathode material by re-dissolution and co-precipitation of hydroxide precursors by addition of, e.g., sodium hydroxide. The metal hydroxides are washed and dried and then mixed with lithium salt and calcined into new cathode material. The citric acid sol–gel method provides a flexible alternative approach to synthesize LIB cathode materials, using the salts from the recycling process [4]. Although this method has been in use for many years, there is a need to further increase the understanding of the effect of key parameters in the sol–gel method on the material quality and battery performance [4].

Carboxylic acids may be environmentally sustainable alternatives to mineral acids in leaching of black mass [19]. Citric acid is a cost-efficient solvent that does not produce toxic emissions. Although a weak acid, it has a powerful chelating ability, and therefore good leaching capabilities. Even if the chelating property can also introduce challenges in the efficient separation and recovery of elements, recent studies report feasible solvent extraction and stripping of Ni, Co and Mn in citric acid media [20]. Metals in the form of metal carboxylates can be recovered from the strip solution by antisolvent crystallization; the metal carboxylates can then be calcined into new active cathode material [21,22]. By this new approach, the precursors are produced directly from the recycling stream, eliminating the hydroxide co-precipitation step. Furthermore, possible contamination by sulfate and sodium ions is avoided. Antisolvent crystallization is a technique widely used in the pharmaceutical industry, based on reducing the solubility of the solute and allowing for supersaturation by adding a water-miscible organic solvent to force the solute to precipitate [21,22,23]. Evaporative crystallization, used to produce the metal sulfates in the conventional LIB recycling process, is an energy-intensive process. In antisolvent crystallization, salt–water separation can be achieved without evaporation of water, which substantially improves the efficiency of the process. Moreover, controlling the supersaturation is easier using antisolvent crystallization compared to evaporative crystallization, producing crystals of higher quality concerning size, shape and purity that are of importance in precursor production. Antisolvent crystallization has been proven to be useful for the recovery of battery-grade nickel sulfate [24]. The antisolvent, usually an alcohol, can be recovered by distillation, where the evaporation of alcohol consumes much less energy than the evaporation of water, and both the acid and the alcohol can be recycled in the process. However, to be able to synthesize cathode material of high quality by antisolvent crystallization of metal carboxylates from carboxylic acid, more research is needed.

In this context, the following points will be addressed among others in this Special Issue of the journal Metals:

• The supply of raw material for the sustainable production of LIBs, outside the context of mining activity, for the extraction of nickel, cobalt, manganese and lithium;
• Sustainable production of LIBs;
• Recycling and reuse
• Business model in the context of the circular economy.