The diversity within the types of anodes used in LIBs is extremely subdued due to the market dominance of graphite. Graphite was first commercialized 20 y ago after the development of the carbon anode and due to its high natural abundance, low cost, and moderate energy density it quickly became the primary anode material in LIBs [9].

Although graphite remains by far the most used material commercially available for battery anodes, an alternate material that has garnered recent attention has been Lithium Titanium Oxide (LTO). While the LTO anode composed in part by Titanium carries a much higher cost in production, the LTO anode has developed some competitive advantages such as its high volumetric capacity and longer life cycle [9]. The LTO anode exhibited capacity retention of 95% after 30,000 that was experimentally achieved during a high-rate cycle life test which suggests that the capacity fading rate of LTO systems are inherently dependent on the cathode potential during storage [10]. Furthermore, LTO is also considered to be a safer anode since it does not have the potential to form Lithium dendrite; a compound that can contribute to spontaneous combustion in LIBs [9].

While the LTO represents a potential future competitor for graphite, the already heightened costs of the battery itself will likely discourage both industry and Governments from investing. Instead, composition modifications of the graphite anode have been the focus of industry efforts: namely the Silicon Oxide anode.

One of the most important properties when discussing anode materials is the extent of volume change that occurs due to the migration of ions that causes the cyclic expansion and contraction of the electrodes. Graphite has a complex carbon crystal structure allowing it to hold up to 372 (miliamp h/g carbon) while experiencing a volume increase of around 10% when fully lithiated. Conversely, Silicon has the capability of holding significantly more Li-ions with a theoretical capacity of 4,200 (milliamp h/g carbon), however, it has been found to swell up to 300% [11, 12]. Additionally, the long-term expansion and contraction of the electrodes has been found to lead to crack initiation and fracturing of particles which ultimately reduce the lifetime of the battery. These experimental phenomena may be discouraging many industry players from investing in Silicon-Graphite anodes and to instead stick with the most established and cost-effective anode material: Graphite. These material properties highlight the difficulty behind incorporating advantageous properties of alternative anodes such as Silicon into LIBs.

Fig. 2 highlights that Silicon has an extremely high potential specific capacity potential in LIBs and offer improvements behind potential voltage as well. Furthermore, HF acid is a by-product of water and Silicon particles and after continuous cycling the polymer matrix and thin elastomer membrane gradually break down leading to a breakdown in capacity and functionality [13].

While Silicon-Graphite anodes are not commonly utilized in industry that has not prevented companies such as Tesla from realizing its advantages as they have been gradually increasing the Silicon content in their EV anodes [11]. In reality, the percent composition of Silicon in commercially available Silicon Oxide anodes remains significantly low compared to Graphite.

The Cathode is one of the most selective materials considered during LIB development. There exists a plethora of commercially tested cathode compositions that should be chosen based on their type of application. The main categories considered are: energy density, power density, cost and lifetime. It is important to define the difference between the first two categories: Energy density defines the batteries capacity by weight (Wh/kg) whereas power density describes the instantaneous output of energy based on its volume [14]. The table below has been developed comparing the previous mentioned parameters along with a variety of other key technical aspects in the Cathode.

Table 1 demonstrates that there isn’t one standout cathode composition that dominates the global markets. Depending on the application, a producer may want to maximize average specific capacity over average voltage. For example, NCA cathodes are frequently used in electric train batteries as they have the largest average specific capacity standing at 200 (mAh/g). These numbers can be explained in part from the fact that electric train batteries are designed with trip distance (capacity) being a priority as opposed to power density or lifetime. Conversely, cathode compositions employed in simpler devices such as cell phones and laptops tend to focus on elements such as volumetric capacity as they are designed to run for at least one day.

An additional way of analyzing the effectiveness of the varying cathodes is through cradle-to-gate analysis of the energy consumption, GHG emission and SO_x production associated with each of the cathodes. Dunn et al. [15] use Argonne’s GHG, Regulated Emissions, and Energy use in Transportation (GREET) to investigate the energy and emissions intensity of cathode preparation [15]. LCO cathodes appear to require the most energy consumption for production in both methods analyzed in the article (solid state and hydrothermal).

Moving forward, battery chemistry is increasingly being driven through innovation and a rise in demand for electric cars. Fierce competition between LIB producers will continue as the market is still in its infancy stage and significant improvements in terms of energy density, power density as well as the lifetime of the battery are expected to increase markedly. The batteries in use today are expected to be strikingly different in the upcoming decades as the industry begins to takeoff.

Fig. 3 demonstrates that batteries are constantly changing as our needs change and develop in different ways. The batteries of today are expected to be phased out by the end of the decade as new innovations within LIB such as the upcoming development of the Li-Mg LIB which is predicted to have significantly better properties compared to modern day LIB.

The electrolyte is a medium that exists inside the battery that serves an extremely important function in terms of acting as the conveyance system for Li-ions to pass back and forth between the cathode and anode. The transfer of Li-ions occurs when the battery is discharging (ions travelling from anode to cathode) and when it is charging (ions travelling from cathode to anode), thus the more efficiently that the ions can travel, the better the battery will perform. The electrolyte is regarded as a substantial safety concern, as it must be able to withstand high temperatures and voltages throughout the entirety of the batteries useful life, without reacting unfavorably [16]. Typically, LIB manufacturers use various types of liquid electrolytes, which differ in energy consumption, power output, safety features and cost [17, 18]. Liquid based electrolytes commonly consist of organic carbons including Propylene Carbonate, Ethylene Carbonate and Di-Methyl Carbonate [19]. Although readily used in most EV’s today, liquid based electrolytes pose various safety risks, including the formation of hydrofluoric acid caused by electrolyte fluid leaking from the battery and reacting with air. Additionally, the potential for spontaneous combustion is significantly higher when the internal battery temperatures become too high [19]. Due to the aforementioned safety concerns, companies are pushing towards new, safer technology, in the form of solid state electrolytes. Solid state electrolytes offer many advantages over their liquid counterparts. The main benefits include: higher energy storage and power capacity, resistance to dendrite formation, chemical stability over a wider voltage range and no risk of spontaneous combustion or explosions [20]. Moreover, solid state electrolytes allow for LIBs to become smaller and more compact, while maintaining the same storage capacity and power output [20]. A LIB using a liquid electrolyte will have limitations with regards to how small it can be constructed, as a reduction in electrode size in this case would result in a loss of power output.

The separator in an LIB is used to encapsulate the electrolyte inside the battery itself. It is a crucial LIB component, as it must facilitate ionic conductivity while preventing direct contact between the anode and cathode [21, 22]. Generally comprised of polyethylene, the separator has no conductivity of its own, rather, it is used to essentially absorb and house the liquid electrolyte [23]. As the separator will be in constant contact with the anode and cathode simultaneously, the Li-ions are able to traverse through the electrolyte inside the separator to ultimately make the battery charge and discharge. Separators are designed to have pores that match the size of Li-ions, so that they can transfer through efficiently. Typically, these pores are 20–25 mm [23]. As mentioned above, the separator acts as a physical barrier between the anode and cathode which prevents them from ever coming in contact. This is critical to battery function, as contact between the anode and cathode could cause a short circuit, resulting in battery failure [24]. A short circuit in a LIB could result in the battery catching fire, so it is essential that the separator exits the production line, free of flaws or defects.

Companies such as Tesla have set directives to create a closed-loop supply system; in their case within North America. In order to do this, they will have to identify more Li sources and to ensure the opening of upcoming large scale manufacturing plants such as the Gigafactory run smoothly. Tesla recently announced an agreement with Pure Energy Minerals for rights to a guaranteed Li supply from Silver Creek mine in Nevada, hours away from the planned Gigafactory [25]. While initiatives such as the arrangement at Silver Peak mine in Nevada are coming to the forefront, it is important to note that Li comprises only 4% [26] of the modern Li battery electrode. It consequently only contributes a limited amount to recent rising battery prices. There are many other elements of the LIB that require a more diversified supply chain. 65% of all Cobalt mined in the world originates from the Democratic Republic of Congo (DRC), a country currently struggling with corporate corruption and political strife [27]. Another common cathode material, Manganese, was subject to a sudden 50% price increase in 2015 after the government banned the export of the metal.

These are but a few examples describing the complex yet limited supply chain behind LIBs but offer a valuable insight and potential explanation towards rising EV battery costs. Furthermore, it remains a clear challenge for the LIB industry to develop a cost effective battery that does not have a large overall negative impact on the cost of the EV. If the industry wants to achieve price parity with combustion engine vehicles, they must find a way to diversify the supply chain in order to address rising battery material costs.

Hydrometallurgy is a chemical-based extraction method for electrode metal recovery in LIBs. The general process involves acid leaching of the in the presence of a reductant and the separation and recovery of the target metals. Common leaching agents include H₂SO₄, HNO₃ or HCl, reductants include H₂SO₃, NH₂OH and H₂O₂ and further recovery using various solvent extraction methods and precipitation using solvents such as PC88A, Cyanex 272, DEHPA, D2EHPA and TOA and NaOH, NH₄OH, citric acid and ammonium oxalate as precipitants [40, 41].

The metal extraction efficiencies and purities are a main advantage to using hydrometallurgical methods, Wang and Friedrich [40] found leaching efficiencies for a LiCoO₂ cathode between 98.6% – 99.9% (Co, Ni, Cu, Li and graphite) using 2 mol/L sulphuric, 4 mol/L hydrochloric acid, 50 g/L hydrogen peroxide and a 100 g/L liquid to solid ratio after 120 min through the development of a product oriented process. The recycling efficiencies of the metals were found to be 98–100%, 97%, 94–99% and 60–95%, 92–99%, 95–98%, and 48–64% for graphite, copper powder, aluminum hydroxide, iron hydroxide, Co, Ni and Mn salts and lithium carbonate, respectively. It was found that the graphite, Co, Ni and Mn obtained was a high enough purity to be used in a new anode, graphite in industrial settlings is usually treated as waste currently. Numerous studies have confirmed these results on a laboratory scale including a paper by Zou [42] who developed a recycling process for a diverse mixture of cathode composition and achieved recovery rate near 100% for Ni, Mn and Co using hydroxide precipitation and 80% for Li in the form of lithium carbonate. Additionally the cathode metals because of their high purity do not need to be separated by solvent extraction since these metals are combined for the manufacture of new cathodes. One suggestion of the study recommended further treatment of the aluminum and iron hydroxides to recover cobalt and nickel. A process by Chen and Zhou [41] who reduced leaching time down from 120 min to 90 min at the same leaching temperature using 2 mol/L citric acid as a leachant and 2 vol.% hydrogen peroxide achieving leaching efficiencies of about 97%, 95%, 94% and 99% for Ni, Co, Mn and Li for LiNi_1/3Co_1/3Mn_1/3O₂ cathodes. Dimethylgloxime and ammonium oxalate were used to precipitate nickel and cobalt, respectively, and oxalic acid was used scrub Mn and Li-ions from the precipitate. D2EHPA was used to recover Mn as MnSO₄ and lastly the Li precipitated using NaPO₄. Using these methods, metal recovery efficiencies of 95%, 97%, 98% and 89% for Ni, Co, Mn and Li, respectively, were achieved. No indication of graphite recovery was mentioned in the process.

The issue with hydrometallurgical methods is the amount of specific and often expensive chemicals needed for the recovery of metals. Additionally, the extraction methods are seen to produce large amounts of spending acid, acid gas, alkali, and solvents which require treatment [43]. There is a significant cost associated with treating secondary pollutions from hydrometallurgy, because these chemicals involved are generally toxic and range in composition, meaning treatment can be complicated and require specialized processes. The research conducted by Chen and Zhou [41] discussed above used an organic leaching acid in citric acid to perform their tests. Leaching with citric acid is desirable due to its low cost, natural degradation and absence of toxic gas production of Cl₂, NO_x and SO₃ common with inorganic acids which are a threat to humans and the environment [41, 44]. Other organic acids such as succinic acid, DL-malic acid (L-and D-enantiomers), aspartic acid, ascorbic acid, oxalic acid, glycine and L-tartaric acid have proven as viable substitutes [45]. The process is a step in the right direction; however, solvent extraction is still commonly utilized to obtain high purity metals which elevate the quantity of secondary pollution from solvents and acid stripping. A study by Li et al. [46] compared metal recovery from LiCoO₂ cathode batteries by leaching in citric, malic and aspartic acid from the recovery of lithium and cobalt. Recoveries of nearly 100% for Li and Co were achieved using 1.25 mol/L citric and 1.5 mol/L malic acid after 30 min and 40 min of leaching time, respectively at 90°C, H₂O₂ at 1.0% and 2.0 vol.%, respectively, and a S:L ratio of 20 g/L. Aspartic acid was proven to have inferior performance to citric and malic acid achieving only 60% recovery in 2 h under the same conditions, this is due to its weak acidity and low solubility in water. Through an environmental assessment of leachates, citric acid is derived from fermentation methods while the other most efficient organic leaching acids malic and aspartic acids are sourced from fossil derived butane [46]. This contributes to much lower energy intensity (FFC) per kilogram produced than the other acids. This shows citric acid to be the most environmentally and economically viable option and thus should be a priority for leachate selection. A study showed high recovery of Li and Co recoveries using 0.5 mol/L citric acid leaching with 0.55 mol/L, a S:L ratio of 25 g/L, temperature of 60°C and leaching time of 6 h [47]. The study also included HCl and H₂SO₄ as leachates, citric acid was seen to be superior in leaching performance compared to the inorganic acids. The process is also ultrasonic assisted which contributes to the high leaching efficiency through cavitation action. Similar methods have been developed by Chen et al. [48] using citric acid and tea waste (as a reductant) to achieve very high leaching efficiencies of Li and Co within 2% of citric acid/H₂O₂ methods after 120 min at similar temperatures. The process also includes citric acid recycling through circulatory leaching with high efficiencies for Co and Li after 5 cycles. Additionally, the only by-products of the process under ideal conditions are water and O_2. Optimal ultrasonic power was determined to be 90 W, which led to 96.13% and 98.4% Co and Li recovery. This process achieves very high leaching efficiencies at low temperatures with the weak point being long leaching times. Additionally further extraction methods of the metals from the leaching solution were not included. Other sustainable methods using citric acid have been developed that uses circulatory leaching to reduce waste acid and use of oxalic acid to precipitate cobalt [44] in replacement of solvent extraction. Ultrasonic washing (NMP) was used and has been seen to be an effective method to wash and separate the cathode. NMP is a toxic solvent, but has the capacity to be recycled which can be applied to the recycling process. The circulatory leaching of acids using filter liquor was found to be effective at 35 min leaching times with leaching efficiencies of 98% and 90% for Li and Co, respectively. Optimal conditions included a leaching temperature of 90°C, L:S of 60 mL/g and 0.9 vol.% H₂O₂. High recoveries of Co (99.5%) and Li (90.2%) were achieved through oxalic acid precipitation and sodium phosphate addition, respectively. Additional cycles of leaching reduce the leaching efficiency however recoveries above 80% can be achieved after 3 cycles. This process is effective at eliminating waste and coupled with high recoveries would make a successful eco-friendly process at the industrial level. The studies using citric acid as a leachate in literature however do not address the recovery of other metals during recovery such as Ni and Mn from the cathode. One study addresses this issue using 2 mol/L L-tartaric acid with 4 vol.% H₂O₂ at 70°C for 30 min to leach Li, Co, Ni and Mn from spent batteries [45]. High leaching efficiencies were found to be 99.31%, 99.07%, 98.64%, and 99.31% for Mn, Li, Co and Mn, respectively, with no recovery methods included in the study. This shows that organic acids can leach all cathode materials at high efficiencies.

LCA of rare earth element (REE) recovery in mining operations through solvent extraction have shown high environmental impact. Specifically, hydrochloric acid and sodium hydroxide were found to have high environmental impacts in various categories used for analysis and were utilized mainly in stripping processes [49] as would occur in hydrometallurgical solvent extraction. A novel process have demonstrated leaching times that have been brought down to 20 min at 70°C while maintaining comparable removal efficiencies using 1.25 mol/L ascorbic acid as the leaching and reducing agent [37]. Leaching efficiencies peaked at 98.5% and 94.8% for Li and Co, respectively. The method also uses NMP, but removes use of H₂O₂ in the leaching process. The shows promise for a low waste recycling method. This method is seen to be the most environmentally sustainable, while obtaining similar leaching and recovery efficiencies of other well established methods currently in use.

Other novel processes such as oxygen-free roasting and wet magnetic separation can successfully separate Li, Co and graphite at recovery efficiency of 98.93%, 95.72% and 91.05%, respectively, and without the creation of secondary pollution from chemical precipitation, solvent extraction or acid leaching [43]. The process is very long with 48 h of magnetic separation stirring times and required temperatures up to 1,400°C for roasting. Additionally, volatile organic compounds (VOCs) were emitted from the electrolyte at high temperatures which is the only pollutant from the process. With high efficiencies this process is desirable; however, more research needs to be conducted to increase process speed to compete with other alternatives.

Promising new research has been done into an ideal zero-waste recovery of metals from LIBs. Marinos and Mishra [50] proposed leaching electrode materials in stirred distilled water and using a floatation cell to wash particles of fine materials and float out plastics. The electrode powders are recovered by wet sieving and hydro cyclone separation while graphite is separated using pine oil. Lithium carbonate is recovered by introducing hydrochloric acid or carbon dioxide which is recycled in the process. The rest are separated using magnetic and eddy-current separation into mesh sizes. Little data is available on the purity, and currently further separation of metals is not part of the system. However, the research is only a few years old and has potential to be a novel, environmentally friendly process with close to 100% Li recovery at specific leaching times.

Pyrometallurgy involves separation of materials through thermal processes, rather than chemical processes as hydrometallurgy does. Through varying degrees of heating of the material, thermal energy initiates reactions to transform the material. The extent and type of changes to the material depends on the severity of the heat applied. Using high temperatures for this process can cause the batteries to be smelted, which produces three products: metallic fraction, slag, and gasses. This recycling technique is often used in industrial furnaces, in which metalliferous materials and metals are produced or cleaned [51]. The main pyrometallurgical methods used for the recovery of metals from electric waste in general are smelting, combustion, pyrolysis, and molten salt processes (such as calcination) [52]. However, for the purpose of LIBs specifically, heat treatment through calcination and pyrolysis is very common.

Calcination thermally decomposes material by use of relatively mild heat, but can rise up to as high as 1,500°C [53]. Usually inorganic reactants are used in calcination. It is a process that is commonly used in the manufacturing of cement, and with material recovery often produces a slag byproduct which can be sold to construction companies for use in cement. The reactions involved are mostly internal reactions that only involve the material itself [51].

Pyrolysis is the thermal decomposition of organic material at elevated temperatures in the absence of oxygen. This causes irreversible reactions that result in the simultaneous change of chemical composition and physical phase of the materials. This leads to the formation of low-molecular byproducts and char by thermal decomposition reactions between 450–1,100°C [52]. These byproducts are often valuable for use as fuel, while the char can contain much of the valuable metals in the battery.

Research completed in the ELIBMA project was done on a process involving calcination in conjunction with pyrolysis, which involved the addition of reducing agents into an oxygen deficient environment for burning at temperatures up to 700°C. The metallic fraction produced contains metals that have low affinity to oxygen and is further refined for the separation of the metals. This mixture is also reduced to balance out the oxidation that occurred during the calcination/pyrolysis stage. This is usually achieved by adding a relatively cheap reducing agent such as coke. The slag produced is a stony mixture of materials that can be sold and used for construction and the production of cement. Finally, the gas fraction that is produced is to be filtered before being released into the atmosphere due to its harmful composition, containing volatile decomposition products and volatile metals such as Hg and Zn [52]. Because pyrometallurgic processes involve immediate burning of material at high temperatures, they are considered fast, and easy to handle [51].

As mentioned above, the cathode and electrode compositions vary greatly in Li-ion EV batteries. One of the major advantage to pyrometallurgy as a material recovery technique is its ability to be effective for a large array for electrode chemistries without altering the method significantly [35]. Additionally, pyrometallurgy consumes about half the water that is consumed during the hydrometallurgy process [35].

Although the metallic smelt produced recovers many of the metals and the stony slag generated can also be applied for secondary uses, the harmful gas fraction poses an environmental risk factor to this process.

A study by Hendrickson et al [54] shows that a 6–56% energy reduction and 23% GHG emission reduction can be expected for pyrometalurgical methods for recycling when compared to virgin production or raw material for EV battery production. This is based off of modeling only two dismantling facilities in the state of California which was shown to be most economically feasible [54]. The study also explains that there are challenges with chemistries such as LMO batteries, as the active materials are relatively cheap and recycling is not considered economical. Additionally, scaling up pyrometallurgical recycling methods can pose serious human health risks in nearby areas. The study shows that NO_x production was 96% from the pyrometallurgy facility, while health risks associated with other pollutants such as PM, SO₂, and VOCs were produced mostly from the depleted EV batteries transportation, marking 99% of the total PM produced. However, a negligible amount of SO₂ was generated during the recycling process while transportation and energy generation processes made up 99% of SO₂ production [54].

LCA was made to show the outputs of EV battery production from pyrometallurgy and hydrometallurgy are compared to that of raw material production [35]. The result showed pyrometallurgy exceeding production from raw materials in electricity consumption and the release of PM_2.5 and VOCs. The electricity consumption and PM_2.5 production was shown to exceed double that of hydrometallurgy [35].

Any pyrometallurgic process involving the heating of the materials until decomposition can result in similar, harmful byproducts that must be considered. However, due to the ease of handling and low cost involved with pyrometallurgy for metal recovery from LIBs, it is currently the method implemented by most battery recycling companies worldwide (Table 3).

Based on the characteristics compared above, hydrometallurgic processes for LIB material recovery are very effective. Currently, the more common method implemented for metal recovery is pyrometallurgy, but as research continues, and LIB are recycled on a larger scale, the economic feasibility of hydrometallurgy is expected to decrease. Based on its lower environmental impact as well as higher recovery and selectivity rates, hydrometallurgical methods are expected to become much more widely implemented.