The transition towards electric vehicles (EVs) has resulted in a proliferating demand for lithium-ion batteries (LIBs). The continuous growth in the EV industry results in a colossal number of LIBs being discarded after reaching their end-of-life. Researchers have carried out numerous investigations on the extraction of valuable metals from spent LIBs. The recycling processes have mainly been concerned with the recovery of the valuable metals of cobalt and nickel, with less attention being placed on lithium recovery. With the imbalance of the supply and demand of lithium resources, research on selective recovery of lithium from spent LIBs has increased in recent years. This paper provides a comprehensive overview of the high-temperature selective conversion, selective leaching, mechanical and electrochemical recycling methods that facilitate selective lithium recovery. It provides recommendations for future research and development to enhance the selective extraction of lithium from spent LIBs.
Contents
1 Introduction
2 Selective lithium extraction from spent LIBs cathode material
2.1 High-temperature transition
2.2 Selective leaching
2.3 Mechanical/electrochemical extraction
2.4 Comparison of advantages and shortcomings of different treatments
Lithium-ion battery is one of the most common energy storage batteries at present. Because of its excellent performance in cost and performance, it is widely used in consumer electronics, electric vehicles and other fields[1,2]. In recent years, with the active promotion of electric vehicles in various countries, electric vehicles are gradually replacing traditional fuel vehicles as the main means of travel, which also makes the market demand for lithium-ion batteries rise rapidly[3,4]. The global lithium-ion battery market was worth $29.86 billion in 2017 and is expected to reach $139.36 billion in 2026[5]. At present, the lithium needed for the production of lithium-ion batteries is mainly extracted from salt lake brine and high-grade lithium ore, but the lithium extracted from these two resources in the future is likely to be unable to meet the growing market demand, and it is expected that the supply will be in short supply in 2023[6][7]. Although lithium-bearing brine is abundant, the concentration of lithium is very low, for example, the content of Li+ in Atacama Salt Lake in Chile is 0.15 wt%, and the extraction of lithium requires a large amount of brine resources[8]. In addition, lithium extraction from salt lake brine has the disadvantages of long production cycle and vulnerability to weather[9]. The lithium content in lithium ores is also not high, for example, the Li content in representative lepidolite and spodumene is 3.84 wt% and 3.73 wt%, respectively[10]. Compared with the extraction of lithium from spent lithium-ion batteries, the extraction of lithium from ores has a greater impact on the environment and consumes more energy in mining and processing[11]. Compared with salt lake brine and lithium ore, the lithium content in cathode materials of spent lithium-ion batteries is about 5 ~ 7 wt%[12,13]. Therefore, extracting lithium from waste lithium-ion batteries not only has higher economic value, but also can reduce the environmental pollution caused by resource exploitation.
The service life of power batteries in lithium-ion batteries is about 5 to 8 years. With the increasing proportion of electric vehicles in travel year by year, a large number of lithium-ion batteries will be retired. It is estimated that the total amount of waste lithium-ion batteries produced in 2040 will reach 8 million tons[14][15]. Previously, the main focus of lithium-ion battery recycling was cobalt and nickel, not lithium extraction[4,16]. For example, the traditional fire smelting is to convert nickel, cobalt, copper and other metal compounds in waste lithium-ion batteries into metal melt at a high temperature above 1000 ℃, and the metal melt and slag are separated due to their insolubility and different densities[17]. However, in this process, lithium will not be reduced to metal, but will be lost in the form of oxide in the slag[18]. The traditional wet process usually adopts the mode of "complete leaching-transition metal separation-lithium recovery". Although this method can recover lithium in the last step, the recovery process is cumbersome, consumes a lot of chemical reagents, and a small amount of lithium will be mixed in the transition metal separation, resulting in the loss of lithium[19]. In addition, the recovery rate of lithium will be further reduced due to the low concentration of lithium in the solution to be extracted after multi-step separation[20]. For example, Yang et al. Used PC88a in the experiment to separate Li+, Ni2+,When the Mn2+ and the Co2+ in the mixed solution of the Co2+ and the Mn2+ are extracted synergistically,It was found that the concentration of Li+ in the mixed solution after extraction decreased from 6. 5 G/L to 0. 55 G/L[21]. Then when Versatic10 was used to extract Ni2+ from the mixed solution of Li+ and Ni2+, it was found that a small amount of Li+ would be entrained into the loaded organic phase, resulting in the loss of lithium. Because there are different degrees of lithium loss in the traditional pyrometallurgical and hydrometallurgical recovery processes, and because lithium and other valuable metals in lithium-ion batteries mainly exist in the positive electrode after discharge,Therefore, in recent years, the research on selective extraction of lithium from waste batteries mainly focuses on cathode materials, while the research on electrolyte and anode is less because of their relatively low recycling value[22,23].
Common types of cathode materials for lithium-ion batteries include LCO(LiCoO2), LMO(LiMn2O4), NCM(LiNixCoyMn1-x-yO2), and LFP(LiFePO4). Their composition, structure, lithium content, main uses, market share, and economic benefits of recycling are shown in Table 1. It can be seen from the table that power batteries used in new energy vehicles, electric bicycles and other fields mainly use NCM, LFP and LMO as cathode materials; Energy storage batteries used in large-scale energy storage power stations and 5G base station markets mainly use LFP as cathode materials. LCO is mainly used in small batteries such as 3C electronic products market and power tool market[24]. Compared with other types of cathode materials, the recovery of valuable metals in LCO and NCM can obtain higher economic benefits, so there are many studies on it at present. Compared with NCM and LCO with layered structure, LFP with olivine structure has lower energy density, but its thermal stability, cycle performance and safety performance are higher, and the research on its recovery is also increasing in recent years[25].
At present, the review on lithium-ion battery recycling has been elaborated in different dimensions, for example, Makuza et al. And Zhao Lei et al. Reviewed and commented on lithium-ion battery recycling from the perspective of pyrometallurgy and hydrometallurgy respectively, and Swain et al. Reviewed the research on lithium recycling from various lithium resources.Kumar et al. Reviewed the selective extraction of lithium from lithium iron phosphate, but there is no article on the systematic review and analysis of the selective extraction of lithium from different types of cathode materials, so it is necessary to sort out the specific methods and principles of selective lithium extraction from lithium-ion batteries in more detail[26][27][6][28]. In this paper, different methods of selective extraction of lithium from cathode materials of spent lithium-ion batteries are summarized. In section 1, the advantages and necessity of selective extraction of lithium from spent lithium-ion batteries and different types of cathode materials for lithium-ion batteries are briefly introduced.In section 2, the different methods of selective extraction of lithium are discussed and compared in detail. In section 3, the recovery of lithium from electrolyte is briefly introduced. Finally, the future development direction of selective extraction of lithium is prospected.
2 Selective extraction of lithium from spent LIBs cathode materials
2.1 High temperature transition
High-temperature transformation includes reduction roasting, salinization roasting and oxidation roasting. Its principle is to selectively convert lithium in cathode materials into compounds with different solubility from other valuable metals such as Ni, Co and Mn through roasting, so that lithium can be selectively separated from other metals in solution during leaching. The reduction roasting methods involved in this paper include carbothermic reduction, hydrogen reduction and aluminothermic reduction. Salinization roasting includes chlorination, sulfation and nitration roasting, and the whole process is shown in Figure 1. The advantages of selective lithium extraction by high temperature transformation are short recovery process and high efficiency. Lithium can be recovered preferentially in the leaching stage, which avoids the loss of lithium caused by multi-step separation and reduces the use of acid, alkali and other reagents in the leaching stage. Therefore, the research on this technology has gradually increased in recent years[30].
2.1.1 Extraction of lithium from slag
Lithium-containing slag is the residual slag after recovering Ni, Co and other metal alloys in lithium-ion batteries by high temperature reduction smelting above 1000 ℃[31]. In the smelting process, lithium in the cathode material will not be reduced into metal simple substance into the alloy phase, but will enter the slag together with the flux CaO, SiO2, etc. In the form of oxide[18,32]. The composition of slag is complex and can not be directly leached and recovered, so it needs to be transformed at high temperature again to extract lithium selectively.
Lithium-containing slag can be roasted by chlorination or sulfation to selectively convert lithium compounds in the slag into water-soluble lithium salts, and then lithium can be recovered by water leaching. For example, in the experiment, Dang et al roasted the prepared pyrometallurgical simulated slag and CaCl2 at 800 ℃ for 60 min according to the molar ratio of Cl-/Li+ of 1. 8 ∶ 1, and then leached at 60 ℃ for 30 min according to the liquid-solid ratio of 30 ∶ 1, and 90. 58 wt% of lithium in the slag was leached[33]. In addition, in the experiment of Dang et al., 97.07% of lithium in the simulated slag was leached out by co-roasting Na2SO4 and NaCl at a molar ratio of 0.474 ∶ 0.526, (S +Cl-)/Li+ at a molar ratio of 5 ∶ 1, and the simulated slag containing lithium at 750 ℃ for 120 min, and then leaching in water at a liquid-solid ratio of 30 ∶ 1 for 5 min at room temperature[34].
In the smelting process, flux is added to make slag, resulting in low lithium content in the slag, for example, the lithium content in the CaO-(FeO)-SiO2-Al2O3 slag system is less than 1. 50 wt%[35]. Therefore, the economy of recovering lithium from slag with complex composition and less lithium content is not good. Ren et al. Adjusted the slag to a MnO-SiO2-Al2O3 system by using pyrolusite as the main flux in the pyrometallurgical smelting process, which increased the Li2O content in the slag to 2.63 wt% and improved the recovery value of lithium-bearing slag[35].
2.1.2 Carbothermic reduction
Carbothermal reduction method is to use carbonaceous materials such as graphite, coke and lignite as reductants to convert Li into Li2CO3 by roasting, and finally achieve the selective separation of Li from other metals by water leaching[36]. Due to the low solubility of Li2CO3 in water, as shown in Fig. 2, the solubility of lithium carbonate at 20 ℃ is only 13.16 G/L, and its solubility decreases further with the increase of temperature, so the method of recovering Li2CO3 by water leaching increases the difficulty of extracting lithium from water leaching solution[37]. Makuza et al. Converted the Li2CO3 into LiHCO3 with high solubility by introducing CO2 into the solution, and the recovery rate of lithium was increased to 92. 25 wt%[38]. Liu et al. Used the carbothermal reduction method to study the effect of different carbon contents in NCM cathode powder on Li recovery[39]. It was found that the highest recovery rate of Li was 93 wt% when the cathode active material was calcined at 650 ℃ for 30 min, the C content was 10 wt%, and the liquid-solid ratio was 30 mL/G. When the C content in the raw material exceeds 10 wt%, the continuous increase of C content has no significant effect on the recovery of Li.
At present, the effect of Al on the recovery of Li in the carbothermal reduction process has also attracted the attention of researchers. When the calcination temperature exceeds 500 ℃, Li and Al in the cathode active material will form a water-insoluble LiAlO2, so that this part of lithium can not be recovered through water immersion into the solution, reducing the recovery rate of Li. Therefore, in order to reduce the formation of LiAlO2, Hu et Al. Used NaOH solution to remove Al from the raw material before calcination[40]. After alkali washing, Li leaching rate of 84.7 wt% was achieved by roasting under optimum conditions with lignite as reductant.
In addition to Al, other impurities in the material will also affect the efficiency of lithium extraction. Caibin Liao et al. Studied the effect of F and P impurities on lithium extraction from ternary cathode active material by carbothermal reduction-water leaching, and found that F and P could partially convert Li in the cathode material into slightly soluble LiF and Li3PO4, reducing the leaching rate of Li[41]. In order to eliminate the influence of F on the selective extraction of lithium, Zhao et al. Used alkali washing, water washing, and calcium oxide or calcium oxide hydrogenation in the pretreatment stage to remove 99% of F in the cathode active material powder[42].
The advantages of carbothermal reduction are short process, simple operation and relatively good selective lithium extraction effect. However, due to a small amount of toxic and harmful tail gas produced in the roasting process and the high liquid-solid ratio in the leaching process, further optimization is needed. At present, NCM ternary cathode powder is the main material for carbothermal reduction, while other cathode materials such as NCA, LCO and LFP are seldom studied.
2.1.3 Hydrogen reduction
The hydrogen reduction method uses hydrogen as a reducing agent to selectively convert lithium into water-soluble lithium oxide or lithium hydroxide during the calcination of the cathode material, and then selectively extract lithium by water leaching. The main reactions occurring in the recovery process are shown in formulas (1-5)[43][44]. Compared with carbothermal reduction, hydrogen reduction can convert lithium in the cathode material into LiOH (125 G/L, 45 ° C), which is more soluble in water than Li2CO3(13 g/L,20℃), and reduce the water consumption in the leaching stage, thereby reducing the energy consumption required for the subsequent evaporation and crystallization stage and reducing the production cost[45][46].
Co3O4 + H2(g) = 3CoO + H2O(g)
CoO + H2(g) = Co + H2O(g)
MnO2+H2(g)=MnO+H2O(g)
NiO+H2(g)=Ni+H2O(g)
Li2O+H2O=2LiOH
The effect of impurity Al on the purity of the product should be considered in the selective extraction of lithium by hydrogen reduction. Liu et al. Used hydrogen reduction method to study the selective extraction of lithium from NCM cathode material, and found that lithium in the cathode material could be selectively converted into LiOH·H2O only by hydrogen reduction roasting at 500 ℃ for 15 min, and the converted roasted product could be directly used to prepare battery-grade LiOH·H2O with a purity of 99.92 wt% by water immersion and evaporation crystallization[47]. However, the product prepared by this method contains 0.008 wt% Al impurity, and the electrochemical performance of the synthesized cathode material is slightly worse than that of other similar cathode materials. Liu Cheng et Al. Also found that the alkaline LiOH solution obtained by water leaching would dissolve Al and Al2O3 in the hydrogen reduction-water leaching ternary cathode material, so that 14.7 wt% of the aluminum in the raw material entered the solution, and the purity of the final LiOH product was 95.4 wt%[48]. Therefore, in order to make the product more pure, the Al3+ in the solution should be removed in advance before evaporative crystallization. In the hydrogen reduction-water immersion experiment, Huang et al. Adjusted the pH value of the solution to 7.0 by 1.2 M HCl, so that the Al3+ in the solution was precipitated in the form of Al(OH)3, and finally obtained the LiCl product with a purity of 99.98 wt%[49].
The removal of impurity Al has an effect on the purity of the product, and the residual binder and graphite in the raw material, the CO2 in the air, the calcination temperature and the thickness of the material layer all affect the process of selective extraction of lithium by hydrogen reduction. For example, the residual binder and graphite in LCO will hinder the process of hydrogen reduction and slow down the reduction rate[50]. LiOH after hydrogen reduction will react with CO2 in the air to form Li2CO3, so that the prepared LiOH product still contains part of Li2CO3, as shown in formula (6)[51]; When the calcination temperature exceeds 1000 ℃, the NCM cathode material will be transformed into water-insoluble LiAlO2 and LiMnO2, which will reduce the recovery rate of Li[49]. The increase of the thickness of the material layer will reduce the effective contact area between hydrogen and the cathode material, and reduce the recovery rate of Li[47]. Therefore, it is necessary to pretreat the cathode material before hydrogen reduction to improve the recovery rate and purity of lithium products.
2LiOH+CO2(g)=Li2CO3+H2O
2.1.4 Aluminothermic reduction
Aluminothermic reduction is to use the aluminum foil in the electrode material itself as a reducing agent to selectively convert lithium in the cathode material into LiAlO2 and Li2O soluble in alkali liquor, so as to realize the selective extraction of lithium, and the recovery process is shown in Fig. 3. In the experiment, Wang et Al. First baked the mixed powder of aluminum foil and LCO at 600 ℃ for 60 min in argon atmosphere, and then leached the baked product with 2. 5 mol/L NaOH solution at 95 ℃ for 150 min, so that 93. 67 wt% of Li and 95. 59 wt% of Al in the cathode material were leached[52]. However, the solution after alkaline leaching contains Al3+ and Na+ in addition to Li+, which needs further separation to obtain pure lithium products. For this purpose, Na3PO4 can be used to precipitate the Li+ in the lye to obtain a Li3PO4 product. The experimental results show that more than 91. 5 wt% of the Li+ can be separated in the form of Li3PO4 precipitation by adding 1. 5 times the theoretical amount of Na3PO4 to the leached alkali liquor at 80 ℃ and stirring for 120 min.
The content of F in the cathode material has an effect on the leaching rate of Li[53]. For example, the polyvinylidene fluoride (PVDF) in the raw material will be decomposed by heating to produce HF, which will react with Li2O to produce slightly soluble LiF that cannot be completely leached by NaOH solution, and the reaction is shown in formula (7)[54]. In addition, the adsorption of Mn oxide may also cause the decrease of Li leaching rate[55].
Li2O+2HF(g)=2LiF+H2O
The advantage of selective lithium extraction by aluminothermic reduction is that it does not need to add additional reducing agent, does not need to separate the cathode material from the aluminum foil in advance, and compared with carbothermic reduction, the temperature required by aluminothermic reduction is lower, which can reduce the energy consumption required by the reaction[52][53]. However, this method adds complexity to the extraction of lithium from the leachate because LiAlO2 is insoluble in water and other metal ions are introduced during the extraction of lithium from LiAlO2 using lye.
2.1.5 Nitrating roasting
The characteristic of nitration roasting method is that nitric acid and positive powder will react when they are mixed, so that lithium and other metal elements in the active material will be converted into highly water-soluble LiNO3(900 g/L,28℃) and other metal nitrates[56]. Because all metal nitrates in the cathode material are soluble in water, lithium cannot be selectively separated by water immersion at this time, so further roasting is needed after nitration to realize the preferential extraction of lithium[57]. The roasting treatment is to take advantage of the difference between the decomposition temperatures of LiNO3 and other metal nitrates, as shown in Table 2, the decomposition temperature of LiNO3 is 913 ± 100 K, while the decomposition temperatures of Ni, Co, Mn, Al, Fe, Cu nitrates are all below 600 K[58]. Therefore, Co(NO3)2, Ni(NO3)2 and Mn(NO3)2 can be decomposed into water-insoluble oxides by controlling the calcination temperature, while LiNO3 still exists in the form of nitrate, and finally the selective separation of lithium and other metal elements can be achieved by water leaching[59].
The roasting process can not only decompose the nitrate, but also make the unnitrated lithium in the cathode material further react with other metal nitrates to form LiNO3, which can improve the leaching rate of Li. However, the LiNO3 in the roasting product may be coated by other metal oxides to limit the leaching of Li, so it is necessary to increase the liquid-solid ratio or adopt multiple leaching in the leaching stage to increase the leaching rate of Li[57]. The advantage of nitration roasting is that the selective conversion of lithium can be achieved at a lower temperature, while the disadvantage is that nitric acid is highly corrosive, which requires high production equipment and the roasting process will form harmful nitrogen oxides to pollute the environment[59][56][59].
2.1.6 Chloridizing roasting
Because LiCl is easily soluble in water (550 G/L, 25 ℃), as long as Li in the cathode material can be selectively converted into LiCl which is easily soluble in water, and other metals exist in the form of oxides or simple metals, the selective leaching of Li can be achieved by water leaching[60]. When C and NH4Cl were used as chlorinating agents and cathode materials for co-calcination, Li and other metal elements would be chlorinated by the chlorinating agents, and the selective extraction of lithium could not be achieved[62]. For example, Barrios et al. Used Cl2 as a chlorinating agent to bake the ternary cathode material at 400 ℃ for 90 min and then leach it in water, only about 40 wt% of Li could be selectively leached[61]. When the roasting temperature is increased to 500 ℃ for 90 min, the leaching rate of Li increases to 55.21 wt%, but 70.81 wt% and 29.22 wt% of Mn and Ni are leached at the same time. When the roasting temperature is increased to 900 ℃, the leaching rates of Li, Ni, Co and Mn all reach 100 wt%. Experiments show that the selective chlorination of lithium can not be achieved by co-calcination of Cl2 and ternary cathode materials. Qu et al. Used NH4Cl as a chlorinating agent to convert 99 wt% of Li and 90 wt% of Co in LCO into water-soluble chlorides by calcination at 300 ° C for 20 min, without achieving selective chlorination of lithium[63].
CaCl2 has been reported to selectively chlorinate NCM cathode materials, and the reaction process is shown in Figure 4[64]. First, the hydrate formed after the CaCl2 absorbs the moisture in the air will be heated and decomposed to produce HCl and Cl2, and at the same time, the cathode material will be heated and decomposed to produce Li2O, which will react with HCl, Cl2 and CaCl2 to obtain LiCl. In this process, NiO, CoO and MnO2 produced by the thermal decomposition of cathode materials will not participate in the reaction. Experiments show that 94 wt% of Li in the cathode material can be leached by water leaching after NCM and CaCl2 are calcined at 800 ℃ for 60 min in air, and the leaching rate of other metals is less than 1 wt%[64]. It is worth noting that due to the similar chemical properties of lithium and calcium, the introduction of Ca2+ in the reaction process will lead to a higher content of calcium impurities in the prepared lithium product, which will affect the purity of the product, and further impurity removal is needed to obtain a higher purity lithium product.
In addition, the atmosphere of roasting has an effect on the chlorination process. Xiao et al. Found that oxygen could control the degree of chlorination reaction in the roasting process and realize the selective chlorination of lithium when they used NH4Cl as chlorinating agent to study the selective extraction of lithium[65]. Under oxygen atmosphere, 91.73 wt% of Li and 6.49 wt% of Mn in the cathode material can be leached into the solution by mixing LMO and NH4Cl at a molar ratio of 1 ∶ 3.5, calcining at 550 ℃ for 30 min, and then leaching in water. Therefore, the selective chlorination of lithium can also be realized by controlling the degree of chlorination reaction by oxygen atmosphere during calcination.
2.1.7 Sulfation roasting
Sulfation roasting is to selectively convert lithium into water-soluble lithium sulfate and nickel, cobalt and manganese into water-insoluble oxides by co-roasting the cathode active material with concentrated sulfuric acid or sulfate, so that lithium is selectively separated from nickel, cobalt and manganese in the leaching stage. For example, when NaHSO4·H2O and LCO are Co-calcined, the SO3 generated by thermal decomposition of NaHSO4·H2O will form water-soluble Li2SO4 and insoluble Co3O4 with LCO at high temperature, so that Li and Co are separated in the water leaching stage, as shown in equations (8 – 9)[66]. Different from the sulfate reaction process, concentrated sulfuric acid and NCM will react differently when they are mixed and roasted.
When they are mixed at room temperature, concentrated sulfuric acid will react with some NCM cathode materials to form Li2SO4, NiSO4, CoSO4 and MnSO4.The Li+ in the positive electrode material which does not react with the concentrated sulfuric acid in the heating process is related to the NiSO4, the CoSO4,The S in the MnSO4 is combined with the to form Li2SO4,Ni which is easy to dissolve in water, Co and Mn are converted into oxides which are insoluble in water,Li is selectively separated from Ni, Co and Mn in the water leaching stage[67].
2NaHSO4·H2O = 3H2O(g) + SO3(g) + Na2SO4
12LiCoO2 + 6SO3(g) = 6Li2SO4 + 4Co3O4 + O2(g)
In the experiment of Lin et al., the LCO cathode powder was mixed with H2SO4, calcined at 800 ℃ for 2 H, and the calcined material was leached in water, and the leaching rate of Li could reach 99. 3 wt%[68]. By controlling the consumption of sulfuric acid, S is recycled in the form of S rather than the discharge of SOx, which can improve the utilization efficiency of S and reduce the harm to the environment. The process principle of this method is shown in Figure 5.
In addition to waste LiCoO2 cathode materials, Lin et al. Studied the selective extraction of lithium from nickel cobalt lithium manganate (NCM523) cathode powder by sulfation roasting[67]. The leaching rate of Li can reach 96. 92 wt%, the leaching rates of Al and Cu are less than 0. 1 wt%, and the leaching rates of Ni, Co and Mn are 0. 15 wt%, 0.33 wt% and 1. 04 wt% respectively after the cathode powder is fully mixed and dried with 98 wt% concentrated sulfuric acid and calcined at 800 ℃ for 120 min, showing good selectivity.
In addition, Wang et al. Mixed LCO battery powder with NaHSO4·H2O in different proportions and calcined at 600 ° C for 0.5 H[66]. It was found that with the increase of the amount of NaHSO4·H2O, the LiCoO2 was gradually transformed into insoluble Co3O4 and soluble LiNa(SO4) during the roasting process, thus realizing the selective separation of Li and Co, and the recovery rate of Li could reach 72. 56 wt% under the optimized conditions, but there were Na+ in the solution obtained by this method, which reduced the purity of Li.
2.1.8 Oxidizing roasting
Oxidizing roasting is suitable for the selective extraction of lithium from LFP, and its principle is that LFP is roasted in an oxidizing atmosphere and converted into Li3Fe2(PO4)3 and Fe2O3, which are then leached with sulfuric acid, and Fe in both of them is selectively separated from lithium in the solution in the form of FePO4 precipitation after being leached by sulfuric acid, as shown in Equation (10 – 11)[69⇓~71]. For example, Tao et al. Roasted LFP powder at 600 ° C in air atmosphere for 1 H, and then leached it at 85 ° C for 4 H with a molar ratio of 0.28 M H2SO4 to the roasted product nLi/nH2SO4=2.63, and the leaching rate of Li reached 98.48 wt%, while only 0.01 wt% of Fe entered the solution[69]. The filtered lithium-containing solution was purified by NaOH, and then Na3PO4 was used as a precipitant to obtain Li3PO4.
The oxidizing roasting of LFP can be transformed into Li3Fe2(PO4)3 and Fe2O3, which can avoid the use of oxidant in the sulfuric acid leaching stage, so that Li and Fe can be effectively separated and the cost of reagents can be reduced. In addition, PVDF and other binders will be decomposed during the oxidation roasting process, so that the cathode material and aluminum foil can be separated while the LFP is oxidized and transformed[70].
2.2 Selective leaching
Selective leaching refers to the method of directly leaching lithium from cathode materials into solution and separating it from other valuable metals. The method avoids the loss of lithium caused by the recovery of lithium in the later stage after the leaching of all components of the cathode material, improves the recovery rate of lithium, and has high research value[72].
2.2.1 Acid leaching
For LFP cathode materials, H2SO4 can selectively leach lithium from LFP, and Li et al. Use H2SO4-H2O2 system to separate Li and Fe from waste LFP powder with H2SO4 as leaching agent and H2O2 as oxidant[73]. The addition of H2O2 and H2SO4,Fe2+ under optimal conditions converted to FePO4 precipitates. The Li3PO4 precipitate was obtained by adding Na3PO4 to the remaining solution, and the final Li leaching rate was 95. 56 wt%.The olivine structure FePO4 could be obtained by calcining the leaching residue at 600 ℃.
Compared with sulfuric acid as leaching agent, phosphoric acid is suitable for a wider range of cathode materials, including LFP, LCO, NCM and other cathode materials. Chen et al. Used H3PO4 and H2O2 to leach LCO in the experiment, in which cobalt was separated from Li+ in the form of Co3(PO4)2 precipitation, and the reaction in this process was shown in equations (12 – 15)[74]. In the leaching process, both Li and Co may exist in the form of ions or precipitates, so it is necessary to control the conditions to make the Li+ enter the solution and avoid the dissolution of cobalt. The results show that the leaching rate of Li is more than 99 wt% and the leaching rate of Co is less than 1 wt% when the LCO cathode powder is leached with 0.7 M H3PO4 and 4 vol%H2O2 as leaching agents at 40 ℃ for 60 min with a liquid-solid ratio of 20 ∶ 1. To further investigate the adaptability of this method to other types of cathode materials, Chen et al. Used H3PO4 to study the selective leaching of LCO, LMO, LFP and NCM[19]. It is experimentally proved that 100 wt%, 92.86 wt%, 97.57 wt% and 98.94 wt% of Li in LCO, LMO, LFP and NCM can be leached using 0.8 M H3PO4 and 4 vol%H2O2 as leaching agents, while only a small amount of Ni2+ and Mn2+ are leached. The small amount of Ni2+ and Mn2+ leached can be removed as NiC2O4 and MnC2O4 precipitates by adding H2C2O4, achieving further purification of the lithium-containing solution.
In addition to inorganic acids such as sulfuric acid and phosphoric acid, organic acids can also realize the selective extraction of lithium, for example, Zeng et al. Used oxalic acid (H2C2O4) to selectively leach lithium from LCO cathode materials, and the reaction process is shown in Equation (16)[75]. Due to the reducibility of oxalic acid solution, the effective leaching of LCO can be achieved without adding reducing agents such as H2O2, and the Co in the leaching solution can be directly separated from the Li+ in the solution in the form of CoC2O4 precipitation. The experimental results show that 98 wt% of Li in LCO can be leached and 97 wt% of Co can be recovered in the form of CoC2O4 precipitation under the conditions of LCO and oxalic acid at a solid-liquid ratio of 15 G/L and a leaching temperature of 95 ℃ for 150 min. Yang et al. Used the organic acid acetic acid as the pickling agent and H2O2 as the oxidant to pickle the LFP cathode powder[76]. The results show that the leaching rate of Li can reach 94. 57 wt% under the optimal conditions. Subsequently, Na2CO3 saturated solution at 95 ° C was used to treat the concentrated leachate to precipitate the Li solution into Li2CO3, and the final recovery rate of Li was 84.76 wt%, and the recovery process is shown in Fig. 6.
5H2C2O4+2LiCoO2=2LiHC2O4+2CoC2O4(s)+4H2O+2CO2(g)
2.2.2 Oxidation leaching
Oxidative leaching is a new method in which only oxidant is added to extract Li preferentially from LFP during leaching. Zhang et al. Used Na2S2O8 to recover Li and FePO4 from waste LFP materials[77]. The experimental results show that 99. 9 wt% of Li can be leached at room temperature by adding 1. 05 times the theoretical amount of Na2S2O8, solid-liquid ratio of 300 G/L and 25 ℃. However, the leaching rates of Al, Fe and P are very small, which are 0.584 wt%, 0.048 wt% and 0.587 wt%, respectively, showing the high selectivity and efficiency of this method.
Compared with the conventional leaching method, the method directly uses Na2S2O8 as a leaching agent and an oxidant, simplifies the types of reagents added in the process, and is easier to realize selective extraction. Without prior separation of the cathode active material and Al foil, this method reduces the influence of Al and leaches very low impurities, so it is a concise and environmentally friendly new method.
2.3 Mechanical/electrochemical strengthening
In addition to the above methods, mechanochemical and electrochemical methods can also be used to selectively extract lithium from cathode materials. The mechanochemical method is to selectively convert lithium in the cathode material into water-soluble lithium salt through a series of physical and chemical reactions between the cathode material and the grinding medium under the action of mechanical forces such as crushing and grinding, so as to realize the selective separation of lithium from other valuable metals in the leaching stage[78]. Wang et al. Experimentally demonstrated that the co-grinding of LCO, PVC (polyvinyl chloride) and iron powder could achieve the selective conversion of lithium[79]. When Fe is co-ground with PVC, reduction reaction will occur, which will promote the dechlorination of PVC and the combination of Li to form LiCl. In this process, Co and Fe are recombined to form magnetic material CoFe4O6. When LCO/PVC/Fe was ground at a mass ratio of 1 ∶ 1:2 for 12 H and then leached in water, the leaching rate of Li was close to 100 wt%, and the leaching rate of Co was 8. 1 wt%. Liu et al. Added Na2S2O8 to the recovered LiFePO4, mechanically activated it with a planetary ball mill, and oxidized the LiFePO4 into FePO4 and soluble Li salt with the Na2S2O8.After washing with water and adding Na3PO4, Li was recovered as a Li3PO4 precipitate, achieving a Li recovery of 99.7 wt% and a Fe recovery of 19.7 wt%, demonstrating good selectivity[80]. The chemical reaction and recovery process involved in the reaction are shown in Formula (17 ~ 18) and Figure 7.
Mechanical ball milling, a solid-solid reaction that does not use water as a medium, can be carried out at room temperature and does not produce acidic wastewater, compared with the previous pyro-wet method, which is a more environmentally friendly and technically easy treatment method. However, this method will produce a lot of noise in the grinding process, as well as the equipment loss and energy consumption caused by long-term grinding, which limits its application in industry.
In addition to mechanical strengthening, electrochemical methods can also be used to selectively extract lithium from cathode materials. For example, Fan et al. Directly prepared metallic lithium foil using an electrochemical method[81]. The method uses LCO or LFP as the anode, copper foil as the cathode, and organic electrolyte to avoid hydrogen evolution during electrolysis. In the process of electrolysis, lithium is removed from the anode and deposited on the cathode to form metallic lithium foil. Based on the combination of LFP charging principle and traditional slurry electrolysis, Li et al. Used an anion diaphragm to divide the electrolyzer into a cathode chamber and an anode chamber, with NaCl as the electrolyte[82]. As shown in Fig. 8, hydrogen evolution reaction occurs in the cathode chamber during electrolysis to produce NaOH, and LFP in the anode chamber successfully desorbs lithium and forms FePO4 under the action of current. Under the optimum conditions, more than 98 wt% of Li was leached and more than 96 wt% of Fe was recovered as FePO4/C, and the olivine-type structure was maintained. In addition, Yu et al. Used [Fe(CN)6]3- as a redox mediator to convert LiFePO4 into FePO4 and Li+,Li+ through a cation exchange membrane to form LiOH at the cathode[83]. The removal rate of Li can reach 99. 8 wt% at room temperature for 50 min, and the purity of the recovered FePO4 is 99. 97 wt%. The redox medium [Fe(CN)6]3- can be continuously regenerated in oxygen, and the electrolyte can be recycled for many times, thereby reducing the cost of the reagent.
The electrochemical lithium extraction method does not consume acidic reagents in the whole recovery process, only uses electric energy, and does not need to separate the aluminum foil and the cathode material in advance before the reaction, so it is a more environmentally friendly extraction method. However, because the cathode active material coated on the aluminum foil is very thin, for example, the thickness of the ultra-thick NCM cathode material studied by Gao et al. Does not exceed 300 μm, the lower lithium content in the cathode sheet per unit area will limit the actual production efficiency[84].
2.4 Comparison of advantages and disadvantages of different selective lithium extraction methods
The advantages of high temperature transformation are large processing capacity, good selectivity, low reagent cost and relatively easy to realize industrialization, but the disadvantages are higher energy consumption compared with selective leaching, and the tail gas generated in the roasting process needs to be absorbed and purified to meet the emission standards. The types of cathode materials suitable for different high-temperature transformation methods, the required calcination temperatures, and the leaching systems required for different calcination products are shown in Figure 9. It can be seen from the figure that the high temperature transition has the following three characteristics: first, different types of cathode materials are suitable for different high temperature transition methods. Reduction roasting and salting roasting are suitable for treating LCO, NCM and LMO materials, while LFP requires oxidizing roasting; Secondly, most of the high temperature transformation reactions need to be carried out at 600 ~ 800 ℃, so the energy consumption of this method is higher than that of selective leaching. Third, water can be directly used as a leaching agent in most methods, and only the LiAlO2 and Li3Fe2(PO4)3 produced by aluminothermic reduction and oxidizing roasting need alkaline and acidic reagents to leach lithium.
The advantages of selective leaching are short process and low energy consumption, while the disadvantage is that the leaching process will consume a large amount of acidic reagents, and the resulting wastewater is easy to cause secondary pollution in the treatment process. In addition, hydrogen peroxide needs to be added as a reducing agent or oxidant to enhance the leaching effect during the acid leaching of the cathode material, which increases the cost. In the selection of leaching agent, organic acid has less impact on the environment than inorganic acid, but the higher price of organic acid will lead to higher treatment costs. However, due to the weak acidity of organic acids, it is often necessary to strengthen leaching under the condition of large liquid-solid ratio (L/S > 50 L/G), which further increases the cost of lithium extraction[85]. The reagents used in the selective leaching process are shown in Table 3.
The method of reduction roasting and selective leaching can also reduce nickel, cobalt and manganese in LCO, NCM and LMO to lower valence state which is easier to leach while extracting lithium, thus reducing the difficulty of subsequent leaching recovery of nickel, cobalt and manganese and the cost of reagents. The advantage of mechanochemical method and electrochemical method is that the process of lithium extraction does not consume acid and alkali, does not produce toxic gases, so it is more environmentally friendly, but the disadvantages of the two methods are also obvious, the reaction time of mechanochemical method is long, the equipment loss is large, and the treatment capacity of electrochemical method is small, so the two methods have not yet been applied on a large scale in industry. See Table 4 for the comparison of the characteristics of lithium extraction by high temperature transformation, selective leaching, mechanochemical method and electrochemical method.
2.5 The effect of impurities
The cathode powder obtained by crushing and sorting of waste lithium-ion batteries will inevitably be mixed with a certain amount of impurities such as aluminum foil, electrolyte and binder, and the cathode powder of lithium iron phosphate may also be mixed with LCO and NCM cathode materials[41]. The impurities contained in these cathode materials will have different effects on the selective extraction of lithium.
The existence of F, P and Al in the process of high temperature transformation will affect the effect of selective lithium extraction. This is due to the fact that Li is easy to form water-insoluble LiF, Li3PO4 and LiAlO2 with it at high temperature, resulting in the decrease of Li leaching rate in the water leaching stage. In addition, the cathode materials containing Cu and Al impurities will transform into water-soluble CuSO4 and Al2(SO4)3 during sulfation roasting, which will enter the solution during water immersion and increase the burden of subsequent impurity removal. In addition, when LCO, NCM and LMO are mixed with LFP impurities, lithium in LFP can not be effectively recovered due to their different recovery principles[41,86,87]. When a certain amount of carbon is mixed into the raw material, salinization roasting can be coupled with carbothermal reduction to improve the reaction efficiency. If the cathode material mixed with a certain amount of carbon is sulfated, the temperature required for the reaction can be reduced, and the nickel, cobalt and manganese can be reduced to divalent which is easier to leach[88]. When 20 wt% carbon is mixed into the LCO powder, the temperature required for the sulfation roasting reaction is reduced from 800 ℃ without carbon to 600 ℃, and Co is reduced from trivalent to divalent, which is more easily leached. The reaction is shown in Equation (19)[89].
4LiCoO2 + 2H2SO4 + C =2Li2SO4 + 4CoO + CO2 + 2H2O
The effect of selective leaching is also affected by the impurity Al mixed in the cathode material. For example, in the acid leaching process, Al mixed in the cathode material will dissolve into Al3+ and enter the solution, and the filtered solution contains both Li+ and Al3+. When Li is recovered in the form of lithium carbonate, Al is easy to form aluminum hydroxide precipitate in this process, which cannot be separated from the precipitated lithium carbonate, resulting in high aluminum impurity content in the product[90,91].
3 Recovery of lithium from spent LIBs electrolyte
Lithium in spent lithium-ion batteries exists not only in the cathode material, but also in the electrolyte in the form of electrolyte lithium salt. The electrolyte is mainly composed of organic solvent, electrolyte lithium salt and a small amount of additives, and the commercial electrolyte is mainly LiPF6. Because the heat generated in the battery grinding process will evaporate most of the organic solvent and decompose the LiPF6 into LiF and PF5, many enterprises do not design the treatment steps for the electrolyte in the recycling process, so that the residual electrolyte in the electrode material will decompose into CO2 and fluorine-containing flue gas during the pyrogenic treatment. Fluorine ions in the electrolyte are dissolved into acid during wet treatment, resulting in air and water pollution[92]. At present, most enterprises have not achieved low-cost and effective recovery of electrolyte, mainly because the electrolyte is mostly dispersed in the pores of electrodes and diaphragms, which is difficult to recover, costly and risky[93,94]. The recovery of lithium from electrolyte is of great significance, which can not only realize the recycling of resources, but also reduce the environmental pollution caused by the recovery process.
The steps of lithium recovery from electrolyte mainly include electrolyte collection, distillation of organic solvent to obtain LiPF6 crude product, and direct purification and recovery of crude product or conversion into stable lithium salt, as shown in Fig. 10. There are four main methods to collect the electrolyte: one is the freezing method in which the electrolyte is converted into a solid state at low temperature and then further separated from the cathode material; The second is the mechanical method of separating the electrolyte from the cathode material under centrifugal force and certain pressure by using a centrifuge or a crushing device; The third is the solvent extraction method which uses organic extractant to extract the electrolyte. And the fourth is a supercritical extraction method for extracting the electrolyte by using a supercritical extractant[95].
After the electrolyte is collected, the lithium in the electrolyte can be recovered by direct recovery method or lithium salt conversion method. The direct recovery method is to separate the organic solvent from the electrolyte lithium salt in the electrolyte by controlling a certain temperature and vacuum degree based on the difference between the boiling points of the organic solvent and the electrolyte lithium salt to obtain a crude product of the LiPF6, and then further purify the crude product to obtain the high-purity LiPF6. The method for purifying the crude product of the LiPF6 comprises the following steps of: firstly, dissolving the crude product of the LiPF6 by using HF solution; and carrying out low-temperature crystallization, filtration, screening and drying on the filtered solution to obtain a high-purity LiPF6 product[96]. The advantage of this method is that the lithium salt recovery process is environmentally friendly and clean, and it is not easy to form secondary pollution, but the vacuum distillation and low-temperature crystallization process need to consume more energy, so the economy is poor[97]. The lithium salt conversion method is to convert the crude product of LiPF6 obtained by distillation or rectification of electrolyte into stable lithium salt as the product to be recovered. The reason for using this method is that LiPF6 is easy to hydrolyze and difficult to purify, while transforming it into stable lithium salt can reduce the requirements of operation conditions and is more conducive to industrial application. Zhu et al. Have disclosed a method for recycling LiPF6 in an electrolyte[98]. The method comprises the following steps of: firstly, mixing the collected electrolyte with dilute sulfuric acid, performing atmospheric azeotropic distillation, and recovering the obtained mixed gas after distillation, wherein the residual concentrated solution is a lithium sulfate solution, and the related reactions are shown in formulas (20-22). And then calcium oxide or calcium hydroxide and other calcium-containing compound are added into that concentrate solution to fix phosphate radicals in the form of Ca3(PO4)2 to obtain a relatively pure lithium sulfate solution. Compared with the direct recovery method, the lithium salt transformation method reduces the difficulty and cost of lithium salt recovery, and has more potential for industrial application.
LiPF6 = PF5(g)+ LiF
PF5(g) + 4H2O = H3PO4 + 5HF(g)
2LiF + H2SO4 = Li2SO4 + 2HF(g)
4 Summary and Prospect
As an important secondary lithium resource, the selective extraction of lithium from spent lithium-ion batteries can not only effectively improve the recovery rate of lithium, alleviate the contradiction between supply and demand of strategic metals, but also reduce the environmental load to a certain extent. In this paper, the research on selective extraction of lithium from cathode materials of spent lithium-ion batteries is systematically sorted out and analyzed, and it is considered that the future research in this field should focus on the following six aspects:
(1) Pay equal attention to environmental protection and safety in the process. The advantages of lithium extraction by carbothermal reduction are low cost and safe production environment, but the recovery rate of lithium is relatively low. In contrast, the hydrogen reduction process does not produce waste gas, and the roasting product can be directly extracted from lithium by water leaching, so the process is clean and environmentally friendly. However, there are certain safety risks in the preparation, transportation, storage and use of hydrogen. Therefore, improving the safety of hydrogen reduction process and developing new efficient, clean and safe processes are worthy of further study.
(2) Improve the degree of automation in the preprocessing stage. At present, the main raw material for selective lithium extraction is cathode active material, but limited by the different specifications of lithium-ion batteries, many links in the pretreatment process still need manual operation to complete. However, manual operation is inefficient and dangerous, so the development of automated and mechanized pretreatment technology will greatly improve the overall efficiency of the waste lithium-ion battery recycling industry.
(3) Reduce the introduction of impurity elements. In the pretreatment stage, some impurities, such as Al, F, P, etc., often enter the cathode active powder during the disassembly, crushing and sorting of the battery. The content of these impurity elements will have a significant impact on the effect of selective extraction of lithium. For example, the residual elements such as Al, F and P in the cathode material during high-temperature transition will form slightly soluble or insoluble LiAlO2, LiF and Li3PO4 with Li, which will reduce the recovery rate of lithium. Therefore, how to reduce the introduction of impurities in the pretreatment stage or remove the impurity elements in the cathode powder in advance before the extraction of valuable metals is worthy of further study.
(4) Increase the recovery rate of Li in the LiFePO4 cell and improve the electrochemical performance of FePO4. The special structure of LiFePO4 can selectively recover Li while maintaining the olivine structure of FePO4, so that it can be reused as cathode material. This method not only takes into account the recovery of lithium, but also realizes the regeneration of electrode materials. With the increasing market share of LiFePO4 batteries, how to improve the recovery rate of Li and the electrochemical performance of recycled FePO4 is one of the future research directions.
And (5) improving the selectivity of Li and the purity of the product. In the process of selective extraction of lithium, the purity of the product is affected by many factors. If the selectivity of the roasting process or the leaching process is not good, impurities such as Ni, Co, Mn, Al and Cu will be introduced into the lithium product. Therefore, the higher the selectivity in the extraction process, the lower the cost of the later purification process.
(6) Explore the method of electrolyte recovery. At present, the research on waste battery recycling mainly focuses on the recycling of electrode materials, and the research on electrolyte recycling is less. In the pretreatment stage, the electrolyte can be effectively recovered, the fluorine-containing gas can be safely disposed, and the comprehensive utilization rate of F element in the battery can be improved, so that the harm of toxic waste gas and waste liquid can be effectively reduced, and the production and environmental risks can be avoided.
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