Basic Research On Novel Deep Eutectic Solvents For Efficient Extraction And Recovery Of Cathode Active Materials From Spent Lithium Ion Batteries

Deep eutectic solvents(DESs),as a new type of green solvent,are room temperature liquids formed by hydrogen bond donors(organic acids,amides,etc.)and hydrogen bond acceptors(choline chloride,organic alcohols,etc.).Due to its strong solubility and recyclability,the DES has become a hot spot in the research of recycling cathode electrode active materials of spent lithium ion batteries(LIBs).However,the reported DESs leaching system has some shortcomings,such as high solvent viscosity,harsh leaching conditions and difficult recovery of valuable metals,which limit its industrial application.The key to solve the problem lies in the cooperative construction of DESs efficient coordination components and selective reduction/oxidation systems.Therefore,in this thesis,a series of new green DESs were designed and synthesized based on the composition characteristics of spent LIBs cathode active materials.The influences of experimental conditions on leaching and separating metal elements were systematically studied.And the efficient and selective recovery mechanism was revealed.Combined with the characteristics of various metal recovery processes,the recycling of DESs,and extraction and recovery of valuable metals were realized.For LiCoO2 and spent LIBs ternary cathode active materials Li14.8Ni1.7Co8.5MnO30.5,the low viscosity component glycol(EG)and the strong coordination component 5-sulfosalicylic acid dihydrate(SAD)were selected.And an acid-based DES(a solvent composed of 12 mol glycol and 1 mol dihydrate 5sulfosalicylic acid,12EG:1 SAD)with strong coordination ability and low viscosity was designed and synthesized,which realized the efficient and gentle recovery of valuable metals.The effects of DES composition and leaching conditions on the leaching efficiency of LiCoO2 were discussed in detail,and the leaching reaction mechanism was clarified.The results showed that under the optimal conditions(110℃,6 h,40 g/L,12EG:1 SAD),all the leaching efficiencies of metal elements in the two active materials were more than 93.5%.Li+was leached from LiCoO2 by the substitution reaction of H+in the solvent.Meanwhile,the intermediate H-Co(Ⅲ)O2 was reduced and coordinated to form soluble complex Co(II)-SA(Co2+-C7H4O6S)to obtain the efficient dissolution of cobalt.The leaching processes of lithium and cobalt were controlled by interfacial chemical reactions,and their apparent activation energies were 77.38 kJ/mol and 79.54 kJ/mol,respectively.Then,combined with oxalic acid precipitation method and distillation process,the efficient recovery of cobalt(99.5%,CoC2O4·2H2O)and regeneration of DES in the leaching solution were realized.The leaching efficiencies of lithium and cobalt(ηLi and ηCo)were always greater than 91.2%when the solvent was recycled three times.The ethylene glycol and lithium were recovered by vacuum distillation and high temperature calcination,then product Li2SO4 was gained.For LiCoO2 and Lii4.8Ni1.7Co8.5MnO30.5,the low viscosity and weak coordination component glycol and the selective coordination precipitated component oxalic acid dihydrate(OAD)were selected as raw materials to design and prepare the bifunctional acid-based DES(A solvent consisting of 5 mol glycol and 1 mol oxalic acid dihydrate,5EG:1OAD).The selective leaching of lithium and separation of transition metal oxalate precipitate were realized in one step.The influences of experimental conditions on the leaching of lithium and transition metals were investigated in detail.and the selective extraction and separation mechanism was clarified.Results demonstrated that under the optimal leaching conditions(90℃,5EG:1OAD,12 h,and 16 g/L),ηLi was greater than 94.1%,whileηNi/Co/Mn was less than 1.2%,and existed as(Ni,Co,Mn)C2O4·2H2O precipitation.The synergistic coordination and post-reduction reaction process of oxalic acid and glycol in DES was the key to efficient extraction and separation in situ ηLi was close to 100%while ηCo was close to 0%after three cycles of leaching solution,indicating that the DES had good cycling stability.In addition,ethylene glycol and lithium(ηLi up to 96.4%,Li2C2O4)were recovered by atmospheric distillation.For LIBs cathode active material LiFePO4,a selective oxidation leaching system of DES(8 ethylene glycol:1 choline chloride,8EG:1CC)combined with ozone was developed and constructed.Lithium was efficiently extracted and separated from iron in one step in situ,and the leaching mechanism of LiFePO4 was explained based on leaching effects.It is found that ozone atmosphere containing moisture had the best leaching performance.Under the optimal conditions(6 h,20 g/L,8EG:1CC,40℃),ηLi>92.2%while ηFe<1.6%,and Fe was recovered as FePO4 precipitate.Under ozone condition,the hydroxyl group in DES component was preoxidized to carboxyl group,meanwhile the intermediate H-Fe(II)PO4 was oxidized to Fe(III)PO4 and precipitated by ozone,which became the core of DES selective synergistic oxidation leaching of LiFePO4.In addition,the mean values of ηLi andηFe were 85.4%and 2.0%,respectively,when LiFePO4 was extracted by 8EG:1CC for ten cycles.After atmospheric distillation treatment(150℃.0.5 h),the trace impurities of iron(FeCl4-、FeCl2+和FeCl3(aq))were removed in the form of FePO4·3H2O,and a high purity LiCl was achieved.In summary,based on the three new synthesized DESs,the efficient extraction and recovery of valuable metals from different cathode active materials of spent lithium ion batteries were achieved,and multiple products such as Li2SO4,Li2C2O4,LiCl,and CoC2O4·2H2O were gained.The whole experimental process avoids the use of strong acids and alkalis,as well as discharge of wastewater,which provides a new way for the efficient and green recycling of spent lithium ion batteries.