Lithium Cobalt Oxide As Cathode Material For Aqueous Lithium-ion Batteries

Aqueous lithium-ion batteries,possessing low cost,high safety,high ionic conductivity,and considerable capacity(in the 1.8 V electrochemical window),are promising large-scale power sources for renewable energy storage systems.Two-dimensional layered cathode materials are suitable for lithium-ion batteries due to their macroscopic delocalization and facile Li⁺diffusion.However,the layered materials,including LiCoO₂,show severe capacity decay in the neutral aqueous electrolyte,which seriously hinders the practical application of aqueous lithium-ion batteries.In this thesis,LiCoO₂ is taken as the research object.The decay mechanism in aqueous electrolytes with various p H values is investigated.It is found that the capacity of LiCoO₂ decreases rapidly due to the unstability of the electrode-electrolyte interface during charging and discharging.These findings can provide fundamental basis for solving the unstability issues of layered materials in aqueous electrolyte.Three effective methods,tailoring the covalency of Co-O,synthesizing hetero-layered H_0.5Li_0.5Co O₂,and synthesizing layered-spinel nanodomains,are proposed for the structural modification of layered LiCoO₂.The specific research contents are as follows,(1)The decay mechanism of LiCoO₂ in a neutral 1 M Li₂SO₄ aqueous electrolyte is investigated.We find that the OH group formation on edge sites of LiCoO₂ such as(104)and(110)surfaces will promote the migration and dissolution of Co element on the surface of LiCoO₂ and accelerate the surface phase transition during charge processes.The great lattice strain and distortion lead to the internal destruction of LiCoO₂.Based on these findings,we tailor the Co 3d-O 2p covalency by introducing excess Li in LiCoO₂ to accommodate the serious structural instability in aqueous electrolyte.The substitution of Li in Co site induces Co O₅ square-based pyramids and shortens Co-O lengths in LiCoO₂.The intercalation of H₂O molecules in LiCoO₂ is observed for the first time.The Li layer distance increased to?6.77?,which can facilitate Li⁺diffusion and realize the reversible Co migration.The lithium-excess Li_1.08Co O₂ shows capacity retention of 88%after 400 cycles at 1 A g^-1,demonstrating excellent structural stability in neutral aqueous electrolyte.The lithium-excess Li_1.08Co O₂ can deliver?130 m Ah g^-1 at 0.1 A g^-1 and?85 m Ah g^-1 at 30 A g^-1,indicating superior electrode kinetics.Our findings and successful intrinsic modification enlighten a new path way for high performance aqueous lithium-ion batteries.(2)Single-or few-layer transition metal(TM)(oxy)hydroxide and oxide nanosheets are promising two-dimensional(2D)nanomaterials for catalysis and energy storage owing to their plentiful active sites.Facile and scalable synthesis of these 2D ultrathin nanosheets,however,is still a great challenge.Here,we report a liquid phase epitaxy(LPE)method to synthesize single-and few-layer TM(oxy)hydroxide and oxide nanosheets(?-Ni(OH)₂,?-Fe OOH,?-Co OOH,and monoclinic Li_xMn O₂)with high-yield and rapid procedure.In the LPE synthesis,the preformed lithium peroxide(Li₂O₂)nanosheets with(001)exposed planes function as substrates,enabling the epitaxial growth of TM hydroxides with coherent interface.The final TM(oxy)hydroxides and oxides ultrathin nanosheets can be obtained by in situ oxidation and removal of Li₂O₂ with simple distilled water washing.To demonstrate their potential applications,the obtained single-layer?-Co OOH ultrathin nanosheets are stacked into hetero-layered hybrids of H_xLi_1-xCo O₂ by hydrothermal ion-exchange reaction at low temperature.Interestingly,the hetero-layered H_0.5Li_0.5Co O₂ shows ultra-high electrochemical stability in 1M Li₂SO₄ solution,and the specific capacity is?120 m Ah g^-1 at a current density of 1 A g^-1,and the capacity retention is?90%after 2000 cycles.The Li layer distance increases to?6?while the H layer distance maintains 4.5?at 1 V(vs.SCE)delithiated state.The increase of Li layer distance can facilitate Li⁺diffusion,suppress the layer to spinel transformation,and enhance the phase change reversibility during Li⁺(de)intercalation process.(3)Three-dimensional(3D)electrodes with freestanding feature represent the state-of-art electrode design for advanced lithium-ion batteries.Construction of 3D architectures for cathode materials,however,is still a great challenge.In this work,freestanding 3D LiCoO₂nanosheets assembled nanorod arrays are grown on carbon fabric via confined dissolution-recrystallization on the Co(CO₃)_1-x(OH)_2x·n H₂O nanowire arrays during hydrothermal treatment.The formed intermediate Co OOH nanosheets can catalyze the decomposition of H₂O₂ and the generated continuous oxygen bubble layer enables self-limiting dissolution-recrystallization.Two single-crystalline layered LiCoO₂ nanosheets assemble with angle of70.2°to form layered-spinel nanodomains.The assembled area shows cubic spinel LiCoO₂symmetry.The heat treatment at a low temperature of 380°C can eliminate some structure distortion caused by proton intercalation and Li⁺occupancy at abnormal sites.The obtained 3D hierarchy LiCoO₂ nanorod arrays with layered-spinel nanodomains exhibit large specific capacity,excellent rate performance,and outstanding cycling stability when tested as cathode for aqueous lithium-ion batteries.By using the 3D hierarchy LiCoO₂ nanorod arrays as cathode,a flexible aqueous LiCoO₂//Li Ti₂(PO₄)₃ lithium-ion battery device is successfully constructed,demonstrating superior rate performance and cycle performance.