Development Of Graphene-based Flexible Current Collectors And Lithium Cobaltate-based Active Materials For Aqueous Energy Storage Devices

At present,the organic electrolytes used in commercial lithium-ion batteries have potential safety hazards.It is very easy to cause thermal runaway in the case of short circuit,battery damage,liquid leakage,etc.,resulting in combustion and even explosion.Aqueous lithium-ion batteries using aqueous electrolytes are one of the effective solutions to this problem.However,different from the organic environment,in aqueous systems,redox environment is more complicated,so the requirements for current collectors and active materials are more stringent.Traditional current collectors cannot meet the needs of application.Therefore,a new type of current collector is urgently needed to cope with harsh working environment in aqueous systems.In addition,the positive and negative electrode materials that can work stably in the aqueous systems also need to be explored.As the positive electrode materials widely used in organic batteries,the large-scale preparation technology of Li Co O₂ has matured.Li Co O₂ has the similar intercalation and deintercalation of Li ions in aqueous systems to organic systems,but its cycle stability is poor.In this paper,aiming at the large-scale preparation of aqueous available current collectors and the stability of the Li Co O₂ cathode active materials,experimental design and data analysis are carried out,and the results obtained are showed as follow:（1）The optimized concentrated sulfuric acid-hydrogen peroxide two-component system successfully inserts between the graphite layers,and lots of graphene nanoplatelets powder is prepared through fast expansion and exfoliation in one step.With the aid of the paper substrate,the obtained graphene nanoplatelets powder is innovatively shaped into films by roll-press,and then calcined at 450℃to obtain flexible graphene films.The flexible graphene films have an ultra-high electrical conductivity of 2.56×10~3 S cm^-1 and light area weight of only 4.20 mg cm^-2.The water contact angle measurement shows that its hydrophilicity is significantly better than commercial carbon cloth and carbon paper.In a 6 M KOH solution,flexible graphene film is used as a current collector to load activated carbon to form a working electrode,which has excellent specific capacity and rate performance.Compared with commercial carbon cloth and carbon paper,flexible graphene film has advantages of performance and cost,and it is expected to become a feasible strategy for large-scale preparation of aqueous available current collectors.（2）Using lithium acetate and cobalt acetate as raw materials and citric acid as the chelating agent,the sol-gel method is used to prepare lithium cobalt oxide active materials at different calcination temperatures.The electrochemical test results prove that the R-3m phase Li Co O₂ calcined at 700℃has the best electrochemical performance in 5 M Li NO₃ aqueous solution.Aiming at the problem of poor cycle stability of pure Li Co O₂,Li Co O₂ is doped and modified with+4 valence of Mn to synthesis Mn-doped Li Co O₂ with excellent crystallinity and structural stability.Because of the successful doping of Mn⁴⁺,the plane spacing is extended,which is beneficial to improve the rapid transport capacity of lithium ions.Moreover,Mn⁴⁺acts as a charge center to inhibit the rearrangement of lithium vacancies and the order-disorder phase transition during the charge and discharge process,making the structure stability of Li Co O₂ improved.Electrochemical tests show that the initial specific capacity of sample with 5%doping of Mn reaches 117.5 m A h g^-1.After 100 cycles at 1 C rate,there is still a capacity retention rate of 63.0%,which is a significant improvement compared to the 40.5%of pure Li Co O₂.