Structural Design And Performances Of Bamboo Cellulose-based Battery Separators

The separator is an indispensable part of the liquid rechargeable battery,and plays an important role in isolating the cathode and anode,storing the electrolyte,etc.However,the existing separators have some shortcomings such as single function and poor performance.For example,Glass Fiber separators have poor mechanical properties and is easily damaged by dendrites,resulting in battery performance degradation;polypropylene separators have poor thermal stability,and severe thermal shrinkage will occur at high temperatures,resulting in short-circuiting of anode and cathode.It is urgent to choose suitable materials to functionally modify the existing separators or construct new high-performance separators to improve the battery performance.As a kind of renewable and degradable green biomass material,cellulose has the advantages of excellent electronic insulation capability,electrolyte wettability,mechanical robustness,chemical/electrochemical stability,organic/inorganic compatibility,etc.It can not only be used to construct separators,but also can be integrated with other materials to construct multifunctional separators.Herein,this thesis takes 2D bamboo cellulose nanosheets as the research object,which can not only be used as functional materials to modify commercial separators,but also can be used to serve as high-performance separators through microstructure control,and further can be integrated with other materials to harvest tailored functions.This thesis focuses on the structural design and composition regulation of cellulose membranes,and studies its performance as battery separators.The specific research contents list as follows:(1)Construction and performance of bamboo cellulose-modified functional separator with regulating Zn deposition.The nonuniformity of Zn deposition can lead to rapid degradation of battery performance,and the growth of Zn dendrites can easily pierce the separator and lead to short-circuit of the battery.In this study,a conductive network of CNT/cellulose nanosheets loaded with Zn@In particles was constructed and be used as modified layers on existing commercial separators to regulate the deposition behavior of Zn anode.A conductive circuit can be formed to obtain electron when the modified layer is in contact with the Zn anode,and loaded Zn@In particles can act as nucleation sites to induce Zn deposition,realizing the opposite growth mode of simultaneous deposition both on the modified layer and Zn anode,thereby forming a dense Zn deposition layer on the surface of the Zn anode.Compared with the un-modified separators,the cycle life of the symmetric battery using separator with bamboo cellulose-modified functional layer is vastly improved from 72 h to 2100 h.(2)Construction and performance of the bamboo cellulose-based separator with high strength and anti-corrosion.Although the performance of the Zn anode was improved by functional modification of the separator,the low strength of the Glass Fiber separator was not changed.Therefore,a high-strength and corrosion-resistant pure bamboo cellulose membrane was constructed with bamboo cellulose nanosheets as a single component.This membrane manifests a unique hierarchical structures with rich nano-scale pores and shows a thickness of is 10 μm with a tensile strength reaching up to 81.0 MPa(vs.0.5 MPa for glass fiber).And benefiting from the amounts of crystalline regions and intermolecular and intramolecular hydrogen bonds within cellulose,it exhibits decent corrosion resistance capability in both acid and alkaline systems(p H: 3.6-13.6).And the nano-scale pores can help to stabilize and homogenize the distribution of Zn ions on the surface of the Zn anode to achieve uniform deposition.Thus,the symmetric battery based on this specially-designed separator can achieve up to 5000 h cycle life in both acid and alkaline electrolytes.(3)Construction and application of safety separators with bamboo cellulose-based nano topological structure.Bamboo cellulose nanosheets can not only be integrated with other functional materials,but also be used as the substrate of separator.Therefore,in view of the safety hazards caused by the use of flammable organic electrolyte and the commercial polypropylene separator with poor thermal stability in non-aqueous battery,a high-safety separator with high thermal stability and flame-retardant function was constructed by using cellulose nanosheets as the separator matrix and flame retardants as the additive.This safety separator shows a unique morphology of the flame retardants particles tightly anchored in the microscroll comprised of cellulose nanosheets.After being ignited,the separator was quickly extinguished after burning for only 0.67 s,presenting good flame retardancy,while polypropylene separator was completely burned out.In electrochemical tests,the safety separator exhibits comparable performance to polypropylene separator.