Lithium-Ion Battery Separators And Electrodes Based On Hydroxyapatite Fibers

Lithium-ion batteries,with high energy density and power density,have been widely used in a variety of fields such as portable devices and electric transportation facilities.However,when lithium-ion batteries in a high-temperature environment(>55°C),the internal short circuit caused by thermal shrinkage of polyolefin separators may cause thermal runaway of the batteries,and the failure of binders in traditional electrodes may cause the fall off of the electrodes,resulting in the serious safety issues of batteries.These problems make the lithium-ion batteries cannot meet the requirements of some fields such as the aerospace,petroleum industry.Therefore,it is urgent to design and prepare safe and stable separators as well as electrodes for lithium-ion batteries towards high-temperature application scenarios.From the perspective of materials design,synthesis,and nano-assembly,hydroxyapatite fibers with good thermal stability were prepared by electrospinning.Thermally stable separators,functionalized separators,and binder-free electrodes using the as-prepared hydroxyapatite fibers were fabricated for high-temperature lithium-ion batteries.As expected,the enhanced electrochemical performances of lithium-ion batteries under high-temperature conditions are successfully achieved.The main contents and results are as follows:(1)The hydroxyapatite fibers were fabricated by the electrospinning method,and the influence of the Ca-/P-sources,solvents,and polymeric concentration on the morphology of precursors were systematically explored.High-temperatures treatments promote the formation of crystalline hydroxyapatite fibers and improve the crystallinity of the final products.Then,with the bacterial cellulose fibers as reinforcing support,the hydroxyapatite-based composite membranes were fabricated by a vacuum filtration process.The influence of the heating temperature and mass ratios of fibers were also investigated.Meanwhile,the physicochemical properties including porosity,electrolyte uptake,ionic conductivities of composite membranes were further studied in detail.The porosity and electrolyte uptake of the composite membranes are optimized to be 63%and 128%,respectively.The as-achieved composite membrane shows a high ionic conductivity of as high as 0.80 m S cm^-1 and outstanding mechanical strength(tensile stress of 15.8 MPa),which holds great promise in lithium-ion batteries.(2)The hydroxyapatite fiber-based composite membranes are used as separators in lithium-ion batteries.The composite separator remains dimensionally stable at 200°C,which prevents the thermal shrinkage of commercial separators at high temperatures.Also,it exhibits good wettability and high ionic conductivity with various types of electrolytes,and the LiFePO₄half-cells assembled with the separators exhibit enhanced rate capability at room temperature.The LiFePO₄/Lihalf-cells deliver a specific capacity of 80 mAh g^-1 at 8 C.Benefitting from the abundant polar group on the surface and the high surface area,the separator shows good wettability to the thermal stable ionic liquid electrolytes,and the contact angle is as small as18°.Also,the assembled LiFePO₄/Lihalf-cell shows superior cyclability with a specific capacity of 140 mAh g^-1 at 80°C.In comparison to commercial polyolefin-based separators,the as-prepared separators based on hydroxyapatite fibers are environmentally benign and easily degradable.(3)Based on the adsorption characteristics of hydroxyapatite,the ability of hydroxyapatite to absorb manganese ions was confirmed by ion adsorption experiments.After that,the hydroxyapatite fiber-based separators are used as functional separators for lithium-ion batteries with the LiMn₂O₄ cathode.Under high-temperature conditions(55°C),the hydroxyapatite fiber-based functional separators are able to adsorb manganese ions in the electrolyte,which avoids the diffusion of dissolved manganese ions to the anode sides.In addition,the separators may react with the trace HF in electrolytes.This reaction prevents the crystallinity of LiMn₂O₄from the corrosion of HF.The adsorption and acid scavenging ability of the hydroxyapatite fiber-based separators improves the cycling stability of the LiMn₂O₄ in lithium-ion batteries at high temperatures,and the LiMn₂O₄/Lihalf-cells have a specific capacity of?60 mAh g^-1 after100 cycles at 55°C.The employment of the bifunctional separators further improves the safety and stability of the battery under high-temperature conditions.At the separator level,this work provides a new strategy to improve the cycling performance of cathode materials under high-temperature conditions.(4)The LiFePO₄ and Li₄Ti₅O₁₂ binder-free electrodes with the hydroxyapatite fiber skeleton were prepared by the vacuum filtration method.The design avoids the failure of the binder in electrodes at high temperatures,which enhances the safety of lithium-ion batteries.The mass-specific capacity of the binder-free electrode is 1.5 times higher than that of the routine coating electrodes.Benefiting from the stability of components and abundant pore structure in electrodes,the LiFePO₄ and Li₄Ti₅O₁₂ binder-free electrodes exhibit excellent thermal stability and electrolyte wettability.The favorable fast electron and ion-transport channels endow the batteries with excellent cyclability and rate capability at room temperature.Moreover,the binder-free electrodes exhibit highly reversible lithium-storage capability at high temperatures.At 55°C,the half-cell shows a specific capacity of?170 mAh g^-1(LiFePO₄)and?130 mAh g^-1(Li₄Ti₅O₁₂)at a rate of 1 C,respectively.Furthermore,the Li₄Ti₅O₁₂//LiFePO₄full cells based on hydroxyapatite fibers are able to operate normally at 100°C,and the cells reach capacity retention of 87%upon 100 cycles at 55°C.