Investigation On Hydroxyapatite Nanowire-based High-temperature-resistant Lithium-ion-battery Materials

For the last several decades,benefiting from their high efficiencies,high working voltages,and relatively light weights,lithium-ion batteries?LIBs?have powered the revolution of personal electronics and thus changed our lives.Nowadays,LIBs are considered as among the most competitive power sources for electric vehicles,grids,and other large-scale energy storage systems,if their safety and energy density issues can be substantially addressed.The development of new battery materials provides new chance and possibility for further improvement in battery performance and battery safety.A typical LIB is mainly composed of four parts,which are cathode,anode,separator and liquid electrolyte.In a LIB,the separator plays a crucial role in preventing the physical contact between the anode and cathode,simultaneously serving as an electrolyte reservoir to enable ion transportation between the electrodes.Even though the separator itself does not take part in any cell reactions directly,its structure and properties can greatly influence the battery performance.Moreover,the thermal stability of the separator also determines the safety property and working-temperature-range of LIBs.The most commonly used separators in LIBs are polyolefin-based organic ones.The advantages of such kind of separators are the low cost,good mechanical strength,and good electrochemical stability.However,there are also some deficiencies such as poor electrolyte wettability,low porosity,and poor thermal stability.The polyolefin-based separators will easily shrink?or melt?at high temperatures,causing safety risks for LIBs.Many studies have focused on the development and modification of electrodes,separators and electrolyte materials,whereas less effort has been dedicated to the design of electrodes and cell architectures.Controlling the electrode architecture is of great importance in optimizing both the ion and electron transportation pathways within the entire electrode,thus in determining the energy delivery of the battery through the electrochemical reaction kinetics.Moreover,the electrode architecture also plays important roles in determining the battery safety as well.However,it is of great challenge in concurrently achieving high mass loading of active materials,good structural integrity,good electrochemical performance as well as easy manufacturing processability of the electrode.Hydroxyapatite?HAP?is the main inorganic component of vertebrate bones and teeth.Owing to its good bioactivity and biocompatibility,HAP has been intensively investigated in biomedical fields.However,the applications of HAP materials in energy-related fields have been rarely reported before.HAP is an insulative material with high thermal stability.The fire-resistant inorganic paper made from ultralong HAP nanowires has high porosity and interconnected pore structure,which are beneficial for the permeation of liquid electrolyte and transportation of ions.Moreover,HAP nanowires possess abundant functional groups?-OH?,which are easily modified and functionalized.In this thesis,we mainly focus on the HAP nanowire-based materials for LIBs.The first part of this thesis is the preparation of network-structured HAP nanowires and their derived high-strength fire-resistant inorganic paper.The second and third part of this thesis are the preparation and investigation of HAP nanowire-based fire-resistant separator,and the HAP nanowire-enabled ultrahigh-capacity and fire-resistant thick LiFePO₄ composite electrode,respectively.1.Preparation of network-structured HAP nanowires and their derivedhigh-strength fire-resistant inorganic paper.In this part,we firstly prepared hydroxyapatite nanowires that can be monodispersed in an aqueous solvent.The hydroxyapatite nanowires have diameters of around 10 nm and lengths about 500 nm.The hydroxyapatite nanowires are modified with oleate functional groups on the surface,which are ideal building blocks for multi-stage ordered self-assembly.Then,starting from the monodisperse hydroxyapatite nanowires,we have obtained highly flexible network-structured hydroxyapatite nanowires through the interfacial self-assembly.The multi-stage self-assembly mechanism is also analyzed.Compared with the bundle-like hydroxyapatite nanowires,the network-structured hydroxyapatite nanowires can effectively improve the bonding strength between the nanowires and are ideal raw materials for constructing the high-strength fire-resistant inorganic paper.Finally,we obtained a composite inorganic fire-resistant paper with high mechanical strength and high-temperature flexibility.2.Preparation and investigation of HAP nanowire-based fire-resistant separatorfor LIBs.In this part,we have successfully developed a novel high-temperature-resistant separator for LIBs based on HAP nanowires.The HAP nanowire-based separator has many advantages,such as high flexibility,good mechanical strength,high porosity,and excellent liquid electrolyte wetting and adsorption properties.The HAP nanowire-based separator also has high thermal stability and fireretardant property.The battery assembled with the HAP nanowire-based high-temperature-resistant separator has better electrochemical performance,cycling stability and rate performance than those assembled with the commercial polypropylene separator.More importantly,the battery assembled with the HAP nanowire-based high-temperature-resistant separator can work in a high-temperature environment?150°C?.However,the battery assembled with the commercial polypropylene separator quickly short-circuited as the temperature rises to 150°C.This work is of great significance in improving the safety and operating temperatures of LIBs.3.Preparation and investigation HAP nanowire-enabled ultrahigh-capacity andfire-resistant thick LiFePO₄ composite electrode.In this part,we have successfully designed and fabricated an ultrahigh-capacity and fire-resistant LiFePO₄?UCFR-LFP?-based nanocomposite cathode through a bottom-up self-assembly strategy using HAP nanowires,Ketjen black nanoparticles,carbon fibers,and LiFePO₄ powder as starting materials.Instead of randomly stacking together,a hierarchical architecture with high electronical conductivity and good liquid electrolyte accessibility is rationally generated through the concerted interactions of building blocks.Benefiting from the structural and chemical uniqueness,the as-prepared UCFR-LFP electrodes demonstrate exceptional improvements in both electrochemical performance,mass loading of active material,thermal stability,and high-temperature operability compared with the conventional slurry-coated electrodes at the same active materials mass loading.Particularly,the UCFR-LFP electrode exhibits enhanced capacity and rate capability due to the improved fast electron/ion transportation,thus promoting better redox reaction kinetics.Notably,an ultrathick UCFR-LFP electrode?1.35 mm?with remarkably high active material mass loading(108 mg cm^-2)and areal capacity(16.4 mAh cm^-2)has been successfully achieved.Such a unique UCFR-LFP electrode with excellent properties offers a promising solution for the next-generation advanced LIBs with high energy density,high safety,and wide operating temperature window.