Controllable Preparation Of Transition Metal Phosphides@carbon Nanofibers Anodes And The Applications In Alkali-metal Secondary Batteries

The fast development of 5G mobile communication and electric vehicles has boosted the upgraded energy storage technology.Traditional lithium(Li)-ion battery(LIB)is limited by the development of critical materials of electrodes and the energy density is expected to reach its theoretical limits(?300 Wh kg^–1).And the cost-reduction is restricted by the raw commodity of LIB.The specific capacity of commercial graphite anode is only 372 m Ah g^–1.Therefore,developing new-generation anode materials with high capacity and low redox potentials is vital to the practical application of high performance rechargeable energy storage devices.As a type of interstitial compound with stable crystalline structure,transition metal phosphides(TMP)show a broad perspective as anode materials for alkali-ion(Li and Na ion)batteries,delivering electrochemical activity,apposite potential range(0.5–1 V vs.Li/Li⁺)and high specific capacity.However,its development still faces great challenges:(1)the complex routes of TMP employs hypertoxic gases as phosphorus sources;(2)The electrochemical conversion reaction mechanism with Li is still unclear.In addition,a large volume change of the electrodes is occurred during the electrochemical reaction process.The tactics of rational design of synthetic routes and hierarchical construction for TMP@carbon composites under safe and controllable preparation can accommodate the volume expansion upon conversion reaction of the electrode,clarify mechanism of electrodes reaction kinetics and mass transfer process and improve the charge transfer,which is crucial to the practical applications in secondary batteries of TMP-based anodes.The ideal Li metal anode(LMA)possesses ultrahigh capacity(3860 m Ah g^–1)and the most electronegative potential(–3.04 V vs.SHE).And Li-metal batteries can readily obtain an energy density of 400 Wh kg^–1 with a Li-metal,which is the ultimate choice for anode materieals in rechargeable batteries.However,detrimental Li dendrites growth during charge/discharge process causes the Li capacity loss and the infinite volumetric expansion of“hostless”Li,and even thermorunaway with safety hazards.Thus,rational design and construction of conductive and lithiophilic three-dimensional(3D)frameworks for composite LMA is capable of mitigating the volumetric expansion of"hostless"Li and regulating its nucleation and growth behavior to obtain high utilization of Li.Based on the high chemical activity and the affinity of TMP to Li,and the high Li⁺conductivity of lithiated Li₃P,it is expected to further develope its application in Li-metal batteries.Building a 3D skeleton with lithiophilic TMP for Li host to guide the uniform Li deposition and mitigate the volume expansion,and further realize dendrite-free and highly reversible Li-metal batteries is in an urgent need.In this thesis,a series of TMP/carbon nanofibers were obtained by electrospinning method combined with a carbothermic in-situ self-reduction process as anode materials for secondary batteries.The main research results in this thesis are as follows:(1)A high performance anode of Mo P@carbon nanofibers(Mo P@NCNFs)were demonstrated,which were prepared from an electrospinning method followed by an in-situ carbothermic self-reduction process under N₂ atmosphere.The well-crystallized Mo P nanoparticles are uniformly distributed in 1D nanofibers,affording a conductive network for fast charge/ion transport.Benefit from the unique structure,the Mo P@NCNFs synthesized at 800°C delivers a reversible capacity of 840 m Ah g^-1 at 100 m A g^-1 after 200 cycles.A capacity of 377 m Ah g^-1 is achieved at 2 A g^-1upto 1300 cycles for Li storage.And a decent performance is also available for sodium storage.The reversible conversion reaction of Mo P is confirmed by in-situ/ex-situ characterization,and the derived Li₃P or Na₃P possessing high ion diffusion rate.Quantitative kinetics results testify that the charge storage is dominated by pseudocapacitance,boosting the high-rate lithium/sodium storage performance.(2)Iron phosphide anode materials based on cheap iron sources is developed for practical application.A flexible Fe₂P@carbon nanofibers film(Fe₂P@NCNF)for LIB and SIB anode was realized under N₂ atmosphere by an electrospinning method and a subsequent in-situ carbothermic reduction process without post phosphorization treatment.The application in anode materials for alkali-ion batteries and its mechanism were demonstrated.Compared with molybdenum based fibers,the flexible Fe₂P@NCNF film can be used as self-supporting electrode.Uniform distribution of well-crystallized Fe₂P in flexible NCNF provides a conductive network for rapid charge/ion transfer and adequate buffer space for volume variation from reaction with prolonged lifespan.A reversible specific capacity of 468 m Ah g^–1 after 250 cycles at 0.1 A g^–1 is obtained for Na storage.Long-term lifespan of 4000 and 10,000 cycles at 5 and 10 A g^–1 is maintained,respectively.A pseudocapacitive dominated Na⁺storage mechanism for the nanosized Fe₂P in NCNF conductive network ensures the high rates capability(82.2%at 1 m V s^–1).And a decent performance for Li storage is also achieved.(3)In order to expand the application range of transition metal phosphides and promote the development of lithium metal batteries,a Co P@carbon nanofibers(Co P@CNF)self-standing substrate for 3D Li host was demonstrated to obtain a Co P@CNF@Li anode to investigate Li migration,nucleation and growth behavior.The high adsorption effect and low migration barrier of Co P to Li⁺lead to the reversible conversion reaction between Li and Co P,enabling an even charge transportation and a homogeneous Li⁺concentration gradient by density functional theory calculations.The uniform nucleation and dense electrodeposition of Li over through the Co P@CNF are demonstrated via combined in-situ/ex-situ morphologic and electrochemical characterizations.Benefiting from the small nucleation and growth overpotential of Li,the symmetrical cell of Co P@CNF@Li can steadily operate under 0.5 m A cm^-2 over 2000 h with a low voltage hysteresis of ca.12 m V.At 5 C(1 C=170 m A g^-1),Co P@CNF@Li||Li Fe PO₄ cell delivers a capacity exceeding 90 m Ah g^-1 up to 1000 cycles,demonstrating the dendritic Li suppression capability of the Co P@CNF@Li composite anode.(4)Although the performance of Co P@CNF@Li anode is decent,the cobalt resources are limited.In response to the"de-cobaltization"in the field of power batteries,a flexible carbon nanofiber membrane modified by nickel phosphide(Ni₂P@CNF)was further proposed as an effective 3D framework to guide Li deposition behaviors.Even and dense deposition of Li is observed via ex-situ/in-situ morphological observations,ascribing to the homogeneous Li nucleation on the lithiophilic Ni₂P crystalline grains.The efficient utilization of Li is achieved benefiting from the synergistic reversible conversion reactions between Li and Ni₂P and plating/stripping of Li.Moreover,the fibrous networks of Ni₂P@CNF remain structural stability upon the prolonged cycling.Thus,an average CE of 97.6%for 200 cycles at 5 m A cm^-2 is available.And an extended lifespan of 2000 h at 0.5 m A cm^-2 for a symmetrical cell of Ni₂P@CNF@Li,and1000 cycles at 5 C with a Li Fe PO₄(10 mg cm^-2,1.5 m Ah cm^-2)cathode are attained,revealing a great potential of practical Li metal anode.