Structural Control Of Novel Nickel-based Materials And Their Lithium Storage Performances

With the rapid development of human society and the wide applicationsof electric vehicles and various portable electronic devices,people's demands for secondary batteries have become stronger.Lithium-ion batteries(LIBs)have the advantages of small size,high output voltage,no memory effect,high energy and power density,etc.Therefore,it has become the most popular secondary battery.Currently commercial graphite anodes have gradually been unable to meet actual development needs due to their low theoretical capacity(372 m Ah g^-1).Therefore,the development of high-capacity and low-cost novel anode materials has become particularly important.The nickel-based transition metal anode materials mainly based on the conversion reaction are favored by the majority of researchers due to their considerable theoretical capacity(572-1340 m Ah g^-1)and lower cost.However,nickel-based anodes also have corresponding problems,including the huge volume expansion and the agglomeration of metal elements during the process of lithiation,poor intrinsic electronic and ionic conductivity,etc.,which limit their practical applications.Therefore,the employment of ion-doped modification and structural design to improve the lithium storage performance of nickel-based materials has become particularly important.The main results in this thesis are summarized as follows:(1)Cu²⁺and Ni³⁺co-doped Ni/Cu layered double hydroxides were prepared by a simple and efficient one-step co-precipitation method.The introduction of Cu²⁺promotes more SO₄^2-and water molecules to be embedded into the interlamination,which increases the interlayer spacing and also improves the stability of the layered structure,thereby promoting the migration of lithium ions.At the same time,the effective doping of Cu²⁺also promotes the formation of Ni³⁺in nickel hydroxide.The in-situ generated Ni³⁺not only shortens the band gap of the materials,but also improves the pseudocapacitance performance.Therefore,the material can contribute an ultra-high first discharge capacity of 1832.5 m Ah g^-1 at a current density of 0.2 A g^-1.In addition,this material can exhibit capacities of 1429.2,1156.8,1014.1,895.2,750.7,736.6 and670.0 m Ah g^-1 at 0.1,0.2,0.5,1,2,3 and 5 A g^-1,respectively.Even after 1000 cycles at a current density of 2 A g^-1,the capacity can still retain 945.2 m Ah g^-1 with a capacity retention rate of 64.8%,which is the best performance of all reported nickel-based hydroxide materials.(2)Preparation of nickel-based oxides with better thermal stability:employing the aforementioned co-doped hydroxide as a precursor,a copper-modified nitrogen-doped carbon-coated NiO composite material was prepared through dopamine coating and subsequent carbonization.This structure not only effectively alleviates the volume change of the material in the process of lithiation,but also enhances the overall conductivity and inhibits the coarsening of the nickel of the materials during the conversion reaction.Therefore,the material can exhibit the capacities of 928.4 m Ah g^-1 and 881.7 m Ah g^-1 after 1000 cycles at 1 and 2 A g^-1,with the capacity retention rates of 95.0%and 92.7%,respectively.(3)Preparation of nickel-based sulfide with better electrochemical activity and reversibility:considering that the composite phase structure can effectively improve the lithium storage performance,the above-mentioned co-doped hydroxide was used as a precursor,combined with dopamine,and then S/N co-doped carbon-coated NiS/Cu₂S composite was prepared by gas-phase vulcanization.Owing to the above-mentioned composite structure,this material can stably contribute 771.7 m Ah g^-1 even after 1800 cycles at a high current density of 2 A g^-1,with an ultra-high capacity retention rate of 98.0%.(4)Although the aforementioned materials can contribute high capacity,their voltage hysteresis is large,which will reduce the actual output voltage of the batteries.Therefore,the preparation of phosphides with the lowest voltage hysteresis has greater practical significance.Using ZIF-67 as the precursor,a nitrogen-doped carbon-coated hollow nickel-cobalt phosphide was prepared.The voltage hysteresis of the prepared material is about 0.4 V according to the CV result,which is obviously lower than the above several nickel-based materials.In addition,the hollow structure provides a larger volume buffer space for lithiation,and promotes the infiltration of electrolyte and the transmission of lithium ions at the same time.The coated carbon layer not only serves as a buffer phase,but also improves the conductivity of the material.This structure makes the material reveal relatively good lithium storage performance,which can exhibit 935.0,704.0,624.8,557.8,502.3 and 462.6 m Ah g^-1 at the current densities of 0.1,0.2,0.5,1,2 and 3 A g^-1.Even it undergoes 1400 cycles at 1 A g^-1,the material can still maintain a capacity of 555.1 m Ah g^-1.In addition,the Li Fe PO₄//Co₂P/Ni Co P@NC full-cell can retain471 m Ah g^-1 capacity after 100 cycles at 0.5A g^-1,with a capacity retention rate of 60%.Meanwhile,we preliminarily tried to prepare phosphorus-rich phase materials with higher phosphorus content.And the results show that the capacity is indeed improved,but its phase structure needs to be adjusted...