Safety And Electrochemical Performance Of Ni/Co/Mn-based Oxides As Electrode Materials For Lithium-ion Batteries

For large-scale applications of lithium-ion batteries(LIBs)in the situations like solar or wind energy plants or grid storage,safety will become major criterion for the general acceptance of this electrochemical technology.Overcharge is the most popular and dangerous problem but not easy to be prevented.In the galvanostatic mode,overcharge often leads to unwanted high-voltages in which truly serious consequences root,including thermal runaway,gas generation,short-circuits and ultimately fire or explosion.In this thesis,to improve the overcharge safety of LIBs,we propose a novel safety strategy to avoid overcharging,i.e.solid-state anti-overcharge additives,an intrinsic overcharge protection mechanism triggered from solid-state composite cathodes but not conventional electrolyte additive.A series of systematic investigations are carried out concentrated on the search of additives,anti-overcharge effects and the anti-overcharge mechanism etc.Additionally,the anti-overcharge effects of this method are demonstrated in several different cathodes.Chapter 1 gives a brief introduction about the key components materials of lithium ion batteries.It focuses mainly on several typical cathode materials and transition-metal oxides of anode materials.In addition,the recent research progresses about the LIBs safety including several key safety issues and corresponding solutions are mainly reviewed.The scope of this thesis is outlined at the end of this chapter.In Chapter 2,the author introduces the experimental reagents,processes and equipments used in the project of this thesis.The general characterization methods for structure,morphology and electrochemical properties of electrode materials,as well as the procedure of test cells fabrication have been elaborated.In Chapter 3,the author proposes and confirms a novel safety strategy to avoid overcharging,i.e.solid-state anti-overcharge additives,an intrinsic overcharge protection mechanism triggered from solid-state composite cathodes.The possible additives including a series of metal or metallic oxides are explored and we have preliminarily verified the effects of two kind of feasible additives,i.e.CuO and Cu2O.It is found that a(LiCoO2+5%CuO)/Li cell can substantially prevent the voltage-runaway and the decomposition of electrolytes consequently improve the overcharge safety,but not hurt the normal cycling performance of the cell.Also,the Cu2O additive presents typical anti-overcharge performance and similar compatibility with components of LIBs.This novel strategy may open a new window to develop safer LIBs systems.In Chapter 4,spinel LiNi0.5Mn1.5O4 powders with excellent electrochemical properties have been successfully synthesized by a facile sol-gel route with RF resin as a chelating and cross-linking agent.The effects of heat-treatment conditions are remarkable on the structure and electrochemical performance of the spinel powders.The lattice parameter and crystallite size rise monotonously with increasing the sintering temperature.Electrochemical measurements suggest that the 850?-sintered(in air)sample(Fd3m phase)performs the best with a high rate discharge capacity of 110 mAh g-1 at 10C,and capacity-retention of 96.3%after 60 cycles at 0.25C and 89%after 200 cycles at 1C.For comparison,the LiNi0.5Mn1.5O4 sample sintered at 850? in O2(P4332 phase)presents limited rate performance(45 mAh g-1 at 10C)and higher values in both AC impedance and DC-method derived resistance.Additionally,the typical anti-overcharge feature is found from the overcharge voltage profile of(LiNi0.5Mn1.5O4+5%CuO)/Li cell,extending the application of CuO as anti-overcharge additive.In Chapter 5,layered LiNi1/3Co1/3Mn1/3O2 powders with high homogeneity and excellent electrochemical properties are successfully synthesized by a one-step radiated polymer gel(RPG)method aided with the y-ray,which can provde the rapid gel formation of acrylic acid(AA)as a chelating agent for metal ions.Herein,some detailed information of the synthesis mechanism is presented in addition to the results of our investigation on the effect of calcination time on electrochemical performance.The unit cell parameter,c,decreases with increasing calcination time of the polymer gel,which is caused by a stronger Li/Ni exchange.Electrochemical measurements suggest that the sample with a calcination time of 25 h performes the best with an initial discharge capacity of 188 mAh g-1(2.8-4.5 V,C/10),high capacity retention(90.1%after 30 cycles)and good rate capability(specific capacity of 153 mAh g-1 at 3 C and 125 mAh g-1 at 6 C).Additionally,the typical anti-overcharge feature is found from the overcharge voltage profile of(LiNi1/3Co1/3Mn1/3O2+5%CuO)/Li cell,extending the application of CuO as anti-overcharge additive.In Chapter 6,bowl-like hollow Co3O4 microspheres are prepared via a simple and low-cost route by thermally treating Co-containing resorcinol-formaldehyde composites gel in air.For such a bowl-like hollow structure,the Co3O4 primary particles assemble to construct the sphere surface accompanied by a certain content of mesopores which can boost mass transport and relieve volume changes and inter-strains.When employed as the anode material for lithium-ion batteries,these bowl-like hollow Co3O4 microspheres exhibit extraordinary cycling performance(111%retention after 50 cycles owing to capacity rise),fairly high rate capacity(650 mAh g-1 at 5C)and enhanced lithium storage capacity.Meanwhile,the Na-storage behavior of Co3O4 as an anode material of Na-ion batteries is initially investigated based on such a hollow structure and it exhibits similar feature of discharge/charge profiles and a high discharge capacity but relatively moderate capacity retention compared with the Li-storage performance.Additionally,the typical anti-overcharge feature is found from the overcharge voltage profile of(LiNi0.5Mn1.5O4+5%CuO)/Co3O4 full cell,extending the application of CuO as anti-overcharge additive.Finally,in Chapter 7,the author gives an overview of the originalities and deficiencies of this thesis.Some prospects and suggestions of the possible future research are also given.