Electrochemical Properties And Reaction Mechanisms Of Nano-Sized Thin Films As Electrode Materials For Lithium Ion Batteries

Pursuing high performance lithium storage materials is one of the primary directions in lithium battery development.Due to their particular dimension effect and large specific surface area,nano-materials are considered as next generation electrode materials for lithium ion batteries.This thesis will focus on the investigations of a series of nano-sized thin film materials fabricated by pulsed laser deposition(PLD) technology.Charge/discharge measurements and cyclic voltammograms were used to investigate cycle performance and electrochemical properties of the materials.X-ray diffraction(XRD),high resolution transmission electron microscopy(HRTEM), selected area electron diffraction(SAED) and X-ray photoelectron spectroscopy(XPS) were employed to detect the composition and structure information of the materials in certain electrochemical states.Thereby the electrochemical reaction mechanism were discussed.The following systems were included:1.Fabrication and electrochemical properties of LiF-M(M = Co,Ni,Fe……) nanocomposite thin films.A series of LiF-M nanocomposite thin films were successfully fabricated by pulsed laser deposition for the first time by adjusting composition of targets and deposition parameters.After characterize their electrochemical properties and detecting composition and structure changes in charge and discharge,we found this system contained an electrochemical process of reversible decomposition and formation of inert LiF in charge and discharge.This kind of electrochemical reaction mechanism is different from the traditional lithium ion insertion and extraction mechanism.It is a reversible conversion reaction based on the reversible transformation between MF₂ and LiF in charge and discharge.2.Fabrication and electrochemical properties of Li₂S-M nanocomposite thin films.We fabricated Li₂S-M nanocomposite thin films by pulsed laser deposition. Electrochemical performance and reaction mechanism were investigated.For Li₂S-Co, we found this thin film yielded best electrochemical performance when the mole ratio of Li₂S and Co was 2:1.It exhibited large first discharge capacity up to 1257 mAh/g in a current density of 28μA/cm².The capacity in second cycle faded quickly. Discharge capacity was 819 mAh/g.However,in the subsequence cycles,capacity fading decreased significantly with better reversibility.After characterize the composition and structure of thin films in charge and discharge,we found it contained multi-step electrochemical reactions.In first charging,not only CoS₂ was formed,but also simple substance S was produced.In the discharging process,CoS₂ transformed to a ternary compound LiCoS,as well as S transformed to Li₂S.3.Fabrication and electrochemical properties of Li₃N-M nanocomposite thin films.Li₃N-M nanocomposite thin films were fabricated by pulsed laser deposition for the first time.Electrochemical characterization results showed that it produced a specific capacity of 515 mAh/g in the first discharge.The reversible capacity in subsequent cycles was about 400 mAh/g.The capacity fading per cycle was less than 0.2%.The results revealed that the conversion between Li₃N and CO₂N was involved. Furthermore,the reversible transformation between Li_2.57Co_0.43N and Co₂N was also discovered.4.Fabrication and electrochemical properties of Li₃N-Si nanocomposite thin films.After the research work above,we have got a deep understanding on the electrochemical reaction mechanism of LiX-M(X = F,S,N;M = Co,Ni,Fe……) as electrode materials for lithium ion batteries.So we started to think about whether Si could also drive the reversible decomposition and formation of LiX.Using pulsed laser deposition technology we fabricated Li₃N-Si nanocomposite thin films with a mole ratio 1:1 of Li₃N and Si.Electrochemical characterization showed good cycle performance except large capacity fading in the first cycle.Li₃N-Si exhibited a large reversible capacity of 700 mAh/g.Capacity fading per cycle was about 0.7%.After charactering the composition and structure of the thin film after charging and discharging,we found the nanosized Si particles drove the decomposition of Li₃N in charging process,then polycrystalline hexagonal Si₃N₄ was formed.In the subsequent cycles,It was a reversible conversion process between Si₃N₄ and Li₅N₃Si.5.Fabrication and electrochemical properties of nanostructured In2O₃ and GeO₂ thin films.Tarascon et al.found the reversible decomposition and formation of Li₂O could be driven by nanosized transition metal particles.Is it possible that nanosized main group metal particles also have this effect? We fabricated nanostructuredâ…¢A metal oxide(In₂O₃) and IVA metal oxide(GeO₂) thin films by pulsed laser deposition, then characterized their electrochemical properties.The results showed that they both exhibited large specific capacity in first discharge,which were 1083 mAh/g and 2073 mAh/g for In₂O₃ and GeO₂,respectively.There is large capacity fading in the second cycle.The discharging capacity decreased to 883 mAh/g and 1336 mAh/g, respectively.However,in the subsequent cycles,the capacity could be maintained very well with excellent reversibility.After characterizing the composition and structure of thin films in charge and discharge,we found the reversible reaction process was mainly due to the reversible conversion between In₂O₃(GeO₂) and Li₂In (Li₁₅Ge₄).This indicated that nanosized metal particles(In and Ge) could drive the decomposition and formation of Li₂O in charging and discharging.Furthermore,they could participate the reversible alloying/dealloying reactions with lithium.This contributed extra reversible capacity for this system.6.Fabrication and electrochemical properties of nanostructured NiF₂ thin films. Nanostructured NiF₂ thin films were fabricated by pulsed laser deposition for the first time.Electrochemical characterization showed a large irreversible capacity in the first cycle.However,the capacity could be well kept from second cycle.The reversible capacity was about 540 mAh/g in the potential range from 0.01 to 3.5 V.After 40 cycles,the specific capacity was around 450 mAh/g.Electrochemical and structural characterizations indicated that the reversible process of nanostructured NiF₂ thin film in charging and discharging contained multi-step reactions.In the range of 1.0 to 3.5 V, it was the reversible conversion between NiF₂ and Li₂NiF₄.In the range of 0.01 to 1.0V,it was the reversible conversion between Li₂NiF₄ and LiF/Ni.The in-depth and systemic investigations of electrochemical properties on nanosized thin films in this paper are very helpful to uncover electrochemical reaction essences,which is on the decomposition of inert matter LiX(X = F,O,S,N) driven by nanosized metal and nonmetal.It has certain directive significance for preparing and developing new kinds of high performance lithium storage materials. In addition,appendix introduced the research work I made as a visiting student in Brookhaven National Laboratory(BNL) last year.It contains two topics:1.Investigations of new high temperature and high voltage electrolyteDimethyl poly(ethylene glycol)(MW=500) was used as electrolyte solvent for the first time.Combining with salt LiPF₆,we prepared a new electrolyte with high boiling point.Cyclic voltammogram test showed a large electrochemical window up to 5.3 V.The conductivity of this electrolyte at room temperature was about 0.39×10^-3 S/cm,which was low due to its high viscosity,but it increased quickly during temperature increasing.2.Thermal stability investigations of cathode materials for lithium batteryBy using in-situ transmission electron microscopy and selected area electron diffraction,we have studied the thermal stability properties of overcharged LiNi_0.8Co_0.15Al_0.05O₂ cathode material.During heating from room temperature to 400℃,we observed the local structure changes of LiNi_0.8Co_0.15Al_0.05O₂ in the real time.It was a process from layered structure(R3m) to spinel structure(Fd3m),then to NaCl structure(Fm3m).We found that a little spinel structure and NaCl structure existed at the room temperature.During heating,both of the structures diffused and expanded from surface to the bulk.Eventually,all of the particles convert to NaCl structure.These results provide some significance and reference value for the safety and reliability for high power lithium ion battery used in HEV and EV.