Study Of Solid Electrolyte LiPON And All Solid-State Thin-Film Lithium-Ion Batteries’ Fabrications And Characteristics

The increasing miniaturization of electronic devices requires the miniaturization of devices that provide energy to them.It keeps all-solid-state thin films lithium（lithium ion）batteries（TFLIB）as a leading role in the energy-storage system.Thin film technologies make the fabrication of all-solid-state TFLIB from the idea into reality.In this thesis,the fabrication and characteristics of solid electrolyte LiPON thin film,solid cathode LiMn₂O₄ thin film,solid anode ZnO thin film and Si thin film are investigated.Then four all-solid-state TFLIBs with different structures are fabricated and studied.In these all-solid-state TFLIBs,LiMn₂O₄ thin film is as the cathode.One the one hand,depending on the reversible reaction mechanism vs.lithium for the anode,these all-solid-state TFLIBs could be divided into two groups.One group is the all-solid-state TFLIBs using the conversion material Zn O as the anode.The other is using the alloying material Si as the anode.On the other hand,these all-solid-state TFLIBs may be separated into two parts according to the different substrates.They are fabricated on the rigid glass substrate and on the flexible polyimide film with the thickness of 125μm,respectively.The main results and innovations are summarized as follows.LiPON thin films are deposited by reactive RF-magnetron sputtering of Li₃PO₄ target.Its metal/insulator/metal（MIM）structures,Al/LiPON/Al,are fabricated to study the key property of solid electrolyte.Through adjusting the deposition parameters,including the substrate-target distance,the sputtering power,the deposition pressure and the ratio between N and Ar atmosphere,the optimized deposition parameters are confirmed.It is found that there are effects of rough interface on impedance of solid LiPON in MIM cells.With increasing the Al films electrode roughness,the deposited LIPON films on these electrodes are correspondingly roughened,which result the interface in MIM cell roughened.Rough LiPON films make their ionic resistance decreasing,the real contact area and the bulk capacitance of MIM cells increasing.These lead to the ionic conductivity of LiPON films improved from 1.09 to 2.70μS/cm.Without changing LiPON components ratio and sputtering conditions for the thin films deposition,we demonstrate an effective way to improve the ionic conductivity.The effect of heat treatment on the ionic conductivity and local structure of LiPON thin films are investigated.It is found that compact LiPON thin films with high ratio of N_t/N_d could be fabricated by a conventional reactive RF magnetron sputtering of a Li₃PO₄ target at a low pure N₂pressure.Annealing on 300℃for 1h on Al/LiPON/Al sandwich structure leads to the maximum improvement of the ionic conductivity from 1.1 to 3.28μS/cm.When annealing temperature increasing to 400℃and 500℃,LiPON films with good quality still has high ionic property.It could help for the application of all-solid-state TFLIB working in the high temperature.The cathode LiMn₂O₄ thin film is fabricated by RF magnetron sputtering without the post-annealing process.It is found that as-deposited LiMn₂O₄ thin film always are amorphous state while changing the Ar/O₂ ratio in the deposition process.In case of using the expensive metal as the current collector,the as-deposited LiMn₂O₄ thin film is suffered in Ar atmosphere annealing to check whether it is crystal.It is found when the annealing process without O₂ atmosphere,the amorphous LiMn₂O₄ thin film is hard to form cubic LiMn₂O₄ spinel.For annealing process in O₂atmosphere,Ti/Pt/LiMn₂O₄ thin film is sputtering.When temperature up to 500℃,the LiMn₂O₄thin film is forming cubic LiMn₂O₄ spinel.With increasing the temperature,the diffraction intensity of manganese oxide spinel is enhanced with the FWHM decreasing.The anode ZnO thin film with high-quality c-oriented（002）,is fabricated by the RF magnetron sputtering.It is found that the c-oriented crystal structure is not changed when thin film suffered heat treatment at 300℃in air.The electrochemical reaction mechanism of ZnO with lithium has two steps in charging/discharging processes.The first step reaction is similar to the oxidation and reduction of metallic oxide.The latter is similar to the classical Li alloying in which Li ions would react with metallic Zn to form zinc lithium alloy.The formation of crystalline thin films can effectively relieve the volume expansion and contraction of material during charging/discharging processes.The anode Si thin film with amorphous state is fabricated by RF magnetron sputtering.When lithium inserting amorphous Si thin film,there is a formation of Li_xSi structure.The electrode materials with thin films state help to relieve the volume expansion and contraction of material during charging/discharging processes,and decrease the migration path of lithium ions in the process of embedding.It will help to improve the electrochemical performance of electrode materials.An all-solid-state thin film（Ti/ZnO/LiPON/LiMn₂O₄/Ti）LiPON lithium-ion battery is fabricated on the glass substrate by magnetron sputtering.This is a first application of ZnO films on the solid state LIBs as we know.In the cell,the crystal ZnO films is anode layer related to the amorphous LiMn₂O₄ films as cathode layer.Amorphous LiPON films is the solid electrolyte with Ti films as the current collectors.The cell presents a reversible capacity of 22μAh/cm between0.5 and 5 V at the current density of 5μA/cm.After 50 charging/discharging cycles,it still presents a discharging capacity of 20μAh/cm.The amorphous LiMn₂O₄ films without heating treatment as cathode still has good specific capacities and stability.Electrochemical impedance spectrum（EIS）shows the inner impedance of cell in cycles increasing.But there is no heavy influenced on the electrical performance.Good rate performance proves the TFLIB with the structure of Ti/ZnO/LiPON/LiMn₂O₄/Ti could be useful for low-power thin film battery applications onto lower-temperature substrate materials and battery stacks without any heat treatments.Sputtering（Pt/LiMn₂O₄/LiPON/ZnO/Pt）LiPON all-solid-state TFLIB suffers a heat treatment at 300℃for 2 h.It is found that all-solid-state TFLIB still has electrical performance after annealing in the high temperature.Specially,the specific capacity is not changed too much.The cell presents a maximum reversible capacity of 20μAh/cm between 0.5 and 5 V at the current density of 5μA/cm.After several cycling,the discharging capacity is decreased to 17μAh/cm.Investigation of EIS proves the interface impedance of cell increased,which results coulombic efficiency and cycling performance being bad.Furthermore,annealing to all-solid-state TFLIB make the electrode materials more passivation related to the initial capacity decaying.However,after activation of 5 cycling,the reversible capacity of cell will be recovered.An all-solid-state TFLIB with the structure of（Ti/Si/LiSiPON/LiMn₂O₄/Pt/Ti）LiSiPON is fabricated on the glass substrate by magnetron sputtering.In this cell,the cubic Li Mn₂O₄ spinel thin films by annealing process at 800℃in O₂ atmosphere is as cathode layer related to the amorphous Si films as anode layer.Amorphous LiSiPON films is the solid electrolyte.The Pt/Ti films are as the current collectors for cathode.Up current collectors for anode is Ti thin films.The cell presents a reversible capacity of 47μAh/cm between 1.5 and 5 V at the current density of20μA/cm.After 50 charging/discharging cycles,it still presents a discharging capacity of41μAh/cm,which means the cell having good electrical performance.When the current density increasing to 80μA/cm,the cell still presents a discharging capacity of 25μAh/cm.Investigation of EIS proves cycling do not have influenced on the electrochemical property.Because the cubic LiMn₂O₄ spinel thin films as cathode layer and the amorphous Si thin films as anode layer help the cell having high specific capacity.And the amorphous Si thin films could restrain the volume expansions during the lithium ion inserting the materials.Thin films also make the interface becoming smooth and reducing the interface impedance.These prove the amorphous Si thin films could be used as the anode in the all-solid-state TFLIB with high specific capacity.An all-solid-state TFLIB is fabricated on the flexible substrate of Polyimide（PI）films by magnetron sputtering.In this cell,the thickness of flexible PI substrate is 125μm.The amorphous Si thin films is anode layer related to the amorphous LiMn₂O₄ films as cathode layer.Amorphous LiSiPON films is the solid electrolyte with Ti films as the current collectors.The cell presents a maximum reversible capacity of 32μAh/cm between 1.0 V and 5.0 V at the current density of 5μA/cm.After 50 charging/discharging cycles,it still presents a discharging capacity of28μAh/cm.The cell shows a high cycling stability and good rate capability.When the current density increased to 20μA/cm,the discharging capacity is up to 20μAh/cm.Additionally,the cell still has a stable electrical performance when it is bending.In this thesis,all functional materials and devices are fabricated by magnetron sputtering.All TFLIBs are covered by a protection layer of LiPON or LiSiPON thin films.Thus the electrochemical tests are performed at the room temperature in air.The conclusions and results are very useful for the practical application of all-solid-state thin-film lithium-ion batteries.