Study On Electrochemical Functional Devices Based On Phase Transition And Electric Double Layer

Reversible ion intercalation-deintercalation,phase transitions and band structure transitions can not only cause electric-chemical energy conversion,but also may be accompanied by the change of carrier type and concentration.The research on the role of these phenomena in the discovery of material properties,control of energy conversion and design of novel functional devices has important academic significance and application prospect.In this thesis,based on the electric double layer phenomenon and the phase transition phenomenon（crystal structure and electronic structure transition）induced by reversible ion intercalation-deintercalation,several important basic science and technology issues（interdisciplinary）in the application of related materials in energy storage devices and synaptic transistor devices have been studied,by utilizing the processes caused by these phenomena,e.g.,the electric energy-chemical energy conversion,carrier density modulation and asymmetric electric double layer behavior.In the meantime,with carbon nanomaterials supplemented appropriately,functional devices such as electrochemical energy storage and artificial synapse are fabricated,and the relevant mechanisms and/or principles are clarified by experiments and simulations.Based on the electric energy-chemical energy conversion caused by reversible ion intercalation-deintercalation,the related study of energy storage devices is carried out.In view of the current situation that it is difficult to obtain both good mechanical strength and high energy density in flexible energy storage devices,an integrated,hierarchical flexible electrode for lithium-ion battery has been prepared based on Li Fe PO₄（LFP）and Li₄Ti₅O₁₂（LTO）.By combination of the excellent mechanical strength and good electronic conductivity of continuous single-walled carbon nanotube（SWCNT）reticulation with the better dispersion ability and lower contact resistance of reduced graphene oxide,a free-standing,hierarchical and flexible lithium-ion battery electrode with high energy density has been constructed with electrochemical active materials.Therefore,by combining the respective advantages of different carbon nanomaterials and making them eliminate each other’s disadvantages,the contradiction between the proportion of active materials and flexibility of electrode can be solved to a certain extent.The as-prepared free-standing electrode only contains～5 wt.%inactive materials（including current collector）,and exhibits energy density up to～480 wh kg^-1 based on the total weight of electrode（which reaches 85%of the theoretical energy density of LFP material）while maintains excellent stability against deformation（up to 20000 cycles of bending）.Flexible lithium-ion battery which can withstand 10000 dynamic bending cycles are fabricated using flexible electrodes and in-situ polymerized gel polymer electrolyte.The physical and electrochemical properties of the flexible electrode and battery in static and dynamic deformation are tested.These results would accumulate data and provide ideas for relevant practical applications.A wearable sensing system is designed and assembled,which is used to demonstrate the application potential of this flexible battery as power supply for downstream devices.Based on the phase transition（crystal structure and electronic structure transition）caused by reversible ion intercalation-deintercalation,the study on synaptic transistors is conducted.Firstly,according to the practical needs of rapid selection of potential materials,a simple and effective method which can track the resistance change of electrochemically active materials is developed,on the base of modified galvanostatic intermittent titration technology.This method is verified by two given materials,Li Co O₂（LCO）and LTO,with the resistance switching behavior of insulator-metal transition（IMT）.The developed method can quickly identify potential materials in different systems:the resistance changing behaviors of Na₂V₆O₁₆（NVO）andα-Mn O₂during ion intercalation-deintercalation are investigated.The results indicate that with the electric field induced intercalation of Zn²⁺or co-intercalation of Zn²⁺and H⁺,the phase transition in NVO andα-Mn O₂ occurs to increase the material resistance.The resistance can also be restored to the initial value after the corresponding ions are stripped.The synaptic transistor devices with these two materials as channel materials respectively are fabricated and showed long-term synaptic plasticity consistent with the prediction results of the selection method.The possible reasons for the instability of NVO channel are elucidated,and an approach is proposed.A gel polymer electrolyte(Zn²⁺and H⁺conductor)is prepared by using the matrix of graphene oxide and polyvinyl alcohol.The stability of NVO is successfully improved with this electrolyte.Furthermore,based on the electric double layer phenomenon near the phase interface between electrolyte and active material,the related research of synaptic transistor devices is performed.This thesis proposes for the first time that the asymmetric electric double layer（AEDL）phenomenon existed in electrolyte-gating transistor devices can be introduced as a charge-based mechanism of volatile information storage.Through comparing the AEDL-related modulation results of the Li gate||LTO channel synaptic transistor with the regulation results of Li gate||α-Al₂O₃ channel and stainless steel gate||LTO channel devices,the main factors are basically clarified:（i）The threshold potential of AEDL modulation is closely related to the potential difference between gate and channel material,rather than the electronic structure of the channel material;（ii）The amplitude of AEDL modulation is positively correlated with the pulse width of stimuli voltage and the voltage difference between stimuli voltage and threshold potential.Thus,in Li gate||LTO channel synaptic transistor,short-term synaptic plasticity is realized by using AEDL phenomenon.The modulation behavior controlled by dual-signals（i.e.,dependent on both gate stimulus and channel current）in a Li gate||LTO channel synaptic transistor is realized for the first time.More importantly,bipolar response with temporal feature is successfully achieved:the channel signal induced by a single gate stimulus can first appear as excitatory or inhibitory,and then spontaneously evolve to inhibitory or excitatory at a particular channel current.Experiments verify the capability of IMT phenomenon in LTO material during reversible intercalation-deintercalation of lithium ions:IMT can change the channel carrier concentration,then affect the channel resistance and realize long-term synaptic plasticity.The contribution of nonvolatile IMT and volatile AEDL to the channel feedback signal induced by a single stimulus is illustrated.At the same time,it is observed in Li/LTO system that the change of channel apparent resistance caused by IMT is always opposite to that of AEDL.It is found that the weight balance of their contribution to the change of channel apparent resistance can be adjusted by channel current,which enables the device to switch between different functional modes:（i）Under the condition of high channel current,the device exhibits IMT dependent modulation;（ii）Under the condition of low channel current,the device shows an AEDL dependent modulation.（iii）Between these two conditions,the device displays the bipolar modulation dominated firstly by AEDL and then by IMT.In the latter mode,the device relies on two phenomena（IMT and AEDL）with different time characteristics and different response directions（exhibitory or inhibitory）to generate channel electrical signals.This is similar to the generation of action potential in mammalian neurons which relies on two ion fluxes,thus can naturally mimic action potentials at hardware level with a single device.The polycrystalline channel with phase transition is modeled with simplification,and a method to simulate the change tendency of resistance of polycrystalline channels in the process of phase transition is developed.The simulation results are consistent with the experimental results of synaptic transistor device based on LTO channel.After analyzing the simulation data,for polycrystalline resistive devices,it is considered that the channel behavior may be related to the statistical properties of polycrystalline grains.Since the depth of discharge at the establishment of first conductive path is very close to the depth of discharge at the boundary of two quasi linear regions of LTO material in the channel modulation experiment,it is believed that the establishment of conductive path may be the cause of the nonlinearity in LTO channel behavior.By comparing the simulation results with different parameters,it is concluded that the resistance evolution tendency and consistency of polycrystalline channel which rely on field induced phase transition may be affected by the following factors:（1）the ratio of resistivity between high and low resistance states of channel materials;（2）the number of intermediates in the phase transition of channel materials;（3）the physical size of the channel and the grains.