Electrolyte And Positive Electrode Thin Film Materials For All-solid-state Lithium Batteries

All-solid-state thin film lithium batteries usually consist of positive electrode, solid electrolyte and negative electrode thin films.Compared with traditional powder materials,thin film materials could show their particular properties in some aspects. The fabrication of thin film form of the traditional materials can't simply satisfy all the demands in the fields of thin film batteries.This thesis will focus on the positive electrode and electrolyte thin film materials applied in all-solid-state thin film lithium batteries.In our initial investigation on positive electrodes,we have considered the thin film fabrication of traditional materials with and without doping for the practical application in thin film batteries.At first,we successfully fabricated nano-crystalline Li_xMn₂O₄(LMO) thin films by R.F.Sputtering.LMO thin film exhibited distinct and symmetrical electrochemistry at 3V and 4V regions,respectively,and it was accordant with the performance of LiMn204 powder electrodes.Due to the absence of Mn²⁺ dissolution in all-solid-state case,the thin film battery with Li/LiPON/LMO layers showed excellent cycling stability in a large voltage range of 2-4.8V,and the reversible capacity could be well kept at 78μAh/cm²-μm.However,in the charge/discharge curves of LMO batteries,the feature of two separated plateaus with 1V step was considered to be unfavorable for practical application.Thus,we tried to add abundant ZrO2 in thin film for the preparation of LMZO to resolve this problem. ZrO₂ characterized by low thermal conductivity,high chemical inertness and high hardness was viewed as a suitable material to maintain the small grain size of LMO after high-temperature annealing or subsequent hundreds of cycles.The thin film battery with Li/LiPON/LMZO layers exhibited utterly different electrochemical behavior from LMO.1.7 Li per LMZO could be inserted at the end of discharge as LMO,and the capacity of Li/LiPON/LMZO could be reversibly maintained at 53μAh/cm²-μm.In fact,for most of positive electrode materials,the high-temperature synthesis process is unavoidable in order to obtain the ideal crystallinity,which is favored for the highly active electrochemistry.However,the high-temperature treatment is not expected in the fabrication process of thin film batteries considering the compatibility with micro-electron techniques.Moreover,the well-crystallized thin films always show quite rough surface with apparent grain boundaries,which is unfavorable for the interface match with solid electrolyte layers.Based on the requirement of on-chip integration of thin film batteries in micro-devices,the amorphous positive electrode thin films are expected from our knowledge due to the low-temperature deposition and the absence of annealing process.The smooth surface of amorphous thin film could largely improve the cycling life and electrode/electrolyte interface,whose degradation even short circuit could be effectively eliminated.According to the above description,LMZO thin film has been demonstrated to show smoother interface, which should be resulted from the control of crystal grain growth during anneal. However,the aim for the fabrication of amorphous thin film without high-temperature treatment hasn't been achieved.Enlightened by the reports of amorphous FePO4 powder materials with active electrochemistry,we fabricated the amorphous FePO₄ and nitrided iron phosphate(FePON) positive electrode thin films for the first time.In the measurement of Li/LiPON/FePO4 cells,the FePO4 thin films deposited at low-temperature showed higher volumetric rate capacity.However,due to dynamic limitation,the reversible capacity was merely kept at about 20μAh/cm²-μm.The higher capacity 63μAh/cm²-μm of as-deposited FePON thin film was obtained when thin film was deposited in pure N₂ ambient.From CV curves of FePON,a set of additional redox peaks located at 1.7V and 2.4V regions was observed,and it was associated with the great capacity enhancement.The Raman and XPS data indicated that the PO₄^3- polyanion framework might be destroyed by the N insertion in FePON thin film.For further investigation on the improvement of thin film electrochemistry after N insertion,we have used the AC impedance technique for quantitive study from a viewpoint of dynamics.Here,two systems based on FePO4 and FePON thin films, sandwich structures with a variety of layer number and all-solid-state battery structures,were adopted.The ionic conductivity of FePON and FePO₄ thin films estimated from sandwich structures was 2.5×10^-8 S/cm and 7.5×10^-10 S/cm, respectively,while the electronic conductivity of FePON and FePO₄ was 2.6×10^-8 S/cm and 2.7×10^-6 S/cm,respectively.The ionic and electronic conductivities of FePON were much more balanced than those of FePO₄.N insertion resulted in the reverse change of thin film ionic and electronic conductivities,which might rise from the oxygen defects.FePON and FePO₄ had the same activation energy of 0.72 eV estimated from the ionic conductivity of various temperatures.The activation energy of interface between electrode and solid electrolyte was relatively small(0.25 eV), indicating the compact contact of amorphous film layers and the facilitated ion-transfer through interface.In all-solid-state battery structures,the charge transfer resistance at various potentials was greatly reduced by the doping of N.The average chemical diffusion coefficient of Li⁺ ion FePON and FePO4 was 1.7×10¹¹ cm²/s and 1.2×10^-13 cm²/s,respectively,indicating that the doping of N facilitated the lithium diffusion in solid electrodes.On the other hand,as known,traditional positive electrodes usually consist of one reactive center.Of course,some materials have more than one active couples. However,the possibly heavy weight or structural collapse during charge/discharge may limit the mass rate capacity.In the fields of thin film electrodes and batteries,the volumetric rate capacity should be the most important performance index.Therefore, some materials with multiple active centers,despite of heavy weight,should be paid enough attention in their thin film form.These compact thin films with high density are expected to improve the volumetric rate capacity of thin film batteries.In this section,we have choosed the materials based on active polyanion framework(WO₄)^2-. CuWO₄ and LiFe(WO₄)₂ positive electrode thin films with two active couples have been successfully fabricated by R.F.Sputtering.For CuWO4 thin films,through the characterization of XRD and SEM,the anorthic phase was made up of nano-sized crystal grains with 20 nm dimension.The initial discharge capacity of 192 mAh/g was achievable for Li/CuWO₄ cells,and two separated plateaus at 2.5 V and 1.6 V regions were detected due to the reactivity of both Cu²⁺/Cu⁰ and W⁶⁺/W⁴⁺.Because of the possible degradation at electrode-electrolyte interface and Cu ion dissolution in liquid electrolyte,CuWO4 electrode underwent gradually structural evolution accompanied with capacity degradation.Based on the TEM and SAED data at different charged/discharged states,a reasonable two-step reaction mechanism of CuWO₄ associated with displacement reaction was suggested.The(WO₄)^2- framework could be rebuilt after reductive decomposition,and well-crystallized CuWO₄ could be reproduced as well during electrochemistry.In Li/LiPON/CuWO₄ cells,the initial discharge capacity was as high as 145μAh/cm²-μm.With the absence of possible degradation phenomena at cathode-electrolyte surface and Cu ion dissolution in all-solid-state system,the electrode structure and electrochemical curves were well kept during the following cycling.Amorphous LiFe(WO₄)₂ positive electrode thin films are expected to achieve both the objects,the design of multiple reactive centers and the deposition fabrication at low-temperature.Li/LiFe(WO₄)₂ cells showed a large initial discharge capacity of 198 mAh/g,and two separated plateaus at 3 V and 1.5 V regions were observed due to the reactivity of two redox couples(Fe³⁺/Fe²⁺ and W⁶⁺/W^x+,where x = 4 or 5).A sustainable discharge capacity of 110 mAh/g could be maintained over 300 cycles. Based on the XPS and TEM data at different charged/discharged states,it has been demonstrated that two redox centers associated with Fe and W were electrochemically reversible.An unavoidable crystallization tendency in as-deposited LiFe(WO4)2 thin film was found after initially several charge/discharge cycles,with concomitant continuous capacity degradation in the potential range between 1 V and 2 V Another advantage of LiFe(WO₄)₂ thin film is its capacity retention under high current density, and a volumetric rate capacity of 60μAh/cm²-μm could be well stabilized under a current density of 50μA/cm².Solid electrolyte thin films are the crucial functional layers in the assembly of thin film batteries.However,the valuable thin film electrolytes have not been plentifully provided as expected.So far,the lithium phosphorous oxynitride(LiPON) thin film has been widely used due to its high ionic conductivity and large electrochemical stability window.In order to enhance the ionic conductivity of LiPON,it is necessary to increase the N insertion quantity in thin films and improve the thin film deposition techniques.Here,we used electron cyclotron resonance(ECR) plasma in R.F.Sputtering equipment to assist the LiPON deposition for increasing the ionic conductivity.The optimization condition was found to be at ECR power of 200 W.Under this fabrication condition,the molar ratio of N/P was estimated to be 0.65,and Li ionic conductivity was obtained to be 8.0×10^-6S/cm.In addition,the R.F.Sputtering technique,which is the best method for LiPON fabrication,has been considered to have some drawbacks,such as low deposition rate and small deposition area.Moreover,some intrinsic disadvantages of LiPON thin film, such as vulnerability to moisture in air,couldn't also be neglected.So we used e-beam evaporation technique to fabricate lithium lanthanum titanate(LLTO) solid electrolyte thin-films instead of LiPON thin film.This fabrication method could greatly enhance the deposition efficiency.E-beam power was a crucial factor for the preparation of high quality LLTO thin films.The as-deposited LLTO thin film at e-beam power of 600 W had the highest ionic conductivity of 1.8×10^-7 S/cm with activation energy of 0.32 eV.All thin films prepared at different e-beam powers showed the amorphous structure but with distinct surface morphologies from XRD and SEM data.XPS measurement indicated that higher e-beam power could keep more La insertion into LLTO thin film.An all-solid-state cell was fabricated using the amorphous LLTO thin film as solid electrolyte and presented good cycle stability at electrochemical window of 3 V-4.4 V.In order to further enhance the capacity of solid micro-batteries,the enlargement of contact area between different functional layers is expected.However,the planar contact area of 2D solid-battery thin films is very limited.The development of 3D electrodes with high rate surface area is being paid much attention today.Recently,the carbon micro-electromechanical systems(C-MEMS) as 3D materials are fashionable. As the extension part of this thesis,the 3D carbon micro-net films(CMNFs) as a prototype of C-MEMS have been primarily investigated for the possible application in 3D solid micro-batteries.CMNFs were fabricated by micro-machining technology, consisting of SU-8 photoresist patterning by photolithography and two-step pyrolysis process.They structurally presented the mixture of both short-distance ordered diamond-like phase and amorphous carbon matrix by TEM.The first discharge capacity for CMNFs was calculated to be as high as 350μAh/cm²,and in the following cycles,the reversible capacity was well maintained at about 100μAh/cm². Compared with graphitized carbonous materials,the pseudocapacitance-like electrochemical behavior and wide CV redox peaks indicated complex electrode reaction with Li⁺.After 100^th cycle,the original 3D shape of CMNFs was well kept without the presence of structural collapse and network rupture.A reasonable Li⁺ moving pathway was concluded according to the size self-regulation of CMNF pattern due to the effect of electrolyte penetration.Under the absence of pyrolysis temperature gradients,most of Li could be intercalated or deintercalated paralleling to the cross-section of CMNF sides.Due to the features of continuity of network structure and fixing of nodes,the CMNFs are expected to provide much interesting information.For the next step,they are aimed for the possible application in future 3D solid micro-batteries.