Structural characterization and electrochemical properties of RF-sputtered nanocrystalline Co3O4 thin-film anode

Nanocrystalline Co₃O₄ thin-film anodes were deposited on Pt-coated silicon and 304 stainless steel by radio frequency (RF) magnetron sputtering. The as-deposited and annealed cobalt oxide thin films showed smooth and crack-free morphologies. Both the as-deposited and annealed films exhibited spinel Co₃O₄ phase with nanocrystalline structure. High-temperature annealing enhanced the crystallinity of RF-sputtered cobalt oxide films due to rearrangement of cobalt and oxygen atoms. Electrochemical characterization of RF-sputtered films was carried out by cyclic voltammetry and charge/discharge tests in the voltage range of 0.3–3.0 V. Cyclic voltammetry plots showed that the RF-sputtered Co₃O₄ thin films were electrochemically active. X-ray photoelectron spectrometer (XPS) showed that the fresh cobalt oxide films had two peaks of Co₃O₄. In addition to the binding energy of cobalt oxide, the XPS spectrum of discharged film presented two additional binding energies correspond to Co metal. The first discharge capacities of as-deposited, 300, 500, and 700 °C-annealed films were 722.8, 772.5, 868.4, and 1059.9 μAh cm⁻² μm⁻¹, respectively. High-temperature annealing could enhance the capacity and cycle retention obviously. After 25 cycles discharging, the annealed films showed better cycle retention than as-deposited film. The 700 °C-annealed film exhibited excellent discharge capacity approximated to the theoretical capacity.     

Structural and electrical properties of LiCoO2 thin-film cathodes deposited on planar and trench structures by liquid-delivery metalorganic chemical vapour deposition

The electrochemical properties of annealed-LiCoO₂ cathodes deposited on planar and trench structures by liquid-delivery metalorganic chemical vapor deposition are investigated for various deposition temperatures and input Li:Co mole ratios. With the planar structure, the best crystallinity of the films is obtained at a deposition temperature of 450 °C and an input Li:Co mole ratio of 1.0. The deposition window for optimum initial discharge capacity and capacity retention is a deposition temperature of 450–500 °C and an input Li:Co mole ratio of 1.0, and an input Li:Co mole ratio of 1.0–1.2 at a deposition temperature of 450 °C. The initial discharge capacity and capacity retention of LiCoO₂ thin films deposited with an input Li:Co mole ratio of 1.2 at 450 °C are approximately 25 μAh/cm² μm and 77%, respectively. The initial discharge capacity of films deposited on a trench structure shows an increase of approximately 130% compared with that of films deposited on a planar structure with an input Li:Co mole ratio of 1.2. The rechargeabilities of films deposited in a trench structure are inferior to those in a planar structure because conformal growth in the trench structure is poor. Thus, a trench structure can improve the initial discharge capacity and capacity retention of lithium microbatteries.     

Effect of annealing temperature on electrochemical performance of thin-film LiMn2O4 cathode

Spinel LiMn₂O₄ thin-film cathodes were obtained by spin-coating the chitosan-containing precursor solution on a Pt-coated silicon substrate followed by a two-stage heat-treatment procedure. The LiMn₂O₄ film calcined at 700 °C for 1 h showed the highest Li-ion diffusion coefficient, 1.55 × 10⁻¹² cm² s⁻¹ (PSCA measurement) among all calcined films. It is attributed to the larger interstitial space and better crystal perfection of LiMn₂O₄ film calcined at 700 °C for 1 h. Consequently, the 700 °C-calcined LiMn₂O₄ film exhibited the best rate performance in comparison with the ones calcined at other temperatures.     

Rapid thermal annealing effect on surface of LiNi1−xCoxO2 cathode film for thin-film microbattery

The effect of rapid thermal annealing (RTA) on the surface of a LiNi_1−xCo_xO₂ cathode film is examined by means of scanning electron microscopy (SEM), atomic force microscopy (AFM) and auger electron spectroscopy (AES). It is found that the as-deposited LiNi_1−xCo_xO₂ film undergoes a surface reaction with oxygen in the air, due to the high activity of lithium in the film. AES spectra indicate that the surface layer consists of lithium and oxygen atoms. The RTA process at 500 °C eliminates the surface layer to some extent. An increase in annealing temperature to 700 °C results in complete elimination of the surface layer. The surface evolution of the LiNi_1−xCo_xO₂ film with increasing annealing time at 700 °C is examined by means of AFM examination. It is found that the surface layer, which is initially present in the form of an amorphous like-film, becomes agglomerated and then vaporizes after 5 min of annealing. A thin-film microbattery (TFB), fabricated by using the LiNi_1−xCo_xO₂ film without a surface layer, exhibits more stable cycliability and a higher specific discharge capacity of 60.2 μAh cm⁻² μm than a TFB with an unannealed LiNi_1−xCo_xO₂ film. Therefore, it is important to completely eliminate the surface layer in order to achieve high performance from all solid-state thin-film microbatteries.     

Vanadium-based antireflection coated on multicrystalline silicon acting as a passivating layer

In this paper, we present important experimental results of a new efficient (ARC), leading to an efficient surface passivation that have not been reported before. Vanadium pentoxide V₂O₅ powder was thermally evaporated onto the front surface of mc-Si substrates, followed by a short annealing duration at 600 °C, 700 °C and 800 °C under an O₂ atmosphere. The chemical composition of the deposited vanadium oxide thin films was analyzed by means of Fourier Transform Infrared Spectroscopy (FTIR). Surface and cross-section morphology were determined by a scanning electron microscope (SEM). The effect of the deposited thin film on the electrical properties was evaluated by means of the internal quantum efficiency (IQE), minority carrier lifetime measurements which have been made using a WTC-120 photoconductance lifetime tester and we used dark current–voltage (I–V) characteristic to measure the defect density at a selected grain boundary (GB) in all samples and compared to an untreated wafer. The results show that the deposited thin film single layer gives the possibility of combining, in one processing step, an antireflection coating deposition along with efficient surface state passivation, as compared to a reference wafer.     

Effect of chitosan on stabilization of acetates-containing solution: A novel precursor for LiMn2O4 film deposition

Due to the adequate viscosity of the chitosan-added precursor solutions, the films deposited from the chitosan-added precursor solution showed a higher deposition rate than the ones from the PVP-added solution under the same coating parameters. Furthermore, the chitosan-added precursor solution remained stable without any precipitation for at least 10 months. On the other hand, without the addition of chitosan, the precursor solution showed apparent precipitation after being stirred for 12 h. The enhanced stability of the precursor solution by the addition of chitosan is attributed to the complexation between metal ions and the –NH₂ groups of chitosan. And the electrochemical behavior for the deposited films calcined at 700 °C for 1 h was also characterized by charge–discharge test. The result revealed that the film deposited from chitosan-containing precursor solution possesses an initial discharge capacity of 134 mAh g⁻¹ and about 9% capacity loss after 50 charge/discharge cycles, which is better than the one deposited from chitosan-free precursor solution with an initial discharge capacity of 108 mAh g⁻¹ and 24% capacity loss after 50 cycles.     

Characterization of a LiCoO2 thick film by screen-printing for a lithium ion micro-battery

A thick film cathode has been fabricated by a screen-printing technique using LiCoO₂ paste to improve the discharge capacity in lithium ion micro-batteries. The LiCoO₂ thick film (about 6 μm) was obtained by screen-printing, but high discharge capacity and a suitable surface roughness of printed LiCoO₂ film cathodes could not be obtained by adding carbon black only to the LiCoO₂ paste. On the other hand, the printed cathode which was prepared using the mixture of carbon-coated LiCoO₂ powders and carbon black showed a typical discharge curve of a LiCoO₂ cathode with a high discharge capacity (179 μAh cm⁻²).     

Structural and electrochemical properties of nanocrystalline LixMn2O4 thin film cathodes (x = 1.0–1.4)

The process optimization of nanocrystalline lithium manganate thin films (Li_xMn₂O₄; x = 1.0–1.4) has been demonstrated by using a cost-effective solution growth technique. Films were first attempted with Pt–Si (Si/SiO₂/TiO₂/Pt) substrates but because of inter-diffusion of TiO₂ buffer layer with Pt at higher annealing temperature, phase impure LiMn₂O₄ films were obtained. Phase pure films on the basis of XRD analysis were found on Pt substrate at specified growth parameters. The annealing temperature and annealing time were varied, the films annealed at 700 °C for 2 h were found to be the best films. The nanocrystalline nature of the films was revealed by the SEM micrographs and the surface morphology studied using AFM. Finally, the electrochemical properties (cyclic voltammetry and constant current measurements) of these films were analyzed using a home made three-electrode cell and Gamry Battery tester instrumentation. The formation of a prominent layer of fluoride species deposited over the cathode surface during the repeated cycling was revealed by XPS measurements. Further experiments are in progress on identifying the exact composition of these unwanted species. The formation of the Jahn-Teller active Mn³⁺ during electrochemical cycling was completely ruled out from the XPS analysis. Also the very consistent value of [Mn³⁺/Mn⁴⁺] ratio before and after electrochemical cycling on the surface of the film revealed good quality of the films. Finally, the formation of the fluoride layer was concluded as a passive layer that causes the initial capacity drop during first few cycles of the cell performance.     

Deposition of ZrO2 film by liquid phase deposition

In this study, undoped ZrO₂ thin films were deposited on single-crystal silicon substrates using liquid phase deposition. The undoped films were formed by hydrolysis of zirconium sulfate (Zr(SO₄)₂·4H₂O) in the presence of H₂O. A continuous oxide film was obtained by controlling adequate (NH₄)₂S₂O₈ concentration. The deposited films were characterized by SEM, FT-IR, XRD and DTA. Typically, the films showed excellent adhesion to the substrate with uniform particle diameter about 150 nm. The thicknesses of ZrO₂ film were about 200 nm after 10 h deposition at 30 °C. These films shows single tetragonal phase after heat treated at 600 °C. High annealing temperature (e.g. 750 °C) may result in the phase transformation of (t)-ZrO₂ into (m)-ZrO₂.     

Characterization and preparation of NiO-V2O5 composite film cathodes

NiO-V₂O₅ composite films have been fabricated by 355 nm pulsed laser reactive deposition using mixed metallic Ni and V targets with different molar ratios. The optimal deposition conditions of NiO-V₂O₅ composite films are found to be the substrate temperature of 300 °C and 100 mTorr O₂ ambient. X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses showed that the crystallinity of the NiO-V₂O₅ composite films gradually decreased with increasing the amount of nickel oxide and transformed to amorphous phase as the molar ratio of NiO/V₂O₅ (x) approaches 0.5. The amorphous (NiO)_0.5V₂O₅ composite film electrode exhibited a specific capacity of 340 mAh/g at a discharge rate of 2 C upon cycling with no obvious fading up to 500 cycles. XRD and X-ray photo-electron spectroscopy (XPS) measurements of NiO-V₂O₅ composite film electrodes revealed that there exists two electrochemical processes upon cycling. During the first discharge, the Li ions insertion process is accompanied by the reduction of NiO into metallic Ni. Then, the reversible processes involving Li ions insertion/extraction in V₂O₅ matrix and oxidation/reduction of Ni and Li₂O take place upon the subsequent cycling.     

Improvement of 12 V overcharge behavior of LiCoO2 cathode material by LiNi0.8Co0.1Mn0.1O2 addition in a Li-ion cell

The 12 V overcharge instability of the LiCoO₂ cathode material was improved by the physical blending it with LiNi_0.8Co_0.1Mn_0.1O₂. Even though a Li-ion cell containing a LiCoO₂ cathode did not exhibit thermal runaway at 12 V at the 1 C overcharging rate, it showed thermal runaway at the 2 C overcharging rate, and the cell surface temperature reached more than 400 °C. However, the LiCoO₂ cell containing 40, 50, and 60 wt.% LiNi_0.8Co_0.1Mn_0.1O₂ did not exhibit thermal runaway at the 2 C overcharging rate. In conclusion, 60 wt.% LiNi_0.8Co_0.1Mn_0.1O₂ in the LiCoO₂ cathode showed the lowest cell surface temperature of <90 °C even at a 3 C overcharging rate.     

Improvement of the rate capability of LiMn2O4 by surface coating with LiCoO2

In order to use LiMn₂O₄ as a cathode material of lithium-secondary battery for an electric vehicle (EV), its rate capability should be improved. To enhance the rate capability of LiMn₂O₄ in this work, the surface of LiMn₂O₄ particle was coated with LiCoO₂ by a sol–gel method. Because LiCoO₂ has a higher electric conductivity than LiMn₂O₄, it is possible to improve the rate capability of LiMn₂O₄. After the surface coating, LiCoO₂-coated LiMn₂O₄ showed a higher discharge capacity of 120 mAh/g than as-received LiMn₂O₄ (115 mAh/g) because LiCoO₂ has a higher capacity than LiMn₂O₄. The rate capability of the coated LiMn₂O₄ improved significantly. While as-received LiMn₂O₄ maintained only 50% of its maximum capacity at a 20C rate (2400 mA/g), the LiCoO₂-coated LiMn₂O₄ maintained more than 80% of maximum capacity. LiCoO₂-coated LiMn₂O₄ with 3 wt.% conducting agent (acetylene black) showed the higher rate capability than as-received LiMn₂O₄ with 20 wt.% conducting agent. From electrochemical impedance spectroscopy (EIS) result that the first and second semicircles of coated LiMn₂O₄ were reduced, the improvement of rate capability is attributed to a decrease of passivation film that acts as an electronic insulating layer and a reduced inter-particle contact resistance. Accordingly, It is proposed that the surface coating of LiMn₂O₄ with LiCoO₂ improve the rate capability as well as the specific and volumetric energy density due to the decrease of conducting agent.     

Preparation of Cu2ZnSnS4 thin films by sulfurizing electroplated precursors

Cu₂ZnSnS₄ (CZTS) thin films were prepared by sulfurizing precursors deposited by electroplating. The precursors (Cu/Sn/Zn stacked layers) were deposited by electroplating sequentially onto Mo-coated glass substrates. Aqueous solutions containing copper sulfate for Cu plating, tin sulfate for Sn plating and zinc sulfate for Zn plating were used as the electrolytes. The precursors were sulfurized by annealing with sulfur at temperatures of 300, 400, 500 and 600 °C in an N₂ gas atmosphere. The X-ray diffraction peaks attributable to CZTS were detected in thin films sulfurized at temperatures above 400 °C. A photovoltaic cell using a CZTS thin film produced by sulfurizing an electroplated Sn-rich precursor at 600 °C exhibited an open-circuit voltage of 262 mV, a short-circuit current of 9.85 mA/cm² and an efficiency of 0.98%.     

Synthesis and electrochemical properties of Mo-doped Li[Ni1/3Mn1/3Co1/3]O2 cathode materials for Li-ion battery

The layered Li[Ni_(1−x)/3Mn_(1−x)/3Co_(1−x)/3Mo_x]O₂ cathode materials (x = 0, 0.005, 0.01, and 0.02) were prepared by a solid-state pyrolysis method (700, 800, 850, and 900 °C). Its structure and electrochemical properties were characterized by XRD, SEM, XPS, cyclic voltammetry, and charge/discharge tests. It can be learned that the doped sample of x = 0.01 calcined at 800 °C shows the highest first discharge capacity of 221.6 mAh g⁻¹ at a current density of 20 mA g⁻¹ in the voltage range of 2.3–4.6 V, and the Mo-doped samples exhibit higher discharge capacity and better cycle-ability than the undoped one at room temperature.     

The influence of target history and deposition geometry on RF magnetron sputtered LiCoO2 thin films

Thin LiCoO₂ films, typically used as cathode layers in thin-film solid-state batteries were RF magnetron sputter-deposited using targets that were either freshly produced, or had seen over 100 h of sputter erosion. The substrates, as received (1 0 0) silicon wafers, were either held stationary or were rocked back and forth under the target. Film texturing, grain size, composition, and thickness were examined using X-ray diffraction (synchrotron light source), inductively coupled plasma-mass spectroscopy (ICP-MS), Rutherfords backscattering spectrometry (RBS) and stylus profilometry. Films that were sputtered from the heavily used target were, on average, lithium-deficient, while films deposited using the fresh target were slightly lithium-rich. Film thickness, composition, and type of crystallographic texture varied radially, in the plane of the film in the stationary substrate case, in a pattern that reflected the sputter target erosion ring. For films deposited with substrate motion, an ovular area was defined on the film in which composition, and texturing were essentially uniform. The Li/Co ratio in the target and subsequent films was found to decrease over many hours of sputtering. Possible causes for the compositional and orientational variations observed are discussed.     

Li-ion transport in all-solid-state lithium batteries with LiCoO2 using NASICON-type glass ceramic electrolytes

LiCoO₂ thin films were deposited on the NASICON-type glass ceramics, Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂, by radio frequency (RF) magnetron sputtering and were annealed at different temperatures. The as-deposited and the annealed LiCoO₂ thin films were characterized by X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). It was found that the films exhibited a (1 0 4) preferred orientation after annealing and Co₃O₄ was observed by annealing over 500 °C due to the reaction between the LiCoO₂ and the glass ceramics. The effect of annealing temperature on the interfacial resistance of glass ceramics/LiCoO₂ and Li-ion transport in the bulk LiCoO₂ thin film was investigated by galvanostatic cycling, cyclic voltammetry (CV), potentiostatic intermittent titration technique (PITT) and electrochemical impedance spectroscopy (EIS) with the Li/PEO/glass ceramics/LiCoO₂ cell. The cell performance was limited by the Li-ion diffusion resistance in Ohara/LiCoO₂ interface as well as in bulk LiCoO₂.     

Wide-optical bandgap with improved conductivity p-μc-Si:Ox:H films prepared by Cat-CVD

Boron-doped hydrogenated microcrystalline silicon oxide (p-μc-Si:O_x:H) films have been deposited using catalytic chemical vapor deposition (Cat-CVD). The single-coiled tungsten catalyst temperature (T_fil) was varied from 1850 to 2100 °C and films were deposited on glass substrates at the temperatures (T_sub) of 100–300 °C. Different catalyst-to-substrate distances of 3–5 cm and deposition pressures from 0.1 to 0.6 Torr were considered.Optical and electrical characterizations have been made for the deposited samples. The sample transmittance measurement shows an optical-bandgap (Eg_opt) variation from 1.74 to 2.10 eV as a function of the catalyst and substrate temperatures. One of the best window materials was obtained at T_sub=100 °C and T_fil=2050 °C, with Eg_opt=2.10 eV, dark conductivity of 3.0×10^?3 S cm^?1 and 0.3 nm s^?1 deposition rate.     

Thermal simulation of large-scale lithium secondary batteries using a graphite–coke hybrid carbon negative electrode and LiNi0.7Co0.3O2 positive electrode

Thermal simulation was applied to 2 Wh-class cells (diameter 14.2 mm, height 50 mm) using LiNi_0.7Co_0.3O₂ or LiCoO₂ as the positive electrode material, in order to clarify the thermal behavior of the cells during charge and discharge. The thermal simulation results for the 2 Wh-class cells showed a good agreement with measured temperature values. The heat generation of a cell using LiNi_0.7Co_0.3O₂ was found to be much less than that using LiCoO₂ during discharge. This difference was considered to be caused by the difference in the change of entropy. A 250 Wh-class cell (diameter 64 mm, height 296 mm) was also constructed using LiNi_0.7Co_0.3O₂ and thermal simulation was applied. We confirmed that the results of the thermal simulation agreed with measured values and that this simulation model is effective for analyzing the thermal behavior of large-scale lithium secondary batteries.     

Ion-implantation modification of lithium–phosphorus oxynitride thin-films

Among various solid electrolytes, the lithium–phosphorus oxynitride (Lipon) electrolyte synthesized by sputtering of Li₃PO₄ in pure N₂ has a good ionic conductivity of 2(±1)×10⁻⁶ S cm⁻¹ at 25° C. As the nitrogen concentration increases in the Lipon electrolyte, the ionic conductivity is reported to increase as a result of a higher degree of cross-links. When Lipon films are deposited by sputtering, however, it is reported that the maximum nitrogen concentration saturates approximately at 6 at.%. By non-equilibrium processes, such as ion-implantation, nitrogen concentration can be controlled over 6 at.%. This study investigates the effect of nitrogen concentration on the ionic conductivity in Lipon films by using ion-implantation. Impedance measurements at 25° C show that the nitrogen-implanted Lipon films enhance or retard the ionic conductivity over a wide range after nitrogen-implantation, when compared with as-deposited thin-films.     

Effect of titanium substitution in layered LiNiO2 cathode material prepared by molten-salt synthesis

LiNi_1−xTi_xO₂ (0 ≤ x ≤ 0.1) compounds have been synthesized by a direct molten-salt method that uses a eutectic mixture of LiNO₃ and LiOH salts. According to X-ray diffraction analysis, these materials have a well-developed layered structure (R3-m) and are an isostructure of LiNiO₂. The LiNi_1−xTi_xO₂ (0 ≤ x ≤ 0.1) compounds have average particle sizes of 1–5 μm depending on the amount of Ti salt. Charge–discharge tests show that a LiNi_1−xTi_xO₂ (0 ≤ x ≤ 0.1) cathode prepared at 700 °C has an initial discharge capacity as high as 171 mA h g⁻¹ and excellent capacity retention in the range 4.3–2.8 V at a current density of 0.2 mA cm⁻².