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Abstract: Investigations were realized on the microstructural and morphological evolution of RF-sputtered vanadium pentoxide thin films during growth. V2O5 thin films at different stages of growth were studied by spectroscopic ellipsometry, X-ray diffraction, atomic force microscopy and scanning electron microscopy. Film grain orientation, roughness and density were found to have notable evolution during growth. Electrochemical tests in liquid and solid electrolyte state configuration showed non-linear relationship between discharge capacity and V2O5 film thickness (<1μm), which could be attributed in parts to the observed morphological and microstructural changes during growth, mainly the existence of a gradient density through film thickness and the pronounced top surface roughness. [Copyright &y& Elsevier]

The electrochemical behavior of all-solid-state lithium microbatteries using crystalline sputtered V2O5 thin films as cathode materials and Li phosphorus oxynitride as solid electrolytes is reported. Vanadium oxide films are deposited by radio-frequency (rf) magnetron sputtering. The results for thin-film batteries using c-V2O5 are discussed and compared with those known for all-solid-state lithium microbatteries usually using amorphous V2O5. Depending on the oxygen flow rate used during deposition, porous or dense crystalline V2O5 films are obtained. The stable discharge capacity of 35 µAh/cm² achieved at a current density of 10 µA/cm² between 3.8 and 2.15 V with a 0.6 µm thick porous film can be improved with the use of a 1 µm thick dense film, which delivers 50 µAh/cm² with high stability upon cycling. At a high current density of 100 µA/cm², a specific capacity of 25 µAh/cm² is obtained without any capacity fading over 100 cycles. This work shows that crystalline V205 thin films prepared by rf magnetron reactive sputtering are attractive cathode materials for rechargeable all-solid-state thin-film lithium microbatteries. [ABSTRACT FROM AUTHOR]

Abstract: LiCoO2 thin films have been prepared on aluminium substrate with RF sputtering using a bias effect combined with a moderate post-annealing treatment at 500 °C and their electrochemical properties investigated in liquid electrolyte by cyclic voltammetry, galvanostatic experiments in various voltage windows, and ac impedance spectroscopy. The positive effect of bias is shown allowing to minimize the presence of the cubic LiCoO2 phase. The lithium extraction process is highly reversible in the biased LiCoO2 films with charge–discharge curves which really superimpose at C/10 rate. A 450 nm thin film is shown to exhibit an initial charge capacity of ≈60 μAh cm−2 μm−1 at C/10 and a high discharge capacity of ≈50 μAh cm−2 μm−1. The rate capability study demonstrates the film can easily sustain high charge–discharge rates and exhibits good cycling properties with capacity retention of 90% after 50 cycles up to a 2C rate and still 73% of the maximum capacity is recovered at 8C in the 4.2 V–3 V potential window. This constitutes remarkable performances both in terms of rate capability and cycling stability. The charge–discharge behaviour as a function of the film thickness in the range 0.25–3.6 μm is investigated and indicates an efficient and homogeneous electrochemical activity with a linear increase of the capacity with thickness. As a consequence an optimization of the capacity is proposed with the use of the thickest deposit of 3.6 μm leading to the value of 200 μAh cm−2, the best one available for a LiCoO2 film in liquid electrolyte. [Copyright &y& Elsevier]

Abstract: Here are reported for the first time electrochemical data on all-solid-state lithium microbatteries using crystalline sputtered V2O5 thin films as cathode materials and LiPON as solid electrolyte. The stable specific capacity of 30µAh/cm2 found with a 2.4µm thick film competes very well with the best values obtained for solid state microbatteries using amorphous films. With the challenge of decreasing the temperature of heat treatment for sputtered LiCoO2 thin films, we show that a temperature of 500°C combined with an optimized bias sputtering (-50V) allows to get highly crystalline deposits, to minimize the presence of Co3O4 and to suppress any trace of the cubic phase. At the same time the theoretical specific capacity is reached in the 4.2V-3V range and a good cycling behaviour is achieved with a high capacity of 50µAh/cm2/µm after 140 cycles at 10µA.cm2. [Copyright &y& Elsevier]

Abstract: The kinetics of the electrochemical lithium insertion reaction in crystalline V2O5 thin films in liquid electrolyte has been investigated using ac impedance spectroscopy. The experimental data are obtained for sputtered films characterized by a common morphology corresponding to an arrangement of V2O5 platelets perpendicular to the substrate (h 00 or 110 preferred orientation). The results are discussed as a function of the Li content for 0< x <1 in Li x V2O5, the film thickness in the range of 0.6–3.6μm and temperature 15–55°C. The moderate evolution of the chemical diffusion coefficient D vs. the lithium content is related with the specific structural response of these pure thin film materials which exhibit a single phase behavior. A comparison of the kinetic parameters for different thickness values allows to indicate the same Li diffusion rate whatever the film thickness and the diffusion pathway does not correspond to the thickness but to the length of the edge (≈1μm) of V2O5 platelets. For the first time, an experimental evaluation of the activation energy for Li diffusion in crystalline V2O5 is obtained. A value of 0.98eV is found for a diffusion phenomenon along the b direction. This work demonstrates the excellent capacity–rate performance as well as the efficient and homogeneous behavior of these oriented films can be explained by their specific microstructure. [Copyright &y& Elsevier]

Abstract: Vanadium pentoxide thin films have been prepared by reactive DC magnetron sputtering from a vanadium metal target without annealing posttreatment. XRD, Raman and electrochemical experiments on 800nm thin films show a high crystallinity of the deposits. Chronopotentiometric measurements performed in the two voltage range 3.8–2.8 and 3.8–2.15V have demonstrated the promoting effect of the h 00 preferred orientation of V2O5 films in terms of polarization, kinetics and rate capability. The same reproducible deposition method is successfully applied to get films thicker than 1μm in order to optimize the specific capacity. Effective high specific capacities can be then obtained with films 2.4μm thick tested at high constant current density (100μAcm−2): a stable capacity of 75μAhcm−2 is available over 100 cycles in the 3.8–2.8V potential range and 130μAhcm−2 are still recovered in the range 3.8–2.15V. These results indicate a promising cycling behaviour can be expected with thick films exhibiting a preferred orientation corresponding to V2O5 planes perpendicular to the substrate. [Copyright &y& Elsevier]

Transparent electrochemical energy storage devices have attracted extensive attention for the power supply of next-generation transparent electronics. In this paper, semitransparent thin film batteries (TFBs) with a grid-structured design have been fabricated on glass substrates using specific photolithography and etching processes to achieve LiCoO 2 /LiPON/Si structures below human eye resolution. UV-vis transmittances up to 60% have been measured for the obtained TFBs. A discharge capacity as high as 0.15 mAh has been recorded upon galvanostatic cycling at the C/2 rate within 4.2-3 V voltage range for the highest transmittances. The capacity variation trend exhibits an initial phase of a gradual decrease with an average capacity loss of 0.15% per cycle and thereafter a second phase with almost stable capacity. Particular attention has been given to the effects of architecture parameters on the TFB optical and electrochemical properties. To the best of our knowledge, this work is the first demonstration of transparent, all inorganic, thin film lithium ion batteries. While reported studies are limited to battery structures involving liquid or polymer materials, our devices will contribute to improve form factor freedom, extend operating ranges, enhance long-term stability, and will be relevant to the integration into various optoelectronic devices.

Lithium cobalt oxide nanobatteries offer exciting prospects in the field of nonvolatile memories and neuromorphic circuits. However, the precise underlying resistive switching (RS) mechanism remains a matter of debate in two-terminal cells. Herein, intriguing results, obtained by secondary ion mass spectroscopy (SIMS) 3D imaging, clearly demonstrate that the RS mechanism corresponds to lithium migration toward the outside of the Li x CoO 2 layer. These observations are very well correlated with the observed insulator-to-metal transition of the oxide. Besides, smaller device area experimentally yields much faster switching kinetics, which is qualitatively well accounted for by a simple numerical simulation. Write/erase endurance is also highly improved with downscaling - much further than the present cycling life of usual lithium-ion batteries. Hence very attractive possibilities can be envisaged for this class of materials in nanoelectronics., (© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The phenomenon of resistive switching (RS), which was initially linked to non-volatile resistive memory applications, has recently also been associated with the concept of memristors, whose adjustable multilevel resistance characteristics open up unforeseen perspectives in cognitive computing. Herein, we demonstrate that the resistance states of Li(x)CoO2 thin film-based metal-insulator-metal (MIM) solid-state cells can be tuned by sequential programming voltage pulses, and that these resistance states are dramatically dependent on the pulses input rate, hence emulating biological synapse plasticity. In addition, we identify the underlying electrochemical processes of RS in our MIM cells, which also reveal a nanobattery-like behavior, leading to the generation of electrical signals that bring an unprecedented new dimension to the connection between memristors and neuromorphic systems. Therefore, these LixCoO2-based MIM devices allow for a combination of possibilities, offering new perspectives of usage in nanoelectronics and bio-inspired neuromorphic circuits.