Solid-state Microbatteries Research Articles

In-Situ Confocal Microscopy to Analyze Volume Expansion of Solid State Microbatteries

The rise of embedded objects put great interests in miniaturized power supply solutions. Solid State Microbatteries (SSM) are the most promising, as they can safely store high energy and power density. SSM have the advantage of being all-solid (no liquid leakage and flammability), to have low self-discharge and good cyclability. Despite the strong interest in the field, the mechanisms of Li plating and stripping at the anode are not well observed and not yet fully understood [1]. The study of these phenomena would bring new insights to mechanical behavior during cycling and therefore may lead to optimization approaches at the cycle life level. In this purpose, we investigated the morphological change of SSM during the charge/discharge cycle. We looked at the influence of the cathode thickness and the packaging. Thick LiCoO2 layers were used for the study with thickness from 5 to 30 µm. Volume variations upon cycling were followed thanks to a Neox microscope 3D (Sensofar) and were correlated to charge/discharge capacity.Microbatteries are consisting of LiCoO2 (LCO) cathode and LiPON electrolyte. The anode-free configuration was adopted for integration compatibility. The anode is formed in-situ during the first charge operation. Wafers with different LCO thickness and back end level have been carried out. Confocal microscope was setup in Ar-filled glovebox to monitor the volume expansion during the charge and discharge operations. SSM cycling was operated through Biologic instruments. Figure 1 depicts a schematic representation of the SSM and the setup. Images were acquired during chrono-amperometric full charge at 4.2 V and galvanostatic full discharge at Idischarge = -3.5 µA within 3-4.2 V.Charge/discharge operations and corresponding in-situ thickness variations are presented in Figure 2 for the samples with different LCO thickness. The complete charge is achieved when current reaches a threshold of 0.5 µA. Charge capacity linearly increased with cathode thickness: Qcharge = 2.0 µA.h for the 5-µm-LCO and Qcharge = 9.6 µA.h for the 30-µm-LCO. Thickness variation increase follows the same evolution as capacity during charge and discharge. Regardless of the cathode thickness, same plating and stripping speed are observed during charge and discharge operation, respectively. According to the Qcharge and the volume variation, a theoretical lithium density at the anode side has been observed for all LCO thickness.Figure 3 depicts 2D images of 5-µm-LCO SSM at the onset of the charge. Pristine SSM presents a smooth surface (Fig. 3.a). Li plating is initiated with nucleated grains (Fig. 3.b). Then, grains grow and tend to coalesce with each other, forming a uniform and smooth Li layer (Fig. 3.c). Figure 4 reports 2D images of 30-µm-LCO SSM at different states of cycling. Bumps at the edge formed during the charge are due to SSM device architecture: electric field bends and the increased ion flux induces a more important Li accumulation on the edge. Otherwise, a uniform and fully dense Li metal deposition is realized in solid-state configuration under zero external pressure [2]. At the end of discharge, the battery thickness is higher than the pristine one (+ 3.7 µm). The Li plating non-reversibility phenomena after the first charge is due to low diffusion coefficient at low state-of-charge [3].Figure 5 shows the electrical behavior (Fig 5.a) and the volume expansion (Fig 5.b) of a SSM over 10 cycles. After the first cycle, cycling is performed at ≈100% coulombic efficiency, demonstrating high reversibility of the phenomena and so high cyclability of SSM [3].Finally, we followed the influence of the encapsulation layers on the volume variation. 2D images of the SSM reveals same volume variation for both samples. Those observations suggest that the encapsulation layers has no significant mechanical effect on the Li plating.In-situ confocal microscopy under Ar atmosphere has been proved efficient to study the volume expansion of SSM upon cycling and is a promising technique to investigate failure mechanisms of SSM.[1] J. Park et al., « In situ atomic force microscopy studies on lithium ( de ) intercalation-induced morphology changes in Li x CoO 2 micro-machined thin fi lm electrodes q », J. Power Sources, vol. 222, p. 417–425, 2013, doi: 10.1016/j.jpowsour.2012.09.017.[2] D. Cheng et al., « Freestanding LiPON: from Fundamental Study to Uniformly Dense Li Metal Deposition Under Zero External Pressure », arXiv.org, 8 août 2022. https://arxiv.org/abs/2208.04402v2.[3] J. Celè, S. Franger, Y. Lamy, et S. Oukassi, « Minimal Architecture Lithium Batteries: Toward High Energy Density Storage Solutions », Small Weinh. Bergstr. Ger., p. e2207657, janv. 2023, doi: 10.1002/smll.202207657. Figure 1

Elucidating Dynamic Conductive State Changes in Amorphous LLTO for Resistive Switching Devices

Resistive random-access memory (ReRAM) is one of the promising memory devices to achieve neuromorphic computing that is a novel system to mimic human brain. ReRAM has a simple metal-insulator-metal structure and has advantages such as low power consumption and low manufacturing cost. Recently, our group has developed amorphous lithium lanthanum titanate (Li3xLa2/3-xTiO3, LLTO) thin film as a solid-state electrolyte for all solid-state thin-film batteries by using pulsed laser deposition, which has high ionic conductivity (≈10-3 S/cm) [1]. Beyond its application to energy storage devices, we found that the amorphous LLTO (a-LLTO) demonstrates a resistive switching response under the stimuli of an external electric field, where LLTO becomes high resistance state (HRS) at pristine state and switches to low resistance state (LRS) by external electric field. However, despite its unique feature, the resistive switching mechanism of a-LLTO is poorly understood.Thus, in this work, we demonstrate successful observation of LLTO resistive switching behavior and optimize voltage range to achieve both resistive switching and cycling stability. Furthermore, electronic conductivity changes that provoke the LLTO switching are investigated through the comparison of thin films with different oxygen composition, under an hypothesis that oxygen vacancy plays a role in changing local LLTO conductivity. First-principles analysis also supports such conductivity change mechanism due to the local composition changes under electrical bias. Based on these experimental and computational results, the switching mechanisms are discussed with a simplified LLTO model. At last, in-situ testing of LLTO nano device under focused ion beam (FIB) setup, which aims for scanning transmission electron microscopy (STEM)-electron energy loss spectroscopy (EELS) to fully elucidate the switching mechanisms of LLTO at nanoscale.Reference[1] Lee, J. Z.; Wang, Z.; Xin, H. L.; Wynn, T. A.; Meng, Y. S. Amorphous Lithium Lanthanum Titanate for Solid-State Microbatteries. J. Electrochem. Soc. 2016, 164 (1), A6268. Figure 1

High-Performance Zn-Ion Microbatteries by Subtractive Manufacturing

Microbatteries are indispensable to power emerging miniaturized electronics for medical, environmental, or stealth applications. However, they are usually fabricated by additive printing processes, limiting their sizes and energy densities. Here, we develop a subtractive manufacturing process by slicing assembled thin-film zinc-ion batteries into microcells in batches using optimized preintercalated MnO2 (K0.27MnO2·0.54H2O, KMO) as cathodes. The composite cathode consisting of KMO and reduced graphene oxide (rGO) delivers a high specific capacity of 323.4 mAh g–1 at 0.1 A g–1 with no capacity loss after 750 cycles at 1 A g–1. Such a high performance renders Zn//KMO/rGO solid-state microbatteries with areal energy densities of 0.244 mWh cm–2, which are comparable to those of the state of the art. This microbattery, with an area of 4 × 4 mm2 and a thickness of 0.56 mm, can be modularly arranged on a chip for a stretchable energy-supply circuit, which can be closely attached to human skin.

3D LiMn2 O4 Thin Film Deposited by ALD: A Road toward High-Capacity Electrode for 3D Li-Ion Microbatteries.

Miniaturized electronics suffer from a lack of energy autonomy. In that context, the fabrication of lithium-ion solid-state microbatteries with high performance is mandatory for powering the next generation of portable electronic devices. Here, the fabrication of a thin film positive electrode for 3D Li-ion microbatteries made by the atomic layer deposition (ALD) method and in situ lithiation step is demonstrated. The 3D electrodes based on spinel LiMn2 O4 films operate at high working potential (4.1 V vs Li/Li+ ) and are capable of delivering a remarkable surface capacity (≈180 μAh cm-2 ) at low C-rate while maintaining more than 40 μAh cm-2 at C/2 (time constant = 2 h). Both the thickness of the electrode material and the 3D gain of the template are carefully tuned to maximize the electrode performance. Advanced characterization techniques such as transmission electron and X-ray transmission microscopies are proposed as perfect tools to study the conformality of the deposited films and the interfaces between each layer: no interdiffusion or segregation are observed. This work represents a major issue towards the fabrication of 3D-lithiated electrode by ALD-without any prelithiation step by electrochemical technique-making it an attractive solution for the fabrication of 3D Li-ion solid-state microbatteries with semiconductor processing methods.

Flash Lamp Annealing Enables Thin-Film Solid-State Batteries on Aluminum Foil

Crystallization of cathode films in solid-state microbatteries requires thermal annealing at high temperatures, restricting the choice of substrate and current collector materials. Here, flash lamp annealing (FLA) is explored to crystallize LiCoO2 (LCO) cathodes on aluminum foils. Millisecond pulses of visible light induce rapid heating of the LCO films up to 900 °C, whereas the aluminum never exceeds the melting point. Microbatteries consisting of an FLA-processed LCO cathode, a LiPON electrolyte, and a Li metal anode are fabricated on flexible aluminum foil, with performance comparable to those on rigid silicon. This method can enable new microbattery designs at lower production costs.

Electrochemically Controlled Deposition of Ultrathin Polymer Electrolyte on Complex Microbattery Electrode Architectures

Solid state microbatteries are highly sought after for emerging microsensor technologies. To overcome the problem of the dwarfing capacity resulting from the miniaturization of the battery, 3D-structured platform consisting of high surface area micropillar-shaped electrodes are used. However, applying a conformal and continuous solid polymer electrolyte films onto the intricate 3D electrodes is a crucial step toward achieving functional microbatteries. In this work, we present our approach for the development of polyethylene oxide (PEO)-acrylate based ion conducting polymer thin films which function as solid polymer electrolyte (SPE) and a separator. The SPEs were electrochemically deposited on the 3D electrodes resulting in ultrathin, continuous, conformal, and pinhole-free polymer films. The electrochemical and Li+ ions transport properties of the SPEs were characterized by EIS measurements and cyclic voltammetry. Furthermore, the homogenous composition of the SPEs at various depths were confirmed by XPS depth profiling techniques.

Development of Low Cost Printable All Solid State Lithium Microbatteries Incorporating Photo-Polymerized Ionogel As Electrolyte

With the advent of the microelectronics and the "smart-devices", the need for micro-sources of energy has risen. To meet these needs, a massive production of microbatteries is required in the next decades. Physical vapor deposition (PVD) is the process commonly used to fabricate thin film all solid state microbatteries. Commercially, LiCoO2 and lithium are used respectively as positive and negative electrodes. LiPON is commonly used as solid electrolyte because of its good ionic conductivity, good thermal and electrochemical stability and also for its good interfacial stability with the electrodes. However, the PVD process requires very expensive deposition machineries, with a high energy cost, which impact consequently the price and the development of the microbatteries. Processes involving liquid step, such as ink jet printing or screen printing propose cheaper machinery and much higher production rate. Moreover, these techniques are precise enough to deposit any pattern either by located drop jetting or by use of specific mask fitting the desired shape. In this communication, we will report our efforts to develop all or partially printed microbatteries. We have worked on replacing LiPON by another solid electrolyte which could be deposited by printing. Ionogels are solid electrolyte in which an ionic liquid (IL) is confined within a solid host network. Although solid like electrolytes, ionogels are formed from a liquid precursor, and are thus appropriate materials for the purpose. The liquid precursor is obtained by mixing the IL with organic, inorganic or hybrid monomers that are then polymerized to build in-situ the confining network. Furthermore, ionogels show good ionic conductivity and also good thermal and electrochemical stability, thus making them attractive solid electrolyte for lithium batteries. The time required to form a fully solidified material can vary depending on the nature of the confining network and the polymerization process. By using photo-polymerization, which is very fast, the manufacturing time is lesser than an hour, while for LiPON it can last more than ten hours. We have identified a promising printable photo-polymerizable ionogel composition with high ionic conductivity, pressure-sensitive-adhesive character, low interface resistance with lithium electrode, which allows stable cycling of LiFePO4/lithium cells, at least up to 1200 cycles at ambient temperature, C/5 rate and for surface capacities typical of microbatteries, e.g. 0.15 mAh cm-2. This ionogel is based on short molecular weight monomers with high content of polymerizable functionalities (triacrylate and diacrylate) and ILs composed of fluorinated sulfonylimide anions combined with alkylated pyrrolidinium cations [1]. We have evaluated the feasibility of making all or partially printed microbatteries with this printable solid electrolyte, not starting from the ground but trying to fit in an existing industrial process that uses PVD for manufacturing microbatteries. Microbatteries assembled from a dense PVD made LiCoO2 layer, a printed photo-polymerized ionogel, and a lithium layer exhibit good performance. However, the photo-polymerized ionogel showed very low compatibility when the lithium electrode is directly deposited by PVD on the top of it. Thus the lithium electrode must be mechanically assembled with the ionogel electrolyte. Microbatteries assembled from a printed porous LiFePO4 composite layer [2], a printed photo-polymerized ionogel, and lithium layer (an already made foil or evaporated layer and mechanically pressed on the ionogel surface) also demonstrated good performance. Acknowledgements Financial funding from the project TOURS 2015 n° O12590-418393 (Programme d’Investissement d’Avenir – FSN – AAP Nanoélectronique n°1) is acknowledged.

Role of the chemical composition and structure on the electrical properties of a solid state electrolyte: Case of a highly conductive LiPON

Lithium Phosphorus Oxynitride glass (LiPON) is the most used electrolyte for lithium solid-state microbatteries. The current main challenge is improving its ionic conductivity without deteriorating its very low electronic conductivity, wide electrochemical window and stability with Li. While the chemical mechanisms of ionic conduction have been intensively studied, they still are controversial and not fully understood to date. Therefore, it is particularly important to well define and understand the influence of the keys parameters acting on the ionic conductivity.In this study, LiPON is deposited by reactive radio frequency (RF) sputtering of Li3PO4 under nitrogen by two ways. LiPON deposited with a non-standard method exhibits an exceptionally high ionic conductivity (6.7 × 10−6 S cm−1) compared to the standard method (1.4 × 10−6 S cm−1). The chemical composition and structure are compared using RBS/NRA and FTIR, respectively. The electrical properties as ionic and electronic conductivities, charge mobility and carrier concentration are measured by impedance spectroscopy. It is demonstrated the positive influence of the disorder created by the phosphate groups on the mobility. This high mobility creates a depletion area at the interfaces which can be suppressed by adding a buffer layer. The interfacial properties are also investigated from both chemical and electrical point of view using ToF-SIMS and impedance spectroscopy.

Improved Electrochemical Performance of an All Solid-State Microbattery By Electrodeposition of Polymer Electrolyte into the Nanostructured Electrodes

The miniaturization of Lithium ion batteries (LIBs) as a power source to drive small devices such as smartcards, medical implants, sensors, radio-frequency identification (RFID) tags etc. has been continuously developed to meet the market requirements of portable applications1. In this direction 3D microbatteries have been considered to satisfy the requirements of these portable devices. All solid-state microbatteries involving self-supported titania nanotubes (TiO2 nts) have been considered, due to their high surface area and short diffusion lengths for Li+ ion transport2. It has been reported that the electropolymerization of the polymer electrolyte through cyclic voltammetry (CV) leads to the conformal deposition of the electrolyte into the 3D TiO2 nts 3,4. So far, the electropolymerization and its influence on the capacity was reported with regard to TiO2 nts only5. This work, along with electropolymerized TiO2 nts, includes the electropolymerization of the polymer electrolyte onto the Lithium nickel manganese oxide (LNMO) composite cathode and its influence on the capacity of the whole microbattery. A drop cast layer of the monomer methyl methacrylate-polyethylene glycol (MA-PEG) serves as the separator and ion conducting medium. Fig.1 a) and b) shows the CV of the electropolymerization of the polymer electrolyte into the TiO2 nts and LNMO respectively. The cathodic current density decreases with cycle number which indicates that the thin polymer layers is deposited onto the TiO2 nts wall which helps in using the effective surface area of the nanotubes. For the LNMO composite electrode, the higher current density observed is due to the high rugosity of the cathode layer. This suggests that the polymer layer formed at the initial cycles allows to follow the rugosity of the LNMO electrode, which is important to establish a good electrode-electrolyte interface. A comparison of the capacities by electropolymerized electrodes (TiO2nts(EP)/Polymer/LNMO(EP)) and non-electropolymerized electrodes (TiO2nts/Polymer/LNMO) shows the positive influence of the electrodeposition of the polymer electrolyte on the capacity of the microbattery which is shown in Fig. 1 c) and d). TiO2nts(EP)/Polymer/LNMO(EP) microbattery delivers a capacity of 169 mAh.g-1 (82 μAh cm−2 μm−1) at the first cycle and 150 mAh.g-1 (70 μAh cm−2 μm−1 ) for the tenth cycle at C/10 rate. Comparison of the capacities at C/10 and C/2 with TiO2nts/Polymer/LNMO showed an increment of around 100% even after 100 cycles.6 Fig. 1 Cyclic voltammetry of the electropolymerization on the (a) TiO2nts and (b) LNMO in aqueous solution of 0.5 M MMA-PEG + 0.5 M LiTFSI vs. Ag/AgCl (saturated) in the potential window -0.35 V to -1 V at a scan rate of 10 mV.s-1. Galvanostatic charge/discharge profiles of different microbatteries at C/10 for (c) TiO2nts(EP)/Polymer/LNMO(EP); (d) TiO2nts/Polymer/LNMO. References 1) B.L. Ellis, P. Knauth, T. Djenizian, Three-Dimensional self-supported metal oxides for advanced energy storage, Adv. Mater. 26 (2014) 3368-3397. 2) T. Djenizian, I. Hanzu, P. Knauth, Nanostructured negative electrodes based on titania for Li-ion microbatteries, J. Mater. Chem. 21 (2011) 9925-9937. 3) G. F. Ortiz, I. Hanzu,, T. Djenizian, P. Lavela,, J. L. Tirado, P. Knauth, Alternative Li-Ion Battery Electrode Based on Self-Organized Titania Nanotubes, Chem. Mater. 21 (2009), 63–67. 4) N. Plylahan, N.A. Kyeremateng, M. Eyraud, F. Dumur, H. Martinez, L. Santinacci, P. Knauth, T. Djenizian, Highly conformal electrodeposition of copolymer electrolytes into titania nanotubes for 3D Li-ion batteries, Nanoscale Res. Lett. 7 (2012) 349-353. 5) N. Plylahan, M. Letiche, M.K.S. Barr, B. Ellis, S. Maria, T.N.T. Phan, E. Bloch, P. Knauth, T. Djenizian, High energy and power density TiO2 nanotube electrodes for single and complete lithium-ion batteries, J. Power Sources 273 (2015) 1182-1188. 6) G. D. Salian , C. Lebouin , A. Demoulin , M.S. Lepihin , S. Maria ,A.K. Galeyeva , A.P. Kurbatov,T. Djenizian, Electrodeposition of polymer electrolyte in nanostructured electrodes for enhanced electrochemical performance of thin-film Li-ion microbatteries, J. Power Sources, 340 (2017), 242-246. Figure 1

Atomic Layer Deposition of Functional Layers for on Chip 3D Li‐Ion All Solid State Microbattery

Nowadays, millimeter scale power sources are key devices for providing autonomy to smart, connected, and miniaturized sensors. However, until now, planar solid state microbatteries do not yet exhibit a sufficient surface energy density. In that context, architectured 3D microbatteries appear therefore to be a good solution to improve the material mass loading while keeping small the footprint area. Beside the design itself of the 3D microbaterry, one important technological barrier to address is the conformal deposition of thin films (lithiated or not) on 3D structures. For that purpose, atomic layer deposition (ALD) technology is a powerful technique that enables conformal coatings of thin film on complex substrate. An original, robust, and highly efficient 3D scaffold is proposed to significantly improve the geometrical surface of miniaturized 3D microbattery. Four functional layers composing the 3D lithium ion microbattery stacking has been successfully deposited on simple and double microtubes 3D templates. In depth synchrotron X‐ray nanotomography and high angle annular dark field transmission electron microscope analyses are used to study the interface between each layer. For the first time, using ALD, anatase TiO2 negative electrode is coated on 3D tubes with Li3PO4 lithium phosphate as electrolyte, opening the way to all solid‐state 3D microbatteries. The surface capacity is significantly increased by the proposed topology (high area enlargement factor – “thick” 3D layer), from 3.5 μA h cm−2 for a planar layer up to 0.37 mA h cm−2 for a 3D thin film (105 times higher).

Fabrication of 3D hexagonal bottle-like Si–SnO2 core–shell nanorod arrays as anode material in on chip micro-lithium-ion-batteries

Three-dimensional (3D) Si–SnO2 composite core–shell nanorod arrays were fabricated as the anode material in lithium ion micro-batteries by nanosphere lithography (NSL) combined with inductive coupled plasma (ICP) dry etching technology. The hexagonal bottle-like Si NR arrays in wafer scale with homogeneous morphology and good mechanical structure provide enough space to accommodate the volume expansion during Li ion insertion/de-insertion processes, while the additionally deposited SnO2 thin film was prepared to successfully improve the capacities and cycle performance by configuring the 3D Si–SnO2 NR composite electrode arrays. This fabrication method has the advantages of simplicity, large scale production, easy size and shape manipulations, low cost and Si-process compatibility. This work will facilitate the configuration of solid state micro-batteries for power supply in micro-electronic devices, such as MEMS devices or smart IC chips.

Microstructural and electrochemical properties of rf-sputtered LiMn2O4 thin film cathodes

Lithium transition metal oxides have received considerable attention in recent years as high voltage positive electrode materials in the fabrication of all solid state microbatteries. Among various lithium-based cathode materials, LiMn2O4 is one of the most promising cathode materials as it offers high energy density, high cell voltage, low cost, and low toxicity over the other electrode materials. Thin films of LiMn2O4 were prepared by radio frequency magnetron sputtering on gold-coated silicon substrates under various substrate temperatures ranging from 373 to 673 K in a partial pressure of 3 × 10−3 mbar with rf power 100 Watts. In the present investigation, the influence of substrate temperature on the growth and microstructural properties was studied. The films deposited at a substrate temperature less than 473 K was found to be X-ray amorphous. The initial crystallization has been observed at a substrate temperature of 523 K. The X-ray diffraction patterns of the films deposited in the substrate temperature range 523–673 K exhibited predominant (111) orientation representing cubic spinel structure with Fd3m symmetry. The grain size was found to be increased with the increase of substrate temperature as evidenced from SEM studies. However, additional impurity phases like Mn3O4 were observed for the films deposited at higher substrate temperatures (>673 K) because of re-evaporation of Li+ ions in the films. The electrochemical (EC) studies were carried for the films deposited at Ts = 673 K in aqueous media in the potential window of 0.0–1.2 V exhibited better electrochemical performance suggesting that the films are well suited as binder free thin film cathode material for commercially viable Li-ion secondary batteries.

Lithiated c-V2O5 thin-film as positive electrode for rocking-chair solid-state lithium microbattery

We report for the first time the use of lithiated crystalline V2O5 thin films as positive electrode in all-solid-state microbatteries. Crystalline LixV2O5 films (x ≈ 0.8 and 1.5) are obtained by vacuum evaporation of metallic lithium deposited on sputtered c-V2O5. An all-solid-state lithium microbattery of Li1.5V2O5/LiPON/Li exhibited a typical reversible capacity of 50 μAh/cm2 in the potential range 3.8/2.15 V which exceeds by far the results known on all-solid-state lithium batteries using amorphous V2O5 films and lithiated amorphous LixV2O5 thin films as positive electrode. Hence, the present work opens the possibility of using high performance crystalline lithiated V2O5 thin films in rocking-chair solid-state microbatteries.

Reaction mechanism of tin nitride (de)lithiation reaction studied by means of 119Sn Mössbauer spectroscopy

Tin nitride thin films have been reported as promising negative electrode materials for lithium-ion solid-state microbatteries. However, the reaction mechanism of this material is not yet fully understood. Results on thin film electrodes pointed out that the conversion mechanism of tin nitride most likely differs from the conversion mechanism usually observed for other oxide and nitride conversion electrode materials. The electrochemical data showed that more than six Li per Sn atom can be reversibly exchanged by this material while about four are expected. In order to investigate in more detail the reaction mechanism of tin nitride, thick film electrodes of two compositions (1:1 and 3:4) have been studied. The as-prepared materials were characterized by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy and 119Sn Mössbauer spectroscopy. Moreover, films (de)lithiated to various extents were analyzed ex situ with Mössbauer spectroscopy. The corresponding results indicate that a more complex reaction mechanism than that generally accepted takes place. During Li-ion insertion, the disappearance of Sn 4+ environments is correlated with the formation of Li–Sn phases, and most likely also of Li 3N. In the case of the SnN x 1:1 composition films, the formation of various Li–Sn phases is evidenced while only the signature of ‘Li 22Sn 5’ is clearly measured for the 3:4 composition. Upon Li-ion extraction, the Li–Sn phases and Li 3N recombine to form octahedrally and tetrahedrally coordinated Sn 4+. The extraction is not fully reversible and the end product consists of a mixture of a tin nitride structure plus a Li y Sn product having the same isomer shift as LiSn but a much higher quadrupole splitting, and most likely some Li 3N.

Electrochemical behaviour of sputtered c-V 2O 5 and LiCoO 2 thin films for solid state lithium microbatteries

Here are reported for the first time electrochemical data on all-solid-state lithium microbatteries using crystalline sputtered V 2O 5 thin films as cathode materials and LiPON as solid electrolyte. The stable specific capacity of 30 µAh/cm 2 found with a 2.4 µm 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 LiCoO 2 thin films, we show that a temperature of 500 °C combined with an optimized bias sputtering (-50 V) allows to get highly crystalline deposits, to minimize the presence of Co 3O 4 and to suppress any trace of the cubic phase. At the same time the theoretical specific capacity is reached in the 4.2 V-3 V range and a good cycling behaviour is achieved with a high capacity of 50 µAh/cm 2/µm after 140 cycles at 10 µA.cm 2.

Tin Nitride Thin Films as Negative Electrode Material for Lithium-Ion Solid-State Batteries

Tin nitride thin films have been reported as promising negative electrode materials for lithium-ion solid-state microbatteries. However, the reaction mechanism of this material has not been thoroughly investigated in the literature. To that purpose, a detailed electrochemical investigation of radio-frequency-sputtered tin nitride electrodes of two compositions (1:1 and 3:4) is presented for several layer thicknesses. The as-prepared thin films have been characterized by Rutherford backscattering spectrometry, inductively coupled plasma optical emission spectrometry, scanning electron microscopy, X-ray diffraction, and transmission electron microscopy. The electrochemical results point out that the conversion mechanism of tin nitride most probably differs from the conversion mechanism usually observed for other oxide and nitride conversion electrode materials. The electrochemical data show that more than 6 Li per Sn atom can be reversibly exchanged by this material, whereas only about 4 are expected. Moreover, the electrochemical performance of the material is discussed, such as electrode cycle life, and a method for improving the cycle life is presented. Finally, thicker films have been characterized by Mössbauer spectroscopy. This technique opens a new route toward determining the conversion reaction mechanism of this promising electrode material.

Optical and electrical properties of laser ablated amorphous LiCoO2 thin film cathodes

Thin films of LiCoO2 were prepared by pulsed laser deposition technique. Two important deposition parameters such as substrate temperature and oxygen partial pressure during the thin film deposition were controlled. The optical absorbance of LiCoO2 thin films have been studied in the vicinity of the fundamental absorption edge. We observed a gradual increase of the optical absorption from 400 to 1000 nm with the appearance of a prominent step at 750 nm. Electrical transport plays an important role in the performance of the electrode in the lithium microbattery. The electrical conductivity of LiCoO2 varies from 10 -9 to 10 -4 S/cm in the temperature range from 180 to 500 K. FTIR supported the optical data. These results suggest that the open and porous structured LiCoO2 PLD films find potential applications as binder free electrode in the fabrication of all solid state microbatteries.

Low-Pressure Chemical Vapor Deposition of LiCoO2 Thin Films: A Systematic Investigation of the Deposition Parameters

The feasibility of volatile precursor low-pressure chemical vapor deposition (LPCVD) for the production of cathodes for all solid-state microbatteries was examined. To test this feasibility, and gain insight into the deposition behavior, the influence of the deposition parameters on the properties of LPCVD grown thin-film cathodes was systematically investigated. The deposition temperature, concentration of the various reactants, and duration of thin-film growth were varied. The resulting layers were subjected to X-ray diffraction, inductively coupled plasma–atomic emission spectrometry, Rutherford backscattering spectroscopy, and electrochemical analyses. Stoichiometry of the films could be controlled by varying the precursor flows. Samples deposited at high temperatures with the optimum stoichiometry showed a high crystallinity and a high electrochemical activity; a storage capacity corresponding to a reversible Li-content around the theoretical value of 0.55 per Co was reached, and a good cycling stability was obtained when using this electrode in combination with a solid-state electrolyte.

Towards High Energy Density 3D-integrated Lithium-ion Micro-batteries

AbstractTo investigate the feasibility of a 3D integrated all-solid-state micro-battery, the deposition of several battery materials was investigated. Deposition techniques where used that are in principle able to deposit step conformally in 3D structures: ALD was used to create a conductive Pt current collector, and LPCVD was applied for the deposition of poly-silicon anodes and LiCoO2 cathodes. The layers, initially deposited on planar substrates, showed the expected physical and electrochemical behavior and are in principle suitable for solid state micro-batteries.

A Combinatorial Study of LiyMnxNi2 − x O 4 Cathode Materials Using Microfabricated Solid-State Electrochemical Cells

A methodology for batch-fabricating hundreds of submillimeter thin-film solid-state batteries was used in conjunction with a combinatorial materials synthesis technique. This approach allowed for the simultaneous creation of many solid-state microbatteries that had cathodes, where x varied continuously from 0.2 to 1.8 and y varied from 2.7 to 3.7 in the as-deposited films. Sputtered lithium phosphorous oxynitride (LiPON) was used as an electrolyte, while evaporated, micropatterned Li metal served as the anode layer. The composition, thickness, and microstructure of the cathodes were examined using X-ray energy-dispersive spectroscopy, synchrotron-based X-ray diffraction, Rutherford backscattering spectroscopy, stylus profilometry, inductively coupled plasma mass spectroscopy, and scanning electron microscopy. Electrochemical cycling data allowed for cell performance to be correlated to cathode composition and structure. Cathodes with a composition of had both the highest specific capacity as well as the longest high-potential (4.7 V) discharge plateau, while cells with higher Mn content had a longer 4 V discharge plateau, a result that qualitatively agrees with similar studies on conventional bulk-fabricated cathodes of the same material. © 2003 The Electrochemical Society. All rights reserved.