A microbattery is a solid state battery (SSB) designed to provide power for small-scale electronic devices. SSB have significant advantages over conventional batteries in terms of performance, sustainability and safety due to their greater thermal and chemical stability, higher energy density, and absence of flammable liquids. Due to the increasing demand of flexible electronic microdevices such as implantable medical devices or skin patches, many researchers have been focused on the deposition of positive and negative electrodes and solid electrolytes for the development of microbatteries. Among micro-SSB, thin-film batteries (TFBs) typically feature a layer-by-layer stacked structure, where various components (cathode, electrolyte, and anode) are sequentially deposited onto a substrate, which can also serve as a current connector. To ensure efficient transfer of both electrical and ionic charge, the electrodes must be extremely thin (with a maximum thickness of a few microns), with the electrolyte even thinner. To achieve this, various deposition techniques have been explored, such as magnetron sputtering, pulsed laser deposition (PLD), thermal evaporation, chemical vapor deposition (CVD), atomic layer deposition (ALD), and printing (Xia et al., 2023). Within these, PLD is widely recognized as a versatile technique for the growth of films. Thanks to the possibility to deposit dense and pure films, PLD has gathered the attention of the scientist for the development of solid state batteries (SSB) (Julien and Mauger, 2019; Fenech and Sharma, 2020). In PLD, a pulsed laser beam ablates material from a solid target, depositing it onto a substrate in form of thin film. The interaction between the laser and target material causes laser photon absorption, leading to the vaporization of the superficial layers of the target and plasma formation, consisting in a mixture of atoms, molecules, ions, electrons, and clusters. The plasma’s composition and expansion are closely linked to deposition parameters, particularly the ambient background (such as vacuum or background gas inert or reactive) and laser parameters (including pulse duration, wavelength, and fluence). Therefore composition, morphology, crystallinity and thickness of the obtained film can be tuned by controlling many experimental parameters during the deposition such as laser wavelength, energy and pulse length, deposition temperature and atmosphere and so on. The role of several PLD parameters on the film’s growth and properties has been discussed (Indrizzi et al., 2020). The advantages and drawbacks of PLD with respect to other chemical and physical deposition methodologies used for the growth of thin electroactive films, were recently reviewed (Karimzadeh et al., 2023; Xia et al., 2023). In brief, among the main advantages of PLD are: i) the ability to deposit any type of material for TFBs, ii) congruent transfer of composition, and iii) low deposition temperatures. However, scaling up production proves challenging as the film maintains its homogeneity over a limited area of the substrate.

This mini-review explores the studies published after 2019 focused on laser technologies for the development of batteries with different chemistry and microbatteries with enriched performances. The cathode, anodes and electrolytes components are separately considered. In particular we have considered studies that report the electrochemical characterization for anodes and cathodes. As the study of solid-state electrolytes is relatively recent compared to other components, only limited investigations on thin film electrolytes deposited via PLD are available, with some focusing solely on optimizing deposition parameters. Therefore, all papers have been considered, despite the absence of electrochemical studies. The experimental details for the deposition of cathodes, anodes and electrolytes are reported in Table 1. A schematic illustration of the sequential deposition process and the cross-section of a thin film battery is reported in Figure 1.

With respect to negative electrodes for secondary batteries, few studies were published in the last years. Commercial anodes for LIBs involve the use of graphite, but its theoretical capacity is too low for the development of high energy density batteries suitable for microelectronic or medical devices. Metallic lithium has high theoretical energy density (−3,680 mAh/g) and low reduction potential (−3.04 V vs. standard hydrogen potential) and can serve as anode for Li Metal Batteries (LMB). However, it presents some serious safety concerns due to the dendrite propagation during the battery cycle life. To tackle the anode issue, thin, dense films of transition metal oxides or multielemental oxides were grown by PLD to serve as anodes or to protect highly reactive anodes. Lithium ferrite nanostructures have been considered as low cost and environmental friendly material. Subash et al. (2023) grown crystalline LiFe₅O₈ films with a thickness of about 100 nm. The ordered ferrite thin films present a specific discharge capacity of 25 µAh/cm² and current density of 10 μA/cm², suggesting their potential use for fabricating sustainable solid batteries.

The capability of PLD technique to deposit films with controlled thickness and morphology resulted useful to study and to limit processes that negatively affect the performances of electrochemical cells such as lithium dendrite growth. In fact, to mitigate the formation and propagation of dendrite that are detrimental for the battery stability Li et al. (2021) deposited on the anode surface a electrically insulating and ionically conductive BN film of about 1.2 µm. The BN film, deposited in argon atmosphere at a pressure of 50 mTorr posses a 3D hierarchical structure providing free space for Li accommodation during Li stripping and plating. The mechanisms involved in the beneficial effect of the coating of Li anodes with Al₂O₃ films were deeply investigated by Kakimi et al. (2023). By depositing smooth Al₂O₃ layers with controlled thickness, the authors compare the cycling stability of coin cells with coated and uncoated electrodes. The Al₂O₃ film thickness was adjusted varying the deposition time from 1 to 3 h, obtaining films 15 nm and 82 nm thick, respectively. The authors observed that the thinner Al₂O₃ coatings assure a more efficient reaction of lithium deposition-dissolution. In fact, lithium grows with a plate-like morphology on the coated electrode, ensuring a good electronic conduction between the electrode and the deposited lithium that is mainly removed from the electrode surface during the dissolution cycle. Moreover, XPS analysis of the coated and uncoated electrodes after cycling, highlight the Al₂O₃ films are able to suppress the formation of lithium carbonate arising for the electrolyte decomposition.

Since TiO₂ have a negligible volume variation during Li intercalation-deintercalation, anatase anodes have been proposed for long cycle life batteries. Although TiO₂ is a low cost and non toxic materials, its major drawbacks are the poor electronic and ionic conductivities. Various strategies have been proposed to overcome these disadvantages, such as combining with other oxides like SnO₂, which has a higher specific capacity. The deposition of a multilayered SnO₂/TiO₂ has been proposed by Wang et al. (2020). The layered anode was grown alternating SnO₂ and TiO₂ layers (20 nm and 5 nm thick, respectively) for total thickness of 300 nm. Since the deposition experiments were carried out at room temperature, authors suggest that the electrode’s amorphous structure can play a beneficial role in its electrochemical performances, in particular considering the cycle stability. In fact, the surface of the coated electrode is characterized by a morphology without any cracks after 200 cycles. A different strategy proposed to overcome the drawbacks of TiO₂ that limit its industrial applications is to tailor size and morphology of TiO₂-based materials. With respect to the possibility to reduce the TiO₂ morphology at the nanoscale, it is well known that during the ablation with femtosecond laser source the ejection of nanoparticles that retains the target stoichiometry take place, getting to the growth of films with a nanomorphology very different with respect to films obtained by nanosecond ablation (De Bonis and Teghil, 2020). To obtain compact, dense and nanostructured TiO₂ films (Figure 2), femtosecond laser ablation of an anatase target has been proposed (Curcio et al., 2021). The authors studied the deposition mechanism and developed and characterised a TiO₂/C layered anode suitable for microbatteries with lower charge transfer resistance, higher apparent Li⁺ diffusion and better cycling stability than reference microelectrodes.

SnO₂ is an efficient anode for SIBs, but the high volume change during cycling due to Na insertion and de-insertion leads to cracking and degradation of electrochemical performance. Biswal et al. (2021) demonstrated the role of the morphology and crystallinity of SnO₂ films on the cycling properties of SIBs. They deposited films at different substrate temperature (300, 400°C and 500°C) with a “cauliflower-like” morphology. With increasing the temperature the nanograins of the structure tend to aggregate and their crystallite size tends to increase as shown by SEM and XRD analysis, respectively. The authors show that the presence of an amorphous phase and nanograin morphology has a positive effect on the film’s ability to resist volume changes, improving its stability compared to crystalline films.

The use of all-state batteries is a promising alternative to conventional batteries. In particular, the use of solid electrolytes (SEs) offers a number of advantages, including higher energy and power density, as well as improved safety. SEs are mainly divided into polymer-based and inorganic types, which include garnets, LISICON, NASICON, and perovskites. As regards garnet Li₇La₃Zr₂O₁₂ (LLZO) a couple of study are focused on the deposition of Ta-doped LLZO, since doping with La stabilizes the cubic form of LLZO, which exhibits higher Li conductivity. In both of them, Ta-doped LLZO target material was enriched in Li content to balance the inevitable Li loss during deposition. Curcio et al. (2023) deposited 2D films at different deposition pressure and post deposition heat treatment. Crystalline LLZTO film was deposited at 10 Pa of O₂ at RT (Figure 3), while post deposition treatment caused Li-poor phase formation (Curcio et al., 2023). While Morozov et al. (2022) studied the correlation among deposition conditions and the microstructure at the interface between LCO and Ta-doped LLZO deposited via PLD.

There are several studies regarding the PLD of lithium phosphates electrolytes (Nishio et al., 2020; Kuwata et al., 2022; Wang et al., 2022), but all investigate only the cathode component in contact with such SE in the form of thin film. Only in one case the properties of a NASICON film are studied. They obtained dense thin films of Li_1+xAl_xTi_2-x(PO₄)₃ (LATP) at high temperature (700°C) and deposition pressure (80 mTorr), however only after post deposition treatment at 800°C the ionic conductivity increased up to 10⁻⁴ S/cm, thanks to the improved contact between crystallites with different orientation as a consequence of the formation of amorphous phase at the grain boundaries (Siller et al., 2021).

Lovett et al. (2022) deposited a vertically aligned nanocomposite of two perovskite-based electrolyte, i.e., Li_3xLa_2/3-xTiO₃ (LLTO) and Li_3xLa_2/3-xNO₃ (LLNO), together with an anodic material: LL(Nb,Ti)O-(Ti,Nb)O₂-anatase. LL(Nb,Ti)O-(Ti,Nb)O₂ film grew epitaxially with high purity and crystallinity on an Nb-STO substrate, exhibiting ion conductivity (2.3x10⁻⁴ S/cm) an order of magnitude higher than bulk LLNO and comparable to the best LLTO film obtained.

Lastly new SE based on hydrides was proposed, in which the lightweight hydrogen atoms in polyanions facilitates their rotation, enabling effective ion transfer through the paddle-wheel mechanism (Pang et al., 2022). Additionally, the highly reductive nature of H^δ− enhances compatibility with metal anodes and leads to more suitable interphases. In particular, Sasahara et al. (2023) deposited a single phase (001)-oriented La₂LiHO₃ epitaxial thin film with an ionic conductivity of 1.3x10⁻⁸ S/cm, an order of magnitude higher than bulk form.

We have reviewed the most recent study on the preparation of thin films for cathodes, electrolytes and anodes for microbatteries application by PLD technique. In order to optimize the electrochemical performances of the deposited electroactive films, the deposition time has been varied, whereas films morphology strongly depends on the properties of the laser source, mainly the laser pulse length. The films’ crystallite size can be controlled varying the temperature of the substrate during the deposition experiments, but it has been show that anodes formed by amorphous films has higher stability with respect to crystalline one due to their capability to resist to volume changes during the intercalation-deintercalation process (Biswal et al., 2021). The ability to finely control the thickness and orientation of crystals in coatings makes it possible to grow model electrodes that have been used to study electrochemical reactions in cells.

PLD is recognized has the technique of choice for deposition of coatings with complex composition, on the other hand the lithium loss during the growth of cathodes and electrolytes thin films require a deep knowledge and a careful control of the deposition conditions and can lead to materials with diminished performances.

Finally, the capability to perform electrochemical characterization of films containing lithium in situ, avoiding air/moisture exposure, holds significant importance and would represent a substantial advancement in the research, facilitating the comparison of data obtained from different laboratories.