The usage of secondary Lithium-ion batteries (LIB) become essential in our day to day life as mobile energy suppliers, portable devices, consumer electronics and electric vehicles. In battery, cathode material plays a vital role to improve overall battery storage performance due to its high operating voltage. Research into various cathode materials in LIB has led to the successful commercialization in the battery market. Lithium cobalt oxide (LiCoO₂) electrode is extensively utilized as a cathode in industrial-scale production of lithium-ion batteries [1]. Unfortunately, the cost and safety issues impede the continuing development in the battery technology and led to search for alternatively electrode materials. A significant amount of research has focused on developing low-cost, energy-efficient, and environmentally friendly Fe-based polyanionic compounds, including LiFePO₄, LiFeSiO₄, LiFeBO₃, and LiFeSO₄OH materials [2–5]. Currently, LiFePO₄ has become the highly attractive iron-based cathode material due to its high storage capacity (170 mAh g^–1), cyclic stability, rate capability and high redox voltage (3.43V vs. Li/Li⁺) [6]. The LiFePO₄ compound had an advantage of stable three-dimensional frameworks containing PO₄ polyanions, which feature strong covalent bonds that contribute to excellent thermal stability and safety [7]. However, its low electrical conductivity and low volumetric density are the main obstacles for the further development in practical applications.

Recently, pyrophosphates Li₂MP₂O₇ (M being transition metal like Fe, Mn, and Co), have identified as high energy storage cathode materials for LIB applications [8–10]. Among them, Li₂FeP₂O₇ compound proposed by Nishimura et.al. is considered as a competent electroactive cathode material for LiFePO₄ due to its thermal stability and high redox voltage of 3.5 V [11]. High operating voltage, low synthesis temperature and structural framework are the main positive attributes to obtain high-capacity, good rate capability and high volumetric energy density over LiFePO₄. In addition, at higher redox potential of 5.2 V, a second Li-atom can be extracted, results in theoretical capacity would reach 220 mAh.g^-1 and the three-dimensional pyrophosphate structure of Li₂FeP₂O₄ allows for greater isotropic lithium-ion transport [12]. On other hand, Li₂FeP₂O₇ electrode possesses poor cycling stability [13]. However, researchers have adopted several strategies to improve the electronic conductivity and storage performance of Li₂FeP₂O₇ electrode, including surface carbon deposition [14], morphology modulation [15], particle size control [16] and cation substitution in iron sites with V [17], Mn [18], Mg [19], Co [19] and Ni [20]. The above said approaches had beneficial effect in improving the electrochemical energy storage performance of Li₂FeP₂O₇ electrode.

Therefore, a study on thin film electrode of Li₂FeP₂O₇ on the current collector deposited by pulsed laser deposition (PLD) could provide new insights on electrochemical performance of pyrophosphates. Thin film electrodes are important as they provide a simplified model for understanding the electrochemical processes of active materials. The planner thin films electrode materials are highly desirable for battery application in order to quantify the true energy storage capacity of the electrode, since no binder or conductive additives are added during preparation. In addition, these cathode materials are crucial for safer energy storage approach i.e. solid-state thin film batteries [21]. Further, thin film batteries are widely used in microelectromechanical systems (MEMS), smart cards, complementary metal oxide semiconductor (CMOS) memory chips, remote sensors, and implantable medical devices [22–24]. This calls for a well-defined thin film batteries for fulfilling the current and future generations requirements. Various experimental techniques have been investigated such as physical vapour deposition techniques (sputtering, pulsed laser deposition, evaporation) chemical vapour deposition techniques (sol-gel, hydrothermal process, solution combustion synthesis), and atomic layer deposition. Among these, PLD is gained great attention as a versatile technique for the growth of thin film nanomaterials, semiconductors, polymers, ferrites and nanocrystalline material [25]. Recently, it is widely adopted by many researchers for the development of solid-state thin film batteries (cathode, electrolyte and anode sequentially fabricated in thin film form) due to the following advantages [26–27] (i) PLD is capable to deposit the thin films from any kind of materials, (ii) it transfer the stoichiometric of multiple components due to rapid heating caused by high laser energy (iii) thin film formation at low temperatures. In addition to that, it was observed that the electrochemical properties has been improved thin films deposited by PLD. Siller et.al., shown the stoichiometric LiMn₂O₄ thin films exhibited the good cycling stability with low volume variation [28]. Trzcinski et.al obtained epitaxial MoO₃ thin films using PLD, which are favourable to ion intercalation, leading to improved electrochemical properties compared to bulk materials [29]. Recently, Lin et.al observed that the thin films of Na-based polyanionic compounds deposited by PLD exhibited superior ion diffusion kinetics than the conventional batteries, results in improved cyclic performance and energy density [30]. The thin films deposited by PLD exhibited outstanding properties such as nanomorphology, dense and compactness, which leads to produce low charge transfer resistance, high Li/Na ion diffusion and better cycling stability. The same as motivated us to work on thin film batteries. In the present work, we report the preparation and energy storage performance of Li₂FeP₂O₇ thin films cathode electrode in Li-ion thin film battery. The Li₂FeP₂O₇ thin film is deposited on stainless steel substrates using PLD technique and used as cathode electrode for LIB. Further, the crystal structure, morphology and energy storage performance of Li₂FeP₂O₇ thin films LIB were analysed. The electrochemical characterizations were evaluated on nanostructured Li₂FeP₂O₇ thin film electrode vs lithium with 100% efficiency till 300 cycle.

X-ray diffraction pattern observed in Fig. 1 revealed the phase purity and crystalline nature of Li₂FeP₂O₇ thin films. There is no evidence for secondary phases and the peaks observed from thin film is well matched with standard ICSD (78-2294) data. Four well defined peaks observed from the sample (shown in inset Fig. 1) along the stainless-steel substrate peaks at 43 and 51°. The pure phase revealed by deposition of Li₂FeP₂O₇ in particular planes as present in bulk, exemplified the thin film deposited is of Li₂FeP₂O₇ only.

Surface morphology and its roughness are an important factor for influence the electrochemical properties of the thin film batteries [25]. Scanning electron microscopy and atomic force microscopy techniques are employed to identify the morphology and roughness. Scanning electron microscopy (SEM) images of the thin film deposited at 600°C on SS substrate is shown in Fig. 2(a). At constant substrate temperature, the adatoms mobility is constant on the surface of the substrate and favours the formation of greater number of crystallites centers rather than the coalescence of islands. The micrographs reveal that the thin film consists of many spherical particles, exhibiting size ranges from 80–100 nm. In Fig. 2a, in-situ deposited film exhibiting uniform distribution, good adhesion to substrate and well-defined particles. The thickness of the film is around 200 nm measured from the cross-sectional SEM image shown as inset in Fig. 2(a). The thickness of thin films can be controlled by varying the deposition time, laser density. Surface morphology of the Li₂FeP₂O₇ thin film further recorded by atomic force microscopy. A 3-dimentional AFM image was captured in tapping mode and shown in Fig. 2(b), which signifies the nature of the crystalline particles, surface roughness (34 nm) and its distribution. Deposited electrode is close packed, fairly homogeneous and nano-sized particles. This type of morphology can decrease particle agglomeration and fasten the ionic migration without trapping at grain boundaries [26]. The deposited Li₂FeP₂O₇ films were also subjected to obtain TEM images and HR-TEM images (Fig. 2c and d, respectively). It is also apparent from TEM image (Fig. 2c) the size and shape of deposited electrode is in nanometer range. In Fig. 2d, HR-TEM clearly affirms the interplanar distance of Li₂FeP₂O₇ which is 1.1 nm corresponding to (100) plane.

It’s well known that PLD is good technique to deposit the multicompound materials, since it transfers ablated material from the target stoichiometrically (without loss of target material stoichiometric). Thus, the deposited material here in particular Li₂FeP₂O₇ should work as cathode as reported in the literature [11–19].

The electrochemical properties of the Li₂FeP₂O₇ thin films were evaluated by CV and galvanostatic charge−discharge tests. Fig. 3 shows the electrochemical properties of the half-cell configuration of these thin films deposited directly over current collector. Cyclic voltammetry measured at different scan rates which is clubbed together and shown in Fig. 3a. The charge-transfer kinetics can be analysed by using the CV which is similar in the present case with bulk Li₂FeP₂O₇ as reported in literatures [11]. In Fig. 3a scan done at 0.3 mV s^–1, the first anodic peak at 3.6 V was due to the oxidation of Fe²⁺/Fe³⁺ while the subsequent cathodic peaks at 3.4 V were ascribed leading to give the average potential of 3.5 V, which is characteristic redox activity value for Li₂FeP₂O₇. Similarly, at other scan rates from 0.5 to 3 mV s^–1 the average potential value is overlapping with 3.5 V which it the redox activity for Fe in present case. Two subsequent cycles during CV at faster scan rate (3 mV s^–1) is represented Fig. 3b to show cycling reversibility at same scan rate.

Galvanostatic charge-discharge profile and corresponding differential capacity (dq/dV) plot of Li half-cell are shown in Fig. 3c and d, respectively. At slow current value (5 μA g^–1) for the first 10 cycles charge-discharge profile of thin film electrode was deduced which showed ~85% of the theoretical capacity. The capacity was reduced from 92 to 88 mAh g^–1 (areal capacity: 37 -35 mAh cm^–2) with no significant increase in polarization between charge and discharge plateaus. Later using this thin film rate capability and cycling performance for 300 cycles were done which is shown in Fig. 4. Coulombic efficiency for the 99 cycles being ~100% which indicated that the cell exhibited high reversible capacity and good cycle performance when tested against different C-rate or current rate. Fig. 4a is the rate performance of the Li₂FeP₂O₇ thin film electrode in cell. Tests were done at different current rates started from 5, 10, 20, 50, 100, 50, 20, 10 and again 5 μA g^–1. Although the capacity decreased with increased current values leaving constant Coulombic efficiency. It noted that the capacity measured after rate testing exceeds the initial value. It is due to the self-activation of the electrode surface during charge-discharge cycles [32]. Thus, the cell exhibited a good rate capacity. Li₂FeP₂O₇ in thin film form along few plane deposition showed a discharge capacity of 70 mAh g^–1 (areal capacity: 28 mAh cm^–2) in the beginning at very slow current rate 5 μA g^–1, which was achieved to 63, 54, 42 and 35, at 10, 20, 50 and 100 μA g^–1 respectively maintaining almost same values at from high to low current rate as shown in Fig. 4b. However, the lowest rate now showed higher capacity (80 mAh g^–1) than the beginning which could be arising due to SEI layer stabilization. Hence, PLD-grown thin films fabricated without the addition of any conductive additives or carbon-based conductive materials exhibited competent electrochemical properties comparable to those reported in the literature [13–16]. For better comparison and cell performance the PLD coated thin film containing cell was tested for 300 cycles at higher current rate and represented in Fig. 4c and d. Fig. 4c shows the charge-discharge values for the electrode till 300 cycles, with firmly good overlap over each other. At 100 μA g^–1 the tests were performed delivering 34 mAh g^–1 (areal capacity: 14 mAh cm^–2) which just reduced to 30 mAh g^–1 (areal capacity: 12 mAh cm^–2) almost 88% capacity retention was observed at 300^th time. Excellent Coulombic efficiency which is almost 100% was observed during charge-discharge for 300 times at high current. Cycling stability is shown in inset of Fig. 4d along with the corresponding charge-discharge profile at 2^nd, 100^th, 200^th and 300^th cycle. These results establish a foundation for the development of Li₂FeP₂O₇ thin films for Li-ion thin film battery application and can be tested against solid state electrolyte.

In summary, in-situ deposition of Li₂FeP₂O₇ thin films on stainless-steel substrates was successfully achieved by pulsed laser deposition (PLD). By obtaining pure phase Li₂FeP₂O₇ in bulk from combustion, pure target was prepared with no impurity which was used to get thin film of Li₂FeP₂O₇ on SS with several other optimization of PLD parameters. Deposited electrode is 200 nm in thickness as revealed by SEM with the interplanar spacing of 1.1 nm. Li₂FeP₂O₇ thin films delivered a reversible capacity of 42 mAh g^–1 (areal capacity: 17 mAh cm^–2) with good rate capability and excellent Coulombic efficiency even till 300 cycles without any conductive coatings. By this approach, we can successfully propose Li₂FeP₂O₇ thin films as cathode for Li-ion thin film battery operated devices, which can deliver up to 85% of its theoretical capacity with excellent Coulombic efficiency. Present study makes this report as first demonstration of PLD deposited Li₂FeP₂O₇ electrochemically active for the construction of lithium-ion thin film batteries.