1 Introduction

Lithium ion batteries have dominated as main power source option for portable electronic devices and electric vehicles. New, unique and improved cathode materials are continuously developed as cathode materials are the highly responsible factor for lithium ion batteries performance. Among them, olivine structured orthophosphates, LiMPO4 (M = Fe, Mn, Co, Ni) have been recognized as one of the suitable cathode materials group for lithium ion batteries with its favorable characteristics such as long cycle life, good thermal stability, environment benignity and safety [1, 2]. Strong (PO4)3− covalent bonding within the structure enables stability [3, 4]. Turning to the potential voltage, LiFePO4 reaches 3.5 V [5–7], LiMnPO4 around 4.1 V [8–10], while LiCoPO4 and LiNiPO4 rises to 4.8 V [11–13] and 5.1 V accordingly [14, 15]. This clearly points out that LiNiPO4 is in the pathway towards promising cathode materials for high voltage lithium batteries.

Nevertheless, its low electronic conductivity, poor lithium diffusion and electrolyte degradation at higher voltage are the main hurdles of LiNiPO4 to achieve high rate electrochemical performance [16, 17]. Generally, numerous modifications such as particle size control [18, 19], metal ion doping [20–23] metal oxide coating [24–26] and carbon coating [27–29] are suggested by researchers to upsurge the efficiency and electronic conductivity of olivine type materials.

Remarkably, synthesis methods and sintering temperatures plays key roles in carbon layer formation and particles growth [30]. In focusing LiNiPO4 cathode materials, notable approaches such as solid state reaction [3, 31], pechini-type polymerizable precursor method [32] and polyol method [15] have been developed for synthesis process. Stefan et al. have synthesized LiNiPO4 and its derivatives LiNi1-y Co y PO4 (y = 0.25, 0.33, 0.66, 1.0) via non aqueous sol–gel route employing ethylene glycol [33]. Capacity about ~ 22 mAh g−1 was reported for LiNiPO4. Gangulibabu et al. have produced LiNiPO4 via citric acid assisted sol–gel method. This work reported on structural characterizations, cyclic voltammetry, and electrochemical impedance studies, no electrochemical charge–discharge included. In another work, LiNiPO4 and copper doped LiNiPO4 cathode materials were prepared by microwave assisted non aqueous sol–gel route [34]. The electrocatalytic activities of the samples were evaluated by cyclic voltammetry studies and no electrochemical tests performed. Dimesso et al. have produced LiNi1-y Mg y PO4 (0 ≤ y ≤ 0.3) using Pechini assisted sol–gel process [35]. LiNiPO4 in this work exhibits initial discharge capacity of 86 mAh g−1 at 0.1 C and it showed capacity fade about 8% at the 10th cycle. A Ornek and co-workers have synthesized pristine LiNiPO4 and Co doped LiNi1-x Co x PO4 (x = 0.2, 0.4, 0.6, 0.8, 1.0) by sol–gel technique utilizing citric acid [36]. LiNiPO4 and LiNiPO4-C samples achieved initial discharge capacity of 18 mAh g−1 and 86 mAh g−1, respectively.

To the best of our knowledge, even though sol–gel technique has been approached to synthesize LiNiPO4 cathode materials but oxalic acid has not been used as carbon source. Simultaneously, fundamental properties such as effect of sintering temperatures and crystallite size of LiNiPO4 cathode materials have not been studied comprehensively. Considering aforementioned facts, LiNiPO4 is synthesized via oxalic acid assisted sol–gel route in this work. Sol–gel route signifies a versatile technique for cathode materials due to its high molecular level mixing, thus widely employed to produce olivines [37–40]. Oxalic acid serves as both chelating agent and carbon source. It turned to carbon during calcination process, thus coated the particles and improve the conductivity of the samples [41, 42]. It also provides high level reactants mixing which yields good product with enhanced performance [43]. Hence, this work will draw insights on influences of sintering temperatures and crystallite size on electrochemical properties of LiNiPO4 cathode materials.

2 Experimental

Stoichiometric amounts of 0.2 m (20.404 g) lithium acetate (LiC2H3O2) (Aldrich), 0.2 m (49.768 g) nickel acetate Ni(CH3COO)2·4H2O (Friendmann Schmidt) and 0.2 m (23.006 g) ammonium dihydrogen phosphate (NH4)H2PO4 (Friendmann Schmidt) were dissolved in 500 mL distilled water under continuous magnetic stirring and heating. 10 wt.% of oxalic acid was added to the solution. The solution was continuously stirred and heated (120 °C) about 4 h until solid precursor formed. The precursor was ground and then calcined at 500 °C, 600, 700, and 800 °C for 3 h. The samples were ground again after the calcination for further structural characterizations.

Crystalline phases of the samples were obtained by X-ray diffraction (XRD) using Siemens D 5000 diffractometer equipped with Cu–Kα radiation (λ = 1.54060 Å). Morphological phases were acquired by field emission scanning electron microscope (FESEM, model JSM 7600-F). Raman spectra were collected from Raman spectrometer (In-via Raman microscope) using wavelength of 532 nm (blue laser).

Electrochemical properties were evaluated by coin cells consist of cathode, lithium metal anode and 1 M LiPF6 dissolved in a mixture of ethylene carbonate/ dimethyl carbonate (1:1 in volume) electrolyte. The cathode was prepared by mixing 62.5% LiNiPO4, 25% teflonized acetyle black, 12.5% graphite and coated on the stainless steel mesh. The coin cells were fabricated in Argon filled glove box. Cycling tests were carried out at current rate of 0.02C (1C=171 mAh g−1) between 2.5 and 4.5 V in a Neware battery system.

3 Results and discussion

XRD patterns of LiNiPO4 materials heat treated at different temperatures are shown in Fig. 1. All the intense peaks in the diagram clearly indicate orthorhombic olivine structure of LiNiPO4 with Pnma space group and good agreement with JCPDS no. 81-1528 [15]. Diffraction peaks of (101), (111), (020), (311), and (121) can be seen at 2θ  = 20.9°, 25.9°, 30.5° 36.4°, and 37.3°, respectively. LiNiPO4 crystallized completely at low sintering temperatures of 500 and 600 °C by oxalic acid assisted sol–gel synthesis even though there is impurity peak which corresponds to Li3PO4 [3]. It also can be noticed that Li3PO4 impurity diminished at higher calcination temperature of 700 and 800 °C which forms pure phase of LiNiPO4.

Fig. 1
figure 1

XRD pattern of LiNiPO4 at different sintering temperatures

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As the calcination temperature increases, the characteristic peaks getting sharper evidently explain that the improved crystallinity achieved at higher temperatures [44]. No obvious diffraction peaks related to residual carbon indicating that carbon amount is low and in amorphous state [45, 46].

Raman spectra as in Fig. 2 presenting dominant band at 948 cm−1 in LiNiPO4 assigned to symmetric Agv1 mode. The peaks located at 1011 cm−1 and 1082 cm−1 denote asymmetric stretching vibration of PO4 tetrahedron. The peak at 290 cm−1 represents asymmetric stretching vibration of Li–O bonds. While the broad peak at 478 cm−1 fits to symmetric Agv2 mode. The other peaks at 597 and 649 cm−1 indicates symmetric Agv4 modes [32, 47].

Fig. 2
figure 2

Raman spectra of LiNiPO4 at different heating temperatures

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The surface morphologies and particle sizes of LiNiPO4 obtained at various calcination temperatures were examined by FESEM and the images are illustrated in Fig. 3. Well-crystallized polyhedral particles are formed with smooth surfaces at all the calcination temperatures. The particles present with less agglomeration and there is no much variation in the particles shape. The particles are within the range of 100–150 nm at the sintering temperatures of 500, 600, and 700 °C. At the sintering temperature of 800 °C, particles are in the size of 100–200 nm.

Fig. 3
figure 3

FESEM micrographs of LiNiPO4 obtained at (a) 500 °C (b) 600 °C (c) 700 °C (d) 800 °C

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Uniform size distribution of LiNiPO4 achieved with the aid of oxalic acid which served as organic fuel during calcination process. Large amount of gas released during oxalate decomposition. Thus, it suppressed agglomeration of particles [48, 49]. Wang et al. [50] have highlighted that chelating agents in sol gel technique able to provide high molecular level mixing, thus results in pure phase structure and morphology.

Fig. 4 depicting the Williamson–Hall plots of β cos θ vs. sin θ for LiNiPO4 samples calcined at different temperatures from 500 to 800 °C. The slope of the graph represents strain value and the line interception provides inverse crystallite size value [51, 52].

Fig. 4
figure 4

Williamson-hall plots of LiNiPO4 produced at sintering temperature of (a) 500 °C (b) 600 °C (c) 700 °C, and (d) 800 °C

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The crystallite size deduced from the graph increases with sintering temperature, this also proved by XRD peaks which getting sharper with rise of heating temperature [53]. At 500 and 600 °C, the crystallite size of LiNiPO4 is 24.8 and 26.2 nm respectively. LiNiPO4 attained at calcination temperature 700 °C has crystallites in the range of 28.9 nm while at sintering temperature of 800 °C contain crystallite sizes of 29.5 nm. Even though, the crystallites sizes seem like moreover within similar range, different strain values are signifying crystal defects that present within the particles [51]. Sintering temperatures of 500 °C and 600 °C recorded the strain values of 2.75 × 10−4 and 1.25 × 10−4 correspondingly. Strain value of crystallites that produced at heating temperature of 700 °C is 5.0 × 10−5, whereas the strain value increased to 2.0 × 10−4 for the sample that obtained at 800 °C. From the results, it can be concluded that LiNiPO4 obtained at sintering temperature of 700 °C has the smallest strain value. Higher strain value for the samples sintered at 500 and 600 °C maybe attributed to impurities that present in the samples as depicted in XRD results. Moreover, high strain value for the samples sintered at 800 °C could be due to the instability of structure formed at high temperature.

Present work demonstrates that cathode materials produced at sintering temperature of 700 °C and 800 °C have good structural properties without any impurities. Hence, these samples were chosen for electrochemical testing. Figure 5 displays first and twenty-fifth charge–discharge curves of LiNiPO4 at a current rate of 0.02 C.

Fig. 5
figure 5

First and twenty-fifth charge–discharge curves of LiNiPO4 obtained at sintering temperature of (a) 700 °C and (b) 800 °C

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Initial charge and discharge capacities of LiNiPO4 obtained at heating temperature of 700 °C were 97.7 mAh g−1 and 97.3 mAh g−1, respectively. Hence, initial cycle performance records high coulombic efficiency value of 97.1% with irreversible capacity loss around 0.4 mAh g−1. While LiNiPO4 produced at calcination temperature of 800 °C exhibited initial charge capacity about 93.7 mAh g−1 and discharge capacity of 91.1 mAh g−1 corresponds to 97.2% coulombic efficiency with irreversible capacity loss 2.6 mAh g−1.

Figure 6 summarizes electrochemical performances of both samples in the voltage range of 2.5 and 4.5 V. It can be observed that discharge capacity of LiNiPO4 processed at calcination temperature of 700 °C higher than 800 °C.

Fig. 6
figure 6

Charge–discharge behavior of LiNiPO4 obtained at sintering temperature of 700 and 800 °C

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In situ carbon coating aided by oxalic acid in synthesis process shows notable enhancement to the first discharge capacity of both samples compared to literature [35, 36]. Among the both samples, LiNiPO4 obtained at calcination temperature 700 °C demonstrates better electrochemical performance than 800 °C, can be accredited to the smaller crystallites. When the crystallite size is reduced, it offer more contact area to electrolyte. It is also beneficial to diminish structural degradation during charging and discharging process [54]. Besides that, shortened electron and lithium ion diffusion paths lead to the enhanced electrochemical properties [55–57]. On contrary, higher strain in sample that obtained at heating temperature of 800 °C impedes smooth lithium intercalation and deintercalation [58, 59]. At the same time, crystal defects that formed at higher sintering temperature affect the cycling performance. It can be concluded that sintering temperatures during synthesis process indirectly influences crystallite size of the samples [60]. Thus, sintering temperature can be optimized to control the size of the crystallites and strain of the particles.

4 Conclusion

Olivine type LiNiPO4 cathode materials have been effectively synthesized by oxalic acid aided sol gel route. Oxalic acid is beneficial as chelating agent and carbon source to form in situ carbon coating on LiNiPO4. Samples sintered at 700 and 800 °C, which demonstrates no impurity and good crystallinity employed as cathode materials in lithium half cells. LiNiPO4 obtained at heating temperature 700 °C exhibits initial discharge capacity 97.3 mAh g−1 and improved electrochemical performance due to its reduced diffusion path by smaller crystallite size and low strain compared to 800 °C. Sintering temperature can be optimized during sol gel synthesis process for enriched final product.