Synthesis, characterization and observation of antisite defects in LiNiPO4 nanomaterials

Structural studies of high voltage cathode materials are necessary to understand their chemistry to improve the electrochemical performance for applications in lithium ion batteries. LiNiPO4 nanorods and nanoplates are synthesized via a one pot synthesis using supercritical fluid process at 450 oC for 10 min. The X-ray diffraction (XRD) analysis confirmed that LiNiPO4 phase is well crystallized, phase purity supported by energy dispersive spectroscopy (EDS) and elemental mapping by scanning electron transmission electron microscopy (STEM). For the first time, we have carried out direct visualization of atom-by-atom structural observation of LiNiPO4 nanomaterials using high-angle annular dark-field (HAADF) and annular bright-field (ABF) scanning transmission electron microscopy (STEM) analysis. The Rietveld refinement analysis was performed to find out the percentage of antisite defects presents in LiNiPO4 nanoplates and about 11% of antisite defects were found. Here, we provide the direct evidence for the presence of Ni atoms in Li sites and Li in Ni sites as an antisite defects are provided for understanding of electrochemical behavior of high voltage Li ion battery cathode materials.

that, controlled size and morphology of cathode materials could improve the electrochemical performances 1,13,14 . To control the shape and morphology of cathode materials solution based synthesis is more suitable 1 . Recently, we have reported size and morphology controlled synthesis of variety of cathode materials such as phosphate, silicates and flurophosphates via a supercritical fluid process [15][16][17][18][19][20][21][22][23] and observe the improvement of electrochemical performances with related to size and shape.
Herein, we report synthesis and characterization of LiNiPO 4 nanoplates prepared via supercritical fluid process. Attempt has been made to investigate the presence of antisite defects in LiNiPO 4 cathode materials, which is also a kind of reason for low capacity issues in high voltage cathodes [24][25][26] .

Results and Discussion
Synthesis and powder X-ray diffraction analysis. Using supercritical fluid process we achieved direct synthesis of phase pure LiNiPO 4 due to the overwhelming advantages of this process as we reported in many of our previously published papers [15][16][17][18][19][20] . The synthesis procedure for LiNiPO 4 cathode materials is shown in Fig. 1. Using same starting materials and by changing reducing agents, LiNiPO 4 with two kinds of morphologies were synthesized.
The XRD pattern of as-synthesized LiNiPO 4 cathode material at 450 o C for 10 min using ascorbic acid and oleylamine are shown in Fig. 2. The crystal structures of as-synthesized materials are identified as LiNiPO 4 and all of reflections are indexed to orthorhombic crystal system and belongs to Pnma space group. From the XRD pattern, it is evident that single phase of LiNiPO 4 was successfully synthesized without any impurities. The two samples showed similar XRD pattern but variations in their peak intensities, where LiNiPO 4 synthesized using oleylamine (Fig. 2b) showed slightly higher intensity than the LiNiPO 4 synthesized using ascorbic acid (Fig. 2a).
Morphologies and size of LiNiPO4 particles. The as-synthesized LiNiPO 4 particles were analyzed using TEM and HRTEM analysis as shown in Fig. 3. LiNiPO 4 particles synthesized using ascorbic acid and oleylamine as reducing agents showed rod and plate like morphologies. The rod like LiNiPO 4 exhibit particle size from 100-200 nm in length, 50-80 nm in diameter as shown in Fig. 3a,b. The plate like LiNiPO 4 exhibit particles size from 250-400 nm in length, 300-600 nm in width, and side thickness of less than 20 nm as shown in Fig. 3d,e. The selected area diffraction pattern taken along [010] axis of rod and plate like particles shown in Fig. 3c,f, confirms that the synthesized LiNiPO 4 are single crystalline in nature. The diffraction pattern is consistent with morphologies of LiNiPO 4 nanorods and nanoplates.
The rod like LiNiPO 4 was obtained in the presence of water-ethanol mixed solvents, where enormous amount of hydroxyl ions are released during crystallization of LiNiPO 4 , which promote one dimensional growth of LiNiPO 4 particles. The ascorbic acid is just worked as reducing agents and not as surfactant. But oleylamine act both as reducing agent and surfactant, that's why plate like particles are obtained using oleylamine, where oleylamine capped on to the specific crystal planes and allows LiNiPO 4 to grow on other specific planes. In the case of plate like morphology, oleylamine capping on b-axis, so that, we obtain plate like particles with less than 20 nm in diameter along b-axis. We have also observed this phenomenon in the synthesis of LiCoPO 4 nanoplates under supercritical conditions 21 . EDS and elemental mapping of LiNiPO4 nanorods and nanoplates. The purity of LiNiPO 4 nanorods and nanoplates were confirmed by STEM analysis, Fig. 4a,b shows the EDS spectra of LiNiPO 4 nanorods and nanoplates, in both the spectra the presence of all the elements such as O, P, and Ni were present and no other impurity was observed, both XRD and EDS supports the purity of LiNiPO 4 nanorods and nanoplates. Furthermore, the elemental mapping was carried out for LiNiPO 4 nanorods and nanoplates, the homogeneous distribution of oxygen, phosphor and nickel elements were clearly observed as shown in Fig. 4c,d.
However, there is no report available on reporting antisite defects in LiNiPO 4 cathodes, and for the first time we have observed antisite defects in LiNiPO 4 nanomaterials. LiNiPO 4 crystal structure is composed of slightly distorted NiO 6 octahedra, P ions are located at the center of PO 4 tetrahedra. Both the lithium and nickel ions occupy the octahedral sites, lithium is located at edge-sharing M1 sites and Ni is located at corner sharing M2 sites in LiNiPO 4 structure as shown in Fig. 5a. It has been reported that, cation exchange will occur between the two octahedral M sites in olivine structured cathode materials as antisite defects [26][27][28][29][30][31][32] . For comparison, two dimensional atomic arrangement of a unit cell structure is superimposed on HAADF-STEM and ABF-STEM image. The bright and dark contrast produced by Ni atoms can be clearly observed in HAADF image and ABF image as shown in Fig. 5b,c. Phosphor atoms are located neighboring to each Ni atoms, which produce low dark and bright contrasts compared to that of Ni atoms. In a unit cell, six Ni atoms form each other a hexagon configuration can be seen in Fig. 5b.
Due to the overlapping of three atomic columns when viewed along [010] projections O columns are not well resolved compared to Ni and P atoms, which is well agreement with the observation of LiFePO 4 crystal structure and LiCoPO 4 crystal structures 26,32 . In HAADF mode, Li atoms are invisible and no contrast could be found along Li columns in by HAADF mode [26][27][28] . However, the bright and dark contrast were observed along Li columns, which clearly indicates that Ni atoms are moved from M2 site to M1 sites and occupy the Li sites (see the dotted square arrow mark), which results in weak contrasts of some Ni columns as shown in Fig. 5c,d. When compared to LiFePO 4 , LiMnPO 4 and LiCoPO 4 structures 26-28 , Li to Ni exchange as an antisite defects in LiNiPO 4 are higher as they exhibit very strong contrast and are homogeneously distributed. In addition, some of the Ni atoms are occupied by Li atoms, which could be noticed due to the weak bright and dark contrast observed at Ni sites (see the circle mark). During electrochemical reaction, the Li ions diffusion through [010] direction are blocked by Ni atoms which results in low discharge capacity of olivine structured cathode materials. The antisite defects are usually occurred in olivine cathode materials synthesized at low temperatures. The low electrochemical performance of high voltage olivine structured materials such as LiCoPO 4 and LiNiPO 4 are due to low electronic conductivity, lack of high voltage electrolytes and also due to the presence of antisite defects. Further, Rietveld refinement analysis was carried out for LiNiPO 4 nanoplates to support the STEM observation of antisite defects and to mention quantitatively the amount of antisite defects. Table 1 shows the parameters obtained from Rietveld refinement analysis. The refined cell parameters of LiNiPO 4 nanoplates are a = 10.0330(4) Å, b = 5.8528(2) Å, and c = 4.6767(2) Å. The refinement analysis showed approximately 5% of Ni in Li site (4a site) and 5% of Li in Ni site (4c site), total 10% of antisite defects are found in LiNiPO 4 nanoplates synthesized via supercritical fluid process. Chung et al. 26 have shown around 1% of antisite defects by Rietveld analysis for LiFePO 4 synthesized at 600 o C and 15% of antisite defects by using quantitative STEM for the same sample. So that, there is difference between antisite defects observation experimentally and by Rietveld refinement. Our STEM observation of LiNiPO 4 showed high percentage of antisite defects as we observed high contrast in lithium columns and low contrast at Ni site. It has been reported that, the antisite defects could increase upon electrochemical cycling, this phenomenon was observed for LiCoPO 4 after few cycles and the pristine sample had 5% of antisite defects 33 . High percentage of antisite defects can be expected for olivine structured materials synthesized at low temperature solution process with nanometer scale.
LiNiPO 4 cathode nanomaterials with nanorods and nanoplates like morphologies were successfully synthesized via one pot synthesis route using supercritical fluid process. The pure phase and phase purity of LiNiPO 4 nanorods and nanoplates were confirmed by XRD and EDS analysis. The rod like LiNiPO 4 exhibited particle size from 100-200 nm in length, 50-80 nm in diameter and the plate like LiNiPO 4 exhibited particles size from 250-400 nm in length, 300-600 nm in width, and side thickness of less than 20 nm. Further, LiNiPO 4 nanoplates were analyzed by HAADF-STEM and ABF-STEM analysis to observe the structure of LiNiPO 4 crystals. The presence of Ni and P atoms are observed with bright and dark contrast. As expected, Ni atoms are found to occupy Li sites as the antisite defects at Li sites and Li in Ni sites. The strong contrast at Li sites confirms the movement of Ni atoms from M2 sites to M1 sites. The Rietveld refinement analysis showed approximately 10% of antisite defects. The antisite distributions are homogeneous and they are probably unavoidable in the olivine cathode materials synthesized at low temperatures.  (Fig. 3(a-c)) and (b) oleylamine (Fig. 3(d-f)) as reducing agent/surfactant via supercritical fluid process.
Scientific RepoRts | 5:11041 | DOi: 10.1038/srep11041 Methods LiNiPO 4 nanorods and nanoplates were synthesized from NiCl 2 , (Wako, Japan) (NH 4 ) 2 HPO 4 (Wako, Japan) and lithium acetyl acetonate (Wako, Japan) in 1:1:1 molar ratio. Oleylamine (Wako, Japan) was used both as surfactant and reducing agent and Ascorbic acid (Wako, Japan) used reducing agent. First, NiCl 2 .6H 2 O was dissolved in a solution of water-ethanol mixed solvents (1:1 volume ratio) and (NH 4 ) 2 H 2 PO 4 was added slowly with constant stirring followed by addition of lithium acetyl acetonate after that ascorbic acid or oleylamine (metal ion to surfactant 1:20) was added. The solution mixture was stirred for about few min after that 5 ml solution was transferred to batch reactors (4 reactors, each 10 ml volume). The batch reactors were heated at 450 o C for 10 min and then reactors were quenched in cold water. The products were recovered by washing and dried in a vacuum for overnight.

Material characterization.
The powder X-ray diffraction (XRD) analysis was carried out using a Bruker AXS D8 Advance instrument with Cu Kα radiation. The XRD pattern was analyzed by the Rietveld method using the program RIETAN 34 The morphology and size of the particles were determined using high-resolution transmission electron microscopy, High angle annular dark field (HAADF) images, elemental mapping and energy dispersive spectroscopy (EDS) were observed using JEM-2010F instrument equipped with a spherical aberration corrector (CEOS) at 200 KeV. The camera length was 6 cm; the BF aperture was 3cm, and HAADF and ABF detectors spanned the ranges of 70-180 and 12-24 mrad, respectively.  Fig. 4(a,b)) and (b) oleylamine ( Fig. 4(c,d)) as reducing agent/surfactant via supercritical fluid process.