15478 | Phys. Chem. Chem. Phys., 2020, 22, 15478--15487 This journal is © the Owner Societies 2020 Cite this: Phys. Chem. Chem. Phys., 2020, 22, 15478 Effect of crystallite size on the phase transition behavior of heterosite FePO 4 † Azeem Banday, a Raza Shahid, b Sher Singh Meena, c S. M. Yusuf cd and Sevi Murugavel * a For advanced lithium-ion battery technology, olivine-based cathodes are considered to be the most dominant and technologically recognized materials. The extraction of lithium ions from olivine LiFePO 4 results in the two-phase mixture with heterosite FePO 4 exhibiting a deintercalation potential of 3.45 V vs. Li + /Li over a wide range of lithium content. Here, we report the synthesis and characterization of chemically deintercalated heterosite FePO 4 with varying crystallite sizes using different analytical techniques. The decrease in the crystallite size of heterosite FePO 4 leads to an increase in the lattice parameters including the unit cell volume. The characteristic behavior in the structural properties of heterosite FePO 4 shows a strong dependency on the crystallite size which is correlated with the change in the chemical bonding. The volume expansion of the nano-sized heterosite FePO 4 with respect to the bulk counterpart is suggested to be a direct consequence of reduced hybridization between the Fe3d and O2p states. Furthermore, the combined X-ray diffraction and Mo ¨ ssbauer spectroscopic studies reveal the appearance of a new phase namely trigonal FePO 4 at the lower crystallite sizes due to the enhanced surface energy kinetics. We also find that the observed trigonal FePO 4 phase is more magnetically active than the paramagnetic olivine FePO 4 . For the unique structural advantage of the heterosite phase as an electrode material, the change in bonding characteristics is very useful and can have strong implications on the electronic properties of heterosite FePO 4 at the nanoscale level. Introduction Lithium-ion batteries (LIB), the most favorable power source for handy electronic and hybrid electric devices, are transforming modern technology by decreasing the dependency on fossil oil and carbon dioxide emission. 1–3 Safety is an important issue in growing portable electronic devices for the usage of LIBs along with high energy and power densities. In addition to the portable electronic devices, it is equally important to develop large-scale devices and to meet the cost and stability under extreme conditions. More specifically, the electrode materials suffer from deteriorations on their repeated usage and beyond usual working potentials due to the irreversible structural changes, which occur due to the insertion and extraction of large amounts of lithium ions. Thus, in order to minimize the irreversible structural modifications, advancements in cathode materials become crucial with respect to its intrinsic physical properties including crystal structure and its interrelation with electrochemical characteristics. 4–6 Olivine-type LiFePO 4 (LFP) is an archetypal cathode material for LIB, which offers a high-level of safety, high capacity, stable charge–discharge characteristics and low cost. 7 Additionally, LFP exhibits a reasonable cyclability owing to the strong P–O bond with the covalent character of PO 4 tetrahedron. The usefulness of the strong covalent characteristic P–O bond in the polyanion (PO 4 ) 3 not only makes LFP a worthy cathode material but also a stable system when the battery is fully charged. However, its potential as a cathode is limited by the low intrinsic conductivities and in order to improve the overall electrochemical properties the underlying mechanism of charge transport needs to be studied atomistically. Mascaro et al. have used an AFM based electrostatic force spectroscopy technique to spatially resolve ionic transport properties at the nanoscale and have found an excellent agreement between experimental and theoretical values. The discrepancy between the experimental and theoretical results is resolved by measuring the collective activation energy (arising due to Coulomb interactions between the ions) and the smaller single-ion bulk hopping barrier. It has also been shown that the lithium ion hopping through the LiFePO 4 /FePO 4 phase boundary possesses a higher hopping a Department of Physics & Astrophysics, University of Delhi, Delhi-110007, India. E-mail: murug@physics.du.ac.in b Department of Physics, Jamia Millia Islamia, New Delhi-110025, India c Solid State Physics Division, Bhabha Atomic Research Centre, Mumbai-400085, India d Homi Bhabha National Institute, Anushaktinagar, Mumbai-400094, India † Electronic supplementary information (ESI) available. See DOI: 10.1039/ d0cp02387f Received 3rd May 2020, Accepted 16th June 2020 DOI: 10.1039/d0cp02387f rsc.li/pccp PCCP PAPER Published on 16 June 2020. Downloaded by Bhabha Atomic Research Centre on 9/1/2020 1:13:37 PM. View Article Online View Journal | View Issue