arXiv subjects

Rajendra S. Dhaka

Publications and source records attributed to Rajendra S. Dhaka.

At least 19 recordsLinked to original sources

arXiv2026

Disentangling octahedral distortion and symmetry breaking in the ordered double perovskite SrLaCoNbO$_6$

We report a comprehensive high temperature structural investigation of the B-site ordered double perovskite SrLaCoNbO$_6$ using combined Raman spectroscopy and x-ray diffraction (XRD) over the range 25--900~$^\circ$C. Two distinct temperature-driven anomalies are identified at $T_\mathrm{OD} = 300\pm 25~^\circ$C and $T_\mathrm{PT} = 750\pm 25~^\circ$C through pronounced renormalizations in the phonon frequency and linewidth, accompanied by the evolution and suppression of characteristic XRD reflections. The low-temperature anomaly at $T_\mathrm{OD}$ is associated with an isostructural octahedral distortion within the monoclinic $P2_1/n$ phase. At $T_\mathrm{OD}$, the radial and angular distortion parameters of the CoO$_6$ and NbO$_6$ octahedra decrease to near-zero values before reversing upon further heating due to an inversion of the in-plane Co--O and Nb--O bond-length hierarchy. The high-temperature anomaly at $T_\mathrm{PT}$ corresponds to a symmetry-breaking structural phase transition from $P2_1/n$ to the monoclinic $I2/m$ phase, confirmed by group-theoretical analysis and Rietveld refinement. These results demonstrate that integrating temperature-dependent Raman spectroscopy with laboratory-based XRD, complemented by quantitative octahedral distortion analysis, provides a robust approach to disentangle isostructural octahedral distortions from symmetry-breaking phase transitions in complex double perovskite oxides without recourse to synchrotron or neutron diffraction facilities.

cond-mat.mtrl-sci↗

arXiv2026

Raman spectroscopic signature of Kitaev magnetism and complex spin-lattice coupling in S = 1/2 antiferromagnet SrLaCoNbO$_6$ double perovskite

We report a detailed analysis of temperature-dependent Raman spectroscopy and Co $K$-edge extended x-ray absorption fine structure (EXAFS) for a pseudospin-$\tilde{S}=1/2$ insulating antiferromagnet SrLaCoNbO$_6$, a B-site--ordered double perovskite hosting Co$^{2+}$ ($3d^7$) ions on an {\it f.c.c.} sublattice. Notably, pronounced anomalies in the phonon frequency, linewidth and spectral weight are observed around 60~K, well above the long-range antiferromagnetic transition at $T_{\rm N} \approx 15$~K. These renormalizations indicate a significant coupling between lattice and spin degrees of freedom, although a purely structural contribution cannot be excluded. Additional modifications of both high- and low-energy Raman modes are detected near 160-180 K, including changes in linewidth and intensity, variations of the Fano asymmetry parameter, and the emergence of an additional low-energy feature. The asymmetric Fano line shape of selected low-energy modes, together with a broad low-energy continuum and quasielastic response, suggests coupling between discrete phonons and fluctuating magnetic excitations. Moreover, the EXAFS analysis reveals correlated changes in bond distances and Debye-Waller factors around the Co ions near 60~K and 160~K, evidencing subtle local structural distortions and possible magnetostrictive effects. The persistence of anomalous lattice dynamics far above $T_{\rm N}$, combined with the excitation-energy--independent continuum, is consistent with fluctuating bond-directional interactions and proximate Kitaev-like correlations.

cond-mat.str-el↗

arXiv2026

Improved Electrochemical Performance and Diffusion kinetics by Boron-doping in Na$_{0.66}$Mn$_{0.8}$Fe$_{0.2}$O$_{2}$ Layered Cathodes for Sodium-Ion Batteries

We report the electrochemical investigation and study the diffusion kinetics of boron doped Na$_{0.66}$Mn$_{0.8}$Fe$_{0.2}$O$_{2}$ (B-NMFO) cathode materials for sodium-ion batteries. Notably, the B-NMFO cathode exhibits improved specific capacity of 163 mAh g$^{-1}$ as compared to 133 mAhg$^{-1}$ at 0.1~C for the NMFO cathode. Further, we observe better capacity retention of 70\% for B-NMFO as compared to the NMFO (60\%) at 1 C after 200 cycles, indicating high structural stability due to the presence of strong B-O bonds. The diffusion coefficient evaluation through galvanostatic intermittent titration technique and cyclic voltammetry, which is found to be in the range of 10$^{-8}$--10$^{-10}$ cm$^{2}$s$^{-1}$. Interestingly, the temperature dependent distribution of relaxation time (DRT) analysis provides a clear understanding about the individual physical processes occurring at different time domains during the electro-chemical testing. Moreover, density functional theory is employed to determine the energetics and the electronic properties of B-NMFO, which suggests that the interstitial tetrahedral sites, especially those next to vacancies, are the dominant incorporation path ways for B in the host structure. Additionally, classical molecular dynamics (MD) simulations are applied to gain insights into the Na-ion transport properties in the bulk structures cathode materials.

cond-mat.mtrl-sci↗

arXiv2026

Order-disorder transition and Na-ion redistribution in NASICON-type Na$_3$FeCr(PO$_4$)$_3$

We report the temperature-dependent synchrotron based X-ray diffraction analysis of NASICON type Na$_3$FeCr(PO$_4$)$_3$ sample, which undergoes a symmetry-lowering structural transition from a monoclinic ($C2/c$) phase with long-range Na-vacancy order to a rhombohedral ($R\bar{3}c$) phase with statistical disordered Na ions. The [FeCr(PO$_4$)$_3$] polyanionic framework remains essentially unchanged, confirming that the transition is governed by redistribution of the Na sublattice rather than by reconstruction of the host framework. The structural evolution is accompanied by a discontinuous increase in the $c$-axis and the unit-cell volume, reflecting the progressive depopulation of the Na(1) sites and transfer of Na ions toward the Na(2) sublattice. The temperature dependence of superstructure intensity found to deviate from mean-field critical behavior, instead, the experimental evolution is accurately captured by a sigmoidal phase-fraction model. The calorimetric measurements show that the enthalpy change for the first transition around 350~K is significantly larger than that of the anomaly around 445 K, indicating the dominant configurational rearrangement of Na ions occurs within the lower-temperature interval. Overall, the diffraction and calorimetric results demonstrate that Na ordering proceeds through an order-disorder transition involving intermediate Na configurations and a finite coexisting regime. The quantitative correlation between Na-vacancy ordering, lattice strain, and symmetry lowering reveals the central role of configurational interactions within the Na conduction channels in governing the phase stability of NASICON-type materials.

cond-mat.mtrl-sci↗

arXiv2026

Revealing the interfacial kinetic mechanisms in high-entropy doped Na$_3$V$_2$(PO$_4$)$_3$ through electrochemical investigation and distribution of relaxation times

We designed a high-entropy doped NASICON cathode, Na$_3$V$_{1.9}$(CrMoAlZrNi)$_{0.1}$(PO$_4$)$_3$ and investigate its electrochemical performance for sodium-ion batteries (SIBs) to understand the diffusion mechanism including distribution of relaxation times analysis of interfacial kinetics. This trace doping induces high-entropy mixing at the vanadium site, tuning the lattice and enhancing specific capacity, activating V$^{4+}$/V$^{5+}$ redox couple 3.95~V. Interestingly, it delivers a reversible capacity of 119~mAh~g$^{-1}$ at 0.1~C, and demonstrate excellent stability of 68\% after 1000 cycles at 10~C. The calculated diffusion coefficient values are found within the range of $10^{-11}$--$10^{-13}~\mathrm{cm^2\,s^{-1}}$. The systematic investigation of temperature and voltage-dependent impedance data using the distribution of relaxation times provides deeper insights into the underlying charge-transfer and transport processes. The full cells with hard carbon delivers 326~Wh~kg$^{-1}$ (with respect to cathode mass) at $\approx$3.2~V and retained $\sim$79\% capacity after 100 cycles at 2~C. Our study opens new avenues for developing high-entropy doped cathodes for enhanced structural stability, extended redox activity, and optimized electrochemical kinetics for practical implementation of SIBs.

cond-mat.mtrl-sci↗

arXiv2026

Magnetic, transport and electronic properties of Ni$_2$FeAl Heusler alloy nanoparticles: Experimental and theoretical investigation

We present a comprehensive investigation of structural, magnetic and transport properties of Ni$_2$FeAl Heusler alloy nanoparticles (NPs) synthesized via template-less chemical route. The NPs exhibit high saturation magnetization of 3.02 $μ_ {\rm B}$/f.u. at 5~K, large magnetic anisotropy of 0.238 MJ/m$^3$, and a Curie temperature of 874~K. Magnetocaloric analysis reveals a magnetic entropy change of 3.1 J.kg$^{-1}$K$^{-1}$ at 70 kOe. Low-temperature transport measurements show a weak resistivity upturn, following a $-T^{1/2}$ dependence, indicative of disorder-enhanced electron-electron interactions. First-principles calculations based on density functional theory yield a magneto-crystalline anisotropy energy of 0.987 MJ/m$^3$, consistent with experiment and demonstrate pronounced surface and finite-size effects through comparison of bulk and nanocluster geometries. The combination of high Curie temperature, sizable perpendicular magnetic anisotropy, and moderate spin polarization and magnetic entropy change make the Ni$_2$FeAl as promising candidate for various applications.

cond-mat.mtrl-sci↗

arXiv2026

Physical and Dielectric Properties of Polycrystalline LaV$_{0.5}$Nb$_{0.5}$O$_4$

We report a detailed investigation of the structural, electronic, vibrational, and dielectric properties of polycrystalline LaV$_{0.5}$Nb$_{0.5}$O$_4$ samples, prepared at two sintering temperatures (1000\degree C and 1250\degree C). The introduction of Nb$^{5+}$ at the V$^{5+}$ site leads to notable structural and vibrational changes, which can be attributed to their isoelectronic nature and the comparatively larger ionic radius of Nb$^{5+}$. The Rietveld refinement of the X-ray diffraction patterns confirms a coexistence of monoclinic ($P$2$_{1}$/$n$) and scheelite-type tetragonal ($I$4$_{1}$/$a$) phases; for example, with a fraction of 4\% and 96\% for the sample annealed at 1250\degree C. The particle morphology has altered from spherical (1000\degree C) to irregular-shaped (1250\degree C) as a result of increase in annealing temperature. The Raman spectroscopy, Fourier Transform Infrared spectroscopy and X-ray Photoemission Spectroscopy have been used to understand the vibrational and electronic properties. An optical band gap of 2.7~eV for the sample sintered at 1250\degree C is calculated using Ultraviolet-vis diffuse reflectance spectroscopy measurements. The dielectric studies shows the higher dielectric permittivity ($ε$$_{r}$) and lower dielectric loss for the sample annealed at 1250\degree C.

cond-mat.mtrl-sci↗

arXiv2026

Case study of an exploratory high voltage NASICON-based Na$_4$NiCr(PO$_4$)$_3$ cathode material for sodium-ion batteries

We examine a new NASICON-type Na$_4$NiCr(PO$_4$)$_3$ material designed for high-voltage and multi-electron reactions for the sodium-ion batteries (SIBs). The Rietveld refinement of the X-ray diffraction pattern, using the R$\bar{3}$c space group, confirmed the stabilization of the rhombohedral NASICON framework. Furthermore, the Raman and Fourier transform infrared spectroscopy are employed to probe the structure and chemical bonding. The core-level photoemission analysis reveals the Cr$^{3+}$ and mixed Ni$^{2+}$/Ni$^{3+}$ oxidation states in the sample. Moreover, the bond valence energy landscape (BVEL) analysis, based on the refined structure, revealed a three-dimensional network of well-connected sodium sites with a migration energy barrier of 0.468 eV. The material delivered a good charge capacity at around 4.5 V, but showed no sodium-ion intercalation during discharge, resulting in negligible discharge capacity. The post-mortem analysis confirmed that the crystal structure remained intact. The calculated energy barrier values indicated a reversal in sodium site stability after cycling, though the barriers can still permit feasible ion migration. This suggests that ion transport alone cannot explain the lack of reversibility, which likely arises from intrinsically poor electronic conductivity. These findings highlight key challenges in achieving stable, reversible capacity in this system and underscore the need for doping, structural modification, and electrolyte optimization to realize its full potential as a high-voltage SIB cathode.

cond-mat.mtrl-sci↗

arXiv2025

Electrochemical performance and diffusion kinetics of a NASICON type Na$_{3.3}$Mn$_{1.2}$Ti$_{0.75}$Mo$_{0.05}$(PO$_4$)$_3$/C cathode for low-cost sodium-ion batteries

We report the electrochemical performance and diffusion kinetics of a newly designed NASICON type Na$_{3.3}$Mn$_{1.2}$Ti$_{0.75}$Mo$_{0.05}$(PO$_4$)$_3$/C composite material as a cathode for cost-effective sodium-ion batteries. A novel strategy of small Mo doping successfully stabilizes the sample having high Mn content in single phase rhombohedral symmerty. The high-resolution microscopy analysis reveals nanocrystallites of around $\sim$18 nm, uniformly embedded within the semi-graphitic carbon matrix, which enhances the surface electronic conductivity and effectively shortens the sodium-ion diffusion path. More importantly, we demonstrate a stable electrochemical behavior, with enhanced discharge capacity of 124 mAh/g at 0.1 C, having good reversibility and retaining 77\% of its capacity after 300 cycles, and 70\% even after 400 cycles at 2 C. The sodium-ion diffusion coefficients, estimated using both galvanostatic intermittent titration technique (GITT) and cyclic voltammetry are found to lie within the range of $10^{-9}$ to $10^{-11}$~cm$^2$/s. Additionally, the bond-valence site energy mapping predicted a sodium-ion migration energy barrier of 0.76 eV. A detailed distribution of relaxation times (DRT) analysis is used to deconvolute the electrochemical impedance spectra into distinct processes based on their characteristic relaxation times. Notably, the solid-state diffusion of sodium ions within the bulk electrode, with a relaxation time of $\sim$50 s, shows a consistent trend with the diffusion coefficients obtained from GITT and Warburg-based evaluations across the state of charge.

physics.chem-ph↗

arXiv2025

Unraveling the storage mechanism of Na$_{3}$V$_{2-x}$Ni$_x$(PO$_4$)$_2$F$_3$/C cathodes for sodium-ion batteries through electrochemical, {\it operando} X-ray diffraction and microscopy studies

The storage mechanism and diffusion kinetics of Na$_{3}$V$_{2-x}$Ni$_{x}$(PO$_{4}$)$_{2}$F$_{3}$/C ($x=$ 0--0.07) cathodes are investigated through electrochemical impedance spectroscopy (EIS), galvanostatic intermittent titration technique (GITT) and cyclic voltammetry (CV). All the samples are prepared through the facile pH-assisted sol-gel route and crystallize in the P4$_{2}$/mnm symmetry. The optimal doping of Ni ($x=$ 0.05) exhibits superior specific capacities of 119 and 100 mAh g$^{-1}$ at 0.1 C and 10 C rates, respectively, along the excellent capacity retention of 78\% after 2000 cycles at 10 C rate with nearly 100\% Coulombic efficiency. The apparent diffusion coefficient values are found to be in the range of 10$^{-9}$--10$^{-10}$ cm$^{2}$ s$^{-1}$ through detailed analysis of CV and GITT. Moreover, we report the reversible structural evolution and morphological changes during charging and discharging under non-equilibrium conditions through the {\it operando} X-ray diffraction and the {\it in-situ} synchrotron based transmission X-ray microscopy, respectively. Further, to understand the stability mechanism and obtain precise polarization values, we performed the distribution of relaxation times (DRT) analysis using the EIS data. The structure and morphology are found to be stable after long cycling.

cond-mat.mtrl-sci↗

arXiv2025

Synergistic Role of Transition Metals and Polyanionic Frameworks in Phosphate-Based Cathode Materials for Sodium-Ion Batteries

Ongoing research in the area of advanced cathode materials for sodium-ion batteries (SIBs) is expected to reduce reliance on lithium-ion batteries (LIBs), providing more affordable and sustainable energy storage solutions. Polyanionic compounds have emerged as promising options due to their stable structure and ability to withstand high-voltage conditions as well as fast charging capabilities. This review offers a thorough discussion of phosphate-based polyanionic cathodes for SIBs, exploring their structure, electrochemical performance with various transition metals, and existing challenges. We discuss different polyanionic frameworks, such as ortho-phosphates, fluoro-phosphates, pyro-phosphates, mix pyro-phosphates, and NASICON-based phosphates, highlighting their unique structural characteristics and ability to perform well across a wide potential range. Further, we delve into the mechanisms governing sodium storage and tunability of redox potentials in polyanionic materials, providing insights into the factors that affect their electrochemical performance. Finally, we outline future research directions and potential avenues for the practical applications of polyanionic high-voltage cathodes in sodium-ion battery technologies.

cond-mat.mtrl-sci↗

arXiv2025

Structural and Electrical Transport Properties of NASICON type Na$_{3}$Zr$_{2-x}$Ti$_{x}$Si$_2$PO$_{\rm 12}$ ($x=$ 0.1-0.4) Solid Electrolyte Materials

We report the structural, resistivity, impedance, and dielectric studies of isovalent substituted Na$_{3}$Zr$_{2-x}$Ti$_{x}$Si$_2$PO$_{\rm 12}$ ($x=$ 0.1--0.4) NASICON type solid electrolyte materials. The Rietveld refinement of XRD patterns shows the monoclinic phase with space group of C 2/c for all the samples. The resistivity analysis shows the Arrhenius-type thermal conduction with an increase in activation energy with doping is explained based on decreased unit cell volume. We use Maxwell-Wagner-Sillars (MWS) relaxation and space charge or interfacial polarization models to explain the frequency and temperature-dependent variations of electric permittivity. The double relaxation peaks in the dielectric loss data show the two types of relaxation mechanisms of different activation energy. The real ($ε^{'}$) and imaginary ($ε^{''}$) parts of permittivity are fitted using the modified Cole-Cole equation, including the conductivity term, which show the non-Debye type relaxation over the measured frequency and temperature range. The impedance analysis shows the contributions from grain and grain boundary relaxation. The fitting performed using the impedance and constant-phase element (CPE) confirm the non-Debye type relaxation. Moreover, the electric modulus analysis confirms the ionic nature having thermally activated relaxation and the modulus scaling analysis shows a similar type of relaxation in the measured temperature range. The modified power law is used to understand the frequency dependence of {\it a.c.} conductivity data. The temperature dependence of exponent ($s$) in modified power law suggests the change in the conduction mechanism from near small polaron tunneling (NSPT) to correlated barrier hopping (CBH) above room temperature. The larger values of $ε$$_{r}$ indicate these materials as a potential candidate for charge-storage devices.

cond-mat.mtrl-sci↗

arXiv2024

Flexible Trilayer Cellulosic Paper Separators engineered with BaTiO$_3$ ferroelectric fillers for High Energy Density Sodium-ion Batteries

We design a full cell configuration having Na$_{3}$V$_{2}$(PO$_{4}$)$_{3}$ as cathode and pre-sodiated hard carbon as an anode with Cellulosic Paper Separators and compare the electrochemical performance of these ceramic-impregnated polymer-coated cellulose paper separators with commercial glass fiber separator. Notably, the paper-based multilayer separators provide desirable characteristics such as excellent electrolyte wettability, thermal stability up to 200\degree C, and ionic conductivity, which are essential for the efficient operation of SIBs. The cellulose separator is coated by a layer of polyvinylidene fluoride polymer, followed by a second layer of styrene butadiene rubber (SBR) polymer in which ferroelectric fillers BaTiO$_{3}$ are integrated, which interacts with the polymer hosts through Lewis acid-base interactions ion and improves the conduction mechanism for the Na$^{+}$ ions. The final lamination is performed by varying the SBR concentrations (0.5, 0.75, and 1.0 w/v\%). The incorporated polymer matrices improve the flexibility, adhesion and dispersion of the nanoparticles and affinity of the electrolyte to the electrode. The morphology of the paper separators shows the uniform interconnected fibers with the porous structure. Interestingly, we find that the paper separator with 0.75 w/v\% content of SBR exhibit decreased interfacial resistance and improved electrochemical performance, having retention of 62\% and nearly 100\% Coulombic efficiency up to 240 cycles, as compared to other concentrations. Moreover, we observe the energy density around 376 Wh kg$^{-1}$ (considering cathode weight), which found to be comparable to the commercially available glass fiber separator. Our results demonstrate the potential of these multilayer paper separators towards achieving sustainability and safety in energy storage systems.

cond-mat.mtrl-sci↗

arXiv2024

Magnetic correlations and Griffith-like phase in Co$_2$TiSi$_{0.5}$Al$_{0.5}$ Heusler alloy

We present a comprehensive study aimed at elucidating the complex magnetic correlations in Co$_2$TiSi$_{0.5}$Al$_{0.5}$ Heusler alloy having the partial B2-type structure amid L2$_1$ cubic main phase. The thermo-magnetization measurements at 100 Oe reveal the presence of two magnetic transitions at T$\rm_{C1}=278~K$ and T$\rm_{C2}=270~K$, respectively, with saturation magnetization of around 1.2 $μ\rm_ B$/f.u. at 5~K. Our magnetic field dependent studies reveal the dominance of T$\rm_{C1}$ transition at lower fields ($μ_0\rm H \leqslant 0.03~Tesla$); however, at higher fields the T$\rm_{C2}$ transition becomes more pronounced. The observation of remnant magnetization above Curie temperature suggests the development of Griffiths-like phase, which is extensively analyzed through {\it ac} and {\it dc}- magnetic susceptibility ($χ$) data. The evaluation of magnetocaloric potential indicates second order phase transition with notable $ΔS_{\rm M}=$ 2.22 Jkg$^{-1}$K$^{-1}$ at 7 Tesla. The low-field ($μ_0\rm H \leqslant 0.1~Tesla$) magnetic entropy ($ΔS_{\rm M}$) curves exhibit two non-identical positive peaks. The nature and range of spin interactions near T$\rm_C$ were scrutinized through rigorous critical phenomenon analysis, and the values of exponents $α, β, γ$ and $δ$ to be $0.063, 0.361, 1.108$ and $3.943$, respectively, which are found to be slightly deviating from mean-field theory towards 3D Heisenberg model. Additionally, the exchange magnetic interactions are found to decay as $J(r) \sim r^{-4.6}$. Furthermore, the density functional theory results reveal the half-metallic nature exhibiting 100\% spin polarization (SP). However, the electronic and magnetic properties are greatly affected by the incorporation of structural disorder, which results in drastic reduction of SP to mere 8.3\%.

cond-mat.str-el↗

arXiv2024

Structural properties, dielectric relaxation and impedance spectroscopy of NASICON type Na$_{3+x}$Zr$_{2-x}$Pr$_{x}$Si$_2$PO$_{\rm 12}$ ceramics

We investigate the dielectric and impedance spectroscopic investigation of Pr-doped NASICON type Na$_{3+x}$Zr$_{2-x}$Pr$_{x}$Si$_2$PO$_{\rm 12}$ ($x=$ 0.05--0.2) samples as a function of temperature and frequency. The Rietveld refinement of x-ray diffraction patterns confirms the monoclinic phase having C2/c space groups for all the samples. The scanning electron microscopy shows the granular-like structure and energy dispersive x-ray analysis confirms the desired compositions. The temperature (90--400~K) and frequency (20 Hz-2 MHz) dependence of electric permittivity are explained using Maxwell-Wagner-Sillars (MWS) polarization and space charge polarization mechanisms. The dielectric relaxation shows nearly equal activation energy for all the samples with a non-Debye type of relaxation in the measured temperature range. The complex impedance analysis shows the presence of broad grain and grain boundary relaxation peaks. The stretched exponent analysis of electric modulus using the Kohlrausch-Williams-Watts (KWW) function further confirms the non-Debye type of relaxation. Moreover, scaling analysis of the electric modulus shows a similar type of relaxation for all the samples. The {\it a.c.} conductivity data are fitted using modified power law, where the temperature dependence of exponent ($s$) confirms the correlated barrier hopping (CBH) type conduction for all the samples. Our results indicate that the Pr doped NASICON samples are potential candidates for charge storage devices due to their large electric permittivity.

cond-mat.mtrl-sci↗

arXiv2024

Improved electrochemical performance of NASICON type Na$_{3}$V$_{2-x}$Co$_x$(PO$_{4}$)$_{3}$/C ($x=$ 0--0.15) cathode for high rate and stable sodium-ion batteries

In recent years, the Na-ion SuperIonic CONductor (NASICON) based polyanionics are considered the pertinent cathode materials in sodium-ion batteries due to their 3D open framework, which can accommodate a wide range of Na content and can offer high ionic conductivity with great structural stability. However, owing to the inferior electronic conductivity, these materials suffer from unappealing rate capability and cyclic stability for practical applications. Therefore, in this work we investigate the effect of Co substitution at V site on the electrochemical performance and diffusion kinetics of Na$_{3}$V$_{2-x}$Co$_x$(PO$_{4}$)$_{3}$/C ($x=$ 0--0.15) cathodes. All the samples are characterized through Rietveld refinement of the x-ray diffraction patterns, Raman spectroscopy, transmission electron microscopy, etc. We demonstrate improved electrochemical performance for the $x=$ 0.05 electrode with reversible capacity of 105 mAh g$^{-1}$ at 0.1 C. Interestingly, the specific capacity of 80 mAh g$^{-1}$ is achieved at 10 C with retention of about 92\% after 500 cycles and 79.5\% after 1500 cycles and having nearly 100\% Coulombic efficiency. The extracted diffusion coefficient values through galvanostatic intermittent titration technique and cyclic voltammetry are found to be in the range of 10$^{-9}$--10$^{-11}$ cm$^{2}$ s$^{-1}$. The postmortem studies show the excellent structural and morphological stability after testing for 500 cycles at 10 C. Our study reveals the role of optimal dopant of Co$^{3+}$ ions at V site to improve the cyclic stability at high current rate.

physics.chem-ph↗

arXiv2024

Mixed Polyanionic NaFe$_{1.6}$V$_{0.4}$(PO$_{4}$)(SO$_{4}$)$_{2}$@CNT Cathode for Sodium-ion Batteries: Electrochemical Diffusion Kinetics and Distribution of Relaxation Time Analysis at Different Temperatures

We report the electrochemical sodium-ion kiinetics and distribution of relaxation time (DRT) analysis of a newly designed mixed polyanionic NaFe$_{1.6}$V$_{0.4}$(PO$_{4}$)(SO$_{4}$)$_{2}$@CNT composite as a cathode. The specific capacity of 104 mAhg$^{-1}$ is observed at 0.1~C with the average working voltage of $\sim$3~V. Intriguingly, a remarkable rate capability and reversibility are demonstrated up to very high current rate of 25~C. The long cycling test up to 10~C shows high capacity retention even after 2000 cycles. The detailed analysis of galvanostatic intermittent titration technique (GITT) and cyclic voltammetry (CV) data reveal the diffusion coefficient of 10$^{-8}$--10$^{-11}$ cm$^{2}$s$^{-1}$. We find excellent stability in the thermal testing between 25--55$^\circ$C temperatures and 80\% capacity retention up to 100 cycles at 5~C. Further, we analyse the individual electrochemical processes in the time domain using the novel DRT technique at different temperatures. The {\it ex-situ} investigation shows the stable and reversible structure, morphology and electronic states of the long cycled cathode material. More importantly, we demonstrate relatively high specific energy of $\approx$155 Wh kg$^{-1}$ (considering the total active material loading of both the electrodes) at 0.2~C for full cell battery having excellent rate capability up to 10~C and long cyclic stability at 1~C.

physics.chem-ph↗

arXiv2024

Temperature dependent conductivity, dielectric relaxation, electrical modulus and impedance spectroscopy of Ni substituted Na$_{3+2x}$Zr$_{2-x}$Ni$_{x}$Si$_2$PO$_{\rm 12}$

We investigate the structural, dielectric relaxation, electric modulus and impedance behavior of Ni-doped NASICON ceramic Na$_{3+2x}$Zr$_{2-x}$Ni$_{x}$Si$_2$PO$_{\rm 12}$ ($x=$ 0.05--0.2) prepared using the solid-state reaction method. The increase in dielectric constant with temperature and decrease with frequency is explained on the basis of space charge polarization using the two-layer model of Maxwell-Wagner relaxation. The dielectric loss peak at lower temperatures follows the Arrhenius-type behavior with frequency having activation energy of 0.27$\pm$0.01~eV of dipolar relaxation, suggests similar type of defects are responsible for all the doped samples. The real ($ε$ $^{'}$) and imaginary ($ε$ $^{''}$) permittivity variation with frequency shows the broad relaxation behavior indicates the non-Debye type of relaxation in the measured temperature range. The permittivity values decrease with the amount of doping due to the increased number of charge carriers upon Ni doping at the Zr site. The grain contributions are observed at higher frequencies, while grain-boundary contributions occur at the lower side of frequencies. The imaginary part of the electric modulus also shows two types of relaxation peaks for all the samples indicating similar activation energy at low temperatures and variable activation energy at higher temperatures. The fitting of the imaginary modulus using KWW function shows the non-Debye type of relaxation. We find that all modulus curves merge with each other at low temperatures showing a similar type of relaxation, while curves at high temperatures show the dispersed behavior above the peak frequency. The {\it a.c.} conductivity data are fitted using the double power law confirming the grain and grain boundary contributions in total conductivity.

cond-mat.mtrl-sci↗