Low-Temperature Properties of the Sodium-Ion Electrolytes Based on EC-DEC, EC-DMC, and EC-DME Binary Solvents

Sodium-ion batteries are a promising class of secondary power sources that can replace some of the lithium-ion, lead–acid, and other types of batteries in large-scale applications. One of the critical parameters for their potential use is high efficiency in a wide temperature range, particularly below 0 °C. This article analyzes the phase equilibria and electrochemical properties of sodium-ion battery electrolytes that are based on NaPF6 solutions in solvent mixtures of ethylene carbonate and diethyl carbonate (EC:DEC), dimethyl carbonate (EC:DMC), and 1,2-dimethoxyethane (EC:DME). All studied electrolytes demonstrate a decrease in conductivity at lower temperatures and transition to a quasi-solid state resembling “wet snow” at certain temperatures: EC:DEC at −8 °C, EC:DMC at −13 °C, and EC:DME at −21 °C for 1 M NaPF6 solutions. This phase transition affects their conductivity to a different degree. The impact is minimal in the case of EC:DEC, although it partially freezes at a higher temperature than other electrolytes. The EC:DMC-based electrolyte demonstrates the best efficiency at temperatures down to −20 °C. However, upon further cooling, 1 M NaPF6 in EC:DEC retains a higher conductivity and lower resistivity in symmetrical Na3V2(PO4)3-based cells. The temperature range from −20 to −40 °C is characterized by the strongest deterioration in the electrochemical properties of electrolytes: for 1 M NaPF6 in EC:DMC, the charge transfer resistance increased 36 times, and for 1 M NaPF6 in EC:DME, 450 times. For 1 M NaPF6 in EC:DEC, the growth of this parameter is much more modest and amounts to only 1.7 times. This allows us to consider the EC:DEC-based electrolyte as a promising basis for the further development of low-temperature sodium-ion batteries

Synthesis and electrochemical performance of Li2Co1−xMxPO4F (M = Fe, Mn) cathode materials

In the search for high-energy materials, novel 3D-fluorophosphates, Li2Co1−xFexPO4F and Li2Co1−xMnxPO4F, have been synthesized. X-ray diffraction and scanning electron microscopy have been applied to analyze the structural and morphological features of the prepared materials. Both systems, Li2Co1−xFexPO4F and Li2Co1−xMnxPO4F, exhibited narrow ranges of solid solutions: x ≤ 0.3 and x ≤ 0.1, respectively. The Li2Co0.9Mn0.1PO4F material demonstrated a reversible electrochemical performance with an initial discharge capacity of 75 mA·h·g−1 (current rate of C/5) upon cycling between 2.5 and 5.5 V in 1 M LiBF4/TMS electrolyte. Galvanostatic measurements along with cyclic voltammetry supported a single-phase de/intercalation mechanism in the Li2Co0.9Mn0.1PO4F material

Phase Transitions in the “Spinel-Layered” Li1+xNi0.5Mn1.5O4 (x = 0, 0.5, 1) Cathodes upon (De)lithiation Studied with Operando Synchrotron X-ray Powder Diffraction

“Spinel-layered” Li1+xNi0.5Mn1.5O4 (x = 0, 0.5, 1) materials are considered as a cobalt-free alternative to currently used positive electrode (cathode) materials for Li-ion batteries. In this work, their electrochemical properties and corresponding phase transitions were studied by means of synchrotron X-ray powder diffraction (SXPD) in operando regime. Within the potential limit of 2.2–4.9 V vs. Li/Li+ LiNi0.5Mn1.5O4 with cubic spinel type structure demonstrates the capacity of 230 mAh·g−1 associated with three first-order phase transitions with significant total volume change of 8.1%. The Li2Ni0.5Mn1.5O4 material exhibits similar capacity value and subsequence of the phase transitions of the spinel phase, although the fraction of the spinel-type phase in this material does not exceed 30 wt.%. The main component of Li2Ni0.5Mn1.5O4 is Li-rich layered oxide Li(Li0.28Mn0.64Ni0.08)O2, which provides nearly half of the capacity with very small unit cell volume change of 0.7%. Lower mechanical stress associated with Li (de)intercalation provides better cycling stability of the spinel-layered complex materials and makes them more perspective for practical applications compared to the single-phase LiNi0.5Mn1.5O4 high-voltage cathode material

β-NaVP2O7 as a Superior Electrode Material for Na-Ion Batteries

Herein, we present a novel β-polymorph of sodium vanadium pyrophosphate NaVP2O7 with the KAlP2O7-type structure obtained via hydrothermal synthesis and further thermal dehydration of a hydrophosphate intermediate. β-NaVP2O7 demonstrates attractive electrochemical behavior as a Na-ion positive electrode (cathode) material with practically achieved reversible capacity of 104 mAh/g at C/10 current density, average operating voltage of 3.9 V vs. Na/Na+ and only 0.5% volume change between the charged and discharged states. Electrode material exhibits excellent C-rate capability and cycling stability, providing the capacity of 90 mAh/g at 20C discharge rate and < 1% capacity loss after 100 charge-discharge cycles. At low voltage region (≈1.5 V vs. Na/Na+), β-NaVP2O7 reversibly intercalates additional sodium cations leading to unprecedented overall Na-ion storage ability exceeding 250 mAh/g within the 1.5 – 4.4 V vs. Na/Na+ voltage region. This material is one of only a few materials that exhibits reversible sodium ion storage capabilities over such a large potential window. </p

New Form of Li₂FePO₄F as Cathode Material for Li-Ion Batteries

New Form of Li₂FePO₄F as Cathode Material for Li-Ion Batterie