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The intrinsic stability of the 5 V LiCoPO4-LiCo2P3O10 thin-film (carbon-free) cathode material coated with MoO3 thin layer is studied using a comprehensive synchrotron electron spectroscopy in situ approach combined with firstprinciple calculations. The atomic-molecular level study demonstrates fully reversible electronic properties of the cathode after the first electrochemical cycle. The polyanionic oxide is not involved in chemical reactions with the tluoroethylene-containing liquid electrolyte even when charged to 5.1 V vs Li+/Li. The high stability of the cathode is explained on the basis of the developed energy level model. In contrast, the chemical composition of the cathode-electrolyte interface evolves continuously by involving MoO3 in the decomposition reaction with consequent leaching of oxide from the surface. The proposed mechanisms of chemical reactions are attributed to external electrolyte oxidation via charge transfer from the relevant electron level to the MoO3 valence band state and internal electrolyte oxidation via proton transfer to the solvents. This study provides a deeper insight into the development of both a doping strategy to enhance the electronic conductivity of high-voltage cathode materials and an efficient surface coating against unfavorable interfacial chemical reactions.

Sodium-ion batteries (SIBs) have developed rapidly owing to the high natural abundance, wide distribution, and low cost of sodium. Among the various materials used in SIBs, sodium superion conductor (NASICON)-based electrode materials with remarkable structural stability and high ionic conductivity are one of the most promising candidates for sodium storage electrodes. Nevertheless, the relatively low electronic conductivity of these materials makes them display poor electrochemical performance, significantly limiting their practical application. In recent years, the strategies of enhancing the inherent conductivity of NASICON-based cathode materials have been extensively studied through coating the active material with a conductive carbon layer, reducing the size of the cathode material, combining the cathode material with various carbon materials, and doping elements in the bulk phase. In this paper, we review the recent progress in the development of NASICON-based cathode materials for SIBs in terms of their synthesis, characterization, functional mechanisms, and performance validation/optimization. The advantages and disadvantages of such SIB cathode materials are analyzed, and the relationship between electrode structures and electrochemical performance as well as the strategies for enhancing their electrical conductivity and structural stability is highlighted. Some technical challenges of NASICON-based cathode materials with respect to SIB performance are analyzed, and several future research directions are also proposed for overcoming the challenges toward practical applications.

Binders are electrochemically inactive components that have a crucial impact on battery aging although being present in only small amounts, typically 1-3% w/w in commercial products. The electrochemical performance of a battery can be tailored via these inactive materials by optimizing the electrode integrity and surface chemistry. Polyacrylonitrile (PAN) for LiNi0.5Mn1.5O4 (LNMO) half-cells is here investigated as a binder material to enable a stable electrode-electrolyte interface. Despite being previously described in the literature as an oxidatively stable polymer, it is shown that PAN degrades and develops resistive layers within the LNMO cathode. We demonstrate continuous internal resistance increase in LNMO-based cells during battery operation using the intermittent current interruption (ICI) technique. Through a combination of on-line electrochemical mass spectrometry (OEMS) and X-ray photoelectron spectroscopy (XPS) characterization techniques, the degradation products can be identified as solid on the LNMO electrode surface, and no excessive gas formation is seen. The increased resistance and parasitic processes are correlated to side-reactions of the PAN, possibly intramolecular cyclization, which can be identified as the main cause of the comparatively fast capacity fade.

Sodium-ion batteries (SIBs) are potential replacements for lithium-ion batteries owing to their comparable energy density and the abundance of sodium. However, the low potential and low stability of their cathode materials have prevented their commercialization. Prussian blue analogs are ideal cathode materials for SIBs owing to the numerous diffusion channels in their 3D structure and their high potential vs. Na/Na+. In this study, we fabricated various Fe-V-incorporated hexacyanoferrates, which are Prussian blue analogs, via a one-step synthesis. These compounds changed their colors from blue to green to yellow with increasing amounts of incorporated V ions. The X-ray photoelectron spectroscopy spectrum revealed that V3+ was oxidized to V4+ in the cubic Prussian blue structure, which enhanced the electrochemical stability and increased the voltage platform. The vanadium ferrocyanide Prussian blue (VFPB1) electrode, which contains V4+ and Fe2+ in the Prussian blue structure, showed Na insertion/extraction potential of 3.26/3.65 V vs. Na/Na+. The cycling test revealed a stable capacity of ~70 mAh g−1 at a rate of 50 mA g−1 and a capacity retention of 82.5% after 100 cycles. We believe that this Fe-V-incorporated Prussian green cathode material is a promising candidate for stable and high-voltage cathodes for SIBs.

Transition metal-based sodium fluoro-perovskite of general formula NaMF3 (M = Fe, Mn, and Co) were investigated as cathode materials for rechargeable Na-ion batteries. Preliminary results indicated Na-ion reversible intercalation but highlighted the need to find optimization strategies to improve conductivity and to modulate the operating voltages within experimentally accessible electrolytes’ stability windows, in order to fully exploit their potential as high-voltage cathodes. In this study, we combined experimental and computational techniques to investigate structures, defects, and intercalation properties of the NaFe1-xMnxF3 and NaCo1-xMnxF3 systems. Through the use of a simple solvothermal synthesis, we demonstrated the possibility to modulate the sample’s morphology in order to obtain fine and dispersed powder samples. The structural results indicated the formations of two solid solutions with a perovskite structure over the entire compositional range investigated. Atomistic simulations suggested that Na-ion diffusion in these systems was characterized by relatively high migration barriers and it was likely to follow three-dimensional paths, thus limiting the effect of anti-site defects. The correlation between structural and computational data highlighted the possibility to modulate both ionic and electronic conductivity as a function of the composition.

Safe and high-capacity cathode materials are a long quest for commercial lithium-ion battery development. Among various searched cathode materials, Li2FeSiO4 has taken the attention due to optimal working voltage, high elemental abundance, and low toxicity. However, as per our understanding and observation, the electrochemical performance of this material is significantly limited by the intrinsic low electronic conductivity and slow lithium-ion diffusion, which limits the practical capacity (a theoretical value of similar to 330 mAh g(-1)). In this report, using first-principles density functional theory based approach, we demonstrate that chlorine doping on oxygen site can enhance the electronic conductivity of the electrode and concurrently improve the electrochemical performance. Experimentally, X-ray diffraction, X-ray photoelectron spectroscopy, and field-emission gun scanning electron microscopy elemental mapping confirms Cl doping in Li(2-x)FeSiO(4-x)C1(x)/C (x < 0.1), while electrochemical cycling performance demonstrated improved performance. The theoretical and experimental studies collectively predict that, via Cl doping, the lithium deinsertion voltage associated with the Fe2+/Fe3+ and Fe3+/Fe4+ redox couples can be reduced and electronic conductivity can be enhanced, which opens up the possibility of utilization of silicate-based cathode with carbonate based commercial electrolyte. In view of potential and electronic conductivity benefits, our results indicate that Cl doping can be a promising low-cost method to improve the electrochemical performance of silicate-based cathode materials.

A LiNi0.35Cu0.1Mn1.45Fe0.1O4 spinel cathode exploiting the multi-metal approach and operating in a lithium battery at 4.7 V is prepared by co-precipitation of metal oxalates followed by annealing. Our investigation demonstrates that the designed electrode formulation including Ni, Cu, Mn and Fe may actually ensure suitable structural and morphological characteristics, as well as an extended stability in cells operating at 25 °C and 55 °C. Indeed, the LiNi0.35Cu0.1Mn1.45Fe0.1O4 cathode shows reversible capacities of 108 mAh g−1 at 0.75C and 91 mAh g−1 at 1.5C (where 1C is 147 mA g−1), with a retention between 84% and 80% after 200 cycles and coulombic efficiency values of about 99%. Notably, optimal cathode composition in terms of metals nature and content avoids material degradation upon cycling and enables stable operation in the lithium cell even at a temperature higher than 25 °C, with satisfactory retention of the initial capacity of 110 mAh g−1, i.e., 77%, after 200 cycles at 0.5C. Therefore, the strategy adopted herein is considered adequate for allowing practical battery application of the high-voltage spinel cathode.

Particle size-tuned platelets of the high-voltage cathode material LiCoPO4 for Li-ion batteries have been synthesized by a simple one-step microwave-assisted solvothermal process using an array of water/ethylene glycol (EG) solvent mixtures. Particle size control was achieved by altering the concentration of the EG co-solvent in the mixture between 0 and 100vol%, with amounts of 0–80vol% EG producing single phase, olivine-type LiCoPO4. The particle sizes of the olivine materials were significantly reduced from about 1.2μm×1.2μm×500nm (0 vol% EG) to 200nm×100nm×50nm (80 vol% EG) with increasing EG content, while specific surface areas increased from 2 to 13m2g-1. The particle size reduction could mainly be attributed to the modified viscosities of the solvent blends. Owing to the soft template effect of EG, the crystals exhibited the smallest dimensions along the [010] direction of the Li diffusion pathways in the olivine crystal structure, resulting in enhanced lithium diffusion properties. The relationship between the synthesis, crystal properties and electrochemical performance was further elucidated, indicating that the electrochemical performances of the as-prepared materials mainly depend on the solvent composition and the respective particle size range. LiCoPO4 products obtained from reaction media with low and high EG contents exhibited good electrochemical performances (initial discharge capacities of 87–124mAhg−1at 0.1C), whereas materials made from medium EG concentrations (40–60vol% EG) showed the highest capacities and gravimetric energy densities (up to 137 mAhg−1 and 658Whkg−1at 0.1C), excellent rate capabilities, and cycle life.

We report in this work a copper-doped Li0.85Ni0.46Cu0.1Mn1.49O4 spinel-structured compound prepared by an easy, two-steps coprecipitation and solid state process and used in a lithium-ion battery in combination with a CuO-based anode. We show that the spinel-type cathode adopts unique morphology, characterized by well-developed, crystalline and aggregated microparticles, that considerably reduces the occurrence of side reactions. This cathode material can operate in a lithium cell at voltages as high as 5.3 V without sign of electrolyte decomposition, delivering a capacity of about 100 mA h g(-1) with high retention and high Coulombic efficiency over prolonged cycling. The combination of the Li0.85Ni0.46Cu0.1Mn1.49O4 cathode with a conversion-type, CuO-MCMB anode results in a new type of lithium ion battery characterized by a voltage value of 3.4 V, a stable capacity of 100 mA h g(-1) and a high Coulombic efficiency (exceeding 95%). Expected low cost, safety, and environmental compatibility are additional advantages of the lithium-ion cell reported here.