Advanced cathodes for potassium-ion battery

Potassium-ion battery (KIB) represents an emerging battery technology. Here in this review, we highlight the research progress of cathode materials for KIBs in recent 2 years. Statuses of four typical cathodes, layered metal oxides, polyanion compounds, Prussian blue analogs, and organic cathodes are discussed. Electrochemical performances of the cathode materials are improved through tailoring of the composition, microstructure, and surface modification of the electrodes. Regulating electrode–electrolyte interface also brings about prominent improvement in the rate capability and cycling stability of the cathodes. In particular, we speculate that both layered metal oxides and polyanion compounds should be of great application potential as cathodes for the future KIB full cells.     

New iron-based mixed-polyanion cathodes for lithium and sodium rechargeable batteries: combined first principles calculations and experimental study

New iron-based mixed-polyanion compounds Li(x)Na(4-x)Fe(3)(PO(4))(2)(P(2)O(7)) (x = 0-3) were synthesized, and their crystal structures were determined. The new compounds contained three-dimensional (3D)sodium/lithium paths supported by P(2)O(7) pillars in the crystal. First principles calculations identified the complex 3D paths with their activation barriers and revealed them as fast ionic conductors. The reversible electrode operation was found in both Li and Na cells with capacities of one-electron reaction per Fe atom, 140 and 129 mAh g(-1), respectively. The redox potential of each phase was ～3.4 V (vs Li) for the Li-ion cell and ～3.2 V (vs Na) for the Na-ion cell. The properties of high power, small volume change, and high thermal stability were also recognized, presenting this new compound as a potential competitor to other iron-based electrodes such as Li(2)FeP(2)O(7), Li(2)FePO(4)F, and LiFePO(4).     

Facile synthesis of vanadium oxide microspheres for lithium-ion battery cathodes

A simple and versatile method for preparation of non-solid and solid V₂O₅ microspheres is developed. Non-solid and solid V₂O₅ microspheres can be controllably prepared via adjusting the mixed solvent volume ratio and reaction time at low temperature. Solid V₂O₅ microspheres display higher discharge capacity and better cycling performance than non-solid V₂O₅ microspheres as a cathode material for lithium-ion batteries, which is ascribed to smaller charge transfer and diffusion resistance.     

Synergetic stability enhancement with magnesium and calcium ion substitution for Ni/Mn-based P2-type sodium-ion battery cathodes

The conventional P2-type cathode material Na_0.67Ni_0.33Mn_0.67O₂ suffers from an irreversible P2–O2 phase transition and serious capacity fading during cycling. Here, we successfully carry out magnesium and calcium ion doping into the transition-metal layers (TM layers) and the alkali-metal layers (AM layers), respectively, of Na_0.67Ni_0.33Mn_0.67O₂. Both Mg and Ca doping can reduce O-type stacking in the high-voltage region, leading to enhanced cycling endurance, however, this is associated with a decrease in capacity. The results of density functional theory (DFT) studies reveal that the introduction of Mg²⁺ and Ca²⁺ make high-voltage reactions (oxygen redox and Ni⁴⁺/Ni³⁺ redox reactions) less accessible. Thanks to the synergetic effect of co-doping with Mg²⁺ and Ca²⁺ ions, the adverse effects on high-voltage reactions involving Ni–O bonding are limited, and the structural stability is further enhanced. The finally obtained P2-type Na_0.62Ca_0.025Ni_0.28Mg_0.05Mn_0.67O₂ exhibits a satisfactory initial energy density of 468.2 W h kg⁻¹ and good capacity retention of 83% after 100 cycles at 50 mA g⁻¹ within the voltage range of 2.2–4.35 V. This work deepens our understanding of the specific effects of Mg²⁺ and Ca²⁺ dopants and provides a stability-enhancing strategy utilizing abundant alkaline earth elements.

A synergetic effect involving Mg and Ca can reduce the adverse impact on redox reactions related to Ni–O bonding in Mg and Ca co-doped P2-Na_0.67Ni_0.33Mn_0.66O₂ material, leading to better overall properties than its singly-doped counterparts.     

Investigating the oxidation state of Fe from LiFePO4-based lithium ion battery cathodes via capillary electrophoresis

A capillary electrophoresis (CE) method with ultraviolet/visible (UV–Vis) spectroscopy for iron speciation in lithium ion battery (LIB) electrolytes was developed. The complexation of Fe²⁺ with 1,10-phenantroline (o-phen) and of Fe³⁺ with ethylenediamine tetraacetic acid (EDTA) revealed effective stabilization of both iron species during sample preparation and CE measurements. For the investigation of small electrolyte volumes from LIB cells, a sample buffer with optimal sample pH was developed to inhibit precipitation of Fe³⁺ during complexation of Fe²⁺ with o-phen. However, the presence of the conducting salt lithium hexafluorophosphate (LiPF₆) in the electrolyte led to the precipitation of the complex [Fe(o-phen)₃](PF₆)₂. Addition of acetonitrile (ACN) to the sample successfully re-dissolved this Fe²⁺-complex to retain the quantification of both species. Further optimization of the method successfully prevented the oxidation of dissolved Fe²⁺ with ambient oxygen during sample preparation, by previously stabilizing the sample with HCl or by working under counterflow of argon. Following dissolution experiments with the positive electrode material LiFePO₄ (LFP) in LIB electrolytes under dry room conditions at 20°C and 60°C mainly revealed iron dissolution at elevated temperatures due to the formation of acidic electrolyte decomposition products. Despite the primary oxidation state of iron in LFP of +2, both iron species were detected in the electrolytes that derive from oxidation of dissolved Fe²⁺ by remaining molecular oxygen in the sample vials during the dissolution experiments.     

LixV2O5 nanobelts for high capacity lithium-ion battery cathodes

5–10 μm long, typically 200–300 nm wide, and several nanometers thick Li_xV₂O₅ (× ～ 0.8) nanobelts with the δ-type crystal structure were synthesized by a hydrothermal treatment of Li⁺-exchanged V₂O₅ gel. When dried at 200 °C under vacuum prior to electrochemical testing, the as-prepared nanobelts underwent the well-known δ → ε → γ-phase transition giving a mixture of ε and γ phases as a nanocomposite electrode material. Such a simple preparation procedure guarantees a yield of material with drastically enhanced initial discharge specific capacity of 490 mAh/g and great cyclability. The enhanced electrochemical performance is attributed to the complex of experimental procedures including post-synthesis treatment of the single-crystalline Li_xV₂O₅ nanobelts.     

Disodium dimolybdate: a potential high-performance anode material for rechargeable sodium ion battery applications

Journal of Solid State Electrochemistry - Na2Mo2O7 was synthesized by solid-state reaction route and explored as possible anode material for sodium ion battery for the first time. The...     

Syntheses,challenges and modifications of single-crystal cathodes for lithium-ion battery

Single-crystal cathodes (SCCs) are promising substitute materials for polycrystal cathodes (PCCs) in lithium-ion batteries (LIBs),because of their unique ordere...     

Synthesis and electrochemical study of Li-Mn-Ni-O cathodes for lithium battery applications

Cathode powders of the Li–Mn–Ni–O system have been prepared at a Mn/(Mn+Ni) ratio varying from 0 to 1. The solid state reaction method was used to obtain the cathode materials by mixing MnO₂, LiCO₃ and NiO. A 20% excess of lithium was used in the precursors. The materials produced were examined by X-rays to identify their structure. Batteries were assembled by using these materials as cathode with a liquid electrolyte consisting of EC/DC 1:1, 1 LiPF₆ and Li anode. Their capacity, cycle fading and charge-discharge conditions were evaluated.Presented at the 3rd International Meeting "Advanced Batteries and Accumulators", June 16th–June 20th 2002, Brno, Czech Republic     

Vanadium-based compounds and heterostructures as functional sulfur catalysts for lithium-sulfur battery cathodes

Lithium-sulfur(Li-S) batteries have attracted wide attention for their high theoretical energy density, low cost, and environmental friendliness. However, the shuttle effect of polysulfides and the insulation of active materials severely restrict the development of Li-S batteries. Constructing conductive sulfur scaffolds with catalytic conversion capability for cathodes is an efficient approach to solving above issues.Vanadium-based compounds and their heterostructures have recently emerged as f...     

A molecular dynamics study on Rh3+ hydration: development and application of a first principles hydrated ion–water interaction potential

The Rh³⁺ aquaion exhibits one of the largest residence times of water molecules in the first hydration shell. The extreme stability of this hexahydrated ion in water solutions makes Rh³⁺ an extremely suitable candidate to be studied using the hydrated ion model. According to this approach, the representative cationic entity in aqueous solution is the ion plus its first hydration shell (i.e. the hydrated ion) and not the bare ion. Our group has successfully applied that concept in the framework of classical statistical simulations based on first principles ion–water interaction potentials. The methodology is now applied to the [Rh(H₂O)₆]³⁺ case based on a previous generalization in which some of the contributions were found to be transferable among the cases already studied (Cr³⁺, Al³⁺, Mg²⁺, Be²⁺). In this contribution a flexible hydrate model is presented, in which rigid first-shell water molecules have rotational and translational degrees of freedom, allowing for internal dynamics of the hydrated ion entity. The potential presented is thoroughly tested by means of a set of molecular dynamics simulations. Structural, dynamical, energetic and spectroscopic information is retrieved from the simulations, allowing the estimation of properties such as ion hydration energy, vibrational spectra of the intermolecular modes, cation mobility, rotational dynamics of the hydrated ion and first-shell water molecules and residence times of the second-shell water molecules. Extension of the Ewald sum to terms r^–4, r^–6 and r^–7 is presented and applied to systems of different size ([Rh(H₂O)₆]³⁺+(n–6)H₂O, n=50, 100, 200, 500, 1000 and 2500) and cutoff radii.Contribution to the Jacopo Tomasi Honorary Issue     

Electrodeposition of polypyrrole for high-performance zinc ion battery

Journal of Solid State Electrochemistry - Zinc-ion batteries are considered as promising energy storage devices for large-scale energy storage due to the simple operation, low cost, and...     

Structure of the water/platinum interface--a first principles simulation under bias potential

Ab initio molecular dynamics simulations have been performed on the water/Pt interface. When the surface is neutral, water is found to form a contact layer directing its O atom toward the surface, i.e., O-down configuration. When the surface is negatively biased, the contact layer shows a significant structural change. The O-down configuration is converted mostly to the H-down configuration. As the surface is biased more strongly, we find that a hydrophobic double layer is formed in the contact layer.     

PVDF-HFP/PMMA-coated PE separator for lithium ion battery

Microporous poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP)/poly(methyl methacryate) (PMMA)-coated polyethylene (PE) separators were prepared by a simple dip-coating process with various compositions of PVDF-HFP/PMMA mixture under 40% relative humidity condition. The results indicate that the porosity, liquid electrolyte uptake, and ionic conductivity of the coated separators are largely affected by a ratio of PVDF-HFP/PMMA mixture and the highest porosity, electrolyte uptake, and ionic conductivity can be achieved at a composition of PVDF-HFP/PMMA (5/5). The results of the cell performance tests also reveal that the PE separator coated with PVDF-HFP/PMMA in a ratio of 5:5 provides better rate capability and cycle stability than other PE separators coated with different ratios.     

Combustion-synthesized sodium manganese (cobalt) oxides as cathodes for sodium ion batteries

We report on the electrochemical properties of layered manganese oxides, with and without cobalt substituents, as cathodes in sodium ion batteries. We fabricated sub-micrometre-sized particles of Na_0.7MnO_2?+?z and Na_0.7Co_0.11Mn_0.89O_2?+?z via combustion synthesis. X-ray diffraction revealed the same layered hexagonal P2-type bronze structure with high crystallinity for both materials. Potentiostatic and galvanostatic charge/discharge cycles in the range 1.5–3.8 V vs. Na | Na⁺ were performed to identify potential-dependent phase transitions, capacity, and capacity retention. After charging to 3.8 V, both materials had an initial discharge capacity of 138 mA?h?g^?1 at a rate of 0.3 C. For the 20th cycle, those values reduced to 75 and 92 mA?h?g^?1 for Co-free and Co-doped samples, respectively. Our findings indicate that earlier works probably underestimated the potential of (doped) P2-type Na_0.7MnO_2?+?z as cathode material for sodium ion batteries in terms of capacity and cycle stability. Apart from doping, a simple optimization parameter seems to be the particle size of the active material.     

Vanadium-based cathodes for aqueous zinc ion batteries: Structure,mechanism and prospects

As an emerging energy storage device with high-safety aqueous electrolytes, low-cost, environmental benignity and large-reserves, the rechargeable aqueous zinc-ion batteries(AZIBs) have attracted more and more attention. Vanadium-based compounds are also supposed as the potential candidate cathode materials for AZIBs due to their wide variety of phases, variable crystal structures and high theoretical capacity. In this review, the recent progress in the development of vanadium-based materials wa...     

Thermal stabilities of delithiated olivine MPO4 (M = Fe,Mn) cathodes investigated using first principles calculations

We present an analysis of the thermal reduction of delithiated LiMnPO₄ and LiFePO₄ based on the quarternary phase diagrams as calculated from first principles. Our results confirm the recent experimental findings that MnPO₄ decomposes at a much lower temperature than FePO₄, thereby potentially posing larger safety issues for LiMnPO₄ cathodes. We find that while substantial oxygen is released as MnPO₄ reduces to Mn₂P₂O₇, the mixed valence phases that form in the decomposition process of FePO₄ limit the amount of oxygen evolved.     

Polydopamine directed MnO@C microstructures as electrode for lithium ion battery

In this work,a facile process was reported to fabricate amorphous carbon-coated MnO micropeanuts(MPs)with 1.8μm in length and 1.0μm in width using hydrothermal reaction followed by heat treatment in the oxygen-free environment.With Mn Cl_2 and KMnO_4 dissolved in the mixture of ethylene glycol and water,MnCO_3 MP precursors were obtained via the hydrothermal reaction with dopamine as surfactant.Then MnCO_3 MP was annealed at 600°C in the N_2 atmosphere and was transformed into MnO MP,and simultaneously the formed polydopamine during the hydrothermal reaction was carbonized to produce amorphous carbon-coating on the MnO MP surface.In contrast,MnCO_3 nanoparticle(NP)precursor was formed without the addition of dopamine and MnO NP agglomerates were prepared after pyrolysis.The carbonization of polydopamine during thermolysis improves the electrical conductivity and thermal stability of the MnO MP and thus its electrochemical performance as electrode materials for lithium ion battery.Hopefully,this facile strategy for fabricating and designing carbon-coated materials would inspire more novel nanostructures and applications thereof.