Insights into electrochemical performance of Li2FeSiO4 from first-principles calculations

Benefits of Ga, Ge and As substitution in Li2FeSiO4: a first-principles exploration of the structural, electrochemical and capacity properties

Herein, the feasibility of Fe substitution by Ga, Ge and As in Li2FeSiO4 in modulating its structural, mechanical, electrochemical, capacity and electronic properties was systematically studied via first-principles calculations based on density functional theory within the generalized gradient approximation with Hubbard corrections (GGA+U). The calculated results show that Ga, Ge and As doping can effectively reduce the range of the cell volume change during Li+ removal, improving the Li+ detachment ability and cycle stability of the system. Meanwhile, the calculated mechanical properties including modulus ratio, B/G, and Poisson ratio, ν, indicate that the doped systems of Ga, Ge and As exhibit excellent mechanical properties. In addition, besides the increase in theoretical average deintercalation voltage induced by the Ga dopant when more than one Li+ ion is removed in the formula unit, the doping of Ga, Ge and As all reduce the theoretical average deintercalation voltage in the process of Li+ extraction. Especially in the case of doping of Ge, when 0.5 Li+ is removed from LiFe0.5Ge0.5SiO4, the theoretical average deintercalation voltage only increases by 0.19 V compared with the case of the removal of one Li+ in Li2Fe0.5Ge0.5SiO4, which causes the cathode material to have a longer and more stable discharge platform. Moreover, in the process of Li+ removal, the doping of Ga, Ge and As can effectively participate in the charge compensation of the system, and Ge and As can provide further charge, increasing the capacity of the Li2FeSiO4 cathode material considerably.

Hollow structured cathode materials for rechargeable batteries

Hollow structuring has been intensively studied as an effective strategy to improve the electrochemical performance of the electrode materials for rechargeable batteries in terms of specific capacity, rate capability, and cycling performance. To date, hollow structured anode materials have been extensively investigated, while hollow structured cathode materials (HSCMs) are relatively less explored because of the difficulties in morphological control as well as the concern of reduced volumetric capacities. In this paper, we provide an overview of the research advances in the synthesis and evolution of HSCMs for metal (Li, Na, etc.) ion batteries. Attributing to the advantages of hollow structures including high surface area, excellent accessibility to active sites, and enhanced mass transport and diffusion, hollow structuring can significantly improve the performance of high-capacity cathode materials with low kinetics, such as lithium rich layered oxides, silicates, and V₂O₅. It is anticipated that the precise and facile control of the spatial configuration can balance the electrochemical performance of HSCMs and the volumetric capacities of HSCMs, leading to practical high-performance batteries.

DFT calculations of the synergistic effect of λ-MnO2/graphene composites for electrochemical adsorption of lithium ions

Recently, the composite of spinel-type manganese oxide (λ-MnO₂)/graphene has drawn wide attention because of its good electrochemical adsorption selectivity for low concentrations of Li⁺ ions from lake brine or seawater to cope with the fast-rising demand of lithium resources. In this composite, the synergistic effect between the good selectivity of λ-MnO₂ for Li⁺ ions and the excellent conductivity of graphene play an important role for the electrochemical adsorption of Li⁺ ions. In order to reveal the synergistic mechanism in the electronic conductivity, the ionic conductivity and the ion selectivity of the λ-MnO₂/graphene composite, density functional theory (DFT) calculations combined with electrochemical adsorption experiments were carried out. The calculation results show that the enhanced electronic conductivity of the composite is due to the decrease of the band gap (E_g) in the λ-MnO₂/graphene composite compared with pure λ-MnO₂. Meanwhile, the graphene composited with λ-MnO₂ decreased the diffusion energy barrier of Li⁺ ions in λ-MnO₂. In addition, the competitive adsorption of Li⁺, Na⁺ and Mg²⁺ ions were investigated by the nudged elastic band (NEB) method and charge distribution analysis. The results show that Li⁺ ions in λ-MnO₂ exist in their pure ion state and have the lowest diffusion energy barrier compared with Na⁺ and Mg²⁺. The results of the DFT calculations were validated by cyclic voltammetry, electrochemical impedance spectroscopy and electrochemical adsorption experiments.

Fe-Si networks and charge/discharge-induced phase transitions in Li2FeSiO4 cathode materials

Structural phase transitions of electrode materials are responsible for poor reversibility during charge/discharge cycling in Li-ion batteries. Using previously developed structural databases, we investigate a structural landscape for LixFeSiO4 systems at x = 1. Starting with low-energy Li2FeSiO4 crystal structures, we explore the crystal structures of the material in different states of charge. The as-prepared Li2FeSiO4 materials adopt low energy structures characterized by two-dimensional (2D) Fe-Si networks. After the removal of one Li per formula unit to form LiFeSiO4, the structures with three-dimensional (3D) diamond-like Fe-Si networks become more energetically favorable without a significant impact on the charge capacity, which agrees with previous experimental and theoretical work. However, we reveal that the structure with a 3D diamond-like Fe-Si network can further transform into a new structure at x = 1. And the Li atom is hard to reinsert into these new structures. Consequently the system is prevented from returning to the Li2FeSiO4 state. We believe that the formation of this new structure plays an important role in the loss of reversible capacity of Li2FeSiO4 electrode materials.

Fe-Si networks in Na2FeSiO4 cathode materials

Using a combination of adaptive genetic algorithm search, motif-network search scheme and first-principles calculations, we have systematically studied the low-energy crystal structures of Na2FeSiO4. We show that the low-energy crystal structures with different space group symmetries can be classified into several families based on the topologies of their Fe-Si networks. In addition to the diamond-like network which is shared by most of the low-energy structures, another three robust Fe-Si networks are also found to be stable during the charge/discharge process. The electrochemical properties of representative structures from these four different Fe-Si networks in Na2FeSiO4 and Li2FeSiO4 are investigated and found to be strongly correlated with the Fe-Si network topologies. Our studies provide a new route to characterize the crystal structures of Na2FeSiO4 and Li2FeSiO4 and offer useful guidance for the design of promising cathodes for Na/Li ion batteries.

First-principles study of structural stability, dynamical and mechanical properties of Li2FeSiO4 polymorphs

Li₂FeSiO₄ is an important alternative cathode for next generation Li-ion batteries due to its high theoretical capacity (330 mA h g⁻¹).

Systematic investigation on Cadmium-incorporation in Li₂FeSiO₄/C cathode material for lithium-ion batteries

Cadmium-incorporated Li₂FeSiO₄/C composites have been successfully synthesized by a solid-state reaction assisted with refluxing. The effect and mechanism of Cd-modification on the electrochemical performance of Li₂FeSiO₄/C were investigated in detail by X-ray powder diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy, Raman spectra, transmission electron microscopy, positron annihilation lifetime spectroscopy and Doppler broadening spectrum, and electrochemical measurements. The results show that Cd not only exists in an amorphous state of CdO on the surface of LFS particles, but also enters into the crystal lattice of LFS. Positron annihilation lifetime spectroscopy and Doppler broadening spectrum analyses verify that Cd-incorporation increases the defect concentration and the electronic conductivity of LFS, thus improve the Li⁺-ion diffusion process. Furthermore, our electrochemical measurements verify that an appropriate amount of Cd-incorporation can achieve a satisfied electrochemical performance for LFS/C cathode material.

Electrochemistry of orthosilicate-based lithium battery cathodes: a perspective

In this perspective we highlight the electrochemical features of lithium metal orthosilicates, investigated by combined in or ex situ XRD and electrochemical measurements.