Design of Nickel-Based Cation-Disordered Rock-Salt Oxides: The Effect of Transition Metal (M = V, Ti, Zr) Substitution in LiNi0.5M0.5O2 Binary Systems

Abstract: Cation-disordered oxides have been ignored as positive electrode material for a long time due to structurally limited lithium insertion/extraction capabilities. In this work, a case study is carried out on nickel-based cation-disordered Fm3̅m LiNi0.5M0.5O2 positive electrode materials. The present investigation targets tailoring the electrochemical properties for nickel-based cation-disordered rock-salt by electronic considerations. The compositional space for binary LiM+3O2 with metals active for +3/+4 redox couples is extended to ternary oxides with LiA0.5B0.5O2 with A = Ni2+ and B = Ti4+, Zr4+, and V+4 to assess the impact of the different transition metals in the isostructural oxides. The direct synthesis of various new unknown ternary nickel-based Fm3m̅ cation- disordered rock-salt positive electrode materials is presented with a particular focus on the LiNi0.5V0.5O2 system. This positive electrode material for Li-ion batteries displays an average voltage of ∼2.55 V and a high discharge capacity of 264 mAhg−1 corresponding to 0.94 Li. For appropriate cutoff voltages, a long cycle life is achieved. The charge compensation mechanism is probed by XANES, confirming the reversible oxidation and reduction of V4+/V5+. The enhancement in the electrochemical performances within the presented compounds stresses the importance of mixed cation-disordered transition metal oxides with different electronic configuration. 

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Reversible Delithiation of Disordered Rock Salt LiVO2

Abstract: A rigid crystal lattice, in which cations occupy specific positions, is generally regarded as a critical requirement to enable Li+ diffusion in the bulk of conventional cathode materials, whereas disorder is generally considered as detrimental. Herein, we demonstrate that facile and reversible insertion and extraction of Li+ is possible with LiVO2, a new cation‐disordered rock salt compound (space group: Fm3 m), which is, to the best of our knowledge, described for the first time. This new polymorph of LiVO2 is synthesized by mechanical alloying. Rietveld refinements of the X‐ray diffractions patterns and SAED (selected‐area electron diffraction) patterns attested the formation of the disordered LiVO2 rock salt phase. Galvanostatic cycling experiments were employed to characterize the electrochemical performance of the material, demonstrating that reversible cycling over 100 cycles with a discharge capacity around 100 mAh g−1 is possible.

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Oxygen Activity in Li-Rich Disordered Rock-Salt Oxide and the Influence of LiNbO3 Surface Modification on the Electrochemical Performance

Abstract: Li-rich disordered rock-salt oxides such as Li1.2Ni1/3Ti1/3Mo2/15O2 are receiving increasing attention as high-capacity cathodes due to their potential as high-energy materials with variable elemental composition. However, the first-cycle oxygen release lowers the cycling performance due to cation densification and structural reconstruction on the surface region. This work explores the influence of lithium excess on the charge compensation mechanism and the effect of surface modification with LiNbO3 on the cycling performance. Moving from a stoichiometric LiNi0.5Ti0.5O2 composition toward Li-rich Li1.2Ni1/3Ti1/3Mo2/15O2, oxygen redox is accompanied by oxygen release. Thereby, cationic charge compensation is governed by the Ni2+/3+ and Mo3+/6+ redox reaction. Contrary to the bulk oxidation state of Mo6+ in the charged state, a mixed Mo valence on the surface is found by XPS. Furthermore, it is observed that smaller particle sizes result in higher specific capacities. Tailoring the surface properties of Li1.2Ni1/3Ti1/3Mo2/15O2 with a solid electrolyte layer of LiNbO3 altered the voltage profile, resulting in a higher average discharge voltage as compared to the unmodified material. The results hint at the interdiffusion of cations from the metal oxide surface coating into the electrode material, leading to bulk composition changes (doping) and a segregated Nb-rich surface. The main finding of this work is the enhanced cycling stability and lower impedance of the surface-modified compound. We argue that surface densification is mitigated by the Nb doping/surface modification.

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CLEASE: a versatile and user-friendly implementation of cluster expansion method

Abstract:  Materials exhibiting a substitutional disorder such as multicomponent alloys and mixed metal oxides/oxyfluorides are of great importance in many scientific and technological sectors. Disordered materials constitute an overwhelmingly large configurational space, which makes it practically impossible to be explored manually using first-principles calculations such as density functional theory due to the high computational costs. Consequently, the use of methods such as cluster expansion (CE) is vital in enhancing our understanding of the disordered materials. CE dramatically reduces the computational cost by mapping the first-principles calculation results on to a Hamiltonian which is much faster to evaluate. In this work, we present our implementation of the CE method, which is integrated as a part of the atomic simulation environment (ASE) open-source package. The versatile and user-friendly code automates the complex set up and construction procedure of CE while giving the users the flexibility to tweak the settings and to import their own structures and previous calculation results. Recent advancements such as regularization techniques from machine learning are implemented in the developed code. The code allows the users to construct CE on any bulk lattice structure, which makes it useful for a wide range of applications involving complex materials. We demonstrate the capabilities of our implementation by analyzing the two example materials with varying complexities: a binary metal alloy and a disordered lithium chromium oxyfluoride.

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Degradation Mechanisms in Li2VO2F Li-Rich Disordered Rock-Salt Cathodes

Abstract:  The increased energy density in Li-ion batteries is particularly dependent on the cathode materials that so far have been limiting the overall battery performance. A new class of materials, Li-rich disordered rock salts, has recently been brought forward as promising candidates for next-generation cathodes because of their ability to reversibly cycle more than one Li-ion per transition metal. Several variants of these Li-rich cathode materials have been developed recently and show promising initial capacities, but challenges concerning capacity fade and voltage decay during cycling are yet to be overcome. Mechanisms behind the significant capacity fade of some materials must be understood to allow for the design of new materials in which detrimental reactions can be mitigated. In this study, the origin of the capacity fade in the Li-rich material Li2VO2F is investigated, and it is shown to begin with degradation of the particle surface that spreads inward with continued cycling.

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Improved cycling stability in high-capacity Li-rich vanadium containing disordered rock salt oxyfluoride cathodes

Abstract:  Lithium-rich transition metal disordered rock salt (DRS) oxyfluorides have the potential to lessen one large bottleneck for lithium ion batteries by improving the cathode capacity. However, irreversible reactions at the electrode/electrolyte interface have so far led to fast capacity fading during electrochemical cycling. Here, we report the synthesis of two new Li-rich transition metal oxyfluorides Li₂V_0.5Ti_0.5O₂F and Li₂V_0.5Fe_0.5O₂F using the mechanochemical ball milling procedure. Both materials show substantially improved cycling stability compared to Li₂VO₂F. Rietveld refinements of synchrotron X-ray diffraction patterns reveal the DRS structure of the materials. Based on density functional theory (DFT) calculations, we demonstrate that substitution of V³⁺ with Ti³⁺ and Fe³⁺ favors disordering of the mixed metastable DRS oxyfluoride phase. Hard X-ray photoelectron spectroscopy shows that the substitution stabilizes the active material electrode particle surface and increases the reversibility of the V³⁺/V⁵⁺ redox couple. This work presents a strategy for stabilization of the DRS structure leading to improved electrochemical cyclability of the materials.

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Suppressing Dissolution of Vanadium from Cation-Disordered Li_2-xVO₂F via a Concentrated Electrolyte Approach

Abstract:  Li₂VO₂F with the cation-disordered rock-salt structure is an attractive high-energy-density positive electrode material but suffers from severe capacity fading upon cycling. The underlying reasons are yet unclear. In this study, we unveil the overlooked role of vanadium dissolution and electrode–electrolyte interactions and provide insight into the failure mechanism. Interfacial reactions, in general, can be tuned by either surface coatings or the modification of the electrolyte chemistry. Here we modify the interfacial reactions through the use of a concentrated electrolyte 5.5 M LiFSI in dimethyl carbonate, effectively reducing vanadium dissolution. Moreover, it results in a lower interfacial resistance build-up as compared to conventional 1.0 M LiPF₆ electrolyte, thus increasing the cycling stability. The solubility of vanadium enhances significantly with higher oxidation states. Furthermore, a chemical prelithiation strategy has been presented, which allows the full lithiation of VO₂F to Li₂VO₂F, with an outlook on the intermediate phases. We argue that the optimization of cathode–electrolyte interactions is of significant importance to improve the cycling performance of disordered rock-salts, where a thorough understanding of the limiting factors is still missing.

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Design and tuning of the electrochemical properties of Vanadium-based cation disordered rock-salt oxide positive electrode material for lithium-ion batteries

Abstract:  Disordered rock-salt compounds are becoming increasingly important due to their potential as high-capacity positive electrode materials for lithium-ion batteries. Thereby, a significant number of studies have focused on increasing the accessible Li capacity, but studies to manipulate the electrochemical potential are limited. This work explores the effect of transition-metal substitution on the electrochemistry of ternary disordered rock-salt-type compounds with LiM²⁺_0.5V_0.5⁴⁺O₂ stoichiometry (M = Mn, Fe, Co) directly synthesized through mechanochemistry. Rietveld refinements of synchrotron X-ray diffraction patterns confirm the disordered rock-salt structures. First-principles density functional theory study is used to predict the impact of the cation substitution on the expected average voltage and the electronic structures of these materials are used to analyze the underlying redox processes. For LiM²⁺_0.5V⁴⁺_0.5O₂ (M = Mn, Fe, Co), discharge voltages increase in the order of Mn < Fe < Co with 2.28, 2.41, and 2.51 V, exhibiting discharge capacities of 219, 207, and 234 mAh g^–1, respectively. In comparison, for the disordered rock-salt Li₂VO_3, an average discharge voltage of ∼2.2 V with V^5+/4+ redox couple has been reported. However, detrimental electrode–electrolyte interactions manifested as transition-metal dissolution has been found to result in severe capacity fading. Thereto, the use of a concentrated 5.5 M LiFSI increased the cycling stability significantly, effectively reducing transition-metal dissolution. The underlying reasons for the capacity fading of disordered rock salts are yet unclear. We stress the importance of cathode–electrolyte interactions, thus opening new directions for the improvement of cation-disordered materials.

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