A synergetic promotion of surface stability for high-voltage LiCoO2 by multi-element surface doping: a first-principles study

The utilization of high-voltage LiCoO2 is an effective approach to break through the bottleneck of practical energy density in lithium ion batteries. However, the structural and interfacial degradations at the deeply delithiated state as well as the associated safety concerns impede the application of high-voltage LiCoO2. Herein, we present a synergetic strategy for promoting the surface stability of LiCoO2 at high voltage by Ti-Mg-Al co-doping and systematically study the effects of the dopants on the surface stability, electronic structure and Li+ diffusion properties of the LiCoO2 (104) surface using first-principles calculations. It is found that Ti, Mg and Al dopants can be facilely introduced into the Co sites of the LiCoO2 (104) surface. Furthermore, the co-doping could significantly stabilize the surface oxygen of LiCoO2 at a high delithiation state. Particularly, by aggregating Ti-Mg-Al co-dopant distribution in the surface layer, surface oxygen loss is dramatically suppressed. In addition, analysis of the electronic structure indicates that Ti-Mg-Al co-doping can enhance the electronic conductivity of the LiCoO2 (104) surface and greatly inhibit the charge deficiency of the superficial lattice O atoms at a highly delithiated state. In spite of a negligible improvement in the surface Li+ diffusion kinetics, the Ti-Mg-Al surface-modified LiCoO2 is expected to exhibit improved electrochemical performance at high voltage due to its superior surface stability. Our results suggest that aggregating Ti, Mg and Al co-dopant distribution in the surface layer is a promising modulation strategy to synergistically promote the surface oxygen stability of LiCoO2 at high voltages.

First-principles study on small polaron and Li diffusion in layered LiCoO2

Li-ion conductivity is one of the essential properties that influences the performance of cathode materials for Li-ion batteries. Here, using density functional theory, we investigate the polaron stability and its effect on the Li-ion diffusion in layered LiCoO2 with various magnetic orderings. We show that the local magnetism promotes the localized Co4+ polaron with the Li-diffusion barrier of ∼0.34 eV. While the Li-ion diffuses, the polaron migrates in the opposite direction to the Li movement. In the non-magnetic structure, on the other hand, the polaron does not form, and the Li diffusion barrier is lowered to 0.21 eV. Although the presence of the polaron raises the diffusion barrier, the magnetically ordered structures are energetically more stable during the migration than the non-magnetic case. Thus, our work advocates the hole polaron migration scenario for Li-ion diffusion. We further demonstrate that the strong electron correlation of Co ions plays an essential role in stabilizing the Co4+ polaron.

Effect of transition metal cations on the local structure and lithium transport in disordered rock-salt oxides

Lithium-excess oxides Li1.2Ti0.4Mn0.4O2 and Li1.3Nb0.3Mn0.4O2 with a disordered rock-salt structure and Mn3+/Mn4+ as a redox couple were compared to analyze the effect of different d0 metal ions on the local structure and Li+ ion migration.

Lithium tracer diffusion in LiNi0.33Mn0.33Co0.33O2 cathode material for lithium-ion batteries

The LiNi0.33Mn0.33Co0.33O2 compound is one of the most interesting cathode materials for Li-ion batteries. Li diffusion in this material directly influences charging/discharging times (and consequently power densities), maximum capacities, stress formation and possible side reactions. In the present study Li tracer self-diffusion is investigated in polycrystalline sintered bulk samples with an average grain size of about 50 nm in the temperature range between 110 and 350 °C. For analysis, stable 6Li tracers are used in combination with Secondary Ion Mass Spectrometry (SIMS). The diffusivities can be described by the Arrhenius law with an activation enthalpy of (0.85 ± 0.03) eV, which is interpreted as the migration energy of a single Li vacancy. Lithium diffuses via structural vacancies whose concentration is fixed by a Li deficiency of about 10%. An extrapolation of the diffusivities to room temperature gives significantly lower values than the diffusivities obtained by electrochemical measurements in literature.

Understanding the sodium ion transport properties, deintercalation mechanism, and phase evolution of a Na2Mn2Si2O7 cathode by atomistic simulation

Molecular dynamics (MD) together with the first principles method (DFT) reveal that Na+ is capable of migrating three dimensionally in a Na2Mn2Si2O7 cathode material. Migration along the a-axis and c-axis have the same mechanism, that is, alternating between the Na1 and Na2 route with a similar local environment and distance. Long-distance hopping between two Na2 atoms or between Na1 and Na2 atoms is crucial for continuous migration along the b-axis. Also, the anti-site phenomenon is identified, and it facilitates the migration of the Na ions. Four intermediate phases are determined according to the formation energy curve and, as a result, the voltage profile is predicted accurately. The state of charge (SOC) dependency of the Na+ energy shows that the mobility of Na+ is highly inhibited in the fully discharged state. Upon the deintercalation of sodium ions, Na+ is activated immediately. A maximal DNa+ value of 3.6 × 10-9 cm2 s-1 and a low energy barrier of ca. 0.26 eV at the deintercalation level of x = 0.25 are observed. Because of the scarcity of Na+, DNa+ experiences a sharp decrease at the end of deintercalation. Despite the low level of Na+ mobility in the range of 0.25 < x < 1, Na2Mn2Si2O7 is still a potential cathode material for use in sodium ion batteries (SIBs).

Highly efficient evaluation of diffusion networks in Li ionic conductors using a 3D-corrugation descriptor

A highly efficient computational approach for the screening of Li ion conducting materials is presented and its performance is demonstrated for olivine-type oxides and thiophosphates. The approach is based on a topological analysis of the electrostatic (Coulomb) potential obtained from a single density functional theory calculation augmented by a Born-Mayer-type repulsive term between Li ions and the anions of the material. This 3D-corrugation descriptor enables the automatic determination of diffusion pathways in one, two, and three dimensions and reproduces migration barriers obtained from density functional theory calculations using nudged elastic band method within approximately 0.1 eV. Importantly, it correlates with Li ion conductivity. This approach thus offers an efficient tool for evaluating, ranking, and optimizing materials with high Li-ion conductivity.

Determination of possible configurations for Li0.5CoO2 delithiated Li-ion battery cathodes via DFT calculations coupled with a multi-objective non-dominated sorting genetic algorithm (NSGA-III)

Here, we propose a new and logical approach to systematically treat the configurational diversity in density functional theory (DFT) calculations. To tackle this issue, we select Li_0.5CoO₂ as a representative example because it is one of the most extensively studied cathodes in Li-ion batteries (LIBs), and it has a huge number of disordered configurations. To delineate the configurations that will match well with the experimentally measured macro-functions of redox potential, band gap energy, and magnetic moment, we adopt a multi-objective, non-dominated sorting, genetic algorithm (NSGA-III) that enables the simultaneous optimization of these three objective functions. The decision variables include configuration of the Li/vacancy, initial input for the magnetic moment distribution reflecting Co³⁺/Co⁴⁺ distribution, and initial input for the lattice parameter and Hubbard U. We use NSGA-III to separate the configurations that exhibit awkward objective function values, which allows us to pinpoint a set of plausible configurations that match the experimentally estimated values of the objective functions. The results reveal a plausible configuration that is a mixture of various ordered/disordered configurations rather than a simple ordered structure.

Ni–Al–Cr superalloy as high temperature cathode current collector for advanced thin film Li batteries

To obtain full advantage of state-of-the-art solid-state lithium-based batteries, produced by sequential deposition of high voltage cathodes and promising oxide-based electrolytes, the current collector must withstand high temperatures (>600 °C) in oxygen atmosphere. This imposes severe restrictions on the choice of materials for the first layer, usually the cathode current collector. It not only must be electrochemically stable at high voltage, but also remain conductive upon deposition and annealing of the subsequent layers without presenting a strong diffusion of its constituent elements into the cathode. A novel cathode current collector based on a Ni–Al–Cr superalloy with target composition Ni_0.72Al_0.18Cr_0.10 is presented here. The suitability of this superalloy as a high voltage current collector was verified by determining its electrochemical stability at high voltage by crystallizing and cycling of LiCoO₂ directly onto it.

A novel cathode current collector based on a Ni–Al–Cr superalloy is presented here. The suitability of this superalloy as a high voltage current collector was verified by crystallizing and cycling of LiCoO₂ directly onto it.

Mechanism of Li intercalation/deintercalation into/from the surface of LiCoO2

Mechanism of Li diffusion at the LiCoO2(101[combining overline]4) surface and in bulk LiCoO2 is studied using density functional theory calculations. We find that there is almost no barrier for the diffusion of Li between the two topmost surface layers. The results show that Li intercalation occurs by the diffusion of Li ions from the first layer to the divacancy of Li sites created by removal of two neighboring Li ions in the first and second layer. However, Li deintercalation occurs by the diffusion of Li ions from the second layer to the missing row of topmost Li sites. The energy barrier for the process of intercalation/deintercalation of Li between the second and third surface layers is also lower than that in the bulk. This finding indicates that nanosized LiCoO2 with a large surface area/volume ratio is a promising cathode material for fast charging/discharging Li-ion batteries.