6 Search Results

Lithium (Li)-excess transition metal oxide materials which crystallize in the cation-disordered rock salt (DRX) structure are promising cathodes for realizing low-cost, high-energy-density Li batteries. However, the state-of-the-art electrolytes for Li-ion batteries cannot meet the high-voltage stability requirement for high-voltage DRX cathodes, thus new electrolytes are urgently demanded. It has been reported that the solvation structures and properties of the electrolytes critically influence the performance and stability of the batteries. In this study, the structure–property relationships of various electrolytes with different solvent-to-diluent ratios are systematically investigated through a combination of theoretical calculations and experimental tests and analyses. This approach guides themore » development of electrolytes with unique solvation structures and characteristics, exhibiting high voltage stability, and enhancing the formation of stable electrode/electrolyte interphases. These electrolytes enable the realization of Li||Li_1.094Mn_0.676Ti_0.228O₂ (LMTO) DRX cells with improved performance compared to the conventional electrolyte. Specifically, Li||LMTO cells with the optimized advanced controlled-solvation electrolyte deliver higher specific capacity and longer cycle life compared to cells with the conventional electrolyte. Additionally, the investigation into the structure–property relationship provides a foundational basis for designing advanced electrolytes, which are crucial for the stable cycling of emerging high-voltage cathodes.« less

Abstract Lithium (Li)‐rich transition metal oxide cathodes with a cation disordered rock salt structure (DRX) are increasingly gaining popularity for advanced Li batteries as they offer high capacity and cost benefits over the commonly used layered Li transition metal oxide cathodes. However, the performance of DRX cathodes and their applications are limited by severe side reactions between the cathode and the state‐of‐the‐art carbonate‐based electrolytes at high voltage of 4.8 V, transition metal dissolution, and structural instability of the cathode particles. In this work, an advanced localized high‐concentration electrolyte (LHCE) is developed to form a stable cathode‐electrolyte interphase and mitigate structural instabilitymore » of the Li _1.13 Mn _0.66 Ti _0.21 O ₂ (LMTO) DRX during electrochemical cycling. Li||LMTO half cells with the LHCE demonstrate increased capacity, cycling stability, and superior rate capability compared with cells containing a conventional carbonate electrolyte. For instance, the Li||LMTO cells cycled in LHCE show a higher initial capacity of 205.2 mAh g ⁻¹ and a better capacity retention of 72.5% after 200 cycles at a current density of 20 mA g ⁻¹ than those with the conventional electrolyte (initial capacity of 187.7 mAh g ⁻¹ and capacity retention of 19.9%). This work paves the way to the development of practical DRX cathode‐based high‐energy Li batteries.« less

We report nickel-rich layered oxides, such as LiNi_0.8Mn_0.1Co_0.1O₂ (NMC 811), are considered as one of the most promising candidates for the next-generation cathode because of their high energy densities and relatively low cost. However, the poor first Coulombic efficiency of NMC 811 leads to around a 15% capacity loss in the first cycle at a cut-off voltage of 4.4 V. Moreover, the structure degradation during cycling results in capacity fading and safety concerns, due to potential oxygen loss after charging. Here, with aluminum substitution for manganese through a developed continuous co-precipitation approach, the electrochemical performance of NMC 811 cathodes hasmore » been greatly enhanced. Among different Al% substituted samples, LiNi_0.8Mn_0.06Co_0.1Al_0.04O₂ cathodes reduced by 50% the first capacity loss of pristine NMC 811(18.0 vs 35.9 mAh g^-1) and improved the capacity retention from 81.4 to 96.4% after 60 cycles at 0.5C in the voltage range of 2.8–4.4 V.« less

Nickel-rich layered metal oxide LiNi_1–y–zMn_yCo_zO₂ (1 – y – z ≥ 0.8) materials are the most promising cathodes for next-generation lithium-ion batteries in electric vehicles. However, they lose more than 10% of their capacity on the first cycle, and interfacial/structural instability causes capacity fading. Coating and substitution are possible direct and effective solutions to solve these challenges. In this Letter, Nb coating and Nb substitution on LiNi_0.8Mn_0.1Co_0.1O₂ (NMC811) is easily produced through a scalable wet chemistry method followed by sintering from 400 to 800 °C. A Li-free Nb oxide treatment is found to remove surface impurities forming a LiNbO₃/Li₃NbO₄ surfacemore » coating, to reduce the first capacity loss and to improve the rate performance. Furthermore, Nb substitution stabilizes the structure, as evidenced by less heat evolution on heating, thus providing better long cycling stability with a 93.2% capacity retention after 250 cycles.« less

The metal to insulator transition of NbO₂ has been predicted to be a result of a structural phase transition (SPT) governed by Peierls physics. However, direct observation of the SPT using experimental techniques is still restricted by the extremely high transition temperature (810 °C) and the proclivity for NbO₂ to oxidize into Nb₂O₅ above 400 °C when exposed to air. In this work, we address these issues and employ temperature-dependent X-ray spectroscopy to describe the SPT of NbO₂ from the bulk to surface. Temperature-dependent extended X-ray absorption fine structure spectroscopy (T-EXAFS) reveals a gradual weakening of the bulk Nb dimersmore » over a large temperature range, which is indicative of a second-order Peierls mechanism. From these measurements, we determine the critical dimer distance to be 2.77 Å. Our T-EXAFS observations are supported by density functional theory of the phonon dispersion and the electronic density of states of NbO₂, which conclude that the dimerization is responsible for the insulating phase. The dimerization does not extend to the topmost layers, where an oxygen rich surface reconstruction is preferred irrespective of temperature even in extremely reducing environments; changes in the low-energy electron diffraction patterns are attributed to oxygen concentration and are independent of the underlying bulk phase transitions of NbO₂.« less

Interest in alkali-rich oxide cathodes has grown in an effort to identify systems that provide high energy densities through reversible oxygen redox. Furthermore, some of the most promising compositions such as those based solely on earth abundant elements, e. g., iron and manganese, suffer from poor capacity retention and large hysteresis. Here, we use the disordered rocksalt cathode, Li_1.3Fe_0.4Nb_0.3O₂, as a model system to identify the underlying origin for the poor performance of Li-rich iron-based cathodes. Using elementally specific spectroscopic probes, we find the first charge is primarily accounted for by iron oxidation to 4+ below 4.25 V and O₂more » gas release beyond 4.25 V with no evidence of bulk oxygen redox. Although the Li_1.3Fe_0.4Nb_0.3O₂ is not a viable oxygen redox cathode, the iron 3+/4+ redox couple can be used reversibly during cycling.« less