From Liquid to Solid-State Batteries: Li-Rich Mn-Based Layered Oxides as Emerging Cathodes with High Energy Density

Li-rich Mn-based (LRMO) cathode materials have attracted widespread attention due to their high specific capacity, energy density, and cost-effectiveness. However, challenges such as poor cycling stability, voltage deca,y and oxygen escape limit their commercial application in liquid Li-ion batteries. Consequently, there is a growing interest in the development of safe and resilient all-solid-state batteries (ASSBs), driven by their remarkable safety features and superior energy density. ASSBs based on LRMO cathodes offer distinct advantages over conventional liquid Li-ion batteries, including long-term cycle stability, thermal and wider electrochemical windows stability, as well as the prevention of transition metal dissolution. This review aims to recapitulate the challenges and fundamental understanding associated with the application of LRMO cathodes in ASSBs. Additionally, it proposes the mechanisms of interfacial mechanical and chemical instability, introduces noteworthy strategies to enhance oxygen redox reversibility, enhances high-voltage interfacial stability, and optimizes Li+ transfer kinetics. Furthermore, it suggests potential research approaches to facilitate the large-scale implementation of LRMO cathodes in ASSBs.

Decoupled alkaline water electrolysis by a K0.5MnO2-Ti mediator via K-ion insertion/extraction

Conventional one-step water electrolyzers generate H₂ accompanied by O₂ evolution, and may cause gas mixing and high cell voltage inputs. Herein, using the potassium ion battery material of K_0.5MnO₂-Ti as a mediator, we effectively decoupled the H₂ and O₂ evolution of alkaline water electrolysis temporally, thereby achieving a membrane-free pathway for H₂ production. As a mediator electrode for charge storage, the K_0.5MnO₂-Ti exhibited a stable capacity of 100 mA h g^-1 at 0.1 A g^-1 owing to the reversible K-ion insertion/extraction mechanism. The decoupled water electrolysis device exhibited the step voltages for hydrogen and oxygen production of 1.02 and 0.57 V at 5 mA, respectively. A nearly unity Faradaic efficiency and sustained production of pure H₂ has been demonstrated at a constant current density. We anticipate that this mediator demonstrated here may provide a route for the practical application of the decoupling strategy.

Impact of Ni Content on the Electrochemical Performance of the Co-Free, Li and Mn-Rich Layered Cathode Materials

Li and Mn-rich layered cathode (LLC) materials show great potential as the next generation cathode materials because of their high, practical and achievable specific capacity of ~250 mAh/g, thermal stability and lower raw material cost. However, LLC materials suffer from degradation of specific capacity, voltage fading due to phase transformation upon cycling and transition-metal dissolution, which presents a significant barrier for commercialization. Here, we report the effects of Ni content on the electrochemical performance, structural and thermal stability of a series of Co-free, LLC materials (Li1.2NixMn0.8-xO2, x = 0.12, 0.18, 0.24, 0.30 and 0.36) synthesized via a sol-gel method. Our study shows that the structure of the material as well as the electrochemical and thermal stability properties of the LLC materials are strongly dependent on the Ni or Mn content. An increase in the Ni to Mn ratio results in an increase in the average discharge voltage and capacity, as well as improved structural stability but decreased thermal stability.

Lithium Antievaporation-Loss Engineering via Sodium/Potassium Doping Enables Superior Electrochemical Performance of High-Nickel Li-Rich Layered Oxide Cathodes

Low-cost Mn- and Li-rich layered oxides suffer from rapid voltage decay, which can be improved by increasing the nickel content to derive high nickel Li-rich layered oxides (HNLO) but is normally accompanied by reduced capacity and inferior cycling stability. Herein, Na or K ions are successfully doped into the lattice of high nickel Li-rich Li_1.2-xM_xNi_0.32Mn_0.48O₂ (M = Na, K) layered oxides via a facile expanded graphite template-sacrificed approach. Both Na- and K-doped samples exhibit excellent rate capability and cycling stability compared with the un-doped one. The Na-doped sample shows a capacity retention of 93% after 200 cycles at 1C, which is quite outstanding for HNLO. The greatly improved electrochemical performances are attributed to the increased effective Li content in the lattice via Li antievaporation-loss engineering, the expanded Li slab, the pillaring effect, the increased C2/m component, and the improved electronic conductivity. Different performances by the introduction of sodium and potassium ions may be ascribed to their different ionic radii, which give rise to their different doping behaviors and threshold doping amounts. This work provides a new idea of enhancing electrochemical performance of HNLO by doping proper alien elements to increase the lattice lithium content effectively.

Synergetic effect of high Ni ratio and low oxygen defect interface zone of single crystals on the capacity retention of lithium rich layered oxides

Li-rich layered oxides (LLOs) are promising cathode materials for Li-ion batteries owing to their high capacities (>250 mAh g^-1), however, they suffered from severe capacity and voltage fading caused by irreversible oxygen loss and phase changes. Herein, the structural stability of single crystalline and polycrystalline Li_1.14Ni_0.32Mn_0.44Co_0.04O₂ was compared in detail. It was found that the stability of oxidized oxygen ions on the near surface was improved in single crystals, which retarded oxygen loss from surface and surficial phase changes, possibly owing to the facet regulating and low surface curvature. In addition, the formation-migration of Mn³⁺, one of the crucial factors that caused capacity fading of LLOs, can be mitigated by increasing Ni³⁺ ratio. Under the synergistic effect of low oxygen defects on the near surface and high Ni³⁺ ratio, stable cycling performances and higher thermal stability were obtained.

Lithium-Rich Cobalt-Free Manganese-Based Layered Cathode Materials for Li-Ion Batteries: Suppressing the Voltage Fading

Lithium-rich layered oxides are recognized as promising materials for Li-ion batteries, owing to higher capacity than the currently available commercialized cathode, for their lower cost. However, their voltage decay and cycling instability during the charge/discharge process are problems that need to be solved before their practical application can be envisioned. These problems are mainly associated with a phase transition of the surface layer from the layered structure to the spinel structure. In this paper, we report the AlF3-coating of the Li-rich Co-free layered Li1.2Ni0.2Mn0.6O2 (LLNMO) oxide as an effective strategy to solve these problems. The samples were synthesized via the hydrothermal route that insures a very good crystallization in the layered structure, probed by XRD, energy-dispersive X-ray (EDX) spectroscopy, and Raman spectroscopy. The hydrothermally synthesized samples before and after AlF3 coating are well crystallized in the layered structure with particle sizes of about 180 nm (crystallites of ~65 nm), with high porosity (pore size 5 nm) determined by Brunauer–Emmett–Teller (BET) specific surface area method. Subsequent improvements in discharge capacity are obtained with a ~5-nm thick coating layer. AlF3-coated Li1.2Ni0.2Mn0.6O2 delivers a capacity of 248 mAh g−1 stable over the 100 cycles, and it exhibits a voltage fading rate of 1.40 mV per cycle. According to the analysis from galvanostatic charge-discharge and electrochemical impedance spectroscopy, the electrochemical performance enhancement is discussed and compared with literature data. Post-mortem analysis confirms that the AlF3 coating is a very efficient surface modification to improve the stability of the layered phase of the Li-rich material, at the origin of the significant improvement of the electrochemical properties.

Atomic Layer Deposition of Lithium–Nickel–Silicon Oxide Cathode Material for Thin-Film Lithium-Ion Batteries

Lithium nickelate (LiNiO2) and materials based on it are attractive positive electrode materials for lithium-ion batteries, owing to their large capacity. In this paper, the results of atomic layer deposition (ALD) of lithium–nickel–silicon oxide thin films using lithium hexamethyldisilazide (LiHMDS) and bis(cyclopentadienyl) nickel (II) (NiCp2) as precursors and remote oxygen plasma as a counter-reagent are reported. Two approaches were studied: ALD using supercycles and ALD of the multilayered structure of lithium oxide, lithium nickel oxide, and nickel oxides followed by annealing. The prepared films were studied by scanning electron microscopy, spectral ellipsometry, X-ray diffraction, X-ray reflectivity, X-ray photoelectron spectroscopy, time-of-flight secondary ion mass spectrometry, energy-dispersive X-ray spectroscopy, transmission electron microscopy, and selected-area electron diffraction. The pulse ratio of LiHMDS/Ni(Cp)2 precursors in one supercycle ranged from 1/1 to 1/10. Silicon was observed in the deposited films, and after annealing, crystalline Li2SiO3 and Li2Si2O5 were formed at 800 °C. Annealing of the multilayered sample caused the partial formation of LiNiO2. The obtained cathode materials possessed electrochemical activity comparable with the results for other thin-film cathodes.

Surface modification of Li-rich manganese-based cathode materials by chemical etching

Chemical etching modifies the surface composition of a Li-rich Mn-based cathode and generates a thin amorphous layer that stabilizes the structure.