Recent Advances in Layered Metal-Oxide Cathodes for Application in Potassium-Ion Batteries

High-Voltage Na0.76Ni0.25-x/2Mgx/2Mn0.75O2-xFx Cathode Improved by One-Step In Situ MgF2 Doping with Superior Low-Temperature Performance and Extra-Stable Air Stability

P2-NaxMnO2 has garnered significant attention due to its favorable Na+ conductivity and structural stability for large-scale energy storage fields. However, achieving a balance between high energy density and extended cycling stability remains a challenge due to the Jahn-Teller distortion of Mn3+ and anionic activity above 4.1 V. Herein, we propose a one-step in situ MgF2 strategy to synthesize a P2-Na0.76Ni0.225Mg0.025Mn0.75O1.95F0.05 cathode with improved Na-storage performance and decent water/air stability. By partially substituting cost-effective Mg for Ni and incorporating extra F for O, the optimized material demonstrates both enhanced capacity and structure stability via promoting Ni2+/Ni4+ and oxygen redox activity. It delivers a high capacity of 132.9 mA h g-1 with an elevated working potential of ≈3.48 V and maintains ≈83.0% capacity retention after 150 cycles at 100 mA g-1 within 2-4.3 V, compared to the 114.9 mA h g-1 capacity and 3.32 V discharging potential of the undoped Na0.76Ni0.25Mn0.75O2. While increasing the charging voltage to 4.5 V, 133.1 mA h g-1 capacity and 3.55 V discharging potential (vs Na/Na+) were achieved with 72.8% capacity retention after 100 cycles, far beyond that of the pristine sample (123.7 mA h g-1, 3.45 V, and 43.8%@100 cycles). Moreover, exceptional low-temperature cycling stability is achieved, with 95.0% after 150 cycles. Finally, the Na-storage mechanism of samples employing various doping strategies was investigated using in situ EIS, in situ XRD, and ex situ XPS techniques.

High Entropy-Induced Kinetics Improvement and Phase Transition Suppression in K-Ion Battery Layered Cathodes

Layered oxides are widely accepted to be promising cathode candidate materials for K-ion batteries (KIBs) in terms of their rich raw materials and low price, while their further applications are restricted by sluggish kinetics and poor structural stability. Here, the high-entropy design concept is introduced into layered KIB cathodes to address the above issues, and an example of high-entropy layered K0.45Mn0.60Ni0.075Fe0.075Co0.075Ti0.10Cu0.05Mg0.025O2 (HE-KMO) is successfully prepared. Benefiting from the high-entropy oxide with multielement doping, the developed HE-KMO exhibits half-metallic oxide features with a narrow bandgap of 0.19 eV. Increased entropy can also reduce the surface energy of the {010} active facets, resulting in about 2.6 times more exposure of the {010} active facets of HE-KMO than the low-entropy K0.45MnO2 (KMO). Both can effectively improve the kinetics in terms of electron conduction and K+ diffusion. Furthermore, high entropy can inhibit space charge ordering during K+ (de)insertion, and the transition metal-oxygen covalent interaction of HE-KMO is also enhanced, leading to suppressed phase transition of HE-KMO in 1.5-4.2 V and better electrochemical stability of HE-KMO (average capacity drop of 0.20%, 200 cycles) than the low-entropy KMO (average capacity drop of 0.41%, 200 cycles) in the wide voltage window.

Bilayer and Trilayer C3N/Blue-Phosphorene Heterostructures as Potential Anode Materials for Potassium-Ion Batteries

Two-dimensional (2D) van der Waals heterostructures outperform conventional anode materials for postlithium-ion batteries in terms of mechanical, thermal, and electrochemical properties. This study systemically investigates the performance of bilayer and trilayer C3N/blue phosphorene (C3N/BlueP) heterostructures as anode materials for potassium-ion batteries (KIBs) using first-principles density functional theory calculations. This study reveals that the adsorption and diffusion of K ions on bilayer and trilayer C3N/BlueP heterostructures are markedly superior to those of their monolayer counterparts. A bilayer heterostructure (C3N/BlueP) effectively reduces the bandgap of the BlueP monolayer (1.98 eV) to 0.02 eV, whereas trilayer heterostructures (bilayer-C3N/BlueP and C3N/bilayer-BlueP) exhibit metallic behavior with no bandgap. Additionally, the theoretical capacity of the bilayer and trilayer heterostructures ranges from 636.7 to 755.5 mA h g-1, considerably higher than the theoretical capacity of other prospective 2D heterostructures for KIBs investigated in the literature. This study also shows that the heterostructures exhibit K-ion diffusion barriers as low as 0.042 eV, ensuring the relatively fast diffusion of K ions.

Layer-Structured Multitransition-Metal Oxide Cathode Materials for Potassium-Ion Batteries with Long Cycling Lifespan and Superior Rate Capability

Manganese (Mn)-based layer-structured transition metal oxides are considered as excellent cathode materials for potassium ion batteries (KIBs) owing to their low theoretical cost and high voltage plateau. The energy density and cycling lifetime, however, cannot simultaneously satisfy the basic requirements of the market for energy storage systems. One of the primary causes results from the complex structural transformation and transition metal migration during the ion intercalation and deintercalation process. The orbital and electronic structure of the octahedral center metal element plays an important role for maintaining the octahedral structural integrity and improving the K+ diffusivity by the introduced heterogeneous [Me-O] chemical bonding. A multitransition metal oxide, P3-type K0.5Mn0.85Co0.05Fe0.05Al0.05O2 (KMCFAO), was synthesized and employed as a cathode material for KIBs. Beneficial from the larger layer spacing for K+ to better accommodate and effectively preventing the irreversible structural transformation in the insertion/extraction process, it can reach a superior capacity retention up to 96.8% after 300 cycles at a current density of 500 mA g-1. The full cell of KMCFAO//hard carbon exhibits an encouraging promising energy density of 113.8 W h kg-1 at 100 mA g-1 and a capacity retention of 72.6% for 500 cycles.

Recent Advances in Sodium-Ion Batteries: Cathode Materials

Emerging energy storage systems have received significant attention along with the development of renewable energy, thereby creating a green energy platform for humans. Lithium-ion batteries (LIBs) are commonly used, such as in smartphones, tablets, earphones, and electric vehicles. However, lithium has certain limitations including safety, cost-effectiveness, and environmental issues. Sodium is believed to be an ideal replacement for lithium owing to its infinite abundance, safety, low cost, environmental friendliness, and energy storage behavior similar to that of lithium. Inhered in the achievement in the development of LIBs, sodium-ion batteries (SIBs) have rapidly evolved to be commercialized. Among the cathode, anode, and electrolyte, the cathode remains a significant challenge for achieving a stable, high-rate, and high-capacity device. In this review, recent advances in the development and optimization of cathode materials, including inorganic, organometallic, and organic materials, are discussed for SIBs. In addition, the challenges and strategies for enhancing the stability and performance of SIBs are highlighted.

Entropy-Stabilized Layered K0.6Ni0.05Fe0.05Mg0.05Ti0.05Mn0.725O2 as a High-Rate and Stable Cathode for Potassium-Ion Batteries

Mn-based layered oxides have been considered the most promising cathode candidates for cost-effective potassium-ion batteries (PIBs). Herein, equiatomic constituents of Ni, Fe, Mg, and Ti have been introduced into the transition metal layers of Mn-based layered oxide to design a high-entropy K0.6Ni0.05Fe0.05Mg0.05Ti0.05Mn0.0725O2 (HE-KMO, S = 1.17R). Consequently, the experimental results manifest that the layered structure of HE-KMO is more stable than conventional low-entropy K0.6MnO2 (LE-KMO, S = 0.66R) during successive cycling and even upon exposure to moisture. Diffraction and electrochemical measurements reveal that HE-KMO undergoes a solid-solution mechanism, contrary to the multistage phase transition processes typically exemplified in K0.6MnO2. Benefiting from the stabilized high-entropy layered framework and the solid-solution K+ storage mechanism, the entropy-stabilized HE-KMO not only demonstrates exceptional rate capability but also shows excellent cyclic stability. Notably, a capacity retention ratio of 86% after 3000 cycles can still be sustained at a remarkable current density of 5000 mA g-1.

Biochar-Derived Hierarchical Porous Carbon as Tellurium Host for High-Performance Potassium-Tellurium Batteries

Potassium-ion battery is promising for its high abundance and low redox potential. As a conversion cathode, Te possesses high conductivity and theoretical volumetric capacity to couple with potassium. The stubborn issues of K-Te battery focus on the large volume change and rapid structure degradation of Te. Herein, we produce biomass carbon from mangosteen shell in a facile method, and obtain a hierarchical porous host with abundance of micropores and mesopores, which is obviously beneficial for hosting Te during K+ storage in K-Te battery. The specific capacity reach to 560 mAh g-1 in the initial cycle at 0.1 A g-1 , and remained 83.8 % after 200 cycles. Impressively, at a high current density of 2.0 A g-1 , the specific capacity still remained 62.6 % after 5000 cycle. These results endow such strategy an efficient way for the development of K-Te batteries.

Enhanced Performance of KVPO4F0.5O0.5 in Potassium Batteries by Carbon Coating Interfaces

Potassium vanadium oxyfluoride phosphate of composition KVPO₄F_0.5O_0.5 was modified by a carbon coating to enhance its electrochemical performance. Two distinct methods were used, first, chemical vapor deposition (CVD) using acetylene gas as a carbon precursor and second, an aqueous route using an abundant, cheap, and green precursor (chitosan) followed by a pyrolysis step. The formation of a 5 to 7 nm-thick carbon coating was confirmed by transmission electron microscopy and it was found to be more homogeneous in the case of CVD using acetylene gas. Indeed, an increase of the specific surface area of one order of magnitude, low content of C sp², and residual oxygen surface functionalities were observed when the coating was obtained using chitosan. Pristine and carbon-coated materials were compared as positive electrode materials in potassium half-cells cycled at a C/5 (C = 26.5 mA g^-1) rate within a potential window of 3 to 5 V vs K⁺/K. The formation by CVD of a uniform carbon coating with the limited presence of surface functions was shown to improve the initial coulombic efficiency up to 87% for KVPFO₄F_0.5O_0.5-C₂H₂ and to mitigate electrolyte decomposition. Thus, performance at high C-rates such as 10 C was significantly improved, with ∼50% of the initial capacity maintained after 10 cycles, whereas a fast capacity loss is observed for the pristine material.

Uniform P2-K0.6CoO2 Microcubes as a High-Energy Cathode Material for Potassium-Ion Batteries

Layered transition-metal (TM) oxides have drawn ever-growing interest as positive electrode materials in potassium-ion batteries (PIBs). Nevertheless, the practical implementation of these positive electrode materials is seriously hampered by their inferior cyclic property and rate performance. Reported here is a self-templating strategy to prepare homogeneous P2-K_0.6CoO₂ (KCO) microcubes. Benefiting from the unusual microcube architecture, the interface between the electrolyte and the active material is considerably diminished. As a result, the KCO microcubes manifest boosted electrochemical properties for potassium storage including large reversible capacity (87.2 mAh g^-1 under 20 mA g^-1), superior rate performance, and ultralong cyclic steady (an improved capacity retention of 86.9% under 40 mA g^-1 after 1000 cycles). More importantly, the fabrication approach can be effectively extended to prepare other layered TM oxide (P3-K_0.5MnO₂, P3-K_0.5Mn_0.8Fe_0.2O₂, P2-K_0.6Co_0.67Mn_0.33O₂, and P2-K_0.6Co_0.66Mn_0.17Ni_0.17O₂) microcubes and nonlayered TM oxide (KFeO₂) microcubes.