Sulfur-Decorated Ti3 C2 TX MXene for High-Performance Sodium/Potassium-Ion Batteries

As post-lithium ion batteries, both sodium-ion batteries (SIBs) and potassium ion batteries (PIBs) possess great potential for large scale energy storage. However, the application of both SIBs and PIBs are hindered by the lack of suitable electrode materials. Here, we synthesized the sulfur decorated Ti3 C2 Tx (S-T3 C2 Tx ) MXene as electrode material for SIBs and PIBs. Thanks to the sulfur functional group and the formation of Ti-S bond, which facilitates the sodium in-/desertion and strengthens the potassium ion adsorption ability, as well as enhances ion reaction kinetics and improved structure stability, the S-T3 C2 Tx exhibit excellent sodium/potassium storage performance, high reversible capacities of 151 and 101 mAh g-1 at 0.1 mA g-1 were achieved for SIBs and PIBs, respectively. Moreover, the S-T3 C2 Tx exhibits remarkable long-term capacity stability at a high density of 500 mA g-1 , providing an impressive storage of 88 mAh g-1 for SIBs and 41 mAh g-1 for PIBs even after 2000 cycles. This work could give a deep comprehension of the heteroatom modification influence on the MXene-based framework and promote the application of MXene electrodes.

Multilayer structure covalent organic frameworks (COFs) linking by double functional groups for advanced K+ batteries

Covalent organic frameworks (COFs) are regarded as the potential and promising anode materials for potassium ion batteries (PIBs) on account of their robust and porous crystalline structure. In this work, multilayer structural COF connected by double functional groups, including imine and amidogent through a simple solvothermalprocess, have been successfully synthesized. The multilayer structure of COF can provide fast charge transfer and combine the merits of imine (the restraint of irreversible dissolution) and amidogent (the supply of more active sites). It presents superior potassium storage performance, including the high reversible capacity of 229.5 mAh g^-1 at 0.2 A g^-1 and outstanding cycling stability of 106.1 mAh g^-1 at the high current density of 5.0 A g^-1 after 2000 cycles, which is superior to the individual COF. The structural advantages of the covalent organic framework linking by double functional groups (d-COF) can develop a new road for that COF anode material for PIBs in further research.

Densified graphene-like carbon nanosheets with enriched heteroatoms enabling superior gravimetric and volumetric potassium storage capacities

Constructing carbon electrodes with abundant heteroatoms and appropriate graphitic interlayer spacing remains a major challenge for achieving high gravimetric and volumetric potassium storage capacities with fast kinetics. Herein, we constructed 3D graphene-like N, F dual-doped carbon sheets induced by Ni template (N, F-CNS-Ni) with dense structure and rich active sites, providing a promising approach to address the facing obstacles. Highly reversible K-ion insertion/extraction is realized in the graphitic carbon structure, and K-adsorption capability is enhanced by introducing N/F heteroatoms. As a result, the N, F-CNS-Ni electrode exhibits ultrahigh gravimetric and volumetric capacities of 404.5 mA h g^-1 and 281.3 mA h cm^-3 at 0.05 A/g, respectively, and a superb capacity of 259.3 mA h g^-1 with a capacity retention ratio of 90 % even after 600 cycles at 5 A/g. This work presents a simple Ni-based template method to prepare graphene-like carbon nanosheets with high packing density and rich heteroatoms, and offers mechanism insight for achieving superior K-ion storage.

3D space-confined Co0.85Se architecture with effective interfacial stress relaxation as anode material reveals robust and highly loading potassium-ion batteries

Conversion-type transition metal chalcogenide anodes could bring relatively high specific capacity in potassium ion storage due to multiple electron transport reactions, but often accompanying huge volume changes and resulting in low cycle life and rapid capacity fading.While electrode materials are closely packed, the contact at the interface during potassiation/depotassiation is similar to point-to-point contact, generating strong stress to make self-aggregation occur. In this work, we constructed a 3D carbon framework to confine Co_0.85Se nanocrystals in three-dimensional space, both fulfilling the requirements of the material's size in the nano-scale and providing the largest contact area for releasing stress. With this optimization, nitrogen-doped carbon confined Co_0.85Se nanocrystals (Co_0.85Se@NC) reach an ultra-stable cycle life over 4000 times with a specific capacity of 190.9 mA h g^-1 at 500 mA g^-1 and provide 155.6 mA h g^-1 at 10 A g^-1 in the rate capability test. It also renders the areal capacity up to 1.03 mA h cm^-2 at 500 mA g^-1 in the high-mass loading test. Furthermore, based on the finite element analysis, the 3D confinement strategy has the lowest interfacial stress, ensuring Co_0.85Se nanocrystals with high structural integrity. This strategy can relieve the stress issue in the conversion-type anode and demonstrate superior electrochemical performance even at high-loading mass electrodes.

Trumpet-like ZnS@C composite for high-performance potassium ion battery anode

ZnS has acquired increasing attention for high-performance PIBs anode because of its remarkable theoretical capacity, and redox reversibility for conversion reaction. However, the larger volume variation and delayed reaction kinetics for the ZnS in the discharge/charge processes lead to pulverization and severe capacity degradation. Herein, the trumpet-like ZnS@C composite was synthesized by template method using sodium citrate as carbon source followed by vulcanization process. As potassium ion batteries (PIB) anode, ZnS@C composite exhibits good rate performance and long life (stable reversible capacity of 107.8 mAh/g over 2000 charge-discharge cycles at 5 A/g and high reversible capacity of 310 mAh/g at 0.1 A/g). The outstanding electrochemical performance of the ZnS@C composite is ascribed to its unique structure, which can mitigate the volume expansion of ZnS in the charge discharge process, expand the contact area between the electrode and electrolyte, and improve the conductivity of electrode materials by the introduction of carbon layer. This method of synthesizing trumpet-like ZnS@C composite provides an important strategy for obtaining potassium ion batteries anode with long cycle.

Multipod Bi(Cu2-xS)n Nanocrystals formed by Dynamic Cation-Ligand Complexation and Their Use as Anodes for Potassium-Ion Batteries

We report the formation of an intermediate lamellar Cu-thiolate complex, and tuning its relative stability using alkylphosphonic acids are crucial to enabling controlled heteronucleation to form Bi(Cu_2-xS)_n heterostructures with a tunable number of Cu_2-xS stems on a Bi core. The denticity of the phosphonic acid group, concentration, and chain length of alkylphosphonic acids are critical factors determining the stability of the Cu-thiolate complex. Increasing the stability of the Cu-thiolate results in single Cu_2-xS stem formation, and decreased stability of the Cu-thiolate complex increases the degree of heteronucleation to form multiple Cu_2-xS stems on the Bi core. Spatially separated multiple Cu_2-xS stems transform into a support network to hold a fragmented Bi core when used as an anode in a K-ion battery, leading to a more stable cycling performance showing a specific capacity of ∼170 mAh·g^-1 after 200 cycles compared to ∼111 mAh·g^-1 for Bi-Cu_2-xS single-stem heterostructures.