Novel layered K0.7Mn0.7Ni0.3O2 cathode material with enlarged diffusion channels for high energy density sodium-ion batteries

As promising, low-cost alternatives of lithium-ion batteries for large-scale electric energy storage, sodium-ion batteries (SIBs) have been studied by many researchers. However, the relatively large size of Na+ leads to sluggish diffusion kinetics and poor cycling stability in most cathode materials, restricting their further applications. In this work, we demonstrated a novel K+-intercalated Mn/Ni-based layered oxide material (K0.7Mn0.7Ni0.3O2, denoted as KMNO) with stabilized and enlarged diffusion channels for high energy density SIBs. A spontaneous ion exchange behavior in forming K0.1Na0.7Mn0.7Ni0.3O2 between the KMNO electrode and the sodium ion electrolyte was clearly revealed by in situ X-ray diffraction and ex situ inductively coupled plasma analysis. The interlayer space varied from 6.90 to 5.76 A, larger than that of Na0.7Mn0.7Ni0.3O2 (5.63 A). The enlarged ionic diffusion channels can effectively increase the ionic diffusion coefficient and simultaneously provide more K+ storage sites in the product framework. As a proof-of-concept application, the SIBs with the as-prepared KMNO as a cathode display a high reversible discharge capacity (161.8 mA h g−1 at 0.1 A g−1), high energy density (459 W h kg−1) and superior rate capability of 71.1 mA h g−1 at 5 A g−1. Our work demonstrates that the K+ pre-intercalation strategy endows the layered metal oxides with excellent sodium storage performance, which provides new directions for the design of cathode materials for various batteries.