ABSTRACT
Sodium-ion batteries have recently aroused the interest of industries as possible replacements for lithium-ion batteries in some areas. With their high theoretical capacities and competitive prices, P2-type layered oxides (NaxTMO2) are among the obvious choices in terms of cathode materials. On the other hand, many of these materials are unstable in air due to their reactivity toward water and carbon dioxide. Here, Na0.67Mn0.9Ni0.1O2 (NMNO), one of such materials, has been synthesized by a classic sol-gel method and then exposed to air for several weeks as a way to allow a simple and reproducible transition toward a Na-rich birnessite phase. The transition between the anhydrous P2 to the hydrated birnessite structure has been followed via periodic XRD analyses, as well as neutron diffraction ones. Extensive electrochemical characterizations of both pristine NMNO and the air-exposed one vs sodium in organic medium showed comparable performances, with capacities fading from 140 to 60 mAh g-1 in around 100 cycles. Structural evolution of the air-exposed NMNO has been investigated both with ex situ synchrotron XRD and Raman. Finally, DFT analyses showed similar charge compensation mechanisms between P2 and birnessite phases, providing a reason for the similarities between the electrochemical properties of both materials.
ABSTRACT
Among the materials for the negative electrodes in Li-ion batteries, oxides capable of reacting with Li+ via intercalation/conversion/alloying are extremely interesting due to their high specific capacities but suffer from poor mechanical stability. A new way to design nanocomposites based on the (Ti/Sn)O2 system is the partial oxidation of the tin-containing MAX phase of Ti3 Al(1-x) Snx O2 composition. Exploiting this strategy, this work develops composite electrodes of (Ti/Sn)O2 and MAX phase capable of withstanding over 600 cycles in half cells with charge efficiencies higher than 99.5% and specific capacities comparable to those of graphite and higher than lithium titanate (Li4 Ti5 O12 ) or MXenes electrodes. These unprecedented electrochemical performances are also demonstrated at full cell level in the presence of a low cobalt content layered oxide and explained through an accurate chemical, morphological, and structural investigation which reveals the intimate contact between the MAX phase and the oxide particles. During the oxidation process, electroactive nanoparticles of TiO2 and Ti(1-y) Sny O2 nucleate on the surface of the unreacted MAX phase which therefore acts both as a conductive agent and as a buffer to preserve the mechanical integrity of the oxide during the lithiation and delithiation cycles.
