Three-Dimensional Compositional and Charge Heterogeneities in Sodium Layered Oxide Cathode Materials

Abstract

Rechargeable energy storage devices have revolutionized the modern electronic industries and electric vehicles. Li-ion batteries with lithium layered oxide cathode materials have mainly dominated the market so far. However, energy storage devices based on sodium layered oxide cathode materials have drawn renewed attention because of the wide availability and cheap sodium resources. These devices are also particularly suitable for some applications such as grid energy storage, which require load leveling and adjusting to intermittent energy production from the renewable energy resources. To date, sodium-ion batteries based on layered oxide cathode materials are far from commercialization because of the low power and energy density as well as low round-trip efficiency because of the bulk phase transformation and surface degradation. Thus, novel design strategies must be undertaken to improve the electrochemical performance of these sodium layered oxide cathode materials. We have designed a highly compositionally heterogeneous Na0.9Cu0.2Fe0.28Mn0.52O2 cathode material for sodium-ion batteries. The material delivers a capacity of 125 mAh/g at C/10 with no capacity fading after 100 cycles and 75 mAh/g at 1C with negligible capacity fading after 200 cycles. In addition to the compositional heterogeneity, we have further demonstrated the heterogeneous nature of redox chemistry of the cathode material. The surface redox behavior of the transition metals is different than that in the bulk. In the bulk, both Cu2+ and Fe3+ play an active role in the redox chemistry of the cathode material whereas the surface redox chemistry is mainly dominated by Mn4+ ion. Furthermore, we also demonstrate that Mn segregation on long term cycling may play a pivotal role for the performance degradation. Through this study, we demonstrate that there is further room for improvement for sodium layered oxide cathode materials by tuning the nano/mesoscale elemental distribution. Tunable elemental distribution may also enable favorable surface chemistry of cathode materials that can account for the Mn segregation on long term cycling and enhance the cycle life of sodium-ion batteries.