Stabilizing High-Voltage Layered Oxide Cathode Materials for Li/Na Ion Batteries

Abstract

Li-ion battery systems have started to extend its roles from portable electronics, power tools to automobile applications such as hybrid and plug-in electric vehicles. However, it is necessary to enhance their electrochemical performance such as energy density and cycle life, for promoting their competition capability as vehicle power system. Obviously, electrode materials, especially positive electrode materials either working voltage or reversible capacity determine the energy density of the Li-ion batteries at mostly [1,2] . In the presentation, some new research progress of stabilizing high-voltage layered oxide cathode materials for Li/Na ion batteries in this group will be reported. Our strategies include two directions [3,4] : one is to improve the interface stability by using high voltage electrolyte additives or surface coating, another is to stabilize the lattice structure by using suitable element doping. For example, when extending the working voltage of LiCoO 2 from 4.2V to 4.4V or even more, we must develop some new electrolyte recipes to stabilize corresponding electrode/electrolyte interface at high voltage. Our recent results [5] show that a synergistic effects of suberonitrile-LiBOB binary additives in high-voltage cathode system, i.e. they could improve the cyclic performance of 4.5V LiCoO 2 effectively. The corresponding working mechanism of the binary additives will be presented. As to develop high energy density Na-ion battery, it is important to develop high-voltage cathode material. It is found that [6,7] the introduction of Zn 2+ in the Na-Ni-Mn-O system can effectively overcome the drawback of voltage decay when charged to a higher cutoff voltage (>4.0 V), and significantly improve capacity retention compared to the unsubstituted material during cycling. In addition, a smoother charge/discharge profile can be observed between 3.0 and 4.0 V for Zn-substituted samples, demonstrating that Na + /vacancy ordering can be suppressed during sodium insertion/extraction. Na 0.66 Ni 0.26 Zn 0.07 Mn 0.67 O 2 can deliver an initial capacity of 132 mAh g –1 at 12 mA g –1 with a high average voltage of 3.6 V and a capacity retention of 89% after 30 cycles. We have used a series of techniques such as in-situ XRD, in-situ XAS, ex-situ solid state NMR and HRTEM techniques to investigate the structural changes during cycling process and working mechanism of Zn 2+ in the corresponding Na 0.66 Ni 0.26 Zn 0.07 Mn 0.67 O 2 cathode materials.