201. IDENTIFYING ELECTROCHEMICAL, STRUCTURAL, AND ELECTRONIC PROPERTIES OF LAYERED NAX[NI1/3MN2/3]O2 (0<X<2/3) CATHODE MATERIALS IN NA-ION BATTERIES: A COMBINATION OF COMPUTATIONAL AND EXPERIMENTAL STUDY

Department: NanoEngineering
Faculty Advisor(s): Ying Meng

Primary Student
Name: Jing Xu
Email: jix012@ucsd.edu
Phone: 858-822-4247
Grad Year: 2015

Abstract
The research on Na-ion battery chemistries has become increasingly intense in recent years, since sodium sources are far more abundant than lithium sources. Limited studies have been conducted recently on cathode materials with different structures to investigate the electrochemical properties of the materials in Na-ion battery systems. It has been reported that intercalation compounds such as NaxCoO2 and Na2/3[Co2/3Mn1/3]O2 exhibited very complicated electrochemical profiles due to their host structural transformation during intercalation and deintercalation of Na-ion. However, few studies focus on layered Nax[NiyMn1-y]O2 (0<x<2/3). Our preliminary work on P2-Na2/3[Ni1/3Mn2/3]O2 indicates that this material exhibits ~170 mAh/g at the 1st charge and ~220 mAh/g at the 1st discharge between the voltage ranges from 1.0 V to 4.5 V at C/100 rate. We believe that this material is a promising candidate as a cathode material for Na-ion batteries. However, the poor rate capability limits its high power application. To address this issue, we use first principles quantum mechanical calculations in the generalized gradient approximation with Hubbard U correction (GGA+U) to the Density Functional Theory (DFT) to obtain proper structural models for layered Nax[Ni1/3Mn2/3]O2 (0<x<2/3). We compare the calculated Na de-intercalation voltages, the Na ion motilities and the phase transformation mechanisms in these two different types of structures, and correlate them to the experimental observations. Based on the combination of our experimental and computational works, we found that the sodium ions are fairly mobile in the P2 structures and the phase stability and the stacking transformation energy barrier is the critical factor for both rate capability and reversibility of layered Nax[Ni1/3Mn2/3]O2 (0<x<2/3) cathode materials.

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