In Operando Strain Evolution in Sodium Chromium Oxide Cathode for Na-Ion Batteries during Battery Cycling
Authors: Wable Minal , Bal Batuhan , Capraz Omer Ozgur Özgür 
Published: September 2023

Layer-structured Na intercalation compounds such as NaxMO2 (M=Co, Mn, Cr) have attracted much attention as cathode materials for sodium-ion batteries due to their high volumetric and gravimetric energy densities. Among them, NaCrO2 with layered rock salt structure is one of the promising cathodes since NaCrO2 has a desirable flat and smooth charge/discharge voltage plateau.1 In addition, NaCrO2 has the highest thermal stability at charged state which makes it a potentially safer cathode material.2 The NaCrO2 exhibits a reversible capacity of 110 mAh g-1 with good cycling performance.3 However, the transition metal oxide (TMO) cathode materials in NIBs undergo severe chemo-mechanical deformations which leads to capacity fade and poor cycling and is the limiting factor of NIBs. The electrochemical characterization and examination of the electrode structure were the primary focus of several investigations. To improve the lifespan and performance of electrode materials for Na-ion batteries, it is vital to comprehend how Na ions impact the chemo-mechanical stability of the electrodes.
In this talk, we will discuss the driving forces behind the structural and interfacial deformations on NaCrO2 cathodes. Digital image correlation measurements were conducted to probe strain evolution in the electrode during cycling. The free-standing composite NaCrO2 electrode was used for stain measurements in custom-cell assembly. The battery was cycled against Na metal in 1 M NaClO4 in PC. The first part of the study involves structural and interfacial deformations in the lower voltage range of 2.3 V to 3.5 V where x<0.5 in NaxCrO2. And the second part focuses on the structural and interfacial deformations in the voltage range of 2.3 V to 4.7 V where x>0.5 in NaxCrO2. In the preliminary studies, we observed that the initial insertion of Na ions leads to negative strain evolution (contraction) in the electrode, followed by expansions in the electrode at a higher state of discharge. Similar phenomena are also observed during charge cycles, where extraction of Na results in an initial contraction in the electrode, followed by expansion at a higher state of charge. Understanding the mechanisms behind chemo-mechanical deformations will allow to tune structure & material property for better electrochemical performance.
Acknowledgement: This work is supported by National Science Foundation’s (NSF) Faculty Early Career Development (CAREER) Program, 2142726.
References:

Bo SH, Li X, Toumar AJ, Ceder G. Layered-to-rock-salt transformation in desodiated Na x CrO2 (x 0.4). Chemistry of Materials. 2016 Mar 8;28(5):1419-29.

Xia X, Dahn JR. NaCrO2 is a fundamentally safe positive electrode material for sodium-ion batteries with liquid electrolytes. Electrochemical and Solid-State Letters. 2011 Nov 18;15(1): A1.

Komaba S, Takei C, Nakayama T, Ogata A, Yabuuchi N. Electrochemical intercalation activity of layered NaCrO2 vs. LiCrO2. Electrochemistry Communications. 2010 Mar 1;12(3):355-8.