Understanding the Cathodic Redox Reactions in Sodium-Ion Batteries: Bulk and Surface Study of NaXCo2/3Mn2/9Ni1/9O2
Authors: Doubaji Siham , Saadoune Ismael , Philippe Bertrand , Valvo Mario , Solhy Abderrahim , Gustafsson Torbjörn , Edström Kristina 
Published: February 2020

Sodium-ion batteries are regarded as a promising energy storage technology due the abundance and the low cost of sodium, as well as its suitable redox potential (E(Na+/Na)=-2.71 V vs. SHE). One main focus for the electrochemical energy storage community is to find suitable high performance sodium-ion insertion electrode materials. A number of potential candidates have been presented as cathode materials showing attractive electrochemical performances with stable cycleability and a good specific capacity. P2-NaxCo2/3Mn2/9Ni1/9O2  
 is one of these cathode materials. Synthesized by a simple sol-gel method, the material crystallizes in a P2-type phase with the P63/mmc space group and it delivers a specific discharge capacity of 110 mAh/g when cycled between 2.0 and 4.2 V vs. Na+/Na with a good capacity retention (11% loss after 90 cycles), an excellent coulombic efficiency exceeding 99.4%. When cycled up to 4.5 V, a new plateau starting after 4.2 V is observed giving a high specific discharge capacity of 140 mAh/g for the Na// NaxCo2/3Mn2/9Ni1/9O2cell. However, cycling at this high cut-off voltage is accompanied with a severe deterioration of the electrochemical performance of the material.1

     Following the structural stability of the material, understanding the chemical composition of the electrode/electrolyte interface and its behavior during cycling, and the determination of oxidation states of the transition metals and their evolution are mandatory to rationalize the electrochemical behavior of the material at different voltages. The structural stability of the material was checked using operando/in-situ measurements. The experiment showed that the P2-type structure is stable when cycled between 2.0 and 4.2V. When the material is cycled up to 4.5V a new phase is formed which showed to have a totally reversible sodium intercalation process. Both in-house XPS (hʋ=1486eV) and HAXPES (hʋ=4000eV) measurements were performed for the pristine material. The evolution of the interface was followed by HAXPES (performed at Helmholtz Zentrum Berlin) measurements at different steps of the charge and discharge of our material. HAXPES measurements revealed that cobalt, nickel, and manganese are all electrochemically active upon cycling between 4.5 and 2.0 V; they are all in the 4+ state at the end of charging. Upon discharge, the reduction to Co3+, Ni3+, and Mn3+ occurs. At low potential there is a partial reversible reduction to Co2+ and Ni2+. By the use of both XPS and HAXPES, a thin layer of Na2CO3 and NaF is distinguished that covers the pristine electrode before any contact with the electrolyte. These compounds are mainly resulting from the reaction of the material with air but also with PVDF which is used as a binder for the electrode processing. Reversible dissolution/reformation of these compounds is observed during the first cycle. The degradation of the salt NaPF6 is driven by the potential where phosphates are the main degradation products at low potential and mainly fluorophosphates (NaPOxFy) are found at a high potential.2

     This combined bulk and surface study allowed us to clearly understand the mechanisms of the reactions occurring during the first electrochemical cycle of composite electrodes based on P2-NaxCo2/3Mn2/9Ni1/9O2 tested in Na half-cells. These results showed also how the electrochemical performance of the cathode material might be hampered by the fast degradation of the electrolyte at high voltage but also of the choice of the binder.

1 S. Doubaji, M. Valvo, I. Saadoune, M. Dahbi, K. Edstrom, Synthesis and characterization of a new layered cathode material for sodium ion batteries.
J. Power Sources, 266, 275-281 (2014).

2 S. Doubaji, B. Philippe, I. Saadoune, M. Gorgoi, T. Gustafsson, A. Solhy, M. Valvo, H. Rensmo and K. Edström, Passivation Layer and Cathodic Redox Reactions in Sodium
-
Ion Batteries Probed by HAXPES. ChemSusChem, 9, 97 – 108 (2016).