Designing High-Performance and Water-Stable ‘Layered’ Na- Transition Metal Oxide Cathode Materials for Na-Ion Batteries By Invoking Fundamental Materials-Electrochemical Principles
Authors: Mukhopadhyay Amartya 
Published: February 2024

To facilitate widespread development of Na-ion battery systems for a variety of applications, not only does the electrochemical behaviour/performances of ‘layered’ Na-TM-oxide based cathode materials need substantial improvements, but also does their (in)stability upon exposure to moisture. The latter renders handling/storage of Na-TM-oxides challenging and also negatively affects their electrochemical performance. More importantly, the water-instability mandates the usage of toxic-hazardous-expensive chemicals likeNMP for electrode preparation, as opposed to the possible usage of water-based slurries. On another note, detrimental structural changes of the Na-TM-oxides upon deep de-sodiation, i.e., charging to high voltages, cause electrochemical instability. From a broader perspective, the above problems are inherent to the ‘layered’ Na-TM-oxides as a class of material, addressing which necessitate tuning the very compositional/structural aspects, invoking fundamental materials science -cum- electrochemical principles; as has been the focus of some of the recent/ongoing research in our group. These have evolved a universal design criterion, paving the way towards successful design and widespread development of environmentally stable and high-performance cathodes for the Na-ion battery system and beyond, as also demonstrated in our works (as in refs. [1-4] here).
In a nut-shell, a ‘layered’ Na-TM-oxide structure is built of alternate ‘slabs’ composed of NaO2 and TMO2, with the O-ions (which bear a net negative charge) shared by the TM-ions and the Na-ions in their respective layers (i.e., O-ion is common to both cation-types). Here, while the TM-O bond is iono-covalent in nature, the Na-O is predominantly ionic. In such a structure, tuning the degree of covalency of the TM-O bond by designing a suitable combination of cations in the TM-layer can tune the net/effective negative charge on the O-ion, which, in turn, can affect the electrostatic attraction between the Na- and O-ions and also the repulsions between the O-ions across the Na-layer. For example, a higher net/effective negative charge on the O-ion due to reduced TM-O covalency will cause accrued electrostatic attraction between the Na-ions and O-ions, rendering the predominantly ionic Na-O bond stronger and shorter. The above should lead to a reduced ‘inter-slab’ spacing (or Na-layer thickness) and can improve the air/water-stability, as well as suppress the occurrence of deleterious structural/phase transition(s) during desodiation/charge in the O3-structured Na-TM-oxide based cathode materials; as demonstrated by us in ref. [1]. This has also allowed the development of high-performance cathodes for Na-ion batteries, which can be prepared via health/environment-friendly and cost-effective ‘aqueous’ processing route; as in refs. [1,4].
By contrast, a lower effective negative charge on the O-ion, as induced by greater TM-O bond covalency, would decrease the electrostatic attraction between the Na- and O-ions, resulting in a weaker-cum-longer Na-O bond and, thus, an enlarged ‘inter-slab’ spacing (i.e., Na-layer thickness). This facilitates faster Na-transport and, thus, enhanced rate-capability of the cathode; as demonstrated in ref. [2], where a highly rate-capable O3-structured Na-TM-oxide based cathode material has been reported.
An allied logic, i.e., increase in the TM-O bond covalency to lessen the effective negative charge on O-ions, as is needed to stabilize the prismatic O-coordination around Na-ions, has been invoked to stabilize the inherently higher rate-capable and electrochemically more stable P2 structure for Na-TM-oxide cathode materials at a considerably higher starting Na-content of ~0.84 p.f.u. (compared to the typically obtained < 0.7 p.f.u.). As demonstrated by us in ref. [3], this newly developed P2-structured Na-TM-oxide cathode material exhibits a very high desodiation capacity of ~178 mAh/g (@ C/5; within 2-4 V vs. Na/Na+), exceptional cyclic stability pertaining to ~98% capacity retention after 500 galvanostatic desodiation/sodiation cycles at a high current density (i.e., 2.5C) and also stability upon exposure to air/water. The cathodes have also exhibited excellent performances in laboratory scale prototype Na-ion ‘full’ cells.
The student contributors, B. S. Kumar, A. Pradeep, I. Biswas, R. Kumar, A. Amardeep, A. Dutta (IIT Bombay), and the collaborators, Prof. A. Chatterjee (IIT Bombay), Dr. V. Srihari, Dr. H. K. Poswal (BARC) (see publications below), are duly acknowledged; and so are SERB and DST, Government of India, for the funding support.

The associated publications (references)

B. S. Kumar, A. Pradeep, V. Srihari, H. K. Poswal, R. Kumar, A. Amardeep, A. Chatterjee and A. Mukhopadhyay; Adv. Energy Mater. (2023) 2204407: 1-15 (https://doi.org/10.1002/aenm.202204407)

I. Biswas, B. S. Kumar, A. Pradeep, A. Das, V. Srihari, H. K. Poswal and A. Mukhopadhyay; Chem. Commun.
59 (2023) 4332-4335

B. S. Kumar, R. Kumar, A. Pradeep, A. Amardeep, V. Srihari, H. K. Poswal, A. Chatterjee and A. Mukhopadhyay; Chem. Mater.
34[23] (2022) 10470–10483

B. S. Kumar, A. Pradeep, A. Dutta and A. Mukhopadhyay; J. Mater. Chem. A
8 (2020) 18064-18078