The LiNiO2 Cathode Active Material: A Comprehensive Study of Calcination Conditions and their Correlation with Physicochemical Properties. Part I. Structural Chemistry

Philipp Kurzhals,Felix Riewald,Jürgen Janek,Hubert A Gasteiger,Heino Sommer,Matteo Bianchini

doi:10.1149/1945-7111/ac33e5

Abstract

Following the demand for increased energy density of lithium-ion batteries, the Ni content of the Nickel-Cobalt-Manganese oxide (NCM) cathode materials has been increased into the direction of LiNiO2 (LNO), which regained the attention of both industry and academia. To understand the correlations between physicochemical parameters and electrochemical performance of LNO, a calcination study was performed with variation of precursor secondary particle size, maximum calcination temperature and Li stoichiometry. The structural properties of the materials were analyzed by means of powder X-ray diffraction, magnetization measurements and half-cell voltage profiles. All three techniques yield good agreement concerning the quantification of Ni excess in the Li layer (1.6%–3.7%). This study reveals that the number of Li equivalents per Ni is the determining factor concerning the final stoichiometry rather than the calcination temperature within the used calcination parameter space. Contrary to widespread belief, the Ni excess shows no correlation to the 1st cycle capacity loss, which indicates that a formerly overlooked physical property of LNO, namely primary particle morphology, has to be considered.

Highlights

The LiNiO2 Cathode Active Material: A Comprehensive Study of Calcination Conditions and their Correlation with Physicochemical Properties
Research showed that LNO always tends to be Li deficient and contains an excess of Ni2+ (“Ni excess” or “off-stoichiometry”) in the Li layers, despite optimized calcination conditions, leading to a stoichiometry of Li1−zNi1+zO2.27–29 Incomplete oxidation and deficiency of Li2O leads to the solid solution of LiNiO2 (z = 0) and NiO (z = 1), where 2z Ni2+ ions are present with half of them located in the Li layer and the other half in the Ni layer
Synchrotron powder X-ray diffraction (PXRD) measurements and refinement.—The layered compound LiNiO2 is known to crystallize in a rhombohedral unit cell, isostructural with α-NaFeO2 with the space group R−3m

Summary

Introduction

The LiNiO2 Cathode Active Material: A Comprehensive Study of Calcination Conditions and their Correlation with Physicochemical Properties. Research showed that LNO always tends to be Li deficient and contains an excess of Ni2+ (“Ni excess” or “off-stoichiometry”) in the Li layers, despite optimized calcination conditions, leading to a stoichiometry of Li1−zNi1+zO2.27–29 Incomplete oxidation and deficiency of Li2O leads to the solid solution of LiNiO2 (z = 0) and NiO (z = 1), where 2z Ni2+ ions are present with half of them located in the Li layer and the other half in the Ni layer This Ni excess was shown to have a significant impact on physical and electrochemical properties, e.g. on magnetic properties and phase transitions during cycling.[30]. In industrial optimization processes narrow parameter ranges are considered in contrast to academia, where a more fundamental understanding is intended

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Conclusion