Ex situ and in situ Raman microscopic investigation of the differences between stoichiometric LiMO2 and high-energy xLi2MnO3·(1–x)LiMO2 (M = Ni, Co, Mn)
Introduction
Thanks to their high-energy density and good cycling stability, lithium-ion batteries have found numerous applications for portable consumer electronics over the past two decades. However, the electric vehicle industry would greatly benefit from further improvements in this field. Since graphite has a relatively high specific charge of 372 mAh/g a lot of attention has been focussed on the search for improved positive electrode materials [1], [2], [3], [4].
In this context, Liu et al. introduced layered mixed transition metal oxides of the type LiMO2 (M = Ni, Co, Mn) [5]. This was followed by the description of Li(Ni1/3Co1/3Mn1/3)O2, also known as stoichiometric NCM, by Ohzuku and Makimura [6]. Kim and Chung further improved the properties of NCM by overlithiation [7]. Thackeray et al. extensively studied this new class of compounds and established the xLi2MnO3·(1–x)LiMO2 notation, based on Li2MnO3 domains with C2/m ordering [8]. These compounds show high specific charges in excess of 200 mAh/g, improved cycling stabilities and higher rate capabilities than stoichiometric NCM [9], [10], [11], [12]. Throughout this article, Li(Ni1/3Co1/3Mn1/3)O2 and commercial xLi2MnO3·(1–x)LiMO2 (M = Ni, Co, Mn; BASF SE; x ≈ 0.5) will be referred to as stoichiometric and high-energy NCM, respectively.
The Li2MnO3 domains have a significant influence on the properties of high-energy NCM. Their activation and subsequent electrochemical cycling have been proposed to be responsible for the high specific charge and the improved cycling stability of high-energy NCM [13], [14]. The corresponding potential plateau observed at 4.5 V vs. Li+/Li during the first charging step has been explained by simultaneous extraction of lithium and oxygen [15], [16], [17], [18]. Tran et al. proposed a mechanism for this activation of Li2MnO3 including extraction of Li2O and subsequent oxygen release [16]. The released oxygen has been detected by differential electrochemical mass spectrometry (DEMS) [11], [19]. Yabuuchi et al. explained some of the extra specific charge by partially reversible redox activity of oxygen-containing surface species resulting from this oxygen release [18]. Bruce et al. assigned part of the electrochemical activity of pure Li2MnO3 to electrolyte oxidation followed by Li+/H+ exchange [20]. Based on diffraction measurements, Koga et al. have questioned the existence of Li2MnO3 domains and suggested a solid-solution model [21]. Furthermore, Koga et al. proposed oxygen loss and densification at the surface, and reversible oxygen oxidation in the bulk [22].
As demonstrated, the reaction mechanisms involved in the activation of high-energy NCM remain hotly debated. The aim of our study was thus to contribute to their further elucidation. Raman spectroscopy was chosen as a powerful tool for such studies due to the possibility of probing the short-range environment of transition metals, even in the absence of extensive long-range order, and due to its suitability for in situ experiments. By direct comparison between the ex situ and in situ Raman spectra of stoichiometric and high-energy NCM cycled under identical conditions, we intended to minimise experimental variations and isolate the true differences between the Raman spectroscopic characteristics of the two compounds. Whereas ex situ Raman experiments are easier to implement, in situ Raman experiments offer the advantages of avoiding relaxation and allowing the determination of the exact potentials at which certain spectroscopic changes occur. In the literature, several ex situ Raman spectra of stoichiometric NCM [23], [24], [25], [26] have been presented. Amalraj et al. recently reported ex situ Raman spectra of high-energy NCM [27]. In corresponding in situ measurements, the emergence of a new band at 544 cm−1 during the first charging step at 4.1–4.3 V vs. Li+/Li was observed [28].
Section snippets
Synthesis of stoichiometric NCM
Stoichiometric NCM was synthesised in-house via the sol-gel method. In a 3 g batch, stoichiometric amounts of metal nitrates (LiNO3, Ni(NO3)2·6H2O, Co(NO3)2·6H2O, Mn(NO3)2·4H2O, all SIGMA-ALDRICH, USA) were dissolved in an aqueous solution (250 ml) of citric acid (4 equiv.). The reaction mixture was continuously stirred and heated to 100 °C (1.5 h), 200 °C (2 h) and 300 °C (2.5 h), yielding a brown powder. The ground powder was transferred to a crucible and calcined under air (temperature ramp to 450 °C
Characterisation of the stoichiometric and the high-energy NCM
The SEM measurements shown in Fig. 1 indicate that the morphologies of the stoichiometric NCM powder (top) and the high-energy NCM powder (bottom) are quite similar. Both samples consist of relatively flat, hexagonal crystallites in the range of a few hundred nanometres (slightly larger for high-energy NCM).
In order to verify the purity of the powders, an XRD study of stoichiometric and high-energy NCM was performed (Fig. 2). All Bragg peaks of stoichiometric NCM (top) could be indexed based on
Conclusions
Stoichiometric NCM can be obtained via the described sol-gel route. SEM measurements showed that both NCM samples consisted of relatively flat hexagonal crystallites in the range of a few hundred nanometres. In agreement with the literature [8], XRD experiments confirmed the expected primary space group R-3 m for both samples, with additional C2/m ordering in high-energy NCM. A series of ex situ Raman measurements of NCMs with increasing overlithiation (stoichiometric NCM < Li1.1(Ni1/3Co1/3Mn1/3)O
Acknowledgements
Financial support from BASF SE is gratefully acknowledged. The authors would also like to thank Dr Heino Sommer for the SEM images, Dr Sofía Pérez for preliminary experiments, Hermann Kaiser for designing the electrochemical cells and Christoph Junker for technical support.
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