Conversion Reaction: TEM and Eels Investigation of MnO Reactivity to Electron at Different Charging State
Authors: Davoisne Carine , Jimenez-Gordon Isabel , Grugeon Sylvie , Laruelle Stephane 
Published: February 2020

The implementation of lithium ion batteries in various applications (electric vehicles, portable electric devices, …) led to the development of both anode and cathode materials with increasing lithium storage capacities1-2. Among the different analytical techniques available, few allow the investigation of both microstructural and chemical evolutions. Transmission electron microscopy (TEM) combined with energy dispersive spectroscopy (EDX) and Electron Energy Loss Spectroscopy (EELS) is one of those powerful tools. However, one of the main drawbacks in the battery field is the high reactivity of the majority of the cycled products to electron irradiation and few studies have been performed so far to fully understand and determine its impact on the microstructure and chemistry of cycled materials. Considering the development of in-situ TEM experiment3-5, knowing the evolution of these electrode materials under electron irradiation is becoming primordial to decipher the real mechanisms going on during cycling from the electron-induced artifacts6-8.

Thus, we chose to investigate the microstructural and chemical evolutions during cycling and electron irradiation of a transition metal oxide material going through a conversion reaction by TEM and EELS9. In this aim, we performed electron irradiation under two ranges of electron dose rate (low dose: 5 -15 103 é.nm-2.s-1, high dose: 2 105 - 5 106 é.nm-2.s-1) on MnO powder at the charged (lithiated) and the discharged (delithiated) state.

As expected after lithiation and using a low dose rate, the micro-sized MnO particles are transformed into agglomerates of nanoparticles (2-3 nm size) surrounded by a Solid Electrolyte Interphase (SEI) with a thickness up to 30 nm (figure 1a). The global shape of the former grains remains the same. The EELS analyses (figure 1e) are in agreement with the presence of Li2O and Mn metal. After increasing the electron dose rate, several microstructural changes were highlighted such as the increase of the nanoparticles size and crystallinity (figure 1b). The pulverization of the SEI coupled with the apparition of a phase on the TEM grid carbon film analyzed as Li2O by EELS (figure 1e) was also observed. The EELS investigation confirmed the presence of Li2O and metal Mn even after irradiation.

After delithiation and using a low dose rate, the microstructure composed of nanoparticles of 2-3 nm size embedded in a matrix and surrounded by a thinner SEI (up to 20 nm) is still maintained (figure 1c). The EELS analyses (figure 1e) are in agreement with the presence of mixture of Li2O and MnO. After increasing the electron dose rate, the observed changes are similar to those observed in the lithiated sample. The EELS and electron diffraction investigation confirmed the presence of manganese oxide as well as Li2O and sometime metal Mn. Under really high irradiation, a new phase appears on the carbon film and was identified by EELS then confirmed by SAED as Mn7C3 (figure 1d and 1e). The apparition of this phase can be explained by an electrical breakdown of the sample.

These irradiation-induced modifications highlight the high sensitivity of cycled materials reacting through conversion processes to the electron beam and how they can lead to misinterpretation in terms of structural and chemical evolution and thus of involved mechanisms. In order to decipher the modifications induced by cycling or irradiation, it is necessary to perform preliminary studies on the electron irradiation effect on the microstructure and chemistry. Then to limit the irradiation impact and perform representative studies of battery materials throughout the cycling, the investigation must be performed in a low dose configuration. In terms of characterization, the electron diffraction could not fully allow the phases determination due to their pseudo-amorphous character. Therefore, its combination with EELS to determine the oxidation states of transition metal was decisive. In conclusion, even if the battery materials are beam sensitive, the use of electron microscopy and associated techniques (EELS, EDX) can be a key to fully understand the mechanisms involved during cycling when carefully controlled.

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Figure 1