Investigation of Structural Changes in Carbon-Coated Li2MoO3-LiCrO2 Cathode Material By Synchrotron-Based X-Ray Techniques
Authors: Lee Hayeon , Kim Taewhan , Lee Wontae , Oh Woong , Yoon Won-Sub 
Published: February 2020

In recent years, layered Li2MoO3 has attracted considerable attention as a cathode material for Li-ion batteries due to its high theoretical capacity (339 mAh g-1), the multiple electron transfer capability of Mo4+/6+ without oxygen release and the unique structure evolution behavior during electrochemical cycling [1,2]. In 2014, Zhou et al. studied the unusual expansion of a(b) lattice parameters during delithiation of Li2MoO3 and found its origin from the expansion of Mo-Mo bond that is not the case of conventional LiMO2 (M=Ni, Co, Mn) layered cathode materials in which the a(b) parameters shrink due to contraction of redox active ionic radii [2]. Li2MoO3 exhibited much smaller c lattice expansion than typical layered cathode materials during charge and did not collapse at all even at high voltage of 4.8V. These were explained on the basis of stabilizing effect of Mo ions in the Li layers. It is known that some amount of cation mixing occurs in Li2MoO3 during delithiation. Meanwhile, one important point is that such structural change behavior can be varied in some materials where Li2MoO3 is incorporated with other components, for example, in the case of Li2MoO3-LiM’O2 (M’=transition metal, etc.) system or Li2MoO3/C composite [3,4,5]. It has been reported that some of these Li2MoO3 related materials more easily disordered than pure Li2MoO3 to form similar to cation disordered rock salt structure [4,5].
Carbon coated Li2MoO3-LiCrO2 (0.7Li4/3Mo2/3O2-0.3LiCrO2; LMCO/C) was first reported by J. Lee et al. in 2014 [4]. It was found that this material has unique structure evolution behavior in which the original layered structure transforms to a disordered rock salt-like structure in the first cycle. Despite of severe cation mixing, LMCO/C showed high and stable capacity over 260mAh/g during the initial 10 cycles in voltage range of 1.5 - 4.3V. Interestingly, very small variation of lattice parameter was observed during delithiation in disordered LMCO/C (Volume change less than 1%). Small volume change can be beneficial in terms of structural stability by introducing less stress along the cathode particles during repeated cycling. Such small volume change implies that the unit cell dimensional change of LMCO/C is not simply affected by the oxidation of transition metal ions but strongly correlated with the local structure including Mo-Mo bonding.
In this study, we revisited LMCO/C cathode material and performed systematic analysis using synchrotron radiation based characterization techniques in order to better understand the detailed redox mechanism and structure evolution. By utilizing combination of XRD and XAS (XANES and EXAFS analysis), we demonstrate that how each transition metal and their local environments affect the overall structural evolution during charge/discharge process. Figure 1 shows the variation of XRD patterns with corresponding voltage profiles during first cycle and second charge process (C/20, 1.4 - 4.3V). It can be seen that disappearing of (003) peak and merging of (018), (110) peaks during first charge are almost irreversible. This indicates that most of the transition metal cations migrating into the Li sites during charge do not return to their original sites during discharge. However, as shown in Figure 2, reversible changes in the Mo K-edge EXAFS spectra indicate that reversible reaction processes exist in local level during electrochemical cycling. The changes in Mo-O peaks during charge indicate that Mo-O6 octahedra are significantly distorted during first charge. These are accompanied by severe cation mixing. During successive discharge/charge, Mo-O6 octahedra restored similar to the original state and distorted again whereas bulk structure remains disordered.
From these results, we will discuss structure evolution behavior of cation disordered LMCO/C cathode material. More detailed results and discussion including reaction process of both Cr and Mo in LMCO/C will be presented in the meeting.
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Figure 1