Influence of Synthesis Routes on the Crystallography, Morphology, and Electrochemistry of Li₂MnO₃

Abstract

With the potential of delivering reversible capacities of up to 300 mAh/g, Li-rich transition-metal oxides hold great promise as cathode materials for future Li-ion batteries. However, a cohesive synthesis–structure–electrochemistry relationship is still lacking for these materials, which impedes progress in the field. This work investigates how and why different synthesis routes, specifically solid-state and modified Pechini sol–gel methods, affect the properties of Li2MnO3, a compositionally simple member of this material system. Through a comprehensive investigation of the synthesis mechanism along with crystallographic, morphological, and electrochemical characterization, the effects of different synthesis routes were found to predominantly influence the degree of stacking faults and particle morphology. That is, the modified Pechini method produced isotropic spherical particles with approximately 57% faulting and the solid-state samples possessed heterogeneous morphology with approximately 43% faulting probability. Inevitably, these differences lead to variations in electrochemical performance. This study accentuates the importance of understanding how synthesis affects the electrochemistry of these materials, which is critical considering the crystallographic and electrochemical complexities of the class of materials more generally. The methodology employed here is extendable to studying synthesis–property relationships of other compositionally complex Li-rich layered oxide systems.

Synthetic Pathway Determines the Non-equilibrium Crystallography of Li- and Mn-rich Layered Oxides

“…Additionally, the presence of organic matter may hinder the formation of a well-layered structure, as shown for sol-gel synthesized Li2MnO3 in our previous work. 16 This work establishes that the phase composition of LMNCO varies significantly depending on the synthetic route. The LMNCO samples of this work were synthesized using limited heat treatment protocols, and thus both structural forms could be metastable.…”

“…While satisfactory, the fit is less good for SS-LMNCO due to the variation of faulting within the structure as previously reported for LMNCO and other Li-rich layered oxides. 7,16 This variation of faulting implies that this material cannot be considered as a 'single-phase', even if in practice a 'single' LMNCO phase model is used for refinements. For SS-LMNCO, the percentage area of the phases (indicative of the phase composition) after refinement was ~65% and ~35% for Li2MnO3 and LiNi0.33Mn0.33Co0.33O2, respectively, indicating an excess of Li2MnO3, further corroborating the EDX data where the phase was found to be over-represented.…”

“…10 The asymmetric broadening of the superstructure reflections (Warren fall 15 ) in the Li-rich materials originate from the disruption of periodicity in the c direction due to TM layer stacking faults. 8,16 This disruption can manifest in multiple ways in LMNCO. For example, in the MP model, stacking faults may be caused when Li2MnO3-like (Li-Mn) stacking is interrupted by a LiNi0.33Mn0.33Co0.33O2-like TM-only layer, in addition to irregular stacking of similar layertypes.…”

Quantum materials with strong spin–orbit coupling: challenges and opportunities for materials chemists

“…71 Furthermore, efforts to study and control stacking faults in α-Li 2 IrO 3 and relatives are paralleled by current efforts in the energy materials field with regard to e.g. Li 2 MnO 3 , 131 another example of the generality of the materials science issues faced by quantum materials researchers.…”

Surface reactivity of Li2MnO3: Structural and morphological impact

“…Li 2 MnO 3 being a Li + and Mn 4+ rich layered oxide (also described as Li[Li 1/3 Mn IV 2/3 ]O 2 ) with lithium in excess in the transition metal layers, 13 it is particularly suitable as a model for understanding the surface reactivity of the high energy Li and Mn-rich layered oxides (the so-called HE-NMC's) expected to be the next generation of positive electrode materials for Lithium-ion batteries as they deliver high reversible discharge capacity involving both cationic and anionic redox. [14][15][16] Moreover, we have to mention that the particle size, Li-Mn site mixing and stacking fault content, controlled by the synthesis route 17 , impact the electrochemical performances 18,19 . Menon et al 18 explained that stacking fault and cation mixing are thought to facilitate the formation of long 3D Li percolation pathways responsible of larger charge/discharge capacity.…”