Thermodynamic accessibility of Li-Mn-Ti-O cation disordered rock-salt phases

Abstract

Disordered rock-salt with Li-excess (DRX) cathode phases within the Li-Mn-Ti-O (LMTO) composition space have recently been extensively studied, as they promise to deliver exceptional energy density at low cost in Li-ion batteries. The continued development of LMTO DRX with improved power density and cycling stability requires optimization of the composition and particle size/morphology, which are determined by synthesis conditions such as annealing temperatures and hold times. These challenges motivate our investigation of the phase diagram of the LMTO rock-salt phase space, with a focus on understanding the stability of DRX by quantifying the order-disorder transition temperature ($T_\text{disord}$) as a function of composition. We harness first-principles calculations and X-ray diffraction experiments to establish the LMTO phase diagram, which lies within the LiMnO$_2$ -- Li$_2$MnO$_3$ -- Li$_2$TiO$_3$ pseudo-ternary. Our calculations predict that the LMTO phase diagram at elevated temperature ($700 - 1300$ C) is composed of three phases: DRX, orthorhombic LiMnO$_2$, and layered Li$_2$Mn$_\text{1-y}$Ti$_\text{y}$O$_3$ ($0 < \text{y} < 1$). $T_\text{disord}$ decreases significantly as off-stoichiometry is introduced to the end-point compositions, resulting in a eutectoid phase diagram. Importantly, a significant range of LMTO compositions containing small to moderate fractions of Li-excess and Ti doping (relative to LiMnO$_2$) have $T_\text{disord}$ spanning $700 - 900$ C. These temperatures are substantially lower than conventional DRX synthesis temperatures ($\geq 1000$ C), suggesting the promise of decreasing synthesis temperatures for specific DRX compositions. The compositions containing moderate to high fractions of Mn$^{4+}$ instead have much greater $T_\text{disord}$ and phase separation to layered Li$_2$MnO$_3$ becomes highly favored.

Figures (8)

• Figure 1: Composition and configuration space of the LMTO rock-salt phases. a) Pseudo-ternary LiMnO2 $-$Li2MnO3 $-$Li2TiO3 composition space. Nominal TM oxidation states and changes in composition along the edge pseudo-binary lines are labeled. b) Unit cell of the FCC rock-salt lattice, with octahedral cation sites that can be occupied by Li$^+$, Mn$^{3+}$, Mn$^{4+}$, or Ti$^{4+}$.
• Figure 2: DFT-computed formation energies at 0 K and Mn magnetic moments of the training set of LMTO rock-salt orderings. a) Formation energies of structures within the LMTO pseudo-ternary --- specifically the lowest energies of each sampled composition. Green circles represent stable phases and squares represent configurations with energy above the convex hull. b) Histogram of absolute values of Mn magnetic moments ($|m_\text{Mn}|$). The vertical line at $|m_\text{Mn}|$$=$$3.35$$\mu_B$ denotes the threshold between Mn$^{3+}$ and Mn$^{4+}$, with all Mn with $|m_\text{Mn}|$$>$ 3.35 $\mu_B$ assigned to be Mn$^{3+}$, while Mn with $|m_\text{Mn}|$$<$ 3.35 assigned as Mn$^{4+}$. Formation energies along the pseudo-binary c) LiMnO2 -- Li2TiO3 (Li$_\text{1+x}$Mn$^\text{3+}_\text{1-3x}$Ti$^\text{4+}_\text{2x}$O$_2$) and d) LiMnO2 -- Li2MnO3 (Li$_\text{1+x}$Mn$^\text{3+}_\text{1-3x}$Mn$^\text{4+}_\text{2x}$O$_2$) composition lines, where green squares are stable phases and orange lines are configurations above the convex hull.
• Figure 3: Temperature $-$ composition phase diagrams of the pseudo-binary composition spaces of a) LiMnO2 -- Li2TiO3, b) LiMnO2 -- Li2MnO3, and c) LiMnO2 -- Li$_2$Mn$_{0.5}$Ti$_{0.5}$O$_3$, calculated from MC simulations (black lines). Gold stars represent $T_\text{disord}$ determined from in situ XRD experiments, which were performed for select compositions along the LiMnO2 -- Li2TiO3 and LiMnO2 -- Li2MnO3 pseudo-binaries only.
• Figure 4: Ex situ XRD patterns of samples quenched from various temperatures. a) Li$_{1.05}$Mn$_{0.85}$Ti$_{0.1}$O$_2$, b) Li$_{1.1}$Mn$_{0.7}$Ti$_{0.2}$O$_2$, c) Li$_{1.25}$Mn$_{0.25}$Ti$_{0.5}$O$_2$, and d) Li$_{1.3}$Mn$_{0.1}$Ti$_{0.6}$O$_2$, each quenched from temperatures above (red) and below (black) their respective $T_\text{disord}$.
• Figure 5: Ex situ XRD patterns of compositions Li$_{1.05}$Mn$_{0.95}$O$_2$, Li$_{1.1}$Mn$_{0.9}$O$_2$, and Li$_{1.25}$Mn$_{0.75}$O$_2$, each quenched from 1100 $\degree$C.