Accelerating the Electrochemical Formation of the δ Phase in Manganese-Rich Rocksalt Cathodes

Abstract

Mn-rich disordered rocksalt materials with Li-excess (DRX) materials have emerged as a promising class of earth-abundant and energy-dense next-generation cathode materials for lithium-ion batteries. Recently, an electrochemical transformation to a spinel-like "δ" phase has been reported in Mn-rich DRX materials, with improved capacity, rate capability, and cycling stability compared with previous DRX compositions. However, this transformation unfolds slowly over the course of cycling, complicating the development and understanding of these materials. In this work, it is reported that the transformation of Mn-rich DRX materials to the promising δ phase can be promoted to occur much more rapidly by electrochemical pulsing at elevated temperature, rate, and voltage. To extend this concept, micron-sized single-crystal DRX particles are also transformed to the δ phase by the same method, possessing greatly improved cycling stability in the first demonstration of cycling for large, single-crystal DRX particles. To shed light on the formation and specific structure of the δ phase, X-ray diffraction, scanning electron nanodiffraction (SEND) and atomic resolution STEM-HAADF are used to reveal a nanodomain spinel structure with minimal remnant disorder.

Keywords: battery; cathode; delta phase; disordered rocksalt; electron microscopy.

Figure 1

Solid‐state synthesis of ${\mathrm{Li}}_{1.1}{\mathrm{Mn}}_{0.8}{\mathrm{Ti}}_{0.1}{\mathrm{O}}_{1.9}{\mathrm{F}}_{0.1}$. a) XRD pattern and Reitveld refinement for disordered rocksalt structure of solid‐state synthesized ${\mathrm{Li}}_{1.1}{\mathrm{Mn}}_{0.8}{\mathrm{Ti}}_{0.1}{\mathrm{O}}_{1.9}{\mathrm{F}}_{0.1}$. b) EDS mapping performed for Mn, Ti, O and F on these samples after shaker‐milling. c,d) SEM images of the resulting powder before (c) and after (d) shaker‐milling with carbon show that the material initially has a wide distribution of particle sizes (submicron to >10 µm), but that this is reduced to 100–500 nm secondary particles during milling.

Figure 2

Pulsing protocol and resulting electrochemistry: a) A representative example of the pulsing process of electrochemical formation, showing an initial charge up to 4.8 V, followed by pulsing in a restricted voltage window at high rate and 50 °C, and subsequent normal cycling at 25 °C and 2.0 – 4.8 V. b) The first charge and discharge curve of materials pulsed in various voltage windows at 100 mA g⁻¹, 50 °C, for 5 days, along with the pristine material, cycled from 2.0 to 4.8 V. c) Associated differential capacity (dQ/dV) for materials transformed with the same pulsing voltage windows, along with the control, showing different extents of the development of the 3 and 4 V features.

Figure 3

Effect of the number of pulses on the transformation: a) The first full charge and discharge cycles (at 25 °C) of materials pulsed different numbers of times at 100 mA g⁻¹, 50 °C, in a window of 3.0–4.5 V along with the pristine material (control), cycled from 2.0 to 4.8 V. b) Differential capacity (dQ/dV) for material pulsed different numbers of times. c) Extended cycling data for material pulsed different numbers of times.

Figure 4

Effect of pulse current rate on the transformation: a) The first charge and discharge curve of materials pulsed 80 times at different rates at 50 °C, in a window of 3.0–4.5 V along with the pristine material, cycled from 2.0 to 4.8 V. b) Differential capacity (dQ/dV) for material pulsed at different rates. c) Extended cycling data for material pulsed at different rates.

Figure 5

Correlation between structural and electrochemical evolution during pulsing and cycling: a) ex situ XRD pattern of shaker‐milled material in pristine electrode and after pulsing 5, 20, and 80 times. The broad spinel‐like features are denoted with S in addition to peaks that are common to the parent rocksalt lattice and spinel which are denoted with D/S. b) Reitveld refinement of the pattern of material pulsed 80 times using a single phase partially disordered spinel model with 16c/16d disorder and a short coherence length. c) Capacity of the 4V feature after pulsing versus domain size obtained from Reitveld refinement of ex situ samples. d) Evolution of the 4V feature in the voltage curve over the course of 100 pulses (using the same color legend used in Figure 5c). e) Differential capacity (dQ/dV) of the 3V feature as a function of total time in pulsing at 100 mA g⁻¹ (colors) and cycling at 20 mA g⁻¹ (black). f) Comparison of material pulsed and cycled an equivalent number of times.

Figure 6

Synthesis and Transformation of Molten Salt Synthesized DRX: a) SEM micrograph of the MS material showing single crystals with a homogenous particle size of 2–3 µm. b) STEM HAADF and EDS performed on the MS sample shows a uniform distribution of Mn, Ti, O, and F in this material. c) Cycling performance of the MS material at 25 °C after 80 pulses with constant voltage holds. d) Cycling performance of the MS material at 50 °C after 80 pulses with constant voltage holds. e) Extended cycling of pulsed MS material at 25 °C and at 50 °C, along with a control cell which has not been pulsed prior to cycling.

Figure 7

Multimodal structural characterization of the $\mathit{\delta }$ phase in the molten salt sample. a) Low magnification STEM‐HAADF image of the pulsed molten salt particle. b) The mean SEND patterns from the region marked in panel a. c) Spatial distribution of the δ phase obtained by virtual imaging the diffraction spots unique to the $\mathrm{Fd}\overline{3}\mathrm{m}$ space group (Scale bar: 5 nm) d) Atomic resolution HAADF‐STEM micrograph collected along the [110] zone axis from the region marked in the red box in panel a (Scale bar: 3 nm). The amplitude of its fast Fourier transform (FFT) is shown in the inset, where the Fourier components belonging to the δ phase are marked in red and blue circles. The Fourier components of the parent DRX phase are marked with white circles. The δ lattice obtained by Fourier filtering of each frequency for the red component e) and blue component f) is overlaid on the DRX parent lattice. In panels d, e, and f the white box and the green dashed box mark a disordered region (neither spinel frequency component present) and a translation in the lattice fringes representative of the antiphase boundary respectively.

Figure 8

Schematic of $\mathit{\delta }$ transformation through pulsing. The accelerated transformation for DRX sample is achieved by a repeated charge and discharge between 4.5 and 3.0 V. This pulsing is carried out at elevated temperature and rate to transform the DRX material into the δ phase more rapidly. The microstructure of δ the phase consists of nanoscale spinel domains which are separated by antiphase boundaries.