Metastable Lithium Cobalt Phosphates:

Co

Li[(OH)

O][(PO

OH)(PO

)

], Pna2

-LiCoPO

, Cmcm-LiCoPO

, and Cmcm-Li

CoPO

Following the investigations on the olivine-type high-voltage cathode material Pnma-LiCoPO4, this part focuses on metastable lithium cobalt phosphates, in specific the Pna21- and Cmcm-LiCoPO4 polymorphs as well as two new phases, Li0.5−δCoPO4, the first Li-deficient derivative of Cmcm-LiCoPO4, and the framework compound Co11Li[(OH)5O][(PO3OH)(PO4)5].

Compared to the thermodynamically stable Pnma-LiCoPO4, the Pna21 and Cmcm analogs have been less studied. Despite the recent advance,[28, 37, 83-84] there are fundamental aspects that have to be further addressed. In both cases, the crystal structure solutions showed issues such as non-standard space groups, apocryphal site occupancies, and unsatisfactory reliability factors. Moreover, the thermal studies left some questions open. In the case of Pna21-LiCoPO4, previous reports^{[28, 37]} presented contradictory results, while the thermal anal-ysis of Cmcm-LiCoPO4 lacked high-temperature data and analysis of the post-TGA material.^[37]

Furthermore, the electrochemical performances of both phases were found to be poor.^{[28, 37]}

However, to date, no attempt was made to optimize (e.g. using particle size and morphology control similar to Pnma-LiCoPO4) or to fully understand the reasons for this unsatisfactory per-formance. In addition, the magnetic properties of Cmcm-LiCoPO4 were not yet reported.

Therefore, new microwave-assisted solvothermal (MWST) or polyol (PO) synthesis pathways towards size- and shape-tuned Pna21- and Cmcm-LiCoPO4 materials have been developed. Based on X-ray and neutron powder diffraction data (PXRD, PND), revised crystal structures are presented. The materials were further characterized using a plethora of techniques, including elemental analysis, scanning electron microscopy (SEM), infrared (IR) and X-ray absorption (XAS) spectroscopy, electrochemical and magnetic measurements, which allows a careful analysis of their structure–property relationships. The thermal properties were examined using a combination of TGA/DSC (thermogravimetric analysis/differential scanning calorimetry) as well as ex and in situ PXRD experiments.

Using kinetically controlled hydrothermal (HT) and polyol (PO) approaches, the mixed-valent Co(II,III) phases Co11Li[(OH)5O][(PO3OH)(PO4)5] and Cmcm-Li0.5−δCoPO4 are reported for the first time. While Co11Li[(OH)5O][(PO3OH)(PO4)5] plays a significant role as a competing phase in the hydrothermal synthesis of Pnma-LiCoPO4, Cmcm-Li0.5−δCoPO4 represents the first Li-deficient derivative of Cmcm-LiCoPO4, which is accessible by a direct polyol synthesis approach. The phases were completely characterized using PXRD (and PND), elemental and thermal analysis, SEM, IR and XAS as well as magnetic measurements.

4.2 Metastable Lithium Cobalt Phosphates: Co11Li[(OH)5O][(PO3OH)(PO4)5], Pna21-LiCoPO4, Cmcm-LiCoPO4, and Cmcm-Li0.5−δCoPO4

4.2.1 Summary: Co

Li[(OH)

O][(PO

OH)(PO

)

], a Lithium-Stabilized, Mixed-Valent Cobalt(II,III) Hydroxide Phosphate Framework

Jennifer Ludwig, Stephan Geprägs, Dennis Nordlund, Marca M. Doeff, and Tom Nilges Manuscript submitted for publication, 2017. see Chapter 6.5

Figure 4.5 A new metastable, lithium-stabilized cobalt(II,III) hydroxide phosphate framework compound with the crystal-chemical composition Co11.0(1)Li1.0(2)[(OH)5O][(PO3OH)(PO4)5] (space group: P31m, Z = 1, simplified bulk composition: Co1.84(2)Li0.16(3)(OH)PO4) was prepared by hydrothermal synthesis. Because the synthesis is highly pH-sensitive, the formation of Co3(OH)2(PO3OH)2 and olivine-type Pnma-LiCoPO4 competes in the process, and a pH value of 5.0 is crucial for obtaining the pure, single-phase title compound (top). The violet powder (center) consists of crystals with dimensions of about 15 µm × 30 µm that exhibit a unique elongated triangular pyramid morphology (bottom left). The main structural feature are alternating double chains of [M2O8(OH)] (M = Co, Li) dimer units, which run along the [001] direction and are connected via [PO4] and [PO3(OH)] tetrahedra (bottom center and right). The graphic was adapted from reference [85]/the manuscript in Chapter 6.5.

While optimizing a hydrothermal process for producing olivine-type Pnma-LiCoPO4 in the precursor system LiOH ∙ H2O – Co(CH3COO)2 ∙ 4 H2O – (NH4)2HPO4 (for experimental details see Chapter 3.2.4), the pH value of the reaction medium was identified as a key parameter that controls the phase formation. Whereas pure Pnma-LiCoPO4 was only obtained under alkaline conditions at pH = 8.0, Co3(OH)2(PO3OH)2[86] was formed in acidic media with

4 Results and Discussion

were dissolved under highly acidic conditions.) In the intermediate pH region, a novel lithium-stabilized cobalt(II,III) hydroxide phosphate compound with the crystal-chemical composition Co11.0(1)Li1.0(2)[(OH)5O][(PO3OH)(PO4)5] was found, which competes with Pnma-LiCoPO4 as reaction product at 5.5 ≤ pH ≤ 7.5 and can be obtained in phase pure form in a very narrow region at pH = 5.0 (cf. Figure 4.5, top). The synthesis conditions, the crystal structure, and ma-terial properties were discussed.

The violet crystals exhibit a unique triangular pyramid morphology (dimensions:

15 µm × 30 µm) with a nanosheet-like primary structure (Figure 4.5, bottom left). PXRD exper-iments reveal that the phase with the simplified empirical formula Co1.84(2)Li0.16(3)(OH)PO4 is isostructural with the Fe- and Mg-bearing phosphate minerals satterlyite^[87] and holtedahlite.^[88]

It crystallizes trigonally, in the space group P31m (a = 11.2533(4) Å, c = 4.9945(2) Å, V = 547.75(3) ų, Z = 1) and features a partial Li substitution on both Co sites, which was fur-ther substantiated by elemental analysis. First experiments suggested that the Li substitution is crucial for the stabilization of the framework since the synthesis of a Li-free P31m-type Co2(OH)PO4 phase proved unsuccessful. The dominant structural motif are alternating double chains running along the [001] direction, which are built from octahedral [M2O8(OH)] (M = Co, Li) dimer units and interconnected by tetrahedral [PO4] and [PO3(OH)] groups (Figure 4.5, bottom center and right). The occurrence of three independent OH groups was confirmed by infrared spectroscopy. Co L-edge X-ray absorption spectroscopy revealed that the framework hosts Co ions that have a mixed valence state (+II/+III). Based on charge-balance arguments, the occurrence of (6 ± 2)% Co³⁺ (cf. theoretically expected value: ~9%) is the result of the incorporation of Li⁺ ions in the structure. Magnetic measurements demonstrated a paramag-netic to antiferromagparamag-netic transition at T = 25 K and a spin-glass-like behavior with a blocking temperature of T ~9 K. Thermal analysis revealed that the phase is metastable and shows a complex two-step decomposition mechanism. Driven by a redox reaction and the intrinsic in-stability of Co³⁺, the framework decomposes into CoO,^[89] Co3(PO4)2,^[90] and olivine-type Pnma-LiCoPO4 with release of H2O and O2. Surprisingly, the oxidation of oxide ions to oxygen was found to be the first (558 °C, exothermic) and the dehydration the second (633 °C, endother-mic) step of the decomposition.

Author contributions: J. Ludwig conceived and designed this work, and carried out the synthesis, material characterization (PXRD, IR, SEM/EDS), and data analysis. S. Geprägs and D. Nordlund performed and analyzed magnetic and XAS measurements, respectively.

J. Ludwig wrote the manuscript. All authors read and approved the final version of the manuscript.

4.2 Metastable Lithium Cobalt Phosphates: Co11Li[(OH)5O][(PO3OH)(PO4)5], Pna21-LiCoPO4, Cmcm-LiCoPO4, and Cmcm-Li0.5−δCoPO4

4.2.2 Summary: Synthesis and Characterization of Metastable, 20 nm-Sized Pna2

-LiCoPO

Nanospheres

Jennifer Ludwig, Dennis Nordlund, Marca M. Doeff, and Tom Nilges

J. Solid State Chem. 2017, 248, 9–17. see Chapter 6.6 DOI: 10.1016/j.jssc.2017.01.015

Figure 4.6 Overview on the structure–property relationships of 15–20 nm-sized Pna21-LiCoPO4 nanospheres (SEM image: top right) made by a microwave-assisted solvothermal (MWST) process. The crystal structure (bottom left) features tetrahedral [CoO4] (yellow) and [PO4] (blue) units with Co–Li antisite defects on the tetrahedrally coordi-nated Li sites (red). The local tetrahedral symmetry of the Co²⁺ ions in the structure was confirmed by Co L2,3-edge X-ray absorption spectroscopy (top left). As demonstrated by DSC data (bottom right), the blue powder (center) transforms to violet, olivine-type Pnma-LiCoPO4 at 527 °C (exothermic). The graphic was adapted from reference [29], Copyright (2017), with permission from Elsevier Inc.

Based on the study on the particle size-controlled microwave-assisted solvothermal synthesis of olivine-type Pnma-LiCoPO4 upon variation of the ethylene glycol (EG) concen-tration of the binary H2O/EG solvent (Chapter 4.1.2), a novel synthesis route towards the meta-stable, exclusively tetrahedrally coordinated (crystal structure see Figure 4.6, bottom left) Pna21-LiCoPO4 polymorph was found by using pure (100 vol%) EG as a solvent (for experi-mental details refer to Chapter 3.2.5). Since the particle size was found to be reduced with

4 Results and Discussion

EG yielded uniform, spherical Pna21-LiCoPO4 nanoparticles with diameters of about 15–20 nm (Figure 4.6, top right). The structure–property relationships of the dark blue nanomaterial (Figure 4.6, center) were studied comprehensively with the help of PXRD, elemental and thermal analysis, SEM, BET surface area analysis, IR and XAS spectroscopy, as well as electrochemical measurements. Furthermore, the kinetically controlled formation and crystal growth mechanisms were discussed.

In contrast to previous reports^{[28, 37]} on Pna21-type LiCoPO4, the results indicated that the compound with the empirical formula Li0.95(1)Co1.03(1)PO4 is non-stoichiometric and exhibits an excess of cobalt. A redetermination of the crystal structure showed that the material fea-tures a disordered cation substructure with 4.8(8)% Co mixing on the Li sites. Since Li migra-tion is hindered due to the defects, the occurrence of Co–Li antisite mixing provided an expla-nation for the poor electrochemical performance of Pna21-LiCoPO4, which demonstrated a dis-charge capacity of only 6 mAh∙g⁻¹.^[28] Another explanation is the high surface area of the material (~61 m²∙g⁻¹), which promotes parasitic side reactions at high voltage (as discussed in detail in Chapter 4.1.4). Co L2,3-edge soft X-ray absorption spectroscopy confirmed the tetra-hedral coordination of the divalent Co²⁺ ions in the structure in the form of [CoO4] units (Figure 4.6, top left).

A thorough investigation of the thermal stability using TGA/DSC and temperature-dependent in situ PXRD experiments revealed that the compound undergoes several phase transitions upon heating. The thermal behavior of Pna21-LiCoPO4 is hence more complex than previously assumed.^{[28, 37]} At 527 °C, the material converts to olivine-type Pnma-LiCoPO4

(Figure 4.6, bottom right). Interestingly, it was found that the Pna21 phase re-emerges as a stable high-temperature modification at temperatures above 800 °C since a partial and reversible transformation of Pnma-LiCoPO4 back to Pna21-LiCoPO4 was observed. After cooling to ambient temperature, single-phase Pnma-LiCoPO4 was obtained. It was further shown that the temperature of the Pna21–Pnma phase transition strongly depends on the atmosphere used upon heating (air vs. Ar) as well as the particle size of the material (nano- vs. micron-sized particles), with the transition temperature decreasing with increasing crystal dimensions.

Author contributions: J. Ludwig conceived and designed this work, and performed the material synthesis and characterization using ex and in situ PXRD, EDS, IR, and galvanostatic cycling (under the supervision of T. Nilges and M. M. Doeff). D. Nordlund collected XAS spectra. J. Ludwig interpreted the data and wrote the manuscript. All authors read and approved the final version of the manuscript.

4.2 Metastable Lithium Cobalt Phosphates: Co11Li[(OH)5O][(PO3OH)(PO4)5], Pna21-LiCoPO4, Cmcm-LiCoPO4, and Cmcm-Li0.5−δCoPO4

4.2.3 Summary: In Situ Studies and Magnetic Properties of the

Cmcm Polymorph of LiCoPO

with a Hierarchical Dumbbell-Like