Standard Microwave-Assisted Solvothermal (MWST) Synthesis of Pnma-LiCoPO 4

Taking a synthesis process for the production of LiMn0.7Fe0.3PO4 nanorods from the literature^[8] as a starting point, a novel microwave-assisted solvothermal (MWST) process for the synthesis of Pnma-LiCoPO4 was developed. In order to produce phase pure olivine-type LiCoPO4, the process was optimized in the precursor system LiOH ∙ H2O – CoSO4 ∙ 7 H2O – H3PO4 with respect to a number of reaction parameters, such as reaction time and tempera-ture, power of the microwave irradiation, stirring speed during the synthesis, pH value of the reaction mixture, the application of reducing agents as well as the concentration, molar n(Li):n(Co):n(P) ratio, and mixing sequence of the starting materials. For these investigations, which are not included in this work, only one synthesis parameter was changed while keeping all other reaction conditions constant in order to be able to evaluate the effect of the individual parameter. Since an aqueous component in the solvent was found to be crucial to obtain Pnma-LiCoPO4 (possibly related to the increased vapor pressure during the synthesis) and a polyol-type co-solvent proved to be necessary to allow for a defined particle morphology (due to its soft template effect^[9-10]) and reasonable electrochemical performance, 30 mL of a binary 1:1 (v:v, by volume) water/ethylene glycol (EG) solvent blend with 50 vol% H2O and 50 vol%

EG were used for the investigations. The sum of these efforts resulted in a standardized basic MWST process towards Pnma-LiCoPO4 particles with a hexagonal platelet morphology (Chap-ter 4.1.1), which is described in the following and shown in Scheme 3.1 and Figure 3.4, re-spectively. The process served as a starting point for the further optimization of the Pnma-LiCoPO4 material from an electrochemical point of view by tuning the size (Chapter 3.2.2) and morphology (Chapter 3.2.3) of the particles. It was also used for the production of nanoparti-cles of the metastable Pna21-LiCoPO4 polymorph (Chapter 3.2.5).

Scheme 3.1 Reaction scheme of the microwave-assisted solvothermal (MWST) standard process for the synthesis of Pnma-LiCoPO4 particles with a hexagonal platelet morphology using a binary 1:1 (v:v) water/ethylene glycol (EG)

3 Experimental Methods

The MWST standard process produces Pnma-LiCoPO4 in a single step without further post-heat treatment at a moderate temperature of 250 °C and a short reaction time of only 30 min, using 30 mL of a binary 1:1 (v:v) water/ethylene glycol (EG) mixture as a solvent (Scheme 3.1). The molar ratio n(Li):n(Co):n(P) of the starting materials LiOH ∙ H2O, CoSO4 ∙ 7 H2O, and H3PO4 was 3:1:1, with the two additional equivalents of Li being required to bind the SO42− ions as Li2SO4 ∙ H2O in a stoichiometric reaction. The highly water-soluble

[11-12] lithium sulfate side phase was subsequently removed by washing (for further discussions on the Li2SO4 phase, please refer to the respective publication and supporting materials in Chapter 6.2). Furthermore, ascorbic acid was used as reducing agent to prevent the oxidation of Co²⁺ to Co³⁺ in the aqueous solvent phase.^[13-14]

In the first step of the process (Figure 3.4), 22.5 mmol LiOH ∙ H2O were dissolved in 15 mL deionized water. Then, 7.5 mmol H3PO4 (85 wt% solution) were added dropwise under stirring to the clear solution with pH = 14, which produced a white suspension of Li3PO4 with pH = 10.5. Subsequently, 15 mL ethylene glycol, 7.5 mmol CoSO4 ∙ 7 H2O and 0.075 g ascor-bic acid were added in this sequence. The resulting blue-violet gel (pH = 5.5), which consists of a mixture of Co3(PO4)2 ∙ 8 H2O, Li2SO4 ∙ H2O, and Li3PO4 according to further studies not included in this work, was homogenized for 20 min and then transferred into a 75 mL capacity TFM–PTFE vessel (HTV-75). The solvothermal reaction was performed using the Ethos One microwave synthesis system equipped with the MR-8 HT monobloc rotor as described in Chapter 3.1.3. The sample temperature (T1) was ramped to 250 °C within 15 min and kept at 250 °C for 30 min under stirring (speed setting: 50%), with a maximum microwave power of Pmax = 600 W being applied (cf. also Figure 3.3e, Chapter 3.1.3). After natural cooling to room temperature (RT), the violet precipitate was separated from the mother solution (pH = 5.0) by suction filtration, and washed five times with 50 mL deionized water and one time with 50 mL absolute ethanol in order to remove the lithium sulfate side product and organic residues of the EG co-solvent, the ascorbic acid additive, and their decomposition products. The light pink powder (see bottom left of Figure 3.4 or Figure 4.1, Chapter 4.1.1) was finally dried in air at 150 °C overnight.

3.2 Synthesis

Figure 3.4 Flow chart demonstrating the microwave-assisted solvothermal (MWST) process. The standard synthe-sis of Pnma-LiCoPO4 (Chapter 3.2.1) was based on a binary solvent blend of water and ethylene glycol (EG) as a co-solvent (50 vol%). It was further modified for tuning the size (variation of the EG content between 0 and 80 vol%;

Chapter 3.2.2) and morphology (variation of the type of co-solvent, 50 vol%: DEG, diethylene glycol; TEG,

3 Experimental Methods

3.2.2 Microwave-Assisted Solvothermal (MWST) Synthesis of Size-Controlled Pnma-LiCoPO

Particles

Scheme 3.2 Reaction scheme of the microwave-assisted solvothermal (MWST) process for the synthesis of size-tuned Pnma-LiCoPO4 particles by using ethylene glycol (EG) concentrations of 0–80 vol% (increment step:

10 vol%) in the binary water/ethylene glycol solvent blend.

Size-tuned Pnma-LiCoPO4 particles (Chapter 4.1.2) were obtained using the standard MWST process described in Chapter 3.2.1 by altering the ethylene glycol (EG) concentration of the binary H2O/EG solvent mixture between 0 vol% EG (i.e. using pure water in a micro-wave-assisted hydrothermal (MWHT) process) and 80 vol% EG with an increment step of 10 vol% (Scheme 3.2). All other parameters were kept constant in order to ensure reliable and comparable results. To further investigate the influence of the solvent composition on the par-ticle size, the viscosities of the respective solvent mixtures were determined by rheometry (for details refer to Chapter 3.3.7). The Pnma-LiCoPO4 (LCP) samples were denoted by the amount of EG in vol% used in the solvent blend, i.e. LCP-0, LCP-10, (…), and LCP-80, respec-tively. The pink color of the powders became lighter and more bluish going from 0 to LCP-80 (cf. bottom of Figure 3.4 or the respective publication in Chapter 6.3), which can be ex-plained by a reduction of the particle size.^[15] Note that at EG concentrations of 90 vol%, a mix-ture of the Pnma- and Pna21-LiCoPO4 polymorphs was obtained. The synthesis in pure (100 vol%) EG yielded single-phase Pna21-LiCoPO4, which was investigated in another study (see Chapter 3.2.5).

3.2 Synthesis

3.2.3 Microwave-Assisted Solvothermal (MWST) Synthesis of Morphology-Controlled Pnma-LiCoPO

Particles

Scheme 3.3 Reaction scheme of the microwave-assisted solvothermal (MWST) process for the synthesis of morphology-tuned Pnma-LiCoPO4 particles by using various co-solvents in a binary (1:1, v:v) mixture with water.

Specifically, 50 vol% ethylene glycol (EG), diethylene glycol (DEG), triethylene glycol (TEG), tetraethylene glycol (TTEG), polyethylene glycol 400 (PEG), or benzyl alcohol (BA) were employed as co-solvents.

For the production of Pnma-LiCoPO4 particles with tuned morphologies (Chap-ter 4.1.3), the standard MWST process (see Chap(Chap-ter 3.2.1 and Figure 3.4) was al(Chap-tered by using a number of different 1:1 (v:v) binary solvent blends (Scheme 3.3). The mixtures were composed of deionized water (50 vol%) and the following polyol-type co-solvents (50 vol%) that feature different amounts of hydroxyl groups (for the structural formulas, please refer to Chapter 3.1.1, Figure 3.1): ethylene glycol (EG), diethylene glycol (DEG) triethylene glycol (TEG), tetraethylene glycol (TTEG), polyethylene glycol 400 (PEG), and benzyl alcohol (BA).

In order to ensure that only the effect of the co-solvent was evaluated, all other reaction con-ditions were kept constant. The corresponding Pnma-LiCoPO4 (LCP) samples were denoted by the co-solvent used as follows: LCP-EG, LCP-DEG, LCP-TEG, LCP-TTEG, LCP-PEG, and LCP-BA. The color of the powders ranged from light pink TTEG) to dark violet (LCP-PEG; cf. Figure 4.3, Chapter 4.1.3), which was related to particle size effects with the color becoming darker and more intense for bigger crystallites.^[16]

3 Experimental Methods

3.2.4 Hydrothermal (HT) Synthesis of Co

Li[(OH)

O][(PO

OH)(PO

)

]

Scheme 3.4 Reaction scheme of the hydrothermal (HT) process towards Co11Li[(OH)5O][(PO3OH)(PO4)5]. Because the pH-dependent formation of other phases such as Co3(OH)2(PO3OH)2 and Pnma-LiCoPO4 competes in the pro-cess, adjusting the pH value of the reaction mixture to a value of 5.0 by adding HCl is crucial to obtain a single-phase material. Since the starting materials are not quantitatively incorporated in the single-phase in stoichiometric amounts, it is not possible to provide the exact molar ratios of the precursors and theoretical side products of the process. To keep the scheme simple, the nominal molar amount of the starting materials (n(Li):n(Co):n(P) molar ratio of 2:1:1) that was employed is provided, and possible side products have been omitted.

Co11Li[(OH)5O][(PO3OH)(PO4)5] (Chapter 4.2.1) was prepared by hydrothermal continuous stirring. The pH value of the resulting purple suspension with pH = 10.0 was further lowered to 5.0 by adding hydrochloric acid (37 wt% solution). Since investigations on the influ-ence of the pH value of the precursor mixture showed that the phase formation is highly pH-sensitive and competes with other phases such as Co3(OH)2(PO3OH)2 (at pH = 3.0–4.5) and Pnma-LiCoPO4 (at pH = 8.0; for specific details refer to Chapter 6.5), adjusting the pH to a value of 5.0 proved to be a crucial step to obtain single-phase Co11Li[(OH)5O][(PO3OH)(PO4)5].

The mixture was homogenized, transferred to a teflon-lined stainless steel pressure vessel, and sealed quickly. The vessel was heated to 220 °C within 2 h and kept at that temperature for a period of 20 h. After natural cooling to ambient temperature, the violet crystals (cf. Fig-ure 4.5, Chapter 4.2.1) were separated from the mother solution (pH = 5.0) by suction filtration, and washed five times with 25 mL deionized water and 25 mL absolute ethanol. The powder was dried in air at 150 °C for 12 h.

3.2 Synthesis

3.2.5 Microwave-Assisted Solvothermal (MWST) Synthesis of Nano-Sized Pna2

-LiCoPO

Particles

Scheme 3.5 Reaction scheme of the microwave-assisted solvothermal (MWST) process used for the synthesis of nano-sized Pna21-LiCoPO4 particles by using pure (100 vol%) ethylene glycol (EG) as a solvent.

Pna21-LiCoPO4 nanoparticles (Chapter 4.2.2) were obtained from a modified micro-wave-assisted solvothermal (MWST) process as described in Chapter 3.2.1, using 30 mL of pure ethylene glycol (EG; i.e. 100 vol%) as a solvent instead of an aqueous binary solvent mixture (Scheme 3.5). All other synthesis parameters and steps remained unchanged. The precipitate exhibited an intense dark blue color as demonstrated at the bottom right of Figure 3.4, Chapter 3.2.1 and in Figure 4.6, Chapter 4.2.2.