Summary: Direct Synthesis and Characterization of Mixed-Valent Li 0.5−δ CoPO 4 , a Li-Deficient Derivative of the Cmcm Polymorph of

Cmcm Polymorph of LiCoPO 4 with a Hierarchical Dumbbell-Like Morphology Synthesized by Easy Single-Step Polyol Synthesis

4.2.4 Summary: Direct Synthesis and Characterization of Mixed-Valent Li 0.5−δ CoPO 4 , a Li-Deficient Derivative of the Cmcm Polymorph of

LiCoPO

Jennifer Ludwig, Carlos Alarcón-Suesca, Stephan Geprägs, Dennis Nordlund, Marca M. Doeff, Inés Puente Orench, and Tom Nilges

RSC Adv., 2017, 7, 28069–28081. see Chapter 6.8 DOI: 10.1039/c7ra04043a

Figure 4.8 A Li-deficient, mixed-valent Co(II,III) derivative of the Cmcm-LiCoPO4 polymorph with the nominal com-position Li0.5−δCoPO4 has been prepared by polyol synthesis. Upon heating, the pale pink Cmcm-Li0.5−δCoPO4 (here:

simplified with δ = 0) material (top left) decomposes to Pnma-LiCoPO4 and α-Co2P2O7 in a two-step mechanism (bottom). Cmcm-Li1−γCoPO4 (here: γ = 0) is formed as an intermediate under release of oxygen, which was revealed by TGA/DSC analysis (top center). Adapted from reference [93]. – Published by The Royal Society of Chemistry.

Based on the study on Cmcm-LiCoPO4 presented in Chapter 4.2.3, a direct polyol synthesis route towards the first Li-deficient structural derivative with the nominal composition Li0.5−δCoPO4 was developed. In contrast to the sub-stoichiometric phases LixCoPO4 (x = 0; ⅔) derived from olivine-type Pnma-LiCoPO4, which are only accessible by chemical or electro-chemical delithiation in a top-down approach,^[94-98] the Cmcm derivative was obtained by a

4 Results and Discussion

in the process (for experimental details see Chapter 3.2.7). The light pink Cmcm-type Li0.5−δCoPO4 material (Figure 4.8, top left) was fully characterized using powder X-ray (PXRD) and neutron diffraction (PND), elemental analysis, SEM, IR and XAS spectroscopy, as well as electrochemical and magnetic measurements. The thermal stability was evaluated with the help of TGA/DSC and in situ PXRD experiments. The results were discussed in context of the

‘fully’ lithiated Cmcm-LiCoPO4 phase, for which an improved structure solution (revealing a sub-stoichiometry reflected by the revised sum formula Li1−γCoPO4) as well as Co L-edge XAS data were presented for the first time.

PXRD experiments and elemental analysis of the novel Cmcm-Li0.5−δCoPO4 (with δ = 0.11(2) (PXRD) and 0.05(5) (AAS), respectively) and its ‘fully’ lithiated counterpart Li1−γCoPO4 (γ = 0.06(2); 0.07(5)) indicated that both structures are non-stoichiometric and fea-ture vacancies on both cation sites. The sub-stoichiometry was further confirmed by neutron powder diffraction experiments. Co L-edge X-ray absorption spectroscopy indicated that, un-like Cmcm-Li1−γCoPO4, which features only octahedrally coordinated Co²⁺ ions as expected, the Li-deficient structure Cmcm-Li0.5−δCoPO4 bears about (71 ± 3)% Co²⁺ and (29 ± 3)% Co³⁺

ions, which is in agreement with charge-balance arguments. According to SEM studies, both materials showed a similar hierarchical, dumbbell-like morphology. Despite the nanostructure of the primary particles, both compounds demonstrated poor electrochemical performances, with initial discharge capacities of only ~3 mAh∙g⁻¹ being reached. This was mainly related to the intrinsically low Li conductivity caused by the lack of suitable Li migration pathways in the crystal structure. No evidence was found that Cmcm-Li0.5−δCoPO4 represents an intermediate phase upon delithiation of Cmcm-Li1−γCoPO4.

A comprehensive investigation of the thermal stability revealed that Cmcm- Li0.5−δCoPO4 shows a complex, two-step decomposition mechanism upon heating (Figure 4.8, center and bottom). In the first step (I, exothermic) at 394 °C, it decomposes to α-Co2P2O7

(P21/c)^[99] and Li1−γCoPO4 (Cmcm)^{[83, 93]} under release of oxygen. This redox reaction is driven by the reduction of Co³⁺ in the Li0.5−δCoPO4 structure by O²⁻ ions during the pyrophosphate formation (i.e., the coupling of isolated [PO4] units to [P2O7]). The resulting Co²⁺-only phase Cmcm-Li1−γCoPO4 then irreversibly converts to the thermodynamically more stable olivine Pnma-LiCoPO4 structure at 686 °C (II) in an endothermic process.

Author contributions: J. Ludwig performed the material characterization using in situ and ex situ PXRD, SEM/EDS, IR spectroscopy, and electrochemical measurements, and analyzed the data. C. Alarcón-Suesca performed the material synthesis. S. Geprägs and D. Nordlund performed and analyzed magnetic and XAS measurements, respectively. I. Puente Orench and C. Alarcón-Suesca conducted neutron diffraction experiments. J. Ludwig wrote the man-uscript. All authors read and approved the final version of the manman-uscript.

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

4.2.5 Discussion

Several metastable lithium cobalt phosphate phases, the novel framework structure Co11Li[(OH)5O][(PO3OH)(PO4)5] as well as the Pna21- and Cmcm-LiCoPO4 polymorphs have been explored, which have been considerably less studied than the olivine-type Pnma-LiCoPO4 phase. Moreover, the first Li-deficient derivative of the Cmcm-LiCoPO4 structure, Li0.5−δCoPO4, was obtained from a direct polyol synthesis approach. The studies provided insights into the structure–property relationships of this class of materials from a fundamental point of view, which might help to understand and modify the material properties with regard to potential future applications.

The trigonal Co11Li[(OH)5O][(PO3OH)(PO4)5] framework was found to compete as a product of a hydrothermal process towards Pnma-LiCoPO4 over a wide pH range (pH = 5.5–

7.5), and hence plays a significant role as a side phase in the synthesis of the high-voltage cathode material. In fact, the phase was also obtained in the MWST syntheses discussed in Chapters 4.1.1–4.1.3 when the synthesis conditions, especially the pH value, were not opti-mized (studies not included in this work). Reflections of the phase were also evident in the PXRD patterns of other Pnma-LiCoPO4 materials in the literature,^[27] however, they have not been assigned correctly but ascribed to a mixture of LiP5, Li0.62CoO2, and CoO. For that matter, the discovery and investigation of this phase makes a significant contribution towards a deeper understanding and optimization of the wet-chemical production of Pnma-LiCoPO4 and possibly other Co-based cathode materials as well. Whether or not the isostructural Fe, Mn, and Ni homologues of the phase exist (note that the iron phase would be a synthetic analog to the mineral satterlyite;^{[87, 100]} see below) and also represent side products of the hydrothermal synthesis of the other members of the olivine family, LiMPO4 (M = Fe, Mn, Ni), will have to be explored in further experiments.

Although the simplified empirical formula Co1.84(2)Li0.16(3)(OH)PO4 suggests that the trigonal (space group: P31m) phase represents a new polymorph of Co2(OH)PO4, for which orthorhombic (Pnnm)^[101] and tetragonal (I41/amd)^[102] structures are known, it does not represent a Co2(OH)PO4 polymorph in the strict sense due to structural reasons, in particular the proton distribution (for details, please refer to the manuscript in Chapter 6.5). Instead, the phase is isostructural with the natural phosphate minerals satterlyite^{[87, 100]}

Fe9.24Mg2.76(OH,O)6(PO3OH)(PO4)5 (found at the Big Fish River area, Yukon Territory, Canada), and holtedahlite^[88] Mg12(OH,O)6(CO3)0.24(PO3OH)0.76(PO4)5 (from Modum, Norway), which are found only in very limited geographical regions. The unit cell volume of 547.70(3) Å³ ranks between the Fe (V = 562.71(9) Å³)^[87] and Mg (V = 539.3(3) Å³)^[103] counterparts,

4 Results and Discussion

Co²⁺/Li⁺ (HS: 0.745 Å), and Mg²⁺ (0.72 Å).^[104] Apart from synthetic holtedahlite Mg12(OH,O)6(PO3OH)(PO4)5[103], the novel Co analog represents the only synthetic phase of that structure family, which underlines that the hydrothermal technique, mimicking nature, is a powerful tool to obtain compounds isostructural with hydrothermal minerals as well as original structures^[105] and hence, allows to further explore the class of transition-metal phosphates.

In agreement with investigations on satterlyite, which appears to be stabilized by a cer-tain Mg content,^[87] the Co framework is likely stabilized by Li substitution on both Co sites, which was substantiated by elemental analysis and PXRD studies. The assumption that Li has a stabilizing effect is further supported by the diagonal relationship between lithium and mag-nesium,^[106] which suggests that both elements exhibit similar chemical properties, as well as the fact that both ions exhibit similar ionic radii (Mg²⁺: 0.72 Å, Li⁺: 0.76 Å; CN = 6)^[104] and charge densities. In fact, within the scope this work, all attempts to prepare a Li-free P31m-framework proved unsuccessful and resulted in the formation of single-phase Co3(OH)2(PO3OH)2.^[86] However, it was possible to prepare solid solutions of the composition Co2−xLix(OH)PO4 (x = 0.15(1)–0.35(3); based on PXRD data), which indicated that there is a certain phase width. As a result of the Li substitution (charge-balance) and supported by XAS studies, the octahedrally coordinated Co ions adopt a mixed valence state (+II, +III; both high spin), which significantly affects the magnetic and thermal behavior of the phase.

Magnetic measurements on the mixed-valent P31m-Co1.84(2)Li0.16(3)(OH)PO4 phase re-vealed that the finite Li⁺ and Co³⁺ contents in the Co2(OH)PO4 framework result in a reduction of the paramagnetic to antiferromagnetic transition as well as the blocking temperature of the spin-glass-like behavior compared to the Li-free Pnnm- and I41/amd-Co2(OH)PO4 phases.^[102,

107] At high temperature (T > 100 K), the new framework exhibits a Curie–Weiss behavior with a Weiss temperature of (−68  2) K, which is similar to the reported Co2(OH)PO4 poly-morphs^{[102, 107]} and indicates that the interaction between the neighboring magnetic ions is pre-dominantly antiferromagnetic. The low-temperature behavior suggests a paramagnetic to anti-ferromagnetic transition at T ~25 K. Whereas this temperature is close to the transition tem-perature found for the I41/amd modification (20 K),^[102] it is much lower than that of Pnnm-Co2(OH)PO4 (71 K).^[107] A second maximum of the magnetic susceptibility was found at T ~9 K, below which the phase exhibited a magnetic hysteresis with a finite remanent magnetization.

However, no saturation of the magnetization was observed up to 7 T. Similar to the Pnnm phase below 15 K,^[107] this indicates a spin-glass-like behavior with a blocking temperature of around 9 K. However, in order to elucidate the structure-related magnetic details of the frame-work in detail, further experiments will be required.

The investigations on the thermal stability showed that the phase undergoes a complex, two-step decomposition to CoO,^[89] Co3(PO4)2,^[90] and Pnma-LiCoPO4 with release of oxygen and water. The formation of CoO and Co (PO ) as decomposition products is consistent with

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

a report^[101] on Pnnm-Co2(OH)PO4. In the case of the I41/amd polymorph, no decomposition products have been reported.^[102] The formation of Pnma-LiCoPO4 as an additional Li-contain-ing decomposition product provides further evidence for the partial Li substitution of the P31m-type Co2(OH)PO4 framework in Co11Li[(OH)5O][(PO3OH)(PO4)5]. Surprisingly, the oxidation, which is driven by the intrinsic instability of the Co³⁺ ions in the framework, occurs at lower temperature (558 °C, mass loss: 0.6 wt%) than the dehydration step (633 °C, mass loss:

4.6 wt%). This finding possibly provides further insights into the thermal stability of Pnnm-Co2(OH)PO4 as well, since the TGA data of this material showed two very similar weight loss steps (initial: 0.4 wt%, ~600 °C: 3.8 wt%). Whereas the second step was attributed to the de-hydration of Pnnm-Co2(OH)PO4, the first step was explained by so-called ‘surface effects’.^[101]

Because the Pnnm material was prepared by hydrothermal synthesis and Co²⁺ is prone to oxidation in aqueous media, however, it is possible that the initial step is also related to O2

release due to the presence of a small amount of Co³⁺. Therefore, the oxidation state of the Co ions in Pnnm-type Co2(OH)PO4 should be reassessed. Because of the presence of mixed-valent Co ions in a complex hydroxide–hydrogen phosphate–phosphate matrix and its inter-esting thermal properties, Co11Li[(OH)5O][(PO3OH)(PO4)5] may act as a good oxygen-evolving catalyst (OEC) for example for water splitting, similar to the well-established Co–Pi (cobalt–

phosphate) catalyst system.^[108-111] Consecutive studies on the new phase should therefore focus onto exploring its catalytic properties.

The investigations on the Pna21- and Cmcm-LiCoPO4 polymorphs addressed important aspects that remained unclear or untouched in other reports.[28, 37, 83] While previous synthesis methods, especially for producing Cmcm-LiCoPO4, required special equipment and complex procedures (cf. high-pressure high-temperature solid-state and microwave-assisted solvo-thermal syntheses under water-free conditions),^{[37, 83]} two alternative, one-step soft-chemical approaches at low temperature, using solvothermal (ST) and polyol (PO) methods, have been developed. The use of a variety of polyol solvents in these new pathways (DEG, TEG, TTEG, and PEG) further allows for morphological tuning of the material, even towards hierarchically organized structures, which has hitherto not been explored. In the case of Pna21-LiCoPO4, the use of pure ethylene glycol (MWST process) instead of pure water as described for the MWHT process by Jähne et al.^[28] resulted in a significant particle size reduction towards a nanostruc-tured (d = 15–20 nm) material. With the help of rheological measurements, the size reduction could clearly be related to the increased viscosity of EG compared to water (EG: 15.7(2) mPa∙s vs. H2O: 0.89(5) mPa∙s; measured at 25 °C and a shear rate of 100 s⁻¹;^{[29, 36]} for a thorough discussion please refer to Chapter 4.1.4). Further studies (for details see Chapter 6.6) demon-strated that by using lower process temperatures, morphological and size tuning of Pna21 -LiCoPO is not restricted to water-free solvents.

4 Results and Discussion

Since both the Pna21- and Cmcm-LiCoPO4 phases undergo a transition to the thermo-dynamically stable Pnma structure upon heating, the materials might be used as intermediates to produce particle size- and morphology-controlled Pnma-LiCoPO4 by post-annealing, a syn-thesis pathway that has only been explored for Pna21-LiCoPO4 to date.^[79] Since the phase transition temperatures towards Pnma-LiCoPO4 (T ~500–600 °C) are significantly lower than the temperatures required for the direct solid-state synthesis of the olivine (T = 800–900 °C),

[112] and because nanoparticles with diameters < 50 nm have only been accessible using high energy-consuming ball-milling procedures,^[113] this approach could pave the way for an easy and scalable production of the high-voltage cathode material Pnma-LiCoPO4 with significant energy savings. In that respect, the comprehensive investigations on the thermal behavior of the metastable LiCoPO4 polymorphs by using a combined approach of TGA/TSC as well as ex situ and temperature-dependent in situ PXRD investigations make a significant contribution.

In the case of Cmcm-LiCoPO4, it was demonstrated that the transformation to Pnma-LiCoPO4 is exothermic and occurs at 575 °C, which is considerably higher than previously assumed.^[37] Interestingly, studies in the high-temperature region, which has not been inves-tigated prior to this work, revealed that Pnma-LiCoPO4 is only stable up to 625 °C. At 650 °C, no reflections were visible, indicating that the material undergoes amorphization, while at 675 °C Pna21-LiCoPO4 is formed. After cooling to room temperature, the stable Pnma-LiCoPO4 was obtained. A similar behavior was observed for the nanostructured Pna21 -LiCoPO4. An exothermic phase transition to the Pnma phase was found to occur at 527 °C, which is comparable to the transition temperature of Cmcm-LiCoPO4 (575 °C) but significantly higher than stated for Pna21-LiCoPO4 by other groups (cf. Jähne et al.:^[28] 221 °C, exothermic;

Kreder et al.:^[37] 340 °C, endothermic). Furthermore, it could be clarified that the transition is in fact exothermic. The discrepancy in transition temperature could be explained by further stud-ies on a micron-sized Pna21 material (also produced by the MWST process, for details see Chapter 6.6), which revealed that the temperature of the Pna21–Pnma phase transition in-creases with the particle size. This is also in line with the findings in the literature (cf. Jähne^[28]:

~5 µm, Kreder^[37]: ~200 nm × 1 µm, this work: ~15–20 nm). An explanation for this observation are probably crystal strains, which are more likely to occur in bigger particles than in small ones. Similar to the thermal behavior of Cmcm-LiCoPO4, the Pna21-LiCoPO4 phase re-emerges when the Pnma-LiCoPO4 intermediate is heated to higher temperatures (T > 800 °C), but is then transformed back to Pnma-LiCoPO4 upon cooling. The investigations on the high-temperature behavior of the Cmcm and Pna21 as well as the Pnma phase (cf. Chapter 4.1.1) give thus new impulses to critically review the thermodynamic relation of all three LiCoPO4

polymorphs. Since all polymorphs demonstrated a high-temperature transformation to the Pna21 structure, it can be concluded that Pna21-LiCoPO4 represents a stable high-temperature modification whereas Pnma-LiCoPO4 becomes metastable in this temperature range. The

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

findings are in contrast to all previous reports^{[28, 37]} which suggested – on the basis of low-temperature and ex situ data – that the Pna21-LiCoPO4 polymorph is a metastable low-tem-perature modification only since Pnma-LiCoPO4, which represents the thermodynamically stable phase at room temperature, was found as an end-product of the transition. Our studies impressively demonstrate the drawbacks of relying solely on post-TGA/DSC analyses and at the same time the significance to study the thermal behavior in situ in order to get a thorough understanding of the underlying processes.

In addition to the studies on morphological tuning and the thermal stability, the redeter-mination of the crystal structures of the two polymorphs provided further insights. Whereas previous studies^{[28, 37]} suggested that the phases are stoichiometric, i.e. exhibit an ideal molar ratio of n(Li):n(Co):n(P) = 1:1:1, we found that both compounds are in fact non-stoichiometric.

The significant deviations of the empirical formulas derived from elemental analysis (Pna21: Li0.95(2)Co1.05(1)P1.00(2)O4, Cmcm: Li0.93(5)Co0.91(3)P1.00(2)O4) from the ideal composition Li1Co1PO4

prompted us to further investigate the crystal structures. In the case of the Cmcm polymorph, the deficit both in Li and Co suggested a defective structure that features vacancies in the cation substructure. This was confirmed by a refinement of the occupancy factors of the Li and Co sites on the basis of X-ray and neutron powder diffraction data (X-ray: 94(2)% Li and 95.5(5)% Co; neutron: 90(3)% Li and 95(6)% Co). Note that there was no indication for antisite defects in Cmcm-Li1−γCoPO4. Since the compositions found by the three techniques (elemental analysis, PXRD, PND) differed to a certain extent, the revised sum formula of the Cmcm poly-morph was denoted as Cmcm-Li1−γCoPO4 to best reflect the off-stoichiometries within three standard deviations. The refinement of the occupancy on the Li site of Pna21-LiCoPO4 resulted in a statistically significant over-occupation of 124(8)% whereas the Co site was found to be fully occupied within standard deviations. The high electron density strongly indicated that there is a partial substitution of the Li site with Co. An over-occupation of 122% Li was also reported by Jähne and co-workers,^[28] but no Co occupancy on this site was assumed. Similar to the isostructural phase δ1-LiZnPO4 (space group: Pna21), for which a disordered Li–Zn distri-bution was described for both the Li and Zn sites (8% Zn on the Li position, and vice versa),^[114]

it could be concluded that Co–Li antisite mixing appears on the Li site (95.2(8)% Li and 4.8(8)%

Co) of the Pna21-LiCoPO4 structure.

Despite the off-stoichiometry of both the Pna21 and Cmcm phases, it is noteworthy that the compositions are charge-balanced within standard deviations and that the structures con-tain only Co²⁺ ions. In addition to the local symmetries (Td coordination in Pna21-LiCoPO4 and Oh in Cmcm-Li1−γCoPO4) of the Co ions, the occurrence of Co²⁺ was confirmed by Co L-edge X-ray absorption spectra, which were reported for the first time. In the case of Cmcm-Li CoPO, the broadening of the spectrum compared to a CoO reference^[115] provided further

4 Results and Discussion

Both materials demonstrated poor electrochemical performances, with capacities of only 3–6 mAh∙g⁻¹ being reached. This could be related to the intrinsically low electrical and ionic conductivities as both structures lack suitable Li migration pathways. The Li–Li distances being rather large in both cases (~5 Å; i.e. twice as large as in the olivine structure), the Li sites are practically isolated and the activation energy for Li migration will be very high. In the case of Pna21-LiCoPO4, the conductivity is additionally reduced due to the Co–Li antisite defects with Co ions blocking Li diffusion paths. This is likely also reason for the generally poor performance observed in other electrochemical studies about Pna21-LiCoPO4[28, 37] and is in line with re-ports[17, 45, 57-58, 60-61] on olivine-LiFePO4 showing that cation-mixing significantly reduces the performance. The blocking of Li channels also explains why the particle size reduction ap-proach, a common strategy to overcome diffusional limitations and to improve the electro-chemical performance (as outlined in Chapter 1.2.1), did not succeed in this case. Hence, to achieve further progress in the field, it will be crucial to find a synthesis pathway towards a stoichiometric Pna21-LiCoPO4 material without antisite defects (e.g. by increasing the synthe-sis temperature). However, similar to the findings on the effect of the particle size on the performance of Pnma-LiCoPO4 (discussion see Chapter 4.1.4), the decomposition of the elec-trolyte at the high operating voltage (5.0 V vs. Li/Li⁺)^[28] has to be considered, because side

Im Dokument Conventional and Microwave-Assisted Solvothermal Synthesis, Characterization, and Optimization of the High-Voltage Cathode Material LiCoPO4 for Lithium-Ion Batteries (Seite 120-144)