Article pubs.acs.org/cm
Cite This: Chem. Mater. 2018, 30, 874−878
Orientation-Dependent Lithium Miscibility Gap in LiFePO4 Zhaojin Li,†,‡ Jinxing Yang,†,∥ Changji Li,† Sucheng Wang,† Lei Zhang,† Kongjun Zhu,§ and Xiaohui Wang*,† †
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China ‡ University of Chinese Academy of Sciences, Beijing 100039, China ∥ School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China § State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China S Supporting Information *
ABSTRACT: [100]Pnma is believed to be a tough diffusion direction for Li+ in LiFePO4, leading to the belief that the rate performance of [100]-oriented LiFePO4 is poor. Recent work revealed that reducing the dimension of the LiFePO4 phase to 12 nm in the [100] direction increased the extent of Li solid solution between LiFePO4 and FePO4, which produced an increase in the active population and excellent rate performance, but the lithiation/delithiation mechanism for this interesting phenomenon remains to be unraveled. Here we report, by using operando X-ray diffraction, the phase transition path involved in the lithiation/delithiation process is singlephase featured. This work presents one of the first experimental demonstrations that decreasing the dimension of LiFePO4 in the [100] direction to the LiFePO4/FePO4 equilibrium phase boundary width can improve the solid solubility of both the end solid solutions (LiαFePO4 and Li1−βFePO4) and decrease the miscibility gap of LiFePO4, demonstrating a highly orientation-dependent lithium miscibility gap.
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INTRODUCTION Upon charging/discharging, phase transition is constantly induced as a result of the Li compositional changes within the electrode material in a Li-ion battery. Generally, the pathway in which the new phase forms is the most critical limiting factor for achieving high-rate performance. For this reason, the phase transition behavior of LiFePO4 (LFP), a model cathode material, has long been considered as the most fundamental topic, since first reported by Padhi et al. in 1997.1 In 2006, Yamada and co-workers2 first depicted the equilibrium phase diagram at room temperature which involves a Li-poor LiαFePO4 phase region, a Li-rich Li1−βFePO4 phase region, and a miscibility gap region (two-phase region), as schematically depicted in Figure S1, Supporting Information. The solid solubility was further determined to be α = 0.05 and 1 − β = 0.89.3 Due to the very limited Li solubility between LFP and FePO4, the lithiation mechanism in (Li)FePO4 is therefore interpreted as a two-phase reaction.4−8 Specifically, the lithium insertion/extraction process preferentially undergoes a 1D transmission along the [010]Pnma direction, leading to a twophase equilibrium composed of LiαFePO4 and Li1−βFePO4 divided at the (100) plane, where the LiαFePO4: Li1−βFePO4 phase ratio changed by a moving phase boundary.2,9−12 Further in-depth studies indicate that the solid solubility as well as © 2018 American Chemical Society
miscibility gap is not well established. In fact, these values depend not only on the intrinsic properties of LFP crystals13−17 like strain,13,18 interface energy,15 and coherency strain13,17−19 but also on external factors such as current density.12 Consequently, as the host particle size decreases, the miscibility gap between the lithium-rich and lithium-poor phases shrinks significantly (i.e., the tendency for phase separation is reduced).16,20 To be exact, the above concepts are based on the thermodynamic equilibrium state. While, under the working state, the electrochemical processes operate at an overpotential, which results in a deviation from the thermodynamic equilibrium of the electrode materials.21 This deviation is of practical interest. For example, as proposed by Ceder and coworkers,22 if the applied overpotential accesses the transition barrier height, Δμb (Figure S2), LFP can bypass nucleation and growth of a second phase, allowing the system to undergo a single-phase transition. Compared with the two-phase transition, the single-phase transition can not only facilitate the transition facile but also result in a higher active population (the Received: October 26, 2017 Revised: January 17, 2018 Published: January 18, 2018 874
DOI: 10.1021/acs.chemmater.7b04463 Chem. Mater. 2018, 30, 874−878
Article
Chemistry of Materials
thin film electrode fabrication for electrochemical characterization. Interestingly, the thick [100]-LFP exhibits much smaller specific discharge capacities of 161, 136, 125, and 113 mAh g−1 at 0.1, 0.5, 1, and 5C (Figure S5), respectively, compared with the 12 nm [100]-LFP and MA-LFP. What is more, the active population calculated from the potentiostatic intermittent titration technique shows that both activation rate n and average filling speed m in the thick [100]-LFP are all much smaller than those in the [100]-oriented nanoflakes (Figure S6). The above distinction in electrochemical performance might originate from the difference in lithiation/delithiation pathway upon charge−discharge. To address this essential issue, we conducted operando XRD examinations on the three samples with different morphological features. The optical photograph of the operando XRD test platform is presented in Figure S7. The cell that sets up for the operando XRD is similar to the coin cells which are commonly used in the laboratory. A hole with a diameter of 3.7 mm is drilled through the stainless steel shell close to the LFP cathode side, and the electrode film with an Al foil current collector with a thickness of 5 μm was pasted on top of the stainless steel shell with epoxy resin glue (Figure S8). The ultrathin character of the Al foil guarantees the penetration of X-ray, enabling the detection of LFP behind the Al foil. As shown in Figure S9, most of the diffraction peaks are in good agreement with the standard olivine LFP phase (ICDD Card no. 40-1499) with a few minor peaks, which may be from the current collector or the inactive materials inside the as-assembled coin cell. According to recent operando XRD investigations, the lithiation/delithiation pathways involved in LFP highly depend on current density. At high current density, a continuous evolution of lattice parameters would occur25 or, alternatively, the appearance of a metastable phase.26 While, at low (≈0.1C) 27 and moderate (1C) 28 current density, the investigations of smaller particles demonstrated only small deviations in stoichiometry from LFP and FePO4 during electrochemical cycling. In this work, the cells for operando XRD investigation were (dis)charged at a low rate of 0.2C. The XRD patterns were acquired continuously every 30 min with a relatively short recording time period of 5 min. The galvanostatic charge/discharge cycle data with the time-points of collected XRD patterns for the 12 nm [100]-LFP/Li cell are shown in Figure 2a. Figure 2b plots the operando XRD patterns collected during nonequilibrium battery operation. From the patterns, it is striking and interesting to note that the phase evolutions for the three types of LFP are quite different even at a low current density (0.2C). Prior to charging, a well-defined peak centered at 34.7° (labeled as peak1) is evident, which corresponds to diffractions from the (211) and (020) planes of LFP. Upon charging, this characteristic peak of LFP shifts in a continuous manner for the 12 nm [100]-LFP. At the termination of charging, two peaks centered at 35.3° (peak2) and 35.9° (peak3), which are assigned to the (211) and (020) planes of the FePO4 phase, respectively, appear. On discharge, we further observed a progressive shift of these two peaks. This continuous change in peak position indicates a solid solutionfeatured phase evolution for the 12 nm [100]-LFP due to the insertion of Li+ into the FePO4 lattice. In stark contrast, the initial delithiation reaction upon charging causes only a slight shift of peak1 corresponding to (211) and (020) reflections of LFP for the thick [100]-LFP. Subsequent charging causes the
fraction of actively intercalating particles),23 which can be increased by decreasing the Δμb.24 In recent work, a high active population associated with excellent rate performance had been achieved by decreasing the dimension of LFP in the [100] direction to ∼12 nm.23 However, the lithiation/delithiation mechanism for this interesting phenomenon remains to be unraveled. In this work, we show the first direct experimental evidence that decreasing the dimension in the [100] direction can improve the solid solubility and decrease the lithium miscibility gap of LFP even at a low charge/discharge rate, shedding light on the lithiation/delithiation mechanism for the [100]-oriented LFP ultrathin nanoflakes. This is achieved by taking advantage of an operando X-ray diffraction (XRD) setup with high X-ray intensity.
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RESULTS AND DISCUSSION For comparative purposes, besides the [100]-oriented LFP ultrathin nanoflakes, we also synthesized two other kinds of LFP, i.e., microwave-assisted nanorods (designated as MALFP) and thick [100]-oriented nanoplates. These three kinds of LFP are different in size and orientation (Figure 1). Their
Figure 1. Statistical sizes of the thinnest axis (a,c,e) and TEM images (b,d,f) of 12 nm [100]-LFP (a,b), MA-LFP (c,d), and thick [100]-LFP (e,f). The insets in (b,d,f) show the corresponding diagrammatic drawings.
statistical dimensions are 12 × 134 × 280 nm (a × b × c), 26 × 26 × 52 nm (a × b × c), and 46 × 734 × 1734 nm (a × b × c), respectively, for 12 nm [100]-LFP, MA-LFP, and thick [100]LFP (Figure 1a,c,e; Figure S3; Figure S4, respectively). The characteristics of the three samples investigated in this work are sumarized in Table S1. The three samples followed the same procedures as those described in ref 23 for carbon coating and 875
DOI: 10.1021/acs.chemmater.7b04463 Chem. Mater. 2018, 30, 874−878
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Chemistry of Materials
Figure 2. (a) Charge/discharge galvanostatic data at 0.2C with the time-points of collected XRD spectra labeled for the 12 nm [100]-LFP. (b) Operando XRD measurements of coin cells with the 12 nm [100]-LFP, MA-LFP, and thick [100]-LFP as cathode materials. The phase transition for the three types of LFP is quite different. For the 12 nm [100]-LFP, a continuous shift of peak1 happens, implying this transition is more toward the single-phase transformation. As for the two others, the appearance of peak2 and peak3 indicates the formation of FePO4, which is quite compatible with the well-known two-phase reaction.
Figure 3. Unit-cell parameters of (a) a, (b) b, and (c) V as a function of x lithium in the 12 nm [100]-LFP, MA-LFP, and thick [100]-LFP. Regardless of whether it is during charging or discharging, 12 nm [100]-LFP showed a much greater change of lattice parameters than those of the MA-LFP, implying a larger solid solubility and smaller lithium miscibility gap for the 12 nm [100]-LFP.
stage of charging before the phase separation occurs,29 the solubility β can be quantitatively assessed by the change extent of V according to the equation
appearance of peak2 and peak3 corresponding to FePO4. This transition feature is quite compatible with the well-known twophase reaction in which two-phase (LFP/FePO4) transition is involved. In the case of the MA-LFP, the peak1 of LFP shifts more than that of the thick [100]-LFP. However, the appearance of the peak2 and peak3 at 35.3° and 35.9° during charging suggests the generation of the FePO4 phase. Based on a careful analysis, the phase transition characteristics of the MALFP fall in between the 12 nm [100]-LFP and the thick [100]LFP. To get deeper insight into the lithiation/delithiation mechanism upon charge/discharge, we indexed the entire collected XRD patterns. The results of such fits (Figure 3; Figure S10) indicate definitely that the lattice parameter variations of the three samples are quite different. On charging, the unit-cell parameters of a, b, and V decrease, while c increases gradually as the Li-ion is extracted from the 12 nm [100]-LFP, which supports the solid solution mechanism.29 In contrast, these parameters decrease or increase only in the initial stage of charging for the thick [100]-LFP and MA-LFP. The gradual decrease of a and b in the early charging stage implies a continuous Li-ion extraction from the LFP, and the later unchanged period demonstrates the saturation of the solid solution for the LFP phase. Since the unit-cell volume V decreases gradually with the Li+ extraction process at the early
β=
VLFP − Vβ VLFP − VFP
(1)
where VLFP is the unit-cell volume of LFP, VFP is the unit-cell volume of FePO4, and Vβ is the minimum unit-cell volume of Li1−βFePO4 before the phase separation. The β values for 12 nm [100]-LFP, MA-LFP, and thick [100]-LFP are calculated to be 0.22, 0.09, and 0.04, respectively. On discharge, the unit-cell parameters of a, b, and V increase, while c decreases initially for all three samples, followed by increases at different fractions of inserted Li+. Thus, variable amounts of Li solid solution occur for the three different samples. Once the solid solution limit is reached, the subsequent insertion of Li-ion would give rise to the formation of LFP, following a phase-separation mechanism. Similar to the charging process, the solubility α during discharging can be quantitatively assessed by the change extent of V according to the equation α= 876
Vα − VFP VLFP − VFP
(2) DOI: 10.1021/acs.chemmater.7b04463 Chem. Mater. 2018, 30, 874−878
Article
Chemistry of Materials where Vα is the maximum unit-cell volume of LiαFePO4 before the phase separation takes place. The α values for 12 nm [100]LFP, MA-LFP, and thick [100]-LFP are determined to be 0.21, 0.13, and 0.13, respectively. Accompanying the evolution of lattice parameters, on discharge, the phase evolution for the three kinds of LFP displays similar behaviors as during the charging process. However, the diffraction peak intensity changed by different amounts, as did the full width at half-maximum (fwhm), especially for the 12 nm [100]-LFP sample. The changes in fwhm for the three samples in this study are shown in Figure S11. As one can see, no matter whether it is during charging or discharging, the increase in fwhm for the 12 nm [100]-LFP is much more remarkable than those of the two counterparts. This demonstrates a greater tendency for the transition to a single-phase solid to occur, even at a low galvanostatic current density. The present work confirms the computational assumption20 that the miscibility gap and spinodal gap both decrease as the particle size is decreased to the scale of the diffuse interphase thickness. As seen in Figure 3, no matter whether it is during charging or discharging, 12 nm [100]-LFP shows a much bigger change in lattice parameters than those of the MA-LFP, implying a bigger solid solubility range and a smaller miscibility gap for the 12 nm [100]-LFP (Figure 4). It should be noted that, although
gap. Considering that the 12 nm [100]-LFP and MA-LFP have comparable specific surface areas, the improved solid solubility and decreased miscibility gap for the 12 nm [100]-LFP is reasonably attributed to the decreased dimension in the [100] direction which is close to the equilibrium phase boundary width17 (around 12 nm). Because of the smaller changes of lattice parameters for the thick [100]-LFP compared to those of the 12 nm [100]-LFP and MA-LFP samples (Figure 3), it shows that not all the [100]-oriented LFP have a small miscibility gap. However, decreasing the dimension in the [100] direction can improve the solid solubility and shrink the lithium miscibility gap of LFP.
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CONCLUSION
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ASSOCIATED CONTENT
In summary, the phase transition mechanism and miscibility gap in LFP with different sizes and orientations during nonequilibrium battery operation which involves an overpotential were investigated in an operando manner. It is demonstrated that the phase transition paths are not strictly single- or two-phase insertion processes but are highly orientation-dependent even at a low charge/discharge rate. This work presents the first direct experimental evidence that decreasing the dimension in the [100] direction can remarkably improve the solid solubility of both the end solid solutions (LiαFePO4 and Li1−βFePO4) and decrease the lithium miscibility gap of LFP. The identification of an orientationdependent miscibility gap enriches the understanding of the transition mechanism in LFP and highlights the importance of orientation-controlled crystal synthesis to enable high-rate charge and discharge, rather than focusing solely on decreasing the size in the [010] direction along which lithium undergoes a curved one-dimensional chain motion31 by cooperative filling of the (bc)Pnma plane.32 S Supporting Information *
Figure 4. Schematic derivation of the FePO4−LFP binary phase diagram of the 12 nm [100]-LFP, MA-LFP. and thick [100]-LFP, which indicates an orientation-dependent lithium miscibility gap in LFP.
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04463. Experimental details and additional characterization data (PDF)
the 12 nm [100]-LFP sample has a decreased miscibility gap, the voltage is very stable over nearly the entire range of Li extraction or insertion (Figure 2a). This is because in a battery there are many individual particles that are connected to each other, which have the same chemical potential in equilibrium.30 When some of the particles arrive at the maximum potential point A (Figure S12), the other particles reach the same potential through polarization. From then on, a stable voltage will keep up until all the particles reach the single lithium-poor phase state at the point B (Figure S12). These processes are involved nearly in the entire range of Li extraction. As a result, although the 12 nm [100]-LFP sample has a decreased miscibility gap, it remains a longer voltage stable region than the miscibility gap due to the multiple-particle nature in the electrode. As predicted by Burch and Bazant,20 the miscibility gap decreases and eventually disappears as the particle size is decreased to the scale of the diffuse interphase thickness. The present work, by means of real-time tracking of phase transition using time-resolved XRD, confirms the early prediction. In fact, the phase transition processes for the three kinds of materials are not strictly single- or two-phase insertion processes but differ in the extent of solid solution and width of the miscibility
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Kongjun Zhu: 0000-0003-0804-8044 Xiaohui Wang: 0000-0001-7271-2662 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the Youth Innovation Promotion Association, Chinese Academy of Sciences (CAS) under grant No. 2011152, and Shenyang National Laboratory for Materials Science, Institute of Metal Research, CAS and by the Special Program for Applied Research on Super Computation of the NSFC-Guangdong Joint Fund (the second phase) under Grant No. U1501501. 877
DOI: 10.1021/acs.chemmater.7b04463 Chem. Mater. 2018, 30, 874−878
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Chemistry of Materials
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