[100]-Oriented LiFePO4 Nanoflakes toward High Rate Li-Ion Battery

Subscriber access provided by Lulea University of Technology

Communication 4

[100]-Oriented LiFePO Nanoflakes toward High Rate Li-Ion Battery Cathode Zhaojin Li, Zhenzhen Peng, Hui Zhang, Tao Hu, Minmin Hu, Kongjun Zhu, and Xiaohui Wang Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b04855 • Publication Date (Web): 22 Dec 2015 Downloaded from http://pubs.acs.org on December 27, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Nano Letters is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

[100]-Oriented LiFePO4 Nanoflakes toward High Rate Li-Ion Battery Cathode Zhaojin Li,†,‡ Zhenzhen Peng,†,‡ Hui Zhang,†,‡ Tao Hu,†,‡ Minmin Hu,†,‡ Kongjun Zhu,§ and Xiaohui Wang*,† †

Shenyang National Laboratory for Materials Science, Institute of Metal Research,

Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, China. ‡

University of Chinese Academy of Sciences, Beijing 100039, China.

§

State Key Laboratory of Mechanics and Control of Mechanical Structures, Nanjing

University of Aeronautics and Astronautics, Nanjing 210016, China. KEYWORDS: Cathode materials, LiFePO4, solvothermal, crystal orientation, Li-ion battery

ABSTRACT: [100] 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. Here we report the fabrication of 12-nm-thick [100]-oriented LiFePO4 nanoflakes by a simple one-pot solvothermal method. The nanoflakes exhibit unexpectedly excellent electrochemical performance, in stark contrast to what was previously believed. Such an exceptional result is attributed to a decreased thermodynamic transformation barrier height (∆µb) associated with increased active population.

1 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 23

Finding ways to improve the electrochemical performance of cathode materials for Li-ion batteries is a crucial issue in chemical energy storage,1–5 and has strongly motivated research efforts toward understanding the lithium (de)intercalation mechanism. Much attention has been directed to the representative cathode material, olive-structured LiFePO4 (LFP),4,6–14 because of its high energy density, excellent thermal stability, environmental friendliness, and low raw-materials cost. In order to understand the lithium (de)intercalation, several representative models6,15–19 such as the core-shell,6,20 mosaic,21 domino cascade,22 and single-phase model17,23 have been proposed. Despite these disputes on lithium (de)intercalation, the prevailing belief is that Li+ diffuses along the [010] direction.24–26 The [100] direction, in contrast, is believed to be an extremely difficult diffusion direction for Li+, due to the prohibitively high Li+ migration energy.25 In accordance with this belief, [100]-oriented LFP always exhibits poor electrochemical performance including low discharge capacity and a short cycle life compared with its counterpart of [010]-oriented LFP.27–29 Therefore, few efforts have been devoted to the synthesis of [100]-oriented LFP. It is noteworthy that the above views are all based on single-particle (de)intercalation. In fact, LFP cathodes in practical batteries are composed of ensembles of particles.30 Chueh and co-workers30 noted that the current density, or current per active internal surface area, is nearly invariant with the global electrode cycling rate. According to this thought-provoking argument, the electrochemical performance of cathode materials is ultimately determined by the fraction of actively intercalating particles, 2 ACS Paragon Plus Environment

Page 3 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

which can be increased by decreasing the transformation barrier height (∆µb) in the LFP.30,31 Reduction of ∆µb can be achieved by increasing the coherency strain energy because the increased coherency strain energy facilitates the single-phase transformation,32–34 which requires a lower ∆µb.17 Coherency strain originates from the difference in lattice parameters between isostructural FePO4 and LFP involved in the two-phase transformation. As for the location of coherency strain, it has been pointed out that a phase boundary of thickness λ between full and empty channels in the two-phase transformation process would form along the [100] direction.22,35–37 Therefore, the strategy used to increase the coherency strain depends on the following process: simultaneously increasing the area of (100) lattice plane and decreasing the crystal size in the [100] direction. Furthermore, the equilibrium phase boundary width simulated with a reaction-limited phase-field model is about 12 nm,32 which is in agreement with the width of 12–15 nm measured by STEM/EELS.36 A single-phase transformation associated with sufficiently low ∆µb can thus be expected if one can control the dimension in the [100] direction of LFP crystal to keep it close to the equilibrium phase boundary width (around 12 nm). Consequently, [100]-oriented LFP platelets, being thin enough, may exhibit extremely low ∆µb and a large active population. Herein, we report a simple yet efficient strategy to synthesize ultrathin [100]-oriented LFP nanoflakes at large scale by a solvothermal approach via creating an extreme environment that is as anhydrous as possible. In spite of the [100] orientation, the cathode comprising the ultrathin nanoflakes exhibits extremely low 3 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 23

∆µb, a large active population, and hence an unexpectedly excellent electrochemical performance. This work offers substantial experimental support for the recently proposed theory wherein improving the electrochemical performance of cathode materials relies on increasing the fraction of active particles,30 and

the work also

demonstrates a new way to novel nanostructures for high-performance LFP. Experimentally, [100]-oriented LFP platelets can be synthesized in an acid environment.27,29,38 Besides, sufficiently small LFP crystallites can be expected by reducing the water content in the solvothermal reaction system.38 Therefore, the synthetic strategy employed in this work is to create an acid environment that is as waterless as possible in order to obtain [100]-oriented LFP crystallites with the desired nanostructure. To achieve this goal, we took two measures against water. First, anhydrous Li3PO4 was used to replace LiOH⋅H2O and H3PO4 (85%) as raw materials. Second, for the first time, the frequently used FeSO4⋅7H2O was dehydrated under vacuum into FeSO4⋅H2O, as confirmed by powder X-ray diffraction (XRD) (Supporting

Information

(SI)

Figure

S1)

and

simultaneous

thermogravimetry/differential scanning calorimetry (SI Figure S2) tests. The 500-µm-sized particles became porous due to the release of crystal water upon dehydration (SI Figure S3.a–d). The fabrication of ultrathin LFP nanoflakes was implemented by a simple one-pot solvothermal method in a home-made pure titanium autoclave. By keeping the suspension with Li3PO4, H3PO4 (85%), and FeSO4⋅H2O at 180 °C for 1 h (monitored by a Pt100 temperature sensor; SI Figure S4), ultrathin LFP nanoflakes were obtained 4 ACS Paragon Plus Environment

Page 5 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(see Methods in the SI for further details). By adopting an in situ temperature measuring device, the reaction path for LFP is revealed in the combination of the evolution of the phase and morphology (see SI S5 for further details). Figure 1a shows the XRD pattern of the as-synthesized powder. It can be indexed with the orthorhombic structure of olivine LFP (space group: Pnma, JCPDS No. 83−2092). The as-prepared LFP exhibits staggered structures composed of ultrathin rectangular nanoflakes (Figure 1c,d; SI Figure S10). To determine the crystallite orientation of LFP, the nanoflakes were first ultrasonically dispersed in ethanol and then dried on an amorphous silicon substrate, as schematically illustrated in Figure 1b. In contrast to the as-synthesized powder with random orientation, the dispersed sample exhibits a strong [100] texture, which is apparently indicated by the dramatic increase of I(200)/I(020) from 0.2 in the as-synthesized powder to 2.4 in the dispersed sample. The degree of [100] texture was quantified using the Lotgering factor,39 f, which is defined by

f = ( p − p0 ) (1 − p0 ) where p = ∑ I h 00

∑I

hkl

for the dispersed sample and p0 = ∑ I h00

(1)

∑I

hkl

for the

as-synthesized powder. The Lotgering factor value for the dispersed sample is as high as 32%. The strong [100] texture in the dispersed sample implies that the LFP particles have a [100] orientation, which is further confirmed by selected-area electron diffraction (SAED) combined with morphological analysis (Figure 1d,e). Statistically, the nanoflakes have average dimensions of 12 × 134 × 280 nm (a × b × c) (Figure 1f–


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 23

h), endowing them with a high Brunauer–Emmett–Teller (BET) specific surface area of 33.0 m2 g–1 (SI Figure S11). In order to increase the electronic conductivity of LFP, the as-synthesized LFP powders were carbon-coated by pyrolysis of sucrose to obtain an LFP/C composite (see Methods in the SI for further details), wherein the morphology of nanoflakes with the (100) facets exposed is well preserved (Figure 2a,b). With the formation of an amorphous carbon coating (3 nm in thickness, Figure 2c), the BET specific surface area of the LFP/C composite increases somewhat to 46.4 m2 g–1 (SI Figure S12). The lithium-ion migration in LFP is determined not only by the particle size and diffusion pathway but also by the point defect concentration.40,41

FeLi•

antisite defects

are innately formed in LFP prepared by a hydrothermal/solvothermal method at low temperatures, forming a barrier to the migration of Li+. According to a previous report by Qin et al.,42 the asymmetric stretching P−O vibration peak of the PO4 tetrahedron in defect-free LFP is located at 957 cm−1. As shown in Figure 2d, the infrared absorption band corresponding to the asymmetric stretching P−O vibration mode is located at 951 and 954 cm–1 for LFP and LFP/C, respectively, indicating low defect concentrations in both the as-synthesized LFP and the carbon-coated LFP/C. Figure 3a shows the charge/discharge curves of the electrode comprising [100]-oriented LFP/C from 0.1 C (1 C = 170 mA g–1) to 20 C between 2.3 and 4.2 V at 25 °C. The electrode exhibits an impressive rate capability with discharge capacities of 164, 159, 155, 144, 135, and 122 mAh g–1 at 0.1, 0.5, 1, 5, 10, and 20 C, respectively. The exceptional rate performance seems to contradict the previously 6 ACS Paragon Plus Environment

Page 7 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

held belief that [100]-oriented LFP is doomed to poor electrochemical performance by the prohibitively higher Li+ migration energy required for the pathway along [100] compared to the [010] channel.25 For instance, micrometer-sized LFP with the [100] orientation exhibits specific discharge capacities of 148, 120, 109, 90, 70, 58, and 42 mAh g–1 at 0.1, 0.5, 1, 2, 5, 10, and 20 C, respectively (Figure 3b).38 Huang et al.27 reported that [100]-oriented LFP with dimensions of 25 × 150 × 250 nm (a × b × c) has a specific discharge capacity of 82 mAh g–1 at 10 C (Figure 3b). In the present study, [100]-oriented LFP with ultrashort length along the a axis (12 nm) exhibits a much higher capacity of 135 mAh g–1. In addition to the ultrathin feature in the [100] direction, the [100]-oriented LFP crystallites in other directions ([010] and [001]) have dimensions of hundreds of nanometers. To unambiguously identify the cause of the excellent electrochemical performance of the 12-nm thickness of [100]-oriented LFP, we designed and synthesized nano-sized LFP with an increased dimension in the [100] direction but a reduced dimension in the [010] direction by a microwave-assisted (MA) hydrothermal synthesis approach (see Methods in the SI for further details). The MA-synthesized LFP (denoted as MA-LFP) has dimensions of about 26 × 26 × 52 nm (a × b × c) (SI Figure S13). It has a BET specific surface area of 36.1 m2 g–1, which is comparable to that of the [100]-oriented LFP (SI Figure S14). Carbon coating and thin film electrode fabrication for both samples with comparable specific surface areas were conducted by the same procedures to exclude the influence of other factors as much as possible. Under identical electrochemical test conditions, remarkably, MA-LFP exhibits much 7 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 23

smaller specific discharge capacities of 156, 138, 131, 110, 99, and 86 mAh g–1 at 0.1, 0.5, 1, 5, 10, and 20 C, respectively (Figure 3c). Since MA-LFP has reduced sizes in the [010] and [001] directions but increased size in the [100] direction as compared with the [100]-oriented ultrathin LFP, the remarkably improved rate performance of the [100]-oriented ultrathin LFP cannot be attributed to the decreased dimension in the [010] or [001] direction. In addition, the nano-size effect can be excluded because these two samples have comparable specific surface areas. Based on the above discussion, it is therefore believed that decreasing the dimension to ∼12 nm in the [100] direction is the cause of the excellent rate performance of the [100]-oriented ultrathin LFP. The [100]-oriented ultrathin LFP has also a striking feature, i.e., its voltage gap between the galvanostatic charge/discharge hysteresis is observably narrower than that for the MA-LFP (Figure 3d,e). According to Chueh and co-workers30, the voltage gap is correlated with active population, which is discussed in detail in the following. More interestingly, the ultrathin [100]-oriented LFP shows excellent cycling stability. As presented in Figure 3f, a capacity of 90% remains even after 1000 cycles at 10 C. In comparison, MA-LFP with a larger size along the a axis retains a capacity of only 70%. Therefore, the improved cycling stability of the [100]-oriented sample can most likely be attributed to the ultra-small thickness (∼10 nm) along the a axis. Heretofore, high-performance LFP has always been achieved by shortening the lithium-ion diffusion pathway along the b axis ([010] orientation).28,40,43 However, in this study, it is demonstrated that LFP with excellent performance can also be 8 ACS Paragon Plus Environment

Page 9 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

obtained by shortening the prohibitive lithium-ion diffusion pathway along the a axis to ∼10 nm. To understand this interesting phenomenon, it is instructive to inspect the voltage gap44 resulting from the galvanostatic charge/discharge hysteresis first. As proposed by Ceder and co-workers,17 the hysteresis emanates from the non-monotone lithium diffusional chemical potential of µLi in a single crystal, which contains a transformation barrier ∆µb (Figure 4c).30 The value of ∆µb scales with the voltage gap. Consequently, the smaller the gap is, the lower the ∆µb is and the larger the active population becomes.30 For LFP, a voltage gap of about 40 mV has been observed by Dreyer et al.44 at a rate of C/20. As the current approaches zero (C/1000), the gap narrows to about 20 mV. In this study, three types of LFP crystallites, that is, [100]-oriented nanoflakes, MA-LFP, and [010]-oriented LFP38 were synthesized (see Methods for these three materials in the SI for further details). These crystallites have dimensions of about 12 × 134 × 280 nm, 26 × 26 × 52 nm, and 100 × 80 × 250 nm (a × b × c), respectively (Figure 4a). When cycled at a progressively decreasing rate of charge/discharge current down to C/100, the decreasing rate results in a decrease in the voltage gap between the charge and discharge curves. For the [100]-oriented nanoflakes in this study, the voltage gap is only 9 mV at a slow rate of C/100. This value is observably smaller than those of for [010]-oriented LFP and MA-LFP (Figure 4b). These two samples are characterized by having an increased dimension in the [100] direction but reduced dimension in the [010] direction. Given that [100]-oriented nanoflakes with the smallest dimension in the [100] direction exhibit the best rate performance among 9 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 23

the three samples investigated in this study, the superior performance can be reasonably attributed to the ultrathin feature in the [100] direction rather than a reduced dimension in the [010] direction. It is therefore believed that the ultrathin feature in the [100] direction gives rise to a significant decrease in ∆µb (Figure 4c) and, as a result, an increased active population accounting for the superior rate performance. In addition to the voltage hysteresis experiments, the increased active population is also supported experimentally by the potentiostatic intermittent titration technique _ (PITT). According to Bai and Tian,45 the activation rate n (the probability of formation of active particles per untransformed particle per unit time) and average _ filling speed m (the probability of finding a fully transformed particle per active nanoparticle per unit time) can be calculated from PITT experiments by a simple mathematical model. Figure 4d depicts the fitting results of the [100]-oriented nanoflakes and MA-LFP electrodes. No matter what the voltage step was, that is, 10 _ _ mV (from 3.42 to 3.41 V) or 150 mV (from 3.5 to 3.35 V), both n and m in the [100]-oriented nanoflake electrode are all significantly larger than those in the MA-LFP electrode. Such results indicate a higher percentage of active population for the [100]-oriented nanoflake electrode. As discussed above, the decreased ∆µb associated with the increased active population originates, in essence, from the ultrathin structural feature along the [100] orientation. This feature increases the coherency strain energy for LFP, as the two-phase reaction takes place on the bc plane,22,35–37 and the [100]-oriented 10 ACS Paragon Plus Environment

Page 11 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

nanoflakes in this work with the (100) facets exposed have a larger bc cross-sectional area.

The

increased

coherency

strain

energy

facilitates

the

single-phase

transformation,32–34 which requires a lower ∆µb.17 Consequently, the ultrathin [100]-oriented structural feature with increased coherency strain energy gives rise to a low ∆µb. In summary, we have explored a simple, one-pot, template-free way to synthesize [100]-oriented LFP nanoflakes as thin as 12 nm. With the [100] orientation well preserved, the LFP/C composite delivers a discharge capacity of 122 mAh g–1 at a current rate of 20 C and retains a capacity of 90% after 1000 cycles at 10 C. The surprising electrochemical performance defies the previously widely held belief that [100]-oriented LFP is synonymous with ‘poor performance’ because of the extremely unfavorable

lithium-ion

transmission

along

[100]

axis.

The

exceptional

electrochemical performance derives from the decreased transformation barrier height associated with increased active population, which can improve the electrode’s cycle life because it is directly related to the local current density. This work offers the new idea that decreasing the Li+ diffusion direction along the b axis is not the only way to design high-performance LFP cathode materials; rather, they can also be obtained by reducing the distance along the a axis to ∼10 nm through increasing the active population.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 23

Figure Legends

Figure 1. (a) XRD patterns of the as-synthesized particles and those first dispersed in ethanol and then slowly dried on an amorphous silicon substrate. (b) Schematic illustration of the as-synthesized LFP nanoflakes and the dispersed ones. (c, d) Transmission electron microscopy (TEM) images of LFP nanoflakes. (e) The SAED pattern corresponding to (d). Statistics of the sizes along (f) the a axis, with an average size of 12 nm, (g) the b axis, with an average size of 134 nm, and (h) the c axis, with an average size of 280 nm, using Gaussian fitting.


Page 13 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 2. (a) Scanning electron microscopy image of the as-prepared LFP/C composite. (b) TEM morphology. The corresponding SAED pattern in the inset indicates that the predominantly exposed facet is (100). (c) High-resolution TEM image. (d) Fourier transform infrared spectra of the as-synthesized LFP nanoflakes and LFP/C composite.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 23

Figure 3. (a) Typical charge/discharge profiles of the synthesized [100]-oriented LFP/C at current rates of 0.1, 0.5, 1, 5, 10, and 20 C. The electrode delivers specific capacities of 164 and 122 mAh g–1 at 0.1 and 20 C, respectively. (b) Comparison of the specific discharge capacities of [100]-oriented LFP/C composite prepared via the methods presented in this work (12 nm along the a axis), in Ref. 38 (micrometer-scale), and in Ref. 27 (25 nm along the a axis). (c) Typical charge/discharge profiles of the MA-synthesized LFP/C at current rates of 0.1, 0.5, 1, 5, 10, and 20 C. (d) Definition of a voltage gap on the example of typical galvanostatic charge/discharge curves for [100]-oriented LFP at a current rate of 1 C. (e) Voltage gaps of [100]-oriented and MA-synthesized LFP/C at current rates of 1, 5, 10, and 20 C. (f) Cyclability of the synthesized [100]-oriented and MA-synthesized LFP/C at a current rate of 10 C. 14 ACS Paragon Plus Environment

Page 15 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 4. (a) Diagrammatic drawing of ultrathin [100]-oriented, MA-synthesized and [010]-oriented LFP. (b) Voltage gap resulting from hysteresis at different charge/discharge currents in the range from C/2 to C/100 for [100]-oriented and MA-synthesized and [010]-oriented LFP/C electrodes. (c) Chemical potential of Li as a function of the particle’s lithiation fraction. µLi,LFP contains a transformation barrier (∆µb), defined as the difference between the local maxima and the chemical potential at the center of the miscibility gap. (d) Fitting results of the PITT experimental data of ultrathin [100]-oriented and MA-synthesized LFP. The coefficients of determination for [100]-oriented LFP are R2 = 0.9928 at a step of 10 mV and R2 = 0.9892 at a step of 150 mV, while those for MA-synthesized LFP are R2 = 0.9507 at a step of 10 mV and R2 = 0.9386 at a step of 150 mV.

ASSOCIATED CONTENT


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 23

Supporting Information

Supporting Information Available: materials, methods, specimen preparations and characterizations. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author * Email: [email protected] (X.H.W.)

Notes The authors declare no competing financial interests.

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.

REFERENCES

(1) Tarascon, J. M.; Armand, M. Nature 2001, 414, 359–367. (2) Armand, M.; Tarascon, J. M. Nature 2008, 451, 652–657. (3) Goodenough, J. B.; Kim, Y. Chem. Mater. 2010, 22, 587–603. (4) Scrosati, B.; Garche, J. J. Power Sources 2010, 195, 2419–2430. (5) Etacheri, V.; Marom, R.; Elazari, R.; Salitra, G.; Aurbach, D. Energy Environ. Sci. 2011, 4, 3243– 3262. (6) Padhi, A. K.; Nanjundaswamy, K. S.; Goodenough, J. B. J. Electrochem. Soc. 1997, 144, 1188– 1194. (7) Kang, B.; Ceder, G. Nature 2009, 458, 190–193. (8) Chung, S. Y.; Bloking, J. T.; Chiang, Y. M. Nat. Mater. 2002, 1, 123–128. (9) Yamada, A.; Chung, S. C.; Hinokuma, K. J. Electrochem. Soc. 2001, 148, A224–A229.


Page 17 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

(10) Delacourt, C.; Poizot, P.; Tarascon, J. M.; Masquelier, C. Nat. Mater. 2005, 4, 254–260. (11) Wang, J.; Sun, X. Energy Environ. Sci. 2015, 8, 1110–1138. (12) Wagemaker, M.; Singh, D. P.; Borghols, W. J. H.; Lafont, U.; Haverkate, L.; Peterson, V. K.; Mulder, F. M. J. Am. Chem. Soc. 2011, 133, 10222–10228. (13) Liu, X.; Wang, D.; Liu, G.; Srinivasan, V.; Liu, Z.; Hussain, Z.; Yang, W. Nat. Commun. 2013, 4, 2568–2575. (14) Ohmer, N.; Fenk, B.; Samuelis, D.; Chen, C. C.; Maier, J.; Weigand, M.; Goering, E.; Schutz, G. Nat. Commun. 2015, 6, 6045–6051. (15) Han, B. C.; Van der Ven, A.; Morgan, D.; Ceder, G. Electrochim. Acta 2004, 49, 4691–4699. (16) Bai, P.; Cogswell, D. A.; Bazant, M. Z. Nano Lett. 2011, 11, 4890–4896. (17) Malik, R.; Zhou, F.; Ceder, G. Nat. Mater. 2011, 10, 587–590. (18) Ferguson, T. R.; Bazant, M. Z. J. Electrochem. Soc. 2012, 159, A1967–A1985. (19) Oyama, G.; Yamada, Y.; Natsui, R.; Nishimura, S.; Yamada, A. J. Phys. Chem. C 2012, 116, 7306– 7311. (20) Srinivasan, V.; Newman, J. J. Electrochem. Soc. 2004, 151, A1517–A1529. (21) Andersson, A. S.; Thomas, J. O. J. Power Sources 2001, 97-98, 498–502. (22) Delmas, C.; Maccario, M.; Croguennec, L.; Le Cras, F.; Weill, F. Nat. Mater. 2008, 7, 665–671. (23) Malik, R.; Abdellahi, A.; Ceder, G. J. Electrochem. Soc. 2013, 160, A3179–A3197. (24) Islam, M. S.; Driscoll, D. J.; Fisher, C. A. J.; Slater, P. R. Chem. Mater. 2005, 17, 5085–5092. (25) Morgan, D.; Van der Ven, A.; Ceder, G. Electrochem. Solid-State Lett. 2004, 7, A30–A32. (26) Nishimura, S.; Kobayashi, G.; Ohoyama, K.; Kanno, R.; Yashima, M.; Yamada, A. Nat. Mater. 2008, 7, 707–711. (27) Huang, X.; He, X.; Jiang, C.; Tian, G. RSC Adv. 2014, 4, 56074–56083. (28) Wang, L.; He, X.; Sun, W.; Wang, J.; Li, Y.; Fan, S. Nano Lett. 2012, 12, 5632–5636. (29) Dokko, K.; Koizumi, S.; Nakano, H.; Kanamura, K. J. Mater. Chem. 2007, 17, 4803–4810. (30) Li, Y.; El Gabaly, F.; Ferguson, T. R.; Smith, R. B.; Bartelt, N. C.; Sugar, J. D.; Fenton, K. R.; Cogswell, D. A.; Kilcoyne, A. L.; Tyliszczak, T.; Bazant, M. Z.; Chueh, W. C. Nat. Mater. 2014, 13, 1149–1156. (31) Wang, J.; Chen-Wiegart, Y. K.; Wang, J. Nat. Commun. 2014, 5, 4570–4579. (32) Cogswell, D. A.; Bazant, M. Z. ACS Nano 2012, 6, 2215–2225. (33) Burch, D.; Bazant, M. Z. Nano Lett. 2009, 9, 3795–3800. (34) Wagemaker, M.; Mulder, F. M.; Van der Ven, A. Adv. Mater. 2009, 21, 2703–2709. (35) Chen, G. Y.; Song, X. Y.; Richardson, T. J. Electrochem. Solid-State Lett. 2006, 9, A295–A298. (36) Laffont, L.; Delacourt, C.; Gibot, P.; Wu, M. Y.; Kooyman, P.; Masquelier, C.; Tarascon, J. M. Chem. Mater. 2006, 18, 5520–5529. (37) Van der Ven, A.; Garikipati, K.; Kim, S.; Wagemaker, M. J. Electrochem. Soc. 2009, 156, A949– A957. (38) Li, Z. J.; Zhu, K. J.; Lia, J. L.; Wang, X. H. Crystengcomm 2014, 16, 10112–10122. (39) Loygering, F. K. J. Inorg. Nucl. Chem. 1959, 9, 113–123. (40) Malik, R.; Burch, D.; Bazant, M.; Ceder, G. Nano Lett. 2010, 10, 4123–4127. (41) Axmann, P.; Stinner, C.; Wohlfahrt-Mehrens, M.; Mauger, A.; Gendron, F.; Julien, C. M. Chem. Mater. 2009, 21, 1636–1644. (42) Qin, X.; Wang, J.; Xie, J.; Li, F.; Wen, L.; Wang, X. Phys. Chem. Chem. Phys. 2012, 14, 2669– 2677.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 23

(43) Zhao, Y.; Peng, L.; Liu, B.; Yu, G. Nano Lett. 2014, 14, 2849–2853. (44) Dreyer, W.; Jamnik, J.; Guhlke, C.; Huth, R.; Moskon, J.; Gaberscek, M. Nat. Mater. 2010, 9, 448– 453. (45) Bai, P.; Tian, G. Electrochim. Acta 2013, 89, 644–651.


Page 19 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Table of Contents

Twelve-nanometer-thick [100]-oriented LiFePO4 nanoflakes exhibit unexpectedly excellent electrochemical performance, in stark contrast to the widely held belief that [100] is a difficult diffusion direction for Li+.


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 1. (a) XRD patterns of the as-synthesized particles and those first dispersed in ethanol and then slowly dried on an amorphous silicon substrate. (b) Schematic illustration of the as-synthesized LFP nanoflakes and the dispersed ones. (c, d) Transmission electron microscopy (TEM) images of LFP nanoflakes. (e) The SAED pattern corresponding to (d). Statistics of the sizes along (f) the a axis, with an average size of 12 nm, (g) the b axis, with an average size of 134 nm, and (h) the c axis, with an average size of 280 nm, using Gaussian fitting. 114x127mm (300 x 300 DPI)

ACS Paragon Plus Environment

Page 20 of 23

Page 21 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 2. (a) Scanning electron microscopy image of the as-prepared LFP/C composite. (b) TEM morphology. The corresponding SAED pattern in the inset indicates that the predominantly exposed facet is (100). (c) High-resolution TEM image. (d) Fourier transform infrared spectra of the as-synthesized LFP nanoflakes and LFP/C composite. 119x87mm (300 x 300 DPI)


Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 3. (a) Typical charge/discharge profiles of the synthesized [100]-oriented LFP/C at current rates of 0.1, 0.5, 1, 5, 10, and 20 C. The electrode delivers specific capacities of 164 and 122 mAh g–1 at 0.1 and 20 C, respectively. (b) Comparison of the specific discharge capacities of [100]-oriented LFP/C composite prepared via the methods presented in this work (12 nm along the a axis), in Ref. 38 (micrometer-scale), and in Ref. 27 (25 nm along the a axis). (c) Typical charge/discharge profiles of the MA-synthesized LFP/C at current rates of 0.1, 0.5, 1, 5, 10, and 20 C. (d) Definition of a voltage gap on the example of typical galvanostatic charge/discharge curves for [100]-oriented LFP at a current rate of 1 C. (e) Voltage gaps of [100]-oriented and MA-synthesized LFP/C at current rates of 1, 5, 10, and 20 C. (f) Cyclability of the synthesized [100]-oriented and MA-synthesized LFP/C at a current rate of 10 C. 123x119mm (300 x 300 DPI)


Page 22 of 23

Page 23 of 23

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

Figure 4. (a) Diagrammatic drawing of ultrathin [100]-oriented, MA-synthesized and [010]-oriented LFP. (b) Voltage gap resulting from hysteresis at different charge/discharge currents in the range from C/2 to C/100 for [100]-oriented and MA-synthesized and [010]-oriented LFP/C electrodes. (c) Chemical potential of Li as a function of the particle’s lithiation fraction. µLi,LFP contains a transformation barrier (∆µb), defined as the difference between the local maxima and the chemical potential at the center of the miscibility gap. (d) Fitting results of the PITT experimental data of ultrathin [100]-oriented and MA-synthesized LFP. The coefficients of determination for [100]-oriented LFP are R2 = 0.9928 at a step of 10 mV and R2 = 0.9892 at a step of 150 mV, while those for MA-synthesized LFP are R2 = 0.9507 at a step of 10 mV and R2 = 0.9386 at a step of 150 mV. 119x86mm (300 x 300 DPI)


[100]-Oriented LiFePO4 Nanoflakes toward High Rate Li-Ion Battery

Recommend Documents