- Email: [email protected]
A novel Li3P-VP nanocomposite fabricated by pulsed laser deposition as anode material for high-capacity lithium ion batteries
Journal of Electroanalytical Chemistry 841 (2019) 21–25
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Short communication
A novel Li3P-VP nanocomposite fabricated by pulsed laser deposition as anode material for high-capacity lithium ion batteries
T
Hailong Wua, Kaiyuan Weia, Binghua Tanga, Yixiu Cuia, Yu Zhaoa, , Mingzhe Xueb, , Chilin Lic, ⁎ Yanhua Cuia, ⁎
⁎
a
Institute of Electronic Engineering, China Academy of Engineering Physics, Mianyang 621000, PR China Clean Energy Automotive Engineering Center, School of Automotive Studies, Tongji University, Shanghai 201804, PR China c State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China b
ARTICLE INFO
ABSTRACT
Keywords: Li3P-VP nanocomposite Thin films Energy storage and conversion Anode
Li3P-VP nanocomposite has been formed by a homemade pulsed laser deposition route and its electrochemical characteristic with lithium is reported for the first time. It displays high lithium storage capability as electrode material for secondary lithium ion batteries. The Li3P-VP nanocomposite exhibits high specific discharge capacity of 1040.4 mAh g−1 between 0.01 and 4.0 V and the specific capacity can still maintain 95% after 50 cycles. Ex situ transmission electron microscopy reveals that VP2 is generated from Li3P and VP after the first charging process to 4.0 V, while Li3P and V are found to be the final discharge products. In the recharging process, VP and VP2 are formed as the delithiated products.
1. Introduction The traditional Li ion batteries (LIBs) are mainly composed of transition metal oxide or phosphate cathode (e.g. LiCoO2, LiFePO4) and graphite anode. However, the redox operations of both electrodes (vs. lithium) always follow insertion reactions, which limits the energy density [1]. Therefore, it is crucial to seek next generation of cathodes and anodes with high energy density. In 2000, Poizot et al. reported that high capacity was achieved in several transition metal oxides based on conversion reaction mechanism [2]. Afterward, various transition metal compounds have been explored [3]. Among them, transition metal phosphides have been received numerous interests due to their high reversibility and large capacities [4]. However, most works were focused on late transition metal phosphides [5–8]. Early transition metal phosphides such as TieP, VeP, CreP and MneP were rarely concerned. Compared with late transition metal phosphides, early transition metal phosphides are expected to display more favorable electrochemical properties due to their multi-valence characteristics. In case of VeP system, VP [9], VP2 [10] and VP4 [11] were successfully fabricated and explored as host materials for rechargeable lithium ion batteries. By using a thin film electrode, the intrinsic properties of electrode material can be investigated due to the absence of Li-storage inactive
⁎
conductive additive and binder. It might be an “ideal” model for fundamental investigation. To enrich the lithium electrochemistry of VeP, in this work, pulsed laser deposition (PLD) was employed to fabricate Li3P-VP nanocomposites. As far as we know, to obtain higher content of transition metal compound using PLD, a trick throughout the route of synthesis must be done [12], especially for the desired compound in this work. Interestingly, it might be possible to obtain VP2 in two steps instead of one-step fabrication. In that case, the excessive lithium in Li3P-VP is helpful to stabilize the microstructure of VP2 after the first charged process. Overall, their electrochemical properties and structural characteristics were investigated systemically by various advanced analytical methods, which demonstrated that Li3P-VP nanocomposite might be a promising lithium host for high capacity LIBs. 2. Experimental A self-made equipment was employed for PLD [6]. Commercial vanadium metal reagent (AR, 99.9%) and red phosphorus reagent (AR, 99.9%) were mixed with a molar ratio of n(V):n(P) = 1:3 and pressed into a laser-ablated target with a diameter of 13 mm. Excessive red phosphorus powders were used in target to compensate for the tittle volatilization of P during laser ablation under vacuum conditions. A lithium sheet was placed on top of target to cover half of it. A 355 nm
Corresponding authors. E-mail addresses: [email protected] (Y. Zhao), [email protected] (M. Xue), [email protected] (Y. Cui).
https://doi.org/10.1016/j.jelechem.2019.03.070 Received 31 October 2018; Received in revised form 26 March 2019; Accepted 27 March 2019 Available online 27 March 2019 1572-6657/ © 2019 Published by Elsevier B.V.
Journal of Electroanalytical Chemistry 841 (2019) 21–25
H. Wu, et al.
Fig. 1. (a) First three CV curves, (b) first three voltage profiles and (c) cycling performance of as-deposited thin film.
SS substrate
electrode and two sheets of metallic lithium were used as reference and counter electrodes, respectively. The nonaqueous solution containing 1 mol L−1 LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (v(EC)/v(DMC) = 1:1) was used as electrolyte. Galvanostatic cycling measurements between 0.01 and 4.0 V (vs. Li+/ Li) at a current density of 5 μA cm−2 were conducted at 25 °C with battery testers (Land CT2001A). The cyclic voltammetry (CV) tests were executed between 0.01 and 4.0 V (vs. Li+/Li) on a CHI660D potentiostat working station at a collecting rate of 0.1 mV s−1. X-ray photoelectron spectroscopy (XPS, Kratos XSAM800) was used to analyze the surface composition of V and P in samples of the pristine, first charged to 4.0 V and first discharged to 0.01 V using Al–Kα (1486.6 eV) radiation as the primary excitation source operated at 180 W. The binding energies were calibrated using the C 1s level (284.6 eV) as an internal reference. In the interest of finding the working processes of Li3P-VP with lithium ions, ex situ TEM investigations were conducted on the lithiated/ delithiated films during the initial cycle. The cells were disassembled in an Ar-filled glove box to get cycled electrodes. After washed by DMC, the active materials were removed from stainless steel substrate and homogeneously dispersed in ethanol. The sample was deposited onto a copper grid drop by drop, which was promptly transferred into chambers to avoid exposure to oxygen or water.
*
Intensity
* as-deposited charging to 4.0 V discharging to 0.01 V
10
20
30
40
50
60
o
2Theta ( ) Fig. 2. XRD patterns of LieVeP nanocomposite thin film at different states.
laser beam with 2 J cm−2 energy intensity was produced by a 5 Hz Nd:YAG laser (Spectra Physics GCR-150) and concentrated on the target surface with an incident angle of 45°. The Ar pressure in vacuum chamber during deposition was 10 Pa. A stainless steel (SS) substrate was placed 3 cm away the target and kept at 400 °C. The weight of the obtained thin films (excluding substrate) was about 0.10 mg per 1.0 cm2. X-ray diffraction (XRD) patterns were collected by a Bruker D8 Focus diffractometer with Cu Kα1 radiation (λ = 0.15406 nm). For ex situ investigation, as-lithiated/delithiated products were directly taken from the cells to perform the data collection, A JEOL 2010 transmission electron microscope (TEM) was used for TEM measurement of lithiated and delithiated products. U-type three-electrode cells were constructed in a glove box filled with high purity argon. The as-achieved thin film was acted as working
3. Results and discussion Fig. 1a exhibits the initial three CV profiles of the as-received LieVeP nanocomposite thin film. In the first anodic process, a distinct peak at 3.67 V was observed, indicating the release of Li from nanocomposite and providing evidence to the existence of lithium containing compound [13]. On the other hand, in the first cathodic process, two reduction peaks at 1.17 and 0.71 V were attributed to the 22
Journal of Electroanalytical Chemistry 841 (2019) 21–25
H. Wu, et al.
Fig. 3. HRTEM and SAED images of LieVeP thin film: (a) and (b) pristine, (c) and (d) charged to 4.0 V, (e) and (f) discharged to 0.01 V.
decomposition reactions of delithiated Li3P-VP [10,11,13]. In the following cycles, these two peaks slightly shifted to 1.38 and 0.75 V and curves of both cathodic and anodic process became almost overlapped, demonstrating good reversibility. Fig. 1b shows the voltage profiles of LieVeP nanocomposite thin film for the initial three cycles. A slope plateau in the first charge process was attributed a delithiation process and was consistent with the anodic peak at 3.67 V in CV curves [10,11,13]. The first discharge process showed a slope plateau from 1.3 to 0.01 V and a high initial discharge capacity of 1671.6 mAh g−1. The second charge and discharge process of the cell yielded a reversible discharge capacity of 1040.4 mAh g−1. Subsequent cycle still kept similar charge/discharge curves as the second one, showing good reversibility. After 50 cycles, a high discharge capacity of 987.2 mAh g−1 was delivered,
corresponding to 94.9% of the specific capacity, exhibiting excellent stability (Fig. 1c). To understand the electrochemical reaction of LieVeP nanocomposite thin film, two questions should be clarified. The first lies in the composition of the as-deposited nanocomposite thin film to be LieVeP ternary compound or the mixture of LieP and VeP. The second is how to reversibly extract and uptake lithium into nanocomposite thin film during cycling. Ex-situ XRD technique was first adopted to check the structural and compositional evolution during electrochemical process. Nevertheless, due to the nanocrystalline or amorphous feature of the thin films before and after reaction, except diffraction peaks from SS substrate, no apparent peaks were observed in all patterns (Fig. 2). Hence, ex situ TEM measurements were further applied. Fig. 3a and b show the high-resolution TEM (HRTEM) image and selected-area 23
Journal of Electroanalytical Chemistry 841 (2019) 21–25
H. Wu, et al.
test samples, especially for the discharged product. It is necessary to take several hours or minutes to dissemble cells, prepare samples, and transfer them to TEM chamber. Although all processes were carried out without air contact, rapid change inevitably occurred since nanosized metal particles were highly active. This might be the reason for the absence of metallic vanadium in TEM and SAED analysis in this work. To further validate the TEM and SAED results, as well as the reaction equations, XPS tests for V and P of the as-prepared samples have been carried out. According to the XPS spectra of P, the overlapping signals of P 2p3/2 and P 2p1/2 regions appearing at a maximum of 129 eV can be attributed to PeV bond in VP. In the first charged sample, the virtually disappearances peak of PeLi bond at around 130.8 eV indicates the oxidation of Li3P, which finally produces VP2 as TEM analysis presented. After the discharged process, the peak of PeLi bond recurs with high intensity, which implies the abundant yield of Li3P. The peak at 132.2 eV in pristine sample should be due to the PeO bond, which could be attributed to the formation of phosphate at the surface of test sample [15]. As is observed from Fig. 4b, the V2p region shows two peaks centered at about 511 and 519 eV, which match well with V2p3/2 and V2p1/2, respectively. The main peak of V 2p3/2 located at about 511.2 eV may be ascribed to the decomposition of VP in the pristine sample. Under the high-resolution laser, vaporized V would fill in the vacuum chamber, and part of the dissociated particles of V could accumulate at the surface of substrate. After the first charge to 4.0 V, V3+ virtually transforms into high value V (e.g. V3+/V4+/V5+), which addresses the shift to higher value. Both V3+ and V4+ peaks remain in a relative low intensity at 512.2 and 513.4 eV respectively, after the first discharge to 0.01 V. The reoccur of V at 511.2 eV indicates that most of high value vanadium have transformed into metallic vanadium. These results can provide a hint for the proposed reaction equations as shown later. Based on the ex situ TEM results, XPS spectra and electrochemical lithiated/delithiated characteristics of samples, the following electrochemical reaction mechanism was proposed for Li3P-VP nanocomposite.
Table 1 d-Spacings derived from SAED analysis of pristine, first charged to 4.0 V, and first discharged to 0.01 V LieVeP thin film electrodes. d-Spacings from JCPDS card no. 65–3512 (Li3P), 65–1897 (VP) and 65–2310 (VP2) were listed for comparison. Pristine LieVeP thin film Our results (Å)
d (hkl) VP
3.79 2.74 2.48 1.85 1.58
3.30 1.94 1.68 1.28 1.12
3.80 (002)
2.75 (100) 2.52 (101)
1.86 (112)
1.59 (110)
Charged to 4.0 V Our results (Å)
d (hkl) Li3P, P63/mmc
d (hkl) VP, P63/ mmc
1.66 (103) 1.26 (202) 1.11 (114)
Discharged to 0.01 V d (hkl) VP2, C2/m
Our results (Å)
3.32 (2 02) 1.94 (112)
3.71 2.64 1.65 1.35
d (hkl) Li3P, P63/ mmc 3.70 2.65 1.66 1.38
(100) (102) (202) (211)
electron diffraction (SAED) pattern of pristine LieVeP nanocomposite thin film. Lattice fringes (see white arrow on figure) on Fig. 3a confirmed the nanocrystalline nature. Diffraction rings on Fig. 3b could be attributed to the (100), (101), (110) crystalline planes of VP and (100), (112) crystalline planes of Li3P (Table 1), revealing the composition of Li3P-VP mixture for the as-deposited nanocomposite. Element analysis performed by an inductively coupled plasma mass spectrometer (i CAP Q, Thermo) showed the molar ratio of V:P in nanocomposite was 7:9. After first charging to 4.0 V, although nanocrystalline state was kept (Fig. 3c), the composition of the nanocomposite changed. The diffractions rings from Li3P disappeared (Fig. 3d). Instead, diffraction rings from VP2 emerged. All five rings on the pattern could be assigned to the (202), (112) crystalline planes of VP2 and (103), (202), (114) crystalline planes of VP, indicating that the extraction of lithium from Li3P left phosphorus in nanocomposite and drove further phosphorization of VP to obtain VP2. When the nanocomposite was fully lithiated to 0.01 V, all d-spacings derived from SAED pattern belonged to Li3P (Fig. 3f), as shown in Table 1, indicating the fully decomposition of VP and VP2. No diffraction assignable to vanadium crystallites was observed. In an earlier work reported by Pralong et al., metallic cobalt was also not detected in the discharged product of CoP3 by ex situ TEM and SAED [14]. They proposed that the metal must be very highly dispersed or weakly bonded to the matrix as highly metal-rich phosphide clusters also existed [14]. We inferred that a rapid change might occur for the
Li3P + VP
3 Li + VP2
(1)
VP2 + 6Li
V + 2 Li3P
(2)
VP + 3 Li
V + Li3P
(3)
As shown in Fig. 5, Reaction (1) occurs in the first charging process. It removes lithium from Li3P-VP nanocomposite to form VP-VP2 nanocomposite. In the following cycles, the reversible decomposition and formation of two kinds of vanadium phosphides (Reactions (2) and (3)) are responsible for the excellent reversibility and a high discharge capacity of 987.2 mAh g−1 was maintained after 50 cycles, making Li3PVP nanocomposite a potential anode candidate for LIBs.
Fig. 4. XPS spectra of (a) P and (b) V for the LieVeP thin films in pristine, first charged to 4.0 V and first discharged to 0.01 V. 24
Journal of Electroanalytical Chemistry 841 (2019) 21–25
H. Wu, et al.
Fig. 5. The lithiation/delithiation mechanism in Li3P-VP nanocomposite during the initial cycles.
4. Conclusions
electrodes for lithium rechargeable batteries, Acc. Chem. Res. 51 (2018) 273–281. [4] L. Feng, H. Xue, Advances in transition-metal phosphide applications in electrochemical energy storage and catalysis, ChemElectroChem. 4 (2017) 20–34. [5] Y.-M. Chun, H.-C. Shin, Electrochemical synthesis of iron phosphides as anode materials for lithium secondary batteries, Electrochim. Acta 209 (2016) 369–378. [6] Y.-H. Cui, M.-Z. Xue, Z.-W. Fu, X.-L. Wang, X.-J. Liu, Nanocrystalline CoP thin film as a new anode material for lithium ion battery, J. Alloys Compd. 555 (2013) 283–290. [7] Q. Li, J. Ma, H. Wang, X. Yang, R. Yuan, Y. Chai, Interconnected Ni2P nanorods grown on nickel foam for binder free lithium ion batteries, Electrochim. Acta 213 (2016) 201–206. [8] G.-A. Li, C.-Y. Wang, W.-C. Chang, H.-Y. Tuan, Phosphorus-rich copper phosphide nanowires for field-effect transistors and lithium-ion batteries, ACS Nano 10 (2016) 8632–8644. [9] C.-M. Park, Y.-U. Kim, H.-J. Sohn, Topotactic Li insertion/extraction in hexagonal vanadium monophosphide, Chem. Mater. 21 (2009) 5566–5568. [10] F. Gillot, M. Ménétrier, E. Bekaert, L. Dupont, M. Morcrette, L. Monconduit, J.M. Tarascon, Vanadium diphosphides as negative electrodes for secondary Li-ion batteries, J. Power Sources 172 (2007) 877–885. [11] Y.-U. Kim, B.W. Cho, H.-J. Sohn, The reaction mechanism of lithium insertion in vanadium tetraphosphide, a possible anode material in lithium-ion batteries, J. Electrochem. Soc. 152 (2005) A1475–A1478. [12] J. Ma, L. Yu, Z.W. Fu, Electrochemical and theoretical investigation on the reaction of transition metals with Li3N, J. Electrochimica Acta 51 (2006) 4802–4814. [13] Y.-N. Zhou, W.-Y. Liu, M.-Z. Xue, L. Yu, C.-L. Wu, X.-J. Wu, Z.-W. Fu, LiF/Co nanocomposite as a new Li storage material, J. Electrochem. and Solid-state Lett. 9 (3) (2006) A147–A150. [14] V. Pralong, D.C.S. Souza, K.T. Leung, L.F. Nazar, Reversible lithium uptake by CoP3 at low potential: role of the anion, Electrochem. Commun. 4 (2002) 516–520. [15] Z.X. He, Y.Q. Jiang, J. Zhu, Y.H. Li, L. Dai, W. Meng, L. Wang, S.Q. Liu, Phosphorus doped multi-walled carbon nanotubes: an excellent electrocatalyst for VO2+/VO2+ redox reaction, Chem. Electro. Chem. 5 (2018) 2464–2474.
Via a homemade device, Li3P-VP nanocomposite was fabricated at the first time by a PLD route. Performed as a lithium host, a high specific discharge capacity of 1040.4 mAh g−1 was found for Li3P-VP nanocomposite between 0.01 and 4.0 V and 95% of it was firmly maintained after 50 cycles, showing the potential to be anode material for high capacity LIBs. Observed from the ex situ TEM, along with the XPS spectra of V and P, a reversible conversion mechanism to V and Li3P was suggested for VP-VP2, a nanocomposite generated from the delithiated reaction of Li3P-VP in the first charging process to 4.0 V. Acknowledgements This work was financially supported by Laboratory of Precision Manufacturing Technology, China Academy of Engineering Physics (Grant Nos. ZD17006， ZZ16002) and National Natural Science Foundation of China (Grant No. U1730136). References [1] J.W. Choi, D. Aurbach, Promise and reality of post-lithium-ion batteries with high energy densities, Nat. Rev. Mat. 1 (2016) 1–16. [2] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nano-sized transitionmetal oxides as negative-electrode materials for lithium-ion batteries, Nature 407 (2000) 496–499. [3] S.-H. Yu, X. Feng, N. Zhang, J. Seok, H.D. Abruña, Understanding conversion-type
25
Contents lists available at ScienceDirect
Journal of Electroanalytical Chemistry journal homepage: www.elsevier.com/locate/jelechem
Short communication
A novel Li3P-VP nanocomposite fabricated by pulsed laser deposition as anode material for high-capacity lithium ion batteries
T
Hailong Wua, Kaiyuan Weia, Binghua Tanga, Yixiu Cuia, Yu Zhaoa, , Mingzhe Xueb, , Chilin Lic, ⁎ Yanhua Cuia, ⁎
⁎
a
Institute of Electronic Engineering, China Academy of Engineering Physics, Mianyang 621000, PR China Clean Energy Automotive Engineering Center, School of Automotive Studies, Tongji University, Shanghai 201804, PR China c State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China b
ARTICLE INFO
ABSTRACT
Keywords: Li3P-VP nanocomposite Thin films Energy storage and conversion Anode
Li3P-VP nanocomposite has been formed by a homemade pulsed laser deposition route and its electrochemical characteristic with lithium is reported for the first time. It displays high lithium storage capability as electrode material for secondary lithium ion batteries. The Li3P-VP nanocomposite exhibits high specific discharge capacity of 1040.4 mAh g−1 between 0.01 and 4.0 V and the specific capacity can still maintain 95% after 50 cycles. Ex situ transmission electron microscopy reveals that VP2 is generated from Li3P and VP after the first charging process to 4.0 V, while Li3P and V are found to be the final discharge products. In the recharging process, VP and VP2 are formed as the delithiated products.
1. Introduction The traditional Li ion batteries (LIBs) are mainly composed of transition metal oxide or phosphate cathode (e.g. LiCoO2, LiFePO4) and graphite anode. However, the redox operations of both electrodes (vs. lithium) always follow insertion reactions, which limits the energy density [1]. Therefore, it is crucial to seek next generation of cathodes and anodes with high energy density. In 2000, Poizot et al. reported that high capacity was achieved in several transition metal oxides based on conversion reaction mechanism [2]. Afterward, various transition metal compounds have been explored [3]. Among them, transition metal phosphides have been received numerous interests due to their high reversibility and large capacities [4]. However, most works were focused on late transition metal phosphides [5–8]. Early transition metal phosphides such as TieP, VeP, CreP and MneP were rarely concerned. Compared with late transition metal phosphides, early transition metal phosphides are expected to display more favorable electrochemical properties due to their multi-valence characteristics. In case of VeP system, VP [9], VP2 [10] and VP4 [11] were successfully fabricated and explored as host materials for rechargeable lithium ion batteries. By using a thin film electrode, the intrinsic properties of electrode material can be investigated due to the absence of Li-storage inactive
⁎
conductive additive and binder. It might be an “ideal” model for fundamental investigation. To enrich the lithium electrochemistry of VeP, in this work, pulsed laser deposition (PLD) was employed to fabricate Li3P-VP nanocomposites. As far as we know, to obtain higher content of transition metal compound using PLD, a trick throughout the route of synthesis must be done [12], especially for the desired compound in this work. Interestingly, it might be possible to obtain VP2 in two steps instead of one-step fabrication. In that case, the excessive lithium in Li3P-VP is helpful to stabilize the microstructure of VP2 after the first charged process. Overall, their electrochemical properties and structural characteristics were investigated systemically by various advanced analytical methods, which demonstrated that Li3P-VP nanocomposite might be a promising lithium host for high capacity LIBs. 2. Experimental A self-made equipment was employed for PLD [6]. Commercial vanadium metal reagent (AR, 99.9%) and red phosphorus reagent (AR, 99.9%) were mixed with a molar ratio of n(V):n(P) = 1:3 and pressed into a laser-ablated target with a diameter of 13 mm. Excessive red phosphorus powders were used in target to compensate for the tittle volatilization of P during laser ablation under vacuum conditions. A lithium sheet was placed on top of target to cover half of it. A 355 nm
Corresponding authors. E-mail addresses: [email protected] (Y. Zhao), [email protected] (M. Xue), [email protected] (Y. Cui).
https://doi.org/10.1016/j.jelechem.2019.03.070 Received 31 October 2018; Received in revised form 26 March 2019; Accepted 27 March 2019 Available online 27 March 2019 1572-6657/ © 2019 Published by Elsevier B.V.
Journal of Electroanalytical Chemistry 841 (2019) 21–25
H. Wu, et al.
Fig. 1. (a) First three CV curves, (b) first three voltage profiles and (c) cycling performance of as-deposited thin film.
SS substrate
electrode and two sheets of metallic lithium were used as reference and counter electrodes, respectively. The nonaqueous solution containing 1 mol L−1 LiPF6 in a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) (v(EC)/v(DMC) = 1:1) was used as electrolyte. Galvanostatic cycling measurements between 0.01 and 4.0 V (vs. Li+/ Li) at a current density of 5 μA cm−2 were conducted at 25 °C with battery testers (Land CT2001A). The cyclic voltammetry (CV) tests were executed between 0.01 and 4.0 V (vs. Li+/Li) on a CHI660D potentiostat working station at a collecting rate of 0.1 mV s−1. X-ray photoelectron spectroscopy (XPS, Kratos XSAM800) was used to analyze the surface composition of V and P in samples of the pristine, first charged to 4.0 V and first discharged to 0.01 V using Al–Kα (1486.6 eV) radiation as the primary excitation source operated at 180 W. The binding energies were calibrated using the C 1s level (284.6 eV) as an internal reference. In the interest of finding the working processes of Li3P-VP with lithium ions, ex situ TEM investigations were conducted on the lithiated/ delithiated films during the initial cycle. The cells were disassembled in an Ar-filled glove box to get cycled electrodes. After washed by DMC, the active materials were removed from stainless steel substrate and homogeneously dispersed in ethanol. The sample was deposited onto a copper grid drop by drop, which was promptly transferred into chambers to avoid exposure to oxygen or water.
*
Intensity
* as-deposited charging to 4.0 V discharging to 0.01 V
10
20
30
40
50
60
o
2Theta ( ) Fig. 2. XRD patterns of LieVeP nanocomposite thin film at different states.
laser beam with 2 J cm−2 energy intensity was produced by a 5 Hz Nd:YAG laser (Spectra Physics GCR-150) and concentrated on the target surface with an incident angle of 45°. The Ar pressure in vacuum chamber during deposition was 10 Pa. A stainless steel (SS) substrate was placed 3 cm away the target and kept at 400 °C. The weight of the obtained thin films (excluding substrate) was about 0.10 mg per 1.0 cm2. X-ray diffraction (XRD) patterns were collected by a Bruker D8 Focus diffractometer with Cu Kα1 radiation (λ = 0.15406 nm). For ex situ investigation, as-lithiated/delithiated products were directly taken from the cells to perform the data collection, A JEOL 2010 transmission electron microscope (TEM) was used for TEM measurement of lithiated and delithiated products. U-type three-electrode cells were constructed in a glove box filled with high purity argon. The as-achieved thin film was acted as working
3. Results and discussion Fig. 1a exhibits the initial three CV profiles of the as-received LieVeP nanocomposite thin film. In the first anodic process, a distinct peak at 3.67 V was observed, indicating the release of Li from nanocomposite and providing evidence to the existence of lithium containing compound [13]. On the other hand, in the first cathodic process, two reduction peaks at 1.17 and 0.71 V were attributed to the 22
Journal of Electroanalytical Chemistry 841 (2019) 21–25
H. Wu, et al.
Fig. 3. HRTEM and SAED images of LieVeP thin film: (a) and (b) pristine, (c) and (d) charged to 4.0 V, (e) and (f) discharged to 0.01 V.
decomposition reactions of delithiated Li3P-VP [10,11,13]. In the following cycles, these two peaks slightly shifted to 1.38 and 0.75 V and curves of both cathodic and anodic process became almost overlapped, demonstrating good reversibility. Fig. 1b shows the voltage profiles of LieVeP nanocomposite thin film for the initial three cycles. A slope plateau in the first charge process was attributed a delithiation process and was consistent with the anodic peak at 3.67 V in CV curves [10,11,13]. The first discharge process showed a slope plateau from 1.3 to 0.01 V and a high initial discharge capacity of 1671.6 mAh g−1. The second charge and discharge process of the cell yielded a reversible discharge capacity of 1040.4 mAh g−1. Subsequent cycle still kept similar charge/discharge curves as the second one, showing good reversibility. After 50 cycles, a high discharge capacity of 987.2 mAh g−1 was delivered,
corresponding to 94.9% of the specific capacity, exhibiting excellent stability (Fig. 1c). To understand the electrochemical reaction of LieVeP nanocomposite thin film, two questions should be clarified. The first lies in the composition of the as-deposited nanocomposite thin film to be LieVeP ternary compound or the mixture of LieP and VeP. The second is how to reversibly extract and uptake lithium into nanocomposite thin film during cycling. Ex-situ XRD technique was first adopted to check the structural and compositional evolution during electrochemical process. Nevertheless, due to the nanocrystalline or amorphous feature of the thin films before and after reaction, except diffraction peaks from SS substrate, no apparent peaks were observed in all patterns (Fig. 2). Hence, ex situ TEM measurements were further applied. Fig. 3a and b show the high-resolution TEM (HRTEM) image and selected-area 23
Journal of Electroanalytical Chemistry 841 (2019) 21–25
H. Wu, et al.
test samples, especially for the discharged product. It is necessary to take several hours or minutes to dissemble cells, prepare samples, and transfer them to TEM chamber. Although all processes were carried out without air contact, rapid change inevitably occurred since nanosized metal particles were highly active. This might be the reason for the absence of metallic vanadium in TEM and SAED analysis in this work. To further validate the TEM and SAED results, as well as the reaction equations, XPS tests for V and P of the as-prepared samples have been carried out. According to the XPS spectra of P, the overlapping signals of P 2p3/2 and P 2p1/2 regions appearing at a maximum of 129 eV can be attributed to PeV bond in VP. In the first charged sample, the virtually disappearances peak of PeLi bond at around 130.8 eV indicates the oxidation of Li3P, which finally produces VP2 as TEM analysis presented. After the discharged process, the peak of PeLi bond recurs with high intensity, which implies the abundant yield of Li3P. The peak at 132.2 eV in pristine sample should be due to the PeO bond, which could be attributed to the formation of phosphate at the surface of test sample [15]. As is observed from Fig. 4b, the V2p region shows two peaks centered at about 511 and 519 eV, which match well with V2p3/2 and V2p1/2, respectively. The main peak of V 2p3/2 located at about 511.2 eV may be ascribed to the decomposition of VP in the pristine sample. Under the high-resolution laser, vaporized V would fill in the vacuum chamber, and part of the dissociated particles of V could accumulate at the surface of substrate. After the first charge to 4.0 V, V3+ virtually transforms into high value V (e.g. V3+/V4+/V5+), which addresses the shift to higher value. Both V3+ and V4+ peaks remain in a relative low intensity at 512.2 and 513.4 eV respectively, after the first discharge to 0.01 V. The reoccur of V at 511.2 eV indicates that most of high value vanadium have transformed into metallic vanadium. These results can provide a hint for the proposed reaction equations as shown later. Based on the ex situ TEM results, XPS spectra and electrochemical lithiated/delithiated characteristics of samples, the following electrochemical reaction mechanism was proposed for Li3P-VP nanocomposite.
Table 1 d-Spacings derived from SAED analysis of pristine, first charged to 4.0 V, and first discharged to 0.01 V LieVeP thin film electrodes. d-Spacings from JCPDS card no. 65–3512 (Li3P), 65–1897 (VP) and 65–2310 (VP2) were listed for comparison. Pristine LieVeP thin film Our results (Å)
d (hkl) VP
3.79 2.74 2.48 1.85 1.58
3.30 1.94 1.68 1.28 1.12
3.80 (002)
2.75 (100) 2.52 (101)
1.86 (112)
1.59 (110)
Charged to 4.0 V Our results (Å)
d (hkl) Li3P, P63/mmc
d (hkl) VP, P63/ mmc
1.66 (103) 1.26 (202) 1.11 (114)
Discharged to 0.01 V d (hkl) VP2, C2/m
Our results (Å)
3.32 (2 02) 1.94 (112)
3.71 2.64 1.65 1.35
d (hkl) Li3P, P63/ mmc 3.70 2.65 1.66 1.38
(100) (102) (202) (211)
electron diffraction (SAED) pattern of pristine LieVeP nanocomposite thin film. Lattice fringes (see white arrow on figure) on Fig. 3a confirmed the nanocrystalline nature. Diffraction rings on Fig. 3b could be attributed to the (100), (101), (110) crystalline planes of VP and (100), (112) crystalline planes of Li3P (Table 1), revealing the composition of Li3P-VP mixture for the as-deposited nanocomposite. Element analysis performed by an inductively coupled plasma mass spectrometer (i CAP Q, Thermo) showed the molar ratio of V:P in nanocomposite was 7:9. After first charging to 4.0 V, although nanocrystalline state was kept (Fig. 3c), the composition of the nanocomposite changed. The diffractions rings from Li3P disappeared (Fig. 3d). Instead, diffraction rings from VP2 emerged. All five rings on the pattern could be assigned to the (202), (112) crystalline planes of VP2 and (103), (202), (114) crystalline planes of VP, indicating that the extraction of lithium from Li3P left phosphorus in nanocomposite and drove further phosphorization of VP to obtain VP2. When the nanocomposite was fully lithiated to 0.01 V, all d-spacings derived from SAED pattern belonged to Li3P (Fig. 3f), as shown in Table 1, indicating the fully decomposition of VP and VP2. No diffraction assignable to vanadium crystallites was observed. In an earlier work reported by Pralong et al., metallic cobalt was also not detected in the discharged product of CoP3 by ex situ TEM and SAED [14]. They proposed that the metal must be very highly dispersed or weakly bonded to the matrix as highly metal-rich phosphide clusters also existed [14]. We inferred that a rapid change might occur for the
Li3P + VP
3 Li + VP2
(1)
VP2 + 6Li
V + 2 Li3P
(2)
VP + 3 Li
V + Li3P
(3)
As shown in Fig. 5, Reaction (1) occurs in the first charging process. It removes lithium from Li3P-VP nanocomposite to form VP-VP2 nanocomposite. In the following cycles, the reversible decomposition and formation of two kinds of vanadium phosphides (Reactions (2) and (3)) are responsible for the excellent reversibility and a high discharge capacity of 987.2 mAh g−1 was maintained after 50 cycles, making Li3PVP nanocomposite a potential anode candidate for LIBs.
Fig. 4. XPS spectra of (a) P and (b) V for the LieVeP thin films in pristine, first charged to 4.0 V and first discharged to 0.01 V. 24
Journal of Electroanalytical Chemistry 841 (2019) 21–25
H. Wu, et al.
Fig. 5. The lithiation/delithiation mechanism in Li3P-VP nanocomposite during the initial cycles.
4. Conclusions
electrodes for lithium rechargeable batteries, Acc. Chem. Res. 51 (2018) 273–281. [4] L. Feng, H. Xue, Advances in transition-metal phosphide applications in electrochemical energy storage and catalysis, ChemElectroChem. 4 (2017) 20–34. [5] Y.-M. Chun, H.-C. Shin, Electrochemical synthesis of iron phosphides as anode materials for lithium secondary batteries, Electrochim. Acta 209 (2016) 369–378. [6] Y.-H. Cui, M.-Z. Xue, Z.-W. Fu, X.-L. Wang, X.-J. Liu, Nanocrystalline CoP thin film as a new anode material for lithium ion battery, J. Alloys Compd. 555 (2013) 283–290. [7] Q. Li, J. Ma, H. Wang, X. Yang, R. Yuan, Y. Chai, Interconnected Ni2P nanorods grown on nickel foam for binder free lithium ion batteries, Electrochim. Acta 213 (2016) 201–206. [8] G.-A. Li, C.-Y. Wang, W.-C. Chang, H.-Y. Tuan, Phosphorus-rich copper phosphide nanowires for field-effect transistors and lithium-ion batteries, ACS Nano 10 (2016) 8632–8644. [9] C.-M. Park, Y.-U. Kim, H.-J. Sohn, Topotactic Li insertion/extraction in hexagonal vanadium monophosphide, Chem. Mater. 21 (2009) 5566–5568. [10] F. Gillot, M. Ménétrier, E. Bekaert, L. Dupont, M. Morcrette, L. Monconduit, J.M. Tarascon, Vanadium diphosphides as negative electrodes for secondary Li-ion batteries, J. Power Sources 172 (2007) 877–885. [11] Y.-U. Kim, B.W. Cho, H.-J. Sohn, The reaction mechanism of lithium insertion in vanadium tetraphosphide, a possible anode material in lithium-ion batteries, J. Electrochem. Soc. 152 (2005) A1475–A1478. [12] J. Ma, L. Yu, Z.W. Fu, Electrochemical and theoretical investigation on the reaction of transition metals with Li3N, J. Electrochimica Acta 51 (2006) 4802–4814. [13] Y.-N. Zhou, W.-Y. Liu, M.-Z. Xue, L. Yu, C.-L. Wu, X.-J. Wu, Z.-W. Fu, LiF/Co nanocomposite as a new Li storage material, J. Electrochem. and Solid-state Lett. 9 (3) (2006) A147–A150. [14] V. Pralong, D.C.S. Souza, K.T. Leung, L.F. Nazar, Reversible lithium uptake by CoP3 at low potential: role of the anion, Electrochem. Commun. 4 (2002) 516–520. [15] Z.X. He, Y.Q. Jiang, J. Zhu, Y.H. Li, L. Dai, W. Meng, L. Wang, S.Q. Liu, Phosphorus doped multi-walled carbon nanotubes: an excellent electrocatalyst for VO2+/VO2+ redox reaction, Chem. Electro. Chem. 5 (2018) 2464–2474.
Via a homemade device, Li3P-VP nanocomposite was fabricated at the first time by a PLD route. Performed as a lithium host, a high specific discharge capacity of 1040.4 mAh g−1 was found for Li3P-VP nanocomposite between 0.01 and 4.0 V and 95% of it was firmly maintained after 50 cycles, showing the potential to be anode material for high capacity LIBs. Observed from the ex situ TEM, along with the XPS spectra of V and P, a reversible conversion mechanism to V and Li3P was suggested for VP-VP2, a nanocomposite generated from the delithiated reaction of Li3P-VP in the first charging process to 4.0 V. Acknowledgements This work was financially supported by Laboratory of Precision Manufacturing Technology, China Academy of Engineering Physics (Grant Nos. ZD17006， ZZ16002) and National Natural Science Foundation of China (Grant No. U1730136). References [1] J.W. Choi, D. Aurbach, Promise and reality of post-lithium-ion batteries with high energy densities, Nat. Rev. Mat. 1 (2016) 1–16. [2] P. Poizot, S. Laruelle, S. Grugeon, L. Dupont, J.-M. Tarascon, Nano-sized transitionmetal oxides as negative-electrode materials for lithium-ion batteries, Nature 407 (2000) 496–499. [3] S.-H. Yu, X. Feng, N. Zhang, J. Seok, H.D. Abruña, Understanding conversion-type
25