- Email: [email protected]
One-pot synthesis of polymeric LiPON
Journal Pre-proof One-Pot Synthesis of polymeric LiPON
Gideon Abels, Ingo Bardenhagen, Julian Schwenzel PII:
S0032-3861(20)30138-5
DOI:
https://doi.org/10.1016/j.polymer.2020.122300
Reference:
JPOL 122300
To appear in:
Polymer
Received Date:
30 November 2019
Accepted Date:
16 February 2020
Please cite this article as: Gideon Abels, Ingo Bardenhagen, Julian Schwenzel, One-Pot Synthesis of polymeric LiPON, Polymer (2020), https://doi.org/10.1016/j.polymer.2020.122300
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof
Journal Pre-proof
One-Pot Synthesis of polymeric LiPON Gideon Abelsa,b,*, Ingo Bardenhagenb,**, Julian Schwenzelb,** Faculty of Production Engineering, University of Bremen, Badgasteiner Straße, 28359 Bremen, Germany b Fraunhofer Institute for Manufacturing Technology and Advanced Materials, Wiener Straße 12, 28359 Bremen, Germany * Corresponding author: Tel.: +49 421 2246 7303 E-mail address: [email protected] ** Co-authors mail addresses: [email protected] [email protected] a
Abstract Lithium metal is a promising anode material for high energy lithium ion batteries due to its high specific capacity. Inherent difficulties such as the chemical reactivity and dendrite growth during cycling have kept it from being operational in energy storage systems so far. One approach to overcome these challenges is the use of a stable interlayer on top of the lithium. The glassy solid-state electrolyte lithium phosphorus oxynitride (LiPON) is a promising candidate to achieve this. However, it is prepared by a time and energy consuming physical vapor deposition technique. As an alternative to this, we developed the synthesis of a modified polyphosphazene having a similar chemical formula as the glassy LiPON which allows for a large scale application of such a stable interface with an easier and cheaper processing. Following a two-step one pot synthesis starting from poly(dichlorophosphazene), this polymeric LiPON could be successfully isolated and showed solubility in different solvents which distinguishes it clearly from sputtered LiPON.
1. Introduction Lithium metal is a long-known promising anode material for high energy density lithium ion batteries, since it has the lowest electrode potential of all anode materials and a high theoretical capacity.[1] But even after several decades of research, its inherent problems have not been solved yet.[1,2] On the one hand, metallic lithium reacts with every known electrolyte, causing safety problems due to strong heat development or formation of volatile products, especially in case of flammable liquid electrolytes.[3,4] On the other hand, strong volume changes of the anode during cyclisation cause a continuous breakage of the protective solid-electrolyte interface (SEI) on the surface as well as dendrite formation due to nonuniform deposition of lithium, which can lead to a shortcut followed by a thermal runaway. [4–8] So Li+-ion batteries with lithium metal anodes require an electrolyte that not only has a high lithium ion conductivity but also forms a stable interface with the anode. In order to find such an electrolyte numerous electrolytes have been developed in the past decades, and among them lithium phosphorus oxynitride (LiPON) stands out.[9] First of all, as a solid-state electrolyte it this safer to use in combination with lithium metal anodes than liquid electrolytes. But
Journal Pre-proof more importantly, it decomposes in contact with metallic lithium, forming Li3N, Li3P and Li2O,[10] which are stable against metallic lithium, have a high lithium ion conductivity and are electrical insulating.[11– 13] Also, due to its stable and self-healing SEI, LiPON is already established in commercial all-solid-state batteries.[14] However, its low lithium ion conductivity of 10-6 S·cm-1 limits the use of LiPON to thin-filmbatteries where the electrolyte layer is only a few microns thick. And since LiPON is formed in a physical vapor deposition process requiring high vacuum, it can only be applied on existing surfaces in a small scale.[9,15] To facilitate the fabrication of this material and therefore allow for easier production a material with the same molecular formula as LiPON needs to be synthesized. Ideally this material can be directly applied to production processes. Therefore a thermoplastic polymeric system seems to be favorable. A suitable material class for this approach are the polyphosphazenes. They exhibit a phosphorousnitrogen backbone with the general chemical formula (NPR2)n, The sum formula of glass-ceramic LiPON is LixPOyNz (x = 2.6 - 3.5; y = 1.9 - 3.8; z = 0.1 - 1.36).[9,10,15] By modifying the organic substituent R by O-Li groups the polyphosphazene with a molecular formula of (Li2PO2N)n may be received (Figure 3). This polymeric LiPON should be a thermoplastic, allowing melt processing or solution-based filmcasting on a larger scale, contrary to the PVD process used for glassy LiPON. To our knowledge, there are no references to such a material in the literature so far.
2. Experimental 2.1. Material and methods Lithium bis(trimethylsilyl)amide (LiN(SiMe3)2), phosphorus trichloride (PCl3), phosphorus pentachloride (PCl5) and thionyl chloride (SO2Cl2) and 2.5 M n-buthyllithium in toluene were received by Sigma Aldrich and Celite 545 from Carl Roth. Toluene, dimethyl sulfoxide (DMSO), acetonitrile and diethyl ether were bought in anhydrous quality from VWR Chemicals. Celite 545 was dried prior to use at 110 °C for 2 days, all other chemicals were used as received. All reactions were performed in an argon atmosphere. FTIR data were measured on a Vertex 70 FTIR spectrometer from Bruker equipped with a Platinum ATR unit. 31P-NMR spectra were recorded with a DPX-200 from Bruker, using 85% H3PO4 in D2O as a reference. Temperature modulated differential scanning calorimetry (DSC) measurements were performed on a Discovery DSC from TA Instruments using aluminums pans with lid. Electrochemical impedance spectroscopy were performed with a Gamry Interface 1010E Potentiostat/Galvanostat/ZRA to determine the ionic conductivity of the final polymer.
2.2. Poly(dichlorophosphazene)
Cl PCl3
+
LiNSi(CH3)2
1. SO2Cl2 2. PCl5
N
P Cl
(NPCl2)n Figure 1: Synthesis of poly(dichlorophosphazene).
(1) n
Journal Pre-proof Synthesis of this precursor was performed following the approach of Wang.[16] LiNSi(CH3)2 (5.17 g, 30.9 mmol) was dissolved in 120 ml anhydrous toluene and cooled down to 0 °C. Then, PCl3 (2.7 ml, 30.9 mmol) was added dropwise over 10 minutes and the reaction mixture stirred 30 minutes at 0 °C followed by 1 h at room temperature. After cooling to 0°C again, SO2Cl2 (2.55 ml, 31.5 mmol) was added dropwise over 10 minutes and the mixture stirred 1 h at 0°C, before adding PCl5 (316 mg, 1.52 mmol) and stirring for 18 h at room temperature. The mixture was then filtered through Celite 545 and the volatiles were removed at the rotary evaporator. The resulting yellow-orange, viscous mass was dried in an oil pump vacuum. Yield: 2.9 g, 25.2 mmol, 81 %
2.3.1. Poly(phosphoramidic acid) Cl N
O DMSO
P Cl
n
-H3CSCH2Cl
H N
P
(2)
OH n (H2PO2N)n
Figure 2: Synthesis of poly(phosphoramidic acid). Anhydrous DMSO (15 ml) was carefully added to the dried poly(dichlorophosphazene) (1 g, 8,63 mmol) in a 100 ml flask in a water bath under slow stirring at room temperature. After maintaining these conditions for 2 h, the clear yellow solution was heated up to 40 °C and stirred for 48 h. The reaction was monitored by 31P-NMR spectroscopy to ensure complete conversion. Then, the oil bath was removed and a colorless solid formed at the inside surface of the flask was scraped back into the solution. The mixture was stored in an ultrasonic bath for 10 minutes and then stirred for 24 h at room temperature. Afterwards, to remove the chloromethyl methyl sulfide side product, the mixture was first stored in vacuum for three hours. Then, it was refilled with 3 ml of anhydrous DMSO and stirred for 18 h at room temperature before it was washed four times with 15 ml anhydrous diethyl ether. Remaining volatile substances were removed in vacuum. The purified solution was then usually used for the next step of the one pot synthesis, so no yield is given here for the intermediate product (H2PO2N)n. However, in order to isolate the intermediate for analysis, 10 ml of the reactions solution were washed four times with 10 ml anhydrous diethyl ether after two days at 40 °C. Then the solution was mixed with 20 ml anhydrous acetonitrile and stored in an ultrasonic bath for 10 minutes. The resulting colorless solid was filtered out, washed with 5 ml anhydrous acetonitrile and dried in vacuum.
Journal Pre-proof 2.3.2 Poly(dilithium phosphoramidate) O
O H N
n-BuLi
P OH
Toluene n
Li N
P
(3)
OLi n (Li2PO2N)n
Figure 3: Synthesis steps of polymeric LiPON. First, the freshly degassed poly(phosphoramidic acid) solution was diluted with 15 ml anhydrous DMSO and put into a cold water bath, before 2.5 M n-buthyllithium in toluene (7.6 ml, 19 mmol, 2.2 eq.) was added dropwise. The dirty-yellow reaction mixture stirred for 96 h at room temperature. Afterwards, volatile compounds were removed in vacuum and the solution was washed three times with 30 ml anhydrous diethyl ether. After adding 60 ml anhydrous acetonitrile the mixture was stored in an ultrasonic bath for 10 minutes. The precipitated colorless solid was filtered out, washed with 6 ml of anhydrous acetonitrile and dried in vacuum. Yield: 454 mg, 5 mmol, 58% [relating to (NPCl2)n]
2.3.3. Conductivity measurements A pellet of polymeric LiPON pressed at 60 bar was placed between two stainless steel electrodes in a custom made electrochemical glass cell. The cell was put in an oven and heated up to 80°C. Then, an impedance spectrum was measured in the frequency range from 106 Hz to 1 Hz with an amplitude of 200 mV.
3. Results and Discussion The synthesis of polymeric LiPON was carried out in three steps. First, poly(dichlorophosphazene) (1) was synthesized which is a common precursor for polyphosphazenes (Figure 1). Following the synthesis procedure of Wang resulted in a gel-like yellow-orange solid. Its FTIR spectrum shows two bands at 1208 cm-1 and 741 cm-1 (Figure 4a) which can be assigned to the P=N-bond and the P-Clbond.[17] In addition, the 31P NMR spectrum of 1 shows one signal at -16.8 ppm that is in good agreement with the literature value, so the synthesis was successful (Figure 4c). [16,17] Since no other signals appeared in the spectra and the product could be completely dissolved in toluene or THF, no crosslinking or other side reaction took place.
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Figure 4: FTIR spectra of (NPCl2)n (a) and (H2PO2N)n (b); 31P-NMR spectra of (NPCl2)n and (H2PO2N)n (c). For the next step, DMSO was used as a chlorine-oxygen transfer reactant as found by Walsh and coworkers (Figure 2).[18] Poly(dichlorophosphazene) (1) was slowly dissolved in a large amount of DMSO within a water bath, since the reaction showed a very exothermal behavior. This way, DMSO acts as both solvent and reactant, consuming the P-Cl-groups upon dissolving. This is necessary because the formed P-OH groups can react with the educt forming crosslinking P-O-P groups whereby the thermoplastic behavior of the material will be lost, as it is the case if the reaction is performed in THF with an equivalent amount of DMSO. By using pure DMSO this side reaction is suppressed. The resulting (H2PO2N)n could be isolated and its FTIR spectrum measured. Compared to (1) the bands of the P-Cl and P=N-groups do not appear anymore, indicating that a complete hydroxylation has taken place followed by a rearrangement of the double-bond system. Instead, the FTIR spectrum of (2) reveals several new bands at 931 cm-1, 1090 cm-1, 1237 cm-1 and 1678 cm-1 which can be assigned to P-OH, P-O, P=O and O-H-functional groups respectively (Figure 4b).[19] According to literature, the bands for the N-H group bridging the phosphor atoms should be located between 3200 cm-1 and 2900 cm-1 and between 1440 cm-1 and 1220 cm-1.[20] So they are most likely overlain by the C-H stretching and deformation vibrations around 2950 cm-1 and 1400 cm-1 of remaining impurities in the product. These and other bands like the one at 960 cm-1 or the shoulder at 1110 cm-1 indicate that there is still some DMSO or chloromethyl methyl sulfide left in the [H2PO2N]n, but this impurities are minimal due to the low intensity of the C-H stretching vibration around 2950 cm-1. However, a distinct assignment for the respective bands was not possible, most likely because both chloromethyl methyl sulfide and DMSO form adducts with [H2PO2N]n which may cause their absorption bands to shift.[18,21–23] Furthermore, it cannot be determined if crosslinking reaction takes place since the P-OH band overlays
Journal Pre-proof the band of the P-O-P-group around 900 cm-1.[19] But since the product was still soluble in DMSO it is likely that this side reaction either does not take place or only to a very small degree. The assumption of a full conversion of the educt is further supported by the 31P-NMR spectrum which shows no educt signal at 16.8 ppm. Instead, one signal at 0.3 ppm appears that is very similar to that of H3PO4 in DMSO-d6 and imidodiphosphate in D2O (Figure 4c).[24] So the phosphorus atoms in our product have a similar chemical environment as in these compounds. The structure of (2) is very alike to that of phosphoric acid except for the nitrogen, but since both oxygen and nitrogen have a very similar electronic effect on the phosphorus the expected difference of its chemical environment in the same deuterated solvent should be rather minor. Combined with the FTIR data this indicates that the signal at 0.3 ppm can be assigned to the desired polymer structure and that the synthesis was successful. Thus, the completely conversed and purified (H2PO2N)n-DMSO solution could then be lithiated in the last step using n-buthyllithium (Figure 3). This reactant can lithiate the product either directly or indirectly by lithiating the DMSO, resulting in an organic super-base that reacts with the protons of the polymer.[25] After lithiation, the 31P-NMR spectrum of the isolated product shows one signal around 0.6 ppm which is very similar to that of the educt (Figure 5b). Such a minor difference can be the result of lithiation and indicates that there are no changes to the bonds at the phosphorus atoms. Therefore, the backbone of the polymer is intact and reactions can only have taken place at the proton sides which are not directly bond to the phosphorus. This is confirmed by the FTIR spectrum of the product in Figure 5a.
Figure 5: FTIR and NMR spectra of (Li2PO2N)n. It shows just one band at 1016 cm-1 that can be assigned to PO--bonds and thus to PO-Li+ groups, as in Li3PO4. [19,26] The extreme broadness of this band may indicate that the negative charge is distributed between both oxygen atoms, so the state of both P-O-bonds is between a single and a double bond instead of one double bond and one single bond. Thus, lithiation took place. By comparing the integrals of the band from the remaining P-OH-groups at 956 cm-1 and from the PO--band a degree of lithiation of around 95% was determined. This is in accordance with the nearly completely absent bands of O-H and N-H groups which also indicates a high degree of lithiation at the nitrogen site. Since the P-OHband is nearly gone in the FTIR spectrum of the product, it was also possible to examine if the resulting material shows crosslinking from the hydroxylation step. Both bands at 883 cm-1 and 712 cm-1 that are assigned to P-O-P-bonds can be found in the spectrum of (3), but since they are very small compared
Journal Pre-proof to the PO--band the material should only show minor crosslinking which shouldn’t affect the thermoplastic behavior of the polymeric LiPON (Inset Figure 5a).[19]
Figure 6: Reversing heat flow of polymeric LiPON in a temperature modulated DSC (a), and a film of polymeric LiPON, cast from a THF solution on a glass substrate (b). This is also proven by the fact that the material can still be dissolved in DMSO, Toluene and THF and a film of the polymer can be casted with the latter two (Figure 6b), although these are fragile and easy to damage and their surface is rather rough. However, the polymeric LiPON shows no sign of melting up to 300°C, and this is most likely because of the great number of ionic interactions between the polymer chains rather than crosslinking. Since the synthesized material is a powder, it is also possible to press it into pellets or other shapes, thus offering further processing options besides solution-based ones. Furthermore, the chemical synthesis offers the possibility of upscaling in contrast to the PVDprocess used for glassy LiPON. However, the conductivity measurements of a 400 µm thick pellet polymeric LiPON resulted in a value of 1.7·10-10 Scm-1 at 80°C which is probably electric conductivity. So although there is lithium in the polymer it is immobile, most likely because there are too many interactions with the polymer backbone, similar to the observation for the melting behavior. And the polymer chains are also very rigid since the DSC data of polymeric LiPON shows no glass transition between -90°C and 250°C which decreases the mobility of the lithium even further (Figure 6a). So the Li+-conductivity of polymeric LiPON is lower than that of glassy LiPON. But much more interesting is the interface between polymeric LiPON and lithium metal. The reason why glassy LiPON can be used in thin film batteries with lithium metal anodes is due to its wellconducting and very stable SEI consisting of Li2O, Li3N and Li3P. Especially the latter two show high Li+conductivities in the range of 10-3 to 10-4 Scm-1 which is several magnitudes better than for both LiPONs. Since polymeric LiPON has a similar molecular formula per monomer unit it should form the same SEI with lithium metal. There is currently work in progress to confirm this hypothesis because even if polymeric LiPON itself is no Li+-conductor, it could form a highly conductive layer upon decomposition. In this case, its addition to polymer electrolytes or its use in copolymers could also drastically improve the SEI of other solid state electrolytes and benefit their performance.
Journal Pre-proof 4. Conclusion In this work, we successfully synthesized polymeric LiPON. Following a two-step one pot synthesis we first modified poly(dichlorophosphazene) with oxygen-containing functional groups and then substituted all protons by lithium ions. The resulting polymer had a similar molecular formula per monomer unit as glassy LiPON, but could be isolated as a powder and dissolved in various solvents. Furthermore, the chemical synthesis is scalable and offers even larger yields, a clear advantage to sputter processes. Although the polymer is not suitable Li+-electrolyte itself due to the similar molecular formula it is likely to decompose into the same binary salts Li2O, Li3P and Li3N against lithium metal as glassy LiPON. Since the latter two have high lithium ion conductivities between 10-3 and 10-4 Scm-1 the synthesized polymer could form a good Li+-electrolyte upon decomposition against lithium metal which is currently investigated.
Acknowledgements The authors thank Katharina Koschek of the Fraunhofer Institute IFAM for providing a laboratory environment and the necessary equipment and Iris Gottschalk of the Fraunhofer Institute IFAM for performing the temperature modulated DSC measurement.
Reference [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21]
W. Xu, J. Wang, F. Ding, X. Chen, E. Nasybulin, Y. Zhang, J.-G. Zhang, Energy Environ. Sci. 2014, 7, 513. R. Selim, P. Bro, J. Electrochem. Soc. 1974, 121, 1457. Zhu, Yizhiou,He, Xingfeng, Mo, Yifei, ACS applied materials & interfaces 2015, 7, 23685. D. Aurbach, E. Zinigrad, Y. Cohen, H. Teller, Solid State Ionics 2002, 148. Y. Liu, D. Lin, P. Y. Yuen, K. Liu, J. Xie, R. H. Dauskardt, Y. Cui, Advanced Materials 2017, 29. Y. Liu, D. Lin, Z. Liang, J. Zhao, K. Yan, Y. Cui, Nature Communications 2016, 7, 1. C. Brissot, M. Rosso, J.-N. Chazalviel, S. Lascaud, Journal of Power Sources 1999, 81-82, 925. R. Khurana, J. L. Schaefer, L. A. Archer, G. W. Coates, Journal of the American Chemical Society 2014, 136, 7395. J. BATES, Solid State Ionics 1992, 53-56, 647. A. Schwöbel, R. Hausbrand, W. Jaegermann, Solid State Ionics 2015, 273, 51. G. Nazri, Solid State Ionics 1989, 34, 97. U. v. Alpen, A. Rabenau, G. H. Talat, Appl. Phys. Lett. 1977, 30, 621. B. Put, P. M. Vereecken, A. Stesmans, J. Mater. Chem. A 2018, 6, 4848. S. R. Anton, A. Erturk, D. J. Inman, Smart Mater. Struct. 2010, 19, 115021. Y. Su, J. Falgenhauer, A. Polity, T. Leichtweiß, A. Kronenberger, J. Obel, S. Zhou, D. Schlettwein, J. Janek, B. K. Meyer, Solid State Ionics 2015, 282, 63. B. Wang, Macromolecules 2005, 38, 643. E. Carolina Martínez Ceballos, R. Vera Graziano, G. Martínez Barrera, O. Olea Mejía, International Journal of Polymer Science 2013, 2013. E. J. Walsh, S. Kaluzene, T. Jubach, Journal of Inorganic and Nuclear Chemistry 1976, 38, 397. D. E. C. Corbridge, E. J. Lowe, Journal of the Chemical Society (Resumed) 1954, 493. J. V. Pustinger, W. T. Cave, M. L. Nielsen, Spectrochimica Acta 1959, 15, 909. F. Boberg, G. Winter, J. Moos, Justus Liebigs Ann. Chem. 1958, 616, 1.
Journal Pre-proof [22] M. Hayashi, Nippon Kagaku Zasshi 1959, 80, 1073. [23] W. Zhang, Y. Jiang, Y. Ding, M. Zheng, S. Wu, X. Lu, X. Gao, Q. Wang, G. Zhou, J. Liu et al., Opt. Mater. Express 2017, 7, 2150. [24] M. Watanabe, Y. Morii, S. Sato, BCSJ 1984, 57, 2914. [25] E. J. Corey, M. Chaykovsky, Journal of the American Chemical Society 1965, 87, 1345. [26] A. Xiao, L. Yang, B. L. Lucht, S.-H. Kang, D. P. Abraham, J. Electrochem. Soc. 2009, 156, A318A327.
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CRediT author statement Gideon Abels: Conceptualization, Investigation, Writing – Original Draft Ingo Bardenhagen: Resources, Validation, Writing – Review & Editing Julian Schwenzel: Supervision
Journal Pre-proof
Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Highlights - Polymeric LiPON was synthesized as an alternative to glassy LiPON with the same molecular formula. - Preparation took place in a two-step one pot synthesis from poly(dichlorophosphazene). - Compared to glassy LiPON, the polymer can be dissolved in different organic solvents enabling film casting
Gideon Abels, Ingo Bardenhagen, Julian Schwenzel PII:
S0032-3861(20)30138-5
DOI:
https://doi.org/10.1016/j.polymer.2020.122300
Reference:
JPOL 122300
To appear in:
Polymer
Received Date:
30 November 2019
Accepted Date:
16 February 2020
Please cite this article as: Gideon Abels, Ingo Bardenhagen, Julian Schwenzel, One-Pot Synthesis of polymeric LiPON, Polymer (2020), https://doi.org/10.1016/j.polymer.2020.122300
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier.
Journal Pre-proof
Journal Pre-proof
One-Pot Synthesis of polymeric LiPON Gideon Abelsa,b,*, Ingo Bardenhagenb,**, Julian Schwenzelb,** Faculty of Production Engineering, University of Bremen, Badgasteiner Straße, 28359 Bremen, Germany b Fraunhofer Institute for Manufacturing Technology and Advanced Materials, Wiener Straße 12, 28359 Bremen, Germany * Corresponding author: Tel.: +49 421 2246 7303 E-mail address: [email protected] ** Co-authors mail addresses: [email protected] [email protected] a
Abstract Lithium metal is a promising anode material for high energy lithium ion batteries due to its high specific capacity. Inherent difficulties such as the chemical reactivity and dendrite growth during cycling have kept it from being operational in energy storage systems so far. One approach to overcome these challenges is the use of a stable interlayer on top of the lithium. The glassy solid-state electrolyte lithium phosphorus oxynitride (LiPON) is a promising candidate to achieve this. However, it is prepared by a time and energy consuming physical vapor deposition technique. As an alternative to this, we developed the synthesis of a modified polyphosphazene having a similar chemical formula as the glassy LiPON which allows for a large scale application of such a stable interface with an easier and cheaper processing. Following a two-step one pot synthesis starting from poly(dichlorophosphazene), this polymeric LiPON could be successfully isolated and showed solubility in different solvents which distinguishes it clearly from sputtered LiPON.
1. Introduction Lithium metal is a long-known promising anode material for high energy density lithium ion batteries, since it has the lowest electrode potential of all anode materials and a high theoretical capacity.[1] But even after several decades of research, its inherent problems have not been solved yet.[1,2] On the one hand, metallic lithium reacts with every known electrolyte, causing safety problems due to strong heat development or formation of volatile products, especially in case of flammable liquid electrolytes.[3,4] On the other hand, strong volume changes of the anode during cyclisation cause a continuous breakage of the protective solid-electrolyte interface (SEI) on the surface as well as dendrite formation due to nonuniform deposition of lithium, which can lead to a shortcut followed by a thermal runaway. [4–8] So Li+-ion batteries with lithium metal anodes require an electrolyte that not only has a high lithium ion conductivity but also forms a stable interface with the anode. In order to find such an electrolyte numerous electrolytes have been developed in the past decades, and among them lithium phosphorus oxynitride (LiPON) stands out.[9] First of all, as a solid-state electrolyte it this safer to use in combination with lithium metal anodes than liquid electrolytes. But
Journal Pre-proof more importantly, it decomposes in contact with metallic lithium, forming Li3N, Li3P and Li2O,[10] which are stable against metallic lithium, have a high lithium ion conductivity and are electrical insulating.[11– 13] Also, due to its stable and self-healing SEI, LiPON is already established in commercial all-solid-state batteries.[14] However, its low lithium ion conductivity of 10-6 S·cm-1 limits the use of LiPON to thin-filmbatteries where the electrolyte layer is only a few microns thick. And since LiPON is formed in a physical vapor deposition process requiring high vacuum, it can only be applied on existing surfaces in a small scale.[9,15] To facilitate the fabrication of this material and therefore allow for easier production a material with the same molecular formula as LiPON needs to be synthesized. Ideally this material can be directly applied to production processes. Therefore a thermoplastic polymeric system seems to be favorable. A suitable material class for this approach are the polyphosphazenes. They exhibit a phosphorousnitrogen backbone with the general chemical formula (NPR2)n, The sum formula of glass-ceramic LiPON is LixPOyNz (x = 2.6 - 3.5; y = 1.9 - 3.8; z = 0.1 - 1.36).[9,10,15] By modifying the organic substituent R by O-Li groups the polyphosphazene with a molecular formula of (Li2PO2N)n may be received (Figure 3). This polymeric LiPON should be a thermoplastic, allowing melt processing or solution-based filmcasting on a larger scale, contrary to the PVD process used for glassy LiPON. To our knowledge, there are no references to such a material in the literature so far.
2. Experimental 2.1. Material and methods Lithium bis(trimethylsilyl)amide (LiN(SiMe3)2), phosphorus trichloride (PCl3), phosphorus pentachloride (PCl5) and thionyl chloride (SO2Cl2) and 2.5 M n-buthyllithium in toluene were received by Sigma Aldrich and Celite 545 from Carl Roth. Toluene, dimethyl sulfoxide (DMSO), acetonitrile and diethyl ether were bought in anhydrous quality from VWR Chemicals. Celite 545 was dried prior to use at 110 °C for 2 days, all other chemicals were used as received. All reactions were performed in an argon atmosphere. FTIR data were measured on a Vertex 70 FTIR spectrometer from Bruker equipped with a Platinum ATR unit. 31P-NMR spectra were recorded with a DPX-200 from Bruker, using 85% H3PO4 in D2O as a reference. Temperature modulated differential scanning calorimetry (DSC) measurements were performed on a Discovery DSC from TA Instruments using aluminums pans with lid. Electrochemical impedance spectroscopy were performed with a Gamry Interface 1010E Potentiostat/Galvanostat/ZRA to determine the ionic conductivity of the final polymer.
2.2. Poly(dichlorophosphazene)
Cl PCl3
+
LiNSi(CH3)2
1. SO2Cl2 2. PCl5
N
P Cl
(NPCl2)n Figure 1: Synthesis of poly(dichlorophosphazene).
(1) n
Journal Pre-proof Synthesis of this precursor was performed following the approach of Wang.[16] LiNSi(CH3)2 (5.17 g, 30.9 mmol) was dissolved in 120 ml anhydrous toluene and cooled down to 0 °C. Then, PCl3 (2.7 ml, 30.9 mmol) was added dropwise over 10 minutes and the reaction mixture stirred 30 minutes at 0 °C followed by 1 h at room temperature. After cooling to 0°C again, SO2Cl2 (2.55 ml, 31.5 mmol) was added dropwise over 10 minutes and the mixture stirred 1 h at 0°C, before adding PCl5 (316 mg, 1.52 mmol) and stirring for 18 h at room temperature. The mixture was then filtered through Celite 545 and the volatiles were removed at the rotary evaporator. The resulting yellow-orange, viscous mass was dried in an oil pump vacuum. Yield: 2.9 g, 25.2 mmol, 81 %
2.3.1. Poly(phosphoramidic acid) Cl N
O DMSO
P Cl
n
-H3CSCH2Cl
H N
P
(2)
OH n (H2PO2N)n
Figure 2: Synthesis of poly(phosphoramidic acid). Anhydrous DMSO (15 ml) was carefully added to the dried poly(dichlorophosphazene) (1 g, 8,63 mmol) in a 100 ml flask in a water bath under slow stirring at room temperature. After maintaining these conditions for 2 h, the clear yellow solution was heated up to 40 °C and stirred for 48 h. The reaction was monitored by 31P-NMR spectroscopy to ensure complete conversion. Then, the oil bath was removed and a colorless solid formed at the inside surface of the flask was scraped back into the solution. The mixture was stored in an ultrasonic bath for 10 minutes and then stirred for 24 h at room temperature. Afterwards, to remove the chloromethyl methyl sulfide side product, the mixture was first stored in vacuum for three hours. Then, it was refilled with 3 ml of anhydrous DMSO and stirred for 18 h at room temperature before it was washed four times with 15 ml anhydrous diethyl ether. Remaining volatile substances were removed in vacuum. The purified solution was then usually used for the next step of the one pot synthesis, so no yield is given here for the intermediate product (H2PO2N)n. However, in order to isolate the intermediate for analysis, 10 ml of the reactions solution were washed four times with 10 ml anhydrous diethyl ether after two days at 40 °C. Then the solution was mixed with 20 ml anhydrous acetonitrile and stored in an ultrasonic bath for 10 minutes. The resulting colorless solid was filtered out, washed with 5 ml anhydrous acetonitrile and dried in vacuum.
Journal Pre-proof 2.3.2 Poly(dilithium phosphoramidate) O
O H N
n-BuLi
P OH
Toluene n
Li N
P
(3)
OLi n (Li2PO2N)n
Figure 3: Synthesis steps of polymeric LiPON. First, the freshly degassed poly(phosphoramidic acid) solution was diluted with 15 ml anhydrous DMSO and put into a cold water bath, before 2.5 M n-buthyllithium in toluene (7.6 ml, 19 mmol, 2.2 eq.) was added dropwise. The dirty-yellow reaction mixture stirred for 96 h at room temperature. Afterwards, volatile compounds were removed in vacuum and the solution was washed three times with 30 ml anhydrous diethyl ether. After adding 60 ml anhydrous acetonitrile the mixture was stored in an ultrasonic bath for 10 minutes. The precipitated colorless solid was filtered out, washed with 6 ml of anhydrous acetonitrile and dried in vacuum. Yield: 454 mg, 5 mmol, 58% [relating to (NPCl2)n]
2.3.3. Conductivity measurements A pellet of polymeric LiPON pressed at 60 bar was placed between two stainless steel electrodes in a custom made electrochemical glass cell. The cell was put in an oven and heated up to 80°C. Then, an impedance spectrum was measured in the frequency range from 106 Hz to 1 Hz with an amplitude of 200 mV.
3. Results and Discussion The synthesis of polymeric LiPON was carried out in three steps. First, poly(dichlorophosphazene) (1) was synthesized which is a common precursor for polyphosphazenes (Figure 1). Following the synthesis procedure of Wang resulted in a gel-like yellow-orange solid. Its FTIR spectrum shows two bands at 1208 cm-1 and 741 cm-1 (Figure 4a) which can be assigned to the P=N-bond and the P-Clbond.[17] In addition, the 31P NMR spectrum of 1 shows one signal at -16.8 ppm that is in good agreement with the literature value, so the synthesis was successful (Figure 4c). [16,17] Since no other signals appeared in the spectra and the product could be completely dissolved in toluene or THF, no crosslinking or other side reaction took place.
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Figure 4: FTIR spectra of (NPCl2)n (a) and (H2PO2N)n (b); 31P-NMR spectra of (NPCl2)n and (H2PO2N)n (c). For the next step, DMSO was used as a chlorine-oxygen transfer reactant as found by Walsh and coworkers (Figure 2).[18] Poly(dichlorophosphazene) (1) was slowly dissolved in a large amount of DMSO within a water bath, since the reaction showed a very exothermal behavior. This way, DMSO acts as both solvent and reactant, consuming the P-Cl-groups upon dissolving. This is necessary because the formed P-OH groups can react with the educt forming crosslinking P-O-P groups whereby the thermoplastic behavior of the material will be lost, as it is the case if the reaction is performed in THF with an equivalent amount of DMSO. By using pure DMSO this side reaction is suppressed. The resulting (H2PO2N)n could be isolated and its FTIR spectrum measured. Compared to (1) the bands of the P-Cl and P=N-groups do not appear anymore, indicating that a complete hydroxylation has taken place followed by a rearrangement of the double-bond system. Instead, the FTIR spectrum of (2) reveals several new bands at 931 cm-1, 1090 cm-1, 1237 cm-1 and 1678 cm-1 which can be assigned to P-OH, P-O, P=O and O-H-functional groups respectively (Figure 4b).[19] According to literature, the bands for the N-H group bridging the phosphor atoms should be located between 3200 cm-1 and 2900 cm-1 and between 1440 cm-1 and 1220 cm-1.[20] So they are most likely overlain by the C-H stretching and deformation vibrations around 2950 cm-1 and 1400 cm-1 of remaining impurities in the product. These and other bands like the one at 960 cm-1 or the shoulder at 1110 cm-1 indicate that there is still some DMSO or chloromethyl methyl sulfide left in the [H2PO2N]n, but this impurities are minimal due to the low intensity of the C-H stretching vibration around 2950 cm-1. However, a distinct assignment for the respective bands was not possible, most likely because both chloromethyl methyl sulfide and DMSO form adducts with [H2PO2N]n which may cause their absorption bands to shift.[18,21–23] Furthermore, it cannot be determined if crosslinking reaction takes place since the P-OH band overlays
Journal Pre-proof the band of the P-O-P-group around 900 cm-1.[19] But since the product was still soluble in DMSO it is likely that this side reaction either does not take place or only to a very small degree. The assumption of a full conversion of the educt is further supported by the 31P-NMR spectrum which shows no educt signal at 16.8 ppm. Instead, one signal at 0.3 ppm appears that is very similar to that of H3PO4 in DMSO-d6 and imidodiphosphate in D2O (Figure 4c).[24] So the phosphorus atoms in our product have a similar chemical environment as in these compounds. The structure of (2) is very alike to that of phosphoric acid except for the nitrogen, but since both oxygen and nitrogen have a very similar electronic effect on the phosphorus the expected difference of its chemical environment in the same deuterated solvent should be rather minor. Combined with the FTIR data this indicates that the signal at 0.3 ppm can be assigned to the desired polymer structure and that the synthesis was successful. Thus, the completely conversed and purified (H2PO2N)n-DMSO solution could then be lithiated in the last step using n-buthyllithium (Figure 3). This reactant can lithiate the product either directly or indirectly by lithiating the DMSO, resulting in an organic super-base that reacts with the protons of the polymer.[25] After lithiation, the 31P-NMR spectrum of the isolated product shows one signal around 0.6 ppm which is very similar to that of the educt (Figure 5b). Such a minor difference can be the result of lithiation and indicates that there are no changes to the bonds at the phosphorus atoms. Therefore, the backbone of the polymer is intact and reactions can only have taken place at the proton sides which are not directly bond to the phosphorus. This is confirmed by the FTIR spectrum of the product in Figure 5a.
Figure 5: FTIR and NMR spectra of (Li2PO2N)n. It shows just one band at 1016 cm-1 that can be assigned to PO--bonds and thus to PO-Li+ groups, as in Li3PO4. [19,26] The extreme broadness of this band may indicate that the negative charge is distributed between both oxygen atoms, so the state of both P-O-bonds is between a single and a double bond instead of one double bond and one single bond. Thus, lithiation took place. By comparing the integrals of the band from the remaining P-OH-groups at 956 cm-1 and from the PO--band a degree of lithiation of around 95% was determined. This is in accordance with the nearly completely absent bands of O-H and N-H groups which also indicates a high degree of lithiation at the nitrogen site. Since the P-OHband is nearly gone in the FTIR spectrum of the product, it was also possible to examine if the resulting material shows crosslinking from the hydroxylation step. Both bands at 883 cm-1 and 712 cm-1 that are assigned to P-O-P-bonds can be found in the spectrum of (3), but since they are very small compared
Journal Pre-proof to the PO--band the material should only show minor crosslinking which shouldn’t affect the thermoplastic behavior of the polymeric LiPON (Inset Figure 5a).[19]
Figure 6: Reversing heat flow of polymeric LiPON in a temperature modulated DSC (a), and a film of polymeric LiPON, cast from a THF solution on a glass substrate (b). This is also proven by the fact that the material can still be dissolved in DMSO, Toluene and THF and a film of the polymer can be casted with the latter two (Figure 6b), although these are fragile and easy to damage and their surface is rather rough. However, the polymeric LiPON shows no sign of melting up to 300°C, and this is most likely because of the great number of ionic interactions between the polymer chains rather than crosslinking. Since the synthesized material is a powder, it is also possible to press it into pellets or other shapes, thus offering further processing options besides solution-based ones. Furthermore, the chemical synthesis offers the possibility of upscaling in contrast to the PVDprocess used for glassy LiPON. However, the conductivity measurements of a 400 µm thick pellet polymeric LiPON resulted in a value of 1.7·10-10 Scm-1 at 80°C which is probably electric conductivity. So although there is lithium in the polymer it is immobile, most likely because there are too many interactions with the polymer backbone, similar to the observation for the melting behavior. And the polymer chains are also very rigid since the DSC data of polymeric LiPON shows no glass transition between -90°C and 250°C which decreases the mobility of the lithium even further (Figure 6a). So the Li+-conductivity of polymeric LiPON is lower than that of glassy LiPON. But much more interesting is the interface between polymeric LiPON and lithium metal. The reason why glassy LiPON can be used in thin film batteries with lithium metal anodes is due to its wellconducting and very stable SEI consisting of Li2O, Li3N and Li3P. Especially the latter two show high Li+conductivities in the range of 10-3 to 10-4 Scm-1 which is several magnitudes better than for both LiPONs. Since polymeric LiPON has a similar molecular formula per monomer unit it should form the same SEI with lithium metal. There is currently work in progress to confirm this hypothesis because even if polymeric LiPON itself is no Li+-conductor, it could form a highly conductive layer upon decomposition. In this case, its addition to polymer electrolytes or its use in copolymers could also drastically improve the SEI of other solid state electrolytes and benefit their performance.
Journal Pre-proof 4. Conclusion In this work, we successfully synthesized polymeric LiPON. Following a two-step one pot synthesis we first modified poly(dichlorophosphazene) with oxygen-containing functional groups and then substituted all protons by lithium ions. The resulting polymer had a similar molecular formula per monomer unit as glassy LiPON, but could be isolated as a powder and dissolved in various solvents. Furthermore, the chemical synthesis is scalable and offers even larger yields, a clear advantage to sputter processes. Although the polymer is not suitable Li+-electrolyte itself due to the similar molecular formula it is likely to decompose into the same binary salts Li2O, Li3P and Li3N against lithium metal as glassy LiPON. Since the latter two have high lithium ion conductivities between 10-3 and 10-4 Scm-1 the synthesized polymer could form a good Li+-electrolyte upon decomposition against lithium metal which is currently investigated.
Acknowledgements The authors thank Katharina Koschek of the Fraunhofer Institute IFAM for providing a laboratory environment and the necessary equipment and Iris Gottschalk of the Fraunhofer Institute IFAM for performing the temperature modulated DSC measurement.
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CRediT author statement Gideon Abels: Conceptualization, Investigation, Writing – Original Draft Ingo Bardenhagen: Resources, Validation, Writing – Review & Editing Julian Schwenzel: Supervision
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Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:
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Highlights - Polymeric LiPON was synthesized as an alternative to glassy LiPON with the same molecular formula. - Preparation took place in a two-step one pot synthesis from poly(dichlorophosphazene). - Compared to glassy LiPON, the polymer can be dissolved in different organic solvents enabling film casting