Pyrophosphate Chemistry toward Safe Rechargeable Batteries

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Pyrophosphate Chemistry toward Safe Rechargeable Batteries Mao Tamaru, Sai-Cheong Chung, Daisuke Shimizu, Shin-ichi Nishimura, and Atsuo Yamada Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm4010739 • Publication Date (Web): 22 May 2013 Downloaded from http://pubs.acs.org on May 24, 2013

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Pyrophosphate Chemistry toward Safe Rechargeable Batteries Mao Tamaru †‡, Sai Cheong Chung†, Daisuke Shimizu†, Shin-ichi Nishimura†, and Atsuo Yamada†* †

Department of Chemical System Engineering, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-Ku, Tokyo 113-8656 , Japan ‡

Mitsubishi Motors Corporation, 1 Nakashinkiri, Hashime-Cho, Okazaki-shi, Aichi 444-8501, Japan

* [email protected] Phone: +81-3-5841-7295 Fax: +81-3-5841-7488


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Abstract We demonstrate that pyrophosphate anion can result in metal pyrophosphate cathode materials with high thermal stabilities. High temperature behaviors for the delithiated states of Li2FeP2O7 and Li2MnP2O7 in the P21/c symmetry are studied. Above 540 oC, the singly delithiated structure LiFeP2O7 undergoes an irreversible phase transformation to the ground state polymorph with a symmetry of P21. Intermediate delithiated compounds Li2-xFeP2O7 ( 0 < x < 1 ) convert to a mixture of LiFeP2O7 in the P21 symmetry and Li2FeP2O7 in the P21/c symmetry. No decomposition is observed for both the singly and partially delithiated compounds until 600 oC showing the high thermal stabilities of the compounds. Analysis of phase stabilities reveals that LiFeP2O7 (P21/c) is intrinsically more stable than FePO4 (olivine) against reduction (high temperature). Similar high thermal stability is also observed for Li1.4MnP2O7. It decomposes to Li2MnP2O7, Mn2P2O7, LiPO3 and O2 at 450 oC, much higher than the olivine counterpart MnPO4. The high stability of these metal pyrophosphates is rationalized by the stability of the P2O74- anion.

Keywords Lithium-ion battery, Cathode, Thermal Stability, Pyrophosphate, Phase Diagram



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Introduction Applications of lithium-ion-batteries as household items raise a pertinent concern about their safety, in particular for large size devices such as automobile batteries where large amount of materials are involved. Typical lithium battery cells contain highly flammable organic solvents as constituents of the electrolyte and are one of the main concerns of safety. Charged cathodes are very oxidizing, they may undergo self-decompositions at elevated temperature with the release of oxygen. An accidental overheat may initiate the cathode breakdown and ignition of solvents by the released O2 could constitute a positive feedback which eventually may lead to a thermal runaway. For charged states of common oxide a stability trend of LixMn2O4 > LixCoO2 > LixNiO2 can be found1. Heat generation from cathode decomposition can be large e.g. for LixNiO2 heat of about 160 kJ/mol has been measured2 and temperature of reaction can be as low as 100 oC. Strategies to remedy include partial substitution of unstable transition metals with stable ones or coating of the oxides with stable phosphates such as AlPO4. Another strategy may resolve to polyanionic materials. It is commonly believed that the strong covalent bonds in polyanions will hold the oxygen atoms strongly through the covalent bonds and therefore stabilize the structure against O2 release. However, high throughput computational studies of series of oxides and phosphates found the contrary3. The trend is that oxides are thermodynamically slightly more stable than phosphates for the same metal in a given oxidation state. Another theory states that voltage and partial pressure of oxygen constitute a linear relation4,5, namely, a higher voltage implies a less stable cathode. Computational study found similar trend3, the transition metals and their oxidation states are the main factor accounting for such correlation. However, large scatterings from the linear trend are observed. Clearly a more detailed elucidation of the influence of anions on the stability of cathode is desirable. Among polyanionic materials olivine structure is the most studied. The stabilities of the charged electrodes follow the order LiFePO4 > LiMnPO4 > LiCoPO4 ~ LiNiPO4, which coincides with the cell voltages of 3.42 V6,7, 4.1 V6, 4.8 V8 and > 5 V9 respectively. This correlation of cathode stability to cell voltage is found to be quite general4,10. FePO4 (delithiated state of LiFePO4) transforms11 to the trigonal ground state at a high temperature of about 600-700 oC. However, intermediate structures LixFePO4 (x < 1) show lower stabilities, above 500 oC they decompose to


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Fe3(P2O7)211, Fe7(PO4)611,12, Fe3(PO4)213 and Fe2P2O713 with the loss of oxygen. In general, the conversion temperature of above 500 oC is high and the amount of oxygen release is small. However, the stability of the manganese counterpart MnPO4 is lower. Upon heating it decomposes to Mn2P2O714–16 and Mn3(PO4)212,13 with the release of O2. It is controversial about the temperature of the decomposition. Majority of reports stated temperature of 210-250 oC12–14, but as high as 480 oC17 has also been claimed. High voltage olivine structures are much less stable, CoPO4 decomposes to Co2P2O7 readily at 100-200 oC18. While pyrophosphate is usually the product of phosphate decomposition, it can be utilized as anion of a cathode material19. Recently, pyrophosphate19 Li2FeP2O720,21 and related compounds21,22 have draw increasing attentions. Among other advantages, pyrophosphates consistently deliver the highest voltages among polyanionic cathodes22,23. Li2FeP2O7 will be delithiated to a metastable structure of LiFeP2O7. The high lying in energy for the delithiated state has the benefit of delivering high cell voltage (> 3.5 V), in contrast it also raises concerns about their thermal stability. Here we report a detailed study about the thermal behaviors of Li2MP2O7 and it’s charged states Li2-xMP2O7 (0 ≤ x< 1), M = Fe, Mn. We show that the high voltages of pyrophosphates do not diminish their stabilities, instead, they are substantially more stable than phosphates. As a result we propose that pyrophosphate and in general condensed polyanions may be the anions of choice for designing safe cathode materials.



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Experimental Methods Li2FeP2O7 product was prepared by conventional solid-state synthesis using a stoichiometric mixture of Li2CO3 (Wako, 99+%), FeC2O4·2H2O (Kojundo, 99+%) and (NH4)2HPO4 (Wako, 99+%), which mixed by planetary ball-milling for 2 h (400 rpm). For milling, Cr-hardened stainless steel (Cr-SS) milling media and container were used. The precursor mixture was then calcined at 600 °C for 12 h in a tubular furnace under a steady Ar flow. Chemical oxidation (reduction) was carried out in acetonitrile by using NO2BF4 (LiI) as oxidizer (reductant) and can be represented by the following equations: Li2FeP2O7 + NO2BF4 → LiFeP2O7 + LiBF4 + NO2 LiFeP2O7 + x(3/2)LiI → Li2-xFeP2O7 + x(1/2)LiI3. Li2MnP2O7 was synthesized by the same method as Li2FeP2O7 except that dehydrated MnC2O4 was used as starting material. Powder X-ray diffraction (XRD) patterns were measured by a Rigaku RINTTTR III powder diffractometer equipped with Cu Kα radiation operating at 50 kV and 300mA. A typical scan was performed in the 2θ range of 10-80˚ with intermittent step of 0.02˚. High temperature XRD (HT-XRD) measurements were carried out by attaching a Rigaku Reactor-X to the powder diffractometer, they were performed by keeping the temperature for at least 1 hour under N2 flow (100 cc/min). Mössbauer spectroscopy was measured by a Topologic System Inc. spectrometer equipped with a 57

Co γ-ray source with due calibration done by employing an α-Fe foil as standard.

Thermogravimetric analysis (TG) was measured by a SII Corp. TG-DTA analyzer, differential scanning calorimetry (DSC) was measured by a Rigaku DSC analyzer. These thermal analyses were performed up to 600˚C with 5˚C/min under Ar flow (200 cc/min). For electrochemical charge-discharge measurements, the cathode pellet was formulated by mixing 85 wt% Li2FeP2O7, 10 wt% carbon black and 5 wt% polytetrafluoroethylene (PTFE) binder. The cathode pellet was pressed on an Al film current collector and dried in vacuum at 120 °C. It was then assembled to 2032-type coin cell inside an Ar-filled glove box with Li metal foil as anode. Separator is a polypropylene film soaked with 1M LiPF6 dissolved in ethylene carbonate/diethyl carbonate solvents (EC/DEC, 3/7 v/v). Galvanostatic charge-discharge cycling was conducted in the voltage range 2.0−4.5 V at a rate of C/20 at 25 °C. To prepare the


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pellet for the case of Mn, 75 wt% Li2MnP2O7 and 20 wt% SuperP carbon black were mixed with 5 wt% of PTFE. The charge-discharge cycling was performed in the voltage range 2.0−4.8 V at a rate of C/100 at 40 °C. The rest were identical to the Fe case. The phase diagrams were constructed using the method and mainly the data of Ong et al.23. In order to obtain the grand potentials from the reported formation energies23 the atomic energy of oxygen atom (-4.229575 eV) in O2 was added accordingly back to the reported data. The stable phases at different chemical potentials of O2 were obtained by finding the convex hull of the grand potentials23.



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Results Crystal Structures Pyrophosphate as cathode for lithium-ion battery was first conducted24,25 by electrochemically inserting lithium into the charged state (LiFeP2O7) that has the LiInP2O7 structure (space group P21). The recent application20,21 is realized by starting from the discharged state Li2FeP2O7 having the P21/c symmetry. Both reactions are topotatic therefore both the charged and the discharged states are different. The change in crystal structure allows a substantial increase in the cell voltage from 2.9 V (P21) to 3.52 V (P21/c). Figure 1 shows the charge discharge profile of Li2FeP2O7 (P21/c). The charging is near 100 % which indicates the resulting active material can be designated as LiFeP2O7. The crystal structures of LiFeP2O7 in the two symmetries are shown in Figure 2. The P21/c structure was not totally solved experimentally, therefore the theoretically optimized structure is shown. The structure shows alternative Li and FeP-O layers along the b axis (Figure 2). The Li ions are in either tetrahedral or pyramidal sites and the Fe ions occupy an octahedral and a pyramidal site. In the lithiated phase Li2FeP2O7, the octahedral and pyramidal Fe share edges with each other and in turn share edges with a common pyramidal site where lithium is located. The two pyramidal sites show considerable exchange between the Li and Fe ions. Li2MnP2O7 adopts similar structure but without the site exchange. When single Li is removed, in LiFeP2O7, some of the pyramidal Fe ions migrate to Li sites further away from the octahedral Fe ion. This migration of the Fe ion upon charging have also been predicted theoretically26,27 in the edge-sharing triplite phase of LiFeSO4F. In the P21 structure the Fe atoms are all octahedrally coordinated. The octahedra are connected indirectly through sharing corners with 5 different pyrophosphate anions (Figure 2), one of the pyrophosphate share corners with the same metal similar to a chelation. In the delithiated structure, lithium atoms are located at highly distorted tetrahedral sites. When lithiated the additional lithium ion is computed to be at a tetrahedral site face sharing with an iron octahedron. Figure 2 also shows the XRD patterns of the delithiated structures in the two symmetries. Thermal Behavior Figure 3a shows the TG and DSC scan of LiFeP2O7 obtained from chemical oxidation of Li2FeP2O7. The electrochemically charged sample essentially shows the same features (Supporting Information Figure S1), however, it contains the binder


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PTFE that starts to decompose at a temperature of interest here (~500 oC)28 so we will use the chemically oxidized sample for discussion. The DSC curve shows a small exothermic peak at about 250 oC, integration of the peak gives a negligible heat of about 1kJ/mol. Another more exothermic reaction started at about 520 oC and peaked at 540 oC, integration shows a moderate evolved heat of about 21 kJ/mol, similar DSC peak at 510 oC has also been observed before29. TG graph shows no change in weight at both 250 oC and 540 oC where DSC peaks are found. In the presence of electrolyte the DSC curves show additional peaks at about 220 and 280 oC, for both Li2FeP2O7 and LiFeP2O7 (Supporting Information Figure S2), these temperatures coincide with the intrinsic thermal stability of the electrolyte30. Since we are interested in the intrinsic stability of the cathode material, in the following we will only discuss the samples without electrolyte. We further studied the transformation by temperature dependent XRD performed in dry nitrogen. Figure 4a shows the evolution of the XRD patterns with temperature. The patterns characterizing the P21/c symmetry of LiFeP2O7 persist below 250 oC. Starting from 250 oC the XRD starts to broaden and then sharpens again at about 450 oC. New peaks emerge again at about 540 oC and the transformation is completed at 600 oC. Comparing with Figure 2 indicates that the final phase corresponds to the P21 symmetry of LiFeP2O7. No trace of peaks from compounds other than that of LiFeP2O7 remains at 600 oC showing that the transformation at intermediate temperatures (250 – 450 oC) does not bring about decomposition of the structure. This is also supported by Mössbauer spectroscopy of the sample that were heat treated at 400 oC, it shows that the Fe3+ ions are not reduced (Supporting Information Figure S4). Furthermore, the marginal heat released (about 1 kJ/mol) is also unlikely to originate from a decomposition reaction. These three facts hint at a polymorphic transformation. The exact crystal structure of this intermediate temperature polymorph is under investigation. After establishing that the exothermic reaction at 540oC corresponds to the phase transformation of LiFeP2O7 to the P21 symmetry we examine the intermediate lithiated structures Li2-xFeP2O7. For olivine, although the totally deliathiated state FePO4 does not decompose, unfavorable oxygen evolution reaction may occur for the nonstoichiometric structures11. Figure 3b shows the DSC and TG profiles of Li1.5FeP2O7.

There is no noticeable thermal behavior until 450oC. The phase

transition peak to P21 symmetry as shown by DSC is again observed but is lower than



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that of LiFeP2O7 and located at about 490oC. TG result again shows no weight change in the whole temperature range. In situ XRD was also conducted and is shown in Figure 4b. The XRD pattern of Li1.5FeP2O7 has been characterized before31 as that of a solid solution. From 200 oC to 460oC we see the same pattern changes as LiFeP2O7 (Figure 4b). At temperature near the DSC peak the XRD has noticeable change. Above 500oC the XRD totally converts to the new patterns indicating that the transformation is completed. We can identify that the new XRD patterns correspond to a mixture of the P21/c phase of Li2FeP2O7 and the P21 phase of LiFeP2O7 (Figure 4b). Figure 5 shows variation of the transformation peak temperatures for the pyrophosphates in different charged states, Li2-xFeP2O7 (0

Pyrophosphate Chemistry toward Safe Rechargeable Batteries

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