Molecular Engineered Safer Organic Battery through the Incorporation

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Molecular Engineered Safer Organic Battery through the Incorporation of Flame Retarding Organophosphonate Moiety Hyun Ho Lee, Dongsik Nam, Choon-Ki Kim, Koeun Kim, Yongwon Lee, Young Jun Ahn, Jae Bin Lee, Ja Hun Kwak, Wonyoung Choe, Nam-Soon Choi, and Sung You Hong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19349 • Publication Date (Web): 02 Mar 2018 Downloaded from http://pubs.acs.org on March 3, 2018

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Molecular Engineered Safer Organic Battery through the Incorporation of Flame Retarding Organophosphonate Moiety









Hyun Ho Lee, Dongsik Nam, Choon-Ki Kim, Koeun Kim, Yongwon Lee,



Young Jun Ahn,† Jae Bin Lee,† Ja Hun Kwak,† Wonyoung Choe,*,‡ Nam-Soon Choi,*,† and Sung You Hong*,†



School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulsan 44919, Republic of Korea ‡

Department of Chemistry, Ulsan National Institute of Science and Technology, 50 UNIST-gil, Ulsan 44919, Republic of Korea

*Corresponding Authors [email protected] (W.C.); [email protected] (N.-S.C.); [email protected] (S.Y.H.)

KEYWORDS: lithium-ion batteries, electrodes, metal-organic frameworks, organophosphorus compound, safety

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ABSTRACT Here we report the first electrochemical assessment of organophosphonate-based compound as a safe electrode material for lithium-ion batteries, which highlights reversible redox activity and inherent flame retarding property. Dinickel 1,4-benzenediphosphonate delivers a high reversible capacity of 585 mA h g−1 with stable cycle performance. It expands the scope of organic batteries, which have been mainly dominated by organic carbonyl family to date. The redox chemistry is elucidated by X-ray absorption spectroscopy and solid-state

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P NMR

investigations. Differential scanning calorimetry profiles of the lithiated electrode material exhibit suppressed heat release, delayed onset temperature, and endothermic behavior in the elevated temperature zone.

1. INTRODUCTION Safety is perceived as one of the major designing factors for high-capacity lithium-ion batteries (LIBs). Harsh operating conditions occasionally trigger thermal runaway of flammable electrolytes.1–3 To reduce safety hazards, several strategies have been utilized: (i) Flame retardant (FR) additives including organophosphorus compounds are formulated along with conventional carbonate solvents.4–8 (ii) Polymer, gel, or solid electrolytes can be used to replace liquid electrolytes.9–11 (iii) Surface modifications of electrodes have also been conducted to alleviate the consecutive flame propagation processes at the electrode/electrolyte interface.12–16 However, they often compromise electrochemical performance owing to the increased interfacial impedance.1 Although the balanced inherent safety and highperformance are highly desirable, the molecular engineering to implement FR moiety within the active material while retaining reversible redox activity has been a daunting challenge. Among the various potential electrode materials, organic compounds have often been referred to as emerging components of next-generation rechargeable batteries due to marked structural diversity and well-established synthetic protocols.17–22 Redox active functionalities 2 ACS Paragon Plus Environment

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including thiocarboxylate-, organosulfur-, free radical-, and carbonyl-based functional groups have been extensively studied.17–25 In particular, a variety of organic carbonyl compounds including carboxylates, anhydrides, imides, and quinones have been evaluated.22,25–31 However, redox active organic molecules often suffer from their poor thermal stability at elevated temperatures restricting their practical applications, unless inverse-Wurster type salt structures are employed.22 We speculated that the utilization of self-extinguishing compounds as smart electrodes might inherently minimize thermal safety hazards. Yet, the prime task associated with this approach is the judicious selection of FR moiety possessing reversible redox activity. Herein we disclose the utilization of dinickel 1,4-benzenediphosphonate (Ni2BDP) as a dual-functional electrode material for organic-based LIBs. Scheme 1 represents an unusual redox chemistry of π-conjugated organophosphonate expanding the scope of organic batteries, primarily led by organic carbonyl family. The metal-organic coordination scaffold ensures thermal safety and reliability of LIBs.

Scheme 1. (a) Redox Chemistry of BDP ligand and (b) Synthesis of Ni2BDP.

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2. EXPERIMENTAL SECTION 2.1. Characterization. Ex-situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data were recorded using the 6D UNIST beam line at the Pohang Light Source (Pohang, Korea). It operates at an electron energy of 3 GeV and a stored top-up current of ~300 mA. The X-rays coming from the bending magnet are monochromated using a Si (111) double crystal monochromator (Kohzu Precision Co. Ltd., Japan) with an energy resolution (∆E/E) of 2 × 10−4. Higher order harmonic contaminations were suppressed by detuning the monochromator to reduce the incident X-ray intensity by ~20%. The incidence and transmitted X-ray intensities were monitored using IC Spec Ionization Chambers (FMB Oxford, UK). Energy calibration was simultaneously carried out for each measurement with a reference metal foil placed in front of the third ionization chamber. The Fourier transform was calculated for EXAFS data ranging from 3.0 to 12.0 Å-1 (wavenumber scale). The data were measured in transmission mode using gas-filled ionization chambers as detectors. Solid-state

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P NMR spectra were obtained from a Varian

600 spectrometer at a spin rate of 25 kHz with a 1.6 mm Agilent Fast MAS probe (one pulse mode). Proton and phosphorus nuclear magnetic resonance (1H and 31P NMR) spectra were obtained from a Varian 600 spectrometer; chemical shifts are given in the δ-scale in ppm, and residual solvent peaks were used as references. Fourier transform infrared (FTIR) spectra were measured on a Nicolet FT-IR 200 (ATR, Thermo Scientific). Absorption bands were recorded in wavenumbers (cm−1). Thermal analysis was performed at a heating rate of 10 °C per min under nitrogen atmosphere using a Q600 thermogravimetric analyzer (TA Instruments). Powder X-ray diffraction (XRD) data were collected on a Rigaku D/MAX2500V/PC powder diffractometer using Cu K-α radiation (λ = 1.5405 Å). The ex-situ XRD samples were prepared in an argon-filled glovebox. The samples were obtained from cell disassembly step, and covered on beryllium windows. Scanning electron microscopy 4 ACS Paragon Plus Environment

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(SEM) samples were examined in a Nano 230 field-emission SEM instrument. Porosity was evaluated by N2 adsorption isotherm using Micromeritics ASAP 2020 at 77 K.32 The electrode samples for differential scanning calorimeter (Mettler Toledo DSC1) measurements were retrieved after lithiation process till 0.0 V versus Li/Li+ with DMC washing. DSC measurements were performed with coexisting electrolyte (0.8 M LiPF6 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1, v/v)) in a hermetic stainless steel pan at a heating rate of 5 °C min-1 in the dry room under the N2 purging conditions. The areal mass loading for Ni2BDP electrode was 1.24–1.44 mg/cm2. The electrode sample was fabricated by mixing active material, Super P, and cellulose sodium salt (CMC) binder in a 4:3:1 weight ratio, and thus the areal mass loading of active material was 0.62–0.72 mg/cm2. NiTP was prepared according to the previously reported method.33,34 Nickel oxide was purchased from the standard supplier (Sigma-Aldrich; nanopowder; particle size < 50 nm; 99.8%). Along with Ni2BDP sample, NiTP, nickel oxide, graphite electrodes were constructed by mixing the corresponding active material, Super P, and CMC in the same weight ratio (4:3:1). The mass loading values of NiTP, NiO, and graphite were 0.62–0.69, 0.59–0.62, and 0.67–0.72 mg/cm2, respectively. 2.2. Electrochemical Measurements. Active material was mixed with Super P and CMC in a 4:3:1 weight ratio. The electrochemical performance was evaluated using 2032 coin-type cells with a lithium metal anode and 0.8 M LiPF6 in a mixture of EC and DEC (1:1, v/v) electrolyte solution. Galvanostatic experiments were performed at 25 °C under specific current densities of 100 mA g-1. 2.3. Synthesis of Compounds. The compounds have been prepared according to the previously reported methods.35–37 Tetraethyl 1,4-benzenebisphosphonate 1:35 1,4-Dibromobenzene (6.0 g, 25.4 mmol) and NiBr2 (4.45 g, 20.3 mmol) in 1,3-diisopropylbenzene (50 mL) were mixed under Ar

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atmosphere. The solution was heated to 200 °C connected with a condenser, then P(OEt)3 (33.8 g, 203 mmol) in 1,3-diisoporpylbenzene (10 mL) was added dropwise for 10 h. Then, the reaction mixture was stirred at 200 °C for 48 h under an argon atmosphere. After extraction with diethyl ether (200 mL×3) and washing with brine (200 mL), the combined organic layer was dried over Na2SO4, filtered, and concentrated in vacuo at 60 °C. Flash column chromatography (ethyl acetate:methanol:acetic acid, 90:7:3) afforded the desired product as a white solid. Yield: 6.98 g (78%); 1H NMR (600 MHz, CDCl3) δH = 1.32-1.35 (t, 12H, 4×CH3), 4.1-4.18 (m, 8H, 4×CH2), 7.88-7.92 (m, 4H, 4×Ar-H);

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P NMR (243 MHz,

CDCl3) δP = 16.73. 1,4-Benzenediphosphoric acid (H4BDP):36 Bromotrimethylsilane (24.3 g, 159 mmol) was slowly added to compound 1 (5.5 g, 15.9 mmol) in dichloromethane (70 mL). The solution was stirred for 15 h at room temperature (rt), and concentrated. DI water (25 mL) was added and stirred for additional 24 h at rt. Then, the mixture was concentrated to afford a white solid. Yield: 3.45 g (91%); 1H NMR (600 MHz, DMSO-d6) δH = 7.72-7.76 (m, 4H, 4×Ar-H);

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NMR (243 MHz, DMSO-d6) δP = 11.68. Dinickel 1,4-benzenediphosphonate (Ni2BDP; Ni2C6H4(PO3)22H2O):37 Ni(NO3)2·6H2O (735 mg, 2.52 mmol) and H4BDP (588 mg, 2.47 mmol) were dissolved in DI water (70 mL) with 1N NaOH (3.7 mL). The solution was transferred to a Teflon-lined autoclave (250 cm3 volume) and heated at 140 °C for 54 h. The light green powder was collected and washed with DI water for several times, then dried at rt in vacuo. Yield: 369 mg (43%).

3. RESULTS AND DISCUSSION Phosphonate ester was prepared from 1,4-dibromobenzene and triethyl phosphite in the presence of NiBr2 catalyst by the Michaelis–Arbuzov reaction (Scheme 1b).35 The precursor was then hydrolyzed promoted by trimethylsilyl bromide in dichloromethane/water to afford 6 ACS Paragon Plus Environment

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H4BDP.36 Nickel complexation to furnish Ni2C6H4(PO3)22H2O, Ni2BDP, was then accomplished according to the previously reported hydrothermal method.37 The FTIR spectrum of Ni2BDP exhibited characteristic phosphonate vibration peaks at 1150, 1093, and 953 cm−1 (Figure 1b). The complete removal of starting organic ligand was confirmed by the disappearance of the FTIR peaks from H4BDP. Since H4BDP can be deprotonated under the basic conditions of the hydrothermal processes, we also prepared Na4BDP through simple acid-base reaction. The clear peak shift of Na4BDP was observed with the phosphonate vibration peaks at 1060 and 964 cm-1 (Figure S2 in the Supporting Information). In addition, the cell parameters of metal-organic framework-type Ni2BDP obtained from the experimental patterns were well-matched with simulated powder XRD patterns. They indicate a pillared layered structure, which crystallizes in the orthorhombic space group of Pnnm (Figure 1c).37,38 TGA and DSC traces show unusually high dehydration temperature of ~320 °C. It is noteworthy that the profiles reveal distinct thermal stability of Ni2BDP (no decomposition up to ~750 °C, see Figure 1d).

Figure 1. (a)

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P NMR (243 MHz, CDCl3) spectrum of H4BDP, (b) FTIR spectra of H4BDP and

Ni2BDP, (c) XRD patterns of Ni2BDP, and (d) TGA and DSC data of Ni2BDP.

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In order to verify the redox feasibility of as-prepared Ni2BDP for LIBs, we explored the electrochemical behaviors of the electrode by galvanostatic charge/discharge versus Li/Li+ at a current density of 100 mA g−1 (see Figure 2). Based on the voltage profile and the corresponding dQ/dV measurement, successive two stages of lithiation in the first cycle could be distinguished (Figure S5a). The plateau ~1.2 V in the first cycle is attributed to the reduction of NiII to Ni0 state (Figure 2a).33,39 Notably, the subsequent slope is attributed to electrochemical lithiation to the π-conjugated BDP ligand, which is an unusual organic redox couple expanding the scope of organic-based rechargeable batteries. The redox mechanism will be discussed further vide infra (i.e., solid-state 31P NMR spectroscopic analysis). Super P lithiation (Figure S5b, Supporting Information) and formation of SEI may result in a low initial Coulombic efficiency (69.3%). However, the Coulombic efficiency was retained at ~99.1% from the second cycle (Figure 2b). In addition, the Ni2BDP electrode exhibited stable cycle performance with a reversible specific capacity of 750 mA h g−1 in the 10th cycle. The rate capability was further investigated. The capacity of pristine Ni2BDP at a high current density of 2000 mA g−1 was ~70% of the initial value at 100 mA g-1 (Figure 2c). In addition, anhydrous Ni2BDP was prepared through dehydration at 400 °C (Figure S6) to precisely determine the number of inserted lithiums. The electrochemical performance of this anhydrous electrode exhibited a reversible capacity of 862 mA h g−1 in the 10th cycle due to reduced molecular weight (Figure S7). After the capacity delivered by Super P (277 mA h g−1 at 100 mA g−1) was subtracted, it was found that pure Ni2BDP exhibited a reversible capacity of 585 mA h g−1 at a current density of 100 mA g−1 corresponding to 7.7 Li+ (Figure S8).

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Figure 2. Electrochemical studies of Ni2BDP. (a) Voltage-capacity traces, (b) cycle performance, and (c) rate performance.

At the outset of redox chemistry study, the nickel oxidation states of Ni2BDP were monitored by ex-situ XANES analysis on the basis of the redox peaks from the dQ/dV plot (Figures 3a and 3b, and Figure S5a in SI). Ni K-edge XANES spectrum of pristine Ni2BDP showed a characteristic weak pre-edge peak at 8333 eV, assigned to the 1s → 3d transition.40 In a plateau region-(ii), the intensity of white line was suppressed while showing the resembled spectroscopic features of the nickel foil used as a reference. This reflects that NiII is partially reduced to metallic phase. After passing through the plateau region-(iii), the white line was further shifted to the nickel foil, indicating the completely converted metallic state. 9 ACS Paragon Plus Environment

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With continued lithiation, no additional variation was detected. In the reverse process approaching 3.0 V, signals remain unaffected until region-(vii): metallic nickel did not participate directly in the electrochemical reaction before region-(vii). When the potential reached 3.0 V, the white line was shifted to the right side, but not to the original pristine state. This finding suggests that a fraction of reduced Ni0 is reverted to NiII.

Figure 3. (a) Voltage-capacity trace, (b) corresponding Ni K-edge XANES spectra of Ni2BDP, and (c) corresponding Ni K-edge EXAFS patterns: the green/dotted line indicates the Ni foil reference. The radial distribution function is uncorrected for phase shifts.

To get better insight into the redox mechanism, EXAFS analysis was then performed (Figure 3c). The EXAFS spectrum of Ni2BDP showed a strong Ni–O distance. In a plateau region-(ii), Ni–Ni distance unambiguously appeared with the remained Ni–O distance. After passing through the plateau region-(iii), the relative intensity of the Ni–Ni increased. In the subsequent lithiation up to region-(v), no pronounced signal changes were observed. During the delithiation process in the region between (vii) and (viii), no new peak appeared. However, the relative intensity of the Ni–Ni abruptly decreased. The weak intensity of Ni–Ni distance describes the incomplete conversion of Ni0 to NiII. The X-ray absorption spectroscopy (XAS) 10 ACS Paragon Plus Environment

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results are also in good agreement with the partially recovered capacity derived from the single nickel (~160 mA h g−1) as shown in Figure 3a. Although two NiII ions in Ni2BDP were initially reduced, only single NiII was recovered. With continued cycling, the reversible involvement of one equivalent NiII/Ni0 was observed (Figure S9). Notably the further reoxidation of the remaining Ni0 could be achieved, once forced delithiation conditions were applied beyond 3.5 V.41 Enhanced nickel reduction profile was observed during the second lithiation process (Figure S10). The redox process of the π-conjugated BDP ligand was then examined via solid-state 31P NMR spectroscopic analysis. At region-(ii), the resonance line of Ni2BDP emerged at 16.7 ppm (Figure 4). The chemical shift at 31.5–28.5 ppm may originate from the decomposition of LiPF6 from electrolytes or the derived SEI layer during cycling. After full discharge to 0.0 V at region-(v), an additional resonance band arose in an upfield region centered at 9.5 ppm. It indicates that the more reduced/saturated molecular structures are generated by the excessive lithiation.20,31,33 During the delithiation process approaching 1.1 V, the resonance line in the upfield disappeared while showing the recovered initial signal at 16.3 ppm. This reflects the involvement of the organophosphonate in the reversible lithiation–delithiation process.

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Figure 4. Solid-state 31P NMR (243 MHz) spectra of Ni2BDP upon discharge-charge processes.

On the basis of the combined XAS and NMR studies, the multi-electron reactions are suggested to involve binary metal- and organophosphonate redox couples. During the first discharge, two NiII-ions were fully reduced to Ni0 phase in the plateau region ~1.2 V. The successive lithiation of organophosphonate occurred in the remaining sloping region. During delithiation to 2.4 V, the reduced organic scaffolds were reverted to the initial π-conjugation state. After full recharging to 3.0 V, only one equivalent Ni0 is recovered to NiII state. During the subsequent cycles, the fractional nickel and the organic scaffold participated reversibly in electrochemical processes. The DSC traces (Figure 5) were then monitored to reveal the thermal characteristics, by comparing lithiated active materials including graphite, nickel oxide (NiO), and nickel terephthalate (NiTP). The peaks near 100–130 °C are attributed to the breakdown of metastable SEI layers.42,43 A fully lithiated Ni2BDP generated less exothermic heat than lithiated graphite and NiO. This finding suggests that more robust SEI layer is constructed on 12 ACS Paragon Plus Environment

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Ni2BDP during lithiation. The peaks around 200–300 °C typically indicate exothermic reactions, where flammable electrolytes and binders can be decomposed on the exposed electrode.43 Compared with lithiated NiTP (552.9 J g−1 at 228 °C), Ni2BDP revealed delayed onset temperature of 243 °C with a relatively low thermal release (324.4 J g−1). Moreover, lithiated Ni2BDP having higher lithium uptake exhibited better thermal stability than lithiated graphite and NiO. Remarkably, a distinct endothermic peak was also observed at 254 °C. Endothermic reactions are associated with phase transitions (e.g. melting and boiling) and molecular decompositions. Flame retardants including the inorganic elements (e.g. P, B, Al, and Mg) can undergo decomposition reactions in the range of 150–400 °C, while exhibiting the endothermic release of non-flammable vapor-phase molecules.44 Phosphorous-based FRs participate in the radical scavenging steps while removing OH and H radicals that are generated by the thermal decomposition of electrolyte solvents (EC and DEC) at elevated temperatures.1,4–8 The released nonflammable compounds and the formation of char residue retard the flammable conditions.45,46

Figure 5. DSC traces of lithiated electrode samples in the presence of electrolyte: 0.8 M LiPF6 in EC:DEC (1:1, v/v).

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4. CONCLUSIONS In summary, we have demonstrated the utilization of dinickel 1,4-benzenediphosphonate as electrode material for safe LIBs. The scope of organic batteries is expanded by employing flame retarding organophosphonate moiety as rare organic redox couple. This molecular engineered electrode delivers a high capacity of 585 mA h g−1 through multi-electron redox processes. The in-built thermal stability features a delayed onset temperature, reduced heat release, and endothermic peak. We expect that this synthetic organic approach coupled with battery technology will contribute to the development of molecular engineering for the multifunctional active materials.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional NMR, FTIR, and XANES spectroscopic data; SEM image of Ni2BDP; N2 adsorption isotherm of Ni2BDP; electrochemical study; thermal analysis; XRD data (PDF)

AUTHOR INFORMATION Corresponding Authors *[email protected] (W.C.); [email protected] (N.-S.C.); [email protected] (S.Y.H.) ORCID Wonyoung Choe: 0000-0003-0957-1187 Nam-Soon Choi: 0000-0003-1183-5735 Sung You Hong: 0000-0002-5785-4475 Notes The authors declare no competing financial interests.

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ACKNOWLEDGMENT We acknowledge the support from the National Research Foundation of Korea (NRF) grant (NRF-2016R1A2B4015497). This work was also supported by the Korean Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20172410100140).

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Table of Contents/Abstract Graphic

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Table of Contents/Abstract Graphic 150x100mm (150 x 150 DPI)

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Scheme 1. (a) Redox Chemistry of BDP ligand and (b) Synthesis of Ni2BDP. 214x145mm (150 x 150 DPI)

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Figure 1. (a) 31P NMR (243 MHz, CDCl3) spectrum of H4BDP, (b) FTIR spectra of H4BDP and Ni2BDP, (c) XRD patterns of Ni2BDP, and (d) TGA and DSC data of Ni2BDP. 270x194mm (150 x 150 DPI)

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Figure 2. Electrochemical studies of Ni2BDP. (a) Voltage-capacity traces, (b) cycle performance, and (c) rate performance. 173x381mm (150 x 150 DPI)

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Figure 3. (a) Voltage-capacity trace, (b) corresponding Ni K-edge XANES spectra of Ni2BDP, and (c) corresponding Ni K-edge EXAFS patterns: the green/dotted line indicates the Ni foil reference. The radial distribution function is uncorrected for phase shifts. 449x249mm (150 x 150 DPI)

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Figure 4. Solid-state 31P NMR (243 MHz) spectra of Ni2BDP during discharge-charge processes. 144x191mm (150 x 150 DPI)

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Figure 5. DSC traces of lithiated electrode samples in the presence of electrolyte: 0.8 M LiPF6 in EC:DEC (1:1, v/v). 181x149mm (150 x 150 DPI)

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