Recycling cellulose waste paper to graphitic carbon membrane
Before pyrolysis, a predegradation process was carried for the cellulose waste paper collected from the lab in order to get hydrocellulose with higher O content for Oxygen-doping. The purities mainly of Ca2+, Mg2+ were eliminated by dilute hydrochloric aqeuous solution (0.1 M HCl). As seen in Figure S 1a, uniform paper pulp would be achieved by redispersing the treated cellulose waste paper in deionized water, and CWPDGC membranes were prepared by annealing the paper membranes gotten from vacuum filtration of the paper pulp (Figure S 1b and 1c). The powder X-ray diffraction (XRD) pattern of CWPDGC (Figure 1a) exhibited two wide peaks at 22.2º and 43.6º (2q) ascribed to the diffraction of (002) and (101) planes for graphite-2H structure (PDF No. 41-1487). The marked shift of the peak for (002) planes diffraction to lower 2q indicates an enlarged lattice spacings (0.39 nm vs. 0.34 nm for standard graphite) which is attributed to deformed layers for sp2 hybrid carbon due to O-doping common for various carbohydrates derived carbon nanostructures.19-20 The scanning electron microscope (SEM) images (Figure 1b and 1c) showed CWPDGC appeared as flexible fibre skeletons with the length of several hundred microns and width of a few microns coiled from waved nanosheets. The high angle annular dark field scanning transmission electron microscope (HAADF-STEM) image (Figure 1d) verified the nanosheets full of nanopores. O-doping was also proved in the energy dispersive X-ray (EDX) spectrum (Figure 1e) with only C and O elements in the graphitic carbon. Clear diffraction rings in the select area electron diffraction (SAED) image (Figure 1f) attributed to the diffraction of (002), (101), and (110) planes substantiated its polycrystalline graphite-2H structure, which was also confirmed by the well-defined lattice fringes of ~2-5 layers in the high resolution transmission electron microscope (HRTEM) image (Figure 1g) characteristic of a layer spacing of ~0.39 nm analyzed by the fast fourier transform (FFT) and intenisity profiles of the selected crystallized zones (Figrue 1h and 1i) conforming to the XRD result.
Primary CWPDGC Ca-metal batteries and the electrochemical mechanism
To assemble primary Ca//CWPDGC batteres, tape casting electrodes of CWPDGC and the membranes were used as the cathodes, while commercial Ca was used as the anode. The electrolyte (denoted as Ca-soaked electrolyte) was made by forced ion exchange between fresh Ca and 1M LiPF6 ethylene carbonate (EC)/diethyl carbonate (DEC) (v/v=1:1) electrolyte. The components of Ca-soaked electrolyte was roughly determined by the EDX spectrum of a drop electrolyte drying on a silicon wafer. As shown in Figure S2, Ca2+ amounts to ~7.8% (at.%) among all the cations of Li+ and Ca2+ according to the calculation based on the content of P. At first, the batteries were galvanostatically tested at a contant window voltage like LIBs. A high initial discharge capacity of 306.3 mAh g-1 was obtained at 0.2 A g-1 in a window voltage of 0.005-4.5 V, however, it quickly run out in less than 100 cycles (Figure S3a). Even if the current rate rised to 0.5 A g-1, only 14% capacity was retained after 100 cycles (164 mAh g-1 vs. 23 mAh g-1). To address this issue, we also carried the test at constant charge durations. Specifically, at the current rates of 20 mA g-1 and 200 mA g-1, the current rates of 20 mA g-1 and 200 mA g-1, the primary Ca//CWPDGC batteries delivered high reversible capacities of 186 mAh g-1 and 106 mAh g-1 respectively when the upper voltage approached 4.9 V. The discharge profiles showed three different parts at around 4.73-1.80 V, 1.80-0.73 V, and 0.73-0.005 V which were consistent with the cathodic peaks ~ 3.88 and 3.08 V, 1.73 and 0.60 V, and 0.32 and 0.03 V correspondingly in the differential capacity curves (Figure 2b). Importantly, the impressive capacity lasted reversibly for 50 cycles at 20 mA g-1 with the coulombic efficiencies roughly around 100% (Figure 2c). Moreover, the large storage capability was also held at various current rates. Specifically, high energy density of 517.5, 433.2, 251.4, and 86.7 Wh kg-1 were achieved at the rates of 0.1, 0.2, 0.5 and 1.0 A g-1 (Figure 2d). These corresponded to impressive specific capacities of 410.5, 250.3, 100.8, and 41.5 mAh g-1 respectively (Figure S 3b and 3c). The superior storage capability and energy performance are comparable to some most advanced energy storage devices including Al-based batteries and Sn-graphite Ca-ion batteries.2, 21-23 Enhanced cycling performance was obtained by performing series of charge durations from 28 to 34 minutes at 0.2 A g-1 (Figure S 3d). The best cycling perfomance (Figure 2e) consisted of initial reversible capacity of ~113 mAh g-1 ( at 2nd cycle), good capacity reteniton of 90.2% for 100 cycles, and high coulombic efficiencies above 90.9%. Although obvious potential shift was observed in the voltage-time (inset view of Figure 2e) and discharge profiles (Figure 2f, Figure S 3e and 3f ), the good cycling performance at such wide window voltage has never been seen for other cathodes. Besides, steady average working voltage around 2.0 V and energy density above 200 Wh kg-1 were held for more 100 cycles for both the tests at 0.1 and 0.2 A g-1 (Figure 2g).
Ex situ characterization of CWPDGC at various stages of charge/discharge (SOCs) were performed to explore the quite different electrochemistry process. As seen in the XRD patterns (Figure 3a-c), diffraction peak of (002) plane shifted to lower 2q range at initial discharge stage to 0.005 V (denoted as D-1st-0.005) due to insertion reaction of cations, i.e., Li+ and Ca2+ like that of Li+ intercalation into graphite or other carbon nanostructures happening in LIBs, which recovered at the subsequent stage of charge process (~2.5 V, denoted as C-1st-2.5) because of the extraction reaction of the intercalated cations.20, 24-25 In the meantime, Ca stripping and Li/Ca plating proceeded at the anode. For further stages charge to 4.9 V (denoted as C-1st-4.9), the diffraction peak increasingly shifted to lower 2q range again resulted from anions insertion reaction similar with dual-ion batteries (DIBs), meanwhile cations continued to plate on the Ca anode.23, 26-27 In subseqent discharge process to 2.0 V (denoted as D-2nd-2), intercalated anions took out of the graphitic layers, leading to the recovery of the diffraction peak. To further discharge to 0.005V (denoted as D-2nd-0.005), cation intercalation occurred repeatly as the initial discharge. The regular shift of diffraction peak during charge/discharge can be also clearly observed in the corresponding contour color map of the XRD profiles. The variation of the element content for F, P, and Ca in the EDX spectra provided us indirect clues of the change of various ions (Figure 3d-f). The lowering of peaks for F and P elements in the depth of discharge below 64.5% reflected the extraction of , and afterwards, Ca2+ insertion was indicated by increasing intensity for the peak of Ca element in fully discharge depth to 100%. Since the ionic radius of lithium (~0.76 Å) is smaller than that of calcium (~1.00 Å), it’s reasonable to conclude that Ca insertion was accompanied by the insertion of Li, although we could’t present the variation of Li element limited by the characterization technique.1, 7, 28
Relating the cathodic peaks mentioned above with the analysis in XRD and EDX results, we can readily concluded that the cathodic peaks at 3.08 and 3.88 V was resulted from extraction reaction of absorded in surface or intercalated in graphitic layers, while the peaks at 1.73, 0.6, 0.32, and 0.03 V were attribtued to adsorption or intercalation reaction from Li+ and Ca2+ respectively. Moreover, the relay insertion/extraction of cations and anions consisting of adsorption-desorption and intercalation/de-intercalation chemistry processes were also demonstrated in the ex situ HAADF-STEM, element mapping, and HRTEM images (Figure 4). At D-1st-0.005 stage (Figure 4a-e), obvious gathering of Ca element was observed in the mapping images, and the initial layer spacing of 0.39 nm for (002) planes was stretched to ~ 0.42 nm due to insertion of Ca2+ and Li+. In constrat, the color of Ca map changed lighter at C-1st-2.5 stage (Figure 4f-j), meanwhile the lattice spacing returned to ~0.39 nm after the extraction of Ca and Li. It’s to note that the color of P and F maps didn’t change obviouly after insertion/extraction reaction of probably because of their poor discrimination and high content of F and P in the solid electrolyte interphases (SEIs), although the HRTEM images displayed severe change of lattice spacings from 0.39 nm to ~0.46 nm (Figure 4k-t). Comparing with the 1st discharge, larger lattice spacings were resulted ( ~0.45 nm vs 0.42 nm) due to the ongoing activation by repeated insertion/extraction processes (Figure 4u-y).29-30 Overally, in the CWPDGC Ca-metal batteries, both cations and anions are fully utilized through relay insertion/extraction mechanism quite different from that only cations in LIBs or anions in DIBs, which accounts for the higher storge capability.
Voltage tailorable CWPDGC Ca-metal batteries
Except for the self-characters such as electropositivity, ionic radius, and charges etc., it’s generally accepted that metal plating/stripping potentials are dependent on the solvation/desolvation properties, which are highly influenced by the ion concentration, type of solvents, substrates, and temperatures.7-8, 31-32 In the case of Ca-metal, the situations seem more complicated due to anion corrosion and large internal resistance resulted from the extremely insulative passivation layers.4-5, 7 Anyway, we found that the polarization potentials for Ca deposition/dissolution could be largely lowered through the increase of Ca2+ concentration by adding Ca(BF4)2. As shown in the potential-capacity profiles (Figure 5a), the potential for steady deposition of Ca happened at ~ -1.36 V (vs. Ca/Ca2+) for Ca//Ca symmetrical cells with Ca-soaked electrolyte, which was increased by ~ 0.65 V when using Ca-soaked electrolyte with calcium tetrafluoroborate of 1% (weight ratio). According to the quantitative analysis based on the EDX spectrum (Figure S4), the content of Ca2+ roughly amounts to ~83.2 % (at.%) among all the cations in the modified electrolyte, which indicates an increase more than tenfold (vs. ~7.8%). More importantly, they showed similar cycling stability up to nearly 400 hours with relatively smaller polarization potential shift (<300 mV) (Figure 5b).
Based on the change of Ca plating/stripping potentials being enabled by the electrolyte modification strategy, voltage tailorable CWPDGC Ca-metal batteries with enhanced storage capability were readily realized. As shown by the typical voltage profiles (Figure 5c), the reversible capacities of 51 mAh g-1 at 2.0-4.7 V and 66 mAh g-1 at 1.15-4.60 V were raised to 101 mAh g-1 and 194 mAh g-1 respectively at 0.1 A g-1 after electrolyte modification. Morevoer, the average working voltages have been also largely elevated up to ~3.2 V at the window voltage of 2.0-4.7 V (Figure S5).
Interestingly, the discharge profiles always displayed similar three characteristic features indicating the same cation-anion relay insertion/extraction mechanism no matter how the voltage has been tailored. The main disparity came from the boundary voltage dividing the cation insertion/extraction and anion insertion/extraction processes, which shifted to higher potentials as the lower cutoff voltage was elevated. The speculation was also revealed in the corresponding differential capacity curves (Figure 5d). Besides an elevation of the output energy (high voltage and superior storage capability), the cycling performance was also enhanced. After being tested at 100 mA g-1 for 100 cycles (Figure 5e), high capacities of ~78 mAh g-1 at 2.0-4.7 V and ~123 mAh g-1 at 1.15-4.60 V were maintained corresponding to retention of ~77% and ~50% respectively with enhanced coulombic efficiencies more than 93% after the 20th cycle. Meanwhile, voltage tailorable CWPDGC Ca-metal batteries also afforded to suffer successive fast-charge tests of 2 A g-1, 1 A g-1, 0.7 A g-1, 0.5 A g-1, and 0.2 A g-1, more than 80-83% of the capacity could be output at 0.2 A g-1 in the window voltage of 2.0-4.7 V (Figure 5f). Afterwards, the battery could stably run for more than 400 cycles at 0.2 A g-1 and another 450 cycles at 0.1 A g-1 with the capacity retention roughly above ~80% (~42 mAh g-1). The impressive storage capability, energy performance, and excellent cycling stability should not only be attributed to the novel relay insertion/extraction for both cations and anions, but also be inseparable from the improved Ca plating/stripping processes and the more abundant free ions of Ca2+ and which possess more favorable radius of polarization and molecular weight in comparison to .29-30, 33