High Performance Voltage Tailorable Room-temperature Ca-metal Batteries


 Calcium metal battery as one of the promising alternatives beyond Li-metal technology is challenged by the lack of suitable cathodes with considerable energy performance, and stable Ca anode of long-term stability and lower polorization potentials for Ca-plating/stripping. Here, by recycling cellulose waste paper of wide sources in our daily life, we develop feasible cathodes for Ca-metal batteries of good high-voltage and wide-window-voltage adaptability (0.005-4.9 V vs. Ca/Ca2+), except for considerable energy performance (~517.5 Wh kg-1 at 0.1 A g-1). Meanwhile, through tailorable Ca-plating/stripping potentials (∆V= ~0.65 V) induced by electrolyte modification, proof-of-concept Ca-metal batteries not only delivered enhanced storage capability (101 mAh g-1 vs. 51 mAh g-1 at 0.1 A g-1 in the window voltage of 2.0-4.7 V ) and cycling stability (~77% capacity retention for 100 cycles), but also simutaneously held high output average working voltage of ~3.2V.


Main Text
The research of Ca-based rechargeable batteries has been highly attractive due to low redox potential (Cã -2.87 V vs. standard hydrogen electrode, SHE), small ionic radius similar with that of sodium (Ca 2+ 1.00 Å vs. Na +~1 .02 Å), and high natural abundance and good eco-friendliness of the Ca element. [1][2] Actually, early work about the deposition of Ca at several organic electrolytes revealed a lm-controlled process impossible for application in room temperature circumustances. 3 Later on, the surface lm was con rmed as electron insulating passiviation layer functioning as the solid electrolyte interphases (SEIs) in lithium ion batteries (LIBs), but initial porous and accessible for anions leading to continuous corrosion (veri ed as anion oxidation forming CaF 2 or CaH 2 ) of freshly exposed calcium and nally densifying as passivation layer insulative for both electrons and ions. [4][5][6][7] The anion oxidation phenomenon could be alleviated at relatively high temperature or by changing the electrolyte salts from Ca(PF 6 ) 2 , Ca(BF 4 ) 2 , and Ca(BH 4 ) 2 to corresponding uorinated alkyl borates. 4,[8][9] However, due to this issue arising from the character of calcium itself, it still needs long-term collaboration from both electrolyte chemistry and calcium-metal surface/interface engineering to fundmentally solve this bottleneck problem as that in lithium-metal technology.
Skipping calcium metal straight to its full batteries is currently a reasonable alternative strategy that are accelerating the pace of research on calcium-based battery technology. In this regard, the key is to screen suitable host materials for reversible Ca 2+ insertion/extraction or adsorption/desorption reactions. For example, Prussian blue analgues, layered transition metal oxides and Van der Waals-bound layered transition metal sul des featured with large tunnel structure or interlayer spacings have been reported as good cathodes, while tin, graphite carbon microbeads (MCMBs), and atitivated carbon cloth served as the anodes. [10][11][12][13][14][15][16] For example, MnFe(CN) 6 //calciated-Sn battery held a capacity of ~50 mAh g -1 (based on the weight of the cathode) at 0-4 V after 30 cycles through a Ca 2+ /Na + hybrid intercalation mechanism. 10 A reversible capacity of ~ 75 mAh g -1 due to Ca 2+ insertion/extraction at 0.01-2V stably persisting for 100 cycles was reported for Na-doped ammonium vanadium oxide (NH 4 V 4 O 10 ) cathode and Ni-based framework anode system. 12 In particular, Mai's group presented a Mg 0.25 V 2 O 5 ·H 2 O//activated carbon cloth Ca-ion battery demonstrting high capacity retention ~87% for 500 cycles (~61 mAh g -1 ) at 0.1 A g -1 in the window voltage of -2.0-1.4 V. 15 Despite important progresses, an obvious shortcoming is the low operation voltage, even below zero, due to inapproprite anodes and sluggish kinetics quite different from that of LIBs. Fortunately, through assembling dual-ion batteries, the low working voltage issue can be largely improved by utilizing the high voltage for anion insertion/exaction reaction. 17 The representative work, which utilized the insertion of into the expanded graphite cathode and Sn-Ca alloying reaction in the anode, was carried in Tang and Chen's group. 2 The type Ca-ion batteries veri ed high window voltage of 3.0-5.0 V, impressive averge working voltage above 4 V, and an initial caapcity of 72 mAh g -1 at 0.1 A g -1 , in spite of the apprently uctuated cycling stability and coulombic e ciencies to be improved. 2 And hence, it is essential to search new cathodes to further improve the storage capability and cycling stability. In combination, it's still a long run to realize practical Ca-metal batteries in accompanying with severe challenges from fast degenerated plating/stripping and insertion/extraction chemistries and their sluggish kinetics and severe polorizations. 7,18 Here, we developed an inexpensive, freestanding membrane cathode of fast charge-kinetics by recycling cellulose waste paper. Except for high anodic stability up to 4.9 V, the cellulose waste paper derived graphitic carbon (CWPDGC) cathodes also exhibited high storage capability up to 410 mAh g -1 at 0.1 A g -1 , equivalent to a high energy capability of 517.5 Wh kg -1 . Through electrolyte optimization, the polarization potential for Ca-plating/stripping was reduced by ~0.65 V, meanwhile the CWPDGC Ca-metal batteries with tailorable window voltages and stable cycling performance were also readily realized.

Results And Discussion
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 Ca 2+ , Mg 2+ 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 ltration 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 sp 2 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 exible bre skeletons with the length of several hundred microns and width of a few microns coiled from waved nanosheets. The high angle annular dark eld scanning transmission electron microscope (HAADF-STEM) image ( Figure 1d) veri ed 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 con rmed by the well-de ned 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 pro les 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 Casoaked electrolyte) was made by forced ion exchange between fresh Ca and 1M LiPF 6 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, Ca 2+ amounts to ~7.8% (at.%) among all the cations of Li + and Ca 2+ according to the calculation based on the content of P. At rst, 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. Speci cally, 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 pro les 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 e ciencies roughly around 100% (Figure 2c). Moreover, the large storage capability was also held at various current rates. Speci cally, high energy density of 517.  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-1 st -0.005) due to insertion reaction of cations, i.e., Li + and Ca 2+ 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-1 st -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-1 st -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-2 nd -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-2 nd -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 pro les. 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% re ected the extraction of , and afterwards, Ca 2+ 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 Ca 2+ respectively. Moreover, the relay insertion/extraction of cations and anions consisting of adsorption-desorption and intercalation/deintercalation chemistry processes were also demonstrated in the ex situ HAADF-STEM, element mapping, and HRTEM images (Figure 4). At D-1 st -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 Ca 2+ and Li + . In constrat, the color of Ca map changed lighter at C-1 st -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 1 st 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 in uenced 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 Ca 2+ concentration by adding Ca(BF 4 ) 2 . As shown in the potential-capacity pro les (Figure 5a), the potential for steady deposition of Ca happened at ~ -1.36 V (vs. Ca/Ca 2+ ) for Ca//Ca symmetrical cells with Ca-soaked electrolyte, which was increased by ~ 0.65 V when using Ca-soaked electrolyte with calcium tetra uoroborate of 1% (weight ratio). According to the quantitative analysis based on the EDX spectrum ( Figure S4), the content of Ca 2+ roughly amounts to ~83.2 % (at.%) among all the cations in the modi ed 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 modi cation strategy, voltage tailorable CWPDGC Ca-metal batteries with enhanced storage capability were readily realized. As shown by the typical voltage pro les (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 modi cation. 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 pro les 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 e ciencies more than 93% after the 20 th 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 Ca 2+ and which possess more favorable radius of polarization and molecular weight in comparison to . [29][30]33 Methods Materials. Cellulose waste paper collected from our lab was primarily degraded in deionized water under magneton stirring for one day. Pure paper pulp would be obtained by subsequently rinsing the degradation product with 0.1 M HCl aqueous solution for three times, and then deionized water and ethanol each time to eliminate the purities. Afterwards, the paper pulp was pumped into thin paper membranes by controlling its concentration and volume. After drying for 6h at 80℃, these paper membranes were pyrolyzed at 1000 ℃ under vacuum circumstance for 6 hours in a tube furnace with a heating ramp rate of 10 ℃ min -1 . CWPDGC membranes would be achieved after natural cooling. Besides membranes themselves as electrodes, they were also pulverized in a degassed polyethylene (PE) bag for fabricating tape-casting electrodes used for electrochemical tests and mechanism characterization. Ca-soaked electrolyte was made by repeatedly dipping fresh Ca plates in 1 M LiPF 6 EC/DEC (1:1, v/v) electrolyte for a week. 1% Ca(BF 4 ) 2 (w%) was added into Ca-soaked electrolyte to prepare the modi ed electrolyte (Ca-soaked electrolyte + 1% Ca(BF 4 ) 2 ).
Physicochemical characterizations. Microstructure, composition, and morphology of CWPDGC and corresponding electrodes were characterized by thermal eld emission SEM (Quanta 400F, 20 kV), eld emission TEM (FEI Tecnai G2 F30, 300 kV), and EDX. Phase characterization was carried out on the powder X-ray diffractometer (XRD) (D/MAX 2200, Cu kα1, 5°/min). All the electrodes for characterization were washed with DEC to eliminate the residual electrolytes.
Electrochemical characterizations. For Ca//Ca symmetrical batteries, commercial Ca granules (99.9%, Sigma Aldrich) were pressed to plates (thickness, ~100-150 μm) which were directly used as both the working and counter electrodes. Ca-soaked electrolyte and its Ca(BF 4 ) 2 modi ed counterpart were used as the electrolytes, and glass micro ber lters (Whatman) were used as the separator. For Ca//CWPDGC batteries, CWPDGC membranes or CWPDGC tape casting electrodes (with a weight ratio of CWPDGC/PVDF=4/1) was tailored to an appropriate size and directly used as the cathodes, the weight was ~0.3-1 mg cm -2 . Standard cells (CR2032) with the above tailored electrodes, glass micro ber lter separator, and 75 μL electrolyte were assembled in an Ar-lled universal glove box with the Oxygen and water vapor pressure less than 0.3 ppm.
The plating-stripping test of Ca//Ca symmetrical batteries were galvanostatically tested at 0.02 mA cm -2 with the capacity of 0.06, 0.135 mAh cm -2 respectively. The Ca// CWPDGC batteries were galvanostatically tested with various window voltage of 0.005-4.5 V, 0.005-4.9 V, 1.1-4 V, 1.15-4.6 V, and 2.0-4.7 V respectively. Cycling performance with different charge durations was also tested at a current density of 100 mA g -1 and 200 mA g -1 respectively. Fast-charge and long-term cycling performance test was also carried at varying rates from 2 A g -1 to 0.1 A g -1 . All the tests were performed on a multichannel Neware battery testing system. Cells for mechanism analysis were galvanostatically discharged and/or charged to different cutoff voltages at 0.1 A g -1 .  Ca//CWPDGC batteries at wide window voltages. a) typical voltage pro les at 20 and 200 mA g-1, b) the corresponding differential capacity curve at 20 mA g-1 in the window voltage of 0.005-4.9 V, c) cycling performance at 20 mA g-1, d) rate performance in contrast to other batteries, e) cycling performance and f) the corresponding discharge pro les, g) output energy performance and average working voltages at 0.1 and 0.2 A g-1. Ex situ structure characterizations of CWPDGC cathodes, a) voltage pro les with marked SOCs, b) the XRD pro les, and c) the corresponding contour color map of the XRD pro les, and ex situ element characterizations of WCPDGC cathodes, d) discharge pro les with marked SOCs, e) the EDX spectra, and f) the corresponding contour color map of the EDX spectra.