milerssliu/battery-device-data-qa huggingface dataset:
Another critical property of an electrode material toward practical application is the ease of handling against a moist environment. Most of the O3-type transition metal oxide cathodes reported so far are moisture sensitive because of the low redox potential associated with 3d-metals. Once the cathode is exposed to air or moisture, water oxidizes the transition metal ions with the concomitant removal of Na+ ions, which further react with CO2 and H2O in air to form Na2CO3 or NaOH. Such undesirable reactions usually cause structural degradation and poor electrochemical performances and thus make them difficult to handle. As revealed in Fig. 7, after exposure to air with a relative humidity of ≈55%, the Na[Ni0.5Mn0.5]O2 exhibited an apparent structural change to a Na-deficient monoclinic O′3 phase together with formation of Na2CO3·nH2O on the surface. In contrast, Na0.98Ca0.01[Ni0.5Mn0.5]O2 remarkably retarded spontaneous phase transition and retained the original O3 structure. To confirm in detail the structural stability against a humid atmosphere, a comparative study of the XRD evolution of the Na0.98Ca0.01[Ni0.5Mn0.5]O2 and Na[Ni0.5Mn0.5]O2 cathodes was conducted as a function of exposure time (1, 3, and 5 days and 1 week) (Fig. S16†). After 1 day, there were no significant structural changes in either cathode. However, after three days, the Na[Ni0.5Mn0.5]O2 cathode was readily oxidized in the presence of moisture, and subsequently, the intensity of (003)hex. and (104)hex. peaks decreased in the XRD patterns. After 5 days, splitting of the (003)hex. and (006)hex. diffraction lines was observed, indicating a slightly enlarged interslab distance for the new monoclinic O3 phase. Finally, after 1 week, the phase transformation from the hexagonal O3 to the monoclinic O′3 phase occurred in the Na[Ni0.5Mn0.5]O2 cathode. This monoclinic O′3 phase transformation can be observed in the sodium deficient structure when the sodium ions are extracted down to 20 mol% in the original O3 structure. In contrast, throughout the week, the intensified peaks, (003)hex., (006)hex., (101)hex., (102)hex., and (104)hex., in the O3 phase were well retained persistently for the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode, indicating less reactivity against moisture. These results suggest that the Ca2+ ions on the Na sites with strong Ca–O bonding prevent the removal of Na+ ions from the structure when exposed to air. As expected, in Fig. S17,† the Na0.98Ca0.01[Ni0.5Mn0.5]O2 cathode demonstrated higher reversible capacity with lower voltage polarization and better cycling stability after prolonged exposure, while the Na[Ni0.5Mn0.5]O2 cathode showed poor electrochemical performance and even exhibited an irreversible reaction during charging, due to the decomposition of Na2CO3 which is a by-product of removed Na+ ions and CO2 in air. Such a comparison of the thermal properties and air-stability strengthens the merits of Ca-substituted cathodes in practical application.

Question: What's the cathode?
Answer: Na[Ni0.5Mn0.5]O2
Unlike DC voltage exfoliation, square-wave voltage uses bulk antimony, bismuth and their compounds as the anode and platinum wire or foil as the cathode, and changes the voltage direction to perform electrochemical exfoliation in an electrolytic solution of Na2SO4 or Li2SO4. This process mainly uses oxygen-containing free radicals produced by electrolysed water to assault large pieces of antimony or bismuth or their compounds in the anode, and SO42− intercalates into the interlayer. Subsequently, the gas generated between the layers of the bulk layered material further expands, and finally 2D antimony or bismuth or its compound is obtained in the electrolyte. In 2020, Marzo et al. elucidated the possible mechanism of changing the voltage exfoliation of 2D Sb and Sb2Te3. As shown in Fig. 4b, this exfoliation process comprises four different stages. In the first stage, oxygen-containing free radicals produced by the electrolysis of water on the anode bulk 2D material react with the edges and boundaries, creating bumps, cracks and holes in the bulk 2D material. Next, the small cations (H+, Li+ and Na+) intercalate into the gaps and holes formed in the previous bulk 2D material, initializing a slight and preliminary expansion of the 2D sandwich. When the conversion voltage is −5 V, the SO42− intercalation layer enters the interlayer of the bulk 2D material. Finally, the gases (H2, O2 and SO2 formed in these redox processes) generated by the insertion of ions between the 2D bulk material layers promote the exfoliation of the 2D material and disperses it into the electrolyte (Fig. 4b).

Question: What's the cathode?
Answer:  platinum wire or foil
Unlike DC voltage exfoliation, square-wave voltage uses bulk antimony, bismuth and their compounds as the anode and platinum wire or foil as the cathode, and changes the voltage direction to perform electrochemical exfoliation in an electrolytic solution of Na2SO4 or Li2SO4. This process mainly uses oxygen-containing free radicals produced by electrolysed water to assault large pieces of antimony or bismuth or their compounds in the anode, and SO42− intercalates into the interlayer. Subsequently, the gas generated between the layers of the bulk layered material further expands, and finally 2D antimony or bismuth or its compound is obtained in the electrolyte. In 2020, Marzo et al. elucidated the possible mechanism of changing the voltage exfoliation of 2D Sb and Sb2Te3. As shown in Fig. 4b, this exfoliation process comprises four different stages. In the first stage, oxygen-containing free radicals produced by the electrolysis of water on the anode bulk 2D material react with the edges and boundaries, creating bumps, cracks and holes in the bulk 2D material. Next, the small cations (H+, Li+ and Na+) intercalate into the gaps and holes formed in the previous bulk 2D material, initializing a slight and preliminary expansion of the 2D sandwich. When the conversion voltage is −5 V, the SO42− intercalation layer enters the interlayer of the bulk 2D material. Finally, the gases (H2, O2 and SO2 formed in these redox processes) generated by the insertion of ions between the 2D bulk material layers promote the exfoliation of the 2D material and disperses it into the electrolyte (Fig. 4b).

Question: What's the anode?
Answer:  bulk antimony, bismuth and their compounds
The development of high-capacity rechargeable lithium–sulfur batteries is an ongoing challenge because of the shuttle effect, which causes rapid capacity fade, limited rate capability, and slow kinetics. Herein, conductive hollow cobalt nitride/carbon (Co5.47Nx–C) spheres with nitrogen vacancies are developed as a high-performance cathode material for use in lithium–sulfur batteries. Nitrogen vacancies are formed by annealing a zeolite imidazole framework (ZIF-67) precursor in ammonia. Benefiting from its Co–N bonds and nitrogen-vacancy sites, the Co5.47Nx–C composite achieves strong anchoring of polysulfides, fast polysulfide conversion, and accelerated lithium-ion transport. The strong anchoring effect of Co5.47Nx is confirmed by experimental measurements and density functional theory (DFT) calculations. Because of its high conductivity and nitrogen vacancies, the Co5.47Nx cathode exhibits faster redox reaction kinetics and lower polarization than a Co5.47N cathode without nitrogen vacancies, thus realizing promising rate and cycling performance. The optimized Co5.47Nx–C electrode delivers a capacity of 850 mA h g−1 at 0.5C and a rate performance of 320 mA h g−1 at 10C. This high-performance, high-rate lithium–sulfur battery is promising for widespread application in electric vehicles and intelligent devices.

Question: What's the cathode?
Answer: Co5.47Nx
The development of high-capacity rechargeable lithium–sulfur batteries is an ongoing challenge because of the shuttle effect, which causes rapid capacity fade, limited rate capability, and slow kinetics. Herein, conductive hollow cobalt nitride/carbon (Co5.47Nx–C) spheres with nitrogen vacancies are developed as a high-performance cathode material for use in lithium–sulfur batteries. Nitrogen vacancies are formed by annealing a zeolite imidazole framework (ZIF-67) precursor in ammonia. Benefiting from its Co–N bonds and nitrogen-vacancy sites, the Co5.47Nx–C composite achieves strong anchoring of polysulfides, fast polysulfide conversion, and accelerated lithium-ion transport. The strong anchoring effect of Co5.47Nx is confirmed by experimental measurements and density functional theory (DFT) calculations. Because of its high conductivity and nitrogen vacancies, the Co5.47Nx cathode exhibits faster redox reaction kinetics and lower polarization than a Co5.47N cathode without nitrogen vacancies, thus realizing promising rate and cycling performance. The optimized Co5.47Nx–C electrode delivers a capacity of 850 mA h g−1 at 0.5C and a rate performance of 320 mA h g−1 at 10C. This high-performance, high-rate lithium–sulfur battery is promising for widespread application in electric vehicles and intelligent devices.

Question: What's the cathode?
Answer: Co5.47N
The carefully designed combination of TM-ions and non-TM-ions suppressed the occurrences of phase transformations during electrochemical cycling in Na ‘half’ cells. More importantly, in addition to air-cum-water stability, Ti-substitution for Mn-ions was found to drastically improve the cyclic stability, with the completely Ti-substituted Na-TM-oxide exhibiting an excellent first cycle reversible capacity of ∼140 mA h g−1 (within 2.0–4.0 V, at C/5), negligible voltage hysteresis and a capacity retention of ∼80% after 100 cycles. In the presence of Mn-ions, formation of Mn3+ at the particle surfaces, concomitant dissolution of Mn-ions (upon disproportionation) in electrolyte, steep rise in cathode/cell impedance and associated rapid increase in voltage hysteresis upon cycling were found to be the major causes for rapid capacity fading, which were suppressed/eliminated upon partial/complete Ti-substitution. The incorporation of functionalized MWCNTs, via simple physical mixing with the cathode-active particles, further improved the capacity retentions to ∼87% after 100 cycles and ∼75% after 200 cycles, along with bestowing excellent rate capability, viz., capacities at 2C and 5C being ∼65% and ∼50% of capacity at C/10, respectively.

Question: What's the cathode?
Answer: MWCNTs
Sodium-ion batteries (SIBs) have recently attracted increasing attention as an alternative to lithium-ion batteries, especially for large-scale energy storage in light of the low cost and high abundance of Na resources. Layer structured sodium transition metal oxides are an important group of cathode materials for SIBs due to their high theoretical capacities. However, the poor cycling stability at high voltage hinders their practical applications, which is due to the multiple phase transition induced structural instability. Our recent work reveals that phase transition induced cracking is a major cause of the performance decay for layered cathodes. Bulk elemental doping has been demonstrated to be an effective approach to improve the cycling performance, because doping can effectively suppress phase transition induced volume changes and even realize a zero-strain cathode or phase transition free cathode. Furthermore, dopants can act as pillars to prevent the layered structure from collapsing and they can tailor the plane spacing to achieve superior diffusion kinetics of alkaline ions.

Question: What's the cathode?
Answer: Layer structured sodium transition metal oxides
Polymers. Using polymeric analogs of small molecules might be an effective strategy to circumvent the solubility issue of organic redox-active materials. The first polymeric cathode material for potassium batteries was poly(anthraquinonyl sulfide) (PAQS, P1). With an ether-based electrolyte (0.5 M KTFSI in DME:DOL) it showed a reversible Qm of 200 mA h g−1 at 20 mA g−1. A capacity fading of 25% was observed after 50 cycles, while the cyclability with a carbonate-based electrolyte was inferior.

Question: What's the cathode?
Answer: polymeric
Polymers. Using polymeric analogs of small molecules might be an effective strategy to circumvent the solubility issue of organic redox-active materials. The first polymeric cathode material for potassium batteries was poly(anthraquinonyl sulfide) (PAQS, P1). With an ether-based electrolyte (0.5 M KTFSI in DME:DOL) it showed a reversible Qm of 200 mA h g−1 at 20 mA g−1. A capacity fading of 25% was observed after 50 cycles, while the cyclability with a carbonate-based electrolyte was inferior.

Question: What's the electrolyte?
Answer: (0.5 M KTFSI in DME:DOL)
Electrospun CNFs/Nb2O5 composite nanofibers were prepared by electrospinning a solution containing the precursors of Nb(C2O4H)5, PAN and TEOS, followed by carbonization and SiO2-etching.Fig. 7f and g show the structural features of the prepared CNFs/Nb2O5 nanocomposites, which reveal a continuous mesoporous network architecture. Compared with solid CNFs/Nb2O5 composites without using TEOS in spinnable solution and bulk C/Nb2O5 prepared via directly annealing the Nb(C2O4H)5, the mesoporous CNFs/Nb2O5 composites revealed much superior electrochemical reactivity. In detail, the mesoporous Nb2O5/CNF showed outstanding cyclability (94% retention after 10000 cycles at 100C, 1C = 200 mA g−1), and superior rate capability with specific capacities of 287 mA h g−1 at 0.5C and 172 mA h g−1 at 150C, which was much higher than that of pure CNFs (159.1 mA h g−1 at 0.5C and 64.9 mA h g−1 at 100C). In addition, a hybrid Na-ion capacitor was fabricated by employing mesoporous CNFs/Nb2O5 as the anode and CNFs/graphene as the cathode. This device showed both large energy densities (124 W h kg−1, 11.2 mW h cm−3) and impressive power densities (60 kW kg−1, 5.4 W h cm−3), indicating that both the introducing of Nb2O5 and the unique continuous mesoporous network architecture played a crucial role in the enhancement of the electrochemical properties.

Question: What's the cathode?
Answer:  CNFs/graphene
Electrospun CNFs/Nb2O5 composite nanofibers were prepared by electrospinning a solution containing the precursors of Nb(C2O4H)5, PAN and TEOS, followed by carbonization and SiO2-etching.Fig. 7f and g show the structural features of the prepared CNFs/Nb2O5 nanocomposites, which reveal a continuous mesoporous network architecture. Compared with solid CNFs/Nb2O5 composites without using TEOS in spinnable solution and bulk C/Nb2O5 prepared via directly annealing the Nb(C2O4H)5, the mesoporous CNFs/Nb2O5 composites revealed much superior electrochemical reactivity. In detail, the mesoporous Nb2O5/CNF showed outstanding cyclability (94% retention after 10000 cycles at 100C, 1C = 200 mA g−1), and superior rate capability with specific capacities of 287 mA h g−1 at 0.5C and 172 mA h g−1 at 150C, which was much higher than that of pure CNFs (159.1 mA h g−1 at 0.5C and 64.9 mA h g−1 at 100C). In addition, a hybrid Na-ion capacitor was fabricated by employing mesoporous CNFs/Nb2O5 as the anode and CNFs/graphene as the cathode. This device showed both large energy densities (124 W h kg−1, 11.2 mW h cm−3) and impressive power densities (60 kW kg−1, 5.4 W h cm−3), indicating that both the introducing of Nb2O5 and the unique continuous mesoporous network architecture played a crucial role in the enhancement of the electrochemical properties.

Question: What's the anode?
Answer: CNFs/Nb2O5
A good dopant must endow the cathode material with superior cyclability. According to Fig. 1(a), among Cu, Ti, Mg and Zn, Mg and Zn are better dopants. From structural view, a good dopant should be able to segregate into precipitates during cycling, offering better material integration. However, when cycled at a lower upper cutoff voltage of 4.3 V, Mg dopant segregation cannot be activated. Therefore, Mg-doped P2-NMM10 shows comparable cyclability to Cu-doped P2-NMC10, which is shown in Fig. S10.† Intriguingly, when cycled at 4.3 V, the Zn-doped sample shows even better cycling stability (91% retention after 100 cycles). Fig. 5(a) shows the capacity retentions of the P2-NMZ10 electrodes at different upper cutoff voltages, from which we can see that Zn-doped P2-NMZ10 shows superior capacity retentions in all voltage windows. The corresponding TEM analysis on the cycled P2-NMZ10 samples shows that the Zn-dopant can form precipitates under all cycling conditions as shown in Fig. 5(b–i), which explains why P2-NMZ10 shows much higher cycling stability. Although increasing the upper cutoff voltage can accelerate the precipitate formation process, material degradation is also aggravated during high voltage cycling. Therefore, a good dopant should enable dopant precipitate formation easily, especially at low cycling voltage.

Question: What's the cathode?
Supercapacitors have high power density and a long lifespan but poor energy density in contrast with rechargeable batteries, restricting their widespread applications. Adding soluble redox-active ingredients into electrolyte is an effective strategy to increase specific energy. However, an ion-selective membrane is generally needed in such supercapacitors to avoid the mixing of anolyte and catholyte, which significantly increases the cost. Here we report a supercapacitor that consists of a modified solid Ti3C2Tx anode and an active catholyte containing Mn2+, where the conversion between soluble Mn2+ and solid MnO2 occurs at the cathode, and the redox of Ti–O with the bonding/de-bonding of H3O+ occurs at the anode. Impressively, this hybrid supercapacitor displays a gratifying specific energy of 43.4 W h kg−1, without using any ion-selective membrane, and excellent cycling stability over 20000 cycles. Moreover, we also demonstrate its superior low-temperature performance even though the electrolyte has been frozen at −70 °C.

Question: What's the cathode?
Although STEM-HAADF imaging has become the most widely used imaging mode in STEM, the strong atomic number dependency of STEM-HAADF contrast also means that the imaging contrast of light atoms is easily submerged by the adjacent heavy atoms. In contrast, STEM-ABF phase-contrast imaging based on wave interference provides better visualization of light elements with the presence of heavy elements, such as oxygen in the SrTiO3 crystal, lithium in LiCoO2 and even hydrogen in metal hydrides. An elegant example was reported by Gu et al., in which aberration-corrected STEM-ABF imaging was applied to directly visualize lithium columns in LiFePO4 and identify an intriguing lithium staging structure in the partially delithiated LixFePO4 (x ≈ 0.5) crystal. Fig. 6(a–c) present the atomic-resolution STEM-ABF images of the LiFePO4 cathode at different charging states. Compared with the pristine and fully-charged structures of LiFePO4 in Fig. 6(a and b), half-delithiated LiFePO4 in Fig. 6(c) demonstrated that part of the lithium remains in the lattice (yellow circles) at every other row of Li-extracted sites (orange circles). This ordered structure contradicted the previous model of LiFePO4/FePO4 two-phase separation, but was analogous to the stage-II phase in some layer compounds, and contributed to minimizing the Li–Li repulsive interaction to stabilize this intermediate phase. This result indicated that a continuous phase transition occurred in partially delithiated LixFePO4 through a metastable phase and helped to understand the high-rate capability of LiFePO4 cathode.

Question: What's the cathode?
Answer:  LiFePO4
Although STEM-HAADF imaging has become the most widely used imaging mode in STEM, the strong atomic number dependency of STEM-HAADF contrast also means that the imaging contrast of light atoms is easily submerged by the adjacent heavy atoms. In contrast, STEM-ABF phase-contrast imaging based on wave interference provides better visualization of light elements with the presence of heavy elements, such as oxygen in the SrTiO3 crystal, lithium in LiCoO2 and even hydrogen in metal hydrides. An elegant example was reported by Gu et al., in which aberration-corrected STEM-ABF imaging was applied to directly visualize lithium columns in LiFePO4 and identify an intriguing lithium staging structure in the partially delithiated LixFePO4 (x ≈ 0.5) crystal. Fig. 6(a–c) present the atomic-resolution STEM-ABF images of the LiFePO4 cathode at different charging states. Compared with the pristine and fully-charged structures of LiFePO4 in Fig. 6(a and b), half-delithiated LiFePO4 in Fig. 6(c) demonstrated that part of the lithium remains in the lattice (yellow circles) at every other row of Li-extracted sites (orange circles). This ordered structure contradicted the previous model of LiFePO4/FePO4 two-phase separation, but was analogous to the stage-II phase in some layer compounds, and contributed to minimizing the Li–Li repulsive interaction to stabilize this intermediate phase. This result indicated that a continuous phase transition occurred in partially delithiated LixFePO4 through a metastable phase and helped to understand the high-rate capability of LiFePO4 cathode.

Question: What's the cathode?
Answer: LiFePO4
By comparing the C 1s, Li 1s, and O 1s XPS spectra and the corresponding area proportions, we found corroborative results. Firstly, a great amount of residual carbonate on the pristine LiNiO2 surface illustrates the continued challenge in avoiding carbonate formation during calcination and quick transferal. There was a clear increase in inorganic carbonates (C 1s, Li 1s, and O 1s XPS spectra) concurrent with the decrease of lattice Li and O proportions after exposure to ambient environment, which was amplified under continuous human exhalation (lattice Li and O are almost negligible on the surface) (Fig. 1). This is indicative of the continuous formation of inorganic carbonates, which strongly attenuates the photoelectron signals from underneath LiNiO2 layers. Our observation is consistent with previous studies suggesting that dense concentration of water and CO2 on the oxide cathode surface facilitates chemical reactions to yield increased amounts of surface inorganic carbonates. Previous studies have shown the surface chemical bonding and structure of NMC materials are readily transformed by certain environments, including CO2, H2O, O2, and other reactive gases in a glove box. These gases react with layered oxides by extracting lithium from the lattice, further generating LiOH and/or Li2CO3 as well as oxygen vacancies. Lithium residuals on NMC and NCA materials also have been confirmed by infrared spectroscopy, Raman spectroscopy, and TEM. It is a consensus that surface instability is one of the common challenges for many battery materials. Other materials, such as sodium-containing layered cathode materials and Li-rich materials, also encounter surface-air instability, which leads to Na/Li residual formation and elevated pH on the particle surfaces. For sodium-containing layered cathode materials, water intercalation or Na+/H+ exchange also takes place, which can disrupt the layered structure. These changes would trigger slurry alkalization, degassing problem, and electrolyte decomposition. Based on our results and previous findings, we recommend that the details about sample handling should be specified in the journal publications, especially those involving delicate surface chemical analysis.

Question: What's the cathode?
Answer: sodium-containing layered