Investigation of Na<sub>x</sub>Mg<sub>x/2</sub>Mn<sub>1-x/2</sub>O<sub>2</sub> (1.0 > x > 1/3) for Na-Ion Battery Positive Electrodes
Bibliographic record
Abstract
Introduction Manganese-based layered oxide materials are promising positive electrode for sodium ion batteries. It has been shown that the partial substitution of Mn with Mg can smooth the voltage curve and improve the cycling stability.1 Mg substitution can also induce the activation of the redox reaction of oxide ions.2 In this study, the NaxMgx/2Mn1-x/2O2 (1.0 ≥ x ≥ 1/3) series of layered oxides were synthesized and characterized in Na cells. Experimental Stoichiometric amounts of MgO, MnO and Na2CO3 were combined by ball milling, then heated at 900 ºC in air to obtain samples of NaxMgx/2Mn1-x/2O2. Electrodes were made with active material, PVDF binder, carbon black at a ratio of 8:1:1 and dried under vacuum at 120 ℃ overnight. 1M NaPF6 in a solution of EC, DEC, FEC (volume ratio 3:6:1) was used as electrolyte and Na foil was used as counter/reference electrode. X-ray diffraction (XRD) patterns were measured with a Rigaku Ultima IV X-ray diffractometer equipped with a Cu anode X-ray tube and a diffracted beam monochromator. Results and discussion Figure 1 shows the XRD patterns of NaxMgx/2Mn1-x/2O2 (1.0 ≥ x ≥ 1/3). All the samples were found to be P2-dominant, even when x approachs 1. The sample with x = 2/3 has the highest P2 phase purity. The further x deviates from 2/3, the more impurities are present in the XRD patterns. Figure 2 shows the voltage curve of Na2/3Mg1/3Mn2/3O2. Na2/3Mg1/3Mn2/3O2 was found to have a reversible capacity of ~250 mAh/g (corresponding to a theoretical energy density of approximately 520 Wh/kg) with 80% capacity retention after 20 cycles. The voltage curve exhibits a plateau at > 4 V during the first charge, and a slope between 1.5 V and 3.5 V during discharge. The loss of capacity of the 4 V plateau during cycling indicates irreversible structure changes upon first charging, possibly due to oxygen loss. After about 10 cycles, the 4 V plateau becomes stabilized and the material cycles reversibly. The voltage curve of Na2/3Mg1/3Mn2/3O2 is consistent with the voltage curve of Li2MnO3, wherein all the Mn is also in the 4+ oxidation state. This result is the first report of the reversible sodium extraction from layered manganese oxide materials where all the Mn exist as Mn4+. References 1. J. Billaud et al., Energy Environ. Sci., 7, 1387-1391 (2014) 2. N. Yabuuchi et al., J. Mater. Chem. A, 2, 16851-16855 (2014) Figure 1
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.001 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".