Sodium‐ion batteries have gained much attention for their potential application in large‐scale stationary energy storage due to the low cost and abundant sodium sources in the earth. However, the electrochemical performance of sodium‐ion full cells (SIFCs) suffers severely from the irreversible consumption of sodium ions of cathode during the solid electrolyte interphase (SEI) formation of hard carbon anode. Here, a high‐efficiency cathode sodiation compensation reagent, sodium oxalate (Na2C2O4), which possesses both a high theoretical capacity of 400 mA h g−1 and a capacity utilization as high as 99%, is proposed. The implementation of Na2C2O4 as sacrificial sodium species is successfully realized by decreasing its oxidation potential from 4.41 to 3.97 V through tuning conductive additives with different physicochemical features, and the corresponding mechanism of oxidation potential manipulation is analyzed. Electrochemical results show that in the full cell based on a hard carbon anode and a P2‐Na2/3Ni1/3Mn1/3Ti1/3O2 cathode with Na2C2O4 as a sodium reservoir to compensate for sodium loss during SEI formation, the capacity retention is increased from 63% to 85% after 200 cycles and the energy density is improved from 129.2 to 172.6 W h kg−1. This work can provide a new avenue for accelerating the development of SIFCs.

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Na-ion cathode materials operating at high voltage with a stable cycling behavior are needed to develop future high-energy Na-ion cells. However, the irreversible oxygen redox reaction at the high-voltage region in sodium layered cathode materials generates structural instability and poor capacity retention upon cycling. Here, we report a doping strategy by incorporating light-weight boron into the cathode active material lattice to decrease the irreversible oxygen oxidation at high voltages (i.e., >4.0 V vs. Na+/Na). The presence of covalent B–O bonds and the negative charges of the oxygen atoms ensures a robust ligand framework for the NaLi1/9Ni2/9Fe2/9Mn4/9O2 cathode material while mitigating the excessive oxidation of oxygen for charge compensation and avoiding irreversible structural changes during cell operation. The B-doped cathode material promotes reversible transition metal redox reaction enabling a room-temperature capacity of 160.5 mAh g−1 at 25 mA g−1 and capacity retention of 82.8% after 200 cycles at 250 mA g−1. A 71.28 mAh single-coated lab-scale Na-ion pouch cell comprising a pre-sodiated hard carbon-based anode and B-doped cathode material is also reported as proof of concept.

ion batteries (LIBs) by virtue of their similar charge storage mechanism, as well as the abundance and low cost of Na resources. [1][2][3][4] Compared with lithium ions, sodium ions have a larger radius and heavier mass (1.02 vs. 0.76 Å and ≈23 vs. ≈6.9 g mol −1 ), which makes SIBs compete unfavorably in terms of energy density with LIBs, thus limiting their applications in portable electronics and electric vehicles. However, SIBs show great promise in the applications where cost and sustainability are top priority, such as large-scale energy storage. [5][6][7][8][9] Like LIBs, cathode materials are also the main factor limiting the energy density and cost of SIBs. Finding suitable sodium intercalation hosts is pressing. Until now many types of materials have been explored, including layered transition metal oxides, polyanionic compounds, Prussian blue-based compounds, and organic compounds. [10][11][12] Among these candidates, layered transition metal oxides Na x TMO 2 (TM referring to transition metals) is extremely promising on account of its simple structure, high compositional diversity, easy synthesis and attractive electrochemical performance. According to the oxygen stacking sequence and the Na + coordination environment, Layered transition metal oxide P2-Na 2/3 Ni 1/3 Mn 2/3 O 2 usually suffers from largevolume phase transitions and different Na-vacancy ordering during sodium (de)intercalation, incurring rapid capacity decline and poor rate capability. Herein, an effective strategy based on synergetic effect of selected multiple metal ions is designed for P2-type cathodes with improved performance. The role of tetravalent titanium provides high redox potential, inactive divalent magnesium stabilizes the structure, and the monovalent lithium smooths the electrochemical curves. The combined analysis of in operando X-ray diffraction, in operando X-ray absorption spectroscopy and density functional theory calculations demonstrates the contribution of multi-metal ions converts the unfavorable and large-volume P2 to O2 transition into a moderate "Z"-intergrowth structure by increasing the energy barrier of transition metal slab gliding. As a consequence, the resultant P2-Na 0.7 Li 0.03 Mg 0.03 Ni 0.27 Mn 0.6 Ti 0.07 O 2 electrode delivers a reversible capacity of 134 mAh g −1 , a working voltage of 3.57 V, excellent cycling stability (82% of capacity retention after 200 cycles), and superior rate performance (110 mAh g −1 at 4 C). Full cells fabricated with a hard carbon anode achieve an energy density of 296 Wh kg −1 . This study presents a route to rationally design cathode materials with this functionalization to improve the cell performance for sodium-ion batteries.

Sulfide-based solid-state electrolytes (SSEs) matched with alloy anodes are considered as promising candidates for application in all-solid-state batteries (ASSBs) to overcome the bottlenecks of the lithium (Li) anode. However, an understanding of the dynamic electrochemical processes on alloy anode in SSE is still elusive. Herein, in situ atomic force microscopy gives insights into the block-formation and stack-accumulation behaviors of Li precipitation on an Li electrode, uncovering the morphological evolution of nanoscale Li deposition/dissolution in ASSBs. Furthermore, two-dimensional Li–indium (In) alloy lamellae and the homogeneous solid electrolyte interphase (SEI) shell on the In electrode reveal the precipitation mechanism microscopically regulated by the alloy anode. The flexible and wrinkle-structure SEI shell further enables the electrode protection and inner Li accommodation upon cycles, elucidating the functional influences of SEI shell on the cycling behaviors. Such on-site tracking of the morphological evolution and dynamic mechanism provide an in-depth understanding and thus benefit the optimizations of alloy-based ASSBs.

As one of the fascinating high capacity cathodes, O3-type layered oxides usually suffer from their intrinsic air sensitivity and sluggish kinetics originating from the spontaneous lattice Na extraction during air exposure and high tetrahedral site energy of Na + diffusion transition state. What is worse, the improvement on the two handicaps is hard to simultaneously realize because of the contradiction between Na containment suggested in air stability mechanism and enhanced Na diffusion mentioned in kinetics strategy. Herein, it is shown that a simple strategy of introducing proper Na vacancies into lattice can simultaneously realize a dual performance improvement. Na vacancies decrease the charge density on transitional metal ions and enhance the antioxidative capability of material, ensuring a stable lattice Na containment for Na 0.93 Li 0.12 Ni 0.25 Fe 0.15 Mn 0.48 O 2 when exposed to air. Additionally, more Na + diffusional sites and enlarged Na layer spacing are obtained and result in a significantly decreased energy barrier from ≈1000 to 300 meV and a high rate capability of 70.8% retention at 2000 mA g −1 .Remarkably, such a strategy can be easily realized by either pre-or posttreating, which exhibits excellent universality for various O3 materials, implying its enormous potential to promote the commercial application of O3-type cathodes.

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