Elevating Energy Density for Sodium-Ion Batteries through Multielectron Reactions

Abstract

It remains a great challenge to explore desirable cathodes for sodium-ion batteries to satisfy the ever-increasing demand for large-scale energy storage systems. In this Letter, we report a NASICON-structured Na4MnCr­(PO4)3 cathode with high specific capacity and operation potential. The reversible access of the Mn2+/Mn3+ (3.75/3.4 V), Mn3+/Mn4+ (4.25/4.1 V), and Cr3+/Cr4+ (4.4/4.3 V vs Na/Na+) redox couples in a Na4MnCr­(PO4)3 cathode endows a distinct three-electron redox reaction during the insertion/extraction process. The highly stable NASICON structure with a small volume variation upon cycling ensures long-time cycling stability (73.3% capacity retention after 500 cycles within the potential region of 2.5–4.6 V). The impedance analysis and interface characterization indicate that the evolution of a cathode electrolyte interphase at high potential is correlated with the capacity fading, while the robustness of the NASICON framework is redemonstrated.

A NASICON‐typed Na4Mn0.5Fe0.5Al(PO4)3 cathode for low‐cost and high‐energy sodium‐ion batteries

“…The lattice parameters a (= b ) and c both changed gradually during Na + insertion and extraction processes, and the overall changes of a and c were 2.8% and 0.6%, respectively. Also, the total unit volume variation (Figure 3e) was calculated as 5.3%, which is smaller than that of common NASICON‐type Na 3 V 2 (PO 4 ) 3 (~8.3%), [ 24 ] Na 3 V 1.5 Cr 0.5 (PO 4 ) 3 (~7.2%), [ 25 ] Na 4 MnCr(PO 4 ) 3 (6.3%), [ 11 ] and Na 3 MnTi(PO 4 ) 3 (~8.1%) [ 26 ] reported for SIBs.…”

Stabilized Multi‐Electron Reactions in a High‐Energy Na4Mn0.9CrMg0.1(PO4)3 Sodium‐Storage Cathode Enabled by the Pinning Effect

“…At the pristine state, the Mn 2p 3/2 peak located at 641.68 eV and the Cr 2p 3/2 peak at 578.54 eV suggests the bivalence of manganese and trivalence of chromium in the as‐prepared Na 4 Mn 0.9 CrMg 0.1 (PO 4 ) 3 /C, respectively. [ 39,40 ] When charged to 3.8 V, the Mn 2p 3/2 peak moved to higher binding energy of 642.38 eV, indicating the oxidation of Mn 2+ to Mn 3+ . In contrast, the peak position of Cr 2p 3/2 remained unchanged, implying that Cr still maintains trivalent at this voltage.…”

“…Upon further charging to 4.3 V, the Mn 2p 3/2 peak shifted to 642.80 eV, manifesting the full oxidation to Mn 4+ , then the peak remained at the same position even when charged to 4.5 V. [ 27,43 ] In the case of the Cr 2p 3/2 peak, it slightly shifted to 578.87 eV upon charging to 4.3 V and further moved to 579.22 eV at the end of the charge process, suggesting the gradual transformation from Cr 3+ to Cr 4+ . [ 39 ] During the subsequent discharge process, both Mn 2p and Cr 2p peaks restored to the initial states, reflecting the high reversibility of the electrochemical reactions. The above results demonstrate that Mn 2+ /Mn 3+ , Mn 3+ /Mn 4+ , and Cr 3+ /Cr 4+ redox couples jointly contribute to the high energy density of Na 4 Mn 0.9 CrMg 0.1 (PO 4 ) 3 , coinciding well with the CV observations.…”

“…The fascinating rate performance is further highlighted in the inset of Figure 2c, which demonstrates great superiority over those of the undoped Na 4 MnCr(PO 4 ) 3 and other previously reported Mn-based NASICON-type phosphate cathodes for SIBs. [25,27,31,[38][39][40][41][42] Such an impressive rate of performance is attributed to the combination of enhanced electronic conductivity due to Mg 2+ doping and the conductive network formed by in situ carbon coating (EIS spectra in Figure S16, Supporting Information). It is worth mentioning that the specific capacity in this study is calculated based on the mass of Na 4 Mn 0.9 CrMg 0.1 (PO 4 ) 3 because the carbon matrix contributes negligible capacity to the electrode (Figure S17, Supporting Information).…”

The prospect and challenges of sodium‐ion batteries for low‐temperature conditions

“…[105] Cationic substitution is another effective method to optimize the electrochemical performance of NASICON-type polyanionic compounds. [106][107][108] For example, Na 3 VCr(PO 4 ) 3 (NVCP) cathode was prepared by replacing V 3+ with Cr 3+ , and a 1.5-electron transfer reaction was achieved during cycling. [109] Furthermore, because the irreversibility of the phase transition was largely suppressed at low temperature, NVCP exhibited excellent cycling stability with capacity retention of 92% after 200 cycles at 0.5 C under −15°C.…”